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+The Project Gutenberg EBook of Darwinism (1889), by Alfred Russel Wallace
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Darwinism (1889)
+
+Author: Alfred Russel Wallace
+
+Release Date: January 2, 2005 [EBook #14558]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK DARWINISM (1889) ***
+
+
+
+
+Produced by StevenGibbs and the Online Distributed Proofreading Team
+
+
+
+
+
+    DARWINISM
+
+    AN EXPOSITION OF THE
+
+    THEORY OF NATURAL SELECTION
+
+    WITH SOME OF ITS APPLICATIONS
+
+    BY
+
+    ALFRED RUSSEL WALLACE
+
+    LL.D., F.L.S., ETC.
+
+
+    WITH A PORTRAIT OF THE AUTHOR, MAP AND ILLUSTRATIONS
+
+    MACMILLAN AND CO.
+    LONDON AND NEW YORK
+    [Second Edition] 1889
+
+
+       *       *       *       *       *
+
+[Illustration: Alfred R. Wallace]
+
+       *       *       *       *       *
+
+
+
+
+
+
+PREFACE TO SECOND EDITION
+
+
+The present edition is a reprint of the first, with a few verbal
+corrections and the alteration of some erroneous or doubtful statements.
+Of these latter the following are the most important:--
+
+_P._ 30. The statement as to the fulmar petrel, which Professor A.
+Newton assures me is erroneous, has been modified.
+
+_P._ 34. A note is added as to Darwin's statement about the missel and
+song-thrushes in Scotland.
+
+_P._ 172. An error as to the differently-coloured herds of cattle in the
+Falkland Islands, is corrected.
+
+
+    PARKSTONE, DORSET
+    _August, 1889_.
+
+
+
+
+
+
+
+PREFACE TO FIRST EDITION
+
+
+The present work treats the problem of the Origin of Species on the same
+general lines as were adopted by Darwin; but from the standpoint reached
+after nearly thirty years of discussion, with an abundance of new facts
+and the advocacy of many new or old theories.
+
+While not attempting to deal, even in outline, with the vast subject of
+evolution in general, an endeavour has been made to give such an account
+of the theory of Natural Selection as may enable any intelligent reader
+to obtain a clear conception of Darwin's work, and to understand
+something of the power and range of his great principle.
+
+Darwin wrote for a generation which had not accepted evolution, and
+which poured contempt on those who upheld the derivation of species from
+species by any natural law of descent. He did his work so well that
+"descent with modification" is now universally accepted as the order of
+nature in the organic world; and the rising generation of naturalists
+can hardly realise the novelty of this idea, or that their fathers
+considered it a scientific heresy to be condemned rather than seriously
+discussed.
+
+The objections now made to Darwin's theory apply, solely, to the
+particular means by which the change of species has been brought about,
+not to the fact of that change. The objectors seek to minimise the
+agency of natural selection and to subordinate it to laws of variation,
+of use and disuse, of intelligence, and of heredity. These views and
+objections are urged with much force and more confidence, and for the
+most part by the modern school of laboratory naturalists, to whom the
+peculiarities and distinctions of species, as such, their distribution
+and their affinities, have little interest as compared with the problems
+of histology and embryology, of physiology and morphology. Their work in
+these departments is of the greatest interest and of the highest
+importance, but it is not the kind of work which, by itself, enables one
+to form a sound judgment on the questions involved in the action of the
+law of natural selection. These rest mainly on the external and vital
+relations of species to species in a state of nature--on what has been
+well termed by Semper the "physiology of organisms," rather than on the
+anatomy or physiology of organs.
+
+       *       *       *       *       *
+
+It has always been considered a weakness in Darwin's work that he based
+his theory, primarily, on the evidence of variation in domesticated
+animals and cultivated plants. I have endeavoured to secure a firm
+foundation for the theory in the variations of organisms in a state of
+nature; and as the exact amount and precise character of these
+variations is of paramount importance in the numerous problems that
+arise when we apply the theory to explain the facts of nature, I have
+endeavoured, by means of a series of diagrams, to exhibit to the eye the
+actual variations as they are found to exist in a sufficient number of
+species. By doing this, not only does the reader obtain a better and
+more precise idea of variation than can be given by any number of
+tabular statements or cases of extreme individual variation, but we
+obtain a basis of fact by which to test the statements and objections
+usually put forth on the subject of specific variability; and it will be
+found that, throughout the work, I have frequently to appeal to these
+diagrams and the facts they illustrate, just as Darwin was accustomed to
+appeal to the facts of variation among dogs and pigeons.
+
+I have also made what appears to me an important change in the
+arrangement of the subject. Instead of treating first the comparatively
+difficult and unfamiliar details of variation, I commence with the
+Struggle for Existence, which is really the fundamental phenomenon on
+which natural selection depends, while the particular facts which
+illustrate it are comparatively familiar and very interesting. It has
+the further advantage that, after discussing variation and the effects
+of artificial selection, we proceed at once to explain how natural
+selection acts.
+
+Among the subjects of novelty or interest discussed in this volume, and
+which have important bearings on the theory of natural selection, are:
+(1) A proof that all _specific_ characters are (or once have been)
+either useful in themselves or correlated with useful characters (Chap.
+VI); (2) a proof that natural selection can, in certain cases, increase
+the sterility of crosses (Chap. VII); (3) a fuller discussion of the
+colour relations of animals, with additional facts and arguments on the
+origin of sexual differences of colour (Chaps. VIII-X); (4) an attempted
+solution of the difficulty presented by the occurrence of both very
+simple and very complex modes of securing the cross-fertilisation of
+plants (Chap. XI); (5) some fresh facts and arguments on the
+wind-carriage of seeds, and its bearing on the wide dispersal of many
+arctic and alpine plants (Chap. XII); (6) some new illustrations of the
+non-heredity of acquired characters, and a proof that the effects of use
+and disuse, even if inherited, must be overpowered by natural selection
+(Chap. XIV); and (7) a new argument as to the nature and origin of the
+moral and intellectual faculties of man (Chap. XV).
+
+       *       *       *       *       *
+
+Although I maintain, and even enforce, my differences from some of
+Darwin's views, my whole work tends forcibly to illustrate the
+overwhelming importance of Natural Selection over all other agencies in
+the production of new species. I thus take up Darwin's earlier
+position, from which he somewhat receded in the later editions of his
+works, on account of criticisms and objections which I have endeavoured
+to show are unsound. Even in rejecting that phase of sexual selection
+depending on female choice, I insist on the greater efficacy of natural
+selection. This is pre-eminently the Darwinian doctrine, and I therefore
+claim for my book the position of being the advocate of pure Darwinism.
+
+I wish to express my obligation to Mr. Francis Darwin for lending me
+some of his father's unused notes, and to many other friends for facts
+or information, which have, I believe, been acknowledged either in the
+text or footnotes. Mr. James Sime has kindly read over the proofs and
+given me many useful suggestions; and I have to thank Professor Meldola,
+Mr. Hemsley, and Mr. E.B. Poulton for valuable notes or corrections in
+the later chapters in which their special subjects are touched upon.
+
+GODALMING, _March 1889_.
+
+
+
+
+CONTENTS
+
+
+CHAPTER I
+
+WHAT ARE "SPECIES" AND WHAT IS MEANT BY THEIR "ORIGIN"
+
+
+    Definition of species--Special creation--The early
+    transmutationists--Scientific opinion before Darwin--The problem
+    before Darwin--The change of opinion effected by Darwin--The
+    Darwinian theory--Proposed mode of treatment of the subject
+
+
+
+CHAPTER II
+
+THE STRUGGLE FOR EXISTENCE
+
+
+    Its importance--The struggle among plants--Among
+    animals--Illustrative cases--Succession of trees in forests of
+    Denmark--The struggle for existence on the Pampas--Increase of
+    organisms in a geometrical ratio--Examples of rapid increase of
+    animals--Rapid increase and wide spread of plants--Great
+    fertility not essential to rapid increase--Struggle between
+    closely allied species most severe--The ethical aspect of the
+    struggle for existence
+
+
+
+CHAPTER III
+
+THE VARIABILITY OF SPECIES IN A STATE OF NATURE
+
+
+   Importance of variability--Popular ideas regarding
+   it--Variability of the lower animals--The variability of
+   insects--Variation among lizards--Variation among
+   birds--Diagrams of bird-variation--Number of varying
+   individuals--Variation in the mammalia--Variation in internal
+   organs--Variations in the skull--Variations in the habits of
+   animals--The variability of plants--Species which vary
+   little--Concluding remarks
+
+
+
+CHAPTER IV
+
+VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS
+
+
+    The facts of variation and artificial selection--Proofs of the
+    generality of variation--Variations of apples and
+    melons--Variations of flowers--Variations of domestic
+    animals--Domestic pigeons--Acclimatisation--Circumstances
+    favourable to selection by man--Conditions favourable to
+    variation--Concluding remarks
+
+
+
+CHAPTER V
+
+NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST
+
+
+    Effect of struggle for existence under unchanged conditions--The
+    effect under change of conditions--Divergence of character--In
+    insects--In birds--In mammalia--Divergence leads to a maximum of
+    life in each area--Closely allied species inhabit distinct
+    areas--Adaptation to conditions at various periods of life--The
+    continued existence of low forms of life--Extinction of low
+    types among the higher animals--Circumstances favourable to the
+    origin of new species--Probable origin of the dippers--The
+    importance of isolation--On the advance of organisation by
+    natural selection--Summary of the first five chapters
+
+
+
+CHAPTER VI
+
+DIFFICULTIES AND OBJECTIONS
+
+
+    Difficulty as to smallness of variations--As to the right
+    variations occurring when required--The beginnings of important
+    organs--The mammary glands--The eyes of flatfish--Origin of the
+    eye--Useless or non-adaptive characters--Recent extension of the
+    region of utility in plants--The same in animals--Uses of
+    tails--Of the horns of deer--Of the scale-ornamentation of
+    reptiles--Instability of non-adaptive characters--Delboeuf's
+    law--No "specific" character proved to be useless--The swamping
+    effects of intercrossing--Isolation as preventing
+    intercrossing--Gulick on the effects of isolation--Cases in
+    which isolation is ineffective
+
+
+
+CHAPTER VII
+
+ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
+STERILITY OF THEIR HYBRID OFFSPRING
+
+
+    Statement of the problem--Extreme susceptibility of the
+    reproductive functions--Reciprocal crosses--Individual
+    differences in respect to cross-fertilisation--Dimorphism and
+    trimorphism among plants--Cases of the fertility of hybrids and
+    of the infertility of mongrels--The effects of close
+    interbreeding--Mr. Huth's objections--Fertile hybrids among
+    animals--Fertility of hybrids among plants--Cases of sterility
+    of mongrels--Parallelism between crossing and change of
+    conditions--Remarks on the facts of hybridity--Sterility due to
+    changed conditions and usually correlated with other
+    characters--Correlation of colour with constitutional
+    peculiarities--The isolation of varieties by selective
+    association--The influence of natural selection upon sterility
+    and fertility--Physiological selection--Summary and concluding
+    remarks
+
+
+
+CHAPTER VIII
+
+THE ORIGIN AND USES OF COLOUR IN ANIMALS
+
+
+    The Darwinian theory threw new light on organic colour--The
+    problem to be solved--The constancy of animal colour indicates
+    utility--Colour and environment--Arctic animals
+    white--Exceptions prove the rule--Desert, forest, nocturnal, and
+    oceanic animals--General theories of animal colour--Variable
+    protective colouring--Mr. Poulton's experiments--Special or
+    local colour adaptations--Imitation of particular objects--How
+    they have been produced--Special protective colouring of
+    butterflies--Protective resemblance among marine
+    animals--Protection by terrifying enemies--Alluring
+    coloration--The coloration of birds' eggs--Colour as a means of
+    recognition--Summary of the preceding exposition--Influence of
+    locality or of climate on colour--Concluding remarks
+
+
+
+CHAPTER IX
+
+WARNING COLORATION AND MIMICRY
+
+
+    The skunk as an example of warning coloration--Warning colours
+    among insects--Butterflies--Caterpillars--Mimicry--How mimicry
+    has been produced--Heliconidae--Perfection of the
+    imitation--Other cases of mimicry among Lepidoptera--Mimicry
+    among protected groups--Its explanation--Extension of the
+    principle--Mimicry in other orders of insects--Mimicry among the
+    vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among
+    birds--Objections to the theory of mimicry--Concluding remarks
+    on warning colours and mimicry
+
+
+
+CHAPTER X
+
+COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
+
+
+    Sex colours in the mollusca and crustacea--In insects--In
+    butterflies and moths--Probable causes of these colours--Sexual
+    selection as a supposed cause--Sexual coloration of birds--Cause
+    of dull colours of female birds--Relation of sex colour to
+    nesting habits--Sexual colours of other vertebrates--Sexual
+    selection by the struggles of males--Sexual characters due to
+    natural selection--Decorative plumage of males and its effect on
+    the females--Display of decorative plumage by the males--A
+    theory of animal coloration--The origin of accessory
+    plumes--Development of accessory plumes and their display--The
+    effect of female preference will be neutralised by natural
+    selection--General laws of animal coloration--Concluding remarks
+
+
+
+CHAPTER XI
+
+THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE
+
+
+    The general colour relations of plants--Colours of fruits--The
+    meaning of nuts--Edible or attractive fruits--The colours of
+    flowers--Modes of securing cross-fertilisation--The
+    interpretation of the facts--Summary of additional facts
+    bearing on insect fertilisation--Fertilisation of flowers by
+    birds--Self-fertilisation of flowers--Difficulties and
+    contradictions--Intercrossing not necessarily
+    advantageous--Supposed evil results of close interbreeding--How
+    the struggle for existence acts among flowers--Flowers the
+    product of insect agency--Concluding remarks on colour in nature
+
+
+
+CHAPTER XII
+
+THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
+
+
+    The facts to be explained--The conditions which have determined
+    distribution--The permanence of oceans--Oceanic and continental
+    areas--Madagascar and New Zealand--The teachings of the
+    thousand-fathom line--The distribution of marsupials--The
+    distribution of tapirs--Powers of dispersal as illustrated by
+    insular organisms--Birds and insects at sea--Insects at great
+    altitudes--The dispersal of plants--Dispersal of seeds by the
+    wind--Mineral matter carried by the wind--Objections to the
+    theory of wind-dispersal answered--Explanation of north
+    temperate plants in the southern hemisphere--No proof of
+    glaciation in the tropics--Lower temperature not needed to
+    explain the facts--Concluding remarks
+
+
+
+CHAPTER XIII
+
+THE GEOLOGICAL EVIDENCES OF EVOLUTION
+
+
+    What we may expect--The number of known species of extinct
+    animals--Causes of the imperfection of the geological
+    record--Geological evidences of
+    evolution--Shells--Crocodiles--The rhinoceros tribe--The
+    pedigree of the horse tribe--Development of deer's horns--Brain
+    development--Local relations of fossil and living animals--Cause
+    of extinction of large animals--Indications of general progress
+    in plants and animals--The progressive development of
+    plants--Possible cause of sudden late appearance of
+    exogens--Geological distribution of insects--Geological
+    succession of vertebrata--Concluding remarks
+
+
+
+CHAPTER XIV
+
+FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
+
+
+    Fundamental difficulties and objections--Mr. Herbert Spencer's
+    factors of organic evolution--Disuse and effects of withdrawal
+    of natural selection--Supposed effects of disuse among wild
+    animals--Difficulty as to co-adaptation of parts by variation
+    and selection--Direct action of the environment--The American
+    school of evolutionists--Origin of the feet of the
+    ungulates--Supposed action of animal intelligence--Semper on the
+    direct influence of the environment--Professor Geddes's theory
+    of variation in plants--Objections to the theory--On the origin
+    of spines--Variation and selection overpower the effects of use
+    and disuse--Supposed action of the environment in imitating
+    variations--Weismann's theory of heredity--The cause of
+    variation--The non-heredity of acquired characters--The theory
+    of instinct--Concluding remarks
+
+
+
+CHAPTER XV
+
+DARWINISM APPLIED TO MAN
+
+
+    General identity of human and animal structure--Rudiments and
+    variations showing relation of man to other mammals--The
+    embryonic development of man and other mammalia--Diseases common
+    to man and the lower animals--The animals most nearly allied to
+    man--The brains of man and apes--External differences of man and
+    apes--Summary of the animal characteristics of man--The
+    geological antiquity of man--The probable birthplace of man--The
+    origin of the moral and intellectual nature of man--The argument
+    from continuity--The origin of the mathematical faculty--The
+    origin of the musical and artistic faculties--Independent proof
+    that these faculties have not been developed by natural
+    selection--The interpretation of the facts--Concluding remarks
+
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+
+    PORTRAIT OF AUTHOR
+    MAP SHOWING THE 1000-FATHOM LINE
+    1.  DIAGRAM OF VARIATIONS OF LACERTA MURALIS
+    2.     "       VARIATION OF LIZARDS
+    3.     "       VARIATION OF WINGS AND TAIL OF BIRDS
+    4.     "       VARIATION OF DOLICHONYX ORYZIVORUS
+    5.     "       VARIATION OF AGELAEUS PHOENICEUS
+    6.     "       VARIATION OF CARDINALIS VIRGINIANUS
+    7.     "       VARIATION OF TARSUS AND TOES
+    8.     "       VARIATION OF BIRDS IN LEYDEN MUSEUM
+    9.     "       VARIATION OF ICTERUS BALTIMORE
+    10.    "       VARIATION OF AGELAEUS PHOENICEUS
+    11.    "       CURVES OF VARIATION
+    12.    "       VARIATION OF CARDINALIS VIRGINIANUS
+    13.    "       VARIATION OF SCIURUS CAROLINENSIS
+    14.    "       VARIATION OF SKULLS OF WOLF
+    15.    "       VARIATION OF SKULLS OF URSUS LABIATUS
+    16.    "       VARIATION OF SKULLS OF SUS CRISTATUS
+    17. PRIMULA VERIS (Cowslip). From Darwin's _Forms of Flowers_
+    18. GAZELLA SOEMMERRINGI (to show recognition marks)
+    19. RECOGNITION MARKS OF AFRICAN PLOVERS
+          (from Seebohm's _Charadriadae_)
+    20. RECOGNITION OF OEDICNEMUS VERMICULATUS AND OE. SENEGALENSIS
+          (from Seebohm's _Charadriadae_)
+    21. RECOGNITION OF CURSORIUS CHALCOPTERUS AND C. GALLICUS
+          (from Seebohm's _Charadriadae_)
+    22. RECOGNITION OF SCOLOPAX MEGALA AND S. STENURA
+          (from Seebohm's _Charadriadae_)
+    23. METHONA PSIDII AND LEPTALIS ORISE
+    24. OPTHALMIS LINCEA AND ARTAXA SIMULANS
+          (from the Official _Narrative of the Voyage of the Challenger_)
+    25. WINGS OF ITUNA ILIONE AND THYRIDIA MEGISTO
+          (from _Proceedings of the Entomological Society_)
+    26. MYGNIMIA AVICULUS AND COLOBORHOMBUS FASCIATIPENNIS
+    27. MIMICKING INSECTS FROM THE PHILIPPINES
+          (from Semper's _Animal Life_)
+    28. MALVA SYLVESTRIS AND M. ROTUNDIFOLIA
+          (from Lubbock's _British Wild Flowers in Relation to Insects_)
+    29. LYTHRUM SALICARIA, THREE FORMS OF
+          (from Lubbock's _British Wild Flowers in Relation to Insects_)
+    30. ORCHIS PYRAMIDALIS (from Darwin's _Fertilisation of Orchids_)
+    31. HUMMING-BIRD FERTILISING MARCGRAVIA NEPENTHOIDES
+    32. DIAGRAM OF MEAN HEIGHT OF LAND AND DEPTH OF OCEANS
+    33. GEOLOGICAL DEVELOPMENT OF THE HORSE TRIBE
+          (from Huxley's _American Addresses_)
+    34. DIAGRAM ILLUSTRATING THE GEOLOGICAL DISTRIBUTION OF PLANTS
+          (from Ward's _Sketch of Palaeobotany_)
+    35. TRANSFORMATION OF ARTEMIA SALINA TO A. MILHAUSENII
+          (from Semper's _Animal Life_)
+    36. BRANCHIPUS STAGNALIS AND ARTEMIA SALINA
+          (from Semper's _Animal Life_)
+    37. CHIMPANZEE (TROGLODYTES NIGER)
+
+
+
+
+
+CHAPTER I
+
+WHAT ARE "SPECIES," AND WHAT IS MEANT BY THEIR "ORIGIN"
+
+
+    Definition of species--Special creation--The early
+    Transmutationists--Scientific opinion before Darwin--The problem
+    before Darwin--The change of opinion effected by Darwin--The
+    Darwinian theory--Proposed mode of treatment of the subject.
+
+
+
+The title of Mr. Darwin's great work is--_On the Origin of Species by
+means of Natural Selection and the Preservation of Favoured Races in the
+Struggle for Life_. In order to appreciate fully the aim and object of
+this work, and the change which it has effected not only in natural
+history but in many other sciences, it is necessary to form a clear
+conception of the meaning of the term "species," to know what was the
+general belief regarding them at the time when Mr. Darwin's book first
+appeared, and to understand what he meant, and what was generally meant,
+by discovering their "origin." It is for want of this preliminary
+knowledge that the majority of educated persons who are not naturalists
+are so ready to accept the innumerable objections, criticisms, and
+difficulties of its opponents as proofs that the Darwinian theory is
+unsound, while it also renders them unable to appreciate, or even to
+comprehend, the vast change which that theory has effected in the whole
+mass of thought and opinion on the great question of evolution.
+
+The term "species" was thus defined by the celebrated botanist De
+Candolle: "A species is a collection of all the individuals which
+resemble each other more than they resemble anything else, which can by
+mutual fecundation produce fertile individuals, and which reproduce
+themselves by generation, in such a manner that we may from analogy
+suppose them all to have sprung from one single individual." And the
+zoologist Swainson gives a somewhat similar definition: "A species, in
+the usual acceptation of the term, is an animal which, in a state of
+nature, is distinguished by certain peculiarities of form, size, colour,
+or other circumstances, from another animal. It propagates, 'after its
+kind,' individuals perfectly resembling the parent; its peculiarities,
+therefore, are permanent."[1]
+
+To illustrate these definitions we will take two common English birds,
+the rook (Corvus frugilegus) and the crow (Corvus corone). These are
+distinct _species_, because, in the first place, they always differ from
+each other in certain slight peculiarities of structure, form, and
+habits, and, in the second place, because rooks always produce rooks,
+and crows produce crows, and they do not interbreed. It was therefore
+concluded that all the rooks in the world had descended from a single
+pair of rooks, and the crows in like manner from a single pair of crows,
+while it was considered impossible that crows could have descended from
+rooks or _vice versā_. The "origin" of the first pair of each kind was a
+mystery. Similar remarks may be applied to our two common plants, the
+sweet violet (Viola odorata) and the dog violet (Viola canina). These
+also produce their like and never produce each other or intermingle, and
+they were therefore each supposed to have sprung from a single
+individual whose "origin" was unknown. But besides the crow and the rook
+there are about thirty other kinds of birds in various parts of the
+world, all so much like our species that they receive the common name of
+crows; and some of them differ less from each other than does our crow
+from our rook. These are all _species_ of the genus Corvus, and were
+therefore believed to have been always as distinct as they are now,
+neither more nor less, and to have each descended from one pair of
+ancestral crows of the same identical species, which themselves had an
+unknown "origin." Of violets there are more than a hundred different
+kinds in various parts of the world, all differing very slightly from
+each other and forming distinct _species_ of the genus Viola. But, as
+these also each produce their like and do not intermingle, it was
+believed that every one of them had always been as distinct from all the
+others as it is now, that all the individuals of each kind had descended
+from one ancestor, but that the "origin" of these hundred slightly
+differing ancestors was unknown. In the words of Sir John Herschel,
+quoted by Mr. Darwin, the origin of such species was "the mystery of
+mysteries."
+
+
+_The Early Transmutationists_.
+
+A few great naturalists, struck by the very slight difference between
+many of these species, and the numerous links that exist between the
+most different forms of animals and plants, and also observing that a
+great many species do vary considerably in their forms, colours, and
+habits, conceived the idea that they might be all produced one from the
+other. The most eminent of these writers was a great French naturalist,
+Lamarck, who published an elaborate work, the _Philosophie Zoologique_,
+in which he endeavoured to prove that all animals whatever are descended
+from other species of animals. He attributed the change of species
+chiefly to the effect of changes in the conditions of life--such as
+climate, food, etc.--and especially to the desires and efforts of the
+animals themselves to improve their condition, leading to a modification
+of form or size in certain parts, owing to the well-known physiological
+law that all organs are strengthened by constant use, while they are
+weakened or even completely lost by disuse. The arguments of Lamarck did
+not, however, satisfy naturalists, and though a few adopted the view
+that closely allied species had descended from each other, the general
+belief of the educated public was, that each species was a "special
+creation" quite independent of all others; while the great body of
+naturalists equally held, that the change from one species to another by
+any known law or cause was impossible, and that the "origin of species"
+was an unsolved and probably insoluble problem. The only other important
+work dealing with the question was the celebrated _Vestiges of
+Creation_, published anonymously, but now acknowledged to have been
+written by the late Robert Chambers. In this work the action of general
+laws was traced throughout the universe as a system of growth and
+development, and it was argued that the various species of animals and
+plants had been produced in orderly succession from each other by the
+action of unknown laws of development aided by the action of external
+conditions. Although this work had a considerable effect in influencing
+public opinion as to the extreme improbability of the doctrine of the
+independent "special creation" of each species, it had little effect
+upon naturalists, because it made no attempt to grapple with the problem
+in detail, or to show in any single case how the allied species of a
+genus could have arisen, and have preserved their numerous slight and
+apparently purposeless differences from each other. No clue whatever was
+afforded to a law which should produce from any one species one or more
+slightly differing but yet permanently distinct species, nor was any
+reason given why such slight yet constant differences should exist at
+all.
+
+
+_Scientific Opinion before Darwin._
+
+In order to show how little effect these writers had upon the public
+mind, I will quote a few passages from the writings of Sir Charles
+Lyell, as representing the opinions of the most advanced thinkers in the
+period immediately preceding that of Darwin's work. When recapitulating
+the facts and arguments in favour of the invariability and permanence of
+species, he says: "The entire variation from the original type which any
+given kind of change can produce may usually be effected in a brief
+period of time, after which no further deviation can be obtained by
+continuing to alter the circumstances, though ever so gradually,
+indefinite divergence either in the way of improvement or deterioration
+being prevented, and the least possible excess beyond the defined limits
+being fatal to the existence of the individual." In another place he
+maintains that "varieties of some species may differ more than other
+species do from each other without shaking our confidence in the reality
+of species." He further adduces certain facts in geology as being, in
+his opinion, "fatal to the theory of progressive development," and he
+explains the fact that there are so often distinct species in countries
+of similar climate and vegetation by "special creations" in each
+country; and these conclusions were arrived at after a careful study of
+Lamarck's work, a full abstract of which is given in the earlier
+editions of the _Principles of Geology_.[2]
+
+Professor Agassiz, one of the greatest naturalists of the last
+generation, went even further, and maintained not only that each species
+was specially created, but that it was created in the proportions and in
+the localities in which we now find it to exist. The following extract
+from his very instructive book on Lake Superior explains this view:
+"There are in animals peculiar adaptations which are characteristic of
+their species, and which cannot be supposed to have arisen from
+subordinate influences. Those which live in shoals cannot be supposed to
+have been created in single pairs. Those which are made to be the food
+of others cannot have been created in the same proportions as those
+which live upon them. Those which are everywhere found in innumerable
+specimens must have been introduced in numbers capable of maintaining
+their normal proportions to those which live isolated and are
+comparatively and constantly fewer. For we know that this harmony in the
+numerical proportions between animals is one of the great laws of
+nature. The circumstance that species occur within definite limits where
+no obstacles prevent their wider distribution leads to the further
+inference that these limits were assigned to them from the beginning,
+and so we should come to the final conclusion that the order which
+prevails throughout nature is intentional, that it is regulated by the
+limits marked out on the first day of creation, and that it has been
+maintained unchanged through ages with no other modifications than those
+which the higher intellectual powers of man enable him to impose on some
+few animals more closely connected with him."[3]
+
+These opinions of some of the most eminent and influential writers of
+the pre-Darwinian age seem to us, now, either altogether obsolete or
+positively absurd; but they nevertheless exhibit the mental condition of
+even the most advanced section of scientific men on the problem of the
+nature and origin of species. They render it clear that,
+notwithstanding the vast knowledge and ingenious reasoning of Lamarck,
+and the more general exposition of the subject by the author of the
+_Vestiges of Creation_, the first step had not been taken towards a
+satisfactory explanation of the derivation of any one species from any
+other. Such eminent naturalists as Geoffroy Saint Hilaire, Dean Herbert,
+Professor Grant, Von Buch, and some others, had expressed their belief
+that species arose as simple varieties, and that the species of each
+genus were all descended from a common ancestor; but none of them gave a
+clue as to the law or the method by which the change had been effected.
+This was still "the great mystery." As to the further question--how far
+this common descent could be carried; whether distinct families, such as
+crows and thrushes, could possibly have descended from each other; or,
+whether all birds, including such widely distinct types as wrens,
+eagles, ostriches, and ducks, could all be the modified descendants of a
+common ancestor; or, still further, whether mammalia, birds, reptiles,
+and fishes, could all have had a common origin;--these questions had
+hardly come up for discussion at all, for it was felt that, while the
+very first step along the road of "transmutation of species" (as it was
+then called) had not been made, it was quite useless to speculate as to
+how far it might be possible to travel in the same direction, or where
+the road would ultimately lead to.
+
+
+_The Problem before Darwin_.
+
+It is clear, then, that what was understood by the "origin" or the
+"transmutation" of species before Darwin's work appeared, was the
+comparatively simple question whether the allied species of each genus
+had or had not been derived from one another and, remotely, from some
+common ancestor, by the ordinary method of reproduction and by means of
+laws and conditions still in action and capable of being thoroughly
+investigated. If any naturalist had been asked at that day whether,
+supposing it to be clearly shown that all the different species of each
+genus had been derived from some one ancestral species, and that a full
+and complete explanation were to be given of how each minute difference
+in form, colour, or structure might have originated, and how the
+several peculiarities of habit and of geographical distribution might
+have been brought about--whether, if this were done, the "origin of
+species" would be discovered, the great mystery solved, he would
+undoubtedly have replied in the affirmative. He would probably have
+added that he never expected any such marvellous discovery to be made in
+his lifetime. But so much as this assuredly Mr. Darwin has done, not
+only in the opinion of his disciples and admirers, but by the admissions
+of those who doubt the completeness of his explanations. For almost all
+their objections and difficulties apply to those larger differences
+which separate genera, families, and orders from each other, not to
+those which separate one species from the species to which it is most
+nearly allied, and from the remaining species of the same genus. They
+adduce such difficulties as the first development of the eye, or of the
+milk-producing glands of the mammalia; the wonderful instincts of bees
+and of ants; the complex arrangements for the fertilisation of orchids,
+and numerous other points of structure or habit, as not being
+satisfactorily explained. But it is evident that these peculiarities had
+their origin at a very remote period of the earth's history, and no
+theory, however complete, can do more than afford a probable conjecture
+as to how they were produced. Our ignorance of the state of the earth's
+surface and of the conditions of life at those remote periods is very
+great; thousands of animals and plants must have existed of which we
+have no record; while we are usually without any information as to the
+habits and general life-history even of those of which we possess some
+fragmentary remains; so that the truest and most complete theory would
+not enable us to solve _all_ the difficult problems which the whole
+course of the development of life upon our globe presents to us.
+
+What we may expect a true theory to do is to enable us to comprehend and
+follow out in some detail those changes in the form, structure, and
+relations of animals and plants which are effected in short periods of
+time, geologically speaking, and which are now going on around us. We
+may expect it to explain satisfactorily most of the lesser and
+superficial differences which distinguish one species from another. We
+may expect it to throw light on the mutual relations of the animals and
+plants which live together in any one country, and to give some rational
+account of the phenomena presented by their distribution in different
+parts of the world. And, lastly, we may expect it to explain many
+difficulties and to harmonise many incongruities in the excessively
+complex affinities and relations of living things. All this the
+Darwinian theory undoubtedly does. It shows us how, by means of some of
+the most universal and ever-acting laws in nature, new species are
+necessarily produced, while the old species become extinct; and it
+enables us to understand how the continuous action of these laws during
+the long periods with which geology makes us acquainted is calculated to
+bring about those greater differences presented by the distinct genera,
+families, and orders into which all living things are classified by
+naturalists. The differences which these present are all of the same
+_nature_ as those presented by the species of many large genera, but
+much greater in _amount_; and they can all be explained by the action of
+the same general laws and by the extinction of a larger or smaller
+number of intermediate species. Whether the distinctions between the
+higher groups termed Classes and Sub-kingdoms may be accounted for in
+the same way is a much more difficult question. The differences which
+separate the mammals, birds, reptiles, and fishes from each other,
+though vast, yet seem of the same nature as those which distinguish a
+mouse from an elephant or a swallow from a goose. But the vertebrate
+animals, the mollusca, and the insects, are so radically distinct in
+their whole organisation and in the very plan of their structure, that
+objectors may not unreasonably doubt whether they can all have been
+derived from a common ancestor by means of the very same laws as have
+sufficed for the differentiation of the various species of birds or of
+reptiles.
+
+
+_The Change of Opinion effected by Darwin_.
+
+The point I wish especially to urge is this. Before Darwin's work
+appeared, the great majority of naturalists, and almost without
+exception the whole literary and scientific world, held firmly to the
+belief that _species_ were realities, and had not been derived from
+other species by any process accessible to us; the different species of
+crow and of violet they are now, and to have originated by some totally
+unknown process so far removed from ordinary reproduction that it was
+usually spoken of as "special creation." There was, then, no question of
+the origin of families, orders, and classes, because the very first step
+of all, the "origin of species," was believed to be an insoluble
+problem. But now this is all changed. The whole scientific and literary
+world, even the whole educated public, accepts, as a matter of common
+knowledge, the origin of species from other allied species by the
+ordinary process of natural birth. The idea of special creation or any
+altogether exceptional mode of production is absolutely extinct! Yet
+more: this is held also to apply to many higher groups as well as to the
+species of a genus, and not even Mr. Darwin's severest critics venture
+to suggest that the primeval bird, reptile, or fish must have been
+"specially created." And this vast, this totally unprecedented change in
+public opinion has been the result of the work of one man, and was
+brought about in the short space of twenty years! This is the answer to
+those who continue to maintain that the "origin of species" is not yet
+discovered; that there are still doubts and difficulties; that there are
+divergencies of structure so great that we cannot understand how they
+had their beginning. We may admit all this, just as we may admit that
+there are enormous difficulties in the way of a complete comprehension
+of the origin and nature of all the parts of the solar system and of the
+stellar universe. But we claim for Darwin that he is the Newton of
+natural history, and that, just so surely as that the discovery and
+demonstration by Newton of the law of gravitation established order in
+place of chaos and laid a sure foundation for all future study of the
+starry heavens, so surely has Darwin, by his discovery of the law of
+natural selection and his demonstration of the great principle of the
+preservation of useful variations in the struggle for life, not only
+thrown a flood of light on the process of development of the whole
+organic world, but also established a firm foundation for all future
+study of nature.
+
+In order to show the view Darwin took of his own work, and what it was
+that he alone claimed to have done, the concluding passage of the
+introduction to the _Origin of_ _Species_ should be carefully
+considered. It is as follows: "Although much remains obscure, and will
+long remain obscure, I can entertain no doubt, after the most deliberate
+and dispassionate judgment of which I am capable, that the view which
+most naturalists until recently entertained and which I formerly
+entertained--namely, that each species has been independently
+created--is erroneous. I am fully convinced that species are not
+immutable; but that those belonging to what are called the same genera
+are lineal descendants of some other and generally extinct species, in
+the same manner as the acknowledged varieties of any one species are the
+descendants of that species. Furthermore, I am convinced that Natural
+Selection has been the most important, but not the exclusive, means of
+modification."
+
+It should be especially noted that all which is here claimed is now
+almost universally admitted, while the criticisms of Darwin's works
+refer almost exclusively to those numerous questions which, as he
+himself says, "will long remain obscure."
+
+
+_The Darwinian Theory_.
+
+As it will be necessary, in the following chapters, to set forth a
+considerable body of facts in almost every department of natural
+history, in order to establish the fundamental propositions on which the
+theory of natural selection rests, I propose to give a preliminary
+statement of what the theory really is, in order that the reader may
+better appreciate the necessity for discussing so many details, and may
+thus feel a more enlightened interest in them. Many of the facts to be
+adduced are so novel and so curious that they are sure to be appreciated
+by every one who takes an interest in nature, but unless the need of
+them is clearly seen it may be thought that time is being wasted on mere
+curious details and strange facts which have little bearing on the
+question at issue.
+
+The theory of natural selection rests on two main classes of facts which
+apply to all organised beings without exception, and which thus take
+rank as fundamental principles or laws. The first is, the power of rapid
+multiplication in a geometrical progression; the second, that the
+offspring always vary slightly from the parents, though generally very
+closely resembling them. From the first fact or law there follows,
+necessarily, a constant struggle for existence; because, while the
+offspring always exceed the parents in number, generally to an enormous
+extent, yet the total number of living organisms in the world does not,
+and cannot, increase year by year. Consequently every year, on the
+average, as many die as are born, plants as well as animals; and the
+majority die premature deaths. They kill each other in a thousand
+different ways; they starve each other by some consuming the food that
+others want; they are destroyed largely by the powers of nature--by cold
+and heat, by rain and storm, by flood and fire. There is thus a
+perpetual struggle among them which shall live and which shall die; and
+this struggle is tremendously severe, because so few can possibly remain
+alive--one in five, one in ten, often only one in a hundred or even one
+in a thousand.
+
+Then comes the question, Why do some live rather than others? If all the
+individuals of each species were exactly alike in every respect, we
+could only say it is a matter of chance. But they are not alike. We find
+that they vary in many different ways. Some are stronger, some swifter,
+some hardier in constitution, some more cunning. An obscure colour may
+render concealment more easy for some, keener sight may enable others to
+discover prey or escape from an enemy better than their fellows. Among
+plants the smallest differences may be useful or the reverse. The
+earliest and strongest shoots may escape the slug; their greater vigour
+may enable them to flower and seed earlier in a wet autumn; plants best
+armed with spines or hairs may escape being devoured; those whose
+flowers are most conspicuous may be soonest fertilised by insects. We
+cannot doubt that, on the whole, any beneficial variations will give the
+possessors of it a greater probability of living through the tremendous
+ordeal they have to undergo. There may be something left to chance, but
+on the whole _the fittest will survive_.
+
+Then we have another important fact to consider, the principle of
+heredity or transmission of variations. If we grow plants from seed or
+breed any kind of animals year after year, consuming or giving away all
+the increase we do not wish to keep just as they come to hand, our
+plants or animals will continue much the same; but if every year we
+carefully save the best seed to sow and the finest or brightest
+coloured animals to breed from, we shall soon find that an improvement
+will take place, and that the average quality of our stock will be
+raised. This is the way in which all our fine garden fruits and
+vegetables and flowers have been produced, as well as all our splendid
+breeds of domestic animals; and they have thus become in many cases so
+different from the wild races from which they originally sprang as to be
+hardly recognisable as the same. It is therefore proved that if any
+particular kind of variation is preserved and bred from, the variation
+itself goes on increasing in amount to an enormous extent; and the
+bearing of this on the question of the origin of species is most
+important. For if in each generation of a given animal or plant the
+fittest survive to continue the breed, then whatever may be the special
+peculiarity that causes "fitness" in the particular case, that
+peculiarity will go on increasing and strengthening _so long as it is
+useful to the species_. But the moment it has reached its maximum of
+usefulness, and some other quality or modification would help in the
+struggle, then the individuals which vary in the new direction will
+survive; and thus a species may be gradually modified, first in one
+direction, then in another, till it differs from the original parent
+form as much as the greyhound differs from any wild dog or the
+cauliflower from any wild plant. But animals or plants which thus differ
+in a state of nature are always classed as distinct species, and thus we
+see how, by the continuous survival of the fittest or the preservation
+of favoured races in the struggle for life, new species may be
+originated.
+
+This self-acting process which, by means of a few easily demonstrated
+groups of facts, brings about change in the organic world, and keeps
+each species in harmony with the conditions of its existence, will
+appear to some persons so clear and simple as to need no further
+demonstration. But to the great majority of naturalists and men of
+science endless difficulties and objections arise, owing to the
+wonderful variety of animal and vegetable forms, and the intricate
+relations of the different species and groups of species with each
+other; and it was to answer as many of these objections as possible, and
+to show that the more we know of nature the more we find it to
+harmonise with the development hypothesis, that Darwin devoted the whole
+of his life to collecting facts and making experiments, the record of a
+portion of which he has given us in a series of twelve masterly volumes.
+
+
+_Proposed Mode of Treatment of the Subject_.
+
+It is evidently of the most vital importance to any theory that its
+foundations should be absolutely secure. It is therefore necessary to
+show, by a wide and comprehensive array of facts, that animals and
+plants _do_ perpetually vary in the manner and to the amount requisite;
+and that this takes place in wild animals as well as in those which are
+domesticated. It is necessary also to prove that all organisms _do_ tend
+to increase at the great rate alleged, and that this increase actually
+occurs, under favourable conditions. We have to prove, further, that
+variations of all kinds can be increased and accumulated by selection;
+and that the struggle for existence to the extent here indicated
+actually occurs in nature, and leads to the continued preservation of
+favourable variations.
+
+These matters will be discussed in the four succeeding chapters, though
+in a somewhat different order--the struggle for existence and the power
+of rapid multiplication, which is its cause, occupying the first place,
+as comprising those facts which are the most fundamental and those which
+can be perfectly explained without any reference to the less generally
+understood facts of variation. These chapters will be followed by a
+discussion of certain difficulties, and of the vexed question of
+hybridity. Then will come a rather full account of the more important of
+the complex relations of organisms to each other and to the earth
+itself, which are either fully explained or greatly elucidated by the
+theory. The concluding chapter will treat of the origin of man and his
+relations to the lower animals.
+
+FOOTNOTES:
+
+[Footnote 1: _Geography and Classification of Animals_, p. 350.]
+
+[Footnote 2: These expressions occur in Chapter IX. of the earlier
+editions (to the ninth) of the _Principles of Geology_.]
+
+[Footnote 3: L. Agassiz, _Lake Superior_, p. 377.]
+
+
+
+
+CHAPTER II
+
+THE STRUGGLE FOR EXISTENCE
+
+
+    Its importance--The struggle among plants--Among
+    animals--Illustrative cases--Succession of trees in forests of
+    Denmark--The struggle for existence on the Pampas--Increase of
+    organisms in a geometrical ratio--Examples of great powers of
+    increase of animals--Rapid increase and wide spread of
+    plants--Great fertility not essential to rapid
+    increase--Struggle between closely allied species most
+    severe--The ethical aspect of the struggle for existence.
+
+
+
+There is perhaps no phenomenon of nature that is at once so important,
+so universal; and so little understood, as the struggle for existence
+continually going on among all organised beings. To most persons nature
+appears calm, orderly, and peaceful. They see the birds singing in the
+trees, the insects hovering over the flowers, the squirrel climbing
+among the tree-tops, and all living things in the possession of health
+and vigour, and in the enjoyment of a sunny existence. But they do not
+see, and hardly ever think of, the means by which this beauty and
+harmony and enjoyment is brought about. They do not see the constant and
+daily search after food, the failure to obtain which means weakness or
+death; the constant effort to escape enemies; the ever-recurring
+struggle against the forces of nature. This daily and hourly struggle,
+this incessant warfare, is nevertheless the very means by which much of
+the beauty and harmony and enjoyment in nature is produced, and also
+affords one of the most important elements in bringing about the origin
+of species. We must, therefore, devote some time to the consideration of
+its various aspects and of the many curious phenomena to which it gives
+rise.
+
+It is a matter of common observation that if weeds are allowed to grow
+unchecked in a garden they will soon destroy a number of the flowers.
+It is not so commonly known that if a garden is left to become
+altogether wild, the weeds that first take possession of it, often
+covering the whole surface of the ground with two or three different
+kinds, will themselves be supplanted by others, so that in a few years
+many of the original flowers and of the earliest weeds may alike have
+disappeared. This is one of the very simplest cases of the struggle for
+existence, resulting in the successive displacement of one set of
+species by another; but the exact causes of this displacement are by no
+means of such a simple nature. All the plants concerned may be perfectly
+hardy, all may grow freely from seed, yet when left alone for a number
+of years, each set is in turn driven out by a succeeding set, till at
+the end of a considerable period--a century or a few centuries
+perhaps--hardly one of the plants which first monopolised the ground
+would be found there.
+
+Another phenomenon of an analogous kind is presented by the different
+behaviour of introduced wild plants or animals into countries apparently
+quite as well suited to them as those which they naturally inhabit.
+Agassiz, in his work on Lake Superior, states that the roadside weeds of
+the northeastern United States, to the number of 130 species, are all
+European, the native weeds having disappeared westwards; and in New
+Zealand there are no less than 250 species of naturalised European
+plants, more than 100 species of which have spread widely over the
+country, often displacing the native vegetation. On the other hand, of
+the many hundreds of hardy plants which produce seed freely in our
+gardens, very few ever run wild, and hardly any have become common. Even
+attempts to naturalise suitable plants usually fail; for A. de Candolle
+states that several botanists of Paris, Geneva, and especially of
+Montpellier, have sown the seeds of many hundreds of species of hardy
+exotic plants in what appeared to be the most favourable situations, but
+that, in hardly a single case, has any one of them become
+naturalised.[4] Even a plant like the potato--so widely cultivated, so
+hardy, and so well adapted to spread by means of its many-eyed
+tubers--has not established itself in a wild state in any part of
+Europe. It would be thought that Australian plants would easily run
+wild in New Zealand. But Sir Joseph Hooker informs us that the late Mr.
+Bidwell habitually scattered Australian seeds during his extensive
+travels in New Zealand, yet only two or three Australian plants appear
+to have established themselves in that country, and these only in
+cultivated or newly moved soil.
+
+These few illustrations sufficiently show that all the plants of a
+country are, as De Candolle says, at war with each other, each one
+struggling to occupy ground at the expense of its neighbour. But,
+besides this direct competition, there is one not less powerful arising
+from the exposure of almost all plants to destruction by animals. The
+buds are destroyed by birds, the leaves by caterpillars, the seeds by
+weevils; some insects bore into the trunk, others burrow in the twigs
+and leaves; slugs devour the young seedlings and the tender shoots,
+wire-worms gnaw the roots. Herbivorous mammals devour many species
+bodily, while some uproot and devour the buried tubers.
+
+In animals, it is the eggs or the very young that suffer most from their
+various enemies; in plants, the tender seedlings when they first appear
+above the ground. To illustrate this latter point Mr. Darwin cleared and
+dug a piece of ground three feet long and two feet wide, and then marked
+all the seedlings of weeds and other plants which came up, noting what
+became of them. The total number was 357, and out of these no less than
+295 were destroyed by slugs and insects. The direct strife of plant with
+plant is almost equally fatal when the stronger are allowed to smother
+the weaker. When turf is mown or closely browsed by animals, a number of
+strong and weak plants live together, because none are allowed to grow
+much beyond the rest; but Mr. Darwin found that when the plants which
+compose such turf are allowed to grow up freely, the stronger kill the
+weaker. In a plot of turf three feet by four, twenty distinct species of
+plants were found to be growing, and no less than nine of these perished
+altogether when the other species were allowed to grow up to their full
+size.[5]
+
+But besides having to protect themselves against competing plants and
+against destructive animals, there is a yet deadlier enemy in the
+forces of inorganic nature. Each species can sustain a certain amount of
+heat and cold, each requires a certain amount of moisture at the right
+season, each wants a proper amount of light or of direct sunshine, each
+needs certain elements in the soil; the failure of a due proportion in
+these inorganic conditions causes weakness, and thus leads to speedy
+death. The struggle for existence in plants is, therefore, threefold in
+character and infinite in complexity, and the result is seen in their
+curiously irregular distribution over the face of the earth. Not only
+has each country its distinct plants, but every valley, every hillside,
+almost every hedgerow, has a different set of plants from its adjacent
+valley, hillside, or hedgerow--if not always different in the actual
+species yet very different in comparative abundance, some which are rare
+in the one being common in the other. Hence it happens that slight
+changes of conditions often produce great changes in the flora of a
+country. Thus in 1740 and the two following years the larva of a moth
+(Phalaena graminis) committed such destruction in many of the meadows of
+Sweden that the grass was greatly diminished in quantity, and many
+plants which were before choked by the grass sprang up, and the ground
+became variegated with a multitude of different species of flowers. The
+introduction of goats into the island of St. Helena led to the entire
+destruction of the native forests, consisting of about a hundred
+distinct species of trees and shrubs, the young plants being devoured by
+the goats as fast as they grew up. The camel is a still greater enemy to
+woody vegetation than the goat, and Mr. Marsh believes that forests
+would soon cover considerable tracts of the Arabian and African deserts
+if the goat and the camel were removed from them.[6] Even in many parts
+of our own country the existence of trees is dependent on the absence of
+cattle. Mr. Darwin observed, on some extensive heaths near Farnham, in
+Surrey, a few clumps of old Scotch firs, but no young trees over
+hundreds of acres. Some portions of the heath had, however, been
+enclosed a few years before, and these enclosures were crowded with
+young fir-trees growing too close together for all to live; and these
+were not sown or planted, nothing having been done to the ground beyond
+enclosing it so as to keep out cattle. On ascertaining this, Mr. Darwin
+was so much surprised that he searched among the heather in the
+unenclosed parts, and there he found multitudes of little trees and
+seedlings which had been perpetually browsed down by the cattle. In one
+square yard, at a point about a hundred yards from one of the old clumps
+of firs, he counted thirty-two little trees, and one of them had
+twenty-six rings of growth, showing that it had for many years tried to
+raise its head above the stems of the heather and had failed. Yet this
+heath was very extensive and very barren, and, as Mr. Darwin remarks, no
+one would ever have imagined that cattle would have so closely and so
+effectually searched it for food.
+
+In the case of animals, the competition and struggle are more obvious.
+The vegetation of a given district can only support a certain number of
+animals, and the different kinds of plant-eaters will compete together
+for it. They will also have insects for their competitors, and these
+insects will be kept down by birds, which will thus assist the mammalia.
+But there will also be carnivora destroying the herbivora; while small
+rodents, like the lemming and some of the field-mice, often destroy so
+much vegetation as materially to affect the food of all the other groups
+of animals. Droughts, floods, severe winters, storms and hurricanes will
+injure these in various degrees, but no one species can be diminished in
+numbers without the effect being felt in various complex ways by all the
+rest. A few illustrations of this reciprocal action must be given.
+
+
+_Illustrative Cases of the Struggle for Life_.
+
+Sir Charles Lyell observes that if, by the attacks of seals or other
+marine foes, salmon are reduced in numbers, the consequence will be that
+otters, living far inland, will be deprived of food and will then
+destroy many young birds or quadrupeds, so that the increase of a marine
+animal may cause the destruction of many land animals hundreds of miles
+away. Mr. Darwin carefully observed the effects produced by planting a
+few hundred acres of Scotch fir, in Staffordshire, on part of a very
+extensive heath which had never been cultivated. After the planted
+portion was about twenty-five years old he observed that the change in
+the native vegetation was greater than is often seen in passing from
+one quite different soil to another. Besides a great change in the
+proportional numbers of the native heath-plants, twelve species which
+could not be found on the heath flourished in the plantations. The
+effect on the insect life must have been still greater, for six
+insectivorous birds which were very common in the plantations were not
+to be seen on the heath, which was, however, frequented by two or three
+different species of insectivorous birds. It would have required
+continued study for several years to determine all the differences in
+the organic life of the two areas, but the facts stated by Mr. Darwin
+are sufficient to show how great a change may be effected by the
+introduction of a single kind of tree and the keeping out of cattle.
+
+The next case I will give in Mr. Darwin's own words: "In several parts
+of the world insects determine the existence of cattle. Perhaps Paraguay
+offers the most curious instance of this; for here neither cattle nor
+horses nor dogs have ever run wild, though they swarm southward and
+northward in a feral state; and Azara and Rengger have shown that this
+is caused by the greater numbers, in Paraguay, of a certain fly which
+lays its eggs in the navels of these animals when first born. The
+increase of these flies, numerous as they are, must be habitually
+checked by some means, probably by other parasitic insects. Hence, if
+certain insectivorous birds were to decrease in Paraguay, the parasitic
+insects would probably increase; and this would lessen the number of the
+navel-frequenting flies--then cattle and horses would become feral, and
+this would greatly alter (as indeed I have observed in parts of South
+America) the vegetation: this again would largely affect the insects,
+and this, as we have just seen in Staffordshire, the insectivorous
+birds, and so onward in ever-increasing circles of complexity. Not that
+under nature the relations will ever be as simple as this. Battle within
+battle must be continually recurring with varying success; and yet in
+the long run the forces are so nicely balanced, that the face of nature
+remains for a long time uniform, though assuredly the merest trifle
+would give the victory to one organic being over another."[7]
+
+Such cases as the above may perhaps be thought exceptional, but there
+is good reason to believe that they are by no means rare, but are
+illustrations of what is going on in every part of the world, only it is
+very difficult for us to trace out the complex reactions that are
+everywhere occurring. The general impression of the ordinary observer
+seems to be that wild animals and plants live peaceful lives and have
+few troubles, each being exactly suited to its place and surroundings,
+and therefore having no difficulty in maintaining itself. Before showing
+that this view is, everywhere and always, demonstrably untrue, we will
+consider one other case of the complex relations of distinct organisms
+adduced by Mr. Darwin, and often quoted for its striking and almost
+eccentric character. It is now well known that many flowers require to
+be fertilised by insects in order to produce seed, and this
+fertilisation can, in some cases, only be effected by one particular
+species of insect to which the flower has become specially adapted. Two
+of our common plants, the wild heart's-ease (Viola tricolor) and the red
+clover (Trifolium pratense), are thus fertilised by humble-bees almost
+exclusively, and if these insects are prevented from visiting the
+flowers, they produce either no seed at all or exceedingly few. Now it
+is known that field-mice destroy the combs and nests of humble-bees, and
+Colonel Newman, who has paid great attention to these insects, believes
+that more than two-thirds of all the humble-bees' nests in England are
+thus destroyed. But the number of mice depends a good deal on the number
+of cats; and the same observer says that near villages and towns he has
+found the nests of humble-bees more numerous than elsewhere, which he
+attributes to the number of cats that destroy the mice. Hence it
+follows, that the abundance of red clover and wild heart's-ease in a
+district will depend on a good supply of cats to kill the mice, which
+would otherwise destroy and keep down the humble-bees and prevent them
+from fertilising the flowers. A chain of connection has thus been found
+between such totally distinct organisms as flesh-eating mammalia and
+sweet-smelling flowers, the abundance or scarcity of the one closely
+corresponding to that of the other!
+
+The following account of the struggle between trees in the forests of
+Denmark, from the researches of M. Hansten-Blangsted, strikingly
+illustrates our subject.[8] The chief combatants are the beech and the
+birch, the former being everywhere successful in its invasions. Forests
+composed wholly of birch are now only found in sterile, sandy tracts;
+everywhere else the trees are mixed, and wherever the soil is favourable
+the beech rapidly drives out the birch. The latter loses its branches at
+the touch of the beech, and devotes all its strength to the upper part
+where it towers above the beech. It may live long in this way, but it
+succumbs ultimately in the fight--of old age if of nothing else, for the
+life of the birch in Denmark is shorter than that of the beech. The
+writer believes that light (or rather shade) is the cause of the
+superiority of the latter, for it has a greater development of its
+branches than the birch, which is more open and thus allows the rays of
+the sun to pass through to the soil below, while the tufted, bushy top
+of the beech preserves a deep shade at its base. Hardly any young plants
+can grow under the beech except its own shoots; and while the beech can
+nourish under the shade of the birch, the latter dies immediately under
+the beech. The birch has only been saved from total extermination by the
+facts that it had possession of the Danish forests long before the beech
+ever reached the country, and that certain districts are unfavourable to
+the growth of the latter. But wherever the soil has been enriched by the
+decomposition of the leaves of the birch the battle begins. The birch
+still flourishes on the borders of lakes and other marshy places, where
+its enemy cannot exist. In the same way, in the forests of Zeeland, the
+fir forests are disappearing before the beech. Left to themselves, the
+firs are soon displaced by the beech. The struggle between the latter
+and the oak is longer and more stubborn, for the branches and foliage of
+the oak are thicker, and offer much resistance to the passage of light.
+The oak, also, has greater longevity; but, sooner or later, it too
+succumbs, because it cannot develop in the shadow of the beech. The
+earliest forests of Denmark were mainly composed of aspens, with which
+the birch was apparently associated; gradually the soil was raised, and
+the climate grew milder; then the fir came and formed large forests.
+This tree ruled for centuries, and then ceded the first place to the
+holm-oak, which is now giving way to the beech. Aspen, birch, fir, oak,
+and beech appear to be the steps in the struggle for the survival of the
+fittest among the forest-trees of Denmark.
+
+It may be added that in the time of the Romans the beech was the
+principal forest-tree of Denmark as it is now, while in the much earlier
+bronze age, represented by the later remains found in the peat bogs,
+there were no beech-trees, or very few, the oak being the prevailing
+tree, while in the still earlier stone period the fir was the most
+abundant. The beech is a tree essentially of the temperate zone, having
+its northern limit considerably southward of the oak, fir, birch, or
+aspen, and its entrance into Denmark was no doubt due to the
+amelioration of the climate after the glacial epoch had entirely passed
+away. We thus see how changes of climate, which are continually
+occurring owing either to cosmical or geographical causes, may initiate
+a struggle among plants which may continue for thousands of years, and
+which must profoundly modify the relations of the animal world, since
+the very existence of innumerable insects, and even of many birds and
+mammals, is dependent more or less completely on certain species of
+plants.
+
+
+_The Struggle for Existence on the Pampas_.
+
+Another illustration of the struggle for existence, in which both plants
+and animals are implicated, is afforded by the pampas of the southern
+part of South America. The absence of trees from these vast plains has
+been imputed by Mr. Darwin to the supposed inability of the tropical and
+sub-tropical forms of South America to thrive on them, and there being
+no other source from which they could obtain a supply; and that
+explanation was adopted by such eminent botanists as Mr. Ball and
+Professor Asa Gray. This explanation has always seemed to me
+unsatisfactory, because there are ample forests both in the temperate
+regions of the Andes and on the whole west coast down to Terra del
+Fuego; and it is inconsistent with what we know of the rapid variation
+and adaptation of species to new conditions. What seems a more
+satisfactory explanation has been given by Mr. Edwin Clark, a civil
+engineer, who resided nearly two years in the country and paid much
+attention to its natural history. He says: "The peculiar characteristics
+of these vast level plains which descend from the Andes to the great
+river basin in unbroken monotony, are the absence of rivers or
+water-storage, and the periodical occurrence of droughts, or 'siccos,'
+in the summer months. These conditions determine the singular character
+both of its flora and fauna.
+
+"The soil is naturally fertile and favourable for the growth of trees,
+and they grow luxuriantly wherever they are protected. The eucalyptus is
+covering large tracts wherever it is enclosed, and willows, poplars, and
+the fig surround every estancia when fenced in.
+
+"The open plains are covered with droves of horses and cattle, and
+overrun by numberless wild rodents, the original tenants of the pampas.
+During the long periods of drought, which are so great a scourge to the
+country, these animals are starved by thousands, destroying, in their
+efforts to live, every vestige of vegetation. In one of these 'siccos,'
+at the time of my visit, no less than 50,000 head of oxen and sheep and
+horses perished from starvation and thirst, after tearing deep out of
+the soil every trace of vegetation, including the wiry roots of the
+pampas-grass. Under such circumstances the existence of an unprotected
+tree is impossible. The only plants that hold their own, in addition to
+the indestructible thistles, grasses, and clover, are a little
+herbaceous oxalis, producing viviparous buds of extraordinary vitality,
+a few poisonous species, such as the hemlock, and a few tough, thorny
+dwarf-acacias and wiry rushes, which even a starving rat refuses.
+
+"Although the cattle are a modern introduction, the numberless
+indigenous rodents must always have effectually prevented the
+introduction of any other species of plants; large tracts are still
+honeycombed by the ubiquitous biscacho, a gigantic rabbit; and numerous
+other rodents still exist, including rats and mice, pampas-hares, and
+the great nutria and carpincho (capybara) on the river banks."[9]
+
+Mr. Clark further remarks on the desperate struggle for existence which
+characterises the bordering fertile zones, where rivers and marshy
+plains permit a more luxuriant and varied vegetable and animal life.
+After describing how the river sometimes rose 30 feet in eight hours,
+doing immense destruction, and the abundance of the larger carnivora and
+large reptiles on its banks, he goes on: "But it was among the flora
+that the principle of natural selection was most prominently displayed.
+In such a district--overrun with rodents and escaped cattle, subject to
+floods that carried away whole islands of botany, and especially to
+droughts that dried up the lakes and almost the river itself--no
+ordinary plant could live, even on this rich and watered alluvial
+debris. The only plants that escaped the cattle were such as were either
+poisonous, or thorny, or resinous, or indestructibly tough. Hence we had
+only a great development of solanums, talas, acacias, euphorbias, and
+laurels. The buttercup is replaced by the little poisonous yellow oxalis
+with its viviparous buds; the passion-flowers, asclepiads, bignonias,
+convolvuluses, and climbing leguminous plants escape both floods and
+cattle by climbing the highest trees and towering overhead in a flood of
+bloom. The ground plants are the portulacas, turneras, and cenotheras,
+bitter and ephemeral, on the bare rock, and almost independent of any
+other moisture than the heavy dews. The pontederias, alismas, and
+plantago, with grasses and sedges, derive protection from the deep and
+brilliant pools; and though at first sight the 'monte' doubtless
+impresses the traveller as a scene of the wildest confusion and ruin,
+yet, on closer examination, we found it far more remarkable as a
+manifestation of harmony and law, and a striking example of the
+marvellous power which plants, like animals, possess, of adapting
+themselves to the local peculiarities of their habitat, whether in the
+fertile shades of the luxuriant 'monte' or on the arid, parched-up
+plains of the treeless pampas."
+
+A curious example of the struggle between plants has been communicated
+to me by Mr. John Ennis, a resident in New Zealand. The English
+water-cress grows so luxuriantly in that country as to completely choke
+up the rivers, sometimes leading to disastrous floods, and necessitating
+great outlay to keep the stream open. But a natural remedy has now been
+found in planting willows on the banks. The roots of these trees
+penetrate the bed of the stream in every direction, and the water-cress,
+unable to obtain the requisite amount of nourishment, gradually
+disappears.
+
+_Increase of Organisms in a Geometrical Ratio_.
+
+The facts which have now been adduced, sufficiently prove that there is
+a continual competition, and struggle, and war going on in nature, and
+that each species of animal and plant affects many others in complex and
+often unexpected ways. We will now proceed to show the fundamental cause
+of this struggle, and to prove that it is ever acting over the whole
+field of nature, and that no single species of animal or plant can
+possibly escape from it. This results from the fact of the rapid
+increase, in a geometrical ratio, of all the species of animals and
+plants. In the lower orders this increase is especially rapid, a single
+flesh-fly (Musca carnaria) producing 20,000 larvae, and these growing so
+quickly that they reach their full size in five days; hence the great
+Swedish naturalist, Linnaeus, asserted that a dead horse would be
+devoured by three of these flies as quickly as by a lion. Each of these
+larvae remains in the pupa state about five or six days, so that each
+parent fly may be increased ten thousand-fold in a fortnight. Supposing
+they went on increasing at this rate during only three months of summer,
+there would result one hundred millions of millions of millions for each
+fly at the commencement of summer,--a number greater probably than
+exists at any one time in the whole world. And this is only one species,
+while there are thousands of other species increasing also at an
+enormous rate; so that, if they were unchecked, the whole atmosphere
+would be dense with flies, and all animal food and much of animal life
+would be destroyed by them. To prevent this tremendous increase there
+must be incessant war against these insects, by insectivorous birds and
+reptiles as well as by other insects, in the larva as well as in the
+perfect state, by the action of the elements in the form of rain, hail,
+or drought, and by other unknown causes; yet we see nothing of this
+ever-present war, though by its means alone, perhaps, we are saved from
+famine and pestilence.
+
+Let us now consider a less extreme and more familiar case. We possess a
+considerable number of birds which, like the redbreast, sparrow, the
+four common titmice, the thrush, and the blackbird, stay with us all the
+year round These lay on an average six eggs, but, as several of them
+have two or more broods a year, ten will be below the average of the
+year's increase. Such birds as these often live from fifteen to twenty
+years in confinement, and we cannot suppose them to live shorter lives
+in a state of nature, if unmolested; but to avoid possible exaggeration
+we will take only ten years as the average duration of their lives. Now,
+if we start with a single pair, and these are allowed to live and breed,
+unmolested, till they die at the end of ten years,--as they might do if
+turned loose into a good-sized island with ample vegetable and insect
+food, but no other competing or destructive birds or quadrupeds--their
+numbers would amount to more than twenty millions. But we know very well
+that our bird population is no greater, on the average, now than it was
+ten years ago. Year by year it may fluctuate a little according as the
+winters are more or less severe, or from other causes, but on the whole
+there is no increase. What, then, becomes of the enormous surplus
+population annually produced? It is evident they must all die or be
+killed, somehow; and as the increase is, on the average, about five to
+one, it follows that, if the average number of birds of all kinds in our
+islands is taken at ten millions--and this is probably far under the
+mark--then about fifty millions of birds, including eggs as possible
+birds, must annually die or be destroyed. Yet we see nothing, or almost
+nothing, of this tremendous slaughter of the innocents going on all
+around us. In severe winters a few birds are found dead, and a few
+feathers or mangled remains show us where a wood-pigeon or some other
+bird has been destroyed by a hawk, but no one would imagine that five
+times as many birds as the total number in the country in early spring
+die every year. No doubt a considerable proportion of these do not die
+here but during or after migration to other countries, but others which
+are bred in distant countries come here, and thus balance the account.
+Again, as the average number of young produced is four or five times
+that of the parents, we ought to have at least five times as many birds
+in the country at the end of summer as at the beginning, and there is
+certainly no such enormous disproportion as this. The fact is, that the
+destruction commences, and is probably most severe, with nestling birds,
+which are often killed by heavy rains or blown away by severe storms, or
+left to die of hunger if either of the parents is killed; while they
+offer a defenceless prey to jackdaws, jays, and magpies, and not a few
+are ejected from their nests by their foster-brothers the cuckoos. As
+soon as they are fledged and begin to leave the nest great numbers are
+destroyed by buzzards, sparrow-hawks, and shrikes. Of those which
+migrate in autumn a considerable proportion are probably lost at sea or
+otherwise destroyed before they reach a place of safety; while those
+which remain with us are greatly thinned by cold and starvation during
+severe winters. Exactly the same thing goes on with every species of
+wild animal and plant from the lowest to the highest. All breed at such
+a rate, that in a few years the progeny of any one species would, if
+allowed to increase unchecked, alone monopolise the land; but all alike
+are kept within bounds by various destructive agencies, so that, though
+the numbers of each may fluctuate, they can never permanently increase
+except at the expense of some others, which must proportionately
+decrease.
+
+
+_Cases showing the Great Powers of Increase of Animals._
+
+As the facts now stated are the very foundation of the theory we are
+considering, and the enormous increase and perpetual destruction
+continually going on require to be kept ever present in the mind, some
+direct evidence of actual cases of increase must be adduced. That even
+the larger animals, which breed comparatively slowly, increase
+enormously when placed under favourable conditions in new countries, is
+shown by the rapid spread of cattle and horses in America. Columbus, in
+his second voyage, left a few black cattle at St. Domingo, and these ran
+wild and increased so much that, twenty-seven years afterwards, herds of
+from 4000 to 8000 head were not uncommon. Cattle were afterwards taken
+from this island to Mexico and to other parts of America, and in 1587,
+sixty-five years after the conquest of Mexico, the Spaniards exported
+64,350 hides from that country and 35,444 from St. Domingo, an
+indication of the vast numbers of these animals which must then have
+existed there, since those captured and killed could have been only a
+small portion of the whole. In the pampas of Buenos Ayres there were, at
+the end of the last century, about twelve million cows and three million
+horses, besides great numbers in all other parts of America where open
+pastures offered suitable conditions. Asses, about fifty years after
+their introduction, ran wild and multiplied so amazingly in Quito, that
+the Spanish traveller Ulloa describes them as being a nuisance. They
+grazed together in great herds, defending themselves with their mouths,
+and if a horse strayed among them they all fell upon him and did not
+cease biting and kicking till they left him dead. Hogs were turned out
+in St. Domingo by Columbus in 1493, and the Spaniards took them to other
+places where they settled, the result being, that in about half a
+century these animals were found in great numbers over a large part of
+America, from 25° north to 40° south latitude. More recently, in New
+Zealand, pigs have multiplied so greatly in a wild state as to be a
+serious nuisance and injury to agriculture. To give some idea of their
+numbers, it is stated that in the province of Nelson there were killed
+in twenty months 25,000 wild pigs.[10] Now, in the case of all these
+animals, we know that in their native countries, and even in America at
+the present time, they do not increase at all in numbers; therefore the
+whole normal increase must be kept down, year by year, by natural or
+artificial means of destruction.
+
+
+_Rapid Increase and Wide Spread of Plants_.
+
+In the case of plants, the power of increase is even greater and its
+effects more distinctly visible. Hundreds of square miles of the plains
+of La Plata are now covered with two or three species of European
+thistle, often to the exclusion of almost every other plant; but in the
+native countries of these thistles they occupy, except in cultivated or
+waste ground, a very subordinate part in the vegetation. Some American
+plants, like the cotton-weed (Asclepias cuiussayica), have now become
+common weeds over a large portion of the tropics. White clover
+(Trifolium repens) spreads over all the temperate regions of the world,
+and in New Zealand is exterminating many native species, including even
+the native flax (Phormium tenax), a large plant with iris-like leaves 5
+or 6 feet high. Mr. W.L. Travers has paid much attention to the effects
+of introduced plants in New Zealand, and notes the following species as
+being especially remarkable. The common knotgrass (Polygonum aviculare)
+grows most luxuriantly, single plants covering a space 4 or 5 feet in
+diameter, and sending their roots 3 or 4 feet deep. A large sub-aquatic
+dock (Rumex obtusifolius) abounds in every river-bed, even far up among
+the mountains. The common sow-thistle (Sonchus oleraceus) grows all over
+the country up to an elevation of 6000 feet. The water-cress (Nasturtium
+officinale) grows with amazing vigour in many of the rivers, forming
+stems 12 feet long and 3/4 inch in diameter, and completely choking them
+up. It cost £300 a year to keep the Avon at Christchurch free from it.
+The sorrel (Rumex acetosella) covers hundreds of acres with a sheet of
+red. It forms a dense mat, exterminating other plants, and preventing
+cultivation. It can, however, be itself exterminated by sowing the
+ground with red clover, which will also vanquish the Polygonum
+aviculare. The most noxious weed in New Zealand appears, however, to be
+the Hypochaeris radicata, a coarse yellow-flowered composite not
+uncommon in our meadows and waste places. This has been introduced with
+grass seeds from England, and is very destructive. It is stated that
+excellent pasture was in three years destroyed by this weed, which
+absolutely displaced every other plant on the ground. It grows in every
+kind of soil, and is said even to drive out the white clover, which is
+usually so powerful in taking possession of the soil.
+
+In Australia another composite plant, called there the Cape-weed
+(Cryptostemma calendulaceum), did much damage, and was noticed by Baron
+Von Hugel in 1833 as "an unexterminable weed"; but, after forty years'
+occupation, it was found to give way to the dense herbage formed by
+lucerne and choice grasses.
+
+In Ceylon we are told by Mr. Thwaites, in his _Enumeration of Ceylon
+Plants_, that a plant introduced into the island less than fifty years
+ago is helping to alter the character of the vegetation up to an
+elevation of 3000 feet. This is the Lantana mixta, a verbenaceous plant
+introduced from the West Indies, which appears to have found in Ceylon
+a soil and climate exactly suited to it. It now covers thousands of
+acres with its dense masses of foliage, taking complete possession of
+land where cultivation has been neglected or abandoned, preventing the
+growth of any other plants, and even destroying small trees, the tops of
+which its subscandent stems are able to reach. The fruit of this plant
+is so acceptable to frugivorous birds of all kinds that, through their
+instrumentality, it is spreading rapidly, to the complete exclusion of
+the indigenous vegetation where it becomes established.
+
+
+_Great Fertility not essential to Rapid Increase_.
+
+The not uncommon circumstance of slow-breeding animals being very
+numerous, shows that it is usually the amount of destruction which an
+animal or plant is exposed to, not its rapid multiplication, that
+determines its numbers in any country. The passenger-pigeon (Ectopistes
+migratorius) is, or rather was, excessively abundant in a certain area
+in North America, and its enormous migrating flocks darkening the sky
+for hours have often been described; yet this bird lays only two eggs.
+The fulmar petrel exists in myriads at St. Kilda and other haunts of the
+species, yet it lays only one egg. On the other hand the great shrike,
+the tree-creeper, the nut-hatch, the nut-cracker, the hoopoe, and many
+other birds, lay from four to six or seven eggs, and yet are never
+abundant. So in plants, the abundance of a species bears little or no
+relation to its seed-producing power. Some of the grasses and sedges,
+the wild hyacinth, and many buttercups occur in immense profusion over
+extensive areas, although each plant produces comparatively few seeds;
+while several species of bell-flowers, gentians, pinks, and mulleins,
+and even some of the composite, which produce an abundance of minute
+seeds, many of which are easily scattered by the wind, are yet rare
+species that never spread beyond a very limited area.
+
+The above-mentioned passenger-pigeon affords such an excellent example
+of an enormous bird-population kept up by a comparatively slow rate of
+increase, and in spite of its complete helplessness and the great
+destruction which it suffers from its numerous enemies, that the
+following account of one of its breeding-places and migrations by the
+celebrated American naturalist, Alexander Wilson, will be read with
+interest:--
+
+"Not far from Shelbyville, in the State of Kentucky, about five years
+ago, there was one of these breeding-places, which stretched through the
+woods in nearly a north and south direction, was several miles in
+breadth, and was said to be upwards of 40 miles in extent. In this tract
+almost every tree was furnished with nests wherever the branches could
+accommodate them. The pigeons made their first appearance there about
+the 10th of April, and left it altogether with their young before the
+25th of May. As soon as the young were fully grown and before they left
+the nests, numerous parties of the inhabitants from all parts of the
+adjacent country came with waggons, axes, beds, cooking utensils, many
+of them accompanied by the greater part of their families, and encamped
+for several days at this immense nursery. Several of them informed me
+that the noise was so great as to terrify their horses, and that it was
+difficult for one person to hear another without bawling in his ear. The
+ground was strewed with broken limbs of trees, eggs, and young squab
+pigeons, which had been precipitated from above, and on which herds of
+hogs were fattening. Hawks, buzzards, and eagles were sailing about in
+great numbers, and seizing the squabs from the nests at pleasure; while,
+from 20 feet upwards to the top of the trees, the view through the woods
+presented a perpetual tumult of crowding and fluttering multitudes of
+pigeons, their wings roaring like thunder, mingled with the frequent
+crash of falling timber; for now the axemen were at work cutting down
+those trees that seemed most crowded with nests, and contrived to fell
+them in such a manner, that in their descent they might bring down
+several others; by which means the falling of one large tree sometimes
+produced 200 squabs little inferior in size to the old birds, and almost
+one heap of fat. On some single trees upwards of a hundred nests were
+found, each containing one squab only; a circumstance in the history of
+the bird not generally known to naturalists.[11] It was dangerous to
+walk under these flying and fluttering millions, from the frequent fall
+of large branches, broken down by the weight of the multitudes above,
+and which in their descent often destroyed numbers of the birds
+themselves; while the clothes of those engaged in traversing the woods
+were completely covered with the excrements of the pigeons.
+
+"These circumstances were related to me by many of the most respectable
+part of the community in that quarter, and were confirmed in part by
+what I myself witnessed. I passed for several miles through this same
+breeding-place, where every tree was spotted with nests, the remains of
+those above described. In many instances I counted upwards of ninety
+nests on a single tree; but the pigeons had abandoned this place for
+another, 60 or 80 miles off, towards Green River, where they were said
+at that time to be equally numerous. From the great numbers that were
+constantly passing over our heads to or from that quarter, I had no
+doubt of the truth of this statement. The mast had been chiefly consumed
+in Kentucky; and the pigeons, every morning a little before sunrise, set
+out for the Indiana territory, the nearest part of which was about sixty
+miles distant. Many of these returned before ten o'clock, and the great
+body generally appeared on their return a little after noon. I had left
+the public road to visit the remains of the breeding-place near
+Shelbyville, and was traversing the woods with my gun, on my way to
+Frankfort, when about ten o'clock the pigeons which I had observed
+flying the greater part of the morning northerly, began to return in
+such immense numbers as I never before had witnessed. Coming to an
+opening by the side of a creek, where I had a more uninterrupted view, I
+was astonished at their appearance: they were flying with great
+steadiness and rapidity, at a height beyond gunshot, in several strata
+deep, and so close together that, could shot have reached them, one
+discharge could not have failed to bring down several individuals. From
+right to left, as far as the eye could reach, the breadth of this vast
+procession extended, seeming everywhere equally crowded. Curious to
+determine how long this appearance would continue, I took out my watch
+to note the time, and sat down to observe them. It was then half-past
+one; I sat for more than an hour, but instead of a diminution of this
+prodigious procession, it seemed rather to increase, both in numbers and
+rapidity; and anxious to reach Frankfort before night, I rose and went
+on. About four o'clock in the afternoon I crossed Kentucky River, at the
+town of Frankfort, at which time the living torrent above my head seemed
+as numerous and as extensive as ever. Long after this I observed them in
+large bodies that continued to pass for six or eight minutes, and these
+again were followed by other detached bodies, all moving in the same
+south-east direction, till after six o'clock in the evening. The great
+breadth of front which this mighty multitude preserved would seem to
+intimate a corresponding breadth of their breeding-place, which, by
+several gentlemen who had lately passed through part of it, was stated
+to me at several miles."
+
+From these various observations, Wilson calculated that the number of
+birds contained in the mass of pigeons which he saw on this occasion was
+at least two thousand millions, while this was only one of many similar
+aggregations known to exist in various parts of the United States. The
+picture here given of these defenceless birds, and their still more
+defenceless young, exposed to the attacks of numerous rapacious enemies,
+brings vividly before us one of the phases of the unceasing struggle for
+existence ever going on; but when we consider the slow rate of increase
+of these birds, and the enormous population they are nevertheless able
+to maintain, we must be convinced that in the case of the majority of
+birds which multiply far more rapidly, and yet are never able to attain
+such numbers, the struggle against their numerous enemies and against
+the adverse forces of nature must be even more severe or more
+continuous.
+
+
+_Struggle for Life between, closely allied Animals and Plants often the
+most severe._
+
+The struggle we have hitherto been considering has been mainly that
+between an animal or plant and its direct enemies, whether these enemies
+are other animals which devour it, or the forces of nature which destroy
+it. But there is another kind of struggle often going on at the same
+time between closely related species, which almost always terminates in
+the destruction of one of them. As an example of what is meant, Darwin
+states that the recent increase of the missel-thrush in parts of
+Scotland has caused the decrease of the song-thrush.[12] The black rat
+(Mus rattus) was the common rat of Europe till, in the beginning of the
+eighteenth century, the large brown rat (Mus decumanus) appeared on the
+Lower Volga, and thence spread more or less rapidly till it overran all
+Europe, and generally drove out the black rat, which in most parts is
+now comparatively rare or quite extinct. This invading rat has now been
+carried by commerce all over the world, and in New Zealand has
+completely extirpated a native rat, which the Maoris allege they brought
+with them from their home in the Pacific; and in the same country a
+native fly is being supplanted by the European house-fly. In Russia the
+small Asiatic cockroach has driven away a larger native species; and in
+Australia the imported hive-bee is exterminating the small stingless
+native bee.
+
+The reason why this kind of struggle goes on is apparent if we consider
+that the allied species fill nearly the same place in the economy of
+nature. They require nearly the same kind of food, are exposed to the
+same enemies and the same dangers. Hence, if one has ever so slight an
+advantage over the other in procuring food or in avoiding danger, in its
+rapidity of multiplication or its tenacity of life, it will increase
+more rapidly, and by that very fact will cause the other to decrease and
+often become altogether extinct. In some cases, no doubt, there is
+actual war between the two, the stronger killing the weaker; but this is
+by no means necessary, and there may be cases in which the weaker
+species, physically, may prevail, by its power of more rapid
+multiplication, its better withstanding vicissitudes of climates, or its
+greater cunning in escaping the attacks of the common enemies. The same
+principle is seen at work in the fact that certain mountain varieties of
+sheep will starve out other mountain varieties, so that the two cannot
+be kept together. In plants the same thing occurs. If several distinct
+varieties of wheat are sown together, and the mixed seed resown, some of
+the varieties which best suit the soil and climate, or are naturally the
+most fertile, will beat the others and so yield more seed, and will
+consequently in a few years supplant the other varieties.
+
+As an effect of this principle, we seldom find closely allied species
+of animals or plants living together, but often in distinct though
+adjacent districts where the conditions of life are somewhat different.
+Thus we may find cowslips (Primula veris) growing in a meadow, and
+primroses (P. vulgaris) in an adjoining wood, each in abundance, but not
+often intermingled. And for the same reason the old turf of a pasture or
+heath consists of a great variety of plants matted together, so much so
+that in a patch little more than a yard square Mr. Darwin found twenty
+distinct species, belonging to eighteen distinct genera and to eight
+natural orders, thus showing their extreme diversity of organisation.
+For the same reason a number of distinct grasses and clovers are sown in
+order to make a good lawn instead of any one species; and the quantity
+of hay produced has been found to be greater from a variety of very
+distinct grasses than from any one species of grass.
+
+It may be thought that forests are an exception to this rule, since in
+the north-temperate and arctic regions we find extensive forests of
+pines or of oaks. But these are, after all, exceptional, and
+characterise those regions only where the climate is little favourable
+to forest vegetation. In the tropical and all the warm temperate parts
+of the earth, where there is a sufficient supply of moisture, the
+forests present the same variety of species as does the turf of our old
+pastures; and in the equatorial virgin forests there is so great a
+variety of forms, and they are so thoroughly intermingled, that the
+traveller often finds it difficult to discover a second specimen of any
+particular species which he has noticed. Even the forests of the
+temperate zones, in all favourable situations, exhibit a considerable
+variety of trees of distinct genera and families, and it is only when we
+approach the outskirts of forest vegetation, where either drought or
+winds or the severity of the winter is adverse to the existence of most
+trees, that we find extensive tracts monopolised by one or two species.
+Even Canada has more than sixty different forest trees and the Eastern
+United States a hundred and fifty; Europe is rather poor, containing
+about eighty trees only; while the forests of Eastern Asia, Japan, and
+Manchuria are exceedingly rich, about a hundred and seventy species
+being already known. And in all these countries the trees grow
+intermingled, so that in every extensive forest we have a considerable
+variety, as may be seen in the few remnants of our primitive woods in
+some parts of Epping Forest and the New Forest.
+
+Among animals the same law prevails, though, owing to their constant
+movements and power of concealment, it is not so readily observed. As
+illustrations we may refer to the wolf, ranging over Europe and Northern
+Asia, while the jackal inhabits Southern Asia and Northern Africa; the
+tree-porcupines, of which there are two closely allied species, one
+inhabiting the eastern, the other the western half of North America; the
+common hare (Lepus timidus) in Central and Southern Europe, while all
+Northern Europe is inhabited by the variable hare (Lepus variabilis);
+the common jay (Garrulus glandarius) inhabiting all Europe, while
+another species (Garrulus Brandti) is found all across Asia from the
+Urals to Japan; and many species of birds in the Eastern United States
+are replaced by closely allied species in the west. Of course there are
+also numbers of closely related species in the same country, but it will
+almost always be found that they frequent different stations and have
+somewhat different habits, and so do not come into direct competition
+with each other; just as closely allied plants may inhabit the same
+districts, when one prefers meadows the other woods, one a chalky soil
+the other sand, one a damp situation the other a dry one. With plants,
+fixed as they are to the earth, we easily note these peculiarities of
+station; but with wild animals, which we see only on rare occasions, it
+requires close and long-continued observation to detect the
+peculiarities in their mode of life which may prevent all direct
+competition between closely allied species dwelling in the same area.
+
+
+_The Ethical Aspect of the Struggle for Existence_.
+
+Our exposition of the phenomena presented by the struggle for existence
+may be fitly concluded by a few remarks on its ethical aspect. Now that
+the war of nature is better known, it has been dwelt upon by many
+writers as presenting so vast an amount of cruelty and pain as to be
+revolting to our instincts of humanity, while it has proved a
+stumbling-block in the way of those who would fain believe in an
+all-wise and benevolent ruler of the universe. Thus, a brilliant writer
+says: "Pain, grief, disease, and death, are these the inventions of a
+loving God? That no animal shall rise to excellence except by being
+fatal to the life of others, is this the law of a kind Creator? It is
+useless to say that pain has its benevolence, that massacre has its
+mercy. Why is it so ordained that bad should be the raw material of
+good? Pain is not the less pain because it is useful; murder is not less
+murder because it is conducive to development. Here is blood upon the
+hand still, and all the perfumes of Arabia will not sweeten it."[13]
+
+Even so thoughtful a writer as Professor Huxley adopts similar views. In
+a recent article on "The Struggle for Existence" he speaks of the
+myriads of generations of herbivorous animals which "have been tormented
+and devoured by carnivores"; of the carnivores and herbivores alike
+"subject to all the miseries incidental to old age, disease, and
+over-multiplication"; and of the "more or less enduring suffering,"
+which is the meed of both vanquished and victor. And he concludes that,
+since thousands of times a minute, were our ears sharp enough, we should
+hear sighs and groans of pain like those heard by Dante at the gate of
+hell, the world cannot be governed by what we call benevolence.[14]
+
+Now there is, I think, good reason to believe that all this is greatly
+exaggerated; that the supposed "torments" and "miseries" of animals have
+little real existence, but are the reflection of the imagined sensations
+of cultivated men and women in similar circumstances; and that the
+amount of actual suffering caused by the struggle for existence among
+animals is altogether insignificant. Let us, therefore, endeavour to
+ascertain what are the real facts on which these tremendous accusations
+are founded.
+
+In the first place, we must remember that animals are entirely spared
+the pain we suffer in the anticipation of death--a pain far greater, in
+most cases, than the reality. This leads, probably, to an almost
+perpetual enjoyment of their lives; since their constant watchfulness
+against danger, and even their actual flight from an enemy, will be the
+enjoyable exercise of the powers and faculties they possess, unmixed
+with any serious dread. There is, in the next place, much evidence to
+show that violent deaths, if not too prolonged, are painless and easy;
+even in the case of man, whose nervous system is in all probability much
+more susceptible to pain than that of most animals. In all cases in
+which persons have escaped after being seized by a lion or tiger, they
+declare that they suffered little or no pain, physical or mental. A
+well-known instance is that of Livingstone, who thus describes his
+sensations when seized by a lion: "Starting and looking half round, I
+saw the lion just in the act of springing on me. I was upon a little
+height; he caught my shoulder as he sprang, and we both came to the
+ground below together. Growling horribly close to my ear, he shook me as
+a terrier-dog does a rat. The shock produced a stupor similar to that
+which seems to be felt by a mouse after the first shake of the cat. It
+causes a sort of dreaminess, _in which there was no sense of pain or
+feeling of terror_, though I was quite conscious of all that was
+happening. It was like what patients partially under the influence of
+chloroform describe, who see all the operation, but feel not the knife.
+This singular condition was not the result of any mental process. The
+shake annihilated fear, and allowed no sense of horror in looking round
+at the beast."
+
+This absence of pain is not peculiar to those seized by wild beasts, but
+is equally produced by any accident which causes a general shock to the
+system. Mr. Whymper describes an accident to himself during one of his
+preliminary explorations of the Matterhorn, when he fell several hundred
+feet, bounding from rock to rock, till fortunately embedded in a
+snow-drift near the edge of a tremendous precipice. He declares that
+while falling and feeling blow after blow, he neither lost consciousness
+nor suffered pain, merely thinking, calmly, that a few more blows would
+finish him. We have therefore a right to conclude, that when death
+follows soon after any great shock it is as easy and painless a death as
+possible; and this is certainly what happens when an animal is seized by
+a beast of prey. For the enemy is one which hunts for food, not for
+pleasure or excitement; and it is doubtful whether any carnivorous
+animal in a state of nature begins to seek after prey till driven to do
+so by hunger. When an animal is caught, therefore, it is very soon
+devoured, and thus the first shock is followed by an almost painless
+death. Neither do those which die of cold or hunger suffer much. Cold is
+generally severest at night and has a tendency to produce sleep and
+painless extinction. Hunger, on the other hand, is hardly felt during
+periods of excitement, and when food is scarce the excitement of seeking
+for it is at its greatest. It is probable, also, that when hunger
+presses, most animals will devour anything to stay their hunger, and
+will die of gradual exhaustion and weakness not necessarily painful, if
+they do not fall an earlier prey to some enemy or to cold.[15]
+
+Now let us consider what are the enjoyments of the lives of most
+animals. As a rule they come into existence at a time of year when food
+is most plentiful and the climate most suitable, that is in the spring
+of the temperate zone and at the commencement of the dry season in the
+tropics. They grow vigorously, being supplied with abundance of food;
+and when they reach maturity their lives are a continual round of
+healthy excitement and exercise, alternating with complete repose. The
+daily search for the daily food employs all their faculties and
+exercises every organ of their bodies, while this exercise leads to the
+satisfaction of all their physical needs. In our own case, we can give
+no more perfect definition of happiness, than this exercise and this
+satisfaction; and we must therefore conclude that animals, as a rule,
+enjoy all the happiness of which they are capable. And this normal state
+of happiness is not alloyed, as with us, by long periods--whole lives
+often--of poverty or ill-health, and of the unsatisfied longing for
+pleasures which others enjoy but to which we cannot attain. Illness, and
+what answers to poverty in animals--continued hunger--are quickly
+followed by unanticipated and almost painless extinction. Where we err
+is, in giving to animals feelings and emotions which they do not
+possess. To us the very sight of blood and of torn or mangled limbs is
+painful, while the idea of the suffering implied by it is heartrending.
+We have a horror of all violent and sudden death, because we think of
+the life full of promise cut short, of hopes and expectations
+unfulfilled, and of the grief of mourning relatives. But all this is
+quite out of place in the case of animals, for whom a violent and a
+sudden death is in every way the best. Thus the poet's picture of
+
+
+    "Nature red in tooth and claw
+         With ravine"
+
+
+is a picture the evil of which is read into it by our imaginations, the
+reality being made up of full and happy lives, usually terminated by the
+quickest and least painful of deaths.
+
+On the whole, then, we conclude that the popular idea of the struggle
+for existence entailing misery and pain on the animal world is the very
+reverse of the truth. What it really brings about, is, the maximum of
+life and of the enjoyment of life with the minimum of suffering and
+pain. Given the necessity of death and reproduction--and without these
+there could have been no progressive development of the organic
+world,--and it is difficult even to imagine a system by which a greater
+balance of happiness could have been secured. And this view was
+evidently that of Darwin himself, who thus concludes his chapter on the
+struggle for existence: "When we reflect on this struggle, we may
+console ourselves with the full belief that the war of nature is not
+incessant, that no fear is felt, that death is generally prompt, and
+that the vigorous, the healthy, and the happy survive and multiply."
+
+FOOTNOTES:
+
+[Footnote 4: _Géographic Botanique_, p. 798.]
+
+[Footnote 5: _The Origin of Species_, p. 53.]
+
+[Footnote 6: _The Earth as Modified by Human Action_, p. 51.]
+
+[Footnote 7: _The Origin of Species_, p. 56.]
+
+[Footnote 8: See _Nature_, vol. xxxi. p. 63.]
+
+[Footnote 9: _A Visit to South America_, 1878; also _Nature_, vol. xxxi.
+pp. 263-339.]
+
+[Footnote 10: Still more remarkable is the increase of rabbits both in
+New Zealand and Australia. No less than seven millions of rabbit-skins
+have been exported from the former country in a single year, their value
+being £67,000. In both countries, sheep-runs have been greatly
+deteriorated in value by the abundance of rabbits, which destroy the
+herbage; and in some cases they have had to be abandoned altogether.]
+
+[Footnote 11: Later observers have proved that two eggs are laid and
+usually two young produced, but it may be that in most cases only one of
+these comes to maturity.]
+
+[Footnote 12: _Origin of Species_, p. 59. Professor A. Newton, however,
+informs me that these species do not interfere with one another in the
+way here stated.]
+
+[Footnote 13: Winwood Reade's _Martyrdom of Man,_ p. 520.]
+
+[Footnote 14: _Nineteenth Century,_ February 1888, pp. 162, 163.]
+
+[Footnote 15: The Kestrel, which usually feeds on mice, birds, and
+frogs, sometimes stays its hunger with earthworms, as do some of the
+American buzzards. The Honey-buzzard sometimes eats not only earthworms
+and slugs, but even corn; and the Buteo borealis of North America, whose
+usual food is small mammals and birds, sometimes eats crayfish.]
+
+
+
+
+CHAPTER III
+
+THE VARIABILITY OF SPECIES IN A STATE OF NATURE
+
+
+    Importance of variability--Popular ideas regarding
+    it--Variability of the lower animals--The variability of
+    insects--Variation among lizards--Variation among
+    birds--Diagrams of bird-variation--Number of varying
+    individuals--Variation in the mammalia--Variation in internal
+    organs--Variations in the skull--Variations in the habits of
+    Animals--The Variability of plants--Species which vary
+    little--Concluding remarks.
+
+
+
+The foundation of the Darwinian theory is the variability of species,
+and it is quite useless to attempt even to understand that theory, much
+less to appreciate the completeness of the proof of it, unless we first
+obtain a clear conception of the nature and extent of this variability.
+The most frequent and the most misleading of the objections to the
+efficacy of natural selection arise from ignorance of this subject, an
+ignorance shared by many naturalists, for it is only since Mr. Darwin
+has taught us their importance that varieties have been systematically
+collected and recorded; and even now very few collectors or students
+bestow upon them the attention they deserve. By the older naturalists,
+indeed, varieties--especially if numerous, small, and of frequent
+occurrence--were looked upon as an unmitigated nuisance, because they
+rendered it almost impossible to give precise definitions of species,
+then considered the chief end of systematic natural history. Hence it
+was the custom to describe what was supposed to be the "typical form" of
+species, and most collectors were satisfied if they possessed this
+typical form in their cabinets. Now, however, a collection is valued in
+proportion as it contains illustrative specimens of all the varieties
+that occur in each species, and in some cases these have been carefully
+described, so that we possess a considerable mass of information on the
+subject. Utilising this information we will now endeavour to give some
+idea of the nature and extent of variation in the species of animals and
+plants.
+
+It is very commonly objected that the widespread and constant
+variability which is admitted to be a characteristic of domesticated
+animals and cultivated plants is largely due to the unnatural conditions
+of their existence, and that we have no proof of any corresponding
+amount of variation occurring in a state of nature. Wild animals and
+plants, it is said, are usually stable, and when variations occur these
+are alleged to be small in amount and to affect superficial characters
+only; or if larger and more important, to occur so rarely as not to
+afford any aid in the supposed formation of new species.
+
+This objection, as will be shown, is utterly unfounded; but as it is one
+which goes to the very root of the problem, it is necessary to enter at
+some length into the various proofs of variation in a state of nature.
+This is the more necessary because the materials collected by Mr. Darwin
+bearing on this question have never been published, and comparatively
+few of them have been cited in _The Origin of Species_; while a
+considerable body of facts has been made known since the publication of
+the last edition of that work.
+
+
+_Variability of the Lower Animals_.
+
+Among the lowest and most ancient marine organisms are the Foraminifera,
+little masses of living jelly, apparently structureless, but which
+secrete beautiful shelly coverings, often perfectly symmetrical, as
+varied in form as those of the mollusca and far more complicated. These
+have been studied with great care by many eminent naturalists, and the
+late Dr. W.B. Carpenter in his great work--the _Introduction to the
+Study of the Foraminifera_--thus refers to their variability: "There is
+not a single species of plant or animal of which the range of variation
+has been studied by the collocation and comparison of so large a number
+of specimens as have passed under the review of Messrs. Williamson,
+Parker, Rupert Jones, and myself in our studies of the types of this
+group;" and he states as the result of this extensive comparison of
+specimens: "The range of variation is so great among the Foraminifera
+as to include not merely those differential characters which have been
+usually accounted _specific_, but also those upon which the greater part
+of the _genera_, of this group have been founded, and even in some
+instances those of its _orders_."[16]
+
+Coming now to a higher group--the Sea-Anemones--Mr. P.H. Gosse and other
+writers on these creatures often refer to variations in size, in the
+thickness and length of the tentacles, the form of the disc and of the
+mouth, and the character of surface of the column, while the colour
+varies enormously in a great number of the species. Similar variations
+occur in all the various groups of marine invertebrata, and in the great
+sub-kingdom of the mollusca they are especially numerous. Thus, Dr. S.P.
+Woodward states that many present a most perplexing amount of variation,
+resulting (as he supposes) from supply of food, variety of depth and of
+saltness of the water; but we know that many variations are quite
+independent of such causes, and we will now consider a few cases among
+the land-mollusca in which they have been more carefully studied.
+
+In the small forest region of Oahu, one of the Sandwich Islands, there
+have been found about 175 species of land-shells represented by 700 or
+800 varieties; and we are told by the Rev. J.T. Gulick, who studied them
+carefully, that "we frequently find a genus represented in several
+successive valleys by allied species, sometimes feeding on the same,
+sometimes on different plants. In every such case the valleys that are
+nearest to each other furnish the most nearly allied forms; _and a full
+set of the varieties of each species presents a minute gradation of
+forms between the more divergent types found in the more widely
+separated localities_."
+
+In most land-shells there is a considerable amount of variation in
+colour, markings, size, form, and texture or striation of the surface,
+even in specimens collected in the same locality. Thus, a French author
+has enumerated no less than 198 varieties of the common wood-snail
+(Helix nemoralis), while of the equally common garden-snail (Helix
+hortensis) ninety varieties have been described. Fresh-water shells are
+also subject to great variation, so that there is much uncertainty as
+to the number of species; and variations are especially frequent in the
+Planorbidae, which exhibit many eccentric deviations from the usual form
+of the species--deviations which must often affect the form of the
+living animal. In Mr. Ingersoll's Report on the Recent Mollusca of
+Colorado many of these extraordinary variations are referred to, and it
+is stated that a shell (Helisonia trivolvis) abundant in some small
+ponds and lakes, had scarcely two specimens alike, and many of them
+closely resembled other and altogether distinct species.[17]
+
+
+_The Variability of Insects_.
+
+Among Insects there is a large amount of variation, though very few
+entomologists devote themselves to its investigation. Our first examples
+will be taken from the late Mr. T. Vernon Wollaston's book, _On the
+Variation of Species_, and they must be considered as indications of
+very widespread though little noticed phenomena. He speaks of the
+curious little carabideous beetles of the genus Notiophilus as being
+"extremely unstable both in their sculpture and hue;" of the common
+Calathus mollis as having "the hind wings at one time ample, at another
+rudimentary, and at a third nearly obsolete;" and of the same
+irregularity as to the wings being characteristic of many Orthoptera and
+of the Homopterous Fulgoridae. Mr. Westwood in his _Modern
+Classification of Insects_ states that "the species of Gerris,
+Hydrometra, and Velia are mostly found perfectly apterous, though
+occasionally with full-sized wings."
+
+It is, however, among the Lepidoptera (butterflies and moths) that the
+most numerous cases of variation have been observed, and every good
+collection of these insects affords striking examples. I will first
+adduce the testimony of Mr. Bates, who speaks of the butterflies of the
+Amazon valley exhibiting innumerable local varieties or races, while
+some species showed great individual variability. Of the beautiful
+Mechanitis Polymnia he says, that at Ega on the Upper Amazons, "it
+varies not only in general colour and pattern, but also very
+considerably in the shape of the wings, especially in the male sex."
+Again, at St. Paulo, Ithomia Orolina exhibits four distinct varieties,
+all occurring together, and these differ not only in colour but in form,
+one variety being described as having the fore wings much elongated in
+the male, while another is much larger and has "the hind wings in the
+male different in shape." Of Heliconius Numata Mr. Bates says: "This
+species is so variable that it is difficult to find two examples exactly
+alike," while "it varies in structure as well as in colours. The wings
+are sometimes broader, sometimes narrower; and their edges are simple in
+some examples and festooned in others." Of another species of the same
+genus, H. melpomene, ten distinct varieties are described all more or
+less connected by intermediate forms, and four of these varieties were
+obtained at one locality, Serpa on the north bank of the Amazon.
+Ceratina Ninonia is another of these very unstable species exhibiting
+many local varieties which are, however, incomplete and connected by
+intermediate forms; while the several species of the genus Lycorea all
+vary to such an extent as almost to link them together, so that Mr.
+Bates thinks they might all fairly be considered as varieties of one
+species only.
+
+Turning to the Eastern Hemisphere we have in Papilio Severus a species
+which exhibits a large amount of simple variation, in the presence or
+absence of a pale patch on the upper wings, in the brown submarginal
+marks on the lower wings, in the form and extent of the yellow band, and
+in the size of the specimens. The most extreme forms, as well as the
+intermediate ones, are often found in one locality and in company with
+each other. A small butterfly (Terias hecabe) ranges over the whole of
+the Indian and Malayan regions to Australia, and everywhere exhibits
+great variations, many of which have been described as distinct species;
+but a gentleman in Australia bred two of these distinct forms (T. hecabe
+and T. Aesiope), with several intermediates, from one batch of
+caterpillars found feeding together on the same plant.[18] It is
+therefore very probable that a considerable number of supposed distinct
+species are only individual varieties.
+
+Cases of variation similar to those now adduced among butterflies might
+be increased indefinitely, but it is as well to note that such important
+characters as the neuration of the wings, on which generic and family
+distinctions are often established, are also subject to variation. The
+Rev. R.P. Murray, in 1872, laid before the Entomological Society
+examples of such variation in six species of butterflies, and other
+cases have been since described. The larvae of butterflies and moths are
+also very variable, and one observer recorded in the _Proceedings of the
+Entomological Society for_ 1870 no less than sixteen varieties of the
+caterpillar of the bedstraw hawk-moth (Deilephela galii).
+
+
+_Variation among Lizards_.
+
+Passing on from the lower animals to the vertebrata, we find more
+abundant and more definite evidence as to the extent and amount of
+individual variation. I will first give a case among the Reptilia from
+some of Mr. Darwin's unpublished MSS., which have been kindly lent me by
+Mr. Francis Darwin.
+
+"M. Milne Edwards (_Annales des Sci. Nat._, I ser., tom. xvi. p. 50) has
+given a curious table of measurements of fourteen specimens of Lacerta
+muralis; and, taking the length of the head as a standard, he finds the
+neck, trunk, tail, front and hind legs, colour, and femoral pores, all
+varying wonderfully; and so it is more or less with other species. So
+apparently trifling a character as the scales on the head affording
+almost the only constant characters."
+
+[Illustration: FIG. 1.--Variations of Lacerta muralis.]
+
+[Illustration: FIG. 2.--Variation of Lizards.]
+
+As the table of measurements above referred to would give no clear
+conception of the nature and amount of the variation without a laborious
+study and comparison of the figures, I have endeavoured to find a method
+of presenting the facts to the eye, so that they may be easily grasped
+and appreciated. In the diagram opposite, the comparative variations of
+the different organs of this species are given by means of variously
+bent lines. The head is represented by a straight line because it
+presented (apparently) no variation. The body is next given, the
+specimens being arranged in the order of their size from No. 1, the
+smallest, to No. 14, the largest, the actual lengths being laid down
+from a base line at a suitable distance below, in this case two inches
+below the centre, the mean length of the body of the fourteen specimens
+being two inches. The respective lengths of the neck, legs, and toe of
+each specimen are then laid down in the same manner at convenient
+distances apart for comparison; and we see that their variations bear no
+definite relation to those of the body, and not much to those of each
+other. With the exception of No. 5, in which all the parts agree in
+being large, there is a marked independence of each part, shown by the
+lines often curving in opposite directions; which proves that in those
+specimens one part is large while the other is small. The actual amount
+of the variation is very great, ranging from one-sixth of the mean
+length in the neck to considerably more than a fourth in the hind leg,
+and this among only fourteen examples which happen to be in a particular
+museum.
+
+To prove that this is not an isolated case, Professor Milne Edwards also
+gives a table showing the amount of variation in the museum specimens of
+six common species of lizards, also taking the head as the standard, so
+that the comparative variation of each part to the head is given. In the
+accompanying diagram (Fig. 2) the variations are exhibited by means of
+lines of varying length. It will be understood that, however much the
+specimens varied in _size_, if they had kept the same _proportions_, the
+variation line would have been in every case reduced to a point, as in
+the neck of L. velox which exhibits no variation. The different
+proportions of the variation lines for each species may show a distinct
+mode of variation, or may be merely due to the small and differing
+number of specimens; for it is certain that whatever amount of variation
+occurs among a few specimens will be greatly increased when a much
+larger number of specimens are examined. That the amount of variation is
+large, may be seen by comparing it with the actual length of the head
+(given below the diagram) which was used as a standard in determining
+the variation, but which itself seems not to have varied.[19]
+
+
+_Variation among Birds_.
+
+Coming now to the class of Birds, we find much more copious evidence of
+variation. This is due partly to the fact that Ornithology has perhaps a
+larger body of devotees than any other branch of natural history (except
+entomology); to the moderate size of the majority of birds; and to the
+circumstance that the form and dimensions of the wings, tail, beak, and
+feet offer the best generic and specific characters and can all be
+easily measured and compared. The most systematic observations on the
+individual variation of birds have been made by Mr. J.A. Allen, in his
+remarkable memoir: "On the Mammals and Winter Birds of East Florida,
+with an examination of certain assumed specific characters in Birds, and
+a sketch of the Bird Faunae of Eastern North America," published in the
+_Bulletin of the Museum of Comparative Zoology_ at Harvard College,
+Cambridge, Massachusetts, in 1871. In this work exact measurements are
+given of all the chief external parts of a large number of species of
+common American birds, from twenty to sixty or more specimens of each
+species being measured, so that we are able to determine with some
+precision the nature and extent of the variation that usually occurs.
+Mr. Allen says: "The facts of the case show that a variation of from 15
+to 20 per cent in general size, and an equal degree of variation in the
+relative size of different parts, may be ordinarily expected among
+specimens of the same species and sex, taken at the same locality, while
+in some cases the variation is even greater than this." He then goes on
+to show that each part varies to a considerable extent independently of
+the other parts; so that when the size varies, the proportions of all
+the parts vary, often to a much greater amount. The wing and tail, for
+example, besides varying in length, vary in the proportionate length of
+each feather, and this causes their outline to vary considerably in
+shape. The bill also varies in length, width, depth, and curvature. The
+tarsus varies in length, as does each toe separately and independently;
+and all this not to a minute degree requiring very careful measurement
+to detect it at all, but to an amount easily seen without any
+measurement, as it averages one-sixth of the whole length and often
+reaches one-fourth. In twelve species of common perching birds the wing
+varied (in from twenty-five to thirty specimens) from 14 to 21 per cent
+of the mean length, and the tail from 13.8 to 23.4 per cent. The
+variation of the form of the wing can be very easily tested by noting
+which feather is longest, which next in length, and so on, the
+respective feathers being indicated by the numbers 1, 2, 3, etc.,
+commencing with the outer one. As an example of the irregular variation
+constantly met with, the following occurred among twenty-five specimens
+of Dendroeca coronata. Numbers bracketed imply that the corresponding
+feathers were of equal length.[20]
+
+
+    RELATIVE LENGTHS OF PRIMARY WING FEATHERS OF
+    DENDROECA CORONATA.
+    ---------+-----------+----------+-----------+----------+----------
+    Longest. | Second in | Third in | Fourth in | Fifth in | Sixth in
+             |  Length.  |  Length. |  Length.  |  Length. |  Length.
+    ---------+-----------+----------+-----------+----------+----------
+    2        |     3     |     1    |     4     |     5    |     6
+    3        |     2     |     4    |     1     |     5    |     6
+             |  /  2     |          |           |          |
+    3        | {         |     1    |     5     |     6    |     7
+             |  \  4     |          |           |          |
+    2    \   |           |          |           |          |
+          }  |     4     |     1    |     5     |     6    |     7
+    3    /   |           |          |           |          |
+    2    \   |           |          |           |          |
+    1     |  |           |          |           |          |
+           } |     5     |     6    |     7     |     8    |     9
+    3     |  |           |          |           |          |
+    4    /   |           |          |           |          |
+    ---------+-----------+----------+-----------+----------+----------
+
+
+Here we have five very distinct proportionate lengths of the wing
+feathers, any one of which is often thought sufficient to characterise a
+distinct species of bird; and though this is rather an extreme case, Mr.
+Allen assures us that "the comparison, extended in the table to only a
+few species, has been carried to scores of others with similar results."
+
+Along with this variation in size and proportions there occurs a large
+amount of variation in colour and markings. "The difference in intensity
+of colour between the extremes of a series of fifty or one hundred
+specimens of any species, collected at a single locality, and nearly at
+the same season of the year, is often as great as occurs between truly
+distinct species." But there is also a great amount of individual
+variability in the markings of the same species. Birds having the
+plumage varied with streaks and spots differ exceedingly in different
+individuals of the same species in respect to the size, shape, and
+number of these marks, and in the general aspect of the plumage
+resulting from such variations. "In the common song sparrow (Melospiza
+melodia), the fox-coloured sparrow (Passerella iliaca), the swamp
+sparrow (Melospiza palustris), the black and white creeper (Mniotilta
+varia), the water-wagtail (Seiurus novaeboracencis), in Turdus
+fuscescens and its allies, the difference in the size of the streaks is
+often very considerable. In the song sparrow they vary to such an extent
+that in some cases they are reduced to narrow lines; in others so
+enlarged as to cover the greater part of the breast and sides of the
+body, sometimes uniting on the middle of the breast into a nearly
+continuous patch."
+
+Mr. Allen then goes on to particularise several species in which such
+variations occur, giving cases in which two specimens taken at the same
+place on the same day exhibited the two extremes of coloration. Another
+set of variations is thus described: "The white markings so common on
+the wings and tails of birds, as the bars formed by the white tips of
+the greater wing-coverts, the white patch occasionally present at the
+base of the primary quills, or the white band crossing them, and the
+white patch near the end of the outer tail-feathers are also extremely
+liable to variation in respect to their extent and the number of
+feathers to which, in the same species, these markings extend." It is to
+be especially noted that all these varieties are distinct from those
+which depend on season, on age, or on sex, and that they are such as
+have in many other species been considered to be of specific value.
+
+These variations of colour could not be presented to the eye without a
+series of carefully engraved plates, but in order to bring Mr. Allen's
+_measurements_, illustrating variations of size and proportion, more
+clearly before the reader, I have prepared a series of diagrams
+illustrating the more important facts and their bearings on the
+Darwinian theory.
+
+The first of these is intended, mainly, to show the actual amount of the
+variation, as it gives the true length of the wing and tail in the
+extreme cases among thirty specimens of each of three species. The
+shaded portion shows the minimum length, the unshaded portion the
+additional length in the maximum. The point to be specially noted here
+is, that in each of these common species there is about the same amount
+of variation, and that it is so great as to be obvious at a glance.
+
+[Illustration: FIG. 3.--Variation of Wings and Tail.]
+
+There is here no question of "minute" or "infinitesimal" variation,
+which many people suppose to be the only kind of variation that exists.
+It cannot even be called small; yet from all the evidence we now possess
+it seems to be the amount which characterises most of the common species
+of birds.
+
+It may be said, however, that these are the extreme variations, and only
+occur in one or two individuals, while the great majority exhibit little
+or no difference. Other diagrams will show that this is not the case;
+but even if it were so, it would be no objection at all, because these
+are the extremes among thirty specimens only. We may safely assume that
+these thirty specimens, taken by chance, are not, in the case of all
+these species, exceptional lots, and therefore we might expect at least
+two similarly varying specimens in each additional thirty. But the
+number of individuals, even in a very rare species, is probably thirty
+thousand or more, and in a common species thirty, or even three hundred,
+millions. Even one individual in each thirty, varying to the amount
+shown in the diagram, would give at least a million in the total
+population of any common bird, and among this million many would vary
+much more than the extreme among thirty only. We should thus have a vast
+body of individuals varying to a large extent in the length of the wings
+and tail, and offering ample material for the modification of these
+organs by natural selection. We will now proceed to show that other
+parts of the body vary, simultaneously, but independently, to an equal
+amount.
+
+[Illustration: FIG. 4.--Dolichonyx oryzivorus. 20 Males.]
+
+[Illustration: FIG. 5.--Agelaeus phoeniceus. 40 Males.]
+
+The first bird taken is the common Bob-o-link or Rice-bird (Dolichonyx
+oryzivorus), and the Diagram, Fig. 4, exhibits the variations of seven
+important characters in twenty male adult specimens.[21] These
+characters are--the lengths of the body, wing, tail, tarsus, middle toe,
+outer toe, and hind toe, being as many as can be conveniently exhibited
+in one diagram. The length of the body is not given by Mr. Allen, but as
+it forms a convenient standard of comparison, it has been obtained by
+deducting the length of the tail from the total length of the birds as
+given by him. The diagram has been constructed as follows:--The twenty
+specimens are first arranged in a series according to the body-lengths
+(which may be considered to give the size of the bird), from the
+shortest to the longest, and the same number of vertical lines are
+drawn, numbered from one to twenty. In this case (and wherever
+practicable) the body-length is measured from the lower line of the
+diagram, so that the actual length of the bird is exhibited as well as
+the actual variations of length. These can be well estimated by means of
+the horizontal line drawn at the mean between the two extremes, and it
+will be seen that one-fifth of the total number of specimens taken on
+either side exhibits a very large amount of variation, which would of
+course be very much greater if a hundred or more specimens were
+compared. The lengths of the wing, tail, and other parts are then laid
+down, and the diagram thus exhibits at a glance the comparative
+variation of these parts in every specimen as well as the actual amount
+of variation in the twenty specimens; and we are thus enabled to arrive
+at some important conclusions.
+
+We note, first, that the variations of none of the parts follow the
+variations of the body, but are sometimes almost in an opposite
+direction. Thus the longest wing corresponds to a rather small body, the
+longest tail to a medium body, while the longest leg and toes belong to
+only a moderately large body. Again, even related parts do not
+constantly vary together but present many instances of independent
+variation, as shown by the want of parallelism in their respective
+variation-lines. In No. 5 (see Fig. 4) the wing is very long, the tail
+moderately so; while in No. 6 the wing is much shorter while the tail is
+considerably longer. The tarsus presents comparatively little variation;
+and although the three toes may be said to vary in general together,
+there are many divergencies; thus, in passing from No. 9 to No. 10, the
+outer toe becomes longer, while the hind toe becomes considerably
+shorter; while in Nos. 3 and 4 the middle toe varies in an opposite way
+to the outer and the hind toes.
+
+[Illustration: FIG. 6.--Cardinalis virginianus. 31 Males.]
+
+In the next diagram (Fig. 5) we have the variations in forty males of
+the Red-winged Blackbird (Agelaeus phoeniceus), and here we see the same
+general features. One-fifth of the whole number of specimens offer a
+large amount of variation either below or above the mean; while the
+wings, tail, and head vary quite independently of the body. The wing and
+tail too, though showing some amount of correlated variation, yet in
+no less than nine cases vary in opposite directions as compared with the
+preceding species.
+
+The next diagram (Fig. 6), showing the variations of thirty-one males of
+the Cardinal bird (Cardinalis virginianus), exhibits these features much
+more strongly. The amount of variation in proportion to the size of the
+bird is very much greater; while the variations of the wing and tail not
+only have no correspondence with that of the body but very little with
+each other. In no less than twelve or thirteen instances they vary in
+opposite directions, while even where they correspond in direction the
+amount of the variation is often very disproportionate.
+
+As the proportions of the tarsi and toes of birds have great influence
+on their mode of life and habits and are often used as specific or even
+generic characters, I have prepared a diagram (Fig. 7) to show the
+variation in these parts only, among twenty specimens of each of four
+species of birds, four or five of the most variable alone being given.
+The extreme divergence of each of the lines in a vertical direction
+shows the actual amount of variation; and if we consider the small
+length of the toes of these small birds, averaging about three-quarters
+of an inch, we shall see that the variation is really very large; while
+the diverging curves and angles show that each part varies, to a great
+extent, independently. It is evident that if we compared some thousands
+of individuals instead of only twenty, we should have an amount of
+independent variation occurring each year which would enable almost any
+modification of these important organs to be rapidly effected.
+
+[Illustration: FIG. 7.--Variation of Tarsus and Toes.]
+
+[Illustration: FIG. 8.--Variation of Birds in Leyden Museum.]
+
+In order to meet the objection that the large amount of variability here
+shown depends chiefly on the observations of one person and on the birds
+of a single country, I have examined Professor Schlegel's Catalogue of
+the Birds in the Leyden Museum, in which he usually gives the range of
+variation of the specimens in the museum (which are commonly less than a
+dozen and rarely over twenty) as regards some of their more important
+dimensions. These fully support the statement of Mr. Allen, since they
+show an equal amount of variability when the numbers compared are
+sufficient, which, however, is not often the case. The accompanying
+diagram exhibits the actual differences of size in five organs which
+occur in five species taken almost at random from this catalogue. Here,
+again, we perceive that the variation is decidedly large, even among a
+very small number of specimens; while the facts all show that there is
+no ground whatever for the common assumption that natural species
+consist of individuals which are nearly all alike, or that the
+variations which occur are "infinitesimal" or even "small."
+
+
+_The proportionate Number of Individuals which present a considerable
+amount of Variation._
+
+The notion that variation is a comparatively exceptional phenomenon, and
+that in any case considerable variations occur very rarely in proportion
+to the number of individuals which do not vary, is so deeply rooted that
+it is necessary to show by every possible method of illustration how
+completely opposed it is to the facts of nature. I have therefore
+prepared some diagrams in which each of the individual birds measured is
+represented by a spot, placed at a proportionate distance, right and
+left, from the median line accordingly as it varies in excess or defect
+of the mean length as regards the particular part compared. As the
+object in this set of diagrams is to show the number of individuals
+which vary considerably in proportion to those which vary little or not
+at all, the scale has been enlarged in order to allow room for placing
+the spots without overlapping each other.
+
+In the diagram opposite twenty males of Icterus Baltimore are
+registered, so as to exhibit to the eye the proportionate number of
+specimens which vary, to a greater or less amount, in the length of the
+tail, wing, tarsus, middle toe, hind toe, and bill. It will be noticed
+that there is usually no very great accumulation of dots about the
+median line which shows the average dimensions, but that a considerable
+number are spread at varying distances on each side of it.
+
+In the next diagram (Fig. 10), showing the variation among forty males
+of Agelaeeus phoeniceus, this approach to an equable spreading of the
+variations is still more apparent; while in Fig. 12, where fifty-eight
+specimens of Cardinalis virginianus are registered, we see a remarkable
+spreading out of the spots, showing in some of the characters a tendency
+to segregation into two or more groups of individuals, each varying
+considerably from the mean.
+
+[Illustration: FIG. 9]
+
+[Illustration: FIG. 10.]
+
+[Illustration: FIG. 11.]
+
+In order fully to appreciate the teaching of these diagrams, we must
+remember, that, whatever kind and amount of variations are exhibited by
+the few specimens here compared, would be greatly extended and brought
+into symmetrical form if large numbers--thousands or millions--were
+subjected to the same process of measurement and registration. We know,
+from the general law which governs variations from a mean value, that
+with increasing numbers the range of variation of each part would
+increase also, at first rather rapidly and then more slowly; while gaps
+and irregularities would be gradually filled up, and at length the
+distribution of the dots would indicate a tolerably regular curve of
+double curvature like those shown in Fig. 11. The great divergence of
+the dots, when even a few specimens are compared, shows that the curve,
+with high numbers, would be a flat one like the lower curve in the
+illustration here given. This being the case it would follow that a very
+large proportion of the total number of individuals constituting a
+species would diverge considerably from its average condition as regards
+each part or organ; and as we know from the previous diagrams of
+variation (Figs. 1 to 7) that each part varies to a considerable extent,
+_independently_, the materials constantly ready for natural selection to
+act upon are abundant in quantity and very varied in kind. Almost any
+combination of variations of distinct parts will be available, where
+required; and this, as we shall see further on, obviates one of the most
+weighty objections which have been urged against the efficiency of
+natural selection in producing new species, genera, and higher groups.
+
+[Illustration: FIG. 12.]
+
+
+_Variation in the Mammalia._
+
+Owing to the generally large size of this class of animals, and the
+comparatively small number of naturalists who study them, large series
+of specimens are only occasionally examined and compared, and thus the
+materials for determining the question of their variability in a state
+of nature are comparatively scanty. The fact that our domestic animals
+belonging to this group, especially dogs, present extreme varieties not
+surpassed even by pigeons and poultry among birds, renders it almost
+certain that an equal amount of variability exists in the wild state;
+and this is confirmed by the example of a species of squirrel (Sciurus
+carolinensis), of which sixteen specimens, all males and all taken in
+Florida, were measured and tabulated by Mr. Allen. The diagram here
+given shows, that, both the general amount of the variation and the
+independent variability of the several members of the body, accord
+completely with the variations so common in the class of birds; while
+their amount and their independence of each other are even greater than
+usual.
+
+
+_Variation in the Internal Organs of Animals._
+
+In case it should be objected that the cases of variation hitherto
+adduced are in the external parts only, and that there is no proof that
+the internal organs vary in the same manner, it will be advisable to
+show that such varieties also occur. It is, however, impossible to
+adduce the same amount of evidence in this class of variation, because
+the great labour of dissecting large numbers of specimens of the same
+species is rarely undertaken, and we have to trust to the chance
+observations of anatomists recorded in their regular course of study.
+
+It must, however, be noted that a very large proportion of the
+variations already recorded in the external parts of animals necessarily
+imply corresponding internal variations. When feet and legs vary in
+size, it is because the bones vary; when the head, body, limbs, and tail
+change their proportions, the bony skeleton must also change; and even
+when the wing or tail feathers of birds become longer or more numerous,
+there is sure to be a corresponding change in the bones which support
+and the muscles which move them. I will, however, give a few cases of
+variations which have been directly observed.
+
+[Illustration: FIG. 13.--Sciurus carolinensis. 32 specimens. Florida.]
+
+Mr. Frank E. Beddard has kindly communicated to me some remarkable
+variations he has observed in the internal organs of a species of
+earthworm (Perionyx excavatus). The normal characters of this species
+are--
+
+
+    Setae forming a complete row round each segment.
+
+    Two pairs of spermathecae--spherical pouches without
+    diverticulae--in segments 8 and 9.
+
+    Two pairs of testes in segments 11 and 12.
+
+    Ovaries, a single pair in segment 13.
+
+    Oviducts open by a common pore in the middle of segment 14.
+
+    Vasa deferentia open separately in segment 18, each furnished at
+    its termination with a large prostate gland.
+
+
+Between two and three hundred specimens were examined, and among them
+thirteen specimens exhibited the following marked variations:--
+
+
+    (1) The number of the spermathecae varied from two to three or
+    four pairs, their position also varying.
+
+    (2) There were occasionally two pairs of ovaries, each with its
+    own oviduct; the external apertures of these varied in position,
+    being upon segments 13 and 14, 14 and 15, or 15 and 16.
+    Occasionally when there was only the normal single oviduct pore
+    present it varied in position, once occurring on the 10th, and
+    once on the 11th segment.
+
+    (3) The male generative pores varied in position from segments
+    14 to 20. In one instance there were two pairs instead of the
+    normal single pair, and in this case each of the four apertures
+    had its own prostate gland.
+
+
+Mr. Beddard remarks that all, or nearly all, the above variations are
+found _normally_ in other genera and species.
+
+When we consider the enormous number of earthworms and the comparatively
+very small number of individuals examined, we may be sure, not only that
+such variations as these occur with considerable frequency, but also
+that still more extraordinary deviations from the normal structure may
+often exist.
+
+The next example is taken from Mr. Darwin's unpublished MSS.
+
+
+    "In some species of Shrews (Sorex) and in some field-mice
+    (Arvicola), the Rev. L. Jenyns (_Ann. Nat. Hist._, vol. vii. pp.
+    267, 272) found the proportional length of the intestinal canal
+    to vary considerably. He found the same variability in the
+    number of the caudal vertebrae. In three specimens of an
+    Arvicola he found the gall-bladder having a very different
+    degree of development, and there is reason to believe it is
+    sometimes absent. Professor Owen has shown that this is the case
+    with the gall-bladder of the giraffe."
+
+
+Dr. Crisp (_Proc. Zool. Soc._, 1862, p. 137) found the gall-bladder
+present in some specimens of Cervus superciliaris while absent in
+others; and he found it to be absent in three giraffes which he
+dissected. A double gall-bladder was found in a sheep, and in a small
+mammal preserved in the Hunterian Museum there are three distinct
+gall-bladders.
+
+The length of the alimentary canal varies greatly. In three adult
+giraffes described by Professor Owen it was from 124 to 136 feet long;
+one dissected in France had this canal 211 feet long; while Dr. Crisp
+measured one of the extraordinary length of 254 feet, and similar
+variations are recorded in other animals.[22]
+
+The number of ribs varies in many animals. Mr. St. George Mivart says:
+"In the highest forms of the Primates, the number of true ribs is seven,
+but in Hylobates there are sometimes eight pairs. In Semnopithecus and
+Colobus there are generally seven, but sometimes eight pairs of true
+ribs. In the Cebidae there are generally seven or eight pairs, but in
+Ateles sometimes nine" (_Proc. Zool. Soc._, 1865, p. 568). In the same
+paper it is stated that the number of dorsal vertebrae in man is
+normally twelve, very rarely thirteen. In the Chimpanzee there are
+normally thirteen dorsal vertebrae, but occasionally there are fourteen
+or only twelve.
+
+
+_Variations in the Skull._
+
+[Illustration: FIG. 14.--Variation of Skull of Wolf. 10 specimens.]
+
+Among the nine adult male Orang-utans, collected by myself in Borneo,
+the skulls differed remarkably in size and proportions. The orbits
+varied in width and height, the cranial ridge was either single or
+double, either much or little developed, and the zygomatic aperture
+varied considerably in size. I noted particularly that these
+variations bore no necessary relation to each other, so that a large
+temporal muscle and zygomatic aperture might exist either with a large
+or a small cranium; and thus was explained the curious difference
+between the single-crested and the double-crested skulls, which had been
+supposed to characterise distinct species. As an instance of the amount
+of variation in the skulls of fully adult male orangs, I found the width
+between the orbits externally to be only 4 inches in one specimen and
+fully 5 inches in another.
+
+Exact measurements of large series of comparable skulls of the mammalia
+are not easily found, but from those available I have prepared three
+diagrams (Figs. 14, 15, and 16), in order to exhibit the facts of
+variation in this very important organ. The first shows the variation in
+ten specimens of the common wolf (Canis lupus) from one district in
+North America, and we see that it is not only large in amount, but that
+each part exhibits a considerable independent variability.[23]
+
+In Diagram 15 we have the variations of eight skulls of the Indian
+Honey-bear (Ursus labiatus), as tabulated by the late Dr. J.E. Gray of
+the British Museum. For such a small number of specimens the amount of
+variation is very large--from one-eighth to one-fifth of the mean
+size,--while there are an extraordinary number of instances of
+independent variability. In Diagram 16 we have the length and width of
+twelve skulls of adult males of the Indian wild boar (Sus cristatus),
+also given by Dr. Gray, exhibiting in both sets of measurements a
+variation of more than one-sixth, combined with a very considerable
+amount of independent variability.[24]
+
+[Illustration: FIG. 15.--Variation of 8 skulls (Ursus labiatus).]
+
+[Illustration: FIG. 16.]
+
+The few facts now given, as to variations of the internal parts of
+animals, might be multiplied indefinitely by a search through the
+voluminous writings of comparative anatomists. But the evidence already
+adduced, taken in conjunction with the much fuller evidence of variation
+in all external organs, leads us to the conclusion that wherever
+variations are looked for among a considerable number of individuals of
+the more common species they are sure to be found; that they are
+everywhere of considerable amount, often reaching 20 per cent of the
+size of the part implicated; and that they are to a great extent
+independent of each other, and thus afford almost any combination of
+variations that may be needed.
+
+It must be particularly noticed that the whole series of
+variation-diagrams here given (except the three which illustrate the
+number of varying individuals) in every case represent the actual amount
+of the variation, not on any reduced or enlarged scale, but as it were
+life-size. Whatever number of inches or decimals of an inch the species
+varies in any of its parts is marked on the diagrams, so that with the
+help of an ordinary divided rule or a pair of compasses the variation of
+the different parts can be ascertained and compared just as if the
+specimens themselves were before the reader, but with much greater ease.
+
+In my lectures on the Darwinian theory in America and in this country I
+used diagrams constructed on a different plan, equally illustrating the
+large amount of independent variability, but less simple and less
+intelligible. The present method is a modification of that used by Mr.
+Francis Galton in his researches on the theory of variability, the upper
+line (showing the variability of the body) in Diagrams 4, 5, 6, and 13,
+being laid down on the method he has used in his experiments with
+sweet-peas and in pedigree moth-breeding.[25] I believe, after much
+consideration, and many tedious experiments in diagram-making, that no
+better method can be adopted for bringing before the eye, both the
+amount and the peculiar features of individual variability.
+
+
+_Variations of the Habits of Animals._
+
+Closely connected with those variations of internal and external
+structure which have been already described, are the changes of habits
+which often occur in certain individuals or in whole species, since
+these must necessarily depend upon some corresponding change in the
+brain or in other parts of the organism; and as these changes are of
+great importance in relation to the theory of instinct, a few examples
+of them will be now adduced.
+
+The Kea (Nestor notabilis) is a curious parrot inhabiting the mountain
+ranges of the Middle Island of New Zealand. It belongs to the family of
+Brush-tongued parrots, and naturally feeds on the honey of flowers and
+the insects which frequent them, together with such fruits or berries as
+are found in the region. Till quite recently this comprised its whole
+diet, but since the country it inhabits has become occupied by Europeans
+it has developed a taste for a carnivorous diet, with alarming results.
+It began by picking the sheepskins hung out to dry or the meat in
+process of being cured. About 1868 it was first observed to attack
+living sheep, which had frequently been found with raw and bleeding
+wounds on their backs. Since then it is stated that the bird actually
+burrows into the living sheep, eating its way down to the kidneys, which
+form its special delicacy. As a natural consequence, the bird is being
+destroyed as rapidly as possible, and one of the rare and curious
+members of the New Zealand fauna will no doubt shortly cease to exist.
+The case affords a remarkable instance of how the climbing feet and
+powerful hooked beak developed for one set of purposes can be applied to
+another altogether different purpose, and it also shows how little real
+stability there may be in what appear to us the most fixed habits of
+life. A somewhat similar change of diet has been recorded by the Duke of
+Argyll, in which a goose, reared by a golden eagle, was taught by its
+foster-parent to eat flesh, which it continued to do regularly and
+apparently with great relish.[26]
+
+Change of habits appears to be often a result of imitation, of which Mr.
+Tegetmeier gives some good examples. He states that if pigeons are
+reared exclusively with small grain, as wheat or barley, they will
+starve before eating beans. But when they are thus starving, if a
+bean-eating pigeon is put among them, they follow its example, and
+thereafter adopt the habit. So fowls sometimes refuse to eat maize, but
+on seeing others eat it, they do the same and become excessively fond of
+it. Many persons have found that their yellow crocuses were eaten by
+sparrows, while the blue, purple, and white coloured varieties were left
+untouched; but Mr. Tegetmeier, who grows only these latter colours,
+found that after two years the sparrows began to attack them, and
+thereafter destroyed them quite as readily as the yellow ones; and he
+believes it was merely because some bolder sparrow than the rest set the
+example. On this subject Mr. Charles C. Abbott well remarks: "In
+studying the habits of our American birds--and I suppose it is true of
+birds everywhere--it must at all times be remembered that there is less
+stability in the habits of birds than is usually supposed; and no
+account of the habits of any one species will exactly detail the various
+features of its habits as they really are, in every portion of the
+territory it inhabits."[27]
+
+Mr. Charles Dixon has recorded a remarkable change in the mode of
+nest-building of some common chaffinches which were taken to New Zealand
+and turned out there. He says: "The cup of the nest is small, loosely
+put together, apparently lined with feathers, and the walls of the
+structure are prolonged for about 18 inches, and hang loosely down the
+side of the supporting branch. The whole structure bears some
+resemblance to the nests of the hangnests (Icteridae), with the
+exception that the cavity is at the top. Clearly these New Zealand
+chaffinches were at a loss for a design when fabricating their nest.
+They had no standard to work by, no nests of their own kind to copy, no
+older birds to give them any instruction, and the result is the abnormal
+structure I have just described."[28]
+
+These few examples are sufficient to show that both the habits and
+instincts of animals are subject to variation; and had we a sufficient
+number of detailed observations we should probably find that these
+variations were as numerous, as diverse in character, as large in
+amount, and as independent of each other as those which we have seen to
+characterise their bodily structure.
+
+
+_The Variability of Plants._
+
+The variability of plants is notorious, being proved not only by the
+endless variations which occur whenever a species is largely grown by
+horticulturists, but also by the great difficulty that is felt by
+botanists in determining the limits of species in many large genera. As
+examples we may take the roses, the brambles, and the willows as well
+illustrating this fact. In Mr. Baker's _Revision of the British Roses_
+(published by the Linnean Society in 1863), he includes under the single
+species, Rosa canina--the common dog-rose--no less than twenty-eight
+named _varieties_ distinguished by more or less constant characters and
+often confined to special localities, and to these are referred about
+seventy of the _species_ of British and continental botanists. Of the
+genus Rubus or bramble, _five_ British species are given in Bentham's
+_Handbook of the British Flora_, while in the fifth edition of
+Babington's _Manual of British Botany_, published about the same time,
+no less than _forty-five_ species are described. Of willows (Salix) the
+same two works enumerate _fifteen_ and _thirty-one_ species
+respectively. The hawkweeds (Hieracium) are equally puzzling, for while
+Mr. Bentham admits only seven British species, Professor Babington
+describes no less than thirty-two, besides several named varieties.
+
+A French botanist, Mons. A. Jordan, has collected numerous forms of a
+common little plant, the spring whitlow-grass (Draba verna); he has
+cultivated these for several successive years, and declares that they
+preserve their peculiarities unchanged; he also says that they each come
+true from seed, and thus possess all the characteristics of true
+species. He has described no less than fifty-two such species or
+permanent varieties, all found in the south of France; and he urges
+botanists to follow his example in collecting, describing, and
+cultivating all such varieties as may occur in their respective
+districts. Now, as the plant is very common almost all over Europe and
+ranges from North America to the Himalayas, the number of similar forms
+over this wide area would probably have to be reckoned by hundreds if
+not by thousands.
+
+The class of facts now adduced must certainly be held to prove that in
+many large genera and in some single species there is a very large
+amount of variation, which renders it quite impossible for experts to
+agree upon the limits of species. We will now adduce a few striking
+cases of individual variation.
+
+The distinguished botanist, Alp. de Candolle, made a special study of
+the oaks of the whole world, and has stated some remarkable facts as to
+their variability. He declares that on the same branch of oak he has
+noted the following variations: (1) In the length of the petiole, as one
+to three; (2) in the form of the leaf, being either elliptical or
+obovoid; (3) in the margin being entire, or notched, or even pinnatifid;
+(4) in the extremity being acute or blunt; (5) in the base being sharp,
+blunt, or cordate; (6) in the surface being pubescent or smooth; (7) the
+perianth varies in depth and lobing; (8) the stamens vary in number,
+independently; (9) the anthers are mucronate or blunt; (10) the fruit
+stalks vary greatly in length, often as one to three; (11) the number of
+fruits varies; (12) the form of the base of the cup varies; (13) the
+scales of the cup vary in form; (14) the proportions of the acorns vary;
+(15) the times of the acorns ripening and falling vary.
+
+Besides this, many species exhibit well-marked varieties which have been
+described and named, and these are most numerous in the best-known
+species. Our British oak (Quercus robur) has twenty-eight varieties;
+Quercus Lusitanica has eleven; Quercus calliprinos has ten; and Quercus
+coccifera eight.
+
+A most remarkable case of variation in the parts of a common flower has
+been given by Dr. Hermann Müller. He examined two hundred flowers of
+Myosurus minimus, among which he found _thirty-five_ different
+proportions of the sepals, petals, and anthers, the first varying from
+four to seven, the second from two to five, and the third from two to
+ten. Five sepals occurred in one hundred and eighty-nine out of the two
+hundred, but of these one hundred and five had three petals, forty-six
+had four petals, and twenty-six had five petals; but in each of these
+sets the anthers varied in number from three to eight, or from two to
+nine. We have here an example of the same amount of "independent
+variability" that, as we have seen, occurs in the various dimensions of
+birds and mammals; and it may be taken as an illustration of the kind
+and degree of variability that may be expected to occur among small and
+little specialised flowers.[29]
+
+In the common wind-flower (Anemone nemorosa) an almost equal amount of
+variation occurs; and I have myself gathered in one locality flowers
+varying from 7/8 inch to 1-3/4 inch in diameter; the bracts varying from
+1-1/2 inch to 4 inches across; and the petaloid sepals either broad or
+narrow, and varying in number from five to ten. Though generally pure
+white on their upper surface, some specimens are a full pink, while
+others have a decided bluish tinge.
+
+Mr. Darwin states that he carefully examined a large number of plants of
+Geranium phaeum and G. pyrenaicum (not perhaps truly British but
+frequently found wild), which had escaped from cultivation, and had
+spread by seed in an open plantation; and he declares that "the
+seedlings varied in almost every single character, both in their flowers
+and foliage, to a degree which I have never seen exceeded; yet they
+could not have been exposed to any great change of their
+conditions."[30]
+
+The following examples of variation in important parts of plants were
+collected by Mr. Darwin and have been copied from his unpublished
+MSS.:--
+
+"De Candolle (_Mem. Soc. Phys. de Genčve_, tom. ii. part ii. p. 217)
+states that Papaver bracteatum and P. orientale present indifferently
+two sepals and four petals, or three sepals and six petals, which is
+sufficiently rare with other species of the genus."
+
+"In the Primulacae and in the great class to which this family belongs
+the unilocular ovarium is free, but M. Dubury (_Mem. Soc. Phys. de
+Genčve_, tom. ii. p. 406) has often found individuals in Cyclamen
+hederaefolium, in which the base of the ovary was connected for a third
+part of its length with the inferior part of the calyx."
+
+"M. Aug. St. Hilaire (Sur la Gynobase, _Mem. des Mus. d'Hist. Nat._,
+tom. x. p. 134), speaking of some bushes of the Gomphia oleaefolia,
+which he at first thought formed a quite distinct species, says: 'Voilą
+donc dans un mźme individu des loges et un style qui se rattachent
+tantōt a un axe vertical, et tantōt a un gynobase; donc celui-ci n'est
+qu'un axe veritable; mais cet axe est deprimé au lieu d'źtre vertical."
+He adds (p. 151), 'Does not all this indicate that nature has tried, in
+a manner, in the family of Rutaceae to produce from a single
+multilocular ovary, one-styled and symmetrical, several unilocular
+ovaries, each with its own style.' And he subsequently shows that, in
+Xanthoxylum monogynum, 'it often happens that on the same plant, on the
+same panicle, we find flowers with one or with two ovaries;' and that
+this is an important character is shown by the Rutaceae (to which
+Xanthoxylum belongs), being placed in a group of natural orders
+characterised by having a solitary ovary."
+
+"De Candolle has divided the Cruciferae into five sub-orders in
+accordance with the position of the radicle and cotyledons, yet Mons. T.
+Gay (_Ann. des Scien. Nat._, ser. i. tom. vii. p. 389) found in sixteen
+seeds of Petrocallis Pyrenaica the form of the embryo so uncertain that
+he could not tell whether it ought to be placed in the sub-orders
+'Pleurorhizée' or 'Notor-hizée'; so again (p. 400) in Cochlearia
+saxatilis M. Gay examined twenty-nine embryos, and of these sixteen were
+vigorously 'pleurorhizées,' nine had characters intermediate between
+pleuro-and notor-hizées, and four were pure notor-hizées."
+
+"M. Raspail asserts (_Ann. des Scien. Nat._, ser. i. tom. v. p. 440)
+that a grass (Nostus Borbonicus) is so eminently variable in its floral
+organisation, that the varieties might serve to make a family with
+sufficiently numerous genera and tribes--a remark which shows that
+important organs must be here variable."
+
+
+_Species which vary little._
+
+The preceding statements, as to the great amount of variation occurring
+in animals and plants, do not prove that all species vary to the same
+extent, or even vary at all, but, merely, that a considerable number of
+species in every class, order, and family do so vary. It will have been
+observed that the examples of great variability have all been taken from
+common species, or species which have a wide range and are abundant in
+individuals. Now Mr. Darwin concludes, from an elaborate examination of
+the floras and faunas of several distinct regions, that common, wide
+ranging species, as a rule, vary most, while those that are confined to
+special districts and are therefore comparatively limited in number of
+individuals vary least. By a similar comparison it is shown that species
+of large genera vary more than species of small genera. These facts
+explain, to some extent, why the opinion has been so prevalent that
+variation is very limited in amount and exceptional in character. For
+naturalists of the old school, and all mere collectors, were interested
+in species in proportion to their rarity, and would often have in their
+collections a larger number of specimens of a rare species than of a
+species that was very common. Now as these rare species do really vary
+much less than the common species, and in many cases hardly vary at all,
+it was very natural that a belief in the fixity of species should
+prevail. It is not, however, as we shall see presently, the rare, but
+the common and widespread species which become the parents of new forms,
+and thus the non-variability of any number of rare or local species
+offers no difficulty whatever in the way of the theory of evolution.
+
+
+_Concluding Remarks._
+
+We have now shown in some detail, at the risk of being tedious, that
+individual variability is a general character of all common and
+widespread species of animals or plants; and, further, that this
+variability extends, so far as we know, to every part and organ, whether
+external or internal, as well as to every mental faculty. Yet more
+important is the fact that each part or organ varies to a considerable
+extent independently of other parts. Again, we have shown, by abundant
+evidence, that the variation that occurs is very large in
+amount--usually reaching 10 or 20, and sometimes even 25 per cent of the
+average size of the varying part; while not one or two only, but from 5
+to 10 per cent of the specimens examined exhibit nearly as large an
+amount of variation. These facts have been brought clearly before the
+reader by means of numerous diagrams, drawn to scale and exhibiting the
+actual variations in inches, so that there can be no possibility of
+denying either their generality or their amount. The importance of this
+full exposition of the subject will be seen in future chapters, when we
+shall frequently have to refer to the facts here set forth, especially
+when we deal with the various theories of recent writers and the
+criticisms that have been made of the Darwinian theory.
+
+A full exposition of the facts of variation among wild animals and
+plants is the more necessary, because comparatively few of them were
+published in Mr. Darwin's works, while the more important have only been
+made known since the last edition of _The Origin of Species_ was
+prepared; and it is clear that Mr. Darwin himself did not fully
+recognise the enormous amount of variability that actually exists. This
+is indicated by his frequent reference to the extreme slowness of the
+changes for which variation furnishes the materials, and also by his use
+of such expressions as the following: "A variety when once formed must
+again, _perhaps after a long interval of time_, vary or present
+individual differences of the same favourable nature as before"
+(_Origin_, p. 66). And again, after speaking of changed conditions
+"affording a better chance of the occurrence of favourable variations,"
+he adds: "_Unless such occur natural selection can do nothing_"
+(_Origin_, p. 64). These expressions are hardly consistent with the fact
+of the constant and large amount of variation, of every part, in all
+directions, which evidently occurs in each generation of all the more
+abundant species, and which must afford an ample supply of favourable
+variations whenever required; and they have been seized upon and
+exaggerated by some writers as proofs of the extreme difficulties in the
+way of the theory. It is to show that such difficulties do not exist,
+and in the full conviction that an adequate knowledge of the facts of
+variation affords the only sure foundation for the Darwinian theory of
+the origin of species, that this chapter has been written.
+
+FOOTNOTES:
+
+[Footnote 16: _Foraminifera_, preface, p. x.]
+
+[Footnote 17: _United States Geological Survey of the Territories_,
+1874.]
+
+[Footnote 18: _Proceedings of the Entomological Society of London_,
+1875, p. vii.]
+
+[Footnote 19: _Ann. des Sci. Nat._, tom. xvi. p. 50.]
+
+[Footnote 20: See _Winter Birds of Florida_, p. 206, Table F.]
+
+[Footnote 21: See Table I, p. 211, of Allen's _Winter Birds of
+Florida_.]
+
+[Footnote 22: _Proc. Zool. Soc._, 1864, p. 64.]
+
+[Footnote 23: J.A. Allen, on Geographical Variation among North American
+Mammals, _Bull. U.S. Geol. and Geog. Survey_, vol. ii. p. 314 (1876).]
+
+[Footnote 24: _Proc. Zool. Soc. Lond._, 1864, p. 700, and 1868, p. 28.]
+
+[Footnote 25: See _Trans. Entomological Society of London_, 1887, p.
+24.]
+
+[Footnote 26: _Nature_, vol. xix. p. 554.]
+
+[Footnote 27: _Nature_, vol. xvi. p. 163; and vol. xi. p. 227.]
+
+[Footnote 28: _Ibid._, vol. xxxi. (1885), p. 533.]
+
+[Footnote 29: _Nature_, vol. xxvi. p. 81.]
+
+[Footnote 30: _Animals and Plants under Domestication_, vol. ii. p.
+258.]
+
+
+
+
+CHAPTER IV
+
+VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS
+
+
+    The facts of variation and artificial selection--Proofs of the
+    generality of variation--Variations of apples and
+    melons--Variations of flowers--Variations of domestic
+    animals--Domestic pigeons--Acclimatisation--Circumstances
+    favourable to selection by man--Conditions favourable to
+    variation--Concluding remarks.
+
+
+
+Having so fully discussed variation under nature it will be unnecessary
+to devote so much space to domesticated animals and cultivated plants,
+especially as Mr. Darwin has published two remarkable volumes on the
+subject where those who desire it may obtain ample information. A
+general sketch of the more important facts will, however, be given, for
+the purpose of showing how closely they correspond with those described
+in the preceding chapter, and also to point out the general principles
+which they illustrate. It will also be necessary to explain how these
+variations have been increased and accumulated by artificial selection,
+since we are thereby better enabled to understand the action of natural
+selection, to be discussed in the succeeding chapter.
+
+
+_The facts of Variation and Artificial Selection._
+
+Every one knows that in each litter of kittens or of puppies no two are
+alike. Even in the case in which several are exactly alike in colours,
+other differences are always perceptible to those who observe them
+closely. They will differ in size, in the proportions of their bodies
+and limbs, in the length or texture of their hairy covering, and notably
+in their disposition. They each possess, too, an individual
+countenance, almost as varied when closely studied as that of a human
+being; not only can a shepherd distinguish every sheep in his flock, but
+we all know that each kitten in the successive families of our old
+favourite cat has a face of its own, with an expression and
+individuality distinct from all its brothers and sisters. Now this
+individual variability exists among all creatures whatever, which we can
+closely observe, even when the two parents are very much alike and have
+been matched in order to preserve some special breed. The same thing
+occurs in the vegetable kingdom. All plants raised from seed differ more
+or less from each other. In every bed of flowers or of vegetables we
+shall find, if we look closely, that there are countless small
+differences, in the size, in the mode of growth, in the shape or colour
+of the leaves, in the form, colour, or markings of the flowers, or in
+the size, form, colour, or flavour of the fruit. These differences are
+usually small, but are yet easily seen, and in their extremes are very
+considerable; and they have this important quality, that they have a
+tendency to be reproduced, and thus by careful breeding any particular
+variation or group of variations can be increased to an enormous
+extent--apparently to any extent not incompatible with the life, growth,
+and reproduction of the plant or animal.
+
+The way this is done is by artificial selection, and it is very
+important to understand this process and its results. Suppose we have a
+plant with a small edible seed, and we want to increase the size of that
+seed. We grow as large a quantity of it as possible, and when the crop
+is ripe we carefully choose a few of the very largest seeds, or we may
+by means of a sieve sort out a quantity of the largest seeds. Next year
+we sow only these large seeds, taking care to give them suitable soil
+and manure, and the result is found to be that the _average_ size of the
+seeds is larger than in the first crop, and that the largest seeds are
+now somewhat larger and more numerous. Again sowing these, we obtain a
+further slight increase of size, and in a very few years we obtain a
+greatly improved race, which will always produce larger seeds than the
+unimproved race, even if cultivated without any special care. In this
+way all our fine sorts of vegetables, fruits, and flowers have been
+obtained, all our choice breeds of cattle or of poultry, our wonderful
+race-horses, and our endless varieties of dogs. It is a very common but
+mistaken idea that this improvement is due to crossing and feeding in
+the case of animals, and to improved cultivation in the case of plants.
+Crossing is occasionally used in order to obtain a combination of
+qualities found in two distinct breeds, and also because it is found to
+increase the constitutional vigour; but every breed possessing any
+exceptional quality is the result of the selection of variations
+occurring year after year and accumulated in the manner just described.
+Purity of breed, with repeated selection of the best varieties of that
+breed, is the foundation of all improvement in our domestic animals and
+cultivated plants.
+
+
+_Proofs of the Generality of Variation._
+
+Another very common error is, that variation is the exception, and
+rather a rare exception, and that it occurs only in one direction at a
+time--that is, that only one or two of the numerous possible modes of
+variation occur at the same time. The experience of breeders and
+cultivators, however, proves that variation is the rule instead of the
+exception, and that it occurs, more or less, in almost every direction.
+This is shown by the fact that different species of plants and animals
+have required different _kinds_ of modification to adapt them to our
+use, and we have never failed to meet with variation _in that particular
+direction_, so as to enable us to accumulate it and so to produce
+ultimately a large amount of change in the required direction. Our
+gardens furnish us with numberless examples of this property of plants.
+In the cabbage and lettuce we have found variation in the size and mode
+of growth of the leaf, enabling us to produce by selection the almost
+innumerable varieties, some with solid heads of foliage quite unlike any
+plant in a state of nature, others with curiously wrinkled leaves like
+the savoy, others of a deep purple colour used for pickling. From the
+very same species as the cabbage (Brassica oleracea) have arisen the
+broccoli and cauliflower, in which the leaves have undergone little
+alteration, while the branching heads of flowers grow into a compact
+mass forming one of our most delicate vegetables. The brussels sprouts
+are another form of the same plant, in which the whole mode of growth
+has been altered, numerous little heads of leaves being produced on the
+stem. In other varieties the ribs of the leaves are thickened so as to
+become themselves a culinary vegetable; while, in the Kohlrabi, the stem
+grows into a turnip-like mass just above ground. Now all these
+extraordinarily distinct plants come from one original species which
+still grows wild on our coasts; and it must have varied in all these
+directions, otherwise variations could not have been accumulated to the
+extent we now see them. The flowers and seeds of all these plants have
+remained nearly stationary, because no attempt has been made to
+accumulate the slight variations that no doubt occur in them.
+
+If now we turn to another set of plants, the turnips, radishes, carrots,
+and potatoes, we find that the roots or underground tubers have been
+wonderfully enlarged and improved, and also altered in shape and colour,
+while the stems, leaves, flowers, and fruits have remained almost
+unchanged. In the various kinds of peas and beans it is the pod or fruit
+and the seed that has been subjected to selection, and therefore greatly
+modified; and it is here very important to notice that while all these
+plants have undergone cultivation in a great variety of soils and
+climates, with different manures and under different systems, yet the
+flowers have remained but little altered, those of the broad bean, the
+scarlet-runner, and the garden-pea, being nearly the same in all the
+varieties. This shows us how little change is produced by mere
+cultivation, or even by variety of soil and climate, if there is no
+_selection_ to preserve and accumulate the small variations that are
+continually occurring. When, however, a great amount of modification has
+been effected in one country, change to another country produces a
+decided effect. Thus it has been found that some of the numerous
+varieties of maize produced and cultivated in the United States change
+considerably, not only in their size and colour, but even in the shape
+of the seed when grown for a few successive years in Germany.[31] In all
+our cultivated fruit trees the fruits vary immensely in shape, size,
+colour, flavour, time of ripening, and other qualities, while the leaves
+and flowers usually differ so little that they are hardly
+distinguishable except to a very close observer.
+
+
+_Variations of Apples and of Melons._
+
+The most remarkable varieties are afforded by the apple and the melon,
+and some account of these will be given as illustrating the effects of
+slight variations accumulated by selection. All our apples are known to
+have descended from the common crab of our hedges (Pyrus malus), and
+from this at least a thousand distinct varieties have been produced.
+These differ greatly in the size and form of the fruit, in its colour,
+and in the texture of the skin. They further differ in the time of
+ripening, in their flavour, and in their keeping properties; but apple
+trees also differ in many other ways. The foliage of the different
+varieties can often be distinguished by peculiarities of form and
+colour, and it varies considerably in the time of its appearance; in
+some hardly a leaf appears till the tree is in full bloom, while others
+produce their leaves so early as almost to hide the flowers. The flowers
+differ in size and colour, and in one case in structure also, that of
+the St. Valery apple having a double calyx with ten divisions, and
+fourteen styles with oblique stigmas, but without stamens or corolla.
+The flowers, therefore, have to be fertilised with the pollen from other
+varieties in order to produce fruit. The pips or seeds differ also in
+shape, size, and colour; some varieties are liable to canker more than
+others, while the Winter Majetin and one or two others have the strange
+constitutional peculiarity of never being attacked by the mealy bug even
+when all the other trees in the same orchard are infested with it.
+
+All the cucumbers and gourds vary immensely, but the melon (Cucumis
+melo) exceeds them all. A French botanist, M. Naudin, devoted six years
+to their study. He found that previous botanists had described thirty
+distinct species, as they thought, which were really only varieties of
+melons. They differ chiefly in their fruits, but also very much in
+foliage and mode of growth. Some melons are only as large as small
+plums, others weigh as much as sixty-six pounds. One variety has a
+scarlet fruit. Another is not more than an inch in diameter, but
+sometimes more than a yard in length, twisting about in all directions
+like a serpent. Some melons are exactly like cucumbers; and an Algerian
+variety, when ripe, cracks and falls to pieces, just as occurs in a
+wild gourd (C. momordica).[32]
+
+
+_Variations of Flowers._
+
+Turning to flowers, we find that in the same genus as our currant and
+gooseberry, which we have cultivated for their fruits, there are some
+ornamental species, as the Ribes sanguinea, and in these the flowers
+have been selected so as to produce deep red, pink, or white varieties.
+When any particular flower becomes fashionable and is grown in large
+quantities, variations are always met with sufficient to produce great
+varieties of tint or marking, as shown by our roses, auriculas, and
+geraniums. When varied leaves are required, it is found that a number of
+plants vary sufficiently in this direction also, and we have zonal
+geraniums, variegated ivies, gold and silver marked hollies, and many
+others.
+
+
+_Variations of Domestic Animals._
+
+Coming now to our domesticated animals, we find still more extraordinary
+cases; and it appears as if any special quality or modification in an
+animal can be obtained if we only breed it in sufficient quantity, watch
+carefully for the required variations, and carry on selection with
+patience and skill for a sufficiently long period. Thus, in sheep we
+have enormously increased the wool, and have obtained the power of
+rapidly forming flesh and fat; in cows we have increased the production
+of milk; in horses we have obtained strength, endurance, or speed, and
+have greatly modified size, form, and colour; in poultry we have secured
+various colours of plumage, increase of size, and almost perpetual
+egg-laying. But it is in dogs and pigeons that the most marvellous
+changes have been effected, and these require our special attention.
+
+Our various domestic dogs are believed to have originated from several
+distinct wild species, because in every part of the world the native
+dogs resemble some wild dogs or wolves of the same country. Thus perhaps
+several species of wolves and jackals were domesticated in very early
+times, and from breeds derived from these, crossed and improved by
+selection, our existing dogs have descended. But this intermixture of
+distinct species will go a very little way in accounting for the
+peculiarities of the different breeds of dogs, many of which are totally
+unlike any wild animal. Such is the case with greyhounds, bloodhounds,
+bulldogs, Blenheim spaniels, terriers, pugs, turnspits, pointers, and
+many others; and these differ so greatly in size, shape, colour, and
+habits, as well as in the form and proportions of all the different
+parts of the body, that it seems impossible that they could have
+descended from any of the known wild dogs, wolves, or allied animals,
+none of which differ nearly so much in size, form, and proportions. We
+have here a remarkable proof that variation is not confined to
+superficial characters--to the colour, hair, or external appendages,
+when we see how the entire skeletons of such forms as the greyhound and
+the bulldog have been gradually changed in opposite directions till they
+are both completely unlike that of any known wild animal, recent or
+extinct. These changes have been the result of some thousands of years
+of domestication and selection, different breeds being used and
+preserved for different purposes; but some of the best breeds are known
+to have been improved and perfected in modern times. About the middle of
+the last century a new and improved kind of foxhound was produced; the
+greyhound was also greatly improved at the end of the last century,
+while the true bulldog was brought to perfection about the same period.
+The Newfoundland dog has been so much changed since it was first
+imported that it is now quite unlike any existing native dog in that
+island.[33]
+
+
+_Domestic Pigeons._
+
+The most remarkable and instructive example of variation produced by
+human selection is afforded by the various races and breeds of domestic
+pigeons, not only because the variations produced are often most
+extraordinary in amount and diverse in character, but because in this
+case there is no doubt whatever that all have been derived from one wild
+species, the common rock-pigeon (Columba livia). As this is a very
+important point it is well to state the evidence on which the belief is
+founded. The wild rock-pigeon is of a slaty-blue colour, the tail has a
+dark band across the end, the wings have two black bands, and the outer
+tail-feathers are edged with white at the base. No other wild pigeon in
+the world has this combination of characters. Now in every one of the
+domestic varieties, even the most extreme, all the above marks, even to
+the white edging of the outer tail-feathers, are sometimes found
+perfectly developed. When birds belonging to two distinct breeds are
+crossed one or more times, neither of the parents being blue, or having
+any of the above-named marks, the mongrel offspring are very apt to
+acquire some of these characters. Mr. Darwin gives instances which he
+observed himself. He crossed some white fantails with some black barbs,
+and the mongrels were black, brown, or mottled. He also crossed a barb
+with a spot, which is a white bird with a red tail and red spot on the
+forehead, and the mongrel offspring were dusky and mottled. On now
+crossing these two sets of mongrels with each other, he obtained a bird
+of a beautiful blue colour, with the barred and white edged tail, and
+double-banded wings, so as almost exactly to resemble a wild
+rock-pigeon. This bird was descended in the second generation from a
+pure white and pure black bird, both of which when unmixed breed their
+kind remarkably true. These facts, well known to experienced
+pigeon-fanciers, together with the habits of the birds, which all like
+to nest in holes, or dovecots, not in trees like the great majority of
+wild pigeons, have led to the general belief in the single origin of all
+the different kinds.
+
+In order to afford some idea of the great differences which exist among
+domesticated pigeons, it will be well to give a brief abstract of Mr.
+Darwin's account of them. He divides them into eleven distinct races,
+most of which have several sub-races.
+
+RACE I. _Pouters_.--These are especially distinguished by the enormously
+enlarged crop, which can be so inflated in some birds as almost to
+conceal the beak. They are very long in the body and legs and stand
+almost upright, so as to present a very distinct appearance. Their
+skeleton has become modified, the ribs being broader and the vertebrae
+more numerous than in other pigeons.
+
+RACE II. _Carriers_.--These are large, long-necked birds, with a long
+pointed beak, and the eyes surrounded with a naked carunculated skin or
+wattle, which is also largely developed at the base of the beak. The
+opening of the mouth is unusually wide. There are several sub-races, one
+being called Dragons.
+
+RACE III. _Runts_.--These are very large-bodied, long-beaked pigeons,
+with naked skin round the eyes. The wings are usually very long, the
+legs long, and the feet large, and the skin of the neck is often red.
+There are several sub-races, and these differ very much, forming a
+series of links between the wild rock-pigeon and the carrier.
+
+RACE IV. _Barbs_.--These are remarkable for their very short and thick
+beak, so unlike that of most pigeons that fanciers compare it with that
+of a bullfinch. They have also a naked carunculated skin round the eyes,
+and the skin over the nostrils swollen.
+
+RACE V. _Fantails_.--Short-bodied and rather small-beaked pigeons, with
+an enormously developed tail, consisting usually of from fourteen to
+forty feathers instead of twelve, the regular number in all other
+pigeons, wild and tame. The tail spreads out like a fan and is usually
+carried erect, and the bird bends back its slender neck, so that in
+highly-bred varieties the head touches the tail. The feet are small, and
+they walk stiffly.
+
+RACE VI. _Turbits and Owls_.--These are characterised by the feathers of
+the middle of neck and breast in front spreading out irregularly so as
+to form a frill. The Turbits also have a crest on the head, and both
+have the beak exceedingly short.
+
+RACE VII. _Tumblers_.--- These have a small body and short beak, but
+they are specially distinguished by the singular habit of tumbling over
+backwards during flight. One of the sub-races, the Indian Lotan or
+Ground tumbler, if slightly shaken and placed on the ground, will
+immediately begin tumbling head over heels until taken up and soothed.
+If not taken up, some of them will go on tumbling till they die. Some
+English tumblers are almost equally persistent. A writer, quoted by Mr.
+Darwin, says that these birds generally begin to tumble almost as soon
+as they can fly; "at three months old they tumble well, but still fly
+strong; at five or six months they tumble excessively; and in the second
+year they mostly give up flying, on account of their tumbling so much
+and so close to the ground. Some fly round with the flock, throwing a
+clean summersault every few yards till they are obliged to settle from
+giddiness and exhaustion. These are called Air-tumblers, and they
+commonly throw from twenty to thirty summersaults in a minute, each
+clear and clean. I have one red cock that I have on two or three
+occasions timed by my watch, and counted forty summersaults in the
+minute. At first they throw a single summersault, then it is double,
+till it becomes a continuous roll, which puts an end to flying, for if
+they fly a few yards over they go, and roll till they reach the ground.
+Thus I had one kill herself, and another broke his leg. Many of them
+turn over only a few inches from the ground, and will tumble two or
+three times in flying across their loft. These are called House-tumblers
+from tumbling in the house. The act of tumbling seems to be one over
+which they have no control, an involuntary movement which they seem to
+try to prevent. I have seen a bird sometimes in his struggles fly a yard
+or two straight upwards, the impulse forcing him backwards while he
+struggles to go forwards."[34]
+
+The Short-faced tumblers are an improved sub-race which have almost lost
+the power of tumbling, but are valued for possessing some other
+characteristics in an extreme degree. They are very small, have almost
+globular heads, and a very minute beak, so that fanciers say the head of
+a perfect bird should resemble a cherry with a barleycorn stuck in it.
+Some of these weigh less than seven ounces, whereas the wild rock-pigeon
+weighs about fourteen ounces. The feet, too, are very short and small,
+and the middle toe has twelve or thirteen instead of fourteen or fifteen
+scutellae. They have often only nine primary wing-feathers instead of
+ten as in all other pigeons.
+
+RACE VIII. _Indian Frill-back_.--In these birds the beak is very short,
+and the feathers of the whole body are reversed or turn backwards.
+
+RACE IX. _Jacobin_.--These curious birds have a hood of feathers almost
+enclosing the head and meeting in front of the neck. The wings and tail
+are unusually long.
+
+RACE X. _Trumpeter_.--Distinguished by a tuft of feathers curling
+forwards over the beak, and the feet very much feathered. They obtain
+their name from the peculiar voice unlike that of any other pigeon. The
+coo is rapidly repeated, and is continued for several minutes. The feet
+are covered with feathers so large as often to appear like little wings.
+
+RACE XI. comprises _Laughers_, _Frill-backs_, _Nuns_, _Spots_, _and
+Swallows_.--They are all very like the common rock-pigeon, but have each
+some slight peculiarity. The Laughers have a peculiar voice, supposed to
+resemble a laugh. The Nuns are white, with the head, tail, and primary
+wing-feathers black or red. The Spots are white, with the tail and a
+spot on the forehead red. The Swallows are slender, white in colour,
+with the head and wings of some darker colour.
+
+Besides these races and sub-races a number of other kinds have been
+described, and about one hundred and fifty varieties can be
+distinguished. It is interesting to note that almost every part of the
+bird, whose variations can be noted and selected, has led to variations
+of a considerable extent, and many of these have necessitated changes in
+the plumage and in the skeleton quite as great as any that occur in the
+numerous distinct species of large genera. The form of the skull and
+beak varies enormously, so that the skulls of the Short-faced tumbler
+and some of the Carriers differ more than any wild pigeons, even those
+classed in distinct genera. The breadth and number of the ribs vary, as
+well as the processes on them; the number of the vertebrae and the
+length of the sternum also vary; and the perforations in the sternum
+vary in size and shape. The oil gland varies in development, and is
+sometimes absent. The number of the wing-feathers varies, and those of
+the tail to an enormous extent. The proportions of the leg and feet and
+the number of the scutellae also vary. The eggs also vary somewhat in
+size and shape; and the amount of downy clothing on the young bird, when
+first hatched, differs very considerably. Finally, the attitude of the
+body, the manner of walking, the mode of flight, and the voice, all
+exhibit modifications of the most remarkable kind.[35]
+
+
+_Acclimatisation_.
+
+A very important kind of variation is that constitutional change termed
+acclimatisation, which enables any organism to become gradually adapted
+to a different climate from the parent stock. As closely allied species
+often inhabit different countries possessing very different climates, we
+should expect to find cases illustrating this change among our
+domesticated animals and cultivated plants. A few examples will
+therefore be adduced showing that such constitutional variation does
+occur.
+
+Among animals the cases are not numerous, because no systematic attempt
+has been made to select varieties for this special quality. It has,
+however, been observed that, though no European dogs thrive well in
+India, the Newfoundland dog, originating from a severe climate, can
+hardly be kept alive. A better case, perhaps, is furnished by merino
+sheep, which, when imported directly from England, do not thrive, while
+those which have been bred in the intermediate climate of the Cape of
+Good Hope do much better. When geese were first introduced into Bogota,
+they laid few eggs at long intervals, and few of the young survived. By
+degrees, however, the fecundity improved, and in about twenty years
+became equal to what it is in Europe. According to Garcilaso, when fowls
+were first introduced into Peru they were not fertile, whereas now they
+are as much so as in Europe.
+
+Plants furnish much more important evidence. Our nurserymen distinguish
+in their catalogues varieties of fruit-trees which are more or less
+hardy, and this is especially the case in America, where certain
+varieties only will stand the severe climate of Canada. There is one
+variety of pear, the Forelle, which both in England and France withstood
+frosts that killed the flowers and buds of all other kinds of pears.
+Wheat, which is grown over so large a portion of the world, has become
+adapted to special climates. Wheat imported from India and sown in good
+wheat soil in England produced the most meagre ears; while wheat taken
+from France to the West Indian Islands produced either wholly barren
+spikes or spikes furnished with two or three miserable seeds, while West
+Indian seed by its side yielded an enormous harvest. The orange was very
+tender when first introduced into Italy, and continued so as long as it
+was propagated by grafts, but when trees were raised from seed many of
+these were found to be hardier, and the orange is now perfectly
+acclimatised in Italy. Sweet-peas (Lathyrus odoratus) imported from
+England to the Calcutta Botanic Gardens produced few blossoms and no
+seed; those from France flowered a little better, but still produced no
+seed, but plants raised from seed brought from Darjeeling in the
+Himalayas, but originally derived from England, flower and seed
+profusely in Calcutta.[36]
+
+An observation by Mr. Darwin himself is perhaps even more instructive.
+He says: "On 24th May 1864 there was a severe frost in Kent, and two
+rows of scarlet runners (Phaseolus multiflorus) in my garden, containing
+390 plants of the same age and equally exposed, were all blackened and
+killed except about a dozen plants. In an adjoining row of Fulmer's
+dwarf bean (Phaseolus vulgaris) one single plant escaped. A still more
+severe frost occurred four days afterwards, and of the dozen plants
+which had previously escaped only three survived; these were not taller
+or more vigorous than the other young plants, but they escaped
+completely, with not even the tips of their leaves browned. It was
+impossible to behold these three plants, with their blackened, withered,
+and dead brethren all around them, and not see at a glance that they
+differed widely in their constitutional power of resisting frost."
+
+The preceding sketch of the variation that occurs among domestic animals
+and cultivated plants shows how wide it is in range and how great in
+amount; and we have good reason to believe that similar variation
+extends to all organised beings. In the class of fishes, for example, we
+have one kind which has been long domesticated in the East, the gold
+and silver carps; and these present great variation, not only of colour
+but in the form and structure of the fins and other external organs. In
+like manner, the only domesticated insects, hive bees and silkworm
+moths, present numbers of remarkable varieties which have been produced
+by the selection of chance variations just as in the case of plants and
+the higher animals.
+
+
+_Circumstances favourable to Selection by Man._
+
+It may be supposed, that the systematic selection which has been
+employed for the purpose of improving the races of animals or plants
+useful to man is of comparatively recent origin, though some of the
+different races are known to have been in existence in very early times.
+But Mr. Darwin has pointed out, that unconscious selection must have
+begun to produce an effect as soon as plants were cultivated or animals
+domesticated by man. It would have been very soon observed that animals
+and plants produced their like, that seed of early wheat produced early
+wheat, that the offspring of very swift dogs were also swift, and as
+every one would try to have a good rather than a bad sort this would
+necessarily lead to the slow but steady improvement of all useful plants
+and animals subject to man's care. Soon there would arise distinct
+breeds, owing to the varying uses to which the animals and plants were
+put. Dogs would be wanted chiefly to hunt one kind of game in one part
+of the country and another kind elsewhere; for one purpose scent would
+be more important, for another swiftness, for another strength and
+courage, for yet another watchfulness and intelligence, and this would
+soon lead to the formation of very distinct races. In the case of
+vegetables and fruits, different varieties would be found to succeed
+best in certain soils and climates; some might be preferred on account
+of the quantity of food they produced, others for their sweetness and
+tenderness, while others might be more useful on account of their
+ripening at a particular season, and thus again distinct varieties would
+be established. An instance of unconscious selection leading to distinct
+results in modern times is afforded by two flocks of Leicester sheep
+which both originated from the same stock, and were then bred pure for
+upwards of fifty years by two gentlemen, Mr. Buckley and Mr. Burgess.
+Mr. Youatt, one of the greatest authorities on breeding domestic
+animals, says: "There is not a suspicion existing in the mind of any one
+at all acquainted with the subject that the owner of either of them has
+deviated in any one instance from the pure blood of Mr. Bakewell's
+original flock, and yet the difference between the sheep possessed by
+these two gentlemen is so great that they have the appearance of being
+quite different varieties." In this case there was no desire to deviate
+from the original breed, and the difference must have arisen from some
+slight difference of taste or judgment in selecting, each year, the
+parents for the next year's stock, combined perhaps with some direct
+effect of the slight differences of climate and soil on the two farms.
+
+Most of our domesticated animals and cultivated plants have come to us
+from the earliest seats of civilisation in Western Asia or Egypt, and
+have therefore been the subjects of human care and selection for some
+thousands of years, the result being that, in many cases, we do not know
+the wild stock from which they originally sprang. The horse, the camel,
+and the common bull and cow are nowhere found in a wild state, and they
+have all been domesticated from remote antiquity. The original of the
+domestic fowl is still wild in India and the Malay Islands, and it was
+domesticated in India and China before 1400 B.C. It was introduced into
+Europe about 600 B.C. Several distinct breeds were known to the Romans
+about the commencement of the Christian era, and they have since spread
+all over the civilised world and been subjected to a vast amount of
+conscious and unconscious selection, to many varieties of climate and to
+differences of food; the result being seen in the wonderful diversity of
+breeds which differ quite as remarkably as do the different races of
+pigeons already described.
+
+In the vegetable kingdom, most of the cereals--wheat, barley, etc.--are
+unknown as truly wild plants; and the same is the case with many
+vegetables, for De Candolle states that out of 157 useful cultivated
+plants thirty-two are quite unknown in a wild state, and that forty more
+are of doubtful origin. It is not improbable that most of these do exist
+wild, but they have been so profoundly changed by thousands of years of
+cultivation as to be quite unrecognisable. The peach is unknown in a
+wild state, unless it is derived from the common almond, on which point
+there is much difference of opinion among botanists and horticulturists.
+
+The immense antiquity of most of our cultivated plants sufficiently
+explains the apparent absence of such useful productions in Australia
+and the Cape of Good Hope, notwithstanding that they both possess an
+exceedingly rich and varied flora. These countries having been, until a
+comparatively recent period, inhabited only by uncivilised men, neither
+cultivation nor selection has been carried on for a sufficiently long
+time. In North America, however, where there was evidently a very
+ancient if low form of civilisation, as indicated by the remarkable
+mounds, earthworks, and other prehistoric remains, maize was cultivated,
+though it was probably derived from Peru; and the ancient civilisation
+of that country and of Mexico has given rise to no fewer than
+thirty-three useful cultivated plants.
+
+
+_Conditions favourable to the production of Variations._
+
+In order that plants and animals may be improved and modified to any
+considerable extent, it is of course essential that suitable variations
+should occur with tolerable frequency. There seem to be three conditions
+which are especially favourable to the production of variations: (1)
+That the particular species or variety should be kept in very large
+numbers; (2) that it should be spread over a wide area and thus
+subjected to a considerable diversity of physical conditions; and (3)
+that it should be occasionally crossed with some distinct but closely
+allied race. The first of these conditions is perhaps the most
+important, the chance of variations of any particular kind being
+increased in proportion to the quantity of the original stock and of its
+annual offspring. It has been remarked that only those breeders who keep
+large flocks can effect much improvement; and it is for the same reason
+that pigeons and fowls, which can be so easily and rapidly increased,
+and which have been kept in such large numbers by so great a number of
+persons, have produced such strange and numerous varieties. In like
+manner, nurserymen who grow fruit and flowers in large quantities have a
+great advantage over private amateurs in the production of new
+varieties.
+
+Although I believe, for reasons which will be given further on, that
+some amount of variability is a constant and necessary property of all
+organisms, yet there appears to be good evidence to show that changed
+conditions of life tend to increase it, both by a direct action on the
+organisation and by indirectly affecting the reproductive system. Hence
+the extension of civilisation, by favouring domestication under altered
+conditions, facilitates the process of modification. Yet this change
+does not seem to be an essential condition, for nowhere has the
+production of extreme varieties of plants and flowers been carried
+farther than in Japan, where careful selection continued for many
+generations must have been the chief factor. The effect of occasional
+crosses often results in a great amount of variation, but it also leads
+to instability of character, and is therefore very little employed in
+the production of fixed and well-marked races. For this purpose, in
+fact, it has to be carefully avoided, as it is only by isolation and
+pure breeding that any specially desired qualities can be increased by
+selection. It is for this reason that among savage peoples, whose
+animals run half wild, little improvement takes place; and the
+difficulty of isolation also explains why distinct and pure breeds of
+cats are so rarely met with. The wide distribution of useful animals and
+plants from a very remote epoch has, no doubt, been a powerful cause of
+modification, because the particular breed first introduced into each
+country has often been kept pure for many years, and has also been
+subjected to slight differences of conditions. It will also usually have
+been selected for a somewhat different purpose in each locality, and
+thus very distinct races would soon originate.
+
+The important physiological effects of crossing breeds or strains, and
+the part this plays in the economy of nature, will be explained in a
+future chapter.
+
+
+_Concluding Remarks._
+
+The examples of variation now adduced--and these might have been almost
+indefinitely increased--will suffice to show that there is hardly an
+organ or a quality in plants or animals which has not been observed to
+vary; and further, that whenever any of these variations have been
+useful to man he has been able to increase them to a marvellous extent
+by the simple process of always preserving the best varieties to breed
+from. Along with these larger variations others of smaller amount
+occasionally appear, sometimes in external, sometimes in internal
+characters, the very bones of the skeleton often changing slightly in
+form, size, or number; but as these secondary characters have been of no
+use to man, and have not been specially selected by him, they have,
+usually, not been developed to any great amount except when they have
+been closely dependent on those external characters which he has largely
+modified.
+
+As man has considered only utility to himself, or the satisfaction of
+his love of beauty, of novelty, or merely of something strange or
+amusing, the variations he has thus produced have something of the
+character of monstrosities. Not only are they often of no use to the
+animals or plants themselves, but they are not unfrequently injurious to
+them. In the Tumbler pigeons, for instance, the habit of tumbling is
+sometimes so excessive as to injure or kill the bird; and many of our
+highly-bred animals have such delicate constitutions that they are very
+liable to disease, while their extreme peculiarities of form or
+structure would often render them quite unfit to live in a wild state.
+In plants, many of our double flowers, and some fruits, have lost the
+power of producing seed, and the race can thus be continued only by
+means of cuttings or grafts. This peculiar character of domestic
+productions distinguishes them broadly from wild species and varieties,
+which, as will be seen by and by, are necessarily adapted in every part
+of their organisation to the conditions under which they have to live.
+Their importance for our present inquiry depends on their demonstrating
+the occurrence of incessant slight variations in all parts of an
+organism, with the transmission to the offspring of the special
+characteristics of the parents; and also, that all such slight
+variations are capable of being accumulated by selection till they
+present very large and important divergencies from the ancestral stock.
+
+We thus see, that the evidence as to variation afforded by animals and
+plants under domestication strikingly accords with that which we have
+proved to exist in a state of nature. And it is not at all surprising
+that it should be so, since all the species were in a state of nature
+when first domesticated or cultivated by man, and whatever variations
+occur must be due to purely natural causes. Moreover, on comparing the
+variations which occur in any one generation of domesticated animals
+with those which we know to occur in wild animals, we find no evidence
+of greater individual variation in the former than in the latter. The
+results of man's selection are more striking to us because we have
+always considered the varieties of each domestic animal to be
+essentially identical, while those which we observe in a wild state are
+held to be essentially diverse. The greyhound and the spaniel seem
+wonderful, as varieties of one animal produced by man's selection; while
+we think little of the diversities of the fox and the wolf, or the horse
+and the zebra, because we have been accustomed to look upon them as
+radically distinct animals, not as the results of nature's selection of
+the varieties of a common ancestor.
+
+FOOTNOTES:
+
+[Footnote 31: Darwin, _Animals and Plants under Domestication_, vol. i.
+p. 322.]
+
+[Footnote 32: These facts are taken from Darwin's _Domesticated Animals
+and Cultivated Plants_, vol. i. pp. 359, 360, 392-401; vol. ii. pp. 231,
+275, 330.]
+
+[Footnote 33: See Darwin's _Animals and Plants under Domestication_,
+vol. i. pp. 40-42.]
+
+[Footnote 34: Mr. Brent in _Journal of Horticulture_, 1861, p. 76;
+quoted by Darwin, _Animals and Plants under Domestication_, vol. i. p.
+151.]
+
+[Footnote 35: This account of domestic pigeons is greatly condensed from
+Mr. Darwin's work already referred to.]
+
+[Footnote 36: _Animals and Plants under Domestication_, vol. ii. pp.
+307-311.]
+
+
+
+
+CHAPTER V
+
+NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST
+
+
+    Effect of struggle for existence under unchanged conditions--The
+    effect under change of conditions--Divergence of character--In
+    insects--In birds--In mammalia--Divergence leads to a maximum of
+    life in each area--Closely allied species inhabit distinct
+    areas--Adaptation to conditions at various periods of life--The
+    continued existence of low forms of life--Extinction of low
+    types among the higher animals--Circumstances favourable to the
+    origin of new species--Probable origin of the dippers--The
+    importance of isolation--On the advance of organisation by
+    natural selection--Summary of the first five chapters.
+
+
+
+In the preceding chapters we have accumulated a body of facts and
+arguments which will enable us now to deal with the very core of our
+subject--the formation of species by means of natural selection. We have
+seen how tremendous is the struggle for existence always going on in
+nature owing to the great powers of increase of all organisms; we have
+ascertained the fact of variability extending to every part and organ,
+each of which varies simultaneously and for the most part independently;
+and we have seen that this variability is both large in its amount in
+proportion to the size of each part, and usually affects a considerable
+proportion of the individuals in the large and dominant species. And,
+lastly, we have seen how similar variations, occurring in cultivated
+plants and domestic animals, are capable of being perpetuated and
+accumulated by artificial selection, till they have resulted in all the
+wonderful varieties of our fruits, flowers, and vegetables, our domestic
+animals and household pets, many of which differ from each other far
+more in external characters, habits, and instincts than do species in a
+state of nature. We have now to inquire whether there is any analogous
+process in nature, by which wild animals and plants can be permanently
+modified and new races or new species produced.
+
+
+_Effect of Struggle for Existence under Unchanged Conditions._
+
+Let us first consider what will be the effect of the struggle for
+existence upon the animals and plants which we see around us, under
+conditions which do not perceptibly vary from year to year or from
+century to century. We have seen that every species is exposed to
+numerous and varied dangers throughout its entire existence, and that it
+is only by means of the exact adaptation of its organisation--including
+its instincts and habits--to its surroundings that it is enabled to live
+till it produces offspring which may take its place when it ceases to
+exist. We have seen also that, of the whole annual increase only a very
+small fraction survives; and though the survival in individual cases may
+sometimes be due rather to accident than to any real superiority, yet we
+cannot doubt that, in the long run, those survive which are best fitted
+by their perfect organisation to escape the dangers that surround them.
+This "survival of the fittest" is what Darwin termed "natural
+selection," because it leads to the same results in nature as are
+produced by man's selection among domestic animals and cultivated
+plants. Its primary effect will, clearly, be to keep each species in the
+most perfect health and vigour, with every part of its organisation in
+full harmony with the conditions of its existence. It prevents any
+possible deterioration in the organic world, and produces that
+appearance of exuberant life and enjoyment, of health and beauty, that
+affords us so much pleasure, and which might lead a superficial observer
+to suppose that peace and quietude reigned throughout nature.
+
+
+_The Effect under changed Conditions._
+
+But the very same process which, so long as conditions remain
+substantially the same, secures the continuance of each species of
+animal or plant in its full perfection, will usually, under changed
+conditions, bring about whatever change of structure or habits may be
+necessitated by them. The changed conditions to which we refer are such
+as we know have occurred throughout all geological time and in every
+part of the world. Land and water have been continually shifting their
+positions; some regions are undergoing subsidence with diminution of
+area, others elevation with extension of area; dry land has been
+converted into marshes, while marshes have been drained or have even
+been elevated into plateaux. Climate too has changed again and again,
+either through the elevation of mountains in high latitudes leading to
+the accumulation of snow and ice, or by a change in the direction of
+winds and ocean currents produced by the subsidence or elevation of
+lands which connected continents and divided oceans. Again, along with
+all these changes have come not less important changes in the
+distribution of species. Vegetation has been greatly modified by changes
+of climate and of altitude; while every union of lands before separated
+has led to extensive migrations of animals into new countries,
+disturbing the balance that before existed among its forms of life,
+leading to the extermination of some species and the increase of others.
+
+When such physical changes as these have taken place, it is evident that
+many species must either become modified or cease to exist. When the
+vegetation has changed in character the herbivorous animals must become
+able to live on new and perhaps less nutritious food; while the change
+from a damp to a dry climate may necessitate migration at certain
+periods to escape destruction by drought. This will expose the species
+to new dangers, and require special modifications of structure to meet
+them. Greater swiftness, increased cunning, nocturnal habits, change of
+colour, or the power of climbing trees and living for a time on their
+foliage or fruit, may be the means adopted by different species to bring
+themselves into harmony with the new conditions; and by the continued
+survival of those individuals, only, which varied sufficiently in the
+right direction, the necessary modifications of structure or of function
+would be brought about, just as surely as man has been able to breed the
+greyhound to hunt by sight and the foxhound by scent, or has produced
+from the same wild plant such distinct forms as the cauliflower and the
+brussels sprouts.
+
+We will now consider the special characteristics of the changes in
+species that are likely to be effected, and how far they agree with what
+we observe in nature.
+
+
+_Divergence of Character._
+
+In species which have a wide range the struggle for existence will often
+cause some individuals or groups of individuals to adopt new habits in
+order to seize upon vacant places in nature where the struggle is less
+severe. Some, living among extensive marshes, may adopt a more aquatic
+mode of life; others, living where forests abound, may become more
+arboreal. In either case we cannot doubt that the changes of structure
+needed to adapt them to their new habits would soon be brought about,
+because we know that variations in all the external organs and all their
+separate parts are very abundant and are also considerable in amount.
+That such divergence of character has actually occurred we have some
+direct evidence. Mr. Darwin informs us that in the Catskill Mountains in
+the United States there are two varieties of wolves, one with a light
+greyhound-like form which pursues deer, the other more bulky with
+shorter legs, which more frequently attacks sheep.[37] Another good
+example is that of the insects in the island of Madeira, many of which
+have either lost their wings or have had them so much reduced as to be
+useless for flight, while the very same species on the continent of
+Europe possess fully developed wings. In other cases the wingless
+Madeira species are distinct from, but closely allied to, winged species
+of Europe. The explanation of this change is, that Madeira, like many
+oceanic islands in the temperate zone, is much exposed to sudden gales
+of wind, and as most of the fertile land is on the coast, insects which
+flew much would be very liable to be blown out to sea and lost. Year
+after year, therefore, those individuals which had shorter wings, or
+which used them least, were preserved; and thus, in time, terrestrial,
+wingless, or imperfectly winged races or species have been produced.
+That this is the true explanation of this singular fact is proved by
+much corroborative evidence. There are some few flower-frequenting
+insects in Madeira to whom wings are essential, and in these the wings
+are somewhat larger than in the same species on the mainland. We thus
+see that there is no general tendency to the abortion of wings in
+Madeira, but that it is simply a case of adaptation to new conditions.
+Those insects to whom wings were not absolutely essential escaped a
+serious danger by not using them, and the wings therefore became reduced
+or were completely lost. But when they were essential they were enlarged
+and strengthened, so that the insect could battle against the winds and
+save itself from destruction at sea. Many flying insects, not varying
+fast enough, would be destroyed before they could establish themselves,
+and thus we may explain the total absence from Madeira of several whole
+families of winged insects which must have had many opportunities of
+reaching the islands. Such are the large groups of the tiger-beetles
+(Cicindelidae), the chafers (Melolonthidae), the click-beetles
+(Elateridae), and many others.
+
+But the most curious and striking confirmation of this portion of Mr.
+Darwin's theory is afforded by the case of Kerguelen Island. This island
+was visited by the _Transit of Venus_ expedition. It is one of the
+stormiest places on the globe, being subject to almost perpetual gales,
+while, there being no wood, it is almost entirely without shelter. The
+Rev. A.E. Eaton, an experienced entomologist, was naturalist to the
+expedition, and he assiduously collected the few insects that were to be
+found. All were incapable of flight, and most of them entirely without
+wings. They included a moth, several flies, and numerous beetles. As
+these insects could hardly have reached the islands in a wingless state,
+even if there were any other known land inhabited by them--which there
+is not--we must assume that, like the Madeiran insects, they were
+originally winged, and lost their power of flight because its possession
+was injurious to them.
+
+It is no doubt due to the same cause that some butterflies on small and
+exposed islands have their wings reduced in size, as is strikingly the
+case with the small tortoise-shell butterfly (Vanessa urticae)
+inhabiting the Isle of Man, which is only about half the size of the
+same species in England or Ireland; and Mr. Wollaston notes that Vanessa
+callirhoe--a closely allied South European form of our red-admiral
+butterfly--is permanently smaller in the small and bare island of Porto
+Santo than in the larger and more wooded adjacent island of Madeira.
+
+A very good example of comparatively recent divergence of character, in
+accordance with new conditions of life, is afforded by our red grouse.
+This bird, the Lagopus scoticus of naturalists, is entirely confined to
+the British Isles. It is, however, very closely allied to the willow
+grouse (Lagopus albus), a bird which ranges all over Europe, Northern
+Asia, and North America, but which, unlike our species, changes to white
+in winter. No difference in form or structure can be detected between
+the two birds, but as they differ so decidedly in colour--our species
+being usually rather darker in winter than in summer, while there are
+also slight differences in the call-note and in habits,--the two species
+are generally considered to be distinct. The differences, however, are
+so clearly adaptations to changed conditions that we can hardly doubt
+that, during the early part of the glacial period, when our islands were
+united to the continent, our grouse was identical with that of the rest
+of Europe. But when the cold passed away and our islands became
+permanently separated from the mainland, with a mild and equable climate
+and very little snow in winter, the change to white at that season
+became hurtful, rendering the birds more conspicuous instead of serving
+as a means of concealment. The colour was, therefore, gradually changed
+by the process of variation and natural selection; and as the birds
+obtained ample shelter among the heather which clothes so many of our
+moorlands, it became useful for them to assimilate with its brown and
+dusky stems and withered flowers rather than with the snow of the higher
+mountains. An interesting confirmation of this change having really
+occurred is afforded by the occasional occurrence in Scotland of birds
+with a considerable amount of white in the winter plumage. This is
+considered to be a case of reversion to the ancestral type, just as the
+slaty colours and banded wings of the wild rock-pigeon sometimes
+reappear in our fancy breeds of domestic pigeons.[38]
+
+The principle of "divergence of character" pervades all nature from the
+lowest groups to the highest, as may be well seen in the class of birds.
+Among our native species we see it well marked in the different species
+of titmice, pipits, and chats. The great titmouse (Parus major) by its
+larger size and stronger bill is adapted to feed on larger insects, and
+is even said sometimes to kill small and weak birds. The smaller and
+weaker coal titmouse (Parus ater) has adopted a more vegetarian diet,
+eating seeds as well as insects, and feeding on the ground as well as
+among trees. The delicate little blue titmouse (Parus coeruleus), with
+its very small bill, feeds on the minutest insects and grubs which it
+extracts from crevices of bark and from the buds of fruit-trees. The
+marsh titmouse, again (Parus palustris), has received its name from the
+low and marshy localities it frequents; while the crested titmouse
+(Parus cristatus) is a northern bird frequenting especially pine
+forests, on the seeds of which trees it partially feeds. Then, again,
+our three common pipits--the tree-pipit (Anthus arboreus), the
+meadow-pipit (Anthus pratensis), and the rock-pipit or sea-lark (Anthus
+obscurus) have each occupied a distinct place in nature to which they
+have become specially adapted, as indicated by the different form and
+size of the hind toe and claw in each species. So, the stone-chat
+(Saxicola rubicola), the whin-chat (S. rubetra), and the wheat-ear (S.
+oenanthe) are more or less divergent forms of one type, with
+modifications in the shape of the wing, feet, and bill adapting them to
+slightly different modes of life. The whin-chat is the smallest, and
+frequents furzy commons, fields, and lowlands, feeding on worms,
+insects, small molluscs, and berries; the stone-chat is next in size,
+and is especially active and lively, frequenting heaths and uplands, and
+is a permanent resident with us, the two other species being migrants;
+while the larger and more conspicuous wheat-ear, besides feeding on
+grubs, beetles, etc., is able to capture flying insects on the wing,
+something after the manner of true flycatchers.
+
+These examples sufficiently indicate how divergence of character has
+acted, and has led to the adaptation of numerous allied species, each to
+a more or less special mode of life, with the variety of food, of
+habits, and of enemies which must necessarily accompany such diversity.
+And when we extend our inquiries to higher groups we find the same
+indications of divergence and special adaptation, often to a still more
+marked extent. Thus we have the larger falcons, which prey upon birds,
+while some of the smaller species, like the hobby (Falco subbuteo), live
+largely on insects. The true falcons capture their prey in the air,
+while the hawks usually seize it on or near the ground, feeding on
+hares, rabbits, squirrels, grouse, pigeons, and poultry. Kites and
+buzzards, on the other hand, seize their prey upon the ground, and the
+former feed largely on reptiles and offal as well as on birds and
+quadrupeds. Others have adopted fish as their chief food, and the osprey
+snatches its prey from the water with as much facility as a gull or a
+petrel; while the South American caracaras (Polyborus) have adopted the
+habits of vultures and live altogether on carrion. In every great group
+there is the same divergence of habits. There are ground-pigeons,
+rock-pigeons, and wood-pigeons,--seed-eating pigeons and fruit-eating
+pigeons; there are carrion-eating, insect-eating, and fruit-eating
+crows. Even kingfishers are, some aquatic, some terrestrial in their
+habits; some live on fish, some on insects, some on reptiles. Lastly,
+among the primary divisions of birds we find a purely terrestrial
+group--the Ratitae, including the ostriches, cassowaries, etc.; other
+great groups, including the ducks, cormorants, gulls, penguins, etc.,
+are aquatic; while the bulk of the Passerine birds are aerial and
+arboreal. The same general facts can be detected in all other classes of
+animals. In the mammalia, for example, we have in the common rat a
+fish-eater and flesh-eater as well as a grain-eater, which has no doubt
+helped to give it the power of spreading over the world and driving away
+the native rats of other countries. Throughout the Rodent tribe we find
+everywhere aquatic, terrestrial, and arboreal forms. In the weasel and
+cat tribes some live more in trees, others on the ground; squirrels have
+diverged into terrestrial, arboreal, and flying species; and finally, in
+the bats we have a truly aerial, and in the whales a truly aquatic order
+of mammals. We thus see that, beginning with different varieties of the
+same species, we have allied species, genera, families, and orders, with
+similarly divergent habits, and adaptations to different modes of life,
+indicating some general principle in nature which has been operative in
+the development of the organic world. But in order to be thus operative
+it must be a generally useful principle, and Mr. Darwin has very clearly
+shown us in what this utility consists.
+
+
+_Divergence leads to a Maximum of Organic Forms in each Area._
+
+Divergence of character has a double purpose and use. In the first place
+it enables a species which is being overcome by rivals, or is in
+process of extinction by enemies, to save itself by adopting new habits
+or by occupying vacant places in nature. This is the immediate and
+obvious effect of all the numerous examples of divergence of character
+which we have pointed out. But there is another and less obvious result,
+which is, that the greater the diversity in the organisms inhabiting a
+country or district the greater will be the total amount of life that
+can be supported there. Hence the continued action of the struggle for
+existence will tend to bring about more and more diversity in each area,
+which may be shown to be the case by several kinds of evidence. As an
+example, a piece of turf, three feet by four in size, was found by Mr.
+Darwin to contain twenty species of plants, and these twenty species
+belonged to eighteen genera and to eight orders, showing how greatly
+they differed from each other. Farmers find that a greater quantity of
+hay is obtained from ground sown with a variety of genera of grasses,
+clover, etc., than from similar land sown with one or two species only;
+and the same principle applies to rotation of crops, plants differing
+very widely from each other giving the best results. So, in small and
+uniform islands, and in small ponds of fresh water, the plants and
+insects, though few in number, are found to be wonderfully varied in
+character.
+
+The same principle is seen in the naturalisation of plants and animals
+by man's agency in distant lands, for the species that thrive best and
+establish themselves permanently are not only very varied among
+themselves but differ greatly from the native inhabitants. Thus, in the
+Northern United States there are, according to Dr. Asa Gray, 260
+naturalised flowering plants which belong to no less than 162 genera;
+and of these, 100 genera are not natives of the United States. So, in
+Australia, the rabbit, though totally unlike any native animal, has
+increased so much that it probably outnumbers in individuals all the
+native mammals of the country; and in New Zealand the rabbit and the pig
+have equally multiplied. Darwin remarks that this "advantage of
+diversification of structure in the inhabitants of the same region is,
+in fact, the same as that of the physiological division of labour in the
+organs of the same body. No physiologist doubts that a stomach adapted
+to digest vegetable matter alone, or flesh alone, draws more nutriment
+from these substances. So, in the general economy of any land, the more
+widely and perfectly the animals and plants are diversified for
+different habits of life, so will a greater number of individuals be
+capable of there supporting themselves."[39]
+
+
+_The most closely allied Species inhabit distinct Areas._
+
+One of the curious results of the general action of this principle in
+nature is, that the most closely allied species--those whose differences
+though often real and important are hardly perceptible to any one but a
+naturalist--are usually not found in the same but in widely separated
+countries. Thus, the nearest allies to our European golden plover are
+found in North America and East Asia; the nearest ally of our European
+jay is found in Japan, although there are several other species of jays
+in Western Asia and North Africa; and though we have several species of
+titmice in England they are not very closely allied to each other. The
+form most akin to our blue tit is the azure tit of Central Asia (Parus
+azureus); the Parus ledouci of Algeria is very near our coal tit, and
+the Parus lugubris of South-Eastern Europe and Asia Minor is nearest to
+our marsh tit. So, our four species of wild pigeons--the ring-dove,
+stock-dove, rock-pigeon, and turtle-dove--are not closely allied to each
+other, but each of them belongs, according to some ornithologists, to a
+separate genus or subgenus, and has its nearest relatives in distant
+parts of Asia and Africa. In mammalia the same thing occurs. Each
+mountain region of Europe and Asia has usually its own species of wild
+sheep and goat, and sometimes of antelope and deer; so that in each
+region there is found the greatest diversity in this class of animals,
+while the closest allies inhabit quite distinct and often distant areas.
+In plants we find the same phenomenon prevalent. Distinct species of
+columbine are found in Central Europe (Aguilegia vulgaris), in Eastern
+Europe, and Siberia (A. glandulosa), in the Alps (A. Alpina), in the
+Pyrenees (A. pyrenaiea), in the Greek mountains (A. ottonis), and in
+Corsica (A. Bernardi), but rarely are two species found in the same
+area. So, each part of the world has its own peculiar forms of pines,
+firs, and cedars, but the closely allied species or varieties are in
+almost every case inhabitants of distinct areas. Examples are the deodar
+of the Himalayas, the cedar of Lebanon, and that of North Africa, all
+very closely allied but confined to distinct areas; and the numerous
+closely allied species of true pine (genus Pinus), which almost always
+inhabit different countries or occupy different stations. We will now
+consider some other modes in which natural selection will act, to adapt
+organisms to changed conditions.
+
+
+_Adaptation to Conditions at Various Periods of Life._
+
+It is found, that, in domestic animals and cultivated plants, variations
+occurring at any one period of life reappear in the offspring at the
+same period, and can be perpetuated and increased by selection without
+modifying other parts of the organisation. Thus, variations in the
+caterpillar or the cocoon of the silkworm, in the eggs of poultry, and
+in the seeds or young shoots of many culinary vegetables, have been
+accumulated till those parts have become greatly modified and, for man's
+purposes, improved. Owing to this fact it is easy for organisms to
+become so modified as to avoid dangers that occur at any one period of
+life. Thus it is that so many seeds have become adapted to various modes
+of dissemination or protection. Some are winged, or have down or hairs
+attached to them, so as to enable them to be carried long distances in
+the air; others have curious hooks and prickles, which cause them to be
+attached firmly to the fur of mammals or the feathers of birds; while
+others are buried within sweet or juicy and brightly coloured fruits,
+which are seen and devoured by birds, the hard smooth seeds passing
+through their bodies in a fit state for germination. In the struggle for
+existence it must benefit a plant to have increased means of dispersing
+its seeds, and of thus having young plants produced in a greater variety
+of soils, aspects, and surroundings, with a greater chance of some of
+them escaping their numerous enemies and arriving at maturity. The
+various differences referred to would, therefore, be brought about by
+variation and survival of the fittest, just as surely as the length and
+quality of cotton on the seed of the cotton-plant have been increased
+by man's selection.
+
+The larvae of insects have thus been wonderfully modified in order to
+escape the numerous enemies to whose attacks they are exposed at this
+period of their existence. Their colours and markings have become
+marvellously adapted to conceal them among the foliage of the plant they
+live upon, and this colour often changes completely after the last
+moult, when the creature has to descend to the ground for its change to
+the pupa state, during which period a brown instead of a green colour is
+protective. Others have acquired curious attitudes and large ocelli,
+which cause them to resemble the head of some reptile, or they have
+curious horns or coloured ejectile processes which frighten away
+enemies; while a great number have acquired secretions which render them
+offensive to the taste of their enemies, and these are always adorned
+with very conspicuous markings or brilliant colours, which serve as a
+sign of inedibility and prevent their being needlessly attacked. This,
+however, is a portion of the very large subject of organic colour and
+marking, which will be fully discussed and illustrated in a separate
+chapter.
+
+In this way every possible modification of an animal or plant, whether
+in colour, form, structure, or habits, which would be serviceable to it
+or to its progeny at any period of its existence, may be readily brought
+about. There are some curious organs which are used only once in a
+creature's life, but which are yet essential to its existence, and thus
+have very much the appearance of design by an intelligent designer. Such
+are, the great jaws possessed by some insects, used exclusively for
+opening the cocoon, and the hard tip to the beak of unhatched birds used
+for breaking the eggshell. The increase in thickness or hardness of the
+cocoons or the eggs being useful for protection against enemies or to
+avoid accidents, it is probable that the change has been very gradual,
+because it would be constantly checked by the necessity for a
+corresponding change in the young insects or birds enabling them to
+overcome the additional obstacle of a tougher cocoon or a harder
+eggshell. As we have seen, however, that every part of the organism
+appears to be varying independently, at the same time, though to
+different amounts, there seems no reason to believe that the necessity
+for two or more coincident variations would prevent the required change
+from taking place.
+
+
+_The Continued Existence of Low Forms of Life._
+
+Since species are continually undergoing modifications giving them some
+superiority over other species or enabling them to occupy fresh places
+in nature, it may be asked--Why do any low forms continue to exist? Why
+have they not long since been improved and developed into higher forms?
+The answer, probably, is, that these low forms occupy places in nature
+which cannot be filled by higher forms, and that they have few or no
+competitors; they therefore continue to exist. Thus, earthworms are
+adapted to their mode of life better than they would be if more highly
+organised. So, in the ocean, the minute foraminifera and infusoria, and
+the larger sponges and corals, occupy places which more highly developed
+creatures could not fill. They form, as it were, the base of the great
+structure of animal life, on which the next higher forms rest; and
+though in the course of ages they may undergo some changes, and
+diversification of form and structure, in accordance with changed
+conditions, their essential nature has probably remained the same from
+the very dawn of life on the earth. The low aquatic diatomaceae and
+confervae, together with the lowest fungi and lichens, occupy a similar
+position in the vegetable kingdom, filling places in nature which would
+be left vacant if only highly organised plants existed. There is,
+therefore, no motive power to destroy or seriously to modify them; and
+they have thus probably persisted, under slightly varying forms, through
+all geological time.
+
+
+_Extinction of Lower Types among the Higher Animals._
+
+So soon; however, as we approach the higher and more fully developed
+groups, we see indications of the often repeated extinction of lower by
+higher forms. This is shown by the great gaps that separate the
+mammalia, birds, reptiles, and fishes from each other; while the lowest
+forms of each are always few in number and confined to limited areas.
+Such are the lowest mammals--the echidna and ornithorhynchus of
+Australia; the lowest birds--the apteryx of New Zealand and the
+cassowaries of the New Guinea region; while the lowest fish--the
+amphioxus or lancelet, is completely isolated, and has apparently
+survived only by its habit of burrowing in the sand. The great
+distinctness of the carnivora, ruminants, rodents, whales, bats, and
+other orders of mammalia; of the accipitres, pigeons, and parrots, among
+birds; and of the beetles, bees, flies, and moths, among insects, all
+indicate an enormous amount of extinction among the comparatively low
+forms by which, on any theory of evolution, these higher and more
+specialised groups must have been preceded.
+
+
+_Circumstances favourable to the Origin of New Species by Natural
+Selection._
+
+We have already seen that, when there is no change in the physical or
+organic conditions of a country, the effect of natural selection is to
+keep all the species inhabiting it in a state of perfect health and full
+development, and to preserve the balance that already exists between the
+different groups of organisms. But, whenever the physical or organic
+conditions change, to however small an extent, some corresponding change
+will be produced in the flora and fauna, since, considering the severe
+struggle for existence and the complex relations of the various
+organisms, it is hardly possible that the change should not be
+beneficial to some species and hurtful to others. The most common
+effect, therefore, will be that some species will increase and others
+will diminish; and in cases where a species was already small in numbers
+a further diminution might lead to extinction. This would afford room
+for the increase of other species, and thus a considerable readjustment
+of the proportions of the several species might take place. When,
+however, the change was of a more important character, directly
+affecting the existence of many species so as to render it difficult for
+them to maintain themselves without some considerable change in
+structure or habits, that change would, in some cases, be brought about
+by variation and natural selection, and thus new varieties or new
+species might be formed. We have to consider, then, which are the
+species that would be most likely to be so modified, while others, not
+becoming modified, would succumb to the changed conditions and become
+extinct.
+
+The most important condition of all is, undoubtedly, that variations
+should occur of sufficient amount, of a sufficiently diverse character,
+and in a large number of individuals, so as to afford ample materials
+for natural selection to act upon; and this, we have seen, does occur in
+most, if not in all, large, wide-ranging, and dominant species. From
+some of these, therefore, the new species adapted to the changed
+conditions would usually be derived; and this would especially be the
+case when the change of conditions was rather rapid, and when a
+correspondingly rapid modification could alone save some species from
+extinction. But when the change was very gradual, then even less
+abundant and less widely distributed species might become modified into
+new forms, more especially if the extinction of many of the rarer
+species left vacant places in the economy of nature.
+
+
+_Probable Origin of the Dippers._
+
+An excellent example of how a limited group of species has been able to
+maintain itself by adaptation to one of these "vacant places" in nature,
+is afforded by the curious little birds called dippers or water-ouzels,
+forming the genus Cinclus and the family Cinclidae of naturalists. These
+birds are something like small thrushes, with very short wings and tail,
+and very dense plumage. They frequent, exclusively, mountain torrents in
+the northern hemisphere, and obtain their food entirely in the water,
+consisting, as it does, of water-beetles, caddis-worms and other
+insect-larvae, as well as numerous small freshwater shells. These birds,
+although not far removed in structure from thrushes and wrens, have the
+extraordinary power of flying under water; for such, according to the
+best observers, is their process of diving in search of their prey,
+their dense and somewhat fibrous plumage retaining so much air that the
+water is prevented from touching their bodies or even from wetting their
+feathers to any great extent. Their powerful feet and long curved claws
+enable them to hold on to stones at the bottom, and thus to retain their
+position while picking up insects, shells, etc. As they frequent
+chiefly the most rapid and boisterous torrents, among rocks, waterfalls,
+and huge boulders, the water is never frozen over, and they are thus
+able to live during the severest winters. Only a very few species of
+dipper are known, all those of the old world being so closely allied to
+our British bird that some ornithologists consider them to be merely
+local races of one species; while in North America and the northern
+Andes there are two other species.
+
+Here then we have a bird, which, in its whole structure, shows a close
+affinity to the smaller typical perching birds, but which has departed
+from all its allies in its habits and mode of life, and has secured for
+itself a place in nature where it has few competitors and few enemies.
+We may well suppose, that, at some remote period, a bird which was
+perhaps the common and more generalised ancestor of most of our
+thrushes, warblers, wrens, etc., had spread widely over the great
+northern continent, and had given rise to numerous varieties adapted to
+special conditions of life. Among these some took to feeding on the
+borders of clear streams, picking out such larvae and molluscs as they
+could reach in shallow water. When food became scarce they would attempt
+to pick them out of deeper and deeper water, and while doing this in
+cold weather many would become frozen and starved. But any which
+possessed denser and more hairy plumage than usual, which was able to
+keep out the water, would survive; and thus a race would be formed which
+would depend more and more on this kind of food. Then, following up the
+frozen streams into the mountains, they would be able to live there
+during the winter; and as such places afforded them much protection from
+enemies and ample shelter for their nests and young, further adaptations
+would occur, till the wonderful power of diving and flying under water
+was acquired by a true land-bird.
+
+That such habits might be acquired under stress of need is rendered
+highly probable by the facts stated by the well-known American
+naturalist, Dr. Abbott. He says that "the water-thrushes (Seiurus sp.)
+all wade in water, and often, seeing minute mollusca on the bottom of
+the stream, plunge both head and neck beneath the surface, so that
+often, for several seconds, a large part of the body is submerged. Now
+these birds still have the plumage pervious to water, and so are liable
+to be drenched and sodden; but they have also the faculty of giving
+these drenched feathers such a good shaking that flight is practicable a
+moment after leaving the water. Certainly the water-thrushes (Seiurus
+ludovicianus, S. auricapillus, and S. noveboracensis) have taken many
+preliminary steps to becoming as aquatic as the dipper; and the
+winter-wren, and even the Maryland yellow-throat are not far
+behind."[40]
+
+Another curious example of the way in which species have been modified
+to occupy new places in nature, is afforded by the various animals which
+inhabit the water-vessels formed by the leaves of many epiphytal species
+of Bromelia. Fritz Müller has described a caddis-fly larva which lives
+among these leaves, and which has been modified in the pupa state in
+accordance with its surroundings. The pupae of caddis-flies inhabiting
+streams have fringes of hair on the tarsi to enable them to reach the
+surface on leaving their cases. But in the species inhabiting bromelia
+leaves there is no need for swimming, and accordingly we find the tarsi
+entirely bare. In the same plants are found curious little Entomostraca,
+very abundant there but found nowhere else. These form a new genus, but
+are most nearly allied to Cythere, a marine type. It is believed that
+the transmission of this species from one tree to another must be
+effected by the young crustacea, which are very minute, clinging to
+beetles, many of which, both terrestrial and aquatic, also inhabit the
+bromelia leaves; and as some water-beetles are known to frequent the
+sea, it is perhaps by these means that the first emigrants established
+themselves in this strange new abode. Bromeliae are often very abundant
+on trees growing on the water's edge, and this would facilitate the
+transition from a marine to an arboreal habitat. Fritz Müller has also
+found, among the bromelia leaves, a small frog bearing its eggs on its
+back, and having some other peculiarities of structure. Several
+beautiful little aquatic plants of the genus Utricularia or bladder-wort
+also inhabit bromelia leaves; and these send runners out to neighbouring
+plants and thus spread themselves with great rapidity.
+
+
+_The Importance of Isolation._
+
+Isolation is no doubt an important aid to natural selection, as shown by
+the fact that islands so often present a number of peculiar species; and
+the same thing is seen on the two sides of a great mountain range or on
+opposite coasts of a continent. The importance of isolation is twofold.
+In the first place, it leads to a body of individuals of each species
+being limited in their range and thus subjected to uniform conditions
+for long spaces of time. Both the direct action of the environment and
+the natural selection of such varieties only as are suited to the
+conditions, will, therefore, be able to produce their full effect. In
+the second place, the process of change will not be interfered with by
+intercrossing with other individuals which are becoming adapted to
+somewhat different conditions in an adjacent area. But this question of
+the swamping effects of intercrossing will be considered in another
+chapter.
+
+Mr. Darwin was of opinion that, on the whole, the largeness of the area
+occupied by a species was of more importance than isolation, as a factor
+in the production of new species, and in this I quite agree with him. It
+must, too, be remembered, that isolation will often be produced in a
+continuous area whenever a species becomes modified in accordance with
+varied conditions or diverging habits. For example, a wide-ranging
+species may in the northern and colder part of its area become modified
+in one direction, and in the southern part in another direction; and
+though for a long time an intermediate form may continue to exist in the
+intervening area, this will be likely soon to die out, both because its
+numbers will be small, and it will be more or less pressed upon in
+varying seasons by the modified varieties, each better able to endure
+extremes of climate. So, when one portion of a terrestrial species takes
+to a more arboreal or to a more aquatic mode of life, the change of
+habit itself leads to the isolation of each portion. Again, as will be
+more fully explained in a future chapter, any difference of habits or of
+haunts usually leads to some modification of colour or marking, as a
+means of concealment from enemies; and there is reason to believe that
+this difference will be intensified by natural selection as a means of
+identification and recognition by members of the same variety or
+incipient species. It has also been observed that each differently
+coloured variety of wild animals, or of domesticated animals which have
+run wild, keep together, and refuse to pair with individuals of the
+other colours; and this must of itself act to keep the races separate as
+completely as physical isolation.
+
+
+_On the Advance of Organisation by Natural Selection._
+
+As natural selection acts solely by the preservation of useful
+variations, or those which are beneficial to the organism under the
+conditions to which it is exposed, the result must necessarily be that
+each species or group tends to become more and more improved in relation
+to its conditions. Hence we should expect that the larger groups in each
+class of animals and plants--those which have persisted and have been
+abundant throughout geological ages--would, almost necessarily, have
+arrived at a high degree of organisation, both physical and mental.
+Illustrations of this are to be seen everywhere. Among mammalia we have
+the carnivora, which from Eocene times have been becoming more and more
+specialised, till they have culminated in the cat and dog tribes, which
+have reached a degree of perfection both in structure and intelligence
+fully equal to that of any other animals. In another line of
+development, the herbivora have been specialised for living solely on
+vegetable food till they have culminated in the sheep, the cattle, the
+deer, and the antelopes. The horse tribe, commencing with an early
+four-toed ancestor in the Eocene age, has increased in size and in
+perfect adaptation of feet and teeth to a life on open plains, and has
+reached its highest perfection in the horse, the ass, and the zebra. In
+birds, also, we see an advance from the imperfect tooth-billed and
+reptile-tailed birds of the secondary epoch, to the wonderfully
+developed falcons, crows, and swallows of our time. So, the ferns,
+lycopods, conifers, and monocotyledons of the palaeozoic and mesozoic
+rocks, have developed into the marvellous wealth of forms of the higher
+dicotyledons that now adorn the earth.
+
+But this remarkable advance in the higher and larger groups does not
+imply any universal law of progress in organisation, because we have at
+the same time numerous examples (as has been already pointed out) of the
+persistence of lowly organised forms, and also of absolute degradation
+or degeneration. Serpents, for example, have been developed from some
+lizard-like type which has lost its limbs; and though this loss has
+enabled them to occupy fresh places in nature and to increase and
+flourish to a marvellous extent, yet it must be considered to be a
+retrogression rather than an advance in organisation. The same remark
+will apply to the whale tribe among mammals; to the blind amphibia and
+insects of the great caverns; and among plants to the numerous cases in
+which flowers, once specially adapted to be fertilised by insects, have
+lost their gay corollas and their special adaptations, and have become
+degraded into wind-fertilised forms. Such are our plantains, our meadow
+burnet, and even, as some botanists maintain, our rushes, sedges, and
+grasses. The causes which have led to this degeneration will be
+discussed in a future chapter; but the facts are undisputed, and they
+show us that although variation and the struggle for existence may lead,
+on the whole, to a continued advance of organisation; yet they also lead
+in many cases to a retrogression, when such retrogression may aid in the
+preservation of any form under new conditions. They also lead to the
+persistence, with slight modifications, of numerous lowly organised
+forms which are suited to places which higher forms could not fully
+occupy, or to conditions under which they could not exist. Such are the
+ocean depths, the soil of the earth, the mud of rivers, deep caverns,
+subterranean waters, etc.; and it is in such places as these, as well as
+in some oceanic islands which competing higher forms have not been able
+to reach, that we find many curious relics of an earlier world, which,
+in the free air and sunlight and in the great continents, have long
+since been driven out or exterminated by higher types.
+
+
+_Summary of the first Five Chapters._
+
+We have now passed in review, in more or less detail, the main facts on
+which the theory of "the origin of species by means of natural
+selection" is founded. In future chapters we shall have to deal mainly
+with the application of the theory to explain the varied and complex
+phenomena presented by the organic world; and, also, to discuss some of
+the theories put forth by modern writers, either as being more
+fundamental than that of Darwin or as supplementary to it. Before doing
+this, however, it will be well briefly to summarise the facts and
+arguments already set forth, because it is only by a clear comprehension
+of these that the full importance of the theory can be appreciated and
+its further applications understood.
+
+The theory itself is exceedingly simple, and the facts on which it
+rests--though excessively numerous individually, and coextensive with
+the entire organic world--yet come under a few simple and easily
+understood classes. These facts are,--first, the enormous powers of
+increase in geometrical progression possessed by all organisms, and the
+inevitable struggle for existence among them; and, in the second place,
+the occurrence of much individual variation combined with the hereditary
+transmission of such variations. From these two great classes of facts,
+which are universal and indisputable, there necessarily arises, as
+Darwin termed it, the "preservation of favoured races in the struggle
+for life," the continuous action of which, under the ever-changing
+conditions both of the inorganic and organic universe, necessarily leads
+to the formation or development of new species.
+
+But, although this general statement is complete and indisputable, yet
+to see its applications under all the complex conditions that actually
+occur in nature, it is necessary always to bear in mind the tremendous
+power and universality of the agencies at work. We must never for an
+instant lose sight of the fact of the enormously rapid increase of all
+organisms, which has been illustrated by actual cases, given in our
+second chapter, no less than by calculations of the results of unchecked
+increase for a few years. Then, never forgetting that the animal and
+plant population of any country is, on the whole, stationary, we must be
+always trying to realise the ever-recurring destruction of the enormous
+annual increase, and asking ourselves what determines, in each
+individual case, the death of the many, the survival of the few. We must
+think over all the causes of destruction to each organism,--to the seed,
+the young shoot, the growing plant, the full-grown tree, or shrub, or
+herb, and again the fruit and seed; and among animals, to the egg or
+new-born young, to the youthful, and to the adults. Then, we must always
+bear in mind that what goes on in the case of the individual or family
+group we may observe or think of, goes on also among the millions and
+scores of millions of individuals which are comprised in almost every
+species; and must get rid of the idea that _chance_ determines which
+shall live and which die. For, although in many individual cases death
+may be due to chance rather than to any inferiority in those which die
+first, yet we cannot possibly believe that this can be the case on the
+large scale on which nature works. A plant, for instance, cannot be
+increased unless there are suitable vacant places its seeds can grow in,
+or stations where it can overcome other less vigorous and healthy
+plants. The seeds of all plants, by their varied modes of dispersal, may
+be said to be seeking out such places in which to grow; and we cannot
+doubt that, in the long run, those individuals whose seeds are the most
+numerous, have the greatest powers of dispersal, and the greatest vigour
+of growth, will leave more descendants than the individuals of the same
+species which are inferior in all these respects, although now and then
+some seed of an inferior individual may _chance_ to be carried to a spot
+where it can grow and survive. The same rule will apply to every period
+of life and to every danger to which plants or animals are exposed. The
+best organised, or the most healthy, or the most active, or the best
+protected, or the most intelligent, will inevitably, in the long run,
+gain an advantage over those which are inferior in these qualities; that
+is, _the fittest will survive_, the fittest being, in each particular
+case, those which are superior in the special qualities on which safety
+depends. At one period of life, or to escape one kind of danger,
+concealment may be necessary; at another time, to escape another danger,
+swiftness; at another, intelligence or cunning; at another, the power to
+endure rain or cold or hunger; and those which possess all these
+faculties in the fullest perfection will generally survive.
+
+Having fully grasped these facts in all their fulness and in their
+endless and complex results, we have next to consider the phenomena of
+variation, discussed in the third and fourth chapters; and it is here
+that perhaps the greatest difficulty will be felt in appreciating the
+full importance of the evidence as set forth. It has been so generally
+the practice to speak of variation as something exceptional and
+comparatively rare--as an abnormal deviation from the uniformity and
+stability of the characters of a species--and so few even among
+naturalists have ever compared, accurately, considerable numbers of
+individuals, that the conception of variability as a general
+characteristic of all dominant and widespread species, large in its
+amount and affecting, not a few, but considerable masses of the
+individuals which make up the species, will be to many entirely new.
+Equally important is the fact that the variability extends to every
+organ and every structure, external and internal; while perhaps most
+important of all is the independent variability of these several parts,
+each one varying without any constant or even usual dependence on, or
+correlation with, other parts. No doubt there is some such correlation
+in the differences that exist between species and species--more
+developed wings usually accompanying smaller feet and _vice versā_--but
+this is, generally, a useful adaptation which has been brought about by
+natural selection, and does not apply to the individual variability
+which occurs within the species.
+
+It is because these facts of variation are so important and so little
+understood, that they have been discussed in what will seem to some
+readers wearisome and unnecessary detail. Many naturalists, however,
+will hold that even more evidence is required; and more, to almost any
+amount, could easily have been given. The character and variety of that
+already adduced will, however, I trust, convince most readers that the
+facts are as stated; while they have been drawn from a sufficiently wide
+area to indicate a general principle throughout nature.
+
+If, now, we fully realise these facts of variation, along with those of
+rapid multiplication and the struggle for existence, most of the
+difficulties in the way of comprehending how species have originated
+through natural selection will disappear. For whenever, through changes
+of climate, or of altitude, or of the nature of the soil, or of the area
+of the country, any species are exposed to new dangers, and have to
+maintain themselves and provide for the safety of their offspring under
+new and more arduous conditions, then, in the variability of all parts,
+organs, and structures, no less than of habits and intelligence, we have
+the means of producing modifications which will certainly bring the
+species into harmony with its new conditions. And if we remember that
+all such physical changes are slow and gradual in their operation, we
+shall see that the amount of variation which we know occurs in every new
+generation will be quite sufficient to enable modification and
+adaptation to go on at the same rate. Mr. Darwin was rather inclined to
+exaggerate the necessary slowness of the action of natural selection;
+but with the knowledge we now possess of the great amount and range of
+individual variation, there seems no difficulty in an amount of change,
+quite equivalent to that which usually distinguishes allied species,
+sometimes taking place in less than a century, should any rapid change
+of conditions necessitate an equally rapid adaptation. This may often
+have occurred, either to immigrants into a new land, or to residents
+whose country has been cut off by subsidence from a larger and more
+varied area over which they had formerly roamed. When no change of
+conditions occurs, species may remain unchanged for very long periods,
+and thus produce that appearance of stability of species which is even
+now often adduced as an argument against evolution by natural selection,
+but which is really quite in harmony with it.
+
+On the principles, and by the light of the facts, now briefly
+summarised, we have been able, in the present chapter, to indicate how
+natural selection acts, how divergence of character is set up, how
+adaptation to conditions at various periods of life has been effected,
+how it is that low forms of life continue to exist, what kind of
+circumstances are most favourable to the formation of new species, and,
+lastly, to what extent the advance of organisation to higher types is
+produced by natural selection. We will now pass on to consider some of
+the more important objections and difficulties which have been advanced
+by eminent naturalists.
+
+FOOTNOTES:
+
+[Footnote 37: _Origin of Species_, p. 71.]
+
+[Footnote 38: Yarrell's _British Birds_, fourth edition, vol. iii. p.
+77.]
+
+[Footnote 39: _Origin of Species_, p. 89.]
+
+[Footnote 40: _Nature_, vol. xxx. p. 30.]
+
+
+
+
+CHAPTER VI
+
+DIFFICULTIES AND OBJECTIONS
+
+
+    Difficulty as to smallness of variations--As to the right
+    variations occurring when required--The beginnings of important
+    organs--The mammary glands--The eyes of flatfish--Origin of the
+    eye--Useless or non-adaptive characters--Recent extension of the
+    region of utility in plants--The same in animals--Uses of
+    tails--Of the horns of deer--Of the scale-ornamentation of
+    reptiles--Instability of non-adaptive characters--Delboeuf's
+    law--No "specific" character proved to be useless--The swamping
+    effects of intercrossing--Isolation as preventing
+    intercrossing--Gulick on the effects of isolation--Cases in
+    which isolation is ineffective.
+
+
+
+In the present chapter I propose to discuss the more obvious and often
+repeated objections to Darwin's theory, and to show how far they affect
+its character as a true and sufficient explanation of the origin of
+species. The more recondite difficulties, affecting such fundamental
+questions as the causes and laws of variability, will be left for a
+future chapter, after we have become better acquainted with the
+applications of the theory to the more important adaptations and
+correlations of animal and plant life.
+
+One of the earliest and most often repeated objections was, that it was
+difficult "to imagine a reason why variations tending in an
+infinitesimal degree in any special direction should be preserved," or
+to believe that the complex adaptation of living organisms could have
+been produced "by infinitesimal beginnings." Now this term
+"infinitesimal," used by a well-known early critic of the _Origin of
+Species_, was never made use of by Darwin himself, who spoke only of
+variations being "slight," and of the "small amount" of the variations
+that might be selected. Even in using these terms he undoubtedly
+afforded grounds for the objection above made, that such small and
+slight variations could be of no real use, and would not determine the
+survival of the individuals possessing them. We have seen, however, in
+our third chapter, that even Darwin's terms were hardly justified; and
+that the variability of many important species is of considerable
+amount, and may very often be properly described as large. As this is
+found to be the case both in animals and plants, and in all their chief
+groups and subdivisions, and also to apply to all the separate parts and
+organs that have been compared, we must take it as proved that the
+average _amount_ of variability presents no difficulty whatever in the
+way of the action of natural selection. It may be here mentioned that,
+up to the time of the preparation of the last edition of _The Origin of
+Species_, Darwin had not seen the work of Mr. J.A. Allen of Harvard
+University (then only just published), which gave us the first body of
+accurate comparisons and measurements demonstrating this large amount of
+variability. Since then evidence of this nature has been accumulating,
+and we are, therefore, now in a far better position to appreciate the
+facilities for natural selection, in this respect, than was Mr. Darwin
+himself.
+
+Another objection of a similar nature is, that the chances are immensely
+against the right variation or combination of variations occurring just
+when required; and further, that no variation can be perpetuated that is
+not accompanied by several concomitant variations of dependent
+parts--greater length of a wing in a bird, for example, would be of
+little use if unaccompanied by increased volume or contractility of the
+muscles which move it. This objection seemed a very strong one so long
+as it was supposed that variations occurred singly and at considerable
+intervals; but it ceases to have any weight now we know that they occur
+simultaneously in various parts of the organism, and also in a large
+proportion of the individuals which make up the species. A considerable
+number of individuals will, therefore, every year possess the required
+combination of characters; and it may also be considered probable that
+when the two characters are such that they always _act_ together, there
+will be such a correlation between them that they will frequently _vary_
+together. But there is another consideration that seems to show that
+this coincident variation is not essential. All animals in a state of
+nature are kept, by the constant struggle for existence and the survival
+of the fittest, in such a state of perfect health and usually
+superabundant vigour, that in all ordinary circumstances they possess a
+surplus power in every important organ--a surplus only drawn upon in
+cases of the direst necessity when their very existence is at stake. It
+follows, therefore, that _any_ additional power given to one of the
+component parts of an organ must be useful--an increase, for example,
+either in the wing muscles or in the form or length of the wing might
+give _some_ increased powers of flight; and thus alternate
+variations--in one generation in the muscles, in another generation in
+the wing itself--might be as effective in permanently improving the
+powers of flight as coincident variations at longer intervals. On either
+supposition, however, this objection appears to have little weight if we
+take into consideration the large amount of coincident variability that
+has been shown to exist.
+
+
+_The Beginnings of Important Organs._
+
+We now come to an objection which has perhaps been more frequently urged
+than any other, and which Darwin himself felt to have much weight--the
+first beginnings of important organs, such, for example, as wings, eyes,
+mammary glands, and numerous other structures. It is urged, that it is
+almost impossible to conceive how the first rudiments of these could
+have been of any use, and, if not of use they could not have been
+preserved and further developed by natural selection.
+
+Now, the first remark to be made on objections of this nature is, that
+they are really outside the question of the origin of all existing
+species from allied species not very far removed from them, which is all
+that Darwin undertook to _prove_ by means of his theory. Organs and
+structures such as those above mentioned all date back to a very remote
+past, when the world and its inhabitants were both very different from
+what they are now. To ask of a new theory that it shall reveal to us
+exactly what took place in remote geological epochs, and how it took
+place, is unreasonable. The most that should be asked is, that some
+probable or possible mode of origination should be pointed out in some
+at least of these difficult cases, and this Mr. Darwin has done. One or
+two of these may be briefly given here, but the whole series should be
+carefully read by any one who wishes to see how many curious facts and
+observations have been required in order to elucidate them; whence we
+may conclude that further knowledge will probably throw light on any
+difficulties that still remain.[41]
+
+In the case of the mammary glands Mr. Darwin remarks that it is admitted
+that the ancestral mammals were allied to the marsupials. Now in the
+very earliest mammals, almost before they really deserved that name, the
+young may have been nourished by a fluid secreted by the interior
+surface of the marsupial sack, as is believed to be the case with the
+fish (Hippocampus) whose eggs are hatched within a somewhat similar
+sack. This being the case, those individuals which secreted a more
+nutritious fluid, and those whose young were able to obtain and swallow
+a more constant supply by suction, would be more likely to live and come
+to a healthy maturity, and would therefore be preserved by natural
+selection.
+
+In another case which has been adduced as one of special difficulty, a
+more complete explanation is given. Soles, turbots, and other flatfish
+are, as is well known, unsymmetrical. They live and move on their sides,
+the under side being usually differently coloured from that which is
+kept uppermost. Now the eyes of these fish are curiously distorted in
+order that both eyes may be on the upper side, where alone they would be
+of any use. It was objected by Mr. Mivart that a sudden transformation
+of the eye from one side to the other was inconceivable, while, if the
+transit were gradual the first step could be of no use, since this would
+not remove the eye from the lower side. But, as Mr. Darwin shows by
+reference to the researches of Malm and others, the young of these fish
+are quite symmetrical, and during their growth exhibit to us the whole
+process of change. This begins by the fish (owing to the increasing
+depth of the body) being unable to maintain the vertical position, so
+that it falls on one side. It then twists the lower eye as much as
+possible towards the upper side; and, the whole bony structure of the
+head being at this time soft and flexible, the constant repetition of
+this effort causes the eye gradually to move round the head till it
+comes to the upper side. Now if we suppose this process, which in the
+young is completed in a few days or weeks, to have been spread over
+thousands of generations during the development of these fish, those
+usually surviving whose eyes retained more and more of the position into
+which the young fish tried to twist them, the change becomes
+intelligible; though it still remains one of the most extraordinary
+cases of degeneration, by which symmetry--which is so universal a
+characteristic of the higher animals--is lost, in order that the
+creature may be adapted to a new mode of life, whereby it is enabled the
+better to escape danger and continue its existence.
+
+The most difficult case of all, that of the eye--the thought of which
+even to the last, Mr. Darwin says, "gave him a cold shiver"--is
+nevertheless shown to be not unintelligible; granting of course the
+sensitiveness to light of some forms of nervous tissue. For he shows
+that there are, in several of the lower animals, rudiments of eyes,
+consisting merely of pigment cells covered with a translucent skin,
+which may possibly serve to distinguish light from darkness, but nothing
+more. Then we have an optic nerve and pigment cells; then we find a
+hollow filled with gelatinous substance of a convex form--the first
+rudiment of a lens. Many of the succeeding steps are lost, as would
+necessarily be the case, owing to the great advantage of each
+modification which gave increased distinctness of vision, the creatures
+possessing it inevitably surviving, while those below them became
+extinct. But we can well understand how, after the first step was taken,
+every variation tending to more complete vision would be preserved till
+we reached the perfect eye of birds and mammals. Even this, as we know,
+is not absolutely, but only relatively, perfect. Neither the chromatic
+nor the spherical aberration is absolutely corrected; while long-and
+short-sightedness, and the various diseases and imperfections to which
+the eye is liable, may be looked upon as relics of the imperfect
+condition from which the eye has been raised by variation and natural
+selection.
+
+These few examples of difficulties as to the origin of remarkable or
+complex organs must suffice here; but the reader who wishes further
+information on the matter may study carefully the whole of the sixth
+and seventh chapters of the last edition of _The Origin of Species_, in
+which these and many other cases are discussed in considerable detail.
+
+
+_Useless or non-adaptive Characters._
+
+Many naturalists seem to be of opinion that a considerable number of the
+characters which distinguish species are of no service whatever to their
+possessors, and therefore cannot have been produced or increased by
+natural selection. Professors Bronn and Broca have urged this objection
+on the continent. In America, Dr. Cope, the well-known palaeontologist,
+has long since put forth the same objection, declaring that non-adaptive
+characters are as numerous as those which are adaptive; but he differs
+completely from most who hold the same general opinion in considering
+that they occur chiefly "in the characters of the classes, orders,
+families, and other higher groups;" and the objection, therefore, is
+quite distinct from that in which it is urged that "specific characters"
+are mostly useless. More recently, Professor G.J. Romanes has urged this
+difficulty in his paper on "Physiological Selection" (_Journ. Linn.
+Soc._, vol. xix. pp. 338, 344). He says that the characters "which serve
+to distinguish allied species are frequently, if not usually, of a kind
+with which natural selection can have had nothing to do," being without
+any utilitarian significance. Again he speaks of "the enormous number,"
+and further on of "the innumerable multitude" of specific peculiarities
+which are useless; and he finally declares that the question needs no
+further arguing, "because in the later editions of his works Mr. Darwin
+freely acknowledges that a large proportion of specific distinctions
+must be conceded to be useless to the species presenting them."
+
+I have looked in vain in Mr. Darwin's works to find any such
+acknowledgment, and I think Mr. Romanes has not sufficiently
+distinguished between "useless characters" and "useless specific
+distinctions." On referring to all the passages indicated by him I find
+that, in regard to specific characters, Mr. Darwin is very cautious in
+admitting inutility. His most pronounced "admissions" on this question
+are the following: "But when, from the nature of the organism and of the
+conditions, modifications have been induced which are unimportant for
+the welfare of the species, they may be, and apparently often have been,
+transmitted in nearly the same state _to numerous, otherwise modified,
+descendants_" (_Origin_, p. 175). The words I have here italicised
+clearly show that such characters are usually not "specific," in the
+sense that they are such as distinguish species from each other, but are
+found in numerous allied species. Again: "Thus a large yet undefined
+extension may safely be given to the direct and indirect results of
+natural selection; but I now admit, after reading the essay of Nägeli on
+plants, and the remarks by various authors with respect to animals, more
+especially those recently made by Professor Broca, that in the earlier
+editions of my _Origin of Species_ I perhaps attributed too much to the
+action of natural selection or the survival of the fittest. I have
+altered the fifth edition of the _Origin_ so as to confine my remarks to
+adaptive changes of structure, _but I am convinced, from the light
+gained during even the last few years, that very many structures which
+now appear to us useless, will hereafter be proved to be useful, and
+will therefore come within the range of natural selection_. Nevertheless
+I did not formerly consider sufficiently the existence of structures
+which, _as far as we can at present judge_, are neither beneficial nor
+injurious; and this I believe to be one of the greatest oversights as
+yet detected in my work." Now it is to be remarked that neither in these
+passages nor in any of the other less distinct expressions of opinion on
+this question, does Darwin ever admit that "specific characters"--that
+is, the particular characters which serve to distinguish one species
+from another--are ever useless, much less that "a large proportion of
+them" are so, as Mr. Romanes makes him "freely acknowledge." On the
+other hand, in the passage which I have italicised he strongly expresses
+his view that much of what we suppose to be useless is due to our
+ignorance; and as I hold myself that, as regards many of the supposed
+useless characters, this is the true explanation, it may be well to give
+a brief sketch of the progress of knowledge in transferring characters
+from the one category to the other.
+
+We have only to go back a single generation, and not even the most acute
+botanist could have suggested a reasonable use, for each species of
+plant, of the infinitely varied forms, sizes, and colours of the
+flowers, the shapes and arrangement of the leaves, and the numerous
+other external characters of the whole plant. But since Mr. Darwin
+showed that plants gained both in vigour and in fertility by being
+crossed with other individuals of the same species, and that this
+crossing was usually effected by insects which, in search of nectar or
+pollen, carried the pollen from one plant to the flowers of another
+plant, almost every detail is found to have a purpose and a use. The
+shape, the size, and the colour of the petals, even the streaks and
+spots with which they are adorned, the position in which they stand, the
+movements of the stamens and pistil at various times, especially at the
+period of, and just after, fertilisation, have been proved to be
+strictly adaptive in so many cases that botanists now believe that all
+the external characters of flowers either are or have been of use to the
+species.
+
+It has also been shown, by Kerner and other botanists, that another set
+of characteristics have relation to the prevention of ants, slugs, and
+other animals from reaching the flowers, because these creatures would
+devour or injure them without effecting fertilisation. The spines,
+hairs, or sticky glands on the stem or flower-stalk, the curious hairs
+or processes shutting up the flower, or sometimes even the extreme
+smoothness and polish of the outside of the petals so that few insects
+can hang to the part, have been shown to be related to the possible
+intrusion of these "unbidden guests."[42] And, still more recently,
+attempts have been made by Grant Allen and Sir John Lubbock to account
+for the innumerable forms, textures, and groupings of leaves, by their
+relation to the needs of the plants themselves; and there can be little
+doubt that these attempts will be ultimately successful. Again, just as
+flowers have been adapted to secure fertilisation or
+cross-fertilisation, fruits have been developed to assist in the
+dispersal of seeds; and their forms, sizes, juices, and colours can be
+shown to be specially adapted to secure such dispersal by the agency of
+birds and mammals; while the same end is secured in other cases by
+downy seeds to be wafted through the air, or by hooked or sticky
+seed-vessels to be carried away, attached to skin, wool, or feathers.
+
+Here, then, we have an enormous extension of the region of utility in
+the vegetable kingdom, and one, moreover, which includes almost all the
+specific characters of plants. For the species of plants are usually
+characterised either by differences in the form, size, and colour of the
+flowers, or of the fruits; or, by peculiarities in the shape, size,
+dentation, or arrangement of the leaves; or by peculiarities in the
+spines, hairs, or down with which various parts of the plant are
+clothed. In the case of plants it must certainly be admitted that
+"specific" characters are pre-eminently adaptive; and though there may
+be some which are not so, yet all those referred to by Darwin as having
+been adduced by various botanists as useless, either pertain to genera
+or higher groups, or are found in some plants of a species only--that
+is, are individual variations not specific characters.
+
+In the case of animals, the most recent wide extension of the sphere of
+utility has been in the matter of their colours and markings. It was of
+course always known that certain creatures gained protection by their
+resemblance to their normal surroundings, as in the case of white arctic
+animals, the yellow or brown tints of those living in deserts, and the
+green hues of many birds and insects surrounded by tropical vegetation.
+But of late years these cases have been greatly increased both in number
+and variety, especially in regard to those which closely imitate special
+objects among which they live; and there are other kinds of coloration
+which long appeared to have no use. Large numbers of animals, more
+especially insects, are gaudily coloured, either with vivid hues or with
+striking patterns, so as to be very easily seen. Now it has been found,
+that in almost all these cases the creatures possess some special
+quality which prevents their being attacked by the enemies of their kind
+whenever the peculiarity is known; and the brilliant or conspicuous
+colours or markings serve as a warning or signal flag against attack.
+Large numbers of insects thus coloured are nauseous and inedible;
+others, like wasps and bees, have stings; others are too hard to be
+eaten by small birds; while snakes with poisonous fangs often have some
+characteristic either of rattle, hood, or unusual colour, which
+indicates that they had better be left alone.
+
+But there is yet another form of coloration, which consists in special
+markings--bands, spots, or patches of white, or of bright colour, which
+vary in every species, and are often concealed when the creature is at
+rest but displayed when in motion,--as in the case of the bands and
+spots so frequent on the wings and tails of birds. Now these specific
+markings are believed, with good reason, to serve the purpose of
+enabling each species to be quickly recognised, even at a distance, by
+its fellows, especially the parents by their young and the two sexes by
+each other; and this recognition must often be an important factor in
+securing the safety of individuals, and therefore the wellbeing and
+continuance of the species. These interesting peculiarities will be more
+fully described in a future chapter, but they are briefly referred to
+here in order to show that the most common of all the characters by
+which species are distinguished from each other--their colours and
+markings--can be shown to be adaptive or utilitarian in their nature.
+
+But besides colour there are almost always some structural characters
+which distinguish species from species, and, as regards many of these
+also, an adaptive character can be often discerned. In birds, for
+instance, we have differences in the size or shape of the bill or the
+feet, in the length of the wing or the tail, and in the proportions of
+the several feathers of which these organs are composed. All these
+differences in the organs on which the very existence of birds depends,
+which determine the character of flight, facility for running or
+climbing, for inhabiting chiefly the ground or trees, and the kind of
+food that can be most easily obtained for themselves and their
+offspring, must surely be in the highest degree utilitarian; although in
+each individual case we, in our ignorance of the minutiae of their
+life-history, may be quite unable to see the use. In mammalia specific
+differences other than colour usually consist in the length or shape of
+the ears and tail, in the proportions of the limbs, or in the length and
+quality of the hair on different parts of the body. As regards the ears
+and tail, one of the objections by Professor Bronn relates to this very
+point. He states that the length of these organs differ in the various
+species of hares and of mice, and he considers that this difference can
+be of no service whatever to their possessors. But to this objection
+Darwin replies, that it has been shown by Dr. Schöbl that the ears of
+mice "are supplied in an extraordinary manner with nerves, so that they
+no doubt serve as tactile organs." Hence, when we consider the life of
+mice, either nocturnal or seeking their food in dark and confined
+places, the length of the ears may be in each case adapted to the
+particular habits and surroundings of the species. Again, the tail, in
+the larger mammals, often serves the purpose of driving off flies and
+other insects from the body; and when we consider in how many parts of
+the world flies are injurious or even fatal to large mammals, we see
+that the peculiar characteristics of this organ may in each case have
+been adapted to its requirements in the particular area where the
+species was developed. The tail is also believed to have some use as a
+balancing organ, which assists an animal to turn easily and rapidly,
+much as our arms are used when running; while in whole groups it is a
+prehensile organ, and has become modified in accordance with the habits
+and needs of each species. In the case of mice it is thus used by the
+young. Darwin informs us that the late Professor Henslow kept some
+harvest-mice in confinement, and observed that they frequently curled
+their tails round the branches of a bush placed in the cage, and thus
+aided themselves in climbing; while Dr. Günther has actually seen a
+mouse suspend itself by the tail (_Origin_, p. 189).
+
+Again, Mr. Lawson Tait has called attention to the use of the tail in
+the cat, squirrel, yak, and many other animals as a means of preserving
+the heat of the body during the nocturnal and the winter sleep. He says,
+that in cold weather animals with long or bushy tails will be found
+lying curled up, with their tails carefully laid over their feet like a
+rug, and with their noses buried in the fur of the tail, which is thus
+used exactly in the same way and for the same purpose as we use
+respirators.[43]
+
+Another illustration is furnished by the horns of deer which, especially
+when very large, have been supposed to be a danger to the animal in
+passing rapidly through dense thickets. But Sir James Hector states,
+that the wapiti, in North America, throws back its head, thus placing
+the horns along the sides of the back, and is then enabled to rush
+through the thickest forest with great rapidity. The brow-antlers
+protect the face and eyes, while the widely spreading horns prevent
+injury to the neck or flanks. Thus an organ which was certainly
+developed as a sexual weapon, has been so guided and modified during its
+increase in size as to be of use in other ways. A similar use of the
+antlers of deer has been observed in India.[44]
+
+The various classes of facts now referred to serve to show us that, in
+the case of the two higher groups--mammalia and birds--almost all the
+characters by which species are distinguished from each other are, or
+may be, adaptive. It is these two classes of animals which have been
+most studied and whose life-histories are supposed to be most fully
+known, yet even here the assertion of inutility, by an eminent
+naturalist, in the case of two important organs, has been sufficiently
+met by minute details either in the anatomy or in the habits of the
+groups referred to. Such a fact as this, together with the extensive
+series of characters already enumerated which have been of late years
+transferred from the "useless" to the "useful" class, should convince
+us, that the assertion of "inutility" in the case of any organ or
+peculiarity which is not a rudiment or a correlation, is not, and can
+never be, the statement of a fact, but merely an expression of our
+ignorance of its purpose or origin.[45]
+
+
+_Instability of Non-adaptive Characters._
+
+One very weighty objection to the theory that _specific_ characters can
+ever be wholly useless (or wholly unconnected with useful organs by
+correlation of growth) appears to have been overlooked by those who have
+maintained the frequency of such characters, and that is, their almost
+necessary instability. Darwin has remarked on the extreme variability of
+secondary sexual characters--such as the horns, crests, plumes, etc.,
+which are found in males only,--the reason being, that, although of some
+use, they are not of such direct and vital importance as those adaptive
+characters on which the wellbeing and very existence of the animals
+depend. But in the case of wholly useless structures, which are not
+rudiments of once useful organs, we cannot see what there is to ensure
+any amount of constancy or stability. One of the cases on which Mr.
+Romanes lays great stress in his paper on "Physiological Selection"
+(_Journ. Linn. Soc._, vol. xix. p. 384) is that of the fleshy appendages
+on the corners of the jaw of Normandy pigs and of some other breeds. But
+it is expressly stated that they are not constant; they appear
+"frequently," or "occasionally," they are "not strictly inherited, for
+they occur or fail in animals of the same litter;" and they are not
+always symmetrical, sometimes appearing on one side of the face alone.
+Now whatever may be the cause or explanation of these anomalous
+appendages they cannot be classed with "specific characters," the most
+essential features of which are, that they _are_ symmetrical, that they
+_are_ inherited, and that they _are_ constant. Admitting that this
+peculiar appendage is (as Mr. Romanes says rather confidently, "we
+happen to know it to be") wholly useless and meaningless, the fact would
+be rather an argument against specific characters being also
+meaningless, because the latter never have the characteristics which
+this particular variation possesses.
+
+These useless or non-adaptive characters are, apparently, of the same
+nature as the "sports" that arise in our domestic productions, but
+which, as Mr. Darwin says, without the aid of selection would soon
+disappear; while some of them may be correlations with other characters
+which are or have been useful. Some of these correlations are very
+curious. Mr. Tegetmeier informed Mr. Darwin that the young of white,
+yellow, or dun-coloured pigeons are born almost naked, whereas other
+coloured pigeons are born well clothed with down. Now, if this
+difference occurred between wild species of different colours, it might
+be said that the nakedness of the young could not be of any use. But the
+colour with which it is correlated might, as has been shown, be useful
+in many ways. The skin and its various appendages, as horns, hoofs,
+hair, feathers, and teeth, are homologous parts, and are subject to very
+strange correlations of growth. In Paraguay, horses with curled hair
+occur, and these always have hoofs exactly like those of a mule, while
+the hair of the mane and tail is much shorter than usual. Now, if any
+one of these characters were useful, the others correlated with it might
+be themselves useless, but would still be tolerably constant because
+dependent on a useful organ. So the tusks and the bristles of the boar
+are correlated and vary in development together, and the former only may
+be useful, or both may be useful in unequal degrees.
+
+The difficulty as to how individual differences or sports can become
+fixed and perpetuated, if altogether useless, is evaded by those who
+hold that such characters are exceedingly common. Mr. Romanes says that,
+upon his theory of physiological selection, "it is quite intelligible
+that when a varietal form is differentiated from its parent form by the
+bar of sterility, any little meaningless peculiarities of structure or
+of instinct _should at first be allowed to arise_, and that they should
+then _be allowed to perpetuate themselves_ by heredity," until they are
+finally eliminated by disuse. But this is entirely begging the
+question. Do meaningless peculiarities, which we admit often arise as
+spontaneous variations, ever perpetuate themselves in all the
+individuals constituting a variety or race, without selection either
+human or natural? Such characters present themselves as unstable
+variations, and as such they remain, unless preserved and accumulated by
+selection; and they can therefore never become "specific" characters
+unless they are strictly correlated with some useful and important
+peculiarities.
+
+As bearing upon this question we may refer to what is termed Delboeuf's
+law, which has been thus briefly stated by Mr. Murphy in his work on
+_Habit and Intelligence_, p. 241.
+
+
+    "If, in any species, a number of individuals, bearing a ratio
+    not infinitely small to the entire number of births, are in
+    every generation born with a particular variation which is
+    neither beneficial nor injurious, and if it is not counteracted
+    by reversion, then the proportion of the new variety to the
+    original form will increase till it approaches indefinitely near
+    to equality."
+
+
+It is not impossible that some definite varieties, such as the melanic
+form of the jaguar and the bridled variety of the guillemot are due to
+this cause; but from their very nature such varieties are unstable, and
+are continually reproduced in varying proportions from the parent forms.
+They can, therefore, never constitute species unless the variation in
+question becomes beneficial, when it will be fixed by natural selection.
+Darwin, it is true, says--"There can be little doubt that the tendency
+to vary in the same manner has often been so strong that all the
+individuals of the same species have been similarly modified without the
+aid of any form of selection."[46] But no proof whatever is offered of
+this statement, and it is so entirely opposed to all we know of the
+facts of variation as given by Darwin himself, that the important word
+"all" is probably an oversight.
+
+On the whole, then, I submit, not only has it not been proved that an
+"enormous number of specific peculiarities" are useless, and that, as a
+logical result, natural selection is "not a theory of the origin of
+species," but only of the origin of adaptations which are usually
+common to many species, or, more commonly, to genera and families; but,
+I urge further, it has not even been proved that any truly "specific"
+characters--those which either singly or in combination distinguish each
+species from its nearest allies--are entirely unadaptive, useless, and
+meaningless; while a great body of facts on the one hand, and some
+weighty arguments on the other, alike prove that specific characters
+have been, and could only have been, developed and fixed by natural
+selection because of their utility. We may admit, that among the great
+number of variations and sports which continually arise many are
+altogether useless without being hurtful; but no cause or influence has
+been adduced adequate to render such characters fixed and constant
+throughout the vast number of individuals which constitute any of the
+more dominant species.[47]
+
+
+_The Swamping Effects of Intercrossing._
+
+This supposed insuperable difficulty was first advanced in an article in
+the _North British Review_ in 1867, and much attention has been
+attracted to it by the acknowledgment of Mr. Darwin that it proved to
+him that "single variations," or what are usually termed "sports," could
+very rarely, if ever, be perpetuated in a state of nature, as he had at
+first thought might occasionally be the case. But he had always
+considered that the chief part, and latterly the whole, of the materials
+with which natural selection works, was afforded by individual
+variations, or that amount of ever fluctuating variability which exists
+in all organisms and in all their parts. Other writers have urged the
+same objection, even as against individual variability, apparently in
+total ignorance of its amount and range; and quite recently Professor
+G.J. Romanes has adduced it as one of the difficulties which can alone
+be overcome by his theory of physiological selection. He urges, that the
+same variation does not occur simultaneously in a number of individuals
+inhabiting the same area, and that it is mere assumption to say it does;
+while he admits that "if the assumption were granted there would be an
+end of the present difficulty; for if a sufficient number of individuals
+were thus simultaneously and similarly modified, there need be no longer
+any danger of the variety becoming swamped by intercrossing." I must
+again refer my readers to my third chapter for the proof that such
+simultaneous variability is not an assumption but a fact; but, even
+admitting this to be proved, the problem is not altogether solved, and
+there is so much misconception regarding variation, and the actual
+process of the origin of new species is so obscure, that some further
+discussion and elucidation of the subject are desirable.
+
+In one of the preliminary chapters of Mr. Seebohm's recent work on the
+_Charadriidae_, he discusses the differentiation of species; and he
+expresses a rather widespread view among naturalists when, speaking of
+the swamping effects of intercrossing, he adds: "This is unquestionably
+a very grave difficulty, to my mind an absolutely fatal one, to the
+theory of accidental variation." And in another passage he says: "The
+simultaneous appearance, and its repetition in successive generations,
+of a beneficial variation, in a large number of individuals in the same
+locality, cannot possibly be ascribed to chance." These remarks appear
+to me to exhibit an entire misconception of the facts of variation as
+they actually occur, and as they have been utilised by natural selection
+in the modification of species. I have already shown that every part of
+the organism, in common species, does vary to a very considerable
+amount, in a large number of individuals, and in the same locality; the
+only point that remains to be discussed is, whether any or most of these
+variations are "beneficial." But every one of these variations consists
+either in increase or diminution of size or power of the organ or
+faculty that varies; they can all be divided into a more effective and a
+less effective group--that is, into one that is more beneficial or less
+beneficial. If less size of body would be beneficial, then, as half the
+variations in size are above and half below the mean or existing
+standard of the species, there would be ample beneficial variations; if
+a darker colour or a longer beak or wing were required, there are always
+a considerable number of individuals darker and lighter in colour than
+the average, with longer or with shorter beaks and wings, and thus the
+beneficial variation must always be present. And so with every other
+part, organ, function, or habit; because, as variation, so far as we
+know, is and always must be in the two directions of excess and defect
+in relation to the mean amount, whichever kind of variation is wanted is
+always present in some degree, and thus the difficulty as to
+"beneficial" variations occurring, as if they were a special and rare
+class, falls to the ground. No doubt some organs may vary in three or
+perhaps more directions, as in the length, breadth, thickness, or
+curvature of the bill. But these may be taken as separate variations,
+each of which again occurs as "more" or "less"; and thus the "right" or
+"beneficial" or "useful" variation must always be present so long as any
+variation at all occurs; and it has not yet been proved that in any
+large or dominant species, or in any part, organ, or faculty of such
+species, there is no variation. And even were such a case found it would
+prove nothing, so long as in numerous other species variation was shown
+to exist; because we know that great numbers of species and groups
+throughout all geological time have died out, leaving no descendants;
+and the obvious and sufficient explanation of this fact is, that they
+did _not_ vary enough at the time when variation was required to bring
+them into harmony with changed conditions. The objection as to the
+"right" or "beneficial" variation occurring when required, seems
+therefore to have no weight in view of the actual facts of variation.
+
+
+_Isolation to prevent Intercrossing._
+
+Most writers on the subject consider the isolation of a portion of a
+species a very important factor in the formation of new species, while
+others maintain it to be absolutely essential. This latter view has
+arisen from an exaggerated opinion as to the power of intercrossing to
+keep down any variety or incipient species, and merge it in the parent
+stock. But it is evident that this can only occur with varieties which
+are not useful, or which, if useful, occur in very small numbers; and
+from this kind of variations it is clear that new species do not arise.
+Complete isolation, as in an oceanic island, will no doubt enable
+natural selection to act more rapidly, for several reasons. In the first
+place, the absence of competition will for some time allow the new
+immigrants to increase rapidly till they reach the limits of
+subsistence. They will then struggle among themselves, and by survival
+of the fittest will quickly become adapted to the new conditions of
+their environment. Organs which they formerly needed, to defend
+themselves against, or to escape from, enemies, being no longer
+required, would be encumbrances to be got rid of, while the power of
+appropriating and digesting new and varied food would rise in
+importance. Thus we may explain the origin of so many flightless and
+rather bulky birds in oceanic islands, as the dodo, the cassowary, and
+the extinct moas. Again, while this process was going on, the complete
+isolation would prevent its being checked by the immigration of new
+competitors or enemies, which would be very likely to occur in a
+continuous area; while, of course, any intercrossing with the original
+unmodified stock would be absolutely prevented. If, now, before this
+change has gone very far, the variety spreads into adjacent but rather
+distant islands, the somewhat different conditions in each may lead to
+the development of distinct forms constituting what are termed
+representative species; and these we find in the separate islands of the
+Galapagos, the West Indies, and other ancient groups of islands.
+
+But such cases as these will only lead to the production of a few
+peculiar species, descended from the original settlers which happened to
+reach the islands; whereas, in wide areas, and in continents, we have
+variation and adaptation on a much larger scale; and, whenever important
+physical changes demand them, with even greater rapidity. The far
+greater complexity of the environment, together with the occurrence of
+variations in constitution and habits, will often allow of effective
+isolation, even here, producing all the results of actual physical
+isolation. As we have already explained, one of the most frequent modes
+in which natural selection acts is, by adapting some individuals of a
+species to a somewhat different mode of life, whereby they are able to
+seize upon unappropriated places in nature, and in so doing they become
+practically isolated from their parent form. Let us suppose, for
+example, that one portion of a species usually living in forests ranges
+into the open plains, and finding abundance of food remains there
+permanently. So long as the struggle for existence is not exceptionally
+severe, these two portions of the species may remain almost unchanged;
+but suppose some fresh enemies are attracted to the plains by the
+presence of these new immigrants, then variation and natural selection
+would lead to the preservation of those individuals best able to cope
+with the difficulty, and thus the open country form would become
+modified into a marked variety or into a distinct species; and there
+would evidently be little chance of this modification being checked by
+intercrossing with the parent form which remained in the forest.
+
+Another mode of isolation is brought about by the variety--either owing
+to habits, climate, or constitutional change--breeding at a slightly
+different time from the parent species. This is known to produce
+complete isolation in the case of many varieties of plants. Yet another
+mode of isolation is brought about by changes of colour, and by the fact
+that in a wild state animals of similar colours prefer to keep together
+and refuse to pair with individuals of another colour. The probable
+reason and utility of this habit will be explained in another chapter,
+but the fact is well illustrated by the cattle which have run wild in
+the Falkland Islands. These are of several different colours, but each
+colour keeps in a separate herd, often restricted to one part of the
+island; and one of these varieties--the mouse-coloured--is said to breed
+a month earlier than the others; so that if this variety inhabited a
+larger area it might very soon be established as a distinct race or
+species.[48] Of course where the change of habits or of station is still
+greater, as when a terrestrial animal becomes sub-aquatic, or when
+aquatic animals come to live in tree-tops, as with the frogs and
+Crustacea described at p. 118, the danger of intercrossing is reduced to
+a minimum.
+
+Several writers, however, not content with the indirect effects of
+isolation here indicated, maintain that it is in itself a cause of
+modification, and ultimately of the origination of new species. This
+was the keynote of Mr. Vernon Wollaston's essay on "Variation of
+Species," published in 1856, and it is adopted by the Rev. J.G. Gulick
+in his paper on "Diversity of Evolution under one Set of External
+Conditions" (_Journ. Linn. Soc. Zool._, vol. xi. p. 496). The idea seems
+to be that there is an inherent tendency to variation in certain
+divergent lines, and that when one portion of a species is isolated,
+even though under identical conditions, that tendency sets up a
+divergence which carries that portion farther and farther away from the
+original species. This view is held to be supported by the case of the
+land shells of the Sandwich Islands, which certainly present some very
+remarkable phenomena. In this comparatively small area there are about
+300 species of land shells, almost all of which belong to one family (or
+sub-family), the Achatinellidae, found nowhere else in the world. The
+interesting point is the extreme restriction of the species and
+varieties. The average range of each species is only five or six miles,
+while some are restricted to but one or two square miles, and only a
+very few range over a whole island. The forest region that extends over
+one of the mountain-ranges of the island of Oahu, is about forty miles
+in length and five or six miles in breadth; and this small territory
+furnishes about 175 species, represented by 700 or 800 varieties. Mr.
+Gulick states, that the vegetation of the different valleys on the same
+side of this range is much the same, yet each has a molluscan fauna
+differing in some degree from that of any other. "We frequently find a
+genus represented in several successive valleys by allied species,
+sometimes feeding on the same, sometimes on different plants. In every
+such case the valleys that are nearest to each other furnish the most
+nearly allied forms; and a full set of the varieties of each species
+presents a minute gradation of forms between the more divergent types
+found in the more widely separated localities." He urges, that these
+constant differences cannot be attributed to natural selection, because
+they occur in different valleys on the same side of the mountain, where
+food, climate, and enemies are the same; and also, because there is no
+greater difference in passing from the rainy to the dry side of the
+mountains than in passing from one valley to another on the same side
+an equal distance apart. In a very lengthy paper, presented to the
+Linnean Society last year, on "Divergent Evolution through Cumulative
+Segregation," Mr. Gulick endeavours to work out his views into a
+complete theory, the main point of which may perhaps be indicated by the
+following passage: "No two portions of a species possess exactly the
+same average character, and the initial differences are for ever
+reacting on the environment and on each other in such a way as to ensure
+increasing divergence in each successive generation as long as the
+individuals of the two groups are kept from intercrossing."[49]
+
+It need hardly be said that the views of Mr. Darwin and myself are
+inconsistent with the notion that, if the environment were absolutely
+similar for the two isolated portions of the species, any such necessary
+and constant divergence would take place. It is an error to assume that
+what seem to us identical conditions are really identical to such small
+and delicate organisms as these land molluscs, of whose needs and
+difficulties at each successive stage of their existence, from the
+freshly-laid egg up to the adult animal, we are so profoundly ignorant.
+The exact proportions of the various species of plants, the numbers of
+each kind of insect or of bird, the peculiarities of more or less
+exposure to sunshine or to wind at certain critical epochs, and other
+slight differences which to us are absolutely immaterial and
+unrecognisable, may be of the highest significance to these humble
+creatures, and be quite sufficient to require some slight adjustments of
+size, form, or colour, which natural selection will bring about. All we
+know of the facts of variation leads us to believe that, without this
+action of natural selection, there would be produced over the whole area
+a series of inconstant varieties mingled together, not a distinct
+segregation of forms each confined to its own limited area.
+
+Mr. Darwin has shown that, in the distribution and modification of
+species, the biological is of more importance than the physical
+environment, the struggle with other organisms being often more severe
+than that with the forces of nature. This is particularly evident in the
+case of plants, many of which, when protected from competition, thrive
+in a soil, climate, and atmosphere widely different from those of their
+native habitat. Thus, many alpine plants only found near perpetual snow
+thrive well in our gardens at the level of the sea; as do the tritomas
+from the sultry plains of South Africa, the yuccas from the arid hills
+of Texas and Mexico, and the fuchsias from the damp and dreary shores of
+the Straits of Magellan. It has been well said that plants do not live
+where they like, but where they can; and the same remark will apply to
+the animal world. Horses and cattle run wild and thrive both in North
+and South America; rabbits, once confined to the south of Europe, have
+established themselves in our own country and in Australia; while the
+domestic fowl, a native of tropical India, thrives well in every part of
+the temperate zone.
+
+If, then, we admit that when one portion of a species is separated from
+the rest, there will necessarily be a slight difference in the average
+characters of the two portions, it does not follow that this difference
+has much if any effect upon the characteristics that are developed by a
+long period of isolation. In the first place, the difference itself will
+necessarily be very slight unless there is an exceptional amount of
+variability in the species; and in the next place, if the average
+characters of the species are the expression of its exact adaptation to
+its whole environment, then, given a precisely similar environment, and
+the isolated portion will inevitably be brought back to the same average
+of characters. But, as a matter of fact, it is impossible that the
+environment of the isolated portion can be exactly like that of the bulk
+of the species. It cannot be so physically, since no two separated areas
+can be absolutely alike in climate and soil; and even if these are the
+same, the geographical features, size, contour, and relation to winds,
+seas, and rivers, would certainly differ. Biologically, the differences
+are sure to be considerable. The isolated portion of a species will
+almost always be in a much smaller area than that occupied by the
+species as a whole, hence it is at once in a different position as
+regards its own kind. The proportions of all the other species of
+animals and plants are also sure to differ in the two areas, and some
+species will almost always be absent in the smaller which are present in
+the larger country. These differences will act and react on the
+isolated portion of the species. The struggle for existence will differ
+in its severity and in its incidence from that which affects the bulk of
+the species. The absence of some one insect or other creature inimical
+to the young animal or plant may cause a vast difference in its
+conditions of existence, and may necessitate a modification of its
+external or internal characters in quite a different direction from that
+which happened to be present in the average of the individuals which
+were first isolated.
+
+On the whole, then, we conclude that, while isolation is an important
+factor in effecting some modification of species, it is so, not on
+account of any effect produced, or influence exerted by isolation _per
+se_, but because it is always and necessarily accompanied by a change of
+environment, both physical and biological. Natural selection will then
+begin to act in adapting the isolated portion to its new conditions, and
+will do this the more quickly and the more effectually because of the
+isolation. We have, however, seen reason to believe that geographical or
+local isolation is by no means essential to the differentiation of
+species, because the same result is brought about by the incipient
+species acquiring different habits or frequenting a different station;
+and also by the fact that different varieties of the same species are
+known to prefer to pair with their like, and thus to bring about a
+physiological isolation of the most effective kind. This part of the
+subject will be again referred to when the very difficult problems
+presented by hybridity are discussed.[50]
+
+
+_Cases in which Isolation is Ineffective._
+
+One objection to the views of those who, like Mr. Gulick, believe
+isolation itself to be a cause of modification of species deserves
+attention, namely, the entire absence of change where, if this were a
+_vera causa_, we should expect to find it. In Ireland we have an
+excellent test case, for we know that it has been separated from Britain
+since the end of the glacial epoch, certainly many thousand years. Yet
+hardly one of its mammals, reptiles, or land molluscs has undergone the
+slightest change, even although there is certainly a distinct difference
+in the environment both inorganic and organic. That changes have not
+occurred through natural selection, is perhaps due to the less severe
+struggle for existence owing to the smaller number of competing species;
+but, if isolation itself were an efficient cause, acting continuously
+and cumulatively, it is incredible that a decided change should not have
+been produced in thousands of years. That no such change has occurred in
+this, and many other cases of isolation, seems to prove that it is not
+in itself a cause of modification.
+
+There yet remain a number of difficulties and objections relating to the
+question of hybridity, which are so important as to require a separate
+chapter for their adequate discussion.
+
+FOOTNOTES:
+
+[Footnote 41: See _Origin of Species_, pp. 176-198.]
+
+[Footnote 42: See Kerner's _Flowers and their Unbidden Guests_ for
+numerous other structures and peculiarities of plants which are shown to
+be adaptive and useful.]
+
+[Footnote 43: _Nature_, vol. xx. p. 603.]
+
+[Footnote 44: _Nature_, vol. xxxviii. p. 328.]
+
+[Footnote 45: A very remarkable illustration of function in an
+apparently useless ornament is given by Semper. He says, "It is known
+that the skin of reptiles encloses the body with scales. These scales
+are distinguished by very various sculpturings, highly characteristic of
+the different species. Irrespective of their systematic significance
+they appear to be of no value in the life of the animal; indeed, they
+are viewed as ornamental without regard to the fact that they are
+microscopic and much too delicate to be visible to other animals of
+their own species. It might, therefore, seem hopeless to show the
+necessity for their existence on Darwinian principles, and to prove that
+they are physiologically active organs. Nevertheless, recent
+investigations on this point have furnished evidence that this is
+possible.
+
+"It is known that many reptiles, and above all the snakes, cast off the
+whole skin at once, whereas human beings do so by degrees. If by any
+accident they are prevented doing so, they infallibly die, because the
+old skin has grown so tough and hard that it hinders the increase in
+volume which is inseparable from the growth of the animal. The casting
+of the skin is induced by the formation on the surface of the inner
+epidermis, of a layer of very fine and equally distributed hairs, which
+evidently serve the purpose of mechanically raising the old skin by
+their rigidity and position. These hairs then may be designated as
+_casting hairs_. That they are destined and calculated for this end is
+evident to me from the fact established by Dr. Braun, that the casting
+of the shells of the river crayfish is induced in exactly the same
+manner by the formation of a coating of hairs which mechanically loosens
+the old skin or shell from the new. Now the researches of Braun and
+Cartier have shown that these casting hairs--which serve the same
+purpose in two groups of animals so far apart in the systematic
+scale--after the casting, are partly transformed into the concentric
+stripes, sharp spikes, ridges, or warts which ornament the outer edges
+of the skin-scales of reptiles or the carapace of crabs."[1] Professor
+Semper adds that this example, with many others that might be quoted,
+shows that we need not abandon the hope of explaining morphological
+characters on Darwinian principles, although their nature is often
+difficult to understand.
+
+During a recent discussion of this question in the pages of _Nature_,
+Mr. St. George Mivart adduces several examples of what he deems useless
+specific characters. Among them are the aborted index finger of the
+lemurine Potto, and the thumbless hands of Colobus and Ateles, the
+"life-saving action" of either of which he thinks incredible. These
+cases suggest two remarks. In the first place, they involve _generic_,
+not _specific_, characters; and the three genera adduced are somewhat
+isolated, implying considerable antiquity and the extinction of many
+allied forms. This is important, because it affords ample time for great
+changes of conditions since the structures in question originated; and
+without a knowledge of these changes we can never safely assert that any
+detail of structure could not have been useful. In the second place, all
+three are cases of aborted or rudimentary organs; and these are admitted
+to be explained by non-use, leading to diminution of size, a further
+reduction being brought about by the action of the principle of economy
+of growth. But, when so reduced, the rudiment might be inconvenient or
+even hurtful, and then natural selection would aid in its complete
+abortion; in other words, the abortion of the part would be _useful_,
+and would therefore be subject to the law of survival of the fittest.
+The genera Ateles and Colobus are two of the most purely arboreal types
+of monkeys, and it is not difficult to conceive that the constant use of
+the elongated fingers for climbing from tree to tree, and catching on to
+branches while making great leaps, might require all the nervous energy
+and muscular growth to be directed to the fingers, the small thumb
+remaining useless. The case of the Potto is more difficult, both because
+it is, presumably, a more ancient type, and its actual life-history and
+habits are completely unknown. These cases are, therefore, not at all to
+the point as proving that positive specific characters--not mere
+rudiments characterising whole genera--are in any case useless.
+
+Mr. Mivart further objects to the alleged rigidity of the action of
+natural selection, because wounded or malformed animals have been found
+which had evidently lived a considerable time in their imperfect
+condition. But this simply proves that they were living under a
+temporarily favourable environment, and that the real struggle for
+existence, in their case, had not yet taken place. We must surely admit
+that, when the pinch came, and when perfectly formed stoats were dying
+for want of food, the one-footed animal, referred to by Mr. Mivart,
+would be among the first to succumb; and the same remark will apply to
+his abnormally toothed hares and rheumatic monkeys, which might,
+nevertheless, get on very well under favourable conditions. The struggle
+for existence, under which all animals and plants have been developed,
+is intermittent, and exceedingly irregular in its incidence and
+severity. It is most severe and fatal to the young; but when an animal
+has once reached maturity, and especially when it has gained experience
+by several years of an eventful existence, it may be able to maintain
+itself under conditions which would be fatal to a young and
+inexperienced creature of the same species. The examples adduced by Mr.
+Mivart do not, therefore, in any way impugn the hardness of nature as a
+taskmaster, or the extreme severity of the recurring struggle for
+existence. (See _Nature_, vol. xxxix. p. 127.)]
+
+[Footnote 46: _Origin of Species,_ p. 72.]
+
+[Footnote 47: Darwin's latest expression of opinion on this question is
+interesting, since it shows that he was inclined to return to his
+earlier view of the general, or universal, utility of specific
+characters. In a letter to Semper (30th Nov. 1878) he writes: "As our
+knowledge advances, very slight differences, considered by systematists
+as of no importance in structure, are continually found to be
+functionally important; and I have been especially struck with this fact
+in the case of plants, to which my observations have, of late years,
+been confined. Therefore it seems to me rather rash to consider slight
+differences between representative species, for instance, those
+inhabiting the different islands of the same archipelago, as of no
+functional importance, and as not in any way due to natural selection"
+_(Life of Darwin_, vol. iii. p. 161).]
+
+[Footnote 48: See _Variation of Animals and Plants_, vol. i. p. 86.]
+
+[Footnote 49: _Journal of the Linnean Society, Zoology,_ vol. xx. p.
+215.]
+
+[Footnote 50: In Mr. Gulick's last paper (_Journal of Linn. Soc. Zool._,
+vol. xx. pp. 189-274) he discusses the various forms of isolation above
+referred to, under no less than thirty-eight different divisions and
+subdivisions, with an elaborate terminology, and he argues that these
+will frequently bring about divergent evolution without any change in
+the environment or any action of natural selection. The discussion of
+the problem here given will, I believe, sufficiently expose the fallacy
+of his contention; but his illustration of the varied and often
+recondite modes by which practical isolation may be brought about, may
+help to remove one of the popular difficulties in the way of the action
+of natural selection in the origination of species.]
+
+
+
+
+CHAPTER VII
+
+ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
+STERILITY OF THEIR HYBRID OFFSPRING
+
+
+    Statement of the problem--Extreme susceptibility of the
+    reproductive functions--Reciprocal crosses--Individual
+    differences in respect to cross-fertilisation--Dimorphism and
+    trimorphism among plants--Cases of the fertility of hybrids and
+    of the infertility of mongrels--The effects of close
+    interbreeding--Mr. Huth's objections--Fertile hybrids among
+    animals--Fertility of hybrids among plants--Cases of sterility
+    of mongrels--Parallelism between crossing and change of
+    conditions--Remarks on the facts of hybridity--Sterility due to
+    changed conditions and usually correlated with other
+    characters--Correlation of colour with constitutional
+    peculiarities--The isolation of varieties by selective
+    association--The influence of natural selection upon sterility
+    and fertility--Physiological selection--Summary and concluding
+    remarks.
+
+
+
+One of the greatest, or perhaps we may say the greatest, of all the
+difficulties in the way of accepting the theory of natural selection as
+a complete explanation of the origin of species, has been the remarkable
+difference between varieties and species in respect of fertility when
+crossed. Generally speaking, it may be said that the varieties of any
+one species, however different they may be in external appearance, are
+perfectly fertile when crossed, and their mongrel offspring are equally
+fertile when bred among themselves; while distinct species, on the other
+hand, however closely they may resemble each other externally, are
+usually infertile when crossed, and their hybrid offspring absolutely
+sterile. This used to be considered a fixed law of nature, constituting
+the absolute test and criterion of a _species_ as distinct from a
+_variety_; and so long as it was believed that species were separate
+creations, or at all events had an origin quite distinct from that of
+varieties, this law could have no exceptions, because, if any two
+species had been found to be fertile when crossed and their hybrid
+offspring to be also fertile, this fact would have been held to prove
+them to be not _species_ but _varieties_. On the other hand, if two
+varieties had been found to be infertile, or their mongrel offspring to
+be sterile, then it would have been said: These are not varieties but
+true species. Thus the old theory led to inevitable reasoning in a
+circle; and what might be only a rather common fact was elevated into a
+law which had no exceptions.
+
+The elaborate and careful examination of the whole subject by Mr.
+Darwin, who has brought together a vast mass of evidence from the
+experience of agriculturists and horticulturists, as well as from
+scientific experimenters, has demonstrated that there is no such fixed
+law in nature as was formerly supposed. He shows us that crosses between
+some varieties are infertile or even sterile, while crosses between some
+species are quite fertile; and that there are besides a number of
+curious phenomena connected with the subject which render it impossible
+to believe that sterility is anything more than an incidental property
+of species, due to the extreme delicacy and susceptibility of the
+reproductive powers, and dependent on physiological causes we have not
+yet been able to trace. Nevertheless, the fact remains that most species
+which have hitherto been crossed produce sterile hybrids, as in the
+well-known case of the mule; while almost all domestic varieties, when
+crossed, produce offspring which are perfectly fertile among themselves.
+I will now endeavour to give such a sketch of the subject as may enable
+the reader to see something of the complexity of the problem, referring
+him to Mr. Darwin's works for fuller details.
+
+
+_Extreme Susceptibility of the Reproductive Functions._
+
+One of the most interesting facts, as showing how susceptible to changed
+conditions or to slight constitutional changes are the reproductive
+powers of animals, is the very general difficulty of getting those which
+are kept in confinement to breed; and this is frequently the only bar to
+domesticating wild species. Thus, elephants, bears, foxes, and numbers
+of species of rodents, very rarely breed in confinement; while other
+species do so more or less freely. Hawks, vultures, and owls hardly ever
+breed in confinement; neither did the falcons kept for hawking ever
+breed. Of the numerous small seed-eating birds kept in aviaries, hardly
+any breed, neither do parrots. Gallinaceous birds usually breed freely
+in confinement, but some do not; and even the guans and curassows, kept
+tame by the South American Indians, never breed. This shows that change
+of climate has nothing to do with the phenomenon; and, in fact, the same
+species that refuse to breed in Europe do so, in almost every case, when
+tamed or confined in their native countries. This inability to reproduce
+is not due to ill-health, since many of these creatures are perfectly
+vigorous and live very long.
+
+With our true domestic animals, on the other hand, fertility is perfect,
+and is very little affected by changed conditions. Thus, we see the
+common fowl, a native of tropical India, living and multiplying in
+almost every part of the world; and the same is the case with our
+cattle, sheep, and goats, our dogs and horses, and especially with
+domestic pigeons. It therefore seems probable, that this facility for
+breeding under changed conditions was an original property of the
+species which man has domesticated--a property which, more than any
+other, enabled him to domesticate them. Yet, even with these, there is
+evidence that great changes of conditions affect the fertility. In the
+hot valleys of the Andes sheep are less fertile; while geese taken to
+the high plateau of Bogota were at first almost sterile, but after some
+generations recovered their fertility. These and many other facts seem
+to show that, with the majority of animals, even a slight change of
+conditions may produce infertility or sterility; and also that after a
+time, when the animal has become thoroughly acclimatised, as it were, to
+the new conditions, the infertility is in some cases diminished or
+altogether ceases. It is stated by Bechstein that the canary was long
+infertile, and it is only of late years that good breeding birds have
+become common; but in this case no doubt selection has aided the change.
+
+As showing that these phenomena depend on deep-seated causes and are of
+a very general nature, it is interesting to note that they occur also
+in the vegetable kingdom. Allowing for all the circumstances which are
+known to prevent the production of seed, such as too great luxuriance of
+foliage, too little or too much heat, or the absence of insects to
+cross-fertilise the flowers, Mr. Darwin shows that many species which
+grow and flower with us, apparently in perfect health, yet never produce
+seed. Other plants are affected by very slight changes of conditions,
+producing seed freely in one soil and not in another, though apparently
+growing equally well in both; while, in some cases, a difference of
+position even in the same garden produces a similar result.[51]
+
+
+_Reciprocal Crosses._
+
+Another indication of the extreme delicacy of the adjustment between the
+sexes, which is necessary to produce fertility, is afforded by the
+behaviour of many species and varieties when reciprocally crossed. This
+will be best illustrated by a few of the examples furnished us by Mr.
+Darwin. The two distinct species of plants, Mirabilis jalapa and M.
+longiflora, can be easily crossed, and will produce healthy and fertile
+hybrids when the pollen of the latter is applied to the stigma of the
+former plant. But the same experimenter, Kölreuter, tried in vain, more
+than two hundred times during eight years, to cross them by applying the
+pollen of M. jalapa to the stigma of M. longiflora. In other cases two
+plants are so closely allied that some botanists class them as varieties
+(as with Matthiola annua and M. glabra), and yet there is the same great
+difference in the result when they are reciprocally crossed.
+
+
+_Individual Differences in respect to Cross-Fertilisation._
+
+A still more remarkable illustration of the delicate balance of
+organisation needful for reproduction, is afforded by the individual
+differences of animals and plants, as regards both their power of
+intercrossing with other individuals or other species, and the fertility
+of the offspring thus produced. Among domestic animals, Darwin states
+that it is by no means rare to find certain males and females which will
+not breed together, though both are known to be perfectly fertile with
+other males and females. Cases of this kind have occurred among horses,
+cattle, pigs, dogs, and pigeons; and the experiment has been tried so
+frequently that there can be no doubt of the fact. Professor G.J.
+Romanes states that he has a number of additional cases of this
+individual incompatibility, or of absolute sterility, between two
+individuals, each of which is perfectly fertile with other individuals.
+
+During the numerous experiments that have been made on the hybridisation
+of plants similar peculiarities have been noticed, some individuals
+being capable, others incapable, of being crossed with a distinct
+species. The same individual peculiarities are found in varieties,
+species, and genera. Kölreuter crossed five varieties of the common
+tobacco (Nicotiana tabacum) with a distinct species, Nicotiana
+glutinosa, and they all yielded very sterile hybrids; but those raised
+from one variety were less sterile, in all the experiments, than the
+hybrids from the four other varieties. Again, most of the species of the
+genus Nicotiana have been crossed, and freely produce hybrids; but one
+species, N. acuminata, not particularly distinct from the others, could
+neither fertilise, nor be fertilised by, any of the eight other species
+experimented on. Among genera we find some--such as Hippeastrum, Crinum,
+Calceolaria, Dianthus--almost all the species of which will fertilise
+other species and produce hybrid offspring; while other allied genera,
+as Zephyranthes and Silene, notwithstanding the most persevering
+efforts, have not produced a single hybrid even between the most closely
+allied species.
+
+
+_Dimorphism and Trimorphism._
+
+Peculiarities in the reproductive system affecting individuals of the
+same species reach their maximum in what are called heterostyled, or
+dimorphic and trimorphic flowers, the phenomena presented by which form
+one of the most remarkable of Mr. Darwin's many discoveries. Our common
+cowslip and primrose, as well as many other species of the genus
+Primula, have two kinds of flowers in about equal proportions. In one
+kind the stamens are short, being situated about the middle of the tube
+of the corolla, while the style is long, the globular stigma appearing
+just in the centre of the open flower. In the other kind the stamens are
+long, appearing in the centre or throat of the flower, while the style
+is short, the stigma being situated halfway down the tube at the same
+level as the stamens in the other form. These two forms have long been
+known to florists as the "pin-eyed" and the "thrum-eyed," but they are
+called by Darwin the long-styled and short-styled forms (see woodcut).
+
+[Illustration: FIG. 17.--Primula veris (Cowslip).]
+
+The meaning and use of these different forms was quite unknown till
+Darwin discovered, first, that cowslips and primroses are absolutely
+barren if insects are prevented from visiting them, and then, what is
+still more extraordinary, that each form is almost sterile when
+fertilised by its own pollen, and comparatively infertile when crossed
+with any other plant of its own form, but is perfectly fertile when the
+pollen of a long-styled is carried to the stigma of a short-styled
+plant, or _vice versā_. It will be seen, by the figures, that the
+arrangement is such that a bee visiting the flowers will carry the
+pollen from the long anthers of the short-styled form to the stigma of
+the long-styled form, while it would never reach the stigma of another
+plant of the short-styled form. But an insect visiting, first, a
+long-styled plant, would deposit the pollen on the stigma of another
+plant of the same kind if it were next visited; and this is probably the
+reason why the wild short-styled plants were found to be almost always
+most productive of seed, since they must be all fertilised by the other
+form, whereas the long-styled plants might often be fertilised by their
+own form. The whole arrangement, however, ensures cross-fertilisation;
+and this, as Mr. Darwin has shown by copious experiments, adds both to
+the vigour and fertility of almost all plants as well as animals.
+
+Besides the primrose family, many other plants of several distinct
+natural orders present similar phenomena, one or two of the most curious
+of which must be referred to. The beautiful crimson flax (Linum
+grandiflorum) has also two forms, the styles only differing in length;
+and in this case Mr. Darwin found by numerous experiments, which have
+since been repeated and confirmed by other observers, that each form is
+absolutely sterile with pollen from another plant of its own form, but
+abundantly fertile when crossed with any plant of the other form. In
+this case the pollen of the two forms cannot be distinguished under the
+microscope (whereas that of the two forms of Primula differs in size and
+shape), yet it has the remarkable property of being absolutely powerless
+on the stigmas of half the plants of its own species. The crosses
+between the opposite forms, which are fertile, are termed by Mr. Darwin
+"legitimate," and those between similar forms, which are sterile,
+"illegitimate"; and he remarks that we have here, within the limits of
+the same species, a degree of sterility which rarely occurs except
+between plants or animals not only of different _species_ but of
+different _genera_.
+
+But there is another set of plants, the trimorphic, in which the styles
+and stamens have each three forms--long, medium, and short, and in these
+it is possible to have eighteen different crosses. By an elaborate
+series of experiments it was shown that the six legitimate unions--that
+is, when a plant was fertilised by pollen from stamens of length
+corresponding to that of its style in the two other forms--were all
+abundantly fertile; while the twelve illegitimate unions, when a plant
+was fertilised by pollen from stamens of a different length from its
+own style, in any of the three forms, were either comparatively or
+wholly sterile.[52]
+
+We have here a wonderful amount of constitutional difference of the
+reproductive organs within a single species, greater than usually occurs
+within the numerous distinct species of a genus or group of genera; and
+all this diversity appears to have arisen for a purpose which has been
+obtained by many other, and apparently simpler, changes of structure or
+of function, in other plants. This seems to show us, in the first place,
+that variations in the mutual relations of the reproductive organs of
+different individuals must be as frequent as structural variations have
+been shown to be; and, also, that sterility in itself can be no test of
+specific distinctness. But this point will be better considered when we
+have further illustrated and discussed the complex phenomena of
+hybridity.
+
+
+_Cases of the Fertility of Hybrids, and of the Infertility of Mongrels._
+
+I now propose to adduce a few cases in which it has been proved, by
+experiment, that hybrids between two distinct species are fertile _inter
+se_; and then to consider why it is that such cases are so few in
+number.
+
+The common domestic goose (Anser ferns) and the Chinese goose (A.
+cygnoides) are very distinct species, so distinct that some naturalists
+have placed them in different genera; yet they have bred together, and
+Mr. Eyton raised from a pair of these hybrids a brood of eight. This
+fact was confirmed by Mr. Darwin himself, who raised several fine birds
+from a pair of hybrids which were sent him.[53] In India, according to
+Mr. Blyth and Captain Hutton, whole flocks of these hybrid geese are
+kept in various parts of the country where neither of the pure parent
+species exists, and as they are kept for profit they must certainly be
+fully fertile.
+
+Another equally striking case is that of the Indian humped and the
+common cattle, species which differ osteologically, and also in habits,
+form, voice, and constitution, so that they are by no means closely
+allied; yet Mr. Darwin assures us that he has received decisive
+evidence that the hybrids between these are perfectly fertile _inter
+se_.
+
+Dogs have been frequently crossed with wolves and with jackals, and
+their hybrid offspring have been found to be fertile _inter se_ to the
+third or fourth generation, and then usually to show some signs of
+sterility or of deterioration. The wolf and dog may be originally the
+same species, but the jackal is certainly distinct; and the appearance
+of infertility or of weakness is probably due to the fact that, in
+almost all these experiments, the offspring of a single pair--themselves
+usually from the same litter--- were bred in-and-in, and this alone
+sometimes produces the most deleterious effects. Thus, Mr. Low in his
+great work on the _Domesticated Animals of Great Britain_, says: "If we
+shall breed a pair of dogs from the same litter, and unite again the
+offspring of this pair, we shall produce at once a feeble race of
+creatures; and the process being repeated for one or two generations
+more, the family will die out, or be incapable of propagating their
+race. A gentleman of Scotland made the experiment on a large scale with
+certain foxhounds, and he found that the race actually became monstrous
+and perished utterly." The same writer tells us that hogs have been made
+the subject of similar experiments: "After a few generations the victims
+manifest the change induced in the system. They become of diminished
+size; the bristles are changed into hairs; the limbs become feeble and
+short; the litters diminish in frequency, and in the number of the young
+produced; the mother becomes unable to nourish them, and, if the
+experiment be carried as far as the case will allow, the feeble, and
+frequently monstrous offspring, will be incapable of being reared up,
+and the miserable race will utterly perish."[54]
+
+These precise statements, by one of the greatest authorities on our
+domesticated animals, are sufficient to show that the fact of
+infertility or degeneracy appearing in the offspring of hybrids after a
+few generations need not be imputed to the fact of the first parents
+being distinct species, since exactly the same phenomena appear when
+individuals of the same species are bred under similar adverse
+conditions. But in almost all the experiments that have hitherto been
+made in crossing distinct species, no care has been taken to avoid close
+interbreeding by securing several hybrids from quite distinct stocks to
+start with, and by having two or more sets of experiments carried on at
+once, so that crosses between the hybrids produced may be occasionally
+made. Till this is done no experiments, such as those hitherto tried,
+can be held to prove that hybrids are in all cases infertile _inter se_.
+
+It has, however, been denied by Mr. A.H. Huth, in his interesting work
+on _The Marriage of Near Kin_, that any amount of breeding in-and-in is
+in itself hurtful; and he quotes the evidence of numerous breeders whose
+choicest stocks have always been so bred, as well as cases like the
+Porto Santo rabbits, the goats of Juan Fernandez, and other cases in
+which animals allowed to run wild have increased prodigiously and
+continued in perfect health and vigour, although all derived from a
+single pair. But in all these cases there has been rigid selection by
+which the weak or the infertile have been eliminated, and with such
+selection there is no doubt that the ill effects of close interbreeding
+can be prevented for a long time; but this by no means proves that no
+ill effects are produced. Mr. Huth himself quotes M. Allié, M. Aubé,
+Stephens, Giblett, Sir John Sebright, Youatt, Druce, Lord Weston, and
+other eminent breeders, as finding from experience that close
+interbreeding _does_ produce bad effects; and it cannot be supposed that
+there would be such a consensus of opinion on this point if the evil
+were altogether imaginary. Mr. Huth argues, that the evil results which
+do occur do not depend on the close interbreeding itself, but on the
+tendency it has to perpetuate any constitutional weakness or other
+hereditary taints; and he attempts to prove this by the argument that
+"if crosses act by virtue of being a cross, and not by virtue of
+removing an hereditary taint, then the greater the difference between
+the two animals crossed the more beneficial will that act be." He then
+shows that, the wider the difference the less is the benefit, and
+concludes that a cross, as such, has no beneficial effect. A parallel
+argument would be, that change of air, as from inland to the sea-coast,
+or from a low to an elevated site, is not beneficial in itself, because,
+if so, a change to the tropics or to the polar regions should be more
+beneficial. In both these cases it may well be that no benefit would
+accrue to a person in perfect health; but then there is no such thing
+as "perfect health" in man, and probably no such thing as absolute
+freedom from constitutional taint in animals. The experiments of Mr.
+Darwin, showing the great and immediate good effects of a cross between
+distinct strains in plants, cannot be explained away; neither can the
+innumerable arrangements to secure cross-fertilisation by insects, the
+real use and purport of which will be discussed in our eleventh chapter.
+On the whole, then, the evidence at our command proves that, whatever
+may be its ultimate cause, close interbreeding _does_ usually produce
+bad results; and it is only by the most rigid selection, whether natural
+or artificial, that the danger can be altogether obviated.
+
+
+_Fertile Hybrids among Animals._
+
+One or two more cases of fertile hybrids may be given before we pass on
+to the corresponding experiments in plants. Professor Alfred Newton
+received from a friend a pair of hybrid ducks, bred from a common duck
+(Anas boschas), and a pintail (Dafila acuta). From these he obtained
+four ducklings, but these latter, when grown up, proved infertile, and
+did not breed again. In this case we have the results of close
+interbreeding, with too great a difference between the original species,
+combining to produce infertility, yet the fact of a hybrid from such a
+pair producing healthy offspring is itself noteworthy.
+
+Still more extraordinary is the following statement of Mr. Low: "It has
+been long known to shepherds, though questioned by naturalists, that the
+progeny of the cross between the sheep and goat is fertile. Breeds of
+this mixed race are numerous in the north of Europe."[55] Nothing
+appears to be known of such hybrids either in Scandinavia or in Italy;
+but Professor Giglioli of Florence has kindly given me some useful
+references to works in which they are described. The following extract
+from his letter is very interesting: "I need not tell you that there
+being such hybrids is now generally accepted as a fact. Buffon
+(_Supplements_, tom. iii. p. 7, 1756) obtained one such hybrid in 1751
+and eight in 1752. Sanson (_La Culture_, vol. vi. p. 372, 1865) mentions
+a case observed in the Vosges, France. Geoff. St. Hilaire (_Hist. Nat.
+Gén. des reg. org._, vol. iii. p. 163) was the first to mention, I
+believe, that in different parts of South America the ram is more
+usually crossed with the she-goat than the sheep with the he-goat. The
+well-known 'pellones' of Chile are produced by the second and third
+generation of such hybrids (Gay, 'Hist, de Chile,' vol. i. p. 466,
+_Agriculture_, 1862). Hybrids bred from goat and sheep are called
+'chabin' in French, and 'cabruno' in Spanish. In Chile such hybrids are
+called 'carneros lanudos'; their breeding _inter se_ appears to be not
+always successful, and often the original cross has to be recommenced to
+obtain the proportion of three-eighths of he-goat and five-eighths of
+sheep, or of three-eighths of ram and five-eighths of she-goat; such
+being the reputed best hybrids."
+
+With these numerous facts recorded by competent observers we can hardly
+doubt that races of hybrids between these very distinct species have
+been produced, and that such hybrids are fairly fertile _inter se_; and
+the analogous facts already given lead us to believe that whatever
+amount of infertility may at first exist could be eliminated by careful
+selection, if the crossed races were bred in large numbers and over a
+considerable area of country. This case is especially valuable, as
+showing how careful we should be in assuming the infertility of hybrids
+when experiments have been made with the progeny of a single pair, and
+have been continued only for one or two generations.
+
+Among insects one case only appears to have been recorded. The hybrids
+of two moths (Bombyx cynthia and B. arrindia) were proved in Paris,
+according to M. Quatrefages, to be fertile _inter se_ for eight
+generations.
+
+
+_Fertility of Hybrids among Plants._
+
+Among plants the cases of fertile hybrids are more numerous, owing, in
+part, to the large scale on which they are grown by gardeners and
+nurserymen, and to the greater facility with which experiments can be
+made. Darwin tells us that Kölreuter found ten cases in which two plants
+considered by botanists to be distinct species were quite fertile
+together, and he therefore ranked them all as varieties of each other.
+In some cases these were grown for six to ten successive generations,
+but after a time the fertility decreased, as we saw to be the case in
+animals, and presumably from the same cause, too close interbreeding.
+
+Dean Herbert, who carried on experiments with great care and skill for
+many years, found numerous cases of hybrids which were perfectly fertile
+_inter se_. Crinum capense, fertilised by three other species--C.
+pedunculatum, C. canaliculatum, or C. defixum--all very distinct from
+it, produced perfectly fertile hybrids; while other species less
+different in appearance were quite sterile with the same C. capense.
+
+All the species of the genus Hippeastrum produce hybrid offspring which
+are invariably fertile. Lobelia syphylitica and L. fulgens, two very
+distinct species, have produced a hybrid which has been named Lobelia
+speciosa, and which reproduces itself abundantly. Many of the beautiful
+pelargoniums of our greenhouses are hybrids, such as P. ignescens from a
+cross between P. citrinodorum and P. fulgidum, which is quite fertile,
+and has become the parent of innumerable varieties of beautiful plants.
+All the varied species of Calceolaria, however different in appearance,
+intermix with the greatest readiness, and the hybrids are all more or
+less fertile. But the most remarkable case is that of two species of
+Petunia, of which Dean Herbert says: "It is very remarkable that,
+although there is a great difference in the form of the flower,
+especially of the tube, of P. nyctanigenaeflora and P. phoenicea the
+mules between them are not only fertile, but I have found them seed much
+more freely with me than either parent.... From a pod of the
+above-mentioned mule, to which no pollen but its own had access, I had a
+large batch of seedlings in which there was no variability or difference
+from itself; and it is evident that the mule planted by itself, in a
+congenial climate, would reproduce itself as a species; at least as much
+deserving to be so considered as the various Calceolarias of different
+districts of South America."[56]
+
+Darwin was informed by Mr. C. Noble that he raises stocks for grafting
+from a hybrid between Rhododendron ponticum and R. catawbiense, and that
+this hybrid seeds as freely as it is possible to imagine. He adds that
+horticulturists raise large beds of the same hybrid, and such alone are
+fairly treated; for, by insect agency, the several individuals are
+freely crossed with each other, and the injurious influence of close
+interbreeding is thus prevented. Had hybrids, when fairly treated,
+always gone on decreasing in fertility in each successive generation, as
+Gartner believed to be the case, the fact would have been notorious to
+nurserymen.[57]
+
+
+_Cases of Sterility of Mongrels._
+
+The reverse phenomenon to the fertility of hybrids, the sterility of
+mongrels or of the crosses between _varieties_ of the same species, is a
+comparatively rare one, yet some undoubted cases have occurred. Gartner,
+who believed in the absolute distinctness of species and varieties, had
+two varieties of maize--one dwarf with yellow seeds, the other taller
+with red seeds; yet they never naturally crossed, and, when fertilised
+artificially, only a single head produced any seeds, and this one only
+five grains. Yet these few seeds were fertile; so that in this case the
+first cross was almost sterile, though the hybrid when at length
+produced was fertile. In like manner, dissimilarly coloured varieties of
+Verbascum or mullein have been found by two distinct observers to be
+comparatively infertile. The two pimpernels (Anagallis arvensis and A.
+coerulea), classed by most botanists as varieties of one species, have
+been found, after repeated trials, to be perfectly sterile when crossed.
+
+No cases of this kind are recorded among animals; but this is not to be
+wondered at, when we consider how very few experiments have been made
+with natural varieties; while there is good reason for believing that
+domestic varieties are exceptionally fertile, partly because one of the
+conditions of domestication was fertility under changed conditions, and
+also because long continued domestication is believed to have the effect
+of increasing fertility and eliminating whatever sterility may exist.
+This is shown by the fact that, in many cases, domestic animals are
+descended from two or more distinct species. This is almost certainly
+the case with the dog, and probably with the hog, the ox, and the sheep;
+yet the various breeds are now all perfectly fertile, although we have
+every reason to suppose that there would be some degree of infertility
+if the several aboriginal species were crossed together for the first
+time.
+
+
+_Parallelism between Crossing and Change of Conditions._
+
+In the whole series of these phenomena, from the beneficial effects of
+the crossing of different stocks and the evil effects of close
+interbreeding, up to the partial or complete sterility induced by
+crosses between species belonging to different genera, we have, as Mr.
+Darwin points out, a curious parallelism with the effects produced by
+change of physical conditions. It is well known that slight changes in
+the conditions of life are beneficial to all living things. Plants, if
+constantly grown in one soil and locality from their own seeds, are
+greatly benefited by the importation of seed from some other locality.
+The same thing happens with animals; and the benefit we ourselves
+experience from "change of air" is an illustration of the same
+phenomenon. But the amount of the change which is beneficial has its
+limits, and then a greater amount is injurious. A change to a climate a
+few degrees warmer or colder may be good, while a change to the tropics
+or to the arctic regions might be injurious.
+
+Thus we see that, both slight changes of conditions and a slight amount
+of crossing, are beneficial; while extreme changes, and crosses between
+individuals too far removed in structure or constitution, are injurious.
+And there is not only a parallelism but an actual connection between the
+two classes of facts, for, as we have already shown, many species of
+animals and plants are rendered infertile, or altogether sterile, by the
+change from their natural conditions which occurs in confinement or in
+cultivation; while, on the other hand, the increased vigour or fertility
+which is invariably produced by a judicious cross may be also effected
+by a judicious change of climate and surroundings. We shall see in a
+subsequent chapter, that this interchangeability of the beneficial
+effects of crossing and of new conditions, serves to explain some very
+puzzling phenomena in the forms and economy of flowers.
+
+
+_Remarks on the Facts of Hybridity._
+
+The facts that have now been adduced, though not very numerous, are
+sufficiently conclusive to prove that the old belief, of the universal
+sterility of hybrids and fertility of mongrels, is incorrect. The
+doctrine that such a universal law existed was never more than a
+plausible generalisation, founded on a few inconclusive facts derived
+from domesticated animals and cultivated plants. The facts were, and
+still are, inconclusive for several reasons. They are founded,
+primarily, on what occurs among animals in domestication; and it has
+been shown that domestication both tends to increase fertility, and was
+itself rendered possible by the fertility of those particular species
+being little affected by changed conditions. The exceptional fertility
+of all the varieties of domesticated animals does not prove that a
+similar fertility exists among natural varieties. In the next place, the
+generalisation is founded on too remote crosses, as in the case of the
+horse and the ass, the two most distinct and widely separated species of
+the genus Equus, so distinct indeed that they have been held by some
+naturalists to form distinct genera. Crosses between the two species of
+zebra, or even between the zebra and the quagga, or the quagga and the
+ass, might have led to a very different result. Again, in pre-Darwinian
+times it was so universally the practice to argue in a circle, and
+declare that the fertility of the offspring of a cross proved the
+identity of species of the parents, that experiments in hybridity were
+usually made between very remote species and even between species of
+different genera, to avoid the possibility of the reply: "They are both
+really the same species;" and the sterility of the hybrid offspring of
+such remote crosses of course served to strengthen the popular belief.
+
+Now that we have arrived at a different standpoint, and look upon a
+species, not as a distinct entity due to special creation, but as an
+assemblage of individuals which have become somewhat modified in
+structure, form, and constitution so as to adapt them to slightly
+different conditions of life; which can be differentiated from other
+allied assemblages; which reproduce their like, and which usually breed
+together--we require a fresh set of experiments calculated to determine
+the matter of fact,--whether such species crossed with their near allies
+do always produce offspring which are more or less sterile _inter se_.
+Ample materials for such experiments exist, in the numerous
+"representative species" inhabiting distinct areas on a continent or
+different islands of a group; or even in those found in the same area
+but frequenting somewhat different stations.
+
+To carry out these experiments with any satisfactory result, it will be
+necessary to avoid the evil effects of confinement and of too close
+interbreeding. If birds are experimented with, they should be allowed as
+much liberty as possible, a plot of ground with trees and bushes being
+enclosed with wire netting overhead so as to form a large open aviary.
+The species experimented with should be obtained in considerable
+numbers, and by two separate persons, each making the opposite
+reciprocal cross, as explained at p. 155. In the second generation these
+two stocks might be themselves crossed to prevent the evil effects of
+too close interbreeding. By such experiments, carefully carried out with
+different groups of animals and plants, we should obtain a body of facts
+of a character now sadly wanting, and without which it is hopeless to
+expect to arrive at a complete solution of this difficult problem. There
+are, however, some other aspects of the question that need to be
+considered, and some theoretical views which require to be carefully
+examined, having done which we shall be in a condition to state the
+general conclusions to which the facts and reasonings at our command
+seem to point.
+
+
+_Sterility due to changed Conditions and usually correlated with other
+Characters, especially with Colour._
+
+The evidence already adduced as to the extreme susceptibility of the
+reproductive system, and the curious irregularity with which infertility
+or sterility appears in the crosses between some varieties or species
+while quite absent in those between others, seem to indicate that
+sterility is a characteristic which has a constant tendency to appear,
+either by itself or in correlation with other characters. It is known to
+be especially liable to occur under changed conditions of life; and, as
+such change is usually the starting-point and cause of the development
+of new species, we have already found a reason why it should so often
+appear when species become fully differentiated.
+
+In almost all the cases of infertility or sterility between varieties or
+species, we have some external differences with which it is correlated;
+and though these differences are sometimes slight, and the amount of the
+infertility is not always, or even usually, proportionate to the
+external difference between the two forms crossed, we must believe that
+there is some connection between the two classes of facts. This is
+especially the case as regards colour; and Mr. Darwin has collected a
+body of facts which go far to prove that colour, instead of being an
+altogether trifling and unimportant character, as was supposed by the
+older naturalists, is really one of great significance, since it is
+undoubtedly often correlated with important constitutional differences.
+Now colour is one of the characters that most usually distinguishes
+closely allied species; and when we hear that the most closely allied
+species of plants are infertile together, while those more remote are
+fertile, the meaning usually is that the former differ chiefly in the
+_colour_ of their flowers, while the latter differ in the form of the
+flowers or foliage, in habit, or in other structural characters.
+
+It is therefore a most curious and suggestive fact, that in all the
+recorded cases, in which a decided infertility occurs between varieties
+of the same species, those varieties are distinguished by a difference
+of colour. The infertile varieties of Verbascum were white and yellow
+flowered respectively; the infertile varieties of maize were red and
+yellow seeded; while the infertile pimpernels were the red and the blue
+flowered varieties. So, the differently coloured varieties of
+hollyhocks, though grown close together, each reproduce their own colour
+from seed, showing that they are not capable of freely intercrossing.
+Yet Mr. Darwin assures us that the agency of bees is necessary to carry
+the pollen from one plant to another, because in each flower the pollen
+is shed before the stigma is ready to receive it. We have here,
+therefore, either almost complete sterility between varieties of
+different colours, or a prepotent effect of pollen from a flower of the
+same colour, bringing about the same result.
+
+Similar phenomena have not been recorded among animals; but this is not
+to be wondered at when we consider that most of our pure and valued
+domestic breeds are characterised by definite colours which constitute
+one of their distinctive marks, and they are, therefore, seldom crossed
+with these of another colour; and even when they are so crossed, no
+notice would be taken of any slight diminution of fertility, since this
+is liable to occur from many causes. We have also reason to believe that
+fertility has been increased by long domestication, in addition to the
+fact of the original stocks being exceptionally fertile; and no
+experiments have been made on the differently coloured varieties of wild
+animals. There are, however, a number of very curious facts showing that
+colour in animals, as in plants, is often correlated with constitutional
+differences of a remarkable kind, and as these have a close relation to
+the subject we are discussing, a brief summary of them will be here
+given.
+
+
+_Correlation of Colour with Constitutional Peculiarities._
+
+The correlation of a white colour and blue eyes in male cats with
+deafness, and of the tortoise-shell marking with the female sex of the
+same animal, are two well-known but most extraordinary cases. Equally
+remarkable is the fact, communicated to Darwin by Mr. Tegetmeier, that
+white, yellow, pale blue, or dun pigeons, of all breeds, have the young
+birds born naked, while in all other colours they are well covered with
+down. Here we have a case in which colour seems of more physiological
+importance than all the varied structural differences between the
+varieties and breeds of pigeons. In Virginia there is a plant called the
+paint-root (Lachnanthes tinctoria), which, when eaten by pigs, colours
+their bones pink, and causes the hoofs of all but the black varieties to
+drop off; so that black pigs only can be kept in the district.[58]
+Buckwheat in flower is also said to be injurious to white pigs but not
+to black. In the Tarentino, black sheep are not injured by eating the
+Hypericum crispum--a species of St. John's-wort--which kills white
+sheep. White terriers suffer most from distemper; white chickens from
+the gapes. White-haired horses or cattle are subject to cutaneous
+diseases from which the dark coloured are free; while, both in Thuringia
+and the West Indies, it has been noticed that white or pale coloured
+cattle are much more troubled by flies than are those which are brown or
+black. The same law even extends to insects, for it is found that
+silkworms which produce white cocoons resist the fungus disease much
+better than do those which produce yellow cocoons.[59] Among plants, we
+have in North America green and yellow-fruited plums not affected by a
+disease that attacked the purple-fruited varieties. Yellow-fleshed
+peaches suffer more from disease than white-fleshed kinds. In Mauritius,
+white sugar-canes were attacked by a disease from which the red canes
+were free. White onions and verbenas are most liable to mildew; and
+red-flowered hyacinths were more injured by the cold during a severe
+winter in Holland than any other kinds.[60]
+
+These curious and inexplicable correlations of colour with
+constitutional peculiarities, both in animals and plants, render it
+probable that the correlation of colour with infertility, which has been
+detected in several cases in plants, may also extend to animals in a
+state of nature; and if so, the fact is of the highest importance as
+throwing light on the origin of the infertility of many allied species.
+This will be better understood after considering the facts which will be
+now described.
+
+
+_The Isolation of Varieties by Selective Association._
+
+In the last chapter I have shown that the importance of geographical
+isolation for the formation of new species by natural selection has been
+greatly exaggerated, because the very change of conditions, which is
+the initial power in starting such new forms, leads also to a local or
+stational segregation of the forms acted upon. But there is also a very
+powerful cause of isolation in the mental nature--the likes and
+dislikes--of animals; and to this is probably due the fact of the
+comparative rarity of hybrids in a state of nature. The differently
+coloured herds of cattle in the Falkland Islands, each of which keeps
+separate, have been already mentioned; and it may be added, that the
+mouse-coloured variety seem to have already developed a physiological
+peculiarity in breeding a month earlier than the others. Similar facts
+occur, however, among our domestic animals and are well known to
+breeders. Professor Low, one of the greatest authorities on our
+domesticated animals, says: "The female of the dog, when not under
+restraint, makes selection of her mate, the mastiff selecting the
+mastiff, the terrier the terrier, and so on." And again: "The Merino
+sheep and Heath sheep of Scotland, if two flocks are mixed together,
+each will breed with its own variety." Mr. Darwin has collected many
+facts illustrating this point. One of the chief pigeon-fanciers in
+England informed him that, if free to choose, each breed would prefer
+pairing with its own kind. Among the wild horses in Paraguay those of
+the same colour and size associate together; while in Circassia there
+are three races of horses which have received special names, and which,
+when living a free life, almost always refuse to mingle and cross, and
+will even attack one another. On one of the Faroe Islands, not more than
+half a mile in diameter, the half-wild native black sheep do not readily
+mix with imported white sheep. In the Forest of Dean, and in the New
+Forest, the dark and pale coloured herds of fallow deer have never been
+known to mingle; and even the curious Ancon sheep of quite modern origin
+have been observed to keep together, separating themselves from the rest
+of the flock when put into enclosures with other sheep. The same rule
+applies to birds, for Darwin was informed by the Rev. W.D. Fox that his
+flocks of white and Chinese geese kept distinct.[61]
+
+This constant preference of animals for their like, even in the case of
+slightly different varieties of the same species, is evidently a fact
+of great importance in considering the origin of species by natural
+selection, since it shows us that, so soon as a slight differentiation
+of form or colour has been effected, isolation will at once arise by the
+selective association of the animals themselves; and thus the great
+stumbling-block of "the swamping effects of intercrossing," which has
+been so prominently brought forward by many naturalists, will be
+completely obviated.
+
+If now we combine with this fact the correlation of colour with
+important constitutional peculiarities, and, in some cases, with
+infertility; and consider, further, the curious parallelism that has
+been shown to exist between the effects of changed conditions and the
+intercrossing of varieties in producing either an increase or a decrease
+of fertility, we shall have obtained, at all events, a starting-point
+for the production of that infertility which is so characteristic a
+feature of distinct species when intercrossed. All we need, now, is some
+means of increasing or accumulating this initial tendency; and to a
+discussion of this problem we will therefore address ourselves.
+
+
+_The Influence of Natural Selection upon Sterility and Fertility._
+
+It will occur to many persons that, as the infertility or sterility of
+incipient species would be useful to them when occupying the same or
+adjacent areas, by neutralising the effects of intercrossing, this
+infertility might have been increased by the action of natural
+selection; and this will be thought the more probable if we admit, as we
+have seen reason to do, that variations in fertility occur, perhaps as
+frequently as other variations. Mr. Darwin tells us that, at one time,
+this appeared to him probable, but he found the problem to be one of
+extreme complexity; and he was also influenced against the view by many
+considerations which seemed to render such an origin of the sterility or
+infertility of species when intercrossed very improbable. The fact that
+species which occupy distinct areas, and which nowhere come in contact
+with each other, are often sterile when crossed, is one of the
+difficulties; but this may perhaps be overcome by the consideration
+that, though now isolated, they may, and often must, have been in
+contact at their origination. More important is the objection that
+natural selection could not possibly have produced the difference that
+often occurs between reciprocal crosses, one of these being sometimes
+fertile, while the other is sterile. The extremely different amounts of
+infertility or sterility between different species of the same genus,
+the infertility often bearing no proportion to the difference between
+the species crossed, is also an important objection. But none of these
+objections would have much weight if it could be clearly shown that
+natural selection _is_ able to increase the infertility variations of
+incipient species, as it is certainly able to increase and develop all
+useful variations of form, structure, instincts, or habits. Ample causes
+of infertility have been shown to exist, in the nature of the organism
+and the laws of correlation; the agency of natural selection is only
+needed to accumulate the effects produced by these causes, and to render
+their final results more uniform and more in accordance with the facts
+that exist.
+
+About twenty years ago I had much correspondence and discussion with Mr.
+Darwin on this question. I then believed that I was able to demonstrate
+the action of natural selection in accumulating infertility; but I could
+not convince him, owing to the extreme complexity of the process under
+the conditions which he thought most probable. I have recently returned
+to the question; and, with the fuller knowledge of the facts of
+variation we now possess, I think it may be shown that natural selection
+_is_, in some probable cases at all events, able to accumulate
+variations in infertility between incipient species.
+
+The simplest case to consider, will be that in which two forms or
+varieties of a species, occupying an extensive area, are in process of
+adaptation to somewhat different modes of life within the same area. If
+these two forms freely intercross with each other, and produce mongrel
+offspring which are quite fertile _inter se_, then the further
+differentiation of the forms into two distinct species will be retarded,
+or perhaps entirely prevented; for the offspring of the crossed unions
+will be, perhaps, more vigorous on account of the cross, although less
+perfectly adapted to the conditions of existence than either of the pure
+breeds; and this would certainly establish a powerful antagonistic
+influence to the further differentiation of the two forms.
+
+Now, let us suppose that a partial sterility of the hybrids between the
+two forms arises, in correlation with the different modes of life and
+the slight external or internal peculiarities that exist between them,
+both of which we have seen to be real causes of infertility. The result
+will be that, even if the hybrids between the two forms are still freely
+produced, these hybrids will not themselves increase so rapidly as the
+two pure forms; and as these latter are, by the terms of the problem,
+better suited to their conditions of life than are the hybrids between
+them, they will not only increase more rapidly, but will also tend to
+supplant the hybrids altogether whenever the struggle for existence
+becomes exceptionally severe. Thus, the more complete the sterility of
+the hybrids the more rapidly will they die out and leave the two parent
+forms pure. Hence it will follow that, if there is greater infertility
+between the two forms in one part of the area than the other, these
+forms will be kept more pure wherever this greater infertility prevails,
+will therefore have an advantage at each recurring period of severe
+struggle for existence, and will thus ultimately supplant the less
+infertile or completely fertile forms that may exist in other portions
+of the area. It thus appears that, in such a case as here supposed,
+natural selection would preserve those portions of the two breeds which
+were most infertile with each other, or whose hybrid offspring were most
+infertile; and would, therefore, if variations in fertility continued to
+arise, tend to increase that infertility. It must particularly be noted
+that this effect would result, not by the preservation of the infertile
+variations on account of their infertility, but by the inferiority of
+the hybrid offspring, both as being fewer in numbers, less able to
+continue their race, and less adapted to the conditions of existence
+than either of the pure forms. It is this inferiority of the hybrid
+offspring that is the essential point; and as the number of these
+hybrids will be permanently less where the infertility is greatest,
+therefore those portions of the two forms in which infertility is
+greatest will have the advantage, and will ultimately survive in the
+struggle for existence.
+
+The differentiation of the two forms into distinct species, with the
+increase of infertility between them, would be greatly assisted by two
+other important factors in the problem. It has already been shown that,
+with each modification of form and habits, and especially with
+modifications of colour, there arises a disinclination of the two forms
+to pair together; and this would produce an amount of isolation which
+would greatly assist the specialisation of the forms in adaptation to
+their different conditions of life. Again, evidence has been adduced
+that change of conditions or of mode of life is a potent cause of
+disturbance of the reproductive system, and, consequently, of
+infertility. We may therefore assume that, as the two forms adopted more
+and more different modes of life, and perhaps acquired also decided
+peculiarities of form and coloration, the infertility between them would
+increase or become more general; and as we have seen that every such
+increase of infertility would give that portion of the species in which
+it arose an advantage over the remaining portions in which the two
+varieties were more fertile together, all this induced infertility would
+maintain itself, and still further increase the general infertility
+between the two forms of the species.
+
+It follows, then, that specialisation to separate conditions of life,
+differentiation of external characters, disinclination to cross-unions,
+and the infertility of the hybrid produce of these unions, would all
+proceed _pari passu_, and would ultimately lead to the production of two
+distinct forms having all the characteristics, physiological as well as
+structural, of true species.
+
+In the case now discussed it has been supposed, that some amount of
+general infertility might arise in correlation with the different modes
+of life of two varieties or incipient species. A considerable body of
+facts already adduced renders it probable that this _is_ the mode in
+which any widespread infertility would arise; and, if so, it has been
+shown that, by the influence of natural selection and the known laws
+which affect varieties, the infertility would be gradually increased.
+But, if we suppose the infertility to arise sporadically within the two
+forms, and to affect only a small proportion of the individuals in any
+area, it will be difficult, if not impossible, to show that such
+infertility would have any tendency to increase, or would produce any
+but a prejudicial effect. If, for example, five per cent of each form
+thus varied so as to be infertile with the other form, the result would
+be hardly perceptible, because the individuals which formed cross-unions
+and produced hybrids would constitute a very small portion of the whole
+species; and the hybrid offspring, being at a disadvantage in the
+struggle for existence and being themselves infertile, would soon die
+out, while the much more numerous fertile portion of the two forms would
+increase rapidly, and furnish a sufficient number of pure-bred offspring
+of each form to take the place of the somewhat inferior hybrids between
+them whenever the struggle for existence became severe. We must suppose
+that the normal fertile forms would transmit their fertility to their
+progeny, and the few infertile forms their infertility; but the latter
+would necessarily lose half their proper increase by the sterility of
+their hybrid offspring whenever they crossed with the other form, and
+when they bred with their own form the tendency to sterility would die
+out except in the very minute proportion of the five per cent
+(one-twentieth) that chance would lead to pair together. Under these
+circumstances the incipient sterility between the two forms would
+rapidly be eliminated, and could never rise much above the numbers which
+were produced by sporadic variation each year.
+
+It was, probably, by a consideration of some such case as this that Mr.
+Darwin came to the conclusion that infertility arising between incipient
+species could not be increased by natural selection; and this is the
+more likely, as he was always disposed to minimise both the frequency
+and the amount even of structural variations.
+
+We have yet to notice another mode of action of natural selection in
+favouring and perpetuating any infertility that may arise between two
+incipient species. If several distinct species are undergoing
+modification at the same time and in the same area, to adapt them to
+some new conditions that have arisen there, then any species in which
+the structural or colour differences that have arisen between it and its
+varieties or close allies were correlated with infertility of the
+crosses between them, would have an advantage over the corresponding
+varieties of other species in which there was no such physiological
+peculiarity. Thus, incipient species which were infertile together would
+have an advantage over other incipient species which were fertile, and,
+whenever the struggle for existence became severe, would prevail over
+them and take their place. Such infertility, being correlated with
+constitutional or structural differences, would probably, as already
+suggested, go on increasing as these differences increased; and thus, by
+the time the new species became fully differentiated from its parent
+form (or brother variety) the infertility might have become as well
+marked as we usually find it to be between distinct species.
+
+This discussion has led us to some conclusions of the greatest
+importance as bearing on the difficult problem of the cause of the
+sterility of the hybrids between distinct species. Accepting, as highly
+probable, the fact of variations in fertility occurring in correlation
+with variations in habits, colour, or structure, we see, that so long as
+such variations occurred only sporadically, and affected but a small
+proportion of the individuals in any area, the infertility could not be
+increased by natural selection, but would tend to die out almost as fast
+as it was produced. If, however, it was so closely correlated with
+physical variations or diverse modes of life as to affect, even in a
+small degree, a considerable proportion of the individuals of the two
+forms in definite areas, it would be preserved by natural selection, and
+the portion of the varying species thus affected would increase at the
+expense of those portions which were more fertile when crossed. Each
+further variation towards infertility between the two forms would be
+again preserved, and thus the incipient infertility of the hybrid
+offspring might be increased till it became so great as almost to amount
+to sterility. Yet further, we have seen that if several competing
+species in the same area were being simultaneously modified, those
+between whose varieties infertility arose would have an advantage over
+those whose varieties remained fertile _inter se_, and would ultimately
+supplant them.
+
+The preceding argument, it will be seen, depends entirely upon the
+assumption that some amount of infertility characterises the distinct
+varieties which are in process of differentiation into species; and it
+may be objected that of such infertility there is no proof. This is
+admitted; but it is urged that facts have been adduced which render such
+infertility probable, at least in some cases, and this is all that is
+required. It is by no means necessary that _all_ varieties should
+exhibit incipient infertility, but only, some varieties; for we know
+that, of the innumerable varieties that occur but few become developed
+into distinct species, and it may be that the absence of infertility, to
+obviate the effects of intercrossing, is one of the usual causes of
+their failure. All I have attempted to show is, that _when_ incipient
+infertility does occur in correlation with other varietal differences,
+that infertility can be, and in fact must be, increased by natural
+selection; and this, it appears to me, is a decided step in advance in
+the solution of the problem.[62]
+
+
+_Physiological Selection._
+
+Another form of infertility has been suggested by Professor G.J. Romanes
+as having aided in bringing about the characteristic infertility or
+sterility of hybrids. It is founded on the fact, already noticed, that
+certain individuals of some species possess what may be termed selective
+sterility--that is, while fertile with some individuals of the species
+they are sterile with others, and this altogether independently of any
+differences of form, colour, or structure. The phenomenon, in the only
+form in which it has been observed, is that of "infertility or absolute
+sterility between two individuals, each of which is perfectly fertile
+with all other individuals;" but Mr. Romanes thinks that "it would not
+be nearly so remarkable, or physiologically improbable, that such
+incompatibility should run through a whole race or strain."[63]
+Admitting that this may be so, though we have at present no evidence
+whatever in support of it, it remains to be considered whether such
+physiological varieties could maintain themselves, or whether, as in the
+cases of sporadic infertility already discussed, they would necessarily
+die out unless correlated with useful characters. Mr. Romanes thinks
+that they would persist, and urges that "whenever this one kind of
+variation occurs _it cannot escape the preserving agency_ of
+physiological selection. Hence, even if it be granted that the variation
+which affects the reproductive system in this particular way is a
+variation of comparatively rare occurrence, still, as _it must always be
+preserved_ whenever it does occur, its influence in the manufacture of
+specific types _must be cumulative_." The very positive statements which
+I have italicised would lead most readers to believe that the alleged
+fact had been demonstrated by a careful working out of the process in
+some definite supposed cases. This, however, has nowhere been done in
+Mr. Romanes' paper; and as it is _the_ vital theoretical point on which
+any possible value of the new theory rests, and as it appears so opposed
+to the self-destructive effects of simple infertility, which we have
+already demonstrated when it occurs between the intermingled portion of
+two varieties, it must be carefully examined. In doing so, I will
+suppose that the required variation is not of "rare occurrence," but of
+considerable amount, and that it appears afresh each year to about the
+same extent, thus giving the theory every possible advantage.
+
+Let us then suppose that a given species consists of 100,000 individuals
+of each sex, with only the usual amount of fluctuating external
+variability. Let a physiological variation arise, so that 10 per cent of
+the whole number--10,000 individuals of each sex--while remaining
+fertile _inter se_ become quite sterile with the remaining 90,000. This
+peculiarity is not correlated with any external differences of form or
+colour, or with inherent peculiarities of likes or dislikes leading to
+any choice as to the pairing of the two sets of individuals. We have now
+to inquire, What would be the result?
+
+Taking, first, the 10,000 pairs of the physiological or abnormal
+variety, we find that each male of these might pair with any one of the
+whole 100,000 of the opposite sex. If, therefore, there was nothing to
+limit their choice to particular individuals of either variety, the
+probabilities are that 9000 of them would pair with the opposite
+variety, and only 1000 with their own variety--that is, that 9000 would
+form sterile unions, and only _one_ thousand would form fertile unions.
+
+Taking, next, the 90,000 normal individuals of either sex, we find, that
+each male of these has also a choice of 100,000 to pair with. The
+probabilities are, therefore, that nine-tenths of them--that is,
+81,000--would pair with their normal fellows, while 9000 would pair with
+the opposite abnormal variety forming the above-mentioned sterile
+unions.
+
+Now, as the number of individuals forming a species remains constant,
+generally speaking, from year to year, we shall have next year also
+100,000 pairs, of which the two physiological varieties will be in the
+proportion of eighty-one to one, or 98,780 pairs of the normal variety
+to 1220[64] of the abnormal, that being the proportion of the fertile
+unions of each. In this year we shall find, by the same rule of
+probabilities, that only 15 males of the abnormal variety will pair with
+their like and be fertile, the remaining 1205 forming sterile unions
+with some of the normal variety. The following year the total 100,000
+pairs will consist of 99,984 of the normal, and only 16 of the abnormal
+variety; and the probabilities, of course, are, that the whole of these
+latter will pair with some of the enormous preponderance of normal
+individuals, and, their unions being sterile, the physiological variety
+will become extinct in the third year.
+
+If now in the second and each succeeding year a similar proportion as at
+first (10 per cent) of the physiological variety is produced afresh from
+the ranks of the normal variety, the same rate of diminution will go on,
+and it will be found that, on the most favourable estimate, the
+physiological variety can never exceed 12,000 to the 88,000 of the
+normal form of the species, as shown by the following table:--
+
+
+    1st Year. 10,000 of physiological variety to 90,000 of normal variety.
+    2d   "     1,220 + 10,000 again produced.
+    3d   "        16 +  1,220 + 10,000 do.               = 11,236
+    4th  "         O +     16 +  1,220 + 10,000 do.      = 11,236
+    5th  "                  O +     16 +  1,220 + 10,000 = 11,236
+    and so on for any number of generations.
+
+
+In the preceding discussion we have given the theory the advantage of
+the large proportion of 10 per cent of this very exceptional variety
+arising in its midst year by year, and we have seen that, even under
+these favourable conditions, it is unable to increase its numbers much
+above its starting-point, and that it remains wholly dependent on the
+continued renewal of the variety for its existence beyond a few years.
+It appears, then, that this form of inter-specific sterility cannot be
+increased by natural or any other known form of selection, but that it
+contains within itself its own principle of destruction. If it is
+proposed to get over the difficulty by postulating a larger percentage
+of the variety annually arising within the species, we shall not affect
+the law of decrease until we approach equality in the numbers of the two
+varieties. But with any such increase of the physiological variety the
+species itself would inevitably suffer by the large proportion of
+sterile unions in its midst, and would thus be at a great disadvantage
+in competition with other species which were fertile throughout. Thus,
+natural selection will always tend to weed out any species with too
+great a tendency to sterility among its own members, and will therefore
+prevent such sterility from becoming the general characteristic of
+varying species, which this theory demands should be the case.
+
+On the whole, then, it appears clear that no form of infertility or
+sterility between the individuals of a species, can be increased by
+natural selection unless correlated with some useful variation, while
+all infertility not so correlated has a constant tendency to effect its
+own elimination. But the opposite property, fertility, is of vital
+importance to every species, and gives the offspring of the individuals
+which possess it, in consequence of their superior numbers, a greater
+chance of survival in the battle of life. It is, therefore, directly
+under the control of natural selection, which acts both by the
+self-preservation of fertile and the self-destruction of infertile
+stocks--except always where correlated as above, when they become
+useful, and therefore subject to be increased by natural selection.
+
+
+_Summary and Concluding Remarks on Hybridity._
+
+The facts which are of the greatest importance to a comprehension of
+this very difficult subject are those which show the extreme
+susceptibility of the reproductive system both in plants and animals. We
+have seen how both these classes of organisms may be rendered infertile,
+by a change of conditions which does not affect their general health, by
+captivity, or by too close interbreeding. We have seen, also, that
+infertility is frequently correlated with a difference of colour, or
+with other characters; that it is not proportionate to divergence of
+structure; that it varies in reciprocal crosses between pairs of the
+same species; while in the cases of dimorphic and trimorphic plants the
+different crosses between the same pair of individuals may be fertile or
+sterile at the same time. It appears as if fertility depended on such a
+delicate adjustment of the male and female elements to each other, that,
+unless constantly kept up by the preservation of the most fertile
+individuals, sterility is always liable to arise. This preservation
+always occurs within the limits of each species, both because fertility
+is of the highest importance to the continuance of the race, and also
+because sterility (and to a less extent infertility) is self-destructive
+as well as injurious to the species.
+
+So long therefore as a species remains undivided, and in occupation of a
+continuous area, its fertility is kept up by natural selection; but the
+moment it becomes separated, either by geographical or selective
+isolation, or by diversity of station or of habits, then, while each
+portion must be kept fertile _inter se_, there is nothing to prevent
+infertility arising between the two separated portions. As the two
+portions will necessarily exist under somewhat different conditions of
+life, and will usually have acquired some diversity of form and
+colour--both which circumstances we know to be either the cause of
+infertility or to be correlated with it,--the fact of some degree of
+infertility usually appearing between closely allied but locally or
+physiologically segregated species is exactly what we should expect.
+
+The reason why varieties do not usually exhibit a similar amount of
+infertility is not difficult to explain. The popular conclusions on this
+matter have been drawn chiefly from what occurs among domestic animals,
+and we have seen that the very first essential to their becoming
+domesticated was that they should continue fertile under changed
+conditions of life. During the slow process of the formation of new
+varieties by conscious or unconscious selection, fertility has always
+been an essential character, and has thus been invariably preserved or
+increased; while there is some evidence to show that domestication
+itself tends to increase fertility.
+
+Among plants, wild species and varieties have been more frequently
+experimented on than among animals, and we accordingly find numerous
+cases in which distinct species of plants are perfectly fertile when
+crossed, their hybrid offspring being also fertile _inter se_. We also
+find some few examples of the converse fact--varieties of the same
+species which when crossed are infertile or even sterile.
+
+The idea that either infertility or geographical isolation is absolutely
+essential to the formation of new species, in order to prevent the
+swamping effects of intercrossing, has been shown to be unsound, because
+the varieties or incipient species will, in most cases, be sufficiently
+isolated by having adopted different habits or by frequenting different
+stations; while selective association, which is known to be general
+among distinct varieties or breeds of the same species, will produce an
+effective isolation even when the two forms occupy the same area.
+
+From the various considerations now adverted to, Mr. Darwin arrived at
+the conclusion that the sterility or infertility of species with each
+other, whether manifested in the difficulty of obtaining first crosses
+between them or in the sterility of the hybrids thus obtained, is not a
+constant or necessary result of specific difference, but is incidental
+on unknown peculiarities of the reproductive system. These peculiarities
+constantly tend to arise under changed conditions owing to the extreme
+susceptibility of that system, and they are usually correlated with
+variations of form or of colour. Hence, as fixed differences of form and
+colour, slowly gained by natural selection in adaptation to changed
+conditions, are what essentially characterise distinct species, some
+amount of infertility between species is the usual result.
+
+Here the problem was left by Mr. Darwin; but we have shown that its
+solution may be carried a step further. If we accept the association of
+some degree of infertility, however slight, as a not unfrequent
+accompaniment of the external differences which always arise in a state
+of nature between varieties and incipient species, it has been shown
+that natural selection _has_ power to increase that infertility just as
+it has power to increase other favourable variations. Such an increase
+of infertility will be beneficial, whenever new species arise in the
+same area with the parent form; and we thus see how, out of the
+fluctuating and very unequal amounts of infertility correlated with
+physical variations, there may have arisen that larger and more constant
+amount which appears usually to characterise well-marked species.
+
+The great body of facts of which a condensed account has been given in
+the present chapter, although from an experimental point of view very
+insufficient, all point to the general conclusion we have now reached,
+and afford us a not unsatisfactory solution of the great problem of
+hybridism in relation to the origin of species by means of natural
+selection. Further experimental research is needed in order to complete
+the elucidation of the subject; but until these additional facts are
+forthcoming no new theory seems required for the explanation of the
+phenomena.
+
+FOOTNOTES:
+
+[Footnote 51: Darwin's _Animals and Plants under Domestication_, vol.
+ii. pp. 163-170.]
+
+[Footnote 52: For a full account of these interesting facts and of the
+various problems to which they give rise, the reader must consult
+Darwin's volume on _The Different Forms of Flowers in Plants of the same
+Species_, chaps, i.-iv.]
+
+[Footnote 53: See _Nature_, vol. xxi. p. 207.]
+
+[Footnote 54: Low's _Domesticated Animals of Great Britain_,
+Introduction, p. lxiv.]
+
+[Footnote 55: Low's _Domesticated Animals_, p. 28.]
+
+[Footnote 56: _Amaryllidaceae_, by the Hon. and Rev. William Herbert, p.
+379.]
+
+[Footnote 57: _Origin of Species_, p. 239.]
+
+[Footnote 58: _Origin of Species_, sixth edition, p. 9.]
+
+[Footnote 59: In the _Medico-Chirurgical Transactions_, vol. liii.
+(1870), Dr. Ogle has adduced some curious physiological facts bearing on
+the presence or absence of white colours in the higher animals. He
+states that a dark pigment in the olfactory region of the nostrils is
+essential to perfect smell, and that this pigment is rarely deficient
+except when the whole animal is pure white, and the creature is then
+almost without smell or taste. He observes that there is no proof that,
+in any of the cases given above, the black animals actually eat the
+poisonous root or plant; and that the facts are readily understood if
+the senses of smell and taste are dependent on a pigment which is absent
+in the white animals, who therefore eat what those gifted with normal
+senses avoid. This explanation however hardly seems to cover the facts.
+We cannot suppose that almost all the sheep in the world (which are
+mostly white) are without smell or taste. The cutaneous disease on the
+white patches of hair on horses, the special liability of white terriers
+to distemper, of white chickens to the gapes, and of silkworms which
+produce yellow silk to the fungus, are not explained by it. The
+analogous facts in plants also indicate a real constitutional relation
+with colour, not an affection of the sense of smell and taste only.]
+
+[Footnote 60: For all these facts, see _Animals and Plants under
+Domestication_, vol. ii. pp. 335-338.]
+
+[Footnote 61: _Animals and Plants under Domestication_, vol. ii. pp.
+102, 103.]
+
+[Footnote 62: As this argument is a rather difficult one to follow,
+while its theoretical importance is very great, I add here the following
+briefer exposition of it, in a series of propositions; being, with a few
+verbal alterations, a copy of what I wrote on the subject about twenty
+years back. Some readers may find this easier to follow than the fuller
+discussion in the text:--
+
+
+    _Can Sterility of Hybrids have been Produced by Natural
+    Selection?_
+
+    1. Let there be a species which has varied into _two forms_ each
+    adapted to certain existing conditions better than the parent
+    form, which they soon supplant.
+
+    2. If these _two forms_, which are supposed to coexist in the
+    same district, do not intercross, natural selection will
+    accumulate all favourable variations till they become well
+    suited to their conditions of life, and form two slightly
+    differing species.
+
+    3. But if these _two forms_ freely intercross with each other,
+    and produce hybrids, which are also quite fertile _inter se_,
+    then the formation of the two distinct races or species will be
+    retarded, or perhaps entirely prevented; for the offspring of
+    the crossed unions will be _more vigorous_ owing to the cross,
+    although _less adapted_ to their conditions of life than either
+    of the pure breeds.
+
+    4. Now, let a partial sterility of the hybrids of some
+    considerable proportion of these two forms arise; and, as this
+    would probably be due to some special conditions of life, we may
+    fairly suppose it to arise in some definite portion of the area
+    occupied by the two forms.
+
+    5. The result will be that, in that area, the hybrids (although
+    continually produced by first crosses almost as freely as
+    before) will not themselves increase so rapidly as the two pure
+    forms; and as the two pure forms are, by the terms of the
+    problem, better suited to their several conditions of life than
+    the hybrids, they will inevitably increase more rapidly, and
+    will continually tend to supplant the hybrids altogether at
+    every recurrent severe struggle for existence.
+
+    6. We may fairly suppose, also, that as soon as any sterility
+    appears some disinclination to _cross unions_ will appear, and
+    this will further tend to the diminution of the production of
+    hybrids.
+
+    7. In the other part of the area, however, where hybridism
+    occurs with perfect freedom, hybrids of various degrees may
+    increase till they equal or even exceed in number the pure
+    species--that is, the incipient species will be liable to be
+    swamped by intercrossing.
+
+    8. The first result, then, of a partial sterility of crosses
+    appearing in one part of the area occupied by the two forms,
+    will be--that the great majority of the individuals will there
+    consist of the two pure forms only, while in the remaining part
+    these will be in a minority,--which is the same as saying that
+    the new _physiological variety_ of the two forms will be better
+    suited to the conditions of existence than the remaining portion
+    which has not varied physiologically.
+
+    9. But when the struggle for existence becomes severe, that
+    variety which is best adapted to the conditions of existence
+    always supplants that which is imperfectly adapted; therefore,
+    _by natural selection_ the _varieties_ which are _sterile_ when
+    crossed will become established as the only ones.
+
+    10. Now let variations in the _amount of sterility_ and in
+    the _disinclination to crossed unions_ continue to occur--also
+    in certain parts of the area: exactly the same result must
+    recur, and the progeny of this new physiological variety will in
+    time occupy the whole area.
+
+    11. There is yet another consideration that would facilitate the
+    process. It seems probable that the _sterility variations_
+    would, to some extent, concur with, and perhaps depend upon, the
+    _specific variations_; so that, just in proportion as the _two
+    forms_ diverged and became better adapted to the conditions of
+    existence, they would become more sterile when intercrossed. If
+    this were the case, then natural selection would act with double
+    strength; and those which were better adapted to survive both
+    structurally and physiologically would certainly do so.]
+
+[Footnote 63: Cases of this kind are referred to at p. 155. It must,
+however, be noted, that such sterility in first crosses appears to be
+equally rare between different species of the same genus as between
+individuals of the same species. Mules and other hybrids are freely
+produced between very distinct species, but are themselves infertile or
+quite sterile; and it is this infertility or sterility of the hybrids
+that is the characteristic--and was once thought to be the criterion--of
+species, not the sterility of their first crosses. Hence we should not
+expect to find any constant infertility in the first crosses between the
+distinct strains or varieties that formed the starting-point of new
+species, but only a slight amount of infertility in their mongrel
+offspring. It follows, that Mr. Romanes' theory of _Physiological
+Selection_--which assumes sterility or infertility between first crosses
+as the fundamental fact in the origin of species--does not accord with
+the general phenomena of hybridism in nature.]
+
+[Footnote 64: The exact number is 1219.51, but the fractions are omitted
+for clearness.]
+
+
+
+
+CHAPTER VIII
+
+THE ORIGIN AND USES OF COLOUR IN ANIMALS
+
+
+    The Darwinian theory threw new light on organic colour--The
+    problem to be solved--The constancy of animal colour indicates
+    utility--Colour and environment--Arctic animals
+    white--Exceptions prove the rule--Desert, forest, nocturnal, and
+    oceanic animals--General theories of animal colour--Variable
+    protective colouring--Mr. Poulton's experiments--Special or
+    local colour adaptations--Imitation of particular objects--How
+    they have been produced--Special protective colouring of
+    butterflies--Protective resemblance among marine
+    animals--Protection by terrifying enemies--Alluring
+    coloration--The coloration of birds' eggs--Colour as a means of
+    recognition--Summary of the preceding exposition--Influence of
+    locality or of climate on colour--Concluding remarks.
+
+
+
+Among the numerous applications of the Darwinian theory in the
+interpretation of the complex phenomena presented by the organic world,
+none have been more successful, or are more interesting, than those
+which deal with the colours of animals and plants. To the older school
+of naturalists colour was a trivial character, eminently unstable and
+untrustworthy in the determination of species; and it appeared to have,
+in most cases, no use or meaning to the objects which displayed it. The
+bright and often gorgeous coloration of insect, bird, or flower, was
+either looked upon as having been created for the enjoyment of mankind,
+or as due to unknown and perhaps undiscoverable laws of nature.
+
+But the researches of Mr. Darwin totally changed our point of view in
+this matter. He showed, clearly, that some of the colours of animals are
+useful, some hurtful to them; and he believed that many of the most
+brilliant colours were developed by sexual choice; while his great
+general principle, that all the fixed characters of organic beings have
+been developed under the action of the law of utility, led to the
+inevitable conclusion that so remarkable and conspicuous a character as
+colour, which so often constitutes the most obvious distinction of
+species from species, or group from group, must also have arisen from
+survival of the fittest, and must, therefore, in most cases have some
+relation to the wellbeing of its possessors. Continuous observation and
+research, carried on by multitudes of observers during the last thirty
+years, have shown this to be the case; but the problem is found to be
+far more complex than was at first supposed. The modes in which colour
+is of use to different classes of organisms is very varied, and have
+probably not yet been all discovered; while the infinite variety and
+marvellous beauty of some of its developments are such as to render it
+hopeless to arrive at a complete and satisfactory explanation of every
+individual case. So much, however, has been achieved, so many curious
+facts have been explained, and so much light has been thrown on some of
+the most obscure phenomena of nature, that the subject deserves a
+prominent place in any account of the Darwinian theory.
+
+
+_The Problem to be Solved._
+
+Before dealing with the various modifications of colour in the animal
+world it is necessary to say a few words on colour in general, on its
+prevalence in nature, and how it is that the colours of animals and
+plants require any special explanation. What we term colour is a
+subjective phenomenon, due to the constitution of our mind and nervous
+system; while, objectively, it consists of light-vibrations of different
+wave-lengths emitted by, or reflected from, various objects. Every
+visible object must be coloured, because to be visible it must send rays
+of light to our eye. The kind of light it sends is modified by the
+molecular constitution or the surface texture of the object. Pigments
+absorb certain rays and reflect the remainder, and this reflected
+portion has to our eyes a definite colour, according to the portion of
+the rays constituting white light which are absorbed. Interference
+colours are produced either by thin films or by very fine striae on the
+surfaces of bodies, which cause rays of certain wave-lengths to
+neutralise each other, leaving the remainder to produce the effects of
+colour. Such are the colours of soap-bubbles, or of steel or glass on
+which extremely fine lines have been ruled; and these colours often
+produce the effect of metallic lustre, and are the cause of most of the
+metallic hues of birds and insects.
+
+As colour thus depends on molecular or chemical constitution or on the
+minute surface texture of bodies, and, as the matter of which organic
+beings are composed consists of chemical compounds of great complexity
+and extreme instability, and is also subject to innumerable changes
+during growth and development, we might naturally expect the phenomena
+of colour to be more varied here than in less complex and more stable
+compounds. Yet even in the inorganic world we find abundant and varied
+colours; in the earth and in the water; in metals, gems, and minerals;
+in the sky and in the ocean; in sunset clouds and in the many-tinted
+rainbow. Here we can have no question of _use_ to the coloured object,
+and almost as little perhaps in the vivid red of blood, in the brilliant
+colours of red snow and other low algae and fungi, or even in the
+universal mantle of green which clothes so large a portion of the
+earth's surface. The presence of some colour, or even of many brilliant
+colours, in animals and plants would require no other explanation than
+does that of the sky or the ocean, of the ruby or the emerald--that is,
+it would require a purely physical explanation only. It is the wonderful
+individuality of the colours of animals and plants that attracts our
+attention--the fact that the colours are localised in definite patterns,
+sometimes in accordance with structural characters, sometimes altogether
+independent of them; while often differing in the most striking and
+fantastic manner in allied species. We are thus compelled to look upon
+colour not merely as a physical but also as a biological characteristic,
+which has been differentiated and specialised by natural selection, and
+must, therefore, find its explanation in the principle of adaptation or
+utility.
+
+
+_The Constancy of Animal Colour indicates Utility._
+
+That the colours and markings of animals have been acquired under the
+fundamental law of utility is indicated by a general fact which has
+received very little attention. As a rule, colour and marking are
+constant in each species of wild animal, while, in almost every
+domesticated animal, there arises great variability. We see this in our
+horses and cattle, our dogs and cats, our pigeons and poultry. Now, the
+essential difference between the conditions of life of domesticated and
+wild animals is, that the former are protected by man, while the latter
+have to protect themselves. The extreme variations in colour that
+immediately arise under domestication indicate a tendency to vary in
+this way, and the occasional occurrence of white or piebald or other
+exceptionally coloured individuals of many species in a state of nature,
+shows that this tendency exists there also; and, as these exceptionally
+coloured individuals rarely or never increase, there must be some
+constant power at work to keep it in check. This power can only be
+natural selection or the survival of the fittest, which again implies
+that some colours are useful, some injurious, in each particular case.
+With this principle as our guide, let us see how far we can account both
+for the general and special colours of the animal world.
+
+
+_Colour and Environment._
+
+The fact that first strikes us in our examination of the colours of
+animals as a whole, is the close relation that exists between these
+colours and the general environment. Thus, white prevails among arctic
+animals; yellow or brown in desert species; while green is only a common
+colour in tropical evergreen forests. If we consider these cases
+somewhat carefully we shall find, that they afford us excellent
+materials for forming a judgment on the various theories that have been
+suggested to account for the colours of the animal world.
+
+In the arctic regions there are a number of animals which are wholly
+white all the year round, or which only turn white in winter. Among the
+former are the polar bear and the American polar hare, the snowy owl and
+the Greenland falcon; among the latter the arctic fox, the arctic hare,
+the ermine, and the ptarmigan. Those which are permanently white remain
+among the snow nearly all the year round, while those which change their
+colour inhabit regions which are free from snow in summer. The obvious
+explanation of this style of coloration is, that it is protective,
+serving to conceal the herbivorous species from their enemies, and
+enabling carnivorous animals to approach their prey unperceived. Two
+other explanations have, however, been suggested. One is, that the
+prevalent white of the arctic regions has a direct effect in producing
+the white colour in animals, either by some photographic or chemical
+action on the skin or by a reflex action through vision. The other is,
+that the white colour is chiefly beneficial as a means of checking
+radiation and so preserving animal heat during the severity of an arctic
+winter. The first is part of the general theory that colour is the
+effect of coloured light on the objects--a pure hypothesis which has, I
+believe, no facts whatever to support it. The second suggestion is also
+an hypothesis merely, since it has not been proved by experiment that a
+white colour, _per se_, independently of the fur or feathers which is so
+coloured, has any effect whatever in checking the radiation of low-grade
+heat like that of the animal body. But both alike are sufficiently
+disproved by the interesting exceptions to the rule of white coloration
+in the arctic regions, which exceptions are, nevertheless, quite in
+harmony with the theory of protection.
+
+Whenever we find arctic animals which, from whatever cause, do not
+require protection by the white colour, then neither the cold nor the
+snow-glare has any effect upon their coloration. The sable retains its
+rich brown fur throughout the Siberian winter; but it frequents trees at
+that season and not only feeds partially on fruits or seeds, but is able
+to catch birds among the branches of the fir-trees, with the bark of
+which its colour assimilates. Then we have that thoroughly arctic
+animal, the musk-sheep, which is brown and conspicuous; but this animal
+is gregarious, and its safety depends on its association in small herds.
+It is, therefore, of more importance for it to be able to recognise its
+kind at a distance than to be concealed from its enemies, against which
+it can well protect itself so long as it keeps together in a compact
+body. But the most striking example is that of the common raven, which
+is a true arctic bird, and is found even in mid-winter as far north as
+any known bird or mammal. Yet it always retains its black coat, and the
+reason, from our point of view, is obvious. The raven is a powerful bird
+and fears no enemy, while, being a carrion-feeder, it has no need for
+concealment in order to approach its prey. The colour of the raven and
+of the musk-sheep are, therefore, both inconsistent with any other
+theory than that the white colour of arctic animals has been acquired
+for concealment, and to that theory both afford a strong support. Here
+we have a striking example of the exception proving the rule.
+
+In the desert regions of the earth we find an even more general
+accordance of colour with surroundings. The lion, the camel, and all the
+desert antelopes have more or less the colour of the sand or rock among
+which they live. The Egyptian cat and the Pampas cat are sandy or earth
+coloured. The Australian kangaroos are of similar tints, and the
+original colour of the wild horse is supposed to have been sandy or clay
+coloured. Birds are equally well protected by assimilative hues; the
+larks, quails, goatsuckers, and grouse which abound in the North African
+and Asiatic deserts are all tinted or mottled so as closely to resemble
+the average colour of the soil in the districts they inhabit. Canon
+Tristram, who knows these regions and their natural history so well,
+says, in an often quoted passage: "In the desert, where neither trees,
+brushwood, nor even undulations of the surface afford the slightest
+protection to its foes, a modification of colour which shall be
+assimilated to that of the surrounding country is absolutely necessary.
+Hence, without exception, the upper plumage of every bird, whether lark,
+chat, sylvain, or sand-grouse, and also the fur of all the smaller
+mammals, and the skin of all the snakes and lizards, is of one uniform
+isabelline or sand colour."
+
+Passing on to the tropical regions, it is among their evergreen forests
+alone that we find whole groups of birds whose ground colour is green.
+Parrots are very generally green, and in the East we have an extensive
+group of green fruit-eating pigeons; while the barbets, bee-eaters,
+turacos, leaf-thrushes (Phyllornis), white-eyes (Zosterops), and many
+other groups, have so much green in their plumage as to tend greatly to
+their concealment among the dense foliage. There can be no doubt that
+these colours have been acquired as a protection, when we see that in
+all the temperate regions, where the leaves are deciduous, the ground
+colour of the great majority of birds, especially on the upper surface,
+is a rusty brown of various shades, well corresponding with the bark,
+withered leaves, ferns, and bare thickets among which they live in
+autumn and winter, and especially in early spring when so many of them
+build their nests.
+
+Nocturnal animals supply another illustration of the same rule, in the
+dusky colours of mice, rats, bats, and moles, and in the soft mottled
+plumage of owls and goatsuckers which, while almost equally
+inconspicuous in the twilight, are such as to favour their concealment
+in the daytime.
+
+An additional illustration of general assimilation of colour to the
+surroundings of animals, is furnished by the inhabitants of the deep
+oceans. Professor Moseley of the Challenger Expedition, in his British
+Association lecture on this subject, says: "Most characteristic of
+pelagic animals is the almost crystalline transparency of their bodies.
+So perfect is this transparency that very many of them are rendered
+almost entirely invisible when floating in the water, while some, even
+when caught and held up in a glass globe, are hardly to be seen. The
+skin, nerves, muscles, and other organs are absolutely hyaline and
+transparent, but the liver and digestive tract often remain opaque and
+of a yellow or brown colour, and exactly resemble when seen in the water
+small pieces of floating seaweed." Such marine organisms, however, as
+are of larger size, and either occasionally or habitually float on the
+surface, are beautifully tinged with blue above, thus harmonising with
+the colour of the sea as seen by hovering birds; while they are white
+below, and are thus invisible against the wave-foam and clouds as seen
+by enemies beneath the surface. Such are the tints of the beautiful
+nudibranchiate mollusc, Glaucus atlanticus, and many others.
+
+
+_General Theories of Animal Colour._
+
+We are now in a position to test the general theories, or, to speak more
+correctly, the popular notions, as to the origin of animal coloration,
+before proceeding to apply the principle of utility to the explanation
+of some among the many extraordinary manifestations of colour in the
+animal world. The most generally received theory undoubtedly is, that
+brilliancy and variety of colour are due to the direct action of light
+and heat; a theory no doubt derived from the abundance of
+bright-coloured birds, insects, and flowers which are brought from
+tropical regions. There are, however, two strong arguments against this
+theory. We have already seen how generally bright coloration is wanting
+in desert animals, yet here heat and light are both at a maximum, and if
+these alone were the agents in the production of colour, desert animals
+should be the most brilliant. Again, all naturalists who have lived in
+tropical regions know that the proportion of bright to dull coloured
+species is little if any greater there than in the temperate zone, while
+there are many tropical groups in which bright colours are almost
+entirely unknown. No part of the world presents so many brilliant birds
+as South America, yet there are extensive families, containing many
+hundreds of species, which are as plainly coloured as our average
+temperate birds. Such are the families of the bush-shrikes and
+ant-thrushes (Formicariidae), the tyrant-shrikes (Tyrannidae), the
+American creepers (Dendrocolaptidae), together with a large proportion
+of the wood-warblers (Mniotiltidae), the finches, the wrens, and some
+other groups. In the eastern hemisphere, also, we have the
+babbling-thrushes (Timaliidae), the cuckoo-shrikes (Campephagidae), the
+honey-suckers (Meliphagidae), and several other smaller groups which are
+certainly not coloured above the average standard of temperate birds.
+
+Again, there are many families of birds which spread over the whole
+world, temperate and tropical, and among these the tropical species
+rarely present any exceptional brilliancy of colour. Such are the
+thrushes, goatsuckers, hawks, plovers, and ducks; and in the last-named
+group it is the temperate and arctic zones that afford the most
+brilliant coloration.
+
+The same general facts are found to prevail among insects. Although
+tropical insects present some of the most gorgeous coloration in the
+whole realm of nature, yet there are thousands and tens of thousands of
+species which are as dull coloured as any in our cloudy land. The
+extensive family of the carnivorous ground-beetles (Carabidae) attains
+its greatest brilliancy in the temperate zone; while by far the larger
+proportion of the great families of the longicorns and the weevils, are
+of obscure colours even in the tropics. In butterflies, there is
+undoubtedly a larger proportion of brilliant colour in the tropics; but
+if we compare families which are almost equally developed over the
+globe--as the Pieridae or whites and yellows, and the Satyridae or
+ringlets--we shall find no great disproportion in colour between those
+of temperate and tropical regions.
+
+The various facts which have now briefly been noticed are sufficient to
+indicate that the light and heat of the sun are not the direct causes of
+the colours of animals, although they may favour the production of
+colour when, as in tropical regions, the persistent high temperature
+favours the development of the maximum of life. We will now consider the
+next suggestion, that light reflected from surrounding coloured objects
+tends to produce corresponding colours in the animal world.
+
+This theory is founded on a number of very curious facts which prove,
+that such a change does sometimes occur and is directly dependent on the
+colours of surrounding objects; but these facts are comparatively rare
+and exceptional in their nature, and the same theory will certainly not
+apply to the infinitely varied colours of the higher animals, many of
+which are exposed to a constantly varying amount of light and colour
+during their active existence. A brief sketch of these dependent changes
+of colour may, however, be advantageously given here.
+
+
+_Variable Protective Colouring._
+
+There are two distinct kinds of change of colour in animals due to the
+colouring of the environment. In one case the change is caused by reflex
+action set up by the animal _seeing_ the colour to be imitated, and the
+change produced can be altered or repeated as the animal changes its
+position. In the other case the change occurs but once, and is probably
+not due to any conscious or sense action, but to some direct influence
+on the surface tissues while the creature is undergoing a moult or
+change to the pupa form.
+
+The most striking example of the first class is that of the chameleon,
+which changes to white, brown, yellowish, or green, according to the
+colour of the object on which it rests. This change is brought about by
+means of two layers of pigment cells, deeply seated in the skin, and of
+bluish and yellowish colours. By suitable muscles these cells can be
+forced upwards so as to modify the colour of the skin, which, when they
+are not brought into action, is a dirty white. These animals are
+excessively sluggish and defenceless, and the power of changing their
+colour to that of their immediate surroundings is no doubt of great
+service to them. Many of the flatfish are also capable of changing their
+colour according to the colour of the bottom they rest on; and frogs
+have a similar power to a limited extent. Some crustacea also change
+colour, and the power is much developed in the Chameleon shrimp (Mysis
+Chamaeleon) which is gray when on sand, but brown or green when among
+brown or green seaweed. It has been proved by experiment that when this
+animal is blinded the change does not occur. In all these cases,
+therefore, we have some form of reflex or sense action by which the
+change is produced, probably by means of pigment cells beneath the skin
+as in the chameleon.
+
+The second class consists of certain larvae, and pupae, which undergo
+changes of colour when exposed to differently coloured surroundings.
+This subject has been carefully investigated by Mr. E.B. Poulton, who
+has communicated the results of his experiments to the Royal
+Society.[65] It had been noticed that some species of larvae which fed
+on several different plants had colours more or less corresponding to
+the particular plant the individual fed on. Numerous cases are given in
+Professor Meldola's article on "Variable Protective Colouring" (_Proc.
+Zool. Soc._, 1873, p. 153), and while the general green coloration was
+attributed to the presence of chlorophyll beneath the skin, the
+particular change in correspondence to each food-plant was attributed to
+a special function which had been developed by natural selection. Later
+on, in a note to his translation of Weissmann's _Theory of Descent_,
+Professor Meldola seemed disposed to think that the variations of colour
+of some of the species might be phytophagic--that is, due to the direct
+action of the differently coloured leaves on which the insect fed. Mr.
+Poulton's experiments have thrown much light on this question, since he
+has conclusively proved that, in the case of the sphinx caterpillar of
+Smerinthus ocellatus, the change of colour is not due to the food but to
+the coloured light reflected from the leaves.
+
+This was shown by feeding two sets of larvae on the same plant but
+exposed to differently coloured surroundings, obtained by sewing the
+leaves together, so that in one case only the dark upper surface, in the
+other the whitish under surface was exposed to view. The result in each
+case was a corresponding change of colour in the larvae, confirming the
+experiments on different individuals of the same batch of larvae which
+had been supplied with different food-plants or exposed to a different
+coloured light.
+
+An even more interesting series of experiments was made on the colours
+of pupae, which in many cases were known to be affected by the material
+on which they underwent their transformations. The late Mr. T.W. Wood
+proved, in 1867, that the pupae of the common cabbage butterflies
+(Pieris brassicae and P. rapae) were either light, or dark, or green,
+according to the coloured boxes they were kept in, or the colours of the
+fences, walls, etc., against which they were suspended. Mrs. Barber in
+South Africa found that the pupae of Papilio Nireus underwent a similar
+change, being deep green when attached to orange leaves of the same
+tint, pale yellowish-green when on a branch of the bottle-brush tree
+whose half-dried leaves were of this colour, and yellowish when attached
+to the wooden frame of a box. A few other observers noted similar
+phenomena, but nothing more was done till Mr. Poulton's elaborate series
+of experiments with the larvae of several of our common butterflies were
+the means of clearing up several important points. He showed that the
+action of the coloured light did not affect the pupa itself but the
+larva, and that only for a limited period of time. After a caterpillar
+has done feeding it wanders about seeking a suitable place to undergo
+its transformation. When this is found it rests quietly for a day or
+two, spinning the web from which it is to suspend itself; and it is
+during this period of quiescence, and perhaps also the first hour or two
+after its suspension, that the action of the surrounding coloured
+surfaces determines, to a considerable extent, the colour of the pupa.
+By the application of various surrounding colours during this period,
+Mr. Poulton was able to modify the colour of the pupa of the common
+tortoise-shell butterfly from nearly black to pale, or to a brilliant
+golden; and that of Pieris rapae from dusky through pinkish to pale
+green. It is interesting to note, that the colours produced were in all
+cases such only as assimilated with the surroundings usually occupied by
+the species, and also, that colours which did not occur in such
+surroundings, as dark red or blue, only produced the same effects as
+dusky or black.
+
+Careful experiments were made to ascertain whether the effect was
+produced through the sight of the caterpillar. The ocelli were covered
+with black varnish, but neither this, nor cutting off the spines of the
+tortoise-shell larva to ascertain whether they might be sense-organs,
+produced any effect on the resulting colour. Mr. Poulton concludes,
+therefore, that the colour-action probably occurs over the whole surface
+of the body, setting up physiological processes which result in the
+corresponding colour-change of the pupa. Such changes are, however, by
+no means universal, or even common, in protectively coloured pupae,
+since in Papilio machaon and some others which have been experimented
+on, both in this country and abroad, no change can be produced on the
+pupa by any amount of exposure to differently coloured surroundings. It
+is a curious point that, with the small tortoise-shell larva, exposure
+to light from gilded surfaces produced pupae with a brilliant golden
+lustre; and the explanation is supposed to be that mica abounded in the
+original habitat of the species, and that the pupae thus obtained
+protection when suspended against micaceous rock. Looking, however, at
+the wide range of the species and the comparatively limited area in
+which micaceous rocks occur, this seems a rather improbable explanation,
+and the occurrence of this metallic appearance is still a difficulty. It
+does not, however, commonly occur in this country in a natural state.
+
+The two classes of variable colouring here discussed are evidently
+exceptional, and can have little if any relation to the colours of those
+more active creatures which are continually changing their position with
+regard to surrounding objects, and whose colours and markings are nearly
+constant throughout the life of the individual, and (with the exception
+of sexual differences) in all the individuals of the species. We will
+now briefly pass in review the various characteristics and uses of the
+colours which more generally prevail in nature; and having already
+discussed those protective colours which serve to harmonise animals with
+their general environment, we have to consider only those cases in which
+the colour resemblance is more local or special in its character.
+
+
+_Special or Local Colour Adaptations._
+
+This form of colour adaptation is generally manifested by markings
+rather than by colour alone, and is extremely prevalent both among
+insects and vertebrates, so that we shall be able to notice only a few
+illustrative cases. Among our native birds we have the snipe and
+woodcock, whose markings and tints strikingly accord with the dead marsh
+vegetation among which they live; the ptarmigan in its summer dress is
+mottled and tinted exactly like the lichens which cover the stones of
+the higher mountains; while young unfledged plovers are spotted so as
+exactly to resemble the beach pebbles among which they crouch for
+protection, as beautifully exhibited in one of the cases of British
+birds in the Natural History Museum at South Kensington.
+
+In mammalia, we notice the frequency of rounded spots on forest or tree
+haunting animals of large size, as the forest deer and the forest cats;
+while those that frequent reedy or grassy places are striped vertically,
+as the marsh antelopes and the tiger. I had long been of opinion that
+the brilliant yellow and black stripes of the tiger were adaptive, but
+have only recently obtained proof that it is so. An experienced
+tiger-hunter, Major Walford, states in a letter, that the haunts of the
+tiger are invariably full of the long grass, dry and pale yellow for at
+least nine months of the year, which covers the ground wherever there is
+water in the rainy season, and he adds: "I once, while following up a
+wounded tiger, failed for at least a minute to see him under a tree in
+grass at a distance of about twenty yards--jungle open--but the natives
+saw him, and I eventually made him out well enough to shoot him, but
+even then I could not see at what part of him I was aiming. There can be
+no doubt whatever that the colour of both the tiger and the panther
+renders them almost invisible, especially in a strong blaze of light,
+when among grass, and one does not seem to notice stripes or spots till
+they are dead." It is the black shadows of the vegetation that
+assimilate with the black stripes of the tiger; and, in like manner,
+the spotty shadows of leaves in the forest so harmonise with the spots
+of ocelots, jaguars, tiger-cats, and spotted deer as to afford them a
+very perfect concealment.
+
+In some cases the concealment is effected by colours and markings which
+are so striking and peculiar that no one who had not seen the creature
+in its native haunts would imagine them to be protective. An example of
+this is afforded by the banded fruit pigeon of Timor, whose pure white
+head and neck, black wings and back, yellow belly, and deeply-curved
+black band across the breast, render it a very handsome and conspicuous
+bird. Yet this is what Mr. H.O. Forbes says of it: "On the trees the
+white-headed fruit pigeon (Ptilopus cinctus) sate motionless during the
+heat of the day in numbers, on well-exposed branches; but it was with
+the utmost difficulty that I or my sharp-eyed native servant could ever
+detect them, even in trees where we knew they were sitting."[66] The
+trees referred to are species of Eucalyptus which abound in Timor. They
+have whitish or yellowish bark and very open foliage, and it is the
+intense sunlight casting black curved shadows of one branch upon
+another, with the white and yellow bark and deep blue sky seen through
+openings of the foliage, that produces the peculiar combination of
+colours and shadows to which the colours and markings of this bird have
+become so closely assimilated.
+
+Even such brilliant and gorgeously coloured birds as the sun-birds of
+Africa are, according to an excellent observer, often protectively
+coloured. Mrs. M.E. Barber remarks that "A casual observer would
+scarcely imagine that the highly varnished and magnificently coloured
+plumage of the various species of Noctarinea could be of service to
+them, yet this is undoubtedly the case. The most unguarded moments of
+the lives of these birds are those that are spent amongst the flowers,
+and it is then that they are less wary than at any other time. The
+different species of aloes, which blossom in succession, form the
+principal sources of their winter supplies of food; and a legion of
+other gay flowering plants in spring and summer, the aloe blossoms
+especially, are all brilliantly coloured, and they harmonise admirably
+with the gay plumage of the different species of sun-birds. Even the
+keen eye of a hawk will fail to detect them, so closely do they resemble
+the flowers they frequent. The sun-birds are fully aware of this fact,
+for no sooner have they relinquished the flowers than they become
+exceedingly wary and rapid in flight, darting arrow-like through the air
+and seldom remaining in exposed situations. The black sun-bird
+(Nectarinea amethystina) is never absent from that magnificent
+forest-tree, the 'Kaffir Boom' (Erythrina caffra); all day long the
+cheerful notes of these birds may be heard amongst its spreading
+branches, yet the general aspect of the tree, which consists of a huge
+mass of scarlet and purple-black blossoms without a single green leaf,
+blends and harmonises with the colours of the black sun-bird to such an
+extent that a dozen of them may be feeding amongst its blossoms without
+being conspicuous, or even visible."[67]
+
+Some other cases will still further illustrate how the colours of even
+very conspicuous animals may be adapted to their peculiar haunts.
+
+The late Mr. Swinhoe says of the Kerivoula picta, which he observed in
+Formosa: "The body of this bat was of an orange colour, but the wings
+were painted with orange-yellow and black. It was caught suspended, head
+downwards, on a cluster of the fruit of the longan tree (Nephelium
+longanum). Now this tree is an evergreen, and all the year round some
+portion of its foliage is undergoing decay, the particular leaves being,
+in such a stage, partially orange and black. This bat can, therefore, at
+all seasons suspend from its branches and elude its enemies by its
+resemblance to the leaves of the tree."[68]
+
+Even more curious is the case of the sloths--defenceless animals which
+feed upon leaves, and hang from the branches of trees with their back
+downwards. Most of the species have a curious buff-coloured spot on the
+back, rounded or oval in shape and often with a darker border, which
+seems placed there on purpose to make them conspicuous; and this was a
+great puzzle to naturalists, because the long coarse gray or greenish
+hair was evidently like tree-moss and therefore protective. But an old
+writer, Baron von Slack, in his _Voyage_ _to Surinam_ (1810), had
+already explained the matter. He says: "The colour and even the shape of
+the hair are much like withered moss, and serve to hide the animal in
+the trees, but particularly when it has that orange-coloured spot
+between the shoulders and lies close to the tree; it looks then exactly
+like a piece of branch where the rest has been broken off, by which the
+hunters are often deceived." Even such a huge animal as the giraffe is
+said to be perfectly concealed by its colour and form when standing
+among the dead and broken trees that so often occur on the outskirts of
+the thickets where it feeds. The large blotch-like spots on the skin and
+the strange shape of the head and horns, like broken branches, so tend
+to its concealment that even the keen-eyed natives have been known to
+mistake trees for giraffes or giraffes for trees.
+
+Innumerable examples of this kind of protective colouring occur among
+insects; beetles mottled like the bark of trees or resembling the sand
+or rock or moss on which they live, with green caterpillars of the exact
+general tints of the foliage they feed on; but there are also many cases
+of detailed imitation of particular objects by insects that must be
+briefly described.[69]
+
+
+_Protective Imitation of Particular Objects._
+
+The insects which present this kind of imitation most perfectly are the
+Phasmidae, or stick and leaf insects. The well-known leaf-insects of
+Ceylon and of Java, species of Phyllium, are so wonderfully coloured and
+veined, with leafy expansions on the legs and thorax, that not one
+person in ten can see them when resting on the food-plant close beneath
+their eyes. Others resemble pieces of stick with all the minutiae of
+knots and branches, formed by the insects' legs, which are stuck out
+rigidly and unsymmetrically. I have often been unable to distinguish
+between one of these insects and a real piece of stick, till I satisfied
+myself by touching it and found it to be alive. One species, which was
+brought me in Borneo, was covered with delicate semitransparent green
+foliations, exactly resembling the hepaticae which cover pieces of
+rotten stick in the damp forests. Others resemble dead leaves in all
+their varieties of colour and form; and to show how perfect is the
+protection obtained and how important it is to the possessors of it, the
+following incident, observed by Mr. Belt in Nicaragua, is most
+instructive. Describing the armies of foraging ants in the forest which
+devour every insect they can catch, he says: "I was much surprised with
+the behaviour of a green leaf-like locust. This insect stood immovably
+among a host of ants, many of which ran over its legs without ever
+discovering there was food within their reach. So fixed was its
+instinctive knowledge that its safety depended on its immovability, that
+it allowed me to pick it up and replace it among the ants without making
+a single effort to escape. This species closely resembles a green
+leaf."[70]
+
+Caterpillars also exhibit a considerable amount of detailed resemblance
+to the plants on which they live. Grass-feeders are striped
+longitudinally, while those on ordinary leaves are always striped
+obliquely. Some very beautiful protective resemblances are shown among
+the caterpillars figured in Smith and Abbott's _Lepidopterous Insects of
+Georgia_, a work published in the early part of the century, before any
+theories of protection were started. The plates in this work are most
+beautifully executed from drawings made by Mr. Abbott, representing the
+insects, in every case, on the plants which they frequented, and no
+reference is made in the descriptions to the remarkable protective
+details which appear upon the plates. We have, first, the larva of
+Sphinx fuciformis feeding on a plant with linear grass-like leaves and
+small blue flowers; and we find the insect of the same green as the
+leaves, striped longitudinally in accordance with the linear leaves, and
+with the head blue corresponding both in size and colour with the
+flowers. Another species (Sphinx tersa) is represented feeding on a
+plant with small red flowers situated in the axils of the leaves; and
+the larva has a row of seven red spots, unequal in size, and
+corresponding very closely with the colour and size of the flowers. Two
+other figures of sphinx larvae are very curious. That of Sphinx
+pampinatrix feeds on a wild vine (Vitis indivisa), having green
+tendrils, and in this species the curved horn on the tail is green, and
+closely imitates in its curve the tip of the tendril. But in another
+species (Sphinx cranta), which feeds on the fox-grape (Vitis vulpina),
+the horn is very long and red, corresponding with the long red-tipped
+tendrils of the plant. Both these larvae are green with oblique stripes,
+to harmonise with the veined leaves of the vines; but a figure is also
+given of the last-named species after it has done feeding, when it is of
+a decided brown colour and has entirely lost its horn. This is because
+it then descends to the ground to bury itself, and the green colour and
+red horn would be conspicuous and dangerous; it therefore loses both at
+the last moult. Such a change of colour occurs in many species of
+caterpillars. Sometimes the change is seasonal; and, in those which
+hibernate with us, the colour of some species, which is brownish in
+autumn in adaptation to the fading foliage, becomes green in spring to
+harmonise with the newly-opened leaves at that season.[71]
+
+Some of the most curious examples of minute imitation are afforded by
+the caterpillars of the geometer moths, which are always brown or
+reddish, and resemble in form little twigs of the plant on which they
+feed. They have the habit, when at rest, of standing out obliquely from
+the branch, to which they hold on by their hind pair of prolegs or
+claspers, and remain motionless for hours. Speaking of these protective
+resemblances Mr. Jenner Weir says: "After being thirty years an
+entomologist I was deceived myself, and took out my pruning scissors to
+cut from a plum tree a spur which I thought I had overlooked. This
+turned out to be the larva of a geometer two inches long. I showed it
+to several members of my family, and defined a space of four inches in
+which it was to be seen, but none of them could perceive that it was a
+caterpillar."[72]
+
+One more example of a protected caterpillar must be given. Mr. A.
+Everett, writing from Sarawak, Borneo, says: "I had a caterpillar
+brought me, which, being mixed by my boy with some other things, I took
+to be a bit of moss with two exquisite pinky-white seed-capsules; but I
+soon saw that it moved, and examining it more closely found out its real
+character: it is covered with hair, with two little pink spots on the
+upper surface, the general hue being more green. Its motions are very
+slow, and when eating the head is withdrawn beneath a fleshy mobile
+hood, so that the action of feeding does not produce any movement
+externally. It was found in the limestone hills at Busan, the situation
+of all others where mosses are most plentiful and delicate, and where
+they partially clothe most of the protruding masses of rock."
+
+
+_How these Imitations have been Produced._
+
+To many persons it will seem impossible that such beautiful and detailed
+resemblances as those now described--and these are only samples of
+thousands that occur in all parts of the world--can have been brought
+about by the preservation of accidental useful variations. But this will
+not seem so surprising if we keep in mind the facts set forth in our
+earlier chapters--the rapid multiplication, the severe struggle for
+existence, and the constant variability of these and all other
+organisms. And, further, we must remember that these delicate
+adjustments are the result of a process which has been going on for
+millions of years, and that we now see the small percentage of successes
+among the myriads of failures. From the very first appearance of insects
+and their various kinds of enemies the need of protection arose, and was
+usually most easily met by modifications of colour. Hence, we may be
+sure that the earliest leaf-eating insects acquired a green colour as
+one of the necessities of their existence; and, as the species became
+modified and specialised, those feeding on particular species of plants
+would rapidly acquire the peculiar tints and markings best adapted to
+conceal them upon those plants. Then, every little variation that, once
+in a hundred years perhaps, led to the preservation of some larva which
+was thereby rather better concealed than its fellows, would form the
+starting-point of a further development, leading ultimately to that
+perfection of imitation in details which now astonishes us. The
+researches of Dr. Weismann illustrate this progressive adaptation. The
+very young larvae of several species are green or yellowish without any
+markings; they then, in subsequent moults, obtain certain markings, some
+of which are often lost again before the larva is fully grown. The early
+stages of those species which, like elephant hawk-moths (Chaerocampa),
+have the anterior segments elongated and retractile, with large eye-like
+spots to imitate the head of a vertebrate, are at first like those of
+non-retractile species, the anterior segments being as large as the
+rest. After the first moult they become smaller, comparatively; but it
+is only after the second moult that the ocelli begin to appear, and
+these are not fully defined till after the third moult. This progressive
+development of the individual--the ontogeny--gives us a clue to the
+ancestral development of the whole race--the phylogeny; and we are
+enabled to picture to ourselves the very slow and gradual steps by which
+the existing perfect adaptation has been brought about. In many larvae
+great variability still exists, and in some there are two or more
+distinctly-coloured forms--usually a dark and a light or a brown and a
+green form. The larva of the humming-bird hawk-moth (Macroglossa
+stellatarum) varies in this manner, and Dr. Weismann raised five
+varieties from a batch of eggs from one moth. It feeds on species of
+bedstraw (Galium verum and G. mollugo), and as the green forms are less
+abundant than the brown, it has probably undergone some recent change of
+food-plant or of habits which renders brown the more protective colour.
+
+
+_Special Protective Colouring of Butterflies._
+
+We will now consider a few cases of special protective colouring in the
+perfect butterfly or moth. Mr. Mansel Weale states that in South Africa
+there is a great prevalence of white and silvery foliage or bark,
+sometimes of dazzling brilliancy, and that many insects and their larvae
+have brilliant silvery tints which are protective, among them being
+three species of butterflies whose undersides are silvery, and which are
+thus effectually protected when at rest.[73] A common African butterfly
+(Aterica meleagris) always settles on the ground with closed wings,
+which so closely resemble the soil of the district that it can with
+difficulty be seen, and the colour varies with the soil in different
+localities. Thus specimens from Senegambia were dull brown, the soil
+being reddish sand and iron-clay; those from Calabar and Cameroons were
+light brown with numerous small white spots, the soil of those countries
+being light brown clay with small quartz pebbles; while in other
+localities where the colours of the soil were more varied the colours of
+the butterfly varied also. Here we have variation in a single species
+which has become specialised in certain areas to harmonise with the
+colour of the soil.[74]
+
+Many butterflies, in all parts of the world, resemble dead leaves on
+their under side, but those in which this form of protection is carried
+to the greatest perfection are the species of the Eastern genus Kallima.
+In India K. inachis, and in the larger Malay islands K. paralekta, are
+very common. They are rather large and showy butterflies, orange and
+bluish on the upper side, with a very rapid flight, and frequenting dry
+forests. Their habit is to settle always where there is some dead or
+decaying foliage, and the shape and colour of the wings (on the under
+surface), together with the attitude of the insect, is such as to
+produce an absolutely perfect imitation of a dead leaf. This is effected
+by the butterfly always settling on a twig, with the short tail of the
+hind wings just touching it and forming the leaf-stalk. From this a dark
+curved line runs across to the elongated tip of the upper wings,
+imitating the midrib, on both sides of which are oblique lines, formed
+partly by the nervures and partly by markings, which give the effect of
+the usual veining of a leaf. The head and antennae fit exactly between
+the closed upper wings so as not to interfere with the outline, which
+has just that amount of irregular curvature that is seen in dry and
+withered leaves. The colour is very remarkable for its extreme amount of
+variability, from deep reddish-brown to olive or pale yellow, hardly two
+specimens being exactly alike, but all coming within the range of colour
+of leaves in various stages of decay. Still more curious is the fact
+that the paler wings, which imitate leaves most decayed, are usually
+covered with small black dots, often gathered into circular groups, and
+so exactly resembling the minute fungi on decaying leaves that it is
+hard at first to believe that the insects themselves are not attacked by
+some such fungus. The concealment produced by this wonderful imitation
+is most complete, and in Sumatra I have often seen one enter a bush and
+then disappear like magic. Once I was so fortunate as to see the exact
+spot on which the insect settled; but even then I lost sight of it for
+some time, and only after a persistent search discovered that it was
+close before my eyes.[75] Here we have a kind of imitation, which is
+very common in a less developed form, carried to extreme perfection,
+with the result that the species is very abundant over a considerable
+area of country.
+
+
+_Protective Resemblance among Marine Animals._
+
+Among marine animals this form of protection is very common. Professor
+Moseley tells us that all the inhabitants of the Gulf-weed are most
+remarkably coloured, for purposes of protection and concealment, exactly
+like the weed itself. "The shrimps and crabs which swarm in the weed are
+of exactly the same shade of yellow as the weed, and have white markings
+upon their bodies to represent the patches of Membranipora. The small
+fish, Antennarius, is in the same way weed-colour with white spots. Even
+a Planarian worm, which lives in the weed, is similarly yellow-coloured,
+and also a mollusc, Scyllaea pelagica." The same writer tells us that "a
+number of little crabs found clinging to the floats of the blue-shelled
+mollusc, Ianthina, were all coloured of a corresponding blue for
+concealment."[76]
+
+Professor E.S. Morse of Salem, Mass., found that most of the New
+England marine mollusca were protectively coloured; instancing among
+others a little red chiton on rocks clothed with red calcareous algae,
+and Crepidula plana, living within the apertures of the shells of larger
+species of Gasteropods and of a pure white colour corresponding to its
+habitat, while allied species living on seaweed or on the outside of
+dark shells were dark brown.[77] A still more interesting case has been
+recorded by Mr. George Brady. He says: "Amongst the Nullipore which
+matted together the laminaria roots in the Firth of Clyde were living
+numerous small starfishes (Ophiocoma bellis) which, except when their
+writhing movements betrayed them, were quite undistinguishable from the
+calcareous branches of the alga; their rigid angularly twisted rays had
+all the appearance of the coralline, and exactly assimilated to its dark
+purple colour, so that though I held in my hand a root in which were
+half a dozen of the starfishes, I was really unable to detect them until
+revealed by their movements."[78]
+
+These few examples are sufficient to show that the principle of
+protective coloration extends to the ocean as well as over the earth;
+and if we consider how completely ignorant we are of the habits and
+surroundings of most marine animals, it may well happen that many of the
+colours of tropical fishes, which seem to us so strange and so
+conspicuous, are really protective, owing to the number of equally
+strange and brilliant forms of corals, sea-anemones, sponges, and
+seaweeds among which they live.
+
+
+_Protection by Terrifying Enemies._
+
+A considerable number of quite defenceless insects obtain protection
+from some of their enemies by having acquired a resemblance to dangerous
+animals, or by some threatening or unusual appearance. This is obtained
+either by a modification of shape, of habits, of colour, or of all
+combined. The simplest form of this protection is the aggressive
+attitude of the caterpillars of the Sphingidae, the forepart of the body
+being erected so as to produce a rude resemblance to the figure of a
+sphinx, hence the name of the family. The protection is carried further
+by those species which retract the first three segments and have large
+ocelli on each side of the fourth segment, thus giving to the
+caterpillar, when the forepart of its body is elevated, the appearance
+of a snake in a threatening attitude.
+
+The blood-red forked tentacle, thrown out of the neck of the larvae of
+the genus Papilio when alarmed, is, no doubt, a protection against the
+attacks of ichneumons, and may, perhaps, also frighten small birds; and
+the habit of turning up the tail possessed by the harmless rove-beetles
+(Staphylinidae), giving the idea that they can sting, has, probably, a
+similar use. Even an unusual angular form, like a crooked twig or
+inorganic substance, may be protective; as Mr. Poulton thinks is the
+case with the curious caterpillar of Notodonta ziczac, which, by means
+of a few slight protuberances on its body, is able to assume an angular
+and very unorganic-looking appearance. But perhaps the most perfect
+example of this kind of protection is exhibited by the large caterpillar
+of the Royal Persimmon moth (Bombyx regia), a native of the southern
+states of North America, and known there as the "Hickory-horned devil."
+It is a large green caterpillar, often six inches long, ornamented with
+an immense crown of orange-red tubercles, which, if disturbed, it erects
+and shakes from side to side in a very alarming manner. In its native
+country the negroes believe it to be as deadly as a rattlesnake, whereas
+it is perfectly innocuous. The green colour of the body suggests that
+its ancestors were once protectively coloured; but, growing too large to
+be effectually concealed, it acquired the habit of shaking its head
+about in order to frighten away its enemies, and ultimately developed
+the crown of tentacles as an addition to its terrifying powers. This
+species is beautifully figured in Abbott and Smith's _Lepidopterous
+Insects of Georgia_.
+
+
+_Alluring Coloration._
+
+Besides those numerous insects which obtain protection through their
+resemblance to the natural objects among which they live, there are some
+whose disguise is not used for concealment, but as a direct means of
+securing their prey by attracting them within the enemy's reach. Only a
+few cases of this kind of coloration have yet been observed, chiefly
+among spiders and mantidae; but, no doubt, if attention were given to
+the subject in tropical countries, many more would be discovered. Mr.
+H.O. Forbes has described a most interesting example of this kind of
+simulation in Java. While pursuing a large butterfly through the jungle,
+he was stopped by a dense bush, on a leaf of which he observed one of
+the skipper butterflies sitting on a bird's dropping. "I had often," he
+says, "observed small Blues at rest on similar spots on the ground, and
+have wondered what such a refined and beautiful family as the Lycaenidae
+could find to enjoy, in food apparently so incongruous for a butterfly.
+I approached with gentle steps, but ready net, to see if possible how
+the present species was engaged. It permitted me to get quite close, and
+even to seize it between my fingers; to my surprise, however, part of
+the body remained behind, adhering as I thought to the excreta. I looked
+closely, and finally touched with my finger the excreta to find if it
+were glutinous. To my delighted astonishment I found that my eyes had
+been most perfectly deceived, and that what seemed to be the excreta was
+a most artfully coloured spider, lying on its back with its feet crossed
+over and closely adpressed to the body." Mr. Forbes then goes on to
+describe the exact appearance of such excreta, and how the various parts
+of the spider are coloured to produce the imitation, even to the liquid
+portion which usually runs a little down the leaf. This is exactly
+imitated by a portion of the thin web which the spider first spins to
+secure himself firmly to the leaf; thus producing, as Mr. Forbes
+remarks, a living bait for butterflies and other insects so artfully
+contrived as to deceive a pair of human eyes, even when intently
+examining it.[79]
+
+A native species of spider (Thomisus citreus) exhibits a somewhat
+similar alluring protection by its close resemblance to buds of the
+wayfaring tree, Viburnum lantana. It is pure creamy-white, the abdomen
+exactly resembling in shape and colour the unopened buds of the flowers
+among which it takes its station; and it has been seen to capture flies
+which came to the flowers.
+
+But the most curious and beautiful case of alluring protection is that
+of a wingless Mantis in India, which is so formed and coloured as to
+resemble a pink orchis or some other fantastic flower. The whole insect
+is of a bright pink colour, the large and oval abdomen looking like the
+labellum of an orchid. On each side, the two posterior legs have
+immensely dilated and flattened thighs which represent the petals of a
+flower, while the neck and forelegs imitate the upper sepal and column
+of an orchid. The insect rests motionless, in this symmetrical attitude,
+among bright green foliage, being of course very conspicuous, but so
+exactly resembling a flower that butterflies and other insects settle
+upon it and are instantly captured. It is a living trap, baited in the
+most alluring manner to catch the unwary flower-haunting insects.[80]
+
+
+_The Coloration of Birds' Eggs._
+
+The colours of birds' eggs have long been a difficulty on the theory of
+adaptive coloration, because, in so many cases it has not been easy to
+see what can be the use of the particular colours, which are often so
+bright and conspicuous that they seem intended to attract attention
+rather than to be concealed. A more careful consideration of the subject
+in all its bearings shows, however, that here too, in a great number of
+cases, we have examples of protective coloration. When, therefore, we
+cannot see the meaning of the colour, we may suppose that it has been
+protective in some ancestral form, and, not being hurtful, has persisted
+under changed conditions which rendered the protection needless.
+
+We may divide all eggs, for our present purpose, into two great
+divisions; those which are white or nearly so, and those which are
+distinctly coloured or spotted. Egg-shells being composed mainly of
+carbonate of lime, we may assume that the primitive colour of birds'
+eggs was white, a colour that prevails now among the other egg-bearing
+vertebrates--lizards, crocodiles, turtles, and snakes; and we might,
+therefore, expect that this colour would continue where its presence had
+no disadvantages. Now, as a matter of fact, we find that in all the
+groups of birds which lay their eggs in concealed places, whether in
+holes of trees or in the ground, or in domed or covered nests, the eggs
+are either pure white or of very pale uniform coloration. Such is the
+case with kingfishers, bee-eaters, penguins, and puffins, which nest in
+holes in the ground; with the great parrot family, the woodpeckers, the
+rollers, hoopoes, trogons, owls, and some others, which build in holes
+in trees or other concealed places; while martins, wrens,
+willow-warblers, and Australian finches, build domed or covered nests,
+and usually have white eggs.
+
+There are, however, many other birds which lay their white eggs in open
+nests; and these afford some very interesting examples of the varied
+modes by which concealment may be obtained. All the duck tribe, the
+grebes, and the pheasants belong to this class; but these birds all have
+the habit of covering their eggs with dead leaves or other material
+whenever they leave the nest, so as effectually to conceal them. Other
+birds, as the short-eared owl, the goatsucker, the partridge, and some
+of the Australian ground pigeons, lay their white or pale eggs on the
+bare soil; but in these cases the birds themselves are protectively
+coloured, so that, when sitting, they are almost invisible; and they
+have the habit of sitting close and almost continuously, thus
+effectually concealing their eggs.
+
+Pigeons and doves offer a very curious case of the protection of exposed
+eggs. They usually build very slight and loose nests of sticks and
+twigs, so open that light can be seen through them from below, while
+they are generally well concealed by foliage above. Their eggs are white
+and shining; yet it is a difficult matter to discover, from beneath,
+whether there are eggs in the nest or not, while they are well hidden by
+the thick foliage above. The Australian podargihuge goatsuckers--build
+very similar nests, and their white eggs are protected in the same
+manner. Some large and powerful birds, as the swans, herons, pelicans,
+cormorants, and storks, lay white eggs in open nests; but they keep
+careful watch over them, and are able to drive away intruders. On the
+whole, then, we see that, while white eggs are conspicuous, and
+therefore especially liable to attack by egg-eating animals, they are
+concealed from observation in many and various ways. We may, therefore,
+assume that, in cases where there seems to be no such concealment, we
+are too ignorant of the whole of the conditions to form a correct
+judgment.
+
+We now come to the large class of coloured or richly spotted eggs, and
+here we have a more difficult task, though many of them decidedly
+exhibit protective tints or markings. There are two birds which nest on
+sandy shores--the lesser tern and the ringed plover,--and both lay
+sand-coloured eggs, the former spotted so as to harmonise with coarse
+shingle, the latter minutely speckled like fine sand, which are the
+kinds of ground the two birds choose respectively for their nests. "The
+common sandpipers' eggs assimilate so closely with the tints around them
+as to make their discovery a matter of no small difficulty, as every
+oologist can testify who has searched for them. The pewits' eggs, dark
+in ground colour and boldly marked, are in strict harmony with the sober
+tints of moor and fallow, and on this circumstance alone their
+concealment and safety depend. The divers' eggs furnish another example
+of protective colour; they are generally laid close to the water's edge,
+amongst drift and shingle, where their dark tints and black spots
+conceal them by harmonising closely with surrounding objects. The snipes
+and the great army of sandpipers furnish innumerable instances of
+protectively coloured eggs. In all the instances given the sitting-bird
+invariably leaves the eggs uncovered when it quits them, and
+consequently their safety depends solely on the colours which adorn
+them."[81] The wonderful range of colour and marking in the eggs of the
+guillemot may be imputed to the inaccessible rocks on which it breeds,
+giving it complete protection from enemies. Thus the pale or bluish
+ground colour of the eggs of its allies, the auks and puffins, has
+become intensified and blotched and spotted in the most marvellous
+variety of patterns, owing to there being no selective agency to prevent
+individual variation having full sway.
+
+The common black coot (Fulica atra) has eggs which are coloured in a
+specially protective manner. Dr. William Marshall writes, that it only
+breeds in certain localities where a large water reed (Phragmites
+arundinacea) abounds. The eggs of the coot are stained and spotted with
+black on a yellowish-gray ground, and the dead leaves of the reed are of
+the same colour, and are stained black by small parasitic fungi of the
+Uredo family; and these leaves form the bed on which the eggs are laid.
+The eggs and the leaves agree so closely in colour and markings that it
+is a difficult thing to distinguish the eggs at any distance. It is to
+be noted that the coot never covers up its eggs, as its ally the
+moor-hen usually does.
+
+The beautiful blue or greenish eggs of the hedge-sparrow, the
+song-thrush, and sometimes those of the blackbird, seem at first sight
+especially calculated to attract attention, but it is very doubtful
+whether they are really so conspicuous when seen at a little distance
+among their usual surroundings. For the nests of these birds are either
+in evergreens, as holly or ivy, or surrounded by the delicate green
+tints of our early spring vegetation, and may thus harmonise very well
+with the colours around them. The great majority of the eggs of our
+smaller birds are so spotted or streaked with brown or black on
+variously tinted grounds that, when lying in the shadow of the nest and
+surrounded by the many colours and tints of bark and moss, of purple
+buds and tender green or yellow foliage, with all the complex glittering
+lights and mottled shades produced among these by the spring sunshine
+and by sparkling raindrops, they must have a quite different aspect from
+that which they possess when we observe them torn from their natural
+surroundings. We have here, probably, a similar case of general
+protective harmony to that of the green caterpillars with beautiful
+white or purple bands and spots, which, though gaudily conspicuous when
+seen alone, become practically invisible among the complex lights and
+shadows of the foliage they feed upon.
+
+In the case of the cuckoo, which lays its eggs in the nests of a variety
+of other birds, the eggs themselves are subject to considerable
+variations of colour, the most common type, however, resembling those of
+the pipits, wagtails, or warblers, in whose nests they are most
+frequently laid. It also often lays in the nest of the hedge-sparrow,
+whose bright blue eggs are usually not at all nearly matched, although
+they are sometimes said to be so on the Continent. It is the opinion of
+many ornithologists that each female cuckoo lays the same coloured eggs,
+and that it usually chooses a nest the owners of which lay somewhat
+similar eggs, though this is by no means universally the case. Although
+birds which have cuckoos' eggs imposed upon them do not seem to neglect
+them on account of any difference of colour, yet they probably do so
+occasionally; and if, as seems probable, each bird's eggs are to some
+extent protected by their harmony of colour with their surroundings, the
+presence of a larger and very differently coloured egg in the nest might
+be dangerous, and lead to the destruction of the whole set. Those
+cuckoos, therefore, which most frequently placed their eggs among the
+kinds which they resembled, would in the long run leave most progeny,
+and thus the very frequent accord in colour might have been brought
+about.
+
+Some writers have suggested that the varied colours of birds' eggs are
+primarily due to the effect of surrounding coloured objects on the
+female bird during the period preceding incubation; and have expended
+much ingenuity in suggesting the objects that may have caused the eggs
+of one bird to be blue, another brown, and another pink.[82] But no
+evidence has been presented to prove that any effects whatever are
+produced by this cause, while there seems no difficulty in accounting
+for the facts by individual variability and the action of natural
+selection. The changes that occur in the conditions of existence of
+birds must sometimes render the concealment less perfect than it may
+once have been; and when any danger arises from this cause, it may be
+met either by some change in the colour of the eggs, or in the
+structure or position of the nest, or by the increased care which the
+parents bestow upon the eggs. In this way the various divergences which
+now so often puzzle us may have arisen.
+
+
+_Colour as a Means of Recognition._
+
+If we consider the habits and life-histories of those animals which are
+more or less gregarious, comprising a large proportion of the herbivora,
+some carnivora, and a considerable number of all orders of birds, we
+shall see that a means of ready recognition of its own kind, at a
+distance or during rapid motion, in the dusk of twilight or in partial
+cover, must be of the greatest advantage and often lead to the
+preservation of life. Animals of this kind will not usually receive a
+stranger into their midst. While they keep together they are generally
+safe from attack, but a solitary straggler becomes an easy prey to the
+enemy; it is, therefore, of the highest importance that, in such a case,
+the wanderer should have every facility for discovering its companions
+with certainty at any distance within the range of vision.
+
+Some means of easy recognition must be of vital importance to the young
+and inexperienced of each flock, and it also enables the sexes to
+recognise their kind and thus avoid the evils of infertile crosses; and
+I am inclined to believe that its necessity has had a more widespread
+influence in determining the diversities of animal coloration than any
+other cause whatever. To it may probably be imputed the singular fact
+that, whereas bilateral symmetry of coloration is very frequently lost
+among domesticated animals, it almost universally prevails in a state of
+nature; for if the two sides of an animal were unlike, and the diversity
+of coloration among domestic animals occurred in a wild state, easy
+recognition would be impossible among numerous closely allied forms.[83]
+The wonderful diversity of colour and of marking that prevails,
+especially in birds and insects, may be due to the fact that one of the
+first needs of a new species would be, to keep separate from its nearest
+allies, and this could be most readily done by some easily seen external
+mark of difference. A few illustrations will serve to show how this
+principle acts in nature.
+
+My attention was first called to the subject by a remark of Mr. Darwin's
+that, though, "the hare on her form is a familiar instance of
+concealment through colour, yet the principle partly fails in a closely
+allied species, the rabbit; for when running to its burrow it is made
+conspicuous to the sportsman, and no doubt to all beasts of prey, by its
+upturned white tail."[84] But a little consideration of the habits of
+the animal will show that the white upturned tail is of the greatest
+value, and is really, as it has been termed by a writer in _The Field_,
+a "signal flag of danger." For the rabbit is usually a crepuscular
+animal, feeding soon after sunset or on moonlight nights. When disturbed
+or alarmed it makes for its burrow, and the white upturned tails of
+those in front serve as guides and signals to those more remote from
+home, to the young and the feeble; and thus each following the one or
+two before it, all are able with the least possible delay to regain a
+place of comparative safety. The apparent danger, therefore, becomes a
+most important means of security.
+
+The same general principle enables us to understand the singular, and
+often conspicuous, markings on so many gregarious herbivora which are
+yet, on the whole, protectively coloured. Thus, the American prong-buck
+has a white patch behind and a black muzzle. The Tartarian antelope, the
+Ovis poli of High Asia, the Java wild ox, several species of deer, and a
+large number of antelopes have a similar conspicuous white patch behind,
+which, in contrast to the dusky body, must enable them to be seen and
+followed from a distance by their fellows. Where there are many species
+of nearly the same general size and form inhabiting the same region--as
+with the antelopes of Africa--we find many distinctive markings of a
+similar kind. The gazelles have variously striped and banded faces,
+besides white patches behind and on the flanks, as shown in the woodcut.
+The spring-bok has a white patch on the face and one on the sides, with
+a curiously distinctive white stripe above the tail, which is nearly
+concealed when the animal is at rest by a fold of skin but comes into
+full view when it is in motion, being thus quite analogous to the
+upturned white tail of the rabbit. In the pallah the white rump-mark is
+bordered with black, and the peculiar shape of the horns distinguishes
+it when seen from the front. The sable-antelope, the gems-bok, the oryx,
+the hart-beest, the bonte-bok, and the addax have each peculiar white
+markings; and they are besides characterised by horns so remarkably
+different in each species and so conspicuous, that it seems probable
+that the peculiarities in length, twist, and curvature have been
+differentiated for the purpose of recognition, rather than for any
+speciality of defence in species whose general habits are so similar.
+
+[Illustration: FIG. 18.--Gazella soemmerringi.]
+
+It is interesting to note that these markings for recognition are very
+slightly developed in the antelopes of the woods and marshes. Thus, the
+grys-bok is nearly uniform in colour, except the long black-tipped ears;
+and it frequents the wooded mountains. The duyker-bok and the rhoode-bok
+are wary bush-haunters, and have no marks but the small white patch
+behind. The wood-haunting bosch-bok goes in pairs, and has hardly any
+distinctive marks on its dusky chestnut coat, but the male alone is
+horned. The large and handsome koodoo frequents brushwood, and its
+vertical white stripes are no doubt protective, while its magnificent
+spiral horns afford easy recognition. The eland, which is an inhabitant
+of the open country, is uniformly coloured, being sufficiently
+recognisable by its large size and distinctive form; but the Derbyan
+eland is a forest animal, and has a protectively striped coat. In like
+manner, the fine Speke's antelope, which lives entirely in the swamps
+and among reeds, has pale vertical stripes on the sides (protective),
+with white markings on face and breast for recognition. An inspection of
+the figures of antelopes and other animals in Wood's _Natural History_,
+or in other illustrated works, will give a better idea of the
+peculiarities of recognition markings than any amount of description.
+
+Other examples of such coloration are to be seen in the dusky tints of
+the musk-sheep and the reindeer, to whom recognition at a distance on
+the snowy plains is of more importance than concealment from their few
+enemies. The conspicuous stripes and bands of the zebra and the quagga
+are probably due to the same cause, as may be the singular crests and
+face-marks of several of the monkeys and lemurs.[85]
+
+[Illustration: FIG. 19--Recognition marks of three African plovers.]
+
+Among birds, these recognition marks are especially numerous and
+suggestive. Species which inhabit open districts are usually
+protectively coloured; but they generally possess some distinctive
+markings for the purpose of being easily recognised by their kind, both
+when at rest and during flight. Such are, the white bands or patches on
+the breast or belly of many birds, but more especially the head and neck
+markings in the form of white or black caps, collars, eye-marks or
+frontal patches, examples of which are seen in the three species of
+African plovers figured on page 221.
+
+Recognition marks during flight are very important for all birds which
+congregate in flocks or which migrate together; and it is essential
+that, while being as conspicuous as possible, the marks shall not
+interfere with the general protective tints of the species when at rest.
+Hence they usually consist of well-contrasted markings on the wings and
+tail, which are concealed during repose but become fully visible when
+the bird takes flight. Such markings are well seen in our four British
+species of shrikes, each having quite different white marks on the
+expanded wings and on the tail feathers; and the same is the case with
+our three species of Saxicola--the stone-chat, whin-chat, and
+wheat-ear--which are thus easily recognisable on the wing, especially
+when seen from above, as they would be by stragglers looking out for
+their companions. The figures opposite, of the wings of two African
+species of stone-curlew which are sometimes found in the same districts,
+well illustrates these specific recognition marks. Though not very
+greatly different to our eyes, they are no doubt amply so to the sharp
+vision of the birds themselves.
+
+Besides the white patches on the primaries here shown, the secondary
+feathers are, in some cases, so coloured as to afford very distinctive
+markings during flight, as seen in the central secondary quills of two
+African coursers (Fig. 21).
+
+[Illustration: FIG. 20.--Oedicnemus vermiculatus (above). Oe.
+senegalensis (below).]
+
+Most characteristic of all, however, are the varied markings of the
+outer tail-feathers, whose purpose is so well shown by their being
+almost always covered during repose by the two middle feathers, which
+are themselves quite unmarked and protectively tinted like the rest of
+the upper surface of the body. The figures of the expanded tails of two
+species of East Asiatic snipe, whose geographical ranges overlap each
+other, will serve to illustrate this difference; which is frequently
+much greater and modified in an endless variety of ways (Fig. 22).
+
+Numbers of species of pigeons, hawks, finches, warblers, ducks, and
+innumerable other birds possess this class of markings; and they
+correspond so exactly in general character with those of the mammalia,
+already described, that we cannot doubt they serve a similar
+purpose.[86]
+
+[Illustration: FIG. 21.--Secondary quills.]
+
+[Illustration: FIG. 22.--Scolopax megala (upper). S. stenura (lower).]
+
+Those birds which are inhabitants of tropical forests, and which need
+recognition marks that shall be at all times visible among the dense
+foliage, and not solely or chiefly during flight, have usually small but
+brilliant patches of colour on the head or neck, often not interfering
+with the generally protective character of their plumage. Such are the
+bright patches of blue, red, or yellow, by which the usually green
+Eastern barbets are distinguished; and similar bright patches of colour
+characterise the separate species of small green fruit-doves. To this
+necessity for specialisation in colour, by which each bird may easily
+recognise its kind, is probably due that marvellous variety in the
+peculiar beauties of some groups of birds. The Duke of Argyll, speaking
+of the humming birds, made the objection that "A crest of topaz is no
+better in the struggle for existence than a crest of sapphire. A frill
+ending in spangles of the emerald is no better in the battle of life
+than a frill ending in spangles of the ruby. A tail is not affected for
+the purposes of flight, whether its marginal or its central feathers are
+decorated with white;" and he goes on to urge that mere beauty and
+variety for their own sake are the only causes of these differences.
+But, on the principles here suggested, the divergence itself is useful,
+and must have been produced _pari passu_ with the structural differences
+on which the differentiation of species depends; and thus we have
+explained the curious fact that prominent differences of colour often
+distinguish species otherwise very closely allied to each other.
+
+Among insects, the principle of distinctive coloration for recognition
+has probably been at work in the production of the wonderful diversity
+of colour and marking we find everywhere, more especially among the
+butterflies and moths; and here its chief function may have been to
+secure the pairing together of individuals of the same species. In some
+of the moths this has been secured by a peculiar odour, which attracts
+the males to the females from a distance; but there is no evidence that
+this is universal or even general, and among butterflies, especially,
+the characteristic colour and marking, aided by size and form, afford
+the most probable means of recognition. That this is so is shown by the
+fact that "the common white butterfly often flies down to a bit of paper
+on the ground, no doubt mistaking it for one of its own species;" while,
+according to Mr. Collingwood, in the Malay Archipelago, "a dead
+butterfly pinned upon a conspicuous twig will often arrest an insect of
+the same species in its headlong flight, and bring it down within easy
+reach of the net, especially if it be of the opposite sex."[87] In a
+great number of insects, no doubt, form, motions, stridulating sounds,
+or peculiar odours, serve to distinguish allied species from each other,
+and this must be especially the case with nocturnal insects, or with
+those whose colours are nearly uniform and are determined by the need of
+protection; but by far the larger number of day-flying and active
+insects exhibit varieties of colour and marking, forming the most
+obvious distinction between allied species, and which have, therefore,
+in all probability been acquired in the process of differentiation for
+the purpose of checking the intercrossing of closely allied forms.[88]
+
+Whether this principle extends to any of the less highly organised
+animals is doubtful, though it may perhaps have affected the higher
+mollusca. But in marine animals it seems probable that the colours,
+however beautiful, varied, and brilliant they may often be, are in most
+cases protective, assimilating them to the various bright-coloured
+seaweeds, or to some other animals which it is advantageous for them to
+imitate.[89]
+
+
+_Summary of the Preceding Exposition._
+
+Before proceeding to discuss some of the more recondite phenomena of
+animal coloration, it will be well to consider for a moment the extent
+of the ground we have already covered. Protective coloration, in some of
+its varied forms, has not improbably modified the appearance of one-half
+of the animals living on the globe. The white of arctic animals, the
+yellowish tints of the desert forms, the dusky hues of crepuscular and
+nocturnal species, the transparent or bluish tints of oceanic creatures,
+represent a vast host in themselves; but we have an equally numerous
+body whose tints are adapted to tropical foliage, to the bark of trees,
+or to the soil or dead leaves on or among which they habitually live.
+Then we have the innumerable special adaptations to the tints and forms
+of leaves, or twigs, or flowers; to bark or moss; to rock or pebble; by
+which such vast numbers of the insect tribes obtain protection; and we
+have seen that these various forms of coloration are equally prevalent
+in the waters of the seas and oceans, and are thus coextensive with the
+domain of life upon the earth. The comparatively small numbers which
+possess "terrifying" or "alluring" coloration may be classed under the
+general head of the protectively coloured.
+
+But under the next head--colour for recognition--we have a totally
+distinct category, to some extent antagonistic or complementary to the
+last, since its essential principle is visibility rather than
+concealment. Yet it has been shown, I think, that this mode of
+coloration is almost equally important, since it not only aids in the
+preservation of existing species and in the perpetuation of pure races,
+but was, perhaps, in its earlier stages, a not unimportant factor in
+their development. To it we owe most of the variety and much of the
+beauty in the colours of animals; it has caused at once bilateral
+symmetry and general permanence of type; and its range of action has
+been perhaps equally extensive with that of coloration for concealment.
+
+
+_Influence of Locality or of Climate on Colour._
+
+Certain relations between locality and coloration have long been
+noticed. Mr. Gould observed that birds from inland or continental
+localities were more brightly coloured than those living near the
+sea-coast or on islands, and he supposed that the more brilliant
+atmosphere of the inland stations was the explanation of the
+phenomenon.[90] Many American naturalists have observed similar facts,
+and they assert that the intensity of the colours of birds and mammals
+increases from north to south, and also with the increase of humidity.
+This change is imputed by Mr. J.A. Allen to the direct action of the
+environment. He says: "In respect to the correlation of intensity of
+colour in animals with the degree of humidity, it would perhaps be more
+in accordance with cause and effect to express the law of correlation as
+a _decrease_ of intensity of colour with a _decrease_ of humidity, the
+paleness evidently resulting from exposure and the blanching effect of
+intense sunlight, and a dry, often intensely heated atmosphere. With the
+decrease of the aqueous precipitation the forest growth and the
+protection afforded by arborescent vegetation gradually also decreases,
+as of course does also the protection afforded by clouds, the
+excessively humid regions being also regions of extreme cloudiness,
+while the dry regions are comparatively cloudless districts."[91] Almost
+identical changes occur in birds, and are imputed by Mr. Allen to
+similar causes.
+
+It will be seen that Mr. Gould and Mr. Allen impute opposite effects to
+the same cause, brilliancy or intensity of colour being due to a
+brilliant atmosphere according to the former, while paleness of colour
+is imputed by the latter to a too brilliant sun. According to the
+principles which have been established by the consideration of arctic,
+desert, and forest animals respectively, we shall be led to conclude
+that there has been no direct action in this case, but that the effects
+observed are due to the greater or less need of protection. The pale
+colour that is prevalent in arid districts is in harmony with the
+general tints of the surface; while the brighter tints or more intense
+coloration, both southward and in humid districts, are sufficiently
+explained by the greater shelter due to a more luxuriant vegetation and
+a shorter winter. The advocates of the theory that intensity of light
+directly affects the colours of organisms, are led into perpetual
+inconsistencies. At one time the brilliant colours of tropical birds and
+insects are imputed to the intensity of a tropical sun, while the same
+intensity of sunlight is now said to have a "bleaching" effect. The
+comparatively dull and sober hues of our northern fauna were once
+supposed to be the result of our cloudy skies; but now we are told that
+cloudy skies and a humid atmosphere intensify colour.
+
+In my _Tropical Nature_ (pp. 257-264) I have called attention to what is
+perhaps the most curious and decided relation of colour to locality
+which has yet been observed--the prevalence of white markings in the
+butterflies and birds of islands.
+
+So many cases are adduced from so many different islands, both in the
+eastern and western hemisphere, that it is impossible to doubt the
+existence of some common cause; and it seems probable to me now, after a
+fuller consideration of the whole subject of colour, that here too we
+have one of the almost innumerable results of the principle of
+protective coloration. White is, as a rule, an uncommon colour in
+animals, but probably only because it is so conspicuous. Whenever it
+becomes protective, as in the case of arctic animals and aquatic birds,
+it appears freely enough; while we know that white varieties of many
+species occur occasionally in the wild state, and that, under
+domestication, white or parti-coloured breeds are freely produced. Now
+in all the islands in which exceptionally white-marked birds and
+butterflies have been observed, we find two features which would tend to
+render the conspicuous white markings less injurious--a luxuriant
+tropical vegetation, and a decided scarcity of rapacious mammals and
+birds. White colours, therefore, would not be eliminated by natural
+selection; but variations in this direction would bear their part in
+producing the recognition marks which are everywhere essential, and
+which, in these islands, need not be so small or so inconspicuous as
+elsewhere.
+
+
+_Concluding Remarks._
+
+On a review of the whole subject, then, we must conclude that there is
+no evidence of the individual or prevalent colours of organisms being
+directly determined by the amount of light, or heat, or moisture, to
+which they are exposed; while, on the other hand, the two great
+principles of the need of concealment from enemies or from their prey,
+and of recognition by their own kind, are so wide-reaching in their
+application that they appear at first sight to cover almost the whole
+ground of animal coloration. But, although they are indeed wonderfully
+general and have as yet been very imperfectly studied, we are acquainted
+with other modes of coloration which have a different origin. These
+chiefly appertain to the very singular class of warning colours, from
+which arise the yet more extraordinary phenomena of mimicry; and they
+open up so curious a field of inquiry and present so many interesting
+problems, that a chapter must be devoted to them. Yet another chapter
+will be required by the subject of sexual differentiation of colour and
+ornament, as to the origin and meaning of which I have arrived at
+different conclusions from Mr. Darwin. These various forms of coloration
+having been discussed and illustrated, we shall be in a position to
+attempt a brief sketch of the fundamental laws which have determined the
+general coloration of the animal world.
+
+FOOTNOTES:
+
+[Footnote 65: _Proceedings of the Royal Society_, No. 243, 1886;
+_Transactions of the Royal Society_, vol. clxxviii. B. pp. 311-441.]
+
+[Footnote 66: _A Naturalist's Wanderings in the Eastern Archipelago_, p.
+460.]
+
+[Footnote 67: _Trans. Phil. Soc._ (? _of S. Africa_), 1878, part iv, p.
+27.]
+
+[Footnote 68: _Proc. Zool. Soc._, 1862 p. 357.]
+
+[Footnote 69: With reference to this general resemblance of insects to
+their environment the following remarks by Mr. Poulton are very
+instructive. He says: "Holding the larva of Sphinx ligustri in one hand
+and a twig of its food-plant in the other, the wonder we feel is, not at
+the resemblance but at the difference; we are surprised at the
+difficulty experienced in detecting so conspicuous an object. And yet
+the protection is very real, for the larvae will be passed over by those
+who are not accustomed to their appearance, although the searcher may be
+told of the presence of a large caterpillar. An experienced entomologist
+may also fail to find the larvae till after a considerable search. This
+is general protective resemblance, and it depends upon a general harmony
+between the appearance of the organism and its whole environment. It is
+impossible to understand the force of this protection for any larva,
+without seeing it on its food-plant and in an entirely normal condition.
+The artistic effect of green foliage is more complex than we often
+imagine; numberless modifications are wrought by varied lights and
+shadows upon colours which are in themselves far from uniform. In the
+larva of Papilio machaon the protection is very real when the larva is
+on the food-plant, and can hardly be appreciated at all when the two are
+apart." Numerous other examples are given in the chapter on "Mimicry and
+other Protective Resemblances among Animals," in my _Contributions to
+the Theory of Natural Selection_.]
+
+[Footnote 70: _The Naturalist in Nicaragua_, p. 19.]
+
+[Footnote 71: R. Meldola, in _Proc. Zool. Soc._, 1873, p. 155.]
+
+[Footnote 72: _Nature_, vol. iii. p. 166.]
+
+[Footnote 73: _Trans. Ent. Soc. Lond._, 1878, p. 185.]
+
+[Footnote 74: _Ibid._ (_Proceedings_, p. xlii.)]
+
+[Footnote 75: Wallace's _Malay Archipelago_, vol. i. p. 204 (fifth
+edition, p. 130), with figure.]
+
+[Footnote 76: Moseley's _Notes by a Naturalist on the Challenger_.]
+
+[Footnote 77: _Proceedings of the Boston Soc. of Nat. Hist._, vol. xiv.
+1871.]
+
+[Footnote 78: _Nature_, 1870, p. 376.]
+
+[Footnote 79: _A Naturalist's Wanderings in the Eastern Archipelago_, p.
+63.]
+
+[Footnote 80: A beautiful drawing of this rare insect, Hymenopus
+bicornis (in the nymph or active pupa state), was kindly sent me by Mr.
+Wood-Mason, Curator of the Indian Museum at Calcutta. A species, very
+similar to it, inhabits Java, where it is said to resemble a pink
+orchid. Other Mantidae, of the genus Gongylus, have the anterior part of
+the thorax dilated and coloured either white, pink, or purple; and they
+so closely resemble flowers that, according to Mr. Wood-Mason, one of
+them, having a bright violet-blue prothoracic shield, was found in Pegu
+by a botanist, and was for a moment mistaken by him for a flower. See
+_Proc. Ent. Soc. Lond._, 1878, p. liii.]
+
+[Footnote 81: C. Dixon, in Seebohm's _History of British Birds_, vol.
+ii. Introduction, p. xxvi. Many of the other examples here cited are
+taken from the same valuable work.]
+
+[Footnote 82: See A.H.S. Lucas, in _Proceedings of Royal Society of
+Victoria_, 1887, p. 56.]
+
+[Footnote 83: Professor Wm.H. Brewer of Yale College has shown that the
+white marks or the spots of domesticated animals are rarely symmetrical,
+but have a tendency to appear more frequently on the left side. This is
+the case with horses, cattle, dogs, and swine. Among wild animals the
+skunk varies considerably in the amount of white on the body, and this
+too was found to be usually greatest on the left side. A close
+examination of numerous striped or spotted species, as tigers, leopards,
+jaguars, zebras, etc., showed that the bilateral symmetry was not exact,
+although the general effect of the two sides was the same. This is
+precisely what we should expect if the symmetry is not the result of a
+general law of the organisation, but has been, in part at least,
+produced and preserved for the useful purpose of recognition by the
+animal's fellows of the same species, and especially by the sexes and
+the young. See _Proc. of the Am. Ass. for Advancement of Science_, vol.
+xxx. p. 246.]
+
+[Footnote 84: _Descent of Man_, p. 542.]
+
+[Footnote 85: It may be thought that such extremely conspicuous markings
+as those of the zebra would be a great danger in a country abounding
+with lions, leopards, and other beasts of prey; but it is not so. Zebras
+usually go in bands, and are so swift and wary that they are in little
+danger during the day. It is in the evening, or on moonlight nights,
+when they go to drink, that they are chiefly exposed to attack; and Mr.
+Francis Galton, who has studied these animals in their native haunts,
+assures me, that in twilight they are not at all conspicuous, the
+stripes of white and black so merging together into a gray tint that it
+is very difficult to see them at a little distance. We have here an
+admirable illustration of how a glaringly conspicuous style of marking
+for recognition may be so arranged as to become also protective at the
+time when protection is most needed; and we may also learn how
+impossible it is for us to decide on the inutility of any kind of
+coloration without a careful study of the habits of the species in its
+native country.]
+
+[Footnote 86: The principle of colouring for recognition was, I believe,
+first stated in my article on "The Colours of Animals and Plants" in
+Macmillan's _Magazine_, and more fully in my volume on _Tropical
+Nature_. Subsequently Mrs. Barber gave a few examples under the head of
+"Indicative or Banner Colours," but she applied it to the distinctive
+colours of the males of birds, which I explain on another principle,
+though this may assist.]
+
+[Footnote 87: Quoted by Darwin in _Descent of Man_, p. 317.]
+
+[Footnote 88: In the _American Naturalist_ of March 1888, Mr. J.E. Todd
+has an article on "Directive Coloration in Animals," in which he
+recognises many of the cases here referred to, and suggests a few
+others, though I think he includes many forms of coloration--as
+"paleness of belly and inner side of legs"--which do not belong to this
+class.]
+
+[Footnote 89: For numerous examples of this protective colouring of
+marine animals see Moseley's _Voyage of the Challenger_, and Dr. E.S.
+Morse in _Proc. of Bost. Soc. of Nat. Hist._, vol. xiv. 1871.]
+
+[Footnote 90: See _Origin of Species_, p. 107.]
+
+[Footnote 91: The "Geographical Variation of North American Squirrels,"
+_Proc. Bost. Soc. of Nat. Hist._, 1874, p. 284; and _Mammals and Winter
+Birds of Florida_, pp. 233-241.]
+
+
+
+
+CHAPTER IX
+
+WARNING COLORATION AND MIMICRY
+
+
+    The skunk as an example of warning coloration--Warning colours
+    among insects--Butterflies--Caterpillars--Mimicry--How mimicry
+    has been produced--Heliconidae--Perfection of the
+    imitation--Other cases of mimicry among Lepidoptera--Mimicry
+    among protected groups--Its explanation--Extension of the
+    principle--Mimicry in other orders of insects--Mimicry among the
+    vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among
+    birds--Objections to the theory of mimicry--Concluding remarks
+    on warning colours and mimicry.
+
+
+
+We have now to deal with a class of colours which are the very opposite
+of those we have hitherto considered, since, instead of serving to
+conceal the animals that possess them or as recognition marks to their
+associates, they are developed for the express purpose of rendering the
+species conspicuous. The reason of this is that the animals in question
+are either the possessors of some deadly weapons, as stings or poison
+fangs, or they are uneatable, and are thus so disagreeable to the usual
+enemies of their kind that they are never attacked when their peculiar
+powers or properties are known. It is, therefore, important that they
+should not be mistaken for defenceless or eatable species of the same
+class or order, since in that case they might suffer injury, or even
+death, before their enemies discovered the danger or the uselessness of
+the attack. They require some signal or danger-flag which shall serve as
+a warning to would-be enemies not to attack them, and they have usually
+obtained this in the form of conspicuous or brilliant coloration, very
+distinct from the protective tints of the defenceless animals allied to
+them.
+
+_The Skunk as illustrating Warning Coloration._
+
+While staying a few days, in July 1887, at the Summit Hotel on the
+Central Pacific Railway, I strolled out one evening after dinner, and on
+the road, not fifty yards from the house, I saw a pretty little white
+and black animal with a bushy tail coming towards me. As it came on at a
+slow pace and without any fear, although it evidently saw me, I thought
+at first that it must be some tame creature, when it suddenly occurred
+to me that it was a skunk. It came on till within five or six yards of
+me, then quietly climbed over a dwarf wall and disappeared under a small
+outhouse, in search of chickens, as the landlord afterwards told me.
+This animal possesses, as is well known, a most offensive secretion,
+which it has the power of ejecting over its enemies, and which
+effectually protects it from attack. The odour of this substance is so
+penetrating that it taints, and renders useless, everything it touches,
+or in its vicinity. Provisions near it become uneatable, and clothes
+saturated with it will retain the smell for several weeks, even though
+they are repeatedly washed and dried. A drop of the liquid in the eyes
+will cause blindness, and Indians are said not unfrequently to lose
+their sight from this cause. Owing to this remarkable power of offence
+the skunk is rarely attacked by other animals, and its black and white
+fur, and the bushy white tail carried erect when disturbed, form the
+danger-signals by which it is easily distinguished in the twilight or
+moonlight from unprotected animals. Its consciousness that it needs only
+to be seen to be avoided gives it that slowness of motion and
+fearlessness of aspect which are, as we shall see, characteristic of
+most creatures so protected.
+
+
+_Warning Colours among Insects._
+
+It is among insects that warning colours are best developed, and most
+abundant. We all know how well marked and conspicuous are the colours
+and forms of the stinging wasps and bees, no one of which in any part of
+the world is known to be protectively coloured like the majority of
+defenceless insects. Most of the great tribe of Malacoderms among
+beetles are distasteful to insect-eating animals. Our red and black
+Telephoridae, commonly called "soldiers and sailors," were found, by Mr.
+Jenner Weir, to be refused by small birds. These and the allied
+Lampyridae (the fireflies and glow-worms) in Nicaragua, were rejected by
+Mr. Belt's tame monkey and by his fowls, though most other insects were
+greedily eaten by them. The Coccinellidae or lady-birds are another
+uneatable group, and their conspicuous and singularly spotted bodies
+serve to distinguish them at a glance from all other beetles.
+
+These uneatable insects are probably more numerous than is supposed,
+although we already know immense numbers that are so protected. The most
+remarkable are the three families of butterflies--Heliconidae, Danaidae,
+and Acraeidae--comprising more than a thousand species, and
+characteristic respectively of the three great tropical regions--South
+America, Southern Asia, and Africa. All these butterflies have
+peculiarities which serve to distinguish them from every other group in
+their respective regions. They all have ample but rather weak wings, and
+fly slowly; they are always very abundant; and they all have conspicuous
+colours or markings, so distinct from those of other families that, in
+conjunction with their peculiar outline and mode of flight, they can
+usually be recognised at a glance. Other distinctive features are, that
+their colours are always nearly the same on the under surface of their
+wings as on the upper; they never try to conceal themselves, but rest on
+the upper surfaces of leaves or flowers; and, lastly, they all have
+juices which exhale a powerful scent, so that when one kills them by
+pinching the body, the liquid that exudes stains the fingers yellow, and
+leaves an odour that can only be removed by repeated washings.
+
+Now, there is much direct evidence to show that this odour, though not
+very offensive to us, is so to most insect-eating creatures. Mr. Bates
+observed that, when set out to dry, specimens of Heliconidae were less
+subject to the attacks of vermin; while both he and I noticed that they
+were not attacked by insect-eating birds or dragonflies, and that their
+wings were not found in the forest paths among the numerous wings of
+other butterflies whose bodies had been devoured. Mr. Belt once observed
+a pair of birds capturing insects for their young; and although the
+Heliconidae swarmed in the vicinity, and from their slow flight could
+have been easily caught, not one was ever pursued, although other
+butterflies did not escape. His tame monkey also, which would greedily
+munch up other butterflies, would never eat the Heliconidae. It would
+sometimes smell them, but always rolled them up in its hand and then
+dropped them.
+
+We have also some corresponding evidence as to the distastefulness of
+the Eastern Danaidae. The Hon. Mr. Justice Newton, who assiduously
+collected and took notes upon the Lepidoptera of Bombay, informed Mr.
+Butler of the British Museum that the large and swift-flying butterfly
+Charaxes psaphon, was continually persecuted by the bulbul, so that he
+rarely caught a specimen of this species which had not a piece snipped
+out of the hind wings. He offered one to a bulbul which he had in a
+cage, and it was greedily devoured, whilst it was only by repeated
+persecution that he succeeded in inducing the bird to touch a
+Danais.[92]
+
+Besides these three families of butterflies, there are certain groups of
+the great genus Papilio--the true swallow-tailed butterflies--which have
+all the characteristics of uneatable insects. They have a special
+coloration, usually red and black (at least in the females), they fly
+slowly, they are very abundant, and they possess a peculiar odour
+somewhat like that of the Heliconidae. One of these groups is common in
+tropical America, another in tropical Asia, and it is curious that,
+although not very closely allied, they have each the same red and black
+colours, and are very distinct from all the other butterflies of their
+respective countries. There is reason to believe also that many of the
+brilliantly coloured and weak-flying diurnal moths, like the fine
+tropical Agaristidae and burnet-moths, are similarly protected, and that
+their conspicuous colours serve as a warning of inedibility. The common
+burnet-moth (Anthrocera filipendula) and the equally conspicuous
+ragwort-moth (Euchelia jacobeae) have been proved to be distasteful to
+insect-eating creatures.
+
+The most interesting and most conclusive example of warning coloration
+is, however, furnished by caterpillars, because in this case the facts
+have been carefully ascertained experimentally by competent observers.
+In the year 1866, when Mr. Darwin was collecting evidence as to the
+supposed effect of sexual selection in bringing about the brilliant
+coloration of the higher animals, he was struck by the fact that many
+caterpillars have brilliant and conspicuous colours, in the production
+of which sexual selection could have no place. We have numbers of such
+caterpillars in this country, and they are characterised not only by
+their gay colours but by not concealing themselves. Such are the mullein
+and the gooseberry caterpillars, the larvae of the spurge hawk-moth, of
+the buff-tip, and many others. Some of these caterpillars are
+wonderfully conspicuous, as in the case of that noticed by Mr. Bates in
+South America, which was four inches long, banded across with black and
+yellow, and with bright red head, legs, and tail. Hence it caught the
+eye of any one who passed by, even at the distance of many yards.
+
+Mr. Darwin asked me to try and suggest some explanation of this
+coloration; and, having been recently interested in the question of the
+warning coloration of butterflies, I suggested that this was probably a
+similar case,--that these conspicuous caterpillars were distasteful to
+birds and other insect-eating creatures, and that their bright
+non-protective colours and habit of exposing themselves to view, enabled
+their enemies to distinguish them at a glance from the edible kinds and
+thus learn not to touch them; for it must be remembered that the bodies
+of caterpillars while growing are so delicate, that a wound from a
+bird's beak would be perhaps as fatal as if they were devoured.[93] At
+this time not a single experiment or observation had been made on the
+subject, but after I had brought the matter before the Entomological
+Society, two gentlemen, who kept birds and other tame animals, undertook
+to make experiments with a variety of caterpillars.
+
+Mr. Jenner Weir was the first to experiment with ten species of small
+birds in his aviary, and he found that none of them would eat the
+following smooth-skinned conspicuous caterpillars--Abraxas
+grossulariata, Diloba caeruleocephala, Anthrocera filipendula, and
+Cucullia verbasci. He also found that they would not touch any hairy or
+spiny larvae, and he was satisfied that it was not the hairs or the
+spines, but the unpleasant taste that caused them to be rejected,
+because in one case a young smooth larva of a hairy species, and in
+another case the pupa of a spiny larva, were equally rejected. On the
+other hand, all green or brown caterpillars as well as those that
+resemble twigs were greedily devoured.[94]
+
+Mr. A.G. Butler also made experiments with some green lizards (Lacerta
+viridis), which greedily ate all kinds of food, including flies of many
+kinds, spiders, bees, butterflies, and green caterpillars; but they
+would not touch the caterpillar of the gooseberry-moth (Abraxas
+grossulariata), or the imago of the burnet-moth (Anthrocera
+filipendula). The same thing happened with frogs. When the gooseberry
+caterpillars were first given to them, "they sprang forward and licked
+them eagerly into their mouths; no sooner, however, had they done so,
+than they seemed to become aware of the mistake that they had made, and
+sat with gaping mouths, rolling their tongues about, until they had got
+quit of the nauseous morsels, which seemed perfectly uninjured, and
+walked off as briskly as ever." Spiders seemed equally to dislike them.
+This and another conspicuous caterpillar (Halia wavaria) were rejected
+by two species--the geometrical garden spider (Epeira diadema) and a
+hunting spider.[95]
+
+Some further experiments with lizards were made by Professor Weismann,
+quite confirming the previous observations; and in 1886 Mr. E.B. Poulton
+of Oxford undertook a considerable series of experiments, with many
+other species of larvae and fresh kinds of lizards and frogs. Mr.
+Poulton then reviewed the whole subject, incorporating all recorded
+facts, as well as some additional observations made by Mr. Jenner Weir
+in 1886. More than a hundred species of larvae or of perfect insects of
+various orders have now been made the subject of experiment, and the
+results completely confirm my original suggestion. In almost every case
+the protectively coloured larvae have been greedily eaten by all kinds
+of insectivorous animals, while, in the immense majority of cases, the
+conspicuous, hairy, or brightly coloured larvae have been rejected by
+some or all of them. In some instances the inedibility of the larvae
+extends to the perfect insect, but not in others. In the former cases
+the perfect insect is usually adorned with conspicuous colours, as the
+burnet and ragwort moths; but in the case of the buff-tip, the moth
+resembles a broken piece of rotten stick, yet it is partly inedible,
+being refused by lizards. It is, however, very doubtful whether these
+are its chief enemies, and its protective form and colour may be needed
+against insectivorous birds or mammals.
+
+Mr. Samuel H. Scudder, who has largely bred North American butterflies,
+has found so many of the eggs and larvae destroyed by hymenopterous and
+dipterous parasites that he thinks at least nine-tenths, perhaps a
+greater proportion, never reach maturity. Yet he has never found any
+evidence that such parasites attack either the egg or the larva of the
+inedible Danais archippus, so that in this case the insect is
+distasteful to its most dangerous foes in all the stages of its
+existence, a fact which serves to explain its great abundance and its
+extension over almost the whole world.[96]
+
+One case has been found of a protectively coloured larva,--one,
+moreover, which in all its habits shows that it trusts to concealment to
+escape its enemies--which was yet always rejected by lizards after they
+had seized it, evidently under the impression that from its colour it
+would be eatable. This is the caterpillar of the very common moth Mania
+typica; and Mr. Poulton thinks that, in this case, the unpleasant taste
+is an incidental result of some physiological processes in the organism,
+and is itself a merely useless character. It is evident that the insect
+would not conceal itself so carefully as it does if it had not some
+enemies, and these are probably birds or small mammals, as its
+food-plants are said to be dock and willow-herb, not suggestive of
+places frequented by lizards; and it has been found by experiment that
+lizards and birds have not always the same likes and dislikes. The case
+is interesting, because it shows that nauseous fluids sometimes occur
+sporadically, and may thus be intensified by natural selection when
+required for the purpose of protection. Another exceptional case is
+that of the very conspicuous caterpillar of the spurge hawk-moth
+(Deilephila euphorbiae), which was at once eaten by a lizard, although,
+as it exposes itself on its food-plant in the daytime and is very
+abundant in some localities, it must almost certainly be disliked by
+birds or by some animals who would otherwise devour it. If disturbed
+while feeding it is said to turn round with fury and eject a quantity of
+green liquid, of an acid and disagreeable smell similar to that of the
+spurge milk, only worse.[97]
+
+These facts, and Mr. Poulton's evidence that some larvae rejected by
+lizards at first will be eaten if the lizards are very hungry, show that
+there are differences in the amount of the distastefulness, and render
+it probable that if other food were wanting many of these conspicuous
+insects would be eaten. It is the abundance of the eatable kinds that
+gives value to the inedibility of the smaller number; and this is
+probably the reason why so many insects rely on protective colouring
+rather than on the acquisition of any kind of defensive weapons. In the
+long run the powers of attack and defence must balance each other. Hence
+we see that even the powerful stings of bees and wasps only protect them
+against some enemies, since a tribe of birds, the bee-eaters, have been
+developed which feed upon them, and some frogs and lizards do so
+occasionally.
+
+The preceding outline will sufficiently explain the characteristics of
+"warning coloration" and the end it serves in nature. There are many
+other curious modifications of it, but these will be best appreciated
+after we have discussed the remarkable phenomenon of "mimicry," which is
+bound up with and altogether depends upon "warning colour," and is in
+some cases the chief indication we have of the possession of some
+offensive weapon to secure the safety of the species imitated.
+
+
+_Mimicry._
+
+This term has been given to a form of protective resemblance, in which
+one species so closely resembles another in external form and colouring
+as to be mistaken for it, although the two may not be really allied and
+often belong to distinct families or orders. One creature seems
+disguised in order to be made like another; hence the terms "mimic" and
+mimicry, which imply no voluntary action on the part of the imitator. It
+has long been known that such resemblances do occur, as, for example,
+the clear-winged moths of the families Sesiidae and Aegeriidae, many of
+which resemble bees, wasps, ichneumons, or saw-flies, and have received
+names expressive of the resemblance; and the parasitic flies (Volucella)
+which closely resemble bees, on whose larvae the larvae of the flies
+feed.
+
+The great bulk of such cases remained, however, unnoticed, and the
+subject was looked upon as one of the inexplicable curiosities of
+nature, till Mr. Bates studied the phenomenon among the butterflies of
+the Amazon, and, on his return home, gave the first rational explanation
+of it.[98] The facts are, briefly, these. Everywhere in that fertile
+region for the entomologist the brilliantly coloured Heliconidae abound,
+with all the characteristics which I have already referred to when
+describing them as illustrative of "warning coloration." But along with
+them other butterflies were occasionally captured, which, though often
+mistaken for them, on account of their close resemblance in form,
+colour, and mode of flight, were found on examination to belong to a
+very distinct family, the Pieridae. Mr. Bates notices fifteen distinct
+species of Pieridae, belonging to the genera Leptalis and Euterpe, each
+of which closely imitates some one species of Heliconidae, inhabiting
+the same region and frequenting the same localities. It must be
+remembered that the two families are altogether distinct in structure.
+The larvae of the Heliconidae are tubercled or spined, the pupae
+suspended head downwards, and the imago has imperfect forelegs in the
+male; while the larvae of the Pieridae are smooth, the pupae are
+suspended with a brace to keep the head erect, and the forefeet are
+fully developed in both sexes. These differences are as large and as
+important as those between pigs and sheep, or between swallows and
+sparrows; while English entomologists will best understand the case by
+supposing that a species of Pieris in this country was coloured and
+shaped like a small tortoise-shell, while another species on the
+Continent was equally like a Camberwell beauty--so like in both cases
+as to be mistaken when on the wing, and the difference only to be
+detected by close examination. As an example of the resemblance,
+woodcuts are given of one pair in which the colours are simple, being
+olive, yellow, and black, while the very distinct neuration of the wings
+and form of the head and body can be easily seen.
+
+[Illustration: FIG. 23.--Methona psidii (Heliconidae). Leptalis orise
+(Pieridae).]
+
+Besides these Pieridae, Mr. Bates found four true Papilios, seven
+Erycinidae, three Castnias (a genus of day-flying moths), and fourteen
+species of diurnal Bombycidae, all imitating some species of Heliconidae
+which inhabited the same district; and it is to be especially noted that
+none of these insects were so abundant as the Heliconidae they
+resembled, generally they were far less common, so that Mr. Bates
+estimated the proportion in some cases as not one to a thousand. Before
+giving an account of the numerous remarkable cases of mimicry in other
+parts of the world, and between various groups of insects and of higher
+animals, it will be well to explain briefly the use and purport of the
+phenomenon, and also the mode by which it has been brought about.
+
+
+_How Mimicry has been Produced._
+
+The fact has been now established that the Heliconidae possess an
+offensive odour and taste, which lead to their being almost entirely
+free from attack by insectivorous creatures; they possess a peculiar
+form and mode of flight, and do not seek concealment; while their
+colours--although very varied, ranging from deep blue-black, with white,
+yellow, or vivid red bands and spots, to the most delicate
+semitransparent wings adorned with pale brown or yellow markings--are
+yet always very distinctive, and unlike those of all the other families
+of butterflies in the same country. It is, therefore, clear that if any
+other butterflies in the same region, which are eatable and suffer great
+persecution from insectivorous animals, should come to resemble any of
+these uneatable species so closely as to be mistaken for them by their
+enemies, they will obtain thereby immunity from persecution. This is the
+obvious and sufficient reason why the imitation is useful, and therefore
+why it occurs in nature. We have now to explain how it has probably been
+brought about, and also why a still larger number of persecuted groups
+have not availed themselves of this simple means of protection.
+
+From the great abundance of the Heliconidae[99] all over tropical
+America, the vast number of their genera and species, and their marked
+distinctions from all other butterflies, it follows that they constitute
+a group of high antiquity, which in the course of ages has become more
+and more specialised, and owing to its peculiar advantages has now
+become a dominant and aggressive race. But when they first arose from
+some ancestral species or group which, owing to the food of the larvae
+or some other cause, possessed disagreeable juices that caused them to
+be disliked by the usual enemies of their kind, they were in all
+probability not very different either in form or coloration from many
+other butterflies. They would at that time be subject to repeated
+attacks by insect-eaters, and, even if finally rejected, would often
+receive a fatal injury. Hence arose the necessity for some
+distinguishing mark, by which the devourers of butterflies in general
+might learn that these particular butterflies were uneatable; and every
+variation leading to such distinction, whether by form, colour, or mode
+of flight, was preserved and accumulated by natural selection, till the
+ancestral Heliconoids became well distinguished from eatable
+butterflies, and thenceforth comparatively free from persecution. Then
+they had a good time of it. They acquired lazy habits, and flew about
+slowly. They increased abundantly and spread all over the country, their
+larvae feeding on many plants and acquiring different habits; while the
+butterflies themselves varied greatly, and colour being useful rather
+than injurious to them, gradually diverged into the many coloured and
+beautifully varied forms we now behold.
+
+But, during the early stages of this process, some of the Pieridae,
+inhabiting the same district, happened to be sufficiently like some of
+the Heliconidae to be occasionally mistaken for them. These, of course,
+survived while their companions were devoured. Those among their
+descendants that were still more like Heliconidae again survived, and at
+length the imitation would become tolerably perfect. Thereafter, as the
+protected group diverged into distinct species of many different
+colours, the imitative group would occasionally be able to follow it
+with similar variations,--a process that is going on now, for Mr. Bates
+informs us that in each fresh district he visited he found closely
+allied representative species or varieties of Heliconidae, and along
+with them species of Leptalis (Pieridae), which had varied in the same
+way so as still to be exact imitations. But this process of imitation
+would be subject to check by the increasing acuteness of birds and other
+animals which, whenever the eatable Leptalis became numerous, would
+surely find them out, and would then probably attack both these and
+their friends the Heliconidae in order to devour the former and reject
+the latter. The Pieridae would, however, usually be less numerous,
+because their larvae are often protectively coloured and therefore
+edible, while the larvae of the Heliconidae are adorned with warning
+colours, spines, or tubercles, and are uneatable. It seems probable that
+the larvae and pupae of the Heliconidae were the first to acquire the
+protective distastefulness, both because in this stage they are more
+defenceless and more liable to fatal injury, and also because we now
+find many instances in which the larvae are distasteful while the
+perfect insects are eatable, but I believe none in which the reverse is
+the case. The larvae of the Pieridae are now beginning to acquire
+offensive juices, but have not yet obtained the corresponding
+conspicuous colours; while the perfect insects remain eatable, except
+perhaps in some Eastern groups, the under sides of whose wings are
+brilliantly coloured although this is the part which is exposed when at
+rest.
+
+It is clear that if a large majority of the larvae of Lepidoptera, as
+well as the perfect insects, acquired these distasteful properties, so
+as seriously to diminish the food supply of insectivorous and nestling
+birds, these latter would be forced by necessity to acquire
+corresponding tastes, and to eat with pleasure what some of them now eat
+only under pressure of hunger; and variation and natural selection would
+soon bring about this change.
+
+Many writers have denied the possibility of such wonderful resemblances
+being produced by the accumulation of fortuitous variations, but if the
+reader will call to mind the large amount of variability that has been
+shown to exist in all organisms, the exceptional power of rapid increase
+possessed by insects, and the tremendous struggle for existence always
+going on, the difficulty will vanish, especially when we remember that
+nature has the same fundamental groundwork to act upon in the two
+groups, general similarity of forms, wings of similar texture and
+outline, and probably some original similarity of colour and marking.
+Yet there is evidently considerable difficulty in the process, or with
+these great resources at her command nature would have produced more of
+these mimicking forms than she has done. One reason of this deficiency
+probably is, that the imitators, being always fewer in number, have not
+been able to keep pace with the variations of the much more numerous
+imitated form; another reason may be the ever-increasing acuteness of
+the enemies, which have again and again detected the imposture and
+exterminated the feeble race before it has had time to become further
+modified. The result of this growing acuteness of enemies has been, that
+those mimics that now survive exhibit, as Mr. Bates well remarks, "a
+palpably intentional likeness that is perfectly staggering," and also
+"that those features of the portrait are most attended to by nature
+which produce the most effective deception when the insects are seen in
+nature." No one, in fact, can understand the perfection of the imitation
+who has not seen these species in their native wilds. So complete is it
+in general effect that in almost every box of butterflies, brought from
+tropical America by amateurs, are to be found some species of the
+mimicking Pieridae, Erycinidae, or moths, and the mimicked Heliconidae,
+placed together under the impression that they are the same species. Yet
+more extraordinary, it sometimes deceives the very insects themselves.
+Mr. Trimen states that the male Danais chrysippus is sometimes deceived
+by the female Diadema bolina which mimics that species. Dr. Fritz
+Müller, writing from Brazil to Professor Meldola, says, "One of the most
+interesting of our mimicking butterflies is Leptalis melite. The female
+alone of this species imitates one of our common white Pieridae, which
+she copies so well that even her own male is often deceived; for I have
+repeatedly seen the male pursuing the mimicked species, till, after
+closely approaching and becoming aware of his error, he suddenly
+returned."[100] This is evidently not a case of true mimicry, since the
+species imitated is not protected; but it may be that the less abundant
+Leptalis is able to mingle with the female Pieridae and thus obtain
+partial immunity from attack. Mr. Kirby of the insect department of the
+British Museum informs me that there are several species of South
+American Pieridae which the female Leptalis melite very nearly
+resembles. The case, however, is interesting as showing that the
+butterflies are themselves deceived by a resemblance which is not so
+great as that of some mimicking species.
+
+
+_Other Examples of Mimicry among Lepidoptera._
+
+In tropical Asia, and eastward to the Pacific Islands, the Danaidae take
+the place of the Heliconidae of America, in their abundance, their
+conspicuousness, their slow flight, and their being the subjects of
+mimicry. They exist under three principal forms or genera. The genus
+Euploea is the most abundant both in species and individuals, and
+consists of fine broad-winged butterflies of a glossy or metallic
+blue-black colour, adorned with pure white, or rich blue, or dusky
+markings situated round the margins of the wings. Danais has generally
+more lengthened wings, of a semitransparent greenish or a rich brown
+colour, with radial or marginal pale spots; while the fine Hestias are
+of enormous size, of a papery or semitransparent white colour, with
+dusky or black spots and markings. Each of these groups is mimicked by
+various species of the genus Papilio, usually with such accuracy that it
+is impossible to distinguish them on the wing.[101] Several species of
+Diadema, a genus of butterflies allied to our Vanessas, also mimic
+species of Danais, but in this case the females only are affected, a
+subject which will be discussed in another chapter.
+
+Another protected group in the Eastern tropics is that of the beautiful
+day-flying moths forming the family Agaristidae. These are usually
+adorned with the most brilliant colours or conspicuous markings, they
+fly slowly in forests among the butterflies and other diurnal insects,
+and their great abundance sufficiently indicates their possession of
+some distastefulness which saves them from attack. Under these
+conditions we may expect to find other moths which are not so protected
+imitating them, and this is the case. One of the common and wide-ranging
+species (Opthalmis lincea), found in the islands from Amboyna to New
+Ireland, is mimicked in a wonderful manner by one of the Liparidae (the
+family to which our common "tussock" and "vapourer" moths belong). This
+is a new species collected at Amboyna during the voyage of the
+_Challenger_, and has been named Artaxa simulans. Both insects are
+black, with the apex of the fore wings ochre coloured, and the outer
+half of the hind wings bright orange. The accompanying woodcuts (for the
+use of which I am indebted to Mr. John Murray of the _Challenger_
+Office) well exhibit their striking resemblance to each other.
+
+[Illustration: FIG. 24.--Opthalmis lincea (Agaristidae). Artaxa simulans
+(Liparidae).]
+
+In Africa exactly similar phenomena recur, species of Papilio and of
+Diadema mimicking Danaidae or Acraeidae with the most curious accuracy.
+Mr. Trimen, who studied this subject in South Africa, has recorded eight
+species or varieties of Diadema, and eight of Papilio, which each mimic
+some species of Danais; while eight species or varieties of Panopaea
+(another genus of Nymphalidae), three of Melanitis (Eurytelidae), and
+two of Papilio, resemble with equal accuracy some species of
+Acraea.[102] He has also independently observed the main facts on which
+the explanation of the phenomenon rests,--the unpleasant odour of the
+Danais and Acraea, extending to their larvae and pupae; their great
+abundance, slow flight, and disregard of concealment; and he states that
+while lizards, mantidae, and dragonflies all hunt butterflies, and the
+rejected wings are to be found abundantly at some of their
+feeding-places, those of the two genera Danais and Acraea were never
+among them.
+
+The two groups of the great genus Papilio (the true swallow-tailed
+butterflies) which have been already referred to as having the special
+characteristics of uneatable insects, have also their imitators in other
+groups; and thus, the belief in their inedibility--derived mainly from
+their style of warning coloration and their peculiar habits--is
+confirmed. In South America, several species of the "Aeneas" group of
+these butterflies are mimicked by Pieridae and by day-flying moths of
+the genera Castnia and Pericopis. In the East, Papilio hector, P.
+diphilus, and P. liris, all belonging to the inedible group, are
+mimicked by the females of other species of Papilio belonging to very
+distinct groups; while in Northern India and China, many fine day-flying
+moths (Epicopeia) have acquired the strange forms and peculiar colours
+of some of the large inedible Papilios of the same regions.
+
+In North America, the large and handsome Danais archippus, with rich
+reddish-brown wings, is very common; and it is closely imitated by
+Limenitis misippus, a butterfly allied to our "white admiral," but which
+has acquired a colour quite distinct from that of the great bulk of its
+allies. In the same country there is a still more interesting case. The
+beautiful dark bronzy green butterfly, Papilio philenor, is inedible
+both in larva and perfect insect, and it is mimicked by the equally dark
+Limenitis ursula. There is also in the Southern and Western States a
+dark female form of the yellow Papilio turnus, which in all probability
+obtains protection from its general resemblance to P. philenor. Mr. W.H.
+Edwards has found, by extensive experiment, that both the dark and
+yellow females produce their own kinds, with very few exceptions; and he
+thinks that the dark form has the advantage in the more open regions and
+in the prairies, where insectivorous birds abound. But in open country
+the dark form would be quite as conspicuous as the yellow form, if not
+more so, so that the resemblance to an inedible species would be there
+more needed.[103]
+
+The only probable case of mimicry in this country is that of the moth,
+Diaphora mendica, whose female only is white, while the larva is of
+protective colours, and therefore almost certainly edible. A much more
+abundant moth, of about the same size and appearing about the same time,
+is Spilosoma menthrasti, also white, but in this case both it and its
+larva have been proved to be inedible. The white colour of the female
+Diaphora, although it must be very conspicuous at night, may, therefore,
+have been acquired in order to resemble the uneatable Spilosoma, and
+thus gain some protection.[104]
+
+
+_Mimicry among Protected (Uneatable) Genera._
+
+Before giving some account of the numerous other cases of warning
+colours and of mimicry that occur in the animal kingdom, it will be well
+to notice a curious phenomenon which long puzzled entomologists, but
+which has at length received a satisfactory explanation.
+
+We have hitherto considered, that mimicry could only occur when a
+comparatively scarce and much persecuted species obtained protection by
+its close external resemblance to a much more abundant uneatable species
+inhabiting its own district; and this rule undoubtedly prevails among
+the great majority of mimicking species all over the world. But Mr.
+Bates also found a number of pairs of species of different genera of
+Heliconidae, which resembled each other quite as closely as did the
+other mimicking species he has described; and since all these insects
+appear to be equally protected by their inedibility, and to be equally
+free from persecution, it was not easy to see why this curious
+resemblance existed, or how it had been brought about. That it is not
+due to close affinity is shown by the fact that the resemblance occurs
+most frequently between the two distinct sub-families into which (as Mr.
+Bates first pointed out) the Heliconidae are naturally divided on
+account of very important structural differences. One of these
+sub-families (the true Heliconinae) consists of two genera only,
+Heliconius and Eueides, the other (the Danaoid Heliconinae) of no less
+than sixteen genera; and, in the instances of mimicry we are now
+discussing, one of the pairs or triplets that resemble each other is
+usually a species of the large and handsome genus Heliconius, the others
+being species of the genera Mechanitis, Melinaea, or Tithorea, though
+several species of other Danaoid genera also imitate each other. The
+following lists will give some idea of the number of these curious
+imitative forms, and of their presence in every part of the Neotropical
+area. The bracketed species are those that resemble each other so
+closely that the difference is not perceptible when they are on the
+wing.
+
+In the Lower Amazon region are found--
+
+
+    { Heliconius sylvana.
+    { Melinaea egina.
+
+    { Heliconius numata.
+    { Melinaea mneme.
+    { Tithorea harmonia.
+
+    { Methona psidii.
+    { Thyridia ino.
+
+    { Ceratina ninonia.
+    { Melinaea mnasias.
+
+
+In Central America are found--
+
+
+              { Heliconius zuleika.
+    Nicaragua { Melinaea hezia.
+              { Mechanitis sp.
+
+              { Heliconius formosus.
+              { Tithorea penthias.
+
+    Guatemala { Heliconius telchina.
+              { Melinaea imitata.
+
+
+
+In the Upper Amazon region--
+
+
+    { Heliconius pardalinus.
+    { Melinaea pardalis.
+
+    { Heliconius aurora.
+    { Melinaea lucifer.
+
+
+In New Grenada--
+
+
+    { Heliconius ismenius.
+    { Melinaea messatis.
+
+    { Heliconius messene.
+    { Melinaea mesenina.
+    { (?) Mechanitis sp.
+
+    { Heliconius hecalesia.
+    { Tithorea hecalesina.
+
+    { Heliconius hecuba.
+    { Tithorea bonplandi.
+
+
+In Eastern Peru and Bolivia--
+
+
+    { Heliconius aristona.
+    { Melinaea cydippe.
+    { (?) Mechanitis mothone.
+
+
+In Pernambuco--
+
+
+    { Heliconius ethra.
+    { Mechanitis nesaea.
+
+
+In Rio Janeiro--
+
+
+    { Helieonius eucrate.
+    { Mechanitis lysimnia.
+
+
+In South Brazil--
+
+
+    { Thyridia megisto.
+    { Ituna ilione.
+
+    { Acraea thalia.
+    { Eueides pavana.
+
+
+Besides these, a number of species of Ithomia and Napeogenes, and of
+Napeogenes and Mechanitis, resemble each other with equal accuracy, so
+that they are liable to be mistaken for each other when on the wing; and
+no doubt many other equally remarkable cases are yet unnoticed.
+
+[Illustration: FIG. 25.--Wings of Ituna Ilione, female. Wings of
+Thyridia megisto, female.]
+
+The figures above of the fore and hind wings of two of these mimicking
+species, from Dr. Fritz Müller's original paper in _Kosmos_, will serve
+to show the considerable amount of difference, in the important
+character of the neuration of the wings, between these butterflies,
+which really belong to very distinct and not at all closely allied
+genera. Other important characters are--(1) The existence of a small
+basal cell in the hind wings of Ituna which is wanting in Thyridia; (2)
+the division of the cell between the veins 1_b_ and 2 of the hind wings
+in the former genus, while it is undivided in the latter; and (3) the
+existence in Thyridia of scent-producing tufts of hair on the upper edge
+of the hind wing, while in Ituna these are wanting; but in place of them
+are extensible processes at the end of the abdomen, also emitting a
+powerful scent. These differences characterise two marked subdivisions
+of the Danaoid Heliconinae, each containing several distinct genera; and
+these subdivisions are further distinguished by very different forms of
+larvae, that to which Ituna belongs having from two to four long
+threadlike tentacles on the back, while in that containing Thyridia
+these are always absent. The former usually feed on Asclepiadeae, the
+latter on Solanaceae or Scrophulariaceae.
+
+The two species figured, though belonging to such distinct and even
+remote genera, have acquired almost identical tints and markings so as
+to be deceptively alike. The surface of the wings is, in both,
+transparent yellowish, with black transverse bands and white marginal
+spots, while both have similar black-and white-marked bodies and long
+yellow antennae. Dr. Müller states that they both show a preference for
+the same flowers growing on the edges of the forest paths.[105]
+
+We will now proceed to give the explanation of these curious
+similarities, which have remained a complete puzzle for twenty years.
+Mr. Bates, when first describing them, suggested that they might be due
+to some form of parallel variation dependent on climatic influences; and
+I myself adduced other cases of coincident local modifications of
+colour, which did not appear to be explicable by any form of
+mimicry.[106] But we neither of us hit upon the simple explanation given
+by Dr. Fritz Müller in 1879.
+
+His theory is founded on the assumed, but probable, fact, that
+insect-eating birds only learn by experience to distinguish the edible
+from the inedible butterflies, and in doing so necessarily sacrifice a
+certain number of the latter. The quantity of insectivorous birds in
+tropical America is enormous; and the number of young birds which every
+year have to learn wisdom by experience, as regards the species of
+butterflies to be caught or to be avoided, is so great that the
+sacrifice of life of the inedible species must be considerable, and, to
+a comparatively weak or scarce species, of vital importance. The number
+thus sacrificed will be fixed by the quantity of young birds, and by the
+number of experiences requisite to cause them to avoid the inedible
+species for the future, and not at all by the numbers of individuals of
+which each species consists. Hence, if two species are so much alike as
+to be mistaken for one another, the fixed number annually sacrificed by
+inexperienced birds will be divided between them, and both will benefit.
+But if the two species are very unequal in numbers, the benefit will be
+comparatively slight for the more abundant species, but very great for
+the rare one. To the latter it may make all the difference between
+safety and destruction.
+
+To give a rough numerical example. Let us suppose that in a given
+limited district there are two species of Heliconidae, one consisting of
+only 1000, the other of 100,000 individuals, and that the quota required
+annually in the same district for the instruction of young insectivorous
+birds is 500. By the larger species this loss will be hardly felt; to
+the smaller it will mean the most dreadful persecution resulting in a
+loss of half the total population. But, let the two species become
+superficially alike, so that the birds see no difference between them.
+The quota of 500 will now be taken from a combined population of 101,000
+butterflies, and if proportionate numbers of each suffer, then the weak
+species will only lose five individuals instead of 500 as it did before.
+Now we know that the different species of Heliconidae are not equally
+abundant, some being quite rare; so that the benefit to be derived in
+these latter cases would be very important. A slight inferiority in
+rapidity of flight or in powers of eluding attack might also be a cause
+of danger to an inedible species of scanty numbers, and in this case too
+the being merged in another much more abundant species, by similarity
+of external appearance, would be an advantage.
+
+The question of fact remains. Do young birds pursue and capture these
+distasteful butterflies till they have learned by bitter experience what
+species to avoid? On this point Dr. Müller has fortunately been able to
+obtain some direct evidence, by capturing several Acraeas and
+Heliconidae which had evidently been seized by birds but had afterwards
+escaped, as they had pieces torn out of the wing, sometimes
+symmetrically out of both wings, showing that the insect had been seized
+when at rest and with the two pairs of wings in contact. There is,
+however, a general impression that this knowledge is hereditary, and
+does not need to be acquired by young birds; in support of which view
+Mr. Jenner Weir states that his birds always disregarded inedible
+caterpillars. When, day by day, he threw into his aviary various larvae,
+those which were edible were eaten immediately, those which were
+inedible were no more noticed than if a pebble had been thrown before
+the birds.
+
+The cases, however, are not strictly comparable. The birds were not
+young birds of the first year; and, what is more important, edible
+larvae have a comparatively simple coloration, being always brown or
+green and smooth. Uneatable larvae, on the other hand, comprise all that
+are of conspicuous colours and are hairy or spiny. But with butterflies
+there is no such simplicity of contrast. The eatable butterflies
+comprise not only brown or white species, but hundreds of Nymphalidae,
+Papilionidae, Lycaenidae, etc., which are gaily coloured and of an
+immense variety of patterns. The colours and patterns of the inedible
+kinds are also greatly varied, while they are often equally gay; and it
+is quite impossible to suppose that any amount of instinct or inherited
+habit (if such a thing exists) could enable young insectivorous birds to
+distinguish all the species of one kind from all those of the other.
+There is also some evidence to show that animals do learn by experience
+what to eat and what to avoid. Mr. Poulton was assured by Rev. G.J.
+Bursch that very young chickens peck at insects which they afterwards
+avoid. Lizards, too, often seized larvae which they were unable to eat
+and ultimately rejected.
+
+Although the Heliconidae present, on the whole, many varieties of
+coloration and pattern, yet, in proportion to the number of distinct
+species in each district, the types of coloration are few and very well
+marked, and thus it becomes easier for a bird or other animal to learn
+that all belonging to such types are uneatable. This must be a decided
+advantage to the family in question, because, not only do fewer
+individuals of each species need to be sacrificed in order that their
+enemies may learn the lesson of their inedibility, but they are more
+easily recognised at a distance, and thus escape even pursuit. There is
+thus a kind of mimicry between closely allied species as well as between
+species of distinct genera, all tending to the same beneficial end. This
+may be seen in the four or five distinct species of the genus Heliconius
+which all have the same peculiar type of coloration--a yellow band
+across the upper wings and radiating red stripes on the lower,--and are
+all found in the same forests of the Lower Amazon; in the numerous very
+similar species of Ithomia with transparent wings, found in every
+locality of the same region; and in the very numerous species of Papilio
+of the "Aeneas" group, all having a similar style of marking, the
+resemblance being especially close in the females. The very uniform type
+of colouring of the blue-black Euplaeas and of the fulvous Acraeas is of
+the same character.[107] In all these cases the similarity of the allied
+species is so great, that, when they are on the wing at some distance
+off, it is difficult to distinguish one species from another. But this
+close external resemblance is not always a sign of very near affinity;
+for minute examination detects differences in the form and scalloping of
+the wings, in the markings on the body, and in those on the under
+surface of the wings, which do not usually characterise the closest
+allies. It is to be further noted, that the presence of groups of very
+similar species of the same genus, in one locality, is not at all a
+common phenomenon among unprotected groups. Usually the species of a
+genus found in one locality are each well marked and belong to somewhat
+distinct types, while the closely allied forms--those that require
+minute examination to discriminate them as distinct species--are most
+generally found in separate areas, and are what are termed
+representative forms.
+
+The extension we have now given to the theory of mimicry is important,
+since it enables us to explain a much wider range of colour phenomena
+than those which were first imputed to mimicry. It is in the richest
+butterfly region in the world--the Amazon valley--that we find the most
+abundant evidence of the three distinct sets of facts, all depending on
+the same general principle. The form of mimicry first elucidated by Mr.
+Bates is characterised by the presence in each locality of certain
+butterflies, or other insects, themselves edible and belonging to edible
+groups, which derived protection from having acquired a deceptive
+resemblance to some of the inedible butterflies in the same localities,
+which latter were believed to be wholly free from the attacks of
+insectivorous birds. Then came the extension of the principle, by Dr. F.
+Müller, to the case of species of distinct genera of the inedible
+butterflies resembling each other quite as closely as in the former
+cases, and like them always found in the same localities. They derive
+mutual benefit from becoming, in appearance, one species, from which a
+certain toll is taken annually to teach the young insectivorous birds
+that they are uneatable. Even when the two or more species are
+approximately equal in numbers, they each derive a considerable benefit
+from thus combining their forces; but when one of the species is scarce
+or verging on extinction, the benefit becomes exceedingly great, being,
+in fact, exactly apportioned to the need of the species.
+
+The third extension of the same principle explains the grouping of
+allied species of the same genera of inedible butterflies into sets,
+each having a distinct type of coloration, and each consisting of a
+number of species which can hardly be distinguished on the wing. This
+must be useful exactly in the same way as in the last case, since it
+divides the inevitable toll to insectivorous birds and other animals
+among a number of species. It also explains the fact of the great
+similarity of many species of inedible insects in the same locality--a
+similarity which does not obtain to anything like the same extent among
+the edible species. The explanation of the various phenomena of
+resemblance and mimicry, presented by the distasteful butterflies, may
+now be considered tolerably complete.
+
+
+_Mimicry in other Orders of Insects._
+
+A very brief sketch of these phenomena will be given, chiefly to show
+that the same principle prevails throughout nature, and that, wherever a
+rather extensive group is protected, either by distastefulness or
+offensive weapons, there are usually some species of edible and
+inoffensive groups that gain protection by imitating them. It has been
+already stated that the Telephoridae, Lampyridae, and other families of
+soft-winged beetles, are distasteful; and as they abound in all parts of
+the world, and especially in the tropics, it is not surprising that
+insects of many other groups should imitate them. This is especially the
+case with the longicorn beetles, which are much persecuted by
+insectivorous birds; and everywhere in tropical regions some of these
+are to be found so completely disguised as to be mistaken for species of
+the protected groups. Numbers of these imitations have been already
+recorded by Mr. Bates and myself, but I will here refer to a few others.
+
+In the recently published volumes on the Longicorn and Malacoderm
+beetles of Central America[108] there are numbers of beautifully
+coloured figures of the new species; and on looking over them we are
+struck by the curious resemblance of some of the Longicorns to species
+of the Malacoderm group. In some cases we discover perfect mimics, and
+on turning to the descriptions we always find these pairs to come from
+the same locality. Thus the Otheostethus melanurus, one of the
+Prionidae, imitates the malacoderm, Lucidota discolor, in form, peculiar
+coloration, and size, and both are found at Chontales in Nicaragua, the
+species mimicked having, however, as is usual, a wider range. The
+curious and very rare little longicorn, Tethlimmena aliena, quite unlike
+its nearest allies in the same country, is an exact copy on a somewhat
+smaller scale of a malacoderm, Lygistopterus amabilis, both found at
+Chontales. The pretty longicorn, Callia albicornis, closely resembles
+two species of malacoderms (Silis chalybeipennis and Colyphus
+signaticollis), all being small beetles with red head and thorax and
+bright blue elytra, and all three have been found at Panama. Many other
+species of Callia also resemble other malacoderms; and the longicorn
+genus Lycidola has been named from its resemblance to various species of
+the Lycidae, one of the species here figured (Lycidola belti) being a
+good mimic of Calopteron corrugatum and of several other allied species,
+all being of about the same size and found at Chontales. In these cases,
+and in most others, the longicorn beetles have lost the general form and
+aspect of their allies to take on the appearance of a distinct tribe.
+Some other groups of beetles, as the Elateridae and Eucnemidae, also
+deceptively mimic malacoderms.
+
+Wasps and bees are often closely imitated by insects of other orders.
+Many longicorn beetles in the tropics exactly mimic wasps, bees, or
+ants. In Borneo a large black wasp, whose wings have a broad white patch
+near the apex (Mygnimia aviculus), is closely imitated by a heteromerous
+beetle (Coloborhombus fasciatipennis), which, contrary to the general
+habit of beetles, keeps its wings expanded in order to show the white
+patch on their apex, the wing-coverts being reduced to small oval
+scales, as shown in the figure. This is a most remarkable instance of
+mimicry, because the beetle has had to acquire so many characters which
+are unknown among its allies (except in another species from Java)--the
+expanded wings, the white band on them, and the oval scale-like
+elytra.[109] Another remarkable case has been noted by Mr. Neville
+Goodman, in Egypt, where a common hornet (Vespa orientalis) is exactly
+imitated in colour, size, shape, attitude when at rest, and mode of
+flight, by a beetle of the genus Laphria.[110]
+
+The tiger-beetles (Cicindelidae) are also the subjects of mimicry by
+more harmless insects. In the Malay Islands I found a heteromerous
+beetle which exactly resembled a Therates, both being found running on
+the trunks of trees. A longicorn (Collyrodes Lacordairei) mimics
+Collyris, another genus of the same family; while in the Philippine
+Islands there is a cricket (Condylodeira tricondyloides), which so
+closely resembles a tiger-beetle of the genus Tricondyla that the
+experienced entomologist, Professor Westwood, at first placed it in his
+cabinet among those beetles.
+
+[Illustration: FIG. 26.--Mygnimia aviculus (Wasp). Coloborhombus
+fasciatipennis (Beetle).]
+
+[Illustration: FIG. 27.
+a. Doliops sp. (Longicorn)
+   mimics Pachyrhynchus orbifae, (b) (a hard curculio).
+c. Doliops curculionoides mimics (d) Pachyrhynchus sp.
+e. Scepastus pachyrhynchoides (a grasshopper),
+   mimics (f) Apocyrtus sp. (a hard curculio).
+g. Doliops sp. mimics (h) Pachyrhynchus sp.
+i. Phoraspis (grasshopper) mimics (k) a Coccinella.
+
+All the above are from the Philippines. The exact correspondence of the
+colours of the insects themselves renders the mimicry much more complete
+in nature than it appears in the above figures.]
+
+One of the characters by which some beetles are protected is excessive
+hardness of the elytra and integuments. Several genera of weevils
+(Curculionidae) are thus saved from attack, and these are often mimicked
+by species of softer and more eatable groups. In South America, the
+genus Heilipus is one of these hard groups, and both Mr. Bates and M.
+Roelofs, a Belgian entomologist, have noticed that species of other
+genera exactly mimic them. So, in the Philippines, there is a group of
+Curculionidae, forming the genus Pachyrhynchus, in which all the species
+are adorned with the most brilliant metallic colours, banded and spotted
+in a curious manner, and are very smooth and hard. Other genera of
+Curculionidae (Desmidophorus, Alcides), which are usually very
+differently coloured, have species in the Philippines which mimic the
+Pachyrhynchi; and there are also several longicorn beetles (Aprophata,
+Doliops, Acronia, and Agnia), which also mimic them. Besides these,
+there are some longicorns and cetonias which reproduce the same colours
+and markings; and there is even a cricket (Scepastus pachyrhynchoides),
+which has taken on the form and peculiar coloration of these beetles in
+order to escape from enemies, which then avoid them as uneatable.[111]
+The figures on the opposite page exhibit several other examples of these
+mimicking insects.
+
+Innumerable other cases of mimicry occur among tropical insects; but we
+must now pass on to consider a few of the very remarkable, but much
+rarer instances, that are found among the higher animals.
+
+
+_Mimicry among the Vertebrata._
+
+Perhaps the most remarkable cases yet known are those of certain
+harmless snakes which mimic poisonous species. The genus Elaps, in
+tropical America, consists of poisonous snakes which do not belong to
+the viper family (in which are included the rattlesnakes and most of
+those which are poisonous), and which do not possess the broad
+triangular head which characterises the latter. They have a peculiar
+style of coloration, consisting of alternate rings of red and black, or
+red, black, and yellow, of different widths and grouped in various ways
+in the different species; and it is a style of coloration which does not
+occur in any other group of snakes in the world. But in the same regions
+are found three genera of harmless snakes, belonging to other families,
+some few species of which mimic the poisonous Elaps, often so exactly
+that it is with difficulty one can be distinguished from the other. Thus
+Elaps fulvius in Guatemala is imitated by the harmless Pliocerus
+equalis; Elaps corallinus in Mexico is mimicked by the harmless
+Homalocranium semicinctum; and Elaps lemniscatus in Brazil is copied by
+Oxyrhopus trigeminus; while in other parts of South America similar
+cases of mimicry occur, sometimes two harmless species imitating the
+same poisonous snake.
+
+A few other instances of mimicry in this group have been recorded. There
+is in South Africa an egg-eating snake (Dasypeltis scaber), which has
+neither fangs nor teeth, yet it is very like the Berg adder (Clothos
+atropos), and when alarmed renders itself still more like by flattening
+out its head and darting forward with a hiss as if to strike a foe.[112]
+Dr. A.B. Meyer has also discovered that, while some species of the genus
+Callophis (belonging to the same family as the American Elaps) have
+large poison fangs, other species of the same genus have none; and that
+one of the latter (C. gracilis) resembles a poisonous species (C.
+intestinalis) so closely, that only an exact comparison will discover
+the difference of colour and marking. A similar kind of resemblance is
+said to exist between another harmless snake, Megaerophis flaviceps, and
+the poisonous Callophis bivirgatus; and in both these cases the harmless
+snake is less abundant than the poisonous one, as occurs in all examples
+of true mimicry.[113]
+
+In the genus Elaps, above referred to, the very peculiar style of colour
+and marking is evidently a "warning colour" for the purpose of
+indicating to snake-eating birds and mammals that these species are
+poisonous; and this throws light on the long-disputed question of the
+use of the rattle of the rattlesnake. This reptile is really both
+sluggish and timid, and is very easily captured by those who know its
+habits. If gently tapped on the head with a stick, it will coil itself
+up and lie still, only raising its tail and rattling. It may then be
+easily caught. This shows that the rattle is a warning to its enemies
+that it is dangerous to proceed to extremities; and the creature has
+probably acquired this structure and habit because it frequents open or
+rocky districts where protective colour is needful to save it from being
+pounced upon by buzzards or other snake-eaters. Quite parallel in
+function is the expanded hood of the Indian cobra, a poisonous snake
+which belongs also to the Elapidae. This is, no doubt, a warning to its
+foes, not an attempt to terrify its prey; and the hood has been
+acquired, as in the case of the rattlesnake, because, protective
+coloration being on the whole useful, some mark was required to
+distinguish it from other protectively coloured, but harmless, snakes.
+Both these species feed on active creatures capable of escaping if their
+enemy were visible at a moderate distance.
+
+
+_Mimicry among Birds._
+
+The varied forms and habits of birds do not favour the production among
+them of the phenomena of warning colours or of mimicry; and the extreme
+development of their instincts and reasoning powers, as well as their
+activity and their power of flight, usually afford them other means of
+evading their enemies. Yet there are a few imperfect, and one or two
+very perfect cases of true mimicry to be found among them. The less
+perfect examples are those presented by several species of cuckoos, an
+exceedingly weak and defenceless group of birds. Our own cuckoo is, in
+colour and markings, very like a sparrow-hawk. In the East, several of
+the small black cuckoos closely resemble the aggressive drongo-shrikes
+of the same country, and the small metallic cuckoos are like glossy
+starlings; while a large ground-cuckoo of Borneo (Carpococcyx radiatus)
+resembles one of the fine pheasants (Euplocamus) of the same country,
+both in form and in its rich metallic colours.
+
+More perfect cases of mimicry occur between some of the dull-coloured
+orioles in the Malay Archipelago and a genus of large honey-suckers--the
+Tropidorhynchi or "Friar-birds." These latter are powerful and noisy
+birds which go in small flocks. They have long, curved, and sharp beaks,
+and powerful grasping claws; and they are quite able to defend
+themselves, often driving away crows and hawks which venture to approach
+them too nearly. The orioles, on the other hand, are weak and timid
+birds, and trust chiefly to concealment and to their retiring habits to
+escape persecution. In each of the great islands of the Austro-Malayan
+region there is a distinct species of Tropidorhynchus, and there is
+always along with it an oriole that exactly mimics it. All the
+Tropidorhynchi have a patch of bare black skin round the eyes, and a
+ruff of curious pale recurved feathers on the nape, whence their name of
+Friar-birds, the ruff being supposed to resemble the cowl of a friar.
+These peculiarities are imitated in the orioles by patches of feathers
+of corresponding colours; while the different tints of the two species
+in each island are exactly the same. Thus in Bouru both are earthy
+brown; in Ceram they are both washed with yellow ochre; in Timor the
+under surface is pale and the throat nearly white, and Mr. H.O. Forbes
+has recently discovered another pair in the island of Timor Laut. The
+close resemblance of these several pairs of birds, of widely different
+families, is quite comparable with that of many of the insects already
+described. It is so close that the preserved specimens have even
+deceived naturalists; for, in the great French work, _Voyage de
+l'Astrolabe_, the oriole of Bouru is actually described and figured as a
+honey-sucker; and Mr. Forbes tells us that, when his birds were
+submitted to Dr. Sclater for description, the oriole and the
+honey-sucker were, previous to close examination, considered to be the
+same species.
+
+
+_Objections to the Theory of Mimicry._
+
+To set forth adequately the varied and surprising facts of mimicry would
+need a large and copiously illustrated volume; and no more interesting
+subject could be taken up by a naturalist who has access to our great
+collections and can devote the necessary time to search out the many
+examples of mimicry that lie hidden in our museums. The brief sketch of
+the subject that has been here given will, however, serve to indicate
+its nature, and to show the weakness of the objections that were at
+first made to it. It was urged that the action of "like conditions,"
+with "accidental resemblances" and "reversion to ancestral types," would
+account for the facts. If, however, we consider the actual phenomena as
+here set forth, and the very constant conditions under which they occur,
+we shall see how utterly inadequate are these causes, either singly or
+combined. These constant conditions are--
+
+
+    1. That the imitative species occur in the same area and occupy
+    the very same station as the imitated.
+
+    2. That the imitators are always the more defenceless.
+
+    3. That the imitators are always less numerous in individuals.
+
+    4. That the imitators differ from the bulk of their allies.
+
+    5. That the imitation, however minute, is _external_ and
+    _visible_ only, never extending to internal characters or to
+    such as do not affect the external appearance.
+
+
+These five characteristic features of mimicry show us that it is really
+an exceptional form of protective resemblance. Different species in the
+same group of organisms may obtain protection in different ways: some by
+a general resemblance to their environment; some by more exactly
+imitating the objects that surround them--bark, or leaf, or flower;
+while others again gain an equal protection by resembling some species
+which, from whatever cause, is almost as free from attack as if it were
+a leaf or a flower. This immunity may depend on its being uneatable, or
+dangerous, or merely strong; and it is the resemblance to such creatures
+for the purpose of sharing in their safety that constitutes mimicry.
+
+
+_Concluding Remarks on Warning Colours and Mimicry._
+
+Colours which have been acquired for the purpose of serving as a warning
+of inedibility, or of the possession of dangerous offensive weapons, are
+probably more numerous than have been hitherto supposed; and, if so, we
+shall be able to explain a considerable amount of colour in nature for
+which no use has hitherto been conjectured. The brilliant and varied
+colours of sea-anemones and of many coral animals will probably come
+under this head, since we know that many of them possess the power of
+ejecting stinging threads from various parts of their bodies which
+render them quite uneatable to most animals. Mr. Gosse describes how, on
+putting an Anthea into a tank containing a half-grown bullhead (Cottus
+bubalis) which had not been fed for some time, the fish opened his mouth
+and sucked in the morsel, but instantly shot it out again. He then
+seized it a second time, and after rolling it about in his mouth for a
+moment shot it out again, and then darted away to hide himself in a
+hole. Some tropical fishes, however, of the genera Tetrodon,
+Pseudoscarus, Astracion, and a few others, seem to have acquired the
+power of feeding on corals and medusae; and the beautiful bands and
+spots and bright colours with which they are frequently adorned, may be
+either protective when feeding in the submarine coral groves, or may, in
+some cases, be warning colours to show that they themselves are
+poisonous and uneatable.
+
+A remarkable illustration of the wide extension of warning colours, and
+their very definite purpose in nature, is afforded by what may now be
+termed "Mr. Belt's frog." Frogs in all parts of the world are, usually,
+protectively coloured with greens or browns; and the little tree-frogs
+are either green like the leaves they rest upon, or curiously mottled to
+imitate bark or dead leaves. But there are a certain number of very
+gaily coloured frogs, and these do not conceal themselves as frogs
+usually do. Such was the small toad found by Darwin at Bahia Blanca,
+which was intense black and bright vermilion, and crawled about in the
+sunshine over dry sand-hills and arid plains. And in Nicaragua, Mr. Belt
+found a little frog gorgeously dressed in a livery of red and blue,
+which did not attempt concealment and was very abundant, a combination
+of characters which convinced him that it was uneatable. He, therefore,
+took a few specimens home with him and gave them to his fowls and ducks,
+but none would touch them. At last, by throwing down pieces of meat, for
+which there was a great competition among the poultry, he managed to
+entice a young duck into snatching up one of the little frogs. Instead
+of swallowing it, however, the duck instantly threw it out of its mouth,
+and went about jerking its head as if trying to get rid of some
+unpleasant taste.[114]
+
+The power of predicting what will happen in a given case is always
+considered to be a crucial test of a true theory, and if so, the theory
+of warning colours, and with it that of mimicry, must be held to be well
+established. Among the creatures which probably have warning colours as
+a sign of inedibility are, the brilliantly coloured nudibranchiate
+molluscs, those curious annelids the Nereis and the Aphrodite or
+sea-mouse, and many other marine animals. The brilliant colours of the
+scallops (Pecten) and some other bivalve shells are perhaps an
+indication of their hardness and consequent inedibility, as in the case
+of the hard beetles; and it is not improbable that some of the
+phosphorescent fishes and other marine organisms may, like the
+glow-worm, hold out their lamp as a warning to enemies.[115] In
+Queensland there is an exceedingly poisonous spider, whose bite will
+kill a dog, and cause severe illness with excruciating pain in man. It
+is black, with a bright vermilion patch on the middle of the body; and
+it is so well recognised by this conspicuous coloration that even the
+spider-hunting wasps avoid it.[116]
+
+Locusts and grasshoppers are generally of green protective tints, but
+there are many tropical species most gaudily decorated with red, blue,
+and black colours. On the same general grounds as those by which Mr.
+Belt predicted the inedibility of his conspicuous frog, we might safely
+predict the same for these insects; but we have fortunately a proof that
+they are so protected, since Mr. Charles Home states that one of the
+bright coloured Indian locusts was invariably rejected when offered to
+birds and lizards.[117]
+
+       *       *       *       *       *
+
+The examples now given lead us to the conclusion that colours acquired
+for the purpose of serving as a danger-signal to enemies are very
+widespread in nature, and, with the corresponding colours of the species
+which mimic them, furnish us with a rational explanation of a
+considerable portion of the coloration of animals which is outside the
+limits of those colours that have been acquired for either protection or
+recognition. There remains, however, another set of colours, chiefly
+among the higher animals, which, being connected with some of the most
+interesting and most disputed questions in natural history, must be
+discussed in a separate chapter.
+
+FOOTNOTES:
+
+[Footnote 92: _Nature_, vol. iii. p. 165. Professor Meldola observed
+that specimens of Danais and Euplaea in collections were less subject to
+the attacks of mites _(Proc. Ent. Soc._, 1877, p. xii.); and this was
+corroborated by Mr. Jenner Weir. _Entomologist_, 1882, vol. xv. p. 160.]
+
+[Footnote 93: See Darwin's _Descent of Man_, p. 325.]
+
+[Footnote 94: _Transactions of the Entomological Society of London_,
+1869, p. 21.]
+
+[Footnote 95: _Ibid._, p. 27.]
+
+[Footnote 96: _Nature_, vol. iii. p. 147.]
+
+[Footnote 97: Stainton's _Manual of Butterflies and Moths_, vol. i. p.
+93; E.B. Poulton, _Proceedings of the Zool. Soc. of London_, 1887, pp.
+191-274.]
+
+[Footnote 98: See _Transactions of the Linnean Society_, vol. xxiii. pp.
+495-566, coloured plates.]
+
+[Footnote 99: These butterflies are now divided into two sub-families,
+one of which is placed with the Danaidae; but to avoid confusion I shall
+always speak of the American genera under the old term Heliconidae.]
+
+[Footnote 100: R. Meldola in _Ann. and Mag. of Nat. Hist._, Feb. 1878,
+p. 158.]
+
+[Footnote 101: See _Trans. Linn. Soc._, vol. xxv. Wallace, on Variation
+of Malayan Papilionidae; and, Wallace's _Contributions to Natural
+Selection_ chaps. iii. and iv., where full details are given.]
+
+[Footnote 102: See _Trans. Linn. Soc._, vol. xxvi., with two coloured
+plates illustrating cases of mimicry.]
+
+[Footnote 103: Edwards's _Butterflies of North America_, second series,
+part vi.]
+
+[Footnote 104: Professor Meldola informs me that he has recorded another
+case of mimicry among British moths, in which Acidalia subsericata
+imitates Asthena candidata. See _Ent. Mo. Mag._, vol. iv. p. 163.]
+
+[Footnote 105: From Professor Meldola's translation of Dr. F. Müller's
+paper, in _Proc. Ent. Soc. Lond._, 1879, p. xx.]
+
+[Footnote 106: _Island Life_, p. 255.]
+
+[Footnote 107: This extension of the theory of mimicry was pointed out
+by Professor Meldola in the paper already referred to; and he has
+answered the objections to Dr. F. Müller's theory with great force in
+the _Annals and Mag. of Nat. Hist._, 1882, p. 417.]
+
+[Footnote 108: Godman and Salvin's _Biologia Centrali-Americana,
+Insecta, Coleoptera_, vol. iii. part ii., and vol. v.]
+
+[Footnote 109: _Trans. Ent. Soc._, 1885, p. 369.]
+
+[Footnote 110: _Proc. Cambridge Phil. Soc._, vol. iii. part ii., 1877.]
+
+[Footnote 111: _Compte-Rendu de la Société Entomologique de Belgaue_,
+series ii., No. 59, 1878.]
+
+[Footnote 112: _Nature_, vol. xxxiv. p. 547.]
+
+[Footnote 113: _Proceedings of the Zool. Soc. of London_, 1870, p. 369.]
+
+[Footnote 114: _The Naturalist in Nicaragua_, p. 321.]
+
+[Footnote 115: Mr. Belt first suggested this use of the light of the
+Lampyridae (fireflies and glow-worms)--_Naturalist in Nicaragua_, p.
+320. Mr. Verrill and Professor Meldola made the same suggestion in the
+case of medusae and other phosphorescent marine organisms (_Nature_,
+vol. xxx. pp. 281, 289).]
+
+[Footnote 116: W.E. Armit, in _Nature_, vol. xviii. p. 642.]
+
+[Footnote 117: _Proc. Ent. Soc._, 1869, p. xiii.]
+
+
+
+
+CHAPTER X
+
+COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
+
+
+    Sex colours in the mollusca and crustacea--In insects--In
+    butterflies and moths--Probable causes of these colours--Sexual
+    selection as a supposed cause--Sexual coloration of birds--Cause
+    of dull colours of female birds--Relation of sex colour to
+    nesting habits--Sexual colours of other vertebrates--Sexual
+    selection by the struggles of males--Sexual characters due to
+    natural selection--Decorative plumage of males and its effect on
+    the females--Display of decorative plumage by the males--A
+    theory of animal coloration--The origin of accessory
+    plumes--Development of accessory plumes and their display--The
+    effect of female preference will be neutralised by natural
+    selection--General laws of animal coloration--Concluding
+    remarks.
+
+
+
+In the preceding chapters we have dealt chiefly with the coloration of
+animals as distinctive of the several species; and we have seen that, in
+an enormous number of cases, the colours can be shown to have a definite
+purpose, and to be useful either as a means of protection or
+concealment, of warning to enemies, or of recognition by their own kind.
+We have now to consider a subordinate but very widespread
+phenomenon---the differences of colour or of ornamental appendages in
+the two sexes. These differences are found to have special relations
+with the three classes of coloration above referred to, in many cases
+confirming the explanation already given of their purport and use, and
+furnishing us with important aid in formulating a general theory of
+animal coloration.
+
+In comparing the colours of the two sexes we find a perfect gradation,
+from absolute identity of colour up to such extreme difference that it
+is difficult to believe that the two forms can belong to the same
+species; and this diversity in the colours of the sexes does not bear
+any constant relation to affinity or systematic position. In both
+insects and birds we find examples of complete identity and extreme
+diversity of the sexes; and these differences occur sometimes in the
+same tribe or family, and sometimes even in the same genus.
+
+It is only among the higher and more active animals that sexual
+differences of colour acquire any prominence. In the mollusca the two
+sexes, when separated, are always alike in colour, and only very rarely
+present slight differences in the form of the shell. In the extensive
+group of crustacea the two sexes as a rule are identical in colour,
+though there are often differences in the form of the prehensile organs;
+but in a very few cases there are differences of colour also. Thus, in a
+Brazilian species of shore-crab (Gelasimus) the female is grayish-brown,
+while in the male the posterior part of the cephalo-thorax is pure
+white, with the anterior part of a rich green. This colour is only
+acquired by the males when they become mature, and is liable to rapid
+change in a few minutes to dusky tints.[118] In some of the freshwater
+fleas (Daphnoidae) the males are ornamented with red and blue spots,
+while in others similar colours occur in both sexes. In spiders also,
+though as a rule the two sexes are alike in colour, there are a few
+exceptions, the males being ornamented with brilliant colours on the
+abdomen, while the female is dull coloured.
+
+
+_Sexual Coloration in Insects._
+
+It is only when we come to the winged insects that we find any large
+amount of peculiarity in sexual coloration, and even here it is only
+developed in certain orders. Flies (Diptera), field-bugs (Hemiptera),
+cicadas (Homoptera), and the grasshoppers, locusts, and crickets
+(Orthoptera) present very few and unimportant sexual differences of
+colour; but the last two groups have special musical organs very fully
+developed in the males of some of the species, and these no doubt enable
+the sexes to discover and recognise each other. In some cases, however,
+when the female is protectively coloured, as in the well-known
+leaf-insects already referred to (p. 207), the male is smaller and much
+less protectively formed and coloured. In the bees and wasps
+(Hymenoptera) it is also the rule that the sexes are alike in colour,
+though there are several cases among solitary bees where they differ;
+the female being black, and the male brown in Anthophora retusa, while
+in Andraena fulva the female is more brightly coloured than the male. Of
+the great order of beetles (Coleoptera) the same thing may be said.
+Though often so rich and varied in their colours the sexes are usually
+alike, and Mr. Darwin was only able to find about a dozen cases in which
+there was any conspicuous difference between them.[119] They exhibit,
+however, numerous sexual characters, in the length of the antennae, and
+in horns, legs, or jaws remarkably enlarged or curiously modified in the
+male sex.
+
+It is in the family of dragonflies (order Neuroptera) that we first meet
+with numerous cases of distinctive sexual coloration. In some of the
+Agrionidae the males have the bodies rich blue and the wings black,
+while the females have the bodies green and the wings transparent. In
+the North American genus Hetaerina the males alone have a carmine spot
+at the base of each wing; but in some other genera the sexes hardly
+differ at all.
+
+The great order of Lepidoptera, including the butterflies and moths,
+affords us the most numerous and striking examples of diversity of
+sexual colouring. Among the moths the difference is usually but slight,
+being manifested in a greater intensity of the colour of the smaller
+winged male; but in a few cases there is a decided difference, as in the
+ghost-moth (Hepialus humuli), in which the male is pure white, while the
+female is yellow with darker markings. This may be a recognition colour,
+enabling the female more readily to discover her mate; and this view
+receives some support from the fact that in the Shetland Islands the
+male is almost as yellow as the female, since it has been suggested that
+at midsummer, when this moth appears, there is in that high latitude
+sufficient twilight all night to render any special coloration
+unnecessary.[120]
+
+Butterflies present us with a wonderful amount of sexual difference of
+colour, in many cases so remarkable that the two sexes of the same
+species remained for many years under different names and were thought
+to be quite distinct species. We find, however, every gradation from
+perfect identity to complete diversity, and in some cases we are able to
+see a reason for this difference. Beginning with the most extraordinary
+cases of diversity--as in Diadema misippus, where the male is black,
+ornamented with a large white spot on each wing margined with rich
+changeable blue, while the female is orange-brown with black spots and
+stripes--we find the explanation in the fact that the female mimics an
+uneatable Danais, and thus gains protection while laying its eggs on low
+plants in company with that insect. In the allied species, Diadema
+bolina, the females are also very different from the males, but are of
+dusky brown tints, evidently protective and very variable, some
+specimens having a general resemblance to the uneatable Euplaeas; so
+that we see here some of the earlier stages of both forms of protection.
+The remarkable differences in some South American Pieridae are similarly
+explained. The males of Pieris pyrrha, P. lorena, and several others,
+are white with a few black bands and marginal spots like so many of
+their allies, while the females are gaily coloured with yellow and
+brown, and exactly resemble some species of the uneatable Heliconidae of
+the same district. Similarly, in the Malay Archipelago, the female of
+Diadema anomala is glossy metallic blue, while the male is brown; the
+reason for this reversal of the usual rule being, that the female
+exactly mimics the brilliant colouring of the common and uneatable
+Euplaea midamus, and thus secures protection. In the fine Adolias
+dirtea, the male is black with a few specks of ochre-yellow and a broad
+marginal band of rich metallic greenish-blue, while the female is
+brownish-black entirely covered with rows of ochre-yellow spots. This
+latter coloration does not appear to be protective when the insect is
+seen in the cabinet, but it really is so. I have observed the female of
+this butterfly in Sumatra, where it settles on the ground in the forest,
+and its yellow spots so harmonise with the flickering gleams of sunlight
+on the dead leaves that it can only be detected with the greatest
+difficulty.
+
+A hundred other cases might be quoted in which the female is either
+more obscurely coloured than the male, or gains protection by imitating
+some inedible species; and any one who has watched these female insects
+flying slowly along in search of the plants on which to deposit their
+eggs, will understand how important it must be to them not to attract
+the attention of insect-eating birds by too conspicuous colours. The
+number of birds which capture insects on the wing is much greater in
+tropical regions than in Europe; and this is perhaps the reason why many
+of our showy species are alike, or almost alike, in both sexes, while
+they are protectively coloured on the under side which is exposed to
+view when they are at rest. Such are our peacock, tortoise-shell, and
+red admiral butterflies; while in the tropics we more commonly find that
+the females are less conspicuous on the upper surface even when
+protectively coloured beneath.
+
+We may here remark, that the cases already quoted prove clearly that
+either male or female may be modified in colour apart from the opposite
+sex. In Pieris pyrrha and its allies the male retains the usual type of
+coloration of the whole genus, while the female has acquired a distinct
+and peculiar style of colouring. In Adolias dirtea, on the other hand,
+the female appears to retain something like the primitive colour and
+markings of the two sexes, modified perhaps for more perfect protection;
+while the male has acquired more and more intense and brilliant colours,
+only showing his original markings by the few small yellow spots that
+remain near the base of the wings. In the more gaily coloured Pieridae,
+of which our orange-tip butterfly may be taken as a type, we see in the
+female the plain ancestral colours of the group, while the male has
+acquired the brilliant orange tip to its wings, probably as a
+recognition mark.
+
+In those species in which the under surface is protectively coloured, we
+often find the upper surface alike in both sexes, the tint of colour
+being usually more intense in the male. But in some cases this leads to
+the female being more conspicuous, as in some of the Lycaenidae, where
+the female is bright blue and the male of a blue so much deeper and
+soberer in tint as to appear the less brilliantly coloured of the two.
+
+
+_Probable Causes of these Colours._
+
+In the production of these varied results there have probably been
+several causes at work. There seems to be a constant tendency in the
+male of most animals--but especially of birds and insects--to develop
+more and more intensity of colour, often culminating in brilliant
+metallic blues or greens or the most splendid iridescent hues; while, at
+the same time, natural selection is constantly at work, preventing the
+female from acquiring these same tints, or modifying her colours in
+various directions to secure protection by assimilating her to her
+surroundings, or by producing mimicry of some protected form. At the
+same time, the need for recognition must be satisfied; and this seems to
+have led to diversities of colour in allied species, sometimes the
+female, sometimes the male undergoing the greatest change according as
+one or other could be modified with the greatest ease, and so as to
+interfere least with the welfare of the race. Hence it is that sometimes
+the males of allied species vary most, as in the different species of
+Epicalia; sometimes the females, as in the magnificent green species of
+Ornithoptera and the "Aeneas" group of Papilio.
+
+The importance of the two principles--the need of protection and
+recognition--in modifying the comparative coloration of the sexes among
+butterflies, is beautifully illustrated in the case of the groups which
+are protected by their distastefulness, and whose females do not,
+therefore, need the protection afforded by sober colours.
+
+In the great families, Heliconidae and Acraeidae, we find that the two
+sexes are almost always alike; and, in the very few exceptions, that the
+female, though differently, is not less gaily or less conspicuously
+coloured. In the Danaidae the same general rule prevails, but the cases
+in which the male exhibits greater intensity of colour than the female
+are perhaps more numerous than in the other two families. There is,
+however, a curious difference in this respect between the Oriental and
+the American groups of distasteful Papilios with warning colours, both
+of which are the subjects of mimicry. In the Eastern groups--of which P.
+hector and P. coon may be taken as types--the two sexes are nearly
+alike, the male being sometimes more intensely coloured and with fewer
+pale markings; but in the American groups--represented by P. aeneas, P.
+sesostris, and allies--there is a wonderful diversity, the males having
+a rich green or bluish patch on the fore wings, while the females have a
+band or spots of pure white, not always corresponding in position to the
+green spot of the males. There are, however, transitional forms, by
+which a complete series can be traced, from close similarity to great
+diversity of colouring between the sexes; and this may perhaps be only
+an extreme example of the intenser colour and more concentrated markings
+which are a very prevalent characteristic of male butterflies.
+
+There are, in fact, many indications of a regular succession of tints in
+which colour development has occurred in the various groups of
+butterflies, from an original grayish or brownish neutral tint. Thus in
+the "Aeneas" group of Papilios we have the patch on the upper wings
+yellowish in P. triopas, olivaceous in P. bolivar, bronzy-gray with a
+white spot in P. erlaces, more greenish and buff in P. iphidamas,
+gradually changing to the fine blue of P. brissonius, and the
+magnificent green of P. sesostris. In like manner, the intense crimson
+spots of the lower wings can be traced step by step from a yellow or
+buff tint, which is one of the most widespread colours in the whole
+order. The greater purity and intensity of colour seem to be usually
+associated with more pointed wings, indicating greater vigour and more
+rapid flight.
+
+
+_Sexual Selection as a supposed Cause of Colour Development._
+
+Mr. Darwin, as is well known, imputed most of the brilliant colours and
+varied patterns of butterflies' wings to sexual selection--that is, to a
+constant preference, by female butterflies, for the more brilliant
+males; the colours thus produced being sometimes transmitted to the
+males alone, sometimes to both sexes. This view has always seemed to me
+to be unsupported by evidence, while it is also quite inadequate to
+account for the facts. The only direct evidence, as set forth with his
+usual fairness by Mr. Darwin himself, is opposed to his views. Several
+entomologists assured him that, in moths, the females evince not the
+least choice of their partners; and Dr. Wallace of Colchester, who has
+largely bred the fine Bombyx cynthia, confirmed this statement. Among
+butterflies, several males often pursue one female, and Mr. Darwin says,
+that, unless the female exerts a choice the pairing must be left to
+chance. But, surely, it may be the most vigorous or most persevering
+male that is chosen, not necessarily one more brightly or differently
+coloured, and this will be true "natural selection." Butterflies have
+been noticed to prefer some coloured flowers to others; but that does
+not prove, or even render probable, any preference for the colour
+itself, but only for flowers of certain colours, on account of the more
+agreeable or more abundant nectar obtained from them. Dr. Schulte called
+Mr. Darwin's attention to the fact, that in the Diadema bolina the
+brilliant blue colour surrounding the white spots is only visible when
+we look towards the insect's head, and this is true of many of the
+iridescent colours of butterflies, and probably depends upon the
+direction of the striae on the scales. It is suggested, however, that
+this display of colour will be seen by the female as the male is
+approaching her, and that it has been developed by sexual
+selection.[121] But in the majority of cases the males _follow_ the
+female, hovering over her in a position which would render it almost
+impossible for her to see the particular colours or patterns on his
+upper surface; to do so the female should mount higher than the male,
+and fly towards him--being the seeker instead of the sought, and this is
+quite opposed to the actual facts. I cannot, therefore, think that this
+suggestion adds anything whatever to the evidence for sexual selection
+of colour by female butterflies. This question will, however, be again
+touched upon after we have considered the phenomena of sexual colour
+among the vertebrata.
+
+
+_Sexual Coloration of Birds._
+
+The general rule among vertebrates, as regards colour, is, for the two
+sexes to be alike. This prevails, with only a few exceptions, in fishes,
+reptiles, and mammalia; but in birds diversity of sexual colouring is
+exceedingly frequent, and is, not improbably, present in a greater or
+less degree in more than half of the known species. It is this class,
+therefore, that will afford us the best materials for a discussion of
+the problem, and that may perhaps lead us to a satisfactory explanation
+of the causes to which sexual colour is due.
+
+The most fundamental characteristic of birds, from our present point of
+view, is a greater intensity of colour in the male. This is the case in
+hawks and falcons; in many thrushes, warblers, and finches; in pigeons,
+partridges, rails, plovers, and many others. When the plumage is highly
+protective or of dull uniform tints, as in many of the thrushes and
+warblers, the sexes are almost or quite identical in colour; but when
+any rich markings or bright tints are acquired, they are almost always
+wanting or much fainter in the female, as we see in the black-cap among
+warblers, and the chaffinch among finches.
+
+It is in tropical regions, where from a variety of causes colour has
+been, developed to its fullest extent, that we find the most remarkable
+examples of sexual divergence of colour. The most gorgeously coloured
+birds known are the birds of paradise, the chatterers, the tanagers, the
+humming-birds, and the pheasant-tribe, including the peacocks. In all
+these the females are much less brilliant, and, in the great majority of
+cases, exceptionally plain and dull coloured birds. Not only are the
+remarkable plumes, crests, and gorgets of the birds of paradise entirely
+wanting in the females, but these latter are usually without any bright
+colour at all, and rank no higher than our thrushes in ornamental
+plumage. Of the humming-birds the same may be said, except that the
+females are often green, and sometimes slightly metallic, but from their
+small size and uniform tints are never conspicuous. The glorious blues
+and purples, the pure whites and intense crimsons of the male chatterers
+are represented in the females by olive-greens or dull browns, as are
+the infinitely varied tints of the male tanagers. And in pheasants, the
+splendour of plumage which characterises the males is entirely absent in
+the females, which, though often ornamental, have always comparatively
+sober and protective tints. The same thing occurs with many other
+groups. In the Eastern tropics are many brilliant birds belonging to the
+families of the warblers, flycatchers, shrikes, etc., but the female is
+always much less brilliant than the male and often quite dull coloured.
+
+
+_Cause of Dull Colours of Female Birds._
+
+The reason of this phenomenon is not difficult to find, if we consider
+the essential conditions of a bird's existence, and the most important
+function it has to fulfil. In order that the species may be continued,
+young birds must be produced, and the female birds have to sit
+assiduously on their eggs. While doing this they are exposed to
+observation and attack by the numerous devourers of eggs and birds, and
+it is of vital importance that they should be protectively coloured in
+all those parts of the body which are exposed during incubation. To
+secure this end all the bright colours and showy ornaments which
+decorate the male have not been acquired by the female, who often
+remains clothed in the sober hues which were probably once common to the
+whole order to which she belongs. The different amounts of colour
+acquired by the females have no doubt depended on peculiarities of
+habits and of environment, and on the powers of defence or of
+concealment possessed by the species. Mr. Darwin has taught us that
+natural selection cannot produce absolute, but only relative perfection;
+and as a protective colour is only one out of many means by which the
+female birds are able to provide for the safety of their young, those
+which are best endowed in other respects will have been allowed to
+acquire more colour than those with whom the struggle for existence is
+more severe.
+
+
+_Relation of Sex Colour to Nesting Habits._
+
+This principle is strikingly illustrated by the existence of
+considerable numbers of birds in which both sexes are similarly and
+brilliantly coloured,--in some cases as brilliantly as the males of many
+of the groups above referred to. Such are the extensive families of the
+kingfishers, the woodpeckers, the toucans, the parrots, the turacos, the
+hangnests, the starlings, and many other smaller groups, all the species
+of which are conspicuously or brilliantly coloured, while in all of them
+the females are either coloured exactly like the males, or, when
+differently coloured, are equally conspicuous. When searching for some
+cause for this singular apparent exception to the rule of female
+protective colouring, I came upon a fact which beautifully explains it;
+for in all these cases, without exception, the species either nests in
+holes in the ground or in trees, or builds a domed or covered nest, so
+as completely to conceal the sitting-bird. We have here a case exactly
+parallel to that of the butterflies protected by distastefulness, whose
+females are either exactly like the males, or, if different, are equally
+conspicuous. We can hardly believe that so exact a parallel should exist
+between such remote classes of animals, except under the influence of a
+general law; and, in the need of protection by all defenceless animals,
+and especially by most female insects and birds, we have such a law,
+which has been proved to have influenced the colours of a considerable
+proportion of the animal kingdom.[122]
+
+The general relation which exists between the mode of nesting and the
+coloration of the sexes in those groups of birds which need protection
+from enemies, may be thus expressed: When both sexes are brilliant or
+conspicuous, the nest is such as to conceal the sitting-bird; but when
+the male is brightly coloured and the female sits exposed on the nest,
+she is always less brilliant and generally of quite sober and protective
+hues.
+
+It must be understood that the mode of nesting has influenced the
+colour, not that the colour has determined the mode of nesting; and
+this, I believe, has been generally, though not perhaps universally, the
+case. For we know that colour varies more rapidly, and can be more
+easily modified and fixed by selection, than any other character;
+whereas habits, especially when connected with structure, and when they
+pervade a whole group, are much more persistent and more difficult to
+change, as shown by the habit of the dog turning round two or three
+times before lying down, believed to be that of the wild ancestral form
+which thus smoothed down the herbage so as to form a comfortable bed. We
+see, too, that the general mode of nesting is characteristic of whole
+families differing widely in size, form, and colours. Thus, all the
+kingfishers and their allies in every part of the world nest in holes,
+usually in banks, but sometimes in trees. The motmots and the puff-birds
+(Bucconidae) build in similar places; while the toucans, barbets,
+trogons, woodpeckers, and parrots all make their nests in hollow trees.
+This habit, pervading all the members of extensive families, must
+therefore be extremely ancient, more especially as it evidently depends
+in some degree on the structure of the birds, the bills, and especially
+the feet, of all these groups being unfitted for the construction of
+woven arboreal nests.[123] But in all these families the colour varies
+greatly from species to species, being constant only in the one
+character of the similarity of the sexes, or, at all events, in their
+being equally conspicuous even though differently coloured.
+
+When I first put forward this view of the connection between the mode of
+nesting and the coloration of female birds, I expressed the law in
+somewhat different terms, which gave rise to some misunderstanding, and
+led to numerous criticisms and objections. Several cases were brought
+forward in which the females were far less brilliant than the males,
+although the nest was covered. This is the case with the Maluridae, or
+superb warblers of Australia, in which the males are very brilliant
+during the pairing season and the females quite plain, yet they build
+domed nests. Here, there can be little doubt, the covered nest is a
+protection from rain or from some special enemies to the eggs; while the
+birds themselves are protectively coloured in both sexes, except for a
+short time during the breeding season when the male acquires brilliant
+colours; and this is probably connected with the fact of their
+inhabiting the open plains and thin scrub of Australia, where protective
+colours are as generally advantageous as they are in our north-temperate
+zones.
+
+As I have now stated the law, I do not think there are any exceptions to
+it, while there are an overwhelming number of cases which give it a
+strong support. It has been objected that the domed nests of many birds
+are as conspicuous as the birds themselves would be, and would,
+therefore, be of no use as a protection to the birds and young. But, as
+a matter of fact, they do protect from attack, for hawks or crows do not
+pluck such nests to pieces, as in doing so they would be exposed to the
+attack of the whole colony; whereas a hawk or falcon could carry off a
+sitting-bird or the young at a swoop, and entirely avoid attack.
+Moreover, each kind of covered nest is doubtless directed against the
+attacks of the most dangerous enemies of the species, the purse-like
+nests, often a yard long, suspended from the extremity of thin twigs,
+being useful against the attacks of snakes, which, if they attempted to
+enter them, would be easily made to lose their hold and fall to the
+ground. Such birds as jays, crows, magpies, hawks, and other birds of
+prey, have also been urged as an exception; but these are all aggressive
+birds, able to protect themselves, and thus do not need any special
+protection for their females during nidification. Some birds which build
+in covered nests are comparatively dull coloured, like many of the
+weaver birds, but in others the colours are more showy, and in all the
+sexes are alike; so that none of these are in any way opposed to the
+rule. The golden orioles have, however, been adduced as a decided
+exception, since the females are showy and build in an open nest. But
+even here the females are less brilliant than the males, and are
+sometimes greenish or olivaceous on the upper surface; while they very
+carefully conceal their nests among dense foliage, and the male is
+sufficiently watchful and pugnacious to drive off most intruders.
+
+On the other hand, how remarkable it is that the only small and brightly
+coloured birds of our own country in which the male and female are
+alike--the tits and starlings--either build in holes or construct
+covered nests; while the beautiful hangnests (Icteridae) of South
+America, which always build covered or purse-shaped nests, are equally
+showy in both sexes, in striking contrast with the chatterers and
+tanagers of the same country, whose females are invariably less
+conspicuous than the males. On a rough estimate, there are about 1200
+species of birds in the class of showy males and females, with concealed
+nidification; while there are probably, from an equally rough estimate,
+about the same number in the contrasted class of showy males and dull
+females, with open nests. This will leave the great bulk of known birds
+in the classes of those which are more or less protectively coloured in
+both sexes; or which, from their organisation and habits, do not
+require special protective coloration, such as many of the birds of
+prey, the larger waders, and the oceanic birds.
+
+There are a few very curious cases in which the female bird is actually
+more brilliant than the male, and which yet have open nests. Such are
+the dotterel (Eudromias morinellus), several species of phalarope, an
+Australian creeper (Climacteris erythropus), and a few others; but in
+every one of these cases the relation of the sexes in regard to
+nidification is reversed, the male performing the duties of incubation,
+while the female is the stronger and more pugnacious. This curious case,
+therefore, quite accords with the general law of coloration.[124]
+
+
+_Sexual Colours of other Vertebrates._
+
+We may consider a few of the cases of sexual colouring of other classes
+of vertebrates, as given by Mr. Darwin. In fishes, though the sexes are
+usually alike, there are several species in which the males are more
+brightly coloured, and have more elongated fins, spines, or other
+appendages, and in some few cases the colours are decidedly different.
+The males often fight together, and are altogether more vivacious and
+excitable than the females during the breeding season; and with this we
+may connect a greater intensity of coloration.
+
+In frogs and toads the colours are usually alike, or a little more
+intense in the males, and the same may be said of most snakes. It is in
+lizards that we first meet with considerable sexual differences, many of
+the species having gular pouches, frills, dorsal crests, or horns,
+either confined to the males, or more developed in them than in the
+females, and these ornaments are often brightly coloured. In most cases,
+however, the tints of lizards are protective, the male being usually a
+little more intense in coloration; and the difference in extreme cases
+may be partly due to the need of protection for the female, which, when
+laden with eggs, must be less active and less able to escape from
+enemies than the male, and may, therefore, have retained more protective
+colours, as so many insects and birds have certainly done.[125]
+
+In mammalia there is often a somewhat greater intensity of colour in
+the male, but rarely a decided difference. The female of the great red
+kangaroo, however, is a delicate gray; while in the Lemur macaco of
+Madagascar the male is jet-black and the female brown. In many monkeys
+also there are some differences of colour, especially on the face. The
+sexual weapons and ornaments of male mammalia, as horns, crests, manes,
+and dewlaps, are well known, and are very numerous and remarkable.
+Having thus briefly reviewed the facts, we will now consider the
+theories to which they have given rise.
+
+
+_Sexual Selection by the Struggles of Males._
+
+Among the higher animals it is a very general fact that the males fight
+together for the possession of the females. This leads, in polygamous
+animals especially, to the stronger or better armed males becoming the
+parents of the next generation, which inherits the peculiarities of the
+parents; and thus vigour and offensive weapons are continually increased
+in the males, resulting in the strength and horns of the bull, the tusks
+of the boar, the antlers of the stag, and the spurs and fighting
+instinct of the gamecock. But almost all male animals fight together,
+though not specially armed; even hares, moles, squirrels, and beavers
+fight to the death, and are often found to be scarred and wounded. The
+same rule applies to almost all male birds; and these battles have been
+observed in such different groups as humming-birds, finches,
+goatsuckers, woodpeckers, ducks, and waders. Among reptiles, battles of
+the males are known to occur in the cases of crocodiles, lizards, and
+tortoises; among fishes, in those of salmon and sticklebats. Even among
+insects the same law prevails; and male spiders, beetles of many groups,
+crickets, and butterflies often fight together.
+
+From this very general phenomenon there necessarily results a form of
+natural selection which increases the vigour and fighting power of the
+male animal, since, in every case, the weaker are either killed,
+wounded, or driven away. This selection would be more powerful if males
+were always in excess of females, but after much research Mr. Darwin
+could not obtain any satisfactory evidence that this was the case. The
+same effect, however, is produced in some cases by constitution or
+habits; thus male insects usually emerge first from the pupa, and among
+migrating birds the males arrive first both in this country and in North
+America. The struggle is thus intensified, and the most vigorous males
+are the first to have offspring. This in all probability is a great
+advantage, as the early breeders have the start in securing food, and
+the young are strong enough to protect themselves while the later broods
+are being produced.
+
+It is to this form of male rivalry that Mr. Darwin first applied the
+term "sexual selection." It is evidently a real power in nature; and to
+it we must impute the development of the exceptional strength, size, and
+activity of the male, together with the possession of special offensive
+and defensive weapons, and of all other characters which arise from the
+development of these or are correlated with them. But he has extended
+the principle into a totally different field of action, which has none
+of that character of constancy and of inevitable result that attaches to
+natural selection, including male rivalry; for by far the larger portion
+of the phenomena, which he endeavours to explain by the direct action of
+sexual selection, can only be so explained on the hypothesis that the
+immediate agency is female choice or preference. It is to this that he
+imputes the origin of all secondary sexual characters other than weapons
+of offence and defence, of all the ornamental crests and accessory
+plumes of birds, the stridulating sounds of insects, the crests and
+beards of monkeys and other mammals, and the brilliant colours and
+patterns of male birds and butterflies. He even goes further, and
+imputes to it a large portion of the brilliant colour that occurs in
+both sexes, on the principle that variations occurring in one sex are
+sometimes transmitted to the same sex only, sometimes to both, owing to
+peculiarities in the laws of inheritance. In this extension of sexual
+selection to include the action of female choice or preference, and in
+the attempt to give to that choice such wide-reaching effects, I am
+unable to follow him more than a very little way; and I will now state
+some of the reasons why I think his views are unsound.
+
+
+_Sexual Characters due to Natural Selection._
+
+Besides the acquisition of weapons by the male for the purpose of
+fighting with other males, there are some other sexual characters which
+may have been produced by natural selection. Such are the various sounds
+and odours which are peculiar to the male, and which serve as a call to
+the female or as an indication of his presence. These are evidently a
+valuable addition to the means of recognition of the two sexes, and are
+a further indication that the pairing season has arrived; and the
+production, intensification, and differentiation of these sounds and
+odours are clearly within the power of natural selection. The same
+remark will apply to the peculiar calls of birds, and even to the
+singing of the males. These may well have originated merely as a means
+of recognition between the two sexes of a species, and as an invitation
+from the male to the female bird. When the individuals of a species are
+widely scattered, such a call must be of great importance in enabling
+pairing to take place as early as possible, and thus the clearness,
+loudness, and individuality of the song becomes a useful character, and
+therefore the subject of natural selection. Such is especially the case
+with the cuckoo, and with all solitary birds, and it may have been
+equally important at some period of the development of all birds. The
+act of singing is evidently a pleasurable one; and it probably serves as
+an outlet for superabundant nervous energy and excitement, just as
+dancing, singing, and field sports do with us. It is suggestive of this
+view that the exercise of the vocal power seems to be complementary to
+the development of accessory plumes and ornaments, all our finest
+singing birds being plainly coloured, and with no crests, neck or tail
+plumes to display; while the gorgeously ornamented birds of the tropics
+have no song, and those which expend much energy in display of plumage,
+as the turkey, peacocks, birds of paradise, and humming-birds, have
+comparatively an insignificant development of voice. Some birds have, in
+the wings or tail, peculiarly developed feathers which produce special
+sounds. In some of the little manakins of Brazil, two or three of the
+wing-feathers are curiously shaped and stiffened in the male, so that
+the bird is able to produce with them a peculiar snapping or cracking
+sound; and the tail-feathers of several species of snipe are so narrowed
+as to produce distinct drumming, whistling, or switching sounds when the
+birds descend rapidly from a great height. All these are probably
+recognition and call notes, useful to each species in relation to the
+most important function of their lives, and thus capable of being
+developed by the agency of natural selection.
+
+
+_Decorative Plumage of Birds and its Display._
+
+Mr. Darwin has devoted four chapters of his _Descent of Man_ to the
+colours of birds, their decorative plumage, and its display at the
+pairing season; and it is on this latter circumstance that he founds his
+theory, that both the plumage and the colours have been developed by the
+preference of the females, the more ornamented males becoming the
+parents of each successive generation. Any one who reads these most
+interesting chapters will admit, that the fact of the display is
+demonstrated; and it may also be admitted, as highly probable, that the
+female is pleased or excited by the display. But it by no means follows
+that slight differences in the shape, pattern, or colours of the
+ornamental plumes are what lead a female to give the preference to one
+male over another; still less that all the females of a species, or the
+great majority of them, over a wide area of country, and for many
+successive generations, prefer exactly the same modification of the
+colour or ornament.
+
+The evidence on this matter is very scanty, and in most cases not at all
+to the point. Some peahens preferred an old pied peacock; albino birds
+in a state of nature have never been seen paired with other birds; a
+Canada goose paired with a Bernicle gander; a male widgeon preferred a
+pintail duck to its own species; a hen canary preferred a male
+greenfinch to either linnet, goldfinch, siskin, or chaffinch. These
+cases are evidently exceptional, and are not such as generally occur in
+nature; and they only prove that the female does exert some choice
+between very different males, and some observations on birds in a state
+of nature prove the same thing; but there is no evidence that slight
+variations in the colour or plumes, in the way of increased intensity or
+complexity, are what determines the choice. On the other hand, Mr.
+Darwin gives much evidence that it is _not_ so determined. He tells us
+that Messrs. Hewitt, Tegetmeier, and Brent, three of the highest
+authorities and best observers, "do not believe that the females prefer
+certain males on account of the beauty of their plumage." Mr. Hewitt was
+convinced "that the female almost invariably prefers the most vigorous,
+defiant, and mettlesome male;" and Mr. Tegetmeier, "that a gamecock,
+though disfigured by being dubbed, and with his hackles trimmed, would
+be accepted as readily as a male retaining all his natural
+ornaments."[126] Evidence is adduced that a female pigeon will sometimes
+turn antipathy to a particular male without any assignable cause; or, in
+other cases, will take a strong fancy to some one bird, and will desert
+her own mate for him; but it is not stated that superiority or
+inferiority of plumage has anything to do with these fancies. Two
+instances are indeed given, of male birds being rejected, which had lost
+their ornamental plumage; but in both cases (a widow-finch and a silver
+pheasant) the long tail-plumes are the indication of sexual maturity.
+Such cases do not support the idea that males with the tail-feathers a
+trifle longer, or the colours a trifle brighter, are generally
+preferred, and that those which are only a little inferior are as
+generally rejected,--and this is what is absolutely needed to establish
+the theory of the development of these plumes by means of the choice of
+the female.
+
+It will be seen, that female birds have unaccountable likes and dislikes
+in the matter of their partners, just as we have ourselves, and this may
+afford us an illustration. A young man, when courting, brushes or curls
+his hair, and has his moustache, beard, or whiskers in perfect order,
+and no doubt his sweetheart admires them; but this does not prove that
+she marries him on account of these ornaments, still less that hair,
+beard, whiskers, and moustache were developed by the continued
+preferences of the female sex. So, a girl likes to see her lover well
+and fashionably dressed, and he always dresses as well as he can when he
+visits her; but we cannot conclude from this that the whole series of
+male costumes, from the brilliantly coloured, puffed, and slashed
+doublet and hose of the Elizabethan period, through the gorgeous coats,
+long waistcoats, and pigtails of the early Georgian era, down to the
+funereal dress-suit of the present day, are the direct result of female
+preference. In like manner, female birds may be charmed or excited by
+the fine display of plumage by the males; but there is no proof whatever
+that slight differences in that display have any effect in determining
+their choice of a partner.
+
+
+_Display of Decorative Plumage._
+
+The extraordinary manner in which most birds display their plumage at
+the time of courtship, apparently with the full knowledge that it is
+beautiful, constitutes one of Mr. Darwin's strongest arguments. It is,
+no doubt, a very curious and interesting phenomenon, and indicates a
+connection between the exertion of particular muscles and the
+development of colour and ornament; but, for the reasons just given, it
+does not prove that the ornament has been developed by female choice.
+During excitement, and when the organism develops superabundant energy,
+many animals find it pleasurable to exercise their various muscles,
+often in fantastic ways, as seen in the gambols of kittens, lambs, and
+other young animals. But at the time of pairing, male birds are in a
+state of the most perfect development, and possess an enormous store of
+vitality; and under the excitement of the sexual passion they perform
+strange antics or rapid flights, as much probably from an internal
+impulse to motion and exertion as with any desire to please their mates.
+Such are the rapid descent of the snipe, the soaring and singing of the
+lark, and the dances of the cock-of-the-rock and of many other birds.
+
+It is very suggestive that similar strange movements are performed by
+many birds which have no ornamental plumage to display. Goatsuckers,
+geese, carrion vultures, and many other birds of plain plumage have been
+observed to dance, spread their wings or tails, and perform strange
+love-antics. The courtship of the great albatross, a most unwieldy and
+dull coloured bird, has been thus described by Professor Moseley: "The
+male, standing by the female on the nest, raises his wings, spreads his
+tail and elevates it, throws up his head with the bill in the air, or
+stretches it straight out, or forwards, as far as he can, and then
+utters a curious cry."[127] Mr. Jenner Weir informs me that "the male
+blackbird is full of action, spreads out his glossy wing and tail, turns
+his rich golden beak towards the female, and chuckles with delight,"
+while he has never seen the more plain coloured thrush demonstrative to
+the female. The linnet distends his rosy breast, and slightly expands
+his brown wings and tail; while the various gay coloured Australian
+finches adopt such attitudes and postures as, in every case, to show off
+their variously coloured plumage to the best advantage.[128]
+
+
+_A Theory of Animal Coloration._
+
+Having rejected Mr. Darwin's theory of female choice as incompetent to
+account for the brilliant colours and markings of the higher animals,
+the preponderance of these colours and markings in the male sex, and
+their display during periods of activity or excitement, I may be asked
+what explanation I have to offer as a preferable substitute. In my
+_Tropical Nature_ I have already indicated such a theory, which I will
+now briefly explain, supporting it by some additional facts and
+arguments, which appear to me to have great weight, and for which I am
+mainly indebted to a most interesting and suggestive posthumous work by
+Mr. Alfred Tylor.[129]
+
+The fundamental or ground colours of animals ar has been shown in
+preceding chapters, very largely protective, and it is not improbable
+that the primitive colours of all animals were so. During the long
+course of animal development other modes of protection than concealment
+by harmony of colour arose, and thenceforth the normal development of
+colour due to the complex chemical and structural changes ever going on
+in the organism, had full play; and the colours thus produced were again
+and again modified by natural selection for purposes of warning,
+recognition, mimicry, or special protection, as has been already fully
+explained in the preceding chapters.
+
+Mr. Taylor has, however, called attention to an important principle
+which underlies the various patterns or ornamental markings of
+animals--namely, that diversified coloration follows the chief lines of
+structure, and changes at points, such as the joints, where function
+changes. He says, "If we take highly decorated species--that is, animals
+marked by alternate dark or light bands or spots, such as the zebra,
+some deer, or the carnivora, we find, first, that the region of the
+spinal column is marked by a dark stripe; secondly, that the regions of
+the appendages, or limbs, are differently marked; thirdly, that the
+flanks are striped or spotted, along or between the regions of the lines
+of the ribs; fourthly, that the shoulder and hip regions are marked by
+curved lines; fifthly, that the pattern changes, and the direction of
+the lines, or spots, at the head, neck, and every joint of the limbs;
+and lastly, that the tips of the ears, nose, tail, and feet, and the eye
+are emphasised in colour. In spotted animals the greatest length of the
+spot is generally in the direction of the largest development of the
+skeleton."
+
+This structural decoration is well seen in many insects. In
+caterpillars, similar spots and markings are repeated in each segment,
+except where modified for some form of protection. In butterflies, the
+spots and bands usually have reference to the form of the wing and the
+arrangement of the nervures; and there is much evidence to show that the
+primitive markings are always spots in the cells, or between the
+nervures, or at the junctions of nervures, the extension and coalescence
+of these spots forming borders, bands, or blotches, which have become
+modified in infinitely varied ways for protection, warning, or
+recognition. Even in birds, the distribution of colours and markings
+follows generally the same law. The crown of the head, the throat, the
+ear-coverts, and the eyes have usually distinct tints in all highly
+coloured birds; the region of the furcula has often a distinct patch of
+colour, as have the pectoral muscles, the uropygium or root of the tail,
+and the under tail-coverts.[130]
+
+Mr. Tylor was of opinion the primitive form of ornamentation consisted
+of spots, the confluence of these in certain directions forming lines or
+bands; and, these again, sometimes coalescing into blotches, or into
+more or less uniform tints covering a large portion of the surface of
+the body. The young lion and tiger are both spotted; and in the Java hog
+(Sus vittatus) very young animals are banded, but have spots over the
+shoulders and thighs. These spots run into stripes as the animal grows
+older; then the stripes expand, and at last, meeting together, the adult
+animal becomes of a uniform dark brown colour. So many of the species of
+deer are spotted when young, that Darwin concludes the ancestral form,
+from which all deer are derived, must have been spotted. Pigs and tapirs
+are banded or spotted when young; an imported young specimen of Tapirus
+Bairdi was covered with white spots in longitudinal rows, here and there
+forming short stripes.[131] Even the horse, which Darwin supposes to be
+descended from a striped animal, is often spotted, as in dappled horses;
+and great numbers show a tendency to spottiness, especially on the
+haunches.
+
+Ocelli may also be developed from spots, or from bars, as pointed out by
+Mr. Darwin. Spots are an ordinary form of marking in disease, and these
+spots sometimes run together, forming blotches. There is evidence that
+colour markings are in some way dependent on nerve distribution. In the
+disease known as frontal herpes, an eruption occurs which corresponds
+exactly to the distribution of the ophthalmic division of the fifth
+cranial nerve, mapping out all its little branches even to the one which
+goes to the tip of the nose. In a Hindoo suffering from herpes the
+pigment was destroyed in the arm along the course of the ulnar nerve,
+with its branches along both sides of one finger and the half of
+another. In the leg the sciatic and scaphenous nerves were partly mapped
+out, giving to the patient the appearance of an anatomical diagram.[132]
+
+These facts are very interesting, because they help to explain the
+general dependence of marking on structure which has been already
+pointed out. For, as the nerves everywhere follow the muscles, and these
+are attached to the various bones, we see how it happens, that the
+tracts in which distinct developments of colour appear, should so often
+be marked out by the chief divisions of the bony structure in
+vertebrates, and by the segments in the annulosa. There is, however,
+another correspondence of even greater interest and importance.
+Brilliant colours usually appear just in proportion to the development
+of tegumentary appendages. Among birds the most brilliant colours are
+possessed by those which have developed frills, crests, and elongated
+tails like the humming-birds; immense tail-coverts like the peacock;
+enormously expanded wing-feathers, as in the argus-pheasant; or
+magnificent plumes from the region of the coracoids in many of the birds
+of paradise. It is to be noted, also, that all these accessory plumes
+spring from parts of the body which, in other species, are distinguished
+by patches of colour; so that we may probably impute the development of
+colour and of accessory plumage to the same fundamental cause.
+
+Among insects, the most brilliant and varied coloration occurs in the
+butterflies and moths, groups in which the wing-membranes have received
+their greatest expansion, and whose specialisation has been carried
+furthest in the marvellous scaly covering which is the seat of the
+colour. It is suggestive, that the only other group in which functional
+wings are much coloured is that of the dragonflies, where the membrane
+is exceedingly expanded. In like manner, the colours of beetles, though
+greatly inferior to those of the lepidoptera, occur in a group in which
+the anterior pair of wings has been thickened and modified in order to
+protect the vital parts, and in which these wing-covers (elytra), in the
+course of development in the different groups, must have undergone great
+changes, and have been the seat of very active growth.
+
+
+_The Origin of Accessory Plumes._
+
+Mr. Darwin supposes, that these have in almost every case been developed
+by the preference of female birds for such males as possessed them in a
+higher degree than others; but this theory does not account for the fact
+that these plumes usually appear in a few definite parts of the body. We
+require some cause to initiate the development in one part rather than
+in another. Now, the view that colour has arisen over surfaces where
+muscular and nervous development is considerable, and the fact that it
+appears especially upon the accessory or highly developed plumes, leads
+us to inquire whether the same cause has not primarily determined the
+development of these plumes. The immense tuft of golden plumage in the
+best known birds of paradise (Paradisea apoda and P. minor) springs
+from a very small area on the side of the breast. Mr. Frank E. Beddard,
+who has kindly examined a specimen for me, says that "this area lies
+upon the pectoral muscles, and near to the point where the fibres of the
+muscle converge towards their attachment to the humerus. The plumes
+arise, therefore, close to the most powerful muscle of the body, and
+near to where the activities of that muscle would be at a maximum.
+Furthermore, the area of attachment of the plumes is just above the
+point where the arteries and nerves for the supply of the pectoral
+muscles, and neighbouring regions, leave the interior of the body. The
+area of attachment of the plume is, also, as you say in your letter,
+just above the junction of the coracoid and sternum." Ornamental plumes
+of considerable size rise from the same part in many other species of
+paradise birds, sometimes extending laterally in front, so as to form
+breast shields. They also occur in many humming-birds, and in some
+sun-birds and honey-suckers; and in all these cases there is a wonderful
+amount of activity and rapid movement, indicating a surplus of vitality,
+which is able to manifest itself in the development of these accessory
+plumes.[133]
+
+In a quite distinct set of birds, the gallinaceae, we find the
+ornamental plumage usually arising from very different parts, in the
+form of elongated tail-feathers or tail-coverts, and of ruffs or hackles
+from the neck. Here the wings are comparatively little used, the most
+constant activities depending on the legs, since the gallinaceae are
+pre-eminently walking, running, and scratching birds. Now the
+magnificent train of the peacock--the grandest development of accessory
+plumes in this order--springs from an oval or circular area, about three
+inches in diameter, just above the base of the tail, and, therefore,
+situated over the lower part of the spinal column near the insertion of
+the powerful muscles which move the hind limbs and elevate the tail. The
+very frequent presence of neck-ruffs or breast-shields in the males of
+birds with accessory plumes may be partly due to selection, because they
+must serve as a protection in their mutual combats, just as does the
+lion's or the horse's mane. The enormously lengthened plumes of the bird
+of paradise and of the peacock can, however, have no such use, but must
+be rather injurious than beneficial in the bird's ordinary life. The
+fact that they have been developed to so great an extent in a few
+species is an indication of such perfect adaptation to the conditions of
+existence, such complete success in the battle for life, that there is,
+in the adult male at all events, a surplus of strength, vitality, and
+growth-power which is able to expend itself in this way without injury.
+That such is the case is shown by the great abundance of most of the
+species which possess these wonderful superfluities of plumage. Birds of
+paradise are among the commonest birds in New Guinea, and their loud
+voices can be often heard when the birds themselves are invisible in the
+depths of the forest; while Indian sportsmen have described the peafowl
+as being so abundant, that from twelve to fifteen hundred have been seen
+within an hour at one spot; and they range over the whole country from
+the Himalayas to Ceylon. Why, in allied species, the development of
+accessory plumes has taken different forms, we are unable to say, except
+that it may be due to that individual variability which has served as
+the starting-point for so much of what seems to us strange in form, or
+fantastic in colour, both in the animal and vegetable world.
+
+
+_Development of Accessory Plumes and their Display._
+
+If we have found a _vera causa_ for the origin of ornamental appendages
+of birds and other animals in a surplus of vital energy, leading to
+abnormal growths in those parts of the integument where muscular and
+nervous action are greatest, the continuous development of these
+appendages will result from the ordinary action of natural selection in
+preserving the most healthy and vigorous individuals, and the still
+further selective agency of sexual struggle in giving to the very
+strongest and most energetic the parentage of the next generation. And,
+as all the evidence goes to show that, so far as female birds exercise
+any choice, it is of "the most vigorous, defiant, and mettlesome male,"
+this form of sexual selection will act in the same direction, and help
+to carry on the process of plume development to its culmination. That
+culmination will be reached when the excessive length or abundance of
+the plumes begins to be injurious to the bearer of them; and it may be
+this check to the further lengthening of the peacock's train that has
+led to the broadening of the feathers at the ends, and the consequent
+production of the magnificent eye-spots which now form its crowning
+ornament.
+
+The display of these plumes will result from the same causes which led
+to their production. Just in proportion as the feathers themselves
+increased in length and abundance, the skin-muscles which serve to
+elevate them would increase also; and the nervous development as well as
+the supply of blood to these parts being at a maximum, the erection of
+the plumes would become a habit at all periods of nervous or sexual
+excitement. The display of the plumes, like the existence of the plumes
+themselves, would be the chief external indication of the maturity and
+vigour of the male, and would, therefore, be necessarily attractive to
+the female. We have, thus, no reason for imputing to her any of those
+aesthetic emotions which are excited in us, by the beauty of form,
+colour, and pattern of these plumes; or the still more improbable
+aesthetic tastes, which would cause her to choose her mate on account of
+minute differences in their forms, colours, or patterns.
+
+As co-operating causes in the production of accessory ornamental plumes,
+I have elsewhere suggested[134] that crests and other erectile feathers
+may have been useful in making the bird more formidable in appearance,
+and thus serving to frighten away enemies; while long tail or wing
+feathers might serve to distract the aim of a bird of prey. But though
+this might be of some use in the earlier stages of their development, it
+is probably of little importance compared with the vigour and pugnacity
+of which the plumes are the indication, and which enable most of their
+possessors to defend themselves against the enemies which are dangerous
+to weaker and more timid birds. Even the tiny humming-birds are said to
+attack birds of prey that approach too near to their nests.
+
+
+_The Effect of Female Preference will be Neutralised by Natural
+Selection._
+
+The various facts and arguments now briefly set forth, afford an
+explanation of the phenomena of male ornament, as being due to the
+general laws of growth and development, and make it unnecessary to call
+to our aid so hypothetical a cause as the cumulative action of female
+preference. There remains, however, a general argument, arising from the
+action of natural selection itself, which renders it almost
+inconceivable that female preference could have been effective in the
+way suggested; while the same argument strongly supports the view here
+set forth. Natural selection, as we have seen in our earlier chapters,
+acts perpetually and on an enormous scale in weeding out the "unfit" at
+every stage of existence, and preserving only those which are in all
+respects the very best. Each year, only a small percentage of young
+birds survive to take the place of the old birds which die; and the
+survivors will be those which are best able to maintain existence from
+the egg onwards, an important factor being that their parents should be
+well able to feed and protect them, while they themselves must in turn
+be equally able to feed and protect their own offspring. Now this
+extremely rigid action of natural selection must render any attempt to
+select mere ornament utterly nugatory, unless the most ornamented always
+coincide with "the fittest" in every other respect; while, if they do so
+coincide, then any selection of ornament is altogether superfluous. If
+the most brightly coloured and fullest plumaged males are _not_ the most
+healthy and vigorous, have _not_ the best instincts for the proper
+construction and concealment of the nest, and for the care and
+protection of the young, they are certainly not the fittest, and will
+not survive, or be the parents of survivors. If, on the other hand,
+there _is_ generally this correlation--if, as has been here argued,
+ornament is the natural product and direct outcome of superabundant
+health and vigour, then no other mode of selection is needed to account
+for the presence of such ornament. The action of natural selection does
+not indeed disprove the existence of female selection of ornament as
+ornament, but it renders it entirely ineffective; and as the direct
+evidence for any such female selection is almost _nil_, while the
+objections to it are certainly weighty, there can be no longer any
+reason for upholding a theory which was provisionally useful in calling
+attention to a most curious and suggestive body of facts, but which is
+now no longer tenable. The term "sexual selection" must, therefore, be
+restricted to the direct results of male struggle and combat. This is
+really a form of natural selection, and is a matter of direct
+observation; while its results are as clearly deducible as those of any
+of the other modes in which selection acts. And if this restriction of
+the term is needful in the case of the higher animals it is much more so
+with the lower. In butterflies the weeding out by natural selection
+takes place to an enormous extent in the egg, larva, and pupa states;
+and perhaps not more than one in a hundred of the eggs laid produces a
+perfect insect which lives to breed. Here, then, the impotence of female
+selection, if it exist, must be complete; for, unless the most
+brilliantly coloured males are those which produce the best protected
+eggs, larvae, and pupae, and unless the particular eggs, larvae, and
+pupae, which are able to survive, are those which produce the most
+brilliantly coloured butterflies, any choice the female might make must
+be completely swamped. If, on the other hand, there _is_ this
+correlation between colour development and perfect adaptation to
+conditions in all stages, then this development will necessarily proceed
+by the agency of natural selection and the general laws which determine
+the production of colour and of ornamental appendages.[135]
+
+
+_General Laws of Animal Coloration._
+
+The condensed account which has now been given of the phenomena of
+colour in the animal world will sufficiently show the wonderful
+complexity and extreme interest of the subject; while it affords an
+admirable illustration of the importance of the great principle of
+utility, and of the effect of the theories of natural selection and
+development in giving a new interest to the most familiar facts of
+nature. Much yet remains to be done, both in the observation of new
+facts as to the relations between the colours of animals and their
+habits or economy, and, more especially, in the elucidation of the laws
+of growth which determine changes of colour in the various groups; but
+so much is already known that we are able, with some confidence, to
+formulate the general principles which have brought about all the beauty
+and variety of colour which everywhere delight us in our contemplation
+of animated nature. A brief statement of these principles will fitly
+conclude our exposition of the subject.
+
+1. Colour may be looked upon as a necessary result of the highly complex
+chemical constitution of animal tissues and fluids. The blood, the bile,
+the bones, the fat, and other tissues have characteristic, and often
+brilliant colours, which we cannot suppose to have been determined for
+any special purpose, as colours, since they are usually concealed. The
+external organs, with their various appendages and integuments, would,
+by the same general laws, naturally give rise to a greater variety of
+colour.
+
+2. We find it to be the fact that colour increases in variety and
+intensity as external structures and dermal appendages become more
+differentiated and developed. It is on scales, hair, and especially on
+the more highly specialised feathers, that colour is most varied and
+beautiful; while among insects colour is most fully developed in those
+whose wing membranes are most expanded, and, as in the lepidoptera, are
+clothed with highly specialised scales. Here, too, we find an additional
+mode of colour production in transparent lamellae or in fine surface
+striae which, by the laws of interference, produce the wonderful
+metallic hues of so many birds and insects.
+
+3. There are indications of a progressive change of colour, perhaps in
+some definite order, accompanying the development of tissues or
+appendages. Thus spots spread and fuse into bands, and when a lateral or
+centrifugal expansion has occurred--as in the termination of the
+peacocks' train feathers, the outer web of the secondary quills of the
+Argus pheasant, or the broad and rounded wings of many butterflies--into
+variously shaded or coloured ocelli. The fact that we find gradations of
+colour in many of the more extensive groups, from comparatively dull or
+simple to brilliant and varied hues, is an indication of some such law
+of development, due probably to progressive local segregation in the
+tissues of identical chemical or organic molecules, and dependent on
+laws of growth yet to be investigated.
+
+4. The colours thus produced, and subject to much individual variation,
+have been modified in innumerable ways for the benefit of each species.
+The most general modification has been in such directions as to favour
+concealment when at rest in the usual surroundings of the species,
+sometimes carried on by successive steps till it has resulted in the
+most minute imitation of some inanimate object or exact mimicry of some
+other animal. In other cases bright colours or striking contrasts have
+been preserved, to serve as a warning of inedibility or of dangerous
+powers of attack. Most frequent of all has been the specialisation of
+each distinct form by some tint or marking for purposes of easy
+recognition, especially in the case of gregarious animals whose safety
+largely depends upon association and mutual defence.
+
+5. As a general rule the colours of the two sexes are alike; but in the
+higher animals there appears a tendency to deeper or more intense
+colouring in the male, due probably to his greater vigour and
+excitability. In many groups in which this superabundant vitality is at
+a maximum, the development of dermal appendages and brilliant colours
+has gone on increasing till it has resulted in a great diversity between
+the sexes; and in most of these cases there is evidence to show that
+natural selection has caused the female to retain the primitive and more
+sober colours of the group for purposes of protection.
+
+
+_Concluding Remarks._
+
+The general principles of colour development now sketched out enable us
+to give some rational explanation of the wonderful amount of brilliant
+colour which occurs among tropical animals. Looking on colour as a
+normal product of organisation, which has either been allowed free play,
+or has been checked and modified for the benefit of the species, we can
+see at once that the luxuriant and perennial vegetation of the tropics,
+by affording much more constant means of concealment, has rendered
+brilliant colour less hurtful there than in the temperate and colder
+regions. Again, this perennial vegetation supplies abundance of both
+vegetable and insect food throughout the year, and thus a greater
+abundance and greater variety of the forms of life are rendered
+possible, than where recurrent seasons of cold and scarcity reduce the
+possibilities of life to a minimum. Geology furnishes us with another
+reason, in the fact, that throughout the tertiary period tropical
+conditions prevailed far into the temperate regions, so that the
+possibilities of colour development were still greater than they are at
+the present time. The tropics, therefore, present to us the results of
+animal development in a much larger area and under more favourable
+conditions than prevail to-day. We see in them samples of the
+productions of an earlier and a better world, from an animal point of
+view; and this probably gives a greater variety and a finer display of
+colour than would have been produced, had conditions always been what
+they are now. The temperate zones, on the other hand, have recently
+suffered the effects of a glacial period of extreme severity, with the
+result that almost the only gay coloured birds they now possess are
+summer visitors from tropical or sub-tropical lands. It is to the
+unbroken and almost unchecked course of development from remote
+geological times that has prevailed in the tropics, favoured by abundant
+food and perennial shelter, that we owe such superb developments as the
+frills and crests and jewelled shields of the humming-birds, the golden
+plumes of the birds of paradise, and the resplendent train of the
+peacock. This last exhibits to us the culmination of that marvel and
+mystery of animal colour which is so well expressed by a poet-artist in
+the following lines. The marvel will ever remain to the sympathetic
+student of nature, but I venture to hope that in the preceding chapters
+I have succeeded in lifting--if only by one of its corners--the veil of
+mystery which has for long shrouded this department of nature.
+
+
+_On a Peacock's Feather._
+
+    In Nature's workshop but a shaving,
+      Of her poem but a word,
+    But a tint brushed from her palette,
+      This feather of a bird!
+    Yet set it in the sun glance,
+      Display it in the shine,
+    Take graver's lens, explore it,
+      Note filament and line,
+    Mark amethyst to sapphire,
+      And sapphire to gold,
+    And gold to emerald changing
+      The archetype unfold!
+    Tone, tint, thread, tissue, texture,
+      Through every atom scan,
+    Conforming still, developing,
+      Obedient to plan.
+    This but to form a pattern
+      On the garment of a bird!
+    What then must be the poem,
+      This but its lightest word!
+    Sit before it; ponder o'er it,
+      'Twill thy mind advantage more,
+    Than a treatise, than a sermon,
+      Than a library of lore.
+
+FOOTNOTES:
+
+[Footnote 118: Darwin's _Descent of Man_, p. 271.]
+
+[Footnote 119: Darwin's _Descent of Man_, p. 294, and footnote.]
+
+[Footnote 120: _Nature_, 1871, p. 489.]
+
+[Footnote 121: Darwin in _Nature_, 1880, p. 237.]
+
+[Footnote 122: See the author's _Contributions to Natural Selection_,
+chap. vii. in which these facts were first brought forward.]
+
+[Footnote 123: On this point see the author's _Contributions to Natural
+Selection_, chap. v. i.]
+
+[Footnote 124: Seebohm's _History of British Birds_, vol. ii.,
+introduction, p. xiii.]
+
+[Footnote 125: For details see Darwin's _Descent of Man_, chap. xii.]
+
+[Footnote 126: _Descent of Man_, pp. 417, 418, 420.]
+
+[Footnote 127: _Notes of a Naturalist on the Challenger._]
+
+[Footnote 128: _Descent of Man_, pp. 401, 402.]
+
+[Footnote 129: _Coloration in Animals and Plants_, London, 1886.]
+
+[Footnote 130: _Coloration of Animals_, Pl. X, p. 90; and Pls. II, III,
+and IV, pp. 30, 40, 42.]
+
+[Footnote 131: See coloured Fig. in _Proc. Zool. Soc._, 1871, p. 626.]
+
+[Footnote 132: A. Tylor's _Coloration_, p. 40; and Photograph in
+Hutchinson's _Illustrations of Clinical Surgery_, quoted by Tylor.]
+
+[Footnote 133: For activity and pugnacity of humming-birds, see
+_Tropical Nature_, pp. 130, 213.]
+
+[Footnote 134: _Tropical Nature_, p. 209. In Chapter V of this work the
+views here advocated were first set forth, and the reader is referred
+there for further details.]
+
+[Footnote 135: The Rev. O. Pickard-Cambridge, who has devoted himself to
+the study of spiders, has kindly sent me the following extract from a
+letter, written in 1869, in which he states his views on this
+question:--
+
+
+    "I myself doubt that particular application of the Darwinian
+    theory which attributes male peculiarities of form, structure,
+    colour, and ornament to female appetency or predilection. There
+    is, it seems to me, undoubtedly something in the male
+    organisation of a special, and sexual nature, which, of its own
+    vital force, develops the remarkable male peculiarities so
+    commonly seen, and of no imaginable use to that sex. In as far
+    as these peculiarities show a great vital power, they point out
+    to us the finest and strongest individuals of the sex, and show
+    us which of them would most certainly appropriate to themselves
+    the best and greatest number of females, and leave behind them
+    the strongest and greatest number of progeny. And here would
+    come in, as it appears to me, the proper application of Darwin's
+    theory of Natural Selection; for the possessors of greatest
+    vital power being those most frequently produced and reproduced,
+    the external signs of it would go on developing in an
+    ever-increasing exaggeration, only to be checked where it became
+    really detrimental in some respect or other to the individual."
+
+
+This passage, giving the independent views of a close observer--one,
+moreover, who has studied the species of an extensive group of animals
+both in the field and in the laboratory--very nearly accords with my own
+conclusions above given; and, so far as the matured opinions of a
+competent naturalist have any weight, afford them an important support.]
+
+
+
+
+CHAPTER XI
+
+THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE
+
+
+    The general colour relations of plants--Colours of fruits--The
+    meaning of nuts--Edible or attractive fruits--The colours of
+    flowers--Modes of securing cross-fertilisation--The
+    interpretation of the facts--Summary of additional facts bearing
+    on insect fertilisation--Fertilisation of flowers by
+    birds--Self-fertilisation of flowers--Difficulties and
+    contradictions--Intercrossing not necessarily
+    advantageous--Supposed evil results of close interbreeding--How
+    the struggle for existence acts among flowers--Flowers the
+    product of insect agency--Concluding remarks on colour in
+    nature.
+
+
+
+The colours of plants are both less definite and less complex than are
+those of animals, and their interpretation on the principle of utility
+is, on the whole, more direct and more easy. Yet here, too, we find that
+in our investigation of the uses of the various colours of fruits and
+flowers, we are introduced to some of the most obscure recesses of
+nature's workshop, and are confronted with problems of the deepest
+interest and of the utmost complexity.
+
+So much has been written on this interesting subject since Mr. Darwin
+first called attention to it, and its main facts have become so
+generally known by means of lectures, articles, and popular books, that
+I shall give here a mere outline sketch, for the purpose of leading up
+to a discussion of some of the more fundamental problems which arise out
+of the facts, and which have hitherto received less attention than they
+deserve.
+
+
+_The General Colour Relations of Plants._
+
+The green colour of the foliage of leafy plants is due to the existence
+of a substance called chlorophyll, which is almost universally developed
+in the leaves under the action of light. It is subject to definite
+chemical changes during the processes of growth and of decay, and it is
+owing to these changes that we have the delicate tints of spring
+foliage, and the more varied, intense, and gorgeous hues of autumn. But
+these all belong to the class of intrinsic or normal colours, due to the
+chemical constitution of the organism; as colours they are unadaptive,
+and appear to have no more relation to the wellbeing of the plants
+themselves than have the colours of gems and minerals. We may also
+include in the same category those algae and fungi which have bright
+colours--the "red snow" of the arctic regions, the red, green, or purple
+seaweeds, the brilliant scarlet, yellow, white, or black agarics, and
+other fungi. All these colours are probably the direct results of
+chemical composition or molecular structure, and, being thus normal
+products of the vegetable organism, need no special explanation from our
+present point of view; and the same remark will apply to the varied
+tints of the bark of trunks, branches, and twigs, which are often of
+various shades of brown and green, or even vivid reds or yellows.
+
+There are, however, a few cases in which the need of protection, which
+we have found to be so important an agency in modifying the colours of
+animals, has also determined those of some of the smaller members of the
+vegetable kingdom. Dr. Burchell found a mesembryanthomum in South Africa
+like a curiously shaped pebble, closely resembling the stones among
+which it grew;[136] and Mr. J.P. Mansel Weale states that in the same
+country one of the Asclepiadeae has tubers growing above ground among
+stones which they exactly resemble, and that, when not in leaf, they are
+for this reason quite invisible.[137] It is clear that such resemblances
+must be highly useful to these plants, inhabiting an arid country
+abounding in herbivorous mammalia, which, in times of drought or
+scarcity, will devour everything in the shape of a fleshy stem or tuber.
+
+True mimicry is very rare in plants, though adaptation to like
+conditions often produces in foliage and habit a similarity that is
+deceiving. Euphorbias growing in deserts often closely resemble cacti.
+Seaside plants and high alpine plants of different orders are often much
+alike; and innumerable resemblances of this kind are recorded in the
+names of plants, as Veronica epacridea (the veronica like an epacris),
+Limnanthemum nymphaeoides (the limnanthemum like a nymphaea), the
+resembling species in each case belonging to totally distinct families.
+But in these cases, and in most others that have been observed, the
+essential features of true mimicry are absent, inasmuch as the one plant
+cannot be supposed to derive any benefit from its close resemblance to
+the other, and this is still more certain from the fact that the two
+species usually inhabit different localities. A few cases exist,
+however, in which there does seem to be the necessary accordance and
+utility. Mr. Mansel Weale mentions a labiate plant (Ajuga ophrydis), the
+only species of the genus Ajuga in South Africa, which is strikingly
+like an orchid of the same country; while a balsam (Impatiens capensis),
+also a solitary species of the genus in that country, is equally like an
+orchid, growing in the same locality and visited by the same insects. As
+both these genera of plants are specialised for insect fertilisation,
+and both of the plants in question are isolated species of their
+respective genera, we may suppose that, when they first reached South
+Africa they were neglected by the insects of the country; but, being
+both remotely like orchids in form of flower, those varieties that
+approached nearest to the familiar species of the country were visited
+by insects and cross-fertilised, and thus a closer resemblance would at
+length be brought about. Another case of close general resemblance, is
+that of our common white dead-nettle (Lamium album) to the
+stinging-nettle (Urtica dioica); and Sir John Lubbock thinks that this
+is a case of true mimicry, the dead-nettle being benefited by being
+mistaken by grazing animals for the stinging-nettle.[138]
+
+
+_Colours of Fruits._
+
+It is when we come to the essential parts of plants on which their
+perpetuation and distribution depends, that we find colour largely
+utilised for a distinct purpose in flowers and fruits. In the former we
+find attractive colours and guiding marks to secure cross-fertilisation
+by insects; in the latter attractive or protective coloration, the first
+to attract birds or other animals when the fruits are intended to be
+eaten, the second to enable them to escape being eaten when it would be
+injurious to the species. The colour phenomena of fruits being much the
+most simple will be considered first.
+
+The perpetuation and therefore the very existence of each species of
+flowering plant depend upon its seeds being preserved from destruction
+and more or less effectually dispersed over a considerable area. The
+dispersal is effected either mechanically or by the agency of animals.
+Mechanical dispersal is chiefly by means of air-currents, and large
+numbers of seeds are specially adapted to be so carried, either by being
+clothed with down or pappus, as in the well-known thistle and dandelion
+seeds; by having wings or other appendages, as in the sycamore, birch,
+and many other trees; by being thrown to a considerable distance by the
+splitting of the seed-vessel, and by many other curious devices.[139]
+Very large numbers of seeds, however, are so small and light that they
+can be carried enormous distances by gales of wind, more especially as
+most of this kind are flattened or curved, so as to expose a large
+surface in proportion to their weight. Those which are carried by
+animals have their surfaces, or that of the seed-vessel, armed with
+minute hooks, or some prickly covering which attaches itself to the hair
+of mammalia or the feathers of birds, as in the burdock, cleavers, and
+many other species. Others again are sticky, as in Plumbago europaea,
+mistletoe, and many foreign plants.
+
+All the seeds or seed-vessels which are adapted to be dispersed in any
+of these ways are of dull protective tints, so that when they fall on
+the ground they are almost indistinguishable; besides which, they are
+usually small, hard, and altogether unattractive, never having any
+soft, juicy pulp; while the edible seeds often bear such a small
+proportion to the hard, dry envelopes or appendages, that few animals
+would care to eat them.
+
+
+_The Meaning of Nuts._
+
+There is, however, another class of fruits or seeds, usually termed
+nuts, in which there is a large amount of edible matter, often very
+agreeable to the taste, and especially attractive and nourishing to a
+large number of animals. But when eaten, the seed is destroyed and the
+existence of the species endangered. It is evident, therefore, that it
+is by a kind of accident that these nuts are eatable; and that they are
+not intended to be eaten is shown by the special care nature seems to
+have taken to conceal or to protect them. We see that all our common
+nuts are green when on the tree, so as not easily to be distinguished
+from the leaves; but when ripe they turn brown, so that when they fall
+on to the ground they are equally indistinguishable among the dead
+leaves and twigs, or on the brown earth. Then they are almost always
+protected by hard coverings, as in hazel-nuts, which are concealed by
+the enlarged leafy involucre, and in the large tropical brazil-nuts and
+cocoa-nuts by such a hard and tough case as to be safe from almost every
+animal. Others have an external bitter rind, as in the walnut; while in
+the chestnuts and beech-nuts two or three fruits are enclosed in a
+prickly involucre.
+
+Notwithstanding all these precautions, nuts are largely devoured by
+mammalia and birds; but as they are chiefly the product of trees or
+shrubs of considerable longevity, and are generally produced in great
+profusion, the perpetuation of the species is not endangered. In some
+cases the devourers of nuts may aid in their dispersal, as they probably
+now and then swallow the seed whole, or not sufficiently crushed to
+prevent germination; while squirrels have been observed to bury nuts,
+many of which are forgotten and afterwards grow in places they could not
+have otherwise reached.[140] Nuts, especially the larger kinds which are
+so well protected by their hard, nearly globular cases, have their
+dispersal facilitated by rolling down hill, and more especially by
+floating in rivers and lakes, and thus reaching other localities. During
+the elevation of land areas this method would be very effective, as the
+new land would always be at a lower level than that already covered with
+vegetation, and therefore in the best position for being stocked with
+plants from it.
+
+The other modes of dispersal of seeds are so clearly adapted to their
+special wants, that we feel sure they must have been acquired by the
+process of variation and natural selection. The hooked and sticky seeds
+are always those of such herbaceous plants as are likely, from their
+size, to come in contact with the wool of sheep or the hair of cattle;
+while seeds of this kind never occur on forest trees, on aquatic plants,
+or even on very dwarf creepers or trailers. The winged seed-vessels or
+seeds, on the other hand, mostly belong to trees and to tall shrubs or
+climbers. We have, therefore, a very exact adaptation to conditions in
+these different modes of dispersal; while, when we come to consider
+individual cases, we find innumerable other adaptations, some of which
+the reader will find described in the little work by Sir John Lubbock
+already referred to.
+
+
+_Edible or Attractive Fruits._
+
+It is, however, when we come to true fruits (in a popular sense) that we
+find varied colours evidently intended to attract animals, in order that
+the fruits may be eaten, while the seeds pass through the body
+undigested and are then in the fittest state for germination. This end
+has been gained in a great variety of ways, and with so many
+corresponding adaptations as to leave no doubt as to the value of the
+result. Fruits are pulpy or juicy, and usually sweet, and form the
+favourite food of innumerable birds and some mammals. They are always
+coloured so as to contrast with the foliage or surroundings, red being
+the most common as it is certainly the most conspicuous colour, but
+yellow, purple, black, or white being not uncommon. The edible portion
+of fruits is developed from different parts of the floral envelopes, or
+of the ovary, in the various orders and genera. Sometimes the calyx
+becomes enlarged and fleshy, as in the apple and pear tribe; more often
+the integuments of the ovary itself are enlarged, as in the plum, peach,
+grape, etc.; the receptacle is enlarged and forms the fruit of the
+strawberry; while the mulberry, pineapple, and fig are examples of
+compound fruits formed in various ways from a dense mass of flowers.
+
+In all cases the seeds themselves are protected from injury by various
+devices. They are small and hard in the strawberry, raspberry, currant,
+etc., and are readily swallowed among the copious pulp. In the grape
+they are hard and bitter; in the rose (hip) disagreeably hairy; in the
+orange tribe very bitter; and all these have a smooth, glutinous
+exterior which facilitates their being swallowed. When the seeds are
+larger and are eatable, they are enclosed in an excessively hard and
+thick covering, as in the various kinds of "stone" fruit (plums,
+peaches, etc.), or in a very tough core, as in the apple. In the nutmeg
+of the Eastern Archipelago we have a curious adaptation to a single
+group of birds. The fruit is yellow, somewhat like an oval peach, but
+firm and hardly eatable. This splits open and shows the glossy black
+covering of the seed or nutmeg, over which spreads the bright scarlet
+arillus or "mace," an adventitious growth of no use to the plant except
+to attract attention. Large fruit pigeons pluck out this seed and
+swallow it entire for the sake of the mace, while the large nutmeg
+passes through their bodies and germinates; and this has led to the wide
+distribution of wild nutmegs over New Guinea and the surrounding
+islands.
+
+In the restriction of bright colour to those edible fruits the eating of
+which is beneficial to the plant, we see the undoubted result of natural
+selection; and this is the more evident when we find that the colour
+never appears till the fruit is ripe--that is, till the seeds within it
+are fully matured and in the best state for germination. Some
+brilliantly coloured fruits are poisonous, as in our bitter-sweet
+(Solanum dulcamara), cuckoo-pint (Arum) and the West Indian manchineel.
+Many of these are, no doubt, eaten by animals to whom they are harmless;
+and it has been suggested that even if some animals are poisoned by them
+the plant is benefited, since it not only gets dispersed, but finds, in
+the decaying body of its victim, a rich manure heap.[141] The particular
+colours of fruits are not, so far as we know, of any use to them other
+than as regards conspicuousness, hence a tendency to _any_ decided
+colour has been preserved and accumulated as serving to render the fruit
+easily visible among its surroundings of leaves or herbage. Out of 134
+fruit-bearing plants in Mongredien's _Trees and Shrubs_, and Hooker's
+_British Flora_, the fruits of no less than sixty-eight, or rather more
+than half, are red, forty-five are black, fourteen yellow, and seven
+white. The great prevalence of red fruits is almost certainly due to
+their greater conspicuousness having favoured their dispersal, though it
+may also have arisen in part from the chemical changes of chlorophyll
+during ripening and decay producing red tints as in many fading leaves.
+Yet the comparative scarcity of yellow in fruits, while it is the most
+common tint of fading leaves, is against this supposition.
+
+There are, however, a few instances of coloured fruits which do not seem
+to be intended to be eaten; such are the colocynth plant (Cucumis
+colocynthus), which has a beautiful fruit the size and colour of an
+orange, but nauseous beyond description to the taste. It has a hard
+rind, and may perhaps be dispersed by being blown along the ground, the
+colour being an adventitious product; but it is quite possible,
+notwithstanding its repulsiveness to us, that it may be eaten by some
+animals. With regard to the fruit of another plant, Calotropis procera,
+there is less doubt, as it is dry and full of thin, flat-winged seeds,
+with fine silky filaments, eminently adapted for wind-dispersal; yet it
+is of a bright yellow colour, as large as an apple, and therefore very
+conspicuous. Here, therefore, we seem to have colour which is a mere
+byproduct of the organism and of no use to it; but such cases are
+exceedingly rare, and this rarity, when compared with the great
+abundance of cases in which there is an obvious purpose in the colour,
+adds weight to the evidence in favour of the theory of the attractive
+coloration of edible fruits in order that birds and other animals may
+assist in their dispersal. Both the above-named plants are natives of
+Palestine and the adjacent arid countries.[142]
+
+
+_The Colours of Flowers._
+
+Flowers are much more varied in their colours than fruits, as they are
+more complex and more varied in form and structure; yet there is some
+parallelism between them in both respects. Flowers are frequently
+adapted to attract insects as fruits are to attract birds, the object
+being in the former to secure cross-fertilisation, in the latter
+dispersal; while just as colour is an index of the edibility of fruits
+which supply pulp or juice to birds, so are the colours of flowers an
+indication of the presence of nectar or of pollen which are devoured by
+insects.
+
+The main facts and many of the details, as to the relation of insects to
+flowers, were discovered by Sprengel in 1793. He noticed the curious
+adaptation of the structure of many flowers to the particular insects
+which visit them; he proved that insects do cross-fertilise flowers, and
+he believed that this was the object of the adaptations, while the
+presence of nectar and pollen ensured the continuance of their visits;
+yet he missed discovering the _use_ of this cross-fertilisation. Several
+writers at a later period obtained evidence that cross-fertilisation of
+plants was a benefit to them; but the wide generality of this fact and
+its intimate connection with the numerous and curious adaptations
+discovered by Sprengel, was first shown by Mr. Darwin, and has since
+been demonstrated by a vast mass of observations, foremost among which
+are his own researches on orchids, primulas, and other plants.[143]
+
+By an elaborate series of experiments carried on for many years Mr.
+Darwin demonstrated the great value of cross-fertilisation in increasing
+the rapidity of growth, the strength and vigour of the plant, and in
+adding to its fertility. This effect is produced immediately, not as he
+expected would be the case, after several generations of crosses. He
+planted seeds from cross-fertilised and self-fertilised plants on two
+sides of the same pot exposed to exactly similar conditions, and in most
+cases the difference in size and vigour was amazing, while the plants
+from cross-fertilised parents also produced more and finer seeds. These
+experiments entirely confirmed the experience of breeders of animals
+already referred to (p. 160), and led him to enunciate his famous
+aphorism, "Nature abhors perpetual self-fertilisation".[144] In this
+principle we appear to have a sufficient reason for the various
+contrivances by which so many flowers secure cross-fertilisation, either
+constantly or occasionally. These contrivances are so numerous, so
+varied, and often so highly complex and extraordinary, that they have
+formed the subject of many elaborate treatises, and have also been amply
+popularised in lectures and handbooks. It will be unnecessary,
+therefore, to give details here, but the main facts will be summarised
+in order to call attention to some difficulties of the theory which seem
+to require further elucidation.
+
+
+_Modes of securing Cross-Fertilisation._
+
+When we examine the various modes in which the cross-fertilisation of
+flowers is brought about, we find that some are comparatively simple in
+their operation and needful adjustments, others highly complex. The
+simple methods belong to four principal classes:--(1) By dichogamy--that
+is, by the anthers and the stigma becoming mature or in a fit state for
+fertilisation at slightly different times on the same plant. The result
+of this is that, as plants in different stations, on different soils, or
+exposed to different aspects flower earlier or later, the mature pollen
+of one plant can only fertilise some plant exposed to somewhat different
+conditions or of different constitution, whose stigma will be mature at
+the same time; and this difference has been shown by Darwin to be that
+which is adapted to secure the fullest benefit of cross-fertilisation.
+This occurs in Geranium pratense, Thymus serpyllum, Arum maculatum, and
+many others. (2) By the flower being self-sterile with its own pollen,
+as in the crimson flax. This absolutely prevents self-fertilisation. (3)
+By the stamens and anthers being so placed that the pollen cannot fall
+upon the stigma, while it does fall upon a visiting insect which carries
+it to the stigma of another flower. This effect is produced in a variety
+of very simple ways, and is often aided by the motion of the stamens
+which bend down out of the way of the stigmas before the pollen is ripe,
+as in Malva sylvestris (see Fig. 28). (4) By the male and female flowers
+being on different plants, forming the class Dioecia of Linnaeus. In
+these cases the pollen may be carried to the stigmas either by the wind
+or by the agency of insects.
+
+[Illustration: FIG. 28.
+
+Malva sylvestris, adapted for insect-fertilisation.
+
+Malva rotundifolia, adapted for self-fertilisation.]
+
+Now these four methods are all apparently very simple, and easily
+produced by variation and selection. They are applicable to flowers of
+any shape, requiring only such size and colour as to attract insects,
+and some secretion of nectar to ensure their repeated visits, characters
+common to the great majority of flowers. All these methods are common,
+except perhaps the second; but there are many flowers in which the
+pollen from another plant is prepotent over the pollen from
+fertilisation, the same flower, and this has nearly the same effect as
+self-sterility if the flowers are frequently crossed by insects. We
+cannot help asking, therefore, why have other and much more elaborate
+methods been needed? And how have the more complex arrangements of so
+many flowers been brought about? Before attempting to answer these
+questions, and in order that the reader may appreciate the difficulty of
+the problem and the nature of the facts to be explained, it will be
+necessary to give a summary of the more elaborate modes of securing
+cross-fertilisation.
+
+(1) We first have dimorphism and heteromorphism, the phenomena of which
+have been already sketched in our seventh chapter.
+
+Here we have both a mechanical and a physiological modification, the
+stamens and pistil being variously modified in length and position,
+while the different stamens in the same flower have widely different
+degrees of fertility when applied to the same stigma,--a phenomenon
+which, if it were not so well established, would have appeared in the
+highest degree improbable. The most remarkable case is that of the three
+different forms of the loosestrife (Lythrum salicaria) here figured
+(Fig. 29 on next page).
+
+(2) Some flowers have irritable stamens which, when their bases are
+touched by an insect, spring up and dust it with pollen. This occurs in
+our common berberry.
+
+[Illustration: FIG. 29.--Lythrum salicaria (Purple loosestrife).]
+
+(3) In others there are levers or processes by which the anthers are
+mechanically brought down on to the head or back of an insect entering
+the flower, in such a position as to be carried to the stigma of the
+next flower it visits. This may be well seen in many species of Salvia
+and Erica.
+
+(4) In some there is a sticky secretion which, getting on to the
+proboscis of an insect, carries away the pollen, and applies it to the
+stigma of another flower. This occurs in our common milkwort (Polygala
+vulgaris).
+
+(5) In papilionaceous plants there are many complex adjustments, such as
+the squeezing out of pollen from a receptacle on to an insect, as in
+Lotus corniculatus, or the sudden springing out and exploding of the
+anthers so as thoroughly to dust the insect, as in Medicago falcata,
+this occurring after the stigma has touched the insect and taken off
+some pollen from the last flower.
+
+(6) Some flowers or spathes form closed boxes in which insects find
+themselves entrapped, and when they have fertilised the flower, the
+fringe of hairs opens and allows them to escape. This occurs in many
+species of Arum and Aristolochia.
+
+(7) Still more remarkable are the traps in the flower of Asclepias which
+catch flies, butterflies, and wasps by the legs, and the wonderfully
+complex arrangements of the orchids. One of these, our common Orchis
+pyramidalis, may be briefly described to show how varied and beautiful
+are the arrangements to secure cross-fertilisation. The broad trifid lip
+of the flower offers a support to the moth which is attracted by its
+sweet odour, and two ridges at the base guide the proboscis with
+certainty to the narrow entrance of the nectary. When the proboscis has
+reached the end of the spur, its basal portion depresses the little
+hinged rostellum that covers the saddle-shaped sticky glands to which
+the pollen masses (pollinia) are attached. On the proboscis being
+withdrawn, the two pollinia stand erect and parallel, firmly attached to
+the proboscis. In this position, however, they would be useless, as they
+would miss the stigmatic surface of the next flower visited by the moth.
+But as soon as the proboscis is withdrawn, the two pollen masses begin
+to diverge till they are exactly as far apart as are the stigmas of the
+flower; and then commences a second movement which brings them down
+till they project straight forward nearly at right angles to their first
+position, so as exactly to hit against the stigmatic surfaces of the
+next flower visited on which they leave a portion of their pollen. The
+whole of these motions take about half a minute, and in that time the
+moth will usually have flown to another plant, and thus effect the most
+beneficial kind of cross-fertilisation.[145] This description will be
+better understood by referring to the illustration opposite, from
+Darwin's _Fertilisation of Orchids_(Fig. 30).
+
+[Illustration: FIG. 30.--Orchis pyramidalis.]
+
+
+_The Interpretation of these Facts._
+
+Having thus briefly indicated the general character of the more complex
+adaptations for cross-fertilisation, the details of which are to be
+found in any of the numerous works on the subject,[146] we find
+ourselves confronted with the very puzzling question--Why were these
+innumerable highly complex adaptations produced, when the very same
+result may be effected--and often is effected--by extremely simple
+means? Supposing, as we must do, that all flowers were once of simple
+and regular forms, like a buttercup or a rose, how did such irregular
+and often complicated flowers as the papilionaceous or pea family, the
+labiates or sage family, and the infinitely varied and fantastic orchids
+ever come into existence? No cause has yet been suggested but the need
+of attracting insects to cross-fertilise them; yet the attractiveness of
+regular flowers with bright colours and an ample supply of nectar is
+equally great, and cross-fertilisation can be quite as effectively
+secured in these by any of the four simple methods already described.
+Before attempting to suggest a possible solution of this difficult
+problem, we have yet to pass in review a large body of curious
+adaptations connected with insect fertilisation, and will first call
+attention to that portion of the phenomena which throw some light upon
+the special colours of flowers in their relation to the various kinds of
+insects which visit them. For these facts we are largely indebted to
+the exact and long-continued researches of Professor Hermann Müller.
+
+
+_Summary of Additional Facts bearing on Insect Fertilisation._
+
+1. That the size and colour of a flower are important factors in
+determining the visits of insects, is shown by the general fact of more
+insects visiting conspicuous than inconspicuous flowers. As a single
+instance, the handsome Geranium palustre was observed by Professor
+Müller to be visited by sixteen different species of insects, the
+equally showy G. pratense by thirteen species, while the smaller and
+much less conspicuous G. molle was visited by eight species, and G.
+pusillum by only one. In many cases, however, a flower may be very
+attractive to only a few species of insects; and Professor Müller
+states, as the result of many years' assiduous observation, that "a
+species of flower is the more visited by insects the more conspicuous it
+is."
+
+2. Sweet odour is usually supplementary to the attraction of colour.
+Thus it is rarely present in the largest and most gaudily coloured
+flowers which inhabit open places, such as poppies, paeonies,
+sunflowers, and many others; while it is often the accompaniment of
+inconspicuous flowers, as the mignonette; of such as grow in shady
+places, as the violet and primrose; and especially of white or yellowish
+flowers, as the white jasmine, clematis, stephanotis, etc.
+
+3. White flowers are often fertilised by moths, and very frequently give
+out their scent only by night, as in our butterfly-orchis (Habenaria
+chlorantha); and they sometimes open only at night, as do many of the
+evening primroses and other flowers. These flowers are often long tubed
+in accordance with the length of the moths' probosces, as in the genus
+Pancratium, our butterfly orchis, white jasmine, and a host of others.
+
+4. Bright red flowers are very attractive to butterflies, and are
+sometimes specially adapted to be fertilised by them, as in many pinks
+(Dianthus deltoides, D. superbus, D. atrorubens), the corn-cockle
+(Lychnis Githago), and many others. Blue flowers are especially
+attractive to bees and other hymenoptera (though they frequent flowers
+of all colours), no less than sixty-seven species of this order having
+been observed to visit the common "sheep's-bit" (Jasione montana). Dull
+yellow or brownish flowers, some of which smell like carrion, are
+attractive to flies, as the Arum and Aristolochia; while the dull
+purplish flowers of the Scrophularia are specially attractive to wasps.
+
+5. Some flowers have neither scent nor nectar, and yet attract insects
+by sham nectaries! In the herb-paris (Paris quadrifolia) the ovary
+glistens as if moist, and flies alight on it and carry away pollen to
+another flower; while in grass of parnassus (Parnassia palustris) there
+are a number of small stalked yellow balls near the base of the flower,
+which look like drops of honey but are really dry. In this case there is
+a little nectar lower down, but the special attraction is a sham; and as
+there are fresh broods of insects every year, it takes time for them to
+learn by experience, and thus enough are always deceived to effect
+cross-fertilisation.[147] This is analogous to the case of the young
+birds, which have to learn by experience the insects that are inedible,
+as explained at page 253.
+
+6. Many flowers change their colour as soon as fertilised; and this is
+beneficial, as it enables bees to avoid wasting time in visiting those
+blossoms which have been already fertilised and their nectar exhausted.
+The common lungwort (Pulmonaria officinalis), is at first red, but later
+turns blue; and H. Müller observed bees visiting many red flowers in
+succession, but neglecting the blue. In South Brazil there is a species
+of Lantana, whose flowers are yellow the first day, orange the second,
+and purple the third; and Dr. Fritz Müller observed that many
+butterflies visited the yellow flowers only, some both the yellow and
+the orange flowers, but none the purple.
+
+7. Many flowers have markings which serve as guides to insects; in some
+cases a bright central eye, as in the borage and forget-me-not; or lines
+or spots converging to the centre, as in geraniums, pinks, and many
+others. This enables insects to go quickly and directly to the opening
+of the flower, and is equally important in aiding them to obtain a
+better supply of food, and to fertilise a larger number of flowers.
+
+8. Flowers have been specially adapted to the kinds of insects that
+most abound where they grow. Thus the gentians of the lowlands are
+adapted to bees, those of the high alps to butterflies only; and while
+most species of Rhinanthus (a genus to which our common "yellow rattle"
+belongs) are bee-flowers, one high alpine species (R. alpinus) has been
+also adapted for fertilisation by butterflies only. The reason of this
+is, that in the high alps butterflies are immensely more plentiful than
+bees, and flowers adapted to be fertilised by bees can often have their
+nectar extracted by butterflies without effecting cross-fertilisation.
+It is, therefore, important to have a modification of structure which
+shall make butterflies the fertilisers, and this in many cases has been
+done.[148]
+
+9. Economy of time is very important both to the insects and the
+flowers, because the fine working days are comparatively few, and if no
+time is wasted the bees will get more honey, and in doing so will
+fertilise more flowers. Now, it has been ascertained by several
+observers that many insects, bees especially, keep to one kind of flower
+at a time, visiting hundreds of blossoms in succession, and passing over
+other species that may be mixed with them. They thus acquire quickness
+in going at once to the nectar, and the change of colour in the flower,
+or incipient withering when fertilised, enables them to avoid those
+flowers that have already had their honey exhausted. It is probably to
+assist the insects in keeping to one flower at a time, which is of vital
+importance to the perpetuation of the species, that the flowers which
+bloom intermingled at the same season are usually very distinct both in
+form and colour. In the sandy districts of Surrey, in the early spring,
+the copses are gay with three flowers--the primrose, the wood-anemone,
+and the lesser celandine, forming a beautiful contrast, while at the
+same time the purple and the white dead-nettles abound on hedge banks. A
+little later, in the same copses, we have the blue wild hyacinth (Scilla
+nutans), the red campion (Lychnis dioica), the pure white great starwort
+(Stellaria Holosteum), and the yellow dead-nettle (Lamium Galeobdolon),
+all distinct and well-contrasted flowers. In damp meadows in summer we
+have the ragged robin (Lychnis Floscuculi), the spotted orchis (O.
+maculata), and the yellow rattle (Rhinanthus Crista-galli); while in
+drier meadows we have cowslips, ox-eye daisies, and buttercups, all very
+distinct both in form and colour. So in cornfields we have the scarlet
+poppies, the purple corn-cockle, the yellow corn-marygold, and the blue
+cornflower; while on our moors the purple heath and the dwarf gorse make
+a gorgeous contrast. Thus the difference of colour which enables the
+insect to visit with rapidity and unerring aim a number of flowers of
+the same kind in succession, serves to adorn our meadows, banks, woods,
+and heaths with a charming variety of floral colour and form at each
+season of the year.[149]
+
+
+_Fertilisation of Flowers by Birds._
+
+In the temperate regions of the Northern Hemisphere, insects are the
+chief agents in cross-fertilisation when this is not effected by the
+wind; but in warmer regions, and in the Southern hemisphere, birds are
+found to take a considerable part in the operation, and have in many
+cases led to modifications in the form and colour of flowers. Each part
+of the globe has special groups of birds which are flower-haunters.
+America has the humming-birds (Trochilidae), and the smaller group of
+the sugar-birds (Caerebidae). In the Eastern tropics the sun-birds
+(Nectarineidae) take the place of the humming-birds, and another small
+group, the flower-peckers (Dicaeidae), assist them. In the Australian
+region there are also two flower-feeding groups, the Meliphagidae, or
+honey-suckers, and the brush-tongued lories (Trichoglossidae). Recent
+researches by American naturalists have shown that many flowers are
+fertilised by humming-birds, such as passion-flowers, trumpet-flowers,
+fuchsias, and lobelias; while some, as the Salvia splendens of Mexico,
+are specially adapted to their visits. We may thus perhaps explain the
+number of very large tubular flowers in the tropics, such as the huge
+brugmansias and bignonias; while in the Andes and in Chile, where
+humming-birds are especially plentiful, we find great numbers of red
+tubular flowers, often of large size and apparently adapted to these
+little creatures. Such are the beautiful Lapageria and Philesia, the
+grand Pitcairneas, and the genera Fuchsia, Mitraria, Embothrium,
+Escallonia, Desfontainea, Eccremocarpus, and many Gesneraceae. Among the
+most extraordinary modifications of flower structure adapted to bird
+fertilisation are the species of Marcgravia, in which the pedicels and
+bracts of the terminal portion of a pendent bunch of flowers have been
+modified into pitchers which secrete nectar and attract insects, while
+birds feeding on the nectar, or insects, have the pollen of the
+overhanging flowers dusted on their backs, and, carrying it to other
+flowers, thus cross-fertilise them (see Illustration).
+
+[Illustration: FIG. 31.--Humming-bird fertilising Marcgravia
+nepenthoides.]
+
+In Australia and New Zealand the fine "glory peas" (Clianthus), the
+Sophora, Loranthus, many Epacrideae and Myrtaceae, and the large flowers
+of the New Zealand flax (Phormium tenax), are cross-fertilised by
+birds; while in Natal the fine trumpet-creeper (Tecoma capensis) is
+fertilised by Nectarineas.
+
+The great extent to which insect and bird agency is necessary to flowers
+is well shown by the case of New Zealand. The entire country is
+comparatively poor in species of insects, especially in bees and
+butterflies which are the chief flower fertilisers; yet according to the
+researches of local botanists no less than one-fourth of all the
+flowering plants are incapable of self-fertilisation, and, therefore,
+wholly dependent on insect or bird agency for the continuance of the
+species.
+
+The facts as to the cross-fertilisation of flowers which have now been
+very briefly summarised, taken in connection with Darwin's experiments
+proving the increased vigour and fertility given by cross-fertilisation,
+seem amply to justify his aphorism that "Nature abhors
+self-fertilisation," and his more precise statement, that, "No plant is
+perpetually self-fertilised;" and this view has been upheld by
+Hildebrand, Delpino, and other botanists.[150]
+
+
+_Self-Fertilisation of Flowers._
+
+But all this time we have been only looking at one side of the question,
+for there exists an abundance of facts which seem to imply, just as
+surely, the utter uselessness of cross-fertilisation. Let us, then, see
+what these facts are before proceeding further.
+
+1. An immense variety of plants are habitually self-fertilised, and
+their numbers probably far exceed those which are habitually
+cross-fertilised by insects. Almost all the very small or obscure
+flowered plants with hermaphrodite flowers are of this kind. Most of
+these, however, may be insect fertilised occasionally, and may,
+therefore, come under the rule that no species are perpetually
+self-fertilised.
+
+2. There are many plants, however, in which special arrangements exist
+to secure self-fertilisation. Sometimes the corolla closes and brings
+the anthers and stigma into contact; in others the anthers cluster round
+the stigmas, both maturing together, as in many buttercups, stitchwort
+(Stellaria media), sandwort (Spergula), and some willow-herbs
+(Epilobium); or they arch over the pistil, as in Galium aparine and
+Alisma Plantago. The style is also modified to bring it into contact
+with the anthers, as in the dandelion, groundsel, and many other
+plants.[151] All these, however, may be occasionally cross-fertilised.
+
+3. In other cases precautions are taken to prevent cross-fertilisation,
+as in the numerous cleistogamous or closed flowers. These occur in no
+less than fifty-five different genera, belonging to twenty-four natural
+orders, and in thirty-two of these genera the normal flowers are
+irregular, and have therefore been specially modified for insect
+fertilisation.[152] These flowers appear to be degradations of the
+normal flowers, and are closed up by various modifications of the petals
+or other parts, so that it is impossible for insects to reach the
+interior, yet they produce seed in abundance, and are often the chief
+means by which the species is continued. Thus, in our common dog-violet
+the perfect flowers rarely produce seed, while the rudimentary
+cleistogamic flowers do so in abundance. The sweet violet also produces
+abundance of seed from its cleistogamic flowers, and few from its
+perfect flowers; but in Liguria it produces only perfect flowers which
+seed abundantly. No case appears to be known of a plant which has
+cleistogamic flowers only, but a small rush (Juncus bufonius) is in this
+condition in some parts of Russia, while in other parts perfect flowers
+are also produced.[153] Our common henbit dead-nettle (Lamium
+amplexicaule) produces cleistogamic flowers, as do also some orchids.
+The advantage gained by the plant is great economy of specialised
+material, since with very small flowers and very little expenditure of
+pollen an abundance of seed is produced.
+
+4. A considerable number of plants which have evidently been specially
+modified for insect fertilisation have, by further modification, become
+quite self-fertile. This is the case with the garden-pea, and also with
+our beautiful bee-orchis, in which the pollen-masses constantly fall on
+to the stigmas, and the flower, being thus self-fertilised, produces
+abundance of capsules and of seed. Yet in many of its close allies
+insect agency is absolutely required; but in one of these, the
+fly-orchis, comparatively very little seed is produced, and
+self-fertilisation would therefore be advantageous to it. When
+garden-peas were artificially cross-fertilised by Mr. Darwin, it seemed
+to do them no good, as the seeds from these crosses produced less
+vigorous plants than seed from those which were self-fertilised; a fact
+directly opposed to what usually occurs in cross-fertilised plants.
+
+5. As opposed to the theory that there is any absolute need for
+cross-fertilisation, it has been urged by Mr. Henslow and others that
+many self-fertilised plants are exceptionally vigorous, such as
+groundsel, chickweed, sow-thistle, buttercups, and other common weeds;
+while most plants of world-wide distribution are self-fertilised, and
+these have proved themselves to be best fitted to survive in the battle
+of life. More than fifty species of common British plants are very
+widely distributed, and all are habitually self-fertilised.[154] That
+self-fertilisation has some great advantage is shown by the fact that it
+is usually the species which have the smallest and least conspicuous
+flowers which have spread widely, while the large and showy flowered
+species of the same genera or families, which require insects to
+cross-fertilise them, have a much more limited distribution.
+
+6. It is now believed by some botanists that many inconspicuous and
+imperfect flowers, including those that are wind-fertilised, such as
+plantains, nettles, sedges, and grasses, do not represent primitive or
+undeveloped forms, but are degradations from more perfect flowers which
+were once adapted to insect fertilisation. In almost every order we find
+some plants which have become thus reduced or degraded for wind or
+self-fertilisation, as Poterium and Sanguisorba among the Rosaceae;
+while this has certainly been the case in the cleistogamic flowers. In
+most of the above-mentioned plants there are distinct rudiments of
+petals or other floral organs, and as the chief use of these is to
+attract insects, they could hardly have existed in primitive
+flowers.[155] We know, moreover, that when the petals cease to be
+required for the attraction of insects, they rapidly diminish in size,
+lose their bright colour or almost wholly disappear.[156]
+
+
+_Difficulties and Contradictions._
+
+The very bare summary that has now been given of the main facts relating
+to the fertilisation of flowers, will have served to show the vast
+extent and complexity of the inquiry, and the extraordinary
+contradictions and difficulties which it presents. We have direct proof
+of the beneficial results of intercrossing in a great number of cases;
+we have an overwhelming mass of facts as to the varied and complex
+structure of flowers evidently adapted to secure this intercrossing by
+insect agency; yet we see many of the most vigorous plants which spread
+widely over the globe, with none of these adaptations, and evidently
+depending on self-fertilisation for their continued existence and
+success in the battle of life. Yet more extraordinary is it to find
+numerous cases in which the special arrangements for cross-fertilisation
+appear to have been a failure, since they have either been supplemented
+by special means for self-fertilisation, or have reverted back in
+various degrees to simpler forms in which self-fertilisation becomes the
+rule. There is also a further difficulty in the highly complex modes by
+which cross-fertilisation is often brought about; for we have seen that
+there are several very effective yet very simple modes of securing
+intercrossing, involving a minimum of change in the form and structure
+of the flower; and when we consider that the result attained with so
+much cost of structural modification is by no means an unmixed good, and
+is far less certain in securing the perpetuation of the species than is
+self-fertilisation, it is most puzzling to find such complex methods
+resorted to, sometimes to the extent of special precautions against the
+possibility of self-fertilisation ever taking place. Let us now see
+whether any light can be thrown on these various anomalies and
+contradictions.
+
+
+_Intercrossing not necessarily Advantageous._
+
+No one was more fully impressed than Mr. Darwin with the beneficial
+effects of intercrossing on the vigour and fertility of the species or
+race, yet he clearly saw that it was not always and necessarily
+advantageous. He says: "The most important conclusion at which I have
+arrived is, that the mere act of intercrossing by itself does no good.
+The good depends on the individuals which are crossed differing slightly
+in constitution, owing to their progenitors having been subjected during
+several generations to slightly different conditions. This conclusion,
+as we shall hereafter see, is closely connected with various important
+physiological problems, such as the benefit derived from slight changes
+in the conditions of life."[157] Mr. Darwin has also adduced much direct
+evidence proving that slight changes in the conditions of life are
+beneficial to both animals and plants, maintaining or restoring their
+vigour and fertility in the same way as a favourable cross seems to
+restore it.[158] It is, I believe, by a careful consideration of these
+two classes of facts that we shall find the clue to the labyrinth in
+which this subject has appeared to involve us.
+
+
+_Supposed Evil Results of Close Interbreeding._
+
+Just as we have seen that intercrossing is not necessarily good, we
+shall be forced to admit that close interbreeding is not necessarily
+bad. Our finest breeds of domestic animals have been thus produced, and
+by a careful statistical inquiry Mr. George Darwin has shown that the
+most constant and long-continued intermarriages among the British
+aristocracy have produced no prejudicial results. The rabbits on Porto
+Santo are all the produce of a single female; they have lived on the
+same small island for 470 years, and they still abound there and appear
+to be vigorous and healthy (see p. 161).
+
+We have, however, on the other hand, overwhelming evidence that in many
+cases, among our domestic animals and cultivated plants, close
+interbreeding does produce bad results, and the apparent contradiction
+may perhaps be explained on the same general principles, and under
+similar limitations, as were found to be necessary in defining the value
+of intercrossing. It appears probable, then, that it is not
+interbreeding in itself that is hurtful, but interbreeding without
+rigid selection or some change of conditions. Under nature, as in the
+case of the Porto Santo rabbits, the rapid increase of these animals
+would in a very few years stock the island with a full population, and
+thereafter natural selection would act powerfully in the preservation
+only of the healthiest and the most fertile, and under these conditions
+no deterioration would occur. Among the aristocracy there has been a
+constant selection of beauty, which is generally synonymous with health,
+while any constitutional infertility has led to the extinction of the
+family. With domestic animals the selection practised is usually neither
+severe enough nor of the right kind. There is no natural struggle for
+existence, but certain points of form and colour characteristic of the
+breed are considered essential, and thus the most vigorous or the most
+fertile are not always those which are selected to continue the stock.
+In nature, too, the species always extends over a larger area and
+consists of much greater numbers, and thus a difference of constitution
+soon arises in different parts of the area, which is wanting in the
+limited numbers of pure bred domestic animals. From a consideration of
+these varied facts we conclude that an occasional disturbance of the
+organic equilibrium is what is essential to keep up the vigour and
+fertility of any organism, and that this disturbance may be equally well
+produced either by a cross between individuals of somewhat different
+constitutions, or by occasional slight changes in the conditions of
+life. Now plants which have great powers of dispersal enjoy a constant
+change of conditions, and can, therefore, exist permanently, or at all
+events, for very long periods, without intercrossing; while those which
+have limited powers of dispersal, and are restricted to a comparatively
+small and uniform area, need an occasional cross to keep up their
+fertility and general vigour. We should, therefore, expect that those
+groups of plants which are adapted both for cross-and
+self-fertilisation, which have showy flowers and possess great powers of
+seed-dispersal, would be the most abundant and most widely distributed;
+and this we find to be the case, the Compositae possessing all these
+characteristics in the highest degree, and being the most generally
+abundant group of plants with conspicuous flowers in all parts of the
+world.
+
+
+_How the Struggle for Existence Acts among Flowers._
+
+Let us now consider what will be the action of the struggle for
+existence under the conditions we have seen to exist.
+
+Everywhere and at all times some species of plants will be dominant and
+aggressive; while others will be diminishing in numbers, reduced to
+occupy a smaller area, and generally having a hard struggle to maintain
+themselves. Whenever a self-fertilising plant is thus reduced in numbers
+it will be in danger of extinction, because, being limited to a small
+area, it will suffer from the effects of too uniform conditions which
+will produce weakness and infertility. But while this change is in
+progress, any crosses between individuals of slightly different
+constitution will be beneficial, and all variations favouring either
+insect agency on the one hand, or wind-dispersal of pollen on the other,
+will lead to the production of a somewhat stronger and more fertile
+stock. Increased size or greater brilliancy of the flower, more abundant
+nectar, sweeter odour, or adaptations for more effectual
+cross-fertilisation would all be preserved, and thus would be initiated
+some form of specialisation for insect agency in cross-fertilisation;
+and in every different species so circumstanced the result would be
+different, depending as it would on many and complex combinations of
+variation of parts of the flower, and of the insect species which most
+abounded in the district.
+
+Species thus favourably modified might begin a new era of development,
+and, while spreading over a somewhat wider area, give rise to new
+varieties or species, all adapted in various degrees and modes to secure
+cross-fertilisation by insect agency. But in course of ages some change
+of conditions might prove adverse. Either the insects required might
+diminish in numbers or be attracted by other competing flowers, or a
+change of climate might give the advantage to other more vigorous
+plants. Then self-fertilisation with greater means of dispersal might be
+more advantageous; the flowers might become smaller and more numerous;
+the seeds smaller and lighter so as to be more easily dispersed by the
+wind, while some of the special adaptations for insect fertilisation
+being useless would, by the absence of selection and by the law of
+economy of growth, be reduced to a rudimentary form. With these
+modifications the species might extend its range into new districts,
+thereby obtaining increased vigour by the change of conditions, as
+appears to have been the case with so many of the small flowered
+self-fertilised plants. Thus it might continue to exist for a long
+series of ages, till under other changes--geographical or biological--it
+might again suffer from competition or from other adverse circumstances,
+and be at length again confined to a limited area, or reduced to very
+scanty numbers.
+
+But when this cycle of change had taken place, the species would be very
+different from the original form. The flower would have been at one time
+modified to favour the visits of insects and to secure
+cross-fertilisation by their aid, and when the need for this passed
+away, some portions of these structures would remain, though in a
+reduced or rudimentary condition. But when insect agency became of
+importance a second time, the new modifications would start from a
+different or more advanced basis, and thus a more complex result might
+be produced. Owing to the unequal rates at which the reduction of the
+various parts might occur, some amount of irregularity in the flower
+might arise, and on a second development towards insect
+cross-fertilisation this irregularity, if useful, might be increased by
+variation and selection.
+
+The rapidity and comparative certainty with which such changes as are
+here supposed do really take place, are well shown by the great
+differences in floral structure, as regards the mode of fertilisation,
+in allied genera and species, and even in some cases in varieties of the
+same species. Thus in the Ranunculaceae we find the conspicuous part of
+the flower to be the petals in Ranunculus, the sepals in Helleborus,
+Anemone, etc., and the stamens in most species of Thalictrum. In all
+these we have a simple regular flower, but in Aquilegia it is made
+complex by the spurred petals, and in Delphinium and Aconitum it becomes
+quite irregular. In the more simple class self-fertilisation occurs
+freely, but it is prevented in the more complex flowers by the stamens
+maturing before the pistil. In the Caprifoliaceae we have small and
+regular greenish flowers, as in the moschatel (Adoxa); more conspicuous
+regular open flowers without honey, as in the elder (Sambucus); and
+tubular flowers increasing in length and irregularity, till in some,
+like our common honeysuckle, they are adapted for fertilisation by moths
+only, with abundant honey and delicious perfume to attract them. In the
+Scrophulariaceae we find open, almost regular flowers, as Veronica and
+Verbascum, fertilised by flies and bees, but also self-fertilised;
+Scrophularia adapted in form and colour to be fertilised by wasps; and
+the more complex and irregular flowers of Linaria, Rhinanthus,
+Melampyrum, Pedicularis, etc., mostly adapted to be fertilised by bees.
+
+In the genera Geranium, Polygonum, Veronica, and several others there is
+a gradation of forms from large and bright to small and obscure coloured
+flowers, and in every case the former are adapted for insect
+fertilisation, often exclusively, while in the latter self-fertilisation
+constantly occurs. In the yellow rattle (Rhinanthus Crista-galli) there
+are two forms (which have been named _major_ and _minor_), the larger
+and more conspicuous adapted to insect fertilisation only, the smaller
+capable of self-fertilisation; and two similar forms exist in the
+eyebright (Euphrasia officinalis). In both these cases there are special
+modifications in the length and curvature of the style as well as in the
+size and shape of the corolla; and the two forms are evidently becoming
+each adapted to special conditions, since in some districts the one, in
+other districts the other is most abundant.[159]
+
+These examples show us that the kind of change suggested above is
+actually going on, and has presumably always been going on in nature
+throughout the long geological epochs during which the development of
+flowers has been progressing. The two great modes of gaining increased
+vigour and fertility--intercrossing and dispersal over wider areas--have
+been resorted to again and again, under the pressure of a constant
+struggle for existence and the need for adaptation to ever-changing
+conditions. During all the modifications that ensued, useless parts were
+reduced or suppressed, owing to the absence of selection and the
+principle of economy of growth; and thus at each fresh adaptation some
+rudiments of old structures were re-developed, but not unfrequently in
+a different form and for a distinct purpose.
+
+The chief types of flowering plants have existed during the millions of
+ages of the whole tertiary period, and during this enormous lapse of
+time many of them may have been modified in the direction of insect
+fertilisation, and again into that of self-fertilisation, not once or
+twice only, but perhaps scores or even hundreds of times; and at each
+such modification a difference in the environment may have led to a
+distinct line of development. At one epoch the highest specialisation of
+structure in adaptation to a single species or group of insects may have
+saved a plant from extinction; while, at other times, the simplest mode
+of self-fertilisation, combined with greater powers of dispersal and a
+constitution capable of supporting diverse physical conditions, may have
+led to a similar result. With some groups the tendency seems to have
+been almost continuously to greater and greater specialisation, while
+with others a tendency to simplification and degradation has resulted in
+such plants as the grasses and sedges.
+
+We are now enabled dimly to perceive how the curious anomaly of very
+simple and very complex methods of securing cross-fertilisation--both
+equally effective--may have been brought about. The simple modes may be
+the result of a comparatively direct modification from the more
+primitive types of flowers, which were occasionally, and, as it were,
+accidentally visited and fertilised by insects; while the more complex
+modes, existing for the most part in the highly irregular flowers, may
+result from those cases in which adaptation to insect-fertilisation, and
+partial or complete degradation to self-fertilisation or to
+wind-fertilisation, have again and again recurred, each time producing
+some additional complexity, arising from the working up of old rudiments
+for new purposes, till there have been reached the marvellous flower
+structures of the papilionaceous tribes, of the asclepiads, or of the
+orchids.
+
+We thus see that the existing diversity of colour and of structure in
+flowers is probably the ultimate result of the ever-recurring struggle
+for existence, combined with the ever-changing relations between the
+vegetable and animal kingdoms during countless ages. The constant
+variability of every part and organ, with the enormous powers of
+increase possessed by plants, have enabled them to become again and
+again readjusted to each change of condition as it occurred, resulting
+in that endless variety, that marvellous complexity, and that exquisite
+colouring which excite our admiration in the realm of flowers, and
+constitute them the perennial charm and crowning glory of nature.
+
+
+_Flowers the Product of Insect Agency._
+
+In his _Origin of Species_, Mr. Darwin first stated that flowers had
+been rendered conspicuous and beautiful in order to attract insects,
+adding: "Hence we may conclude that, if insects had not been developed
+on the earth, our plants would not have been decked with beautiful
+flowers, but would have produced only such poor flowers as we see on our
+fir, oak, nut, and ash trees, on grasses, docks, and nettles, which are
+all fertilised through the agency of the wind." The argument in favour
+of this view is now much stronger than when he wrote; for not only have
+we reason to believe that most of these wind-fertilised flowers are
+degraded forms of flowers which have once been insect fertilised, but we
+have abundant evidence that whenever insect agency becomes comparatively
+ineffective, the colours of the flowers become less bright, their size
+and beauty diminish, till they are reduced to such small, greenish,
+inconspicuous flowers as those of the rupture-wort (Herniaria glabra),
+the knotgrass (Polygonum aviculare), or the cleistogamic flowers of the
+violet. There is good reason to believe, therefore, not only that
+flowers have been developed in order to attract insects to aid in their
+fertilisation, but that, having been once produced, in however great
+profusion, if the insect races were all to become extinct, flowers (in
+the temperate zones at all events) would soon dwindle away, and that
+ultimately all floral beauty would vanish from the earth.
+
+We cannot, therefore, deny the vast change which insects have produced
+upon the earth's surface, and which has been thus forcibly and
+beautifully delineated by Mr. Grant Allen: "While man has only tilled a
+few level plains, a few great river valleys, a few peninsular mountain
+slopes, leaving the vast mass of earth untouched by his hand, the insect
+has spread himself over every land in a thousand shapes, and has made
+the whole flowering creation subservient to his daily wants. His
+buttercup, his dandelion, and his meadow-sweet grow thick in every
+English field. His thyme clothes the hillside; his heather purples the
+bleak gray moorland. High up among the alpine heights his gentian
+spreads its lakes of blue; amid the snows of the Himalayas his
+rhododendrons gleam with crimson light. Even the wayside pond yields him
+the white crowfoot and the arrowhead, while the broad expanses of
+Brazilian streams are beautified by his gorgeous water-lilies. The
+insect has thus turned the whole surface of the earth into a boundless
+flower-garden, which supplies him from year to year with pollen or
+honey, and itself in turn gains perpetuation by the baits that it offers
+for his allurement."[160]
+
+
+_Concluding Remarks on Colour in Nature._
+
+In the last four chapters I have endeavoured to give a general and
+systematic, though necessarily condensed view of the part which is
+played by colour in the organic world. We have seen in what infinitely
+varied ways the need of concealment has led to the modification of
+animal colours, whether among polar snows or sandy deserts, in tropical
+forests or in the abysses of the ocean. We next find these general
+adaptations giving way to more specialised types of coloration, by which
+each species has become more and more harmonised with its immediate
+surroundings, till we reach the most curiously minute resemblances to
+natural objects in the leaf and stick insects, and those which are so
+like flowers or moss or birds' droppings that they deceive the acutest
+eye. We have learnt, further, that these varied forms of protective
+colouring are far more numerous than has been usually suspected,
+because, what appear to be very conspicuous colours or markings when the
+species is observed in a museum or in a menagerie, are often highly
+protective when the creature is seen under the natural conditions of its
+existence. From these varied classes of facts it seems not improbable
+that fully one-half of the species in the animal kingdom possess colours
+which have been more or less adapted to secure for them concealment or
+protection.
+
+Passing onward we find the explanation of a distinct type of colour or
+marking, often superimposed upon protective tints, in the importance of
+easy recognition by many animals of their fellows, their parents, or
+their mates. By this need we have been able to account for markings that
+seem calculated to make the animal conspicuous, when the general tints
+and well-known habits of the whole group demonstrate the need of
+concealment. Thus also we are able to explain the constant symmetry in
+the markings of wild animals, as well as the numerous cases in which the
+conspicuous colours are concealed when at rest and only become visible
+during rapid motion. In striking contrast to ordinary protective
+coloration we have "warning colours," usually very conspicuous and often
+brilliant or gaudy, which serve to indicate that their possessors are
+either dangerous or uneatable to the usual enemies of their tribe. This
+kind of coloration is probably more prevalent than has been hitherto
+supposed, because in the case of many tropical animals we are quite
+unacquainted with their special and most dangerous enemies, and are also
+unable to determine whether they are or are not distasteful to those
+enemies. As a kind of corollary to the "warning colours," we find the
+extraordinary phenomena of "mimicry," in which defenceless species
+obtain protection by being mistaken for those which, from any cause,
+possess immunity from attack. Although a large number of instances of
+warning colour and of mimicry are now recorded, it is probably still an
+almost unworked field of research, more especially in tropical regions
+and among the inhabitants of the ocean.
+
+The phenomena of sexual diversities of coloration next engaged our
+attention, and the reasons why Mr. Darwin's theory of "sexual
+selection," as regards colour and ornament, could not be accepted were
+stated at some length, together with the theory of animal coloration and
+ornament we propose to substitute for it. This theory is held to be in
+harmony with the general facts of animal coloration, while it entirely
+dispenses with the very hypothetical and inadequate agency of female
+choice in producing the detailed colours, patterns, and ornaments, which
+in so many cases distinguish the male sex.
+
+If my arguments on this point are sound, they will dispose also of Mr.
+Grant Allen's view of the direct action of the colour sense on the
+animal integuments.[161] He argues that the colours of insects and birds
+reproduce generally the colours of the flowers they frequent or the
+fruits they eat, and he adduces numerous cases in which flower-haunting
+insects and fruit-eating birds are gaily coloured. This he supposes to
+be due to the colour-taste, developed by the constant presence of bright
+flowers and fruits, being applied to the selection of each variation
+towards brilliancy in their mates; thus in time producing the gorgeous
+and varied hues they now possess. Mr. Allen maintains that "insects are
+bright where bright flowers exist in numbers, and dull where flowers are
+rare or inconspicuous;" and he urges that "we can hardly explain this
+wide coincidence otherwise than by supposing that a taste for colour is
+produced through the constant search for food among entomophilous
+blossoms, and that this taste has reacted upon its possessors through
+the action of unconscious sexual selection."
+
+The examples Mr. Allen quotes of bright insects being associated with
+bright flowers seem very forcible, but are really deceptive or
+erroneous; and quite as many cases could be quoted which prove the very
+opposite. For example, in the dense equatorial forests flowers are
+exceedingly scarce, and there is no comparison with the amount of floral
+colour to be met with in our temperate meadows, woods, and hillsides.
+The forests about Para in the lower Amazon are typical in this respect,
+yet they abound with the most gorgeously coloured butterflies, almost
+all of which frequent the forest depths, keeping near the ground, where
+there is the greatest deficiency of brilliant flowers. In contrast with
+this let us take the Cape of Good Hope--the most flowery region probably
+that exists upon the globe,--where the country is a complete
+flower-garden of heaths, pelargoniums, mesembryanthemus, exquisite
+iridaceous and other bulbs, and numerous flowering shrubs and trees; yet
+the Cape butterflies are hardly equal, either in number or variety, to
+those of any country in South Europe, and are utterly insignificant when
+compared with those of the comparatively flowerless forest-depths of the
+Amazon or of New Guinea. Neither is there any relation between the
+colours of other insects and their haunts. Few are more gorgeous than
+some of the tiger-beetles and the carabi, yet these are all carnivorous;
+while many of the most brilliant metallic buprestidae and longicorns are
+always found on the bark of fallen trees. So with the humming-birds;
+their brilliant metallic tints can only be compared with metals or gems,
+and are totally unlike the delicate pinks and purples, yellows and reds
+of the majority of flowers. Again, the Australian honey-suckers
+(Meliphagidae) are genuine flower-haunters, and the Australian flora is
+more brilliant in colour display than that of most tropical regions, yet
+these birds are, as a rule, of dull colours, not superior on the average
+to our grain-eating finches. Then, again, we have the grand pheasant
+family, including the gold and the silver pheasants, the gorgeous
+fire-backed and ocellated pheasants, and the resplendent peacock, all
+feeding on the ground on grain or seeds or insects, yet adorned with the
+most gorgeous colours.
+
+There is, therefore, no adequate basis of facts for this theory to rest
+upon, even if there were the slightest reason to believe that not only
+birds, but butterflies and beetles, take any delight in colour for its
+own sake, apart from the food-supply of which it indicates the presence.
+All that has been proved or that appears to be probable is, that they
+are able to perceive differences of colour, and to associate each colour
+with the particular flowers or fruits which best satisfy their wants.
+Colour being in its nature diverse, it has been beneficial for them to
+be able to distinguish all its chief varieties, as manifested more
+particularly in the vegetable kingdom, and among the different species
+of their own group; and the fact that certain species of insects show
+some preference for a particular colour may be explained by their having
+found flowers of that colour to yield them a more abundant supply of
+nectar or of pollen. In those cases in which butterflies frequent
+flowers of their own colour, the habit may well have been acquired from
+the protection it affords them.
+
+It appears to me that, in imputing to insects and birds the same love of
+colour for its own sake and the same aesthetic tastes as we ourselves
+possess, we may be as far from the truth as were those writers who held
+that the bee was a good mathematician, and that the honeycomb was
+constructed throughout to satisfy its refined mathematical instincts;
+whereas it is now generally admitted to be the result of the simple
+principle of economy of material applied to a primitive cylindrical
+cell.[162]
+
+In studying the phenomena of colour in the organic world we have been
+led to realise the wonderful complexity of the adaptations which bring
+each species into harmonious relation with all those which surround it,
+and which thus link together the whole of nature in a network of
+relations of marvellous intricacy. Yet all this is but, as it were, the
+outward show and garment of nature, behind which lies the inner
+structure--the framework, the vessels, the cells, the circulating
+fluids, and the digestive and reproductive processes,--and behind these
+again those mysterious chemical, electrical, and vital forces which
+constitute what we term Life. These forces appear to be fundamentally
+the same for all organisms, as is the material of which all are
+constructed; and we thus find behind the outer diversities an inner
+relationship which binds together the myriad forms of life.
+
+Each species of animal or plant thus forms part of one harmonious whole,
+carrying in all the details of its complex structure the record of the
+long story of organic development; and it was with a truly inspired
+insight that our great philosophical poet apostrophised the humble
+weed--
+
+
+    Flower in the crannied wall,
+    I pluck you out of the crannies,
+    I hold you here, root and all, in my hand,
+    Little flower--but _if_ I could understand
+    What you are, root and all, and all in all,
+    I should know what God and man is.
+
+
+FOOTNOTES:
+
+[Footnote 136: Burchell's _Travels_, vol. i. p. 10.]
+
+[Footnote 137: _Nature_, vol. iii. p. 507.]
+
+[Footnote 138: _Flowers, Fruits, and Leaves_, p. 128 (Fig. 79).]
+
+[Footnote 139: For a popular sketch of these, see Sir J. Lubbock's
+_Flowers, Fruits, and Leaves_, or any general botanical work.]
+
+[Footnote 140: _Nature_, vol. xv. p, 117.]
+
+[Footnote 141: Grant Allen's _Colour Sense_, p. 113.]
+
+[Footnote 142: Canon Tristram's _Natural History of the Bible_, pp. 483,
+484.]
+
+[Footnote 143: For a complete historical account of this subject with
+full references to all the works upon it, see the Introduction to
+Hermann Müller's _Fertilisation of Flowers_, translated by D'Arcy W.
+Thompson.]
+
+[Footnote 144: For the full detail of his experiments, see _Cross-and
+Self-Fertilisation of Plants_, 1876.]
+
+[Footnote 145: See Darwin's _Fertilisation of Orchids_ for the many
+extraordinary and complex arrangements in these plants.]
+
+[Footnote 146: The English reader may consult Sir John Lubbock's
+_British Wild Flowers in Relation to Insects_, and H. Müller's great and
+original work, _The Fertilisation of Flowers_.]
+
+[Footnote 147: Müller's _Fertilisation of Flowers_, p. 248.]
+
+[Footnote 148: "Alpenblumen," by D.H. Müller. See _Nature_, vol. xxiii.
+p. 333.]
+
+[Footnote 149: This peculiarity of local distribution of colour in
+flowers may be compared, as regards its purpose, with the recognition
+colours of animals. Just as these latter colours enable the sexes to
+recognise each other, and thus avoid sterile unions of distinct species,
+so the distinctive form and colour of each species of flower, as
+compared with those that usually grow around it, enables the fertilising
+insects to avoid carrying the pollen of one flower to the stigma of a
+distinct species.]
+
+[Footnote 150: See H. Müller's _Fertilisation of Flowers_, p. 18.]
+
+[Footnote 151: The above examples are taken from Rev. G. Henslow's paper
+on "Self-Fertilisation of Plants," in _Trans. Linn. Soc._ Second series,
+_Botany_, vol. i. pp. 317-398, with plate. Mr. H.O. Forbes has shown
+that the same thing occurs among tropical orchids, in his paper "On the
+Contrivances for insuring Self-Fertilisation in some Tropical Orchids,"
+_Journ. Linn. Soc._, xxi. p. 538.]
+
+[Footnote 152: These are the numbers given by Darwin, but I am informed
+by Mr. Hemsley that many additions have been since made to the list, and
+that cleistogamic flowers probably occur in nearly all the natural
+orders.]
+
+[Footnote 153: For a full account of cleistogamic flowers, see Darwin's
+_Forms of Flowers_, chap. viii.]
+
+[Footnote 154: Henslow's "Self-Fertilisation," _Trans. Linn. Soc._
+Second series, _Botany_, vol. i. p. 391.]
+
+[Footnote 155: The Rev. George Henslow, in his _Origin of Floral
+Structures_, says: "There is little doubt but that all wind-fertilised
+angiosperms are degradations from insect-fertilised flowers....
+_Poterium sanguisorba_ is anemophilous; and _Sanguisorba officinalis_
+presumably was so formerly, but has reacquired an entomophilous habit;
+the whole tribe Poterieae being, in fact, a degraded group which has
+descended from Potentilleae. Plantains retain their corolla but in a
+degraded form. Junceae are degraded Lilies; while Cyperaceae and
+Gramineae among monocotyledons may be ranked with Amentiferae among
+dicotyledons, as representing orders which have retrograded very far
+from the entomophilous forms from which they were possibly and probably
+descended" (p. 266).
+
+"The genus Plantago, like _Thalictrum minus_, Poterium, and others, well
+illustrate the change from an entomophilous to the anemophilous state.
+_P. lanceolata_ has polymorphic flowers, and is visited by
+pollen-seeking insects, so that it can be fertilised either by insects
+or the wind. _P. media_ illustrates transitions in point of structure,
+as the filaments are pink, the anthers motionless, and the pollen grains
+aggregated, and it is regularly visited by _Bombus terrestris_. On the
+other hand, the slender filaments, versatile anthers, powdery pollen,
+and elongated protogynous style are features of other species indicating
+anemophily; while the presence of a degraded corolla shows its ancestors
+to have been entomophilous. _P. media_, therefore, illustrates, not a
+primitive entomophilous condition, but a return to it; just as is the
+case with _Sanguisorba officinalis_ and _Salix Caprea_; but these show
+no capacity of restoring the corolla, the attractive features having to
+be borne by the calyx, which is purplish in Sanguisorba, by the pink
+filaments of Plantago, and by the yellow anthers in the Sallow willow"
+(p. 271).
+
+"The interpretation, then, I would offer of inconspicuousness and all
+kinds of degradations is the exact opposite to that of conspicuousness
+and great differentiations; namely, that species with minute flowers,
+rarely or never visited by insects, and habitually self-fertilised, have
+primarily arisen through the neglect of insects, and have in consequence
+assumed their present floral structures" (p. 282).
+
+In a letter just received from Mr. Henslow, he gives a few additional
+illustrations of his views, of which the following are the most
+important: "Passing to Incompletae, the orders known collectively as
+'Cyclospermeae' are related to Caryophylleae; and to my mind are
+degradations from it, of which Orache is anemophilous. Cupuliferae have
+an inferior ovary and rudimentary calyx-limb on the top. These, as far
+as I know, cannot be interpreted except as degradations. The whole of
+Monocotyledons appear to me (from anatomical reasons especially) to be
+degradations from Dicotyledons, and primarily through the agency of
+growth in water. Many subsequently became terrestrial, but retained the
+effects of their primitive habitat through heredity. The 3-merous [sic]
+perianth of grasses, the parts of the flower being in whorls, point to a
+degradation from a sub-liliaceous condition."
+
+Mr. Henslow informs me that he has long held these views, but, as far as
+he knows, alone. Mr. Grant Allen, however, set forth a similar theory in
+his _Vignettes from Nature_ (p. 15) and more fully in _The Colours of
+Flowers_ (chap. v.), where he develops it fully and uses similar
+arguments to those of Mr. Henslow.]
+
+[Footnote 156: H. Müller gives ample proof of this in his _Fertilisation
+of Flowers_.]
+
+[Footnote 157: _Cross-and Self-Fertilisation_, p. 27.]
+
+[Footnote 158: _Animals and Plants_, vol. ii. p. 145.]
+
+[Footnote 159: Müller's _Fertilisation of Flowers_, pp. 448, 455. Other
+cases of recent degradation and readaptation to insect-fertilisation are
+given by Professor Henslow (see footnote, p. 324).]
+
+[Footnote 160: _The Colour Sense_, by Grant Allen, p. 95.]
+
+[Footnote 161: _The Colour Sense_, chap. ix.]
+
+[Footnote 162: See _Origin of Species_, sixth edition, p. 220.]
+
+
+
+
+CHAPTER XII
+
+THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
+
+
+    The facts to be explained--The conditions which have determined
+    distribution--The permanence of oceans--Oceanic and continental
+    areas--Madagascar and New Zealand--The teachings of the
+    thousand-fathom line--The distribution of marsupials--The
+    distribution of tapirs--Powers of dispersal as illustrated by
+    insular organisms--Birds and insects at sea--Insects at great
+    altitudes--The dispersal of plants--Dispersal of seeds by the
+    wind--Mineral matter carried by the wind--Objections to the
+    theory of wind-dispersal answered--Explanation of north
+    temperate plants in the southern hemisphere--No proof of
+    glaciation in the tropics--Lower temperature not needed to
+    explain the facts--Concluding remarks.
+
+
+
+The theory which we may now take as established--that all the existing
+forms of life have been derived from other forms by a natural process of
+descent with modification, and that this same process has been in action
+during past geological time--should enable us to give a rational account
+not only of the peculiarities of form and structure presented by animals
+and plants, but also of their grouping together in certain areas, and
+their general distribution over the earth's surface.
+
+In the absence of any exact knowledge of the facts of distribution, a
+student of the theory of evolution might naturally anticipate that all
+groups of allied organisms would be found in the same region, and that,
+as he travelled farther and farther from any given centre, the forms of
+life would differ more and more from those which prevailed at the
+starting-point, till, in the remotest regions to which he could
+penetrate, he would find an entirely new assemblage of animals and
+plants, altogether unlike those with which he was familiar. He would
+also anticipate that diversities of climate would always be associated
+with a corresponding diversity in the forms of life.
+
+Now these anticipations are to a considerable extent justified.
+Remoteness on the earth's surface is usually an indication of diversity
+in the fauna and flora, while strongly contrasted climates are always
+accompanied by a considerable contrast in the forms of life. But this
+correspondence is by no means exact or proportionate, and the converse
+propositions are often quite untrue. Countries which are near to each
+other often differ radically in their animal and vegetable productions;
+while similarity of climate, together with moderate geographical
+proximity, are often accompanied by marked diversities in the prevailing
+forms of life. Again, while many groups of animals--genera, families,
+and sometimes even orders--are confined to limited regions, most of the
+families, many genera, and even some species are found in every part of
+the earth. An enumeration of a few of these anomalies will better
+illustrate the nature of the problem we have to solve.
+
+As examples of extreme diversity, notwithstanding geographical
+proximity, we may adduce Madagascar and Africa, whose animal and
+vegetable productions are far less alike than are those of Great Britain
+and Japan at the remotest extremities of the great northern continent;
+while an equal, or perhaps even a still greater, diversity exists
+between Australia and New Zealand. On the other hand, Northern Africa
+and South Europe, though separated by the Mediterranean Sea, have faunas
+and floras which do not differ from each other more than do the various
+countries of Europe. As a proof that similarity of climate and general
+adaptability have had but a small part in determining the forms of life
+in each country, we have the fact of the enormous increase of rabbits
+and pigs in Australia and New Zealand, of horses and cattle in South
+America, and of the common sparrow in North America, though in none of
+these cases are the animals natives of the countries in which they
+thrive so well. And lastly, in illustration of the fact that allied
+forms are not always found in adjacent regions, we have the tapirs,
+which are found only on opposite sides of the globe, in tropical America
+and the Malayan Islands; the camels of the Asiatic deserts, whose
+nearest allies are the llamas and alpacas of the Andes; and the
+marsupials, only found in Australia and on the opposite side of the
+globe, in America. Yet, again, although mammalia may be said to be
+universally distributed over the globe, being found abundantly on all
+the continents and on a great many of the larger islands, yet they are
+entirely wanting in New Zealand, and in a considerable number of other
+islands which are, nevertheless, perfectly able to support them when
+introduced.
+
+Now most of these difficulties can be solved by means of well-known
+geographical and geological facts. When the productions of remote
+countries resemble each other, there is almost always continuity of land
+with similarity of climate between them. When adjacent countries differ
+greatly in their productions, we find them separated by a sea or strait
+whose great depth is an indication of its antiquity or permanence. When
+a group of animals inhabits two countries or regions separated by wide
+oceans, it is found that in past geological times the same group was
+much more widely distributed, and may have reached the countries it
+inhabits from an intermediate region in which it is now extinct. We
+know, also, that countries now united by land were divided by arms of
+the sea at a not very remote epoch; while there is good reason to
+believe that others now entirely isolated by a broad expanse of sea were
+formerly united and formed a single land area. There is also another
+important factor to be taken account of in considering how animals and
+plants have acquired their present peculiarities of
+distribution,--changes of climate. We know that quite recently a glacial
+epoch extended over much of what are now the temperate regions of the
+northern hemisphere, and that consequently the organisms which inhabit
+those parts must be, comparatively speaking, recent immigrants from more
+southern lands. But it is a yet more important fact that, down to middle
+Tertiary times at all events, an equable temperate climate, with a
+luxuriant vegetation, extended to far within the arctic circle, over
+what are now barren wastes, covered for ten months of the year with snow
+and ice. The arctic zone has, therefore, been in past times capable of
+supporting almost all the forms of life of our temperate regions; and we
+must take account of this condition of things whenever we have to
+speculate on the possible migrations of organisms between the old and
+new continents.
+
+
+_The Conditions which have determined Distribution._
+
+When we endeavour to explain in detail the facts of the existing
+distribution of organic beings, we are confronted by several preliminary
+questions, upon the solution of which will depend our treatment of the
+phenomena presented to us. Upon the theory of descent which we have
+adopted, all the different species of a genus, as well as all the genera
+which compose a family or higher group, have descended from some common
+ancestor, and must therefore, at some remote epoch, have occupied the
+same area, from which their descendants have spread to the regions they
+now inhabit. In the numerous cases in which the same group now occupies
+countries separated by oceans or seas, by lofty mountain-chains, by wide
+deserts, or by inhospitable climates, we have to consider how the
+migration which must certainly have taken place has been effected. It is
+possible that during some portion of the time which has elapsed since
+the origin of the group the interposing barriers have not been in
+existence; or, on the other hand, the particular organisms we are
+dealing with may have the power of overpassing the barriers, and thus
+reaching their present remote dwelling-places. As this is really the
+fundamental question of distribution on which the solution of all its
+more difficult problems depends, we have to inquire, in the first place,
+what is the nature of, and what are the limits to, the changes of the
+earth's surface, especially during the Tertiary and latter part of the
+Secondary periods, as it was during those periods that most of the
+existing types of the higher animals and plants came into existence;
+and, in the next place, what are the extreme limits of the powers of
+dispersal possessed by the chief groups of animals and plants. We will
+first consider the question of barriers, more especially those formed by
+seas and oceans.
+
+
+_The Permanence of Oceans._
+
+It was formerly a very general belief, even amongst geologists, that the
+great features of the earth's surface, no less than the smaller ones,
+were subject to continual mutations, and that during the course of
+known geological time the continents and great oceans had again and
+again changed places with each other. Sir Charles Lyell, in the last
+edition of his _Principles of Geology_ (1872), said: "Continents,
+therefore, although permanent for whole geological epochs, shift their
+positions entirely in the course of ages;" and this may be said to have
+been the orthodox opinion down to the very recent period when, by means
+of deep-sea soundings, the nature of the ocean bottom was made known.
+The first person to throw doubt on this view appears to have been the
+veteran American geologist, Professor Dana. In 1849, in the Report of
+Wilke's Exploring Expedition, he adduced the argument against a former
+continent in the Pacific during the Tertiary period, from the absence of
+all native quadrupeds. In 1856, in articles in the _American Journal_,
+he discussed the development of the American continent, and argued for
+its general permanence; and in his _Manual of Geology_ in 1863 and later
+editions, the same views were more fully enforced and were latterly
+applied to all continents. Darwin, in his _Journal of Researches_,
+published in 1845, called attention to the fact that all the small
+islands far from land in the Pacific, Indian, and Atlantic Oceans are
+either of coralline or volcanic formation. He excepted, however, the
+Seychelles and St. Paul's rocks; but the former have since been shown to
+be no exception, as they consist entirely of coral rock; and although
+Darwin himself spent a few hours on St. Paul's rocks on his outward
+voyage in the _Beagle_, and believed he had found some portions of them
+to be of a "cherty," and others of a "felspathic" nature, this also has
+been shown to be erroneous, and the careful examination of the rocks by
+the Abbé Renard clearly proves them to be wholly of volcanic
+origin.[163] We have, therefore, at the present time, absolutely no
+exception whatever to the remarkable fact that all the oceanic islands
+of the globe are either of volcanic or coral formation; and there is,
+further, good reason to believe that those of the latter class in every
+case rest upon a volcanic foundation.
+
+In his _Origin of Species_, Darwin further showed that no true oceanic
+island had any native mammals or batrachia when first discovered, this
+fact constituting the test of the class to which an island belongs;
+whence he argued that none of them had ever been connected with
+continents, but all had originated in mid-ocean. These considerations
+alone render it almost certain that the areas now occupied by the great
+oceans have never, during known geological time, been occupied by
+continents, since it is in the highest degree improbable that every
+fragment of those continents should have completely disappeared, and
+have been replaced by volcanic islands rising out of profound oceanic
+abysses; but recent research into the depth of the oceans and the nature
+of the deposits now forming on their floors, adds greatly to the
+evidence in this direction, and renders it almost a certainty that they
+represent very ancient if not primaeval features of the earth's surface.
+A very brief outline of the nature of this evidence will be now given.
+
+The researches of the _Challenger_ expedition into the nature of the
+sea-bottom show, that the whole of the land debris brought down by
+rivers to the ocean (with the exception of pumice and other floating
+matter), is deposited comparatively near to the shores, and that the
+fineness of the material is an indication of the distance to which it
+has been carried. Everything in the nature of gravel and sand is laid
+down within a very few miles of land, only the finer muddy sediments
+being carried out for 20 or 50 miles, and the very finest of all, under
+the most favourable conditions, rarely extending beyond 150, or at the
+utmost, 300 miles from land into the deep ocean.[164] Beyond these
+distances, and covering the entire ocean floor, are various oozes formed
+wholly from the debris of marine organisms; while intermingled with
+these are found various volcanic products which have been either carried
+through the air or floated on the surface, and a small but perfectly
+recognisable quantity of meteoric matter. Ice-borne rocks are also found
+abundantly scattered over the ocean bottom within a definite distance of
+the arctic and antarctic circles, clearly marking out the limit of
+floating icebergs in recent geological times.
+
+Now the whole series of marine stratified rocks, from the earliest
+Palaeozoic to the most recent Tertiary beds, consist of materials
+closely corresponding to the land debris now being deposited within a
+narrow belt round the shores of all continents; while no rocks have been
+found which can be identified with the various oozes now forming in the
+deep abysses of the ocean. It follows, therefore, that all the
+geological formations have been formed in comparatively shallow water,
+and always adjacent to the continental land of the period. The great
+thickness of some of the formations is no indication of a deep sea, but
+only of slow subsidence during the time that the deposition was in
+progress. This view is now adopted by many of the most experienced
+geologists, especially by Dr. Archibald Geikie, Director of the
+Geological Survey of Great Britain, who, in his lecture on "Geographical
+Evolution," says: "From all this evidence we may legitimately conclude
+that the present land of the globe, though consisting in great measure
+of marine formations, has never lain under the deep sea; but that its
+site must always have been near land. Even its thick marine limestones
+are the deposits of comparatively shallow water."[165]
+
+But besides these geological and physical considerations, there is a
+mechanical difficulty in the way of repeated change of position of
+oceans and continents which has not yet received the attention it
+deserves. According to the recent careful estimate by Mr. John Murray,
+the land area of the globe is to the water area as ·28 to ·72. The mean
+height of the land above sea-level is 2250 feet, while the mean depth of
+the ocean is 14,640 feet. Hence the bulk of dry land is 23,450,000 cubic
+miles, and that of the waters of the ocean 323,800,000 cubic miles; and
+it follows that if the whole of the solid matter of the earth's surface
+were reduced to one level, it would be everywhere covered by an ocean
+about two miles deep. The accompanying diagram will serve to render
+these figures more intelligible. The length of the sections of land and
+ocean are in the proportion of their respective areas, while the mean
+height of the land and the mean depth of the ocean are exhibited on a
+greatly increased vertical scale. If we considered the continents and
+their adjacent oceans separately they would differ a little, but not
+very materially, from this diagram; in some cases the proportion of land
+to ocean would be a little greater, in others a little less.
+
+[Illustration: FIG. 32.]
+
+Now, if we try to imagine a process of elevation and depression by which
+the sea and land shall completely change places, we shall be met by
+insuperable difficulties. We must, in the first place, assume a general
+equality between elevation and subsidence during any given period,
+because if the elevation over any extensive continental area were not
+balanced by some subsidence of approximately equal amount, an
+unsupported hollow would be left under the earth's crust. Let us now
+suppose a continental area to sink, and an adjacent oceanic area to
+rise, it will be seen that the greater part of the land will disappear
+long before the new land has approached the surface of the ocean. This
+difficulty will not be removed by supposing a portion of a continent to
+subside, and the immediately adjacent portion of the ocean on the other
+side of the continent to rise, because in almost every case we find that
+within a comparatively short distance from the shores of all existing
+continents, the ocean floor sinks rapidly to a depth of from 2000 to
+3000 fathoms, and maintains a similar depth, generally speaking, over a
+large portion of the oceanic areas. In order, therefore, that any area
+of continental extent be upraised from the great oceans, there must be a
+subsidence of a land area five or six times as great, unless it can be
+shown that an extensive elevation of the ocean floor up to and far
+above the surface could occur without an equivalent depression
+elsewhere. The fact that the waters of the ocean are sufficient to cover
+the whole globe to a depth of two miles, is alone sufficient to indicate
+that the great ocean basins are permanent features of the earth's
+surface, since any process of alternation of these with the land areas
+would have been almost certain to result again and again in the total
+disappearance of large portions, if not of all, of the dry land of the
+globe. But the continuity of terrestrial life since the Devonian and
+Carboniferous periods, and the existence of very similar forms in the
+corresponding deposits of every continent--as well as the occurrence of
+sedimentary rocks, indicating the proximity of land at the time of their
+deposit, over a large portion of the surface of all the continents, and
+in every geological period--assure us that no such disappearance has
+ever occurred.
+
+
+_Oceanic and Continental Areas._
+
+When we speak of the permanence of oceanic and continental areas as one
+of the established facts of modern research, we do not mean that
+existing continents and oceans have always maintained the exact areas
+and outlines that they now present, but merely, that while all of them
+have been undergoing changes in outline and extent from age to age, they
+have yet maintained substantially the same positions, and have never
+actually changed places with each other. There are, moreover, certain
+physical and biological facts which enable us to mark out these areas
+with some confidence.
+
+We have seen that there are a large number of islands which may be
+classed as oceanic, because they have never formed parts of continents,
+but have originated in mid-ocean, and have derived their forms of life
+by migration across the sea. Their peculiarities are seen to be very
+marked in comparison with those islands which there is good reason to
+believe are really fragments of more extensive land areas, and are hence
+termed "continental." These continental islands consist in every case of
+a variety of stratified rocks of various ages, thus corresponding
+closely with the usual structure of continents; although many of the
+islands are small like Jersey or the Shetland Islands, or far from
+continental land like the Falkland Islands or New Zealand. They all
+contain indigenous mammalia or batrachia, and generally a much greater
+variety of birds, reptiles, insects, and plants, than do the oceanic
+islands. From these various characteristics we conclude that they have
+all once formed parts of continents, or at all events of much larger
+land areas, and have become isolated, either by subsidence of the
+intervening land or by the effects of long-continued marine denudation.
+
+Now, if we trace the thousand-fathom line around all our existing
+continents we find that, with only two exceptions, every island which
+can be classed as "continental" falls within this line, while all that
+lie beyond it have the undoubted characteristics of "oceanic" islands.
+We, therefore, conclude that the thousand-fathom line marks out,
+approximately, the "continental area,"--that is, the limits within which
+continental development and change throughout known geological time have
+gone on. There may, of course, have been some extensions of land beyond
+this limit, while some areas within it may always have been ocean; but
+so far as we have any direct evidence, this line may be taken to mark
+out, approximately, the most probable boundary between the "continental
+area," which has always consisted of land and shallow sea in varying
+proportions, and the great oceanic basins, within the limits of which
+volcanic activity has been building up numerous islands, but whose
+profound depths have apparently undergone little change.
+
+
+_Madagascar and New Zealand._
+
+The two exceptions just referred to are Madagascar and New Zealand, and
+all the evidence goes to show that in these cases the land connection
+with the nearest continental area was very remote in time. The
+extraordinary isolation of the productions of Madagascar--almost all the
+most characteristic forms of mammalia, birds, and reptiles of Africa
+being absent from it--renders it certain that it must have been
+separated from that continent very early in the Tertiary, if not as far
+back as the latter part of the Secondary period; and this extreme
+antiquity is indicated by a depth of considerably more than a thousand
+fathoms in the Mozambique Channel, though this deep portion is less than
+a hundred miles wide between the Comoro Islands and the mainland.[166]
+Madagascar is the only island on the globe with a fairly rich mammalian
+fauna which is separated from a continent by a depth greater than a
+thousand fathoms; and no other island presents so many peculiarities in
+these animals, or has preserved so many lowly organised and archaic
+forms. The exceptional character of its productions agrees exactly with
+its exceptional isolation by means of a very deep arm of the sea.
+
+New Zealand possesses no known mammals and only a single species of
+batrachian; but its geological structure is perfectly continental. There
+is also much evidence that it does possess one mammal, although no
+specimens have been yet obtained.[167] Its reptiles and birds are highly
+peculiar and more numerous than in any truly oceanic island. Now the sea
+which directly separates New Zealand from Australia is more than 2000
+fathoms deep, but in a north-west direction there is an extensive bank
+under 1000 fathoms, extending to and including Lord Howe's Island, while
+north of this are other banks of the same depth, approaching towards a
+submarine extension of Queensland on the one hand, and New Caledonia on
+the other, and altogether suggestive of a land union with Australia at
+some very remote period. Now the peculiar relations of the New Zealand
+fauna and flora with those of Australia and of the tropical Pacific
+Islands to the northward indicate such a connection, probably during the
+Cretaceous period; and here, again, we have the exceptional depth of the
+dividing sea and the form of the ocean bottom according well with the
+altogether exceptional isolation of New Zealand, an isolation which has
+been held by some naturalists to be great enough to justify its claim to
+be one of the primary Zoological Regions.
+
+
+_The Teachings of the Thousand-Fathom Line._
+
+If now we accept the annexed map as showing us approximately how far
+beyond their present limits our continents may have extended during any
+portion of the Tertiary and Secondary periods, we shall obtain a
+foundation of inestimable value for our inquiries into those migrations
+of animals and plants during past ages which have resulted in their
+present peculiarities of distribution. We see, for instance, that the
+South American and African continents have always been separated by
+nearly as wide an ocean as at present, and that whatever similarities
+there may be in their productions must be due to the similar forms
+having been derived from a common origin in one of the great northern
+continents. The radical difference between the higher forms of life of
+the two continents accords perfectly with their permanent separation. If
+there had been any direct connection between them during Tertiary times,
+we should hardly have found the deep-seated differences between the
+Quadrumana of the two regions--no family even being common to both; nor
+the peculiar Insectivora of the one continent, and the equally peculiar
+Edentata of the other. The very numerous families of birds quite
+peculiar to one or other of these continents, many of which, by their
+structural isolation and varied development of generic and specific
+forms, indicate a high antiquity, equally suggest that there has been no
+near approach to a land connection during the same epoch.
+
+Looking to the two great northern continents, we see indications of a
+possible connection between them both in the North Atlantic and the
+North Pacific oceans; and when we remember that from middle Tertiary
+times backward--so far as we know continuously to the earliest
+Palaeozoic epoch--a temperate and equable climate, with abundant woody
+vegetation, prevailed up to and within the arctic circle, we see what
+facilities may have been afforded for migration from one continent to
+the other, sometimes between America and Europe, sometimes between
+America and Asia. Admitting these highly probable connections, no
+bridging of the Atlantic in more southern latitudes (of which there is
+not a particle of evidence) will have been necessary to account for all
+the intermigration that has occurred between the two continents. If, on
+the other hand, we remember how long must have been the route, and how
+diverse must always have been the conditions between the more northern
+and the more southern portions of the American and Euro-Asiatic
+continents, we shall not be surprised that many widespread forms in
+either continent have not crossed into the other; and that while the
+skunks (Mephitis), the pouched rats (Saccomyidae), and the turkeys
+(Meleagris) are confined to America, the pigs and the hedgehogs, the
+true flycatchers and the pheasants are found only in the Euro-Asiatic
+continent. But, just as there have been periods which facilitated
+intermigration between America and the Old World, there have almost
+certainly been periods, perhaps of long duration even geologically, when
+these continents have been separated by seas as wide as, or even wider
+than, those of the present day; and thus may be explained such curious
+anomalies as the origination of the camel-tribe in America, and its
+entrance into Asia in comparatively recent Tertiary times, while the
+introduction of oxen and bears into America from the Euro-Asiatic
+continent appears to have been equally recent.[168]
+
+We shall find on examination that this view of the general permanence of
+the oceanic and continental areas, with constant minor fluctuations of
+land and sea over the whole extent of the latter, enables us to
+understand, and offer a rational explanation of, most of the difficult
+problems of geographical distribution; and further, that our power of
+doing this is in direct proportion to our acquaintance with the
+distribution of fossil forms of life during the Tertiary period. We
+must, also, take due note of many other facts of almost equal importance
+for a due appreciation of the problems presented for solution, the most
+essential being, the various powers of dispersal possessed by the
+different groups of animals and plants, the geological antiquity of the
+species and genera, and the width and depth of the seas which separate
+the countries they, inhabit. A few illustrations will now be given of
+the way in which these branches of knowledge enable us to deal with the
+difficulties and anomalies that present themselves.
+
+
+_The Distribution of Marsupials._
+
+This singular and lowly organised type of mammals constitutes almost the
+sole representative of the class in Australia and New Guinea, while it
+is entirely unknown in Asia, Africa, or Europe. It reappears in America,
+where several species of opossums are found; and it was long thought
+necessary to postulate a direct southern connection of these distant
+countries, in order to account for this curious fact of distribution.
+When, however, we look to what is known of the geological history of the
+marsupials the difficulty vanishes. In the Upper Eocene deposits of
+Western Europe the remains of several animals closely allied to the
+American opossums have been found; and as, at this period, a very mild
+climate prevailed far up into the arctic regions, there is no difficulty
+in supposing that the ancestors of the group entered America from Europe
+or Northern Asia during early Tertiary times.
+
+But we must go much further back for the origin of the Australian
+marsupials. All the chief types of the higher mammalia were in existence
+in the Eocene, if not in the preceding Cretaceous period, and as we find
+none of these in Australia, that country must have been finally
+separated from the Asiatic continent during the Secondary or Mesozoic
+period. Now during that period, in the Upper and the Lower Oolite and in
+the still older Trias, the jaw-bones of numerous small mammalia have
+been found, forming eight distinct genera, which are believed to have
+been either marsupials or some allied lowly forms. In North America
+also, in beds of the Jurassic and Triassic formations, the remains of an
+equally great variety of these small mammalia have been discovered; and
+from the examination of more than sixty specimens, belonging to at least
+six distinct genera, Professor Marsh is of opinion that they represent a
+generalised type, from which the more specialised marsupials and
+insectivora were developed.
+
+From the fact that very similar mammals occur both in Europe and America
+at corresponding periods, and in beds which represent a long succession
+of geological time, and that during the whole of this time no fragments
+of any higher forms have been discovered, it seems probable that both
+the northern continents (or the larger portion of their area) were then
+inhabited by no other mammalia than these, with perhaps other equally
+low types. It was, probably, not later than the Jurassic age when some
+of these primitive marsupials were able to enter Australia, where they
+have since remained almost completely isolated; and, being free from
+the competition of higher forms, they have developed into the great
+variety of types we now behold there. These occupy the place, and have
+to some extent acquired the form and structure of distinct orders of the
+higher mammals--the rodents, the insectivora, and the carnivora,--while
+still preserving the essential characteristics and lowly organisation of
+the marsupials. At a much later period--probably in late Tertiary
+times--the ancestors of the various species of rats and mice which now
+abound in Australia, and which, with the aerial bats, constitute its
+only forms of placental mammals, entered the country from some of the
+adjacent islands. For this purpose a land connection was not necessary,
+as these small creatures might easily be conveyed among the branches or
+in the crevices of trees uprooted by floods and carried down to the sea,
+and then floated to a shore many miles distant. That no actual land
+connection with, or very close approximation to, an Asiatic island has
+occurred in recent times, is sufficiently proved by the fact that no
+squirrel, pig, civet, or other widespread mammal of the Eastern
+hemisphere has been able to reach the Australian continent.
+
+
+_The Distribution of Tapirs._
+
+These curious animals form one of the puzzles of geographical
+distribution, being now confined to two very remote regions of the
+globe--the Malay Peninsula and adjacent islands of Sumatra and Borneo,
+inhabited by one species, and tropical America, where there are three or
+four species, ranging from Brazil to Ecuador and Guatemala. If we
+considered these living forms only, we should be obliged to speculate on
+enormous changes of land and sea in order that these tropical animals
+might have passed from one country to the other. But geological
+discoveries have rendered all such hypothetical changes unnecessary.
+During Miocene and Pliocene times tapirs abounded over the whole of
+Europe and Asia, their remains having been found in the tertiary
+deposits of France, India, Burmah, and China. In both North and South
+America fossil remains of tapirs occur only in caves and deposits of
+Post-Pliocene age, showing that they are comparatively recent immigrants
+into that continent. They perhaps entered by the route of Kamchatka and
+Alaska, where the climate, even now so much milder and more equable than
+on the north-east of America, might have been warm enough in late
+Pliocene times to have allowed the migration of these animals. In Asia
+they were driven southwards by the competition of numerous higher and
+more powerful forms, but have found a last resting-place in the swampy
+forests of the Malay region.
+
+
+_What these Facts Prove._
+
+Now these two cases, of the marsupials and the tapirs, are in the
+highest degree instructive, because they show us that, without any
+hypothetical bridging of deep oceans, and with only such changes of sea
+and land as are indicated by the extent of the comparatively shallow
+seas surrounding and connecting the existing continents, we are able to
+account for the anomaly of allied forms occurring only in remote and
+widely separated areas. These examples really constitute crucial tests,
+because, of all classes of animals, mammalia are least able to surmount
+physical barriers. They are obviously unable to pass over wide arms of
+the sea, while the necessity for constant supplies of food and water
+renders sandy deserts or snow-clad plains equally impassable. Then,
+again, the peculiar kinds of food on which alone many of them can
+subsist, and their liability to the attacks of other animals, put a
+further check upon their migrations. In these respects almost all other
+organisms have great advantages over mammals. Birds can often fly long
+distances, and can thus cross arms of the sea, deserts, or mountain
+ranges; insects not only fly, but are frequently carried great distances
+by gales of wind, as shown by the numerous cases of their visits to
+ships hundreds of miles from land. Reptiles, though slow of movement,
+have advantages in their greater capacity for enduring hunger or thirst,
+their power of resisting cold or drought in a state of torpidity, and
+they have also some facilities for migration across the sea by means of
+their eggs, which may be conveyed in crevices of timber or among masses
+of floating vegetable matter. And when we come to the vegetable kingdom,
+the means of transport are at their maximum, numbers of seeds having
+special adaptations for being carried by mammalia or birds, and for
+floating in the water, or through the air, while many are so small and
+so light that there is practically no limit to the distances they may be
+carried by gales and hurricanes.
+
+We may, therefore, feel quite certain that the means of distribution
+that have enabled the larger mammalia to reach the most remote regions
+from a common starting-point, will be at least as efficacious, and
+usually far more efficacious, with all other land animals and plants;
+and if in every case the existing distribution of this class can be
+explained on the theory of oceanic and continental permanence, with the
+limited changes of sea and land already referred to, no valid objections
+can be taken against this theory founded on anomalies of distribution in
+other orders. Yet nothing is more common than for students of this or
+that group to assort that the theory of oceanic permanence is quite
+inconsistent with the distribution of its various species and genera.
+Because a few Indian genera and closely allied species of birds are
+found in Madagascar, a land termed "Lemuria" has been supposed to have
+united the two countries during a comparatively recent geological epoch;
+while the similarity of fossil plants and reptiles, from the Permian and
+Miocene formations of India and South Africa, has been adduced as
+further evidence of this connection. But there are also genera of
+snakes, of insects, and of plants, common to Madagascar and South
+America only, which have been held to necessitate a direct land
+connection between these countries. These views evidently refute
+themselves, because any such land connections must have led to a far
+greater similarity in the productions of the several countries than
+actually exists, and would besides render altogether inexplicable the
+absence of all the chief types of African and Indian mammalia from
+Madagascar, and its marvellous individuality in every department of the
+organic world.[169]
+
+
+_Powers of Dispersal as illustrated by Insular Organisms._
+
+Having arrived at the conclusion that our existing oceans have remained
+practically unaltered throughout the Tertiary and Secondary periods of
+geology, and that the distribution of the mammalia is such as might
+have been brought about by their known powers of dispersal, and by such
+changes of land and sea as have probably or certainly occurred, we are,
+of course, restricted to similar causes to explain the much wider and
+sometimes more eccentric distribution of other classes of animals and of
+plants. In doing so, we have to rely partly on direct evidence of
+dispersal, afforded by the land organisms that have been observed far
+out at sea, or which have taken refuge on ships, as well as by the
+periodical visitants to remote islands; but very largely on indirect
+evidence, afforded by the frequent presence of certain groups on remote
+oceanic islands, which some ancestral forms must, therefore, have
+reached by transmission across the ocean from distant lands.
+
+
+_Birds._
+
+These vary much in their powers of flight, and their capability of
+traversing wide seas and oceans. Many swimming and wading birds can
+continue long on the wing, fly swiftly, and have, besides, the power of
+resting safely on the surface of the water. These would hardly be
+limited by any width of ocean, except for the need of food; and many of
+them, as the gulls, petrels, and divers, find abundance of food on the
+surface of the sea itself. These groups have a wide distribution
+_across_ the oceans; while waders--especially plovers, sandpipers,
+snipes, and herons--are equally cosmopolitan, travelling _along_ the
+coasts of all the continents, and across the narrow seas which separate
+them. Many of these birds seem unaffected by climate, and as the
+organisms on which they feed are equally abundant on arctic, temperate,
+and tropical shores, there is hardly any limit to the range even of some
+of the species.
+
+Land-birds are much more restricted in their range, owing to their
+usually limited powers of flight, their inability to rest on the surface
+of the sea or to obtain food from it, and their greater specialisation,
+which renders them less able to maintain themselves in the new countries
+they may occasionally reach. Many of them are adapted to live only in
+woods, or in marshes, or in deserts; they need particular kinds of food
+or a limited range of temperature; and they are adapted to cope only
+with the special enemies or the particular group of competitors among
+which they have been developed. Such birds as these may pass again and
+again to a new country, but are never able to establish themselves in
+it; and it is this organic barrier, as it is termed, rather than any
+physical barrier, which, in many cases, determines the presence of a
+species in one area and its absence from another. We must always
+remember, therefore, that, although the presence of a species in a
+remote oceanic island clearly proves that its ancestors must at one time
+have found their way there, the absence of a species does not prove the
+contrary, since it also may have reached the island, but have been
+unable to maintain itself, owing to the inorganic or organic conditions
+not being suitable to it. This general principle applies to all classes
+of organisms, and there are many striking illustrations of it. In the
+Azores there are eighteen species of land-birds which are permanent
+residents, but there are also several others which reach the islands
+almost every year after great storms, but have never been able to
+establish themselves. In Bermuda the facts are still more striking,
+since there are only ten species of resident birds, while no less than
+twenty other species of land-birds and more than a hundred species of
+waders and aquatics are frequent visitors, often in great numbers, but
+are never able to establish themselves. On the same principle we account
+for the fact that, of the many continental insects and birds that have
+been let loose, or have escaped from confinement, in this country,
+hardly one has been able to maintain itself, and the same phenomenon is
+still more striking in the case of plants. Of the thousands of hardy
+plants which grow easily in our gardens, very few have ever run wild,
+and when the experiment is purposely tried it invariably fails. Thus A.
+de Candolle informs us that several botanists of Paris, Geneva, and
+especially of Montpellier, have sown the seeds of many hundreds of
+species of exotic hardy plants, in what appeared to be the most
+favourable situations, but that in hardly a single case has any one of
+them become naturalised.[170] Still more, then, in plants than in
+animals the absence of a species does not prove that it has never
+reached the locality, but merely that it has not been able to maintain
+itself in competition with the native productions. In other cases, as
+we have seen, facts of an exactly opposite nature occur. The rat, the
+pig, and the rabbit, the water-cress, the clover, and many other plants,
+when introduced into New Zealand, nourish exceedingly, and even
+exterminate their native competitors; so that in these cases we may feel
+sure that the species in question did not exist in New Zealand simply
+because they had been unable to reach that country by their natural
+means of dispersal. I will now give a few cases, in addition to those
+recorded in my previous works, of birds and insects which have been
+observed far from any land.
+
+
+_Birds and Insects at Sea._
+
+Captain D. Fullarton of the ship _Timaru_ recorded in his log the
+occurrence of a great number of small land-birds about the ship on 15th
+March 1886, when in Lat. 48° 31' N., Long. 8° 16' W. He says: "A great
+many small land-birds about us; put about sixty into a coop, evidently
+tired out." And two days later, 17th March, "Over fifty of the birds
+cooped on 15th died, though fed. Sparrows, finches, water-wagtails, two
+small birds, name unknown, one kind like a linnet, and a large bird like
+a starling. In all there have been on board over seventy birds, besides
+some that hovered about us for some time and then fell into the sea
+exhausted." Easterly winds and severe weather were experienced at the
+time.[171] The spot where this remarkable flight of birds was met with
+is about 160 miles due west of Brest, and this is the least distance the
+birds must have been carried. It is interesting to note that the
+position of the ship is nearly in the line from the English and French
+coasts to the Azores, where, after great storms, so many bird stragglers
+arrive annually. These birds were probably blown out to sea during their
+spring migration along the south coast of England to Wales and Ireland.
+During the autumnal migration, however, great flocks of
+birds--especially starlings, thrushes, and fieldfares--have been
+observed every year flying out to sea from the west coast of Ireland,
+almost the whole of which must perish. At the Nash Lighthouse, in the
+Bristol Channel on the coast of Glamorganshire, an enormous number of
+small birds were observed on 3d September, including nightjars,
+buntings, white-throats, willow-wrens, cuckoos, house-sparrows, robins,
+wheatears, and blackbirds. These had probably crossed from
+Somersetshire, and had they been caught by a storm the larger portion of
+them must have been blown out to sea.[172]
+
+These facts enable us to account sufficiently well for the birds of
+oceanic islands, the number and variety of which are seen to be
+proportionate to their facilities for reaching the island and
+maintaining themselves in it. Thus, though more birds yearly reach
+Bermuda than the Azores, the number of residents in the latter islands
+is much larger, due to the greater extent of the islands, their number,
+and their more varied surface. In the Galapagos the land-birds are still
+more numerous, due in part to their larger area and greater proximity to
+the continent, but chiefly to the absence of storms, so that the birds
+which originally reached the islands have remained long isolated and
+have developed into many closely allied species adapted to the special
+conditions. All the species of the Galapagos but one are peculiar to the
+islands, while the Azores possess only one peculiar species, and Bermuda
+none--a fact which is clearly due to the continual immigration of fresh
+individuals keeping up the purity of the breed by intercrossing. In the
+Sandwich Islands, which are extremely isolated, being more than 2000
+miles from any continent or large island, we have a condition of things
+similar to what prevails in the Galapagos, the land-birds, eighteen in
+number, being all peculiar, and belonging, except one, to peculiar
+genera. These birds have probably all descended from three or four
+original types which reached the islands at some remote period, probably
+by means of intervening islets that have since disappeared. In St.
+Helena we have a degree of permanent isolation which has prevented any
+land-birds from reaching the island; for although its distance from the
+continent, 1100 miles, is not so great as in the case of the Sandwich
+Islands, it is situated in an ocean almost entirely destitute of small
+islands, while its position within the tropics renders it free from
+violent storms. Neither is there, on the nearest part of the coast of
+Africa, a perpetual stream of migrating birds like that which supplies
+the innumerable stragglers which every year reach Bermuda and the
+Azores.
+
+
+_Insects._
+
+Winged insects have been mainly dispersed in the same way as birds, by
+their power of flight, aided by violent or long-continued winds. Being
+so small, and of such low specific gravity, they are occasionally
+carried to still greater distances; and thus no islands, however remote,
+are altogether without them. The eggs of insects, being often deposited
+in borings or in crevices of timber, may have been conveyed long
+distances by floating trees, as may the larvae of those species which
+feed on wood. Several cases have been published of insects coming on
+board ships at great distances from land; and Darwin records having
+caught a large grasshopper when the ship was 370 miles from the coast of
+Africa, whence the insect had probably come.
+
+In the _Entomologists' Monthly Magazine_ for June 1885, Mr. MacLachlan
+has recorded the occurrence of a swarm of moths in the Atlantic ocean,
+from the log of the ship _Pleione_. The vessel was homeward bound from
+New Zealand, and in Lat. 6° 47' N., Long. 32° 50' W., hundreds of moths
+appeared about the ship, settling in numbers on the spars and rigging.
+The wind for four days previously had been very light from north,
+north-west, or north-east, and sometimes calm. The north-east trade wind
+occasionally extends to the ship's position at that time of year. The
+captain adds that "frequently, in that part of the ocean, he has had
+moths and butterflies come on board." The position is 960 miles
+south-west of the Cape Verde Islands, and about 440 north-east of the
+South American coast. The specimen preserved is Deiopeia pulchella, a
+very common species in dry localities in the Eastern tropics, and rarely
+found in Britain, but, Mr. MacLachlan thinks, not found in South
+America. They must have come, therefore, from the Cape Verde Islands, or
+from some parts of the African coast, and must have traversed about a
+thousand miles of ocean with the assistance, no doubt, of a strong
+north-east trade wind for a great part of the distance. In the British
+Museum collection there is a specimen of the same moth caught at sea
+during the voyage of the _Rattlesnake_, in Lat. 6° N., Long. 22-1/2°
+W., being between the former position and Sierra Leone, thus rendering
+it probable that the moths came from that part of the African coast, in
+which case the swarm encountered by the _Pleione_ must have travelled
+more than 1200 miles.
+
+A similar case was recorded by Mr. F.A. Lucas in the American periodical
+_Science_ of 8th April 1887. He states that in 1870 he met with numerous
+moths of many species while at sea in the South Atlantic (Lat. 25° S.,
+Long. 24° W.), about 1000 miles from the coast of Brazil. As this
+position is just beyond the south-east trades, the insects may have been
+brought from the land by a westerly gale. In the _Zoologist_ (1864, p.
+8920) is the record of a small longicorn beetle which flew on board a
+ship 500 miles off the west coast of Africa. Numerous other cases are
+recorded of insects at less distances from land, and, taken in
+connection with those already given, they are sufficient to show that
+great numbers must be continually carried out to sea, and that
+occasionally they are able to reach enormous distances. But the
+reproductive powers of insects are so great that all we require, in
+order to stock a remote island, is that some few specimens shall reach
+it even once in a century, or once in a thousand years.
+
+
+_Insects at great Altitudes._
+
+Equally important is the proof we possess that insects are often carried
+to great altitudes by upward currents of air. Humboldt noticed them up
+to heights of 15,000 and 18,000 feet in South America, and Mr. Albert
+Müller has collected many interesting cases of the same character in
+Europe.[173] A moth (Plusia gamma) has been found on the summit of Mont
+Blanc; small hymenoptera and moths have been seen on the Pyrenees at a
+height of 11,000 feet, while numerous flies and beetles, some of
+considerable size, have been caught on the glaciers and snow-fields of
+various parts of the Alps. Upward currents of air, whirlwinds and
+tornadoes, occur in all parts of the world, and large numbers of insects
+are thus carried up into the higher regions of the atmosphere, where
+they are liable to be caught by strong winds, and thus conveyed enormous
+distances over seas or continents. With such powerful means of
+dispersal the distribution of insects over the entire globe, and their
+presence in the most remote oceanic islands, offer no difficulties.
+
+
+_The Dispersal of Plants._
+
+The dispersal of seeds is effected in a greater variety of ways than are
+available in the case of any animals. Some fruits or seed-vessels, and
+some seeds, will float for many weeks, and after immersion in salt water
+for that period the seeds will often germinate. Extreme cases are the
+double cocoa-nut of the Seychelles, which has been found on the coast of
+Sumatra, about 3000 miles distant; the fruits of the Sapindus saponaria
+(soap-berry), which has been brought to Bermuda by the Gulf Stream from
+the West Indies, and has grown after a journey in the sea of about 1500
+miles; and the West Indian bean, Entada scandens, which reached the
+Azores from the West Indies, a distance of full 3000 miles, and
+afterwards germinated at Kew. By these means we can account for the
+similarity in the shore flora of the Malay Archipelago and most of the
+islands of the Pacific; and from an examination of the fruits and seeds,
+collected among drift during the voyage of the _Challenger_, Mr. Hemsley
+has compiled a list of 121 species which are probably widely dispersed
+by this means.
+
+A still larger number of species owe their dispersal to birds in several
+distinct ways. An immense number of fruits in all parts of the world are
+devoured by birds, and have been attractively coloured (as we have
+seen), in order to be so devoured, because the seeds pass through the
+birds' bodies and germinate where they fall. We have seen how frequently
+birds are forced by gales of wind across a wide expanse of ocean, and
+thus seeds must be occasionally carried. It is a very suggestive fact,
+that all the trees and shrubs in the Azores bear berries or small fruits
+which are eaten by birds; while all those which bear larger fruits, or
+are eaten chiefly by mammals--such as oaks, beeches, hazels, crabs,
+etc.--are entirely wanting. Game-birds and waders often have portions of
+mud attached to their feet, and Mr. Darwin has proved by experiment that
+such mud frequently contains seeds. One partridge had such a quantity of
+mud attached to its foot as to contain seeds from which eighty-two
+plants germinated; this proves that a very small portion of mud may
+serve to convey seeds, and such an occurrence repeated even at long
+intervals may greatly aid in stocking remote islands with vegetation.
+Many seeds also adhere to the feathers of birds, and thus, again, may be
+conveyed as far as birds are ever carried. Dr. Guppy found a small hard
+seed in the gizzard of a Cape Petrel, taken about 550 miles east of
+Tristan da Cunha.
+
+
+_Dispersal of Seeds by the Wind._
+
+In the preceding cases we have been able to obtain direct evidence of
+transportal; but although we know that many seeds are specially adapted
+to be dispersed by the wind, we cannot obtain direct proof that they are
+so carried for hundreds or thousands of miles across the sea, owing to
+the difficulty of detecting single objects which are so small and
+inconspicuous. It is probable, however, that the wind as an agent of
+dispersal is really more effective than any of those we have hitherto
+considered, because a very large number of plants have seeds which are
+very small and light, and are often of such a form as to facilitate
+aerial carriage for enormous distances. It is evident that such seeds
+are especially liable to be transported by violent winds, because they
+become ripe in autumn at the time when storms are most prevalent, while
+they either lie upon the surface of the ground, or are disposed in dry
+capsules on the plant ready to be blown away. If inorganic particles
+comparable in weight, size, or form with such seeds are carried for
+great distances, we may be sure that seeds will also be occasionally
+carried in the same way. It will, therefore, be necessary to give a few
+examples of wind-carriage of small objects.
+
+On 27th July 1875 a remarkable shower of small pieces of hay occurred at
+Monkstown, near Dublin. They appeared floating slowly down from a great
+height, as if falling from a dark cloud which hung overhead. The pieces
+picked up were wet, and varied from single blades of grass to tufts
+weighing one or two ounces. A similar shower occurred a few days earlier
+in Denbighshire, and was observed to travel in a direction contrary to
+that of the wind in the lower atmosphere.[174] There is no evidence of
+the distance from which the hay was brought, but as it had been carried
+to a great height, it was in a position to be conveyed to almost any
+distance by a violent wind, had such occurred at the time.
+
+
+_Mineral Matter carried by the Wind._
+
+The numerous cases of sand and volcanic dust being carried enormous
+distances through the atmosphere sufficiently prove the importance of
+wind as a carrier of solid matter, but unfortunately the matter
+collected has not been hitherto examined with a view to determine the
+maximum size and weight of the particles. A few facts, however, have
+been kindly furnished me by Professor Judd, F.R.S. Some dust which fell
+at Genoa on 15th October 1885, and was believed to have been brought
+from the African desert, consisted of quartz, hornblende, and other
+minerals, and contained particles having a diameter of 1/500 inch, each
+weighing 1/200,000 grain. This dust had probably travelled over 600
+miles. In the dust from Krakatoa, which fell at Batavia, about 100 miles
+distant, during the great eruption, there are many solid particles even
+larger than those mentioned above. Some of this dust was given me by
+Professor Judd, and I found in it several ovoid particles of a much
+larger size, being 1/50 inch long, and 1/70 wide and deep. The dust from
+the same eruption, which fell on board the ship _Arabella_, 970 miles
+from the volcano, also contained solid particles 1/500 inch diameter.
+Mr. John Murray of the _Challenger_ Expedition writes to me that he
+finds in the deep sea deposits 500 and even 700 miles west of the coast
+of Africa, rounded particles of quartz, having a diameter of 1/250 inch,
+and similar particles are found at equally great distances from the
+south-west coasts of Australia; and he considers these to be atmospheric
+dust carried to that distance by the wind. Taking the sp. gr. of quartz
+at 2.6, these particles would weigh about 1/25,000 grain each. These
+interesting facts can, however, by no means be taken as indicating the
+extreme limits of the power of wind in carrying solid particles. During
+the Krakatoa eruption no gale of special violence occurred, and the
+region is one of comparative calms. The grains of quartz found by Mr.
+Murray more nearly indicate the limit, but the very small portions of
+matter brought up by the dredge, as compared with the enormous areas of
+sea-bottom, over which the atmospheric dust must have been scattered,
+render it in the highest degree improbable that the maximum limit either
+of size of particles, or of distance from land has been reached.
+
+Let us, however, assume that the quartz grains, found by Mr. Murray in
+the deep-sea ooze 700 miles from land, give us the extreme limit of the
+power of the atmosphere as a carrier of solid particles, and let us
+compare with these the weights of some seeds. From a small collection of
+the seeds of thirty species of herbaceous plants sent me from Kew, those
+in the above table were selected, and small portions of eight of them
+carefully weighed in a chemical balance.[175] By counting these portions
+I was able to estimate the number of seeds weighing one grain. The three
+very minute species, whose numbers are marked with an asterisk (*), were
+estimated by the comparison of their sizes with those of the smaller
+weighed seeds.
+
+
+No|    Species.         |Approximate |      Approximate  |  Remarks.
+  |                     |No. of Seeds|      Dimensions.  |
+  |                     |In one Grain|                   |
+  |                     |            | in.    in.    in. |
+ 1|Draba verna          |  1,800     |1/60 x 1/90 x 1/160|Oval, flat.
+ 2|Hypericum perforatum |    520     |      1/30 x 1/80  |Cylindrical.
+ 3|Astilbe rivularis    |  4,500     |      1/50 x 1/100 |Elongate, flat, tailed,
+  |                     |            |                   |   wavy.
+ 4|Saxifraga coriophylla|    750     |      1/40 x 1/75  |Surface rough, adhere
+  |                     |            |                   |   to the dry capsules.
+ 5|Oenothera rosea      |    640     |  1/40 x 1/80      |Ovate.
+ 6|Hypericum hirsutum   |    700     |      1/30 x 1/100 |Cylindrical, rough.
+ 7|Mimulus luteus       |  2,900     |      1/60 x 1/100 |Oval, minute.
+ 8|Penthorum sedoides   |  8,000*    |      1/70 x 1/150 |Flattened, very minute.
+ 9|Sagina procumbens    | 12,000*    |          1/120    |Sub-triangular, flat.
+10|Orchis maculata      | 15,000*    |           ---     |Margined, flat,
+  |                     |            |                   | very minute.
+11|Gentiana purpurea    |     35     |           1/25    |Wavy, rough, with this
+  |                     |            |                   |   coriaceous margins.
+12|Silene alpina        |   ---      |           1/30    |Flat, with fringed
+  |                     |            |                   |   margins.
+13|Adenophora communis  |   ---      |       1/20 x 1/40 |Very thin, wavy, light.
+  |Quartz grains        | 25,000     |           1/250   |Deep sea ... 700 miles.
+  |Do.                  |200,000     |           1/500   |Genoa ... 600 miles.
+
+
+If now we compare the seeds with the quartz grains, we find that
+several are from twice to three times the weight of the grains found by
+Mr. Murray, and others five times, eight times, and fifteen times as
+heavy; but they are proportionately very much larger, and, being usually
+irregular in shape or compressed, they expose a very much larger surface
+to the air. The surface is often rough, and several have dilated margins
+or tailed appendages, increasing friction and rendering the uniform rate
+of falling through still air immensely less than in the case of the
+smooth, rounded, solid quartz grains. With these advantages it is a
+moderate estimate that seeds ten times the weight of the quartz grains
+could be carried quite as far through the air by a violent gale and
+under the most favourable conditions. These limits will include five of
+the seeds here given, as well as hundreds of others which do not exceed
+them in weight; and to these we may add some larger seeds which have
+other favourable characteristics, as is the case with numbers 11-13,
+which, though very much larger than the rest, are so formed as in all
+probability to be still more easily carried great distances by a gale of
+wind. It appears, therefore, to be absolutely certain that every
+autumnal gale capable of conveying solid mineral particles to great
+distances, must also carry numbers of small seeds at least as far; and
+if this is so, the wind alone will form one of the most effective agents
+in the dispersal of plants.
+
+Hitherto this mode of conveyance, as applying to the transmission of
+seeds for great distances across the ocean, has been rejected by
+botanists, for two reasons. In the first place, there is said to be no
+direct evidence of such conveyance; and, secondly, the peculiar plants
+of remote oceanic islands do not appear to have seeds specially adapted
+for aerial transmission. I will consider briefly each of these
+objections.
+
+
+_Objection to the Theory of Wind-Dispersal._
+
+To obtain direct evidence of the transmission of such minute and
+perishable objects, which do not exist in great quantities, and are
+probably carried to the greatest distances but rarely and as single
+specimens, is extremely difficult. A bird or insect can be seen if it
+comes on board ship, but who would ever detect the seeds of Mimulus or
+Orchis even if a score of them fell on a ship's deck? Yet if but one
+such seed per century were carried to an oceanic island, that island
+might become rapidly overrun by the plant, if the conditions were
+favourable to its growth and reproduction. It is further objected that
+search has been made for such seeds, and they have not been found.
+Professor Kerner of Innsbruck examined the snow on the surface of
+glaciers, and assiduously collected all the seeds he could find, and
+these were all of plants which grew in the adjacent mountains or in the
+same district. In like manner, the plants growing on moraines were found
+to be those of the adjacent mountains, plateaux, or lowlands. Hence he
+concluded that the prevalent opinion that seeds may be carried through
+the air for very great distances "is not supported by fact."[176] The
+opinion is certainly not supported by Kerner's facts, but neither is it
+opposed by them. It is obvious that the seeds that would be carried by
+the wind to moraines or to the surface of glaciers would be, first and
+in the greatest abundance, those of the immediately surrounding
+district; then, very much more rarely, those from more remote mountains;
+and lastly, in extreme rarity, those from distant countries or
+altogether distinct mountain ranges. Let us suppose the first to be so
+abundant that a single seed could be found by industrious search on each
+square yard of the surface of the glacier; the second so scarce that
+only one could possibly be found in a hundred yards square; while to
+find one of the third class it would be necessary exhaustively to
+examine a square mile of surface. Should we expect that _one_ ever to be
+found, and should the fact that it could not be found be taken as a
+proof that it was not there? Besides, a glacier is altogether in a bad
+position to receive such remote wanderers, since it is generally
+surrounded by lofty mountains, often range behind range, which would
+intercept the few air-borne seeds that might have been carried from a
+distant land. The conditions in an oceanic island, on the other hand,
+are the most favourable, since the land, especially if high, will
+intercept objects carried by the wind, and will thus cause more of the
+solid matter to fall on it than on an equal area of ocean. We know that
+winds at sea often blow violently for days together, and the rate of
+motion is indicated by the fact that 72 miles an hour was the average
+velocity of the wind observed during twelve hours at the Ben Nevis
+observatory, while the velocity sometimes rises to 120 miles an hour. A
+twelve hours' gale might, therefore, carry light seeds a thousand miles
+as easily and certainly as it could carry quartz-grains of much greater
+specific gravity, rotundity, and smoothness, 500 or even 100 miles; and
+it is difficult even to imagine a sufficient reason why they should not
+be so carried--perhaps very rarely and under exceptionally favourable
+conditions,--but this is all that is required.
+
+As regards the second objection, it has been observed that orchideae,
+which have often exceedingly small and light seeds, are remarkably
+absent from oceanic islands. This, however, may be very largely due to
+their extreme specialisation and dependence on insect agency for their
+fertilisation; while the fact that they do occur in such very remote
+islands as the Azores, Tahiti, and the Sandwich Islands, proves that
+they must have once reached these localities either by the agency of
+birds or by transmission through the air; and the facts I have given
+above render the latter mode at least as probable as the former. Sir
+Joseph Hooker remarks on the composite plant of Kerguelen Island (Cotula
+plumosa) being found also on Lord Auckland and MacQuarrie Islands, and
+yet having no pappus, while other species of the genus possess it. This
+is certainly remarkable, and proves that the plant must have, or once
+have had, some other means of dispersal across wide oceans.[177] One of
+the most widely dispersed species in the whole world (Sonchus oleraceus)
+possesses pappus, as do four out of five of the species which are common
+to Europe and New Zealand, all of which have a very wide distribution.
+The same author remarks on the limited area occupied by most species of
+Compositae, notwithstanding their facilities for dispersal by means of
+their feathered seeds; but it has been already shown that limitations
+of area are almost always due to the competition of allied forms,
+facilities for dispersal being only one of many factors in determining
+the wide range of species. It is, however, a specially important factor
+in the case of the inhabitants of remote oceanic islands, since, whether
+they are peculiar species or not, they or their remote ancestors must at
+some time or other have reached their present position by natural means.
+
+I have already shown elsewhere, that the flora of the Azores strikingly
+supports the view of the species having been introduced by aerial
+transmission only, that is, by the agency of birds and the wind, because
+all plants that could not possibly have been carried by these means are
+absent.[178] In the same way we may account for the extreme rarity of
+Leguminosae in all oceanic islands. Mr. Hemsley, in his Report on
+Insular Floras, says that they "are wanting in a large number of oceanic
+islands where there is no true littoral flora," as St. Helena, Juan
+Fernandez, and all the islands of the South Atlantic and South Indian
+Oceans. Even in the tropical islands, such as Mauritius and Bourbon,
+there are no endemic species, and very few in the Galapagos and the
+remoter Pacific Islands. All these facts are quite in accordance with
+the absence of facilities for transmission through the air, either by
+birds or the wind, owing to the comparatively large size and weight of
+the seeds; and an additional proof is thus afforded of the extreme
+rarity of the successful floating of seeds for great distances across
+the ocean.[179]
+
+
+_Explanation of North Temperate Plants in the Southern Hemisphere._
+
+If we now admit that many seeds which are either minute in size, of thin
+texture or wavy form, or so fringed or margined as to afford a good hold
+to the air, are capable of being carried for many hundreds of miles by
+exceptionally violent and long-continued gales of wind, we shall not
+only be better able to account for the floras of some of the remotest
+oceanic islands, but shall also find in the fact a sufficient
+explanation of the wide diffusion of many genera, and even species, of
+arctic and north temperate plants in the southern hemisphere or on the
+summits of tropical mountains. Nearly fifty of the flowering plants of
+Tierra-del-Fuego are found also in North America or Europe, but in no
+intermediate country; while fifty-eight species are common to New
+Zealand and Northern Europe; thirty-eight to Australia, Northern Europe,
+and Asia; and no less than seventy-seven common to New Zealand,
+Australia, and South America.[180] On lofty mountains far removed from
+each other, identical or closely allied plants often occur. Thus the
+fine Primula imperialis of a single mountain peak in Java has been found
+(or a closely allied species) in the Himalayas; and many other plants of
+the high mountains of Java, Ceylon, and North India are either identical
+or closely allied forms. So, in Africa, some species, found on the
+summits of the Cameroons and Fernando Po in West Africa, are closely
+allied to species in the Abyssinian highlands and in Temperate Europe;
+while other Abyssinian and Cameroons species have recently been found on
+the mountains of Madagascar. Some peculiar Australian forms have been
+found represented on the summit of Kini Balu in Borneo. Again, on the
+summit of the Organ mountains in Brazil there are species allied to
+those of the Andes, but not found in the intervening lowlands.
+
+
+_No Proof of Recent Lower Temperature in the Tropics._
+
+Now all these facts, and numerous others of like character, were
+supposed by Mr. Darwin to be due to a lowering of temperature during
+glacial epochs, which allowed these temperate forms to migrate across
+the intervening tropical lowlands. But any such change within the epoch
+of existing species is almost inconceivable. In the first place, it
+would necessitate the extinction of much of the tropical flora (and with
+it of the insect life), because without such extinction alpine
+herbaceous plants could certainly never spread over tropical forest
+lowlands; and, in the next place, there is not a particle of direct
+evidence that any such lowering of temperature in inter-tropical
+lowlands ever took place. The only alleged evidence of the kind is that
+adduced by the late Professor Agassiz and Mr. Hartt; but I am informed
+by my friend, Mr. J.C. Branner (now State Geologist of Arkansas, U.S.),
+who succeeded Mr. Hartt, and spent several years completing the
+geological survey of Brazil, that the supposed moraines and glaciated
+granite rocks near Rio Janeiro and elsewhere, as well as the so-called
+boulder-clay of the same region, are entirely explicable as the results
+of sub-aerial denudation and weathering, and that there is no proof
+whatever of glaciation in any part of Brazil.
+
+
+_Lower Temperature not needed to Explain the Facts._
+
+But any such vast physical change as that suggested by Darwin, involving
+as it does such tremendous issues as regards its effects on the tropical
+fauna and flora of the whole world, is really quite uncalled for,
+because the facts to be explained are of the same essential nature as
+those presented by remote oceanic islands, between which and the nearest
+continents no temperate land connection is postulated. In proportion to
+their limited area and extreme isolation, the Azores, St. Helena, the
+Galapagos, and the Sandwich Islands, each possess a fairly rich--the
+last a very rich--indigenous flora; and the means which sufficed to
+stock them with a great variety of plants would probably suffice to
+transmit others from mountain-top to mountain-top in various parts of
+the globe. In the case of the Azores, we have large numbers of species
+identical with those of Europe, and others closely allied, forming an
+exactly parallel case to the species found on the various mountain
+summits which have been referred to. The distances from Madagascar to
+the South African mountains and to Kilimandjaro, and from the latter to
+Abyssinia, are no greater than from Spain to the Azores, while there are
+other equatorial mountains forming stepping-stones at about an equal
+distance to the Cameroons. Between Java and the Himalayas we have the
+lofty mountains of Sumatra and of North-western Burma, forming steps at
+about the same distance apart; while between Kini Balu and the
+Australian Alps we have the unexplored snow mountains of New Guinea,
+the Bellenden Ker mountains in Queensland, and the New England and Blue
+Mountains of New South Wales. Between Brazil and Bolivia the distances
+are no greater; while the unbroken range of mountains from Arctic
+America to Tierra-del-Fuego offers the greatest facilities for
+transmission, the partial gap between the lofty peak of Chiriqui and the
+high Andes of New Grenada being far less than from Spain to the Azores.
+Thus, whatever means have sufficed for stocking oceanic islands must
+have been to some extent effective in transmitting northern forms from
+mountain to mountain, across the equator, to the southern hemisphere;
+while for this latter form of dispersal there are special facilities, in
+the abundance of fresh and unoccupied surfaces always occurring in
+mountain regions, owing to avalanches, torrents, mountain-slides, and
+rock-falls, thus affording stations on which air-borne seeds may
+germinate and find a temporary home till driven out by the inroads of
+the indigenous vegetation. These temporary stations may be at much lower
+altitudes than the original habitat of the species, if other conditions
+are favourable. Alpine plants often descend into the valleys on glacial
+moraines, while some arctic species grow equally well on mountain
+summits and on the seashore. The distances above referred to between the
+loftier mountains may thus be greatly reduced by the occurrence of
+suitable conditions at lower altitudes, and the facilities for
+transmission by means of aerial currents proportionally increased.[181]
+
+
+_Facts Explained by the Wind-Carriage of Seeds._
+
+But if we altogether reject aerial transmission of seeds for great
+distances, except by the agency of birds, it will be difficult, if not
+impossible, to account for the presence of so many identical species of
+plants on remote mountain summits, or for that "continuous current of
+vegetation" described by Sir Joseph Hooker as having apparently long
+existed from the northern to the southern hemisphere. It may be admitted
+that we can, possibly, account for the greater portion of the floras of
+remote oceanic islands by the agency of birds alone; because, when blown
+out to sea land-birds must reach some island or perish, and all which
+come within sight of an island will struggle to reach it as their only
+refuge. But, with mountain summits the case is altogether different,
+because, being surrounded by land instead of by sea, no bird would need
+to fly, or to be carried by the wind, for several hundred miles at a
+stretch to another mountain summit, but would find a refuge in the
+surrounding uplands, ridges, valleys, or plains. As a rule the birds
+that frequent lofty mountain tops are peculiar species, allied to those
+of the surrounding district; and there is no indication whatever of the
+passage of birds from one remote mountain to another in any way
+comparable with the flights of birds which are known to reach the Azores
+annually, or even with the few regular migrants from Australia to New
+Zealand. It is almost impossible to conceive that the seeds of the
+Himalayan primula should have been thus carried to Java; but, by means
+of gales of wind, and intermediate stations from fifty to a few hundred
+miles apart, where the seeds might vegetate for a year or two and
+produce fresh seed to be again carried on in the same manner, the
+transmission might, after many failures, be at last effected.
+
+A very important consideration is the vastly larger scale on which
+wind-carriage of seeds must act, as compared with bird-carriage. It can
+only be a few birds which carry seeds attached to their feathers or
+feet. A very small proportion of these would carry the seeds of Alpine
+plants; while an almost infinitesimal fraction of these latter would
+convey the few seeds attached to them safely to an oceanic island or
+remote mountain. But winds, in the form of whirlwinds or tornadoes,
+gales or hurricanes, are perpetually at work over large areas of land
+and sea. Insects and light particles of matter are often carried up to
+the tops of high mountains; and, from the very nature and origin of
+winds, they usually consist of ascending or descending currents, the
+former capable of suspending such small and light objects as are many
+seeds long enough for them to be carried enormous distances. For each
+single seed carried away by external attachment to the feet or feathers
+of a bird, countless millions are probably carried away by violent
+winds; and the chance of conveyance to a great distance and in a
+definite direction must be many times greater by the latter mode than
+by the former.[182] We have seen that inorganic particles of much
+greater specific gravity than seeds, and nearly as heavy as the smallest
+kinds, are carried to great distances through the air, and we can
+therefore hardly doubt that some seeds are carried as far. The direct
+agency of the wind, as a supplement to bird-transport, will help to
+explain the presence in oceanic islands of plants growing in dry or
+rocky places whose small seeds are not likely to become attached to
+birds; while it seems to be the only effective agency possible in the
+dispersal of those species of alpine or sub-alpine plants found on the
+summits of distant mountains, or still more widely separated in the
+temperate zones of the northern and southern hemispheres.
+
+
+_Concluding Remarks._
+
+On the general principles that have been now laid down, it will be found
+that all the chief facts of the geographical distribution of animals and
+plants can be sufficiently understood. There will, of course, be many
+cases of difficulty and some seeming anomalies, but these can usually be
+seen to depend on our ignorance of some of the essential factors of the
+problem. Either we do not know the distribution of the group in recent
+geological times, or we are still ignorant of the special methods by
+which the organisms are able to cross the sea. The latter difficulty
+applies especially to the lizard tribe, which are found in almost all
+the tropical oceanic islands; but the particular mode in which they are
+able to traverse a wide expanse of ocean, which is a perfect barrier to
+batrachia and almost so to snakes, has not yet been discovered. Lizards
+are found in all the larger Pacific Islands as far as Tahiti, while
+snakes do not extend beyond the Fiji Islands; and the latter are also
+absent from Mauritius and Bourbon, where lizards of seven or eight
+species abound. Naturalists resident in the Pacific Islands would make a
+valuable contribution to our science by studying the life-history of the
+native lizards, and endeavouring to ascertain the special facilities
+they possess for crossing over wide spaces of ocean.
+
+FOOTNOTES:
+
+[Footnote 163: See A. Agassiz, _Three Cruises of the Blake_ (Cambridge,
+Mass., 1888), vol. i. p. 127, footnote.]
+
+[Footnote 164: Even the extremely fine Mississippi mud is nowhere found
+beyond a hundred miles from the mouths of the river in the Gulf of
+Mexico (A. Agassiz, _Three Cruises of the Blake_, vol. i. p. 128).]
+
+[Footnote 165: I have given a full summary of the evidence for the
+permanence of oceanic and continental areas in my _Island Life_, chap.
+vi.]
+
+[Footnote 166: For a full account of the peculiarities of the Madagascar
+fauna, see my _Island Life_, chap. xix.]
+
+[Footnote 167: See _Island Life_, p. 446, and the whole of chaps. xxi.
+xxii. More recent soundings have shown that the Map at p. 443, as well
+as that of the Madagascar group at p. 387, are erroneous, the ocean
+around Norfolk Island and in the Straits of Mozambique being more than
+1000 fathoms deep. The general argument is, however, unaffected.]
+
+[Footnote 168: For some details of these migrations, see the author's
+_Geographical Distribution of Animals_, vol. i. p. 140; also Heilprin's
+_Geographical and Geological Distribution of Animals_.]
+
+[Footnote 169: For a full discussion of this question, see _Island
+Life_, pp. 390-420.]
+
+[Footnote 170: _Géographie Botanique_, p. 798.]
+
+[Footnote 171: _Nature_, 1st April 1886.]
+
+[Footnote 172: Report of the Brit. Assoc. Committee on Migration of
+Birds during 1886.]
+
+[Footnote 173: _Trans. Ent. Soc._, 1871, p. 184.]
+
+[Footnote 174: _Nature_ (1875), vol. xii. pp. 279, 298.]
+
+[Footnote 175: I am indebted to Professor R. Meldola of the Finsbury
+Technical Institute, and Rev. T.D. Titmas of Charterhouse for furnishing
+me with the weights required.]
+
+[Footnote 176: See _Nature_, vol. vi. p. 164, for a summary of Kerner's
+paper.]
+
+[Footnote 177: It seems quite possible that the absence of pappus in
+this case is a recent adaptation, and that it has been brought about by
+causes similar to those which have reduced or aborted the wings of
+insects in oceanic islands. For when a plant has once reached one of the
+storm-swept islands of the southern ocean, the pappus will be injurious
+for the same reason that the wings of insects are injurious, since it
+will lead to the seeds being blown out to sea and destroyed. The seeds
+which are heaviest and have least pappus will have the best chance of
+falling on the ground and remaining there to germinate, and this process
+of selection might rapidly lead to the entire disappearance of the
+pappus.]
+
+[Footnote 178: See _Island Life_, p. 251.]
+
+[Footnote 179: Mr. Hemsley suggests that it is not so much the
+difficulty of transmission by floating, as the bad conditions the seeds
+are usually exposed to when they reach land. Many, even if they
+germinate, are destroyed by the waves, as Burchell noticed at St.
+Helena; while even a flat and sheltered shore would be an unsuitable
+position for many inland plants. Air-borne seeds, on the other hand, may
+be carried far inland, and so scattered that some of them are likely to
+reach suitable stations.]
+
+[Footnote 180: For fuller particulars, see Sir J. Hooker's _Introduction
+to Floras of New Zealand and Australia_, and a summary in my _Island
+Life_, chaps. xxii. xxiii.]
+
+[Footnote 181: For a fuller discussion of this subject, see my _Island
+Life_, chap. xxiii.]
+
+[Footnote 182: A very remarkable case of wind conveyance of seeds on a
+large scale is described in a letter from Mr. Thomas Hanbury to his
+brother, the late Daniel Hanbury, which has been kindly communicated to
+me by Mr. Hemsley of Kew. The letter is dated "Shanghai, 1st May 1856,"
+and the passage referred to is as follows:--
+
+
+"For the past three days we have had very warm weather for this time of
+year, in fact almost as warm as the middle of summer. Last evening the
+wind suddenly changed round to the north and blew all night with
+considerable violence, making a great change in the atmosphere.
+
+"This morning, myriads of small white particles are floating about in
+the air; there is not a single cloud and no mist, yet the sun is quite
+obscured by this substance, and it looks like a white fog in England. I
+enclose thee a sample, thinking it may interest. It is evidently a
+vegetable production; I think, apparently, some kind of seed."
+
+
+Mr. Hemsley adds, that this substance proves to be the plumose seeds of
+a poplar or willow. In order to produce the effects described--_quite
+obscuring the sun like a white fog_,--the seeds must have filled the air
+to a very great height; and they must have been brought from some
+district where there were extensive tracts covered with the tree which
+produced them.]
+
+
+
+
+CHAPTER XIII
+
+THE GEOLOGICAL EVIDENCES OF EVOLUTION
+
+
+    What we may expect--The number of known species of extinct
+    animals--Causes of the imperfection of the geological
+    record--Geological evidences of
+    evolution--Shells--Crocodiles--The rhinoceros tribe--The
+    pedigree of the horse tribe--Development of deer's horns--Brain
+    development--Local relations of fossil and living animals--Cause
+    of extinction of large animals--Indications of general progress
+    in plants and animals--The progressive development of
+    plants--Possible cause of sudden late appearance of
+    exogens--Geological distribution of insects--Geological
+    succession of vertebrata--Concluding remarks.
+
+
+
+The theory of evolution in the organic world necessarily implies that
+the forms of animals and plants have, broadly speaking, progressed from
+a more generalised to a more specialised structure, and from simpler to
+more complex forms. We know, however, that this progression has been by
+no means regular, but has been accompanied by repeated degradation and
+degeneration; while extinction on an enormous scale has again and again
+stopped all progress in certain directions, and has often compelled a
+fresh start in development from some comparatively low and imperfect
+type.
+
+The enormous extension of geological research in recent times has made
+us acquainted with a vast number of extinct organisms, so vast that in
+some important groups--such as the mollusca--the fossil are more
+numerous than the living species; while in the mammalia they are not
+much less numerous, the preponderance of living species being chiefly in
+the smaller and in the arboreal forms which have not been so well
+preserved as the members of the larger groups. With such a wealth of
+material to illustrate the successive stages through which animals have
+passed, it will naturally be expected that we should find important
+evidence of evolution. We should hope to learn the steps by which some
+isolated forms have been connected with their nearest allies, and in
+many cases to have the gaps filled up which now separate genus from
+genus, or species from species. In some cases these expectations are
+fulfilled, but in many other cases we seek in vain for evidence of the
+kind we desire; and this absence of evidence with such an apparent
+wealth of material is held by many persons to throw doubt on the theory
+of evolution itself. They urge, with much appearance of reason, that all
+the arguments we have hitherto adduced fall short of demonstration, and
+that the crucial test consists in being able to show, in a great number
+of cases, those connecting links which we say must have existed. Many of
+the gaps that still remain are so vast that it seems incredible to these
+writers that they could ever have been filled up by a close succession
+of species, since these must have spread over so many ages, and have
+existed in such numbers, that it seems impossible to account for their
+total absence from deposits in which great numbers of species belonging
+to other groups are preserved and have been discovered. In order to
+appreciate the force, or weakness, of these objections, we must inquire
+into the character and completeness of that record of the past life of
+the earth which geology has unfolded, and ascertain the nature and
+amount of the evidence which, under actual conditions, we may expect to
+find.
+
+
+_The Number of known Species of Extinct Animals._
+
+When we state that the known fossil mollusca are considerably more
+numerous than those which now live on the earth, it appears at first
+sight that our knowledge is very complete, but this is far from being
+the case. The species have been continually changing throughout
+geological time, and at each period have probably been as numerous as
+they are now. If we divide the fossiliferous strata into twelve great
+divisions--the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias,
+Trias, Permian, Carboniferous, Devonian, Silurian, and Cambrian,--we
+find not only that each has a very distinct and characteristic molluscan
+fauna, but that the different subdivisions often present a widely
+different series of species; so that although a certain number of
+species are common to two or more of the great divisions, the totality
+of the species that have lived upon the earth must be very much more
+than twelve times--perhaps even thirty or forty times--the number now
+living. In like manner, although the species of fossil mammals now
+recognised by more or less fragmentary fossil remains may not be much
+less numerous than the living species, yet the duration of existence of
+these was comparatively so short that they were almost completely
+changed, perhaps six or seven times, during the Tertiary period; and
+this is certainly only a fragment of the geological time during which
+mammalia existed on the globe.
+
+There is also reason to believe that the higher animals were much more
+abundant in species during past geological epochs than now, owing to the
+greater equability of the climate which rendered even the arctic regions
+as habitable as the temperate zones are in our time.
+
+The same equable climate would probably cause a more uniform
+distribution of moisture, and render what are now desert regions capable
+of supporting abundance of animal life. This is indicated by the number
+and variety of the species of large animals that have been found fossil
+in very limited areas which they evidently inhabited at one period. M.
+Albert Gaudry found, in the deposits of a mountain stream at Pikermi in
+Greece, an abundance of large mammalia such as are nowhere to be found
+living together at the present time. Among them were two species of
+Mastodon, two different rhinoceroses, a gigantic wild boar, a camel and
+a giraffe larger than those now living, several monkeys, carnivora
+ranging from martens and civets to lions and hyaenas of the largest
+size, numerous antelopes of at least five distinct genera, and besides
+these many forms altogether extinct. Such were the great herds of
+Hipparion, an ancestral form of horse; the Helladotherium, a huge animal
+bigger than the giraffe; the Ancylotherium, one of the Edentata; the
+huge Dinotherium; the Aceratherium, allied to the rhinoceros; and the
+monstrous Chalicotherium, allied to the swine and ruminants, but as
+large as a rhinoceros; and to prey upon these, the great Machairodus or
+sabre-toothed tiger. And all these remains were found in a space 300
+paces long by 60 paces broad, many of the species existing in enormous
+quantities.
+
+The Pikermi fossils belong to the Upper Miocene formation, but an
+equally rich deposit of Upper Eocene age has been discovered in
+South-Western France at Quercy, where M. Filhol has determined the
+presence of no less than forty-two species of beasts of prey alone.
+Equally remarkable are the various discoveries of mammalian fossils in
+North America, especially in the old lake bottoms now forming what are
+called the "bad lands" of Dakota and Nebraska, belonging to the Miocene
+period. Here are found an enormous assemblage of remains, often perfect
+skeletons, of herbivora and carnivora, as varied and interesting as
+those from the localities already referred to in Europe; but altogether
+distinct, and far exceeding, in number and variety of species of the
+larger animals, the whole existing fauna of North America. Very similar
+phenomena occur in South America and in Australia, leading us to the
+conclusion that the earth at the present time is impoverished as regards
+the larger animals, and that at each successive period of Tertiary time,
+at all events, it contained a far greater number of species than now
+inhabit it. The very richness and abundance of the remains which we find
+in limited areas, serve to convince us how imperfect and fragmentary
+must be our knowledge of the earth's fauna at any one past epoch; since
+we cannot believe that all, or nearly all, of the animals which
+inhabited any district were entombed in a single lake, or overwhelmed by
+the floods of a single river.
+
+But the spots where such rich deposits occur are exceedingly few and far
+between when compared with the vast areas of continental land, and we
+have every reason to believe that in past ages, as now, numbers of
+curious species were rare or local, the commoner and more abundant
+species giving a very imperfect idea of the existing series of animal
+forms. Yet more important, as showing the imperfection of our knowledge,
+is the enormous lapse of time between the several formations in which we
+find organic remains in any abundance, so vast that in many cases we
+find ourselves almost in a new world, all the species and most of the
+genera of the higher animals having undergone a complete change.
+
+
+_Causes of the Imperfection of the Geological Record._
+
+These facts are quite in accordance with the conclusions of geologists
+as to the necessary imperfection of the geological record, since it
+requires the concurrence of a number of favourable conditions to
+preserve any adequate representation of the life of a given epoch. In
+the first place, the animals to be preserved must not die a natural
+death by disease, or old age, or by being the prey of other animals, but
+must be destroyed by some accident which shall lead to their being
+embedded in the soil. They must be either carried away by floods, sink
+into bogs or quicksands, or be enveloped in the mud or ashes of a
+volcanic eruption; and when thus embedded they must remain undisturbed
+amid all the future changes of the earth's surface.
+
+But the chances against this are enormous, because denudation is always
+going on, and the rocks we now find at the earth's surface are only a
+small fragment of those which were originally laid down. The
+alternations of marine and freshwater deposits, and the frequent
+unconformability of strata with those which overlie them, tell us
+plainly of repeated elevations and depressions of the surface, and of
+denudation on an enormous scale. Almost every mountain range, with its
+peaks, ridges, and valleys, is but the remnant of some vast plateau
+eaten away by sub-aerial agencies; every range of sea-cliffs tell us of
+long slopes of land destroyed by the waves; while almost all the older
+rocks which now form the surface of the earth have been once covered
+with newer deposits which have long since disappeared. Nowhere are the
+evidences of this denudation more apparent than in North and South
+America, where granitic or metamorphic rocks cover an area hardly less
+than that of all Europe. The same rocks are largely developed in Central
+Africa and Eastern Asia; while, besides those portions that appear
+exposed on the surface, areas of unknown extent are buried under strata
+which rest on them uncomformably, and could not, therefore, constitute
+the original capping under which the whole of these rocks must once have
+been deeply buried; because granite can only be formed, and metamorphism
+can only go on, deep down in the crust of the earth. What an
+overwhelming idea does this give us of the destruction of whole piles
+of rock, miles in thickness and covering areas comparable with those of
+continents; and how great must have been the loss of the innumerable
+fossil forms which those rocks contained! In view of such destruction we
+are forced to conclude that our palaeontological collections, rich
+though they may appear, are really but small and random samples, giving
+no adequate idea of the mighty series of organism which have lived upon
+the earth.[183]
+
+Admitting, however, the extreme imperfection of the geological record as
+a whole, it may be urged that certain limited portions of it are fairly
+complete--as, for example, the various Miocene deposits of India,
+Europe, and North America,--and that in these we ought to find many
+examples of species and genera linked together by intermediate forms. It
+may be replied that in several cases this really occurs; and the reason
+why it does not occur more often is, that the theory of evolution
+requires that distinct genera should be linked together, not by a direct
+passage, but by the descent of both from a common ancestor, which may
+have lived in some much earlier age the record of which is either
+wanting or very incomplete. An illustration given by Mr. Darwin will
+make this more clear to those who have not studied the subject. The
+fantail and pouter pigeons are two very distinct and unlike breeds,
+which we yet know to have been both derived from the common wild
+rock-pigeon. Now, if we had every variety of living pigeon before us, or
+even all those which have lived during the present century, we should
+find no intermediate types between these two--none combining in any
+degree the characters of the pouter with that of the fantail. Neither
+should we ever find such an intermediate form, even had there been
+preserved a specimen of every breed of pigeon since the ancestral
+rock-pigeon was first tamed by man--a period of probably several
+thousand years. We thus see that a complete passage from one very
+distinct species to another could not be expected even had we a complete
+record of the life of any one period. What we require is a complete
+record of all the species that have existed since the two forms began
+to diverge from their common ancestor, and this the known imperfection
+of the record renders it almost impossible that we should ever attain.
+All that we have a right to expect is, that, as we multiply the fossil
+forms in any group, the gaps that at first existed in that group shall
+become less wide and less numerous; and also that, in some cases, a
+tolerably direct series shall be found, by which the more specialised
+forms of the present day shall be connected with more generalised
+ancestral types. We might also expect that when a country is now
+characterised by special groups of animals, the fossil forms that
+immediately preceded them shall, for the most part, belong to the same
+groups; and further, that, comparing the more ancient with the more
+modern types, we should find indications of progression, the earlier
+forms being, on the whole, lower in organisation, and less specialised
+in structure than the later. Now evidence of evolution of these varied
+kinds is what we do find, and almost every fresh discovery adds to their
+number and cogency. In order, therefore, to show that the testimony
+given by geology is entirely in favour of the theory of descent with
+modification, some of the more striking of the facts will now be given.
+
+
+_Geological Evidences of Evolution._
+
+In an article in _Nature_ (vol. xiv. p. 275), Professor Judd calls
+attention to some recent discoveries in the Hungarian plains, of fossil
+lacustrine shells, and their careful study by Dr. Neumayr and M. Paul of
+the Austrian Geological Survey. The beds in which they occur have
+accumulated to the thickness of 2000 feet, containing throughout
+abundance of fossils, and divisible into eight zones, each of which
+exhibits a well-marked and characteristic fauna. Professor Judd then
+describes the bearing of these discoveries as follows--
+
+
+    "The group of shells which affords the most interesting evidence
+    of the origin of new forms through descent with modification is
+    that of the genus Vivipara or Paludina, which occurs in
+    prodigious abundance throughout the whole series of freshwater
+    strata. We shall not, of course, attempt in this place to enter
+    into any details concerning the forty distinct _forms_ of this
+    genus (Dr. Neumayr very properly hesitates to call them all
+    _species_), which are named and described in this monograph,
+    and between which, as the authors show, so many connecting
+    links, clearly illustrating the derivation of the newer from the
+    older types, have been detected. On the minds of those who
+    carefully examine the admirably engraved figures given in the
+    plates accompanying this valuable memoir, or still better, the
+    very large series of specimens from among which the subjects of
+    these figures are selected, and which are now in the museum of
+    the Reichsanstalt of Vienna, but little doubt will, we suspect,
+    remain that the authors have fully made out their case, and have
+    demonstrated that, beyond all controversy, the series with
+    highly complicated ornamentation were variously derived by
+    descent--the lines of which are in most cases perfectly clear
+    and obvious--from the simple and unornamented Vivipara
+    achatinoides of the Congerien-Schichten (the lower division of
+    the series of strata). It is interesting to notice that a large
+    portion of these unquestionably derived forms depart so widely
+    from the type of the genus Vivipara, that they have been
+    separated on so high an authority as that of Sandberger, as a
+    new genus, under the name of Tulotoma. And hence we are led to
+    the conclusion that a vast number of forms, certainly exhibiting
+    specific distinctions, and according to some naturalists,
+    differences even entitled to be regarded of generic value, have
+    all a common ancestry."
+
+
+It is, as Professor Judd remarks, owing to the exceptionally favourable
+circumstances of a long-continued and unbroken series of deposits being
+formed under physical conditions either identical or very slowly
+changing, that we owe so complete a record of the process of organic
+change. Usually, some disturbing elements, such as a sudden change of
+physical conditions, or the immigration of new sets of forms from other
+areas and the consequent retreat or partial extinction of the older
+fauna, interferes with the continuity of organic development, and
+produces those puzzling discordances so generally met with in geological
+formations of marine origin. While a case of the kind now described
+affords evidence of the origin of species complete and conclusive,
+though on a necessarily very limited scale, the very rarity of the
+conditions which are essential to such completeness serves to explain
+why it is that in most cases the direct evidence of evolution is not to
+be obtained.
+
+Another illustration of the filling up of gaps between existing groups
+is afforded by Professor Huxley's researches on fossil crocodiles. The
+gap between the existing crocodiles and the lizards is very wide, but as
+we go back in geological time we meet with fossil forms which are to
+some extent intermediate and form a connected series. The three living
+genera--Crocodilus, Alligator, and Gavialis--are found in the Eocene
+formation, and allied forms of another genus, Holops, in the Chalk. From
+the Chalk backward to the Lias another group of genera occurs, having
+anatomical characteristics intermediate between the living crocodiles
+and the most ancient forms. These, forming two genera Belodon and
+Stagonolepis, are found in a still older formation, the Trias. They have
+characters resembling some lizards, especially the remarkable Hatteria
+of New Zealand, and have also some resemblances to the
+Dinosaurians--reptiles which in some respects approach birds.
+Considering how comparatively few are the remains of this group of
+animals, the evidence which it affords of progressive development is
+remarkably clear.[184]
+
+Among the higher animals the rhinoceros, the horse, and the deer afford
+good evidence of advance in organisation and of the filling up of the
+gaps which separate the living forms from their nearest allies. The
+earliest ancestral forms of the rhinoceroses occur in the Middle Eocene
+of the United States, and were to some extent intermediate between the
+rhinoceros and tapir families, having like the latter four toes to the
+front feet, and three to those behind. These are followed in the Upper
+Eocene by the genus Amynodon, in which the skull assumes more distinctly
+the rhinocerotic type. Following this in the Lower Miocene we have the
+Aceratherium, like the last in its feet, but still more decidedly a
+rhinoceros in its general structure. From this there are two diverging
+lines--one in the Old World, the other in the New. In the former, to
+which the Aceratherium is supposed to have migrated in early Miocene
+times, when a mild climate and luxuriant vegetation prevailed far within
+the arctic circle, it gave rise to the Ceratorhinus and the various
+horned rhinoceroses of late Tertiary times and of those now living. In
+America a number of large hornless rhinoceroses were developed--they
+are found in the Upper Miocene, Pliocene, and Post-Pliocene
+formations--and then became extinct. The true rhinoceroses have three
+toes on all the feet.[185]
+
+
+_The Pedigree of the Horse Tribe._
+
+Yet more remarkable is the evidence afforded by the ancestral forms of
+the horse tribe which have been discovered in the American tertiaries.
+The family Equidae, comprising the living horse, asses, and zebras,
+differ widely from all other mammals in the peculiar structure of the
+feet, all of which terminate in a single large toe forming the hoof.
+They have forty teeth, the molars being formed of hard and soft material
+in crescentic folds, so as to be a powerful agent in grinding up hard
+grasses and other vegetable food. The former peculiarities depend upon
+modifications of the skeleton, which have been thus described by
+Professor Huxley:--
+
+
+    "Let us turn in the first place to the fore-limb. In most
+    quadrupeds, as in ourselves, the fore-arm contains distinct
+    bones, called the radius and the ulna. The corresponding region
+    in the horse seems at first to possess but one bone. Careful
+    observation, however, enables us to distinguish in this bone a
+    part which clearly answers to the upper end of the ulna. This is
+    closely united with the chief mass of the bone which represents
+    the radius, and runs out into a slender shaft, which may be
+    traced for some distance downwards upon the back of the radius,
+    and then in most cases thins out and vanishes. It takes still
+    more trouble to make sure of what is nevertheless the fact, that
+    a small part of the lower end of the bone of a horse's fore-arm,
+    which is only distinct in a very young foal, is really the lower
+    extremity of the ulna.
+
+    "What is commonly called the knee of a horse is its wrist. The
+    'cannon bone' answers to the middle bone of the five metacarpal
+    bones which support the palm of the hand in ourselves. The
+    pastern, coronary, and coffin bones of veterinarians answer to
+    the joints of our middle fingers, while the hoof is simply a
+    greatly enlarged and thickened nail. But if what lies below the
+    horse's 'knee' thus corresponds to the middle finger in
+    ourselves, what has become of the four other fingers or digits?
+    We find in the places of the second and fourth digits only two
+    slender splintlike bones, about two-thirds as long as the cannon
+    bone, which gradually taper to their lower ends and bear no
+    finger joints, or, as they are termed, phalanges. Sometimes,
+    small bony or gristly nodules are to be found at the bases of
+    these two metacarpal splints, and it is probable that these
+    represent rudiments of the first and fifth toes. Thus, the part
+    of the horse's skeleton which corresponds with that of the human
+    hand, contains one overgrown middle digit, and at least two
+    imperfect lateral digits; and these answer, respectively, to the
+    third, the second, and the fourth fingers in man.
+
+    "Corresponding modifications are found in the hind limb. In
+    ourselves, and in most quadrupeds, the leg contains two distinct
+    bones, a large bone, the tibia, and a smaller and more slender
+    bone, the fibula. But, in the horse, the fibula seems, at first,
+    to be reduced to its upper end; a short slender bone united with
+    the tibia, and ending in a point below, occupying its place.
+    Examination of the lower end of a young foal's shin-bone,
+    however, shows a distinct portion of osseous matter which is the
+    lower end of the fibula; so that the, apparently single, lower
+    end of the shin-bone is really made up of the coalesced ends of
+    the tibia and fibula, just as the, apparently single, lower end
+    of the fore-arm bone is composed of the coalesced radius and
+    ulna.
+
+    "The heel of the horse is the part commonly known as the hock.
+    The hinder cannon bone answers to the middle metatarsal bone of
+    the human foot, the pastern, coronary, and coffin bones, to the
+    middle toe bones; the hind hoof to the nail; as in the forefoot.
+    And, as in the forefoot, there are merely two splints to
+    represent the second and the fourth toes. Sometimes a rudiment
+    of a fifth toe appears to be traceable.
+
+    "The teeth of a horse are not less peculiar than its limbs. The
+    living engine, like all others, must be well stoked if it is to
+    do its work; and the horse, if it is to make good its wear and
+    tear, and to exert the enormous amount of force required for its
+    propulsion, must be well and rapidly fed. To this end, good
+    cutting instruments and powerful and lasting crushers are
+    needful. Accordingly, the twelve cutting teeth of a horse are
+    close-set and concentrated in the forepart of its mouth, like so
+    many adzes or chisels. The grinders or molars are large, and
+    have an extremely complicated structure, being composed of a
+    number of different substances of unequal hardness. The
+    consequence of this is that they wear away at different rates;
+    and, hence, the surface of each grinder is always as uneven as
+    that of a good millstone."[186]
+
+
+We thus see that the Equidae differ very widely in structure from most
+other mammals. Assuming the truth of the theory of evolution, we should
+expect to find traces among extinct animals of the steps by which this
+great modification has been effected; and we do really find traces of
+these steps, imperfectly among European fossils, but far more completely
+among those of America.
+
+It is a singular fact that, although no horse inhabited America when
+discovered by Europeans, yet abundance of remains of extinct horses have
+been found both in North and South America in Post-Tertiary and Upper
+Pliocene deposits; and from these an almost continuous series of
+modified forms can be traced in the Tertiary formation, till we reach,
+at the very base of the series, a primitive form so unlike our perfected
+animal, that, had we not the intermediate links, few persons would
+believe that the one was the ancestor of the other. The tracing out of
+this marvellous history we owe chiefly to Professor Marsh of Yale
+College, who has himself discovered no less than thirty species of
+fossil Equidae; and we will allow him to tell the story of the
+development of the horse from a humble progenitor in his own words.
+
+
+    "The oldest representative of the horse at present known is the
+    diminutive Eohippus from the Lower Eocene. Several species have
+    been found, all about the size of a fox. Like most of the early
+    mammals, these ungulates had forty-four teeth, the molars with
+    short crowns and quite distinct in form from the premolars. The
+    ulna and fibula were entire and distinct, and there were four
+    well-developed toes and a rudiment of another on the forefeet,
+    and three toes behind. In the structure of the feet and teeth,
+    the Eohippus unmistakably indicates that the direct ancestral
+    line to the modern horse has already separated from the other
+    perissodactyles, or odd-toed ungulates.
+
+    "In the next higher division of the Eocene another genus,
+    Orohippus, makes its appearance, replacing Eohippus, and showing
+    a greater, though still distant, resemblance to the equine type.
+    The rudimentary first digit of the forefoot has disappeared, and
+    the last premolar has gone over to the molar series. Orohippus
+    was but little larger than Eohippus, and in most other respects
+    very similar. Several species have been found, but none occur
+    later than the Upper Eocene.
+
+    "Near the base of the Miocene, we find a third closely allied
+    genus, Mesohippus, which is about as large as a sheep, and one
+    stage nearer the horse. There are only three toes and a
+    rudimentary splint on the forefeet, and three toes behind. Two
+    of the premolar teeth are quite like the molars. The ulna is no
+    longer distinct or the fibula entire, and other characters show
+    clearly that the transition is advancing.
+
+    "In the Upper Miocene Mesohippus is not found, but in its place
+    a fourth form, Miohippus, continues the line. This genus is near
+    the Anchitherium of Europe, but presents several important
+    differences. The three toes in each foot are more nearly of a
+    size, and a rudiment of the fifth metacarpal bone is retained.
+    All the known species of this genus are larger than those of
+    Mesohippus, and none of them pass above the Miocene formation.
+
+    "The genus Protohippus of the Lower Pliocene is yet more equine,
+    and some of its species equalled the ass in size. There are
+    still three toes on each foot, but only the middle one,
+    corresponding to the single toe of the horse, comes to the
+    ground. This genus resembles most nearly the Hipparion of
+    Europe.
+
+    "In the Pliocene we have the last stage of the series before
+    reaching the horse, in the genus Pliohippus, which has lost the
+    small hooflets, and in other respects is very equine. Only in
+    the Upper Pliocene does the true Equus appear and complete the
+    genealogy of the horse, which in the Post-Tertiary roamed over
+    the whole of North and South America, and soon after became
+    extinct. This occurred long before the discovery of the
+    continent by Europeans, and no satisfactory reason for the
+    extinction has yet been given. Besides the characters I have
+    mentioned, there are many others in the skeleton, skull, teeth,
+    and brain of the forty or more intermediate species, which show
+    that the transition from the Eocene Eohippus to the modern Equus
+    has taken place in the order indicated"[187] (see Fig. 33).
+
+
+[Illustration: FIG. 33.--Geological development of the horse tribe
+(Eohippus since discovered).]
+
+Well may Professor Huxley say that this is demonstrative evidence of
+evolution; the doctrine resting upon exactly as secure a foundation as
+did the Copernican theory of the motions of the heavenly bodies at the
+time of its promulgation. Both have the same basis--the coincidence of
+the observed facts with the theoretical requirements.
+
+
+_Development of Deer's Horns._
+
+Another clear and unmistakable proof of evolution is afforded by one of
+the highest and latest developed tribes of mammals--the true deer. These
+differ from all other ruminants in possessing solid deciduous horns
+which are always more or less branched. They first appear in the Middle
+Miocene formation, and continue down to our time; and their development
+has been carefully traced by Professor Boyd Dawkins, who thus summarises
+his results:--
+
+
+    "In the middle stage of the Miocene the cervine antler consists
+    merely of a simple forked crown (as in Cervus dicroceros), which
+    increases in size in the Upper Miocene, although it still
+    remains small and erect, like that of the roe. In Cervus
+    Matheroni it measures 11·4 inches, and throws off not more than
+    four tines, all small. The deer living in Auvergne in the
+    succeeding or Pliocene age, present us with another stage in the
+    history of antler development. There, for the first time, we see
+    antlers of the Axis and Rusa type, larger and longer, and more
+    branching than any antlers were before, and possessing three or
+    more well-developed tines. Deer of this type abounded in
+    Pliocene Europe. They belong to the Oriental division of the
+    Cervidae, and their presence in Europe confirms the evidence of
+    the flora, brought forward by the Comte de Saporta, that the
+    Pliocene climate was warm. They have probably disappeared from
+    Europe in consequence of the lowering of the temperature in the
+    Pleistocene age, while their descendants have found a congenial
+    home in the warmer regions of Eastern Asia.
+
+    "In the latest stage of the Pliocene--the Upper Pliocene of the
+    Val d'Arno--the Cervus dicranios of Nesti presents us with
+    antlers much smaller than those of the Irish elk, but very
+    complicated in their branching. This animal survived into the
+    succeeding age, and is found in the pre-glacial forest bed of
+    Norfolk, being described by Dr. Falconer under the name of
+    Sedgwick's deer. The Irish elk, moose, stag, reindeer, and
+    fallow deer appear in Europe in the Pleistocene age, all with
+    highly complicated antlers in the adult, and the first
+    possessing the largest antlers yet known. Of these the Irish elk
+    disappeared in the Prehistoric age, after having lived in
+    countless herds in Ireland, while the rest have lived on into
+    our own times in Euro-Asia, and, with the exception of the last,
+    also in North America.
+
+    "From this survey it is obvious that the cervine antlers have
+    increased in size and complexity from the Mid-Miocene to the
+    Pleistocene age, and that their successive changes are analogous
+    to those which are observed in the development of antlers in the
+    living deer, which begin with a simple point, and increase in
+    number of tines till their limit of growth be reached. In other
+    words, the development of antlers indicated at successive and
+    widely-separated pages of the geological record is the same as
+    that observed in the history of a single living species. It is
+    also obvious that the progressive diminution of size and
+    complexity in the antlers, from the present time back into the
+    early Tertiary age, shows that we are approaching the zero of
+    antler development in the Mid-Miocene. No trace of any
+    antler-bearing ruminant has been met with in the lower Miocenes,
+    either of Europe or the United States."[188]
+
+
+
+_Progressive Brain-Development._
+
+The three illustrations now given sufficiently prove that, whenever the
+geological record approaches to completeness, we have evidence of the
+progressive change of species in definite directions, and from less
+developed to more developed types--exactly such a change as we may
+expect to find if the evolution theory be the true one. Many other
+illustrations of a similar change could be given, but the animal groups
+in which they occur being less familiar, the details would be less
+interesting, and perhaps hardly intelligible. There is, however, one
+very remarkable proof of development that must be briefly noticed--that
+afforded by the steady increase in the size of the brain. This may be
+best stated in the words of Professor Marsh:--
+
+
+    "The real progress of mammalian life in America, from the
+    beginning of the Tertiary to the present, is well illustrated by
+    the brain-growth, in which we have the key to many other
+    changes. The earliest known Tertiary mammals all had very small
+    brains, and in some forms this organ was proportionally less
+    than in certain reptiles. There was a gradual increase in the
+    size of the brain during this period, and it is interesting to
+    find that this growth was mainly confined to the cerebral
+    hemispheres, or higher portion of the brain. In most groups of
+    mammals the brain has gradually become more convoluted, and thus
+    increased in quality as well as quantity. In some also the
+    cerebellum and olfactory lobes, the lower parts of the brain,
+    have even diminished in size. In the long struggle for existence
+    during Tertiary time the big brains won, then as now; and the
+    increasing power thus gained rendered useless many structures
+    inherited from primitive ancestors, but no longer adapted to new
+    conditions."
+
+
+This remarkable proof of development in the organ of the mental
+faculties, forms a fitting climax to the evidence already adduced of the
+progressive evolution of the general structure of the body, as
+illustrated by the bony skeleton. We now pass on to another class of
+facts equally suggestive of evolution.
+
+
+_The Local Relations of Fossil and Living Animals._
+
+If all existing animals have been produced from ancestral forms--mostly
+extinct--under the law of variation and natural selection, we may expect
+to find in most cases a close relation between the living forms of each
+country and those which inhabited it in the immediately preceding epoch.
+But if species have originated in some quite different way, either by
+any kind of special creation, or by sudden advances of organisation in
+the offspring of preceding types, such close relationship would not be
+found; and facts of this kind become, therefore, to some extent a test
+of evolution under natural selection or some other law of gradual
+change. Of course the relationship will not appear when extensive
+migration has occurred, by which the inhabitants of one region have been
+able to take possession of another region, and destroy or drive out its
+original inhabitants, as has sometimes happened. But such cases are
+comparatively rare, except where great changes of climate are known to
+have occurred; and we usually do find a remarkable continuity between
+the existing fauna and flora of a country and those of the immediately
+preceding age. A few of the more remarkable of these cases will now be
+briefly noticed.
+
+The mammalian fauna of Australia consists, as is well known, wholly of
+the lowest forms--the Marsupials and Monotremata--except only a few
+species of mice. This is accounted for by the complete isolation of the
+country from the Asiatic continent during the whole period of the
+development of the higher animals. At some earlier epoch the ancestral
+marsupials, which abounded both in Europe and North America in the
+middle of the Secondary period, entered the country, and have since
+remained there, free from the competition of higher forms, and have
+undergone a special development in accordance with the peculiar
+conditions of a limited area. While in the large continents higher forms
+of mammalia have been developed, which have almost or wholly
+exterminated the less perfect marsupials, in Australia these latter have
+become modified into such varied forms as the leaping kangaroos, the
+burrowing wombats, the arboreal phalangers, the insectivorous
+bandicoots, and the carnivorous Dasyuridae or native cats, culminating
+in the Thylacinus or "tiger-wolf" of Tasmania--animals as unlike each
+other as our sheep, rabbits, squirrels, and dogs, but all retaining the
+characteristic features of the marsupial type.
+
+Now in the caves and late Tertiary or Post-Tertiary deposits of
+Australia the remains of many extinct mammalia have been found, but all
+are marsupials. There are many kangaroos, some larger than any living
+species, and others more allied to the tree-kangaroos of New Guinea; a
+large wombat as large as a tapir; the Diprotodon, a thick-limbed
+kangaroo the size of a rhinoceros or small elephant; and a quite
+different animal, the Nototherium, nearly as large. The carnivorous
+Thylacinus of Tasmania is also found fossil; and a huge phalanger,
+Thylacoleo, the size of a lion, believed by Professor Owen and by
+Professor Oscar Schmidt to have been equally carnivorous and
+destructive.[189] Besides these, there are many other species more
+resembling the living forms both in size and structure, of which they
+may be, in some cases, the direct ancestors. Two species of extinct
+Echidna, belonging to the very low Monotremata, have also been found in
+New South Wales.
+
+Next to Australia, South America possesses the most remarkable
+assemblage of peculiar mammals, in its numerous Edentata--the sloths,
+ant-eaters, and armadillos; its rodents, such as the cavies and
+chinchillas; its marsupial opossums, and its quadrumana of the family
+Cebidae. Remains of extinct species of all these have been found in the
+caves of Brazil, of Post-Pliocene age; while in the earlier Pliocene
+deposits of the pampas many distinct genera of these groups have been
+found, some of gigantic size and extraordinary form. There are
+armadillos of many types, some being as large as elephants; gigantic
+sloths of the genera Megatherium, Megalonyx, Mylodon, Lestodon, and many
+others; rodents belonging to the American families Cavidae and
+Chinchillidae; and ungulates allied to the llama; besides many other
+extinct forms of intermediate types or of uncertain affinities.[190] The
+extinct Moas of New Zealand--huge wingless birds allied to the living
+Apteryx--illustrate the same general law.
+
+The examples now quoted, besides illustrating and enforcing the general
+fact of evolution, throw some light on the usual character of the
+modification and progression of animal forms. In the cases where the
+geological record is tolerably complete, we find a continuous
+development of some kind--either in complexity of ornamentation, as in
+the fossil Paludinas of the Hungarian lake-basins; in size and in the
+specialisation of the feet and teeth, as in the American fossil horses;
+or in the increased development of the branching horns, as in the true
+deer. In each of these cases specialisation and adaptation to the
+conditions of the environment appear to have reached their limits, and
+any change of these conditions, especially if it be at all rapid or
+accompanied by the competition of less developed but more adaptable
+forms, is liable to cause the extinction of the most highly developed
+groups. Such we know was the case with the horse tribe in America, which
+totally disappeared in that continent at an epoch so recent that we
+cannot be sure that the disappearance was not witnessed, perhaps caused,
+by man; while even in the Eastern hemisphere it is the smaller
+species--the asses and the zebras--that have persisted, while the larger
+and more highly developed true horses have almost, if not quite,
+disappeared in a state of nature. So we find, both in Australia and
+South America, that in a quite recent period many of the largest and
+most specialised forms have become extinct, while only the smaller types
+have survived to our day; and a similar fact is to be observed in many
+of the earlier geological epochs, a group progressing and reaching a
+maximum of size or complexity and then dying out, or leaving at most but
+few and pigmy representatives.
+
+
+_Cause of Extinction of Large Animals._
+
+Now there are several reasons for the repeated extinction of large
+rather than of small animals. In the first place, animals of great bulk
+require a proportionate supply of food, and any adverse change of
+conditions would affect them more seriously than it would smaller
+animals. In the next place, the extreme specialisation of many of these
+large animals would render it less easy for them to be modified in any
+new direction suited to changed conditions. Still more important,
+perhaps, is the fact that very large animals always increase slowly as
+compared with small ones--the elephant producing a single young one
+every three years, while a rabbit may have a litter of seven or eight
+young two or three times a year. Now the probability of favourable
+variations will be in direct proportion to the population of the
+species, and as the smaller animals are not only many hundred times more
+numerous than the largest, but also increase perhaps a hundred times as
+rapidly, they are able to become quickly modified by variation and
+natural selection in harmony with changed conditions, while the large
+and bulky species, being unable to vary quickly enough, are obliged to
+succumb in the struggle for existence. As Professor Marsh well observes:
+"In every vigorous primitive type which was destined to survive many
+geological changes, there seems to have been a tendency to throw off
+lateral branches, which became highly specialised and soon died out,
+because they were unable to adapt themselves to new conditions." And he
+goes on to show how the whole narrow path of the persistent Suilline
+type, throughout the entire series of the American tertiaries, is
+strewed with the remains of such ambitious offshoots, many of them
+attaining the size of a rhinoceros; "while the typical pig, with an
+obstinacy never lost, has held on in spite of catastrophes and
+evolution, and still lives in America to-day."
+
+
+_Indications of General Progression in Plants and Animals._
+
+One of the most powerful arguments formerly adduced against evolution
+was, that geology afforded no evidence of the gradual development of
+organic forms, but that whole tribes and classes appeared suddenly at
+definite epochs, and often in great variety and exhibiting a very
+perfect organisation. The mammalia, for example, were long thought to
+have first appeared in Tertiary times, where they are represented in
+some of the earlier deposits by all the great divisions of the class
+fully developed--carnivora, rodents, insectivora, marsupials, and even
+the perissodactyle and artiodactyle divisions of the ungulata--as
+clearly defined as at the present day. The discovery in 1818 of a single
+lower jaw in the Stonesfield Slate of Oxfordshire hardly threw doubt on
+the generalisation, since either its mammalian character was denied, or
+the geological position of the strata, in which it was found, was held
+to have been erroneously determined. But since then, at intervals of
+many years, other remains of mammalia have been discovered in the
+Secondary strata, ranging from the Upper Oolite to the Upper Trias both
+in Europe and the United States, and one even (Tritylodon) in the Trias
+of South Africa. All these are either marsupials, or of some still lower
+type of mammalia; but they consist of many distinct forms classed in
+about twenty genera. Nevertheless, a great gap still exists between
+these mammals and those of the Tertiary strata, since no mammal of any
+kind has been found in any part of the Cretaceous formation, although in
+several of its subdivisions abundance of land plants, freshwater shells,
+and air-breathing reptiles have been discovered. So with fishes. In the
+last century none had been obtained lower than the Carboniferous
+formation; thirty years later they were found to be very abundant in the
+Devonian rocks, and later still they were discovered in the Upper Ludlow
+and Lower Ludlow beds of the Silurian formation.
+
+We thus see that such sudden appearances are deceptive, and are, in
+fact, only what we ought to expect from the known imperfection of the
+geological record. The conditions favourable to the fossilisation of any
+group of animals occur comparatively rarely, and only in very limited
+areas; while the conditions essential for their permanent preservation
+in the rocks, amid all the destruction caused by denudation or
+metamorphism, are still more exceptional. And when they are thus
+preserved to our day, the particular part of the rocks in which they lie
+hidden may not be on the surface but buried down deep under other
+strata, and may thus, except in the case of mineral-bearing deposits, be
+altogether out of our reach. Then, again, how large a proportion of the
+earth consists of wild and uncivilised regions in which no exploration
+of the rocks has been yet made, so that whether we shall find the
+fossilised remains of any particular group of animals which lived during
+a limited period of the earth's history, and in a limited area, depends
+upon at least a fivefold combination of chances. Now, if we take each of
+these chances separately as only ten to one against us (and some are
+certainly more than this), then the actual chance against our finding
+the fossil remains, say of any one order of mammalia, or of land plants,
+at any particular geological horizon, will be about a hundred thousand
+to one.
+
+It may be said, if the chances are so great, how is it that we find such
+immense numbers of fossil species exceeding in number, in some groups,
+all those that are now living? But this is exactly what we should
+expect, because the number of species of organisms that have ever lived
+upon the earth, since the earliest geological times, will probably be
+many hundred times greater than those now existing of which we have any
+knowledge; and hence the enormous gaps and chasms in the geological
+record of extinct forms is not to be wondered at. Yet, notwithstanding
+these chasms in our knowledge, if evolution is true, there ought to have
+been, on the whole, progression in all the chief types of life. The
+higher and more specialised forms should have come into existence later
+than the lower and more generalised forms; and however fragmentary the
+portions we possess of the whole tree of life upon the earth, they ought
+to show us broadly that such a progressive evolution has taken place. We
+have seen that in some special groups, already referred to, such a
+progression is clearly visible, and we will now cast a hasty glance over
+the entire series of fossil forms, in order to see if a similar
+progression is manifested by them as a whole.
+
+
+_The Progressive Development of Plants._
+
+Ever since fossil plants have been collected and studied, the broad fact
+has been apparent that the early plants--those of the Coal
+formation--were mainly cryptogamous, while in the Tertiary deposits the
+higher flowering plants prevailed. In the intermediate secondary epoch
+the gymnosperms--cycads and coniferae--formed a prominent part of the
+vegetation, and as these have usually been held to be a kind of
+transition form between the flowerless and flowering plants, the
+geological succession has always, broadly speaking, been in accordance
+with the theory of evolution. Beyond this, however, the facts were very
+puzzling. The highest cryptogams--ferns, lycopods, and
+equisetaceae--appeared suddenly, and in immense profusion in the Coal
+formation, at which period they attained a development they have never
+since surpassed or even equalled; while the highest plants--the
+dicotyledonous and monocotyledonous angiosperms--which now form the bulk
+of the vegetation of the world, and exhibit the most wonderful
+modifications of form and structure, were almost unknown till the
+Tertiary period, when they suddenly appeared in full development, and,
+for the most part, under the same generic forms as now exist.
+
+During the latter half of the present century, however, great additions
+have been made to our knowledge of fossil plants; and although there
+are still indications of vast gaps in our knowledge, due, no doubt, to
+the very exceptional conditions required for the preservation of plant
+remains, we now possess evidence of a more continuous development of the
+various types of vegetation. According to Mr. Lester F. Ward, between
+8000 and 9000 species of fossil plants have been described or indicated;
+and, owing to the careful study of the nervation of leaves, a large
+number of these are referable to their proper orders or genera, and
+therefore give us some notion--which, though very imperfect, is probably
+accurate in its main outlines--of the progressive development of
+vegetation on the earth.[191] The following is a summary of the facts as
+given by Mr. Ward:--
+
+The lowest forms of vegetable life--the cellular plants--have been found
+in Lower Silurian deposits in the form of three species of marine algae;
+and in the whole Silurian formation fifty species have been recognised.
+We cannot for a moment suppose, however, that this indicates the first
+appearance of vegetable life upon the earth, for in these same Lower
+Silurian beds the more highly organised vascular cryptogams appear in
+the form of rhizocarps--plants allied to Marsilea and Azolla,--and a
+very little higher, ferns, lycopods, and even conifers appear. We have
+indications, however, of a still more ancient vegetation, in the
+carbonaceous shales and thick beds of graphite far down in the Middle
+Laurentian, since there is no other known agency than the vegetable cell
+by means of which carbon can be extracted from the atmosphere and fixed
+in the solid state. These great beds of graphite, therefore, imply the
+existence of abundance of vegetable life at the very commencement of the
+era of which we have any geological record.[192]
+
+Ferns, as already stated, begin in the Middle Silurian formation with
+the Eopteris Morrieri. In the Devonian, we have 79 species, in the
+Carboniferous 627, and in the Permian 186 species; after which fossil
+ferns diminish greatly, though they are found in every formation; and
+the fact that fully 3000 living species are known, while the richest
+portion of the Tertiary in fossil plants--the Miocene--- has only
+produced 87 species, will serve to indicate the extreme imperfection of
+the geological record.
+
+The Equisetaceae (horsetails) which also first appear in the Silurian and
+reach their maximum development in the Coal formation, are, in all
+succeeding formations, far less numerous than ferns, and only thirty
+living species are known. Lycopodiaceae, though still more abundant in
+the Coal formation, are very rarely found in any succeeding deposit,
+though the living species are tolerably numerous, about 500 having been
+described. As we cannot suppose them to have really diminished and then
+increased again in this extraordinary manner, we have another indication
+of the exceptional nature of plant preservation and the extreme and
+erratic character of the imperfection of the record.
+
+Passing now to the next higher division of plants--the gymnosperms--we
+find Coniferae appearing in the Upper Silurian, becoming tolerably
+abundant in the Devonian, and reaching a maximum in the Carboniferous,
+from which formation more than 300 species are known, equal to the
+number recorded as now living. They occur in all succeeding formations,
+being abundant in the Oolite, and excessively so in the Miocene, from
+which 250 species have been described. The allied family of gymnosperms,
+the Cycadaceae, first appear in the Carboniferous era, but very
+scantily; are most abundant in the Oolite, from which formation 116
+species are known, and then steadily diminish to the Tertiary, although
+there are seventy-five living species.
+
+We now come to the true flowering plants, and we first meet with
+monocotyledons in the Carboniferous and Permian formations. The
+character of these fossils was long disputed, but is now believed to be
+well established; and the sub-class continues to be present in small
+numbers in all succeeding deposits, becoming rather plentiful in the
+Upper Cretaceous, and very abundant in the Eocene and Miocene. In the
+latter formation 272 species have been discovered; but the 116 species
+in the Eocene form a larger proportion of the total vegetation of the
+period.
+
+True dicotyledons appear very much later, in the Cretaceous period, and
+only in its upper division, if we except a single species from the
+Urgonian beds of Greenland. The remarkable thing is that we here find
+the sub-class fully developed and in great luxuriance of types, all the
+three divisions--Apetalae, Polypetalae, and Gamopetalae--being
+represented, with a total of no less than 770 species. Among them are
+such familiar forms as the poplar, the birch, the beech, the sycamore,
+and the oak; as well as the fig, the true laurel, the sassafras, the
+persimmon, the maple, the walnut, the magnolia, and even the apple and
+the plum tribes. Passing on to the Tertiary period the numbers increase,
+till they reach their maximum in the Miocene, where more than 2000
+species of dicotyledons have been discovered. Among these the
+proportionate number of the higher gamopetalae has slightly increased,
+but is considerably less than at the present day.
+
+
+_Possible Cause of sudden late Appearance of Exogens._
+
+The sudden appearance of fully developed exogenous flowering plants in
+the Cretaceous period is very analogous to the equally sudden appearance
+of all the chief types of placental mammalia in the Eocene; and in both
+cases we must feel sure that this suddenness is only apparent, due to
+unknown conditions which have prevented their preservation (or their
+discovery) in earlier formations. The case of the dicotyledonous plants
+is in some respects the most extraordinary, because in the earlier
+Mesozoic formations we appear to have a fair representation of the flora
+of the period, including such varied forms as ferns, equisetums, cycads,
+conifers, and monocotyledons. The only hint at an explanation of this
+anomaly has been given by Mr. Ball, who supposes that all these groups
+inhabited the lowlands, where there was not only excessive heat and
+moisture, but also a superabundance of carbonic acid in the
+atmosphere--conditions under which these groups had been developed, but
+which were prejudicial to the dicotyledons. These latter are supposed to
+have originated on the high table-lands and mountain ranges, in a rarer
+and drier atmosphere in which the quantity of carbonic acid gas was much
+less; and any deposits formed in lake beds at high altitudes and at such
+a remote epoch have been destroyed by denudation, and hence we have no
+record of their existence.[193]
+
+During a few weeks spent recently in the Rocky Mountains, I was struck
+by the great scarcity of monocotyledons and ferns in comparison with
+dicotyledons--a scarcity due apparently to the dryness and rarity of the
+atmosphere favouring the higher groups. If we compare Coulter's _Rocky
+Mountain Botany_ with Gray's _Botany of the Northern (East) United
+States_, we have two areas which differ chiefly in the points of
+altitude and atmospheric moisture. Unfortunately, in neither of these
+works are the species consecutively numbered; but by taking the pages
+occupied by the two divisions of dicotyledons on the one hand,
+monocotyledons and ferns on the other, we can obtain a good
+approximation. In this way we find that in the flora of the
+North-Eastern States the monocotyledons and ferns are to the
+dicotyledons in the proportion of 45 to 100; in the Rocky Mountains they
+are in the proportion of only 34 to 100; while if we take an exclusively
+Alpine flora, as given by Mr. Ball, there are not one-fifth as many
+monocotyledons as dicotyledons. These facts show that even at the
+present day elevated plateaux and mountains are more favourable to
+dicotyledons than to monocotyledons, and we may, therefore, well suppose
+that the former originated within such elevated areas, and were for long
+ages confined to them. It is interesting to note that their richest
+early remains have been found in the central regions of the North
+American continent, where they now, proportionally, most abound, and
+where the conditions of altitude and a dry atmosphere were probably
+present at a very early period.
+
+[Illustration: FIG. 34.--Diagram illustrating the Geological
+Distribution of Plants.]
+
+The diagram (Fig. 34), slightly modified from one given by Mr. Ward,
+will illustrate our present knowledge of the development of the
+vegetable kingdom in geological time. The shaded vertical bands exhibit
+the proportions of the fossil forms actually discovered, while the
+outline extensions are intended to show what we may fairly presume to
+have been the approximate periods of origin, and progressive increase of
+the number of species, of the chief divisions of the vegetable kingdom.
+These seem to accord fairly well with their respective grades of
+development, and thus offer no obstacle to the acceptance of the belief
+in their progressive evolution.
+
+
+_Geological Distribution of Insects._
+
+The marvellous development of insects into such an endless variety of
+forms, their extreme specialisation, and their adaptation to almost
+every possible condition of life, would almost necessarily imply an
+extreme antiquity. Owing, however, to their small size, their lightness,
+and their usually aerial habits, no class of animals has been so
+scantily preserved in the rocks; and it is only recently that the whole
+of the scattered material relating to fossil insects and their allies
+have been brought together by Mr. Samuel H. Scudder of Boston, and we
+have thus learned their bearing on the theory of evolution.[194]
+
+The most striking fact which presents itself on a glance at the
+distribution of fossil insects, is the completeness of the
+representation of all the chief types far back in the Secondary period,
+at which time many of the existing families appear to have been
+perfectly differentiated. Thus in the Lias we find dragonflies
+"apparently as highly specialised as to-day, no less than four tribes
+being present." Of beetles we have undoubted Curculionidae from the Lias
+and Trias; Chrysomelidae in the same deposits; Cerambycidae in the
+Oolites; Scarabaeidae in the Lias; Buprestidae in the Trias; Elateridae,
+Trogositidae, and Nitidulidae in the Lias; Staphylinidae in the English
+Purbecks; while Hydrophilidae, Gyrinidae, and Carabidae occur in the
+Lias. All these forms are well represented, but there are many other
+families doubtfully identified in equally ancient rocks. Diptera of the
+families Empidae, Asilidae, and Tipulidae have been found as far back as
+the Lias. Of Lepidoptera, Sphingidae and Tineidae have been found in
+the Oolite; while ants, representing the highly specialised Hymenoptera,
+have occurred in the Purbeck and Lias.
+
+This remarkable identity of the families of very ancient with those of
+existing insects is quite comparable with the apparently sudden
+appearance of existing genera of trees in the Cretaceous epoch. In both
+cases we feel certain that we must go very much farther back in order to
+find the ancestral forms from which they were developed, and that at any
+moment some fresh discovery may revolutionise our ideas as to the
+antiquity of certain groups. Such a discovery was made while Mr.
+Scudder's work was passing through the press. Up to that date all the
+existing orders of true insects appeared to have originated in the
+Trias, the alleged moth and beetle of the Coal formation having been
+incorrectly determined. But now, undoubted remains of beetles have been
+found in the Coal measures of Silesia, thus supporting the
+interpretation of the borings in carboniferous trees as having been made
+by insects of this order, and carrying back this highly specialised form
+of insect life well into Palaeozoic times. Such a discovery renders all
+speculation as to the origin of true insects premature, because we may
+feel sure that all the other orders of insects, except perhaps
+hymenoptera and lepidoptera, were contemporaneous with the highly
+specialised beetles.
+
+The less highly organised terrestrial arthropoda--the Arachnida and
+Myriapoda--are, as might be expected, much more ancient. A fossil spider
+has been found in the Carboniferous, and scorpions in the Upper Silurian
+rocks of Scotland, Sweden, and the United States. Myriapoda have been
+found abundantly in the Carboniferous and Devonian formations; but all
+are of extinct orders, exhibiting a more generalised structure than
+living forms.
+
+Much more extraordinary, however, is the presence in the Palaeozoic
+formations of ancestral forms of true insects, termed by Mr. Scudder
+Palaeodictyoptera. They consist of generalised cockroaches and
+walking-stick insects (Orthopteroidea); ancient mayflies and allied
+forms, of which there are six families and more than thirty genera
+(Neuropteroidea); three genera of Hemipteroidea resembling various
+Homoptera and Hemiptera, mostly from the Carboniferous formation, a few
+from the Devonian, and one ancestral cockroach (Palaeoblattina) from
+the Middle Silurian sandstone of France. If this occurrence of a true
+hexapod insect from the Middle Silurian be really established, taken in
+connection with the well-defined Coleoptera from the Carboniferous, the
+origin of the entire group of terrestrial arthropoda is necessarily
+thrown back into the Cambrian epoch, if not earlier. And this cannot be
+considered improbable in view of the highly differentiated land
+plants--ferns, equisetums, and lycopods--in the Middle or Lower
+Silurian, and even a conifer (Cordaites Robbii) in the Upper Silurian;
+while the beds of graphite in the Laurentian were probably formed from
+terrestrial vegetation.
+
+On the whole, then, we may affirm that, although the geological record
+of the insect life of the earth is exceptionally imperfect, it yet
+decidedly supports the evolution hypothesis. The most specialised order,
+Lepidoptera, is the most recent, only dating back to the Oolite; the
+Hymenoptera, Diptera, and Homoptera go as far as the Lias; while the
+Orthoptera and Neuroptera extend to the Trias. The recent discovery of
+Coleoptera in the Carboniferous shows, however, that the preceding
+limits are not absolute, and will probably soon be overpassed. Only the
+more generalised ancestral forms of winged insects have been traced back
+to Silurian time, and along with them the less highly organised
+scorpions; facts which serve to show us the extreme imperfection of our
+knowledge, and indicate possibilities of a world of terrestrial life in
+the remotest Palaeozoic times.
+
+
+_Geological Succession of Vertebrata._
+
+The lowest forms of vertebrates are the fishes, and these appear first
+in the geological record in the Upper Silurian formation. The most
+ancient known fish is a Pteraspis, one of the bucklered ganoids or
+plated fishes--by no means a very low type--allied to the sturgeon
+(Accipenser) and alligator-gar (Lepidosteus), but, as a group, now
+nearly extinct. Almost equally ancient are the sharks, which under
+various forms still abound in our seas. We cannot suppose these to be
+nearly the earliest fishes, especially as the two lowest orders, now
+represented by the Amphioxus or lancelet and the lampreys, have not yet
+been found fossil. The ganoids were greatly developed in the Devonian
+era, and continued till the Cretaceous, when they gave way to the true
+osseous fishes, which had first appeared in the Jurassic period, and
+have continued to increase till the present day. This much later
+appearance of the higher osseous fishes is quite in accordance with
+evolution, although some of the very lowest forms, the lancelet and the
+lampreys, together with the archaic ceratodus, have survived to our
+time.
+
+The Amphibia, represented by the extinct labyrinthodons, appear first in
+the Carboniferous rocks, and these peculiar forms became extinct early
+in the Secondary period. The labyrinthodons were, however, highly
+specialised, and do not at all indicate the origin of the class, which
+may be as ancient as the lower forms of fishes. Hardly any recognisable
+remains of our existing groups--the frogs, toads, and salamanders--are
+found before the Tertiary period, a fact which indicates the extreme
+imperfection of the record as regards this class of animals.
+
+True reptiles have not been found till we reach the Permian where
+Prohatteria and Proterosaurus occur, the former closely allied to the
+lizard-like Sphenodon of New Zealand, the latter having its nearest
+allies in the same group of reptiles--Rhyncocephala, other forms of
+which occur in the Trias. In this last-named formation the earliest
+crocodiles--Phytosaurus (Belodon) and Stagonolepis occur, as well as the
+earliest tortoises--Chelytherium, Proganochelys, and Psephoderma.[195]
+Fossil serpents have been first found in the Cretaceous formation, but
+the conditions for the preservation of these forms have evidently been
+unfavourable, and the record is correspondingly incomplete. The marine
+Plesiosauri and Ichthyosauri, the flying Pterodactyles, the terrestrial
+Iguanodon of Europe, and the huge Atlantosaurus of Colorado--the largest
+land animal that has ever lived upon the earth[196]--all belong to
+special developments of the reptilian type which flourished during the
+Secondary epoch, and then became extinct.
+
+Birds are among the rarest of fossils, due, no doubt, to their aerial
+habits removing them from the ordinary dangers of flood, bog, or ice
+which overwhelm mammals and reptiles, and also to their small specific
+gravity which keeps them floating on the surface of water till devoured.
+Their remains were long confined to Tertiary deposits, where many living
+genera and a few extinct forms have been found. The only birds yet known
+from the older rocks are the toothed birds (Odontornithes) of the
+Cretaceous beds of the United States, belonging to two distinct families
+and many genera; a penguin-like form (Enaliornis) from the Upper
+Greensand of Cambridge; and the well-known long-tailed Archaeopteryx
+from the Upper Oolite of Bavaria. The record is thus imperfect and
+fragmentary in the extreme; but it yet shows us, in the few birds
+discovered in the older rocks, more primitive and generalised types,
+while the Tertiary birds had already become specialised like those
+living, and had lost both the teeth and the long vertebral tail, which
+indicate reptilian affinities in the earlier Mammalia have been found,
+as already stated, as far back as the Trias formation, in Europe in the
+United States and in South Africa, all being very small, and belonging
+either to the Marsupial order, or to some still lower and more
+generalised type, out of which both Marsupials and Insectivora were
+developed. Other allied forms have been found in the Lower and Upper
+Oolite both of Europe and the United States. But there is then a great
+gap in the whole Cretaceous formation, from which no mammal has been
+obtained, although both in the Wealden and the Upper Chalk in Europe,
+and in the Upper Cretaceous deposits of the United States an abundant
+and well-preserved terrestrial flora has been discovered. Why no mammals
+have left their remains here it is impossible to say. We can only
+suppose that the limited areas in which land plants have been so
+abundantly preserved, did not present the conditions which are needed
+for the fossilisation and preservation of mammalian remains.
+
+When we come to the Tertiary formation, we find mammals in abundance;
+but a wonderful change has taken place. The obscure early types have
+disappeared, and we discover in their place a whole series of forms
+belonging to existing orders, and even sometimes to existing families.
+Thus, in the Eocene we have remains of the opossum family; bats
+apparently belonging to living genera; rodents allied to the South
+American cavies and to dormice and squirrels; hoofed animals belonging
+to the odd-toed and even-toed groups; and ancestral forms of cats,
+civets, dogs, with a number of more generalised forms of carnivora.
+Besides these there are whales, lemurs, and many strange ancestral forms
+of proboscidea.[197]
+
+The great diversity of forms and structures at so remote an epoch would
+require for their development an amount of time, which, judging by the
+changes that have occurred in other groups, would carry us back far into
+the Mesozoic period. In order to understand why we have no record of
+these changes in any part of the world, we must fall back upon some such
+supposition as we made in the case of the dicotyledonous plants.
+Perhaps, indeed, the two cases are really connected, and the upland
+regions of the primeval world, which saw the development of our higher
+vegetation, may have also afforded the theatre for the gradual
+development of the varied mammalian types which surprise us by their
+sudden appearance in Tertiary times.
+
+[Illustration: GEOLOGICAL DISTRIBUTION OF MAMMALIA.]
+
+Notwithstanding these irregularities and gaps in the record, the
+accompanying table, summarising our actual knowledge of the geological
+distribution of the five classes of vertebrata, exhibits a steady
+progression from lower to higher types, excepting only the deficiency in
+the bird record which is easily explained. The comparative perfection of
+type in which each of these classes first appears, renders it certain
+that the origin of each and all of them must be sought much farther back
+than any records which have yet been discovered. The researches of
+palaeontologists and embryologists indicate a reptilian origin for birds
+and mammals, while reptiles and amphibia arose, perhaps independently,
+from fishes.
+
+
+_Concluding Remarks._
+
+The brief review we have now taken of the more suggestive facts
+presented by the geological succession of organic forms, is sufficient
+to show that most, if not all, of the supposed difficulties which it
+presents in the way of evolution, are due either to imperfections in the
+geological record itself, or to our still very incomplete knowledge of
+what is really recorded in the earth's crust. We learn, however, that
+just as discovery progresses, gaps are filled up and difficulties
+disappear; while, in the case of many individual groups, we have already
+obtained all the evidence of progressive development that can reasonably
+be expected. We conclude, therefore, that the geological difficulty has
+now disappeared; and that this noble science, when properly understood,
+affords clear and weighty evidence of evolution.
+
+FOOTNOTES:
+
+[Footnote 183: The reader who desires to understand this subject more
+fully, should study chap. x. of the _Origin of Species_, and chap. xiv.
+of Sir Charles Lyell's _Principles of Geology_.]
+
+[Footnote 184: On "Stagonolepis Robertsoni and on the Evolution of the
+Crocodilia," in _Q.J. of Geological Society_, 1875; and abstract in
+_Nature_, vol. xii. p. 38.]
+
+[Footnote 185: From a paper by Messrs. Scott and Osborne, "On the Origin
+and Development of the Rhinoceros Group," read before the British
+Association in 1883.]
+
+[Footnote 186: American Addresses, pp. 73-76.]
+
+[Footnote 187: Lecture on the Introduction and Succession of Vertebrate
+Life in America, _Nature_, vol. xvi. p. 471.]
+
+[Footnote 188: _Nature_, vol. xxv. p. 84.]
+
+[Footnote 189: See _The Mammalia in their Relation to Primeval Times_,
+p. 102.]
+
+[Footnote 190: For a brief enumeration and description of these fossils,
+see the author's _Geographical Distribution of Animals_, vol. i. p.
+146.]
+
+[Footnote 191: Sketch of Palaeobotany in Fifth Annual Report of U.S.
+Geological Survey, 1883-84, pp. 363-452, with diagrams. Sir J. William
+Dawson, speaking of the value of leaves for the determination of fossil
+plants, says: "In my own experience I have often found determinations of
+the leaves of trees confirmed by the discovery of their fruits or of the
+structure of their stems. Thus, in the rich cretaceous plant-beds of the
+Dunvegan series, we have beech-nuts associated in the same bed with
+leaves referred to _Fagus_. In the Laramie beds I determined many years
+ago nuts of the _Trapa_ or water-chestnut, and subsequently Lesquereux
+found in beds in the United States leaves which he referred to the same
+genus. Later, I found in collections made on the Red Deer River of
+Canada my fruits and Lesquereux's leaves on the same slab. The presence
+of trees of the genera _Carya_ and _Juglans_ in the same formation was
+inferred from their leaves, and specimens have since been obtained of
+silicified wood with the microscopic structure of the modern butternut.
+Still we are willing to admit that determinations from leaves alone are
+liable to doubt."--_The Geological History of Plants_, p. 196.]
+
+[Footnote 192: Sir J. William Dawson's _Geological History of Plants_,
+p. 18.]
+
+[Footnote 193: "On the Origin of the Flora of the European Alps," _Proc.
+of Roy. Geog. Society_, vol. i. (1879), pp. 564-588.]
+
+[Footnote 194: Systematic Review of our Present Knowledge of Fossil
+Insects, including Myriapods and Arachnids (_Bull. of U.S. Geol.
+Survey_, No. 31, Washington, 1886).]
+
+[Footnote 195: For the facts as to the early appearance of the above
+named groups of reptiles I am indebted to Mr. E. Lydekker of the
+Geological Department of the Natural History Museum.]
+
+[Footnote 196: According to Professor Marsh this creature was 50 or 60
+feet long, and when erect, at least 30 feet in height. It fed upon the
+foliage of the mountain forests of the Cretaceous epoch, the remains of
+which are preserved with it.]
+
+[Footnote 197: For fuller details, see the author's _Geographical
+Distribution of Animals_, and Heilprin's _Geographical and Geological
+Distribution of Animals_.]
+
+
+
+
+CHAPTER XIV
+
+FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
+
+
+    Fundamental difficulties and objections--Mr. Herbert Spencer's
+    factors of organic evolution--Disuse and effects of withdrawal
+    of natural selection--Supposed effects of disuse among wild
+    animals--Difficulty as to co-adaptation of parts by variation
+    and selection--Direct action of the environment--The American
+    school of evolutionists--Origin of the feet of the
+    ungulates--Supposed action of animal intelligence--Semper on the
+    direct influence of the environment--Professor Geddes's theory
+    of variation in plants--Objections to the theory--On the origin
+    of spines--Variation and selection overpower the effects of use
+    and disuse--Supposed action of the environment in imitating
+    variations--Weismann's theory of heredity--The cause of
+    variation--The non-heredity of acquired characters--The theory
+    of instinct--Concluding remarks.
+
+
+
+Having now set forth and illustrated at some length the most important
+of the applications of the development hypothesis in the explanation of
+the broader and more generally interesting phenomena presented by the
+organic world, we propose to discuss some of the more fundamental
+problems and difficulties which have recently been adduced by eminent
+naturalists. It is the more necessary to do this, because there is now a
+tendency to minimise the action of natural selection in the production
+of organic forms, and to set up in its place certain fundamental
+principles of variation or laws of growth, which it is urged are the
+real originators of the several lines of development, and of most of the
+variety of form and structure in the vegetable and animal kingdoms.
+These views have, moreover, been seized upon by popular writers to throw
+doubt and discredit on the whole theory of evolution, and especially on
+Darwin's presentation of that theory, to the bewilderment of the general
+public, who are quite unable to decide how far the new views, even if
+well established, tend to subvert the Darwinian theory, or whether they
+are really more than subsidiary parts of it, and quite powerless without
+it to produce any effect whatever.
+
+The writers whose special views we now propose to consider are: (1) Mr.
+Herbert Spencer, on modification of structures arising from modification
+of functions, as set forth in his _Factors of Organic Evolution_. (2)
+Dr. E.D. Cope, who advocates similar views in detail, in his work
+entitled _The Origin of the Fittest_, and may be considered the head of
+a school of American naturalists who minimise the agency of natural
+selection. (3) Dr. Karl Semper, who has especially studied the direct
+influence of the environment in the whole animal kingdom, and has set
+forth his views in a volume on _The Natural Conditions of Existence as
+they Affect Animal Life_. (4) Mr. Patrick Geddes, who urges that
+fundamental laws of growth, and the antagonism of vegetative and
+reproductive forces, account for much that has been imputed to natural
+selection.
+
+We will now endeavour to ascertain what are the more important facts and
+arguments adduced by each of the above writers, and how far they offer a
+substitute for the action of natural selection; having done which, a
+brief account will be given of the views of Dr. Aug. Weismann, whose
+theory of heredity will, if established, strike at the very root of the
+arguments of the first three of the writers above referred to.
+
+
+_Mr. Herbert Spencer's Factors of Organic Evolution._
+
+Mr. Spencer, while fully recognising the importance and wide range of
+the principle of natural selection, thinks that sufficient weight has
+not been given to the effects of use and disuse as a factor in
+evolution, or to the direct action of the environment in determining or
+modifying organic structures. As examples of the former class of
+actions, he adduces the decreased size of the jaws in the civilised
+races of mankind, the inheritance of nervous disease produced by
+overwork, the great and inherited development of the udders in cows and
+goats, and the shortened legs, jaws, and snout in improved races of
+pigs--the two latter examples being quoted from Mr. Darwin,--and other
+cases of like nature. As examples of the latter, Mr. Darwin is again
+quoted as admitting that there are many cases in which the action of
+similar conditions appears to have produced corresponding changes in
+different species; and we have a very elaborate discussion of the direct
+action of the medium in modifying the protoplasm of simple organisms, so
+as to bring about the difference between the outer surface and the inner
+part that characterises the cells or other units of which they are
+formed.
+
+Now, although this essay did little more than bring together facts which
+had been already adduced by Mr. Darwin or by Mr. Spencer himself, and
+lay stress upon their importance, its publication in a popular review
+was immediately seized upon as "an avowed and definite declaration
+against some of the leading ideas on which the Mechanical Philosophy
+depends," and as being "fatal to the adequacy of the Mechanical
+Philosophy as any explanation of organic evolution,"[198]--an expression
+of opinion which would be repudiated by every Darwinian. For, even
+admitting the interpretation which Mr. Spencer puts on the facts he
+adduces, they are all included in the causes which Darwin himself
+recognised as having acted in bringing about the infinitude of forms in
+the organic world. In the concluding chapter of the _Origin of Species_
+he says: "I have now recapitulated the facts and considerations which
+have thoroughly convinced me that species have been modified during a
+long course of descent. This has been effected chiefly through the
+natural selection of numerous successive, slight, favourable variations;
+aided in an important manner by the inherited effects of the use and
+disuse of parts; and in an unimportant manner--that is, in relation to
+adaptive structures whether past or present, by the direct action of
+external conditions, and by variations which seem to us, in our
+ignorance, to arise spontaneously." This passage, summarising Darwin's
+whole inquiry, and explaining his final point of view, shows how very
+inaccurate may be the popular notion, as expressed by the Duke of
+Argyll, of any supposed additions to the causes of change of species as
+recognised by Darwin.
+
+But, as we shall see presently, there is now much reason to believe
+that the supposed inheritance of acquired modifications--that is, of the
+effects of use and disuse, or of the direct influence of the
+environment--is not a fact; and if so, the very foundation is taken away
+from the whole class of objections on which so much stress is now laid.
+It therefore becomes important to inquire whether the facts adduced by
+Darwin, Spencer, and others, do really necessitate such inheritance, or
+whether any other interpretation of them is possible. I believe there is
+such an interpretation; and we will first consider the cases of disuse
+on which Mr. Spencer lays most stress.
+
+The cases Mr. Spencer adduces as demonstrating the effects of disuse in
+diminishing the size and strength of organs are, the diminished size of
+the jaws in the races of civilised men, and the diminution of the
+muscles used in closing the jaws in the case of pet-dogs fed for
+generations on soft food. He argues that the minute reduction in any one
+generation could not possibly have been useful, and, therefore, not the
+subject of natural selection; and against the theory of correlation of
+the diminished jaw with increased brain in man, he urges that there are
+cases of large brain development, accompanied by jaws above the average
+size. Against the theory of economy of nutrition in the case of the
+pet-dogs, he places the abundant food of these animals which would
+render such economy needless.
+
+But neither he nor Mr. Darwin has considered the effects of the
+withdrawal of the action of natural selection in keeping up the parts in
+question to their full dimensions, which, of itself, seems to me quite
+adequate to produce the results observed. Recurring to the evidence,
+adduced in Chapter III, of the constant variation occurring in all parts
+of the organism, while selection is constantly acting on these
+variations in eliminating all that fall below the best working standard,
+and preserving only those that are fully up to it; and, remembering
+further, that, of the whole number of the increase produced annually,
+only a small percentage of the best adapted can be preserved, we shall
+see that every useful organ will be kept up nearly to its higher limit
+of size and efficiency. Now Mr. Galton has proved experimentally that,
+when any part has thus been increased (or diminished) by selection,
+there is in the offspring a strong tendency to revert to a mean or
+average size, which tends to check further increase. And this mean
+appears to be, not the mean of the actual existing individuals but a
+lower mean, or that from which they had been recently raised by
+selection.[199] He calls this the law of "Regression towards
+Mediocrity," and it has been proved by experiments with vegetables and
+by observations on mankind. This regression, in every generation, takes
+place even when both parents have been selected for their high
+development of the organ in question; but when there is no such
+selection, and crosses are allowed among individuals of every grade of
+development, the deterioration will be very rapid; and after a time not
+only will the average size of the part be greatly reduced, but the
+instances of full development will become very rare. Thus what Weismann
+terms "panmixia," or free intercrossing, will co-operate with Galton's
+law of "regression towards mediocrity," and the result will be that,
+whenever selection ceases to act on any part or organ which has
+heretofore been kept up to a maximum of size and efficiency, the organ
+in question will rapidly decrease till it reaches a mean value
+considerably below the mean of the progeny that has usually been
+produced each year, and very greatly below the mean of that portion
+which has survived annually; and this will take place by the general law
+of heredity, and quite irrespective of any _use_ or _disuse_ of the part
+in question. Now, no observations have been adduced by Mr. Spencer or
+others, showing that the average amount of change supposed to be due to
+_disuse_ is greater than that due to the law of regression towards
+mediocrity; while even if it were somewhat greater, we can see many
+possible contributory causes to its production. In the case of civilised
+man's diminished jaw, there may well be some correlation between the jaw
+and the brain, seeing that increased mental activity would lead to the
+withdrawal of blood and of nervous energy from adjacent parts, and might
+thus lead to diminished growth of those parts in the individual. And in
+the case of pet-dogs, the selection of small or short-headed individuals
+would imply the unconscious selection of those with less massive
+temporal muscles, and thus lead to the concomitant reduction of those
+muscles. The amount of reduction observed by Darwin in the wing-bones of
+domestic ducks and poultry, and in the hind legs of tame rabbits, is
+very small, and is certainly no greater than the above causes will well
+account for; while so many of the external characters of all our
+domestic animals have been subject to long-continued artificial
+selection, and we are so ignorant of the possible correlations of
+different parts, that the phenomena presented by them seem sufficiently
+explained without recurrence to the assumption that any changes in the
+individual, due to disuse, are inherited by the offspring.
+
+
+_Supposed Effects of Disuse among Wild Animals._
+
+It may be urged, however, that among wild animals we have many undoubted
+results of disuse much more pronounced than those among domestic kinds,
+results which cannot be explained by the causes already adduced. Such
+are the reduced size of the wings of many birds on oceanic islands; the
+abortion of the eyes in many cave animals, and in some which live
+underground; and the loss of the hind limbs in whales and in some
+lizards. These cases differ greatly in the amount of the reduction of
+parts which has taken place, and may be due to different causes. It is
+remarkable that in some of the birds of oceanic islands the reduction is
+little if at all greater than in domestic birds, as in the water-hen of
+Tristan d'Acunha. Now if the reduction of wing were due to the
+hereditary effects of disuse, we should expect a very much greater
+effect in a bird inhabiting an oceanic island than in a domestic bird,
+where the disuse has been in action for an indefinitely shorter period.
+In the case of many other birds, however--as some of the New Zealand
+rails and the extinct dodo of Mauritius--the wings have been reduced to
+a much more rudimentary condition, though it is still obvious that they
+were once organs of flight; and in these cases we certainly require some
+other causes than those which have reduced the wings of our domestic
+fowls. One such cause may have been of the same nature as that which has
+been so efficient in reducing the wings of the insects of oceanic
+islands--the destruction of those which, during the occasional use of
+their wings, were carried out to sea. This form of natural selection may
+well have acted in the case of birds whose powers of flight were
+already somewhat reduced, and to whom, there being no enemies to escape
+from, their use was only a source of danger. We may thus, perhaps,
+account for the fact that many of these birds retain small but useless
+wings with which they never fly; for, the wings having been reduced to
+this functionless condition, no power could reduce them further except
+correlation of growth or economy of nutrition, causes which only rarely
+come into play.
+
+The complete loss of eyes in some cave animals may, perhaps, be
+explained in a somewhat similar way. Whenever, owing to the total
+darkness, they became useless, they might also become injurious, on
+account of their delicacy of organisation and liability to accidents and
+disease; in which case natural selection would begin to act to reduce,
+and finally abort them; and this explains why, in some cases, the
+rudimentary eye remains, although completely covered by a protective
+outer skin. Whales, like moas and cassowaries, carry us back to a remote
+past, of whose conditions we know too little for safe speculation. We
+are quite ignorant of the ancestral forms of either of these groups, and
+are therefore without the materials needful for determining the steps by
+which the change took place, or the causes which brought it about.[200]
+
+On a review of the various examples that have been given by Mr. Darwin
+and others of organs that have been reduced or aborted, there seems too
+much diversity in the results for all to be due to so direct and uniform
+a cause as the individual effects of disuse accumulated by heredity. For
+if that were the only or chief efficient cause, and a cause capable of
+producing a decided effect during the comparatively short period of the
+existence of animals in a state of domestication, we should expect to
+find that, in wild species, all unused parts or organs had been reduced
+to the smallest rudiments, or had wholly disappeared. Instead of this we
+find various grades of reduction, indicating the probable result of
+several distinct causes, sometimes acting separately, sometimes in
+combination, such as those we have already pointed out.
+
+And if we find no positive evidence of _disuse_, acting by its direct
+effect on the individual, being transmitted to the offspring, still less
+can we find such evidence in the case of the _use_ of organs. For here
+the very fact of _use_, in a wild state, implies _utility_, and utility
+is the constant subject for the action of natural selection; while among
+domestic animals those parts which are exceptionally used are so used in
+the service of man, and have thus become the subjects of artificial
+selection. Thus "the great and inherited development of the udders in
+cows and goats," quoted by Spencer from Darwin, really affords no proof
+of inheritance of the increase due to use, because, from the earliest
+period of the domestication of these animals, abundant milk-production
+has been highly esteemed, and has thus been the subject of selection;
+while there are no cases among wild animals that may not be better
+explained by variation and natural selection.
+
+
+_Difficulty as to Co-adaptation of Parts by Variation and Selection._
+
+Mr. Spencer again brings forward this difficulty, as he did in his
+_Principles of Biology_ twenty-five years ago, and urges that all the
+adjustments of bones, muscles, blood-vessels, and nerves which would be
+required during, for example, the development of the neck and fore-limbs
+of the giraffe, could not have been effected by "simultaneous fortunate
+spontaneous variations." But this difficulty is fully disposed of by the
+facts of simultaneous variation adduced in our third chapter, and has
+also been specially considered in Chapter VI, p. 127. The best answer to
+this objection may, perhaps, be found in the fact that the very thing
+said to be impossible by variation and natural selection has been again
+and again effected by variation and artificial selection. During the
+process of formation of such breeds as the greyhound or the bulldog, of
+the race-horse and carthorse, of the fantail pigeon or the otter-sheep,
+many co-ordinate adjustments have been produced; and no difficulty has
+occurred, whether the change has been effected by a single variation--as
+in the last case named--or by slow steps, as in all the others. It seems
+to be forgotten that most animals have such a surplus of vitality and
+strength for all the ordinary occasions of life that any slight
+superiority in one part can be at once utilised; while the moment any
+want of balance occurs, variations in the insufficiently developed parts
+will be selected to bring back the harmony of the whole organisation.
+The fact that, in all domestic animals, variations do occur, rendering
+them swifter or stronger, larger or smaller, stouter or slenderer, and
+that such variations can be separately selected and accumulated for
+man's purposes, is sufficient to render it certain that similar or even
+greater changes may be effected by natural selection, which, as Darwin
+well remarks, "acts on every internal organ, on every shade of
+constitututional difference, on the whole machinery of life." The
+difficulty as to co-adaptation of parts by variation and natural
+selection appears to me, therefore, to be a wholly imaginary difficulty
+which has no place whatever in the operations of nature.
+
+
+_Direct Action of the Environment._
+
+Mr. Spencer's last objection to the wide scope given by Darwinians to
+the agency of natural selection is, that organisms are acted upon by the
+environment, which produces in them definite changes, and that these
+changes in the individual are transmitted by inheritance, and thus
+become increased in successive generations. That such changes are
+produced in the individual there is ample evidence, but that they are
+inherited independently of any form of selection or of reversion is
+exceedingly doubtful, and Darwin nowhere expresses himself as satisfied
+with the evidence. The two very strongest cases he mentions are the
+twenty-nine species of American trees which all differed in a
+corresponding way from their nearest European allies; and the American
+maize which became changed after three generations in Europe. But in the
+case of the trees the differences alleged may be partly due to
+correlation with constitutional peculiarities dependent on climate,
+especially as regards the deeper tint of the fading leaves and the
+smaller size of the buds and seeds in America than in Europe; while the
+less deeply toothed or serrated leaves in the American species are, in
+our present complete ignorance of the causes and uses of serration,
+quite as likely to be due to some form of adaptation as to any direct
+action of the climate. Again, we are not told how many of the allied
+species do not vary in this particular manner, and this is certainly an
+important factor in any conclusion we may form on the question.
+
+In the case of the maize it appears that one of the more remarkable and
+highly selected American varieties was cultivated in Germany, and in
+three years nearly all resemblance to the original parent was lost; and
+in the sixth year it closely resembled a common European variety, but
+was of somewhat more vigorous growth. In this case no selection appears
+to have been practised, and the effects may have been due to that
+"reversion to mediocrity" which invariably occurs, and is more
+especially marked in the case of varieties which have been rapidly
+produced by artificial selection. It may be considered as a partial
+reversion to the wild or unimproved stock; and the same thing would
+probably have occurred, though perhaps less rapidly, in America itself.
+As this is stated by Darwin to be the most remarkable case known to him
+"of the direct and prompt action of climate on a plant," we must
+conclude that such direct effects have not been proved to be accumulated
+by inheritance, independently of reversion or selection.
+
+The remaining part of Mr. Spencer's essay is devoted to a consideration
+of the hypothetical action of the environment on the lower organisms
+which consist of simple cells or formless masses of protoplasm; and he
+shows with great elaboration that the outer and inner parts of these
+are necessarily subject to different conditions; and that the outer
+actions of air or water lead to the formation of integuments, and
+sometimes to other definite modifications of the surface, whence arise
+permanent differences of structure. Although in these cases also it is
+very difficult to determine how much is due to direct modification by
+external agencies transmitted and accumulated by inheritance, and how
+much to spontaneous variations accumulated by natural selection, the
+probabilities in favour of the former mode of action are here greater,
+because there is no differentiation of nutritive and reproductive cells
+in these simple organisms; and it can be readily seen that any change
+produced in the latter will almost certainly affect the next
+generation.[201] We are thus carried back almost to the origin of life,
+and can only vaguely speculate on what took place under conditions of
+which we know so little.
+
+
+_The American School of Evolutionists._
+
+The tentative views of Mr. Spencer which we have just discussed, are
+carried much further, and attempts have been made to work them out in
+great detail, by many American naturalists, whose best representative is
+Dr. E.D. Cope of Philadelphia.[202] This school endeavours to explain
+all the chief modifications of form in the animal kingdom by fundamental
+laws of growth and the inherited effects of use and effort, returning,
+in fact, to the teachings of Lamarck as being at least equally important
+with those of Darwin.
+
+The following extract will serve to show the high position claimed by
+this school as original discoverers, and as having made important
+additions to the theory of evolution:
+
+"Wallace and Darwin have propounded as the cause of modification in
+descent their law of natural selection. This law has been epitomised by
+Spencer as the 'survival of the fittest.' This neat expression no doubt
+covers the case, but it leaves the origin of the fittest entirely
+untouched. Darwin assumes a 'tendency to variation' in nature, and it is
+plainly necessary to do this, in order that materials for the exercise
+of a selection should exist. Darwin and Wallace's law is then only
+restrictive, directive, conservative, or destructive of something
+already created. I propose, then, to seek for the originative laws by
+which these subjects are furnished; in other words, for the causes of
+the origin of the fittest."[203]
+
+Mr. Cope lays great stress on the existence of a special developmental
+force termed "bathmism" or growth-force, which acts by means of
+retardation and acceleration "without any reference to fitness at all;"
+that "instead of being controlled by fitness it is the controller of
+fitness." He argues that "all the characteristics of generalised groups
+from genera up (excepting, perhaps, families) have been evolved under
+the law of acceleration and retardation," combined with some
+intervention of natural selection; and that specific characters, or
+species, have been evolved by natural selection with some assistance
+from the higher law. He, therefore, makes species and genera two
+absolutely distinct things, the latter not developed out of the former;
+generic characters and specific characters are, in his opinion,
+fundamentally different, and have had different origins, and whole
+groups of species have been simultaneously modified, so as to belong to
+another genus; whence he thinks it "highly probable that the same
+specific form has existed through a succession of genera, and perhaps in
+different epochs of geologic time."
+
+Useful characters, he concludes, have been produced by the special
+location of growth-force by use; useless ones have been produced by
+location of growth-force without the influence of use. Another element
+which determines the direction of growth-force, and which precedes use,
+is effort; and "it is thought that effort becomes incorporated into the
+metaphysical acquisitions of the parent, and is inherited with other
+metaphysical qualities by the young, which, during the period of growth,
+is much more susceptible to modifying influences, and is likely to
+exhibit structural change in consequence."[204]
+
+From these few examples of their teachings, it is clear that these
+American evolutionists have departed very widely from the views of Mr.
+Darwin, and in place of the well-established causes and admitted laws to
+which he appeals have introduced theoretical conceptions which have not
+yet been tested by experiments or facts, as well as metaphysical
+conceptions which are incapable of proof. And when they come to
+illustrate these views by an appeal to palaeontology or morphology, we
+find that a far simpler and more complete explanation of the facts is
+afforded by the established principles of variation and natural
+selection. The confidence with which these new ideas are enunciated, and
+the repeated assertion that without them Darwinism is powerless to
+explain the origin of organic forms, renders it necessary to bestow a
+little more time on the explanations they give us of well-known
+phenomena with which, they assert, other theories are incompetent to
+grapple.
+
+As examples of use producing structural change, Mr. Cope adduces the
+hooked and toothed beaks of the falcons and the butcher-birds, and he
+argues that the fact of these birds belonging to widely different groups
+proves that similarity of use has produced a similar structural result.
+But no attempt is made to show any direct causal connection between the
+use of a bill to cut or tear flesh and the development of a tooth on the
+mandible. Such use might conceivably strengthen the bill or increase its
+size, but not cause a special tooth-like outgrowth which was not present
+in the ancestral thrush-like forms of the butcher-bird. On the other
+hand, it is clear that any variations of the bill tending towards a hook
+or tooth would give the possessor some advantage in seizing and tearing
+its prey, and would thus be preserved and increased by natural
+selection. Again, Mr. Cope urges the effects of a supposed "law of polar
+or centrifugal growth" to counteract a tendency to unsymmetrical growth,
+where one side of the body is used more than the other. But the
+undoubted hurtfulness of want of symmetry in many important actions or
+functions would rapidly eliminate any such tendency. When, however, it
+has become useful, as in the case of the single enlarged claw of many
+Crustacea, it has been preserved by natural selection.
+
+
+_Origin of the Feet of the Ungulates._
+
+Perhaps the most original and suggestive of Mr. Cope's applications of
+the theory of use and effort in modifying structure are, his chapters
+"On the Origin of the Foot-Structure of the Ungulates;" and that "On the
+Effect of Impacts and Strains on the Feet of Mammalia;" and they will
+serve also to show the comparative merits of this theory and that of
+natural selection in explaining a difficult case of modification,
+especially as it is an explanation claimed as new and original when
+first enunciated in 1881. Let us, then, see how he deals with the
+problem.
+
+The remarkable progressive change of a four or five-toed ancestor into
+the one-toed horse, and the equally remarkable division of the whole
+group of ungulate animals into the odd-toed and even-toed divisions, Mr.
+Cope attempts to explain by the effects of impact and use among animals
+which frequented hard or swampy ground respectively. On hard ground, it
+is urged, the long middle toe would be most used and subjected to the
+greatest strains, and would therefore acquire both strength and
+development. It would then be still more exclusively used, and the extra
+nourishment required by it would be drawn from the adjacent less-used
+toes, which would accordingly diminish in size, till, after a long
+series of changes, the records of which are so well preserved in the
+American tertiary rocks, the true one-toed horse was developed. In soft
+or swampy ground, on the other hand, the tendency would be to spread out
+the foot so that there were two toes on each side. The two middle toes
+would thus be most used and most subject to strains, and would,
+therefore, increase at the expense of the lateral toes. There would be,
+no doubt, an advantage in these two functional toes being of equal size,
+so as to prevent twisting of the foot while walking; and variations
+tending to bring this about would be advantageous, and would therefore
+be preserved. Thus, by a parallel series of changes in another
+direction, adapted to a distinct set of conditions, we should arrive at
+the symmetrical divided hoofs of our deer and cattle. The fact that
+sheep and goats are specially mountain and rock-loving animals may be
+explained by their being a later modification, since the divided hoof
+once formed is evidently well adapted to secure a firm footing on rugged
+and precipitous ground, although it could hardly have been first
+developed in such localities. Mr. Cope thus concludes: "Certain it is
+that the length of the bones in the feet of the ungulate orders has a
+direct relation to the dryness of the ground they inhabit, and the
+possibility of speed which their habit permits them or necessarily
+imposes on them."[205]
+
+If there is any truth in the explanation here briefly summarised, it
+must entirely depend on the fact of individual modifications thus
+produced being hereditary, and we yet await the proof of this. In the
+meantime it is clear that the very same results could have been brought
+about by variation and natural selection. For the toes, like all other
+organs, vary in size and proportions, and in their degree of union or
+separation; and if in one group of animals it was beneficial to have the
+middle toe larger and longer, and in another set to have the two middle
+toes of the same size, nothing can be more certain than that these
+particular modifications would be continuously preserved, and the very
+results we see ultimately produced.
+
+The oft-repeated objections that the cause of variations is unknown,
+that there must be something to determine variations in the right
+direction; that "natural selection includes no actively progressive
+principle, but must wait for the development of variation, and then,
+after securing the survival of the best, wait again for the best to
+project its own variations for selection," we have already sufficiently
+answered by showing that variation--in abundant or typical species--is
+always present in ample amount; that it exists in all parts and organs;
+that these vary, for the most part, independently, so that any required
+combination of variations can be secured; and finally, that all
+variation is necessarily either in excess or defect of the mean
+condition, and that, consequently, the right or favourable variations
+are so frequently present that the unerring power of natural selection
+never wants materials to work upon.
+
+_Supposed Action of Animal Intelligence._
+
+The following passage briefly summarises Mr. Cope's position:
+"Intelligence is a conservative principle, and will always direct effort
+and use into lines which will be beneficial to its possessor. Here we
+have the source of the fittest, _i.e._ addition of parts by increase and
+location of growth-force, directed by the influence of various kinds of
+compulsion in the lower, and intelligent option among higher animals.
+Thus intelligent choice, taking advantage of the successive evolution of
+physical conditions, may be regarded as the _originator of the fittest_,
+while natural selection is the tribunal to which all results of
+accelerated growth are submitted. This preserves or destroys them, and
+determines the new points of departure on which accelerated growth shall
+build."[206]
+
+This notion of "intelligence"--the intelligence of the animal
+itself--determining its own variation, is so evidently a very partial
+theory, inapplicable to the whole vegetable kingdom, and almost so to
+all the lower forms of animals, amongst which, nevertheless, there is
+the very same adaptation and co-ordination of parts and functions as
+among the highest, that it is strange to see it put forward with such
+confidence as necessary for the completion of Darwin's theory. If "the
+various kinds of compulsion"--by which are apparently meant the laws of
+variation, growth, and reproduction, the struggle for existence, and the
+actions necessary to preserve life under the conditions of the animal's
+environment--are sufficient to have developed the varied forms of the
+lower animals and of plants, we can see no reason why the same
+"compulsion" should not have carried on the development of the higher
+animals also. The action of this "intelligent option" is altogether
+unproved; while the acknowledgment that natural selection is the
+tribunal which either preserves or destroys the variations submitted to
+it, seems quite inconsistent with the statement that intelligent choice
+is the "orginator of the fittest," since whatever is really "the
+fittest" can never be destroyed by natural selection, which is but
+another name for the survival of the fittest. If "the fittest" is always
+definitely produced by some other power, then natural selection is not
+wanted. If, on the other hand, both fit and unfit are produced, and
+natural selection decides between them, that is pure Darwinism, and Mr.
+Cope's theories have added nothing to it.
+
+[Illustration: FIG. 35.--Transformation of Artemia salina to A.
+Milhausenii; 1, tail-lobe of A. salina, and its transition through
+2,3,4,5, to 6, into that of A. Milhausenii; 7, post-abdomen of A.
+salina; 8, post-abdomen of a form bred in brackish water; 9, gill of A.
+Milhausenii; 10, gill of A. salina. (From Schmankewitsch.)]
+
+
+_Semper on the Direct Influence of the Environment._
+
+Another eminent naturalist, Professor Karl Semper of Würzburg, also
+adopts the view of the direct transforming power of the environment, and
+has brought together an immense body of interesting facts showing the
+influence of food, of light, of temperature, of still water and moving
+water, of the atmosphere and its currents, of gravitation, and of other
+organisms, in modifying the forms and other characteristics of
+animals.[207] He believes that these various influences produce a direct
+and important effect, and that this effect is accumulated by
+inheritance; yet he acknowledges that we have no direct evidence of
+this, and there is hardly a single case adduced in the book which is not
+equally well explained by adaptation, brought about by the survival of
+beneficial variations. Perhaps the most remarkable case he has brought
+forward is that of the transformation of species of crustaceans by a
+change in the saltness of the water (see Fig. 35). Artemia salina lives
+in brackish water, while A. Milhausenii inhabits water which is much
+salter. They differ greatly in the form of the tail-lobes, and in the
+presence or absence of spines upon the tail, and had always been
+considered perfectly distinct species. Yet either was transformed into
+the other in a few generations, during which the saltness of the water
+was gradually altered. Yet more, A. salina was gradually accustomed to
+fresher water, and in the course of a few generations, when the water
+had become perfectly fresh, the species was changed into Branchipus
+stagnalis, which had always been considered to belong to a different
+genus on account of differences in the form of the antennae and of the
+posterior segments of the body (see Fig. 36). This certainly appears to
+be a proof of change of conditions producing a change of form
+independently of selection, and of that change of form, while remaining
+under the same conditions, being inherited. Yet there is this
+peculiarity in the case, that there is a chemical change in the water,
+and that this water permeates the whole body, and must be absorbed by
+the tissues, and thus affect the ova and even the reproductive
+elements, and in this way may profoundly modify the whole organisation.
+Why and how the external effects are limited to special details of the
+structure we do not know; but it does not seem as if any far-reaching
+conclusions as to the cumulative effect of external conditions on the
+higher terrestrial animals and plants, can be drawn from such an
+exceptional phenomenon. It seems rather analogous to those effects of
+external influences on the very lowest organisms in which the vegetative
+and reproductive organs are hardly differentiated, in which case such
+effects are doubtless inherited.[208]
+
+[Illustration: FIG. 36. _a._ Branchipus stagnalis. _b._ Artemia salina.]
+
+
+_Professor Geddes's Theory of Variation in Plants._
+
+In a paper read before the Edinburgh Botanical Society in 1886 Mr.
+Patrick Geddes laid down the outlines of a fundamental theory of plant
+variation, which he has further extended in the article "Variation and
+Selection" in the _Encydopaedia Britannica_, and in a paper read before
+the Linnaean Society but not yet published.
+
+A theory of variation should deal alike with the origin of specific
+distinctions and with those vaster differences which characterise the
+larger groups, and he thinks it should answer such questions as--How an
+axis comes to be arrested to form a flower? how the various forms of
+inflorescence were evolved? how did perigynous or epigynous flowers
+arise from hypogynous flowers? and many others equally fundamental.
+Natural selection acting upon numerous accidental variations will not,
+he urges, account for such general facts as these, which must depend on
+some constant law of variation. This law he believes to be the
+well-known antagonism of vegetative and reproductive growth acting
+throughout the whole course of plant development; and he uses it to
+explain many of the most characteristic features of the structure of
+flowers and fruits.
+
+Commencing with the origin of the flower, which all botanists agree in
+regarding as a shortened branch, he explains this shortening as an
+inevitable physiological fact, since the cost of the development of the
+reproductive elements is so great as necessarily to check vegetative
+growth. In the same manner the shortening of the inflorescence from
+raceme to spike or umbel, and thence to the capitulum or dense
+flower-head of the composite plants is brought about. This shortening,
+carried still further, produces the flattened leaf-like receptacle of
+Dorstenia, and further still the deeply hollowed fruity receptacle of
+the fig.
+
+The flower itself undergoes a parallel modification due to a similar
+cause. It is formed by a series of modified leaves arranged round a
+shortened axis. In its earlier stages the number of these modified
+leaves is indefinite, as in many Ranunculaceae; and the axis itself is
+not greatly shortened, as in Myosurus. The first advance is to a
+definite number of parts and a permanently shortened axis, in the
+arrangement termed hypogynous, in which all the whorls are quite
+distinct from each other. In the next stage there is a further
+shortening of the central axis, leaving the outer portion as a ring on
+which the petals are inserted, producing the arrangement termed
+perigynous. A still further advance is made by the contraction of the
+axis, so as to leave the central part forming the ovary quite below the
+flower, which is then termed epigynous.
+
+These several modifications are said to be parallel and definite, and to
+be determined by the continuous checking of vegetation by reproduction
+along what is an absolute groove of progressive change. This being the
+case, the importance of natural selection is greatly diminished. Instead
+of selecting and accumulating spontaneous indefinite variations, its
+function is to retard them after the stage of maximum utility has been
+independently reached. The same simple conception is said to unlock
+innumerable problems of vegetable morphology, large and small alike. It
+explains the inevitable development of gymnosperm into angiosperm by the
+checked vegetative growth of the ovule-bearing leaf or carpel; while
+such minor adaptations as the splitting fruit of the geranium or the
+cupped stigma of the pansy, can be no longer looked upon as achievements
+of natural selection, but must be regarded as naturally traceable to
+the vegetative checking of their respective types of leaf organ. Again,
+a detailed examination of spiny plants practically excludes the
+hypothesis of mammalian selection altogether, and shows spines to arise
+as an expression of the diminishing vegetativeness--in fact, the ebbing
+vitality of a species.[209]
+
+
+_Objections to the Theory._
+
+The theory here sketched out is enticing, and at first sight seems
+calculated to throw much light on the history of plant development; but
+on further consideration, it seems wanting in definiteness, while it is
+beset with difficulties at every step. Take first the shortening of the
+raceme into the umbel and the capitulum, said to be caused by arrest of
+vegetative growth, due to the antagonism of reproduction. If this were
+the whole explanation of the phenomenon, we should expect the quantity
+of seed to increase as this vegetative growth diminished, since the seed
+is the product of the reproductive energy of the plant, and its quantity
+the best measure of that energy. But is this the case? The ranunculus
+has comparatively few seeds, and the flowers are not numerous; while in
+the same order the larkspur and the columbine have far more seeds as
+well as more flowers, but there is no shortening of the raceme or
+diminution of the foliage, although the flowers are large and complex.
+So, the extremely shortened and compressed flower-heads of the
+compositae produce comparatively few seeds--one only to each flower;
+while the foxglove, with its long spike of showy flowers, produces an
+enormous number.
+
+Again, if the shortening of the central axis in the successive stages of
+hypogynous, perigynous, and epigynous flowers were an indication of
+preponderant reproduction and diminished vegetation, we should find
+everywhere some clear indications of this fact. The plants with
+hypogynous flowers should, as a rule, have less seed and more vigorous
+and abundant foliage than those at the other extreme with epigynous
+flowers. But the hypogynous poppies, pinks, and St. John's worts have
+abundance of seed and rather scanty foliage; while the epigynous
+dogwoods and honeysuckles have few seeds and abundant foliage. If,
+instead of the number of the seeds, we take the size of the fruit as an
+indication of reproductive energy, we find this at a maximum in the
+gourd family, yet their rapid and luxuriant growth shows no diminution
+of vegetative power. So that the statement that plant modifications
+proceed "along an absolute groove of progressive change" is contradicted
+by innumerable facts indicating advance and regression, improvement or
+degradation, according as the ever-changing environment renders one form
+more advantageous than the other. As one instance I may mention the
+Anonaceae or custard-apple tribe, which are certainly an advance from
+the Ranunculaceae; yet in the genus Polyalthea the fruit consists of a
+number of separate carpels, each borne on a long stalk, as if reverting
+to the primitive stalked carpellary leaves.
+
+
+_On the Origin of Spines._
+
+But perhaps the most extraordinary application of the theory is that
+which considers spines to be an indication of the "ebbing vitality of a
+species," and which excludes "mammalian selection altogether." If this
+were true, spines should occur mainly in feeble, rare, and dying-out
+species, instead of which we have the hawthorn, one of our most vigorous
+shrubs or trees, with abundant vitality and an extensive range over the
+whole Palaearctic region, showing that it is really a dominant species.
+In North America the numerous thorny species of Crataegus are equally
+vigorous, as are the false acacia (Robinia) and the honey-locust
+(Gleditschia). Neither have the numerous species of very spiny Acacias
+been noticed to be rarer or less vigorous than the unarmed kinds.
+
+On the other point--that spines are not due to mammalian selection--we
+are able to adduce what must be considered direct and conclusive
+evidence. For if spines, admittedly produced by aborted branches,
+petioles, or peduncles, are due solely or mainly to diminished
+vegetativeness or ebbing vitality, they ought to occur in all countries
+alike, or at all events in all whose similar conditions tend to check
+vegetation; whereas, if they are, solely or mainly, developed as a
+protection against the attacks of herbivorous mammals, they ought to be
+most abundant where these are plentiful, and rare or absent where
+indigenous mammalia are wanting. Oceanic islands, as compared with
+continents, would thus furnish a crucial test of the two theories; and
+Mr. Hemsley of Kew, who has specially studied insular floras, has given
+me some valuable information on this point. He says: "There are no spiny
+or prickly plants in the indigenous element of the St. Helena flora. The
+relatively rich flora of the Sandwich Isles is not absolutely without a
+prickly plant, but almost so. All the endemic genera are unarmed, and
+the endemic species of almost every other genus. Even such genera as
+Zanthoxylon, Acacia, Xylosoma, Lycium, and Solanum, of which there are
+many armed species in other countries, are only represented by unarmed
+species. The two endemic Rubi have the prickles reduced to the setaceous
+condition, and the two palms are unarmed.
+
+"The flora of the Galapagos includes a number of prickly plants, among
+them several cacti (these have not been investigated and may be American
+species), but I do not think one of the known endemic species of any
+family is prickly or spiny.
+
+"Spiny and prickly plants are also rare in New Zealand, but there are
+the formidably armed species of wild Spaniard (Aciphylla), one species
+of Rubus, the pungent-leaved Epacrideae and a few others."
+
+Mr. J.G. Baker of Kew, who has specially studied the flora of Mauritius
+and the adjacent islands, also writes me on this point. He says: "Taking
+Mauritius alone, I do not call to mind a single species that is a
+spinose endemic tree or shrub. If you take the whole group of islands
+(Mauritius, Bourbon, Seychelles, and Rodriguez), there will be about a
+dozen species, but then nine of these are palms. Leaving out palms, the
+trees and shrubs of that part of the world are exceptionally
+non-spinose."
+
+These are certainly remarkable facts, and quite inexplicable on the
+theory of spines being caused solely by checked vegetative growth, due
+to weakness of constitution or to an arid soil and climate. For the
+Galapagos and many parts of the Sandwich Islands are very arid, as is a
+considerable part of the North Island of New Zealand. Yet in our own
+moist climate and with our very limited number of trees and shrubs we
+have about eighteen spiny or prickly species, more, apparently, than in
+the whole endemic floras of the Mauritius, Sandwich Islands, and
+Galapagos, though these are all especially rich in shrubby and arboreal
+species. In New Zealand the prickly Rubus is a leafless trailing plant,
+and its prickles are probably a protection against the large snails of
+the country, several of which have shells from two to three and a half
+inches long.[210] The "wild Spaniards" are very spiny herbaceous
+Umbelliferae, and may have gained their spines to preserve them from
+being trodden down or eaten by the Moas, which, for countless ages, took
+the place of mammals in New Zealand. The exact use or meaning of the
+spines in palms is more doubtful, though they are, no doubt, protective
+against some animals; but it is certainly an extraordinary fact that in
+the entire flora of the Mauritius, so largely consisting of trees and
+shrubs, not a single endemic species should be thorny or spiny.
+
+If now we consider that every continental flora produces a considerable
+proportion of spiny and thorny species, and that these rise to a maximum
+in South Africa, where herbivorous mammalia were (before the settlement
+of the country), perhaps, more abundant and varied than in any other
+part of the world; while another district, remarkable for well-armed
+vegetation, is Chile, where the camel-like vicugnas, llamas, and
+alpacas, and an abundance of large rodents wage perpetual war against
+shrubby vegetation, we shall see the full significance of the almost
+total absence of thorny and spiny plants in the chief oceanic islands;
+and so far from "excluding the hypothesis of mammalian selection
+altogether," we shall find in this hypothesis the only satisfactory
+explanation of the facts.
+
+From the brief consideration of Professor Geddes's theory now given, we
+conclude that, although the antagonism between vegetative and
+reproductive growth is a real agency, and must be taken account of in
+our endeavour to explain many of the fundamental facts in the structure
+and form of plants, yet it is so overpowered and directed at every step
+by the natural selection of favourable variations, that the results of
+its exclusive and unmodified action are nowhere to be found in nature.
+It may be allowed to rank as one of those "laws of growth," of which so
+many have now been indicated, and which were always recognised by Darwin
+as underlying all variation; but unless we bear in mind that its action
+must always be subordinated to natural selection, and that it is
+continually checked, or diverted, or even reversed by the necessity of
+adaptation to the environment, we shall be liable to fall into such
+glaring errors as the imputing to "ebbing vitality" alone such a
+widespread phenomenon as the occurrence of spines and thorns, while
+ignoring altogether the influence of the organic environment in their
+production.[211]
+
+The sketch now given of the chief attempts that have been made to prove
+that either the direct action of the environment or certain fundamental
+laws of variation are independent causes of modification of species,
+shows us that their authors have, in every case, failed to establish
+their contention. Any direct action of the environment, or any
+characters acquired by use or disuse, can have no effect whatever upon
+the race unless they are inherited; and that they are inherited in any
+case, except when they directly affect the reproductive cells, has not
+been proved. On the other hand, as we shall presently show, there is
+much reason for believing that such acquired characters are in their
+nature non-heritable.
+
+
+_Variation and Selection Overpower the Effects of Use and Disuse._
+
+But there is another objection to this theory arising from the very
+nature of the effects produced. In each generation the effects of use or
+disuse, or of effort, will certainly be very small, while of this small
+effect it is not maintained that the whole will be always inherited by
+the next generation. How small the effect is we have no means of
+determining, except in the case of disuse, which Mr. Darwin investigated
+carefully. He found that in twelve fancy breeds of pigeons, which are
+often kept in aviaries, or if free fly but little, the sternum had been
+reduced by about one-seventh or one-eighth of its entire length, and
+that of the scapula about one-ninth. In domestic ducks the weight of the
+wing-bones in proportion to that of the whole skeleton had decreased
+about one-tenth. In domestic rabbits the bones of the legs were found to
+have increased in weight in due proportion to the increased weight of
+the body, but those of the hind legs were rather less in proportion to
+those of the fore legs than in the wild animal, a difference which may
+be imputed to their being less used in rapid motion. The pigeons,
+therefore, afford the greatest amount of reduction by
+disuse--one-seventh of the length of the sternum. But the pigeon has
+certainly been domesticated four or five thousand years; and if the
+reduction of the wings by disuse has only been going on for the last
+thousand years, the amount of reduction in each generation would be
+absolutely imperceptible, and quite within the limits of the reduction
+due to the absence of selection, as already explained. But, as we have
+seen in Chapter III, the fortuitous variation of every part or organ
+usually amounts to one-tenth, and often to one-sixth of the average
+dimensions--that is, the fortuitous variation in one generation among a
+limited number of the individuals of a species is as great as the
+cumulative effects of disuse in a thousand generations! If we assume
+that the effects of use or of effort in the individual are equal to the
+effects of disuse, or even ten or a hundred times greater, they will
+even then not equal, in each generation, the amount of the fortuitous
+variations of the same part. If it be urged that the effects of use
+would modify all the individuals of a species, while the fortuitous
+variations to the amount named only apply to a portion of them, it may
+be replied, that that portion is sufficiently large to afford ample
+materials for selection, since it often equals the numbers that can
+annually survive; while the recurrence in each successive generation of
+a like amount of variation would render possible such a rapid adjustment
+to new conditions that the effects of use or disuse would be as nothing
+in comparison. It follows, that even admitting the modifying effects of
+the environment, and that such modifications are inherited, they would
+yet be entirely swamped by the greater effects of fortuitous variation,
+and the far more rapid cumulative results of the selection of such
+variations.
+
+
+_Supposed Action of the Environment in Initiating Variations._
+
+It is, however, urged that the reaction of the environment initiates
+variations, which without it would never arise; such, for instance, as
+the origin of horns through the pressures and irritations caused by
+butting, or otherwise using the head as a weapon or for defence.
+Admitting, for the sake of argument, that this is so, all the evidence
+we possess shows that, from the very first appearance of the rudiment of
+such an organ, it would vary to a greater extent than the amount of
+growth directly produced by use; and these variations would be subject
+to selection, and would thus modify the organ in ways which use alone
+would never bring about. We have seen that this has been the case with
+the branching antlers of the stag, which have been modified by
+selection, so as to become useful in other ways than as a mere weapon;
+and the same has almost certainly been the case with the variously
+curved and twisted horns of antelopes. In like manner, every conceivable
+rudiment would, from its first appearance, be subject to the law of
+variation and selection, to which, thenceforth, the direct effect of the
+environment would be altogether subordinate.
+
+A very similar mode of reasoning will apply to the other branch of the
+subject--the initiation of structures and organs by the action of the
+fundamental laws of growth. Admitting that such laws have determined
+some of the main divisions of the animal and vegetable kingdom, have
+originated certain important organs, and have been the fundamental cause
+of certain lines of development, yet at every step of the process these
+laws must have acted in entire subordination to the law of natural
+selection. No modification thus initiated could have advanced a single
+step, unless it were, on the whole, a useful modification; while its
+entire future course would be necessarily subject to the laws of
+variation and selection, by which it would be sometimes checked,
+sometimes hastened on, sometimes diverted to one purpose, sometimes to
+another, according as the needs of the organism, under the special
+conditions of its existence, required such modification. We need not
+deny that such laws and influences may have acted in the manner
+suggested, but what we do deny is that they could possibly escape from
+the ever-present and all-powerful modifying effects of variation and
+natural selection.[212]
+
+
+_Weismann's Theory of Heredity._
+
+Professor August Weismann has put forth a new theory of heredity founded
+upon the "continuity of the germ-plasm," one of the logical consequences
+of which is, that acquired characters of whatever kind are not
+transmitted from parent to offspring. As this is a matter of vital
+importance to the theory of natural selection, and as, if well founded,
+it strikes away the foundations of most of the theories discussed in the
+present chapter, a brief outline of Weismann's views must be attempted,
+although it is very difficult to make them intelligible to persons
+unfamiliar with the main facts of modern embryology.[213]
+
+The problem is thus stated by Weismann: "How is it that in the case of
+all higher animals and plants a single cell is able to separate itself
+from amongst the millions of most various kinds of which an organism is
+composed, and by division and complicated differentiation to reconstruct
+a new individual with marvellous likeness, unchanged in many cases even
+throughout whole geological periods?" Darwin attempted to solve the
+problem by his theory of "Pangenesis," which supposed that every
+individual cell in the body gave off gemmules or germs capable of
+reproducing themselves, and that portions of these germs of each of the
+almost infinite number of cells permeate the whole body and become
+collected in the generative cells, and are thus able to reproduce the
+whole organism. This theory is felt to be so ponderously complex and
+difficult that it has met with no general acceptance among
+physiologists.
+
+The fact that the germ-cells _do_ reproduce with wonderful accuracy not
+only the general characters of the species, but many of the individual
+characteristics of the parents or more remote ancestors, and that this
+process is continued from generation to generation, can be accounted
+for, Weismann thinks, only on two suppositions which are physiologically
+possible. Either the substance of the parent germ-cell, after passing
+through a cycle of changes required for the construction of a new
+individual, possesses the capability of producing anew germ-cells
+identical with those from which that individual was developed, or _the
+new germ-cells arise, as far as their essential and characteristic
+substance is concerned, not at all out of the body of the individual,
+but direct from the parent germ-cell_. This latter view Weismann holds
+to be the correct one, and, on this theory, heredity depends on the fact
+that a substance of special molecular composition passes over from one
+generation to another. This is the "germ-plasm," the power of which to
+develop itself into a perfect organism depends on the extraordinary
+complication of its minutest structure. At every new birth a portion of
+the specific germ-plasm, which the parent egg-cell contains, is not used
+up in producing the offspring, but is reserved unchanged to produce the
+germ-cells of the following generation. Thus the germ-cells--so far as
+regards their essential part the germ-plasm--are not a product of the
+body itself, but are related to one another in the same way as are a
+series of generations of unicellular organisms derived from one another
+by a continuous course of simple division. Thus the question of heredity
+is reduced to one of growth. A minute portion of the very same
+germ-plasm from which, first the germ-cell, and then the whole organism
+of the parent, were developed, becomes the starting-point of the growth
+of the child.
+
+
+_The Cause of Variation._
+
+But if this were all, the offspring would reproduce the parent exactly,
+in every detail of form and structure; and here we see the importance of
+sex, for each new germ grows out of the united germ-plasms of two
+parents, whence arises a mingling of their characters in the offspring.
+This occurs in each generation; hence every individual is a complex
+result reproducing in ever-varying degrees the diverse characteristics
+of his two parents, four grandparents, eight great-grandparents, and
+other more remote ancestors; and that ever-present individual variation
+arises which furnishes the material for natural selection to act upon.
+Diversity of sex becomes, therefore, of primary importance as _the cause
+of variation_. Where asexual generation prevails, the characteristics of
+the individual alone are reproduced, and there are thus no means of
+effecting the change of form or structure required by changed conditions
+of existence. Under such changed conditions a complex organism, if only
+asexually propagated, would become extinct. But when a complex organism
+is sexually propagated, there is an ever-present cause of change which,
+though slight in any one generation, is cumulative, and under the
+influence of selection is sufficient to keep up the harmony between the
+organism and its slowly changing environment.[214]
+
+
+_The Non-Heredity of Acquired Characters._
+
+Certain observations on the embryology of the lower animals are held to
+afford direct proof of this theory of heredity, but they are too
+technical to be made clear to ordinary readers. A logical result of the
+theory is the impossibility of the transmission of acquired characters,
+since the molecular structure of the germ-plasm is already determined
+within the embryo; and Weismann holds that there are no facts which
+really prove that acquired characters can be inherited, although their
+inheritance has, by most writers, been considered so probable as hardly
+to stand in need of direct proof.
+
+We have already shown, in the earlier part of this chapter, that many
+instances of change, imputed to the inheritance of acquired variations,
+are really cases of selection; while the very fact that _use_ implies
+_usefulness_ renders it almost impossible to eliminate the action of
+selection in a state of nature. As regards mutilations, it is generally
+admitted that they are not hereditary, and there is ample evidence on
+this point. When it was the fashion to dock horses' tails, it was not
+found that horses were born with short tails; nor are Chinese women born
+with distorted feet; nor are any of the numerous forms of racial
+mutilation in man, which have in some cases been carried on for hundreds
+of generations, inherited. Nevertheless, a few cases of apparent
+inheritance of mutilations have been recorded,[215] and these, if
+trustworthy, are difficulties in the way of the theory. The undoubted
+inheritance of disease is hardly a difficulty, because the
+predisposition to disease is a congenital, not an acquired character,
+and as such would be the subject of inheritance. The often-quoted case
+of a disease induced by mutilation being inherited (Brown-Sequard's
+epileptic guinea-pigs) has been discussed by Professor Weismann, and
+shown to be not conclusive. The mutilation itself--a section of certain
+nerves--was never inherited, but the resulting epilepsy, or a general
+state of weakness, deformity, or sores, was sometimes inherited. It is,
+however, possible that the mere injury introduced and encouraged the
+growth of certain microbes, which, spreading through the organism,
+sometimes reached the germ-cells, and thus transmitted a diseased
+condition to the offspring. Such a transference of microbes is believed
+to occur in syphilis and tuberculosis, and has been ascertained to occur
+in the case of the muscardine silkworm disease.[216]
+
+
+_The Theory of Instinct._
+
+The theory now briefly outlined cannot be said to be proved, but it
+commends itself to many physiologists as being inherently probable, and
+as furnishing a good working hypothesis till displaced by a better. We
+cannot, therefore, accept any arguments against the agency of natural
+selection which are based upon the opposite and equally unproved theory
+that acquired characters are inherited; and as this applies to the whole
+school of what may be termed Neo-Lamarckians, their speculations cease
+to have any weight.
+
+The same remark applies to the popular theory of instincts as being
+inherited habits; though Darwin gave very little weight to this, but
+derived almost all instincts from spontaneous useful variations which,
+like other spontaneous variations, are of course inherited. At first
+sight it appears as if the acquired habits of our trained
+dogs--pointers, retrievers, etc.--are certainly inherited; but this need
+not be the case, because there must be some structural or psychical
+peculiarities, such as modifications in the attachments of muscles,
+increased delicacy of smell or sight, or peculiar likes and dislikes,
+which are inherited; and from these, peculiar habits follow as a natural
+consequence, or are easily acquired. Now, as selection has been
+constantly at work in improving all our domestic animals, we have
+unconsciously modified the structure, while preserving only those
+animals which best served our purpose in their peculiar faculties,
+instincts, or habits.
+
+Much of the mystery of instinct arises from the persistent refusal to
+recognise the agency of imitation, memory, observation, and reason as
+often forming part of it. Yet there is ample evidence that such agency
+must be taken into account. Both Wilson and Leroy state that young birds
+build inferior nests to old ones, and the latter author observes that
+the best nests are made by birds whose young remain longest in the nest.
+So, migration is now well ascertained to be effected by means of vision,
+long flights being made on bright moonlight nights when the birds fly
+very high, while on cloudy nights they fly low, and then often lose
+their way. Thousands annually fly out to sea and perish, showing that
+the instinct to migrate is imperfect, and is not a good substitute for
+reason and observation.
+
+Again, much of the perfection of instinct is due to the extreme severity
+of the selection during its development, any failure involving
+destruction. The chick which cannot break the eggshell, the caterpillar
+that fails to suspend itself properly or to spin a safe cocoon, the bees
+that lose their way or that fail to store honey, inevitably perish. So
+the birds that fail to feed and protect their young, or the butterflies
+that lay their eggs on the wrong food-plant, leave no offspring, and the
+race with imperfect instincts perishes. Now, during the long and very
+slow course of development of each organism, this rigid selection at
+every step of progress has led to the preservation of every detail of
+structure, faculty, or habit that has been necessary for the
+preservation of the race, and has thus gradually built up the various
+instincts which seem so marvellous to us, but which can yet be shown to
+be in many cases still imperfect. Here, as everywhere else in nature, we
+find comparative, not absolute perfection, with every gradation from
+what is clearly due to imitation or reason up to what seems to us
+perfect instinct--that in which a complex action is performed without
+any previous experience or instruction.[217]
+
+
+_Concluding Remarks._
+
+Having now passed in review the more important of the recent objections
+to, or criticisms of, the theory of natural selection, we have arrived
+at the conclusion that in no one case have the writers in question been
+able materially to diminish its importance, or to show that any of the
+laws or forces to which they appeal can act otherwise than in strict
+subordination to it. The direct action of the environment as set forth
+by Mr. Herbert Spencer, Dr. Cope, and Dr. Karl Semper, even if we admit
+that its effects on the individual are transmitted by inheritance, are
+so small in comparison with the amount of spontaneous variation of every
+part of the organism that they must be quite overshadowed by the latter.
+And if such direct action may, in some cases, have initiated certain
+organs or outgrowths, these must from their very first beginnings have
+been subject to variation and natural selection, and their further
+development have been almost wholly due to these ever-present and
+powerful causes. The same remark applies to the views of Professor
+Geddes on the laws of growth which have determined certain essential
+features in the morphology of plants and animals. The attempt to
+substitute these laws for those of variation and natural selection has
+failed in cases where we can apply a definite test, as in that of the
+origin of spines on trees and shrubs; while the extreme diversity of
+vegetable structure and form among the plants of the same country and of
+the same natural order, of itself affords a proof of the preponderating
+influence of variation and natural selection in keeping the many diverse
+forms in harmony with the highly complex and ever-changing environment.
+
+Lastly, we have seen that Professor Weismann's theory of the continuity
+of the germ-plasm and the consequent non-heredity of acquired
+characters, while in perfect harmony with all the well-ascertained facts
+of heredity and development, adds greatly to the importance of natural
+selection as the one invariable and ever-present factor in all organic
+change, and that which can alone have produced the temporary fixity
+combined with the secular modification of species. While admitting, as
+Darwin always admitted, the co-operation of the fundamental laws of
+growth and variation, of correlation and heredity, in determining the
+direction of lines of variation or in the initiation of peculiar organs,
+we find that variation and natural selection are ever-present agencies,
+which take possession, as it were, of every minute change originated by
+these fundamental causes, check or favour their further development, or
+modify them in countless varied ways according to the varying needs of
+the organism. Whatever other causes have been at work, Natural Selection
+is supreme, to an extent which even Darwin himself hesitated to claim
+for it. The more we study it the more we are convinced of its
+overpowering importance, and the more confidently we claim, in Darwin's
+own words, that it "has been the most important, but not the exclusive,
+means of modification."
+
+FOOTNOTES:
+
+[Footnote 198: See the Duke of Argyll's letter in _Nature_, vol. xxxiv.
+p. 336.]
+
+[Footnote 199: _Journal of the Anthropological Institute,_ vol. xv. pp.
+246-260.]
+
+[Footnote 200: The idea of the non-heredity of acquired variations was
+suggested by the summary of Professor Weismann's views, in _Nature_,
+referred to later on. But since this chapter was written I have, through
+the kindness of Mr. E.B. Poulton, seen some of the proofs of the
+forthcoming translation of Weismann's Essays on Heredity, in which he
+sets forth an explanation very similar to that here given. On the
+difficult question of the almost entire disappearance of organs, as in
+the limbs of snakes and of some lizards, he adduces "a certain form of
+correlation, which Roux calls 'the struggle of the parts in the
+organism,'" as playing an important part. Atrophy following disuse is
+nearly always attended by the corresponding increase of other organs:
+blind animals possess more developed organs of touch, hearing, and
+smell; the loss of power in the wings is accompanied by increased
+strength of the legs, etc. Now as these latter characters, being useful,
+will be selected, it is easy to understand that a congenital increase of
+these will be accompanied by a corresponding congenital diminution of
+the unused organ; and in cases where the means of nutrition are
+deficient, every diminution of these useless parts will be a gain to the
+whole organism, and thus their complete disappearance will, in some
+cases, be brought about directly by natural selection. This corresponds
+with what we know of these rudimentary organs.
+
+It must, however, be pointed out that the non-heredity of acquired
+characters was maintained by Mr. Francis Galton more than twelve years
+ago, on theoretical considerations almost identical with those urged by
+Professor Weismann; while the insufficiency of the evidence for their
+hereditary transmission was shown, by similar arguments to those used
+above and in the work of Professor Weismann already referred to (see "A
+Theory of Heredity," in _Journ. Anthrop. Instit._, vol. v. pp.
+343-345).]
+
+[Footnote 201: This explanation is derived from Weismann's Theory of the
+Continuity of the Germ-Plasm as summarised in _Nature_.]
+
+[Footnote 202: See a collection of his essays under the title, _The
+Origin of the Fittest: Essays on Evolution_, D. Appleton and Co. New
+York. 1887.]
+
+[Footnote 203: _Origin of the Fittest_, p. 174.]
+
+[Footnote 204: _Ibid._ p. 29. It may be here noted that Darwin found
+these theories unintelligible. In a letter to Professor E.T. Morse in
+1877, he writes: "There is one point which I regret you did not make
+clear in your Address, namely, what is the meaning and importance of
+Professors Cope and Hyatt's views on acceleration and retardation? I
+have endeavoured, and given up in despair, the attempt to grasp their
+meaning" (_Life and Letters_, vol. iii. p. 233).]
+
+[Footnote 205: _Origin of the Fittest_, p. 374.]
+
+[Footnote 206: _Origin of the Fittest_, p. 40.]
+
+[Footnote 207: _The Natural Conditions of Existence as they Affect
+Animal Life._ London, 1883.]
+
+[Footnote 208: In Dr. Weismann's essay on "Heredity," already referred
+to, he considers it not improbable that changes in organisms produced by
+climatic influences may be inherited, because, as these changes do not
+affect the external parts of an organism only, but often, as in the case
+of warmth or moisture permeate the whole structure, they may possibly
+modify the germ-plasm itself, and thus induce variations in the next
+generation. In this way, he thinks, may possibly be explained the
+climatic varieties of certain butterflies, and some other changes which
+seem to be effected by change of climate in a few generations.]
+
+[Footnote 209: This brief indication of Professor Geddes's views is
+taken from the article "Variation and Selection" in the _Encyclopedia
+Britannica_, and a paper "On the Nature and Causes of Variation in
+Plants" in _Trans. and Proc. of the Edinburgh Botanical Society_, 1886;
+and is, for the most part, expressed in his own words.]
+
+[Footnote 210: Placostylis bovinus, 3½ inches long; Paryphanta Busbyi, 3
+in. diam.; P. Hochstetteri, 2¾ in. diam.]
+
+[Footnote 211: The general arguments and objections here set forth will
+apply with equal force to Professor G. Henslow's theory of the origin of
+the various forms and structures of flowers as due to "the responsive
+actions of the protoplasm in consequence of the irritations set up by
+the weights, pressures, thrusts, tensions, etc., of the insect visitors"
+(_The Origin of Floral Structures through Insect and other Agencies_, p.
+340). On the assumption that acquired characters are inherited, such
+irritations may have had something to do with the initiation of
+variations and with the production of certain details of structure, but
+they are clearly incompetent to have brought about the more important
+structural and functional modifications of flowers. Such are, the
+various adjustments of length and position of the stamens to bring the
+pollen to the insect and from the insect to the stigma; the various
+motions of stamens and styles at the right time and the right direction;
+the physiological adjustments bringing about fertility or sterility in
+heterostyled plants; the traps, springs, and complex movements of
+various parts of orchids; and innumerable other remarkable phenomena.
+
+For the explanation of these we have no resource but variation and
+selection, to the effects of which, acting alternately with regression
+or degradation as above explained (p. 328) must be imputed the
+development of the countless floral structures we now behold. Even the
+primitive flowers, whose initiation may, perhaps, have been caused, or
+rendered possible, by the irritation set up by insects' visits, must,
+from their very origin, have been modified, in accordance with the
+supreme law of utility, by means of variation and survival of the
+fittest.]
+
+[Footnote 212: In an essay on "The Duration of Life," forming part of
+the translation of Dr. Weismann's papers already referred to, the author
+still further extends the sphere of natural selection by showing that
+the average duration of life in each species has been determined by it.
+A certain length of life is essential in order that the species may
+produce offspring sufficient to ensure its continuance under the most
+unfavourable conditions; and it is shown that the remarkable
+inequalities of longevity in different species and groups may be thus
+accounted for. Yet more, the occurrence of death in the higher
+organisms, in place of the continued survival of the unicellular
+organisms however much they may increase by subdivision, may be traced
+to the same great law of utility for the race and survival of the
+fittest. The whole essay is of exceeding interest, and will repay a
+careful perusal. A similar idea occurred to the present writer about
+twenty years back, and was briefly noted down at the time, but
+subsequently forgotten.]
+
+[Footnote 213: The outline here given is derived from two articles in
+_Nature_, vol. xxxiii. p. 154, and vol. xxxiv. p. 629, in which
+Weismann's papers are summarised and partly translated.]
+
+[Footnote 214: There are many indications that this explanation of the
+cause of variation is the true one. Mr. E.B. Poulton suggests one, in
+the fact that parthenogenetic reproduction only occurs in isolated
+species, not in groups of related species; as this shows that
+parthenogenesis cannot lead to the evolution of new forms. Again, in
+parthenogenetic females the complete apparatus for fertilisation remains
+unreduced; but if these varied as do sexually produced animals, the
+organs referred to, being unused, would become rudimentary.
+
+Even more important is the significance of the "polar bodies," as
+explained by Weismann in one of his _Essays_; since, if his
+interpretation of them be correct, variability is a necessary
+consequence of sexual generation.]
+
+[Footnote 215: Darwin's _Animals and Plants_, vol. ii. pp. 23, 24.]
+
+[Footnote 216: In his essay on "Heredity," Dr. Weismann discusses many
+other cases of supposed inheritance of acquired characters, and shows
+that they can all be explained in other ways. Shortsightedness among
+civilised nations, for example, is due partly to the absence of
+selection and consequent regression towards a mean, and partly to its
+individual production by constant reading.]
+
+[Footnote 217: Weismann explains instinct on similar lines, and gives
+many interesting illustrations (see _Essays on Heredity_). He holds
+"that all instinct is entirely due to the operation of natural
+selection, and has its foundation, not upon inherited experiences, but
+upon variations of the germ." Many interesting and difficult cases of
+instinct are discussed by Darwin in Chapter VIII of the _Origin of
+Species_, which should be read in connection with the above remarks.
+
+Since this chapter was written my attention has been directed to Mr.
+Francis Galton's _Theory of Heredity_ (already referred to at p. 417)
+which was published thirteen years ago as an alternative for Darwin's
+theory of pangenesis.
+
+Mr. Galton's theory, although it attracted little attention, appears to
+me to be substantially the same as that of Professor Weismann. Galton's
+"stirp" is Weismann's "germ-plasm." Galton supposes the sexual elements
+in the offspring to be directly formed from the residue of the _stirp_
+not used up in the development of the body of the parent--Weismann's
+"continuity of the germ-plasm." Galton also draws many of the same
+conclusions from his theory. He maintains that characters acquired by
+the individual as the result of external influences cannot be inherited,
+unless such influences act directly on the reproductive
+elements--instancing the possible heredity of alcoholism, because the
+alcohol permeates the tissues and may reach the sexual elements. He
+discusses the supposed heredity of effects produced by use or disuse,
+and explains them much in the same manner as does Weismann. Galton is an
+anthropologist, and applies the theory, mainly, to explain the
+peculiarities of hereditary transmission in man, many of which
+peculiarities he discusses and elucidates. Weismann is a biologist, and
+is mostly concerned with the application of the theory to explain
+variation and instinct, and to the further development of the theory of
+evolution. He has worked it out more thoroughly, and has adduced
+embryological evidence in its support; but the views of both writers are
+substantially the same, and their theories were arrived at quite
+independently. The names of Galton and Weismann should therefore be
+associated as discoverers of what may be considered (if finally
+established) the most important contribution to the evolution theory
+since the appearance of the _Origin of Species_.]
+
+
+
+
+CHAPTER XV
+
+DARWINISM APPLIED TO MAN
+
+
+    General identity of human and animal structure--Rudiments and
+    variations showing relation of man to other mammals--The
+    embryonic development of man and other mammalia--Diseases common
+    to man and the lower animals--The animals most nearly allied to
+    man--The brains of man and apes--External differences of man and
+    apes--Summary of the animal characteristics of man--The
+    geological antiquity of man--The probable birthplace of man--The
+    origin of the moral and intellectual nature of man--The argument
+    from continuity--The origin of the mathematical faculty--The
+    origin of the musical and artistic faculties--Independent proof
+    that these faculties have not been developed by natural
+    selection--The interpretation of the facts--Concluding remarks.
+
+
+
+Our review of modern Darwinism might fitly have terminated with the
+preceding chapter; but the immense interest that attaches to the origin
+of the human race, and the amount of misconception which prevails
+regarding the essential teachings of Darwin's theory on this question,
+as well as regarding my own special views upon it, induce me to devote a
+final chapter to its discussion.
+
+To any one who considers the structure of man's body, even in the most
+superficial manner, it must be evident that it is the body of an animal,
+differing greatly, it is true, from the bodies of all other animals, but
+agreeing with them in all essential features. The bony structure of man
+classes him as a vertebrate; the mode of suckling his young classes him
+as a mammal; his blood, his muscles, and his nerves, the structure of
+his heart with its veins and arteries, his lungs and his whole
+respiratory and circulatory systems, all closely correspond to those of
+other mammals, and are often almost identical with them. He possesses
+the same number of limbs terminating in the same number of digits as
+belong fundamentally to the mammalian class. His senses are identical
+with theirs, and his organs of sense are the same in number and occupy
+the same relative position. Every detail of structure which is common to
+the mammalia as a class is found also in man, while he only differs from
+them in such ways and degrees as the various species or groups of
+mammals differ from each other. If, then, we have good reason to believe
+that every existing group of mammalia has descended from some common
+ancestral form--as we saw to be so completely demonstrated in the case
+of the horse tribe,--and that each family, each order, and even the
+whole class must similarly have descended from some much more ancient
+and more generalised type, it would be in the highest degree
+improbable--so improbable as to be almost inconceivable--that man,
+agreeing with them so closely in every detail of his structure, should
+have had some quite distinct mode of origin. Let us, then, see what
+other evidence bears upon the question, and whether it is sufficient to
+convert the probability of his animal origin into a practical certainty.
+
+
+_Rudiments and Variations as Indicating the Relation of Man to other
+Mammals._
+
+All the higher animals present rudiments of organs which, though useless
+to them, are useful in some allied group, and are believed to have
+descended from a common ancestor in which they were useful. Thus there
+are in ruminants rudiments of incisor teeth which, in some species,
+never cut through the gums; many lizards have external rudimentary legs;
+while many birds, as the Apteryx, have quite rudimentary wings. Now man
+possesses similar rudiments, sometimes constantly, sometimes only
+occasionally present, which serve intimately to connect his bodily
+structure with that of the lower animals. Many animals, for example,
+have a special muscle for moving or twitching the skin. In man there are
+remnants of this in certain parts of the body, especially in the
+forehead, enabling us to raise our eyebrows; but some persons have it in
+other parts. A few persons are able to move the whole scalp so as to
+throw off any object placed on the head, and this property has been
+proved, in one case, to be inherited. In the outer fold of the ear there
+is sometimes a projecting point, corresponding in position to the
+pointed ear of many animals, and believed to be a rudiment of it. In the
+alimentary canal there is a rudiment--the vermiform appendage of the
+caecum--which is not only useless, but is sometimes a cause of disease
+and death in man; yet in many vegetable feeding animals it is very long,
+and even in the orang-utan it is of considerable length and convoluted.
+So, man possesses rudimentary bones of a tail concealed beneath the
+skin, and, in some rare cases, this forms a minute external tail.
+
+The variability of every part of man's structure is very great, and many
+of these variations tend to approximate towards the structure of other
+animals. The courses of the arteries are eminently variable, so that for
+surgical purposes it has been necessary to determine the probable
+proportion of each variation. The muscles are so variable that in fifty
+cases the muscles of the foot were found to be not strictly alike in any
+two, and in some the deviations were considerable; while in thirty-six
+subjects Mr. J. Wood observed no fewer than 558 muscular variations. The
+same author states that in a single male subject there were no fewer
+than seven muscular variations, all of which plainly represented muscles
+proper to various kinds of apes. The muscles of the hands and
+arms--parts which are so eminently characteristic of man--are extremely
+liable to vary, so as to resemble the corresponding muscles of the lower
+animals. That such variations are due to reversion to a former state of
+existence Mr. Darwin thinks highly probable, and he adds: "It is quite
+incredible that a man should, through mere accident, abnormally resemble
+certain apes in no less than seven of his muscles, if there had been no
+genetic connection between them. On the other hand, if man is descended
+from some ape-like creature, no valid reason can be assigned why certain
+muscles should not suddenly reappear after an interval of many thousand
+generations, in the same manner as, with horses, asses, and mules, dark
+coloured stripes suddenly reappear on the legs and shoulders, after an
+interval of hundreds, or more probably of thousands of
+generations."[218]
+
+
+_The Embryonic Development of Man and other Mammalia._
+
+The progressive development of any vertebrate from the ovum or minute
+embryonic egg affords one of the most marvellous chapters in Natural
+History. We see the contents of the ovum undergoing numerous definite
+changes, its interior dividing and subdividing till it consists of a
+mass of cells, then a groove appears marking out the median line or
+vertebral column of the future animal, and thereafter are slowly
+developed the various essential organs of the body. After describing in
+some detail what takes place in the case of the ovum of the dog,
+Professor Huxley continues: "The history of the development of any other
+vertebrate animal, lizard, snake, frog, or fish tells the same story.
+There is always to begin with, an egg having the same essential
+structure as that of the dog; the yelk of that egg undergoes division or
+segmentation, as it is called, the ultimate products of that
+segmentation constitute the building materials for the body of the young
+animal; and this is built up round a primitive groove, in the floor of
+which a notochord is developed. Furthermore, there is a period in which
+the young of all these animals resemble one another, not merely in
+outward form, but in all essentials of structure, so closely, that the
+differences between them are inconsiderable, while in their subsequent
+course they diverge more and more widely from one another. And it is a
+general law that the more closely any animals resemble one another in
+adult structure, the larger and the more intimately do their embryos
+resemble one another; so that, for example, the embryos of a snake and
+of a lizard remain like one another longer than do those of a snake and
+a bird; and the embryos of a dog and of a cat remain like one another
+for a far longer period than do those of a dog and a bird, or of a dog
+and an opossum, or even than those of a dog and a monkey."[219]
+
+We thus see that the study of development affords a test of affinity in
+animals that are externally very much unlike each other; and we
+naturally ask how this applies to man. Is he developed in a different
+way from other mammals, as we should certainly expect if he has had a
+distinct and altogether different origin? "The reply," says Professor
+Huxley, "is not doubtful for a moment. Without question, the mode of
+origin and the early stages of the development of man are identical with
+those of the animals immediately below him in the scale." And again he
+tells us: "It is very long before the body of the young human being can
+be readily discriminated from that of the young puppy; but at a
+tolerably early period the two become distinguishable by the different
+forms of their adjuncts, the yelk-sac and the allantois;" and after
+describing these differences he continues: "But exactly in those
+respects in which the developing man differs from the dog, he resembles
+the ape.... So that it is only quite in the latter stages of development
+that the young human being presents marked differences from the young
+ape, while the latter departs as much from the dog in its development as
+the man does. Startling as this last assertion may appear to be, it is
+demonstrably true, and it alone appears to me sufficient to place beyond
+all doubt the structural unity of man with the rest of the animal world,
+and more particularly and closely with the apes."[220]
+
+A few of the curious details in which man passes through stages common
+to the lower animals may be mentioned. At one stage the os coccyx
+projects like a true tail, extending considerably beyond the rudimentary
+legs. In the seventh month the convolutions of the brain resemble those
+of an adult baboon. The great toe, so characteristic of man, forming the
+fulcrum which most assists him in standing erect, in an early stage of
+the embryo is much shorter than the other toes, and instead of being
+parallel with them, projects at an angle from the side of the foot, thus
+corresponding with its permanent condition in the quadrumana. Numerous
+other examples might be quoted, all illustrating the same general law.
+
+
+_Diseases Common to Man and the Lower Animals._
+
+Though the fact is so well known, it is certainly one of profound
+significance that many animal diseases can be communicated to man, since
+it shows similarity, if not identity, in the minute structure of the
+tissues, the nature of the blood, the nerves, and the brain. Such
+diseases as hydrophobia, variola, the glanders, cholera, herpes, etc.,
+can be transmitted from animals to man or the reverse; while monkeys are
+liable to many of the same non-contagious diseases as we are. Rengger,
+who carefully observed the common monkey (Cebus Azarae) in Paraguay,
+found it liable to catarrh, with the usual symptoms, terminating
+sometimes in consumption. These monkeys also suffered from apoplexy,
+inflammation of the bowels, and cataract in the eye. Medicines produced
+the same effect upon them as upon us. Many kinds of monkeys have a
+strong taste for tea, coffee, spirits, and even tobacco. These facts
+show the similarity of the nerves of taste in monkeys and in ourselves,
+and that their whole nervous system is affected in a similar way. Even
+the parasites, both external and internal, that affect man are not
+altogether peculiar to him, but belong to the same families or genera as
+those which infest animals, and in one case, scabies, even the same
+species.[221] These curious facts seem quite inconsistent with the idea
+that man's bodily structure and nature are altogether distinct from
+those of animals, and have had a different origin; while the facts are
+just what we should expect if he has been produced by descent with
+modification from some common ancestor.
+
+
+_The Animals most nearly Allied to Man._
+
+By universal consent we see in the monkey tribe a caricature of
+humanity. Their faces, their hands, their actions and expressions
+present ludicrous resemblances to our own. But there is one group of
+this great tribe in which this resemblance is greatest, and they have
+hence been called the anthropoid or man-like apes. These are few in
+number, and inhabit only the equatorial regions of Africa and Asia,
+countries where the climate is most uniform, the forests densest, and
+the supply of fruit abundant throughout the year. These animals are now
+comparatively well known, consisting of the orang-utan of Borneo and
+Sumatra, the chimpanzee and the gorilla of West Africa, and the group of
+gibbons or long-armed apes, consisting of many species and inhabiting
+South-Eastern Asia and the larger Malay Islands. These last are far
+less like man than the other three, one or other of which has at various
+times been claimed to be the most man-like of the apes and our nearest
+relations in the animal kingdom. The question of the degree of
+resemblance of these animals to ourselves is one of great interest,
+leading, as it does, to some important conclusions as to our origin and
+geological antiquity, and we will therefore briefly consider it.
+
+If we compare the skeletons of the orang or chimpanzee with that of man,
+we find them to be a kind of distorted copy, every bone corresponding
+(with very few exceptions), but altered somewhat in size, proportions,
+and position. So great is this resemblance that it led Professor Owen to
+remark: "I cannot shut my eyes to the significance of that all-pervading
+similitude of structure--every tooth, every bone, strictly
+homologous--which makes the determination of the difference between
+_Homo_ and _Pithecus_ the anatomist's difficulty."
+
+The actual differences in the skeletons of these apes and that of
+man--that is, differences dependent on the presence or absence of
+certain bones, and not on their form or position--have been enumerated
+by Mr. Mivart as follows:--(1) In the breast-bone consisting of but two
+bones, man agrees with the gibbons; the chimpanzee and gorilla having
+this part consisting of seven bones in a single series, while in the
+orang they are arranged in a double series of ten bones. (2) The normal
+number of the ribs in the orang and some gibbons is twelve pairs, as in
+man, while in the chimpanzee and gorilla there are thirteen pairs. (3)
+The orang and the gibbons also agree with man in having five lumbar
+vertebrae, while in the gorilla and the chimpanzee there are but four,
+and sometimes only three. (4) The gorilla and chimpanzee agree with man
+in having eight small bones in the wrist, while the orang and the
+gibbons, as well as all other monkeys, have nine.[222]
+
+The differences in the form, size, and attachments of the various bones,
+muscles, and other organs of these apes and man are very numerous and
+exceedingly complex, sometimes one species, sometimes another agreeing
+most nearly with ourselves, thus presenting a tangled web of affinities
+which it is very difficult to unravel. Estimated by the skeleton alone,
+the chimpanzee and gorilla seem nearer to man than the orang, which last
+is also inferior as presenting certain aberrations in the muscles. In
+the form of the ear the gorilla is more human than any other ape, while
+in the tongue the orang is the more man-like. In the stomach and liver
+the gibbons approach nearest to man, then come the orang and chimpanzee,
+while the gorilla has a degraded liver more resembling that of the lower
+monkeys and baboons.
+
+
+_The Brains of Man and Apes._
+
+We come now to that part of his organisation in which man is so much
+higher than all the lower animals--the brain; and here, Mr. Mivart
+informs us, the orang stands highest in rank. The height of the orang's
+cerebrum in front is greater in proportion than in either the chimpanzee
+or the gorilla. "On comparing the brain of man with the brains of the
+orang, chimpanzee, and baboon, we find a successive decrease in the
+frontal lobe, and a successive and very great increase in the relative
+size of the occipital lobe. Concomitantly with this increase and
+decrease, certain folds of brain substance, called 'bridging
+convolutions,' which in man are conspicuously interposed between the
+parietal and occipital lobes, seem as utterly to disappear in the
+chimpanzee, as they do in the baboon. In the orang, however, though much
+reduced, they are still to be distinguished.... The actual and absolute
+mass of the brain is, however, slightly greater in the chimpanzee than
+in the orang, as is the relative vertical extent of the middle part of
+the cerebrum, although, as already stated, the frontal portion is higher
+in the orang; while, according to M. Gratiolet, the gorilla is not only
+inferior to the orang in cerebral development, but even to his smaller
+African congener, the chimpanzee."[223]
+
+On the whole, then, we find that no one of the great apes can be
+positively asserted to be nearest to man in structure. Each of them
+approaches him in certain characteristics, while in others it is widely
+removed, giving the idea, so consonant with the theory of evolution as
+developed by Darwin, that all are derived from a common ancestor, from
+which the existing anthropoid apes as well as man have diverged. When,
+however, we turn from the details of anatomy to peculiarities of
+external form and motions, we find that, in a variety of characters, all
+these apes resemble each other and differ from man, so that we may
+fairly say that, while they have diverged somewhat from each other, they
+have diverged much more widely from ourselves. Let us briefly enumerate
+some of these differences.
+
+
+_External Differences of Man and Apes._
+
+All apes have large canine teeth, while in man these are no longer than
+the adjacent incisors or premolars, the whole forming a perfectly even
+series. In apes the arms are proportionately much longer than in man,
+while the thighs are much shorter. No ape stands really erect, a posture
+which is natural in man. The thumb is proportionately larger in man, and
+more perfectly opposable than in that of any ape. The foot of man
+differs largely from that of all apes, in the horizontal sole, the
+projecting heel, the short toes, and the powerful great toe firmly
+attached parallel to the other toes; all perfectly adapted for
+maintaining the erect posture, and for free motion without any aid from
+the arms or hands. In apes the foot is formed almost exactly like our
+hand, with a large thumb-like great toe quite free from the other toes,
+and so articulated as to be opposable to them; forming with the long
+finger-like toes a perfect grasping hand. The sole cannot be placed
+horizontally on the ground; but when standing on a level surface the
+animal rests on the outer edge of the foot with the finger and
+thumb-like toes partly closed, while the hands are placed on the ground
+resting on the knuckles. The illustration on the next page (Fig. 37)
+shows, fairly well, the peculiarities of the hands and feet of the
+chimpanzee, and their marked differences, both in form and use, from
+those of man.
+
+The four limbs, with the peculiarly formed feet and hands, are those of
+arboreal animals which only occasionally and awkwardly move on level
+ground. The arms are used in progression equally with the feet, and the
+hands are only adapted for uses similar to those of our hands when the
+animal is at rest, and then but clumsily. Lastly, the apes are all hairy
+animals, like the majority of other mammals, man alone having a smooth
+and almost naked skin. These numerous and striking differences, even
+more than those of the skeleton and internal anatomy, point to an
+enormously remote epoch when the race that was ultimately to develop
+into man diverged from that other stock which continued the animal type
+and ultimately produced the existing varieties of anthropoid apes.
+
+[Illustration: FIG. 37.--Chimpanzee (Troglodytes niger).]
+
+
+_Summary of the Animal Characteristics of Man._
+
+The facts now very briefly summarised amount almost to a demonstration
+that man, in his bodily structure, has been derived from the lower
+animals, of which he is the culminating development. In his possession
+of rudimentary structures which are functional in some of the mammalia;
+in the numerous variations of his muscles and other organs agreeing with
+characters which are constant in some apes; in his embryonic
+development, absolutely identical in character with that of mammalia in
+general, and closely resembling in its details that of the higher
+quadrumana; in the diseases which he has in common with other mammalia;
+and in the wonderful approximation of his skeleton to those of one or
+other of the anthropoid apes, we have an amount of evidence in this
+direction which it seems impossible to explain away. And this evidence
+will appear more forcible if we consider for a moment what the rejection
+of it implies. For the only alternative supposition is, that man has
+been specially created--that is to say, has been produced in some quite
+different way from other animals and altogether independently of them.
+But in that case the rudimentary structures, the animal-like variations,
+the identical course of development, and all the other animal
+characteristics he possesses are deceptive, and inevitably lead us, as
+thinking beings making use of the reason which is our noblest and most
+distinctive feature, into gross error.
+
+We cannot believe, however, that a careful study of the facts of nature
+leads to conclusions directly opposed to the truth; and, as we seek in
+vain, in our physical structure and the course of its development, for
+any indication of an origin independent of the rest of the animal world,
+we are compelled to reject the idea of "special creation" for man, as
+being entirely unsupported by facts as well as in the highest degree
+improbable.
+
+
+_The Geological Antiquity of Man._
+
+The evidence we now possess of the exact nature of the resemblance of
+man to the various species of anthropoid apes, shows us that he has
+little special affinity for any one rather than another species, while
+he differs from them all in several important characters in which they
+agree with each other. The conclusion to be drawn from these facts is,
+that his points of affinity connect him with the whole group, while his
+special peculiarities equally separate him from the whole group, and
+that he must, therefore, have diverged from the common ancestral form
+before the existing types of anthropoid apes had diverged from each
+other. Now, this divergence almost certainly took place as early as the
+Miocene period, because in the Upper Miocene deposits of Western Europe
+remains of two species of ape have been found allied to the gibbons, one
+of them, Dryopithecus, nearly as large as a man, and believed by M.
+Lartet to have approached man in its dentition more than the existing
+apes. We seem hardly, therefore, to have reached, in the Upper Miocene,
+the epoch of the common ancestor of man and the anthropoids.
+
+The evidence of the antiquity of man himself is also scanty, and takes
+us but very little way back into the past. We have clear proof of his
+existence in Europe in the latter stages of the glacial epoch, with many
+indications of his presence in interglacial or even pre-glacial times;
+while both the actual remains and the works of man found in the
+auriferous gravels of California deep under lava-flows of Pliocene age,
+show that he existed in the New World at least as early as in the
+Old.[224] These earliest remains of man have been received with doubt,
+and even with ridicule, as if there were some extreme improbability in
+them. But, in point of fact, the wonder is that human remains have not
+been found more frequently in pre-glacial deposits. Referring to the
+most ancient fossil remains found in Europe--the Engis and Neanderthal
+crania,--Professor Huxley makes the following weighty remark: "In
+conclusion, I may say, that the fossil remains of Man hitherto
+discovered do not seem to me to take us appreciably nearer to that lower
+pithecoid form, by the modification of which he has, probably, become
+what he is." The Californian remains and works of art, above referred
+to, give no indication of a specially low form of man; and it remains an
+unsolved problem why no traces of the long line of man's ancestors, back
+to the remote period when he first branched off from the pithecoid type,
+have yet been discovered.
+
+It has been objected by some writers--notably by Professor Boyd
+Dawkins--that man did not probably exist in Pliocene times, because
+almost all the known mammalia of that epoch are distinct species from
+those now living on the earth, and that the same changes of the
+environment which led to the modification of other mammalian species
+would also have led to a change in man. But this argument overlooks the
+fact that man differs essentially from all other mammals in this
+respect, that whereas any important adaptation to new conditions can be
+effected in them only by a change in bodily structure, man is able to
+adapt himself to much greater changes of conditions by a mental
+development leading him to the use of fire, of tools, of clothing, of
+improved dwellings, of nets and snares, and of agriculture. By the help
+of these, without any change whatever in his bodily structure, he has
+been able to spread over and occupy the whole earth; to dwell securely
+in forest, plain, or mountain; to inhabit alike the burning desert or
+the arctic wastes; to cope with every kind of wild beast, and to provide
+himself with food in districts where, as an animal trusting to nature's
+unaided productions, he would have starved.[225]
+
+It follows, therefore, that from the time when the ancestral man first
+walked erect, with hands freed from any active part in locomotion, and
+when his brain-power became sufficient to cause him to use his hands in
+making weapons and tools, houses and clothing, to use fire for cooking,
+and to plant seeds or roots to supply himself with stores of food, the
+power of natural selection would cease to act in producing modifications
+of his body, but would continuously advance his mind through the
+development of its organ, the brain. Hence man may have become truly
+man--the species, Homo sapiens--even in the Miocene period; and while
+all other mammals were becoming modified from age to age under the
+influence of ever-changing physical and biological conditions, he would
+be advancing mainly in intelligence, but perhaps also in stature, and by
+that advance alone would be able to maintain himself as the master of
+all other animals and as the most widespread occupier of the earth. It
+is quite in accordance with this view that we find the most pronounced
+distinction between man and the anthropoid apes in the size and
+complexity of his brain. Thus, Professor Huxley tells us that "it may be
+doubted whether a healthy human adult brain ever weighed less than 31
+or 32 ounces, or that the heaviest gorilla brain has exceeded 20
+ounces," although "a full-grown gorilla is probably pretty nearly twice
+as heavy as a Bosjes man, or as many an European woman."[226] The
+average human brain, however, weighs 48 or 49 ounces, and if we take the
+average ape brain at only 2 ounces less than the very largest gorilla's
+brain, or 18 ounces, we shall see better the enormous increase which has
+taken place in the brain of man since the time when he branched off from
+the apes; and this increase will be still greater if we consider that
+the brains of apes, like those of all other mammals, have also increased
+from earlier to later geological times.
+
+If these various considerations are taken into account, we must conclude
+that the essential features of man's structure as compared with that of
+apes--his erect posture and free hands--were acquired at a comparatively
+early period, and were, in fact, the characteristics which gave him his
+superiority over other mammals, and started him on the line of
+development which has led to his conquest of the world. But during this
+long and steady development of brain and intellect, mankind must have
+continuously increased in numbers and in the area which they
+occupied--they must have formed what Darwin terms a "dominant race." For
+had they been few in numbers and confined to a limited area, they could
+hardly have successfully struggled against the numerous fierce carnivora
+of that period, and against those adverse influences which led to the
+extinction of so many more powerful animals. A large population spread
+over an extensive area is also needed to supply an adequate number of
+brain variations for man's progressive improvement. But this large
+population and long-continued development in a single line of advance
+renders it the more difficult to account for the complete absence of
+human or pre-human remains in all those deposits which have furnished,
+in such rich abundance, the remains of other land animals. It is true
+that the remains of apes are also very rare, and we may well suppose
+that the superior intelligence of man led him to avoid that extensive
+destruction by flood or in morass which seems to have often overwhelmed
+other animals. Yet, when we consider that, even in our own day, men are
+not unfrequently overwhelmed by volcanic eruptions, as in Java and
+Japan, or carried away in vast numbers by floods, as in Bengal and
+China, it seems impossible but that ample remains of Miocene and
+Pliocene man do exist buried in the most recent layers of the earth's
+crust, and that more extended research or some fortunate discovery will
+some day bring them to light.
+
+
+_The Probable Birthplace of Man._
+
+It has usually been considered that the ancestral form of man originated
+in the tropics, where vegetation is most abundant and the climate most
+equable. But there are some important objections to this view. The
+anthropoid apes, as well as most of the monkey tribe, are essentially
+arboreal in their structure, whereas the great distinctive character of
+man is his special adaptation to terrestrial locomotion. We can hardly
+suppose, therefore, that he originated in a forest region, where fruits
+to be obtained by climbing are the chief vegetable food. It is more
+probable that he began his existence on the open plains or high plateaux
+of the temperate or sub-tropical zone, where the seeds of indigenous
+cereals and numerous herbivora, rodents, and game-birds, with fishes and
+molluscs in the lakes, rivers, and seas supplied him with an abundance
+of varied food. In such a region he would develop skill as a hunter,
+trapper, or fisherman, and later as a herdsman and cultivator,--a
+succession of which we find indications in the palaeolithic and
+neolithic races of Europe.
+
+In seeking to determine the particular areas in which his earliest
+traces are likely to be found, we are restricted to some portion of the
+Eastern hemisphere, where alone the anthropoid apes exist, or have
+apparently ever existed.
+
+There is good reason to believe, also, that Africa must be excluded,
+because it is known to have been separated from the northern continent
+in early tertiary times, and to have acquired its existing fauna of the
+higher mammalia by a later union with that continent after the
+separation from it of Madagascar, an island which has preserved for us a
+sample, as it were, of the early African mammalian fauna, from which not
+only the anthropoid apes, but all the higher quadrumana are
+absent.[227] There remains only the great Euro-Asiatic continent; and
+its enormous plateaux, extending from Persia right across Tibet and
+Siberia to Manchuria, afford an area, some part or other of which
+probably offered suitable conditions, in late Miocene or early Pliocene
+times, for the development of ancestral man.
+
+It is in this area that we still find that type of mankind--the
+Mongolian--which retains a colour of the skin midway between the black
+or brown-black of the negro, and the ruddy or olive-white of the
+Caucasian types, a colour which still prevails over all Northern Asia,
+over the American continents, and over much of Polynesia. From this
+primary tint arose, under the influence of varied conditions, and
+probably in correlation with constitutional changes adapted to peculiar
+climates, the varied tints which still exist among mankind. If the
+reasoning by which this conclusion is reached be sound, and all the
+earlier stages of man's development from an animal form occurred in the
+area now indicated, we can better understand how it is that we have as
+yet met with no traces of the missing links, or even of man's existence
+during late tertiary times, because no part of the world is so entirely
+unexplored by the geologist as this very region. The area in question is
+sufficiently extensive and varied to admit of primeval man having
+attained to a considerable population, and having developed his full
+human characteristics, both physical and mental, before there was any
+need for him to migrate beyond its limits. One of his earliest important
+migrations was probably into Africa, where, spreading westward, he
+became modified in colour and hair in correlation with physiological
+changes adapting him to the climate of the equatorial lowlands.
+Spreading north-westward into Europe the moist and cool climate led to a
+modification of an opposite character, and thus may have arisen the
+three great human types which still exist. Somewhat later, probably, he
+spread eastward into North-West America and soon scattered himself over
+the whole continent; and all this may well have occurred in early or
+middle Pliocene times. Thereafter, at very long intervals, successive
+waves of migration carried him into every part of the habitable world,
+and by conquest and intermixture led ultimately to that puzzling
+gradation of types which the ethnologist in vain seeks to unravel.
+
+
+_The Origin of the Moral and Intellectual Nature of Man._
+
+From the foregoing discussion it will be seen that I fully accept Mr.
+Darwin's conclusion as to the essential identity of man's bodily
+structure with that of the higher mammalia, and his descent from some
+ancestral form common to man and the anthropoid apes. The evidence of
+such descent appears to me to be overwhelming and conclusive. Again, as
+to the cause and method of such descent and modification, we may admit,
+at all events provisionally, that the laws of variation and natural
+selection, acting through the struggle for existence and the continual
+need of more perfect adaptation to the physical and biological
+environments, may have brought about, first that perfection of bodily
+structure in which he is so far above all other animals, and in
+co-ordination with it the larger and more developed brain, by means of
+which he has been able to utilise that structure in the more and more
+complete subjection of the whole animal and vegetable kingdoms to his
+service.
+
+But this is only the beginning of Mr. Darwin's work, since he goes on to
+discuss the moral nature and mental faculties of man, and derives these
+too by gradual modification and development from the lower animals.
+Although, perhaps, nowhere distinctly formulated, his whole argument
+tends to the conclusion that man's entire nature and all his faculties,
+whether moral, intellectual, or spiritual, have been derived from their
+rudiments in the lower animals, in the same manner and by the action of
+the same general laws as his physical structure has been derived. As
+this conclusion appears to me not to be supported by adequate evidence,
+and to be directly opposed to many well-ascertained facts, I propose to
+devote a brief space to its discussion.
+
+
+_The Argument from Continuity._
+
+Mr. Darwin's mode of argument consists in showing that the rudiments of
+most, if not of all, the mental and moral faculties of man can be
+detected in some animals. The manifestations of intelligence, amounting
+in some cases to distinct acts of reasoning, in many animals, are
+adduced as exhibiting in a much less degree the intelligence and reason
+of man. Instances of curiosity, imitation, attention, wonder, and memory
+are given; while examples are also adduced which may be interpreted as
+proving that animals exhibit kindness to their fellows, or manifest
+pride, contempt, and shame. Some are said to have the rudiments of
+language, because they utter several different sounds, each of which has
+a definite meaning to their fellows or to their young; others the
+rudiments of arithmetic, because they seem to count and remember up to
+three, four, or even five. A sense of beauty is imputed to them on
+account of their own bright colours or the use of coloured objects in
+their nests; while dogs, cats, and horses are said to have imagination,
+because they appear to be disturbed by dreams. Even some distant
+approach to the rudiments of religion is said to be found in the deep
+love and complete submission of a dog to his master.[228]
+
+Turning from animals to man, it is shown that in the lowest savages many
+of these faculties are very little advanced from the condition in which
+they appear in the higher animals; while others, although fairly well
+exhibited, are yet greatly inferior to the point of development they
+have reached in civilised races. In particular, the moral sense is said
+to have been developed from the social instincts of savages, and to
+depend mainly on the enduring discomfort produced by any action which
+excites the general disapproval of the tribe. Thus, every act of an
+individual which is believed to be contrary to the interests of the
+tribe, excites its unvarying disapprobation and is held to be immoral;
+while every act, on the other hand, which is, as a rule, beneficial to
+the tribe, is warmly and constantly approved, and is thus considered to
+be right or moral. From the mental struggle, when an act that would
+benefit self is injurious to the tribe, there arises conscience; and
+thus the social instincts are the foundation of the moral sense and of
+the fundamental principles of morality.[229]
+
+The question of the origin and nature of the moral sense and of
+conscience is far too vast and complex to be discussed here, and a
+reference to it has been introduced only to complete the sketch of Mr.
+Darwin's view of the continuity and gradual development of all human
+faculties from the lower animals up to savages, and from savage up to
+civilised man. The point to which I wish specially to call attention is,
+that to prove continuity and the progressive development of the
+intellectual and moral faculties from animals to man, is not the same as
+proving that these faculties have been developed by natural selection;
+and this last is what Mr. Darwin has hardly attempted, although to
+support his theory it was absolutely essential to prove it. Because
+man's physical structure has been developed from an animal form by
+natural selection, it does not necessarily follow that his mental
+nature, even though developed _pari passu_ with it, has been developed
+by the same causes only. To illustrate by a physical analogy. Upheaval
+and depression of land, combined with sub-aerial denudation by wind and
+frost, rain and rivers, and marine denudation on coastlines, were long
+thought to account for all the modelling of the earth's surface not
+directly due to volcanic action; and in the early editions of Lyell's
+_Principles of Geology_ these are the sole causes appealed to. But when
+the action of glaciers was studied and the recent occurrence of a
+glacial epoch demonstrated as a fact, many phenomena--such as moraines
+and other gravel deposits, boulder clay, erratic boulders, grooved and
+rounded rocks, and Alpine lake basins--were seen to be due to this
+altogether distinct cause. There was no breach of continuity, no sudden
+catastrophe; the cold period came on and passed away in the most gradual
+manner, and its effects often passed insensibly into those produced by
+denudation or upheaval; yet none the less a new agency appeared at a
+definite time, and new effects were produced which, though continuous
+with preceding effects, were not due to the same causes. It is not,
+therefore, to be assumed, without proof or against independent evidence,
+that the later stages of an apparently continuous development are
+necessarily due to the same causes only as the earlier stages. Applying
+this argument to the case of man's intellectual and moral nature, I
+propose to show that certain definite portions of it could not have been
+developed by variation and natural selection alone, and that, therefore,
+some other influence, law, or agency is required to account for them.
+If this can be clearly shown for any one or more of the special
+faculties of intellectual man, we shall be justified in assuming that
+the same unknown cause or power may have had a much wider influence, and
+may have profoundly influenced the whole course of his development.
+
+
+_The Origin of the Mathematical Faculty._
+
+We have ample evidence that, in all the lower races of man, what may be
+termed the mathematical faculty is, either absent, or, if present, quite
+unexercised. The Bushmen and the Brazilian Wood-Indians are said not to
+count beyond two. Many Australian tribes only have words for one and
+two, which are combined to make three, four, five, or six, beyond which
+they do not count. The Damaras of South Africa only count to three; and
+Mr. Galton gives a curious description of how one of them was hopelessly
+puzzled when he had sold two sheep for two sticks of tobacco each, and
+received four sticks in payment. He could only find out that he was
+correctly paid by taking two sticks and then giving one sheep, then
+receiving two sticks more and giving the other sheep. Even the
+comparatively intellectual Zulus can only count up to ten by using the
+hands and fingers. The Ahts of North-West America count in nearly the
+same manner, and most of the tribes of South America are no further
+advanced.[230] The Kaffirs have great herds of cattle, and if one is
+lost they miss it immediately, but this is not by counting, but by
+noticing the absence of one they know; just as in a large family or a
+school a boy is missed without going through the process of counting.
+Somewhat higher races, as the Esquimaux, can count up to twenty by using
+the hands and the feet; and other races get even further than this by
+saying "one man" for twenty, "two men" for forty, and so on, equivalent
+to our rural mode of reckoning by scores. From the fact that so many of
+the existing savage races can only count to four or five, Sir John
+Lubbock thinks it improbable that our earliest ancestors could have
+counted as high as ten.[231]
+
+When we turn to the more civilised races, we find the use of numbers
+and the art of counting greatly extended. Even the Tongas of the South
+Sea islands are said to have been able to count as high as 100,000. But
+mere counting does not imply either the possession or the use of
+anything that can be really called the mathematical faculty, the
+exercise of which in any broad sense has only been possible since the
+introduction of the decimal notation. The Greeks, the Romans, the
+Egyptians, the Jews, and the Chinese had all such cumbrous systems, that
+anything like a science of arithmetic, beyond very simple operations,
+was impossible; and the Roman system, by which the year 1888 would be
+written MDCCCLXXXVIII, was that in common use in Europe down to the
+fourteenth or fifteenth centuries, and even much later in some places.
+Algebra, which was invented by the Hindoos, from whom also came the
+decimal notation, was not introduced into Europe till the thirteenth
+century, although the Greeks had some acquaintance with it; and it
+reached Western Europe from Italy only in the sixteenth century.[232] It
+was, no doubt, owing to the absence of a sound system of numeration that
+the mathematical talent of the Greeks was directed chiefly to geometry,
+in which science Euclid, Archimedes, and others made such brilliant
+discoveries. It is, however, during the last three centuries only that
+the civilised world appears to have become conscious of the possession
+of a marvellous faculty which, when supplied with the necessary tools in
+the decimal notation, the elements of algebra and geometry, and the
+power of rapidly communicating discoveries and ideas by the art of
+printing, has developed to an extent, the full grandeur of which can be
+appreciated only by those who have devoted some time (even if
+unsuccessfully) to the study.
+
+The facts now set forth as to the almost total absence of mathematical
+faculty in savages and its wonderful development in quite recent times,
+are exceedingly suggestive, and in regard to them we are limited to two
+possible theories. Either prehistoric and savage man did not possess
+this faculty at all (or only in its merest rudiments); or they did
+possess it, but had neither the means nor the incitements for its
+exercise. In the former case we have to ask by what means has this
+faculty been so rapidly developed in all civilised races, many of which
+a few centuries back were, in this respect, almost savages themselves;
+while in the latter case the difficulty is still greater, for we have to
+assume the existence of a faculty which had never been used either by
+the supposed possessors of it or by their ancestors.
+
+Let us take, then, the least difficult supposition--that savages
+possessed only the mere rudiments of the faculty, such as their ability
+to count, sometimes up to ten, but with an utter inability to perform
+the very simplest processes of arithmetic or of geometry--and inquire
+how this rudimentary faculty became rapidly developed into that of a
+Newton, a La Place, a Gauss, or a Cayley. We will admit that there is
+every possible gradation between these extremes, and that there has been
+perfect continuity in the development of the faculty; but we ask, What
+motive power caused its development?
+
+It must be remembered we are here dealing solely with the capability of
+the Darwinian theory to account for the origin of the _mind_, as well as
+it accounts for the origin of the _body_ of man, and we must, therefore,
+recall the essential features of that theory. These are, the
+preservation of useful variations in the struggle for life; that no
+creature can be improved beyond its necessities for the time being; that
+the law acts by life and death, and by the survival of the fittest. We
+have to ask, therefore, what relation the successive stages of
+improvement of the mathematical faculty had to the life or death of its
+possessors; to the struggles of tribe with tribe, or nation with nation;
+or to the ultimate survival of one race and the extinction of another.
+If it cannot possibly have had any such effects, then it cannot have
+been produced by natural selection.
+
+It is evident that in the struggles of savage man with the elements and
+with wild beasts, or of tribe with tribe, this faculty can have had no
+influence. It had nothing to do with the early migrations of man, or
+with the conquest and extermination of weaker by more powerful peoples.
+The Greeks did not successfully resist the Persian invaders by any aid
+from their few mathematicians, but by military training, patriotism, and
+self-sacrifice. The barbarous conquerors of the East, Timurlane and
+Gengkhis Khan, did not owe their success to any superiority of intellect
+or of mathematical faculty in themselves or their followers. Even if the
+great conquests of the Romans were, in part, due to their systematic
+military organisation, and to their skill in making roads and
+encampments, which may, perhaps, be imputed to some exercise of the
+mathematical faculty, that did not prevent them from being conquered in
+turn by barbarians, in whom it was almost entirely absent. And if we
+take the most civilised peoples of the ancient world--the Hindoos, the
+Arabs, the Greeks, and the Romans, all of whom had some amount of
+mathematical talent--we find that it is not these, but the descendants
+of the barbarians of those days--the Celts, the Teutons, and the
+Slavs--who have proved themselves the fittest to survive in the great
+struggle of races, although we cannot trace their steadily growing
+success during past centuries either to the possession of any
+exceptional mathematical faculty or to its exercise. They have indeed
+proved themselves, to-day, to be possessed of a marvellous endowment of
+the mathematical faculty; but their success at home and abroad, as
+colonists or as conquerors, as individuals or as nations, can in no way
+be traced to this faculty, since they were almost the last who devoted
+themselves to its exercise. We conclude, then, that the present gigantic
+development of the mathematical faculty is wholly unexplained by the
+theory of natural selection, and must be due to some altogether distinct
+cause.
+
+
+_The Origin of the Musical and Artistic Faculties._
+
+These distinctively human faculties follow very closely the lines of the
+mathematical faculty in their progressive development, and serve to
+enforce the same argument. Among the lower savages music, as we
+understand it, hardly exists, though they all delight in rude musical
+sounds, as of drums, tom-toms, or gongs; and they also sing in
+monotonous chants. Almost exactly as they advance in general intellect,
+and in the arts of social life, their appreciation of music appears to
+rise in proportion; and we find among them rude stringed instruments and
+whistles, till, in Java, we have regular bands of skilled performers
+probably the successors of Hindoo musicians of the age before the
+Mahometan conquest. The Egyptians are believed to have been the earliest
+musicians, and from them the Jews and the Greeks, no doubt, derived
+their knowledge of the art; but it seems to be admitted that neither the
+latter nor the Romans knew anything of harmony or of the essential
+features of modern music.[233] Till the fifteenth century little
+progress appears to have been made in the science or the practice of
+music; but since that era it has advanced with marvellous rapidity, its
+progress being curiously parallel with that of mathematics, inasmuch as
+great musical geniuses appeared suddenly among different nations, equal
+in their possession of this special faculty to any that have since
+arisen.
+
+As with the mathematical, so with the musical faculty, it is impossible
+to trace any connection between its possession and survival in the
+struggle for existence. It seems to have arisen as a _result_ of social
+and intellectual advancement, not as a _cause_; and there is some
+evidence that it is latent in the lower races, since under European
+training native military bands have been formed in many parts of the
+world, which have been able to perform creditably the best modern music.
+
+The artistic faculty has run a somewhat different course, though
+analogous to that of the faculties already discussed. Most savages
+exhibit some rudiments of it, either in drawing or carving human or
+animal figures; but, almost without exception, these figures are rude
+and such as would be executed by the ordinary inartistic child. In fact,
+modern savages are, in this respect hardly equal to those prehistoric
+men who represented the mammoth and the reindeer on pieces of horn or
+bone. With any advance in the arts of social life, we have a
+corresponding advance in artistic skill and taste, rising very high in
+the art of Japan and India, but culminating in the marvellous sculpture
+of the best period of Grecian history. In the Middle Ages art was
+chiefly manifested in ecclesiastical architecture and the illumination
+of manuscripts, but from the thirteenth to the fifteenth centuries
+pictorial art revived in Italy and attained to a degree of perfection
+which has never been surpassed. This revival was followed closely by the
+schools of Germany, the Netherlands, Spain, France, and England, showing
+that the true artistic faculty belonged to no one nation, but was fairly
+distributed among the various European races.
+
+These several developments of the artistic faculty, whether manifested
+in sculpture, painting, or architecture, are evidently outgrowths of the
+human intellect which have no immediate influence on the survival of
+individuals or of tribes, or on the success of nations in their
+struggles for supremacy or for existence. The glorious art of Greece did
+not prevent the nation from falling under the sway of the less advanced
+Roman; while we ourselves, among whom art was the latest to arise, have
+taken the lead in the colonisation of the world, thus proving our mixed
+race to be the fittest to survive.
+
+
+_Independent Proof that the Mathematical, Musical, and Artistic
+Faculties have not been Developed under the Law of Natural Selection._
+
+The law of Natural Selection or the survival of the fittest is, as its
+name implies, a rigid law, which acts by the life or death of the
+individuals submitted to its action. From its very nature it can act
+only on useful or hurtful characteristics, eliminating the latter and
+keeping up the former to a fairly general level of efficiency. Hence it
+necessarily follows that the characters developed by its means will be
+present in all the individuals of a species, and, though varying, will
+not vary very widely from a common standard. The amount of variation we
+found, in our third chapter, to be about one-fifth or one-sixth of the
+mean value--that is, if the mean value were taken at 100, the variations
+would reach from 80 to 120, or somewhat more, if very large numbers were
+compared. In accordance with this law we find, that all those characters
+in man which were certainly essential to him during his early stages of
+development, exist in all savages with some approach to equality. In the
+speed of running, in bodily strength, in skill with weapons, in
+acuteness of vision, or in power of following a trail, all are fairly
+proficient, and the differences of endowment do not probably exceed the
+limits of variation in animals above referred to. So, in animal instinct
+or intelligence, we find the same general level of development. Every
+wren makes a fairly good nest like its fellows; every fox has an average
+amount of the sagacity of its race; while all the higher birds and
+mammals have the necessary affections and instincts needful for the
+protection and bringing-up of their offspring.
+
+But in those specially developed faculties of civilised man which we
+have been considering, the case is very different. They exist only in a
+small proportion of individuals, while the difference of capacity
+between these favoured individuals and the average of mankind is
+enormous. Taking first the mathematical faculty, probably fewer than one
+in a hundred really possess it, the great bulk of the population having
+no natural ability for the study, or feeling the slightest interest in
+it.[234] And if we attempt to measure the amount of variation in the
+faculty itself between a first-class mathematician and the ordinary run
+of people who find any kind of calculation confusing and altogether
+devoid of interest, it is probable that the former could not be
+estimated at less than a hundred times the latter, and perhaps a
+thousand times would more nearly measure the difference between them.
+
+The artistic faculty appears to agree pretty closely with the
+mathematical in its frequency. The boys and girls who, going beyond the
+mere conventional designs of children, draw what they _see_, not what
+they _know_ to be the shape of things; who naturally sketch in
+perspective, because it is thus they see objects; who see, and represent
+in their sketches, the light and shade as well as the mere outlines of
+objects; and who can draw recognisable sketches of every one they know,
+are certainly very few compared with those who are totally incapable of
+anything of the kind. From some inquiries I have made in schools, and
+from my own observation, I believe that those who are endowed with this
+natural artistic talent do not exceed, even if they come up to, one per
+cent of the whole population.
+
+The variations in the amount of artistic faculty are certainly very
+great, even if we do not take the extremes. The gradations of power
+between the ordinary man or woman "who does not draw," and whose
+attempts at representing any object, animate or inanimate, would be
+laughable, and the average good artist who, with a few bold strokes, can
+produce a recognisable and even effective sketch of a landscape, a
+street, or an animal, are very numerous; and we can hardly measure the
+difference between them at less than fifty or a hundred fold.
+
+The musical faculty is undoubtedly, in its lower forms, less uncommon
+than either of the preceding, but it still differs essentially from the
+necessary or useful faculties in that it is almost entirely wanting in
+one-half even of civilised men. For every person who draws, as it were
+instinctively, there are probably five or ten who sing or play without
+having been taught and from mere innate love and perception of melody
+and harmony.[235] On the other hand, there are probably about as many
+who seem absolutely deficient in musical perception, who take little
+pleasure in it, who cannot perceive discords or remember tunes, and who
+could not learn to sing or play with any amount of study. The
+gradations, too, are here quite as great as in mathematics or pictorial
+art, and the special faculty of the great musical composer must be
+reckoned many hundreds or perhaps thousands of times greater than that
+of the ordinary "unmusical" person above referred to.
+
+It appears then, that, both on account of the limited number of persons
+gifted with the mathematical, the artistic, or the musical faculty, as
+well as from the enormous variations in its development, these mental
+powers differ widely from those which are essential to man, and are, for
+the most part, common to him and the lower animals; and that they could
+not, therefore, possibly have been developed in him by means of the law
+of natural selection.
+
+       *       *       *       *       *
+
+We have thus shown, by two distinct lines of argument, that faculties
+are developed in civilised man which, both in their mode of origin,
+their function, and their variations, are altogether distinct from those
+other characters and faculties which are essential to him, and which
+have been brought to their actual state of efficiency by the necessities
+of his existence. And besides the three which have been specially
+referred to, there are others which evidently belong to the same class.
+Such is the metaphysical faculty, which enables us to form abstract
+conceptions of a kind the most remote from all practical applications,
+to discuss the ultimate causes of things, the nature and qualities of
+matter, motion, and force, of space and time, of cause and effect, of
+will and conscience. Speculations on these abstract and difficult
+questions are impossible to savages, who seem to have no mental faculty
+enabling them to grasp the essential ideas or conceptions; yet whenever
+any race attains to civilisation, and comprises a body of people who,
+whether as priests or philosophers, are relieved from the necessity of
+labour or of taking an active part in war or government, the
+metaphysical faculty appears to spring suddenly into existence,
+although, like the other faculties we have referred to, it is always
+confined to a very limited proportion of the population.
+
+In the same class we may place the peculiar faculty of wit and humour,
+an altogether natural gift whose development appears to be parallel with
+that of the other exceptional faculties. Like them, it is almost unknown
+among savages, but appears more or less frequently as civilisation
+advances and the interests of life become more numerous and more
+complex. Like them, too, it is altogether removed from utility in the
+struggle for life, and appears sporadically in a very small percentage
+of the population; the majority being, as is well known, totally unable
+to say a witty thing or make a pun even to save their lives.[236]
+
+
+_The Interpretation of the Facts._
+
+The facts now set forth prove the existence of a number of mental
+faculties which either do not exist at all or exist in a very
+rudimentary condition in savages, but appear almost suddenly and in
+perfect development in the higher civilised races. These same faculties
+are further distinguished by their sporadic character, being well
+developed only in a very small proportion of the community; and by the
+enormous amount of variation in their development, the higher
+manifestations of them being many times--perhaps a hundred or a thousand
+times--stronger than the lower. Each of these characteristics is totally
+inconsistent with any action of the law of natural selection in the
+production of the faculties referred to; and the facts, taken in their
+entirety, compel us to recognise some origin for them wholly distinct
+from that which has served to account for the animal
+characteristics--whether bodily or mental--of man.
+
+The special faculties we have been discussing clearly point to the
+existence in man of something which he has not derived from his animal
+progenitors--something which we may best refer to as being of a
+spiritual essence or nature, capable of progressive development under
+favourable conditions. On the hypothesis of this spiritual nature,
+superadded to the animal nature of man, we are able to understand much
+that is otherwise mysterious or unintelligible in regard to him,
+especially the enormous influence of ideas, principles, and beliefs over
+his whole life and actions. Thus alone we can understand the constancy
+of the martyr, the unselfishness of the philanthropist, the devotion of
+the patriot, the enthusiasm of the artist, and the resolute and
+persevering search of the scientific worker after nature's secrets. Thus
+we may perceive that the love of truth, the delight in beauty, the
+passion for justice, and the thrill of exultation with which we hear of
+any act of courageous self-sacrifice, are the workings within us of a
+higher nature which has not been developed by means of the struggle for
+material existence.
+
+It will, no doubt, be urged that the admitted continuity of man's
+progress from the brute does not admit of the introduction of new
+causes, and that we have no evidence of the sudden change of nature
+which such introduction would bring about. The fallacy as to new causes
+involving any breach of continuity, or any sudden or abrupt change, in
+the effects, has already been shown; but we will further point out that
+there are at least three stages in the development of the organic world
+when some new cause or power must necessarily have come into action.
+
+The first stage is the change from inorganic to organic, when the
+earliest vegetable cell, or the living protoplasm out of which it arose,
+first appeared. This is often imputed to a mere increase of complexity
+of chemical compounds; but increase of complexity, with consequent
+instability, even if we admit that it may have produced protoplasm as a
+chemical compound, could certainly not have produced _living_
+protoplasm--protoplasm which has the power of growth and of
+reproduction, and of that continuous process of development which has
+resulted in the marvellous variety and complex organisation of the whole
+vegetable kingdom. There is in all this something quite beyond and
+apart from chemical changes, however complex; and it has been well said
+that the first vegetable cell was a new thing in the world, possessing
+altogether new powers--that of extracting and fixing carbon from the
+carbon-dioxide of the atmosphere, that of indefinite reproduction, and,
+still more marvellous, the power of variation and of reproducing those
+variations till endless complications of structure and varieties of form
+have been the result. Here, then, we have indications of a new power at
+work, which we may term _vitality_, since it gives to certain forms of
+matter all those characters and properties which constitute Life.
+
+The next stage is still more marvellous, still more completely beyond
+all possibility of explanation by matter, its laws and forces. It is the
+introduction of sensation or consciousness, constituting the fundamental
+distinction between the animal and vegetable kingdoms. Here all idea of
+mere complication of structure producing the result is out of the
+question. We feel it to be altogether preposterous to assume that at a
+certain stage of complexity of atomic constitution, and as a necessary
+result of that complexity alone, an _ego_ should start into existence, a
+thing that _feels_, that is _conscious_ of its own existence. Here we
+have the certainty that something new has arisen, a being whose nascent
+consciousness has gone on increasing in power and definiteness till it
+has culminated in the higher animals. No verbal explanation or attempt
+at explanation--such as the statement that life is the result of the
+molecular forces of the protoplasm, or that the whole existing organic
+universe from the amaeba up to man was latent in the fire-mist from
+which the solar system was developed--can afford any mental
+satisfaction, or help us in any way to a solution of the mystery.
+
+The third stage is, as we have seen, the existence in man of a number of
+his most characteristic and noblest faculties, those which raise him
+furthest above the brutes and open up possibilities of almost indefinite
+advancement. These faculties could not possibly have been developed by
+means of the same laws which have determined the progressive development
+of the organic world in general, and also of man's physical
+organism.[237]
+
+These three distinct stages of progress from the inorganic world of
+matter and motion up to man, point clearly to an unseen universe--to a
+world of spirit, to which the world of matter is altogether subordinate.
+To this spiritual world we may refer the marvellously complex forces
+which we know as gravitation, cohesion, chemical force, radiant force,
+and electricity, without which the material universe could not exist for
+a moment in its present form, and perhaps not at all, since without
+these forces, and perhaps others which may be termed atomic, it is
+doubtful whether matter itself could have any existence. And still more
+surely can we refer to it those progressive manifestations of Life in
+the vegetable, the animal, and man--which we may classify as
+unconscious, conscious, and intellectual life,--and which probably
+depend upon different degrees of spiritual influx. I have already shown
+that this involves no necessary infraction of the law of continuity in
+physical or mental evolution; whence it follows that any difficulty we
+may find in discriminating the inorganic from the organic, the lower
+vegetable from the lower animal organisms, or the higher animals from
+the lowest types of man, has no bearing at all upon the question. This
+is to be decided by showing that a change in essential nature (due,
+probably, to causes of a higher order than those of the material
+universe) took place at the several stages of progress which I have
+indicated; a change which may be none the less real because absolutely
+imperceptible at its point of origin, as is the change that takes place
+in the curve in which a body is moving when the application of some new
+force causes the curve to be slightly altered.
+
+
+_Concluding Remarks._
+
+Those who admit my interpretation of the evidence now adduced--strictly
+scientific evidence in its appeal to facts which are clearly what ought
+_not_ to be on the materialistic theory--will be able to accept the
+spiritual nature of man, as not in any way inconsistent with the theory
+of evolution, but as dependent on those fundamental laws and causes
+which furnish the very materials for evolution to work with. They will
+also be relieved from the crushing mental burthen imposed upon those
+who--maintaining that we, in common with the rest of nature, are but
+products of the blind eternal forces of the universe, and believing also
+that the time must come when the sun will lose his heat and all life on
+the earth necessarily cease--have to contemplate a not very distant
+future in which all this glorious earth--which for untold millions of
+years has been slowly developing forms of life and beauty to culminate
+at last in man--shall be as if it had never existed; who are compelled
+to suppose that all the slow growths of our race struggling towards a
+higher life, all the agony of martyrs, all the groans of victims, all
+the evil and misery and undeserved suffering of the ages, all the
+struggles for freedom, all the efforts towards justice, all the
+aspirations for virtue and the wellbeing of humanity, shall absolutely
+vanish, and, "like the baseless fabric of a vision, leave not a wrack
+behind."
+
+As contrasted with this hopeless and soul-deadening belief, we, who
+accept the existence of a spiritual world, can look upon the universe as
+a grand consistent whole adapted in all its parts to the development of
+spiritual beings capable of indefinite life and perfectibility. To us,
+the whole purpose, the only _raison d'źtre_ of the world--with all its
+complexities of physical structure, with its grand geological progress,
+the slow evolution of the vegetable and animal kingdoms, and the
+ultimate appearance of man--was the development of the human spirit in
+association with the human body. From the fact that the spirit of
+man--the man himself--_is_ so developed, we may well believe that this
+is the only, or at least the best, way for its development; and we may
+even see in what is usually termed "evil" on the earth, one of the most
+efficient means of its growth. For we know that the noblest faculties of
+man are strengthened and perfected by struggle and effort; it is by
+unceasing warfare against physical evils and in the midst of difficulty
+and danger that energy, courage, self-reliance, and industry have become
+the common qualities of the northern races; it is by the battle with
+moral evil in all its hydra-headed forms, that the still nobler
+qualities of justice and mercy and humanity and self-sacrifice have been
+steadily increasing in the world. Beings thus trained and strengthened
+by their surroundings, and possessing latent faculties capable of such
+noble development, are surely destined for a higher and more permanent
+existence; and we may confidently believe with our greatest living
+poet--
+
+
+    That life is not as idle ore,
+
+    But iron dug from central gloom,
+        And heated hot with burning fears,
+        And dipt in baths of hissing tears,
+    And batter'd with the shocks of doom
+
+    To shape and use.
+
+
+We thus find that the Darwinian theory, even when carried out to its
+extreme logical conclusion, not only does not oppose, but lends a
+decided support to, a belief in the spiritual nature of man. It shows us
+how man's body may have been developed from that of a lower animal form
+under the law of natural selection; but it also teaches us that we
+possess intellectual and moral faculties which could not have been so
+developed, but must have had another origin; and for this origin we can
+only find an adequate cause in the unseen universe of Spirit.
+
+FOOTNOTES:
+
+[Footnote 218: _Descent of Man_, pp. 41-43; also pp. 13-15.]
+
+[Footnote 219: _Man's Place in Nature_, p. 64.]
+
+[Footnote 220: _Man's Place in Nature_, p. 67. See Figs. of Embryos of
+Man and Dog in Darwin's _Descent of Man_, p. 10.]
+
+[Footnote 221: _The Descent of Man_, pp. 7, 8.]
+
+[Footnote 222: _Man and Apes._ By St. George Mivart, F.R.S., 1873. It is
+an interesting fact (for which I am indebted to Mr. E.B. Poulton) that
+the human embryo possesses the extra rib and wrist-bone referred to
+above in (2) and (4) as occurring in some of the apes.]
+
+[Footnote 223: _Man and Apes_, pp. 138, 144.]
+
+[Footnote 224: For a sketch of the evidence of Man's Antiquity in
+America, see _The Nineteenth Century_ for November 1887.]
+
+[Footnote 225: This subject was first discussed in an article in the
+_Anthropological Review_, May 1864, and republished in my _Contributions
+to Natural Selection_, chap, ix, in 1870.]
+
+[Footnote 226: _Man's Place in Nature_, p. 102.]
+
+[Footnote 227: For a full discussion of this question, see the author's
+_Geographical Distribution of Animals_, vol. i. p. 285.]
+
+[Footnote 228: For a full discussion of all these points, see _Descent
+of Man_, chap. iii.]
+
+[Footnote 229: _Descent of Man_, chap. iv.]
+
+[Footnote 230: Lubbock's _Origin of Civilisation_, fourth edition, pp.
+434-440; Tylor's _Primitive Culture_, chap. vii.]
+
+[Footnote 231: It has been recently stated that some of these facts are
+erroneous, and that some Australians can keep accurate reckoning up to
+100, or more, when required. But this does not alter the general fact
+that many low races, including the Australians, have no words for high
+numbers and never require to use them. If they are now, with a little
+practice, able to count much higher, this indicates the possession of a
+faculty which could not have been developed under the law of utility
+only, since the absence of words for such high numbers shows that they
+were neither used nor required.]
+
+[Footnote 232: Article Arithmetic in _Eng. Cyc. of Arts and Sciences_.]
+
+[Footnote 233: See "History of Music," in _Eng. Cyc._, Science and Arts
+Division.]
+
+[Footnote 234: This is the estimate furnished me by two mathematical
+masters in one of our great public schools of the proportion of boys who
+have any special taste or capacity for mathematical studies. Many more,
+of course, can be drilled into a fair knowledge of elementary
+mathematics, but only this small proportion possess the natural faculty
+which renders it possible for them ever to rank high as mathematicians,
+to take any pleasure in it, or to do any original mathematical work.]
+
+[Footnote 235: I am informed, however, by a music master in a large
+school that only about one per cent have real or decided musical talent,
+corresponding curiously with the estimate of the mathematicians.]
+
+[Footnote 236: In the latter part of his essay on Heredity (pp. 91-93 of
+the volume of _Essays_), Dr. Weismann refers to this question of the
+origin of "talents" in man, and, like myself, comes to the conclusion
+that they could not be developed under the law of natural selection. He
+says: "It may be objected that, in man, in addition to the instincts
+inherent in every individual, special individual predispositions are
+also found, of such a nature that it is impossible they can have arisen
+by individual variations of the germ-plasm. On the other hand, these
+predispositions--which we call talents--cannot have arisen through
+natural selection, because life is in no way dependent on their
+presence, and there seems to be no way of explaining their origin except
+by an assumption of the summation of the skill attained by exercise in
+the course of each single life. In this case, therefore, we seem at
+first sight to be compelled to accept the transmission of acquired
+characters." Weismann then goes on to show that the facts do not support
+this view; that the mathematical, musical, or artistic faculties often
+appear suddenly in a family whose other members and ancestors were in no
+way distinguished; and that even when hereditary in families, the talent
+often appears at its maximum at the commencement or in the middle of the
+series, not increasing to the end, as it should do if it depended in any
+way on the transmission of acquired skill. Gauss was not the son of a
+mathematician, nor Handel of a musician, nor Titian of a painter, and
+there is no proof of any special talent in the ancestors of these men of
+genius, who at once developed the most marvellous pre-eminence in their
+respective talents. And after showing that such great men only appear at
+certain stages of human development, and that two or more of the special
+talents are not unfrequently combined in one individual, he concludes
+thus--
+
+
+    "Upon this subject I only wish to add that, in my opinion,
+    talents do not appear to depend upon the improvement of any
+    special mental quality by continued practice, but they are the
+    expression, and to a certain extent the bye-product, of the
+    human mind, which is so highly developed in all directions."
+
+
+It will, I think, be admitted that this view hardly accounts for the
+existence of the highly peculiar human faculties in question.]
+
+[Footnote 237: For an earlier discussion of this subject, with some
+wider applications, see the author's _Contributions to the Theory of
+Natural Selection_, chap. x.]
+
+
+
+
+INDEX
+
+
+
+=A=
+
+Abbott, Dr. C.C., instability of habits of birds, 76
+  on American water-thrushes (Seiurus), 117
+  Mr., drawings of caterpillars and their food plants, 203
+
+Accessory plumes, development and display of, 293
+
+Acclimatisation, 94
+
+Achatinellidae, Gulick on variations in, 147
+
+Acquired characters, non-heredity of, 440
+
+Acraeidae, mimicry of, 247
+
+Adaptation to conditions at various periods of life, 112
+
+Adolias dirtea, sexual diversity of, 271
+
+Aegeriidae, mimicry by, 240
+
+Agaristidae, mimicry of, 246
+
+Agassiz, on species, 5
+  on North American weeds, 15.
+
+Agelaeus phoeniceus, diagram showing variations of, 56;
+  proportionate numbers which vary, 64
+
+Albatross, courtship of great, 287
+
+Allen, Mr. Grant, on forms of leaves, 133
+  on degradation of wind-fertilised from insect-fertilised flowers,
+  325 (note)
+  on insects and flowers, 332
+  on production of colour through the agency of the colour sense, 334
+  Mr. J.A., on the variability of birds, 50
+
+Allen, Mr. J.A.,  on colour as influenced by climate, 228
+
+Alluring coloration, 210
+
+American school of evolutionists, 420
+
+Anemone nemorosa, variability of, 78
+
+Animal coloration, a theory of, 288
+  general laws of, 296
+  intelligence, supposed action of, 425
+  characteristics of man, 454
+
+Animals, the struggle among, 18
+  wild, their enjoyment of life, 39
+  usually die painless deaths, 38
+  constitutional variation of, 94
+  uses of colours of, 134
+  supposed effects of disuse in wild, 415
+  most allied to man, 450
+
+Antelopes, recognition marks of, 219
+
+Anthrocera filipendula inedible, 235
+
+Apples, variations of, 87
+
+Arctic animals, supposed causes of white colour of, 191
+
+Argyll, Duke of, on goose reared by a golden eagle, 75
+
+Artemia salina and A. milhausenii, 426
+
+Asclepias curassavica, spread of, 28
+
+Asses running wild in Quito, 28
+
+Attractive fruits, 306
+
+Australia, spread of the Cape-weed in, 29
+  fossil and recent mammals of, 392
+
+Azara, on cause of horses and cattle not running wild in Paraguay, 19
+
+Azores, flora of, supports aerial transmission of seeds, 368
+
+
+=B=
+
+Baker, Mr. J.G., on rarity of spiny plants in Mauritius, 432
+
+Ball, Mr., on cause of late appearance of exogens, 400
+
+Barber, Mrs., on variable colouring of pupae of Papilio nireus, 197
+  on protective colours of African sun-birds, 200
+
+Barbs, 91
+
+Barriers, importance of, in questions of distribution, 341
+
+Bates, Mr. H.W., on varieties of butterflies, 44
+  on inedibility of Heliconidae, 234
+  on a conspicuous caterpillar, 236
+  on mimicry, 240, 243, 249
+
+Bathmism or growth-force, Cope on, 421
+
+Beddard, Mr. F.E., variations of earthworms, 67
+  on plumes of bird of paradise, 292
+
+Beech trees, aggressive in Denmark, 21
+
+Beetle and wasp (figs.), 259
+
+Beetle, fossil in coal measures of Silesia, 404
+
+Beginnings of important organs, 128
+
+Belt, Mr., on leaf-like locust, 203
+  on birds avoiding Heliconidae, 234
+
+Belt's frog, 266
+
+Birds, rate of increase of, 25
+  how destroyed, 26
+  variation among, 49
+  variation of markings of, 52
+  variation of wings and tails of, 53
+  diagram showing variation of tarsus and toes, 60
+  use of structural peculiarities of, 135
+  eggs, coloration of, 212
+  recognition marks of, 222
+  and butterflies, white in tropical islands, 230
+  sometimes seize inedible butterflies, 255
+  mimicry among, 263
+
+Birds, sexual coloration of, 275
+  cause of dull colour of female, 277
+  choice of female not known to be determined by colour, etc., 285
+  decorative plumage of, 285
+  antics of unornamented, 287
+  which fertilise flowers, 319
+  colours of, not dependent on the colours of flowers, 336
+  no proof of aesthetic tastes in, 336
+  dispersal of, 355
+  and insects at sea, 357
+  of oceanic islands, 358
+  carrying seeds on their feet, 361
+  ancestral forms of, 407
+
+Birthplace, probable, of man, 459
+
+Bombyx regia, protective form of larva of, 210
+
+Boyd Dawkins, on development of deer's horns, 389
+  on origin of man, 456
+
+Brady, Mr. George, on protective colouring of starfishes, 209
+
+Brain development, progressive, 390
+
+Brains of man and apes, 452
+
+Branner, Mr. J.C., on supposed proofs of glaciation in Brazil, 370
+
+Brazil, supposed proof of glaciation in, 370
+
+Brewer, Professor W.H., on want of symmetry in colours of animals, 217
+
+Bromelia, animals inhabiting leaves of, 118
+
+Bronn, Professor, on supposed uselessness of variations of ears
+  and tails, 136
+
+Butler, Mr. A.G., on inedibility of conspicuous caterpillars, 237
+
+Butterflies, varieties of, 44
+  small, of Isle of Man, 106
+  special protective colouring of, 206
+  recognition by, 226
+  inedibility of some, 234
+  mimicry among, 240, 249
+  colour development of, 274
+  sexual coloration of, 271
+
+
+=C=
+
+Caddis-fly larvae inhabiting bromelia leaves, 118
+
+Callophis, harmless mimicking poisonous species, 262
+
+Candolle, Alp. de, on variation in oaks, 77
+  on variability of Papaver bracteatum, 79
+
+Cardinalis virginianus, diagram showing proportionate numbers
+  which vary, 65;
+  variations of, 58
+
+Carpenter, Dr. W.B., on variation in the Foraminifera, 43
+
+Carriers, 91
+
+Caterpillars, resemblance of, to their food plants, 203-205
+  inedible, 236
+
+Cattle, how they prevent the growth of trees, 18
+  increase of, in St. Domingo, Mexico, and the pampas, 27
+
+Ceylon, spread of Lantana mixta in, 29
+
+Chaffinch, change of habit of, in New Zealand, 76
+
+Chambers, Robert, on origin of species, 3
+
+Chance rarely determines survival, 123
+
+Change of conditions, utility of, 326
+
+Characters, non-adaptive, 131
+  transferred from useless to useful class, 132
+
+Charaxes psaphon persecuted by a bird, 235
+
+Chile, numerous red tubular flowers in, 320
+
+Chimpanzee, figure of, 454
+
+Clark, Mr. Edwin, on cause of absence of forests on the pampas, 23
+  on the struggle for life in the South American valleys, 24
+
+Cleistogamous flowers, 322
+
+Close interbreeding, supposed evil results of, 326
+
+Clover, white, spread of, in New Zealand, 28
+
+Co-adaptation of parts by variation, no real difficulty, 418
+
+Cobra, use of hood of, 262
+
+Coccinella mimicked by grasshopper, (figure), 260
+
+Collingwood, Mr., on butterflies recognising their kind, 226
+
+Coloration, alluring, 210
+  of birds' eggs, 212
+  a theory of animal, 288
+
+Colour correlated with sterility, 169
+  correlated with constitutional peculiarities, 170
+  in nature, the problem to be solved, 188
+  constancy, in animals indicates utility, 189
+  and environment, 190
+  general theories of animal, 193
+  animal, supposed causes of, 193
+  obscure, of many tropical animals, 194
+  produced by surrounding objects, 195
+  adaptations, local, 199
+  for recognition, 217
+  of wild animals not quite symmetrical, 217 (note)
+  as influenced by locality or climate, 228
+  development in butterflies, 274
+  more variable than habits, 278
+  and nerve distribution, 290
+  and tegumentary appendages, 291
+  of flowers, 308
+  change of, in flowers when fertilised, 317
+  in nature, concluding remarks on, 299, 333
+  of fruits, 304
+  of flowers growing together contrasted, 318
+
+Complexity of flowers due to alternate adaptation to insect
+  and self-fertilisation, 328
+
+Composite, a, widely dispersed without pappus, 367
+
+Confinement, affecting fertility, 154
+
+Continental and oceanic areas, 346
+
+Continents and oceans cannot have changed places, 345
+  possible connections between, 349
+
+Continuity does not prove identity of origin, 463
+
+Cope, Dr. E.D., on non-adaptive characters, 131
+  on fundamental laws of growth, 420
+  on bathmism or growth-force, 421
+  on use producing structural change, 422
+  on law of centrifugal growth, 422
+  on origin of the feet of ungulates, 423
+  on action of animal intelligence, 425
+
+Correlations in pigeons, horses, etc., 140
+
+Corvus frugilegus, 2
+  corone, 2
+
+Coursers, figures of secondary quills, 224
+
+Cowslip, two forms of, 157
+
+Crab, sexual diversity of colour of, 269
+
+Cretaceous period, dicotyledons of, 400
+
+Crisp, Dr., on variations of gall bladder and alimentary canal, 69
+
+Crosses, a cause of variation, 99
+  reciprocal, 155
+
+Cross-fertilisation, modes of securing, 310
+  difference in, 155
+
+Crossing and changed conditions,
+  parallelism of, 166
+
+Cruciferae, variations of structure in, 80
+
+Cuckoo, eggs of, 216
+
+Cuckoos mimick hawks, 263
+
+Cultivated plants, origin of useful, 97
+
+Curculionidae mimicked by various insects (figs.), 260
+
+Curves of variation, 64
+
+
+=D=
+
+Dana, Professor, on the permanence of continents, 342
+
+Danaidae little attacked by mites, 235
+  mimicry of, 246
+
+Darwin, change of opinion effected by, 8
+  the Newton of Natural History, 9
+  his view of his own work, 10
+  on the enemies of plants, 16
+  on fir-trees destroyed by cattle, 17
+  on change of plants and animals caused by planting, 18
+  on absence of wild cattle in Paraguay, 19
+  on cats and red clover, 20
+  on variety of plants in old turf, 35
+  on the beneficent action of the struggle for existence, 40
+  on variability of wild geraniums, 79
+  on variability of common species, 80
+     his non-recognition of extreme variability of wild species, 82
+  on races of domestic pigeon, 90
+  on constitutional variation in plants, 95
+  on unconscious selection, 96
+  on a case of divergence, 105
+  on advantage of diversification of structure in inhabitants
+     of one region, 110
+  on species of plants in turf, 110
+  on isolation, 119
+  on origin of mammary glands, 129
+  on eyes of flatfish, 129
+  on origin of the eye, 130
+  on useless characters, 131
+  on use of ears and tails, 136
+  on disappearance of sports, 140
+  on tendency to vary in one direction, 141
+  on rare perpetuation of sports, 142
+  on utility of specific characters, 142 (note)
+  on importance of biological environment, 148
+  on variable fertility of plants, 155
+  on fertile hybrids among plants, 164
+
+Darwin, on correlation of sterility and colour, 169
+  on selective association, 172
+  on infertility and natural selection, 174
+  on cause of infertility of hybrids, 185
+  on white tail of rabbit, 218
+  on conspicuous caterpillars, 236
+  on sexual selection in insects, 274
+  on decorative plumage of male birds, 285
+  on development of ocelli, 290
+  on value of cross-fertilisation, 309
+  on limits  to  utility  of  intercrossing, 326
+  on flowers due to insects, 332
+  on oceanic islands, 342
+  on effects of disuse in domestic animals, 415, 435
+  on direct action of environment, 419
+  on unintelligibility of theory of retardation and acceleration,
+     421 (note)
+  on origin of man's moral nature, 461
+  Mr. George, on intermarriages of British aristocracy, 326
+
+Darwinian theory, statement of, 10
+  not opposed to spiritual nature of man, 478
+Dawkins, Professor Boyd, on development of deer's horns, 389
+  on recent origin of man, 456
+Dawson, Sir W., on determination of fossil plants by leaves, 398 (note)
+
+Death of wild animals usually painless, 38
+
+De Candolle, definition of species, 1
+  on difficulty of naturalising plants, 15
+  on war between plants, 16
+  on origin of useful cultivated plants, 97
+
+Deer's horns, development of, 389
+
+Degeneration, 121
+
+Delboeuf's law of variation, 141
+
+Dendraeca coronata, variation of wing-feathers of, 51
+
+Denmark, struggle between trees in, 20
+
+Denudation, evidences of, 379
+
+Desert animals, colour of, 192
+
+Deserts, effect of goats and camels in destroying vegetation in, 17
+
+Development and display of accessory plumes, 293
+
+Diadema anomala, 271
+  misippus, great diversity of sexes in, 271
+
+Diaphora mendica mimics Spilosoma menthrasti, 249
+
+Difficulties in the facts of fertilisation of flowers, 325
+
+Dimorphism and trimorphism, 156
+
+Dippers, probable origin of, 116
+
+Disease and markings, 290
+
+Diseases common to man and animals, 449
+
+Display of decorative plumage, 287
+
+Distribution of organisms should be explained by theory of descent, 338
+  conditions which have determined the, 341
+  of marsupials, 350
+  of tapirs, 352
+
+Disuse, effects of, among wild animals, 415
+  no proof that the effects of, are inherited, 417
+
+Divergence of character, 105-109
+  leads to maximum of forms of life in each area, 109
+
+Diversity of fauna and flora with geographical proximity, 339
+
+Dixon, Mr. C, changed habits of chaffinch in New Zealand, 76
+
+Dogs, origin of, 88
+  varieties of, 89
+
+Dolichonyx oryzivorus, diagram showing variations of, 55
+
+Domestic animals, varieties of, 88
+
+Draba verna, varieties of, 77
+
+Dress of men not determined by female choice, 286
+
+Dust from Krakatoa, size of particles of, 363
+
+
+=E=
+
+Eastern butterflies, variation of, 45
+
+Eaton, Rev. A.E., on Kerguelen insects, 106
+
+Edwards, Mr. W.H., on dark forms of Papilio turnus, 248
+
+Eggs protectively coloured, 214, 215
+  theory of varied colours of, 216
+
+Elaps mimicked by harmless snakes, 261
+
+Embryonic development of man and other mammalia, 448
+
+Ennis, Mr. John, on willows driving out watercresses from
+  rivers of New Zealand, 24
+
+Entomostraca, in bromelia leaves, 118
+
+Environment never identical for two species, 149
+  direct action of, 418
+  direct influence of, 426
+  as initiator of variations, 436
+  action of, overpowered by natural selection, 437
+
+Ethical aspect of the struggle for existence, 36
+
+Euchelia jacobeae inedible, 235
+
+Everett, Mr. A., on a caterpillar resembling moss, 205
+
+Evidence of evolution that may be expected among fossil forms, 380
+
+Evolutionists, American school of, 420
+
+Exogens, possible cause of sudden late appearance of, 400
+
+External differences of man and apes, 453
+
+Extinct animals, number of species of, 376
+
+Extinction of large animals, cause of, 394
+
+Eye, origin of, 130
+
+Eyes, explanation of loss of in cave animals, 416
+
+
+=F=
+
+Facts of natural selection, summary of, 122
+
+Falcons illustrating divergence, 108
+  and butcher birds, hooked and toothed beaks of, 422
+
+Fantails, 91
+
+Female birds, why often dull coloured, 277
+
+Female birds, what their choice of mates is determined by, 286
+  butterflies, why dull coloured, 272
+  brighter than male bird, 281
+  choice a doubtful agent in selection, 283
+  preference neutralised by natural selection, 294
+
+Fertility of domestic animals, 154
+
+Flatfish, eyes of, 129
+
+Flesh-fly, enormous increase of, 25
+
+Floral structure, great differences of, in allied genera and species, 329
+
+Flowers, variations of, 88
+  colours of, 308
+  with sham nectaries, 317
+  changing colour when fertilised, 317
+  adapted to bees or to butterflies, 318
+  contrasted colours of, at same season and locality, 318
+  fertilisation of, by birds, 319
+  self-fertilisation of, 321
+  once insect-fertilised now self-fertile, 323
+  how the struggle for existence acts among, 328
+  repeatedly modified during whole Tertiary period, 331
+  the product of insect agency, 332
+
+Forbes, Mr. H.O., on protective colour of a pigeon, 200
+  on spider imitating birds' dropping, 211
+
+Fossil shells, complete series of transitional forms of, 381
+  crocodiles afford evidence of evolution, 383
+  horses in America, 386
+  and living animals, local relations of, 391
+
+Fowl, early domestication of, 97
+
+Frill-back, Indian, 93
+
+Frog inhabiting bromelia leaves, 118
+
+Fruits, use of characters of, 133
+  colours of, 304
+  edible or attractive, 306
+  poisonous, 307
+
+Fulica atra, protectively coloured eggs of, 215
+
+Fulmar petrel, abundance of, 30
+
+
+=G=
+
+Gallinaceae, ornamental plumes of, 292
+
+Galton, Mr. F., diagrams of variability used by, 74
+  on markings of zebra, 220 (note)
+  on regression towards  mediocrity, 414
+  theory of heredity by, 443 (note)
+  on imperfect counting of the Damaras, 464
+
+Gaudry on extinct animals at Pikermi, 377
+
+Gay, Mons. T., on variations of structure in Cruciferae, 80
+
+Gazella soemmerringi (figure), 219
+
+Gazelles, recognition marks of, 218
+
+Geddes, Professor, on variation in plants, 428
+  objection to theory of, 430
+
+Geikie, Dr. Archibald, on formation of marine stratified rocks, 344
+
+Geoffroy St. Hilaire, on species, 6
+
+Geological evidences of evolution, 376, 381
+  record, causes of imperfection of, 379
+  distribution of insects, 403
+  antiquity of man, 455
+
+Ghost-moth, colours of, 270
+
+Glaciation, no proofs of, in Brazil, 370
+
+Glow-worm, light a warning of inedibility, 287
+
+Gomphia oleaefolia, variability of, 79
+
+Goose eating flesh, 75
+
+Gosse, Mr. P.H., on variation in the sea-anemones, 43
+  on sea-anemone and bullhead, 265
+
+Gould, Mr., on colours of coast and inland birds, 228
+
+Grant Allen, on forms of leaves, 133
+  on insects and flowers, 332
+
+Graphite in Laurentian implies abundant plant life, 398
+
+Gray, Dr. Asa, on naturalised plants in the United States, 110
+  Dr. J.E., on variation of skulls of mammalia, 71
+
+Great fertility not essential to rapid increase, 30
+
+Great powers of increase of animals, 27
+
+Green colour of birds in tropical forests, 192
+
+Grouse, red, recent divergence of, 106
+
+Gulick, Rev. J.T., on variation of land-shells, 43
+  on isolation and variation, 147, 150
+  on divergent evolution, 148
+
+
+=H=
+
+Habits  of animals, variability of, 74
+
+Hairy caterpillars inedible, 237
+
+Hanbury, Mr. Thomas, on a remarkable case of wind
+  conveyance of seed, 373 (note)
+
+Hansten-Blangsted, on succession of trees in Denmark, 21
+
+Harvest mice, prehensile tails of young, 136
+
+Hawkweed, species and varieties of British, 77
+
+Hector, Sir James, use of horns of deer, 137
+
+Heliconidae, warning colours of, 234
+  mimicry of, 240
+
+Helix nemoralis, varieties of, 43
+  hortensis, varieties of, 43
+
+Hemsley, Mr., on rarity of spines in oceanic islands, 432
+
+Henslow, Professor G., on vigour of self-fertilised plants, 323
+  on wind-fertilised as degradations from insect-fertilised flowers, 324
+  on origin of forms and structures of flowers, 434 (note)
+
+Herbert, Dean, on species, 6
+  on plant hybrids, 164
+
+Herbivora, recognition marks of, 218
+
+Heredity, 11
+  Weismann's theory of, 437
+
+Herschel, Sir John, on species, 3
+
+Hooker, Sir Joseph, on attempts at naturalising Australian
+  plants in New Zealand, 16
+
+Home, Mr. C, on inedibility of an Indian locust, 267
+
+Horns of deer, uses of, 136
+
+Horse tribe, pedigree of, 384
+  ancestral forms of, 386
+
+Humming-birds, recognition marks of, 226
+
+Huth, Mr., on close interbreeding, 160
+
+Huxley, Professor on the struggle for existence, 37
+  on fossil crocodiles, 383
+  on anatomical peculiarities of the horse tribe, 384
+  on development of vertebrates, 448
+  on early man, 456
+  on brains of man and the gorilla, 457
+
+Hybridity, remarks on facts of, 166
+  summary on, 184
+
+Hybrids, infertility of, supposed test of distinct species, 152
+  fertility of, 159
+  fertile among animals, 162
+  between sheep and goat, 162
+  fertile between distinct species of moths, 163
+  fertile among plants, 163
+
+Hymenopus bicornis, resembling flower, 212
+
+
+=I=
+
+Icterus Baltimore, diagram showing proportionate numbers which
+vary, 63
+
+Imitative resemblances, how produced, 205
+
+Increase of organisms in a geometrical ratio, 25
+
+Inedible fruits rarely coloured, 308
+
+Insect and self-fertilisation, alternation of, in flowers, 328
+
+Insect-fertilisation, facts relating to, 316
+
+Insects, coloured for recognition, 226
+  warning colours of, 233
+  sexual coloration of, 269
+  importance of dull colours to female, 272
+  visiting one kind of flower at a time, 318
+  and flowers, the most brilliant not found together, 335
+
+Insects, no proof of love of colour by, 336
+  and birds at sea, 357
+  in mid-ocean, 359
+  at great altitudes, 360
+  geological distribution of, 403
+  ancestral in Silurian, 405
+  fossil support evolution, 405
+
+Instability of useless characters, 138
+
+Instinct, the theory of, 441
+
+Insular organisms illustrate powers of dispersal, 354
+
+Interbreeding, close, injurious effects of, 160
+  supposed evil results of close, 326
+
+Intercrossing, swamping effects of, 142
+  not necessarily useful, 325
+
+Intermediate forms, why not found, 380
+
+Islands, all oceanic are volcanic or coralline, 342
+
+Isle of Man, small butterflies of, 106
+
+Isolation, the importance of, 119
+  to prevent intercrossing, 144
+  by variations of habits, etc., 145
+  Rev. J.G. Gulick on, 147
+  when ineffective, 150
+
+Ituna Ilione and Thyridia megisto, figures of wings of, 251
+
+
+=J=
+
+Jacobin, 93
+
+Jenyns, Rev. L., on internal variations of mammalia, 69
+
+Jordan, Mons. A., on varieties of Draba verna, 77
+
+Judd, Professor, on dust fallen at Genoa, 363
+  on Hungarian fossil lacustrine shells, 381
+
+
+=K=
+
+Kerguelen Island, wingless insects of, 106
+
+Kerivoula picta, protective colour of, 201
+
+Kerner, Professor, on use of external characters of plants, 133
+  on seeds found on glaciers, 366
+
+Kingfishers illustrating divergence of character, 109
+
+
+=L=
+
+Lacerta muralis, diagram of variation of, 47
+
+Lagopus scoticus, origin of, 107
+
+Lamarck, on origin of species, 3
+
+Land debris deposited near coasts, 343
+  and ocean, diagram showing comparative height and depth of, 345
+
+Large animals, cause of extinction of, 394
+
+Larvae of moths, variability of, 46
+
+Laughers, Frill-backs, Nuns, Spots, and Swallows, 93
+
+Law of relation of colour and nest, 278, 279
+
+Laws of animal coloration, 296
+
+Lawson Tait, on uses of tails, 136
+
+Leaf-butterflies, 207
+
+Leguminosae, rare in oceanic islands, 368
+
+Lemuria, an unsound hypothesis, 354
+
+Lepidoptera, variation of, 44
+
+Leyden Museum, diagram showing variability of birds in, 61
+
+Life, Weismann on duration of, 437 (note)
+
+Limenitis misippus mimics Danais archippus, 248
+  ursula mimics Papilio philenor, 248
+
+Linnaeus, on rapid increase of the flesh-fly, 25
+
+Livingstone, his sensations when seized by a lion, 38
+
+Lizards, variation among, 46
+  diagram of variation of, 48
+  sexual colours of, 281
+
+Local colour adaptations, 199
+
+Locusts with warning colours inedible, 267
+
+Longicorns mimic Malacoderms, 257
+
+Low, Mr., on effects of close interbreeding, 160
+
+Low, Mr., on fertile crosses between sheep and goat, 162
+  on selective association, 172
+
+Low forms of life, continued existence of, explained, 114
+  forms, persistence of, 121
+  temperature of tropics not needed to explain plant dispersal, 370
+
+Lower types, extinction of, among the higher animals, 114
+
+Lubbock, Sir John, on forms of leaves, 133
+  on imperfect counting of early man, 464
+
+Lyell, Sir Charles, on variation of species, 4
+  on the shifting of continents, 342
+
+
+=M=
+
+Madagascar and New Zealand, 347
+
+Madeira, wingless beetles of, 105
+
+Maize, origin of, 98
+
+Male rivalry, a real cause of selection, 283
+
+Males of many animals fights together, 282
+
+Malm, on eyes of flatfish, 129
+
+Mammalia, variation of, 65
+  sexual colours of, 281, 282
+  afford crucial tests of theories of distribution, 353
+  early forms of, 407
+  geological distribution of, 408
+
+Mammary glands, supposed origin of, 129
+
+Man, summary of animal characteristics of, 454
+  geological antiquity of, 455
+  early remains of, in California, 456
+  probably as old as the Miocene, 457
+  probable birthplace of, 459
+  origin of moral and intellectual nature of, 461
+  possesses mental qualities not derived exclusively
+    from his animal progenitors, 474
+
+Man's body that of an animal, 444
+  development similar to that of animals, 449
+  structure compared with that of the anthropoid apes, 451
+
+Mania typica refused by lizards, 238
+
+Mantidae resembling flowers, 212
+
+Marcgravia nepenthoides fertilised by birds (woodcut), 320
+
+Marine animals, protective resemblance among, 208
+  with warning colours, 266
+
+Marsh, Mr., on destructiveness to vegetation of goats and camels, 17
+  Professor O., on the development of the horse tribe, 386
+  on brain development of Tertiary mammals, 391
+  on specialised forms dying out, 395
+
+Marsupials, distribution of, 350
+
+Mathematical faculty, the origin of the, 464
+  how developed, 466
+  not developed by law of natural selection, 469
+
+Mathematics, late development of, 465
+
+Meldola, Professor R., on variable protective colouring, 196
+  on mimicry among British moths, 249 (note)
+  on an extension of the theory of mimicry, 255 (note)
+
+Melons, variations of, 87
+
+Methona psidii and Leptalis orise (figs.), 241
+
+Meyer, Dr. A.B., on mimicry of snakes, 262
+
+Milne Edwards, on variation of lizards, 46
+
+Mimicking birds deceive naturalists, 264
+  butterfly, figure of, 241
+
+Mimicry, 239
+  how it has been produced, 242
+  among protected genera, 249
+  extension of, 255
+  in various orders of insects, 257
+  among vertebrata, 261
+  among birds, 263
+  objections to theory of, 264
+
+Mineral particles carried by wind, 363
+
+Miocene fossils of North America, 378
+
+Missing links, character of, 380
+
+Mivart, Dr. St. George, on variation of ribs and dorsal vertebrae, 69
+  on supposed useless characters, 138 (note)
+  on resemblance of man and apes, 451
+
+Modifications for special purposes, 113
+
+Mongrels, sterility of, 165
+
+Monkeys affected by medicines as are men, 450
+
+Monocotyledons degradations from dicotyledons, 325 (note)
+  scarcity of, in Rocky Mountains, 401
+  scarcity of, in Alpine flora, 401
+
+Moral nature of man, origin of, 461
+
+Morse, Professor E.T., on protective colouring of marine mollusca, 209
+
+Moseley, Professor, on protective resemblance among marine
+animals, 208
+  on courtship of Great Albatross, 287
+
+Moths, protected groups of, 235
+
+Mountains, remote, with identical plants, 369
+
+Müller, Dr. Fritz, on inhabitants of bromelia leaves, 118
+  on butterfly, deceived by its mimic, 245
+  his explanation of mimicry among protected genera, 252
+  Dr. Hermann, on variability of Myosurus minimus, 78
+
+Murray, Mr. John, on bulk of land and ocean, 344
+  on quartz particles on ocean floor, 363
+  Rev. R.P., variation in the neuration of butterflies' wings, 45
+
+Musical and artistic faculties, origin of, 467
+
+Myosurns minimus, variability of, 78
+
+
+=N=
+
+Natural selection with changed and unchanged conditions, 103
+  and sterility, 173
+  overpowers effects of use and disuse, 435
+  the most important agency in modifying species, 444
+
+Naturalist deceived by a mimicking insect, 259
+  by mimicking birds, 264
+
+Naudin, M., on varieties of melons, 87
+
+Nectarinea amethystina, protective colouring of, 201
+
+Nestor notabilis, variation of habits of, 75
+
+Nests of birds influence the colour of females, 278
+
+New species, conditions favourable to origin of, 115
+
+Newton, Professor A., on fertile hybrid ducks, 162
+
+New Zealand, European plants in, 15
+  spread of white clover in, 28
+  effects of introduced plants in, 29
+  native rat and fly exterminated by European species, 34
+  many plants of, incapable of self-fertilisation, 321
+  fauna of, 348
+  few spiny plants in, 433
+
+Nocturnal animals, colours of, 193
+
+Non-adaptive characters, instability of, 138
+
+Normandy pigs, fleshy appendages to jaws of, 139
+
+North America, Miocene fossils of, 378
+
+Northern plants in southern hemisphere, 368
+
+Nostus Borbonicus, variability of, 80
+
+Number of individuals which vary, 62
+
+Nutmeg, how dispersed, 307
+
+Nuts, not meant to be eaten, 305
+
+
+=O=
+
+Oaks, great variability of, 78
+
+Objections to Darwin's theory, 126
+
+Ocean floor, deposits on, 343
+
+Oceanic animals, colours of, 193
+  and continental areas, 346
+  islands have no mammals or batrachia, 342
+
+Oceans, the permanence of, 341
+
+Oedicnemus, figures of wings of, 223
+
+Opthalmis lincea and Artaxa simulans (figs.), 247
+
+Orang-utans, variations of skull of, 69
+
+Orchideae, why scarce on oceanic islands, 367
+
+Orchis pyramidalis, mode of fertilisation of, 314
+  figures illustrating fertilisation of, 315
+
+Organic development, three stages of, involving new cause or power, 474
+  world, the development of, implies a spiritual world, 476
+
+Organisation, advance of, by natural selection, 120
+  degradation in, 121
+
+Origin of species, objections, 7
+  of accessory plumes, 291
+
+Orioles mimicking honey-suckers, 263
+
+Ornamental plumes and vitality, 293
+
+
+=P=
+
+Pachyrhynchi subjects of mimicry, 261
+
+Pampas, effects of drought in, 23
+
+Papaver bracteatum, variability of, 79
+
+Papilio, use of forked tentacle of larva of, 210
+  protected groups of, 235
+  mimicry of, 247
+
+Paraguay, absence of wild cattle and horses, 19
+
+Parnassia palustris, sham nectaries of, 317
+
+Parrot, change of habits of New Zealand, 75
+
+Parus, species of, illustrate divergence, of character, 107
+
+Passenger-pigeon, account of its breeding-places and numbers, 31
+
+Pelagic animals, colours of, 193
+
+Phasmidae, resemblance of, to sticks and leaves, 203
+
+Physiological selection, 180
+
+Pickard-Cambridge, Rev. O., on sexual selection, 296 (note)
+
+Pieridae, sexual diversity among, 271
+
+Pigeons, varieties of, 89
+  domestic, derived from wild rock-pigeons, 90
+  curious correlations in, 140
+  white eggs of, protective, 213
+
+Pigs, great increase of, in South America and New Zealand, 28
+
+Pikermi, extinct animals of, 377
+
+Pipits as illustrating divergence, 108
+
+Planorbidae, variations of, 44
+
+Plants, the enemies of, 16
+  variability of, 76
+  constitutional variation of, 94
+  colour relations of, 302
+  true mimicry rare in, 303
+  exotic rarely naturalised in Europe, 356
+  dispersal of, 361
+  northern, in southern hemisphere, 368
+  identical on summits of remote mountains, 369
+  progressive development of, 397
+  geological development of (diagram), 402
+
+Plovers, recognition marks of (figure), 221
+
+Plumes, origin of accessory, 291
+  muscular relation of ornamental, 292
+
+Poisonous fruits, 307
+
+Porto Santo, rabbits of, 326
+
+Poulton, Mr. E.B., on variable colours of larvae and pupae, 196, 198
+  on concealments of insects by resemblance to environment, 202
+  on protective form of Notodonta ziczac, 210
+  on inedibility of conspicuous larvae, 237
+
+Pouters, 90
+
+Primulaceae, variations of structure in, 79
+
+Problem, the, before Darwin, 6
+
+Problems in variation and heredity, 410
+
+Progression in plants and animals, 395
+
+Protection by terrifying enemies, 209
+
+Protective colouring, variable, 195
+  of white-headed fruit-pigeon, 200
+  of African sun-birds, 200
+  of Kerivoula picta, 201
+  of sloths, 201
+  of larva of Sphinx ligustri, 202
+  of stick and leaf insects, 203
+  of caterpillars, 203, 205
+  of butterflies, 206
+
+Ptilopus cinctus, protective colour of, 200
+
+Pugnacity of birds with accessory plumes, 294
+
+
+=R=
+
+Rabbit, use of white tail of, 218
+
+Rapid increase of plants, 28
+
+Raspail, M., on variability in a grass, 80
+
+Rat, black, spread of, 34
+
+Rattlesnake, use of rattle of, 262
+
+Raven, why black in arctic regions, 191
+
+Reciprocal crosses, 155
+
+Recognition marks of herbivora, 218
+  of birds, 222
+  of tropical forest birds, 224
+  of insects, 226
+
+Reproductive functions, susceptibility of, 153
+
+Reptiles, geological distribution of, 406
+
+Rhinoceroses, evidence of evolution afforded by fossil, 383
+
+Rocks, all stratified formed in shallow water, 344
+
+Rocky Mountains, scarcity of monocotyledons in, 401
+
+Rodents, prevent woody vegetation in the pampas, 23
+
+Romanes, Professor G.J., on useless characters, 131, 139
+  on meaningless peculiarities of structure, 140
+  on supposed absence of simultaneous variations, 142
+  on physiological selection, 180
+
+Rook and crow, 2
+
+Roses, Mr. Baker on varieties of, 77
+
+Rubus, Bentham and Babington on species and varieties of, 77
+
+Rudiments and variations in man, 446
+
+Runts, 91
+
+Rutaceae, variation of structure in, 79
+
+
+=S=
+
+St. Helena, destruction of forests by goats, 17
+
+St. Hilaire, M. Aug., variability of Gomphia oleaefolia, 79
+
+Saxicola, divergence of character in species of, 108
+  recognition marks of, 222
+
+Scientific opinion before Darwin, 4
+
+Scolopax, figures of tails of, 225
+
+Scudder, Mr. S.H., on inedibility of Danais archippus, 238
+  on fossil insects, 403
+
+Seebohm, Mr., on swamping effects of intercrossing, 143
+
+Seeds, how dispersed, 306
+  how protected, 307
+  floating great distances, 361
+  dispersal of, by wind, 362
+  weight and dimensions of, 364
+  importance of wind-carriage of, 372
+  remarkable case of wind-carriage of, 373
+
+Seiurus carolinensis, diagram of variation, 67
+  sp., habits of, 117
+
+Selection, artificial, 84
+  by man, circumstances favourable to, 96
+  unconscious, 96
+
+Selective association, isolation by, 171
+
+Self-fertilisation of flowers, 321
+
+Semper, Professor, on casting hairs of reptiles and Crustacea, 137 (note)
+  on direct influence of environment, 426
+
+Sesiidae, mimicry by, 240
+
+Sex colour and nests of birds, 277
+
+Sex, colours characteristic of, 269
+
+Sexual colours of insects, probable causes of, 273
+  of birds, 275
+  characters due to natural selection, 283
+  diversity the cause of variation, 439
+
+Sexual selection and colour, 274
+  by struggles of males, 282
+  neutralised by natural selection, 294-296
+  restricted to male struggles, 296
+
+Shetland Islands, variety of ghost-moth in, 270
+
+Shrews and field-mice, internal variations of, 69
+
+Shrikes, recognition marks of, 222
+
+Similarity of forms of life not due to similarity of conditions, 339
+
+Singing of male birds, use of, 284
+
+Skull of wolf, diagram of variations of, 70
+  of Ursus labiatus, diagram of variations of, 72
+  of Sus cristatus, diagram of variations of, 73
+
+Skunk an illustration of warning colour, 233
+
+Slack, Baron von, on protective markings of sloths, 201
+
+Sloth, protective colour and marking of, 201
+
+Snakes, mimicry of poisonous, 261
+
+Snipe, tails of two species (figs.), 225
+
+Sounds and odours peculiar to male,
+  how useful, 284
+  produced by peculiar feathers, 284
+
+South America, fossil and recent mammals of, 393
+
+Species, definition of, 1, 2
+  origin of, 2, 6
+  Lyell on, 4
+  Agassiz on, 5
+  transmutation of, 6
+  Geoffroy St. Hilaire on, 6
+  Dean Herbert on, 6
+  Professor Grant on, 6
+  Von Buch on, 6
+  allied, found in distinct areas, 36
+
+Species, which vary little, 80
+  closely allied inhabit distinct areas, 111
+  vigour and fertility of, how kept up, 327
+
+Spencer, Mr. Herbert, on factors of organic evolution, 411
+  on effects of disuse, 413
+  on difficulty as to co-adaptation of parts, 417
+  on direct action of environment, 418
+
+Sphingidae, protective attitudes of larvae, 210
+
+Sphinx ligustri, general resemblance of larva to food plant, 202
+
+Spider, alluring coloration of, 211
+
+Spines, on origin of, 431
+  rarity of, in oceanic islands, 432
+
+Spiny plants abundant in South Africa and Chile, 433
+
+Spots a primitive ornamentation of animals, 289
+
+Sprengel on flowers and insects, 309
+
+Staphylinidae, protective habit of, 210
+
+Sterility of mongrels, 165
+  correlated with colour, etc., 168
+  and natural selection, 173
+  of hybrids produced by natural selection, 179
+
+Struggle for existence, 14
+  among plants, 15
+  for life, illustrations of, 18
+  for existence on the pampas, 22
+  for life between closely allied forms most severe, 33
+  for existence, ethics of, 36
+  how it acts among flowers, 328
+
+Summary of facts of colouring for protection and recognition, 227
+
+Survival of the fittest, 11, 122, 123
+
+Swainson, definition of species, 2
+
+Swamping effects of intercrossing, 142
+
+Sweden, destruction of grass by larvae of moths in, 17
+
+Swinhoe, Mr., on protective colouring of a bat, 201
+
+Symmetry, bilateral in colours of animals needful for recognition, 217
+
+
+=T=
+
+Tails used as respirators, 136
+
+Tapirs, distribution of, 352
+
+Tegetmeier, Mr., on feeding habits of pigeons and fowls, 75
+  on sparrows and crocuses, 75
+  on curious correlations in pigeons, 140
+
+Tegumentary appendages and colour, 291
+
+Thousand-fathom line divides oceanic from continental islands, 347
+  the teachings of, 348
+  map showing, 349
+
+Thwaites, Mr., on spread of Lantana mixta in Ceylon, 30
+
+Tiger, use of stripes of, 199
+
+Titmice as illustrating divergence, 107
+
+Transformation of species of crustacea, 427
+
+Transmutationists, the early, 3
+
+Travers, Mr. W.L., on effects of introduced plants in New Zealand, 29
+
+Trees, great variety of, in many forests, 36
+
+Trimen, Mr., on butterfly deceived by its mimic, 245
+  on mimicry, 247
+
+Tropical animals, why brilliantly coloured, 299
+
+Tropics, no proof of lower temperature of, 369
+
+Tropidorhynchi mimicked by orioles, 263
+
+Trumpeter, 93
+
+Tumblers, 91
+
+Turbits and owls, 91
+
+Tylor, Mr. A., on _Coloration in Animals and Plants_, 285
+
+
+=U=
+
+Ungulates, origin of feet of, 423
+
+Use and disuse, effects of, overpowered by natural selection, 435
+
+Useless characters, 131
+  not specific, 132
+
+Useless specific characters, no proof of existence of, 141
+
+Utriculariae inhabiting bromelias, 118
+
+
+=V=
+
+Vanessa callirhoe, small variety in Porto Santo, 106
+
+Variability of the lower animals, 42
+  of the Foraminifera, 43
+  of sea-anemones, 43
+  of land mollusca, 43
+  of insects, 44
+  of lizards, 46
+  of birds, 49
+  of primary wing-feathers, 51
+  of wings and tail, 53
+  of Dolichonyx oryzivorus, 55
+  of Agelaeus phoeniceus, 56
+  of Cardinalis virginianus, 58
+  of tarsus and toes, 60
+  of birds in Leyden Museum, 61
+  of Sciurus carolinensis, 67
+  of skulls of wolf, 70
+  of skulls of a bear, 72
+  of skulls of Sus cristatus, 73
+  of plants, 76
+  of oaks, 77
+
+Variation, Lyell on, 4
+  in internal organs, 66
+  the facts of, 83
+  proofs of generality of, 85
+  of vegetables and fruits, 86
+  of apples and melons, 87
+  under domestication accords with that under nature, 100
+  coincident not necessary, 127
+  and heredity, problems of, 410
+  Professor Geddes's theory of, 428
+  the cause of, 439
+
+Variations of flowers, 88
+  of domestic animals, 88
+  of domestic pigeons, 89
+  conditions favourable to production of, 98
+  beneficial, 143
+
+Varieties, importance of, 41
+  of same species adapted to self or to insect-fertilisation, 330
+
+Vegetables, variation of, 86
+
+Vegetation and reproduction, antagonism of, 428
+
+Vertebrata, mimicry among, 261
+  geological succession of, 405
+
+_Vestiges of Creation_, 3
+
+Viola odorata, 2
+  canina, 2
+
+Violets, as illustrating species, 2
+
+Von Buch on species, 6
+
+
+=W=
+
+Wallace, Dr. Alexander, on absence of choice by female moths, 275
+
+Ward, Mr. Lester F., on progressive development of plants, 398
+
+Warning coloration, 232
+
+Warning colours of marine animals, 265
+
+Wasps and bees, mimicry of, 258
+  poisonous with warning colours, 287
+
+Water-cress, chokes rivers in New Zealand, 24
+  driven out by willows, 24
+
+Water-ouzels, probable origin of, 116
+
+Weale, Mr. Mansel, on protective colours of butterflies, 206
+
+Weeds of United States, 15
+
+Weir, Mr. Jenner, on deceptive resemblance of a caterpillar to
+a twig, 204
+  on inedibility of conspicuous caterpillars, 236
+  on birds disregarding inedible larvae, 254
+
+Weismann on progressive adaptation of colours of larva, 206
+  on non-heredity of acquired characters, 440
+  and Galton's theories of heredity almost identical, 443 (note)
+  on origin of the mathematical faculty, 472 (note)
+
+Weismann's theory of heredity, 437
+
+Westwood, Professor, on variation of insects, 44
+  deceived by a mimicking cricket, 259
+
+White coloration of insular birds and butterflies, 230
+
+Whymper, Mr., his sensations when falling on the Matterhorn, 38
+
+Willows, species and varieties of British, 77
+
+Wilson, Alexander, his account of the passenger-pigeon in North
+  America, 31
+
+Wind-carriage of seeds explains many facts of plant distribution,
+  371
+
+Wind-dispersal of seeds, objections to, 365
+
+Wind-fertilised degraded from insect-fertilised flowers, 324
+
+Wings of stone-curlews (figure), 223 why small but useless are
+  retained, 416
+
+Wit and humour, origin of faculties of, 472
+
+Wollaston, Mr. T.W., on variation of beetles, 44 on small
+butterfly in Porto Santo, 106
+
+Wolves, varieties of in Catskill Mountains, 105
+
+Wood, Mr. J., on muscular variations, 447
+
+Mr. T.W., on variable colouring of pupae of cabbage butterflies,
+  197
+
+Woodward, Dr. S.P., on variation of mollusca, 43
+
+
+=Y=
+
+Youatt, on breeds of sheep, 97
+
+Young animals often spotted, 289
+
+
+=Z=
+
+Zebra, markings for recognition and protection, 220 (note)
+
+
+
+THE END
+
+
+
+
+
+End of Project Gutenberg's Darwinism (1889), by Alfred Russel Wallace
+
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+
+The Project Gutenberg EBook of Darwinism (1889), by Alfred Russel Wallace
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Darwinism (1889)
+
+Author: Alfred Russel Wallace
+
+Release Date: January 2, 2005 [EBook #14558]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK DARWINISM (1889) ***
+
+
+
+
+Produced by StevenGibbs and the Online Distributed Proofreading Team
+
+
+
+
+
+</pre>
+
+
+  <h1><a name="Page_-15" id="Page_-15"></a>DARWINISM</h1>
+
+  <h2>AN EXPOSITION OF THE<br />
+  THEORY OF NATURAL SELECTION<br />
+  WITH SOME OF ITS APPLICATIONS</h2>
+
+  <h3>BY</h3>
+
+  <h2>ALFRED RUSSEL WALLACE<br />
+  <span style='font-size: smaller;'>LL.D., F.L.S., ETC.</span></h2>
+
+  <h4>WITH A PORTRAIT OF THE AUTHOR, MAP AND ILLUSTRATIONS</h4>
+
+  <h5>MACMILLAN AND CO.<br />
+  LONDON AND NEW YORK<br />
+  1889</h5>
+  <hr style='width: 45%;' />
+
+  <div class="figcenter" style="width: 298px;">
+    <a name="frontispiece" id="frontispiece"></a> <img src=
+    "images/frontispiece.jpg" width="298" height="472" alt=
+    "Alfred R. Wallace" title="Alfred R. Wallace" /> <b>Alfred R.
+    Wallace</b>
+  </div>
+  <hr style='width: 45%;' />
+
+  <h2><a name="Page_-14" id="Page_-14"></a><a name=
+  "PREFACE_TO_SECOND_EDITION" id=
+  "PREFACE_TO_SECOND_EDITION"></a>PREFACE TO SECOND EDITION</h2>
+
+  <p><a name="Page_-13" id="Page_-13"></a>The present edition is a
+  reprint of the first, with a few verbal corrections and the
+  alteration of some erroneous or doubtful statements. Of these
+  latter the following are the most important:&mdash;</p>
+
+  <p><i>P.</i> <a href='#Page_30'>30</a>. The statement as to the
+  fulmar petrel, which Professor A. Newton assures me is erroneous,
+  has been modified.</p>
+
+  <p><i>P.</i> <a href='#Page_34'>34</a>. A note is added as to
+  Darwin's statement about the missel and song-thrushes in
+  Scotland.</p>
+
+  <p><i>P.</i> <a href='#Page_172'>172</a>. An error as to the
+  differently-coloured herds of cattle in the Falkland Islands, is
+  corrected.</p>
+
+  <p><span style="margin-left: 2em;">PARKSTONE, DORSET</span><br />
+  <span style="margin-left: 2em;"><i>August,
+  1889</i>.</span><br /></p>
+  <hr style="width: 65%;" />
+
+  <h2><a name="Page_-12" id="Page_-12"></a><a name="Page_-11" id=
+  "Page_-11"></a><a name="PREFACE_TO_FIRST_EDITION" id=
+  "PREFACE_TO_FIRST_EDITION"></a>PREFACE TO FIRST EDITION</h2>
+
+  <p>The present work treats the problem of the Origin of Species
+  on the same general lines as were adopted by Darwin; but from the
+  standpoint reached after nearly thirty years of discussion, with
+  an abundance of new facts and the advocacy of many new or old
+  theories.</p>
+
+  <p>While not attempting to deal, even in outline, with the vast
+  subject of evolution in general, an endeavour has been made to
+  give such an account of the theory of Natural Selection as may
+  enable any intelligent reader to obtain a clear conception of
+  Darwin's work, and to understand something of the power and range
+  of his great principle.</p>
+
+  <p>Darwin wrote for a generation which had not accepted
+  evolution, and which poured contempt on those who upheld the
+  derivation of species from species by any natural law of descent.
+  He did his work so well that "descent with modification" is now
+  universally accepted as the order of nature in the organic world;
+  and the rising generation of naturalists can hardly realise the
+  novelty of this idea, or that their fathers considered it a
+  scientific heresy to be condemned rather than seriously
+  discussed.</p>
+
+  <p>The objections now made to Darwin's theory apply, solely, to
+  the particular means by which the change of species has been
+  brought about, not to the fact of that change. The objectors seek
+  to minimise the agency of natural selection and to subordinate it
+  to laws of variation, of use and disuse, of intelligence, and of
+  heredity. These views and objections are urged with much force
+  and more confidence, and for the<a name="Page_-10" id=
+  "Page_-10"></a> most part by the modern school of laboratory
+  naturalists, to whom the peculiarities and distinctions of
+  species, as such, their distribution and their affinities, have
+  little interest as compared with the problems of histology and
+  embryology, of physiology and morphology. Their work in these
+  departments is of the greatest interest and of the highest
+  importance, but it is not the kind of work which, by itself,
+  enables one to form a sound judgment on the questions involved in
+  the action of the law of natural selection. These rest mainly on
+  the external and vital relations of species to species in a state
+  of nature&mdash;on what has been well termed by Semper the
+  "physiology of organisms," rather than on the anatomy or
+  physiology of organs.</p>
+  <hr style='width: 45%;' />
+
+  <p>It has always been considered a weakness in Darwin's work that
+  he based his theory, primarily, on the evidence of variation in
+  domesticated animals and cultivated plants. I have endeavoured to
+  secure a firm foundation for the theory in the variations of
+  organisms in a state of nature; and as the exact amount and
+  precise character of these variations is of paramount importance
+  in the numerous problems that arise when we apply the theory to
+  explain the facts of nature, I have endeavoured, by means of a
+  series of diagrams, to exhibit to the eye the actual variations
+  as they are found to exist in a sufficient number of species. By
+  doing this, not only does the reader obtain a better and more
+  precise idea of variation than can be given by any number of
+  tabular statements or cases of extreme individual variation, but
+  we obtain a basis of fact by which to test the statements and
+  objections usually put forth on the subject of specific
+  variability; and it will be found that, throughout the work, I
+  have frequently to appeal to these diagrams and the facts they
+  illustrate, just as Darwin was accustomed to appeal to the facts
+  of variation among dogs and pigeons.</p>
+
+  <p><a name="Page_-9" id="Page_-9"></a>I have also made what
+  appears to me an important change in the arrangement of the
+  subject. Instead of treating first the comparatively difficult
+  and unfamiliar details of variation, I commence with the Struggle
+  for Existence, which is really the fundamental phenomenon on
+  which natural selection depends, while the particular facts which
+  illustrate it are comparatively familiar and very interesting. It
+  has the further advantage that, after discussing variation and
+  the effects of artificial selection, we proceed at once to
+  explain how natural selection acts.</p>
+
+  <p>Among the subjects of novelty or interest discussed in this
+  volume, and which have important bearings on the theory of
+  natural selection, are: (1) A proof that all <i>specific</i>
+  characters are (or once have been) either useful in themselves or
+  correlated with useful characters (Chap. VI); (2) a proof that
+  natural selection can, in certain cases, increase the sterility
+  of crosses (Chap. VII); (3) a fuller discussion of the colour
+  relations of animals, with additional facts and arguments on the
+  origin of sexual differences of colour (Chaps. VIII-X); (4) an
+  attempted solution of the difficulty presented by the occurrence
+  of both very simple and very complex modes of securing the
+  cross-fertilisation of plants (Chap. XI); (5) some fresh facts
+  and arguments on the wind-carriage of seeds, and its bearing on
+  the wide dispersal of many arctic and alpine plants (Chap. XII);
+  (6) some new illustrations of the non-heredity of acquired
+  characters, and a proof that the effects of use and disuse, even
+  if inherited, must be overpowered by natural selection (Chap.
+  XIV); and (7) a new argument as to the nature and origin of the
+  moral and intellectual faculties of man (Chap. XV).</p>
+  <hr style='width: 45%;' />
+
+  <p>Although I maintain, and even enforce, my differences from
+  some of Darwin's views, my whole work tends forcibly to
+  illustrate the overwhelming importance of Natural Selection over
+  all other agencies in the production of new species. <a name=
+  "Page_-8" id="Page_-8"></a>I thus take up Darwin's earlier
+  position, from which he somewhat receded in the later editions of
+  his works, on account of criticisms and objections which I have
+  endeavoured to show are unsound. Even in rejecting that phase of
+  sexual selection depending on female choice, I insist on the
+  greater efficacy of natural selection. This is pre-eminently the
+  Darwinian doctrine, and I therefore claim for my book the
+  position of being the advocate of pure Darwinism.</p>
+
+  <p>I wish to express my obligation to Mr. Francis Darwin for
+  lending me some of his father's unused notes, and to many other
+  friends for facts or information, which have, I believe, been
+  acknowledged either in the text or footnotes. Mr. James Sime has
+  kindly read over the proofs and given me many useful suggestions;
+  and I have to thank Professor Meldola, Mr. Hemsley, and Mr. E.B.
+  Poulton for valuable notes or corrections in the later chapters
+  in which their special subjects are touched upon.</p>
+
+  <p>GODALMING, <i>March 1889</i>.</p>
+  <hr style="width: 65%;" />
+
+  <h2><a name="Page_-7" id="Page_-7"></a>CONTENTS</h2>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#PREFACE_TO_SECOND_EDITION"><b>PREFACE TO SECOND
+  EDITION</b></a></p>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#PREFACE_TO_FIRST_EDITION"><b>PREFACE TO FIRST
+  EDITION</b></a></p>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#LIST_OF_ILLUSTRATIONS"><b>LIST OF
+  ILLUSTRATIONS</b></a></p>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_I"><b>CHAPTER I</b></a></p>
+
+  <p style='font-weight:bold;'>WHAT ARE "SPECIES" AND WHAT IS MEANT
+  BY THEIR "ORIGIN"</p>
+
+  <div class="blockquot">
+    <p>Definition of species&mdash;Special creation&mdash;The early
+    transmutationists&mdash;Scientific opinion before
+    Darwin&mdash;The problem before Darwin&mdash;The change of
+    opinion effected by Darwin&mdash;The Darwinian
+    theory&mdash;Proposed mode of treatment of the subject</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_II"><b>CHAPTER II</b></a></p>
+
+  <p style='font-weight:bold;'>THE STRUGGLE FOR EXISTENCE</p>
+
+  <div class="blockquot">
+    <p>Its importance&mdash;The struggle among plants&mdash;Among
+    animals&mdash;Illustrative cases&mdash;Succession of trees in
+    forests of Denmark&mdash;The struggle for existence on the
+    Pampas&mdash;Increase of organisms in a geometrical
+    ratio&mdash;Examples of rapid increase of animals&mdash;Rapid
+    increase and wide spread of plants&mdash;Great fertility not
+    essential to rapid increase&mdash;Struggle between closely
+    allied species most severe&mdash;The ethical aspect of the
+    struggle for existence</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_III"><b>CHAPTER III</b></a></p>
+
+  <p style='font-weight:bold;'>THE VARIABILITY OF SPECIES IN A
+  STATE OF NATURE</p>
+
+  <div class="blockquot">
+    <p>Importance of variability&mdash;Popular ideas regarding
+    it&mdash;Variability of the lower animals&mdash;The variability
+    of insects&mdash;Variation among lizards<a name="Page_-6" id=
+    "Page_-6"></a>Variation among birds&mdash;Diagrams of
+    bird-variation&mdash;Number of varying
+    individuals&mdash;Variation in the mammalia&mdash;Variation in
+    internal organs&mdash;Variations in the skull&mdash;Variations
+    in the habits of animals&mdash;The variability of
+    plants&mdash;Species which vary little&mdash;Concluding
+    remarks</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_IV"><b>CHAPTER IV</b></a></p>
+
+  <p style='font-weight:bold;'>VARIATION OF DOMESTICATED ANIMALS
+  AND CULTIVATED PLANTS</p>
+
+  <div class="blockquot">
+    <p>The facts of variation and artificial selection&mdash;Proofs
+    of the generality of variation&mdash;Variations of apples and
+    melons&mdash;Variations of flowers&mdash;Variations of domestic
+    animals&mdash;Domestic
+    pigeons&mdash;Acclimatisation&mdash;Circumstances favourable to
+    selection by man&mdash;Conditions favourable to
+    variation&mdash;Concluding remarks</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_V"><b>CHAPTER V</b></a></p>
+
+  <p style='font-weight:bold;'>NATURAL SELECTION BY VARIATION AND
+  SURVIVAL OF THE FITTEST</p>
+
+  <div class="blockquot">
+    <p>Effect of struggle for existence under unchanged
+    conditions&mdash;The effect under change of
+    conditions&mdash;Divergence of character&mdash;In
+    insects&mdash;In birds&mdash;In mammalia&mdash;Divergence leads
+    to a maximum of life in each area&mdash;Closely allied species
+    inhabit distinct areas&mdash;Adaptation to conditions at
+    various periods of life&mdash;The continued existence of low
+    forms of life&mdash;Extinction of low types among the higher
+    animals&mdash;Circumstances favourable to the origin of new
+    species&mdash;Probable origin of the dippers&mdash;The
+    importance of isolation&mdash;On the advance of organisation by
+    natural selection&mdash;Summary of the first five chapters</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_VI"><b>CHAPTER VI</b></a></p>
+
+  <p style='font-weight:bold;'>DIFFICULTIES AND OBJECTIONS</p>
+
+  <div class="blockquot">
+    <p>Difficulty as to smallness of variations&mdash;As to the
+    right variations occurring when required&mdash;The beginnings
+    of important organs&mdash;The mammary glands&mdash;The eyes of
+    flatfish&mdash;Origin of the eye&mdash;Useless or non-adaptive
+    characters&mdash;Recent extension of the region of utility in
+    <a name="Page_-5" id="Page_-5"></a>plants&mdash;The same in
+    animals&mdash;Uses of tails&mdash;Of the horns of deer&mdash;Of
+    the scale-ornamentation of reptiles&mdash;Instability of
+    non-adaptive characters&mdash;Delboeuf's law&mdash;No
+    "specific" character proved to be useless&mdash;The swamping
+    effects of intercrossing&mdash;Isolation as preventing
+    intercrossing&mdash;Gulick on the effects of
+    isolation&mdash;Cases in which isolation is ineffective</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_VII"><b>CHAPTER VII</b></a></p>
+
+  <p style='font-weight:bold;'>ON THE INFERTILITY OF CROSSES
+  BETWEEN DISTINCT SPECIES AND THE USUAL STERILITY OF THEIR HYBRID
+  OFFSPRING</p>
+
+  <div class="blockquot">
+    <p>Statement of the problem&mdash;Extreme susceptibility of the
+    reproductive functions&mdash;Reciprocal
+    crosses&mdash;Individual differences in respect to
+    cross-fertilisation&mdash;Dimorphism and trimorphism among
+    plants&mdash;Cases of the fertility of hybrids and of the
+    infertility of mongrels&mdash;The effects of close
+    interbreeding&mdash;Mr. Huth's objections&mdash;Fertile hybrids
+    among animals&mdash;Fertility of hybrids among
+    plants&mdash;Cases of sterility of mongrels&mdash;Parallelism
+    between crossing and change of conditions&mdash;Remarks on the
+    facts of hybridity&mdash;Sterility due to changed conditions
+    and usually correlated with other characters&mdash;Correlation
+    of colour with constitutional peculiarities&mdash;The isolation
+    of varieties by selective association&mdash;The influence of
+    natural selection upon sterility and
+    fertility&mdash;Physiological selection&mdash;Summary and
+    concluding remarks</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_VIII"><b>CHAPTER VIII</b></a></p>
+
+  <p style='font-weight:bold;'>THE ORIGIN AND USES OF COLOUR IN
+  ANIMALS</p>
+
+  <div class="blockquot">
+    <p>The Darwinian theory threw new light on organic
+    colour&mdash;The problem to be solved&mdash;The constancy of
+    animal colour indicates utility&mdash;Colour and
+    environment&mdash;Arctic animals white&mdash;Exceptions prove
+    the rule&mdash;Desert, forest, nocturnal, and oceanic
+    animals&mdash;General theories of animal colour&mdash;Variable
+    protective colouring&mdash;Mr. Poulton's
+    experiments&mdash;Special or local colour
+    adaptations&mdash;Imitation of particular objects&mdash;How
+    they have been produced&mdash;Special protective colouring of
+    butterflies&mdash;Protective resemblance among marine
+    animals&mdash;Protection by terrifying enemies&mdash;Alluring
+    coloration&mdash;The coloration of birds' eggs&mdash;Colour as
+    a means of recognition&mdash;Summary of the preceding
+    exposition&mdash;Influence of locality or of climate on
+    colour&mdash;Concluding remarks</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a name="Page_-4" id="Page_-4"></a><a href=
+  "#CHAPTER_IX"><b>CHAPTER IX</b></a></p>
+
+  <p style='font-weight:bold;'>WARNING COLORATION AND MIMICRY</p>
+
+  <div class="blockquot">
+    <p>The skunk as an example of warning coloration&mdash;Warning
+    colours among
+    insects&mdash;Butterflies&mdash;Caterpillars&mdash;Mimicry&mdash;How
+    mimicry has been produced&mdash;Heliconidae&mdash;Perfection of
+    the imitation&mdash;Other cases of mimicry among
+    Lepidoptera&mdash;Mimicry among protected groups&mdash;Its
+    explanation&mdash;Extension of the principle&mdash;Mimicry in
+    other orders of insects&mdash;Mimicry among the
+    vertebrata&mdash;Snakes&mdash;The rattlesnake and the
+    cobra&mdash;Mimicry among birds&mdash;Objections to the theory
+    of mimicry&mdash;Concluding remarks on warning colours and
+    mimicry</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_X"><b>CHAPTER X</b></a></p>
+
+  <p style='font-weight:bold;'>COLOURS AND ORNAMENTS CHARACTERISTIC
+  OF SEX</p>
+
+  <div class="blockquot">
+    <p>Sex colours in the mollusca and crustacea&mdash;In
+    insects&mdash;In butterflies and moths&mdash;Probable causes of
+    these colours&mdash;Sexual selection as a supposed
+    cause&mdash;Sexual coloration of birds&mdash;Cause of dull
+    colours of female birds&mdash;Relation of sex colour to nesting
+    habits&mdash;Sexual colours of other vertebrates&mdash;Sexual
+    selection by the struggles of males&mdash;Sexual characters due
+    to natural selection&mdash;Decorative plumage of males and its
+    effect on the females&mdash;Display of decorative plumage by
+    the males&mdash;A theory of animal coloration&mdash;The origin
+    of accessory plumes&mdash;Development of accessory plumes and
+    their display&mdash;The effect of female preference will be
+    neutralised by natural selection&mdash;General laws of animal
+    coloration&mdash;Concluding remarks</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_XI"><b>CHAPTER XI</b></a></p>
+
+  <p style='font-weight:bold;'>THE SPECIAL COLOURS OF PLANTS: THEIR
+  ORIGIN AND PURPOSE</p>
+
+  <div class="blockquot">
+    <p>The general colour relations of plants&mdash;Colours of
+    fruits&mdash;The meaning of nuts&mdash;Edible or attractive
+    fruits&mdash;The colours of flowers&mdash;Modes of securing
+    cross-fertilisation&mdash;The interpretation of the
+    facts&mdash;Summary <a name="Page_-3" id="Page_-3"></a>of
+    additional facts bearing on insect
+    fertilisation&mdash;Fertilisation of flowers by
+    birds&mdash;Self-fertilisation of flowers&mdash;Difficulties
+    and contradictions&mdash;Intercrossing not necessarily
+    advantageous&mdash;Supposed evil results of close
+    interbreeding&mdash;How the struggle for existence acts among
+    flowers&mdash;Flowers the product of insect
+    agency&mdash;Concluding remarks on colour in nature</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_XII"><b>CHAPTER XII</b></a></p>
+
+  <p style='font-weight:bold;'>THE GEOGRAPHICAL DISTRIBUTION OF
+  ORGANISMS</p>
+
+  <div class="blockquot">
+    <p>The facts to be explained&mdash;The conditions which have
+    determined distribution&mdash;The permanence of
+    oceans&mdash;Oceanic and continental areas&mdash;Madagascar and
+    New Zealand&mdash;The teachings of the thousand-fathom
+    line&mdash;The distribution of marsupials&mdash;The
+    distribution of tapirs&mdash;Powers of dispersal as illustrated
+    by insular organisms&mdash;Birds and insects at
+    sea&mdash;Insects at great altitudes&mdash;The dispersal of
+    plants&mdash;Dispersal of seeds by the wind&mdash;Mineral
+    matter carried by the wind&mdash;Objections to the theory of
+    wind-dispersal answered&mdash;Explanation of north temperate
+    plants in the southern hemisphere&mdash;No proof of glaciation
+    in the tropics&mdash;Lower temperature not needed to explain
+    the facts&mdash;Concluding remarks</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_XIII"><b>CHAPTER XIII</b></a></p>
+
+  <p style='font-weight:bold;'>THE GEOLOGICAL EVIDENCES OF
+  EVOLUTION</p>
+
+  <div class="blockquot">
+    <p>What we may expect&mdash;The number of known species of
+    extinct animals&mdash;Causes of the imperfection of the
+    geological record&mdash;Geological evidences of
+    evolution&mdash;Shells&mdash;Crocodiles&mdash;The rhinoceros
+    tribe&mdash;The pedigree of the horse tribe&mdash;Development
+    of deer's horns&mdash;Brain development&mdash;Local relations
+    of fossil and living animals&mdash;Cause of extinction of large
+    animals&mdash;Indications of general progress in plants and
+    animals&mdash;The progressive development of
+    plants&mdash;Possible cause of sudden late appearance of
+    exogens&mdash;Geological distribution of
+    insects&mdash;Geological succession of
+    vertebrata&mdash;Concluding remarks</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a name="Page_-2" id="Page_-2"></a><a href=
+  "#CHAPTER_XIV"><b>CHAPTER XIV</b></a></p>
+
+  <p style='font-weight:bold;'>FUNDAMENTAL PROBLEMS IN RELATION TO
+  VARIATION AND HEREDITY</p>
+
+  <div class="blockquot">
+    <p>Fundamental difficulties and objections&mdash;Mr. Herbert
+    Spencer's factors of organic evolution&mdash;Disuse and effects
+    of withdrawal of natural selection&mdash;Supposed effects of
+    disuse among wild animals&mdash;Difficulty as to co-adaptation
+    of parts by variation and selection&mdash;Direct action of the
+    environment&mdash;The American school of
+    evolutionists&mdash;Origin of the feet of the
+    ungulates&mdash;Supposed action of animal
+    intelligence&mdash;Semper on the direct influence of the
+    environment&mdash;Professor Geddes's theory of variation in
+    plants&mdash;Objections to the theory&mdash;On the origin of
+    spines&mdash;Variation and selection overpower the effects of
+    use and disuse&mdash;Supposed action of the environment in
+    imitating variations&mdash;Weismann's theory of
+    heredity&mdash;The cause of variation&mdash;The non-heredity of
+    acquired characters&mdash;The theory of
+    instinct&mdash;Concluding remarks</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#CHAPTER_XV"><b>CHAPTER XV</b></a></p>
+
+  <p style='font-weight:bold;'>DARWINISM APPLIED TO MAN</p>
+
+  <div class="blockquot">
+    <p>General identity of human and animal
+    structure&mdash;Rudiments and variations showing relation of
+    man to other mammals&mdash;The embryonic development of man and
+    other mammalia&mdash;Diseases common to man and the lower
+    animals&mdash;The animals most nearly allied to man&mdash;The
+    brains of man and apes&mdash;External differences of man and
+    apes&mdash;Summary of the animal characteristics of
+    man&mdash;The geological antiquity of man&mdash;The probable
+    birthplace of man&mdash;The origin of the moral and
+    intellectual nature of man&mdash;The argument from
+    continuity&mdash;The origin of the mathematical
+    faculty&mdash;The origin of the musical and artistic
+    faculties&mdash;Independent proof that these faculties have not
+    been developed by natural selection&mdash;The interpretation of
+    the facts&mdash;Concluding remarks</p>
+  </div>
+
+  <p style='font-weight:bold; margin-top:1.5em'>
+  <a href="#INDEX"><b>INDEX</b></a></p>
+  <hr style="width: 65%;" />
+
+  <h2><a name="LIST_OF_ILLUSTRATIONS" id=
+  "LIST_OF_ILLUSTRATIONS"></a><a name="Page_-1" id=
+  "Page_-1"></a>LIST OF ILLUSTRATIONS</h2>
+
+  <p><span style="margin-left: 2em;"><a href=
+  "#frontispiece">PORTRAIT OF AUTHOR</a></span><br />
+  <span style="margin-left: 2em;">MAP SHOWING THE 1000-FATHOM
+  LINE</span><br />
+  <span style="margin-left: 2em;"><a href="#fig01">1.</a>&nbsp;
+  DIAGRAM OF VARIATIONS OF LACERTA MURALIS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig02">2.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF LIZARDS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig03">3.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF WINGS AND TAIL OF
+  BIRDS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig04">4.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF DOLICHONYX
+  ORYZIVORUS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig05">5.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF AGELAEUS
+  PHOENICEUS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig06">6.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF CARDINALIS
+  VIRGINIANUS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig07">7.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF TARSUS AND
+  TOES</span><br />
+  <span style="margin-left: 2em;"><a href="#fig08">8.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF BIRDS IN LEYDEN
+  MUSEUM</span><br />
+  <span style="margin-left: 2em;"><a href="#fig09">9.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF ICTERUS
+  BALTIMORE</span><br />
+  <span style="margin-left: 2em;"><a href="#fig10">10.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF AGELAEUS
+  PHOENICEUS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig11">11.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; CURVES OF VARIATION</span><br />
+  <span style="margin-left: 2em;"><a href="#fig12">12.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF CARDINALIS
+  VIRGINIANUS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig13">13.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF SCIURUS
+  CAROLINENSIS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig14">14.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF SKULLS OF
+  WOLF</span><br />
+  <span style="margin-left: 2em;"><a href="#fig15">15.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF SKULLS OF URSUS
+  LABIATUS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig16">16.</a>&nbsp;
+  &nbsp; "&nbsp; &nbsp; &nbsp; VARIATION OF SKULLS OF SUS
+  CRISTATUS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig17">17.</a> PRIMULA
+  VERIS (Cowslip). From Darwin's <i>Forms of
+  Flowers</i></span><br />
+  <span style="margin-left: 2em;"><a href="#fig18">18.</a> GAZELLA
+  SOEMMERRINGI (to show recognition marks)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig19">19.</a>
+  RECOGNITION MARKS OF AFRICAN PLOVERS</span><br />
+  <span style="margin-left: 4em;">(from Seebohm's
+  <i>Charadriadae</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig20">20.</a>
+  RECOGNITION OF OEDICNEMUS VERMICULATUS AND OE.
+  SENEGALENSIS</span><br />
+  <a name="Page_0" id="Page_0"></a> <span style=
+  "margin-left: 4em;">(from Seebohm's
+  <i>Charadriadae</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig21">21.</a>
+  RECOGNITION OF CURSORIUS CHALCOPTERUS AND C.
+  GALLICUS</span><br />
+  <span style="margin-left: 4em;">(from Seebohm's
+  <i>Charadriadae</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig22">22.</a>
+  RECOGNITION OF SCOLOPAX MEGALA AND S. STENURA</span><br />
+  <span style="margin-left: 4em;">(from Seebohm's
+  <i>Charadriadae</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig23">23.</a> METHONA
+  PSIDII AND LEPTALIS ORISE</span><br />
+  <span style="margin-left: 2em;"><a href="#fig24">24.</a>
+  OPTHALMIS LINCEA AND ARTAXA SIMULANS</span><br />
+  <span style="margin-left: 4em;">(from the Official <i>Narrative
+  of the Voyage of the Challenger</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig25">25.</a> WINGS OF
+  ITUNA ILIONE AND THYRIDIA MEGISTO</span><br />
+  <span style="margin-left: 4em;">(from <i>Proceedings of the
+  Entomological Society</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig26">26.</a> MYGNIMIA
+  AVICULUS AND COLOBORHOMBUS FASCIATIPENNIS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig27">27.</a>
+  MIMICKING INSECTS FROM THE PHILIPPINES</span><br />
+  <span style="margin-left: 4em;">(from Semper's <i>Animal
+  Life</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig28">28.</a> MALVA
+  SYLVESTRIS AND M. ROTUNDIFOLIA</span><br />
+  <span style="margin-left: 4em;">(from Lubbock's <i>British Wild
+  Flowers in Relation to Insects</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig29">29.</a> LYTHRUM
+  SALICARIA, THREE FORMS OF</span><br />
+  <span style="margin-left: 4em;">(from Lubbock's <i>British Wild
+  Flowers in Relation to Insects</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig30">30.</a> ORCHIS
+  PYRAMIDALIS (from Darwin's <i>Fertilisation of
+  Orchids</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig31">31.</a>
+  HUMMING-BIRD FERTILISING MARCGRAVIA NEPENTHOIDES</span><br />
+  <span style="margin-left: 2em;"><a href="#fig32">32.</a> DIAGRAM
+  OF MEAN HEIGHT OF LAND AND DEPTH OF OCEANS</span><br />
+  <span style="margin-left: 2em;"><a href="#fig33">33.</a>
+  GEOLOGICAL DEVELOPMENT OF THE HORSE TRIBE</span><br />
+  <span style="margin-left: 4em;">(from Huxley's <i>American
+  Addresses</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig34">34.</a> DIAGRAM
+  ILLUSTRATING THE GEOLOGICAL DISTRIBUTION OF PLANTS</span><br />
+  <span style="margin-left: 4em;">(from Ward's <i>Sketch of
+  Palaeobotany</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig35">35.</a>
+  TRANSFORMATION OF ARTEMIA SALINA TO A. MILHAUSENII</span><br />
+  <span style="margin-left: 4em;">(from Semper's <i>Animal
+  Life</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig36">36.</a>
+  BRANCHIPUS STAGNALIS AND ARTEMIA SALINA</span><br />
+  <span style="margin-left: 4em;">(from Semper's <i>Animal
+  Life</i>)</span><br />
+  <span style="margin-left: 2em;"><a href="#fig37">37.</a>
+  CHIMPANZEE (TROGLODYTES NIGER)</span><br /></p>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_I" id="CHAPTER_I"></a><a name="Page_1" id=
+  "Page_1"></a>CHAPTER I</h2>
+
+  <p style='font-weight:bold;'>WHAT ARE "SPECIES," AND WHAT IS
+  MEANT BY THEIR "ORIGIN"</p>
+
+  <div class="blockquot">
+    <p>Definition of species&mdash;Special creation&mdash;The early
+    Transmutationists&mdash;Scientific opinion before
+    Darwin&mdash;The problem before Darwin&mdash;The change of
+    opinion effected by Darwin&mdash;The Darwinian
+    theory&mdash;Proposed mode of treatment of the subject.</p>
+  </div>
+
+  <p>The title of Mr. Darwin's great work is&mdash;<i>On the Origin
+  of Species by means of Natural Selection and the Preservation of
+  Favoured Races in the Struggle for Life</i>. In order to
+  appreciate fully the aim and object of this work, and the change
+  which it has effected not only in natural history but in many
+  other sciences, it is necessary to form a clear conception of the
+  meaning of the term "species," to know what was the general
+  belief regarding them at the time when Mr. Darwin's book first
+  appeared, and to understand what he meant, and what was generally
+  meant, by discovering their "origin." It is for want of this
+  preliminary knowledge that the majority of educated persons who
+  are not naturalists are so ready to accept the innumerable
+  objections, criticisms, and difficulties of its opponents as
+  proofs that the Darwinian theory is unsound, while it also
+  renders them unable to appreciate, or even to comprehend, the
+  vast change which that theory has effected in the whole mass of
+  thought and opinion on the great question of evolution.</p>
+
+  <p>The term "species" was thus defined by the celebrated botanist
+  De Candolle: "A species is a collection of all the individuals
+  which resemble each other more than they resemble anything else,
+  which can by mutual fecundation <a name="Page_2" id=
+  "Page_2"></a>produce fertile individuals, and which reproduce
+  themselves by generation, in such a manner that we may from
+  analogy suppose them all to have sprung from one single
+  individual." And the zoologist Swainson gives a somewhat similar
+  definition: "A species, in the usual acceptation of the term, is
+  an animal which, in a state of nature, is distinguished by
+  certain peculiarities of form, size, colour, or other
+  circumstances, from another animal. It propagates, 'after its
+  kind,' individuals perfectly resembling the parent; its
+  peculiarities, therefore, are permanent."<a name="FNanchor_1_1"
+  id="FNanchor_1_1"></a><a href="#Footnote_1_1" class=
+  "fnanchor">[1]</a></p>
+
+  <p>To illustrate these definitions we will take two common
+  English birds, the rook (Corvus frugilegus) and the crow (Corvus
+  corone). These are distinct <i>species</i>, because, in the first
+  place, they always differ from each other in certain slight
+  peculiarities of structure, form, and habits, and, in the second
+  place, because rooks always produce rooks, and crows produce
+  crows, and they do not interbreed. It was therefore concluded
+  that all the rooks in the world had descended from a single pair
+  of rooks, and the crows in like manner from a single pair of
+  crows, while it was considered impossible that crows could have
+  descended from rooks or <i>vice vers&acirc;</i>. The "origin" of
+  the first pair of each kind was a mystery. Similar remarks may be
+  applied to our two common plants, the sweet violet (Viola
+  odorata) and the dog violet (Viola canina). These also produce
+  their like and never produce each other or intermingle, and they
+  were therefore each supposed to have sprung from a single
+  individual whose "origin" was unknown. But besides the crow and
+  the rook there are about thirty other kinds of birds in various
+  parts of the world, all so much like our species that they
+  receive the common name of crows; and some of them differ less
+  from each other than does our crow from our rook. These are all
+  <i>species</i> of the genus Corvus, and were therefore believed
+  to have been always as distinct as they are now, neither more nor
+  less, and to have each descended from one pair of ancestral crows
+  of the same identical species, which themselves had an unknown
+  "origin." Of violets there are more than a hundred different
+  kinds in various parts of the world, all differing very slightly
+  from each other and forming distinct <a name="Page_3" id=
+  "Page_3"></a><i>species</i> of the genus Viola. But, as these
+  also each produce their like and do not intermingle, it was
+  believed that every one of them had always been as distinct from
+  all the others as it is now, that all the individuals of each
+  kind had descended from one ancestor, but that the "origin" of
+  these hundred slightly differing ancestors was unknown. In the
+  words of Sir John Herschel, quoted by Mr. Darwin, the origin of
+  such species was "the mystery of mysteries."</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Early Transmutationists</i>.</p>
+
+  <p>A few great naturalists, struck by the very slight difference
+  between many of these species, and the numerous links that exist
+  between the most different forms of animals and plants, and also
+  observing that a great many species do vary considerably in their
+  forms, colours, and habits, conceived the idea that they might be
+  all produced one from the other. The most eminent of these
+  writers was a great French naturalist, Lamarck, who published an
+  elaborate work, the <i>Philosophie Zoologique</i>, in which he
+  endeavoured to prove that all animals whatever are descended from
+  other species of animals. He attributed the change of species
+  chiefly to the effect of changes in the conditions of
+  life&mdash;such as climate, food, etc.&mdash;and especially to
+  the desires and efforts of the animals themselves to improve
+  their condition, leading to a modification of form or size in
+  certain parts, owing to the well-known physiological law that all
+  organs are strengthened by constant use, while they are weakened
+  or even completely lost by disuse. The arguments of Lamarck did
+  not, however, satisfy naturalists, and though a few adopted the
+  view that closely allied species had descended from each other,
+  the general belief of the educated public was, that each species
+  was a "special creation" quite independent of all others; while
+  the great body of naturalists equally held, that the change from
+  one species to another by any known law or cause was impossible,
+  and that the "origin of species" was an unsolved and probably
+  insoluble problem. The only other important work dealing with the
+  question was the celebrated <i>Vestiges of Creation</i>,
+  published anonymously, but now acknowledged to have been written
+  by the late Robert Chambers. In this work the action of general
+  laws was traced throughout the <a name="Page_4" id=
+  "Page_4"></a>universe as a system of growth and development, and
+  it was argued that the various species of animals and plants had
+  been produced in orderly succession from each other by the action
+  of unknown laws of development aided by the action of external
+  conditions. Although this work had a considerable effect in
+  influencing public opinion as to the extreme improbability of the
+  doctrine of the independent "special creation" of each species,
+  it had little effect upon naturalists, because it made no attempt
+  to grapple with the problem in detail, or to show in any single
+  case how the allied species of a genus could have arisen, and
+  have preserved their numerous slight and apparently purposeless
+  differences from each other. No clue whatever was afforded to a
+  law which should produce from any one species one or more
+  slightly differing but yet permanently distinct species, nor was
+  any reason given why such slight yet constant differences should
+  exist at all.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Scientific Opinion before Darwin.</i></p>
+
+  <p>In order to show how little effect these writers had upon the
+  public mind, I will quote a few passages from the writings of Sir
+  Charles Lyell, as representing the opinions of the most advanced
+  thinkers in the period immediately preceding that of Darwin's
+  work. When recapitulating the facts and arguments in favour of
+  the invariability and permanence of species, he says: "The entire
+  variation from the original type which any given kind of change
+  can produce may usually be effected in a brief period of time,
+  after which no further deviation can be obtained by continuing to
+  alter the circumstances, though ever so gradually, indefinite
+  divergence either in the way of improvement or deterioration
+  being prevented, and the least possible excess beyond the defined
+  limits being fatal to the existence of the individual." In
+  another place he maintains that "varieties of some species may
+  differ more than other species do from each other without shaking
+  our confidence in the reality of species." He further adduces
+  certain facts in geology as being, in his opinion, "fatal to the
+  theory of progressive development," and he explains the fact that
+  there are so often distinct species in countries of similar
+  climate and vegetation by "<a name="Page_5" id=
+  "Page_5"></a>special creations" in each country; and these
+  conclusions were arrived at after a careful study of Lamarck's
+  work, a full abstract of which is given in the earlier editions
+  of the <i>Principles of Geology</i>.<a name="FNanchor_2_2" id=
+  "FNanchor_2_2"></a><a href="#Footnote_2_2" class=
+  "fnanchor">[2]</a></p>
+
+  <p>Professor Agassiz, one of the greatest naturalists of the last
+  generation, went even further, and maintained not only that each
+  species was specially created, but that it was created in the
+  proportions and in the localities in which we now find it to
+  exist. The following extract from his very instructive book on
+  Lake Superior explains this view: "There are in animals peculiar
+  adaptations which are characteristic of their species, and which
+  cannot be supposed to have arisen from subordinate influences.
+  Those which live in shoals cannot be supposed to have been
+  created in single pairs. Those which are made to be the food of
+  others cannot have been created in the same proportions as those
+  which live upon them. Those which are everywhere found in
+  innumerable specimens must have been introduced in numbers
+  capable of maintaining their normal proportions to those which
+  live isolated and are comparatively and constantly fewer. For we
+  know that this harmony in the numerical proportions between
+  animals is one of the great laws of nature. The circumstance that
+  species occur within definite limits where no obstacles prevent
+  their wider distribution leads to the further inference that
+  these limits were assigned to them from the beginning, and so we
+  should come to the final conclusion that the order which prevails
+  throughout nature is intentional, that it is regulated by the
+  limits marked out on the first day of creation, and that it has
+  been maintained unchanged through ages with no other
+  modifications than those which the higher intellectual powers of
+  man enable him to impose on some few animals more closely
+  connected with him."<a name="FNanchor_3_3" id=
+  "FNanchor_3_3"></a><a href="#Footnote_3_3" class=
+  "fnanchor">[3]</a></p>
+
+  <p>These opinions of some of the most eminent and influential
+  writers of the pre-Darwinian age seem to us, now, either
+  altogether obsolete or positively absurd; but they nevertheless
+  exhibit the mental condition of even the most advanced section of
+  scientific men on the problem of the <a name="Page_6" id=
+  "Page_6"></a>nature and origin of species. They render it clear
+  that, notwithstanding the vast knowledge and ingenious reasoning
+  of Lamarck, and the more general exposition of the subject by the
+  author of the <i>Vestiges of Creation</i>, the first step had not
+  been taken towards a satisfactory explanation of the derivation
+  of any one species from any other. Such eminent naturalists as
+  Geoffroy Saint Hilaire, Dean Herbert, Professor Grant, Von Buch,
+  and some others, had expressed their belief that species arose as
+  simple varieties, and that the species of each genus were all
+  descended from a common ancestor; but none of them gave a clue as
+  to the law or the method by which the change had been effected.
+  This was still "the great mystery." As to the further
+  question&mdash;how far this common descent could be carried;
+  whether distinct families, such as crows and thrushes, could
+  possibly have descended from each other; or, whether all birds,
+  including such widely distinct types as wrens, eagles, ostriches,
+  and ducks, could all be the modified descendants of a common
+  ancestor; or, still further, whether mammalia, birds, reptiles,
+  and fishes, could all have had a common origin;&mdash;these
+  questions had hardly come up for discussion at all, for it was
+  felt that, while the very first step along the road of
+  "transmutation of species" (as it was then called) had not been
+  made, it was quite useless to speculate as to how far it might be
+  possible to travel in the same direction, or where the road would
+  ultimately lead to.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Problem before Darwin</i>.</p>
+
+  <p>It is clear, then, that what was understood by the "origin" or
+  the "transmutation" of species before Darwin's work appeared, was
+  the comparatively simple question whether the allied species of
+  each genus had or had not been derived from one another and,
+  remotely, from some common ancestor, by the ordinary method of
+  reproduction and by means of laws and conditions still in action
+  and capable of being thoroughly investigated. If any naturalist
+  had been asked at that day whether, supposing it to be clearly
+  shown that all the different species of each genus had been
+  derived from some one ancestral species, and that a full and
+  complete explanation were to be given of how each minute
+  difference in form, colour, or structure might have originated,
+  and how the <a name="Page_7" id="Page_7"></a>several
+  peculiarities of habit and of geographical distribution might
+  have been brought about&mdash;whether, if this were done, the
+  "origin of species" would be discovered, the great mystery
+  solved, he would undoubtedly have replied in the affirmative. He
+  would probably have added that he never expected any such
+  marvellous discovery to be made in his lifetime. But so much as
+  this assuredly Mr. Darwin has done, not only in the opinion of
+  his disciples and admirers, but by the admissions of those who
+  doubt the completeness of his explanations. For almost all their
+  objections and difficulties apply to those larger differences
+  which separate genera, families, and orders from each other, not
+  to those which separate one species from the species to which it
+  is most nearly allied, and from the remaining species of the same
+  genus. They adduce such difficulties as the first development of
+  the eye, or of the milk-producing glands of the mammalia; the
+  wonderful instincts of bees and of ants; the complex arrangements
+  for the fertilisation of orchids, and numerous other points of
+  structure or habit, as not being satisfactorily explained. But it
+  is evident that these peculiarities had their origin at a very
+  remote period of the earth's history, and no theory, however
+  complete, can do more than afford a probable conjecture as to how
+  they were produced. Our ignorance of the state of the earth's
+  surface and of the conditions of life at those remote periods is
+  very great; thousands of animals and plants must have existed of
+  which we have no record; while we are usually without any
+  information as to the habits and general life-history even of
+  those of which we possess some fragmentary remains; so that the
+  truest and most complete theory would not enable us to solve
+  <i>all</i> the difficult problems which the whole course of the
+  development of life upon our globe presents to us.</p>
+
+  <p>What we may expect a true theory to do is to enable us to
+  comprehend and follow out in some detail those changes in the
+  form, structure, and relations of animals and plants which are
+  effected in short periods of time, geologically speaking, and
+  which are now going on around us. We may expect it to explain
+  satisfactorily most of the lesser and superficial differences
+  which distinguish one species from another. We may expect it to
+  throw light on the mutual relations of the <a name="Page_8" id=
+  "Page_8"></a>animals and plants which live together in any one
+  country, and to give some rational account of the phenomena
+  presented by their distribution in different parts of the world.
+  And, lastly, we may expect it to explain many difficulties and to
+  harmonise many incongruities in the excessively complex
+  affinities and relations of living things. All this the Darwinian
+  theory undoubtedly does. It shows us how, by means of some of the
+  most universal and ever-acting laws in nature, new species are
+  necessarily produced, while the old species become extinct; and
+  it enables us to understand how the continuous action of these
+  laws during the long periods with which geology makes us
+  acquainted is calculated to bring about those greater differences
+  presented by the distinct genera, families, and orders into which
+  all living things are classified by naturalists. The differences
+  which these present are all of the same <i>nature</i> as those
+  presented by the species of many large genera, but much greater
+  in <i>amount</i>; and they can all be explained by the action of
+  the same general laws and by the extinction of a larger or
+  smaller number of intermediate species. Whether the distinctions
+  between the higher groups termed Classes and Sub-kingdoms may be
+  accounted for in the same way is a much more difficult question.
+  The differences which separate the mammals, birds, reptiles, and
+  fishes from each other, though vast, yet seem of the same nature
+  as those which distinguish a mouse from an elephant or a swallow
+  from a goose. But the vertebrate animals, the mollusca, and the
+  insects, are so radically distinct in their whole organisation
+  and in the very plan of their structure, that objectors may not
+  unreasonably doubt whether they can all have been derived from a
+  common ancestor by means of the very same laws as have sufficed
+  for the differentiation of the various species of birds or of
+  reptiles.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Change of Opinion effected by Darwin</i>.</p>
+
+  <p>The point I wish especially to urge is this. Before Darwin's
+  work appeared, the great majority of naturalists, and almost
+  without exception the whole literary and scientific world, held
+  firmly to the belief that <i>species</i> were realities, and had
+  not been derived from other species by any process accessible to
+  us; the different species of crow and of violet <a name="Page_9"
+  id="Page_9"></a>they are now, and to have originated by some
+  totally unknown process so far removed from ordinary reproduction
+  that it was usually spoken of as "special creation." There was,
+  then, no question of the origin of families, orders, and classes,
+  because the very first step of all, the "origin of species," was
+  believed to be an insoluble problem. But now this is all changed.
+  The whole scientific and literary world, even the whole educated
+  public, accepts, as a matter of common knowledge, the origin of
+  species from other allied species by the ordinary process of
+  natural birth. The idea of special creation or any altogether
+  exceptional mode of production is absolutely extinct! Yet more:
+  this is held also to apply to many higher groups as well as to
+  the species of a genus, and not even Mr. Darwin's severest
+  critics venture to suggest that the primeval bird, reptile, or
+  fish must have been "specially created." And this vast, this
+  totally unprecedented change in public opinion has been the
+  result of the work of one man, and was brought about in the short
+  space of twenty years! This is the answer to those who continue
+  to maintain that the "origin of species" is not yet discovered;
+  that there are still doubts and difficulties; that there are
+  divergencies of structure so great that we cannot understand how
+  they had their beginning. We may admit all this, just as we may
+  admit that there are enormous difficulties in the way of a
+  complete comprehension of the origin and nature of all the parts
+  of the solar system and of the stellar universe. But we claim for
+  Darwin that he is the Newton of natural history, and that, just
+  so surely as that the discovery and demonstration by Newton of
+  the law of gravitation established order in place of chaos and
+  laid a sure foundation for all future study of the starry
+  heavens, so surely has Darwin, by his discovery of the law of
+  natural selection and his demonstration of the great principle of
+  the preservation of useful variations in the struggle for life,
+  not only thrown a flood of light on the process of development of
+  the whole organic world, but also established a firm foundation
+  for all future study of nature.</p>
+
+  <p>In order to show the view Darwin took of his own work, and
+  what it was that he alone claimed to have done, the concluding
+  passage of the introduction to the <i>Origin of</i> <a name=
+  "Page_10" id="Page_10"></a><i>Species</i> should be carefully
+  considered. It is as follows: "Although much remains obscure, and
+  will long remain obscure, I can entertain no doubt, after the
+  most deliberate and dispassionate judgment of which I am capable,
+  that the view which most naturalists until recently entertained
+  and which I formerly entertained&mdash;namely, that each species
+  has been independently created&mdash;is erroneous. I am fully
+  convinced that species are not immutable; but that those
+  belonging to what are called the same genera are lineal
+  descendants of some other and generally extinct species, in the
+  same manner as the acknowledged varieties of any one species are
+  the descendants of that species. Furthermore, I am convinced that
+  Natural Selection has been the most important, but not the
+  exclusive, means of modification."</p>
+
+  <p>It should be especially noted that all which is here claimed
+  is now almost universally admitted, while the criticisms of
+  Darwin's works refer almost exclusively to those numerous
+  questions which, as he himself says, "will long remain
+  obscure."</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Darwinian Theory</i>.</p>
+
+  <p>As it will be necessary, in the following chapters, to set
+  forth a considerable body of facts in almost every department of
+  natural history, in order to establish the fundamental
+  propositions on which the theory of natural selection rests, I
+  propose to give a preliminary statement of what the theory really
+  is, in order that the reader may better appreciate the necessity
+  for discussing so many details, and may thus feel a more
+  enlightened interest in them. Many of the facts to be adduced are
+  so novel and so curious that they are sure to be appreciated by
+  every one who takes an interest in nature, but unless the need of
+  them is clearly seen it may be thought that time is being wasted
+  on mere curious details and strange facts which have little
+  bearing on the question at issue.</p>
+
+  <p>The theory of natural selection rests on two main classes of
+  facts which apply to all organised beings without exception, and
+  which thus take rank as fundamental principles or laws. The first
+  is, the power of rapid multiplication in a geometrical
+  progression; the second, that the offspring always vary slightly
+  from the parents, though generally very closely resembling
+  <a name="Page_11" id="Page_11"></a>them. From the first fact or
+  law there follows, necessarily, a constant struggle for
+  existence; because, while the offspring always exceed the parents
+  in number, generally to an enormous extent, yet the total number
+  of living organisms in the world does not, and cannot, increase
+  year by year. Consequently every year, on the average, as many
+  die as are born, plants as well as animals; and the majority die
+  premature deaths. They kill each other in a thousand different
+  ways; they starve each other by some consuming the food that
+  others want; they are destroyed largely by the powers of
+  nature&mdash;by cold and heat, by rain and storm, by flood and
+  fire. There is thus a perpetual struggle among them which shall
+  live and which shall die; and this struggle is tremendously
+  severe, because so few can possibly remain alive&mdash;one in
+  five, one in ten, often only one in a hundred or even one in a
+  thousand.</p>
+
+  <p>Then comes the question, Why do some live rather than others?
+  If all the individuals of each species were exactly alike in
+  every respect, we could only say it is a matter of chance. But
+  they are not alike. We find that they vary in many different
+  ways. Some are stronger, some swifter, some hardier in
+  constitution, some more cunning. An obscure colour may render
+  concealment more easy for some, keener sight may enable others to
+  discover prey or escape from an enemy better than their fellows.
+  Among plants the smallest differences may be useful or the
+  reverse. The earliest and strongest shoots may escape the slug;
+  their greater vigour may enable them to flower and seed earlier
+  in a wet autumn; plants best armed with spines or hairs may
+  escape being devoured; those whose flowers are most conspicuous
+  may be soonest fertilised by insects. We cannot doubt that, on
+  the whole, any beneficial variations will give the possessors of
+  it a greater probability of living through the tremendous ordeal
+  they have to undergo. There may be something left to chance, but
+  on the whole <i>the fittest will survive</i>.</p>
+
+  <p>Then we have another important fact to consider, the principle
+  of heredity or transmission of variations. If we grow plants from
+  seed or breed any kind of animals year after year, consuming or
+  giving away all the increase we do not wish to keep just as they
+  come to hand, our plants or animals will continue much the same;
+  but if every year we <a name="Page_12" id="Page_12"></a>carefully
+  save the best seed to sow and the finest or brightest coloured
+  animals to breed from, we shall soon find that an improvement
+  will take place, and that the average quality of our stock will
+  be raised. This is the way in which all our fine garden fruits
+  and vegetables and flowers have been produced, as well as all our
+  splendid breeds of domestic animals; and they have thus become in
+  many cases so different from the wild races from which they
+  originally sprang as to be hardly recognisable as the same. It is
+  therefore proved that if any particular kind of variation is
+  preserved and bred from, the variation itself goes on increasing
+  in amount to an enormous extent; and the bearing of this on the
+  question of the origin of species is most important. For if in
+  each generation of a given animal or plant the fittest survive to
+  continue the breed, then whatever may be the special peculiarity
+  that causes "fitness" in the particular case, that peculiarity
+  will go on increasing and strengthening <i>so long as it is
+  useful to the species</i>. But the moment it has reached its
+  maximum of usefulness, and some other quality or modification
+  would help in the struggle, then the individuals which vary in
+  the new direction will survive; and thus a species may be
+  gradually modified, first in one direction, then in another, till
+  it differs from the original parent form as much as the greyhound
+  differs from any wild dog or the cauliflower from any wild plant.
+  But animals or plants which thus differ in a state of nature are
+  always classed as distinct species, and thus we see how, by the
+  continuous survival of the fittest or the preservation of
+  favoured races in the struggle for life, new species may be
+  originated.</p>
+
+  <p>This self-acting process which, by means of a few easily
+  demonstrated groups of facts, brings about change in the organic
+  world, and keeps each species in harmony with the conditions of
+  its existence, will appear to some persons so clear and simple as
+  to need no further demonstration. But to the great majority of
+  naturalists and men of science endless difficulties and
+  objections arise, owing to the wonderful variety of animal and
+  vegetable forms, and the intricate relations of the different
+  species and groups of species with each other; and it was to
+  answer as many of these objections as possible, and to show that
+  the more we know of nature the more we <a name="Page_13" id=
+  "Page_13"></a>find it to harmonise with the development
+  hypothesis, that Darwin devoted the whole of his life to
+  collecting facts and making experiments, the record of a portion
+  of which he has given us in a series of twelve masterly
+  volumes.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Proposed Mode of Treatment of the Subject</i>.</p>
+
+  <p>It is evidently of the most vital importance to any theory
+  that its foundations should be absolutely secure. It is therefore
+  necessary to show, by a wide and comprehensive array of facts,
+  that animals and plants <i>do</i> perpetually vary in the manner
+  and to the amount requisite; and that this takes place in wild
+  animals as well as in those which are domesticated. It is
+  necessary also to prove that all organisms <i>do</i> tend to
+  increase at the great rate alleged, and that this increase
+  actually occurs, under favourable conditions. We have to prove,
+  further, that variations of all kinds can be increased and
+  accumulated by selection; and that the struggle for existence to
+  the extent here indicated actually occurs in nature, and leads to
+  the continued preservation of favourable variations.</p>
+
+  <p>These matters will be discussed in the four succeeding
+  chapters, though in a somewhat different order&mdash;the struggle
+  for existence and the power of rapid multiplication, which is its
+  cause, occupying the first place, as comprising those facts which
+  are the most fundamental and those which can be perfectly
+  explained without any reference to the less generally understood
+  facts of variation. These chapters will be followed by a
+  discussion of certain difficulties, and of the vexed question of
+  hybridity. Then will come a rather full account of the more
+  important of the complex relations of organisms to each other and
+  to the earth itself, which are either fully explained or greatly
+  elucidated by the theory. The concluding chapter will treat of
+  the origin of man and his relations to the lower animals.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href=
+      "#FNanchor_1_1"><span class="label">[1]</span></a>
+      <i>Geography and Classification of Animals</i>, p. 350.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href=
+      "#FNanchor_2_2"><span class="label">[2]</span></a> These
+      expressions occur in Chapter IX. of the earlier editions (to
+      the ninth) of the <i>Principles of Geology</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href=
+      "#FNanchor_3_3"><span class="label">[3]</span></a> L.
+      Agassiz, <i>Lake Superior</i>, p. 377.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_II" id="CHAPTER_II"></a><a name="Page_14"
+  id="Page_14"></a>CHAPTER II</h2>
+
+  <p style='font-weight:bold;'>THE STRUGGLE FOR EXISTENCE</p>
+
+  <div class="blockquot">
+    <p>Its importance&mdash;The struggle among plants&mdash;Among
+    animals&mdash;Illustrative cases&mdash;Succession of trees in
+    forests of Denmark&mdash;The struggle for existence on the
+    Pampas&mdash;Increase of organisms in a geometrical
+    ratio&mdash;Examples of great powers of increase of
+    animals&mdash;Rapid increase and wide spread of
+    plants&mdash;Great fertility not essential to rapid
+    increase&mdash;Struggle between closely allied species most
+    severe&mdash;The ethical aspect of the struggle for
+    existence.</p>
+  </div>
+
+  <p>There is perhaps no phenomenon of nature that is at once so
+  important, so universal; and so little understood, as the
+  struggle for existence continually going on among all organised
+  beings. To most persons nature appears calm, orderly, and
+  peaceful. They see the birds singing in the trees, the insects
+  hovering over the flowers, the squirrel climbing among the
+  tree-tops, and all living things in the possession of health and
+  vigour, and in the enjoyment of a sunny existence. But they do
+  not see, and hardly ever think of, the means by which this beauty
+  and harmony and enjoyment is brought about. They do not see the
+  constant and daily search after food, the failure to obtain which
+  means weakness or death; the constant effort to escape enemies;
+  the ever-recurring struggle against the forces of nature. This
+  daily and hourly struggle, this incessant warfare, is
+  nevertheless the very means by which much of the beauty and
+  harmony and enjoyment in nature is produced, and also affords one
+  of the most important elements in bringing about the origin of
+  species. We must, therefore, devote some time to the
+  consideration of its various aspects and of the many curious
+  phenomena to which it gives rise.</p>
+
+  <p>It is a matter of common observation that if weeds are allowed
+  to grow unchecked in a garden they will soon destroy <a name=
+  "Page_15" id="Page_15"></a>a number of the flowers. It is not so
+  commonly known that if a garden is left to become altogether
+  wild, the weeds that first take possession of it, often covering
+  the whole surface of the ground with two or three different
+  kinds, will themselves be supplanted by others, so that in a few
+  years many of the original flowers and of the earliest weeds may
+  alike have disappeared. This is one of the very simplest cases of
+  the struggle for existence, resulting in the successive
+  displacement of one set of species by another; but the exact
+  causes of this displacement are by no means of such a simple
+  nature. All the plants concerned may be perfectly hardy, all may
+  grow freely from seed, yet when left alone for a number of years,
+  each set is in turn driven out by a succeeding set, till at the
+  end of a considerable period&mdash;a century or a few centuries
+  perhaps&mdash;hardly one of the plants which first monopolised
+  the ground would be found there.</p>
+
+  <p>Another phenomenon of an analogous kind is presented by the
+  different behaviour of introduced wild plants or animals into
+  countries apparently quite as well suited to them as those which
+  they naturally inhabit. Agassiz, in his work on Lake Superior,
+  states that the roadside weeds of the northeastern United States,
+  to the number of 130 species, are all European, the native weeds
+  having disappeared westwards; and in New Zealand there are no
+  less than 250 species of naturalised European plants, more than
+  100 species of which have spread widely over the country, often
+  displacing the native vegetation. On the other hand, of the many
+  hundreds of hardy plants which produce seed freely in our
+  gardens, very few ever run wild, and hardly any have become
+  common. Even attempts to naturalise suitable plants usually fail;
+  for A. de Candolle states that several botanists of Paris,
+  Geneva, and especially of Montpellier, have sown the seeds of
+  many hundreds of species of hardy exotic plants in what appeared
+  to be the most favourable situations, but that, in hardly a
+  single case, has any one of them become naturalised.<a name=
+  "FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4"
+  class="fnanchor">[4]</a> Even a plant like the potato&mdash;so
+  widely cultivated, so hardy, and so well adapted to spread by
+  means of its many-eyed tubers&mdash;has not established itself in
+  a wild state in any part of Europe. It would be thought that
+  Australian plants would easily run <a name="Page_16" id=
+  "Page_16"></a>wild in New Zealand. But Sir Joseph Hooker informs
+  us that the late Mr. Bidwell habitually scattered Australian
+  seeds during his extensive travels in New Zealand, yet only two
+  or three Australian plants appear to have established themselves
+  in that country, and these only in cultivated or newly moved
+  soil.</p>
+
+  <p>These few illustrations sufficiently show that all the plants
+  of a country are, as De Candolle says, at war with each other,
+  each one struggling to occupy ground at the expense of its
+  neighbour. But, besides this direct competition, there is one not
+  less powerful arising from the exposure of almost all plants to
+  destruction by animals. The buds are destroyed by birds, the
+  leaves by caterpillars, the seeds by weevils; some insects bore
+  into the trunk, others burrow in the twigs and leaves; slugs
+  devour the young seedlings and the tender shoots, wire-worms gnaw
+  the roots. Herbivorous mammals devour many species bodily, while
+  some uproot and devour the buried tubers.</p>
+
+  <p>In animals, it is the eggs or the very young that suffer most
+  from their various enemies; in plants, the tender seedlings when
+  they first appear above the ground. To illustrate this latter
+  point Mr. Darwin cleared and dug a piece of ground three feet
+  long and two feet wide, and then marked all the seedlings of
+  weeds and other plants which came up, noting what became of them.
+  The total number was 357, and out of these no less than 295 were
+  destroyed by slugs and insects. The direct strife of plant with
+  plant is almost equally fatal when the stronger are allowed to
+  smother the weaker. When turf is mown or closely browsed by
+  animals, a number of strong and weak plants live together,
+  because none are allowed to grow much beyond the rest; but Mr.
+  Darwin found that when the plants which compose such turf are
+  allowed to grow up freely, the stronger kill the weaker. In a
+  plot of turf three feet by four, twenty distinct species of
+  plants were found to be growing, and no less than nine of these
+  perished altogether when the other species were allowed to grow
+  up to their full size.<a name="FNanchor_5_5" id=
+  "FNanchor_5_5"></a><a href="#Footnote_5_5" class=
+  "fnanchor">[5]</a></p>
+
+  <p>But besides having to protect themselves against competing
+  plants and against destructive animals, there is a yet deadlier
+  <a name="Page_17" id="Page_17"></a>enemy in the forces of
+  inorganic nature. Each species can sustain a certain amount of
+  heat and cold, each requires a certain amount of moisture at the
+  right season, each wants a proper amount of light or of direct
+  sunshine, each needs certain elements in the soil; the failure of
+  a due proportion in these inorganic conditions causes weakness,
+  and thus leads to speedy death. The struggle for existence in
+  plants is, therefore, threefold in character and infinite in
+  complexity, and the result is seen in their curiously irregular
+  distribution over the face of the earth. Not only has each
+  country its distinct plants, but every valley, every hillside,
+  almost every hedgerow, has a different set of plants from its
+  adjacent valley, hillside, or hedgerow&mdash;if not always
+  different in the actual species yet very different in comparative
+  abundance, some which are rare in the one being common in the
+  other. Hence it happens that slight changes of conditions often
+  produce great changes in the flora of a country. Thus in 1740 and
+  the two following years the larva of a moth (Phalaena graminis)
+  committed such destruction in many of the meadows of Sweden that
+  the grass was greatly diminished in quantity, and many plants
+  which were before choked by the grass sprang up, and the ground
+  became variegated with a multitude of different species of
+  flowers. The introduction of goats into the island of St. Helena
+  led to the entire destruction of the native forests, consisting
+  of about a hundred distinct species of trees and shrubs, the
+  young plants being devoured by the goats as fast as they grew up.
+  The camel is a still greater enemy to woody vegetation than the
+  goat, and Mr. Marsh believes that forests would soon cover
+  considerable tracts of the Arabian and African deserts if the
+  goat and the camel were removed from them.<a name="FNanchor_6_6"
+  id="FNanchor_6_6"></a><a href="#Footnote_6_6" class=
+  "fnanchor">[6]</a> Even in many parts of our own country the
+  existence of trees is dependent on the absence of cattle. Mr.
+  Darwin observed, on some extensive heaths near Farnham, in
+  Surrey, a few clumps of old Scotch firs, but no young trees over
+  hundreds of acres. Some portions of the heath had, however, been
+  enclosed a few years before, and these enclosures were crowded
+  with young fir-trees growing too close together for all to live;
+  and these were not sown or planted, nothing having been done to
+  the ground beyond enclosing it <a name="Page_18" id=
+  "Page_18"></a>so as to keep out cattle. On ascertaining this, Mr.
+  Darwin was so much surprised that he searched among the heather
+  in the unenclosed parts, and there he found multitudes of little
+  trees and seedlings which had been perpetually browsed down by
+  the cattle. In one square yard, at a point about a hundred yards
+  from one of the old clumps of firs, he counted thirty-two little
+  trees, and one of them had twenty-six rings of growth, showing
+  that it had for many years tried to raise its head above the
+  stems of the heather and had failed. Yet this heath was very
+  extensive and very barren, and, as Mr. Darwin remarks, no one
+  would ever have imagined that cattle would have so closely and so
+  effectually searched it for food.</p>
+
+  <p>In the case of animals, the competition and struggle are more
+  obvious. The vegetation of a given district can only support a
+  certain number of animals, and the different kinds of
+  plant-eaters will compete together for it. They will also have
+  insects for their competitors, and these insects will be kept
+  down by birds, which will thus assist the mammalia. But there
+  will also be carnivora destroying the herbivora; while small
+  rodents, like the lemming and some of the field-mice, often
+  destroy so much vegetation as materially to affect the food of
+  all the other groups of animals. Droughts, floods, severe
+  winters, storms and hurricanes will injure these in various
+  degrees, but no one species can be diminished in numbers without
+  the effect being felt in various complex ways by all the rest. A
+  few illustrations of this reciprocal action must be given.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Illustrative Cases of the Struggle for Life</i>.</p>
+
+  <p>Sir Charles Lyell observes that if, by the attacks of seals or
+  other marine foes, salmon are reduced in numbers, the consequence
+  will be that otters, living far inland, will be deprived of food
+  and will then destroy many young birds or quadrupeds, so that the
+  increase of a marine animal may cause the destruction of many
+  land animals hundreds of miles away. Mr. Darwin carefully
+  observed the effects produced by planting a few hundred acres of
+  Scotch fir, in Staffordshire, on part of a very extensive heath
+  which had never been cultivated. After the planted portion was
+  about twenty-five years old he observed that the change in the
+  native vegetation <a name="Page_19" id="Page_19"></a>was greater
+  than is often seen in passing from one quite different soil to
+  another. Besides a great change in the proportional numbers of
+  the native heath-plants, twelve species which could not be found
+  on the heath flourished in the plantations. The effect on the
+  insect life must have been still greater, for six insectivorous
+  birds which were very common in the plantations were not to be
+  seen on the heath, which was, however, frequented by two or three
+  different species of insectivorous birds. It would have required
+  continued study for several years to determine all the
+  differences in the organic life of the two areas, but the facts
+  stated by Mr. Darwin are sufficient to show how great a change
+  may be effected by the introduction of a single kind of tree and
+  the keeping out of cattle.</p>
+
+  <p>The next case I will give in Mr. Darwin's own words: "In
+  several parts of the world insects determine the existence of
+  cattle. Perhaps Paraguay offers the most curious instance of
+  this; for here neither cattle nor horses nor dogs have ever run
+  wild, though they swarm southward and northward in a feral state;
+  and Azara and Rengger have shown that this is caused by the
+  greater numbers, in Paraguay, of a certain fly which lays its
+  eggs in the navels of these animals when first born. The increase
+  of these flies, numerous as they are, must be habitually checked
+  by some means, probably by other parasitic insects. Hence, if
+  certain insectivorous birds were to decrease in Paraguay, the
+  parasitic insects would probably increase; and this would lessen
+  the number of the navel-frequenting flies&mdash;then cattle and
+  horses would become feral, and this would greatly alter (as
+  indeed I have observed in parts of South America) the vegetation:
+  this again would largely affect the insects, and this, as we have
+  just seen in Staffordshire, the insectivorous birds, and so
+  onward in ever-increasing circles of complexity. Not that under
+  nature the relations will ever be as simple as this. Battle
+  within battle must be continually recurring with varying success;
+  and yet in the long run the forces are so nicely balanced, that
+  the face of nature remains for a long time uniform, though
+  assuredly the merest trifle would give the victory to one organic
+  being over another."<a name="FNanchor_7_7" id=
+  "FNanchor_7_7"></a><a href="#Footnote_7_7" class=
+  "fnanchor">[7]</a></p>
+
+  <p><a name="Page_20" id="Page_20"></a>Such cases as the above may
+  perhaps be thought exceptional, but there is good reason to
+  believe that they are by no means rare, but are illustrations of
+  what is going on in every part of the world, only it is very
+  difficult for us to trace out the complex reactions that are
+  everywhere occurring. The general impression of the ordinary
+  observer seems to be that wild animals and plants live peaceful
+  lives and have few troubles, each being exactly suited to its
+  place and surroundings, and therefore having no difficulty in
+  maintaining itself. Before showing that this view is, everywhere
+  and always, demonstrably untrue, we will consider one other case
+  of the complex relations of distinct organisms adduced by Mr.
+  Darwin, and often quoted for its striking and almost eccentric
+  character. It is now well known that many flowers require to be
+  fertilised by insects in order to produce seed, and this
+  fertilisation can, in some cases, only be effected by one
+  particular species of insect to which the flower has become
+  specially adapted. Two of our common plants, the wild
+  heart's-ease (Viola tricolor) and the red clover (Trifolium
+  pratense), are thus fertilised by humble-bees almost exclusively,
+  and if these insects are prevented from visiting the flowers,
+  they produce either no seed at all or exceedingly few. Now it is
+  known that field-mice destroy the combs and nests of humble-bees,
+  and Colonel Newman, who has paid great attention to these
+  insects, believes that more than two-thirds of all the
+  humble-bees' nests in England are thus destroyed. But the number
+  of mice depends a good deal on the number of cats; and the same
+  observer says that near villages and towns he has found the nests
+  of humble-bees more numerous than elsewhere, which he attributes
+  to the number of cats that destroy the mice. Hence it follows,
+  that the abundance of red clover and wild heart's-ease in a
+  district will depend on a good supply of cats to kill the mice,
+  which would otherwise destroy and keep down the humble-bees and
+  prevent them from fertilising the flowers. A chain of connection
+  has thus been found between such totally distinct organisms as
+  flesh-eating mammalia and sweet-smelling flowers, the abundance
+  or scarcity of the one closely corresponding to that of the
+  other!</p>
+
+  <p>The following account of the struggle between trees in the
+  forests of Denmark, from the researches of M. Hansten-<a name=
+  "Page_21" id="Page_21"></a>Blangsted, strikingly illustrates our
+  subject.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href=
+  "#Footnote_8_8" class="fnanchor">[8]</a> The chief combatants are
+  the beech and the birch, the former being everywhere successful
+  in its invasions. Forests composed wholly of birch are now only
+  found in sterile, sandy tracts; everywhere else the trees are
+  mixed, and wherever the soil is favourable the beech rapidly
+  drives out the birch. The latter loses its branches at the touch
+  of the beech, and devotes all its strength to the upper part
+  where it towers above the beech. It may live long in this way,
+  but it succumbs ultimately in the fight&mdash;of old age if of
+  nothing else, for the life of the birch in Denmark is shorter
+  than that of the beech. The writer believes that light (or rather
+  shade) is the cause of the superiority of the latter, for it has
+  a greater development of its branches than the birch, which is
+  more open and thus allows the rays of the sun to pass through to
+  the soil below, while the tufted, bushy top of the beech
+  preserves a deep shade at its base. Hardly any young plants can
+  grow under the beech except its own shoots; and while the beech
+  can nourish under the shade of the birch, the latter dies
+  immediately under the beech. The birch has only been saved from
+  total extermination by the facts that it had possession of the
+  Danish forests long before the beech ever reached the country,
+  and that certain districts are unfavourable to the growth of the
+  latter. But wherever the soil has been enriched by the
+  decomposition of the leaves of the birch the battle begins. The
+  birch still flourishes on the borders of lakes and other marshy
+  places, where its enemy cannot exist. In the same way, in the
+  forests of Zeeland, the fir forests are disappearing before the
+  beech. Left to themselves, the firs are soon displaced by the
+  beech. The struggle between the latter and the oak is longer and
+  more stubborn, for the branches and foliage of the oak are
+  thicker, and offer much resistance to the passage of light. The
+  oak, also, has greater longevity; but, sooner or later, it too
+  succumbs, because it cannot develop in the shadow of the beech.
+  The earliest forests of Denmark were mainly composed of aspens,
+  with which the birch was apparently associated; gradually the
+  soil was raised, and the climate grew milder; then the fir came
+  and formed large forests. This tree ruled for centuries, and then
+  ceded the <a name="Page_22" id="Page_22"></a>first place to the
+  holm-oak, which is now giving way to the beech. Aspen, birch,
+  fir, oak, and beech appear to be the steps in the struggle for
+  the survival of the fittest among the forest-trees of
+  Denmark.</p>
+
+  <p>It may be added that in the time of the Romans the beech was
+  the principal forest-tree of Denmark as it is now, while in the
+  much earlier bronze age, represented by the later remains found
+  in the peat bogs, there were no beech-trees, or very few, the oak
+  being the prevailing tree, while in the still earlier stone
+  period the fir was the most abundant. The beech is a tree
+  essentially of the temperate zone, having its northern limit
+  considerably southward of the oak, fir, birch, or aspen, and its
+  entrance into Denmark was no doubt due to the amelioration of the
+  climate after the glacial epoch had entirely passed away. We thus
+  see how changes of climate, which are continually occurring owing
+  either to cosmical or geographical causes, may initiate a
+  struggle among plants which may continue for thousands of years,
+  and which must profoundly modify the relations of the animal
+  world, since the very existence of innumerable insects, and even
+  of many birds and mammals, is dependent more or less completely
+  on certain species of plants.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Struggle for Existence on the Pampas</i>.</p>
+
+  <p>Another illustration of the struggle for existence,in which
+  both plants and animals are implicated, is afforded by the pampas
+  of the southern part of South America. The absence of trees from
+  these vast plains has been imputed by Mr. Darwin to the supposed
+  inability of the tropical and sub-tropical forms of South America
+  to thrive on them, and there being no other source from which
+  they could obtain a supply; and that explanation was adopted by
+  such eminent botanists as Mr. Ball and Professor Asa Gray. This
+  explanation has always seemed to me unsatisfactory, because there
+  are ample forests both in the temperate regions of the Andes and
+  on the whole west coast down to Terra del Fuego; and it is
+  inconsistent with what we know of the rapid variation and
+  adaptation of species to new conditions. What seems a more
+  satisfactory explanation has been given by Mr. Edwin Clark, a
+  civil engineer, who resided nearly two years in the country and
+  <a name="Page_23" id="Page_23"></a>paid much attention to its
+  natural history. He says: "The peculiar characteristics of these
+  vast level plains which descend from the Andes to the great river
+  basin in unbroken monotony, are the absence of rivers or
+  water-storage, and the periodical occurrence of droughts, or
+  'siccos,' in the summer months. These conditions determine the
+  singular character both of its flora and fauna.</p>
+
+  <p>"The soil is naturally fertile and favourable for the growth
+  of trees, and they grow luxuriantly wherever they are protected.
+  The eucalyptus is covering large tracts wherever it is enclosed,
+  and willows, poplars, and the fig surround every estancia when
+  fenced in.</p>
+
+  <p>"The open plains are covered with droves of horses and cattle,
+  and overrun by numberless wild rodents, the original tenants of
+  the pampas. During the long periods of drought, which are so
+  great a scourge to the country, these animals are starved by
+  thousands, destroying, in their efforts to live, every vestige of
+  vegetation. In one of these 'siccos,' at the time of my visit, no
+  less than 50,000 head of oxen and sheep and horses perished from
+  starvation and thirst, after tearing deep out of the soil every
+  trace of vegetation, including the wiry roots of the
+  pampas-grass. Under such circumstances the existence of an
+  unprotected tree is impossible. The only plants that hold their
+  own, in addition to the indestructible thistles, grasses, and
+  clover, are a little herbaceous oxalis, producing viviparous buds
+  of extraordinary vitality, a few poisonous species, such as the
+  hemlock, and a few tough, thorny dwarf-acacias and wiry rushes,
+  which even a starving rat refuses.</p>
+
+  <p>"Although the cattle are a modern introduction, the numberless
+  indigenous rodents must always have effectually prevented the
+  introduction of any other species of plants; large tracts are
+  still honeycombed by the ubiquitous biscacho, a gigantic rabbit;
+  and numerous other rodents still exist, including rats and mice,
+  pampas-hares, and the great nutria and carpincho (capybara) on
+  the river banks."<a name="FNanchor_9_9" id=
+  "FNanchor_9_9"></a><a href="#Footnote_9_9" class=
+  "fnanchor">[9]</a></p>
+
+  <p>Mr. Clark further remarks on the desperate struggle for
+  existence which characterises the bordering fertile zones, where
+  rivers and marshy plains permit a more luxuriant and varied
+  vegetable and animal life. After describing how the <a name=
+  "Page_24" id="Page_24"></a>river sometimes rose 30 feet in eight
+  hours, doing immense destruction, and the abundance of the larger
+  carnivora and large reptiles on its banks, he goes on: "But it
+  was among the flora that the principle of natural selection was
+  most prominently displayed. In such a district&mdash;overrun with
+  rodents and escaped cattle, subject to floods that carried away
+  whole islands of botany, and especially to droughts that dried up
+  the lakes and almost the river itself&mdash;no ordinary plant
+  could live, even on this rich and watered alluvial debris. The
+  only plants that escaped the cattle were such as were either
+  poisonous, or thorny, or resinous, or indestructibly tough. Hence
+  we had only a great development of solanums, talas, acacias,
+  euphorbias, and laurels. The buttercup is replaced by the little
+  poisonous yellow oxalis with its viviparous buds; the
+  passion-flowers, asclepiads, bignonias, convolvuluses, and
+  climbing leguminous plants escape both floods and cattle by
+  climbing the highest trees and towering overhead in a flood of
+  bloom. The ground plants are the portulacas, turneras, and
+  cenotheras, bitter and ephemeral, on the bare rock, and almost
+  independent of any other moisture than the heavy dews. The
+  pontederias, alismas, and plantago, with grasses and sedges,
+  derive protection from the deep and brilliant pools; and though
+  at first sight the 'monte' doubtless impresses the traveller as a
+  scene of the wildest confusion and ruin, yet, on closer
+  examination, we found it far more remarkable as a manifestation
+  of harmony and law, and a striking example of the marvellous
+  power which plants, like animals, possess, of adapting themselves
+  to the local peculiarities of their habitat, whether in the
+  fertile shades of the luxuriant 'monte' or on the arid,
+  parched-up plains of the treeless pampas."</p>
+
+  <p>A curious example of the struggle between plants has been
+  communicated to me by Mr. John Ennis, a resident in New Zealand.
+  The English water-cress grows so luxuriantly in that country as
+  to completely choke up the rivers, sometimes leading to
+  disastrous floods, and necessitating great outlay to keep the
+  stream open. But a natural remedy has now been found in planting
+  willows on the banks. The roots of these trees penetrate the bed
+  of the stream in every direction, and the water-cress, unable to
+  obtain the requisite amount of nourishment, gradually
+  disappears.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_25" id="Page_25"></a><i>Increase of Organisms in a
+  Geometrical Ratio</i>.</p>
+
+  <p>The facts which have now been adduced, sufficiently prove that
+  there is a continual competition, and struggle, and war going on
+  in nature, and that each species of animal and plant affects many
+  others in complex and often unexpected ways. We will now proceed
+  to show the fundamental cause of this struggle, and to prove that
+  it is ever acting over the whole field of nature, and that no
+  single species of animal or plant can possibly escape from it.
+  This results from the fact of the rapid increase, in a
+  geometrical ratio, of all the species of animals and plants. In
+  the lower orders this increase is especially rapid, a single
+  flesh-fly (Musca carnaria) producing 20,000 larvae, and these
+  growing so quickly that they reach their full size in five days;
+  hence the great Swedish naturalist, Linnaeus, asserted that a
+  dead horse would be devoured by three of these flies as quickly
+  as by a lion. Each of these larvae remains in the pupa state
+  about five or six days, so that each parent fly may be increased
+  ten thousand-fold in a fortnight. Supposing they went on
+  increasing at this rate during only three months of summer, there
+  would result one hundred millions of millions of millions for
+  each fly at the commencement of summer,&mdash;a number greater
+  probably than exists at any one time in the whole world. And this
+  is only one species, while there are thousands of other species
+  increasing also at an enormous rate; so that, if they were
+  unchecked, the whole atmosphere would be dense with flies, and
+  all animal food and much of animal life would be destroyed by
+  them. To prevent this tremendous increase there must be incessant
+  war against these insects, by insectivorous birds and reptiles as
+  well as by other insects, in the larva as well as in the perfect
+  state, by the action of the elements in the form of rain, hail,
+  or drought, and by other unknown causes; yet we see nothing of
+  this ever-present war, though by its means alone, perhaps, we are
+  saved from famine and pestilence.</p>
+
+  <p>Let us now consider a less extreme and more familiar case. We
+  possess a considerable number of birds which, like the redbreast,
+  sparrow, the four common titmice, the thrush, and the blackbird,
+  stay with us all the year round These lay on an average six eggs,
+  but, as several of them have <a name="Page_26" id=
+  "Page_26"></a>two or more broods a year, ten will be below the
+  average of the year's increase. Such birds as these often live
+  from fifteen to twenty years in confinement, and we cannot
+  suppose them to live shorter lives in a state of nature, if
+  unmolested; but to avoid possible exaggeration we will take only
+  ten years as the average duration of their lives. Now, if we
+  start with a single pair, and these are allowed to live and
+  breed, unmolested, till they die at the end of ten
+  years,&mdash;as they might do if turned loose into a good-sized
+  island with ample vegetable and insect food, but no other
+  competing or destructive birds or quadrupeds&mdash;their numbers
+  would amount to more than twenty millions. But we know very well
+  that our bird population is no greater, on the average, now than
+  it was ten years ago. Year by year it may fluctuate a little
+  according as the winters are more or less severe, or from other
+  causes, but on the whole there is no increase. What, then,
+  becomes of the enormous surplus population annually produced? It
+  is evident they must all die or be killed, somehow; and as the
+  increase is, on the average, about five to one, it follows that,
+  if the average number of birds of all kinds in our islands is
+  taken at ten millions&mdash;and this is probably far under the
+  mark&mdash;then about fifty millions of birds, including eggs as
+  possible birds, must annually die or be destroyed. Yet we see
+  nothing, or almost nothing, of this tremendous slaughter of the
+  innocents going on all around us. In severe winters a few birds
+  are found dead, and a few feathers or mangled remains show us
+  where a wood-pigeon or some other bird has been destroyed by a
+  hawk, but no one would imagine that five times as many birds as
+  the total number in the country in early spring die every year.
+  No doubt a considerable proportion of these do not die here but
+  during or after migration to other countries, but others which
+  are bred in distant countries come here, and thus balance the
+  account. Again, as the average number of young produced is four
+  or five times that of the parents, we ought to have at least five
+  times as many birds in the country at the end of summer as at the
+  beginning, and there is certainly no such enormous disproportion
+  as this. The fact is, that the destruction commences, and is
+  probably most severe, with nestling birds, which are often killed
+  by heavy rains or blown away by severe storms, or left to die of
+  hunger if either of <a name="Page_27" id="Page_27"></a>the
+  parents is killed; while they offer a defenceless prey to
+  jackdaws, jays, and magpies, and not a few are ejected from their
+  nests by their foster-brothers the cuckoos. As soon as they are
+  fledged and begin to leave the nest great numbers are destroyed
+  by buzzards, sparrow-hawks, and shrikes. Of those which migrate
+  in autumn a considerable proportion are probably lost at sea or
+  otherwise destroyed before they reach a place of safety; while
+  those which remain with us are greatly thinned by cold and
+  starvation during severe winters. Exactly the same thing goes on
+  with every species of wild animal and plant from the lowest to
+  the highest. All breed at such a rate, that in a few years the
+  progeny of any one species would, if allowed to increase
+  unchecked, alone monopolise the land; but all alike are kept
+  within bounds by various destructive agencies, so that, though
+  the numbers of each may fluctuate, they can never permanently
+  increase except at the expense of some others, which must
+  proportionately decrease.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Cases showing the Great Powers of Increase of Animals.</i></p>
+
+  <p>As the facts now stated are the very foundation of the theory
+  we are considering, and the enormous increase and perpetual
+  destruction continually going on require to be kept ever present
+  in the mind, some direct evidence of actual cases of increase
+  must be adduced. That even the larger animals, which breed
+  comparatively slowly, increase enormously when placed under
+  favourable conditions in new countries, is shown by the rapid
+  spread of cattle and horses in America. Columbus, in his second
+  voyage, left a few black cattle at St. Domingo, and these ran
+  wild and increased so much that, twenty-seven years afterwards,
+  herds of from 4000 to 8000 head were not uncommon. Cattle were
+  afterwards taken from this island to Mexico and to other parts of
+  America, and in 1587, sixty-five years after the conquest of
+  Mexico, the Spaniards exported 64,350 hides from that country and
+  35,444 from St. Domingo, an indication of the vast numbers of
+  these animals which must then have existed there, since those
+  captured and killed could have been only a small portion of the
+  whole. In the pampas of Buenos Ayres there were, at the end of
+  the last century, about twelve million cows and three million
+  horses, besides great numbers in all other parts <a name=
+  "Page_28" id="Page_28"></a>of America where open pastures offered
+  suitable conditions. Asses, about fifty years after their
+  introduction, ran wild and multiplied so amazingly in Quito, that
+  the Spanish traveller Ulloa describes them as being a nuisance.
+  They grazed together in great herds, defending themselves with
+  their mouths, and if a horse strayed among them they all fell
+  upon him and did not cease biting and kicking till they left him
+  dead. Hogs were turned out in St. Domingo by Columbus in 1493,
+  and the Spaniards took them to other places where they settled,
+  the result being, that in about half a century these animals were
+  found in great numbers over a large part of America, from 25&deg;
+  north to 40&deg; south latitude. More recently, in New Zealand,
+  pigs have multiplied so greatly in a wild state as to be a
+  serious nuisance and injury to agriculture. To give some idea of
+  their numbers, it is stated that in the province of Nelson there
+  were killed in twenty months 25,000 wild pigs.<a name=
+  "FNanchor_10_10" id="FNanchor_10_10"></a><a href=
+  "#Footnote_10_10" class="fnanchor">[10]</a> Now, in the case of
+  all these animals, we know that in their native countries, and
+  even in America at the present time, they do not increase at all
+  in numbers; therefore the whole normal increase must be kept
+  down, year by year, by natural or artificial means of
+  destruction.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Rapid Increase and Wide Spread of Plants</i>.</p>
+
+  <p>In the case of plants, the power of increase is even greater
+  and its effects more distinctly visible. Hundreds of square miles
+  of the plains of La Plata are now covered with two or three
+  species of European thistle, often to the exclusion of almost
+  every other plant; but in the native countries of these thistles
+  they occupy, except in cultivated or waste ground, a very
+  subordinate part in the vegetation. Some American plants, like
+  the cotton-weed (Asclepias cuiussayica), have now become common
+  weeds over a large portion of the tropics. White clover
+  (Trifolium repens) spreads over all the temperate regions of the
+  world, and in New Zealand is exterminating many native species,
+  including even the native flax (Phormium <a name="Page_29" id=
+  "Page_29"></a>tenax), a large plant with iris-like leaves 5 or 6
+  feet high. Mr. W.L. Travers has paid much attention to the
+  effects of introduced plants in New Zealand, and notes the
+  following species as being especially remarkable. The common
+  knotgrass (Polygonum aviculare) grows most luxuriantly, single
+  plants covering a space 4 or 5 feet in diameter, and sending
+  their roots 3 or 4 feet deep. A large sub-aquatic dock (Rumex
+  obtusifolius) abounds in every river-bed, even far up among the
+  mountains. The common sow-thistle (Sonchus oleraceus) grows all
+  over the country up to an elevation of 6000 feet. The water-cress
+  (Nasturtium officinale) grows with amazing vigour in many of the
+  rivers, forming stems 12 feet long and 3/4 inch in diameter, and
+  completely choking them up. It cost &pound;300 a year to keep the
+  Avon at Christchurch free from it. The sorrel (Rumex acetosella)
+  covers hundreds of acres with a sheet of red. It forms a dense
+  mat, exterminating other plants, and preventing cultivation. It
+  can, however, be itself exterminated by sowing the ground with
+  red clover, which will also vanquish the Polygonum aviculare. The
+  most noxious weed in New Zealand appears, however, to be the
+  Hypochaeris radicata, a coarse yellow-flowered composite not
+  uncommon in our meadows and waste places. This has been
+  introduced with grass seeds from England, and is very
+  destructive. It is stated that excellent pasture was in three
+  years destroyed by this weed, which absolutely displaced every
+  other plant on the ground. It grows in every kind of soil, and is
+  said even to drive out the white clover, which is usually so
+  powerful in taking possession of the soil.</p>
+
+  <p>In Australia another composite plant, called there the
+  Cape-weed (Cryptostemma calendulaceum), did much damage, and was
+  noticed by Baron Von Hugel in 1833 as "an unexterminable weed";
+  but, after forty years' occupation, it was found to give way to
+  the dense herbage formed by lucerne and choice grasses.</p>
+
+  <p>In Ceylon we are told by Mr. Thwaites, in his <i>Enumeration
+  of Ceylon Plants</i>, that a plant introduced into the island
+  less than fifty years ago is helping to alter the character of
+  the vegetation up to an elevation of 3000 feet. This is the
+  Lantana mixta, a verbenaceous plant introduced <a name="Page_30"
+  id="Page_30"></a>from the West Indies, which appears to have
+  found in Ceylon a soil and climate exactly suited to it. It now
+  covers thousands of acres with its dense masses of foliage,
+  taking complete possession of land where cultivation has been
+  neglected or abandoned, preventing the growth of any other
+  plants, and even destroying small trees, the tops of which its
+  subscandent stems are able to reach. The fruit of this plant is
+  so acceptable to frugivorous birds of all kinds that, through
+  their instrumentality, it is spreading rapidly, to the complete
+  exclusion of the indigenous vegetation where it becomes
+  established.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Great Fertility not essential to Rapid Increase</i>.</p>
+
+  <p>The not uncommon circumstance of slow-breeding animals being
+  very numerous, shows that it is usually the amount of destruction
+  which an animal or plant is exposed to, not its rapid
+  multiplication, that determines its numbers in any country. The
+  passenger-pigeon (Ectopistes migratorius) is, or rather was,
+  excessively abundant in a certain area in North America, and its
+  enormous migrating flocks darkening the sky for hours have often
+  been described; yet this bird lays only two eggs. The fulmar
+  petrel exists in myriads at St. Kilda and other haunts of the
+  species, yet it lays only one egg. On the other hand the great
+  shrike, the tree-creeper, the nut-hatch, the nut-cracker, the
+  hoopoe, and many other birds, lay from four to six or seven eggs,
+  and yet are never abundant. So in plants, the abundance of a
+  species bears little or no relation to its seed-producing power.
+  Some of the grasses and sedges, the wild hyacinth, and many
+  buttercups occur in immense profusion over extensive areas,
+  although each plant produces comparatively few seeds; while
+  several species of bell-flowers, gentians, pinks, and mulleins,
+  and even some of the composite, which produce an abundance of
+  minute seeds, many of which are easily scattered by the wind, are
+  yet rare species that never spread beyond a very limited
+  area.</p>
+
+  <p>The above-mentioned passenger-pigeon affords such an excellent
+  example of an enormous bird-population kept up by a comparatively
+  slow rate of increase, and in spite of its complete helplessness
+  and the great destruction which it suffers from its numerous
+  enemies, that the following account of one of its breeding-places
+  and migrations by the celebrated <a name="Page_31" id=
+  "Page_31"></a>American naturalist, Alexander Wilson, will be read
+  with interest:&mdash;</p>
+
+  <p>"Not far from Shelbyville, in the State of Kentucky, about
+  five years ago, there was one of these breeding-places, which
+  stretched through the woods in nearly a north and south
+  direction, was several miles in breadth, and was said to be
+  upwards of 40 miles in extent. In this tract almost every tree
+  was furnished with nests wherever the branches could accommodate
+  them. The pigeons made their first appearance there about the
+  10th of April, and left it altogether with their young before the
+  25th of May. As soon as the young were fully grown and before
+  they left the nests, numerous parties of the inhabitants from all
+  parts of the adjacent country came with waggons, axes, beds,
+  cooking utensils, many of them accompanied by the greater part of
+  their families, and encamped for several days at this immense
+  nursery. Several of them informed me that the noise was so great
+  as to terrify their horses, and that it was difficult for one
+  person to hear another without bawling in his ear. The ground was
+  strewed with broken limbs of trees, eggs, and young squab
+  pigeons, which had been precipitated from above, and on which
+  herds of hogs were fattening. Hawks, buzzards, and eagles were
+  sailing about in great numbers, and seizing the squabs from the
+  nests at pleasure; while, from 20 feet upwards to the top of the
+  trees, the view through the woods presented a perpetual tumult of
+  crowding and fluttering multitudes of pigeons, their wings
+  roaring like thunder, mingled with the frequent crash of falling
+  timber; for now the axemen were at work cutting down those trees
+  that seemed most crowded with nests, and contrived to fell them
+  in such a manner, that in their descent they might bring down
+  several others; by which means the falling of one large tree
+  sometimes produced 200 squabs little inferior in size to the old
+  birds, and almost one heap of fat. On some single trees upwards
+  of a hundred nests were found, each containing one squab only; a
+  circumstance in the history of the bird not generally known to
+  naturalists.<a name="FNanchor_11_11" id=
+  "FNanchor_11_11"></a><a href="#Footnote_11_11" class=
+  "fnanchor">[11]</a> It was dangerous to walk <a name="Page_32"
+  id="Page_32"></a>under these flying and fluttering millions, from
+  the frequent fall of large branches, broken down by the weight of
+  the multitudes above, and which in their descent often destroyed
+  numbers of the birds themselves; while the clothes of those
+  engaged in traversing the woods were completely covered with the
+  excrements of the pigeons.</p>
+
+  <p>"These circumstances were related to me by many of the most
+  respectable part of the community in that quarter, and were
+  confirmed in part by what I myself witnessed. I passed for
+  several miles through this same breeding-place, where every tree
+  was spotted with nests, the remains of those above described. In
+  many instances I counted upwards of ninety nests on a single
+  tree; but the pigeons had abandoned this place for another, 60 or
+  80 miles off, towards Green River, where they were said at that
+  time to be equally numerous. From the great numbers that were
+  constantly passing over our heads to or from that quarter, I had
+  no doubt of the truth of this statement. The mast had been
+  chiefly consumed in Kentucky; and the pigeons, every morning a
+  little before sunrise, set out for the Indiana territory, the
+  nearest part of which was about sixty miles distant. Many of
+  these returned before ten o'clock, and the great body generally
+  appeared on their return a little after noon. I had left the
+  public road to visit the remains of the breeding-place near
+  Shelbyville, and was traversing the woods with my gun, on my way
+  to Frankfort, when about ten o'clock the pigeons which I had
+  observed flying the greater part of the morning northerly, began
+  to return in such immense numbers as I never before had
+  witnessed. Coming to an opening by the side of a creek, where I
+  had a more uninterrupted view, I was astonished at their
+  appearance: they were flying with great steadiness and rapidity,
+  at a height beyond gunshot, in several strata deep, and so close
+  together that, could shot have reached them, one discharge could
+  not have failed to bring down several individuals. From right to
+  left, as far as the eye could reach, the breadth of this vast
+  procession extended, seeming everywhere equally crowded. Curious
+  to determine how long this appearance would continue, I took out
+  my watch to note the time, and sat down to observe them. It was
+  then half-past one; I sat for more than an hour, but <a name=
+  "Page_33" id="Page_33"></a>instead of a diminution of this
+  prodigious procession, it seemed rather to increase, both in
+  numbers and rapidity; and anxious to reach Frankfort before
+  night, I rose and went on. About four o'clock in the afternoon I
+  crossed Kentucky River, at the town of Frankfort, at which time
+  the living torrent above my head seemed as numerous and as
+  extensive as ever. Long after this I observed them in large
+  bodies that continued to pass for six or eight minutes, and these
+  again were followed by other detached bodies, all moving in the
+  same south-east direction, till after six o'clock in the evening.
+  The great breadth of front which this mighty multitude preserved
+  would seem to intimate a corresponding breadth of their
+  breeding-place, which, by several gentlemen who had lately passed
+  through part of it, was stated to me at several miles."</p>
+
+  <p>From these various observations, Wilson calculated that the
+  number of birds contained in the mass of pigeons which he saw on
+  this occasion was at least two thousand millions, while this was
+  only one of many similar aggregations known to exist in various
+  parts of the United States. The picture here given of these
+  defenceless birds, and their still more defenceless young,
+  exposed to the attacks of numerous rapacious enemies, brings
+  vividly before us one of the phases of the unceasing struggle for
+  existence ever going on; but when we consider the slow rate of
+  increase of these birds, and the enormous population they are
+  nevertheless able to maintain, we must be convinced that in the
+  case of the majority of birds which multiply far more rapidly,
+  and yet are never able to attain such numbers, the struggle
+  against their numerous enemies and against the adverse forces of
+  nature must be even more severe or more continuous.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Struggle for Life between, closely allied Animals and Plants
+  often the most severe.</i></p>
+
+  <p>The struggle we have hitherto been considering has been mainly
+  that between an animal or plant and its direct enemies, whether
+  these enemies are other animals which devour it, or the forces of
+  nature which destroy it. But there is another kind of struggle
+  often going on at the same time between closely related species,
+  which almost always terminates in the destruction of one of them.
+  As an example of what is <a name="Page_34" id=
+  "Page_34"></a>meant, Darwin states that the recent increase of
+  the missel-thrush in parts of Scotland has caused the decrease of
+  the song-thrush.<a name="FNanchor_12_12" id=
+  "FNanchor_12_12"></a><a href="#Footnote_12_12" class=
+  "fnanchor">[12]</a> The black rat (Mus rattus) was the common rat
+  of Europe till, in the beginning of the eighteenth century, the
+  large brown rat (Mus decumanus) appeared on the Lower Volga, and
+  thence spread more or less rapidly till it overran all Europe,
+  and generally drove out the black rat, which in most parts is now
+  comparatively rare or quite extinct. This invading rat has now
+  been carried by commerce all over the world, and in New Zealand
+  has completely extirpated a native rat, which the Maoris allege
+  they brought with them from their home in the Pacific; and in the
+  same country a native fly is being supplanted by the European
+  house-fly. In Russia the small Asiatic cockroach has driven away
+  a larger native species; and in Australia the imported hive-bee
+  is exterminating the small stingless native bee.</p>
+
+  <p>The reason why this kind of struggle goes on is apparent if we
+  consider that the allied species fill nearly the same place in
+  the economy of nature. They require nearly the same kind of food,
+  are exposed to the same enemies and the same dangers. Hence, if
+  one has ever so slight an advantage over the other in procuring
+  food or in avoiding danger, in its rapidity of multiplication or
+  its tenacity of life, it will increase more rapidly, and by that
+  very fact will cause the other to decrease and often become
+  altogether extinct. In some cases, no doubt, there is actual war
+  between the two, the stronger killing the weaker; but this is by
+  no means necessary, and there may be cases in which the weaker
+  species, physically, may prevail, by its power of more rapid
+  multiplication, its better withstanding vicissitudes of climates,
+  or its greater cunning in escaping the attacks of the common
+  enemies. The same principle is seen at work in the fact that
+  certain mountain varieties of sheep will starve out other
+  mountain varieties, so that the two cannot be kept together. In
+  plants the same thing occurs. If several distinct varieties of
+  wheat are sown together, and the mixed seed resown, some of the
+  varieties which best suit the soil and climate, or are naturally
+  the most fertile, will beat the others and so yield more seed,
+  and will consequently in a few years supplant the other
+  varieties.</p>
+
+  <p><a name="Page_35" id="Page_35"></a>As an effect of this
+  principle, we seldom find closely allied species of animals or
+  plants living together, but often in distinct though adjacent
+  districts where the conditions of life are somewhat different.
+  Thus we may find cowslips (Primula veris) growing in a meadow,
+  and primroses (P. vulgaris) in an adjoining wood, each in
+  abundance, but not often intermingled. And for the same reason
+  the old turf of a pasture or heath consists of a great variety of
+  plants matted together, so much so that in a patch little more
+  than a yard square Mr. Darwin found twenty distinct species,
+  belonging to eighteen distinct genera and to eight natural
+  orders, thus showing their extreme diversity of organisation. For
+  the same reason a number of distinct grasses and clovers are sown
+  in order to make a good lawn instead of any one species; and the
+  quantity of hay produced has been found to be greater from a
+  variety of very distinct grasses than from any one species of
+  grass.</p>
+
+  <p>It may be thought that forests are an exception to this rule,
+  since in the north-temperate and arctic regions we find extensive
+  forests of pines or of oaks. But these are, after all,
+  exceptional, and characterise those regions only where the
+  climate is little favourable to forest vegetation. In the
+  tropical and all the warm temperate parts of the earth, where
+  there is a sufficient supply of moisture, the forests present the
+  same variety of species as does the turf of our old pastures; and
+  in the equatorial virgin forests there is so great a variety of
+  forms, and they are so thoroughly intermingled, that the
+  traveller often finds it difficult to discover a second specimen
+  of any particular species which he has noticed. Even the forests
+  of the temperate zones, in all favourable situations, exhibit a
+  considerable variety of trees of distinct genera and families,
+  and it is only when we approach the outskirts of forest
+  vegetation, where either drought or winds or the severity of the
+  winter is adverse to the existence of most trees, that we find
+  extensive tracts monopolised by one or two species. Even Canada
+  has more than sixty different forest trees and the Eastern United
+  States a hundred and fifty; Europe is rather poor, containing
+  about eighty trees only; while the forests of Eastern Asia,
+  Japan, and Manchuria are exceedingly rich, about a hundred and
+  seventy species being already known. And in all these countries
+  the trees grow inter<a name="Page_36" id="Page_36"></a>mingled,
+  so that in every extensive forest we have a considerable variety,
+  as may be seen in the few remnants of our primitive woods in some
+  parts of Epping Forest and the New Forest.</p>
+
+  <p>Among animals the same law prevails, though, owing to their
+  constant movements and power of concealment, it is not so readily
+  observed. As illustrations we may refer to the wolf, ranging over
+  Europe and Northern Asia, while the jackal inhabits Southern Asia
+  and Northern Africa; the tree-porcupines, of which there are two
+  closely allied species, one inhabiting the eastern, the other the
+  western half of North America; the common hare (Lepus timidus) in
+  Central and Southern Europe, while all Northern Europe is
+  inhabited by the variable hare (Lepus variabilis); the common jay
+  (Garrulus glandarius) inhabiting all Europe, while another
+  species (Garrulus Brandti) is found all across Asia from the
+  Urals to Japan; and many species of birds in the Eastern United
+  States are replaced by closely allied species in the west. Of
+  course there are also numbers of closely related species in the
+  same country, but it will almost always be found that they
+  frequent different stations and have somewhat different habits,
+  and so do not come into direct competition with each other; just
+  as closely allied plants may inhabit the same districts, when one
+  prefers meadows the other woods, one a chalky soil the other
+  sand, one a damp situation the other a dry one. With plants,
+  fixed as they are to the earth, we easily note these
+  peculiarities of station; but with wild animals, which we see
+  only on rare occasions, it requires close and long-continued
+  observation to detect the peculiarities in their mode of life
+  which may prevent all direct competition between closely allied
+  species dwelling in the same area.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Ethical Aspect of the Struggle for Existence</i>.</p>
+
+  <p>Our exposition of the phenomena presented by the struggle for
+  existence may be fitly concluded by a few remarks on its ethical
+  aspect. Now that the war of nature is better known, it has been
+  dwelt upon by many writers as presenting so vast an amount of
+  cruelty and pain as to be revolting to our instincts of humanity,
+  while it has proved a stumbling-block in the way of those who
+  would fain believe in an all-wise and <a name="Page_37" id=
+  "Page_37"></a>benevolent ruler of the universe. Thus, a brilliant
+  writer says: "Pain, grief, disease, and death, are these the
+  inventions of a loving God? That no animal shall rise to
+  excellence except by being fatal to the life of others, is this
+  the law of a kind Creator? It is useless to say that pain has its
+  benevolence, that massacre has its mercy. Why is it so ordained
+  that bad should be the raw material of good? Pain is not the less
+  pain because it is useful; murder is not less murder because it
+  is conducive to development. Here is blood upon the hand still,
+  and all the perfumes of Arabia will not sweeten it."<a name=
+  "FNanchor_13_13" id="FNanchor_13_13"></a><a href=
+  "#Footnote_13_13" class="fnanchor">[13]</a></p>
+
+  <p>Even so thoughtful a writer as Professor Huxley adopts similar
+  views. In a recent article on "The Struggle for Existence" he
+  speaks of the myriads of generations of herbivorous animals which
+  "have been tormented and devoured by carnivores"; of the
+  carnivores and herbivores alike "subject to all the miseries
+  incidental to old age, disease, and over-multiplication"; and of
+  the "more or less enduring suffering," which is the meed of both
+  vanquished and victor. And he concludes that, since thousands of
+  times a minute, were our ears sharp enough, we should hear sighs
+  and groans of pain like those heard by Dante at the gate of hell,
+  the world cannot be governed by what we call benevolence.<a name=
+  "FNanchor_14_14" id="FNanchor_14_14"></a><a href=
+  "#Footnote_14_14" class="fnanchor">[14]</a></p>
+
+  <p>Now there is, I think, good reason to believe that all this is
+  greatly exaggerated; that the supposed "torments" and "miseries"
+  of animals have little real existence, but are the reflection of
+  the imagined sensations of cultivated men and women in similar
+  circumstances; and that the amount of actual suffering caused by
+  the struggle for existence among animals is altogether
+  insignificant. Let us, therefore, endeavour to ascertain what are
+  the real facts on which these tremendous accusations are
+  founded.</p>
+
+  <p>In the first place, we must remember that animals are entirely
+  spared the pain we suffer in the anticipation of death&mdash;a
+  pain far greater, in most cases, than the reality. This leads,
+  probably, to an almost perpetual enjoyment of their lives; since
+  their constant watchfulness against danger, and even their actual
+  flight from an enemy, will be the enjoyable <a name="Page_38" id=
+  "Page_38"></a>exercise of the powers and faculties they possess,
+  unmixed with any serious dread. There is, in the next place, much
+  evidence to show that violent deaths, if not too prolonged, are
+  painless and easy; even in the case of man, whose nervous system
+  is in all probability much more susceptible to pain than that of
+  most animals. In all cases in which persons have escaped after
+  being seized by a lion or tiger, they declare that they suffered
+  little or no pain, physical or mental. A well-known instance is
+  that of Livingstone, who thus describes his sensations when
+  seized by a lion: "Starting and looking half round, I saw the
+  lion just in the act of springing on me. I was upon a little
+  height; he caught my shoulder as he sprang, and we both came to
+  the ground below together. Growling horribly close to my ear, he
+  shook me as a terrier-dog does a rat. The shock produced a stupor
+  similar to that which seems to be felt by a mouse after the first
+  shake of the cat. It causes a sort of dreaminess, <i>in which
+  there was no sense of pain or feeling of terror</i>, though I was
+  quite conscious of all that was happening. It was like what
+  patients partially under the influence of chloroform describe,
+  who see all the operation, but feel not the knife. This singular
+  condition was not the result of any mental process. The shake
+  annihilated fear, and allowed no sense of horror in looking round
+  at the beast."</p>
+
+  <p>This absence of pain is not peculiar to those seized by wild
+  beasts, but is equally produced by any accident which causes a
+  general shock to the system. Mr. Whymper describes an accident to
+  himself during one of his preliminary explorations of the
+  Matterhorn, when he fell several hundred feet, bounding from rock
+  to rock, till fortunately embedded in a snow-drift near the edge
+  of a tremendous precipice. He declares that while falling and
+  feeling blow after blow, he neither lost consciousness nor
+  suffered pain, merely thinking, calmly, that a few more blows
+  would finish him. We have therefore a right to conclude, that
+  when death follows soon after any great shock it is as easy and
+  painless a death as possible; and this is certainly what happens
+  when an animal is seized by a beast of prey. For the enemy is one
+  which hunts for food, not for pleasure or excitement; and it is
+  doubtful whether any carnivorous animal in a state of nature
+  begins to seek after <a name="Page_39" id="Page_39"></a>prey till
+  driven to do so by hunger. When an animal is caught, therefore,
+  it is very soon devoured, and thus the first shock is followed by
+  an almost painless death. Neither do those which die of cold or
+  hunger suffer much. Cold is generally severest at night and has a
+  tendency to produce sleep and painless extinction. Hunger, on the
+  other hand, is hardly felt during periods of excitement, and when
+  food is scarce the excitement of seeking for it is at its
+  greatest. It is probable, also, that when hunger presses, most
+  animals will devour anything to stay their hunger, and will die
+  of gradual exhaustion and weakness not necessarily painful, if
+  they do not fall an earlier prey to some enemy or to
+  cold.<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href=
+  "#Footnote_15_15" class="fnanchor">[15]</a></p>
+
+  <p>Now let us consider what are the enjoyments of the lives of
+  most animals. As a rule they come into existence at a time of
+  year when food is most plentiful and the climate most suitable,
+  that is in the spring of the temperate zone and at the
+  commencement of the dry season in the tropics. They grow
+  vigorously, being supplied with abundance of food; and when they
+  reach maturity their lives are a continual round of healthy
+  excitement and exercise, alternating with complete repose. The
+  daily search for the daily food employs all their faculties and
+  exercises every organ of their bodies, while this exercise leads
+  to the satisfaction of all their physical needs. In our own case,
+  we can give no more perfect definition of happiness, than this
+  exercise and this satisfaction; and we must therefore conclude
+  that animals, as a rule, enjoy all the happiness of which they
+  are capable. And this normal state of happiness is not alloyed,
+  as with us, by long periods&mdash;whole lives often&mdash;of
+  poverty or ill-health, and of the unsatisfied longing for
+  pleasures which others enjoy but to which we cannot attain.
+  Illness, and what answers to poverty in animals&mdash;continued
+  hunger&mdash;are quickly followed by unanticipated and almost
+  painless extinction. Where we err is, in giving to animals
+  feelings and emotions which they do not possess. To us the very
+  sight of blood and of torn or mangled limbs is painful, while the
+  idea of the suffering implied by it <a name="Page_40" id=
+  "Page_40"></a>is heartrending. We have a horror of all violent
+  and sudden death, because we think of the life full of promise
+  cut short, of hopes and expectations unfulfilled, and of the
+  grief of mourning relatives. But all this is quite out of place
+  in the case of animals, for whom a violent and a sudden death is
+  in every way the best. Thus the poet's picture of</p>
+
+  <p><span style="margin-left: 2em;">"Nature red in tooth and
+  claw</span><br />
+  <span style="margin-left: 4.5em;">With ravine"</span><br /></p>
+
+  <p>is a picture the evil of which is read into it by our
+  imaginations, the reality being made up of full and happy lives,
+  usually terminated by the quickest and least painful of
+  deaths.</p>
+
+  <p>On the whole, then, we conclude that the popular idea of the
+  struggle for existence entailing misery and pain on the animal
+  world is the very reverse of the truth. What it really brings
+  about, is, the maximum of life and of the enjoyment of life with
+  the minimum of suffering and pain. Given the necessity of death
+  and reproduction&mdash;and without these there could have been no
+  progressive development of the organic world,&mdash;and it is
+  difficult even to imagine a system by which a greater balance of
+  happiness could have been secured. And this view was evidently
+  that of Darwin himself, who thus concludes his chapter on the
+  struggle for existence: "When we reflect on this struggle, we may
+  console ourselves with the full belief that the war of nature is
+  not incessant, that no fear is felt, that death is generally
+  prompt, and that the vigorous, the healthy, and the happy survive
+  and multiply."</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href=
+      "#FNanchor_4_4"><span class="label">[4]</span></a>
+      <i>G&eacute;ographic Botanique</i>, p. 798.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href=
+      "#FNanchor_5_5"><span class="label">[5]</span></a> <i>The
+      Origin of Species</i>, p. 53.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href=
+      "#FNanchor_6_6"><span class="label">[6]</span></a> <i>The
+      Earth as Modified by Human Action</i>, p. 51.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href=
+      "#FNanchor_7_7"><span class="label">[7]</span></a> <i>The
+      Origin of Species</i>, p. 56.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href=
+      "#FNanchor_8_8"><span class="label">[8]</span></a> See
+      <i>Nature</i>, vol. xxxi. p. 63.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href=
+      "#FNanchor_9_9"><span class="label">[9]</span></a> <i>A Visit
+      to South America</i>, 1878; also <i>Nature</i>, vol. xxxi.
+      pp. 263-339.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_10_10" id="Footnote_10_10"></a><a href=
+      "#FNanchor_10_10"><span class="label">[10]</span></a> Still
+      more remarkable is the increase of rabbits both in New
+      Zealand and Australia. No less than seven millions of
+      rabbit-skins have been exported from the former country in a
+      single year, their value being &pound;67,000. In both
+      countries, sheep-runs have been greatly deteriorated in value
+      by the abundance of rabbits, which destroy the herbage; and
+      in some cases they have had to be abandoned altogether.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_11_11" id="Footnote_11_11"></a><a href=
+      "#FNanchor_11_11"><span class="label">[11]</span></a> Later
+      observers have proved that two eggs are laid and usually two
+      young produced, but it may be that in most cases only one of
+      these comes to maturity.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_12_12" id="Footnote_12_12"></a><a href=
+      "#FNanchor_12_12"><span class="label">[12]</span></a>
+      <i>Origin of Species</i>, p. 59. Professor A. Newton,
+      however, informs me that these species do not interfere with
+      one another in the way here stated.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_13_13" id="Footnote_13_13"></a><a href=
+      "#FNanchor_13_13"><span class="label">[13]</span></a> Winwood
+      Reade's <i>Martyrdom of Man,</i> p. 520.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_14_14" id="Footnote_14_14"></a><a href=
+      "#FNanchor_14_14"><span class="label">[14]</span></a>
+      <i>Nineteenth Century,</i> February 1888, pp. 162, 163.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_15_15" id="Footnote_15_15"></a><a href=
+      "#FNanchor_15_15"><span class="label">[15]</span></a> The
+      Kestrel, which usually feeds on mice, birds, and frogs,
+      sometimes stays its hunger with earthworms, as do some of the
+      American buzzards. The Honey-buzzard sometimes eats not only
+      earthworms and slugs, but even corn; and the Buteo borealis
+      of North America, whose usual food is small mammals and
+      birds, sometimes eats crayfish.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_III" id="CHAPTER_III"></a><a name="Page_41"
+  id="Page_41"></a>CHAPTER III</h2>
+
+  <p style='font-weight:bold;'>THE VARIABILITY OF SPECIES IN A
+  STATE OF NATURE</p>
+
+  <div class="blockquot">
+    <p>Importance of variability&mdash;Popular ideas regarding
+    it&mdash;Variability of the lower animals&mdash;The variability
+    of insects&mdash;Variation among lizards&mdash;Variation among
+    birds&mdash;Diagrams of bird-variation&mdash;Number of varying
+    individuals&mdash;Variation in the mammalia&mdash;Variation in
+    internal organs&mdash;Variations in the skull&mdash;Variations
+    in the habits of Animals&mdash;The Variability of
+    plants&mdash;Species which vary little&mdash;Concluding
+    remarks.</p>
+  </div>
+
+  <p>The foundation of the Darwinian theory is the variability of
+  species, and it is quite useless to attempt even to understand
+  that theory, much less to appreciate the completeness of the
+  proof of it, unless we first obtain a clear conception of the
+  nature and extent of this variability. The most frequent and the
+  most misleading of the objections to the efficacy of natural
+  selection arise from ignorance of this subject, an ignorance
+  shared by many naturalists, for it is only since Mr. Darwin has
+  taught us their importance that varieties have been
+  systematically collected and recorded; and even now very few
+  collectors or students bestow upon them the attention they
+  deserve. By the older naturalists, indeed,
+  varieties&mdash;especially if numerous, small, and of frequent
+  occurrence&mdash;were looked upon as an unmitigated nuisance,
+  because they rendered it almost impossible to give precise
+  definitions of species, then considered the chief end of
+  systematic natural history. Hence it was the custom to describe
+  what was supposed to be the "typical form" of species, and most
+  collectors were satisfied if they possessed this typical form in
+  their cabinets. Now, however, a collection is valued in
+  proportion as it contains illustrative specimens of all the
+  varieties that occur in each species, and in some cases these
+  <a name="Page_42" id="Page_42"></a>have been carefully described,
+  so that we possess a considerable mass of information on the
+  subject. Utilising this information we will now endeavour to give
+  some idea of the nature and extent of variation in the species of
+  animals and plants.</p>
+
+  <p>It is very commonly objected that the widespread and constant
+  variability which is admitted to be a characteristic of
+  domesticated animals and cultivated plants is largely due to the
+  unnatural conditions of their existence, and that we have no
+  proof of any corresponding amount of variation occurring in a
+  state of nature. Wild animals and plants, it is said, are usually
+  stable, and when variations occur these are alleged to be small
+  in amount and to affect superficial characters only; or if larger
+  and more important, to occur so rarely as not to afford any aid
+  in the supposed formation of new species.</p>
+
+  <p>This objection, as will be shown, is utterly unfounded; but as
+  it is one which goes to the very root of the problem, it is
+  necessary to enter at some length into the various proofs of
+  variation in a state of nature. This is the more necessary
+  because the materials collected by Mr. Darwin bearing on this
+  question have never been published, and comparatively few of them
+  have been cited in <i>The Origin of Species</i>; while a
+  considerable body of facts has been made known since the
+  publication of the last edition of that work.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variability of the Lower Animals</i>.</p>
+
+  <p>Among the lowest and most ancient marine organisms are the
+  Foraminifera, little masses of living jelly, apparently
+  structureless, but which secrete beautiful shelly coverings,
+  often perfectly symmetrical, as varied in form as those of the
+  mollusca and far more complicated. These have been studied with
+  great care by many eminent naturalists, and the late Dr. W.B.
+  Carpenter in his great work&mdash;the <i>Introduction to the
+  Study of the Foraminifera</i>&mdash;thus refers to their
+  variability: "There is not a single species of plant or animal of
+  which the range of variation has been studied by the collocation
+  and comparison of so large a number of specimens as have passed
+  under the review of Messrs. Williamson, Parker, Rupert Jones, and
+  myself in our studies of the types of this group;" and he states
+  as the result of this extensive comparison of <a name="Page_43"
+  id="Page_43"></a>specimens: "The range of variation is so great
+  among the Foraminifera as to include not merely those
+  differential characters which have been usually accounted
+  <i>specific</i>, but also those upon which the greater part of
+  the <i>genera</i>, of this group have been founded, and even in
+  some instances those of its <i>orders</i>."<a name=
+  "FNanchor_16_16" id="FNanchor_16_16"></a><a href=
+  "#Footnote_16_16" class="fnanchor">[16]</a></p>
+
+  <p>Coming now to a higher group&mdash;the Sea-Anemones&mdash;Mr.
+  P.H. Gosse and other writers on these creatures often refer to
+  variations in size, in the thickness and length of the tentacles,
+  the form of the disc and of the mouth, and the character of
+  surface of the column, while the colour varies enormously in a
+  great number of the species. Similar variations occur in all the
+  various groups of marine invertebrata, and in the great
+  sub-kingdom of the mollusca they are especially numerous. Thus,
+  Dr. S.P. Woodward states that many present a most perplexing
+  amount of variation, resulting (as he supposes) from supply of
+  food, variety of depth and of saltness of the water; but we know
+  that many variations are quite independent of such causes, and we
+  will now consider a few cases among the land-mollusca in which
+  they have been more carefully studied.</p>
+
+  <p>In the small forest region of Oahu, one of the Sandwich
+  Islands, there have been found about 175 species of land-shells
+  represented by 700 or 800 varieties; and we are told by the Rev.
+  J.T. Gulick, who studied them carefully, that "we frequently find
+  a genus represented in several successive valleys by allied
+  species, sometimes feeding on the same, sometimes on different
+  plants. In every such case the valleys that are nearest to each
+  other furnish the most nearly allied forms; <i>and a full set of
+  the varieties of each species presents a minute gradation of
+  forms between the more divergent types found in the more widely
+  separated localities</i>."</p>
+
+  <p>In most land-shells there is a considerable amount of
+  variation in colour, markings, size, form, and texture or
+  striation of the surface, even in specimens collected in the same
+  locality. Thus, a French author has enumerated no less than 198
+  varieties of the common wood-snail (Helix nemoralis), while of
+  the equally common garden-snail (Helix hortensis) ninety
+  varieties have been described. Fresh-water shells are also
+  <a name="Page_44" id="Page_44"></a>subject to great variation, so
+  that there is much uncertainty as to the number of species; and
+  variations are especially frequent in the Planorbidae, which
+  exhibit many eccentric deviations from the usual form of the
+  species&mdash;deviations which must often affect the form of the
+  living animal. In Mr. Ingersoll's Report on the Recent Mollusca
+  of Colorado many of these extraordinary variations are referred
+  to, and it is stated that a shell (Helisonia trivolvis) abundant
+  in some small ponds and lakes, had scarcely two specimens alike,
+  and many of them closely resembled other and altogether distinct
+  species.<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href=
+  "#Footnote_17_17" class="fnanchor">[17]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Variability of Insects</i>.</p>
+
+  <p>Among Insects there is a large amount of variation, though
+  very few entomologists devote themselves to its investigation.
+  Our first examples will be taken from the late Mr. T. Vernon
+  Wollaston's book, <i>On the Variation of Species</i>, and they
+  must be considered as indications of very widespread though
+  little noticed phenomena. He speaks of the curious little
+  carabideous beetles of the genus Notiophilus as being "extremely
+  unstable both in their sculpture and hue;" of the common Calathus
+  mollis as having "the hind wings at one time ample, at another
+  rudimentary, and at a third nearly obsolete;" and of the same
+  irregularity as to the wings being characteristic of many
+  Orthoptera and of the Homopterous Fulgoridae. Mr. Westwood in his
+  <i>Modern Classification of Insects</i> states that "the species
+  of Gerris, Hydrometra, and Velia are mostly found perfectly
+  apterous, though occasionally with full-sized wings."</p>
+
+  <p>It is, however, among the Lepidoptera (butterflies and moths)
+  that the most numerous cases of variation have been observed, and
+  every good collection of these insects affords striking examples.
+  I will first adduce the testimony of Mr. Bates, who speaks of the
+  butterflies of the Amazon valley exhibiting innumerable local
+  varieties or races, while some species showed great individual
+  variability. Of the beautiful Mechanitis Polymnia he says, that
+  at Ega on the Upper Amazons, "it varies not only in general
+  colour and pattern, but also very considerably in the shape of
+  the wings, especially in the male sex." Again, at St. Paulo,
+  Ithomia</p>
+
+  <p><a name="Page_45" id="Page_45"></a>Orolina exhibits four
+  distinct varieties, all occurring together, and these differ not
+  only in colour but in form, one variety being described as having
+  the fore wings much elongated in the male, while another is much
+  larger and has "the hind wings in the male different in shape."
+  Of Heliconius Numata Mr. Bates says: "This species is so variable
+  that it is difficult to find two examples exactly alike," while
+  "it varies in structure as well as in colours. The wings are
+  sometimes broader, sometimes narrower; and their edges are simple
+  in some examples and festooned in others." Of another species of
+  the same genus, H. melpomene, ten distinct varieties are
+  described all more or less connected by intermediate forms, and
+  four of these varieties were obtained at one locality, Serpa on
+  the north bank of the Amazon. Ceratina Ninonia is another of
+  these very unstable species exhibiting many local varieties which
+  are, however, incomplete and connected by intermediate forms;
+  while the several species of the genus Lycorea all vary to such
+  an extent as almost to link them together, so that Mr. Bates
+  thinks they might all fairly be considered as varieties of one
+  species only.</p>
+
+  <p>Turning to the Eastern Hemisphere we have in Papilio Severus a
+  species which exhibits a large amount of simple variation, in the
+  presence or absence of a pale patch on the upper wings, in the
+  brown submarginal marks on the lower wings, in the form and
+  extent of the yellow band, and in the size of the specimens. The
+  most extreme forms, as well as the intermediate ones, are often
+  found in one locality and in company with each other. A small
+  butterfly (Terias hecabe) ranges over the whole of the Indian and
+  Malayan regions to Australia, and everywhere exhibits great
+  variations, many of which have been described as distinct
+  species; but a gentleman in Australia bred two of these distinct
+  forms (T. hecabe and T. Aesiope), with several intermediates,
+  from one batch of caterpillars found feeding together on the same
+  plant.<a name="FNanchor_18_18" id="FNanchor_18_18"></a><a href=
+  "#Footnote_18_18" class="fnanchor">[18]</a> It is therefore very
+  probable that a considerable number of supposed distinct species
+  are only individual varieties.</p>
+
+  <p>Cases of variation similar to those now adduced among
+  butterflies might be increased indefinitely, but it is as well to
+  note that such important characters as the neuration of the
+  <a name="Page_46" id="Page_46"></a>wings, on which generic and
+  family distinctions are often established, are also subject to
+  variation. The Rev. R.P. Murray, in 1872, laid before the
+  Entomological Society examples of such variation in six species
+  of butterflies, and other cases have been since described. The
+  larvae of butterflies and moths are also very variable, and one
+  observer recorded in the <i>Proceedings of the Entomological
+  Society for</i> 1870 no less than sixteen varieties of the
+  caterpillar of the bedstraw hawk-moth (Deilephela galii).</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variation among Lizards</i>.</p>
+
+  <p>Passing on from the lower animals to the vertebrata, we find
+  more abundant and more definite evidence as to the extent and
+  amount of individual variation. I will first give a case among
+  the Reptilia from some of Mr. Darwin's unpublished MSS., which
+  have been kindly lent me by Mr. Francis Darwin.</p>
+
+  <p>"M. Milne Edwards (<i>Annales des Sci. Nat.</i>, I ser., tom.
+  xvi. p. 50) has given a curious table of measurements of fourteen
+  specimens of Lacerta muralis; and, taking the length of the head
+  as a standard, he finds the neck, trunk, tail, front and hind
+  legs, colour, and femoral pores, all varying wonderfully; and so
+  it is more or less with other species. So apparently trifling a
+  character as the scales on the head affording almost the only
+  constant characters."</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig01" id="fig01"></a> <img src="images/fig01.jpg"
+    width="400" height="645" alt=
+    "FIG. 1.&mdash;Variations of Lacerta muralis." title=
+    "FIG. 1.&mdash;Variations of Lacerta muralis." /> <b>FIG.
+    1.&mdash;Variations of Lacerta muralis.</b>
+  </div>
+
+  <div class="figcenter" style="width: 438px;">
+    <a name="fig02" id="fig02"></a> <img src="images/fig02.jpg"
+    width="438" height="698" alt=
+    "FIG. 2.&mdash;Variation of Lizards." title=
+    "FIG. 2.&mdash;Variation of Lizards." /> <b>FIG.
+    2.&mdash;Variation of Lizards.</b>
+  </div>
+
+  <p>As the table of measurements above referred to would give no
+  clear conception of the nature and amount of the variation
+  without a laborious study and comparison of the figures, I have
+  endeavoured to find a method of presenting the facts to the eye,
+  so that they may be easily grasped and appreciated. In the
+  diagram opposite, the comparative variations of the different
+  organs of this species are given by means of variously bent
+  lines. The head is represented by a straight line because it
+  presented (apparently) no variation. The body is next given, the
+  specimens being arranged in the order of their size from No. 1,
+  the smallest, to No. 14, the largest, the actual lengths being
+  laid down from a base line at a suitable distance below, in this
+  case two inches below the centre, the mean length of the body of
+  the fourteen specimens being two inches. The respective lengths
+  of the neck, legs, and toe of <a name="Page_47" id=
+  "Page_47"></a><a name="Page_48" id="Page_48"></a><a name=
+  "Page_49" id="Page_49"></a>each specimen are then laid down in
+  the same manner at convenient distances apart for comparison; and
+  we see that their variations bear no definite relation to those
+  of the body, and not much to those of each other. With the
+  exception of No. 5, in which all the parts agree in being large,
+  there is a marked independence of each part, shown by the lines
+  often curving in opposite directions; which proves that in those
+  specimens one part is large while the other is small. The actual
+  amount of the variation is very great, ranging from one-sixth of
+  the mean length in the neck to considerably more than a fourth in
+  the hind leg, and this among only fourteen examples which happen
+  to be in a particular museum.</p>
+
+  <p>To prove that this is not an isolated case, Professor Milne
+  Edwards also gives a table showing the amount of variation in the
+  museum specimens of six common species of lizards, also taking
+  the head as the standard, so that the comparative variation of
+  each part to the head is given. In the accompanying diagram (Fig.
+  2) the variations are exhibited by means of lines of varying
+  length. It will be understood that, however much the specimens
+  varied in <i>size</i>, if they had kept the same
+  <i>proportions</i>, the variation line would have been in every
+  case reduced to a point, as in the neck of L. velox which
+  exhibits no variation. The different proportions of the variation
+  lines for each species may show a distinct mode of variation, or
+  may be merely due to the small and differing number of specimens;
+  for it is certain that whatever amount of variation occurs among
+  a few specimens will be greatly increased when a much larger
+  number of specimens are examined. That the amount of variation is
+  large, may be seen by comparing it with the actual length of the
+  head (given below the diagram) which was used as a standard in
+  determining the variation, but which itself seems not to have
+  varied.<a name="FNanchor_19_19" id="FNanchor_19_19"></a><a href=
+  "#Footnote_19_19" class="fnanchor">[19]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variation among Birds</i>.</p>
+
+  <p>Coming now to the class of Birds, we find much more copious
+  evidence of variation. This is due partly to the fact that
+  Ornithology has perhaps a larger body of devotees than any other
+  branch of natural history (except entomology); to the moderate
+  size of the majority of birds; and to the circum<a name="Page_50"
+  id="Page_50"></a>stance that the form and dimensions of the
+  wings, tail, beak, and feet offer the best generic and specific
+  characters and can all be easily measured and compared. The most
+  systematic observations on the individual variation of birds have
+  been made by Mr. J.A. Allen, in his remarkable memoir: "On the
+  Mammals and Winter Birds of East Florida, with an examination of
+  certain assumed specific characters in Birds, and a sketch of the
+  Bird Faunae of Eastern North America," published in the
+  <i>Bulletin of the Museum of Comparative Zoology</i> at Harvard
+  College, Cambridge, Massachusetts, in 1871. In this work exact
+  measurements are given of all the chief external parts of a large
+  number of species of common American birds, from twenty to sixty
+  or more specimens of each species being measured, so that we are
+  able to determine with some precision the nature and extent of
+  the variation that usually occurs. Mr. Allen says: "The facts of
+  the case show that a variation of from 15 to 20 per cent in
+  general size, and an equal degree of variation in the relative
+  size of different parts, may be ordinarily expected among
+  specimens of the same species and sex, taken at the same
+  locality, while in some cases the variation is even greater than
+  this." He then goes on to show that each part varies to a
+  considerable extent independently of the other parts; so that
+  when the size varies, the proportions of all the parts vary,
+  often to a much greater amount. The wing and tail, for example,
+  besides varying in length, vary in the proportionate length of
+  each feather, and this causes their outline to vary considerably
+  in shape. The bill also varies in length, width, depth, and
+  curvature. The tarsus varies in length, as does each toe
+  separately and independently; and all this not to a minute degree
+  requiring very careful measurement to detect it at all, but to an
+  amount easily seen without any measurement, as it averages
+  one-sixth of the whole length and often reaches one-fourth. In
+  twelve species of common perching birds the wing varied (in from
+  twenty-five to thirty specimens) from 14 to 21 per cent of the
+  mean length, and the tail from 13.8 to 23.4 per cent. The
+  variation of the form of the wing can be very easily tested by
+  noting which feather is longest, which next in length, and so on,
+  the respective feathers being indicated by the numbers 1, 2, 3,
+  etc., com<a name="Page_51" id="Page_51"></a>mencing with the
+  outer one. As an example of the irregular variation constantly
+  met with, the following occurred among twenty-five specimens of
+  Dendroeca coronata. Numbers bracketed imply that the
+  corresponding feathers were of equal length.<a name=
+  "FNanchor_20_20" id="FNanchor_20_20"></a><a href=
+  "#Footnote_20_20" class="fnanchor">[20]</a></p>
+
+  <pre>
+    RELATIVE LENGTHS OF PRIMARY WING FEATHERS OF
+    DENDROECA CORONATA.
+    ---------+-----------+----------+-----------+----------+----------
+    Longest. | Second in | Third in | Fourth in | Fifth in | Sixth in
+             |  Length.  |  Length. |  Length.  |  Length. |  Length.
+    ---------+-----------+----------+-----------+----------+----------
+    2        |     3     |     1    |     4     |     5    |     6
+    3        |     2     |     4    |     1     |     5    |     6
+             |  /  2     |          |           |          |
+    3        | {         |     1    |     5     |     6    |     7
+             |  \  4     |          |           |          |
+    2    \   |           |          |           |          |
+          }  |     4     |     1    |     5     |     6    |     7
+    3    /   |           |          |           |          |
+    2    \   |           |          |           |          |
+    1     |  |           |          |           |          |
+           } |     5     |     6    |     7     |     8    |     9
+    3     |  |           |          |           |          |
+    4    /   |           |          |           |          |
+    ---------+-----------+----------+-----------+----------+----------
+  </pre>
+
+  <p>Here we have five very distinct proportionate lengths of the
+  wing feathers, any one of which is often thought sufficient to
+  characterise a distinct species of bird; and though this is
+  rather an extreme case, Mr. Allen assures us that "the
+  comparison, extended in the table to only a few species, has been
+  carried to scores of others with similar results."</p>
+
+  <p>Along with this variation in size and proportions there occurs
+  a large amount of variation in colour and markings. "The
+  difference in intensity of colour between the extremes of a
+  series of fifty or one hundred specimens of any species,
+  collected at a single locality, and nearly at the same season of
+  the year, is often as great as occurs between truly distinct
+  species." But there is also a great amount of individual
+  variability in the markings of the same species. Birds having the
+  plumage varied with streaks and spots differ exceedingly in
+  different individuals of the same species in respect to the size,
+  shape, and number of these marks, and in the general aspect of
+  the plumage resulting from such variations. "In the common
+  <a name="Page_52" id="Page_52"></a>song sparrow (Melospiza
+  melodia), the fox-coloured sparrow (Passerella iliaca), the swamp
+  sparrow (Melospiza palustris), the black and white creeper
+  (Mniotilta varia), the water-wagtail (Seiurus novaeboracencis),
+  in Turdus fuscescens and its allies, the difference in the size
+  of the streaks is often very considerable. In the song sparrow
+  they vary to such an extent that in some cases they are reduced
+  to narrow lines; in others so enlarged as to cover the greater
+  part of the breast and sides of the body, sometimes uniting on
+  the middle of the breast into a nearly continuous patch."</p>
+
+  <p>Mr. Allen then goes on to particularise several species in
+  which such variations occur, giving cases in which two specimens
+  taken at the same place on the same day exhibited the two
+  extremes of coloration. Another set of variations is thus
+  described: "The white markings so common on the wings and tails
+  of birds, as the bars formed by the white tips of the greater
+  wing-coverts, the white patch occasionally present at the base of
+  the primary quills, or the white band crossing them, and the
+  white patch near the end of the outer tail-feathers are also
+  extremely liable to variation in respect to their extent and the
+  number of feathers to which, in the same species, these markings
+  extend." It is to be especially noted that all these varieties
+  are distinct from those which depend on season, on age, or on
+  sex, and that they are such as have in many other species been
+  considered to be of specific value.</p>
+
+  <p>These variations of colour could not be presented to the eye
+  without a series of carefully engraved plates, but in order to
+  bring Mr. Allen's <i>measurements</i>, illustrating variations of
+  size and proportion, more clearly before the reader, I have
+  prepared a series of diagrams illustrating the more important
+  facts and their bearings on the Darwinian theory.</p>
+
+  <p>The first of these is intended, mainly, to show the actual
+  amount of the variation, as it gives the true length of the wing
+  and tail in the extreme cases among thirty specimens of each of
+  three species. The shaded portion shows the minimum length, the
+  unshaded portion the additional length in the maximum. The point
+  to be specially noted here is, that in each of these common
+  species there is about the same amount of variation, and that it
+  is so great as to be obvious at a glance.</p>
+
+  <div class="figcenter" style="width: 500px;">
+    <a name="fig03" id="fig03"></a> <a name="Page_53" id=
+    "Page_53"></a><img src="images/fig03.jpg" width="500" height=
+    "328" alt="FIG. 3.&mdash;Variation of Wings and Tail." title=
+    "FIG. 3.&mdash;Variation of Wings and Tail." /> <b>FIG.
+    3.&mdash;Variation of Wings and Tail.</b>
+  </div>
+
+  <p><a name="Page_54" id="Page_54"></a>There is here no question
+  of "minute" or "infinitesimal" variation, which many people
+  suppose to be the only kind of variation that exists. It cannot
+  even be called small; yet from all the evidence we now possess it
+  seems to be the amount which characterises most of the common
+  species of birds.</p>
+
+  <p>It may be said, however, that these are the extreme
+  variations, and only occur in one or two individuals, while the
+  great majority exhibit little or no difference. Other diagrams
+  will show that this is not the case; but even if it were so, it
+  would be no objection at all, because these are the extremes
+  among thirty specimens only. We may safely assume that these
+  thirty specimens, taken by chance, are not, in the case of all
+  these species, exceptional lots, and therefore we might expect at
+  least two similarly varying specimens in each additional thirty.
+  But the number of individuals, even in a very rare species, is
+  probably thirty thousand or more, and in a common species thirty,
+  or even three hundred, millions. Even one individual in each
+  thirty, varying to the amount shown in the diagram, would give at
+  least a million in the total population of any common bird, and
+  among this million many would vary much more than the extreme
+  among thirty only. We should thus have a vast body of individuals
+  varying to a large extent in the length of the wings and tail,
+  and offering ample material for the modification of these organs
+  by natural selection. We will now proceed to show that other
+  parts of the body vary, simultaneously, but independently, to an
+  equal amount.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig04" id="fig04"></a> <img src="images/fig04.jpg"
+    width="400" height="701" alt=
+    "FIG. 4.&mdash;Dolichonyx oryzivorus. 20 Males." title=
+    "FIG. 4.&mdash;Dolichonyx oryzivorus. 20 Males." /> <b>FIG.
+    4.&mdash;Dolichonyx oryzivorus. 20 Males.</b>
+  </div>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig05" id="fig05"></a> <img src="images/fig05.jpg"
+    width="400" height="632" alt=
+    "FIG. 5.&mdash;Agelaeus phoeniceus. 40 Males." title=
+    "FIG. 5.&mdash;Agelaeus phoeniceus. 40 Males." /> <b>FIG.
+    5.&mdash;Agelaeus phoeniceus. 40 Males.</b>
+  </div>
+
+  <p>The first bird taken is the common Bob-o-link or Rice-bird
+  (Dolichonyx oryzivorus), and the Diagram, Fig. 4, exhibits the
+  variations of seven important characters in twenty male adult
+  specimens.<a name="FNanchor_21_21" id=
+  "FNanchor_21_21"></a><a href="#Footnote_21_21" class=
+  "fnanchor">[21]</a> These characters are&mdash;the lengths of the
+  body, wing, tail, tarsus, middle toe, outer toe, and hind toe,
+  being as many as can be conveniently exhibited in one diagram.
+  The length of the body is not given by Mr. Allen, but as it forms
+  a convenient standard of comparison, it has been obtained by
+  deducting the length of the tail from the total length of the
+  birds as given by him. The diagram has been constructed as
+  follows:&mdash;The twenty specimens are first arranged in a
+  series according to the body-lengths (which may be con<a name=
+  "Page_55" id="Page_55"></a><a name="Page_56" id=
+  "Page_56"></a><a name="Page_57" id="Page_57"></a>sidered to give
+  the size of the bird), from the shortest to the longest, and the
+  same number of vertical lines are drawn, numbered from one to
+  twenty. In this case (and wherever practicable) the body-length
+  is measured from the lower line of the diagram, so that the
+  actual length of the bird is exhibited as well as the actual
+  variations of length. These can be well estimated by means of the
+  horizontal line drawn at the mean between the two extremes, and
+  it will be seen that one-fifth of the total number of specimens
+  taken on either side exhibits a very large amount of variation,
+  which would of course be very much greater if a hundred or more
+  specimens were compared. The lengths of the wing, tail, and other
+  parts are then laid down, and the diagram thus exhibits at a
+  glance the comparative variation of these parts in every specimen
+  as well as the actual amount of variation in the twenty
+  specimens; and we are thus enabled to arrive at some important
+  conclusions.</p>
+
+  <p>We note, first, that the variations of none of the parts
+  follow the variations of the body, but are sometimes almost in an
+  opposite direction. Thus the longest wing corresponds to a rather
+  small body, the longest tail to a medium body, while the longest
+  leg and toes belong to only a moderately large body. Again, even
+  related parts do not constantly vary together but present many
+  instances of independent variation, as shown by the want of
+  parallelism in their respective variation-lines. In No. 5 (see
+  Fig. 4) the wing is very long, the tail moderately so; while in
+  No. 6 the wing is much shorter while the tail is considerably
+  longer. The tarsus presents comparatively little variation; and
+  although the three toes may be said to vary in general together,
+  there are many divergencies; thus, in passing from No. 9 to No.
+  10, the outer toe becomes longer, while the hind toe becomes
+  considerably shorter; while in Nos. 3 and 4 the middle toe varies
+  in an opposite way to the outer and the hind toes.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig06" id="fig06"></a> <img src="images/fig06.jpg"
+    width="400" height="667" alt=
+    "FIG. 6.&mdash;Cardinalis virginianus. 31 Males." title=
+    "FIG. 6.&mdash;Cardinalis virginianus. 31 Males." /> <b>FIG.
+    6.&mdash;Cardinalis virginianus. 31 Males.</b>
+  </div>
+
+  <p>In the next diagram (Fig. 5) we have the variations in forty
+  males of the Red-winged Blackbird (Agelaeus phoeniceus), and here
+  we see the same general features. One-fifth of the whole number
+  of specimens offer a large amount of variation either below or
+  above the mean; while the wings, tail, and head vary quite
+  independently of the body. The wing and tail too, <a name=
+  "Page_58" id="Page_58"></a><a name="Page_59" id=
+  "Page_59"></a>though showing some amount of correlated variation,
+  yet in no less than nine cases vary in opposite directions as
+  compared with the preceding species.</p>
+
+  <p>The next diagram (Fig. 6), showing the variations of
+  thirty-one males of the Cardinal bird (Cardinalis virginianus),
+  exhibits these features much more strongly. The amount of
+  variation in proportion to the size of the bird is very much
+  greater; while the variations of the wing and tail not only have
+  no correspondence with that of the body but very little with each
+  other. In no less than twelve or thirteen instances they vary in
+  opposite directions, while even where they correspond in
+  direction the amount of the variation is often very
+  disproportionate.</p>
+
+  <p>As the proportions of the tarsi and toes of birds have great
+  influence on their mode of life and habits and are often used as
+  specific or even generic characters, I have prepared a diagram
+  (Fig. 7) to show the variation in these parts only, among twenty
+  specimens of each of four species of birds, four or five of the
+  most variable alone being given. The extreme divergence of each
+  of the lines in a vertical direction shows the actual amount of
+  variation; and if we consider the small length of the toes of
+  these small birds, averaging about three-quarters of an inch, we
+  shall see that the variation is really very large; while the
+  diverging curves and angles show that each part varies, to a
+  great extent, independently. It is evident that if we compared
+  some thousands of individuals instead of only twenty, we should
+  have an amount of independent variation occurring each year which
+  would enable almost any modification of these important organs to
+  be rapidly effected.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig07" id="fig07"></a> <img src="images/fig07.jpg"
+    width="400" height="650" alt=
+    "FIG. 7.&mdash;Variation of Tarsus and Toes." title=
+    "FIG. 7.&mdash;Variation of Tarsus and Toes." /> <b>FIG.
+    7.&mdash;Variation of Tarsus and Toes.</b>
+  </div>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig08" id="fig08"></a> <img src="images/fig08.jpg"
+    width="400" height="607" alt=
+    "FIG. 8.&mdash;Variation of Birds in Leyden Museum." title=
+    "FIG. 8.&mdash;Variation of Birds in Leyden Museum." /> <b>FIG.
+    8.&mdash;Variation of Birds in Leyden Museum.</b>
+  </div>
+
+  <p>In order to meet the objection that the large amount of
+  variability here shown depends chiefly on the observations of one
+  person and on the birds of a single country, I have examined
+  Professor Schlegel's Catalogue of the Birds in the Leyden Museum,
+  in which he usually gives the range of variation of the specimens
+  in the museum (which are commonly less than a dozen and rarely
+  over twenty) as regards some of their more important dimensions.
+  These fully support the statement of Mr. Allen, since they show
+  an equal amount of variability when the numbers compared are
+  <a name="Page_60" id="Page_60"></a><a name="Page_61" id=
+  "Page_61"></a><a name="Page_62" id="Page_62"></a>sufficient,
+  which, however, is not often the case. The accompanying diagram
+  exhibits the actual differences of size in five organs which
+  occur in five species taken almost at random from this catalogue.
+  Here, again, we perceive that the variation is decidedly large,
+  even among a very small number of specimens; while the facts all
+  show that there is no ground whatever for the common assumption
+  that natural species consist of individuals which are nearly all
+  alike, or that the variations which occur are "infinitesimal" or
+  even "small."</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The proportionate Number of Individuals which present a
+  considerable amount of Variation.</i></p>
+
+  <p>The notion that variation is a comparatively exceptional
+  phenomenon, and that in any case considerable variations occur
+  very rarely in proportion to the number of individuals which do
+  not vary, is so deeply rooted that it is necessary to show by
+  every possible method of illustration how completely opposed it
+  is to the facts of nature. I have therefore prepared some
+  diagrams in which each of the individual birds measured is
+  represented by a spot, placed at a proportionate distance, right
+  and left, from the median line accordingly as it varies in excess
+  or defect of the mean length as regards the particular part
+  compared. As the object in this set of diagrams is to show the
+  number of individuals which vary considerably in proportion to
+  those which vary little or not at all, the scale has been
+  enlarged in order to allow room for placing the spots without
+  overlapping each other.</p>
+
+  <p>In the diagram opposite twenty males of Icterus Baltimore are
+  registered, so as to exhibit to the eye the proportionate number
+  of specimens which vary, to a greater or less amount, in the
+  length of the tail, wing, tarsus, middle toe, hind toe, and bill.
+  It will be noticed that there is usually no very great
+  accumulation of dots about the median line which shows the
+  average dimensions, but that a considerable number are spread at
+  varying distances on each side of it.</p>
+
+  <p>In the next diagram (Fig. 10), showing the variation among
+  forty males of Agelaeeus phoeniceus, this approach to an equable
+  spreading of the variations is still more apparent; while in Fig.
+  12, where fifty-eight specimens of Cardinalis <a name="Page_63"
+  id="Page_63"></a>virginianus are registered, we see a remarkable
+  spreading out of the spots, showing in some of the characters a
+  tendency to segregation into two or more groups of individuals,
+  each varying considerably from the mean.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig09" id="fig09"></a> <img src="images/fig09.jpg"
+    width="400" height="676" alt="FIG. 9" title="FIG. 9" /> <b>FIG.
+    9</b>
+  </div>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig10" id="fig10"></a> <img src="images/fig10.jpg"
+    width="400" height="492" alt="FIG. 10." title="FIG. 10." />
+    <b>FIG. 10.</b>
+  </div>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig11" id="fig11"></a> <img src="images/fig11.jpg"
+    width="400" height="80" alt="FIG. 11." title="FIG. 11." />
+    <b>FIG. 11.</b>
+  </div>
+
+  <p>In order fully to appreciate the teaching of these diagrams,
+  we must remember, that, whatever kind and amount of variations
+  are exhibited by the few specimens here compared, would be
+  greatly extended and brought into symmetrical form if large
+  numbers&mdash;thousands or millions&mdash;were subjected to the
+  same process of measurement and registration. We know, from the
+  general law which governs variations from a mean value, that with
+  increasing numbers the range <a name="Page_64" id=
+  "Page_64"></a>of variation of each part would increase also, at
+  first rather rapidly and then more slowly; while gaps and
+  irregularities would be gradually filled up, and at length the
+  distribution of the dots would indicate a tolerably regular curve
+  of double curvature like those shown in Fig. 11. The great
+  divergence <a name="Page_65" id="Page_65"></a>of the dots, when
+  even a few specimens are compared, shows that the curve, with
+  high numbers, would be a flat one like the lower curve in the
+  illustration here given. This being the case it would follow that
+  a very large proportion of the total number of individuals
+  constituting a species would diverge considerably from its
+  average condition as regards each part or organ; and as we know
+  from the previous diagrams of variation (Figs. 1 to 7) that each
+  part varies to a considerable extent, <i>independently</i>, the
+  materials constantly ready for natural selection to act upon are
+  abundant in quantity and very varied in kind. Almost any
+  combination of variations of distinct parts will be available,
+  where required; and this, as we shall see further on, obviates
+  one of the most weighty objections which have been urged against
+  the efficiency of natural selection in producing new species,
+  genera, and higher groups.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig12" id="fig12"></a> <img src="images/fig12.jpg"
+    width="400" height="287" alt="FIG. 12." title="FIG. 12." />
+    <b>FIG. 12.</b>
+  </div>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variation in the Mammalia.</i></p>
+
+  <p>Owing to the generally large size of this class of animals,
+  and the comparatively small number of naturalists who study them,
+  large series of specimens are only occasionally examined <a name=
+  "Page_66" id="Page_66"></a>and compared, and thus the materials
+  for determining the question of their variability in a state of
+  nature are comparatively scanty. The fact that our domestic
+  animals belonging to this group, especially dogs, present extreme
+  varieties not surpassed even by pigeons and poultry among birds,
+  renders it almost certain that an equal amount of variability
+  exists in the wild state; and this is confirmed by the example of
+  a species of squirrel (Sciurus carolinensis), of which sixteen
+  specimens, all males and all taken in Florida, were measured and
+  tabulated by Mr. Allen. The diagram here given shows, that, both
+  the general amount of the variation and the independent
+  variability of the several members of the body, accord completely
+  with the variations so common in the class of birds; while their
+  amount and their independence of each other are even greater than
+  usual.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variation in the Internal Organs of Animals.</i></p>
+
+  <p>In case it should be objected that the cases of variation
+  hitherto adduced are in the external parts only, and that there
+  is no proof that the internal organs vary in the same manner, it
+  will be advisable to show that such varieties also occur. It is,
+  however, impossible to adduce the same amount of evidence in this
+  class of variation, because the great labour of dissecting large
+  numbers of specimens of the same species is rarely undertaken,
+  and we have to trust to the chance observations of anatomists
+  recorded in their regular course of study.</p>
+
+  <p>It must, however, be noted that a very large proportion of the
+  variations already recorded in the external parts of animals
+  necessarily imply corresponding internal variations. When feet
+  and legs vary in size, it is because the bones vary; when the
+  head, body, limbs, and tail change their proportions, the bony
+  skeleton must also change; and even when the wing or tail
+  feathers of birds become longer or more numerous, there is sure
+  to be a corresponding change in the bones which support and the
+  muscles which move them. I will, however, give a few cases of
+  variations which have been directly observed.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig13" id="fig13"></a> <img src="images/fig13.jpg"
+    width="400" height="664" alt=
+    "FIG. 13.&mdash;Sciurus carolinensis. 32 specimens. Florida."
+    title=
+    "FIG. 13.&mdash;Sciurus carolinensis. 32 specimens. Florida." />
+     <b>FIG. 13.&mdash;Sciurus carolinensis. 32 specimens.
+    Florida.</b>
+  </div>
+
+  <p>Mr. Frank E. Beddard has kindly communicated to me some
+  remarkable variations he has observed in the internal <a name=
+  "Page_67" id="Page_67"></a><a name="Page_68" id=
+  "Page_68"></a>organs of a species of earthworm (Perionyx
+  excavatus). The normal characters of this species are&mdash;</p>
+
+  <div class="blockquot">
+    <p>Setae forming a complete row round each segment.</p>
+
+    <p>Two pairs of spermathecae&mdash;spherical pouches without
+    diverticulae&mdash;in segments 8 and 9.</p>
+
+    <p>Two pairs of testes in segments 11 and 12.</p>
+
+    <p>Ovaries, a single pair in segment 13.</p>
+
+    <p>Oviducts open by a common pore in the middle of segment
+    14.</p>
+
+    <p>Vasa deferentia open separately in segment 18, each
+    furnished at its termination with a large prostate gland.</p>
+  </div>
+
+  <p>Between two and three hundred specimens were examined, and
+  among them thirteen specimens exhibited the following marked
+  variations:&mdash;</p>
+
+  <div class="blockquot">
+    <p>(1) The number of the spermathecae varied from two to three
+    or four pairs, their position also varying.</p>
+
+    <p>(2) There were occasionally two pairs of ovaries, each with
+    its own oviduct; the external apertures of these varied in
+    position, being upon segments 13 and 14, 14 and 15, or 15 and
+    16. Occasionally when there was only the normal single oviduct
+    pore present it varied in position, once occurring on the 10th,
+    and once on the 11th segment.</p>
+
+    <p>(3) The male generative pores varied in position from
+    segments 14 to 20. In one instance there were two pairs instead
+    of the normal single pair, and in this case each of the four
+    apertures had its own prostate gland.</p>
+  </div>
+
+  <p>Mr. Beddard remarks that all, or nearly all, the above
+  variations are found <i>normally</i> in other genera and
+  species.</p>
+
+  <p>When we consider the enormous number of earthworms and the
+  comparatively very small number of individuals examined, we may
+  be sure, not only that such variations as these occur with
+  considerable frequency, but also that still more extraordinary
+  deviations from the normal structure may often exist.</p>
+
+  <p>The next example is taken from Mr. Darwin's unpublished
+  MSS.</p>
+
+  <div class="blockquot">
+    <p>"In some species of Shrews (Sorex) and in some
+    field-mice<a name="Page_69" id="Page_69"></a> (Arvicola), the
+    Rev. L. Jenyns (<i>Ann. Nat. Hist.</i>, vol. vii. pp. 267, 272)
+    found the proportional length of the intestinal canal to vary
+    considerably. He found the same variability in the number of
+    the caudal vertebrae. In three specimens of an Arvicola he
+    found the gall-bladder having a very different degree of
+    development, and there is reason to believe it is sometimes
+    absent. Professor Owen has shown that this is the case with the
+    gall-bladder of the giraffe."</p>
+  </div>
+
+  <p>Dr. Crisp (<i>Proc. Zool. Soc.</i>, 1862, p. 137) found the
+  gall-bladder present in some specimens of Cervus superciliaris
+  while absent in others; and he found it to be absent in three
+  giraffes which he dissected. A double gall-bladder was found in a
+  sheep, and in a small mammal preserved in the Hunterian Museum
+  there are three distinct gall-bladders.</p>
+
+  <p>The length of the alimentary canal varies greatly. In three
+  adult giraffes described by Professor Owen it was from 124 to 136
+  feet long; one dissected in France had this canal 211 feet long;
+  while Dr. Crisp measured one of the extraordinary length of 254
+  feet, and similar variations are recorded in other
+  animals.<a name="FNanchor_22_22" id="FNanchor_22_22"></a><a href=
+  "#Footnote_22_22" class="fnanchor">[22]</a></p>
+
+  <p>The number of ribs varies in many animals. Mr. St. George
+  Mivart says: "In the highest forms of the Primates, the number of
+  true ribs is seven, but in Hylobates there are sometimes eight
+  pairs. In Semnopithecus and Colobus there are generally seven,
+  but sometimes eight pairs of true ribs. In the Cebidae there are
+  generally seven or eight pairs, but in Ateles sometimes nine"
+  (<i>Proc. Zool. Soc.</i>, 1865, p. 568). In the same paper it is
+  stated that the number of dorsal vertebrae in man is normally
+  twelve, very rarely thirteen. In the Chimpanzee there are
+  normally thirteen dorsal vertebrae, but occasionally there are
+  fourteen or only twelve.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variations in the Skull.</i></p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig14" id="fig14"></a> <img src="images/fig14.jpg"
+    width="400" height="715" alt=
+    "FIG. 14.&mdash;Variation of Skull of Wolf. 10 specimens."
+    title=
+    "FIG. 14.&mdash;Variation of Skull of Wolf. 10 specimens." />
+    <b>FIG. 14.&mdash;Variation of Skull of Wolf. 10 specimens.</b>
+  </div>
+
+  <p>Among the nine adult male Orang-utans, collected by myself in
+  Borneo, the skulls differed remarkably in size and proportions.
+  The orbits varied in width and height, the cranial ridge was
+  either single or double, either much or little developed, and the
+  zygomatic aperture varied considerably in <a name="Page_70" id=
+  "Page_70"></a><a name="Page_71" id="Page_71"></a>size. I noted
+  particularly that these variations bore no necessary relation to
+  each other, so that a large temporal muscle and zygomatic
+  aperture might exist either with a large or a small cranium; and
+  thus was explained the curious difference between the
+  single-crested and the double-crested skulls, which had been
+  supposed to characterise distinct species. As an instance of the
+  amount of variation in the skulls of fully adult male orangs, I
+  found the width between the orbits externally to be only 4 inches
+  in one specimen and fully 5 inches in another.</p>
+
+  <p>Exact measurements of large series of comparable skulls of the
+  mammalia are not easily found, but from those available I have
+  prepared three diagrams (Figs. 14, 15, and 16), in order to
+  exhibit the facts of variation in this very important organ. The
+  first shows the variation in ten specimens of the common wolf
+  (Canis lupus) from one district in North America, and we see that
+  it is not only large in amount, but that each part exhibits a
+  considerable independent variability.<a name="FNanchor_23_23" id=
+  "FNanchor_23_23"></a><a href="#Footnote_23_23" class=
+  "fnanchor">[23]</a></p>
+
+  <p>In Diagram 15 we have the variations of eight skulls of the
+  Indian Honey-bear (Ursus labiatus), as tabulated by the late Dr.
+  J.E. Gray of the British Museum. For such a small number of
+  specimens the amount of variation is very large&mdash;from
+  one-eighth to one-fifth of the mean size,&mdash;while there are
+  an extraordinary number of instances of independent variability.
+  In Diagram 16 we have the length and width of twelve skulls of
+  adult males of the Indian wild boar (Sus cristatus), also given
+  by Dr. Gray, exhibiting in both sets of measurements a variation
+  of more than one-sixth, combined with a very considerable amount
+  of independent variability.<a name="FNanchor_24_24" id=
+  "FNanchor_24_24"></a><a href="#Footnote_24_24" class=
+  "fnanchor">[24]</a></p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig15" id="fig15"></a> <img src="images/fig15.jpg"
+    width="400" height="609" alt=
+    "FIG. 15.&mdash;Variation of 8 skulls (Ursus labiatus)." title=
+    "FIG. 15.&mdash;Variation of 8 skulls (Ursus labiatus)." />
+    <b>FIG. 15.&mdash;Variation of 8 skulls (Ursus labiatus).</b>
+  </div>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig16" id="fig16"></a> <img src="images/fig16.jpg"
+    width="400" height="298" alt="FIG. 16." title="FIG. 16." />
+    <b>FIG. 16.</b>
+  </div>
+
+  <p>The few facts now given, as to variations of the internal
+  parts of animals, might be multiplied indefinitely by a search
+  through the voluminous writings of comparative anatomists. But
+  the evidence already adduced, taken in conjunction with the much
+  fuller evidence of variation in all external organs, leads us to
+  the conclusion that wherever variations are looked for among a
+  considerable number of individuals of the more <a name="Page_72"
+  id="Page_72"></a><a name="Page_73" id="Page_73"></a><a name=
+  "Page_74" id="Page_74"></a>common species they are sure to be
+  found; that they are everywhere of considerable amount, often
+  reaching 20 per cent of the size of the part implicated; and that
+  they are to a great extent independent of each other, and thus
+  afford almost any combination of variations that may be
+  needed.</p>
+
+  <p>It must be particularly noticed that the whole series of
+  variation-diagrams here given (except the three which illustrate
+  the number of varying individuals) in every case represent the
+  actual amount of the variation, not on any reduced or enlarged
+  scale, but as it were life-size. Whatever number of inches or
+  decimals of an inch the species varies in any of its parts is
+  marked on the diagrams, so that with the help of an ordinary
+  divided rule or a pair of compasses the variation of the
+  different parts can be ascertained and compared just as if the
+  specimens themselves were before the reader, but with much
+  greater ease.</p>
+
+  <p>In my lectures on the Darwinian theory in America and in this
+  country I used diagrams constructed on a different plan, equally
+  illustrating the large amount of independent variability, but
+  less simple and less intelligible. The present method is a
+  modification of that used by Mr. Francis Galton in his researches
+  on the theory of variability, the upper line (showing the
+  variability of the body) in Diagrams 4, 5, 6, and 13, being laid
+  down on the method he has used in his experiments with sweet-peas
+  and in pedigree moth-breeding.<a name="FNanchor_25_25" id=
+  "FNanchor_25_25"></a><a href="#Footnote_25_25" class=
+  "fnanchor">[25]</a> I believe, after much consideration, and many
+  tedious experiments in diagram-making, that no better method can
+  be adopted for bringing before the eye, both the amount and the
+  peculiar features of individual variability.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variations of the Habits of Animals.</i></p>
+
+  <p>Closely connected with those variations of internal and
+  external structure which have been already described, are the
+  changes of habits which often occur in certain individuals or in
+  whole species, since these must necessarily depend upon some
+  corresponding change in the brain or in other parts of the
+  organism; and as these changes are of great importance in
+  relation to the theory of instinct, a few examples of them will
+  be now adduced.</p>
+
+  <p><a name="Page_75" id="Page_75"></a>The Kea (Nestor notabilis)
+  is a curious parrot inhabiting the mountain ranges of the Middle
+  Island of New Zealand. It belongs to the family of Brush-tongued
+  parrots, and naturally feeds on the honey of flowers and the
+  insects which frequent them, together with such fruits or berries
+  as are found in the region. Till quite recently this comprised
+  its whole diet, but since the country it inhabits has become
+  occupied by Europeans it has developed a taste for a carnivorous
+  diet, with alarming results. It began by picking the sheepskins
+  hung out to dry or the meat in process of being cured. About 1868
+  it was first observed to attack living sheep, which had
+  frequently been found with raw and bleeding wounds on their
+  backs. Since then it is stated that the bird actually burrows
+  into the living sheep, eating its way down to the kidneys, which
+  form its special delicacy. As a natural consequence, the bird is
+  being destroyed as rapidly as possible, and one of the rare and
+  curious members of the New Zealand fauna will no doubt shortly
+  cease to exist. The case affords a remarkable instance of how the
+  climbing feet and powerful hooked beak developed for one set of
+  purposes can be applied to another altogether different purpose,
+  and it also shows how little real stability there may be in what
+  appear to us the most fixed habits of life. A somewhat similar
+  change of diet has been recorded by the Duke of Argyll, in which
+  a goose, reared by a golden eagle, was taught by its
+  foster-parent to eat flesh, which it continued to do regularly
+  and apparently with great relish.<a name="FNanchor_26_26" id=
+  "FNanchor_26_26"></a><a href="#Footnote_26_26" class=
+  "fnanchor">[26]</a></p>
+
+  <p>Change of habits appears to be often a result of imitation, of
+  which Mr. Tegetmeier gives some good examples. He states that if
+  pigeons are reared exclusively with small grain, as wheat or
+  barley, they will starve before eating beans. But when they are
+  thus starving, if a bean-eating pigeon is put among them, they
+  follow its example, and thereafter adopt the habit. So fowls
+  sometimes refuse to eat maize, but on seeing others eat it, they
+  do the same and become excessively fond of it. Many persons have
+  found that their yellow crocuses were eaten by sparrows, while
+  the blue, purple, and white coloured varieties were left
+  untouched; but Mr. Tegetmeier, who grows only these latter
+  colours, found that after <a name="Page_76" id="Page_76"></a>two
+  years the sparrows began to attack them, and thereafter destroyed
+  them quite as readily as the yellow ones; and he believes it was
+  merely because some bolder sparrow than the rest set the example.
+  On this subject Mr. Charles C. Abbott well remarks: "In studying
+  the habits of our American birds&mdash;and I suppose it is true
+  of birds everywhere&mdash;it must at all times be remembered that
+  there is less stability in the habits of birds than is usually
+  supposed; and no account of the habits of any one species will
+  exactly detail the various features of its habits as they really
+  are, in every portion of the territory it inhabits."<a name=
+  "FNanchor_27_27" id="FNanchor_27_27"></a><a href=
+  "#Footnote_27_27" class="fnanchor">[27]</a></p>
+
+  <p>Mr. Charles Dixon has recorded a remarkable change in the mode
+  of nest-building of some common chaffinches which were taken to
+  New Zealand and turned out there. He says: "The cup of the nest
+  is small, loosely put together, apparently lined with feathers,
+  and the walls of the structure are prolonged for about 18 inches,
+  and hang loosely down the side of the supporting branch. The
+  whole structure bears some resemblance to the nests of the
+  hangnests (Icteridae), with the exception that the cavity is at
+  the top. Clearly these New Zealand chaffinches were at a loss for
+  a design when fabricating their nest. They had no standard to
+  work by, no nests of their own kind to copy, no older birds to
+  give them any instruction, and the result is the abnormal
+  structure I have just described."<a name="FNanchor_28_28" id=
+  "FNanchor_28_28"></a><a href="#Footnote_28_28" class=
+  "fnanchor">[28]</a></p>
+
+  <p>These few examples are sufficient to show that both the habits
+  and instincts of animals are subject to variation; and had we a
+  sufficient number of detailed observations we should probably
+  find that these variations were as numerous, as diverse in
+  character, as large in amount, and as independent of each other
+  as those which we have seen to characterise their bodily
+  structure.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Variability of Plants.</i></p>
+
+  <p>The variability of plants is notorious, being proved not only
+  by the endless variations which occur whenever a species is
+  largely grown by horticulturists, but also by the great
+  difficulty that is felt by botanists in determining the limits of
+  species in <a name="Page_77" id="Page_77"></a>many large genera.
+  As examples we may take the roses, the brambles, and the willows
+  as well illustrating this fact. In Mr. Baker's <i>Revision of the
+  British Roses</i> (published by the Linnean Society in 1863), he
+  includes under the single species, Rosa canina&mdash;the common
+  dog-rose&mdash;no less than twenty-eight named <i>varieties</i>
+  distinguished by more or less constant characters and often
+  confined to special localities, and to these are referred about
+  seventy of the <i>species</i> of British and continental
+  botanists. Of the genus Rubus or bramble, <i>five</i> British
+  species are given in Bentham's <i>Handbook of the British
+  Flora</i>, while in the fifth edition of Babington's <i>Manual of
+  British Botany</i>, published about the same time, no less than
+  <i>forty-five</i> species are described. Of willows (Salix) the
+  same two works enumerate <i>fifteen</i> and <i>thirty-one</i>
+  species respectively. The hawkweeds (Hieracium) are equally
+  puzzling, for while Mr. Bentham admits only seven British
+  species, Professor Babington describes no less than thirty-two,
+  besides several named varieties.</p>
+
+  <p>A French botanist, Mons. A. Jordan, has collected numerous
+  forms of a common little plant, the spring whitlow-grass (Draba
+  verna); he has cultivated these for several successive years, and
+  declares that they preserve their peculiarities unchanged; he
+  also says that they each come true from seed, and thus possess
+  all the characteristics of true species. He has described no less
+  than fifty-two such species or permanent varieties, all found in
+  the south of France; and he urges botanists to follow his example
+  in collecting, describing, and cultivating all such varieties as
+  may occur in their respective districts. Now, as the plant is
+  very common almost all over Europe and ranges from North America
+  to the Himalayas, the number of similar forms over this wide area
+  would probably have to be reckoned by hundreds if not by
+  thousands.</p>
+
+  <p>The class of facts now adduced must certainly be held to prove
+  that in many large genera and in some single species there is a
+  very large amount of variation, which renders it quite impossible
+  for experts to agree upon the limits of species. We will now
+  adduce a few striking cases of individual variation.</p>
+
+  <p>The distinguished botanist, Alp. de Candolle, made a special
+  study of the oaks of the whole world, and has stated some
+  <a name="Page_78" id="Page_78"></a>remarkable facts as to their
+  variability. He declares that on the same branch of oak he has
+  noted the following variations: (1) In the length of the petiole,
+  as one to three; (2) in the form of the leaf, being either
+  elliptical or obovoid; (3) in the margin being entire, or
+  notched, or even pinnatifid; (4) in the extremity being acute or
+  blunt; (5) in the base being sharp, blunt, or cordate; (6) in the
+  surface being pubescent or smooth; (7) the perianth varies in
+  depth and lobing; (8) the stamens vary in number, independently;
+  (9) the anthers are mucronate or blunt; (10) the fruit stalks
+  vary greatly in length, often as one to three; (11) the number of
+  fruits varies; (12) the form of the base of the cup varies; (13)
+  the scales of the cup vary in form; (14) the proportions of the
+  acorns vary; (15) the times of the acorns ripening and falling
+  vary.</p>
+
+  <p>Besides this, many species exhibit well-marked varieties which
+  have been described and named, and these are most numerous in the
+  best-known species. Our British oak (Quercus robur) has
+  twenty-eight varieties; Quercus Lusitanica has eleven; Quercus
+  calliprinos has ten; and Quercus coccifera eight.</p>
+
+  <p>A most remarkable case of variation in the parts of a common
+  flower has been given by Dr. Hermann M&uuml;ller. He examined two
+  hundred flowers of Myosurus minimus, among which he found
+  <i>thirty-five</i> different proportions of the sepals, petals,
+  and anthers, the first varying from four to seven, the second
+  from two to five, and the third from two to ten. Five sepals
+  occurred in one hundred and eighty-nine out of the two hundred,
+  but of these one hundred and five had three petals, forty-six had
+  four petals, and twenty-six had five petals; but in each of these
+  sets the anthers varied in number from three to eight, or from
+  two to nine. We have here an example of the same amount of
+  "independent variability" that, as we have seen, occurs in the
+  various dimensions of birds and mammals; and it may be taken as
+  an illustration of the kind and degree of variability that may be
+  expected to occur among small and little specialised
+  flowers.<a name="FNanchor_29_29" id="FNanchor_29_29"></a><a href=
+  "#Footnote_29_29" class="fnanchor">[29]</a></p>
+
+  <p>In the common wind-flower (Anemone nemorosa) an almost equal
+  amount of variation occurs; and I have myself gathered <a name=
+  "Page_79" id="Page_79"></a>in one locality flowers varying from
+  7/8 inch to 1-3/4 inch in diameter; the bracts varying from 1-1/2
+  inch to 4 inches across; and the petaloid sepals either broad or
+  narrow, and varying in number from five to ten. Though generally
+  pure white on their upper surface, some specimens are a full
+  pink, while others have a decided bluish tinge.</p>
+
+  <p>Mr. Darwin states that he carefully examined a large number of
+  plants of Geranium phaeum and G. pyrenaicum (not perhaps truly
+  British but frequently found wild), which had escaped from
+  cultivation, and had spread by seed in an open plantation; and he
+  declares that "the seedlings varied in almost every single
+  character, both in their flowers and foliage, to a degree which I
+  have never seen exceeded; yet they could not have been exposed to
+  any great change of their conditions."<a name="FNanchor_30_30"
+  id="FNanchor_30_30"></a><a href="#Footnote_30_30" class=
+  "fnanchor">[30]</a></p>
+
+  <p>The following examples of variation in important parts of
+  plants were collected by Mr. Darwin and have been copied from his
+  unpublished MSS.:&mdash;</p>
+
+  <p>"De Candolle (<i>Mem. Soc. Phys. de Gen&egrave;ve</i>, tom.
+  ii. part ii. p. 217) states that Papaver bracteatum and P.
+  orientale present indifferently two sepals and four petals, or
+  three sepals and six petals, which is sufficiently rare with
+  other species of the genus."</p>
+
+  <p>"In the Primulacae and in the great class to which this family
+  belongs the unilocular ovarium is free, but M. Dubury (<i>Mem.
+  Soc. Phys. de Gen&egrave;ve</i>, tom. ii. p. 406) has often found
+  individuals in Cyclamen hederaefolium, in which the base of the
+  ovary was connected for a third part of its length with the
+  inferior part of the calyx."</p>
+
+  <p>"M. Aug. St. Hilaire (Sur la Gynobase, <i>Mem. des Mus.
+  d'Hist. Nat.</i>, tom. x. p. 134), speaking of some bushes of the
+  Gomphia oleaefolia, which he at first thought formed a quite
+  distinct species, says: 'Voil&agrave; donc dans un m&ecirc;me
+  individu des loges et un style qui se rattachent tant&ocirc;t a
+  un axe vertical, et tant&ocirc;t a un gynobase; donc celui-ci
+  n'est qu'un axe veritable; mais cet axe est deprim&eacute; au
+  lieu d'&ecirc;tre vertical." He adds (p. 151), 'Does not all this
+  indicate that nature has tried, in a manner, in the family of
+  Rutaceae to produce from a single multilocular ovary, one-styled
+  and symmetrical, several unilocular ovaries, each with its own
+  style.' And he <a name="Page_80" id="Page_80"></a>subsequently
+  shows that, in Xanthoxylum monogynum, 'it often happens that on
+  the same plant, on the same panicle, we find flowers with one or
+  with two ovaries;' and that this is an important character is
+  shown by the Rutaceae (to which Xanthoxylum belongs), being
+  placed in a group of natural orders characterised by having a
+  solitary ovary."</p>
+
+  <p>"De Candolle has divided the Cruciferae into five sub-orders
+  in accordance with the position of the radicle and cotyledons,
+  yet Mons. T. Gay (<i>Ann. des Scien. Nat.</i>, ser. i. tom. vii.
+  p. 389) found in sixteen seeds of Petrocallis Pyrenaica the form
+  of the embryo so uncertain that he could not tell whether it
+  ought to be placed in the sub-orders 'Pleurorhiz&eacute;e' or
+  'Notor-hiz&eacute;e'; so again (p. 400) in Cochlearia saxatilis
+  M. Gay examined twenty-nine embryos, and of these sixteen were
+  vigorously 'pleurorhiz&eacute;es,' nine had characters
+  intermediate between pleuro-and notor-hiz&eacute;es, and four
+  were pure notor-hiz&eacute;es."</p>
+
+  <p>"M. Raspail asserts (<i>Ann. des Scien. Nat.</i>, ser. i. tom.
+  v. p. 440) that a grass (Nostus Borbonicus) is so eminently
+  variable in its floral organisation, that the varieties might
+  serve to make a family with sufficiently numerous genera and
+  tribes&mdash;a remark which shows that important organs must be
+  here variable."</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Species which vary little.</i></p>
+
+  <p>The preceding statements, as to the great amount of variation
+  occurring in animals and plants, do not prove that all species
+  vary to the same extent, or even vary at all, but, merely, that a
+  considerable number of species in every class, order, and family
+  do so vary. It will have been observed that the examples of great
+  variability have all been taken from common species, or species
+  which have a wide range and are abundant in individuals. Now Mr.
+  Darwin concludes, from an elaborate examination of the floras and
+  faunas of several distinct regions, that common, wide ranging
+  species, as a rule, vary most, while those that are confined to
+  special districts and are therefore comparatively limited in
+  number of individuals vary least. By a similar comparison it is
+  shown that species of large genera vary more than species of
+  small genera. These facts explain, to some extent, why the
+  opinion has been so prevalent that variation is very limited in
+  amount and exceptional in character. For <a name="Page_81" id=
+  "Page_81"></a>naturalists of the old school, and all mere
+  collectors, were interested in species in proportion to their
+  rarity, and would often have in their collections a larger number
+  of specimens of a rare species than of a species that was very
+  common. Now as these rare species do really vary much less than
+  the common species, and in many cases hardly vary at all, it was
+  very natural that a belief in the fixity of species should
+  prevail. It is not, however, as we shall see presently, the rare,
+  but the common and widespread species which become the parents of
+  new forms, and thus the non-variability of any number of rare or
+  local species offers no difficulty whatever in the way of the
+  theory of evolution.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Concluding Remarks.</i></p>
+
+  <p>We have now shown in some detail, at the risk of being
+  tedious, that individual variability is a general character of
+  all common and widespread species of animals or plants; and,
+  further, that this variability extends, so far as we know, to
+  every part and organ, whether external or internal, as well as to
+  every mental faculty. Yet more important is the fact that each
+  part or organ varies to a considerable extent independently of
+  other parts. Again, we have shown, by abundant evidence, that the
+  variation that occurs is very large in amount&mdash;usually
+  reaching 10 or 20, and sometimes even 25 per cent of the average
+  size of the varying part; while not one or two only, but from 5
+  to 10 per cent of the specimens examined exhibit nearly as large
+  an amount of variation. These facts have been brought clearly
+  before the reader by means of numerous diagrams, drawn to scale
+  and exhibiting the actual variations in inches, so that there can
+  be no possibility of denying either their generality or their
+  amount. The importance of this full exposition of the subject
+  will be seen in future chapters, when we shall frequently have to
+  refer to the facts here set forth, especially when we deal with
+  the various theories of recent writers and the criticisms that
+  have been made of the Darwinian theory.</p>
+
+  <p>A full exposition of the facts of variation among wild animals
+  and plants is the more necessary, because comparatively few of
+  them were published in Mr. Darwin's works, while the more
+  important have only been made known since <a name="Page_82" id=
+  "Page_82"></a>the last edition of <i>The Origin of Species</i>
+  was prepared; and it is clear that Mr. Darwin himself did not
+  fully recognise the enormous amount of variability that actually
+  exists. This is indicated by his frequent reference to the
+  extreme slowness of the changes for which variation furnishes the
+  materials, and also by his use of such expressions as the
+  following: "A variety when once formed must again, <i>perhaps
+  after a long interval of time</i>, vary or present individual
+  differences of the same favourable nature as before"
+  (<i>Origin</i>, p. 66). And again, after speaking of changed
+  conditions "affording a better chance of the occurrence of
+  favourable variations," he adds: "<i>Unless such occur natural
+  selection can do nothing</i>" (<i>Origin</i>, p. 64). These
+  expressions are hardly consistent with the fact of the constant
+  and large amount of variation, of every part, in all directions,
+  which evidently occurs in each generation of all the more
+  abundant species, and which must afford an ample supply of
+  favourable variations whenever required; and they have been
+  seized upon and exaggerated by some writers as proofs of the
+  extreme difficulties in the way of the theory. It is to show that
+  such difficulties do not exist, and in the full conviction that
+  an adequate knowledge of the facts of variation affords the only
+  sure foundation for the Darwinian theory of the origin of
+  species, that this chapter has been written.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_16_16" id="Footnote_16_16"></a><a href=
+      "#FNanchor_16_16"><span class="label">[16]</span></a>
+      <i>Foraminifera</i>, preface, p. x.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_17_17" id="Footnote_17_17"></a><a href=
+      "#FNanchor_17_17"><span class="label">[17]</span></a>
+      <i>United States Geological Survey of the Territories</i>,
+      1874.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_18_18" id="Footnote_18_18"></a><a href=
+      "#FNanchor_18_18"><span class="label">[18]</span></a>
+      <i>Proceedings of the Entomological Society of London</i>,
+      1875, p. vii.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_19_19" id="Footnote_19_19"></a><a href=
+      "#FNanchor_19_19"><span class="label">[19]</span></a> <i>Ann.
+      des Sci. Nat.</i>, tom. xvi. p. 50.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_20_20" id="Footnote_20_20"></a><a href=
+      "#FNanchor_20_20"><span class="label">[20]</span></a> See
+      <i>Winter Birds of Florida</i>, p. 206, Table F.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_21_21" id="Footnote_21_21"></a><a href=
+      "#FNanchor_21_21"><span class="label">[21]</span></a> See
+      Table I, p. 211, of Allen's <i>Winter Birds of
+      Florida</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_22_22" id="Footnote_22_22"></a><a href=
+      "#FNanchor_22_22"><span class="label">[22]</span></a>
+      <i>Proc. Zool. Soc.</i>, 1864, p. 64.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_23_23" id="Footnote_23_23"></a><a href=
+      "#FNanchor_23_23"><span class="label">[23]</span></a> J.A.
+      Allen, on Geographical Variation among North American
+      Mammals, <i>Bull. U.S. Geol. and Geog. Survey</i>, vol. ii.
+      p. 314 (1876).</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_24_24" id="Footnote_24_24"></a><a href=
+      "#FNanchor_24_24"><span class="label">[24]</span></a>
+      <i>Proc. Zool. Soc. Lond.</i>, 1864, p. 700, and 1868, p.
+      28.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_25_25" id="Footnote_25_25"></a><a href=
+      "#FNanchor_25_25"><span class="label">[25]</span></a> See
+      <i>Trans. Entomological Society of London</i>, 1887, p.
+      24.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_26_26" id="Footnote_26_26"></a><a href=
+      "#FNanchor_26_26"><span class="label">[26]</span></a>
+      <i>Nature</i>, vol. xix. p. 554.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_27_27" id="Footnote_27_27"></a><a href=
+      "#FNanchor_27_27"><span class="label">[27]</span></a>
+      <i>Nature</i>, vol. xvi. p. 163; and vol. xi. p. 227.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_28_28" id="Footnote_28_28"></a><a href=
+      "#FNanchor_28_28"><span class="label">[28]</span></a>
+      <i>Ibid.</i>, vol. xxxi. (1885), p. 533.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_29_29" id="Footnote_29_29"></a><a href=
+      "#FNanchor_29_29"><span class="label">[29]</span></a>
+      <i>Nature</i>, vol. xxvi. p. 81.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_30_30" id="Footnote_30_30"></a><a href=
+      "#FNanchor_30_30"><span class="label">[30]</span></a>
+      <i>Animals and Plants under Domestication</i>, vol. ii. p.
+      258.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a><a name="Page_83"
+  id="Page_83"></a>CHAPTER IV</h2>
+
+  <p style='font-weight:bold;'>VARIATION OF DOMESTICATED ANIMALS
+  AND CULTIVATED PLANTS</p>
+
+  <div class="blockquot">
+    <p>The facts of variation and artificial selection&mdash;Proofs
+    of the generality of variation&mdash;Variations of apples and
+    melons&mdash;Variations of flowers&mdash;Variations of domestic
+    animals&mdash;Domestic
+    pigeons&mdash;Acclimatisation&mdash;Circumstances favourable to
+    selection by man&mdash;Conditions favourable to
+    variation&mdash;Concluding remarks.</p>
+  </div>
+
+  <p>Having so fully discussed variation under nature it will be
+  unnecessary to devote so much space to domesticated animals and
+  cultivated plants, especially as Mr. Darwin has published two
+  remarkable volumes on the subject where those who desire it may
+  obtain ample information. A general sketch of the more important
+  facts will, however, be given, for the purpose of showing how
+  closely they correspond with those described in the preceding
+  chapter, and also to point out the general principles which they
+  illustrate. It will also be necessary to explain how these
+  variations have been increased and accumulated by artificial
+  selection, since we are thereby better enabled to understand the
+  action of natural selection, to be discussed in the succeeding
+  chapter.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The facts of Variation and Artificial Selection.</i></p>
+
+  <p>Every one knows that in each litter of kittens or of puppies
+  no two are alike. Even in the case in which several are exactly
+  alike in colours, other differences are always perceptible to
+  those who observe them closely. They will differ in size, in the
+  proportions of their bodies and limbs, in the length or texture
+  of their hairy covering, and notably in their disposition. They
+  each possess, too, an individual <a name="Page_84" id=
+  "Page_84"></a>countenance, almost as varied when closely studied
+  as that of a human being; not only can a shepherd distinguish
+  every sheep in his flock, but we all know that each kitten in the
+  successive families of our old favourite cat has a face of its
+  own, with an expression and individuality distinct from all its
+  brothers and sisters. Now this individual variability exists
+  among all creatures whatever, which we can closely observe, even
+  when the two parents are very much alike and have been matched in
+  order to preserve some special breed. The same thing occurs in
+  the vegetable kingdom. All plants raised from seed differ more or
+  less from each other. In every bed of flowers or of vegetables we
+  shall find, if we look closely, that there are countless small
+  differences, in the size, in the mode of growth, in the shape or
+  colour of the leaves, in the form, colour, or markings of the
+  flowers, or in the size, form, colour, or flavour of the fruit.
+  These differences are usually small, but are yet easily seen, and
+  in their extremes are very considerable; and they have this
+  important quality, that they have a tendency to be reproduced,
+  and thus by careful breeding any particular variation or group of
+  variations can be increased to an enormous
+  extent&mdash;apparently to any extent not incompatible with the
+  life, growth, and reproduction of the plant or animal.</p>
+
+  <p>The way this is done is by artificial selection, and it is
+  very important to understand this process and its results.
+  Suppose we have a plant with a small edible seed, and we want to
+  increase the size of that seed. We grow as large a quantity of it
+  as possible, and when the crop is ripe we carefully choose a few
+  of the very largest seeds, or we may by means of a sieve sort out
+  a quantity of the largest seeds. Next year we sow only these
+  large seeds, taking care to give them suitable soil and manure,
+  and the result is found to be that the <i>average</i> size of the
+  seeds is larger than in the first crop, and that the largest
+  seeds are now somewhat larger and more numerous. Again sowing
+  these, we obtain a further slight increase of size, and in a very
+  few years we obtain a greatly improved race, which will always
+  produce larger seeds than the unimproved race, even if cultivated
+  without any special care. In this way all our fine sorts of
+  vegetables, fruits, and flowers have been obtained, all our
+  choice breeds <a name="Page_85" id="Page_85"></a>of cattle or of
+  poultry, our wonderful race-horses, and our endless varieties of
+  dogs. It is a very common but mistaken idea that this improvement
+  is due to crossing and feeding in the case of animals, and to
+  improved cultivation in the case of plants. Crossing is
+  occasionally used in order to obtain a combination of qualities
+  found in two distinct breeds, and also because it is found to
+  increase the constitutional vigour; but every breed possessing
+  any exceptional quality is the result of the selection of
+  variations occurring year after year and accumulated in the
+  manner just described. Purity of breed, with repeated selection
+  of the best varieties of that breed, is the foundation of all
+  improvement in our domestic animals and cultivated plants.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Proofs of the Generality of Variation.</i></p>
+
+  <p>Another very common error is, that variation is the exception,
+  and rather a rare exception, and that it occurs only in one
+  direction at a time&mdash;that is, that only one or two of the
+  numerous possible modes of variation occur at the same time. The
+  experience of breeders and cultivators, however, proves that
+  variation is the rule instead of the exception, and that it
+  occurs, more or less, in almost every direction. This is shown by
+  the fact that different species of plants and animals have
+  required different <i>kinds</i> of modification to adapt them to
+  our use, and we have never failed to meet with variation <i>in
+  that particular direction</i>, so as to enable us to accumulate
+  it and so to produce ultimately a large amount of change in the
+  required direction. Our gardens furnish us with numberless
+  examples of this property of plants. In the cabbage and lettuce
+  we have found variation in the size and mode of growth of the
+  leaf, enabling us to produce by selection the almost innumerable
+  varieties, some with solid heads of foliage quite unlike any
+  plant in a state of nature, others with curiously wrinkled leaves
+  like the savoy, others of a deep purple colour used for pickling.
+  From the very same species as the cabbage (Brassica oleracea)
+  have arisen the broccoli and cauliflower, in which the leaves
+  have undergone little alteration, while the branching heads of
+  flowers grow into a compact mass forming one of our most delicate
+  vegetables. The brussels sprouts are another form of the same
+  plant, in <a name="Page_86" id="Page_86"></a>which the whole mode
+  of growth has been altered, numerous little heads of leaves being
+  produced on the stem. In other varieties the ribs of the leaves
+  are thickened so as to become themselves a culinary vegetable;
+  while, in the Kohlrabi, the stem grows into a turnip-like mass
+  just above ground. Now all these extraordinarily distinct plants
+  come from one original species which still grows wild on our
+  coasts; and it must have varied in all these directions,
+  otherwise variations could not have been accumulated to the
+  extent we now see them. The flowers and seeds of all these plants
+  have remained nearly stationary, because no attempt has been made
+  to accumulate the slight variations that no doubt occur in
+  them.</p>
+
+  <p>If now we turn to another set of plants, the turnips,
+  radishes, carrots, and potatoes, we find that the roots or
+  underground tubers have been wonderfully enlarged and improved,
+  and also altered in shape and colour, while the stems, leaves,
+  flowers, and fruits have remained almost unchanged. In the
+  various kinds of peas and beans it is the pod or fruit and the
+  seed that has been subjected to selection, and therefore greatly
+  modified; and it is here very important to notice that while all
+  these plants have undergone cultivation in a great variety of
+  soils and climates, with different manures and under different
+  systems, yet the flowers have remained but little altered, those
+  of the broad bean, the scarlet-runner, and the garden-pea, being
+  nearly the same in all the varieties. This shows us how little
+  change is produced by mere cultivation, or even by variety of
+  soil and climate, if there is no <i>selection</i> to preserve and
+  accumulate the small variations that are continually occurring.
+  When, however, a great amount of modification has been effected
+  in one country, change to another country produces a decided
+  effect. Thus it has been found that some of the numerous
+  varieties of maize produced and cultivated in the United States
+  change considerably, not only in their size and colour, but even
+  in the shape of the seed when grown for a few successive years in
+  Germany.<a name="FNanchor_31_31" id="FNanchor_31_31"></a><a href=
+  "#Footnote_31_31" class="fnanchor">[31]</a> In all our cultivated
+  fruit trees the fruits vary immensely in shape, size, colour,
+  flavour, time of ripening, and other qualities, while the leaves
+  and flowers usually differ so little that they are hardly
+  distinguishable except to a very close observer.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_87" id="Page_87"></a><i>Variations of Apples and of
+  Melons.</i></p>
+
+  <p>The most remarkable varieties are afforded by the apple and
+  the melon, and some account of these will be given as
+  illustrating the effects of slight variations accumulated by
+  selection. All our apples are known to have descended from the
+  common crab of our hedges (Pyrus malus), and from this at least a
+  thousand distinct varieties have been produced. These differ
+  greatly in the size and form of the fruit, in its colour, and in
+  the texture of the skin. They further differ in the time of
+  ripening, in their flavour, and in their keeping properties; but
+  apple trees also differ in many other ways. The foliage of the
+  different varieties can often be distinguished by peculiarities
+  of form and colour, and it varies considerably in the time of its
+  appearance; in some hardly a leaf appears till the tree is in
+  full bloom, while others produce their leaves so early as almost
+  to hide the flowers. The flowers differ in size and colour, and
+  in one case in structure also, that of the St. Valery apple
+  having a double calyx with ten divisions, and fourteen styles
+  with oblique stigmas, but without stamens or corolla. The
+  flowers, therefore, have to be fertilised with the pollen from
+  other varieties in order to produce fruit. The pips or seeds
+  differ also in shape, size, and colour; some varieties are liable
+  to canker more than others, while the Winter Majetin and one or
+  two others have the strange constitutional peculiarity of never
+  being attacked by the mealy bug even when all the other trees in
+  the same orchard are infested with it.</p>
+
+  <p>All the cucumbers and gourds vary immensely, but the melon
+  (Cucumis melo) exceeds them all. A French botanist, M. Naudin,
+  devoted six years to their study. He found that previous
+  botanists had described thirty distinct species, as they thought,
+  which were really only varieties of melons. They differ chiefly
+  in their fruits, but also very much in foliage and mode of
+  growth. Some melons are only as large as small plums, others
+  weigh as much as sixty-six pounds. One variety has a scarlet
+  fruit. Another is not more than an inch in diameter, but
+  sometimes more than a yard in length, twisting about in all
+  directions like a serpent. Some melons are exactly like
+  cucumbers; and an Algerian variety, when ripe, <a name="Page_88"
+  id="Page_88"></a>cracks and falls to pieces, just as occurs in a
+  wild gourd (C. momordica).<a name="FNanchor_32_32" id=
+  "FNanchor_32_32"></a><a href="#Footnote_32_32" class=
+  "fnanchor">[32]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variations of Flowers.</i></p>
+
+  <p>Turning to flowers, we find that in the same genus as our
+  currant and gooseberry, which we have cultivated for their
+  fruits, there are some ornamental species, as the Ribes
+  sanguinea, and in these the flowers have been selected so as to
+  produce deep red, pink, or white varieties. When any particular
+  flower becomes fashionable and is grown in large quantities,
+  variations are always met with sufficient to produce great
+  varieties of tint or marking, as shown by our roses, auriculas,
+  and geraniums. When varied leaves are required, it is found that
+  a number of plants vary sufficiently in this direction also, and
+  we have zonal geraniums, variegated ivies, gold and silver marked
+  hollies, and many others.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variations of Domestic Animals.</i></p>
+
+  <p>Coming now to our domesticated animals, we find still more
+  extraordinary cases; and it appears as if any special quality or
+  modification in an animal can be obtained if we only breed it in
+  sufficient quantity, watch carefully for the required variations,
+  and carry on selection with patience and skill for a sufficiently
+  long period. Thus, in sheep we have enormously increased the
+  wool, and have obtained the power of rapidly forming flesh and
+  fat; in cows we have increased the production of milk; in horses
+  we have obtained strength, endurance, or speed, and have greatly
+  modified size, form, and colour; in poultry we have secured
+  various colours of plumage, increase of size, and almost
+  perpetual egg-laying. But it is in dogs and pigeons that the most
+  marvellous changes have been effected, and these require our
+  special attention.</p>
+
+  <p>Our various domestic dogs are believed to have originated from
+  several distinct wild species, because in every part of the world
+  the native dogs resemble some wild dogs or wolves of the same
+  country. Thus perhaps several species of wolves and jackals were
+  domesticated in very early times, and from breeds derived from
+  these, crossed and improved by selection, <a name="Page_89" id=
+  "Page_89"></a>our existing dogs have descended. But this
+  intermixture of distinct species will go a very little way in
+  accounting for the peculiarities of the different breeds of dogs,
+  many of which are totally unlike any wild animal. Such is the
+  case with greyhounds, bloodhounds, bulldogs, Blenheim spaniels,
+  terriers, pugs, turnspits, pointers, and many others; and these
+  differ so greatly in size, shape, colour, and habits, as well as
+  in the form and proportions of all the different parts of the
+  body, that it seems impossible that they could have descended
+  from any of the known wild dogs, wolves, or allied animals, none
+  of which differ nearly so much in size, form, and proportions. We
+  have here a remarkable proof that variation is not confined to
+  superficial characters&mdash;to the colour, hair, or external
+  appendages, when we see how the entire skeletons of such forms as
+  the greyhound and the bulldog have been gradually changed in
+  opposite directions till they are both completely unlike that of
+  any known wild animal, recent or extinct. These changes have been
+  the result of some thousands of years of domestication and
+  selection, different breeds being used and preserved for
+  different purposes; but some of the best breeds are known to have
+  been improved and perfected in modern times. About the middle of
+  the last century a new and improved kind of foxhound was
+  produced; the greyhound was also greatly improved at the end of
+  the last century, while the true bulldog was brought to
+  perfection about the same period. The Newfoundland dog has been
+  so much changed since it was first imported that it is now quite
+  unlike any existing native dog in that island.<a name=
+  "FNanchor_33_33" id="FNanchor_33_33"></a><a href=
+  "#Footnote_33_33" class="fnanchor">[33]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Domestic Pigeons.</i></p>
+
+  <p>The most remarkable and instructive example of variation
+  produced by human selection is afforded by the various races and
+  breeds of domestic pigeons, not only because the variations
+  produced are often most extraordinary in amount and diverse in
+  character, but because in this case there is no doubt whatever
+  that all have been derived from one wild species, the common
+  rock-pigeon (Columba livia). As this is a very important point it
+  is well to state the evidence on which the belief is founded. The
+  wild rock-pigeon is of a slaty-blue <a name="Page_90" id=
+  "Page_90"></a>colour, the tail has a dark band across the end,
+  the wings have two black bands, and the outer tail-feathers are
+  edged with white at the base. No other wild pigeon in the world
+  has this combination of characters. Now in every one of the
+  domestic varieties, even the most extreme, all the above marks,
+  even to the white edging of the outer tail-feathers, are
+  sometimes found perfectly developed. When birds belonging to two
+  distinct breeds are crossed one or more times, neither of the
+  parents being blue, or having any of the above-named marks, the
+  mongrel offspring are very apt to acquire some of these
+  characters. Mr. Darwin gives instances which he observed himself.
+  He crossed some white fantails with some black barbs, and the
+  mongrels were black, brown, or mottled. He also crossed a barb
+  with a spot, which is a white bird with a red tail and red spot
+  on the forehead, and the mongrel offspring were dusky and
+  mottled. On now crossing these two sets of mongrels with each
+  other, he obtained a bird of a beautiful blue colour, with the
+  barred and white edged tail, and double-banded wings, so as
+  almost exactly to resemble a wild rock-pigeon. This bird was
+  descended in the second generation from a pure white and pure
+  black bird, both of which when unmixed breed their kind
+  remarkably true. These facts, well known to experienced
+  pigeon-fanciers, together with the habits of the birds, which all
+  like to nest in holes, or dovecots, not in trees like the great
+  majority of wild pigeons, have led to the general belief in the
+  single origin of all the different kinds.</p>
+
+  <p>In order to afford some idea of the great differences which
+  exist among domesticated pigeons, it will be well to give a brief
+  abstract of Mr. Darwin's account of them. He divides them into
+  eleven distinct races, most of which have several sub-races.</p>
+
+  <p>RACE I. <i>Pouters</i>.&mdash;These are especially
+  distinguished by the enormously enlarged crop, which can be so
+  inflated in some birds as almost to conceal the beak. They are
+  very long in the body and legs and stand almost upright, so as to
+  present a very distinct appearance. Their skeleton has become
+  modified, the ribs being broader and the vertebrae more numerous
+  than in other pigeons.</p>
+
+  <p><a name="Page_91" id="Page_91"></a>RACE II.
+  <i>Carriers</i>.&mdash;These are large, long-necked birds, with a
+  long pointed beak, and the eyes surrounded with a naked
+  carunculated skin or wattle, which is also largely developed at
+  the base of the beak. The opening of the mouth is unusually wide.
+  There are several sub-races, one being called Dragons.</p>
+
+  <p>RACE III. <i>Runts</i>.&mdash;These are very large-bodied,
+  long-beaked pigeons, with naked skin round the eyes. The wings
+  are usually very long, the legs long, and the feet large, and the
+  skin of the neck is often red. There are several sub-races, and
+  these differ very much, forming a series of links between the
+  wild rock-pigeon and the carrier.</p>
+
+  <p>RACE IV. <i>Barbs</i>.&mdash;These are remarkable for their
+  very short and thick beak, so unlike that of most pigeons that
+  fanciers compare it with that of a bullfinch. They have also a
+  naked carunculated skin round the eyes, and the skin over the
+  nostrils swollen.</p>
+
+  <p>RACE V. <i>Fantails</i>.&mdash;Short-bodied and rather
+  small-beaked pigeons, with an enormously developed tail,
+  consisting usually of from fourteen to forty feathers instead of
+  twelve, the regular number in all other pigeons, wild and tame.
+  The tail spreads out like a fan and is usually carried erect, and
+  the bird bends back its slender neck, so that in highly-bred
+  varieties the head touches the tail. The feet are small, and they
+  walk stiffly.</p>
+
+  <p>RACE VI. <i>Turbits and Owls</i>.&mdash;These are
+  characterised by the feathers of the middle of neck and breast in
+  front spreading out irregularly so as to form a frill. The
+  Turbits also have a crest on the head, and both have the beak
+  exceedingly short.</p>
+
+  <p>RACE VII. <i>Tumblers</i>.&mdash;These have a small body and
+  short beak, but they are specially distinguished by the singular
+  habit of tumbling over backwards during flight. One of the
+  sub-races, the Indian Lotan or Ground tumbler, if slightly shaken
+  and placed on the ground, will immediately begin tumbling head
+  over heels until taken up and soothed. If not taken up, some of
+  them will go on tumbling till they die. <a name="Page_92" id=
+  "Page_92"></a>Some English tumblers are almost equally
+  persistent. A writer, quoted by Mr. Darwin, says that these birds
+  generally begin to tumble almost as soon as they can fly; "at
+  three months old they tumble well, but still fly strong; at five
+  or six months they tumble excessively; and in the second year
+  they mostly give up flying, on account of their tumbling so much
+  and so close to the ground. Some fly round with the flock,
+  throwing a clean summersault every few yards till they are
+  obliged to settle from giddiness and exhaustion. These are called
+  Air-tumblers, and they commonly throw from twenty to thirty
+  summersaults in a minute, each clear and clean. I have one red
+  cock that I have on two or three occasions timed by my watch, and
+  counted forty summersaults in the minute. At first they throw a
+  single summersault, then it is double, till it becomes a
+  continuous roll, which puts an end to flying, for if they fly a
+  few yards over they go, and roll till they reach the ground. Thus
+  I had one kill herself, and another broke his leg. Many of them
+  turn over only a few inches from the ground, and will tumble two
+  or three times in flying across their loft. These are called
+  House-tumblers from tumbling in the house. The act of tumbling
+  seems to be one over which they have no control, an involuntary
+  movement which they seem to try to prevent. I have seen a bird
+  sometimes in his struggles fly a yard or two straight upwards,
+  the impulse forcing him backwards while he struggles to go
+  forwards."<a name="FNanchor_34_34" id=
+  "FNanchor_34_34"></a><a href="#Footnote_34_34" class=
+  "fnanchor">[34]</a></p>
+
+  <p>The Short-faced tumblers are an improved sub-race which have
+  almost lost the power of tumbling, but are valued for possessing
+  some other characteristics in an extreme degree. They are very
+  small, have almost globular heads, and a very minute beak, so
+  that fanciers say the head of a perfect bird should resemble a
+  cherry with a barleycorn stuck in it. Some of these weigh less
+  than seven ounces, whereas the wild rock-pigeon weighs about
+  fourteen ounces. The feet, too, are very short and small, and the
+  middle toe has twelve or thirteen instead of fourteen or fifteen
+  scutellae. They have often only nine primary wing-feathers
+  instead of ten as in all other pigeons.</p>
+
+  <p><a name="Page_93" id="Page_93"></a>RACE VIII. <i>Indian
+  Frill-back</i>.&mdash;In these birds the beak is very short, and
+  the feathers of the whole body are reversed or turn
+  backwards.</p>
+
+  <p>RACE IX. <i>Jacobin</i>.&mdash;These curious birds have a hood
+  of feathers almost enclosing the head and meeting in front of the
+  neck. The wings and tail are unusually long.</p>
+
+  <p>RACE X. <i>Trumpeter</i>.&mdash;Distinguished by a tuft of
+  feathers curling forwards over the beak, and the feet very much
+  feathered. They obtain their name from the peculiar voice unlike
+  that of any other pigeon. The coo is rapidly repeated, and is
+  continued for several minutes. The feet are covered with feathers
+  so large as often to appear like little wings.</p>
+
+  <p>RACE XI. comprises <i>Laughers</i>, <i>Frill-backs</i>,
+  <i>Nuns</i>, <i>Spots</i>, <i>and Swallows</i>.&mdash;They are
+  all very like the common rock-pigeon, but have each some slight
+  peculiarity. The Laughers have a peculiar voice, supposed to
+  resemble a laugh. The Nuns are white, with the head, tail, and
+  primary wing-feathers black or red. The Spots are white, with the
+  tail and a spot on the forehead red. The Swallows are slender,
+  white in colour, with the head and wings of some darker
+  colour.</p>
+
+  <p>Besides these races and sub-races a number of other kinds have
+  been described, and about one hundred and fifty varieties can be
+  distinguished. It is interesting to note that almost every part
+  of the bird, whose variations can be noted and selected, has led
+  to variations of a considerable extent, and many of these have
+  necessitated changes in the plumage and in the skeleton quite as
+  great as any that occur in the numerous distinct species of large
+  genera. The form of the skull and beak varies enormously, so that
+  the skulls of the Short-faced tumbler and some of the Carriers
+  differ more than any wild pigeons, even those classed in distinct
+  genera. The breadth and number of the ribs vary, as well as the
+  processes on them; the number of the vertebrae and the length of
+  the sternum also vary; and the perforations in the sternum vary
+  in size and shape. The oil gland varies in development, and is
+  sometimes absent. The number of the wing-feathers varies, and
+  those of the tail to an enormous extent. The proportions of the
+  leg and feet and the number <a name="Page_94" id="Page_94"></a>of
+  the scutellae also vary. The eggs also vary somewhat in size and
+  shape; and the amount of downy clothing on the young bird, when
+  first hatched, differs very considerably. Finally, the attitude
+  of the body, the manner of walking, the mode of flight, and the
+  voice, all exhibit modifications of the most remarkable
+  kind.<a name="FNanchor_35_35" id="FNanchor_35_35"></a><a href=
+  "#Footnote_35_35" class="fnanchor">[35]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Acclimatisation</i>.</p>
+
+  <p>A very important kind of variation is that constitutional
+  change termed acclimatisation, which enables any organism to
+  become gradually adapted to a different climate from the parent
+  stock. As closely allied species often inhabit different
+  countries possessing very different climates, we should expect to
+  find cases illustrating this change among our domesticated
+  animals and cultivated plants. A few examples will therefore be
+  adduced showing that such constitutional variation does
+  occur.</p>
+
+  <p>Among animals the cases are not numerous, because no
+  systematic attempt has been made to select varieties for this
+  special quality. It has, however, been observed that, though no
+  European dogs thrive well in India, the Newfoundland dog,
+  originating from a severe climate, can hardly be kept alive. A
+  better case, perhaps, is furnished by merino sheep, which, when
+  imported directly from England, do not thrive, while those which
+  have been bred in the intermediate climate of the Cape of Good
+  Hope do much better. When geese were first introduced into
+  Bogota, they laid few eggs at long intervals, and few of the
+  young survived. By degrees, however, the fecundity improved, and
+  in about twenty years became equal to what it is in Europe.
+  According to Garcilaso, when fowls were first introduced into
+  Peru they were not fertile, whereas now they are as much so as in
+  Europe.</p>
+
+  <p>Plants furnish much more important evidence. Our nurserymen
+  distinguish in their catalogues varieties of fruit-trees which
+  are more or less hardy, and this is especially the case in
+  America, where certain varieties only will stand the severe
+  climate of Canada. There is one variety of pear, the Forelle,
+  which both in England and France withstood frosts <a name=
+  "Page_95" id="Page_95"></a>that killed the flowers and buds of
+  all other kinds of pears. Wheat, which is grown over so large a
+  portion of the world, has become adapted to special climates.
+  Wheat imported from India and sown in good wheat soil in England
+  produced the most meagre ears; while wheat taken from France to
+  the West Indian Islands produced either wholly barren spikes or
+  spikes furnished with two or three miserable seeds, while West
+  Indian seed by its side yielded an enormous harvest. The orange
+  was very tender when first introduced into Italy, and continued
+  so as long as it was propagated by grafts, but when trees were
+  raised from seed many of these were found to be hardier, and the
+  orange is now perfectly acclimatised in Italy. Sweet-peas
+  (Lathyrus odoratus) imported from England to the Calcutta Botanic
+  Gardens produced few blossoms and no seed; those from France
+  flowered a little better, but still produced no seed, but plants
+  raised from seed brought from Darjeeling in the Himalayas, but
+  originally derived from England, flower and seed profusely in
+  Calcutta.<a name="FNanchor_36_36" id=
+  "FNanchor_36_36"></a><a href="#Footnote_36_36" class=
+  "fnanchor">[36]</a></p>
+
+  <p>An observation by Mr. Darwin himself is perhaps even more
+  instructive. He says: "On 24th May 1864 there was a severe frost
+  in Kent, and two rows of scarlet runners (Phaseolus multiflorus)
+  in my garden, containing 390 plants of the same age and equally
+  exposed, were all blackened and killed except about a dozen
+  plants. In an adjoining row of Fulmer's dwarf bean (Phaseolus
+  vulgaris) one single plant escaped. A still more severe frost
+  occurred four days afterwards, and of the dozen plants which had
+  previously escaped only three survived; these were not taller or
+  more vigorous than the other young plants, but they escaped
+  completely, with not even the tips of their leaves browned. It
+  was impossible to behold these three plants, with their
+  blackened, withered, and dead brethren all around them, and not
+  see at a glance that they differed widely in their constitutional
+  power of resisting frost."</p>
+
+  <p>The preceding sketch of the variation that occurs among
+  domestic animals and cultivated plants shows how wide it is in
+  range and how great in amount; and we have good reason to believe
+  that similar variation extends to all organised beings. In the
+  class of fishes, for example, we have one kind which has <a name=
+  "Page_96" id="Page_96"></a>been long domesticated in the East,
+  the gold and silver carps; and these present great variation, not
+  only of colour but in the form and structure of the fins and
+  other external organs. In like manner, the only domesticated
+  insects, hive bees and silkworm moths, present numbers of
+  remarkable varieties which have been produced by the selection of
+  chance variations just as in the case of plants and the higher
+  animals.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Circumstances favourable to Selection by Man.</i></p>
+
+  <p>It may be supposed, that the systematic selection which has
+  been employed for the purpose of improving the races of animals
+  or plants useful to man is of comparatively recent origin, though
+  some of the different races are known to have been in existence
+  in very early times. But Mr. Darwin has pointed out, that
+  unconscious selection must have begun to produce an effect as
+  soon as plants were cultivated or animals domesticated by man. It
+  would have been very soon observed that animals and plants
+  produced their like, that seed of early wheat produced early
+  wheat, that the offspring of very swift dogs were also swift, and
+  as every one would try to have a good rather than a bad sort this
+  would necessarily lead to the slow but steady improvement of all
+  useful plants and animals subject to man's care. Soon there would
+  arise distinct breeds, owing to the varying uses to which the
+  animals and plants were put. Dogs would be wanted chiefly to hunt
+  one kind of game in one part of the country and another kind
+  elsewhere; for one purpose scent would be more important, for
+  another swiftness, for another strength and courage, for yet
+  another watchfulness and intelligence, and this would soon lead
+  to the formation of very distinct races. In the case of
+  vegetables and fruits, different varieties would be found to
+  succeed best in certain soils and climates; some might be
+  preferred on account of the quantity of food they produced,
+  others for their sweetness and tenderness, while others might be
+  more useful on account of their ripening at a particular season,
+  and thus again distinct varieties would be established. An
+  instance of unconscious selection leading to distinct results in
+  modern times is afforded by two flocks of Leicester sheep which
+  both originated from the same stock, and were then bred pure for
+  upwards of fifty years by two gentlemen, Mr. Buckley <a name=
+  "Page_97" id="Page_97"></a>and Mr. Burgess. Mr. Youatt, one of
+  the greatest authorities on breeding domestic animals, says:
+  "There is not a suspicion existing in the mind of any one at all
+  acquainted with the subject that the owner of either of them has
+  deviated in any one instance from the pure blood of Mr.
+  Bakewell's original flock, and yet the difference between the
+  sheep possessed by these two gentlemen is so great that they have
+  the appearance of being quite different varieties." In this case
+  there was no desire to deviate from the original breed, and the
+  difference must have arisen from some slight difference of taste
+  or judgment in selecting, each year, the parents for the next
+  year's stock, combined perhaps with some direct effect of the
+  slight differences of climate and soil on the two farms.</p>
+
+  <p>Most of our domesticated animals and cultivated plants have
+  come to us from the earliest seats of civilisation in Western
+  Asia or Egypt, and have therefore been the subjects of human care
+  and selection for some thousands of years, the result being that,
+  in many cases, we do not know the wild stock from which they
+  originally sprang. The horse, the camel, and the common bull and
+  cow are nowhere found in a wild state, and they have all been
+  domesticated from remote antiquity. The original of the domestic
+  fowl is still wild in India and the Malay Islands, and it was
+  domesticated in India and China before 1400 B.C. It was
+  introduced into Europe about 600 B.C. Several distinct breeds
+  were known to the Romans about the commencement of the Christian
+  era, and they have since spread all over the civilised world and
+  been subjected to a vast amount of conscious and unconscious
+  selection, to many varieties of climate and to differences of
+  food; the result being seen in the wonderful diversity of breeds
+  which differ quite as remarkably as do the different races of
+  pigeons already described.</p>
+
+  <p>In the vegetable kingdom, most of the cereals&mdash;wheat,
+  barley, etc.&mdash;are unknown as truly wild plants; and the same
+  is the case with many vegetables, for De Candolle states that out
+  of 157 useful cultivated plants thirty-two are quite unknown in a
+  wild state, and that forty more are of doubtful origin. It is not
+  improbable that most of these do exist wild, but they have been
+  so profoundly changed by thousands of years of cultivation as to
+  be quite unrecognisable. The <a name="Page_98" id=
+  "Page_98"></a>peach is unknown in a wild state, unless it is
+  derived from the common almond, on which point there is much
+  difference of opinion among botanists and horticulturists.</p>
+
+  <p>The immense antiquity of most of our cultivated plants
+  sufficiently explains the apparent absence of such useful
+  productions in Australia and the Cape of Good Hope,
+  notwithstanding that they both possess an exceedingly rich and
+  varied flora. These countries having been, until a comparatively
+  recent period, inhabited only by uncivilised men, neither
+  cultivation nor selection has been carried on for a sufficiently
+  long time. In North America, however, where there was evidently a
+  very ancient if low form of civilisation, as indicated by the
+  remarkable mounds, earthworks, and other prehistoric remains,
+  maize was cultivated, though it was probably derived from Peru;
+  and the ancient civilisation of that country and of Mexico has
+  given rise to no fewer than thirty-three useful cultivated
+  plants.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Conditions favourable to the production of Variations.</i></p>
+
+  <p>In order that plants and animals may be improved and modified
+  to any considerable extent, it is of course essential that
+  suitable variations should occur with tolerable frequency. There
+  seem to be three conditions which are especially favourable to
+  the production of variations: (1) That the particular species or
+  variety should be kept in very large numbers; (2) that it should
+  be spread over a wide area and thus subjected to a considerable
+  diversity of physical conditions; and (3) that it should be
+  occasionally crossed with some distinct but closely allied race.
+  The first of these conditions is perhaps the most important, the
+  chance of variations of any particular kind being increased in
+  proportion to the quantity of the original stock and of its
+  annual offspring. It has been remarked that only those breeders
+  who keep large flocks can effect much improvement; and it is for
+  the same reason that pigeons and fowls, which can be so easily
+  and rapidly increased, and which have been kept in such large
+  numbers by so great a number of persons, have produced such
+  strange and numerous varieties. In like manner, nurserymen who
+  grow fruit and flowers in large quantities have a great advantage
+  over private amateurs in the production of new varieties.</p>
+
+  <p><a name="Page_99" id="Page_99"></a>Although I believe, for
+  reasons which will be given further on, that some amount of
+  variability is a constant and necessary property of all
+  organisms, yet there appears to be good evidence to show that
+  changed conditions of life tend to increase it, both by a direct
+  action on the organisation and by indirectly affecting the
+  reproductive system. Hence the extension of civilisation, by
+  favouring domestication under altered conditions, facilitates the
+  process of modification. Yet this change does not seem to be an
+  essential condition, for nowhere has the production of extreme
+  varieties of plants and flowers been carried farther than in
+  Japan, where careful selection continued for many generations
+  must have been the chief factor. The effect of occasional crosses
+  often results in a great amount of variation, but it also leads
+  to instability of character, and is therefore very little
+  employed in the production of fixed and well-marked races. For
+  this purpose, in fact, it has to be carefully avoided, as it is
+  only by isolation and pure breeding that any specially desired
+  qualities can be increased by selection. It is for this reason
+  that among savage peoples, whose animals run half wild, little
+  improvement takes place; and the difficulty of isolation also
+  explains why distinct and pure breeds of cats are so rarely met
+  with. The wide distribution of useful animals and plants from a
+  very remote epoch has, no doubt, been a powerful cause of
+  modification, because the particular breed first introduced into
+  each country has often been kept pure for many years, and has
+  also been subjected to slight differences of conditions. It will
+  also usually have been selected for a somewhat different purpose
+  in each locality, and thus very distinct races would soon
+  originate.</p>
+
+  <p>The important physiological effects of crossing breeds or
+  strains, and the part this plays in the economy of nature, will
+  be explained in a future chapter.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Concluding Remarks.</i></p>
+
+  <p>The examples of variation now adduced&mdash;and these might
+  have been almost indefinitely increased&mdash;will suffice to
+  show that there is hardly an organ or a quality in plants or
+  animals which has not been observed to vary; and further, that
+  whenever any of these variations have been useful to man he has
+  <a name="Page_100" id="Page_100"></a>been able to increase them
+  to a marvellous extent by the simple process of always preserving
+  the best varieties to breed from. Along with these larger
+  variations others of smaller amount occasionally appear,
+  sometimes in external, sometimes in internal characters, the very
+  bones of the skeleton often changing slightly in form, size, or
+  number; but as these secondary characters have been of no use to
+  man, and have not been specially selected by him, they have,
+  usually, not been developed to any great amount except when they
+  have been closely dependent on those external characters which he
+  has largely modified.</p>
+
+  <p>As man has considered only utility to himself, or the
+  satisfaction of his love of beauty, of novelty, or merely of
+  something strange or amusing, the variations he has thus produced
+  have something of the character of monstrosities. Not only are
+  they often of no use to the animals or plants themselves, but
+  they are not unfrequently injurious to them. In the Tumbler
+  pigeons, for instance, the habit of tumbling is sometimes so
+  excessive as to injure or kill the bird; and many of our
+  highly-bred animals have such delicate constitutions that they
+  are very liable to disease, while their extreme peculiarities of
+  form or structure would often render them quite unfit to live in
+  a wild state. In plants, many of our double flowers, and some
+  fruits, have lost the power of producing seed, and the race can
+  thus be continued only by means of cuttings or grafts. This
+  peculiar character of domestic productions distinguishes them
+  broadly from wild species and varieties, which, as will be seen
+  by and by, are necessarily adapted in every part of their
+  organisation to the conditions under which they have to live.
+  Their importance for our present inquiry depends on their
+  demonstrating the occurrence of incessant slight variations in
+  all parts of an organism, with the transmission to the offspring
+  of the special characteristics of the parents; and also, that all
+  such slight variations are capable of being accumulated by
+  selection till they present very large and important divergencies
+  from the ancestral stock.</p>
+
+  <p>We thus see, that the evidence as to variation afforded by
+  animals and plants under domestication strikingly accords with
+  that which we have proved to exist in a state of nature. <a name=
+  "Page_101" id="Page_101"></a>And it is not at all surprising that
+  it should be so, since all the species were in a state of nature
+  when first domesticated or cultivated by man, and whatever
+  variations occur must be due to purely natural causes. Moreover,
+  on comparing the variations which occur in any one generation of
+  domesticated animals with those which we know to occur in wild
+  animals, we find no evidence of greater individual variation in
+  the former than in the latter. The results of man's selection are
+  more striking to us because we have always considered the
+  varieties of each domestic animal to be essentially identical,
+  while those which we observe in a wild state are held to be
+  essentially diverse. The greyhound and the spaniel seem
+  wonderful, as varieties of one animal produced by man's
+  selection; while we think little of the diversities of the fox
+  and the wolf, or the horse and the zebra, because we have been
+  accustomed to look upon them as radically distinct animals, not
+  as the results of nature's selection of the varieties of a common
+  ancestor.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_31_31" id="Footnote_31_31"></a><a href=
+      "#FNanchor_31_31"><span class="label">[31]</span></a> Darwin,
+      <i>Animals and Plants under Domestication</i>, vol. i. p.
+      322.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_32_32" id="Footnote_32_32"></a><a href=
+      "#FNanchor_32_32"><span class="label">[32]</span></a> These
+      facts are taken from Darwin's <i>Domesticated Animals and
+      Cultivated Plants</i>, vol. i. pp. 359, 360, 392-401; vol.
+      ii. pp. 231, 275, 330.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_33_33" id="Footnote_33_33"></a><a href=
+      "#FNanchor_33_33"><span class="label">[33]</span></a> See
+      Darwin's <i>Animals and Plants under Domestication</i>, vol.
+      i. pp. 40-42.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_34_34" id="Footnote_34_34"></a><a href=
+      "#FNanchor_34_34"><span class="label">[34]</span></a> Mr.
+      Brent in <i>Journal of Horticulture</i>, 1861, p. 76; quoted
+      by Darwin, <i>Animals and Plants under Domestication</i>,
+      vol. i. p. 151.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_35_35" id="Footnote_35_35"></a><a href=
+      "#FNanchor_35_35"><span class="label">[35]</span></a> This
+      account of domestic pigeons is greatly condensed from Mr.
+      Darwin's work already referred to.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_36_36" id="Footnote_36_36"></a><a href=
+      "#FNanchor_36_36"><span class="label">[36]</span></a>
+      <i>Animals and Plants under Domestication</i>, vol. ii. pp.
+      307-311.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_V" id="CHAPTER_V"></a><a name="Page_102" id=
+  "Page_102"></a>CHAPTER V</h2>
+
+  <p style='font-weight:bold;'>NATURAL SELECTION BY VARIATION AND
+  SURVIVAL OF THE FITTEST</p>
+
+  <div class="blockquot">
+    <p>Effect of struggle for existence under unchanged
+    conditions&mdash;The effect under change of
+    conditions&mdash;Divergence of character&mdash;In
+    insects&mdash;In birds&mdash;In mammalia&mdash;Divergence leads
+    to a maximum of life in each area&mdash;Closely allied species
+    inhabit distinct areas&mdash;Adaptation to conditions at
+    various periods of life&mdash;The continued existence of low
+    forms of life&mdash;Extinction of low types among the higher
+    animals&mdash;Circumstances favourable to the origin of new
+    species&mdash;Probable origin of the dippers&mdash;The
+    importance of isolation&mdash;On the advance of organisation by
+    natural selection&mdash;Summary of the first five chapters.</p>
+  </div>
+
+  <p>In the preceding chapters we have accumulated a body of facts
+  and arguments which will enable us now to deal with the very core
+  of our subject&mdash;the formation of species by means of natural
+  selection. We have seen how tremendous is the struggle for
+  existence always going on in nature owing to the great powers of
+  increase of all organisms; we have ascertained the fact of
+  variability extending to every part and organ, each of which
+  varies simultaneously and for the most part independently; and we
+  have seen that this variability is both large in its amount in
+  proportion to the size of each part, and usually affects a
+  considerable proportion of the individuals in the large and
+  dominant species. And, lastly, we have seen how similar
+  variations, occurring in cultivated plants and domestic animals,
+  are capable of being perpetuated and accumulated by artificial
+  selection, till they have resulted in all the wonderful varieties
+  of our fruits, flowers, and vegetables, our domestic animals and
+  household pets, many of which differ from each other far more in
+  external characters, habits, and instincts than do species in
+  <a name="Page_103" id="Page_103"></a>a state of nature. We have
+  now to inquire whether there is any analogous process in nature,
+  by which wild animals and plants can be permanently modified and
+  new races or new species produced.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Effect of Struggle for Existence under Unchanged
+  Conditions.</i></p>
+
+  <p>Let us first consider what will be the effect of the struggle
+  for existence upon the animals and plants which we see around us,
+  under conditions which do not perceptibly vary from year to year
+  or from century to century. We have seen that every species is
+  exposed to numerous and varied dangers throughout its entire
+  existence, and that it is only by means of the exact adaptation
+  of its organisation&mdash;including its instincts and
+  habits&mdash;to its surroundings that it is enabled to live till
+  it produces offspring which may take its place when it ceases to
+  exist. We have seen also that, of the whole annual increase only
+  a very small fraction survives; and though the survival in
+  individual cases may sometimes be due rather to accident than to
+  any real superiority, yet we cannot doubt that, in the long run,
+  those survive which are best fitted by their perfect organisation
+  to escape the dangers that surround them. This "survival of the
+  fittest" is what Darwin termed "natural selection," because it
+  leads to the same results in nature as are produced by man's
+  selection among domestic animals and cultivated plants. Its
+  primary effect will, clearly, be to keep each species in the most
+  perfect health and vigour, with every part of its organisation in
+  full harmony with the conditions of its existence. It prevents
+  any possible deterioration in the organic world, and produces
+  that appearance of exuberant life and enjoyment, of health and
+  beauty, that affords us so much pleasure, and which might lead a
+  superficial observer to suppose that peace and quietude reigned
+  throughout nature.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Effect under changed Conditions.</i></p>
+
+  <p>But the very same process which, so long as conditions remain
+  substantially the same, secures the continuance of each species
+  of animal or plant in its full perfection, will usually, under
+  changed conditions, bring about whatever change of structure or
+  habits may be necessitated by them. The changed conditions to
+  which we refer are such as we know have occurred <a name=
+  "Page_104" id="Page_104"></a>throughout all geological time and
+  in every part of the world. Land and water have been continually
+  shifting their positions; some regions are undergoing subsidence
+  with diminution of area, others elevation with extension of area;
+  dry land has been converted into marshes, while marshes have been
+  drained or have even been elevated into plateaux. Climate too has
+  changed again and again, either through the elevation of
+  mountains in high latitudes leading to the accumulation of snow
+  and ice, or by a change in the direction of winds and ocean
+  currents produced by the subsidence or elevation of lands which
+  connected continents and divided oceans. Again, along with all
+  these changes have come not less important changes in the
+  distribution of species. Vegetation has been greatly modified by
+  changes of climate and of altitude; while every union of lands
+  before separated has led to extensive migrations of animals into
+  new countries, disturbing the balance that before existed among
+  its forms of life, leading to the extermination of some species
+  and the increase of others.</p>
+
+  <p>When such physical changes as these have taken place, it is
+  evident that many species must either become modified or cease to
+  exist. When the vegetation has changed in character the
+  herbivorous animals must become able to live on new and perhaps
+  less nutritious food; while the change from a damp to a dry
+  climate may necessitate migration at certain periods to escape
+  destruction by drought. This will expose the species to new
+  dangers, and require special modifications of structure to meet
+  them. Greater swiftness, increased cunning, nocturnal habits,
+  change of colour, or the power of climbing trees and living for a
+  time on their foliage or fruit, may be the means adopted by
+  different species to bring themselves into harmony with the new
+  conditions; and by the continued survival of those individuals,
+  only, which varied sufficiently in the right direction, the
+  necessary modifications of structure or of function would be
+  brought about, just as surely as man has been able to breed the
+  greyhound to hunt by sight and the foxhound by scent, or has
+  produced from the same wild plant such distinct forms as the
+  cauliflower and the brussels sprouts.</p>
+
+  <p>We will now consider the special characteristics of the
+  changes in species that are likely to be effected, and how far
+  they agree with what we observe in nature.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_105" id="Page_105"></a><i>Divergence of
+  Character.</i></p>
+
+  <p>In species which have a wide range the struggle for existence
+  will often cause some individuals or groups of individuals to
+  adopt new habits in order to seize upon vacant places in nature
+  where the struggle is less severe. Some, living among extensive
+  marshes, may adopt a more aquatic mode of life; others, living
+  where forests abound, may become more arboreal. In either case we
+  cannot doubt that the changes of structure needed to adapt them
+  to their new habits would soon be brought about, because we know
+  that variations in all the external organs and all their separate
+  parts are very abundant and are also considerable in amount. That
+  such divergence of character has actually occurred we have some
+  direct evidence. Mr. Darwin informs us that in the Catskill
+  Mountains in the United States there are two varieties of wolves,
+  one with a light greyhound-like form which pursues deer, the
+  other more bulky with shorter legs, which more frequently attacks
+  sheep.<a name="FNanchor_37_37" id="FNanchor_37_37"></a><a href=
+  "#Footnote_37_37" class="fnanchor">[37]</a> Another good example
+  is that of the insects in the island of Madeira, many of which
+  have either lost their wings or have had them so much reduced as
+  to be useless for flight, while the very same species on the
+  continent of Europe possess fully developed wings. In other cases
+  the wingless Madeira species are distinct from, but closely
+  allied to, winged species of Europe. The explanation of this
+  change is, that Madeira, like many oceanic islands in the
+  temperate zone, is much exposed to sudden gales of wind, and as
+  most of the fertile land is on the coast, insects which flew much
+  would be very liable to be blown out to sea and lost. Year after
+  year, therefore, those individuals which had shorter wings, or
+  which used them least, were preserved; and thus, in time,
+  terrestrial, wingless, or imperfectly winged races or species
+  have been produced. That this is the true explanation of this
+  singular fact is proved by much corroborative evidence. There are
+  some few flower-frequenting insects in Madeira to whom wings are
+  essential, and in these the wings are somewhat larger than in the
+  same species on the mainland. We thus see that there is no
+  general tendency to the abortion of wings in Madeira, but that it
+  is simply a case of adaptation to new conditions. Those insects
+  <a name="Page_106" id="Page_106"></a>to whom wings were not
+  absolutely essential escaped a serious danger by not using them,
+  and the wings therefore became reduced or were completely lost.
+  But when they were essential they were enlarged and strengthened,
+  so that the insect could battle against the winds and save itself
+  from destruction at sea. Many flying insects, not varying fast
+  enough, would be destroyed before they could establish
+  themselves, and thus we may explain the total absence from
+  Madeira of several whole families of winged insects which must
+  have had many opportunities of reaching the islands. Such are the
+  large groups of the tiger-beetles (Cicindelidae), the chafers
+  (Melolonthidae), the click-beetles (Elateridae), and many
+  others.</p>
+
+  <p>But the most curious and striking confirmation of this portion
+  of Mr. Darwin's theory is afforded by the case of Kerguelen
+  Island. This island was visited by the <i>Transit of Venus</i>
+  expedition. It is one of the stormiest places on the globe, being
+  subject to almost perpetual gales, while, there being no wood, it
+  is almost entirely without shelter. The Rev. A.E. Eaton, an
+  experienced entomologist, was naturalist to the expedition, and
+  he assiduously collected the few insects that were to be found.
+  All were incapable of flight, and most of them entirely without
+  wings. They included a moth, several flies, and numerous beetles.
+  As these insects could hardly have reached the islands in a
+  wingless state, even if there were any other known land inhabited
+  by them&mdash;which there is not&mdash;we must assume that, like
+  the Madeiran insects, they were originally winged, and lost their
+  power of flight because its possession was injurious to them.</p>
+
+  <p>It is no doubt due to the same cause that some butterflies on
+  small and exposed islands have their wings reduced in size, as is
+  strikingly the case with the small tortoise-shell butterfly
+  (Vanessa urticae) inhabiting the Isle of Man, which is only about
+  half the size of the same species in England or Ireland; and Mr.
+  Wollaston notes that Vanessa callirhoe&mdash;a closely allied
+  South European form of our red-admiral butterfly&mdash;is
+  permanently smaller in the small and bare island of Porto Santo
+  than in the larger and more wooded adjacent island of
+  Madeira.</p>
+
+  <p>A very good example of comparatively recent divergence of
+  character, in accordance with new conditions of life, is afforded
+  by our red grouse. This bird, the Lagopus scoticus of <a name=
+  "Page_107" id="Page_107"></a>naturalists, is entirely confined to
+  the British Isles. It is, however, very closely allied to the
+  willow grouse (Lagopus albus), a bird which ranges all over
+  Europe, Northern Asia, and North America, but which, unlike our
+  species, changes to white in winter. No difference in form or
+  structure can be detected between the two birds, but as they
+  differ so decidedly in colour&mdash;our species being usually
+  rather darker in winter than in summer, while there are also
+  slight differences in the call-note and in habits,&mdash;the two
+  species are generally considered to be distinct. The differences,
+  however, are so clearly adaptations to changed conditions that we
+  can hardly doubt that, during the early part of the glacial
+  period, when our islands were united to the continent, our grouse
+  was identical with that of the rest of Europe. But when the cold
+  passed away and our islands became permanently separated from the
+  mainland, with a mild and equable climate and very little snow in
+  winter, the change to white at that season became hurtful,
+  rendering the birds more conspicuous instead of serving as a
+  means of concealment. The colour was, therefore, gradually
+  changed by the process of variation and natural selection; and as
+  the birds obtained ample shelter among the heather which clothes
+  so many of our moorlands, it became useful for them to assimilate
+  with its brown and dusky stems and withered flowers rather than
+  with the snow of the higher mountains. An interesting
+  confirmation of this change having really occurred is afforded by
+  the occasional occurrence in Scotland of birds with a
+  considerable amount of white in the winter plumage. This is
+  considered to be a case of reversion to the ancestral type, just
+  as the slaty colours and banded wings of the wild rock-pigeon
+  sometimes reappear in our fancy breeds of domestic
+  pigeons.<a name="FNanchor_38_38" id="FNanchor_38_38"></a><a href=
+  "#Footnote_38_38" class="fnanchor">[38]</a></p>
+
+  <p>The principle of "divergence of character" pervades all nature
+  from the lowest groups to the highest, as may be well seen in the
+  class of birds. Among our native species we see it well marked in
+  the different species of titmice, pipits, and chats. The great
+  titmouse (Parus major) by its larger size and stronger bill is
+  adapted to feed on larger insects, and is even said sometimes to
+  kill small and weak birds. The smaller and weaker coal titmouse
+  (Parus ater) has adopted a <a name="Page_108" id=
+  "Page_108"></a>more vegetarian diet, eating seeds as well as
+  insects, and feeding on the ground as well as among trees. The
+  delicate little blue titmouse (Parus coeruleus), with its very
+  small bill, feeds on the minutest insects and grubs which it
+  extracts from crevices of bark and from the buds of fruit-trees.
+  The marsh titmouse, again (Parus palustris), has received its
+  name from the low and marshy localities it frequents; while the
+  crested titmouse (Parus cristatus) is a northern bird frequenting
+  especially pine forests, on the seeds of which trees it partially
+  feeds. Then, again, our three common pipits&mdash;the tree-pipit
+  (Anthus arboreus), the meadow-pipit (Anthus pratensis), and the
+  rock-pipit or sea-lark (Anthus obscurus) have each occupied a
+  distinct place in nature to which they have become specially
+  adapted, as indicated by the different form and size of the hind
+  toe and claw in each species. So, the stone-chat (Saxicola
+  rubicola), the whin-chat (S. rubetra), and the wheat-ear (S.
+  oenanthe) are more or less divergent forms of one type, with
+  modifications in the shape of the wing, feet, and bill adapting
+  them to slightly different modes of life. The whin-chat is the
+  smallest, and frequents furzy commons, fields, and lowlands,
+  feeding on worms, insects, small molluscs, and berries; the
+  stone-chat is next in size, and is especially active and lively,
+  frequenting heaths and uplands, and is a permanent resident with
+  us, the two other species being migrants; while the larger and
+  more conspicuous wheat-ear, besides feeding on grubs, beetles,
+  etc., is able to capture flying insects on the wing, something
+  after the manner of true flycatchers.</p>
+
+  <p>These examples sufficiently indicate how divergence of
+  character has acted, and has led to the adaptation of numerous
+  allied species, each to a more or less special mode of life, with
+  the variety of food, of habits, and of enemies which must
+  necessarily accompany such diversity. And when we extend our
+  inquiries to higher groups we find the same indications of
+  divergence and special adaptation, often to a still more marked
+  extent. Thus we have the larger falcons, which prey upon birds,
+  while some of the smaller species, like the hobby (Falco
+  subbuteo), live largely on insects. The true falcons capture
+  their prey in the air, while the hawks usually seize it on or
+  near the ground, feeding on hares, rabbits, squirrels, grouse,
+  pigeons, and poultry. Kites and buzzards, on the <a name=
+  "Page_109" id="Page_109"></a>other hand, seize their prey upon
+  the ground, and the former feed largely on reptiles and offal as
+  well as on birds and quadrupeds. Others have adopted fish as
+  their chief food, and the osprey snatches its prey from the water
+  with as much facility as a gull or a petrel; while the South
+  American caracaras (Polyborus) have adopted the habits of
+  vultures and live altogether on carrion. In every great group
+  there is the same divergence of habits. There are ground-pigeons,
+  rock-pigeons, and wood-pigeons,&mdash;seed-eating pigeons and
+  fruit-eating pigeons; there are carrion-eating, insect-eating,
+  and fruit-eating crows. Even kingfishers are, some aquatic, some
+  terrestrial in their habits; some live on fish, some on insects,
+  some on reptiles. Lastly, among the primary divisions of birds we
+  find a purely terrestrial group&mdash;the Ratitae, including the
+  ostriches, cassowaries, etc.; other great groups, including the
+  ducks, cormorants, gulls, penguins, etc., are aquatic; while the
+  bulk of the Passerine birds are aerial and arboreal. The same
+  general facts can be detected in all other classes of animals. In
+  the mammalia, for example, we have in the common rat a fish-eater
+  and flesh-eater as well as a grain-eater, which has no doubt
+  helped to give it the power of spreading over the world and
+  driving away the native rats of other countries. Throughout the
+  Rodent tribe we find everywhere aquatic, terrestrial, and
+  arboreal forms. In the weasel and cat tribes some live more in
+  trees, others on the ground; squirrels have diverged into
+  terrestrial, arboreal, and flying species; and finally, in the
+  bats we have a truly aerial, and in the whales a truly aquatic
+  order of mammals. We thus see that, beginning with different
+  varieties of the same species, we have allied species, genera,
+  families, and orders, with similarly divergent habits, and
+  adaptations to different modes of life, indicating some general
+  principle in nature which has been operative in the development
+  of the organic world. But in order to be thus operative it must
+  be a generally useful principle, and Mr. Darwin has very clearly
+  shown us in what this utility consists.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Divergence leads to a Maximum of Organic Forms in each
+  Area.</i></p>
+
+  <p>Divergence of character has a double purpose and use. In the
+  first place it enables a species which is being overcome <a name=
+  "Page_110" id="Page_110"></a>by rivals, or is in process of
+  extinction by enemies, to save itself by adopting new habits or
+  by occupying vacant places in nature. This is the immediate and
+  obvious effect of all the numerous examples of divergence of
+  character which we have pointed out. But there is another and
+  less obvious result, which is, that the greater the diversity in
+  the organisms inhabiting a country or district the greater will
+  be the total amount of life that can be supported there. Hence
+  the continued action of the struggle for existence will tend to
+  bring about more and more diversity in each area, which may be
+  shown to be the case by several kinds of evidence. As an example,
+  a piece of turf, three feet by four in size, was found by Mr.
+  Darwin to contain twenty species of plants, and these twenty
+  species belonged to eighteen genera and to eight orders, showing
+  how greatly they differed from each other. Farmers find that a
+  greater quantity of hay is obtained from ground sown with a
+  variety of genera of grasses, clover, etc., than from similar
+  land sown with one or two species only; and the same principle
+  applies to rotation of crops, plants differing very widely from
+  each other giving the best results. So, in small and uniform
+  islands, and in small ponds of fresh water, the plants and
+  insects, though few in number, are found to be wonderfully varied
+  in character.</p>
+
+  <p>The same principle is seen in the naturalisation of plants and
+  animals by man's agency in distant lands, for the species that
+  thrive best and establish themselves permanently are not only
+  very varied among themselves but differ greatly from the native
+  inhabitants. Thus, in the Northern United States there are,
+  according to Dr. Asa Gray, 260 naturalised flowering plants which
+  belong to no less than 162 genera; and of these, 100 genera are
+  not natives of the United States. So, in Australia, the rabbit,
+  though totally unlike any native animal, has increased so much
+  that it probably outnumbers in individuals all the native mammals
+  of the country; and in New Zealand the rabbit and the pig have
+  equally multiplied. Darwin remarks that this "advantage of
+  diversification of structure in the inhabitants of the same
+  region is, in fact, the same as that of the physiological
+  division of labour in the organs of the same body. No
+  physiologist doubts that a <a name="Page_111" id=
+  "Page_111"></a>stomach adapted to digest vegetable matter alone,
+  or flesh alone, draws more nutriment from these substances. So,
+  in the general economy of any land, the more widely and perfectly
+  the animals and plants are diversified for different habits of
+  life, so will a greater number of individuals be capable of there
+  supporting themselves."<a name="FNanchor_39_39" id=
+  "FNanchor_39_39"></a><a href="#Footnote_39_39" class=
+  "fnanchor">[39]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The most closely allied Species inhabit distinct
+  Areas.</i></p>
+
+  <p>One of the curious results of the general action of this
+  principle in nature is, that the most closely allied
+  species&mdash;those whose differences though often real and
+  important are hardly perceptible to any one but a
+  naturalist&mdash;are usually not found in the same but in widely
+  separated countries. Thus, the nearest allies to our European
+  golden plover are found in North America and East Asia; the
+  nearest ally of our European jay is found in Japan, although
+  there are several other species of jays in Western Asia and North
+  Africa; and though we have several species of titmice in England
+  they are not very closely allied to each other. The form most
+  akin to our blue tit is the azure tit of Central Asia (Parus
+  azureus); the Parus ledouci of Algeria is very near our coal tit,
+  and the Parus lugubris of South-Eastern Europe and Asia Minor is
+  nearest to our marsh tit. So, our four species of wild
+  pigeons&mdash;the ring-dove, stock-dove, rock-pigeon, and
+  turtle-dove&mdash;are not closely allied to each other, but each
+  of them belongs, according to some ornithologists, to a separate
+  genus or subgenus, and has its nearest relatives in distant parts
+  of Asia and Africa. In mammalia the same thing occurs. Each
+  mountain region of Europe and Asia has usually its own species of
+  wild sheep and goat, and sometimes of antelope and deer; so that
+  in each region there is found the greatest diversity in this
+  class of animals, while the closest allies inhabit quite distinct
+  and often distant areas. In plants we find the same phenomenon
+  prevalent. Distinct species of columbine are found in Central
+  Europe (Aguilegia vulgaris), in Eastern Europe, and Siberia (A.
+  glandulosa), in the Alps (A. Alpina), in the Pyrenees (A.
+  pyrenaiea), in the Greek mountains (A. ottonis), and in Corsica
+  (A. Bernardi), but rarely are two <a name="Page_112" id=
+  "Page_112"></a>species found in the same area. So, each part of
+  the world has its own peculiar forms of pines, firs, and cedars,
+  but the closely allied species or varieties are in almost every
+  case inhabitants of distinct areas. Examples are the deodar of
+  the Himalayas, the cedar of Lebanon, and that of North Africa,
+  all very closely allied but confined to distinct areas; and the
+  numerous closely allied species of true pine (genus Pinus), which
+  almost always inhabit different countries or occupy different
+  stations. We will now consider some other modes in which natural
+  selection will act, to adapt organisms to changed conditions.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Adaptation to Conditions at Various Periods of Life.</i></p>
+
+  <p>It is found, that, in domestic animals and cultivated plants,
+  variations occurring at any one period of life reappear in the
+  offspring at the same period, and can be perpetuated and
+  increased by selection without modifying other parts of the
+  organisation. Thus, variations in the caterpillar or the cocoon
+  of the silkworm, in the eggs of poultry, and in the seeds or
+  young shoots of many culinary vegetables, have been accumulated
+  till those parts have become greatly modified and, for man's
+  purposes, improved. Owing to this fact it is easy for organisms
+  to become so modified as to avoid dangers that occur at any one
+  period of life. Thus it is that so many seeds have become adapted
+  to various modes of dissemination or protection. Some are winged,
+  or have down or hairs attached to them, so as to enable them to
+  be carried long distances in the air; others have curious hooks
+  and prickles, which cause them to be attached firmly to the fur
+  of mammals or the feathers of birds; while others are buried
+  within sweet or juicy and brightly coloured fruits, which are
+  seen and devoured by birds, the hard smooth seeds passing through
+  their bodies in a fit state for germination. In the struggle for
+  existence it must benefit a plant to have increased means of
+  dispersing its seeds, and of thus having young plants produced in
+  a greater variety of soils, aspects, and surroundings, with a
+  greater chance of some of them escaping their numerous enemies
+  and arriving at maturity. The various differences referred to
+  would, therefore, be brought about by variation and survival of
+  the fittest, just as surely as the length and quality <a name=
+  "Page_113" id="Page_113"></a>of cotton on the seed of the
+  cotton-plant have been increased by man's selection.</p>
+
+  <p>The larvae of insects have thus been wonderfully modified in
+  order to escape the numerous enemies to whose attacks they are
+  exposed at this period of their existence. Their colours and
+  markings have become marvellously adapted to conceal them among
+  the foliage of the plant they live upon, and this colour often
+  changes completely after the last moult, when the creature has to
+  descend to the ground for its change to the pupa state, during
+  which period a brown instead of a green colour is protective.
+  Others have acquired curious attitudes and large ocelli, which
+  cause them to resemble the head of some reptile, or they have
+  curious horns or coloured ejectile processes which frighten away
+  enemies; while a great number have acquired secretions which
+  render them offensive to the taste of their enemies, and these
+  are always adorned with very conspicuous markings or brilliant
+  colours, which serve as a sign of inedibility and prevent their
+  being needlessly attacked. This, however, is a portion of the
+  very large subject of organic colour and marking, which will be
+  fully discussed and illustrated in a separate chapter.</p>
+
+  <p>In this way every possible modification of an animal or plant,
+  whether in colour, form, structure, or habits, which would be
+  serviceable to it or to its progeny at any period of its
+  existence, may be readily brought about. There are some curious
+  organs which are used only once in a creature's life, but which
+  are yet essential to its existence, and thus have very much the
+  appearance of design by an intelligent designer. Such are, the
+  great jaws possessed by some insects, used exclusively for
+  opening the cocoon, and the hard tip to the beak of unhatched
+  birds used for breaking the eggshell. The increase in thickness
+  or hardness of the cocoons or the eggs being useful for
+  protection against enemies or to avoid accidents, it is probable
+  that the change has been very gradual, because it would be
+  constantly checked by the necessity for a corresponding change in
+  the young insects or birds enabling them to overcome the
+  additional obstacle of a tougher cocoon or a harder eggshell. As
+  we have seen, however, that every part of the organism appears to
+  be varying independently, at the same time, though to different
+  <a name="Page_114" id="Page_114"></a>amounts, there seems no
+  reason to believe that the necessity for two or more coincident
+  variations would prevent the required change from taking
+  place.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Continued Existence of Low Forms of Life.</i></p>
+
+  <p>Since species are continually undergoing modifications giving
+  them some superiority over other species or enabling them to
+  occupy fresh places in nature, it may be asked&mdash;Why do any
+  low forms continue to exist? Why have they not long since been
+  improved and developed into higher forms? The answer, probably,
+  is, that these low forms occupy places in nature which cannot be
+  filled by higher forms, and that they have few or no competitors;
+  they therefore continue to exist. Thus, earthworms are adapted to
+  their mode of life better than they would be if more highly
+  organised. So, in the ocean, the minute foraminifera and
+  infusoria, and the larger sponges and corals, occupy places which
+  more highly developed creatures could not fill. They form, as it
+  were, the base of the great structure of animal life, on which
+  the next higher forms rest; and though in the course of ages they
+  may undergo some changes, and diversification of form and
+  structure, in accordance with changed conditions, their essential
+  nature has probably remained the same from the very dawn of life
+  on the earth. The low aquatic diatomaceae and confervae, together
+  with the lowest fungi and lichens, occupy a similar position in
+  the vegetable kingdom, filling places in nature which would be
+  left vacant if only highly organised plants existed. There is,
+  therefore, no motive power to destroy or seriously to modify
+  them; and they have thus probably persisted, under slightly
+  varying forms, through all geological time.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Extinction of Lower Types among the Higher Animals.</i></p>
+
+  <p>So soon; however, as we approach the higher and more fully
+  developed groups, we see indications of the often repeated
+  extinction of lower by higher forms. This is shown by the great
+  gaps that separate the mammalia, birds, reptiles, and fishes from
+  each other; while the lowest forms of each are always few in
+  number and confined to limited areas. Such <a name="Page_115" id=
+  "Page_115"></a>are the lowest mammals&mdash;the echidna and
+  ornithorhynchus of Australia; the lowest birds&mdash;the apteryx
+  of New Zealand and the cassowaries of the New Guinea region;
+  while the lowest fish&mdash;the amphioxus or lancelet, is
+  completely isolated, and has apparently survived only by its
+  habit of burrowing in the sand. The great distinctness of the
+  carnivora, ruminants, rodents, whales, bats, and other orders of
+  mammalia; of the accipitres, pigeons, and parrots, among birds;
+  and of the beetles, bees, flies, and moths, among insects, all
+  indicate an enormous amount of extinction among the comparatively
+  low forms by which, on any theory of evolution, these higher and
+  more specialised groups must have been preceded.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Circumstances favourable to the Origin of New Species by
+  Natural Selection.</i></p>
+
+  <p>We have already seen that, when there is no change in the
+  physical or organic conditions of a country, the effect of
+  natural selection is to keep all the species inhabiting it in a
+  state of perfect health and full development, and to preserve the
+  balance that already exists between the different groups of
+  organisms. But, whenever the physical or organic conditions
+  change, to however small an extent, some corresponding change
+  will be produced in the flora and fauna, since, considering the
+  severe struggle for existence and the complex relations of the
+  various organisms, it is hardly possible that the change should
+  not be beneficial to some species and hurtful to others. The most
+  common effect, therefore, will be that some species will increase
+  and others will diminish; and in cases where a species was
+  already small in numbers a further diminution might lead to
+  extinction. This would afford room for the increase of other
+  species, and thus a considerable readjustment of the proportions
+  of the several species might take place. When, however, the
+  change was of a more important character, directly affecting the
+  existence of many species so as to render it difficult for them
+  to maintain themselves without some considerable change in
+  structure or habits, that change would, in some cases, be brought
+  about by variation and natural selection, and thus new varieties
+  or new species might be formed. We have to consider, then, which
+  <a name="Page_116" id="Page_116"></a>are the species that would
+  be most likely to be so modified, while others, not becoming
+  modified, would succumb to the changed conditions and become
+  extinct.</p>
+
+  <p>The most important condition of all is, undoubtedly, that
+  variations should occur of sufficient amount, of a sufficiently
+  diverse character, and in a large number of individuals, so as to
+  afford ample materials for natural selection to act upon; and
+  this, we have seen, does occur in most, if not in all, large,
+  wide-ranging, and dominant species. From some of these,
+  therefore, the new species adapted to the changed conditions
+  would usually be derived; and this would especially be the case
+  when the change of conditions was rather rapid, and when a
+  correspondingly rapid modification could alone save some species
+  from extinction. But when the change was very gradual, then even
+  less abundant and less widely distributed species might become
+  modified into new forms, more especially if the extinction of
+  many of the rarer species left vacant places in the economy of
+  nature.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Probable Origin of the Dippers.</i></p>
+
+  <p>An excellent example of how a limited group of species has
+  been able to maintain itself by adaptation to one of these
+  "vacant places" in nature, is afforded by the curious little
+  birds called dippers or water-ouzels, forming the genus Cinclus
+  and the family Cinclidae of naturalists. These birds are
+  something like small thrushes, with very short wings and tail,
+  and very dense plumage. They frequent, exclusively, mountain
+  torrents in the northern hemisphere, and obtain their food
+  entirely in the water, consisting, as it does, of water-beetles,
+  caddis-worms and other insect-larvae, as well as numerous small
+  freshwater shells. These birds, although not far removed in
+  structure from thrushes and wrens, have the extraordinary power
+  of flying under water; for such, according to the best observers,
+  is their process of diving in search of their prey, their dense
+  and somewhat fibrous plumage retaining so much air that the water
+  is prevented from touching their bodies or even from wetting
+  their feathers to any great extent. Their powerful feet and long
+  curved claws enable them to hold on to stones at the bottom, and
+  thus to retain their position while picking up insects, shells,
+  <a name="Page_117" id="Page_117"></a>etc. As they frequent
+  chiefly the most rapid and boisterous torrents, among rocks,
+  waterfalls, and huge boulders, the water is never frozen over,
+  and they are thus able to live during the severest winters. Only
+  a very few species of dipper are known, all those of the old
+  world being so closely allied to our British bird that some
+  ornithologists consider them to be merely local races of one
+  species; while in North America and the northern Andes there are
+  two other species.</p>
+
+  <p>Here then we have a bird, which, in its whole structure, shows
+  a close affinity to the smaller typical perching birds, but which
+  has departed from all its allies in its habits and mode of life,
+  and has secured for itself a place in nature where it has few
+  competitors and few enemies. We may well suppose, that, at some
+  remote period, a bird which was perhaps the common and more
+  generalised ancestor of most of our thrushes, warblers, wrens,
+  etc., had spread widely over the great northern continent, and
+  had given rise to numerous varieties adapted to special
+  conditions of life. Among these some took to feeding on the
+  borders of clear streams, picking out such larvae and molluscs as
+  they could reach in shallow water. When food became scarce they
+  would attempt to pick them out of deeper and deeper water, and
+  while doing this in cold weather many would become frozen and
+  starved. But any which possessed denser and more hairy plumage
+  than usual, which was able to keep out the water, would survive;
+  and thus a race would be formed which would depend more and more
+  on this kind of food. Then, following up the frozen streams into
+  the mountains, they would be able to live there during the
+  winter; and as such places afforded them much protection from
+  enemies and ample shelter for their nests and young, further
+  adaptations would occur, till the wonderful power of diving and
+  flying under water was acquired by a true land-bird.</p>
+
+  <p>That such habits might be acquired under stress of need is
+  rendered highly probable by the facts stated by the well-known
+  American naturalist, Dr. Abbott. He says that "the water-thrushes
+  (Seiurus sp.) all wade in water, and often, seeing minute
+  mollusca on the bottom of the stream, plunge both head and neck
+  beneath the surface, so that often, for <a name="Page_118" id=
+  "Page_118"></a>several seconds, a large part of the body is
+  submerged. Now these birds still have the plumage pervious to
+  water, and so are liable to be drenched and sodden; but they have
+  also the faculty of giving these drenched feathers such a good
+  shaking that flight is practicable a moment after leaving the
+  water. Certainly the water-thrushes (Seiurus ludovicianus, S.
+  auricapillus, and S. noveboracensis) have taken many preliminary
+  steps to becoming as aquatic as the dipper; and the winter-wren,
+  and even the Maryland yellow-throat are not far behind."<a name=
+  "FNanchor_40_40" id="FNanchor_40_40"></a><a href=
+  "#Footnote_40_40" class="fnanchor">[40]</a></p>
+
+  <p>Another curious example of the way in which species have been
+  modified to occupy new places in nature, is afforded by the
+  various animals which inhabit the water-vessels formed by the
+  leaves of many epiphytal species of Bromelia. Fritz M&uuml;ller
+  has described a caddis-fly larva which lives among these leaves,
+  and which has been modified in the pupa state in accordance with
+  its surroundings. The pupae of caddis-flies inhabiting streams
+  have fringes of hair on the tarsi to enable them to reach the
+  surface on leaving their cases. But in the species inhabiting
+  bromelia leaves there is no need for swimming, and accordingly we
+  find the tarsi entirely bare. In the same plants are found
+  curious little Entomostraca, very abundant there but found
+  nowhere else. These form a new genus, but are most nearly allied
+  to Cythere, a marine type. It is believed that the transmission
+  of this species from one tree to another must be effected by the
+  young crustacea, which are very minute, clinging to beetles, many
+  of which, both terrestrial and aquatic, also inhabit the bromelia
+  leaves; and as some water-beetles are known to frequent the sea,
+  it is perhaps by these means that the first emigrants established
+  themselves in this strange new abode. Bromeliae are often very
+  abundant on trees growing on the water's edge, and this would
+  facilitate the transition from a marine to an arboreal habitat.
+  Fritz M&uuml;ller has also found, among the bromelia leaves, a
+  small frog bearing its eggs on its back, and having some other
+  peculiarities of structure. Several beautiful little aquatic
+  plants of the genus Utricularia or bladder-wort also inhabit
+  bromelia leaves; and these send runners out to neighbouring
+  plants and thus spread themselves with great rapidity.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_119" id="Page_119"></a><i>The Importance of
+  Isolation.</i></p>
+
+  <p>Isolation is no doubt an important aid to natural selection,
+  as shown by the fact that islands so often present a number of
+  peculiar species; and the same thing is seen on the two sides of
+  a great mountain range or on opposite coasts of a continent. The
+  importance of isolation is twofold. In the first place, it leads
+  to a body of individuals of each species being limited in their
+  range and thus subjected to uniform conditions for long spaces of
+  time. Both the direct action of the environment and the natural
+  selection of such varieties only as are suited to the conditions,
+  will, therefore, be able to produce their full effect. In the
+  second place, the process of change will not be interfered with
+  by intercrossing with other individuals which are becoming
+  adapted to somewhat different conditions in an adjacent area. But
+  this question of the swamping effects of intercrossing will be
+  considered in another chapter.</p>
+
+  <p>Mr. Darwin was of opinion that, on the whole, the largeness of
+  the area occupied by a species was of more importance than
+  isolation, as a factor in the production of new species, and in
+  this I quite agree with him. It must, too, be remembered, that
+  isolation will often be produced in a continuous area whenever a
+  species becomes modified in accordance with varied conditions or
+  diverging habits. For example, a wide-ranging species may in the
+  northern and colder part of its area become modified in one
+  direction, and in the southern part in another direction; and
+  though for a long time an intermediate form may continue to exist
+  in the intervening area, this will be likely soon to die out,
+  both because its numbers will be small, and it will be more or
+  less pressed upon in varying seasons by the modified varieties,
+  each better able to endure extremes of climate. So, when one
+  portion of a terrestrial species takes to a more arboreal or to a
+  more aquatic mode of life, the change of habit itself leads to
+  the isolation of each portion. Again, as will be more fully
+  explained in a future chapter, any difference of habits or of
+  haunts usually leads to some modification of colour or marking,
+  as a means of concealment from enemies; and there is reason to
+  believe that this difference will be intensified by natural
+  selection as a means of identification <a name="Page_120" id=
+  "Page_120"></a>and recognition by members of the same variety or
+  incipient species. It has also been observed that each
+  differently coloured variety of wild animals, or of domesticated
+  animals which have run wild, keep together, and refuse to pair
+  with individuals of the other colours; and this must of itself
+  act to keep the races separate as completely as physical
+  isolation.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>On the Advance of Organisation by Natural Selection.</i></p>
+
+  <p>As natural selection acts solely by the preservation of useful
+  variations, or those which are beneficial to the organism under
+  the conditions to which it is exposed, the result must
+  necessarily be that each species or group tends to become more
+  and more improved in relation to its conditions. Hence we should
+  expect that the larger groups in each class of animals and
+  plants&mdash;those which have persisted and have been abundant
+  throughout geological ages&mdash;would, almost necessarily, have
+  arrived at a high degree of organisation, both physical and
+  mental. Illustrations of this are to be seen everywhere. Among
+  mammalia we have the carnivora, which from Eocene times have been
+  becoming more and more specialised, till they have culminated in
+  the cat and dog tribes, which have reached a degree of perfection
+  both in structure and intelligence fully equal to that of any
+  other animals. In another line of development, the herbivora have
+  been specialised for living solely on vegetable food till they
+  have culminated in the sheep, the cattle, the deer, and the
+  antelopes. The horse tribe, commencing with an early four-toed
+  ancestor in the Eocene age, has increased in size and in perfect
+  adaptation of feet and teeth to a life on open plains, and has
+  reached its highest perfection in the horse, the ass, and the
+  zebra. In birds, also, we see an advance from the imperfect
+  tooth-billed and reptile-tailed birds of the secondary epoch, to
+  the wonderfully developed falcons, crows, and swallows of our
+  time. So, the ferns, lycopods, conifers, and monocotyledons of
+  the palaeozoic and mesozoic rocks, have developed into the
+  marvellous wealth of forms of the higher dicotyledons that now
+  adorn the earth.</p>
+
+  <p>But this remarkable advance in the higher and larger groups
+  does not imply any universal law of progress in organisation,
+  because we have at the same time numerous examples (as has been
+  already pointed out) of the persistence of lowly organised
+  <a name="Page_121" id="Page_121"></a>forms, and also of absolute
+  degradation or degeneration. Serpents, for example, have been
+  developed from some lizard-like type which has lost its limbs;
+  and though this loss has enabled them to occupy fresh places in
+  nature and to increase and flourish to a marvellous extent, yet
+  it must be considered to be a retrogression rather than an
+  advance in organisation. The same remark will apply to the whale
+  tribe among mammals; to the blind amphibia and insects of the
+  great caverns; and among plants to the numerous cases in which
+  flowers, once specially adapted to be fertilised by insects, have
+  lost their gay corollas and their special adaptations, and have
+  become degraded into wind-fertilised forms. Such are our
+  plantains, our meadow burnet, and even, as some botanists
+  maintain, our rushes, sedges, and grasses. The causes which have
+  led to this degeneration will be discussed in a future chapter;
+  but the facts are undisputed, and they show us that although
+  variation and the struggle for existence may lead, on the whole,
+  to a continued advance of organisation; yet they also lead in
+  many cases to a retrogression, when such retrogression may aid in
+  the preservation of any form under new conditions. They also lead
+  to the persistence, with slight modifications, of numerous lowly
+  organised forms which are suited to places which higher forms
+  could not fully occupy, or to conditions under which they could
+  not exist. Such are the ocean depths, the soil of the earth, the
+  mud of rivers, deep caverns, subterranean waters, etc.; and it is
+  in such places as these, as well as in some oceanic islands which
+  competing higher forms have not been able to reach, that we find
+  many curious relics of an earlier world, which, in the free air
+  and sunlight and in the great continents, have long since been
+  driven out or exterminated by higher types.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Summary of the first Five Chapters.</i></p>
+
+  <p>We have now passed in review, in more or less detail, the main
+  facts on which the theory of "the origin of species by means of
+  natural selection" is founded. In future chapters we shall have
+  to deal mainly with the application of the theory to explain the
+  varied and complex phenomena presented by the organic world; and,
+  also, to discuss some of the theories put forth by modern
+  writers, either as being more fundamental than <a name="Page_122"
+  id="Page_122"></a>that of Darwin or as supplementary to it.
+  Before doing this, however, it will be well briefly to summarise
+  the facts and arguments already set forth, because it is only by
+  a clear comprehension of these that the full importance of the
+  theory can be appreciated and its further applications
+  understood.</p>
+
+  <p>The theory itself is exceedingly simple, and the facts on
+  which it rests&mdash;though excessively numerous individually,
+  and coextensive with the entire organic world&mdash;yet come
+  under a few simple and easily understood classes. These facts
+  are,&mdash;first, the enormous powers of increase in geometrical
+  progression possessed by all organisms, and the inevitable
+  struggle for existence among them; and, in the second place, the
+  occurrence of much individual variation combined with the
+  hereditary transmission of such variations. From these two great
+  classes of facts, which are universal and indisputable, there
+  necessarily arises, as Darwin termed it, the "preservation of
+  favoured races in the struggle for life," the continuous action
+  of which, under the ever-changing conditions both of the
+  inorganic and organic universe, necessarily leads to the
+  formation or development of new species.</p>
+
+  <p>But, although this general statement is complete and
+  indisputable, yet to see its applications under all the complex
+  conditions that actually occur in nature, it is necessary always
+  to bear in mind the tremendous power and universality of the
+  agencies at work. We must never for an instant lose sight of the
+  fact of the enormously rapid increase of all organisms, which has
+  been illustrated by actual cases, given in our second chapter, no
+  less than by calculations of the results of unchecked increase
+  for a few years. Then, never forgetting that the animal and plant
+  population of any country is, on the whole, stationary, we must
+  be always trying to realise the ever-recurring destruction of the
+  enormous annual increase, and asking ourselves what determines,
+  in each individual case, the death of the many, the survival of
+  the few. We must think over all the causes of destruction to each
+  organism,&mdash;to the seed, the young shoot, the growing plant,
+  the full-grown tree, or shrub, or herb, and again the fruit and
+  seed; and among animals, to the egg or new-born young, to the
+  youthful, and to the adults. Then, we must always bear in mind
+  that what goes on in the case of the individual or family group
+  we may <a name="Page_123" id="Page_123"></a>observe or think of,
+  goes on also among the millions and scores of millions of
+  individuals which are comprised in almost every species; and must
+  get rid of the idea that <i>chance</i> determines which shall
+  live and which die. For, although in many individual cases death
+  may be due to chance rather than to any inferiority in those
+  which die first, yet we cannot possibly believe that this can be
+  the case on the large scale on which nature works. A plant, for
+  instance, cannot be increased unless there are suitable vacant
+  places its seeds can grow in, or stations where it can overcome
+  other less vigorous and healthy plants. The seeds of all plants,
+  by their varied modes of dispersal, may be said to be seeking out
+  such places in which to grow; and we cannot doubt that, in the
+  long run, those individuals whose seeds are the most numerous,
+  have the greatest powers of dispersal, and the greatest vigour of
+  growth, will leave more descendants than the individuals of the
+  same species which are inferior in all these respects, although
+  now and then some seed of an inferior individual may
+  <i>chance</i> to be carried to a spot where it can grow and
+  survive. The same rule will apply to every period of life and to
+  every danger to which plants or animals are exposed. The best
+  organised, or the most healthy, or the most active, or the best
+  protected, or the most intelligent, will inevitably, in the long
+  run, gain an advantage over those which are inferior in these
+  qualities; that is, <i>the fittest will survive</i>, the fittest
+  being, in each particular case, those which are superior in the
+  special qualities on which safety depends. At one period of life,
+  or to escape one kind of danger, concealment may be necessary; at
+  another time, to escape another danger, swiftness; at another,
+  intelligence or cunning; at another, the power to endure rain or
+  cold or hunger; and those which possess all these faculties in
+  the fullest perfection will generally survive.</p>
+
+  <p>Having fully grasped these facts in all their fulness and in
+  their endless and complex results, we have next to consider the
+  phenomena of variation, discussed in the third and fourth
+  chapters; and it is here that perhaps the greatest difficulty
+  will be felt in appreciating the full importance of the evidence
+  as set forth. It has been so generally the practice to speak of
+  variation as something exceptional and comparatively
+  rare&mdash;as an abnormal deviation from the uniformity and
+  stability of the <a name="Page_124" id="Page_124"></a>characters
+  of a species&mdash;and so few even among naturalists have ever
+  compared, accurately, considerable numbers of individuals, that
+  the conception of variability as a general characteristic of all
+  dominant and widespread species, large in its amount and
+  affecting, not a few, but considerable masses of the individuals
+  which make up the species, will be to many entirely new. Equally
+  important is the fact that the variability extends to every organ
+  and every structure, external and internal; while perhaps most
+  important of all is the independent variability of these several
+  parts, each one varying without any constant or even usual
+  dependence on, or correlation with, other parts. No doubt there
+  is some such correlation in the differences that exist between
+  species and species&mdash;more developed wings usually
+  accompanying smaller feet and <i>vice vers&acirc;</i>&mdash;but
+  this is, generally, a useful adaptation which has been brought
+  about by natural selection, and does not apply to the individual
+  variability which occurs within the species.</p>
+
+  <p>It is because these facts of variation are so important and so
+  little understood, that they have been discussed in what will
+  seem to some readers wearisome and unnecessary detail. Many
+  naturalists, however, will hold that even more evidence is
+  required; and more, to almost any amount, could easily have been
+  given. The character and variety of that already adduced will,
+  however, I trust, convince most readers that the facts are as
+  stated; while they have been drawn from a sufficiently wide area
+  to indicate a general principle throughout nature.</p>
+
+  <p>If, now, we fully realise these facts of variation, along with
+  those of rapid multiplication and the struggle for existence,
+  most of the difficulties in the way of comprehending how species
+  have originated through natural selection will disappear. For
+  whenever, through changes of climate, or of altitude, or of the
+  nature of the soil, or of the area of the country, any species
+  are exposed to new dangers, and have to maintain themselves and
+  provide for the safety of their offspring under new and more
+  arduous conditions, then, in the variability of all parts,
+  organs, and structures, no less than of habits and intelligence,
+  we have the means of producing modifications which will certainly
+  bring the species into harmony with its <a name="Page_125" id=
+  "Page_125"></a>new conditions. And if we remember that all such
+  physical changes are slow and gradual in their operation, we
+  shall see that the amount of variation which we know occurs in
+  every new generation will be quite sufficient to enable
+  modification and adaptation to go on at the same rate. Mr. Darwin
+  was rather inclined to exaggerate the necessary slowness of the
+  action of natural selection; but with the knowledge we now
+  possess of the great amount and range of individual variation,
+  there seems no difficulty in an amount of change, quite
+  equivalent to that which usually distinguishes allied species,
+  sometimes taking place in less than a century, should any rapid
+  change of conditions necessitate an equally rapid adaptation.
+  This may often have occurred, either to immigrants into a new
+  land, or to residents whose country has been cut off by
+  subsidence from a larger and more varied area over which they had
+  formerly roamed. When no change of conditions occurs, species may
+  remain unchanged for very long periods, and thus produce that
+  appearance of stability of species which is even now often
+  adduced as an argument against evolution by natural selection,
+  but which is really quite in harmony with it.</p>
+
+  <p>On the principles, and by the light of the facts, now briefly
+  summarised, we have been able, in the present chapter, to
+  indicate how natural selection acts, how divergence of character
+  is set up, how adaptation to conditions at various periods of
+  life has been effected, how it is that low forms of life continue
+  to exist, what kind of circumstances are most favourable to the
+  formation of new species, and, lastly, to what extent the advance
+  of organisation to higher types is produced by natural selection.
+  We will now pass on to consider some of the more important
+  objections and difficulties which have been advanced by eminent
+  naturalists.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_37_37" id="Footnote_37_37"></a><a href=
+      "#FNanchor_37_37"><span class="label">[37]</span></a>
+      <i>Origin of Species</i>, p. 71.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_38_38" id="Footnote_38_38"></a><a href=
+      "#FNanchor_38_38"><span class="label">[38]</span></a>
+      Yarrell's <i>British Birds</i>, fourth edition, vol. iii. p.
+      77.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_39_39" id="Footnote_39_39"></a><a href=
+      "#FNanchor_39_39"><span class="label">[39]</span></a>
+      <i>Origin of Species</i>, p. 89.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_40_40" id="Footnote_40_40"></a><a href=
+      "#FNanchor_40_40"><span class="label">[40]</span></a>
+      <i>Nature</i>, vol. xxx. p. 30.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a><a name="Page_126"
+  id="Page_126"></a>CHAPTER VI</h2>
+
+  <p style='font-weight:bold;'>DIFFICULTIES AND OBJECTIONS</p>
+
+  <div class="blockquot">
+    <p>Difficulty as to smallness of variations&mdash;As to the
+    right variations occurring when required&mdash;The beginnings
+    of important organs&mdash;The mammary glands&mdash;The eyes of
+    flatfish&mdash;Origin of the eye&mdash;Useless or non-adaptive
+    characters&mdash;Recent extension of the region of utility in
+    plants&mdash;The same in animals&mdash;Uses of tails&mdash;Of
+    the horns of deer&mdash;Of the scale-ornamentation of
+    reptiles&mdash;Instability of non-adaptive
+    characters&mdash;Delboeuf's law&mdash;No "specific" character
+    proved to be useless&mdash;The swamping effects of
+    intercrossing&mdash;Isolation as preventing
+    intercrossing&mdash;Gulick on the effects of
+    isolation&mdash;Cases in which isolation is ineffective.</p>
+  </div>
+
+  <p>In the present chapter I propose to discuss the more obvious
+  and often repeated objections to Darwin's theory, and to show how
+  far they affect its character as a true and sufficient
+  explanation of the origin of species. The more recondite
+  difficulties, affecting such fundamental questions as the causes
+  and laws of variability, will be left for a future chapter, after
+  we have become better acquainted with the applications of the
+  theory to the more important adaptations and correlations of
+  animal and plant life.</p>
+
+  <p>One of the earliest and most often repeated objections was,
+  that it was difficult "to imagine a reason why variations tending
+  in an infinitesimal degree in any special direction should be
+  preserved," or to believe that the complex adaptation of living
+  organisms could have been produced "by infinitesimal beginnings."
+  Now this term "infinitesimal," used by a well-known early critic
+  of the <i>Origin of Species</i>, was never made use of by Darwin
+  himself, who spoke only of variations being "slight," and of the
+  "small amount" of the variations that might be selected. Even in
+  using these terms he undoubtedly afforded <a name="Page_127" id=
+  "Page_127"></a>grounds for the objection above made, that such
+  small and slight variations could be of no real use, and would
+  not determine the survival of the individuals possessing them. We
+  have seen, however, in our third chapter, that even Darwin's
+  terms were hardly justified; and that the variability of many
+  important species is of considerable amount, and may very often
+  be properly described as large. As this is found to be the case
+  both in animals and plants, and in all their chief groups and
+  subdivisions, and also to apply to all the separate parts and
+  organs that have been compared, we must take it as proved that
+  the average <i>amount</i> of variability presents no difficulty
+  whatever in the way of the action of natural selection. It may be
+  here mentioned that, up to the time of the preparation of the
+  last edition of <i>The Origin of Species</i>, Darwin had not seen
+  the work of Mr. J.A. Allen of Harvard University (then only just
+  published), which gave us the first body of accurate comparisons
+  and measurements demonstrating this large amount of variability.
+  Since then evidence of this nature has been accumulating, and we
+  are, therefore, now in a far better position to appreciate the
+  facilities for natural selection, in this respect, than was Mr.
+  Darwin himself.</p>
+
+  <p>Another objection of a similar nature is, that the chances are
+  immensely against the right variation or combination of
+  variations occurring just when required; and further, that no
+  variation can be perpetuated that is not accompanied by several
+  concomitant variations of dependent parts&mdash;greater length of
+  a wing in a bird, for example, would be of little use if
+  unaccompanied by increased volume or contractility of the muscles
+  which move it. This objection seemed a very strong one so long as
+  it was supposed that variations occurred singly and at
+  considerable intervals; but it ceases to have any weight now we
+  know that they occur simultaneously in various parts of the
+  organism, and also in a large proportion of the individuals which
+  make up the species. A considerable number of individuals will,
+  therefore, every year possess the required combination of
+  characters; and it may also be considered probable that when the
+  two characters are such that they always <i>act</i> together,
+  there will be such a correlation between them that they will
+  frequently <i>vary</i> together. But there is another
+  consideration that seems to show that this coincident <a name=
+  "Page_128" id="Page_128"></a>variation is not essential. All
+  animals in a state of nature are kept, by the constant struggle
+  for existence and the survival of the fittest, in such a state of
+  perfect health and usually superabundant vigour, that in all
+  ordinary circumstances they possess a surplus power in every
+  important organ&mdash;a surplus only drawn upon in cases of the
+  direst necessity when their very existence is at stake. It
+  follows, therefore, that <i>any</i> additional power given to one
+  of the component parts of an organ must be useful&mdash;an
+  increase, for example, either in the wing muscles or in the form
+  or length of the wing might give <i>some</i> increased powers of
+  flight; and thus alternate variations&mdash;in one generation in
+  the muscles, in another generation in the wing itself&mdash;might
+  be as effective in permanently improving the powers of flight as
+  coincident variations at longer intervals. On either supposition,
+  however, this objection appears to have little weight if we take
+  into consideration the large amount of coincident variability
+  that has been shown to exist.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Beginnings of Important Organs.</i></p>
+
+  <p>We now come to an objection which has perhaps been more
+  frequently urged than any other, and which Darwin himself felt to
+  have much weight&mdash;the first beginnings of important organs,
+  such, for example, as wings, eyes, mammary glands, and numerous
+  other structures. It is urged, that it is almost impossible to
+  conceive how the first rudiments of these could have been of any
+  use, and, if not of use they could not have been preserved and
+  further developed by natural selection.</p>
+
+  <p>Now, the first remark to be made on objections of this nature
+  is, that they are really outside the question of the origin of
+  all existing species from allied species not very far removed
+  from them, which is all that Darwin undertook to <i>prove</i> by
+  means of his theory. Organs and structures such as those above
+  mentioned all date back to a very remote past, when the world and
+  its inhabitants were both very different from what they are now.
+  To ask of a new theory that it shall reveal to us exactly what
+  took place in remote geological epochs, and how it took place, is
+  unreasonable. The most that should be asked is, that some
+  probable or possible mode of origination should be pointed out in
+  some at least of these <a name="Page_129" id=
+  "Page_129"></a>difficult cases, and this Mr. Darwin has done. One
+  or two of these may be briefly given here, but the whole series
+  should be carefully read by any one who wishes to see how many
+  curious facts and observations have been required in order to
+  elucidate them; whence we may conclude that further knowledge
+  will probably throw light on any difficulties that still
+  remain.<a name="FNanchor_41_41" id="FNanchor_41_41"></a><a href=
+  "#Footnote_41_41" class="fnanchor">[41]</a></p>
+
+  <p>In the case of the mammary glands Mr. Darwin remarks that it
+  is admitted that the ancestral mammals were allied to the
+  marsupials. Now in the very earliest mammals, almost before they
+  really deserved that name, the young may have been nourished by a
+  fluid secreted by the interior surface of the marsupial sack, as
+  is believed to be the case with the fish (Hippocampus) whose eggs
+  are hatched within a somewhat similar sack. This being the case,
+  those individuals which secreted a more nutritious fluid, and
+  those whose young were able to obtain and swallow a more constant
+  supply by suction, would be more likely to live and come to a
+  healthy maturity, and would therefore be preserved by natural
+  selection.</p>
+
+  <p>In another case which has been adduced as one of special
+  difficulty, a more complete explanation is given. Soles, turbots,
+  and other flatfish are, as is well known, unsymmetrical. They
+  live and move on their sides, the under side being usually
+  differently coloured from that which is kept uppermost. Now the
+  eyes of these fish are curiously distorted in order that both
+  eyes may be on the upper side, where alone they would be of any
+  use. It was objected by Mr. Mivart that a sudden transformation
+  of the eye from one side to the other was inconceivable, while,
+  if the transit were gradual the first step could be of no use,
+  since this would not remove the eye from the lower side. But, as
+  Mr. Darwin shows by reference to the researches of Malm and
+  others, the young of these fish are quite symmetrical, and during
+  their growth exhibit to us the whole process of change. This
+  begins by the fish (owing to the increasing depth of the body)
+  being unable to maintain the vertical position, so that it falls
+  on one side. It then twists the lower eye as much as possible
+  towards the upper side; and, the whole bony structure of the head
+  being at <a name="Page_130" id="Page_130"></a>this time soft and
+  flexible, the constant repetition of this effort causes the eye
+  gradually to move round the head till it comes to the upper side.
+  Now if we suppose this process, which in the young is completed
+  in a few days or weeks, to have been spread over thousands of
+  generations during the development of these fish, those usually
+  surviving whose eyes retained more and more of the position into
+  which the young fish tried to twist them, the change becomes
+  intelligible; though it still remains one of the most
+  extraordinary cases of degeneration, by which
+  symmetry&mdash;which is so universal a characteristic of the
+  higher animals&mdash;is lost, in order that the creature may be
+  adapted to a new mode of life, whereby it is enabled the better
+  to escape danger and continue its existence.</p>
+
+  <p>The most difficult case of all, that of the eye&mdash;the
+  thought of which even to the last, Mr. Darwin says, "gave him a
+  cold shiver"&mdash;is nevertheless shown to be not
+  unintelligible; granting of course the sensitiveness to light of
+  some forms of nervous tissue. For he shows that there are, in
+  several of the lower animals, rudiments of eyes, consisting
+  merely of pigment cells covered with a translucent skin, which
+  may possibly serve to distinguish light from darkness, but
+  nothing more. Then we have an optic nerve and pigment cells; then
+  we find a hollow filled with gelatinous substance of a convex
+  form&mdash;the first rudiment of a lens. Many of the succeeding
+  steps are lost, as would necessarily be the case, owing to the
+  great advantage of each modification which gave increased
+  distinctness of vision, the creatures possessing it inevitably
+  surviving, while those below them became extinct. But we can well
+  understand how, after the first step was taken, every variation
+  tending to more complete vision would be preserved till we
+  reached the perfect eye of birds and mammals. Even this, as we
+  know, is not absolutely, but only relatively, perfect. Neither
+  the chromatic nor the spherical aberration is absolutely
+  corrected; while long-and short-sightedness, and the various
+  diseases and imperfections to which the eye is liable, may be
+  looked upon as relics of the imperfect condition from which the
+  eye has been raised by variation and natural selection.</p>
+
+  <p>These few examples of difficulties as to the origin of
+  remarkable or complex organs must suffice here; but the reader
+  who wishes further information on the matter may study carefully
+  <a name="Page_131" id="Page_131"></a>the whole of the sixth and
+  seventh chapters of the last edition of <i>The Origin of
+  Species</i>, in which these and many other cases are discussed in
+  considerable detail.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Useless or non-adaptive Characters.</i></p>
+
+  <p>Many naturalists seem to be of opinion that a considerable
+  number of the characters which distinguish species are of no
+  service whatever to their possessors, and therefore cannot have
+  been produced or increased by natural selection. Professors Bronn
+  and Broca have urged this objection on the continent. In America,
+  Dr. Cope, the well-known palaeontologist, has long since put
+  forth the same objection, declaring that non-adaptive characters
+  are as numerous as those which are adaptive; but he differs
+  completely from most who hold the same general opinion in
+  considering that they occur chiefly "in the characters of the
+  classes, orders, families, and other higher groups;" and the
+  objection, therefore, is quite distinct from that in which it is
+  urged that "specific characters" are mostly useless. More
+  recently, Professor G.J. Romanes has urged this difficulty in his
+  paper on "Physiological Selection" (<i>Journ. Linn. Soc.</i>,
+  vol. xix. pp. 338, 344). He says that the characters "which serve
+  to distinguish allied species are frequently, if not usually, of
+  a kind with which natural selection can have had nothing to do,"
+  being without any utilitarian significance. Again he speaks of
+  "the enormous number," and further on of "the innumerable
+  multitude" of specific peculiarities which are useless; and he
+  finally declares that the question needs no further arguing,
+  "because in the later editions of his works Mr. Darwin freely
+  acknowledges that a large proportion of specific distinctions
+  must be conceded to be useless to the species presenting
+  them."</p>
+
+  <p>I have looked in vain in Mr. Darwin's works to find any such
+  acknowledgment, and I think Mr. Romanes has not sufficiently
+  distinguished between "useless characters" and "useless specific
+  distinctions." On referring to all the passages indicated by him
+  I find that, in regard to specific characters, Mr. Darwin is very
+  cautious in admitting inutility. His most pronounced "admissions"
+  on this question are the following: "But when, from the nature of
+  the organism and of the conditions, modifications have been
+  induced which are <a name="Page_132" id=
+  "Page_132"></a>unimportant for the welfare of the species, they
+  may be, and apparently often have been, transmitted in nearly the
+  same state <i>to numerous, otherwise modified, descendants</i>"
+  (<i>Origin</i>, p. 175). The words I have here italicised clearly
+  show that such characters are usually not "specific," in the
+  sense that they are such as distinguish species from each other,
+  but are found in numerous allied species. Again: "Thus a large
+  yet undefined extension may safely be given to the direct and
+  indirect results of natural selection; but I now admit, after
+  reading the essay of N&auml;geli on plants, and the remarks by
+  various authors with respect to animals, more especially those
+  recently made by Professor Broca, that in the earlier editions of
+  my <i>Origin of Species</i> I perhaps attributed too much to the
+  action of natural selection or the survival of the fittest. I
+  have altered the fifth edition of the <i>Origin</i> so as to
+  confine my remarks to adaptive changes of structure, <i>but I am
+  convinced, from the light gained during even the last few years,
+  that very many structures which now appear to us useless, will
+  hereafter be proved to be useful, and will therefore come within
+  the range of natural selection</i>. Nevertheless I did not
+  formerly consider sufficiently the existence of structures which,
+  <i>as far as we can at present judge</i>, are neither beneficial
+  nor injurious; and this I believe to be one of the greatest
+  oversights as yet detected in my work." Now it is to be remarked
+  that neither in these passages nor in any of the other less
+  distinct expressions of opinion on this question, does Darwin
+  ever admit that "specific characters"&mdash;that is, the
+  particular characters which serve to distinguish one species from
+  another&mdash;are ever useless, much less that "a large
+  proportion of them" are so, as Mr. Romanes makes him "freely
+  acknowledge." On the other hand, in the passage which I have
+  italicised he strongly expresses his view that much of what we
+  suppose to be useless is due to our ignorance; and as I hold
+  myself that, as regards many of the supposed useless characters,
+  this is the true explanation, it may be well to give a brief
+  sketch of the progress of knowledge in transferring characters
+  from the one category to the other.</p>
+
+  <p>We have only to go back a single generation, and not even the
+  most acute botanist could have suggested a reasonable use, for
+  each species of plant, of the infinitely varied forms, sizes,
+  <a name="Page_133" id="Page_133"></a>and colours of the flowers,
+  the shapes and arrangement of the leaves, and the numerous other
+  external characters of the whole plant. But since Mr. Darwin
+  showed that plants gained both in vigour and in fertility by
+  being crossed with other individuals of the same species, and
+  that this crossing was usually effected by insects which, in
+  search of nectar or pollen, carried the pollen from one plant to
+  the flowers of another plant, almost every detail is found to
+  have a purpose and a use. The shape, the size, and the colour of
+  the petals, even the streaks and spots with which they are
+  adorned, the position in which they stand, the movements of the
+  stamens and pistil at various times, especially at the period of,
+  and just after, fertilisation, have been proved to be strictly
+  adaptive in so many cases that botanists now believe that all the
+  external characters of flowers either are or have been of use to
+  the species.</p>
+
+  <p>It has also been shown, by Kerner and other botanists, that
+  another set of characteristics have relation to the prevention of
+  ants, slugs, and other animals from reaching the flowers, because
+  these creatures would devour or injure them without effecting
+  fertilisation. The spines, hairs, or sticky glands on the stem or
+  flower-stalk, the curious hairs or processes shutting up the
+  flower, or sometimes even the extreme smoothness and polish of
+  the outside of the petals so that few insects can hang to the
+  part, have been shown to be related to the possible intrusion of
+  these "unbidden guests."<a name="FNanchor_42_42" id=
+  "FNanchor_42_42"></a><a href="#Footnote_42_42" class=
+  "fnanchor">[42]</a> And, still more recently, attempts have been
+  made by Grant Allen and Sir John Lubbock to account for the
+  innumerable forms, textures, and groupings of leaves, by their
+  relation to the needs of the plants themselves; and there can be
+  little doubt that these attempts will be ultimately successful.
+  Again, just as flowers have been adapted to secure fertilisation
+  or cross-fertilisation, fruits have been developed to assist in
+  the dispersal of seeds; and their forms, sizes, juices, and
+  colours can be shown to be specially adapted to secure such
+  dispersal by the agency of birds and mammals; while the same end
+  is secured in other <a name="Page_134" id="Page_134"></a>cases by
+  downy seeds to be wafted through the air, or by hooked or sticky
+  seed-vessels to be carried away, attached to skin, wool, or
+  feathers.</p>
+
+  <p>Here, then, we have an enormous extension of the region of
+  utility in the vegetable kingdom, and one, moreover, which
+  includes almost all the specific characters of plants. For the
+  species of plants are usually characterised either by differences
+  in the form, size, and colour of the flowers, or of the fruits;
+  or, by peculiarities in the shape, size, dentation, or
+  arrangement of the leaves; or by peculiarities in the spines,
+  hairs, or down with which various parts of the plant are clothed.
+  In the case of plants it must certainly be admitted that
+  "specific" characters are pre-eminently adaptive; and though
+  there may be some which are not so, yet all those referred to by
+  Darwin as having been adduced by various botanists as useless,
+  either pertain to genera or higher groups, or are found in some
+  plants of a species only&mdash;that is, are individual variations
+  not specific characters.</p>
+
+  <p>In the case of animals, the most recent wide extension of the
+  sphere of utility has been in the matter of their colours and
+  markings. It was of course always known that certain creatures
+  gained protection by their resemblance to their normal
+  surroundings, as in the case of white arctic animals, the yellow
+  or brown tints of those living in deserts, and the green hues of
+  many birds and insects surrounded by tropical vegetation. But of
+  late years these cases have been greatly increased both in number
+  and variety, especially in regard to those which closely imitate
+  special objects among which they live; and there are other kinds
+  of coloration which long appeared to have no use. Large numbers
+  of animals, more especially insects, are gaudily coloured, either
+  with vivid hues or with striking patterns, so as to be very
+  easily seen. Now it has been found, that in almost all these
+  cases the creatures possess some special quality which prevents
+  their being attacked by the enemies of their kind whenever the
+  peculiarity is known; and the brilliant or conspicuous colours or
+  markings serve as a warning or signal flag against attack. Large
+  numbers of insects thus coloured are nauseous and inedible;
+  others, like wasps and bees, have stings; others are too hard to
+  be eaten by small birds; while snakes with <a name="Page_135" id=
+  "Page_135"></a>poisonous fangs often have some characteristic
+  either of rattle, hood, or unusual colour, which indicates that
+  they had better be left alone.</p>
+
+  <p>But there is yet another form of coloration, which consists in
+  special markings&mdash;bands, spots, or patches of white, or of
+  bright colour, which vary in every species, and are often
+  concealed when the creature is at rest but displayed when in
+  motion,&mdash;as in the case of the bands and spots so frequent
+  on the wings and tails of birds. Now these specific markings are
+  believed, with good reason, to serve the purpose of enabling each
+  species to be quickly recognised, even at a distance, by its
+  fellows, especially the parents by their young and the two sexes
+  by each other; and this recognition must often be an important
+  factor in securing the safety of individuals, and therefore the
+  wellbeing and continuance of the species. These interesting
+  peculiarities will be more fully described in a future chapter,
+  but they are briefly referred to here in order to show that the
+  most common of all the characters by which species are
+  distinguished from each other&mdash;their colours and
+  markings&mdash;can be shown to be adaptive or utilitarian in
+  their nature.</p>
+
+  <p>But besides colour there are almost always some structural
+  characters which distinguish species from species, and, as
+  regards many of these also, an adaptive character can be often
+  discerned. In birds, for instance, we have differences in the
+  size or shape of the bill or the feet, in the length of the wing
+  or the tail, and in the proportions of the several feathers of
+  which these organs are composed. All these differences in the
+  organs on which the very existence of birds depends, which
+  determine the character of flight, facility for running or
+  climbing, for inhabiting chiefly the ground or trees, and the
+  kind of food that can be most easily obtained for themselves and
+  their offspring, must surely be in the highest degree
+  utilitarian; although in each individual case we, in our
+  ignorance of the minutiae of their life-history, may be quite
+  unable to see the use. In mammalia specific differences other
+  than colour usually consist in the length or shape of the ears
+  and tail, in the proportions of the limbs, or in the length and
+  quality of the hair on different parts of the body. As regards
+  the ears and tail, one of the objections by Professor <a name=
+  "Page_136" id="Page_136"></a>Bronn relates to this very point. He
+  states that the length of these organs differ in the various
+  species of hares and of mice, and he considers that this
+  difference can be of no service whatever to their possessors. But
+  to this objection Darwin replies, that it has been shown by Dr.
+  Sch&ouml;bl that the ears of mice "are supplied in an
+  extraordinary manner with nerves, so that they no doubt serve as
+  tactile organs." Hence, when we consider the life of mice, either
+  nocturnal or seeking their food in dark and confined places, the
+  length of the ears may be in each case adapted to the particular
+  habits and surroundings of the species. Again, the tail, in the
+  larger mammals, often serves the purpose of driving off flies and
+  other insects from the body; and when we consider in how many
+  parts of the world flies are injurious or even fatal to large
+  mammals, we see that the peculiar characteristics of this organ
+  may in each case have been adapted to its requirements in the
+  particular area where the species was developed. The tail is also
+  believed to have some use as a balancing organ, which assists an
+  animal to turn easily and rapidly, much as our arms are used when
+  running; while in whole groups it is a prehensile organ, and has
+  become modified in accordance with the habits and needs of each
+  species. In the case of mice it is thus used by the young. Darwin
+  informs us that the late Professor Henslow kept some harvest-mice
+  in confinement, and observed that they frequently curled their
+  tails round the branches of a bush placed in the cage, and thus
+  aided themselves in climbing; while Dr. G&uuml;nther has actually
+  seen a mouse suspend itself by the tail (<i>Origin</i>, p.
+  189).</p>
+
+  <p>Again, Mr. Lawson Tait has called attention to the use of the
+  tail in the cat, squirrel, yak, and many other animals as a means
+  of preserving the heat of the body during the nocturnal and the
+  winter sleep. He says, that in cold weather animals with long or
+  bushy tails will be found lying curled up, with their tails
+  carefully laid over their feet like a rug, and with their noses
+  buried in the fur of the tail, which is thus used exactly in the
+  same way and for the same purpose as we use respirators.<a name=
+  "FNanchor_43_43" id="FNanchor_43_43"></a><a href=
+  "#Footnote_43_43" class="fnanchor">[43]</a></p>
+
+  <p>Another illustration is furnished by the horns of deer which,
+  especially when very large, have been supposed to be <a name=
+  "Page_137" id="Page_137"></a>a danger to the animal in passing
+  rapidly through dense thickets. But Sir James Hector states, that
+  the wapiti, in North America, throws back its head, thus placing
+  the horns along the sides of the back, and is then enabled to
+  rush through the thickest forest with great rapidity. The
+  brow-antlers protect the face and eyes, while the widely
+  spreading horns prevent injury to the neck or flanks. Thus an
+  organ which was certainly developed as a sexual weapon, has been
+  so guided and modified during its increase in size as to be of
+  use in other ways. A similar use of the antlers of deer has been
+  observed in India.<a name="FNanchor_44_44" id=
+  "FNanchor_44_44"></a><a href="#Footnote_44_44" class=
+  "fnanchor">[44]</a></p>
+
+  <p>The various classes of facts now referred to serve to show us
+  that, in the case of the two higher groups&mdash;mammalia and
+  birds&mdash;almost all the characters by which species are
+  distinguished from each other are, or may be, adaptive. It is
+  these two classes of animals which have been most studied and
+  whose life-histories are supposed to be most fully known, yet
+  even here the assertion of inutility, by an eminent naturalist,
+  in the case of two important organs, has been sufficiently met by
+  minute details either in the anatomy or in the habits of the
+  groups referred to. Such a fact as this, together with the
+  extensive series of characters already enumerated which have been
+  of late years transferred from the "useless" to the "useful"
+  class, should convince us, that the assertion of "inutility" in
+  the case of any organ or peculiarity which is not a rudiment or a
+  correlation, is not, and can never be, the statement of a fact,
+  but merely an expression of our ignorance of its purpose or
+  origin.<a name="FNanchor_45_45" id="FNanchor_45_45"></a><a href=
+  "#Footnote_45_45" class="fnanchor">[45]</a><a name="Page_138" id=
+  "Page_138"></a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Instability of Non-adaptive Characters.</i></p>
+
+  <p>One very weighty objection to the theory that <i>specific</i>
+  characters can ever be wholly useless (or wholly unconnected with
+  useful organs by correlation of growth) appears to have been
+  overlooked by those who have maintained the frequency of such
+  characters, and that is, their almost necessary instability.
+  Darwin has remarked on the extreme variability of secondary
+  sexual characters&mdash;such as the horns, crests, plumes, etc.,
+  which are found in males only,&mdash;the reason being, that,
+  although of some use, they are not of such direct and vital
+  importance as those adaptive characters on which the wellbeing
+  and very existence of the animals depend. But in the case of
+  wholly useless structures, <a name="Page_139" id=
+  "Page_139"></a>which are not rudiments of once useful organs, we
+  cannot see what there is to ensure any amount of constancy or
+  stability. One of the cases on which Mr. Romanes lays great
+  stress in his paper on "Physiological Selection" (<i>Journ. Linn.
+  Soc.</i>, vol. xix. p. 384) is that of the fleshy appendages on
+  the corners of the jaw of Normandy pigs and of some other breeds.
+  But it is expressly stated that they are not constant; they
+  appear "frequently," or "occasionally," they are "not strictly
+  inherited, for they occur or fail in animals of the same litter;"
+  and they are not always symmetrical, sometimes appearing on one
+  side of the face alone. Now whatever may be the cause or
+  explanation of these anomalous appendages they cannot be classed
+  with "specific characters," the most essential features of which
+  are, that they <i>are</i> symmetrical, <a name="Page_140" id=
+  "Page_140"></a>that they <i>are</i> inherited, and that they
+  <i>are</i> constant. Admitting that this peculiar appendage is
+  (as Mr. Romanes says rather confidently, "we happen to know it to
+  be") wholly useless and meaningless, the fact would be rather an
+  argument against specific characters being also meaningless,
+  because the latter never have the characteristics which this
+  particular variation possesses.</p>
+
+  <p>These useless or non-adaptive characters are, apparently, of
+  the same nature as the "sports" that arise in our domestic
+  productions, but which, as Mr. Darwin says, without the aid of
+  selection would soon disappear; while some of them may be
+  correlations with other characters which are or have been useful.
+  Some of these correlations are very curious. Mr. Tegetmeier
+  informed Mr. Darwin that the young of white, yellow, or
+  dun-coloured pigeons are born almost naked, whereas other
+  coloured pigeons are born well clothed with down. Now, if this
+  difference occurred between wild species of different colours, it
+  might be said that the nakedness of the young could not be of any
+  use. But the colour with which it is correlated might, as has
+  been shown, be useful in many ways. The skin and its various
+  appendages, as horns, hoofs, hair, feathers, and teeth, are
+  homologous parts, and are subject to very strange correlations of
+  growth. In Paraguay, horses with curled hair occur, and these
+  always have hoofs exactly like those of a mule, while the hair of
+  the mane and tail is much shorter than usual. Now, if any one of
+  these characters were useful, the others correlated with it might
+  be themselves useless, but would still be tolerably constant
+  because dependent on a useful organ. So the tusks and the
+  bristles of the boar are correlated and vary in development
+  together, and the former only may be useful, or both may be
+  useful in unequal degrees.</p>
+
+  <p>The difficulty as to how individual differences or sports can
+  become fixed and perpetuated, if altogether useless, is evaded by
+  those who hold that such characters are exceedingly common. Mr.
+  Romanes says that, upon his theory of physiological selection,
+  "it is quite intelligible that when a varietal form is
+  differentiated from its parent form by the bar of sterility, any
+  little meaningless peculiarities of structure or of instinct
+  <i>should at first be allowed to arise</i>, and that they should
+  then <i>be allowed to perpetuate themselves</i> by heredity,"
+  until they are finally <a name="Page_141" id=
+  "Page_141"></a>eliminated by disuse. But this is entirely begging
+  the question. Do meaningless peculiarities, which we admit often
+  arise as spontaneous variations, ever perpetuate themselves in
+  all the individuals constituting a variety or race, without
+  selection either human or natural? Such characters present
+  themselves as unstable variations, and as such they remain,
+  unless preserved and accumulated by selection; and they can
+  therefore never become "specific" characters unless they are
+  strictly correlated with some useful and important
+  peculiarities.</p>
+
+  <p>As bearing upon this question we may refer to what is termed
+  Delboeuf's law, which has been thus briefly stated by Mr. Murphy
+  in his work on <i>Habit and Intelligence</i>, p. 241.</p>
+
+  <div class="blockquot">
+    <p>"If, in any species, a number of individuals, bearing a
+    ratio not infinitely small to the entire number of births, are
+    in every generation born with a particular variation which is
+    neither beneficial nor injurious, and if it is not counteracted
+    by reversion, then the proportion of the new variety to the
+    original form will increase till it approaches indefinitely
+    near to equality."</p>
+  </div>
+
+  <p>It is not impossible that some definite varieties, such as the
+  melanic form of the jaguar and the bridled variety of the
+  guillemot are due to this cause; but from their very nature such
+  varieties are unstable, and are continually reproduced in varying
+  proportions from the parent forms. They can, therefore, never
+  constitute species unless the variation in question becomes
+  beneficial, when it will be fixed by natural selection. Darwin,
+  it is true, says&mdash;"There can be little doubt that the
+  tendency to vary in the same manner has often been so strong that
+  all the individuals of the same species have been similarly
+  modified without the aid of any form of selection."<a name=
+  "FNanchor_46_46" id="FNanchor_46_46"></a><a href=
+  "#Footnote_46_46" class="fnanchor">[46]</a> But no proof whatever
+  is offered of this statement, and it is so entirely opposed to
+  all we know of the facts of variation as given by Darwin himself,
+  that the important word "all" is probably an oversight.</p>
+
+  <p>On the whole, then, I submit, not only has it not been proved
+  that an "enormous number of specific peculiarities" are useless,
+  and that, as a logical result, natural selection is "not a theory
+  of the origin of species," but only of the origin <a name=
+  "Page_142" id="Page_142"></a>of adaptations which are usually
+  common to many species, or, more commonly, to genera and
+  families; but, I urge further, it has not even been proved that
+  any truly "specific" characters&mdash;those which either singly
+  or in combination distinguish each species from its nearest
+  allies&mdash;are entirely unadaptive, useless, and meaningless;
+  while a great body of facts on the one hand, and some weighty
+  arguments on the other, alike prove that specific characters have
+  been, and could only have been, developed and fixed by natural
+  selection because of their utility. We may admit, that among the
+  great number of variations and sports which continually arise
+  many are altogether useless without being hurtful; but no cause
+  or influence has been adduced adequate to render such characters
+  fixed and constant throughout the vast number of individuals
+  which constitute any of the more dominant species.<a name=
+  "FNanchor_47_47" id="FNanchor_47_47"></a><a href=
+  "#Footnote_47_47" class="fnanchor">[47]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Swamping Effects of Intercrossing.</i></p>
+
+  <p>This supposed insuperable difficulty was first advanced in an
+  article in the <i>North British Review</i> in 1867, and much
+  attention has been attracted to it by the acknowledgment of Mr.
+  Darwin that it proved to him that "single variations," or what
+  are usually termed "sports," could very rarely, if ever, be
+  perpetuated in a state of nature, as he had at first thought
+  might occasionally be the case. But he had always considered that
+  the chief part, and latterly the whole, of the materials with
+  which natural selection works, was afforded by individual
+  variations, or that amount of ever fluctuating variability which
+  exists in all organisms and in all their parts. Other writers
+  have urged the same objection, even as against individual
+  variability, apparently in total ignorance of its amount and
+  range; and quite recently Professor G.J. Romanes has adduced
+  <a name="Page_143" id="Page_143"></a>it as one of the
+  difficulties which can alone be overcome by his theory of
+  physiological selection. He urges, that the same variation does
+  not occur simultaneously in a number of individuals inhabiting
+  the same area, and that it is mere assumption to say it does;
+  while he admits that "if the assumption were granted there would
+  be an end of the present difficulty; for if a sufficient number
+  of individuals were thus simultaneously and similarly modified,
+  there need be no longer any danger of the variety becoming
+  swamped by intercrossing." I must again refer my readers to my
+  third chapter for the proof that such simultaneous variability is
+  not an assumption but a fact; but, even admitting this to be
+  proved, the problem is not altogether solved, and there is so
+  much misconception regarding variation, and the actual process of
+  the origin of new species is so obscure, that some further
+  discussion and elucidation of the subject are desirable.</p>
+
+  <p>In one of the preliminary chapters of Mr. Seebohm's recent
+  work on the <i>Charadriidae</i>, he discusses the differentiation
+  of species; and he expresses a rather widespread view among
+  naturalists when, speaking of the swamping effects of
+  intercrossing, he adds: "This is unquestionably a very grave
+  difficulty, to my mind an absolutely fatal one, to the theory of
+  accidental variation." And in another passage he says: "The
+  simultaneous appearance, and its repetition in successive
+  generations, of a beneficial variation, in a large number of
+  individuals in the same locality, cannot possibly be ascribed to
+  chance." These remarks appear to me to exhibit an entire
+  misconception of the facts of variation as they actually occur,
+  and as they have been utilised by natural selection in the
+  modification of species. I have already shown that every part of
+  the organism, in common species, does vary to a very considerable
+  amount, in a large number of individuals, and in the same
+  locality; the only point that remains to be discussed is, whether
+  any or most of these variations are "beneficial." But every one
+  of these variations consists either in increase or diminution of
+  size or power of the organ or faculty that varies; they can all
+  be divided into a more effective and a less effective
+  group&mdash;that is, into one that is more beneficial or less
+  beneficial. If less size of body would be beneficial, then, as
+  half the variations in size are above and half below the mean or
+  existing standard of the species, there <a name="Page_144" id=
+  "Page_144"></a>would be ample beneficial variations; if a darker
+  colour or a longer beak or wing were required, there are always a
+  considerable number of individuals darker and lighter in colour
+  than the average, with longer or with shorter beaks and wings,
+  and thus the beneficial variation must always be present. And so
+  with every other part, organ, function, or habit; because, as
+  variation, so far as we know, is and always must be in the two
+  directions of excess and defect in relation to the mean amount,
+  whichever kind of variation is wanted is always present in some
+  degree, and thus the difficulty as to "beneficial" variations
+  occurring, as if they were a special and rare class, falls to the
+  ground. No doubt some organs may vary in three or perhaps more
+  directions, as in the length, breadth, thickness, or curvature of
+  the bill. But these may be taken as separate variations, each of
+  which again occurs as "more" or "less"; and thus the "right" or
+  "beneficial" or "useful" variation must always be present so long
+  as any variation at all occurs; and it has not yet been proved
+  that in any large or dominant species, or in any part, organ, or
+  faculty of such species, there is no variation. And even were
+  such a case found it would prove nothing, so long as in numerous
+  other species variation was shown to exist; because we know that
+  great numbers of species and groups throughout all geological
+  time have died out, leaving no descendants; and the obvious and
+  sufficient explanation of this fact is, that they did <i>not</i>
+  vary enough at the time when variation was required to bring them
+  into harmony with changed conditions. The objection as to the
+  "right" or "beneficial" variation occurring when required, seems
+  therefore to have no weight in view of the actual facts of
+  variation.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Isolation to prevent Intercrossing.</i></p>
+
+  <p>Most writers on the subject consider the isolation of a
+  portion of a species a very important factor in the formation of
+  new species, while others maintain it to be absolutely essential.
+  This latter view has arisen from an exaggerated opinion as to the
+  power of intercrossing to keep down any variety or incipient
+  species, and merge it in the parent stock. But it is evident that
+  this can only occur with varieties which are not useful, or
+  which, if useful, occur in very small numbers; and from this kind
+  of variations it is clear that <a name="Page_145" id=
+  "Page_145"></a>new species do not arise. Complete isolation, as
+  in an oceanic island, will no doubt enable natural selection to
+  act more rapidly, for several reasons. In the first place, the
+  absence of competition will for some time allow the new
+  immigrants to increase rapidly till they reach the limits of
+  subsistence. They will then struggle among themselves, and by
+  survival of the fittest will quickly become adapted to the new
+  conditions of their environment. Organs which they formerly
+  needed, to defend themselves against, or to escape from, enemies,
+  being no longer required, would be encumbrances to be got rid of,
+  while the power of appropriating and digesting new and varied
+  food would rise in importance. Thus we may explain the origin of
+  so many flightless and rather bulky birds in oceanic islands, as
+  the dodo, the cassowary, and the extinct moas. Again, while this
+  process was going on, the complete isolation would prevent its
+  being checked by the immigration of new competitors or enemies,
+  which would be very likely to occur in a continuous area; while,
+  of course, any intercrossing with the original unmodified stock
+  would be absolutely prevented. If, now, before this change has
+  gone very far, the variety spreads into adjacent but rather
+  distant islands, the somewhat different conditions in each may
+  lead to the development of distinct forms constituting what are
+  termed representative species; and these we find in the separate
+  islands of the Galapagos, the West Indies, and other ancient
+  groups of islands.</p>
+
+  <p>But such cases as these will only lead to the production of a
+  few peculiar species, descended from the original settlers which
+  happened to reach the islands; whereas, in wide areas, and in
+  continents, we have variation and adaptation on a much larger
+  scale; and, whenever important physical changes demand them, with
+  even greater rapidity. The far greater complexity of the
+  environment, together with the occurrence of variations in
+  constitution and habits, will often allow of effective isolation,
+  even here, producing all the results of actual physical
+  isolation. As we have already explained, one of the most frequent
+  modes in which natural selection acts is, by adapting some
+  individuals of a species to a somewhat different mode of life,
+  whereby they are able to seize upon unappropriated places in
+  nature, and in so doing they become practically <a name=
+  "Page_146" id="Page_146"></a>isolated from their parent form. Let
+  us suppose, for example, that one portion of a species usually
+  living in forests ranges into the open plains, and finding
+  abundance of food remains there permanently. So long as the
+  struggle for existence is not exceptionally severe, these two
+  portions of the species may remain almost unchanged; but suppose
+  some fresh enemies are attracted to the plains by the presence of
+  these new immigrants, then variation and natural selection would
+  lead to the preservation of those individuals best able to cope
+  with the difficulty, and thus the open country form would become
+  modified into a marked variety or into a distinct species; and
+  there would evidently be little chance of this modification being
+  checked by intercrossing with the parent form which remained in
+  the forest.</p>
+
+  <p>Another mode of isolation is brought about by the
+  variety&mdash;either owing to habits, climate, or constitutional
+  change&mdash;breeding at a slightly different time from the
+  parent species. This is known to produce complete isolation in
+  the case of many varieties of plants. Yet another mode of
+  isolation is brought about by changes of colour, and by the fact
+  that in a wild state animals of similar colours prefer to keep
+  together and refuse to pair with individuals of another colour.
+  The probable reason and utility of this habit will be explained
+  in another chapter, but the fact is well illustrated by the
+  cattle which have run wild in the Falkland Islands. These are of
+  several different colours, but each colour keeps in a separate
+  herd, often restricted to one part of the island; and one of
+  these varieties&mdash;the mouse-coloured&mdash;is said to breed a
+  month earlier than the others; so that if this variety inhabited
+  a larger area it might very soon be established as a distinct
+  race or species.<a name="FNanchor_48_48" id=
+  "FNanchor_48_48"></a><a href="#Footnote_48_48" class=
+  "fnanchor">[48]</a> Of course where the change of habits or of
+  station is still greater, as when a terrestrial animal becomes
+  sub-aquatic, or when aquatic animals come to live in tree-tops,
+  as with the frogs and Crustacea described at p. 118, the danger
+  of intercrossing is reduced to a minimum.</p>
+
+  <p>Several writers, however, not content with the indirect
+  effects of isolation here indicated, maintain that it is in
+  itself a cause of modification, and ultimately of the origination
+  of <a name="Page_147" id="Page_147"></a>new species. This was the
+  keynote of Mr. Vernon Wollaston's essay on "Variation of
+  Species," published in 1856, and it is adopted by the Rev. J.G.
+  Gulick in his paper on "Diversity of Evolution under one Set of
+  External Conditions" (<i>Journ. Linn. Soc. Zool.</i>, vol. xi. p.
+  496). The idea seems to be that there is an inherent tendency to
+  variation in certain divergent lines, and that when one portion
+  of a species is isolated, even though under identical conditions,
+  that tendency sets up a divergence which carries that portion
+  farther and farther away from the original species. This view is
+  held to be supported by the case of the land shells of the
+  Sandwich Islands, which certainly present some very remarkable
+  phenomena. In this comparatively small area there are about 300
+  species of land shells, almost all of which belong to one family
+  (or sub-family), the Achatinellidae, found nowhere else in the
+  world. The interesting point is the extreme restriction of the
+  species and varieties. The average range of each species is only
+  five or six miles, while some are restricted to but one or two
+  square miles, and only a very few range over a whole island. The
+  forest region that extends over one of the mountain-ranges of the
+  island of Oahu, is about forty miles in length and five or six
+  miles in breadth; and this small territory furnishes about 175
+  species, represented by 700 or 800 varieties. Mr. Gulick states,
+  that the vegetation of the different valleys on the same side of
+  this range is much the same, yet each has a molluscan fauna
+  differing in some degree from that of any other. "We frequently
+  find a genus represented in several successive valleys by allied
+  species, sometimes feeding on the same, sometimes on different
+  plants. In every such case the valleys that are nearest to each
+  other furnish the most nearly allied forms; and a full set of the
+  varieties of each species presents a minute gradation of forms
+  between the more divergent types found in the more widely
+  separated localities." He urges, that these constant differences
+  cannot be attributed to natural selection, because they occur in
+  different valleys on the same side of the mountain, where food,
+  climate, and enemies are the same; and also, because there is no
+  greater difference in passing from the rainy to the dry side of
+  the mountains than in passing from one valley to <a name=
+  "Page_148" id="Page_148"></a>another on the same side an equal
+  distance apart. In a very lengthy paper, presented to the Linnean
+  Society last year, on "Divergent Evolution through Cumulative
+  Segregation," Mr. Gulick endeavours to work out his views into a
+  complete theory, the main point of which may perhaps be indicated
+  by the following passage: "No two portions of a species possess
+  exactly the same average character, and the initial differences
+  are for ever reacting on the environment and on each other in
+  such a way as to ensure increasing divergence in each successive
+  generation as long as the individuals of the two groups are kept
+  from intercrossing."<a name="FNanchor_49_49" id=
+  "FNanchor_49_49"></a><a href="#Footnote_49_49" class=
+  "fnanchor">[49]</a></p>
+
+  <p>It need hardly be said that the views of Mr. Darwin and myself
+  are inconsistent with the notion that, if the environment were
+  absolutely similar for the two isolated portions of the species,
+  any such necessary and constant divergence would take place. It
+  is an error to assume that what seem to us identical conditions
+  are really identical to such small and delicate organisms as
+  these land molluscs, of whose needs and difficulties at each
+  successive stage of their existence, from the freshly-laid egg up
+  to the adult animal, we are so profoundly ignorant. The exact
+  proportions of the various species of plants, the numbers of each
+  kind of insect or of bird, the peculiarities of more or less
+  exposure to sunshine or to wind at certain critical epochs, and
+  other slight differences which to us are absolutely immaterial
+  and unrecognisable, may be of the highest significance to these
+  humble creatures, and be quite sufficient to require some slight
+  adjustments of size, form, or colour, which natural selection
+  will bring about. All we know of the facts of variation leads us
+  to believe that, without this action of natural selection, there
+  would be produced over the whole area a series of inconstant
+  varieties mingled together, not a distinct segregation of forms
+  each confined to its own limited area.</p>
+
+  <p>Mr. Darwin has shown that, in the distribution and
+  modification of species, the biological is of more importance
+  than the physical environment, the struggle with other organisms
+  being often more severe than that with the forces of nature. This
+  is particularly evident in the case of plants, many of which,
+  when protected from competition, thrive in a <a name="Page_149"
+  id="Page_149"></a>soil, climate, and atmosphere widely different
+  from those of their native habitat. Thus, many alpine plants only
+  found near perpetual snow thrive well in our gardens at the level
+  of the sea; as do the tritomas from the sultry plains of South
+  Africa, the yuccas from the arid hills of Texas and Mexico, and
+  the fuchsias from the damp and dreary shores of the Straits of
+  Magellan. It has been well said that plants do not live where
+  they like, but where they can; and the same remark will apply to
+  the animal world. Horses and cattle run wild and thrive both in
+  North and South America; rabbits, once confined to the south of
+  Europe, have established themselves in our own country and in
+  Australia; while the domestic fowl, a native of tropical India,
+  thrives well in every part of the temperate zone.</p>
+
+  <p>If, then, we admit that when one portion of a species is
+  separated from the rest, there will necessarily be a slight
+  difference in the average characters of the two portions, it does
+  not follow that this difference has much if any effect upon the
+  characteristics that are developed by a long period of isolation.
+  In the first place, the difference itself will necessarily be
+  very slight unless there is an exceptional amount of variability
+  in the species; and in the next place, if the average characters
+  of the species are the expression of its exact adaptation to its
+  whole environment, then, given a precisely similar environment,
+  and the isolated portion will inevitably be brought back to the
+  same average of characters. But, as a matter of fact, it is
+  impossible that the environment of the isolated portion can be
+  exactly like that of the bulk of the species. It cannot be so
+  physically, since no two separated areas can be absolutely alike
+  in climate and soil; and even if these are the same, the
+  geographical features, size, contour, and relation to winds,
+  seas, and rivers, would certainly differ. Biologically, the
+  differences are sure to be considerable. The isolated portion of
+  a species will almost always be in a much smaller area than that
+  occupied by the species as a whole, hence it is at once in a
+  different position as regards its own kind. The proportions of
+  all the other species of animals and plants are also sure to
+  differ in the two areas, and some species will almost always be
+  absent in the smaller which are present in the larger country.
+  These differences will act and react on <a name="Page_150" id=
+  "Page_150"></a>the isolated portion of the species. The struggle
+  for existence will differ in its severity and in its incidence
+  from that which affects the bulk of the species. The absence of
+  some one insect or other creature inimical to the young animal or
+  plant may cause a vast difference in its conditions of existence,
+  and may necessitate a modification of its external or internal
+  characters in quite a different direction from that which
+  happened to be present in the average of the individuals which
+  were first isolated.</p>
+
+  <p>On the whole, then, we conclude that, while isolation is an
+  important factor in effecting some modification of species, it is
+  so, not on account of any effect produced, or influence exerted
+  by isolation <i>per se</i>, but because it is always and
+  necessarily accompanied by a change of environment, both physical
+  and biological. Natural selection will then begin to act in
+  adapting the isolated portion to its new conditions, and will do
+  this the more quickly and the more effectually because of the
+  isolation. We have, however, seen reason to believe that
+  geographical or local isolation is by no means essential to the
+  differentiation of species, because the same result is brought
+  about by the incipient species acquiring different habits or
+  frequenting a different station; and also by the fact that
+  different varieties of the same species are known to prefer to
+  pair with their like, and thus to bring about a physiological
+  isolation of the most effective kind. This part of the subject
+  will be again referred to when the very difficult problems
+  presented by hybridity are discussed.<a name="FNanchor_50_50" id=
+  "FNanchor_50_50"></a><a href="#Footnote_50_50" class=
+  "fnanchor">[50]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Cases in which Isolation is Ineffective.</i></p>
+
+  <p>One objection to the views of those who, like Mr. Gulick,
+  believe isolation itself to be a cause of modification of species
+  deserves attention, namely, the entire absence of change where,
+  <a name="Page_151" id="Page_151"></a>if this were a <i>vera
+  causa</i>, we should expect to find it. In Ireland we have an
+  excellent test case, for we know that it has been separated from
+  Britain since the end of the glacial epoch, certainly many
+  thousand years. Yet hardly one of its mammals, reptiles, or land
+  molluscs has undergone the slightest change, even although there
+  is certainly a distinct difference in the environment both
+  inorganic and organic. That changes have not occurred through
+  natural selection, is perhaps due to the less severe struggle for
+  existence owing to the smaller number of competing species; but,
+  if isolation itself were an efficient cause, acting continuously
+  and cumulatively, it is incredible that a decided change should
+  not have been produced in thousands of years. That no such change
+  has occurred in this, and many other cases of isolation, seems to
+  prove that it is not in itself a cause of modification.</p>
+
+  <p>There yet remain a number of difficulties and objections
+  relating to the question of hybridity, which are so important as
+  to require a separate chapter for their adequate discussion.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_41_41" id="Footnote_41_41"></a><a href=
+      "#FNanchor_41_41"><span class="label">[41]</span></a> See
+      <i>Origin of Species</i>, pp. 176-198.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_42_42" id="Footnote_42_42"></a><a href=
+      "#FNanchor_42_42"><span class="label">[42]</span></a> See
+      Kerner's <i>Flowers and their Unbidden Guests</i> for
+      numerous other structures and peculiarities of plants which
+      are shown to be adaptive and useful.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_43_43" id="Footnote_43_43"></a><a href=
+      "#FNanchor_43_43"><span class="label">[43]</span></a>
+      <i>Nature</i>, vol. xx. p. 603.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_44_44" id="Footnote_44_44"></a><a href=
+      "#FNanchor_44_44"><span class="label">[44]</span></a>
+      <i>Nature</i>, vol. xxxviii. p. 328.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_45_45" id="Footnote_45_45"></a><a href=
+      "#FNanchor_45_45"><span class="label">[45]</span></a> A very
+      remarkable illustration of function in an apparently useless
+      ornament is given by Semper. He says, "It is known that the
+      skin of reptiles encloses the body with scales. These scales
+      are distinguished by very various sculpturings, highly
+      characteristic of the different species. Irrespective of
+      their systematic significance they appear to be of no value
+      in the life of the animal; indeed, they are viewed as
+      ornamental without regard to the fact that they are
+      microscopic and much too delicate to be visible to other
+      animals of their own species. It might, therefore, seem
+      hopeless to show the necessity for their existence on
+      Darwinian principles, and to prove that they are
+      physiologically active organs. Nevertheless, recent
+      investigations on this point have furnished evidence that
+      this is possible.</p>
+
+      <p>"It is known that many reptiles, and above all the snakes,
+      cast off the whole skin at once, whereas human beings do so
+      by degrees. If by any accident they are prevented doing so,
+      they infallibly die, because the old skin has grown so tough
+      and hard that it hinders the increase in volume which is
+      inseparable from the growth of the animal. The casting of the
+      skin is induced by the formation on the surface of the inner
+      epidermis, of a layer of very fine and equally distributed
+      hairs, which evidently serve the purpose of mechanically
+      raising the old skin by their rigidity and position. These
+      hairs then may be designated as <i>casting hairs</i>. That
+      they are destined and calculated for this end is evident to
+      me from the fact established by Dr. Braun, that the casting
+      of the shells of the river crayfish is induced in exactly the
+      same manner by the formation of a coating of hairs which
+      mechanically loosens the old skin or shell from the new. Now
+      the researches of Braun and Cartier have shown that these
+      casting hairs&mdash;which serve the same purpose in two
+      groups of animals so far apart in the systematic
+      scale&mdash;after the casting, are partly transformed into
+      the concentric stripes, sharp spikes, ridges, or warts which
+      ornament the outer edges of the skin-scales of reptiles or
+      the carapace of crabs." [<i>The Natural Conditions of
+      Existance as they affect Animal Life</i>, p. 19.] Professor
+      Semper adds that this example, with many others that might be
+      quoted, shows that we need not abandon the hope of explaining
+      morphological characters on Darwinian principles, although
+      their nature is often difficult to understand.</p>
+
+      <p>During a recent discussion of this question in the pages
+      of <i>Nature</i>, Mr. St. George Mivart adduces several
+      examples of what he deems useless specific characters. Among
+      them are the aborted index finger of the lemurine Potto, and
+      the thumbless hands of Colobus and Ateles, the "life-saving
+      action" of either of which he thinks incredible. These cases
+      suggest two remarks. In the first place, they involve
+      <i>generic</i>, not <i>specific</i>, characters; and the
+      three genera adduced are somewhat isolated, implying
+      considerable antiquity and the extinction of many allied
+      forms. This is important, because it affords ample time for
+      great changes of conditions since the structures in question
+      originated; and without a knowledge of these changes we can
+      never safely assert that any detail of structure could not
+      have been useful. In the second place, all three are cases of
+      aborted or rudimentary organs; and these are admitted to be
+      explained by non-use, leading to diminution of size, a
+      further reduction being brought about by the action of the
+      principle of economy of growth. But, when so reduced, the
+      rudiment might be inconvenient or even hurtful, and then
+      natural selection would aid in its complete abortion; in
+      other words, the abortion of the part would be <i>useful</i>,
+      and would therefore be subject to the law of survival of the
+      fittest. The genera Ateles and Colobus are two of the most
+      purely arboreal types of monkeys, and it is not difficult to
+      conceive that the constant use of the elongated fingers for
+      climbing from tree to tree, and catching on to branches while
+      making great leaps, might require all the nervous energy and
+      muscular growth to be directed to the fingers, the small
+      thumb remaining useless. The case of the Potto is more
+      difficult, both because it is, presumably, a more ancient
+      type, and its actual life-history and habits are completely
+      unknown. These cases are, therefore, not at all to the point
+      as proving that positive specific characters&mdash;not mere
+      rudiments characterising whole genera&mdash;are in any case
+      useless.</p>
+
+      <p>Mr. Mivart further objects to the alleged rigidity of the
+      action of natural selection, because wounded or malformed
+      animals have been found which had evidently lived a
+      considerable time in their imperfect condition. But this
+      simply proves that they were living under a temporarily
+      favourable environment, and that the real struggle for
+      existence, in their case, had not yet taken place. We must
+      surely admit that, when the pinch came, and when perfectly
+      formed stoats were dying for want of food, the one-footed
+      animal, referred to by Mr. Mivart, would be among the first
+      to succumb; and the same remark will apply to his abnormally
+      toothed hares and rheumatic monkeys, which might,
+      nevertheless, get on very well under favourable conditions.
+      The struggle for existence, under which all animals and
+      plants have been developed, is intermittent, and exceedingly
+      irregular in its incidence and severity. It is most severe
+      and fatal to the young; but when an animal has once reached
+      maturity, and especially when it has gained experience by
+      several years of an eventful existence, it may be able to
+      maintain itself under conditions which would be fatal to a
+      young and inexperienced creature of the same species. The
+      examples adduced by Mr. Mivart do not, therefore, in any way
+      impugn the hardness of nature as a taskmaster, or the extreme
+      severity of the recurring struggle for existence. [See
+      <i>Nature</i>, vol. xxxix. p. 127.]</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_46_46" id="Footnote_46_46"></a><a href=
+      "#FNanchor_46_46"><span class="label">[46]</span></a>
+      <i>Origin of Species,</i> p. 72.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_47_47" id="Footnote_47_47"></a><a href=
+      "#FNanchor_47_47"><span class="label">[47]</span></a>
+      Darwin's latest expression of opinion on this question is
+      interesting, since it shows that he was inclined to return to
+      his earlier view of the general, or universal, utility of
+      specific characters. In a letter to Semper (30th Nov. 1878)
+      he writes: "As our knowledge advances, very slight
+      differences, considered by systematists as of no importance
+      in structure, are continually found to be functionally
+      important; and I have been especially struck with this fact
+      in the case of plants, to which my observations have, of late
+      years, been confined. Therefore it seems to me rather rash to
+      consider slight differences between representative species,
+      for instance, those inhabiting the different islands of the
+      same archipelago, as of no functional importance, and as not
+      in any way due to natural selection" <i>(Life of Darwin</i>,
+      vol. iii. p. 161).</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_48_48" id="Footnote_48_48"></a><a href=
+      "#FNanchor_48_48"><span class="label">[48]</span></a> See
+      <i>Variation of Animals and Plants</i>, vol. i. p. 86.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_49_49" id="Footnote_49_49"></a><a href=
+      "#FNanchor_49_49"><span class="label">[49]</span></a>
+      <i>Journal of the Linnean Society, Zoology,</i> vol. xx. p.
+      215.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_50_50" id="Footnote_50_50"></a><a href=
+      "#FNanchor_50_50"><span class="label">[50]</span></a> In Mr.
+      Gulick's last paper (<i>Journal of Linn. Soc. Zool.</i>, vol.
+      xx. pp. 189-274) he discusses the various forms of isolation
+      above referred to, under no less than thirty-eight different
+      divisions and subdivisions, with an elaborate terminology,
+      and he argues that these will frequently bring about
+      divergent evolution without any change in the environment or
+      any action of natural selection. The discussion of the
+      problem here given will, I believe, sufficiently expose the
+      fallacy of his contention; but his illustration of the varied
+      and often recondite modes by which practical isolation may be
+      brought about, may help to remove one of the popular
+      difficulties in the way of the action of natural selection in
+      the origination of species.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a><a name="Page_152"
+  id="Page_152"></a>CHAPTER VII</h2>
+
+  <p style='font-weight:bold;'>ON THE INFERTILITY OF CROSSES
+  BETWEEN DISTINCT SPECIES AND THE USUAL STERILITY OF THEIR HYBRID
+  OFFSPRING</p>
+
+  <div class="blockquot">
+    <p>Statement of the problem&mdash;Extreme susceptibility of the
+    reproductive functions&mdash;Reciprocal
+    crosses&mdash;Individual differences in respect to
+    cross-fertilisation&mdash;Dimorphism and trimorphism among
+    plants&mdash;Cases of the fertility of hybrids and of the
+    infertility of mongrels&mdash;The effects of close
+    interbreeding&mdash;Mr. Huth's objections&mdash;Fertile hybrids
+    among animals&mdash;Fertility of hybrids among
+    plants&mdash;Cases of sterility of mongrels&mdash;Parallelism
+    between crossing and change of conditions&mdash;Remarks on the
+    facts of hybridity&mdash;Sterility due to changed conditions
+    and usually correlated with other characters&mdash;Correlation
+    of colour with constitutional peculiarities&mdash;The isolation
+    of varieties by selective association&mdash;The influence of
+    natural selection upon sterility and
+    fertility&mdash;Physiological selection&mdash;Summary and
+    concluding remarks.</p>
+  </div>
+
+  <p>One of the greatest, or perhaps we may say the greatest, of
+  all the difficulties in the way of accepting the theory of
+  natural selection as a complete explanation of the origin of
+  species, has been the remarkable difference between varieties and
+  species in respect of fertility when crossed. Generally speaking,
+  it may be said that the varieties of any one species, however
+  different they may be in external appearance, are perfectly
+  fertile when crossed, and their mongrel offspring are equally
+  fertile when bred among themselves; while distinct species, on
+  the other hand, however closely they may resemble each other
+  externally, are usually infertile when crossed, and their hybrid
+  offspring absolutely sterile. This used to be considered a fixed
+  law of nature, constituting the absolute test and criterion of a
+  <i>species</i> as distinct from a <i>variety</i>; and so long as
+  it was believed that species were separate creations, or <a name=
+  "Page_153" id="Page_153"></a>at all events had an origin quite
+  distinct from that of varieties, this law could have no
+  exceptions, because, if any two species had been found to be
+  fertile when crossed and their hybrid offspring to be also
+  fertile, this fact would have been held to prove them to be not
+  <i>species</i> but <i>varieties</i>. On the other hand, if two
+  varieties had been found to be infertile, or their mongrel
+  offspring to be sterile, then it would have been said: These are
+  not varieties but true species. Thus the old theory led to
+  inevitable reasoning in a circle; and what might be only a rather
+  common fact was elevated into a law which had no exceptions.</p>
+
+  <p>The elaborate and careful examination of the whole subject by
+  Mr. Darwin, who has brought together a vast mass of evidence from
+  the experience of agriculturists and horticulturists, as well as
+  from scientific experimenters, has demonstrated that there is no
+  such fixed law in nature as was formerly supposed. He shows us
+  that crosses between some varieties are infertile or even
+  sterile, while crosses between some species are quite fertile;
+  and that there are besides a number of curious phenomena
+  connected with the subject which render it impossible to believe
+  that sterility is anything more than an incidental property of
+  species, due to the extreme delicacy and susceptibility of the
+  reproductive powers, and dependent on physiological causes we
+  have not yet been able to trace. Nevertheless, the fact remains
+  that most species which have hitherto been crossed produce
+  sterile hybrids, as in the well-known case of the mule; while
+  almost all domestic varieties, when crossed, produce offspring
+  which are perfectly fertile among themselves. I will now
+  endeavour to give such a sketch of the subject as may enable the
+  reader to see something of the complexity of the problem,
+  referring him to Mr. Darwin's works for fuller details.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Extreme Susceptibility of the Reproductive Functions.</i></p>
+
+  <p>One of the most interesting facts, as showing how susceptible
+  to changed conditions or to slight constitutional changes are the
+  reproductive powers of animals, is the very general difficulty of
+  getting those which are kept in confinement to breed; and this is
+  frequently the only bar to domesticating wild species. Thus,
+  elephants, bears, foxes, <a name="Page_154" id="Page_154"></a>and
+  numbers of species of rodents, very rarely breed in confinement;
+  while other species do so more or less freely. Hawks, vultures,
+  and owls hardly ever breed in confinement; neither did the
+  falcons kept for hawking ever breed. Of the numerous small
+  seed-eating birds kept in aviaries, hardly any breed, neither do
+  parrots. Gallinaceous birds usually breed freely in confinement,
+  but some do not; and even the guans and curassows, kept tame by
+  the South American Indians, never breed. This shows that change
+  of climate has nothing to do with the phenomenon; and, in fact,
+  the same species that refuse to breed in Europe do so, in almost
+  every case, when tamed or confined in their native countries.
+  This inability to reproduce is not due to ill-health, since many
+  of these creatures are perfectly vigorous and live very long.</p>
+
+  <p>With our true domestic animals, on the other hand, fertility
+  is perfect, and is very little affected by changed conditions.
+  Thus, we see the common fowl, a native of tropical India, living
+  and multiplying in almost every part of the world; and the same
+  is the case with our cattle, sheep, and goats, our dogs and
+  horses, and especially with domestic pigeons. It therefore seems
+  probable, that this facility for breeding under changed
+  conditions was an original property of the species which man has
+  domesticated&mdash;a property which, more than any other, enabled
+  him to domesticate them. Yet, even with these, there is evidence
+  that great changes of conditions affect the fertility. In the hot
+  valleys of the Andes sheep are less fertile; while geese taken to
+  the high plateau of Bogota were at first almost sterile, but
+  after some generations recovered their fertility. These and many
+  other facts seem to show that, with the majority of animals, even
+  a slight change of conditions may produce infertility or
+  sterility; and also that after a time, when the animal has become
+  thoroughly acclimatised, as it were, to the new conditions, the
+  infertility is in some cases diminished or altogether ceases. It
+  is stated by Bechstein that the canary was long infertile, and it
+  is only of late years that good breeding birds have become
+  common; but in this case no doubt selection has aided the
+  change.</p>
+
+  <p>As showing that these phenomena depend on deep-seated causes
+  and are of a very general nature, it is interesting <a name=
+  "Page_155" id="Page_155"></a>to note that they occur also in the
+  vegetable kingdom. Allowing for all the circumstances which are
+  known to prevent the production of seed, such as too great
+  luxuriance of foliage, too little or too much heat, or the
+  absence of insects to cross-fertilise the flowers, Mr. Darwin
+  shows that many species which grow and flower with us, apparently
+  in perfect health, yet never produce seed. Other plants are
+  affected by very slight changes of conditions, producing seed
+  freely in one soil and not in another, though apparently growing
+  equally well in both; while, in some cases, a difference of
+  position even in the same garden produces a similar
+  result.<a name="FNanchor_51_51" id="FNanchor_51_51"></a><a href=
+  "#Footnote_51_51" class="fnanchor">[51]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Reciprocal Crosses.</i></p>
+
+  <p>Another indication of the extreme delicacy of the adjustment
+  between the sexes, which is necessary to produce fertility, is
+  afforded by the behaviour of many species and varieties when
+  reciprocally crossed. This will be best illustrated by a few of
+  the examples furnished us by Mr. Darwin. The two distinct species
+  of plants, Mirabilis jalapa and M. longiflora, can be easily
+  crossed, and will produce healthy and fertile hybrids when the
+  pollen of the latter is applied to the stigma of the former
+  plant. But the same experimenter, K&ouml;lreuter, tried in vain,
+  more than two hundred times during eight years, to cross them by
+  applying the pollen of M. jalapa to the stigma of M. longiflora.
+  In other cases two plants are so closely allied that some
+  botanists class them as varieties (as with Matthiola annua and M.
+  glabra), and yet there is the same great difference in the result
+  when they are reciprocally crossed.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Individual Differences in respect to
+  Cross-Fertilisation.</i></p>
+
+  <p>A still more remarkable illustration of the delicate balance
+  of organisation needful for reproduction, is afforded by the
+  individual differences of animals and plants, as regards both
+  their power of intercrossing with other individuals or other
+  species, and the fertility of the offspring thus produced. Among
+  domestic animals, Darwin states that it is by no means rare to
+  find certain males and females which will not breed <a name=
+  "Page_156" id="Page_156"></a>together, though both are known to
+  be perfectly fertile with other males and females. Cases of this
+  kind have occurred among horses, cattle, pigs, dogs, and pigeons;
+  and the experiment has been tried so frequently that there can be
+  no doubt of the fact. Professor G.J. Romanes states that he has a
+  number of additional cases of this individual incompatibility, or
+  of absolute sterility, between two individuals, each of which is
+  perfectly fertile with other individuals.</p>
+
+  <p>During the numerous experiments that have been made on the
+  hybridisation of plants similar peculiarities have been noticed,
+  some individuals being capable, others incapable, of being
+  crossed with a distinct species. The same individual
+  peculiarities are found in varieties, species, and genera.
+  K&ouml;lreuter crossed five varieties of the common tobacco
+  (Nicotiana tabacum) with a distinct species, Nicotiana glutinosa,
+  and they all yielded very sterile hybrids; but those raised from
+  one variety were less sterile, in all the experiments, than the
+  hybrids from the four other varieties. Again, most of the species
+  of the genus Nicotiana have been crossed, and freely produce
+  hybrids; but one species, N. acuminata, not particularly distinct
+  from the others, could neither fertilise, nor be fertilised by,
+  any of the eight other species experimented on. Among genera we
+  find some&mdash;such as Hippeastrum, Crinum, Calceolaria,
+  Dianthus&mdash;almost all the species of which will fertilise
+  other species and produce hybrid offspring; while other allied
+  genera, as Zephyranthes and Silene, notwithstanding the most
+  persevering efforts, have not produced a single hybrid even
+  between the most closely allied species.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Dimorphism and Trimorphism.</i></p>
+
+  <p>Peculiarities in the reproductive system affecting individuals
+  of the same species reach their maximum in what are called
+  heterostyled, or dimorphic and trimorphic flowers, the phenomena
+  presented by which form one of the most remarkable of Mr.
+  Darwin's many discoveries. Our common cowslip and primrose, as
+  well as many other species of the genus Primula, have two kinds
+  of flowers in about equal proportions. In one kind the stamens
+  are short, being situated about the middle of the tube of the
+  corolla, while the <a name="Page_157" id="Page_157"></a>style is
+  long, the globular stigma appearing just in the centre of the
+  open flower. In the other kind the stamens are long, appearing in
+  the centre or throat of the flower, while the style is short, the
+  stigma being situated halfway down the tube at the same level as
+  the stamens in the other form. These two forms have long been
+  known to florists as the "pin-eyed" and the "thrum-eyed," but
+  they are called by Darwin the long-styled and short-styled forms
+  (see woodcut).</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig17" id="fig17"></a> <img src="images/fig17.jpg"
+    width="400" height="338" alt=
+    "FIG. 17.&mdash;Primula veris (Cowslip)." title=
+    "FIG. 17.&mdash;Primula veris (Cowslip)." /> <b>FIG.
+    17.&mdash;Primula veris (Cowslip).</b>
+  </div>
+
+  <p>The meaning and use of these different forms was quite unknown
+  till Darwin discovered, first, that cowslips and primroses are
+  absolutely barren if insects are prevented from visiting them,
+  and then, what is still more extraordinary, that each form is
+  almost sterile when fertilised by its own pollen, and
+  comparatively infertile when crossed with any other plant of its
+  own form, but is perfectly fertile when the pollen of a
+  long-styled is carried to the stigma of a short-styled plant, or
+  <i>vice vers&acirc;</i>. It will be seen, by the figures, that
+  the arrangement is such that a bee visiting the flowers will
+  carry the pollen from the long anthers of the short-styled form
+  to the stigma of the long-styled form, while it would never reach
+  the stigma of another plant of the short-styled form. <a name=
+  "Page_158" id="Page_158"></a>But an insect visiting, first, a
+  long-styled plant, would deposit the pollen on the stigma of
+  another plant of the same kind if it were next visited; and this
+  is probably the reason why the wild short-styled plants were
+  found to be almost always most productive of seed, since they
+  must be all fertilised by the other form, whereas the long-styled
+  plants might often be fertilised by their own form. The whole
+  arrangement, however, ensures cross-fertilisation; and this, as
+  Mr. Darwin has shown by copious experiments, adds both to the
+  vigour and fertility of almost all plants as well as animals.</p>
+
+  <p>Besides the primrose family, many other plants of several
+  distinct natural orders present similar phenomena, one or two of
+  the most curious of which must be referred to. The beautiful
+  crimson flax (Linum grandiflorum) has also two forms, the styles
+  only differing in length; and in this case Mr. Darwin found by
+  numerous experiments, which have since been repeated and
+  confirmed by other observers, that each form is absolutely
+  sterile with pollen from another plant of its own form, but
+  abundantly fertile when crossed with any plant of the other form.
+  In this case the pollen of the two forms cannot be distinguished
+  under the microscope (whereas that of the two forms of Primula
+  differs in size and shape), yet it has the remarkable property of
+  being absolutely powerless on the stigmas of half the plants of
+  its own species. The crosses between the opposite forms, which
+  are fertile, are termed by Mr. Darwin "legitimate," and those
+  between similar forms, which are sterile, "illegitimate"; and he
+  remarks that we have here, within the limits of the same species,
+  a degree of sterility which rarely occurs except between plants
+  or animals not only of different <i>species</i> but of different
+  <i>genera</i>.</p>
+
+  <p>But there is another set of plants, the trimorphic, in which
+  the styles and stamens have each three forms&mdash;long, medium,
+  and short, and in these it is possible to have eighteen different
+  crosses. By an elaborate series of experiments it was shown that
+  the six legitimate unions&mdash;that is, when a plant was
+  fertilised by pollen from stamens of length corresponding to that
+  of its style in the two other forms&mdash;were all abundantly
+  fertile; while the twelve illegitimate unions, when a plant was
+  fertilised by pollen from stamens of a different length from its
+  <a name="Page_159" id="Page_159"></a>own style, in any of the
+  three forms, were either comparatively or wholly sterile.<a name=
+  "FNanchor_52_52" id="FNanchor_52_52"></a><a href=
+  "#Footnote_52_52" class="fnanchor">[52]</a></p>
+
+  <p>We have here a wonderful amount of constitutional difference
+  of the reproductive organs within a single species, greater than
+  usually occurs within the numerous distinct species of a genus or
+  group of genera; and all this diversity appears to have arisen
+  for a purpose which has been obtained by many other, and
+  apparently simpler, changes of structure or of function, in other
+  plants. This seems to show us, in the first place, that
+  variations in the mutual relations of the reproductive organs of
+  different individuals must be as frequent as structural
+  variations have been shown to be; and, also, that sterility in
+  itself can be no test of specific distinctness. But this point
+  will be better considered when we have further illustrated and
+  discussed the complex phenomena of hybridity.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Cases of the Fertility of Hybrids, and of the Infertility of
+  Mongrels.</i></p>
+
+  <p>I now propose to adduce a few cases in which it has been
+  proved, by experiment, that hybrids between two distinct species
+  are fertile <i>inter se</i>; and then to consider why it is that
+  such cases are so few in number.</p>
+
+  <p>The common domestic goose (Anser ferns) and the Chinese goose
+  (A. cygnoides) are very distinct species, so distinct that some
+  naturalists have placed them in different genera; yet they have
+  bred together, and Mr. Eyton raised from a pair of these hybrids
+  a brood of eight. This fact was confirmed by Mr. Darwin himself,
+  who raised several fine birds from a pair of hybrids which were
+  sent him.<a name="FNanchor_53_53" id=
+  "FNanchor_53_53"></a><a href="#Footnote_53_53" class=
+  "fnanchor">[53]</a> In India, according to Mr. Blyth and Captain
+  Hutton, whole flocks of these hybrid geese are kept in various
+  parts of the country where neither of the pure parent species
+  exists, and as they are kept for profit they must certainly be
+  fully fertile.</p>
+
+  <p>Another equally striking case is that of the Indian humped and
+  the common cattle, species which differ osteologically, and also
+  in habits, form, voice, and constitution, so that they are by no
+  means closely allied; yet Mr. Darwin assures us that he <a name=
+  "Page_160" id="Page_160"></a>has received decisive evidence that
+  the hybrids between these are perfectly fertile <i>inter
+  se</i>.</p>
+
+  <p>Dogs have been frequently crossed with wolves and with
+  jackals, and their hybrid offspring have been found to be fertile
+  <i>inter se</i> to the third or fourth generation, and then
+  usually to show some signs of sterility or of deterioration. The
+  wolf and dog may be originally the same species, but the jackal
+  is certainly distinct; and the appearance of infertility or of
+  weakness is probably due to the fact that, in almost all these
+  experiments, the offspring of a single pair&mdash;themselves
+  usually from the same litter&mdash;- were bred in-and-in, and
+  this alone sometimes produces the most deleterious effects. Thus,
+  Mr. Low in his great work on the <i>Domesticated Animals of Great
+  Britain</i>, says: "If we shall breed a pair of dogs from the
+  same litter, and unite again the offspring of this pair, we shall
+  produce at once a feeble race of creatures; and the process being
+  repeated for one or two generations more, the family will die
+  out, or be incapable of propagating their race. A gentleman of
+  Scotland made the experiment on a large scale with certain
+  foxhounds, and he found that the race actually became monstrous
+  and perished utterly." The same writer tells us that hogs have
+  been made the subject of similar experiments: "After a few
+  generations the victims manifest the change induced in the
+  system. They become of diminished size; the bristles are changed
+  into hairs; the limbs become feeble and short; the litters
+  diminish in frequency, and in the number of the young produced;
+  the mother becomes unable to nourish them, and, if the experiment
+  be carried as far as the case will allow, the feeble, and
+  frequently monstrous offspring, will be incapable of being reared
+  up, and the miserable race will utterly perish."<a name=
+  "FNanchor_54_54" id="FNanchor_54_54"></a><a href=
+  "#Footnote_54_54" class="fnanchor">[54]</a></p>
+
+  <p>These precise statements, by one of the greatest authorities
+  on our domesticated animals, are sufficient to show that the fact
+  of infertility or degeneracy appearing in the offspring of
+  hybrids after a few generations need not be imputed to the fact
+  of the first parents being distinct species, since exactly the
+  same phenomena appear when individuals of the same species are
+  bred under similar adverse conditions. But in almost all the
+  experiments that have hitherto been made in crossing distinct
+  species, no care has been taken to avoid close inter<a name=
+  "Page_161" id="Page_161"></a>breeding by securing several hybrids
+  from quite distinct stocks to start with, and by having two or
+  more sets of experiments carried on at once, so that crosses
+  between the hybrids produced may be occasionally made. Till this
+  is done no experiments, such as those hitherto tried, can be held
+  to prove that hybrids are in all cases infertile <i>inter
+  se</i>.</p>
+
+  <p>It has, however, been denied by Mr. A.H. Huth, in his
+  interesting work on <i>The Marriage of Near Kin</i>, that any
+  amount of breeding in-and-in is in itself hurtful; and he quotes
+  the evidence of numerous breeders whose choicest stocks have
+  always been so bred, as well as cases like the Porto Santo
+  rabbits, the goats of Juan Fernandez, and other cases in which
+  animals allowed to run wild have increased prodigiously and
+  continued in perfect health and vigour, although all derived from
+  a single pair. But in all these cases there has been rigid
+  selection by which the weak or the infertile have been
+  eliminated, and with such selection there is no doubt that the
+  ill effects of close interbreeding can be prevented for a long
+  time; but this by no means proves that no ill effects are
+  produced. Mr. Huth himself quotes M. Alli&eacute;, M.
+  Aub&eacute;, Stephens, Giblett, Sir John Sebright, Youatt, Druce,
+  Lord Weston, and other eminent breeders, as finding from
+  experience that close interbreeding <i>does</i> produce bad
+  effects; and it cannot be supposed that there would be such a
+  consensus of opinion on this point if the evil were altogether
+  imaginary. Mr. Huth argues, that the evil results which do occur
+  do not depend on the close interbreeding itself, but on the
+  tendency it has to perpetuate any constitutional weakness or
+  other hereditary taints; and he attempts to prove this by the
+  argument that "if crosses act by virtue of being a cross, and not
+  by virtue of removing an hereditary taint, then the greater the
+  difference between the two animals crossed the more beneficial
+  will that act be." He then shows that, the wider the difference
+  the less is the benefit, and concludes that a cross, as such, has
+  no beneficial effect. A parallel argument would be, that change
+  of air, as from inland to the sea-coast, or from a low to an
+  elevated site, is not beneficial in itself, because, if so, a
+  change to the tropics or to the polar regions should be more
+  beneficial. In both these cases it may well be that no benefit
+  would accrue to a person in perfect health; but then there is no
+  <a name="Page_162" id="Page_162"></a>such thing as "perfect
+  health" in man, and probably no such thing as absolute freedom
+  from constitutional taint in animals. The experiments of Mr.
+  Darwin, showing the great and immediate good effects of a cross
+  between distinct strains in plants, cannot be explained away;
+  neither can the innumerable arrangements to secure
+  cross-fertilisation by insects, the real use and purport of which
+  will be discussed in our eleventh chapter. On the whole, then,
+  the evidence at our command proves that, whatever may be its
+  ultimate cause, close interbreeding <i>does</i> usually produce
+  bad results; and it is only by the most rigid selection, whether
+  natural or artificial, that the danger can be altogether
+  obviated.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Fertile Hybrids among Animals.</i></p>
+
+  <p>One or two more cases of fertile hybrids may be given before
+  we pass on to the corresponding experiments in plants. Professor
+  Alfred Newton received from a friend a pair of hybrid ducks, bred
+  from a common duck (Anas boschas), and a pintail (Dafila acuta).
+  From these he obtained four ducklings, but these latter, when
+  grown up, proved infertile, and did not breed again. In this case
+  we have the results of close interbreeding, with too great a
+  difference between the original species, combining to produce
+  infertility, yet the fact of a hybrid from such a pair producing
+  healthy offspring is itself noteworthy.</p>
+
+  <p>Still more extraordinary is the following statement of Mr.
+  Low: "It has been long known to shepherds, though questioned by
+  naturalists, that the progeny of the cross between the sheep and
+  goat is fertile. Breeds of this mixed race are numerous in the
+  north of Europe."<a name="FNanchor_55_55" id=
+  "FNanchor_55_55"></a><a href="#Footnote_55_55" class=
+  "fnanchor">[55]</a> Nothing appears to be known of such hybrids
+  either in Scandinavia or in Italy; but Professor Giglioli of
+  Florence has kindly given me some useful references to works in
+  which they are described. The following extract from his letter
+  is very interesting: "I need not tell you that there being such
+  hybrids is now generally accepted as a fact. Buffon
+  (<i>Supplements</i>, tom. iii. p. 7, 1756) obtained one such
+  hybrid in 1751 and eight in 1752. Sanson (<i>La Culture</i>, vol.
+  vi. p. 372, 1865) mentions a case observed in the Vosges, France.
+  Geoff. St. Hilaire (<i>Hist. Nat. G&eacute;n. des reg. org.</i>,
+  vol. iii. p. <a name="Page_163" id="Page_163"></a>163) was the
+  first to mention, I believe, that in different parts of South
+  America the ram is more usually crossed with the she-goat than
+  the sheep with the he-goat. The well-known 'pellones' of Chile
+  are produced by the second and third generation of such hybrids
+  (Gay, 'Hist, de Chile,' vol. i. p. 466, <i>Agriculture</i>,
+  1862). Hybrids bred from goat and sheep are called 'chabin' in
+  French, and 'cabruno' in Spanish. In Chile such hybrids are
+  called 'carneros lanudos'; their breeding <i>inter se</i> appears
+  to be not always successful, and often the original cross has to
+  be recommenced to obtain the proportion of three-eighths of
+  he-goat and five-eighths of sheep, or of three-eighths of ram and
+  five-eighths of she-goat; such being the reputed best
+  hybrids."</p>
+
+  <p>With these numerous facts recorded by competent observers we
+  can hardly doubt that races of hybrids between these very
+  distinct species have been produced, and that such hybrids are
+  fairly fertile <i>inter se</i>; and the analogous facts already
+  given lead us to believe that whatever amount of infertility may
+  at first exist could be eliminated by careful selection, if the
+  crossed races were bred in large numbers and over a considerable
+  area of country. This case is especially valuable, as showing how
+  careful we should be in assuming the infertility of hybrids when
+  experiments have been made with the progeny of a single pair, and
+  have been continued only for one or two generations.</p>
+
+  <p>Among insects one case only appears to have been recorded. The
+  hybrids of two moths (Bombyx cynthia and B. arrindia) were proved
+  in Paris, according to M. Quatrefages, to be fertile <i>inter
+  se</i> for eight generations.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Fertility of Hybrids among Plants.</i></p>
+
+  <p>Among plants the cases of fertile hybrids are more numerous,
+  owing, in part, to the large scale on which they are grown by
+  gardeners and nurserymen, and to the greater facility with which
+  experiments can be made. Darwin tells us that K&ouml;lreuter
+  found ten cases in which two plants considered by botanists to be
+  distinct species were quite fertile together, and he therefore
+  ranked them all as varieties of each other. In some cases these
+  were grown for six to ten successive generations, but after a
+  time the fertility decreased, as we saw to be the case in
+  <a name="Page_164" id="Page_164"></a>animals, and presumably from
+  the same cause, too close interbreeding.</p>
+
+  <p>Dean Herbert, who carried on experiments with great care and
+  skill for many years, found numerous cases of hybrids which were
+  perfectly fertile <i>inter se</i>. Crinum capense, fertilised by
+  three other species&mdash;C. pedunculatum, C. canaliculatum, or
+  C. defixum&mdash;all very distinct from it, produced perfectly
+  fertile hybrids; while other species less different in appearance
+  were quite sterile with the same C. capense.</p>
+
+  <p>All the species of the genus Hippeastrum produce hybrid
+  offspring which are invariably fertile. Lobelia syphylitica and
+  L. fulgens, two very distinct species, have produced a hybrid
+  which has been named Lobelia speciosa, and which reproduces
+  itself abundantly. Many of the beautiful pelargoniums of our
+  greenhouses are hybrids, such as P. ignescens from a cross
+  between P. citrinodorum and P. fulgidum, which is quite fertile,
+  and has become the parent of innumerable varieties of beautiful
+  plants. All the varied species of Calceolaria, however different
+  in appearance, intermix with the greatest readiness, and the
+  hybrids are all more or less fertile. But the most remarkable
+  case is that of two species of Petunia, of which Dean Herbert
+  says: "It is very remarkable that, although there is a great
+  difference in the form of the flower, especially of the tube, of
+  P. nyctanigenaeflora and P. phoenicea the mules between them are
+  not only fertile, but I have found them seed much more freely
+  with me than either parent.... From a pod of the above-mentioned
+  mule, to which no pollen but its own had access, I had a large
+  batch of seedlings in which there was no variability or
+  difference from itself; and it is evident that the mule planted
+  by itself, in a congenial climate, would reproduce itself as a
+  species; at least as much deserving to be so considered as the
+  various Calceolarias of different districts of South
+  America."<a name="FNanchor_56_56" id=
+  "FNanchor_56_56"></a><a href="#Footnote_56_56" class=
+  "fnanchor">[56]</a></p>
+
+  <p>Darwin was informed by Mr. C. Noble that he raises stocks for
+  grafting from a hybrid between Rhododendron ponticum and R.
+  catawbiense, and that this hybrid seeds as freely as it is
+  possible to imagine. He adds that horticulturists raise large
+  beds of the same hybrid, and such alone are fairly treated; for,
+  by insect agency, the several individuals are freely <a name=
+  "Page_165" id="Page_165"></a>crossed with each other, and the
+  injurious influence of close interbreeding is thus prevented. Had
+  hybrids, when fairly treated, always gone on decreasing in
+  fertility in each successive generation, as Gartner believed to
+  be the case, the fact would have been notorious to
+  nurserymen.<a name="FNanchor_57_57" id=
+  "FNanchor_57_57"></a><a href="#Footnote_57_57" class=
+  "fnanchor">[57]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Cases of Sterility of Mongrels.</i></p>
+
+  <p>The reverse phenomenon to the fertility of hybrids, the
+  sterility of mongrels or of the crosses between <i>varieties</i>
+  of the same species, is a comparatively rare one, yet some
+  undoubted cases have occurred. Gartner, who believed in the
+  absolute distinctness of species and varieties, had two varieties
+  of maize&mdash;one dwarf with yellow seeds, the other taller with
+  red seeds; yet they never naturally crossed, and, when fertilised
+  artificially, only a single head produced any seeds, and this one
+  only five grains. Yet these few seeds were fertile; so that in
+  this case the first cross was almost sterile, though the hybrid
+  when at length produced was fertile. In like manner, dissimilarly
+  coloured varieties of Verbascum or mullein have been found by two
+  distinct observers to be comparatively infertile. The two
+  pimpernels (Anagallis arvensis and A. coerulea), classed by most
+  botanists as varieties of one species, have been found, after
+  repeated trials, to be perfectly sterile when crossed.</p>
+
+  <p>No cases of this kind are recorded among animals; but this is
+  not to be wondered at, when we consider how very few experiments
+  have been made with natural varieties; while there is good reason
+  for believing that domestic varieties are exceptionally fertile,
+  partly because one of the conditions of domestication was
+  fertility under changed conditions, and also because long
+  continued domestication is believed to have the effect of
+  increasing fertility and eliminating whatever sterility may
+  exist. This is shown by the fact that, in many cases, domestic
+  animals are descended from two or more distinct species. This is
+  almost certainly the case with the dog, and probably with the
+  hog, the ox, and the sheep; yet the various breeds are now all
+  perfectly fertile, although we have every reason to suppose that
+  there would be some degree of infertility if the several
+  aboriginal species were crossed together for the first time.</p>
+
+  <p><a name="Page_166" id="Page_166"></a><i>Parallelism between
+  Crossing and Change of Conditions.</i></p>
+
+  <p>In the whole series of these phenomena, from the beneficial
+  effects of the crossing of different stocks and the evil effects
+  of close interbreeding, up to the partial or complete sterility
+  induced by crosses between species belonging to different genera,
+  we have, as Mr. Darwin points out, a curious parallelism with the
+  effects produced by change of physical conditions. It is well
+  known that slight changes in the conditions of life are
+  beneficial to all living things. Plants, if constantly grown in
+  one soil and locality from their own seeds, are greatly benefited
+  by the importation of seed from some other locality. The same
+  thing happens with animals; and the benefit we ourselves
+  experience from "change of air" is an illustration of the same
+  phenomenon. But the amount of the change which is beneficial has
+  its limits, and then a greater amount is injurious. A change to a
+  climate a few degrees warmer or colder may be good, while a
+  change to the tropics or to the arctic regions might be
+  injurious.</p>
+
+  <p>Thus we see that, both slight changes of conditions and a
+  slight amount of crossing, are beneficial; while extreme changes,
+  and crosses between individuals too far removed in structure or
+  constitution, are injurious. And there is not only a parallelism
+  but an actual connection between the two classes of facts, for,
+  as we have already shown, many species of animals and plants are
+  rendered infertile, or altogether sterile, by the change from
+  their natural conditions which occurs in confinement or in
+  cultivation; while, on the other hand, the increased vigour or
+  fertility which is invariably produced by a judicious cross may
+  be also effected by a judicious change of climate and
+  surroundings. We shall see in a subsequent chapter, that this
+  interchangeability of the beneficial effects of crossing and of
+  new conditions, serves to explain some very puzzling phenomena in
+  the forms and economy of flowers.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Remarks on the Facts of Hybridity.</i></p>
+
+  <p>The facts that have now been adduced, though not very
+  numerous, are sufficiently conclusive to prove that the old
+  belief, of the universal sterility of hybrids and fertility of
+  mongrels, is incorrect. The doctrine that such a universal
+  <a name="Page_167" id="Page_167"></a>law existed was never more
+  than a plausible generalisation, founded on a few inconclusive
+  facts derived from domesticated animals and cultivated plants.
+  The facts were, and still are, inconclusive for several reasons.
+  They are founded, primarily, on what occurs among animals in
+  domestication; and it has been shown that domestication both
+  tends to increase fertility, and was itself rendered possible by
+  the fertility of those particular species being little affected
+  by changed conditions. The exceptional fertility of all the
+  varieties of domesticated animals does not prove that a similar
+  fertility exists among natural varieties. In the next place, the
+  generalisation is founded on too remote crosses, as in the case
+  of the horse and the ass, the two most distinct and widely
+  separated species of the genus Equus, so distinct indeed that
+  they have been held by some naturalists to form distinct genera.
+  Crosses between the two species of zebra, or even between the
+  zebra and the quagga, or the quagga and the ass, might have led
+  to a very different result. Again, in pre-Darwinian times it was
+  so universally the practice to argue in a circle, and declare
+  that the fertility of the offspring of a cross proved the
+  identity of species of the parents, that experiments in hybridity
+  were usually made between very remote species and even between
+  species of different genera, to avoid the possibility of the
+  reply: "They are both really the same species;" and the sterility
+  of the hybrid offspring of such remote crosses of course served
+  to strengthen the popular belief.</p>
+
+  <p>Now that we have arrived at a different standpoint, and look
+  upon a species, not as a distinct entity due to special creation,
+  but as an assemblage of individuals which have become somewhat
+  modified in structure, form, and constitution so as to adapt them
+  to slightly different conditions of life; which can be
+  differentiated from other allied assemblages; which reproduce
+  their like, and which usually breed together&mdash;we require a
+  fresh set of experiments calculated to determine the matter of
+  fact,&mdash;whether such species crossed with their near allies
+  do always produce offspring which are more or less sterile
+  <i>inter se</i>. Ample materials for such experiments exist, in
+  the numerous "representative species" inhabiting distinct areas
+  on a continent or different islands of a group; or even <a name=
+  "Page_168" id="Page_168"></a>in those found in the same area but
+  frequenting somewhat different stations.</p>
+
+  <p>To carry out these experiments with any satisfactory result,
+  it will be necessary to avoid the evil effects of confinement and
+  of too close interbreeding. If birds are experimented with, they
+  should be allowed as much liberty as possible, a plot of ground
+  with trees and bushes being enclosed with wire netting overhead
+  so as to form a large open aviary. The species experimented with
+  should be obtained in considerable numbers, and by two separate
+  persons, each making the opposite reciprocal cross, as explained
+  at p. 155. In the second generation these two stocks might be
+  themselves crossed to prevent the evil effects of too close
+  interbreeding. By such experiments, carefully carried out with
+  different groups of animals and plants, we should obtain a body
+  of facts of a character now sadly wanting, and without which it
+  is hopeless to expect to arrive at a complete solution of this
+  difficult problem. There are, however, some other aspects of the
+  question that need to be considered, and some theoretical views
+  which require to be carefully examined, having done which we
+  shall be in a condition to state the general conclusions to which
+  the facts and reasonings at our command seem to point.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Sterility due to changed Conditions and usually correlated
+  with other Characters, especially with Colour.</i></p>
+
+  <p>The evidence already adduced as to the extreme susceptibility
+  of the reproductive system, and the curious irregularity with
+  which infertility or sterility appears in the crosses between
+  some varieties or species while quite absent in those between
+  others, seem to indicate that sterility is a characteristic which
+  has a constant tendency to appear, either by itself or in
+  correlation with other characters. It is known to be especially
+  liable to occur under changed conditions of life; and, as such
+  change is usually the starting-point and cause of the development
+  of new species, we have already found a reason why it should so
+  often appear when species become fully differentiated.</p>
+
+  <p>In almost all the cases of infertility or sterility between
+  varieties or species, we have some external differences with
+  <a name="Page_169" id="Page_169"></a>which it is correlated; and
+  though these differences are sometimes slight, and the amount of
+  the infertility is not always, or even usually, proportionate to
+  the external difference between the two forms crossed, we must
+  believe that there is some connection between the two classes of
+  facts. This is especially the case as regards colour; and Mr.
+  Darwin has collected a body of facts which go far to prove that
+  colour, instead of being an altogether trifling and unimportant
+  character, as was supposed by the older naturalists, is really
+  one of great significance, since it is undoubtedly often
+  correlated with important constitutional differences. Now colour
+  is one of the characters that most usually distinguishes closely
+  allied species; and when we hear that the most closely allied
+  species of plants are infertile together, while those more remote
+  are fertile, the meaning usually is that the former differ
+  chiefly in the <i>colour</i> of their flowers, while the latter
+  differ in the form of the flowers or foliage, in habit, or in
+  other structural characters.</p>
+
+  <p>It is therefore a most curious and suggestive fact, that in
+  all the recorded cases, in which a decided infertility occurs
+  between varieties of the same species, those varieties are
+  distinguished by a difference of colour. The infertile varieties
+  of Verbascum were white and yellow flowered respectively; the
+  infertile varieties of maize were red and yellow seeded; while
+  the infertile pimpernels were the red and the blue flowered
+  varieties. So, the differently coloured varieties of hollyhocks,
+  though grown close together, each reproduce their own colour from
+  seed, showing that they are not capable of freely intercrossing.
+  Yet Mr. Darwin assures us that the agency of bees is necessary to
+  carry the pollen from one plant to another, because in each
+  flower the pollen is shed before the stigma is ready to receive
+  it. We have here, therefore, either almost complete sterility
+  between varieties of different colours, or a prepotent effect of
+  pollen from a flower of the same colour, bringing about the same
+  result.</p>
+
+  <p>Similar phenomena have not been recorded among animals; but
+  this is not to be wondered at when we consider that most of our
+  pure and valued domestic breeds are characterised by definite
+  colours which constitute one of their <a name="Page_170" id=
+  "Page_170"></a>distinctive marks, and they are, therefore, seldom
+  crossed with these of another colour; and even when they are so
+  crossed, no notice would be taken of any slight diminution of
+  fertility, since this is liable to occur from many causes. We
+  have also reason to believe that fertility has been increased by
+  long domestication, in addition to the fact of the original
+  stocks being exceptionally fertile; and no experiments have been
+  made on the differently coloured varieties of wild animals. There
+  are, however, a number of very curious facts showing that colour
+  in animals, as in plants, is often correlated with constitutional
+  differences of a remarkable kind, and as these have a close
+  relation to the subject we are discussing, a brief summary of
+  them will be here given.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Correlation of Colour with Constitutional
+  Peculiarities.</i></p>
+
+  <p>The correlation of a white colour and blue eyes in male cats
+  with deafness, and of the tortoise-shell marking with the female
+  sex of the same animal, are two well-known but most extraordinary
+  cases. Equally remarkable is the fact, communicated to Darwin by
+  Mr. Tegetmeier, that white, yellow, pale blue, or dun pigeons, of
+  all breeds, have the young birds born naked, while in all other
+  colours they are well covered with down. Here we have a case in
+  which colour seems of more physiological importance than all the
+  varied structural differences between the varieties and breeds of
+  pigeons. In Virginia there is a plant called the paint-root
+  (Lachnanthes tinctoria), which, when eaten by pigs, colours their
+  bones pink, and causes the hoofs of all but the black varieties
+  to drop off; so that black pigs only can be kept in the
+  district.<a name="FNanchor_58_58" id=
+  "FNanchor_58_58"></a><a href="#Footnote_58_58" class=
+  "fnanchor">[58]</a> Buckwheat in flower is also said to be
+  injurious to white pigs but not to black. In the Tarentino, black
+  sheep are not injured by eating the Hypericum crispum&mdash;a
+  species of St. John's-wort&mdash;which kills white sheep. White
+  terriers suffer most from distemper; white chickens from the
+  gapes. White-haired horses or cattle are subject to cutaneous
+  diseases from which the dark coloured are free; while, both in
+  Thuringia and the West Indies, it has been noticed that white or
+  pale coloured cattle are much more troubled by flies than are
+  those which are brown or black. The same law even extends
+  <a name="Page_171" id="Page_171"></a>to insects, for it is found
+  that silkworms which produce white cocoons resist the fungus
+  disease much better than do those which produce yellow
+  cocoons.<a name="FNanchor_59_59" id="FNanchor_59_59"></a><a href=
+  "#Footnote_59_59" class="fnanchor">[59]</a> Among plants, we have
+  in North America green and yellow-fruited plums not affected by a
+  disease that attacked the purple-fruited varieties.
+  Yellow-fleshed peaches suffer more from disease than
+  white-fleshed kinds. In Mauritius, white sugar-canes were
+  attacked by a disease from which the red canes were free. White
+  onions and verbenas are most liable to mildew; and red-flowered
+  hyacinths were more injured by the cold during a severe winter in
+  Holland than any other kinds.<a name="FNanchor_60_60" id=
+  "FNanchor_60_60"></a><a href="#Footnote_60_60" class=
+  "fnanchor">[60]</a></p>
+
+  <p>These curious and inexplicable correlations of colour with
+  constitutional peculiarities, both in animals and plants, render
+  it probable that the correlation of colour with infertility,
+  which has been detected in several cases in plants, may also
+  extend to animals in a state of nature; and if so, the fact is of
+  the highest importance as throwing light on the origin of the
+  infertility of many allied species. This will be better
+  understood after considering the facts which will be now
+  described.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Isolation of Varieties by Selective Association.</i></p>
+
+  <p>In the last chapter I have shown that the importance of
+  geographical isolation for the formation of new species by
+  natural selection has been greatly exaggerated, because the
+  <a name="Page_172" id="Page_172"></a>very change of conditions,
+  which is the initial power in starting such new forms, leads also
+  to a local or stational segregation of the forms acted upon. But
+  there is also a very powerful cause of isolation in the mental
+  nature&mdash;the likes and dislikes&mdash;of animals; and to this
+  is probably due the fact of the comparative rarity of hybrids in
+  a state of nature. The differently coloured herds of cattle in
+  the Falkland Islands, each of which keeps separate, have been
+  already mentioned; and it may be added, that the mouse-coloured
+  variety seem to have already developed a physiological
+  peculiarity in breeding a month earlier than the others. Similar
+  facts occur, however, among our domestic animals and are well
+  known to breeders. Professor Low, one of the greatest authorities
+  on our domesticated animals, says: "The female of the dog, when
+  not under restraint, makes selection of her mate, the mastiff
+  selecting the mastiff, the terrier the terrier, and so on." And
+  again: "The Merino sheep and Heath sheep of Scotland, if two
+  flocks are mixed together, each will breed with its own variety."
+  Mr. Darwin has collected many facts illustrating this point. One
+  of the chief pigeon-fanciers in England informed him that, if
+  free to choose, each breed would prefer pairing with its own
+  kind. Among the wild horses in Paraguay those of the same colour
+  and size associate together; while in Circassia there are three
+  races of horses which have received special names, and which,
+  when living a free life, almost always refuse to mingle and
+  cross, and will even attack one another. On one of the Faroe
+  Islands, not more than half a mile in diameter, the half-wild
+  native black sheep do not readily mix with imported white sheep.
+  In the Forest of Dean, and in the New Forest, the dark and pale
+  coloured herds of fallow deer have never been known to mingle;
+  and even the curious Ancon sheep of quite modern origin have been
+  observed to keep together, separating themselves from the rest of
+  the flock when put into enclosures with other sheep. The same
+  rule applies to birds, for Darwin was informed by the Rev. W.D.
+  Fox that his flocks of white and Chinese geese kept
+  distinct.<a name="FNanchor_61_61" id=
+  "FNanchor_61_61"></a><a href="#Footnote_61_61" class=
+  "fnanchor">[61]</a></p>
+
+  <p>This constant preference of animals for their like, even in
+  the case of slightly different varieties of the same species, is
+  evidently <a name="Page_173" id="Page_173"></a>a fact of great
+  importance in considering the origin of species by natural
+  selection, since it shows us that, so soon as a slight
+  differentiation of form or colour has been effected, isolation
+  will at once arise by the selective association of the animals
+  themselves; and thus the great stumbling-block of "the swamping
+  effects of intercrossing," which has been so prominently brought
+  forward by many naturalists, will be completely obviated.</p>
+
+  <p>If now we combine with this fact the correlation of colour
+  with important constitutional peculiarities, and, in some cases,
+  with infertility; and consider, further, the curious parallelism
+  that has been shown to exist between the effects of changed
+  conditions and the intercrossing of varieties in producing either
+  an increase or a decrease of fertility, we shall have obtained,
+  at all events, a starting-point for the production of that
+  infertility which is so characteristic a feature of distinct
+  species when intercrossed. All we need, now, is some means of
+  increasing or accumulating this initial tendency; and to a
+  discussion of this problem we will therefore address
+  ourselves.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Influence of Natural Selection upon Sterility and
+  Fertility.</i></p>
+
+  <p>It will occur to many persons that, as the infertility or
+  sterility of incipient species would be useful to them when
+  occupying the same or adjacent areas, by neutralising the effects
+  of intercrossing, this infertility might have been increased by
+  the action of natural selection; and this will be thought the
+  more probable if we admit, as we have seen reason to do, that
+  variations in fertility occur, perhaps as frequently as other
+  variations. Mr. Darwin tells us that, at one time, this appeared
+  to him probable, but he found the problem to be one of extreme
+  complexity; and he was also influenced against the view by many
+  considerations which seemed to render such an origin of the
+  sterility or infertility of species when intercrossed very
+  improbable. The fact that species which occupy distinct areas,
+  and which nowhere come in contact with each other, are often
+  sterile when crossed, is one of the difficulties; but this may
+  perhaps be overcome by the consideration that, though now
+  isolated, they may, and often must, have been in contact at their
+  origination. More important is the objection that natural
+  selection could not <a name="Page_174" id="Page_174"></a>possibly
+  have produced the difference that often occurs between reciprocal
+  crosses, one of these being sometimes fertile, while the other is
+  sterile. The extremely different amounts of infertility or
+  sterility between different species of the same genus, the
+  infertility often bearing no proportion to the difference between
+  the species crossed, is also an important objection. But none of
+  these objections would have much weight if it could be clearly
+  shown that natural selection <i>is</i> able to increase the
+  infertility variations of incipient species, as it is certainly
+  able to increase and develop all useful variations of form,
+  structure, instincts, or habits. Ample causes of infertility have
+  been shown to exist, in the nature of the organism and the laws
+  of correlation; the agency of natural selection is only needed to
+  accumulate the effects produced by these causes, and to render
+  their final results more uniform and more in accordance with the
+  facts that exist.</p>
+
+  <p>About twenty years ago I had much correspondence and
+  discussion with Mr. Darwin on this question. I then believed that
+  I was able to demonstrate the action of natural selection in
+  accumulating infertility; but I could not convince him, owing to
+  the extreme complexity of the process under the conditions which
+  he thought most probable. I have recently returned to the
+  question; and, with the fuller knowledge of the facts of
+  variation we now possess, I think it may be shown that natural
+  selection <i>is</i>, in some probable cases at all events, able
+  to accumulate variations in infertility between incipient
+  species.</p>
+
+  <p>The simplest case to consider, will be that in which two forms
+  or varieties of a species, occupying an extensive area, are in
+  process of adaptation to somewhat different modes of life within
+  the same area. If these two forms freely intercross with each
+  other, and produce mongrel offspring which are quite fertile
+  <i>inter se</i>, then the further differentiation of the forms
+  into two distinct species will be retarded, or perhaps entirely
+  prevented; for the offspring of the crossed unions will be,
+  perhaps, more vigorous on account of the cross, although less
+  perfectly adapted to the conditions of existence than either of
+  the pure breeds; and this would certainly establish a powerful
+  antagonistic influence to the further differentiation of the two
+  forms.</p>
+
+  <p><a name="Page_175" id="Page_175"></a>Now, let us suppose that
+  a partial sterility of the hybrids between the two forms arises,
+  in correlation with the different modes of life and the slight
+  external or internal peculiarities that exist between them, both
+  of which we have seen to be real causes of infertility. The
+  result will be that, even if the hybrids between the two forms
+  are still freely produced, these hybrids will not themselves
+  increase so rapidly as the two pure forms; and as these latter
+  are, by the terms of the problem, better suited to their
+  conditions of life than are the hybrids between them, they will
+  not only increase more rapidly, but will also tend to supplant
+  the hybrids altogether whenever the struggle for existence
+  becomes exceptionally severe. Thus, the more complete the
+  sterility of the hybrids the more rapidly will they die out and
+  leave the two parent forms pure. Hence it will follow that, if
+  there is greater infertility between the two forms in one part of
+  the area than the other, these forms will be kept more pure
+  wherever this greater infertility prevails, will therefore have
+  an advantage at each recurring period of severe struggle for
+  existence, and will thus ultimately supplant the less infertile
+  or completely fertile forms that may exist in other portions of
+  the area. It thus appears that, in such a case as here supposed,
+  natural selection would preserve those portions of the two breeds
+  which were most infertile with each other, or whose hybrid
+  offspring were most infertile; and would, therefore, if
+  variations in fertility continued to arise, tend to increase that
+  infertility. It must particularly be noted that this effect would
+  result, not by the preservation of the infertile variations on
+  account of their infertility, but by the inferiority of the
+  hybrid offspring, both as being fewer in numbers, less able to
+  continue their race, and less adapted to the conditions of
+  existence than either of the pure forms. It is this inferiority
+  of the hybrid offspring that is the essential point; and as the
+  number of these hybrids will be permanently less where the
+  infertility is greatest, therefore those portions of the two
+  forms in which infertility is greatest will have the advantage,
+  and will ultimately survive in the struggle for existence.</p>
+
+  <p>The differentiation of the two forms into distinct species,
+  with the increase of infertility between them, would be <a name=
+  "Page_176" id="Page_176"></a>greatly assisted by two other
+  important factors in the problem. It has already been shown that,
+  with each modification of form and habits, and especially with
+  modifications of colour, there arises a disinclination of the two
+  forms to pair together; and this would produce an amount of
+  isolation which would greatly assist the specialisation of the
+  forms in adaptation to their different conditions of life. Again,
+  evidence has been adduced that change of conditions or of mode of
+  life is a potent cause of disturbance of the reproductive system,
+  and, consequently, of infertility. We may therefore assume that,
+  as the two forms adopted more and more different modes of life,
+  and perhaps acquired also decided peculiarities of form and
+  coloration, the infertility between them would increase or become
+  more general; and as we have seen that every such increase of
+  infertility would give that portion of the species in which it
+  arose an advantage over the remaining portions in which the two
+  varieties were more fertile together, all this induced
+  infertility would maintain itself, and still further increase the
+  general infertility between the two forms of the species.</p>
+
+  <p>It follows, then, that specialisation to separate conditions
+  of life, differentiation of external characters, disinclination
+  to cross-unions, and the infertility of the hybrid produce of
+  these unions, would all proceed <i>pari passu</i>, and would
+  ultimately lead to the production of two distinct forms having
+  all the characteristics, physiological as well as structural, of
+  true species.</p>
+
+  <p>In the case now discussed it has been supposed, that some
+  amount of general infertility might arise in correlation with the
+  different modes of life of two varieties or incipient species. A
+  considerable body of facts already adduced renders it probable
+  that this <i>is</i> the mode in which any widespread infertility
+  would arise; and, if so, it has been shown that, by the influence
+  of natural selection and the known laws which affect varieties,
+  the infertility would be gradually increased. But, if we suppose
+  the infertility to arise sporadically within the two forms, and
+  to affect only a small proportion of the individuals in any area,
+  it will be difficult, if not impossible, to show that such
+  infertility would have any tendency to increase, or would produce
+  any but a <a name="Page_177" id="Page_177"></a>prejudicial
+  effect. If, for example, five per cent of each form thus varied
+  so as to be infertile with the other form, the result would be
+  hardly perceptible, because the individuals which formed
+  cross-unions and produced hybrids would constitute a very small
+  portion of the whole species; and the hybrid offspring, being at
+  a disadvantage in the struggle for existence and being themselves
+  infertile, would soon die out, while the much more numerous
+  fertile portion of the two forms would increase rapidly, and
+  furnish a sufficient number of pure-bred offspring of each form
+  to take the place of the somewhat inferior hybrids between them
+  whenever the struggle for existence became severe. We must
+  suppose that the normal fertile forms would transmit their
+  fertility to their progeny, and the few infertile forms their
+  infertility; but the latter would necessarily lose half their
+  proper increase by the sterility of their hybrid offspring
+  whenever they crossed with the other form, and when they bred
+  with their own form the tendency to sterility would die out
+  except in the very minute proportion of the five per cent
+  (one-twentieth) that chance would lead to pair together. Under
+  these circumstances the incipient sterility between the two forms
+  would rapidly be eliminated, and could never rise much above the
+  numbers which were produced by sporadic variation each year.</p>
+
+  <p>It was, probably, by a consideration of some such case as this
+  that Mr. Darwin came to the conclusion that infertility arising
+  between incipient species could not be increased by natural
+  selection; and this is the more likely, as he was always disposed
+  to minimise both the frequency and the amount even of structural
+  variations.</p>
+
+  <p>We have yet to notice another mode of action of natural
+  selection in favouring and perpetuating any infertility that may
+  arise between two incipient species. If several distinct species
+  are undergoing modification at the same time and in the same
+  area, to adapt them to some new conditions that have arisen
+  there, then any species in which the structural or colour
+  differences that have arisen between it and its varieties or
+  close allies were correlated with infertility of the crosses
+  between them, would have an advantage over the corresponding
+  varieties of other species in which there was no such <a name=
+  "Page_178" id="Page_178"></a>physiological peculiarity. Thus,
+  incipient species which were infertile together would have an
+  advantage over other incipient species which were fertile, and,
+  whenever the struggle for existence became severe, would prevail
+  over them and take their place. Such infertility, being
+  correlated with constitutional or structural differences, would
+  probably, as already suggested, go on increasing as these
+  differences increased; and thus, by the time the new species
+  became fully differentiated from its parent form (or brother
+  variety) the infertility might have become as well marked as we
+  usually find it to be between distinct species.</p>
+
+  <p>This discussion has led us to some conclusions of the greatest
+  importance as bearing on the difficult problem of the cause of
+  the sterility of the hybrids between distinct species. Accepting,
+  as highly probable, the fact of variations in fertility occurring
+  in correlation with variations in habits, colour, or structure,
+  we see, that so long as such variations occurred only
+  sporadically, and affected but a small proportion of the
+  individuals in any area, the infertility could not be increased
+  by natural selection, but would tend to die out almost as fast as
+  it was produced. If, however, it was so closely correlated with
+  physical variations or diverse modes of life as to affect, even
+  in a small degree, a considerable proportion of the individuals
+  of the two forms in definite areas, it would be preserved by
+  natural selection, and the portion of the varying species thus
+  affected would increase at the expense of those portions which
+  were more fertile when crossed. Each further variation towards
+  infertility between the two forms would be again preserved, and
+  thus the incipient infertility of the hybrid offspring might be
+  increased till it became so great as almost to amount to
+  sterility. Yet further, we have seen that if several competing
+  species in the same area were being simultaneously modified,
+  those between whose varieties infertility arose would have an
+  advantage over those whose varieties remained fertile <i>inter
+  se</i>, and would ultimately supplant them.</p>
+
+  <p>The preceding argument, it will be seen, depends entirely upon
+  the assumption that some amount of infertility characterises the
+  distinct varieties which are in process of differentiation into
+  species; and it may be objected that of <a name="Page_179" id=
+  "Page_179"></a>such infertility there is no proof. This is
+  admitted; but it is urged that facts have been adduced which
+  render such infertility probable, at least in some cases, and
+  this is all that is required. It is by no means necessary that
+  <i>all</i> varieties should exhibit incipient infertility, but
+  only, some varieties; for we know that, of the innumerable
+  varieties that occur but few become developed into distinct
+  species, and it may be that the absence of infertility, to
+  obviate the effects of intercrossing, is one of the usual causes
+  of their failure. All I have attempted to show is, that
+  <i>when</i> incipient infertility does occur in correlation with
+  other varietal differences, that infertility can be, and in fact
+  must be, increased by natural selection; and this, it appears to
+  me, is a decided step in advance in the solution of the
+  problem.<a name="FNanchor_62_62" id="FNanchor_62_62"></a><a href=
+  "#Footnote_62_62" class="fnanchor">[62]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_180" id="Page_180"></a><i>Physiological
+  Selection.</i></p>
+
+  <p>Another form of infertility has been suggested by Professor
+  G.J. Romanes as having aided in bringing about the characteristic
+  infertility or sterility of hybrids. It is founded on the fact,
+  already noticed, that certain individuals of some species possess
+  what may be termed selective sterility&mdash;that is, while
+  fertile with some individuals of the species they are sterile
+  with others, and this altogether independently of any differences
+  of form, colour, or structure. The phenomenon, in the only form
+  in which it has been observed, is that of "infertility or
+  absolute sterility between two individuals, each of which is
+  perfectly fertile with all other individuals;" but Mr. Romanes
+  thinks that "it would not be nearly so remarkable, or
+  physiologically improbable, that such incompatibility should run
+  through a whole race or strain."<a name="FNanchor_63_63" id=
+  "FNanchor_63_63"></a><a href="#Footnote_63_63" class=
+  "fnanchor">[63]</a> Admitting that this may be <a name="Page_181"
+  id="Page_181"></a>so, though we have at present no evidence
+  whatever in support of it, it remains to be considered whether
+  such physiological varieties could maintain themselves, or
+  whether, as in the cases of sporadic infertility already
+  discussed, they would necessarily die out unless correlated with
+  useful characters. Mr. Romanes thinks that they would persist,
+  and urges that "whenever this one kind of variation occurs <i>it
+  cannot escape the preserving agency</i> of physiological
+  selection. Hence, even if it be granted that the variation which
+  affects the reproductive system in this particular way is a
+  variation of comparatively rare occurrence, still, as <i>it must
+  always be preserved</i> whenever it does occur, its influence in
+  the manufacture of specific types <i>must be cumulative</i>." The
+  very positive statements which I have italicised would lead most
+  readers to believe that the alleged fact had been demonstrated by
+  a careful working out of the process in some definite supposed
+  cases. This, however, has nowhere been done in Mr. Romanes'
+  paper; and as it is <i>the</i> vital theoretical point on which
+  any possible value of the new theory rests, and as it appears so
+  opposed to the self-destructive effects of simple infertility,
+  which we have already demonstrated when it occurs between the
+  intermingled portion of two varieties, it must be carefully
+  examined. In doing so, I will suppose that the required variation
+  is not of "rare occurrence," but of considerable amount, and that
+  it appears afresh each year to about the same extent, thus giving
+  the theory every possible advantage.</p>
+
+  <p>Let us then suppose that a given species consists of 100,000
+  individuals of each sex, with only the usual amount of
+  fluctuating external variability. Let a physiological variation
+  arise, so that 10 per cent of the whole number&mdash;10,000
+  individuals of each sex&mdash;while remaining fertile <i>inter
+  se</i> become quite sterile with the remaining 90,000. This
+  peculiarity is not correlated with any external differences of
+  <a name="Page_182" id="Page_182"></a>form or colour, or with
+  inherent peculiarities of likes or dislikes leading to any choice
+  as to the pairing of the two sets of individuals. We have now to
+  inquire, What would be the result?</p>
+
+  <p>Taking, first, the 10,000 pairs of the physiological or
+  abnormal variety, we find that each male of these might pair with
+  any one of the whole 100,000 of the opposite sex. If, therefore,
+  there was nothing to limit their choice to particular individuals
+  of either variety, the probabilities are that 9000 of them would
+  pair with the opposite variety, and only 1000 with their own
+  variety&mdash;that is, that 9000 would form sterile unions, and
+  only <i>one</i> thousand would form fertile unions.</p>
+
+  <p>Taking, next, the 90,000 normal individuals of either sex, we
+  find, that each male of these has also a choice of 100,000 to
+  pair with. The probabilities are, therefore, that nine-tenths of
+  them&mdash;that is, 81,000&mdash;would pair with their normal
+  fellows, while 9000 would pair with the opposite abnormal variety
+  forming the above-mentioned sterile unions.</p>
+
+  <p>Now, as the number of individuals forming a species remains
+  constant, generally speaking, from year to year, we shall have
+  next year also 100,000 pairs, of which the two physiological
+  varieties will be in the proportion of eighty-one to one, or
+  98,780 pairs of the normal variety to 1220<a name=
+  "FNanchor_64_64" id="FNanchor_64_64"></a><a href=
+  "#Footnote_64_64" class="fnanchor">[64]</a> of the abnormal, that
+  being the proportion of the fertile unions of each. In this year
+  we shall find, by the same rule of probabilities, that only 15
+  males of the abnormal variety will pair with their like and be
+  fertile, the remaining 1205 forming sterile unions with some of
+  the normal variety. The following year the total 100,000 pairs
+  will consist of 99,984 of the normal, and only 16 of the abnormal
+  variety; and the probabilities, of course, are, that the whole of
+  these latter will pair with some of the enormous preponderance of
+  normal individuals, and, their unions being sterile, the
+  physiological variety will become extinct in the third year.</p>
+
+  <p>If now in the second and each succeeding year a similar
+  proportion as at first (10 per cent) of the physiological variety
+  is produced afresh from the ranks of the normal variety, the same
+  rate of diminution will go on, and it will be found that,
+  <a name="Page_183" id="Page_183"></a>on the most favourable
+  estimate, the physiological variety can never exceed 12,000 to
+  the 88,000 of the normal form of the species, as shown by the
+  following table:&mdash;</p>
+
+  <pre>
+    1st Year. 10,000 of physiological variety to 90,000 of normal variety.
+    2d   "     1,220 + 10,000 again produced.
+    3d   "        16 +  1,220 + 10,000 do.               = 11,236
+    4th  "         O +     16 +  1,220 + 10,000 do.      = 11,236
+    5th  "                  O +     16 +  1,220 + 10,000 = 11,236
+    and so on for any number of generations.
+  </pre>
+
+  <p>In the preceding discussion we have given the theory the
+  advantage of the large proportion of 10 per cent of this very
+  exceptional variety arising in its midst year by year, and we
+  have seen that, even under these favourable conditions, it is
+  unable to increase its numbers much above its starting-point, and
+  that it remains wholly dependent on the continued renewal of the
+  variety for its existence beyond a few years. It appears, then,
+  that this form of inter-specific sterility cannot be increased by
+  natural or any other known form of selection, but that it
+  contains within itself its own principle of destruction. If it is
+  proposed to get over the difficulty by postulating a larger
+  percentage of the variety annually arising within the species, we
+  shall not affect the law of decrease until we approach equality
+  in the numbers of the two varieties. But with any such increase
+  of the physiological variety the species itself would inevitably
+  suffer by the large proportion of sterile unions in its midst,
+  and would thus be at a great disadvantage in competition with
+  other species which were fertile throughout. Thus, natural
+  selection will always tend to weed out any species with too great
+  a tendency to sterility among its own members, and will therefore
+  prevent such sterility from becoming the general characteristic
+  of varying species, which this theory demands should be the
+  case.</p>
+
+  <p>On the whole, then, it appears clear that no form of
+  infertility or sterility between the individuals of a species,
+  can be increased by natural selection unless correlated with some
+  useful variation, while all infertility not so correlated has a
+  constant tendency to effect its own elimination. But the opposite
+  property, fertility, is of vital importance to every species, and
+  gives the offspring of the individuals which possess it, in
+  consequence of their superior numbers, a greater <a name=
+  "Page_184" id="Page_184"></a>chance of survival in the battle of
+  life. It is, therefore, directly under the control of natural
+  selection, which acts both by the self-preservation of fertile
+  and the self-destruction of infertile stocks&mdash;except always
+  where correlated as above, when they become useful, and therefore
+  subject to be increased by natural selection.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Summary and Concluding Remarks on Hybridity.</i></p>
+
+  <p>The facts which are of the greatest importance to a
+  comprehension of this very difficult subject are those which show
+  the extreme susceptibility of the reproductive system both in
+  plants and animals. We have seen how both these classes of
+  organisms may be rendered infertile, by a change of conditions
+  which does not affect their general health, by captivity, or by
+  too close interbreeding. We have seen, also, that infertility is
+  frequently correlated with a difference of colour, or with other
+  characters; that it is not proportionate to divergence of
+  structure; that it varies in reciprocal crosses between pairs of
+  the same species; while in the cases of dimorphic and trimorphic
+  plants the different crosses between the same pair of individuals
+  may be fertile or sterile at the same time. It appears as if
+  fertility depended on such a delicate adjustment of the male and
+  female elements to each other, that, unless constantly kept up by
+  the preservation of the most fertile individuals, sterility is
+  always liable to arise. This preservation always occurs within
+  the limits of each species, both because fertility is of the
+  highest importance to the continuance of the race, and also
+  because sterility (and to a less extent infertility) is
+  self-destructive as well as injurious to the species.</p>
+
+  <p>So long therefore as a species remains undivided, and in
+  occupation of a continuous area, its fertility is kept up by
+  natural selection; but the moment it becomes separated, either by
+  geographical or selective isolation, or by diversity of station
+  or of habits, then, while each portion must be kept fertile
+  <i>inter se</i>, there is nothing to prevent infertility arising
+  between the two separated portions. As the two portions will
+  necessarily exist under somewhat different conditions of life,
+  and will usually have acquired some diversity of form and
+  colour&mdash;both which circumstances we know to be either the
+  cause of infertility or to be correlated with it,&mdash;the fact
+  of <a name="Page_185" id="Page_185"></a>some degree of
+  infertility usually appearing between closely allied but locally
+  or physiologically segregated species is exactly what we should
+  expect.</p>
+
+  <p>The reason why varieties do not usually exhibit a similar
+  amount of infertility is not difficult to explain. The popular
+  conclusions on this matter have been drawn chiefly from what
+  occurs among domestic animals, and we have seen that the very
+  first essential to their becoming domesticated was that they
+  should continue fertile under changed conditions of life. During
+  the slow process of the formation of new varieties by conscious
+  or unconscious selection, fertility has always been an essential
+  character, and has thus been invariably preserved or increased;
+  while there is some evidence to show that domestication itself
+  tends to increase fertility.</p>
+
+  <p>Among plants, wild species and varieties have been more
+  frequently experimented on than among animals, and we accordingly
+  find numerous cases in which distinct species of plants are
+  perfectly fertile when crossed, their hybrid offspring being also
+  fertile <i>inter se</i>. We also find some few examples of the
+  converse fact&mdash;varieties of the same species which when
+  crossed are infertile or even sterile.</p>
+
+  <p>The idea that either infertility or geographical isolation is
+  absolutely essential to the formation of new species, in order to
+  prevent the swamping effects of intercrossing, has been shown to
+  be unsound, because the varieties or incipient species will, in
+  most cases, be sufficiently isolated by having adopted different
+  habits or by frequenting different stations; while selective
+  association, which is known to be general among distinct
+  varieties or breeds of the same species, will produce an
+  effective isolation even when the two forms occupy the same
+  area.</p>
+
+  <p>From the various considerations now adverted to, Mr. Darwin
+  arrived at the conclusion that the sterility or infertility of
+  species with each other, whether manifested in the difficulty of
+  obtaining first crosses between them or in the sterility of the
+  hybrids thus obtained, is not a constant or necessary result of
+  specific difference, but is incidental on unknown peculiarities
+  of the reproductive system. These peculiarities constantly tend
+  to arise under changed conditions owing to the extreme
+  susceptibility of that system, and they <a name="Page_186" id=
+  "Page_186"></a>are usually correlated with variations of form or
+  of colour. Hence, as fixed differences of form and colour, slowly
+  gained by natural selection in adaptation to changed conditions,
+  are what essentially characterise distinct species, some amount
+  of infertility between species is the usual result.</p>
+
+  <p>Here the problem was left by Mr. Darwin; but we have shown
+  that its solution may be carried a step further. If we accept the
+  association of some degree of infertility, however slight, as a
+  not unfrequent accompaniment of the external differences which
+  always arise in a state of nature between varieties and incipient
+  species, it has been shown that natural selection <i>has</i>
+  power to increase that infertility just as it has power to
+  increase other favourable variations. Such an increase of
+  infertility will be beneficial, whenever new species arise in the
+  same area with the parent form; and we thus see how, out of the
+  fluctuating and very unequal amounts of infertility correlated
+  with physical variations, there may have arisen that larger and
+  more constant amount which appears usually to characterise
+  well-marked species.</p>
+
+  <p>The great body of facts of which a condensed account has been
+  given in the present chapter, although from an experimental point
+  of view very insufficient, all point to the general conclusion we
+  have now reached, and afford us a not unsatisfactory solution of
+  the great problem of hybridism in relation to the origin of
+  species by means of natural selection. Further experimental
+  research is needed in order to complete the elucidation of the
+  subject; but until these additional facts are forthcoming no new
+  theory seems required for the explanation of the phenomena.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_51_51" id="Footnote_51_51"></a><a href=
+      "#FNanchor_51_51"><span class="label">[51]</span></a>
+      Darwin's <i>Animals and Plants under Domestication</i>, vol.
+      ii. pp. 163-170.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_52_52" id="Footnote_52_52"></a><a href=
+      "#FNanchor_52_52"><span class="label">[52]</span></a> For a
+      full account of these interesting facts and of the various
+      problems to which they give rise, the reader must consult
+      Darwin's volume on <i>The Different Forms of Flowers in
+      Plants of the same Species</i>, chaps, i.-iv.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_53_53" id="Footnote_53_53"></a><a href=
+      "#FNanchor_53_53"><span class="label">[53]</span></a> See
+      <i>Nature</i>, vol. xxi. p. 207.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_54_54" id="Footnote_54_54"></a><a href=
+      "#FNanchor_54_54"><span class="label">[54]</span></a> Low's
+      <i>Domesticated Animals of Great Britain</i>, Introduction,
+      p. lxiv.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_55_55" id="Footnote_55_55"></a><a href=
+      "#FNanchor_55_55"><span class="label">[55]</span></a> Low's
+      <i>Domesticated Animals</i>, p. 28.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_56_56" id="Footnote_56_56"></a><a href=
+      "#FNanchor_56_56"><span class="label">[56]</span></a>
+      <i>Amaryllidaceae</i>, by the Hon. and Rev. William Herbert,
+      p. 379.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_57_57" id="Footnote_57_57"></a><a href=
+      "#FNanchor_57_57"><span class="label">[57]</span></a>
+      <i>Origin of Species</i>, p. 239.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_58_58" id="Footnote_58_58"></a><a href=
+      "#FNanchor_58_58"><span class="label">[58]</span></a>
+      <i>Origin of Species</i>, sixth edition, p. 9.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_59_59" id="Footnote_59_59"></a><a href=
+      "#FNanchor_59_59"><span class="label">[59]</span></a> In the
+      <i>Medico-Chirurgical Transactions</i>, vol. liii. (1870),
+      Dr. Ogle has adduced some curious physiological facts bearing
+      on the presence or absence of white colours in the higher
+      animals. He states that a dark pigment in the olfactory
+      region of the nostrils is essential to perfect smell, and
+      that this pigment is rarely deficient except when the whole
+      animal is pure white, and the creature is then almost without
+      smell or taste. He observes that there is no proof that, in
+      any of the cases given above, the black animals actually eat
+      the poisonous root or plant; and that the facts are readily
+      understood if the senses of smell and taste are dependent on
+      a pigment which is absent in the white animals, who therefore
+      eat what those gifted with normal senses avoid. This
+      explanation however hardly seems to cover the facts. We
+      cannot suppose that almost all the sheep in the world (which
+      are mostly white) are without smell or taste. The cutaneous
+      disease on the white patches of hair on horses, the special
+      liability of white terriers to distemper, of white chickens
+      to the gapes, and of silkworms which produce yellow silk to
+      the fungus, are not explained by it. The analogous facts in
+      plants also indicate a real constitutional relation with
+      colour, not an affection of the sense of smell and taste
+      only.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_60_60" id="Footnote_60_60"></a><a href=
+      "#FNanchor_60_60"><span class="label">[60]</span></a> For all
+      these facts, see <i>Animals and Plants under
+      Domestication</i>, vol. ii. pp. 335-338.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_61_61" id="Footnote_61_61"></a><a href=
+      "#FNanchor_61_61"><span class="label">[61]</span></a>
+      <i>Animals and Plants under Domestication</i>, vol. ii. pp.
+      102, 103.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_62_62" id="Footnote_62_62"></a><a href=
+      "#FNanchor_62_62"><span class="label">[62]</span></a> As this
+      argument is a rather difficult one to follow, while its
+      theoretical importance is very great, I add here the
+      following briefer exposition of it, in a series of
+      propositions; being, with a few verbal alterations, a copy of
+      what I wrote on the subject about twenty years back. Some
+      readers may find this easier to follow than the fuller
+      discussion in the text:&mdash;</p>
+
+      <div class="blockquot" style='font-size: 100%;'>
+        <p style=
+        'text-align:center; font-weight:bold; margin-top:1.5em'>
+        <i>Can Sterility of Hybrids have been Produced by Natural
+        Selection?</i></p>
+
+        <p>1. Let there be a species which has varied into <i>two
+        forms</i> each adapted to certain existing conditions
+        better than the parent form, which they soon supplant.</p>
+
+        <p>2. If these <i>two forms</i>, which are supposed to
+        coexist in the same district, do not intercross, natural
+        selection will accumulate all favourable variations till
+        they become well suited to their conditions of life, and
+        form two slightly differing species.</p>
+
+        <p>3. But if these <i>two forms</i> freely intercross with
+        each other, and produce hybrids, which are also quite
+        fertile <i>inter se</i>, then the formation of the two
+        distinct races or species will be retarded, or perhaps
+        entirely prevented; for the offspring of the crossed unions
+        will be <i>more vigorous</i> owing to the cross, although
+        <i>less adapted</i> to their conditions of life than either
+        of the pure breeds.</p>
+
+        <p>4. Now, let a partial sterility of the hybrids of some
+        considerable proportion of these two forms arise; and, as
+        this would probably be due to some special conditions of
+        life, we may fairly suppose it to arise in some definite
+        portion of the area occupied by the two forms.</p>
+
+        <p>5. The result will be that, in that area, the hybrids
+        (although continually produced by first crosses almost as
+        freely as before) will not themselves increase so rapidly
+        as the two pure forms; and as the two pure forms are, by
+        the terms of the problem, better suited to their several
+        conditions of life than the hybrids, they will inevitably
+        increase more rapidly, and will continually tend to
+        supplant the hybrids altogether at every recurrent severe
+        struggle for existence.</p>
+
+        <p>6. We may fairly suppose, also, that as soon as any
+        sterility appears some disinclination to <i>cross
+        unions</i> will appear, and this will further tend to the
+        diminution of the production of hybrids.</p>
+
+        <p>7. In the other part of the area, however, where
+        hybridism occurs with perfect freedom, hybrids of various
+        degrees may increase till they equal or even exceed in
+        number the pure species&mdash;that is, the incipient
+        species will be liable to be swamped by intercrossing.</p>
+
+        <p>8. The first result, then, of a partial sterility of
+        crosses appearing in one part of the area occupied by the
+        two forms, will be&mdash;that the great majority of the
+        individuals will there consist of the two pure forms only,
+        while in the remaining part these will be in a
+        minority,&mdash;which is the same as saying that the new
+        <i>physiological variety</i> of the two forms will be
+        better suited to the conditions of existence than the
+        remaining portion which has not varied physiologically.</p>
+
+        <p>9. But when the struggle for existence becomes severe,
+        that variety which is best adapted to the conditions of
+        existence always supplants that which is imperfectly
+        adapted; therefore, <i>by natural selection</i> the
+        <i>varieties</i> which are <i>sterile</i> when crossed will
+        become established as the only ones.</p>
+
+        <p>10. Now let variations in the <i>amount of
+        sterility</i>and in the <i>disinclination to crossed
+        unions</i> continue to occur&mdash;also in certain parts of
+        the area: exactly the same result must recur, and the
+        progeny of this new physiological variety will in time
+        occupy the whole area.</p>
+
+        <p>11. There is yet another consideration that would
+        facilitate the process. It seems probable that the
+        <i>sterility variations</i> would, to some extent, concur
+        with, and perhaps depend upon, the <i>specific
+        variations</i>; so that, just in proportion as the <i>two
+        forms</i> diverged and became better adapted to the
+        conditions of existence, they would become more sterile
+        when intercrossed. If this were the case, then natural
+        selection would act with double strength; and those which
+        were better adapted to survive both structurally and
+        physiologically would certainly do so.</p>
+      </div>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_63_63" id="Footnote_63_63"></a><a href=
+      "#FNanchor_63_63"><span class="label">[63]</span></a> Cases
+      of this kind are referred to at p. 155. It must, however, be
+      noted, that such sterility in first crosses appears to be
+      equally rare between different species of the same genus as
+      between individuals of the same species. Mules and other
+      hybrids are freely produced between very distinct species,
+      but are themselves infertile or quite sterile; and it is this
+      infertility or sterility of the hybrids that is the
+      characteristic&mdash;and was once thought to be the
+      criterion&mdash;of species, not the sterility of their first
+      crosses. Hence we should not expect to find any constant
+      infertility in the first crosses between the distinct strains
+      or varieties that formed the starting-point of new species,
+      but only a slight amount of infertility in their mongrel
+      offspring. It follows, that Mr. Romanes' theory of
+      <i>Physiological Selection</i>&mdash;which assumes sterility
+      or infertility between first crosses as the fundamental fact
+      in the origin of species&mdash;does not accord with the
+      general phenomena of hybridism in nature.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_64_64" id="Footnote_64_64"></a><a href=
+      "#FNanchor_64_64"><span class="label">[64]</span></a> The
+      exact number is 1219.51, but the fractions are omitted for
+      clearness.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a><a name=
+  "Page_187" id="Page_187"></a>CHAPTER VIII</h2>
+
+  <p style='font-weight:bold;'>THE ORIGIN AND USES OF COLOUR IN
+  ANIMALS</p>
+
+  <div class="blockquot">
+    <p>The Darwinian theory threw new light on organic
+    colour&mdash;The problem to be solved&mdash;The constancy of
+    animal colour indicates utility&mdash;Colour and
+    environment&mdash;Arctic animals white&mdash;Exceptions prove
+    the rule&mdash;Desert, forest, nocturnal, and oceanic
+    animals&mdash;General theories of animal colour&mdash;Variable
+    protective colouring&mdash;Mr. Poulton's
+    experiments&mdash;Special or local colour
+    adaptations&mdash;Imitation of particular objects&mdash;How
+    they have been produced&mdash;Special protective colouring of
+    butterflies&mdash;Protective resemblance among marine
+    animals&mdash;Protection by terrifying enemies&mdash;Alluring
+    coloration&mdash;The coloration of birds' eggs&mdash;Colour as
+    a means of recognition&mdash;Summary of the preceding
+    exposition&mdash;Influence of locality or of climate on
+    colour&mdash;Concluding remarks.</p>
+  </div>
+
+  <p>Among the numerous applications of the Darwinian theory in the
+  interpretation of the complex phenomena presented by the organic
+  world, none have been more successful, or are more interesting,
+  than those which deal with the colours of animals and plants. To
+  the older school of naturalists colour was a trivial character,
+  eminently unstable and untrustworthy in the determination of
+  species; and it appeared to have, in most cases, no use or
+  meaning to the objects which displayed it. The bright and often
+  gorgeous coloration of insect, bird, or flower, was either looked
+  upon as having been created for the enjoyment of mankind, or as
+  due to unknown and perhaps undiscoverable laws of nature.</p>
+
+  <p>But the researches of Mr. Darwin totally changed our point of
+  view in this matter. He showed, clearly, that some of the colours
+  of animals are useful, some hurtful to them; and he believed that
+  many of the most brilliant colours were developed by sexual
+  choice; while his great general principle, that all <a name=
+  "Page_188" id="Page_188"></a>the fixed characters of organic
+  beings have been developed under the action of the law of
+  utility, led to the inevitable conclusion that so remarkable and
+  conspicuous a character as colour, which so often constitutes the
+  most obvious distinction of species from species, or group from
+  group, must also have arisen from survival of the fittest, and
+  must, therefore, in most cases have some relation to the
+  wellbeing of its possessors. Continuous observation and research,
+  carried on by multitudes of observers during the last thirty
+  years, have shown this to be the case; but the problem is found
+  to be far more complex than was at first supposed. The modes in
+  which colour is of use to different classes of organisms is very
+  varied, and have probably not yet been all discovered; while the
+  infinite variety and marvellous beauty of some of its
+  developments are such as to render it hopeless to arrive at a
+  complete and satisfactory explanation of every individual case.
+  So much, however, has been achieved, so many curious facts have
+  been explained, and so much light has been thrown on some of the
+  most obscure phenomena of nature, that the subject deserves a
+  prominent place in any account of the Darwinian theory.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Problem to be Solved.</i></p>
+
+  <p>Before dealing with the various modifications of colour in the
+  animal world it is necessary to say a few words on colour in
+  general, on its prevalence in nature, and how it is that the
+  colours of animals and plants require any special explanation.
+  What we term colour is a subjective phenomenon, due to the
+  constitution of our mind and nervous system; while, objectively,
+  it consists of light-vibrations of different wave-lengths emitted
+  by, or reflected from, various objects. Every visible object must
+  be coloured, because to be visible it must send rays of light to
+  our eye. The kind of light it sends is modified by the molecular
+  constitution or the surface texture of the object. Pigments
+  absorb certain rays and reflect the remainder, and this reflected
+  portion has to our eyes a definite colour, according to the
+  portion of the rays constituting white light which are absorbed.
+  Interference colours are produced either by thin films or by very
+  fine striae on the surfaces of bodies, which cause rays of
+  certain wave-lengths to neutralise each other, leaving the
+  remainder to produce the effects of colour. Such <a name=
+  "Page_189" id="Page_189"></a>are the colours of soap-bubbles, or
+  of steel or glass on which extremely fine lines have been ruled;
+  and these colours often produce the effect of metallic lustre,
+  and are the cause of most of the metallic hues of birds and
+  insects.</p>
+
+  <p>As colour thus depends on molecular or chemical constitution
+  or on the minute surface texture of bodies, and, as the matter of
+  which organic beings are composed consists of chemical compounds
+  of great complexity and extreme instability, and is also subject
+  to innumerable changes during growth and development, we might
+  naturally expect the phenomena of colour to be more varied here
+  than in less complex and more stable compounds. Yet even in the
+  inorganic world we find abundant and varied colours; in the earth
+  and in the water; in metals, gems, and minerals; in the sky and
+  in the ocean; in sunset clouds and in the many-tinted rainbow.
+  Here we can have no question of <i>use</i> to the coloured
+  object, and almost as little perhaps in the vivid red of blood,
+  in the brilliant colours of red snow and other low algae and
+  fungi, or even in the universal mantle of green which clothes so
+  large a portion of the earth's surface. The presence of some
+  colour, or even of many brilliant colours, in animals and plants
+  would require no other explanation than does that of the sky or
+  the ocean, of the ruby or the emerald&mdash;that is, it would
+  require a purely physical explanation only. It is the wonderful
+  individuality of the colours of animals and plants that attracts
+  our attention&mdash;the fact that the colours are localised in
+  definite patterns, sometimes in accordance with structural
+  characters, sometimes altogether independent of them; while often
+  differing in the most striking and fantastic manner in allied
+  species. We are thus compelled to look upon colour not merely as
+  a physical but also as a biological characteristic, which has
+  been differentiated and specialised by natural selection, and
+  must, therefore, find its explanation in the principle of
+  adaptation or utility.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Constancy of Animal Colour indicates Utility.</i></p>
+
+  <p>That the colours and markings of animals have been acquired
+  under the fundamental law of utility is indicated by a general
+  fact which has received very little attention. As a rule, colour
+  and marking are constant in each species of wild animal, while,
+  in almost every domesticated animal, there arises <a name=
+  "Page_190" id="Page_190"></a>great variability. We see this in
+  our horses and cattle, our dogs and cats, our pigeons and
+  poultry. Now, the essential difference between the conditions of
+  life of domesticated and wild animals is, that the former are
+  protected by man, while the latter have to protect themselves.
+  The extreme variations in colour that immediately arise under
+  domestication indicate a tendency to vary in this way, and the
+  occasional occurrence of white or piebald or other exceptionally
+  coloured individuals of many species in a state of nature, shows
+  that this tendency exists there also; and, as these exceptionally
+  coloured individuals rarely or never increase, there must be some
+  constant power at work to keep it in check. This power can only
+  be natural selection or the survival of the fittest, which again
+  implies that some colours are useful, some injurious, in each
+  particular case. With this principle as our guide, let us see how
+  far we can account both for the general and special colours of
+  the animal world.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Colour and Environment.</i></p>
+
+  <p>The fact that first strikes us in our examination of the
+  colours of animals as a whole, is the close relation that exists
+  between these colours and the general environment. Thus, white
+  prevails among arctic animals; yellow or brown in desert species;
+  while green is only a common colour in tropical evergreen
+  forests. If we consider these cases somewhat carefully we shall
+  find, that they afford us excellent materials for forming a
+  judgment on the various theories that have been suggested to
+  account for the colours of the animal world.</p>
+
+  <p>In the arctic regions there are a number of animals which are
+  wholly white all the year round, or which only turn white in
+  winter. Among the former are the polar bear and the American
+  polar hare, the snowy owl and the Greenland falcon; among the
+  latter the arctic fox, the arctic hare, the ermine, and the
+  ptarmigan. Those which are permanently white remain among the
+  snow nearly all the year round, while those which change their
+  colour inhabit regions which are free from snow in summer. The
+  obvious explanation of this style of coloration is, that it is
+  protective, serving to conceal the herbivorous species from their
+  enemies, and enabling carnivorous animals to approach their prey
+  unperceived. Two other explanations have, how<a name="Page_191"
+  id="Page_191"></a>ever, been suggested. One is, that the
+  prevalent white of the arctic regions has a direct effect in
+  producing the white colour in animals, either by some
+  photographic or chemical action on the skin or by a reflex action
+  through vision. The other is, that the white colour is chiefly
+  beneficial as a means of checking radiation and so preserving
+  animal heat during the severity of an arctic winter. The first is
+  part of the general theory that colour is the effect of coloured
+  light on the objects&mdash;a pure hypothesis which has, I
+  believe, no facts whatever to support it. The second suggestion
+  is also an hypothesis merely, since it has not been proved by
+  experiment that a white colour, <i>per se</i>, independently of
+  the fur or feathers which is so coloured, has any effect whatever
+  in checking the radiation of low-grade heat like that of the
+  animal body. But both alike are sufficiently disproved by the
+  interesting exceptions to the rule of white coloration in the
+  arctic regions, which exceptions are, nevertheless, quite in
+  harmony with the theory of protection.</p>
+
+  <p>Whenever we find arctic animals which, from whatever cause, do
+  not require protection by the white colour, then neither the cold
+  nor the snow-glare has any effect upon their coloration. The
+  sable retains its rich brown fur throughout the Siberian winter;
+  but it frequents trees at that season and not only feeds
+  partially on fruits or seeds, but is able to catch birds among
+  the branches of the fir-trees, with the bark of which its colour
+  assimilates. Then we have that thoroughly arctic animal, the
+  musk-sheep, which is brown and conspicuous; but this animal is
+  gregarious, and its safety depends on its association in small
+  herds. It is, therefore, of more importance for it to be able to
+  recognise its kind at a distance than to be concealed from its
+  enemies, against which it can well protect itself so long as it
+  keeps together in a compact body. But the most striking example
+  is that of the common raven, which is a true arctic bird, and is
+  found even in mid-winter as far north as any known bird or
+  mammal. Yet it always retains its black coat, and the reason,
+  from our point of view, is obvious. The raven is a powerful bird
+  and fears no enemy, while, being a carrion-feeder, it has no need
+  for concealment in order to approach its prey. The colour of the
+  raven and of the musk-sheep are, therefore, <a name="Page_192"
+  id="Page_192"></a>both inconsistent with any other theory than
+  that the white colour of arctic animals has been acquired for
+  concealment, and to that theory both afford a strong support.
+  Here we have a striking example of the exception proving the
+  rule.</p>
+
+  <p>In the desert regions of the earth we find an even more
+  general accordance of colour with surroundings. The lion, the
+  camel, and all the desert antelopes have more or less the colour
+  of the sand or rock among which they live. The Egyptian cat and
+  the Pampas cat are sandy or earth coloured. The Australian
+  kangaroos are of similar tints, and the original colour of the
+  wild horse is supposed to have been sandy or clay coloured. Birds
+  are equally well protected by assimilative hues; the larks,
+  quails, goatsuckers, and grouse which abound in the North African
+  and Asiatic deserts are all tinted or mottled so as closely to
+  resemble the average colour of the soil in the districts they
+  inhabit. Canon Tristram, who knows these regions and their
+  natural history so well, says, in an often quoted passage: "In
+  the desert, where neither trees, brushwood, nor even undulations
+  of the surface afford the slightest protection to its foes, a
+  modification of colour which shall be assimilated to that of the
+  surrounding country is absolutely necessary. Hence, without
+  exception, the upper plumage of every bird, whether lark, chat,
+  sylvain, or sand-grouse, and also the fur of all the smaller
+  mammals, and the skin of all the snakes and lizards, is of one
+  uniform isabelline or sand colour."</p>
+
+  <p>Passing on to the tropical regions, it is among their
+  evergreen forests alone that we find whole groups of birds whose
+  ground colour is green. Parrots are very generally green, and in
+  the East we have an extensive group of green fruit-eating
+  pigeons; while the barbets, bee-eaters, turacos, leaf-thrushes
+  (Phyllornis), white-eyes (Zosterops), and many other groups, have
+  so much green in their plumage as to tend greatly to their
+  concealment among the dense foliage. There can be no doubt that
+  these colours have been acquired as a protection, when we see
+  that in all the temperate regions, where the leaves are
+  deciduous, the ground colour of the great majority of birds,
+  especially on the upper surface, is a rusty brown of various
+  shades, well corresponding with the bark, withered leaves, ferns,
+  and bare thickets among which <a name="Page_193" id=
+  "Page_193"></a>they live in autumn and winter, and especially in
+  early spring when so many of them build their nests.</p>
+
+  <p>Nocturnal animals supply another illustration of the same
+  rule, in the dusky colours of mice, rats, bats, and moles, and in
+  the soft mottled plumage of owls and goatsuckers which, while
+  almost equally inconspicuous in the twilight, are such as to
+  favour their concealment in the daytime.</p>
+
+  <p>An additional illustration of general assimilation of colour
+  to the surroundings of animals, is furnished by the inhabitants
+  of the deep oceans. Professor Moseley of the Challenger
+  Expedition, in his British Association lecture on this subject,
+  says: "Most characteristic of pelagic animals is the almost
+  crystalline transparency of their bodies. So perfect is this
+  transparency that very many of them are rendered almost entirely
+  invisible when floating in the water, while some, even when
+  caught and held up in a glass globe, are hardly to be seen. The
+  skin, nerves, muscles, and other organs are absolutely hyaline
+  and transparent, but the liver and digestive tract often remain
+  opaque and of a yellow or brown colour, and exactly resemble when
+  seen in the water small pieces of floating seaweed." Such marine
+  organisms, however, as are of larger size, and either
+  occasionally or habitually float on the surface, are beautifully
+  tinged with blue above, thus harmonising with the colour of the
+  sea as seen by hovering birds; while they are white below, and
+  are thus invisible against the wave-foam and clouds as seen by
+  enemies beneath the surface. Such are the tints of the beautiful
+  nudibranchiate mollusc, Glaucus atlanticus, and many others.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>General Theories of Animal Colour.</i></p>
+
+  <p>We are now in a position to test the general theories, or, to
+  speak more correctly, the popular notions, as to the origin of
+  animal coloration, before proceeding to apply the principle of
+  utility to the explanation of some among the many extraordinary
+  manifestations of colour in the animal world. The most generally
+  received theory undoubtedly is, that brilliancy and variety of
+  colour are due to the direct action of light and heat; a theory
+  no doubt derived from the abundance of bright-coloured birds,
+  insects, and flowers which are brought from tropical regions.
+  There are, however, <a name="Page_194" id="Page_194"></a>two
+  strong arguments against this theory. We have already seen how
+  generally bright coloration is wanting in desert animals, yet
+  here heat and light are both at a maximum, and if these alone
+  were the agents in the production of colour, desert animals
+  should be the most brilliant. Again, all naturalists who have
+  lived in tropical regions know that the proportion of bright to
+  dull coloured species is little if any greater there than in the
+  temperate zone, while there are many tropical groups in which
+  bright colours are almost entirely unknown. No part of the world
+  presents so many brilliant birds as South America, yet there are
+  extensive families, containing many hundreds of species, which
+  are as plainly coloured as our average temperate birds. Such are
+  the families of the bush-shrikes and ant-thrushes
+  (Formicariidae), the tyrant-shrikes (Tyrannidae), the American
+  creepers (Dendrocolaptidae), together with a large proportion of
+  the wood-warblers (Mniotiltidae), the finches, the wrens, and
+  some other groups. In the eastern hemisphere, also, we have the
+  babbling-thrushes (Timaliidae), the cuckoo-shrikes
+  (Campephagidae), the honey-suckers (Meliphagidae), and several
+  other smaller groups which are certainly not coloured above the
+  average standard of temperate birds.</p>
+
+  <p>Again, there are many families of birds which spread over the
+  whole world, temperate and tropical, and among these the tropical
+  species rarely present any exceptional brilliancy of colour. Such
+  are the thrushes, goatsuckers, hawks, plovers, and ducks; and in
+  the last-named group it is the temperate and arctic zones that
+  afford the most brilliant coloration.</p>
+
+  <p>The same general facts are found to prevail among insects.
+  Although tropical insects present some of the most gorgeous
+  coloration in the whole realm of nature, yet there are thousands
+  and tens of thousands of species which are as dull coloured as
+  any in our cloudy land. The extensive family of the carnivorous
+  ground-beetles (Carabidae) attains its greatest brilliancy in the
+  temperate zone; while by far the larger proportion of the great
+  families of the longicorns and the weevils, are of obscure
+  colours even in the tropics. In butterflies, there is undoubtedly
+  a larger proportion of brilliant colour in the tropics; but if we
+  compare families which are almost equally developed over the
+  globe&mdash;as the Pieridae or <a name="Page_195" id=
+  "Page_195"></a>whites and yellows, and the Satyridae or
+  ringlets&mdash;we shall find no great disproportion in colour
+  between those of temperate and tropical regions.</p>
+
+  <p>The various facts which have now briefly been noticed are
+  sufficient to indicate that the light and heat of the sun are not
+  the direct causes of the colours of animals, although they may
+  favour the production of colour when, as in tropical regions, the
+  persistent high temperature favours the development of the
+  maximum of life. We will now consider the next suggestion, that
+  light reflected from surrounding coloured objects tends to
+  produce corresponding colours in the animal world.</p>
+
+  <p>This theory is founded on a number of very curious facts which
+  prove, that such a change does sometimes occur and is directly
+  dependent on the colours of surrounding objects; but these facts
+  are comparatively rare and exceptional in their nature, and the
+  same theory will certainly not apply to the infinitely varied
+  colours of the higher animals, many of which are exposed to a
+  constantly varying amount of light and colour during their active
+  existence. A brief sketch of these dependent changes of colour
+  may, however, be advantageously given here.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variable Protective Colouring.</i></p>
+
+  <p>There are two distinct kinds of change of colour in animals
+  due to the colouring of the environment. In one case the change
+  is caused by reflex action set up by the animal <i>seeing</i> the
+  colour to be imitated, and the change produced can be altered or
+  repeated as the animal changes its position. In the other case
+  the change occurs but once, and is probably not due to any
+  conscious or sense action, but to some direct influence on the
+  surface tissues while the creature is undergoing a moult or
+  change to the pupa form.</p>
+
+  <p>The most striking example of the first class is that of the
+  chameleon, which changes to white, brown, yellowish, or green,
+  according to the colour of the object on which it rests. This
+  change is brought about by means of two layers of pigment cells,
+  deeply seated in the skin, and of bluish and yellowish colours.
+  By suitable muscles these cells can be forced upwards so as to
+  modify the colour of the skin, which, <a name="Page_196" id=
+  "Page_196"></a>when they are not brought into action, is a dirty
+  white. These animals are excessively sluggish and defenceless,
+  and the power of changing their colour to that of their immediate
+  surroundings is no doubt of great service to them. Many of the
+  flatfish are also capable of changing their colour according to
+  the colour of the bottom they rest on; and frogs have a similar
+  power to a limited extent. Some crustacea also change colour, and
+  the power is much developed in the Chameleon shrimp (Mysis
+  Chamaeleon) which is gray when on sand, but brown or green when
+  among brown or green seaweed. It has been proved by experiment
+  that when this animal is blinded the change does not occur. In
+  all these cases, therefore, we have some form of reflex or sense
+  action by which the change is produced, probably by means of
+  pigment cells beneath the skin as in the chameleon.</p>
+
+  <p>The second class consists of certain larvae, and pupae, which
+  undergo changes of colour when exposed to differently coloured
+  surroundings. This subject has been carefully investigated by Mr.
+  E.B. Poulton, who has communicated the results of his experiments
+  to the Royal Society.<a name="FNanchor_65_65" id=
+  "FNanchor_65_65"></a><a href="#Footnote_65_65" class=
+  "fnanchor">[65]</a> It had been noticed that some species of
+  larvae which fed on several different plants had colours more or
+  less corresponding to the particular plant the individual fed on.
+  Numerous cases are given in Professor Meldola's article on
+  "Variable Protective Colouring" (<i>Proc. Zool. Soc.</i>, 1873,
+  p. 153), and while the general green coloration was attributed to
+  the presence of chlorophyll beneath the skin, the particular
+  change in correspondence to each food-plant was attributed to a
+  special function which had been developed by natural selection.
+  Later on, in a note to his translation of Weissmann's <i>Theory
+  of Descent</i>, Professor Meldola seemed disposed to think that
+  the variations of colour of some of the species might be
+  phytophagic&mdash;that is, due to the direct action of the
+  differently coloured leaves on which the insect fed. Mr.
+  Poulton's experiments have thrown much light on this question,
+  since he has conclusively proved that, in the case of the sphinx
+  caterpillar of Smerinthus ocellatus, the change of colour is not
+  due to the food but to the coloured light reflected from the
+  leaves.</p>
+
+  <p><a name="Page_197" id="Page_197"></a>This was shown by feeding
+  two sets of larvae on the same plant but exposed to differently
+  coloured surroundings, obtained by sewing the leaves together, so
+  that in one case only the dark upper surface, in the other the
+  whitish under surface was exposed to view. The result in each
+  case was a corresponding change of colour in the larvae,
+  confirming the experiments on different individuals of the same
+  batch of larvae which had been supplied with different
+  food-plants or exposed to a different coloured light.</p>
+
+  <p>An even more interesting series of experiments was made on the
+  colours of pupae, which in many cases were known to be affected
+  by the material on which they underwent their transformations.
+  The late Mr. T.W. Wood proved, in 1867, that the pupae of the
+  common cabbage butterflies (Pieris brassicae and P. rapae) were
+  either light, or dark, or green, according to the coloured boxes
+  they were kept in, or the colours of the fences, walls, etc.,
+  against which they were suspended. Mrs. Barber in South Africa
+  found that the pupae of Papilio Nireus underwent a similar
+  change, being deep green when attached to orange leaves of the
+  same tint, pale yellowish-green when on a branch of the
+  bottle-brush tree whose half-dried leaves were of this colour,
+  and yellowish when attached to the wooden frame of a box. A few
+  other observers noted similar phenomena, but nothing more was
+  done till Mr. Poulton's elaborate series of experiments with the
+  larvae of several of our common butterflies were the means of
+  clearing up several important points. He showed that the action
+  of the coloured light did not affect the pupa itself but the
+  larva, and that only for a limited period of time. After a
+  caterpillar has done feeding it wanders about seeking a suitable
+  place to undergo its transformation. When this is found it rests
+  quietly for a day or two, spinning the web from which it is to
+  suspend itself; and it is during this period of quiescence, and
+  perhaps also the first hour or two after its suspension, that the
+  action of the surrounding coloured surfaces determines, to a
+  considerable extent, the colour of the pupa. By the application
+  of various surrounding colours during this period, Mr. Poulton
+  was able to modify the colour of the pupa of the common
+  tortoise-shell butterfly from nearly black to pale, or to a
+  brilliant golden; and that of Pieris rapae <a name="Page_198" id=
+  "Page_198"></a>from dusky through pinkish to pale green. It is
+  interesting to note, that the colours produced were in all cases
+  such only as assimilated with the surroundings usually occupied
+  by the species, and also, that colours which did not occur in
+  such surroundings, as dark red or blue, only produced the same
+  effects as dusky or black.</p>
+
+  <p>Careful experiments were made to ascertain whether the effect
+  was produced through the sight of the caterpillar. The ocelli
+  were covered with black varnish, but neither this, nor cutting
+  off the spines of the tortoise-shell larva to ascertain whether
+  they might be sense-organs, produced any effect on the resulting
+  colour. Mr. Poulton concludes, therefore, that the colour-action
+  probably occurs over the whole surface of the body, setting up
+  physiological processes which result in the corresponding
+  colour-change of the pupa. Such changes are, however, by no means
+  universal, or even common, in protectively coloured pupae, since
+  in Papilio machaon and some others which have been experimented
+  on, both in this country and abroad, no change can be produced on
+  the pupa by any amount of exposure to differently coloured
+  surroundings. It is a curious point that, with the small
+  tortoise-shell larva, exposure to light from gilded surfaces
+  produced pupae with a brilliant golden lustre; and the
+  explanation is supposed to be that mica abounded in the original
+  habitat of the species, and that the pupae thus obtained
+  protection when suspended against micaceous rock. Looking,
+  however, at the wide range of the species and the comparatively
+  limited area in which micaceous rocks occur, this seems a rather
+  improbable explanation, and the occurrence of this metallic
+  appearance is still a difficulty. It does not, however, commonly
+  occur in this country in a natural state.</p>
+
+  <p>The two classes of variable colouring here discussed are
+  evidently exceptional, and can have little if any relation to the
+  colours of those more active creatures which are continually
+  changing their position with regard to surrounding objects, and
+  whose colours and markings are nearly constant throughout the
+  life of the individual, and (with the exception of sexual
+  differences) in all the individuals of the species. We will now
+  briefly pass in review the various characteristics and uses of
+  the colours which more generally prevail in nature; <a name=
+  "Page_199" id="Page_199"></a>and having already discussed those
+  protective colours which serve to harmonise animals with their
+  general environment, we have to consider only those cases in
+  which the colour resemblance is more local or special in its
+  character.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Special or Local Colour Adaptations.</i></p>
+
+  <p>This form of colour adaptation is generally manifested by
+  markings rather than by colour alone, and is extremely prevalent
+  both among insects and vertebrates, so that we shall be able to
+  notice only a few illustrative cases. Among our native birds we
+  have the snipe and woodcock, whose markings and tints strikingly
+  accord with the dead marsh vegetation among which they live; the
+  ptarmigan in its summer dress is mottled and tinted exactly like
+  the lichens which cover the stones of the higher mountains; while
+  young unfledged plovers are spotted so as exactly to resemble the
+  beach pebbles among which they crouch for protection, as
+  beautifully exhibited in one of the cases of British birds in the
+  Natural History Museum at South Kensington.</p>
+
+  <p>In mammalia, we notice the frequency of rounded spots on
+  forest or tree haunting animals of large size, as the forest deer
+  and the forest cats; while those that frequent reedy or grassy
+  places are striped vertically, as the marsh antelopes and the
+  tiger. I had long been of opinion that the brilliant yellow and
+  black stripes of the tiger were adaptive, but have only recently
+  obtained proof that it is so. An experienced tiger-hunter, Major
+  Walford, states in a letter, that the haunts of the tiger are
+  invariably full of the long grass, dry and pale yellow for at
+  least nine months of the year, which covers the ground wherever
+  there is water in the rainy season, and he adds: "I once, while
+  following up a wounded tiger, failed for at least a minute to see
+  him under a tree in grass at a distance of about twenty
+  yards&mdash;jungle open&mdash;but the natives saw him, and I
+  eventually made him out well enough to shoot him, but even then I
+  could not see at what part of him I was aiming. There can be no
+  doubt whatever that the colour of both the tiger and the panther
+  renders them almost invisible, especially in a strong blaze of
+  light, when among grass, and one does not seem to notice stripes
+  or spots till they are dead." It is the black shadows of the
+  vegetation that <a name="Page_200" id="Page_200"></a>assimilate
+  with the black stripes of the tiger; and, in like manner, the
+  spotty shadows of leaves in the forest so harmonise with the
+  spots of ocelots, jaguars, tiger-cats, and spotted deer as to
+  afford them a very perfect concealment.</p>
+
+  <p>In some cases the concealment is effected by colours and
+  markings which are so striking and peculiar that no one who had
+  not seen the creature in its native haunts would imagine them to
+  be protective. An example of this is afforded by the banded fruit
+  pigeon of Timor, whose pure white head and neck, black wings and
+  back, yellow belly, and deeply-curved black band across the
+  breast, render it a very handsome and conspicuous bird. Yet this
+  is what Mr. H.O. Forbes says of it: "On the trees the
+  white-headed fruit pigeon (Ptilopus cinctus) sate motionless
+  during the heat of the day in numbers, on well-exposed branches;
+  but it was with the utmost difficulty that I or my sharp-eyed
+  native servant could ever detect them, even in trees where we
+  knew they were sitting."<a name="FNanchor_66_66" id=
+  "FNanchor_66_66"></a><a href="#Footnote_66_66" class=
+  "fnanchor">[66]</a> The trees referred to are species of
+  Eucalyptus which abound in Timor. They have whitish or yellowish
+  bark and very open foliage, and it is the intense sunlight
+  casting black curved shadows of one branch upon another, with the
+  white and yellow bark and deep blue sky seen through openings of
+  the foliage, that produces the peculiar combination of colours
+  and shadows to which the colours and markings of this bird have
+  become so closely assimilated.</p>
+
+  <p>Even such brilliant and gorgeously coloured birds as the
+  sun-birds of Africa are, according to an excellent observer,
+  often protectively coloured. Mrs. M.E. Barber remarks that "A
+  casual observer would scarcely imagine that the highly varnished
+  and magnificently coloured plumage of the various species of
+  Noctarinea could be of service to them, yet this is undoubtedly
+  the case. The most unguarded moments of the lives of these birds
+  are those that are spent amongst the flowers, and it is then that
+  they are less wary than at any other time. The different species
+  of aloes, which blossom in succession, form the principal sources
+  of their winter supplies of food; and a legion of other gay
+  flowering plants in spring and summer, the aloe blossoms
+  especially, are all brilliantly coloured, and they harmonise
+  admirably with the gay plumage <a name="Page_201" id=
+  "Page_201"></a>of the different species of sun-birds. Even the
+  keen eye of a hawk will fail to detect them, so closely do they
+  resemble the flowers they frequent. The sun-birds are fully aware
+  of this fact, for no sooner have they relinquished the flowers
+  than they become exceedingly wary and rapid in flight, darting
+  arrow-like through the air and seldom remaining in exposed
+  situations. The black sun-bird (Nectarinea amethystina) is never
+  absent from that magnificent forest-tree, the 'Kaffir Boom'
+  (Erythrina caffra); all day long the cheerful notes of these
+  birds may be heard amongst its spreading branches, yet the
+  general aspect of the tree, which consists of a huge mass of
+  scarlet and purple-black blossoms without a single green leaf,
+  blends and harmonises with the colours of the black sun-bird to
+  such an extent that a dozen of them may be feeding amongst its
+  blossoms without being conspicuous, or even visible."<a name=
+  "FNanchor_67_67" id="FNanchor_67_67"></a><a href=
+  "#Footnote_67_67" class="fnanchor">[67]</a></p>
+
+  <p>Some other cases will still further illustrate how the colours
+  of even very conspicuous animals may be adapted to their peculiar
+  haunts.</p>
+
+  <p>The late Mr. Swinhoe says of the Kerivoula picta, which he
+  observed in Formosa: "The body of this bat was of an orange
+  colour, but the wings were painted with orange-yellow and black.
+  It was caught suspended, head downwards, on a cluster of the
+  fruit of the longan tree (Nephelium longanum). Now this tree is
+  an evergreen, and all the year round some portion of its foliage
+  is undergoing decay, the particular leaves being, in such a
+  stage, partially orange and black. This bat can, therefore, at
+  all seasons suspend from its branches and elude its enemies by
+  its resemblance to the leaves of the tree."<a name=
+  "FNanchor_68_68" id="FNanchor_68_68"></a><a href=
+  "#Footnote_68_68" class="fnanchor">[68]</a></p>
+
+  <p>Even more curious is the case of the sloths&mdash;defenceless
+  animals which feed upon leaves, and hang from the branches of
+  trees with their back downwards. Most of the species have a
+  curious buff-coloured spot on the back, rounded or oval in shape
+  and often with a darker border, which seems placed there on
+  purpose to make them conspicuous; and this was a great puzzle to
+  naturalists, because the long coarse gray or greenish hair was
+  evidently like tree-moss and therefore protective. But an old
+  writer, Baron von Slack, in his <i>Voyage</i> <a name="Page_202"
+  id="Page_202"></a><i>to Surinam</i> (1810), had already explained
+  the matter. He says: "The colour and even the shape of the hair
+  are much like withered moss, and serve to hide the animal in the
+  trees, but particularly when it has that orange-coloured spot
+  between the shoulders and lies close to the tree; it looks then
+  exactly like a piece of branch where the rest has been broken
+  off, by which the hunters are often deceived." Even such a huge
+  animal as the giraffe is said to be perfectly concealed by its
+  colour and form when standing among the dead and broken trees
+  that so often occur on the outskirts of the thickets where it
+  feeds. The large blotch-like spots on the skin and the strange
+  shape of the head and horns, like broken branches, so tend to its
+  concealment that even the keen-eyed natives have been known to
+  mistake trees for giraffes or giraffes for trees.</p>
+
+  <p>Innumerable examples of this kind of protective colouring
+  occur among insects; beetles mottled like the bark of trees or
+  resembling the sand or rock or moss on which they live, with
+  green caterpillars of the exact general tints of the foliage they
+  feed on; but there are also many cases of detailed imitation of
+  particular objects by insects that must be briefly
+  described.<a name="FNanchor_69_69" id=
+  "FNanchor_69_69"></a><a href="#Footnote_69_69" class=
+  "fnanchor">[69]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Protective Imitation of Particular Objects.</i></p>
+
+  <p>The insects which present this kind of imitation most
+  perfectly are the Phasmidae, or stick and leaf insects. The
+  well-<a name="Page_203" id="Page_203"></a>known leaf-insects of
+  Ceylon and of Java, species of Phyllium, are so wonderfully
+  coloured and veined, with leafy expansions on the legs and
+  thorax, that not one person in ten can see them when resting on
+  the food-plant close beneath their eyes. Others resemble pieces
+  of stick with all the minutiae of knots and branches, formed by
+  the insects' legs, which are stuck out rigidly and
+  unsymmetrically. I have often been unable to distinguish between
+  one of these insects and a real piece of stick, till I satisfied
+  myself by touching it and found it to be alive. One species,
+  which was brought me in Borneo, was covered with delicate
+  semitransparent green foliations, exactly resembling the
+  hepaticae which cover pieces of rotten stick in the damp forests.
+  Others resemble dead leaves in all their varieties of colour and
+  form; and to show how perfect is the protection obtained and how
+  important it is to the possessors of it, the following incident,
+  observed by Mr. Belt in Nicaragua, is most instructive.
+  Describing the armies of foraging ants in the forest which devour
+  every insect they can catch, he says: "I was much surprised with
+  the behaviour of a green leaf-like locust. This insect stood
+  immovably among a host of ants, many of which ran over its legs
+  without ever discovering there was food within their reach. So
+  fixed was its instinctive knowledge that its safety depended on
+  its immovability, that it allowed me to pick it up and replace it
+  among the ants without making a single effort to escape. This
+  species closely resembles a green leaf."<a name="FNanchor_70_70"
+  id="FNanchor_70_70"></a><a href="#Footnote_70_70" class=
+  "fnanchor">[70]</a></p>
+
+  <p>Caterpillars also exhibit a considerable amount of detailed
+  resemblance to the plants on which they live. Grass-feeders are
+  striped longitudinally, while those on ordinary leaves are always
+  striped obliquely. Some very beautiful protective resemblances
+  are shown among the caterpillars figured in Smith and Abbott's
+  <i>Lepidopterous Insects of Georgia</i>, a work published in the
+  early part of the century, before any theories of protection were
+  started. The plates in this work are most beautifully executed
+  from drawings made by Mr. Abbott, representing the insects, in
+  every case, on the plants which they frequented, and no reference
+  is made in the descriptions to the remarkable protective details
+  which appear upon the plates. We have, first, the larva of Sphinx
+  fuciformis feeding <a name="Page_204" id="Page_204"></a>on a
+  plant with linear grass-like leaves and small blue flowers; and
+  we find the insect of the same green as the leaves, striped
+  longitudinally in accordance with the linear leaves, and with the
+  head blue corresponding both in size and colour with the flowers.
+  Another species (Sphinx tersa) is represented feeding on a plant
+  with small red flowers situated in the axils of the leaves; and
+  the larva has a row of seven red spots, unequal in size, and
+  corresponding very closely with the colour and size of the
+  flowers. Two other figures of sphinx larvae are very curious.
+  That of Sphinx pampinatrix feeds on a wild vine (Vitis indivisa),
+  having green tendrils, and in this species the curved horn on the
+  tail is green, and closely imitates in its curve the tip of the
+  tendril. But in another species (Sphinx cranta), which feeds on
+  the fox-grape (Vitis vulpina), the horn is very long and red,
+  corresponding with the long red-tipped tendrils of the plant.
+  Both these larvae are green with oblique stripes, to harmonise
+  with the veined leaves of the vines; but a figure is also given
+  of the last-named species after it has done feeding, when it is
+  of a decided brown colour and has entirely lost its horn. This is
+  because it then descends to the ground to bury itself, and the
+  green colour and red horn would be conspicuous and dangerous; it
+  therefore loses both at the last moult. Such a change of colour
+  occurs in many species of caterpillars. Sometimes the change is
+  seasonal; and, in those which hibernate with us, the colour of
+  some species, which is brownish in autumn in adaptation to the
+  fading foliage, becomes green in spring to harmonise with the
+  newly-opened leaves at that season.<a name="FNanchor_71_71" id=
+  "FNanchor_71_71"></a><a href="#Footnote_71_71" class=
+  "fnanchor">[71]</a></p>
+
+  <p>Some of the most curious examples of minute imitation are
+  afforded by the caterpillars of the geometer moths, which are
+  always brown or reddish, and resemble in form little twigs of the
+  plant on which they feed. They have the habit, when at rest, of
+  standing out obliquely from the branch, to which they hold on by
+  their hind pair of prolegs or claspers, and remain motionless for
+  hours. Speaking of these protective resemblances Mr. Jenner Weir
+  says: "After being thirty years an entomologist I was deceived
+  myself, and took out my pruning scissors to cut from a plum tree
+  a spur which I thought I had overlooked. This turned out to be
+  the larva <a name="Page_205" id="Page_205"></a>of a geometer two
+  inches long. I showed it to several members of my family, and
+  defined a space of four inches in which it was to be seen, but
+  none of them could perceive that it was a caterpillar."<a name=
+  "FNanchor_72_72" id="FNanchor_72_72"></a><a href=
+  "#Footnote_72_72" class="fnanchor">[72]</a></p>
+
+  <p>One more example of a protected caterpillar must be given. Mr.
+  A. Everett, writing from Sarawak, Borneo, says: "I had a
+  caterpillar brought me, which, being mixed by my boy with some
+  other things, I took to be a bit of moss with two exquisite
+  pinky-white seed-capsules; but I soon saw that it moved, and
+  examining it more closely found out its real character: it is
+  covered with hair, with two little pink spots on the upper
+  surface, the general hue being more green. Its motions are very
+  slow, and when eating the head is withdrawn beneath a fleshy
+  mobile hood, so that the action of feeding does not produce any
+  movement externally. It was found in the limestone hills at
+  Busan, the situation of all others where mosses are most
+  plentiful and delicate, and where they partially clothe most of
+  the protruding masses of rock."</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>How these Imitations have been Produced.</i></p>
+
+  <p>To many persons it will seem impossible that such beautiful
+  and detailed resemblances as those now described&mdash;and these
+  are only samples of thousands that occur in all parts of the
+  world&mdash;can have been brought about by the preservation of
+  accidental useful variations. But this will not seem so
+  surprising if we keep in mind the facts set forth in our earlier
+  chapters&mdash;the rapid multiplication, the severe struggle for
+  existence, and the constant variability of these and all other
+  organisms. And, further, we must remember that these delicate
+  adjustments are the result of a process which has been going on
+  for millions of years, and that we now see the small percentage
+  of successes among the myriads of failures. From the very first
+  appearance of insects and their various kinds of enemies the need
+  of protection arose, and was usually most easily met by
+  modifications of colour. Hence, we may be sure that the earliest
+  leaf-eating insects acquired a green colour as one of the
+  necessities of their existence; and, as the species became
+  modified and specialised, <a name="Page_206" id=
+  "Page_206"></a>those feeding on particular species of plants
+  would rapidly acquire the peculiar tints and markings best
+  adapted to conceal them upon those plants. Then, every little
+  variation that, once in a hundred years perhaps, led to the
+  preservation of some larva which was thereby rather better
+  concealed than its fellows, would form the starting-point of a
+  further development, leading ultimately to that perfection of
+  imitation in details which now astonishes us. The researches of
+  Dr. Weismann illustrate this progressive adaptation. The very
+  young larvae of several species are green or yellowish without
+  any markings; they then, in subsequent moults, obtain certain
+  markings, some of which are often lost again before the larva is
+  fully grown. The early stages of those species which, like
+  elephant hawk-moths (Chaerocampa), have the anterior segments
+  elongated and retractile, with large eye-like spots to imitate
+  the head of a vertebrate, are at first like those of
+  non-retractile species, the anterior segments being as large as
+  the rest. After the first moult they become smaller,
+  comparatively; but it is only after the second moult that the
+  ocelli begin to appear, and these are not fully defined till
+  after the third moult. This progressive development of the
+  individual&mdash;the ontogeny&mdash;gives us a clue to the
+  ancestral development of the whole race&mdash;the phylogeny; and
+  we are enabled to picture to ourselves the very slow and gradual
+  steps by which the existing perfect adaptation has been brought
+  about. In many larvae great variability still exists, and in some
+  there are two or more distinctly-coloured forms&mdash;usually a
+  dark and a light or a brown and a green form. The larva of the
+  humming-bird hawk-moth (Macroglossa stellatarum) varies in this
+  manner, and Dr. Weismann raised five varieties from a batch of
+  eggs from one moth. It feeds on species of bedstraw (Galium verum
+  and G. mollugo), and as the green forms are less abundant than
+  the brown, it has probably undergone some recent change of
+  food-plant or of habits which renders brown the more protective
+  colour.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Special Protective Colouring of Butterflies.</i></p>
+
+  <p>We will now consider a few cases of special protective
+  colouring in the perfect butterfly or moth. Mr. Mansel Weale
+  states that in South Africa there is a great prevalence <a name=
+  "Page_207" id="Page_207"></a>of white and silvery foliage or
+  bark, sometimes of dazzling brilliancy, and that many insects and
+  their larvae have brilliant silvery tints which are protective,
+  among them being three species of butterflies whose undersides
+  are silvery, and which are thus effectually protected when at
+  rest.<a name="FNanchor_73_73" id="FNanchor_73_73"></a><a href=
+  "#Footnote_73_73" class="fnanchor">[73]</a> A common African
+  butterfly (Aterica meleagris) always settles on the ground with
+  closed wings, which so closely resemble the soil of the district
+  that it can with difficulty be seen, and the colour varies with
+  the soil in different localities. Thus specimens from Senegambia
+  were dull brown, the soil being reddish sand and iron-clay; those
+  from Calabar and Cameroons were light brown with numerous small
+  white spots, the soil of those countries being light brown clay
+  with small quartz pebbles; while in other localities where the
+  colours of the soil were more varied the colours of the butterfly
+  varied also. Here we have variation in a single species which has
+  become specialised in certain areas to harmonise with the colour
+  of the soil.<a name="FNanchor_74_74" id=
+  "FNanchor_74_74"></a><a href="#Footnote_74_74" class=
+  "fnanchor">[74]</a></p>
+
+  <p>Many butterflies, in all parts of the world, resemble dead
+  leaves on their under side, but those in which this form of
+  protection is carried to the greatest perfection are the species
+  of the Eastern genus Kallima. In India K. inachis, and in the
+  larger Malay islands K. paralekta, are very common. They are
+  rather large and showy butterflies, orange and bluish on the
+  upper side, with a very rapid flight, and frequenting dry
+  forests. Their habit is to settle always where there is some dead
+  or decaying foliage, and the shape and colour of the wings (on
+  the under surface), together with the attitude of the insect, is
+  such as to produce an absolutely perfect imitation of a dead
+  leaf. This is effected by the butterfly always settling on a
+  twig, with the short tail of the hind wings just touching it and
+  forming the leaf-stalk. From this a dark curved line runs across
+  to the elongated tip of the upper wings, imitating the midrib, on
+  both sides of which are oblique lines, formed partly by the
+  nervures and partly by markings, which give the effect of the
+  usual veining of a leaf. The head and antennae fit exactly
+  between the closed upper wings so as not to interfere with the
+  outline, <a name="Page_208" id="Page_208"></a>which has just that
+  amount of irregular curvature that is seen in dry and withered
+  leaves. The colour is very remarkable for its extreme amount of
+  variability, from deep reddish-brown to olive or pale yellow,
+  hardly two specimens being exactly alike, but all coming within
+  the range of colour of leaves in various stages of decay. Still
+  more curious is the fact that the paler wings, which imitate
+  leaves most decayed, are usually covered with small black dots,
+  often gathered into circular groups, and so exactly resembling
+  the minute fungi on decaying leaves that it is hard at first to
+  believe that the insects themselves are not attacked by some such
+  fungus. The concealment produced by this wonderful imitation is
+  most complete, and in Sumatra I have often seen one enter a bush
+  and then disappear like magic. Once I was so fortunate as to see
+  the exact spot on which the insect settled; but even then I lost
+  sight of it for some time, and only after a persistent search
+  discovered that it was close before my eyes.<a name=
+  "FNanchor_75_75" id="FNanchor_75_75"></a><a href=
+  "#Footnote_75_75" class="fnanchor">[75]</a> Here we have a kind
+  of imitation, which is very common in a less developed form,
+  carried to extreme perfection, with the result that the species
+  is very abundant over a considerable area of country.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Protective Resemblance among Marine Animals.</i></p>
+
+  <p>Among marine animals this form of protection is very common.
+  Professor Moseley tells us that all the inhabitants of the
+  Gulf-weed are most remarkably coloured, for purposes of
+  protection and concealment, exactly like the weed itself. "The
+  shrimps and crabs which swarm in the weed are of exactly the same
+  shade of yellow as the weed, and have white markings upon their
+  bodies to represent the patches of Membranipora. The small fish,
+  Antennarius, is in the same way weed-colour with white spots.
+  Even a Planarian worm, which lives in the weed, is similarly
+  yellow-coloured, and also a mollusc, Scyllaea pelagica." The same
+  writer tells us that "a number of little crabs found clinging to
+  the floats of the blue-shelled mollusc, Ianthina, were all
+  coloured of a corresponding blue for concealment."<a name=
+  "FNanchor_76_76" id="FNanchor_76_76"></a><a href=
+  "#Footnote_76_76" class="fnanchor">[76]</a></p>
+
+  <p><a name="Page_209" id="Page_209"></a>Professor E.S. Morse of
+  Salem, Mass., found that most of the New England marine mollusca
+  were protectively coloured; instancing among others a little red
+  chiton on rocks clothed with red calcareous algae, and Crepidula
+  plana, living within the apertures of the shells of larger
+  species of Gasteropods and of a pure white colour corresponding
+  to its habitat, while allied species living on seaweed or on the
+  outside of dark shells were dark brown.<a name="FNanchor_77_77"
+  id="FNanchor_77_77"></a><a href="#Footnote_77_77" class=
+  "fnanchor">[77]</a> A still more interesting case has been
+  recorded by Mr. George Brady. He says: "Amongst the Nullipore
+  which matted together the laminaria roots in the Firth of Clyde
+  were living numerous small starfishes (Ophiocoma bellis) which,
+  except when their writhing movements betrayed them, were quite
+  undistinguishable from the calcareous branches of the alga; their
+  rigid angularly twisted rays had all the appearance of the
+  coralline, and exactly assimilated to its dark purple colour, so
+  that though I held in my hand a root in which were half a dozen
+  of the starfishes, I was really unable to detect them until
+  revealed by their movements."<a name="FNanchor_78_78" id=
+  "FNanchor_78_78"></a><a href="#Footnote_78_78" class=
+  "fnanchor">[78]</a></p>
+
+  <p>These few examples are sufficient to show that the principle
+  of protective coloration extends to the ocean as well as over the
+  earth; and if we consider how completely ignorant we are of the
+  habits and surroundings of most marine animals, it may well
+  happen that many of the colours of tropical fishes, which seem to
+  us so strange and so conspicuous, are really protective, owing to
+  the number of equally strange and brilliant forms of corals,
+  sea-anemones, sponges, and seaweeds among which they live.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Protection by Terrifying Enemies.</i></p>
+
+  <p>A considerable number of quite defenceless insects obtain
+  protection from some of their enemies by having acquired a
+  resemblance to dangerous animals, or by some threatening or
+  unusual appearance. This is obtained either by a modification of
+  shape, of habits, of colour, or of all combined. The simplest
+  form of this protection is the aggressive attitude of the
+  caterpillars of the Sphingidae, the forepart of the body <a name=
+  "Page_210" id="Page_210"></a>being erected so as to produce a
+  rude resemblance to the figure of a sphinx, hence the name of the
+  family. The protection is carried further by those species which
+  retract the first three segments and have large ocelli on each
+  side of the fourth segment, thus giving to the caterpillar, when
+  the forepart of its body is elevated, the appearance of a snake
+  in a threatening attitude.</p>
+
+  <p>The blood-red forked tentacle, thrown out of the neck of the
+  larvae of the genus Papilio when alarmed, is, no doubt, a
+  protection against the attacks of ichneumons, and may, perhaps,
+  also frighten small birds; and the habit of turning up the tail
+  possessed by the harmless rove-beetles (Staphylinidae), giving
+  the idea that they can sting, has, probably, a similar use. Even
+  an unusual angular form, like a crooked twig or inorganic
+  substance, may be protective; as Mr. Poulton thinks is the case
+  with the curious caterpillar of Notodonta ziczac, which, by means
+  of a few slight protuberances on its body, is able to assume an
+  angular and very unorganic-looking appearance. But perhaps the
+  most perfect example of this kind of protection is exhibited by
+  the large caterpillar of the Royal Persimmon moth (Bombyx regia),
+  a native of the southern states of North America, and known there
+  as the "Hickory-horned devil." It is a large green caterpillar,
+  often six inches long, ornamented with an immense crown of
+  orange-red tubercles, which, if disturbed, it erects and shakes
+  from side to side in a very alarming manner. In its native
+  country the negroes believe it to be as deadly as a rattlesnake,
+  whereas it is perfectly innocuous. The green colour of the body
+  suggests that its ancestors were once protectively coloured; but,
+  growing too large to be effectually concealed, it acquired the
+  habit of shaking its head about in order to frighten away its
+  enemies, and ultimately developed the crown of tentacles as an
+  addition to its terrifying powers. This species is beautifully
+  figured in Abbott and Smith's <i>Lepidopterous Insects of
+  Georgia</i>.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Alluring Coloration.</i></p>
+
+  <p>Besides those numerous insects which obtain protection through
+  their resemblance to the natural objects among which they live,
+  there are some whose disguise is not used for <a name="Page_211"
+  id="Page_211"></a>concealment, but as a direct means of securing
+  their prey by attracting them within the enemy's reach. Only a
+  few cases of this kind of coloration have yet been observed,
+  chiefly among spiders and mantidae; but, no doubt, if attention
+  were given to the subject in tropical countries, many more would
+  be discovered. Mr. H.O. Forbes has described a most interesting
+  example of this kind of simulation in Java. While pursuing a
+  large butterfly through the jungle, he was stopped by a dense
+  bush, on a leaf of which he observed one of the skipper
+  butterflies sitting on a bird's dropping. "I had often," he says,
+  "observed small Blues at rest on similar spots on the ground, and
+  have wondered what such a refined and beautiful family as the
+  Lycaenidae could find to enjoy, in food apparently so incongruous
+  for a butterfly. I approached with gentle steps, but ready net,
+  to see if possible how the present species was engaged. It
+  permitted me to get quite close, and even to seize it between my
+  fingers; to my surprise, however, part of the body remained
+  behind, adhering as I thought to the excreta. I looked closely,
+  and finally touched with my finger the excreta to find if it were
+  glutinous. To my delighted astonishment I found that my eyes had
+  been most perfectly deceived, and that what seemed to be the
+  excreta was a most artfully coloured spider, lying on its back
+  with its feet crossed over and closely adpressed to the body."
+  Mr. Forbes then goes on to describe the exact appearance of such
+  excreta, and how the various parts of the spider are coloured to
+  produce the imitation, even to the liquid portion which usually
+  runs a little down the leaf. This is exactly imitated by a
+  portion of the thin web which the spider first spins to secure
+  himself firmly to the leaf; thus producing, as Mr. Forbes
+  remarks, a living bait for butterflies and other insects so
+  artfully contrived as to deceive a pair of human eyes, even when
+  intently examining it.<a name="FNanchor_79_79" id=
+  "FNanchor_79_79"></a><a href="#Footnote_79_79" class=
+  "fnanchor">[79]</a></p>
+
+  <p>A native species of spider (Thomisus citreus) exhibits a
+  somewhat similar alluring protection by its close resemblance to
+  buds of the wayfaring tree, Viburnum lantana. It is pure
+  creamy-white, the abdomen exactly resembling in shape and colour
+  the unopened buds of the flowers among which it takes <a name=
+  "Page_212" id="Page_212"></a>its station; and it has been seen to
+  capture flies which came to the flowers.</p>
+
+  <p>But the most curious and beautiful case of alluring protection
+  is that of a wingless Mantis in India, which is so formed and
+  coloured as to resemble a pink orchis or some other fantastic
+  flower. The whole insect is of a bright pink colour, the large
+  and oval abdomen looking like the labellum of an orchid. On each
+  side, the two posterior legs have immensely dilated and flattened
+  thighs which represent the petals of a flower, while the neck and
+  forelegs imitate the upper sepal and column of an orchid. The
+  insect rests motionless, in this symmetrical attitude, among
+  bright green foliage, being of course very conspicuous, but so
+  exactly resembling a flower that butterflies and other insects
+  settle upon it and are instantly captured. It is a living trap,
+  baited in the most alluring manner to catch the unwary
+  flower-haunting insects.<a name="FNanchor_80_80" id=
+  "FNanchor_80_80"></a><a href="#Footnote_80_80" class=
+  "fnanchor">[80]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Coloration of Birds' Eggs.</i></p>
+
+  <p>The colours of birds' eggs have long been a difficulty on the
+  theory of adaptive coloration, because, in so many cases it has
+  not been easy to see what can be the use of the particular
+  colours, which are often so bright and conspicuous that they seem
+  intended to attract attention rather than to be concealed. A more
+  careful consideration of the subject in all its bearings shows,
+  however, that here too, in a great number of cases, we have
+  examples of protective coloration. When, therefore, we cannot see
+  the meaning of the colour, we may suppose that it has been
+  protective in some ancestral form, and, not being hurtful, has
+  persisted under changed conditions which rendered the protection
+  needless.</p>
+
+  <p>We may divide all eggs, for our present purpose, into two
+  <a name="Page_213" id="Page_213"></a>great divisions; those which
+  are white or nearly so, and those which are distinctly coloured
+  or spotted. Egg-shells being composed mainly of carbonate of
+  lime, we may assume that the primitive colour of birds' eggs was
+  white, a colour that prevails now among the other egg-bearing
+  vertebrates&mdash;lizards, crocodiles, turtles, and snakes; and
+  we might, therefore, expect that this colour would continue where
+  its presence had no disadvantages. Now, as a matter of fact, we
+  find that in all the groups of birds which lay their eggs in
+  concealed places, whether in holes of trees or in the ground, or
+  in domed or covered nests, the eggs are either pure white or of
+  very pale uniform coloration. Such is the case with kingfishers,
+  bee-eaters, penguins, and puffins, which nest in holes in the
+  ground; with the great parrot family, the woodpeckers, the
+  rollers, hoopoes, trogons, owls, and some others, which build in
+  holes in trees or other concealed places; while martins, wrens,
+  willow-warblers, and Australian finches, build domed or covered
+  nests, and usually have white eggs.</p>
+
+  <p>There are, however, many other birds which lay their white
+  eggs in open nests; and these afford some very interesting
+  examples of the varied modes by which concealment may be
+  obtained. All the duck tribe, the grebes, and the pheasants
+  belong to this class; but these birds all have the habit of
+  covering their eggs with dead leaves or other material whenever
+  they leave the nest, so as effectually to conceal them. Other
+  birds, as the short-eared owl, the goatsucker, the partridge, and
+  some of the Australian ground pigeons, lay their white or pale
+  eggs on the bare soil; but in these cases the birds themselves
+  are protectively coloured, so that, when sitting, they are almost
+  invisible; and they have the habit of sitting close and almost
+  continuously, thus effectually concealing their eggs.</p>
+
+  <p>Pigeons and doves offer a very curious case of the protection
+  of exposed eggs. They usually build very slight and loose nests
+  of sticks and twigs, so open that light can be seen through them
+  from below, while they are generally well concealed by foliage
+  above. Their eggs are white and shining; yet it is a difficult
+  matter to discover, from beneath, whether there are eggs in the
+  nest or not, while they are well hidden by the thick foliage
+  above. The Australian podargi<a name="Page_214" id=
+  "Page_214"></a>huge goatsuckers&mdash;build very similar nests,
+  and their white eggs are protected in the same manner. Some large
+  and powerful birds, as the swans, herons, pelicans, cormorants,
+  and storks, lay white eggs in open nests; but they keep careful
+  watch over them, and are able to drive away intruders. On the
+  whole, then, we see that, while white eggs are conspicuous, and
+  therefore especially liable to attack by egg-eating animals, they
+  are concealed from observation in many and various ways. We may,
+  therefore, assume that, in cases where there seems to be no such
+  concealment, we are too ignorant of the whole of the conditions
+  to form a correct judgment.</p>
+
+  <p>We now come to the large class of coloured or richly spotted
+  eggs, and here we have a more difficult task, though many of them
+  decidedly exhibit protective tints or markings. There are two
+  birds which nest on sandy shores&mdash;the lesser tern and the
+  ringed plover,&mdash;and both lay sand-coloured eggs, the former
+  spotted so as to harmonise with coarse shingle, the latter
+  minutely speckled like fine sand, which are the kinds of ground
+  the two birds choose respectively for their nests. "The common
+  sandpipers' eggs assimilate so closely with the tints around them
+  as to make their discovery a matter of no small difficulty, as
+  every oologist can testify who has searched for them. The pewits'
+  eggs, dark in ground colour and boldly marked, are in strict
+  harmony with the sober tints of moor and fallow, and on this
+  circumstance alone their concealment and safety depend. The
+  divers' eggs furnish another example of protective colour; they
+  are generally laid close to the water's edge, amongst drift and
+  shingle, where their dark tints and black spots conceal them by
+  harmonising closely with surrounding objects. The snipes and the
+  great army of sandpipers furnish innumerable instances of
+  protectively coloured eggs. In all the instances given the
+  sitting-bird invariably leaves the eggs uncovered when it quits
+  them, and consequently their safety depends solely on the colours
+  which adorn them."<a name="FNanchor_81_81" id=
+  "FNanchor_81_81"></a><a href="#Footnote_81_81" class=
+  "fnanchor">[81]</a> The wonderful range of colour and marking in
+  the eggs of the guillemot may be imputed to the inaccessible
+  rocks on which <a name="Page_215" id="Page_215"></a>it breeds,
+  giving it complete protection from enemies. Thus the pale or
+  bluish ground colour of the eggs of its allies, the auks and
+  puffins, has become intensified and blotched and spotted in the
+  most marvellous variety of patterns, owing to there being no
+  selective agency to prevent individual variation having full
+  sway.</p>
+
+  <p>The common black coot (Fulica atra) has eggs which are
+  coloured in a specially protective manner. Dr. William Marshall
+  writes, that it only breeds in certain localities where a large
+  water reed (Phragmites arundinacea) abounds. The eggs of the coot
+  are stained and spotted with black on a yellowish-gray ground,
+  and the dead leaves of the reed are of the same colour, and are
+  stained black by small parasitic fungi of the Uredo family; and
+  these leaves form the bed on which the eggs are laid. The eggs
+  and the leaves agree so closely in colour and markings that it is
+  a difficult thing to distinguish the eggs at any distance. It is
+  to be noted that the coot never covers up its eggs, as its ally
+  the moor-hen usually does.</p>
+
+  <p>The beautiful blue or greenish eggs of the hedge-sparrow, the
+  song-thrush, and sometimes those of the blackbird, seem at first
+  sight especially calculated to attract attention, but it is very
+  doubtful whether they are really so conspicuous when seen at a
+  little distance among their usual surroundings. For the nests of
+  these birds are either in evergreens, as holly or ivy, or
+  surrounded by the delicate green tints of our early spring
+  vegetation, and may thus harmonise very well with the colours
+  around them. The great majority of the eggs of our smaller birds
+  are so spotted or streaked with brown or black on variously
+  tinted grounds that, when lying in the shadow of the nest and
+  surrounded by the many colours and tints of bark and moss, of
+  purple buds and tender green or yellow foliage, with all the
+  complex glittering lights and mottled shades produced among these
+  by the spring sunshine and by sparkling raindrops, they must have
+  a quite different aspect from that which they possess when we
+  observe them torn from their natural surroundings. We have here,
+  probably, a similar case of general protective harmony to that of
+  the green caterpillars with beautiful white or purple bands and
+  spots, which, though gaudily conspicuous when seen alone,
+  <a name="Page_216" id="Page_216"></a>become practically invisible
+  among the complex lights and shadows of the foliage they feed
+  upon.</p>
+
+  <p>In the case of the cuckoo, which lays its eggs in the nests of
+  a variety of other birds, the eggs themselves are subject to
+  considerable variations of colour, the most common type, however,
+  resembling those of the pipits, wagtails, or warblers, in whose
+  nests they are most frequently laid. It also often lays in the
+  nest of the hedge-sparrow, whose bright blue eggs are usually not
+  at all nearly matched, although they are sometimes said to be so
+  on the Continent. It is the opinion of many ornithologists that
+  each female cuckoo lays the same coloured eggs, and that it
+  usually chooses a nest the owners of which lay somewhat similar
+  eggs, though this is by no means universally the case. Although
+  birds which have cuckoos' eggs imposed upon them do not seem to
+  neglect them on account of any difference of colour, yet they
+  probably do so occasionally; and if, as seems probable, each
+  bird's eggs are to some extent protected by their harmony of
+  colour with their surroundings, the presence of a larger and very
+  differently coloured egg in the nest might be dangerous, and lead
+  to the destruction of the whole set. Those cuckoos, therefore,
+  which most frequently placed their eggs among the kinds which
+  they resembled, would in the long run leave most progeny, and
+  thus the very frequent accord in colour might have been brought
+  about.</p>
+
+  <p>Some writers have suggested that the varied colours of birds'
+  eggs are primarily due to the effect of surrounding coloured
+  objects on the female bird during the period preceding
+  incubation; and have expended much ingenuity in suggesting the
+  objects that may have caused the eggs of one bird to be blue,
+  another brown, and another pink.<a name="FNanchor_82_82" id=
+  "FNanchor_82_82"></a><a href="#Footnote_82_82" class=
+  "fnanchor">[82]</a> But no evidence has been presented to prove
+  that any effects whatever are produced by this cause, while there
+  seems no difficulty in accounting for the facts by individual
+  variability and the action of natural selection. The changes that
+  occur in the conditions of existence of birds must sometimes
+  render the concealment less perfect than it may once have been;
+  and when any danger arises from this cause, it may be met either
+  <a name="Page_217" id="Page_217"></a>by some change in the colour
+  of the eggs, or in the structure or position of the nest, or by
+  the increased care which the parents bestow upon the eggs. In
+  this way the various divergences which now so often puzzle us may
+  have arisen.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Colour as a Means of Recognition.</i></p>
+
+  <p>If we consider the habits and life-histories of those animals
+  which are more or less gregarious, comprising a large proportion
+  of the herbivora, some carnivora, and a considerable number of
+  all orders of birds, we shall see that a means of ready
+  recognition of its own kind, at a distance or during rapid
+  motion, in the dusk of twilight or in partial cover, must be of
+  the greatest advantage and often lead to the preservation of
+  life. Animals of this kind will not usually receive a stranger
+  into their midst. While they keep together they are generally
+  safe from attack, but a solitary straggler becomes an easy prey
+  to the enemy; it is, therefore, of the highest importance that,
+  in such a case, the wanderer should have every facility for
+  discovering its companions with certainty at any distance within
+  the range of vision.</p>
+
+  <p>Some means of easy recognition must be of vital importance to
+  the young and inexperienced of each flock, and it also enables
+  the sexes to recognise their kind and thus avoid the evils of
+  infertile crosses; and I am inclined to believe that its
+  necessity has had a more widespread influence in determining the
+  diversities of animal coloration than any other cause whatever.
+  To it may probably be imputed the singular fact that, whereas
+  bilateral symmetry of coloration is very frequently lost among
+  domesticated animals, it almost universally prevails in a state
+  of nature; for if the two sides of an animal were unlike, and the
+  diversity of coloration among domestic animals occurred in a wild
+  state, easy recognition would be impossible among numerous
+  closely allied forms.<a name="FNanchor_83_83" id=
+  "FNanchor_83_83"></a><a href="#Footnote_83_83" class=
+  "fnanchor">[83]</a> <a name="Page_218" id="Page_218"></a>The
+  wonderful diversity of colour and of marking that prevails,
+  especially in birds and insects, may be due to the fact that one
+  of the first needs of a new species would be, to keep separate
+  from its nearest allies, and this could be most readily done by
+  some easily seen external mark of difference. A few illustrations
+  will serve to show how this principle acts in nature.</p>
+
+  <p>My attention was first called to the subject by a remark of
+  Mr. Darwin's that, though, "the hare on her form is a familiar
+  instance of concealment through colour, yet the principle partly
+  fails in a closely allied species, the rabbit; for when running
+  to its burrow it is made conspicuous to the sportsman, and no
+  doubt to all beasts of prey, by its upturned white tail."<a name=
+  "FNanchor_84_84" id="FNanchor_84_84"></a><a href=
+  "#Footnote_84_84" class="fnanchor">[84]</a> But a little
+  consideration of the habits of the animal will show that the
+  white upturned tail is of the greatest value, and is really, as
+  it has been termed by a writer in <i>The Field</i>, a "signal
+  flag of danger." For the rabbit is usually a crepuscular animal,
+  feeding soon after sunset or on moonlight nights. When disturbed
+  or alarmed it makes for its burrow, and the white upturned tails
+  of those in front serve as guides and signals to those more
+  remote from home, to the young and the feeble; and thus each
+  following the one or two before it, all are able with the least
+  possible delay to regain a place of comparative safety. The
+  apparent danger, therefore, becomes a most important means of
+  security.</p>
+
+  <p>The same general principle enables us to understand the
+  singular, and often conspicuous, markings on so many gregarious
+  herbivora which are yet, on the whole, protectively coloured.
+  Thus, the American prong-buck has a white patch behind and a
+  black muzzle. The Tartarian antelope, the Ovis poli of High Asia,
+  the Java wild ox, several species of deer, and a large number of
+  antelopes have a similar conspicuous white patch behind, which,
+  in contrast to the dusky body, must enable them to be seen and
+  followed from a distance by their fellows. Where there are many
+  species of nearly the same general size and form inhabiting the
+  same region&mdash;as with the antelopes <a name="Page_219" id=
+  "Page_219"></a>of Africa&mdash;we find many distinctive markings
+  of a similar kind. The gazelles have variously striped and banded
+  faces, besides white patches behind and on the flanks, as shown
+  in the woodcut. The spring-bok has a white patch on the face and
+  one on the sides, with a curiously distinctive white stripe above
+  the tail, which is nearly concealed when the animal is at rest by
+  a fold of skin but comes into full view when it is in motion,
+  being thus quite analogous to the upturned white tail of the
+  rabbit. In the pallah the white rump-mark is bordered with black,
+  and the peculiar shape of the horns distinguishes it when seen
+  from the front. The sable-antelope, the gems-bok, the oryx, the
+  hart-beest, the bonte-bok, and the addax have each peculiar white
+  markings; and they are besides characterised by horns so
+  remarkably different in each species and so conspicuous, that it
+  seems probable that the peculiarities in length, twist, and
+  curvature have been differentiated for the purpose of
+  recognition, rather than for any speciality of defence in species
+  whose general habits are so similar.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig18" id="fig18"></a> <img src="images/fig18.jpg"
+    width="400" height="468" alt=
+    "FIG. 18.&mdash;Gazella soemmerringi." title=
+    "FIG. 18.&mdash;Gazella soemmerringi." /> <b>FIG.
+    18.&mdash;Gazella soemmerringi.</b>
+  </div>
+
+  <p><a name="Page_220" id="Page_220"></a>It is interesting to note
+  that these markings for recognition are very slightly developed
+  in the antelopes of the woods and marshes. Thus, the grys-bok is
+  nearly uniform in colour, except the long black-tipped ears; and
+  it frequents the wooded mountains. The duyker-bok and the
+  rhoode-bok are wary bush-haunters, and have no marks but the
+  small white patch behind. The wood-haunting bosch-bok goes in
+  pairs, and has hardly any distinctive marks on its dusky chestnut
+  coat, but the male alone is horned. The large and handsome koodoo
+  frequents brushwood, and its vertical white stripes are no doubt
+  protective, while its magnificent spiral horns afford easy
+  recognition. The eland, which is an inhabitant of the open
+  country, is uniformly coloured, being sufficiently recognisable
+  by its large size and distinctive form; but the Derbyan eland is
+  a forest animal, and has a protectively striped coat. In like
+  manner, the fine Speke's antelope, which lives entirely in the
+  swamps and among reeds, has pale vertical stripes on the sides
+  (protective), with white markings on face and breast for
+  recognition. An inspection of the figures of antelopes and other
+  animals in Wood's <i>Natural History</i>, or in other illustrated
+  works, will give a better idea of the peculiarities of
+  recognition markings than any amount of description.</p>
+
+  <p>Other examples of such coloration are to be seen in the dusky
+  tints of the musk-sheep and the reindeer, to whom recognition at
+  a distance on the snowy plains is of more importance than
+  concealment from their few enemies. The conspicuous stripes and
+  bands of the zebra and the quagga are probably due to the same
+  cause, as may be the singular crests and face-marks of several of
+  the monkeys and lemurs.<a name="FNanchor_85_85" id=
+  "FNanchor_85_85"></a><a href="#Footnote_85_85" class=
+  "fnanchor">[85]</a></p>
+
+  <div class="figcenter" style="width: 500px;">
+    <a name="fig19" id="fig19"></a> <a name="Page_221" id=
+    "Page_221"></a><img src="images/fig19.jpg" width="500" height=
+    "300" alt=
+    "FIG. 19&mdash;Recognition marks of three African plovers."
+    title=
+    "FIG. 19&mdash;Recognition marks of three African plovers." />
+    <b>FIG. 19&mdash;Recognition marks of three African
+    plovers.</b>
+  </div>
+
+  <p><a name="Page_222" id="Page_222"></a>Among birds, these
+  recognition marks are especially numerous and suggestive. Species
+  which inhabit open districts are usually protectively coloured;
+  but they generally possess some distinctive markings for the
+  purpose of being easily recognised by their kind, both when at
+  rest and during flight. Such are, the white bands or patches on
+  the breast or belly of many birds, but more especially the head
+  and neck markings in the form of white or black caps, collars,
+  eye-marks or frontal patches, examples of which are seen in the
+  three species of African plovers figured on page 221.</p>
+
+  <p>Recognition marks during flight are very important for all
+  birds which congregate in flocks or which migrate together; and
+  it is essential that, while being as conspicuous as possible, the
+  marks shall not interfere with the general protective tints of
+  the species when at rest. Hence they usually consist of
+  well-contrasted markings on the wings and tail, which are
+  concealed during repose but become fully visible when the bird
+  takes flight. Such markings are well seen in our four British
+  species of shrikes, each having quite different white marks on
+  the expanded wings and on the tail feathers; and the same is the
+  case with our three species of Saxicola&mdash;the stone-chat,
+  whin-chat, and wheat-ear&mdash;which are thus easily recognisable
+  on the wing, especially when seen from above, as they would be by
+  stragglers looking out for their companions. The figures
+  opposite, of the wings of two African species of stone-curlew
+  which are sometimes found in the same districts, well illustrates
+  these specific recognition marks. Though not very greatly
+  different to our eyes, they are no doubt amply so to the sharp
+  vision of the birds themselves.</p>
+
+  <p>Besides the white patches on the primaries here shown, the
+  secondary feathers are, in some cases, so coloured as to afford
+  very distinctive markings during flight, as seen in the central
+  secondary quills of two African coursers (Fig. 21).</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig20" id="fig20"></a> <img src="images/fig20.jpg"
+    width="400" height="560" alt=
+    "FIG. 20.&mdash;Oedicnemus vermiculatus (above). Oe. senegalensis (below)."
+    title=
+    "FIG. 20.&mdash;Oedicnemus vermiculatus (above). Oe. senegalensis (below)." />
+     <b>FIG. 20.&mdash;Oedicnemus vermiculatus (above). Oe.
+    senegalensis (below).</b>
+  </div>
+
+  <p>Most characteristic of all, however, are the varied markings
+  of the outer tail-feathers, whose purpose is so well shown by
+  their being almost always covered during repose by the two middle
+  feathers, which are themselves quite unmarked and protectively
+  tinted like the rest of the upper surface of the body. The
+  figures of the expanded tails of two species of East Asiatic
+  snipe, whose geographical ranges overlap each other, <a name=
+  "Page_223" id="Page_223"></a><a name="Page_224" id=
+  "Page_224"></a>will serve to illustrate this difference; which is
+  frequently much greater and modified in an endless variety of
+  ways (Fig. 22).</p>
+
+  <p>Numbers of species of pigeons, hawks, finches, warblers,
+  ducks, and innumerable other birds possess this class of
+  markings; and they correspond so exactly in general character
+  with those of the mammalia, already described, that we cannot
+  doubt they serve a similar purpose.<a name="FNanchor_86_86" id=
+  "FNanchor_86_86"></a><a href="#Footnote_86_86" class=
+  "fnanchor">[86]</a></p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig21" id="fig21"></a> <img src="images/fig21.jpg"
+    width="400" height="532" alt="FIG. 21.&mdash;Secondary quills."
+    title="FIG. 21.&mdash;Secondary quills." /> <b>FIG.
+    21.&mdash;Secondary quills.</b>
+  </div>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig22" id="fig22"></a> <img src="images/fig22.jpg"
+    width="400" height="558" alt=
+    "FIG. 22.&mdash;Scolopax megala (upper). S. stenura (lower)."
+    title=
+    "FIG. 22.&mdash;Scolopax megala (upper). S. stenura (lower)." />
+     <b>FIG. 22.&mdash;Scolopax megala (upper). S. stenura
+    (lower).</b>
+  </div>
+
+  <p>Those birds which are inhabitants of tropical forests, and
+  which need recognition marks that shall be at all times visible
+  among the dense foliage, and not solely or chiefly during flight,
+  have usually small but brilliant patches of colour <a name=
+  "Page_225" id="Page_225"></a><a name="Page_226" id=
+  "Page_226"></a>on the head or neck, often not interfering with
+  the generally protective character of their plumage. Such are the
+  bright patches of blue, red, or yellow, by which the usually
+  green Eastern barbets are distinguished; and similar bright
+  patches of colour characterise the separate species of small
+  green fruit-doves. To this necessity for specialisation in
+  colour, by which each bird may easily recognise its kind, is
+  probably due that marvellous variety in the peculiar beauties of
+  some groups of birds. The Duke of Argyll, speaking of the humming
+  birds, made the objection that "A crest of topaz is no better in
+  the struggle for existence than a crest of sapphire. A frill
+  ending in spangles of the emerald is no better in the battle of
+  life than a frill ending in spangles of the ruby. A tail is not
+  affected for the purposes of flight, whether its marginal or its
+  central feathers are decorated with white;" and he goes on to
+  urge that mere beauty and variety for their own sake are the only
+  causes of these differences. But, on the principles here
+  suggested, the divergence itself is useful, and must have been
+  produced <i>pari passu</i> with the structural differences on
+  which the differentiation of species depends; and thus we have
+  explained the curious fact that prominent differences of colour
+  often distinguish species otherwise very closely allied to each
+  other.</p>
+
+  <p>Among insects, the principle of distinctive coloration for
+  recognition has probably been at work in the production of the
+  wonderful diversity of colour and marking we find everywhere,
+  more especially among the butterflies and moths; and here its
+  chief function may have been to secure the pairing together of
+  individuals of the same species. In some of the moths this has
+  been secured by a peculiar odour, which attracts the males to the
+  females from a distance; but there is no evidence that this is
+  universal or even general, and among butterflies, especially, the
+  characteristic colour and marking, aided by size and form, afford
+  the most probable means of recognition. That this is so is shown
+  by the fact that "the common white butterfly often flies down to
+  a bit of paper on the ground, no doubt mistaking it for one of
+  its own species;" while, according to Mr. Collingwood, in the
+  Malay Archipelago, "a dead butterfly pinned upon a conspicuous
+  twig will often arrest an insect of the same species in its
+  headlong flight, and <a name="Page_227" id="Page_227"></a>bring
+  it down within easy reach of the net, especially if it be of the
+  opposite sex."<a name="FNanchor_87_87" id=
+  "FNanchor_87_87"></a><a href="#Footnote_87_87" class=
+  "fnanchor">[87]</a> In a great number of insects, no doubt, form,
+  motions, stridulating sounds, or peculiar odours, serve to
+  distinguish allied species from each other, and this must be
+  especially the case with nocturnal insects, or with those whose
+  colours are nearly uniform and are determined by the need of
+  protection; but by far the larger number of day-flying and active
+  insects exhibit varieties of colour and marking, forming the most
+  obvious distinction between allied species, and which have,
+  therefore, in all probability been acquired in the process of
+  differentiation for the purpose of checking the intercrossing of
+  closely allied forms.<a name="FNanchor_88_88" id=
+  "FNanchor_88_88"></a><a href="#Footnote_88_88" class=
+  "fnanchor">[88]</a></p>
+
+  <p>Whether this principle extends to any of the less highly
+  organised animals is doubtful, though it may perhaps have
+  affected the higher mollusca. But in marine animals it seems
+  probable that the colours, however beautiful, varied, and
+  brilliant they may often be, are in most cases protective,
+  assimilating them to the various bright-coloured seaweeds, or to
+  some other animals which it is advantageous for them to
+  imitate.<a name="FNanchor_89_89" id="FNanchor_89_89"></a><a href=
+  "#Footnote_89_89" class="fnanchor">[89]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Summary of the Preceding Exposition.</i></p>
+
+  <p>Before proceeding to discuss some of the more recondite
+  phenomena of animal coloration, it will be well to consider for a
+  moment the extent of the ground we have already covered.
+  Protective coloration, in some of its varied forms, has not
+  improbably modified the appearance of one-half of the animals
+  living on the globe. The white of arctic animals, the yellowish
+  tints of the desert forms, the dusky hues of crepuscular and
+  nocturnal species, the transparent or bluish tints of oceanic
+  creatures, represent a vast host in themselves; but we have an
+  equally numerous body whose tints are adapted to tropical
+  foliage, to the bark of trees, or to the soil <a name="Page_228"
+  id="Page_228"></a>or dead leaves on or among which they
+  habitually live. Then we have the innumerable special adaptations
+  to the tints and forms of leaves, or twigs, or flowers; to bark
+  or moss; to rock or pebble; by which such vast numbers of the
+  insect tribes obtain protection; and we have seen that these
+  various forms of coloration are equally prevalent in the waters
+  of the seas and oceans, and are thus coextensive with the domain
+  of life upon the earth. The comparatively small numbers which
+  possess "terrifying" or "alluring" coloration may be classed
+  under the general head of the protectively coloured.</p>
+
+  <p>But under the next head&mdash;colour for recognition&mdash;we
+  have a totally distinct category, to some extent antagonistic or
+  complementary to the last, since its essential principle is
+  visibility rather than concealment. Yet it has been shown, I
+  think, that this mode of coloration is almost equally important,
+  since it not only aids in the preservation of existing species
+  and in the perpetuation of pure races, but was, perhaps, in its
+  earlier stages, a not unimportant factor in their development. To
+  it we owe most of the variety and much of the beauty in the
+  colours of animals; it has caused at once bilateral symmetry and
+  general permanence of type; and its range of action has been
+  perhaps equally extensive with that of coloration for
+  concealment.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Influence of Locality or of Climate on Colour.</i></p>
+
+  <p>Certain relations between locality and coloration have long
+  been noticed. Mr. Gould observed that birds from inland or
+  continental localities were more brightly coloured than those
+  living near the sea-coast or on islands, and he supposed that the
+  more brilliant atmosphere of the inland stations was the
+  explanation of the phenomenon.<a name="FNanchor_90_90" id=
+  "FNanchor_90_90"></a><a href="#Footnote_90_90" class=
+  "fnanchor">[90]</a> Many American naturalists have observed
+  similar facts, and they assert that the intensity of the colours
+  of birds and mammals increases from north to south, and also with
+  the increase of humidity. This change is imputed by Mr. J.A.
+  Allen to the direct action of the environment. He says: "In
+  respect to the correlation of intensity of colour in animals with
+  the degree of humidity, it would perhaps be more in accordance
+  with cause and effect to express the law of correlation as a
+  <i>decrease</i> of intensity of colour with <a name="Page_229"
+  id="Page_229"></a>a <i>decrease</i> of humidity, the paleness
+  evidently resulting from exposure and the blanching effect of
+  intense sunlight, and a dry, often intensely heated atmosphere.
+  With the decrease of the aqueous precipitation the forest growth
+  and the protection afforded by arborescent vegetation gradually
+  also decreases, as of course does also the protection afforded by
+  clouds, the excessively humid regions being also regions of
+  extreme cloudiness, while the dry regions are comparatively
+  cloudless districts."<a name="FNanchor_91_91" id=
+  "FNanchor_91_91"></a><a href="#Footnote_91_91" class=
+  "fnanchor">[91]</a> Almost identical changes occur in birds, and
+  are imputed by Mr. Allen to similar causes.</p>
+
+  <p>It will be seen that Mr. Gould and Mr. Allen impute opposite
+  effects to the same cause, brilliancy or intensity of colour
+  being due to a brilliant atmosphere according to the former,
+  while paleness of colour is imputed by the latter to a too
+  brilliant sun. According to the principles which have been
+  established by the consideration of arctic, desert, and forest
+  animals respectively, we shall be led to conclude that there has
+  been no direct action in this case, but that the effects observed
+  are due to the greater or less need of protection. The pale
+  colour that is prevalent in arid districts is in harmony with the
+  general tints of the surface; while the brighter tints or more
+  intense coloration, both southward and in humid districts, are
+  sufficiently explained by the greater shelter due to a more
+  luxuriant vegetation and a shorter winter. The advocates of the
+  theory that intensity of light directly affects the colours of
+  organisms, are led into perpetual inconsistencies. At one time
+  the brilliant colours of tropical birds and insects are imputed
+  to the intensity of a tropical sun, while the same intensity of
+  sunlight is now said to have a "bleaching" effect. The
+  comparatively dull and sober hues of our northern fauna were once
+  supposed to be the result of our cloudy skies; but now we are
+  told that cloudy skies and a humid atmosphere intensify
+  colour.</p>
+
+  <p>In my <i>Tropical Nature</i> (pp. 257-264) I have called
+  attention to what is perhaps the most curious and decided
+  relation of colour to locality which has yet been
+  observed&mdash;the prevalence of white markings in the
+  butterflies and birds of islands.</p>
+
+  <p><a name="Page_230" id="Page_230"></a>So many cases are adduced
+  from so many different islands, both in the eastern and western
+  hemisphere, that it is impossible to doubt the existence of some
+  common cause; and it seems probable to me now, after a fuller
+  consideration of the whole subject of colour, that here too we
+  have one of the almost innumerable results of the principle of
+  protective coloration. White is, as a rule, an uncommon colour in
+  animals, but probably only because it is so conspicuous. Whenever
+  it becomes protective, as in the case of arctic animals and
+  aquatic birds, it appears freely enough; while we know that white
+  varieties of many species occur occasionally in the wild state,
+  and that, under domestication, white or parti-coloured breeds are
+  freely produced. Now in all the islands in which exceptionally
+  white-marked birds and butterflies have been observed, we find
+  two features which would tend to render the conspicuous white
+  markings less injurious&mdash;a luxuriant tropical vegetation,
+  and a decided scarcity of rapacious mammals and birds. White
+  colours, therefore, would not be eliminated by natural selection;
+  but variations in this direction would bear their part in
+  producing the recognition marks which are everywhere essential,
+  and which, in these islands, need not be so small or so
+  inconspicuous as elsewhere.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Concluding Remarks.</i></p>
+
+  <p>On a review of the whole subject, then, we must conclude that
+  there is no evidence of the individual or prevalent colours of
+  organisms being directly determined by the amount of light, or
+  heat, or moisture, to which they are exposed; while, on the other
+  hand, the two great principles of the need of concealment from
+  enemies or from their prey, and of recognition by their own kind,
+  are so wide-reaching in their application that they appear at
+  first sight to cover almost the whole ground of animal
+  coloration. But, although they are indeed wonderfully general and
+  have as yet been very imperfectly studied, we are acquainted with
+  other modes of coloration which have a different origin. These
+  chiefly appertain to the very singular class of warning colours,
+  from which arise the yet more extraordinary phenomena of mimicry;
+  and they open up so curious a field of inquiry and present so
+  many interesting problems, that a chapter must be devoted to
+  them. Yet another chapter will <a name="Page_231" id=
+  "Page_231"></a>be required by the subject of sexual
+  differentiation of colour and ornament, as to the origin and
+  meaning of which I have arrived at different conclusions from Mr.
+  Darwin. These various forms of coloration having been discussed
+  and illustrated, we shall be in a position to attempt a brief
+  sketch of the fundamental laws which have determined the general
+  coloration of the animal world.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_65_65" id="Footnote_65_65"></a><a href=
+      "#FNanchor_65_65"><span class="label">[65]</span></a>
+      <i>Proceedings of the Royal Society</i>, No. 243, 1886;
+      <i>Transactions of the Royal Society</i>, vol. clxxviii. B.
+      pp. 311-441.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_66_66" id="Footnote_66_66"></a><a href=
+      "#FNanchor_66_66"><span class="label">[66]</span></a> <i>A
+      Naturalist's Wanderings in the Eastern Archipelago</i>, p.
+      460.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_67_67" id="Footnote_67_67"></a><a href=
+      "#FNanchor_67_67"><span class="label">[67]</span></a>
+      <i>Trans. Phil. Soc.</i> (? <i>of S. Africa</i>), 1878, part
+      iv, p. 27.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_68_68" id="Footnote_68_68"></a><a href=
+      "#FNanchor_68_68"><span class="label">[68]</span></a>
+      <i>Proc. Zool. Soc.</i>, 1862 p. 357.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_69_69" id="Footnote_69_69"></a><a href=
+      "#FNanchor_69_69"><span class="label">[69]</span></a> With
+      reference to this general resemblance of insects to their
+      environment the following remarks by Mr. Poulton are very
+      instructive. He says: "Holding the larva of Sphinx ligustri
+      in one hand and a twig of its food-plant in the other, the
+      wonder we feel is, not at the resemblance but at the
+      difference; we are surprised at the difficulty experienced in
+      detecting so conspicuous an object. And yet the protection is
+      very real, for the larvae will be passed over by those who
+      are not accustomed to their appearance, although the searcher
+      may be told of the presence of a large caterpillar. An
+      experienced entomologist may also fail to find the larvae
+      till after a considerable search. This is general protective
+      resemblance, and it depends upon a general harmony between
+      the appearance of the organism and its whole environment. It
+      is impossible to understand the force of this protection for
+      any larva, without seeing it on its food-plant and in an
+      entirely normal condition. The artistic effect of green
+      foliage is more complex than we often imagine; numberless
+      modifications are wrought by varied lights and shadows upon
+      colours which are in themselves far from uniform. In the
+      larva of Papilio machaon the protection is very real when the
+      larva is on the food-plant, and can hardly be appreciated at
+      all when the two are apart." Numerous other examples are
+      given in the chapter on "Mimicry and other Protective
+      Resemblances among Animals," in my <i>Contributions to the
+      Theory of Natural Selection</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_70_70" id="Footnote_70_70"></a><a href=
+      "#FNanchor_70_70"><span class="label">[70]</span></a> <i>The
+      Naturalist in Nicaragua</i>, p. 19.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_71_71" id="Footnote_71_71"></a><a href=
+      "#FNanchor_71_71"><span class="label">[71]</span></a> R.
+      Meldola, in <i>Proc. Zool. Soc.</i>, 1873, p. 155.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_72_72" id="Footnote_72_72"></a><a href=
+      "#FNanchor_72_72"><span class="label">[72]</span></a>
+      <i>Nature</i>, vol. iii. p. 166.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_73_73" id="Footnote_73_73"></a><a href=
+      "#FNanchor_73_73"><span class="label">[73]</span></a>
+      <i>Trans. Ent. Soc. Lond.</i>, 1878, p. 185.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_74_74" id="Footnote_74_74"></a><a href=
+      "#FNanchor_74_74"><span class="label">[74]</span></a>
+      <i>Ibid.</i> (<i>Proceedings</i>, p. xlii.)</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_75_75" id="Footnote_75_75"></a><a href=
+      "#FNanchor_75_75"><span class="label">[75]</span></a>
+      Wallace's <i>Malay Archipelago</i>, vol. i. p. 204 (fifth
+      edition, p. 130), with figure.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_76_76" id="Footnote_76_76"></a><a href=
+      "#FNanchor_76_76"><span class="label">[76]</span></a>
+      Moseley's <i>Notes by a Naturalist on the Challenger</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_77_77" id="Footnote_77_77"></a><a href=
+      "#FNanchor_77_77"><span class="label">[77]</span></a>
+      <i>Proceedings of the Boston Soc. of Nat. Hist.</i>, vol.
+      xiv. 1871.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_78_78" id="Footnote_78_78"></a><a href=
+      "#FNanchor_78_78"><span class="label">[78]</span></a>
+      <i>Nature</i>, 1870, p. 376.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_79_79" id="Footnote_79_79"></a><a href=
+      "#FNanchor_79_79"><span class="label">[79]</span></a> <i>A
+      Naturalist's Wanderings in the Eastern Archipelago</i>, p.
+      63.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_80_80" id="Footnote_80_80"></a><a href=
+      "#FNanchor_80_80"><span class="label">[80]</span></a> A
+      beautiful drawing of this rare insect, Hymenopus bicornis (in
+      the nymph or active pupa state), was kindly sent me by Mr.
+      Wood-Mason, Curator of the Indian Museum at Calcutta. A
+      species, very similar to it, inhabits Java, where it is said
+      to resemble a pink orchid. Other Mantidae, of the genus
+      Gongylus, have the anterior part of the thorax dilated and
+      coloured either white, pink, or purple; and they so closely
+      resemble flowers that, according to Mr. Wood-Mason, one of
+      them, having a bright violet-blue prothoracic shield, was
+      found in Pegu by a botanist, and was for a moment mistaken by
+      him for a flower. See <i>Proc. Ent. Soc. Lond.</i>, 1878, p.
+      liii.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_81_81" id="Footnote_81_81"></a><a href=
+      "#FNanchor_81_81"><span class="label">[81]</span></a> C.
+      Dixon, in Seebohm's <i>History of British Birds</i>, vol. ii.
+      Introduction, p. xxvi. Many of the other examples here cited
+      are taken from the same valuable work.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_82_82" id="Footnote_82_82"></a><a href=
+      "#FNanchor_82_82"><span class="label">[82]</span></a> See
+      A.H.S. Lucas, in <i>Proceedings of Royal Society of
+      Victoria</i>, 1887, p. 56.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_83_83" id="Footnote_83_83"></a><a href=
+      "#FNanchor_83_83"><span class="label">[83]</span></a>
+      Professor Wm.H. Brewer of Yale College has shown that the
+      white marks or the spots of domesticated animals are rarely
+      symmetrical, but have a tendency to appear more frequently on
+      the left side. This is the case with horses, cattle, dogs,
+      and swine. Among wild animals the skunk varies considerably
+      in the amount of white on the body, and this too was found to
+      be usually greatest on the left side. A close examination of
+      numerous striped or spotted species, as tigers, leopards,
+      jaguars, zebras, etc., showed that the bilateral symmetry was
+      not exact, although the general effect of the two sides was
+      the same. This is precisely what we should expect if the
+      symmetry is not the result of a general law of the
+      organisation, but has been, in part at least, produced and
+      preserved for the useful purpose of recognition by the
+      animal's fellows of the same species, and especially by the
+      sexes and the young. See <i>Proc. of the Am. Ass. for
+      Advancement of Science</i>, vol. xxx. p. 246.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_84_84" id="Footnote_84_84"></a><a href=
+      "#FNanchor_84_84"><span class="label">[84]</span></a>
+      <i>Descent of Man</i>, p. 542.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_85_85" id="Footnote_85_85"></a><a href=
+      "#FNanchor_85_85"><span class="label">[85]</span></a> It may
+      be thought that such extremely conspicuous markings as those
+      of the zebra would be a great danger in a country abounding
+      with lions, leopards, and other beasts of prey; but it is not
+      so. Zebras usually go in bands, and are so swift and wary
+      that they are in little danger during the day. It is in the
+      evening, or on moonlight nights, when they go to drink, that
+      they are chiefly exposed to attack; and Mr. Francis Galton,
+      who has studied these animals in their native haunts, assures
+      me, that in twilight they are not at all conspicuous, the
+      stripes of white and black so merging together into a gray
+      tint that it is very difficult to see them at a little
+      distance. We have here an admirable illustration of how a
+      glaringly conspicuous style of marking for recognition may be
+      so arranged as to become also protective at the time when
+      protection is most needed; and we may also learn how
+      impossible it is for us to decide on the inutility of any
+      kind of coloration without a careful study of the habits of
+      the species in its native country.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_86_86" id="Footnote_86_86"></a><a href=
+      "#FNanchor_86_86"><span class="label">[86]</span></a> The
+      principle of colouring for recognition was, I believe, first
+      stated in my article on "The Colours of Animals and Plants"
+      in Macmillan's <i>Magazine</i>, and more fully in my volume
+      on <i>Tropical Nature</i>. Subsequently Mrs. Barber gave a
+      few examples under the head of "Indicative or Banner
+      Colours," but she applied it to the distinctive colours of
+      the males of birds, which I explain on another principle,
+      though this may assist.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_87_87" id="Footnote_87_87"></a><a href=
+      "#FNanchor_87_87"><span class="label">[87]</span></a> Quoted
+      by Darwin in <i>Descent of Man</i>, p. 317.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_88_88" id="Footnote_88_88"></a><a href=
+      "#FNanchor_88_88"><span class="label">[88]</span></a> In the
+      <i>American Naturalist</i> of March 1888, Mr. J.E. Todd has
+      an article on "Directive Coloration in Animals," in which he
+      recognises many of the cases here referred to, and suggests a
+      few others, though I think he includes many forms of
+      coloration&mdash;as "paleness of belly and inner side of
+      legs"&mdash;which do not belong to this class.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_89_89" id="Footnote_89_89"></a><a href=
+      "#FNanchor_89_89"><span class="label">[89]</span></a> For
+      numerous examples of this protective colouring of marine
+      animals see Moseley's <i>Voyage of the Challenger</i>, and
+      Dr. E.S. Morse in <i>Proc. of Bost. Soc. of Nat. Hist.</i>,
+      vol. xiv. 1871.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_90_90" id="Footnote_90_90"></a><a href=
+      "#FNanchor_90_90"><span class="label">[90]</span></a> See
+      <i>Origin of Species</i>, p. 107.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_91_91" id="Footnote_91_91"></a><a href=
+      "#FNanchor_91_91"><span class="label">[91]</span></a> The
+      "Geographical Variation of North American Squirrels,"
+      <i>Proc. Bost. Soc. of Nat. Hist.</i>, 1874, p. 284; and
+      <i>Mammals and Winter Birds of Florida</i>, pp. 233-241.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_IX" id="CHAPTER_IX"></a><a name="Page_232"
+  id="Page_232"></a>CHAPTER IX</h2>
+
+  <p style='font-weight:bold;'>WARNING COLORATION AND MIMICRY</p>
+
+  <div class="blockquot">
+    <p>The skunk as an example of warning coloration&mdash;Warning
+    colours among
+    insects&mdash;Butterflies&mdash;Caterpillars&mdash;Mimicry&mdash;How
+    mimicry has been produced&mdash;Heliconidae&mdash;Perfection of
+    the imitation&mdash;Other cases of mimicry among
+    Lepidoptera&mdash;Mimicry among protected groups&mdash;Its
+    explanation&mdash;Extension of the principle&mdash;Mimicry in
+    other orders of insects&mdash;Mimicry among the
+    vertebrata&mdash;Snakes&mdash;The rattlesnake and the
+    cobra&mdash;Mimicry among birds&mdash;Objections to the theory
+    of mimicry&mdash;Concluding remarks on warning colours and
+    mimicry.</p>
+  </div>
+
+  <p>We have now to deal with a class of colours which are the very
+  opposite of those we have hitherto considered, since, instead of
+  serving to conceal the animals that possess them or as
+  recognition marks to their associates, they are developed for the
+  express purpose of rendering the species conspicuous. The reason
+  of this is that the animals in question are either the possessors
+  of some deadly weapons, as stings or poison fangs, or they are
+  uneatable, and are thus so disagreeable to the usual enemies of
+  their kind that they are never attacked when their peculiar
+  powers or properties are known. It is, therefore, important that
+  they should not be mistaken for defenceless or eatable species of
+  the same class or order, since in that case they might suffer
+  injury, or even death, before their enemies discovered the danger
+  or the uselessness of the attack. They require some signal or
+  danger-flag which shall serve as a warning to would-be enemies
+  not to attack them, and they have usually obtained this in the
+  form of conspicuous or brilliant coloration, very distinct from
+  the protective tints of the defenceless animals allied to
+  them.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_233" id="Page_233"></a><i>The Skunk as illustrating
+  Warning Coloration.</i></p>
+
+  <p>While staying a few days, in July 1887, at the Summit Hotel on
+  the Central Pacific Railway, I strolled out one evening after
+  dinner, and on the road, not fifty yards from the house, I saw a
+  pretty little white and black animal with a bushy tail coming
+  towards me. As it came on at a slow pace and without any fear,
+  although it evidently saw me, I thought at first that it must be
+  some tame creature, when it suddenly occurred to me that it was a
+  skunk. It came on till within five or six yards of me, then
+  quietly climbed over a dwarf wall and disappeared under a small
+  outhouse, in search of chickens, as the landlord afterwards told
+  me. This animal possesses, as is well known, a most offensive
+  secretion, which it has the power of ejecting over its enemies,
+  and which effectually protects it from attack. The odour of this
+  substance is so penetrating that it taints, and renders useless,
+  everything it touches, or in its vicinity. Provisions near it
+  become uneatable, and clothes saturated with it will retain the
+  smell for several weeks, even though they are repeatedly washed
+  and dried. A drop of the liquid in the eyes will cause blindness,
+  and Indians are said not unfrequently to lose their sight from
+  this cause. Owing to this remarkable power of offence the skunk
+  is rarely attacked by other animals, and its black and white fur,
+  and the bushy white tail carried erect when disturbed, form the
+  danger-signals by which it is easily distinguished in the
+  twilight or moonlight from unprotected animals. Its consciousness
+  that it needs only to be seen to be avoided gives it that
+  slowness of motion and fearlessness of aspect which are, as we
+  shall see, characteristic of most creatures so protected.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Warning Colours among Insects.</i></p>
+
+  <p>It is among insects that warning colours are best developed,
+  and most abundant. We all know how well marked and conspicuous
+  are the colours and forms of the stinging wasps and bees, no one
+  of which in any part of the world is known to be protectively
+  coloured like the majority of defenceless insects. Most of the
+  great tribe of Malacoderms among beetles are distasteful to
+  insect-eating animals. Our red and <a name="Page_234" id=
+  "Page_234"></a>black Telephoridae, commonly called "soldiers and
+  sailors," were found, by Mr. Jenner Weir, to be refused by small
+  birds. These and the allied Lampyridae (the fireflies and
+  glow-worms) in Nicaragua, were rejected by Mr. Belt's tame monkey
+  and by his fowls, though most other insects were greedily eaten
+  by them. The Coccinellidae or lady-birds are another uneatable
+  group, and their conspicuous and singularly spotted bodies serve
+  to distinguish them at a glance from all other beetles.</p>
+
+  <p>These uneatable insects are probably more numerous than is
+  supposed, although we already know immense numbers that are so
+  protected. The most remarkable are the three families of
+  butterflies&mdash;Heliconidae, Danaidae, and
+  Acraeidae&mdash;comprising more than a thousand species, and
+  characteristic respectively of the three great tropical
+  regions&mdash;South America, Southern Asia, and Africa. All these
+  butterflies have peculiarities which serve to distinguish them
+  from every other group in their respective regions. They all have
+  ample but rather weak wings, and fly slowly; they are always very
+  abundant; and they all have conspicuous colours or markings, so
+  distinct from those of other families that, in conjunction with
+  their peculiar outline and mode of flight, they can usually be
+  recognised at a glance. Other distinctive features are, that
+  their colours are always nearly the same on the under surface of
+  their wings as on the upper; they never try to conceal
+  themselves, but rest on the upper surfaces of leaves or flowers;
+  and, lastly, they all have juices which exhale a powerful scent,
+  so that when one kills them by pinching the body, the liquid that
+  exudes stains the fingers yellow, and leaves an odour that can
+  only be removed by repeated washings.</p>
+
+  <p>Now, there is much direct evidence to show that this odour,
+  though not very offensive to us, is so to most insect-eating
+  creatures. Mr. Bates observed that, when set out to dry,
+  specimens of Heliconidae were less subject to the attacks of
+  vermin; while both he and I noticed that they were not attacked
+  by insect-eating birds or dragonflies, and that their wings were
+  not found in the forest paths among the numerous wings of other
+  butterflies whose bodies had been devoured. Mr. Belt once
+  observed a pair of birds capturing insects for <a name="Page_235"
+  id="Page_235"></a>their young; and although the Heliconidae
+  swarmed in the vicinity, and from their slow flight could have
+  been easily caught, not one was ever pursued, although other
+  butterflies did not escape. His tame monkey also, which would
+  greedily munch up other butterflies, would never eat the
+  Heliconidae. It would sometimes smell them, but always rolled
+  them up in its hand and then dropped them.</p>
+
+  <p>We have also some corresponding evidence as to the
+  distastefulness of the Eastern Danaidae. The Hon. Mr. Justice
+  Newton, who assiduously collected and took notes upon the
+  Lepidoptera of Bombay, informed Mr. Butler of the British Museum
+  that the large and swift-flying butterfly Charaxes psaphon, was
+  continually persecuted by the bulbul, so that he rarely caught a
+  specimen of this species which had not a piece snipped out of the
+  hind wings. He offered one to a bulbul which he had in a cage,
+  and it was greedily devoured, whilst it was only by repeated
+  persecution that he succeeded in inducing the bird to touch a
+  Danais.<a name="FNanchor_92_92" id="FNanchor_92_92"></a><a href=
+  "#Footnote_92_92" class="fnanchor">[92]</a></p>
+
+  <p>Besides these three families of butterflies, there are certain
+  groups of the great genus Papilio&mdash;the true swallow-tailed
+  butterflies&mdash;which have all the characteristics of uneatable
+  insects. They have a special coloration, usually red and black
+  (at least in the females), they fly slowly, they are very
+  abundant, and they possess a peculiar odour somewhat like that of
+  the Heliconidae. One of these groups is common in tropical
+  America, another in tropical Asia, and it is curious that,
+  although not very closely allied, they have each the same red and
+  black colours, and are very distinct from all the other
+  butterflies of their respective countries. There is reason to
+  believe also that many of the brilliantly coloured and
+  weak-flying diurnal moths, like the fine tropical Agaristidae and
+  burnet-moths, are similarly protected, and that their conspicuous
+  colours serve as a warning of inedibility. The common burnet-moth
+  (Anthrocera filipendula) and the equally conspicuous ragwort-moth
+  (Euchelia jacobeae) have been proved to be distasteful to
+  insect-eating creatures.</p>
+
+  <p><a name="Page_236" id="Page_236"></a>The most interesting and
+  most conclusive example of warning coloration is, however,
+  furnished by caterpillars, because in this case the facts have
+  been carefully ascertained experimentally by competent observers.
+  In the year 1866, when Mr. Darwin was collecting evidence as to
+  the supposed effect of sexual selection in bringing about the
+  brilliant coloration of the higher animals, he was struck by the
+  fact that many caterpillars have brilliant and conspicuous
+  colours, in the production of which sexual selection could have
+  no place. We have numbers of such caterpillars in this country,
+  and they are characterised not only by their gay colours but by
+  not concealing themselves. Such are the mullein and the
+  gooseberry caterpillars, the larvae of the spurge hawk-moth, of
+  the buff-tip, and many others. Some of these caterpillars are
+  wonderfully conspicuous, as in the case of that noticed by Mr.
+  Bates in South America, which was four inches long, banded across
+  with black and yellow, and with bright red head, legs, and tail.
+  Hence it caught the eye of any one who passed by, even at the
+  distance of many yards.</p>
+
+  <p>Mr. Darwin asked me to try and suggest some explanation of
+  this coloration; and, having been recently interested in the
+  question of the warning coloration of butterflies, I suggested
+  that this was probably a similar case,&mdash;that these
+  conspicuous caterpillars were distasteful to birds and other
+  insect-eating creatures, and that their bright non-protective
+  colours and habit of exposing themselves to view, enabled their
+  enemies to distinguish them at a glance from the edible kinds and
+  thus learn not to touch them; for it must be remembered that the
+  bodies of caterpillars while growing are so delicate, that a
+  wound from a bird's beak would be perhaps as fatal as if they
+  were devoured.<a name="FNanchor_93_93" id=
+  "FNanchor_93_93"></a><a href="#Footnote_93_93" class=
+  "fnanchor">[93]</a> At this time not a single experiment or
+  observation had been made on the subject, but after I had brought
+  the matter before the Entomological Society, two gentlemen, who
+  kept birds and other tame animals, undertook to make experiments
+  with a variety of caterpillars.</p>
+
+  <p>Mr. Jenner Weir was the first to experiment with ten species
+  of small birds in his aviary, and he found that none of them
+  would eat the following smooth-skinned conspicuous cater<a name=
+  "Page_237" id="Page_237"></a>pillars&mdash;Abraxas grossulariata,
+  Diloba caeruleocephala, Anthrocera filipendula, and Cucullia
+  verbasci. He also found that they would not touch any hairy or
+  spiny larvae, and he was satisfied that it was not the hairs or
+  the spines, but the unpleasant taste that caused them to be
+  rejected, because in one case a young smooth larva of a hairy
+  species, and in another case the pupa of a spiny larva, were
+  equally rejected. On the other hand, all green or brown
+  caterpillars as well as those that resemble twigs were greedily
+  devoured.<a name="FNanchor_94_94" id=
+  "FNanchor_94_94"></a><a href="#Footnote_94_94" class=
+  "fnanchor">[94]</a></p>
+
+  <p>Mr. A.G. Butler also made experiments with some green lizards
+  (Lacerta viridis), which greedily ate all kinds of food,
+  including flies of many kinds, spiders, bees, butterflies, and
+  green caterpillars; but they would not touch the caterpillar of
+  the gooseberry-moth (Abraxas grossulariata), or the imago of the
+  burnet-moth (Anthrocera filipendula). The same thing happened
+  with frogs. When the gooseberry caterpillars were first given to
+  them, "they sprang forward and licked them eagerly into their
+  mouths; no sooner, however, had they done so, than they seemed to
+  become aware of the mistake that they had made, and sat with
+  gaping mouths, rolling their tongues about, until they had got
+  quit of the nauseous morsels, which seemed perfectly uninjured,
+  and walked off as briskly as ever." Spiders seemed equally to
+  dislike them. This and another conspicuous caterpillar (Halia
+  wavaria) were rejected by two species&mdash;the geometrical
+  garden spider (Epeira diadema) and a hunting spider.<a name=
+  "FNanchor_95_95" id="FNanchor_95_95"></a><a href=
+  "#Footnote_95_95" class="fnanchor">[95]</a></p>
+
+  <p>Some further experiments with lizards were made by Professor
+  Weismann, quite confirming the previous observations; and in 1886
+  Mr. E.B. Poulton of Oxford undertook a considerable series of
+  experiments, with many other species of larvae and fresh kinds of
+  lizards and frogs. Mr. Poulton then reviewed the whole subject,
+  incorporating all recorded facts, as well as some additional
+  observations made by Mr. Jenner Weir in 1886. More than a hundred
+  species of larvae or of perfect insects of various orders have
+  now been made the subject of experiment, and the results
+  completely confirm my original suggestion. In almost every case
+  the protectively coloured larvae have been greedily eaten by all
+  kinds of insectivorous <a name="Page_238" id=
+  "Page_238"></a>animals, while, in the immense majority of cases,
+  the conspicuous, hairy, or brightly coloured larvae have been
+  rejected by some or all of them. In some instances the
+  inedibility of the larvae extends to the perfect insect, but not
+  in others. In the former cases the perfect insect is usually
+  adorned with conspicuous colours, as the burnet and ragwort
+  moths; but in the case of the buff-tip, the moth resembles a
+  broken piece of rotten stick, yet it is partly inedible, being
+  refused by lizards. It is, however, very doubtful whether these
+  are its chief enemies, and its protective form and colour may be
+  needed against insectivorous birds or mammals.</p>
+
+  <p>Mr. Samuel H. Scudder, who has largely bred North American
+  butterflies, has found so many of the eggs and larvae destroyed
+  by hymenopterous and dipterous parasites that he thinks at least
+  nine-tenths, perhaps a greater proportion, never reach maturity.
+  Yet he has never found any evidence that such parasites attack
+  either the egg or the larva of the inedible Danais archippus, so
+  that in this case the insect is distasteful to its most dangerous
+  foes in all the stages of its existence, a fact which serves to
+  explain its great abundance and its extension over almost the
+  whole world.<a name="FNanchor_96_96" id=
+  "FNanchor_96_96"></a><a href="#Footnote_96_96" class=
+  "fnanchor">[96]</a></p>
+
+  <p>One case has been found of a protectively coloured
+  larva,&mdash;one, moreover, which in all its habits shows that it
+  trusts to concealment to escape its enemies&mdash;which was yet
+  always rejected by lizards after they had seized it, evidently
+  under the impression that from its colour it would be eatable.
+  This is the caterpillar of the very common moth Mania typica; and
+  Mr. Poulton thinks that, in this case, the unpleasant taste is an
+  incidental result of some physiological processes in the
+  organism, and is itself a merely useless character. It is evident
+  that the insect would not conceal itself so carefully as it does
+  if it had not some enemies, and these are probably birds or small
+  mammals, as its food-plants are said to be dock and willow-herb,
+  not suggestive of places frequented by lizards; and it has been
+  found by experiment that lizards and birds have not always the
+  same likes and dislikes. The case is interesting, because it
+  shows that nauseous fluids sometimes occur sporadically, and may
+  thus be intensified by natural selection when required for the
+  purpose <a name="Page_239" id="Page_239"></a>of protection.
+  Another exceptional case is that of the very conspicuous
+  caterpillar of the spurge hawk-moth (Deilephila euphorbiae),
+  which was at once eaten by a lizard, although, as it exposes
+  itself on its food-plant in the daytime and is very abundant in
+  some localities, it must almost certainly be disliked by birds or
+  by some animals who would otherwise devour it. If disturbed while
+  feeding it is said to turn round with fury and eject a quantity
+  of green liquid, of an acid and disagreeable smell similar to
+  that of the spurge milk, only worse.<a name="FNanchor_97_97" id=
+  "FNanchor_97_97"></a><a href="#Footnote_97_97" class=
+  "fnanchor">[97]</a></p>
+
+  <p>These facts, and Mr. Poulton's evidence that some larvae
+  rejected by lizards at first will be eaten if the lizards are
+  very hungry, show that there are differences in the amount of the
+  distastefulness, and render it probable that if other food were
+  wanting many of these conspicuous insects would be eaten. It is
+  the abundance of the eatable kinds that gives value to the
+  inedibility of the smaller number; and this is probably the
+  reason why so many insects rely on protective colouring rather
+  than on the acquisition of any kind of defensive weapons. In the
+  long run the powers of attack and defence must balance each
+  other. Hence we see that even the powerful stings of bees and
+  wasps only protect them against some enemies, since a tribe of
+  birds, the bee-eaters, have been developed which feed upon them,
+  and some frogs and lizards do so occasionally.</p>
+
+  <p>The preceding outline will sufficiently explain the
+  characteristics of "warning coloration" and the end it serves in
+  nature. There are many other curious modifications of it, but
+  these will be best appreciated after we have discussed the
+  remarkable phenomenon of "mimicry," which is bound up with and
+  altogether depends upon "warning colour," and is in some cases
+  the chief indication we have of the possession of some offensive
+  weapon to secure the safety of the species imitated.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Mimicry.</i></p>
+
+  <p>This term has been given to a form of protective resemblance,
+  in which one species so closely resembles another in external
+  form and colouring as to be mistaken for it, although the two may
+  not be really allied and often belong to distinct <a name=
+  "Page_240" id="Page_240"></a>families or orders. One creature
+  seems disguised in order to be made like another; hence the terms
+  "mimic" and mimicry, which imply no voluntary action on the part
+  of the imitator. It has long been known that such resemblances do
+  occur, as, for example, the clear-winged moths of the families
+  Sesiidae and Aegeriidae, many of which resemble bees, wasps,
+  ichneumons, or saw-flies, and have received names expressive of
+  the resemblance; and the parasitic flies (Volucella) which
+  closely resemble bees, on whose larvae the larvae of the flies
+  feed.</p>
+
+  <p>The great bulk of such cases remained, however, unnoticed, and
+  the subject was looked upon as one of the inexplicable
+  curiosities of nature, till Mr. Bates studied the phenomenon
+  among the butterflies of the Amazon, and, on his return home,
+  gave the first rational explanation of it.<a name=
+  "FNanchor_98_98" id="FNanchor_98_98"></a><a href=
+  "#Footnote_98_98" class="fnanchor">[98]</a> The facts are,
+  briefly, these. Everywhere in that fertile region for the
+  entomologist the brilliantly coloured Heliconidae abound, with
+  all the characteristics which I have already referred to when
+  describing them as illustrative of "warning coloration." But
+  along with them other butterflies were occasionally captured,
+  which, though often mistaken for them, on account of their close
+  resemblance in form, colour, and mode of flight, were found on
+  examination to belong to a very distinct family, the Pieridae.
+  Mr. Bates notices fifteen distinct species of Pieridae, belonging
+  to the genera Leptalis and Euterpe, each of which closely
+  imitates some one species of Heliconidae, inhabiting the same
+  region and frequenting the same localities. It must be remembered
+  that the two families are altogether distinct in structure. The
+  larvae of the Heliconidae are tubercled or spined, the pupae
+  suspended head downwards, and the imago has imperfect forelegs in
+  the male; while the larvae of the Pieridae are smooth, the pupae
+  are suspended with a brace to keep the head erect, and the
+  forefeet are fully developed in both sexes. These differences are
+  as large and as important as those between pigs and sheep, or
+  between swallows and sparrows; while English entomologists will
+  best understand the case by supposing that a species of Pieris in
+  this country was coloured and shaped like a small tortoise-shell,
+  while another species on the Continent was equally like a
+  Camberwell beauty&mdash;so like in both <a name="Page_241" id=
+  "Page_241"></a>cases as to be mistaken when on the wing, and the
+  difference only to be detected by close examination. As an
+  example of the resemblance, woodcuts are given of one pair in
+  which the colours are simple, being olive, yellow, and black,
+  while the very distinct neuration of the wings and form of the
+  head and body can be easily seen.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig23" id="fig23"></a> <img src="images/fig23.jpg"
+    width="400" height="429" alt=
+    "FIG. 23.&mdash;Methona psidii (Heliconidae). Leptalis orise (Pieridae)."
+    title=
+    "FIG. 23.&mdash;Methona psidii (Heliconidae). Leptalis orise (Pieridae)." />
+     <b>FIG. 23.&mdash;Methona psidii (Heliconidae).<br />
+    Leptalis orise (Pieridae).</b>
+  </div>
+
+  <p>Besides these Pieridae, Mr. Bates found four true Papilios,
+  seven Erycinidae, three Castnias (a genus of day-flying moths),
+  and fourteen species of diurnal Bombycidae, all imitating some
+  species of Heliconidae which inhabited the same district; and it
+  is to be especially noted that none of these insects were so
+  abundant as the Heliconidae they resembled, generally they
+  <a name="Page_242" id="Page_242"></a>were far less common, so
+  that Mr. Bates estimated the proportion in some cases as not one
+  to a thousand. Before giving an account of the numerous
+  remarkable cases of mimicry in other parts of the world, and
+  between various groups of insects and of higher animals, it will
+  be well to explain briefly the use and purport of the phenomenon,
+  and also the mode by which it has been brought about.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>How Mimicry has been Produced.</i></p>
+
+  <p>The fact has been now established that the Heliconidae possess
+  an offensive odour and taste, which lead to their being almost
+  entirely free from attack by insectivorous creatures; they
+  possess a peculiar form and mode of flight, and do not seek
+  concealment; while their colours&mdash;although very varied,
+  ranging from deep blue-black, with white, yellow, or vivid red
+  bands and spots, to the most delicate semitransparent wings
+  adorned with pale brown or yellow markings&mdash;are yet always
+  very distinctive, and unlike those of all the other families of
+  butterflies in the same country. It is, therefore, clear that if
+  any other butterflies in the same region, which are eatable and
+  suffer great persecution from insectivorous animals, should come
+  to resemble any of these uneatable species so closely as to be
+  mistaken for them by their enemies, they will obtain thereby
+  immunity from persecution. This is the obvious and sufficient
+  reason why the imitation is useful, and therefore why it occurs
+  in nature. We have now to explain how it has probably been
+  brought about, and also why a still larger number of persecuted
+  groups have not availed themselves of this simple means of
+  protection.</p>
+
+  <p>From the great abundance of the Heliconidae<a name=
+  "FNanchor_99_99" id="FNanchor_99_99"></a><a href=
+  "#Footnote_99_99" class="fnanchor">[99]</a> all over tropical
+  America, the vast number of their genera and species, and their
+  marked distinctions from all other butterflies, it follows that
+  they constitute a group of high antiquity, which in the course of
+  ages has become more and more specialised, and owing to its
+  peculiar advantages has now become a dominant and aggressive
+  race. But when they first arose from some ancestral species or
+  group which, owing to the food <a name="Page_243" id=
+  "Page_243"></a>of the larvae or some other cause, possessed
+  disagreeable juices that caused them to be disliked by the usual
+  enemies of their kind, they were in all probability not very
+  different either in form or coloration from many other
+  butterflies. They would at that time be subject to repeated
+  attacks by insect-eaters, and, even if finally rejected, would
+  often receive a fatal injury. Hence arose the necessity for some
+  distinguishing mark, by which the devourers of butterflies in
+  general might learn that these particular butterflies were
+  uneatable; and every variation leading to such distinction,
+  whether by form, colour, or mode of flight, was preserved and
+  accumulated by natural selection, till the ancestral Heliconoids
+  became well distinguished from eatable butterflies, and
+  thenceforth comparatively free from persecution. Then they had a
+  good time of it. They acquired lazy habits, and flew about
+  slowly. They increased abundantly and spread all over the
+  country, their larvae feeding on many plants and acquiring
+  different habits; while the butterflies themselves varied
+  greatly, and colour being useful rather than injurious to them,
+  gradually diverged into the many coloured and beautifully varied
+  forms we now behold.</p>
+
+  <p>But, during the early stages of this process, some of the
+  Pieridae, inhabiting the same district, happened to be
+  sufficiently like some of the Heliconidae to be occasionally
+  mistaken for them. These, of course, survived while their
+  companions were devoured. Those among their descendants that were
+  still more like Heliconidae again survived, and at length the
+  imitation would become tolerably perfect. Thereafter, as the
+  protected group diverged into distinct species of many different
+  colours, the imitative group would occasionally be able to follow
+  it with similar variations,&mdash;a process that is going on now,
+  for Mr. Bates informs us that in each fresh district he visited
+  he found closely allied representative species or varieties of
+  Heliconidae, and along with them species of Leptalis (Pieridae),
+  which had varied in the same way so as still to be exact
+  imitations. But this process of imitation would be subject to
+  check by the increasing acuteness of birds and other animals
+  which, whenever the eatable Leptalis became numerous, would
+  surely find them out, and would then probably attack both these
+  and their friends the Heliconidae in order to devour <a name=
+  "Page_244" id="Page_244"></a>the former and reject the latter.
+  The Pieridae would, however, usually be less numerous, because
+  their larvae are often protectively coloured and therefore
+  edible, while the larvae of the Heliconidae are adorned with
+  warning colours, spines, or tubercles, and are uneatable. It
+  seems probable that the larvae and pupae of the Heliconidae were
+  the first to acquire the protective distastefulness, both because
+  in this stage they are more defenceless and more liable to fatal
+  injury, and also because we now find many instances in which the
+  larvae are distasteful while the perfect insects are eatable, but
+  I believe none in which the reverse is the case. The larvae of
+  the Pieridae are now beginning to acquire offensive juices, but
+  have not yet obtained the corresponding conspicuous colours;
+  while the perfect insects remain eatable, except perhaps in some
+  Eastern groups, the under sides of whose wings are brilliantly
+  coloured although this is the part which is exposed when at
+  rest.</p>
+
+  <p>It is clear that if a large majority of the larvae of
+  Lepidoptera, as well as the perfect insects, acquired these
+  distasteful properties, so as seriously to diminish the food
+  supply of insectivorous and nestling birds, these latter would be
+  forced by necessity to acquire corresponding tastes, and to eat
+  with pleasure what some of them now eat only under pressure of
+  hunger; and variation and natural selection would soon bring
+  about this change.</p>
+
+  <p>Many writers have denied the possibility of such wonderful
+  resemblances being produced by the accumulation of fortuitous
+  variations, but if the reader will call to mind the large amount
+  of variability that has been shown to exist in all organisms, the
+  exceptional power of rapid increase possessed by insects, and the
+  tremendous struggle for existence always going on, the difficulty
+  will vanish, especially when we remember that nature has the same
+  fundamental groundwork to act upon in the two groups, general
+  similarity of forms, wings of similar texture and outline, and
+  probably some original similarity of colour and marking. Yet
+  there is evidently considerable difficulty in the process, or
+  with these great resources at her command nature would have
+  produced more of these mimicking forms than she has done. One
+  reason of this deficiency probably is, that the imitators, being
+  always fewer in number, have <a name="Page_245" id=
+  "Page_245"></a>not been able to keep pace with the variations of
+  the much more numerous imitated form; another reason may be the
+  ever-increasing acuteness of the enemies, which have again and
+  again detected the imposture and exterminated the feeble race
+  before it has had time to become further modified. The result of
+  this growing acuteness of enemies has been, that those mimics
+  that now survive exhibit, as Mr. Bates well remarks, "a palpably
+  intentional likeness that is perfectly staggering," and also
+  "that those features of the portrait are most attended to by
+  nature which produce the most effective deception when the
+  insects are seen in nature." No one, in fact, can understand the
+  perfection of the imitation who has not seen these species in
+  their native wilds. So complete is it in general effect that in
+  almost every box of butterflies, brought from tropical America by
+  amateurs, are to be found some species of the mimicking Pieridae,
+  Erycinidae, or moths, and the mimicked Heliconidae, placed
+  together under the impression that they are the same species. Yet
+  more extraordinary, it sometimes deceives the very insects
+  themselves. Mr. Trimen states that the male Danais chrysippus is
+  sometimes deceived by the female Diadema bolina which mimics that
+  species. Dr. Fritz M&uuml;ller, writing from Brazil to Professor
+  Meldola, says, "One of the most interesting of our mimicking
+  butterflies is Leptalis melite. The female alone of this species
+  imitates one of our common white Pieridae, which she copies so
+  well that even her own male is often deceived; for I have
+  repeatedly seen the male pursuing the mimicked species, till,
+  after closely approaching and becoming aware of his error, he
+  suddenly returned."<a name="FNanchor_100_100" id=
+  "FNanchor_100_100"></a><a href="#Footnote_100_100" class=
+  "fnanchor">[100]</a> This is evidently not a case of true
+  mimicry, since the species imitated is not protected; but it may
+  be that the less abundant Leptalis is able to mingle with the
+  female Pieridae and thus obtain partial immunity from attack. Mr.
+  Kirby of the insect department of the British Museum informs me
+  that there are several species of South American Pieridae which
+  the female Leptalis melite very nearly resembles. The case,
+  however, is interesting as showing that the butterflies are
+  themselves deceived by a resemblance which is not so great as
+  that of some mimicking species.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_246" id="Page_246"></a><i>Other Examples of Mimicry
+  among Lepidoptera.</i></p>
+
+  <p>In tropical Asia, and eastward to the Pacific Islands, the
+  Danaidae take the place of the Heliconidae of America, in their
+  abundance, their conspicuousness, their slow flight, and their
+  being the subjects of mimicry. They exist under three principal
+  forms or genera. The genus Euploea is the most abundant both in
+  species and individuals, and consists of fine broad-winged
+  butterflies of a glossy or metallic blue-black colour, adorned
+  with pure white, or rich blue, or dusky markings situated round
+  the margins of the wings. Danais has generally more lengthened
+  wings, of a semitransparent greenish or a rich brown colour, with
+  radial or marginal pale spots; while the fine Hestias are of
+  enormous size, of a papery or semitransparent white colour, with
+  dusky or black spots and markings. Each of these groups is
+  mimicked by various species of the genus Papilio, usually with
+  such accuracy that it is impossible to distinguish them on the
+  wing.<a name="FNanchor_101_101" id=
+  "FNanchor_101_101"></a><a href="#Footnote_101_101" class=
+  "fnanchor">[101]</a> Several species of Diadema, a genus of
+  butterflies allied to our Vanessas, also mimic species of Danais,
+  but in this case the females only are affected, a subject which
+  will be discussed in another chapter.</p>
+
+  <p>Another protected group in the Eastern tropics is that of the
+  beautiful day-flying moths forming the family Agaristidae. These
+  are usually adorned with the most brilliant colours or
+  conspicuous markings, they fly slowly in forests among the
+  butterflies and other diurnal insects, and their great abundance
+  sufficiently indicates their possession of some distastefulness
+  which saves them from attack. Under these conditions we may
+  expect to find other moths which are not so protected imitating
+  them, and this is the case. One of the common and wide-ranging
+  species (Opthalmis lincea), found in the islands from Amboyna to
+  New Ireland, is mimicked in a wonderful manner by one of the
+  Liparidae (the family to which our common "tussock" and
+  "vapourer" moths belong). This is a new species collected at
+  Amboyna during the voyage of the <i>Challenger</i>, and has been
+  named Artaxa simulans. Both <a name="Page_247" id=
+  "Page_247"></a>insects are black, with the apex of the fore wings
+  ochre coloured, and the outer half of the hind wings bright
+  orange. The accompanying woodcuts (for the use of which I am
+  indebted to Mr. John Murray of the <i>Challenger</i> Office) well
+  exhibit their striking resemblance to each other.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig24" id="fig24"></a> <img src="images/fig24.jpg"
+    width="400" height="333" alt=
+    "FIG. 24.&mdash;Opthalmis lincea (Agaristidae). Artaxa simulans (Liparidae)."
+    title=
+    "FIG. 24.&mdash;Opthalmis lincea (Agaristidae). Artaxa simulans (Liparidae)." />
+     <b>FIG. 24.&mdash;Opthalmis lincea (Agaristidae).<br />
+    Artaxa simulans (Liparidae).</b>
+  </div>
+
+  <p>In Africa exactly similar phenomena recur, species of Papilio
+  and of Diadema mimicking Danaidae or Acraeidae with the most
+  curious accuracy. Mr. Trimen, who studied this subject in South
+  Africa, has recorded eight species or varieties of Diadema, and
+  eight of Papilio, which each mimic some species of Danais; while
+  eight species or varieties of Panopaea (another genus of
+  Nymphalidae), three of Melanitis (Eurytelidae), and two of
+  Papilio, resemble with equal accuracy some species of
+  Acraea.<a name="FNanchor_102_102" id=
+  "FNanchor_102_102"></a><a href="#Footnote_102_102" class=
+  "fnanchor">[102]</a> He has also independently observed the main
+  facts on which the explanation of the phenomenon rests,&mdash;the
+  unpleasant odour of the Danais and Acraea, extending to their
+  larvae and pupae; their great abundance, slow flight, and
+  disregard of concealment; and he states that while lizards,
+  mantidae, and dragonflies all hunt butterflies, and the rejected
+  wings are to be found abundantly at some of their <a name=
+  "Page_248" id="Page_248"></a>feeding-places, those of the two
+  genera Danais and Acraea were never among them.</p>
+
+  <p>The two groups of the great genus Papilio (the true
+  swallow-tailed butterflies) which have been already referred to
+  as having the special characteristics of uneatable insects, have
+  also their imitators in other groups; and thus, the belief in
+  their inedibility&mdash;derived mainly from their style of
+  warning coloration and their peculiar habits&mdash;is confirmed.
+  In South America, several species of the "Aeneas" group of these
+  butterflies are mimicked by Pieridae and by day-flying moths of
+  the genera Castnia and Pericopis. In the East, Papilio hector, P.
+  diphilus, and P. liris, all belonging to the inedible group, are
+  mimicked by the females of other species of Papilio belonging to
+  very distinct groups; while in Northern India and China, many
+  fine day-flying moths (Epicopeia) have acquired the strange forms
+  and peculiar colours of some of the large inedible Papilios of
+  the same regions.</p>
+
+  <p>In North America, the large and handsome Danais archippus,
+  with rich reddish-brown wings, is very common; and it is closely
+  imitated by Limenitis misippus, a butterfly allied to our "white
+  admiral," but which has acquired a colour quite distinct from
+  that of the great bulk of its allies. In the same country there
+  is a still more interesting case. The beautiful dark bronzy green
+  butterfly, Papilio philenor, is inedible both in larva and
+  perfect insect, and it is mimicked by the equally dark Limenitis
+  ursula. There is also in the Southern and Western States a dark
+  female form of the yellow Papilio turnus, which in all
+  probability obtains protection from its general resemblance to P.
+  philenor. Mr. W.H. Edwards has found, by extensive experiment,
+  that both the dark and yellow females produce their own kinds,
+  with very few exceptions; and he thinks that the dark form has
+  the advantage in the more open regions and in the prairies, where
+  insectivorous birds abound. But in open country the dark form
+  would be quite as conspicuous as the yellow form, if not more so,
+  so that the resemblance to an inedible species would be there
+  more needed.<a name="FNanchor_103_103" id=
+  "FNanchor_103_103"></a><a href="#Footnote_103_103" class=
+  "fnanchor">[103]</a></p>
+
+  <p>The only probable case of mimicry in this country is that of
+  the moth, Diaphora mendica, whose female only is white, <a name=
+  "Page_249" id="Page_249"></a>while the larva is of protective
+  colours, and therefore almost certainly edible. A much more
+  abundant moth, of about the same size and appearing about the
+  same time, is Spilosoma menthrasti, also white, but in this case
+  both it and its larva have been proved to be inedible. The white
+  colour of the female Diaphora, although it must be very
+  conspicuous at night, may, therefore, have been acquired in order
+  to resemble the uneatable Spilosoma, and thus gain some
+  protection.<a name="FNanchor_104_104" id=
+  "FNanchor_104_104"></a><a href="#Footnote_104_104" class=
+  "fnanchor">[104]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Mimicry among Protected (Uneatable) Genera.</i></p>
+
+  <p>Before giving some account of the numerous other cases of
+  warning colours and of mimicry that occur in the animal kingdom,
+  it will be well to notice a curious phenomenon which long puzzled
+  entomologists, but which has at length received a satisfactory
+  explanation.</p>
+
+  <p>We have hitherto considered, that mimicry could only occur
+  when a comparatively scarce and much persecuted species obtained
+  protection by its close external resemblance to a much more
+  abundant uneatable species inhabiting its own district; and this
+  rule undoubtedly prevails among the great majority of mimicking
+  species all over the world. But Mr. Bates also found a number of
+  pairs of species of different genera of Heliconidae, which
+  resembled each other quite as closely as did the other mimicking
+  species he has described; and since all these insects appear to
+  be equally protected by their inedibility, and to be equally free
+  from persecution, it was not easy to see why this curious
+  resemblance existed, or how it had been brought about. That it is
+  not due to close affinity is shown by the fact that the
+  resemblance occurs most frequently between the two distinct
+  sub-families into which (as Mr. Bates first pointed out) the
+  Heliconidae are naturally divided on account of very important
+  structural differences. One of these sub-families (the true
+  Heliconinae) consists of two genera only, Heliconius and Eueides,
+  the other (the Danaoid Heliconinae) of no less than sixteen
+  genera; and, in the instances of mimicry we are now discussing,
+  one of the pairs or <a name="Page_250" id="Page_250"></a>triplets
+  that resemble each other is usually a species of the large and
+  handsome genus Heliconius, the others being species of the genera
+  Mechanitis, Melinaea, or Tithorea, though several species of
+  other Danaoid genera also imitate each other. The following lists
+  will give some idea of the number of these curious imitative
+  forms, and of their presence in every part of the Neotropical
+  area. The bracketed species are those that resemble each other so
+  closely that the difference is not perceptible when they are on
+  the wing.</p>
+
+  <p>In the Lower Amazon region are found&mdash;</p>
+
+  <p><span style="margin-left: 2em;">{ Heliconius
+  sylvana.</span><br />
+  <span style="margin-left: 2em;">{ Melinaea egina.</span><br />
+  <br />
+  <span style="margin-left: 2em;">{ Heliconius numata.</span><br />
+  <span style="margin-left: 2em;">{ Melinaea mneme.</span><br />
+  <span style="margin-left: 2em;">{ Tithorea harmonia.</span><br />
+  <br />
+  <span style="margin-left: 2em;">{ Methona psidii.</span><br />
+  <span style="margin-left: 2em;">{ Thyridia ino.</span><br />
+  <br />
+  <span style="margin-left: 2em;">{ Ceratina ninonia.</span><br />
+  <span style="margin-left: 2em;">{ Melinaea
+  mnasias.</span><br /></p>
+
+  <p>In Central America are found&mdash;</p>
+
+  <p><span style="margin-left: 7em;">{ Heliconius
+  zuleika.</span><br />
+  <span style="margin-left: 2em;">Nicaragua { Melinaea
+  hezia.</span><br />
+  <span style="margin-left: 7em;">{ Mechanitis sp.</span><br />
+  <br />
+  <span style="margin-left: 7em;">{ Heliconius
+  formosus.</span><br />
+  <span style="margin-left: 7em;">{ Tithorea penthias.</span><br />
+  <br />
+  <span style="margin-left: 2em;">Guatemala { Heliconius
+  telchina.</span><br />
+  <span style="margin-left: 7em;">{ Melinaea imitata.</span><br />
+  <br /></p>
+
+  <p>In the Upper Amazon region&mdash;</p>
+
+  <p><span style="margin-left: 2em;">{ Heliconius
+  pardalinus.</span><br />
+  <span style="margin-left: 2em;">{ Melinaea pardalis.</span><br />
+  <br />
+  <span style="margin-left: 2em;">{ Heliconius aurora.</span><br />
+  <span style="margin-left: 2em;">{ Melinaea
+  lucifer.</span><br /></p>
+
+  <p>In New Grenada&mdash;</p>
+
+  <p><span style="margin-left: 2em;">{ Heliconius
+  ismenius.</span><br />
+  <span style="margin-left: 2em;">{ Melinaea messatis.</span><br />
+  <br />
+  <span style="margin-left: 2em;">{ Heliconius
+  messene.</span><br />
+  <span style="margin-left: 2em;">{ Melinaea mesenina.</span><br />
+  <span style="margin-left: 2em;">{ (?) Mechanitis sp.</span><br />
+  <br />
+  <span style="margin-left: 2em;">{ Heliconius
+  hecalesia.</span><br />
+  <span style="margin-left: 2em;">{ Tithorea
+  hecalesina.</span><br />
+  <br />
+  <span style="margin-left: 2em;">{ Heliconius hecuba.</span><br />
+  <span style="margin-left: 2em;">{ Tithorea
+  bonplandi.</span><br /></p>
+
+  <p><a name="Page_251" id="Page_251"></a>In Eastern Peru and
+  Bolivia&mdash;</p>
+
+  <p><span style="margin-left: 2em;">{ Heliconius
+  aristona.</span><br />
+  <span style="margin-left: 2em;">{ Melinaea cydippe.</span><br />
+  <span style="margin-left: 2em;">{ (?) Mechanitis
+  mothone.</span><br /></p>
+
+  <p>In Pernambuco&mdash;</p>
+
+  <p><span style="margin-left: 2em;">{ Heliconius
+  ethra.</span><br />
+  <span style="margin-left: 2em;">{ Mechanitis
+  nesaea.</span><br /></p>
+
+  <p>In Rio Janeiro&mdash;</p>
+
+  <p><span style="margin-left: 2em;">{ Helieonius
+  eucrate.</span><br />
+  <span style="margin-left: 2em;">{ Mechanitis
+  lysimnia.</span><br /></p>
+
+  <p>In South Brazil&mdash;</p>
+
+  <p><span style="margin-left: 2em;">{ Thyridia
+  megisto.</span><br />
+  <span style="margin-left: 2em;">{ Ituna ilione.</span><br />
+  <br />
+  <span style="margin-left: 2em;">{ Acraea thalia.</span><br />
+  <span style="margin-left: 2em;">{ Eueides
+  pavana.</span><br /></p>
+
+  <p>Besides these, a number of species of Ithomia and Napeogenes,
+  and of Napeogenes and Mechanitis, resemble each other with equal
+  accuracy, so that they are liable to be mistaken for each other
+  when on the wing; and no doubt many other equally remarkable
+  cases are yet unnoticed.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig25" id="fig25"></a> <img src="images/fig25.jpg"
+    width="400" height="251" alt=
+    "FIG. 25.&mdash;Wings of Ituna Ilione, female. Wings of Thyridia megisto, female."
+    title=
+    "FIG. 25.&mdash;Wings of Ituna Ilione, female. Wings of Thyridia megisto, female." />
+     <b>FIG. 25.&mdash;Wings of Ituna Ilione, female.<br />
+    Wings of Thyridia megisto, female.</b>
+  </div>
+
+  <p>The figures above of the fore and hind wings of two of these
+  mimicking species, from Dr. Fritz M&uuml;ller's original paper in
+  <i>Kosmos</i>, will serve to show the considerable amount of
+  <a name="Page_252" id="Page_252"></a>difference, in the important
+  character of the neuration of the wings, between these
+  butterflies, which really belong to very distinct and not at all
+  closely allied genera. Other important characters are&mdash;(1)
+  The existence of a small basal cell in the hind wings of Ituna
+  which is wanting in Thyridia; (2) the division of the cell
+  between the veins 1<i>b</i> and 2 of the hind wings in the former
+  genus, while it is undivided in the latter; and (3) the existence
+  in Thyridia of scent-producing tufts of hair on the upper edge of
+  the hind wing, while in Ituna these are wanting; but in place of
+  them are extensible processes at the end of the abdomen, also
+  emitting a powerful scent. These differences characterise two
+  marked subdivisions of the Danaoid Heliconinae, each containing
+  several distinct genera; and these subdivisions are further
+  distinguished by very different forms of larvae, that to which
+  Ituna belongs having from two to four long threadlike tentacles
+  on the back, while in that containing Thyridia these are always
+  absent. The former usually feed on Asclepiadeae, the latter on
+  Solanaceae or Scrophulariaceae.</p>
+
+  <p>The two species figured, though belonging to such distinct and
+  even remote genera, have acquired almost identical tints and
+  markings so as to be deceptively alike. The surface of the wings
+  is, in both, transparent yellowish, with black transverse bands
+  and white marginal spots, while both have similar black-and
+  white-marked bodies and long yellow antennae. Dr. M&uuml;ller
+  states that they both show a preference for the same flowers
+  growing on the edges of the forest paths.<a name=
+  "FNanchor_105_105" id="FNanchor_105_105"></a><a href=
+  "#Footnote_105_105" class="fnanchor">[105]</a></p>
+
+  <p>We will now proceed to give the explanation of these curious
+  similarities, which have remained a complete puzzle for twenty
+  years. Mr. Bates, when first describing them, suggested that they
+  might be due to some form of parallel variation dependent on
+  climatic influences; and I myself adduced other cases of
+  coincident local modifications of colour, which did not appear to
+  be explicable by any form of mimicry.<a name="FNanchor_106_106"
+  id="FNanchor_106_106"></a><a href="#Footnote_106_106" class=
+  "fnanchor">[106]</a> But we neither of us hit upon the simple
+  explanation given by Dr. Fritz M&uuml;ller in 1879.</p>
+
+  <p>His theory is founded on the assumed, but probable, <a name=
+  "Page_253" id="Page_253"></a>fact, that insect-eating birds only
+  learn by experience to distinguish the edible from the inedible
+  butterflies, and in doing so necessarily sacrifice a certain
+  number of the latter. The quantity of insectivorous birds in
+  tropical America is enormous; and the number of young birds which
+  every year have to learn wisdom by experience, as regards the
+  species of butterflies to be caught or to be avoided, is so great
+  that the sacrifice of life of the inedible species must be
+  considerable, and, to a comparatively weak or scarce species, of
+  vital importance. The number thus sacrificed will be fixed by the
+  quantity of young birds, and by the number of experiences
+  requisite to cause them to avoid the inedible species for the
+  future, and not at all by the numbers of individuals of which
+  each species consists. Hence, if two species are so much alike as
+  to be mistaken for one another, the fixed number annually
+  sacrificed by inexperienced birds will be divided between them,
+  and both will benefit. But if the two species are very unequal in
+  numbers, the benefit will be comparatively slight for the more
+  abundant species, but very great for the rare one. To the latter
+  it may make all the difference between safety and
+  destruction.</p>
+
+  <p>To give a rough numerical example. Let us suppose that in a
+  given limited district there are two species of Heliconidae, one
+  consisting of only 1000, the other of 100,000 individuals, and
+  that the quota required annually in the same district for the
+  instruction of young insectivorous birds is 500. By the larger
+  species this loss will be hardly felt; to the smaller it will
+  mean the most dreadful persecution resulting in a loss of half
+  the total population. But, let the two species become
+  superficially alike, so that the birds see no difference between
+  them. The quota of 500 will now be taken from a combined
+  population of 101,000 butterflies, and if proportionate numbers
+  of each suffer, then the weak species will only lose five
+  individuals instead of 500 as it did before. Now we know that the
+  different species of Heliconidae are not equally abundant, some
+  being quite rare; so that the benefit to be derived in these
+  latter cases would be very important. A slight inferiority in
+  rapidity of flight or in powers of eluding attack might also be a
+  cause of danger to an inedible species of scanty numbers, and in
+  this case too the being <a name="Page_254" id=
+  "Page_254"></a>merged in another much more abundant species, by
+  similarity of external appearance, would be an advantage.</p>
+
+  <p>The question of fact remains. Do young birds pursue and
+  capture these distasteful butterflies till they have learned by
+  bitter experience what species to avoid? On this point Dr.
+  M&uuml;ller has fortunately been able to obtain some direct
+  evidence, by capturing several Acraeas and Heliconidae which had
+  evidently been seized by birds but had afterwards escaped, as
+  they had pieces torn out of the wing, sometimes symmetrically out
+  of both wings, showing that the insect had been seized when at
+  rest and with the two pairs of wings in contact. There is,
+  however, a general impression that this knowledge is hereditary,
+  and does not need to be acquired by young birds; in support of
+  which view Mr. Jenner Weir states that his birds always
+  disregarded inedible caterpillars. When, day by day, he threw
+  into his aviary various larvae, those which were edible were
+  eaten immediately, those which were inedible were no more noticed
+  than if a pebble had been thrown before the birds.</p>
+
+  <p>The cases, however, are not strictly comparable. The birds
+  were not young birds of the first year; and, what is more
+  important, edible larvae have a comparatively simple coloration,
+  being always brown or green and smooth. Uneatable larvae, on the
+  other hand, comprise all that are of conspicuous colours and are
+  hairy or spiny. But with butterflies there is no such simplicity
+  of contrast. The eatable butterflies comprise not only brown or
+  white species, but hundreds of Nymphalidae, Papilionidae,
+  Lycaenidae, etc., which are gaily coloured and of an immense
+  variety of patterns. The colours and patterns of the inedible
+  kinds are also greatly varied, while they are often equally gay;
+  and it is quite impossible to suppose that any amount of instinct
+  or inherited habit (if such a thing exists) could enable young
+  insectivorous birds to distinguish all the species of one kind
+  from all those of the other. There is also some evidence to show
+  that animals do learn by experience what to eat and what to
+  avoid. Mr. Poulton was assured by Rev. G.J. Bursch that very
+  young chickens peck at insects which they afterwards avoid.
+  Lizards, too, often seized larvae which they were unable to eat
+  and ultimately rejected.</p>
+
+  <p><a name="Page_255" id="Page_255"></a>Although the Heliconidae
+  present, on the whole, many varieties of coloration and pattern,
+  yet, in proportion to the number of distinct species in each
+  district, the types of coloration are few and very well marked,
+  and thus it becomes easier for a bird or other animal to learn
+  that all belonging to such types are uneatable. This must be a
+  decided advantage to the family in question, because, not only do
+  fewer individuals of each species need to be sacrificed in order
+  that their enemies may learn the lesson of their inedibility, but
+  they are more easily recognised at a distance, and thus escape
+  even pursuit. There is thus a kind of mimicry between closely
+  allied species as well as between species of distinct genera, all
+  tending to the same beneficial end. This may be seen in the four
+  or five distinct species of the genus Heliconius which all have
+  the same peculiar type of coloration&mdash;a yellow band across
+  the upper wings and radiating red stripes on the lower,&mdash;and
+  are all found in the same forests of the Lower Amazon; in the
+  numerous very similar species of Ithomia with transparent wings,
+  found in every locality of the same region; and in the very
+  numerous species of Papilio of the "Aeneas" group, all having a
+  similar style of marking, the resemblance being especially close
+  in the females. The very uniform type of colouring of the
+  blue-black Euplaeas and of the fulvous Acraeas is of the same
+  character.<a name="FNanchor_107_107" id=
+  "FNanchor_107_107"></a><a href="#Footnote_107_107" class=
+  "fnanchor">[107]</a> In all these cases the similarity of the
+  allied species is so great, that, when they are on the wing at
+  some distance off, it is difficult to distinguish one species
+  from another. But this close external resemblance is not always a
+  sign of very near affinity; for minute examination detects
+  differences in the form and scalloping of the wings, in the
+  markings on the body, and in those on the under surface of the
+  wings, which do not usually characterise the closest allies. It
+  is to be further noted, that the presence of groups of very
+  similar species of the same genus, in one locality, is not at all
+  a common phenomenon among unprotected groups. Usually the species
+  of a genus found in one locality are each well marked and belong
+  to somewhat distinct types, while the <a name="Page_256" id=
+  "Page_256"></a>closely allied forms&mdash;those that require
+  minute examination to discriminate them as distinct
+  species&mdash;are most generally found in separate areas, and are
+  what are termed representative forms.</p>
+
+  <p>The extension we have now given to the theory of mimicry is
+  important, since it enables us to explain a much wider range of
+  colour phenomena than those which were first imputed to mimicry.
+  It is in the richest butterfly region in the world&mdash;the
+  Amazon valley&mdash;that we find the most abundant evidence of
+  the three distinct sets of facts, all depending on the same
+  general principle. The form of mimicry first elucidated by Mr.
+  Bates is characterised by the presence in each locality of
+  certain butterflies, or other insects, themselves edible and
+  belonging to edible groups, which derived protection from having
+  acquired a deceptive resemblance to some of the inedible
+  butterflies in the same localities, which latter were believed to
+  be wholly free from the attacks of insectivorous birds. Then came
+  the extension of the principle, by Dr. F. M&uuml;ller, to the
+  case of species of distinct genera of the inedible butterflies
+  resembling each other quite as closely as in the former cases,
+  and like them always found in the same localities. They derive
+  mutual benefit from becoming, in appearance, one species, from
+  which a certain toll is taken annually to teach the young
+  insectivorous birds that they are uneatable. Even when the two or
+  more species are approximately equal in numbers, they each derive
+  a considerable benefit from thus combining their forces; but when
+  one of the species is scarce or verging on extinction, the
+  benefit becomes exceedingly great, being, in fact, exactly
+  apportioned to the need of the species.</p>
+
+  <p>The third extension of the same principle explains the
+  grouping of allied species of the same genera of inedible
+  butterflies into sets, each having a distinct type of coloration,
+  and each consisting of a number of species which can hardly be
+  distinguished on the wing. This must be useful exactly in the
+  same way as in the last case, since it divides the inevitable
+  toll to insectivorous birds and other animals among a number of
+  species. It also explains the fact of the great similarity of
+  many species of inedible insects in the same locality&mdash;a
+  similarity which does not obtain to anything <a name="Page_257"
+  id="Page_257"></a>like the same extent among the edible species.
+  The explanation of the various phenomena of resemblance and
+  mimicry, presented by the distasteful butterflies, may now be
+  considered tolerably complete.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Mimicry in other Orders of Insects.</i></p>
+
+  <p>A very brief sketch of these phenomena will be given, chiefly
+  to show that the same principle prevails throughout nature, and
+  that, wherever a rather extensive group is protected, either by
+  distastefulness or offensive weapons, there are usually some
+  species of edible and inoffensive groups that gain protection by
+  imitating them. It has been already stated that the Telephoridae,
+  Lampyridae, and other families of soft-winged beetles, are
+  distasteful; and as they abound in all parts of the world, and
+  especially in the tropics, it is not surprising that insects of
+  many other groups should imitate them. This is especially the
+  case with the longicorn beetles, which are much persecuted by
+  insectivorous birds; and everywhere in tropical regions some of
+  these are to be found so completely disguised as to be mistaken
+  for species of the protected groups. Numbers of these imitations
+  have been already recorded by Mr. Bates and myself, but I will
+  here refer to a few others.</p>
+
+  <p>In the recently published volumes on the Longicorn and
+  Malacoderm beetles of Central America<a name="FNanchor_108_108"
+  id="FNanchor_108_108"></a><a href="#Footnote_108_108" class=
+  "fnanchor">[108]</a> there are numbers of beautifully coloured
+  figures of the new species; and on looking over them we are
+  struck by the curious resemblance of some of the Longicorns to
+  species of the Malacoderm group. In some cases we discover
+  perfect mimics, and on turning to the descriptions we always find
+  these pairs to come from the same locality. Thus the Otheostethus
+  melanurus, one of the Prionidae, imitates the malacoderm,
+  Lucidota discolor, in form, peculiar coloration, and size, and
+  both are found at Chontales in Nicaragua, the species mimicked
+  having, however, as is usual, a wider range. The curious and very
+  rare little longicorn, Tethlimmena aliena, quite unlike its
+  nearest allies in the same country, is an exact copy on a
+  somewhat smaller scale of a malacoderm, Lygistopterus amabilis,
+  both <a name="Page_258" id="Page_258"></a>found at Chontales. The
+  pretty longicorn, Callia albicornis, closely resembles two
+  species of malacoderms (Silis chalybeipennis and Colyphus
+  signaticollis), all being small beetles with red head and thorax
+  and bright blue elytra, and all three have been found at Panama.
+  Many other species of Callia also resemble other malacoderms; and
+  the longicorn genus Lycidola has been named from its resemblance
+  to various species of the Lycidae, one of the species here
+  figured (Lycidola belti) being a good mimic of Calopteron
+  corrugatum and of several other allied species, all being of
+  about the same size and found at Chontales. In these cases, and
+  in most others, the longicorn beetles have lost the general form
+  and aspect of their allies to take on the appearance of a
+  distinct tribe. Some other groups of beetles, as the Elateridae
+  and Eucnemidae, also deceptively mimic malacoderms.</p>
+
+  <p>Wasps and bees are often closely imitated by insects of other
+  orders. Many longicorn beetles in the tropics exactly mimic
+  wasps, bees, or ants. In Borneo a large black wasp, whose wings
+  have a broad white patch near the apex (Mygnimia aviculus), is
+  closely imitated by a heteromerous beetle (Coloborhombus
+  fasciatipennis), which, contrary to the general habit of beetles,
+  keeps its wings expanded in order to show the white patch on
+  their apex, the wing-coverts being reduced to small oval scales,
+  as shown in the figure. This is a most remarkable instance of
+  mimicry, because the beetle has had to acquire so many characters
+  which are unknown among its allies (except in another species
+  from Java)&mdash;the expanded wings, the white band on them, and
+  the oval scale-like elytra.<a name="FNanchor_109_109" id=
+  "FNanchor_109_109"></a><a href="#Footnote_109_109" class=
+  "fnanchor">[109]</a> Another remarkable case has been noted by
+  Mr. Neville Goodman, in Egypt, where a common hornet (Vespa
+  orientalis) is exactly imitated in colour, size, shape, attitude
+  when at rest, and mode of flight, by a beetle of the genus
+  Laphria.<a name="FNanchor_110_110" id=
+  "FNanchor_110_110"></a><a href="#Footnote_110_110" class=
+  "fnanchor">[110]</a></p>
+
+  <p>The tiger-beetles (Cicindelidae) are also the subjects of
+  mimicry by more harmless insects. In the Malay Islands I found a
+  heteromerous beetle which exactly resembled a Therates, both
+  being found running on the trunks of trees. A longicorn
+  (Collyrodes Lacordairei) mimics Collyris, another genus of the
+  same family; while in the Philippine Islands <a name="Page_259"
+  id="Page_259"></a>there is a cricket (Condylodeira
+  tricondyloides), which so closely resembles a tiger-beetle of the
+  genus Tricondyla that the experienced entomologist, Professor
+  Westwood, at first placed it in his cabinet among those
+  beetles.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig26" id="fig26"></a> <img src="images/fig26.jpg"
+    width="400" height="485" alt=
+    "FIG. 26.&mdash;Mygnimia aviculus (Wasp). Coloborhombus fasciatipennis (Beetle)."
+    title=
+    "FIG. 26.&mdash;Mygnimia aviculus (Wasp). Coloborhombus fasciatipennis (Beetle)." />
+     <b>FIG. 26.&mdash;Mygnimia aviculus (Wasp).<br />
+    Coloborhombus fasciatipennis (Beetle).</b>
+  </div>
+
+  <p><a name="Page_260" id="Page_260"></a></p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig27" id="fig27"></a> <img src="images/fig27.jpg"
+    width="400" height="502" alt=
+    "FIG. 27.&mdash;a. Doliops sp. (Longicorn) mimics Pachyrhynchus orbifae."
+    title=
+    "FIG. 27. a. Doliops sp. (Longicorn) mimics Pachyrhynchus orbifae." />
+
+    <p>FIG. 27.</p>
+
+    <p style=
+    'font-size:smaller; font-weight:bolder; text-align:left;'>a.
+    Doliops sp. (Longicorn) mimics Pachyrhynchus orbifae, (b) (a
+    hard curculio).<br />
+    c. Doliops curculionoides mimics (d) Pachyrhynchus sp.<br />
+    e. Scepastus pachyrhynchoides (a grasshopper), mimics (f)
+    Apocyrtus sp. (a hard curculio).<br />
+    g. Doliops sp. mimics (h) Pachyrhynchus sp.<br />
+    i. Phoraspis (grasshopper) mimics (k) a Coccinella.<br />
+    <br />
+    All the above are from the Philippines. The exact
+    correspondence of the colours of the insects themselves renders
+    the mimicry much more complete in nature than it appears in the
+    above figures.</p>
+  </div>
+
+  <p>One of the characters by which some beetles are protected is
+  excessive hardness of the elytra and integuments. Several genera
+  of weevils (Curculionidae) are thus saved from attack, and these
+  are often mimicked by species of softer and more eatable groups.
+  In South America, the genus Heilipus is one of these hard groups,
+  and both Mr. Bates and M. Roelofs, a Belgian entomologist, have
+  noticed that species of other genera exactly mimic them. So, in
+  the Philippines, there <a name="Page_261" id="Page_261"></a>is a
+  group of Curculionidae, forming the genus Pachyrhynchus, in which
+  all the species are adorned with the most brilliant metallic
+  colours, banded and spotted in a curious manner, and are very
+  smooth and hard. Other genera of Curculionidae (Desmidophorus,
+  Alcides), which are usually very differently coloured, have
+  species in the Philippines which mimic the Pachyrhynchi; and
+  there are also several longicorn beetles (Aprophata, Doliops,
+  Acronia, and Agnia), which also mimic them. Besides these, there
+  are some longicorns and cetonias which reproduce the same colours
+  and markings; and there is even a cricket (Scepastus
+  pachyrhynchoides), which has taken on the form and peculiar
+  coloration of these beetles in order to escape from enemies,
+  which then avoid them as uneatable.<a name="FNanchor_111_111" id=
+  "FNanchor_111_111"></a><a href="#Footnote_111_111" class=
+  "fnanchor">[111]</a> The figures on the opposite page exhibit
+  several other examples of these mimicking insects.</p>
+
+  <p>Innumerable other cases of mimicry occur among tropical
+  insects; but we must now pass on to consider a few of the very
+  remarkable, but much rarer instances, that are found among the
+  higher animals.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Mimicry among the Vertebrata.</i></p>
+
+  <p>Perhaps the most remarkable cases yet known are those of
+  certain harmless snakes which mimic poisonous species. The genus
+  Elaps, in tropical America, consists of poisonous snakes which do
+  not belong to the viper family (in which are included the
+  rattlesnakes and most of those which are poisonous), and which do
+  not possess the broad triangular head which characterises the
+  latter. They have a peculiar style of coloration, consisting of
+  alternate rings of red and black, or red, black, and yellow, of
+  different widths and grouped in various ways in the different
+  species; and it is a style of coloration which does not occur in
+  any other group of snakes in the world. But in the same regions
+  are found three genera of harmless snakes, belonging to other
+  families, some few species of which mimic the poisonous Elaps,
+  often so exactly that it is with difficulty one can be
+  distinguished from the other. Thus Elaps fulvius in Guatemala is
+  imitated by the harmless Pliocerus equalis; Elaps corallinus in
+  Mexico is mimicked by the <a name="Page_262" id=
+  "Page_262"></a>harmless Homalocranium semicinctum; and Elaps
+  lemniscatus in Brazil is copied by Oxyrhopus trigeminus; while in
+  other parts of South America similar cases of mimicry occur,
+  sometimes two harmless species imitating the same poisonous
+  snake.</p>
+
+  <p>A few other instances of mimicry in this group have been
+  recorded. There is in South Africa an egg-eating snake
+  (Dasypeltis scaber), which has neither fangs nor teeth, yet it is
+  very like the Berg adder (Clothos atropos), and when alarmed
+  renders itself still more like by flattening out its head and
+  darting forward with a hiss as if to strike a foe.<a name=
+  "FNanchor_112_112" id="FNanchor_112_112"></a><a href=
+  "#Footnote_112_112" class="fnanchor">[112]</a> Dr. A.B. Meyer has
+  also discovered that, while some species of the genus Callophis
+  (belonging to the same family as the American Elaps) have large
+  poison fangs, other species of the same genus have none; and that
+  one of the latter (C. gracilis) resembles a poisonous species (C.
+  intestinalis) so closely, that only an exact comparison will
+  discover the difference of colour and marking. A similar kind of
+  resemblance is said to exist between another harmless snake,
+  Megaerophis flaviceps, and the poisonous Callophis bivirgatus;
+  and in both these cases the harmless snake is less abundant than
+  the poisonous one, as occurs in all examples of true
+  mimicry.<a name="FNanchor_113_113" id=
+  "FNanchor_113_113"></a><a href="#Footnote_113_113" class=
+  "fnanchor">[113]</a></p>
+
+  <p>In the genus Elaps, above referred to, the very peculiar style
+  of colour and marking is evidently a "warning colour" for the
+  purpose of indicating to snake-eating birds and mammals that
+  these species are poisonous; and this throws light on the
+  long-disputed question of the use of the rattle of the
+  rattlesnake. This reptile is really both sluggish and timid, and
+  is very easily captured by those who know its habits. If gently
+  tapped on the head with a stick, it will coil itself up and lie
+  still, only raising its tail and rattling. It may then be easily
+  caught. This shows that the rattle is a warning to its enemies
+  that it is dangerous to proceed to extremities; and the creature
+  has probably acquired this structure and habit because it
+  frequents open or rocky districts where protective colour is
+  needful to save it from being pounced upon by buzzards or other
+  snake-eaters. Quite parallel in function is the expanded hood of
+  the Indian cobra, a <a name="Page_263" id=
+  "Page_263"></a>poisonous snake which belongs also to the
+  Elapidae. This is, no doubt, a warning to its foes, not an
+  attempt to terrify its prey; and the hood has been acquired, as
+  in the case of the rattlesnake, because, protective coloration
+  being on the whole useful, some mark was required to distinguish
+  it from other protectively coloured, but harmless, snakes. Both
+  these species feed on active creatures capable of escaping if
+  their enemy were visible at a moderate distance.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Mimicry among Birds.</i></p>
+
+  <p>The varied forms and habits of birds do not favour the
+  production among them of the phenomena of warning colours or of
+  mimicry; and the extreme development of their instincts and
+  reasoning powers, as well as their activity and their power of
+  flight, usually afford them other means of evading their enemies.
+  Yet there are a few imperfect, and one or two very perfect cases
+  of true mimicry to be found among them. The less perfect examples
+  are those presented by several species of cuckoos, an exceedingly
+  weak and defenceless group of birds. Our own cuckoo is, in colour
+  and markings, very like a sparrow-hawk. In the East, several of
+  the small black cuckoos closely resemble the aggressive
+  drongo-shrikes of the same country, and the small metallic
+  cuckoos are like glossy starlings; while a large ground-cuckoo of
+  Borneo (Carpococcyx radiatus) resembles one of the fine pheasants
+  (Euplocamus) of the same country, both in form and in its rich
+  metallic colours.</p>
+
+  <p>More perfect cases of mimicry occur between some of the
+  dull-coloured orioles in the Malay Archipelago and a genus of
+  large honey-suckers&mdash;the Tropidorhynchi or "Friar-birds."
+  These latter are powerful and noisy birds which go in small
+  flocks. They have long, curved, and sharp beaks, and powerful
+  grasping claws; and they are quite able to defend themselves,
+  often driving away crows and hawks which venture to approach them
+  too nearly. The orioles, on the other hand, are weak and timid
+  birds, and trust chiefly to concealment and to their retiring
+  habits to escape persecution. In each of the great islands of the
+  Austro-Malayan region there is a distinct species of
+  Tropidorhynchus, and there is always along with it an oriole that
+  exactly mimics it. All the Tropidorhynchi <a name="Page_264" id=
+  "Page_264"></a>have a patch of bare black skin round the eyes,
+  and a ruff of curious pale recurved feathers on the nape, whence
+  their name of Friar-birds, the ruff being supposed to resemble
+  the cowl of a friar. These peculiarities are imitated in the
+  orioles by patches of feathers of corresponding colours; while
+  the different tints of the two species in each island are exactly
+  the same. Thus in Bouru both are earthy brown; in Ceram they are
+  both washed with yellow ochre; in Timor the under surface is pale
+  and the throat nearly white, and Mr. H.O. Forbes has recently
+  discovered another pair in the island of Timor Laut. The close
+  resemblance of these several pairs of birds, of widely different
+  families, is quite comparable with that of many of the insects
+  already described. It is so close that the preserved specimens
+  have even deceived naturalists; for, in the great French work,
+  <i>Voyage de l'Astrolabe</i>, the oriole of Bouru is actually
+  described and figured as a honey-sucker; and Mr. Forbes tells us
+  that, when his birds were submitted to Dr. Sclater for
+  description, the oriole and the honey-sucker were, previous to
+  close examination, considered to be the same species.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Objections to the Theory of Mimicry.</i></p>
+
+  <p>To set forth adequately the varied and surprising facts of
+  mimicry would need a large and copiously illustrated volume; and
+  no more interesting subject could be taken up by a naturalist who
+  has access to our great collections and can devote the necessary
+  time to search out the many examples of mimicry that lie hidden
+  in our museums. The brief sketch of the subject that has been
+  here given will, however, serve to indicate its nature, and to
+  show the weakness of the objections that were at first made to
+  it. It was urged that the action of "like conditions," with
+  "accidental resemblances" and "reversion to ancestral types,"
+  would account for the facts. If, however, we consider the actual
+  phenomena as here set forth, and the very constant conditions
+  under which they occur, we shall see how utterly inadequate are
+  these causes, either singly or combined. These constant
+  conditions are&mdash;</p>
+
+  <div class="blockquot">
+    <p>1. That the imitative species occur in the same area and
+    occupy the very same station as the imitated.</p>
+
+    <p>2. That the imitators are always the more defenceless.</p>
+
+    <p><a name="Page_265" id="Page_265"></a>3. That the imitators
+    are always less numerous in individuals.</p>
+
+    <p>4. That the imitators differ from the bulk of their
+    allies.</p>
+
+    <p>5. That the imitation, however minute, is <i>external</i>
+    and <i>visible</i> only, never extending to internal characters
+    or to such as do not affect the external appearance.</p>
+  </div>
+
+  <p>These five characteristic features of mimicry show us that it
+  is really an exceptional form of protective resemblance.
+  Different species in the same group of organisms may obtain
+  protection in different ways: some by a general resemblance to
+  their environment; some by more exactly imitating the objects
+  that surround them&mdash;bark, or leaf, or flower; while others
+  again gain an equal protection by resembling some species which,
+  from whatever cause, is almost as free from attack as if it were
+  a leaf or a flower. This immunity may depend on its being
+  uneatable, or dangerous, or merely strong; and it is the
+  resemblance to such creatures for the purpose of sharing in their
+  safety that constitutes mimicry.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Concluding Remarks on Warning Colours and Mimicry.</i></p>
+
+  <p>Colours which have been acquired for the purpose of serving as
+  a warning of inedibility, or of the possession of dangerous
+  offensive weapons, are probably more numerous than have been
+  hitherto supposed; and, if so, we shall be able to explain a
+  considerable amount of colour in nature for which no use has
+  hitherto been conjectured. The brilliant and varied colours of
+  sea-anemones and of many coral animals will probably come under
+  this head, since we know that many of them possess the power of
+  ejecting stinging threads from various parts of their bodies
+  which render them quite uneatable to most animals. Mr. Gosse
+  describes how, on putting an Anthea into a tank containing a
+  half-grown bullhead (Cottus bubalis) which had not been fed for
+  some time, the fish opened his mouth and sucked in the morsel,
+  but instantly shot it out again. He then seized it a second time,
+  and after rolling it about in his mouth for a moment shot it out
+  again, and then darted away to hide himself in a hole. Some
+  tropical fishes, however, of the genera Tetrodon, Pseudoscarus,
+  Astracion, and a few others, seem <a name="Page_266" id=
+  "Page_266"></a>to have acquired the power of feeding on corals
+  and medusae; and the beautiful bands and spots and bright colours
+  with which they are frequently adorned, may be either protective
+  when feeding in the submarine coral groves, or may, in some
+  cases, be warning colours to show that they themselves are
+  poisonous and uneatable.</p>
+
+  <p>A remarkable illustration of the wide extension of warning
+  colours, and their very definite purpose in nature, is afforded
+  by what may now be termed "Mr. Belt's frog." Frogs in all parts
+  of the world are, usually, protectively coloured with greens or
+  browns; and the little tree-frogs are either green like the
+  leaves they rest upon, or curiously mottled to imitate bark or
+  dead leaves. But there are a certain number of very gaily
+  coloured frogs, and these do not conceal themselves as frogs
+  usually do. Such was the small toad found by Darwin at Bahia
+  Blanca, which was intense black and bright vermilion, and crawled
+  about in the sunshine over dry sand-hills and arid plains. And in
+  Nicaragua, Mr. Belt found a little frog gorgeously dressed in a
+  livery of red and blue, which did not attempt concealment and was
+  very abundant, a combination of characters which convinced him
+  that it was uneatable. He, therefore, took a few specimens home
+  with him and gave them to his fowls and ducks, but none would
+  touch them. At last, by throwing down pieces of meat, for which
+  there was a great competition among the poultry, he managed to
+  entice a young duck into snatching up one of the little frogs.
+  Instead of swallowing it, however, the duck instantly threw it
+  out of its mouth, and went about jerking its head as if trying to
+  get rid of some unpleasant taste.<a name="FNanchor_114_114" id=
+  "FNanchor_114_114"></a><a href="#Footnote_114_114" class=
+  "fnanchor">[114]</a></p>
+
+  <p>The power of predicting what will happen in a given case is
+  always considered to be a crucial test of a true theory, and if
+  so, the theory of warning colours, and with it that of mimicry,
+  must be held to be well established. Among the creatures which
+  probably have warning colours as a sign of inedibility are, the
+  brilliantly coloured nudibranchiate molluscs, those curious
+  annelids the Nereis and the Aphrodite or sea-mouse, and many
+  other marine animals. The brilliant colours of the scallops
+  (Pecten)and some other bivalve shells are perhaps <a name=
+  "Page_267" id="Page_267"></a>an indication of their hardness and
+  consequent inedibility, as in the case of the hard beetles; and
+  it is not improbable that some of the phosphorescent fishes and
+  other marine organisms may, like the glow-worm, hold out their
+  lamp as a warning to enemies.<a name="FNanchor_115_115" id=
+  "FNanchor_115_115"></a><a href="#Footnote_115_115" class=
+  "fnanchor">[115]</a> In Queensland there is an exceedingly
+  poisonous spider, whose bite will kill a dog, and cause severe
+  illness with excruciating pain in man. It is black, with a bright
+  vermilion patch on the middle of the body; and it is so well
+  recognised by this conspicuous coloration that even the
+  spider-hunting wasps avoid it.<a name="FNanchor_116_116" id=
+  "FNanchor_116_116"></a><a href="#Footnote_116_116" class=
+  "fnanchor">[116]</a></p>
+
+  <p>Locusts and grasshoppers are generally of green protective
+  tints, but there are many tropical species most gaudily decorated
+  with red, blue, and black colours. On the same general grounds as
+  those by which Mr. Belt predicted the inedibility of his
+  conspicuous frog, we might safely predict the same for these
+  insects; but we have fortunately a proof that they are so
+  protected, since Mr. Charles Home states that one of the bright
+  coloured Indian locusts was invariably rejected when offered to
+  birds and lizards.<a name="FNanchor_117_117" id=
+  "FNanchor_117_117"></a><a href="#Footnote_117_117" class=
+  "fnanchor">[117]</a></p>
+  <hr style='width: 45%;' />
+
+  <p>The examples now given lead us to the conclusion that colours
+  acquired for the purpose of serving as a danger-signal to enemies
+  are very widespread in nature, and, with the corresponding
+  colours of the species which mimic them, furnish us with a
+  rational explanation of a considerable portion of the coloration
+  of animals which is outside the limits of those colours that have
+  been acquired for either protection or recognition. There
+  remains, however, another set of colours, chiefly among the
+  higher animals, which, being connected with some of the most
+  interesting and most disputed questions in natural history, must
+  be discussed in a separate chapter.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_92_92" id="Footnote_92_92"></a><a href=
+      "#FNanchor_92_92"><span class="label">[92]</span></a>
+      <i>Nature</i>, vol. iii. p. 165. Professor Meldola observed
+      that specimens of Danais and Euplaea in collections were less
+      subject to the attacks of mites <i>(Proc. Ent. Soc.</i>,
+      1877, p. xii.); and this was corroborated by Mr. Jenner Weir.
+      <i>Entomologist</i>, 1882, vol. xv. p. 160.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_93_93" id="Footnote_93_93"></a><a href=
+      "#FNanchor_93_93"><span class="label">[93]</span></a> See
+      Darwin's <i>Descent of Man</i>, p. 325.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_94_94" id="Footnote_94_94"></a><a href=
+      "#FNanchor_94_94"><span class="label">[94]</span></a>
+      <i>Transactions of the Entomological Society of London</i>,
+      1869, p. 21.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_95_95" id="Footnote_95_95"></a><a href=
+      "#FNanchor_95_95"><span class="label">[95]</span></a>
+      <i>Ibid.</i>, p. 27.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_96_96" id="Footnote_96_96"></a><a href=
+      "#FNanchor_96_96"><span class="label">[96]</span></a>
+      <i>Nature</i>, vol. iii. p. 147.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_97_97" id="Footnote_97_97"></a><a href=
+      "#FNanchor_97_97"><span class="label">[97]</span></a>
+      Stainton's <i>Manual of Butterflies and Moths</i>, vol. i. p.
+      93; E.B. Poulton, <i>Proceedings of the Zool. Soc. of
+      London</i>, 1887, pp. 191-274.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_98_98" id="Footnote_98_98"></a><a href=
+      "#FNanchor_98_98"><span class="label">[98]</span></a> See
+      <i>Transactions of the Linnean Society</i>, vol. xxiii. pp.
+      495-566, coloured plates.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_99_99" id="Footnote_99_99"></a><a href=
+      "#FNanchor_99_99"><span class="label">[99]</span></a> These
+      butterflies are now divided into two sub-families, one of
+      which is placed with the Danaidae; but to avoid confusion I
+      shall always speak of the American genera under the old term
+      Heliconidae.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_100_100" id=
+      "Footnote_100_100"></a><a href=
+      "#FNanchor_100_100"><span class="label">[100]</span></a> R.
+      Meldola in <i>Ann. and Mag. of Nat. Hist.</i>, Feb. 1878, p.
+      158.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_101_101" id=
+      "Footnote_101_101"></a><a href=
+      "#FNanchor_101_101"><span class="label">[101]</span></a> See
+      <i>Trans. Linn. Soc.</i>, vol. xxv. Wallace, on Variation of
+      Malayan Papilionidae; and, Wallace's <i>Contributions to
+      Natural Selection</i> chaps. iii. and iv., where full details
+      are given.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_102_102" id=
+      "Footnote_102_102"></a><a href=
+      "#FNanchor_102_102"><span class="label">[102]</span></a> See
+      <i>Trans. Linn. Soc.</i>, vol. xxvi., with two coloured
+      plates illustrating cases of mimicry.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_103_103" id=
+      "Footnote_103_103"></a><a href=
+      "#FNanchor_103_103"><span class="label">[103]</span></a>
+      Edwards's <i>Butterflies of North America</i>, second series,
+      part vi.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_104_104" id=
+      "Footnote_104_104"></a><a href=
+      "#FNanchor_104_104"><span class="label">[104]</span></a>
+      Professor Meldola informs me that he has recorded another
+      case of mimicry among British moths, in which Acidalia
+      subsericata imitates Asthena candidata. See <i>Ent. Mo.
+      Mag.</i>, vol. iv. p. 163.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_105_105" id=
+      "Footnote_105_105"></a><a href=
+      "#FNanchor_105_105"><span class="label">[105]</span></a> From
+      Professor Meldola's translation of Dr. F. M&uuml;ller's
+      paper, in <i>Proc. Ent. Soc. Lond.</i>, 1879, p. xx.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_106_106" id=
+      "Footnote_106_106"></a><a href=
+      "#FNanchor_106_106"><span class="label">[106]</span></a>
+      <i>Island Life</i>, p. 255.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_107_107" id=
+      "Footnote_107_107"></a><a href=
+      "#FNanchor_107_107"><span class="label">[107]</span></a> This
+      extension of the theory of mimicry was pointed out by
+      Professor Meldola in the paper already referred to; and he
+      has answered the objections to Dr. F. M&uuml;ller's theory
+      with great force in the <i>Annals and Mag. of Nat. Hist.</i>,
+      1882, p. 417.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_108_108" id=
+      "Footnote_108_108"></a><a href=
+      "#FNanchor_108_108"><span class="label">[108]</span></a>
+      Godman and Salvin's <i>Biologia Centrali-Americana, Insecta,
+      Coleoptera</i>, vol. iii. part ii., and vol. v.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_109_109" id=
+      "Footnote_109_109"></a><a href=
+      "#FNanchor_109_109"><span class="label">[109]</span></a>
+      <i>Trans. Ent. Soc.</i>, 1885, p. 369.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_110_110" id=
+      "Footnote_110_110"></a><a href=
+      "#FNanchor_110_110"><span class="label">[110]</span></a>
+      <i>Proc. Cambridge Phil. Soc.</i>, vol. iii. part ii.,
+      1877.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_111_111" id=
+      "Footnote_111_111"></a><a href=
+      "#FNanchor_111_111"><span class="label">[111]</span></a>
+      <i>Compte-Rendu de la Soci&eacute;t&eacute; Entomologique de
+      Belgaue</i>, series ii., No. 59, 1878.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_112_112" id=
+      "Footnote_112_112"></a><a href=
+      "#FNanchor_112_112"><span class="label">[112]</span></a>
+      <i>Nature</i>, vol. xxxiv. p. 547.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_113_113" id=
+      "Footnote_113_113"></a><a href=
+      "#FNanchor_113_113"><span class="label">[113]</span></a>
+      <i>Proceedings of the Zool. Soc. of London</i>, 1870, p.
+      369.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_114_114" id=
+      "Footnote_114_114"></a><a href=
+      "#FNanchor_114_114"><span class="label">[114]</span></a>
+      <i>The Naturalist in Nicaragua</i>, p. 321.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_115_115" id=
+      "Footnote_115_115"></a><a href=
+      "#FNanchor_115_115"><span class="label">[115]</span></a> Mr.
+      Belt first suggested this use of the light of the Lampyridae
+      (fireflies and glow-worms)&mdash;<i>Naturalist in
+      Nicaragua</i>, p. 320. Mr. Verrill and Professor Meldola made
+      the same suggestion in the case of medusae and other
+      phosphorescent marine organisms (<i>Nature</i>, vol. xxx. pp.
+      281, 289).</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_116_116" id=
+      "Footnote_116_116"></a><a href=
+      "#FNanchor_116_116"><span class="label">[116]</span></a> W.E.
+      Armit, in <i>Nature</i>, vol. xviii. p. 642.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_117_117" id=
+      "Footnote_117_117"></a><a href=
+      "#FNanchor_117_117"><span class="label">[117]</span></a>
+      <i>Proc. Ent. Soc.</i>, 1869, p. xiii.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_X" id="CHAPTER_X"></a><a name="Page_268" id=
+  "Page_268"></a>CHAPTER X</h2>
+
+  <p style='font-weight:bold;'>COLOURS AND ORNAMENTS CHARACTERISTIC
+  OF SEX</p>
+
+  <div class="blockquot">
+    <p>Sex colours in the mollusca and crustacea&mdash;In
+    insects&mdash;In butterflies and moths&mdash;Probable causes of
+    these colours&mdash;Sexual selection as a supposed
+    cause&mdash;Sexual coloration of birds&mdash;Cause of dull
+    colours of female birds&mdash;Relation of sex colour to nesting
+    habits&mdash;Sexual colours of other vertebrates&mdash;Sexual
+    selection by the struggles of males&mdash;Sexual characters due
+    to natural selection&mdash;Decorative plumage of males and its
+    effect on the females&mdash;Display of decorative plumage by
+    the males&mdash;A theory of animal coloration&mdash;The origin
+    of accessory plumes&mdash;Development of accessory plumes and
+    their display&mdash;The effect of female preference will be
+    neutralised by natural selection&mdash;General laws of animal
+    coloration&mdash;Concluding remarks.</p>
+  </div>
+
+  <p>In the preceding chapters we have dealt chiefly with the
+  coloration of animals as distinctive of the several species; and
+  we have seen that, in an enormous number of cases, the colours
+  can be shown to have a definite purpose, and to be useful either
+  as a means of protection or concealment, of warning to enemies,
+  or of recognition by their own kind. We have now to consider a
+  subordinate but very widespread phenomenon&mdash;-the differences
+  of colour or of ornamental appendages in the two sexes. These
+  differences are found to have special relations with the three
+  classes of coloration above referred to, in many cases confirming
+  the explanation already given of their purport and use, and
+  furnishing us with important aid in formulating a general theory
+  of animal coloration.</p>
+
+  <p>In comparing the colours of the two sexes we find a perfect
+  gradation, from absolute identity of colour up to such extreme
+  difference that it is difficult to believe that the two forms can
+  belong to the same species; and this diversity in the <a name=
+  "Page_269" id="Page_269"></a>colours of the sexes does not bear
+  any constant relation to affinity or systematic position. In both
+  insects and birds we find examples of complete identity and
+  extreme diversity of the sexes; and these differences occur
+  sometimes in the same tribe or family, and sometimes even in the
+  same genus.</p>
+
+  <p>It is only among the higher and more active animals that
+  sexual differences of colour acquire any prominence. In the
+  mollusca the two sexes, when separated, are always alike in
+  colour, and only very rarely present slight differences in the
+  form of the shell. In the extensive group of crustacea the two
+  sexes as a rule are identical in colour, though there are often
+  differences in the form of the prehensile organs; but in a very
+  few cases there are differences of colour also. Thus, in a
+  Brazilian species of shore-crab (Gelasimus) the female is
+  grayish-brown, while in the male the posterior part of the
+  cephalo-thorax is pure white, with the anterior part of a rich
+  green. This colour is only acquired by the males when they become
+  mature, and is liable to rapid change in a few minutes to dusky
+  tints.<a name="FNanchor_118_118" id=
+  "FNanchor_118_118"></a><a href="#Footnote_118_118" class=
+  "fnanchor">[118]</a> In some of the freshwater fleas (Daphnoidae)
+  the males are ornamented with red and blue spots, while in others
+  similar colours occur in both sexes. In spiders also, though as a
+  rule the two sexes are alike in colour, there are a few
+  exceptions, the males being ornamented with brilliant colours on
+  the abdomen, while the female is dull coloured.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Sexual Coloration in Insects.</i></p>
+
+  <p>It is only when we come to the winged insects that we find any
+  large amount of peculiarity in sexual coloration, and even here
+  it is only developed in certain orders. Flies (Diptera),
+  field-bugs (Hemiptera), cicadas (Homoptera), and the
+  grasshoppers, locusts, and crickets (Orthoptera) present very few
+  and unimportant sexual differences of colour; but the last two
+  groups have special musical organs very fully developed in the
+  males of some of the species, and these no doubt enable the sexes
+  to discover and recognise each other. In some cases, however,
+  when the female is protectively coloured, as in the well-known
+  leaf-insects already referred to (p. 207), the male <a name=
+  "Page_270" id="Page_270"></a>is smaller and much less
+  protectively formed and coloured. In the bees and wasps
+  (Hymenoptera) it is also the rule that the sexes are alike in
+  colour, though there are several cases among solitary bees where
+  they differ; the female being black, and the male brown in
+  Anthophora retusa, while in Andraena fulva the female is more
+  brightly coloured than the male. Of the great order of beetles
+  (Coleoptera) the same thing may be said. Though often so rich and
+  varied in their colours the sexes are usually alike, and Mr.
+  Darwin was only able to find about a dozen cases in which there
+  was any conspicuous difference between them.<a name=
+  "FNanchor_119_119" id="FNanchor_119_119"></a><a href=
+  "#Footnote_119_119" class="fnanchor">[119]</a> They exhibit,
+  however, numerous sexual characters, in the length of the
+  antennae, and in horns, legs, or jaws remarkably enlarged or
+  curiously modified in the male sex.</p>
+
+  <p>It is in the family of dragonflies (order Neuroptera) that we
+  first meet with numerous cases of distinctive sexual coloration.
+  In some of the Agrionidae the males have the bodies rich blue and
+  the wings black, while the females have the bodies green and the
+  wings transparent. In the North American genus Hetaerina the
+  males alone have a carmine spot at the base of each wing; but in
+  some other genera the sexes hardly differ at all.</p>
+
+  <p>The great order of Lepidoptera, including the butterflies and
+  moths, affords us the most numerous and striking examples of
+  diversity of sexual colouring. Among the moths the difference is
+  usually but slight, being manifested in a greater intensity of
+  the colour of the smaller winged male; but in a few cases there
+  is a decided difference, as in the ghost-moth (Hepialus humuli),
+  in which the male is pure white, while the female is yellow with
+  darker markings. This may be a recognition colour, enabling the
+  female more readily to discover her mate; and this view receives
+  some support from the fact that in the Shetland Islands the male
+  is almost as yellow as the female, since it has been suggested
+  that at midsummer, when this moth appears, there is in that high
+  latitude sufficient twilight all night to render any special
+  coloration unnecessary.<a name="FNanchor_120_120" id=
+  "FNanchor_120_120"></a><a href="#Footnote_120_120" class=
+  "fnanchor">[120]</a></p>
+
+  <p>Butterflies present us with a wonderful amount of sexual
+  <a name="Page_271" id="Page_271"></a>difference of colour, in
+  many cases so remarkable that the two sexes of the same species
+  remained for many years under different names and were thought to
+  be quite distinct species. We find, however, every gradation from
+  perfect identity to complete diversity, and in some cases we are
+  able to see a reason for this difference. Beginning with the most
+  extraordinary cases of diversity&mdash;as in Diadema misippus,
+  where the male is black, ornamented with a large white spot on
+  each wing margined with rich changeable blue, while the female is
+  orange-brown with black spots and stripes&mdash;we find the
+  explanation in the fact that the female mimics an uneatable
+  Danais, and thus gains protection while laying its eggs on low
+  plants in company with that insect. In the allied species,
+  Diadema bolina, the females are also very different from the
+  males, but are of dusky brown tints, evidently protective and
+  very variable, some specimens having a general resemblance to the
+  uneatable Euplaeas; so that we see here some of the earlier
+  stages of both forms of protection. The remarkable differences in
+  some South American Pieridae are similarly explained. The males
+  of Pieris pyrrha, P. lorena, and several others, are white with a
+  few black bands and marginal spots like so many of their allies,
+  while the females are gaily coloured with yellow and brown, and
+  exactly resemble some species of the uneatable Heliconidae of the
+  same district. Similarly, in the Malay Archipelago, the female of
+  Diadema anomala is glossy metallic blue, while the male is brown;
+  the reason for this reversal of the usual rule being, that the
+  female exactly mimics the brilliant colouring of the common and
+  uneatable Euplaea midamus, and thus secures protection. In the
+  fine Adolias dirtea, the male is black with a few specks of
+  ochre-yellow and a broad marginal band of rich metallic
+  greenish-blue, while the female is brownish-black entirely
+  covered with rows of ochre-yellow spots. This latter coloration
+  does not appear to be protective when the insect is seen in the
+  cabinet, but it really is so. I have observed the female of this
+  butterfly in Sumatra, where it settles on the ground in the
+  forest, and its yellow spots so harmonise with the flickering
+  gleams of sunlight on the dead leaves that it can only be
+  detected with the greatest difficulty.</p>
+
+  <p><a name="Page_272" id="Page_272"></a>A hundred other cases
+  might be quoted in which the female is either more obscurely
+  coloured than the male, or gains protection by imitating some
+  inedible species; and any one who has watched these female
+  insects flying slowly along in search of the plants on which to
+  deposit their eggs, will understand how important it must be to
+  them not to attract the attention of insect-eating birds by too
+  conspicuous colours. The number of birds which capture insects on
+  the wing is much greater in tropical regions than in Europe; and
+  this is perhaps the reason why many of our showy species are
+  alike, or almost alike, in both sexes, while they are
+  protectively coloured on the under side which is exposed to view
+  when they are at rest. Such are our peacock, tortoise-shell, and
+  red admiral butterflies; while in the tropics we more commonly
+  find that the females are less conspicuous on the upper surface
+  even when protectively coloured beneath.</p>
+
+  <p>We may here remark, that the cases already quoted prove
+  clearly that either male or female may be modified in colour
+  apart from the opposite sex. In Pieris pyrrha and its allies the
+  male retains the usual type of coloration of the whole genus,
+  while the female has acquired a distinct and peculiar style of
+  colouring. In Adolias dirtea, on the other hand, the female
+  appears to retain something like the primitive colour and
+  markings of the two sexes, modified perhaps for more perfect
+  protection; while the male has acquired more and more intense and
+  brilliant colours, only showing his original markings by the few
+  small yellow spots that remain near the base of the wings. In the
+  more gaily coloured Pieridae, of which our orange-tip butterfly
+  may be taken as a type, we see in the female the plain ancestral
+  colours of the group, while the male has acquired the brilliant
+  orange tip to its wings, probably as a recognition mark.</p>
+
+  <p>In those species in which the under surface is protectively
+  coloured, we often find the upper surface alike in both sexes,
+  the tint of colour being usually more intense in the male. But in
+  some cases this leads to the female being more conspicuous, as in
+  some of the Lycaenidae, where the female is bright blue and the
+  male of a blue so much deeper and soberer in tint as to appear
+  the less brilliantly coloured of the two.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_273" id="Page_273"></a><i>Probable Causes of these
+  Colours.</i></p>
+
+  <p>In the production of these varied results there have probably
+  been several causes at work. There seems to be a constant
+  tendency in the male of most animals&mdash;but especially of
+  birds and insects&mdash;to develop more and more intensity of
+  colour, often culminating in brilliant metallic blues or greens
+  or the most splendid iridescent hues; while, at the same time,
+  natural selection is constantly at work, preventing the female
+  from acquiring these same tints, or modifying her colours in
+  various directions to secure protection by assimilating her to
+  her surroundings, or by producing mimicry of some protected form.
+  At the same time, the need for recognition must be satisfied; and
+  this seems to have led to diversities of colour in allied
+  species, sometimes the female, sometimes the male undergoing the
+  greatest change according as one or other could be modified with
+  the greatest ease, and so as to interfere least with the welfare
+  of the race. Hence it is that sometimes the males of allied
+  species vary most, as in the different species of Epicalia;
+  sometimes the females, as in the magnificent green species of
+  Ornithoptera and the "Aeneas" group of Papilio.</p>
+
+  <p>The importance of the two principles&mdash;the need of
+  protection and recognition&mdash;in modifying the comparative
+  coloration of the sexes among butterflies, is beautifully
+  illustrated in the case of the groups which are protected by
+  their distastefulness, and whose females do not, therefore, need
+  the protection afforded by sober colours.</p>
+
+  <p>In the great families, Heliconidae and Acraeidae, we find that
+  the two sexes are almost always alike; and, in the very few
+  exceptions, that the female, though differently, is not less
+  gaily or less conspicuously coloured. In the Danaidae the same
+  general rule prevails, but the cases in which the male exhibits
+  greater intensity of colour than the female are perhaps more
+  numerous than in the other two families. There is, however, a
+  curious difference in this respect between the Oriental and the
+  American groups of distasteful Papilios with warning colours,
+  both of which are the subjects of mimicry. In the Eastern
+  groups&mdash;of which P. hector and P. coon may be taken <a name=
+  "Page_274" id="Page_274"></a>as types&mdash;the two sexes are
+  nearly alike, the male being sometimes more intensely coloured
+  and with fewer pale markings; but in the American
+  groups&mdash;represented by P. aeneas, P. sesostris, and
+  allies&mdash;there is a wonderful diversity, the males having a
+  rich green or bluish patch on the fore wings, while the females
+  have a band or spots of pure white, not always corresponding in
+  position to the green spot of the males. There are, however,
+  transitional forms, by which a complete series can be traced,
+  from close similarity to great diversity of colouring between the
+  sexes; and this may perhaps be only an extreme example of the
+  intenser colour and more concentrated markings which are a very
+  prevalent characteristic of male butterflies.</p>
+
+  <p>There are, in fact, many indications of a regular succession
+  of tints in which colour development has occurred in the various
+  groups of butterflies, from an original grayish or brownish
+  neutral tint. Thus in the "Aeneas" group of Papilios we have the
+  patch on the upper wings yellowish in P. triopas, olivaceous in
+  P. bolivar, bronzy-gray with a white spot in P. erlaces, more
+  greenish and buff in P. iphidamas, gradually changing to the fine
+  blue of P. brissonius, and the magnificent green of P. sesostris.
+  In like manner, the intense crimson spots of the lower wings can
+  be traced step by step from a yellow or buff tint, which is one
+  of the most widespread colours in the whole order. The greater
+  purity and intensity of colour seem to be usually associated with
+  more pointed wings, indicating greater vigour and more rapid
+  flight.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Sexual Selection as a supposed Cause of Colour
+  Development.</i></p>
+
+  <p>Mr. Darwin, as is well known, imputed most of the brilliant
+  colours and varied patterns of butterflies' wings to sexual
+  selection&mdash;that is, to a constant preference, by female
+  butterflies, for the more brilliant males; the colours thus
+  produced being sometimes transmitted to the males alone,
+  sometimes to both sexes. This view has always seemed to me to be
+  unsupported by evidence, while it is also quite inadequate to
+  account for the facts. The only direct evidence, as set forth
+  with his usual fairness by Mr. Darwin himself, is opposed to his
+  views. Several entomologists assured him that, in moths, the
+  females evince not the least choice of their <a name="Page_275"
+  id="Page_275"></a>partners; and Dr. Wallace of Colchester, who
+  has largely bred the fine Bombyx cynthia, confirmed this
+  statement. Among butterflies, several males often pursue one
+  female, and Mr. Darwin says, that, unless the female exerts a
+  choice the pairing must be left to chance. But, surely, it may be
+  the most vigorous or most persevering male that is chosen, not
+  necessarily one more brightly or differently coloured, and this
+  will be true "natural selection." Butterflies have been noticed
+  to prefer some coloured flowers to others; but that does not
+  prove, or even render probable, any preference for the colour
+  itself, but only for flowers of certain colours, on account of
+  the more agreeable or more abundant nectar obtained from them.
+  Dr. Schulte called Mr. Darwin's attention to the fact, that in
+  the Diadema bolina the brilliant blue colour surrounding the
+  white spots is only visible when we look towards the insect's
+  head, and this is true of many of the iridescent colours of
+  butterflies, and probably depends upon the direction of the
+  striae on the scales. It is suggested, however, that this display
+  of colour will be seen by the female as the male is approaching
+  her, and that it has been developed by sexual selection.<a name=
+  "FNanchor_121_121" id="FNanchor_121_121"></a><a href=
+  "#Footnote_121_121" class="fnanchor">[121]</a> But in the
+  majority of cases the males <i>follow</i> the female, hovering
+  over her in a position which would render it almost impossible
+  for her to see the particular colours or patterns on his upper
+  surface; to do so the female should mount higher than the male,
+  and fly towards him&mdash;being the seeker instead of the sought,
+  and this is quite opposed to the actual facts. I cannot,
+  therefore, think that this suggestion adds anything whatever to
+  the evidence for sexual selection of colour by female
+  butterflies. This question will, however, be again touched upon
+  after we have considered the phenomena of sexual colour among the
+  vertebrata.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Sexual Coloration of Birds.</i></p>
+
+  <p>The general rule among vertebrates, as regards colour, is, for
+  the two sexes to be alike. This prevails, with only a few
+  exceptions, in fishes, reptiles, and mammalia; but in birds
+  diversity of sexual colouring is exceedingly frequent, and is,
+  not improbably, present in a greater or less degree in more
+  <a name="Page_276" id="Page_276"></a>than half of the known
+  species. It is this class, therefore, that will afford us the
+  best materials for a discussion of the problem, and that may
+  perhaps lead us to a satisfactory explanation of the causes to
+  which sexual colour is due.</p>
+
+  <p>The most fundamental characteristic of birds, from our present
+  point of view, is a greater intensity of colour in the male. This
+  is the case in hawks and falcons; in many thrushes, warblers, and
+  finches; in pigeons, partridges, rails, plovers, and many others.
+  When the plumage is highly protective or of dull uniform tints,
+  as in many of the thrushes and warblers, the sexes are almost or
+  quite identical in colour; but when any rich markings or bright
+  tints are acquired, they are almost always wanting or much
+  fainter in the female, as we see in the black-cap among warblers,
+  and the chaffinch among finches.</p>
+
+  <p>It is in tropical regions, where from a variety of causes
+  colour has been, developed to its fullest extent, that we find
+  the most remarkable examples of sexual divergence of colour. The
+  most gorgeously coloured birds known are the birds of paradise,
+  the chatterers, the tanagers, the humming-birds, and the
+  pheasant-tribe, including the peacocks. In all these the females
+  are much less brilliant, and, in the great majority of cases,
+  exceptionally plain and dull coloured birds. Not only are the
+  remarkable plumes, crests, and gorgets of the birds of paradise
+  entirely wanting in the females, but these latter are usually
+  without any bright colour at all, and rank no higher than our
+  thrushes in ornamental plumage. Of the humming-birds the same may
+  be said, except that the females are often green, and sometimes
+  slightly metallic, but from their small size and uniform tints
+  are never conspicuous. The glorious blues and purples, the pure
+  whites and intense crimsons of the male chatterers are
+  represented in the females by olive-greens or dull browns, as are
+  the infinitely varied tints of the male tanagers. And in
+  pheasants, the splendour of plumage which characterises the males
+  is entirely absent in the females, which, though often
+  ornamental, have always comparatively sober and protective tints.
+  The same thing occurs with many other groups. In the Eastern
+  tropics are many brilliant birds belonging to the families of the
+  warblers, flycatchers, shrikes, etc., but the female is always
+  <a name="Page_277" id="Page_277"></a>much less brilliant than the
+  male and often quite dull coloured.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Cause of Dull Colours of Female Birds.</i></p>
+
+  <p>The reason of this phenomenon is not difficult to find, if we
+  consider the essential conditions of a bird's existence, and the
+  most important function it has to fulfil. In order that the
+  species may be continued, young birds must be produced, and the
+  female birds have to sit assiduously on their eggs. While doing
+  this they are exposed to observation and attack by the numerous
+  devourers of eggs and birds, and it is of vital importance that
+  they should be protectively coloured in all those parts of the
+  body which are exposed during incubation. To secure this end all
+  the bright colours and showy ornaments which decorate the male
+  have not been acquired by the female, who often remains clothed
+  in the sober hues which were probably once common to the whole
+  order to which she belongs. The different amounts of colour
+  acquired by the females have no doubt depended on peculiarities
+  of habits and of environment, and on the powers of defence or of
+  concealment possessed by the species. Mr. Darwin has taught us
+  that natural selection cannot produce absolute, but only relative
+  perfection; and as a protective colour is only one out of many
+  means by which the female birds are able to provide for the
+  safety of their young, those which are best endowed in other
+  respects will have been allowed to acquire more colour than those
+  with whom the struggle for existence is more severe.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Relation of Sex Colour to Nesting Habits.</i></p>
+
+  <p>This principle is strikingly illustrated by the existence of
+  considerable numbers of birds in which both sexes are similarly
+  and brilliantly coloured,&mdash;in some cases as brilliantly as
+  the males of many of the groups above referred to. Such are the
+  extensive families of the kingfishers, the woodpeckers, the
+  toucans, the parrots, the turacos, the hangnests, the starlings,
+  and many other smaller groups, all the species of which are
+  conspicuously or brilliantly coloured, while in all of them the
+  females are either coloured exactly like the males, or, when
+  differently coloured, are equally conspicuous. When <a name=
+  "Page_278" id="Page_278"></a>searching for some cause for this
+  singular apparent exception to the rule of female protective
+  colouring, I came upon a fact which beautifully explains it; for
+  in all these cases, without exception, the species either nests
+  in holes in the ground or in trees, or builds a domed or covered
+  nest, so as completely to conceal the sitting-bird. We have here
+  a case exactly parallel to that of the butterflies protected by
+  distastefulness, whose females are either exactly like the males,
+  or, if different, are equally conspicuous. We can hardly believe
+  that so exact a parallel should exist between such remote classes
+  of animals, except under the influence of a general law; and, in
+  the need of protection by all defenceless animals, and especially
+  by most female insects and birds, we have such a law, which has
+  been proved to have influenced the colours of a considerable
+  proportion of the animal kingdom.<a name="FNanchor_122_122" id=
+  "FNanchor_122_122"></a><a href="#Footnote_122_122" class=
+  "fnanchor">[122]</a></p>
+
+  <p>The general relation which exists between the mode of nesting
+  and the coloration of the sexes in those groups of birds which
+  need protection from enemies, may be thus expressed: When both
+  sexes are brilliant or conspicuous, the nest is such as to
+  conceal the sitting-bird; but when the male is brightly coloured
+  and the female sits exposed on the nest, she is always less
+  brilliant and generally of quite sober and protective hues.</p>
+
+  <p>It must be understood that the mode of nesting has influenced
+  the colour, not that the colour has determined the mode of
+  nesting; and this, I believe, has been generally, though not
+  perhaps universally, the case. For we know that colour varies
+  more rapidly, and can be more easily modified and fixed by
+  selection, than any other character; whereas habits, especially
+  when connected with structure, and when they pervade a whole
+  group, are much more persistent and more difficult to change, as
+  shown by the habit of the dog turning round two or three times
+  before lying down, believed to be that of the wild ancestral form
+  which thus smoothed down the herbage so as to form a comfortable
+  bed. We see, too, that the general mode of nesting is
+  characteristic of whole families differing widely in size, form,
+  and colours. Thus, all the kingfishers and their allies in every
+  part of the world nest <a name="Page_279" id="Page_279"></a>in
+  holes, usually in banks, but sometimes in trees. The motmots and
+  the puff-birds (Bucconidae) build in similar places; while the
+  toucans, barbets, trogons, woodpeckers, and parrots all make
+  their nests in hollow trees. This habit, pervading all the
+  members of extensive families, must therefore be extremely
+  ancient, more especially as it evidently depends in some degree
+  on the structure of the birds, the bills, and especially the
+  feet, of all these groups being unfitted for the construction of
+  woven arboreal nests.<a name="FNanchor_123_123" id=
+  "FNanchor_123_123"></a><a href="#Footnote_123_123" class=
+  "fnanchor">[123]</a> But in all these families the colour varies
+  greatly from species to species, being constant only in the one
+  character of the similarity of the sexes, or, at all events, in
+  their being equally conspicuous even though differently
+  coloured.</p>
+
+  <p>When I first put forward this view of the connection between
+  the mode of nesting and the coloration of female birds, I
+  expressed the law in somewhat different terms, which gave rise to
+  some misunderstanding, and led to numerous criticisms and
+  objections. Several cases were brought forward in which the
+  females were far less brilliant than the males, although the nest
+  was covered. This is the case with the Maluridae, or superb
+  warblers of Australia, in which the males are very brilliant
+  during the pairing season and the females quite plain, yet they
+  build domed nests. Here, there can be little doubt, the covered
+  nest is a protection from rain or from some special enemies to
+  the eggs; while the birds themselves are protectively coloured in
+  both sexes, except for a short time during the breeding season
+  when the male acquires brilliant colours; and this is probably
+  connected with the fact of their inhabiting the open plains and
+  thin scrub of Australia, where protective colours are as
+  generally advantageous as they are in our north-temperate
+  zones.</p>
+
+  <p>As I have now stated the law, I do not think there are any
+  exceptions to it, while there are an overwhelming number of cases
+  which give it a strong support. It has been objected that the
+  domed nests of many birds are as conspicuous as the birds
+  themselves would be, and would, therefore, be of no use as a
+  protection to the birds and young. But, as a matter of fact, they
+  do protect from attack, for hawks or crows do not pluck such
+  nests to pieces, as in doing so they would be <a name="Page_280"
+  id="Page_280"></a>exposed to the attack of the whole colony;
+  whereas a hawk or falcon could carry off a sitting-bird or the
+  young at a swoop, and entirely avoid attack. Moreover, each kind
+  of covered nest is doubtless directed against the attacks of the
+  most dangerous enemies of the species, the purse-like nests,
+  often a yard long, suspended from the extremity of thin twigs,
+  being useful against the attacks of snakes, which, if they
+  attempted to enter them, would be easily made to lose their hold
+  and fall to the ground. Such birds as jays, crows, magpies,
+  hawks, and other birds of prey, have also been urged as an
+  exception; but these are all aggressive birds, able to protect
+  themselves, and thus do not need any special protection for their
+  females during nidification. Some birds which build in covered
+  nests are comparatively dull coloured, like many of the weaver
+  birds, but in others the colours are more showy, and in all the
+  sexes are alike; so that none of these are in any way opposed to
+  the rule. The golden orioles have, however, been adduced as a
+  decided exception, since the females are showy and build in an
+  open nest. But even here the females are less brilliant than the
+  males, and are sometimes greenish or olivaceous on the upper
+  surface; while they very carefully conceal their nests among
+  dense foliage, and the male is sufficiently watchful and
+  pugnacious to drive off most intruders.</p>
+
+  <p>On the other hand, how remarkable it is that the only small
+  and brightly coloured birds of our own country in which the male
+  and female are alike&mdash;the tits and starlings&mdash;either
+  build in holes or construct covered nests; while the beautiful
+  hangnests (Icteridae) of South America, which always build
+  covered or purse-shaped nests, are equally showy in both sexes,
+  in striking contrast with the chatterers and tanagers of the same
+  country, whose females are invariably less conspicuous than the
+  males. On a rough estimate, there are about 1200 species of birds
+  in the class of showy males and females, with concealed
+  nidification; while there are probably, from an equally rough
+  estimate, about the same number in the contrasted class of showy
+  males and dull females, with open nests. This will leave the
+  great bulk of known birds in the classes of those which are more
+  or less protectively coloured in both sexes; or which, from their
+  organisation and habits, do <a name="Page_281" id=
+  "Page_281"></a>not require special protective coloration, such as
+  many of the birds of prey, the larger waders, and the oceanic
+  birds.</p>
+
+  <p>There are a few very curious cases in which the female bird is
+  actually more brilliant than the male, and which yet have open
+  nests. Such are the dotterel (Eudromias morinellus), several
+  species of phalarope, an Australian creeper (Climacteris
+  erythropus), and a few others; but in every one of these cases
+  the relation of the sexes in regard to nidification is reversed,
+  the male performing the duties of incubation, while the female is
+  the stronger and more pugnacious. This curious case, therefore,
+  quite accords with the general law of coloration.<a name=
+  "FNanchor_124_124" id="FNanchor_124_124"></a><a href=
+  "#Footnote_124_124" class="fnanchor">[124]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Sexual Colours of other Vertebrates.</i></p>
+
+  <p>We may consider a few of the cases of sexual colouring of
+  other classes of vertebrates, as given by Mr. Darwin. In fishes,
+  though the sexes are usually alike, there are several species in
+  which the males are more brightly coloured, and have more
+  elongated fins, spines, or other appendages, and in some few
+  cases the colours are decidedly different. The males often fight
+  together, and are altogether more vivacious and excitable than
+  the females during the breeding season; and with this we may
+  connect a greater intensity of coloration.</p>
+
+  <p>In frogs and toads the colours are usually alike, or a little
+  more intense in the males, and the same may be said of most
+  snakes. It is in lizards that we first meet with considerable
+  sexual differences, many of the species having gular pouches,
+  frills, dorsal crests, or horns, either confined to the males, or
+  more developed in them than in the females, and these ornaments
+  are often brightly coloured. In most cases, however, the tints of
+  lizards are protective, the male being usually a little more
+  intense in coloration; and the difference in extreme cases may be
+  partly due to the need of protection for the female, which, when
+  laden with eggs, must be less active and less able to escape from
+  enemies than the male, and may, therefore, have retained more
+  protective colours, as so many insects and birds have certainly
+  done.<a name="FNanchor_125_125" id=
+  "FNanchor_125_125"></a><a href="#Footnote_125_125" class=
+  "fnanchor">[125]</a></p>
+
+  <p>In mammalia there is often a somewhat greater intensity
+  <a name="Page_282" id="Page_282"></a>of colour in the male, but
+  rarely a decided difference. The female of the great red
+  kangaroo, however, is a delicate gray; while in the Lemur macaco
+  of Madagascar the male is jet-black and the female brown. In many
+  monkeys also there are some differences of colour, especially on
+  the face. The sexual weapons and ornaments of male mammalia, as
+  horns, crests, manes, and dewlaps, are well known, and are very
+  numerous and remarkable. Having thus briefly reviewed the facts,
+  we will now consider the theories to which they have given
+  rise.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Sexual Selection by the Struggles of Males.</i></p>
+
+  <p>Among the higher animals it is a very general fact that the
+  males fight together for the possession of the females. This
+  leads, in polygamous animals especially, to the stronger or
+  better armed males becoming the parents of the next generation,
+  which inherits the peculiarities of the parents; and thus vigour
+  and offensive weapons are continually increased in the males,
+  resulting in the strength and horns of the bull, the tusks of the
+  boar, the antlers of the stag, and the spurs and fighting
+  instinct of the gamecock. But almost all male animals fight
+  together, though not specially armed; even hares, moles,
+  squirrels, and beavers fight to the death, and are often found to
+  be scarred and wounded. The same rule applies to almost all male
+  birds; and these battles have been observed in such different
+  groups as humming-birds, finches, goatsuckers, woodpeckers,
+  ducks, and waders. Among reptiles, battles of the males are known
+  to occur in the cases of crocodiles, lizards, and tortoises;
+  among fishes, in those of salmon and sticklebats. Even among
+  insects the same law prevails; and male spiders, beetles of many
+  groups, crickets, and butterflies often fight together.</p>
+
+  <p>From this very general phenomenon there necessarily results a
+  form of natural selection which increases the vigour and fighting
+  power of the male animal, since, in every case, the weaker are
+  either killed, wounded, or driven away. This selection would be
+  more powerful if males were always in excess of females, but
+  after much research Mr. Darwin could not obtain any satisfactory
+  evidence that this was the case. The same effect, however, is
+  produced in some cases by constitution or habits; thus male
+  insects usually emerge first from <a name="Page_283" id=
+  "Page_283"></a>the pupa, and among migrating birds the males
+  arrive first both in this country and in North America. The
+  struggle is thus intensified, and the most vigorous males are the
+  first to have offspring. This in all probability is a great
+  advantage, as the early breeders have the start in securing food,
+  and the young are strong enough to protect themselves while the
+  later broods are being produced.</p>
+
+  <p>It is to this form of male rivalry that Mr. Darwin first
+  applied the term "sexual selection." It is evidently a real power
+  in nature; and to it we must impute the development of the
+  exceptional strength, size, and activity of the male, together
+  with the possession of special offensive and defensive weapons,
+  and of all other characters which arise from the development of
+  these or are correlated with them. But he has extended the
+  principle into a totally different field of action, which has
+  none of that character of constancy and of inevitable result that
+  attaches to natural selection, including male rivalry; for by far
+  the larger portion of the phenomena, which he endeavours to
+  explain by the direct action of sexual selection, can only be so
+  explained on the hypothesis that the immediate agency is female
+  choice or preference. It is to this that he imputes the origin of
+  all secondary sexual characters other than weapons of offence and
+  defence, of all the ornamental crests and accessory plumes of
+  birds, the stridulating sounds of insects, the crests and beards
+  of monkeys and other mammals, and the brilliant colours and
+  patterns of male birds and butterflies. He even goes further, and
+  imputes to it a large portion of the brilliant colour that occurs
+  in both sexes, on the principle that variations occurring in one
+  sex are sometimes transmitted to the same sex only, sometimes to
+  both, owing to peculiarities in the laws of inheritance. In this
+  extension of sexual selection to include the action of female
+  choice or preference, and in the attempt to give to that choice
+  such wide-reaching effects, I am unable to follow him more than a
+  very little way; and I will now state some of the reasons why I
+  think his views are unsound.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Sexual Characters due to Natural Selection.</i></p>
+
+  <p>Besides the acquisition of weapons by the male for the purpose
+  of fighting with other males, there are some other <a name=
+  "Page_284" id="Page_284"></a>sexual characters which may have
+  been produced by natural selection. Such are the various sounds
+  and odours which are peculiar to the male, and which serve as a
+  call to the female or as an indication of his presence. These are
+  evidently a valuable addition to the means of recognition of the
+  two sexes, and are a further indication that the pairing season
+  has arrived; and the production, intensification, and
+  differentiation of these sounds and odours are clearly within the
+  power of natural selection. The same remark will apply to the
+  peculiar calls of birds, and even to the singing of the males.
+  These may well have originated merely as a means of recognition
+  between the two sexes of a species, and as an invitation from the
+  male to the female bird. When the individuals of a species are
+  widely scattered, such a call must be of great importance in
+  enabling pairing to take place as early as possible, and thus the
+  clearness, loudness, and individuality of the song becomes a
+  useful character, and therefore the subject of natural selection.
+  Such is especially the case with the cuckoo, and with all
+  solitary birds, and it may have been equally important at some
+  period of the development of all birds. The act of singing is
+  evidently a pleasurable one; and it probably serves as an outlet
+  for superabundant nervous energy and excitement, just as dancing,
+  singing, and field sports do with us. It is suggestive of this
+  view that the exercise of the vocal power seems to be
+  complementary to the development of accessory plumes and
+  ornaments, all our finest singing birds being plainly coloured,
+  and with no crests, neck or tail plumes to display; while the
+  gorgeously ornamented birds of the tropics have no song, and
+  those which expend much energy in display of plumage, as the
+  turkey, peacocks, birds of paradise, and humming-birds, have
+  comparatively an insignificant development of voice. Some birds
+  have, in the wings or tail, peculiarly developed feathers which
+  produce special sounds. In some of the little manakins of Brazil,
+  two or three of the wing-feathers are curiously shaped and
+  stiffened in the male, so that the bird is able to produce with
+  them a peculiar snapping or cracking sound; and the tail-feathers
+  of several species of snipe are so narrowed as to produce
+  distinct drumming, whistling, or switching sounds when the birds
+  <a name="Page_285" id="Page_285"></a>descend rapidly from a great
+  height. All these are probably recognition and call notes, useful
+  to each species in relation to the most important function of
+  their lives, and thus capable of being developed by the agency of
+  natural selection.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Decorative Plumage of Birds and its Display.</i></p>
+
+  <p>Mr. Darwin has devoted four chapters of his <i>Descent of
+  Man</i> to the colours of birds, their decorative plumage, and
+  its display at the pairing season; and it is on this latter
+  circumstance that he founds his theory, that both the plumage and
+  the colours have been developed by the preference of the females,
+  the more ornamented males becoming the parents of each successive
+  generation. Any one who reads these most interesting chapters
+  will admit, that the fact of the display is demonstrated; and it
+  may also be admitted, as highly probable, that the female is
+  pleased or excited by the display. But it by no means follows
+  that slight differences in the shape, pattern, or colours of the
+  ornamental plumes are what lead a female to give the preference
+  to one male over another; still less that all the females of a
+  species, or the great majority of them, over a wide area of
+  country, and for many successive generations, prefer exactly the
+  same modification of the colour or ornament.</p>
+
+  <p>The evidence on this matter is very scanty, and in most cases
+  not at all to the point. Some peahens preferred an old pied
+  peacock; albino birds in a state of nature have never been seen
+  paired with other birds; a Canada goose paired with a Bernicle
+  gander; a male widgeon preferred a pintail duck to its own
+  species; a hen canary preferred a male greenfinch to either
+  linnet, goldfinch, siskin, or chaffinch. These cases are
+  evidently exceptional, and are not such as generally occur in
+  nature; and they only prove that the female does exert some
+  choice between very different males, and some observations on
+  birds in a state of nature prove the same thing; but there is no
+  evidence that slight variations in the colour or plumes, in the
+  way of increased intensity or complexity, are what determines the
+  choice. On the other hand, Mr. Darwin gives much evidence that it
+  is <i>not</i> so determined. He tells us that Messrs. Hewitt,
+  Tegetmeier, and Brent, three of the highest authorities and best
+  observers, "do not believe that <a name="Page_286" id=
+  "Page_286"></a>the females prefer certain males on account of the
+  beauty of their plumage." Mr. Hewitt was convinced "that the
+  female almost invariably prefers the most vigorous, defiant, and
+  mettlesome male;" and Mr. Tegetmeier, "that a gamecock, though
+  disfigured by being dubbed, and with his hackles trimmed, would
+  be accepted as readily as a male retaining all his natural
+  ornaments."<a name="FNanchor_126_126" id=
+  "FNanchor_126_126"></a><a href="#Footnote_126_126" class=
+  "fnanchor">[126]</a> Evidence is adduced that a female pigeon
+  will sometimes tn antipathy to a particular male without any
+  assignable cause; or, in other cases, will take a strong fancy to
+  some one bird, and will desert her own mate for him; but it is
+  not stated that superiority or inferiority of plumage has
+  anything to do with these fancies. Two instances are indeed
+  given, of male birds being rejected, which had lost their
+  ornamental plumage; but in both cases (a widow-finch and a silver
+  pheasant) the long tail-plumes are the indication of sexual
+  maturity. Such cases do not support the idea that males with the
+  tail-feathers a trifle longer, or the colours a trifle brighter,
+  are generally preferred, and that those which are only a little
+  inferior are as generally rejected,&mdash;and this is what is
+  absolutely needed to establish the theory of the development of
+  these plumes by means of the choice of the female.</p>
+
+  <p>It will be seen, that female birds have unaccountable likes
+  and dislikes in the matter of their partners, just as we have
+  ourselves, and this may afford us an illustration. A young man,
+  when courting, brushes or curls his hair, and has his moustache,
+  beard, or whiskers in perfect order, and no doubt his sweetheart
+  admires them; but this does not prove that she marries him on
+  account of these ornaments, still less that hair, beard,
+  whiskers, and moustache were developed by the continued
+  preferences of the female sex. So, a girl likes to see her lover
+  well and fashionably dressed, and he always dresses as well as he
+  can when he visits her; but we cannot conclude from this that the
+  whole series of male costumes, from the brilliantly coloured,
+  puffed, and slashed doublet and hose of the Elizabethan period,
+  through the gorgeous coats, long waistcoats, and pigtails of the
+  early Georgian era, down to the funereal dress-suit of the
+  present day, are the direct result of female preference. In like
+  manner, female birds may be <a name="Page_287" id=
+  "Page_287"></a>charmed or excited by the fine display of plumage
+  by the males; but there is no proof whatever that slight
+  differences in that display have any effect in determining their
+  choice of a partner.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Display of Decorative Plumage.</i></p>
+
+  <p>The extraordinary manner in which most birds display their
+  plumage at the time of courtship, apparently with the full
+  knowledge that it is beautiful, constitutes one of Mr. Darwin's
+  strongest arguments. It is, no doubt, a very curious and
+  interesting phenomenon, and indicates a connection between the
+  exertion of particular muscles and the development of colour and
+  ornament; but, for the reasons just given, it does not prove that
+  the ornament has been developed by female choice. During
+  excitement, and when the organism develops superabundant energy,
+  many animals find it pleasurable to exercise their various
+  muscles, often in fantastic ways, as seen in the gambols of
+  kittens, lambs, and other young animals. But at the time of
+  pairing, male birds are in a state of the most perfect
+  development, and possess an enormous store of vitality; and under
+  the excitement of the sexual passion they perform strange antics
+  or rapid flights, as much probably from an internal impulse to
+  motion and exertion as with any desire to please their mates.
+  Such are the rapid descent of the snipe, the soaring and singing
+  of the lark, and the dances of the cock-of-the-rock and of many
+  other birds.</p>
+
+  <p>It is very suggestive that similar strange movements are
+  performed by many birds which have no ornamental plumage to
+  display. Goatsuckers, geese, carrion vultures, and many other
+  birds of plain plumage have been observed to dance, spread their
+  wings or tails, and perform strange love-antics. The courtship of
+  the great albatross, a most unwieldy and dull coloured bird, has
+  been thus described by Professor Moseley: "The male, standing by
+  the female on the nest, raises his wings, spreads his tail and
+  elevates it, throws up his head with the bill in the air, or
+  stretches it straight out, or forwards, as far as he can, and
+  then utters a curious cry."<a name="FNanchor_127_127" id=
+  "FNanchor_127_127"></a><a href="#Footnote_127_127" class=
+  "fnanchor">[127]</a> Mr. Jenner Weir informs me that "the male
+  blackbird is full of action, spreads out his glossy wing and
+  tail, turns his rich golden <a name="Page_288" id=
+  "Page_288"></a>beak towards the female, and chuckles with
+  delight," while he has never seen the more plain coloured thrush
+  demonstrative to the female. The linnet distends his rosy breast,
+  and slightly expands his brown wings and tail; while the various
+  gay coloured Australian finches adopt such attitudes and postures
+  as, in every case, to show off their variously coloured plumage
+  to the best advantage.<a name="FNanchor_128_128" id=
+  "FNanchor_128_128"></a><a href="#Footnote_128_128" class=
+  "fnanchor">[128]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>A Theory of Animal Coloration.</i></p>
+
+  <p>Having rejected Mr. Darwin's theory of female choice as
+  incompetent to account for the brilliant colours and markings of
+  the higher animals, the preponderance of these colours and
+  markings in the male sex, and their display during periods of
+  activity or excitement, I may be asked what explanation I have to
+  offer as a preferable substitute. In my <i>Tropical Nature</i> I
+  have already indicated such a theory, which I will now briefly
+  explain, supporting it by some additional facts and arguments,
+  which appear to me to have great weight, and for which I am
+  mainly indebted to a most interesting and suggestive posthumous
+  work by Mr. Alfred Tylor.<a name="FNanchor_129_129" id=
+  "FNanchor_129_129"></a><a href="#Footnote_129_129" class=
+  "fnanchor">[129]</a></p>
+
+  <p>The fundamental or ground colours of animals ar has been shown
+  in preceding chapters, very largely protective, and it is not
+  improbable that the primitive colours of all animals were so.
+  During the long course of animal development other modes of
+  protection than concealment by harmony of colour arose, and
+  thenceforth the normal development of colour due to the complex
+  chemical and structural changes ever going on in the organism,
+  had full play; and the colours thus produced were again and again
+  modified by natural selection for purposes of warning,
+  recognition, mimicry, or special protection, as has been already
+  fully explained in the preceding chapters.</p>
+
+  <p>Mr. Taylor has, however, called attention to an important
+  principle which underlies the various patterns or ornamental
+  markings of animals&mdash;namely, that diversified coloration
+  follows the chief lines of structure, and changes at points, such
+  as the joints, where function changes. He says, "If we take
+  highly decorated species&mdash;that is, animals marked by
+  <a name="Page_289" id="Page_289"></a>alternate dark or light
+  bands or spots, such as the zebra, some deer, or the carnivora,
+  we find, first, that the region of the spinal column is marked by
+  a dark stripe; secondly, that the regions of the appendages, or
+  limbs, are differently marked; thirdly, that the flanks are
+  striped or spotted, along or between the regions of the lines of
+  the ribs; fourthly, that the shoulder and hip regions are marked
+  by curved lines; fifthly, that the pattern changes, and the
+  direction of the lines, or spots, at the head, neck, and every
+  joint of the limbs; and lastly, that the tips of the ears, nose,
+  tail, and feet, and the eye are emphasised in colour. In spotted
+  animals the greatest length of the spot is generally in the
+  direction of the largest development of the skeleton."</p>
+
+  <p>This structural decoration is well seen in many insects. In
+  caterpillars, similar spots and markings are repeated in each
+  segment, except where modified for some form of protection. In
+  butterflies, the spots and bands usually have reference to the
+  form of the wing and the arrangement of the nervures; and there
+  is much evidence to show that the primitive markings are always
+  spots in the cells, or between the nervures, or at the junctions
+  of nervures, the extension and coalescence of these spots forming
+  borders, bands, or blotches, which have become modified in
+  infinitely varied ways for protection, warning, or recognition.
+  Even in birds, the distribution of colours and markings follows
+  generally the same law. The crown of the head, the throat, the
+  ear-coverts, and the eyes have usually distinct tints in all
+  highly coloured birds; the region of the furcula has often a
+  distinct patch of colour, as have the pectoral muscles, the
+  uropygium or root of the tail, and the under
+  tail-coverts.<a name="FNanchor_130_130" id=
+  "FNanchor_130_130"></a><a href="#Footnote_130_130" class=
+  "fnanchor">[130]</a></p>
+
+  <p>Mr. Tylor was of opinion the primitive form of ornamentation
+  consisted of spots, the confluence of these in certain directions
+  forming lines or bands; and, these again, sometimes coalescing
+  into blotches, or into more or less uniform tints covering a
+  large portion of the surface of the body. The young lion and
+  tiger are both spotted; and in the Java hog (Sus vittatus) very
+  young animals are banded, but have spots over the shoulders and
+  thighs. These spots run into stripes <a name="Page_290" id=
+  "Page_290"></a>as the animal grows older; then the stripes
+  expand, and at last, meeting together, the adult animal becomes
+  of a uniform dark brown colour. So many of the species of deer
+  are spotted when young, that Darwin concludes the ancestral form,
+  from which all deer are derived, must have been spotted. Pigs and
+  tapirs are banded or spotted when young; an imported young
+  specimen of Tapirus Bairdi was covered with white spots in
+  longitudinal rows, here and there forming short stripes.<a name=
+  "FNanchor_131_131" id="FNanchor_131_131"></a><a href=
+  "#Footnote_131_131" class="fnanchor">[131]</a> Even the horse,
+  which Darwin supposes to be descended from a striped animal, is
+  often spotted, as in dappled horses; and great numbers show a
+  tendency to spottiness, especially on the haunches.</p>
+
+  <p>Ocelli may also be developed from spots, or from bars, as
+  pointed out by Mr. Darwin. Spots are an ordinary form of marking
+  in disease, and these spots sometimes run together, forming
+  blotches. There is evidence that colour markings are in some way
+  dependent on nerve distribution. In the disease known as frontal
+  herpes, an eruption occurs which corresponds exactly to the
+  distribution of the ophthalmic division of the fifth cranial
+  nerve, mapping out all its little branches even to the one which
+  goes to the tip of the nose. In a Hindoo suffering from herpes
+  the pigment was destroyed in the arm along the course of the
+  ulnar nerve, with its branches along both sides of one finger and
+  the half of another. In the leg the sciatic and scaphenous nerves
+  were partly mapped out, giving to the patient the appearance of
+  an anatomical diagram.<a name="FNanchor_132_132" id=
+  "FNanchor_132_132"></a><a href="#Footnote_132_132" class=
+  "fnanchor">[132]</a></p>
+
+  <p>These facts are very interesting, because they help to explain
+  the general dependence of marking on structure which has been
+  already pointed out. For, as the nerves everywhere follow the
+  muscles, and these are attached to the various bones, we see how
+  it happens, that the tracts in which distinct developments of
+  colour appear, should so often be marked out by the chief
+  divisions of the bony structure in vertebrates, and by the
+  segments in the annulosa. There is, however, another
+  correspondence of even greater interest and importance. Brilliant
+  colours usually appear just in proportion to the <a name=
+  "Page_291" id="Page_291"></a>development of tegumentary
+  appendages. Among birds the most brilliant colours are possessed
+  by those which have developed frills, crests, and elongated tails
+  like the humming-birds; immense tail-coverts like the peacock;
+  enormously expanded wing-feathers, as in the argus-pheasant; or
+  magnificent plumes from the region of the coracoids in many of
+  the birds of paradise. It is to be noted, also, that all these
+  accessory plumes spring from parts of the body which, in other
+  species, are distinguished by patches of colour; so that we may
+  probably impute the development of colour and of accessory
+  plumage to the same fundamental cause.</p>
+
+  <p>Among insects, the most brilliant and varied coloration occurs
+  in the butterflies and moths, groups in which the wing-membranes
+  have received their greatest expansion, and whose specialisation
+  has been carried furthest in the marvellous scaly covering which
+  is the seat of the colour. It is suggestive, that the only other
+  group in which functional wings are much coloured is that of the
+  dragonflies, where the membrane is exceedingly expanded. In like
+  manner, the colours of beetles, though greatly inferior to those
+  of the lepidoptera, occur in a group in which the anterior pair
+  of wings has been thickened and modified in order to protect the
+  vital parts, and in which these wing-covers (elytra), in the
+  course of development in the different groups, must have
+  undergone great changes, and have been the seat of very active
+  growth.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Origin of Accessory Plumes.</i></p>
+
+  <p>Mr. Darwin supposes, that these have in almost every case been
+  developed by the preference of female birds for such males as
+  possessed them in a higher degree than others; but this theory
+  does not account for the fact that these plumes usually appear in
+  a few definite parts of the body. We require some cause to
+  initiate the development in one part rather than in another. Now,
+  the view that colour has arisen over surfaces where muscular and
+  nervous development is considerable, and the fact that it appears
+  especially upon the accessory or highly developed plumes, leads
+  us to inquire whether the same cause has not primarily determined
+  the development of these plumes. The immense tuft of golden
+  plumage in the best known birds of paradise (Paradisea apoda and
+  P. minor) <a name="Page_292" id="Page_292"></a>springs from a
+  very small area on the side of the breast. Mr. Frank E. Beddard,
+  who has kindly examined a specimen for me, says that "this area
+  lies upon the pectoral muscles, and near to the point where the
+  fibres of the muscle converge towards their attachment to the
+  humerus. The plumes arise, therefore, close to the most powerful
+  muscle of the body, and near to where the activities of that
+  muscle would be at a maximum. Furthermore, the area of attachment
+  of the plumes is just above the point where the arteries and
+  nerves for the supply of the pectoral muscles, and neighbouring
+  regions, leave the interior of the body. The area of attachment
+  of the plume is, also, as you say in your letter, just above the
+  junction of the coracoid and sternum." Ornamental plumes of
+  considerable size rise from the same part in many other species
+  of paradise birds, sometimes extending laterally in front, so as
+  to form breast shields. They also occur in many humming-birds,
+  and in some sun-birds and honey-suckers; and in all these cases
+  there is a wonderful amount of activity and rapid movement,
+  indicating a surplus of vitality, which is able to manifest
+  itself in the development of these accessory plumes.<a name=
+  "FNanchor_133_133" id="FNanchor_133_133"></a><a href=
+  "#Footnote_133_133" class="fnanchor">[133]</a></p>
+
+  <p>In a quite distinct set of birds, the gallinaceae, we find the
+  ornamental plumage usually arising from very different parts, in
+  the form of elongated tail-feathers or tail-coverts, and of ruffs
+  or hackles from the neck. Here the wings are comparatively little
+  used, the most constant activities depending on the legs, since
+  the gallinaceae are pre-eminently walking, running, and
+  scratching birds. Now the magnificent train of the
+  peacock&mdash;the grandest development of accessory plumes in
+  this order&mdash;springs from an oval or circular area, about
+  three inches in diameter, just above the base of the tail, and,
+  therefore, situated over the lower part of the spinal column near
+  the insertion of the powerful muscles which move the hind limbs
+  and elevate the tail. The very frequent presence of neck-ruffs or
+  breast-shields in the males of birds with accessory plumes may be
+  partly due to selection, because they must serve as a protection
+  in their mutual combats, just as does the lion's or the horse's
+  mane. The enormously lengthened plumes of the bird of paradise
+  and of the peacock can, however, have no such use, <a name=
+  "Page_293" id="Page_293"></a>but must be rather injurious than
+  beneficial in the bird's ordinary life. The fact that they have
+  been developed to so great an extent in a few species is an
+  indication of such perfect adaptation to the conditions of
+  existence, such complete success in the battle for life, that
+  there is, in the adult male at all events, a surplus of strength,
+  vitality, and growth-power which is able to expend itself in this
+  way without injury. That such is the case is shown by the great
+  abundance of most of the species which possess these wonderful
+  superfluities of plumage. Birds of paradise are among the
+  commonest birds in New Guinea, and their loud voices can be often
+  heard when the birds themselves are invisible in the depths of
+  the forest; while Indian sportsmen have described the peafowl as
+  being so abundant, that from twelve to fifteen hundred have been
+  seen within an hour at one spot; and they range over the whole
+  country from the Himalayas to Ceylon. Why, in allied species, the
+  development of accessory plumes has taken different forms, we are
+  unable to say, except that it may be due to that individual
+  variability which has served as the starting-point for so much of
+  what seems to us strange in form, or fantastic in colour, both in
+  the animal and vegetable world.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Development of Accessory Plumes and their Display.</i></p>
+
+  <p>If we have found a <i>vera causa</i>for the origin of
+  ornamental appendages of birds and other animals in a surplus of
+  vital energy, leading to abnormal growths in those parts of the
+  integument where muscular and nervous action are greatest, the
+  continuous development of these appendages will result from the
+  ordinary action of natural selection in preserving the most
+  healthy and vigorous individuals, and the still further selective
+  agency of sexual struggle in giving to the very strongest and
+  most energetic the parentage of the next generation. And, as all
+  the evidence goes to show that, so far as female birds exercise
+  any choice, it is of "the most vigorous, defiant, and mettlesome
+  male," this form of sexual selection will act in the same
+  direction, and help to carry on the process of plume development
+  to its culmination. That culmination will be reached when the
+  excessive length or abundance of the plumes begins to be
+  injurious to the bearer of them; and it may be this check to the
+  further lengthening of the peacock's <a name="Page_294" id=
+  "Page_294"></a>train that has led to the broadening of the
+  feathers at the ends, and the consequent production of the
+  magnificent eye-spots which now form its crowning ornament.</p>
+
+  <p>The display of these plumes will result from the same causes
+  which led to their production. Just in proportion as the feathers
+  themselves increased in length and abundance, the skin-muscles
+  which serve to elevate them would increase also; and the nervous
+  development as well as the supply of blood to these parts being
+  at a maximum, the erection of the plumes would become a habit at
+  all periods of nervous or sexual excitement. The display of the
+  plumes, like the existence of the plumes themselves, would be the
+  chief external indication of the maturity and vigour of the male,
+  and would, therefore, be necessarily attractive to the female. We
+  have, thus, no reason for imputing to her any of those aesthetic
+  emotions which are excited in us, by the beauty of form, colour,
+  and pattern of these plumes; or the still more improbable
+  aesthetic tastes, which would cause her to choose her mate on
+  account of minute differences in their forms, colours, or
+  patterns.</p>
+
+  <p>As co-operating causes in the production of accessory
+  ornamental plumes, I have elsewhere suggested<a name=
+  "FNanchor_134_134" id="FNanchor_134_134"></a><a href=
+  "#Footnote_134_134" class="fnanchor">[134]</a> that crests and
+  other erectile feathers may have been useful in making the bird
+  more formidable in appearance, and thus serving to frighten away
+  enemies; while long tail or wing feathers might serve to distract
+  the aim of a bird of prey. But though this might be of some use
+  in the earlier stages of their development, it is probably of
+  little importance compared with the vigour and pugnacity of which
+  the plumes are the indication, and which enable most of their
+  possessors to defend themselves against the enemies which are
+  dangerous to weaker and more timid birds. Even the tiny
+  humming-birds are said to attack birds of prey that approach too
+  near to their nests.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Effect of Female Preference will be Neutralised by Natural
+  Selection.</i></p>
+
+  <p>The various facts and arguments now briefly set forth, afford
+  an explanation of the phenomena of male ornament, <a name=
+  "Page_295" id="Page_295"></a>as being due to the general laws of
+  growth and development, and make it unnecessary to call to our
+  aid so hypothetical a cause as the cumulative action of female
+  preference. There remains, however, a general argument, arising
+  from the action of natural selection itself, which renders it
+  almost inconceivable that female preference could have been
+  effective in the way suggested; while the same argument strongly
+  supports the view here set forth. Natural selection, as we have
+  seen in our earlier chapters, acts perpetually and on an enormous
+  scale in weeding out the "unfit" at every stage of existence, and
+  preserving only those which are in all respects the very best.
+  Each year, only a small percentage of young birds survive to take
+  the place of the old birds which die; and the survivors will be
+  those which are best able to maintain existence from the egg
+  onwards, an important factor being that their parents should be
+  well able to feed and protect them, while they themselves must in
+  turn be equally able to feed and protect their own offspring. Now
+  this extremely rigid action of natural selection must render any
+  attempt to select mere ornament utterly nugatory, unless the most
+  ornamented always coincide with "the fittest" in every other
+  respect; while, if they do so coincide, then any selection of
+  ornament is altogether superfluous. If the most brightly coloured
+  and fullest plumaged males are <i>not</i> the most healthy and
+  vigorous, have <i>not</i> the best instincts for the proper
+  construction and concealment of the nest, and for the care and
+  protection of the young, they are certainly not the fittest, and
+  will not survive, or be the parents of survivors. If, on the
+  other hand, there <i>is</i> generally this correlation&mdash;if,
+  as has been here argued, ornament is the natural product and
+  direct outcome of superabundant health and vigour, then no other
+  mode of selection is needed to account for the presence of such
+  ornament. The action of natural selection does not indeed
+  disprove the existence of female selection of ornament as
+  ornament, but it renders it entirely ineffective; and as the
+  direct evidence for any such female selection is almost
+  <i>nil</i>, while the objections to it are certainly weighty,
+  there can be no longer any reason for upholding a theory which
+  was provisionally useful in calling attention to a most curious
+  and suggestive body of facts, but which is now no longer tenable.
+  <a name="Page_296" id="Page_296"></a>The term "sexual selection"
+  must, therefore, be restricted to the direct results of male
+  struggle and combat. This is really a form of natural selection,
+  and is a matter of direct observation; while its results are as
+  clearly deducible as those of any of the other modes in which
+  selection acts. And if this restriction of the term is needful in
+  the case of the higher animals it is much more so with the lower.
+  In butterflies the weeding out by natural selection takes place
+  to an enormous extent in the egg, larva, and pupa states; and
+  perhaps not more than one in a hundred of the eggs laid produces
+  a perfect insect which lives to breed. Here, then, the impotence
+  of female selection, if it exist, must be complete; for, unless
+  the most brilliantly coloured males are those which produce the
+  best protected eggs, larvae, and pupae, and unless the particular
+  eggs, larvae, and pupae, which are able to survive, are those
+  which produce the most brilliantly coloured butterflies, any
+  choice the female might make must be completely swamped. If, on
+  the other hand, there <i>is</i> this correlation between colour
+  development and perfect adaptation to conditions in all stages,
+  then this development will necessarily proceed by the agency of
+  natural selection and the general laws which determine the
+  production of colour and of ornamental appendages.<a name=
+  "FNanchor_135_135" id="FNanchor_135_135"></a><a href=
+  "#Footnote_135_135" class="fnanchor">[135]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>General Laws of Animal Coloration.</i></p>
+
+  <p>The condensed account which has now been given of the
+  phenomena of colour in the animal world will sufficiently show
+  the wonderful complexity and extreme interest of the subject;
+  while it affords an admirable illustration of the importance of
+  the great principle of utility, and of the effect of the theories
+  of natural selection and development in giving a new interest
+  <a name="Page_297" id="Page_297"></a>to the most familiar facts
+  of nature. Much yet remains to be done, both in the observation
+  of new facts as to the relations between the colours of animals
+  and their habits or economy, and, more especially, in the
+  elucidation of the laws of growth which determine changes of
+  colour in the various groups; but so much is already known that
+  we are able, with some confidence, to formulate the general
+  principles which have brought about all the beauty and variety of
+  colour which everywhere delight us in our contemplation of
+  animated nature. A brief statement of these principles will fitly
+  conclude our exposition of the subject.</p>
+
+  <p>1. Colour may be looked upon as a necessary result of the
+  highly complex chemical constitution of animal tissues and
+  fluids. The blood, the bile, the bones, the fat, and other
+  tissues have characteristic, and often brilliant colours, which
+  we cannot suppose to have been determined for any special
+  purpose, as colours, since they are usually concealed. The
+  external organs, with their various appendages and integuments,
+  would, by the same general laws, naturally give rise to a greater
+  variety of colour.</p>
+
+  <p>2. We find it to be the fact that colour increases in variety
+  and intensity as external structures and dermal appendages become
+  more differentiated and developed. It is on scales, hair, and
+  especially on the more highly specialised feathers, that colour
+  is most varied and beautiful; while among insects colour is most
+  fully developed in those whose wing membranes are most expanded,
+  and, as in the lepidoptera, are clothed with highly specialised
+  scales. Here, too, we find an additional mode of colour
+  production in transparent lamellae or in fine surface striae
+  which, by the laws of interference, produce the wonderful
+  metallic hues of so many birds and insects.</p>
+
+  <p><a name="Page_298" id="Page_298"></a>3. There are indications
+  of a progressive change of colour, perhaps in some definite
+  order, accompanying the development of tissues or appendages.
+  Thus spots spread and fuse into bands, and when a lateral or
+  centrifugal expansion has occurred&mdash;as in the termination of
+  the peacocks' train feathers, the outer web of the secondary
+  quills of the Argus pheasant, or the broad and rounded wings of
+  many butterflies&mdash;into variously shaded or coloured ocelli.
+  The fact that we find gradations of colour in many of the more
+  extensive groups, from comparatively dull or simple to brilliant
+  and varied hues, is an indication of some such law of
+  development, due probably to progressive local segregation in the
+  tissues of identical chemical or organic molecules, and dependent
+  on laws of growth yet to be investigated.</p>
+
+  <p>4. The colours thus produced, and subject to much individual
+  variation, have been modified in innumerable ways for the benefit
+  of each species. The most general modification has been in such
+  directions as to favour concealment when at rest in the usual
+  surroundings of the species, sometimes carried on by successive
+  steps till it has resulted in the most minute imitation of some
+  inanimate object or exact mimicry of some other animal. In other
+  cases bright colours or striking contrasts have been preserved,
+  to serve as a warning of inedibility or of dangerous powers of
+  attack. Most frequent of all has been the specialisation of each
+  distinct form by some tint or marking for purposes of easy
+  recognition, especially in the case of gregarious animals whose
+  safety largely depends upon association and mutual defence.</p>
+
+  <p>5. As a general rule the colours of the two sexes are alike;
+  but in the higher animals there appears a tendency to deeper or
+  more intense colouring in the male, due probably to his greater
+  vigour and excitability. In many groups in which this
+  superabundant vitality is at a maximum, the development of dermal
+  appendages and brilliant colours has gone on increasing till it
+  has resulted in a great diversity between the sexes; and in most
+  of these cases there is evidence to show that natural selection
+  has caused the female to retain the primitive and more sober
+  colours of the group for purposes of protection.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_299" id="Page_299"></a><i>Concluding
+  Remarks.</i></p>
+
+  <p>The general principles of colour development now sketched out
+  enable us to give some rational explanation of the wonderful
+  amount of brilliant colour which occurs among tropical animals.
+  Looking on colour as a normal product of organisation, which has
+  either been allowed free play, or has been checked and modified
+  for the benefit of the species, we can see at once that the
+  luxuriant and perennial vegetation of the tropics, by affording
+  much more constant means of concealment, has rendered brilliant
+  colour less hurtful there than in the temperate and colder
+  regions. Again, this perennial vegetation supplies abundance of
+  both vegetable and insect food throughout the year, and thus a
+  greater abundance and greater variety of the forms of life are
+  rendered possible, than where recurrent seasons of cold and
+  scarcity reduce the possibilities of life to a minimum. Geology
+  furnishes us with another reason, in the fact, that throughout
+  the tertiary period tropical conditions prevailed far into the
+  temperate regions, so that the possibilities of colour
+  development were still greater than they are at the present time.
+  The tropics, therefore, present to us the results of animal
+  development in a much larger area and under more favourable
+  conditions than prevail to-day. We see in them samples of the
+  productions of an earlier and a better world, from an animal
+  point of view; and this probably gives a greater variety and a
+  finer display of colour than would have been produced, had
+  conditions always been what they are now. The temperate zones, on
+  the other hand, have recently suffered the effects of a glacial
+  period of extreme severity, with the result that almost the only
+  gay coloured birds they now possess are summer visitors from
+  tropical or sub-tropical lands. It is to the unbroken and almost
+  unchecked course of development from remote geological times that
+  has prevailed in the tropics, favoured by abundant food and
+  perennial shelter, that we owe such superb developments as the
+  frills and crests and jewelled shields of the humming-birds, the
+  golden plumes of the birds of paradise, and the resplendent train
+  of the peacock. This last exhibits to us the culmination of that
+  marvel and mystery of animal colour which is so well expressed by
+  a poet-artist in the following <a name="Page_300" id=
+  "Page_300"></a>lines. The marvel will ever remain to the
+  sympathetic student of nature, but I venture to hope that in the
+  preceding chapters I have succeeded in lifting&mdash;if only by
+  one of its corners&mdash;the veil of mystery which has for long
+  shrouded this department of nature.</p>
+
+  <div class="poem">
+    <p style='font-weight:bold;margin-top:1.5em'><i>On a Peacock's
+    Feather.</i></p>
+
+    <div class="stanza">
+      <span>In Nature's workshop but a shaving,<br /></span>
+      <span class="i2">Of her poem but a word,<br /></span>
+      <span>But a tint brushed from her palette,<br /></span>
+      <span class="i2">This feather of a bird!<br /></span>
+      <span>Yet set it in the sun glance,<br /></span> <span class=
+      "i2">Display it in the shine,<br /></span> <span>Take
+      graver's lens, explore it,<br /></span> <span class="i2">Note
+      filament and line,<br /></span> <span>Mark amethyst to
+      sapphire,<br /></span> <span class="i2">And sapphire to
+      gold,<br /></span> <span>And gold to emerald
+      changing<br /></span> <span class="i2">The archetype
+      unfold!<br /></span> <span>Tone, tint, thread, tissue,
+      texture,<br /></span> <span class="i2">Through every atom
+      scan,<br /></span> <span>Conforming still,
+      developing,<br /></span> <span class="i2">Obedient to
+      plan.<br /></span> <span>This but to form a
+      pattern<br /></span> <span class="i2">On the garment of a
+      bird!<br /></span> <span>What then must be the
+      poem,<br /></span> <span class="i2">This but its lightest
+      word!<br /></span> <span>Sit before it; ponder o'er
+      it,<br /></span> <span class="i2">'Twill thy mind advantage
+      more,<br /></span> <span>Than a treatise, than a
+      sermon,<br /></span> <span class="i2">Than a library of
+      lore.<br /></span>
+    </div>
+  </div>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_118_118" id=
+      "Footnote_118_118"></a><a href=
+      "#FNanchor_118_118"><span class="label">[118]</span></a>
+      Darwin's <i>Descent of Man</i>, p. 271.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_119_119" id=
+      "Footnote_119_119"></a><a href=
+      "#FNanchor_119_119"><span class="label">[119]</span></a>
+      Darwin's <i>Descent of Man</i>, p. 294, and footnote.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_120_120" id=
+      "Footnote_120_120"></a><a href=
+      "#FNanchor_120_120"><span class="label">[120]</span></a>
+      <i>Nature</i>, 1871, p. 489.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_121_121" id=
+      "Footnote_121_121"></a><a href=
+      "#FNanchor_121_121"><span class="label">[121]</span></a>
+      Darwin in <i>Nature</i>, 1880, p. 237.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_122_122" id=
+      "Footnote_122_122"></a><a href=
+      "#FNanchor_122_122"><span class="label">[122]</span></a> See
+      the author's <i>Contributions to Natural Selection</i>, chap.
+      vii. in which these facts were first brought forward.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_123_123" id=
+      "Footnote_123_123"></a><a href=
+      "#FNanchor_123_123"><span class="label">[123]</span></a> On
+      this point see the author's <i>Contributions to Natural
+      Selection</i>, chap. v. i.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_124_124" id=
+      "Footnote_124_124"></a><a href=
+      "#FNanchor_124_124"><span class="label">[124]</span></a>
+      Seebohm's <i>History of British Birds</i>, vol. ii.,
+      introduction, p. xiii.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_125_125" id=
+      "Footnote_125_125"></a><a href=
+      "#FNanchor_125_125"><span class="label">[125]</span></a> For
+      details see Darwin's <i>Descent of Man</i>, chap. xii.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_126_126" id=
+      "Footnote_126_126"></a><a href=
+      "#FNanchor_126_126"><span class="label">[126]</span></a>
+      <i>Descent of Man</i>, pp. 417, 418, 420.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_127_127" id=
+      "Footnote_127_127"></a><a href=
+      "#FNanchor_127_127"><span class="label">[127]</span></a>
+      <i>Notes of a Naturalist on the Challenger.</i></p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_128_128" id=
+      "Footnote_128_128"></a><a href=
+      "#FNanchor_128_128"><span class="label">[128]</span></a>
+      <i>Descent of Man</i>, pp. 401, 402.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_129_129" id=
+      "Footnote_129_129"></a><a href=
+      "#FNanchor_129_129"><span class="label">[129]</span></a>
+      <i>Coloration in Animals and Plants</i>, London, 1886.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_130_130" id=
+      "Footnote_130_130"></a><a href=
+      "#FNanchor_130_130"><span class="label">[130]</span></a>
+      <i>Coloration of Animals</i>, Pl. X, p. 90; and Pls. II, III,
+      and IV, pp. 30, 40, 42.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_131_131" id=
+      "Footnote_131_131"></a><a href=
+      "#FNanchor_131_131"><span class="label">[131]</span></a> See
+      coloured Fig. in <i>Proc. Zool. Soc.</i>, 1871, p. 626.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_132_132" id=
+      "Footnote_132_132"></a><a href=
+      "#FNanchor_132_132"><span class="label">[132]</span></a> A.
+      Tylor's <i>Coloration</i>, p. 40; and Photograph in
+      Hutchinson's <i>Illustrations of Clinical Surgery</i>, quoted
+      by Tylor.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_133_133" id=
+      "Footnote_133_133"></a><a href=
+      "#FNanchor_133_133"><span class="label">[133]</span></a> For
+      activity and pugnacity of humming-birds, see <i>Tropical
+      Nature</i>, pp. 130, 213.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_134_134" id=
+      "Footnote_134_134"></a><a href=
+      "#FNanchor_134_134"><span class="label">[134]</span></a>
+      <i>Tropical Nature</i>, p. 209. In Chapter V of this work the
+      views here advocated were first set forth, and the reader is
+      referred there for further details.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_135_135" id=
+      "Footnote_135_135"></a><a href=
+      "#FNanchor_135_135"><span class="label">[135]</span></a> The
+      Rev. O. Pickard-Cambridge, who has devoted himself to the
+      study of spiders, has kindly sent me the following extract
+      from a letter, written in 1869, in which he states his views
+      on this question:&mdash;</p>
+
+      <div class="blockquot" style='font-size: 100%'>
+        <p>"I myself doubt that particular application of the
+        Darwinian theory which attributes male peculiarities of
+        form, structure, colour, and ornament to female appetency
+        or predilection. There is, it seems to me, undoubtedly
+        something in the male organisation of a special, and sexual
+        nature, which, of its own vital force, develops the
+        remarkable male peculiarities so commonly seen, and of no
+        imaginable use to that sex. In as far as these
+        peculiarities show a great vital power, they point out to
+        us the finest and strongest individuals of the sex, and
+        show us which of them would most certainly appropriate to
+        themselves the best and greatest number of females, and
+        leave behind them the strongest and greatest number of
+        progeny. And here would come in, as it appears to me, the
+        proper application of Darwin's theory of Natural Selection;
+        for the possessors of greatest vital power being those most
+        frequently produced and reproduced, the external signs of
+        it would go on developing in an ever-increasing
+        exaggeration, only to be checked where it became really
+        detrimental in some respect or other to the
+        individual."</p>
+      </div>
+
+      <p>This passage, giving the independent views of a close
+      observer&mdash;one, moreover, who has studied the species of
+      an extensive group of animals both in the field and in the
+      laboratory&mdash;very nearly accords with my own conclusions
+      above given; and, so far as the matured opinions of a
+      competent naturalist have any weight, afford them an
+      important support.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_XI" id="CHAPTER_XI"></a><a name="Page_301"
+  id="Page_301"></a>CHAPTER XI</h2>
+
+  <p style='font-weight:bold;'>THE SPECIAL COLOURS OF PLANTS: THEIR
+  ORIGIN AND PURPOSE</p>
+
+  <div class="blockquot">
+    <p>The general colour relations of plants&mdash;Colours of
+    fruits&mdash;The meaning of nuts&mdash;Edible or attractive
+    fruits&mdash;The colours of flowers&mdash;Modes of securing
+    cross-fertilisation&mdash;The interpretation of the
+    facts&mdash;Summary of additional facts bearing on insect
+    fertilisation&mdash;Fertilisation of flowers by
+    birds&mdash;Self-fertilisation of flowers&mdash;Difficulties
+    and contradictions&mdash;Intercrossing not necessarily
+    advantageous&mdash;Supposed evil results of close
+    interbreeding&mdash;How the struggle for existence acts among
+    flowers&mdash;Flowers the product of insect
+    agency&mdash;Concluding remarks on colour in nature.</p>
+  </div>
+
+  <p>The colours of plants are both less definite and less complex
+  than are those of animals, and their interpretation on the
+  principle of utility is, on the whole, more direct and more easy.
+  Yet here, too, we find that in our investigation of the uses of
+  the various colours of fruits and flowers, we are introduced to
+  some of the most obscure recesses of nature's workshop, and are
+  confronted with problems of the deepest interest and of the
+  utmost complexity.</p>
+
+  <p>So much has been written on this interesting subject since Mr.
+  Darwin first called attention to it, and its main facts have
+  become so generally known by means of lectures, articles, and
+  popular books, that I shall give here a mere outline sketch, for
+  the purpose of leading up to a discussion of some of the more
+  fundamental problems which arise out of the facts, and which have
+  hitherto received less attention than they deserve.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_302" id="Page_302"></a><i>The General Colour
+  Relations of Plants.</i></p>
+
+  <p>The green colour of the foliage of leafy plants is due to the
+  existence of a substance called chlorophyll, which is almost
+  universally developed in the leaves under the action of light. It
+  is subject to definite chemical changes during the processes of
+  growth and of decay, and it is owing to these changes that we
+  have the delicate tints of spring foliage, and the more varied,
+  intense, and gorgeous hues of autumn. But these all belong to the
+  class of intrinsic or normal colours, due to the chemical
+  constitution of the organism; as colours they are unadaptive, and
+  appear to have no more relation to the wellbeing of the plants
+  themselves than have the colours of gems and minerals. We may
+  also include in the same category those algae and fungi which
+  have bright colours&mdash;the "red snow" of the arctic regions,
+  the red, green, or purple seaweeds, the brilliant scarlet,
+  yellow, white, or black agarics, and other fungi. All these
+  colours are probably the direct results of chemical composition
+  or molecular structure, and, being thus normal products of the
+  vegetable organism, need no special explanation from our present
+  point of view; and the same remark will apply to the varied tints
+  of the bark of trunks, branches, and twigs, which are often of
+  various shades of brown and green, or even vivid reds or
+  yellows.</p>
+
+  <p>There are, however, a few cases in which the need of
+  protection, which we have found to be so important an agency in
+  modifying the colours of animals, has also determined those of
+  some of the smaller members of the vegetable kingdom. Dr.
+  Burchell found a mesembryanthomum in South Africa like a
+  curiously shaped pebble, closely resembling the stones among
+  which it grew;<a name="FNanchor_136_136" id=
+  "FNanchor_136_136"></a><a href="#Footnote_136_136" class=
+  "fnanchor">[136]</a> and Mr. J.P. Mansel Weale states that in the
+  same country one of the Asclepiadeae has tubers growing above
+  ground among stones which they exactly resemble, and that, when
+  not in leaf, they are for this reason quite invisible.<a name=
+  "FNanchor_137_137" id="FNanchor_137_137"></a><a href=
+  "#Footnote_137_137" class="fnanchor">[137]</a> It is clear that
+  such resemblances must be highly useful to these plants,
+  inhabiting an arid country abounding in herbivorous mammalia,
+  which, <a name="Page_303" id="Page_303"></a>in times of drought
+  or scarcity, will devour everything in the shape of a fleshy stem
+  or tuber.</p>
+
+  <p>True mimicry is very rare in plants, though adaptation to like
+  conditions often produces in foliage and habit a similarity that
+  is deceiving. Euphorbias growing in deserts often closely
+  resemble cacti. Seaside plants and high alpine plants of
+  different orders are often much alike; and innumerable
+  resemblances of this kind are recorded in the names of plants, as
+  Veronica epacridea (the veronica like an epacris), Limnanthemum
+  nymphaeoides (the limnanthemum like a nymphaea), the resembling
+  species in each case belonging to totally distinct families. But
+  in these cases, and in most others that have been observed, the
+  essential features of true mimicry are absent, inasmuch as the
+  one plant cannot be supposed to derive any benefit from its close
+  resemblance to the other, and this is still more certain from the
+  fact that the two species usually inhabit different localities. A
+  few cases exist, however, in which there does seem to be the
+  necessary accordance and utility. Mr. Mansel Weale mentions a
+  labiate plant (Ajuga ophrydis), the only species of the genus
+  Ajuga in South Africa, which is strikingly like an orchid of the
+  same country; while a balsam (Impatiens capensis), also a
+  solitary species of the genus in that country, is equally like an
+  orchid, growing in the same locality and visited by the same
+  insects. As both these genera of plants are specialised for
+  insect fertilisation, and both of the plants in question are
+  isolated species of their respective genera, we may suppose that,
+  when they first reached South Africa they were neglected by the
+  insects of the country; but, being both remotely like orchids in
+  form of flower, those varieties that approached nearest to the
+  familiar species of the country were visited by insects and
+  cross-fertilised, and thus a closer resemblance would at length
+  be brought about. Another case of close general resemblance, is
+  that of our common white dead-nettle (Lamium album) to the
+  stinging-nettle (Urtica dioica); and Sir John Lubbock thinks that
+  this is a case of true mimicry, the dead-nettle being benefited
+  by being mistaken by grazing animals for the
+  stinging-nettle.<a name="FNanchor_138_138" id=
+  "FNanchor_138_138"></a><a href="#Footnote_138_138" class=
+  "fnanchor">[138]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_304" id="Page_304"></a><i>Colours of
+  Fruits.</i></p>
+
+  <p>It is when we come to the essential parts of plants on which
+  their perpetuation and distribution depends, that we find colour
+  largely utilised for a distinct purpose in flowers and fruits. In
+  the former we find attractive colours and guiding marks to secure
+  cross-fertilisation by insects; in the latter attractive or
+  protective coloration, the first to attract birds or other
+  animals when the fruits are intended to be eaten, the second to
+  enable them to escape being eaten when it would be injurious to
+  the species. The colour phenomena of fruits being much the most
+  simple will be considered first.</p>
+
+  <p>The perpetuation and therefore the very existence of each
+  species of flowering plant depend upon its seeds being preserved
+  from destruction and more or less effectually dispersed over a
+  considerable area. The dispersal is effected either mechanically
+  or by the agency of animals. Mechanical dispersal is chiefly by
+  means of air-currents, and large numbers of seeds are specially
+  adapted to be so carried, either by being clothed with down or
+  pappus, as in the well-known thistle and dandelion seeds; by
+  having wings or other appendages, as in the sycamore, birch, and
+  many other trees; by being thrown to a considerable distance by
+  the splitting of the seed-vessel, and by many other curious
+  devices.<a name="FNanchor_139_139" id=
+  "FNanchor_139_139"></a><a href="#Footnote_139_139" class=
+  "fnanchor">[139]</a> Very large numbers of seeds, however, are so
+  small and light that they can be carried enormous distances by
+  gales of wind, more especially as most of this kind are flattened
+  or curved, so as to expose a large surface in proportion to their
+  weight. Those which are carried by animals have their surfaces,
+  or that of the seed-vessel, armed with minute hooks, or some
+  prickly covering which attaches itself to the hair of mammalia or
+  the feathers of birds, as in the burdock, cleavers, and many
+  other species. Others again are sticky, as in Plumbago europaea,
+  mistletoe, and many foreign plants.</p>
+
+  <p>All the seeds or seed-vessels which are adapted to be
+  dispersed in any of these ways are of dull protective tints, so
+  that when they fall on the ground they are almost
+  indistinguishable; besides which, they are usually small, hard,
+  and <a name="Page_305" id="Page_305"></a>altogether unattractive,
+  never having any soft, juicy pulp; while the edible seeds often
+  bear such a small proportion to the hard, dry envelopes or
+  appendages, that few animals would care to eat them.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Meaning of Nuts.</i></p>
+
+  <p>There is, however, another class of fruits or seeds, usually
+  termed nuts, in which there is a large amount of edible matter,
+  often very agreeable to the taste, and especially attractive and
+  nourishing to a large number of animals. But when eaten, the seed
+  is destroyed and the existence of the species endangered. It is
+  evident, therefore, that it is by a kind of accident that these
+  nuts are eatable; and that they are not intended to be eaten is
+  shown by the special care nature seems to have taken to conceal
+  or to protect them. We see that all our common nuts are green
+  when on the tree, so as not easily to be distinguished from the
+  leaves; but when ripe they turn brown, so that when they fall on
+  to the ground they are equally indistinguishable among the dead
+  leaves and twigs, or on the brown earth. Then they are almost
+  always protected by hard coverings, as in hazel-nuts, which are
+  concealed by the enlarged leafy involucre, and in the large
+  tropical brazil-nuts and cocoa-nuts by such a hard and tough case
+  as to be safe from almost every animal. Others have an external
+  bitter rind, as in the walnut; while in the chestnuts and
+  beech-nuts two or three fruits are enclosed in a prickly
+  involucre.</p>
+
+  <p>Notwithstanding all these precautions, nuts are largely
+  devoured by mammalia and birds; but as they are chiefly the
+  product of trees or shrubs of considerable longevity, and are
+  generally produced in great profusion, the perpetuation of the
+  species is not endangered. In some cases the devourers of nuts
+  may aid in their dispersal, as they probably now and then swallow
+  the seed whole, or not sufficiently crushed to prevent
+  germination; while squirrels have been observed to bury nuts,
+  many of which are forgotten and afterwards grow in places they
+  could not have otherwise reached.<a name="FNanchor_140_140" id=
+  "FNanchor_140_140"></a><a href="#Footnote_140_140" class=
+  "fnanchor">[140]</a> Nuts, especially the larger kinds which are
+  so well protected by their hard, nearly globular cases, have
+  their dispersal facilitated by rolling down hill, and more
+  especially <a name="Page_306" id="Page_306"></a>by floating in
+  rivers and lakes, and thus reaching other localities. During the
+  elevation of land areas this method would be very effective, as
+  the new land would always be at a lower level than that already
+  covered with vegetation, and therefore in the best position for
+  being stocked with plants from it.</p>
+
+  <p>The other modes of dispersal of seeds are so clearly adapted
+  to their special wants, that we feel sure they must have been
+  acquired by the process of variation and natural selection. The
+  hooked and sticky seeds are always those of such herbaceous
+  plants as are likely, from their size, to come in contact with
+  the wool of sheep or the hair of cattle; while seeds of this kind
+  never occur on forest trees, on aquatic plants, or even on very
+  dwarf creepers or trailers. The winged seed-vessels or seeds, on
+  the other hand, mostly belong to trees and to tall shrubs or
+  climbers. We have, therefore, a very exact adaptation to
+  conditions in these different modes of dispersal; while, when we
+  come to consider individual cases, we find innumerable other
+  adaptations, some of which the reader will find described in the
+  little work by Sir John Lubbock already referred to.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Edible or Attractive Fruits.</i></p>
+
+  <p>It is, however, when we come to true fruits (in a popular
+  sense) that we find varied colours evidently intended to attract
+  animals, in order that the fruits may be eaten, while the seeds
+  pass through the body undigested and are then in the fittest
+  state for germination. This end has been gained in a great
+  variety of ways, and with so many corresponding adaptations as to
+  leave no doubt as to the value of the result. Fruits are pulpy or
+  juicy, and usually sweet, and form the favourite food of
+  innumerable birds and some mammals. They are always coloured so
+  as to contrast with the foliage or surroundings, red being the
+  most common as it is certainly the most conspicuous colour, but
+  yellow, purple, black, or white being not uncommon. The edible
+  portion of fruits is developed from different parts of the floral
+  envelopes, or of the ovary, in the various orders and genera.
+  Sometimes the calyx becomes enlarged and fleshy, as in the apple
+  and pear tribe; more often the integuments of the ovary itself
+  are enlarged, as in the plum, peach, grape, etc.; the receptacle
+  is enlarged and <a name="Page_307" id="Page_307"></a>forms the
+  fruit of the strawberry; while the mulberry, pineapple, and fig
+  are examples of compound fruits formed in various ways from a
+  dense mass of flowers.</p>
+
+  <p>In all cases the seeds themselves are protected from injury by
+  various devices. They are small and hard in the strawberry,
+  raspberry, currant, etc., and are readily swallowed among the
+  copious pulp. In the grape they are hard and bitter; in the rose
+  (hip) disagreeably hairy; in the orange tribe very bitter; and
+  all these have a smooth, glutinous exterior which facilitates
+  their being swallowed. When the seeds are larger and are eatable,
+  they are enclosed in an excessively hard and thick covering, as
+  in the various kinds of "stone" fruit (plums, peaches, etc.), or
+  in a very tough core, as in the apple. In the nutmeg of the
+  Eastern Archipelago we have a curious adaptation to a single
+  group of birds. The fruit is yellow, somewhat like an oval peach,
+  but firm and hardly eatable. This splits open and shows the
+  glossy black covering of the seed or nutmeg, over which spreads
+  the bright scarlet arillus or "mace," an adventitious growth of
+  no use to the plant except to attract attention. Large fruit
+  pigeons pluck out this seed and swallow it entire for the sake of
+  the mace, while the large nutmeg passes through their bodies and
+  germinates; and this has led to the wide distribution of wild
+  nutmegs over New Guinea and the surrounding islands.</p>
+
+  <p>In the restriction of bright colour to those edible fruits the
+  eating of which is beneficial to the plant, we see the undoubted
+  result of natural selection; and this is the more evident when we
+  find that the colour never appears till the fruit is
+  ripe&mdash;that is, till the seeds within it are fully matured
+  and in the best state for germination. Some brilliantly coloured
+  fruits are poisonous, as in our bitter-sweet (Solanum dulcamara),
+  cuckoo-pint (Arum) and the West Indian manchineel. Many of these
+  are, no doubt, eaten by animals to whom they are harmless; and it
+  has been suggested that even if some animals are poisoned by them
+  the plant is benefited, since it not only gets dispersed, but
+  finds, in the decaying body of its victim, a rich manure
+  heap.<a name="FNanchor_141_141" id=
+  "FNanchor_141_141"></a><a href="#Footnote_141_141" class=
+  "fnanchor">[141]</a> The particular colours of fruits are not, so
+  far as we know, of any use to them other <a name="Page_308" id=
+  "Page_308"></a>than as regards conspicuousness, hence a tendency
+  to <i>any</i> decided colour has been preserved and accumulated
+  as serving to render the fruit easily visible among its
+  surroundings of leaves or herbage. Out of 134 fruit-bearing
+  plants in Mongredien's <i>Trees and Shrubs</i>, and Hooker's
+  <i>British Flora</i>, the fruits of no less than sixty-eight, or
+  rather more than half, are red, forty-five are black, fourteen
+  yellow, and seven white. The great prevalence of red fruits is
+  almost certainly due to their greater conspicuousness having
+  favoured their dispersal, though it may also have arisen in part
+  from the chemical changes of chlorophyll during ripening and
+  decay producing red tints as in many fading leaves. Yet the
+  comparative scarcity of yellow in fruits, while it is the most
+  common tint of fading leaves, is against this supposition.</p>
+
+  <p>There are, however, a few instances of coloured fruits which
+  do not seem to be intended to be eaten; such are the colocynth
+  plant (Cucumis colocynthus), which has a beautiful fruit the size
+  and colour of an orange, but nauseous beyond description to the
+  taste. It has a hard rind, and may perhaps be dispersed by being
+  blown along the ground, the colour being an adventitious product;
+  but it is quite possible, notwithstanding its repulsiveness to
+  us, that it may be eaten by some animals. With regard to the
+  fruit of another plant, Calotropis procera, there is less doubt,
+  as it is dry and full of thin, flat-winged seeds, with fine silky
+  filaments, eminently adapted for wind-dispersal; yet it is of a
+  bright yellow colour, as large as an apple, and therefore very
+  conspicuous. Here, therefore, we seem to have colour which is a
+  mere byproduct of the organism and of no use to it; but such
+  cases are exceedingly rare, and this rarity, when compared with
+  the great abundance of cases in which there is an obvious purpose
+  in the colour, adds weight to the evidence in favour of the
+  theory of the attractive coloration of edible fruits in order
+  that birds and other animals may assist in their dispersal. Both
+  the above-named plants are natives of Palestine and the adjacent
+  arid countries.<a name="FNanchor_142_142" id=
+  "FNanchor_142_142"></a><a href="#Footnote_142_142" class=
+  "fnanchor">[142]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Colours of Flowers.</i></p>
+
+  <p>Flowers are much more varied in their colours than fruits,
+  <a name="Page_309" id="Page_309"></a>as they are more complex and
+  more varied in form and structure; yet there is some parallelism
+  between them in both respects. Flowers are frequently adapted to
+  attract insects as fruits are to attract birds, the object being
+  in the former to secure cross-fertilisation, in the latter
+  dispersal; while just as colour is an index of the edibility of
+  fruits which supply pulp or juice to birds, so are the colours of
+  flowers an indication of the presence of nectar or of pollen
+  which are devoured by insects.</p>
+
+  <p>The main facts and many of the details, as to the relation of
+  insects to flowers, were discovered by Sprengel in 1793. He
+  noticed the curious adaptation of the structure of many flowers
+  to the particular insects which visit them; he proved that
+  insects do cross-fertilise flowers, and he believed that this was
+  the object of the adaptations, while the presence of nectar and
+  pollen ensured the continuance of their visits; yet he missed
+  discovering the <i>use</i> of this cross-fertilisation. Several
+  writers at a later period obtained evidence that
+  cross-fertilisation of plants was a benefit to them; but the wide
+  generality of this fact and its intimate connection with the
+  numerous and curious adaptations discovered by Sprengel, was
+  first shown by Mr. Darwin, and has since been demonstrated by a
+  vast mass of observations, foremost among which are his own
+  researches on orchids, primulas, and other plants.<a name=
+  "FNanchor_143_143" id="FNanchor_143_143"></a><a href=
+  "#Footnote_143_143" class="fnanchor">[143]</a></p>
+
+  <p>By an elaborate series of experiments carried on for many
+  years Mr. Darwin demonstrated the great value of
+  cross-fertilisation in increasing the rapidity of growth, the
+  strength and vigour of the plant, and in adding to its fertility.
+  This effect is produced immediately, not as he expected would be
+  the case, after several generations of crosses. He planted seeds
+  from cross-fertilised and self-fertilised plants on two sides of
+  the same pot exposed to exactly similar conditions, and in most
+  cases the difference in size and vigour was amazing, while the
+  plants from cross-fertilised parents also produced more and finer
+  seeds. These experiments entirely confirmed the experience of
+  breeders of animals already referred to (p. 160), and led him to
+  enunciate his famous aphorism, "<a name="Page_310" id=
+  "Page_310"></a>Nature abhors perpetual
+  self-fertilisation".<a name="FNanchor_144_144" id=
+  "FNanchor_144_144"></a><a href="#Footnote_144_144" class=
+  "fnanchor">[144]</a> In this principle we appear to have a
+  sufficient reason for the various contrivances by which so many
+  flowers secure cross-fertilisation, either constantly or
+  occasionally. These contrivances are so numerous, so varied, and
+  often so highly complex and extraordinary, that they have formed
+  the subject of many elaborate treatises, and have also been amply
+  popularised in lectures and handbooks. It will be unnecessary,
+  therefore, to give details here, but the main facts will be
+  summarised in order to call attention to some difficulties of the
+  theory which seem to require further elucidation.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Modes of securing Cross-Fertilisation.</i></p>
+
+  <p>When we examine the various modes in which the
+  cross-fertilisation of flowers is brought about, we find that
+  some are comparatively simple in their operation and needful
+  adjustments, others highly complex. The simple methods belong to
+  four principal classes:&mdash;(1) By dichogamy&mdash;that is, by
+  the anthers and the stigma becoming mature or in a fit state for
+  fertilisation at slightly different times on the same plant. The
+  result of this is that, as plants in different stations, on
+  different soils, or exposed to different aspects flower earlier
+  or later, the mature pollen of one plant can only fertilise some
+  plant exposed to somewhat different conditions or of different
+  constitution, whose stigma will be mature at the same time; and
+  this difference has been shown by Darwin to be that which is
+  adapted to secure the fullest benefit of cross-fertilisation.
+  This occurs in Geranium pratense, Thymus serpyllum, Arum
+  maculatum, and many others. (2) By the flower being self-sterile
+  with its own pollen, as in the crimson flax. This absolutely
+  prevents self-fertilisation. (3) By the stamens and anthers being
+  so placed that the pollen cannot fall upon the stigma, while it
+  does fall upon a visiting insect which carries it to the stigma
+  of another flower. This effect is produced in a variety of very
+  simple ways, and is often aided by the motion of the stamens
+  which bend down out of the way of the stigmas before the pollen
+  is ripe, as in Malva sylvestris (see Fig. 28). (4) By the male
+  and female flowers being on <a name="Page_311" id=
+  "Page_311"></a>different plants, forming the class Dioecia of
+  Linnaeus. In these cases the pollen may be carried to the stigmas
+  either by the wind or by the agency of insects.</p>
+
+  <div class="figcenter" style="width: 300px;">
+    <a name="fig28" id="fig28"></a> <img src="images/fig28.jpg"
+    width="300" height="340" alt=
+    "FIG. 28. Malva sylvestris, adapted for insect-fertilisation. Malva rotundifolia, adapted for self-fertilisation."
+    title=
+    "FIG. 28. Malva sylvestris, adapted for insect-fertilisation. Malva rotundifolia, adapted for self-fertilisation." />
+     <b>FIG. 28. Malva sylvestris, adapted for
+    insect-fertilisation.<br />
+    Malva rotundifolia, adapted for self-fertilisation.</b>
+  </div>
+
+  <p>Now these four methods are all apparently very simple, and
+  easily produced by variation and selection. They are applicable
+  to flowers of any shape, requiring only such size and colour as
+  to attract insects, and some secretion of nectar to ensure their
+  repeated visits, characters common to the great majority of
+  flowers. All these methods are common, except perhaps the second;
+  but there are many flowers in which the pollen from another plant
+  is prepotent over the pollen from fertilisation, the same flower,
+  and this has nearly the same effect as self-sterility if the
+  flowers are frequently crossed by insects. We cannot help asking,
+  therefore, why have other and much more elaborate methods been
+  needed? And how have the more complex arrangements of so many
+  flowers been brought about? Before attempting to answer these
+  questions, and in order that the reader may appreciate the
+  difficulty of the problem and the nature of the facts to be
+  explained, it will be necessary to give a summary of the more
+  elaborate modes of securing cross-fertilisation.</p>
+
+  <p>(1) We first have dimorphism and heteromorphism, the phenomena
+  of which have been already sketched in our seventh chapter.</p>
+
+  <p>Here we have both a mechanical and a physiological
+  modification, the stamens and pistil being variously modified in
+  length and position, while the different stamens in the same
+  flower have widely different degrees of fertility when applied to
+  the same stigma,&mdash;a phenomenon which, if it were not so well
+  established, would have appeared in the highest degree
+  improbable. The most remarkable case is that of the three
+  different forms of the loosestrife (Lythrum salicaria) here
+  figured (Fig. 29 on next page).</p>
+
+  <p><a name="Page_312" id="Page_312"></a>(2) Some flowers have
+  irritable stamens which, when their bases are touched by an
+  insect, spring up and dust it with pollen. This occurs in our
+  common berberry.</p>
+
+  <div class="figleft" style="width: 500px;">
+    <a name="fig29" id="fig29"></a> <img src="images/fig29.jpg"
+    width="500" height="334" alt=
+    "FIG. 29.&mdash;Lythrum salicaria (Purple loosestrife)." title=
+    "FIG. 29.&mdash;Lythrum salicaria (Purple loosestrife)." />
+    <b>FIG. 29.&mdash;Lythrum salicaria (Purple loosestrife).</b>
+  </div>
+
+  <p><a name="Page_313" id="Page_313"></a>(3) In others there are
+  levers or processes by which the anthers are mechanically brought
+  down on to the head or back of an insect entering the flower, in
+  such a position as to be carried to the stigma of the next flower
+  it visits. This may be well seen in many species of Salvia and
+  Erica.</p>
+
+  <p>(4) In some there is a sticky secretion which, getting on to
+  the proboscis of an insect, carries away the pollen, and applies
+  it to the stigma of another flower. This occurs in our common
+  milkwort (Polygala vulgaris).</p>
+
+  <p>(5) In papilionaceous plants there are many complex
+  adjustments, such as the squeezing out of pollen from a
+  receptacle on to an insect, as in Lotus corniculatus, or the
+  sudden springing out and exploding of the anthers so as
+  thoroughly to dust the insect, as in Medicago falcata, this
+  occurring after the stigma has touched the insect and taken off
+  some pollen from the last flower.</p>
+
+  <p>(6) Some flowers or spathes form closed boxes in which insects
+  find themselves entrapped, and when they have fertilised the
+  flower, the fringe of hairs opens and allows them to escape. This
+  occurs in many species of Arum and Aristolochia.</p>
+
+  <p>(7) Still more remarkable are the traps in the flower of
+  Asclepias which catch flies, butterflies, and wasps by the legs,
+  and the wonderfully complex arrangements of the orchids. One of
+  these, our common Orchis pyramidalis, may be briefly described to
+  show how varied and beautiful are the arrangements to secure
+  cross-fertilisation. The broad trifid lip of the flower offers a
+  support to the moth which is attracted by its sweet odour, and
+  two ridges at the base guide the proboscis with certainty to the
+  narrow entrance of the nectary. When the proboscis has reached
+  the end of the spur, its basal portion depresses the little
+  hinged rostellum that covers the saddle-shaped sticky glands to
+  which the pollen masses (pollinia) are attached. On the proboscis
+  being withdrawn, the two pollinia stand erect and parallel,
+  firmly attached to the proboscis. In this position, however, they
+  would be useless, as they would miss the stigmatic surface of the
+  next flower visited by the moth. But as soon as the proboscis is
+  withdrawn, the two pollen masses begin to diverge till they are
+  exactly as far apart as are the stigmas of the flower; and then
+  commences a second move<a name="Page_314" id=
+  "Page_314"></a><a name="Page_315" id="Page_315"></a>ment which
+  brings them down till they project straight forward nearly at
+  right angles to their first position, so as exactly to hit
+  against the stigmatic surfaces of the next flower visited on
+  which they leave a portion of their pollen. The whole of these
+  motions take about half a minute, and in that time the moth will
+  usually have flown to another plant, and thus effect the most
+  beneficial kind of cross-fertilisation.<a name="FNanchor_145_145"
+  id="FNanchor_145_145"></a><a href="#Footnote_145_145" class=
+  "fnanchor">[145]</a> This description will be better understood
+  by referring to the illustration opposite, from Darwin's
+  <i>Fertilisation of Orchids</i>(Fig. 30).</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig30" id="fig30"></a> <img src="images/fig30.jpg"
+    width="400" height="666" alt=
+    "FIG. 30.&mdash;Orchis pyramidalis." title=
+    "FIG. 30.&mdash;Orchis pyramidalis." /> <b>FIG.
+    30.&mdash;Orchis pyramidalis.</b>
+  </div>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Interpretation of these Facts.</i></p>
+
+  <p>Having thus briefly indicated the general character of the
+  more complex adaptations for cross-fertilisation, the details of
+  which are to be found in any of the numerous works on the
+  subject,<a name="FNanchor_146_146" id=
+  "FNanchor_146_146"></a><a href="#Footnote_146_146" class=
+  "fnanchor">[146]</a> we find ourselves confronted with the very
+  puzzling question&mdash;Why were these innumerable highly complex
+  adaptations produced, when the very same result may be
+  effected&mdash;and often is effected&mdash;by extremely simple
+  means? Supposing, as we must do, that all flowers were once of
+  simple and regular forms, like a buttercup or a rose, how did
+  such irregular and often complicated flowers as the
+  papilionaceous or pea family, the labiates or sage family, and
+  the infinitely varied and fantastic orchids ever come into
+  existence? No cause has yet been suggested but the need of
+  attracting insects to cross-fertilise them; yet the
+  attractiveness of regular flowers with bright colours and an
+  ample supply of nectar is equally great, and cross-fertilisation
+  can be quite as effectively secured in these by any of the four
+  simple methods already described. Before attempting to suggest a
+  possible solution of this difficult problem, we have yet to pass
+  in review a large body of curious adaptations connected with
+  insect fertilisation, and will first call attention to that
+  portion of the phenomena which throw some light upon the special
+  colours of flowers in their relation to the various kinds of
+  insects which visit them. For these facts we are largely
+  in<a name="Page_316" id="Page_316"></a>debted to the exact and
+  long-continued researches of Professor Hermann M&uuml;ller.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Summary of Additional Facts bearing on Insect
+  Fertilisation.</i></p>
+
+  <p>1. That the size and colour of a flower are important factors
+  in determining the visits of insects, is shown by the general
+  fact of more insects visiting conspicuous than inconspicuous
+  flowers. As a single instance, the handsome Geranium palustre was
+  observed by Professor M&uuml;ller to be visited by sixteen
+  different species of insects, the equally showy G. pratense by
+  thirteen species, while the smaller and much less conspicuous G.
+  molle was visited by eight species, and G. pusillum by only one.
+  In many cases, however, a flower may be very attractive to only a
+  few species of insects; and Professor M&uuml;ller states, as the
+  result of many years' assiduous observation, that "a species of
+  flower is the more visited by insects the more conspicuous it
+  is."</p>
+
+  <p>2. Sweet odour is usually supplementary to the attraction of
+  colour. Thus it is rarely present in the largest and most gaudily
+  coloured flowers which inhabit open places, such as poppies,
+  paeonies, sunflowers, and many others; while it is often the
+  accompaniment of inconspicuous flowers, as the mignonette; of
+  such as grow in shady places, as the violet and primrose; and
+  especially of white or yellowish flowers, as the white jasmine,
+  clematis, stephanotis, etc.</p>
+
+  <p>3. White flowers are often fertilised by moths, and very
+  frequently give out their scent only by night, as in our
+  butterfly-orchis (Habenaria chlorantha); and they sometimes open
+  only at night, as do many of the evening primroses and other
+  flowers. These flowers are often long tubed in accordance with
+  the length of the moths' probosces, as in the genus Pancratium,
+  our butterfly orchis, white jasmine, and a host of others.</p>
+
+  <p>4. Bright red flowers are very attractive to butterflies, and
+  are sometimes specially adapted to be fertilised by them, as in
+  many pinks (Dianthus deltoides, D. superbus, D. atrorubens), the
+  corn-cockle (Lychnis Githago), and many others. Blue flowers are
+  especially attractive to bees and other hymenoptera (though they
+  frequent flowers of all colours), no less than sixty-seven
+  species of this order having been observed to visit the common
+  "sheep's-bit" (Jasione montana). Dull yellow or <a name=
+  "Page_317" id="Page_317"></a>brownish flowers, some of which
+  smell like carrion, are attractive to flies, as the Arum and
+  Aristolochia; while the dull purplish flowers of the Scrophularia
+  are specially attractive to wasps.</p>
+
+  <p>5. Some flowers have neither scent nor nectar, and yet attract
+  insects by sham nectaries! In the herb-paris (Paris quadrifolia)
+  the ovary glistens as if moist, and flies alight on it and carry
+  away pollen to another flower; while in grass of parnassus
+  (Parnassia palustris) there are a number of small stalked yellow
+  balls near the base of the flower, which look like drops of honey
+  but are really dry. In this case there is a little nectar lower
+  down, but the special attraction is a sham; and as there are
+  fresh broods of insects every year, it takes time for them to
+  learn by experience, and thus enough are always deceived to
+  effect cross-fertilisation.<a name="FNanchor_147_147" id=
+  "FNanchor_147_147"></a><a href="#Footnote_147_147" class=
+  "fnanchor">[147]</a> This is analogous to the case of the young
+  birds, which have to learn by experience the insects that are
+  inedible, as explained at page 253.</p>
+
+  <p>6. Many flowers change their colour as soon as fertilised; and
+  this is beneficial, as it enables bees to avoid wasting time in
+  visiting those blossoms which have been already fertilised and
+  their nectar exhausted. The common lungwort (Pulmonaria
+  officinalis), is at first red, but later turns blue; and H.
+  M&uuml;ller observed bees visiting many red flowers in
+  succession, but neglecting the blue. In South Brazil there is a
+  species of Lantana, whose flowers are yellow the first day,
+  orange the second, and purple the third; and Dr. Fritz
+  M&uuml;ller observed that many butterflies visited the yellow
+  flowers only, some both the yellow and the orange flowers, but
+  none the purple.</p>
+
+  <p>7. Many flowers have markings which serve as guides to
+  insects; in some cases a bright central eye, as in the borage and
+  forget-me-not; or lines or spots converging to the centre, as in
+  geraniums, pinks, and many others. This enables insects to go
+  quickly and directly to the opening of the flower, and is equally
+  important in aiding them to obtain a better supply of food, and
+  to fertilise a larger number of flowers.</p>
+
+  <p>8. Flowers have been specially adapted to the kinds of
+  <a name="Page_318" id="Page_318"></a>insects that most abound
+  where they grow. Thus the gentians of the lowlands are adapted to
+  bees, those of the high alps to butterflies only; and while most
+  species of Rhinanthus (a genus to which our common "yellow
+  rattle" belongs) are bee-flowers, one high alpine species (R.
+  alpinus) has been also adapted for fertilisation by butterflies
+  only. The reason of this is, that in the high alps butterflies
+  are immensely more plentiful than bees, and flowers adapted to be
+  fertilised by bees can often have their nectar extracted by
+  butterflies without effecting cross-fertilisation. It is,
+  therefore, important to have a modification of structure which
+  shall make butterflies the fertilisers, and this in many cases
+  has been done.<a name="FNanchor_148_148" id=
+  "FNanchor_148_148"></a><a href="#Footnote_148_148" class=
+  "fnanchor">[148]</a></p>
+
+  <p>9. Economy of time is very important both to the insects and
+  the flowers, because the fine working days are comparatively few,
+  and if no time is wasted the bees will get more honey, and in
+  doing so will fertilise more flowers. Now, it has been
+  ascertained by several observers that many insects, bees
+  especially, keep to one kind of flower at a time, visiting
+  hundreds of blossoms in succession, and passing over other
+  species that may be mixed with them. They thus acquire quickness
+  in going at once to the nectar, and the change of colour in the
+  flower, or incipient withering when fertilised, enables them to
+  avoid those flowers that have already had their honey exhausted.
+  It is probably to assist the insects in keeping to one flower at
+  a time, which is of vital importance to the perpetuation of the
+  species, that the flowers which bloom intermingled at the same
+  season are usually very distinct both in form and colour. In the
+  sandy districts of Surrey, in the early spring, the copses are
+  gay with three flowers&mdash;the primrose, the wood-anemone, and
+  the lesser celandine, forming a beautiful contrast, while at the
+  same time the purple and the white dead-nettles abound on hedge
+  banks. A little later, in the same copses, we have the blue wild
+  hyacinth (Scilla nutans), the red campion (Lychnis dioica), the
+  pure white great starwort (Stellaria Holosteum), and the yellow
+  dead-nettle (Lamium Galeobdolon), all distinct and
+  well-contrasted flowers. In damp meadows in summer we have the
+  ragged robin (Lychnis Floscuculi), the spotted orchis (O.
+  maculata), and the yellow rattle (Rhinanthus <a name="Page_319"
+  id="Page_319"></a>Crista-galli); while in drier meadows we have
+  cowslips, ox-eye daisies, and buttercups, all very distinct both
+  in form and colour. So in cornfields we have the scarlet poppies,
+  the purple corn-cockle, the yellow corn-marygold, and the blue
+  cornflower; while on our moors the purple heath and the dwarf
+  gorse make a gorgeous contrast. Thus the difference of colour
+  which enables the insect to visit with rapidity and unerring aim
+  a number of flowers of the same kind in succession, serves to
+  adorn our meadows, banks, woods, and heaths with a charming
+  variety of floral colour and form at each season of the
+  year.<a name="FNanchor_149_149" id=
+  "FNanchor_149_149"></a><a href="#Footnote_149_149" class=
+  "fnanchor">[149]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Fertilisation of Flowers by Birds.</i></p>
+
+  <p>In the temperate regions of the Northern Hemisphere, insects
+  are the chief agents in cross-fertilisation when this is not
+  effected by the wind; but in warmer regions, and in the Southern
+  hemisphere, birds are found to take a considerable part in the
+  operation, and have in many cases led to modifications in the
+  form and colour of flowers. Each part of the globe has special
+  groups of birds which are flower-haunters. America has the
+  humming-birds (Trochilidae), and the smaller group of the
+  sugar-birds (Caerebidae). In the Eastern tropics the sun-birds
+  (Nectarineidae) take the place of the humming-birds, and another
+  small group, the flower-peckers (Dicaeidae), assist them. In the
+  Australian region there are also two flower-feeding groups, the
+  Meliphagidae, or honey-suckers, and the brush-tongued lories
+  (Trichoglossidae). Recent researches by American naturalists have
+  shown that many flowers are fertilised by humming-birds, such as
+  passion-flowers, trumpet-flowers, fuchsias, and lobelias; while
+  some, as the Salvia splendens of Mexico, are specially adapted to
+  their visits. We may thus perhaps explain the number of very
+  large tubular flowers in the tropics, such as the huge
+  brugmansias and bignonias; while in the Andes and in <a name=
+  "Page_320" id="Page_320"></a>Chile, where humming-birds are
+  especially plentiful, we find great numbers of red tubular
+  flowers, often of large size and apparently adapted to these
+  little creatures. Such are the beautiful Lapageria and Philesia,
+  the grand Pitcairneas, and the genera Fuchsia, Mitraria,
+  Embothrium, Escallonia, Desfontainea, Eccremocarpus, and many
+  Gesneraceae. Among the most extraordinary modifications of flower
+  structure adapted to bird fertilisation are the species of
+  Marcgravia, in which the pedicels and bracts of the terminal
+  portion of a pendent bunch of flowers have been modified into
+  pitchers which secrete nectar and attract insects, while birds
+  feeding on the nectar, or insects, have the pollen of the
+  overhanging flowers dusted on their backs, and, carrying it to
+  other flowers, thus cross-fertilise them (see Illustration).</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig31" id="fig31"></a> <img src="images/fig31.jpg"
+    width="400" height="372" alt=
+    "FIG. 31.&mdash;Humming-bird fertilising Marcgravia nepenthoides."
+    title=
+    "FIG. 31.&mdash;Humming-bird fertilising Marcgravia nepenthoides." />
+     <b>FIG. 31.&mdash;Humming-bird fertilising Marcgravia
+    nepenthoides.</b>
+  </div>
+
+  <p>In Australia and New Zealand the fine "glory peas"
+  (Clianthus), the Sophora, Loranthus, many Epacrideae and
+  Myrtaceae, and the large flowers of the New Zealand flax <a name=
+  "Page_321" id="Page_321"></a>(Phormium tenax), are
+  cross-fertilised by birds; while in Natal the fine
+  trumpet-creeper (Tecoma capensis) is fertilised by
+  Nectarineas.</p>
+
+  <p>The great extent to which insect and bird agency is necessary
+  to flowers is well shown by the case of New Zealand. The entire
+  country is comparatively poor in species of insects, especially
+  in bees and butterflies which are the chief flower fertilisers;
+  yet according to the researches of local botanists no less than
+  one-fourth of all the flowering plants are incapable of
+  self-fertilisation, and, therefore, wholly dependent on insect or
+  bird agency for the continuance of the species.</p>
+
+  <p>The facts as to the cross-fertilisation of flowers which have
+  now been very briefly summarised, taken in connection with
+  Darwin's experiments proving the increased vigour and fertility
+  given by cross-fertilisation, seem amply to justify his aphorism
+  that "Nature abhors self-fertilisation," and his more precise
+  statement, that, "No plant is perpetually self-fertilised;" and
+  this view has been upheld by Hildebrand, Delpino, and other
+  botanists.<a name="FNanchor_150_150" id=
+  "FNanchor_150_150"></a><a href="#Footnote_150_150" class=
+  "fnanchor">[150]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Self-Fertilisation of Flowers.</i></p>
+
+  <p>But all this time we have been only looking at one side of the
+  question, for there exists an abundance of facts which seem to
+  imply, just as surely, the utter uselessness of
+  cross-fertilisation. Let us, then, see what these facts are
+  before proceeding further.</p>
+
+  <p>1. An immense variety of plants are habitually
+  self-fertilised, and their numbers probably far exceed those
+  which are habitually cross-fertilised by insects. Almost all the
+  very small or obscure flowered plants with hermaphrodite flowers
+  are of this kind. Most of these, however, may be insect
+  fertilised occasionally, and may, therefore, come under the rule
+  that no species are perpetually self-fertilised.</p>
+
+  <p>2. There are many plants, however, in which special
+  arrangements exist to secure self-fertilisation. Sometimes the
+  corolla closes and brings the anthers and stigma into contact; in
+  others the anthers cluster round the stigmas, both maturing
+  together, as in many buttercups, stitchwort (Stellaria media),
+  <a name="Page_322" id="Page_322"></a>sandwort (Spergula), and
+  some willow-herbs (Epilobium); or they arch over the pistil, as
+  in Galium aparine and Alisma Plantago. The style is also modified
+  to bring it into contact with the anthers, as in the dandelion,
+  groundsel, and many other plants.<a name="FNanchor_151_151" id=
+  "FNanchor_151_151"></a><a href="#Footnote_151_151" class=
+  "fnanchor">[151]</a> All these, however, may be occasionally
+  cross-fertilised.</p>
+
+  <p>3. In other cases precautions are taken to prevent
+  cross-fertilisation, as in the numerous cleistogamous or closed
+  flowers. These occur in no less than fifty-five different genera,
+  belonging to twenty-four natural orders, and in thirty-two of
+  these genera the normal flowers are irregular, and have therefore
+  been specially modified for insect fertilisation.<a name=
+  "FNanchor_152_152" id="FNanchor_152_152"></a><a href=
+  "#Footnote_152_152" class="fnanchor">[152]</a> These flowers
+  appear to be degradations of the normal flowers, and are closed
+  up by various modifications of the petals or other parts, so that
+  it is impossible for insects to reach the interior, yet they
+  produce seed in abundance, and are often the chief means by which
+  the species is continued. Thus, in our common dog-violet the
+  perfect flowers rarely produce seed, while the rudimentary
+  cleistogamic flowers do so in abundance. The sweet violet also
+  produces abundance of seed from its cleistogamic flowers, and few
+  from its perfect flowers; but in Liguria it produces only perfect
+  flowers which seed abundantly. No case appears to be known of a
+  plant which has cleistogamic flowers only, but a small rush
+  (Juncus bufonius) is in this condition in some parts of Russia,
+  while in other parts perfect flowers are also produced.<a name=
+  "FNanchor_153_153" id="FNanchor_153_153"></a><a href=
+  "#Footnote_153_153" class="fnanchor">[153]</a> Our common henbit
+  dead-nettle (Lamium amplexicaule) produces cleistogamic flowers,
+  as do also some orchids. The advantage gained by the plant is
+  great economy of specialised material, since with very small
+  flowers and very little expenditure of pollen an abundance of
+  seed is produced.</p>
+
+  <p>4. A considerable number of plants which have evidently been
+  specially modified for insect fertilisation have, by further
+  <a name="Page_323" id="Page_323"></a>modification, become quite
+  self-fertile. This is the case with the garden-pea, and also with
+  our beautiful bee-orchis, in which the pollen-masses constantly
+  fall on to the stigmas, and the flower, being thus
+  self-fertilised, produces abundance of capsules and of seed. Yet
+  in many of its close allies insect agency is absolutely required;
+  but in one of these, the fly-orchis, comparatively very little
+  seed is produced, and self-fertilisation would therefore be
+  advantageous to it. When garden-peas were artificially
+  cross-fertilised by Mr. Darwin, it seemed to do them no good, as
+  the seeds from these crosses produced less vigorous plants than
+  seed from those which were self-fertilised; a fact directly
+  opposed to what usually occurs in cross-fertilised plants.</p>
+
+  <p>5. As opposed to the theory that there is any absolute need
+  for cross-fertilisation, it has been urged by Mr. Henslow and
+  others that many self-fertilised plants are exceptionally
+  vigorous, such as groundsel, chickweed, sow-thistle, buttercups,
+  and other common weeds; while most plants of world-wide
+  distribution are self-fertilised, and these have proved
+  themselves to be best fitted to survive in the battle of life.
+  More than fifty species of common British plants are very widely
+  distributed, and all are habitually self-fertilised.<a name=
+  "FNanchor_154_154" id="FNanchor_154_154"></a><a href=
+  "#Footnote_154_154" class="fnanchor">[154]</a> That
+  self-fertilisation has some great advantage is shown by the fact
+  that it is usually the species which have the smallest and least
+  conspicuous flowers which have spread widely, while the large and
+  showy flowered species of the same genera or families, which
+  require insects to cross-fertilise them, have a much more limited
+  distribution.</p>
+
+  <p>6. It is now believed by some botanists that many
+  inconspicuous and imperfect flowers, including those that are
+  wind-fertilised, such as plantains, nettles, sedges, and grasses,
+  do not represent primitive or undeveloped forms, but are
+  degradations from more perfect flowers which were once adapted to
+  insect fertilisation. In almost every order we find some plants
+  which have become thus reduced or degraded for wind or
+  self-fertilisation, as Poterium and Sanguisorba among the
+  Rosaceae; while this has certainly been the case in the
+  cleistogamic flowers. In most of the above-mentioned plants there
+  are distinct rudiments of petals or other floral organs, <a name=
+  "Page_324" id="Page_324"></a>and as the chief use of these is to
+  attract insects, they could hardly have existed in primitive
+  flowers.<a name="FNanchor_155_155" id=
+  "FNanchor_155_155"></a><a href="#Footnote_155_155" class=
+  "fnanchor">[155]</a> We know, moreover, that when the petals
+  cease to be required for the attraction of <a name="Page_325" id=
+  "Page_325"></a>insects, they rapidly diminish in size, lose their
+  bright colour or almost wholly disappear.<a name=
+  "FNanchor_156_156" id="FNanchor_156_156"></a><a href=
+  "#Footnote_156_156" class="fnanchor">[156]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Difficulties and Contradictions.</i></p>
+
+  <p>The very bare summary that has now been given of the main
+  facts relating to the fertilisation of flowers, will have served
+  to show the vast extent and complexity of the inquiry, and the
+  extraordinary contradictions and difficulties which it presents.
+  We have direct proof of the beneficial results of intercrossing
+  in a great number of cases; we have an overwhelming mass of facts
+  as to the varied and complex structure of flowers evidently
+  adapted to secure this intercrossing by insect agency; yet we see
+  many of the most vigorous plants which spread widely over the
+  globe, with none of these adaptations, and evidently depending on
+  self-fertilisation for their continued existence and success in
+  the battle of life. Yet more extraordinary is it to find numerous
+  cases in which the special arrangements for cross-fertilisation
+  appear to have been a failure, since they have either been
+  supplemented by special means for self-fertilisation, or have
+  reverted back in various degrees to simpler forms in which
+  self-fertilisation becomes the rule. There is also a further
+  difficulty in the highly complex modes by which
+  cross-fertilisation is often brought about; for we have seen that
+  there are several very effective yet very simple modes of
+  securing intercrossing, involving a minimum of change in the form
+  and structure of the flower; and when we consider that the result
+  attained with so much cost of structural modification is by no
+  means an unmixed good, and is far less certain in securing the
+  perpetuation of the species than is self-fertilisation, it is
+  most puzzling to find such complex methods resorted to, sometimes
+  to the extent of special precautions against the possibility of
+  self-fertilisation ever taking place. Let us now see whether any
+  light can be thrown on these various anomalies and
+  contradictions.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Intercrossing not necessarily Advantageous.</i></p>
+
+  <p>No one was more fully impressed than Mr. Darwin with the
+  beneficial effects of intercrossing on the vigour and fertility
+  <a name="Page_326" id="Page_326"></a>of the species or race, yet
+  he clearly saw that it was not always and necessarily
+  advantageous. He says: "The most important conclusion at which I
+  have arrived is, that the mere act of intercrossing by itself
+  does no good. The good depends on the individuals which are
+  crossed differing slightly in constitution, owing to their
+  progenitors having been subjected during several generations to
+  slightly different conditions. This conclusion, as we shall
+  hereafter see, is closely connected with various important
+  physiological problems, such as the benefit derived from slight
+  changes in the conditions of life."<a name="FNanchor_157_157" id=
+  "FNanchor_157_157"></a><a href="#Footnote_157_157" class=
+  "fnanchor">[157]</a> Mr. Darwin has also adduced much direct
+  evidence proving that slight changes in the conditions of life
+  are beneficial to both animals and plants, maintaining or
+  restoring their vigour and fertility in the same way as a
+  favourable cross seems to restore it.<a name="FNanchor_158_158"
+  id="FNanchor_158_158"></a><a href="#Footnote_158_158" class=
+  "fnanchor">[158]</a> It is, I believe, by a careful consideration
+  of these two classes of facts that we shall find the clue to the
+  labyrinth in which this subject has appeared to involve us.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Supposed Evil Results of Close Interbreeding.</i></p>
+
+  <p>Just as we have seen that intercrossing is not necessarily
+  good, we shall be forced to admit that close interbreeding is not
+  necessarily bad. Our finest breeds of domestic animals have been
+  thus produced, and by a careful statistical inquiry Mr. George
+  Darwin has shown that the most constant and long-continued
+  intermarriages among the British aristocracy have produced no
+  prejudicial results. The rabbits on Porto Santo are all the
+  produce of a single female; they have lived on the same small
+  island for 470 years, and they still abound there and appear to
+  be vigorous and healthy (see p. 161*).</p>
+
+  <p>We have, however, on the other hand, overwhelming evidence
+  that in many cases, among our domestic animals and cultivated
+  plants, close interbreeding does produce bad results, and the
+  apparent contradiction may perhaps be explained on the same
+  general principles, and under similar limitations, as were found
+  to be necessary in defining the value of intercrossing. It
+  appears probable, then, that it is not interbreeding in itself
+  that is hurtful, but interbreeding without <a name="Page_327" id=
+  "Page_327"></a>rigid selection or some change of conditions.
+  Under nature, as in the case of the Porto Santo rabbits, the
+  rapid increase of these animals would in a very few years stock
+  the island with a full population, and thereafter natural
+  selection would act powerfully in the preservation only of the
+  healthiest and the most fertile, and under these conditions no
+  deterioration would occur. Among the aristocracy there has been a
+  constant selection of beauty, which is generally synonymous with
+  health, while any constitutional infertility has led to the
+  extinction of the family. With domestic animals the selection
+  practised is usually neither severe enough nor of the right kind.
+  There is no natural struggle for existence, but certain points of
+  form and colour characteristic of the breed are considered
+  essential, and thus the most vigorous or the most fertile are not
+  always those which are selected to continue the stock. In nature,
+  too, the species always extends over a larger area and consists
+  of much greater numbers, and thus a difference of constitution
+  soon arises in different parts of the area, which is wanting in
+  the limited numbers of pure bred domestic animals. From a
+  consideration of these varied facts we conclude that an
+  occasional disturbance of the organic equilibrium is what is
+  essential to keep up the vigour and fertility of any organism,
+  and that this disturbance may be equally well produced either by
+  a cross between individuals of somewhat different constitutions,
+  or by occasional slight changes in the conditions of life. Now
+  plants which have great powers of dispersal enjoy a constant
+  change of conditions, and can, therefore, exist permanently, or
+  at all events, for very long periods, without intercrossing;
+  while those which have limited powers of dispersal, and are
+  restricted to a comparatively small and uniform area, need an
+  occasional cross to keep up their fertility and general vigour.
+  We should, therefore, expect that those groups of plants which
+  are adapted both for cross-and self-fertilisation, which have
+  showy flowers and possess great powers of seed-dispersal, would
+  be the most abundant and most widely distributed; and this we
+  find to be the case, the Compositae possessing all these
+  characteristics in the highest degree, and being the most
+  generally abundant group of plants with conspicuous flowers in
+  all parts of the world.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_328" id="Page_328"></a><i>How the Struggle for
+  Existence Acts among Flowers.</i></p>
+
+  <p>Let us now consider what will be the action of the struggle
+  for existence under the conditions we have seen to exist.</p>
+
+  <p>Everywhere and at all times some species of plants will be
+  dominant and aggressive; while others will be diminishing in
+  numbers, reduced to occupy a smaller area, and generally having a
+  hard struggle to maintain themselves. Whenever a self-fertilising
+  plant is thus reduced in numbers it will be in danger of
+  extinction, because, being limited to a small area, it will
+  suffer from the effects of too uniform conditions which will
+  produce weakness and infertility. But while this change is in
+  progress, any crosses between individuals of slightly different
+  constitution will be beneficial, and all variations favouring
+  either insect agency on the one hand, or wind-dispersal of pollen
+  on the other, will lead to the production of a somewhat stronger
+  and more fertile stock. Increased size or greater brilliancy of
+  the flower, more abundant nectar, sweeter odour, or adaptations
+  for more effectual cross-fertilisation would all be preserved,
+  and thus would be initiated some form of specialisation for
+  insect agency in cross-fertilisation; and in every different
+  species so circumstanced the result would be different, depending
+  as it would on many and complex combinations of variation of
+  parts of the flower, and of the insect species which most
+  abounded in the district.</p>
+
+  <p>Species thus favourably modified might begin a new era of
+  development, and, while spreading over a somewhat wider area,
+  give rise to new varieties or species, all adapted in various
+  degrees and modes to secure cross-fertilisation by insect agency.
+  But in course of ages some change of conditions might prove
+  adverse. Either the insects required might diminish in numbers or
+  be attracted by other competing flowers, or a change of climate
+  might give the advantage to other more vigorous plants. Then
+  self-fertilisation with greater means of dispersal might be more
+  advantageous; the flowers might become smaller and more numerous;
+  the seeds smaller and lighter so as to be more easily dispersed
+  by the wind, while some of the special adaptations for insect
+  fertilisation being useless would, by the absence of selection
+  and by the law of economy of growth, be reduced to a rudimentary
+  <a name="Page_329" id="Page_329"></a>form. With these
+  modifications the species might extend its range into new
+  districts, thereby obtaining increased vigour by the change of
+  conditions, as appears to have been the case with so many of the
+  small flowered self-fertilised plants. Thus it might continue to
+  exist for a long series of ages, till under other
+  changes&mdash;geographical or biological&mdash;it might again
+  suffer from competition or from other adverse circumstances, and
+  be at length again confined to a limited area, or reduced to very
+  scanty numbers.</p>
+
+  <p>But when this cycle of change had taken place, the species
+  would be very different from the original form. The flower would
+  have been at one time modified to favour the visits of insects
+  and to secure cross-fertilisation by their aid, and when the need
+  for this passed away, some portions of these structures would
+  remain, though in a reduced or rudimentary condition. But when
+  insect agency became of importance a second time, the new
+  modifications would start from a different or more advanced
+  basis, and thus a more complex result might be produced. Owing to
+  the unequal rates at which the reduction of the various parts
+  might occur, some amount of irregularity in the flower might
+  arise, and on a second development towards insect
+  cross-fertilisation this irregularity, if useful, might be
+  increased by variation and selection.</p>
+
+  <p>The rapidity and comparative certainty with which such changes
+  as are here supposed do really take place, are well shown by the
+  great differences in floral structure, as regards the mode of
+  fertilisation, in allied genera and species, and even in some
+  cases in varieties of the same species. Thus in the Ranunculaceae
+  we find the conspicuous part of the flower to be the petals in
+  Ranunculus, the sepals in Helleborus, Anemone, etc., and the
+  stamens in most species of Thalictrum. In all these we have a
+  simple regular flower, but in Aquilegia it is made complex by the
+  spurred petals, and in Delphinium and Aconitum it becomes quite
+  irregular. In the more simple class self-fertilisation occurs
+  freely, but it is prevented in the more complex flowers by the
+  stamens maturing before the pistil. In the Caprifoliaceae we have
+  small and regular greenish flowers, as in the moschatel (Adoxa);
+  more conspicuous regular open flowers without honey, as in the
+  elder (Sambucus); and <a name="Page_330" id=
+  "Page_330"></a>tubular flowers increasing in length and
+  irregularity, till in some, like our common honeysuckle, they are
+  adapted for fertilisation by moths only, with abundant honey and
+  delicious perfume to attract them. In the Scrophulariaceae we
+  find open, almost regular flowers, as Veronica and Verbascum,
+  fertilised by flies and bees, but also self-fertilised;
+  Scrophularia adapted in form and colour to be fertilised by
+  wasps; and the more complex and irregular flowers of Linaria,
+  Rhinanthus, Melampyrum, Pedicularis, etc., mostly adapted to be
+  fertilised by bees.</p>
+
+  <p>In the genera Geranium, Polygonum, Veronica, and several
+  others there is a gradation of forms from large and bright to
+  small and obscure coloured flowers, and in every case the former
+  are adapted for insect fertilisation, often exclusively, while in
+  the latter self-fertilisation constantly occurs. In the yellow
+  rattle (Rhinanthus Crista-galli) there are two forms (which have
+  been named <i>major</i> and <i>minor</i>), the larger and more
+  conspicuous adapted to insect fertilisation only, the smaller
+  capable of self-fertilisation; and two similar forms exist in the
+  eyebright (Euphrasia officinalis). In both these cases there are
+  special modifications in the length and curvature of the style as
+  well as in the size and shape of the corolla; and the two forms
+  are evidently becoming each adapted to special conditions, since
+  in some districts the one, in other districts the other is most
+  abundant.<a name="FNanchor_159_159" id=
+  "FNanchor_159_159"></a><a href="#Footnote_159_159" class=
+  "fnanchor">[159]</a></p>
+
+  <p>These examples show us that the kind of change suggested above
+  is actually going on, and has presumably always been going on in
+  nature throughout the long geological epochs during which the
+  development of flowers has been progressing. The two great modes
+  of gaining increased vigour and fertility&mdash;intercrossing and
+  dispersal over wider areas&mdash;have been resorted to again and
+  again, under the pressure of a constant struggle for existence
+  and the need for adaptation to ever-changing conditions. During
+  all the modifications that ensued, useless parts were reduced or
+  suppressed, owing to the absence of selection and the principle
+  of economy of growth; and thus at each fresh adaptation some
+  rudiments of old structures were <a name="Page_331" id=
+  "Page_331"></a>re-developed, but not unfrequently in a different
+  form and for a distinct purpose.</p>
+
+  <p>The chief types of flowering plants have existed during the
+  millions of ages of the whole tertiary period, and during this
+  enormous lapse of time many of them may have been modified in the
+  direction of insect fertilisation, and again into that of
+  self-fertilisation, not once or twice only, but perhaps scores or
+  even hundreds of times; and at each such modification a
+  difference in the environment may have led to a distinct line of
+  development. At one epoch the highest specialisation of structure
+  in adaptation to a single species or group of insects may have
+  saved a plant from extinction; while, at other times, the
+  simplest mode of self-fertilisation, combined with greater powers
+  of dispersal and a constitution capable of supporting diverse
+  physical conditions, may have led to a similar result. With some
+  groups the tendency seems to have been almost continuously to
+  greater and greater specialisation, while with others a tendency
+  to simplification and degradation has resulted in such plants as
+  the grasses and sedges.</p>
+
+  <p>We are now enabled dimly to perceive how the curious anomaly
+  of very simple and very complex methods of securing
+  cross-fertilisation&mdash;both equally effective&mdash;may have
+  been brought about. The simple modes may be the result of a
+  comparatively direct modification from the more primitive types
+  of flowers, which were occasionally, and, as it were,
+  accidentally visited and fertilised by insects; while the more
+  complex modes, existing for the most part in the highly irregular
+  flowers, may result from those cases in which adaptation to
+  insect-fertilisation, and partial or complete degradation to
+  self-fertilisation or to wind-fertilisation, have again and again
+  recurred, each time producing some additional complexity, arising
+  from the working up of old rudiments for new purposes, till there
+  have been reached the marvellous flower structures of the
+  papilionaceous tribes, of the asclepiads, or of the orchids.</p>
+
+  <p>We thus see that the existing diversity of colour and of
+  structure in flowers is probably the ultimate result of the
+  ever-recurring struggle for existence, combined with the
+  ever-changing relations between the vegetable and animal kingdoms
+  during countless ages. The constant variability of every part
+  <a name="Page_332" id="Page_332"></a>and organ, with the enormous
+  powers of increase possessed by plants, have enabled them to
+  become again and again readjusted to each change of condition as
+  it occurred, resulting in that endless variety, that marvellous
+  complexity, and that exquisite colouring which excite our
+  admiration in the realm of flowers, and constitute them the
+  perennial charm and crowning glory of nature.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Flowers the Product of Insect Agency.</i></p>
+
+  <p>In his <i>Origin of Species</i>, Mr. Darwin first stated that
+  flowers had been rendered conspicuous and beautiful in order to
+  attract insects, adding: "Hence we may conclude that, if insects
+  had not been developed on the earth, our plants would not have
+  been decked with beautiful flowers, but would have produced only
+  such poor flowers as we see on our fir, oak, nut, and ash trees,
+  on grasses, docks, and nettles, which are all fertilised through
+  the agency of the wind." The argument in favour of this view is
+  now much stronger than when he wrote; for not only have we reason
+  to believe that most of these wind-fertilised flowers are
+  degraded forms of flowers which have once been insect fertilised,
+  but we have abundant evidence that whenever insect agency becomes
+  comparatively ineffective, the colours of the flowers become less
+  bright, their size and beauty diminish, till they are reduced to
+  such small, greenish, inconspicuous flowers as those of the
+  rupture-wort (Herniaria glabra), the knotgrass (Polygonum
+  aviculare), or the cleistogamic flowers of the violet. There is
+  good reason to believe, therefore, not only that flowers have
+  been developed in order to attract insects to aid in their
+  fertilisation, but that, having been once produced, in however
+  great profusion, if the insect races were all to become extinct,
+  flowers (in the temperate zones at all events) would soon dwindle
+  away, and that ultimately all floral beauty would vanish from the
+  earth.</p>
+
+  <p>We cannot, therefore, deny the vast change which insects have
+  produced upon the earth's surface, and which has been thus
+  forcibly and beautifully delineated by Mr. Grant Allen: "While
+  man has only tilled a few level plains, a few great river
+  valleys, a few peninsular mountain slopes, leaving the vast mass
+  of earth untouched by his hand, the insect has spread himself
+  over every land in a thousand shapes, and has made the whole
+  <a name="Page_333" id="Page_333"></a>flowering creation
+  subservient to his daily wants. His buttercup, his dandelion, and
+  his meadow-sweet grow thick in every English field. His thyme
+  clothes the hillside; his heather purples the bleak gray
+  moorland. High up among the alpine heights his gentian spreads
+  its lakes of blue; amid the snows of the Himalayas his
+  rhododendrons gleam with crimson light. Even the wayside pond
+  yields him the white crowfoot and the arrowhead, while the broad
+  expanses of Brazilian streams are beautified by his gorgeous
+  water-lilies. The insect has thus turned the whole surface of the
+  earth into a boundless flower-garden, which supplies him from
+  year to year with pollen or honey, and itself in turn gains
+  perpetuation by the baits that it offers for his
+  allurement."<a name="FNanchor_160_160" id=
+  "FNanchor_160_160"></a><a href="#Footnote_160_160" class=
+  "fnanchor">[160]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Concluding Remarks on Colour in Nature.</i></p>
+
+  <p>In the last four chapters I have endeavoured to give a general
+  and systematic, though necessarily condensed view of the part
+  which is played by colour in the organic world. We have seen in
+  what infinitely varied ways the need of concealment has led to
+  the modification of animal colours, whether among polar snows or
+  sandy deserts, in tropical forests or in the abysses of the
+  ocean. We next find these general adaptations giving way to more
+  specialised types of coloration, by which each species has become
+  more and more harmonised with its immediate surroundings, till we
+  reach the most curiously minute resemblances to natural objects
+  in the leaf and stick insects, and those which are so like
+  flowers or moss or birds' droppings that they deceive the acutest
+  eye. We have learnt, further, that these varied forms of
+  protective colouring are far more numerous than has been usually
+  suspected, because, what appear to be very conspicuous colours or
+  markings when the species is observed in a museum or in a
+  menagerie, are often highly protective when the creature is seen
+  under the natural conditions of its existence. From these varied
+  classes of facts it seems not improbable that fully one-half of
+  the species in the animal kingdom possess colours which have been
+  more or less adapted to secure for them concealment or
+  protection.</p>
+
+  <p>Passing onward we find the explanation of a distinct type
+  <a name="Page_334" id="Page_334"></a>of colour or marking, often
+  superimposed upon protective tints, in the importance of easy
+  recognition by many animals of their fellows, their parents, or
+  their mates. By this need we have been able to account for
+  markings that seem calculated to make the animal conspicuous,
+  when the general tints and well-known habits of the whole group
+  demonstrate the need of concealment. Thus also we are able to
+  explain the constant symmetry in the markings of wild animals, as
+  well as the numerous cases in which the conspicuous colours are
+  concealed when at rest and only become visible during rapid
+  motion. In striking contrast to ordinary protective coloration we
+  have "warning colours," usually very conspicuous and often
+  brilliant or gaudy, which serve to indicate that their possessors
+  are either dangerous or uneatable to the usual enemies of their
+  tribe. This kind of coloration is probably more prevalent than
+  has been hitherto supposed, because in the case of many tropical
+  animals we are quite unacquainted with their special and most
+  dangerous enemies, and are also unable to determine whether they
+  are or are not distasteful to those enemies. As a kind of
+  corollary to the "warning colours," we find the extraordinary
+  phenomena of "mimicry," in which defenceless species obtain
+  protection by being mistaken for those which, from any cause,
+  possess immunity from attack. Although a large number of
+  instances of warning colour and of mimicry are now recorded, it
+  is probably still an almost unworked field of research, more
+  especially in tropical regions and among the inhabitants of the
+  ocean.</p>
+
+  <p>The phenomena of sexual diversities of coloration next engaged
+  our attention, and the reasons why Mr. Darwin's theory of "sexual
+  selection," as regards colour and ornament, could not be accepted
+  were stated at some length, together with the theory of animal
+  coloration and ornament we propose to substitute for it. This
+  theory is held to be in harmony with the general facts of animal
+  coloration, while it entirely dispenses with the very
+  hypothetical and inadequate agency of female choice in producing
+  the detailed colours, patterns, and ornaments, which in so many
+  cases distinguish the male sex.</p>
+
+  <p>If my arguments on this point are sound, they will dispose
+  also of Mr. Grant Allen's view of the direct action of the
+  <a name="Page_335" id="Page_335"></a>colour sense on the animal
+  integuments.<a name="FNanchor_161_161" id=
+  "FNanchor_161_161"></a><a href="#Footnote_161_161" class=
+  "fnanchor">[161]</a> He argues that the colours of insects and
+  birds reproduce generally the colours of the flowers they
+  frequent or the fruits they eat, and he adduces numerous cases in
+  which flower-haunting insects and fruit-eating birds are gaily
+  coloured. This he supposes to be due to the colour-taste,
+  developed by the constant presence of bright flowers and fruits,
+  being applied to the selection of each variation towards
+  brilliancy in their mates; thus in time producing the gorgeous
+  and varied hues they now possess. Mr. Allen maintains that
+  "insects are bright where bright flowers exist in numbers, and
+  dull where flowers are rare or inconspicuous;" and he urges that
+  "we can hardly explain this wide coincidence otherwise than by
+  supposing that a taste for colour is produced through the
+  constant search for food among entomophilous blossoms, and that
+  this taste has reacted upon its possessors through the action of
+  unconscious sexual selection."</p>
+
+  <p>The examples Mr. Allen quotes of bright insects being
+  associated with bright flowers seem very forcible, but are really
+  deceptive or erroneous; and quite as many cases could be quoted
+  which prove the very opposite. For example, in the dense
+  equatorial forests flowers are exceedingly scarce, and there is
+  no comparison with the amount of floral colour to be met with in
+  our temperate meadows, woods, and hillsides. The forests about
+  Para in the lower Amazon are typical in this respect, yet they
+  abound with the most gorgeously coloured butterflies, almost all
+  of which frequent the forest depths, keeping near the ground,
+  where there is the greatest deficiency of brilliant flowers. In
+  contrast with this let us take the Cape of Good Hope&mdash;the
+  most flowery region probably that exists upon the
+  globe,&mdash;where the country is a complete flower-garden of
+  heaths, pelargoniums, mesembryanthemus, exquisite iridaceous and
+  other bulbs, and numerous flowering shrubs and trees; yet the
+  Cape butterflies are hardly equal, either in number or variety,
+  to those of any country in South Europe, and are utterly
+  insignificant when compared with those of the comparatively
+  flowerless forest-depths of the Amazon or of New Guinea. Neither
+  is there any relation between the colours of other insects and
+  their haunts. Few <a name="Page_336" id="Page_336"></a>are more
+  gorgeous than some of the tiger-beetles and the carabi, yet these
+  are all carnivorous; while many of the most brilliant metallic
+  buprestidae and longicorns are always found on the bark of fallen
+  trees. So with the humming-birds; their brilliant metallic tints
+  can only be compared with metals or gems, and are totally unlike
+  the delicate pinks and purples, yellows and reds of the majority
+  of flowers. Again, the Australian honey-suckers (Meliphagidae)
+  are genuine flower-haunters, and the Australian flora is more
+  brilliant in colour display than that of most tropical regions,
+  yet these birds are, as a rule, of dull colours, not superior on
+  the average to our grain-eating finches. Then, again, we have the
+  grand pheasant family, including the gold and the silver
+  pheasants, the gorgeous fire-backed and ocellated pheasants, and
+  the resplendent peacock, all feeding on the ground on grain or
+  seeds or insects, yet adorned with the most gorgeous colours.</p>
+
+  <p>There is, therefore, no adequate basis of facts for this
+  theory to rest upon, even if there were the slightest reason to
+  believe that not only birds, but butterflies and beetles, take
+  any delight in colour for its own sake, apart from the
+  food-supply of which it indicates the presence. All that has been
+  proved or that appears to be probable is, that they are able to
+  perceive differences of colour, and to associate each colour with
+  the particular flowers or fruits which best satisfy their wants.
+  Colour being in its nature diverse, it has been beneficial for
+  them to be able to distinguish all its chief varieties, as
+  manifested more particularly in the vegetable kingdom, and among
+  the different species of their own group; and the fact that
+  certain species of insects show some preference for a particular
+  colour may be explained by their having found flowers of that
+  colour to yield them a more abundant supply of nectar or of
+  pollen. In those cases in which butterflies frequent flowers of
+  their own colour, the habit may well have been acquired from the
+  protection it affords them.</p>
+
+  <p>It appears to me that, in imputing to insects and birds the
+  same love of colour for its own sake and the same aesthetic
+  tastes as we ourselves possess, we may be as far from the truth
+  as were those writers who held that the bee was a good
+  mathematician, and that the honeycomb was constructed throughout
+  to satisfy its refined mathematical instincts; whereas it is now
+  <a name="Page_337" id="Page_337"></a>generally admitted to be the
+  result of the simple principle of economy of material applied to
+  a primitive cylindrical cell.<a name="FNanchor_162_162" id=
+  "FNanchor_162_162"></a><a href="#Footnote_162_162" class=
+  "fnanchor">[162]</a></p>
+
+  <p>In studying the phenomena of colour in the organic world we
+  have been led to realise the wonderful complexity of the
+  adaptations which bring each species into harmonious relation
+  with all those which surround it, and which thus link together
+  the whole of nature in a network of relations of marvellous
+  intricacy. Yet all this is but, as it were, the outward show and
+  garment of nature, behind which lies the inner
+  structure&mdash;the framework, the vessels, the cells, the
+  circulating fluids, and the digestive and reproductive
+  processes,&mdash;and behind these again those mysterious
+  chemical, electrical, and vital forces which constitute what we
+  term Life. These forces appear to be fundamentally the same for
+  all organisms, as is the material of which all are constructed;
+  and we thus find behind the outer diversities an inner
+  relationship which binds together the myriad forms of life.</p>
+
+  <p>Each species of animal or plant thus forms part of one
+  harmonious whole, carrying in all the details of its complex
+  structure the record of the long story of organic development;
+  and it was with a truly inspired insight that our great
+  philosophical poet apostrophised the humble weed&mdash;</p>
+
+  <p><span style="margin-left: 2em;">Flower in the crannied
+  wall,</span><br />
+  <span style="margin-left: 2em;">I pluck you out of the
+  crannies,</span><br />
+  <span style="margin-left: 2em;">I hold you here, root and all, in
+  my hand,</span><br />
+  <span style="margin-left: 2em;">Little flower&mdash;but <i>if</i>
+  I could understand</span><br />
+  <span style="margin-left: 2em;">What you are, root and all, and
+  all in all,</span><br />
+  <span style="margin-left: 2em;">I should know what God and man
+  is.</span><br /></p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_136_136" id=
+      "Footnote_136_136"></a><a href=
+      "#FNanchor_136_136"><span class="label">[136]</span></a>
+      Burchell's <i>Travels</i>, vol. i. p. 10.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_137_137" id=
+      "Footnote_137_137"></a><a href=
+      "#FNanchor_137_137"><span class="label">[137]</span></a>
+      <i>Nature</i>, vol. iii. p. 507.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_138_138" id=
+      "Footnote_138_138"></a><a href=
+      "#FNanchor_138_138"><span class="label">[138]</span></a>
+      <i>Flowers, Fruits, and Leaves</i>, p. 128 (Fig. 79).</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_139_139" id=
+      "Footnote_139_139"></a><a href=
+      "#FNanchor_139_139"><span class="label">[139]</span></a> For
+      a popular sketch of these, see Sir J. Lubbock's <i>Flowers,
+      Fruits, and Leaves</i>, or any general botanical work.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_140_140" id=
+      "Footnote_140_140"></a><a href=
+      "#FNanchor_140_140"><span class="label">[140]</span></a>
+      <i>Nature</i>, vol. xv. p, 117.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_141_141" id=
+      "Footnote_141_141"></a><a href=
+      "#FNanchor_141_141"><span class="label">[141]</span></a>
+      Grant Allen's <i>Colour Sense</i>, p. 113.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_142_142" id=
+      "Footnote_142_142"></a><a href=
+      "#FNanchor_142_142"><span class="label">[142]</span></a>
+      Canon Tristram's <i>Natural History of the Bible</i>, pp.
+      483, 484.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_143_143" id=
+      "Footnote_143_143"></a><a href=
+      "#FNanchor_143_143"><span class="label">[143]</span></a> For
+      a complete historical account of this subject with full
+      references to all the works upon it, see the Introduction to
+      Hermann M&uuml;ller's <i>Fertilisation of Flowers</i>,
+      translated by D'Arcy W. Thompson.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_144_144" id=
+      "Footnote_144_144"></a><a href=
+      "#FNanchor_144_144"><span class="label">[144]</span></a> For
+      the full detail of his experiments, see <i>Cross-and
+      Self-Fertilisation of Plants</i>, 1876.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_145_145" id=
+      "Footnote_145_145"></a><a href=
+      "#FNanchor_145_145"><span class="label">[145]</span></a> See
+      Darwin's <i>Fertilisation of Orchids</i> for the many
+      extraordinary and complex arrangements in these plants.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_146_146" id=
+      "Footnote_146_146"></a><a href=
+      "#FNanchor_146_146"><span class="label">[146]</span></a> The
+      English reader may consult Sir John Lubbock's <i>British Wild
+      Flowers in Relation to Insects</i>, and H. M&uuml;ller's
+      great and original work, <i>The Fertilisation of
+      Flowers</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_147_147" id=
+      "Footnote_147_147"></a><a href=
+      "#FNanchor_147_147"><span class="label">[147]</span></a>
+      M&uuml;ller's <i>Fertilisation of Flowers</i>, p. 248.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_148_148" id=
+      "Footnote_148_148"></a><a href=
+      "#FNanchor_148_148"><span class="label">[148]</span></a>
+      "Alpenblumen," by D.H. M&uuml;ller. See <i>Nature</i>, vol.
+      xxiii. p. 333.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_149_149" id=
+      "Footnote_149_149"></a><a href=
+      "#FNanchor_149_149"><span class="label">[149]</span></a> This
+      peculiarity of local distribution of colour in flowers may be
+      compared, as regards its purpose, with the recognition
+      colours of animals. Just as these latter colours enable the
+      sexes to recognise each other, and thus avoid sterile unions
+      of distinct species, so the distinctive form and colour of
+      each species of flower, as compared with those that usually
+      grow around it, enables the fertilising insects to avoid
+      carrying the pollen of one flower to the stigma of a distinct
+      species.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_150_150" id=
+      "Footnote_150_150"></a><a href=
+      "#FNanchor_150_150"><span class="label">[150]</span></a> See
+      H. M&uuml;ller's <i>Fertilisation of Flowers</i>, p. 18.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_151_151" id=
+      "Footnote_151_151"></a><a href=
+      "#FNanchor_151_151"><span class="label">[151]</span></a> The
+      above examples are taken from Rev. G. Henslow's paper on
+      "Self-Fertilisation of Plants," in <i>Trans. Linn. Soc.</i>
+      Second series, <i>Botany</i>, vol. i. pp. 317-398, with
+      plate. Mr. H.O. Forbes has shown that the same thing occurs
+      among tropical orchids, in his paper "On the Contrivances for
+      insuring Self-Fertilisation in some Tropical Orchids,"
+      <i>Journ. Linn. Soc.</i>, xxi. p. 538.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_152_152" id=
+      "Footnote_152_152"></a><a href=
+      "#FNanchor_152_152"><span class="label">[152]</span></a>
+      These are the numbers given by Darwin, but I am informed by
+      Mr. Hemsley that many additions have been since made to the
+      list, and that cleistogamic flowers probably occur in nearly
+      all the natural orders.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_153_153" id=
+      "Footnote_153_153"></a><a href=
+      "#FNanchor_153_153"><span class="label">[153]</span></a> For
+      a full account of cleistogamic flowers, see Darwin's <i>Forms
+      of Flowers</i>, chap. viii.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_154_154" id=
+      "Footnote_154_154"></a><a href=
+      "#FNanchor_154_154"><span class="label">[154]</span></a>
+      Henslow's "Self-Fertilisation," <i>Trans. Linn. Soc.</i>
+      Second series, <i>Botany</i>, vol. i. p. 391.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_155_155" id=
+      "Footnote_155_155"></a><a href=
+      "#FNanchor_155_155"><span class="label">[155]</span></a> The
+      Rev. George Henslow, in his <i>Origin of Floral
+      Structures</i>, says: "There is little doubt but that all
+      wind-fertilised angiosperms are degradations from
+      insect-fertilised flowers.... <i>Poterium sanguisorba</i> is
+      anemophilous; and <i>Sanguisorba officinalis</i> presumably
+      was so formerly, but has reacquired an entomophilous habit;
+      the whole tribe Poterieae being, in fact, a degraded group
+      which has descended from Potentilleae. Plantains retain their
+      corolla but in a degraded form. Junceae are degraded Lilies;
+      while Cyperaceae and Gramineae among monocotyledons may be
+      ranked with Amentiferae among dicotyledons, as representing
+      orders which have retrograded very far from the entomophilous
+      forms from which they were possibly and probably descended"
+      (p. 266).</p>
+
+      <p>"The genus Plantago, like <i>Thalictrum minus</i>,
+      Poterium, and others, well illustrate the change from an
+      entomophilous to the anemophilous state. <i>P. lanceolata</i>
+      has polymorphic flowers, and is visited by pollen-seeking
+      insects, so that it can be fertilised either by insects or
+      the wind. <i>P. media</i> illustrates transitions in point of
+      structure, as the filaments are pink, the anthers motionless,
+      and the pollen grains aggregated, and it is regularly visited
+      by <i>Bombus terrestris</i>. On the other hand, the slender
+      filaments, versatile anthers, powdery pollen, and elongated
+      protogynous style are features of other species indicating
+      anemophily; while the presence of a degraded corolla shows
+      its ancestors to have been entomophilous. <i>P. media</i>,
+      therefore, illustrates, not a primitive entomophilous
+      condition, but a return to it; just as is the case with
+      <i>Sanguisorba officinalis</i> and <i>Salix Caprea</i>; but
+      these show no capacity of restoring the corolla, the
+      attractive features having to be borne by the calyx, which is
+      purplish in Sanguisorba, by the pink filaments of Plantago,
+      and by the yellow anthers in the Sallow willow" (p. 271).</p>
+
+      <p>"The interpretation, then, I would offer of
+      inconspicuousness and all kinds of degradations is the exact
+      opposite to that of conspicuousness and great
+      differentiations; namely, that species with minute flowers,
+      rarely or never visited by insects, and habitually
+      self-fertilised, have primarily arisen through the neglect of
+      insects, and have in consequence assumed their present floral
+      structures" (p. 282).</p>
+
+      <p>In a letter just received from Mr. Henslow, he gives a few
+      additional illustrations of his views, of which the following
+      are the most important: "Passing to Incompletae, the orders
+      known collectively as 'Cyclospermeae' are related to
+      Caryophylleae; and to my mind are degradations from it, of
+      which Orache is anemophilous. Cupuliferae have an inferior
+      ovary and rudimentary calyx-limb on the top. These, as far as
+      I know, cannot be interpreted except as degradations. The
+      whole of Monocotyledons appear to me (from anatomical reasons
+      especially) to be degradations from Dicotyledons, and
+      primarily through the agency of growth in water. Many
+      subsequently became terrestrial, but retained the effects of
+      their primitive habitat through heredity. The 3-merous [sic]
+      perianth of grasses, the parts of the flower being in whorls,
+      point to a degradation from a sub-liliaceous condition."</p>
+
+      <p>Mr. Henslow informs me that he has long held these views,
+      but, as far as he knows, alone. Mr. Grant Allen, however, set
+      forth a similar theory in his <i>Vignettes from Nature</i>
+      (p. 15) and more fully in <i>The Colours of Flowers</i>
+      (chap. v.), where he develops it fully and uses similar
+      arguments to those of Mr. Henslow.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_156_156" id=
+      "Footnote_156_156"></a><a href=
+      "#FNanchor_156_156"><span class="label">[156]</span></a> H.
+      M&uuml;ller gives ample proof of this in his <i>Fertilisation
+      of Flowers</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_157_157" id=
+      "Footnote_157_157"></a><a href=
+      "#FNanchor_157_157"><span class="label">[157]</span></a>
+      <i>Cross-and Self-Fertilisation</i>, p. 27.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_158_158" id=
+      "Footnote_158_158"></a><a href=
+      "#FNanchor_158_158"><span class="label">[158]</span></a>
+      <i>Animals and Plants</i>, vol. ii. p. 145.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_159_159" id=
+      "Footnote_159_159"></a><a href=
+      "#FNanchor_159_159"><span class="label">[159]</span></a>
+      M&uuml;ller's <i>Fertilisation of Flowers</i>, pp. 448, 455.
+      Other cases of recent degradation and readaptation to
+      insect-fertilisation are given by Professor Henslow (see
+      footnote, p. 324).</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_160_160" id=
+      "Footnote_160_160"></a><a href=
+      "#FNanchor_160_160"><span class="label">[160]</span></a>
+      <i>The Colour Sense</i>, by Grant Allen, p. 95.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_161_161" id=
+      "Footnote_161_161"></a><a href=
+      "#FNanchor_161_161"><span class="label">[161]</span></a>
+      <i>The Colour Sense</i>, chap. ix.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_162_162" id=
+      "Footnote_162_162"></a><a href=
+      "#FNanchor_162_162"><span class="label">[162]</span></a> See
+      <i>Origin of Species</i>, sixth edition, p. 220.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_XII" id="CHAPTER_XII"></a><a name="Page_338"
+  id="Page_338"></a>CHAPTER XII</h2>
+
+  <p style='font-weight:bold;'>THE GEOGRAPHICAL DISTRIBUTION OF
+  ORGANISMS</p>
+
+  <div class="blockquot">
+    <p>The facts to be explained&mdash;The conditions which have
+    determined distribution&mdash;The permanence of
+    oceans&mdash;Oceanic and continental areas&mdash;Madagascar and
+    New Zealand&mdash;The teachings of the thousand-fathom
+    line&mdash;The distribution of marsupials&mdash;The
+    distribution of tapirs&mdash;Powers of dispersal as illustrated
+    by insular organisms&mdash;Birds and insects at
+    sea&mdash;Insects at great altitudes&mdash;The dispersal of
+    plants&mdash;Dispersal of seeds by the wind&mdash;Mineral
+    matter carried by the wind&mdash;Objections to the theory of
+    wind-dispersal answered&mdash;Explanation of north temperate
+    plants in the southern hemisphere&mdash;No proof of glaciation
+    in the tropics&mdash;Lower temperature not needed to explain
+    the facts&mdash;Concluding remarks.</p>
+  </div>
+
+  <p>The theory which we may now take as established&mdash;that all
+  the existing forms of life have been derived from other forms by
+  a natural process of descent with modification, and that this
+  same process has been in action during past geological
+  time&mdash;should enable us to give a rational account not only
+  of the peculiarities of form and structure presented by animals
+  and plants, but also of their grouping together in certain areas,
+  and their general distribution over the earth's surface.</p>
+
+  <p>In the absence of any exact knowledge of the facts of
+  distribution, a student of the theory of evolution might
+  naturally anticipate that all groups of allied organisms would be
+  found in the same region, and that, as he travelled farther and
+  farther from any given centre, the forms of life would differ
+  more and more from those which prevailed at the starting-point,
+  till, in the remotest regions to which he could penetrate, he
+  would find an entirely new assemblage of animals and plants,
+  altogether unlike those with which he was <a name="Page_339" id=
+  "Page_339"></a>familiar. He would also anticipate that
+  diversities of climate would always be associated with a
+  corresponding diversity in the forms of life.</p>
+
+  <p>Now these anticipations are to a considerable extent
+  justified. Remoteness on the earth's surface is usually an
+  indication of diversity in the fauna and flora, while strongly
+  contrasted climates are always accompanied by a considerable
+  contrast in the forms of life. But this correspondence is by no
+  means exact or proportionate, and the converse propositions are
+  often quite untrue. Countries which are near to each other often
+  differ radically in their animal and vegetable productions; while
+  similarity of climate, together with moderate geographical
+  proximity, are often accompanied by marked diversities in the
+  prevailing forms of life. Again, while many groups of
+  animals&mdash;genera, families, and sometimes even
+  orders&mdash;are confined to limited regions, most of the
+  families, many genera, and even some species are found in every
+  part of the earth. An enumeration of a few of these anomalies
+  will better illustrate the nature of the problem we have to
+  solve.</p>
+
+  <p>As examples of extreme diversity, notwithstanding geographical
+  proximity, we may adduce Madagascar and Africa, whose animal and
+  vegetable productions are far less alike than are those of Great
+  Britain and Japan at the remotest extremities of the great
+  northern continent; while an equal, or perhaps even a still
+  greater, diversity exists between Australia and New Zealand. On
+  the other hand, Northern Africa and South Europe, though
+  separated by the Mediterranean Sea, have faunas and floras which
+  do not differ from each other more than do the various countries
+  of Europe. As a proof that similarity of climate and general
+  adaptability have had but a small part in determining the forms
+  of life in each country, we have the fact of the enormous
+  increase of rabbits and pigs in Australia and New Zealand, of
+  horses and cattle in South America, and of the common sparrow in
+  North America, though in none of these cases are the animals
+  natives of the countries in which they thrive so well. And
+  lastly, in illustration of the fact that allied forms are not
+  always found in adjacent regions, we have the tapirs, which are
+  found only on opposite sides of the globe, in tropical America
+  and the Malayan Islands; the camels of <a name="Page_340" id=
+  "Page_340"></a>the Asiatic deserts, whose nearest allies are the
+  llamas and alpacas of the Andes; and the marsupials, only found
+  in Australia and on the opposite side of the globe, in America.
+  Yet, again, although mammalia may be said to be universally
+  distributed over the globe, being found abundantly on all the
+  continents and on a great many of the larger islands, yet they
+  are entirely wanting in New Zealand, and in a considerable number
+  of other islands which are, nevertheless, perfectly able to
+  support them when introduced.</p>
+
+  <p>Now most of these difficulties can be solved by means of
+  well-known geographical and geological facts. When the
+  productions of remote countries resemble each other, there is
+  almost always continuity of land with similarity of climate
+  between them. When adjacent countries differ greatly in their
+  productions, we find them separated by a sea or strait whose
+  great depth is an indication of its antiquity or permanence. When
+  a group of animals inhabits two countries or regions separated by
+  wide oceans, it is found that in past geological times the same
+  group was much more widely distributed, and may have reached the
+  countries it inhabits from an intermediate region in which it is
+  now extinct. We know, also, that countries now united by land
+  were divided by arms of the sea at a not very remote epoch; while
+  there is good reason to believe that others now entirely isolated
+  by a broad expanse of sea were formerly united and formed a
+  single land area. There is also another important factor to be
+  taken account of in considering how animals and plants have
+  acquired their present peculiarities of
+  distribution,&mdash;changes of climate. We know that quite
+  recently a glacial epoch extended over much of what are now the
+  temperate regions of the northern hemisphere, and that
+  consequently the organisms which inhabit those parts must be,
+  comparatively speaking, recent immigrants from more southern
+  lands. But it is a yet more important fact that, down to middle
+  Tertiary times at all events, an equable temperate climate, with
+  a luxuriant vegetation, extended to far within the arctic circle,
+  over what are now barren wastes, covered for ten months of the
+  year with snow and ice. The arctic zone has, therefore, been in
+  past times capable of supporting almost all the forms of life of
+  our temperate regions; and we <a name="Page_341" id=
+  "Page_341"></a>must take account of this condition of things
+  whenever we have to speculate on the possible migrations of
+  organisms between the old and new continents.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Conditions which have determined Distribution.</i></p>
+
+  <p>When we endeavour to explain in detail the facts of the
+  existing distribution of organic beings, we are confronted by
+  several preliminary questions, upon the solution of which will
+  depend our treatment of the phenomena presented to us. Upon the
+  theory of descent which we have adopted, all the different
+  species of a genus, as well as all the genera which compose a
+  family or higher group, have descended from some common ancestor,
+  and must therefore, at some remote epoch, have occupied the same
+  area, from which their descendants have spread to the regions
+  they now inhabit. In the numerous cases in which the same group
+  now occupies countries separated by oceans or seas, by lofty
+  mountain-chains, by wide deserts, or by inhospitable climates, we
+  have to consider how the migration which must certainly have
+  taken place has been effected. It is possible that during some
+  portion of the time which has elapsed since the origin of the
+  group the interposing barriers have not been in existence; or, on
+  the other hand, the particular organisms we are dealing with may
+  have the power of overpassing the barriers, and thus reaching
+  their present remote dwelling-places. As this is really the
+  fundamental question of distribution on which the solution of all
+  its more difficult problems depends, we have to inquire, in the
+  first place, what is the nature of, and what are the limits to,
+  the changes of the earth's surface, especially during the
+  Tertiary and latter part of the Secondary periods, as it was
+  during those periods that most of the existing types of the
+  higher animals and plants came into existence; and, in the next
+  place, what are the extreme limits of the powers of dispersal
+  possessed by the chief groups of animals and plants. We will
+  first consider the question of barriers, more especially those
+  formed by seas and oceans.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Permanence of Oceans.</i></p>
+
+  <p>It was formerly a very general belief, even amongst
+  geologists, that the great features of the earth's surface, no
+  less than the smaller ones, were subject to continual mutations,
+  <a name="Page_342" id="Page_342"></a>and that during the course
+  of known geological time the continents and great oceans had
+  again and again changed places with each other. Sir Charles
+  Lyell, in the last edition of his <i>Principles of Geology</i>
+  (1872), said: "Continents, therefore, although permanent for
+  whole geological epochs, shift their positions entirely in the
+  course of ages;" and this may be said to have been the orthodox
+  opinion down to the very recent period when, by means of deep-sea
+  soundings, the nature of the ocean bottom was made known. The
+  first person to throw doubt on this view appears to have been the
+  veteran American geologist, Professor Dana. In 1849, in the
+  Report of Wilke's Exploring Expedition, he adduced the argument
+  against a former continent in the Pacific during the Tertiary
+  period, from the absence of all native quadrupeds. In 1856, in
+  articles in the <i>American Journal</i>, he discussed the
+  development of the American continent, and argued for its general
+  permanence; and in his <i>Manual of Geology</i> in 1863 and later
+  editions, the same views were more fully enforced and were
+  latterly applied to all continents. Darwin, in his <i>Journal of
+  Researches</i>, published in 1845, called attention to the fact
+  that all the small islands far from land in the Pacific, Indian,
+  and Atlantic Oceans are either of coralline or volcanic
+  formation. He excepted, however, the Seychelles and St. Paul's
+  rocks; but the former have since been shown to be no exception,
+  as they consist entirely of coral rock; and although Darwin
+  himself spent a few hours on St. Paul's rocks on his outward
+  voyage in the <i>Beagle</i>, and believed he had found some
+  portions of them to be of a "cherty," and others of a
+  "felspathic" nature, this also has been shown to be erroneous,
+  and the careful examination of the rocks by the Abb&eacute;
+  Renard clearly proves them to be wholly of volcanic
+  origin.<a name="FNanchor_163_163" id=
+  "FNanchor_163_163"></a><a href="#Footnote_163_163" class=
+  "fnanchor">[163]</a> We have, therefore, at the present time,
+  absolutely no exception whatever to the remarkable fact that all
+  the oceanic islands of the globe are either of volcanic or coral
+  formation; and there is, further, good reason to believe that
+  those of the latter class in every case rest upon a volcanic
+  foundation.</p>
+
+  <p>In his <i>Origin of Species</i>, Darwin further showed that no
+  true oceanic island had any native mammals or batrachia <a name=
+  "Page_343" id="Page_343"></a>when first discovered, this fact
+  constituting the test of the class to which an island belongs;
+  whence he argued that none of them had ever been connected with
+  continents, but all had originated in mid-ocean. These
+  considerations alone render it almost certain that the areas now
+  occupied by the great oceans have never, during known geological
+  time, been occupied by continents, since it is in the highest
+  degree improbable that every fragment of those continents should
+  have completely disappeared, and have been replaced by volcanic
+  islands rising out of profound oceanic abysses; but recent
+  research into the depth of the oceans and the nature of the
+  deposits now forming on their floors, adds greatly to the
+  evidence in this direction, and renders it almost a certainty
+  that they represent very ancient if not primaeval features of the
+  earth's surface. A very brief outline of the nature of this
+  evidence will be now given.</p>
+
+  <p>The researches of the <i>Challenger</i> expedition into the
+  nature of the sea-bottom show, that the whole of the land debris
+  brought down by rivers to the ocean (with the exception of pumice
+  and other floating matter), is deposited comparatively near to
+  the shores, and that the fineness of the material is an
+  indication of the distance to which it has been carried.
+  Everything in the nature of gravel and sand is laid down within a
+  very few miles of land, only the finer muddy sediments being
+  carried out for 20 or 50 miles, and the very finest of all, under
+  the most favourable conditions, rarely extending beyond 150, or
+  at the utmost, 300 miles from land into the deep ocean.<a name=
+  "FNanchor_164_164" id="FNanchor_164_164"></a><a href=
+  "#Footnote_164_164" class="fnanchor">[164]</a> Beyond these
+  distances, and covering the entire ocean floor, are various oozes
+  formed wholly from the debris of marine organisms; while
+  intermingled with these are found various volcanic products which
+  have been either carried through the air or floated on the
+  surface, and a small but perfectly recognisable quantity of
+  meteoric matter. Ice-borne rocks are also found abundantly
+  scattered over the ocean bottom within a definite distance of the
+  arctic and antarctic circles, clearly marking out the limit of
+  floating icebergs in recent geological times.</p>
+
+  <p><a name="Page_344" id="Page_344"></a>Now the whole series of
+  marine stratified rocks, from the earliest Palaeozoic to the most
+  recent Tertiary beds, consist of materials closely corresponding
+  to the land debris now being deposited within a narrow belt round
+  the shores of all continents; while no rocks have been found
+  which can be identified with the various oozes now forming in the
+  deep abysses of the ocean. It follows, therefore, that all the
+  geological formations have been formed in comparatively shallow
+  water, and always adjacent to the continental land of the period.
+  The great thickness of some of the formations is no indication of
+  a deep sea, but only of slow subsidence during the time that the
+  deposition was in progress. This view is now adopted by many of
+  the most experienced geologists, especially by Dr. Archibald
+  Geikie, Director of the Geological Survey of Great Britain, who,
+  in his lecture on "Geographical Evolution," says: "From all this
+  evidence we may legitimately conclude that the present land of
+  the globe, though consisting in great measure of marine
+  formations, has never lain under the deep sea; but that its site
+  must always have been near land. Even its thick marine limestones
+  are the deposits of comparatively shallow water."<a name=
+  "FNanchor_165_165" id="FNanchor_165_165"></a><a href=
+  "#Footnote_165_165" class="fnanchor">[165]</a></p>
+
+  <p>But besides these geological and physical considerations,
+  there is a mechanical difficulty in the way of repeated change of
+  position of oceans and continents which has not yet received the
+  attention it deserves. According to the recent careful estimate
+  by Mr. John Murray, the land area of the globe is to the water
+  area as &middot;28 to &middot;72. The mean height of the land
+  above sea-level is 2250 feet, while the mean depth of the ocean
+  is 14,640 feet. Hence the bulk of dry land is 23,450,000 cubic
+  miles, and that of the waters of the ocean 323,800,000 cubic
+  miles; and it follows that if the whole of the solid matter of
+  the earth's surface were reduced to one level, it would be
+  everywhere covered by an ocean about two miles deep. The
+  accompanying diagram will serve to render these figures more
+  intelligible. The length of the sections of land and ocean are in
+  the proportion of their respective areas, while the mean height
+  of the land and the mean depth of the ocean are exhibited on a
+  greatly increased <a name="Page_345" id="Page_345"></a>vertical
+  scale. If we considered the continents and their adjacent oceans
+  separately they would differ a little, but not very materially,
+  from this diagram; in some cases the proportion of land to ocean
+  would be a little greater, in others a little less.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig32" id="fig32"></a> <img src="images/fig32.jpg"
+    width="400" height="184" alt="FIG. 32." title="FIG. 32." />
+    <b>FIG. 32.</b>
+  </div>
+
+  <p>Now, if we try to imagine a process of elevation and
+  depression by which the sea and land shall completely change
+  places, we shall be met by insuperable difficulties. We must, in
+  the first place, assume a general equality between elevation and
+  subsidence during any given period, because if the elevation over
+  any extensive continental area were not balanced by some
+  subsidence of approximately equal amount, an unsupported hollow
+  would be left under the earth's crust. Let us now suppose a
+  continental area to sink, and an adjacent oceanic area to rise,
+  it will be seen that the greater part of the land will disappear
+  long before the new land has approached the surface of the ocean.
+  This difficulty will not be removed by supposing a portion of a
+  continent to subside, and the immediately adjacent portion of the
+  ocean on the other side of the continent to rise, because in
+  almost every case we find that within a comparatively short
+  distance from the shores of all existing continents, the ocean
+  floor sinks rapidly to a depth of from 2000 to 3000 fathoms, and
+  maintains a similar depth, generally speaking, over a large
+  portion of the oceanic areas. In order, therefore, that any area
+  of continental extent be upraised from the great oceans, there
+  must be a subsidence of a land area five or six times as great,
+  unless it can be shown that an extensive elevation of the ocean
+  floor up to and far <a name="Page_346" id="Page_346"></a>above
+  the surface could occur without an equivalent depression
+  elsewhere. The fact that the waters of the ocean are sufficient
+  to cover the whole globe to a depth of two miles, is alone
+  sufficient to indicate that the great ocean basins are permanent
+  features of the earth's surface, since any process of alternation
+  of these with the land areas would have been almost certain to
+  result again and again in the total disappearance of large
+  portions, if not of all, of the dry land of the globe. But the
+  continuity of terrestrial life since the Devonian and
+  Carboniferous periods, and the existence of very similar forms in
+  the corresponding deposits of every continent&mdash;as well as
+  the occurrence of sedimentary rocks, indicating the proximity of
+  land at the time of their deposit, over a large portion of the
+  surface of all the continents, and in every geological
+  period&mdash;assure us that no such disappearance has ever
+  occurred.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Oceanic and Continental Areas.</i></p>
+
+  <p>When we speak of the permanence of oceanic and continental
+  areas as one of the established facts of modern research, we do
+  not mean that existing continents and oceans have always
+  maintained the exact areas and outlines that they now present,
+  but merely, that while all of them have been undergoing changes
+  in outline and extent from age to age, they have yet maintained
+  substantially the same positions, and have never actually changed
+  places with each other. There are, moreover, certain physical and
+  biological facts which enable us to mark out these areas with
+  some confidence.</p>
+
+  <p>We have seen that there are a large number of islands which
+  may be classed as oceanic, because they have never formed parts
+  of continents, but have originated in mid-ocean, and have derived
+  their forms of life by migration across the sea. Their
+  peculiarities are seen to be very marked in comparison with those
+  islands which there is good reason to believe are really
+  fragments of more extensive land areas, and are hence termed
+  "continental." These continental islands consist in every case of
+  a variety of stratified rocks of various ages, thus corresponding
+  closely with the usual structure of continents; although many of
+  the islands are small like Jersey or the Shetland Islands, or far
+  from continental land like the Falkland Islands or New Zealand.
+  They all <a name="Page_347" id="Page_347"></a>contain indigenous
+  mammalia or batrachia, and generally a much greater variety of
+  birds, reptiles, insects, and plants, than do the oceanic
+  islands. From these various characteristics we conclude that they
+  have all once formed parts of continents, or at all events of
+  much larger land areas, and have become isolated, either by
+  subsidence of the intervening land or by the effects of
+  long-continued marine denudation.</p>
+
+  <p>Now, if we trace the thousand-fathom line around all our
+  existing continents we find that, with only two exceptions, every
+  island which can be classed as "continental" falls within this
+  line, while all that lie beyond it have the undoubted
+  characteristics of "oceanic" islands. We, therefore, conclude
+  that the thousand-fathom line marks out, approximately, the
+  "continental area,"&mdash;that is, the limits within which
+  continental development and change throughout known geological
+  time have gone on. There may, of course, have been some
+  extensions of land beyond this limit, while some areas within it
+  may always have been ocean; but so far as we have any direct
+  evidence, this line may be taken to mark out, approximately, the
+  most probable boundary between the "continental area," which has
+  always consisted of land and shallow sea in varying proportions,
+  and the great oceanic basins, within the limits of which volcanic
+  activity has been building up numerous islands, but whose
+  profound depths have apparently undergone little change.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Madagascar and New Zealand.</i></p>
+
+  <p>The two exceptions just referred to are Madagascar and New
+  Zealand, and all the evidence goes to show that in these cases
+  the land connection with the nearest continental area was very
+  remote in time. The extraordinary isolation of the productions of
+  Madagascar&mdash;almost all the most characteristic forms of
+  mammalia, birds, and reptiles of Africa being absent from
+  it&mdash;renders it certain that it must have been separated from
+  that continent very early in the Tertiary, if not as far back as
+  the latter part of the Secondary period; and this extreme
+  antiquity is indicated by a depth of considerably more than a
+  thousand fathoms in the Mozambique Channel, though this deep
+  portion is less than a hundred miles wide between the Comoro
+  Islands and the main<a name="Page_348" id=
+  "Page_348"></a>land.<a name="FNanchor_166_166" id=
+  "FNanchor_166_166"></a><a href="#Footnote_166_166" class=
+  "fnanchor">[166]</a> Madagascar is the only island on the globe
+  with a fairly rich mammalian fauna which is separated from a
+  continent by a depth greater than a thousand fathoms; and no
+  other island presents so many peculiarities in these animals, or
+  has preserved so many lowly organised and archaic forms. The
+  exceptional character of its productions agrees exactly with its
+  exceptional isolation by means of a very deep arm of the sea.</p>
+
+  <p>New Zealand possesses no known mammals and only a single
+  species of batrachian; but its geological structure is perfectly
+  continental. There is also much evidence that it does possess one
+  mammal, although no specimens have been yet obtained.<a name=
+  "FNanchor_167_167" id="FNanchor_167_167"></a><a href=
+  "#Footnote_167_167" class="fnanchor">[167]</a> Its reptiles and
+  birds are highly peculiar and more numerous than in any truly
+  oceanic island. Now the sea which directly separates New Zealand
+  from Australia is more than 2000 fathoms deep, but in a
+  north-west direction there is an extensive bank under 1000
+  fathoms, extending to and including Lord Howe's Island, while
+  north of this are other banks of the same depth, approaching
+  towards a submarine extension of Queensland on the one hand, and
+  New Caledonia on the other, and altogether suggestive of a land
+  union with Australia at some very remote period. Now the peculiar
+  relations of the New Zealand fauna and flora with those of
+  Australia and of the tropical Pacific Islands to the northward
+  indicate such a connection, probably during the Cretaceous
+  period; and here, again, we have the exceptional depth of the
+  dividing sea and the form of the ocean bottom according well with
+  the altogether exceptional isolation of New Zealand, an isolation
+  which has been held by some naturalists to be great enough to
+  justify its claim to be one of the primary Zoological
+  Regions.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Teachings of the Thousand-Fathom Line.</i></p>
+
+  <p>If now we accept the annexed map as showing us approximately
+  how far beyond their present limits our continents may <a name=
+  "Page_349" id="Page_349"></a>have extended during any portion of
+  the Tertiary and Secondary periods, we shall obtain a foundation
+  of inestimable value for our inquiries into those migrations of
+  animals and plants during past ages which have resulted in their
+  present peculiarities of distribution. We see, for instance, that
+  the South American and African continents have always been
+  separated by nearly as wide an ocean as at present, and that
+  whatever similarities there may be in their productions must be
+  due to the similar forms having been derived from a common origin
+  in one of the great northern continents. The radical difference
+  between the higher forms of life of the two continents accords
+  perfectly with their permanent separation. If there had been any
+  direct connection between them during Tertiary times, we should
+  hardly have found the deep-seated differences between the
+  Quadrumana of the two regions&mdash;no family even being common
+  to both; nor the peculiar Insectivora of the one continent, and
+  the equally peculiar Edentata of the other. The very numerous
+  families of birds quite peculiar to one or other of these
+  continents, many of which, by their structural isolation and
+  varied development of generic and specific forms, indicate a high
+  antiquity, equally suggest that there has been no near approach
+  to a land connection during the same epoch.</p>
+
+  <p>Looking to the two great northern continents, we see
+  indications of a possible connection between them both in the
+  North Atlantic and the North Pacific oceans; and when we remember
+  that from middle Tertiary times backward&mdash;so far as we know
+  continuously to the earliest Palaeozoic epoch&mdash;a temperate
+  and equable climate, with abundant woody vegetation, prevailed up
+  to and within the arctic circle, we see what facilities may have
+  been afforded for migration from one continent to the other,
+  sometimes between America and Europe, sometimes between America
+  and Asia. Admitting these highly probable connections, no
+  bridging of the Atlantic in more southern latitudes (of which
+  there is not a particle of evidence) will have been necessary to
+  account for all the intermigration that has occurred between the
+  two continents. If, on the other hand, we remember how long must
+  have been the route, and how diverse must always have been the
+  conditions between the more northern and the more southern
+  portions of the American and Euro-Asiatic continents, we shall
+  not be <a name="Page_350" id="Page_350"></a>surprised that many
+  widespread forms in either continent have not crossed into the
+  other; and that while the skunks (Mephitis), the pouched rats
+  (Saccomyidae), and the turkeys (Meleagris) are confined to
+  America, the pigs and the hedgehogs, the true flycatchers and the
+  pheasants are found only in the Euro-Asiatic continent. But, just
+  as there have been periods which facilitated intermigration
+  between America and the Old World, there have almost certainly
+  been periods, perhaps of long duration even geologically, when
+  these continents have been separated by seas as wide as, or even
+  wider than, those of the present day; and thus may be explained
+  such curious anomalies as the origination of the camel-tribe in
+  America, and its entrance into Asia in comparatively recent
+  Tertiary times, while the introduction of oxen and bears into
+  America from the Euro-Asiatic continent appears to have been
+  equally recent.<a name="FNanchor_168_168" id=
+  "FNanchor_168_168"></a><a href="#Footnote_168_168" class=
+  "fnanchor">[168]</a></p>
+
+  <p>We shall find on examination that this view of the general
+  permanence of the oceanic and continental areas, with constant
+  minor fluctuations of land and sea over the whole extent of the
+  latter, enables us to understand, and offer a rational
+  explanation of, most of the difficult problems of geographical
+  distribution; and further, that our power of doing this is in
+  direct proportion to our acquaintance with the distribution of
+  fossil forms of life during the Tertiary period. We must, also,
+  take due note of many other facts of almost equal importance for
+  a due appreciation of the problems presented for solution, the
+  most essential being, the various powers of dispersal possessed
+  by the different groups of animals and plants, the geological
+  antiquity of the species and genera, and the width and depth of
+  the seas which separate the countries they, inhabit. A few
+  illustrations will now be given of the way in which these
+  branches of knowledge enable us to deal with the difficulties and
+  anomalies that present themselves.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Distribution of Marsupials.</i></p>
+
+  <p>This singular and lowly organised type of mammals constitutes
+  almost the sole representative of the class in Australia <a name=
+  "Page_351" id="Page_351"></a>and New Guinea, while it is entirely
+  unknown in Asia, Africa, or Europe. It reappears in America,
+  where several species of opossums are found; and it was long
+  thought necessary to postulate a direct southern connection of
+  these distant countries, in order to account for this curious
+  fact of distribution. When, however, we look to what is known of
+  the geological history of the marsupials the difficulty vanishes.
+  In the Upper Eocene deposits of Western Europe the remains of
+  several animals closely allied to the American opossums have been
+  found; and as, at this period, a very mild climate prevailed far
+  up into the arctic regions, there is no difficulty in supposing
+  that the ancestors of the group entered America from Europe or
+  Northern Asia during early Tertiary times.</p>
+
+  <p>But we must go much further back for the origin of the
+  Australian marsupials. All the chief types of the higher mammalia
+  were in existence in the Eocene, if not in the preceding
+  Cretaceous period, and as we find none of these in Australia,
+  that country must have been finally separated from the Asiatic
+  continent during the Secondary or Mesozoic period. Now during
+  that period, in the Upper and the Lower Oolite and in the still
+  older Trias, the jaw-bones of numerous small mammalia have been
+  found, forming eight distinct genera, which are believed to have
+  been either marsupials or some allied lowly forms. In North
+  America also, in beds of the Jurassic and Triassic formations,
+  the remains of an equally great variety of these small mammalia
+  have been discovered; and from the examination of more than sixty
+  specimens, belonging to at least six distinct genera, Professor
+  Marsh is of opinion that they represent a generalised type, from
+  which the more specialised marsupials and insectivora were
+  developed.</p>
+
+  <p>From the fact that very similar mammals occur both in Europe
+  and America at corresponding periods, and in beds which represent
+  a long succession of geological time, and that during the whole
+  of this time no fragments of any higher forms have been
+  discovered, it seems probable that both the northern continents
+  (or the larger portion of their area) were then inhabited by no
+  other mammalia than these, with perhaps other equally low types.
+  It was, probably, not later than the Jurassic age when some of
+  these primitive marsupials were able to enter Australia, where
+  they have since <a name="Page_352" id="Page_352"></a>remained
+  almost completely isolated; and, being free from the competition
+  of higher forms, they have developed into the great variety of
+  types we now behold there. These occupy the place, and have to
+  some extent acquired the form and structure of distinct orders of
+  the higher mammals&mdash;the rodents, the insectivora, and the
+  carnivora,&mdash;while still preserving the essential
+  characteristics and lowly organisation of the marsupials. At a
+  much later period&mdash;probably in late Tertiary times&mdash;the
+  ancestors of the various species of rats and mice which now
+  abound in Australia, and which, with the aerial bats, constitute
+  its only forms of placental mammals, entered the country from
+  some of the adjacent islands. For this purpose a land connection
+  was not necessary, as these small creatures might easily be
+  conveyed among the branches or in the crevices of trees uprooted
+  by floods and carried down to the sea, and then floated to a
+  shore many miles distant. That no actual land connection with, or
+  very close approximation to, an Asiatic island has occurred in
+  recent times, is sufficiently proved by the fact that no
+  squirrel, pig, civet, or other widespread mammal of the Eastern
+  hemisphere has been able to reach the Australian continent.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Distribution of Tapirs.</i></p>
+
+  <p>These curious animals form one of the puzzles of geographical
+  distribution, being now confined to two very remote regions of
+  the globe&mdash;the Malay Peninsula and adjacent islands of
+  Sumatra and Borneo, inhabited by one species, and tropical
+  America, where there are three or four species, ranging from
+  Brazil to Ecuador and Guatemala. If we considered these living
+  forms only, we should be obliged to speculate on enormous changes
+  of land and sea in order that these tropical animals might have
+  passed from one country to the other. But geological discoveries
+  have rendered all such hypothetical changes unnecessary. During
+  Miocene and Pliocene times tapirs abounded over the whole of
+  Europe and Asia, their remains having been found in the tertiary
+  deposits of France, India, Burmah, and China. In both North and
+  South America fossil remains of tapirs occur only in caves and
+  deposits of Post-Pliocene age, showing that they are
+  comparatively recent immigrants into that continent. They perhaps
+  <a name="Page_353" id="Page_353"></a>entered by the route of
+  Kamchatka and Alaska, where the climate, even now so much milder
+  and more equable than on the north-east of America, might have
+  been warm enough in late Pliocene times to have allowed the
+  migration of these animals. In Asia they were driven southwards
+  by the competition of numerous higher and more powerful forms,
+  but have found a last resting-place in the swampy forests of the
+  Malay region.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>What these Facts Prove.</i></p>
+
+  <p>Now these two cases, of the marsupials and the tapirs, are in
+  the highest degree instructive, because they show us that,
+  without any hypothetical bridging of deep oceans, and with only
+  such changes of sea and land as are indicated by the extent of
+  the comparatively shallow seas surrounding and connecting the
+  existing continents, we are able to account for the anomaly of
+  allied forms occurring only in remote and widely separated areas.
+  These examples really constitute crucial tests, because, of all
+  classes of animals, mammalia are least able to surmount physical
+  barriers. They are obviously unable to pass over wide arms of the
+  sea, while the necessity for constant supplies of food and water
+  renders sandy deserts or snow-clad plains equally impassable.
+  Then, again, the peculiar kinds of food on which alone many of
+  them can subsist, and their liability to the attacks of other
+  animals, put a further check upon their migrations. In these
+  respects almost all other organisms have great advantages over
+  mammals. Birds can often fly long distances, and can thus cross
+  arms of the sea, deserts, or mountain ranges; insects not only
+  fly, but are frequently carried great distances by gales of wind,
+  as shown by the numerous cases of their visits to ships hundreds
+  of miles from land. Reptiles, though slow of movement, have
+  advantages in their greater capacity for enduring hunger or
+  thirst, their power of resisting cold or drought in a state of
+  torpidity, and they have also some facilities for migration
+  across the sea by means of their eggs, which may be conveyed in
+  crevices of timber or among masses of floating vegetable matter.
+  And when we come to the vegetable kingdom, the means of transport
+  are at their maximum, numbers of seeds having special adaptations
+  <a name="Page_354" id="Page_354"></a>for being carried by
+  mammalia or birds, and for floating in the water, or through the
+  air, while many are so small and so light that there is
+  practically no limit to the distances they may be carried by
+  gales and hurricanes.</p>
+
+  <p>We may, therefore, feel quite certain that the means of
+  distribution that have enabled the larger mammalia to reach the
+  most remote regions from a common starting-point, will be at
+  least as efficacious, and usually far more efficacious, with all
+  other land animals and plants; and if in every case the existing
+  distribution of this class can be explained on the theory of
+  oceanic and continental permanence, with the limited changes of
+  sea and land already referred to, no valid objections can be
+  taken against this theory founded on anomalies of distribution in
+  other orders. Yet nothing is more common than for students of
+  this or that group to assort that the theory of oceanic
+  permanence is quite inconsistent with the distribution of its
+  various species and genera. Because a few Indian genera and
+  closely allied species of birds are found in Madagascar, a land
+  termed "Lemuria" has been supposed to have united the two
+  countries during a comparatively recent geological epoch; while
+  the similarity of fossil plants and reptiles, from the Permian
+  and Miocene formations of India and South Africa, has been
+  adduced as further evidence of this connection. But there are
+  also genera of snakes, of insects, and of plants, common to
+  Madagascar and South America only, which have been held to
+  necessitate a direct land connection between these countries.
+  These views evidently refute themselves, because any such land
+  connections must have led to a far greater similarity in the
+  productions of the several countries than actually exists, and
+  would besides render altogether inexplicable the absence of all
+  the chief types of African and Indian mammalia from Madagascar,
+  and its marvellous individuality in every department of the
+  organic world.<a name="FNanchor_169_169" id=
+  "FNanchor_169_169"></a><a href="#Footnote_169_169" class=
+  "fnanchor">[169]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Powers of Dispersal as illustrated by Insular
+  Organisms.</i></p>
+
+  <p>Having arrived at the conclusion that our existing oceans have
+  remained practically unaltered throughout the Tertiary and
+  Secondary periods of geology, and that the distribution of the
+  <a name="Page_355" id="Page_355"></a>mammalia is such as might
+  have been brought about by their known powers of dispersal, and
+  by such changes of land and sea as have probably or certainly
+  occurred, we are, of course, restricted to similar causes to
+  explain the much wider and sometimes more eccentric distribution
+  of other classes of animals and of plants. In doing so, we have
+  to rely partly on direct evidence of dispersal, afforded by the
+  land organisms that have been observed far out at sea, or which
+  have taken refuge on ships, as well as by the periodical
+  visitants to remote islands; but very largely on indirect
+  evidence, afforded by the frequent presence of certain groups on
+  remote oceanic islands, which some ancestral forms must,
+  therefore, have reached by transmission across the ocean from
+  distant lands.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Birds.</i></p>
+
+  <p>These vary much in their powers of flight, and their
+  capability of traversing wide seas and oceans. Many swimming and
+  wading birds can continue long on the wing, fly swiftly, and
+  have, besides, the power of resting safely on the surface of the
+  water. These would hardly be limited by any width of ocean,
+  except for the need of food; and many of them, as the gulls,
+  petrels, and divers, find abundance of food on the surface of the
+  sea itself. These groups have a wide distribution <i>across</i>
+  the oceans; while waders&mdash;especially plovers, sandpipers,
+  snipes, and herons&mdash;are equally cosmopolitan, travelling
+  <i>along</i> the coasts of all the continents, and across the
+  narrow seas which separate them. Many of these birds seem
+  unaffected by climate, and as the organisms on which they feed
+  are equally abundant on arctic, temperate, and tropical shores,
+  there is hardly any limit to the range even of some of the
+  species.</p>
+
+  <p>Land-birds are much more restricted in their range, owing to
+  their usually limited powers of flight, their inability to rest
+  on the surface of the sea or to obtain food from it, and their
+  greater specialisation, which renders them less able to maintain
+  themselves in the new countries they may occasionally reach. Many
+  of them are adapted to live only in woods, or in marshes, or in
+  deserts; they need particular kinds of food or a limited range of
+  temperature; and they are adapted to cope only with the special
+  enemies or the particular group of <a name="Page_356" id=
+  "Page_356"></a>competitors among which they have been developed.
+  Such birds as these may pass again and again to a new country,
+  but are never able to establish themselves in it; and it is this
+  organic barrier, as it is termed, rather than any physical
+  barrier, which, in many cases, determines the presence of a
+  species in one area and its absence from another. We must always
+  remember, therefore, that, although the presence of a species in
+  a remote oceanic island clearly proves that its ancestors must at
+  one time have found their way there, the absence of a species
+  does not prove the contrary, since it also may have reached the
+  island, but have been unable to maintain itself, owing to the
+  inorganic or organic conditions not being suitable to it. This
+  general principle applies to all classes of organisms, and there
+  are many striking illustrations of it. In the Azores there are
+  eighteen species of land-birds which are permanent residents, but
+  there are also several others which reach the islands almost
+  every year after great storms, but have never been able to
+  establish themselves. In Bermuda the facts are still more
+  striking, since there are only ten species of resident birds,
+  while no less than twenty other species of land-birds and more
+  than a hundred species of waders and aquatics are frequent
+  visitors, often in great numbers, but are never able to establish
+  themselves. On the same principle we account for the fact that,
+  of the many continental insects and birds that have been let
+  loose, or have escaped from confinement, in this country, hardly
+  one has been able to maintain itself, and the same phenomenon is
+  still more striking in the case of plants. Of the thousands of
+  hardy plants which grow easily in our gardens, very few have ever
+  run wild, and when the experiment is purposely tried it
+  invariably fails. Thus A. de Candolle informs us that several
+  botanists of Paris, Geneva, and especially of Montpellier, have
+  sown the seeds of many hundreds of species of exotic hardy
+  plants, in what appeared to be the most favourable situations,
+  but that in hardly a single case has any one of them become
+  naturalised.<a name="FNanchor_170_170" id=
+  "FNanchor_170_170"></a><a href="#Footnote_170_170" class=
+  "fnanchor">[170]</a> Still more, then, in plants than in animals
+  the absence of a species does not prove that it has never reached
+  the locality, but merely that it has not been able to maintain
+  itself in com<a name="Page_357" id="Page_357"></a>petition with
+  the native productions. In other cases, as we have seen, facts of
+  an exactly opposite nature occur. The rat, the pig, and the
+  rabbit, the water-cress, the clover, and many other plants, when
+  introduced into New Zealand, nourish exceedingly, and even
+  exterminate their native competitors; so that in these cases we
+  may feel sure that the species in question did not exist in New
+  Zealand simply because they had been unable to reach that country
+  by their natural means of dispersal. I will now give a few cases,
+  in addition to those recorded in my previous works, of birds and
+  insects which have been observed far from any land.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Birds and Insects at Sea.</i></p>
+
+  <p>Captain D. Fullarton of the ship <i>Timaru</i> recorded in his
+  log the occurrence of a great number of small land-birds about
+  the ship on 15th March 1886, when in Lat. 48&deg; 31' N., Long.
+  8&deg; 16' W. He says: "A great many small land-birds about us;
+  put about sixty into a coop, evidently tired out." And two days
+  later, 17th March, "Over fifty of the birds cooped on 15th died,
+  though fed. Sparrows, finches, water-wagtails, two small birds,
+  name unknown, one kind like a linnet, and a large bird like a
+  starling. In all there have been on board over seventy birds,
+  besides some that hovered about us for some time and then fell
+  into the sea exhausted." Easterly winds and severe weather were
+  experienced at the time.<a name="FNanchor_171_171" id=
+  "FNanchor_171_171"></a><a href="#Footnote_171_171" class=
+  "fnanchor">[171]</a> The spot where this remarkable flight of
+  birds was met with is about 160 miles due west of Brest, and this
+  is the least distance the birds must have been carried. It is
+  interesting to note that the position of the ship is nearly in
+  the line from the English and French coasts to the Azores, where,
+  after great storms, so many bird stragglers arrive annually.
+  These birds were probably blown out to sea during their spring
+  migration along the south coast of England to Wales and Ireland.
+  During the autumnal migration, however, great flocks of
+  birds&mdash;especially starlings, thrushes, and
+  fieldfares&mdash;have been observed every year flying out to sea
+  from the west coast of Ireland, almost the whole of which must
+  perish. At the Nash Lighthouse, in the Bristol Channel on the
+  coast of Glamorganshire, an enormous number of small birds were
+  observed on 3d September, includ<a name="Page_358" id=
+  "Page_358"></a>ing nightjars, buntings, white-throats,
+  willow-wrens, cuckoos, house-sparrows, robins, wheatears, and
+  blackbirds. These had probably crossed from Somersetshire, and
+  had they been caught by a storm the larger portion of them must
+  have been blown out to sea.<a name="FNanchor_172_172" id=
+  "FNanchor_172_172"></a><a href="#Footnote_172_172" class=
+  "fnanchor">[172]</a></p>
+
+  <p>These facts enable us to account sufficiently well for the
+  birds of oceanic islands, the number and variety of which are
+  seen to be proportionate to their facilities for reaching the
+  island and maintaining themselves in it. Thus, though more birds
+  yearly reach Bermuda than the Azores, the number of residents in
+  the latter islands is much larger, due to the greater extent of
+  the islands, their number, and their more varied surface. In the
+  Galapagos the land-birds are still more numerous, due in part to
+  their larger area and greater proximity to the continent, but
+  chiefly to the absence of storms, so that the birds which
+  originally reached the islands have remained long isolated and
+  have developed into many closely allied species adapted to the
+  special conditions. All the species of the Galapagos but one are
+  peculiar to the islands, while the Azores possess only one
+  peculiar species, and Bermuda none&mdash;a fact which is clearly
+  due to the continual immigration of fresh individuals keeping up
+  the purity of the breed by intercrossing. In the Sandwich
+  Islands, which are extremely isolated, being more than 2000 miles
+  from any continent or large island, we have a condition of things
+  similar to what prevails in the Galapagos, the land-birds,
+  eighteen in number, being all peculiar, and belonging, except
+  one, to peculiar genera. These birds have probably all descended
+  from three or four original types which reached the islands at
+  some remote period, probably by means of intervening islets that
+  have since disappeared. In St. Helena we have a degree of
+  permanent isolation which has prevented any land-birds from
+  reaching the island; for although its distance from the
+  continent, 1100 miles, is not so great as in the case of the
+  Sandwich Islands, it is situated in an ocean almost entirely
+  destitute of small islands, while its position within the tropics
+  renders it free from violent storms. Neither is there, on the
+  nearest part of the coast of Africa, a perpetual stream of
+  migrating birds like that which <a name="Page_359" id=
+  "Page_359"></a>supplies the innumerable stragglers which every
+  year reach Bermuda and the Azores.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Insects.</i></p>
+
+  <p>Winged insects have been mainly dispersed in the same way as
+  birds, by their power of flight, aided by violent or
+  long-continued winds. Being so small, and of such low specific
+  gravity, they are occasionally carried to still greater
+  distances; and thus no islands, however remote, are altogether
+  without them. The eggs of insects, being often deposited in
+  borings or in crevices of timber, may have been conveyed long
+  distances by floating trees, as may the larvae of those species
+  which feed on wood. Several cases have been published of insects
+  coming on board ships at great distances from land; and Darwin
+  records having caught a large grasshopper when the ship was 370
+  miles from the coast of Africa, whence the insect had probably
+  come.</p>
+
+  <p>In the <i>Entomologists' Monthly Magazine</i> for June 1885,
+  Mr. MacLachlan has recorded the occurrence of a swarm of moths in
+  the Atlantic ocean, from the log of the ship <i>Pleione</i>. The
+  vessel was homeward bound from New Zealand, and in Lat. 6&deg;
+  47' N., Long. 32&deg; 50' W., hundreds of moths appeared about
+  the ship, settling in numbers on the spars and rigging. The wind
+  for four days previously had been very light from north,
+  north-west, or north-east, and sometimes calm. The north-east
+  trade wind occasionally extends to the ship's position at that
+  time of year. The captain adds that "frequently, in that part of
+  the ocean, he has had moths and butterflies come on board." The
+  position is 960 miles south-west of the Cape Verde Islands, and
+  about 440 north-east of the South American coast. The specimen
+  preserved is Deiopeia pulchella, a very common species in dry
+  localities in the Eastern tropics, and rarely found in Britain,
+  but, Mr. MacLachlan thinks, not found in South America. They must
+  have come, therefore, from the Cape Verde Islands, or from some
+  parts of the African coast, and must have traversed about a
+  thousand miles of ocean with the assistance, no doubt, of a
+  strong north-east trade wind for a great part of the distance. In
+  the British Museum collection there is a specimen of the same
+  moth caught at sea during the voyage of the <i>Rattlesnake</i>,
+  <a name="Page_360" id="Page_360"></a>in Lat. 6&deg; N., Long.
+  22-1/2&deg; W., being between the former position and Sierra
+  Leone, thus rendering it probable that the moths came from that
+  part of the African coast, in which case the swarm encountered by
+  the <i>Pleione</i> must have travelled more than 1200 miles.</p>
+
+  <p>A similar case was recorded by Mr. F.A. Lucas in the American
+  periodical <i>Science</i> of 8th April 1887. He states that in
+  1870 he met with numerous moths of many species while at sea in
+  the South Atlantic (Lat. 25&deg; S., Long. 24&deg; W.), about
+  1000 miles from the coast of Brazil. As this position is just
+  beyond the south-east trades, the insects may have been brought
+  from the land by a westerly gale. In the <i>Zoologist</i> (1864,
+  p. 8920) is the record of a small longicorn beetle which flew on
+  board a ship 500 miles off the west coast of Africa. Numerous
+  other cases are recorded of insects at less distances from land,
+  and, taken in connection with those already given, they are
+  sufficient to show that great numbers must be continually carried
+  out to sea, and that occasionally they are able to reach enormous
+  distances. But the reproductive powers of insects are so great
+  that all we require, in order to stock a remote island, is that
+  some few specimens shall reach it even once in a century, or once
+  in a thousand years.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Insects at great Altitudes.</i></p>
+
+  <p>Equally important is the proof we possess that insects are
+  often carried to great altitudes by upward currents of air.
+  Humboldt noticed them up to heights of 15,000 and 18,000 feet in
+  South America, and Mr. Albert M&uuml;ller has collected many
+  interesting cases of the same character in Europe.<a name=
+  "FNanchor_173_173" id="FNanchor_173_173"></a><a href=
+  "#Footnote_173_173" class="fnanchor">[173]</a> A moth (Plusia
+  gamma) has been found on the summit of Mont Blanc; small
+  hymenoptera and moths have been seen on the Pyrenees at a height
+  of 11,000 feet, while numerous flies and beetles, some of
+  considerable size, have been caught on the glaciers and
+  snow-fields of various parts of the Alps. Upward currents of air,
+  whirlwinds and tornadoes, occur in all parts of the world, and
+  large numbers of insects are thus carried up into the higher
+  regions of the atmosphere, where they are liable to be caught by
+  strong winds, and thus conveyed enormous distances over seas or
+  continents. With such <a name="Page_361" id=
+  "Page_361"></a>powerful means of dispersal the distribution of
+  insects over the entire globe, and their presence in the most
+  remote oceanic islands, offer no difficulties.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Dispersal of Plants.</i></p>
+
+  <p>The dispersal of seeds is effected in a greater variety of
+  ways than are available in the case of any animals. Some fruits
+  or seed-vessels, and some seeds, will float for many weeks, and
+  after immersion in salt water for that period the seeds will
+  often germinate. Extreme cases are the double cocoa-nut of the
+  Seychelles, which has been found on the coast of Sumatra, about
+  3000 miles distant; the fruits of the Sapindus saponaria
+  (soap-berry), which has been brought to Bermuda by the Gulf
+  Stream from the West Indies, and has grown after a journey in the
+  sea of about 1500 miles; and the West Indian bean, Entada
+  scandens, which reached the Azores from the West Indies, a
+  distance of full 3000 miles, and afterwards germinated at Kew. By
+  these means we can account for the similarity in the shore flora
+  of the Malay Archipelago and most of the islands of the Pacific;
+  and from an examination of the fruits and seeds, collected among
+  drift during the voyage of the <i>Challenger</i>, Mr. Hemsley has
+  compiled a list of 121 species which are probably widely
+  dispersed by this means.</p>
+
+  <p>A still larger number of species owe their dispersal to birds
+  in several distinct ways. An immense number of fruits in all
+  parts of the world are devoured by birds, and have been
+  attractively coloured (as we have seen), in order to be so
+  devoured, because the seeds pass through the birds' bodies and
+  germinate where they fall. We have seen how frequently birds are
+  forced by gales of wind across a wide expanse of ocean, and thus
+  seeds must be occasionally carried. It is a very suggestive fact,
+  that all the trees and shrubs in the Azores bear berries or small
+  fruits which are eaten by birds; while all those which bear
+  larger fruits, or are eaten chiefly by mammals&mdash;such as
+  oaks, beeches, hazels, crabs, etc.&mdash;are entirely wanting.
+  Game-birds and waders often have portions of mud attached to
+  their feet, and Mr. Darwin has proved by experiment that such mud
+  frequently contains seeds. One partridge had such a quantity of
+  mud attached to its foot as to contain seeds from which
+  eighty-two plants germinated; this proves that <a name="Page_362"
+  id="Page_362"></a>a very small portion of mud may serve to convey
+  seeds, and such an occurrence repeated even at long intervals may
+  greatly aid in stocking remote islands with vegetation. Many
+  seeds also adhere to the feathers of birds, and thus, again, may
+  be conveyed as far as birds are ever carried. Dr. Guppy found a
+  small hard seed in the gizzard of a Cape Petrel, taken about 550
+  miles east of Tristan da Cunha.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Dispersal of Seeds by the Wind.</i></p>
+
+  <p>In the preceding cases we have been able to obtain direct
+  evidence of transportal; but although we know that many seeds are
+  specially adapted to be dispersed by the wind, we cannot obtain
+  direct proof that they are so carried for hundreds or thousands
+  of miles across the sea, owing to the difficulty of detecting
+  single objects which are so small and inconspicuous. It is
+  probable, however, that the wind as an agent of dispersal is
+  really more effective than any of those we have hitherto
+  considered, because a very large number of plants have seeds
+  which are very small and light, and are often of such a form as
+  to facilitate aerial carriage for enormous distances. It is
+  evident that such seeds are especially liable to be transported
+  by violent winds, because they become ripe in autumn at the time
+  when storms are most prevalent, while they either lie upon the
+  surface of the ground, or are disposed in dry capsules on the
+  plant ready to be blown away. If inorganic particles comparable
+  in weight, size, or form with such seeds are carried for great
+  distances, we may be sure that seeds will also be occasionally
+  carried in the same way. It will, therefore, be necessary to give
+  a few examples of wind-carriage of small objects.</p>
+
+  <p>On 27th July 1875 a remarkable shower of small pieces of hay
+  occurred at Monkstown, near Dublin. They appeared floating slowly
+  down from a great height, as if falling from a dark cloud which
+  hung overhead. The pieces picked up were wet, and varied from
+  single blades of grass to tufts weighing one or two ounces. A
+  similar shower occurred a few days earlier in Denbighshire, and
+  was observed to travel in a direction contrary to that of the
+  wind in the lower atmosphere.<a name="FNanchor_174_174" id=
+  "FNanchor_174_174"></a><a href="#Footnote_174_174" class=
+  "fnanchor">[174]</a> There is no evidence of the distance from
+  which the hay was <a name="Page_363" id="Page_363"></a>brought,
+  but as it had been carried to a great height, it was in a
+  position to be conveyed to almost any distance by a violent wind,
+  had such occurred at the time.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Mineral Matter carried by the Wind.</i></p>
+
+  <p>The numerous cases of sand and volcanic dust being carried
+  enormous distances through the atmosphere sufficiently prove the
+  importance of wind as a carrier of solid matter, but
+  unfortunately the matter collected has not been hitherto examined
+  with a view to determine the maximum size and weight of the
+  particles. A few facts, however, have been kindly furnished me by
+  Professor Judd, F.R.S. Some dust which fell at Genoa on 15th
+  October 1885, and was believed to have been brought from the
+  African desert, consisted of quartz, hornblende, and other
+  minerals, and contained particles having a diameter of 1/500
+  inch, each weighing 1/200,000 grain. This dust had probably
+  travelled over 600 miles. In the dust from Krakatoa, which fell
+  at Batavia, about 100 miles distant, during the great eruption,
+  there are many solid particles even larger than those mentioned
+  above. Some of this dust was given me by Professor Judd, and I
+  found in it several ovoid particles of a much larger size, being
+  1/50 inch long, and 1/70 wide and deep. The dust from the same
+  eruption, which fell on board the ship <i>Arabella</i>, 970 miles
+  from the volcano, also contained solid particles 1/500 inch
+  diameter. Mr. John Murray of the <i>Challenger</i> Expedition
+  writes to me that he finds in the deep sea deposits 500 and even
+  700 miles west of the coast of Africa, rounded particles of
+  quartz, having a diameter of 1/250 inch, and similar particles
+  are found at equally great distances from the south-west coasts
+  of Australia; and he considers these to be atmospheric dust
+  carried to that distance by the wind. Taking the sp. gr. of
+  quartz at 2.6, these particles would weigh about 1/25,000 grain
+  each. These interesting facts can, however, by no means be taken
+  as indicating the extreme limits of the power of wind in carrying
+  solid particles. During the Krakatoa eruption no gale of special
+  violence occurred, and the region is one of comparative calms.
+  The grains of quartz found by Mr. Murray more nearly indicate the
+  limit, but the very small portions of matter brought up by the
+  dredge, as com<a name="Page_364" id="Page_364"></a>pared with the
+  enormous areas of sea-bottom, over which the atmospheric dust
+  must have been scattered, render it in the highest degree
+  improbable that the maximum limit either of size of particles, or
+  of distance from land has been reached.</p>
+
+  <p>Let us, however, assume that the quartz grains, found by Mr.
+  Murray in the deep-sea ooze 700 miles from land, give us the
+  extreme limit of the power of the atmosphere as a carrier of
+  solid particles, and let us compare with these the weights of
+  some seeds. From a small collection of the seeds of thirty
+  species of herbaceous plants sent me from Kew, those in the above
+  table were selected, and small portions of eight of them
+  carefully weighed in a chemical balance.<a name=
+  "FNanchor_175_175" id="FNanchor_175_175"></a><a href=
+  "#Footnote_175_175" class="fnanchor">[175]</a> By counting these
+  portions I was able to estimate the number of seeds weighing one
+  grain. The three very minute species, whose numbers are marked
+  with an asterisk (*), were estimated by the comparison of their
+  sizes with those of the smaller weighed seeds.</p>
+
+<pre>
+No|    Species.         |Approximate |      Approximate  |  Remarks.
+  |                     |No. of Seeds|      Dimensions.  |
+  |                     |In one Grain|                   |
+  |                     |            | in.    in.    in. |
+ 1|Draba verna          |  1,800     |1/60 x 1/90 x 1/160|Oval, flat.
+ 2|Hypericum perforatum |    520     |      1/30 x 1/80  |Cylindrical.
+ 3|Astilbe rivularis    |  4,500     |      1/50 x 1/100 |Elongate, flat, tailed,
+  |                     |            |                   |   wavy.
+ 4|Saxifraga coriophylla|    750     |      1/40 x 1/75  |Surface rough, adhere
+  |                     |            |                   |   to the dry capsules.
+ 5|Oenothera rosea      |    640     |  1/40 x 1/80      |Ovate.
+ 6|Hypericum hirsutum   |    700     |      1/30 x 1/100 |Cylindrical, rough.
+ 7|Mimulus luteus       |  2,900     |      1/60 x 1/100 |Oval, minute.
+ 8|Penthorum sedoides   |  8,000*    |      1/70 x 1/150 |Flattened, very minute.
+ 9|Sagina procumbens    | 12,000*    |          1/120    |Sub-triangular, flat.
+10|Orchis maculata      | 15,000*    |           ---     |Margined, flat,
+  |                     |            |                   | very minute.
+11|Gentiana purpurea    |     35     |           1/25    |Wavy, rough, with this
+  |                     |            |                   |   coriaceous margins.
+12|Silene alpina        |   ---      |           1/30    |Flat, with fringed
+  |                     |            |                   |   margins.
+13|Adenophora communis  |   ---      |       1/20 x 1/40 |Very thin, wavy, light.
+  |Quartz grains        | 25,000     |           1/250   |Deep sea ... 700 miles.
+  |Do.                  |200,000     |           1/500   |Genoa ... 600 miles.
+  </pre>
+
+  <p>If now we compare the seeds with the quartz grains, we
+  <a name="Page_365" id="Page_365"></a>find that several are from
+  twice to three times the weight of the grains found by Mr.
+  Murray, and others five times, eight times, and fifteen times as
+  heavy; but they are proportionately very much larger, and, being
+  usually irregular in shape or compressed, they expose a very much
+  larger surface to the air. The surface is often rough, and
+  several have dilated margins or tailed appendages, increasing
+  friction and rendering the uniform rate of falling through still
+  air immensely less than in the case of the smooth, rounded, solid
+  quartz grains. With these advantages it is a moderate estimate
+  that seeds ten times the weight of the quartz grains could be
+  carried quite as far through the air by a violent gale and under
+  the most favourable conditions. These limits will include five of
+  the seeds here given, as well as hundreds of others which do not
+  exceed them in weight; and to these we may add some larger seeds
+  which have other favourable characteristics, as is the case with
+  numbers 11-13, which, though very much larger than the rest, are
+  so formed as in all probability to be still more easily carried
+  great distances by a gale of wind. It appears, therefore, to be
+  absolutely certain that every autumnal gale capable of conveying
+  solid mineral particles to great distances, must also carry
+  numbers of small seeds at least as far; and if this is so, the
+  wind alone will form one of the most effective agents in the
+  dispersal of plants.</p>
+
+  <p>Hitherto this mode of conveyance, as applying to the
+  transmission of seeds for great distances across the ocean, has
+  been rejected by botanists, for two reasons. In the first place,
+  there is said to be no direct evidence of such conveyance; and,
+  secondly, the peculiar plants of remote oceanic islands do not
+  appear to have seeds specially adapted for aerial transmission. I
+  will consider briefly each of these objections.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Objection to the Theory of Wind-Dispersal.</i></p>
+
+  <p>To obtain direct evidence of the transmission of such minute
+  and perishable objects, which do not exist in great quantities,
+  and are probably carried to the greatest distances but rarely and
+  as single specimens, is extremely difficult. A bird or insect can
+  be seen if it comes on board ship, but who would ever detect the
+  seeds of Mimulus or Orchis even if a score of them fell on a
+  ship's deck? Yet if but one such seed <a name="Page_366" id=
+  "Page_366"></a>per century were carried to an oceanic island,
+  that island might become rapidly overrun by the plant, if the
+  conditions were favourable to its growth and reproduction. It is
+  further objected that search has been made for such seeds, and
+  they have not been found. Professor Kerner of Innsbruck examined
+  the snow on the surface of glaciers, and assiduously collected
+  all the seeds he could find, and these were all of plants which
+  grew in the adjacent mountains or in the same district. In like
+  manner, the plants growing on moraines were found to be those of
+  the adjacent mountains, plateaux, or lowlands. Hence he concluded
+  that the prevalent opinion that seeds may be carried through the
+  air for very great distances "is not supported by fact."<a name=
+  "FNanchor_176_176" id="FNanchor_176_176"></a><a href=
+  "#Footnote_176_176" class="fnanchor">[176]</a> The opinion is
+  certainly not supported by Kerner's facts, but neither is it
+  opposed by them. It is obvious that the seeds that would be
+  carried by the wind to moraines or to the surface of glaciers
+  would be, first and in the greatest abundance, those of the
+  immediately surrounding district; then, very much more rarely,
+  those from more remote mountains; and lastly, in extreme rarity,
+  those from distant countries or altogether distinct mountain
+  ranges. Let us suppose the first to be so abundant that a single
+  seed could be found by industrious search on each square yard of
+  the surface of the glacier; the second so scarce that only one
+  could possibly be found in a hundred yards square; while to find
+  one of the third class it would be necessary exhaustively to
+  examine a square mile of surface. Should we expect that
+  <i>one</i> ever to be found, and should the fact that it could
+  not be found be taken as a proof that it was not there? Besides,
+  a glacier is altogether in a bad position to receive such remote
+  wanderers, since it is generally surrounded by lofty mountains,
+  often range behind range, which would intercept the few air-borne
+  seeds that might have been carried from a distant land. The
+  conditions in an oceanic island, on the other hand, are the most
+  favourable, since the land, especially if high, will intercept
+  objects carried by the wind, and will thus cause more of the
+  solid matter to fall on it than on an equal area of ocean. We
+  know that winds at sea often blow violently for days together,
+  and the rate of motion is indicated by the fact that 72 miles an
+  hour was the average velocity <a name="Page_367" id=
+  "Page_367"></a>of the wind observed during twelve hours at the
+  Ben Nevis observatory, while the velocity sometimes rises to 120
+  miles an hour. A twelve hours' gale might, therefore, carry light
+  seeds a thousand miles as easily and certainly as it could carry
+  quartz-grains of much greater specific gravity, rotundity, and
+  smoothness, 500 or even 100 miles; and it is difficult even to
+  imagine a sufficient reason why they should not be so
+  carried&mdash;perhaps very rarely and under exceptionally
+  favourable conditions,&mdash;but this is all that is
+  required.</p>
+
+  <p>As regards the second objection, it has been observed that
+  orchideae, which have often exceedingly small and light seeds,
+  are remarkably absent from oceanic islands. This, however, may be
+  very largely due to their extreme specialisation and dependence
+  on insect agency for their fertilisation; while the fact that
+  they do occur in such very remote islands as the Azores, Tahiti,
+  and the Sandwich Islands, proves that they must have once reached
+  these localities either by the agency of birds or by transmission
+  through the air; and the facts I have given above render the
+  latter mode at least as probable as the former. Sir Joseph Hooker
+  remarks on the composite plant of Kerguelen Island (Cotula
+  plumosa) being found also on Lord Auckland and MacQuarrie
+  Islands, and yet having no pappus, while other species of the
+  genus possess it. This is certainly remarkable, and proves that
+  the plant must have, or once have had, some other means of
+  dispersal across wide oceans.<a name="FNanchor_177_177" id=
+  "FNanchor_177_177"></a><a href="#Footnote_177_177" class=
+  "fnanchor">[177]</a> One of the most widely dispersed species in
+  the whole world (Sonchus oleraceus) possesses pappus, as do four
+  out of five of the species which are common to Europe and New
+  Zealand, all of which have a very wide distribution. The same
+  author remarks on the limited area occupied by most species of
+  Compositae, notwithstanding their facilities for dispersal by
+  means of their feathered seeds; but it has been <a name=
+  "Page_368" id="Page_368"></a>already shown that limitations of
+  area are almost always due to the competition of allied forms,
+  facilities for dispersal being only one of many factors in
+  determining the wide range of species. It is, however, a
+  specially important factor in the case of the inhabitants of
+  remote oceanic islands, since, whether they are peculiar species
+  or not, they or their remote ancestors must at some time or other
+  have reached their present position by natural means.</p>
+
+  <p>I have already shown elsewhere, that the flora of the Azores
+  strikingly supports the view of the species having been
+  introduced by aerial transmission only, that is, by the agency of
+  birds and the wind, because all plants that could not possibly
+  have been carried by these means are absent.<a name=
+  "FNanchor_178_178" id="FNanchor_178_178"></a><a href=
+  "#Footnote_178_178" class="fnanchor">[178]</a> In the same way we
+  may account for the extreme rarity of Leguminosae in all oceanic
+  islands. Mr. Hemsley, in his Report on Insular Floras, says that
+  they "are wanting in a large number of oceanic islands where
+  there is no true littoral flora," as St. Helena, Juan Fernandez,
+  and all the islands of the South Atlantic and South Indian
+  Oceans. Even in the tropical islands, such as Mauritius and
+  Bourbon, there are no endemic species, and very few in the
+  Galapagos and the remoter Pacific Islands. All these facts are
+  quite in accordance with the absence of facilities for
+  transmission through the air, either by birds or the wind, owing
+  to the comparatively large size and weight of the seeds; and an
+  additional proof is thus afforded of the extreme rarity of the
+  successful floating of seeds for great distances across the
+  ocean.<a name="FNanchor_179_179" id=
+  "FNanchor_179_179"></a><a href="#Footnote_179_179" class=
+  "fnanchor">[179]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Explanation of North Temperate Plants in the Southern
+  Hemisphere.</i></p>
+
+  <p>If we now admit that many seeds which are either minute in
+  size, of thin texture or wavy form, or so fringed or margined as
+  to afford a good hold to the air, are capable of being carried
+  for many hundreds of miles by exceptionally <a name="Page_369"
+  id="Page_369"></a>violent and long-continued gales of wind, we
+  shall not only be better able to account for the floras of some
+  of the remotest oceanic islands, but shall also find in the fact
+  a sufficient explanation of the wide diffusion of many genera,
+  and even species, of arctic and north temperate plants in the
+  southern hemisphere or on the summits of tropical mountains.
+  Nearly fifty of the flowering plants of Tierra-del-Fuego are
+  found also in North America or Europe, but in no intermediate
+  country; while fifty-eight species are common to New Zealand and
+  Northern Europe; thirty-eight to Australia, Northern Europe, and
+  Asia; and no less than seventy-seven common to New Zealand,
+  Australia, and South America.<a name="FNanchor_180_180" id=
+  "FNanchor_180_180"></a><a href="#Footnote_180_180" class=
+  "fnanchor">[180]</a> On lofty mountains far removed from each
+  other, identical or closely allied plants often occur. Thus the
+  fine Primula imperialis of a single mountain peak in Java has
+  been found (or a closely allied species) in the Himalayas; and
+  many other plants of the high mountains of Java, Ceylon, and
+  North India are either identical or closely allied forms. So, in
+  Africa, some species, found on the summits of the Cameroons and
+  Fernando Po in West Africa, are closely allied to species in the
+  Abyssinian highlands and in Temperate Europe; while other
+  Abyssinian and Cameroons species have recently been found on the
+  mountains of Madagascar. Some peculiar Australian forms have been
+  found represented on the summit of Kini Balu in Borneo. Again, on
+  the summit of the Organ mountains in Brazil there are species
+  allied to those of the Andes, but not found in the intervening
+  lowlands.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>No Proof of Recent Lower Temperature in the Tropics.</i></p>
+
+  <p>Now all these facts, and numerous others of like character,
+  were supposed by Mr. Darwin to be due to a lowering of
+  temperature during glacial epochs, which allowed these temperate
+  forms to migrate across the intervening tropical lowlands. But
+  any such change within the epoch of existing species is almost
+  inconceivable. In the first place, it would necessitate the
+  extinction of much of the tropical flora (and with it of the
+  insect life), because without such extinction alpine herbaceous
+  plants could certainly never spread over tropical forest
+  low<a name="Page_370" id="Page_370"></a>lands; and, in the next
+  place, there is not a particle of direct evidence that any such
+  lowering of temperature in inter-tropical lowlands ever took
+  place. The only alleged evidence of the kind is that adduced by
+  the late Professor Agassiz and Mr. Hartt; but I am informed by my
+  friend, Mr. J.C. Branner (now State Geologist of Arkansas, U.S.),
+  who succeeded Mr. Hartt, and spent several years completing the
+  geological survey of Brazil, that the supposed moraines and
+  glaciated granite rocks near Rio Janeiro and elsewhere, as well
+  as the so-called boulder-clay of the same region, are entirely
+  explicable as the results of sub-aerial denudation and
+  weathering, and that there is no proof whatever of glaciation in
+  any part of Brazil.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Lower Temperature not needed to Explain the Facts.</i></p>
+
+  <p>But any such vast physical change as that suggested by Darwin,
+  involving as it does such tremendous issues as regards its
+  effects on the tropical fauna and flora of the whole world, is
+  really quite uncalled for, because the facts to be explained are
+  of the same essential nature as those presented by remote oceanic
+  islands, between which and the nearest continents no temperate
+  land connection is postulated. In proportion to their limited
+  area and extreme isolation, the Azores, St. Helena, the
+  Galapagos, and the Sandwich Islands, each possess a fairly
+  rich&mdash;the last a very rich&mdash;indigenous flora; and the
+  means which sufficed to stock them with a great variety of plants
+  would probably suffice to transmit others from mountain-top to
+  mountain-top in various parts of the globe. In the case of the
+  Azores, we have large numbers of species identical with those of
+  Europe, and others closely allied, forming an exactly parallel
+  case to the species found on the various mountain summits which
+  have been referred to. The distances from Madagascar to the South
+  African mountains and to Kilimandjaro, and from the latter to
+  Abyssinia, are no greater than from Spain to the Azores, while
+  there are other equatorial mountains forming stepping-stones at
+  about an equal distance to the Cameroons. Between Java and the
+  Himalayas we have the lofty mountains of Sumatra and of
+  North-western Burma, forming steps at about the same distance
+  apart; while between Kini Balu and the Australian Alps we
+  <a name="Page_371" id="Page_371"></a>have the unexplored snow
+  mountains of New Guinea, the Bellenden Ker mountains in
+  Queensland, and the New England and Blue Mountains of New South
+  Wales. Between Brazil and Bolivia the distances are no greater;
+  while the unbroken range of mountains from Arctic America to
+  Tierra-del-Fuego offers the greatest facilities for transmission,
+  the partial gap between the lofty peak of Chiriqui and the high
+  Andes of New Grenada being far less than from Spain to the
+  Azores. Thus, whatever means have sufficed for stocking oceanic
+  islands must have been to some extent effective in transmitting
+  northern forms from mountain to mountain, across the equator, to
+  the southern hemisphere; while for this latter form of dispersal
+  there are special facilities, in the abundance of fresh and
+  unoccupied surfaces always occurring in mountain regions, owing
+  to avalanches, torrents, mountain-slides, and rock-falls, thus
+  affording stations on which air-borne seeds may germinate and
+  find a temporary home till driven out by the inroads of the
+  indigenous vegetation. These temporary stations may be at much
+  lower altitudes than the original habitat of the species, if
+  other conditions are favourable. Alpine plants often descend into
+  the valleys on glacial moraines, while some arctic species grow
+  equally well on mountain summits and on the seashore. The
+  distances above referred to between the loftier mountains may
+  thus be greatly reduced by the occurrence of suitable conditions
+  at lower altitudes, and the facilities for transmission by means
+  of aerial currents proportionally increased.<a name=
+  "FNanchor_181_181" id="FNanchor_181_181"></a><a href=
+  "#Footnote_181_181" class="fnanchor">[181]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Facts Explained by the Wind-Carriage of Seeds.</i></p>
+
+  <p>But if we altogether reject aerial transmission of seeds for
+  great distances, except by the agency of birds, it will be
+  difficult, if not impossible, to account for the presence of so
+  many identical species of plants on remote mountain summits, or
+  for that "continuous current of vegetation" described by Sir
+  Joseph Hooker as having apparently long existed from the northern
+  to the southern hemisphere. It may be admitted that we can,
+  possibly, account for the greater portion of the floras of remote
+  oceanic islands by the agency of birds alone; because, when blown
+  out to sea land-birds must reach some island <a name="Page_372"
+  id="Page_372"></a>or perish, and all which come within sight of
+  an island will struggle to reach it as their only refuge. But,
+  with mountain summits the case is altogether different, because,
+  being surrounded by land instead of by sea, no bird would need to
+  fly, or to be carried by the wind, for several hundred miles at a
+  stretch to another mountain summit, but would find a refuge in
+  the surrounding uplands, ridges, valleys, or plains. As a rule
+  the birds that frequent lofty mountain tops are peculiar species,
+  allied to those of the surrounding district; and there is no
+  indication whatever of the passage of birds from one remote
+  mountain to another in any way comparable with the flights of
+  birds which are known to reach the Azores annually, or even with
+  the few regular migrants from Australia to New Zealand. It is
+  almost impossible to conceive that the seeds of the Himalayan
+  primula should have been thus carried to Java; but, by means of
+  gales of wind, and intermediate stations from fifty to a few
+  hundred miles apart, where the seeds might vegetate for a year or
+  two and produce fresh seed to be again carried on in the same
+  manner, the transmission might, after many failures, be at last
+  effected.</p>
+
+  <p>A very important consideration is the vastly larger scale on
+  which wind-carriage of seeds must act, as compared with
+  bird-carriage. It can only be a few birds which carry seeds
+  attached to their feathers or feet. A very small proportion of
+  these would carry the seeds of Alpine plants; while an almost
+  infinitesimal fraction of these latter would convey the few seeds
+  attached to them safely to an oceanic island or remote mountain.
+  But winds, in the form of whirlwinds or tornadoes, gales or
+  hurricanes, are perpetually at work over large areas of land and
+  sea. Insects and light particles of matter are often carried up
+  to the tops of high mountains; and, from the very nature and
+  origin of winds, they usually consist of ascending or descending
+  currents, the former capable of suspending such small and light
+  objects as are many seeds long enough for them to be carried
+  enormous distances. For each single seed carried away by external
+  attachment to the feet or feathers of a bird, countless millions
+  are probably carried away by violent winds; and the chance of
+  conveyance to a great distance and in a definite direction must
+  be many <a name="Page_373" id="Page_373"></a>times greater by the
+  latter mode than by the former.<a name="FNanchor_182_182" id=
+  "FNanchor_182_182"></a><a href="#Footnote_182_182" class=
+  "fnanchor">[182]</a> We have seen that inorganic particles of
+  much greater specific gravity than seeds, and nearly as heavy as
+  the smallest kinds, are carried to great distances through the
+  air, and we can therefore hardly doubt that some seeds are
+  carried as far. The direct agency of the wind, as a supplement to
+  bird-transport, will help to explain the presence in oceanic
+  islands of plants growing in dry or rocky places whose small
+  seeds are not likely to become attached to birds; while it seems
+  to be the only effective agency possible in the dispersal of
+  those species of alpine or sub-alpine plants found on the summits
+  of distant mountains, or still more widely separated in the
+  temperate zones of the northern and southern hemispheres.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Concluding Remarks.</i></p>
+
+  <p>On the general principles that have been now laid down, it
+  will be found that all the chief facts of the geographical
+  distribution of animals and plants can be sufficiently
+  understood. There will, of course, be many cases of difficulty
+  and some seeming anomalies, but these can usually be seen to
+  depend on our ignorance of some of the essential factors of the
+  problem. Either we do not know the distribution of the group in
+  recent geological times, or we are still ignorant of the special
+  methods by which the organisms are able to cross the sea. The
+  latter difficulty applies especially to the lizard tribe, which
+  are found <a name="Page_374" id="Page_374"></a>in almost all the
+  tropical oceanic islands; but the particular mode in which they
+  are able to traverse a wide expanse of ocean, which is a perfect
+  barrier to batrachia and almost so to snakes, has not yet been
+  discovered. Lizards are found in all the larger Pacific Islands
+  as far as Tahiti, while snakes do not extend beyond the Fiji
+  Islands; and the latter are also absent from Mauritius and
+  Bourbon, where lizards of seven or eight species abound.
+  Naturalists resident in the Pacific Islands would make a valuable
+  contribution to our science by studying the life-history of the
+  native lizards, and endeavouring to ascertain the special
+  facilities they possess for crossing over wide spaces of
+  ocean.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_163_163" id=
+      "Footnote_163_163"></a><a href=
+      "#FNanchor_163_163"><span class="label">[163]</span></a> See
+      A. Agassiz, <i>Three Cruises of the Blake</i> (Cambridge,
+      Mass., 1888), vol. i. p. 127, footnote.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_164_164" id=
+      "Footnote_164_164"></a><a href=
+      "#FNanchor_164_164"><span class="label">[164]</span></a> Even
+      the extremely fine Mississippi mud is nowhere found beyond a
+      hundred miles from the mouths of the river in the Gulf of
+      Mexico (A. Agassiz, <i>Three Cruises of the Blake</i>, vol.
+      i. p. 128).</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_165_165" id=
+      "Footnote_165_165"></a><a href=
+      "#FNanchor_165_165"><span class="label">[165]</span></a> I
+      have given a full summary of the evidence for the permanence
+      of oceanic and continental areas in my <i>Island Life</i>,
+      chap. vi.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_166_166" id=
+      "Footnote_166_166"></a><a href=
+      "#FNanchor_166_166"><span class="label">[166]</span></a> For
+      a full account of the peculiarities of the Madagascar fauna,
+      see my <i>Island Life</i>, chap. xix.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_167_167" id=
+      "Footnote_167_167"></a><a href=
+      "#FNanchor_167_167"><span class="label">[167]</span></a> See
+      <i>Island Life</i>, p. 446, and the whole of chaps. xxi.
+      xxii. More recent soundings have shown that the Map at p.
+      443, as well as that of the Madagascar group at p. 387, are
+      erroneous, the ocean around Norfolk Island and in the Straits
+      of Mozambique being more than 1000 fathoms deep. The general
+      argument is, however, unaffected.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_168_168" id=
+      "Footnote_168_168"></a><a href=
+      "#FNanchor_168_168"><span class="label">[168]</span></a> For
+      some details of these migrations, see the author's
+      <i>Geographical Distribution of Animals</i>, vol. i. p. 140;
+      also Heilprin's <i>Geographical and Geological Distribution
+      of Animals</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_169_169" id=
+      "Footnote_169_169"></a><a href=
+      "#FNanchor_169_169"><span class="label">[169]</span></a> For
+      a full discussion of this question, see <i>Island Life</i>,
+      pp. 390-420.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_170_170" id=
+      "Footnote_170_170"></a><a href=
+      "#FNanchor_170_170"><span class="label">[170]</span></a>
+      <i>G&eacute;ographie Botanique</i>, p. 798.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_171_171" id=
+      "Footnote_171_171"></a><a href=
+      "#FNanchor_171_171"><span class="label">[171]</span></a>
+      <i>Nature</i>, 1st April 1886.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_172_172" id=
+      "Footnote_172_172"></a><a href=
+      "#FNanchor_172_172"><span class="label">[172]</span></a>
+      Report of the Brit. Assoc. Committee on Migration of Birds
+      during 1886.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_173_173" id=
+      "Footnote_173_173"></a><a href=
+      "#FNanchor_173_173"><span class="label">[173]</span></a>
+      <i>Trans. Ent. Soc.</i>, 1871, p. 184.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_174_174" id=
+      "Footnote_174_174"></a><a href=
+      "#FNanchor_174_174"><span class="label">[174]</span></a>
+      <i>Nature</i> (1875), vol. xii. pp. 279, 298.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_175_175" id=
+      "Footnote_175_175"></a><a href=
+      "#FNanchor_175_175"><span class="label">[175]</span></a> I am
+      indebted to Professor R. Meldola of the Finsbury Technical
+      Institute, and Rev. T.D. Titmas of Charterhouse for
+      furnishing me with the weights required.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_176_176" id=
+      "Footnote_176_176"></a><a href=
+      "#FNanchor_176_176"><span class="label">[176]</span></a> See
+      <i>Nature</i>, vol. vi. p. 164, for a summary of Kerner's
+      paper.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_177_177" id=
+      "Footnote_177_177"></a><a href=
+      "#FNanchor_177_177"><span class="label">[177]</span></a> It
+      seems quite possible that the absence of pappus in this case
+      is a recent adaptation, and that it has been brought about by
+      causes similar to those which have reduced or aborted the
+      wings of insects in oceanic islands. For when a plant has
+      once reached one of the storm-swept islands of the southern
+      ocean, the pappus will be injurious for the same reason that
+      the wings of insects are injurious, since it will lead to the
+      seeds being blown out to sea and destroyed. The seeds which
+      are heaviest and have least pappus will have the best chance
+      of falling on the ground and remaining there to germinate,
+      and this process of selection might rapidly lead to the
+      entire disappearance of the pappus.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_178_178" id=
+      "Footnote_178_178"></a><a href=
+      "#FNanchor_178_178"><span class="label">[178]</span></a> See
+      <i>Island Life</i>, p. 251.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_179_179" id=
+      "Footnote_179_179"></a><a href=
+      "#FNanchor_179_179"><span class="label">[179]</span></a> Mr.
+      Hemsley suggests that it is not so much the difficulty of
+      transmission by floating, as the bad conditions the seeds are
+      usually exposed to when they reach land. Many, even if they
+      germinate, are destroyed by the waves, as Burchell noticed at
+      St. Helena; while even a flat and sheltered shore would be an
+      unsuitable position for many inland plants. Air-borne seeds,
+      on the other hand, may be carried far inland, and so
+      scattered that some of them are likely to reach suitable
+      stations.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_180_180" id=
+      "Footnote_180_180"></a><a href=
+      "#FNanchor_180_180"><span class="label">[180]</span></a> For
+      fuller particulars, see Sir J. Hooker's <i>Introduction to
+      Floras of New Zealand and Australia</i>, and a summary in my
+      <i>Island Life</i>, chaps. xxii. xxiii.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_181_181" id=
+      "Footnote_181_181"></a><a href=
+      "#FNanchor_181_181"><span class="label">[181]</span></a> For
+      a fuller discussion of this subject, see my <i>Island
+      Life</i>, chap. xxiii.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_182_182" id=
+      "Footnote_182_182"></a><a href=
+      "#FNanchor_182_182"><span class="label">[182]</span></a> A
+      very remarkable case of wind conveyance of seeds on a large
+      scale is described in a letter from Mr. Thomas Hanbury to his
+      brother, the late Daniel Hanbury, which has been kindly
+      communicated to me by Mr. Hemsley of Kew. The letter is dated
+      "Shanghai, 1st May 1856," and the passage referred to is as
+      follows:&mdash;</p>
+
+      <div class="blockquot" style='font-size: 100%;'>
+        <p>"For the past three days we have had very warm weather
+        for this time of year, in fact almost as warm as the middle
+        of summer. Last evening the wind suddenly changed round to
+        the north and blew all night with considerable violence,
+        making a great change in the atmosphere.</p>
+
+        <p>"This morning, myriads of small white particles are
+        floating about in the air; there is not a single cloud and
+        no mist, yet the sun is quite obscured by this substance,
+        and it looks like a white fog in England. I enclose thee a
+        sample, thinking it may interest. It is evidently a
+        vegetable production; I think, apparently, some kind of
+        seed."</p>
+      </div>
+
+      <p>Mr. Hemsley adds, that this substance proves to be the
+      plumose seeds of a poplar or willow. In order to produce the
+      effects described&mdash;<i>quite obscuring the sun like a
+      white fog</i>,&mdash;the seeds must have filled the air to a
+      very great height; and they must have been brought from some
+      district where there were extensive tracts covered with the
+      tree which produced them.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_XIII" id="CHAPTER_XIII"></a><a name=
+  "Page_375" id="Page_375"></a>CHAPTER XIII</h2>
+
+  <p style='font-weight:bold;'>THE GEOLOGICAL EVIDENCES OF
+  EVOLUTION</p>
+
+  <div class="blockquot">
+    <p>What we may expect&mdash;The number of known species of
+    extinct animals&mdash;Causes of the imperfection of the
+    geological record&mdash;Geological evidences of
+    evolution&mdash;Shells&mdash;Crocodiles&mdash;The rhinoceros
+    tribe&mdash;The pedigree of the horse tribe&mdash;Development
+    of deer's horns&mdash;Brain development&mdash;Local relations
+    of fossil and living animals&mdash;Cause of extinction of large
+    animals&mdash;Indications of general progress in plants and
+    animals&mdash;The progressive development of
+    plants&mdash;Possible cause of sudden late appearance of
+    exogens&mdash;Geological distribution of
+    insects&mdash;Geological succession of
+    vertebrata&mdash;Concluding remarks.</p>
+  </div>
+
+  <p>The theory of evolution in the organic world necessarily
+  implies that the forms of animals and plants have, broadly
+  speaking, progressed from a more generalised to a more
+  specialised structure, and from simpler to more complex forms. We
+  know, however, that this progression has been by no means
+  regular, but has been accompanied by repeated degradation and
+  degeneration; while extinction on an enormous scale has again and
+  again stopped all progress in certain directions, and has often
+  compelled a fresh start in development from some comparatively
+  low and imperfect type.</p>
+
+  <p>The enormous extension of geological research in recent times
+  has made us acquainted with a vast number of extinct organisms,
+  so vast that in some important groups&mdash;such as the
+  mollusca&mdash;the fossil are more numerous than the living
+  species; while in the mammalia they are not much less numerous,
+  the preponderance of living species being chiefly in the smaller
+  and in the arboreal forms which have not been so well preserved
+  as the members of the larger groups. With such a wealth of
+  material to illustrate the successive stages <a name="Page_376"
+  id="Page_376"></a>through which animals have passed, it will
+  naturally be expected that we should find important evidence of
+  evolution. We should hope to learn the steps by which some
+  isolated forms have been connected with their nearest allies, and
+  in many cases to have the gaps filled up which now separate genus
+  from genus, or species from species. In some cases these
+  expectations are fulfilled, but in many other cases we seek in
+  vain for evidence of the kind we desire; and this absence of
+  evidence with such an apparent wealth of material is held by many
+  persons to throw doubt on the theory of evolution itself. They
+  urge, with much appearance of reason, that all the arguments we
+  have hitherto adduced fall short of demonstration, and that the
+  crucial test consists in being able to show, in a great number of
+  cases, those connecting links which we say must have existed.
+  Many of the gaps that still remain are so vast that it seems
+  incredible to these writers that they could ever have been filled
+  up by a close succession of species, since these must have spread
+  over so many ages, and have existed in such numbers, that it
+  seems impossible to account for their total absence from deposits
+  in which great numbers of species belonging to other groups are
+  preserved and have been discovered. In order to appreciate the
+  force, or weakness, of these objections, we must inquire into the
+  character and completeness of that record of the past life of the
+  earth which geology has unfolded, and ascertain the nature and
+  amount of the evidence which, under actual conditions, we may
+  expect to find.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Number of known Species of Extinct Animals.</i></p>
+
+  <p>When we state that the known fossil mollusca are considerably
+  more numerous than those which now live on the earth, it appears
+  at first sight that our knowledge is very complete, but this is
+  far from being the case. The species have been continually
+  changing throughout geological time, and at each period have
+  probably been as numerous as they are now. If we divide the
+  fossiliferous strata into twelve great divisions&mdash;the
+  Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias, Trias,
+  Permian, Carboniferous, Devonian, Silurian, and
+  Cambrian,&mdash;we find not only that each has a very distinct
+  and characteristic molluscan fauna, but that the different
+  sub<a name="Page_377" id="Page_377"></a>divisions often present a
+  widely different series of species; so that although a certain
+  number of species are common to two or more of the great
+  divisions, the totality of the species that have lived upon the
+  earth must be very much more than twelve times&mdash;perhaps even
+  thirty or forty times&mdash;the number now living. In like
+  manner, although the species of fossil mammals now recognised by
+  more or less fragmentary fossil remains may not be much less
+  numerous than the living species, yet the duration of existence
+  of these was comparatively so short that they were almost
+  completely changed, perhaps six or seven times, during the
+  Tertiary period; and this is certainly only a fragment of the
+  geological time during which mammalia existed on the globe.</p>
+
+  <p>There is also reason to believe that the higher animals were
+  much more abundant in species during past geological epochs than
+  now, owing to the greater equability of the climate which
+  rendered even the arctic regions as habitable as the temperate
+  zones are in our time.</p>
+
+  <p>The same equable climate would probably cause a more uniform
+  distribution of moisture, and render what are now desert regions
+  capable of supporting abundance of animal life. This is indicated
+  by the number and variety of the species of large animals that
+  have been found fossil in very limited areas which they evidently
+  inhabited at one period. M. Albert Gaudry found, in the deposits
+  of a mountain stream at Pikermi in Greece, an abundance of large
+  mammalia such as are nowhere to be found living together at the
+  present time. Among them were two species of Mastodon, two
+  different rhinoceroses, a gigantic wild boar, a camel and a
+  giraffe larger than those now living, several monkeys, carnivora
+  ranging from martens and civets to lions and hyaenas of the
+  largest size, numerous antelopes of at least five distinct
+  genera, and besides these many forms altogether extinct. Such
+  were the great herds of Hipparion, an ancestral form of horse;
+  the Helladotherium, a huge animal bigger than the giraffe; the
+  Ancylotherium, one of the Edentata; the huge Dinotherium; the
+  Aceratherium, allied to the rhinoceros; and the monstrous
+  Chalicotherium, allied to the swine and ruminants, but as large
+  as a rhinoceros; and to prey upon these, the great Machairodus or
+  sabre-toothed tiger. And all these remains were <a name=
+  "Page_378" id="Page_378"></a>found in a space 300 paces long by
+  60 paces broad, many of the species existing in enormous
+  quantities.</p>
+
+  <p>The Pikermi fossils belong to the Upper Miocene formation, but
+  an equally rich deposit of Upper Eocene age has been discovered
+  in South-Western France at Quercy, where M. Filhol has determined
+  the presence of no less than forty-two species of beasts of prey
+  alone. Equally remarkable are the various discoveries of
+  mammalian fossils in North America, especially in the old lake
+  bottoms now forming what are called the "bad lands" of Dakota and
+  Nebraska, belonging to the Miocene period. Here are found an
+  enormous assemblage of remains, often perfect skeletons, of
+  herbivora and carnivora, as varied and interesting as those from
+  the localities already referred to in Europe; but altogether
+  distinct, and far exceeding, in number and variety of species of
+  the larger animals, the whole existing fauna of North America.
+  Very similar phenomena occur in South America and in Australia,
+  leading us to the conclusion that the earth at the present time
+  is impoverished as regards the larger animals, and that at each
+  successive period of Tertiary time, at all events, it contained a
+  far greater number of species than now inhabit it. The very
+  richness and abundance of the remains which we find in limited
+  areas, serve to convince us how imperfect and fragmentary must be
+  our knowledge of the earth's fauna at any one past epoch; since
+  we cannot believe that all, or nearly all, of the animals which
+  inhabited any district were entombed in a single lake, or
+  overwhelmed by the floods of a single river.</p>
+
+  <p>But the spots where such rich deposits occur are exceedingly
+  few and far between when compared with the vast areas of
+  continental land, and we have every reason to believe that in
+  past ages, as now, numbers of curious species were rare or local,
+  the commoner and more abundant species giving a very imperfect
+  idea of the existing series of animal forms. Yet more important,
+  as showing the imperfection of our knowledge, is the enormous
+  lapse of time between the several formations in which we find
+  organic remains in any abundance, so vast that in many cases we
+  find ourselves almost in a new world, all the species and most of
+  the genera of the higher animals having undergone a complete
+  change.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_379" id="Page_379"></a><i>Causes of the
+  Imperfection of the Geological Record.</i></p>
+
+  <p>These facts are quite in accordance with the conclusions of
+  geologists as to the necessary imperfection of the geological
+  record, since it requires the concurrence of a number of
+  favourable conditions to preserve any adequate representation of
+  the life of a given epoch. In the first place, the animals to be
+  preserved must not die a natural death by disease, or old age, or
+  by being the prey of other animals, but must be destroyed by some
+  accident which shall lead to their being embedded in the soil.
+  They must be either carried away by floods, sink into bogs or
+  quicksands, or be enveloped in the mud or ashes of a volcanic
+  eruption; and when thus embedded they must remain undisturbed
+  amid all the future changes of the earth's surface.</p>
+
+  <p>But the chances against this are enormous, because denudation
+  is always going on, and the rocks we now find at the earth's
+  surface are only a small fragment of those which were originally
+  laid down. The alternations of marine and freshwater deposits,
+  and the frequent unconformability of strata with those which
+  overlie them, tell us plainly of repeated elevations and
+  depressions of the surface, and of denudation on an enormous
+  scale. Almost every mountain range, with its peaks, ridges, and
+  valleys, is but the remnant of some vast plateau eaten away by
+  sub-aerial agencies; every range of sea-cliffs tell us of long
+  slopes of land destroyed by the waves; while almost all the older
+  rocks which now form the surface of the earth have been once
+  covered with newer deposits which have long since disappeared.
+  Nowhere are the evidences of this denudation more apparent than
+  in North and South America, where granitic or metamorphic rocks
+  cover an area hardly less than that of all Europe. The same rocks
+  are largely developed in Central Africa and Eastern Asia; while,
+  besides those portions that appear exposed on the surface, areas
+  of unknown extent are buried under strata which rest on them
+  uncomformably, and could not, therefore, constitute the original
+  capping under which the whole of these rocks must once have been
+  deeply buried; because granite can only be formed, and
+  metamorphism can only go on, deep down in the crust of the earth.
+  What an over<a name="Page_380" id="Page_380"></a>whelming idea
+  does this give us of the destruction of whole piles of rock,
+  miles in thickness and covering areas comparable with those of
+  continents; and how great must have been the loss of the
+  innumerable fossil forms which those rocks contained! In view of
+  such destruction we are forced to conclude that our
+  palaeontological collections, rich though they may appear, are
+  really but small and random samples, giving no adequate idea of
+  the mighty series of organism which have lived upon the
+  earth.<a name="FNanchor_183_183" id=
+  "FNanchor_183_183"></a><a href="#Footnote_183_183" class=
+  "fnanchor">[183]</a></p>
+
+  <p>Admitting, however, the extreme imperfection of the geological
+  record as a whole, it may be urged that certain limited portions
+  of it are fairly complete&mdash;as, for example, the various
+  Miocene deposits of India, Europe, and North America,&mdash;and
+  that in these we ought to find many examples of species and
+  genera linked together by intermediate forms. It may be replied
+  that in several cases this really occurs; and the reason why it
+  does not occur more often is, that the theory of evolution
+  requires that distinct genera should be linked together, not by a
+  direct passage, but by the descent of both from a common
+  ancestor, which may have lived in some much earlier age the
+  record of which is either wanting or very incomplete. An
+  illustration given by Mr. Darwin will make this more clear to
+  those who have not studied the subject. The fantail and pouter
+  pigeons are two very distinct and unlike breeds, which we yet
+  know to have been both derived from the common wild rock-pigeon.
+  Now, if we had every variety of living pigeon before us, or even
+  all those which have lived during the present century, we should
+  find no intermediate types between these two&mdash;none combining
+  in any degree the characters of the pouter with that of the
+  fantail. Neither should we ever find such an intermediate form,
+  even had there been preserved a specimen of every breed of pigeon
+  since the ancestral rock-pigeon was first tamed by man&mdash;a
+  period of probably several thousand years. We thus see that a
+  complete passage from one very distinct species to another could
+  not be expected even had we a complete record of the life of any
+  one period. What we require is a complete <a name="Page_381" id=
+  "Page_381"></a>record of all the species that have existed since
+  the two forms began to diverge from their common ancestor, and
+  this the known imperfection of the record renders it almost
+  impossible that we should ever attain. All that we have a right
+  to expect is, that, as we multiply the fossil forms in any group,
+  the gaps that at first existed in that group shall become less
+  wide and less numerous; and also that, in some cases, a tolerably
+  direct series shall be found, by which the more specialised forms
+  of the present day shall be connected with more generalised
+  ancestral types. We might also expect that when a country is now
+  characterised by special groups of animals, the fossil forms that
+  immediately preceded them shall, for the most part, belong to the
+  same groups; and further, that, comparing the more ancient with
+  the more modern types, we should find indications of progression,
+  the earlier forms being, on the whole, lower in organisation, and
+  less specialised in structure than the later. Now evidence of
+  evolution of these varied kinds is what we do find, and almost
+  every fresh discovery adds to their number and cogency. In order,
+  therefore, to show that the testimony given by geology is
+  entirely in favour of the theory of descent with modification,
+  some of the more striking of the facts will now be given.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Geological Evidences of Evolution.</i></p>
+
+  <p>In an article in <i>Nature</i> (vol. xiv. p. 275), Professor
+  Judd calls attention to some recent discoveries in the Hungarian
+  plains, of fossil lacustrine shells, and their careful study by
+  Dr. Neumayr and M. Paul of the Austrian Geological Survey. The
+  beds in which they occur have accumulated to the thickness of
+  2000 feet, containing throughout abundance of fossils, and
+  divisible into eight zones, each of which exhibits a well-marked
+  and characteristic fauna. Professor Judd then describes the
+  bearing of these discoveries as follows&mdash;</p>
+
+  <div class="blockquot">
+    <p>"The group of shells which affords the most interesting
+    evidence of the origin of new forms through descent with
+    modification is that of the genus Vivipara or Paludina, which
+    occurs in prodigious abundance throughout the whole series of
+    freshwater strata. We shall not, of course, attempt in this
+    place to enter into any details concerning the forty distinct
+    <i>forms</i> of this genus (Dr. Neumayr very properly hesitates
+    to call them all <a name="Page_382" id=
+    "Page_382"></a><i>species</i>), which are named and described
+    in this monograph, and between which, as the authors show, so
+    many connecting links, clearly illustrating the derivation of
+    the newer from the older types, have been detected. On the
+    minds of those who carefully examine the admirably engraved
+    figures given in the plates accompanying this valuable memoir,
+    or still better, the very large series of specimens from among
+    which the subjects of these figures are selected, and which are
+    now in the museum of the Reichsanstalt of Vienna, but little
+    doubt will, we suspect, remain that the authors have fully made
+    out their case, and have demonstrated that, beyond all
+    controversy, the series with highly complicated ornamentation
+    were variously derived by descent&mdash;the lines of which are
+    in most cases perfectly clear and obvious&mdash;from the simple
+    and unornamented Vivipara achatinoides of the
+    Congerien-Schichten (the lower division of the series of
+    strata). It is interesting to notice that a large portion of
+    these unquestionably derived forms depart so widely from the
+    type of the genus Vivipara, that they have been separated on so
+    high an authority as that of Sandberger, as a new genus, under
+    the name of Tulotoma. And hence we are led to the conclusion
+    that a vast number of forms, certainly exhibiting specific
+    distinctions, and according to some naturalists, differences
+    even entitled to be regarded of generic value, have all a
+    common ancestry."</p>
+  </div>
+
+  <p>It is, as Professor Judd remarks, owing to the exceptionally
+  favourable circumstances of a long-continued and unbroken series
+  of deposits being formed under physical conditions either
+  identical or very slowly changing, that we owe so complete a
+  record of the process of organic change. Usually, some disturbing
+  elements, such as a sudden change of physical conditions, or the
+  immigration of new sets of forms from other areas and the
+  consequent retreat or partial extinction of the older fauna,
+  interferes with the continuity of organic development, and
+  produces those puzzling discordances so generally met with in
+  geological formations of marine origin. While a case of the kind
+  now described affords evidence of the origin of species complete
+  and conclusive, though on a necessarily very limited scale, the
+  very rarity of the conditions which are essential to such
+  completeness serves to explain why it is that in most cases the
+  direct evidence of evolution is not to be obtained.</p>
+
+  <p><a name="Page_383" id="Page_383"></a>Another illustration of
+  the filling up of gaps between existing groups is afforded by
+  Professor Huxley's researches on fossil crocodiles. The gap
+  between the existing crocodiles and the lizards is very wide, but
+  as we go back in geological time we meet with fossil forms which
+  are to some extent intermediate and form a connected series. The
+  three living genera&mdash;Crocodilus, Alligator, and
+  Gavialis&mdash;are found in the Eocene formation, and allied
+  forms of another genus, Holops, in the Chalk. From the Chalk
+  backward to the Lias another group of genera occurs, having
+  anatomical characteristics intermediate between the living
+  crocodiles and the most ancient forms. These, forming two genera
+  Belodon and Stagonolepis, are found in a still older formation,
+  the Trias. They have characters resembling some lizards,
+  especially the remarkable Hatteria of New Zealand, and have also
+  some resemblances to the Dinosaurians&mdash;reptiles which in
+  some respects approach birds. Considering how comparatively few
+  are the remains of this group of animals, the evidence which it
+  affords of progressive development is remarkably clear.<a name=
+  "FNanchor_184_184" id="FNanchor_184_184"></a><a href=
+  "#Footnote_184_184" class="fnanchor">[184]</a></p>
+
+  <p>Among the higher animals the rhinoceros, the horse, and the
+  deer afford good evidence of advance in organisation and of the
+  filling up of the gaps which separate the living forms from their
+  nearest allies. The earliest ancestral forms of the rhinoceroses
+  occur in the Middle Eocene of the United States, and were to some
+  extent intermediate between the rhinoceros and tapir families,
+  having like the latter four toes to the front feet, and three to
+  those behind. These are followed in the Upper Eocene by the genus
+  Amynodon, in which the skull assumes more distinctly the
+  rhinocerotic type. Following this in the Lower Miocene we have
+  the Aceratherium, like the last in its feet, but still more
+  decidedly a rhinoceros in its general structure. From this there
+  are two diverging lines&mdash;one in the Old World, the other in
+  the New. In the former, to which the Aceratherium is supposed to
+  have migrated in early Miocene times, when a mild climate and
+  luxuriant vegetation prevailed far within the arctic circle, it
+  gave rise to the Ceratorhinus and the various horned rhinoceroses
+  of late Tertiary times and of those now living. In America a
+  <a name="Page_384" id="Page_384"></a>number of large hornless
+  rhinoceroses were developed&mdash;they are found in the Upper
+  Miocene, Pliocene, and Post-Pliocene formations&mdash;and then
+  became extinct. The true rhinoceroses have three toes on all the
+  feet.<a name="FNanchor_185_185" id=
+  "FNanchor_185_185"></a><a href="#Footnote_185_185" class=
+  "fnanchor">[185]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Pedigree of the Horse Tribe.</i></p>
+
+  <p>Yet more remarkable is the evidence afforded by the ancestral
+  forms of the horse tribe which have been discovered in the
+  American tertiaries. The family Equidae, comprising the living
+  horse, asses, and zebras, differ widely from all other mammals in
+  the peculiar structure of the feet, all of which terminate in a
+  single large toe forming the hoof. They have forty teeth, the
+  molars being formed of hard and soft material in crescentic
+  folds, so as to be a powerful agent in grinding up hard grasses
+  and other vegetable food. The former peculiarities depend upon
+  modifications of the skeleton, which have been thus described by
+  Professor Huxley:&mdash;</p>
+
+  <div class="blockquot">
+    <p>"Let us turn in the first place to the fore-limb. In most
+    quadrupeds, as in ourselves, the fore-arm contains distinct
+    bones, called the radius and the ulna. The corresponding region
+    in the horse seems at first to possess but one bone. Careful
+    observation, however, enables us to distinguish in this bone a
+    part which clearly answers to the upper end of the ulna. This
+    is closely united with the chief mass of the bone which
+    represents the radius, and runs out into a slender shaft, which
+    may be traced for some distance downwards upon the back of the
+    radius, and then in most cases thins out and vanishes. It takes
+    still more trouble to make sure of what is nevertheless the
+    fact, that a small part of the lower end of the bone of a
+    horse's fore-arm, which is only distinct in a very young foal,
+    is really the lower extremity of the ulna.</p>
+
+    <p>"What is commonly called the knee of a horse is its wrist.
+    The 'cannon bone' answers to the middle bone of the five
+    metacarpal bones which support the palm of the hand in
+    ourselves. The pastern, coronary, and coffin bones of
+    veterinarians answer to the joints of our middle fingers, while
+    the hoof is simply a greatly enlarged and thickened nail. But
+    if <a name="Page_385" id="Page_385"></a>what lies below the
+    horse's 'knee' thus corresponds to the middle finger in
+    ourselves, what has become of the four other fingers or digits?
+    We find in the places of the second and fourth digits only two
+    slender splintlike bones, about two-thirds as long as the
+    cannon bone, which gradually taper to their lower ends and bear
+    no finger joints, or, as they are termed, phalanges. Sometimes,
+    small bony or gristly nodules are to be found at the bases of
+    these two metacarpal splints, and it is probable that these
+    represent rudiments of the first and fifth toes. Thus, the part
+    of the horse's skeleton which corresponds with that of the
+    human hand, contains one overgrown middle digit, and at least
+    two imperfect lateral digits; and these answer, respectively,
+    to the third, the second, and the fourth fingers in man.</p>
+
+    <p>"Corresponding modifications are found in the hind limb. In
+    ourselves, and in most quadrupeds, the leg contains two
+    distinct bones, a large bone, the tibia, and a smaller and more
+    slender bone, the fibula. But, in the horse, the fibula seems,
+    at first, to be reduced to its upper end; a short slender bone
+    united with the tibia, and ending in a point below, occupying
+    its place. Examination of the lower end of a young foal's
+    shin-bone, however, shows a distinct portion of osseous matter
+    which is the lower end of the fibula; so that the, apparently
+    single, lower end of the shin-bone is really made up of the
+    coalesced ends of the tibia and fibula, just as the, apparently
+    single, lower end of the fore-arm bone is composed of the
+    coalesced radius and ulna.</p>
+
+    <p>"The heel of the horse is the part commonly known as the
+    hock. The hinder cannon bone answers to the middle metatarsal
+    bone of the human foot, the pastern, coronary, and coffin
+    bones, to the middle toe bones; the hind hoof to the nail; as
+    in the forefoot. And, as in the forefoot, there are merely two
+    splints to represent the second and the fourth toes. Sometimes
+    a rudiment of a fifth toe appears to be traceable.</p>
+
+    <p>"The teeth of a horse are not less peculiar than its limbs.
+    The living engine, like all others, must be well stoked if it
+    is to do its work; and the horse, if it is to make good its
+    wear and tear, and to exert the enormous amount of force
+    required for its propulsion, must be well and rapidly fed. To
+    this end, <a name="Page_386" id="Page_386"></a>good cutting
+    instruments and powerful and lasting crushers are needful.
+    Accordingly, the twelve cutting teeth of a horse are close-set
+    and concentrated in the forepart of its mouth, like so many
+    adzes or chisels. The grinders or molars are large, and have an
+    extremely complicated structure, being composed of a number of
+    different substances of unequal hardness. The consequence of
+    this is that they wear away at different rates; and, hence, the
+    surface of each grinder is always as uneven as that of a good
+    millstone."<a name="FNanchor_186_186" id=
+    "FNanchor_186_186"></a><a href="#Footnote_186_186" class=
+    "fnanchor">[186]</a></p>
+  </div>
+
+  <p>We thus see that the Equidae differ very widely in structure
+  from most other mammals. Assuming the truth of the theory of
+  evolution, we should expect to find traces among extinct animals
+  of the steps by which this great modification has been effected;
+  and we do really find traces of these steps, imperfectly among
+  European fossils, but far more completely among those of
+  America.</p>
+
+  <p>It is a singular fact that, although no horse inhabited
+  America when discovered by Europeans, yet abundance of remains of
+  extinct horses have been found both in North and South America in
+  Post-Tertiary and Upper Pliocene deposits; and from these an
+  almost continuous series of modified forms can be traced in the
+  Tertiary formation, till we reach, at the very base of the
+  series, a primitive form so unlike our perfected animal, that,
+  had we not the intermediate links, few persons would believe that
+  the one was the ancestor of the other. The tracing out of this
+  marvellous history we owe chiefly to Professor Marsh of Yale
+  College, who has himself discovered no less than thirty species
+  of fossil Equidae; and we will allow him to tell the story of the
+  development of the horse from a humble progenitor in his own
+  words.</p>
+
+  <div class="blockquot">
+    <p>"The oldest representative of the horse at present known is
+    the diminutive Eohippus from the Lower Eocene. Several species
+    have been found, all about the size of a fox. Like most of the
+    early mammals, these ungulates had forty-four teeth, the molars
+    with short crowns and quite distinct in form from the
+    premolars. The ulna and fibula were entire and distinct, and
+    there were four well-developed toes and a rudiment of another
+    on the forefeet, and three toes behind. In the structure of the
+    feet and teeth, the Eohippus unmistak<a name="Page_387" id=
+    "Page_387"></a>ably indicates that the direct ancestral line to
+    the modern horse has already separated from the other
+    perissodactyles, or odd-toed ungulates.</p>
+
+    <p>"In the next higher division of the Eocene another genus,
+    Orohippus, makes its appearance, replacing Eohippus, and
+    showing a greater, though still distant, resemblance to the
+    equine type. The rudimentary first digit of the forefoot has
+    disappeared, and the last premolar has gone over to the molar
+    series. Orohippus was but little larger than Eohippus, and in
+    most other respects very similar. Several species have been
+    found, but none occur later than the Upper Eocene.</p>
+
+    <p>"Near the base of the Miocene, we find a third closely
+    allied genus, Mesohippus, which is about as large as a sheep,
+    and one stage nearer the horse. There are only three toes and a
+    rudimentary splint on the forefeet, and three toes behind. Two
+    of the premolar teeth are quite like the molars. The ulna is no
+    longer distinct or the fibula entire, and other characters show
+    clearly that the transition is advancing.</p>
+
+    <p>"In the Upper Miocene Mesohippus is not found, but in its
+    place a fourth form, Miohippus, continues the line. This genus
+    is near the Anchitherium of Europe, but presents several
+    important differences. The three toes in each foot are more
+    nearly of a size, and a rudiment of the fifth metacarpal bone
+    is retained. All the known species of this genus are larger
+    than those of Mesohippus, and none of them pass above the
+    Miocene formation.</p>
+
+    <p>"The genus Protohippus of the Lower Pliocene is yet more
+    equine, and some of its species equalled the ass in size. There
+    are still three toes on each foot, but only the middle one,
+    corresponding to the single toe of the horse, comes to the
+    ground. This genus resembles most nearly the Hipparion of
+    Europe.</p>
+
+    <p>"In the Pliocene we have the last stage of the series before
+    reaching the horse, in the genus Pliohippus, which has lost the
+    small hooflets, and in other respects is very equine. Only in
+    the Upper Pliocene does the true Equus appear and complete the
+    genealogy of the horse, which in the Post-Tertiary roamed over
+    the whole of North and South America, and soon after became
+    extinct. This occurred long before the discovery of the
+    continent by Europeans, and no satisfactory <a name="Page_388"
+    id="Page_388"></a><a name="Page_389" id="Page_389"></a>reason
+    for the extinction has yet been given. Besides the characters I
+    have mentioned, there are many others in the skeleton, skull,
+    teeth, and brain of the forty or more intermediate species,
+    which show that the transition from the Eocene Eohippus to the
+    modern Equus has taken place in the order indicated"<a name=
+    "FNanchor_187_187" id="FNanchor_187_187"></a><a href=
+    "#Footnote_187_187" class="fnanchor">[187]</a> (see Fig.
+    33).</p>
+  </div>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig33" id="fig33"></a> <img src="images/fig33.jpg"
+    width="400" height="664" alt=
+    "FIG. 33.&mdash;Geological development of the horse tribe (Eohippus since discovered)."
+    title=
+    "FIG. 33.&mdash;Geological development of the horse tribe (Eohippus since discovered)." />
+     <b>FIG. 33.&mdash;Geological development of the horse tribe
+    (Eohippus since discovered).</b>
+  </div>
+
+  <p>Well may Professor Huxley say that this is demonstrative
+  evidence of evolution; the doctrine resting upon exactly as
+  secure a foundation as did the Copernican theory of the motions
+  of the heavenly bodies at the time of its promulgation. Both have
+  the same basis&mdash;the coincidence of the observed facts with
+  the theoretical requirements.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Development of Deer's Horns.</i></p>
+
+  <p>Another clear and unmistakable proof of evolution is afforded
+  by one of the highest and latest developed tribes of
+  mammals&mdash;the true deer. These differ from all other
+  ruminants in possessing solid deciduous horns which are always
+  more or less branched. They first appear in the Middle Miocene
+  formation, and continue down to our time; and their development
+  has been carefully traced by Professor Boyd Dawkins, who thus
+  summarises his results:&mdash;</p>
+
+  <div class="blockquot">
+    <p>"In the middle stage of the Miocene the cervine antler
+    consists merely of a simple forked crown (as in Cervus
+    dicroceros), which increases in size in the Upper Miocene,
+    although it still remains small and erect, like that of the
+    roe. In Cervus Matheroni it measures 11&middot;4 inches, and
+    throws off not more than four tines, all small. The deer living
+    in Auvergne in the succeeding or Pliocene age, present us with
+    another stage in the history of antler development. There, for
+    the first time, we see antlers of the Axis and Rusa type,
+    larger and longer, and more branching than any antlers were
+    before, and possessing three or more well-developed tines. Deer
+    of this type abounded in Pliocene Europe. They belong to the
+    Oriental division of the Cervidae, and their presence in Europe
+    confirms the evidence of the flora, brought forward by the
+    Comte de Saporta, that the Pliocene climate was warm. They have
+    probably disappeared from Europe in consequence <a name=
+    "Page_390" id="Page_390"></a>of the lowering of the temperature
+    in the Pleistocene age, while their descendants have found a
+    congenial home in the warmer regions of Eastern Asia.</p>
+
+    <p>"In the latest stage of the Pliocene&mdash;the Upper
+    Pliocene of the Val d'Arno&mdash;the Cervus dicranios of Nesti
+    presents us with antlers much smaller than those of the Irish
+    elk, but very complicated in their branching. This animal
+    survived into the succeeding age, and is found in the
+    pre-glacial forest bed of Norfolk, being described by Dr.
+    Falconer under the name of Sedgwick's deer. The Irish elk,
+    moose, stag, reindeer, and fallow deer appear in Europe in the
+    Pleistocene age, all with highly complicated antlers in the
+    adult, and the first possessing the largest antlers yet known.
+    Of these the Irish elk disappeared in the Prehistoric age,
+    after having lived in countless herds in Ireland, while the
+    rest have lived on into our own times in Euro-Asia, and, with
+    the exception of the last, also in North America.</p>
+
+    <p>"From this survey it is obvious that the cervine antlers
+    have increased in size and complexity from the Mid-Miocene to
+    the Pleistocene age, and that their successive changes are
+    analogous to those which are observed in the development of
+    antlers in the living deer, which begin with a simple point,
+    and increase in number of tines till their limit of growth be
+    reached. In other words, the development of antlers indicated
+    at successive and widely-separated pages of the geological
+    record is the same as that observed in the history of a single
+    living species. It is also obvious that the progressive
+    diminution of size and complexity in the antlers, from the
+    present time back into the early Tertiary age, shows that we
+    are approaching the zero of antler development in the
+    Mid-Miocene. No trace of any antler-bearing ruminant has been
+    met with in the lower Miocenes, either of Europe or the United
+    States."<a name="FNanchor_188_188" id=
+    "FNanchor_188_188"></a><a href="#Footnote_188_188" class=
+    "fnanchor">[188]</a></p>
+  </div>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Progressive Brain-Development.</i></p>
+
+  <p>The three illustrations now given sufficiently prove that,
+  whenever the geological record approaches to completeness, we
+  have evidence of the progressive change of species in definite
+  directions, and from less developed to more de<a name="Page_391"
+  id="Page_391"></a>veloped types&mdash;exactly such a change as we
+  may expect to find if the evolution theory be the true one. Many
+  other illustrations of a similar change could be given, but the
+  animal groups in which they occur being less familiar, the
+  details would be less interesting, and perhaps hardly
+  intelligible. There is, however, one very remarkable proof of
+  development that must be briefly noticed&mdash;that afforded by
+  the steady increase in the size of the brain. This may be best
+  stated in the words of Professor Marsh:&mdash;</p>
+
+  <div class="blockquot">
+    <p>"The real progress of mammalian life in America, from the
+    beginning of the Tertiary to the present, is well illustrated
+    by the brain-growth, in which we have the key to many other
+    changes. The earliest known Tertiary mammals all had very small
+    brains, and in some forms this organ was proportionally less
+    than in certain reptiles. There was a gradual increase in the
+    size of the brain during this period, and it is interesting to
+    find that this growth was mainly confined to the cerebral
+    hemispheres, or higher portion of the brain. In most groups of
+    mammals the brain has gradually become more convoluted, and
+    thus increased in quality as well as quantity. In some also the
+    cerebellum and olfactory lobes, the lower parts of the brain,
+    have even diminished in size. In the long struggle for
+    existence during Tertiary time the big brains won, then as now;
+    and the increasing power thus gained rendered useless many
+    structures inherited from primitive ancestors, but no longer
+    adapted to new conditions."</p>
+  </div>
+
+  <p>This remarkable proof of development in the organ of the
+  mental faculties, forms a fitting climax to the evidence already
+  adduced of the progressive evolution of the general structure of
+  the body, as illustrated by the bony skeleton. We now pass on to
+  another class of facts equally suggestive of evolution.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Local Relations of Fossil and Living Animals.</i></p>
+
+  <p>If all existing animals have been produced from ancestral
+  forms&mdash;mostly extinct&mdash;under the law of variation and
+  natural selection, we may expect to find in most cases a close
+  relation between the living forms of each country and those which
+  inhabited it in the immediately preceding epoch. But if species
+  have originated in some quite different way, either by <a name=
+  "Page_392" id="Page_392"></a>any kind of special creation, or by
+  sudden advances of organisation in the offspring of preceding
+  types, such close relationship would not be found; and facts of
+  this kind become, therefore, to some extent a test of evolution
+  under natural selection or some other law of gradual change. Of
+  course the relationship will not appear when extensive migration
+  has occurred, by which the inhabitants of one region have been
+  able to take possession of another region, and destroy or drive
+  out its original inhabitants, as has sometimes happened. But such
+  cases are comparatively rare, except where great changes of
+  climate are known to have occurred; and we usually do find a
+  remarkable continuity between the existing fauna and flora of a
+  country and those of the immediately preceding age. A few of the
+  more remarkable of these cases will now be briefly noticed.</p>
+
+  <p>The mammalian fauna of Australia consists, as is well known,
+  wholly of the lowest forms&mdash;the Marsupials and
+  Monotremata&mdash;except only a few species of mice. This is
+  accounted for by the complete isolation of the country from the
+  Asiatic continent during the whole period of the development of
+  the higher animals. At some earlier epoch the ancestral
+  marsupials, which abounded both in Europe and North America in
+  the middle of the Secondary period, entered the country, and have
+  since remained there, free from the competition of higher forms,
+  and have undergone a special development in accordance with the
+  peculiar conditions of a limited area. While in the large
+  continents higher forms of mammalia have been developed, which
+  have almost or wholly exterminated the less perfect marsupials,
+  in Australia these latter have become modified into such varied
+  forms as the leaping kangaroos, the burrowing wombats, the
+  arboreal phalangers, the insectivorous bandicoots, and the
+  carnivorous Dasyuridae or native cats, culminating in the
+  Thylacinus or "tiger-wolf" of Tasmania&mdash;animals as unlike
+  each other as our sheep, rabbits, squirrels, and dogs, but all
+  retaining the characteristic features of the marsupial type.</p>
+
+  <p>Now in the caves and late Tertiary or Post-Tertiary deposits
+  of Australia the remains of many extinct mammalia have been
+  found, but all are marsupials. There are many kangaroos, some
+  larger than any living species, and others more allied to
+  <a name="Page_393" id="Page_393"></a>the tree-kangaroos of New
+  Guinea; a large wombat as large as a tapir; the Diprotodon, a
+  thick-limbed kangaroo the size of a rhinoceros or small elephant;
+  and a quite different animal, the Nototherium, nearly as large.
+  The carnivorous Thylacinus of Tasmania is also found fossil; and
+  a huge phalanger, Thylacoleo, the size of a lion, believed by
+  Professor Owen and by Professor Oscar Schmidt to have been
+  equally carnivorous and destructive.<a name="FNanchor_189_189"
+  id="FNanchor_189_189"></a><a href="#Footnote_189_189" class=
+  "fnanchor">[189]</a> Besides these, there are many other species
+  more resembling the living forms both in size and structure, of
+  which they may be, in some cases, the direct ancestors. Two
+  species of extinct Echidna, belonging to the very low
+  Monotremata, have also been found in New South Wales.</p>
+
+  <p>Next to Australia, South America possesses the most remarkable
+  assemblage of peculiar mammals, in its numerous
+  Edentata&mdash;the sloths, ant-eaters, and armadillos; its
+  rodents, such as the cavies and chinchillas; its marsupial
+  opossums, and its quadrumana of the family Cebidae. Remains of
+  extinct species of all these have been found in the caves of
+  Brazil, of Post-Pliocene age; while in the earlier Pliocene
+  deposits of the pampas many distinct genera of these groups have
+  been found, some of gigantic size and extraordinary form. There
+  are armadillos of many types, some being as large as elephants;
+  gigantic sloths of the genera Megatherium, Megalonyx, Mylodon,
+  Lestodon, and many others; rodents belonging to the American
+  families Cavidae and Chinchillidae; and ungulates allied to the
+  llama; besides many other extinct forms of intermediate types or
+  of uncertain affinities.<a name="FNanchor_190_190" id=
+  "FNanchor_190_190"></a><a href="#Footnote_190_190" class=
+  "fnanchor">[190]</a> The extinct Moas of New Zealand&mdash;huge
+  wingless birds allied to the living Apteryx&mdash;illustrate the
+  same general law.</p>
+
+  <p>The examples now quoted, besides illustrating and enforcing
+  the general fact of evolution, throw some light on the usual
+  character of the modification and progression of animal forms. In
+  the cases where the geological record is tolerably complete, we
+  find a continuous development of some kind&mdash;either in
+  complexity of ornamentation, as in the fossil Paludinas of the
+  Hungarian lake-basins; in size and in the specialisation of the
+  <a name="Page_394" id="Page_394"></a>feet and teeth, as in the
+  American fossil horses; or in the increased development of the
+  branching horns, as in the true deer. In each of these cases
+  specialisation and adaptation to the conditions of the
+  environment appear to have reached their limits, and any change
+  of these conditions, especially if it be at all rapid or
+  accompanied by the competition of less developed but more
+  adaptable forms, is liable to cause the extinction of the most
+  highly developed groups. Such we know was the case with the horse
+  tribe in America, which totally disappeared in that continent at
+  an epoch so recent that we cannot be sure that the disappearance
+  was not witnessed, perhaps caused, by man; while even in the
+  Eastern hemisphere it is the smaller species&mdash;the asses and
+  the zebras&mdash;that have persisted, while the larger and more
+  highly developed true horses have almost, if not quite,
+  disappeared in a state of nature. So we find, both in Australia
+  and South America, that in a quite recent period many of the
+  largest and most specialised forms have become extinct, while
+  only the smaller types have survived to our day; and a similar
+  fact is to be observed in many of the earlier geological epochs,
+  a group progressing and reaching a maximum of size or complexity
+  and then dying out, or leaving at most but few and pigmy
+  representatives.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Cause of Extinction of Large Animals.</i></p>
+
+  <p>Now there are several reasons for the repeated extinction of
+  large rather than of small animals. In the first place, animals
+  of great bulk require a proportionate supply of food, and any
+  adverse change of conditions would affect them more seriously
+  than it would smaller animals. In the next place, the extreme
+  specialisation of many of these large animals would render it
+  less easy for them to be modified in any new direction suited to
+  changed conditions. Still more important, perhaps, is the fact
+  that very large animals always increase slowly as compared with
+  small ones&mdash;the elephant producing a single young one every
+  three years, while a rabbit may have a litter of seven or eight
+  young two or three times a year. Now the probability of
+  favourable variations will be in direct proportion to the
+  population of the species, and as the smaller animals are not
+  only many hundred times more numerous than the largest, but also
+  increase perhaps a hundred times as <a name="Page_395" id=
+  "Page_395"></a>rapidly, they are able to become quickly modified
+  by variation and natural selection in harmony with changed
+  conditions, while the large and bulky species, being unable to
+  vary quickly enough, are obliged to succumb in the struggle for
+  existence. As Professor Marsh well observes: "In every vigorous
+  primitive type which was destined to survive many geological
+  changes, there seems to have been a tendency to throw off lateral
+  branches, which became highly specialised and soon died out,
+  because they were unable to adapt themselves to new conditions."
+  And he goes on to show how the whole narrow path of the
+  persistent Suilline type, throughout the entire series of the
+  American tertiaries, is strewed with the remains of such
+  ambitious offshoots, many of them attaining the size of a
+  rhinoceros; "while the typical pig, with an obstinacy never lost,
+  has held on in spite of catastrophes and evolution, and still
+  lives in America to-day."</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Indications of General Progression in Plants and
+  Animals.</i></p>
+
+  <p>One of the most powerful arguments formerly adduced against
+  evolution was, that geology afforded no evidence of the gradual
+  development of organic forms, but that whole tribes and classes
+  appeared suddenly at definite epochs, and often in great variety
+  and exhibiting a very perfect organisation. The mammalia, for
+  example, were long thought to have first appeared in Tertiary
+  times, where they are represented in some of the earlier deposits
+  by all the great divisions of the class fully
+  developed&mdash;carnivora, rodents, insectivora, marsupials, and
+  even the perissodactyle and artiodactyle divisions of the
+  ungulata&mdash;as clearly defined as at the present day. The
+  discovery in 1818 of a single lower jaw in the Stonesfield Slate
+  of Oxfordshire hardly threw doubt on the generalisation, since
+  either its mammalian character was denied, or the geological
+  position of the strata, in which it was found, was held to have
+  been erroneously determined. But since then, at intervals of many
+  years, other remains of mammalia have been discovered in the
+  Secondary strata, ranging from the Upper Oolite to the Upper
+  Trias both in Europe and the United States, and one even
+  (Tritylodon) in the Trias of South Africa. All these are either
+  marsupials, or of some still lower type of mammalia; but they
+  consist of many distinct forms classed in <a name="Page_396" id=
+  "Page_396"></a>about twenty genera. Nevertheless, a great gap
+  still exists between these mammals and those of the Tertiary
+  strata, since no mammal of any kind has been found in any part of
+  the Cretaceous formation, although in several of its subdivisions
+  abundance of land plants, freshwater shells, and air-breathing
+  reptiles have been discovered. So with fishes. In the last
+  century none had been obtained lower than the Carboniferous
+  formation; thirty years later they were found to be very abundant
+  in the Devonian rocks, and later still they were discovered in
+  the Upper Ludlow and Lower Ludlow beds of the Silurian
+  formation.</p>
+
+  <p>We thus see that such sudden appearances are deceptive, and
+  are, in fact, only what we ought to expect from the known
+  imperfection of the geological record. The conditions favourable
+  to the fossilisation of any group of animals occur comparatively
+  rarely, and only in very limited areas; while the conditions
+  essential for their permanent preservation in the rocks, amid all
+  the destruction caused by denudation or metamorphism, are still
+  more exceptional. And when they are thus preserved to our day,
+  the particular part of the rocks in which they lie hidden may not
+  be on the surface but buried down deep under other strata, and
+  may thus, except in the case of mineral-bearing deposits, be
+  altogether out of our reach. Then, again, how large a proportion
+  of the earth consists of wild and uncivilised regions in which no
+  exploration of the rocks has been yet made, so that whether we
+  shall find the fossilised remains of any particular group of
+  animals which lived during a limited period of the earth's
+  history, and in a limited area, depends upon at least a fivefold
+  combination of chances. Now, if we take each of these chances
+  separately as only ten to one against us (and some are certainly
+  more than this), then the actual chance against our finding the
+  fossil remains, say of any one order of mammalia, or of land
+  plants, at any particular geological horizon, will be about a
+  hundred thousand to one.</p>
+
+  <p>It may be said, if the chances are so great, how is it that we
+  find such immense numbers of fossil species exceeding in number,
+  in some groups, all those that are now living? But this is
+  exactly what we should expect, because the number of species of
+  organisms that have ever lived upon the earth, since <a name=
+  "Page_397" id="Page_397"></a>the earliest geological times, will
+  probably be many hundred times greater than those now existing of
+  which we have any knowledge; and hence the enormous gaps and
+  chasms in the geological record of extinct forms is not to be
+  wondered at. Yet, notwithstanding these chasms in our knowledge,
+  if evolution is true, there ought to have been, on the whole,
+  progression in all the chief types of life. The higher and more
+  specialised forms should have come into existence later than the
+  lower and more generalised forms; and however fragmentary the
+  portions we possess of the whole tree of life upon the earth,
+  they ought to show us broadly that such a progressive evolution
+  has taken place. We have seen that in some special groups,
+  already referred to, such a progression is clearly visible, and
+  we will now cast a hasty glance over the entire series of fossil
+  forms, in order to see if a similar progression is manifested by
+  them as a whole.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Progressive Development of Plants.</i></p>
+
+  <p>Ever since fossil plants have been collected and studied, the
+  broad fact has been apparent that the early plants&mdash;those of
+  the Coal formation&mdash;were mainly cryptogamous, while in the
+  Tertiary deposits the higher flowering plants prevailed. In the
+  intermediate secondary epoch the gymnosperms&mdash;cycads and
+  coniferae&mdash;formed a prominent part of the vegetation, and as
+  these have usually been held to be a kind of transition form
+  between the flowerless and flowering plants, the geological
+  succession has always, broadly speaking, been in accordance with
+  the theory of evolution. Beyond this, however, the facts were
+  very puzzling. The highest cryptogams&mdash;ferns, lycopods, and
+  equisetaceae&mdash;appeared suddenly, and in immense profusion in
+  the Coal formation, at which period they attained a development
+  they have never since surpassed or even equalled; while the
+  highest plants&mdash;the dicotyledonous and monocotyledonous
+  angiosperms&mdash;which now form the bulk of the vegetation of
+  the world, and exhibit the most wonderful modifications of form
+  and structure, were almost unknown till the Tertiary period, when
+  they suddenly appeared in full development, and, for the most
+  part, under the same generic forms as now exist.</p>
+
+  <p>During the latter half of the present century, however, great
+  additions have been made to our knowledge of fossil <a name=
+  "Page_398" id="Page_398"></a>plants; and although there are still
+  indications of vast gaps in our knowledge, due, no doubt, to the
+  very exceptional conditions required for the preservation of
+  plant remains, we now possess evidence of a more continuous
+  development of the various types of vegetation. According to Mr.
+  Lester F. Ward, between 8000 and 9000 species of fossil plants
+  have been described or indicated; and, owing to the careful study
+  of the nervation of leaves, a large number of these are referable
+  to their proper orders or genera, and therefore give us some
+  notion&mdash;which, though very imperfect, is probably accurate
+  in its main outlines&mdash;of the progressive development of
+  vegetation on the earth.<a name="FNanchor_191_191" id=
+  "FNanchor_191_191"></a><a href="#Footnote_191_191" class=
+  "fnanchor">[191]</a> The following is a summary of the facts as
+  given by Mr. Ward:&mdash;</p>
+
+  <p>The lowest forms of vegetable life&mdash;the cellular
+  plants&mdash;have been found in Lower Silurian deposits in the
+  form of three species of marine algae; and in the whole Silurian
+  formation fifty species have been recognised. We cannot for a
+  moment suppose, however, that this indicates the first appearance
+  of vegetable life upon the earth, for in these same Lower
+  Silurian beds the more highly organised vascular cryptogams
+  appear in the form of rhizocarps&mdash;plants allied to Marsilea
+  and Azolla,&mdash;and a very little higher, ferns, lycopods, and
+  even conifers appear. We have indications, however, of a still
+  more ancient vegetation, in the carbonaceous shales and thick
+  beds of graphite far down in the Middle Laurentian, since there
+  is no other known agency than the vegetable cell by means of
+  which carbon can be extracted from the atmo<a name="Page_399" id=
+  "Page_399"></a>sphere and fixed in the solid state. These great
+  beds of graphite, therefore, imply the existence of abundance of
+  vegetable life at the very commencement of the era of which we
+  have any geological record.<a name="FNanchor_192_192" id=
+  "FNanchor_192_192"></a><a href="#Footnote_192_192" class=
+  "fnanchor">[192]</a></p>
+
+  <p>Ferns, as already stated, begin in the Middle Silurian
+  formation with the Eopteris Morrieri. In the Devonian, we have 79
+  species, in the Carboniferous 627, and in the Permian 186
+  species; after which fossil ferns diminish greatly, though they
+  are found in every formation; and the fact that fully 3000 living
+  species are known, while the richest portion of the Tertiary in
+  fossil plants&mdash;the Miocene&mdash;- has only produced 87
+  species, will serve to indicate the extreme imperfection of the
+  geological record.</p>
+
+  <p>The Equisetaceae(horsetails) which also first appear in the
+  Silurian and reach their maximum development in the Coal
+  formation, are, in all succeeding formations, far less numerous
+  than ferns, and only thirty living species are known.
+  Lycopodiaceae, though still more abundant in the Coal formation,
+  are very rarely found in any succeeding deposit, though the
+  living species are tolerably numerous, about 500 having been
+  described. As we cannot suppose them to have really diminished
+  and then increased again in this extraordinary manner, we have
+  another indication of the exceptional nature of plant
+  preservation and the extreme and erratic character of the
+  imperfection of the record.</p>
+
+  <p>Passing now to the next higher division of plants&mdash;the
+  gymnosperms&mdash;we find Coniferae appearing in the Upper
+  Silurian, becoming tolerably abundant in the Devonian, and
+  reaching a maximum in the Carboniferous, from which formation
+  more than 300 species are known, equal to the number recorded as
+  now living. They occur in all succeeding formations, being
+  abundant in the Oolite, and excessively so in the Miocene, from
+  which 250 species have been described. The allied family of
+  gymnosperms, the Cycadaceae, first appear in the Carboniferous
+  era, but very scantily; are most abundant in the Oolite, from
+  which formation 116 species are known, and then steadily diminish
+  to the Tertiary, although there are seventy-five living
+  species.</p>
+
+  <p>We now come to the true flowering plants, and we first
+  <a name="Page_400" id="Page_400"></a>meet with monocotyledons in
+  the Carboniferous and Permian formations. The character of these
+  fossils was long disputed, but is now believed to be well
+  established; and the sub-class continues to be present in small
+  numbers in all succeeding deposits, becoming rather plentiful in
+  the Upper Cretaceous, and very abundant in the Eocene and
+  Miocene. In the latter formation 272 species have been
+  discovered; but the 116 species in the Eocene form a larger
+  proportion of the total vegetation of the period.</p>
+
+  <p>True dicotyledons appear very much later, in the Cretaceous
+  period, and only in its upper division, if we except a single
+  species from the Urgonian beds of Greenland. The remarkable thing
+  is that we here find the sub-class fully developed and in great
+  luxuriance of types, all the three divisions&mdash;Apetalae,
+  Polypetalae, and Gamopetalae&mdash;being represented, with a
+  total of no less than 770 species. Among them are such familiar
+  forms as the poplar, the birch, the beech, the sycamore, and the
+  oak; as well as the fig, the true laurel, the sassafras, the
+  persimmon, the maple, the walnut, the magnolia, and even the
+  apple and the plum tribes. Passing on to the Tertiary period the
+  numbers increase, till they reach their maximum in the Miocene,
+  where more than 2000 species of dicotyledons have been
+  discovered. Among these the proportionate number of the higher
+  gamopetalae has slightly increased, but is considerably less than
+  at the present day.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Possible Cause of sudden late Appearance of Exogens.</i></p>
+
+  <p>The sudden appearance of fully developed exogenous flowering
+  plants in the Cretaceous period is very analogous to the equally
+  sudden appearance of all the chief types of placental mammalia in
+  the Eocene; and in both cases we must feel sure that this
+  suddenness is only apparent, due to unknown conditions which have
+  prevented their preservation (or their discovery) in earlier
+  formations. The case of the dicotyledonous plants is in some
+  respects the most extraordinary, because in the earlier Mesozoic
+  formations we appear to have a fair representation of the flora
+  of the period, including such varied forms as ferns, equisetums,
+  cycads, conifers, and monocotyledons. The only hint at an
+  explanation of this anomaly has been given by Mr. Ball, who
+  supposes <a name="Page_401" id="Page_401"></a>that all these
+  groups inhabited the lowlands, where there was not only excessive
+  heat and moisture, but also a superabundance of carbonic acid in
+  the atmosphere&mdash;conditions under which these groups had been
+  developed, but which were prejudicial to the dicotyledons. These
+  latter are supposed to have originated on the high table-lands
+  and mountain ranges, in a rarer and drier atmosphere in which the
+  quantity of carbonic acid gas was much less; and any deposits
+  formed in lake beds at high altitudes and at such a remote epoch
+  have been destroyed by denudation, and hence we have no record of
+  their existence.<a name="FNanchor_193_193" id=
+  "FNanchor_193_193"></a><a href="#Footnote_193_193" class=
+  "fnanchor">[193]</a></p>
+
+  <p>During a few weeks spent recently in the Rocky Mountains, I
+  was struck by the great scarcity of monocotyledons and ferns in
+  comparison with dicotyledons&mdash;a scarcity due apparently to
+  the dryness and rarity of the atmosphere favouring the higher
+  groups. If we compare Coulter's <i>Rocky Mountain Botany</i> with
+  Gray's <i>Botany of the Northern (East) United States</i>, we
+  have two areas which differ chiefly in the points of altitude and
+  atmospheric moisture. Unfortunately, in neither of these works
+  are the species consecutively numbered; but by taking the pages
+  occupied by the two divisions of dicotyledons on the one hand,
+  monocotyledons and ferns on the other, we can obtain a good
+  approximation. In this way we find that in the flora of the
+  North-Eastern States the monocotyledons and ferns are to the
+  dicotyledons in the proportion of 45 to 100; in the Rocky
+  Mountains they are in the proportion of only 34 to 100; while if
+  we take an exclusively Alpine flora, as given by Mr. Ball, there
+  are not one-fifth as many monocotyledons as dicotyledons. These
+  facts show that even at the present day elevated plateaux and
+  mountains are more favourable to dicotyledons than to
+  monocotyledons, and we may, therefore, well suppose that the
+  former originated within such elevated areas, and were for long
+  ages confined to them. It is interesting to note that their
+  richest early remains have been found in the central regions of
+  the North American continent, where they now, proportionally,
+  most abound, and where the conditions of altitude and a dry
+  atmosphere were probably present at a very early period.</p>
+
+  <p><a name="Page_402" id="Page_402"></a></p>
+
+  <div class="figcenter" style="width: 500px;">
+    <a name="fig34" id="fig34"></a> <img src="images/fig34.jpg"
+    width="500" height="358" alt=
+    "FIG. 34.&mdash;Diagram illustrating the Geological Distribution of Plants."
+    title=
+    "FIG. 34.&mdash;Diagram illustrating the Geological Distribution of Plants." />
+     <b>FIG. 34.&mdash;Diagram illustrating the Geological
+    Distribution of Plants.</b>
+  </div>
+
+  <p>The diagram (Fig. 34), slightly modified from one given by Mr.
+  Ward, will illustrate our present knowledge of the development of
+  the vegetable kingdom in geological time. <a name="Page_403" id=
+  "Page_403"></a>The shaded vertical bands exhibit the proportions
+  of the fossil forms actually discovered, while the outline
+  extensions are intended to show what we may fairly presume to
+  have been the approximate periods of origin, and progressive
+  increase of the number of species, of the chief divisions of the
+  vegetable kingdom. These seem to accord fairly well with their
+  respective grades of development, and thus offer no obstacle to
+  the acceptance of the belief in their progressive evolution.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Geological Distribution of Insects.</i></p>
+
+  <p>The marvellous development of insects into such an endless
+  variety of forms, their extreme specialisation, and their
+  adaptation to almost every possible condition of life, would
+  almost necessarily imply an extreme antiquity. Owing, however, to
+  their small size, their lightness, and their usually aerial
+  habits, no class of animals has been so scantily preserved in the
+  rocks; and it is only recently that the whole of the scattered
+  material relating to fossil insects and their allies have been
+  brought together by Mr. Samuel H. Scudder of Boston, and we have
+  thus learned their bearing on the theory of evolution.<a name=
+  "FNanchor_194_194" id="FNanchor_194_194"></a><a href=
+  "#Footnote_194_194" class="fnanchor">[194]</a></p>
+
+  <p>The most striking fact which presents itself on a glance at
+  the distribution of fossil insects, is the completeness of the
+  representation of all the chief types far back in the Secondary
+  period, at which time many of the existing families appear to
+  have been perfectly differentiated. Thus in the Lias we find
+  dragonflies "apparently as highly specialised as to-day, no less
+  than four tribes being present." Of beetles we have undoubted
+  Curculionidae from the Lias and Trias; Chrysomelidae in the same
+  deposits; Cerambycidae in the Oolites; Scarabaeidae in the Lias;
+  Buprestidae in the Trias; Elateridae, Trogositidae, and
+  Nitidulidae in the Lias; Staphylinidae in the English Purbecks;
+  while Hydrophilidae, Gyrinidae, and Carabidae occur in the Lias.
+  All these forms are well represented, but there are many other
+  families doubtfully identified in equally ancient rocks. Diptera
+  of the families Empidae, Asilidae, and Tipulidae have been found
+  as far back as the Lias. Of Lepidoptera, Sphingidae and Tineidae
+  have been found <a name="Page_404" id="Page_404"></a>in the
+  Oolite; while ants, representing the highly specialised
+  Hymenoptera, have occurred in the Purbeck and Lias.</p>
+
+  <p>This remarkable identity of the families of very ancient with
+  those of existing insects is quite comparable with the apparently
+  sudden appearance of existing genera of trees in the Cretaceous
+  epoch. In both cases we feel certain that we must go very much
+  farther back in order to find the ancestral forms from which they
+  were developed, and that at any moment some fresh discovery may
+  revolutionise our ideas as to the antiquity of certain groups.
+  Such a discovery was made while Mr. Scudder's work was passing
+  through the press. Up to that date all the existing orders of
+  true insects appeared to have originated in the Trias, the
+  alleged moth and beetle of the Coal formation having been
+  incorrectly determined. But now, undoubted remains of beetles
+  have been found in the Coal measures of Silesia, thus supporting
+  the interpretation of the borings in carboniferous trees as
+  having been made by insects of this order, and carrying back this
+  highly specialised form of insect life well into Palaeozoic
+  times. Such a discovery renders all speculation as to the origin
+  of true insects premature, because we may feel sure that all the
+  other orders of insects, except perhaps hymenoptera and
+  lepidoptera, were contemporaneous with the highly specialised
+  beetles.</p>
+
+  <p>The less highly organised terrestrial arthropoda&mdash;the
+  Arachnida and Myriapoda&mdash;are, as might be expected, much
+  more ancient. A fossil spider has been found in the
+  Carboniferous, and scorpions in the Upper Silurian rocks of
+  Scotland, Sweden, and the United States. Myriapoda have been
+  found abundantly in the Carboniferous and Devonian formations;
+  but all are of extinct orders, exhibiting a more generalised
+  structure than living forms.</p>
+
+  <p>Much more extraordinary, however, is the presence in the
+  Palaeozoic formations of ancestral forms of true insects, termed
+  by Mr. Scudder Palaeodictyoptera. They consist of generalised
+  cockroaches and walking-stick insects (Orthopteroidea); ancient
+  mayflies and allied forms, of which there are six families and
+  more than thirty genera (Neuropteroidea); three genera of
+  Hemipteroidea resembling various Homoptera and Hemiptera, mostly
+  from the Carboniferous formation, a few from the Devonian, and
+  one ancestral cockroach (Palaeoblattina) <a name="Page_405" id=
+  "Page_405"></a>from the Middle Silurian sandstone of France. If
+  this occurrence of a true hexapod insect from the Middle Silurian
+  be really established, taken in connection with the well-defined
+  Coleoptera from the Carboniferous, the origin of the entire group
+  of terrestrial arthropoda is necessarily thrown back into the
+  Cambrian epoch, if not earlier. And this cannot be considered
+  improbable in view of the highly differentiated land
+  plants&mdash;ferns, equisetums, and lycopods&mdash;in the Middle
+  or Lower Silurian, and even a conifer (Cordaites Robbii) in the
+  Upper Silurian; while the beds of graphite in the Laurentian were
+  probably formed from terrestrial vegetation.</p>
+
+  <p>On the whole, then, we may affirm that, although the
+  geological record of the insect life of the earth is
+  exceptionally imperfect, it yet decidedly supports the evolution
+  hypothesis. The most specialised order, Lepidoptera, is the most
+  recent, only dating back to the Oolite; the Hymenoptera, Diptera,
+  and Homoptera go as far as the Lias; while the Orthoptera and
+  Neuroptera extend to the Trias. The recent discovery of
+  Coleoptera in the Carboniferous shows, however, that the
+  preceding limits are not absolute, and will probably soon be
+  overpassed. Only the more generalised ancestral forms of winged
+  insects have been traced back to Silurian time, and along with
+  them the less highly organised scorpions; facts which serve to
+  show us the extreme imperfection of our knowledge, and indicate
+  possibilities of a world of terrestrial life in the remotest
+  Palaeozoic times.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Geological Succession of Vertebrata.</i></p>
+
+  <p>The lowest forms of vertebrates are the fishes, and these
+  appear first in the geological record in the Upper Silurian
+  formation. The most ancient known fish is a Pteraspis, one of the
+  bucklered ganoids or plated fishes&mdash;by no means a very low
+  type&mdash;allied to the sturgeon (Accipenser) and alligator-gar
+  (Lepidosteus), but, as a group, now nearly extinct. Almost
+  equally ancient are the sharks, which under various forms still
+  abound in our seas. We cannot suppose these to be nearly the
+  earliest fishes, especially as the two lowest orders, now
+  represented by the Amphioxus or lancelet and the lampreys, have
+  not yet been found fossil. The ganoids were greatly developed in
+  the Devonian era, and continued till the <a name="Page_406" id=
+  "Page_406"></a>Cretaceous, when they gave way to the true osseous
+  fishes, which had first appeared in the Jurassic period, and have
+  continued to increase till the present day. This much later
+  appearance of the higher osseous fishes is quite in accordance
+  with evolution, although some of the very lowest forms, the
+  lancelet and the lampreys, together with the archaic ceratodus,
+  have survived to our time.</p>
+
+  <p>The Amphibia, represented by the extinct labyrinthodons,
+  appear first in the Carboniferous rocks, and these peculiar forms
+  became extinct early in the Secondary period. The labyrinthodons
+  were, however, highly specialised, and do not at all indicate the
+  origin of the class, which may be as ancient as the lower forms
+  of fishes. Hardly any recognisable remains of our existing
+  groups&mdash;the frogs, toads, and salamanders&mdash;are found
+  before the Tertiary period, a fact which indicates the extreme
+  imperfection of the record as regards this class of animals.</p>
+
+  <p>True reptiles have not been found till we reach the Permian
+  where Prohatteria and Proterosaurus occur, the former closely
+  allied to the lizard-like Sphenodon of New Zealand, the latter
+  having its nearest allies in the same group of
+  reptiles&mdash;Rhyncocephala, other forms of which occur in the
+  Trias. In this last-named formation the earliest
+  crocodiles&mdash;Phytosaurus (Belodon) and Stagonolepis occur, as
+  well as the earliest tortoises&mdash;Chelytherium, Proganochelys,
+  and Psephoderma.<a name="FNanchor_195_195" id=
+  "FNanchor_195_195"></a><a href="#Footnote_195_195" class=
+  "fnanchor">[195]</a> Fossil serpents have been first found in the
+  Cretaceous formation, but the conditions for the preservation of
+  these forms have evidently been unfavourable, and the record is
+  correspondingly incomplete. The marine Plesiosauri and
+  Ichthyosauri, the flying Pterodactyles, the terrestrial Iguanodon
+  of Europe, and the huge Atlantosaurus of Colorado&mdash;the
+  largest land animal that has ever lived upon the earth<a name=
+  "FNanchor_196_196" id="FNanchor_196_196"></a><a href=
+  "#Footnote_196_196" class="fnanchor">[196]</a>&mdash;all belong
+  to special developments of the reptilian type which flourished
+  during the Secondary epoch, and then became extinct.</p>
+
+  <p><a name="Page_407" id="Page_407"></a>Birds are among the
+  rarest of fossils, due, no doubt, to their aerial habits removing
+  them from the ordinary dangers of flood, bog, or ice which
+  overwhelm mammals and reptiles, and also to their small specific
+  gravity which keeps them floating on the surface of water till
+  devoured. Their remains were long confined to Tertiary deposits,
+  where many living genera and a few extinct forms have been found.
+  The only birds yet known from the older rocks are the toothed
+  birds (Odontornithes) of the Cretaceous beds of the United
+  States, belonging to two distinct families and many genera; a
+  penguin-like form (Enaliornis) from the Upper Greensand of
+  Cambridge; and the well-known long-tailed Archaeopteryx from the
+  Upper Oolite of Bavaria. The record is thus imperfect and
+  fragmentary in the extreme; but it yet shows us, in the few birds
+  discovered in the older rocks, more primitive and generalised
+  types, while the Tertiary birds had already become specialised
+  like those living, and had lost both the teeth and the long
+  vertebral tail, which indicate reptilian affinities in the
+  earlier</p>
+
+  <p>Mammalia have been found, as already stated, as far back as
+  the Trias formation, in Europe in the United States and in South
+  Africa, all being very small, and belonging either to the
+  Marsupial order, or to some still lower and more generalised
+  type, out of which both Marsupials and Insectivora were
+  developed. Other allied forms have been found in the Lower and
+  Upper Oolite both of Europe and the United States. But there is
+  then a great gap in the whole Cretaceous formation, from which no
+  mammal has been obtained, although both in the Wealden and the
+  Upper Chalk in Europe, and in the Upper Cretaceous deposits of
+  the United States an abundant and well-preserved terrestrial
+  flora has been discovered. Why no mammals have left their remains
+  here it is impossible to say. We can only suppose that the
+  limited areas in which land plants have been so abundantly
+  preserved, did not present the conditions which are needed for
+  the fossilisation and preservation of mammalian remains.</p>
+
+  <p>When we come to the Tertiary formation, we find mammals in
+  abundance; but a wonderful change has taken place. The obscure
+  early types have disappeared, and we discover in their place a
+  whole series of forms belonging to existing orders, <a name=
+  "Page_408" id="Page_408"></a>and even sometimes to existing
+  families. Thus, in the Eocene we have remains of the opossum
+  family; bats apparently belonging to living genera; rodents
+  allied to the South American cavies and to dormice and squirrels;
+  hoofed animals belonging to the odd-toed and even-toed groups;
+  and ancestral forms of cats, civets, dogs, with a number of more
+  generalised forms of carnivora. Besides these there are whales,
+  lemurs, and many strange ancestral forms of proboscidea.<a name=
+  "FNanchor_197_197" id="FNanchor_197_197"></a><a href=
+  "#Footnote_197_197" class="fnanchor">[197]</a></p>
+
+  <p>The great diversity of forms and structures at so remote an
+  epoch would require for their development an amount of time,
+  which, judging by the changes that have occurred in other groups,
+  would carry us back far into the Mesozoic period. In order to
+  understand why we have no record of these changes in any part of
+  the world, we must fall back upon some such supposition as we
+  made in the case of the dicotyledonous plants. Perhaps, indeed,
+  the two cases are really connected, and the upland regions of the
+  primeval world, which saw the development of our higher
+  vegetation, may have also afforded the theatre for the gradual
+  development of the varied mammalian types which surprise us by
+  their sudden appearance in Tertiary times.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <img src="images/p408.jpg" width="400" height="209" alt=
+    "GEOLOGICAL DISTRIBUTION OF MAMMALIA." title=
+    "GEOLOGICAL DISTRIBUTION OF MAMMALIA." /> <b>GEOLOGICAL
+    DISTRIBUTION OF MAMMALIA.</b>
+  </div>
+
+  <p>Notwithstanding these irregularities and gaps in the record,
+  the accompanying table, summarising our actual knowledge of the
+  geological distribution of the five classes of vertebrata,
+  <a name="Page_409" id="Page_409"></a>exhibits a steady
+  progression from lower to higher types, excepting only the
+  deficiency in the bird record which is easily explained. The
+  comparative perfection of type in which each of these classes
+  first appears, renders it certain that the origin of each and all
+  of them must be sought much farther back than any records which
+  have yet been discovered. The researches of palaeontologists and
+  embryologists indicate a reptilian origin for birds and mammals,
+  while reptiles and amphibia arose, perhaps independently, from
+  fishes.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Concluding Remarks.</i></p>
+
+  <p>The brief review we have now taken of the more suggestive
+  facts presented by the geological succession of organic forms, is
+  sufficient to show that most, if not all, of the supposed
+  difficulties which it presents in the way of evolution, are due
+  either to imperfections in the geological record itself, or to
+  our still very incomplete knowledge of what is really recorded in
+  the earth's crust. We learn, however, that just as discovery
+  progresses, gaps are filled up and difficulties disappear; while,
+  in the case of many individual groups, we have already obtained
+  all the evidence of progressive development that can reasonably
+  be expected. We conclude, therefore, that the geological
+  difficulty has now disappeared; and that this noble science, when
+  properly understood, affords clear and weighty evidence of
+  evolution.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_183_183" id=
+      "Footnote_183_183"></a><a href=
+      "#FNanchor_183_183"><span class="label">[183]</span></a> The
+      reader who desires to understand this subject more fully,
+      should study chap. x. of the <i>Origin of Species</i>, and
+      chap. xiv. of Sir Charles Lyell's <i>Principles of
+      Geology</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_184_184" id=
+      "Footnote_184_184"></a><a href=
+      "#FNanchor_184_184"><span class="label">[184]</span></a> On
+      "Stagonolepis Robertsoni and on the Evolution of the
+      Crocodilia," in <i>Q.J. of Geological Society</i>, 1875; and
+      abstract in <i>Nature</i>, vol. xii. p. 38.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_185_185" id=
+      "Footnote_185_185"></a><a href=
+      "#FNanchor_185_185"><span class="label">[185]</span></a> From
+      a paper by Messrs. Scott and Osborne, "On the Origin and
+      Development of the Rhinoceros Group," read before the British
+      Association in 1883.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_186_186" id=
+      "Footnote_186_186"></a><a href=
+      "#FNanchor_186_186"><span class="label">[186]</span></a>
+      American Addresses, pp. 73-76.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_187_187" id=
+      "Footnote_187_187"></a><a href=
+      "#FNanchor_187_187"><span class="label">[187]</span></a>
+      Lecture on the Introduction and Succession of Vertebrate Life
+      in America, <i>Nature</i>, vol. xvi. p. 471.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_188_188" id=
+      "Footnote_188_188"></a><a href=
+      "#FNanchor_188_188"><span class="label">[188]</span></a>
+      <i>Nature</i>, vol. xxv. p. 84.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_189_189" id=
+      "Footnote_189_189"></a><a href=
+      "#FNanchor_189_189"><span class="label">[189]</span></a> See
+      <i>The Mammalia in their Relation to Primeval Times</i>, p.
+      102.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_190_190" id=
+      "Footnote_190_190"></a><a href=
+      "#FNanchor_190_190"><span class="label">[190]</span></a> For
+      a brief enumeration and description of these fossils, see the
+      author's <i>Geographical Distribution of Animals</i>, vol. i.
+      p. 146.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_191_191" id=
+      "Footnote_191_191"></a><a href=
+      "#FNanchor_191_191"><span class="label">[191]</span></a>
+      Sketch of Palaeobotany in Fifth Annual Report of U.S.
+      Geological Survey, 1883-84, pp. 363-452, with diagrams. Sir
+      J. William Dawson, speaking of the value of leaves for the
+      determination of fossil plants, says: "In my own experience I
+      have often found determinations of the leaves of trees
+      confirmed by the discovery of their fruits or of the
+      structure of their stems. Thus, in the rich cretaceous
+      plant-beds of the Dunvegan series, we have beech-nuts
+      associated in the same bed with leaves referred to
+      <i>Fagus</i>. In the Laramie beds I determined many years ago
+      nuts of the <i>Trapa</i> or water-chestnut, and subsequently
+      Lesquereux found in beds in the United States leaves which he
+      referred to the same genus. Later, I found in collections
+      made on the Red Deer River of Canada my fruits and
+      Lesquereux's leaves on the same slab. The presence of trees
+      of the genera <i>Carya</i> and <i>Juglans</i> in the same
+      formation was inferred from their leaves, and specimens have
+      since been obtained of silicified wood with the microscopic
+      structure of the modern butternut. Still we are willing to
+      admit that determinations from leaves alone are liable to
+      doubt."&mdash;<i>The Geological History of Plants</i>, p.
+      196.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_192_192" id=
+      "Footnote_192_192"></a><a href=
+      "#FNanchor_192_192"><span class="label">[192]</span></a> Sir
+      J. William Dawson's <i>Geological History of Plants</i>, p.
+      18.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_193_193" id=
+      "Footnote_193_193"></a><a href=
+      "#FNanchor_193_193"><span class="label">[193]</span></a> "On
+      the Origin of the Flora of the European Alps," <i>Proc. of
+      Roy. Geog. Society</i>, vol. i. (1879), pp. 564-588.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_194_194" id=
+      "Footnote_194_194"></a><a href=
+      "#FNanchor_194_194"><span class="label">[194]</span></a>
+      Systematic Review of our Present Knowledge of Fossil Insects,
+      including Myriapods and Arachnids (<i>Bull. of U.S. Geol.
+      Survey</i>, No. 31, Washington, 1886).</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_195_195" id=
+      "Footnote_195_195"></a><a href=
+      "#FNanchor_195_195"><span class="label">[195]</span></a> For
+      the facts as to the early appearance of the above named
+      groups of reptiles I am indebted to Mr. E. Lydekker of the
+      Geological Department of the Natural History Museum.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_196_196" id=
+      "Footnote_196_196"></a><a href=
+      "#FNanchor_196_196"><span class="label">[196]</span></a>
+      According to Professor Marsh this creature was 50 or 60 feet
+      long, and when erect, at least 30 feet in height. It fed upon
+      the foliage of the mountain forests of the Cretaceous epoch,
+      the remains of which are preserved with it.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_197_197" id=
+      "Footnote_197_197"></a><a href=
+      "#FNanchor_197_197"><span class="label">[197]</span></a> For
+      fuller details, see the author's <i>Geographical Distribution
+      of Animals</i>, and Heilprin's <i>Geographical and Geological
+      Distribution of Animals</i>.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_XIV" id="CHAPTER_XIV"></a><a name="Page_410"
+  id="Page_410"></a>CHAPTER XIV</h2>
+
+  <p style='font-weight:bold;'>FUNDAMENTAL PROBLEMS IN RELATION TO
+  VARIATION AND HEREDITY</p>
+
+  <div class="blockquot">
+    <p>Fundamental difficulties and objections&mdash;Mr. Herbert
+    Spencer's factors of organic evolution&mdash;Disuse and effects
+    of withdrawal of natural selection&mdash;Supposed effects of
+    disuse among wild animals&mdash;Difficulty as to co-adaptation
+    of parts by variation and selection&mdash;Direct action of the
+    environment&mdash;The American school of
+    evolutionists&mdash;Origin of the feet of the
+    ungulates&mdash;Supposed action of animal
+    intelligence&mdash;Semper on the direct influence of the
+    environment&mdash;Professor Geddes's theory of variation in
+    plants&mdash;Objections to the theory&mdash;On the origin of
+    spines&mdash;Variation and selection overpower the effects of
+    use and disuse&mdash;Supposed action of the environment in
+    imitating variations&mdash;Weismann's theory of
+    heredity&mdash;The cause of variation&mdash;The non-heredity of
+    acquired characters&mdash;The theory of
+    instinct&mdash;Concluding remarks.</p>
+  </div>
+
+  <p>Having now set forth and illustrated at some length the most
+  important of the applications of the development hypothesis in
+  the explanation of the broader and more generally interesting
+  phenomena presented by the organic world, we propose to discuss
+  some of the more fundamental problems and difficulties which have
+  recently been adduced by eminent naturalists. It is the more
+  necessary to do this, because there is now a tendency to minimise
+  the action of natural selection in the production of organic
+  forms, and to set up in its place certain fundamental principles
+  of variation or laws of growth, which it is urged are the real
+  originators of the several lines of development, and of most of
+  the variety of form and structure in the vegetable and animal
+  kingdoms. These views have, moreover, been seized upon by popular
+  writers to throw doubt and discredit on the whole theory of
+  <a name="Page_411" id="Page_411"></a>evolution, and especially on
+  Darwin's presentation of that theory, to the bewilderment of the
+  general public, who are quite unable to decide how far the new
+  views, even if well established, tend to subvert the Darwinian
+  theory, or whether they are really more than subsidiary parts of
+  it, and quite powerless without it to produce any effect
+  whatever.</p>
+
+  <p>The writers whose special views we now propose to consider
+  are: (1) Mr. Herbert Spencer, on modification of structures
+  arising from modification of functions, as set forth in his
+  <i>Factors of Organic Evolution</i>. (2) Dr. E.D. Cope, who
+  advocates similar views in detail, in his work entitled <i>The
+  Origin of the Fittest</i>, and may be considered the head of a
+  school of American naturalists who minimise the agency of natural
+  selection. (3) Dr. Karl Semper, who has especially studied the
+  direct influence of the environment in the whole animal kingdom,
+  and has set forth his views in a volume on <i>The Natural
+  Conditions of Existence as they Affect Animal Life</i>. (4) Mr.
+  Patrick Geddes, who urges that fundamental laws of growth, and
+  the antagonism of vegetative and reproductive forces, account for
+  much that has been imputed to natural selection.</p>
+
+  <p>We will now endeavour to ascertain what are the more important
+  facts and arguments adduced by each of the above writers, and how
+  far they offer a substitute for the action of natural selection;
+  having done which, a brief account will be given of the views of
+  Dr. Aug. Weismann, whose theory of heredity will, if established,
+  strike at the very root of the arguments of the first three of
+  the writers above referred to.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Mr. Herbert Spencer's Factors of Organic Evolution.</i></p>
+
+  <p>Mr. Spencer, while fully recognising the importance and wide
+  range of the principle of natural selection, thinks that
+  sufficient weight has not been given to the effects of use and
+  disuse as a factor in evolution, or to the direct action of the
+  environment in determining or modifying organic structures. As
+  examples of the former class of actions, he adduces the decreased
+  size of the jaws in the civilised races of mankind, the
+  inheritance of nervous disease produced by overwork, the great
+  and inherited development of the udders in cows and goats, and
+  the shortened legs, jaws, and snout in <a name="Page_412" id=
+  "Page_412"></a>improved races of pigs&mdash;the two latter
+  examples being quoted from Mr. Darwin,&mdash;and other cases of
+  like nature. As examples of the latter, Mr. Darwin is again
+  quoted as admitting that there are many cases in which the action
+  of similar conditions appears to have produced corresponding
+  changes in different species; and we have a very elaborate
+  discussion of the direct action of the medium in modifying the
+  protoplasm of simple organisms, so as to bring about the
+  difference between the outer surface and the inner part that
+  characterises the cells or other units of which they are
+  formed.</p>
+
+  <p>Now, although this essay did little more than bring together
+  facts which had been already adduced by Mr. Darwin or by Mr.
+  Spencer himself, and lay stress upon their importance, its
+  publication in a popular review was immediately seized upon as
+  "an avowed and definite declaration against some of the leading
+  ideas on which the Mechanical Philosophy depends," and as being
+  "fatal to the adequacy of the Mechanical Philosophy as any
+  explanation of organic evolution,"<a name="FNanchor_198_198" id=
+  "FNanchor_198_198"></a><a href="#Footnote_198_198" class=
+  "fnanchor">[198]</a>&mdash;an expression of opinion which would
+  be repudiated by every Darwinian. For, even admitting the
+  interpretation which Mr. Spencer puts on the facts he adduces,
+  they are all included in the causes which Darwin himself
+  recognised as having acted in bringing about the infinitude of
+  forms in the organic world. In the concluding chapter of the
+  <i>Origin of Species</i> he says: "I have now recapitulated the
+  facts and considerations which have thoroughly convinced me that
+  species have been modified during a long course of descent. This
+  has been effected chiefly through the natural selection of
+  numerous successive, slight, favourable variations; aided in an
+  important manner by the inherited effects of the use and disuse
+  of parts; and in an unimportant manner&mdash;that is, in relation
+  to adaptive structures whether past or present, by the direct
+  action of external conditions, and by variations which seem to
+  us, in our ignorance, to arise spontaneously." This passage,
+  summarising Darwin's whole inquiry, and explaining his final
+  point of view, shows how very inaccurate may be the popular
+  notion, as expressed by the Duke of Argyll, of any supposed
+  additions to the causes of change of species as recognised by
+  Darwin.</p>
+
+  <p><a name="Page_413" id="Page_413"></a>But, as we shall see
+  presently, there is now much reason to believe that the supposed
+  inheritance of acquired modifications&mdash;that is, of the
+  effects of use and disuse, or of the direct influence of the
+  environment&mdash;is not a fact; and if so, the very foundation
+  is taken away from the whole class of objections on which so much
+  stress is now laid. It therefore becomes important to inquire
+  whether the facts adduced by Darwin, Spencer, and others, do
+  really necessitate such inheritance, or whether any other
+  interpretation of them is possible. I believe there is such an
+  interpretation; and we will first consider the cases of disuse on
+  which Mr. Spencer lays most stress.</p>
+
+  <p>The cases Mr. Spencer adduces as demonstrating the effects of
+  disuse in diminishing the size and strength of organs are, the
+  diminished size of the jaws in the races of civilised men, and
+  the diminution of the muscles used in closing the jaws in the
+  case of pet-dogs fed for generations on soft food. He argues that
+  the minute reduction in any one generation could not possibly
+  have been useful, and, therefore, not the subject of natural
+  selection; and against the theory of correlation of the
+  diminished jaw with increased brain in man, he urges that there
+  are cases of large brain development, accompanied by jaws above
+  the average size. Against the theory of economy of nutrition in
+  the case of the pet-dogs, he places the abundant food of these
+  animals which would render such economy needless.</p>
+
+  <p>But neither he nor Mr. Darwin has considered the effects of
+  the withdrawal of the action of natural selection in keeping up
+  the parts in question to their full dimensions, which, of itself,
+  seems to me quite adequate to produce the results observed.
+  Recurring to the evidence, adduced in Chapter III, of the
+  constant variation occurring in all parts of the organism, while
+  selection is constantly acting on these variations in eliminating
+  all that fall below the best working standard, and preserving
+  only those that are fully up to it; and, remembering further,
+  that, of the whole number of the increase produced annually, only
+  a small percentage of the best adapted can be preserved, we shall
+  see that every useful organ will be kept up nearly to its higher
+  limit of size and efficiency. Now Mr. Galton has proved
+  experimentally that, when any part has <a name="Page_414" id=
+  "Page_414"></a>thus been increased (or diminished) by selection,
+  there is in the offspring a strong tendency to revert to a mean
+  or average size, which tends to check further increase. And this
+  mean appears to be, not the mean of the actual existing
+  individuals but a lower mean, or that from which they had been
+  recently raised by selection.<a name="FNanchor_199_199" id=
+  "FNanchor_199_199"></a><a href="#Footnote_199_199" class=
+  "fnanchor">[199]</a> He calls this the law of "Regression towards
+  Mediocrity," and it has been proved by experiments with
+  vegetables and by observations on mankind. This regression, in
+  every generation, takes place even when both parents have been
+  selected for their high development of the organ in question; but
+  when there is no such selection, and crosses are allowed among
+  individuals of every grade of development, the deterioration will
+  be very rapid; and after a time not only will the average size of
+  the part be greatly reduced, but the instances of full
+  development will become very rare. Thus what Weismann terms
+  "panmixia," or free intercrossing, will co-operate with Galton's
+  law of "regression towards mediocrity," and the result will be
+  that, whenever selection ceases to act on any part or organ which
+  has heretofore been kept up to a maximum of size and efficiency,
+  the organ in question will rapidly decrease till it reaches a
+  mean value considerably below the mean of the progeny that has
+  usually been produced each year, and very greatly below the mean
+  of that portion which has survived annually; and this will take
+  place by the general law of heredity, and quite irrespective of
+  any <i>use</i> or <i>disuse</i> of the part in question. Now, no
+  observations have been adduced by Mr. Spencer or others, showing
+  that the average amount of change supposed to be due to
+  <i>disuse</i> is greater than that due to the law of regression
+  towards mediocrity; while even if it were somewhat greater, we
+  can see many possible contributory causes to its production. In
+  the case of civilised man's diminished jaw, there may well be
+  some correlation between the jaw and the brain, seeing that
+  increased mental activity would lead to the withdrawal of blood
+  and of nervous energy from adjacent parts, and might thus lead to
+  diminished growth of those parts in the individual. And in the
+  case of pet-dogs, the selection of small or short-headed
+  individuals would imply the unconscious selection of those with
+  less massive temporal muscles, and thus lead to the concomitant
+  <a name="Page_415" id="Page_415"></a>reduction of those muscles.
+  The amount of reduction observed by Darwin in the wing-bones of
+  domestic ducks and poultry, and in the hind legs of tame rabbits,
+  is very small, and is certainly no greater than the above causes
+  will well account for; while so many of the external characters
+  of all our domestic animals have been subject to long-continued
+  artificial selection, and we are so ignorant of the possible
+  correlations of different parts, that the phenomena presented by
+  them seem sufficiently explained without recurrence to the
+  assumption that any changes in the individual, due to disuse, are
+  inherited by the offspring.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Supposed Effects of Disuse among Wild Animals.</i></p>
+
+  <p>It may be urged, however, that among wild animals we have many
+  undoubted results of disuse much more pronounced than those among
+  domestic kinds, results which cannot be explained by the causes
+  already adduced. Such are the reduced size of the wings of many
+  birds on oceanic islands; the abortion of the eyes in many cave
+  animals, and in some which live underground; and the loss of the
+  hind limbs in whales and in some lizards. These cases differ
+  greatly in the amount of the reduction of parts which has taken
+  place, and may be due to different causes. It is remarkable that
+  in some of the birds of oceanic islands the reduction is little
+  if at all greater than in domestic birds, as in the water-hen of
+  Tristan d'Acunha. Now if the reduction of wing were due to the
+  hereditary effects of disuse, we should expect a very much
+  greater effect in a bird inhabiting an oceanic island than in a
+  domestic bird, where the disuse has been in action for an
+  indefinitely shorter period. In the case of many other birds,
+  however&mdash;as some of the New Zealand rails and the extinct
+  dodo of Mauritius&mdash;the wings have been reduced to a much
+  more rudimentary condition, though it is still obvious that they
+  were once organs of flight; and in these cases we certainly
+  require some other causes than those which have reduced the wings
+  of our domestic fowls. One such cause may have been of the same
+  nature as that which has been so efficient in reducing the wings
+  of the insects of oceanic islands&mdash;the destruction of those
+  which, during the occasional use of their wings, were carried out
+  to sea. This form of natural selection may well have acted in the
+  case of <a name="Page_416" id="Page_416"></a>birds whose powers
+  of flight were already somewhat reduced, and to whom, there being
+  no enemies to escape from, their use was only a source of danger.
+  We may thus, perhaps, account for the fact that many of these
+  birds retain small but useless wings with which they never fly;
+  for, the wings having been reduced to this functionless
+  condition, no power could reduce them further except correlation
+  of growth or economy of nutrition, causes which only rarely come
+  into play.</p>
+
+  <p>The complete loss of eyes in some cave animals may, perhaps,
+  be explained in a somewhat similar way. Whenever, owing to the
+  total darkness, they became useless, they might also become
+  injurious, on account of their delicacy of organisation and
+  liability to accidents and disease; in which case natural
+  selection would begin to act to reduce, and finally abort them;
+  and this explains why, in some cases, the rudimentary eye
+  remains, although completely covered by a protective outer skin.
+  Whales, like moas and cassowaries, carry us back to a remote
+  past, of whose conditions we know too little for safe
+  speculation. We are quite ignorant of the ancestral forms of
+  either of these groups, and are therefore without the materials
+  needful for determining the steps by which the change took place,
+  or the causes which brought it about.<a name="FNanchor_200_200"
+  id="FNanchor_200_200"></a><a href="#Footnote_200_200" class=
+  "fnanchor">[200]</a></p>
+
+  <p>On a review of the various examples that have been given by
+  Mr. Darwin and others of organs that have been reduced or
+  aborted, there seems too much diversity in the results for all to
+  be due to so direct and uniform a cause as the individual effects
+  of disuse accumulated by heredity. For if that were the only or
+  chief efficient cause, and a cause capable of producing a decided
+  effect during the comparatively short period <a name="Page_417"
+  id="Page_417"></a>of the existence of animals in a state of
+  domestication, we should expect to find that, in wild species,
+  all unused parts or organs had been reduced to the smallest
+  rudiments, or had wholly disappeared. Instead of this we find
+  various grades of reduction, indicating the probable result of
+  several distinct causes, sometimes acting separately, sometimes
+  in combination, such as those we have already pointed out.</p>
+
+  <p>And if we find no positive evidence of <i>disuse</i>, acting
+  by its direct effect on the individual, being transmitted to the
+  offspring, still less can we find such evidence in the case of
+  the <i>use</i> of organs. For here the very fact of <i>use</i>,
+  in a wild state, implies <i>utility</i>, and utility is the
+  constant subject for the action of natural selection; while among
+  domestic animals those parts which are exceptionally used are so
+  used in the service of man, and have thus become the subjects of
+  artificial selection. Thus "the great and inherited development
+  of the udders in cows and goats," quoted by Spencer from Darwin,
+  really affords no proof of inheritance of the increase due to
+  use, because, from the earliest period of the domestication of
+  these animals, abundant milk-production has been highly esteemed,
+  and has thus been the subject of selection; while there are no
+  cases among wild animals that may not be better explained by
+  variation and natural selection.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Difficulty as to Co-adaptation of Parts by Variation and
+  Selection.</i></p>
+
+  <p>Mr. Spencer again brings forward this difficulty, as he did in
+  his <i>Principles of Biology</i> twenty-five years ago, and urges
+  that all the adjustments of bones, muscles, blood-vessels, and
+  nerves which would be required during, for example, the
+  development of the neck and fore-limbs of the giraffe, could
+  <a name="Page_418" id="Page_418"></a>not have been effected by
+  "simultaneous fortunate spontaneous variations." But this
+  difficulty is fully disposed of by the facts of simultaneous
+  variation adduced in our third chapter, and has also been
+  specially considered in Chapter VI, p. 127. The best answer to
+  this objection may, perhaps, be found in the fact that the very
+  thing said to be impossible by variation and natural selection
+  has been again and again effected by variation and artificial
+  selection. During the process of formation of such breeds as the
+  greyhound or the bulldog, of the race-horse and carthorse, of the
+  fantail pigeon or the otter-sheep, many co-ordinate adjustments
+  have been produced; and no difficulty has occurred, whether the
+  change has been effected by a single variation&mdash;as in the
+  last case named&mdash;or by slow steps, as in all the others. It
+  seems to be forgotten that most animals have such a surplus of
+  vitality and strength for all the ordinary occasions of life that
+  any slight superiority in one part can be at once utilised; while
+  the moment any want of balance occurs, variations in the
+  insufficiently developed parts will be selected to bring back the
+  harmony of the whole organisation. The fact that, in all domestic
+  animals, variations do occur, rendering them swifter or stronger,
+  larger or smaller, stouter or slenderer, and that such variations
+  can be separately selected and accumulated for man's purposes, is
+  sufficient to render it certain that similar or even greater
+  changes may be effected by natural selection, which, as Darwin
+  well remarks, "acts on every internal organ, on every shade of
+  constitututional difference, on the whole machinery of life." The
+  difficulty as to co-adaptation of parts by variation and natural
+  selection appears to me, therefore, to be a wholly imaginary
+  difficulty which has no place whatever in the operations of
+  nature.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Direct Action of the Environment.</i></p>
+
+  <p>Mr. Spencer's last objection to the wide scope given by
+  Darwinians to the agency of natural selection is, that organisms
+  are acted upon by the environment, which produces in them
+  definite changes, and that these changes in the individual are
+  transmitted by inheritance, and thus become increased in
+  successive generations. That such changes are produced in the
+  individual there is ample evidence, but that they are in<a name=
+  "Page_419" id="Page_419"></a>herited independently of any form of
+  selection or of reversion is exceedingly doubtful, and Darwin
+  nowhere expresses himself as satisfied with the evidence. The two
+  very strongest cases he mentions are the twenty-nine species of
+  American trees which all differed in a corresponding way from
+  their nearest European allies; and the American maize which
+  became changed after three generations in Europe. But in the case
+  of the trees the differences alleged may be partly due to
+  correlation with constitutional peculiarities dependent on
+  climate, especially as regards the deeper tint of the fading
+  leaves and the smaller size of the buds and seeds in America than
+  in Europe; while the less deeply toothed or serrated leaves in
+  the American species are, in our present complete ignorance of
+  the causes and uses of serration, quite as likely to be due to
+  some form of adaptation as to any direct action of the climate.
+  Again, we are not told how many of the allied species do not vary
+  in this particular manner, and this is certainly an important
+  factor in any conclusion we may form on the question.</p>
+
+  <p>In the case of the maize it appears that one of the more
+  remarkable and highly selected American varieties was cultivated
+  in Germany, and in three years nearly all resemblance to the
+  original parent was lost; and in the sixth year it closely
+  resembled a common European variety, but was of somewhat more
+  vigorous growth. In this case no selection appears to have been
+  practised, and the effects may have been due to that "reversion
+  to mediocrity" which invariably occurs, and is more especially
+  marked in the case of varieties which have been rapidly produced
+  by artificial selection. It may be considered as a partial
+  reversion to the wild or unimproved stock; and the same thing
+  would probably have occurred, though perhaps less rapidly, in
+  America itself. As this is stated by Darwin to be the most
+  remarkable case known to him "of the direct and prompt action of
+  climate on a plant," we must conclude that such direct effects
+  have not been proved to be accumulated by inheritance,
+  independently of reversion or selection.</p>
+
+  <p>The remaining part of Mr. Spencer's essay is devoted to a
+  consideration of the hypothetical action of the environment on
+  the lower organisms which consist of simple cells or formless
+  masses of protoplasm; and he shows with great <a name="Page_420"
+  id="Page_420"></a>elaboration that the outer and inner parts of
+  these are necessarily subject to different conditions; and that
+  the outer actions of air or water lead to the formation of
+  integuments, and sometimes to other definite modifications of the
+  surface, whence arise permanent differences of structure.
+  Although in these cases also it is very difficult to determine
+  how much is due to direct modification by external agencies
+  transmitted and accumulated by inheritance, and how much to
+  spontaneous variations accumulated by natural selection, the
+  probabilities in favour of the former mode of action are here
+  greater, because there is no differentiation of nutritive and
+  reproductive cells in these simple organisms; and it can be
+  readily seen that any change produced in the latter will almost
+  certainly affect the next generation.<a name="FNanchor_201_201"
+  id="FNanchor_201_201"></a><a href="#Footnote_201_201" class=
+  "fnanchor">[201]</a> We are thus carried back almost to the
+  origin of life, and can only vaguely speculate on what took place
+  under conditions of which we know so little.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The American School of Evolutionists.</i></p>
+
+  <p>The tentative views of Mr. Spencer which we have just
+  discussed, are carried much further, and attempts have been made
+  to work them out in great detail, by many American naturalists,
+  whose best representative is Dr. E.D. Cope of
+  Philadelphia.<a name="FNanchor_202_202" id=
+  "FNanchor_202_202"></a><a href="#Footnote_202_202" class=
+  "fnanchor">[202]</a> This school endeavours to explain all the
+  chief modifications of form in the animal kingdom by fundamental
+  laws of growth and the inherited effects of use and effort,
+  returning, in fact, to the teachings of Lamarck as being at least
+  equally important with those of Darwin.</p>
+
+  <p>The following extract will serve to show the high position
+  claimed by this school as original discoverers, and as having
+  made important additions to the theory of evolution:</p>
+
+  <p>"Wallace and Darwin have propounded as the cause of
+  modification in descent their law of natural selection. This law
+  has been epitomised by Spencer as the 'survival of the fittest.'
+  This neat expression no doubt covers the case, but it leaves the
+  origin of the fittest entirely untouched. Darwin assumes a
+  'tendency to variation' in nature, and it is plainly <a name=
+  "Page_421" id="Page_421"></a>necessary to do this, in order that
+  materials for the exercise of a selection should exist. Darwin
+  and Wallace's law is then only restrictive, directive,
+  conservative, or destructive of something already created. I
+  propose, then, to seek for the originative laws by which these
+  subjects are furnished; in other words, for the causes of the
+  origin of the fittest."<a name="FNanchor_203_203" id=
+  "FNanchor_203_203"></a><a href="#Footnote_203_203" class=
+  "fnanchor">[203]</a></p>
+
+  <p>Mr. Cope lays great stress on the existence of a special
+  developmental force termed "bathmism" or growth-force, which acts
+  by means of retardation and acceleration "without any reference
+  to fitness at all;" that "instead of being controlled by fitness
+  it is the controller of fitness." He argues that "all the
+  characteristics of generalised groups from genera up (excepting,
+  perhaps, families) have been evolved under the law of
+  acceleration and retardation," combined with some intervention of
+  natural selection; and that specific characters, or species, have
+  been evolved by natural selection with some assistance from the
+  higher law. He, therefore, makes species and genera two
+  absolutely distinct things, the latter not developed out of the
+  former; generic characters and specific characters are, in his
+  opinion, fundamentally different, and have had different origins,
+  and whole groups of species have been simultaneously modified, so
+  as to belong to another genus; whence he thinks it "highly
+  probable that the same specific form has existed through a
+  succession of genera, and perhaps in different epochs of geologic
+  time."</p>
+
+  <p>Useful characters, he concludes, have been produced by the
+  special location of growth-force by use; useless ones have been
+  produced by location of growth-force without the influence of
+  use. Another element which determines the direction of
+  growth-force, and which precedes use, is effort; and "it is
+  thought that effort becomes incorporated into the metaphysical
+  acquisitions of the parent, and is inherited with other
+  metaphysical qualities by the young, which, during the period of
+  growth, is much more susceptible to modifying influences, and is
+  likely to exhibit structural change in consequence."<a name=
+  "FNanchor_204_204" id="FNanchor_204_204"></a><a href=
+  "#Footnote_204_204" class="fnanchor">[204]</a></p>
+
+  <p>From these few examples of their teachings, it is clear that
+  <a name="Page_422" id="Page_422"></a>these American evolutionists
+  have departed very widely from the views of Mr. Darwin, and in
+  place of the well-established causes and admitted laws to which
+  he appeals have introduced theoretical conceptions which have not
+  yet been tested by experiments or facts, as well as metaphysical
+  conceptions which are incapable of proof. And when they come to
+  illustrate these views by an appeal to palaeontology or
+  morphology, we find that a far simpler and more complete
+  explanation of the facts is afforded by the established
+  principles of variation and natural selection. The confidence
+  with which these new ideas are enunciated, and the repeated
+  assertion that without them Darwinism is powerless to explain the
+  origin of organic forms, renders it necessary to bestow a little
+  more time on the explanations they give us of well-known
+  phenomena with which, they assert, other theories are incompetent
+  to grapple.</p>
+
+  <p>As examples of use producing structural change, Mr. Cope
+  adduces the hooked and toothed beaks of the falcons and the
+  butcher-birds, and he argues that the fact of these birds
+  belonging to widely different groups proves that similarity of
+  use has produced a similar structural result. But no attempt is
+  made to show any direct causal connection between the use of a
+  bill to cut or tear flesh and the development of a tooth on the
+  mandible. Such use might conceivably strengthen the bill or
+  increase its size, but not cause a special tooth-like outgrowth
+  which was not present in the ancestral thrush-like forms of the
+  butcher-bird. On the other hand, it is clear that any variations
+  of the bill tending towards a hook or tooth would give the
+  possessor some advantage in seizing and tearing its prey, and
+  would thus be preserved and increased by natural selection.
+  Again, Mr. Cope urges the effects of a supposed "law of polar or
+  centrifugal growth" to counteract a tendency to unsymmetrical
+  growth, where one side of the body is used more than the other.
+  But the undoubted hurtfulness of want of symmetry in many
+  important actions or functions would rapidly eliminate any such
+  tendency. When, however, it has <a name="Page_423" id=
+  "Page_423"></a>become useful, as in the case of the single
+  enlarged claw of many Crustacea, it has been preserved by natural
+  selection.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Origin of the Feet of the Ungulates.</i></p>
+
+  <p>Perhaps the most original and suggestive of Mr. Cope's
+  applications of the theory of use and effort in modifying
+  structure are, his chapters "On the Origin of the Foot-Structure
+  of the Ungulates;" and that "On the Effect of Impacts and Strains
+  on the Feet of Mammalia;" and they will serve also to show the
+  comparative merits of this theory and that of natural selection
+  in explaining a difficult case of modification, especially as it
+  is an explanation claimed as new and original when first
+  enunciated in 1881. Let us, then, see how he deals with the
+  problem.</p>
+
+  <p>The remarkable progressive change of a four or five-toed
+  ancestor into the one-toed horse, and the equally remarkable
+  division of the whole group of ungulate animals into the odd-toed
+  and even-toed divisions, Mr. Cope attempts to explain by the
+  effects of impact and use among animals which frequented hard or
+  swampy ground respectively. On hard ground, it is urged, the long
+  middle toe would be most used and subjected to the greatest
+  strains, and would therefore acquire both strength and
+  development. It would then be still more exclusively used, and
+  the extra nourishment required by it would be drawn from the
+  adjacent less-used toes, which would accordingly diminish in
+  size, till, after a long series of changes, the records of which
+  are so well preserved in the American tertiary rocks, the true
+  one-toed horse was developed. In soft or swampy ground, on the
+  other hand, the tendency would be to spread out the foot so that
+  there were two toes on each side. The two middle toes would thus
+  be most used and most subject to strains, and would, therefore,
+  increase at the expense of the lateral toes. There would be, no
+  doubt, an advantage in these two functional toes being of equal
+  size, so as to prevent twisting of the foot while walking; and
+  variations tending to bring this about would be advantageous, and
+  would therefore be preserved. Thus, by a parallel series of
+  changes in another direction, adapted to a distinct set of
+  conditions, we should arrive at the symmetrical divided hoofs of
+  our deer and cattle. The fact <a name="Page_424" id=
+  "Page_424"></a>that sheep and goats are specially mountain and
+  rock-loving animals may be explained by their being a later
+  modification, since the divided hoof once formed is evidently
+  well adapted to secure a firm footing on rugged and precipitous
+  ground, although it could hardly have been first developed in
+  such localities. Mr. Cope thus concludes: "Certain it is that the
+  length of the bones in the feet of the ungulate orders has a
+  direct relation to the dryness of the ground they inhabit, and
+  the possibility of speed which their habit permits them or
+  necessarily imposes on them."<a name="FNanchor_205_205" id=
+  "FNanchor_205_205"></a><a href="#Footnote_205_205" class=
+  "fnanchor">[205]</a></p>
+
+  <p>If there is any truth in the explanation here briefly
+  summarised, it must entirely depend on the fact of individual
+  modifications thus produced being hereditary, and we yet await
+  the proof of this. In the meantime it is clear that the very same
+  results could have been brought about by variation and natural
+  selection. For the toes, like all other organs, vary in size and
+  proportions, and in their degree of union or separation; and if
+  in one group of animals it was beneficial to have the middle toe
+  larger and longer, and in another set to have the two middle toes
+  of the same size, nothing can be more certain than that these
+  particular modifications would be continuously preserved, and the
+  very results we see ultimately produced.</p>
+
+  <p>The oft-repeated objections that the cause of variations is
+  unknown, that there must be something to determine variations in
+  the right direction; that "natural selection includes no actively
+  progressive principle, but must wait for the development of
+  variation, and then, after securing the survival of the best,
+  wait again for the best to project its own variations for
+  selection," we have already sufficiently answered by showing that
+  variation&mdash;in abundant or typical species&mdash;is always
+  present in ample amount; that it exists in all parts and organs;
+  that these vary, for the most part, independently, so that any
+  required combination of variations can be secured; and finally,
+  that all variation is necessarily either in excess or defect of
+  the mean condition, and that, consequently, the right or
+  favourable variations are so frequently present that the unerring
+  power of natural selection never wants materials to work
+  upon.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_425" id="Page_425"></a><i>Supposed Action of Animal
+  Intelligence.</i></p>
+
+  <p>The following passage briefly summarises Mr. Cope's position:
+  "Intelligence is a conservative principle, and will always direct
+  effort and use into lines which will be beneficial to its
+  possessor. Here we have the source of the fittest, <i>i.e.</i>
+  addition of parts by increase and location of growth-force,
+  directed by the influence of various kinds of compulsion in the
+  lower, and intelligent option among higher animals. Thus
+  intelligent choice, taking advantage of the successive evolution
+  of physical conditions, may be regarded as the <i>originator of
+  the fittest</i>, while natural selection is the tribunal to which
+  all results of accelerated growth are submitted. This preserves
+  or destroys them, and determines the new points of departure on
+  which accelerated growth shall build."<a name="FNanchor_206_206"
+  id="FNanchor_206_206"></a><a href="#Footnote_206_206" class=
+  "fnanchor">[206]</a></p>
+
+  <p>This notion of "intelligence"&mdash;the intelligence of the
+  animal itself&mdash;determining its own variation, is so
+  evidently a very partial theory, inapplicable to the whole
+  vegetable kingdom, and almost so to all the lower forms of
+  animals, amongst which, nevertheless, there is the very same
+  adaptation and co-ordination of parts and functions as among the
+  highest, that it is strange to see it put forward with such
+  confidence as necessary for the completion of Darwin's theory. If
+  "the various kinds of compulsion"&mdash;by which are apparently
+  meant the laws of variation, growth, and reproduction, the
+  struggle for existence, and the actions necessary to preserve
+  life under the conditions of the animal's environment&mdash;are
+  sufficient to have developed the varied forms of the lower
+  animals and of plants, we can see no reason why the same
+  "compulsion" should not have carried on the development of the
+  higher animals also. The action of this "intelligent option" is
+  altogether unproved; while the acknowledgment that natural
+  selection is the tribunal which either preserves or destroys the
+  variations submitted to it, seems quite inconsistent with the
+  statement that intelligent choice is the "orginator of the
+  fittest," since whatever is really "the fittest" can never be
+  destroyed by natural selection, which is but another name for the
+  survival of the fittest. If "the fittest" is always definitely
+  <a name="Page_426" id="Page_426"></a>produced by some other
+  power, then natural selection is not wanted. If, on the other
+  hand, both fit and unfit are produced, and natural selection
+  decides between them, that is pure Darwinism, and Mr. Cope's
+  theories have added nothing to it.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig35" id="fig35"></a> <img src="images/fig35.jpg"
+    width="400" height="494" alt=
+    "FIG. 35.&mdash;Transformation of Artemia salina to A. Milhausenii; 1, tail-lobe of A. salina, and its transition through 2,3,4,5, to 6, into that of A. Milhausenii; 7, post-abdomen of A. salina; 8, post-abdomen of a form bred in brackish water; 9, gill of A. Milhausenii; 10, gill of A. salina. (From Schmankewitsch.)"
+    title=
+    "FIG. 35.&mdash;Transformation of Artemia salina to A. Milhausenii; 1, tail-lobe of A. salina, and its transition through 2,3,4,5, to 6, into that of A. Milhausenii; 7, post-abdomen of A. salina; 8, post-abdomen of a form bred in brackish water; 9, gill of A. Milhausenii; 10, gill of A. salina. (From Schmankewitsch.)" />
+     <b>FIG. 35.&mdash;Transformation of Artemia salina to A.
+    Milhausenii; 1, tail-lobe of A. salina, and its transition
+    through 2,3,4,5, to 6, into that of A. Milhausenii; 7,
+    post-abdomen of A. salina; 8, post-abdomen of a form bred in
+    brackish water; 9, gill of A. Milhausenii; 10, gill of A.
+    salina. (From Schmankewitsch.)</b>
+  </div>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Semper on the Direct Influence of the Environment.</i></p>
+
+  <p>Another eminent naturalist, Professor Karl Semper of
+  W&uuml;rzburg, also adopts the view of the direct transforming
+  power of the environment, and has brought together an <a name=
+  "Page_427" id="Page_427"></a>immense body of interesting facts
+  showing the influence of food, of light, of temperature, of still
+  water and moving water, of the atmosphere and its currents, of
+  gravitation, and of other organisms, in modifying the forms and
+  other characteristics of animals.<a name="FNanchor_207_207" id=
+  "FNanchor_207_207"></a><a href="#Footnote_207_207" class=
+  "fnanchor">[207]</a> He believes that these various influences
+  produce a direct and important effect, and that this effect is
+  accumulated by inheritance; yet he acknowledges that we have no
+  direct evidence of this, and there is hardly a single case
+  adduced in the book which is not equally well explained by
+  adaptation, brought about by the survival of beneficial
+  variations. Perhaps the most remarkable case he has brought
+  forward is that of the transformation of species of crustaceans
+  by a change in the saltness of the water (see Fig. 35). Artemia
+  salina lives in brackish water, while A. Milhausenii inhabits
+  water which is much salter. They differ greatly in the form of
+  the tail-lobes, and in the presence or absence of spines upon the
+  tail, and had always been considered perfectly distinct species.
+  Yet either was transformed into the other in a few generations,
+  during which the saltness of the water was gradually altered. Yet
+  more, A. salina was gradually accustomed to fresher water, and in
+  the course of a few generations, when the water had become
+  perfectly fresh, the species was changed into Branchipus
+  stagnalis, which had always been considered to belong to a
+  different genus on account of differences in the form of the
+  antennae and of the posterior segments of the body (see Fig. 36).
+  This certainly appears to be a proof of change of conditions
+  producing a change of form independently of selection, and of
+  that change of form, while remaining under the same conditions,
+  being inherited. Yet there is this peculiarity in the case, that
+  there is a chemical change in the water, and that this water
+  permeates the whole body, and must be absorbed by the tissues,
+  and thus affect the ova and even the reproductive <a name=
+  "Page_428" id="Page_428"></a>elements, and in this way may
+  profoundly modify the whole organisation. Why and how the
+  external effects are limited to special details of the structure
+  we do not know; but it does not seem as if any far-reaching
+  conclusions as to the cumulative effect of external conditions on
+  the higher terrestrial animals and plants, can be drawn from such
+  an exceptional phenomenon. It seems rather analogous to those
+  effects of external influences on the very lowest organisms in
+  which the vegetative and reproductive organs are hardly
+  differentiated, in which case such effects are doubtless
+  inherited.<a name="FNanchor_208_208" id=
+  "FNanchor_208_208"></a><a href="#Footnote_208_208" class=
+  "fnanchor">[208]</a></p>
+
+  <div class="figcenter" style="width: 414px;">
+    <a name="fig36" id="fig36"></a> <img src="images/fig36.jpg"
+    width="414" height="586" alt=
+    "FIG. 36. a. Branchipus stagnalis. b. Artemia salina." title=
+    "FIG. 36. a. Branchipus stagnalis. b. Artemia salina." />
+    <b>FIG. 36. a. Branchipus stagnalis. b. Artemia salina.</b>
+  </div>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Professor Geddes's Theory of Variation in Plants.</i></p>
+
+  <p>In a paper read before the Edinburgh Botanical Society in 1886
+  Mr. Patrick Geddes laid down the outlines of a fundamental theory
+  of plant variation, which he has further extended in the article
+  "Variation and Selection" in the <i>Encydopaedia Britannica</i>,
+  and in a paper read before the Linnaean Society but not yet
+  published.</p>
+
+  <p>A theory of variation should deal alike with the origin of
+  specific distinctions and with those vaster differences which
+  characterise the larger groups, and he thinks it should answer
+  such questions as&mdash;How an axis comes to be arrested to form
+  a flower? how the various forms of inflorescence were evolved?
+  how did perigynous or epigynous flowers arise from hypogynous
+  flowers? and many others equally fundamental. Natural selection
+  acting upon numerous accidental variations will not, he urges,
+  account for such general facts as these, which must depend on
+  some constant law of variation. This law he believes to be the
+  well-known antagonism of vegetative and reproductive growth
+  acting throughout the whole course of plant development; and he
+  uses it to explain many of the most characteristic features of
+  the structure of flowers and fruits.</p>
+
+  <p><a name="Page_429" id="Page_429"></a>Commencing with the
+  origin of the flower, which all botanists agree in regarding as a
+  shortened branch, he explains this shortening as an inevitable
+  physiological fact, since the cost of the development of the
+  reproductive elements is so great as necessarily to check
+  vegetative growth. In the same manner the shortening of the
+  inflorescence from raceme to spike or umbel, and thence to the
+  capitulum or dense flower-head of the composite plants is brought
+  about. This shortening, carried still further, produces the
+  flattened leaf-like receptacle of Dorstenia, and further still
+  the deeply hollowed fruity receptacle of the fig.</p>
+
+  <p>The flower itself undergoes a parallel modification due to a
+  similar cause. It is formed by a series of modified leaves
+  arranged round a shortened axis. In its earlier stages the number
+  of these modified leaves is indefinite, as in many Ranunculaceae;
+  and the axis itself is not greatly shortened, as in Myosurus. The
+  first advance is to a definite number of parts and a permanently
+  shortened axis, in the arrangement termed hypogynous, in which
+  all the whorls are quite distinct from each other. In the next
+  stage there is a further shortening of the central axis, leaving
+  the outer portion as a ring on which the petals are inserted,
+  producing the arrangement termed perigynous. A still further
+  advance is made by the contraction of the axis, so as to leave
+  the central part forming the ovary quite below the flower, which
+  is then termed epigynous.</p>
+
+  <p>These several modifications are said to be parallel and
+  definite, and to be determined by the continuous checking of
+  vegetation by reproduction along what is an absolute groove of
+  progressive change. This being the case, the importance of
+  natural selection is greatly diminished. Instead of selecting and
+  accumulating spontaneous indefinite variations, its function is
+  to retard them after the stage of maximum utility has been
+  independently reached. The same simple conception is said to
+  unlock innumerable problems of vegetable morphology, large and
+  small alike. It explains the inevitable development of gymnosperm
+  into angiosperm by the checked vegetative growth of the
+  ovule-bearing leaf or carpel; while such minor adaptations as the
+  splitting fruit of the geranium or the cupped stigma of the
+  pansy, can be no longer looked upon as achievements <a name=
+  "Page_430" id="Page_430"></a>of natural selection, but must be
+  regarded as naturally traceable to the vegetative checking of
+  their respective types of leaf organ. Again, a detailed
+  examination of spiny plants practically excludes the hypothesis
+  of mammalian selection altogether, and shows spines to arise as
+  an expression of the diminishing vegetativeness&mdash;in fact,
+  the ebbing vitality of a species.<a name="FNanchor_209_209" id=
+  "FNanchor_209_209"></a><a href="#Footnote_209_209" class=
+  "fnanchor">[209]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Objections to the Theory.</i></p>
+
+  <p>The theory here sketched out is enticing, and at first sight
+  seems calculated to throw much light on the history of plant
+  development; but on further consideration, it seems wanting in
+  definiteness, while it is beset with difficulties at every step.
+  Take first the shortening of the raceme into the umbel and the
+  capitulum, said to be caused by arrest of vegetative growth, due
+  to the antagonism of reproduction. If this were the whole
+  explanation of the phenomenon, we should expect the quantity of
+  seed to increase as this vegetative growth diminished, since the
+  seed is the product of the reproductive energy of the plant, and
+  its quantity the best measure of that energy. But is this the
+  case? The ranunculus has comparatively few seeds, and the flowers
+  are not numerous; while in the same order the larkspur and the
+  columbine have far more seeds as well as more flowers, but there
+  is no shortening of the raceme or diminution of the foliage,
+  although the flowers are large and complex. So, the extremely
+  shortened and compressed flower-heads of the compositae produce
+  comparatively few seeds&mdash;one only to each flower; while the
+  foxglove, with its long spike of showy flowers, produces an
+  enormous number.</p>
+
+  <p>Again, if the shortening of the central axis in the successive
+  stages of hypogynous, perigynous, and epigynous flowers were an
+  indication of preponderant reproduction and diminished
+  vegetation, we should find everywhere some clear indications of
+  this fact. The plants with hypogynous flowers should, as a rule,
+  have less seed and more vigorous and abundant foliage than those
+  at the other extreme with epigynous flowers. But the <a name=
+  "Page_431" id="Page_431"></a>hypogynous poppies, pinks, and St.
+  John's worts have abundance of seed and rather scanty foliage;
+  while the epigynous dogwoods and honeysuckles have few seeds and
+  abundant foliage. If, instead of the number of the seeds, we take
+  the size of the fruit as an indication of reproductive energy, we
+  find this at a maximum in the gourd family, yet their rapid and
+  luxuriant growth shows no diminution of vegetative power. So that
+  the statement that plant modifications proceed "along an absolute
+  groove of progressive change" is contradicted by innumerable
+  facts indicating advance and regression, improvement or
+  degradation, according as the ever-changing environment renders
+  one form more advantageous than the other. As one instance I may
+  mention the Anonaceae or custard-apple tribe, which are certainly
+  an advance from the Ranunculaceae; yet in the genus Polyalthea
+  the fruit consists of a number of separate carpels, each borne on
+  a long stalk, as if reverting to the primitive stalked carpellary
+  leaves.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>On the Origin of Spines.</i></p>
+
+  <p>But perhaps the most extraordinary application of the theory
+  is that which considers spines to be an indication of the "ebbing
+  vitality of a species," and which excludes "mammalian selection
+  altogether." If this were true, spines should occur mainly in
+  feeble, rare, and dying-out species, instead of which we have the
+  hawthorn, one of our most vigorous shrubs or trees, with abundant
+  vitality and an extensive range over the whole Palaearctic
+  region, showing that it is really a dominant species. In North
+  America the numerous thorny species of Crataegus are equally
+  vigorous, as are the false acacia (Robinia) and the honey-locust
+  (Gleditschia). Neither have the numerous species of very spiny
+  Acacias been noticed to be rarer or less vigorous than the
+  unarmed kinds.</p>
+
+  <p>On the other point&mdash;that spines are not due to mammalian
+  selection&mdash;we are able to adduce what must be considered
+  direct and conclusive evidence. For if spines, admittedly
+  produced by aborted branches, petioles, or peduncles, are due
+  solely or mainly to diminished vegetativeness or ebbing vitality,
+  they ought to occur in all countries alike, or at all events in
+  all whose similar conditions tend to check vegetation; whereas,
+  if they are, solely or mainly, developed as a protection against
+  the attacks <a name="Page_432" id="Page_432"></a>of herbivorous
+  mammals, they ought to be most abundant where these are
+  plentiful, and rare or absent where indigenous mammalia are
+  wanting. Oceanic islands, as compared with continents, would thus
+  furnish a crucial test of the two theories; and Mr. Hemsley of
+  Kew, who has specially studied insular floras, has given me some
+  valuable information on this point. He says: "There are no spiny
+  or prickly plants in the indigenous element of the St. Helena
+  flora. The relatively rich flora of the Sandwich Isles is not
+  absolutely without a prickly plant, but almost so. All the
+  endemic genera are unarmed, and the endemic species of almost
+  every other genus. Even such genera as Zanthoxylon, Acacia,
+  Xylosoma, Lycium, and Solanum, of which there are many armed
+  species in other countries, are only represented by unarmed
+  species. The two endemic Rubi have the prickles reduced to the
+  setaceous condition, and the two palms are unarmed.</p>
+
+  <p>"The flora of the Galapagos includes a number of prickly
+  plants, among them several cacti (these have not been
+  investigated and may be American species), but I do not think one
+  of the known endemic species of any family is prickly or
+  spiny.</p>
+
+  <p>"Spiny and prickly plants are also rare in New Zealand, but
+  there are the formidably armed species of wild Spaniard
+  (Aciphylla), one species of Rubus, the pungent-leaved Epacrideae
+  and a few others."</p>
+
+  <p>Mr. J.G. Baker of Kew, who has specially studied the flora of
+  Mauritius and the adjacent islands, also writes me on this point.
+  He says: "Taking Mauritius alone, I do not call to mind a single
+  species that is a spinose endemic tree or shrub. If you take the
+  whole group of islands (Mauritius, Bourbon, Seychelles, and
+  Rodriguez), there will be about a dozen species, but then nine of
+  these are palms. Leaving out palms, the trees and shrubs of that
+  part of the world are exceptionally non-spinose."</p>
+
+  <p>These are certainly remarkable facts, and quite inexplicable
+  on the theory of spines being caused solely by checked vegetative
+  growth, due to weakness of constitution or to an arid soil and
+  climate. For the Galapagos and many parts of the Sandwich Islands
+  are very arid, as is a considerable part of the North Island of
+  New Zealand. Yet in our own moist climate <a name="Page_433" id=
+  "Page_433"></a>and with our very limited number of trees and
+  shrubs we have about eighteen spiny or prickly species, more,
+  apparently, than in the whole endemic floras of the Mauritius,
+  Sandwich Islands, and Galapagos, though these are all especially
+  rich in shrubby and arboreal species. In New Zealand the prickly
+  Rubus is a leafless trailing plant, and its prickles are probably
+  a protection against the large snails of the country, several of
+  which have shells from two to three and a half inches
+  long.<a name="FNanchor_210_210" id=
+  "FNanchor_210_210"></a><a href="#Footnote_210_210" class=
+  "fnanchor">[210]</a> The "wild Spaniards" are very spiny
+  herbaceous Umbelliferae, and may have gained their spines to
+  preserve them from being trodden down or eaten by the Moas,
+  which, for countless ages, took the place of mammals in New
+  Zealand. The exact use or meaning of the spines in palms is more
+  doubtful, though they are, no doubt, protective against some
+  animals; but it is certainly an extraordinary fact that in the
+  entire flora of the Mauritius, so largely consisting of trees and
+  shrubs, not a single endemic species should be thorny or
+  spiny.</p>
+
+  <p>If now we consider that every continental flora produces a
+  considerable proportion of spiny and thorny species, and that
+  these rise to a maximum in South Africa, where herbivorous
+  mammalia were (before the settlement of the country), perhaps,
+  more abundant and varied than in any other part of the world;
+  while another district, remarkable for well-armed vegetation, is
+  Chile, where the camel-like vicugnas, llamas, and alpacas, and an
+  abundance of large rodents wage perpetual war against shrubby
+  vegetation, we shall see the full significance of the almost
+  total absence of thorny and spiny plants in the chief oceanic
+  islands; and so far from "excluding the hypothesis of mammalian
+  selection altogether," we shall find in this hypothesis the only
+  satisfactory explanation of the facts.</p>
+
+  <p>From the brief consideration of Professor Geddes's theory now
+  given, we conclude that, although the antagonism between
+  vegetative and reproductive growth is a real agency, and must be
+  taken account of in our endeavour to explain many of the
+  fundamental facts in the structure and form of plants, yet it is
+  so overpowered and directed at every step by the natural
+  selection of favourable variations, that the results of its
+  <a name="Page_434" id="Page_434"></a>exclusive and unmodified
+  action are nowhere to be found in nature. It may be allowed to
+  rank as one of those "laws of growth," of which so many have now
+  been indicated, and which were always recognised by Darwin as
+  underlying all variation; but unless we bear in mind that its
+  action must always be subordinated to natural selection, and that
+  it is continually checked, or diverted, or even reversed by the
+  necessity of adaptation to the environment, we shall be liable to
+  fall into such glaring errors as the imputing to "ebbing
+  vitality" alone such a widespread phenomenon as the occurrence of
+  spines and thorns, while ignoring altogether the influence of the
+  organic environment in their production.<a name=
+  "FNanchor_211_211" id="FNanchor_211_211"></a><a href=
+  "#Footnote_211_211" class="fnanchor">[211]</a></p>
+
+  <p>The sketch now given of the chief attempts that have been made
+  to prove that either the direct action of the environment or
+  certain fundamental laws of variation are independent causes of
+  modification of species, shows us that their authors have, in
+  every case, failed to establish their contention. Any direct
+  action of the environment, or any characters acquired by use or
+  disuse, can have no effect whatever upon the race unless they are
+  inherited; and that they are inherited in any case, <a name=
+  "Page_435" id="Page_435"></a>except when they directly affect the
+  reproductive cells, has not been proved. On the other hand, as we
+  shall presently show, there is much reason for believing that
+  such acquired characters are in their nature non-heritable.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Variation and Selection Overpower the Effects of Use and
+  Disuse.</i></p>
+
+  <p>But there is another objection to this theory arising from the
+  very nature of the effects produced. In each generation the
+  effects of use or disuse, or of effort, will certainly be very
+  small, while of this small effect it is not maintained that the
+  whole will be always inherited by the next generation. How small
+  the effect is we have no means of determining, except in the case
+  of disuse, which Mr. Darwin investigated carefully. He found that
+  in twelve fancy breeds of pigeons, which are often kept in
+  aviaries, or if free fly but little, the sternum had been reduced
+  by about one-seventh or one-eighth of its entire length, and that
+  of the scapula about one-ninth. In domestic ducks the weight of
+  the wing-bones in proportion to that of the whole skeleton had
+  decreased about one-tenth. In domestic rabbits the bones of the
+  legs were found to have increased in weight in due proportion to
+  the increased weight of the body, but those of the hind legs were
+  rather less in proportion to those of the fore legs than in the
+  wild animal, a difference which may be imputed to their being
+  less used in rapid motion. The pigeons, therefore, afford the
+  greatest amount of reduction by disuse&mdash;one-seventh of the
+  length of the sternum. But the pigeon has certainly been
+  domesticated four or five thousand years; and if the reduction of
+  the wings by disuse has only been going on for the last thousand
+  years, the amount of reduction in each generation would be
+  absolutely imperceptible, and quite within the limits of the
+  reduction due to the absence of selection, as already explained.
+  But, as we have seen in Chapter III, the fortuitous variation of
+  every part or organ usually amounts to one-tenth, and often to
+  one-sixth of the average dimensions&mdash;that is, the fortuitous
+  variation in one generation among a limited number of the
+  individuals of a species is as great as the cumulative effects of
+  disuse in a thousand generations! If we assume that the effects
+  of use or of effort in the individual are equal to the effects of
+  disuse, or even ten or a hundred times greater, they <a name=
+  "Page_436" id="Page_436"></a>will even then not equal, in each
+  generation, the amount of the fortuitous variations of the same
+  part. If it be urged that the effects of use would modify all the
+  individuals of a species, while the fortuitous variations to the
+  amount named only apply to a portion of them, it may be replied,
+  that that portion is sufficiently large to afford ample materials
+  for selection, since it often equals the numbers that can
+  annually survive; while the recurrence in each successive
+  generation of a like amount of variation would render possible
+  such a rapid adjustment to new conditions that the effects of use
+  or disuse would be as nothing in comparison. It follows, that
+  even admitting the modifying effects of the environment, and that
+  such modifications are inherited, they would yet be entirely
+  swamped by the greater effects of fortuitous variation, and the
+  far more rapid cumulative results of the selection of such
+  variations.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Supposed Action of the Environment in Initiating
+  Variations.</i></p>
+
+  <p>It is, however, urged that the reaction of the environment
+  initiates variations, which without it would never arise; such,
+  for instance, as the origin of horns through the pressures and
+  irritations caused by butting, or otherwise using the head as a
+  weapon or for defence. Admitting, for the sake of argument, that
+  this is so, all the evidence we possess shows that, from the very
+  first appearance of the rudiment of such an organ, it would vary
+  to a greater extent than the amount of growth directly produced
+  by use; and these variations would be subject to selection, and
+  would thus modify the organ in ways which use alone would never
+  bring about. We have seen that this has been the case with the
+  branching antlers of the stag, which have been modified by
+  selection, so as to become useful in other ways than as a mere
+  weapon; and the same has almost certainly been the case with the
+  variously curved and twisted horns of antelopes. In like manner,
+  every conceivable rudiment would, from its first appearance, be
+  subject to the law of variation and selection, to which,
+  thenceforth, the direct effect of the environment would be
+  altogether subordinate.</p>
+
+  <p>A very similar mode of reasoning will apply to the other
+  branch of the subject&mdash;the initiation of structures and
+  organs <a name="Page_437" id="Page_437"></a>by the action of the
+  fundamental laws of growth. Admitting that such laws have
+  determined some of the main divisions of the animal and vegetable
+  kingdom, have originated certain important organs, and have been
+  the fundamental cause of certain lines of development, yet at
+  every step of the process these laws must have acted in entire
+  subordination to the law of natural selection. No modification
+  thus initiated could have advanced a single step, unless it were,
+  on the whole, a useful modification; while its entire future
+  course would be necessarily subject to the laws of variation and
+  selection, by which it would be sometimes checked, sometimes
+  hastened on, sometimes diverted to one purpose, sometimes to
+  another, according as the needs of the organism, under the
+  special conditions of its existence, required such modification.
+  We need not deny that such laws and influences may have acted in
+  the manner suggested, but what we do deny is that they could
+  possibly escape from the ever-present and all-powerful modifying
+  effects of variation and natural selection.<a name=
+  "FNanchor_212_212" id="FNanchor_212_212"></a><a href=
+  "#Footnote_212_212" class="fnanchor">[212]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Weismann's Theory of Heredity.</i></p>
+
+  <p>Professor August Weismann has put forth a new theory of
+  heredity founded upon the "continuity of the germ-plasm," one of
+  the logical consequences of which is, that acquired characters of
+  whatever kind are not transmitted from parent to offspring. As
+  this is a matter of vital importance to the theory of natural
+  selection, and as, if well founded, it strikes away the
+  foundations of most of the theories discussed in the present
+  chapter, a brief outline of Weismann's views must be attempted,
+  <a name="Page_438" id="Page_438"></a>although it is very
+  difficult to make them intelligible to persons unfamiliar with
+  the main facts of modern embryology.<a name="FNanchor_213_213"
+  id="FNanchor_213_213"></a><a href="#Footnote_213_213" class=
+  "fnanchor">[213]</a></p>
+
+  <p>The problem is thus stated by Weismann: "How is it that in the
+  case of all higher animals and plants a single cell is able to
+  separate itself from amongst the millions of most various kinds
+  of which an organism is composed, and by division and complicated
+  differentiation to reconstruct a new individual with marvellous
+  likeness, unchanged in many cases even throughout whole
+  geological periods?" Darwin attempted to solve the problem by his
+  theory of "Pangenesis," which supposed that every individual cell
+  in the body gave off gemmules or germs capable of reproducing
+  themselves, and that portions of these germs of each of the
+  almost infinite number of cells permeate the whole body and
+  become collected in the generative cells, and are thus able to
+  reproduce the whole organism. This theory is felt to be so
+  ponderously complex and difficult that it has met with no general
+  acceptance among physiologists.</p>
+
+  <p>The fact that the germ-cells <i>do</i> reproduce with
+  wonderful accuracy not only the general characters of the
+  species, but many of the individual characteristics of the
+  parents or more remote ancestors, and that this process is
+  continued from generation to generation, can be accounted for,
+  Weismann thinks, only on two suppositions which are
+  physiologically possible. Either the substance of the parent
+  germ-cell, after passing through a cycle of changes required for
+  the construction of a new individual, possesses the capability of
+  producing anew germ-cells identical with those from which that
+  individual was developed, or <i>the new germ-cells arise, as far
+  as their essential and characteristic substance is concerned, not
+  at all out of the body of the individual, but direct from the
+  parent germ-cell</i>. This latter view Weismann holds to be the
+  correct one, and, on this theory, heredity depends on the fact
+  that a substance of special molecular composition passes over
+  from one generation to another. This is the "germ-plasm," the
+  power of which to develop itself into a perfect organism depends
+  on the extraordinary complication of its minutest structure. At
+  every new birth a portion <a name="Page_439" id="Page_439"></a>of
+  the specific germ-plasm, which the parent egg-cell contains, is
+  not used up in producing the offspring, but is reserved unchanged
+  to produce the germ-cells of the following generation. Thus the
+  germ-cells&mdash;so far as regards their essential part the
+  germ-plasm&mdash;are not a product of the body itself, but are
+  related to one another in the same way as are a series of
+  generations of unicellular organisms derived from one another by
+  a continuous course of simple division. Thus the question of
+  heredity is reduced to one of growth. A minute portion of the
+  very same germ-plasm from which, first the germ-cell, and then
+  the whole organism of the parent, were developed, becomes the
+  starting-point of the growth of the child.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Cause of Variation.</i></p>
+
+  <p>But if this were all, the offspring would reproduce the parent
+  exactly, in every detail of form and structure; and here we see
+  the importance of sex, for each new germ grows out of the united
+  germ-plasms of two parents, whence arises a mingling of their
+  characters in the offspring. This occurs in each generation;
+  hence every individual is a complex result reproducing in
+  ever-varying degrees the diverse characteristics of his two
+  parents, four grandparents, eight great-grandparents, and other
+  more remote ancestors; and that ever-present individual variation
+  arises which furnishes the material for natural selection to act
+  upon. Diversity of sex becomes, therefore, of primary importance
+  as <i>the cause of variation</i>. Where asexual generation
+  prevails, the characteristics of the individual alone are
+  reproduced, and there are thus no means of effecting the change
+  of form or structure required by changed conditions of existence.
+  Under such changed conditions a complex organism, if only
+  asexually propagated, would become extinct. But when a complex
+  organism is sexually propagated, there is an ever-present cause
+  of change which, though slight in any one generation, is
+  cumulative, and under the influence of selection is sufficient to
+  keep up the harmony between the organism and its slowly changing
+  environment.<a name="FNanchor_214_214" id=
+  "FNanchor_214_214"></a><a href="#Footnote_214_214" class=
+  "fnanchor">[214]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_440" id="Page_440"></a><i>The Non-Heredity of
+  Acquired Characters.</i></p>
+
+  <p>Certain observations on the embryology of the lower animals
+  are held to afford direct proof of this theory of heredity, but
+  they are too technical to be made clear to ordinary readers. A
+  logical result of the theory is the impossibility of the
+  transmission of acquired characters, since the molecular
+  structure of the germ-plasm is already determined within the
+  embryo; and Weismann holds that there are no facts which really
+  prove that acquired characters can be inherited, although their
+  inheritance has, by most writers, been considered so probable as
+  hardly to stand in need of direct proof.</p>
+
+  <p>We have already shown, in the earlier part of this chapter,
+  that many instances of change, imputed to the inheritance of
+  acquired variations, are really cases of selection; while the
+  very fact that <i>use</i> implies <i>usefulness</i> renders it
+  almost impossible to eliminate the action of selection in a state
+  of nature. As regards mutilations, it is generally admitted that
+  they are not hereditary, and there is ample evidence on this
+  point. When it was the fashion to dock horses' tails, it was not
+  found that horses were born with short tails; nor are Chinese
+  women born with distorted feet; nor are any of the numerous forms
+  of racial mutilation in man, which have in some cases been
+  carried on for hundreds of generations, inherited. Nevertheless,
+  a few cases of apparent inheritance of mutilations have been
+  recorded,<a name="FNanchor_215_215" id=
+  "FNanchor_215_215"></a><a href="#Footnote_215_215" class=
+  "fnanchor">[215]</a> and these, if trustworthy, are difficulties
+  in the way of the theory. The undoubted inheritance of disease is
+  hardly a difficulty, because the predisposition to disease is a
+  congenital, not an acquired character, and as such would be the
+  subject of inheritance. The often-quoted case of a disease
+  induced by mutilation being inherited (Brown-Sequard's epileptic
+  guinea-pigs) has been discussed by Professor Weismann, and shown
+  to be not conclusive. The mutilation itself&mdash;a section of
+  certain nerves&mdash;was never inherited, but <a name="Page_441"
+  id="Page_441"></a>the resulting epilepsy, or a general state of
+  weakness, deformity, or sores, was sometimes inherited. It is,
+  however, possible that the mere injury introduced and encouraged
+  the growth of certain microbes, which, spreading through the
+  organism, sometimes reached the germ-cells, and thus transmitted
+  a diseased condition to the offspring. Such a transference of
+  microbes is believed to occur in syphilis and tuberculosis, and
+  has been ascertained to occur in the case of the muscardine
+  silkworm disease.<a name="FNanchor_216_216" id=
+  "FNanchor_216_216"></a><a href="#Footnote_216_216" class=
+  "fnanchor">[216]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Theory of Instinct.</i></p>
+
+  <p>The theory now briefly outlined cannot be said to be proved,
+  but it commends itself to many physiologists as being inherently
+  probable, and as furnishing a good working hypothesis till
+  displaced by a better. We cannot, therefore, accept any arguments
+  against the agency of natural selection which are based upon the
+  opposite and equally unproved theory that acquired characters are
+  inherited; and as this applies to the whole school of what may be
+  termed Neo-Lamarckians, their speculations cease to have any
+  weight.</p>
+
+  <p>The same remark applies to the popular theory of instincts as
+  being inherited habits; though Darwin gave very little weight to
+  this, but derived almost all instincts from spontaneous useful
+  variations which, like other spontaneous variations, are of
+  course inherited. At first sight it appears as if the acquired
+  habits of our trained dogs&mdash;pointers, retrievers,
+  etc.&mdash;are certainly inherited; but this need not be the
+  case, because there must be some structural or psychical
+  peculiarities, such as modifications in the attachments of
+  muscles, increased delicacy of smell or sight, or peculiar likes
+  and dislikes, which are inherited; and from these, peculiar
+  habits follow as a natural consequence, or are easily acquired.
+  Now, as selection has been constantly at work in improving all
+  our domestic animals, we have unconsciously modified the
+  structure, while preserving only those animals which best served
+  our purpose in their peculiar faculties, instincts, or
+  habits.</p>
+
+  <p><a name="Page_442" id="Page_442"></a>Much of the mystery of
+  instinct arises from the persistent refusal to recognise the
+  agency of imitation, memory, observation, and reason as often
+  forming part of it. Yet there is ample evidence that such agency
+  must be taken into account. Both Wilson and Leroy state that
+  young birds build inferior nests to old ones, and the latter
+  author observes that the best nests are made by birds whose young
+  remain longest in the nest. So, migration is now well ascertained
+  to be effected by means of vision, long flights being made on
+  bright moonlight nights when the birds fly very high, while on
+  cloudy nights they fly low, and then often lose their way.
+  Thousands annually fly out to sea and perish, showing that the
+  instinct to migrate is imperfect, and is not a good substitute
+  for reason and observation.</p>
+
+  <p>Again, much of the perfection of instinct is due to the
+  extreme severity of the selection during its development, any
+  failure involving destruction. The chick which cannot break the
+  eggshell, the caterpillar that fails to suspend itself properly
+  or to spin a safe cocoon, the bees that lose their way or that
+  fail to store honey, inevitably perish. So the birds that fail to
+  feed and protect their young, or the butterflies that lay their
+  eggs on the wrong food-plant, leave no offspring, and the race
+  with imperfect instincts perishes. Now, during the long and very
+  slow course of development of each organism, this rigid selection
+  at every step of progress has led to the preservation of every
+  detail of structure, faculty, or habit that has been necessary
+  for the preservation of the race, and has thus gradually built up
+  the various instincts which seem so marvellous to us, but which
+  can yet be shown to be in many cases still imperfect. Here, as
+  everywhere else in nature, we find comparative, not absolute
+  perfection, with every gradation from what is clearly due to
+  imitation or reason up to what seems to us perfect
+  instinct&mdash;that in which a complex action is performed
+  without any previous experience or instruction.<a name=
+  "FNanchor_217_217" id="FNanchor_217_217"></a><a href=
+  "#Footnote_217_217" class="fnanchor">[217]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_443" id="Page_443"></a><i>Concluding
+  Remarks.</i></p>
+
+  <p>Having now passed in review the more important of the recent
+  objections to, or criticisms of, the theory of natural selection,
+  we have arrived at the conclusion that in no one case have the
+  writers in question been able materially to diminish its
+  importance, or to show that any of the laws or forces to which
+  they appeal can act otherwise than in strict subordination to it.
+  The direct action of the environment as set forth by Mr. Herbert
+  Spencer, Dr. Cope, and Dr. Karl Semper, even if we admit that its
+  effects on the individual are transmitted by inheritance, are so
+  small in comparison with the amount of spontaneous variation of
+  every part of the organism that they must be quite overshadowed
+  by the latter. And if such direct action may, in some cases, have
+  initiated certain organs or outgrowths, these must from their
+  very first beginnings have been subject to variation and natural
+  selection, and their further development have been almost wholly
+  due to these ever-present and powerful causes. <a name="Page_444"
+  id="Page_444"></a>The same remark applies to the views of
+  Professor Geddes on the laws of growth which have determined
+  certain essential features in the morphology of plants and
+  animals. The attempt to substitute these laws for those of
+  variation and natural selection has failed in cases where we can
+  apply a definite test, as in that of the origin of spines on
+  trees and shrubs; while the extreme diversity of vegetable
+  structure and form among the plants of the same country and of
+  the same natural order, of itself affords a proof of the
+  preponderating influence of variation and natural selection in
+  keeping the many diverse forms in harmony with the highly complex
+  and ever-changing environment.</p>
+
+  <p>Lastly, we have seen that Professor Weismann's theory of the
+  continuity of the germ-plasm and the consequent non-heredity of
+  acquired characters, while in perfect harmony with all the
+  well-ascertained facts of heredity and development, adds greatly
+  to the importance of natural selection as the one invariable and
+  ever-present factor in all organic change, and that which can
+  alone have produced the temporary fixity combined with the
+  secular modification of species. While admitting, as Darwin
+  always admitted, the co-operation of the fundamental laws of
+  growth and variation, of correlation and heredity, in determining
+  the direction of lines of variation or in the initiation of
+  peculiar organs, we find that variation and natural selection are
+  ever-present agencies, which take possession, as it were, of
+  every minute change originated by these fundamental causes, check
+  or favour their further development, or modify them in countless
+  varied ways according to the varying needs of the organism.
+  Whatever other causes have been at work, Natural Selection is
+  supreme, to an extent which even Darwin himself hesitated to
+  claim for it. The more we study it the more we are convinced of
+  its overpowering importance, and the more confidently we claim,
+  in Darwin's own words, that it "has been the most important, but
+  not the exclusive, means of modification."</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_198_198" id=
+      "Footnote_198_198"></a><a href=
+      "#FNanchor_198_198"><span class="label">[198]</span></a> See
+      the Duke of Argyll's letter in <i>Nature</i>, vol. xxxiv. p.
+      336.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_199_199" id=
+      "Footnote_199_199"></a><a href=
+      "#FNanchor_199_199"><span class="label">[199]</span></a>
+      <i>Journal of the Anthropological Institute,</i> vol. xv. pp.
+      246-260.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_200_200" id=
+      "Footnote_200_200"></a><a href=
+      "#FNanchor_200_200"><span class="label">[200]</span></a> The
+      idea of the non-heredity of acquired variations was suggested
+      by the summary of Professor Weismann's views, in
+      <i>Nature</i>, referred to later on. But since this chapter
+      was written I have, through the kindness of Mr. E.B. Poulton,
+      seen some of the proofs of the forthcoming translation of
+      Weismann's Essays on Heredity, in which he sets forth an
+      explanation very similar to that here given. On the difficult
+      question of the almost entire disappearance of organs, as in
+      the limbs of snakes and of some lizards, he adduces "a
+      certain form of correlation, which Roux calls 'the struggle
+      of the parts in the organism,'" as playing an important part.
+      Atrophy following disuse is nearly always attended by the
+      corresponding increase of other organs: blind animals possess
+      more developed organs of touch, hearing, and smell; the loss
+      of power in the wings is accompanied by increased strength of
+      the legs, etc. Now as these latter characters, being useful,
+      will be selected, it is easy to understand that a congenital
+      increase of these will be accompanied by a corresponding
+      congenital diminution of the unused organ; and in cases where
+      the means of nutrition are deficient, every diminution of
+      these useless parts will be a gain to the whole organism, and
+      thus their complete disappearance will, in some cases, be
+      brought about directly by natural selection. This corresponds
+      with what we know of these rudimentary organs.</p>
+
+      <p>It must, however, be pointed out that the non-heredity of
+      acquired characters was maintained by Mr. Francis Galton more
+      than twelve years ago, on theoretical considerations almost
+      identical with those urged by Professor Weismann; while the
+      insufficiency of the evidence for their hereditary
+      transmission was shown, by similar arguments to those used
+      above and in the work of Professor Weismann already referred
+      to (see "A Theory of Heredity," in <i>Journ. Anthrop.
+      Instit.</i>, vol. v. pp. 343-345).</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_201_201" id=
+      "Footnote_201_201"></a><a href=
+      "#FNanchor_201_201"><span class="label">[201]</span></a> This
+      explanation is derived from Weismann's Theory of the
+      Continuity of the Germ-Plasm as summarised in
+      <i>Nature</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_202_202" id=
+      "Footnote_202_202"></a><a href=
+      "#FNanchor_202_202"><span class="label">[202]</span></a> See
+      a collection of his essays under the title, <i>The Origin of
+      the Fittest: Essays on Evolution</i>, D. Appleton and Co. New
+      York. 1887.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_203_203" id=
+      "Footnote_203_203"></a><a href=
+      "#FNanchor_203_203"><span class="label">[203]</span></a>
+      <i>Origin of the Fittest</i>, p. 174.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_204_204" id=
+      "Footnote_204_204"></a><a href=
+      "#FNanchor_204_204"><span class="label">[204]</span></a>
+      <i>Ibid.</i> p. 29. It may be here noted that Darwin found
+      these theories unintelligible. In a letter to Professor E.T.
+      Morse in 1877, he writes: "There is one point which I regret
+      you did not make clear in your Address, namely, what is the
+      meaning and importance of Professors Cope and Hyatt's views
+      on acceleration and retardation? I have endeavoured, and
+      given up in despair, the attempt to grasp their meaning"
+      (<i>Life and Letters</i>, vol. iii. p. 233).</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_205_205" id=
+      "Footnote_205_205"></a><a href=
+      "#FNanchor_205_205"><span class="label">[205]</span></a>
+      <i>Origin of the Fittest</i>, p. 374.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_206_206" id=
+      "Footnote_206_206"></a><a href=
+      "#FNanchor_206_206"><span class="label">[206]</span></a>
+      <i>Origin of the Fittest</i>, p. 40.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_207_207" id=
+      "Footnote_207_207"></a><a href=
+      "#FNanchor_207_207"><span class="label">[207]</span></a>
+      <i>The Natural Conditions of Existence as they Affect Animal
+      Life.</i> London, 1883.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_208_208" id=
+      "Footnote_208_208"></a><a href=
+      "#FNanchor_208_208"><span class="label">[208]</span></a> In
+      Dr. Weismann's essay on "Heredity," already referred to, he
+      considers it not improbable that changes in organisms
+      produced by climatic influences may be inherited, because, as
+      these changes do not affect the external parts of an organism
+      only, but often, as in the case of warmth or moisture
+      permeate the whole structure, they may possibly modify the
+      germ-plasm itself, and thus induce variations in the next
+      generation. In this way, he thinks, may possibly be explained
+      the climatic varieties of certain butterflies, and some other
+      changes which seem to be effected by change of climate in a
+      few generations.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_209_209" id=
+      "Footnote_209_209"></a><a href=
+      "#FNanchor_209_209"><span class="label">[209]</span></a> This
+      brief indication of Professor Geddes's views is taken from
+      the article "Variation and Selection" in the <i>Encyclopedia
+      Britannica</i>, and a paper "On the Nature and Causes of
+      Variation in Plants" in <i>Trans. and Proc. of the Edinburgh
+      Botanical Society</i>, 1886; and is, for the most part,
+      expressed in his own words.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_210_210" id=
+      "Footnote_210_210"></a><a href=
+      "#FNanchor_210_210"><span class="label">[210]</span></a>
+      Placostylis bovinus, 3&frac12; inches long; Paryphanta
+      Busbyi, 3 in. diam.; P. Hochstetteri, 2&frac34; in. diam.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_211_211" id=
+      "Footnote_211_211"></a><a href=
+      "#FNanchor_211_211"><span class="label">[211]</span></a> The
+      general arguments and objections here set forth will apply
+      with equal force to Professor G. Henslow's theory of the
+      origin of the various forms and structures of flowers as due
+      to "the responsive actions of the protoplasm in consequence
+      of the irritations set up by the weights, pressures, thrusts,
+      tensions, etc., of the insect visitors" (<i>The Origin of
+      Floral Structures through Insect and other Agencies</i>, p.
+      340). On the assumption that acquired characters are
+      inherited, such irritations may have had something to do with
+      the initiation of variations and with the production of
+      certain details of structure, but they are clearly
+      incompetent to have brought about the more important
+      structural and functional modifications of flowers. Such are,
+      the various adjustments of length and position of the stamens
+      to bring the pollen to the insect and from the insect to the
+      stigma; the various motions of stamens and styles at the
+      right time and the right direction; the physiological
+      adjustments bringing about fertility or sterility in
+      heterostyled plants; the traps, springs, and complex
+      movements of various parts of orchids; and innumerable other
+      remarkable phenomena.</p>
+
+      <p>For the explanation of these we have no resource but
+      variation and selection, to the effects of which, acting
+      alternately with regression or degradation as above explained
+      (p. 328) must be imputed the development of the countless
+      floral structures we now behold. Even the primitive flowers,
+      whose initiation may, perhaps, have been caused, or rendered
+      possible, by the irritation set up by insects' visits, must,
+      from their very origin, have been modified, in accordance
+      with the supreme law of utility, by means of variation and
+      survival of the fittest.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_212_212" id=
+      "Footnote_212_212"></a><a href=
+      "#FNanchor_212_212"><span class="label">[212]</span></a> In
+      an essay on "The Duration of Life," forming part of the
+      translation of Dr. Weismann's papers already referred to, the
+      author still further extends the sphere of natural selection
+      by showing that the average duration of life in each species
+      has been determined by it. A certain length of life is
+      essential in order that the species may produce offspring
+      sufficient to ensure its continuance under the most
+      unfavourable conditions; and it is shown that the remarkable
+      inequalities of longevity in different species and groups may
+      be thus accounted for. Yet more, the occurrence of death in
+      the higher organisms, in place of the continued survival of
+      the unicellular organisms however much they may increase by
+      subdivision, may be traced to the same great law of utility
+      for the race and survival of the fittest. The whole essay is
+      of exceeding interest, and will repay a careful perusal. A
+      similar idea occurred to the present writer about twenty
+      years back, and was briefly noted down at the time, but
+      subsequently forgotten.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_213_213" id=
+      "Footnote_213_213"></a><a href=
+      "#FNanchor_213_213"><span class="label">[213]</span></a> The
+      outline here given is derived from two articles in
+      <i>Nature</i>, vol. xxxiii. p. 154, and vol. xxxiv. p. 629,
+      in which Weismann's papers are summarised and partly
+      translated.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_214_214" id=
+      "Footnote_214_214"></a><a href=
+      "#FNanchor_214_214"><span class="label">[214]</span></a>
+      There are many indications that this explanation of the cause
+      of variation is the true one. Mr. E.B. Poulton suggests one,
+      in the fact that parthenogenetic reproduction only occurs in
+      isolated species, not in groups of related species; as this
+      shows that parthenogenesis cannot lead to the evolution of
+      new forms. Again, in parthenogenetic females the complete
+      apparatus for fertilisation remains unreduced; but if these
+      varied as do sexually produced animals, the organs referred
+      to, being unused, would become rudimentary.</p>
+
+      <p>Even more important is the significance of the "polar
+      bodies," as explained by Weismann in one of his
+      <i>Essays</i>; since, if his interpretation of them be
+      correct, variability is a necessary consequence of sexual
+      generation.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_215_215" id=
+      "Footnote_215_215"></a><a href=
+      "#FNanchor_215_215"><span class="label">[215]</span></a>
+      Darwin's <i>Animals and Plants</i>, vol. ii. pp. 23, 24.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_216_216" id=
+      "Footnote_216_216"></a><a href=
+      "#FNanchor_216_216"><span class="label">[216]</span></a> In
+      his essay on "Heredity," Dr. Weismann discusses many other
+      cases of supposed inheritance of acquired characters, and
+      shows that they can all be explained in other ways.
+      Shortsightedness among civilised nations, for example, is due
+      partly to the absence of selection and consequent regression
+      towards a mean, and partly to its individual production by
+      constant reading.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_217_217" id=
+      "Footnote_217_217"></a><a href=
+      "#FNanchor_217_217"><span class="label">[217]</span></a>
+      Weismann explains instinct on similar lines, and gives many
+      interesting illustrations (see <i>Essays on Heredity</i>). He
+      holds "that all instinct is entirely due to the operation of
+      natural selection, and has its foundation, not upon inherited
+      experiences, but upon variations of the germ." Many
+      interesting and difficult cases of instinct are discussed by
+      Darwin in Chapter VIII of the <i>Origin of Species</i>, which
+      should be read in connection with the above remarks.</p>
+
+      <p>Since this chapter was written my attention has been
+      directed to Mr. Francis Galton's <i>Theory of Heredity</i>
+      (already referred to at p. 417) which was published thirteen
+      years ago as an alternative for Darwin's theory of
+      pangenesis.</p>
+
+      <p>Mr. Galton's theory, although it attracted little
+      attention, appears to me to be substantially the same as that
+      of Professor Weismann. Galton's "stirp" is Weismann's
+      "germ-plasm." Galton supposes the sexual elements in the
+      offspring to be directly formed from the residue of the
+      <i>stirp</i> not used up in the development of the body of
+      the parent&mdash;Weismann's "continuity of the germ-plasm."
+      Galton also draws many of the same conclusions from his
+      theory. He maintains that characters acquired by the
+      individual as the result of external influences cannot be
+      inherited, unless such influences act directly on the
+      reproductive elements&mdash;instancing the possible heredity
+      of alcoholism, because the alcohol permeates the tissues and
+      may reach the sexual elements. He discusses the supposed
+      heredity of effects produced by use or disuse, and explains
+      them much in the same manner as does Weismann. Galton is an
+      anthropologist, and applies the theory, mainly, to explain
+      the peculiarities of hereditary transmission in man, many of
+      which peculiarities he discusses and elucidates. Weismann is
+      a biologist, and is mostly concerned with the application of
+      the theory to explain variation and instinct, and to the
+      further development of the theory of evolution. He has worked
+      it out more thoroughly, and has adduced embryological
+      evidence in its support; but the views of both writers are
+      substantially the same, and their theories were arrived at
+      quite independently. The names of Galton and Weismann should
+      therefore be associated as discoverers of what may be
+      considered (if finally established) the most important
+      contribution to the evolution theory since the appearance of
+      the <i>Origin of Species</i>.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="CHAPTER_XV" id="CHAPTER_XV"></a><a name="Page_445"
+  id="Page_445"></a>CHAPTER XV</h2>
+
+  <p style='font-weight:bold;'>DARWINISM APPLIED TO MAN</p>
+
+  <div class="blockquot">
+    <p>General identity of human and animal
+    structure&mdash;Rudiments and variations showing relation of
+    man to other mammals&mdash;The embryonic development of man and
+    other mammalia&mdash;Diseases common to man and the lower
+    animals&mdash;The animals most nearly allied to man&mdash;The
+    brains of man and apes&mdash;External differences of man and
+    apes&mdash;Summary of the animal characteristics of
+    man&mdash;The geological antiquity of man&mdash;The probable
+    birthplace of man&mdash;The origin of the moral and
+    intellectual nature of man&mdash;The argument from
+    continuity&mdash;The origin of the mathematical
+    faculty&mdash;The origin of the musical and artistic
+    faculties&mdash;Independent proof that these faculties have not
+    been developed by natural selection&mdash;The interpretation of
+    the facts&mdash;Concluding remarks.</p>
+  </div>
+
+  <p>Our review of modern Darwinism might fitly have terminated
+  with the preceding chapter; but the immense interest that
+  attaches to the origin of the human race, and the amount of
+  misconception which prevails regarding the essential teachings of
+  Darwin's theory on this question, as well as regarding my own
+  special views upon it, induce me to devote a final chapter to its
+  discussion.</p>
+
+  <p>To any one who considers the structure of man's body, even in
+  the most superficial manner, it must be evident that it is the
+  body of an animal, differing greatly, it is true, from the bodies
+  of all other animals, but agreeing with them in all essential
+  features. The bony structure of man classes him as a vertebrate;
+  the mode of suckling his young classes him as a mammal; his
+  blood, his muscles, and his nerves, the structure of his heart
+  with its veins and arteries, his lungs and his whole respiratory
+  and circulatory systems, all closely correspond to those of other
+  mammals, and are often almost identical with <a name="Page_446"
+  id="Page_446"></a>them. He possesses the same number of limbs
+  terminating in the same number of digits as belong fundamentally
+  to the mammalian class. His senses are identical with theirs, and
+  his organs of sense are the same in number and occupy the same
+  relative position. Every detail of structure which is common to
+  the mammalia as a class is found also in man, while he only
+  differs from them in such ways and degrees as the various species
+  or groups of mammals differ from each other. If, then, we have
+  good reason to believe that every existing group of mammalia has
+  descended from some common ancestral form&mdash;as we saw to be
+  so completely demonstrated in the case of the horse
+  tribe,&mdash;and that each family, each order, and even the whole
+  class must similarly have descended from some much more ancient
+  and more generalised type, it would be in the highest degree
+  improbable&mdash;so improbable as to be almost
+  inconceivable&mdash;that man, agreeing with them so closely in
+  every detail of his structure, should have had some quite
+  distinct mode of origin. Let us, then, see what other evidence
+  bears upon the question, and whether it is sufficient to convert
+  the probability of his animal origin into a practical
+  certainty.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Rudiments and Variations as Indicating the Relation of Man to
+  other Mammals.</i></p>
+
+  <p>All the higher animals present rudiments of organs which,
+  though useless to them, are useful in some allied group, and are
+  believed to have descended from a common ancestor in which they
+  were useful. Thus there are in ruminants rudiments of incisor
+  teeth which, in some species, never cut through the gums; many
+  lizards have external rudimentary legs; while many birds, as the
+  Apteryx, have quite rudimentary wings. Now man possesses similar
+  rudiments, sometimes constantly, sometimes only occasionally
+  present, which serve intimately to connect his bodily structure
+  with that of the lower animals. Many animals, for example, have a
+  special muscle for moving or twitching the skin. In man there are
+  remnants of this in certain parts of the body, especially in the
+  forehead, enabling us to raise our eyebrows; but some persons
+  have it in other parts. A few persons are able to move the whole
+  scalp so as to throw off any object placed on the head, <a name=
+  "Page_447" id="Page_447"></a>and this property has been proved,
+  in one case, to be inherited. In the outer fold of the ear there
+  is sometimes a projecting point, corresponding in position to the
+  pointed ear of many animals, and believed to be a rudiment of it.
+  In the alimentary canal there is a rudiment&mdash;the vermiform
+  appendage of the caecum&mdash;which is not only useless, but is
+  sometimes a cause of disease and death in man; yet in many
+  vegetable feeding animals it is very long, and even in the
+  orang-utan it is of considerable length and convoluted. So, man
+  possesses rudimentary bones of a tail concealed beneath the skin,
+  and, in some rare cases, this forms a minute external tail.</p>
+
+  <p>The variability of every part of man's structure is very
+  great, and many of these variations tend to approximate towards
+  the structure of other animals. The courses of the arteries are
+  eminently variable, so that for surgical purposes it has been
+  necessary to determine the probable proportion of each variation.
+  The muscles are so variable that in fifty cases the muscles of
+  the foot were found to be not strictly alike in any two, and in
+  some the deviations were considerable; while in thirty-six
+  subjects Mr. J. Wood observed no fewer than 558 muscular
+  variations. The same author states that in a single male subject
+  there were no fewer than seven muscular variations, all of which
+  plainly represented muscles proper to various kinds of apes. The
+  muscles of the hands and arms&mdash;parts which are so eminently
+  characteristic of man&mdash;are extremely liable to vary, so as
+  to resemble the corresponding muscles of the lower animals. That
+  such variations are due to reversion to a former state of
+  existence Mr. Darwin thinks highly probable, and he adds: "It is
+  quite incredible that a man should, through mere accident,
+  abnormally resemble certain apes in no less than seven of his
+  muscles, if there had been no genetic connection between them. On
+  the other hand, if man is descended from some ape-like creature,
+  no valid reason can be assigned why certain muscles should not
+  suddenly reappear after an interval of many thousand generations,
+  in the same manner as, with horses, asses, and mules, dark
+  coloured stripes suddenly reappear on the legs and shoulders,
+  after an interval of hundreds, or more probably of thousands of
+  generations."<a name="FNanchor_218_218" id=
+  "FNanchor_218_218"></a><a href="#Footnote_218_218" class=
+  "fnanchor">[218]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_448" id="Page_448"></a><i>The Embryonic Development
+  of Man and other Mammalia.</i></p>
+
+  <p>The progressive development of any vertebrate from the ovum or
+  minute embryonic egg affords one of the most marvellous chapters
+  in Natural History. We see the contents of the ovum undergoing
+  numerous definite changes, its interior dividing and subdividing
+  till it consists of a mass of cells, then a groove appears
+  marking out the median line or vertebral column of the future
+  animal, and thereafter are slowly developed the various essential
+  organs of the body. After describing in some detail what takes
+  place in the case of the ovum of the dog, Professor Huxley
+  continues: "The history of the development of any other
+  vertebrate animal, lizard, snake, frog, or fish tells the same
+  story. There is always to begin with, an egg having the same
+  essential structure as that of the dog; the yelk of that egg
+  undergoes division or segmentation, as it is called, the ultimate
+  products of that segmentation constitute the building materials
+  for the body of the young animal; and this is built up round a
+  primitive groove, in the floor of which a notochord is developed.
+  Furthermore, there is a period in which the young of all these
+  animals resemble one another, not merely in outward form, but in
+  all essentials of structure, so closely, that the differences
+  between them are inconsiderable, while in their subsequent course
+  they diverge more and more widely from one another. And it is a
+  general law that the more closely any animals resemble one
+  another in adult structure, the larger and the more intimately do
+  their embryos resemble one another; so that, for example, the
+  embryos of a snake and of a lizard remain like one another longer
+  than do those of a snake and a bird; and the embryos of a dog and
+  of a cat remain like one another for a far longer period than do
+  those of a dog and a bird, or of a dog and an opossum, or even
+  than those of a dog and a monkey."<a name="FNanchor_219_219" id=
+  "FNanchor_219_219"></a><a href="#Footnote_219_219" class=
+  "fnanchor">[219]</a></p>
+
+  <p>We thus see that the study of development affords a test of
+  affinity in animals that are externally very much unlike each
+  other; and we naturally ask how this applies to man. Is he
+  developed in a different way from other mammals, as we should
+  certainly expect if he has had a distinct and <a name="Page_449"
+  id="Page_449"></a>altogether different origin? "The reply," says
+  Professor Huxley, "is not doubtful for a moment. Without
+  question, the mode of origin and the early stages of the
+  development of man are identical with those of the animals
+  immediately below him in the scale." And again he tells us: "It
+  is very long before the body of the young human being can be
+  readily discriminated from that of the young puppy; but at a
+  tolerably early period the two become distinguishable by the
+  different forms of their adjuncts, the yelk-sac and the
+  allantois;" and after describing these differences he continues:
+  "But exactly in those respects in which the developing man
+  differs from the dog, he resembles the ape.... So that it is only
+  quite in the latter stages of development that the young human
+  being presents marked differences from the young ape, while the
+  latter departs as much from the dog in its development as the man
+  does. Startling as this last assertion may appear to be, it is
+  demonstrably true, and it alone appears to me sufficient to place
+  beyond all doubt the structural unity of man with the rest of the
+  animal world, and more particularly and closely with the
+  apes."<a name="FNanchor_220_220" id=
+  "FNanchor_220_220"></a><a href="#Footnote_220_220" class=
+  "fnanchor">[220]</a></p>
+
+  <p>A few of the curious details in which man passes through
+  stages common to the lower animals may be mentioned. At one stage
+  the os coccyx projects like a true tail, extending considerably
+  beyond the rudimentary legs. In the seventh month the
+  convolutions of the brain resemble those of an adult baboon. The
+  great toe, so characteristic of man, forming the fulcrum which
+  most assists him in standing erect, in an early stage of the
+  embryo is much shorter than the other toes, and instead of being
+  parallel with them, projects at an angle from the side of the
+  foot, thus corresponding with its permanent condition in the
+  quadrumana. Numerous other examples might be quoted, all
+  illustrating the same general law.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Diseases Common to Man and the Lower Animals.</i></p>
+
+  <p>Though the fact is so well known, it is certainly one of
+  profound significance that many animal diseases can be
+  communicated to man, since it shows similarity, if not identity,
+  in <a name="Page_450" id="Page_450"></a>the minute structure of
+  the tissues, the nature of the blood, the nerves, and the brain.
+  Such diseases as hydrophobia, variola, the glanders, cholera,
+  herpes, etc., can be transmitted from animals to man or the
+  reverse; while monkeys are liable to many of the same
+  non-contagious diseases as we are. Rengger, who carefully
+  observed the common monkey (Cebus Azarae) in Paraguay, found it
+  liable to catarrh, with the usual symptoms, terminating sometimes
+  in consumption. These monkeys also suffered from apoplexy,
+  inflammation of the bowels, and cataract in the eye. Medicines
+  produced the same effect upon them as upon us. Many kinds of
+  monkeys have a strong taste for tea, coffee, spirits, and even
+  tobacco. These facts show the similarity of the nerves of taste
+  in monkeys and in ourselves, and that their whole nervous system
+  is affected in a similar way. Even the parasites, both external
+  and internal, that affect man are not altogether peculiar to him,
+  but belong to the same families or genera as those which infest
+  animals, and in one case, scabies, even the same species.<a name=
+  "FNanchor_221_221" id="FNanchor_221_221"></a><a href=
+  "#Footnote_221_221" class="fnanchor">[221]</a> These curious
+  facts seem quite inconsistent with the idea that man's bodily
+  structure and nature are altogether distinct from those of
+  animals, and have had a different origin; while the facts are
+  just what we should expect if he has been produced by descent
+  with modification from some common ancestor.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Animals most nearly Allied to Man.</i></p>
+
+  <p>By universal consent we see in the monkey tribe a caricature
+  of humanity. Their faces, their hands, their actions and
+  expressions present ludicrous resemblances to our own. But there
+  is one group of this great tribe in which this resemblance is
+  greatest, and they have hence been called the anthropoid or
+  man-like apes. These are few in number, and inhabit only the
+  equatorial regions of Africa and Asia, countries where the
+  climate is most uniform, the forests densest, and the supply of
+  fruit abundant throughout the year. These animals are now
+  comparatively well known, consisting of the orang-utan of Borneo
+  and Sumatra, the chimpanzee and the gorilla of West Africa, and
+  the group of gibbons or long-armed apes, consisting of many
+  species and inhabiting South-Eastern <a name="Page_451" id=
+  "Page_451"></a>Asia and the larger Malay Islands. These last are
+  far less like man than the other three, one or other of which has
+  at various times been claimed to be the most man-like of the apes
+  and our nearest relations in the animal kingdom. The question of
+  the degree of resemblance of these animals to ourselves is one of
+  great interest, leading, as it does, to some important
+  conclusions as to our origin and geological antiquity, and we
+  will therefore briefly consider it.</p>
+
+  <p>If we compare the skeletons of the orang or chimpanzee with
+  that of man, we find them to be a kind of distorted copy, every
+  bone corresponding (with very few exceptions), but altered
+  somewhat in size, proportions, and position. So great is this
+  resemblance that it led Professor Owen to remark: "I cannot shut
+  my eyes to the significance of that all-pervading similitude of
+  structure&mdash;every tooth, every bone, strictly
+  homologous&mdash;which makes the determination of the difference
+  between <i>Homo</i> and <i>Pithecus</i> the anatomist's
+  difficulty."</p>
+
+  <p>The actual differences in the skeletons of these apes and that
+  of man&mdash;that is, differences dependent on the presence or
+  absence of certain bones, and not on their form or
+  position&mdash;have been enumerated by Mr. Mivart as
+  follows:&mdash;(1) In the breast-bone consisting of but two
+  bones, man agrees with the gibbons; the chimpanzee and gorilla
+  having this part consisting of seven bones in a single series,
+  while in the orang they are arranged in a double series of ten
+  bones. (2) The normal number of the ribs in the orang and some
+  gibbons is twelve pairs, as in man, while in the chimpanzee and
+  gorilla there are thirteen pairs. (3) The orang and the gibbons
+  also agree with man in having five lumbar vertebrae, while in the
+  gorilla and the chimpanzee there are but four, and sometimes only
+  three. (4) The gorilla and chimpanzee agree with man in having
+  eight small bones in the wrist, while the orang and the gibbons,
+  as well as all other monkeys, have nine.<a name=
+  "FNanchor_222_222" id="FNanchor_222_222"></a><a href=
+  "#Footnote_222_222" class="fnanchor">[222]</a></p>
+
+  <p>The differences in the form, size, and attachments of the
+  various bones, muscles, and other organs of these apes and
+  <a name="Page_452" id="Page_452"></a>man are very numerous and
+  exceedingly complex, sometimes one species, sometimes another
+  agreeing most nearly with ourselves, thus presenting a tangled
+  web of affinities which it is very difficult to unravel.
+  Estimated by the skeleton alone, the chimpanzee and gorilla seem
+  nearer to man than the orang, which last is also inferior as
+  presenting certain aberrations in the muscles. In the form of the
+  ear the gorilla is more human than any other ape, while in the
+  tongue the orang is the more man-like. In the stomach and liver
+  the gibbons approach nearest to man, then come the orang and
+  chimpanzee, while the gorilla has a degraded liver more
+  resembling that of the lower monkeys and baboons.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Brains of Man and Apes.</i></p>
+
+  <p>We come now to that part of his organisation in which man is
+  so much higher than all the lower animals&mdash;the brain; and
+  here, Mr. Mivart informs us, the orang stands highest in rank.
+  The height of the orang's cerebrum in front is greater in
+  proportion than in either the chimpanzee or the gorilla. "On
+  comparing the brain of man with the brains of the orang,
+  chimpanzee, and baboon, we find a successive decrease in the
+  frontal lobe, and a successive and very great increase in the
+  relative size of the occipital lobe. Concomitantly with this
+  increase and decrease, certain folds of brain substance, called
+  'bridging convolutions,' which in man are conspicuously
+  interposed between the parietal and occipital lobes, seem as
+  utterly to disappear in the chimpanzee, as they do in the baboon.
+  In the orang, however, though much reduced, they are still to be
+  distinguished.... The actual and absolute mass of the brain is,
+  however, slightly greater in the chimpanzee than in the orang, as
+  is the relative vertical extent of the middle part of the
+  cerebrum, although, as already stated, the frontal portion is
+  higher in the orang; while, according to M. Gratiolet, the
+  gorilla is not only inferior to the orang in cerebral
+  development, but even to his smaller African congener, the
+  chimpanzee."<a name="FNanchor_223_223" id=
+  "FNanchor_223_223"></a><a href="#Footnote_223_223" class=
+  "fnanchor">[223]</a></p>
+
+  <p>On the whole, then, we find that no one of the great apes can
+  be positively asserted to be nearest to man in structure. Each of
+  them approaches him in certain characteristics, while <a name=
+  "Page_453" id="Page_453"></a>in others it is widely removed,
+  giving the idea, so consonant with the theory of evolution as
+  developed by Darwin, that all are derived from a common ancestor,
+  from which the existing anthropoid apes as well as man have
+  diverged. When, however, we turn from the details of anatomy to
+  peculiarities of external form and motions, we find that, in a
+  variety of characters, all these apes resemble each other and
+  differ from man, so that we may fairly say that, while they have
+  diverged somewhat from each other, they have diverged much more
+  widely from ourselves. Let us briefly enumerate some of these
+  differences.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>External Differences of Man and Apes.</i></p>
+
+  <p>All apes have large canine teeth, while in man these are no
+  longer than the adjacent incisors or premolars, the whole forming
+  a perfectly even series. In apes the arms are proportionately
+  much longer than in man, while the thighs are much shorter. No
+  ape stands really erect, a posture which is natural in man. The
+  thumb is proportionately larger in man, and more perfectly
+  opposable than in that of any ape. The foot of man differs
+  largely from that of all apes, in the horizontal sole, the
+  projecting heel, the short toes, and the powerful great toe
+  firmly attached parallel to the other toes; all perfectly adapted
+  for maintaining the erect posture, and for free motion without
+  any aid from the arms or hands. In apes the foot is formed almost
+  exactly like our hand, with a large thumb-like great toe quite
+  free from the other toes, and so articulated as to be opposable
+  to them; forming with the long finger-like toes a perfect
+  grasping hand. The sole cannot be placed horizontally on the
+  ground; but when standing on a level surface the animal rests on
+  the outer edge of the foot with the finger and thumb-like toes
+  partly closed, while the hands are placed on the ground resting
+  on the knuckles. The illustration on the next page (Fig. 37)
+  shows, fairly well, the peculiarities of the hands and feet of
+  the chimpanzee, and their marked differences, both in form and
+  use, from those of man.</p>
+
+  <p>The four limbs, with the peculiarly formed feet and hands, are
+  those of arboreal animals which only occasionally and awkwardly
+  move on level ground. The arms are used in pro<a name="Page_454"
+  id="Page_454"></a>gression equally with the feet, and the hands
+  are only adapted for uses similar to those of our hands when the
+  animal is at rest, and then but clumsily. Lastly, the apes are
+  all hairy animals, like the majority of other mammals, man alone
+  having a smooth and almost naked skin. These numerous and
+  striking differences, even more than those of the skeleton and
+  internal anatomy, point to an enormously remote epoch when the
+  race that was ultimately to develop into man diverged from that
+  other stock which continued the animal type and ultimately
+  produced the existing varieties of anthropoid apes.</p>
+
+  <div class="figcenter" style="width: 400px;">
+    <a name="fig37" id="fig37"></a> <img src="images/fig37.jpg"
+    width="400" height="455" alt=
+    "FIG. 37.&mdash;Chimpanzee (Troglodytes niger)." title=
+    "FIG. 37.&mdash;Chimpanzee (Troglodytes niger)." /> <b>FIG.
+    37.&mdash;Chimpanzee (Troglodytes niger).</b>
+  </div>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Summary of the Animal Characteristics of Man.</i></p>
+
+  <p>The facts now very briefly summarised amount almost to a
+  demonstration that man, in his bodily structure, has been derived
+  from the lower animals, of which he is the culminating
+  development. In his possession of rudimentary structures <a name=
+  "Page_455" id="Page_455"></a>which are functional in some of the
+  mammalia; in the numerous variations of his muscles and other
+  organs agreeing with characters which are constant in some apes;
+  in his embryonic development, absolutely identical in character
+  with that of mammalia in general, and closely resembling in its
+  details that of the higher quadrumana; in the diseases which he
+  has in common with other mammalia; and in the wonderful
+  approximation of his skeleton to those of one or other of the
+  anthropoid apes, we have an amount of evidence in this direction
+  which it seems impossible to explain away. And this evidence will
+  appear more forcible if we consider for a moment what the
+  rejection of it implies. For the only alternative supposition is,
+  that man has been specially created&mdash;that is to say, has
+  been produced in some quite different way from other animals and
+  altogether independently of them. But in that case the
+  rudimentary structures, the animal-like variations, the identical
+  course of development, and all the other animal characteristics
+  he possesses are deceptive, and inevitably lead us, as thinking
+  beings making use of the reason which is our noblest and most
+  distinctive feature, into gross error.</p>
+
+  <p>We cannot believe, however, that a careful study of the facts
+  of nature leads to conclusions directly opposed to the truth;
+  and, as we seek in vain, in our physical structure and the course
+  of its development, for any indication of an origin independent
+  of the rest of the animal world, we are compelled to reject the
+  idea of "special creation" for man, as being entirely unsupported
+  by facts as well as in the highest degree improbable.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Geological Antiquity of Man.</i></p>
+
+  <p>The evidence we now possess of the exact nature of the
+  resemblance of man to the various species of anthropoid apes,
+  shows us that he has little special affinity for any one rather
+  than another species, while he differs from them all in several
+  important characters in which they agree with each other. The
+  conclusion to be drawn from these facts is, that his points of
+  affinity connect him with the whole group, while his special
+  peculiarities equally separate him from the whole group, and that
+  he must, therefore, have diverged from the common ancestral form
+  before the existing types of anthropoid apes <a name="Page_456"
+  id="Page_456"></a>had diverged from each other. Now, this
+  divergence almost certainly took place as early as the Miocene
+  period, because in the Upper Miocene deposits of Western Europe
+  remains of two species of ape have been found allied to the
+  gibbons, one of them, Dryopithecus, nearly as large as a man, and
+  believed by M. Lartet to have approached man in its dentition
+  more than the existing apes. We seem hardly, therefore, to have
+  reached, in the Upper Miocene, the epoch of the common ancestor
+  of man and the anthropoids.</p>
+
+  <p>The evidence of the antiquity of man himself is also scanty,
+  and takes us but very little way back into the past. We have
+  clear proof of his existence in Europe in the latter stages of
+  the glacial epoch, with many indications of his presence in
+  interglacial or even pre-glacial times; while both the actual
+  remains and the works of man found in the auriferous gravels of
+  California deep under lava-flows of Pliocene age, show that he
+  existed in the New World at least as early as in the Old.<a name=
+  "FNanchor_224_224" id="FNanchor_224_224"></a><a href=
+  "#Footnote_224_224" class="fnanchor">[224]</a> These earliest
+  remains of man have been received with doubt, and even with
+  ridicule, as if there were some extreme improbability in them.
+  But, in point of fact, the wonder is that human remains have not
+  been found more frequently in pre-glacial deposits. Referring to
+  the most ancient fossil remains found in Europe&mdash;the Engis
+  and Neanderthal crania,&mdash;Professor Huxley makes the
+  following weighty remark: "In conclusion, I may say, that the
+  fossil remains of Man hitherto discovered do not seem to me to
+  take us appreciably nearer to that lower pithecoid form, by the
+  modification of which he has, probably, become what he is." The
+  Californian remains and works of art, above referred to, give no
+  indication of a specially low form of man; and it remains an
+  unsolved problem why no traces of the long line of man's
+  ancestors, back to the remote period when he first branched off
+  from the pithecoid type, have yet been discovered.</p>
+
+  <p>It has been objected by some writers&mdash;notably by
+  Professor Boyd Dawkins&mdash;that man did not probably exist in
+  Pliocene times, because almost all the known mammalia of that
+  epoch are distinct species from those now living on the earth,
+  and that the same changes of the environment which led to
+  <a name="Page_457" id="Page_457"></a>the modification of other
+  mammalian species would also have led to a change in man. But
+  this argument overlooks the fact that man differs essentially
+  from all other mammals in this respect, that whereas any
+  important adaptation to new conditions can be effected in them
+  only by a change in bodily structure, man is able to adapt
+  himself to much greater changes of conditions by a mental
+  development leading him to the use of fire, of tools, of
+  clothing, of improved dwellings, of nets and snares, and of
+  agriculture. By the help of these, without any change whatever in
+  his bodily structure, he has been able to spread over and occupy
+  the whole earth; to dwell securely in forest, plain, or mountain;
+  to inhabit alike the burning desert or the arctic wastes; to cope
+  with every kind of wild beast, and to provide himself with food
+  in districts where, as an animal trusting to nature's unaided
+  productions, he would have starved.<a name="FNanchor_225_225" id=
+  "FNanchor_225_225"></a><a href="#Footnote_225_225" class=
+  "fnanchor">[225]</a></p>
+
+  <p>It follows, therefore, that from the time when the ancestral
+  man first walked erect, with hands freed from any active part in
+  locomotion, and when his brain-power became sufficient to cause
+  him to use his hands in making weapons and tools, houses and
+  clothing, to use fire for cooking, and to plant seeds or roots to
+  supply himself with stores of food, the power of natural
+  selection would cease to act in producing modifications of his
+  body, but would continuously advance his mind through the
+  development of its organ, the brain. Hence man may have become
+  truly man&mdash;the species, Homo sapiens&mdash;even in the
+  Miocene period; and while all other mammals were becoming
+  modified from age to age under the influence of ever-changing
+  physical and biological conditions, he would be advancing mainly
+  in intelligence, but perhaps also in stature, and by that advance
+  alone would be able to maintain himself as the master of all
+  other animals and as the most widespread occupier of the earth.
+  It is quite in accordance with this view that we find the most
+  pronounced distinction between man and the anthropoid apes in the
+  size and complexity of his brain. Thus, Professor Huxley tells us
+  that "it may be doubted whether a healthy human adult brain ever
+  weighed <a name="Page_458" id="Page_458"></a>less than 31 or 32
+  ounces, or that the heaviest gorilla brain has exceeded 20
+  ounces," although "a full-grown gorilla is probably pretty nearly
+  twice as heavy as a Bosjes man, or as many an European
+  woman."<a name="FNanchor_226_226" id=
+  "FNanchor_226_226"></a><a href="#Footnote_226_226" class=
+  "fnanchor">[226]</a> The average human brain, however, weighs 48
+  or 49 ounces, and if we take the average ape brain at only 2
+  ounces less than the very largest gorilla's brain, or 18 ounces,
+  we shall see better the enormous increase which has taken place
+  in the brain of man since the time when he branched off from the
+  apes; and this increase will be still greater if we consider that
+  the brains of apes, like those of all other mammals, have also
+  increased from earlier to later geological times.</p>
+
+  <p>If these various considerations are taken into account, we
+  must conclude that the essential features of man's structure as
+  compared with that of apes&mdash;his erect posture and free
+  hands&mdash;were acquired at a comparatively early period, and
+  were, in fact, the characteristics which gave him his superiority
+  over other mammals, and started him on the line of development
+  which has led to his conquest of the world. But during this long
+  and steady development of brain and intellect, mankind must have
+  continuously increased in numbers and in the area which they
+  occupied&mdash;they must have formed what Darwin terms a
+  "dominant race." For had they been few in numbers and confined to
+  a limited area, they could hardly have successfully struggled
+  against the numerous fierce carnivora of that period, and against
+  those adverse influences which led to the extinction of so many
+  more powerful animals. A large population spread over an
+  extensive area is also needed to supply an adequate number of
+  brain variations for man's progressive improvement. But this
+  large population and long-continued development in a single line
+  of advance renders it the more difficult to account for the
+  complete absence of human or pre-human remains in all those
+  deposits which have furnished, in such rich abundance, the
+  remains of other land animals. It is true that the remains of
+  apes are also very rare, and we may well suppose that the
+  superior intelligence of man led him to avoid that extensive
+  destruction by flood or in morass which seems to have often
+  overwhelmed other animals. Yet, when <a name="Page_459" id=
+  "Page_459"></a>we consider that, even in our own day, men are not
+  unfrequently overwhelmed by volcanic eruptions, as in Java and
+  Japan, or carried away in vast numbers by floods, as in Bengal
+  and China, it seems impossible but that ample remains of Miocene
+  and Pliocene man do exist buried in the most recent layers of the
+  earth's crust, and that more extended research or some fortunate
+  discovery will some day bring them to light.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Probable Birthplace of Man.</i></p>
+
+  <p>It has usually been considered that the ancestral form of man
+  originated in the tropics, where vegetation is most abundant and
+  the climate most equable. But there are some important objections
+  to this view. The anthropoid apes, as well as most of the monkey
+  tribe, are essentially arboreal in their structure, whereas the
+  great distinctive character of man is his special adaptation to
+  terrestrial locomotion. We can hardly suppose, therefore, that he
+  originated in a forest region, where fruits to be obtained by
+  climbing are the chief vegetable food. It is more probable that
+  he began his existence on the open plains or high plateaux of the
+  temperate or sub-tropical zone, where the seeds of indigenous
+  cereals and numerous herbivora, rodents, and game-birds, with
+  fishes and molluscs in the lakes, rivers, and seas supplied him
+  with an abundance of varied food. In such a region he would
+  develop skill as a hunter, trapper, or fisherman, and later as a
+  herdsman and cultivator,&mdash;a succession of which we find
+  indications in the palaeolithic and neolithic races of
+  Europe.</p>
+
+  <p>In seeking to determine the particular areas in which his
+  earliest traces are likely to be found, we are restricted to some
+  portion of the Eastern hemisphere, where alone the anthropoid
+  apes exist, or have apparently ever existed.</p>
+
+  <p>There is good reason to believe, also, that Africa must be
+  excluded, because it is known to have been separated from the
+  northern continent in early tertiary times, and to have acquired
+  its existing fauna of the higher mammalia by a later union with
+  that continent after the separation from it of Madagascar, an
+  island which has preserved for us a sample, as it were, of the
+  early African mammalian fauna, from which not only the anthropoid
+  apes, but all the higher quadrumana <a name="Page_460" id=
+  "Page_460"></a>are absent.<a name="FNanchor_227_227" id=
+  "FNanchor_227_227"></a><a href="#Footnote_227_227" class=
+  "fnanchor">[227]</a> There remains only the great Euro-Asiatic
+  continent; and its enormous plateaux, extending from Persia right
+  across Tibet and Siberia to Manchuria, afford an area, some part
+  or other of which probably offered suitable conditions, in late
+  Miocene or early Pliocene times, for the development of ancestral
+  man.</p>
+
+  <p>It is in this area that we still find that type of
+  mankind&mdash;the Mongolian&mdash;which retains a colour of the
+  skin midway between the black or brown-black of the negro, and
+  the ruddy or olive-white of the Caucasian types, a colour which
+  still prevails over all Northern Asia, over the American
+  continents, and over much of Polynesia. From this primary tint
+  arose, under the influence of varied conditions, and probably in
+  correlation with constitutional changes adapted to peculiar
+  climates, the varied tints which still exist among mankind. If
+  the reasoning by which this conclusion is reached be sound, and
+  all the earlier stages of man's development from an animal form
+  occurred in the area now indicated, we can better understand how
+  it is that we have as yet met with no traces of the missing
+  links, or even of man's existence during late tertiary times,
+  because no part of the world is so entirely unexplored by the
+  geologist as this very region. The area in question is
+  sufficiently extensive and varied to admit of primeval man having
+  attained to a considerable population, and having developed his
+  full human characteristics, both physical and mental, before
+  there was any need for him to migrate beyond its limits. One of
+  his earliest important migrations was probably into Africa,
+  where, spreading westward, he became modified in colour and hair
+  in correlation with physiological changes adapting him to the
+  climate of the equatorial lowlands. Spreading north-westward into
+  Europe the moist and cool climate led to a modification of an
+  opposite character, and thus may have arisen the three great
+  human types which still exist. Somewhat later, probably, he
+  spread eastward into North-West America and soon scattered
+  himself over the whole continent; and all this may well have
+  occurred in early or middle Pliocene times. Thereafter, at very
+  long intervals, successive waves of migration carried him into
+  every <a name="Page_461" id="Page_461"></a>part of the habitable
+  world, and by conquest and intermixture led ultimately to that
+  puzzling gradation of types which the ethnologist in vain seeks
+  to unravel.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Origin of the Moral and Intellectual Nature of
+  Man.</i></p>
+
+  <p>From the foregoing discussion it will be seen that I fully
+  accept Mr. Darwin's conclusion as to the essential identity of
+  man's bodily structure with that of the higher mammalia, and his
+  descent from some ancestral form common to man and the anthropoid
+  apes. The evidence of such descent appears to me to be
+  overwhelming and conclusive. Again, as to the cause and method of
+  such descent and modification, we may admit, at all events
+  provisionally, that the laws of variation and natural selection,
+  acting through the struggle for existence and the continual need
+  of more perfect adaptation to the physical and biological
+  environments, may have brought about, first that perfection of
+  bodily structure in which he is so far above all other animals,
+  and in co-ordination with it the larger and more developed brain,
+  by means of which he has been able to utilise that structure in
+  the more and more complete subjection of the whole animal and
+  vegetable kingdoms to his service.</p>
+
+  <p>But this is only the beginning of Mr. Darwin's work, since he
+  goes on to discuss the moral nature and mental faculties of man,
+  and derives these too by gradual modification and development
+  from the lower animals. Although, perhaps, nowhere distinctly
+  formulated, his whole argument tends to the conclusion that man's
+  entire nature and all his faculties, whether moral, intellectual,
+  or spiritual, have been derived from their rudiments in the lower
+  animals, in the same manner and by the action of the same general
+  laws as his physical structure has been derived. As this
+  conclusion appears to me not to be supported by adequate
+  evidence, and to be directly opposed to many well-ascertained
+  facts, I propose to devote a brief space to its discussion.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Argument from Continuity.</i></p>
+
+  <p>Mr. Darwin's mode of argument consists in showing that the
+  rudiments of most, if not of all, the mental and moral faculties
+  of man can be detected in some animals. The <a name="Page_462"
+  id="Page_462"></a>manifestations of intelligence, amounting in
+  some cases to distinct acts of reasoning, in many animals, are
+  adduced as exhibiting in a much less degree the intelligence and
+  reason of man. Instances of curiosity, imitation, attention,
+  wonder, and memory are given; while examples are also adduced
+  which may be interpreted as proving that animals exhibit kindness
+  to their fellows, or manifest pride, contempt, and shame. Some
+  are said to have the rudiments of language, because they utter
+  several different sounds, each of which has a definite meaning to
+  their fellows or to their young; others the rudiments of
+  arithmetic, because they seem to count and remember up to three,
+  four, or even five. A sense of beauty is imputed to them on
+  account of their own bright colours or the use of coloured
+  objects in their nests; while dogs, cats, and horses are said to
+  have imagination, because they appear to be disturbed by dreams.
+  Even some distant approach to the rudiments of religion is said
+  to be found in the deep love and complete submission of a dog to
+  his master.<a name="FNanchor_228_228" id=
+  "FNanchor_228_228"></a><a href="#Footnote_228_228" class=
+  "fnanchor">[228]</a></p>
+
+  <p>Turning from animals to man, it is shown that in the lowest
+  savages many of these faculties are very little advanced from the
+  condition in which they appear in the higher animals; while
+  others, although fairly well exhibited, are yet greatly inferior
+  to the point of development they have reached in civilised races.
+  In particular, the moral sense is said to have been developed
+  from the social instincts of savages, and to depend mainly on the
+  enduring discomfort produced by any action which excites the
+  general disapproval of the tribe. Thus, every act of an
+  individual which is believed to be contrary to the interests of
+  the tribe, excites its unvarying disapprobation and is held to be
+  immoral; while every act, on the other hand, which is, as a rule,
+  beneficial to the tribe, is warmly and constantly approved, and
+  is thus considered to be right or moral. From the mental
+  struggle, when an act that would benefit self is injurious to the
+  tribe, there arises conscience; and thus the social instincts are
+  the foundation of the moral sense and of the fundamental
+  principles of morality.<a name="FNanchor_229_229" id=
+  "FNanchor_229_229"></a><a href="#Footnote_229_229" class=
+  "fnanchor">[229]</a></p>
+
+  <p>The question of the origin and nature of the moral sense and
+  of conscience is far too vast and complex to be discussed
+  <a name="Page_463" id="Page_463"></a>here, and a reference to it
+  has been introduced only to complete the sketch of Mr. Darwin's
+  view of the continuity and gradual development of all human
+  faculties from the lower animals up to savages, and from savage
+  up to civilised man. The point to which I wish specially to call
+  attention is, that to prove continuity and the progressive
+  development of the intellectual and moral faculties from animals
+  to man, is not the same as proving that these faculties have been
+  developed by natural selection; and this last is what Mr. Darwin
+  has hardly attempted, although to support his theory it was
+  absolutely essential to prove it. Because man's physical
+  structure has been developed from an animal form by natural
+  selection, it does not necessarily follow that his mental nature,
+  even though developed <i>pari passu</i> with it, has been
+  developed by the same causes only. To illustrate by a physical
+  analogy. Upheaval and depression of land, combined with
+  sub-aerial denudation by wind and frost, rain and rivers, and
+  marine denudation on coastlines, were long thought to account for
+  all the modelling of the earth's surface not directly due to
+  volcanic action; and in the early editions of Lyell's
+  <i>Principles of Geology</i> these are the sole causes appealed
+  to. But when the action of glaciers was studied and the recent
+  occurrence of a glacial epoch demonstrated as a fact, many
+  phenomena&mdash;such as moraines and other gravel deposits,
+  boulder clay, erratic boulders, grooved and rounded rocks, and
+  Alpine lake basins&mdash;were seen to be due to this altogether
+  distinct cause. There was no breach of continuity, no sudden
+  catastrophe; the cold period came on and passed away in the most
+  gradual manner, and its effects often passed insensibly into
+  those produced by denudation or upheaval; yet none the less a new
+  agency appeared at a definite time, and new effects were produced
+  which, though continuous with preceding effects, were not due to
+  the same causes. It is not, therefore, to be assumed, without
+  proof or against independent evidence, that the later stages of
+  an apparently continuous development are necessarily due to the
+  same causes only as the earlier stages. Applying this argument to
+  the case of man's intellectual and moral nature, I propose to
+  show that certain definite portions of it could not have been
+  developed by variation and natural selection alone, and that,
+  therefore, some other influence, law, or agency is <a name=
+  "Page_464" id="Page_464"></a>required to account for them. If
+  this can be clearly shown for any one or more of the special
+  faculties of intellectual man, we shall be justified in assuming
+  that the same unknown cause or power may have had a much wider
+  influence, and may have profoundly influenced the whole course of
+  his development.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Origin of the Mathematical Faculty.</i></p>
+
+  <p>We have ample evidence that, in all the lower races of man,
+  what may be termed the mathematical faculty is, either absent,
+  or, if present, quite unexercised. The Bushmen and the Brazilian
+  Wood-Indians are said not to count beyond two. Many Australian
+  tribes only have words for one and two, which are combined to
+  make three, four, five, or six, beyond which they do not count.
+  The Damaras of South Africa only count to three; and Mr. Galton
+  gives a curious description of how one of them was hopelessly
+  puzzled when he had sold two sheep for two sticks of tobacco
+  each, and received four sticks in payment. He could only find out
+  that he was correctly paid by taking two sticks and then giving
+  one sheep, then receiving two sticks more and giving the other
+  sheep. Even the comparatively intellectual Zulus can only count
+  up to ten by using the hands and fingers. The Ahts of North-West
+  America count in nearly the same manner, and most of the tribes
+  of South America are no further advanced.<a name=
+  "FNanchor_230_230" id="FNanchor_230_230"></a><a href=
+  "#Footnote_230_230" class="fnanchor">[230]</a> The Kaffirs have
+  great herds of cattle, and if one is lost they miss it
+  immediately, but this is not by counting, but by noticing the
+  absence of one they know; just as in a large family or a school a
+  boy is missed without going through the process of counting.
+  Somewhat higher races, as the Esquimaux, can count up to twenty
+  by using the hands and the feet; and other races get even further
+  than this by saying "one man" for twenty, "two men" for forty,
+  and so on, equivalent to our rural mode of reckoning by scores.
+  From the fact that so many of the existing savage races can only
+  count to four or five, Sir John Lubbock thinks it improbable that
+  our earliest ancestors could have counted as high as ten.<a name=
+  "FNanchor_231_231" id="FNanchor_231_231"></a><a href=
+  "#Footnote_231_231" class="fnanchor">[231]</a></p>
+
+  <p><a name="Page_465" id="Page_465"></a>When we turn to the more
+  civilised races, we find the use of numbers and the art of
+  counting greatly extended. Even the Tongas of the South Sea
+  islands are said to have been able to count as high as 100,000.
+  But mere counting does not imply either the possession or the use
+  of anything that can be really called the mathematical faculty,
+  the exercise of which in any broad sense has only been possible
+  since the introduction of the decimal notation. The Greeks, the
+  Romans, the Egyptians, the Jews, and the Chinese had all such
+  cumbrous systems, that anything like a science of arithmetic,
+  beyond very simple operations, was impossible; and the Roman
+  system, by which the year 1888 would be written MDCCCLXXXVIII,
+  was that in common use in Europe down to the fourteenth or
+  fifteenth centuries, and even much later in some places. Algebra,
+  which was invented by the Hindoos, from whom also came the
+  decimal notation, was not introduced into Europe till the
+  thirteenth century, although the Greeks had some acquaintance
+  with it; and it reached Western Europe from Italy only in the
+  sixteenth century.<a name="FNanchor_232_232" id=
+  "FNanchor_232_232"></a><a href="#Footnote_232_232" class=
+  "fnanchor">[232]</a> It was, no doubt, owing to the absence of a
+  sound system of numeration that the mathematical talent of the
+  Greeks was directed chiefly to geometry, in which science Euclid,
+  Archimedes, and others made such brilliant discoveries. It is,
+  however, during the last three centuries only that the civilised
+  world appears to have become conscious of the possession of a
+  marvellous faculty which, when supplied with the necessary tools
+  in the decimal notation, the elements of algebra and geometry,
+  and the power of rapidly communicating discoveries and ideas by
+  the art of printing, has developed to an extent, the full
+  grandeur of which can be appreciated only by those who have
+  devoted some time (even if unsuccessfully) to the study.</p>
+
+  <p>The facts now set forth as to the almost total absence of
+  mathematical faculty in savages and its wonderful development in
+  quite recent times, are exceedingly suggestive, and in regard
+  <a name="Page_466" id="Page_466"></a>to them we are limited to
+  two possible theories. Either prehistoric and savage man did not
+  possess this faculty at all (or only in its merest rudiments); or
+  they did possess it, but had neither the means nor the
+  incitements for its exercise. In the former case we have to ask
+  by what means has this faculty been so rapidly developed in all
+  civilised races, many of which a few centuries back were, in this
+  respect, almost savages themselves; while in the latter case the
+  difficulty is still greater, for we have to assume the existence
+  of a faculty which had never been used either by the supposed
+  possessors of it or by their ancestors.</p>
+
+  <p>Let us take, then, the least difficult supposition&mdash;that
+  savages possessed only the mere rudiments of the faculty, such as
+  their ability to count, sometimes up to ten, but with an utter
+  inability to perform the very simplest processes of arithmetic or
+  of geometry&mdash;and inquire how this rudimentary faculty became
+  rapidly developed into that of a Newton, a La Place, a Gauss, or
+  a Cayley. We will admit that there is every possible gradation
+  between these extremes, and that there has been perfect
+  continuity in the development of the faculty; but we ask, What
+  motive power caused its development?</p>
+
+  <p>It must be remembered we are here dealing solely with the
+  capability of the Darwinian theory to account for the origin of
+  the <i>mind</i>, as well as it accounts for the origin of the
+  <i>body</i> of man, and we must, therefore, recall the essential
+  features of that theory. These are, the preservation of useful
+  variations in the struggle for life; that no creature can be
+  improved beyond its necessities for the time being; that the law
+  acts by life and death, and by the survival of the fittest. We
+  have to ask, therefore, what relation the successive stages of
+  improvement of the mathematical faculty had to the life or death
+  of its possessors; to the struggles of tribe with tribe, or
+  nation with nation; or to the ultimate survival of one race and
+  the extinction of another. If it cannot possibly have had any
+  such effects, then it cannot have been produced by natural
+  selection.</p>
+
+  <p>It is evident that in the struggles of savage man with the
+  elements and with wild beasts, or of tribe with tribe, this
+  faculty can have had no influence. It had nothing to do with
+  <a name="Page_467" id="Page_467"></a>the early migrations of man,
+  or with the conquest and extermination of weaker by more powerful
+  peoples. The Greeks did not successfully resist the Persian
+  invaders by any aid from their few mathematicians, but by
+  military training, patriotism, and self-sacrifice. The barbarous
+  conquerors of the East, Timurlane and Gengkhis Khan, did not owe
+  their success to any superiority of intellect or of mathematical
+  faculty in themselves or their followers. Even if the great
+  conquests of the Romans were, in part, due to their systematic
+  military organisation, and to their skill in making roads and
+  encampments, which may, perhaps, be imputed to some exercise of
+  the mathematical faculty, that did not prevent them from being
+  conquered in turn by barbarians, in whom it was almost entirely
+  absent. And if we take the most civilised peoples of the ancient
+  world&mdash;the Hindoos, the Arabs, the Greeks, and the Romans,
+  all of whom had some amount of mathematical talent&mdash;we find
+  that it is not these, but the descendants of the barbarians of
+  those days&mdash;the Celts, the Teutons, and the Slavs&mdash;who
+  have proved themselves the fittest to survive in the great
+  struggle of races, although we cannot trace their steadily
+  growing success during past centuries either to the possession of
+  any exceptional mathematical faculty or to its exercise. They
+  have indeed proved themselves, to-day, to be possessed of a
+  marvellous endowment of the mathematical faculty; but their
+  success at home and abroad, as colonists or as conquerors, as
+  individuals or as nations, can in no way be traced to this
+  faculty, since they were almost the last who devoted themselves
+  to its exercise. We conclude, then, that the present gigantic
+  development of the mathematical faculty is wholly unexplained by
+  the theory of natural selection, and must be due to some
+  altogether distinct cause.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>The Origin of the Musical and Artistic Faculties.</i></p>
+
+  <p>These distinctively human faculties follow very closely the
+  lines of the mathematical faculty in their progressive
+  development, and serve to enforce the same argument. Among the
+  lower savages music, as we understand it, hardly exists, though
+  they all delight in rude musical sounds, as of drums, tom-toms,
+  or gongs; and they also sing in monotonous chants. Almost exactly
+  as they advance in general intellect, and in the arts <a name=
+  "Page_468" id="Page_468"></a>of social life, their appreciation
+  of music appears to rise in proportion; and we find among them
+  rude stringed instruments and whistles, till, in Java, we have
+  regular bands of skilled performers probably the successors of
+  Hindoo musicians of the age before the Mahometan conquest. The
+  Egyptians are believed to have been the earliest musicians, and
+  from them the Jews and the Greeks, no doubt, derived their
+  knowledge of the art; but it seems to be admitted that neither
+  the latter nor the Romans knew anything of harmony or of the
+  essential features of modern music.<a name="FNanchor_233_233" id=
+  "FNanchor_233_233"></a><a href="#Footnote_233_233" class=
+  "fnanchor">[233]</a> Till the fifteenth century little progress
+  appears to have been made in the science or the practice of
+  music; but since that era it has advanced with marvellous
+  rapidity, its progress being curiously parallel with that of
+  mathematics, inasmuch as great musical geniuses appeared suddenly
+  among different nations, equal in their possession of this
+  special faculty to any that have since arisen.</p>
+
+  <p>As with the mathematical, so with the musical faculty, it is
+  impossible to trace any connection between its possession and
+  survival in the struggle for existence. It seems to have arisen
+  as a <i>result</i> of social and intellectual advancement, not as
+  a <i>cause</i>; and there is some evidence that it is latent in
+  the lower races, since under European training native military
+  bands have been formed in many parts of the world, which have
+  been able to perform creditably the best modern music.</p>
+
+  <p>The artistic faculty has run a somewhat different course,
+  though analogous to that of the faculties already discussed. Most
+  savages exhibit some rudiments of it, either in drawing or
+  carving human or animal figures; but, almost without exception,
+  these figures are rude and such as would be executed by the
+  ordinary inartistic child. In fact, modern savages are, in this
+  respect hardly equal to those prehistoric men who represented the
+  mammoth and the reindeer on pieces of horn or bone. With any
+  advance in the arts of social life, we have a corresponding
+  advance in artistic skill and taste, rising very high in the art
+  of Japan and India, but culminating in the marvellous sculpture
+  of the best period of Grecian history. In the Middle Ages art was
+  chiefly manifested in <a name="Page_469" id=
+  "Page_469"></a>ecclesiastical architecture and the illumination
+  of manuscripts, but from the thirteenth to the fifteenth
+  centuries pictorial art revived in Italy and attained to a degree
+  of perfection which has never been surpassed. This revival was
+  followed closely by the schools of Germany, the Netherlands,
+  Spain, France, and England, showing that the true artistic
+  faculty belonged to no one nation, but was fairly distributed
+  among the various European races.</p>
+
+  <p>These several developments of the artistic faculty, whether
+  manifested in sculpture, painting, or architecture, are evidently
+  outgrowths of the human intellect which have no immediate
+  influence on the survival of individuals or of tribes, or on the
+  success of nations in their struggles for supremacy or for
+  existence. The glorious art of Greece did not prevent the nation
+  from falling under the sway of the less advanced Roman; while we
+  ourselves, among whom art was the latest to arise, have taken the
+  lead in the colonisation of the world, thus proving our mixed
+  race to be the fittest to survive.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Independent Proof that the Mathematical, Musical, and Artistic
+  Faculties have not been Developed under the Law of Natural
+  Selection.</i></p>
+
+  <p>The law of Natural Selection or the survival of the fittest
+  is, as its name implies, a rigid law, which acts by the life or
+  death of the individuals submitted to its action. From its very
+  nature it can act only on useful or hurtful characteristics,
+  eliminating the latter and keeping up the former to a fairly
+  general level of efficiency. Hence it necessarily follows that
+  the characters developed by its means will be present in all the
+  individuals of a species, and, though varying, will not vary very
+  widely from a common standard. The amount of variation we found,
+  in our third chapter, to be about one-fifth or one-sixth of the
+  mean value&mdash;that is, if the mean value were taken at 100,
+  the variations would reach from 80 to 120, or somewhat more, if
+  very large numbers were compared. In accordance with this law we
+  find, that all those characters in man which were certainly
+  essential to him during his early stages of development, exist in
+  all savages with some approach to equality. In the speed of
+  running, in bodily strength, in skill with weapons, in acuteness
+  of vision, or in power of <a name="Page_470" id=
+  "Page_470"></a>following a trail, all are fairly proficient, and
+  the differences of endowment do not probably exceed the limits of
+  variation in animals above referred to. So, in animal instinct or
+  intelligence, we find the same general level of development.
+  Every wren makes a fairly good nest like its fellows; every fox
+  has an average amount of the sagacity of its race; while all the
+  higher birds and mammals have the necessary affections and
+  instincts needful for the protection and bringing-up of their
+  offspring.</p>
+
+  <p>But in those specially developed faculties of civilised man
+  which we have been considering, the case is very different. They
+  exist only in a small proportion of individuals, while the
+  difference of capacity between these favoured individuals and the
+  average of mankind is enormous. Taking first the mathematical
+  faculty, probably fewer than one in a hundred really possess it,
+  the great bulk of the population having no natural ability for
+  the study, or feeling the slightest interest in it.<a name=
+  "FNanchor_234_234" id="FNanchor_234_234"></a><a href=
+  "#Footnote_234_234" class="fnanchor">[234]</a> And if we attempt
+  to measure the amount of variation in the faculty itself between
+  a first-class mathematician and the ordinary run of people who
+  find any kind of calculation confusing and altogether devoid of
+  interest, it is probable that the former could not be estimated
+  at less than a hundred times the latter, and perhaps a thousand
+  times would more nearly measure the difference between them.</p>
+
+  <p>The artistic faculty appears to agree pretty closely with the
+  mathematical in its frequency. The boys and girls who, going
+  beyond the mere conventional designs of children, draw what they
+  <i>see</i>, not what they <i>know</i> to be the shape of things;
+  who naturally sketch in perspective, because it is thus they see
+  objects; who see, and represent in their sketches, the light and
+  shade as well as the mere outlines of objects; and who can draw
+  recognisable sketches of every one they know, are certainly very
+  few compared with those who are totally incap<a name="Page_471"
+  id="Page_471"></a>able of anything of the kind. From some
+  inquiries I have made in schools, and from my own observation, I
+  believe that those who are endowed with this natural artistic
+  talent do not exceed, even if they come up to, one per cent of
+  the whole population.</p>
+
+  <p>The variations in the amount of artistic faculty are certainly
+  very great, even if we do not take the extremes. The gradations
+  of power between the ordinary man or woman "who does not draw,"
+  and whose attempts at representing any object, animate or
+  inanimate, would be laughable, and the average good artist who,
+  with a few bold strokes, can produce a recognisable and even
+  effective sketch of a landscape, a street, or an animal, are very
+  numerous; and we can hardly measure the difference between them
+  at less than fifty or a hundred fold.</p>
+
+  <p>The musical faculty is undoubtedly, in its lower forms, less
+  uncommon than either of the preceding, but it still differs
+  essentially from the necessary or useful faculties in that it is
+  almost entirely wanting in one-half even of civilised men. For
+  every person who draws, as it were instinctively, there are
+  probably five or ten who sing or play without having been taught
+  and from mere innate love and perception of melody and
+  harmony.<a name="FNanchor_235_235" id=
+  "FNanchor_235_235"></a><a href="#Footnote_235_235" class=
+  "fnanchor">[235]</a> On the other hand, there are probably about
+  as many who seem absolutely deficient in musical perception, who
+  take little pleasure in it, who cannot perceive discords or
+  remember tunes, and who could not learn to sing or play with any
+  amount of study. The gradations, too, are here quite as great as
+  in mathematics or pictorial art, and the special faculty of the
+  great musical composer must be reckoned many hundreds or perhaps
+  thousands of times greater than that of the ordinary "unmusical"
+  person above referred to.</p>
+
+  <p>It appears then, that, both on account of the limited number
+  of persons gifted with the mathematical, the artistic, or the
+  musical faculty, as well as from the enormous variations in its
+  development, these mental powers differ widely from those which
+  are essential to man, and are, for the most part, common to him
+  and the lower animals; and that they could <a name="Page_472" id=
+  "Page_472"></a>not, therefore, possibly have been developed in
+  him by means of the law of natural selection.</p>
+  <hr style='width: 45%;' />
+
+  <p>We have thus shown, by two distinct lines of argument, that
+  faculties are developed in civilised man which, both in their
+  mode of origin, their function, and their variations, are
+  altogether distinct from those other characters and faculties
+  which are essential to him, and which have been brought to their
+  actual state of efficiency by the necessities of his existence.
+  And besides the three which have been specially referred to,
+  there are others which evidently belong to the same class. Such
+  is the metaphysical faculty, which enables us to form abstract
+  conceptions of a kind the most remote from all practical
+  applications, to discuss the ultimate causes of things, the
+  nature and qualities of matter, motion, and force, of space and
+  time, of cause and effect, of will and conscience. Speculations
+  on these abstract and difficult questions are impossible to
+  savages, who seem to have no mental faculty enabling them to
+  grasp the essential ideas or conceptions; yet whenever any race
+  attains to civilisation, and comprises a body of people who,
+  whether as priests or philosophers, are relieved from the
+  necessity of labour or of taking an active part in war or
+  government, the metaphysical faculty appears to spring suddenly
+  into existence, although, like the other faculties we have
+  referred to, it is always confined to a very limited proportion
+  of the population.</p>
+
+  <p>In the same class we may place the peculiar faculty of wit and
+  humour, an altogether natural gift whose development appears to
+  be parallel with that of the other exceptional faculties. Like
+  them, it is almost unknown among savages, but appears more or
+  less frequently as civilisation advances and the interests of
+  life become more numerous and more complex. Like them, too, it is
+  altogether removed from utility in the struggle for life, and
+  appears sporadically in a very small percentage of the
+  population; the majority being, as is well known, totally unable
+  to say a witty thing or make a pun even to save their
+  lives.<a name="FNanchor_236_236" id=
+  "FNanchor_236_236"></a><a href="#Footnote_236_236" class=
+  "fnanchor">[236]</a></p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <a name="Page_473" id="Page_473"></a><i>The Interpretation of the
+  Facts.</i></p>
+
+  <p>The facts now set forth prove the existence of a number of
+  mental faculties which either do not exist at all or exist in a
+  very rudimentary condition in savages, but appear almost suddenly
+  and in perfect development in the higher civilised races. These
+  same faculties are further distinguished by their sporadic
+  character, being well developed only in a very small proportion
+  of the community; and by the enormous amount of variation in
+  their development, the higher manifestations of them being many
+  times&mdash;perhaps a hundred or a thousand times&mdash;stronger
+  than the lower. Each of these characteristics is totally
+  inconsistent with any action of the law of natural selection in
+  the production of the faculties referred to; and the facts, taken
+  in their entirety, compel us to recognise some origin for them
+  wholly distinct from that which has served to account for the
+  animal characteristics&mdash;whether bodily or mental&mdash;of
+  man.</p>
+
+  <p><a name="Page_474" id="Page_474"></a>The special faculties we
+  have been discussing clearly point to the existence in man of
+  something which he has not derived from his animal
+  progenitors&mdash;something which we may best refer to as being
+  of a spiritual essence or nature, capable of progressive
+  development under favourable conditions. On the hypothesis of
+  this spiritual nature, superadded to the animal nature of man, we
+  are able to understand much that is otherwise mysterious or
+  unintelligible in regard to him, especially the enormous
+  influence of ideas, principles, and beliefs over his whole life
+  and actions. Thus alone we can understand the constancy of the
+  martyr, the unselfishness of the philanthropist, the devotion of
+  the patriot, the enthusiasm of the artist, and the resolute and
+  persevering search of the scientific worker after nature's
+  secrets. Thus we may perceive that the love of truth, the delight
+  in beauty, the passion for justice, and the thrill of exultation
+  with which we hear of any act of courageous self-sacrifice, are
+  the workings within us of a higher nature which has not been
+  developed by means of the struggle for material existence.</p>
+
+  <p>It will, no doubt, be urged that the admitted continuity of
+  man's progress from the brute does not admit of the introduction
+  of new causes, and that we have no evidence of the sudden change
+  of nature which such introduction would bring about. The fallacy
+  as to new causes involving any breach of continuity, or any
+  sudden or abrupt change, in the effects, has already been shown;
+  but we will further point out that there are at least three
+  stages in the development of the organic world when some new
+  cause or power must necessarily have come into action.</p>
+
+  <p>The first stage is the change from inorganic to organic, when
+  the earliest vegetable cell, or the living protoplasm out of
+  which it arose, first appeared. This is often imputed to a mere
+  increase of complexity of chemical compounds; but increase of
+  complexity, with consequent instability, even if we admit that it
+  may have produced protoplasm as a chemical compound, could
+  certainly not have produced <i>living</i>
+  protoplasm&mdash;protoplasm which has the power of growth and of
+  reproduction, and of that continuous process of development which
+  has resulted in the marvellous variety and complex organisation
+  of the whole vegetable kingdom. There is in all this something
+  <a name="Page_475" id="Page_475"></a>quite beyond and apart from
+  chemical changes, however complex; and it has been well said that
+  the first vegetable cell was a new thing in the world, possessing
+  altogether new powers&mdash;that of extracting and fixing carbon
+  from the carbon-dioxide of the atmosphere, that of indefinite
+  reproduction, and, still more marvellous, the power of variation
+  and of reproducing those variations till endless complications of
+  structure and varieties of form have been the result. Here, then,
+  we have indications of a new power at work, which we may term
+  <i>vitality</i>, since it gives to certain forms of matter all
+  those characters and properties which constitute Life.</p>
+
+  <p>The next stage is still more marvellous, still more completely
+  beyond all possibility of explanation by matter, its laws and
+  forces. It is the introduction of sensation or consciousness,
+  constituting the fundamental distinction between the animal and
+  vegetable kingdoms. Here all idea of mere complication of
+  structure producing the result is out of the question. We feel it
+  to be altogether preposterous to assume that at a certain stage
+  of complexity of atomic constitution, and as a necessary result
+  of that complexity alone, an <i>ego</i> should start into
+  existence, a thing that <i>feels</i>, that is <i>conscious</i> of
+  its own existence. Here we have the certainty that something new
+  has arisen, a being whose nascent consciousness has gone on
+  increasing in power and definiteness till it has culminated in
+  the higher animals. No verbal explanation or attempt at
+  explanation&mdash;such as the statement that life is the result
+  of the molecular forces of the protoplasm, or that the whole
+  existing organic universe from the amaeba up to man was latent in
+  the fire-mist from which the solar system was developed&mdash;can
+  afford any mental satisfaction, or help us in any way to a
+  solution of the mystery.</p>
+
+  <p>The third stage is, as we have seen, the existence in man of a
+  number of his most characteristic and noblest faculties, those
+  which raise him furthest above the brutes and open up
+  possibilities of almost indefinite advancement. These faculties
+  could not possibly have been developed by means of the same laws
+  which have determined the progressive development of the organic
+  world in general, and also of man's physical organism.<a name=
+  "FNanchor_237_237" id="FNanchor_237_237"></a><a href=
+  "#Footnote_237_237" class="fnanchor">[237]</a></p>
+
+  <p><a name="Page_476" id="Page_476"></a>These three distinct
+  stages of progress from the inorganic world of matter and motion
+  up to man, point clearly to an unseen universe&mdash;to a world
+  of spirit, to which the world of matter is altogether
+  subordinate. To this spiritual world we may refer the
+  marvellously complex forces which we know as gravitation,
+  cohesion, chemical force, radiant force, and electricity, without
+  which the material universe could not exist for a moment in its
+  present form, and perhaps not at all, since without these forces,
+  and perhaps others which may be termed atomic, it is doubtful
+  whether matter itself could have any existence. And still more
+  surely can we refer to it those progressive manifestations of
+  Life in the vegetable, the animal, and man&mdash;which we may
+  classify as unconscious, conscious, and intellectual
+  life,&mdash;and which probably depend upon different degrees of
+  spiritual influx. I have already shown that this involves no
+  necessary infraction of the law of continuity in physical or
+  mental evolution; whence it follows that any difficulty we may
+  find in discriminating the inorganic from the organic, the lower
+  vegetable from the lower animal organisms, or the higher animals
+  from the lowest types of man, has no bearing at all upon the
+  question. This is to be decided by showing that a change in
+  essential nature (due, probably, to causes of a higher order than
+  those of the material universe) took place at the several stages
+  of progress which I have indicated; a change which may be none
+  the less real because absolutely imperceptible at its point of
+  origin, as is the change that takes place in the curve in which a
+  body is moving when the application of some new force causes the
+  curve to be slightly altered.</p>
+
+  <p style='text-align:center; font-weight:bold; margin-top:1.5em'>
+  <i>Concluding Remarks.</i></p>
+
+  <p>Those who admit my interpretation of the evidence now
+  adduced&mdash;strictly scientific evidence in its appeal to facts
+  which are clearly what ought <i>not</i> to be on the
+  materialistic theory&mdash;will be able to accept the spiritual
+  nature of man, as not in any way inconsistent with the theory of
+  evolution, but as dependent on those fundamental laws and causes
+  which furnish the very materials for evolution to work with. They
+  will also be relieved from the crushing mental burthen imposed
+  upon those who&mdash;maintaining that we, in common with the
+  <a name="Page_477" id="Page_477"></a>rest of nature, are but
+  products of the blind eternal forces of the universe, and
+  believing also that the time must come when the sun will lose his
+  heat and all life on the earth necessarily cease&mdash;have to
+  contemplate a not very distant future in which all this glorious
+  earth&mdash;which for untold millions of years has been slowly
+  developing forms of life and beauty to culminate at last in
+  man&mdash;shall be as if it had never existed; who are compelled
+  to suppose that all the slow growths of our race struggling
+  towards a higher life, all the agony of martyrs, all the groans
+  of victims, all the evil and misery and undeserved suffering of
+  the ages, all the struggles for freedom, all the efforts towards
+  justice, all the aspirations for virtue and the wellbeing of
+  humanity, shall absolutely vanish, and, "like the baseless fabric
+  of a vision, leave not a wrack behind."</p>
+
+  <p>As contrasted with this hopeless and soul-deadening belief,
+  we, who accept the existence of a spiritual world, can look upon
+  the universe as a grand consistent whole adapted in all its parts
+  to the development of spiritual beings capable of indefinite life
+  and perfectibility. To us, the whole purpose, the only <i>raison
+  d'&ecirc;tre</i> of the world&mdash;with all its complexities of
+  physical structure, with its grand geological progress, the slow
+  evolution of the vegetable and animal kingdoms, and the ultimate
+  appearance of man&mdash;was the development of the human spirit
+  in association with the human body. From the fact that the spirit
+  of man&mdash;the man himself&mdash;<i>is</i> so developed, we may
+  well believe that this is the only, or at least the best, way for
+  its development; and we may even see in what is usually termed
+  "evil" on the earth, one of the most efficient means of its
+  growth. For we know that the noblest faculties of man are
+  strengthened and perfected by struggle and effort; it is by
+  unceasing warfare against physical evils and in the midst of
+  difficulty and danger that energy, courage, self-reliance, and
+  industry have become the common qualities of the northern races;
+  it is by the battle with moral evil in all its hydra-headed
+  forms, that the still nobler qualities of justice and mercy and
+  humanity and self-sacrifice have been steadily increasing in the
+  world. Beings thus trained and strengthened by their
+  surroundings, and possessing latent faculties capable of such
+  noble development, are surely destined for a higher and more
+  permanent exist<a name="Page_478" id="Page_478"></a>ence; and we
+  may confidently believe with our greatest living poet&mdash;</p>
+
+  <p><span style="margin-left: 2em;">That life is not as idle
+  ore,</span><br />
+  <br />
+  <span style="margin-left: 2em;">But iron dug from central
+  gloom,</span><br />
+  <span style="margin-left: 3em;">And heated hot with burning
+  fears,</span><br />
+  <span style="margin-left: 3em;">And dipt in baths of hissing
+  tears,</span><br />
+  <span style="margin-left: 2em;">And batter'd with the shocks of
+  doom</span><br />
+  <br />
+  <span style="margin-left: 2em;">To shape and
+  use.</span><br /></p>
+
+  <p>We thus find that the Darwinian theory, even when carried out
+  to its extreme logical conclusion, not only does not oppose, but
+  lends a decided support to, a belief in the spiritual nature of
+  man. It shows us how man's body may have been developed from that
+  of a lower animal form under the law of natural selection; but it
+  also teaches us that we possess intellectual and moral faculties
+  which could not have been so developed, but must have had another
+  origin; and for this origin we can only find an adequate cause in
+  the unseen universe of Spirit.</p>
+
+  <div class="footnotes">
+    <h3>FOOTNOTES:</h3>
+
+    <div class="footnote">
+      <p><a name="Footnote_218_218" id=
+      "Footnote_218_218"></a><a href=
+      "#FNanchor_218_218"><span class="label">[218]</span></a>
+      <i>Descent of Man</i>, pp. 41-43; also pp. 13-15.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_219_219" id=
+      "Footnote_219_219"></a><a href=
+      "#FNanchor_219_219"><span class="label">[219]</span></a>
+      <i>Man's Place in Nature</i>, p. 64.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_220_220" id=
+      "Footnote_220_220"></a><a href=
+      "#FNanchor_220_220"><span class="label">[220]</span></a>
+      <i>Man's Place in Nature</i>, p. 67. See Figs. of Embryos of
+      Man and Dog in Darwin's <i>Descent of Man</i>, p. 10.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_221_221" id=
+      "Footnote_221_221"></a><a href=
+      "#FNanchor_221_221"><span class="label">[221]</span></a>
+      <i>The Descent of Man</i>, pp. 7, 8.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_222_222" id=
+      "Footnote_222_222"></a><a href=
+      "#FNanchor_222_222"><span class="label">[222]</span></a>
+      <i>Man and Apes.</i> By St. George Mivart, F.R.S., 1873. It
+      is an interesting fact (for which I am indebted to Mr. E.B.
+      Poulton) that the human embryo possesses the extra rib and
+      wrist-bone referred to above in (2) and (4) as occurring in
+      some of the apes.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_223_223" id=
+      "Footnote_223_223"></a><a href=
+      "#FNanchor_223_223"><span class="label">[223]</span></a>
+      <i>Man and Apes</i>, pp. 138, 144.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_224_224" id=
+      "Footnote_224_224"></a><a href=
+      "#FNanchor_224_224"><span class="label">[224]</span></a> For
+      a sketch of the evidence of Man's Antiquity in America, see
+      <i>The Nineteenth Century</i> for November 1887.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_225_225" id=
+      "Footnote_225_225"></a><a href=
+      "#FNanchor_225_225"><span class="label">[225]</span></a> This
+      subject was first discussed in an article in the
+      <i>Anthropological Review</i>, May 1864, and republished in
+      my <i>Contributions to Natural Selection</i>, chap, ix, in
+      1870.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_226_226" id=
+      "Footnote_226_226"></a><a href=
+      "#FNanchor_226_226"><span class="label">[226]</span></a>
+      <i>Man's Place in Nature</i>, p. 102.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_227_227" id=
+      "Footnote_227_227"></a><a href=
+      "#FNanchor_227_227"><span class="label">[227]</span></a> For
+      a full discussion of this question, see the author's
+      <i>Geographical Distribution of Animals</i>, vol. i. p.
+      285.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_228_228" id=
+      "Footnote_228_228"></a><a href=
+      "#FNanchor_228_228"><span class="label">[228]</span></a> For
+      a full discussion of all these points, see <i>Descent of
+      Man</i>, chap. iii.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_229_229" id=
+      "Footnote_229_229"></a><a href=
+      "#FNanchor_229_229"><span class="label">[229]</span></a>
+      <i>Descent of Man</i>, chap. iv.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_230_230" id=
+      "Footnote_230_230"></a><a href=
+      "#FNanchor_230_230"><span class="label">[230]</span></a>
+      Lubbock's <i>Origin of Civilisation</i>, fourth edition, pp.
+      434-440; Tylor's <i>Primitive Culture</i>, chap. vii.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_231_231" id=
+      "Footnote_231_231"></a><a href=
+      "#FNanchor_231_231"><span class="label">[231]</span></a> It
+      has been recently stated that some of these facts are
+      erroneous, and that some Australians can keep accurate
+      reckoning up to 100, or more, when required. But this does
+      not alter the general fact that many low races, including the
+      Australians, have no words for high numbers and never require
+      to use them. If they are now, with a little practice, able to
+      count much higher, this indicates the possession of a faculty
+      which could not have been developed under the law of utility
+      only, since the absence of words for such high numbers shows
+      that they were neither used nor required.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_232_232" id=
+      "Footnote_232_232"></a><a href=
+      "#FNanchor_232_232"><span class="label">[232]</span></a>
+      Article Arithmetic in <i>Eng. Cyc. of Arts and
+      Sciences</i>.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_233_233" id=
+      "Footnote_233_233"></a><a href=
+      "#FNanchor_233_233"><span class="label">[233]</span></a> See
+      "History of Music," in <i>Eng. Cyc.</i>, Science and Arts
+      Division.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_234_234" id=
+      "Footnote_234_234"></a><a href=
+      "#FNanchor_234_234"><span class="label">[234]</span></a> This
+      is the estimate furnished me by two mathematical masters in
+      one of our great public schools of the proportion of boys who
+      have any special taste or capacity for mathematical studies.
+      Many more, of course, can be drilled into a fair knowledge of
+      elementary mathematics, but only this small proportion
+      possess the natural faculty which renders it possible for
+      them ever to rank high as mathematicians, to take any
+      pleasure in it, or to do any original mathematical work.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_235_235" id=
+      "Footnote_235_235"></a><a href=
+      "#FNanchor_235_235"><span class="label">[235]</span></a> I am
+      informed, however, by a music master in a large school that
+      only about one per cent have real or decided musical talent,
+      corresponding curiously with the estimate of the
+      mathematicians.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_236_236" id=
+      "Footnote_236_236"></a><a href=
+      "#FNanchor_236_236"><span class="label">[236]</span></a> In
+      the latter part of his essay on Heredity (pp. 91-93 of the
+      volume of <i>Essays</i>), Dr. Weismann refers to this
+      question of the origin of "talents" in man, and, like myself,
+      comes to the conclusion that they could not be developed
+      under the law of natural selection. He says: "It may be
+      objected that, in man, in addition to the instincts inherent
+      in every individual, special individual predispositions are
+      also found, of such a nature that it is impossible they can
+      have arisen by individual variations of the germ-plasm. On
+      the other hand, these predispositions&mdash;which we call
+      talents&mdash;cannot have arisen through natural selection,
+      because life is in no way dependent on their presence, and
+      there seems to be no way of explaining their origin except by
+      an assumption of the summation of the skill attained by
+      exercise in the course of each single life. In this case,
+      therefore, we seem at first sight to be compelled to accept
+      the transmission of acquired characters." Weismann then goes
+      on to show that the facts do not support this view; that the
+      mathematical, musical, or artistic faculties often appear
+      suddenly in a family whose other members and ancestors were
+      in no way distinguished; and that even when hereditary in
+      families, the talent often appears at its maximum at the
+      commencement or in the middle of the series, not increasing
+      to the end, as it should do if it depended in any way on the
+      transmission of acquired skill. Gauss was not the son of a
+      mathematician, nor Handel of a musician, nor Titian of a
+      painter, and there is no proof of any special talent in the
+      ancestors of these men of genius, who at once developed the
+      most marvellous pre-eminence in their respective talents. And
+      after showing that such great men only appear at certain
+      stages of human development, and that two or more of the
+      special talents are not unfrequently combined in one
+      individual, he concludes thus&mdash;</p>
+
+      <div class="blockquot" style='font-size: 100%;'>
+        <p>"Upon this subject I only wish to add that, in my
+        opinion, talents do not appear to depend upon the
+        improvement of any special mental quality by continued
+        practice, but they are the expression, and to a certain
+        extent the bye-product, of the human mind, which is so
+        highly developed in all directions."</p>
+      </div>
+
+      <p>It will, I think, be admitted that this view hardly
+      accounts for the existence of the highly peculiar human
+      faculties in question.</p>
+    </div>
+
+    <div class="footnote">
+      <p><a name="Footnote_237_237" id=
+      "Footnote_237_237"></a><a href=
+      "#FNanchor_237_237"><span class="label">[237]</span></a> For
+      an earlier discussion of this subject, with some wider
+      applications, see the author's <i>Contributions to the Theory
+      of Natural Selection</i>, chap. x.</p>
+    </div>
+  </div>
+  <hr style="width: 65%;" />
+
+  <h2><a name="INDEX" id="INDEX"></a><a name="Page_479" id=
+  "Page_479"></a>INDEX</h2>
+
+  <p><span style="margin-left: 1em;"><b>A</b>&nbsp;</span><br />
+  <br />
+  <span style="margin-left: 1em;">Abbott, Dr. C.C., instability of
+  habits of birds, <a href='#Page_76'>76</a></span><br />
+  <span style="margin-left: 2em;">on American water-thrushes
+  (Seiurus), <a href='#Page_117'>117</a></span><br />
+  <span style="margin-left: 2em;">Mr., drawings of caterpillars and
+  their food plants, <a href='#Page_203'>203</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Accessory plumes, development and
+  display of, <a href='#Page_293'>293</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Acclimatisation, <a href=
+  '#Page_94'>94</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Achatinellidae, Gulick on
+  variations in, <a href='#Page_147'>147</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Acquired characters, non-heredity
+  of, <a href='#Page_440'>440</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Acraeidae, mimicry of, <a href=
+  '#Page_247'>247</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Adaptation to conditions at
+  various periods of life, <a href='#Page_112'>112</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Adolias dirtea, sexual diversity
+  of, <a href='#Page_271'>271</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Aegeriidae, mimicry by, <a href=
+  '#Page_240'>240</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Agaristidae, mimicry of, <a href=
+  '#Page_246'>246</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Agassiz, on species, <a href=
+  '#Page_5'>5</a></span><br />
+  <span style="margin-left: 2em;">on North American weeds, <a href=
+  '#Page_15'>15</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Agelaeus phoeniceus, diagram
+  showing variations of, <a href='#Page_56'>56</a>;</span><br />
+  <span style="margin-left: 2em;">proportionate numbers which vary,
+  <a href='#Page_64'>64</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Albatross, courtship of great,
+  <a href='#Page_287'>287</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Allen, Mr. Grant, on forms of
+  leaves, <a href='#Page_133'>133</a></span><br />
+  <span style="margin-left: 2em;">on degradation of wind-fertilised
+  from insect-fertilised flowers, <a href='#Page_325'>325</a>
+  (note)</span><br />
+  <span style="margin-left: 2em;">on insects and flowers, <a href=
+  '#Page_332'>332</a></span><br />
+  <span style="margin-left: 2em;">on production of colour through
+  the agency of the colour sense, <a href=
+  '#Page_334'>334</a></span><br />
+  <span style="margin-left: 2em;">Mr. J.A., on the variability of
+  birds, <a href='#Page_50'>50</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Allen, Mr. J.A.,&nbsp; on colour
+  as influenced by climate, <a href=
+  '#Page_228'>228</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Alluring coloration, <a href=
+  '#Page_210'>210</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">American school of evolutionists,
+  <a href='#Page_420'>420</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Anemone nemorosa, variability of,
+  <a href='#Page_78'>78</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Animal coloration, a theory of,
+  <a href='#Page_288'>288</a></span><br />
+  <span style="margin-left: 2em;">general laws of, <a href=
+  '#Page_296'>296</a></span><br />
+  <span style="margin-left: 2em;">intelligence, supposed action of,
+  <a href='#Page_425'>425</a></span><br />
+  <span style="margin-left: 2em;">characteristics of man, <a href=
+  '#Page_454'>454</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Animals, the struggle among,
+  <a href='#Page_18'>18</a></span><br />
+  <span style="margin-left: 2em;">wild, their enjoyment of life,
+  <a href='#Page_39'>39</a></span><br />
+  <span style="margin-left: 2em;">usually die painless deaths,
+  <a href='#Page_38'>38</a></span><br />
+  <span style="margin-left: 2em;">constitutional variation of,
+  <a href='#Page_94'>94</a></span><br />
+  <span style="margin-left: 2em;">uses of colours of, <a href=
+  '#Page_134'>134</a></span><br />
+  <span style="margin-left: 2em;">supposed effects of disuse in
+  wild, <a href='#Page_415'>415</a></span><br />
+  <span style="margin-left: 2em;">most allied to man, <a href=
+  '#Page_450'>450</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Antelopes, recognition marks of,
+  <a href='#Page_219'>219</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Anthrocera filipendula inedible,
+  <a href='#Page_235'>235</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Apples, variations of, <a href=
+  '#Page_87'>87</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Arctic animals, supposed causes
+  of white colour of, <a href='#Page_191'>191</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Argyll, Duke of, on goose reared
+  by a golden eagle, <a href='#Page_75'>75</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Artemia salina and A.
+  milhausenii, <a href='#Page_426'>426</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Asclepias curassavica, spread of,
+  <a href='#Page_28'>28</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Asses running wild in Quito,
+  <a href='#Page_28'>28</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Attractive fruits, <a href=
+  '#Page_306'>306</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Australia, spread of the
+  Cape-weed in, <a href='#Page_29'>29</a></span><br />
+  <span style="margin-left: 2em;">fossil and recent mammals of,
+  <a href='#Page_392'>392</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Azara, on cause of horses and
+  cattle not running wild in Paraguay, <a href=
+  '#Page_19'>19</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Azores, flora of, supports aerial
+  transmission of seeds, <a href='#Page_368'>368</a></span><br />
+  <br />
+  <a name="Page_480" id="Page_480"></a><br />
+  <span style="margin-left: 1em;"><b>B</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Baker, Mr. J.G., on rarity of
+  spiny plants in Mauritius, <a href=
+  '#Page_432'>432</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ball, Mr., on cause of late
+  appearance of exogens, <a href='#Page_400'>400</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Barber, Mrs., on variable
+  colouring of pupae of Papilio nireus, <a href=
+  '#Page_197'>197</a></span><br />
+  <span style="margin-left: 2em;">on protective colours of African
+  sun-birds, <a href='#Page_200'>200</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Barbs, <a href=
+  '#Page_91'>91</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Barriers, importance of, in
+  questions of distribution, <a href=
+  '#Page_341'>341</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Bates, Mr. H.W., on varieties of
+  butterflies, <a href='#Page_44'>44</a></span><br />
+  <span style="margin-left: 2em;">on inedibility of Heliconidae,
+  <a href='#Page_234'>234</a></span><br />
+  <span style="margin-left: 2em;">on a conspicuous caterpillar,
+  <a href='#Page_236'>236</a></span><br />
+  <span style="margin-left: 2em;">on mimicry, <a href=
+  '#Page_240'>240</a>, <a href='#Page_243'>243</a>, <a href=
+  '#Page_249'>249</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Bathmism or growth-force, Cope
+  on, <a href='#Page_421'>421</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Beddard, Mr. F.E., variations of
+  earthworms, <a href='#Page_67'>67</a></span><br />
+  <span style="margin-left: 2em;">on plumes of bird of paradise,
+  <a href='#Page_292'>292</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Beech trees, aggressive in
+  Denmark, <a href='#Page_21'>21</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Beetle and wasp (figs.), <a href=
+  '#Page_259'>259</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Beetle, fossil in coal measures
+  of Silesia, <a href='#Page_404'>404</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Beginnings of important organs,
+  <a href='#Page_128'>128</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Belt, Mr., on leaf-like locust,
+  <a href='#Page_203'>203</a></span><br />
+  <span style="margin-left: 2em;">on birds avoiding Heliconidae,
+  <a href='#Page_234'>234</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Belt's frog, <a href=
+  '#Page_266'>266</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Birds, rate of increase of,
+  <a href='#Page_25'>25</a></span><br />
+  <span style="margin-left: 2em;">how destroyed, <a href=
+  '#Page_26'>26</a></span><br />
+  <span style="margin-left: 2em;">variation among, <a href=
+  '#Page_49'>49</a></span><br />
+  <span style="margin-left: 2em;">variation of markings of,
+  <a href='#Page_52'>52</a></span><br />
+  <span style="margin-left: 2em;">variation of wings and tails of,
+  <a href='#Page_53'>53</a></span><br />
+  <span style="margin-left: 2em;">diagram showing variation of
+  tarsus and toes, <a href='#Page_60'>60</a></span><br />
+  <span style="margin-left: 2em;">use of structural peculiarities
+  of, <a href='#Page_135'>135</a></span><br />
+  <span style="margin-left: 2em;">eggs, coloration of, <a href=
+  '#Page_212'>212</a></span><br />
+  <span style="margin-left: 2em;">recognition marks of, <a href=
+  '#Page_222'>222</a></span><br />
+  <span style="margin-left: 2em;">and butterflies, white in
+  tropical islands, <a href='#Page_230'>230</a></span><br />
+  <span style="margin-left: 2em;">sometimes seize inedible
+  butterflies, <a href='#Page_255'>255</a></span><br />
+  <span style="margin-left: 2em;">mimicry among, <a href=
+  '#Page_263'>263</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Birds, sexual coloration of,
+  <a href='#Page_275'>275</a></span><br />
+  <span style="margin-left: 2em;">cause of dull colour of female,
+  <a href='#Page_277'>277</a></span><br />
+  <span style="margin-left: 2em;">choice of female not known to be
+  determined by colour, etc., <a href=
+  '#Page_285'>285</a></span><br />
+  <span style="margin-left: 2em;">decorative plumage of, <a href=
+  '#Page_285'>285</a></span><br />
+  <span style="margin-left: 2em;">antics of unornamented, <a href=
+  '#Page_287'>287</a></span><br />
+  <span style="margin-left: 2em;">which fertilise flowers, <a href=
+  '#Page_319'>319</a></span><br />
+  <span style="margin-left: 2em;">colours of, not dependent on the
+  colours of flowers, <a href='#Page_336'>336</a></span><br />
+  <span style="margin-left: 2em;">no proof of aesthetic tastes in,
+  <a href='#Page_336'>336</a></span><br />
+  <span style="margin-left: 2em;">dispersal of, <a href=
+  '#Page_355'>355</a></span><br />
+  <span style="margin-left: 2em;">and insects at sea, <a href=
+  '#Page_357'>357</a></span><br />
+  <span style="margin-left: 2em;">of oceanic islands, <a href=
+  '#Page_358'>358</a></span><br />
+  <span style="margin-left: 2em;">carrying seeds on their feet,
+  <a href='#Page_361'>361</a></span><br />
+  <span style="margin-left: 2em;">ancestral forms of, <a href=
+  '#Page_407'>407</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Birthplace, probable, of man,
+  <a href='#Page_459'>459</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Bombyx regia, protective form of
+  larva of, <a href='#Page_210'>210</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Boyd Dawkins, on development of
+  deer's horns, <a href='#Page_389'>389</a></span><br />
+  <span style="margin-left: 2em;">on origin of man, <a href=
+  '#Page_456'>456</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Brady, Mr. George, on protective
+  colouring of starfishes, <a href='#Page_209'>209</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Brain development, progressive,
+  <a href='#Page_390'>390</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Brains of man and apes, <a href=
+  '#Page_452'>452</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Branner, Mr. J.C., on supposed
+  proofs of glaciation in Brazil, <a href=
+  '#Page_370'>370</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Brazil, supposed proof of
+  glaciation in, <a href='#Page_370'>370</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Brewer, Professor W.H., on want
+  of symmetry in colours of animals, <a href=
+  '#Page_217'>217</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Bromelia, animals inhabiting
+  leaves of, <a href='#Page_118'>118</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Bronn, Professor, on supposed
+  uselessness of variations of ears and tails, <a href=
+  '#Page_136'>136</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Butler, Mr. A.G., on inedibility
+  of conspicuous caterpillars, <a href=
+  '#Page_237'>237</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Butterflies, varieties of,
+  <a href='#Page_44'>44</a></span><br />
+  <span style="margin-left: 2em;">small, of Isle of Man, <a href=
+  '#Page_106'>106</a></span><br />
+  <span style="margin-left: 2em;">special protective colouring of,
+  <a href='#Page_206'>206</a></span><br />
+  <span style="margin-left: 2em;">recognition by, <a href=
+  '#Page_226'>226</a></span><br />
+  <span style="margin-left: 2em;">inedibility of some, <a href=
+  '#Page_234'>234</a></span><br />
+  <span style="margin-left: 2em;">mimicry among, <a href=
+  '#Page_240'>240</a>, <a href='#Page_249'>249</a></span><br />
+  <span style="margin-left: 2em;">colour development of, <a href=
+  '#Page_274'>274</a></span><br />
+  <span style="margin-left: 2em;">sexual coloration of, <a href=
+  '#Page_271'>271</a></span><br />
+  <br />
+  <a name="Page_481" id="Page_481"></a><br />
+  <span style="margin-left: 1em;"><b>C</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Caddis-fly larvae inhabiting
+  bromelia leaves, <a href='#Page_118'>118</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Callophis, harmless mimicking
+  poisonous species, <a href='#Page_262'>262</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Candolle, Alp. de, on variation
+  in oaks, <a href='#Page_77'>77</a></span><br />
+  <span style="margin-left: 2em;">on variability of Papaver
+  bracteatum, <a href='#Page_79'>79</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cardinalis virginianus, diagram
+  showing proportionate numbers which vary, <a href=
+  '#Page_65'>65</a>;</span><br />
+  <span style="margin-left: 2em;">variations of, <a href=
+  '#Page_58'>58</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Carpenter, Dr. W.B., on variation
+  in the Foraminifera, <a href='#Page_43'>43</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Carriers, <a href=
+  '#Page_91'>91</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Caterpillars, resemblance of, to
+  their food plants, <a href='#Page_203'>203</a>-<a href=
+  '#Page_205'>205</a></span><br />
+  <span style="margin-left: 2em;">inedible, <a href=
+  '#Page_236'>236</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cattle, how they prevent the
+  growth of trees, <a href='#Page_18'>18</a></span><br />
+  <span style="margin-left: 2em;">increase of, in St. Domingo,
+  Mexico, and the pampas, <a href='#Page_27'>27</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ceylon, spread of Lantana mixta
+  in, <a href='#Page_29'>29</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Chaffinch, change of habit of, in
+  New Zealand, <a href='#Page_76'>76</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Chambers, Robert, on origin of
+  species, <a href='#Page_3'>3</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Chance rarely determines
+  survival, <a href='#Page_123'>123</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Change of conditions, utility of,
+  <a href='#Page_326'>326</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Characters, non-adaptive,
+  <a href='#Page_131'>131</a></span><br />
+  <span style="margin-left: 2em;">transferred from useless to
+  useful class, <a href='#Page_132'>132</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Charaxes psaphon persecuted by a
+  bird, <a href='#Page_235'>235</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Chile, numerous red tubular
+  flowers in, <a href='#Page_320'>320</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Chimpanzee, figure of, <a href=
+  '#Page_454'>454</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Clark, Mr. Edwin, on cause of
+  absence of forests on the pampas, <a href=
+  '#Page_23'>23</a></span><br />
+  <span style="margin-left: 2em;">on the struggle for life in the
+  South American valleys, <a href='#Page_24'>24</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cleistogamous flowers, <a href=
+  '#Page_322'>322</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Close interbreeding, supposed
+  evil results of, <a href='#Page_326'>326</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Clover, white, spread of, in New
+  Zealand, <a href='#Page_28'>28</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Co-adaptation of parts by
+  variation, no real difficulty, <a href=
+  '#Page_418'>418</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cobra, use of hood of, <a href=
+  '#Page_262'>262</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Coccinella mimicked by
+  grasshopper, (figure), <a href='#Page_260'>260</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Collingwood, Mr., on butterflies
+  recognising their kind, <a href='#Page_226'>226</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Coloration, alluring, <a href=
+  '#Page_210'>210</a></span><br />
+  <span style="margin-left: 2em;">of birds' eggs, <a href=
+  '#Page_212'>212</a></span><br />
+  <span style="margin-left: 2em;">a theory of animal, <a href=
+  '#Page_288'>288</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Colour correlated with sterility,
+  <a href='#Page_169'>169</a></span><br />
+  <span style="margin-left: 2em;">correlated with constitutional
+  peculiarities, <a href='#Page_170'>170</a></span><br />
+  <span style="margin-left: 2em;">in nature, the problem to be
+  solved, <a href='#Page_188'>188</a></span><br />
+  <span style="margin-left: 2em;">constancy, in animals indicates
+  utility, <a href='#Page_189'>189</a></span><br />
+  <span style="margin-left: 2em;">and environment, <a href=
+  '#Page_190'>190</a></span><br />
+  <span style="margin-left: 2em;">general theories of animal,
+  <a href='#Page_193'>193</a></span><br />
+  <span style="margin-left: 2em;">animal, supposed causes of,
+  <a href='#Page_193'>193</a></span><br />
+  <span style="margin-left: 2em;">obscure, of many tropical
+  animals, <a href='#Page_194'>194</a></span><br />
+  <span style="margin-left: 2em;">produced by surrounding objects,
+  <a href='#Page_195'>195</a></span><br />
+  <span style="margin-left: 2em;">adaptations, local, <a href=
+  '#Page_199'>199</a></span><br />
+  <span style="margin-left: 2em;">for recognition, <a href=
+  '#Page_217'>217</a></span><br />
+  <span style="margin-left: 2em;">of wild animals not quite
+  symmetrical, <a href='#Page_217'>217</a> (note)</span><br />
+  <span style="margin-left: 2em;">as influenced by locality or
+  climate, <a href='#Page_228'>228</a></span><br />
+  <span style="margin-left: 2em;">development in butterflies,
+  <a href='#Page_274'>274</a></span><br />
+  <span style="margin-left: 2em;">more variable than habits,
+  <a href='#Page_278'>278</a></span><br />
+  <span style="margin-left: 2em;">and nerve distribution, <a href=
+  '#Page_290'>290</a></span><br />
+  <span style="margin-left: 2em;">and tegumentary appendages,
+  <a href='#Page_291'>291</a></span><br />
+  <span style="margin-left: 2em;">of flowers, <a href=
+  '#Page_308'>308</a></span><br />
+  <span style="margin-left: 2em;">change of, in flowers when
+  fertilised, <a href='#Page_317'>317</a></span><br />
+  <span style="margin-left: 2em;">in nature, concluding remarks on,
+  <a href='#Page_299'>299</a>, <a href=
+  '#Page_333'>333</a></span><br />
+  <span style="margin-left: 2em;">of fruits, <a href=
+  '#Page_304'>304</a></span><br />
+  <span style="margin-left: 2em;">of flowers growing together
+  contrasted, <a href='#Page_318'>318</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Complexity of flowers due to
+  alternate adaptation to insect and self-fertilisation, <a href=
+  '#Page_328'>328</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Composite, a, widely dispersed
+  without pappus, <a href='#Page_367'>367</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Confinement, affecting fertility,
+  <a href='#Page_154'>154</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Continental and oceanic areas,
+  <a href='#Page_346'>346</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Continents and oceans cannot have
+  changed places, <a href='#Page_345'>345</a></span><br />
+  <span style="margin-left: 2em;">possible connections between,
+  <a href='#Page_349'>349</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Continuity does not prove
+  identity of origin, <a href='#Page_463'>463</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cope, Dr. E.D., on non-adaptive
+  characters, <a href='#Page_131'>131</a></span><br />
+  <a name="Page_482" id="Page_482"></a> <span style=
+  "margin-left: 2em;">on fundamental laws of growth, <a href=
+  '#Page_420'>420</a></span><br />
+  <span style="margin-left: 2em;">on bathmism or growth-force,
+  <a href='#Page_421'>421</a></span><br />
+  <span style="margin-left: 2em;">on use producing structural
+  change, <a href='#Page_422'>422</a></span><br />
+  <span style="margin-left: 2em;">on law of centrifugal growth,
+  <a href='#Page_422'>422</a></span><br />
+  <span style="margin-left: 2em;">on origin of the feet of
+  ungulates, <a href='#Page_423'>423</a></span><br />
+  <span style="margin-left: 2em;">on action of animal intelligence,
+  <a href='#Page_425'>425</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Correlations in pigeons, horses,
+  etc., <a href='#Page_140'>140</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Corvus frugilegus, <a href=
+  '#Page_2'>2</a></span><br />
+  <span style="margin-left: 2em;">corone, <a href=
+  '#Page_2'>2</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Coursers, figures of secondary
+  quills, <a href='#Page_224'>224</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cowslip, two forms of, <a href=
+  '#Page_157'>157</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Crab, sexual diversity of colour
+  of, <a href='#Page_269'>269</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cretaceous period, dicotyledons
+  of, <a href='#Page_400'>400</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Crisp, Dr., on variations of gall
+  bladder and alimentary canal, <a href=
+  '#Page_69'>69</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Crosses, a cause of variation,
+  <a href='#Page_99'>99</a></span><br />
+  <span style="margin-left: 2em;">reciprocal, <a href=
+  '#Page_155'>155</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cross-fertilisation, modes of
+  securing, <a href='#Page_310'>310</a></span><br />
+  <span style="margin-left: 2em;">difference in, <a href=
+  '#Page_155'>155</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Crossing and changed
+  conditions,</span><br />
+  <span style="margin-left: 2em;">parallelism of, <a href=
+  '#Page_166'>166</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cruciferae, variations of
+  structure in, <a href='#Page_80'>80</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cuckoo, eggs of, <a href=
+  '#Page_216'>216</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cuckoos mimick hawks, <a href=
+  '#Page_263'>263</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Cultivated plants, origin of
+  useful, <a href='#Page_97'>97</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Curculionidae mimicked by various
+  insects (figs.), <a href='#Page_260'>260</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Curves of variation, <a href=
+  '#Page_64'>64</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>D</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Dana, Professor, on the
+  permanence of continents, <a href=
+  '#Page_342'>342</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Danaidae little attacked by
+  mites, <a href='#Page_235'>235</a></span><br />
+  <span style="margin-left: 2em;">mimicry of, <a href=
+  '#Page_246'>246</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Darwin, change of opinion
+  effected by, <a href='#Page_8'>8</a></span><br />
+  <span style="margin-left: 2em;">the Newton of Natural History,
+  <a href='#Page_9'>9</a></span><br />
+  <span style="margin-left: 2em;">his view of his own work,
+  <a href='#Page_10'>10</a></span><br />
+  <span style="margin-left: 2em;">on the enemies of plants,
+  <a href='#Page_16'>16</a></span><br />
+  <span style="margin-left: 2em;">on fir-trees destroyed by cattle,
+  <a href='#Page_17'>17</a></span><br />
+  <span style="margin-left: 2em;">on change of plants and animals
+  caused by planting, <a href='#Page_18'>18</a></span><br />
+  <span style="margin-left: 2em;">on absence of wild cattle in
+  Paraguay, <a href='#Page_19'>19</a></span><br />
+  <span style="margin-left: 2em;">on cats and red clover, <a href=
+  '#Page_20'>20</a></span><br />
+  <span style="margin-left: 2em;">on variety of plants in old turf,
+  <a href='#Page_35'>35</a></span><br />
+  <span style="margin-left: 2em;">on the beneficent action of the
+  struggle for existence, <a href='#Page_40'>40</a></span><br />
+  <span style="margin-left: 2em;">on variability of wild geraniums,
+  <a href='#Page_79'>79</a></span><br />
+  <span style="margin-left: 2em;">on variability of common species,
+  <a href='#Page_80'>80</a></span><br />
+  <span style="margin-left: 2em;">his non-recognition of extreme
+  variability of wild species, <a href=
+  '#Page_82'>82</a></span><br />
+  <span style="margin-left: 2em;">on races of domestic pigeon,
+  <a href='#Page_90'>90</a></span><br />
+  <span style="margin-left: 2em;">on constitutional variation in
+  plants, <a href='#Page_95'>95</a></span><br />
+  <span style="margin-left: 2em;">on unconscious selection,
+  <a href='#Page_96'>96</a></span><br />
+  <span style="margin-left: 2em;">on a case of divergence, <a href=
+  '#Page_105'>105</a></span><br />
+  <span style="margin-left: 2em;">on advantage of diversification
+  of structure in inhabitants of one region, <a href=
+  '#Page_110'>110</a></span><br />
+  <span style="margin-left: 2em;">on species of plants in turf,
+  <a href='#Page_110'>110</a></span><br />
+  <span style="margin-left: 2em;">on isolation, <a href=
+  '#Page_119'>119</a></span><br />
+  <span style="margin-left: 2em;">on origin of mammary glands,
+  <a href='#Page_129'>129</a></span><br />
+  <span style="margin-left: 2em;">on eyes of flatfish, <a href=
+  '#Page_129'>129</a></span><br />
+  <span style="margin-left: 2em;">on origin of the eye, <a href=
+  '#Page_130'>130</a></span><br />
+  <span style="margin-left: 2em;">on useless characters, <a href=
+  '#Page_131'>131</a></span><br />
+  <span style="margin-left: 2em;">on use of ears and tails,
+  <a href='#Page_136'>136</a></span><br />
+  <span style="margin-left: 2em;">on disappearance of sports,
+  <a href='#Page_140'>140</a></span><br />
+  <span style="margin-left: 2em;">on tendency to vary in one
+  direction, <a href='#Page_141'>141</a></span><br />
+  <span style="margin-left: 2em;">on rare perpetuation of sports,
+  <a href='#Page_142'>142</a></span><br />
+  <span style="margin-left: 2em;">on utility of specific
+  characters, <a href='#Page_142'>142</a> (note)</span><br />
+  <span style="margin-left: 2em;">on importance of biological
+  environment, <a href='#Page_148'>148</a></span><br />
+  <span style="margin-left: 2em;">on variable fertility of plants,
+  <a href='#Page_155'>155</a></span><br />
+  <span style="margin-left: 2em;">on fertile hybrids among plants,
+  <a href='#Page_164'>164</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Darwin, on correlation of
+  sterility and colour, <a href='#Page_169'>169</a></span><br />
+  <a name="Page_483" id="Page_483"></a> <span style=
+  "margin-left: 2em;">on selective association, <a href=
+  '#Page_172'>172</a></span><br />
+  <span style="margin-left: 2em;">on infertility and natural
+  selection, <a href='#Page_174'>174</a></span><br />
+  <span style="margin-left: 2em;">on cause of infertility of
+  hybrids, <a href='#Page_185'>185</a></span><br />
+  <span style="margin-left: 2em;">on white tail of rabbit, <a href=
+  '#Page_218'>218</a></span><br />
+  <span style="margin-left: 2em;">on conspicuous caterpillars,
+  <a href='#Page_236'>236</a></span><br />
+  <span style="margin-left: 2em;">on sexual selection in insects,
+  <a href='#Page_274'>274</a></span><br />
+  <span style="margin-left: 2em;">on decorative plumage of male
+  birds, <a href='#Page_285'>285</a></span><br />
+  <span style="margin-left: 2em;">on development of ocelli,
+  <a href='#Page_290'>290</a></span><br />
+  <span style="margin-left: 2em;">on value of cross-fertilisation,
+  <a href='#Page_309'>309</a></span><br />
+  <span style="margin-left: 2em;">on limits&nbsp; to&nbsp;
+  utility&nbsp; of&nbsp; intercrossing, <a href=
+  '#Page_326'>326</a></span><br />
+  <span style="margin-left: 2em;">on flowers due to insects,
+  <a href='#Page_332'>332</a></span><br />
+  <span style="margin-left: 2em;">on oceanic islands, <a href=
+  '#Page_342'>342</a></span><br />
+  <span style="margin-left: 2em;">on effects of disuse in domestic
+  animals, <a href='#Page_415'>415</a>, <a href=
+  '#Page_435'>435</a></span><br />
+  <span style="margin-left: 2em;">on direct action of environment,
+  <a href='#Page_419'>419</a></span><br />
+  <span style="margin-left: 2em;">on unintelligibility of theory of
+  retardation and acceleration, <a href='#Page_421'>421</a>
+  (note)</span><br />
+  <span style="margin-left: 2em;">on origin of man's moral nature,
+  <a href='#Page_461'>461</a></span><br />
+  <span style="margin-left: 2em;">Mr. George, on intermarriages of
+  British aristocracy, <a href='#Page_326'>326</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Darwinian theory, statement of,
+  <a href='#Page_10'>10</a></span><br />
+  <span style="margin-left: 2em;">not opposed to spiritual nature
+  of man, <a href='#Page_478'>478</a></span><br />
+  <span style="margin-left: 1em;">Dawkins, Professor Boyd, on
+  development of deer's horns, <a href=
+  '#Page_389'>389</a></span><br />
+  <span style="margin-left: 2em;">on recent origin of man, <a href=
+  '#Page_456'>456</a></span><br />
+  <span style="margin-left: 1em;">Dawson, Sir W., on determination
+  of fossil plants by leaves, <a href='#Page_398'>398</a>
+  (note)</span><br />
+  <br />
+  <span style="margin-left: 1em;">Death of wild animals usually
+  painless, <a href='#Page_38'>38</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">De Candolle, definition of
+  species, <a href='#Page_1'>1</a></span><br />
+  <span style="margin-left: 2em;">on difficulty of naturalising
+  plants, <a href='#Page_15'>15</a></span><br />
+  <span style="margin-left: 2em;">on war between plants, <a href=
+  '#Page_16'>16</a></span><br />
+  <span style="margin-left: 2em;">on origin of useful cultivated
+  plants, <a href='#Page_97'>97</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Deer's horns, development of,
+  <a href='#Page_389'>389</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Degeneration, <a href=
+  '#Page_121'>121</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Delboeuf's law of variation,
+  <a href='#Page_141'>141</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Dendraeca coronata, variation of
+  wing-feathers of, <a href='#Page_51'>51</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Denmark, struggle between trees
+  in, <a href='#Page_20'>20</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Denudation, evidences of,
+  <a href='#Page_379'>379</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Desert animals, colour of,
+  <a href='#Page_192'>192</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Deserts, effect of goats and
+  camels in destroying vegetation in, <a href=
+  '#Page_17'>17</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Development and display of
+  accessory plumes, <a href='#Page_293'>293</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Diadema anomala, <a href=
+  '#Page_271'>271</a></span><br />
+  <span style="margin-left: 2em;">misippus, great diversity of
+  sexes in, <a href='#Page_271'>271</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Diaphora mendica mimics Spilosoma
+  menthrasti, <a href='#Page_249'>249</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Difficulties in the facts of
+  fertilisation of flowers, <a href=
+  '#Page_325'>325</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Dimorphism and trimorphism,
+  <a href='#Page_156'>156</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Dippers, probable origin of,
+  <a href='#Page_116'>116</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Disease and markings, <a href=
+  '#Page_290'>290</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Diseases common to man and
+  animals, <a href='#Page_449'>449</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Display of decorative plumage,
+  <a href='#Page_287'>287</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Distribution of organisms should
+  be explained by theory of descent, <a href=
+  '#Page_338'>338</a></span><br />
+  <span style="margin-left: 2em;">conditions which have determined
+  the, <a href='#Page_341'>341</a></span><br />
+  <span style="margin-left: 2em;">of marsupials, <a href=
+  '#Page_350'>350</a></span><br />
+  <span style="margin-left: 2em;">of tapirs, <a href=
+  '#Page_352'>352</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Disuse, effects of, among wild
+  animals, <a href='#Page_415'>415</a></span><br />
+  <span style="margin-left: 2em;">no proof that the effects of, are
+  inherited, <a href='#Page_417'>417</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Divergence of character, <a href=
+  '#Page_105'>105</a>-<a href='#Page_109'>109</a></span><br />
+  <span style="margin-left: 2em;">leads to maximum of forms of life
+  in each area, <a href='#Page_109'>109</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Diversity of fauna and flora with
+  geographical proximity, <a href='#Page_339'>339</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Dixon, Mr. C, changed habits of
+  chaffinch in New Zealand, <a href='#Page_76'>76</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Dogs, origin of, <a href=
+  '#Page_88'>88</a></span><br />
+  <span style="margin-left: 2em;">varieties of, <a href=
+  '#Page_89'>89</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Dolichonyx oryzivorus, diagram
+  showing variations of, <a href='#Page_55'>55</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Domestic animals, varieties of,
+  <a href='#Page_88'>88</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Draba verna, varieties of,
+  <a href='#Page_77'>77</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Dress of men not determined by
+  female choice, <a href='#Page_286'>286</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Dust from Krakatoa, size of
+  particles of, <a href='#Page_363'>363</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>E</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Eastern butterflies, variation
+  of, <a href='#Page_45'>45</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Eaton, Rev. A.E., on Kerguelen
+  insects, <a href='#Page_106'>106</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Edwards, Mr. W.H., on dark forms
+  of Papilio turnus, <a href='#Page_248'>248</a></span><br />
+  <a name="Page_484" id="Page_484"></a><br />
+  <span style="margin-left: 1em;">Eggs protectively coloured,
+  <a href='#Page_214'>214</a>, <a href=
+  '#Page_215'>215</a></span><br />
+  <span style="margin-left: 2em;">theory of varied colours of,
+  <a href='#Page_216'>216</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Elaps mimicked by harmless
+  snakes, <a href='#Page_261'>261</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Embryonic development of man and
+  other mammalia, <a href='#Page_448'>448</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ennis, Mr. John, on willows
+  driving out watercresses from rivers of New Zealand, <a href=
+  '#Page_24'>24</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Entomostraca, in bromelia leaves,
+  <a href='#Page_118'>118</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Environment never identical for
+  two species, <a href='#Page_149'>149</a></span><br />
+  <span style="margin-left: 2em;">direct action of, <a href=
+  '#Page_418'>418</a></span><br />
+  <span style="margin-left: 2em;">direct influence of, <a href=
+  '#Page_426'>426</a></span><br />
+  <span style="margin-left: 2em;">as initiator of variations,
+  <a href='#Page_436'>436</a></span><br />
+  <span style="margin-left: 2em;">action of, overpowered by natural
+  selection, <a href='#Page_437'>437</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ethical aspect of the struggle
+  for existence, <a href='#Page_36'>36</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Euchelia jacobeae inedible,
+  <a href='#Page_235'>235</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Everett, Mr. A., on a caterpillar
+  resembling moss, <a href='#Page_205'>205</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Evidence of evolution that may be
+  expected among fossil forms, <a href=
+  '#Page_380'>380</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Evolutionists, American school
+  of, <a href='#Page_420'>420</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Exogens, possible cause of sudden
+  late appearance of, <a href='#Page_400'>400</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">External differences of man and
+  apes, <a href='#Page_453'>453</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Extinct animals, number of
+  species of, <a href='#Page_376'>376</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Extinction of large animals,
+  cause of, <a href='#Page_394'>394</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Eye, origin of, <a href=
+  '#Page_130'>130</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Eyes, explanation of loss of in
+  cave animals, <a href='#Page_416'>416</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>F</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Facts of natural selection,
+  summary of, <a href='#Page_122'>122</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Falcons illustrating divergence,
+  <a href='#Page_108'>108</a></span><br />
+  <span style="margin-left: 2em;">and butcher birds, hooked and
+  toothed beaks of, <a href='#Page_422'>422</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Fantails, <a href=
+  '#Page_91'>91</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Female birds, why often dull
+  coloured, <a href='#Page_277'>277</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Female birds, what their choice
+  of mates is determined by, <a href=
+  '#Page_286'>286</a></span><br />
+  <span style="margin-left: 2em;">butterflies, why dull coloured,
+  <a href='#Page_272'>272</a></span><br />
+  <span style="margin-left: 2em;">brighter than male bird, <a href=
+  '#Page_281'>281</a></span><br />
+  <span style="margin-left: 2em;">choice a doubtful agent in
+  selection, <a href='#Page_283'>283</a></span><br />
+  <span style="margin-left: 2em;">preference neutralised by natural
+  selection, <a href='#Page_294'>294</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Fertility of domestic animals,
+  <a href='#Page_154'>154</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Flatfish, eyes of, <a href=
+  '#Page_129'>129</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Flesh-fly, enormous increase of,
+  <a href='#Page_25'>25</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Floral structure, great
+  differences of, in allied genera and species, <a href=
+  '#Page_329'>329</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Flowers, variations of, <a href=
+  '#Page_88'>88</a></span><br />
+  <span style="margin-left: 2em;">colours of, <a href=
+  '#Page_308'>308</a></span><br />
+  <span style="margin-left: 2em;">with sham nectaries, <a href=
+  '#Page_317'>317</a></span><br />
+  <span style="margin-left: 2em;">changing colour when fertilised,
+  <a href='#Page_317'>317</a></span><br />
+  <span style="margin-left: 2em;">adapted to bees or to
+  butterflies, <a href='#Page_318'>318</a></span><br />
+  <span style="margin-left: 2em;">contrasted colours of, at same
+  season and locality, <a href='#Page_318'>318</a></span><br />
+  <span style="margin-left: 2em;">fertilisation of, by birds,
+  <a href='#Page_319'>319</a></span><br />
+  <span style="margin-left: 2em;">self-fertilisation of, <a href=
+  '#Page_321'>321</a></span><br />
+  <span style="margin-left: 2em;">once insect-fertilised now
+  self-fertile, <a href='#Page_323'>323</a></span><br />
+  <span style="margin-left: 2em;">how the struggle for existence
+  acts among, <a href='#Page_328'>328</a></span><br />
+  <span style="margin-left: 2em;">repeatedly modified during whole
+  Tertiary period, <a href='#Page_331'>331</a></span><br />
+  <span style="margin-left: 2em;">the product of insect agency,
+  <a href='#Page_332'>332</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Forbes, Mr. H.O., on protective
+  colour of a pigeon, <a href='#Page_200'>200</a></span><br />
+  <span style="margin-left: 2em;">on spider imitating birds'
+  dropping, <a href='#Page_211'>211</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Fossil shells, complete series of
+  transitional forms of, <a href='#Page_381'>381</a></span><br />
+  <span style="margin-left: 2em;">crocodiles afford evidence of
+  evolution, <a href='#Page_383'>383</a></span><br />
+  <span style="margin-left: 2em;">horses in America, <a href=
+  '#Page_386'>386</a></span><br />
+  <span style="margin-left: 2em;">and living animals, local
+  relations of, <a href='#Page_391'>391</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Fowl, early domestication of,
+  <a href='#Page_97'>97</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Frill-back, Indian, <a href=
+  '#Page_93'>93</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Frog inhabiting bromelia leaves,
+  <a href='#Page_118'>118</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Fruits, use of characters of,
+  <a href='#Page_133'>133</a></span><br />
+  <span style="margin-left: 2em;">colours of, <a href=
+  '#Page_304'>304</a></span><br />
+  <span style="margin-left: 2em;">edible or attractive, <a href=
+  '#Page_306'>306</a></span><br />
+  <span style="margin-left: 2em;">poisonous, <a href=
+  '#Page_307'>307</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Fulica atra, protectively
+  coloured eggs of, <a href='#Page_215'>215</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Fulmar petrel, abundance of,
+  <a href='#Page_30'>30</a></span><br />
+  <br />
+  <br />
+  <a name="Page_485" id="Page_485"></a> <span style=
+  "margin-left: 1em;"><b>G</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gallinaceae, ornamental plumes
+  of, <a href='#Page_292'>292</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Galton, Mr. F., diagrams of
+  variability used by, <a href='#Page_74'>74</a></span><br />
+  <span style="margin-left: 2em;">on markings of zebra, <a href=
+  '#Page_220'>220</a> (note)</span><br />
+  <span style="margin-left: 2em;">on regression towards&nbsp;
+  mediocrity, <a href='#Page_414'>414</a></span><br />
+  <span style="margin-left: 2em;">theory of heredity by, <a href=
+  '#Page_443'>443</a> (note)</span><br />
+  <span style="margin-left: 2em;">on imperfect counting of the
+  Damaras, <a href='#Page_464'>464</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gaudry on extinct animals at
+  Pikermi, <a href='#Page_377'>377</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gay, Mons. T., on variations of
+  structure in Cruciferae, <a href='#Page_80'>80</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gazella soemmerringi (figure),
+  <a href='#Page_219'>219</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gazelles, recognition marks of,
+  <a href='#Page_218'>218</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Geddes, Professor, on variation
+  in plants, <a href='#Page_428'>428</a></span><br />
+  <span style="margin-left: 2em;">objection to theory of, <a href=
+  '#Page_430'>430</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Geikie, Dr. Archibald, on
+  formation of marine stratified rocks, <a href=
+  '#Page_344'>344</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Geoffroy St. Hilaire, on species,
+  <a href='#Page_6'>6</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Geological evidences of
+  evolution, <a href='#Page_376'>376</a>, <a href=
+  '#Page_381'>381</a></span><br />
+  <span style="margin-left: 2em;">record, causes of imperfection
+  of, <a href='#Page_379'>379</a></span><br />
+  <span style="margin-left: 2em;">distribution of insects, <a href=
+  '#Page_403'>403</a></span><br />
+  <span style="margin-left: 2em;">antiquity of man, <a href=
+  '#Page_455'>455</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ghost-moth, colours of, <a href=
+  '#Page_270'>270</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Glaciation, no proofs of, in
+  Brazil, <a href='#Page_370'>370</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Glow-worm, light a warning of
+  inedibility, <a href='#Page_287'>287</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gomphia oleaefolia, variability
+  of, <a href='#Page_79'>79</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Goose eating flesh, <a href=
+  '#Page_75'>75</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gosse, Mr. P.H., on variation in
+  the sea-anemones, <a href='#Page_43'>43</a></span><br />
+  <span style="margin-left: 2em;">on sea-anemone and bullhead,
+  <a href='#Page_265'>265</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gould, Mr., on colours of coast
+  and inland birds, <a href='#Page_228'>228</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Grant Allen, on forms of leaves,
+  <a href='#Page_133'>133</a></span><br />
+  <span style="margin-left: 2em;">on insects and flowers, <a href=
+  '#Page_332'>332</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Graphite in Laurentian implies
+  abundant plant life, <a href='#Page_398'>398</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gray, Dr. Asa, on naturalised
+  plants in the United States, <a href=
+  '#Page_110'>110</a></span><br />
+  <span style="margin-left: 2em;">Dr. J.E., on variation of skulls
+  of mammalia, <a href='#Page_71'>71</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Great fertility not essential to
+  rapid increase, <a href='#Page_30'>30</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Great powers of increase of
+  animals, <a href='#Page_27'>27</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Green colour of birds in tropical
+  forests, <a href='#Page_192'>192</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Grouse, red, recent divergence
+  of, <a href='#Page_106'>106</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Gulick, Rev. J.T., on variation
+  of land-shells, <a href='#Page_43'>43</a></span><br />
+  <span style="margin-left: 2em;">on isolation and variation,
+  <a href='#Page_147'>147</a>, <a href=
+  '#Page_150'>150</a></span><br />
+  <span style="margin-left: 2em;">on divergent evolution, <a href=
+  '#Page_148'>148</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>H</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Habits&nbsp; of animals,
+  variability of, <a href='#Page_74'>74</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Hairy caterpillars inedible,
+  <a href='#Page_237'>237</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Hanbury, Mr. Thomas, on a
+  remarkable case of wind conveyance of seed, <a href=
+  '#Page_373'>373</a> (note)</span><br />
+  <br />
+  <span style="margin-left: 1em;">Hansten-Blangsted, on succession
+  of trees in Denmark, <a href='#Page_21'>21</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Harvest mice, prehensile tails of
+  young, <a href='#Page_136'>136</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Hawkweed, species and varieties
+  of British, <a href='#Page_77'>77</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Hector, Sir James, use of horns
+  of deer, <a href='#Page_137'>137</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Heliconidae, warning colours of,
+  <a href='#Page_234'>234</a></span><br />
+  <span style="margin-left: 2em;">mimicry of, <a href=
+  '#Page_240'>240</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Helix nemoralis, varieties of,
+  <a href='#Page_43'>43</a></span><br />
+  <span style="margin-left: 2em;">hortensis, varieties of, <a href=
+  '#Page_43'>43</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Hemsley, Mr., on rarity of spines
+  in oceanic islands, <a href='#Page_432'>432</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Henslow, Professor G., on vigour
+  of self-fertilised plants, <a href=
+  '#Page_323'>323</a></span><br />
+  <span style="margin-left: 2em;">on wind-fertilised as
+  degradations from insect-fertilised flowers, <a href=
+  '#Page_324'>324</a></span><br />
+  <span style="margin-left: 2em;">on origin of forms and structures
+  of flowers, 434 (note)</span><br />
+  <br />
+  <span style="margin-left: 1em;">Herbert, Dean, on species,
+  <a href='#Page_6'>6</a></span><br />
+  <span style="margin-left: 2em;">on plant hybrids, <a href=
+  '#Page_164'>164</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Herbivora, recognition marks of,
+  <a href='#Page_218'>218</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Heredity, <a href=
+  '#Page_11'>11</a></span><br />
+  <span style="margin-left: 2em;">Weismann's theory of, <a href=
+  '#Page_437'>437</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Herschel, Sir John, on species,
+  <a href='#Page_3'>3</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Hooker, Sir Joseph, on attempts
+  at naturalising Australian plants in New Zealand, <a href=
+  '#Page_16'>16</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Home, Mr. C, on inedibility of an
+  Indian locust, <a href='#Page_267'>267</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Horns of deer, uses of, <a href=
+  '#Page_136'>136</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Horse tribe, pedigree of,
+  <a href='#Page_384'>384</a></span><br />
+  <a name="Page_486" id="Page_486"></a> <span style=
+  "margin-left: 2em;">ancestral forms of, <a href=
+  '#Page_386'>386</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Humming-birds, recognition marks
+  of, <a href='#Page_226'>226</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Huth, Mr., on close
+  interbreeding, <a href='#Page_160'>160</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Huxley, Professor on the struggle
+  for existence, <a href='#Page_37'>37</a></span><br />
+  <span style="margin-left: 2em;">on fossil crocodiles, <a href=
+  '#Page_383'>383</a></span><br />
+  <span style="margin-left: 2em;">on anatomical peculiarities of
+  the horse tribe, <a href='#Page_384'>384</a></span><br />
+  <span style="margin-left: 2em;">on development of vertebrates,
+  <a href='#Page_448'>448</a></span><br />
+  <span style="margin-left: 2em;">on early man, <a href=
+  '#Page_456'>456</a></span><br />
+  <span style="margin-left: 2em;">on brains of man and the gorilla,
+  <a href='#Page_457'>457</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Hybridity, remarks on facts of,
+  <a href='#Page_166'>166</a></span><br />
+  <span style="margin-left: 2em;">summary on, <a href=
+  '#Page_184'>184</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Hybrids, infertility of, supposed
+  test of distinct species, <a href=
+  '#Page_152'>152</a></span><br />
+  <span style="margin-left: 2em;">fertility of, <a href=
+  '#Page_159'>159</a></span><br />
+  <span style="margin-left: 2em;">fertile among animals, <a href=
+  '#Page_162'>162</a></span><br />
+  <span style="margin-left: 2em;">between sheep and goat, <a href=
+  '#Page_162'>162</a></span><br />
+  <span style="margin-left: 2em;">fertile between distinct species
+  of moths, <a href='#Page_163'>163</a></span><br />
+  <span style="margin-left: 2em;">fertile among plants, <a href=
+  '#Page_163'>163</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Hymenopus bicornis, resembling
+  flower, <a href='#Page_212'>212</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>I</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Icterus Baltimore, diagram
+  showing proportionate numbers which</span><br />
+  <span style="margin-left: 1em;">vary, <a href=
+  '#Page_63'>63</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Imitative resemblances, how
+  produced, <a href='#Page_205'>205</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Increase of organisms in a
+  geometrical ratio, <a href='#Page_25'>25</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Inedible fruits rarely coloured,
+  <a href='#Page_308'>308</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Insect and self-fertilisation,
+  alternation of, in flowers, <a href=
+  '#Page_328'>328</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Insect-fertilisation, facts
+  relating to, <a href='#Page_316'>316</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Insects, coloured for
+  recognition, <a href='#Page_226'>226</a></span><br />
+  <span style="margin-left: 2em;">warning colours of, <a href=
+  '#Page_233'>233</a></span><br />
+  <span style="margin-left: 2em;">sexual coloration of, <a href=
+  '#Page_269'>269</a></span><br />
+  <span style="margin-left: 2em;">importance of dull colours to
+  female, <a href='#Page_272'>272</a></span><br />
+  <span style="margin-left: 2em;">visiting one kind of flower at a
+  time, <a href='#Page_318'>318</a></span><br />
+  <span style="margin-left: 2em;">and flowers, the most brilliant
+  not found together, <a href='#Page_335'>335</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Insects, no proof of love of
+  colour by, <a href='#Page_336'>336</a></span><br />
+  <span style="margin-left: 2em;">and birds at sea, <a href=
+  '#Page_357'>357</a></span><br />
+  <span style="margin-left: 2em;">in mid-ocean, <a href=
+  '#Page_359'>359</a></span><br />
+  <span style="margin-left: 2em;">at great altitudes, <a href=
+  '#Page_360'>360</a></span><br />
+  <span style="margin-left: 2em;">geological distribution of,
+  <a href='#Page_403'>403</a></span><br />
+  <span style="margin-left: 2em;">ancestral in Silurian, <a href=
+  '#Page_405'>405</a></span><br />
+  <span style="margin-left: 2em;">fossil support evolution,
+  <a href='#Page_405'>405</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Instability of useless
+  characters, <a href='#Page_138'>138</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Instinct, the theory of, <a href=
+  '#Page_441'>441</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Insular organisms illustrate
+  powers of dispersal, <a href='#Page_354'>354</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Interbreeding, close, injurious
+  effects of, <a href='#Page_160'>160</a></span><br />
+  <span style="margin-left: 2em;">supposed evil results of close,
+  <a href='#Page_326'>326</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Intercrossing, swamping effects
+  of, <a href='#Page_142'>142</a></span><br />
+  <span style="margin-left: 2em;">not necessarily useful, <a href=
+  '#Page_325'>325</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Intermediate forms, why not
+  found, <a href='#Page_380'>380</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Islands, all oceanic are volcanic
+  or coralline, <a href='#Page_342'>342</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Isle of Man, small butterflies
+  of, <a href='#Page_106'>106</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Isolation, the importance of,
+  <a href='#Page_119'>119</a></span><br />
+  <span style="margin-left: 2em;">to prevent intercrossing,
+  <a href='#Page_144'>144</a></span><br />
+  <span style="margin-left: 2em;">by variations of habits, etc.,
+  <a href='#Page_145'>145</a></span><br />
+  <span style="margin-left: 2em;">Rev. J.G. Gulick on, <a href=
+  '#Page_147'>147</a></span><br />
+  <span style="margin-left: 2em;">when ineffective, <a href=
+  '#Page_150'>150</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ituna Ilione and Thyridia
+  megisto, figures of wings of, <a href=
+  '#Page_251'>251</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>J</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Jacobin, <a href=
+  '#Page_93'>93</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Jenyns, Rev. L., on internal
+  variations of mammalia, <a href='#Page_69'>69</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Jordan, Mons. A., on varieties of
+  Draba verna, <a href='#Page_77'>77</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Judd, Professor, on dust fallen
+  at Genoa, <a href='#Page_363'>363</a></span><br />
+  <span style="margin-left: 2em;">on Hungarian fossil lacustrine
+  shells, <a href='#Page_381'>381</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>K</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Kerguelen Island, wingless
+  insects of, <a href='#Page_106'>106</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Kerivoula picta, protective
+  colour of, <a href='#Page_201'>201</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Kerner, Professor, on use of
+  external characters of plants, <a href=
+  '#Page_133'>133</a></span><br />
+  <a name="Page_487" id="Page_487"></a> <span style=
+  "margin-left: 2em;">on seeds found on glaciers, <a href=
+  '#Page_366'>366</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Kingfishers illustrating
+  divergence of character, <a href='#Page_109'>109</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>L</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lacerta muralis, diagram of
+  variation of, <a href='#Page_47'>47</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lagopus scoticus, origin of,
+  <a href='#Page_107'>107</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lamarck, on origin of species,
+  <a href='#Page_3'>3</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Land debris deposited near
+  coasts, <a href='#Page_343'>343</a></span><br />
+  <span style="margin-left: 2em;">and ocean, diagram showing
+  comparative height and depth of, <a href=
+  '#Page_345'>345</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Large animals, cause of
+  extinction of, <a href='#Page_394'>394</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Larvae of moths, variability of,
+  <a href='#Page_46'>46</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Laughers, Frill-backs, Nuns,
+  Spots, and Swallows, <a href='#Page_93'>93</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Law of relation of colour and
+  nest, <a href='#Page_278'>278</a>, <a href=
+  '#Page_279'>279</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Laws of animal coloration,
+  <a href='#Page_296'>296</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lawson Tait, on uses of tails,
+  <a href='#Page_136'>136</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Leaf-butterflies, <a href=
+  '#Page_207'>207</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Leguminosae, rare in oceanic
+  islands, <a href='#Page_368'>368</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lemuria, an unsound hypothesis,
+  <a href='#Page_354'>354</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lepidoptera, variation of,
+  <a href='#Page_44'>44</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Leyden Museum, diagram showing
+  variability of birds in, <a href='#Page_61'>61</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Life, Weismann on duration of,
+  437 (note)</span><br />
+  <br />
+  <span style="margin-left: 1em;">Limenitis misippus mimics Danais
+  archippus, <a href='#Page_248'>248</a></span><br />
+  <span style="margin-left: 2em;">ursula mimics Papilio philenor,
+  <a href='#Page_248'>248</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Linnaeus, on rapid increase of
+  the flesh-fly, <a href='#Page_25'>25</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Livingstone, his sensations when
+  seized by a lion, <a href='#Page_38'>38</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lizards, variation among,
+  <a href='#Page_46'>46</a></span><br />
+  <span style="margin-left: 2em;">diagram of variation of, <a href=
+  '#Page_48'>48</a></span><br />
+  <span style="margin-left: 2em;">sexual colours of, <a href=
+  '#Page_281'>281</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Local colour adaptations,
+  <a href='#Page_199'>199</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Locusts with warning colours
+  inedible, <a href='#Page_267'>267</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Longicorns mimic Malacoderms,
+  <a href='#Page_257'>257</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Low, Mr., on effects of close
+  interbreeding, <a href='#Page_160'>160</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Low, Mr., on fertile crosses
+  between sheep and goat, <a href='#Page_162'>162</a></span><br />
+  <span style="margin-left: 2em;">on selective association,
+  <a href='#Page_172'>172</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Low forms of life, continued
+  existence of, explained, <a href='#Page_114'>114</a></span><br />
+  <span style="margin-left: 2em;">forms, persistence of, <a href=
+  '#Page_121'>121</a></span><br />
+  <span style="margin-left: 2em;">temperature of tropics not needed
+  to explain plant dispersal, <a href=
+  '#Page_370'>370</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lower types, extinction of, among
+  the higher animals, <a href='#Page_114'>114</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lubbock, Sir John, on forms of
+  leaves, <a href='#Page_133'>133</a></span><br />
+  <span style="margin-left: 2em;">on imperfect counting of early
+  man, <a href='#Page_464'>464</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Lyell, Sir Charles, on variation
+  of species, <a href='#Page_4'>4</a></span><br />
+  <span style="margin-left: 2em;">on the shifting of continents,
+  <a href='#Page_342'>342</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>M</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Madagascar and New Zealand,
+  <a href='#Page_347'>347</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Madeira, wingless beetles of,
+  <a href='#Page_105'>105</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Maize, origin of, <a href=
+  '#Page_98'>98</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Male rivalry, a real cause of
+  selection, <a href='#Page_283'>283</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Males of many animals fights
+  together, <a href='#Page_282'>282</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Malm, on eyes of flatfish,
+  <a href='#Page_129'>129</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mammalia, variation of, <a href=
+  '#Page_65'>65</a></span><br />
+  <span style="margin-left: 2em;">sexual colours of, <a href=
+  '#Page_281'>281</a>, <a href='#Page_282'>282</a></span><br />
+  <span style="margin-left: 2em;">afford crucial tests of theories
+  of distribution, <a href='#Page_353'>353</a></span><br />
+  <span style="margin-left: 2em;">early forms of, <a href=
+  '#Page_407'>407</a></span><br />
+  <span style="margin-left: 2em;">geological distribution of,
+  <a href='#Page_408'>408</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mammary glands, supposed origin
+  of, <a href='#Page_129'>129</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Man, summary of animal
+  characteristics of, <a href='#Page_454'>454</a></span><br />
+  <span style="margin-left: 2em;">geological antiquity of, <a href=
+  '#Page_455'>455</a></span><br />
+  <span style="margin-left: 2em;">early remains of, in California,
+  <a href='#Page_456'>456</a></span><br />
+  <span style="margin-left: 2em;">probably as old as the Miocene,
+  <a href='#Page_457'>457</a></span><br />
+  <span style="margin-left: 2em;">probable birthplace of, <a href=
+  '#Page_459'>459</a></span><br />
+  <span style="margin-left: 2em;">origin of moral and intellectual
+  nature of, <a href='#Page_461'>461</a></span><br />
+  <span style="margin-left: 2em;">possesses mental qualities not
+  derived exclusively</span><br />
+  <span style="margin-left: 6em;">from his animal progenitors,
+  <a href='#Page_474'>474</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Man's body that of an animal,
+  <a href='#Page_444'>444</a></span><br />
+  <a name="Page_488" id="Page_488"></a> <span style=
+  "margin-left: 2em;">development similar to that of animals,
+  <a href='#Page_449'>449</a></span><br />
+  <span style="margin-left: 2em;">structure compared with that of
+  the anthropoid apes, <a href='#Page_451'>451</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mania typica refused by lizards,
+  <a href='#Page_238'>238</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mantidae resembling flowers,
+  <a href='#Page_212'>212</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Marcgravia nepenthoides
+  fertilised by birds (woodcut), <a href=
+  '#Page_320'>320</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Marine animals, protective
+  resemblance among, <a href='#Page_208'>208</a></span><br />
+  <span style="margin-left: 2em;">with warning colours, <a href=
+  '#Page_266'>266</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Marsh, Mr., on destructiveness to
+  vegetation of goats and camels, <a href=
+  '#Page_17'>17</a></span><br />
+  <span style="margin-left: 2em;">Professor O., on the development
+  of the horse tribe, <a href='#Page_386'>386</a></span><br />
+  <span style="margin-left: 2em;">on brain development of Tertiary
+  mammals, <a href='#Page_391'>391</a></span><br />
+  <span style="margin-left: 2em;">on specialised forms dying out,
+  <a href='#Page_395'>395</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Marsupials, distribution of,
+  <a href='#Page_350'>350</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mathematical faculty, the origin
+  of the, <a href='#Page_464'>464</a></span><br />
+  <span style="margin-left: 2em;">how developed, <a href=
+  '#Page_466'>466</a></span><br />
+  <span style="margin-left: 2em;">not developed by law of natural
+  selection, <a href='#Page_469'>469</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mathematics, late development of,
+  <a href='#Page_465'>465</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Meldola, Professor R., on
+  variable protective colouring, <a href=
+  '#Page_196'>196</a></span><br />
+  <span style="margin-left: 2em;">on mimicry among British moths,
+  249 (note)</span><br />
+  <span style="margin-left: 2em;">on an extension of the theory of
+  mimicry, <a href='#Page_255'>255</a> (note)</span><br />
+  <br />
+  <span style="margin-left: 1em;">Melons, variations of, <a href=
+  '#Page_87'>87</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Methona psidii and Leptalis orise
+  (figs.), <a href='#Page_241'>241</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Meyer, Dr. A.B., on mimicry of
+  snakes, <a href='#Page_262'>262</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Milne Edwards, on variation of
+  lizards, <a href='#Page_46'>46</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mimicking birds deceive
+  naturalists, <a href='#Page_264'>264</a></span><br />
+  <span style="margin-left: 2em;">butterfly, figure of, <a href=
+  '#Page_241'>241</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mimicry, <a href=
+  '#Page_239'>239</a></span><br />
+  <span style="margin-left: 2em;">how it has been produced,
+  <a href='#Page_242'>242</a></span><br />
+  <span style="margin-left: 2em;">among protected genera, <a href=
+  '#Page_249'>249</a></span><br />
+  <span style="margin-left: 2em;">extension of, <a href=
+  '#Page_255'>255</a></span><br />
+  <span style="margin-left: 2em;">in various orders of insects,
+  <a href='#Page_257'>257</a></span><br />
+  <span style="margin-left: 2em;">among vertebrata, <a href=
+  '#Page_261'>261</a></span><br />
+  <span style="margin-left: 2em;">among birds, <a href=
+  '#Page_263'>263</a></span><br />
+  <span style="margin-left: 2em;">objections to theory of, <a href=
+  '#Page_264'>264</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mineral particles carried by
+  wind, <a href='#Page_363'>363</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Miocene fossils of North America,
+  <a href='#Page_378'>378</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Missing links, character of,
+  <a href='#Page_380'>380</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mivart, Dr. St. George, on
+  variation of ribs and dorsal vertebrae, <a href=
+  '#Page_69'>69</a></span><br />
+  <span style="margin-left: 2em;">on supposed useless characters,
+  <a href='#Page_138'>138</a> (note)</span><br />
+  <span style="margin-left: 2em;">on resemblance of man and apes,
+  <a href='#Page_451'>451</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Modifications for special
+  purposes, <a href='#Page_113'>113</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mongrels, sterility of, <a href=
+  '#Page_165'>165</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Monkeys affected by medicines as
+  are men, <a href='#Page_450'>450</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Monocotyledons degradations from
+  dicotyledons, <a href='#Page_325'>325</a> (note)</span><br />
+  <span style="margin-left: 2em;">scarcity of, in Rocky Mountains,
+  <a href='#Page_401'>401</a></span><br />
+  <span style="margin-left: 2em;">scarcity of, in Alpine flora,
+  <a href='#Page_401'>401</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Moral nature of man, origin of,
+  <a href='#Page_461'>461</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Morse, Professor E.T., on
+  protective colouring of marine mollusca, <a href=
+  '#Page_209'>209</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Moseley, Professor, on protective
+  resemblance among marine</span><br />
+  <span style="margin-left: 1em;">animals, <a href=
+  '#Page_208'>208</a></span><br />
+  <span style="margin-left: 2em;">on courtship of Great Albatross,
+  <a href='#Page_287'>287</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Moths, protected groups of,
+  <a href='#Page_235'>235</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mountains, remote, with identical
+  plants, <a href='#Page_369'>369</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">M&uuml;ller, Dr. Fritz, on
+  inhabitants of bromelia leaves, <a href=
+  '#Page_118'>118</a></span><br />
+  <span style="margin-left: 2em;">on butterfly, deceived by its
+  mimic, <a href='#Page_245'>245</a></span><br />
+  <span style="margin-left: 2em;">his explanation of mimicry among
+  protected genera, <a href='#Page_252'>252</a></span><br />
+  <span style="margin-left: 2em;">Dr. Hermann, on variability of
+  Myosurus minimus, <a href='#Page_78'>78</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Murray, Mr. John, on bulk of land
+  and ocean, <a href='#Page_344'>344</a></span><br />
+  <span style="margin-left: 2em;">on quartz particles on ocean
+  floor, <a href='#Page_363'>363</a></span><br />
+  <span style="margin-left: 2em;">Rev. R.P., variation in the
+  neuration of butterflies' wings, <a href=
+  '#Page_45'>45</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Musical and artistic faculties,
+  origin of, <a href='#Page_467'>467</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Myosurns minimus, variability of,
+  <a href='#Page_78'>78</a></span><br />
+  <br />
+  <a name="Page_489" id="Page_489"></a><br />
+  <span style="margin-left: 1em;"><b>N</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Natural selection with changed
+  and unchanged conditions, <a href=
+  '#Page_103'>103</a></span><br />
+  <span style="margin-left: 2em;">and sterility, <a href=
+  '#Page_173'>173</a></span><br />
+  <span style="margin-left: 2em;">overpowers effects of use and
+  disuse, <a href='#Page_435'>435</a></span><br />
+  <span style="margin-left: 2em;">the most important agency in
+  modifying species, <a href='#Page_444'>444</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Naturalist deceived by a
+  mimicking insect, <a href='#Page_259'>259</a></span><br />
+  <span style="margin-left: 2em;">by mimicking birds, <a href=
+  '#Page_264'>264</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Naudin, M., on varieties of
+  melons, <a href='#Page_87'>87</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Nectarinea amethystina,
+  protective colouring of, <a href='#Page_201'>201</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Nestor notabilis, variation of
+  habits of, <a href='#Page_75'>75</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Nests of birds influence the
+  colour of females, <a href='#Page_278'>278</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">New species, conditions
+  favourable to origin of, <a href='#Page_115'>115</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Newton, Professor A., on fertile
+  hybrid ducks, <a href='#Page_162'>162</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">New Zealand, European plants in,
+  <a href='#Page_15'>15</a></span><br />
+  <span style="margin-left: 2em;">spread of white clover in,
+  <a href='#Page_28'>28</a></span><br />
+  <span style="margin-left: 2em;">effects of introduced plants in,
+  <a href='#Page_29'>29</a></span><br />
+  <span style="margin-left: 2em;">native rat and fly exterminated
+  by European species, <a href='#Page_34'>34</a></span><br />
+  <span style="margin-left: 2em;">many plants of, incapable of
+  self-fertilisation, <a href='#Page_321'>321</a></span><br />
+  <span style="margin-left: 2em;">fauna of, <a href=
+  '#Page_348'>348</a></span><br />
+  <span style="margin-left: 2em;">few spiny plants in, <a href=
+  '#Page_433'>433</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Nocturnal animals, colours of,
+  <a href='#Page_193'>193</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Non-adaptive characters,
+  instability of, <a href='#Page_138'>138</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Normandy pigs, fleshy appendages
+  to jaws of, <a href='#Page_139'>139</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">North America, Miocene fossils
+  of, <a href='#Page_378'>378</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Northern plants in southern
+  hemisphere, <a href='#Page_368'>368</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Nostus Borbonicus, variability
+  of, <a href='#Page_80'>80</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Number of individuals which vary,
+  <a href='#Page_62'>62</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Nutmeg, how dispersed, <a href=
+  '#Page_307'>307</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Nuts, not meant to be eaten,
+  <a href='#Page_305'>305</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>O</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Oaks, great variability of,
+  <a href='#Page_78'>78</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Objections to Darwin's theory,
+  <a href='#Page_126'>126</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ocean floor, deposits on,
+  <a href='#Page_343'>343</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Oceanic animals, colours of,
+  <a href='#Page_193'>193</a></span><br />
+  <span style="margin-left: 2em;">and continental areas, <a href=
+  '#Page_346'>346</a></span><br />
+  <span style="margin-left: 2em;">islands have no mammals or
+  batrachia, <a href='#Page_342'>342</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Oceans, the permanence of,
+  <a href='#Page_341'>341</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Oedicnemus, figures of wings of,
+  <a href='#Page_223'>223</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Opthalmis lincea and Artaxa
+  simulans (figs.), <a href='#Page_247'>247</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Orang-utans, variations of skull
+  of, <a href='#Page_69'>69</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Orchideae, why scarce on oceanic
+  islands, <a href='#Page_367'>367</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Orchis pyramidalis, mode of
+  fertilisation of, <a href='#Page_314'>314</a></span><br />
+  <span style="margin-left: 2em;">figures illustrating
+  fertilisation of, <a href='#Page_315'>315</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Organic development, three stages
+  of, involving new cause or power, <a href=
+  '#Page_474'>474</a></span><br />
+  <span style="margin-left: 2em;">world, the development of,
+  implies a spiritual world, <a href=
+  '#Page_476'>476</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Organisation, advance of, by
+  natural selection, <a href='#Page_120'>120</a></span><br />
+  <span style="margin-left: 2em;">degradation in, <a href=
+  '#Page_121'>121</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Origin of species, objections,
+  <a href='#Page_7'>7</a></span><br />
+  <span style="margin-left: 2em;">of accessory plumes, <a href=
+  '#Page_291'>291</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Orioles mimicking honey-suckers,
+  <a href='#Page_263'>263</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ornamental plumes and vitality,
+  <a href='#Page_293'>293</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>P</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pachyrhynchi subjects of mimicry,
+  <a href='#Page_261'>261</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pampas, effects of drought in,
+  <a href='#Page_23'>23</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Papaver bracteatum, variability
+  of, <a href='#Page_79'>79</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Papilio, use of forked tentacle
+  of larva of, <a href='#Page_210'>210</a></span><br />
+  <span style="margin-left: 2em;">protected groups of, <a href=
+  '#Page_235'>235</a></span><br />
+  <span style="margin-left: 2em;">mimicry of, <a href=
+  '#Page_247'>247</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Paraguay, absence of wild cattle
+  and horses, <a href='#Page_19'>19</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Parnassia palustris, sham
+  nectaries of, <a href='#Page_317'>317</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Parrot, change of habits of New
+  Zealand, <a href='#Page_75'>75</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Parus, species of, illustrate
+  divergence, of character, <a href=
+  '#Page_107'>107</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Passenger-pigeon, account of its
+  breeding-places and numbers, <a href=
+  '#Page_31'>31</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pelagic animals, colours of,
+  <a href='#Page_193'>193</a></span><br />
+  <a name="Page_490" id="Page_490"></a><br />
+  <span style="margin-left: 1em;">Phasmidae, resemblance of, to
+  sticks and leaves, <a href='#Page_203'>203</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Physiological selection, <a href=
+  '#Page_180'>180</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pickard-Cambridge, Rev. O., on
+  sexual selection, <a href='#Page_296'>296</a> (note)</span><br />
+  <br />
+  <span style="margin-left: 1em;">Pieridae, sexual diversity among,
+  <a href='#Page_271'>271</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pigeons, varieties of, <a href=
+  '#Page_89'>89</a></span><br />
+  <span style="margin-left: 2em;">domestic, derived from wild
+  rock-pigeons, <a href='#Page_90'>90</a></span><br />
+  <span style="margin-left: 2em;">curious correlations in, <a href=
+  '#Page_140'>140</a></span><br />
+  <span style="margin-left: 2em;">white eggs of, protective,
+  <a href='#Page_213'>213</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pigs, great increase of, in South
+  America and New Zealand, <a href='#Page_28'>28</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pikermi, extinct animals of,
+  <a href='#Page_377'>377</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pipits as illustrating
+  divergence, <a href='#Page_108'>108</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Planorbidae, variations of,
+  <a href='#Page_44'>44</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Plants, the enemies of, <a href=
+  '#Page_16'>16</a></span><br />
+  <span style="margin-left: 2em;">variability of, <a href=
+  '#Page_76'>76</a></span><br />
+  <span style="margin-left: 2em;">constitutional variation of,
+  <a href='#Page_94'>94</a></span><br />
+  <span style="margin-left: 2em;">colour relations of, <a href=
+  '#Page_302'>302</a></span><br />
+  <span style="margin-left: 2em;">true mimicry rare in, <a href=
+  '#Page_303'>303</a></span><br />
+  <span style="margin-left: 2em;">exotic rarely naturalised in
+  Europe, <a href='#Page_356'>356</a></span><br />
+  <span style="margin-left: 2em;">dispersal of, <a href=
+  '#Page_361'>361</a></span><br />
+  <span style="margin-left: 2em;">northern, in southern hemisphere,
+  <a href='#Page_368'>368</a></span><br />
+  <span style="margin-left: 2em;">identical on summits of remote
+  mountains, <a href='#Page_369'>369</a></span><br />
+  <span style="margin-left: 2em;">progressive development of,
+  <a href='#Page_397'>397</a></span><br />
+  <span style="margin-left: 2em;">geological development of
+  (diagram), <a href='#Page_402'>402</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Plovers, recognition marks of
+  (figure), <a href='#Page_221'>221</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Plumes, origin of accessory,
+  <a href='#Page_291'>291</a></span><br />
+  <span style="margin-left: 2em;">muscular relation of ornamental,
+  <a href='#Page_292'>292</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Poisonous fruits, <a href=
+  '#Page_307'>307</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Porto Santo, rabbits of, <a href=
+  '#Page_326'>326</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Poulton, Mr. E.B., on variable
+  colours of larvae and pupae, <a href='#Page_196'>196</a>,
+  <a href='#Page_198'>198</a></span><br />
+  <span style="margin-left: 2em;">on concealments of insects by
+  resemblance to environment, <a href=
+  '#Page_202'>202</a></span><br />
+  <span style="margin-left: 2em;">on protective form of Notodonta
+  ziczac, <a href='#Page_210'>210</a></span><br />
+  <span style="margin-left: 2em;">on inedibility of conspicuous
+  larvae, <a href='#Page_237'>237</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pouters, <a href=
+  '#Page_90'>90</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Primulaceae, variations of
+  structure in, <a href='#Page_79'>79</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Problem, the, before Darwin,
+  <a href='#Page_6'>6</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Problems in variation and
+  heredity, <a href='#Page_410'>410</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Progression in plants and
+  animals, <a href='#Page_395'>395</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Protection by terrifying enemies,
+  <a href='#Page_209'>209</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Protective colouring, variable,
+  <a href='#Page_195'>195</a></span><br />
+  <span style="margin-left: 2em;">of white-headed fruit-pigeon,
+  <a href='#Page_200'>200</a></span><br />
+  <span style="margin-left: 2em;">of African sun-birds, <a href=
+  '#Page_200'>200</a></span><br />
+  <span style="margin-left: 2em;">of Kerivoula picta, <a href=
+  '#Page_201'>201</a></span><br />
+  <span style="margin-left: 2em;">of sloths, <a href=
+  '#Page_201'>201</a></span><br />
+  <span style="margin-left: 2em;">of larva of Sphinx ligustri,
+  <a href='#Page_202'>202</a></span><br />
+  <span style="margin-left: 2em;">of stick and leaf insects,
+  <a href='#Page_203'>203</a></span><br />
+  <span style="margin-left: 2em;">of caterpillars, <a href=
+  '#Page_203'>203</a>, <a href='#Page_205'>205</a></span><br />
+  <span style="margin-left: 2em;">of butterflies, <a href=
+  '#Page_206'>206</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ptilopus cinctus, protective
+  colour of, <a href='#Page_200'>200</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Pugnacity of birds with accessory
+  plumes, <a href='#Page_294'>294</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>R</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rabbit, use of white tail of,
+  <a href='#Page_218'>218</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rapid increase of plants,
+  <a href='#Page_28'>28</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Raspail, M., on variability in a
+  grass, <a href='#Page_80'>80</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rat, black, spread of, <a href=
+  '#Page_34'>34</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rattlesnake, use of rattle of,
+  <a href='#Page_262'>262</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Raven, why black in arctic
+  regions, <a href='#Page_191'>191</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Reciprocal crosses, <a href=
+  '#Page_155'>155</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Recognition marks of herbivora,
+  <a href='#Page_218'>218</a></span><br />
+  <span style="margin-left: 2em;">of birds, <a href=
+  '#Page_222'>222</a></span><br />
+  <span style="margin-left: 2em;">of tropical forest birds,
+  <a href='#Page_224'>224</a></span><br />
+  <span style="margin-left: 2em;">of insects, <a href=
+  '#Page_226'>226</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Reproductive functions,
+  susceptibility of, <a href='#Page_153'>153</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Reptiles, geological distribution
+  of, <a href='#Page_406'>406</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rhinoceroses, evidence of
+  evolution afforded by fossil, <a href=
+  '#Page_383'>383</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rocks, all stratified formed in
+  shallow water, <a href='#Page_344'>344</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rocky Mountains, scarcity of
+  monocotyledons in, <a href='#Page_401'>401</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rodents, prevent woody vegetation
+  in the pampas, <a href='#Page_23'>23</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Romanes, Professor G.J., on
+  useless characters, <a href='#Page_131'>131</a>, <a href=
+  '#Page_139'>139</a></span><br />
+  <span style="margin-left: 2em;">on meaningless peculiarities of
+  structure, <a href='#Page_140'>140</a></span><br />
+  <span style="margin-left: 2em;">on supposed absence of
+  simultaneous variations, <a href='#Page_142'>142</a></span><br />
+  <span style="margin-left: 2em;">on physiological selection,
+  <a href='#Page_180'>180</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rook and crow, <a href=
+  '#Page_2'>2</a></span><br />
+  <a name="Page_491" id="Page_491"></a><br />
+  <span style="margin-left: 1em;">Roses, Mr. Baker on varieties of,
+  <a href='#Page_77'>77</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rubus, Bentham and Babington on
+  species and varieties of, <a href='#Page_77'>77</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rudiments and variations in man,
+  <a href='#Page_446'>446</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Runts, <a href=
+  '#Page_91'>91</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Rutaceae, variation of structure
+  in, <a href='#Page_79'>79</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>S</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">St. Helena, destruction of
+  forests by goats, <a href='#Page_17'>17</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">St. Hilaire, M. Aug., variability
+  of Gomphia oleaefolia, <a href='#Page_79'>79</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Saxicola, divergence of character
+  in species of, <a href='#Page_108'>108</a></span><br />
+  <span style="margin-left: 2em;">recognition marks of, <a href=
+  '#Page_222'>222</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Scientific opinion before Darwin,
+  <a href='#Page_4'>4</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Scolopax, figures of tails of,
+  <a href='#Page_225'>225</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Scudder, Mr. S.H., on inedibility
+  of Danais archippus, <a href='#Page_238'>238</a></span><br />
+  <span style="margin-left: 2em;">on fossil insects, <a href=
+  '#Page_403'>403</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Seebohm, Mr., on swamping effects
+  of intercrossing, <a href='#Page_143'>143</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Seeds, how dispersed, <a href=
+  '#Page_306'>306</a></span><br />
+  <span style="margin-left: 2em;">how protected, <a href=
+  '#Page_307'>307</a></span><br />
+  <span style="margin-left: 2em;">floating great distances,
+  <a href='#Page_361'>361</a></span><br />
+  <span style="margin-left: 2em;">dispersal of, by wind, <a href=
+  '#Page_362'>362</a></span><br />
+  <span style="margin-left: 2em;">weight and dimensions of,
+  <a href='#Page_364'>364</a></span><br />
+  <span style="margin-left: 2em;">importance of wind-carriage of,
+  <a href='#Page_372'>372</a></span><br />
+  <span style="margin-left: 2em;">remarkable case of wind-carriage
+  of, <a href='#Page_373'>373</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Seiurus carolinensis, diagram of
+  variation, <a href='#Page_67'>67</a></span><br />
+  <span style="margin-left: 2em;">sp., habits of, <a href=
+  '#Page_117'>117</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Selection, artificial, <a href=
+  '#Page_84'>84</a></span><br />
+  <span style="margin-left: 2em;">by man, circumstances favourable
+  to, <a href='#Page_96'>96</a></span><br />
+  <span style="margin-left: 2em;">unconscious, <a href=
+  '#Page_96'>96</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Selective association, isolation
+  by, <a href='#Page_171'>171</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Self-fertilisation of flowers,
+  <a href='#Page_321'>321</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Semper, Professor, on casting
+  hairs of reptiles and Crustacea, <a href='#Page_137'>137</a>
+  (note)</span><br />
+  <span style="margin-left: 2em;">on direct influence of
+  environment, <a href='#Page_426'>426</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sesiidae, mimicry by, <a href=
+  '#Page_240'>240</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sex colour and nests of birds,
+  <a href='#Page_277'>277</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sex, colours characteristic of,
+  <a href='#Page_269'>269</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sexual colours of insects,
+  probable causes of, <a href='#Page_273'>273</a></span><br />
+  <span style="margin-left: 2em;">of birds, <a href=
+  '#Page_275'>275</a></span><br />
+  <span style="margin-left: 2em;">characters due to natural
+  selection, <a href='#Page_283'>283</a></span><br />
+  <span style="margin-left: 2em;">diversity the cause of variation,
+  <a href='#Page_439'>439</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sexual selection and colour,
+  <a href='#Page_274'>274</a></span><br />
+  <span style="margin-left: 2em;">by struggles of males, <a href=
+  '#Page_282'>282</a></span><br />
+  <span style="margin-left: 2em;">neutralised by natural selection,
+  <a href='#Page_294'>294</a>-<a href=
+  '#Page_296'>296</a></span><br />
+  <span style="margin-left: 2em;">restricted to male struggles,
+  <a href='#Page_296'>296</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Shetland Islands, variety of
+  ghost-moth in, <a href='#Page_270'>270</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Shrews and field-mice, internal
+  variations of, <a href='#Page_69'>69</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Shrikes, recognition marks of,
+  <a href='#Page_222'>222</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Similarity of forms of life not
+  due to similarity of conditions, <a href=
+  '#Page_339'>339</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Singing of male birds, use of,
+  <a href='#Page_284'>284</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Skull of wolf, diagram of
+  variations of, <a href='#Page_70'>70</a></span><br />
+  <span style="margin-left: 2em;">of Ursus labiatus, diagram of
+  variations of, <a href='#Page_72'>72</a></span><br />
+  <span style="margin-left: 2em;">of Sus cristatus, diagram of
+  variations of, <a href='#Page_73'>73</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Skunk an illustration of warning
+  colour, <a href='#Page_233'>233</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Slack, Baron von, on protective
+  markings of sloths, <a href='#Page_201'>201</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sloth, protective colour and
+  marking of, <a href='#Page_201'>201</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Snakes, mimicry of poisonous,
+  <a href='#Page_261'>261</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Snipe, tails of two species
+  (figs.), <a href='#Page_225'>225</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sounds and odours peculiar to
+  male,</span><br />
+  <span style="margin-left: 2em;">how useful, <a href=
+  '#Page_284'>284</a></span><br />
+  <span style="margin-left: 2em;">produced by peculiar feathers,
+  <a href='#Page_284'>284</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">South America, fossil and recent
+  mammals of, <a href='#Page_393'>393</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Species, definition of, <a href=
+  '#Page_1'>1</a>, <a href='#Page_2'>2</a></span><br />
+  <span style="margin-left: 2em;">origin of, <a href=
+  '#Page_2'>2</a>, <a href='#Page_6'>6</a></span><br />
+  <span style="margin-left: 2em;">Lyell on, <a href=
+  '#Page_4'>4</a></span><br />
+  <span style="margin-left: 2em;">Agassiz on, <a href=
+  '#Page_5'>5</a></span><br />
+  <span style="margin-left: 2em;">transmutation of, <a href=
+  '#Page_6'>6</a></span><br />
+  <span style="margin-left: 2em;">Geoffroy St. Hilaire on, <a href=
+  '#Page_6'>6</a></span><br />
+  <span style="margin-left: 2em;">Dean Herbert on, <a href=
+  '#Page_6'>6</a></span><br />
+  <span style="margin-left: 2em;">Professor Grant on, <a href=
+  '#Page_6'>6</a></span><br />
+  <span style="margin-left: 2em;">Von Buch on, <a href=
+  '#Page_6'>6</a></span><br />
+  <span style="margin-left: 2em;">allied, found in distinct areas,
+  <a href='#Page_36'>36</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Species, which vary little,
+  <a href='#Page_80'>80</a></span><br />
+  <a name="Page_492" id="Page_492"></a> <span style=
+  "margin-left: 2em;">closely allied inhabit distinct areas,
+  <a href='#Page_111'>111</a></span><br />
+  <span style="margin-left: 2em;">vigour and fertility of, how kept
+  up, <a href='#Page_327'>327</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Spencer, Mr. Herbert, on factors
+  of organic evolution, <a href='#Page_411'>411</a></span><br />
+  <span style="margin-left: 2em;">on effects of disuse, <a href=
+  '#Page_413'>413</a></span><br />
+  <span style="margin-left: 2em;">on difficulty as to co-adaptation
+  of parts, <a href='#Page_417'>417</a></span><br />
+  <span style="margin-left: 2em;">on direct action of environment,
+  <a href='#Page_418'>418</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sphingidae, protective attitudes
+  of larvae, <a href='#Page_210'>210</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sphinx ligustri, general
+  resemblance of larva to food plant, <a href=
+  '#Page_202'>202</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Spider, alluring coloration of,
+  <a href='#Page_211'>211</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Spines, on origin of, <a href=
+  '#Page_431'>431</a></span><br />
+  <span style="margin-left: 2em;">rarity of, in oceanic islands,
+  <a href='#Page_432'>432</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Spiny plants abundant in South
+  Africa and Chile, <a href='#Page_433'>433</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Spots a primitive ornamentation
+  of animals, <a href='#Page_289'>289</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sprengel on flowers and insects,
+  <a href='#Page_309'>309</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Staphylinidae, protective habit
+  of, <a href='#Page_210'>210</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sterility of mongrels, <a href=
+  '#Page_165'>165</a></span><br />
+  <span style="margin-left: 2em;">correlated with colour, etc.,
+  <a href='#Page_168'>168</a></span><br />
+  <span style="margin-left: 2em;">and natural selection, <a href=
+  '#Page_173'>173</a></span><br />
+  <span style="margin-left: 2em;">of hybrids produced by natural
+  selection, <a href='#Page_179'>179</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Struggle for existence, <a href=
+  '#Page_14'>14</a></span><br />
+  <span style="margin-left: 2em;">among plants, <a href=
+  '#Page_15'>15</a></span><br />
+  <span style="margin-left: 2em;">for life, illustrations of,
+  <a href='#Page_18'>18</a></span><br />
+  <span style="margin-left: 2em;">for existence on the pampas,
+  <a href='#Page_22'>22</a></span><br />
+  <span style="margin-left: 2em;">for life between closely allied
+  forms most severe, <a href='#Page_33'>33</a></span><br />
+  <span style="margin-left: 2em;">for existence, ethics of,
+  <a href='#Page_36'>36</a></span><br />
+  <span style="margin-left: 2em;">how it acts among flowers,
+  <a href='#Page_328'>328</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Summary of facts of colouring for
+  protection and recognition, <a href=
+  '#Page_227'>227</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Survival of the fittest, <a href=
+  '#Page_11'>11</a>, <a href='#Page_122'>122</a>, <a href=
+  '#Page_123'>123</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Swainson, definition of species,
+  <a href='#Page_2'>2</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Swamping effects of
+  intercrossing, <a href='#Page_142'>142</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Sweden, destruction of grass by
+  larvae of moths in, <a href='#Page_17'>17</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Swinhoe, Mr., on protective
+  colouring of a bat, <a href='#Page_201'>201</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Symmetry, bilateral in colours of
+  animals needful for recognition, <a href=
+  '#Page_217'>217</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>T</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tails used as respirators,
+  <a href='#Page_136'>136</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tapirs, distribution of, <a href=
+  '#Page_352'>352</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tegetmeier, Mr., on feeding
+  habits of pigeons and fowls, <a href=
+  '#Page_75'>75</a></span><br />
+  <span style="margin-left: 2em;">on sparrows and crocuses,
+  <a href='#Page_75'>75</a></span><br />
+  <span style="margin-left: 2em;">on curious correlations in
+  pigeons, <a href='#Page_140'>140</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tegumentary appendages and
+  colour, <a href='#Page_291'>291</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Thousand-fathom line divides
+  oceanic from continental islands, <a href=
+  '#Page_347'>347</a></span><br />
+  <span style="margin-left: 2em;">the teachings of, <a href=
+  '#Page_348'>348</a></span><br />
+  <span style="margin-left: 2em;">map showing, <a href=
+  '#Page_349'>349</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Thwaites, Mr., on spread of
+  Lantana mixta in Ceylon, <a href='#Page_30'>30</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tiger, use of stripes of,
+  <a href='#Page_199'>199</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Titmice as illustrating
+  divergence, <a href='#Page_107'>107</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Transformation of species of
+  crustacea, <a href='#Page_427'>427</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Transmutationists, the early,
+  <a href='#Page_3'>3</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Travers, Mr. W.L., on effects of
+  introduced plants in New Zealand, <a href=
+  '#Page_29'>29</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Trees, great variety of, in many
+  forests, <a href='#Page_36'>36</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Trimen, Mr., on butterfly
+  deceived by its mimic, <a href='#Page_245'>245</a></span><br />
+  <span style="margin-left: 2em;">on mimicry, <a href=
+  '#Page_247'>247</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tropical animals, why brilliantly
+  coloured, <a href='#Page_299'>299</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tropics, no proof of lower
+  temperature of, <a href='#Page_369'>369</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tropidorhynchi mimicked by
+  orioles, <a href='#Page_263'>263</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Trumpeter, <a href=
+  '#Page_93'>93</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tumblers, <a href=
+  '#Page_91'>91</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Turbits and owls, <a href=
+  '#Page_91'>91</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Tylor, Mr. A., on <i>Coloration
+  in Animals and Plants</i>, <a href=
+  '#Page_285'>285</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>U</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ungulates, origin of feet of,
+  <a href='#Page_423'>423</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Use and disuse, effects of,
+  overpowered by natural selection, <a href=
+  '#Page_435'>435</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Useless characters, <a href=
+  '#Page_131'>131</a></span><br />
+  <span style="margin-left: 2em;">not specific, <a href=
+  '#Page_132'>132</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Useless specific characters, no
+  proof of existence of, <a href='#Page_141'>141</a></span><br />
+  <a name="Page_493" id="Page_493"></a><br />
+  <span style="margin-left: 1em;">Utriculariae inhabiting
+  bromelias, <a href='#Page_118'>118</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>V</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Vanessa callirhoe, small variety
+  in Porto Santo, <a href='#Page_106'>106</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Variability of the lower animals,
+  <a href='#Page_42'>42</a></span><br />
+  <span style="margin-left: 2em;">of the Foraminifera, <a href=
+  '#Page_43'>43</a></span><br />
+  <span style="margin-left: 2em;">of sea-anemones, <a href=
+  '#Page_43'>43</a></span><br />
+  <span style="margin-left: 2em;">of land mollusca, <a href=
+  '#Page_43'>43</a></span><br />
+  <span style="margin-left: 2em;">of insects, <a href=
+  '#Page_44'>44</a></span><br />
+  <span style="margin-left: 2em;">of lizards, <a href=
+  '#Page_46'>46</a></span><br />
+  <span style="margin-left: 2em;">of birds, <a href=
+  '#Page_49'>49</a></span><br />
+  <span style="margin-left: 2em;">of primary wing-feathers,
+  <a href='#Page_51'>51</a></span><br />
+  <span style="margin-left: 2em;">of wings and tail, <a href=
+  '#Page_53'>53</a></span><br />
+  <span style="margin-left: 2em;">of Dolichonyx oryzivorus,
+  <a href='#Page_55'>55</a></span><br />
+  <span style="margin-left: 2em;">of Agelaeus phoeniceus, <a href=
+  '#Page_56'>56</a></span><br />
+  <span style="margin-left: 2em;">of Cardinalis virginianus,
+  <a href='#Page_58'>58</a></span><br />
+  <span style="margin-left: 2em;">of tarsus and toes, <a href=
+  '#Page_60'>60</a></span><br />
+  <span style="margin-left: 2em;">of birds in Leyden Museum,
+  <a href='#Page_61'>61</a></span><br />
+  <span style="margin-left: 2em;">of Sciurus carolinensis, <a href=
+  '#Page_67'>67</a></span><br />
+  <span style="margin-left: 2em;">of skulls of wolf, <a href=
+  '#Page_70'>70</a></span><br />
+  <span style="margin-left: 2em;">of skulls of a bear, <a href=
+  '#Page_72'>72</a></span><br />
+  <span style="margin-left: 2em;">of skulls of Sus cristatus,
+  <a href='#Page_73'>73</a></span><br />
+  <span style="margin-left: 2em;">of plants, <a href=
+  '#Page_76'>76</a></span><br />
+  <span style="margin-left: 2em;">of oaks, <a href=
+  '#Page_77'>77</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Variation, Lyell on, <a href=
+  '#Page_4'>4</a></span><br />
+  <span style="margin-left: 2em;">in internal organs, <a href=
+  '#Page_66'>66</a></span><br />
+  <span style="margin-left: 2em;">the facts of, <a href=
+  '#Page_83'>83</a></span><br />
+  <span style="margin-left: 2em;">proofs of generality of, <a href=
+  '#Page_85'>85</a></span><br />
+  <span style="margin-left: 2em;">of vegetables and fruits,
+  <a href='#Page_86'>86</a></span><br />
+  <span style="margin-left: 2em;">of apples and melons, <a href=
+  '#Page_87'>87</a></span><br />
+  <span style="margin-left: 2em;">under domestication accords with
+  that under nature, <a href='#Page_100'>100</a></span><br />
+  <span style="margin-left: 2em;">coincident not necessary,
+  <a href='#Page_127'>127</a></span><br />
+  <span style="margin-left: 2em;">and heredity, problems of,
+  <a href='#Page_410'>410</a></span><br />
+  <span style="margin-left: 2em;">Professor Geddes's theory of,
+  <a href='#Page_428'>428</a></span><br />
+  <span style="margin-left: 2em;">the cause of, <a href=
+  '#Page_439'>439</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Variations of flowers, <a href=
+  '#Page_88'>88</a></span><br />
+  <span style="margin-left: 2em;">of domestic animals, <a href=
+  '#Page_88'>88</a></span><br />
+  <span style="margin-left: 2em;">of domestic pigeons, <a href=
+  '#Page_89'>89</a></span><br />
+  <span style="margin-left: 2em;">conditions favourable to
+  production of, <a href='#Page_98'>98</a></span><br />
+  <span style="margin-left: 2em;">beneficial, <a href=
+  '#Page_143'>143</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Varieties, importance of,
+  <a href='#Page_41'>41</a></span><br />
+  <span style="margin-left: 2em;">of same species adapted to self
+  or to insect-fertilisation, <a href=
+  '#Page_330'>330</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Vegetables, variation of,
+  <a href='#Page_86'>86</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Vegetation and reproduction,
+  antagonism of, <a href='#Page_428'>428</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Vertebrata, mimicry among,
+  <a href='#Page_261'>261</a></span><br />
+  <span style="margin-left: 2em;">geological succession of,
+  <a href='#Page_405'>405</a></span><br />
+  <br />
+  <span style="margin-left: 1em;"><i>Vestiges of Creation</i>,
+  <a href='#Page_3'>3</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Viola odorata, <a href=
+  '#Page_2'>2</a></span><br />
+  <span style="margin-left: 2em;">canina, <a href=
+  '#Page_2'>2</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Violets, as illustrating species,
+  <a href='#Page_2'>2</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Von Buch on species, <a href=
+  '#Page_6'>6</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>W</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wallace, Dr. Alexander, on
+  absence of choice by female moths, <a href=
+  '#Page_275'>275</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Ward, Mr. Lester F., on
+  progressive development of plants, <a href=
+  '#Page_398'>398</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Warning coloration, <a href=
+  '#Page_232'>232</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Warning colours of marine
+  animals, <a href='#Page_265'>265</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wasps and bees, mimicry of,
+  <a href='#Page_258'>258</a></span><br />
+  <span style="margin-left: 2em;">poisonous with warning colours,
+  <a href='#Page_287'>287</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Water-cress, chokes rivers in New
+  Zealand, <a href='#Page_24'>24</a></span><br />
+  <span style="margin-left: 2em;">driven out by willows, <a href=
+  '#Page_24'>24</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Water-ouzels, probable origin of,
+  <a href='#Page_116'>116</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Weale, Mr. Mansel, on protective
+  colours of butterflies, <a href='#Page_206'>206</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Weeds of United States, <a href=
+  '#Page_15'>15</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Weir, Mr. Jenner, on deceptive
+  resemblance of a caterpillar to</span><br />
+  <span style="margin-left: 1em;">a twig, <a href=
+  '#Page_204'>204</a></span><br />
+  <span style="margin-left: 2em;">on inedibility of conspicuous
+  caterpillars, <a href='#Page_236'>236</a></span><br />
+  <span style="margin-left: 2em;">on birds disregarding inedible
+  larvae, <a href='#Page_254'>254</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Weismann on progressive
+  adaptation of colours of larva, <a href=
+  '#Page_206'>206</a></span><br />
+  <span style="margin-left: 2em;">on non-heredity of acquired
+  characters, <a href='#Page_440'>440</a></span><br />
+  <span style="margin-left: 2em;">and Galton's theories of heredity
+  almost identical, 443 (note)</span><br />
+  <span style="margin-left: 2em;">on origin of the mathematical
+  faculty, (<a href="#Footnote_236_236">note</a>)</span><br />
+  <br />
+  <span style="margin-left: 1em;">Weismann's theory of heredity,
+  <a href='#Page_437'>437</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Westwood, Professor, on variation
+  of insects, <a href='#Page_44'>44</a></span><br />
+  <span style="margin-left: 2em;">deceived by a mimicking cricket,
+  <a href='#Page_259'>259</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">White coloration of insular birds
+  and butterflies, <a href='#Page_230'>230</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Whymper, Mr., his sensations when
+  falling on the Matterhorn, <a href='#Page_38'>38</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Willows, species and varieties of
+  British, <a href='#Page_77'>77</a></span><br />
+  <a name="Page_494" id="Page_494"></a><br />
+  <span style="margin-left: 1em;">Wilson, Alexander, his account of
+  the passenger-pigeon in North</span><br />
+  <span style="margin-left: 2em;">America, <a href=
+  '#Page_31'>31</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wind-carriage of seeds explains
+  many facts of plant distribution, <a href=
+  '#Page_371'>371</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wind-dispersal of seeds,
+  objections to, <a href='#Page_365'>365</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wind-fertilised degraded from
+  insect-fertilised flowers, <a href=
+  '#Page_324'>324</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wings of stone-curlews (figure),
+  <a href='#Page_223'>223</a></span><br />
+  <span style="margin-left: 2em;">why small but useless are
+  retained, <a href='#Page_416'>416</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wit and humour, origin of
+  faculties of, <a href='#Page_472'>472</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wollaston, Mr. T.W., on variation
+  of beetles, <a href='#Page_44'>44</a></span><br />
+  <span style="margin-left: 1em;">on small butterfly in Porto
+  Santo, <a href='#Page_106'>106</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wolves, varieties of in Catskill
+  Mountains, <a href='#Page_105'>105</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Wood, Mr. J., on muscular
+  variations, <a href='#Page_447'>447</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Mr. T.W., on variable colouring
+  of pupae of cabbage butterflies,</span><br />
+  <span style="margin-left: 2em;"><a href=
+  '#Page_197'>197</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Woodward, Dr. S.P., on variation
+  of mollusca, <a href='#Page_43'>43</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>Y</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Youatt, on breeds of sheep,
+  <a href='#Page_97'>97</a></span><br />
+  <br />
+  <span style="margin-left: 1em;">Young animals often spotted,
+  <a href='#Page_289'>289</a></span><br />
+  <br />
+  <br />
+  <span style="margin-left: 1em;"><b>Z</b></span><br />
+  <br />
+  <span style="margin-left: 1em;">Zebra, markings for recognition
+  and protection, (<a href=
+  "#Footnote_85_85">note</a>)</span><br /></p>
+
+  <h3>THE END</h3>
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of Project Gutenberg's Darwinism (1889), by Alfred Russel Wallace
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@@ -0,0 +1,19888 @@
+The Project Gutenberg EBook of Darwinism (1889), by Alfred Russel Wallace
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Darwinism (1889)
+
+Author: Alfred Russel Wallace
+
+Release Date: January 2, 2005 [EBook #14558]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK DARWINISM (1889) ***
+
+
+
+
+Produced by StevenGibbs and the Online Distributed Proofreading Team
+
+
+
+
+
+    DARWINISM
+
+    AN EXPOSITION OF THE
+
+    THEORY OF NATURAL SELECTION
+
+    WITH SOME OF ITS APPLICATIONS
+
+    BY
+
+    ALFRED RUSSEL WALLACE
+
+    LL.D., F.L.S., ETC.
+
+
+    WITH A PORTRAIT OF THE AUTHOR, MAP AND ILLUSTRATIONS
+
+    MACMILLAN AND CO.
+    LONDON AND NEW YORK
+    [Second Edition] 1889
+
+
+       *       *       *       *       *
+
+[Illustration: Alfred R. Wallace]
+
+       *       *       *       *       *
+
+
+
+
+
+
+PREFACE TO SECOND EDITION
+
+
+The present edition is a reprint of the first, with a few verbal
+corrections and the alteration of some erroneous or doubtful statements.
+Of these latter the following are the most important:--
+
+_P._ 30. The statement as to the fulmar petrel, which Professor A.
+Newton assures me is erroneous, has been modified.
+
+_P._ 34. A note is added as to Darwin's statement about the missel and
+song-thrushes in Scotland.
+
+_P._ 172. An error as to the differently-coloured herds of cattle in the
+Falkland Islands, is corrected.
+
+
+    PARKSTONE, DORSET
+    _August, 1889_.
+
+
+
+
+
+
+
+PREFACE TO FIRST EDITION
+
+
+The present work treats the problem of the Origin of Species on the same
+general lines as were adopted by Darwin; but from the standpoint reached
+after nearly thirty years of discussion, with an abundance of new facts
+and the advocacy of many new or old theories.
+
+While not attempting to deal, even in outline, with the vast subject of
+evolution in general, an endeavour has been made to give such an account
+of the theory of Natural Selection as may enable any intelligent reader
+to obtain a clear conception of Darwin's work, and to understand
+something of the power and range of his great principle.
+
+Darwin wrote for a generation which had not accepted evolution, and
+which poured contempt on those who upheld the derivation of species from
+species by any natural law of descent. He did his work so well that
+"descent with modification" is now universally accepted as the order of
+nature in the organic world; and the rising generation of naturalists
+can hardly realise the novelty of this idea, or that their fathers
+considered it a scientific heresy to be condemned rather than seriously
+discussed.
+
+The objections now made to Darwin's theory apply, solely, to the
+particular means by which the change of species has been brought about,
+not to the fact of that change. The objectors seek to minimise the
+agency of natural selection and to subordinate it to laws of variation,
+of use and disuse, of intelligence, and of heredity. These views and
+objections are urged with much force and more confidence, and for the
+most part by the modern school of laboratory naturalists, to whom the
+peculiarities and distinctions of species, as such, their distribution
+and their affinities, have little interest as compared with the problems
+of histology and embryology, of physiology and morphology. Their work in
+these departments is of the greatest interest and of the highest
+importance, but it is not the kind of work which, by itself, enables one
+to form a sound judgment on the questions involved in the action of the
+law of natural selection. These rest mainly on the external and vital
+relations of species to species in a state of nature--on what has been
+well termed by Semper the "physiology of organisms," rather than on the
+anatomy or physiology of organs.
+
+       *       *       *       *       *
+
+It has always been considered a weakness in Darwin's work that he based
+his theory, primarily, on the evidence of variation in domesticated
+animals and cultivated plants. I have endeavoured to secure a firm
+foundation for the theory in the variations of organisms in a state of
+nature; and as the exact amount and precise character of these
+variations is of paramount importance in the numerous problems that
+arise when we apply the theory to explain the facts of nature, I have
+endeavoured, by means of a series of diagrams, to exhibit to the eye the
+actual variations as they are found to exist in a sufficient number of
+species. By doing this, not only does the reader obtain a better and
+more precise idea of variation than can be given by any number of
+tabular statements or cases of extreme individual variation, but we
+obtain a basis of fact by which to test the statements and objections
+usually put forth on the subject of specific variability; and it will be
+found that, throughout the work, I have frequently to appeal to these
+diagrams and the facts they illustrate, just as Darwin was accustomed to
+appeal to the facts of variation among dogs and pigeons.
+
+I have also made what appears to me an important change in the
+arrangement of the subject. Instead of treating first the comparatively
+difficult and unfamiliar details of variation, I commence with the
+Struggle for Existence, which is really the fundamental phenomenon on
+which natural selection depends, while the particular facts which
+illustrate it are comparatively familiar and very interesting. It has
+the further advantage that, after discussing variation and the effects
+of artificial selection, we proceed at once to explain how natural
+selection acts.
+
+Among the subjects of novelty or interest discussed in this volume, and
+which have important bearings on the theory of natural selection, are:
+(1) A proof that all _specific_ characters are (or once have been)
+either useful in themselves or correlated with useful characters (Chap.
+VI); (2) a proof that natural selection can, in certain cases, increase
+the sterility of crosses (Chap. VII); (3) a fuller discussion of the
+colour relations of animals, with additional facts and arguments on the
+origin of sexual differences of colour (Chaps. VIII-X); (4) an attempted
+solution of the difficulty presented by the occurrence of both very
+simple and very complex modes of securing the cross-fertilisation of
+plants (Chap. XI); (5) some fresh facts and arguments on the
+wind-carriage of seeds, and its bearing on the wide dispersal of many
+arctic and alpine plants (Chap. XII); (6) some new illustrations of the
+non-heredity of acquired characters, and a proof that the effects of use
+and disuse, even if inherited, must be overpowered by natural selection
+(Chap. XIV); and (7) a new argument as to the nature and origin of the
+moral and intellectual faculties of man (Chap. XV).
+
+       *       *       *       *       *
+
+Although I maintain, and even enforce, my differences from some of
+Darwin's views, my whole work tends forcibly to illustrate the
+overwhelming importance of Natural Selection over all other agencies in
+the production of new species. I thus take up Darwin's earlier
+position, from which he somewhat receded in the later editions of his
+works, on account of criticisms and objections which I have endeavoured
+to show are unsound. Even in rejecting that phase of sexual selection
+depending on female choice, I insist on the greater efficacy of natural
+selection. This is pre-eminently the Darwinian doctrine, and I therefore
+claim for my book the position of being the advocate of pure Darwinism.
+
+I wish to express my obligation to Mr. Francis Darwin for lending me
+some of his father's unused notes, and to many other friends for facts
+or information, which have, I believe, been acknowledged either in the
+text or footnotes. Mr. James Sime has kindly read over the proofs and
+given me many useful suggestions; and I have to thank Professor Meldola,
+Mr. Hemsley, and Mr. E.B. Poulton for valuable notes or corrections in
+the later chapters in which their special subjects are touched upon.
+
+GODALMING, _March 1889_.
+
+
+
+
+CONTENTS
+
+
+CHAPTER I
+
+WHAT ARE "SPECIES" AND WHAT IS MEANT BY THEIR "ORIGIN"
+
+
+    Definition of species--Special creation--The early
+    transmutationists--Scientific opinion before Darwin--The problem
+    before Darwin--The change of opinion effected by Darwin--The
+    Darwinian theory--Proposed mode of treatment of the subject
+
+
+
+CHAPTER II
+
+THE STRUGGLE FOR EXISTENCE
+
+
+    Its importance--The struggle among plants--Among
+    animals--Illustrative cases--Succession of trees in forests of
+    Denmark--The struggle for existence on the Pampas--Increase of
+    organisms in a geometrical ratio--Examples of rapid increase of
+    animals--Rapid increase and wide spread of plants--Great
+    fertility not essential to rapid increase--Struggle between
+    closely allied species most severe--The ethical aspect of the
+    struggle for existence
+
+
+
+CHAPTER III
+
+THE VARIABILITY OF SPECIES IN A STATE OF NATURE
+
+
+   Importance of variability--Popular ideas regarding
+   it--Variability of the lower animals--The variability of
+   insects--Variation among lizards--Variation among
+   birds--Diagrams of bird-variation--Number of varying
+   individuals--Variation in the mammalia--Variation in internal
+   organs--Variations in the skull--Variations in the habits of
+   animals--The variability of plants--Species which vary
+   little--Concluding remarks
+
+
+
+CHAPTER IV
+
+VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS
+
+
+    The facts of variation and artificial selection--Proofs of the
+    generality of variation--Variations of apples and
+    melons--Variations of flowers--Variations of domestic
+    animals--Domestic pigeons--Acclimatisation--Circumstances
+    favourable to selection by man--Conditions favourable to
+    variation--Concluding remarks
+
+
+
+CHAPTER V
+
+NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST
+
+
+    Effect of struggle for existence under unchanged conditions--The
+    effect under change of conditions--Divergence of character--In
+    insects--In birds--In mammalia--Divergence leads to a maximum of
+    life in each area--Closely allied species inhabit distinct
+    areas--Adaptation to conditions at various periods of life--The
+    continued existence of low forms of life--Extinction of low
+    types among the higher animals--Circumstances favourable to the
+    origin of new species--Probable origin of the dippers--The
+    importance of isolation--On the advance of organisation by
+    natural selection--Summary of the first five chapters
+
+
+
+CHAPTER VI
+
+DIFFICULTIES AND OBJECTIONS
+
+
+    Difficulty as to smallness of variations--As to the right
+    variations occurring when required--The beginnings of important
+    organs--The mammary glands--The eyes of flatfish--Origin of the
+    eye--Useless or non-adaptive characters--Recent extension of the
+    region of utility in plants--The same in animals--Uses of
+    tails--Of the horns of deer--Of the scale-ornamentation of
+    reptiles--Instability of non-adaptive characters--Delboeuf's
+    law--No "specific" character proved to be useless--The swamping
+    effects of intercrossing--Isolation as preventing
+    intercrossing--Gulick on the effects of isolation--Cases in
+    which isolation is ineffective
+
+
+
+CHAPTER VII
+
+ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
+STERILITY OF THEIR HYBRID OFFSPRING
+
+
+    Statement of the problem--Extreme susceptibility of the
+    reproductive functions--Reciprocal crosses--Individual
+    differences in respect to cross-fertilisation--Dimorphism and
+    trimorphism among plants--Cases of the fertility of hybrids and
+    of the infertility of mongrels--The effects of close
+    interbreeding--Mr. Huth's objections--Fertile hybrids among
+    animals--Fertility of hybrids among plants--Cases of sterility
+    of mongrels--Parallelism between crossing and change of
+    conditions--Remarks on the facts of hybridity--Sterility due to
+    changed conditions and usually correlated with other
+    characters--Correlation of colour with constitutional
+    peculiarities--The isolation of varieties by selective
+    association--The influence of natural selection upon sterility
+    and fertility--Physiological selection--Summary and concluding
+    remarks
+
+
+
+CHAPTER VIII
+
+THE ORIGIN AND USES OF COLOUR IN ANIMALS
+
+
+    The Darwinian theory threw new light on organic colour--The
+    problem to be solved--The constancy of animal colour indicates
+    utility--Colour and environment--Arctic animals
+    white--Exceptions prove the rule--Desert, forest, nocturnal, and
+    oceanic animals--General theories of animal colour--Variable
+    protective colouring--Mr. Poulton's experiments--Special or
+    local colour adaptations--Imitation of particular objects--How
+    they have been produced--Special protective colouring of
+    butterflies--Protective resemblance among marine
+    animals--Protection by terrifying enemies--Alluring
+    coloration--The coloration of birds' eggs--Colour as a means of
+    recognition--Summary of the preceding exposition--Influence of
+    locality or of climate on colour--Concluding remarks
+
+
+
+CHAPTER IX
+
+WARNING COLORATION AND MIMICRY
+
+
+    The skunk as an example of warning coloration--Warning colours
+    among insects--Butterflies--Caterpillars--Mimicry--How mimicry
+    has been produced--Heliconidae--Perfection of the
+    imitation--Other cases of mimicry among Lepidoptera--Mimicry
+    among protected groups--Its explanation--Extension of the
+    principle--Mimicry in other orders of insects--Mimicry among the
+    vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among
+    birds--Objections to the theory of mimicry--Concluding remarks
+    on warning colours and mimicry
+
+
+
+CHAPTER X
+
+COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
+
+
+    Sex colours in the mollusca and crustacea--In insects--In
+    butterflies and moths--Probable causes of these colours--Sexual
+    selection as a supposed cause--Sexual coloration of birds--Cause
+    of dull colours of female birds--Relation of sex colour to
+    nesting habits--Sexual colours of other vertebrates--Sexual
+    selection by the struggles of males--Sexual characters due to
+    natural selection--Decorative plumage of males and its effect on
+    the females--Display of decorative plumage by the males--A
+    theory of animal coloration--The origin of accessory
+    plumes--Development of accessory plumes and their display--The
+    effect of female preference will be neutralised by natural
+    selection--General laws of animal coloration--Concluding remarks
+
+
+
+CHAPTER XI
+
+THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE
+
+
+    The general colour relations of plants--Colours of fruits--The
+    meaning of nuts--Edible or attractive fruits--The colours of
+    flowers--Modes of securing cross-fertilisation--The
+    interpretation of the facts--Summary of additional facts
+    bearing on insect fertilisation--Fertilisation of flowers by
+    birds--Self-fertilisation of flowers--Difficulties and
+    contradictions--Intercrossing not necessarily
+    advantageous--Supposed evil results of close interbreeding--How
+    the struggle for existence acts among flowers--Flowers the
+    product of insect agency--Concluding remarks on colour in nature
+
+
+
+CHAPTER XII
+
+THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
+
+
+    The facts to be explained--The conditions which have determined
+    distribution--The permanence of oceans--Oceanic and continental
+    areas--Madagascar and New Zealand--The teachings of the
+    thousand-fathom line--The distribution of marsupials--The
+    distribution of tapirs--Powers of dispersal as illustrated by
+    insular organisms--Birds and insects at sea--Insects at great
+    altitudes--The dispersal of plants--Dispersal of seeds by the
+    wind--Mineral matter carried by the wind--Objections to the
+    theory of wind-dispersal answered--Explanation of north
+    temperate plants in the southern hemisphere--No proof of
+    glaciation in the tropics--Lower temperature not needed to
+    explain the facts--Concluding remarks
+
+
+
+CHAPTER XIII
+
+THE GEOLOGICAL EVIDENCES OF EVOLUTION
+
+
+    What we may expect--The number of known species of extinct
+    animals--Causes of the imperfection of the geological
+    record--Geological evidences of
+    evolution--Shells--Crocodiles--The rhinoceros tribe--The
+    pedigree of the horse tribe--Development of deer's horns--Brain
+    development--Local relations of fossil and living animals--Cause
+    of extinction of large animals--Indications of general progress
+    in plants and animals--The progressive development of
+    plants--Possible cause of sudden late appearance of
+    exogens--Geological distribution of insects--Geological
+    succession of vertebrata--Concluding remarks
+
+
+
+CHAPTER XIV
+
+FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
+
+
+    Fundamental difficulties and objections--Mr. Herbert Spencer's
+    factors of organic evolution--Disuse and effects of withdrawal
+    of natural selection--Supposed effects of disuse among wild
+    animals--Difficulty as to co-adaptation of parts by variation
+    and selection--Direct action of the environment--The American
+    school of evolutionists--Origin of the feet of the
+    ungulates--Supposed action of animal intelligence--Semper on the
+    direct influence of the environment--Professor Geddes's theory
+    of variation in plants--Objections to the theory--On the origin
+    of spines--Variation and selection overpower the effects of use
+    and disuse--Supposed action of the environment in imitating
+    variations--Weismann's theory of heredity--The cause of
+    variation--The non-heredity of acquired characters--The theory
+    of instinct--Concluding remarks
+
+
+
+CHAPTER XV
+
+DARWINISM APPLIED TO MAN
+
+
+    General identity of human and animal structure--Rudiments and
+    variations showing relation of man to other mammals--The
+    embryonic development of man and other mammalia--Diseases common
+    to man and the lower animals--The animals most nearly allied to
+    man--The brains of man and apes--External differences of man and
+    apes--Summary of the animal characteristics of man--The
+    geological antiquity of man--The probable birthplace of man--The
+    origin of the moral and intellectual nature of man--The argument
+    from continuity--The origin of the mathematical faculty--The
+    origin of the musical and artistic faculties--Independent proof
+    that these faculties have not been developed by natural
+    selection--The interpretation of the facts--Concluding remarks
+
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+
+    PORTRAIT OF AUTHOR
+    MAP SHOWING THE 1000-FATHOM LINE
+    1.  DIAGRAM OF VARIATIONS OF LACERTA MURALIS
+    2.     "       VARIATION OF LIZARDS
+    3.     "       VARIATION OF WINGS AND TAIL OF BIRDS
+    4.     "       VARIATION OF DOLICHONYX ORYZIVORUS
+    5.     "       VARIATION OF AGELAEUS PHOENICEUS
+    6.     "       VARIATION OF CARDINALIS VIRGINIANUS
+    7.     "       VARIATION OF TARSUS AND TOES
+    8.     "       VARIATION OF BIRDS IN LEYDEN MUSEUM
+    9.     "       VARIATION OF ICTERUS BALTIMORE
+    10.    "       VARIATION OF AGELAEUS PHOENICEUS
+    11.    "       CURVES OF VARIATION
+    12.    "       VARIATION OF CARDINALIS VIRGINIANUS
+    13.    "       VARIATION OF SCIURUS CAROLINENSIS
+    14.    "       VARIATION OF SKULLS OF WOLF
+    15.    "       VARIATION OF SKULLS OF URSUS LABIATUS
+    16.    "       VARIATION OF SKULLS OF SUS CRISTATUS
+    17. PRIMULA VERIS (Cowslip). From Darwin's _Forms of Flowers_
+    18. GAZELLA SOEMMERRINGI (to show recognition marks)
+    19. RECOGNITION MARKS OF AFRICAN PLOVERS
+          (from Seebohm's _Charadriadae_)
+    20. RECOGNITION OF OEDICNEMUS VERMICULATUS AND OE. SENEGALENSIS
+          (from Seebohm's _Charadriadae_)
+    21. RECOGNITION OF CURSORIUS CHALCOPTERUS AND C. GALLICUS
+          (from Seebohm's _Charadriadae_)
+    22. RECOGNITION OF SCOLOPAX MEGALA AND S. STENURA
+          (from Seebohm's _Charadriadae_)
+    23. METHONA PSIDII AND LEPTALIS ORISE
+    24. OPTHALMIS LINCEA AND ARTAXA SIMULANS
+          (from the Official _Narrative of the Voyage of the Challenger_)
+    25. WINGS OF ITUNA ILIONE AND THYRIDIA MEGISTO
+          (from _Proceedings of the Entomological Society_)
+    26. MYGNIMIA AVICULUS AND COLOBORHOMBUS FASCIATIPENNIS
+    27. MIMICKING INSECTS FROM THE PHILIPPINES
+          (from Semper's _Animal Life_)
+    28. MALVA SYLVESTRIS AND M. ROTUNDIFOLIA
+          (from Lubbock's _British Wild Flowers in Relation to Insects_)
+    29. LYTHRUM SALICARIA, THREE FORMS OF
+          (from Lubbock's _British Wild Flowers in Relation to Insects_)
+    30. ORCHIS PYRAMIDALIS (from Darwin's _Fertilisation of Orchids_)
+    31. HUMMING-BIRD FERTILISING MARCGRAVIA NEPENTHOIDES
+    32. DIAGRAM OF MEAN HEIGHT OF LAND AND DEPTH OF OCEANS
+    33. GEOLOGICAL DEVELOPMENT OF THE HORSE TRIBE
+          (from Huxley's _American Addresses_)
+    34. DIAGRAM ILLUSTRATING THE GEOLOGICAL DISTRIBUTION OF PLANTS
+          (from Ward's _Sketch of Palaeobotany_)
+    35. TRANSFORMATION OF ARTEMIA SALINA TO A. MILHAUSENII
+          (from Semper's _Animal Life_)
+    36. BRANCHIPUS STAGNALIS AND ARTEMIA SALINA
+          (from Semper's _Animal Life_)
+    37. CHIMPANZEE (TROGLODYTES NIGER)
+
+
+
+
+
+CHAPTER I
+
+WHAT ARE "SPECIES," AND WHAT IS MEANT BY THEIR "ORIGIN"
+
+
+    Definition of species--Special creation--The early
+    Transmutationists--Scientific opinion before Darwin--The problem
+    before Darwin--The change of opinion effected by Darwin--The
+    Darwinian theory--Proposed mode of treatment of the subject.
+
+
+
+The title of Mr. Darwin's great work is--_On the Origin of Species by
+means of Natural Selection and the Preservation of Favoured Races in the
+Struggle for Life_. In order to appreciate fully the aim and object of
+this work, and the change which it has effected not only in natural
+history but in many other sciences, it is necessary to form a clear
+conception of the meaning of the term "species," to know what was the
+general belief regarding them at the time when Mr. Darwin's book first
+appeared, and to understand what he meant, and what was generally meant,
+by discovering their "origin." It is for want of this preliminary
+knowledge that the majority of educated persons who are not naturalists
+are so ready to accept the innumerable objections, criticisms, and
+difficulties of its opponents as proofs that the Darwinian theory is
+unsound, while it also renders them unable to appreciate, or even to
+comprehend, the vast change which that theory has effected in the whole
+mass of thought and opinion on the great question of evolution.
+
+The term "species" was thus defined by the celebrated botanist De
+Candolle: "A species is a collection of all the individuals which
+resemble each other more than they resemble anything else, which can by
+mutual fecundation produce fertile individuals, and which reproduce
+themselves by generation, in such a manner that we may from analogy
+suppose them all to have sprung from one single individual." And the
+zoologist Swainson gives a somewhat similar definition: "A species, in
+the usual acceptation of the term, is an animal which, in a state of
+nature, is distinguished by certain peculiarities of form, size, colour,
+or other circumstances, from another animal. It propagates, 'after its
+kind,' individuals perfectly resembling the parent; its peculiarities,
+therefore, are permanent."[1]
+
+To illustrate these definitions we will take two common English birds,
+the rook (Corvus frugilegus) and the crow (Corvus corone). These are
+distinct _species_, because, in the first place, they always differ from
+each other in certain slight peculiarities of structure, form, and
+habits, and, in the second place, because rooks always produce rooks,
+and crows produce crows, and they do not interbreed. It was therefore
+concluded that all the rooks in the world had descended from a single
+pair of rooks, and the crows in like manner from a single pair of crows,
+while it was considered impossible that crows could have descended from
+rooks or _vice versa_. The "origin" of the first pair of each kind was a
+mystery. Similar remarks may be applied to our two common plants, the
+sweet violet (Viola odorata) and the dog violet (Viola canina). These
+also produce their like and never produce each other or intermingle, and
+they were therefore each supposed to have sprung from a single
+individual whose "origin" was unknown. But besides the crow and the rook
+there are about thirty other kinds of birds in various parts of the
+world, all so much like our species that they receive the common name of
+crows; and some of them differ less from each other than does our crow
+from our rook. These are all _species_ of the genus Corvus, and were
+therefore believed to have been always as distinct as they are now,
+neither more nor less, and to have each descended from one pair of
+ancestral crows of the same identical species, which themselves had an
+unknown "origin." Of violets there are more than a hundred different
+kinds in various parts of the world, all differing very slightly from
+each other and forming distinct _species_ of the genus Viola. But, as
+these also each produce their like and do not intermingle, it was
+believed that every one of them had always been as distinct from all the
+others as it is now, that all the individuals of each kind had descended
+from one ancestor, but that the "origin" of these hundred slightly
+differing ancestors was unknown. In the words of Sir John Herschel,
+quoted by Mr. Darwin, the origin of such species was "the mystery of
+mysteries."
+
+
+_The Early Transmutationists_.
+
+A few great naturalists, struck by the very slight difference between
+many of these species, and the numerous links that exist between the
+most different forms of animals and plants, and also observing that a
+great many species do vary considerably in their forms, colours, and
+habits, conceived the idea that they might be all produced one from the
+other. The most eminent of these writers was a great French naturalist,
+Lamarck, who published an elaborate work, the _Philosophie Zoologique_,
+in which he endeavoured to prove that all animals whatever are descended
+from other species of animals. He attributed the change of species
+chiefly to the effect of changes in the conditions of life--such as
+climate, food, etc.--and especially to the desires and efforts of the
+animals themselves to improve their condition, leading to a modification
+of form or size in certain parts, owing to the well-known physiological
+law that all organs are strengthened by constant use, while they are
+weakened or even completely lost by disuse. The arguments of Lamarck did
+not, however, satisfy naturalists, and though a few adopted the view
+that closely allied species had descended from each other, the general
+belief of the educated public was, that each species was a "special
+creation" quite independent of all others; while the great body of
+naturalists equally held, that the change from one species to another by
+any known law or cause was impossible, and that the "origin of species"
+was an unsolved and probably insoluble problem. The only other important
+work dealing with the question was the celebrated _Vestiges of
+Creation_, published anonymously, but now acknowledged to have been
+written by the late Robert Chambers. In this work the action of general
+laws was traced throughout the universe as a system of growth and
+development, and it was argued that the various species of animals and
+plants had been produced in orderly succession from each other by the
+action of unknown laws of development aided by the action of external
+conditions. Although this work had a considerable effect in influencing
+public opinion as to the extreme improbability of the doctrine of the
+independent "special creation" of each species, it had little effect
+upon naturalists, because it made no attempt to grapple with the problem
+in detail, or to show in any single case how the allied species of a
+genus could have arisen, and have preserved their numerous slight and
+apparently purposeless differences from each other. No clue whatever was
+afforded to a law which should produce from any one species one or more
+slightly differing but yet permanently distinct species, nor was any
+reason given why such slight yet constant differences should exist at
+all.
+
+
+_Scientific Opinion before Darwin._
+
+In order to show how little effect these writers had upon the public
+mind, I will quote a few passages from the writings of Sir Charles
+Lyell, as representing the opinions of the most advanced thinkers in the
+period immediately preceding that of Darwin's work. When recapitulating
+the facts and arguments in favour of the invariability and permanence of
+species, he says: "The entire variation from the original type which any
+given kind of change can produce may usually be effected in a brief
+period of time, after which no further deviation can be obtained by
+continuing to alter the circumstances, though ever so gradually,
+indefinite divergence either in the way of improvement or deterioration
+being prevented, and the least possible excess beyond the defined limits
+being fatal to the existence of the individual." In another place he
+maintains that "varieties of some species may differ more than other
+species do from each other without shaking our confidence in the reality
+of species." He further adduces certain facts in geology as being, in
+his opinion, "fatal to the theory of progressive development," and he
+explains the fact that there are so often distinct species in countries
+of similar climate and vegetation by "special creations" in each
+country; and these conclusions were arrived at after a careful study of
+Lamarck's work, a full abstract of which is given in the earlier
+editions of the _Principles of Geology_.[2]
+
+Professor Agassiz, one of the greatest naturalists of the last
+generation, went even further, and maintained not only that each species
+was specially created, but that it was created in the proportions and in
+the localities in which we now find it to exist. The following extract
+from his very instructive book on Lake Superior explains this view:
+"There are in animals peculiar adaptations which are characteristic of
+their species, and which cannot be supposed to have arisen from
+subordinate influences. Those which live in shoals cannot be supposed to
+have been created in single pairs. Those which are made to be the food
+of others cannot have been created in the same proportions as those
+which live upon them. Those which are everywhere found in innumerable
+specimens must have been introduced in numbers capable of maintaining
+their normal proportions to those which live isolated and are
+comparatively and constantly fewer. For we know that this harmony in the
+numerical proportions between animals is one of the great laws of
+nature. The circumstance that species occur within definite limits where
+no obstacles prevent their wider distribution leads to the further
+inference that these limits were assigned to them from the beginning,
+and so we should come to the final conclusion that the order which
+prevails throughout nature is intentional, that it is regulated by the
+limits marked out on the first day of creation, and that it has been
+maintained unchanged through ages with no other modifications than those
+which the higher intellectual powers of man enable him to impose on some
+few animals more closely connected with him."[3]
+
+These opinions of some of the most eminent and influential writers of
+the pre-Darwinian age seem to us, now, either altogether obsolete or
+positively absurd; but they nevertheless exhibit the mental condition of
+even the most advanced section of scientific men on the problem of the
+nature and origin of species. They render it clear that,
+notwithstanding the vast knowledge and ingenious reasoning of Lamarck,
+and the more general exposition of the subject by the author of the
+_Vestiges of Creation_, the first step had not been taken towards a
+satisfactory explanation of the derivation of any one species from any
+other. Such eminent naturalists as Geoffroy Saint Hilaire, Dean Herbert,
+Professor Grant, Von Buch, and some others, had expressed their belief
+that species arose as simple varieties, and that the species of each
+genus were all descended from a common ancestor; but none of them gave a
+clue as to the law or the method by which the change had been effected.
+This was still "the great mystery." As to the further question--how far
+this common descent could be carried; whether distinct families, such as
+crows and thrushes, could possibly have descended from each other; or,
+whether all birds, including such widely distinct types as wrens,
+eagles, ostriches, and ducks, could all be the modified descendants of a
+common ancestor; or, still further, whether mammalia, birds, reptiles,
+and fishes, could all have had a common origin;--these questions had
+hardly come up for discussion at all, for it was felt that, while the
+very first step along the road of "transmutation of species" (as it was
+then called) had not been made, it was quite useless to speculate as to
+how far it might be possible to travel in the same direction, or where
+the road would ultimately lead to.
+
+
+_The Problem before Darwin_.
+
+It is clear, then, that what was understood by the "origin" or the
+"transmutation" of species before Darwin's work appeared, was the
+comparatively simple question whether the allied species of each genus
+had or had not been derived from one another and, remotely, from some
+common ancestor, by the ordinary method of reproduction and by means of
+laws and conditions still in action and capable of being thoroughly
+investigated. If any naturalist had been asked at that day whether,
+supposing it to be clearly shown that all the different species of each
+genus had been derived from some one ancestral species, and that a full
+and complete explanation were to be given of how each minute difference
+in form, colour, or structure might have originated, and how the
+several peculiarities of habit and of geographical distribution might
+have been brought about--whether, if this were done, the "origin of
+species" would be discovered, the great mystery solved, he would
+undoubtedly have replied in the affirmative. He would probably have
+added that he never expected any such marvellous discovery to be made in
+his lifetime. But so much as this assuredly Mr. Darwin has done, not
+only in the opinion of his disciples and admirers, but by the admissions
+of those who doubt the completeness of his explanations. For almost all
+their objections and difficulties apply to those larger differences
+which separate genera, families, and orders from each other, not to
+those which separate one species from the species to which it is most
+nearly allied, and from the remaining species of the same genus. They
+adduce such difficulties as the first development of the eye, or of the
+milk-producing glands of the mammalia; the wonderful instincts of bees
+and of ants; the complex arrangements for the fertilisation of orchids,
+and numerous other points of structure or habit, as not being
+satisfactorily explained. But it is evident that these peculiarities had
+their origin at a very remote period of the earth's history, and no
+theory, however complete, can do more than afford a probable conjecture
+as to how they were produced. Our ignorance of the state of the earth's
+surface and of the conditions of life at those remote periods is very
+great; thousands of animals and plants must have existed of which we
+have no record; while we are usually without any information as to the
+habits and general life-history even of those of which we possess some
+fragmentary remains; so that the truest and most complete theory would
+not enable us to solve _all_ the difficult problems which the whole
+course of the development of life upon our globe presents to us.
+
+What we may expect a true theory to do is to enable us to comprehend and
+follow out in some detail those changes in the form, structure, and
+relations of animals and plants which are effected in short periods of
+time, geologically speaking, and which are now going on around us. We
+may expect it to explain satisfactorily most of the lesser and
+superficial differences which distinguish one species from another. We
+may expect it to throw light on the mutual relations of the animals and
+plants which live together in any one country, and to give some rational
+account of the phenomena presented by their distribution in different
+parts of the world. And, lastly, we may expect it to explain many
+difficulties and to harmonise many incongruities in the excessively
+complex affinities and relations of living things. All this the
+Darwinian theory undoubtedly does. It shows us how, by means of some of
+the most universal and ever-acting laws in nature, new species are
+necessarily produced, while the old species become extinct; and it
+enables us to understand how the continuous action of these laws during
+the long periods with which geology makes us acquainted is calculated to
+bring about those greater differences presented by the distinct genera,
+families, and orders into which all living things are classified by
+naturalists. The differences which these present are all of the same
+_nature_ as those presented by the species of many large genera, but
+much greater in _amount_; and they can all be explained by the action of
+the same general laws and by the extinction of a larger or smaller
+number of intermediate species. Whether the distinctions between the
+higher groups termed Classes and Sub-kingdoms may be accounted for in
+the same way is a much more difficult question. The differences which
+separate the mammals, birds, reptiles, and fishes from each other,
+though vast, yet seem of the same nature as those which distinguish a
+mouse from an elephant or a swallow from a goose. But the vertebrate
+animals, the mollusca, and the insects, are so radically distinct in
+their whole organisation and in the very plan of their structure, that
+objectors may not unreasonably doubt whether they can all have been
+derived from a common ancestor by means of the very same laws as have
+sufficed for the differentiation of the various species of birds or of
+reptiles.
+
+
+_The Change of Opinion effected by Darwin_.
+
+The point I wish especially to urge is this. Before Darwin's work
+appeared, the great majority of naturalists, and almost without
+exception the whole literary and scientific world, held firmly to the
+belief that _species_ were realities, and had not been derived from
+other species by any process accessible to us; the different species of
+crow and of violet they are now, and to have originated by some totally
+unknown process so far removed from ordinary reproduction that it was
+usually spoken of as "special creation." There was, then, no question of
+the origin of families, orders, and classes, because the very first step
+of all, the "origin of species," was believed to be an insoluble
+problem. But now this is all changed. The whole scientific and literary
+world, even the whole educated public, accepts, as a matter of common
+knowledge, the origin of species from other allied species by the
+ordinary process of natural birth. The idea of special creation or any
+altogether exceptional mode of production is absolutely extinct! Yet
+more: this is held also to apply to many higher groups as well as to the
+species of a genus, and not even Mr. Darwin's severest critics venture
+to suggest that the primeval bird, reptile, or fish must have been
+"specially created." And this vast, this totally unprecedented change in
+public opinion has been the result of the work of one man, and was
+brought about in the short space of twenty years! This is the answer to
+those who continue to maintain that the "origin of species" is not yet
+discovered; that there are still doubts and difficulties; that there are
+divergencies of structure so great that we cannot understand how they
+had their beginning. We may admit all this, just as we may admit that
+there are enormous difficulties in the way of a complete comprehension
+of the origin and nature of all the parts of the solar system and of the
+stellar universe. But we claim for Darwin that he is the Newton of
+natural history, and that, just so surely as that the discovery and
+demonstration by Newton of the law of gravitation established order in
+place of chaos and laid a sure foundation for all future study of the
+starry heavens, so surely has Darwin, by his discovery of the law of
+natural selection and his demonstration of the great principle of the
+preservation of useful variations in the struggle for life, not only
+thrown a flood of light on the process of development of the whole
+organic world, but also established a firm foundation for all future
+study of nature.
+
+In order to show the view Darwin took of his own work, and what it was
+that he alone claimed to have done, the concluding passage of the
+introduction to the _Origin of_ _Species_ should be carefully
+considered. It is as follows: "Although much remains obscure, and will
+long remain obscure, I can entertain no doubt, after the most deliberate
+and dispassionate judgment of which I am capable, that the view which
+most naturalists until recently entertained and which I formerly
+entertained--namely, that each species has been independently
+created--is erroneous. I am fully convinced that species are not
+immutable; but that those belonging to what are called the same genera
+are lineal descendants of some other and generally extinct species, in
+the same manner as the acknowledged varieties of any one species are the
+descendants of that species. Furthermore, I am convinced that Natural
+Selection has been the most important, but not the exclusive, means of
+modification."
+
+It should be especially noted that all which is here claimed is now
+almost universally admitted, while the criticisms of Darwin's works
+refer almost exclusively to those numerous questions which, as he
+himself says, "will long remain obscure."
+
+
+_The Darwinian Theory_.
+
+As it will be necessary, in the following chapters, to set forth a
+considerable body of facts in almost every department of natural
+history, in order to establish the fundamental propositions on which the
+theory of natural selection rests, I propose to give a preliminary
+statement of what the theory really is, in order that the reader may
+better appreciate the necessity for discussing so many details, and may
+thus feel a more enlightened interest in them. Many of the facts to be
+adduced are so novel and so curious that they are sure to be appreciated
+by every one who takes an interest in nature, but unless the need of
+them is clearly seen it may be thought that time is being wasted on mere
+curious details and strange facts which have little bearing on the
+question at issue.
+
+The theory of natural selection rests on two main classes of facts which
+apply to all organised beings without exception, and which thus take
+rank as fundamental principles or laws. The first is, the power of rapid
+multiplication in a geometrical progression; the second, that the
+offspring always vary slightly from the parents, though generally very
+closely resembling them. From the first fact or law there follows,
+necessarily, a constant struggle for existence; because, while the
+offspring always exceed the parents in number, generally to an enormous
+extent, yet the total number of living organisms in the world does not,
+and cannot, increase year by year. Consequently every year, on the
+average, as many die as are born, plants as well as animals; and the
+majority die premature deaths. They kill each other in a thousand
+different ways; they starve each other by some consuming the food that
+others want; they are destroyed largely by the powers of nature--by cold
+and heat, by rain and storm, by flood and fire. There is thus a
+perpetual struggle among them which shall live and which shall die; and
+this struggle is tremendously severe, because so few can possibly remain
+alive--one in five, one in ten, often only one in a hundred or even one
+in a thousand.
+
+Then comes the question, Why do some live rather than others? If all the
+individuals of each species were exactly alike in every respect, we
+could only say it is a matter of chance. But they are not alike. We find
+that they vary in many different ways. Some are stronger, some swifter,
+some hardier in constitution, some more cunning. An obscure colour may
+render concealment more easy for some, keener sight may enable others to
+discover prey or escape from an enemy better than their fellows. Among
+plants the smallest differences may be useful or the reverse. The
+earliest and strongest shoots may escape the slug; their greater vigour
+may enable them to flower and seed earlier in a wet autumn; plants best
+armed with spines or hairs may escape being devoured; those whose
+flowers are most conspicuous may be soonest fertilised by insects. We
+cannot doubt that, on the whole, any beneficial variations will give the
+possessors of it a greater probability of living through the tremendous
+ordeal they have to undergo. There may be something left to chance, but
+on the whole _the fittest will survive_.
+
+Then we have another important fact to consider, the principle of
+heredity or transmission of variations. If we grow plants from seed or
+breed any kind of animals year after year, consuming or giving away all
+the increase we do not wish to keep just as they come to hand, our
+plants or animals will continue much the same; but if every year we
+carefully save the best seed to sow and the finest or brightest
+coloured animals to breed from, we shall soon find that an improvement
+will take place, and that the average quality of our stock will be
+raised. This is the way in which all our fine garden fruits and
+vegetables and flowers have been produced, as well as all our splendid
+breeds of domestic animals; and they have thus become in many cases so
+different from the wild races from which they originally sprang as to be
+hardly recognisable as the same. It is therefore proved that if any
+particular kind of variation is preserved and bred from, the variation
+itself goes on increasing in amount to an enormous extent; and the
+bearing of this on the question of the origin of species is most
+important. For if in each generation of a given animal or plant the
+fittest survive to continue the breed, then whatever may be the special
+peculiarity that causes "fitness" in the particular case, that
+peculiarity will go on increasing and strengthening _so long as it is
+useful to the species_. But the moment it has reached its maximum of
+usefulness, and some other quality or modification would help in the
+struggle, then the individuals which vary in the new direction will
+survive; and thus a species may be gradually modified, first in one
+direction, then in another, till it differs from the original parent
+form as much as the greyhound differs from any wild dog or the
+cauliflower from any wild plant. But animals or plants which thus differ
+in a state of nature are always classed as distinct species, and thus we
+see how, by the continuous survival of the fittest or the preservation
+of favoured races in the struggle for life, new species may be
+originated.
+
+This self-acting process which, by means of a few easily demonstrated
+groups of facts, brings about change in the organic world, and keeps
+each species in harmony with the conditions of its existence, will
+appear to some persons so clear and simple as to need no further
+demonstration. But to the great majority of naturalists and men of
+science endless difficulties and objections arise, owing to the
+wonderful variety of animal and vegetable forms, and the intricate
+relations of the different species and groups of species with each
+other; and it was to answer as many of these objections as possible, and
+to show that the more we know of nature the more we find it to
+harmonise with the development hypothesis, that Darwin devoted the whole
+of his life to collecting facts and making experiments, the record of a
+portion of which he has given us in a series of twelve masterly volumes.
+
+
+_Proposed Mode of Treatment of the Subject_.
+
+It is evidently of the most vital importance to any theory that its
+foundations should be absolutely secure. It is therefore necessary to
+show, by a wide and comprehensive array of facts, that animals and
+plants _do_ perpetually vary in the manner and to the amount requisite;
+and that this takes place in wild animals as well as in those which are
+domesticated. It is necessary also to prove that all organisms _do_ tend
+to increase at the great rate alleged, and that this increase actually
+occurs, under favourable conditions. We have to prove, further, that
+variations of all kinds can be increased and accumulated by selection;
+and that the struggle for existence to the extent here indicated
+actually occurs in nature, and leads to the continued preservation of
+favourable variations.
+
+These matters will be discussed in the four succeeding chapters, though
+in a somewhat different order--the struggle for existence and the power
+of rapid multiplication, which is its cause, occupying the first place,
+as comprising those facts which are the most fundamental and those which
+can be perfectly explained without any reference to the less generally
+understood facts of variation. These chapters will be followed by a
+discussion of certain difficulties, and of the vexed question of
+hybridity. Then will come a rather full account of the more important of
+the complex relations of organisms to each other and to the earth
+itself, which are either fully explained or greatly elucidated by the
+theory. The concluding chapter will treat of the origin of man and his
+relations to the lower animals.
+
+FOOTNOTES:
+
+[Footnote 1: _Geography and Classification of Animals_, p. 350.]
+
+[Footnote 2: These expressions occur in Chapter IX. of the earlier
+editions (to the ninth) of the _Principles of Geology_.]
+
+[Footnote 3: L. Agassiz, _Lake Superior_, p. 377.]
+
+
+
+
+CHAPTER II
+
+THE STRUGGLE FOR EXISTENCE
+
+
+    Its importance--The struggle among plants--Among
+    animals--Illustrative cases--Succession of trees in forests of
+    Denmark--The struggle for existence on the Pampas--Increase of
+    organisms in a geometrical ratio--Examples of great powers of
+    increase of animals--Rapid increase and wide spread of
+    plants--Great fertility not essential to rapid
+    increase--Struggle between closely allied species most
+    severe--The ethical aspect of the struggle for existence.
+
+
+
+There is perhaps no phenomenon of nature that is at once so important,
+so universal; and so little understood, as the struggle for existence
+continually going on among all organised beings. To most persons nature
+appears calm, orderly, and peaceful. They see the birds singing in the
+trees, the insects hovering over the flowers, the squirrel climbing
+among the tree-tops, and all living things in the possession of health
+and vigour, and in the enjoyment of a sunny existence. But they do not
+see, and hardly ever think of, the means by which this beauty and
+harmony and enjoyment is brought about. They do not see the constant and
+daily search after food, the failure to obtain which means weakness or
+death; the constant effort to escape enemies; the ever-recurring
+struggle against the forces of nature. This daily and hourly struggle,
+this incessant warfare, is nevertheless the very means by which much of
+the beauty and harmony and enjoyment in nature is produced, and also
+affords one of the most important elements in bringing about the origin
+of species. We must, therefore, devote some time to the consideration of
+its various aspects and of the many curious phenomena to which it gives
+rise.
+
+It is a matter of common observation that if weeds are allowed to grow
+unchecked in a garden they will soon destroy a number of the flowers.
+It is not so commonly known that if a garden is left to become
+altogether wild, the weeds that first take possession of it, often
+covering the whole surface of the ground with two or three different
+kinds, will themselves be supplanted by others, so that in a few years
+many of the original flowers and of the earliest weeds may alike have
+disappeared. This is one of the very simplest cases of the struggle for
+existence, resulting in the successive displacement of one set of
+species by another; but the exact causes of this displacement are by no
+means of such a simple nature. All the plants concerned may be perfectly
+hardy, all may grow freely from seed, yet when left alone for a number
+of years, each set is in turn driven out by a succeeding set, till at
+the end of a considerable period--a century or a few centuries
+perhaps--hardly one of the plants which first monopolised the ground
+would be found there.
+
+Another phenomenon of an analogous kind is presented by the different
+behaviour of introduced wild plants or animals into countries apparently
+quite as well suited to them as those which they naturally inhabit.
+Agassiz, in his work on Lake Superior, states that the roadside weeds of
+the northeastern United States, to the number of 130 species, are all
+European, the native weeds having disappeared westwards; and in New
+Zealand there are no less than 250 species of naturalised European
+plants, more than 100 species of which have spread widely over the
+country, often displacing the native vegetation. On the other hand, of
+the many hundreds of hardy plants which produce seed freely in our
+gardens, very few ever run wild, and hardly any have become common. Even
+attempts to naturalise suitable plants usually fail; for A. de Candolle
+states that several botanists of Paris, Geneva, and especially of
+Montpellier, have sown the seeds of many hundreds of species of hardy
+exotic plants in what appeared to be the most favourable situations, but
+that, in hardly a single case, has any one of them become
+naturalised.[4] Even a plant like the potato--so widely cultivated, so
+hardy, and so well adapted to spread by means of its many-eyed
+tubers--has not established itself in a wild state in any part of
+Europe. It would be thought that Australian plants would easily run
+wild in New Zealand. But Sir Joseph Hooker informs us that the late Mr.
+Bidwell habitually scattered Australian seeds during his extensive
+travels in New Zealand, yet only two or three Australian plants appear
+to have established themselves in that country, and these only in
+cultivated or newly moved soil.
+
+These few illustrations sufficiently show that all the plants of a
+country are, as De Candolle says, at war with each other, each one
+struggling to occupy ground at the expense of its neighbour. But,
+besides this direct competition, there is one not less powerful arising
+from the exposure of almost all plants to destruction by animals. The
+buds are destroyed by birds, the leaves by caterpillars, the seeds by
+weevils; some insects bore into the trunk, others burrow in the twigs
+and leaves; slugs devour the young seedlings and the tender shoots,
+wire-worms gnaw the roots. Herbivorous mammals devour many species
+bodily, while some uproot and devour the buried tubers.
+
+In animals, it is the eggs or the very young that suffer most from their
+various enemies; in plants, the tender seedlings when they first appear
+above the ground. To illustrate this latter point Mr. Darwin cleared and
+dug a piece of ground three feet long and two feet wide, and then marked
+all the seedlings of weeds and other plants which came up, noting what
+became of them. The total number was 357, and out of these no less than
+295 were destroyed by slugs and insects. The direct strife of plant with
+plant is almost equally fatal when the stronger are allowed to smother
+the weaker. When turf is mown or closely browsed by animals, a number of
+strong and weak plants live together, because none are allowed to grow
+much beyond the rest; but Mr. Darwin found that when the plants which
+compose such turf are allowed to grow up freely, the stronger kill the
+weaker. In a plot of turf three feet by four, twenty distinct species of
+plants were found to be growing, and no less than nine of these perished
+altogether when the other species were allowed to grow up to their full
+size.[5]
+
+But besides having to protect themselves against competing plants and
+against destructive animals, there is a yet deadlier enemy in the
+forces of inorganic nature. Each species can sustain a certain amount of
+heat and cold, each requires a certain amount of moisture at the right
+season, each wants a proper amount of light or of direct sunshine, each
+needs certain elements in the soil; the failure of a due proportion in
+these inorganic conditions causes weakness, and thus leads to speedy
+death. The struggle for existence in plants is, therefore, threefold in
+character and infinite in complexity, and the result is seen in their
+curiously irregular distribution over the face of the earth. Not only
+has each country its distinct plants, but every valley, every hillside,
+almost every hedgerow, has a different set of plants from its adjacent
+valley, hillside, or hedgerow--if not always different in the actual
+species yet very different in comparative abundance, some which are rare
+in the one being common in the other. Hence it happens that slight
+changes of conditions often produce great changes in the flora of a
+country. Thus in 1740 and the two following years the larva of a moth
+(Phalaena graminis) committed such destruction in many of the meadows of
+Sweden that the grass was greatly diminished in quantity, and many
+plants which were before choked by the grass sprang up, and the ground
+became variegated with a multitude of different species of flowers. The
+introduction of goats into the island of St. Helena led to the entire
+destruction of the native forests, consisting of about a hundred
+distinct species of trees and shrubs, the young plants being devoured by
+the goats as fast as they grew up. The camel is a still greater enemy to
+woody vegetation than the goat, and Mr. Marsh believes that forests
+would soon cover considerable tracts of the Arabian and African deserts
+if the goat and the camel were removed from them.[6] Even in many parts
+of our own country the existence of trees is dependent on the absence of
+cattle. Mr. Darwin observed, on some extensive heaths near Farnham, in
+Surrey, a few clumps of old Scotch firs, but no young trees over
+hundreds of acres. Some portions of the heath had, however, been
+enclosed a few years before, and these enclosures were crowded with
+young fir-trees growing too close together for all to live; and these
+were not sown or planted, nothing having been done to the ground beyond
+enclosing it so as to keep out cattle. On ascertaining this, Mr. Darwin
+was so much surprised that he searched among the heather in the
+unenclosed parts, and there he found multitudes of little trees and
+seedlings which had been perpetually browsed down by the cattle. In one
+square yard, at a point about a hundred yards from one of the old clumps
+of firs, he counted thirty-two little trees, and one of them had
+twenty-six rings of growth, showing that it had for many years tried to
+raise its head above the stems of the heather and had failed. Yet this
+heath was very extensive and very barren, and, as Mr. Darwin remarks, no
+one would ever have imagined that cattle would have so closely and so
+effectually searched it for food.
+
+In the case of animals, the competition and struggle are more obvious.
+The vegetation of a given district can only support a certain number of
+animals, and the different kinds of plant-eaters will compete together
+for it. They will also have insects for their competitors, and these
+insects will be kept down by birds, which will thus assist the mammalia.
+But there will also be carnivora destroying the herbivora; while small
+rodents, like the lemming and some of the field-mice, often destroy so
+much vegetation as materially to affect the food of all the other groups
+of animals. Droughts, floods, severe winters, storms and hurricanes will
+injure these in various degrees, but no one species can be diminished in
+numbers without the effect being felt in various complex ways by all the
+rest. A few illustrations of this reciprocal action must be given.
+
+
+_Illustrative Cases of the Struggle for Life_.
+
+Sir Charles Lyell observes that if, by the attacks of seals or other
+marine foes, salmon are reduced in numbers, the consequence will be that
+otters, living far inland, will be deprived of food and will then
+destroy many young birds or quadrupeds, so that the increase of a marine
+animal may cause the destruction of many land animals hundreds of miles
+away. Mr. Darwin carefully observed the effects produced by planting a
+few hundred acres of Scotch fir, in Staffordshire, on part of a very
+extensive heath which had never been cultivated. After the planted
+portion was about twenty-five years old he observed that the change in
+the native vegetation was greater than is often seen in passing from
+one quite different soil to another. Besides a great change in the
+proportional numbers of the native heath-plants, twelve species which
+could not be found on the heath flourished in the plantations. The
+effect on the insect life must have been still greater, for six
+insectivorous birds which were very common in the plantations were not
+to be seen on the heath, which was, however, frequented by two or three
+different species of insectivorous birds. It would have required
+continued study for several years to determine all the differences in
+the organic life of the two areas, but the facts stated by Mr. Darwin
+are sufficient to show how great a change may be effected by the
+introduction of a single kind of tree and the keeping out of cattle.
+
+The next case I will give in Mr. Darwin's own words: "In several parts
+of the world insects determine the existence of cattle. Perhaps Paraguay
+offers the most curious instance of this; for here neither cattle nor
+horses nor dogs have ever run wild, though they swarm southward and
+northward in a feral state; and Azara and Rengger have shown that this
+is caused by the greater numbers, in Paraguay, of a certain fly which
+lays its eggs in the navels of these animals when first born. The
+increase of these flies, numerous as they are, must be habitually
+checked by some means, probably by other parasitic insects. Hence, if
+certain insectivorous birds were to decrease in Paraguay, the parasitic
+insects would probably increase; and this would lessen the number of the
+navel-frequenting flies--then cattle and horses would become feral, and
+this would greatly alter (as indeed I have observed in parts of South
+America) the vegetation: this again would largely affect the insects,
+and this, as we have just seen in Staffordshire, the insectivorous
+birds, and so onward in ever-increasing circles of complexity. Not that
+under nature the relations will ever be as simple as this. Battle within
+battle must be continually recurring with varying success; and yet in
+the long run the forces are so nicely balanced, that the face of nature
+remains for a long time uniform, though assuredly the merest trifle
+would give the victory to one organic being over another."[7]
+
+Such cases as the above may perhaps be thought exceptional, but there
+is good reason to believe that they are by no means rare, but are
+illustrations of what is going on in every part of the world, only it is
+very difficult for us to trace out the complex reactions that are
+everywhere occurring. The general impression of the ordinary observer
+seems to be that wild animals and plants live peaceful lives and have
+few troubles, each being exactly suited to its place and surroundings,
+and therefore having no difficulty in maintaining itself. Before showing
+that this view is, everywhere and always, demonstrably untrue, we will
+consider one other case of the complex relations of distinct organisms
+adduced by Mr. Darwin, and often quoted for its striking and almost
+eccentric character. It is now well known that many flowers require to
+be fertilised by insects in order to produce seed, and this
+fertilisation can, in some cases, only be effected by one particular
+species of insect to which the flower has become specially adapted. Two
+of our common plants, the wild heart's-ease (Viola tricolor) and the red
+clover (Trifolium pratense), are thus fertilised by humble-bees almost
+exclusively, and if these insects are prevented from visiting the
+flowers, they produce either no seed at all or exceedingly few. Now it
+is known that field-mice destroy the combs and nests of humble-bees, and
+Colonel Newman, who has paid great attention to these insects, believes
+that more than two-thirds of all the humble-bees' nests in England are
+thus destroyed. But the number of mice depends a good deal on the number
+of cats; and the same observer says that near villages and towns he has
+found the nests of humble-bees more numerous than elsewhere, which he
+attributes to the number of cats that destroy the mice. Hence it
+follows, that the abundance of red clover and wild heart's-ease in a
+district will depend on a good supply of cats to kill the mice, which
+would otherwise destroy and keep down the humble-bees and prevent them
+from fertilising the flowers. A chain of connection has thus been found
+between such totally distinct organisms as flesh-eating mammalia and
+sweet-smelling flowers, the abundance or scarcity of the one closely
+corresponding to that of the other!
+
+The following account of the struggle between trees in the forests of
+Denmark, from the researches of M. Hansten-Blangsted, strikingly
+illustrates our subject.[8] The chief combatants are the beech and the
+birch, the former being everywhere successful in its invasions. Forests
+composed wholly of birch are now only found in sterile, sandy tracts;
+everywhere else the trees are mixed, and wherever the soil is favourable
+the beech rapidly drives out the birch. The latter loses its branches at
+the touch of the beech, and devotes all its strength to the upper part
+where it towers above the beech. It may live long in this way, but it
+succumbs ultimately in the fight--of old age if of nothing else, for the
+life of the birch in Denmark is shorter than that of the beech. The
+writer believes that light (or rather shade) is the cause of the
+superiority of the latter, for it has a greater development of its
+branches than the birch, which is more open and thus allows the rays of
+the sun to pass through to the soil below, while the tufted, bushy top
+of the beech preserves a deep shade at its base. Hardly any young plants
+can grow under the beech except its own shoots; and while the beech can
+nourish under the shade of the birch, the latter dies immediately under
+the beech. The birch has only been saved from total extermination by the
+facts that it had possession of the Danish forests long before the beech
+ever reached the country, and that certain districts are unfavourable to
+the growth of the latter. But wherever the soil has been enriched by the
+decomposition of the leaves of the birch the battle begins. The birch
+still flourishes on the borders of lakes and other marshy places, where
+its enemy cannot exist. In the same way, in the forests of Zeeland, the
+fir forests are disappearing before the beech. Left to themselves, the
+firs are soon displaced by the beech. The struggle between the latter
+and the oak is longer and more stubborn, for the branches and foliage of
+the oak are thicker, and offer much resistance to the passage of light.
+The oak, also, has greater longevity; but, sooner or later, it too
+succumbs, because it cannot develop in the shadow of the beech. The
+earliest forests of Denmark were mainly composed of aspens, with which
+the birch was apparently associated; gradually the soil was raised, and
+the climate grew milder; then the fir came and formed large forests.
+This tree ruled for centuries, and then ceded the first place to the
+holm-oak, which is now giving way to the beech. Aspen, birch, fir, oak,
+and beech appear to be the steps in the struggle for the survival of the
+fittest among the forest-trees of Denmark.
+
+It may be added that in the time of the Romans the beech was the
+principal forest-tree of Denmark as it is now, while in the much earlier
+bronze age, represented by the later remains found in the peat bogs,
+there were no beech-trees, or very few, the oak being the prevailing
+tree, while in the still earlier stone period the fir was the most
+abundant. The beech is a tree essentially of the temperate zone, having
+its northern limit considerably southward of the oak, fir, birch, or
+aspen, and its entrance into Denmark was no doubt due to the
+amelioration of the climate after the glacial epoch had entirely passed
+away. We thus see how changes of climate, which are continually
+occurring owing either to cosmical or geographical causes, may initiate
+a struggle among plants which may continue for thousands of years, and
+which must profoundly modify the relations of the animal world, since
+the very existence of innumerable insects, and even of many birds and
+mammals, is dependent more or less completely on certain species of
+plants.
+
+
+_The Struggle for Existence on the Pampas_.
+
+Another illustration of the struggle for existence, in which both plants
+and animals are implicated, is afforded by the pampas of the southern
+part of South America. The absence of trees from these vast plains has
+been imputed by Mr. Darwin to the supposed inability of the tropical and
+sub-tropical forms of South America to thrive on them, and there being
+no other source from which they could obtain a supply; and that
+explanation was adopted by such eminent botanists as Mr. Ball and
+Professor Asa Gray. This explanation has always seemed to me
+unsatisfactory, because there are ample forests both in the temperate
+regions of the Andes and on the whole west coast down to Terra del
+Fuego; and it is inconsistent with what we know of the rapid variation
+and adaptation of species to new conditions. What seems a more
+satisfactory explanation has been given by Mr. Edwin Clark, a civil
+engineer, who resided nearly two years in the country and paid much
+attention to its natural history. He says: "The peculiar characteristics
+of these vast level plains which descend from the Andes to the great
+river basin in unbroken monotony, are the absence of rivers or
+water-storage, and the periodical occurrence of droughts, or 'siccos,'
+in the summer months. These conditions determine the singular character
+both of its flora and fauna.
+
+"The soil is naturally fertile and favourable for the growth of trees,
+and they grow luxuriantly wherever they are protected. The eucalyptus is
+covering large tracts wherever it is enclosed, and willows, poplars, and
+the fig surround every estancia when fenced in.
+
+"The open plains are covered with droves of horses and cattle, and
+overrun by numberless wild rodents, the original tenants of the pampas.
+During the long periods of drought, which are so great a scourge to the
+country, these animals are starved by thousands, destroying, in their
+efforts to live, every vestige of vegetation. In one of these 'siccos,'
+at the time of my visit, no less than 50,000 head of oxen and sheep and
+horses perished from starvation and thirst, after tearing deep out of
+the soil every trace of vegetation, including the wiry roots of the
+pampas-grass. Under such circumstances the existence of an unprotected
+tree is impossible. The only plants that hold their own, in addition to
+the indestructible thistles, grasses, and clover, are a little
+herbaceous oxalis, producing viviparous buds of extraordinary vitality,
+a few poisonous species, such as the hemlock, and a few tough, thorny
+dwarf-acacias and wiry rushes, which even a starving rat refuses.
+
+"Although the cattle are a modern introduction, the numberless
+indigenous rodents must always have effectually prevented the
+introduction of any other species of plants; large tracts are still
+honeycombed by the ubiquitous biscacho, a gigantic rabbit; and numerous
+other rodents still exist, including rats and mice, pampas-hares, and
+the great nutria and carpincho (capybara) on the river banks."[9]
+
+Mr. Clark further remarks on the desperate struggle for existence which
+characterises the bordering fertile zones, where rivers and marshy
+plains permit a more luxuriant and varied vegetable and animal life.
+After describing how the river sometimes rose 30 feet in eight hours,
+doing immense destruction, and the abundance of the larger carnivora and
+large reptiles on its banks, he goes on: "But it was among the flora
+that the principle of natural selection was most prominently displayed.
+In such a district--overrun with rodents and escaped cattle, subject to
+floods that carried away whole islands of botany, and especially to
+droughts that dried up the lakes and almost the river itself--no
+ordinary plant could live, even on this rich and watered alluvial
+debris. The only plants that escaped the cattle were such as were either
+poisonous, or thorny, or resinous, or indestructibly tough. Hence we had
+only a great development of solanums, talas, acacias, euphorbias, and
+laurels. The buttercup is replaced by the little poisonous yellow oxalis
+with its viviparous buds; the passion-flowers, asclepiads, bignonias,
+convolvuluses, and climbing leguminous plants escape both floods and
+cattle by climbing the highest trees and towering overhead in a flood of
+bloom. The ground plants are the portulacas, turneras, and cenotheras,
+bitter and ephemeral, on the bare rock, and almost independent of any
+other moisture than the heavy dews. The pontederias, alismas, and
+plantago, with grasses and sedges, derive protection from the deep and
+brilliant pools; and though at first sight the 'monte' doubtless
+impresses the traveller as a scene of the wildest confusion and ruin,
+yet, on closer examination, we found it far more remarkable as a
+manifestation of harmony and law, and a striking example of the
+marvellous power which plants, like animals, possess, of adapting
+themselves to the local peculiarities of their habitat, whether in the
+fertile shades of the luxuriant 'monte' or on the arid, parched-up
+plains of the treeless pampas."
+
+A curious example of the struggle between plants has been communicated
+to me by Mr. John Ennis, a resident in New Zealand. The English
+water-cress grows so luxuriantly in that country as to completely choke
+up the rivers, sometimes leading to disastrous floods, and necessitating
+great outlay to keep the stream open. But a natural remedy has now been
+found in planting willows on the banks. The roots of these trees
+penetrate the bed of the stream in every direction, and the water-cress,
+unable to obtain the requisite amount of nourishment, gradually
+disappears.
+
+_Increase of Organisms in a Geometrical Ratio_.
+
+The facts which have now been adduced, sufficiently prove that there is
+a continual competition, and struggle, and war going on in nature, and
+that each species of animal and plant affects many others in complex and
+often unexpected ways. We will now proceed to show the fundamental cause
+of this struggle, and to prove that it is ever acting over the whole
+field of nature, and that no single species of animal or plant can
+possibly escape from it. This results from the fact of the rapid
+increase, in a geometrical ratio, of all the species of animals and
+plants. In the lower orders this increase is especially rapid, a single
+flesh-fly (Musca carnaria) producing 20,000 larvae, and these growing so
+quickly that they reach their full size in five days; hence the great
+Swedish naturalist, Linnaeus, asserted that a dead horse would be
+devoured by three of these flies as quickly as by a lion. Each of these
+larvae remains in the pupa state about five or six days, so that each
+parent fly may be increased ten thousand-fold in a fortnight. Supposing
+they went on increasing at this rate during only three months of summer,
+there would result one hundred millions of millions of millions for each
+fly at the commencement of summer,--a number greater probably than
+exists at any one time in the whole world. And this is only one species,
+while there are thousands of other species increasing also at an
+enormous rate; so that, if they were unchecked, the whole atmosphere
+would be dense with flies, and all animal food and much of animal life
+would be destroyed by them. To prevent this tremendous increase there
+must be incessant war against these insects, by insectivorous birds and
+reptiles as well as by other insects, in the larva as well as in the
+perfect state, by the action of the elements in the form of rain, hail,
+or drought, and by other unknown causes; yet we see nothing of this
+ever-present war, though by its means alone, perhaps, we are saved from
+famine and pestilence.
+
+Let us now consider a less extreme and more familiar case. We possess a
+considerable number of birds which, like the redbreast, sparrow, the
+four common titmice, the thrush, and the blackbird, stay with us all the
+year round These lay on an average six eggs, but, as several of them
+have two or more broods a year, ten will be below the average of the
+year's increase. Such birds as these often live from fifteen to twenty
+years in confinement, and we cannot suppose them to live shorter lives
+in a state of nature, if unmolested; but to avoid possible exaggeration
+we will take only ten years as the average duration of their lives. Now,
+if we start with a single pair, and these are allowed to live and breed,
+unmolested, till they die at the end of ten years,--as they might do if
+turned loose into a good-sized island with ample vegetable and insect
+food, but no other competing or destructive birds or quadrupeds--their
+numbers would amount to more than twenty millions. But we know very well
+that our bird population is no greater, on the average, now than it was
+ten years ago. Year by year it may fluctuate a little according as the
+winters are more or less severe, or from other causes, but on the whole
+there is no increase. What, then, becomes of the enormous surplus
+population annually produced? It is evident they must all die or be
+killed, somehow; and as the increase is, on the average, about five to
+one, it follows that, if the average number of birds of all kinds in our
+islands is taken at ten millions--and this is probably far under the
+mark--then about fifty millions of birds, including eggs as possible
+birds, must annually die or be destroyed. Yet we see nothing, or almost
+nothing, of this tremendous slaughter of the innocents going on all
+around us. In severe winters a few birds are found dead, and a few
+feathers or mangled remains show us where a wood-pigeon or some other
+bird has been destroyed by a hawk, but no one would imagine that five
+times as many birds as the total number in the country in early spring
+die every year. No doubt a considerable proportion of these do not die
+here but during or after migration to other countries, but others which
+are bred in distant countries come here, and thus balance the account.
+Again, as the average number of young produced is four or five times
+that of the parents, we ought to have at least five times as many birds
+in the country at the end of summer as at the beginning, and there is
+certainly no such enormous disproportion as this. The fact is, that the
+destruction commences, and is probably most severe, with nestling birds,
+which are often killed by heavy rains or blown away by severe storms, or
+left to die of hunger if either of the parents is killed; while they
+offer a defenceless prey to jackdaws, jays, and magpies, and not a few
+are ejected from their nests by their foster-brothers the cuckoos. As
+soon as they are fledged and begin to leave the nest great numbers are
+destroyed by buzzards, sparrow-hawks, and shrikes. Of those which
+migrate in autumn a considerable proportion are probably lost at sea or
+otherwise destroyed before they reach a place of safety; while those
+which remain with us are greatly thinned by cold and starvation during
+severe winters. Exactly the same thing goes on with every species of
+wild animal and plant from the lowest to the highest. All breed at such
+a rate, that in a few years the progeny of any one species would, if
+allowed to increase unchecked, alone monopolise the land; but all alike
+are kept within bounds by various destructive agencies, so that, though
+the numbers of each may fluctuate, they can never permanently increase
+except at the expense of some others, which must proportionately
+decrease.
+
+
+_Cases showing the Great Powers of Increase of Animals._
+
+As the facts now stated are the very foundation of the theory we are
+considering, and the enormous increase and perpetual destruction
+continually going on require to be kept ever present in the mind, some
+direct evidence of actual cases of increase must be adduced. That even
+the larger animals, which breed comparatively slowly, increase
+enormously when placed under favourable conditions in new countries, is
+shown by the rapid spread of cattle and horses in America. Columbus, in
+his second voyage, left a few black cattle at St. Domingo, and these ran
+wild and increased so much that, twenty-seven years afterwards, herds of
+from 4000 to 8000 head were not uncommon. Cattle were afterwards taken
+from this island to Mexico and to other parts of America, and in 1587,
+sixty-five years after the conquest of Mexico, the Spaniards exported
+64,350 hides from that country and 35,444 from St. Domingo, an
+indication of the vast numbers of these animals which must then have
+existed there, since those captured and killed could have been only a
+small portion of the whole. In the pampas of Buenos Ayres there were, at
+the end of the last century, about twelve million cows and three million
+horses, besides great numbers in all other parts of America where open
+pastures offered suitable conditions. Asses, about fifty years after
+their introduction, ran wild and multiplied so amazingly in Quito, that
+the Spanish traveller Ulloa describes them as being a nuisance. They
+grazed together in great herds, defending themselves with their mouths,
+and if a horse strayed among them they all fell upon him and did not
+cease biting and kicking till they left him dead. Hogs were turned out
+in St. Domingo by Columbus in 1493, and the Spaniards took them to other
+places where they settled, the result being, that in about half a
+century these animals were found in great numbers over a large part of
+America, from 25 deg. north to 40 deg. south latitude. More recently, in New
+Zealand, pigs have multiplied so greatly in a wild state as to be a
+serious nuisance and injury to agriculture. To give some idea of their
+numbers, it is stated that in the province of Nelson there were killed
+in twenty months 25,000 wild pigs.[10] Now, in the case of all these
+animals, we know that in their native countries, and even in America at
+the present time, they do not increase at all in numbers; therefore the
+whole normal increase must be kept down, year by year, by natural or
+artificial means of destruction.
+
+
+_Rapid Increase and Wide Spread of Plants_.
+
+In the case of plants, the power of increase is even greater and its
+effects more distinctly visible. Hundreds of square miles of the plains
+of La Plata are now covered with two or three species of European
+thistle, often to the exclusion of almost every other plant; but in the
+native countries of these thistles they occupy, except in cultivated or
+waste ground, a very subordinate part in the vegetation. Some American
+plants, like the cotton-weed (Asclepias cuiussayica), have now become
+common weeds over a large portion of the tropics. White clover
+(Trifolium repens) spreads over all the temperate regions of the world,
+and in New Zealand is exterminating many native species, including even
+the native flax (Phormium tenax), a large plant with iris-like leaves 5
+or 6 feet high. Mr. W.L. Travers has paid much attention to the effects
+of introduced plants in New Zealand, and notes the following species as
+being especially remarkable. The common knotgrass (Polygonum aviculare)
+grows most luxuriantly, single plants covering a space 4 or 5 feet in
+diameter, and sending their roots 3 or 4 feet deep. A large sub-aquatic
+dock (Rumex obtusifolius) abounds in every river-bed, even far up among
+the mountains. The common sow-thistle (Sonchus oleraceus) grows all over
+the country up to an elevation of 6000 feet. The water-cress (Nasturtium
+officinale) grows with amazing vigour in many of the rivers, forming
+stems 12 feet long and 3/4 inch in diameter, and completely choking them
+up. It cost L300 a year to keep the Avon at Christchurch free from it.
+The sorrel (Rumex acetosella) covers hundreds of acres with a sheet of
+red. It forms a dense mat, exterminating other plants, and preventing
+cultivation. It can, however, be itself exterminated by sowing the
+ground with red clover, which will also vanquish the Polygonum
+aviculare. The most noxious weed in New Zealand appears, however, to be
+the Hypochaeris radicata, a coarse yellow-flowered composite not
+uncommon in our meadows and waste places. This has been introduced with
+grass seeds from England, and is very destructive. It is stated that
+excellent pasture was in three years destroyed by this weed, which
+absolutely displaced every other plant on the ground. It grows in every
+kind of soil, and is said even to drive out the white clover, which is
+usually so powerful in taking possession of the soil.
+
+In Australia another composite plant, called there the Cape-weed
+(Cryptostemma calendulaceum), did much damage, and was noticed by Baron
+Von Hugel in 1833 as "an unexterminable weed"; but, after forty years'
+occupation, it was found to give way to the dense herbage formed by
+lucerne and choice grasses.
+
+In Ceylon we are told by Mr. Thwaites, in his _Enumeration of Ceylon
+Plants_, that a plant introduced into the island less than fifty years
+ago is helping to alter the character of the vegetation up to an
+elevation of 3000 feet. This is the Lantana mixta, a verbenaceous plant
+introduced from the West Indies, which appears to have found in Ceylon
+a soil and climate exactly suited to it. It now covers thousands of
+acres with its dense masses of foliage, taking complete possession of
+land where cultivation has been neglected or abandoned, preventing the
+growth of any other plants, and even destroying small trees, the tops of
+which its subscandent stems are able to reach. The fruit of this plant
+is so acceptable to frugivorous birds of all kinds that, through their
+instrumentality, it is spreading rapidly, to the complete exclusion of
+the indigenous vegetation where it becomes established.
+
+
+_Great Fertility not essential to Rapid Increase_.
+
+The not uncommon circumstance of slow-breeding animals being very
+numerous, shows that it is usually the amount of destruction which an
+animal or plant is exposed to, not its rapid multiplication, that
+determines its numbers in any country. The passenger-pigeon (Ectopistes
+migratorius) is, or rather was, excessively abundant in a certain area
+in North America, and its enormous migrating flocks darkening the sky
+for hours have often been described; yet this bird lays only two eggs.
+The fulmar petrel exists in myriads at St. Kilda and other haunts of the
+species, yet it lays only one egg. On the other hand the great shrike,
+the tree-creeper, the nut-hatch, the nut-cracker, the hoopoe, and many
+other birds, lay from four to six or seven eggs, and yet are never
+abundant. So in plants, the abundance of a species bears little or no
+relation to its seed-producing power. Some of the grasses and sedges,
+the wild hyacinth, and many buttercups occur in immense profusion over
+extensive areas, although each plant produces comparatively few seeds;
+while several species of bell-flowers, gentians, pinks, and mulleins,
+and even some of the composite, which produce an abundance of minute
+seeds, many of which are easily scattered by the wind, are yet rare
+species that never spread beyond a very limited area.
+
+The above-mentioned passenger-pigeon affords such an excellent example
+of an enormous bird-population kept up by a comparatively slow rate of
+increase, and in spite of its complete helplessness and the great
+destruction which it suffers from its numerous enemies, that the
+following account of one of its breeding-places and migrations by the
+celebrated American naturalist, Alexander Wilson, will be read with
+interest:--
+
+"Not far from Shelbyville, in the State of Kentucky, about five years
+ago, there was one of these breeding-places, which stretched through the
+woods in nearly a north and south direction, was several miles in
+breadth, and was said to be upwards of 40 miles in extent. In this tract
+almost every tree was furnished with nests wherever the branches could
+accommodate them. The pigeons made their first appearance there about
+the 10th of April, and left it altogether with their young before the
+25th of May. As soon as the young were fully grown and before they left
+the nests, numerous parties of the inhabitants from all parts of the
+adjacent country came with waggons, axes, beds, cooking utensils, many
+of them accompanied by the greater part of their families, and encamped
+for several days at this immense nursery. Several of them informed me
+that the noise was so great as to terrify their horses, and that it was
+difficult for one person to hear another without bawling in his ear. The
+ground was strewed with broken limbs of trees, eggs, and young squab
+pigeons, which had been precipitated from above, and on which herds of
+hogs were fattening. Hawks, buzzards, and eagles were sailing about in
+great numbers, and seizing the squabs from the nests at pleasure; while,
+from 20 feet upwards to the top of the trees, the view through the woods
+presented a perpetual tumult of crowding and fluttering multitudes of
+pigeons, their wings roaring like thunder, mingled with the frequent
+crash of falling timber; for now the axemen were at work cutting down
+those trees that seemed most crowded with nests, and contrived to fell
+them in such a manner, that in their descent they might bring down
+several others; by which means the falling of one large tree sometimes
+produced 200 squabs little inferior in size to the old birds, and almost
+one heap of fat. On some single trees upwards of a hundred nests were
+found, each containing one squab only; a circumstance in the history of
+the bird not generally known to naturalists.[11] It was dangerous to
+walk under these flying and fluttering millions, from the frequent fall
+of large branches, broken down by the weight of the multitudes above,
+and which in their descent often destroyed numbers of the birds
+themselves; while the clothes of those engaged in traversing the woods
+were completely covered with the excrements of the pigeons.
+
+"These circumstances were related to me by many of the most respectable
+part of the community in that quarter, and were confirmed in part by
+what I myself witnessed. I passed for several miles through this same
+breeding-place, where every tree was spotted with nests, the remains of
+those above described. In many instances I counted upwards of ninety
+nests on a single tree; but the pigeons had abandoned this place for
+another, 60 or 80 miles off, towards Green River, where they were said
+at that time to be equally numerous. From the great numbers that were
+constantly passing over our heads to or from that quarter, I had no
+doubt of the truth of this statement. The mast had been chiefly consumed
+in Kentucky; and the pigeons, every morning a little before sunrise, set
+out for the Indiana territory, the nearest part of which was about sixty
+miles distant. Many of these returned before ten o'clock, and the great
+body generally appeared on their return a little after noon. I had left
+the public road to visit the remains of the breeding-place near
+Shelbyville, and was traversing the woods with my gun, on my way to
+Frankfort, when about ten o'clock the pigeons which I had observed
+flying the greater part of the morning northerly, began to return in
+such immense numbers as I never before had witnessed. Coming to an
+opening by the side of a creek, where I had a more uninterrupted view, I
+was astonished at their appearance: they were flying with great
+steadiness and rapidity, at a height beyond gunshot, in several strata
+deep, and so close together that, could shot have reached them, one
+discharge could not have failed to bring down several individuals. From
+right to left, as far as the eye could reach, the breadth of this vast
+procession extended, seeming everywhere equally crowded. Curious to
+determine how long this appearance would continue, I took out my watch
+to note the time, and sat down to observe them. It was then half-past
+one; I sat for more than an hour, but instead of a diminution of this
+prodigious procession, it seemed rather to increase, both in numbers and
+rapidity; and anxious to reach Frankfort before night, I rose and went
+on. About four o'clock in the afternoon I crossed Kentucky River, at the
+town of Frankfort, at which time the living torrent above my head seemed
+as numerous and as extensive as ever. Long after this I observed them in
+large bodies that continued to pass for six or eight minutes, and these
+again were followed by other detached bodies, all moving in the same
+south-east direction, till after six o'clock in the evening. The great
+breadth of front which this mighty multitude preserved would seem to
+intimate a corresponding breadth of their breeding-place, which, by
+several gentlemen who had lately passed through part of it, was stated
+to me at several miles."
+
+From these various observations, Wilson calculated that the number of
+birds contained in the mass of pigeons which he saw on this occasion was
+at least two thousand millions, while this was only one of many similar
+aggregations known to exist in various parts of the United States. The
+picture here given of these defenceless birds, and their still more
+defenceless young, exposed to the attacks of numerous rapacious enemies,
+brings vividly before us one of the phases of the unceasing struggle for
+existence ever going on; but when we consider the slow rate of increase
+of these birds, and the enormous population they are nevertheless able
+to maintain, we must be convinced that in the case of the majority of
+birds which multiply far more rapidly, and yet are never able to attain
+such numbers, the struggle against their numerous enemies and against
+the adverse forces of nature must be even more severe or more
+continuous.
+
+
+_Struggle for Life between, closely allied Animals and Plants often the
+most severe._
+
+The struggle we have hitherto been considering has been mainly that
+between an animal or plant and its direct enemies, whether these enemies
+are other animals which devour it, or the forces of nature which destroy
+it. But there is another kind of struggle often going on at the same
+time between closely related species, which almost always terminates in
+the destruction of one of them. As an example of what is meant, Darwin
+states that the recent increase of the missel-thrush in parts of
+Scotland has caused the decrease of the song-thrush.[12] The black rat
+(Mus rattus) was the common rat of Europe till, in the beginning of the
+eighteenth century, the large brown rat (Mus decumanus) appeared on the
+Lower Volga, and thence spread more or less rapidly till it overran all
+Europe, and generally drove out the black rat, which in most parts is
+now comparatively rare or quite extinct. This invading rat has now been
+carried by commerce all over the world, and in New Zealand has
+completely extirpated a native rat, which the Maoris allege they brought
+with them from their home in the Pacific; and in the same country a
+native fly is being supplanted by the European house-fly. In Russia the
+small Asiatic cockroach has driven away a larger native species; and in
+Australia the imported hive-bee is exterminating the small stingless
+native bee.
+
+The reason why this kind of struggle goes on is apparent if we consider
+that the allied species fill nearly the same place in the economy of
+nature. They require nearly the same kind of food, are exposed to the
+same enemies and the same dangers. Hence, if one has ever so slight an
+advantage over the other in procuring food or in avoiding danger, in its
+rapidity of multiplication or its tenacity of life, it will increase
+more rapidly, and by that very fact will cause the other to decrease and
+often become altogether extinct. In some cases, no doubt, there is
+actual war between the two, the stronger killing the weaker; but this is
+by no means necessary, and there may be cases in which the weaker
+species, physically, may prevail, by its power of more rapid
+multiplication, its better withstanding vicissitudes of climates, or its
+greater cunning in escaping the attacks of the common enemies. The same
+principle is seen at work in the fact that certain mountain varieties of
+sheep will starve out other mountain varieties, so that the two cannot
+be kept together. In plants the same thing occurs. If several distinct
+varieties of wheat are sown together, and the mixed seed resown, some of
+the varieties which best suit the soil and climate, or are naturally the
+most fertile, will beat the others and so yield more seed, and will
+consequently in a few years supplant the other varieties.
+
+As an effect of this principle, we seldom find closely allied species
+of animals or plants living together, but often in distinct though
+adjacent districts where the conditions of life are somewhat different.
+Thus we may find cowslips (Primula veris) growing in a meadow, and
+primroses (P. vulgaris) in an adjoining wood, each in abundance, but not
+often intermingled. And for the same reason the old turf of a pasture or
+heath consists of a great variety of plants matted together, so much so
+that in a patch little more than a yard square Mr. Darwin found twenty
+distinct species, belonging to eighteen distinct genera and to eight
+natural orders, thus showing their extreme diversity of organisation.
+For the same reason a number of distinct grasses and clovers are sown in
+order to make a good lawn instead of any one species; and the quantity
+of hay produced has been found to be greater from a variety of very
+distinct grasses than from any one species of grass.
+
+It may be thought that forests are an exception to this rule, since in
+the north-temperate and arctic regions we find extensive forests of
+pines or of oaks. But these are, after all, exceptional, and
+characterise those regions only where the climate is little favourable
+to forest vegetation. In the tropical and all the warm temperate parts
+of the earth, where there is a sufficient supply of moisture, the
+forests present the same variety of species as does the turf of our old
+pastures; and in the equatorial virgin forests there is so great a
+variety of forms, and they are so thoroughly intermingled, that the
+traveller often finds it difficult to discover a second specimen of any
+particular species which he has noticed. Even the forests of the
+temperate zones, in all favourable situations, exhibit a considerable
+variety of trees of distinct genera and families, and it is only when we
+approach the outskirts of forest vegetation, where either drought or
+winds or the severity of the winter is adverse to the existence of most
+trees, that we find extensive tracts monopolised by one or two species.
+Even Canada has more than sixty different forest trees and the Eastern
+United States a hundred and fifty; Europe is rather poor, containing
+about eighty trees only; while the forests of Eastern Asia, Japan, and
+Manchuria are exceedingly rich, about a hundred and seventy species
+being already known. And in all these countries the trees grow
+intermingled, so that in every extensive forest we have a considerable
+variety, as may be seen in the few remnants of our primitive woods in
+some parts of Epping Forest and the New Forest.
+
+Among animals the same law prevails, though, owing to their constant
+movements and power of concealment, it is not so readily observed. As
+illustrations we may refer to the wolf, ranging over Europe and Northern
+Asia, while the jackal inhabits Southern Asia and Northern Africa; the
+tree-porcupines, of which there are two closely allied species, one
+inhabiting the eastern, the other the western half of North America; the
+common hare (Lepus timidus) in Central and Southern Europe, while all
+Northern Europe is inhabited by the variable hare (Lepus variabilis);
+the common jay (Garrulus glandarius) inhabiting all Europe, while
+another species (Garrulus Brandti) is found all across Asia from the
+Urals to Japan; and many species of birds in the Eastern United States
+are replaced by closely allied species in the west. Of course there are
+also numbers of closely related species in the same country, but it will
+almost always be found that they frequent different stations and have
+somewhat different habits, and so do not come into direct competition
+with each other; just as closely allied plants may inhabit the same
+districts, when one prefers meadows the other woods, one a chalky soil
+the other sand, one a damp situation the other a dry one. With plants,
+fixed as they are to the earth, we easily note these peculiarities of
+station; but with wild animals, which we see only on rare occasions, it
+requires close and long-continued observation to detect the
+peculiarities in their mode of life which may prevent all direct
+competition between closely allied species dwelling in the same area.
+
+
+_The Ethical Aspect of the Struggle for Existence_.
+
+Our exposition of the phenomena presented by the struggle for existence
+may be fitly concluded by a few remarks on its ethical aspect. Now that
+the war of nature is better known, it has been dwelt upon by many
+writers as presenting so vast an amount of cruelty and pain as to be
+revolting to our instincts of humanity, while it has proved a
+stumbling-block in the way of those who would fain believe in an
+all-wise and benevolent ruler of the universe. Thus, a brilliant writer
+says: "Pain, grief, disease, and death, are these the inventions of a
+loving God? That no animal shall rise to excellence except by being
+fatal to the life of others, is this the law of a kind Creator? It is
+useless to say that pain has its benevolence, that massacre has its
+mercy. Why is it so ordained that bad should be the raw material of
+good? Pain is not the less pain because it is useful; murder is not less
+murder because it is conducive to development. Here is blood upon the
+hand still, and all the perfumes of Arabia will not sweeten it."[13]
+
+Even so thoughtful a writer as Professor Huxley adopts similar views. In
+a recent article on "The Struggle for Existence" he speaks of the
+myriads of generations of herbivorous animals which "have been tormented
+and devoured by carnivores"; of the carnivores and herbivores alike
+"subject to all the miseries incidental to old age, disease, and
+over-multiplication"; and of the "more or less enduring suffering,"
+which is the meed of both vanquished and victor. And he concludes that,
+since thousands of times a minute, were our ears sharp enough, we should
+hear sighs and groans of pain like those heard by Dante at the gate of
+hell, the world cannot be governed by what we call benevolence.[14]
+
+Now there is, I think, good reason to believe that all this is greatly
+exaggerated; that the supposed "torments" and "miseries" of animals have
+little real existence, but are the reflection of the imagined sensations
+of cultivated men and women in similar circumstances; and that the
+amount of actual suffering caused by the struggle for existence among
+animals is altogether insignificant. Let us, therefore, endeavour to
+ascertain what are the real facts on which these tremendous accusations
+are founded.
+
+In the first place, we must remember that animals are entirely spared
+the pain we suffer in the anticipation of death--a pain far greater, in
+most cases, than the reality. This leads, probably, to an almost
+perpetual enjoyment of their lives; since their constant watchfulness
+against danger, and even their actual flight from an enemy, will be the
+enjoyable exercise of the powers and faculties they possess, unmixed
+with any serious dread. There is, in the next place, much evidence to
+show that violent deaths, if not too prolonged, are painless and easy;
+even in the case of man, whose nervous system is in all probability much
+more susceptible to pain than that of most animals. In all cases in
+which persons have escaped after being seized by a lion or tiger, they
+declare that they suffered little or no pain, physical or mental. A
+well-known instance is that of Livingstone, who thus describes his
+sensations when seized by a lion: "Starting and looking half round, I
+saw the lion just in the act of springing on me. I was upon a little
+height; he caught my shoulder as he sprang, and we both came to the
+ground below together. Growling horribly close to my ear, he shook me as
+a terrier-dog does a rat. The shock produced a stupor similar to that
+which seems to be felt by a mouse after the first shake of the cat. It
+causes a sort of dreaminess, _in which there was no sense of pain or
+feeling of terror_, though I was quite conscious of all that was
+happening. It was like what patients partially under the influence of
+chloroform describe, who see all the operation, but feel not the knife.
+This singular condition was not the result of any mental process. The
+shake annihilated fear, and allowed no sense of horror in looking round
+at the beast."
+
+This absence of pain is not peculiar to those seized by wild beasts, but
+is equally produced by any accident which causes a general shock to the
+system. Mr. Whymper describes an accident to himself during one of his
+preliminary explorations of the Matterhorn, when he fell several hundred
+feet, bounding from rock to rock, till fortunately embedded in a
+snow-drift near the edge of a tremendous precipice. He declares that
+while falling and feeling blow after blow, he neither lost consciousness
+nor suffered pain, merely thinking, calmly, that a few more blows would
+finish him. We have therefore a right to conclude, that when death
+follows soon after any great shock it is as easy and painless a death as
+possible; and this is certainly what happens when an animal is seized by
+a beast of prey. For the enemy is one which hunts for food, not for
+pleasure or excitement; and it is doubtful whether any carnivorous
+animal in a state of nature begins to seek after prey till driven to do
+so by hunger. When an animal is caught, therefore, it is very soon
+devoured, and thus the first shock is followed by an almost painless
+death. Neither do those which die of cold or hunger suffer much. Cold is
+generally severest at night and has a tendency to produce sleep and
+painless extinction. Hunger, on the other hand, is hardly felt during
+periods of excitement, and when food is scarce the excitement of seeking
+for it is at its greatest. It is probable, also, that when hunger
+presses, most animals will devour anything to stay their hunger, and
+will die of gradual exhaustion and weakness not necessarily painful, if
+they do not fall an earlier prey to some enemy or to cold.[15]
+
+Now let us consider what are the enjoyments of the lives of most
+animals. As a rule they come into existence at a time of year when food
+is most plentiful and the climate most suitable, that is in the spring
+of the temperate zone and at the commencement of the dry season in the
+tropics. They grow vigorously, being supplied with abundance of food;
+and when they reach maturity their lives are a continual round of
+healthy excitement and exercise, alternating with complete repose. The
+daily search for the daily food employs all their faculties and
+exercises every organ of their bodies, while this exercise leads to the
+satisfaction of all their physical needs. In our own case, we can give
+no more perfect definition of happiness, than this exercise and this
+satisfaction; and we must therefore conclude that animals, as a rule,
+enjoy all the happiness of which they are capable. And this normal state
+of happiness is not alloyed, as with us, by long periods--whole lives
+often--of poverty or ill-health, and of the unsatisfied longing for
+pleasures which others enjoy but to which we cannot attain. Illness, and
+what answers to poverty in animals--continued hunger--are quickly
+followed by unanticipated and almost painless extinction. Where we err
+is, in giving to animals feelings and emotions which they do not
+possess. To us the very sight of blood and of torn or mangled limbs is
+painful, while the idea of the suffering implied by it is heartrending.
+We have a horror of all violent and sudden death, because we think of
+the life full of promise cut short, of hopes and expectations
+unfulfilled, and of the grief of mourning relatives. But all this is
+quite out of place in the case of animals, for whom a violent and a
+sudden death is in every way the best. Thus the poet's picture of
+
+
+    "Nature red in tooth and claw
+         With ravine"
+
+
+is a picture the evil of which is read into it by our imaginations, the
+reality being made up of full and happy lives, usually terminated by the
+quickest and least painful of deaths.
+
+On the whole, then, we conclude that the popular idea of the struggle
+for existence entailing misery and pain on the animal world is the very
+reverse of the truth. What it really brings about, is, the maximum of
+life and of the enjoyment of life with the minimum of suffering and
+pain. Given the necessity of death and reproduction--and without these
+there could have been no progressive development of the organic
+world,--and it is difficult even to imagine a system by which a greater
+balance of happiness could have been secured. And this view was
+evidently that of Darwin himself, who thus concludes his chapter on the
+struggle for existence: "When we reflect on this struggle, we may
+console ourselves with the full belief that the war of nature is not
+incessant, that no fear is felt, that death is generally prompt, and
+that the vigorous, the healthy, and the happy survive and multiply."
+
+FOOTNOTES:
+
+[Footnote 4: _Geographic Botanique_, p. 798.]
+
+[Footnote 5: _The Origin of Species_, p. 53.]
+
+[Footnote 6: _The Earth as Modified by Human Action_, p. 51.]
+
+[Footnote 7: _The Origin of Species_, p. 56.]
+
+[Footnote 8: See _Nature_, vol. xxxi. p. 63.]
+
+[Footnote 9: _A Visit to South America_, 1878; also _Nature_, vol. xxxi.
+pp. 263-339.]
+
+[Footnote 10: Still more remarkable is the increase of rabbits both in
+New Zealand and Australia. No less than seven millions of rabbit-skins
+have been exported from the former country in a single year, their value
+being L67,000. In both countries, sheep-runs have been greatly
+deteriorated in value by the abundance of rabbits, which destroy the
+herbage; and in some cases they have had to be abandoned altogether.]
+
+[Footnote 11: Later observers have proved that two eggs are laid and
+usually two young produced, but it may be that in most cases only one of
+these comes to maturity.]
+
+[Footnote 12: _Origin of Species_, p. 59. Professor A. Newton, however,
+informs me that these species do not interfere with one another in the
+way here stated.]
+
+[Footnote 13: Winwood Reade's _Martyrdom of Man,_ p. 520.]
+
+[Footnote 14: _Nineteenth Century,_ February 1888, pp. 162, 163.]
+
+[Footnote 15: The Kestrel, which usually feeds on mice, birds, and
+frogs, sometimes stays its hunger with earthworms, as do some of the
+American buzzards. The Honey-buzzard sometimes eats not only earthworms
+and slugs, but even corn; and the Buteo borealis of North America, whose
+usual food is small mammals and birds, sometimes eats crayfish.]
+
+
+
+
+CHAPTER III
+
+THE VARIABILITY OF SPECIES IN A STATE OF NATURE
+
+
+    Importance of variability--Popular ideas regarding
+    it--Variability of the lower animals--The variability of
+    insects--Variation among lizards--Variation among
+    birds--Diagrams of bird-variation--Number of varying
+    individuals--Variation in the mammalia--Variation in internal
+    organs--Variations in the skull--Variations in the habits of
+    Animals--The Variability of plants--Species which vary
+    little--Concluding remarks.
+
+
+
+The foundation of the Darwinian theory is the variability of species,
+and it is quite useless to attempt even to understand that theory, much
+less to appreciate the completeness of the proof of it, unless we first
+obtain a clear conception of the nature and extent of this variability.
+The most frequent and the most misleading of the objections to the
+efficacy of natural selection arise from ignorance of this subject, an
+ignorance shared by many naturalists, for it is only since Mr. Darwin
+has taught us their importance that varieties have been systematically
+collected and recorded; and even now very few collectors or students
+bestow upon them the attention they deserve. By the older naturalists,
+indeed, varieties--especially if numerous, small, and of frequent
+occurrence--were looked upon as an unmitigated nuisance, because they
+rendered it almost impossible to give precise definitions of species,
+then considered the chief end of systematic natural history. Hence it
+was the custom to describe what was supposed to be the "typical form" of
+species, and most collectors were satisfied if they possessed this
+typical form in their cabinets. Now, however, a collection is valued in
+proportion as it contains illustrative specimens of all the varieties
+that occur in each species, and in some cases these have been carefully
+described, so that we possess a considerable mass of information on the
+subject. Utilising this information we will now endeavour to give some
+idea of the nature and extent of variation in the species of animals and
+plants.
+
+It is very commonly objected that the widespread and constant
+variability which is admitted to be a characteristic of domesticated
+animals and cultivated plants is largely due to the unnatural conditions
+of their existence, and that we have no proof of any corresponding
+amount of variation occurring in a state of nature. Wild animals and
+plants, it is said, are usually stable, and when variations occur these
+are alleged to be small in amount and to affect superficial characters
+only; or if larger and more important, to occur so rarely as not to
+afford any aid in the supposed formation of new species.
+
+This objection, as will be shown, is utterly unfounded; but as it is one
+which goes to the very root of the problem, it is necessary to enter at
+some length into the various proofs of variation in a state of nature.
+This is the more necessary because the materials collected by Mr. Darwin
+bearing on this question have never been published, and comparatively
+few of them have been cited in _The Origin of Species_; while a
+considerable body of facts has been made known since the publication of
+the last edition of that work.
+
+
+_Variability of the Lower Animals_.
+
+Among the lowest and most ancient marine organisms are the Foraminifera,
+little masses of living jelly, apparently structureless, but which
+secrete beautiful shelly coverings, often perfectly symmetrical, as
+varied in form as those of the mollusca and far more complicated. These
+have been studied with great care by many eminent naturalists, and the
+late Dr. W.B. Carpenter in his great work--the _Introduction to the
+Study of the Foraminifera_--thus refers to their variability: "There is
+not a single species of plant or animal of which the range of variation
+has been studied by the collocation and comparison of so large a number
+of specimens as have passed under the review of Messrs. Williamson,
+Parker, Rupert Jones, and myself in our studies of the types of this
+group;" and he states as the result of this extensive comparison of
+specimens: "The range of variation is so great among the Foraminifera
+as to include not merely those differential characters which have been
+usually accounted _specific_, but also those upon which the greater part
+of the _genera_, of this group have been founded, and even in some
+instances those of its _orders_."[16]
+
+Coming now to a higher group--the Sea-Anemones--Mr. P.H. Gosse and other
+writers on these creatures often refer to variations in size, in the
+thickness and length of the tentacles, the form of the disc and of the
+mouth, and the character of surface of the column, while the colour
+varies enormously in a great number of the species. Similar variations
+occur in all the various groups of marine invertebrata, and in the great
+sub-kingdom of the mollusca they are especially numerous. Thus, Dr. S.P.
+Woodward states that many present a most perplexing amount of variation,
+resulting (as he supposes) from supply of food, variety of depth and of
+saltness of the water; but we know that many variations are quite
+independent of such causes, and we will now consider a few cases among
+the land-mollusca in which they have been more carefully studied.
+
+In the small forest region of Oahu, one of the Sandwich Islands, there
+have been found about 175 species of land-shells represented by 700 or
+800 varieties; and we are told by the Rev. J.T. Gulick, who studied them
+carefully, that "we frequently find a genus represented in several
+successive valleys by allied species, sometimes feeding on the same,
+sometimes on different plants. In every such case the valleys that are
+nearest to each other furnish the most nearly allied forms; _and a full
+set of the varieties of each species presents a minute gradation of
+forms between the more divergent types found in the more widely
+separated localities_."
+
+In most land-shells there is a considerable amount of variation in
+colour, markings, size, form, and texture or striation of the surface,
+even in specimens collected in the same locality. Thus, a French author
+has enumerated no less than 198 varieties of the common wood-snail
+(Helix nemoralis), while of the equally common garden-snail (Helix
+hortensis) ninety varieties have been described. Fresh-water shells are
+also subject to great variation, so that there is much uncertainty as
+to the number of species; and variations are especially frequent in the
+Planorbidae, which exhibit many eccentric deviations from the usual form
+of the species--deviations which must often affect the form of the
+living animal. In Mr. Ingersoll's Report on the Recent Mollusca of
+Colorado many of these extraordinary variations are referred to, and it
+is stated that a shell (Helisonia trivolvis) abundant in some small
+ponds and lakes, had scarcely two specimens alike, and many of them
+closely resembled other and altogether distinct species.[17]
+
+
+_The Variability of Insects_.
+
+Among Insects there is a large amount of variation, though very few
+entomologists devote themselves to its investigation. Our first examples
+will be taken from the late Mr. T. Vernon Wollaston's book, _On the
+Variation of Species_, and they must be considered as indications of
+very widespread though little noticed phenomena. He speaks of the
+curious little carabideous beetles of the genus Notiophilus as being
+"extremely unstable both in their sculpture and hue;" of the common
+Calathus mollis as having "the hind wings at one time ample, at another
+rudimentary, and at a third nearly obsolete;" and of the same
+irregularity as to the wings being characteristic of many Orthoptera and
+of the Homopterous Fulgoridae. Mr. Westwood in his _Modern
+Classification of Insects_ states that "the species of Gerris,
+Hydrometra, and Velia are mostly found perfectly apterous, though
+occasionally with full-sized wings."
+
+It is, however, among the Lepidoptera (butterflies and moths) that the
+most numerous cases of variation have been observed, and every good
+collection of these insects affords striking examples. I will first
+adduce the testimony of Mr. Bates, who speaks of the butterflies of the
+Amazon valley exhibiting innumerable local varieties or races, while
+some species showed great individual variability. Of the beautiful
+Mechanitis Polymnia he says, that at Ega on the Upper Amazons, "it
+varies not only in general colour and pattern, but also very
+considerably in the shape of the wings, especially in the male sex."
+Again, at St. Paulo, Ithomia Orolina exhibits four distinct varieties,
+all occurring together, and these differ not only in colour but in form,
+one variety being described as having the fore wings much elongated in
+the male, while another is much larger and has "the hind wings in the
+male different in shape." Of Heliconius Numata Mr. Bates says: "This
+species is so variable that it is difficult to find two examples exactly
+alike," while "it varies in structure as well as in colours. The wings
+are sometimes broader, sometimes narrower; and their edges are simple in
+some examples and festooned in others." Of another species of the same
+genus, H. melpomene, ten distinct varieties are described all more or
+less connected by intermediate forms, and four of these varieties were
+obtained at one locality, Serpa on the north bank of the Amazon.
+Ceratina Ninonia is another of these very unstable species exhibiting
+many local varieties which are, however, incomplete and connected by
+intermediate forms; while the several species of the genus Lycorea all
+vary to such an extent as almost to link them together, so that Mr.
+Bates thinks they might all fairly be considered as varieties of one
+species only.
+
+Turning to the Eastern Hemisphere we have in Papilio Severus a species
+which exhibits a large amount of simple variation, in the presence or
+absence of a pale patch on the upper wings, in the brown submarginal
+marks on the lower wings, in the form and extent of the yellow band, and
+in the size of the specimens. The most extreme forms, as well as the
+intermediate ones, are often found in one locality and in company with
+each other. A small butterfly (Terias hecabe) ranges over the whole of
+the Indian and Malayan regions to Australia, and everywhere exhibits
+great variations, many of which have been described as distinct species;
+but a gentleman in Australia bred two of these distinct forms (T. hecabe
+and T. Aesiope), with several intermediates, from one batch of
+caterpillars found feeding together on the same plant.[18] It is
+therefore very probable that a considerable number of supposed distinct
+species are only individual varieties.
+
+Cases of variation similar to those now adduced among butterflies might
+be increased indefinitely, but it is as well to note that such important
+characters as the neuration of the wings, on which generic and family
+distinctions are often established, are also subject to variation. The
+Rev. R.P. Murray, in 1872, laid before the Entomological Society
+examples of such variation in six species of butterflies, and other
+cases have been since described. The larvae of butterflies and moths are
+also very variable, and one observer recorded in the _Proceedings of the
+Entomological Society for_ 1870 no less than sixteen varieties of the
+caterpillar of the bedstraw hawk-moth (Deilephela galii).
+
+
+_Variation among Lizards_.
+
+Passing on from the lower animals to the vertebrata, we find more
+abundant and more definite evidence as to the extent and amount of
+individual variation. I will first give a case among the Reptilia from
+some of Mr. Darwin's unpublished MSS., which have been kindly lent me by
+Mr. Francis Darwin.
+
+"M. Milne Edwards (_Annales des Sci. Nat._, I ser., tom. xvi. p. 50) has
+given a curious table of measurements of fourteen specimens of Lacerta
+muralis; and, taking the length of the head as a standard, he finds the
+neck, trunk, tail, front and hind legs, colour, and femoral pores, all
+varying wonderfully; and so it is more or less with other species. So
+apparently trifling a character as the scales on the head affording
+almost the only constant characters."
+
+[Illustration: FIG. 1.--Variations of Lacerta muralis.]
+
+[Illustration: FIG. 2.--Variation of Lizards.]
+
+As the table of measurements above referred to would give no clear
+conception of the nature and amount of the variation without a laborious
+study and comparison of the figures, I have endeavoured to find a method
+of presenting the facts to the eye, so that they may be easily grasped
+and appreciated. In the diagram opposite, the comparative variations of
+the different organs of this species are given by means of variously
+bent lines. The head is represented by a straight line because it
+presented (apparently) no variation. The body is next given, the
+specimens being arranged in the order of their size from No. 1, the
+smallest, to No. 14, the largest, the actual lengths being laid down
+from a base line at a suitable distance below, in this case two inches
+below the centre, the mean length of the body of the fourteen specimens
+being two inches. The respective lengths of the neck, legs, and toe of
+each specimen are then laid down in the same manner at convenient
+distances apart for comparison; and we see that their variations bear no
+definite relation to those of the body, and not much to those of each
+other. With the exception of No. 5, in which all the parts agree in
+being large, there is a marked independence of each part, shown by the
+lines often curving in opposite directions; which proves that in those
+specimens one part is large while the other is small. The actual amount
+of the variation is very great, ranging from one-sixth of the mean
+length in the neck to considerably more than a fourth in the hind leg,
+and this among only fourteen examples which happen to be in a particular
+museum.
+
+To prove that this is not an isolated case, Professor Milne Edwards also
+gives a table showing the amount of variation in the museum specimens of
+six common species of lizards, also taking the head as the standard, so
+that the comparative variation of each part to the head is given. In the
+accompanying diagram (Fig. 2) the variations are exhibited by means of
+lines of varying length. It will be understood that, however much the
+specimens varied in _size_, if they had kept the same _proportions_, the
+variation line would have been in every case reduced to a point, as in
+the neck of L. velox which exhibits no variation. The different
+proportions of the variation lines for each species may show a distinct
+mode of variation, or may be merely due to the small and differing
+number of specimens; for it is certain that whatever amount of variation
+occurs among a few specimens will be greatly increased when a much
+larger number of specimens are examined. That the amount of variation is
+large, may be seen by comparing it with the actual length of the head
+(given below the diagram) which was used as a standard in determining
+the variation, but which itself seems not to have varied.[19]
+
+
+_Variation among Birds_.
+
+Coming now to the class of Birds, we find much more copious evidence of
+variation. This is due partly to the fact that Ornithology has perhaps a
+larger body of devotees than any other branch of natural history (except
+entomology); to the moderate size of the majority of birds; and to the
+circumstance that the form and dimensions of the wings, tail, beak, and
+feet offer the best generic and specific characters and can all be
+easily measured and compared. The most systematic observations on the
+individual variation of birds have been made by Mr. J.A. Allen, in his
+remarkable memoir: "On the Mammals and Winter Birds of East Florida,
+with an examination of certain assumed specific characters in Birds, and
+a sketch of the Bird Faunae of Eastern North America," published in the
+_Bulletin of the Museum of Comparative Zoology_ at Harvard College,
+Cambridge, Massachusetts, in 1871. In this work exact measurements are
+given of all the chief external parts of a large number of species of
+common American birds, from twenty to sixty or more specimens of each
+species being measured, so that we are able to determine with some
+precision the nature and extent of the variation that usually occurs.
+Mr. Allen says: "The facts of the case show that a variation of from 15
+to 20 per cent in general size, and an equal degree of variation in the
+relative size of different parts, may be ordinarily expected among
+specimens of the same species and sex, taken at the same locality, while
+in some cases the variation is even greater than this." He then goes on
+to show that each part varies to a considerable extent independently of
+the other parts; so that when the size varies, the proportions of all
+the parts vary, often to a much greater amount. The wing and tail, for
+example, besides varying in length, vary in the proportionate length of
+each feather, and this causes their outline to vary considerably in
+shape. The bill also varies in length, width, depth, and curvature. The
+tarsus varies in length, as does each toe separately and independently;
+and all this not to a minute degree requiring very careful measurement
+to detect it at all, but to an amount easily seen without any
+measurement, as it averages one-sixth of the whole length and often
+reaches one-fourth. In twelve species of common perching birds the wing
+varied (in from twenty-five to thirty specimens) from 14 to 21 per cent
+of the mean length, and the tail from 13.8 to 23.4 per cent. The
+variation of the form of the wing can be very easily tested by noting
+which feather is longest, which next in length, and so on, the
+respective feathers being indicated by the numbers 1, 2, 3, etc.,
+commencing with the outer one. As an example of the irregular variation
+constantly met with, the following occurred among twenty-five specimens
+of Dendroeca coronata. Numbers bracketed imply that the corresponding
+feathers were of equal length.[20]
+
+
+    RELATIVE LENGTHS OF PRIMARY WING FEATHERS OF
+    DENDROECA CORONATA.
+    ---------+-----------+----------+-----------+----------+----------
+    Longest. | Second in | Third in | Fourth in | Fifth in | Sixth in
+             |  Length.  |  Length. |  Length.  |  Length. |  Length.
+    ---------+-----------+----------+-----------+----------+----------
+    2        |     3     |     1    |     4     |     5    |     6
+    3        |     2     |     4    |     1     |     5    |     6
+             |  /  2     |          |           |          |
+    3        | {         |     1    |     5     |     6    |     7
+             |  \  4     |          |           |          |
+    2    \   |           |          |           |          |
+          }  |     4     |     1    |     5     |     6    |     7
+    3    /   |           |          |           |          |
+    2    \   |           |          |           |          |
+    1     |  |           |          |           |          |
+           } |     5     |     6    |     7     |     8    |     9
+    3     |  |           |          |           |          |
+    4    /   |           |          |           |          |
+    ---------+-----------+----------+-----------+----------+----------
+
+
+Here we have five very distinct proportionate lengths of the wing
+feathers, any one of which is often thought sufficient to characterise a
+distinct species of bird; and though this is rather an extreme case, Mr.
+Allen assures us that "the comparison, extended in the table to only a
+few species, has been carried to scores of others with similar results."
+
+Along with this variation in size and proportions there occurs a large
+amount of variation in colour and markings. "The difference in intensity
+of colour between the extremes of a series of fifty or one hundred
+specimens of any species, collected at a single locality, and nearly at
+the same season of the year, is often as great as occurs between truly
+distinct species." But there is also a great amount of individual
+variability in the markings of the same species. Birds having the
+plumage varied with streaks and spots differ exceedingly in different
+individuals of the same species in respect to the size, shape, and
+number of these marks, and in the general aspect of the plumage
+resulting from such variations. "In the common song sparrow (Melospiza
+melodia), the fox-coloured sparrow (Passerella iliaca), the swamp
+sparrow (Melospiza palustris), the black and white creeper (Mniotilta
+varia), the water-wagtail (Seiurus novaeboracencis), in Turdus
+fuscescens and its allies, the difference in the size of the streaks is
+often very considerable. In the song sparrow they vary to such an extent
+that in some cases they are reduced to narrow lines; in others so
+enlarged as to cover the greater part of the breast and sides of the
+body, sometimes uniting on the middle of the breast into a nearly
+continuous patch."
+
+Mr. Allen then goes on to particularise several species in which such
+variations occur, giving cases in which two specimens taken at the same
+place on the same day exhibited the two extremes of coloration. Another
+set of variations is thus described: "The white markings so common on
+the wings and tails of birds, as the bars formed by the white tips of
+the greater wing-coverts, the white patch occasionally present at the
+base of the primary quills, or the white band crossing them, and the
+white patch near the end of the outer tail-feathers are also extremely
+liable to variation in respect to their extent and the number of
+feathers to which, in the same species, these markings extend." It is to
+be especially noted that all these varieties are distinct from those
+which depend on season, on age, or on sex, and that they are such as
+have in many other species been considered to be of specific value.
+
+These variations of colour could not be presented to the eye without a
+series of carefully engraved plates, but in order to bring Mr. Allen's
+_measurements_, illustrating variations of size and proportion, more
+clearly before the reader, I have prepared a series of diagrams
+illustrating the more important facts and their bearings on the
+Darwinian theory.
+
+The first of these is intended, mainly, to show the actual amount of the
+variation, as it gives the true length of the wing and tail in the
+extreme cases among thirty specimens of each of three species. The
+shaded portion shows the minimum length, the unshaded portion the
+additional length in the maximum. The point to be specially noted here
+is, that in each of these common species there is about the same amount
+of variation, and that it is so great as to be obvious at a glance.
+
+[Illustration: FIG. 3.--Variation of Wings and Tail.]
+
+There is here no question of "minute" or "infinitesimal" variation,
+which many people suppose to be the only kind of variation that exists.
+It cannot even be called small; yet from all the evidence we now possess
+it seems to be the amount which characterises most of the common species
+of birds.
+
+It may be said, however, that these are the extreme variations, and only
+occur in one or two individuals, while the great majority exhibit little
+or no difference. Other diagrams will show that this is not the case;
+but even if it were so, it would be no objection at all, because these
+are the extremes among thirty specimens only. We may safely assume that
+these thirty specimens, taken by chance, are not, in the case of all
+these species, exceptional lots, and therefore we might expect at least
+two similarly varying specimens in each additional thirty. But the
+number of individuals, even in a very rare species, is probably thirty
+thousand or more, and in a common species thirty, or even three hundred,
+millions. Even one individual in each thirty, varying to the amount
+shown in the diagram, would give at least a million in the total
+population of any common bird, and among this million many would vary
+much more than the extreme among thirty only. We should thus have a vast
+body of individuals varying to a large extent in the length of the wings
+and tail, and offering ample material for the modification of these
+organs by natural selection. We will now proceed to show that other
+parts of the body vary, simultaneously, but independently, to an equal
+amount.
+
+[Illustration: FIG. 4.--Dolichonyx oryzivorus. 20 Males.]
+
+[Illustration: FIG. 5.--Agelaeus phoeniceus. 40 Males.]
+
+The first bird taken is the common Bob-o-link or Rice-bird (Dolichonyx
+oryzivorus), and the Diagram, Fig. 4, exhibits the variations of seven
+important characters in twenty male adult specimens.[21] These
+characters are--the lengths of the body, wing, tail, tarsus, middle toe,
+outer toe, and hind toe, being as many as can be conveniently exhibited
+in one diagram. The length of the body is not given by Mr. Allen, but as
+it forms a convenient standard of comparison, it has been obtained by
+deducting the length of the tail from the total length of the birds as
+given by him. The diagram has been constructed as follows:--The twenty
+specimens are first arranged in a series according to the body-lengths
+(which may be considered to give the size of the bird), from the
+shortest to the longest, and the same number of vertical lines are
+drawn, numbered from one to twenty. In this case (and wherever
+practicable) the body-length is measured from the lower line of the
+diagram, so that the actual length of the bird is exhibited as well as
+the actual variations of length. These can be well estimated by means of
+the horizontal line drawn at the mean between the two extremes, and it
+will be seen that one-fifth of the total number of specimens taken on
+either side exhibits a very large amount of variation, which would of
+course be very much greater if a hundred or more specimens were
+compared. The lengths of the wing, tail, and other parts are then laid
+down, and the diagram thus exhibits at a glance the comparative
+variation of these parts in every specimen as well as the actual amount
+of variation in the twenty specimens; and we are thus enabled to arrive
+at some important conclusions.
+
+We note, first, that the variations of none of the parts follow the
+variations of the body, but are sometimes almost in an opposite
+direction. Thus the longest wing corresponds to a rather small body, the
+longest tail to a medium body, while the longest leg and toes belong to
+only a moderately large body. Again, even related parts do not
+constantly vary together but present many instances of independent
+variation, as shown by the want of parallelism in their respective
+variation-lines. In No. 5 (see Fig. 4) the wing is very long, the tail
+moderately so; while in No. 6 the wing is much shorter while the tail is
+considerably longer. The tarsus presents comparatively little variation;
+and although the three toes may be said to vary in general together,
+there are many divergencies; thus, in passing from No. 9 to No. 10, the
+outer toe becomes longer, while the hind toe becomes considerably
+shorter; while in Nos. 3 and 4 the middle toe varies in an opposite way
+to the outer and the hind toes.
+
+[Illustration: FIG. 6.--Cardinalis virginianus. 31 Males.]
+
+In the next diagram (Fig. 5) we have the variations in forty males of
+the Red-winged Blackbird (Agelaeus phoeniceus), and here we see the same
+general features. One-fifth of the whole number of specimens offer a
+large amount of variation either below or above the mean; while the
+wings, tail, and head vary quite independently of the body. The wing and
+tail too, though showing some amount of correlated variation, yet in
+no less than nine cases vary in opposite directions as compared with the
+preceding species.
+
+The next diagram (Fig. 6), showing the variations of thirty-one males of
+the Cardinal bird (Cardinalis virginianus), exhibits these features much
+more strongly. The amount of variation in proportion to the size of the
+bird is very much greater; while the variations of the wing and tail not
+only have no correspondence with that of the body but very little with
+each other. In no less than twelve or thirteen instances they vary in
+opposite directions, while even where they correspond in direction the
+amount of the variation is often very disproportionate.
+
+As the proportions of the tarsi and toes of birds have great influence
+on their mode of life and habits and are often used as specific or even
+generic characters, I have prepared a diagram (Fig. 7) to show the
+variation in these parts only, among twenty specimens of each of four
+species of birds, four or five of the most variable alone being given.
+The extreme divergence of each of the lines in a vertical direction
+shows the actual amount of variation; and if we consider the small
+length of the toes of these small birds, averaging about three-quarters
+of an inch, we shall see that the variation is really very large; while
+the diverging curves and angles show that each part varies, to a great
+extent, independently. It is evident that if we compared some thousands
+of individuals instead of only twenty, we should have an amount of
+independent variation occurring each year which would enable almost any
+modification of these important organs to be rapidly effected.
+
+[Illustration: FIG. 7.--Variation of Tarsus and Toes.]
+
+[Illustration: FIG. 8.--Variation of Birds in Leyden Museum.]
+
+In order to meet the objection that the large amount of variability here
+shown depends chiefly on the observations of one person and on the birds
+of a single country, I have examined Professor Schlegel's Catalogue of
+the Birds in the Leyden Museum, in which he usually gives the range of
+variation of the specimens in the museum (which are commonly less than a
+dozen and rarely over twenty) as regards some of their more important
+dimensions. These fully support the statement of Mr. Allen, since they
+show an equal amount of variability when the numbers compared are
+sufficient, which, however, is not often the case. The accompanying
+diagram exhibits the actual differences of size in five organs which
+occur in five species taken almost at random from this catalogue. Here,
+again, we perceive that the variation is decidedly large, even among a
+very small number of specimens; while the facts all show that there is
+no ground whatever for the common assumption that natural species
+consist of individuals which are nearly all alike, or that the
+variations which occur are "infinitesimal" or even "small."
+
+
+_The proportionate Number of Individuals which present a considerable
+amount of Variation._
+
+The notion that variation is a comparatively exceptional phenomenon, and
+that in any case considerable variations occur very rarely in proportion
+to the number of individuals which do not vary, is so deeply rooted that
+it is necessary to show by every possible method of illustration how
+completely opposed it is to the facts of nature. I have therefore
+prepared some diagrams in which each of the individual birds measured is
+represented by a spot, placed at a proportionate distance, right and
+left, from the median line accordingly as it varies in excess or defect
+of the mean length as regards the particular part compared. As the
+object in this set of diagrams is to show the number of individuals
+which vary considerably in proportion to those which vary little or not
+at all, the scale has been enlarged in order to allow room for placing
+the spots without overlapping each other.
+
+In the diagram opposite twenty males of Icterus Baltimore are
+registered, so as to exhibit to the eye the proportionate number of
+specimens which vary, to a greater or less amount, in the length of the
+tail, wing, tarsus, middle toe, hind toe, and bill. It will be noticed
+that there is usually no very great accumulation of dots about the
+median line which shows the average dimensions, but that a considerable
+number are spread at varying distances on each side of it.
+
+In the next diagram (Fig. 10), showing the variation among forty males
+of Agelaeeus phoeniceus, this approach to an equable spreading of the
+variations is still more apparent; while in Fig. 12, where fifty-eight
+specimens of Cardinalis virginianus are registered, we see a remarkable
+spreading out of the spots, showing in some of the characters a tendency
+to segregation into two or more groups of individuals, each varying
+considerably from the mean.
+
+[Illustration: FIG. 9]
+
+[Illustration: FIG. 10.]
+
+[Illustration: FIG. 11.]
+
+In order fully to appreciate the teaching of these diagrams, we must
+remember, that, whatever kind and amount of variations are exhibited by
+the few specimens here compared, would be greatly extended and brought
+into symmetrical form if large numbers--thousands or millions--were
+subjected to the same process of measurement and registration. We know,
+from the general law which governs variations from a mean value, that
+with increasing numbers the range of variation of each part would
+increase also, at first rather rapidly and then more slowly; while gaps
+and irregularities would be gradually filled up, and at length the
+distribution of the dots would indicate a tolerably regular curve of
+double curvature like those shown in Fig. 11. The great divergence of
+the dots, when even a few specimens are compared, shows that the curve,
+with high numbers, would be a flat one like the lower curve in the
+illustration here given. This being the case it would follow that a very
+large proportion of the total number of individuals constituting a
+species would diverge considerably from its average condition as regards
+each part or organ; and as we know from the previous diagrams of
+variation (Figs. 1 to 7) that each part varies to a considerable extent,
+_independently_, the materials constantly ready for natural selection to
+act upon are abundant in quantity and very varied in kind. Almost any
+combination of variations of distinct parts will be available, where
+required; and this, as we shall see further on, obviates one of the most
+weighty objections which have been urged against the efficiency of
+natural selection in producing new species, genera, and higher groups.
+
+[Illustration: FIG. 12.]
+
+
+_Variation in the Mammalia._
+
+Owing to the generally large size of this class of animals, and the
+comparatively small number of naturalists who study them, large series
+of specimens are only occasionally examined and compared, and thus the
+materials for determining the question of their variability in a state
+of nature are comparatively scanty. The fact that our domestic animals
+belonging to this group, especially dogs, present extreme varieties not
+surpassed even by pigeons and poultry among birds, renders it almost
+certain that an equal amount of variability exists in the wild state;
+and this is confirmed by the example of a species of squirrel (Sciurus
+carolinensis), of which sixteen specimens, all males and all taken in
+Florida, were measured and tabulated by Mr. Allen. The diagram here
+given shows, that, both the general amount of the variation and the
+independent variability of the several members of the body, accord
+completely with the variations so common in the class of birds; while
+their amount and their independence of each other are even greater than
+usual.
+
+
+_Variation in the Internal Organs of Animals._
+
+In case it should be objected that the cases of variation hitherto
+adduced are in the external parts only, and that there is no proof that
+the internal organs vary in the same manner, it will be advisable to
+show that such varieties also occur. It is, however, impossible to
+adduce the same amount of evidence in this class of variation, because
+the great labour of dissecting large numbers of specimens of the same
+species is rarely undertaken, and we have to trust to the chance
+observations of anatomists recorded in their regular course of study.
+
+It must, however, be noted that a very large proportion of the
+variations already recorded in the external parts of animals necessarily
+imply corresponding internal variations. When feet and legs vary in
+size, it is because the bones vary; when the head, body, limbs, and tail
+change their proportions, the bony skeleton must also change; and even
+when the wing or tail feathers of birds become longer or more numerous,
+there is sure to be a corresponding change in the bones which support
+and the muscles which move them. I will, however, give a few cases of
+variations which have been directly observed.
+
+[Illustration: FIG. 13.--Sciurus carolinensis. 32 specimens. Florida.]
+
+Mr. Frank E. Beddard has kindly communicated to me some remarkable
+variations he has observed in the internal organs of a species of
+earthworm (Perionyx excavatus). The normal characters of this species
+are--
+
+
+    Setae forming a complete row round each segment.
+
+    Two pairs of spermathecae--spherical pouches without
+    diverticulae--in segments 8 and 9.
+
+    Two pairs of testes in segments 11 and 12.
+
+    Ovaries, a single pair in segment 13.
+
+    Oviducts open by a common pore in the middle of segment 14.
+
+    Vasa deferentia open separately in segment 18, each furnished at
+    its termination with a large prostate gland.
+
+
+Between two and three hundred specimens were examined, and among them
+thirteen specimens exhibited the following marked variations:--
+
+
+    (1) The number of the spermathecae varied from two to three or
+    four pairs, their position also varying.
+
+    (2) There were occasionally two pairs of ovaries, each with its
+    own oviduct; the external apertures of these varied in position,
+    being upon segments 13 and 14, 14 and 15, or 15 and 16.
+    Occasionally when there was only the normal single oviduct pore
+    present it varied in position, once occurring on the 10th, and
+    once on the 11th segment.
+
+    (3) The male generative pores varied in position from segments
+    14 to 20. In one instance there were two pairs instead of the
+    normal single pair, and in this case each of the four apertures
+    had its own prostate gland.
+
+
+Mr. Beddard remarks that all, or nearly all, the above variations are
+found _normally_ in other genera and species.
+
+When we consider the enormous number of earthworms and the comparatively
+very small number of individuals examined, we may be sure, not only that
+such variations as these occur with considerable frequency, but also
+that still more extraordinary deviations from the normal structure may
+often exist.
+
+The next example is taken from Mr. Darwin's unpublished MSS.
+
+
+    "In some species of Shrews (Sorex) and in some field-mice
+    (Arvicola), the Rev. L. Jenyns (_Ann. Nat. Hist._, vol. vii. pp.
+    267, 272) found the proportional length of the intestinal canal
+    to vary considerably. He found the same variability in the
+    number of the caudal vertebrae. In three specimens of an
+    Arvicola he found the gall-bladder having a very different
+    degree of development, and there is reason to believe it is
+    sometimes absent. Professor Owen has shown that this is the case
+    with the gall-bladder of the giraffe."
+
+
+Dr. Crisp (_Proc. Zool. Soc._, 1862, p. 137) found the gall-bladder
+present in some specimens of Cervus superciliaris while absent in
+others; and he found it to be absent in three giraffes which he
+dissected. A double gall-bladder was found in a sheep, and in a small
+mammal preserved in the Hunterian Museum there are three distinct
+gall-bladders.
+
+The length of the alimentary canal varies greatly. In three adult
+giraffes described by Professor Owen it was from 124 to 136 feet long;
+one dissected in France had this canal 211 feet long; while Dr. Crisp
+measured one of the extraordinary length of 254 feet, and similar
+variations are recorded in other animals.[22]
+
+The number of ribs varies in many animals. Mr. St. George Mivart says:
+"In the highest forms of the Primates, the number of true ribs is seven,
+but in Hylobates there are sometimes eight pairs. In Semnopithecus and
+Colobus there are generally seven, but sometimes eight pairs of true
+ribs. In the Cebidae there are generally seven or eight pairs, but in
+Ateles sometimes nine" (_Proc. Zool. Soc._, 1865, p. 568). In the same
+paper it is stated that the number of dorsal vertebrae in man is
+normally twelve, very rarely thirteen. In the Chimpanzee there are
+normally thirteen dorsal vertebrae, but occasionally there are fourteen
+or only twelve.
+
+
+_Variations in the Skull._
+
+[Illustration: FIG. 14.--Variation of Skull of Wolf. 10 specimens.]
+
+Among the nine adult male Orang-utans, collected by myself in Borneo,
+the skulls differed remarkably in size and proportions. The orbits
+varied in width and height, the cranial ridge was either single or
+double, either much or little developed, and the zygomatic aperture
+varied considerably in size. I noted particularly that these
+variations bore no necessary relation to each other, so that a large
+temporal muscle and zygomatic aperture might exist either with a large
+or a small cranium; and thus was explained the curious difference
+between the single-crested and the double-crested skulls, which had been
+supposed to characterise distinct species. As an instance of the amount
+of variation in the skulls of fully adult male orangs, I found the width
+between the orbits externally to be only 4 inches in one specimen and
+fully 5 inches in another.
+
+Exact measurements of large series of comparable skulls of the mammalia
+are not easily found, but from those available I have prepared three
+diagrams (Figs. 14, 15, and 16), in order to exhibit the facts of
+variation in this very important organ. The first shows the variation in
+ten specimens of the common wolf (Canis lupus) from one district in
+North America, and we see that it is not only large in amount, but that
+each part exhibits a considerable independent variability.[23]
+
+In Diagram 15 we have the variations of eight skulls of the Indian
+Honey-bear (Ursus labiatus), as tabulated by the late Dr. J.E. Gray of
+the British Museum. For such a small number of specimens the amount of
+variation is very large--from one-eighth to one-fifth of the mean
+size,--while there are an extraordinary number of instances of
+independent variability. In Diagram 16 we have the length and width of
+twelve skulls of adult males of the Indian wild boar (Sus cristatus),
+also given by Dr. Gray, exhibiting in both sets of measurements a
+variation of more than one-sixth, combined with a very considerable
+amount of independent variability.[24]
+
+[Illustration: FIG. 15.--Variation of 8 skulls (Ursus labiatus).]
+
+[Illustration: FIG. 16.]
+
+The few facts now given, as to variations of the internal parts of
+animals, might be multiplied indefinitely by a search through the
+voluminous writings of comparative anatomists. But the evidence already
+adduced, taken in conjunction with the much fuller evidence of variation
+in all external organs, leads us to the conclusion that wherever
+variations are looked for among a considerable number of individuals of
+the more common species they are sure to be found; that they are
+everywhere of considerable amount, often reaching 20 per cent of the
+size of the part implicated; and that they are to a great extent
+independent of each other, and thus afford almost any combination of
+variations that may be needed.
+
+It must be particularly noticed that the whole series of
+variation-diagrams here given (except the three which illustrate the
+number of varying individuals) in every case represent the actual amount
+of the variation, not on any reduced or enlarged scale, but as it were
+life-size. Whatever number of inches or decimals of an inch the species
+varies in any of its parts is marked on the diagrams, so that with the
+help of an ordinary divided rule or a pair of compasses the variation of
+the different parts can be ascertained and compared just as if the
+specimens themselves were before the reader, but with much greater ease.
+
+In my lectures on the Darwinian theory in America and in this country I
+used diagrams constructed on a different plan, equally illustrating the
+large amount of independent variability, but less simple and less
+intelligible. The present method is a modification of that used by Mr.
+Francis Galton in his researches on the theory of variability, the upper
+line (showing the variability of the body) in Diagrams 4, 5, 6, and 13,
+being laid down on the method he has used in his experiments with
+sweet-peas and in pedigree moth-breeding.[25] I believe, after much
+consideration, and many tedious experiments in diagram-making, that no
+better method can be adopted for bringing before the eye, both the
+amount and the peculiar features of individual variability.
+
+
+_Variations of the Habits of Animals._
+
+Closely connected with those variations of internal and external
+structure which have been already described, are the changes of habits
+which often occur in certain individuals or in whole species, since
+these must necessarily depend upon some corresponding change in the
+brain or in other parts of the organism; and as these changes are of
+great importance in relation to the theory of instinct, a few examples
+of them will be now adduced.
+
+The Kea (Nestor notabilis) is a curious parrot inhabiting the mountain
+ranges of the Middle Island of New Zealand. It belongs to the family of
+Brush-tongued parrots, and naturally feeds on the honey of flowers and
+the insects which frequent them, together with such fruits or berries as
+are found in the region. Till quite recently this comprised its whole
+diet, but since the country it inhabits has become occupied by Europeans
+it has developed a taste for a carnivorous diet, with alarming results.
+It began by picking the sheepskins hung out to dry or the meat in
+process of being cured. About 1868 it was first observed to attack
+living sheep, which had frequently been found with raw and bleeding
+wounds on their backs. Since then it is stated that the bird actually
+burrows into the living sheep, eating its way down to the kidneys, which
+form its special delicacy. As a natural consequence, the bird is being
+destroyed as rapidly as possible, and one of the rare and curious
+members of the New Zealand fauna will no doubt shortly cease to exist.
+The case affords a remarkable instance of how the climbing feet and
+powerful hooked beak developed for one set of purposes can be applied to
+another altogether different purpose, and it also shows how little real
+stability there may be in what appear to us the most fixed habits of
+life. A somewhat similar change of diet has been recorded by the Duke of
+Argyll, in which a goose, reared by a golden eagle, was taught by its
+foster-parent to eat flesh, which it continued to do regularly and
+apparently with great relish.[26]
+
+Change of habits appears to be often a result of imitation, of which Mr.
+Tegetmeier gives some good examples. He states that if pigeons are
+reared exclusively with small grain, as wheat or barley, they will
+starve before eating beans. But when they are thus starving, if a
+bean-eating pigeon is put among them, they follow its example, and
+thereafter adopt the habit. So fowls sometimes refuse to eat maize, but
+on seeing others eat it, they do the same and become excessively fond of
+it. Many persons have found that their yellow crocuses were eaten by
+sparrows, while the blue, purple, and white coloured varieties were left
+untouched; but Mr. Tegetmeier, who grows only these latter colours,
+found that after two years the sparrows began to attack them, and
+thereafter destroyed them quite as readily as the yellow ones; and he
+believes it was merely because some bolder sparrow than the rest set the
+example. On this subject Mr. Charles C. Abbott well remarks: "In
+studying the habits of our American birds--and I suppose it is true of
+birds everywhere--it must at all times be remembered that there is less
+stability in the habits of birds than is usually supposed; and no
+account of the habits of any one species will exactly detail the various
+features of its habits as they really are, in every portion of the
+territory it inhabits."[27]
+
+Mr. Charles Dixon has recorded a remarkable change in the mode of
+nest-building of some common chaffinches which were taken to New Zealand
+and turned out there. He says: "The cup of the nest is small, loosely
+put together, apparently lined with feathers, and the walls of the
+structure are prolonged for about 18 inches, and hang loosely down the
+side of the supporting branch. The whole structure bears some
+resemblance to the nests of the hangnests (Icteridae), with the
+exception that the cavity is at the top. Clearly these New Zealand
+chaffinches were at a loss for a design when fabricating their nest.
+They had no standard to work by, no nests of their own kind to copy, no
+older birds to give them any instruction, and the result is the abnormal
+structure I have just described."[28]
+
+These few examples are sufficient to show that both the habits and
+instincts of animals are subject to variation; and had we a sufficient
+number of detailed observations we should probably find that these
+variations were as numerous, as diverse in character, as large in
+amount, and as independent of each other as those which we have seen to
+characterise their bodily structure.
+
+
+_The Variability of Plants._
+
+The variability of plants is notorious, being proved not only by the
+endless variations which occur whenever a species is largely grown by
+horticulturists, but also by the great difficulty that is felt by
+botanists in determining the limits of species in many large genera. As
+examples we may take the roses, the brambles, and the willows as well
+illustrating this fact. In Mr. Baker's _Revision of the British Roses_
+(published by the Linnean Society in 1863), he includes under the single
+species, Rosa canina--the common dog-rose--no less than twenty-eight
+named _varieties_ distinguished by more or less constant characters and
+often confined to special localities, and to these are referred about
+seventy of the _species_ of British and continental botanists. Of the
+genus Rubus or bramble, _five_ British species are given in Bentham's
+_Handbook of the British Flora_, while in the fifth edition of
+Babington's _Manual of British Botany_, published about the same time,
+no less than _forty-five_ species are described. Of willows (Salix) the
+same two works enumerate _fifteen_ and _thirty-one_ species
+respectively. The hawkweeds (Hieracium) are equally puzzling, for while
+Mr. Bentham admits only seven British species, Professor Babington
+describes no less than thirty-two, besides several named varieties.
+
+A French botanist, Mons. A. Jordan, has collected numerous forms of a
+common little plant, the spring whitlow-grass (Draba verna); he has
+cultivated these for several successive years, and declares that they
+preserve their peculiarities unchanged; he also says that they each come
+true from seed, and thus possess all the characteristics of true
+species. He has described no less than fifty-two such species or
+permanent varieties, all found in the south of France; and he urges
+botanists to follow his example in collecting, describing, and
+cultivating all such varieties as may occur in their respective
+districts. Now, as the plant is very common almost all over Europe and
+ranges from North America to the Himalayas, the number of similar forms
+over this wide area would probably have to be reckoned by hundreds if
+not by thousands.
+
+The class of facts now adduced must certainly be held to prove that in
+many large genera and in some single species there is a very large
+amount of variation, which renders it quite impossible for experts to
+agree upon the limits of species. We will now adduce a few striking
+cases of individual variation.
+
+The distinguished botanist, Alp. de Candolle, made a special study of
+the oaks of the whole world, and has stated some remarkable facts as to
+their variability. He declares that on the same branch of oak he has
+noted the following variations: (1) In the length of the petiole, as one
+to three; (2) in the form of the leaf, being either elliptical or
+obovoid; (3) in the margin being entire, or notched, or even pinnatifid;
+(4) in the extremity being acute or blunt; (5) in the base being sharp,
+blunt, or cordate; (6) in the surface being pubescent or smooth; (7) the
+perianth varies in depth and lobing; (8) the stamens vary in number,
+independently; (9) the anthers are mucronate or blunt; (10) the fruit
+stalks vary greatly in length, often as one to three; (11) the number of
+fruits varies; (12) the form of the base of the cup varies; (13) the
+scales of the cup vary in form; (14) the proportions of the acorns vary;
+(15) the times of the acorns ripening and falling vary.
+
+Besides this, many species exhibit well-marked varieties which have been
+described and named, and these are most numerous in the best-known
+species. Our British oak (Quercus robur) has twenty-eight varieties;
+Quercus Lusitanica has eleven; Quercus calliprinos has ten; and Quercus
+coccifera eight.
+
+A most remarkable case of variation in the parts of a common flower has
+been given by Dr. Hermann Mueller. He examined two hundred flowers of
+Myosurus minimus, among which he found _thirty-five_ different
+proportions of the sepals, petals, and anthers, the first varying from
+four to seven, the second from two to five, and the third from two to
+ten. Five sepals occurred in one hundred and eighty-nine out of the two
+hundred, but of these one hundred and five had three petals, forty-six
+had four petals, and twenty-six had five petals; but in each of these
+sets the anthers varied in number from three to eight, or from two to
+nine. We have here an example of the same amount of "independent
+variability" that, as we have seen, occurs in the various dimensions of
+birds and mammals; and it may be taken as an illustration of the kind
+and degree of variability that may be expected to occur among small and
+little specialised flowers.[29]
+
+In the common wind-flower (Anemone nemorosa) an almost equal amount of
+variation occurs; and I have myself gathered in one locality flowers
+varying from 7/8 inch to 1-3/4 inch in diameter; the bracts varying from
+1-1/2 inch to 4 inches across; and the petaloid sepals either broad or
+narrow, and varying in number from five to ten. Though generally pure
+white on their upper surface, some specimens are a full pink, while
+others have a decided bluish tinge.
+
+Mr. Darwin states that he carefully examined a large number of plants of
+Geranium phaeum and G. pyrenaicum (not perhaps truly British but
+frequently found wild), which had escaped from cultivation, and had
+spread by seed in an open plantation; and he declares that "the
+seedlings varied in almost every single character, both in their flowers
+and foliage, to a degree which I have never seen exceeded; yet they
+could not have been exposed to any great change of their
+conditions."[30]
+
+The following examples of variation in important parts of plants were
+collected by Mr. Darwin and have been copied from his unpublished
+MSS.:--
+
+"De Candolle (_Mem. Soc. Phys. de Geneve_, tom. ii. part ii. p. 217)
+states that Papaver bracteatum and P. orientale present indifferently
+two sepals and four petals, or three sepals and six petals, which is
+sufficiently rare with other species of the genus."
+
+"In the Primulacae and in the great class to which this family belongs
+the unilocular ovarium is free, but M. Dubury (_Mem. Soc. Phys. de
+Geneve_, tom. ii. p. 406) has often found individuals in Cyclamen
+hederaefolium, in which the base of the ovary was connected for a third
+part of its length with the inferior part of the calyx."
+
+"M. Aug. St. Hilaire (Sur la Gynobase, _Mem. des Mus. d'Hist. Nat._,
+tom. x. p. 134), speaking of some bushes of the Gomphia oleaefolia,
+which he at first thought formed a quite distinct species, says: 'Voila
+donc dans un meme individu des loges et un style qui se rattachent
+tantot a un axe vertical, et tantot a un gynobase; donc celui-ci n'est
+qu'un axe veritable; mais cet axe est deprime au lieu d'etre vertical."
+He adds (p. 151), 'Does not all this indicate that nature has tried, in
+a manner, in the family of Rutaceae to produce from a single
+multilocular ovary, one-styled and symmetrical, several unilocular
+ovaries, each with its own style.' And he subsequently shows that, in
+Xanthoxylum monogynum, 'it often happens that on the same plant, on the
+same panicle, we find flowers with one or with two ovaries;' and that
+this is an important character is shown by the Rutaceae (to which
+Xanthoxylum belongs), being placed in a group of natural orders
+characterised by having a solitary ovary."
+
+"De Candolle has divided the Cruciferae into five sub-orders in
+accordance with the position of the radicle and cotyledons, yet Mons. T.
+Gay (_Ann. des Scien. Nat._, ser. i. tom. vii. p. 389) found in sixteen
+seeds of Petrocallis Pyrenaica the form of the embryo so uncertain that
+he could not tell whether it ought to be placed in the sub-orders
+'Pleurorhizee' or 'Notor-hizee'; so again (p. 400) in Cochlearia
+saxatilis M. Gay examined twenty-nine embryos, and of these sixteen were
+vigorously 'pleurorhizees,' nine had characters intermediate between
+pleuro-and notor-hizees, and four were pure notor-hizees."
+
+"M. Raspail asserts (_Ann. des Scien. Nat._, ser. i. tom. v. p. 440)
+that a grass (Nostus Borbonicus) is so eminently variable in its floral
+organisation, that the varieties might serve to make a family with
+sufficiently numerous genera and tribes--a remark which shows that
+important organs must be here variable."
+
+
+_Species which vary little._
+
+The preceding statements, as to the great amount of variation occurring
+in animals and plants, do not prove that all species vary to the same
+extent, or even vary at all, but, merely, that a considerable number of
+species in every class, order, and family do so vary. It will have been
+observed that the examples of great variability have all been taken from
+common species, or species which have a wide range and are abundant in
+individuals. Now Mr. Darwin concludes, from an elaborate examination of
+the floras and faunas of several distinct regions, that common, wide
+ranging species, as a rule, vary most, while those that are confined to
+special districts and are therefore comparatively limited in number of
+individuals vary least. By a similar comparison it is shown that species
+of large genera vary more than species of small genera. These facts
+explain, to some extent, why the opinion has been so prevalent that
+variation is very limited in amount and exceptional in character. For
+naturalists of the old school, and all mere collectors, were interested
+in species in proportion to their rarity, and would often have in their
+collections a larger number of specimens of a rare species than of a
+species that was very common. Now as these rare species do really vary
+much less than the common species, and in many cases hardly vary at all,
+it was very natural that a belief in the fixity of species should
+prevail. It is not, however, as we shall see presently, the rare, but
+the common and widespread species which become the parents of new forms,
+and thus the non-variability of any number of rare or local species
+offers no difficulty whatever in the way of the theory of evolution.
+
+
+_Concluding Remarks._
+
+We have now shown in some detail, at the risk of being tedious, that
+individual variability is a general character of all common and
+widespread species of animals or plants; and, further, that this
+variability extends, so far as we know, to every part and organ, whether
+external or internal, as well as to every mental faculty. Yet more
+important is the fact that each part or organ varies to a considerable
+extent independently of other parts. Again, we have shown, by abundant
+evidence, that the variation that occurs is very large in
+amount--usually reaching 10 or 20, and sometimes even 25 per cent of the
+average size of the varying part; while not one or two only, but from 5
+to 10 per cent of the specimens examined exhibit nearly as large an
+amount of variation. These facts have been brought clearly before the
+reader by means of numerous diagrams, drawn to scale and exhibiting the
+actual variations in inches, so that there can be no possibility of
+denying either their generality or their amount. The importance of this
+full exposition of the subject will be seen in future chapters, when we
+shall frequently have to refer to the facts here set forth, especially
+when we deal with the various theories of recent writers and the
+criticisms that have been made of the Darwinian theory.
+
+A full exposition of the facts of variation among wild animals and
+plants is the more necessary, because comparatively few of them were
+published in Mr. Darwin's works, while the more important have only been
+made known since the last edition of _The Origin of Species_ was
+prepared; and it is clear that Mr. Darwin himself did not fully
+recognise the enormous amount of variability that actually exists. This
+is indicated by his frequent reference to the extreme slowness of the
+changes for which variation furnishes the materials, and also by his use
+of such expressions as the following: "A variety when once formed must
+again, _perhaps after a long interval of time_, vary or present
+individual differences of the same favourable nature as before"
+(_Origin_, p. 66). And again, after speaking of changed conditions
+"affording a better chance of the occurrence of favourable variations,"
+he adds: "_Unless such occur natural selection can do nothing_"
+(_Origin_, p. 64). These expressions are hardly consistent with the fact
+of the constant and large amount of variation, of every part, in all
+directions, which evidently occurs in each generation of all the more
+abundant species, and which must afford an ample supply of favourable
+variations whenever required; and they have been seized upon and
+exaggerated by some writers as proofs of the extreme difficulties in the
+way of the theory. It is to show that such difficulties do not exist,
+and in the full conviction that an adequate knowledge of the facts of
+variation affords the only sure foundation for the Darwinian theory of
+the origin of species, that this chapter has been written.
+
+FOOTNOTES:
+
+[Footnote 16: _Foraminifera_, preface, p. x.]
+
+[Footnote 17: _United States Geological Survey of the Territories_,
+1874.]
+
+[Footnote 18: _Proceedings of the Entomological Society of London_,
+1875, p. vii.]
+
+[Footnote 19: _Ann. des Sci. Nat._, tom. xvi. p. 50.]
+
+[Footnote 20: See _Winter Birds of Florida_, p. 206, Table F.]
+
+[Footnote 21: See Table I, p. 211, of Allen's _Winter Birds of
+Florida_.]
+
+[Footnote 22: _Proc. Zool. Soc._, 1864, p. 64.]
+
+[Footnote 23: J.A. Allen, on Geographical Variation among North American
+Mammals, _Bull. U.S. Geol. and Geog. Survey_, vol. ii. p. 314 (1876).]
+
+[Footnote 24: _Proc. Zool. Soc. Lond._, 1864, p. 700, and 1868, p. 28.]
+
+[Footnote 25: See _Trans. Entomological Society of London_, 1887, p.
+24.]
+
+[Footnote 26: _Nature_, vol. xix. p. 554.]
+
+[Footnote 27: _Nature_, vol. xvi. p. 163; and vol. xi. p. 227.]
+
+[Footnote 28: _Ibid._, vol. xxxi. (1885), p. 533.]
+
+[Footnote 29: _Nature_, vol. xxvi. p. 81.]
+
+[Footnote 30: _Animals and Plants under Domestication_, vol. ii. p.
+258.]
+
+
+
+
+CHAPTER IV
+
+VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS
+
+
+    The facts of variation and artificial selection--Proofs of the
+    generality of variation--Variations of apples and
+    melons--Variations of flowers--Variations of domestic
+    animals--Domestic pigeons--Acclimatisation--Circumstances
+    favourable to selection by man--Conditions favourable to
+    variation--Concluding remarks.
+
+
+
+Having so fully discussed variation under nature it will be unnecessary
+to devote so much space to domesticated animals and cultivated plants,
+especially as Mr. Darwin has published two remarkable volumes on the
+subject where those who desire it may obtain ample information. A
+general sketch of the more important facts will, however, be given, for
+the purpose of showing how closely they correspond with those described
+in the preceding chapter, and also to point out the general principles
+which they illustrate. It will also be necessary to explain how these
+variations have been increased and accumulated by artificial selection,
+since we are thereby better enabled to understand the action of natural
+selection, to be discussed in the succeeding chapter.
+
+
+_The facts of Variation and Artificial Selection._
+
+Every one knows that in each litter of kittens or of puppies no two are
+alike. Even in the case in which several are exactly alike in colours,
+other differences are always perceptible to those who observe them
+closely. They will differ in size, in the proportions of their bodies
+and limbs, in the length or texture of their hairy covering, and notably
+in their disposition. They each possess, too, an individual
+countenance, almost as varied when closely studied as that of a human
+being; not only can a shepherd distinguish every sheep in his flock, but
+we all know that each kitten in the successive families of our old
+favourite cat has a face of its own, with an expression and
+individuality distinct from all its brothers and sisters. Now this
+individual variability exists among all creatures whatever, which we can
+closely observe, even when the two parents are very much alike and have
+been matched in order to preserve some special breed. The same thing
+occurs in the vegetable kingdom. All plants raised from seed differ more
+or less from each other. In every bed of flowers or of vegetables we
+shall find, if we look closely, that there are countless small
+differences, in the size, in the mode of growth, in the shape or colour
+of the leaves, in the form, colour, or markings of the flowers, or in
+the size, form, colour, or flavour of the fruit. These differences are
+usually small, but are yet easily seen, and in their extremes are very
+considerable; and they have this important quality, that they have a
+tendency to be reproduced, and thus by careful breeding any particular
+variation or group of variations can be increased to an enormous
+extent--apparently to any extent not incompatible with the life, growth,
+and reproduction of the plant or animal.
+
+The way this is done is by artificial selection, and it is very
+important to understand this process and its results. Suppose we have a
+plant with a small edible seed, and we want to increase the size of that
+seed. We grow as large a quantity of it as possible, and when the crop
+is ripe we carefully choose a few of the very largest seeds, or we may
+by means of a sieve sort out a quantity of the largest seeds. Next year
+we sow only these large seeds, taking care to give them suitable soil
+and manure, and the result is found to be that the _average_ size of the
+seeds is larger than in the first crop, and that the largest seeds are
+now somewhat larger and more numerous. Again sowing these, we obtain a
+further slight increase of size, and in a very few years we obtain a
+greatly improved race, which will always produce larger seeds than the
+unimproved race, even if cultivated without any special care. In this
+way all our fine sorts of vegetables, fruits, and flowers have been
+obtained, all our choice breeds of cattle or of poultry, our wonderful
+race-horses, and our endless varieties of dogs. It is a very common but
+mistaken idea that this improvement is due to crossing and feeding in
+the case of animals, and to improved cultivation in the case of plants.
+Crossing is occasionally used in order to obtain a combination of
+qualities found in two distinct breeds, and also because it is found to
+increase the constitutional vigour; but every breed possessing any
+exceptional quality is the result of the selection of variations
+occurring year after year and accumulated in the manner just described.
+Purity of breed, with repeated selection of the best varieties of that
+breed, is the foundation of all improvement in our domestic animals and
+cultivated plants.
+
+
+_Proofs of the Generality of Variation._
+
+Another very common error is, that variation is the exception, and
+rather a rare exception, and that it occurs only in one direction at a
+time--that is, that only one or two of the numerous possible modes of
+variation occur at the same time. The experience of breeders and
+cultivators, however, proves that variation is the rule instead of the
+exception, and that it occurs, more or less, in almost every direction.
+This is shown by the fact that different species of plants and animals
+have required different _kinds_ of modification to adapt them to our
+use, and we have never failed to meet with variation _in that particular
+direction_, so as to enable us to accumulate it and so to produce
+ultimately a large amount of change in the required direction. Our
+gardens furnish us with numberless examples of this property of plants.
+In the cabbage and lettuce we have found variation in the size and mode
+of growth of the leaf, enabling us to produce by selection the almost
+innumerable varieties, some with solid heads of foliage quite unlike any
+plant in a state of nature, others with curiously wrinkled leaves like
+the savoy, others of a deep purple colour used for pickling. From the
+very same species as the cabbage (Brassica oleracea) have arisen the
+broccoli and cauliflower, in which the leaves have undergone little
+alteration, while the branching heads of flowers grow into a compact
+mass forming one of our most delicate vegetables. The brussels sprouts
+are another form of the same plant, in which the whole mode of growth
+has been altered, numerous little heads of leaves being produced on the
+stem. In other varieties the ribs of the leaves are thickened so as to
+become themselves a culinary vegetable; while, in the Kohlrabi, the stem
+grows into a turnip-like mass just above ground. Now all these
+extraordinarily distinct plants come from one original species which
+still grows wild on our coasts; and it must have varied in all these
+directions, otherwise variations could not have been accumulated to the
+extent we now see them. The flowers and seeds of all these plants have
+remained nearly stationary, because no attempt has been made to
+accumulate the slight variations that no doubt occur in them.
+
+If now we turn to another set of plants, the turnips, radishes, carrots,
+and potatoes, we find that the roots or underground tubers have been
+wonderfully enlarged and improved, and also altered in shape and colour,
+while the stems, leaves, flowers, and fruits have remained almost
+unchanged. In the various kinds of peas and beans it is the pod or fruit
+and the seed that has been subjected to selection, and therefore greatly
+modified; and it is here very important to notice that while all these
+plants have undergone cultivation in a great variety of soils and
+climates, with different manures and under different systems, yet the
+flowers have remained but little altered, those of the broad bean, the
+scarlet-runner, and the garden-pea, being nearly the same in all the
+varieties. This shows us how little change is produced by mere
+cultivation, or even by variety of soil and climate, if there is no
+_selection_ to preserve and accumulate the small variations that are
+continually occurring. When, however, a great amount of modification has
+been effected in one country, change to another country produces a
+decided effect. Thus it has been found that some of the numerous
+varieties of maize produced and cultivated in the United States change
+considerably, not only in their size and colour, but even in the shape
+of the seed when grown for a few successive years in Germany.[31] In all
+our cultivated fruit trees the fruits vary immensely in shape, size,
+colour, flavour, time of ripening, and other qualities, while the leaves
+and flowers usually differ so little that they are hardly
+distinguishable except to a very close observer.
+
+
+_Variations of Apples and of Melons._
+
+The most remarkable varieties are afforded by the apple and the melon,
+and some account of these will be given as illustrating the effects of
+slight variations accumulated by selection. All our apples are known to
+have descended from the common crab of our hedges (Pyrus malus), and
+from this at least a thousand distinct varieties have been produced.
+These differ greatly in the size and form of the fruit, in its colour,
+and in the texture of the skin. They further differ in the time of
+ripening, in their flavour, and in their keeping properties; but apple
+trees also differ in many other ways. The foliage of the different
+varieties can often be distinguished by peculiarities of form and
+colour, and it varies considerably in the time of its appearance; in
+some hardly a leaf appears till the tree is in full bloom, while others
+produce their leaves so early as almost to hide the flowers. The flowers
+differ in size and colour, and in one case in structure also, that of
+the St. Valery apple having a double calyx with ten divisions, and
+fourteen styles with oblique stigmas, but without stamens or corolla.
+The flowers, therefore, have to be fertilised with the pollen from other
+varieties in order to produce fruit. The pips or seeds differ also in
+shape, size, and colour; some varieties are liable to canker more than
+others, while the Winter Majetin and one or two others have the strange
+constitutional peculiarity of never being attacked by the mealy bug even
+when all the other trees in the same orchard are infested with it.
+
+All the cucumbers and gourds vary immensely, but the melon (Cucumis
+melo) exceeds them all. A French botanist, M. Naudin, devoted six years
+to their study. He found that previous botanists had described thirty
+distinct species, as they thought, which were really only varieties of
+melons. They differ chiefly in their fruits, but also very much in
+foliage and mode of growth. Some melons are only as large as small
+plums, others weigh as much as sixty-six pounds. One variety has a
+scarlet fruit. Another is not more than an inch in diameter, but
+sometimes more than a yard in length, twisting about in all directions
+like a serpent. Some melons are exactly like cucumbers; and an Algerian
+variety, when ripe, cracks and falls to pieces, just as occurs in a
+wild gourd (C. momordica).[32]
+
+
+_Variations of Flowers._
+
+Turning to flowers, we find that in the same genus as our currant and
+gooseberry, which we have cultivated for their fruits, there are some
+ornamental species, as the Ribes sanguinea, and in these the flowers
+have been selected so as to produce deep red, pink, or white varieties.
+When any particular flower becomes fashionable and is grown in large
+quantities, variations are always met with sufficient to produce great
+varieties of tint or marking, as shown by our roses, auriculas, and
+geraniums. When varied leaves are required, it is found that a number of
+plants vary sufficiently in this direction also, and we have zonal
+geraniums, variegated ivies, gold and silver marked hollies, and many
+others.
+
+
+_Variations of Domestic Animals._
+
+Coming now to our domesticated animals, we find still more extraordinary
+cases; and it appears as if any special quality or modification in an
+animal can be obtained if we only breed it in sufficient quantity, watch
+carefully for the required variations, and carry on selection with
+patience and skill for a sufficiently long period. Thus, in sheep we
+have enormously increased the wool, and have obtained the power of
+rapidly forming flesh and fat; in cows we have increased the production
+of milk; in horses we have obtained strength, endurance, or speed, and
+have greatly modified size, form, and colour; in poultry we have secured
+various colours of plumage, increase of size, and almost perpetual
+egg-laying. But it is in dogs and pigeons that the most marvellous
+changes have been effected, and these require our special attention.
+
+Our various domestic dogs are believed to have originated from several
+distinct wild species, because in every part of the world the native
+dogs resemble some wild dogs or wolves of the same country. Thus perhaps
+several species of wolves and jackals were domesticated in very early
+times, and from breeds derived from these, crossed and improved by
+selection, our existing dogs have descended. But this intermixture of
+distinct species will go a very little way in accounting for the
+peculiarities of the different breeds of dogs, many of which are totally
+unlike any wild animal. Such is the case with greyhounds, bloodhounds,
+bulldogs, Blenheim spaniels, terriers, pugs, turnspits, pointers, and
+many others; and these differ so greatly in size, shape, colour, and
+habits, as well as in the form and proportions of all the different
+parts of the body, that it seems impossible that they could have
+descended from any of the known wild dogs, wolves, or allied animals,
+none of which differ nearly so much in size, form, and proportions. We
+have here a remarkable proof that variation is not confined to
+superficial characters--to the colour, hair, or external appendages,
+when we see how the entire skeletons of such forms as the greyhound and
+the bulldog have been gradually changed in opposite directions till they
+are both completely unlike that of any known wild animal, recent or
+extinct. These changes have been the result of some thousands of years
+of domestication and selection, different breeds being used and
+preserved for different purposes; but some of the best breeds are known
+to have been improved and perfected in modern times. About the middle of
+the last century a new and improved kind of foxhound was produced; the
+greyhound was also greatly improved at the end of the last century,
+while the true bulldog was brought to perfection about the same period.
+The Newfoundland dog has been so much changed since it was first
+imported that it is now quite unlike any existing native dog in that
+island.[33]
+
+
+_Domestic Pigeons._
+
+The most remarkable and instructive example of variation produced by
+human selection is afforded by the various races and breeds of domestic
+pigeons, not only because the variations produced are often most
+extraordinary in amount and diverse in character, but because in this
+case there is no doubt whatever that all have been derived from one wild
+species, the common rock-pigeon (Columba livia). As this is a very
+important point it is well to state the evidence on which the belief is
+founded. The wild rock-pigeon is of a slaty-blue colour, the tail has a
+dark band across the end, the wings have two black bands, and the outer
+tail-feathers are edged with white at the base. No other wild pigeon in
+the world has this combination of characters. Now in every one of the
+domestic varieties, even the most extreme, all the above marks, even to
+the white edging of the outer tail-feathers, are sometimes found
+perfectly developed. When birds belonging to two distinct breeds are
+crossed one or more times, neither of the parents being blue, or having
+any of the above-named marks, the mongrel offspring are very apt to
+acquire some of these characters. Mr. Darwin gives instances which he
+observed himself. He crossed some white fantails with some black barbs,
+and the mongrels were black, brown, or mottled. He also crossed a barb
+with a spot, which is a white bird with a red tail and red spot on the
+forehead, and the mongrel offspring were dusky and mottled. On now
+crossing these two sets of mongrels with each other, he obtained a bird
+of a beautiful blue colour, with the barred and white edged tail, and
+double-banded wings, so as almost exactly to resemble a wild
+rock-pigeon. This bird was descended in the second generation from a
+pure white and pure black bird, both of which when unmixed breed their
+kind remarkably true. These facts, well known to experienced
+pigeon-fanciers, together with the habits of the birds, which all like
+to nest in holes, or dovecots, not in trees like the great majority of
+wild pigeons, have led to the general belief in the single origin of all
+the different kinds.
+
+In order to afford some idea of the great differences which exist among
+domesticated pigeons, it will be well to give a brief abstract of Mr.
+Darwin's account of them. He divides them into eleven distinct races,
+most of which have several sub-races.
+
+RACE I. _Pouters_.--These are especially distinguished by the enormously
+enlarged crop, which can be so inflated in some birds as almost to
+conceal the beak. They are very long in the body and legs and stand
+almost upright, so as to present a very distinct appearance. Their
+skeleton has become modified, the ribs being broader and the vertebrae
+more numerous than in other pigeons.
+
+RACE II. _Carriers_.--These are large, long-necked birds, with a long
+pointed beak, and the eyes surrounded with a naked carunculated skin or
+wattle, which is also largely developed at the base of the beak. The
+opening of the mouth is unusually wide. There are several sub-races, one
+being called Dragons.
+
+RACE III. _Runts_.--These are very large-bodied, long-beaked pigeons,
+with naked skin round the eyes. The wings are usually very long, the
+legs long, and the feet large, and the skin of the neck is often red.
+There are several sub-races, and these differ very much, forming a
+series of links between the wild rock-pigeon and the carrier.
+
+RACE IV. _Barbs_.--These are remarkable for their very short and thick
+beak, so unlike that of most pigeons that fanciers compare it with that
+of a bullfinch. They have also a naked carunculated skin round the eyes,
+and the skin over the nostrils swollen.
+
+RACE V. _Fantails_.--Short-bodied and rather small-beaked pigeons, with
+an enormously developed tail, consisting usually of from fourteen to
+forty feathers instead of twelve, the regular number in all other
+pigeons, wild and tame. The tail spreads out like a fan and is usually
+carried erect, and the bird bends back its slender neck, so that in
+highly-bred varieties the head touches the tail. The feet are small, and
+they walk stiffly.
+
+RACE VI. _Turbits and Owls_.--These are characterised by the feathers of
+the middle of neck and breast in front spreading out irregularly so as
+to form a frill. The Turbits also have a crest on the head, and both
+have the beak exceedingly short.
+
+RACE VII. _Tumblers_.--- These have a small body and short beak, but
+they are specially distinguished by the singular habit of tumbling over
+backwards during flight. One of the sub-races, the Indian Lotan or
+Ground tumbler, if slightly shaken and placed on the ground, will
+immediately begin tumbling head over heels until taken up and soothed.
+If not taken up, some of them will go on tumbling till they die. Some
+English tumblers are almost equally persistent. A writer, quoted by Mr.
+Darwin, says that these birds generally begin to tumble almost as soon
+as they can fly; "at three months old they tumble well, but still fly
+strong; at five or six months they tumble excessively; and in the second
+year they mostly give up flying, on account of their tumbling so much
+and so close to the ground. Some fly round with the flock, throwing a
+clean summersault every few yards till they are obliged to settle from
+giddiness and exhaustion. These are called Air-tumblers, and they
+commonly throw from twenty to thirty summersaults in a minute, each
+clear and clean. I have one red cock that I have on two or three
+occasions timed by my watch, and counted forty summersaults in the
+minute. At first they throw a single summersault, then it is double,
+till it becomes a continuous roll, which puts an end to flying, for if
+they fly a few yards over they go, and roll till they reach the ground.
+Thus I had one kill herself, and another broke his leg. Many of them
+turn over only a few inches from the ground, and will tumble two or
+three times in flying across their loft. These are called House-tumblers
+from tumbling in the house. The act of tumbling seems to be one over
+which they have no control, an involuntary movement which they seem to
+try to prevent. I have seen a bird sometimes in his struggles fly a yard
+or two straight upwards, the impulse forcing him backwards while he
+struggles to go forwards."[34]
+
+The Short-faced tumblers are an improved sub-race which have almost lost
+the power of tumbling, but are valued for possessing some other
+characteristics in an extreme degree. They are very small, have almost
+globular heads, and a very minute beak, so that fanciers say the head of
+a perfect bird should resemble a cherry with a barleycorn stuck in it.
+Some of these weigh less than seven ounces, whereas the wild rock-pigeon
+weighs about fourteen ounces. The feet, too, are very short and small,
+and the middle toe has twelve or thirteen instead of fourteen or fifteen
+scutellae. They have often only nine primary wing-feathers instead of
+ten as in all other pigeons.
+
+RACE VIII. _Indian Frill-back_.--In these birds the beak is very short,
+and the feathers of the whole body are reversed or turn backwards.
+
+RACE IX. _Jacobin_.--These curious birds have a hood of feathers almost
+enclosing the head and meeting in front of the neck. The wings and tail
+are unusually long.
+
+RACE X. _Trumpeter_.--Distinguished by a tuft of feathers curling
+forwards over the beak, and the feet very much feathered. They obtain
+their name from the peculiar voice unlike that of any other pigeon. The
+coo is rapidly repeated, and is continued for several minutes. The feet
+are covered with feathers so large as often to appear like little wings.
+
+RACE XI. comprises _Laughers_, _Frill-backs_, _Nuns_, _Spots_, _and
+Swallows_.--They are all very like the common rock-pigeon, but have each
+some slight peculiarity. The Laughers have a peculiar voice, supposed to
+resemble a laugh. The Nuns are white, with the head, tail, and primary
+wing-feathers black or red. The Spots are white, with the tail and a
+spot on the forehead red. The Swallows are slender, white in colour,
+with the head and wings of some darker colour.
+
+Besides these races and sub-races a number of other kinds have been
+described, and about one hundred and fifty varieties can be
+distinguished. It is interesting to note that almost every part of the
+bird, whose variations can be noted and selected, has led to variations
+of a considerable extent, and many of these have necessitated changes in
+the plumage and in the skeleton quite as great as any that occur in the
+numerous distinct species of large genera. The form of the skull and
+beak varies enormously, so that the skulls of the Short-faced tumbler
+and some of the Carriers differ more than any wild pigeons, even those
+classed in distinct genera. The breadth and number of the ribs vary, as
+well as the processes on them; the number of the vertebrae and the
+length of the sternum also vary; and the perforations in the sternum
+vary in size and shape. The oil gland varies in development, and is
+sometimes absent. The number of the wing-feathers varies, and those of
+the tail to an enormous extent. The proportions of the leg and feet and
+the number of the scutellae also vary. The eggs also vary somewhat in
+size and shape; and the amount of downy clothing on the young bird, when
+first hatched, differs very considerably. Finally, the attitude of the
+body, the manner of walking, the mode of flight, and the voice, all
+exhibit modifications of the most remarkable kind.[35]
+
+
+_Acclimatisation_.
+
+A very important kind of variation is that constitutional change termed
+acclimatisation, which enables any organism to become gradually adapted
+to a different climate from the parent stock. As closely allied species
+often inhabit different countries possessing very different climates, we
+should expect to find cases illustrating this change among our
+domesticated animals and cultivated plants. A few examples will
+therefore be adduced showing that such constitutional variation does
+occur.
+
+Among animals the cases are not numerous, because no systematic attempt
+has been made to select varieties for this special quality. It has,
+however, been observed that, though no European dogs thrive well in
+India, the Newfoundland dog, originating from a severe climate, can
+hardly be kept alive. A better case, perhaps, is furnished by merino
+sheep, which, when imported directly from England, do not thrive, while
+those which have been bred in the intermediate climate of the Cape of
+Good Hope do much better. When geese were first introduced into Bogota,
+they laid few eggs at long intervals, and few of the young survived. By
+degrees, however, the fecundity improved, and in about twenty years
+became equal to what it is in Europe. According to Garcilaso, when fowls
+were first introduced into Peru they were not fertile, whereas now they
+are as much so as in Europe.
+
+Plants furnish much more important evidence. Our nurserymen distinguish
+in their catalogues varieties of fruit-trees which are more or less
+hardy, and this is especially the case in America, where certain
+varieties only will stand the severe climate of Canada. There is one
+variety of pear, the Forelle, which both in England and France withstood
+frosts that killed the flowers and buds of all other kinds of pears.
+Wheat, which is grown over so large a portion of the world, has become
+adapted to special climates. Wheat imported from India and sown in good
+wheat soil in England produced the most meagre ears; while wheat taken
+from France to the West Indian Islands produced either wholly barren
+spikes or spikes furnished with two or three miserable seeds, while West
+Indian seed by its side yielded an enormous harvest. The orange was very
+tender when first introduced into Italy, and continued so as long as it
+was propagated by grafts, but when trees were raised from seed many of
+these were found to be hardier, and the orange is now perfectly
+acclimatised in Italy. Sweet-peas (Lathyrus odoratus) imported from
+England to the Calcutta Botanic Gardens produced few blossoms and no
+seed; those from France flowered a little better, but still produced no
+seed, but plants raised from seed brought from Darjeeling in the
+Himalayas, but originally derived from England, flower and seed
+profusely in Calcutta.[36]
+
+An observation by Mr. Darwin himself is perhaps even more instructive.
+He says: "On 24th May 1864 there was a severe frost in Kent, and two
+rows of scarlet runners (Phaseolus multiflorus) in my garden, containing
+390 plants of the same age and equally exposed, were all blackened and
+killed except about a dozen plants. In an adjoining row of Fulmer's
+dwarf bean (Phaseolus vulgaris) one single plant escaped. A still more
+severe frost occurred four days afterwards, and of the dozen plants
+which had previously escaped only three survived; these were not taller
+or more vigorous than the other young plants, but they escaped
+completely, with not even the tips of their leaves browned. It was
+impossible to behold these three plants, with their blackened, withered,
+and dead brethren all around them, and not see at a glance that they
+differed widely in their constitutional power of resisting frost."
+
+The preceding sketch of the variation that occurs among domestic animals
+and cultivated plants shows how wide it is in range and how great in
+amount; and we have good reason to believe that similar variation
+extends to all organised beings. In the class of fishes, for example, we
+have one kind which has been long domesticated in the East, the gold
+and silver carps; and these present great variation, not only of colour
+but in the form and structure of the fins and other external organs. In
+like manner, the only domesticated insects, hive bees and silkworm
+moths, present numbers of remarkable varieties which have been produced
+by the selection of chance variations just as in the case of plants and
+the higher animals.
+
+
+_Circumstances favourable to Selection by Man._
+
+It may be supposed, that the systematic selection which has been
+employed for the purpose of improving the races of animals or plants
+useful to man is of comparatively recent origin, though some of the
+different races are known to have been in existence in very early times.
+But Mr. Darwin has pointed out, that unconscious selection must have
+begun to produce an effect as soon as plants were cultivated or animals
+domesticated by man. It would have been very soon observed that animals
+and plants produced their like, that seed of early wheat produced early
+wheat, that the offspring of very swift dogs were also swift, and as
+every one would try to have a good rather than a bad sort this would
+necessarily lead to the slow but steady improvement of all useful plants
+and animals subject to man's care. Soon there would arise distinct
+breeds, owing to the varying uses to which the animals and plants were
+put. Dogs would be wanted chiefly to hunt one kind of game in one part
+of the country and another kind elsewhere; for one purpose scent would
+be more important, for another swiftness, for another strength and
+courage, for yet another watchfulness and intelligence, and this would
+soon lead to the formation of very distinct races. In the case of
+vegetables and fruits, different varieties would be found to succeed
+best in certain soils and climates; some might be preferred on account
+of the quantity of food they produced, others for their sweetness and
+tenderness, while others might be more useful on account of their
+ripening at a particular season, and thus again distinct varieties would
+be established. An instance of unconscious selection leading to distinct
+results in modern times is afforded by two flocks of Leicester sheep
+which both originated from the same stock, and were then bred pure for
+upwards of fifty years by two gentlemen, Mr. Buckley and Mr. Burgess.
+Mr. Youatt, one of the greatest authorities on breeding domestic
+animals, says: "There is not a suspicion existing in the mind of any one
+at all acquainted with the subject that the owner of either of them has
+deviated in any one instance from the pure blood of Mr. Bakewell's
+original flock, and yet the difference between the sheep possessed by
+these two gentlemen is so great that they have the appearance of being
+quite different varieties." In this case there was no desire to deviate
+from the original breed, and the difference must have arisen from some
+slight difference of taste or judgment in selecting, each year, the
+parents for the next year's stock, combined perhaps with some direct
+effect of the slight differences of climate and soil on the two farms.
+
+Most of our domesticated animals and cultivated plants have come to us
+from the earliest seats of civilisation in Western Asia or Egypt, and
+have therefore been the subjects of human care and selection for some
+thousands of years, the result being that, in many cases, we do not know
+the wild stock from which they originally sprang. The horse, the camel,
+and the common bull and cow are nowhere found in a wild state, and they
+have all been domesticated from remote antiquity. The original of the
+domestic fowl is still wild in India and the Malay Islands, and it was
+domesticated in India and China before 1400 B.C. It was introduced into
+Europe about 600 B.C. Several distinct breeds were known to the Romans
+about the commencement of the Christian era, and they have since spread
+all over the civilised world and been subjected to a vast amount of
+conscious and unconscious selection, to many varieties of climate and to
+differences of food; the result being seen in the wonderful diversity of
+breeds which differ quite as remarkably as do the different races of
+pigeons already described.
+
+In the vegetable kingdom, most of the cereals--wheat, barley, etc.--are
+unknown as truly wild plants; and the same is the case with many
+vegetables, for De Candolle states that out of 157 useful cultivated
+plants thirty-two are quite unknown in a wild state, and that forty more
+are of doubtful origin. It is not improbable that most of these do exist
+wild, but they have been so profoundly changed by thousands of years of
+cultivation as to be quite unrecognisable. The peach is unknown in a
+wild state, unless it is derived from the common almond, on which point
+there is much difference of opinion among botanists and horticulturists.
+
+The immense antiquity of most of our cultivated plants sufficiently
+explains the apparent absence of such useful productions in Australia
+and the Cape of Good Hope, notwithstanding that they both possess an
+exceedingly rich and varied flora. These countries having been, until a
+comparatively recent period, inhabited only by uncivilised men, neither
+cultivation nor selection has been carried on for a sufficiently long
+time. In North America, however, where there was evidently a very
+ancient if low form of civilisation, as indicated by the remarkable
+mounds, earthworks, and other prehistoric remains, maize was cultivated,
+though it was probably derived from Peru; and the ancient civilisation
+of that country and of Mexico has given rise to no fewer than
+thirty-three useful cultivated plants.
+
+
+_Conditions favourable to the production of Variations._
+
+In order that plants and animals may be improved and modified to any
+considerable extent, it is of course essential that suitable variations
+should occur with tolerable frequency. There seem to be three conditions
+which are especially favourable to the production of variations: (1)
+That the particular species or variety should be kept in very large
+numbers; (2) that it should be spread over a wide area and thus
+subjected to a considerable diversity of physical conditions; and (3)
+that it should be occasionally crossed with some distinct but closely
+allied race. The first of these conditions is perhaps the most
+important, the chance of variations of any particular kind being
+increased in proportion to the quantity of the original stock and of its
+annual offspring. It has been remarked that only those breeders who keep
+large flocks can effect much improvement; and it is for the same reason
+that pigeons and fowls, which can be so easily and rapidly increased,
+and which have been kept in such large numbers by so great a number of
+persons, have produced such strange and numerous varieties. In like
+manner, nurserymen who grow fruit and flowers in large quantities have a
+great advantage over private amateurs in the production of new
+varieties.
+
+Although I believe, for reasons which will be given further on, that
+some amount of variability is a constant and necessary property of all
+organisms, yet there appears to be good evidence to show that changed
+conditions of life tend to increase it, both by a direct action on the
+organisation and by indirectly affecting the reproductive system. Hence
+the extension of civilisation, by favouring domestication under altered
+conditions, facilitates the process of modification. Yet this change
+does not seem to be an essential condition, for nowhere has the
+production of extreme varieties of plants and flowers been carried
+farther than in Japan, where careful selection continued for many
+generations must have been the chief factor. The effect of occasional
+crosses often results in a great amount of variation, but it also leads
+to instability of character, and is therefore very little employed in
+the production of fixed and well-marked races. For this purpose, in
+fact, it has to be carefully avoided, as it is only by isolation and
+pure breeding that any specially desired qualities can be increased by
+selection. It is for this reason that among savage peoples, whose
+animals run half wild, little improvement takes place; and the
+difficulty of isolation also explains why distinct and pure breeds of
+cats are so rarely met with. The wide distribution of useful animals and
+plants from a very remote epoch has, no doubt, been a powerful cause of
+modification, because the particular breed first introduced into each
+country has often been kept pure for many years, and has also been
+subjected to slight differences of conditions. It will also usually have
+been selected for a somewhat different purpose in each locality, and
+thus very distinct races would soon originate.
+
+The important physiological effects of crossing breeds or strains, and
+the part this plays in the economy of nature, will be explained in a
+future chapter.
+
+
+_Concluding Remarks._
+
+The examples of variation now adduced--and these might have been almost
+indefinitely increased--will suffice to show that there is hardly an
+organ or a quality in plants or animals which has not been observed to
+vary; and further, that whenever any of these variations have been
+useful to man he has been able to increase them to a marvellous extent
+by the simple process of always preserving the best varieties to breed
+from. Along with these larger variations others of smaller amount
+occasionally appear, sometimes in external, sometimes in internal
+characters, the very bones of the skeleton often changing slightly in
+form, size, or number; but as these secondary characters have been of no
+use to man, and have not been specially selected by him, they have,
+usually, not been developed to any great amount except when they have
+been closely dependent on those external characters which he has largely
+modified.
+
+As man has considered only utility to himself, or the satisfaction of
+his love of beauty, of novelty, or merely of something strange or
+amusing, the variations he has thus produced have something of the
+character of monstrosities. Not only are they often of no use to the
+animals or plants themselves, but they are not unfrequently injurious to
+them. In the Tumbler pigeons, for instance, the habit of tumbling is
+sometimes so excessive as to injure or kill the bird; and many of our
+highly-bred animals have such delicate constitutions that they are very
+liable to disease, while their extreme peculiarities of form or
+structure would often render them quite unfit to live in a wild state.
+In plants, many of our double flowers, and some fruits, have lost the
+power of producing seed, and the race can thus be continued only by
+means of cuttings or grafts. This peculiar character of domestic
+productions distinguishes them broadly from wild species and varieties,
+which, as will be seen by and by, are necessarily adapted in every part
+of their organisation to the conditions under which they have to live.
+Their importance for our present inquiry depends on their demonstrating
+the occurrence of incessant slight variations in all parts of an
+organism, with the transmission to the offspring of the special
+characteristics of the parents; and also, that all such slight
+variations are capable of being accumulated by selection till they
+present very large and important divergencies from the ancestral stock.
+
+We thus see, that the evidence as to variation afforded by animals and
+plants under domestication strikingly accords with that which we have
+proved to exist in a state of nature. And it is not at all surprising
+that it should be so, since all the species were in a state of nature
+when first domesticated or cultivated by man, and whatever variations
+occur must be due to purely natural causes. Moreover, on comparing the
+variations which occur in any one generation of domesticated animals
+with those which we know to occur in wild animals, we find no evidence
+of greater individual variation in the former than in the latter. The
+results of man's selection are more striking to us because we have
+always considered the varieties of each domestic animal to be
+essentially identical, while those which we observe in a wild state are
+held to be essentially diverse. The greyhound and the spaniel seem
+wonderful, as varieties of one animal produced by man's selection; while
+we think little of the diversities of the fox and the wolf, or the horse
+and the zebra, because we have been accustomed to look upon them as
+radically distinct animals, not as the results of nature's selection of
+the varieties of a common ancestor.
+
+FOOTNOTES:
+
+[Footnote 31: Darwin, _Animals and Plants under Domestication_, vol. i.
+p. 322.]
+
+[Footnote 32: These facts are taken from Darwin's _Domesticated Animals
+and Cultivated Plants_, vol. i. pp. 359, 360, 392-401; vol. ii. pp. 231,
+275, 330.]
+
+[Footnote 33: See Darwin's _Animals and Plants under Domestication_,
+vol. i. pp. 40-42.]
+
+[Footnote 34: Mr. Brent in _Journal of Horticulture_, 1861, p. 76;
+quoted by Darwin, _Animals and Plants under Domestication_, vol. i. p.
+151.]
+
+[Footnote 35: This account of domestic pigeons is greatly condensed from
+Mr. Darwin's work already referred to.]
+
+[Footnote 36: _Animals and Plants under Domestication_, vol. ii. pp.
+307-311.]
+
+
+
+
+CHAPTER V
+
+NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST
+
+
+    Effect of struggle for existence under unchanged conditions--The
+    effect under change of conditions--Divergence of character--In
+    insects--In birds--In mammalia--Divergence leads to a maximum of
+    life in each area--Closely allied species inhabit distinct
+    areas--Adaptation to conditions at various periods of life--The
+    continued existence of low forms of life--Extinction of low
+    types among the higher animals--Circumstances favourable to the
+    origin of new species--Probable origin of the dippers--The
+    importance of isolation--On the advance of organisation by
+    natural selection--Summary of the first five chapters.
+
+
+
+In the preceding chapters we have accumulated a body of facts and
+arguments which will enable us now to deal with the very core of our
+subject--the formation of species by means of natural selection. We have
+seen how tremendous is the struggle for existence always going on in
+nature owing to the great powers of increase of all organisms; we have
+ascertained the fact of variability extending to every part and organ,
+each of which varies simultaneously and for the most part independently;
+and we have seen that this variability is both large in its amount in
+proportion to the size of each part, and usually affects a considerable
+proportion of the individuals in the large and dominant species. And,
+lastly, we have seen how similar variations, occurring in cultivated
+plants and domestic animals, are capable of being perpetuated and
+accumulated by artificial selection, till they have resulted in all the
+wonderful varieties of our fruits, flowers, and vegetables, our domestic
+animals and household pets, many of which differ from each other far
+more in external characters, habits, and instincts than do species in a
+state of nature. We have now to inquire whether there is any analogous
+process in nature, by which wild animals and plants can be permanently
+modified and new races or new species produced.
+
+
+_Effect of Struggle for Existence under Unchanged Conditions._
+
+Let us first consider what will be the effect of the struggle for
+existence upon the animals and plants which we see around us, under
+conditions which do not perceptibly vary from year to year or from
+century to century. We have seen that every species is exposed to
+numerous and varied dangers throughout its entire existence, and that it
+is only by means of the exact adaptation of its organisation--including
+its instincts and habits--to its surroundings that it is enabled to live
+till it produces offspring which may take its place when it ceases to
+exist. We have seen also that, of the whole annual increase only a very
+small fraction survives; and though the survival in individual cases may
+sometimes be due rather to accident than to any real superiority, yet we
+cannot doubt that, in the long run, those survive which are best fitted
+by their perfect organisation to escape the dangers that surround them.
+This "survival of the fittest" is what Darwin termed "natural
+selection," because it leads to the same results in nature as are
+produced by man's selection among domestic animals and cultivated
+plants. Its primary effect will, clearly, be to keep each species in the
+most perfect health and vigour, with every part of its organisation in
+full harmony with the conditions of its existence. It prevents any
+possible deterioration in the organic world, and produces that
+appearance of exuberant life and enjoyment, of health and beauty, that
+affords us so much pleasure, and which might lead a superficial observer
+to suppose that peace and quietude reigned throughout nature.
+
+
+_The Effect under changed Conditions._
+
+But the very same process which, so long as conditions remain
+substantially the same, secures the continuance of each species of
+animal or plant in its full perfection, will usually, under changed
+conditions, bring about whatever change of structure or habits may be
+necessitated by them. The changed conditions to which we refer are such
+as we know have occurred throughout all geological time and in every
+part of the world. Land and water have been continually shifting their
+positions; some regions are undergoing subsidence with diminution of
+area, others elevation with extension of area; dry land has been
+converted into marshes, while marshes have been drained or have even
+been elevated into plateaux. Climate too has changed again and again,
+either through the elevation of mountains in high latitudes leading to
+the accumulation of snow and ice, or by a change in the direction of
+winds and ocean currents produced by the subsidence or elevation of
+lands which connected continents and divided oceans. Again, along with
+all these changes have come not less important changes in the
+distribution of species. Vegetation has been greatly modified by changes
+of climate and of altitude; while every union of lands before separated
+has led to extensive migrations of animals into new countries,
+disturbing the balance that before existed among its forms of life,
+leading to the extermination of some species and the increase of others.
+
+When such physical changes as these have taken place, it is evident that
+many species must either become modified or cease to exist. When the
+vegetation has changed in character the herbivorous animals must become
+able to live on new and perhaps less nutritious food; while the change
+from a damp to a dry climate may necessitate migration at certain
+periods to escape destruction by drought. This will expose the species
+to new dangers, and require special modifications of structure to meet
+them. Greater swiftness, increased cunning, nocturnal habits, change of
+colour, or the power of climbing trees and living for a time on their
+foliage or fruit, may be the means adopted by different species to bring
+themselves into harmony with the new conditions; and by the continued
+survival of those individuals, only, which varied sufficiently in the
+right direction, the necessary modifications of structure or of function
+would be brought about, just as surely as man has been able to breed the
+greyhound to hunt by sight and the foxhound by scent, or has produced
+from the same wild plant such distinct forms as the cauliflower and the
+brussels sprouts.
+
+We will now consider the special characteristics of the changes in
+species that are likely to be effected, and how far they agree with what
+we observe in nature.
+
+
+_Divergence of Character._
+
+In species which have a wide range the struggle for existence will often
+cause some individuals or groups of individuals to adopt new habits in
+order to seize upon vacant places in nature where the struggle is less
+severe. Some, living among extensive marshes, may adopt a more aquatic
+mode of life; others, living where forests abound, may become more
+arboreal. In either case we cannot doubt that the changes of structure
+needed to adapt them to their new habits would soon be brought about,
+because we know that variations in all the external organs and all their
+separate parts are very abundant and are also considerable in amount.
+That such divergence of character has actually occurred we have some
+direct evidence. Mr. Darwin informs us that in the Catskill Mountains in
+the United States there are two varieties of wolves, one with a light
+greyhound-like form which pursues deer, the other more bulky with
+shorter legs, which more frequently attacks sheep.[37] Another good
+example is that of the insects in the island of Madeira, many of which
+have either lost their wings or have had them so much reduced as to be
+useless for flight, while the very same species on the continent of
+Europe possess fully developed wings. In other cases the wingless
+Madeira species are distinct from, but closely allied to, winged species
+of Europe. The explanation of this change is, that Madeira, like many
+oceanic islands in the temperate zone, is much exposed to sudden gales
+of wind, and as most of the fertile land is on the coast, insects which
+flew much would be very liable to be blown out to sea and lost. Year
+after year, therefore, those individuals which had shorter wings, or
+which used them least, were preserved; and thus, in time, terrestrial,
+wingless, or imperfectly winged races or species have been produced.
+That this is the true explanation of this singular fact is proved by
+much corroborative evidence. There are some few flower-frequenting
+insects in Madeira to whom wings are essential, and in these the wings
+are somewhat larger than in the same species on the mainland. We thus
+see that there is no general tendency to the abortion of wings in
+Madeira, but that it is simply a case of adaptation to new conditions.
+Those insects to whom wings were not absolutely essential escaped a
+serious danger by not using them, and the wings therefore became reduced
+or were completely lost. But when they were essential they were enlarged
+and strengthened, so that the insect could battle against the winds and
+save itself from destruction at sea. Many flying insects, not varying
+fast enough, would be destroyed before they could establish themselves,
+and thus we may explain the total absence from Madeira of several whole
+families of winged insects which must have had many opportunities of
+reaching the islands. Such are the large groups of the tiger-beetles
+(Cicindelidae), the chafers (Melolonthidae), the click-beetles
+(Elateridae), and many others.
+
+But the most curious and striking confirmation of this portion of Mr.
+Darwin's theory is afforded by the case of Kerguelen Island. This island
+was visited by the _Transit of Venus_ expedition. It is one of the
+stormiest places on the globe, being subject to almost perpetual gales,
+while, there being no wood, it is almost entirely without shelter. The
+Rev. A.E. Eaton, an experienced entomologist, was naturalist to the
+expedition, and he assiduously collected the few insects that were to be
+found. All were incapable of flight, and most of them entirely without
+wings. They included a moth, several flies, and numerous beetles. As
+these insects could hardly have reached the islands in a wingless state,
+even if there were any other known land inhabited by them--which there
+is not--we must assume that, like the Madeiran insects, they were
+originally winged, and lost their power of flight because its possession
+was injurious to them.
+
+It is no doubt due to the same cause that some butterflies on small and
+exposed islands have their wings reduced in size, as is strikingly the
+case with the small tortoise-shell butterfly (Vanessa urticae)
+inhabiting the Isle of Man, which is only about half the size of the
+same species in England or Ireland; and Mr. Wollaston notes that Vanessa
+callirhoe--a closely allied South European form of our red-admiral
+butterfly--is permanently smaller in the small and bare island of Porto
+Santo than in the larger and more wooded adjacent island of Madeira.
+
+A very good example of comparatively recent divergence of character, in
+accordance with new conditions of life, is afforded by our red grouse.
+This bird, the Lagopus scoticus of naturalists, is entirely confined to
+the British Isles. It is, however, very closely allied to the willow
+grouse (Lagopus albus), a bird which ranges all over Europe, Northern
+Asia, and North America, but which, unlike our species, changes to white
+in winter. No difference in form or structure can be detected between
+the two birds, but as they differ so decidedly in colour--our species
+being usually rather darker in winter than in summer, while there are
+also slight differences in the call-note and in habits,--the two species
+are generally considered to be distinct. The differences, however, are
+so clearly adaptations to changed conditions that we can hardly doubt
+that, during the early part of the glacial period, when our islands were
+united to the continent, our grouse was identical with that of the rest
+of Europe. But when the cold passed away and our islands became
+permanently separated from the mainland, with a mild and equable climate
+and very little snow in winter, the change to white at that season
+became hurtful, rendering the birds more conspicuous instead of serving
+as a means of concealment. The colour was, therefore, gradually changed
+by the process of variation and natural selection; and as the birds
+obtained ample shelter among the heather which clothes so many of our
+moorlands, it became useful for them to assimilate with its brown and
+dusky stems and withered flowers rather than with the snow of the higher
+mountains. An interesting confirmation of this change having really
+occurred is afforded by the occasional occurrence in Scotland of birds
+with a considerable amount of white in the winter plumage. This is
+considered to be a case of reversion to the ancestral type, just as the
+slaty colours and banded wings of the wild rock-pigeon sometimes
+reappear in our fancy breeds of domestic pigeons.[38]
+
+The principle of "divergence of character" pervades all nature from the
+lowest groups to the highest, as may be well seen in the class of birds.
+Among our native species we see it well marked in the different species
+of titmice, pipits, and chats. The great titmouse (Parus major) by its
+larger size and stronger bill is adapted to feed on larger insects, and
+is even said sometimes to kill small and weak birds. The smaller and
+weaker coal titmouse (Parus ater) has adopted a more vegetarian diet,
+eating seeds as well as insects, and feeding on the ground as well as
+among trees. The delicate little blue titmouse (Parus coeruleus), with
+its very small bill, feeds on the minutest insects and grubs which it
+extracts from crevices of bark and from the buds of fruit-trees. The
+marsh titmouse, again (Parus palustris), has received its name from the
+low and marshy localities it frequents; while the crested titmouse
+(Parus cristatus) is a northern bird frequenting especially pine
+forests, on the seeds of which trees it partially feeds. Then, again,
+our three common pipits--the tree-pipit (Anthus arboreus), the
+meadow-pipit (Anthus pratensis), and the rock-pipit or sea-lark (Anthus
+obscurus) have each occupied a distinct place in nature to which they
+have become specially adapted, as indicated by the different form and
+size of the hind toe and claw in each species. So, the stone-chat
+(Saxicola rubicola), the whin-chat (S. rubetra), and the wheat-ear (S.
+oenanthe) are more or less divergent forms of one type, with
+modifications in the shape of the wing, feet, and bill adapting them to
+slightly different modes of life. The whin-chat is the smallest, and
+frequents furzy commons, fields, and lowlands, feeding on worms,
+insects, small molluscs, and berries; the stone-chat is next in size,
+and is especially active and lively, frequenting heaths and uplands, and
+is a permanent resident with us, the two other species being migrants;
+while the larger and more conspicuous wheat-ear, besides feeding on
+grubs, beetles, etc., is able to capture flying insects on the wing,
+something after the manner of true flycatchers.
+
+These examples sufficiently indicate how divergence of character has
+acted, and has led to the adaptation of numerous allied species, each to
+a more or less special mode of life, with the variety of food, of
+habits, and of enemies which must necessarily accompany such diversity.
+And when we extend our inquiries to higher groups we find the same
+indications of divergence and special adaptation, often to a still more
+marked extent. Thus we have the larger falcons, which prey upon birds,
+while some of the smaller species, like the hobby (Falco subbuteo), live
+largely on insects. The true falcons capture their prey in the air,
+while the hawks usually seize it on or near the ground, feeding on
+hares, rabbits, squirrels, grouse, pigeons, and poultry. Kites and
+buzzards, on the other hand, seize their prey upon the ground, and the
+former feed largely on reptiles and offal as well as on birds and
+quadrupeds. Others have adopted fish as their chief food, and the osprey
+snatches its prey from the water with as much facility as a gull or a
+petrel; while the South American caracaras (Polyborus) have adopted the
+habits of vultures and live altogether on carrion. In every great group
+there is the same divergence of habits. There are ground-pigeons,
+rock-pigeons, and wood-pigeons,--seed-eating pigeons and fruit-eating
+pigeons; there are carrion-eating, insect-eating, and fruit-eating
+crows. Even kingfishers are, some aquatic, some terrestrial in their
+habits; some live on fish, some on insects, some on reptiles. Lastly,
+among the primary divisions of birds we find a purely terrestrial
+group--the Ratitae, including the ostriches, cassowaries, etc.; other
+great groups, including the ducks, cormorants, gulls, penguins, etc.,
+are aquatic; while the bulk of the Passerine birds are aerial and
+arboreal. The same general facts can be detected in all other classes of
+animals. In the mammalia, for example, we have in the common rat a
+fish-eater and flesh-eater as well as a grain-eater, which has no doubt
+helped to give it the power of spreading over the world and driving away
+the native rats of other countries. Throughout the Rodent tribe we find
+everywhere aquatic, terrestrial, and arboreal forms. In the weasel and
+cat tribes some live more in trees, others on the ground; squirrels have
+diverged into terrestrial, arboreal, and flying species; and finally, in
+the bats we have a truly aerial, and in the whales a truly aquatic order
+of mammals. We thus see that, beginning with different varieties of the
+same species, we have allied species, genera, families, and orders, with
+similarly divergent habits, and adaptations to different modes of life,
+indicating some general principle in nature which has been operative in
+the development of the organic world. But in order to be thus operative
+it must be a generally useful principle, and Mr. Darwin has very clearly
+shown us in what this utility consists.
+
+
+_Divergence leads to a Maximum of Organic Forms in each Area._
+
+Divergence of character has a double purpose and use. In the first place
+it enables a species which is being overcome by rivals, or is in
+process of extinction by enemies, to save itself by adopting new habits
+or by occupying vacant places in nature. This is the immediate and
+obvious effect of all the numerous examples of divergence of character
+which we have pointed out. But there is another and less obvious result,
+which is, that the greater the diversity in the organisms inhabiting a
+country or district the greater will be the total amount of life that
+can be supported there. Hence the continued action of the struggle for
+existence will tend to bring about more and more diversity in each area,
+which may be shown to be the case by several kinds of evidence. As an
+example, a piece of turf, three feet by four in size, was found by Mr.
+Darwin to contain twenty species of plants, and these twenty species
+belonged to eighteen genera and to eight orders, showing how greatly
+they differed from each other. Farmers find that a greater quantity of
+hay is obtained from ground sown with a variety of genera of grasses,
+clover, etc., than from similar land sown with one or two species only;
+and the same principle applies to rotation of crops, plants differing
+very widely from each other giving the best results. So, in small and
+uniform islands, and in small ponds of fresh water, the plants and
+insects, though few in number, are found to be wonderfully varied in
+character.
+
+The same principle is seen in the naturalisation of plants and animals
+by man's agency in distant lands, for the species that thrive best and
+establish themselves permanently are not only very varied among
+themselves but differ greatly from the native inhabitants. Thus, in the
+Northern United States there are, according to Dr. Asa Gray, 260
+naturalised flowering plants which belong to no less than 162 genera;
+and of these, 100 genera are not natives of the United States. So, in
+Australia, the rabbit, though totally unlike any native animal, has
+increased so much that it probably outnumbers in individuals all the
+native mammals of the country; and in New Zealand the rabbit and the pig
+have equally multiplied. Darwin remarks that this "advantage of
+diversification of structure in the inhabitants of the same region is,
+in fact, the same as that of the physiological division of labour in the
+organs of the same body. No physiologist doubts that a stomach adapted
+to digest vegetable matter alone, or flesh alone, draws more nutriment
+from these substances. So, in the general economy of any land, the more
+widely and perfectly the animals and plants are diversified for
+different habits of life, so will a greater number of individuals be
+capable of there supporting themselves."[39]
+
+
+_The most closely allied Species inhabit distinct Areas._
+
+One of the curious results of the general action of this principle in
+nature is, that the most closely allied species--those whose differences
+though often real and important are hardly perceptible to any one but a
+naturalist--are usually not found in the same but in widely separated
+countries. Thus, the nearest allies to our European golden plover are
+found in North America and East Asia; the nearest ally of our European
+jay is found in Japan, although there are several other species of jays
+in Western Asia and North Africa; and though we have several species of
+titmice in England they are not very closely allied to each other. The
+form most akin to our blue tit is the azure tit of Central Asia (Parus
+azureus); the Parus ledouci of Algeria is very near our coal tit, and
+the Parus lugubris of South-Eastern Europe and Asia Minor is nearest to
+our marsh tit. So, our four species of wild pigeons--the ring-dove,
+stock-dove, rock-pigeon, and turtle-dove--are not closely allied to each
+other, but each of them belongs, according to some ornithologists, to a
+separate genus or subgenus, and has its nearest relatives in distant
+parts of Asia and Africa. In mammalia the same thing occurs. Each
+mountain region of Europe and Asia has usually its own species of wild
+sheep and goat, and sometimes of antelope and deer; so that in each
+region there is found the greatest diversity in this class of animals,
+while the closest allies inhabit quite distinct and often distant areas.
+In plants we find the same phenomenon prevalent. Distinct species of
+columbine are found in Central Europe (Aguilegia vulgaris), in Eastern
+Europe, and Siberia (A. glandulosa), in the Alps (A. Alpina), in the
+Pyrenees (A. pyrenaiea), in the Greek mountains (A. ottonis), and in
+Corsica (A. Bernardi), but rarely are two species found in the same
+area. So, each part of the world has its own peculiar forms of pines,
+firs, and cedars, but the closely allied species or varieties are in
+almost every case inhabitants of distinct areas. Examples are the deodar
+of the Himalayas, the cedar of Lebanon, and that of North Africa, all
+very closely allied but confined to distinct areas; and the numerous
+closely allied species of true pine (genus Pinus), which almost always
+inhabit different countries or occupy different stations. We will now
+consider some other modes in which natural selection will act, to adapt
+organisms to changed conditions.
+
+
+_Adaptation to Conditions at Various Periods of Life._
+
+It is found, that, in domestic animals and cultivated plants, variations
+occurring at any one period of life reappear in the offspring at the
+same period, and can be perpetuated and increased by selection without
+modifying other parts of the organisation. Thus, variations in the
+caterpillar or the cocoon of the silkworm, in the eggs of poultry, and
+in the seeds or young shoots of many culinary vegetables, have been
+accumulated till those parts have become greatly modified and, for man's
+purposes, improved. Owing to this fact it is easy for organisms to
+become so modified as to avoid dangers that occur at any one period of
+life. Thus it is that so many seeds have become adapted to various modes
+of dissemination or protection. Some are winged, or have down or hairs
+attached to them, so as to enable them to be carried long distances in
+the air; others have curious hooks and prickles, which cause them to be
+attached firmly to the fur of mammals or the feathers of birds; while
+others are buried within sweet or juicy and brightly coloured fruits,
+which are seen and devoured by birds, the hard smooth seeds passing
+through their bodies in a fit state for germination. In the struggle for
+existence it must benefit a plant to have increased means of dispersing
+its seeds, and of thus having young plants produced in a greater variety
+of soils, aspects, and surroundings, with a greater chance of some of
+them escaping their numerous enemies and arriving at maturity. The
+various differences referred to would, therefore, be brought about by
+variation and survival of the fittest, just as surely as the length and
+quality of cotton on the seed of the cotton-plant have been increased
+by man's selection.
+
+The larvae of insects have thus been wonderfully modified in order to
+escape the numerous enemies to whose attacks they are exposed at this
+period of their existence. Their colours and markings have become
+marvellously adapted to conceal them among the foliage of the plant they
+live upon, and this colour often changes completely after the last
+moult, when the creature has to descend to the ground for its change to
+the pupa state, during which period a brown instead of a green colour is
+protective. Others have acquired curious attitudes and large ocelli,
+which cause them to resemble the head of some reptile, or they have
+curious horns or coloured ejectile processes which frighten away
+enemies; while a great number have acquired secretions which render them
+offensive to the taste of their enemies, and these are always adorned
+with very conspicuous markings or brilliant colours, which serve as a
+sign of inedibility and prevent their being needlessly attacked. This,
+however, is a portion of the very large subject of organic colour and
+marking, which will be fully discussed and illustrated in a separate
+chapter.
+
+In this way every possible modification of an animal or plant, whether
+in colour, form, structure, or habits, which would be serviceable to it
+or to its progeny at any period of its existence, may be readily brought
+about. There are some curious organs which are used only once in a
+creature's life, but which are yet essential to its existence, and thus
+have very much the appearance of design by an intelligent designer. Such
+are, the great jaws possessed by some insects, used exclusively for
+opening the cocoon, and the hard tip to the beak of unhatched birds used
+for breaking the eggshell. The increase in thickness or hardness of the
+cocoons or the eggs being useful for protection against enemies or to
+avoid accidents, it is probable that the change has been very gradual,
+because it would be constantly checked by the necessity for a
+corresponding change in the young insects or birds enabling them to
+overcome the additional obstacle of a tougher cocoon or a harder
+eggshell. As we have seen, however, that every part of the organism
+appears to be varying independently, at the same time, though to
+different amounts, there seems no reason to believe that the necessity
+for two or more coincident variations would prevent the required change
+from taking place.
+
+
+_The Continued Existence of Low Forms of Life._
+
+Since species are continually undergoing modifications giving them some
+superiority over other species or enabling them to occupy fresh places
+in nature, it may be asked--Why do any low forms continue to exist? Why
+have they not long since been improved and developed into higher forms?
+The answer, probably, is, that these low forms occupy places in nature
+which cannot be filled by higher forms, and that they have few or no
+competitors; they therefore continue to exist. Thus, earthworms are
+adapted to their mode of life better than they would be if more highly
+organised. So, in the ocean, the minute foraminifera and infusoria, and
+the larger sponges and corals, occupy places which more highly developed
+creatures could not fill. They form, as it were, the base of the great
+structure of animal life, on which the next higher forms rest; and
+though in the course of ages they may undergo some changes, and
+diversification of form and structure, in accordance with changed
+conditions, their essential nature has probably remained the same from
+the very dawn of life on the earth. The low aquatic diatomaceae and
+confervae, together with the lowest fungi and lichens, occupy a similar
+position in the vegetable kingdom, filling places in nature which would
+be left vacant if only highly organised plants existed. There is,
+therefore, no motive power to destroy or seriously to modify them; and
+they have thus probably persisted, under slightly varying forms, through
+all geological time.
+
+
+_Extinction of Lower Types among the Higher Animals._
+
+So soon; however, as we approach the higher and more fully developed
+groups, we see indications of the often repeated extinction of lower by
+higher forms. This is shown by the great gaps that separate the
+mammalia, birds, reptiles, and fishes from each other; while the lowest
+forms of each are always few in number and confined to limited areas.
+Such are the lowest mammals--the echidna and ornithorhynchus of
+Australia; the lowest birds--the apteryx of New Zealand and the
+cassowaries of the New Guinea region; while the lowest fish--the
+amphioxus or lancelet, is completely isolated, and has apparently
+survived only by its habit of burrowing in the sand. The great
+distinctness of the carnivora, ruminants, rodents, whales, bats, and
+other orders of mammalia; of the accipitres, pigeons, and parrots, among
+birds; and of the beetles, bees, flies, and moths, among insects, all
+indicate an enormous amount of extinction among the comparatively low
+forms by which, on any theory of evolution, these higher and more
+specialised groups must have been preceded.
+
+
+_Circumstances favourable to the Origin of New Species by Natural
+Selection._
+
+We have already seen that, when there is no change in the physical or
+organic conditions of a country, the effect of natural selection is to
+keep all the species inhabiting it in a state of perfect health and full
+development, and to preserve the balance that already exists between the
+different groups of organisms. But, whenever the physical or organic
+conditions change, to however small an extent, some corresponding change
+will be produced in the flora and fauna, since, considering the severe
+struggle for existence and the complex relations of the various
+organisms, it is hardly possible that the change should not be
+beneficial to some species and hurtful to others. The most common
+effect, therefore, will be that some species will increase and others
+will diminish; and in cases where a species was already small in numbers
+a further diminution might lead to extinction. This would afford room
+for the increase of other species, and thus a considerable readjustment
+of the proportions of the several species might take place. When,
+however, the change was of a more important character, directly
+affecting the existence of many species so as to render it difficult for
+them to maintain themselves without some considerable change in
+structure or habits, that change would, in some cases, be brought about
+by variation and natural selection, and thus new varieties or new
+species might be formed. We have to consider, then, which are the
+species that would be most likely to be so modified, while others, not
+becoming modified, would succumb to the changed conditions and become
+extinct.
+
+The most important condition of all is, undoubtedly, that variations
+should occur of sufficient amount, of a sufficiently diverse character,
+and in a large number of individuals, so as to afford ample materials
+for natural selection to act upon; and this, we have seen, does occur in
+most, if not in all, large, wide-ranging, and dominant species. From
+some of these, therefore, the new species adapted to the changed
+conditions would usually be derived; and this would especially be the
+case when the change of conditions was rather rapid, and when a
+correspondingly rapid modification could alone save some species from
+extinction. But when the change was very gradual, then even less
+abundant and less widely distributed species might become modified into
+new forms, more especially if the extinction of many of the rarer
+species left vacant places in the economy of nature.
+
+
+_Probable Origin of the Dippers._
+
+An excellent example of how a limited group of species has been able to
+maintain itself by adaptation to one of these "vacant places" in nature,
+is afforded by the curious little birds called dippers or water-ouzels,
+forming the genus Cinclus and the family Cinclidae of naturalists. These
+birds are something like small thrushes, with very short wings and tail,
+and very dense plumage. They frequent, exclusively, mountain torrents in
+the northern hemisphere, and obtain their food entirely in the water,
+consisting, as it does, of water-beetles, caddis-worms and other
+insect-larvae, as well as numerous small freshwater shells. These birds,
+although not far removed in structure from thrushes and wrens, have the
+extraordinary power of flying under water; for such, according to the
+best observers, is their process of diving in search of their prey,
+their dense and somewhat fibrous plumage retaining so much air that the
+water is prevented from touching their bodies or even from wetting their
+feathers to any great extent. Their powerful feet and long curved claws
+enable them to hold on to stones at the bottom, and thus to retain their
+position while picking up insects, shells, etc. As they frequent
+chiefly the most rapid and boisterous torrents, among rocks, waterfalls,
+and huge boulders, the water is never frozen over, and they are thus
+able to live during the severest winters. Only a very few species of
+dipper are known, all those of the old world being so closely allied to
+our British bird that some ornithologists consider them to be merely
+local races of one species; while in North America and the northern
+Andes there are two other species.
+
+Here then we have a bird, which, in its whole structure, shows a close
+affinity to the smaller typical perching birds, but which has departed
+from all its allies in its habits and mode of life, and has secured for
+itself a place in nature where it has few competitors and few enemies.
+We may well suppose, that, at some remote period, a bird which was
+perhaps the common and more generalised ancestor of most of our
+thrushes, warblers, wrens, etc., had spread widely over the great
+northern continent, and had given rise to numerous varieties adapted to
+special conditions of life. Among these some took to feeding on the
+borders of clear streams, picking out such larvae and molluscs as they
+could reach in shallow water. When food became scarce they would attempt
+to pick them out of deeper and deeper water, and while doing this in
+cold weather many would become frozen and starved. But any which
+possessed denser and more hairy plumage than usual, which was able to
+keep out the water, would survive; and thus a race would be formed which
+would depend more and more on this kind of food. Then, following up the
+frozen streams into the mountains, they would be able to live there
+during the winter; and as such places afforded them much protection from
+enemies and ample shelter for their nests and young, further adaptations
+would occur, till the wonderful power of diving and flying under water
+was acquired by a true land-bird.
+
+That such habits might be acquired under stress of need is rendered
+highly probable by the facts stated by the well-known American
+naturalist, Dr. Abbott. He says that "the water-thrushes (Seiurus sp.)
+all wade in water, and often, seeing minute mollusca on the bottom of
+the stream, plunge both head and neck beneath the surface, so that
+often, for several seconds, a large part of the body is submerged. Now
+these birds still have the plumage pervious to water, and so are liable
+to be drenched and sodden; but they have also the faculty of giving
+these drenched feathers such a good shaking that flight is practicable a
+moment after leaving the water. Certainly the water-thrushes (Seiurus
+ludovicianus, S. auricapillus, and S. noveboracensis) have taken many
+preliminary steps to becoming as aquatic as the dipper; and the
+winter-wren, and even the Maryland yellow-throat are not far
+behind."[40]
+
+Another curious example of the way in which species have been modified
+to occupy new places in nature, is afforded by the various animals which
+inhabit the water-vessels formed by the leaves of many epiphytal species
+of Bromelia. Fritz Mueller has described a caddis-fly larva which lives
+among these leaves, and which has been modified in the pupa state in
+accordance with its surroundings. The pupae of caddis-flies inhabiting
+streams have fringes of hair on the tarsi to enable them to reach the
+surface on leaving their cases. But in the species inhabiting bromelia
+leaves there is no need for swimming, and accordingly we find the tarsi
+entirely bare. In the same plants are found curious little Entomostraca,
+very abundant there but found nowhere else. These form a new genus, but
+are most nearly allied to Cythere, a marine type. It is believed that
+the transmission of this species from one tree to another must be
+effected by the young crustacea, which are very minute, clinging to
+beetles, many of which, both terrestrial and aquatic, also inhabit the
+bromelia leaves; and as some water-beetles are known to frequent the
+sea, it is perhaps by these means that the first emigrants established
+themselves in this strange new abode. Bromeliae are often very abundant
+on trees growing on the water's edge, and this would facilitate the
+transition from a marine to an arboreal habitat. Fritz Mueller has also
+found, among the bromelia leaves, a small frog bearing its eggs on its
+back, and having some other peculiarities of structure. Several
+beautiful little aquatic plants of the genus Utricularia or bladder-wort
+also inhabit bromelia leaves; and these send runners out to neighbouring
+plants and thus spread themselves with great rapidity.
+
+
+_The Importance of Isolation._
+
+Isolation is no doubt an important aid to natural selection, as shown by
+the fact that islands so often present a number of peculiar species; and
+the same thing is seen on the two sides of a great mountain range or on
+opposite coasts of a continent. The importance of isolation is twofold.
+In the first place, it leads to a body of individuals of each species
+being limited in their range and thus subjected to uniform conditions
+for long spaces of time. Both the direct action of the environment and
+the natural selection of such varieties only as are suited to the
+conditions, will, therefore, be able to produce their full effect. In
+the second place, the process of change will not be interfered with by
+intercrossing with other individuals which are becoming adapted to
+somewhat different conditions in an adjacent area. But this question of
+the swamping effects of intercrossing will be considered in another
+chapter.
+
+Mr. Darwin was of opinion that, on the whole, the largeness of the area
+occupied by a species was of more importance than isolation, as a factor
+in the production of new species, and in this I quite agree with him. It
+must, too, be remembered, that isolation will often be produced in a
+continuous area whenever a species becomes modified in accordance with
+varied conditions or diverging habits. For example, a wide-ranging
+species may in the northern and colder part of its area become modified
+in one direction, and in the southern part in another direction; and
+though for a long time an intermediate form may continue to exist in the
+intervening area, this will be likely soon to die out, both because its
+numbers will be small, and it will be more or less pressed upon in
+varying seasons by the modified varieties, each better able to endure
+extremes of climate. So, when one portion of a terrestrial species takes
+to a more arboreal or to a more aquatic mode of life, the change of
+habit itself leads to the isolation of each portion. Again, as will be
+more fully explained in a future chapter, any difference of habits or of
+haunts usually leads to some modification of colour or marking, as a
+means of concealment from enemies; and there is reason to believe that
+this difference will be intensified by natural selection as a means of
+identification and recognition by members of the same variety or
+incipient species. It has also been observed that each differently
+coloured variety of wild animals, or of domesticated animals which have
+run wild, keep together, and refuse to pair with individuals of the
+other colours; and this must of itself act to keep the races separate as
+completely as physical isolation.
+
+
+_On the Advance of Organisation by Natural Selection._
+
+As natural selection acts solely by the preservation of useful
+variations, or those which are beneficial to the organism under the
+conditions to which it is exposed, the result must necessarily be that
+each species or group tends to become more and more improved in relation
+to its conditions. Hence we should expect that the larger groups in each
+class of animals and plants--those which have persisted and have been
+abundant throughout geological ages--would, almost necessarily, have
+arrived at a high degree of organisation, both physical and mental.
+Illustrations of this are to be seen everywhere. Among mammalia we have
+the carnivora, which from Eocene times have been becoming more and more
+specialised, till they have culminated in the cat and dog tribes, which
+have reached a degree of perfection both in structure and intelligence
+fully equal to that of any other animals. In another line of
+development, the herbivora have been specialised for living solely on
+vegetable food till they have culminated in the sheep, the cattle, the
+deer, and the antelopes. The horse tribe, commencing with an early
+four-toed ancestor in the Eocene age, has increased in size and in
+perfect adaptation of feet and teeth to a life on open plains, and has
+reached its highest perfection in the horse, the ass, and the zebra. In
+birds, also, we see an advance from the imperfect tooth-billed and
+reptile-tailed birds of the secondary epoch, to the wonderfully
+developed falcons, crows, and swallows of our time. So, the ferns,
+lycopods, conifers, and monocotyledons of the palaeozoic and mesozoic
+rocks, have developed into the marvellous wealth of forms of the higher
+dicotyledons that now adorn the earth.
+
+But this remarkable advance in the higher and larger groups does not
+imply any universal law of progress in organisation, because we have at
+the same time numerous examples (as has been already pointed out) of the
+persistence of lowly organised forms, and also of absolute degradation
+or degeneration. Serpents, for example, have been developed from some
+lizard-like type which has lost its limbs; and though this loss has
+enabled them to occupy fresh places in nature and to increase and
+flourish to a marvellous extent, yet it must be considered to be a
+retrogression rather than an advance in organisation. The same remark
+will apply to the whale tribe among mammals; to the blind amphibia and
+insects of the great caverns; and among plants to the numerous cases in
+which flowers, once specially adapted to be fertilised by insects, have
+lost their gay corollas and their special adaptations, and have become
+degraded into wind-fertilised forms. Such are our plantains, our meadow
+burnet, and even, as some botanists maintain, our rushes, sedges, and
+grasses. The causes which have led to this degeneration will be
+discussed in a future chapter; but the facts are undisputed, and they
+show us that although variation and the struggle for existence may lead,
+on the whole, to a continued advance of organisation; yet they also lead
+in many cases to a retrogression, when such retrogression may aid in the
+preservation of any form under new conditions. They also lead to the
+persistence, with slight modifications, of numerous lowly organised
+forms which are suited to places which higher forms could not fully
+occupy, or to conditions under which they could not exist. Such are the
+ocean depths, the soil of the earth, the mud of rivers, deep caverns,
+subterranean waters, etc.; and it is in such places as these, as well as
+in some oceanic islands which competing higher forms have not been able
+to reach, that we find many curious relics of an earlier world, which,
+in the free air and sunlight and in the great continents, have long
+since been driven out or exterminated by higher types.
+
+
+_Summary of the first Five Chapters._
+
+We have now passed in review, in more or less detail, the main facts on
+which the theory of "the origin of species by means of natural
+selection" is founded. In future chapters we shall have to deal mainly
+with the application of the theory to explain the varied and complex
+phenomena presented by the organic world; and, also, to discuss some of
+the theories put forth by modern writers, either as being more
+fundamental than that of Darwin or as supplementary to it. Before doing
+this, however, it will be well briefly to summarise the facts and
+arguments already set forth, because it is only by a clear comprehension
+of these that the full importance of the theory can be appreciated and
+its further applications understood.
+
+The theory itself is exceedingly simple, and the facts on which it
+rests--though excessively numerous individually, and coextensive with
+the entire organic world--yet come under a few simple and easily
+understood classes. These facts are,--first, the enormous powers of
+increase in geometrical progression possessed by all organisms, and the
+inevitable struggle for existence among them; and, in the second place,
+the occurrence of much individual variation combined with the hereditary
+transmission of such variations. From these two great classes of facts,
+which are universal and indisputable, there necessarily arises, as
+Darwin termed it, the "preservation of favoured races in the struggle
+for life," the continuous action of which, under the ever-changing
+conditions both of the inorganic and organic universe, necessarily leads
+to the formation or development of new species.
+
+But, although this general statement is complete and indisputable, yet
+to see its applications under all the complex conditions that actually
+occur in nature, it is necessary always to bear in mind the tremendous
+power and universality of the agencies at work. We must never for an
+instant lose sight of the fact of the enormously rapid increase of all
+organisms, which has been illustrated by actual cases, given in our
+second chapter, no less than by calculations of the results of unchecked
+increase for a few years. Then, never forgetting that the animal and
+plant population of any country is, on the whole, stationary, we must be
+always trying to realise the ever-recurring destruction of the enormous
+annual increase, and asking ourselves what determines, in each
+individual case, the death of the many, the survival of the few. We must
+think over all the causes of destruction to each organism,--to the seed,
+the young shoot, the growing plant, the full-grown tree, or shrub, or
+herb, and again the fruit and seed; and among animals, to the egg or
+new-born young, to the youthful, and to the adults. Then, we must always
+bear in mind that what goes on in the case of the individual or family
+group we may observe or think of, goes on also among the millions and
+scores of millions of individuals which are comprised in almost every
+species; and must get rid of the idea that _chance_ determines which
+shall live and which die. For, although in many individual cases death
+may be due to chance rather than to any inferiority in those which die
+first, yet we cannot possibly believe that this can be the case on the
+large scale on which nature works. A plant, for instance, cannot be
+increased unless there are suitable vacant places its seeds can grow in,
+or stations where it can overcome other less vigorous and healthy
+plants. The seeds of all plants, by their varied modes of dispersal, may
+be said to be seeking out such places in which to grow; and we cannot
+doubt that, in the long run, those individuals whose seeds are the most
+numerous, have the greatest powers of dispersal, and the greatest vigour
+of growth, will leave more descendants than the individuals of the same
+species which are inferior in all these respects, although now and then
+some seed of an inferior individual may _chance_ to be carried to a spot
+where it can grow and survive. The same rule will apply to every period
+of life and to every danger to which plants or animals are exposed. The
+best organised, or the most healthy, or the most active, or the best
+protected, or the most intelligent, will inevitably, in the long run,
+gain an advantage over those which are inferior in these qualities; that
+is, _the fittest will survive_, the fittest being, in each particular
+case, those which are superior in the special qualities on which safety
+depends. At one period of life, or to escape one kind of danger,
+concealment may be necessary; at another time, to escape another danger,
+swiftness; at another, intelligence or cunning; at another, the power to
+endure rain or cold or hunger; and those which possess all these
+faculties in the fullest perfection will generally survive.
+
+Having fully grasped these facts in all their fulness and in their
+endless and complex results, we have next to consider the phenomena of
+variation, discussed in the third and fourth chapters; and it is here
+that perhaps the greatest difficulty will be felt in appreciating the
+full importance of the evidence as set forth. It has been so generally
+the practice to speak of variation as something exceptional and
+comparatively rare--as an abnormal deviation from the uniformity and
+stability of the characters of a species--and so few even among
+naturalists have ever compared, accurately, considerable numbers of
+individuals, that the conception of variability as a general
+characteristic of all dominant and widespread species, large in its
+amount and affecting, not a few, but considerable masses of the
+individuals which make up the species, will be to many entirely new.
+Equally important is the fact that the variability extends to every
+organ and every structure, external and internal; while perhaps most
+important of all is the independent variability of these several parts,
+each one varying without any constant or even usual dependence on, or
+correlation with, other parts. No doubt there is some such correlation
+in the differences that exist between species and species--more
+developed wings usually accompanying smaller feet and _vice versa_--but
+this is, generally, a useful adaptation which has been brought about by
+natural selection, and does not apply to the individual variability
+which occurs within the species.
+
+It is because these facts of variation are so important and so little
+understood, that they have been discussed in what will seem to some
+readers wearisome and unnecessary detail. Many naturalists, however,
+will hold that even more evidence is required; and more, to almost any
+amount, could easily have been given. The character and variety of that
+already adduced will, however, I trust, convince most readers that the
+facts are as stated; while they have been drawn from a sufficiently wide
+area to indicate a general principle throughout nature.
+
+If, now, we fully realise these facts of variation, along with those of
+rapid multiplication and the struggle for existence, most of the
+difficulties in the way of comprehending how species have originated
+through natural selection will disappear. For whenever, through changes
+of climate, or of altitude, or of the nature of the soil, or of the area
+of the country, any species are exposed to new dangers, and have to
+maintain themselves and provide for the safety of their offspring under
+new and more arduous conditions, then, in the variability of all parts,
+organs, and structures, no less than of habits and intelligence, we have
+the means of producing modifications which will certainly bring the
+species into harmony with its new conditions. And if we remember that
+all such physical changes are slow and gradual in their operation, we
+shall see that the amount of variation which we know occurs in every new
+generation will be quite sufficient to enable modification and
+adaptation to go on at the same rate. Mr. Darwin was rather inclined to
+exaggerate the necessary slowness of the action of natural selection;
+but with the knowledge we now possess of the great amount and range of
+individual variation, there seems no difficulty in an amount of change,
+quite equivalent to that which usually distinguishes allied species,
+sometimes taking place in less than a century, should any rapid change
+of conditions necessitate an equally rapid adaptation. This may often
+have occurred, either to immigrants into a new land, or to residents
+whose country has been cut off by subsidence from a larger and more
+varied area over which they had formerly roamed. When no change of
+conditions occurs, species may remain unchanged for very long periods,
+and thus produce that appearance of stability of species which is even
+now often adduced as an argument against evolution by natural selection,
+but which is really quite in harmony with it.
+
+On the principles, and by the light of the facts, now briefly
+summarised, we have been able, in the present chapter, to indicate how
+natural selection acts, how divergence of character is set up, how
+adaptation to conditions at various periods of life has been effected,
+how it is that low forms of life continue to exist, what kind of
+circumstances are most favourable to the formation of new species, and,
+lastly, to what extent the advance of organisation to higher types is
+produced by natural selection. We will now pass on to consider some of
+the more important objections and difficulties which have been advanced
+by eminent naturalists.
+
+FOOTNOTES:
+
+[Footnote 37: _Origin of Species_, p. 71.]
+
+[Footnote 38: Yarrell's _British Birds_, fourth edition, vol. iii. p.
+77.]
+
+[Footnote 39: _Origin of Species_, p. 89.]
+
+[Footnote 40: _Nature_, vol. xxx. p. 30.]
+
+
+
+
+CHAPTER VI
+
+DIFFICULTIES AND OBJECTIONS
+
+
+    Difficulty as to smallness of variations--As to the right
+    variations occurring when required--The beginnings of important
+    organs--The mammary glands--The eyes of flatfish--Origin of the
+    eye--Useless or non-adaptive characters--Recent extension of the
+    region of utility in plants--The same in animals--Uses of
+    tails--Of the horns of deer--Of the scale-ornamentation of
+    reptiles--Instability of non-adaptive characters--Delboeuf's
+    law--No "specific" character proved to be useless--The swamping
+    effects of intercrossing--Isolation as preventing
+    intercrossing--Gulick on the effects of isolation--Cases in
+    which isolation is ineffective.
+
+
+
+In the present chapter I propose to discuss the more obvious and often
+repeated objections to Darwin's theory, and to show how far they affect
+its character as a true and sufficient explanation of the origin of
+species. The more recondite difficulties, affecting such fundamental
+questions as the causes and laws of variability, will be left for a
+future chapter, after we have become better acquainted with the
+applications of the theory to the more important adaptations and
+correlations of animal and plant life.
+
+One of the earliest and most often repeated objections was, that it was
+difficult "to imagine a reason why variations tending in an
+infinitesimal degree in any special direction should be preserved," or
+to believe that the complex adaptation of living organisms could have
+been produced "by infinitesimal beginnings." Now this term
+"infinitesimal," used by a well-known early critic of the _Origin of
+Species_, was never made use of by Darwin himself, who spoke only of
+variations being "slight," and of the "small amount" of the variations
+that might be selected. Even in using these terms he undoubtedly
+afforded grounds for the objection above made, that such small and
+slight variations could be of no real use, and would not determine the
+survival of the individuals possessing them. We have seen, however, in
+our third chapter, that even Darwin's terms were hardly justified; and
+that the variability of many important species is of considerable
+amount, and may very often be properly described as large. As this is
+found to be the case both in animals and plants, and in all their chief
+groups and subdivisions, and also to apply to all the separate parts and
+organs that have been compared, we must take it as proved that the
+average _amount_ of variability presents no difficulty whatever in the
+way of the action of natural selection. It may be here mentioned that,
+up to the time of the preparation of the last edition of _The Origin of
+Species_, Darwin had not seen the work of Mr. J.A. Allen of Harvard
+University (then only just published), which gave us the first body of
+accurate comparisons and measurements demonstrating this large amount of
+variability. Since then evidence of this nature has been accumulating,
+and we are, therefore, now in a far better position to appreciate the
+facilities for natural selection, in this respect, than was Mr. Darwin
+himself.
+
+Another objection of a similar nature is, that the chances are immensely
+against the right variation or combination of variations occurring just
+when required; and further, that no variation can be perpetuated that is
+not accompanied by several concomitant variations of dependent
+parts--greater length of a wing in a bird, for example, would be of
+little use if unaccompanied by increased volume or contractility of the
+muscles which move it. This objection seemed a very strong one so long
+as it was supposed that variations occurred singly and at considerable
+intervals; but it ceases to have any weight now we know that they occur
+simultaneously in various parts of the organism, and also in a large
+proportion of the individuals which make up the species. A considerable
+number of individuals will, therefore, every year possess the required
+combination of characters; and it may also be considered probable that
+when the two characters are such that they always _act_ together, there
+will be such a correlation between them that they will frequently _vary_
+together. But there is another consideration that seems to show that
+this coincident variation is not essential. All animals in a state of
+nature are kept, by the constant struggle for existence and the survival
+of the fittest, in such a state of perfect health and usually
+superabundant vigour, that in all ordinary circumstances they possess a
+surplus power in every important organ--a surplus only drawn upon in
+cases of the direst necessity when their very existence is at stake. It
+follows, therefore, that _any_ additional power given to one of the
+component parts of an organ must be useful--an increase, for example,
+either in the wing muscles or in the form or length of the wing might
+give _some_ increased powers of flight; and thus alternate
+variations--in one generation in the muscles, in another generation in
+the wing itself--might be as effective in permanently improving the
+powers of flight as coincident variations at longer intervals. On either
+supposition, however, this objection appears to have little weight if we
+take into consideration the large amount of coincident variability that
+has been shown to exist.
+
+
+_The Beginnings of Important Organs._
+
+We now come to an objection which has perhaps been more frequently urged
+than any other, and which Darwin himself felt to have much weight--the
+first beginnings of important organs, such, for example, as wings, eyes,
+mammary glands, and numerous other structures. It is urged, that it is
+almost impossible to conceive how the first rudiments of these could
+have been of any use, and, if not of use they could not have been
+preserved and further developed by natural selection.
+
+Now, the first remark to be made on objections of this nature is, that
+they are really outside the question of the origin of all existing
+species from allied species not very far removed from them, which is all
+that Darwin undertook to _prove_ by means of his theory. Organs and
+structures such as those above mentioned all date back to a very remote
+past, when the world and its inhabitants were both very different from
+what they are now. To ask of a new theory that it shall reveal to us
+exactly what took place in remote geological epochs, and how it took
+place, is unreasonable. The most that should be asked is, that some
+probable or possible mode of origination should be pointed out in some
+at least of these difficult cases, and this Mr. Darwin has done. One or
+two of these may be briefly given here, but the whole series should be
+carefully read by any one who wishes to see how many curious facts and
+observations have been required in order to elucidate them; whence we
+may conclude that further knowledge will probably throw light on any
+difficulties that still remain.[41]
+
+In the case of the mammary glands Mr. Darwin remarks that it is admitted
+that the ancestral mammals were allied to the marsupials. Now in the
+very earliest mammals, almost before they really deserved that name, the
+young may have been nourished by a fluid secreted by the interior
+surface of the marsupial sack, as is believed to be the case with the
+fish (Hippocampus) whose eggs are hatched within a somewhat similar
+sack. This being the case, those individuals which secreted a more
+nutritious fluid, and those whose young were able to obtain and swallow
+a more constant supply by suction, would be more likely to live and come
+to a healthy maturity, and would therefore be preserved by natural
+selection.
+
+In another case which has been adduced as one of special difficulty, a
+more complete explanation is given. Soles, turbots, and other flatfish
+are, as is well known, unsymmetrical. They live and move on their sides,
+the under side being usually differently coloured from that which is
+kept uppermost. Now the eyes of these fish are curiously distorted in
+order that both eyes may be on the upper side, where alone they would be
+of any use. It was objected by Mr. Mivart that a sudden transformation
+of the eye from one side to the other was inconceivable, while, if the
+transit were gradual the first step could be of no use, since this would
+not remove the eye from the lower side. But, as Mr. Darwin shows by
+reference to the researches of Malm and others, the young of these fish
+are quite symmetrical, and during their growth exhibit to us the whole
+process of change. This begins by the fish (owing to the increasing
+depth of the body) being unable to maintain the vertical position, so
+that it falls on one side. It then twists the lower eye as much as
+possible towards the upper side; and, the whole bony structure of the
+head being at this time soft and flexible, the constant repetition of
+this effort causes the eye gradually to move round the head till it
+comes to the upper side. Now if we suppose this process, which in the
+young is completed in a few days or weeks, to have been spread over
+thousands of generations during the development of these fish, those
+usually surviving whose eyes retained more and more of the position into
+which the young fish tried to twist them, the change becomes
+intelligible; though it still remains one of the most extraordinary
+cases of degeneration, by which symmetry--which is so universal a
+characteristic of the higher animals--is lost, in order that the
+creature may be adapted to a new mode of life, whereby it is enabled the
+better to escape danger and continue its existence.
+
+The most difficult case of all, that of the eye--the thought of which
+even to the last, Mr. Darwin says, "gave him a cold shiver"--is
+nevertheless shown to be not unintelligible; granting of course the
+sensitiveness to light of some forms of nervous tissue. For he shows
+that there are, in several of the lower animals, rudiments of eyes,
+consisting merely of pigment cells covered with a translucent skin,
+which may possibly serve to distinguish light from darkness, but nothing
+more. Then we have an optic nerve and pigment cells; then we find a
+hollow filled with gelatinous substance of a convex form--the first
+rudiment of a lens. Many of the succeeding steps are lost, as would
+necessarily be the case, owing to the great advantage of each
+modification which gave increased distinctness of vision, the creatures
+possessing it inevitably surviving, while those below them became
+extinct. But we can well understand how, after the first step was taken,
+every variation tending to more complete vision would be preserved till
+we reached the perfect eye of birds and mammals. Even this, as we know,
+is not absolutely, but only relatively, perfect. Neither the chromatic
+nor the spherical aberration is absolutely corrected; while long-and
+short-sightedness, and the various diseases and imperfections to which
+the eye is liable, may be looked upon as relics of the imperfect
+condition from which the eye has been raised by variation and natural
+selection.
+
+These few examples of difficulties as to the origin of remarkable or
+complex organs must suffice here; but the reader who wishes further
+information on the matter may study carefully the whole of the sixth
+and seventh chapters of the last edition of _The Origin of Species_, in
+which these and many other cases are discussed in considerable detail.
+
+
+_Useless or non-adaptive Characters._
+
+Many naturalists seem to be of opinion that a considerable number of the
+characters which distinguish species are of no service whatever to their
+possessors, and therefore cannot have been produced or increased by
+natural selection. Professors Bronn and Broca have urged this objection
+on the continent. In America, Dr. Cope, the well-known palaeontologist,
+has long since put forth the same objection, declaring that non-adaptive
+characters are as numerous as those which are adaptive; but he differs
+completely from most who hold the same general opinion in considering
+that they occur chiefly "in the characters of the classes, orders,
+families, and other higher groups;" and the objection, therefore, is
+quite distinct from that in which it is urged that "specific characters"
+are mostly useless. More recently, Professor G.J. Romanes has urged this
+difficulty in his paper on "Physiological Selection" (_Journ. Linn.
+Soc._, vol. xix. pp. 338, 344). He says that the characters "which serve
+to distinguish allied species are frequently, if not usually, of a kind
+with which natural selection can have had nothing to do," being without
+any utilitarian significance. Again he speaks of "the enormous number,"
+and further on of "the innumerable multitude" of specific peculiarities
+which are useless; and he finally declares that the question needs no
+further arguing, "because in the later editions of his works Mr. Darwin
+freely acknowledges that a large proportion of specific distinctions
+must be conceded to be useless to the species presenting them."
+
+I have looked in vain in Mr. Darwin's works to find any such
+acknowledgment, and I think Mr. Romanes has not sufficiently
+distinguished between "useless characters" and "useless specific
+distinctions." On referring to all the passages indicated by him I find
+that, in regard to specific characters, Mr. Darwin is very cautious in
+admitting inutility. His most pronounced "admissions" on this question
+are the following: "But when, from the nature of the organism and of the
+conditions, modifications have been induced which are unimportant for
+the welfare of the species, they may be, and apparently often have been,
+transmitted in nearly the same state _to numerous, otherwise modified,
+descendants_" (_Origin_, p. 175). The words I have here italicised
+clearly show that such characters are usually not "specific," in the
+sense that they are such as distinguish species from each other, but are
+found in numerous allied species. Again: "Thus a large yet undefined
+extension may safely be given to the direct and indirect results of
+natural selection; but I now admit, after reading the essay of Naegeli on
+plants, and the remarks by various authors with respect to animals, more
+especially those recently made by Professor Broca, that in the earlier
+editions of my _Origin of Species_ I perhaps attributed too much to the
+action of natural selection or the survival of the fittest. I have
+altered the fifth edition of the _Origin_ so as to confine my remarks to
+adaptive changes of structure, _but I am convinced, from the light
+gained during even the last few years, that very many structures which
+now appear to us useless, will hereafter be proved to be useful, and
+will therefore come within the range of natural selection_. Nevertheless
+I did not formerly consider sufficiently the existence of structures
+which, _as far as we can at present judge_, are neither beneficial nor
+injurious; and this I believe to be one of the greatest oversights as
+yet detected in my work." Now it is to be remarked that neither in these
+passages nor in any of the other less distinct expressions of opinion on
+this question, does Darwin ever admit that "specific characters"--that
+is, the particular characters which serve to distinguish one species
+from another--are ever useless, much less that "a large proportion of
+them" are so, as Mr. Romanes makes him "freely acknowledge." On the
+other hand, in the passage which I have italicised he strongly expresses
+his view that much of what we suppose to be useless is due to our
+ignorance; and as I hold myself that, as regards many of the supposed
+useless characters, this is the true explanation, it may be well to give
+a brief sketch of the progress of knowledge in transferring characters
+from the one category to the other.
+
+We have only to go back a single generation, and not even the most acute
+botanist could have suggested a reasonable use, for each species of
+plant, of the infinitely varied forms, sizes, and colours of the
+flowers, the shapes and arrangement of the leaves, and the numerous
+other external characters of the whole plant. But since Mr. Darwin
+showed that plants gained both in vigour and in fertility by being
+crossed with other individuals of the same species, and that this
+crossing was usually effected by insects which, in search of nectar or
+pollen, carried the pollen from one plant to the flowers of another
+plant, almost every detail is found to have a purpose and a use. The
+shape, the size, and the colour of the petals, even the streaks and
+spots with which they are adorned, the position in which they stand, the
+movements of the stamens and pistil at various times, especially at the
+period of, and just after, fertilisation, have been proved to be
+strictly adaptive in so many cases that botanists now believe that all
+the external characters of flowers either are or have been of use to the
+species.
+
+It has also been shown, by Kerner and other botanists, that another set
+of characteristics have relation to the prevention of ants, slugs, and
+other animals from reaching the flowers, because these creatures would
+devour or injure them without effecting fertilisation. The spines,
+hairs, or sticky glands on the stem or flower-stalk, the curious hairs
+or processes shutting up the flower, or sometimes even the extreme
+smoothness and polish of the outside of the petals so that few insects
+can hang to the part, have been shown to be related to the possible
+intrusion of these "unbidden guests."[42] And, still more recently,
+attempts have been made by Grant Allen and Sir John Lubbock to account
+for the innumerable forms, textures, and groupings of leaves, by their
+relation to the needs of the plants themselves; and there can be little
+doubt that these attempts will be ultimately successful. Again, just as
+flowers have been adapted to secure fertilisation or
+cross-fertilisation, fruits have been developed to assist in the
+dispersal of seeds; and their forms, sizes, juices, and colours can be
+shown to be specially adapted to secure such dispersal by the agency of
+birds and mammals; while the same end is secured in other cases by
+downy seeds to be wafted through the air, or by hooked or sticky
+seed-vessels to be carried away, attached to skin, wool, or feathers.
+
+Here, then, we have an enormous extension of the region of utility in
+the vegetable kingdom, and one, moreover, which includes almost all the
+specific characters of plants. For the species of plants are usually
+characterised either by differences in the form, size, and colour of the
+flowers, or of the fruits; or, by peculiarities in the shape, size,
+dentation, or arrangement of the leaves; or by peculiarities in the
+spines, hairs, or down with which various parts of the plant are
+clothed. In the case of plants it must certainly be admitted that
+"specific" characters are pre-eminently adaptive; and though there may
+be some which are not so, yet all those referred to by Darwin as having
+been adduced by various botanists as useless, either pertain to genera
+or higher groups, or are found in some plants of a species only--that
+is, are individual variations not specific characters.
+
+In the case of animals, the most recent wide extension of the sphere of
+utility has been in the matter of their colours and markings. It was of
+course always known that certain creatures gained protection by their
+resemblance to their normal surroundings, as in the case of white arctic
+animals, the yellow or brown tints of those living in deserts, and the
+green hues of many birds and insects surrounded by tropical vegetation.
+But of late years these cases have been greatly increased both in number
+and variety, especially in regard to those which closely imitate special
+objects among which they live; and there are other kinds of coloration
+which long appeared to have no use. Large numbers of animals, more
+especially insects, are gaudily coloured, either with vivid hues or with
+striking patterns, so as to be very easily seen. Now it has been found,
+that in almost all these cases the creatures possess some special
+quality which prevents their being attacked by the enemies of their kind
+whenever the peculiarity is known; and the brilliant or conspicuous
+colours or markings serve as a warning or signal flag against attack.
+Large numbers of insects thus coloured are nauseous and inedible;
+others, like wasps and bees, have stings; others are too hard to be
+eaten by small birds; while snakes with poisonous fangs often have some
+characteristic either of rattle, hood, or unusual colour, which
+indicates that they had better be left alone.
+
+But there is yet another form of coloration, which consists in special
+markings--bands, spots, or patches of white, or of bright colour, which
+vary in every species, and are often concealed when the creature is at
+rest but displayed when in motion,--as in the case of the bands and
+spots so frequent on the wings and tails of birds. Now these specific
+markings are believed, with good reason, to serve the purpose of
+enabling each species to be quickly recognised, even at a distance, by
+its fellows, especially the parents by their young and the two sexes by
+each other; and this recognition must often be an important factor in
+securing the safety of individuals, and therefore the wellbeing and
+continuance of the species. These interesting peculiarities will be more
+fully described in a future chapter, but they are briefly referred to
+here in order to show that the most common of all the characters by
+which species are distinguished from each other--their colours and
+markings--can be shown to be adaptive or utilitarian in their nature.
+
+But besides colour there are almost always some structural characters
+which distinguish species from species, and, as regards many of these
+also, an adaptive character can be often discerned. In birds, for
+instance, we have differences in the size or shape of the bill or the
+feet, in the length of the wing or the tail, and in the proportions of
+the several feathers of which these organs are composed. All these
+differences in the organs on which the very existence of birds depends,
+which determine the character of flight, facility for running or
+climbing, for inhabiting chiefly the ground or trees, and the kind of
+food that can be most easily obtained for themselves and their
+offspring, must surely be in the highest degree utilitarian; although in
+each individual case we, in our ignorance of the minutiae of their
+life-history, may be quite unable to see the use. In mammalia specific
+differences other than colour usually consist in the length or shape of
+the ears and tail, in the proportions of the limbs, or in the length and
+quality of the hair on different parts of the body. As regards the ears
+and tail, one of the objections by Professor Bronn relates to this very
+point. He states that the length of these organs differ in the various
+species of hares and of mice, and he considers that this difference can
+be of no service whatever to their possessors. But to this objection
+Darwin replies, that it has been shown by Dr. Schoebl that the ears of
+mice "are supplied in an extraordinary manner with nerves, so that they
+no doubt serve as tactile organs." Hence, when we consider the life of
+mice, either nocturnal or seeking their food in dark and confined
+places, the length of the ears may be in each case adapted to the
+particular habits and surroundings of the species. Again, the tail, in
+the larger mammals, often serves the purpose of driving off flies and
+other insects from the body; and when we consider in how many parts of
+the world flies are injurious or even fatal to large mammals, we see
+that the peculiar characteristics of this organ may in each case have
+been adapted to its requirements in the particular area where the
+species was developed. The tail is also believed to have some use as a
+balancing organ, which assists an animal to turn easily and rapidly,
+much as our arms are used when running; while in whole groups it is a
+prehensile organ, and has become modified in accordance with the habits
+and needs of each species. In the case of mice it is thus used by the
+young. Darwin informs us that the late Professor Henslow kept some
+harvest-mice in confinement, and observed that they frequently curled
+their tails round the branches of a bush placed in the cage, and thus
+aided themselves in climbing; while Dr. Guenther has actually seen a
+mouse suspend itself by the tail (_Origin_, p. 189).
+
+Again, Mr. Lawson Tait has called attention to the use of the tail in
+the cat, squirrel, yak, and many other animals as a means of preserving
+the heat of the body during the nocturnal and the winter sleep. He says,
+that in cold weather animals with long or bushy tails will be found
+lying curled up, with their tails carefully laid over their feet like a
+rug, and with their noses buried in the fur of the tail, which is thus
+used exactly in the same way and for the same purpose as we use
+respirators.[43]
+
+Another illustration is furnished by the horns of deer which, especially
+when very large, have been supposed to be a danger to the animal in
+passing rapidly through dense thickets. But Sir James Hector states,
+that the wapiti, in North America, throws back its head, thus placing
+the horns along the sides of the back, and is then enabled to rush
+through the thickest forest with great rapidity. The brow-antlers
+protect the face and eyes, while the widely spreading horns prevent
+injury to the neck or flanks. Thus an organ which was certainly
+developed as a sexual weapon, has been so guided and modified during its
+increase in size as to be of use in other ways. A similar use of the
+antlers of deer has been observed in India.[44]
+
+The various classes of facts now referred to serve to show us that, in
+the case of the two higher groups--mammalia and birds--almost all the
+characters by which species are distinguished from each other are, or
+may be, adaptive. It is these two classes of animals which have been
+most studied and whose life-histories are supposed to be most fully
+known, yet even here the assertion of inutility, by an eminent
+naturalist, in the case of two important organs, has been sufficiently
+met by minute details either in the anatomy or in the habits of the
+groups referred to. Such a fact as this, together with the extensive
+series of characters already enumerated which have been of late years
+transferred from the "useless" to the "useful" class, should convince
+us, that the assertion of "inutility" in the case of any organ or
+peculiarity which is not a rudiment or a correlation, is not, and can
+never be, the statement of a fact, but merely an expression of our
+ignorance of its purpose or origin.[45]
+
+
+_Instability of Non-adaptive Characters._
+
+One very weighty objection to the theory that _specific_ characters can
+ever be wholly useless (or wholly unconnected with useful organs by
+correlation of growth) appears to have been overlooked by those who have
+maintained the frequency of such characters, and that is, their almost
+necessary instability. Darwin has remarked on the extreme variability of
+secondary sexual characters--such as the horns, crests, plumes, etc.,
+which are found in males only,--the reason being, that, although of some
+use, they are not of such direct and vital importance as those adaptive
+characters on which the wellbeing and very existence of the animals
+depend. But in the case of wholly useless structures, which are not
+rudiments of once useful organs, we cannot see what there is to ensure
+any amount of constancy or stability. One of the cases on which Mr.
+Romanes lays great stress in his paper on "Physiological Selection"
+(_Journ. Linn. Soc._, vol. xix. p. 384) is that of the fleshy appendages
+on the corners of the jaw of Normandy pigs and of some other breeds. But
+it is expressly stated that they are not constant; they appear
+"frequently," or "occasionally," they are "not strictly inherited, for
+they occur or fail in animals of the same litter;" and they are not
+always symmetrical, sometimes appearing on one side of the face alone.
+Now whatever may be the cause or explanation of these anomalous
+appendages they cannot be classed with "specific characters," the most
+essential features of which are, that they _are_ symmetrical, that they
+_are_ inherited, and that they _are_ constant. Admitting that this
+peculiar appendage is (as Mr. Romanes says rather confidently, "we
+happen to know it to be") wholly useless and meaningless, the fact would
+be rather an argument against specific characters being also
+meaningless, because the latter never have the characteristics which
+this particular variation possesses.
+
+These useless or non-adaptive characters are, apparently, of the same
+nature as the "sports" that arise in our domestic productions, but
+which, as Mr. Darwin says, without the aid of selection would soon
+disappear; while some of them may be correlations with other characters
+which are or have been useful. Some of these correlations are very
+curious. Mr. Tegetmeier informed Mr. Darwin that the young of white,
+yellow, or dun-coloured pigeons are born almost naked, whereas other
+coloured pigeons are born well clothed with down. Now, if this
+difference occurred between wild species of different colours, it might
+be said that the nakedness of the young could not be of any use. But the
+colour with which it is correlated might, as has been shown, be useful
+in many ways. The skin and its various appendages, as horns, hoofs,
+hair, feathers, and teeth, are homologous parts, and are subject to very
+strange correlations of growth. In Paraguay, horses with curled hair
+occur, and these always have hoofs exactly like those of a mule, while
+the hair of the mane and tail is much shorter than usual. Now, if any
+one of these characters were useful, the others correlated with it might
+be themselves useless, but would still be tolerably constant because
+dependent on a useful organ. So the tusks and the bristles of the boar
+are correlated and vary in development together, and the former only may
+be useful, or both may be useful in unequal degrees.
+
+The difficulty as to how individual differences or sports can become
+fixed and perpetuated, if altogether useless, is evaded by those who
+hold that such characters are exceedingly common. Mr. Romanes says that,
+upon his theory of physiological selection, "it is quite intelligible
+that when a varietal form is differentiated from its parent form by the
+bar of sterility, any little meaningless peculiarities of structure or
+of instinct _should at first be allowed to arise_, and that they should
+then _be allowed to perpetuate themselves_ by heredity," until they are
+finally eliminated by disuse. But this is entirely begging the
+question. Do meaningless peculiarities, which we admit often arise as
+spontaneous variations, ever perpetuate themselves in all the
+individuals constituting a variety or race, without selection either
+human or natural? Such characters present themselves as unstable
+variations, and as such they remain, unless preserved and accumulated by
+selection; and they can therefore never become "specific" characters
+unless they are strictly correlated with some useful and important
+peculiarities.
+
+As bearing upon this question we may refer to what is termed Delboeuf's
+law, which has been thus briefly stated by Mr. Murphy in his work on
+_Habit and Intelligence_, p. 241.
+
+
+    "If, in any species, a number of individuals, bearing a ratio
+    not infinitely small to the entire number of births, are in
+    every generation born with a particular variation which is
+    neither beneficial nor injurious, and if it is not counteracted
+    by reversion, then the proportion of the new variety to the
+    original form will increase till it approaches indefinitely near
+    to equality."
+
+
+It is not impossible that some definite varieties, such as the melanic
+form of the jaguar and the bridled variety of the guillemot are due to
+this cause; but from their very nature such varieties are unstable, and
+are continually reproduced in varying proportions from the parent forms.
+They can, therefore, never constitute species unless the variation in
+question becomes beneficial, when it will be fixed by natural selection.
+Darwin, it is true, says--"There can be little doubt that the tendency
+to vary in the same manner has often been so strong that all the
+individuals of the same species have been similarly modified without the
+aid of any form of selection."[46] But no proof whatever is offered of
+this statement, and it is so entirely opposed to all we know of the
+facts of variation as given by Darwin himself, that the important word
+"all" is probably an oversight.
+
+On the whole, then, I submit, not only has it not been proved that an
+"enormous number of specific peculiarities" are useless, and that, as a
+logical result, natural selection is "not a theory of the origin of
+species," but only of the origin of adaptations which are usually
+common to many species, or, more commonly, to genera and families; but,
+I urge further, it has not even been proved that any truly "specific"
+characters--those which either singly or in combination distinguish each
+species from its nearest allies--are entirely unadaptive, useless, and
+meaningless; while a great body of facts on the one hand, and some
+weighty arguments on the other, alike prove that specific characters
+have been, and could only have been, developed and fixed by natural
+selection because of their utility. We may admit, that among the great
+number of variations and sports which continually arise many are
+altogether useless without being hurtful; but no cause or influence has
+been adduced adequate to render such characters fixed and constant
+throughout the vast number of individuals which constitute any of the
+more dominant species.[47]
+
+
+_The Swamping Effects of Intercrossing._
+
+This supposed insuperable difficulty was first advanced in an article in
+the _North British Review_ in 1867, and much attention has been
+attracted to it by the acknowledgment of Mr. Darwin that it proved to
+him that "single variations," or what are usually termed "sports," could
+very rarely, if ever, be perpetuated in a state of nature, as he had at
+first thought might occasionally be the case. But he had always
+considered that the chief part, and latterly the whole, of the materials
+with which natural selection works, was afforded by individual
+variations, or that amount of ever fluctuating variability which exists
+in all organisms and in all their parts. Other writers have urged the
+same objection, even as against individual variability, apparently in
+total ignorance of its amount and range; and quite recently Professor
+G.J. Romanes has adduced it as one of the difficulties which can alone
+be overcome by his theory of physiological selection. He urges, that the
+same variation does not occur simultaneously in a number of individuals
+inhabiting the same area, and that it is mere assumption to say it does;
+while he admits that "if the assumption were granted there would be an
+end of the present difficulty; for if a sufficient number of individuals
+were thus simultaneously and similarly modified, there need be no longer
+any danger of the variety becoming swamped by intercrossing." I must
+again refer my readers to my third chapter for the proof that such
+simultaneous variability is not an assumption but a fact; but, even
+admitting this to be proved, the problem is not altogether solved, and
+there is so much misconception regarding variation, and the actual
+process of the origin of new species is so obscure, that some further
+discussion and elucidation of the subject are desirable.
+
+In one of the preliminary chapters of Mr. Seebohm's recent work on the
+_Charadriidae_, he discusses the differentiation of species; and he
+expresses a rather widespread view among naturalists when, speaking of
+the swamping effects of intercrossing, he adds: "This is unquestionably
+a very grave difficulty, to my mind an absolutely fatal one, to the
+theory of accidental variation." And in another passage he says: "The
+simultaneous appearance, and its repetition in successive generations,
+of a beneficial variation, in a large number of individuals in the same
+locality, cannot possibly be ascribed to chance." These remarks appear
+to me to exhibit an entire misconception of the facts of variation as
+they actually occur, and as they have been utilised by natural selection
+in the modification of species. I have already shown that every part of
+the organism, in common species, does vary to a very considerable
+amount, in a large number of individuals, and in the same locality; the
+only point that remains to be discussed is, whether any or most of these
+variations are "beneficial." But every one of these variations consists
+either in increase or diminution of size or power of the organ or
+faculty that varies; they can all be divided into a more effective and a
+less effective group--that is, into one that is more beneficial or less
+beneficial. If less size of body would be beneficial, then, as half the
+variations in size are above and half below the mean or existing
+standard of the species, there would be ample beneficial variations; if
+a darker colour or a longer beak or wing were required, there are always
+a considerable number of individuals darker and lighter in colour than
+the average, with longer or with shorter beaks and wings, and thus the
+beneficial variation must always be present. And so with every other
+part, organ, function, or habit; because, as variation, so far as we
+know, is and always must be in the two directions of excess and defect
+in relation to the mean amount, whichever kind of variation is wanted is
+always present in some degree, and thus the difficulty as to
+"beneficial" variations occurring, as if they were a special and rare
+class, falls to the ground. No doubt some organs may vary in three or
+perhaps more directions, as in the length, breadth, thickness, or
+curvature of the bill. But these may be taken as separate variations,
+each of which again occurs as "more" or "less"; and thus the "right" or
+"beneficial" or "useful" variation must always be present so long as any
+variation at all occurs; and it has not yet been proved that in any
+large or dominant species, or in any part, organ, or faculty of such
+species, there is no variation. And even were such a case found it would
+prove nothing, so long as in numerous other species variation was shown
+to exist; because we know that great numbers of species and groups
+throughout all geological time have died out, leaving no descendants;
+and the obvious and sufficient explanation of this fact is, that they
+did _not_ vary enough at the time when variation was required to bring
+them into harmony with changed conditions. The objection as to the
+"right" or "beneficial" variation occurring when required, seems
+therefore to have no weight in view of the actual facts of variation.
+
+
+_Isolation to prevent Intercrossing._
+
+Most writers on the subject consider the isolation of a portion of a
+species a very important factor in the formation of new species, while
+others maintain it to be absolutely essential. This latter view has
+arisen from an exaggerated opinion as to the power of intercrossing to
+keep down any variety or incipient species, and merge it in the parent
+stock. But it is evident that this can only occur with varieties which
+are not useful, or which, if useful, occur in very small numbers; and
+from this kind of variations it is clear that new species do not arise.
+Complete isolation, as in an oceanic island, will no doubt enable
+natural selection to act more rapidly, for several reasons. In the first
+place, the absence of competition will for some time allow the new
+immigrants to increase rapidly till they reach the limits of
+subsistence. They will then struggle among themselves, and by survival
+of the fittest will quickly become adapted to the new conditions of
+their environment. Organs which they formerly needed, to defend
+themselves against, or to escape from, enemies, being no longer
+required, would be encumbrances to be got rid of, while the power of
+appropriating and digesting new and varied food would rise in
+importance. Thus we may explain the origin of so many flightless and
+rather bulky birds in oceanic islands, as the dodo, the cassowary, and
+the extinct moas. Again, while this process was going on, the complete
+isolation would prevent its being checked by the immigration of new
+competitors or enemies, which would be very likely to occur in a
+continuous area; while, of course, any intercrossing with the original
+unmodified stock would be absolutely prevented. If, now, before this
+change has gone very far, the variety spreads into adjacent but rather
+distant islands, the somewhat different conditions in each may lead to
+the development of distinct forms constituting what are termed
+representative species; and these we find in the separate islands of the
+Galapagos, the West Indies, and other ancient groups of islands.
+
+But such cases as these will only lead to the production of a few
+peculiar species, descended from the original settlers which happened to
+reach the islands; whereas, in wide areas, and in continents, we have
+variation and adaptation on a much larger scale; and, whenever important
+physical changes demand them, with even greater rapidity. The far
+greater complexity of the environment, together with the occurrence of
+variations in constitution and habits, will often allow of effective
+isolation, even here, producing all the results of actual physical
+isolation. As we have already explained, one of the most frequent modes
+in which natural selection acts is, by adapting some individuals of a
+species to a somewhat different mode of life, whereby they are able to
+seize upon unappropriated places in nature, and in so doing they become
+practically isolated from their parent form. Let us suppose, for
+example, that one portion of a species usually living in forests ranges
+into the open plains, and finding abundance of food remains there
+permanently. So long as the struggle for existence is not exceptionally
+severe, these two portions of the species may remain almost unchanged;
+but suppose some fresh enemies are attracted to the plains by the
+presence of these new immigrants, then variation and natural selection
+would lead to the preservation of those individuals best able to cope
+with the difficulty, and thus the open country form would become
+modified into a marked variety or into a distinct species; and there
+would evidently be little chance of this modification being checked by
+intercrossing with the parent form which remained in the forest.
+
+Another mode of isolation is brought about by the variety--either owing
+to habits, climate, or constitutional change--breeding at a slightly
+different time from the parent species. This is known to produce
+complete isolation in the case of many varieties of plants. Yet another
+mode of isolation is brought about by changes of colour, and by the fact
+that in a wild state animals of similar colours prefer to keep together
+and refuse to pair with individuals of another colour. The probable
+reason and utility of this habit will be explained in another chapter,
+but the fact is well illustrated by the cattle which have run wild in
+the Falkland Islands. These are of several different colours, but each
+colour keeps in a separate herd, often restricted to one part of the
+island; and one of these varieties--the mouse-coloured--is said to breed
+a month earlier than the others; so that if this variety inhabited a
+larger area it might very soon be established as a distinct race or
+species.[48] Of course where the change of habits or of station is still
+greater, as when a terrestrial animal becomes sub-aquatic, or when
+aquatic animals come to live in tree-tops, as with the frogs and
+Crustacea described at p. 118, the danger of intercrossing is reduced to
+a minimum.
+
+Several writers, however, not content with the indirect effects of
+isolation here indicated, maintain that it is in itself a cause of
+modification, and ultimately of the origination of new species. This
+was the keynote of Mr. Vernon Wollaston's essay on "Variation of
+Species," published in 1856, and it is adopted by the Rev. J.G. Gulick
+in his paper on "Diversity of Evolution under one Set of External
+Conditions" (_Journ. Linn. Soc. Zool._, vol. xi. p. 496). The idea seems
+to be that there is an inherent tendency to variation in certain
+divergent lines, and that when one portion of a species is isolated,
+even though under identical conditions, that tendency sets up a
+divergence which carries that portion farther and farther away from the
+original species. This view is held to be supported by the case of the
+land shells of the Sandwich Islands, which certainly present some very
+remarkable phenomena. In this comparatively small area there are about
+300 species of land shells, almost all of which belong to one family (or
+sub-family), the Achatinellidae, found nowhere else in the world. The
+interesting point is the extreme restriction of the species and
+varieties. The average range of each species is only five or six miles,
+while some are restricted to but one or two square miles, and only a
+very few range over a whole island. The forest region that extends over
+one of the mountain-ranges of the island of Oahu, is about forty miles
+in length and five or six miles in breadth; and this small territory
+furnishes about 175 species, represented by 700 or 800 varieties. Mr.
+Gulick states, that the vegetation of the different valleys on the same
+side of this range is much the same, yet each has a molluscan fauna
+differing in some degree from that of any other. "We frequently find a
+genus represented in several successive valleys by allied species,
+sometimes feeding on the same, sometimes on different plants. In every
+such case the valleys that are nearest to each other furnish the most
+nearly allied forms; and a full set of the varieties of each species
+presents a minute gradation of forms between the more divergent types
+found in the more widely separated localities." He urges, that these
+constant differences cannot be attributed to natural selection, because
+they occur in different valleys on the same side of the mountain, where
+food, climate, and enemies are the same; and also, because there is no
+greater difference in passing from the rainy to the dry side of the
+mountains than in passing from one valley to another on the same side
+an equal distance apart. In a very lengthy paper, presented to the
+Linnean Society last year, on "Divergent Evolution through Cumulative
+Segregation," Mr. Gulick endeavours to work out his views into a
+complete theory, the main point of which may perhaps be indicated by the
+following passage: "No two portions of a species possess exactly the
+same average character, and the initial differences are for ever
+reacting on the environment and on each other in such a way as to ensure
+increasing divergence in each successive generation as long as the
+individuals of the two groups are kept from intercrossing."[49]
+
+It need hardly be said that the views of Mr. Darwin and myself are
+inconsistent with the notion that, if the environment were absolutely
+similar for the two isolated portions of the species, any such necessary
+and constant divergence would take place. It is an error to assume that
+what seem to us identical conditions are really identical to such small
+and delicate organisms as these land molluscs, of whose needs and
+difficulties at each successive stage of their existence, from the
+freshly-laid egg up to the adult animal, we are so profoundly ignorant.
+The exact proportions of the various species of plants, the numbers of
+each kind of insect or of bird, the peculiarities of more or less
+exposure to sunshine or to wind at certain critical epochs, and other
+slight differences which to us are absolutely immaterial and
+unrecognisable, may be of the highest significance to these humble
+creatures, and be quite sufficient to require some slight adjustments of
+size, form, or colour, which natural selection will bring about. All we
+know of the facts of variation leads us to believe that, without this
+action of natural selection, there would be produced over the whole area
+a series of inconstant varieties mingled together, not a distinct
+segregation of forms each confined to its own limited area.
+
+Mr. Darwin has shown that, in the distribution and modification of
+species, the biological is of more importance than the physical
+environment, the struggle with other organisms being often more severe
+than that with the forces of nature. This is particularly evident in the
+case of plants, many of which, when protected from competition, thrive
+in a soil, climate, and atmosphere widely different from those of their
+native habitat. Thus, many alpine plants only found near perpetual snow
+thrive well in our gardens at the level of the sea; as do the tritomas
+from the sultry plains of South Africa, the yuccas from the arid hills
+of Texas and Mexico, and the fuchsias from the damp and dreary shores of
+the Straits of Magellan. It has been well said that plants do not live
+where they like, but where they can; and the same remark will apply to
+the animal world. Horses and cattle run wild and thrive both in North
+and South America; rabbits, once confined to the south of Europe, have
+established themselves in our own country and in Australia; while the
+domestic fowl, a native of tropical India, thrives well in every part of
+the temperate zone.
+
+If, then, we admit that when one portion of a species is separated from
+the rest, there will necessarily be a slight difference in the average
+characters of the two portions, it does not follow that this difference
+has much if any effect upon the characteristics that are developed by a
+long period of isolation. In the first place, the difference itself will
+necessarily be very slight unless there is an exceptional amount of
+variability in the species; and in the next place, if the average
+characters of the species are the expression of its exact adaptation to
+its whole environment, then, given a precisely similar environment, and
+the isolated portion will inevitably be brought back to the same average
+of characters. But, as a matter of fact, it is impossible that the
+environment of the isolated portion can be exactly like that of the bulk
+of the species. It cannot be so physically, since no two separated areas
+can be absolutely alike in climate and soil; and even if these are the
+same, the geographical features, size, contour, and relation to winds,
+seas, and rivers, would certainly differ. Biologically, the differences
+are sure to be considerable. The isolated portion of a species will
+almost always be in a much smaller area than that occupied by the
+species as a whole, hence it is at once in a different position as
+regards its own kind. The proportions of all the other species of
+animals and plants are also sure to differ in the two areas, and some
+species will almost always be absent in the smaller which are present in
+the larger country. These differences will act and react on the
+isolated portion of the species. The struggle for existence will differ
+in its severity and in its incidence from that which affects the bulk of
+the species. The absence of some one insect or other creature inimical
+to the young animal or plant may cause a vast difference in its
+conditions of existence, and may necessitate a modification of its
+external or internal characters in quite a different direction from that
+which happened to be present in the average of the individuals which
+were first isolated.
+
+On the whole, then, we conclude that, while isolation is an important
+factor in effecting some modification of species, it is so, not on
+account of any effect produced, or influence exerted by isolation _per
+se_, but because it is always and necessarily accompanied by a change of
+environment, both physical and biological. Natural selection will then
+begin to act in adapting the isolated portion to its new conditions, and
+will do this the more quickly and the more effectually because of the
+isolation. We have, however, seen reason to believe that geographical or
+local isolation is by no means essential to the differentiation of
+species, because the same result is brought about by the incipient
+species acquiring different habits or frequenting a different station;
+and also by the fact that different varieties of the same species are
+known to prefer to pair with their like, and thus to bring about a
+physiological isolation of the most effective kind. This part of the
+subject will be again referred to when the very difficult problems
+presented by hybridity are discussed.[50]
+
+
+_Cases in which Isolation is Ineffective._
+
+One objection to the views of those who, like Mr. Gulick, believe
+isolation itself to be a cause of modification of species deserves
+attention, namely, the entire absence of change where, if this were a
+_vera causa_, we should expect to find it. In Ireland we have an
+excellent test case, for we know that it has been separated from Britain
+since the end of the glacial epoch, certainly many thousand years. Yet
+hardly one of its mammals, reptiles, or land molluscs has undergone the
+slightest change, even although there is certainly a distinct difference
+in the environment both inorganic and organic. That changes have not
+occurred through natural selection, is perhaps due to the less severe
+struggle for existence owing to the smaller number of competing species;
+but, if isolation itself were an efficient cause, acting continuously
+and cumulatively, it is incredible that a decided change should not have
+been produced in thousands of years. That no such change has occurred in
+this, and many other cases of isolation, seems to prove that it is not
+in itself a cause of modification.
+
+There yet remain a number of difficulties and objections relating to the
+question of hybridity, which are so important as to require a separate
+chapter for their adequate discussion.
+
+FOOTNOTES:
+
+[Footnote 41: See _Origin of Species_, pp. 176-198.]
+
+[Footnote 42: See Kerner's _Flowers and their Unbidden Guests_ for
+numerous other structures and peculiarities of plants which are shown to
+be adaptive and useful.]
+
+[Footnote 43: _Nature_, vol. xx. p. 603.]
+
+[Footnote 44: _Nature_, vol. xxxviii. p. 328.]
+
+[Footnote 45: A very remarkable illustration of function in an
+apparently useless ornament is given by Semper. He says, "It is known
+that the skin of reptiles encloses the body with scales. These scales
+are distinguished by very various sculpturings, highly characteristic of
+the different species. Irrespective of their systematic significance
+they appear to be of no value in the life of the animal; indeed, they
+are viewed as ornamental without regard to the fact that they are
+microscopic and much too delicate to be visible to other animals of
+their own species. It might, therefore, seem hopeless to show the
+necessity for their existence on Darwinian principles, and to prove that
+they are physiologically active organs. Nevertheless, recent
+investigations on this point have furnished evidence that this is
+possible.
+
+"It is known that many reptiles, and above all the snakes, cast off the
+whole skin at once, whereas human beings do so by degrees. If by any
+accident they are prevented doing so, they infallibly die, because the
+old skin has grown so tough and hard that it hinders the increase in
+volume which is inseparable from the growth of the animal. The casting
+of the skin is induced by the formation on the surface of the inner
+epidermis, of a layer of very fine and equally distributed hairs, which
+evidently serve the purpose of mechanically raising the old skin by
+their rigidity and position. These hairs then may be designated as
+_casting hairs_. That they are destined and calculated for this end is
+evident to me from the fact established by Dr. Braun, that the casting
+of the shells of the river crayfish is induced in exactly the same
+manner by the formation of a coating of hairs which mechanically loosens
+the old skin or shell from the new. Now the researches of Braun and
+Cartier have shown that these casting hairs--which serve the same
+purpose in two groups of animals so far apart in the systematic
+scale--after the casting, are partly transformed into the concentric
+stripes, sharp spikes, ridges, or warts which ornament the outer edges
+of the skin-scales of reptiles or the carapace of crabs."[1] Professor
+Semper adds that this example, with many others that might be quoted,
+shows that we need not abandon the hope of explaining morphological
+characters on Darwinian principles, although their nature is often
+difficult to understand.
+
+During a recent discussion of this question in the pages of _Nature_,
+Mr. St. George Mivart adduces several examples of what he deems useless
+specific characters. Among them are the aborted index finger of the
+lemurine Potto, and the thumbless hands of Colobus and Ateles, the
+"life-saving action" of either of which he thinks incredible. These
+cases suggest two remarks. In the first place, they involve _generic_,
+not _specific_, characters; and the three genera adduced are somewhat
+isolated, implying considerable antiquity and the extinction of many
+allied forms. This is important, because it affords ample time for great
+changes of conditions since the structures in question originated; and
+without a knowledge of these changes we can never safely assert that any
+detail of structure could not have been useful. In the second place, all
+three are cases of aborted or rudimentary organs; and these are admitted
+to be explained by non-use, leading to diminution of size, a further
+reduction being brought about by the action of the principle of economy
+of growth. But, when so reduced, the rudiment might be inconvenient or
+even hurtful, and then natural selection would aid in its complete
+abortion; in other words, the abortion of the part would be _useful_,
+and would therefore be subject to the law of survival of the fittest.
+The genera Ateles and Colobus are two of the most purely arboreal types
+of monkeys, and it is not difficult to conceive that the constant use of
+the elongated fingers for climbing from tree to tree, and catching on to
+branches while making great leaps, might require all the nervous energy
+and muscular growth to be directed to the fingers, the small thumb
+remaining useless. The case of the Potto is more difficult, both because
+it is, presumably, a more ancient type, and its actual life-history and
+habits are completely unknown. These cases are, therefore, not at all to
+the point as proving that positive specific characters--not mere
+rudiments characterising whole genera--are in any case useless.
+
+Mr. Mivart further objects to the alleged rigidity of the action of
+natural selection, because wounded or malformed animals have been found
+which had evidently lived a considerable time in their imperfect
+condition. But this simply proves that they were living under a
+temporarily favourable environment, and that the real struggle for
+existence, in their case, had not yet taken place. We must surely admit
+that, when the pinch came, and when perfectly formed stoats were dying
+for want of food, the one-footed animal, referred to by Mr. Mivart,
+would be among the first to succumb; and the same remark will apply to
+his abnormally toothed hares and rheumatic monkeys, which might,
+nevertheless, get on very well under favourable conditions. The struggle
+for existence, under which all animals and plants have been developed,
+is intermittent, and exceedingly irregular in its incidence and
+severity. It is most severe and fatal to the young; but when an animal
+has once reached maturity, and especially when it has gained experience
+by several years of an eventful existence, it may be able to maintain
+itself under conditions which would be fatal to a young and
+inexperienced creature of the same species. The examples adduced by Mr.
+Mivart do not, therefore, in any way impugn the hardness of nature as a
+taskmaster, or the extreme severity of the recurring struggle for
+existence. (See _Nature_, vol. xxxix. p. 127.)]
+
+[Footnote 46: _Origin of Species,_ p. 72.]
+
+[Footnote 47: Darwin's latest expression of opinion on this question is
+interesting, since it shows that he was inclined to return to his
+earlier view of the general, or universal, utility of specific
+characters. In a letter to Semper (30th Nov. 1878) he writes: "As our
+knowledge advances, very slight differences, considered by systematists
+as of no importance in structure, are continually found to be
+functionally important; and I have been especially struck with this fact
+in the case of plants, to which my observations have, of late years,
+been confined. Therefore it seems to me rather rash to consider slight
+differences between representative species, for instance, those
+inhabiting the different islands of the same archipelago, as of no
+functional importance, and as not in any way due to natural selection"
+_(Life of Darwin_, vol. iii. p. 161).]
+
+[Footnote 48: See _Variation of Animals and Plants_, vol. i. p. 86.]
+
+[Footnote 49: _Journal of the Linnean Society, Zoology,_ vol. xx. p.
+215.]
+
+[Footnote 50: In Mr. Gulick's last paper (_Journal of Linn. Soc. Zool._,
+vol. xx. pp. 189-274) he discusses the various forms of isolation above
+referred to, under no less than thirty-eight different divisions and
+subdivisions, with an elaborate terminology, and he argues that these
+will frequently bring about divergent evolution without any change in
+the environment or any action of natural selection. The discussion of
+the problem here given will, I believe, sufficiently expose the fallacy
+of his contention; but his illustration of the varied and often
+recondite modes by which practical isolation may be brought about, may
+help to remove one of the popular difficulties in the way of the action
+of natural selection in the origination of species.]
+
+
+
+
+CHAPTER VII
+
+ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
+STERILITY OF THEIR HYBRID OFFSPRING
+
+
+    Statement of the problem--Extreme susceptibility of the
+    reproductive functions--Reciprocal crosses--Individual
+    differences in respect to cross-fertilisation--Dimorphism and
+    trimorphism among plants--Cases of the fertility of hybrids and
+    of the infertility of mongrels--The effects of close
+    interbreeding--Mr. Huth's objections--Fertile hybrids among
+    animals--Fertility of hybrids among plants--Cases of sterility
+    of mongrels--Parallelism between crossing and change of
+    conditions--Remarks on the facts of hybridity--Sterility due to
+    changed conditions and usually correlated with other
+    characters--Correlation of colour with constitutional
+    peculiarities--The isolation of varieties by selective
+    association--The influence of natural selection upon sterility
+    and fertility--Physiological selection--Summary and concluding
+    remarks.
+
+
+
+One of the greatest, or perhaps we may say the greatest, of all the
+difficulties in the way of accepting the theory of natural selection as
+a complete explanation of the origin of species, has been the remarkable
+difference between varieties and species in respect of fertility when
+crossed. Generally speaking, it may be said that the varieties of any
+one species, however different they may be in external appearance, are
+perfectly fertile when crossed, and their mongrel offspring are equally
+fertile when bred among themselves; while distinct species, on the other
+hand, however closely they may resemble each other externally, are
+usually infertile when crossed, and their hybrid offspring absolutely
+sterile. This used to be considered a fixed law of nature, constituting
+the absolute test and criterion of a _species_ as distinct from a
+_variety_; and so long as it was believed that species were separate
+creations, or at all events had an origin quite distinct from that of
+varieties, this law could have no exceptions, because, if any two
+species had been found to be fertile when crossed and their hybrid
+offspring to be also fertile, this fact would have been held to prove
+them to be not _species_ but _varieties_. On the other hand, if two
+varieties had been found to be infertile, or their mongrel offspring to
+be sterile, then it would have been said: These are not varieties but
+true species. Thus the old theory led to inevitable reasoning in a
+circle; and what might be only a rather common fact was elevated into a
+law which had no exceptions.
+
+The elaborate and careful examination of the whole subject by Mr.
+Darwin, who has brought together a vast mass of evidence from the
+experience of agriculturists and horticulturists, as well as from
+scientific experimenters, has demonstrated that there is no such fixed
+law in nature as was formerly supposed. He shows us that crosses between
+some varieties are infertile or even sterile, while crosses between some
+species are quite fertile; and that there are besides a number of
+curious phenomena connected with the subject which render it impossible
+to believe that sterility is anything more than an incidental property
+of species, due to the extreme delicacy and susceptibility of the
+reproductive powers, and dependent on physiological causes we have not
+yet been able to trace. Nevertheless, the fact remains that most species
+which have hitherto been crossed produce sterile hybrids, as in the
+well-known case of the mule; while almost all domestic varieties, when
+crossed, produce offspring which are perfectly fertile among themselves.
+I will now endeavour to give such a sketch of the subject as may enable
+the reader to see something of the complexity of the problem, referring
+him to Mr. Darwin's works for fuller details.
+
+
+_Extreme Susceptibility of the Reproductive Functions._
+
+One of the most interesting facts, as showing how susceptible to changed
+conditions or to slight constitutional changes are the reproductive
+powers of animals, is the very general difficulty of getting those which
+are kept in confinement to breed; and this is frequently the only bar to
+domesticating wild species. Thus, elephants, bears, foxes, and numbers
+of species of rodents, very rarely breed in confinement; while other
+species do so more or less freely. Hawks, vultures, and owls hardly ever
+breed in confinement; neither did the falcons kept for hawking ever
+breed. Of the numerous small seed-eating birds kept in aviaries, hardly
+any breed, neither do parrots. Gallinaceous birds usually breed freely
+in confinement, but some do not; and even the guans and curassows, kept
+tame by the South American Indians, never breed. This shows that change
+of climate has nothing to do with the phenomenon; and, in fact, the same
+species that refuse to breed in Europe do so, in almost every case, when
+tamed or confined in their native countries. This inability to reproduce
+is not due to ill-health, since many of these creatures are perfectly
+vigorous and live very long.
+
+With our true domestic animals, on the other hand, fertility is perfect,
+and is very little affected by changed conditions. Thus, we see the
+common fowl, a native of tropical India, living and multiplying in
+almost every part of the world; and the same is the case with our
+cattle, sheep, and goats, our dogs and horses, and especially with
+domestic pigeons. It therefore seems probable, that this facility for
+breeding under changed conditions was an original property of the
+species which man has domesticated--a property which, more than any
+other, enabled him to domesticate them. Yet, even with these, there is
+evidence that great changes of conditions affect the fertility. In the
+hot valleys of the Andes sheep are less fertile; while geese taken to
+the high plateau of Bogota were at first almost sterile, but after some
+generations recovered their fertility. These and many other facts seem
+to show that, with the majority of animals, even a slight change of
+conditions may produce infertility or sterility; and also that after a
+time, when the animal has become thoroughly acclimatised, as it were, to
+the new conditions, the infertility is in some cases diminished or
+altogether ceases. It is stated by Bechstein that the canary was long
+infertile, and it is only of late years that good breeding birds have
+become common; but in this case no doubt selection has aided the change.
+
+As showing that these phenomena depend on deep-seated causes and are of
+a very general nature, it is interesting to note that they occur also
+in the vegetable kingdom. Allowing for all the circumstances which are
+known to prevent the production of seed, such as too great luxuriance of
+foliage, too little or too much heat, or the absence of insects to
+cross-fertilise the flowers, Mr. Darwin shows that many species which
+grow and flower with us, apparently in perfect health, yet never produce
+seed. Other plants are affected by very slight changes of conditions,
+producing seed freely in one soil and not in another, though apparently
+growing equally well in both; while, in some cases, a difference of
+position even in the same garden produces a similar result.[51]
+
+
+_Reciprocal Crosses._
+
+Another indication of the extreme delicacy of the adjustment between the
+sexes, which is necessary to produce fertility, is afforded by the
+behaviour of many species and varieties when reciprocally crossed. This
+will be best illustrated by a few of the examples furnished us by Mr.
+Darwin. The two distinct species of plants, Mirabilis jalapa and M.
+longiflora, can be easily crossed, and will produce healthy and fertile
+hybrids when the pollen of the latter is applied to the stigma of the
+former plant. But the same experimenter, Koelreuter, tried in vain, more
+than two hundred times during eight years, to cross them by applying the
+pollen of M. jalapa to the stigma of M. longiflora. In other cases two
+plants are so closely allied that some botanists class them as varieties
+(as with Matthiola annua and M. glabra), and yet there is the same great
+difference in the result when they are reciprocally crossed.
+
+
+_Individual Differences in respect to Cross-Fertilisation._
+
+A still more remarkable illustration of the delicate balance of
+organisation needful for reproduction, is afforded by the individual
+differences of animals and plants, as regards both their power of
+intercrossing with other individuals or other species, and the fertility
+of the offspring thus produced. Among domestic animals, Darwin states
+that it is by no means rare to find certain males and females which will
+not breed together, though both are known to be perfectly fertile with
+other males and females. Cases of this kind have occurred among horses,
+cattle, pigs, dogs, and pigeons; and the experiment has been tried so
+frequently that there can be no doubt of the fact. Professor G.J.
+Romanes states that he has a number of additional cases of this
+individual incompatibility, or of absolute sterility, between two
+individuals, each of which is perfectly fertile with other individuals.
+
+During the numerous experiments that have been made on the hybridisation
+of plants similar peculiarities have been noticed, some individuals
+being capable, others incapable, of being crossed with a distinct
+species. The same individual peculiarities are found in varieties,
+species, and genera. Koelreuter crossed five varieties of the common
+tobacco (Nicotiana tabacum) with a distinct species, Nicotiana
+glutinosa, and they all yielded very sterile hybrids; but those raised
+from one variety were less sterile, in all the experiments, than the
+hybrids from the four other varieties. Again, most of the species of the
+genus Nicotiana have been crossed, and freely produce hybrids; but one
+species, N. acuminata, not particularly distinct from the others, could
+neither fertilise, nor be fertilised by, any of the eight other species
+experimented on. Among genera we find some--such as Hippeastrum, Crinum,
+Calceolaria, Dianthus--almost all the species of which will fertilise
+other species and produce hybrid offspring; while other allied genera,
+as Zephyranthes and Silene, notwithstanding the most persevering
+efforts, have not produced a single hybrid even between the most closely
+allied species.
+
+
+_Dimorphism and Trimorphism._
+
+Peculiarities in the reproductive system affecting individuals of the
+same species reach their maximum in what are called heterostyled, or
+dimorphic and trimorphic flowers, the phenomena presented by which form
+one of the most remarkable of Mr. Darwin's many discoveries. Our common
+cowslip and primrose, as well as many other species of the genus
+Primula, have two kinds of flowers in about equal proportions. In one
+kind the stamens are short, being situated about the middle of the tube
+of the corolla, while the style is long, the globular stigma appearing
+just in the centre of the open flower. In the other kind the stamens are
+long, appearing in the centre or throat of the flower, while the style
+is short, the stigma being situated halfway down the tube at the same
+level as the stamens in the other form. These two forms have long been
+known to florists as the "pin-eyed" and the "thrum-eyed," but they are
+called by Darwin the long-styled and short-styled forms (see woodcut).
+
+[Illustration: FIG. 17.--Primula veris (Cowslip).]
+
+The meaning and use of these different forms was quite unknown till
+Darwin discovered, first, that cowslips and primroses are absolutely
+barren if insects are prevented from visiting them, and then, what is
+still more extraordinary, that each form is almost sterile when
+fertilised by its own pollen, and comparatively infertile when crossed
+with any other plant of its own form, but is perfectly fertile when the
+pollen of a long-styled is carried to the stigma of a short-styled
+plant, or _vice versa_. It will be seen, by the figures, that the
+arrangement is such that a bee visiting the flowers will carry the
+pollen from the long anthers of the short-styled form to the stigma of
+the long-styled form, while it would never reach the stigma of another
+plant of the short-styled form. But an insect visiting, first, a
+long-styled plant, would deposit the pollen on the stigma of another
+plant of the same kind if it were next visited; and this is probably the
+reason why the wild short-styled plants were found to be almost always
+most productive of seed, since they must be all fertilised by the other
+form, whereas the long-styled plants might often be fertilised by their
+own form. The whole arrangement, however, ensures cross-fertilisation;
+and this, as Mr. Darwin has shown by copious experiments, adds both to
+the vigour and fertility of almost all plants as well as animals.
+
+Besides the primrose family, many other plants of several distinct
+natural orders present similar phenomena, one or two of the most curious
+of which must be referred to. The beautiful crimson flax (Linum
+grandiflorum) has also two forms, the styles only differing in length;
+and in this case Mr. Darwin found by numerous experiments, which have
+since been repeated and confirmed by other observers, that each form is
+absolutely sterile with pollen from another plant of its own form, but
+abundantly fertile when crossed with any plant of the other form. In
+this case the pollen of the two forms cannot be distinguished under the
+microscope (whereas that of the two forms of Primula differs in size and
+shape), yet it has the remarkable property of being absolutely powerless
+on the stigmas of half the plants of its own species. The crosses
+between the opposite forms, which are fertile, are termed by Mr. Darwin
+"legitimate," and those between similar forms, which are sterile,
+"illegitimate"; and he remarks that we have here, within the limits of
+the same species, a degree of sterility which rarely occurs except
+between plants or animals not only of different _species_ but of
+different _genera_.
+
+But there is another set of plants, the trimorphic, in which the styles
+and stamens have each three forms--long, medium, and short, and in these
+it is possible to have eighteen different crosses. By an elaborate
+series of experiments it was shown that the six legitimate unions--that
+is, when a plant was fertilised by pollen from stamens of length
+corresponding to that of its style in the two other forms--were all
+abundantly fertile; while the twelve illegitimate unions, when a plant
+was fertilised by pollen from stamens of a different length from its
+own style, in any of the three forms, were either comparatively or
+wholly sterile.[52]
+
+We have here a wonderful amount of constitutional difference of the
+reproductive organs within a single species, greater than usually occurs
+within the numerous distinct species of a genus or group of genera; and
+all this diversity appears to have arisen for a purpose which has been
+obtained by many other, and apparently simpler, changes of structure or
+of function, in other plants. This seems to show us, in the first place,
+that variations in the mutual relations of the reproductive organs of
+different individuals must be as frequent as structural variations have
+been shown to be; and, also, that sterility in itself can be no test of
+specific distinctness. But this point will be better considered when we
+have further illustrated and discussed the complex phenomena of
+hybridity.
+
+
+_Cases of the Fertility of Hybrids, and of the Infertility of Mongrels._
+
+I now propose to adduce a few cases in which it has been proved, by
+experiment, that hybrids between two distinct species are fertile _inter
+se_; and then to consider why it is that such cases are so few in
+number.
+
+The common domestic goose (Anser ferns) and the Chinese goose (A.
+cygnoides) are very distinct species, so distinct that some naturalists
+have placed them in different genera; yet they have bred together, and
+Mr. Eyton raised from a pair of these hybrids a brood of eight. This
+fact was confirmed by Mr. Darwin himself, who raised several fine birds
+from a pair of hybrids which were sent him.[53] In India, according to
+Mr. Blyth and Captain Hutton, whole flocks of these hybrid geese are
+kept in various parts of the country where neither of the pure parent
+species exists, and as they are kept for profit they must certainly be
+fully fertile.
+
+Another equally striking case is that of the Indian humped and the
+common cattle, species which differ osteologically, and also in habits,
+form, voice, and constitution, so that they are by no means closely
+allied; yet Mr. Darwin assures us that he has received decisive
+evidence that the hybrids between these are perfectly fertile _inter
+se_.
+
+Dogs have been frequently crossed with wolves and with jackals, and
+their hybrid offspring have been found to be fertile _inter se_ to the
+third or fourth generation, and then usually to show some signs of
+sterility or of deterioration. The wolf and dog may be originally the
+same species, but the jackal is certainly distinct; and the appearance
+of infertility or of weakness is probably due to the fact that, in
+almost all these experiments, the offspring of a single pair--themselves
+usually from the same litter--- were bred in-and-in, and this alone
+sometimes produces the most deleterious effects. Thus, Mr. Low in his
+great work on the _Domesticated Animals of Great Britain_, says: "If we
+shall breed a pair of dogs from the same litter, and unite again the
+offspring of this pair, we shall produce at once a feeble race of
+creatures; and the process being repeated for one or two generations
+more, the family will die out, or be incapable of propagating their
+race. A gentleman of Scotland made the experiment on a large scale with
+certain foxhounds, and he found that the race actually became monstrous
+and perished utterly." The same writer tells us that hogs have been made
+the subject of similar experiments: "After a few generations the victims
+manifest the change induced in the system. They become of diminished
+size; the bristles are changed into hairs; the limbs become feeble and
+short; the litters diminish in frequency, and in the number of the young
+produced; the mother becomes unable to nourish them, and, if the
+experiment be carried as far as the case will allow, the feeble, and
+frequently monstrous offspring, will be incapable of being reared up,
+and the miserable race will utterly perish."[54]
+
+These precise statements, by one of the greatest authorities on our
+domesticated animals, are sufficient to show that the fact of
+infertility or degeneracy appearing in the offspring of hybrids after a
+few generations need not be imputed to the fact of the first parents
+being distinct species, since exactly the same phenomena appear when
+individuals of the same species are bred under similar adverse
+conditions. But in almost all the experiments that have hitherto been
+made in crossing distinct species, no care has been taken to avoid close
+interbreeding by securing several hybrids from quite distinct stocks to
+start with, and by having two or more sets of experiments carried on at
+once, so that crosses between the hybrids produced may be occasionally
+made. Till this is done no experiments, such as those hitherto tried,
+can be held to prove that hybrids are in all cases infertile _inter se_.
+
+It has, however, been denied by Mr. A.H. Huth, in his interesting work
+on _The Marriage of Near Kin_, that any amount of breeding in-and-in is
+in itself hurtful; and he quotes the evidence of numerous breeders whose
+choicest stocks have always been so bred, as well as cases like the
+Porto Santo rabbits, the goats of Juan Fernandez, and other cases in
+which animals allowed to run wild have increased prodigiously and
+continued in perfect health and vigour, although all derived from a
+single pair. But in all these cases there has been rigid selection by
+which the weak or the infertile have been eliminated, and with such
+selection there is no doubt that the ill effects of close interbreeding
+can be prevented for a long time; but this by no means proves that no
+ill effects are produced. Mr. Huth himself quotes M. Allie, M. Aube,
+Stephens, Giblett, Sir John Sebright, Youatt, Druce, Lord Weston, and
+other eminent breeders, as finding from experience that close
+interbreeding _does_ produce bad effects; and it cannot be supposed that
+there would be such a consensus of opinion on this point if the evil
+were altogether imaginary. Mr. Huth argues, that the evil results which
+do occur do not depend on the close interbreeding itself, but on the
+tendency it has to perpetuate any constitutional weakness or other
+hereditary taints; and he attempts to prove this by the argument that
+"if crosses act by virtue of being a cross, and not by virtue of
+removing an hereditary taint, then the greater the difference between
+the two animals crossed the more beneficial will that act be." He then
+shows that, the wider the difference the less is the benefit, and
+concludes that a cross, as such, has no beneficial effect. A parallel
+argument would be, that change of air, as from inland to the sea-coast,
+or from a low to an elevated site, is not beneficial in itself, because,
+if so, a change to the tropics or to the polar regions should be more
+beneficial. In both these cases it may well be that no benefit would
+accrue to a person in perfect health; but then there is no such thing
+as "perfect health" in man, and probably no such thing as absolute
+freedom from constitutional taint in animals. The experiments of Mr.
+Darwin, showing the great and immediate good effects of a cross between
+distinct strains in plants, cannot be explained away; neither can the
+innumerable arrangements to secure cross-fertilisation by insects, the
+real use and purport of which will be discussed in our eleventh chapter.
+On the whole, then, the evidence at our command proves that, whatever
+may be its ultimate cause, close interbreeding _does_ usually produce
+bad results; and it is only by the most rigid selection, whether natural
+or artificial, that the danger can be altogether obviated.
+
+
+_Fertile Hybrids among Animals._
+
+One or two more cases of fertile hybrids may be given before we pass on
+to the corresponding experiments in plants. Professor Alfred Newton
+received from a friend a pair of hybrid ducks, bred from a common duck
+(Anas boschas), and a pintail (Dafila acuta). From these he obtained
+four ducklings, but these latter, when grown up, proved infertile, and
+did not breed again. In this case we have the results of close
+interbreeding, with too great a difference between the original species,
+combining to produce infertility, yet the fact of a hybrid from such a
+pair producing healthy offspring is itself noteworthy.
+
+Still more extraordinary is the following statement of Mr. Low: "It has
+been long known to shepherds, though questioned by naturalists, that the
+progeny of the cross between the sheep and goat is fertile. Breeds of
+this mixed race are numerous in the north of Europe."[55] Nothing
+appears to be known of such hybrids either in Scandinavia or in Italy;
+but Professor Giglioli of Florence has kindly given me some useful
+references to works in which they are described. The following extract
+from his letter is very interesting: "I need not tell you that there
+being such hybrids is now generally accepted as a fact. Buffon
+(_Supplements_, tom. iii. p. 7, 1756) obtained one such hybrid in 1751
+and eight in 1752. Sanson (_La Culture_, vol. vi. p. 372, 1865) mentions
+a case observed in the Vosges, France. Geoff. St. Hilaire (_Hist. Nat.
+Gen. des reg. org._, vol. iii. p. 163) was the first to mention, I
+believe, that in different parts of South America the ram is more
+usually crossed with the she-goat than the sheep with the he-goat. The
+well-known 'pellones' of Chile are produced by the second and third
+generation of such hybrids (Gay, 'Hist, de Chile,' vol. i. p. 466,
+_Agriculture_, 1862). Hybrids bred from goat and sheep are called
+'chabin' in French, and 'cabruno' in Spanish. In Chile such hybrids are
+called 'carneros lanudos'; their breeding _inter se_ appears to be not
+always successful, and often the original cross has to be recommenced to
+obtain the proportion of three-eighths of he-goat and five-eighths of
+sheep, or of three-eighths of ram and five-eighths of she-goat; such
+being the reputed best hybrids."
+
+With these numerous facts recorded by competent observers we can hardly
+doubt that races of hybrids between these very distinct species have
+been produced, and that such hybrids are fairly fertile _inter se_; and
+the analogous facts already given lead us to believe that whatever
+amount of infertility may at first exist could be eliminated by careful
+selection, if the crossed races were bred in large numbers and over a
+considerable area of country. This case is especially valuable, as
+showing how careful we should be in assuming the infertility of hybrids
+when experiments have been made with the progeny of a single pair, and
+have been continued only for one or two generations.
+
+Among insects one case only appears to have been recorded. The hybrids
+of two moths (Bombyx cynthia and B. arrindia) were proved in Paris,
+according to M. Quatrefages, to be fertile _inter se_ for eight
+generations.
+
+
+_Fertility of Hybrids among Plants._
+
+Among plants the cases of fertile hybrids are more numerous, owing, in
+part, to the large scale on which they are grown by gardeners and
+nurserymen, and to the greater facility with which experiments can be
+made. Darwin tells us that Koelreuter found ten cases in which two plants
+considered by botanists to be distinct species were quite fertile
+together, and he therefore ranked them all as varieties of each other.
+In some cases these were grown for six to ten successive generations,
+but after a time the fertility decreased, as we saw to be the case in
+animals, and presumably from the same cause, too close interbreeding.
+
+Dean Herbert, who carried on experiments with great care and skill for
+many years, found numerous cases of hybrids which were perfectly fertile
+_inter se_. Crinum capense, fertilised by three other species--C.
+pedunculatum, C. canaliculatum, or C. defixum--all very distinct from
+it, produced perfectly fertile hybrids; while other species less
+different in appearance were quite sterile with the same C. capense.
+
+All the species of the genus Hippeastrum produce hybrid offspring which
+are invariably fertile. Lobelia syphylitica and L. fulgens, two very
+distinct species, have produced a hybrid which has been named Lobelia
+speciosa, and which reproduces itself abundantly. Many of the beautiful
+pelargoniums of our greenhouses are hybrids, such as P. ignescens from a
+cross between P. citrinodorum and P. fulgidum, which is quite fertile,
+and has become the parent of innumerable varieties of beautiful plants.
+All the varied species of Calceolaria, however different in appearance,
+intermix with the greatest readiness, and the hybrids are all more or
+less fertile. But the most remarkable case is that of two species of
+Petunia, of which Dean Herbert says: "It is very remarkable that,
+although there is a great difference in the form of the flower,
+especially of the tube, of P. nyctanigenaeflora and P. phoenicea the
+mules between them are not only fertile, but I have found them seed much
+more freely with me than either parent.... From a pod of the
+above-mentioned mule, to which no pollen but its own had access, I had a
+large batch of seedlings in which there was no variability or difference
+from itself; and it is evident that the mule planted by itself, in a
+congenial climate, would reproduce itself as a species; at least as much
+deserving to be so considered as the various Calceolarias of different
+districts of South America."[56]
+
+Darwin was informed by Mr. C. Noble that he raises stocks for grafting
+from a hybrid between Rhododendron ponticum and R. catawbiense, and that
+this hybrid seeds as freely as it is possible to imagine. He adds that
+horticulturists raise large beds of the same hybrid, and such alone are
+fairly treated; for, by insect agency, the several individuals are
+freely crossed with each other, and the injurious influence of close
+interbreeding is thus prevented. Had hybrids, when fairly treated,
+always gone on decreasing in fertility in each successive generation, as
+Gartner believed to be the case, the fact would have been notorious to
+nurserymen.[57]
+
+
+_Cases of Sterility of Mongrels._
+
+The reverse phenomenon to the fertility of hybrids, the sterility of
+mongrels or of the crosses between _varieties_ of the same species, is a
+comparatively rare one, yet some undoubted cases have occurred. Gartner,
+who believed in the absolute distinctness of species and varieties, had
+two varieties of maize--one dwarf with yellow seeds, the other taller
+with red seeds; yet they never naturally crossed, and, when fertilised
+artificially, only a single head produced any seeds, and this one only
+five grains. Yet these few seeds were fertile; so that in this case the
+first cross was almost sterile, though the hybrid when at length
+produced was fertile. In like manner, dissimilarly coloured varieties of
+Verbascum or mullein have been found by two distinct observers to be
+comparatively infertile. The two pimpernels (Anagallis arvensis and A.
+coerulea), classed by most botanists as varieties of one species, have
+been found, after repeated trials, to be perfectly sterile when crossed.
+
+No cases of this kind are recorded among animals; but this is not to be
+wondered at, when we consider how very few experiments have been made
+with natural varieties; while there is good reason for believing that
+domestic varieties are exceptionally fertile, partly because one of the
+conditions of domestication was fertility under changed conditions, and
+also because long continued domestication is believed to have the effect
+of increasing fertility and eliminating whatever sterility may exist.
+This is shown by the fact that, in many cases, domestic animals are
+descended from two or more distinct species. This is almost certainly
+the case with the dog, and probably with the hog, the ox, and the sheep;
+yet the various breeds are now all perfectly fertile, although we have
+every reason to suppose that there would be some degree of infertility
+if the several aboriginal species were crossed together for the first
+time.
+
+
+_Parallelism between Crossing and Change of Conditions._
+
+In the whole series of these phenomena, from the beneficial effects of
+the crossing of different stocks and the evil effects of close
+interbreeding, up to the partial or complete sterility induced by
+crosses between species belonging to different genera, we have, as Mr.
+Darwin points out, a curious parallelism with the effects produced by
+change of physical conditions. It is well known that slight changes in
+the conditions of life are beneficial to all living things. Plants, if
+constantly grown in one soil and locality from their own seeds, are
+greatly benefited by the importation of seed from some other locality.
+The same thing happens with animals; and the benefit we ourselves
+experience from "change of air" is an illustration of the same
+phenomenon. But the amount of the change which is beneficial has its
+limits, and then a greater amount is injurious. A change to a climate a
+few degrees warmer or colder may be good, while a change to the tropics
+or to the arctic regions might be injurious.
+
+Thus we see that, both slight changes of conditions and a slight amount
+of crossing, are beneficial; while extreme changes, and crosses between
+individuals too far removed in structure or constitution, are injurious.
+And there is not only a parallelism but an actual connection between the
+two classes of facts, for, as we have already shown, many species of
+animals and plants are rendered infertile, or altogether sterile, by the
+change from their natural conditions which occurs in confinement or in
+cultivation; while, on the other hand, the increased vigour or fertility
+which is invariably produced by a judicious cross may be also effected
+by a judicious change of climate and surroundings. We shall see in a
+subsequent chapter, that this interchangeability of the beneficial
+effects of crossing and of new conditions, serves to explain some very
+puzzling phenomena in the forms and economy of flowers.
+
+
+_Remarks on the Facts of Hybridity._
+
+The facts that have now been adduced, though not very numerous, are
+sufficiently conclusive to prove that the old belief, of the universal
+sterility of hybrids and fertility of mongrels, is incorrect. The
+doctrine that such a universal law existed was never more than a
+plausible generalisation, founded on a few inconclusive facts derived
+from domesticated animals and cultivated plants. The facts were, and
+still are, inconclusive for several reasons. They are founded,
+primarily, on what occurs among animals in domestication; and it has
+been shown that domestication both tends to increase fertility, and was
+itself rendered possible by the fertility of those particular species
+being little affected by changed conditions. The exceptional fertility
+of all the varieties of domesticated animals does not prove that a
+similar fertility exists among natural varieties. In the next place, the
+generalisation is founded on too remote crosses, as in the case of the
+horse and the ass, the two most distinct and widely separated species of
+the genus Equus, so distinct indeed that they have been held by some
+naturalists to form distinct genera. Crosses between the two species of
+zebra, or even between the zebra and the quagga, or the quagga and the
+ass, might have led to a very different result. Again, in pre-Darwinian
+times it was so universally the practice to argue in a circle, and
+declare that the fertility of the offspring of a cross proved the
+identity of species of the parents, that experiments in hybridity were
+usually made between very remote species and even between species of
+different genera, to avoid the possibility of the reply: "They are both
+really the same species;" and the sterility of the hybrid offspring of
+such remote crosses of course served to strengthen the popular belief.
+
+Now that we have arrived at a different standpoint, and look upon a
+species, not as a distinct entity due to special creation, but as an
+assemblage of individuals which have become somewhat modified in
+structure, form, and constitution so as to adapt them to slightly
+different conditions of life; which can be differentiated from other
+allied assemblages; which reproduce their like, and which usually breed
+together--we require a fresh set of experiments calculated to determine
+the matter of fact,--whether such species crossed with their near allies
+do always produce offspring which are more or less sterile _inter se_.
+Ample materials for such experiments exist, in the numerous
+"representative species" inhabiting distinct areas on a continent or
+different islands of a group; or even in those found in the same area
+but frequenting somewhat different stations.
+
+To carry out these experiments with any satisfactory result, it will be
+necessary to avoid the evil effects of confinement and of too close
+interbreeding. If birds are experimented with, they should be allowed as
+much liberty as possible, a plot of ground with trees and bushes being
+enclosed with wire netting overhead so as to form a large open aviary.
+The species experimented with should be obtained in considerable
+numbers, and by two separate persons, each making the opposite
+reciprocal cross, as explained at p. 155. In the second generation these
+two stocks might be themselves crossed to prevent the evil effects of
+too close interbreeding. By such experiments, carefully carried out with
+different groups of animals and plants, we should obtain a body of facts
+of a character now sadly wanting, and without which it is hopeless to
+expect to arrive at a complete solution of this difficult problem. There
+are, however, some other aspects of the question that need to be
+considered, and some theoretical views which require to be carefully
+examined, having done which we shall be in a condition to state the
+general conclusions to which the facts and reasonings at our command
+seem to point.
+
+
+_Sterility due to changed Conditions and usually correlated with other
+Characters, especially with Colour._
+
+The evidence already adduced as to the extreme susceptibility of the
+reproductive system, and the curious irregularity with which infertility
+or sterility appears in the crosses between some varieties or species
+while quite absent in those between others, seem to indicate that
+sterility is a characteristic which has a constant tendency to appear,
+either by itself or in correlation with other characters. It is known to
+be especially liable to occur under changed conditions of life; and, as
+such change is usually the starting-point and cause of the development
+of new species, we have already found a reason why it should so often
+appear when species become fully differentiated.
+
+In almost all the cases of infertility or sterility between varieties or
+species, we have some external differences with which it is correlated;
+and though these differences are sometimes slight, and the amount of the
+infertility is not always, or even usually, proportionate to the
+external difference between the two forms crossed, we must believe that
+there is some connection between the two classes of facts. This is
+especially the case as regards colour; and Mr. Darwin has collected a
+body of facts which go far to prove that colour, instead of being an
+altogether trifling and unimportant character, as was supposed by the
+older naturalists, is really one of great significance, since it is
+undoubtedly often correlated with important constitutional differences.
+Now colour is one of the characters that most usually distinguishes
+closely allied species; and when we hear that the most closely allied
+species of plants are infertile together, while those more remote are
+fertile, the meaning usually is that the former differ chiefly in the
+_colour_ of their flowers, while the latter differ in the form of the
+flowers or foliage, in habit, or in other structural characters.
+
+It is therefore a most curious and suggestive fact, that in all the
+recorded cases, in which a decided infertility occurs between varieties
+of the same species, those varieties are distinguished by a difference
+of colour. The infertile varieties of Verbascum were white and yellow
+flowered respectively; the infertile varieties of maize were red and
+yellow seeded; while the infertile pimpernels were the red and the blue
+flowered varieties. So, the differently coloured varieties of
+hollyhocks, though grown close together, each reproduce their own colour
+from seed, showing that they are not capable of freely intercrossing.
+Yet Mr. Darwin assures us that the agency of bees is necessary to carry
+the pollen from one plant to another, because in each flower the pollen
+is shed before the stigma is ready to receive it. We have here,
+therefore, either almost complete sterility between varieties of
+different colours, or a prepotent effect of pollen from a flower of the
+same colour, bringing about the same result.
+
+Similar phenomena have not been recorded among animals; but this is not
+to be wondered at when we consider that most of our pure and valued
+domestic breeds are characterised by definite colours which constitute
+one of their distinctive marks, and they are, therefore, seldom crossed
+with these of another colour; and even when they are so crossed, no
+notice would be taken of any slight diminution of fertility, since this
+is liable to occur from many causes. We have also reason to believe that
+fertility has been increased by long domestication, in addition to the
+fact of the original stocks being exceptionally fertile; and no
+experiments have been made on the differently coloured varieties of wild
+animals. There are, however, a number of very curious facts showing that
+colour in animals, as in plants, is often correlated with constitutional
+differences of a remarkable kind, and as these have a close relation to
+the subject we are discussing, a brief summary of them will be here
+given.
+
+
+_Correlation of Colour with Constitutional Peculiarities._
+
+The correlation of a white colour and blue eyes in male cats with
+deafness, and of the tortoise-shell marking with the female sex of the
+same animal, are two well-known but most extraordinary cases. Equally
+remarkable is the fact, communicated to Darwin by Mr. Tegetmeier, that
+white, yellow, pale blue, or dun pigeons, of all breeds, have the young
+birds born naked, while in all other colours they are well covered with
+down. Here we have a case in which colour seems of more physiological
+importance than all the varied structural differences between the
+varieties and breeds of pigeons. In Virginia there is a plant called the
+paint-root (Lachnanthes tinctoria), which, when eaten by pigs, colours
+their bones pink, and causes the hoofs of all but the black varieties to
+drop off; so that black pigs only can be kept in the district.[58]
+Buckwheat in flower is also said to be injurious to white pigs but not
+to black. In the Tarentino, black sheep are not injured by eating the
+Hypericum crispum--a species of St. John's-wort--which kills white
+sheep. White terriers suffer most from distemper; white chickens from
+the gapes. White-haired horses or cattle are subject to cutaneous
+diseases from which the dark coloured are free; while, both in Thuringia
+and the West Indies, it has been noticed that white or pale coloured
+cattle are much more troubled by flies than are those which are brown or
+black. The same law even extends to insects, for it is found that
+silkworms which produce white cocoons resist the fungus disease much
+better than do those which produce yellow cocoons.[59] Among plants, we
+have in North America green and yellow-fruited plums not affected by a
+disease that attacked the purple-fruited varieties. Yellow-fleshed
+peaches suffer more from disease than white-fleshed kinds. In Mauritius,
+white sugar-canes were attacked by a disease from which the red canes
+were free. White onions and verbenas are most liable to mildew; and
+red-flowered hyacinths were more injured by the cold during a severe
+winter in Holland than any other kinds.[60]
+
+These curious and inexplicable correlations of colour with
+constitutional peculiarities, both in animals and plants, render it
+probable that the correlation of colour with infertility, which has been
+detected in several cases in plants, may also extend to animals in a
+state of nature; and if so, the fact is of the highest importance as
+throwing light on the origin of the infertility of many allied species.
+This will be better understood after considering the facts which will be
+now described.
+
+
+_The Isolation of Varieties by Selective Association._
+
+In the last chapter I have shown that the importance of geographical
+isolation for the formation of new species by natural selection has been
+greatly exaggerated, because the very change of conditions, which is
+the initial power in starting such new forms, leads also to a local or
+stational segregation of the forms acted upon. But there is also a very
+powerful cause of isolation in the mental nature--the likes and
+dislikes--of animals; and to this is probably due the fact of the
+comparative rarity of hybrids in a state of nature. The differently
+coloured herds of cattle in the Falkland Islands, each of which keeps
+separate, have been already mentioned; and it may be added, that the
+mouse-coloured variety seem to have already developed a physiological
+peculiarity in breeding a month earlier than the others. Similar facts
+occur, however, among our domestic animals and are well known to
+breeders. Professor Low, one of the greatest authorities on our
+domesticated animals, says: "The female of the dog, when not under
+restraint, makes selection of her mate, the mastiff selecting the
+mastiff, the terrier the terrier, and so on." And again: "The Merino
+sheep and Heath sheep of Scotland, if two flocks are mixed together,
+each will breed with its own variety." Mr. Darwin has collected many
+facts illustrating this point. One of the chief pigeon-fanciers in
+England informed him that, if free to choose, each breed would prefer
+pairing with its own kind. Among the wild horses in Paraguay those of
+the same colour and size associate together; while in Circassia there
+are three races of horses which have received special names, and which,
+when living a free life, almost always refuse to mingle and cross, and
+will even attack one another. On one of the Faroe Islands, not more than
+half a mile in diameter, the half-wild native black sheep do not readily
+mix with imported white sheep. In the Forest of Dean, and in the New
+Forest, the dark and pale coloured herds of fallow deer have never been
+known to mingle; and even the curious Ancon sheep of quite modern origin
+have been observed to keep together, separating themselves from the rest
+of the flock when put into enclosures with other sheep. The same rule
+applies to birds, for Darwin was informed by the Rev. W.D. Fox that his
+flocks of white and Chinese geese kept distinct.[61]
+
+This constant preference of animals for their like, even in the case of
+slightly different varieties of the same species, is evidently a fact
+of great importance in considering the origin of species by natural
+selection, since it shows us that, so soon as a slight differentiation
+of form or colour has been effected, isolation will at once arise by the
+selective association of the animals themselves; and thus the great
+stumbling-block of "the swamping effects of intercrossing," which has
+been so prominently brought forward by many naturalists, will be
+completely obviated.
+
+If now we combine with this fact the correlation of colour with
+important constitutional peculiarities, and, in some cases, with
+infertility; and consider, further, the curious parallelism that has
+been shown to exist between the effects of changed conditions and the
+intercrossing of varieties in producing either an increase or a decrease
+of fertility, we shall have obtained, at all events, a starting-point
+for the production of that infertility which is so characteristic a
+feature of distinct species when intercrossed. All we need, now, is some
+means of increasing or accumulating this initial tendency; and to a
+discussion of this problem we will therefore address ourselves.
+
+
+_The Influence of Natural Selection upon Sterility and Fertility._
+
+It will occur to many persons that, as the infertility or sterility of
+incipient species would be useful to them when occupying the same or
+adjacent areas, by neutralising the effects of intercrossing, this
+infertility might have been increased by the action of natural
+selection; and this will be thought the more probable if we admit, as we
+have seen reason to do, that variations in fertility occur, perhaps as
+frequently as other variations. Mr. Darwin tells us that, at one time,
+this appeared to him probable, but he found the problem to be one of
+extreme complexity; and he was also influenced against the view by many
+considerations which seemed to render such an origin of the sterility or
+infertility of species when intercrossed very improbable. The fact that
+species which occupy distinct areas, and which nowhere come in contact
+with each other, are often sterile when crossed, is one of the
+difficulties; but this may perhaps be overcome by the consideration
+that, though now isolated, they may, and often must, have been in
+contact at their origination. More important is the objection that
+natural selection could not possibly have produced the difference that
+often occurs between reciprocal crosses, one of these being sometimes
+fertile, while the other is sterile. The extremely different amounts of
+infertility or sterility between different species of the same genus,
+the infertility often bearing no proportion to the difference between
+the species crossed, is also an important objection. But none of these
+objections would have much weight if it could be clearly shown that
+natural selection _is_ able to increase the infertility variations of
+incipient species, as it is certainly able to increase and develop all
+useful variations of form, structure, instincts, or habits. Ample causes
+of infertility have been shown to exist, in the nature of the organism
+and the laws of correlation; the agency of natural selection is only
+needed to accumulate the effects produced by these causes, and to render
+their final results more uniform and more in accordance with the facts
+that exist.
+
+About twenty years ago I had much correspondence and discussion with Mr.
+Darwin on this question. I then believed that I was able to demonstrate
+the action of natural selection in accumulating infertility; but I could
+not convince him, owing to the extreme complexity of the process under
+the conditions which he thought most probable. I have recently returned
+to the question; and, with the fuller knowledge of the facts of
+variation we now possess, I think it may be shown that natural selection
+_is_, in some probable cases at all events, able to accumulate
+variations in infertility between incipient species.
+
+The simplest case to consider, will be that in which two forms or
+varieties of a species, occupying an extensive area, are in process of
+adaptation to somewhat different modes of life within the same area. If
+these two forms freely intercross with each other, and produce mongrel
+offspring which are quite fertile _inter se_, then the further
+differentiation of the forms into two distinct species will be retarded,
+or perhaps entirely prevented; for the offspring of the crossed unions
+will be, perhaps, more vigorous on account of the cross, although less
+perfectly adapted to the conditions of existence than either of the pure
+breeds; and this would certainly establish a powerful antagonistic
+influence to the further differentiation of the two forms.
+
+Now, let us suppose that a partial sterility of the hybrids between the
+two forms arises, in correlation with the different modes of life and
+the slight external or internal peculiarities that exist between them,
+both of which we have seen to be real causes of infertility. The result
+will be that, even if the hybrids between the two forms are still freely
+produced, these hybrids will not themselves increase so rapidly as the
+two pure forms; and as these latter are, by the terms of the problem,
+better suited to their conditions of life than are the hybrids between
+them, they will not only increase more rapidly, but will also tend to
+supplant the hybrids altogether whenever the struggle for existence
+becomes exceptionally severe. Thus, the more complete the sterility of
+the hybrids the more rapidly will they die out and leave the two parent
+forms pure. Hence it will follow that, if there is greater infertility
+between the two forms in one part of the area than the other, these
+forms will be kept more pure wherever this greater infertility prevails,
+will therefore have an advantage at each recurring period of severe
+struggle for existence, and will thus ultimately supplant the less
+infertile or completely fertile forms that may exist in other portions
+of the area. It thus appears that, in such a case as here supposed,
+natural selection would preserve those portions of the two breeds which
+were most infertile with each other, or whose hybrid offspring were most
+infertile; and would, therefore, if variations in fertility continued to
+arise, tend to increase that infertility. It must particularly be noted
+that this effect would result, not by the preservation of the infertile
+variations on account of their infertility, but by the inferiority of
+the hybrid offspring, both as being fewer in numbers, less able to
+continue their race, and less adapted to the conditions of existence
+than either of the pure forms. It is this inferiority of the hybrid
+offspring that is the essential point; and as the number of these
+hybrids will be permanently less where the infertility is greatest,
+therefore those portions of the two forms in which infertility is
+greatest will have the advantage, and will ultimately survive in the
+struggle for existence.
+
+The differentiation of the two forms into distinct species, with the
+increase of infertility between them, would be greatly assisted by two
+other important factors in the problem. It has already been shown that,
+with each modification of form and habits, and especially with
+modifications of colour, there arises a disinclination of the two forms
+to pair together; and this would produce an amount of isolation which
+would greatly assist the specialisation of the forms in adaptation to
+their different conditions of life. Again, evidence has been adduced
+that change of conditions or of mode of life is a potent cause of
+disturbance of the reproductive system, and, consequently, of
+infertility. We may therefore assume that, as the two forms adopted more
+and more different modes of life, and perhaps acquired also decided
+peculiarities of form and coloration, the infertility between them would
+increase or become more general; and as we have seen that every such
+increase of infertility would give that portion of the species in which
+it arose an advantage over the remaining portions in which the two
+varieties were more fertile together, all this induced infertility would
+maintain itself, and still further increase the general infertility
+between the two forms of the species.
+
+It follows, then, that specialisation to separate conditions of life,
+differentiation of external characters, disinclination to cross-unions,
+and the infertility of the hybrid produce of these unions, would all
+proceed _pari passu_, and would ultimately lead to the production of two
+distinct forms having all the characteristics, physiological as well as
+structural, of true species.
+
+In the case now discussed it has been supposed, that some amount of
+general infertility might arise in correlation with the different modes
+of life of two varieties or incipient species. A considerable body of
+facts already adduced renders it probable that this _is_ the mode in
+which any widespread infertility would arise; and, if so, it has been
+shown that, by the influence of natural selection and the known laws
+which affect varieties, the infertility would be gradually increased.
+But, if we suppose the infertility to arise sporadically within the two
+forms, and to affect only a small proportion of the individuals in any
+area, it will be difficult, if not impossible, to show that such
+infertility would have any tendency to increase, or would produce any
+but a prejudicial effect. If, for example, five per cent of each form
+thus varied so as to be infertile with the other form, the result would
+be hardly perceptible, because the individuals which formed cross-unions
+and produced hybrids would constitute a very small portion of the whole
+species; and the hybrid offspring, being at a disadvantage in the
+struggle for existence and being themselves infertile, would soon die
+out, while the much more numerous fertile portion of the two forms would
+increase rapidly, and furnish a sufficient number of pure-bred offspring
+of each form to take the place of the somewhat inferior hybrids between
+them whenever the struggle for existence became severe. We must suppose
+that the normal fertile forms would transmit their fertility to their
+progeny, and the few infertile forms their infertility; but the latter
+would necessarily lose half their proper increase by the sterility of
+their hybrid offspring whenever they crossed with the other form, and
+when they bred with their own form the tendency to sterility would die
+out except in the very minute proportion of the five per cent
+(one-twentieth) that chance would lead to pair together. Under these
+circumstances the incipient sterility between the two forms would
+rapidly be eliminated, and could never rise much above the numbers which
+were produced by sporadic variation each year.
+
+It was, probably, by a consideration of some such case as this that Mr.
+Darwin came to the conclusion that infertility arising between incipient
+species could not be increased by natural selection; and this is the
+more likely, as he was always disposed to minimise both the frequency
+and the amount even of structural variations.
+
+We have yet to notice another mode of action of natural selection in
+favouring and perpetuating any infertility that may arise between two
+incipient species. If several distinct species are undergoing
+modification at the same time and in the same area, to adapt them to
+some new conditions that have arisen there, then any species in which
+the structural or colour differences that have arisen between it and its
+varieties or close allies were correlated with infertility of the
+crosses between them, would have an advantage over the corresponding
+varieties of other species in which there was no such physiological
+peculiarity. Thus, incipient species which were infertile together would
+have an advantage over other incipient species which were fertile, and,
+whenever the struggle for existence became severe, would prevail over
+them and take their place. Such infertility, being correlated with
+constitutional or structural differences, would probably, as already
+suggested, go on increasing as these differences increased; and thus, by
+the time the new species became fully differentiated from its parent
+form (or brother variety) the infertility might have become as well
+marked as we usually find it to be between distinct species.
+
+This discussion has led us to some conclusions of the greatest
+importance as bearing on the difficult problem of the cause of the
+sterility of the hybrids between distinct species. Accepting, as highly
+probable, the fact of variations in fertility occurring in correlation
+with variations in habits, colour, or structure, we see, that so long as
+such variations occurred only sporadically, and affected but a small
+proportion of the individuals in any area, the infertility could not be
+increased by natural selection, but would tend to die out almost as fast
+as it was produced. If, however, it was so closely correlated with
+physical variations or diverse modes of life as to affect, even in a
+small degree, a considerable proportion of the individuals of the two
+forms in definite areas, it would be preserved by natural selection, and
+the portion of the varying species thus affected would increase at the
+expense of those portions which were more fertile when crossed. Each
+further variation towards infertility between the two forms would be
+again preserved, and thus the incipient infertility of the hybrid
+offspring might be increased till it became so great as almost to amount
+to sterility. Yet further, we have seen that if several competing
+species in the same area were being simultaneously modified, those
+between whose varieties infertility arose would have an advantage over
+those whose varieties remained fertile _inter se_, and would ultimately
+supplant them.
+
+The preceding argument, it will be seen, depends entirely upon the
+assumption that some amount of infertility characterises the distinct
+varieties which are in process of differentiation into species; and it
+may be objected that of such infertility there is no proof. This is
+admitted; but it is urged that facts have been adduced which render such
+infertility probable, at least in some cases, and this is all that is
+required. It is by no means necessary that _all_ varieties should
+exhibit incipient infertility, but only, some varieties; for we know
+that, of the innumerable varieties that occur but few become developed
+into distinct species, and it may be that the absence of infertility, to
+obviate the effects of intercrossing, is one of the usual causes of
+their failure. All I have attempted to show is, that _when_ incipient
+infertility does occur in correlation with other varietal differences,
+that infertility can be, and in fact must be, increased by natural
+selection; and this, it appears to me, is a decided step in advance in
+the solution of the problem.[62]
+
+
+_Physiological Selection._
+
+Another form of infertility has been suggested by Professor G.J. Romanes
+as having aided in bringing about the characteristic infertility or
+sterility of hybrids. It is founded on the fact, already noticed, that
+certain individuals of some species possess what may be termed selective
+sterility--that is, while fertile with some individuals of the species
+they are sterile with others, and this altogether independently of any
+differences of form, colour, or structure. The phenomenon, in the only
+form in which it has been observed, is that of "infertility or absolute
+sterility between two individuals, each of which is perfectly fertile
+with all other individuals;" but Mr. Romanes thinks that "it would not
+be nearly so remarkable, or physiologically improbable, that such
+incompatibility should run through a whole race or strain."[63]
+Admitting that this may be so, though we have at present no evidence
+whatever in support of it, it remains to be considered whether such
+physiological varieties could maintain themselves, or whether, as in the
+cases of sporadic infertility already discussed, they would necessarily
+die out unless correlated with useful characters. Mr. Romanes thinks
+that they would persist, and urges that "whenever this one kind of
+variation occurs _it cannot escape the preserving agency_ of
+physiological selection. Hence, even if it be granted that the variation
+which affects the reproductive system in this particular way is a
+variation of comparatively rare occurrence, still, as _it must always be
+preserved_ whenever it does occur, its influence in the manufacture of
+specific types _must be cumulative_." The very positive statements which
+I have italicised would lead most readers to believe that the alleged
+fact had been demonstrated by a careful working out of the process in
+some definite supposed cases. This, however, has nowhere been done in
+Mr. Romanes' paper; and as it is _the_ vital theoretical point on which
+any possible value of the new theory rests, and as it appears so opposed
+to the self-destructive effects of simple infertility, which we have
+already demonstrated when it occurs between the intermingled portion of
+two varieties, it must be carefully examined. In doing so, I will
+suppose that the required variation is not of "rare occurrence," but of
+considerable amount, and that it appears afresh each year to about the
+same extent, thus giving the theory every possible advantage.
+
+Let us then suppose that a given species consists of 100,000 individuals
+of each sex, with only the usual amount of fluctuating external
+variability. Let a physiological variation arise, so that 10 per cent of
+the whole number--10,000 individuals of each sex--while remaining
+fertile _inter se_ become quite sterile with the remaining 90,000. This
+peculiarity is not correlated with any external differences of form or
+colour, or with inherent peculiarities of likes or dislikes leading to
+any choice as to the pairing of the two sets of individuals. We have now
+to inquire, What would be the result?
+
+Taking, first, the 10,000 pairs of the physiological or abnormal
+variety, we find that each male of these might pair with any one of the
+whole 100,000 of the opposite sex. If, therefore, there was nothing to
+limit their choice to particular individuals of either variety, the
+probabilities are that 9000 of them would pair with the opposite
+variety, and only 1000 with their own variety--that is, that 9000 would
+form sterile unions, and only _one_ thousand would form fertile unions.
+
+Taking, next, the 90,000 normal individuals of either sex, we find, that
+each male of these has also a choice of 100,000 to pair with. The
+probabilities are, therefore, that nine-tenths of them--that is,
+81,000--would pair with their normal fellows, while 9000 would pair with
+the opposite abnormal variety forming the above-mentioned sterile
+unions.
+
+Now, as the number of individuals forming a species remains constant,
+generally speaking, from year to year, we shall have next year also
+100,000 pairs, of which the two physiological varieties will be in the
+proportion of eighty-one to one, or 98,780 pairs of the normal variety
+to 1220[64] of the abnormal, that being the proportion of the fertile
+unions of each. In this year we shall find, by the same rule of
+probabilities, that only 15 males of the abnormal variety will pair with
+their like and be fertile, the remaining 1205 forming sterile unions
+with some of the normal variety. The following year the total 100,000
+pairs will consist of 99,984 of the normal, and only 16 of the abnormal
+variety; and the probabilities, of course, are, that the whole of these
+latter will pair with some of the enormous preponderance of normal
+individuals, and, their unions being sterile, the physiological variety
+will become extinct in the third year.
+
+If now in the second and each succeeding year a similar proportion as at
+first (10 per cent) of the physiological variety is produced afresh from
+the ranks of the normal variety, the same rate of diminution will go on,
+and it will be found that, on the most favourable estimate, the
+physiological variety can never exceed 12,000 to the 88,000 of the
+normal form of the species, as shown by the following table:--
+
+
+    1st Year. 10,000 of physiological variety to 90,000 of normal variety.
+    2d   "     1,220 + 10,000 again produced.
+    3d   "        16 +  1,220 + 10,000 do.               = 11,236
+    4th  "         O +     16 +  1,220 + 10,000 do.      = 11,236
+    5th  "                  O +     16 +  1,220 + 10,000 = 11,236
+    and so on for any number of generations.
+
+
+In the preceding discussion we have given the theory the advantage of
+the large proportion of 10 per cent of this very exceptional variety
+arising in its midst year by year, and we have seen that, even under
+these favourable conditions, it is unable to increase its numbers much
+above its starting-point, and that it remains wholly dependent on the
+continued renewal of the variety for its existence beyond a few years.
+It appears, then, that this form of inter-specific sterility cannot be
+increased by natural or any other known form of selection, but that it
+contains within itself its own principle of destruction. If it is
+proposed to get over the difficulty by postulating a larger percentage
+of the variety annually arising within the species, we shall not affect
+the law of decrease until we approach equality in the numbers of the two
+varieties. But with any such increase of the physiological variety the
+species itself would inevitably suffer by the large proportion of
+sterile unions in its midst, and would thus be at a great disadvantage
+in competition with other species which were fertile throughout. Thus,
+natural selection will always tend to weed out any species with too
+great a tendency to sterility among its own members, and will therefore
+prevent such sterility from becoming the general characteristic of
+varying species, which this theory demands should be the case.
+
+On the whole, then, it appears clear that no form of infertility or
+sterility between the individuals of a species, can be increased by
+natural selection unless correlated with some useful variation, while
+all infertility not so correlated has a constant tendency to effect its
+own elimination. But the opposite property, fertility, is of vital
+importance to every species, and gives the offspring of the individuals
+which possess it, in consequence of their superior numbers, a greater
+chance of survival in the battle of life. It is, therefore, directly
+under the control of natural selection, which acts both by the
+self-preservation of fertile and the self-destruction of infertile
+stocks--except always where correlated as above, when they become
+useful, and therefore subject to be increased by natural selection.
+
+
+_Summary and Concluding Remarks on Hybridity._
+
+The facts which are of the greatest importance to a comprehension of
+this very difficult subject are those which show the extreme
+susceptibility of the reproductive system both in plants and animals. We
+have seen how both these classes of organisms may be rendered infertile,
+by a change of conditions which does not affect their general health, by
+captivity, or by too close interbreeding. We have seen, also, that
+infertility is frequently correlated with a difference of colour, or
+with other characters; that it is not proportionate to divergence of
+structure; that it varies in reciprocal crosses between pairs of the
+same species; while in the cases of dimorphic and trimorphic plants the
+different crosses between the same pair of individuals may be fertile or
+sterile at the same time. It appears as if fertility depended on such a
+delicate adjustment of the male and female elements to each other, that,
+unless constantly kept up by the preservation of the most fertile
+individuals, sterility is always liable to arise. This preservation
+always occurs within the limits of each species, both because fertility
+is of the highest importance to the continuance of the race, and also
+because sterility (and to a less extent infertility) is self-destructive
+as well as injurious to the species.
+
+So long therefore as a species remains undivided, and in occupation of a
+continuous area, its fertility is kept up by natural selection; but the
+moment it becomes separated, either by geographical or selective
+isolation, or by diversity of station or of habits, then, while each
+portion must be kept fertile _inter se_, there is nothing to prevent
+infertility arising between the two separated portions. As the two
+portions will necessarily exist under somewhat different conditions of
+life, and will usually have acquired some diversity of form and
+colour--both which circumstances we know to be either the cause of
+infertility or to be correlated with it,--the fact of some degree of
+infertility usually appearing between closely allied but locally or
+physiologically segregated species is exactly what we should expect.
+
+The reason why varieties do not usually exhibit a similar amount of
+infertility is not difficult to explain. The popular conclusions on this
+matter have been drawn chiefly from what occurs among domestic animals,
+and we have seen that the very first essential to their becoming
+domesticated was that they should continue fertile under changed
+conditions of life. During the slow process of the formation of new
+varieties by conscious or unconscious selection, fertility has always
+been an essential character, and has thus been invariably preserved or
+increased; while there is some evidence to show that domestication
+itself tends to increase fertility.
+
+Among plants, wild species and varieties have been more frequently
+experimented on than among animals, and we accordingly find numerous
+cases in which distinct species of plants are perfectly fertile when
+crossed, their hybrid offspring being also fertile _inter se_. We also
+find some few examples of the converse fact--varieties of the same
+species which when crossed are infertile or even sterile.
+
+The idea that either infertility or geographical isolation is absolutely
+essential to the formation of new species, in order to prevent the
+swamping effects of intercrossing, has been shown to be unsound, because
+the varieties or incipient species will, in most cases, be sufficiently
+isolated by having adopted different habits or by frequenting different
+stations; while selective association, which is known to be general
+among distinct varieties or breeds of the same species, will produce an
+effective isolation even when the two forms occupy the same area.
+
+From the various considerations now adverted to, Mr. Darwin arrived at
+the conclusion that the sterility or infertility of species with each
+other, whether manifested in the difficulty of obtaining first crosses
+between them or in the sterility of the hybrids thus obtained, is not a
+constant or necessary result of specific difference, but is incidental
+on unknown peculiarities of the reproductive system. These peculiarities
+constantly tend to arise under changed conditions owing to the extreme
+susceptibility of that system, and they are usually correlated with
+variations of form or of colour. Hence, as fixed differences of form and
+colour, slowly gained by natural selection in adaptation to changed
+conditions, are what essentially characterise distinct species, some
+amount of infertility between species is the usual result.
+
+Here the problem was left by Mr. Darwin; but we have shown that its
+solution may be carried a step further. If we accept the association of
+some degree of infertility, however slight, as a not unfrequent
+accompaniment of the external differences which always arise in a state
+of nature between varieties and incipient species, it has been shown
+that natural selection _has_ power to increase that infertility just as
+it has power to increase other favourable variations. Such an increase
+of infertility will be beneficial, whenever new species arise in the
+same area with the parent form; and we thus see how, out of the
+fluctuating and very unequal amounts of infertility correlated with
+physical variations, there may have arisen that larger and more constant
+amount which appears usually to characterise well-marked species.
+
+The great body of facts of which a condensed account has been given in
+the present chapter, although from an experimental point of view very
+insufficient, all point to the general conclusion we have now reached,
+and afford us a not unsatisfactory solution of the great problem of
+hybridism in relation to the origin of species by means of natural
+selection. Further experimental research is needed in order to complete
+the elucidation of the subject; but until these additional facts are
+forthcoming no new theory seems required for the explanation of the
+phenomena.
+
+FOOTNOTES:
+
+[Footnote 51: Darwin's _Animals and Plants under Domestication_, vol.
+ii. pp. 163-170.]
+
+[Footnote 52: For a full account of these interesting facts and of the
+various problems to which they give rise, the reader must consult
+Darwin's volume on _The Different Forms of Flowers in Plants of the same
+Species_, chaps, i.-iv.]
+
+[Footnote 53: See _Nature_, vol. xxi. p. 207.]
+
+[Footnote 54: Low's _Domesticated Animals of Great Britain_,
+Introduction, p. lxiv.]
+
+[Footnote 55: Low's _Domesticated Animals_, p. 28.]
+
+[Footnote 56: _Amaryllidaceae_, by the Hon. and Rev. William Herbert, p.
+379.]
+
+[Footnote 57: _Origin of Species_, p. 239.]
+
+[Footnote 58: _Origin of Species_, sixth edition, p. 9.]
+
+[Footnote 59: In the _Medico-Chirurgical Transactions_, vol. liii.
+(1870), Dr. Ogle has adduced some curious physiological facts bearing on
+the presence or absence of white colours in the higher animals. He
+states that a dark pigment in the olfactory region of the nostrils is
+essential to perfect smell, and that this pigment is rarely deficient
+except when the whole animal is pure white, and the creature is then
+almost without smell or taste. He observes that there is no proof that,
+in any of the cases given above, the black animals actually eat the
+poisonous root or plant; and that the facts are readily understood if
+the senses of smell and taste are dependent on a pigment which is absent
+in the white animals, who therefore eat what those gifted with normal
+senses avoid. This explanation however hardly seems to cover the facts.
+We cannot suppose that almost all the sheep in the world (which are
+mostly white) are without smell or taste. The cutaneous disease on the
+white patches of hair on horses, the special liability of white terriers
+to distemper, of white chickens to the gapes, and of silkworms which
+produce yellow silk to the fungus, are not explained by it. The
+analogous facts in plants also indicate a real constitutional relation
+with colour, not an affection of the sense of smell and taste only.]
+
+[Footnote 60: For all these facts, see _Animals and Plants under
+Domestication_, vol. ii. pp. 335-338.]
+
+[Footnote 61: _Animals and Plants under Domestication_, vol. ii. pp.
+102, 103.]
+
+[Footnote 62: As this argument is a rather difficult one to follow,
+while its theoretical importance is very great, I add here the following
+briefer exposition of it, in a series of propositions; being, with a few
+verbal alterations, a copy of what I wrote on the subject about twenty
+years back. Some readers may find this easier to follow than the fuller
+discussion in the text:--
+
+
+    _Can Sterility of Hybrids have been Produced by Natural
+    Selection?_
+
+    1. Let there be a species which has varied into _two forms_ each
+    adapted to certain existing conditions better than the parent
+    form, which they soon supplant.
+
+    2. If these _two forms_, which are supposed to coexist in the
+    same district, do not intercross, natural selection will
+    accumulate all favourable variations till they become well
+    suited to their conditions of life, and form two slightly
+    differing species.
+
+    3. But if these _two forms_ freely intercross with each other,
+    and produce hybrids, which are also quite fertile _inter se_,
+    then the formation of the two distinct races or species will be
+    retarded, or perhaps entirely prevented; for the offspring of
+    the crossed unions will be _more vigorous_ owing to the cross,
+    although _less adapted_ to their conditions of life than either
+    of the pure breeds.
+
+    4. Now, let a partial sterility of the hybrids of some
+    considerable proportion of these two forms arise; and, as this
+    would probably be due to some special conditions of life, we may
+    fairly suppose it to arise in some definite portion of the area
+    occupied by the two forms.
+
+    5. The result will be that, in that area, the hybrids (although
+    continually produced by first crosses almost as freely as
+    before) will not themselves increase so rapidly as the two pure
+    forms; and as the two pure forms are, by the terms of the
+    problem, better suited to their several conditions of life than
+    the hybrids, they will inevitably increase more rapidly, and
+    will continually tend to supplant the hybrids altogether at
+    every recurrent severe struggle for existence.
+
+    6. We may fairly suppose, also, that as soon as any sterility
+    appears some disinclination to _cross unions_ will appear, and
+    this will further tend to the diminution of the production of
+    hybrids.
+
+    7. In the other part of the area, however, where hybridism
+    occurs with perfect freedom, hybrids of various degrees may
+    increase till they equal or even exceed in number the pure
+    species--that is, the incipient species will be liable to be
+    swamped by intercrossing.
+
+    8. The first result, then, of a partial sterility of crosses
+    appearing in one part of the area occupied by the two forms,
+    will be--that the great majority of the individuals will there
+    consist of the two pure forms only, while in the remaining part
+    these will be in a minority,--which is the same as saying that
+    the new _physiological variety_ of the two forms will be better
+    suited to the conditions of existence than the remaining portion
+    which has not varied physiologically.
+
+    9. But when the struggle for existence becomes severe, that
+    variety which is best adapted to the conditions of existence
+    always supplants that which is imperfectly adapted; therefore,
+    _by natural selection_ the _varieties_ which are _sterile_ when
+    crossed will become established as the only ones.
+
+    10. Now let variations in the _amount of sterility_ and in
+    the _disinclination to crossed unions_ continue to occur--also
+    in certain parts of the area: exactly the same result must
+    recur, and the progeny of this new physiological variety will in
+    time occupy the whole area.
+
+    11. There is yet another consideration that would facilitate the
+    process. It seems probable that the _sterility variations_
+    would, to some extent, concur with, and perhaps depend upon, the
+    _specific variations_; so that, just in proportion as the _two
+    forms_ diverged and became better adapted to the conditions of
+    existence, they would become more sterile when intercrossed. If
+    this were the case, then natural selection would act with double
+    strength; and those which were better adapted to survive both
+    structurally and physiologically would certainly do so.]
+
+[Footnote 63: Cases of this kind are referred to at p. 155. It must,
+however, be noted, that such sterility in first crosses appears to be
+equally rare between different species of the same genus as between
+individuals of the same species. Mules and other hybrids are freely
+produced between very distinct species, but are themselves infertile or
+quite sterile; and it is this infertility or sterility of the hybrids
+that is the characteristic--and was once thought to be the criterion--of
+species, not the sterility of their first crosses. Hence we should not
+expect to find any constant infertility in the first crosses between the
+distinct strains or varieties that formed the starting-point of new
+species, but only a slight amount of infertility in their mongrel
+offspring. It follows, that Mr. Romanes' theory of _Physiological
+Selection_--which assumes sterility or infertility between first crosses
+as the fundamental fact in the origin of species--does not accord with
+the general phenomena of hybridism in nature.]
+
+[Footnote 64: The exact number is 1219.51, but the fractions are omitted
+for clearness.]
+
+
+
+
+CHAPTER VIII
+
+THE ORIGIN AND USES OF COLOUR IN ANIMALS
+
+
+    The Darwinian theory threw new light on organic colour--The
+    problem to be solved--The constancy of animal colour indicates
+    utility--Colour and environment--Arctic animals
+    white--Exceptions prove the rule--Desert, forest, nocturnal, and
+    oceanic animals--General theories of animal colour--Variable
+    protective colouring--Mr. Poulton's experiments--Special or
+    local colour adaptations--Imitation of particular objects--How
+    they have been produced--Special protective colouring of
+    butterflies--Protective resemblance among marine
+    animals--Protection by terrifying enemies--Alluring
+    coloration--The coloration of birds' eggs--Colour as a means of
+    recognition--Summary of the preceding exposition--Influence of
+    locality or of climate on colour--Concluding remarks.
+
+
+
+Among the numerous applications of the Darwinian theory in the
+interpretation of the complex phenomena presented by the organic world,
+none have been more successful, or are more interesting, than those
+which deal with the colours of animals and plants. To the older school
+of naturalists colour was a trivial character, eminently unstable and
+untrustworthy in the determination of species; and it appeared to have,
+in most cases, no use or meaning to the objects which displayed it. The
+bright and often gorgeous coloration of insect, bird, or flower, was
+either looked upon as having been created for the enjoyment of mankind,
+or as due to unknown and perhaps undiscoverable laws of nature.
+
+But the researches of Mr. Darwin totally changed our point of view in
+this matter. He showed, clearly, that some of the colours of animals are
+useful, some hurtful to them; and he believed that many of the most
+brilliant colours were developed by sexual choice; while his great
+general principle, that all the fixed characters of organic beings have
+been developed under the action of the law of utility, led to the
+inevitable conclusion that so remarkable and conspicuous a character as
+colour, which so often constitutes the most obvious distinction of
+species from species, or group from group, must also have arisen from
+survival of the fittest, and must, therefore, in most cases have some
+relation to the wellbeing of its possessors. Continuous observation and
+research, carried on by multitudes of observers during the last thirty
+years, have shown this to be the case; but the problem is found to be
+far more complex than was at first supposed. The modes in which colour
+is of use to different classes of organisms is very varied, and have
+probably not yet been all discovered; while the infinite variety and
+marvellous beauty of some of its developments are such as to render it
+hopeless to arrive at a complete and satisfactory explanation of every
+individual case. So much, however, has been achieved, so many curious
+facts have been explained, and so much light has been thrown on some of
+the most obscure phenomena of nature, that the subject deserves a
+prominent place in any account of the Darwinian theory.
+
+
+_The Problem to be Solved._
+
+Before dealing with the various modifications of colour in the animal
+world it is necessary to say a few words on colour in general, on its
+prevalence in nature, and how it is that the colours of animals and
+plants require any special explanation. What we term colour is a
+subjective phenomenon, due to the constitution of our mind and nervous
+system; while, objectively, it consists of light-vibrations of different
+wave-lengths emitted by, or reflected from, various objects. Every
+visible object must be coloured, because to be visible it must send rays
+of light to our eye. The kind of light it sends is modified by the
+molecular constitution or the surface texture of the object. Pigments
+absorb certain rays and reflect the remainder, and this reflected
+portion has to our eyes a definite colour, according to the portion of
+the rays constituting white light which are absorbed. Interference
+colours are produced either by thin films or by very fine striae on the
+surfaces of bodies, which cause rays of certain wave-lengths to
+neutralise each other, leaving the remainder to produce the effects of
+colour. Such are the colours of soap-bubbles, or of steel or glass on
+which extremely fine lines have been ruled; and these colours often
+produce the effect of metallic lustre, and are the cause of most of the
+metallic hues of birds and insects.
+
+As colour thus depends on molecular or chemical constitution or on the
+minute surface texture of bodies, and, as the matter of which organic
+beings are composed consists of chemical compounds of great complexity
+and extreme instability, and is also subject to innumerable changes
+during growth and development, we might naturally expect the phenomena
+of colour to be more varied here than in less complex and more stable
+compounds. Yet even in the inorganic world we find abundant and varied
+colours; in the earth and in the water; in metals, gems, and minerals;
+in the sky and in the ocean; in sunset clouds and in the many-tinted
+rainbow. Here we can have no question of _use_ to the coloured object,
+and almost as little perhaps in the vivid red of blood, in the brilliant
+colours of red snow and other low algae and fungi, or even in the
+universal mantle of green which clothes so large a portion of the
+earth's surface. The presence of some colour, or even of many brilliant
+colours, in animals and plants would require no other explanation than
+does that of the sky or the ocean, of the ruby or the emerald--that is,
+it would require a purely physical explanation only. It is the wonderful
+individuality of the colours of animals and plants that attracts our
+attention--the fact that the colours are localised in definite patterns,
+sometimes in accordance with structural characters, sometimes altogether
+independent of them; while often differing in the most striking and
+fantastic manner in allied species. We are thus compelled to look upon
+colour not merely as a physical but also as a biological characteristic,
+which has been differentiated and specialised by natural selection, and
+must, therefore, find its explanation in the principle of adaptation or
+utility.
+
+
+_The Constancy of Animal Colour indicates Utility._
+
+That the colours and markings of animals have been acquired under the
+fundamental law of utility is indicated by a general fact which has
+received very little attention. As a rule, colour and marking are
+constant in each species of wild animal, while, in almost every
+domesticated animal, there arises great variability. We see this in our
+horses and cattle, our dogs and cats, our pigeons and poultry. Now, the
+essential difference between the conditions of life of domesticated and
+wild animals is, that the former are protected by man, while the latter
+have to protect themselves. The extreme variations in colour that
+immediately arise under domestication indicate a tendency to vary in
+this way, and the occasional occurrence of white or piebald or other
+exceptionally coloured individuals of many species in a state of nature,
+shows that this tendency exists there also; and, as these exceptionally
+coloured individuals rarely or never increase, there must be some
+constant power at work to keep it in check. This power can only be
+natural selection or the survival of the fittest, which again implies
+that some colours are useful, some injurious, in each particular case.
+With this principle as our guide, let us see how far we can account both
+for the general and special colours of the animal world.
+
+
+_Colour and Environment._
+
+The fact that first strikes us in our examination of the colours of
+animals as a whole, is the close relation that exists between these
+colours and the general environment. Thus, white prevails among arctic
+animals; yellow or brown in desert species; while green is only a common
+colour in tropical evergreen forests. If we consider these cases
+somewhat carefully we shall find, that they afford us excellent
+materials for forming a judgment on the various theories that have been
+suggested to account for the colours of the animal world.
+
+In the arctic regions there are a number of animals which are wholly
+white all the year round, or which only turn white in winter. Among the
+former are the polar bear and the American polar hare, the snowy owl and
+the Greenland falcon; among the latter the arctic fox, the arctic hare,
+the ermine, and the ptarmigan. Those which are permanently white remain
+among the snow nearly all the year round, while those which change their
+colour inhabit regions which are free from snow in summer. The obvious
+explanation of this style of coloration is, that it is protective,
+serving to conceal the herbivorous species from their enemies, and
+enabling carnivorous animals to approach their prey unperceived. Two
+other explanations have, however, been suggested. One is, that the
+prevalent white of the arctic regions has a direct effect in producing
+the white colour in animals, either by some photographic or chemical
+action on the skin or by a reflex action through vision. The other is,
+that the white colour is chiefly beneficial as a means of checking
+radiation and so preserving animal heat during the severity of an arctic
+winter. The first is part of the general theory that colour is the
+effect of coloured light on the objects--a pure hypothesis which has, I
+believe, no facts whatever to support it. The second suggestion is also
+an hypothesis merely, since it has not been proved by experiment that a
+white colour, _per se_, independently of the fur or feathers which is so
+coloured, has any effect whatever in checking the radiation of low-grade
+heat like that of the animal body. But both alike are sufficiently
+disproved by the interesting exceptions to the rule of white coloration
+in the arctic regions, which exceptions are, nevertheless, quite in
+harmony with the theory of protection.
+
+Whenever we find arctic animals which, from whatever cause, do not
+require protection by the white colour, then neither the cold nor the
+snow-glare has any effect upon their coloration. The sable retains its
+rich brown fur throughout the Siberian winter; but it frequents trees at
+that season and not only feeds partially on fruits or seeds, but is able
+to catch birds among the branches of the fir-trees, with the bark of
+which its colour assimilates. Then we have that thoroughly arctic
+animal, the musk-sheep, which is brown and conspicuous; but this animal
+is gregarious, and its safety depends on its association in small herds.
+It is, therefore, of more importance for it to be able to recognise its
+kind at a distance than to be concealed from its enemies, against which
+it can well protect itself so long as it keeps together in a compact
+body. But the most striking example is that of the common raven, which
+is a true arctic bird, and is found even in mid-winter as far north as
+any known bird or mammal. Yet it always retains its black coat, and the
+reason, from our point of view, is obvious. The raven is a powerful bird
+and fears no enemy, while, being a carrion-feeder, it has no need for
+concealment in order to approach its prey. The colour of the raven and
+of the musk-sheep are, therefore, both inconsistent with any other
+theory than that the white colour of arctic animals has been acquired
+for concealment, and to that theory both afford a strong support. Here
+we have a striking example of the exception proving the rule.
+
+In the desert regions of the earth we find an even more general
+accordance of colour with surroundings. The lion, the camel, and all the
+desert antelopes have more or less the colour of the sand or rock among
+which they live. The Egyptian cat and the Pampas cat are sandy or earth
+coloured. The Australian kangaroos are of similar tints, and the
+original colour of the wild horse is supposed to have been sandy or clay
+coloured. Birds are equally well protected by assimilative hues; the
+larks, quails, goatsuckers, and grouse which abound in the North African
+and Asiatic deserts are all tinted or mottled so as closely to resemble
+the average colour of the soil in the districts they inhabit. Canon
+Tristram, who knows these regions and their natural history so well,
+says, in an often quoted passage: "In the desert, where neither trees,
+brushwood, nor even undulations of the surface afford the slightest
+protection to its foes, a modification of colour which shall be
+assimilated to that of the surrounding country is absolutely necessary.
+Hence, without exception, the upper plumage of every bird, whether lark,
+chat, sylvain, or sand-grouse, and also the fur of all the smaller
+mammals, and the skin of all the snakes and lizards, is of one uniform
+isabelline or sand colour."
+
+Passing on to the tropical regions, it is among their evergreen forests
+alone that we find whole groups of birds whose ground colour is green.
+Parrots are very generally green, and in the East we have an extensive
+group of green fruit-eating pigeons; while the barbets, bee-eaters,
+turacos, leaf-thrushes (Phyllornis), white-eyes (Zosterops), and many
+other groups, have so much green in their plumage as to tend greatly to
+their concealment among the dense foliage. There can be no doubt that
+these colours have been acquired as a protection, when we see that in
+all the temperate regions, where the leaves are deciduous, the ground
+colour of the great majority of birds, especially on the upper surface,
+is a rusty brown of various shades, well corresponding with the bark,
+withered leaves, ferns, and bare thickets among which they live in
+autumn and winter, and especially in early spring when so many of them
+build their nests.
+
+Nocturnal animals supply another illustration of the same rule, in the
+dusky colours of mice, rats, bats, and moles, and in the soft mottled
+plumage of owls and goatsuckers which, while almost equally
+inconspicuous in the twilight, are such as to favour their concealment
+in the daytime.
+
+An additional illustration of general assimilation of colour to the
+surroundings of animals, is furnished by the inhabitants of the deep
+oceans. Professor Moseley of the Challenger Expedition, in his British
+Association lecture on this subject, says: "Most characteristic of
+pelagic animals is the almost crystalline transparency of their bodies.
+So perfect is this transparency that very many of them are rendered
+almost entirely invisible when floating in the water, while some, even
+when caught and held up in a glass globe, are hardly to be seen. The
+skin, nerves, muscles, and other organs are absolutely hyaline and
+transparent, but the liver and digestive tract often remain opaque and
+of a yellow or brown colour, and exactly resemble when seen in the water
+small pieces of floating seaweed." Such marine organisms, however, as
+are of larger size, and either occasionally or habitually float on the
+surface, are beautifully tinged with blue above, thus harmonising with
+the colour of the sea as seen by hovering birds; while they are white
+below, and are thus invisible against the wave-foam and clouds as seen
+by enemies beneath the surface. Such are the tints of the beautiful
+nudibranchiate mollusc, Glaucus atlanticus, and many others.
+
+
+_General Theories of Animal Colour._
+
+We are now in a position to test the general theories, or, to speak more
+correctly, the popular notions, as to the origin of animal coloration,
+before proceeding to apply the principle of utility to the explanation
+of some among the many extraordinary manifestations of colour in the
+animal world. The most generally received theory undoubtedly is, that
+brilliancy and variety of colour are due to the direct action of light
+and heat; a theory no doubt derived from the abundance of
+bright-coloured birds, insects, and flowers which are brought from
+tropical regions. There are, however, two strong arguments against this
+theory. We have already seen how generally bright coloration is wanting
+in desert animals, yet here heat and light are both at a maximum, and if
+these alone were the agents in the production of colour, desert animals
+should be the most brilliant. Again, all naturalists who have lived in
+tropical regions know that the proportion of bright to dull coloured
+species is little if any greater there than in the temperate zone, while
+there are many tropical groups in which bright colours are almost
+entirely unknown. No part of the world presents so many brilliant birds
+as South America, yet there are extensive families, containing many
+hundreds of species, which are as plainly coloured as our average
+temperate birds. Such are the families of the bush-shrikes and
+ant-thrushes (Formicariidae), the tyrant-shrikes (Tyrannidae), the
+American creepers (Dendrocolaptidae), together with a large proportion
+of the wood-warblers (Mniotiltidae), the finches, the wrens, and some
+other groups. In the eastern hemisphere, also, we have the
+babbling-thrushes (Timaliidae), the cuckoo-shrikes (Campephagidae), the
+honey-suckers (Meliphagidae), and several other smaller groups which are
+certainly not coloured above the average standard of temperate birds.
+
+Again, there are many families of birds which spread over the whole
+world, temperate and tropical, and among these the tropical species
+rarely present any exceptional brilliancy of colour. Such are the
+thrushes, goatsuckers, hawks, plovers, and ducks; and in the last-named
+group it is the temperate and arctic zones that afford the most
+brilliant coloration.
+
+The same general facts are found to prevail among insects. Although
+tropical insects present some of the most gorgeous coloration in the
+whole realm of nature, yet there are thousands and tens of thousands of
+species which are as dull coloured as any in our cloudy land. The
+extensive family of the carnivorous ground-beetles (Carabidae) attains
+its greatest brilliancy in the temperate zone; while by far the larger
+proportion of the great families of the longicorns and the weevils, are
+of obscure colours even in the tropics. In butterflies, there is
+undoubtedly a larger proportion of brilliant colour in the tropics; but
+if we compare families which are almost equally developed over the
+globe--as the Pieridae or whites and yellows, and the Satyridae or
+ringlets--we shall find no great disproportion in colour between those
+of temperate and tropical regions.
+
+The various facts which have now briefly been noticed are sufficient to
+indicate that the light and heat of the sun are not the direct causes of
+the colours of animals, although they may favour the production of
+colour when, as in tropical regions, the persistent high temperature
+favours the development of the maximum of life. We will now consider the
+next suggestion, that light reflected from surrounding coloured objects
+tends to produce corresponding colours in the animal world.
+
+This theory is founded on a number of very curious facts which prove,
+that such a change does sometimes occur and is directly dependent on the
+colours of surrounding objects; but these facts are comparatively rare
+and exceptional in their nature, and the same theory will certainly not
+apply to the infinitely varied colours of the higher animals, many of
+which are exposed to a constantly varying amount of light and colour
+during their active existence. A brief sketch of these dependent changes
+of colour may, however, be advantageously given here.
+
+
+_Variable Protective Colouring._
+
+There are two distinct kinds of change of colour in animals due to the
+colouring of the environment. In one case the change is caused by reflex
+action set up by the animal _seeing_ the colour to be imitated, and the
+change produced can be altered or repeated as the animal changes its
+position. In the other case the change occurs but once, and is probably
+not due to any conscious or sense action, but to some direct influence
+on the surface tissues while the creature is undergoing a moult or
+change to the pupa form.
+
+The most striking example of the first class is that of the chameleon,
+which changes to white, brown, yellowish, or green, according to the
+colour of the object on which it rests. This change is brought about by
+means of two layers of pigment cells, deeply seated in the skin, and of
+bluish and yellowish colours. By suitable muscles these cells can be
+forced upwards so as to modify the colour of the skin, which, when they
+are not brought into action, is a dirty white. These animals are
+excessively sluggish and defenceless, and the power of changing their
+colour to that of their immediate surroundings is no doubt of great
+service to them. Many of the flatfish are also capable of changing their
+colour according to the colour of the bottom they rest on; and frogs
+have a similar power to a limited extent. Some crustacea also change
+colour, and the power is much developed in the Chameleon shrimp (Mysis
+Chamaeleon) which is gray when on sand, but brown or green when among
+brown or green seaweed. It has been proved by experiment that when this
+animal is blinded the change does not occur. In all these cases,
+therefore, we have some form of reflex or sense action by which the
+change is produced, probably by means of pigment cells beneath the skin
+as in the chameleon.
+
+The second class consists of certain larvae, and pupae, which undergo
+changes of colour when exposed to differently coloured surroundings.
+This subject has been carefully investigated by Mr. E.B. Poulton, who
+has communicated the results of his experiments to the Royal
+Society.[65] It had been noticed that some species of larvae which fed
+on several different plants had colours more or less corresponding to
+the particular plant the individual fed on. Numerous cases are given in
+Professor Meldola's article on "Variable Protective Colouring" (_Proc.
+Zool. Soc._, 1873, p. 153), and while the general green coloration was
+attributed to the presence of chlorophyll beneath the skin, the
+particular change in correspondence to each food-plant was attributed to
+a special function which had been developed by natural selection. Later
+on, in a note to his translation of Weissmann's _Theory of Descent_,
+Professor Meldola seemed disposed to think that the variations of colour
+of some of the species might be phytophagic--that is, due to the direct
+action of the differently coloured leaves on which the insect fed. Mr.
+Poulton's experiments have thrown much light on this question, since he
+has conclusively proved that, in the case of the sphinx caterpillar of
+Smerinthus ocellatus, the change of colour is not due to the food but to
+the coloured light reflected from the leaves.
+
+This was shown by feeding two sets of larvae on the same plant but
+exposed to differently coloured surroundings, obtained by sewing the
+leaves together, so that in one case only the dark upper surface, in the
+other the whitish under surface was exposed to view. The result in each
+case was a corresponding change of colour in the larvae, confirming the
+experiments on different individuals of the same batch of larvae which
+had been supplied with different food-plants or exposed to a different
+coloured light.
+
+An even more interesting series of experiments was made on the colours
+of pupae, which in many cases were known to be affected by the material
+on which they underwent their transformations. The late Mr. T.W. Wood
+proved, in 1867, that the pupae of the common cabbage butterflies
+(Pieris brassicae and P. rapae) were either light, or dark, or green,
+according to the coloured boxes they were kept in, or the colours of the
+fences, walls, etc., against which they were suspended. Mrs. Barber in
+South Africa found that the pupae of Papilio Nireus underwent a similar
+change, being deep green when attached to orange leaves of the same
+tint, pale yellowish-green when on a branch of the bottle-brush tree
+whose half-dried leaves were of this colour, and yellowish when attached
+to the wooden frame of a box. A few other observers noted similar
+phenomena, but nothing more was done till Mr. Poulton's elaborate series
+of experiments with the larvae of several of our common butterflies were
+the means of clearing up several important points. He showed that the
+action of the coloured light did not affect the pupa itself but the
+larva, and that only for a limited period of time. After a caterpillar
+has done feeding it wanders about seeking a suitable place to undergo
+its transformation. When this is found it rests quietly for a day or
+two, spinning the web from which it is to suspend itself; and it is
+during this period of quiescence, and perhaps also the first hour or two
+after its suspension, that the action of the surrounding coloured
+surfaces determines, to a considerable extent, the colour of the pupa.
+By the application of various surrounding colours during this period,
+Mr. Poulton was able to modify the colour of the pupa of the common
+tortoise-shell butterfly from nearly black to pale, or to a brilliant
+golden; and that of Pieris rapae from dusky through pinkish to pale
+green. It is interesting to note, that the colours produced were in all
+cases such only as assimilated with the surroundings usually occupied by
+the species, and also, that colours which did not occur in such
+surroundings, as dark red or blue, only produced the same effects as
+dusky or black.
+
+Careful experiments were made to ascertain whether the effect was
+produced through the sight of the caterpillar. The ocelli were covered
+with black varnish, but neither this, nor cutting off the spines of the
+tortoise-shell larva to ascertain whether they might be sense-organs,
+produced any effect on the resulting colour. Mr. Poulton concludes,
+therefore, that the colour-action probably occurs over the whole surface
+of the body, setting up physiological processes which result in the
+corresponding colour-change of the pupa. Such changes are, however, by
+no means universal, or even common, in protectively coloured pupae,
+since in Papilio machaon and some others which have been experimented
+on, both in this country and abroad, no change can be produced on the
+pupa by any amount of exposure to differently coloured surroundings. It
+is a curious point that, with the small tortoise-shell larva, exposure
+to light from gilded surfaces produced pupae with a brilliant golden
+lustre; and the explanation is supposed to be that mica abounded in the
+original habitat of the species, and that the pupae thus obtained
+protection when suspended against micaceous rock. Looking, however, at
+the wide range of the species and the comparatively limited area in
+which micaceous rocks occur, this seems a rather improbable explanation,
+and the occurrence of this metallic appearance is still a difficulty. It
+does not, however, commonly occur in this country in a natural state.
+
+The two classes of variable colouring here discussed are evidently
+exceptional, and can have little if any relation to the colours of those
+more active creatures which are continually changing their position with
+regard to surrounding objects, and whose colours and markings are nearly
+constant throughout the life of the individual, and (with the exception
+of sexual differences) in all the individuals of the species. We will
+now briefly pass in review the various characteristics and uses of the
+colours which more generally prevail in nature; and having already
+discussed those protective colours which serve to harmonise animals with
+their general environment, we have to consider only those cases in which
+the colour resemblance is more local or special in its character.
+
+
+_Special or Local Colour Adaptations._
+
+This form of colour adaptation is generally manifested by markings
+rather than by colour alone, and is extremely prevalent both among
+insects and vertebrates, so that we shall be able to notice only a few
+illustrative cases. Among our native birds we have the snipe and
+woodcock, whose markings and tints strikingly accord with the dead marsh
+vegetation among which they live; the ptarmigan in its summer dress is
+mottled and tinted exactly like the lichens which cover the stones of
+the higher mountains; while young unfledged plovers are spotted so as
+exactly to resemble the beach pebbles among which they crouch for
+protection, as beautifully exhibited in one of the cases of British
+birds in the Natural History Museum at South Kensington.
+
+In mammalia, we notice the frequency of rounded spots on forest or tree
+haunting animals of large size, as the forest deer and the forest cats;
+while those that frequent reedy or grassy places are striped vertically,
+as the marsh antelopes and the tiger. I had long been of opinion that
+the brilliant yellow and black stripes of the tiger were adaptive, but
+have only recently obtained proof that it is so. An experienced
+tiger-hunter, Major Walford, states in a letter, that the haunts of the
+tiger are invariably full of the long grass, dry and pale yellow for at
+least nine months of the year, which covers the ground wherever there is
+water in the rainy season, and he adds: "I once, while following up a
+wounded tiger, failed for at least a minute to see him under a tree in
+grass at a distance of about twenty yards--jungle open--but the natives
+saw him, and I eventually made him out well enough to shoot him, but
+even then I could not see at what part of him I was aiming. There can be
+no doubt whatever that the colour of both the tiger and the panther
+renders them almost invisible, especially in a strong blaze of light,
+when among grass, and one does not seem to notice stripes or spots till
+they are dead." It is the black shadows of the vegetation that
+assimilate with the black stripes of the tiger; and, in like manner,
+the spotty shadows of leaves in the forest so harmonise with the spots
+of ocelots, jaguars, tiger-cats, and spotted deer as to afford them a
+very perfect concealment.
+
+In some cases the concealment is effected by colours and markings which
+are so striking and peculiar that no one who had not seen the creature
+in its native haunts would imagine them to be protective. An example of
+this is afforded by the banded fruit pigeon of Timor, whose pure white
+head and neck, black wings and back, yellow belly, and deeply-curved
+black band across the breast, render it a very handsome and conspicuous
+bird. Yet this is what Mr. H.O. Forbes says of it: "On the trees the
+white-headed fruit pigeon (Ptilopus cinctus) sate motionless during the
+heat of the day in numbers, on well-exposed branches; but it was with
+the utmost difficulty that I or my sharp-eyed native servant could ever
+detect them, even in trees where we knew they were sitting."[66] The
+trees referred to are species of Eucalyptus which abound in Timor. They
+have whitish or yellowish bark and very open foliage, and it is the
+intense sunlight casting black curved shadows of one branch upon
+another, with the white and yellow bark and deep blue sky seen through
+openings of the foliage, that produces the peculiar combination of
+colours and shadows to which the colours and markings of this bird have
+become so closely assimilated.
+
+Even such brilliant and gorgeously coloured birds as the sun-birds of
+Africa are, according to an excellent observer, often protectively
+coloured. Mrs. M.E. Barber remarks that "A casual observer would
+scarcely imagine that the highly varnished and magnificently coloured
+plumage of the various species of Noctarinea could be of service to
+them, yet this is undoubtedly the case. The most unguarded moments of
+the lives of these birds are those that are spent amongst the flowers,
+and it is then that they are less wary than at any other time. The
+different species of aloes, which blossom in succession, form the
+principal sources of their winter supplies of food; and a legion of
+other gay flowering plants in spring and summer, the aloe blossoms
+especially, are all brilliantly coloured, and they harmonise admirably
+with the gay plumage of the different species of sun-birds. Even the
+keen eye of a hawk will fail to detect them, so closely do they resemble
+the flowers they frequent. The sun-birds are fully aware of this fact,
+for no sooner have they relinquished the flowers than they become
+exceedingly wary and rapid in flight, darting arrow-like through the air
+and seldom remaining in exposed situations. The black sun-bird
+(Nectarinea amethystina) is never absent from that magnificent
+forest-tree, the 'Kaffir Boom' (Erythrina caffra); all day long the
+cheerful notes of these birds may be heard amongst its spreading
+branches, yet the general aspect of the tree, which consists of a huge
+mass of scarlet and purple-black blossoms without a single green leaf,
+blends and harmonises with the colours of the black sun-bird to such an
+extent that a dozen of them may be feeding amongst its blossoms without
+being conspicuous, or even visible."[67]
+
+Some other cases will still further illustrate how the colours of even
+very conspicuous animals may be adapted to their peculiar haunts.
+
+The late Mr. Swinhoe says of the Kerivoula picta, which he observed in
+Formosa: "The body of this bat was of an orange colour, but the wings
+were painted with orange-yellow and black. It was caught suspended, head
+downwards, on a cluster of the fruit of the longan tree (Nephelium
+longanum). Now this tree is an evergreen, and all the year round some
+portion of its foliage is undergoing decay, the particular leaves being,
+in such a stage, partially orange and black. This bat can, therefore, at
+all seasons suspend from its branches and elude its enemies by its
+resemblance to the leaves of the tree."[68]
+
+Even more curious is the case of the sloths--defenceless animals which
+feed upon leaves, and hang from the branches of trees with their back
+downwards. Most of the species have a curious buff-coloured spot on the
+back, rounded or oval in shape and often with a darker border, which
+seems placed there on purpose to make them conspicuous; and this was a
+great puzzle to naturalists, because the long coarse gray or greenish
+hair was evidently like tree-moss and therefore protective. But an old
+writer, Baron von Slack, in his _Voyage_ _to Surinam_ (1810), had
+already explained the matter. He says: "The colour and even the shape of
+the hair are much like withered moss, and serve to hide the animal in
+the trees, but particularly when it has that orange-coloured spot
+between the shoulders and lies close to the tree; it looks then exactly
+like a piece of branch where the rest has been broken off, by which the
+hunters are often deceived." Even such a huge animal as the giraffe is
+said to be perfectly concealed by its colour and form when standing
+among the dead and broken trees that so often occur on the outskirts of
+the thickets where it feeds. The large blotch-like spots on the skin and
+the strange shape of the head and horns, like broken branches, so tend
+to its concealment that even the keen-eyed natives have been known to
+mistake trees for giraffes or giraffes for trees.
+
+Innumerable examples of this kind of protective colouring occur among
+insects; beetles mottled like the bark of trees or resembling the sand
+or rock or moss on which they live, with green caterpillars of the exact
+general tints of the foliage they feed on; but there are also many cases
+of detailed imitation of particular objects by insects that must be
+briefly described.[69]
+
+
+_Protective Imitation of Particular Objects._
+
+The insects which present this kind of imitation most perfectly are the
+Phasmidae, or stick and leaf insects. The well-known leaf-insects of
+Ceylon and of Java, species of Phyllium, are so wonderfully coloured and
+veined, with leafy expansions on the legs and thorax, that not one
+person in ten can see them when resting on the food-plant close beneath
+their eyes. Others resemble pieces of stick with all the minutiae of
+knots and branches, formed by the insects' legs, which are stuck out
+rigidly and unsymmetrically. I have often been unable to distinguish
+between one of these insects and a real piece of stick, till I satisfied
+myself by touching it and found it to be alive. One species, which was
+brought me in Borneo, was covered with delicate semitransparent green
+foliations, exactly resembling the hepaticae which cover pieces of
+rotten stick in the damp forests. Others resemble dead leaves in all
+their varieties of colour and form; and to show how perfect is the
+protection obtained and how important it is to the possessors of it, the
+following incident, observed by Mr. Belt in Nicaragua, is most
+instructive. Describing the armies of foraging ants in the forest which
+devour every insect they can catch, he says: "I was much surprised with
+the behaviour of a green leaf-like locust. This insect stood immovably
+among a host of ants, many of which ran over its legs without ever
+discovering there was food within their reach. So fixed was its
+instinctive knowledge that its safety depended on its immovability, that
+it allowed me to pick it up and replace it among the ants without making
+a single effort to escape. This species closely resembles a green
+leaf."[70]
+
+Caterpillars also exhibit a considerable amount of detailed resemblance
+to the plants on which they live. Grass-feeders are striped
+longitudinally, while those on ordinary leaves are always striped
+obliquely. Some very beautiful protective resemblances are shown among
+the caterpillars figured in Smith and Abbott's _Lepidopterous Insects of
+Georgia_, a work published in the early part of the century, before any
+theories of protection were started. The plates in this work are most
+beautifully executed from drawings made by Mr. Abbott, representing the
+insects, in every case, on the plants which they frequented, and no
+reference is made in the descriptions to the remarkable protective
+details which appear upon the plates. We have, first, the larva of
+Sphinx fuciformis feeding on a plant with linear grass-like leaves and
+small blue flowers; and we find the insect of the same green as the
+leaves, striped longitudinally in accordance with the linear leaves, and
+with the head blue corresponding both in size and colour with the
+flowers. Another species (Sphinx tersa) is represented feeding on a
+plant with small red flowers situated in the axils of the leaves; and
+the larva has a row of seven red spots, unequal in size, and
+corresponding very closely with the colour and size of the flowers. Two
+other figures of sphinx larvae are very curious. That of Sphinx
+pampinatrix feeds on a wild vine (Vitis indivisa), having green
+tendrils, and in this species the curved horn on the tail is green, and
+closely imitates in its curve the tip of the tendril. But in another
+species (Sphinx cranta), which feeds on the fox-grape (Vitis vulpina),
+the horn is very long and red, corresponding with the long red-tipped
+tendrils of the plant. Both these larvae are green with oblique stripes,
+to harmonise with the veined leaves of the vines; but a figure is also
+given of the last-named species after it has done feeding, when it is of
+a decided brown colour and has entirely lost its horn. This is because
+it then descends to the ground to bury itself, and the green colour and
+red horn would be conspicuous and dangerous; it therefore loses both at
+the last moult. Such a change of colour occurs in many species of
+caterpillars. Sometimes the change is seasonal; and, in those which
+hibernate with us, the colour of some species, which is brownish in
+autumn in adaptation to the fading foliage, becomes green in spring to
+harmonise with the newly-opened leaves at that season.[71]
+
+Some of the most curious examples of minute imitation are afforded by
+the caterpillars of the geometer moths, which are always brown or
+reddish, and resemble in form little twigs of the plant on which they
+feed. They have the habit, when at rest, of standing out obliquely from
+the branch, to which they hold on by their hind pair of prolegs or
+claspers, and remain motionless for hours. Speaking of these protective
+resemblances Mr. Jenner Weir says: "After being thirty years an
+entomologist I was deceived myself, and took out my pruning scissors to
+cut from a plum tree a spur which I thought I had overlooked. This
+turned out to be the larva of a geometer two inches long. I showed it
+to several members of my family, and defined a space of four inches in
+which it was to be seen, but none of them could perceive that it was a
+caterpillar."[72]
+
+One more example of a protected caterpillar must be given. Mr. A.
+Everett, writing from Sarawak, Borneo, says: "I had a caterpillar
+brought me, which, being mixed by my boy with some other things, I took
+to be a bit of moss with two exquisite pinky-white seed-capsules; but I
+soon saw that it moved, and examining it more closely found out its real
+character: it is covered with hair, with two little pink spots on the
+upper surface, the general hue being more green. Its motions are very
+slow, and when eating the head is withdrawn beneath a fleshy mobile
+hood, so that the action of feeding does not produce any movement
+externally. It was found in the limestone hills at Busan, the situation
+of all others where mosses are most plentiful and delicate, and where
+they partially clothe most of the protruding masses of rock."
+
+
+_How these Imitations have been Produced._
+
+To many persons it will seem impossible that such beautiful and detailed
+resemblances as those now described--and these are only samples of
+thousands that occur in all parts of the world--can have been brought
+about by the preservation of accidental useful variations. But this will
+not seem so surprising if we keep in mind the facts set forth in our
+earlier chapters--the rapid multiplication, the severe struggle for
+existence, and the constant variability of these and all other
+organisms. And, further, we must remember that these delicate
+adjustments are the result of a process which has been going on for
+millions of years, and that we now see the small percentage of successes
+among the myriads of failures. From the very first appearance of insects
+and their various kinds of enemies the need of protection arose, and was
+usually most easily met by modifications of colour. Hence, we may be
+sure that the earliest leaf-eating insects acquired a green colour as
+one of the necessities of their existence; and, as the species became
+modified and specialised, those feeding on particular species of plants
+would rapidly acquire the peculiar tints and markings best adapted to
+conceal them upon those plants. Then, every little variation that, once
+in a hundred years perhaps, led to the preservation of some larva which
+was thereby rather better concealed than its fellows, would form the
+starting-point of a further development, leading ultimately to that
+perfection of imitation in details which now astonishes us. The
+researches of Dr. Weismann illustrate this progressive adaptation. The
+very young larvae of several species are green or yellowish without any
+markings; they then, in subsequent moults, obtain certain markings, some
+of which are often lost again before the larva is fully grown. The early
+stages of those species which, like elephant hawk-moths (Chaerocampa),
+have the anterior segments elongated and retractile, with large eye-like
+spots to imitate the head of a vertebrate, are at first like those of
+non-retractile species, the anterior segments being as large as the
+rest. After the first moult they become smaller, comparatively; but it
+is only after the second moult that the ocelli begin to appear, and
+these are not fully defined till after the third moult. This progressive
+development of the individual--the ontogeny--gives us a clue to the
+ancestral development of the whole race--the phylogeny; and we are
+enabled to picture to ourselves the very slow and gradual steps by which
+the existing perfect adaptation has been brought about. In many larvae
+great variability still exists, and in some there are two or more
+distinctly-coloured forms--usually a dark and a light or a brown and a
+green form. The larva of the humming-bird hawk-moth (Macroglossa
+stellatarum) varies in this manner, and Dr. Weismann raised five
+varieties from a batch of eggs from one moth. It feeds on species of
+bedstraw (Galium verum and G. mollugo), and as the green forms are less
+abundant than the brown, it has probably undergone some recent change of
+food-plant or of habits which renders brown the more protective colour.
+
+
+_Special Protective Colouring of Butterflies._
+
+We will now consider a few cases of special protective colouring in the
+perfect butterfly or moth. Mr. Mansel Weale states that in South Africa
+there is a great prevalence of white and silvery foliage or bark,
+sometimes of dazzling brilliancy, and that many insects and their larvae
+have brilliant silvery tints which are protective, among them being
+three species of butterflies whose undersides are silvery, and which are
+thus effectually protected when at rest.[73] A common African butterfly
+(Aterica meleagris) always settles on the ground with closed wings,
+which so closely resemble the soil of the district that it can with
+difficulty be seen, and the colour varies with the soil in different
+localities. Thus specimens from Senegambia were dull brown, the soil
+being reddish sand and iron-clay; those from Calabar and Cameroons were
+light brown with numerous small white spots, the soil of those countries
+being light brown clay with small quartz pebbles; while in other
+localities where the colours of the soil were more varied the colours of
+the butterfly varied also. Here we have variation in a single species
+which has become specialised in certain areas to harmonise with the
+colour of the soil.[74]
+
+Many butterflies, in all parts of the world, resemble dead leaves on
+their under side, but those in which this form of protection is carried
+to the greatest perfection are the species of the Eastern genus Kallima.
+In India K. inachis, and in the larger Malay islands K. paralekta, are
+very common. They are rather large and showy butterflies, orange and
+bluish on the upper side, with a very rapid flight, and frequenting dry
+forests. Their habit is to settle always where there is some dead or
+decaying foliage, and the shape and colour of the wings (on the under
+surface), together with the attitude of the insect, is such as to
+produce an absolutely perfect imitation of a dead leaf. This is effected
+by the butterfly always settling on a twig, with the short tail of the
+hind wings just touching it and forming the leaf-stalk. From this a dark
+curved line runs across to the elongated tip of the upper wings,
+imitating the midrib, on both sides of which are oblique lines, formed
+partly by the nervures and partly by markings, which give the effect of
+the usual veining of a leaf. The head and antennae fit exactly between
+the closed upper wings so as not to interfere with the outline, which
+has just that amount of irregular curvature that is seen in dry and
+withered leaves. The colour is very remarkable for its extreme amount of
+variability, from deep reddish-brown to olive or pale yellow, hardly two
+specimens being exactly alike, but all coming within the range of colour
+of leaves in various stages of decay. Still more curious is the fact
+that the paler wings, which imitate leaves most decayed, are usually
+covered with small black dots, often gathered into circular groups, and
+so exactly resembling the minute fungi on decaying leaves that it is
+hard at first to believe that the insects themselves are not attacked by
+some such fungus. The concealment produced by this wonderful imitation
+is most complete, and in Sumatra I have often seen one enter a bush and
+then disappear like magic. Once I was so fortunate as to see the exact
+spot on which the insect settled; but even then I lost sight of it for
+some time, and only after a persistent search discovered that it was
+close before my eyes.[75] Here we have a kind of imitation, which is
+very common in a less developed form, carried to extreme perfection,
+with the result that the species is very abundant over a considerable
+area of country.
+
+
+_Protective Resemblance among Marine Animals._
+
+Among marine animals this form of protection is very common. Professor
+Moseley tells us that all the inhabitants of the Gulf-weed are most
+remarkably coloured, for purposes of protection and concealment, exactly
+like the weed itself. "The shrimps and crabs which swarm in the weed are
+of exactly the same shade of yellow as the weed, and have white markings
+upon their bodies to represent the patches of Membranipora. The small
+fish, Antennarius, is in the same way weed-colour with white spots. Even
+a Planarian worm, which lives in the weed, is similarly yellow-coloured,
+and also a mollusc, Scyllaea pelagica." The same writer tells us that "a
+number of little crabs found clinging to the floats of the blue-shelled
+mollusc, Ianthina, were all coloured of a corresponding blue for
+concealment."[76]
+
+Professor E.S. Morse of Salem, Mass., found that most of the New
+England marine mollusca were protectively coloured; instancing among
+others a little red chiton on rocks clothed with red calcareous algae,
+and Crepidula plana, living within the apertures of the shells of larger
+species of Gasteropods and of a pure white colour corresponding to its
+habitat, while allied species living on seaweed or on the outside of
+dark shells were dark brown.[77] A still more interesting case has been
+recorded by Mr. George Brady. He says: "Amongst the Nullipore which
+matted together the laminaria roots in the Firth of Clyde were living
+numerous small starfishes (Ophiocoma bellis) which, except when their
+writhing movements betrayed them, were quite undistinguishable from the
+calcareous branches of the alga; their rigid angularly twisted rays had
+all the appearance of the coralline, and exactly assimilated to its dark
+purple colour, so that though I held in my hand a root in which were
+half a dozen of the starfishes, I was really unable to detect them until
+revealed by their movements."[78]
+
+These few examples are sufficient to show that the principle of
+protective coloration extends to the ocean as well as over the earth;
+and if we consider how completely ignorant we are of the habits and
+surroundings of most marine animals, it may well happen that many of the
+colours of tropical fishes, which seem to us so strange and so
+conspicuous, are really protective, owing to the number of equally
+strange and brilliant forms of corals, sea-anemones, sponges, and
+seaweeds among which they live.
+
+
+_Protection by Terrifying Enemies._
+
+A considerable number of quite defenceless insects obtain protection
+from some of their enemies by having acquired a resemblance to dangerous
+animals, or by some threatening or unusual appearance. This is obtained
+either by a modification of shape, of habits, of colour, or of all
+combined. The simplest form of this protection is the aggressive
+attitude of the caterpillars of the Sphingidae, the forepart of the body
+being erected so as to produce a rude resemblance to the figure of a
+sphinx, hence the name of the family. The protection is carried further
+by those species which retract the first three segments and have large
+ocelli on each side of the fourth segment, thus giving to the
+caterpillar, when the forepart of its body is elevated, the appearance
+of a snake in a threatening attitude.
+
+The blood-red forked tentacle, thrown out of the neck of the larvae of
+the genus Papilio when alarmed, is, no doubt, a protection against the
+attacks of ichneumons, and may, perhaps, also frighten small birds; and
+the habit of turning up the tail possessed by the harmless rove-beetles
+(Staphylinidae), giving the idea that they can sting, has, probably, a
+similar use. Even an unusual angular form, like a crooked twig or
+inorganic substance, may be protective; as Mr. Poulton thinks is the
+case with the curious caterpillar of Notodonta ziczac, which, by means
+of a few slight protuberances on its body, is able to assume an angular
+and very unorganic-looking appearance. But perhaps the most perfect
+example of this kind of protection is exhibited by the large caterpillar
+of the Royal Persimmon moth (Bombyx regia), a native of the southern
+states of North America, and known there as the "Hickory-horned devil."
+It is a large green caterpillar, often six inches long, ornamented with
+an immense crown of orange-red tubercles, which, if disturbed, it erects
+and shakes from side to side in a very alarming manner. In its native
+country the negroes believe it to be as deadly as a rattlesnake, whereas
+it is perfectly innocuous. The green colour of the body suggests that
+its ancestors were once protectively coloured; but, growing too large to
+be effectually concealed, it acquired the habit of shaking its head
+about in order to frighten away its enemies, and ultimately developed
+the crown of tentacles as an addition to its terrifying powers. This
+species is beautifully figured in Abbott and Smith's _Lepidopterous
+Insects of Georgia_.
+
+
+_Alluring Coloration._
+
+Besides those numerous insects which obtain protection through their
+resemblance to the natural objects among which they live, there are some
+whose disguise is not used for concealment, but as a direct means of
+securing their prey by attracting them within the enemy's reach. Only a
+few cases of this kind of coloration have yet been observed, chiefly
+among spiders and mantidae; but, no doubt, if attention were given to
+the subject in tropical countries, many more would be discovered. Mr.
+H.O. Forbes has described a most interesting example of this kind of
+simulation in Java. While pursuing a large butterfly through the jungle,
+he was stopped by a dense bush, on a leaf of which he observed one of
+the skipper butterflies sitting on a bird's dropping. "I had often," he
+says, "observed small Blues at rest on similar spots on the ground, and
+have wondered what such a refined and beautiful family as the Lycaenidae
+could find to enjoy, in food apparently so incongruous for a butterfly.
+I approached with gentle steps, but ready net, to see if possible how
+the present species was engaged. It permitted me to get quite close, and
+even to seize it between my fingers; to my surprise, however, part of
+the body remained behind, adhering as I thought to the excreta. I looked
+closely, and finally touched with my finger the excreta to find if it
+were glutinous. To my delighted astonishment I found that my eyes had
+been most perfectly deceived, and that what seemed to be the excreta was
+a most artfully coloured spider, lying on its back with its feet crossed
+over and closely adpressed to the body." Mr. Forbes then goes on to
+describe the exact appearance of such excreta, and how the various parts
+of the spider are coloured to produce the imitation, even to the liquid
+portion which usually runs a little down the leaf. This is exactly
+imitated by a portion of the thin web which the spider first spins to
+secure himself firmly to the leaf; thus producing, as Mr. Forbes
+remarks, a living bait for butterflies and other insects so artfully
+contrived as to deceive a pair of human eyes, even when intently
+examining it.[79]
+
+A native species of spider (Thomisus citreus) exhibits a somewhat
+similar alluring protection by its close resemblance to buds of the
+wayfaring tree, Viburnum lantana. It is pure creamy-white, the abdomen
+exactly resembling in shape and colour the unopened buds of the flowers
+among which it takes its station; and it has been seen to capture flies
+which came to the flowers.
+
+But the most curious and beautiful case of alluring protection is that
+of a wingless Mantis in India, which is so formed and coloured as to
+resemble a pink orchis or some other fantastic flower. The whole insect
+is of a bright pink colour, the large and oval abdomen looking like the
+labellum of an orchid. On each side, the two posterior legs have
+immensely dilated and flattened thighs which represent the petals of a
+flower, while the neck and forelegs imitate the upper sepal and column
+of an orchid. The insect rests motionless, in this symmetrical attitude,
+among bright green foliage, being of course very conspicuous, but so
+exactly resembling a flower that butterflies and other insects settle
+upon it and are instantly captured. It is a living trap, baited in the
+most alluring manner to catch the unwary flower-haunting insects.[80]
+
+
+_The Coloration of Birds' Eggs._
+
+The colours of birds' eggs have long been a difficulty on the theory of
+adaptive coloration, because, in so many cases it has not been easy to
+see what can be the use of the particular colours, which are often so
+bright and conspicuous that they seem intended to attract attention
+rather than to be concealed. A more careful consideration of the subject
+in all its bearings shows, however, that here too, in a great number of
+cases, we have examples of protective coloration. When, therefore, we
+cannot see the meaning of the colour, we may suppose that it has been
+protective in some ancestral form, and, not being hurtful, has persisted
+under changed conditions which rendered the protection needless.
+
+We may divide all eggs, for our present purpose, into two great
+divisions; those which are white or nearly so, and those which are
+distinctly coloured or spotted. Egg-shells being composed mainly of
+carbonate of lime, we may assume that the primitive colour of birds'
+eggs was white, a colour that prevails now among the other egg-bearing
+vertebrates--lizards, crocodiles, turtles, and snakes; and we might,
+therefore, expect that this colour would continue where its presence had
+no disadvantages. Now, as a matter of fact, we find that in all the
+groups of birds which lay their eggs in concealed places, whether in
+holes of trees or in the ground, or in domed or covered nests, the eggs
+are either pure white or of very pale uniform coloration. Such is the
+case with kingfishers, bee-eaters, penguins, and puffins, which nest in
+holes in the ground; with the great parrot family, the woodpeckers, the
+rollers, hoopoes, trogons, owls, and some others, which build in holes
+in trees or other concealed places; while martins, wrens,
+willow-warblers, and Australian finches, build domed or covered nests,
+and usually have white eggs.
+
+There are, however, many other birds which lay their white eggs in open
+nests; and these afford some very interesting examples of the varied
+modes by which concealment may be obtained. All the duck tribe, the
+grebes, and the pheasants belong to this class; but these birds all have
+the habit of covering their eggs with dead leaves or other material
+whenever they leave the nest, so as effectually to conceal them. Other
+birds, as the short-eared owl, the goatsucker, the partridge, and some
+of the Australian ground pigeons, lay their white or pale eggs on the
+bare soil; but in these cases the birds themselves are protectively
+coloured, so that, when sitting, they are almost invisible; and they
+have the habit of sitting close and almost continuously, thus
+effectually concealing their eggs.
+
+Pigeons and doves offer a very curious case of the protection of exposed
+eggs. They usually build very slight and loose nests of sticks and
+twigs, so open that light can be seen through them from below, while
+they are generally well concealed by foliage above. Their eggs are white
+and shining; yet it is a difficult matter to discover, from beneath,
+whether there are eggs in the nest or not, while they are well hidden by
+the thick foliage above. The Australian podargihuge goatsuckers--build
+very similar nests, and their white eggs are protected in the same
+manner. Some large and powerful birds, as the swans, herons, pelicans,
+cormorants, and storks, lay white eggs in open nests; but they keep
+careful watch over them, and are able to drive away intruders. On the
+whole, then, we see that, while white eggs are conspicuous, and
+therefore especially liable to attack by egg-eating animals, they are
+concealed from observation in many and various ways. We may, therefore,
+assume that, in cases where there seems to be no such concealment, we
+are too ignorant of the whole of the conditions to form a correct
+judgment.
+
+We now come to the large class of coloured or richly spotted eggs, and
+here we have a more difficult task, though many of them decidedly
+exhibit protective tints or markings. There are two birds which nest on
+sandy shores--the lesser tern and the ringed plover,--and both lay
+sand-coloured eggs, the former spotted so as to harmonise with coarse
+shingle, the latter minutely speckled like fine sand, which are the
+kinds of ground the two birds choose respectively for their nests. "The
+common sandpipers' eggs assimilate so closely with the tints around them
+as to make their discovery a matter of no small difficulty, as every
+oologist can testify who has searched for them. The pewits' eggs, dark
+in ground colour and boldly marked, are in strict harmony with the sober
+tints of moor and fallow, and on this circumstance alone their
+concealment and safety depend. The divers' eggs furnish another example
+of protective colour; they are generally laid close to the water's edge,
+amongst drift and shingle, where their dark tints and black spots
+conceal them by harmonising closely with surrounding objects. The snipes
+and the great army of sandpipers furnish innumerable instances of
+protectively coloured eggs. In all the instances given the sitting-bird
+invariably leaves the eggs uncovered when it quits them, and
+consequently their safety depends solely on the colours which adorn
+them."[81] The wonderful range of colour and marking in the eggs of the
+guillemot may be imputed to the inaccessible rocks on which it breeds,
+giving it complete protection from enemies. Thus the pale or bluish
+ground colour of the eggs of its allies, the auks and puffins, has
+become intensified and blotched and spotted in the most marvellous
+variety of patterns, owing to there being no selective agency to prevent
+individual variation having full sway.
+
+The common black coot (Fulica atra) has eggs which are coloured in a
+specially protective manner. Dr. William Marshall writes, that it only
+breeds in certain localities where a large water reed (Phragmites
+arundinacea) abounds. The eggs of the coot are stained and spotted with
+black on a yellowish-gray ground, and the dead leaves of the reed are of
+the same colour, and are stained black by small parasitic fungi of the
+Uredo family; and these leaves form the bed on which the eggs are laid.
+The eggs and the leaves agree so closely in colour and markings that it
+is a difficult thing to distinguish the eggs at any distance. It is to
+be noted that the coot never covers up its eggs, as its ally the
+moor-hen usually does.
+
+The beautiful blue or greenish eggs of the hedge-sparrow, the
+song-thrush, and sometimes those of the blackbird, seem at first sight
+especially calculated to attract attention, but it is very doubtful
+whether they are really so conspicuous when seen at a little distance
+among their usual surroundings. For the nests of these birds are either
+in evergreens, as holly or ivy, or surrounded by the delicate green
+tints of our early spring vegetation, and may thus harmonise very well
+with the colours around them. The great majority of the eggs of our
+smaller birds are so spotted or streaked with brown or black on
+variously tinted grounds that, when lying in the shadow of the nest and
+surrounded by the many colours and tints of bark and moss, of purple
+buds and tender green or yellow foliage, with all the complex glittering
+lights and mottled shades produced among these by the spring sunshine
+and by sparkling raindrops, they must have a quite different aspect from
+that which they possess when we observe them torn from their natural
+surroundings. We have here, probably, a similar case of general
+protective harmony to that of the green caterpillars with beautiful
+white or purple bands and spots, which, though gaudily conspicuous when
+seen alone, become practically invisible among the complex lights and
+shadows of the foliage they feed upon.
+
+In the case of the cuckoo, which lays its eggs in the nests of a variety
+of other birds, the eggs themselves are subject to considerable
+variations of colour, the most common type, however, resembling those of
+the pipits, wagtails, or warblers, in whose nests they are most
+frequently laid. It also often lays in the nest of the hedge-sparrow,
+whose bright blue eggs are usually not at all nearly matched, although
+they are sometimes said to be so on the Continent. It is the opinion of
+many ornithologists that each female cuckoo lays the same coloured eggs,
+and that it usually chooses a nest the owners of which lay somewhat
+similar eggs, though this is by no means universally the case. Although
+birds which have cuckoos' eggs imposed upon them do not seem to neglect
+them on account of any difference of colour, yet they probably do so
+occasionally; and if, as seems probable, each bird's eggs are to some
+extent protected by their harmony of colour with their surroundings, the
+presence of a larger and very differently coloured egg in the nest might
+be dangerous, and lead to the destruction of the whole set. Those
+cuckoos, therefore, which most frequently placed their eggs among the
+kinds which they resembled, would in the long run leave most progeny,
+and thus the very frequent accord in colour might have been brought
+about.
+
+Some writers have suggested that the varied colours of birds' eggs are
+primarily due to the effect of surrounding coloured objects on the
+female bird during the period preceding incubation; and have expended
+much ingenuity in suggesting the objects that may have caused the eggs
+of one bird to be blue, another brown, and another pink.[82] But no
+evidence has been presented to prove that any effects whatever are
+produced by this cause, while there seems no difficulty in accounting
+for the facts by individual variability and the action of natural
+selection. The changes that occur in the conditions of existence of
+birds must sometimes render the concealment less perfect than it may
+once have been; and when any danger arises from this cause, it may be
+met either by some change in the colour of the eggs, or in the
+structure or position of the nest, or by the increased care which the
+parents bestow upon the eggs. In this way the various divergences which
+now so often puzzle us may have arisen.
+
+
+_Colour as a Means of Recognition._
+
+If we consider the habits and life-histories of those animals which are
+more or less gregarious, comprising a large proportion of the herbivora,
+some carnivora, and a considerable number of all orders of birds, we
+shall see that a means of ready recognition of its own kind, at a
+distance or during rapid motion, in the dusk of twilight or in partial
+cover, must be of the greatest advantage and often lead to the
+preservation of life. Animals of this kind will not usually receive a
+stranger into their midst. While they keep together they are generally
+safe from attack, but a solitary straggler becomes an easy prey to the
+enemy; it is, therefore, of the highest importance that, in such a case,
+the wanderer should have every facility for discovering its companions
+with certainty at any distance within the range of vision.
+
+Some means of easy recognition must be of vital importance to the young
+and inexperienced of each flock, and it also enables the sexes to
+recognise their kind and thus avoid the evils of infertile crosses; and
+I am inclined to believe that its necessity has had a more widespread
+influence in determining the diversities of animal coloration than any
+other cause whatever. To it may probably be imputed the singular fact
+that, whereas bilateral symmetry of coloration is very frequently lost
+among domesticated animals, it almost universally prevails in a state of
+nature; for if the two sides of an animal were unlike, and the diversity
+of coloration among domestic animals occurred in a wild state, easy
+recognition would be impossible among numerous closely allied forms.[83]
+The wonderful diversity of colour and of marking that prevails,
+especially in birds and insects, may be due to the fact that one of the
+first needs of a new species would be, to keep separate from its nearest
+allies, and this could be most readily done by some easily seen external
+mark of difference. A few illustrations will serve to show how this
+principle acts in nature.
+
+My attention was first called to the subject by a remark of Mr. Darwin's
+that, though, "the hare on her form is a familiar instance of
+concealment through colour, yet the principle partly fails in a closely
+allied species, the rabbit; for when running to its burrow it is made
+conspicuous to the sportsman, and no doubt to all beasts of prey, by its
+upturned white tail."[84] But a little consideration of the habits of
+the animal will show that the white upturned tail is of the greatest
+value, and is really, as it has been termed by a writer in _The Field_,
+a "signal flag of danger." For the rabbit is usually a crepuscular
+animal, feeding soon after sunset or on moonlight nights. When disturbed
+or alarmed it makes for its burrow, and the white upturned tails of
+those in front serve as guides and signals to those more remote from
+home, to the young and the feeble; and thus each following the one or
+two before it, all are able with the least possible delay to regain a
+place of comparative safety. The apparent danger, therefore, becomes a
+most important means of security.
+
+The same general principle enables us to understand the singular, and
+often conspicuous, markings on so many gregarious herbivora which are
+yet, on the whole, protectively coloured. Thus, the American prong-buck
+has a white patch behind and a black muzzle. The Tartarian antelope, the
+Ovis poli of High Asia, the Java wild ox, several species of deer, and a
+large number of antelopes have a similar conspicuous white patch behind,
+which, in contrast to the dusky body, must enable them to be seen and
+followed from a distance by their fellows. Where there are many species
+of nearly the same general size and form inhabiting the same region--as
+with the antelopes of Africa--we find many distinctive markings of a
+similar kind. The gazelles have variously striped and banded faces,
+besides white patches behind and on the flanks, as shown in the woodcut.
+The spring-bok has a white patch on the face and one on the sides, with
+a curiously distinctive white stripe above the tail, which is nearly
+concealed when the animal is at rest by a fold of skin but comes into
+full view when it is in motion, being thus quite analogous to the
+upturned white tail of the rabbit. In the pallah the white rump-mark is
+bordered with black, and the peculiar shape of the horns distinguishes
+it when seen from the front. The sable-antelope, the gems-bok, the oryx,
+the hart-beest, the bonte-bok, and the addax have each peculiar white
+markings; and they are besides characterised by horns so remarkably
+different in each species and so conspicuous, that it seems probable
+that the peculiarities in length, twist, and curvature have been
+differentiated for the purpose of recognition, rather than for any
+speciality of defence in species whose general habits are so similar.
+
+[Illustration: FIG. 18.--Gazella soemmerringi.]
+
+It is interesting to note that these markings for recognition are very
+slightly developed in the antelopes of the woods and marshes. Thus, the
+grys-bok is nearly uniform in colour, except the long black-tipped ears;
+and it frequents the wooded mountains. The duyker-bok and the rhoode-bok
+are wary bush-haunters, and have no marks but the small white patch
+behind. The wood-haunting bosch-bok goes in pairs, and has hardly any
+distinctive marks on its dusky chestnut coat, but the male alone is
+horned. The large and handsome koodoo frequents brushwood, and its
+vertical white stripes are no doubt protective, while its magnificent
+spiral horns afford easy recognition. The eland, which is an inhabitant
+of the open country, is uniformly coloured, being sufficiently
+recognisable by its large size and distinctive form; but the Derbyan
+eland is a forest animal, and has a protectively striped coat. In like
+manner, the fine Speke's antelope, which lives entirely in the swamps
+and among reeds, has pale vertical stripes on the sides (protective),
+with white markings on face and breast for recognition. An inspection of
+the figures of antelopes and other animals in Wood's _Natural History_,
+or in other illustrated works, will give a better idea of the
+peculiarities of recognition markings than any amount of description.
+
+Other examples of such coloration are to be seen in the dusky tints of
+the musk-sheep and the reindeer, to whom recognition at a distance on
+the snowy plains is of more importance than concealment from their few
+enemies. The conspicuous stripes and bands of the zebra and the quagga
+are probably due to the same cause, as may be the singular crests and
+face-marks of several of the monkeys and lemurs.[85]
+
+[Illustration: FIG. 19--Recognition marks of three African plovers.]
+
+Among birds, these recognition marks are especially numerous and
+suggestive. Species which inhabit open districts are usually
+protectively coloured; but they generally possess some distinctive
+markings for the purpose of being easily recognised by their kind, both
+when at rest and during flight. Such are, the white bands or patches on
+the breast or belly of many birds, but more especially the head and neck
+markings in the form of white or black caps, collars, eye-marks or
+frontal patches, examples of which are seen in the three species of
+African plovers figured on page 221.
+
+Recognition marks during flight are very important for all birds which
+congregate in flocks or which migrate together; and it is essential
+that, while being as conspicuous as possible, the marks shall not
+interfere with the general protective tints of the species when at rest.
+Hence they usually consist of well-contrasted markings on the wings and
+tail, which are concealed during repose but become fully visible when
+the bird takes flight. Such markings are well seen in our four British
+species of shrikes, each having quite different white marks on the
+expanded wings and on the tail feathers; and the same is the case with
+our three species of Saxicola--the stone-chat, whin-chat, and
+wheat-ear--which are thus easily recognisable on the wing, especially
+when seen from above, as they would be by stragglers looking out for
+their companions. The figures opposite, of the wings of two African
+species of stone-curlew which are sometimes found in the same districts,
+well illustrates these specific recognition marks. Though not very
+greatly different to our eyes, they are no doubt amply so to the sharp
+vision of the birds themselves.
+
+Besides the white patches on the primaries here shown, the secondary
+feathers are, in some cases, so coloured as to afford very distinctive
+markings during flight, as seen in the central secondary quills of two
+African coursers (Fig. 21).
+
+[Illustration: FIG. 20.--Oedicnemus vermiculatus (above). Oe.
+senegalensis (below).]
+
+Most characteristic of all, however, are the varied markings of the
+outer tail-feathers, whose purpose is so well shown by their being
+almost always covered during repose by the two middle feathers, which
+are themselves quite unmarked and protectively tinted like the rest of
+the upper surface of the body. The figures of the expanded tails of two
+species of East Asiatic snipe, whose geographical ranges overlap each
+other, will serve to illustrate this difference; which is frequently
+much greater and modified in an endless variety of ways (Fig. 22).
+
+Numbers of species of pigeons, hawks, finches, warblers, ducks, and
+innumerable other birds possess this class of markings; and they
+correspond so exactly in general character with those of the mammalia,
+already described, that we cannot doubt they serve a similar
+purpose.[86]
+
+[Illustration: FIG. 21.--Secondary quills.]
+
+[Illustration: FIG. 22.--Scolopax megala (upper). S. stenura (lower).]
+
+Those birds which are inhabitants of tropical forests, and which need
+recognition marks that shall be at all times visible among the dense
+foliage, and not solely or chiefly during flight, have usually small but
+brilliant patches of colour on the head or neck, often not interfering
+with the generally protective character of their plumage. Such are the
+bright patches of blue, red, or yellow, by which the usually green
+Eastern barbets are distinguished; and similar bright patches of colour
+characterise the separate species of small green fruit-doves. To this
+necessity for specialisation in colour, by which each bird may easily
+recognise its kind, is probably due that marvellous variety in the
+peculiar beauties of some groups of birds. The Duke of Argyll, speaking
+of the humming birds, made the objection that "A crest of topaz is no
+better in the struggle for existence than a crest of sapphire. A frill
+ending in spangles of the emerald is no better in the battle of life
+than a frill ending in spangles of the ruby. A tail is not affected for
+the purposes of flight, whether its marginal or its central feathers are
+decorated with white;" and he goes on to urge that mere beauty and
+variety for their own sake are the only causes of these differences.
+But, on the principles here suggested, the divergence itself is useful,
+and must have been produced _pari passu_ with the structural differences
+on which the differentiation of species depends; and thus we have
+explained the curious fact that prominent differences of colour often
+distinguish species otherwise very closely allied to each other.
+
+Among insects, the principle of distinctive coloration for recognition
+has probably been at work in the production of the wonderful diversity
+of colour and marking we find everywhere, more especially among the
+butterflies and moths; and here its chief function may have been to
+secure the pairing together of individuals of the same species. In some
+of the moths this has been secured by a peculiar odour, which attracts
+the males to the females from a distance; but there is no evidence that
+this is universal or even general, and among butterflies, especially,
+the characteristic colour and marking, aided by size and form, afford
+the most probable means of recognition. That this is so is shown by the
+fact that "the common white butterfly often flies down to a bit of paper
+on the ground, no doubt mistaking it for one of its own species;" while,
+according to Mr. Collingwood, in the Malay Archipelago, "a dead
+butterfly pinned upon a conspicuous twig will often arrest an insect of
+the same species in its headlong flight, and bring it down within easy
+reach of the net, especially if it be of the opposite sex."[87] In a
+great number of insects, no doubt, form, motions, stridulating sounds,
+or peculiar odours, serve to distinguish allied species from each other,
+and this must be especially the case with nocturnal insects, or with
+those whose colours are nearly uniform and are determined by the need of
+protection; but by far the larger number of day-flying and active
+insects exhibit varieties of colour and marking, forming the most
+obvious distinction between allied species, and which have, therefore,
+in all probability been acquired in the process of differentiation for
+the purpose of checking the intercrossing of closely allied forms.[88]
+
+Whether this principle extends to any of the less highly organised
+animals is doubtful, though it may perhaps have affected the higher
+mollusca. But in marine animals it seems probable that the colours,
+however beautiful, varied, and brilliant they may often be, are in most
+cases protective, assimilating them to the various bright-coloured
+seaweeds, or to some other animals which it is advantageous for them to
+imitate.[89]
+
+
+_Summary of the Preceding Exposition._
+
+Before proceeding to discuss some of the more recondite phenomena of
+animal coloration, it will be well to consider for a moment the extent
+of the ground we have already covered. Protective coloration, in some of
+its varied forms, has not improbably modified the appearance of one-half
+of the animals living on the globe. The white of arctic animals, the
+yellowish tints of the desert forms, the dusky hues of crepuscular and
+nocturnal species, the transparent or bluish tints of oceanic creatures,
+represent a vast host in themselves; but we have an equally numerous
+body whose tints are adapted to tropical foliage, to the bark of trees,
+or to the soil or dead leaves on or among which they habitually live.
+Then we have the innumerable special adaptations to the tints and forms
+of leaves, or twigs, or flowers; to bark or moss; to rock or pebble; by
+which such vast numbers of the insect tribes obtain protection; and we
+have seen that these various forms of coloration are equally prevalent
+in the waters of the seas and oceans, and are thus coextensive with the
+domain of life upon the earth. The comparatively small numbers which
+possess "terrifying" or "alluring" coloration may be classed under the
+general head of the protectively coloured.
+
+But under the next head--colour for recognition--we have a totally
+distinct category, to some extent antagonistic or complementary to the
+last, since its essential principle is visibility rather than
+concealment. Yet it has been shown, I think, that this mode of
+coloration is almost equally important, since it not only aids in the
+preservation of existing species and in the perpetuation of pure races,
+but was, perhaps, in its earlier stages, a not unimportant factor in
+their development. To it we owe most of the variety and much of the
+beauty in the colours of animals; it has caused at once bilateral
+symmetry and general permanence of type; and its range of action has
+been perhaps equally extensive with that of coloration for concealment.
+
+
+_Influence of Locality or of Climate on Colour._
+
+Certain relations between locality and coloration have long been
+noticed. Mr. Gould observed that birds from inland or continental
+localities were more brightly coloured than those living near the
+sea-coast or on islands, and he supposed that the more brilliant
+atmosphere of the inland stations was the explanation of the
+phenomenon.[90] Many American naturalists have observed similar facts,
+and they assert that the intensity of the colours of birds and mammals
+increases from north to south, and also with the increase of humidity.
+This change is imputed by Mr. J.A. Allen to the direct action of the
+environment. He says: "In respect to the correlation of intensity of
+colour in animals with the degree of humidity, it would perhaps be more
+in accordance with cause and effect to express the law of correlation as
+a _decrease_ of intensity of colour with a _decrease_ of humidity, the
+paleness evidently resulting from exposure and the blanching effect of
+intense sunlight, and a dry, often intensely heated atmosphere. With the
+decrease of the aqueous precipitation the forest growth and the
+protection afforded by arborescent vegetation gradually also decreases,
+as of course does also the protection afforded by clouds, the
+excessively humid regions being also regions of extreme cloudiness,
+while the dry regions are comparatively cloudless districts."[91] Almost
+identical changes occur in birds, and are imputed by Mr. Allen to
+similar causes.
+
+It will be seen that Mr. Gould and Mr. Allen impute opposite effects to
+the same cause, brilliancy or intensity of colour being due to a
+brilliant atmosphere according to the former, while paleness of colour
+is imputed by the latter to a too brilliant sun. According to the
+principles which have been established by the consideration of arctic,
+desert, and forest animals respectively, we shall be led to conclude
+that there has been no direct action in this case, but that the effects
+observed are due to the greater or less need of protection. The pale
+colour that is prevalent in arid districts is in harmony with the
+general tints of the surface; while the brighter tints or more intense
+coloration, both southward and in humid districts, are sufficiently
+explained by the greater shelter due to a more luxuriant vegetation and
+a shorter winter. The advocates of the theory that intensity of light
+directly affects the colours of organisms, are led into perpetual
+inconsistencies. At one time the brilliant colours of tropical birds and
+insects are imputed to the intensity of a tropical sun, while the same
+intensity of sunlight is now said to have a "bleaching" effect. The
+comparatively dull and sober hues of our northern fauna were once
+supposed to be the result of our cloudy skies; but now we are told that
+cloudy skies and a humid atmosphere intensify colour.
+
+In my _Tropical Nature_ (pp. 257-264) I have called attention to what is
+perhaps the most curious and decided relation of colour to locality
+which has yet been observed--the prevalence of white markings in the
+butterflies and birds of islands.
+
+So many cases are adduced from so many different islands, both in the
+eastern and western hemisphere, that it is impossible to doubt the
+existence of some common cause; and it seems probable to me now, after a
+fuller consideration of the whole subject of colour, that here too we
+have one of the almost innumerable results of the principle of
+protective coloration. White is, as a rule, an uncommon colour in
+animals, but probably only because it is so conspicuous. Whenever it
+becomes protective, as in the case of arctic animals and aquatic birds,
+it appears freely enough; while we know that white varieties of many
+species occur occasionally in the wild state, and that, under
+domestication, white or parti-coloured breeds are freely produced. Now
+in all the islands in which exceptionally white-marked birds and
+butterflies have been observed, we find two features which would tend to
+render the conspicuous white markings less injurious--a luxuriant
+tropical vegetation, and a decided scarcity of rapacious mammals and
+birds. White colours, therefore, would not be eliminated by natural
+selection; but variations in this direction would bear their part in
+producing the recognition marks which are everywhere essential, and
+which, in these islands, need not be so small or so inconspicuous as
+elsewhere.
+
+
+_Concluding Remarks._
+
+On a review of the whole subject, then, we must conclude that there is
+no evidence of the individual or prevalent colours of organisms being
+directly determined by the amount of light, or heat, or moisture, to
+which they are exposed; while, on the other hand, the two great
+principles of the need of concealment from enemies or from their prey,
+and of recognition by their own kind, are so wide-reaching in their
+application that they appear at first sight to cover almost the whole
+ground of animal coloration. But, although they are indeed wonderfully
+general and have as yet been very imperfectly studied, we are acquainted
+with other modes of coloration which have a different origin. These
+chiefly appertain to the very singular class of warning colours, from
+which arise the yet more extraordinary phenomena of mimicry; and they
+open up so curious a field of inquiry and present so many interesting
+problems, that a chapter must be devoted to them. Yet another chapter
+will be required by the subject of sexual differentiation of colour and
+ornament, as to the origin and meaning of which I have arrived at
+different conclusions from Mr. Darwin. These various forms of coloration
+having been discussed and illustrated, we shall be in a position to
+attempt a brief sketch of the fundamental laws which have determined the
+general coloration of the animal world.
+
+FOOTNOTES:
+
+[Footnote 65: _Proceedings of the Royal Society_, No. 243, 1886;
+_Transactions of the Royal Society_, vol. clxxviii. B. pp. 311-441.]
+
+[Footnote 66: _A Naturalist's Wanderings in the Eastern Archipelago_, p.
+460.]
+
+[Footnote 67: _Trans. Phil. Soc._ (? _of S. Africa_), 1878, part iv, p.
+27.]
+
+[Footnote 68: _Proc. Zool. Soc._, 1862 p. 357.]
+
+[Footnote 69: With reference to this general resemblance of insects to
+their environment the following remarks by Mr. Poulton are very
+instructive. He says: "Holding the larva of Sphinx ligustri in one hand
+and a twig of its food-plant in the other, the wonder we feel is, not at
+the resemblance but at the difference; we are surprised at the
+difficulty experienced in detecting so conspicuous an object. And yet
+the protection is very real, for the larvae will be passed over by those
+who are not accustomed to their appearance, although the searcher may be
+told of the presence of a large caterpillar. An experienced entomologist
+may also fail to find the larvae till after a considerable search. This
+is general protective resemblance, and it depends upon a general harmony
+between the appearance of the organism and its whole environment. It is
+impossible to understand the force of this protection for any larva,
+without seeing it on its food-plant and in an entirely normal condition.
+The artistic effect of green foliage is more complex than we often
+imagine; numberless modifications are wrought by varied lights and
+shadows upon colours which are in themselves far from uniform. In the
+larva of Papilio machaon the protection is very real when the larva is
+on the food-plant, and can hardly be appreciated at all when the two are
+apart." Numerous other examples are given in the chapter on "Mimicry and
+other Protective Resemblances among Animals," in my _Contributions to
+the Theory of Natural Selection_.]
+
+[Footnote 70: _The Naturalist in Nicaragua_, p. 19.]
+
+[Footnote 71: R. Meldola, in _Proc. Zool. Soc._, 1873, p. 155.]
+
+[Footnote 72: _Nature_, vol. iii. p. 166.]
+
+[Footnote 73: _Trans. Ent. Soc. Lond._, 1878, p. 185.]
+
+[Footnote 74: _Ibid._ (_Proceedings_, p. xlii.)]
+
+[Footnote 75: Wallace's _Malay Archipelago_, vol. i. p. 204 (fifth
+edition, p. 130), with figure.]
+
+[Footnote 76: Moseley's _Notes by a Naturalist on the Challenger_.]
+
+[Footnote 77: _Proceedings of the Boston Soc. of Nat. Hist._, vol. xiv.
+1871.]
+
+[Footnote 78: _Nature_, 1870, p. 376.]
+
+[Footnote 79: _A Naturalist's Wanderings in the Eastern Archipelago_, p.
+63.]
+
+[Footnote 80: A beautiful drawing of this rare insect, Hymenopus
+bicornis (in the nymph or active pupa state), was kindly sent me by Mr.
+Wood-Mason, Curator of the Indian Museum at Calcutta. A species, very
+similar to it, inhabits Java, where it is said to resemble a pink
+orchid. Other Mantidae, of the genus Gongylus, have the anterior part of
+the thorax dilated and coloured either white, pink, or purple; and they
+so closely resemble flowers that, according to Mr. Wood-Mason, one of
+them, having a bright violet-blue prothoracic shield, was found in Pegu
+by a botanist, and was for a moment mistaken by him for a flower. See
+_Proc. Ent. Soc. Lond._, 1878, p. liii.]
+
+[Footnote 81: C. Dixon, in Seebohm's _History of British Birds_, vol.
+ii. Introduction, p. xxvi. Many of the other examples here cited are
+taken from the same valuable work.]
+
+[Footnote 82: See A.H.S. Lucas, in _Proceedings of Royal Society of
+Victoria_, 1887, p. 56.]
+
+[Footnote 83: Professor Wm.H. Brewer of Yale College has shown that the
+white marks or the spots of domesticated animals are rarely symmetrical,
+but have a tendency to appear more frequently on the left side. This is
+the case with horses, cattle, dogs, and swine. Among wild animals the
+skunk varies considerably in the amount of white on the body, and this
+too was found to be usually greatest on the left side. A close
+examination of numerous striped or spotted species, as tigers, leopards,
+jaguars, zebras, etc., showed that the bilateral symmetry was not exact,
+although the general effect of the two sides was the same. This is
+precisely what we should expect if the symmetry is not the result of a
+general law of the organisation, but has been, in part at least,
+produced and preserved for the useful purpose of recognition by the
+animal's fellows of the same species, and especially by the sexes and
+the young. See _Proc. of the Am. Ass. for Advancement of Science_, vol.
+xxx. p. 246.]
+
+[Footnote 84: _Descent of Man_, p. 542.]
+
+[Footnote 85: It may be thought that such extremely conspicuous markings
+as those of the zebra would be a great danger in a country abounding
+with lions, leopards, and other beasts of prey; but it is not so. Zebras
+usually go in bands, and are so swift and wary that they are in little
+danger during the day. It is in the evening, or on moonlight nights,
+when they go to drink, that they are chiefly exposed to attack; and Mr.
+Francis Galton, who has studied these animals in their native haunts,
+assures me, that in twilight they are not at all conspicuous, the
+stripes of white and black so merging together into a gray tint that it
+is very difficult to see them at a little distance. We have here an
+admirable illustration of how a glaringly conspicuous style of marking
+for recognition may be so arranged as to become also protective at the
+time when protection is most needed; and we may also learn how
+impossible it is for us to decide on the inutility of any kind of
+coloration without a careful study of the habits of the species in its
+native country.]
+
+[Footnote 86: The principle of colouring for recognition was, I believe,
+first stated in my article on "The Colours of Animals and Plants" in
+Macmillan's _Magazine_, and more fully in my volume on _Tropical
+Nature_. Subsequently Mrs. Barber gave a few examples under the head of
+"Indicative or Banner Colours," but she applied it to the distinctive
+colours of the males of birds, which I explain on another principle,
+though this may assist.]
+
+[Footnote 87: Quoted by Darwin in _Descent of Man_, p. 317.]
+
+[Footnote 88: In the _American Naturalist_ of March 1888, Mr. J.E. Todd
+has an article on "Directive Coloration in Animals," in which he
+recognises many of the cases here referred to, and suggests a few
+others, though I think he includes many forms of coloration--as
+"paleness of belly and inner side of legs"--which do not belong to this
+class.]
+
+[Footnote 89: For numerous examples of this protective colouring of
+marine animals see Moseley's _Voyage of the Challenger_, and Dr. E.S.
+Morse in _Proc. of Bost. Soc. of Nat. Hist._, vol. xiv. 1871.]
+
+[Footnote 90: See _Origin of Species_, p. 107.]
+
+[Footnote 91: The "Geographical Variation of North American Squirrels,"
+_Proc. Bost. Soc. of Nat. Hist._, 1874, p. 284; and _Mammals and Winter
+Birds of Florida_, pp. 233-241.]
+
+
+
+
+CHAPTER IX
+
+WARNING COLORATION AND MIMICRY
+
+
+    The skunk as an example of warning coloration--Warning colours
+    among insects--Butterflies--Caterpillars--Mimicry--How mimicry
+    has been produced--Heliconidae--Perfection of the
+    imitation--Other cases of mimicry among Lepidoptera--Mimicry
+    among protected groups--Its explanation--Extension of the
+    principle--Mimicry in other orders of insects--Mimicry among the
+    vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among
+    birds--Objections to the theory of mimicry--Concluding remarks
+    on warning colours and mimicry.
+
+
+
+We have now to deal with a class of colours which are the very opposite
+of those we have hitherto considered, since, instead of serving to
+conceal the animals that possess them or as recognition marks to their
+associates, they are developed for the express purpose of rendering the
+species conspicuous. The reason of this is that the animals in question
+are either the possessors of some deadly weapons, as stings or poison
+fangs, or they are uneatable, and are thus so disagreeable to the usual
+enemies of their kind that they are never attacked when their peculiar
+powers or properties are known. It is, therefore, important that they
+should not be mistaken for defenceless or eatable species of the same
+class or order, since in that case they might suffer injury, or even
+death, before their enemies discovered the danger or the uselessness of
+the attack. They require some signal or danger-flag which shall serve as
+a warning to would-be enemies not to attack them, and they have usually
+obtained this in the form of conspicuous or brilliant coloration, very
+distinct from the protective tints of the defenceless animals allied to
+them.
+
+_The Skunk as illustrating Warning Coloration._
+
+While staying a few days, in July 1887, at the Summit Hotel on the
+Central Pacific Railway, I strolled out one evening after dinner, and on
+the road, not fifty yards from the house, I saw a pretty little white
+and black animal with a bushy tail coming towards me. As it came on at a
+slow pace and without any fear, although it evidently saw me, I thought
+at first that it must be some tame creature, when it suddenly occurred
+to me that it was a skunk. It came on till within five or six yards of
+me, then quietly climbed over a dwarf wall and disappeared under a small
+outhouse, in search of chickens, as the landlord afterwards told me.
+This animal possesses, as is well known, a most offensive secretion,
+which it has the power of ejecting over its enemies, and which
+effectually protects it from attack. The odour of this substance is so
+penetrating that it taints, and renders useless, everything it touches,
+or in its vicinity. Provisions near it become uneatable, and clothes
+saturated with it will retain the smell for several weeks, even though
+they are repeatedly washed and dried. A drop of the liquid in the eyes
+will cause blindness, and Indians are said not unfrequently to lose
+their sight from this cause. Owing to this remarkable power of offence
+the skunk is rarely attacked by other animals, and its black and white
+fur, and the bushy white tail carried erect when disturbed, form the
+danger-signals by which it is easily distinguished in the twilight or
+moonlight from unprotected animals. Its consciousness that it needs only
+to be seen to be avoided gives it that slowness of motion and
+fearlessness of aspect which are, as we shall see, characteristic of
+most creatures so protected.
+
+
+_Warning Colours among Insects._
+
+It is among insects that warning colours are best developed, and most
+abundant. We all know how well marked and conspicuous are the colours
+and forms of the stinging wasps and bees, no one of which in any part of
+the world is known to be protectively coloured like the majority of
+defenceless insects. Most of the great tribe of Malacoderms among
+beetles are distasteful to insect-eating animals. Our red and black
+Telephoridae, commonly called "soldiers and sailors," were found, by Mr.
+Jenner Weir, to be refused by small birds. These and the allied
+Lampyridae (the fireflies and glow-worms) in Nicaragua, were rejected by
+Mr. Belt's tame monkey and by his fowls, though most other insects were
+greedily eaten by them. The Coccinellidae or lady-birds are another
+uneatable group, and their conspicuous and singularly spotted bodies
+serve to distinguish them at a glance from all other beetles.
+
+These uneatable insects are probably more numerous than is supposed,
+although we already know immense numbers that are so protected. The most
+remarkable are the three families of butterflies--Heliconidae, Danaidae,
+and Acraeidae--comprising more than a thousand species, and
+characteristic respectively of the three great tropical regions--South
+America, Southern Asia, and Africa. All these butterflies have
+peculiarities which serve to distinguish them from every other group in
+their respective regions. They all have ample but rather weak wings, and
+fly slowly; they are always very abundant; and they all have conspicuous
+colours or markings, so distinct from those of other families that, in
+conjunction with their peculiar outline and mode of flight, they can
+usually be recognised at a glance. Other distinctive features are, that
+their colours are always nearly the same on the under surface of their
+wings as on the upper; they never try to conceal themselves, but rest on
+the upper surfaces of leaves or flowers; and, lastly, they all have
+juices which exhale a powerful scent, so that when one kills them by
+pinching the body, the liquid that exudes stains the fingers yellow, and
+leaves an odour that can only be removed by repeated washings.
+
+Now, there is much direct evidence to show that this odour, though not
+very offensive to us, is so to most insect-eating creatures. Mr. Bates
+observed that, when set out to dry, specimens of Heliconidae were less
+subject to the attacks of vermin; while both he and I noticed that they
+were not attacked by insect-eating birds or dragonflies, and that their
+wings were not found in the forest paths among the numerous wings of
+other butterflies whose bodies had been devoured. Mr. Belt once observed
+a pair of birds capturing insects for their young; and although the
+Heliconidae swarmed in the vicinity, and from their slow flight could
+have been easily caught, not one was ever pursued, although other
+butterflies did not escape. His tame monkey also, which would greedily
+munch up other butterflies, would never eat the Heliconidae. It would
+sometimes smell them, but always rolled them up in its hand and then
+dropped them.
+
+We have also some corresponding evidence as to the distastefulness of
+the Eastern Danaidae. The Hon. Mr. Justice Newton, who assiduously
+collected and took notes upon the Lepidoptera of Bombay, informed Mr.
+Butler of the British Museum that the large and swift-flying butterfly
+Charaxes psaphon, was continually persecuted by the bulbul, so that he
+rarely caught a specimen of this species which had not a piece snipped
+out of the hind wings. He offered one to a bulbul which he had in a
+cage, and it was greedily devoured, whilst it was only by repeated
+persecution that he succeeded in inducing the bird to touch a
+Danais.[92]
+
+Besides these three families of butterflies, there are certain groups of
+the great genus Papilio--the true swallow-tailed butterflies--which have
+all the characteristics of uneatable insects. They have a special
+coloration, usually red and black (at least in the females), they fly
+slowly, they are very abundant, and they possess a peculiar odour
+somewhat like that of the Heliconidae. One of these groups is common in
+tropical America, another in tropical Asia, and it is curious that,
+although not very closely allied, they have each the same red and black
+colours, and are very distinct from all the other butterflies of their
+respective countries. There is reason to believe also that many of the
+brilliantly coloured and weak-flying diurnal moths, like the fine
+tropical Agaristidae and burnet-moths, are similarly protected, and that
+their conspicuous colours serve as a warning of inedibility. The common
+burnet-moth (Anthrocera filipendula) and the equally conspicuous
+ragwort-moth (Euchelia jacobeae) have been proved to be distasteful to
+insect-eating creatures.
+
+The most interesting and most conclusive example of warning coloration
+is, however, furnished by caterpillars, because in this case the facts
+have been carefully ascertained experimentally by competent observers.
+In the year 1866, when Mr. Darwin was collecting evidence as to the
+supposed effect of sexual selection in bringing about the brilliant
+coloration of the higher animals, he was struck by the fact that many
+caterpillars have brilliant and conspicuous colours, in the production
+of which sexual selection could have no place. We have numbers of such
+caterpillars in this country, and they are characterised not only by
+their gay colours but by not concealing themselves. Such are the mullein
+and the gooseberry caterpillars, the larvae of the spurge hawk-moth, of
+the buff-tip, and many others. Some of these caterpillars are
+wonderfully conspicuous, as in the case of that noticed by Mr. Bates in
+South America, which was four inches long, banded across with black and
+yellow, and with bright red head, legs, and tail. Hence it caught the
+eye of any one who passed by, even at the distance of many yards.
+
+Mr. Darwin asked me to try and suggest some explanation of this
+coloration; and, having been recently interested in the question of the
+warning coloration of butterflies, I suggested that this was probably a
+similar case,--that these conspicuous caterpillars were distasteful to
+birds and other insect-eating creatures, and that their bright
+non-protective colours and habit of exposing themselves to view, enabled
+their enemies to distinguish them at a glance from the edible kinds and
+thus learn not to touch them; for it must be remembered that the bodies
+of caterpillars while growing are so delicate, that a wound from a
+bird's beak would be perhaps as fatal as if they were devoured.[93] At
+this time not a single experiment or observation had been made on the
+subject, but after I had brought the matter before the Entomological
+Society, two gentlemen, who kept birds and other tame animals, undertook
+to make experiments with a variety of caterpillars.
+
+Mr. Jenner Weir was the first to experiment with ten species of small
+birds in his aviary, and he found that none of them would eat the
+following smooth-skinned conspicuous caterpillars--Abraxas
+grossulariata, Diloba caeruleocephala, Anthrocera filipendula, and
+Cucullia verbasci. He also found that they would not touch any hairy or
+spiny larvae, and he was satisfied that it was not the hairs or the
+spines, but the unpleasant taste that caused them to be rejected,
+because in one case a young smooth larva of a hairy species, and in
+another case the pupa of a spiny larva, were equally rejected. On the
+other hand, all green or brown caterpillars as well as those that
+resemble twigs were greedily devoured.[94]
+
+Mr. A.G. Butler also made experiments with some green lizards (Lacerta
+viridis), which greedily ate all kinds of food, including flies of many
+kinds, spiders, bees, butterflies, and green caterpillars; but they
+would not touch the caterpillar of the gooseberry-moth (Abraxas
+grossulariata), or the imago of the burnet-moth (Anthrocera
+filipendula). The same thing happened with frogs. When the gooseberry
+caterpillars were first given to them, "they sprang forward and licked
+them eagerly into their mouths; no sooner, however, had they done so,
+than they seemed to become aware of the mistake that they had made, and
+sat with gaping mouths, rolling their tongues about, until they had got
+quit of the nauseous morsels, which seemed perfectly uninjured, and
+walked off as briskly as ever." Spiders seemed equally to dislike them.
+This and another conspicuous caterpillar (Halia wavaria) were rejected
+by two species--the geometrical garden spider (Epeira diadema) and a
+hunting spider.[95]
+
+Some further experiments with lizards were made by Professor Weismann,
+quite confirming the previous observations; and in 1886 Mr. E.B. Poulton
+of Oxford undertook a considerable series of experiments, with many
+other species of larvae and fresh kinds of lizards and frogs. Mr.
+Poulton then reviewed the whole subject, incorporating all recorded
+facts, as well as some additional observations made by Mr. Jenner Weir
+in 1886. More than a hundred species of larvae or of perfect insects of
+various orders have now been made the subject of experiment, and the
+results completely confirm my original suggestion. In almost every case
+the protectively coloured larvae have been greedily eaten by all kinds
+of insectivorous animals, while, in the immense majority of cases, the
+conspicuous, hairy, or brightly coloured larvae have been rejected by
+some or all of them. In some instances the inedibility of the larvae
+extends to the perfect insect, but not in others. In the former cases
+the perfect insect is usually adorned with conspicuous colours, as the
+burnet and ragwort moths; but in the case of the buff-tip, the moth
+resembles a broken piece of rotten stick, yet it is partly inedible,
+being refused by lizards. It is, however, very doubtful whether these
+are its chief enemies, and its protective form and colour may be needed
+against insectivorous birds or mammals.
+
+Mr. Samuel H. Scudder, who has largely bred North American butterflies,
+has found so many of the eggs and larvae destroyed by hymenopterous and
+dipterous parasites that he thinks at least nine-tenths, perhaps a
+greater proportion, never reach maturity. Yet he has never found any
+evidence that such parasites attack either the egg or the larva of the
+inedible Danais archippus, so that in this case the insect is
+distasteful to its most dangerous foes in all the stages of its
+existence, a fact which serves to explain its great abundance and its
+extension over almost the whole world.[96]
+
+One case has been found of a protectively coloured larva,--one,
+moreover, which in all its habits shows that it trusts to concealment to
+escape its enemies--which was yet always rejected by lizards after they
+had seized it, evidently under the impression that from its colour it
+would be eatable. This is the caterpillar of the very common moth Mania
+typica; and Mr. Poulton thinks that, in this case, the unpleasant taste
+is an incidental result of some physiological processes in the organism,
+and is itself a merely useless character. It is evident that the insect
+would not conceal itself so carefully as it does if it had not some
+enemies, and these are probably birds or small mammals, as its
+food-plants are said to be dock and willow-herb, not suggestive of
+places frequented by lizards; and it has been found by experiment that
+lizards and birds have not always the same likes and dislikes. The case
+is interesting, because it shows that nauseous fluids sometimes occur
+sporadically, and may thus be intensified by natural selection when
+required for the purpose of protection. Another exceptional case is
+that of the very conspicuous caterpillar of the spurge hawk-moth
+(Deilephila euphorbiae), which was at once eaten by a lizard, although,
+as it exposes itself on its food-plant in the daytime and is very
+abundant in some localities, it must almost certainly be disliked by
+birds or by some animals who would otherwise devour it. If disturbed
+while feeding it is said to turn round with fury and eject a quantity of
+green liquid, of an acid and disagreeable smell similar to that of the
+spurge milk, only worse.[97]
+
+These facts, and Mr. Poulton's evidence that some larvae rejected by
+lizards at first will be eaten if the lizards are very hungry, show that
+there are differences in the amount of the distastefulness, and render
+it probable that if other food were wanting many of these conspicuous
+insects would be eaten. It is the abundance of the eatable kinds that
+gives value to the inedibility of the smaller number; and this is
+probably the reason why so many insects rely on protective colouring
+rather than on the acquisition of any kind of defensive weapons. In the
+long run the powers of attack and defence must balance each other. Hence
+we see that even the powerful stings of bees and wasps only protect them
+against some enemies, since a tribe of birds, the bee-eaters, have been
+developed which feed upon them, and some frogs and lizards do so
+occasionally.
+
+The preceding outline will sufficiently explain the characteristics of
+"warning coloration" and the end it serves in nature. There are many
+other curious modifications of it, but these will be best appreciated
+after we have discussed the remarkable phenomenon of "mimicry," which is
+bound up with and altogether depends upon "warning colour," and is in
+some cases the chief indication we have of the possession of some
+offensive weapon to secure the safety of the species imitated.
+
+
+_Mimicry._
+
+This term has been given to a form of protective resemblance, in which
+one species so closely resembles another in external form and colouring
+as to be mistaken for it, although the two may not be really allied and
+often belong to distinct families or orders. One creature seems
+disguised in order to be made like another; hence the terms "mimic" and
+mimicry, which imply no voluntary action on the part of the imitator. It
+has long been known that such resemblances do occur, as, for example,
+the clear-winged moths of the families Sesiidae and Aegeriidae, many of
+which resemble bees, wasps, ichneumons, or saw-flies, and have received
+names expressive of the resemblance; and the parasitic flies (Volucella)
+which closely resemble bees, on whose larvae the larvae of the flies
+feed.
+
+The great bulk of such cases remained, however, unnoticed, and the
+subject was looked upon as one of the inexplicable curiosities of
+nature, till Mr. Bates studied the phenomenon among the butterflies of
+the Amazon, and, on his return home, gave the first rational explanation
+of it.[98] The facts are, briefly, these. Everywhere in that fertile
+region for the entomologist the brilliantly coloured Heliconidae abound,
+with all the characteristics which I have already referred to when
+describing them as illustrative of "warning coloration." But along with
+them other butterflies were occasionally captured, which, though often
+mistaken for them, on account of their close resemblance in form,
+colour, and mode of flight, were found on examination to belong to a
+very distinct family, the Pieridae. Mr. Bates notices fifteen distinct
+species of Pieridae, belonging to the genera Leptalis and Euterpe, each
+of which closely imitates some one species of Heliconidae, inhabiting
+the same region and frequenting the same localities. It must be
+remembered that the two families are altogether distinct in structure.
+The larvae of the Heliconidae are tubercled or spined, the pupae
+suspended head downwards, and the imago has imperfect forelegs in the
+male; while the larvae of the Pieridae are smooth, the pupae are
+suspended with a brace to keep the head erect, and the forefeet are
+fully developed in both sexes. These differences are as large and as
+important as those between pigs and sheep, or between swallows and
+sparrows; while English entomologists will best understand the case by
+supposing that a species of Pieris in this country was coloured and
+shaped like a small tortoise-shell, while another species on the
+Continent was equally like a Camberwell beauty--so like in both cases
+as to be mistaken when on the wing, and the difference only to be
+detected by close examination. As an example of the resemblance,
+woodcuts are given of one pair in which the colours are simple, being
+olive, yellow, and black, while the very distinct neuration of the wings
+and form of the head and body can be easily seen.
+
+[Illustration: FIG. 23.--Methona psidii (Heliconidae). Leptalis orise
+(Pieridae).]
+
+Besides these Pieridae, Mr. Bates found four true Papilios, seven
+Erycinidae, three Castnias (a genus of day-flying moths), and fourteen
+species of diurnal Bombycidae, all imitating some species of Heliconidae
+which inhabited the same district; and it is to be especially noted that
+none of these insects were so abundant as the Heliconidae they
+resembled, generally they were far less common, so that Mr. Bates
+estimated the proportion in some cases as not one to a thousand. Before
+giving an account of the numerous remarkable cases of mimicry in other
+parts of the world, and between various groups of insects and of higher
+animals, it will be well to explain briefly the use and purport of the
+phenomenon, and also the mode by which it has been brought about.
+
+
+_How Mimicry has been Produced._
+
+The fact has been now established that the Heliconidae possess an
+offensive odour and taste, which lead to their being almost entirely
+free from attack by insectivorous creatures; they possess a peculiar
+form and mode of flight, and do not seek concealment; while their
+colours--although very varied, ranging from deep blue-black, with white,
+yellow, or vivid red bands and spots, to the most delicate
+semitransparent wings adorned with pale brown or yellow markings--are
+yet always very distinctive, and unlike those of all the other families
+of butterflies in the same country. It is, therefore, clear that if any
+other butterflies in the same region, which are eatable and suffer great
+persecution from insectivorous animals, should come to resemble any of
+these uneatable species so closely as to be mistaken for them by their
+enemies, they will obtain thereby immunity from persecution. This is the
+obvious and sufficient reason why the imitation is useful, and therefore
+why it occurs in nature. We have now to explain how it has probably been
+brought about, and also why a still larger number of persecuted groups
+have not availed themselves of this simple means of protection.
+
+From the great abundance of the Heliconidae[99] all over tropical
+America, the vast number of their genera and species, and their marked
+distinctions from all other butterflies, it follows that they constitute
+a group of high antiquity, which in the course of ages has become more
+and more specialised, and owing to its peculiar advantages has now
+become a dominant and aggressive race. But when they first arose from
+some ancestral species or group which, owing to the food of the larvae
+or some other cause, possessed disagreeable juices that caused them to
+be disliked by the usual enemies of their kind, they were in all
+probability not very different either in form or coloration from many
+other butterflies. They would at that time be subject to repeated
+attacks by insect-eaters, and, even if finally rejected, would often
+receive a fatal injury. Hence arose the necessity for some
+distinguishing mark, by which the devourers of butterflies in general
+might learn that these particular butterflies were uneatable; and every
+variation leading to such distinction, whether by form, colour, or mode
+of flight, was preserved and accumulated by natural selection, till the
+ancestral Heliconoids became well distinguished from eatable
+butterflies, and thenceforth comparatively free from persecution. Then
+they had a good time of it. They acquired lazy habits, and flew about
+slowly. They increased abundantly and spread all over the country, their
+larvae feeding on many plants and acquiring different habits; while the
+butterflies themselves varied greatly, and colour being useful rather
+than injurious to them, gradually diverged into the many coloured and
+beautifully varied forms we now behold.
+
+But, during the early stages of this process, some of the Pieridae,
+inhabiting the same district, happened to be sufficiently like some of
+the Heliconidae to be occasionally mistaken for them. These, of course,
+survived while their companions were devoured. Those among their
+descendants that were still more like Heliconidae again survived, and at
+length the imitation would become tolerably perfect. Thereafter, as the
+protected group diverged into distinct species of many different
+colours, the imitative group would occasionally be able to follow it
+with similar variations,--a process that is going on now, for Mr. Bates
+informs us that in each fresh district he visited he found closely
+allied representative species or varieties of Heliconidae, and along
+with them species of Leptalis (Pieridae), which had varied in the same
+way so as still to be exact imitations. But this process of imitation
+would be subject to check by the increasing acuteness of birds and other
+animals which, whenever the eatable Leptalis became numerous, would
+surely find them out, and would then probably attack both these and
+their friends the Heliconidae in order to devour the former and reject
+the latter. The Pieridae would, however, usually be less numerous,
+because their larvae are often protectively coloured and therefore
+edible, while the larvae of the Heliconidae are adorned with warning
+colours, spines, or tubercles, and are uneatable. It seems probable that
+the larvae and pupae of the Heliconidae were the first to acquire the
+protective distastefulness, both because in this stage they are more
+defenceless and more liable to fatal injury, and also because we now
+find many instances in which the larvae are distasteful while the
+perfect insects are eatable, but I believe none in which the reverse is
+the case. The larvae of the Pieridae are now beginning to acquire
+offensive juices, but have not yet obtained the corresponding
+conspicuous colours; while the perfect insects remain eatable, except
+perhaps in some Eastern groups, the under sides of whose wings are
+brilliantly coloured although this is the part which is exposed when at
+rest.
+
+It is clear that if a large majority of the larvae of Lepidoptera, as
+well as the perfect insects, acquired these distasteful properties, so
+as seriously to diminish the food supply of insectivorous and nestling
+birds, these latter would be forced by necessity to acquire
+corresponding tastes, and to eat with pleasure what some of them now eat
+only under pressure of hunger; and variation and natural selection would
+soon bring about this change.
+
+Many writers have denied the possibility of such wonderful resemblances
+being produced by the accumulation of fortuitous variations, but if the
+reader will call to mind the large amount of variability that has been
+shown to exist in all organisms, the exceptional power of rapid increase
+possessed by insects, and the tremendous struggle for existence always
+going on, the difficulty will vanish, especially when we remember that
+nature has the same fundamental groundwork to act upon in the two
+groups, general similarity of forms, wings of similar texture and
+outline, and probably some original similarity of colour and marking.
+Yet there is evidently considerable difficulty in the process, or with
+these great resources at her command nature would have produced more of
+these mimicking forms than she has done. One reason of this deficiency
+probably is, that the imitators, being always fewer in number, have not
+been able to keep pace with the variations of the much more numerous
+imitated form; another reason may be the ever-increasing acuteness of
+the enemies, which have again and again detected the imposture and
+exterminated the feeble race before it has had time to become further
+modified. The result of this growing acuteness of enemies has been, that
+those mimics that now survive exhibit, as Mr. Bates well remarks, "a
+palpably intentional likeness that is perfectly staggering," and also
+"that those features of the portrait are most attended to by nature
+which produce the most effective deception when the insects are seen in
+nature." No one, in fact, can understand the perfection of the imitation
+who has not seen these species in their native wilds. So complete is it
+in general effect that in almost every box of butterflies, brought from
+tropical America by amateurs, are to be found some species of the
+mimicking Pieridae, Erycinidae, or moths, and the mimicked Heliconidae,
+placed together under the impression that they are the same species. Yet
+more extraordinary, it sometimes deceives the very insects themselves.
+Mr. Trimen states that the male Danais chrysippus is sometimes deceived
+by the female Diadema bolina which mimics that species. Dr. Fritz
+Mueller, writing from Brazil to Professor Meldola, says, "One of the most
+interesting of our mimicking butterflies is Leptalis melite. The female
+alone of this species imitates one of our common white Pieridae, which
+she copies so well that even her own male is often deceived; for I have
+repeatedly seen the male pursuing the mimicked species, till, after
+closely approaching and becoming aware of his error, he suddenly
+returned."[100] This is evidently not a case of true mimicry, since the
+species imitated is not protected; but it may be that the less abundant
+Leptalis is able to mingle with the female Pieridae and thus obtain
+partial immunity from attack. Mr. Kirby of the insect department of the
+British Museum informs me that there are several species of South
+American Pieridae which the female Leptalis melite very nearly
+resembles. The case, however, is interesting as showing that the
+butterflies are themselves deceived by a resemblance which is not so
+great as that of some mimicking species.
+
+
+_Other Examples of Mimicry among Lepidoptera._
+
+In tropical Asia, and eastward to the Pacific Islands, the Danaidae take
+the place of the Heliconidae of America, in their abundance, their
+conspicuousness, their slow flight, and their being the subjects of
+mimicry. They exist under three principal forms or genera. The genus
+Euploea is the most abundant both in species and individuals, and
+consists of fine broad-winged butterflies of a glossy or metallic
+blue-black colour, adorned with pure white, or rich blue, or dusky
+markings situated round the margins of the wings. Danais has generally
+more lengthened wings, of a semitransparent greenish or a rich brown
+colour, with radial or marginal pale spots; while the fine Hestias are
+of enormous size, of a papery or semitransparent white colour, with
+dusky or black spots and markings. Each of these groups is mimicked by
+various species of the genus Papilio, usually with such accuracy that it
+is impossible to distinguish them on the wing.[101] Several species of
+Diadema, a genus of butterflies allied to our Vanessas, also mimic
+species of Danais, but in this case the females only are affected, a
+subject which will be discussed in another chapter.
+
+Another protected group in the Eastern tropics is that of the beautiful
+day-flying moths forming the family Agaristidae. These are usually
+adorned with the most brilliant colours or conspicuous markings, they
+fly slowly in forests among the butterflies and other diurnal insects,
+and their great abundance sufficiently indicates their possession of
+some distastefulness which saves them from attack. Under these
+conditions we may expect to find other moths which are not so protected
+imitating them, and this is the case. One of the common and wide-ranging
+species (Opthalmis lincea), found in the islands from Amboyna to New
+Ireland, is mimicked in a wonderful manner by one of the Liparidae (the
+family to which our common "tussock" and "vapourer" moths belong). This
+is a new species collected at Amboyna during the voyage of the
+_Challenger_, and has been named Artaxa simulans. Both insects are
+black, with the apex of the fore wings ochre coloured, and the outer
+half of the hind wings bright orange. The accompanying woodcuts (for the
+use of which I am indebted to Mr. John Murray of the _Challenger_
+Office) well exhibit their striking resemblance to each other.
+
+[Illustration: FIG. 24.--Opthalmis lincea (Agaristidae). Artaxa simulans
+(Liparidae).]
+
+In Africa exactly similar phenomena recur, species of Papilio and of
+Diadema mimicking Danaidae or Acraeidae with the most curious accuracy.
+Mr. Trimen, who studied this subject in South Africa, has recorded eight
+species or varieties of Diadema, and eight of Papilio, which each mimic
+some species of Danais; while eight species or varieties of Panopaea
+(another genus of Nymphalidae), three of Melanitis (Eurytelidae), and
+two of Papilio, resemble with equal accuracy some species of
+Acraea.[102] He has also independently observed the main facts on which
+the explanation of the phenomenon rests,--the unpleasant odour of the
+Danais and Acraea, extending to their larvae and pupae; their great
+abundance, slow flight, and disregard of concealment; and he states that
+while lizards, mantidae, and dragonflies all hunt butterflies, and the
+rejected wings are to be found abundantly at some of their
+feeding-places, those of the two genera Danais and Acraea were never
+among them.
+
+The two groups of the great genus Papilio (the true swallow-tailed
+butterflies) which have been already referred to as having the special
+characteristics of uneatable insects, have also their imitators in other
+groups; and thus, the belief in their inedibility--derived mainly from
+their style of warning coloration and their peculiar habits--is
+confirmed. In South America, several species of the "Aeneas" group of
+these butterflies are mimicked by Pieridae and by day-flying moths of
+the genera Castnia and Pericopis. In the East, Papilio hector, P.
+diphilus, and P. liris, all belonging to the inedible group, are
+mimicked by the females of other species of Papilio belonging to very
+distinct groups; while in Northern India and China, many fine day-flying
+moths (Epicopeia) have acquired the strange forms and peculiar colours
+of some of the large inedible Papilios of the same regions.
+
+In North America, the large and handsome Danais archippus, with rich
+reddish-brown wings, is very common; and it is closely imitated by
+Limenitis misippus, a butterfly allied to our "white admiral," but which
+has acquired a colour quite distinct from that of the great bulk of its
+allies. In the same country there is a still more interesting case. The
+beautiful dark bronzy green butterfly, Papilio philenor, is inedible
+both in larva and perfect insect, and it is mimicked by the equally dark
+Limenitis ursula. There is also in the Southern and Western States a
+dark female form of the yellow Papilio turnus, which in all probability
+obtains protection from its general resemblance to P. philenor. Mr. W.H.
+Edwards has found, by extensive experiment, that both the dark and
+yellow females produce their own kinds, with very few exceptions; and he
+thinks that the dark form has the advantage in the more open regions and
+in the prairies, where insectivorous birds abound. But in open country
+the dark form would be quite as conspicuous as the yellow form, if not
+more so, so that the resemblance to an inedible species would be there
+more needed.[103]
+
+The only probable case of mimicry in this country is that of the moth,
+Diaphora mendica, whose female only is white, while the larva is of
+protective colours, and therefore almost certainly edible. A much more
+abundant moth, of about the same size and appearing about the same time,
+is Spilosoma menthrasti, also white, but in this case both it and its
+larva have been proved to be inedible. The white colour of the female
+Diaphora, although it must be very conspicuous at night, may, therefore,
+have been acquired in order to resemble the uneatable Spilosoma, and
+thus gain some protection.[104]
+
+
+_Mimicry among Protected (Uneatable) Genera._
+
+Before giving some account of the numerous other cases of warning
+colours and of mimicry that occur in the animal kingdom, it will be well
+to notice a curious phenomenon which long puzzled entomologists, but
+which has at length received a satisfactory explanation.
+
+We have hitherto considered, that mimicry could only occur when a
+comparatively scarce and much persecuted species obtained protection by
+its close external resemblance to a much more abundant uneatable species
+inhabiting its own district; and this rule undoubtedly prevails among
+the great majority of mimicking species all over the world. But Mr.
+Bates also found a number of pairs of species of different genera of
+Heliconidae, which resembled each other quite as closely as did the
+other mimicking species he has described; and since all these insects
+appear to be equally protected by their inedibility, and to be equally
+free from persecution, it was not easy to see why this curious
+resemblance existed, or how it had been brought about. That it is not
+due to close affinity is shown by the fact that the resemblance occurs
+most frequently between the two distinct sub-families into which (as Mr.
+Bates first pointed out) the Heliconidae are naturally divided on
+account of very important structural differences. One of these
+sub-families (the true Heliconinae) consists of two genera only,
+Heliconius and Eueides, the other (the Danaoid Heliconinae) of no less
+than sixteen genera; and, in the instances of mimicry we are now
+discussing, one of the pairs or triplets that resemble each other is
+usually a species of the large and handsome genus Heliconius, the others
+being species of the genera Mechanitis, Melinaea, or Tithorea, though
+several species of other Danaoid genera also imitate each other. The
+following lists will give some idea of the number of these curious
+imitative forms, and of their presence in every part of the Neotropical
+area. The bracketed species are those that resemble each other so
+closely that the difference is not perceptible when they are on the
+wing.
+
+In the Lower Amazon region are found--
+
+
+    { Heliconius sylvana.
+    { Melinaea egina.
+
+    { Heliconius numata.
+    { Melinaea mneme.
+    { Tithorea harmonia.
+
+    { Methona psidii.
+    { Thyridia ino.
+
+    { Ceratina ninonia.
+    { Melinaea mnasias.
+
+
+In Central America are found--
+
+
+              { Heliconius zuleika.
+    Nicaragua { Melinaea hezia.
+              { Mechanitis sp.
+
+              { Heliconius formosus.
+              { Tithorea penthias.
+
+    Guatemala { Heliconius telchina.
+              { Melinaea imitata.
+
+
+
+In the Upper Amazon region--
+
+
+    { Heliconius pardalinus.
+    { Melinaea pardalis.
+
+    { Heliconius aurora.
+    { Melinaea lucifer.
+
+
+In New Grenada--
+
+
+    { Heliconius ismenius.
+    { Melinaea messatis.
+
+    { Heliconius messene.
+    { Melinaea mesenina.
+    { (?) Mechanitis sp.
+
+    { Heliconius hecalesia.
+    { Tithorea hecalesina.
+
+    { Heliconius hecuba.
+    { Tithorea bonplandi.
+
+
+In Eastern Peru and Bolivia--
+
+
+    { Heliconius aristona.
+    { Melinaea cydippe.
+    { (?) Mechanitis mothone.
+
+
+In Pernambuco--
+
+
+    { Heliconius ethra.
+    { Mechanitis nesaea.
+
+
+In Rio Janeiro--
+
+
+    { Helieonius eucrate.
+    { Mechanitis lysimnia.
+
+
+In South Brazil--
+
+
+    { Thyridia megisto.
+    { Ituna ilione.
+
+    { Acraea thalia.
+    { Eueides pavana.
+
+
+Besides these, a number of species of Ithomia and Napeogenes, and of
+Napeogenes and Mechanitis, resemble each other with equal accuracy, so
+that they are liable to be mistaken for each other when on the wing; and
+no doubt many other equally remarkable cases are yet unnoticed.
+
+[Illustration: FIG. 25.--Wings of Ituna Ilione, female. Wings of
+Thyridia megisto, female.]
+
+The figures above of the fore and hind wings of two of these mimicking
+species, from Dr. Fritz Mueller's original paper in _Kosmos_, will serve
+to show the considerable amount of difference, in the important
+character of the neuration of the wings, between these butterflies,
+which really belong to very distinct and not at all closely allied
+genera. Other important characters are--(1) The existence of a small
+basal cell in the hind wings of Ituna which is wanting in Thyridia; (2)
+the division of the cell between the veins 1_b_ and 2 of the hind wings
+in the former genus, while it is undivided in the latter; and (3) the
+existence in Thyridia of scent-producing tufts of hair on the upper edge
+of the hind wing, while in Ituna these are wanting; but in place of them
+are extensible processes at the end of the abdomen, also emitting a
+powerful scent. These differences characterise two marked subdivisions
+of the Danaoid Heliconinae, each containing several distinct genera; and
+these subdivisions are further distinguished by very different forms of
+larvae, that to which Ituna belongs having from two to four long
+threadlike tentacles on the back, while in that containing Thyridia
+these are always absent. The former usually feed on Asclepiadeae, the
+latter on Solanaceae or Scrophulariaceae.
+
+The two species figured, though belonging to such distinct and even
+remote genera, have acquired almost identical tints and markings so as
+to be deceptively alike. The surface of the wings is, in both,
+transparent yellowish, with black transverse bands and white marginal
+spots, while both have similar black-and white-marked bodies and long
+yellow antennae. Dr. Mueller states that they both show a preference for
+the same flowers growing on the edges of the forest paths.[105]
+
+We will now proceed to give the explanation of these curious
+similarities, which have remained a complete puzzle for twenty years.
+Mr. Bates, when first describing them, suggested that they might be due
+to some form of parallel variation dependent on climatic influences; and
+I myself adduced other cases of coincident local modifications of
+colour, which did not appear to be explicable by any form of
+mimicry.[106] But we neither of us hit upon the simple explanation given
+by Dr. Fritz Mueller in 1879.
+
+His theory is founded on the assumed, but probable, fact, that
+insect-eating birds only learn by experience to distinguish the edible
+from the inedible butterflies, and in doing so necessarily sacrifice a
+certain number of the latter. The quantity of insectivorous birds in
+tropical America is enormous; and the number of young birds which every
+year have to learn wisdom by experience, as regards the species of
+butterflies to be caught or to be avoided, is so great that the
+sacrifice of life of the inedible species must be considerable, and, to
+a comparatively weak or scarce species, of vital importance. The number
+thus sacrificed will be fixed by the quantity of young birds, and by the
+number of experiences requisite to cause them to avoid the inedible
+species for the future, and not at all by the numbers of individuals of
+which each species consists. Hence, if two species are so much alike as
+to be mistaken for one another, the fixed number annually sacrificed by
+inexperienced birds will be divided between them, and both will benefit.
+But if the two species are very unequal in numbers, the benefit will be
+comparatively slight for the more abundant species, but very great for
+the rare one. To the latter it may make all the difference between
+safety and destruction.
+
+To give a rough numerical example. Let us suppose that in a given
+limited district there are two species of Heliconidae, one consisting of
+only 1000, the other of 100,000 individuals, and that the quota required
+annually in the same district for the instruction of young insectivorous
+birds is 500. By the larger species this loss will be hardly felt; to
+the smaller it will mean the most dreadful persecution resulting in a
+loss of half the total population. But, let the two species become
+superficially alike, so that the birds see no difference between them.
+The quota of 500 will now be taken from a combined population of 101,000
+butterflies, and if proportionate numbers of each suffer, then the weak
+species will only lose five individuals instead of 500 as it did before.
+Now we know that the different species of Heliconidae are not equally
+abundant, some being quite rare; so that the benefit to be derived in
+these latter cases would be very important. A slight inferiority in
+rapidity of flight or in powers of eluding attack might also be a cause
+of danger to an inedible species of scanty numbers, and in this case too
+the being merged in another much more abundant species, by similarity
+of external appearance, would be an advantage.
+
+The question of fact remains. Do young birds pursue and capture these
+distasteful butterflies till they have learned by bitter experience what
+species to avoid? On this point Dr. Mueller has fortunately been able to
+obtain some direct evidence, by capturing several Acraeas and
+Heliconidae which had evidently been seized by birds but had afterwards
+escaped, as they had pieces torn out of the wing, sometimes
+symmetrically out of both wings, showing that the insect had been seized
+when at rest and with the two pairs of wings in contact. There is,
+however, a general impression that this knowledge is hereditary, and
+does not need to be acquired by young birds; in support of which view
+Mr. Jenner Weir states that his birds always disregarded inedible
+caterpillars. When, day by day, he threw into his aviary various larvae,
+those which were edible were eaten immediately, those which were
+inedible were no more noticed than if a pebble had been thrown before
+the birds.
+
+The cases, however, are not strictly comparable. The birds were not
+young birds of the first year; and, what is more important, edible
+larvae have a comparatively simple coloration, being always brown or
+green and smooth. Uneatable larvae, on the other hand, comprise all that
+are of conspicuous colours and are hairy or spiny. But with butterflies
+there is no such simplicity of contrast. The eatable butterflies
+comprise not only brown or white species, but hundreds of Nymphalidae,
+Papilionidae, Lycaenidae, etc., which are gaily coloured and of an
+immense variety of patterns. The colours and patterns of the inedible
+kinds are also greatly varied, while they are often equally gay; and it
+is quite impossible to suppose that any amount of instinct or inherited
+habit (if such a thing exists) could enable young insectivorous birds to
+distinguish all the species of one kind from all those of the other.
+There is also some evidence to show that animals do learn by experience
+what to eat and what to avoid. Mr. Poulton was assured by Rev. G.J.
+Bursch that very young chickens peck at insects which they afterwards
+avoid. Lizards, too, often seized larvae which they were unable to eat
+and ultimately rejected.
+
+Although the Heliconidae present, on the whole, many varieties of
+coloration and pattern, yet, in proportion to the number of distinct
+species in each district, the types of coloration are few and very well
+marked, and thus it becomes easier for a bird or other animal to learn
+that all belonging to such types are uneatable. This must be a decided
+advantage to the family in question, because, not only do fewer
+individuals of each species need to be sacrificed in order that their
+enemies may learn the lesson of their inedibility, but they are more
+easily recognised at a distance, and thus escape even pursuit. There is
+thus a kind of mimicry between closely allied species as well as between
+species of distinct genera, all tending to the same beneficial end. This
+may be seen in the four or five distinct species of the genus Heliconius
+which all have the same peculiar type of coloration--a yellow band
+across the upper wings and radiating red stripes on the lower,--and are
+all found in the same forests of the Lower Amazon; in the numerous very
+similar species of Ithomia with transparent wings, found in every
+locality of the same region; and in the very numerous species of Papilio
+of the "Aeneas" group, all having a similar style of marking, the
+resemblance being especially close in the females. The very uniform type
+of colouring of the blue-black Euplaeas and of the fulvous Acraeas is of
+the same character.[107] In all these cases the similarity of the allied
+species is so great, that, when they are on the wing at some distance
+off, it is difficult to distinguish one species from another. But this
+close external resemblance is not always a sign of very near affinity;
+for minute examination detects differences in the form and scalloping of
+the wings, in the markings on the body, and in those on the under
+surface of the wings, which do not usually characterise the closest
+allies. It is to be further noted, that the presence of groups of very
+similar species of the same genus, in one locality, is not at all a
+common phenomenon among unprotected groups. Usually the species of a
+genus found in one locality are each well marked and belong to somewhat
+distinct types, while the closely allied forms--those that require
+minute examination to discriminate them as distinct species--are most
+generally found in separate areas, and are what are termed
+representative forms.
+
+The extension we have now given to the theory of mimicry is important,
+since it enables us to explain a much wider range of colour phenomena
+than those which were first imputed to mimicry. It is in the richest
+butterfly region in the world--the Amazon valley--that we find the most
+abundant evidence of the three distinct sets of facts, all depending on
+the same general principle. The form of mimicry first elucidated by Mr.
+Bates is characterised by the presence in each locality of certain
+butterflies, or other insects, themselves edible and belonging to edible
+groups, which derived protection from having acquired a deceptive
+resemblance to some of the inedible butterflies in the same localities,
+which latter were believed to be wholly free from the attacks of
+insectivorous birds. Then came the extension of the principle, by Dr. F.
+Mueller, to the case of species of distinct genera of the inedible
+butterflies resembling each other quite as closely as in the former
+cases, and like them always found in the same localities. They derive
+mutual benefit from becoming, in appearance, one species, from which a
+certain toll is taken annually to teach the young insectivorous birds
+that they are uneatable. Even when the two or more species are
+approximately equal in numbers, they each derive a considerable benefit
+from thus combining their forces; but when one of the species is scarce
+or verging on extinction, the benefit becomes exceedingly great, being,
+in fact, exactly apportioned to the need of the species.
+
+The third extension of the same principle explains the grouping of
+allied species of the same genera of inedible butterflies into sets,
+each having a distinct type of coloration, and each consisting of a
+number of species which can hardly be distinguished on the wing. This
+must be useful exactly in the same way as in the last case, since it
+divides the inevitable toll to insectivorous birds and other animals
+among a number of species. It also explains the fact of the great
+similarity of many species of inedible insects in the same locality--a
+similarity which does not obtain to anything like the same extent among
+the edible species. The explanation of the various phenomena of
+resemblance and mimicry, presented by the distasteful butterflies, may
+now be considered tolerably complete.
+
+
+_Mimicry in other Orders of Insects._
+
+A very brief sketch of these phenomena will be given, chiefly to show
+that the same principle prevails throughout nature, and that, wherever a
+rather extensive group is protected, either by distastefulness or
+offensive weapons, there are usually some species of edible and
+inoffensive groups that gain protection by imitating them. It has been
+already stated that the Telephoridae, Lampyridae, and other families of
+soft-winged beetles, are distasteful; and as they abound in all parts of
+the world, and especially in the tropics, it is not surprising that
+insects of many other groups should imitate them. This is especially the
+case with the longicorn beetles, which are much persecuted by
+insectivorous birds; and everywhere in tropical regions some of these
+are to be found so completely disguised as to be mistaken for species of
+the protected groups. Numbers of these imitations have been already
+recorded by Mr. Bates and myself, but I will here refer to a few others.
+
+In the recently published volumes on the Longicorn and Malacoderm
+beetles of Central America[108] there are numbers of beautifully
+coloured figures of the new species; and on looking over them we are
+struck by the curious resemblance of some of the Longicorns to species
+of the Malacoderm group. In some cases we discover perfect mimics, and
+on turning to the descriptions we always find these pairs to come from
+the same locality. Thus the Otheostethus melanurus, one of the
+Prionidae, imitates the malacoderm, Lucidota discolor, in form, peculiar
+coloration, and size, and both are found at Chontales in Nicaragua, the
+species mimicked having, however, as is usual, a wider range. The
+curious and very rare little longicorn, Tethlimmena aliena, quite unlike
+its nearest allies in the same country, is an exact copy on a somewhat
+smaller scale of a malacoderm, Lygistopterus amabilis, both found at
+Chontales. The pretty longicorn, Callia albicornis, closely resembles
+two species of malacoderms (Silis chalybeipennis and Colyphus
+signaticollis), all being small beetles with red head and thorax and
+bright blue elytra, and all three have been found at Panama. Many other
+species of Callia also resemble other malacoderms; and the longicorn
+genus Lycidola has been named from its resemblance to various species of
+the Lycidae, one of the species here figured (Lycidola belti) being a
+good mimic of Calopteron corrugatum and of several other allied species,
+all being of about the same size and found at Chontales. In these cases,
+and in most others, the longicorn beetles have lost the general form and
+aspect of their allies to take on the appearance of a distinct tribe.
+Some other groups of beetles, as the Elateridae and Eucnemidae, also
+deceptively mimic malacoderms.
+
+Wasps and bees are often closely imitated by insects of other orders.
+Many longicorn beetles in the tropics exactly mimic wasps, bees, or
+ants. In Borneo a large black wasp, whose wings have a broad white patch
+near the apex (Mygnimia aviculus), is closely imitated by a heteromerous
+beetle (Coloborhombus fasciatipennis), which, contrary to the general
+habit of beetles, keeps its wings expanded in order to show the white
+patch on their apex, the wing-coverts being reduced to small oval
+scales, as shown in the figure. This is a most remarkable instance of
+mimicry, because the beetle has had to acquire so many characters which
+are unknown among its allies (except in another species from Java)--the
+expanded wings, the white band on them, and the oval scale-like
+elytra.[109] Another remarkable case has been noted by Mr. Neville
+Goodman, in Egypt, where a common hornet (Vespa orientalis) is exactly
+imitated in colour, size, shape, attitude when at rest, and mode of
+flight, by a beetle of the genus Laphria.[110]
+
+The tiger-beetles (Cicindelidae) are also the subjects of mimicry by
+more harmless insects. In the Malay Islands I found a heteromerous
+beetle which exactly resembled a Therates, both being found running on
+the trunks of trees. A longicorn (Collyrodes Lacordairei) mimics
+Collyris, another genus of the same family; while in the Philippine
+Islands there is a cricket (Condylodeira tricondyloides), which so
+closely resembles a tiger-beetle of the genus Tricondyla that the
+experienced entomologist, Professor Westwood, at first placed it in his
+cabinet among those beetles.
+
+[Illustration: FIG. 26.--Mygnimia aviculus (Wasp). Coloborhombus
+fasciatipennis (Beetle).]
+
+[Illustration: FIG. 27.
+a. Doliops sp. (Longicorn)
+   mimics Pachyrhynchus orbifae, (b) (a hard curculio).
+c. Doliops curculionoides mimics (d) Pachyrhynchus sp.
+e. Scepastus pachyrhynchoides (a grasshopper),
+   mimics (f) Apocyrtus sp. (a hard curculio).
+g. Doliops sp. mimics (h) Pachyrhynchus sp.
+i. Phoraspis (grasshopper) mimics (k) a Coccinella.
+
+All the above are from the Philippines. The exact correspondence of the
+colours of the insects themselves renders the mimicry much more complete
+in nature than it appears in the above figures.]
+
+One of the characters by which some beetles are protected is excessive
+hardness of the elytra and integuments. Several genera of weevils
+(Curculionidae) are thus saved from attack, and these are often mimicked
+by species of softer and more eatable groups. In South America, the
+genus Heilipus is one of these hard groups, and both Mr. Bates and M.
+Roelofs, a Belgian entomologist, have noticed that species of other
+genera exactly mimic them. So, in the Philippines, there is a group of
+Curculionidae, forming the genus Pachyrhynchus, in which all the species
+are adorned with the most brilliant metallic colours, banded and spotted
+in a curious manner, and are very smooth and hard. Other genera of
+Curculionidae (Desmidophorus, Alcides), which are usually very
+differently coloured, have species in the Philippines which mimic the
+Pachyrhynchi; and there are also several longicorn beetles (Aprophata,
+Doliops, Acronia, and Agnia), which also mimic them. Besides these,
+there are some longicorns and cetonias which reproduce the same colours
+and markings; and there is even a cricket (Scepastus pachyrhynchoides),
+which has taken on the form and peculiar coloration of these beetles in
+order to escape from enemies, which then avoid them as uneatable.[111]
+The figures on the opposite page exhibit several other examples of these
+mimicking insects.
+
+Innumerable other cases of mimicry occur among tropical insects; but we
+must now pass on to consider a few of the very remarkable, but much
+rarer instances, that are found among the higher animals.
+
+
+_Mimicry among the Vertebrata._
+
+Perhaps the most remarkable cases yet known are those of certain
+harmless snakes which mimic poisonous species. The genus Elaps, in
+tropical America, consists of poisonous snakes which do not belong to
+the viper family (in which are included the rattlesnakes and most of
+those which are poisonous), and which do not possess the broad
+triangular head which characterises the latter. They have a peculiar
+style of coloration, consisting of alternate rings of red and black, or
+red, black, and yellow, of different widths and grouped in various ways
+in the different species; and it is a style of coloration which does not
+occur in any other group of snakes in the world. But in the same regions
+are found three genera of harmless snakes, belonging to other families,
+some few species of which mimic the poisonous Elaps, often so exactly
+that it is with difficulty one can be distinguished from the other. Thus
+Elaps fulvius in Guatemala is imitated by the harmless Pliocerus
+equalis; Elaps corallinus in Mexico is mimicked by the harmless
+Homalocranium semicinctum; and Elaps lemniscatus in Brazil is copied by
+Oxyrhopus trigeminus; while in other parts of South America similar
+cases of mimicry occur, sometimes two harmless species imitating the
+same poisonous snake.
+
+A few other instances of mimicry in this group have been recorded. There
+is in South Africa an egg-eating snake (Dasypeltis scaber), which has
+neither fangs nor teeth, yet it is very like the Berg adder (Clothos
+atropos), and when alarmed renders itself still more like by flattening
+out its head and darting forward with a hiss as if to strike a foe.[112]
+Dr. A.B. Meyer has also discovered that, while some species of the genus
+Callophis (belonging to the same family as the American Elaps) have
+large poison fangs, other species of the same genus have none; and that
+one of the latter (C. gracilis) resembles a poisonous species (C.
+intestinalis) so closely, that only an exact comparison will discover
+the difference of colour and marking. A similar kind of resemblance is
+said to exist between another harmless snake, Megaerophis flaviceps, and
+the poisonous Callophis bivirgatus; and in both these cases the harmless
+snake is less abundant than the poisonous one, as occurs in all examples
+of true mimicry.[113]
+
+In the genus Elaps, above referred to, the very peculiar style of colour
+and marking is evidently a "warning colour" for the purpose of
+indicating to snake-eating birds and mammals that these species are
+poisonous; and this throws light on the long-disputed question of the
+use of the rattle of the rattlesnake. This reptile is really both
+sluggish and timid, and is very easily captured by those who know its
+habits. If gently tapped on the head with a stick, it will coil itself
+up and lie still, only raising its tail and rattling. It may then be
+easily caught. This shows that the rattle is a warning to its enemies
+that it is dangerous to proceed to extremities; and the creature has
+probably acquired this structure and habit because it frequents open or
+rocky districts where protective colour is needful to save it from being
+pounced upon by buzzards or other snake-eaters. Quite parallel in
+function is the expanded hood of the Indian cobra, a poisonous snake
+which belongs also to the Elapidae. This is, no doubt, a warning to its
+foes, not an attempt to terrify its prey; and the hood has been
+acquired, as in the case of the rattlesnake, because, protective
+coloration being on the whole useful, some mark was required to
+distinguish it from other protectively coloured, but harmless, snakes.
+Both these species feed on active creatures capable of escaping if their
+enemy were visible at a moderate distance.
+
+
+_Mimicry among Birds._
+
+The varied forms and habits of birds do not favour the production among
+them of the phenomena of warning colours or of mimicry; and the extreme
+development of their instincts and reasoning powers, as well as their
+activity and their power of flight, usually afford them other means of
+evading their enemies. Yet there are a few imperfect, and one or two
+very perfect cases of true mimicry to be found among them. The less
+perfect examples are those presented by several species of cuckoos, an
+exceedingly weak and defenceless group of birds. Our own cuckoo is, in
+colour and markings, very like a sparrow-hawk. In the East, several of
+the small black cuckoos closely resemble the aggressive drongo-shrikes
+of the same country, and the small metallic cuckoos are like glossy
+starlings; while a large ground-cuckoo of Borneo (Carpococcyx radiatus)
+resembles one of the fine pheasants (Euplocamus) of the same country,
+both in form and in its rich metallic colours.
+
+More perfect cases of mimicry occur between some of the dull-coloured
+orioles in the Malay Archipelago and a genus of large honey-suckers--the
+Tropidorhynchi or "Friar-birds." These latter are powerful and noisy
+birds which go in small flocks. They have long, curved, and sharp beaks,
+and powerful grasping claws; and they are quite able to defend
+themselves, often driving away crows and hawks which venture to approach
+them too nearly. The orioles, on the other hand, are weak and timid
+birds, and trust chiefly to concealment and to their retiring habits to
+escape persecution. In each of the great islands of the Austro-Malayan
+region there is a distinct species of Tropidorhynchus, and there is
+always along with it an oriole that exactly mimics it. All the
+Tropidorhynchi have a patch of bare black skin round the eyes, and a
+ruff of curious pale recurved feathers on the nape, whence their name of
+Friar-birds, the ruff being supposed to resemble the cowl of a friar.
+These peculiarities are imitated in the orioles by patches of feathers
+of corresponding colours; while the different tints of the two species
+in each island are exactly the same. Thus in Bouru both are earthy
+brown; in Ceram they are both washed with yellow ochre; in Timor the
+under surface is pale and the throat nearly white, and Mr. H.O. Forbes
+has recently discovered another pair in the island of Timor Laut. The
+close resemblance of these several pairs of birds, of widely different
+families, is quite comparable with that of many of the insects already
+described. It is so close that the preserved specimens have even
+deceived naturalists; for, in the great French work, _Voyage de
+l'Astrolabe_, the oriole of Bouru is actually described and figured as a
+honey-sucker; and Mr. Forbes tells us that, when his birds were
+submitted to Dr. Sclater for description, the oriole and the
+honey-sucker were, previous to close examination, considered to be the
+same species.
+
+
+_Objections to the Theory of Mimicry._
+
+To set forth adequately the varied and surprising facts of mimicry would
+need a large and copiously illustrated volume; and no more interesting
+subject could be taken up by a naturalist who has access to our great
+collections and can devote the necessary time to search out the many
+examples of mimicry that lie hidden in our museums. The brief sketch of
+the subject that has been here given will, however, serve to indicate
+its nature, and to show the weakness of the objections that were at
+first made to it. It was urged that the action of "like conditions,"
+with "accidental resemblances" and "reversion to ancestral types," would
+account for the facts. If, however, we consider the actual phenomena as
+here set forth, and the very constant conditions under which they occur,
+we shall see how utterly inadequate are these causes, either singly or
+combined. These constant conditions are--
+
+
+    1. That the imitative species occur in the same area and occupy
+    the very same station as the imitated.
+
+    2. That the imitators are always the more defenceless.
+
+    3. That the imitators are always less numerous in individuals.
+
+    4. That the imitators differ from the bulk of their allies.
+
+    5. That the imitation, however minute, is _external_ and
+    _visible_ only, never extending to internal characters or to
+    such as do not affect the external appearance.
+
+
+These five characteristic features of mimicry show us that it is really
+an exceptional form of protective resemblance. Different species in the
+same group of organisms may obtain protection in different ways: some by
+a general resemblance to their environment; some by more exactly
+imitating the objects that surround them--bark, or leaf, or flower;
+while others again gain an equal protection by resembling some species
+which, from whatever cause, is almost as free from attack as if it were
+a leaf or a flower. This immunity may depend on its being uneatable, or
+dangerous, or merely strong; and it is the resemblance to such creatures
+for the purpose of sharing in their safety that constitutes mimicry.
+
+
+_Concluding Remarks on Warning Colours and Mimicry._
+
+Colours which have been acquired for the purpose of serving as a warning
+of inedibility, or of the possession of dangerous offensive weapons, are
+probably more numerous than have been hitherto supposed; and, if so, we
+shall be able to explain a considerable amount of colour in nature for
+which no use has hitherto been conjectured. The brilliant and varied
+colours of sea-anemones and of many coral animals will probably come
+under this head, since we know that many of them possess the power of
+ejecting stinging threads from various parts of their bodies which
+render them quite uneatable to most animals. Mr. Gosse describes how, on
+putting an Anthea into a tank containing a half-grown bullhead (Cottus
+bubalis) which had not been fed for some time, the fish opened his mouth
+and sucked in the morsel, but instantly shot it out again. He then
+seized it a second time, and after rolling it about in his mouth for a
+moment shot it out again, and then darted away to hide himself in a
+hole. Some tropical fishes, however, of the genera Tetrodon,
+Pseudoscarus, Astracion, and a few others, seem to have acquired the
+power of feeding on corals and medusae; and the beautiful bands and
+spots and bright colours with which they are frequently adorned, may be
+either protective when feeding in the submarine coral groves, or may, in
+some cases, be warning colours to show that they themselves are
+poisonous and uneatable.
+
+A remarkable illustration of the wide extension of warning colours, and
+their very definite purpose in nature, is afforded by what may now be
+termed "Mr. Belt's frog." Frogs in all parts of the world are, usually,
+protectively coloured with greens or browns; and the little tree-frogs
+are either green like the leaves they rest upon, or curiously mottled to
+imitate bark or dead leaves. But there are a certain number of very
+gaily coloured frogs, and these do not conceal themselves as frogs
+usually do. Such was the small toad found by Darwin at Bahia Blanca,
+which was intense black and bright vermilion, and crawled about in the
+sunshine over dry sand-hills and arid plains. And in Nicaragua, Mr. Belt
+found a little frog gorgeously dressed in a livery of red and blue,
+which did not attempt concealment and was very abundant, a combination
+of characters which convinced him that it was uneatable. He, therefore,
+took a few specimens home with him and gave them to his fowls and ducks,
+but none would touch them. At last, by throwing down pieces of meat, for
+which there was a great competition among the poultry, he managed to
+entice a young duck into snatching up one of the little frogs. Instead
+of swallowing it, however, the duck instantly threw it out of its mouth,
+and went about jerking its head as if trying to get rid of some
+unpleasant taste.[114]
+
+The power of predicting what will happen in a given case is always
+considered to be a crucial test of a true theory, and if so, the theory
+of warning colours, and with it that of mimicry, must be held to be well
+established. Among the creatures which probably have warning colours as
+a sign of inedibility are, the brilliantly coloured nudibranchiate
+molluscs, those curious annelids the Nereis and the Aphrodite or
+sea-mouse, and many other marine animals. The brilliant colours of the
+scallops (Pecten) and some other bivalve shells are perhaps an
+indication of their hardness and consequent inedibility, as in the case
+of the hard beetles; and it is not improbable that some of the
+phosphorescent fishes and other marine organisms may, like the
+glow-worm, hold out their lamp as a warning to enemies.[115] In
+Queensland there is an exceedingly poisonous spider, whose bite will
+kill a dog, and cause severe illness with excruciating pain in man. It
+is black, with a bright vermilion patch on the middle of the body; and
+it is so well recognised by this conspicuous coloration that even the
+spider-hunting wasps avoid it.[116]
+
+Locusts and grasshoppers are generally of green protective tints, but
+there are many tropical species most gaudily decorated with red, blue,
+and black colours. On the same general grounds as those by which Mr.
+Belt predicted the inedibility of his conspicuous frog, we might safely
+predict the same for these insects; but we have fortunately a proof that
+they are so protected, since Mr. Charles Home states that one of the
+bright coloured Indian locusts was invariably rejected when offered to
+birds and lizards.[117]
+
+       *       *       *       *       *
+
+The examples now given lead us to the conclusion that colours acquired
+for the purpose of serving as a danger-signal to enemies are very
+widespread in nature, and, with the corresponding colours of the species
+which mimic them, furnish us with a rational explanation of a
+considerable portion of the coloration of animals which is outside the
+limits of those colours that have been acquired for either protection or
+recognition. There remains, however, another set of colours, chiefly
+among the higher animals, which, being connected with some of the most
+interesting and most disputed questions in natural history, must be
+discussed in a separate chapter.
+
+FOOTNOTES:
+
+[Footnote 92: _Nature_, vol. iii. p. 165. Professor Meldola observed
+that specimens of Danais and Euplaea in collections were less subject to
+the attacks of mites _(Proc. Ent. Soc._, 1877, p. xii.); and this was
+corroborated by Mr. Jenner Weir. _Entomologist_, 1882, vol. xv. p. 160.]
+
+[Footnote 93: See Darwin's _Descent of Man_, p. 325.]
+
+[Footnote 94: _Transactions of the Entomological Society of London_,
+1869, p. 21.]
+
+[Footnote 95: _Ibid._, p. 27.]
+
+[Footnote 96: _Nature_, vol. iii. p. 147.]
+
+[Footnote 97: Stainton's _Manual of Butterflies and Moths_, vol. i. p.
+93; E.B. Poulton, _Proceedings of the Zool. Soc. of London_, 1887, pp.
+191-274.]
+
+[Footnote 98: See _Transactions of the Linnean Society_, vol. xxiii. pp.
+495-566, coloured plates.]
+
+[Footnote 99: These butterflies are now divided into two sub-families,
+one of which is placed with the Danaidae; but to avoid confusion I shall
+always speak of the American genera under the old term Heliconidae.]
+
+[Footnote 100: R. Meldola in _Ann. and Mag. of Nat. Hist._, Feb. 1878,
+p. 158.]
+
+[Footnote 101: See _Trans. Linn. Soc._, vol. xxv. Wallace, on Variation
+of Malayan Papilionidae; and, Wallace's _Contributions to Natural
+Selection_ chaps. iii. and iv., where full details are given.]
+
+[Footnote 102: See _Trans. Linn. Soc._, vol. xxvi., with two coloured
+plates illustrating cases of mimicry.]
+
+[Footnote 103: Edwards's _Butterflies of North America_, second series,
+part vi.]
+
+[Footnote 104: Professor Meldola informs me that he has recorded another
+case of mimicry among British moths, in which Acidalia subsericata
+imitates Asthena candidata. See _Ent. Mo. Mag._, vol. iv. p. 163.]
+
+[Footnote 105: From Professor Meldola's translation of Dr. F. Mueller's
+paper, in _Proc. Ent. Soc. Lond._, 1879, p. xx.]
+
+[Footnote 106: _Island Life_, p. 255.]
+
+[Footnote 107: This extension of the theory of mimicry was pointed out
+by Professor Meldola in the paper already referred to; and he has
+answered the objections to Dr. F. Mueller's theory with great force in
+the _Annals and Mag. of Nat. Hist._, 1882, p. 417.]
+
+[Footnote 108: Godman and Salvin's _Biologia Centrali-Americana,
+Insecta, Coleoptera_, vol. iii. part ii., and vol. v.]
+
+[Footnote 109: _Trans. Ent. Soc._, 1885, p. 369.]
+
+[Footnote 110: _Proc. Cambridge Phil. Soc._, vol. iii. part ii., 1877.]
+
+[Footnote 111: _Compte-Rendu de la Societe Entomologique de Belgaue_,
+series ii., No. 59, 1878.]
+
+[Footnote 112: _Nature_, vol. xxxiv. p. 547.]
+
+[Footnote 113: _Proceedings of the Zool. Soc. of London_, 1870, p. 369.]
+
+[Footnote 114: _The Naturalist in Nicaragua_, p. 321.]
+
+[Footnote 115: Mr. Belt first suggested this use of the light of the
+Lampyridae (fireflies and glow-worms)--_Naturalist in Nicaragua_, p.
+320. Mr. Verrill and Professor Meldola made the same suggestion in the
+case of medusae and other phosphorescent marine organisms (_Nature_,
+vol. xxx. pp. 281, 289).]
+
+[Footnote 116: W.E. Armit, in _Nature_, vol. xviii. p. 642.]
+
+[Footnote 117: _Proc. Ent. Soc._, 1869, p. xiii.]
+
+
+
+
+CHAPTER X
+
+COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
+
+
+    Sex colours in the mollusca and crustacea--In insects--In
+    butterflies and moths--Probable causes of these colours--Sexual
+    selection as a supposed cause--Sexual coloration of birds--Cause
+    of dull colours of female birds--Relation of sex colour to
+    nesting habits--Sexual colours of other vertebrates--Sexual
+    selection by the struggles of males--Sexual characters due to
+    natural selection--Decorative plumage of males and its effect on
+    the females--Display of decorative plumage by the males--A
+    theory of animal coloration--The origin of accessory
+    plumes--Development of accessory plumes and their display--The
+    effect of female preference will be neutralised by natural
+    selection--General laws of animal coloration--Concluding
+    remarks.
+
+
+
+In the preceding chapters we have dealt chiefly with the coloration of
+animals as distinctive of the several species; and we have seen that, in
+an enormous number of cases, the colours can be shown to have a definite
+purpose, and to be useful either as a means of protection or
+concealment, of warning to enemies, or of recognition by their own kind.
+We have now to consider a subordinate but very widespread
+phenomenon---the differences of colour or of ornamental appendages in
+the two sexes. These differences are found to have special relations
+with the three classes of coloration above referred to, in many cases
+confirming the explanation already given of their purport and use, and
+furnishing us with important aid in formulating a general theory of
+animal coloration.
+
+In comparing the colours of the two sexes we find a perfect gradation,
+from absolute identity of colour up to such extreme difference that it
+is difficult to believe that the two forms can belong to the same
+species; and this diversity in the colours of the sexes does not bear
+any constant relation to affinity or systematic position. In both
+insects and birds we find examples of complete identity and extreme
+diversity of the sexes; and these differences occur sometimes in the
+same tribe or family, and sometimes even in the same genus.
+
+It is only among the higher and more active animals that sexual
+differences of colour acquire any prominence. In the mollusca the two
+sexes, when separated, are always alike in colour, and only very rarely
+present slight differences in the form of the shell. In the extensive
+group of crustacea the two sexes as a rule are identical in colour,
+though there are often differences in the form of the prehensile organs;
+but in a very few cases there are differences of colour also. Thus, in a
+Brazilian species of shore-crab (Gelasimus) the female is grayish-brown,
+while in the male the posterior part of the cephalo-thorax is pure
+white, with the anterior part of a rich green. This colour is only
+acquired by the males when they become mature, and is liable to rapid
+change in a few minutes to dusky tints.[118] In some of the freshwater
+fleas (Daphnoidae) the males are ornamented with red and blue spots,
+while in others similar colours occur in both sexes. In spiders also,
+though as a rule the two sexes are alike in colour, there are a few
+exceptions, the males being ornamented with brilliant colours on the
+abdomen, while the female is dull coloured.
+
+
+_Sexual Coloration in Insects._
+
+It is only when we come to the winged insects that we find any large
+amount of peculiarity in sexual coloration, and even here it is only
+developed in certain orders. Flies (Diptera), field-bugs (Hemiptera),
+cicadas (Homoptera), and the grasshoppers, locusts, and crickets
+(Orthoptera) present very few and unimportant sexual differences of
+colour; but the last two groups have special musical organs very fully
+developed in the males of some of the species, and these no doubt enable
+the sexes to discover and recognise each other. In some cases, however,
+when the female is protectively coloured, as in the well-known
+leaf-insects already referred to (p. 207), the male is smaller and much
+less protectively formed and coloured. In the bees and wasps
+(Hymenoptera) it is also the rule that the sexes are alike in colour,
+though there are several cases among solitary bees where they differ;
+the female being black, and the male brown in Anthophora retusa, while
+in Andraena fulva the female is more brightly coloured than the male. Of
+the great order of beetles (Coleoptera) the same thing may be said.
+Though often so rich and varied in their colours the sexes are usually
+alike, and Mr. Darwin was only able to find about a dozen cases in which
+there was any conspicuous difference between them.[119] They exhibit,
+however, numerous sexual characters, in the length of the antennae, and
+in horns, legs, or jaws remarkably enlarged or curiously modified in the
+male sex.
+
+It is in the family of dragonflies (order Neuroptera) that we first meet
+with numerous cases of distinctive sexual coloration. In some of the
+Agrionidae the males have the bodies rich blue and the wings black,
+while the females have the bodies green and the wings transparent. In
+the North American genus Hetaerina the males alone have a carmine spot
+at the base of each wing; but in some other genera the sexes hardly
+differ at all.
+
+The great order of Lepidoptera, including the butterflies and moths,
+affords us the most numerous and striking examples of diversity of
+sexual colouring. Among the moths the difference is usually but slight,
+being manifested in a greater intensity of the colour of the smaller
+winged male; but in a few cases there is a decided difference, as in the
+ghost-moth (Hepialus humuli), in which the male is pure white, while the
+female is yellow with darker markings. This may be a recognition colour,
+enabling the female more readily to discover her mate; and this view
+receives some support from the fact that in the Shetland Islands the
+male is almost as yellow as the female, since it has been suggested that
+at midsummer, when this moth appears, there is in that high latitude
+sufficient twilight all night to render any special coloration
+unnecessary.[120]
+
+Butterflies present us with a wonderful amount of sexual difference of
+colour, in many cases so remarkable that the two sexes of the same
+species remained for many years under different names and were thought
+to be quite distinct species. We find, however, every gradation from
+perfect identity to complete diversity, and in some cases we are able to
+see a reason for this difference. Beginning with the most extraordinary
+cases of diversity--as in Diadema misippus, where the male is black,
+ornamented with a large white spot on each wing margined with rich
+changeable blue, while the female is orange-brown with black spots and
+stripes--we find the explanation in the fact that the female mimics an
+uneatable Danais, and thus gains protection while laying its eggs on low
+plants in company with that insect. In the allied species, Diadema
+bolina, the females are also very different from the males, but are of
+dusky brown tints, evidently protective and very variable, some
+specimens having a general resemblance to the uneatable Euplaeas; so
+that we see here some of the earlier stages of both forms of protection.
+The remarkable differences in some South American Pieridae are similarly
+explained. The males of Pieris pyrrha, P. lorena, and several others,
+are white with a few black bands and marginal spots like so many of
+their allies, while the females are gaily coloured with yellow and
+brown, and exactly resemble some species of the uneatable Heliconidae of
+the same district. Similarly, in the Malay Archipelago, the female of
+Diadema anomala is glossy metallic blue, while the male is brown; the
+reason for this reversal of the usual rule being, that the female
+exactly mimics the brilliant colouring of the common and uneatable
+Euplaea midamus, and thus secures protection. In the fine Adolias
+dirtea, the male is black with a few specks of ochre-yellow and a broad
+marginal band of rich metallic greenish-blue, while the female is
+brownish-black entirely covered with rows of ochre-yellow spots. This
+latter coloration does not appear to be protective when the insect is
+seen in the cabinet, but it really is so. I have observed the female of
+this butterfly in Sumatra, where it settles on the ground in the forest,
+and its yellow spots so harmonise with the flickering gleams of sunlight
+on the dead leaves that it can only be detected with the greatest
+difficulty.
+
+A hundred other cases might be quoted in which the female is either
+more obscurely coloured than the male, or gains protection by imitating
+some inedible species; and any one who has watched these female insects
+flying slowly along in search of the plants on which to deposit their
+eggs, will understand how important it must be to them not to attract
+the attention of insect-eating birds by too conspicuous colours. The
+number of birds which capture insects on the wing is much greater in
+tropical regions than in Europe; and this is perhaps the reason why many
+of our showy species are alike, or almost alike, in both sexes, while
+they are protectively coloured on the under side which is exposed to
+view when they are at rest. Such are our peacock, tortoise-shell, and
+red admiral butterflies; while in the tropics we more commonly find that
+the females are less conspicuous on the upper surface even when
+protectively coloured beneath.
+
+We may here remark, that the cases already quoted prove clearly that
+either male or female may be modified in colour apart from the opposite
+sex. In Pieris pyrrha and its allies the male retains the usual type of
+coloration of the whole genus, while the female has acquired a distinct
+and peculiar style of colouring. In Adolias dirtea, on the other hand,
+the female appears to retain something like the primitive colour and
+markings of the two sexes, modified perhaps for more perfect protection;
+while the male has acquired more and more intense and brilliant colours,
+only showing his original markings by the few small yellow spots that
+remain near the base of the wings. In the more gaily coloured Pieridae,
+of which our orange-tip butterfly may be taken as a type, we see in the
+female the plain ancestral colours of the group, while the male has
+acquired the brilliant orange tip to its wings, probably as a
+recognition mark.
+
+In those species in which the under surface is protectively coloured, we
+often find the upper surface alike in both sexes, the tint of colour
+being usually more intense in the male. But in some cases this leads to
+the female being more conspicuous, as in some of the Lycaenidae, where
+the female is bright blue and the male of a blue so much deeper and
+soberer in tint as to appear the less brilliantly coloured of the two.
+
+
+_Probable Causes of these Colours._
+
+In the production of these varied results there have probably been
+several causes at work. There seems to be a constant tendency in the
+male of most animals--but especially of birds and insects--to develop
+more and more intensity of colour, often culminating in brilliant
+metallic blues or greens or the most splendid iridescent hues; while, at
+the same time, natural selection is constantly at work, preventing the
+female from acquiring these same tints, or modifying her colours in
+various directions to secure protection by assimilating her to her
+surroundings, or by producing mimicry of some protected form. At the
+same time, the need for recognition must be satisfied; and this seems to
+have led to diversities of colour in allied species, sometimes the
+female, sometimes the male undergoing the greatest change according as
+one or other could be modified with the greatest ease, and so as to
+interfere least with the welfare of the race. Hence it is that sometimes
+the males of allied species vary most, as in the different species of
+Epicalia; sometimes the females, as in the magnificent green species of
+Ornithoptera and the "Aeneas" group of Papilio.
+
+The importance of the two principles--the need of protection and
+recognition--in modifying the comparative coloration of the sexes among
+butterflies, is beautifully illustrated in the case of the groups which
+are protected by their distastefulness, and whose females do not,
+therefore, need the protection afforded by sober colours.
+
+In the great families, Heliconidae and Acraeidae, we find that the two
+sexes are almost always alike; and, in the very few exceptions, that the
+female, though differently, is not less gaily or less conspicuously
+coloured. In the Danaidae the same general rule prevails, but the cases
+in which the male exhibits greater intensity of colour than the female
+are perhaps more numerous than in the other two families. There is,
+however, a curious difference in this respect between the Oriental and
+the American groups of distasteful Papilios with warning colours, both
+of which are the subjects of mimicry. In the Eastern groups--of which P.
+hector and P. coon may be taken as types--the two sexes are nearly
+alike, the male being sometimes more intensely coloured and with fewer
+pale markings; but in the American groups--represented by P. aeneas, P.
+sesostris, and allies--there is a wonderful diversity, the males having
+a rich green or bluish patch on the fore wings, while the females have a
+band or spots of pure white, not always corresponding in position to the
+green spot of the males. There are, however, transitional forms, by
+which a complete series can be traced, from close similarity to great
+diversity of colouring between the sexes; and this may perhaps be only
+an extreme example of the intenser colour and more concentrated markings
+which are a very prevalent characteristic of male butterflies.
+
+There are, in fact, many indications of a regular succession of tints in
+which colour development has occurred in the various groups of
+butterflies, from an original grayish or brownish neutral tint. Thus in
+the "Aeneas" group of Papilios we have the patch on the upper wings
+yellowish in P. triopas, olivaceous in P. bolivar, bronzy-gray with a
+white spot in P. erlaces, more greenish and buff in P. iphidamas,
+gradually changing to the fine blue of P. brissonius, and the
+magnificent green of P. sesostris. In like manner, the intense crimson
+spots of the lower wings can be traced step by step from a yellow or
+buff tint, which is one of the most widespread colours in the whole
+order. The greater purity and intensity of colour seem to be usually
+associated with more pointed wings, indicating greater vigour and more
+rapid flight.
+
+
+_Sexual Selection as a supposed Cause of Colour Development._
+
+Mr. Darwin, as is well known, imputed most of the brilliant colours and
+varied patterns of butterflies' wings to sexual selection--that is, to a
+constant preference, by female butterflies, for the more brilliant
+males; the colours thus produced being sometimes transmitted to the
+males alone, sometimes to both sexes. This view has always seemed to me
+to be unsupported by evidence, while it is also quite inadequate to
+account for the facts. The only direct evidence, as set forth with his
+usual fairness by Mr. Darwin himself, is opposed to his views. Several
+entomologists assured him that, in moths, the females evince not the
+least choice of their partners; and Dr. Wallace of Colchester, who has
+largely bred the fine Bombyx cynthia, confirmed this statement. Among
+butterflies, several males often pursue one female, and Mr. Darwin says,
+that, unless the female exerts a choice the pairing must be left to
+chance. But, surely, it may be the most vigorous or most persevering
+male that is chosen, not necessarily one more brightly or differently
+coloured, and this will be true "natural selection." Butterflies have
+been noticed to prefer some coloured flowers to others; but that does
+not prove, or even render probable, any preference for the colour
+itself, but only for flowers of certain colours, on account of the more
+agreeable or more abundant nectar obtained from them. Dr. Schulte called
+Mr. Darwin's attention to the fact, that in the Diadema bolina the
+brilliant blue colour surrounding the white spots is only visible when
+we look towards the insect's head, and this is true of many of the
+iridescent colours of butterflies, and probably depends upon the
+direction of the striae on the scales. It is suggested, however, that
+this display of colour will be seen by the female as the male is
+approaching her, and that it has been developed by sexual
+selection.[121] But in the majority of cases the males _follow_ the
+female, hovering over her in a position which would render it almost
+impossible for her to see the particular colours or patterns on his
+upper surface; to do so the female should mount higher than the male,
+and fly towards him--being the seeker instead of the sought, and this is
+quite opposed to the actual facts. I cannot, therefore, think that this
+suggestion adds anything whatever to the evidence for sexual selection
+of colour by female butterflies. This question will, however, be again
+touched upon after we have considered the phenomena of sexual colour
+among the vertebrata.
+
+
+_Sexual Coloration of Birds._
+
+The general rule among vertebrates, as regards colour, is, for the two
+sexes to be alike. This prevails, with only a few exceptions, in fishes,
+reptiles, and mammalia; but in birds diversity of sexual colouring is
+exceedingly frequent, and is, not improbably, present in a greater or
+less degree in more than half of the known species. It is this class,
+therefore, that will afford us the best materials for a discussion of
+the problem, and that may perhaps lead us to a satisfactory explanation
+of the causes to which sexual colour is due.
+
+The most fundamental characteristic of birds, from our present point of
+view, is a greater intensity of colour in the male. This is the case in
+hawks and falcons; in many thrushes, warblers, and finches; in pigeons,
+partridges, rails, plovers, and many others. When the plumage is highly
+protective or of dull uniform tints, as in many of the thrushes and
+warblers, the sexes are almost or quite identical in colour; but when
+any rich markings or bright tints are acquired, they are almost always
+wanting or much fainter in the female, as we see in the black-cap among
+warblers, and the chaffinch among finches.
+
+It is in tropical regions, where from a variety of causes colour has
+been, developed to its fullest extent, that we find the most remarkable
+examples of sexual divergence of colour. The most gorgeously coloured
+birds known are the birds of paradise, the chatterers, the tanagers, the
+humming-birds, and the pheasant-tribe, including the peacocks. In all
+these the females are much less brilliant, and, in the great majority of
+cases, exceptionally plain and dull coloured birds. Not only are the
+remarkable plumes, crests, and gorgets of the birds of paradise entirely
+wanting in the females, but these latter are usually without any bright
+colour at all, and rank no higher than our thrushes in ornamental
+plumage. Of the humming-birds the same may be said, except that the
+females are often green, and sometimes slightly metallic, but from their
+small size and uniform tints are never conspicuous. The glorious blues
+and purples, the pure whites and intense crimsons of the male chatterers
+are represented in the females by olive-greens or dull browns, as are
+the infinitely varied tints of the male tanagers. And in pheasants, the
+splendour of plumage which characterises the males is entirely absent in
+the females, which, though often ornamental, have always comparatively
+sober and protective tints. The same thing occurs with many other
+groups. In the Eastern tropics are many brilliant birds belonging to the
+families of the warblers, flycatchers, shrikes, etc., but the female is
+always much less brilliant than the male and often quite dull coloured.
+
+
+_Cause of Dull Colours of Female Birds._
+
+The reason of this phenomenon is not difficult to find, if we consider
+the essential conditions of a bird's existence, and the most important
+function it has to fulfil. In order that the species may be continued,
+young birds must be produced, and the female birds have to sit
+assiduously on their eggs. While doing this they are exposed to
+observation and attack by the numerous devourers of eggs and birds, and
+it is of vital importance that they should be protectively coloured in
+all those parts of the body which are exposed during incubation. To
+secure this end all the bright colours and showy ornaments which
+decorate the male have not been acquired by the female, who often
+remains clothed in the sober hues which were probably once common to the
+whole order to which she belongs. The different amounts of colour
+acquired by the females have no doubt depended on peculiarities of
+habits and of environment, and on the powers of defence or of
+concealment possessed by the species. Mr. Darwin has taught us that
+natural selection cannot produce absolute, but only relative perfection;
+and as a protective colour is only one out of many means by which the
+female birds are able to provide for the safety of their young, those
+which are best endowed in other respects will have been allowed to
+acquire more colour than those with whom the struggle for existence is
+more severe.
+
+
+_Relation of Sex Colour to Nesting Habits._
+
+This principle is strikingly illustrated by the existence of
+considerable numbers of birds in which both sexes are similarly and
+brilliantly coloured,--in some cases as brilliantly as the males of many
+of the groups above referred to. Such are the extensive families of the
+kingfishers, the woodpeckers, the toucans, the parrots, the turacos, the
+hangnests, the starlings, and many other smaller groups, all the species
+of which are conspicuously or brilliantly coloured, while in all of them
+the females are either coloured exactly like the males, or, when
+differently coloured, are equally conspicuous. When searching for some
+cause for this singular apparent exception to the rule of female
+protective colouring, I came upon a fact which beautifully explains it;
+for in all these cases, without exception, the species either nests in
+holes in the ground or in trees, or builds a domed or covered nest, so
+as completely to conceal the sitting-bird. We have here a case exactly
+parallel to that of the butterflies protected by distastefulness, whose
+females are either exactly like the males, or, if different, are equally
+conspicuous. We can hardly believe that so exact a parallel should exist
+between such remote classes of animals, except under the influence of a
+general law; and, in the need of protection by all defenceless animals,
+and especially by most female insects and birds, we have such a law,
+which has been proved to have influenced the colours of a considerable
+proportion of the animal kingdom.[122]
+
+The general relation which exists between the mode of nesting and the
+coloration of the sexes in those groups of birds which need protection
+from enemies, may be thus expressed: When both sexes are brilliant or
+conspicuous, the nest is such as to conceal the sitting-bird; but when
+the male is brightly coloured and the female sits exposed on the nest,
+she is always less brilliant and generally of quite sober and protective
+hues.
+
+It must be understood that the mode of nesting has influenced the
+colour, not that the colour has determined the mode of nesting; and
+this, I believe, has been generally, though not perhaps universally, the
+case. For we know that colour varies more rapidly, and can be more
+easily modified and fixed by selection, than any other character;
+whereas habits, especially when connected with structure, and when they
+pervade a whole group, are much more persistent and more difficult to
+change, as shown by the habit of the dog turning round two or three
+times before lying down, believed to be that of the wild ancestral form
+which thus smoothed down the herbage so as to form a comfortable bed. We
+see, too, that the general mode of nesting is characteristic of whole
+families differing widely in size, form, and colours. Thus, all the
+kingfishers and their allies in every part of the world nest in holes,
+usually in banks, but sometimes in trees. The motmots and the puff-birds
+(Bucconidae) build in similar places; while the toucans, barbets,
+trogons, woodpeckers, and parrots all make their nests in hollow trees.
+This habit, pervading all the members of extensive families, must
+therefore be extremely ancient, more especially as it evidently depends
+in some degree on the structure of the birds, the bills, and especially
+the feet, of all these groups being unfitted for the construction of
+woven arboreal nests.[123] But in all these families the colour varies
+greatly from species to species, being constant only in the one
+character of the similarity of the sexes, or, at all events, in their
+being equally conspicuous even though differently coloured.
+
+When I first put forward this view of the connection between the mode of
+nesting and the coloration of female birds, I expressed the law in
+somewhat different terms, which gave rise to some misunderstanding, and
+led to numerous criticisms and objections. Several cases were brought
+forward in which the females were far less brilliant than the males,
+although the nest was covered. This is the case with the Maluridae, or
+superb warblers of Australia, in which the males are very brilliant
+during the pairing season and the females quite plain, yet they build
+domed nests. Here, there can be little doubt, the covered nest is a
+protection from rain or from some special enemies to the eggs; while the
+birds themselves are protectively coloured in both sexes, except for a
+short time during the breeding season when the male acquires brilliant
+colours; and this is probably connected with the fact of their
+inhabiting the open plains and thin scrub of Australia, where protective
+colours are as generally advantageous as they are in our north-temperate
+zones.
+
+As I have now stated the law, I do not think there are any exceptions to
+it, while there are an overwhelming number of cases which give it a
+strong support. It has been objected that the domed nests of many birds
+are as conspicuous as the birds themselves would be, and would,
+therefore, be of no use as a protection to the birds and young. But, as
+a matter of fact, they do protect from attack, for hawks or crows do not
+pluck such nests to pieces, as in doing so they would be exposed to the
+attack of the whole colony; whereas a hawk or falcon could carry off a
+sitting-bird or the young at a swoop, and entirely avoid attack.
+Moreover, each kind of covered nest is doubtless directed against the
+attacks of the most dangerous enemies of the species, the purse-like
+nests, often a yard long, suspended from the extremity of thin twigs,
+being useful against the attacks of snakes, which, if they attempted to
+enter them, would be easily made to lose their hold and fall to the
+ground. Such birds as jays, crows, magpies, hawks, and other birds of
+prey, have also been urged as an exception; but these are all aggressive
+birds, able to protect themselves, and thus do not need any special
+protection for their females during nidification. Some birds which build
+in covered nests are comparatively dull coloured, like many of the
+weaver birds, but in others the colours are more showy, and in all the
+sexes are alike; so that none of these are in any way opposed to the
+rule. The golden orioles have, however, been adduced as a decided
+exception, since the females are showy and build in an open nest. But
+even here the females are less brilliant than the males, and are
+sometimes greenish or olivaceous on the upper surface; while they very
+carefully conceal their nests among dense foliage, and the male is
+sufficiently watchful and pugnacious to drive off most intruders.
+
+On the other hand, how remarkable it is that the only small and brightly
+coloured birds of our own country in which the male and female are
+alike--the tits and starlings--either build in holes or construct
+covered nests; while the beautiful hangnests (Icteridae) of South
+America, which always build covered or purse-shaped nests, are equally
+showy in both sexes, in striking contrast with the chatterers and
+tanagers of the same country, whose females are invariably less
+conspicuous than the males. On a rough estimate, there are about 1200
+species of birds in the class of showy males and females, with concealed
+nidification; while there are probably, from an equally rough estimate,
+about the same number in the contrasted class of showy males and dull
+females, with open nests. This will leave the great bulk of known birds
+in the classes of those which are more or less protectively coloured in
+both sexes; or which, from their organisation and habits, do not
+require special protective coloration, such as many of the birds of
+prey, the larger waders, and the oceanic birds.
+
+There are a few very curious cases in which the female bird is actually
+more brilliant than the male, and which yet have open nests. Such are
+the dotterel (Eudromias morinellus), several species of phalarope, an
+Australian creeper (Climacteris erythropus), and a few others; but in
+every one of these cases the relation of the sexes in regard to
+nidification is reversed, the male performing the duties of incubation,
+while the female is the stronger and more pugnacious. This curious case,
+therefore, quite accords with the general law of coloration.[124]
+
+
+_Sexual Colours of other Vertebrates._
+
+We may consider a few of the cases of sexual colouring of other classes
+of vertebrates, as given by Mr. Darwin. In fishes, though the sexes are
+usually alike, there are several species in which the males are more
+brightly coloured, and have more elongated fins, spines, or other
+appendages, and in some few cases the colours are decidedly different.
+The males often fight together, and are altogether more vivacious and
+excitable than the females during the breeding season; and with this we
+may connect a greater intensity of coloration.
+
+In frogs and toads the colours are usually alike, or a little more
+intense in the males, and the same may be said of most snakes. It is in
+lizards that we first meet with considerable sexual differences, many of
+the species having gular pouches, frills, dorsal crests, or horns,
+either confined to the males, or more developed in them than in the
+females, and these ornaments are often brightly coloured. In most cases,
+however, the tints of lizards are protective, the male being usually a
+little more intense in coloration; and the difference in extreme cases
+may be partly due to the need of protection for the female, which, when
+laden with eggs, must be less active and less able to escape from
+enemies than the male, and may, therefore, have retained more protective
+colours, as so many insects and birds have certainly done.[125]
+
+In mammalia there is often a somewhat greater intensity of colour in
+the male, but rarely a decided difference. The female of the great red
+kangaroo, however, is a delicate gray; while in the Lemur macaco of
+Madagascar the male is jet-black and the female brown. In many monkeys
+also there are some differences of colour, especially on the face. The
+sexual weapons and ornaments of male mammalia, as horns, crests, manes,
+and dewlaps, are well known, and are very numerous and remarkable.
+Having thus briefly reviewed the facts, we will now consider the
+theories to which they have given rise.
+
+
+_Sexual Selection by the Struggles of Males._
+
+Among the higher animals it is a very general fact that the males fight
+together for the possession of the females. This leads, in polygamous
+animals especially, to the stronger or better armed males becoming the
+parents of the next generation, which inherits the peculiarities of the
+parents; and thus vigour and offensive weapons are continually increased
+in the males, resulting in the strength and horns of the bull, the tusks
+of the boar, the antlers of the stag, and the spurs and fighting
+instinct of the gamecock. But almost all male animals fight together,
+though not specially armed; even hares, moles, squirrels, and beavers
+fight to the death, and are often found to be scarred and wounded. The
+same rule applies to almost all male birds; and these battles have been
+observed in such different groups as humming-birds, finches,
+goatsuckers, woodpeckers, ducks, and waders. Among reptiles, battles of
+the males are known to occur in the cases of crocodiles, lizards, and
+tortoises; among fishes, in those of salmon and sticklebats. Even among
+insects the same law prevails; and male spiders, beetles of many groups,
+crickets, and butterflies often fight together.
+
+From this very general phenomenon there necessarily results a form of
+natural selection which increases the vigour and fighting power of the
+male animal, since, in every case, the weaker are either killed,
+wounded, or driven away. This selection would be more powerful if males
+were always in excess of females, but after much research Mr. Darwin
+could not obtain any satisfactory evidence that this was the case. The
+same effect, however, is produced in some cases by constitution or
+habits; thus male insects usually emerge first from the pupa, and among
+migrating birds the males arrive first both in this country and in North
+America. The struggle is thus intensified, and the most vigorous males
+are the first to have offspring. This in all probability is a great
+advantage, as the early breeders have the start in securing food, and
+the young are strong enough to protect themselves while the later broods
+are being produced.
+
+It is to this form of male rivalry that Mr. Darwin first applied the
+term "sexual selection." It is evidently a real power in nature; and to
+it we must impute the development of the exceptional strength, size, and
+activity of the male, together with the possession of special offensive
+and defensive weapons, and of all other characters which arise from the
+development of these or are correlated with them. But he has extended
+the principle into a totally different field of action, which has none
+of that character of constancy and of inevitable result that attaches to
+natural selection, including male rivalry; for by far the larger portion
+of the phenomena, which he endeavours to explain by the direct action of
+sexual selection, can only be so explained on the hypothesis that the
+immediate agency is female choice or preference. It is to this that he
+imputes the origin of all secondary sexual characters other than weapons
+of offence and defence, of all the ornamental crests and accessory
+plumes of birds, the stridulating sounds of insects, the crests and
+beards of monkeys and other mammals, and the brilliant colours and
+patterns of male birds and butterflies. He even goes further, and
+imputes to it a large portion of the brilliant colour that occurs in
+both sexes, on the principle that variations occurring in one sex are
+sometimes transmitted to the same sex only, sometimes to both, owing to
+peculiarities in the laws of inheritance. In this extension of sexual
+selection to include the action of female choice or preference, and in
+the attempt to give to that choice such wide-reaching effects, I am
+unable to follow him more than a very little way; and I will now state
+some of the reasons why I think his views are unsound.
+
+
+_Sexual Characters due to Natural Selection._
+
+Besides the acquisition of weapons by the male for the purpose of
+fighting with other males, there are some other sexual characters which
+may have been produced by natural selection. Such are the various sounds
+and odours which are peculiar to the male, and which serve as a call to
+the female or as an indication of his presence. These are evidently a
+valuable addition to the means of recognition of the two sexes, and are
+a further indication that the pairing season has arrived; and the
+production, intensification, and differentiation of these sounds and
+odours are clearly within the power of natural selection. The same
+remark will apply to the peculiar calls of birds, and even to the
+singing of the males. These may well have originated merely as a means
+of recognition between the two sexes of a species, and as an invitation
+from the male to the female bird. When the individuals of a species are
+widely scattered, such a call must be of great importance in enabling
+pairing to take place as early as possible, and thus the clearness,
+loudness, and individuality of the song becomes a useful character, and
+therefore the subject of natural selection. Such is especially the case
+with the cuckoo, and with all solitary birds, and it may have been
+equally important at some period of the development of all birds. The
+act of singing is evidently a pleasurable one; and it probably serves as
+an outlet for superabundant nervous energy and excitement, just as
+dancing, singing, and field sports do with us. It is suggestive of this
+view that the exercise of the vocal power seems to be complementary to
+the development of accessory plumes and ornaments, all our finest
+singing birds being plainly coloured, and with no crests, neck or tail
+plumes to display; while the gorgeously ornamented birds of the tropics
+have no song, and those which expend much energy in display of plumage,
+as the turkey, peacocks, birds of paradise, and humming-birds, have
+comparatively an insignificant development of voice. Some birds have, in
+the wings or tail, peculiarly developed feathers which produce special
+sounds. In some of the little manakins of Brazil, two or three of the
+wing-feathers are curiously shaped and stiffened in the male, so that
+the bird is able to produce with them a peculiar snapping or cracking
+sound; and the tail-feathers of several species of snipe are so narrowed
+as to produce distinct drumming, whistling, or switching sounds when the
+birds descend rapidly from a great height. All these are probably
+recognition and call notes, useful to each species in relation to the
+most important function of their lives, and thus capable of being
+developed by the agency of natural selection.
+
+
+_Decorative Plumage of Birds and its Display._
+
+Mr. Darwin has devoted four chapters of his _Descent of Man_ to the
+colours of birds, their decorative plumage, and its display at the
+pairing season; and it is on this latter circumstance that he founds his
+theory, that both the plumage and the colours have been developed by the
+preference of the females, the more ornamented males becoming the
+parents of each successive generation. Any one who reads these most
+interesting chapters will admit, that the fact of the display is
+demonstrated; and it may also be admitted, as highly probable, that the
+female is pleased or excited by the display. But it by no means follows
+that slight differences in the shape, pattern, or colours of the
+ornamental plumes are what lead a female to give the preference to one
+male over another; still less that all the females of a species, or the
+great majority of them, over a wide area of country, and for many
+successive generations, prefer exactly the same modification of the
+colour or ornament.
+
+The evidence on this matter is very scanty, and in most cases not at all
+to the point. Some peahens preferred an old pied peacock; albino birds
+in a state of nature have never been seen paired with other birds; a
+Canada goose paired with a Bernicle gander; a male widgeon preferred a
+pintail duck to its own species; a hen canary preferred a male
+greenfinch to either linnet, goldfinch, siskin, or chaffinch. These
+cases are evidently exceptional, and are not such as generally occur in
+nature; and they only prove that the female does exert some choice
+between very different males, and some observations on birds in a state
+of nature prove the same thing; but there is no evidence that slight
+variations in the colour or plumes, in the way of increased intensity or
+complexity, are what determines the choice. On the other hand, Mr.
+Darwin gives much evidence that it is _not_ so determined. He tells us
+that Messrs. Hewitt, Tegetmeier, and Brent, three of the highest
+authorities and best observers, "do not believe that the females prefer
+certain males on account of the beauty of their plumage." Mr. Hewitt was
+convinced "that the female almost invariably prefers the most vigorous,
+defiant, and mettlesome male;" and Mr. Tegetmeier, "that a gamecock,
+though disfigured by being dubbed, and with his hackles trimmed, would
+be accepted as readily as a male retaining all his natural
+ornaments."[126] Evidence is adduced that a female pigeon will sometimes
+turn antipathy to a particular male without any assignable cause; or, in
+other cases, will take a strong fancy to some one bird, and will desert
+her own mate for him; but it is not stated that superiority or
+inferiority of plumage has anything to do with these fancies. Two
+instances are indeed given, of male birds being rejected, which had lost
+their ornamental plumage; but in both cases (a widow-finch and a silver
+pheasant) the long tail-plumes are the indication of sexual maturity.
+Such cases do not support the idea that males with the tail-feathers a
+trifle longer, or the colours a trifle brighter, are generally
+preferred, and that those which are only a little inferior are as
+generally rejected,--and this is what is absolutely needed to establish
+the theory of the development of these plumes by means of the choice of
+the female.
+
+It will be seen, that female birds have unaccountable likes and dislikes
+in the matter of their partners, just as we have ourselves, and this may
+afford us an illustration. A young man, when courting, brushes or curls
+his hair, and has his moustache, beard, or whiskers in perfect order,
+and no doubt his sweetheart admires them; but this does not prove that
+she marries him on account of these ornaments, still less that hair,
+beard, whiskers, and moustache were developed by the continued
+preferences of the female sex. So, a girl likes to see her lover well
+and fashionably dressed, and he always dresses as well as he can when he
+visits her; but we cannot conclude from this that the whole series of
+male costumes, from the brilliantly coloured, puffed, and slashed
+doublet and hose of the Elizabethan period, through the gorgeous coats,
+long waistcoats, and pigtails of the early Georgian era, down to the
+funereal dress-suit of the present day, are the direct result of female
+preference. In like manner, female birds may be charmed or excited by
+the fine display of plumage by the males; but there is no proof whatever
+that slight differences in that display have any effect in determining
+their choice of a partner.
+
+
+_Display of Decorative Plumage._
+
+The extraordinary manner in which most birds display their plumage at
+the time of courtship, apparently with the full knowledge that it is
+beautiful, constitutes one of Mr. Darwin's strongest arguments. It is,
+no doubt, a very curious and interesting phenomenon, and indicates a
+connection between the exertion of particular muscles and the
+development of colour and ornament; but, for the reasons just given, it
+does not prove that the ornament has been developed by female choice.
+During excitement, and when the organism develops superabundant energy,
+many animals find it pleasurable to exercise their various muscles,
+often in fantastic ways, as seen in the gambols of kittens, lambs, and
+other young animals. But at the time of pairing, male birds are in a
+state of the most perfect development, and possess an enormous store of
+vitality; and under the excitement of the sexual passion they perform
+strange antics or rapid flights, as much probably from an internal
+impulse to motion and exertion as with any desire to please their mates.
+Such are the rapid descent of the snipe, the soaring and singing of the
+lark, and the dances of the cock-of-the-rock and of many other birds.
+
+It is very suggestive that similar strange movements are performed by
+many birds which have no ornamental plumage to display. Goatsuckers,
+geese, carrion vultures, and many other birds of plain plumage have been
+observed to dance, spread their wings or tails, and perform strange
+love-antics. The courtship of the great albatross, a most unwieldy and
+dull coloured bird, has been thus described by Professor Moseley: "The
+male, standing by the female on the nest, raises his wings, spreads his
+tail and elevates it, throws up his head with the bill in the air, or
+stretches it straight out, or forwards, as far as he can, and then
+utters a curious cry."[127] Mr. Jenner Weir informs me that "the male
+blackbird is full of action, spreads out his glossy wing and tail, turns
+his rich golden beak towards the female, and chuckles with delight,"
+while he has never seen the more plain coloured thrush demonstrative to
+the female. The linnet distends his rosy breast, and slightly expands
+his brown wings and tail; while the various gay coloured Australian
+finches adopt such attitudes and postures as, in every case, to show off
+their variously coloured plumage to the best advantage.[128]
+
+
+_A Theory of Animal Coloration._
+
+Having rejected Mr. Darwin's theory of female choice as incompetent to
+account for the brilliant colours and markings of the higher animals,
+the preponderance of these colours and markings in the male sex, and
+their display during periods of activity or excitement, I may be asked
+what explanation I have to offer as a preferable substitute. In my
+_Tropical Nature_ I have already indicated such a theory, which I will
+now briefly explain, supporting it by some additional facts and
+arguments, which appear to me to have great weight, and for which I am
+mainly indebted to a most interesting and suggestive posthumous work by
+Mr. Alfred Tylor.[129]
+
+The fundamental or ground colours of animals ar has been shown in
+preceding chapters, very largely protective, and it is not improbable
+that the primitive colours of all animals were so. During the long
+course of animal development other modes of protection than concealment
+by harmony of colour arose, and thenceforth the normal development of
+colour due to the complex chemical and structural changes ever going on
+in the organism, had full play; and the colours thus produced were again
+and again modified by natural selection for purposes of warning,
+recognition, mimicry, or special protection, as has been already fully
+explained in the preceding chapters.
+
+Mr. Taylor has, however, called attention to an important principle
+which underlies the various patterns or ornamental markings of
+animals--namely, that diversified coloration follows the chief lines of
+structure, and changes at points, such as the joints, where function
+changes. He says, "If we take highly decorated species--that is, animals
+marked by alternate dark or light bands or spots, such as the zebra,
+some deer, or the carnivora, we find, first, that the region of the
+spinal column is marked by a dark stripe; secondly, that the regions of
+the appendages, or limbs, are differently marked; thirdly, that the
+flanks are striped or spotted, along or between the regions of the lines
+of the ribs; fourthly, that the shoulder and hip regions are marked by
+curved lines; fifthly, that the pattern changes, and the direction of
+the lines, or spots, at the head, neck, and every joint of the limbs;
+and lastly, that the tips of the ears, nose, tail, and feet, and the eye
+are emphasised in colour. In spotted animals the greatest length of the
+spot is generally in the direction of the largest development of the
+skeleton."
+
+This structural decoration is well seen in many insects. In
+caterpillars, similar spots and markings are repeated in each segment,
+except where modified for some form of protection. In butterflies, the
+spots and bands usually have reference to the form of the wing and the
+arrangement of the nervures; and there is much evidence to show that the
+primitive markings are always spots in the cells, or between the
+nervures, or at the junctions of nervures, the extension and coalescence
+of these spots forming borders, bands, or blotches, which have become
+modified in infinitely varied ways for protection, warning, or
+recognition. Even in birds, the distribution of colours and markings
+follows generally the same law. The crown of the head, the throat, the
+ear-coverts, and the eyes have usually distinct tints in all highly
+coloured birds; the region of the furcula has often a distinct patch of
+colour, as have the pectoral muscles, the uropygium or root of the tail,
+and the under tail-coverts.[130]
+
+Mr. Tylor was of opinion the primitive form of ornamentation consisted
+of spots, the confluence of these in certain directions forming lines or
+bands; and, these again, sometimes coalescing into blotches, or into
+more or less uniform tints covering a large portion of the surface of
+the body. The young lion and tiger are both spotted; and in the Java hog
+(Sus vittatus) very young animals are banded, but have spots over the
+shoulders and thighs. These spots run into stripes as the animal grows
+older; then the stripes expand, and at last, meeting together, the adult
+animal becomes of a uniform dark brown colour. So many of the species of
+deer are spotted when young, that Darwin concludes the ancestral form,
+from which all deer are derived, must have been spotted. Pigs and tapirs
+are banded or spotted when young; an imported young specimen of Tapirus
+Bairdi was covered with white spots in longitudinal rows, here and there
+forming short stripes.[131] Even the horse, which Darwin supposes to be
+descended from a striped animal, is often spotted, as in dappled horses;
+and great numbers show a tendency to spottiness, especially on the
+haunches.
+
+Ocelli may also be developed from spots, or from bars, as pointed out by
+Mr. Darwin. Spots are an ordinary form of marking in disease, and these
+spots sometimes run together, forming blotches. There is evidence that
+colour markings are in some way dependent on nerve distribution. In the
+disease known as frontal herpes, an eruption occurs which corresponds
+exactly to the distribution of the ophthalmic division of the fifth
+cranial nerve, mapping out all its little branches even to the one which
+goes to the tip of the nose. In a Hindoo suffering from herpes the
+pigment was destroyed in the arm along the course of the ulnar nerve,
+with its branches along both sides of one finger and the half of
+another. In the leg the sciatic and scaphenous nerves were partly mapped
+out, giving to the patient the appearance of an anatomical diagram.[132]
+
+These facts are very interesting, because they help to explain the
+general dependence of marking on structure which has been already
+pointed out. For, as the nerves everywhere follow the muscles, and these
+are attached to the various bones, we see how it happens, that the
+tracts in which distinct developments of colour appear, should so often
+be marked out by the chief divisions of the bony structure in
+vertebrates, and by the segments in the annulosa. There is, however,
+another correspondence of even greater interest and importance.
+Brilliant colours usually appear just in proportion to the development
+of tegumentary appendages. Among birds the most brilliant colours are
+possessed by those which have developed frills, crests, and elongated
+tails like the humming-birds; immense tail-coverts like the peacock;
+enormously expanded wing-feathers, as in the argus-pheasant; or
+magnificent plumes from the region of the coracoids in many of the birds
+of paradise. It is to be noted, also, that all these accessory plumes
+spring from parts of the body which, in other species, are distinguished
+by patches of colour; so that we may probably impute the development of
+colour and of accessory plumage to the same fundamental cause.
+
+Among insects, the most brilliant and varied coloration occurs in the
+butterflies and moths, groups in which the wing-membranes have received
+their greatest expansion, and whose specialisation has been carried
+furthest in the marvellous scaly covering which is the seat of the
+colour. It is suggestive, that the only other group in which functional
+wings are much coloured is that of the dragonflies, where the membrane
+is exceedingly expanded. In like manner, the colours of beetles, though
+greatly inferior to those of the lepidoptera, occur in a group in which
+the anterior pair of wings has been thickened and modified in order to
+protect the vital parts, and in which these wing-covers (elytra), in the
+course of development in the different groups, must have undergone great
+changes, and have been the seat of very active growth.
+
+
+_The Origin of Accessory Plumes._
+
+Mr. Darwin supposes, that these have in almost every case been developed
+by the preference of female birds for such males as possessed them in a
+higher degree than others; but this theory does not account for the fact
+that these plumes usually appear in a few definite parts of the body. We
+require some cause to initiate the development in one part rather than
+in another. Now, the view that colour has arisen over surfaces where
+muscular and nervous development is considerable, and the fact that it
+appears especially upon the accessory or highly developed plumes, leads
+us to inquire whether the same cause has not primarily determined the
+development of these plumes. The immense tuft of golden plumage in the
+best known birds of paradise (Paradisea apoda and P. minor) springs
+from a very small area on the side of the breast. Mr. Frank E. Beddard,
+who has kindly examined a specimen for me, says that "this area lies
+upon the pectoral muscles, and near to the point where the fibres of the
+muscle converge towards their attachment to the humerus. The plumes
+arise, therefore, close to the most powerful muscle of the body, and
+near to where the activities of that muscle would be at a maximum.
+Furthermore, the area of attachment of the plumes is just above the
+point where the arteries and nerves for the supply of the pectoral
+muscles, and neighbouring regions, leave the interior of the body. The
+area of attachment of the plume is, also, as you say in your letter,
+just above the junction of the coracoid and sternum." Ornamental plumes
+of considerable size rise from the same part in many other species of
+paradise birds, sometimes extending laterally in front, so as to form
+breast shields. They also occur in many humming-birds, and in some
+sun-birds and honey-suckers; and in all these cases there is a wonderful
+amount of activity and rapid movement, indicating a surplus of vitality,
+which is able to manifest itself in the development of these accessory
+plumes.[133]
+
+In a quite distinct set of birds, the gallinaceae, we find the
+ornamental plumage usually arising from very different parts, in the
+form of elongated tail-feathers or tail-coverts, and of ruffs or hackles
+from the neck. Here the wings are comparatively little used, the most
+constant activities depending on the legs, since the gallinaceae are
+pre-eminently walking, running, and scratching birds. Now the
+magnificent train of the peacock--the grandest development of accessory
+plumes in this order--springs from an oval or circular area, about three
+inches in diameter, just above the base of the tail, and, therefore,
+situated over the lower part of the spinal column near the insertion of
+the powerful muscles which move the hind limbs and elevate the tail. The
+very frequent presence of neck-ruffs or breast-shields in the males of
+birds with accessory plumes may be partly due to selection, because they
+must serve as a protection in their mutual combats, just as does the
+lion's or the horse's mane. The enormously lengthened plumes of the bird
+of paradise and of the peacock can, however, have no such use, but must
+be rather injurious than beneficial in the bird's ordinary life. The
+fact that they have been developed to so great an extent in a few
+species is an indication of such perfect adaptation to the conditions of
+existence, such complete success in the battle for life, that there is,
+in the adult male at all events, a surplus of strength, vitality, and
+growth-power which is able to expend itself in this way without injury.
+That such is the case is shown by the great abundance of most of the
+species which possess these wonderful superfluities of plumage. Birds of
+paradise are among the commonest birds in New Guinea, and their loud
+voices can be often heard when the birds themselves are invisible in the
+depths of the forest; while Indian sportsmen have described the peafowl
+as being so abundant, that from twelve to fifteen hundred have been seen
+within an hour at one spot; and they range over the whole country from
+the Himalayas to Ceylon. Why, in allied species, the development of
+accessory plumes has taken different forms, we are unable to say, except
+that it may be due to that individual variability which has served as
+the starting-point for so much of what seems to us strange in form, or
+fantastic in colour, both in the animal and vegetable world.
+
+
+_Development of Accessory Plumes and their Display._
+
+If we have found a _vera causa_ for the origin of ornamental appendages
+of birds and other animals in a surplus of vital energy, leading to
+abnormal growths in those parts of the integument where muscular and
+nervous action are greatest, the continuous development of these
+appendages will result from the ordinary action of natural selection in
+preserving the most healthy and vigorous individuals, and the still
+further selective agency of sexual struggle in giving to the very
+strongest and most energetic the parentage of the next generation. And,
+as all the evidence goes to show that, so far as female birds exercise
+any choice, it is of "the most vigorous, defiant, and mettlesome male,"
+this form of sexual selection will act in the same direction, and help
+to carry on the process of plume development to its culmination. That
+culmination will be reached when the excessive length or abundance of
+the plumes begins to be injurious to the bearer of them; and it may be
+this check to the further lengthening of the peacock's train that has
+led to the broadening of the feathers at the ends, and the consequent
+production of the magnificent eye-spots which now form its crowning
+ornament.
+
+The display of these plumes will result from the same causes which led
+to their production. Just in proportion as the feathers themselves
+increased in length and abundance, the skin-muscles which serve to
+elevate them would increase also; and the nervous development as well as
+the supply of blood to these parts being at a maximum, the erection of
+the plumes would become a habit at all periods of nervous or sexual
+excitement. The display of the plumes, like the existence of the plumes
+themselves, would be the chief external indication of the maturity and
+vigour of the male, and would, therefore, be necessarily attractive to
+the female. We have, thus, no reason for imputing to her any of those
+aesthetic emotions which are excited in us, by the beauty of form,
+colour, and pattern of these plumes; or the still more improbable
+aesthetic tastes, which would cause her to choose her mate on account of
+minute differences in their forms, colours, or patterns.
+
+As co-operating causes in the production of accessory ornamental plumes,
+I have elsewhere suggested[134] that crests and other erectile feathers
+may have been useful in making the bird more formidable in appearance,
+and thus serving to frighten away enemies; while long tail or wing
+feathers might serve to distract the aim of a bird of prey. But though
+this might be of some use in the earlier stages of their development, it
+is probably of little importance compared with the vigour and pugnacity
+of which the plumes are the indication, and which enable most of their
+possessors to defend themselves against the enemies which are dangerous
+to weaker and more timid birds. Even the tiny humming-birds are said to
+attack birds of prey that approach too near to their nests.
+
+
+_The Effect of Female Preference will be Neutralised by Natural
+Selection._
+
+The various facts and arguments now briefly set forth, afford an
+explanation of the phenomena of male ornament, as being due to the
+general laws of growth and development, and make it unnecessary to call
+to our aid so hypothetical a cause as the cumulative action of female
+preference. There remains, however, a general argument, arising from the
+action of natural selection itself, which renders it almost
+inconceivable that female preference could have been effective in the
+way suggested; while the same argument strongly supports the view here
+set forth. Natural selection, as we have seen in our earlier chapters,
+acts perpetually and on an enormous scale in weeding out the "unfit" at
+every stage of existence, and preserving only those which are in all
+respects the very best. Each year, only a small percentage of young
+birds survive to take the place of the old birds which die; and the
+survivors will be those which are best able to maintain existence from
+the egg onwards, an important factor being that their parents should be
+well able to feed and protect them, while they themselves must in turn
+be equally able to feed and protect their own offspring. Now this
+extremely rigid action of natural selection must render any attempt to
+select mere ornament utterly nugatory, unless the most ornamented always
+coincide with "the fittest" in every other respect; while, if they do so
+coincide, then any selection of ornament is altogether superfluous. If
+the most brightly coloured and fullest plumaged males are _not_ the most
+healthy and vigorous, have _not_ the best instincts for the proper
+construction and concealment of the nest, and for the care and
+protection of the young, they are certainly not the fittest, and will
+not survive, or be the parents of survivors. If, on the other hand,
+there _is_ generally this correlation--if, as has been here argued,
+ornament is the natural product and direct outcome of superabundant
+health and vigour, then no other mode of selection is needed to account
+for the presence of such ornament. The action of natural selection does
+not indeed disprove the existence of female selection of ornament as
+ornament, but it renders it entirely ineffective; and as the direct
+evidence for any such female selection is almost _nil_, while the
+objections to it are certainly weighty, there can be no longer any
+reason for upholding a theory which was provisionally useful in calling
+attention to a most curious and suggestive body of facts, but which is
+now no longer tenable. The term "sexual selection" must, therefore, be
+restricted to the direct results of male struggle and combat. This is
+really a form of natural selection, and is a matter of direct
+observation; while its results are as clearly deducible as those of any
+of the other modes in which selection acts. And if this restriction of
+the term is needful in the case of the higher animals it is much more so
+with the lower. In butterflies the weeding out by natural selection
+takes place to an enormous extent in the egg, larva, and pupa states;
+and perhaps not more than one in a hundred of the eggs laid produces a
+perfect insect which lives to breed. Here, then, the impotence of female
+selection, if it exist, must be complete; for, unless the most
+brilliantly coloured males are those which produce the best protected
+eggs, larvae, and pupae, and unless the particular eggs, larvae, and
+pupae, which are able to survive, are those which produce the most
+brilliantly coloured butterflies, any choice the female might make must
+be completely swamped. If, on the other hand, there _is_ this
+correlation between colour development and perfect adaptation to
+conditions in all stages, then this development will necessarily proceed
+by the agency of natural selection and the general laws which determine
+the production of colour and of ornamental appendages.[135]
+
+
+_General Laws of Animal Coloration._
+
+The condensed account which has now been given of the phenomena of
+colour in the animal world will sufficiently show the wonderful
+complexity and extreme interest of the subject; while it affords an
+admirable illustration of the importance of the great principle of
+utility, and of the effect of the theories of natural selection and
+development in giving a new interest to the most familiar facts of
+nature. Much yet remains to be done, both in the observation of new
+facts as to the relations between the colours of animals and their
+habits or economy, and, more especially, in the elucidation of the laws
+of growth which determine changes of colour in the various groups; but
+so much is already known that we are able, with some confidence, to
+formulate the general principles which have brought about all the beauty
+and variety of colour which everywhere delight us in our contemplation
+of animated nature. A brief statement of these principles will fitly
+conclude our exposition of the subject.
+
+1. Colour may be looked upon as a necessary result of the highly complex
+chemical constitution of animal tissues and fluids. The blood, the bile,
+the bones, the fat, and other tissues have characteristic, and often
+brilliant colours, which we cannot suppose to have been determined for
+any special purpose, as colours, since they are usually concealed. The
+external organs, with their various appendages and integuments, would,
+by the same general laws, naturally give rise to a greater variety of
+colour.
+
+2. We find it to be the fact that colour increases in variety and
+intensity as external structures and dermal appendages become more
+differentiated and developed. It is on scales, hair, and especially on
+the more highly specialised feathers, that colour is most varied and
+beautiful; while among insects colour is most fully developed in those
+whose wing membranes are most expanded, and, as in the lepidoptera, are
+clothed with highly specialised scales. Here, too, we find an additional
+mode of colour production in transparent lamellae or in fine surface
+striae which, by the laws of interference, produce the wonderful
+metallic hues of so many birds and insects.
+
+3. There are indications of a progressive change of colour, perhaps in
+some definite order, accompanying the development of tissues or
+appendages. Thus spots spread and fuse into bands, and when a lateral or
+centrifugal expansion has occurred--as in the termination of the
+peacocks' train feathers, the outer web of the secondary quills of the
+Argus pheasant, or the broad and rounded wings of many butterflies--into
+variously shaded or coloured ocelli. The fact that we find gradations of
+colour in many of the more extensive groups, from comparatively dull or
+simple to brilliant and varied hues, is an indication of some such law
+of development, due probably to progressive local segregation in the
+tissues of identical chemical or organic molecules, and dependent on
+laws of growth yet to be investigated.
+
+4. The colours thus produced, and subject to much individual variation,
+have been modified in innumerable ways for the benefit of each species.
+The most general modification has been in such directions as to favour
+concealment when at rest in the usual surroundings of the species,
+sometimes carried on by successive steps till it has resulted in the
+most minute imitation of some inanimate object or exact mimicry of some
+other animal. In other cases bright colours or striking contrasts have
+been preserved, to serve as a warning of inedibility or of dangerous
+powers of attack. Most frequent of all has been the specialisation of
+each distinct form by some tint or marking for purposes of easy
+recognition, especially in the case of gregarious animals whose safety
+largely depends upon association and mutual defence.
+
+5. As a general rule the colours of the two sexes are alike; but in the
+higher animals there appears a tendency to deeper or more intense
+colouring in the male, due probably to his greater vigour and
+excitability. In many groups in which this superabundant vitality is at
+a maximum, the development of dermal appendages and brilliant colours
+has gone on increasing till it has resulted in a great diversity between
+the sexes; and in most of these cases there is evidence to show that
+natural selection has caused the female to retain the primitive and more
+sober colours of the group for purposes of protection.
+
+
+_Concluding Remarks._
+
+The general principles of colour development now sketched out enable us
+to give some rational explanation of the wonderful amount of brilliant
+colour which occurs among tropical animals. Looking on colour as a
+normal product of organisation, which has either been allowed free play,
+or has been checked and modified for the benefit of the species, we can
+see at once that the luxuriant and perennial vegetation of the tropics,
+by affording much more constant means of concealment, has rendered
+brilliant colour less hurtful there than in the temperate and colder
+regions. Again, this perennial vegetation supplies abundance of both
+vegetable and insect food throughout the year, and thus a greater
+abundance and greater variety of the forms of life are rendered
+possible, than where recurrent seasons of cold and scarcity reduce the
+possibilities of life to a minimum. Geology furnishes us with another
+reason, in the fact, that throughout the tertiary period tropical
+conditions prevailed far into the temperate regions, so that the
+possibilities of colour development were still greater than they are at
+the present time. The tropics, therefore, present to us the results of
+animal development in a much larger area and under more favourable
+conditions than prevail to-day. We see in them samples of the
+productions of an earlier and a better world, from an animal point of
+view; and this probably gives a greater variety and a finer display of
+colour than would have been produced, had conditions always been what
+they are now. The temperate zones, on the other hand, have recently
+suffered the effects of a glacial period of extreme severity, with the
+result that almost the only gay coloured birds they now possess are
+summer visitors from tropical or sub-tropical lands. It is to the
+unbroken and almost unchecked course of development from remote
+geological times that has prevailed in the tropics, favoured by abundant
+food and perennial shelter, that we owe such superb developments as the
+frills and crests and jewelled shields of the humming-birds, the golden
+plumes of the birds of paradise, and the resplendent train of the
+peacock. This last exhibits to us the culmination of that marvel and
+mystery of animal colour which is so well expressed by a poet-artist in
+the following lines. The marvel will ever remain to the sympathetic
+student of nature, but I venture to hope that in the preceding chapters
+I have succeeded in lifting--if only by one of its corners--the veil of
+mystery which has for long shrouded this department of nature.
+
+
+_On a Peacock's Feather._
+
+    In Nature's workshop but a shaving,
+      Of her poem but a word,
+    But a tint brushed from her palette,
+      This feather of a bird!
+    Yet set it in the sun glance,
+      Display it in the shine,
+    Take graver's lens, explore it,
+      Note filament and line,
+    Mark amethyst to sapphire,
+      And sapphire to gold,
+    And gold to emerald changing
+      The archetype unfold!
+    Tone, tint, thread, tissue, texture,
+      Through every atom scan,
+    Conforming still, developing,
+      Obedient to plan.
+    This but to form a pattern
+      On the garment of a bird!
+    What then must be the poem,
+      This but its lightest word!
+    Sit before it; ponder o'er it,
+      'Twill thy mind advantage more,
+    Than a treatise, than a sermon,
+      Than a library of lore.
+
+FOOTNOTES:
+
+[Footnote 118: Darwin's _Descent of Man_, p. 271.]
+
+[Footnote 119: Darwin's _Descent of Man_, p. 294, and footnote.]
+
+[Footnote 120: _Nature_, 1871, p. 489.]
+
+[Footnote 121: Darwin in _Nature_, 1880, p. 237.]
+
+[Footnote 122: See the author's _Contributions to Natural Selection_,
+chap. vii. in which these facts were first brought forward.]
+
+[Footnote 123: On this point see the author's _Contributions to Natural
+Selection_, chap. v. i.]
+
+[Footnote 124: Seebohm's _History of British Birds_, vol. ii.,
+introduction, p. xiii.]
+
+[Footnote 125: For details see Darwin's _Descent of Man_, chap. xii.]
+
+[Footnote 126: _Descent of Man_, pp. 417, 418, 420.]
+
+[Footnote 127: _Notes of a Naturalist on the Challenger._]
+
+[Footnote 128: _Descent of Man_, pp. 401, 402.]
+
+[Footnote 129: _Coloration in Animals and Plants_, London, 1886.]
+
+[Footnote 130: _Coloration of Animals_, Pl. X, p. 90; and Pls. II, III,
+and IV, pp. 30, 40, 42.]
+
+[Footnote 131: See coloured Fig. in _Proc. Zool. Soc._, 1871, p. 626.]
+
+[Footnote 132: A. Tylor's _Coloration_, p. 40; and Photograph in
+Hutchinson's _Illustrations of Clinical Surgery_, quoted by Tylor.]
+
+[Footnote 133: For activity and pugnacity of humming-birds, see
+_Tropical Nature_, pp. 130, 213.]
+
+[Footnote 134: _Tropical Nature_, p. 209. In Chapter V of this work the
+views here advocated were first set forth, and the reader is referred
+there for further details.]
+
+[Footnote 135: The Rev. O. Pickard-Cambridge, who has devoted himself to
+the study of spiders, has kindly sent me the following extract from a
+letter, written in 1869, in which he states his views on this
+question:--
+
+
+    "I myself doubt that particular application of the Darwinian
+    theory which attributes male peculiarities of form, structure,
+    colour, and ornament to female appetency or predilection. There
+    is, it seems to me, undoubtedly something in the male
+    organisation of a special, and sexual nature, which, of its own
+    vital force, develops the remarkable male peculiarities so
+    commonly seen, and of no imaginable use to that sex. In as far
+    as these peculiarities show a great vital power, they point out
+    to us the finest and strongest individuals of the sex, and show
+    us which of them would most certainly appropriate to themselves
+    the best and greatest number of females, and leave behind them
+    the strongest and greatest number of progeny. And here would
+    come in, as it appears to me, the proper application of Darwin's
+    theory of Natural Selection; for the possessors of greatest
+    vital power being those most frequently produced and reproduced,
+    the external signs of it would go on developing in an
+    ever-increasing exaggeration, only to be checked where it became
+    really detrimental in some respect or other to the individual."
+
+
+This passage, giving the independent views of a close observer--one,
+moreover, who has studied the species of an extensive group of animals
+both in the field and in the laboratory--very nearly accords with my own
+conclusions above given; and, so far as the matured opinions of a
+competent naturalist have any weight, afford them an important support.]
+
+
+
+
+CHAPTER XI
+
+THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE
+
+
+    The general colour relations of plants--Colours of fruits--The
+    meaning of nuts--Edible or attractive fruits--The colours of
+    flowers--Modes of securing cross-fertilisation--The
+    interpretation of the facts--Summary of additional facts bearing
+    on insect fertilisation--Fertilisation of flowers by
+    birds--Self-fertilisation of flowers--Difficulties and
+    contradictions--Intercrossing not necessarily
+    advantageous--Supposed evil results of close interbreeding--How
+    the struggle for existence acts among flowers--Flowers the
+    product of insect agency--Concluding remarks on colour in
+    nature.
+
+
+
+The colours of plants are both less definite and less complex than are
+those of animals, and their interpretation on the principle of utility
+is, on the whole, more direct and more easy. Yet here, too, we find that
+in our investigation of the uses of the various colours of fruits and
+flowers, we are introduced to some of the most obscure recesses of
+nature's workshop, and are confronted with problems of the deepest
+interest and of the utmost complexity.
+
+So much has been written on this interesting subject since Mr. Darwin
+first called attention to it, and its main facts have become so
+generally known by means of lectures, articles, and popular books, that
+I shall give here a mere outline sketch, for the purpose of leading up
+to a discussion of some of the more fundamental problems which arise out
+of the facts, and which have hitherto received less attention than they
+deserve.
+
+
+_The General Colour Relations of Plants._
+
+The green colour of the foliage of leafy plants is due to the existence
+of a substance called chlorophyll, which is almost universally developed
+in the leaves under the action of light. It is subject to definite
+chemical changes during the processes of growth and of decay, and it is
+owing to these changes that we have the delicate tints of spring
+foliage, and the more varied, intense, and gorgeous hues of autumn. But
+these all belong to the class of intrinsic or normal colours, due to the
+chemical constitution of the organism; as colours they are unadaptive,
+and appear to have no more relation to the wellbeing of the plants
+themselves than have the colours of gems and minerals. We may also
+include in the same category those algae and fungi which have bright
+colours--the "red snow" of the arctic regions, the red, green, or purple
+seaweeds, the brilliant scarlet, yellow, white, or black agarics, and
+other fungi. All these colours are probably the direct results of
+chemical composition or molecular structure, and, being thus normal
+products of the vegetable organism, need no special explanation from our
+present point of view; and the same remark will apply to the varied
+tints of the bark of trunks, branches, and twigs, which are often of
+various shades of brown and green, or even vivid reds or yellows.
+
+There are, however, a few cases in which the need of protection, which
+we have found to be so important an agency in modifying the colours of
+animals, has also determined those of some of the smaller members of the
+vegetable kingdom. Dr. Burchell found a mesembryanthomum in South Africa
+like a curiously shaped pebble, closely resembling the stones among
+which it grew;[136] and Mr. J.P. Mansel Weale states that in the same
+country one of the Asclepiadeae has tubers growing above ground among
+stones which they exactly resemble, and that, when not in leaf, they are
+for this reason quite invisible.[137] It is clear that such resemblances
+must be highly useful to these plants, inhabiting an arid country
+abounding in herbivorous mammalia, which, in times of drought or
+scarcity, will devour everything in the shape of a fleshy stem or tuber.
+
+True mimicry is very rare in plants, though adaptation to like
+conditions often produces in foliage and habit a similarity that is
+deceiving. Euphorbias growing in deserts often closely resemble cacti.
+Seaside plants and high alpine plants of different orders are often much
+alike; and innumerable resemblances of this kind are recorded in the
+names of plants, as Veronica epacridea (the veronica like an epacris),
+Limnanthemum nymphaeoides (the limnanthemum like a nymphaea), the
+resembling species in each case belonging to totally distinct families.
+But in these cases, and in most others that have been observed, the
+essential features of true mimicry are absent, inasmuch as the one plant
+cannot be supposed to derive any benefit from its close resemblance to
+the other, and this is still more certain from the fact that the two
+species usually inhabit different localities. A few cases exist,
+however, in which there does seem to be the necessary accordance and
+utility. Mr. Mansel Weale mentions a labiate plant (Ajuga ophrydis), the
+only species of the genus Ajuga in South Africa, which is strikingly
+like an orchid of the same country; while a balsam (Impatiens capensis),
+also a solitary species of the genus in that country, is equally like an
+orchid, growing in the same locality and visited by the same insects. As
+both these genera of plants are specialised for insect fertilisation,
+and both of the plants in question are isolated species of their
+respective genera, we may suppose that, when they first reached South
+Africa they were neglected by the insects of the country; but, being
+both remotely like orchids in form of flower, those varieties that
+approached nearest to the familiar species of the country were visited
+by insects and cross-fertilised, and thus a closer resemblance would at
+length be brought about. Another case of close general resemblance, is
+that of our common white dead-nettle (Lamium album) to the
+stinging-nettle (Urtica dioica); and Sir John Lubbock thinks that this
+is a case of true mimicry, the dead-nettle being benefited by being
+mistaken by grazing animals for the stinging-nettle.[138]
+
+
+_Colours of Fruits._
+
+It is when we come to the essential parts of plants on which their
+perpetuation and distribution depends, that we find colour largely
+utilised for a distinct purpose in flowers and fruits. In the former we
+find attractive colours and guiding marks to secure cross-fertilisation
+by insects; in the latter attractive or protective coloration, the first
+to attract birds or other animals when the fruits are intended to be
+eaten, the second to enable them to escape being eaten when it would be
+injurious to the species. The colour phenomena of fruits being much the
+most simple will be considered first.
+
+The perpetuation and therefore the very existence of each species of
+flowering plant depend upon its seeds being preserved from destruction
+and more or less effectually dispersed over a considerable area. The
+dispersal is effected either mechanically or by the agency of animals.
+Mechanical dispersal is chiefly by means of air-currents, and large
+numbers of seeds are specially adapted to be so carried, either by being
+clothed with down or pappus, as in the well-known thistle and dandelion
+seeds; by having wings or other appendages, as in the sycamore, birch,
+and many other trees; by being thrown to a considerable distance by the
+splitting of the seed-vessel, and by many other curious devices.[139]
+Very large numbers of seeds, however, are so small and light that they
+can be carried enormous distances by gales of wind, more especially as
+most of this kind are flattened or curved, so as to expose a large
+surface in proportion to their weight. Those which are carried by
+animals have their surfaces, or that of the seed-vessel, armed with
+minute hooks, or some prickly covering which attaches itself to the hair
+of mammalia or the feathers of birds, as in the burdock, cleavers, and
+many other species. Others again are sticky, as in Plumbago europaea,
+mistletoe, and many foreign plants.
+
+All the seeds or seed-vessels which are adapted to be dispersed in any
+of these ways are of dull protective tints, so that when they fall on
+the ground they are almost indistinguishable; besides which, they are
+usually small, hard, and altogether unattractive, never having any
+soft, juicy pulp; while the edible seeds often bear such a small
+proportion to the hard, dry envelopes or appendages, that few animals
+would care to eat them.
+
+
+_The Meaning of Nuts._
+
+There is, however, another class of fruits or seeds, usually termed
+nuts, in which there is a large amount of edible matter, often very
+agreeable to the taste, and especially attractive and nourishing to a
+large number of animals. But when eaten, the seed is destroyed and the
+existence of the species endangered. It is evident, therefore, that it
+is by a kind of accident that these nuts are eatable; and that they are
+not intended to be eaten is shown by the special care nature seems to
+have taken to conceal or to protect them. We see that all our common
+nuts are green when on the tree, so as not easily to be distinguished
+from the leaves; but when ripe they turn brown, so that when they fall
+on to the ground they are equally indistinguishable among the dead
+leaves and twigs, or on the brown earth. Then they are almost always
+protected by hard coverings, as in hazel-nuts, which are concealed by
+the enlarged leafy involucre, and in the large tropical brazil-nuts and
+cocoa-nuts by such a hard and tough case as to be safe from almost every
+animal. Others have an external bitter rind, as in the walnut; while in
+the chestnuts and beech-nuts two or three fruits are enclosed in a
+prickly involucre.
+
+Notwithstanding all these precautions, nuts are largely devoured by
+mammalia and birds; but as they are chiefly the product of trees or
+shrubs of considerable longevity, and are generally produced in great
+profusion, the perpetuation of the species is not endangered. In some
+cases the devourers of nuts may aid in their dispersal, as they probably
+now and then swallow the seed whole, or not sufficiently crushed to
+prevent germination; while squirrels have been observed to bury nuts,
+many of which are forgotten and afterwards grow in places they could not
+have otherwise reached.[140] Nuts, especially the larger kinds which are
+so well protected by their hard, nearly globular cases, have their
+dispersal facilitated by rolling down hill, and more especially by
+floating in rivers and lakes, and thus reaching other localities. During
+the elevation of land areas this method would be very effective, as the
+new land would always be at a lower level than that already covered with
+vegetation, and therefore in the best position for being stocked with
+plants from it.
+
+The other modes of dispersal of seeds are so clearly adapted to their
+special wants, that we feel sure they must have been acquired by the
+process of variation and natural selection. The hooked and sticky seeds
+are always those of such herbaceous plants as are likely, from their
+size, to come in contact with the wool of sheep or the hair of cattle;
+while seeds of this kind never occur on forest trees, on aquatic plants,
+or even on very dwarf creepers or trailers. The winged seed-vessels or
+seeds, on the other hand, mostly belong to trees and to tall shrubs or
+climbers. We have, therefore, a very exact adaptation to conditions in
+these different modes of dispersal; while, when we come to consider
+individual cases, we find innumerable other adaptations, some of which
+the reader will find described in the little work by Sir John Lubbock
+already referred to.
+
+
+_Edible or Attractive Fruits._
+
+It is, however, when we come to true fruits (in a popular sense) that we
+find varied colours evidently intended to attract animals, in order that
+the fruits may be eaten, while the seeds pass through the body
+undigested and are then in the fittest state for germination. This end
+has been gained in a great variety of ways, and with so many
+corresponding adaptations as to leave no doubt as to the value of the
+result. Fruits are pulpy or juicy, and usually sweet, and form the
+favourite food of innumerable birds and some mammals. They are always
+coloured so as to contrast with the foliage or surroundings, red being
+the most common as it is certainly the most conspicuous colour, but
+yellow, purple, black, or white being not uncommon. The edible portion
+of fruits is developed from different parts of the floral envelopes, or
+of the ovary, in the various orders and genera. Sometimes the calyx
+becomes enlarged and fleshy, as in the apple and pear tribe; more often
+the integuments of the ovary itself are enlarged, as in the plum, peach,
+grape, etc.; the receptacle is enlarged and forms the fruit of the
+strawberry; while the mulberry, pineapple, and fig are examples of
+compound fruits formed in various ways from a dense mass of flowers.
+
+In all cases the seeds themselves are protected from injury by various
+devices. They are small and hard in the strawberry, raspberry, currant,
+etc., and are readily swallowed among the copious pulp. In the grape
+they are hard and bitter; in the rose (hip) disagreeably hairy; in the
+orange tribe very bitter; and all these have a smooth, glutinous
+exterior which facilitates their being swallowed. When the seeds are
+larger and are eatable, they are enclosed in an excessively hard and
+thick covering, as in the various kinds of "stone" fruit (plums,
+peaches, etc.), or in a very tough core, as in the apple. In the nutmeg
+of the Eastern Archipelago we have a curious adaptation to a single
+group of birds. The fruit is yellow, somewhat like an oval peach, but
+firm and hardly eatable. This splits open and shows the glossy black
+covering of the seed or nutmeg, over which spreads the bright scarlet
+arillus or "mace," an adventitious growth of no use to the plant except
+to attract attention. Large fruit pigeons pluck out this seed and
+swallow it entire for the sake of the mace, while the large nutmeg
+passes through their bodies and germinates; and this has led to the wide
+distribution of wild nutmegs over New Guinea and the surrounding
+islands.
+
+In the restriction of bright colour to those edible fruits the eating of
+which is beneficial to the plant, we see the undoubted result of natural
+selection; and this is the more evident when we find that the colour
+never appears till the fruit is ripe--that is, till the seeds within it
+are fully matured and in the best state for germination. Some
+brilliantly coloured fruits are poisonous, as in our bitter-sweet
+(Solanum dulcamara), cuckoo-pint (Arum) and the West Indian manchineel.
+Many of these are, no doubt, eaten by animals to whom they are harmless;
+and it has been suggested that even if some animals are poisoned by them
+the plant is benefited, since it not only gets dispersed, but finds, in
+the decaying body of its victim, a rich manure heap.[141] The particular
+colours of fruits are not, so far as we know, of any use to them other
+than as regards conspicuousness, hence a tendency to _any_ decided
+colour has been preserved and accumulated as serving to render the fruit
+easily visible among its surroundings of leaves or herbage. Out of 134
+fruit-bearing plants in Mongredien's _Trees and Shrubs_, and Hooker's
+_British Flora_, the fruits of no less than sixty-eight, or rather more
+than half, are red, forty-five are black, fourteen yellow, and seven
+white. The great prevalence of red fruits is almost certainly due to
+their greater conspicuousness having favoured their dispersal, though it
+may also have arisen in part from the chemical changes of chlorophyll
+during ripening and decay producing red tints as in many fading leaves.
+Yet the comparative scarcity of yellow in fruits, while it is the most
+common tint of fading leaves, is against this supposition.
+
+There are, however, a few instances of coloured fruits which do not seem
+to be intended to be eaten; such are the colocynth plant (Cucumis
+colocynthus), which has a beautiful fruit the size and colour of an
+orange, but nauseous beyond description to the taste. It has a hard
+rind, and may perhaps be dispersed by being blown along the ground, the
+colour being an adventitious product; but it is quite possible,
+notwithstanding its repulsiveness to us, that it may be eaten by some
+animals. With regard to the fruit of another plant, Calotropis procera,
+there is less doubt, as it is dry and full of thin, flat-winged seeds,
+with fine silky filaments, eminently adapted for wind-dispersal; yet it
+is of a bright yellow colour, as large as an apple, and therefore very
+conspicuous. Here, therefore, we seem to have colour which is a mere
+byproduct of the organism and of no use to it; but such cases are
+exceedingly rare, and this rarity, when compared with the great
+abundance of cases in which there is an obvious purpose in the colour,
+adds weight to the evidence in favour of the theory of the attractive
+coloration of edible fruits in order that birds and other animals may
+assist in their dispersal. Both the above-named plants are natives of
+Palestine and the adjacent arid countries.[142]
+
+
+_The Colours of Flowers._
+
+Flowers are much more varied in their colours than fruits, as they are
+more complex and more varied in form and structure; yet there is some
+parallelism between them in both respects. Flowers are frequently
+adapted to attract insects as fruits are to attract birds, the object
+being in the former to secure cross-fertilisation, in the latter
+dispersal; while just as colour is an index of the edibility of fruits
+which supply pulp or juice to birds, so are the colours of flowers an
+indication of the presence of nectar or of pollen which are devoured by
+insects.
+
+The main facts and many of the details, as to the relation of insects to
+flowers, were discovered by Sprengel in 1793. He noticed the curious
+adaptation of the structure of many flowers to the particular insects
+which visit them; he proved that insects do cross-fertilise flowers, and
+he believed that this was the object of the adaptations, while the
+presence of nectar and pollen ensured the continuance of their visits;
+yet he missed discovering the _use_ of this cross-fertilisation. Several
+writers at a later period obtained evidence that cross-fertilisation of
+plants was a benefit to them; but the wide generality of this fact and
+its intimate connection with the numerous and curious adaptations
+discovered by Sprengel, was first shown by Mr. Darwin, and has since
+been demonstrated by a vast mass of observations, foremost among which
+are his own researches on orchids, primulas, and other plants.[143]
+
+By an elaborate series of experiments carried on for many years Mr.
+Darwin demonstrated the great value of cross-fertilisation in increasing
+the rapidity of growth, the strength and vigour of the plant, and in
+adding to its fertility. This effect is produced immediately, not as he
+expected would be the case, after several generations of crosses. He
+planted seeds from cross-fertilised and self-fertilised plants on two
+sides of the same pot exposed to exactly similar conditions, and in most
+cases the difference in size and vigour was amazing, while the plants
+from cross-fertilised parents also produced more and finer seeds. These
+experiments entirely confirmed the experience of breeders of animals
+already referred to (p. 160), and led him to enunciate his famous
+aphorism, "Nature abhors perpetual self-fertilisation".[144] In this
+principle we appear to have a sufficient reason for the various
+contrivances by which so many flowers secure cross-fertilisation, either
+constantly or occasionally. These contrivances are so numerous, so
+varied, and often so highly complex and extraordinary, that they have
+formed the subject of many elaborate treatises, and have also been amply
+popularised in lectures and handbooks. It will be unnecessary,
+therefore, to give details here, but the main facts will be summarised
+in order to call attention to some difficulties of the theory which seem
+to require further elucidation.
+
+
+_Modes of securing Cross-Fertilisation._
+
+When we examine the various modes in which the cross-fertilisation of
+flowers is brought about, we find that some are comparatively simple in
+their operation and needful adjustments, others highly complex. The
+simple methods belong to four principal classes:--(1) By dichogamy--that
+is, by the anthers and the stigma becoming mature or in a fit state for
+fertilisation at slightly different times on the same plant. The result
+of this is that, as plants in different stations, on different soils, or
+exposed to different aspects flower earlier or later, the mature pollen
+of one plant can only fertilise some plant exposed to somewhat different
+conditions or of different constitution, whose stigma will be mature at
+the same time; and this difference has been shown by Darwin to be that
+which is adapted to secure the fullest benefit of cross-fertilisation.
+This occurs in Geranium pratense, Thymus serpyllum, Arum maculatum, and
+many others. (2) By the flower being self-sterile with its own pollen,
+as in the crimson flax. This absolutely prevents self-fertilisation. (3)
+By the stamens and anthers being so placed that the pollen cannot fall
+upon the stigma, while it does fall upon a visiting insect which carries
+it to the stigma of another flower. This effect is produced in a variety
+of very simple ways, and is often aided by the motion of the stamens
+which bend down out of the way of the stigmas before the pollen is ripe,
+as in Malva sylvestris (see Fig. 28). (4) By the male and female flowers
+being on different plants, forming the class Dioecia of Linnaeus. In
+these cases the pollen may be carried to the stigmas either by the wind
+or by the agency of insects.
+
+[Illustration: FIG. 28.
+
+Malva sylvestris, adapted for insect-fertilisation.
+
+Malva rotundifolia, adapted for self-fertilisation.]
+
+Now these four methods are all apparently very simple, and easily
+produced by variation and selection. They are applicable to flowers of
+any shape, requiring only such size and colour as to attract insects,
+and some secretion of nectar to ensure their repeated visits, characters
+common to the great majority of flowers. All these methods are common,
+except perhaps the second; but there are many flowers in which the
+pollen from another plant is prepotent over the pollen from
+fertilisation, the same flower, and this has nearly the same effect as
+self-sterility if the flowers are frequently crossed by insects. We
+cannot help asking, therefore, why have other and much more elaborate
+methods been needed? And how have the more complex arrangements of so
+many flowers been brought about? Before attempting to answer these
+questions, and in order that the reader may appreciate the difficulty of
+the problem and the nature of the facts to be explained, it will be
+necessary to give a summary of the more elaborate modes of securing
+cross-fertilisation.
+
+(1) We first have dimorphism and heteromorphism, the phenomena of which
+have been already sketched in our seventh chapter.
+
+Here we have both a mechanical and a physiological modification, the
+stamens and pistil being variously modified in length and position,
+while the different stamens in the same flower have widely different
+degrees of fertility when applied to the same stigma,--a phenomenon
+which, if it were not so well established, would have appeared in the
+highest degree improbable. The most remarkable case is that of the three
+different forms of the loosestrife (Lythrum salicaria) here figured
+(Fig. 29 on next page).
+
+(2) Some flowers have irritable stamens which, when their bases are
+touched by an insect, spring up and dust it with pollen. This occurs in
+our common berberry.
+
+[Illustration: FIG. 29.--Lythrum salicaria (Purple loosestrife).]
+
+(3) In others there are levers or processes by which the anthers are
+mechanically brought down on to the head or back of an insect entering
+the flower, in such a position as to be carried to the stigma of the
+next flower it visits. This may be well seen in many species of Salvia
+and Erica.
+
+(4) In some there is a sticky secretion which, getting on to the
+proboscis of an insect, carries away the pollen, and applies it to the
+stigma of another flower. This occurs in our common milkwort (Polygala
+vulgaris).
+
+(5) In papilionaceous plants there are many complex adjustments, such as
+the squeezing out of pollen from a receptacle on to an insect, as in
+Lotus corniculatus, or the sudden springing out and exploding of the
+anthers so as thoroughly to dust the insect, as in Medicago falcata,
+this occurring after the stigma has touched the insect and taken off
+some pollen from the last flower.
+
+(6) Some flowers or spathes form closed boxes in which insects find
+themselves entrapped, and when they have fertilised the flower, the
+fringe of hairs opens and allows them to escape. This occurs in many
+species of Arum and Aristolochia.
+
+(7) Still more remarkable are the traps in the flower of Asclepias which
+catch flies, butterflies, and wasps by the legs, and the wonderfully
+complex arrangements of the orchids. One of these, our common Orchis
+pyramidalis, may be briefly described to show how varied and beautiful
+are the arrangements to secure cross-fertilisation. The broad trifid lip
+of the flower offers a support to the moth which is attracted by its
+sweet odour, and two ridges at the base guide the proboscis with
+certainty to the narrow entrance of the nectary. When the proboscis has
+reached the end of the spur, its basal portion depresses the little
+hinged rostellum that covers the saddle-shaped sticky glands to which
+the pollen masses (pollinia) are attached. On the proboscis being
+withdrawn, the two pollinia stand erect and parallel, firmly attached to
+the proboscis. In this position, however, they would be useless, as they
+would miss the stigmatic surface of the next flower visited by the moth.
+But as soon as the proboscis is withdrawn, the two pollen masses begin
+to diverge till they are exactly as far apart as are the stigmas of the
+flower; and then commences a second movement which brings them down
+till they project straight forward nearly at right angles to their first
+position, so as exactly to hit against the stigmatic surfaces of the
+next flower visited on which they leave a portion of their pollen. The
+whole of these motions take about half a minute, and in that time the
+moth will usually have flown to another plant, and thus effect the most
+beneficial kind of cross-fertilisation.[145] This description will be
+better understood by referring to the illustration opposite, from
+Darwin's _Fertilisation of Orchids_(Fig. 30).
+
+[Illustration: FIG. 30.--Orchis pyramidalis.]
+
+
+_The Interpretation of these Facts._
+
+Having thus briefly indicated the general character of the more complex
+adaptations for cross-fertilisation, the details of which are to be
+found in any of the numerous works on the subject,[146] we find
+ourselves confronted with the very puzzling question--Why were these
+innumerable highly complex adaptations produced, when the very same
+result may be effected--and often is effected--by extremely simple
+means? Supposing, as we must do, that all flowers were once of simple
+and regular forms, like a buttercup or a rose, how did such irregular
+and often complicated flowers as the papilionaceous or pea family, the
+labiates or sage family, and the infinitely varied and fantastic orchids
+ever come into existence? No cause has yet been suggested but the need
+of attracting insects to cross-fertilise them; yet the attractiveness of
+regular flowers with bright colours and an ample supply of nectar is
+equally great, and cross-fertilisation can be quite as effectively
+secured in these by any of the four simple methods already described.
+Before attempting to suggest a possible solution of this difficult
+problem, we have yet to pass in review a large body of curious
+adaptations connected with insect fertilisation, and will first call
+attention to that portion of the phenomena which throw some light upon
+the special colours of flowers in their relation to the various kinds of
+insects which visit them. For these facts we are largely indebted to
+the exact and long-continued researches of Professor Hermann Mueller.
+
+
+_Summary of Additional Facts bearing on Insect Fertilisation._
+
+1. That the size and colour of a flower are important factors in
+determining the visits of insects, is shown by the general fact of more
+insects visiting conspicuous than inconspicuous flowers. As a single
+instance, the handsome Geranium palustre was observed by Professor
+Mueller to be visited by sixteen different species of insects, the
+equally showy G. pratense by thirteen species, while the smaller and
+much less conspicuous G. molle was visited by eight species, and G.
+pusillum by only one. In many cases, however, a flower may be very
+attractive to only a few species of insects; and Professor Mueller
+states, as the result of many years' assiduous observation, that "a
+species of flower is the more visited by insects the more conspicuous it
+is."
+
+2. Sweet odour is usually supplementary to the attraction of colour.
+Thus it is rarely present in the largest and most gaudily coloured
+flowers which inhabit open places, such as poppies, paeonies,
+sunflowers, and many others; while it is often the accompaniment of
+inconspicuous flowers, as the mignonette; of such as grow in shady
+places, as the violet and primrose; and especially of white or yellowish
+flowers, as the white jasmine, clematis, stephanotis, etc.
+
+3. White flowers are often fertilised by moths, and very frequently give
+out their scent only by night, as in our butterfly-orchis (Habenaria
+chlorantha); and they sometimes open only at night, as do many of the
+evening primroses and other flowers. These flowers are often long tubed
+in accordance with the length of the moths' probosces, as in the genus
+Pancratium, our butterfly orchis, white jasmine, and a host of others.
+
+4. Bright red flowers are very attractive to butterflies, and are
+sometimes specially adapted to be fertilised by them, as in many pinks
+(Dianthus deltoides, D. superbus, D. atrorubens), the corn-cockle
+(Lychnis Githago), and many others. Blue flowers are especially
+attractive to bees and other hymenoptera (though they frequent flowers
+of all colours), no less than sixty-seven species of this order having
+been observed to visit the common "sheep's-bit" (Jasione montana). Dull
+yellow or brownish flowers, some of which smell like carrion, are
+attractive to flies, as the Arum and Aristolochia; while the dull
+purplish flowers of the Scrophularia are specially attractive to wasps.
+
+5. Some flowers have neither scent nor nectar, and yet attract insects
+by sham nectaries! In the herb-paris (Paris quadrifolia) the ovary
+glistens as if moist, and flies alight on it and carry away pollen to
+another flower; while in grass of parnassus (Parnassia palustris) there
+are a number of small stalked yellow balls near the base of the flower,
+which look like drops of honey but are really dry. In this case there is
+a little nectar lower down, but the special attraction is a sham; and as
+there are fresh broods of insects every year, it takes time for them to
+learn by experience, and thus enough are always deceived to effect
+cross-fertilisation.[147] This is analogous to the case of the young
+birds, which have to learn by experience the insects that are inedible,
+as explained at page 253.
+
+6. Many flowers change their colour as soon as fertilised; and this is
+beneficial, as it enables bees to avoid wasting time in visiting those
+blossoms which have been already fertilised and their nectar exhausted.
+The common lungwort (Pulmonaria officinalis), is at first red, but later
+turns blue; and H. Mueller observed bees visiting many red flowers in
+succession, but neglecting the blue. In South Brazil there is a species
+of Lantana, whose flowers are yellow the first day, orange the second,
+and purple the third; and Dr. Fritz Mueller observed that many
+butterflies visited the yellow flowers only, some both the yellow and
+the orange flowers, but none the purple.
+
+7. Many flowers have markings which serve as guides to insects; in some
+cases a bright central eye, as in the borage and forget-me-not; or lines
+or spots converging to the centre, as in geraniums, pinks, and many
+others. This enables insects to go quickly and directly to the opening
+of the flower, and is equally important in aiding them to obtain a
+better supply of food, and to fertilise a larger number of flowers.
+
+8. Flowers have been specially adapted to the kinds of insects that
+most abound where they grow. Thus the gentians of the lowlands are
+adapted to bees, those of the high alps to butterflies only; and while
+most species of Rhinanthus (a genus to which our common "yellow rattle"
+belongs) are bee-flowers, one high alpine species (R. alpinus) has been
+also adapted for fertilisation by butterflies only. The reason of this
+is, that in the high alps butterflies are immensely more plentiful than
+bees, and flowers adapted to be fertilised by bees can often have their
+nectar extracted by butterflies without effecting cross-fertilisation.
+It is, therefore, important to have a modification of structure which
+shall make butterflies the fertilisers, and this in many cases has been
+done.[148]
+
+9. Economy of time is very important both to the insects and the
+flowers, because the fine working days are comparatively few, and if no
+time is wasted the bees will get more honey, and in doing so will
+fertilise more flowers. Now, it has been ascertained by several
+observers that many insects, bees especially, keep to one kind of flower
+at a time, visiting hundreds of blossoms in succession, and passing over
+other species that may be mixed with them. They thus acquire quickness
+in going at once to the nectar, and the change of colour in the flower,
+or incipient withering when fertilised, enables them to avoid those
+flowers that have already had their honey exhausted. It is probably to
+assist the insects in keeping to one flower at a time, which is of vital
+importance to the perpetuation of the species, that the flowers which
+bloom intermingled at the same season are usually very distinct both in
+form and colour. In the sandy districts of Surrey, in the early spring,
+the copses are gay with three flowers--the primrose, the wood-anemone,
+and the lesser celandine, forming a beautiful contrast, while at the
+same time the purple and the white dead-nettles abound on hedge banks. A
+little later, in the same copses, we have the blue wild hyacinth (Scilla
+nutans), the red campion (Lychnis dioica), the pure white great starwort
+(Stellaria Holosteum), and the yellow dead-nettle (Lamium Galeobdolon),
+all distinct and well-contrasted flowers. In damp meadows in summer we
+have the ragged robin (Lychnis Floscuculi), the spotted orchis (O.
+maculata), and the yellow rattle (Rhinanthus Crista-galli); while in
+drier meadows we have cowslips, ox-eye daisies, and buttercups, all very
+distinct both in form and colour. So in cornfields we have the scarlet
+poppies, the purple corn-cockle, the yellow corn-marygold, and the blue
+cornflower; while on our moors the purple heath and the dwarf gorse make
+a gorgeous contrast. Thus the difference of colour which enables the
+insect to visit with rapidity and unerring aim a number of flowers of
+the same kind in succession, serves to adorn our meadows, banks, woods,
+and heaths with a charming variety of floral colour and form at each
+season of the year.[149]
+
+
+_Fertilisation of Flowers by Birds._
+
+In the temperate regions of the Northern Hemisphere, insects are the
+chief agents in cross-fertilisation when this is not effected by the
+wind; but in warmer regions, and in the Southern hemisphere, birds are
+found to take a considerable part in the operation, and have in many
+cases led to modifications in the form and colour of flowers. Each part
+of the globe has special groups of birds which are flower-haunters.
+America has the humming-birds (Trochilidae), and the smaller group of
+the sugar-birds (Caerebidae). In the Eastern tropics the sun-birds
+(Nectarineidae) take the place of the humming-birds, and another small
+group, the flower-peckers (Dicaeidae), assist them. In the Australian
+region there are also two flower-feeding groups, the Meliphagidae, or
+honey-suckers, and the brush-tongued lories (Trichoglossidae). Recent
+researches by American naturalists have shown that many flowers are
+fertilised by humming-birds, such as passion-flowers, trumpet-flowers,
+fuchsias, and lobelias; while some, as the Salvia splendens of Mexico,
+are specially adapted to their visits. We may thus perhaps explain the
+number of very large tubular flowers in the tropics, such as the huge
+brugmansias and bignonias; while in the Andes and in Chile, where
+humming-birds are especially plentiful, we find great numbers of red
+tubular flowers, often of large size and apparently adapted to these
+little creatures. Such are the beautiful Lapageria and Philesia, the
+grand Pitcairneas, and the genera Fuchsia, Mitraria, Embothrium,
+Escallonia, Desfontainea, Eccremocarpus, and many Gesneraceae. Among the
+most extraordinary modifications of flower structure adapted to bird
+fertilisation are the species of Marcgravia, in which the pedicels and
+bracts of the terminal portion of a pendent bunch of flowers have been
+modified into pitchers which secrete nectar and attract insects, while
+birds feeding on the nectar, or insects, have the pollen of the
+overhanging flowers dusted on their backs, and, carrying it to other
+flowers, thus cross-fertilise them (see Illustration).
+
+[Illustration: FIG. 31.--Humming-bird fertilising Marcgravia
+nepenthoides.]
+
+In Australia and New Zealand the fine "glory peas" (Clianthus), the
+Sophora, Loranthus, many Epacrideae and Myrtaceae, and the large flowers
+of the New Zealand flax (Phormium tenax), are cross-fertilised by
+birds; while in Natal the fine trumpet-creeper (Tecoma capensis) is
+fertilised by Nectarineas.
+
+The great extent to which insect and bird agency is necessary to flowers
+is well shown by the case of New Zealand. The entire country is
+comparatively poor in species of insects, especially in bees and
+butterflies which are the chief flower fertilisers; yet according to the
+researches of local botanists no less than one-fourth of all the
+flowering plants are incapable of self-fertilisation, and, therefore,
+wholly dependent on insect or bird agency for the continuance of the
+species.
+
+The facts as to the cross-fertilisation of flowers which have now been
+very briefly summarised, taken in connection with Darwin's experiments
+proving the increased vigour and fertility given by cross-fertilisation,
+seem amply to justify his aphorism that "Nature abhors
+self-fertilisation," and his more precise statement, that, "No plant is
+perpetually self-fertilised;" and this view has been upheld by
+Hildebrand, Delpino, and other botanists.[150]
+
+
+_Self-Fertilisation of Flowers._
+
+But all this time we have been only looking at one side of the question,
+for there exists an abundance of facts which seem to imply, just as
+surely, the utter uselessness of cross-fertilisation. Let us, then, see
+what these facts are before proceeding further.
+
+1. An immense variety of plants are habitually self-fertilised, and
+their numbers probably far exceed those which are habitually
+cross-fertilised by insects. Almost all the very small or obscure
+flowered plants with hermaphrodite flowers are of this kind. Most of
+these, however, may be insect fertilised occasionally, and may,
+therefore, come under the rule that no species are perpetually
+self-fertilised.
+
+2. There are many plants, however, in which special arrangements exist
+to secure self-fertilisation. Sometimes the corolla closes and brings
+the anthers and stigma into contact; in others the anthers cluster round
+the stigmas, both maturing together, as in many buttercups, stitchwort
+(Stellaria media), sandwort (Spergula), and some willow-herbs
+(Epilobium); or they arch over the pistil, as in Galium aparine and
+Alisma Plantago. The style is also modified to bring it into contact
+with the anthers, as in the dandelion, groundsel, and many other
+plants.[151] All these, however, may be occasionally cross-fertilised.
+
+3. In other cases precautions are taken to prevent cross-fertilisation,
+as in the numerous cleistogamous or closed flowers. These occur in no
+less than fifty-five different genera, belonging to twenty-four natural
+orders, and in thirty-two of these genera the normal flowers are
+irregular, and have therefore been specially modified for insect
+fertilisation.[152] These flowers appear to be degradations of the
+normal flowers, and are closed up by various modifications of the petals
+or other parts, so that it is impossible for insects to reach the
+interior, yet they produce seed in abundance, and are often the chief
+means by which the species is continued. Thus, in our common dog-violet
+the perfect flowers rarely produce seed, while the rudimentary
+cleistogamic flowers do so in abundance. The sweet violet also produces
+abundance of seed from its cleistogamic flowers, and few from its
+perfect flowers; but in Liguria it produces only perfect flowers which
+seed abundantly. No case appears to be known of a plant which has
+cleistogamic flowers only, but a small rush (Juncus bufonius) is in this
+condition in some parts of Russia, while in other parts perfect flowers
+are also produced.[153] Our common henbit dead-nettle (Lamium
+amplexicaule) produces cleistogamic flowers, as do also some orchids.
+The advantage gained by the plant is great economy of specialised
+material, since with very small flowers and very little expenditure of
+pollen an abundance of seed is produced.
+
+4. A considerable number of plants which have evidently been specially
+modified for insect fertilisation have, by further modification, become
+quite self-fertile. This is the case with the garden-pea, and also with
+our beautiful bee-orchis, in which the pollen-masses constantly fall on
+to the stigmas, and the flower, being thus self-fertilised, produces
+abundance of capsules and of seed. Yet in many of its close allies
+insect agency is absolutely required; but in one of these, the
+fly-orchis, comparatively very little seed is produced, and
+self-fertilisation would therefore be advantageous to it. When
+garden-peas were artificially cross-fertilised by Mr. Darwin, it seemed
+to do them no good, as the seeds from these crosses produced less
+vigorous plants than seed from those which were self-fertilised; a fact
+directly opposed to what usually occurs in cross-fertilised plants.
+
+5. As opposed to the theory that there is any absolute need for
+cross-fertilisation, it has been urged by Mr. Henslow and others that
+many self-fertilised plants are exceptionally vigorous, such as
+groundsel, chickweed, sow-thistle, buttercups, and other common weeds;
+while most plants of world-wide distribution are self-fertilised, and
+these have proved themselves to be best fitted to survive in the battle
+of life. More than fifty species of common British plants are very
+widely distributed, and all are habitually self-fertilised.[154] That
+self-fertilisation has some great advantage is shown by the fact that it
+is usually the species which have the smallest and least conspicuous
+flowers which have spread widely, while the large and showy flowered
+species of the same genera or families, which require insects to
+cross-fertilise them, have a much more limited distribution.
+
+6. It is now believed by some botanists that many inconspicuous and
+imperfect flowers, including those that are wind-fertilised, such as
+plantains, nettles, sedges, and grasses, do not represent primitive or
+undeveloped forms, but are degradations from more perfect flowers which
+were once adapted to insect fertilisation. In almost every order we find
+some plants which have become thus reduced or degraded for wind or
+self-fertilisation, as Poterium and Sanguisorba among the Rosaceae;
+while this has certainly been the case in the cleistogamic flowers. In
+most of the above-mentioned plants there are distinct rudiments of
+petals or other floral organs, and as the chief use of these is to
+attract insects, they could hardly have existed in primitive
+flowers.[155] We know, moreover, that when the petals cease to be
+required for the attraction of insects, they rapidly diminish in size,
+lose their bright colour or almost wholly disappear.[156]
+
+
+_Difficulties and Contradictions._
+
+The very bare summary that has now been given of the main facts relating
+to the fertilisation of flowers, will have served to show the vast
+extent and complexity of the inquiry, and the extraordinary
+contradictions and difficulties which it presents. We have direct proof
+of the beneficial results of intercrossing in a great number of cases;
+we have an overwhelming mass of facts as to the varied and complex
+structure of flowers evidently adapted to secure this intercrossing by
+insect agency; yet we see many of the most vigorous plants which spread
+widely over the globe, with none of these adaptations, and evidently
+depending on self-fertilisation for their continued existence and
+success in the battle of life. Yet more extraordinary is it to find
+numerous cases in which the special arrangements for cross-fertilisation
+appear to have been a failure, since they have either been supplemented
+by special means for self-fertilisation, or have reverted back in
+various degrees to simpler forms in which self-fertilisation becomes the
+rule. There is also a further difficulty in the highly complex modes by
+which cross-fertilisation is often brought about; for we have seen that
+there are several very effective yet very simple modes of securing
+intercrossing, involving a minimum of change in the form and structure
+of the flower; and when we consider that the result attained with so
+much cost of structural modification is by no means an unmixed good, and
+is far less certain in securing the perpetuation of the species than is
+self-fertilisation, it is most puzzling to find such complex methods
+resorted to, sometimes to the extent of special precautions against the
+possibility of self-fertilisation ever taking place. Let us now see
+whether any light can be thrown on these various anomalies and
+contradictions.
+
+
+_Intercrossing not necessarily Advantageous._
+
+No one was more fully impressed than Mr. Darwin with the beneficial
+effects of intercrossing on the vigour and fertility of the species or
+race, yet he clearly saw that it was not always and necessarily
+advantageous. He says: "The most important conclusion at which I have
+arrived is, that the mere act of intercrossing by itself does no good.
+The good depends on the individuals which are crossed differing slightly
+in constitution, owing to their progenitors having been subjected during
+several generations to slightly different conditions. This conclusion,
+as we shall hereafter see, is closely connected with various important
+physiological problems, such as the benefit derived from slight changes
+in the conditions of life."[157] Mr. Darwin has also adduced much direct
+evidence proving that slight changes in the conditions of life are
+beneficial to both animals and plants, maintaining or restoring their
+vigour and fertility in the same way as a favourable cross seems to
+restore it.[158] It is, I believe, by a careful consideration of these
+two classes of facts that we shall find the clue to the labyrinth in
+which this subject has appeared to involve us.
+
+
+_Supposed Evil Results of Close Interbreeding._
+
+Just as we have seen that intercrossing is not necessarily good, we
+shall be forced to admit that close interbreeding is not necessarily
+bad. Our finest breeds of domestic animals have been thus produced, and
+by a careful statistical inquiry Mr. George Darwin has shown that the
+most constant and long-continued intermarriages among the British
+aristocracy have produced no prejudicial results. The rabbits on Porto
+Santo are all the produce of a single female; they have lived on the
+same small island for 470 years, and they still abound there and appear
+to be vigorous and healthy (see p. 161).
+
+We have, however, on the other hand, overwhelming evidence that in many
+cases, among our domestic animals and cultivated plants, close
+interbreeding does produce bad results, and the apparent contradiction
+may perhaps be explained on the same general principles, and under
+similar limitations, as were found to be necessary in defining the value
+of intercrossing. It appears probable, then, that it is not
+interbreeding in itself that is hurtful, but interbreeding without
+rigid selection or some change of conditions. Under nature, as in the
+case of the Porto Santo rabbits, the rapid increase of these animals
+would in a very few years stock the island with a full population, and
+thereafter natural selection would act powerfully in the preservation
+only of the healthiest and the most fertile, and under these conditions
+no deterioration would occur. Among the aristocracy there has been a
+constant selection of beauty, which is generally synonymous with health,
+while any constitutional infertility has led to the extinction of the
+family. With domestic animals the selection practised is usually neither
+severe enough nor of the right kind. There is no natural struggle for
+existence, but certain points of form and colour characteristic of the
+breed are considered essential, and thus the most vigorous or the most
+fertile are not always those which are selected to continue the stock.
+In nature, too, the species always extends over a larger area and
+consists of much greater numbers, and thus a difference of constitution
+soon arises in different parts of the area, which is wanting in the
+limited numbers of pure bred domestic animals. From a consideration of
+these varied facts we conclude that an occasional disturbance of the
+organic equilibrium is what is essential to keep up the vigour and
+fertility of any organism, and that this disturbance may be equally well
+produced either by a cross between individuals of somewhat different
+constitutions, or by occasional slight changes in the conditions of
+life. Now plants which have great powers of dispersal enjoy a constant
+change of conditions, and can, therefore, exist permanently, or at all
+events, for very long periods, without intercrossing; while those which
+have limited powers of dispersal, and are restricted to a comparatively
+small and uniform area, need an occasional cross to keep up their
+fertility and general vigour. We should, therefore, expect that those
+groups of plants which are adapted both for cross-and
+self-fertilisation, which have showy flowers and possess great powers of
+seed-dispersal, would be the most abundant and most widely distributed;
+and this we find to be the case, the Compositae possessing all these
+characteristics in the highest degree, and being the most generally
+abundant group of plants with conspicuous flowers in all parts of the
+world.
+
+
+_How the Struggle for Existence Acts among Flowers._
+
+Let us now consider what will be the action of the struggle for
+existence under the conditions we have seen to exist.
+
+Everywhere and at all times some species of plants will be dominant and
+aggressive; while others will be diminishing in numbers, reduced to
+occupy a smaller area, and generally having a hard struggle to maintain
+themselves. Whenever a self-fertilising plant is thus reduced in numbers
+it will be in danger of extinction, because, being limited to a small
+area, it will suffer from the effects of too uniform conditions which
+will produce weakness and infertility. But while this change is in
+progress, any crosses between individuals of slightly different
+constitution will be beneficial, and all variations favouring either
+insect agency on the one hand, or wind-dispersal of pollen on the other,
+will lead to the production of a somewhat stronger and more fertile
+stock. Increased size or greater brilliancy of the flower, more abundant
+nectar, sweeter odour, or adaptations for more effectual
+cross-fertilisation would all be preserved, and thus would be initiated
+some form of specialisation for insect agency in cross-fertilisation;
+and in every different species so circumstanced the result would be
+different, depending as it would on many and complex combinations of
+variation of parts of the flower, and of the insect species which most
+abounded in the district.
+
+Species thus favourably modified might begin a new era of development,
+and, while spreading over a somewhat wider area, give rise to new
+varieties or species, all adapted in various degrees and modes to secure
+cross-fertilisation by insect agency. But in course of ages some change
+of conditions might prove adverse. Either the insects required might
+diminish in numbers or be attracted by other competing flowers, or a
+change of climate might give the advantage to other more vigorous
+plants. Then self-fertilisation with greater means of dispersal might be
+more advantageous; the flowers might become smaller and more numerous;
+the seeds smaller and lighter so as to be more easily dispersed by the
+wind, while some of the special adaptations for insect fertilisation
+being useless would, by the absence of selection and by the law of
+economy of growth, be reduced to a rudimentary form. With these
+modifications the species might extend its range into new districts,
+thereby obtaining increased vigour by the change of conditions, as
+appears to have been the case with so many of the small flowered
+self-fertilised plants. Thus it might continue to exist for a long
+series of ages, till under other changes--geographical or biological--it
+might again suffer from competition or from other adverse circumstances,
+and be at length again confined to a limited area, or reduced to very
+scanty numbers.
+
+But when this cycle of change had taken place, the species would be very
+different from the original form. The flower would have been at one time
+modified to favour the visits of insects and to secure
+cross-fertilisation by their aid, and when the need for this passed
+away, some portions of these structures would remain, though in a
+reduced or rudimentary condition. But when insect agency became of
+importance a second time, the new modifications would start from a
+different or more advanced basis, and thus a more complex result might
+be produced. Owing to the unequal rates at which the reduction of the
+various parts might occur, some amount of irregularity in the flower
+might arise, and on a second development towards insect
+cross-fertilisation this irregularity, if useful, might be increased by
+variation and selection.
+
+The rapidity and comparative certainty with which such changes as are
+here supposed do really take place, are well shown by the great
+differences in floral structure, as regards the mode of fertilisation,
+in allied genera and species, and even in some cases in varieties of the
+same species. Thus in the Ranunculaceae we find the conspicuous part of
+the flower to be the petals in Ranunculus, the sepals in Helleborus,
+Anemone, etc., and the stamens in most species of Thalictrum. In all
+these we have a simple regular flower, but in Aquilegia it is made
+complex by the spurred petals, and in Delphinium and Aconitum it becomes
+quite irregular. In the more simple class self-fertilisation occurs
+freely, but it is prevented in the more complex flowers by the stamens
+maturing before the pistil. In the Caprifoliaceae we have small and
+regular greenish flowers, as in the moschatel (Adoxa); more conspicuous
+regular open flowers without honey, as in the elder (Sambucus); and
+tubular flowers increasing in length and irregularity, till in some,
+like our common honeysuckle, they are adapted for fertilisation by moths
+only, with abundant honey and delicious perfume to attract them. In the
+Scrophulariaceae we find open, almost regular flowers, as Veronica and
+Verbascum, fertilised by flies and bees, but also self-fertilised;
+Scrophularia adapted in form and colour to be fertilised by wasps; and
+the more complex and irregular flowers of Linaria, Rhinanthus,
+Melampyrum, Pedicularis, etc., mostly adapted to be fertilised by bees.
+
+In the genera Geranium, Polygonum, Veronica, and several others there is
+a gradation of forms from large and bright to small and obscure coloured
+flowers, and in every case the former are adapted for insect
+fertilisation, often exclusively, while in the latter self-fertilisation
+constantly occurs. In the yellow rattle (Rhinanthus Crista-galli) there
+are two forms (which have been named _major_ and _minor_), the larger
+and more conspicuous adapted to insect fertilisation only, the smaller
+capable of self-fertilisation; and two similar forms exist in the
+eyebright (Euphrasia officinalis). In both these cases there are special
+modifications in the length and curvature of the style as well as in the
+size and shape of the corolla; and the two forms are evidently becoming
+each adapted to special conditions, since in some districts the one, in
+other districts the other is most abundant.[159]
+
+These examples show us that the kind of change suggested above is
+actually going on, and has presumably always been going on in nature
+throughout the long geological epochs during which the development of
+flowers has been progressing. The two great modes of gaining increased
+vigour and fertility--intercrossing and dispersal over wider areas--have
+been resorted to again and again, under the pressure of a constant
+struggle for existence and the need for adaptation to ever-changing
+conditions. During all the modifications that ensued, useless parts were
+reduced or suppressed, owing to the absence of selection and the
+principle of economy of growth; and thus at each fresh adaptation some
+rudiments of old structures were re-developed, but not unfrequently in
+a different form and for a distinct purpose.
+
+The chief types of flowering plants have existed during the millions of
+ages of the whole tertiary period, and during this enormous lapse of
+time many of them may have been modified in the direction of insect
+fertilisation, and again into that of self-fertilisation, not once or
+twice only, but perhaps scores or even hundreds of times; and at each
+such modification a difference in the environment may have led to a
+distinct line of development. At one epoch the highest specialisation of
+structure in adaptation to a single species or group of insects may have
+saved a plant from extinction; while, at other times, the simplest mode
+of self-fertilisation, combined with greater powers of dispersal and a
+constitution capable of supporting diverse physical conditions, may have
+led to a similar result. With some groups the tendency seems to have
+been almost continuously to greater and greater specialisation, while
+with others a tendency to simplification and degradation has resulted in
+such plants as the grasses and sedges.
+
+We are now enabled dimly to perceive how the curious anomaly of very
+simple and very complex methods of securing cross-fertilisation--both
+equally effective--may have been brought about. The simple modes may be
+the result of a comparatively direct modification from the more
+primitive types of flowers, which were occasionally, and, as it were,
+accidentally visited and fertilised by insects; while the more complex
+modes, existing for the most part in the highly irregular flowers, may
+result from those cases in which adaptation to insect-fertilisation, and
+partial or complete degradation to self-fertilisation or to
+wind-fertilisation, have again and again recurred, each time producing
+some additional complexity, arising from the working up of old rudiments
+for new purposes, till there have been reached the marvellous flower
+structures of the papilionaceous tribes, of the asclepiads, or of the
+orchids.
+
+We thus see that the existing diversity of colour and of structure in
+flowers is probably the ultimate result of the ever-recurring struggle
+for existence, combined with the ever-changing relations between the
+vegetable and animal kingdoms during countless ages. The constant
+variability of every part and organ, with the enormous powers of
+increase possessed by plants, have enabled them to become again and
+again readjusted to each change of condition as it occurred, resulting
+in that endless variety, that marvellous complexity, and that exquisite
+colouring which excite our admiration in the realm of flowers, and
+constitute them the perennial charm and crowning glory of nature.
+
+
+_Flowers the Product of Insect Agency._
+
+In his _Origin of Species_, Mr. Darwin first stated that flowers had
+been rendered conspicuous and beautiful in order to attract insects,
+adding: "Hence we may conclude that, if insects had not been developed
+on the earth, our plants would not have been decked with beautiful
+flowers, but would have produced only such poor flowers as we see on our
+fir, oak, nut, and ash trees, on grasses, docks, and nettles, which are
+all fertilised through the agency of the wind." The argument in favour
+of this view is now much stronger than when he wrote; for not only have
+we reason to believe that most of these wind-fertilised flowers are
+degraded forms of flowers which have once been insect fertilised, but we
+have abundant evidence that whenever insect agency becomes comparatively
+ineffective, the colours of the flowers become less bright, their size
+and beauty diminish, till they are reduced to such small, greenish,
+inconspicuous flowers as those of the rupture-wort (Herniaria glabra),
+the knotgrass (Polygonum aviculare), or the cleistogamic flowers of the
+violet. There is good reason to believe, therefore, not only that
+flowers have been developed in order to attract insects to aid in their
+fertilisation, but that, having been once produced, in however great
+profusion, if the insect races were all to become extinct, flowers (in
+the temperate zones at all events) would soon dwindle away, and that
+ultimately all floral beauty would vanish from the earth.
+
+We cannot, therefore, deny the vast change which insects have produced
+upon the earth's surface, and which has been thus forcibly and
+beautifully delineated by Mr. Grant Allen: "While man has only tilled a
+few level plains, a few great river valleys, a few peninsular mountain
+slopes, leaving the vast mass of earth untouched by his hand, the insect
+has spread himself over every land in a thousand shapes, and has made
+the whole flowering creation subservient to his daily wants. His
+buttercup, his dandelion, and his meadow-sweet grow thick in every
+English field. His thyme clothes the hillside; his heather purples the
+bleak gray moorland. High up among the alpine heights his gentian
+spreads its lakes of blue; amid the snows of the Himalayas his
+rhododendrons gleam with crimson light. Even the wayside pond yields him
+the white crowfoot and the arrowhead, while the broad expanses of
+Brazilian streams are beautified by his gorgeous water-lilies. The
+insect has thus turned the whole surface of the earth into a boundless
+flower-garden, which supplies him from year to year with pollen or
+honey, and itself in turn gains perpetuation by the baits that it offers
+for his allurement."[160]
+
+
+_Concluding Remarks on Colour in Nature._
+
+In the last four chapters I have endeavoured to give a general and
+systematic, though necessarily condensed view of the part which is
+played by colour in the organic world. We have seen in what infinitely
+varied ways the need of concealment has led to the modification of
+animal colours, whether among polar snows or sandy deserts, in tropical
+forests or in the abysses of the ocean. We next find these general
+adaptations giving way to more specialised types of coloration, by which
+each species has become more and more harmonised with its immediate
+surroundings, till we reach the most curiously minute resemblances to
+natural objects in the leaf and stick insects, and those which are so
+like flowers or moss or birds' droppings that they deceive the acutest
+eye. We have learnt, further, that these varied forms of protective
+colouring are far more numerous than has been usually suspected,
+because, what appear to be very conspicuous colours or markings when the
+species is observed in a museum or in a menagerie, are often highly
+protective when the creature is seen under the natural conditions of its
+existence. From these varied classes of facts it seems not improbable
+that fully one-half of the species in the animal kingdom possess colours
+which have been more or less adapted to secure for them concealment or
+protection.
+
+Passing onward we find the explanation of a distinct type of colour or
+marking, often superimposed upon protective tints, in the importance of
+easy recognition by many animals of their fellows, their parents, or
+their mates. By this need we have been able to account for markings that
+seem calculated to make the animal conspicuous, when the general tints
+and well-known habits of the whole group demonstrate the need of
+concealment. Thus also we are able to explain the constant symmetry in
+the markings of wild animals, as well as the numerous cases in which the
+conspicuous colours are concealed when at rest and only become visible
+during rapid motion. In striking contrast to ordinary protective
+coloration we have "warning colours," usually very conspicuous and often
+brilliant or gaudy, which serve to indicate that their possessors are
+either dangerous or uneatable to the usual enemies of their tribe. This
+kind of coloration is probably more prevalent than has been hitherto
+supposed, because in the case of many tropical animals we are quite
+unacquainted with their special and most dangerous enemies, and are also
+unable to determine whether they are or are not distasteful to those
+enemies. As a kind of corollary to the "warning colours," we find the
+extraordinary phenomena of "mimicry," in which defenceless species
+obtain protection by being mistaken for those which, from any cause,
+possess immunity from attack. Although a large number of instances of
+warning colour and of mimicry are now recorded, it is probably still an
+almost unworked field of research, more especially in tropical regions
+and among the inhabitants of the ocean.
+
+The phenomena of sexual diversities of coloration next engaged our
+attention, and the reasons why Mr. Darwin's theory of "sexual
+selection," as regards colour and ornament, could not be accepted were
+stated at some length, together with the theory of animal coloration and
+ornament we propose to substitute for it. This theory is held to be in
+harmony with the general facts of animal coloration, while it entirely
+dispenses with the very hypothetical and inadequate agency of female
+choice in producing the detailed colours, patterns, and ornaments, which
+in so many cases distinguish the male sex.
+
+If my arguments on this point are sound, they will dispose also of Mr.
+Grant Allen's view of the direct action of the colour sense on the
+animal integuments.[161] He argues that the colours of insects and birds
+reproduce generally the colours of the flowers they frequent or the
+fruits they eat, and he adduces numerous cases in which flower-haunting
+insects and fruit-eating birds are gaily coloured. This he supposes to
+be due to the colour-taste, developed by the constant presence of bright
+flowers and fruits, being applied to the selection of each variation
+towards brilliancy in their mates; thus in time producing the gorgeous
+and varied hues they now possess. Mr. Allen maintains that "insects are
+bright where bright flowers exist in numbers, and dull where flowers are
+rare or inconspicuous;" and he urges that "we can hardly explain this
+wide coincidence otherwise than by supposing that a taste for colour is
+produced through the constant search for food among entomophilous
+blossoms, and that this taste has reacted upon its possessors through
+the action of unconscious sexual selection."
+
+The examples Mr. Allen quotes of bright insects being associated with
+bright flowers seem very forcible, but are really deceptive or
+erroneous; and quite as many cases could be quoted which prove the very
+opposite. For example, in the dense equatorial forests flowers are
+exceedingly scarce, and there is no comparison with the amount of floral
+colour to be met with in our temperate meadows, woods, and hillsides.
+The forests about Para in the lower Amazon are typical in this respect,
+yet they abound with the most gorgeously coloured butterflies, almost
+all of which frequent the forest depths, keeping near the ground, where
+there is the greatest deficiency of brilliant flowers. In contrast with
+this let us take the Cape of Good Hope--the most flowery region probably
+that exists upon the globe,--where the country is a complete
+flower-garden of heaths, pelargoniums, mesembryanthemus, exquisite
+iridaceous and other bulbs, and numerous flowering shrubs and trees; yet
+the Cape butterflies are hardly equal, either in number or variety, to
+those of any country in South Europe, and are utterly insignificant when
+compared with those of the comparatively flowerless forest-depths of the
+Amazon or of New Guinea. Neither is there any relation between the
+colours of other insects and their haunts. Few are more gorgeous than
+some of the tiger-beetles and the carabi, yet these are all carnivorous;
+while many of the most brilliant metallic buprestidae and longicorns are
+always found on the bark of fallen trees. So with the humming-birds;
+their brilliant metallic tints can only be compared with metals or gems,
+and are totally unlike the delicate pinks and purples, yellows and reds
+of the majority of flowers. Again, the Australian honey-suckers
+(Meliphagidae) are genuine flower-haunters, and the Australian flora is
+more brilliant in colour display than that of most tropical regions, yet
+these birds are, as a rule, of dull colours, not superior on the average
+to our grain-eating finches. Then, again, we have the grand pheasant
+family, including the gold and the silver pheasants, the gorgeous
+fire-backed and ocellated pheasants, and the resplendent peacock, all
+feeding on the ground on grain or seeds or insects, yet adorned with the
+most gorgeous colours.
+
+There is, therefore, no adequate basis of facts for this theory to rest
+upon, even if there were the slightest reason to believe that not only
+birds, but butterflies and beetles, take any delight in colour for its
+own sake, apart from the food-supply of which it indicates the presence.
+All that has been proved or that appears to be probable is, that they
+are able to perceive differences of colour, and to associate each colour
+with the particular flowers or fruits which best satisfy their wants.
+Colour being in its nature diverse, it has been beneficial for them to
+be able to distinguish all its chief varieties, as manifested more
+particularly in the vegetable kingdom, and among the different species
+of their own group; and the fact that certain species of insects show
+some preference for a particular colour may be explained by their having
+found flowers of that colour to yield them a more abundant supply of
+nectar or of pollen. In those cases in which butterflies frequent
+flowers of their own colour, the habit may well have been acquired from
+the protection it affords them.
+
+It appears to me that, in imputing to insects and birds the same love of
+colour for its own sake and the same aesthetic tastes as we ourselves
+possess, we may be as far from the truth as were those writers who held
+that the bee was a good mathematician, and that the honeycomb was
+constructed throughout to satisfy its refined mathematical instincts;
+whereas it is now generally admitted to be the result of the simple
+principle of economy of material applied to a primitive cylindrical
+cell.[162]
+
+In studying the phenomena of colour in the organic world we have been
+led to realise the wonderful complexity of the adaptations which bring
+each species into harmonious relation with all those which surround it,
+and which thus link together the whole of nature in a network of
+relations of marvellous intricacy. Yet all this is but, as it were, the
+outward show and garment of nature, behind which lies the inner
+structure--the framework, the vessels, the cells, the circulating
+fluids, and the digestive and reproductive processes,--and behind these
+again those mysterious chemical, electrical, and vital forces which
+constitute what we term Life. These forces appear to be fundamentally
+the same for all organisms, as is the material of which all are
+constructed; and we thus find behind the outer diversities an inner
+relationship which binds together the myriad forms of life.
+
+Each species of animal or plant thus forms part of one harmonious whole,
+carrying in all the details of its complex structure the record of the
+long story of organic development; and it was with a truly inspired
+insight that our great philosophical poet apostrophised the humble
+weed--
+
+
+    Flower in the crannied wall,
+    I pluck you out of the crannies,
+    I hold you here, root and all, in my hand,
+    Little flower--but _if_ I could understand
+    What you are, root and all, and all in all,
+    I should know what God and man is.
+
+
+FOOTNOTES:
+
+[Footnote 136: Burchell's _Travels_, vol. i. p. 10.]
+
+[Footnote 137: _Nature_, vol. iii. p. 507.]
+
+[Footnote 138: _Flowers, Fruits, and Leaves_, p. 128 (Fig. 79).]
+
+[Footnote 139: For a popular sketch of these, see Sir J. Lubbock's
+_Flowers, Fruits, and Leaves_, or any general botanical work.]
+
+[Footnote 140: _Nature_, vol. xv. p, 117.]
+
+[Footnote 141: Grant Allen's _Colour Sense_, p. 113.]
+
+[Footnote 142: Canon Tristram's _Natural History of the Bible_, pp. 483,
+484.]
+
+[Footnote 143: For a complete historical account of this subject with
+full references to all the works upon it, see the Introduction to
+Hermann Mueller's _Fertilisation of Flowers_, translated by D'Arcy W.
+Thompson.]
+
+[Footnote 144: For the full detail of his experiments, see _Cross-and
+Self-Fertilisation of Plants_, 1876.]
+
+[Footnote 145: See Darwin's _Fertilisation of Orchids_ for the many
+extraordinary and complex arrangements in these plants.]
+
+[Footnote 146: The English reader may consult Sir John Lubbock's
+_British Wild Flowers in Relation to Insects_, and H. Mueller's great and
+original work, _The Fertilisation of Flowers_.]
+
+[Footnote 147: Mueller's _Fertilisation of Flowers_, p. 248.]
+
+[Footnote 148: "Alpenblumen," by D.H. Mueller. See _Nature_, vol. xxiii.
+p. 333.]
+
+[Footnote 149: This peculiarity of local distribution of colour in
+flowers may be compared, as regards its purpose, with the recognition
+colours of animals. Just as these latter colours enable the sexes to
+recognise each other, and thus avoid sterile unions of distinct species,
+so the distinctive form and colour of each species of flower, as
+compared with those that usually grow around it, enables the fertilising
+insects to avoid carrying the pollen of one flower to the stigma of a
+distinct species.]
+
+[Footnote 150: See H. Mueller's _Fertilisation of Flowers_, p. 18.]
+
+[Footnote 151: The above examples are taken from Rev. G. Henslow's paper
+on "Self-Fertilisation of Plants," in _Trans. Linn. Soc._ Second series,
+_Botany_, vol. i. pp. 317-398, with plate. Mr. H.O. Forbes has shown
+that the same thing occurs among tropical orchids, in his paper "On the
+Contrivances for insuring Self-Fertilisation in some Tropical Orchids,"
+_Journ. Linn. Soc._, xxi. p. 538.]
+
+[Footnote 152: These are the numbers given by Darwin, but I am informed
+by Mr. Hemsley that many additions have been since made to the list, and
+that cleistogamic flowers probably occur in nearly all the natural
+orders.]
+
+[Footnote 153: For a full account of cleistogamic flowers, see Darwin's
+_Forms of Flowers_, chap. viii.]
+
+[Footnote 154: Henslow's "Self-Fertilisation," _Trans. Linn. Soc._
+Second series, _Botany_, vol. i. p. 391.]
+
+[Footnote 155: The Rev. George Henslow, in his _Origin of Floral
+Structures_, says: "There is little doubt but that all wind-fertilised
+angiosperms are degradations from insect-fertilised flowers....
+_Poterium sanguisorba_ is anemophilous; and _Sanguisorba officinalis_
+presumably was so formerly, but has reacquired an entomophilous habit;
+the whole tribe Poterieae being, in fact, a degraded group which has
+descended from Potentilleae. Plantains retain their corolla but in a
+degraded form. Junceae are degraded Lilies; while Cyperaceae and
+Gramineae among monocotyledons may be ranked with Amentiferae among
+dicotyledons, as representing orders which have retrograded very far
+from the entomophilous forms from which they were possibly and probably
+descended" (p. 266).
+
+"The genus Plantago, like _Thalictrum minus_, Poterium, and others, well
+illustrate the change from an entomophilous to the anemophilous state.
+_P. lanceolata_ has polymorphic flowers, and is visited by
+pollen-seeking insects, so that it can be fertilised either by insects
+or the wind. _P. media_ illustrates transitions in point of structure,
+as the filaments are pink, the anthers motionless, and the pollen grains
+aggregated, and it is regularly visited by _Bombus terrestris_. On the
+other hand, the slender filaments, versatile anthers, powdery pollen,
+and elongated protogynous style are features of other species indicating
+anemophily; while the presence of a degraded corolla shows its ancestors
+to have been entomophilous. _P. media_, therefore, illustrates, not a
+primitive entomophilous condition, but a return to it; just as is the
+case with _Sanguisorba officinalis_ and _Salix Caprea_; but these show
+no capacity of restoring the corolla, the attractive features having to
+be borne by the calyx, which is purplish in Sanguisorba, by the pink
+filaments of Plantago, and by the yellow anthers in the Sallow willow"
+(p. 271).
+
+"The interpretation, then, I would offer of inconspicuousness and all
+kinds of degradations is the exact opposite to that of conspicuousness
+and great differentiations; namely, that species with minute flowers,
+rarely or never visited by insects, and habitually self-fertilised, have
+primarily arisen through the neglect of insects, and have in consequence
+assumed their present floral structures" (p. 282).
+
+In a letter just received from Mr. Henslow, he gives a few additional
+illustrations of his views, of which the following are the most
+important: "Passing to Incompletae, the orders known collectively as
+'Cyclospermeae' are related to Caryophylleae; and to my mind are
+degradations from it, of which Orache is anemophilous. Cupuliferae have
+an inferior ovary and rudimentary calyx-limb on the top. These, as far
+as I know, cannot be interpreted except as degradations. The whole of
+Monocotyledons appear to me (from anatomical reasons especially) to be
+degradations from Dicotyledons, and primarily through the agency of
+growth in water. Many subsequently became terrestrial, but retained the
+effects of their primitive habitat through heredity. The 3-merous [sic]
+perianth of grasses, the parts of the flower being in whorls, point to a
+degradation from a sub-liliaceous condition."
+
+Mr. Henslow informs me that he has long held these views, but, as far as
+he knows, alone. Mr. Grant Allen, however, set forth a similar theory in
+his _Vignettes from Nature_ (p. 15) and more fully in _The Colours of
+Flowers_ (chap. v.), where he develops it fully and uses similar
+arguments to those of Mr. Henslow.]
+
+[Footnote 156: H. Mueller gives ample proof of this in his _Fertilisation
+of Flowers_.]
+
+[Footnote 157: _Cross-and Self-Fertilisation_, p. 27.]
+
+[Footnote 158: _Animals and Plants_, vol. ii. p. 145.]
+
+[Footnote 159: Mueller's _Fertilisation of Flowers_, pp. 448, 455. Other
+cases of recent degradation and readaptation to insect-fertilisation are
+given by Professor Henslow (see footnote, p. 324).]
+
+[Footnote 160: _The Colour Sense_, by Grant Allen, p. 95.]
+
+[Footnote 161: _The Colour Sense_, chap. ix.]
+
+[Footnote 162: See _Origin of Species_, sixth edition, p. 220.]
+
+
+
+
+CHAPTER XII
+
+THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
+
+
+    The facts to be explained--The conditions which have determined
+    distribution--The permanence of oceans--Oceanic and continental
+    areas--Madagascar and New Zealand--The teachings of the
+    thousand-fathom line--The distribution of marsupials--The
+    distribution of tapirs--Powers of dispersal as illustrated by
+    insular organisms--Birds and insects at sea--Insects at great
+    altitudes--The dispersal of plants--Dispersal of seeds by the
+    wind--Mineral matter carried by the wind--Objections to the
+    theory of wind-dispersal answered--Explanation of north
+    temperate plants in the southern hemisphere--No proof of
+    glaciation in the tropics--Lower temperature not needed to
+    explain the facts--Concluding remarks.
+
+
+
+The theory which we may now take as established--that all the existing
+forms of life have been derived from other forms by a natural process of
+descent with modification, and that this same process has been in action
+during past geological time--should enable us to give a rational account
+not only of the peculiarities of form and structure presented by animals
+and plants, but also of their grouping together in certain areas, and
+their general distribution over the earth's surface.
+
+In the absence of any exact knowledge of the facts of distribution, a
+student of the theory of evolution might naturally anticipate that all
+groups of allied organisms would be found in the same region, and that,
+as he travelled farther and farther from any given centre, the forms of
+life would differ more and more from those which prevailed at the
+starting-point, till, in the remotest regions to which he could
+penetrate, he would find an entirely new assemblage of animals and
+plants, altogether unlike those with which he was familiar. He would
+also anticipate that diversities of climate would always be associated
+with a corresponding diversity in the forms of life.
+
+Now these anticipations are to a considerable extent justified.
+Remoteness on the earth's surface is usually an indication of diversity
+in the fauna and flora, while strongly contrasted climates are always
+accompanied by a considerable contrast in the forms of life. But this
+correspondence is by no means exact or proportionate, and the converse
+propositions are often quite untrue. Countries which are near to each
+other often differ radically in their animal and vegetable productions;
+while similarity of climate, together with moderate geographical
+proximity, are often accompanied by marked diversities in the prevailing
+forms of life. Again, while many groups of animals--genera, families,
+and sometimes even orders--are confined to limited regions, most of the
+families, many genera, and even some species are found in every part of
+the earth. An enumeration of a few of these anomalies will better
+illustrate the nature of the problem we have to solve.
+
+As examples of extreme diversity, notwithstanding geographical
+proximity, we may adduce Madagascar and Africa, whose animal and
+vegetable productions are far less alike than are those of Great Britain
+and Japan at the remotest extremities of the great northern continent;
+while an equal, or perhaps even a still greater, diversity exists
+between Australia and New Zealand. On the other hand, Northern Africa
+and South Europe, though separated by the Mediterranean Sea, have faunas
+and floras which do not differ from each other more than do the various
+countries of Europe. As a proof that similarity of climate and general
+adaptability have had but a small part in determining the forms of life
+in each country, we have the fact of the enormous increase of rabbits
+and pigs in Australia and New Zealand, of horses and cattle in South
+America, and of the common sparrow in North America, though in none of
+these cases are the animals natives of the countries in which they
+thrive so well. And lastly, in illustration of the fact that allied
+forms are not always found in adjacent regions, we have the tapirs,
+which are found only on opposite sides of the globe, in tropical America
+and the Malayan Islands; the camels of the Asiatic deserts, whose
+nearest allies are the llamas and alpacas of the Andes; and the
+marsupials, only found in Australia and on the opposite side of the
+globe, in America. Yet, again, although mammalia may be said to be
+universally distributed over the globe, being found abundantly on all
+the continents and on a great many of the larger islands, yet they are
+entirely wanting in New Zealand, and in a considerable number of other
+islands which are, nevertheless, perfectly able to support them when
+introduced.
+
+Now most of these difficulties can be solved by means of well-known
+geographical and geological facts. When the productions of remote
+countries resemble each other, there is almost always continuity of land
+with similarity of climate between them. When adjacent countries differ
+greatly in their productions, we find them separated by a sea or strait
+whose great depth is an indication of its antiquity or permanence. When
+a group of animals inhabits two countries or regions separated by wide
+oceans, it is found that in past geological times the same group was
+much more widely distributed, and may have reached the countries it
+inhabits from an intermediate region in which it is now extinct. We
+know, also, that countries now united by land were divided by arms of
+the sea at a not very remote epoch; while there is good reason to
+believe that others now entirely isolated by a broad expanse of sea were
+formerly united and formed a single land area. There is also another
+important factor to be taken account of in considering how animals and
+plants have acquired their present peculiarities of
+distribution,--changes of climate. We know that quite recently a glacial
+epoch extended over much of what are now the temperate regions of the
+northern hemisphere, and that consequently the organisms which inhabit
+those parts must be, comparatively speaking, recent immigrants from more
+southern lands. But it is a yet more important fact that, down to middle
+Tertiary times at all events, an equable temperate climate, with a
+luxuriant vegetation, extended to far within the arctic circle, over
+what are now barren wastes, covered for ten months of the year with snow
+and ice. The arctic zone has, therefore, been in past times capable of
+supporting almost all the forms of life of our temperate regions; and we
+must take account of this condition of things whenever we have to
+speculate on the possible migrations of organisms between the old and
+new continents.
+
+
+_The Conditions which have determined Distribution._
+
+When we endeavour to explain in detail the facts of the existing
+distribution of organic beings, we are confronted by several preliminary
+questions, upon the solution of which will depend our treatment of the
+phenomena presented to us. Upon the theory of descent which we have
+adopted, all the different species of a genus, as well as all the genera
+which compose a family or higher group, have descended from some common
+ancestor, and must therefore, at some remote epoch, have occupied the
+same area, from which their descendants have spread to the regions they
+now inhabit. In the numerous cases in which the same group now occupies
+countries separated by oceans or seas, by lofty mountain-chains, by wide
+deserts, or by inhospitable climates, we have to consider how the
+migration which must certainly have taken place has been effected. It is
+possible that during some portion of the time which has elapsed since
+the origin of the group the interposing barriers have not been in
+existence; or, on the other hand, the particular organisms we are
+dealing with may have the power of overpassing the barriers, and thus
+reaching their present remote dwelling-places. As this is really the
+fundamental question of distribution on which the solution of all its
+more difficult problems depends, we have to inquire, in the first place,
+what is the nature of, and what are the limits to, the changes of the
+earth's surface, especially during the Tertiary and latter part of the
+Secondary periods, as it was during those periods that most of the
+existing types of the higher animals and plants came into existence;
+and, in the next place, what are the extreme limits of the powers of
+dispersal possessed by the chief groups of animals and plants. We will
+first consider the question of barriers, more especially those formed by
+seas and oceans.
+
+
+_The Permanence of Oceans._
+
+It was formerly a very general belief, even amongst geologists, that the
+great features of the earth's surface, no less than the smaller ones,
+were subject to continual mutations, and that during the course of
+known geological time the continents and great oceans had again and
+again changed places with each other. Sir Charles Lyell, in the last
+edition of his _Principles of Geology_ (1872), said: "Continents,
+therefore, although permanent for whole geological epochs, shift their
+positions entirely in the course of ages;" and this may be said to have
+been the orthodox opinion down to the very recent period when, by means
+of deep-sea soundings, the nature of the ocean bottom was made known.
+The first person to throw doubt on this view appears to have been the
+veteran American geologist, Professor Dana. In 1849, in the Report of
+Wilke's Exploring Expedition, he adduced the argument against a former
+continent in the Pacific during the Tertiary period, from the absence of
+all native quadrupeds. In 1856, in articles in the _American Journal_,
+he discussed the development of the American continent, and argued for
+its general permanence; and in his _Manual of Geology_ in 1863 and later
+editions, the same views were more fully enforced and were latterly
+applied to all continents. Darwin, in his _Journal of Researches_,
+published in 1845, called attention to the fact that all the small
+islands far from land in the Pacific, Indian, and Atlantic Oceans are
+either of coralline or volcanic formation. He excepted, however, the
+Seychelles and St. Paul's rocks; but the former have since been shown to
+be no exception, as they consist entirely of coral rock; and although
+Darwin himself spent a few hours on St. Paul's rocks on his outward
+voyage in the _Beagle_, and believed he had found some portions of them
+to be of a "cherty," and others of a "felspathic" nature, this also has
+been shown to be erroneous, and the careful examination of the rocks by
+the Abbe Renard clearly proves them to be wholly of volcanic
+origin.[163] We have, therefore, at the present time, absolutely no
+exception whatever to the remarkable fact that all the oceanic islands
+of the globe are either of volcanic or coral formation; and there is,
+further, good reason to believe that those of the latter class in every
+case rest upon a volcanic foundation.
+
+In his _Origin of Species_, Darwin further showed that no true oceanic
+island had any native mammals or batrachia when first discovered, this
+fact constituting the test of the class to which an island belongs;
+whence he argued that none of them had ever been connected with
+continents, but all had originated in mid-ocean. These considerations
+alone render it almost certain that the areas now occupied by the great
+oceans have never, during known geological time, been occupied by
+continents, since it is in the highest degree improbable that every
+fragment of those continents should have completely disappeared, and
+have been replaced by volcanic islands rising out of profound oceanic
+abysses; but recent research into the depth of the oceans and the nature
+of the deposits now forming on their floors, adds greatly to the
+evidence in this direction, and renders it almost a certainty that they
+represent very ancient if not primaeval features of the earth's surface.
+A very brief outline of the nature of this evidence will be now given.
+
+The researches of the _Challenger_ expedition into the nature of the
+sea-bottom show, that the whole of the land debris brought down by
+rivers to the ocean (with the exception of pumice and other floating
+matter), is deposited comparatively near to the shores, and that the
+fineness of the material is an indication of the distance to which it
+has been carried. Everything in the nature of gravel and sand is laid
+down within a very few miles of land, only the finer muddy sediments
+being carried out for 20 or 50 miles, and the very finest of all, under
+the most favourable conditions, rarely extending beyond 150, or at the
+utmost, 300 miles from land into the deep ocean.[164] Beyond these
+distances, and covering the entire ocean floor, are various oozes formed
+wholly from the debris of marine organisms; while intermingled with
+these are found various volcanic products which have been either carried
+through the air or floated on the surface, and a small but perfectly
+recognisable quantity of meteoric matter. Ice-borne rocks are also found
+abundantly scattered over the ocean bottom within a definite distance of
+the arctic and antarctic circles, clearly marking out the limit of
+floating icebergs in recent geological times.
+
+Now the whole series of marine stratified rocks, from the earliest
+Palaeozoic to the most recent Tertiary beds, consist of materials
+closely corresponding to the land debris now being deposited within a
+narrow belt round the shores of all continents; while no rocks have been
+found which can be identified with the various oozes now forming in the
+deep abysses of the ocean. It follows, therefore, that all the
+geological formations have been formed in comparatively shallow water,
+and always adjacent to the continental land of the period. The great
+thickness of some of the formations is no indication of a deep sea, but
+only of slow subsidence during the time that the deposition was in
+progress. This view is now adopted by many of the most experienced
+geologists, especially by Dr. Archibald Geikie, Director of the
+Geological Survey of Great Britain, who, in his lecture on "Geographical
+Evolution," says: "From all this evidence we may legitimately conclude
+that the present land of the globe, though consisting in great measure
+of marine formations, has never lain under the deep sea; but that its
+site must always have been near land. Even its thick marine limestones
+are the deposits of comparatively shallow water."[165]
+
+But besides these geological and physical considerations, there is a
+mechanical difficulty in the way of repeated change of position of
+oceans and continents which has not yet received the attention it
+deserves. According to the recent careful estimate by Mr. John Murray,
+the land area of the globe is to the water area as .28 to .72. The mean
+height of the land above sea-level is 2250 feet, while the mean depth of
+the ocean is 14,640 feet. Hence the bulk of dry land is 23,450,000 cubic
+miles, and that of the waters of the ocean 323,800,000 cubic miles; and
+it follows that if the whole of the solid matter of the earth's surface
+were reduced to one level, it would be everywhere covered by an ocean
+about two miles deep. The accompanying diagram will serve to render
+these figures more intelligible. The length of the sections of land and
+ocean are in the proportion of their respective areas, while the mean
+height of the land and the mean depth of the ocean are exhibited on a
+greatly increased vertical scale. If we considered the continents and
+their adjacent oceans separately they would differ a little, but not
+very materially, from this diagram; in some cases the proportion of land
+to ocean would be a little greater, in others a little less.
+
+[Illustration: FIG. 32.]
+
+Now, if we try to imagine a process of elevation and depression by which
+the sea and land shall completely change places, we shall be met by
+insuperable difficulties. We must, in the first place, assume a general
+equality between elevation and subsidence during any given period,
+because if the elevation over any extensive continental area were not
+balanced by some subsidence of approximately equal amount, an
+unsupported hollow would be left under the earth's crust. Let us now
+suppose a continental area to sink, and an adjacent oceanic area to
+rise, it will be seen that the greater part of the land will disappear
+long before the new land has approached the surface of the ocean. This
+difficulty will not be removed by supposing a portion of a continent to
+subside, and the immediately adjacent portion of the ocean on the other
+side of the continent to rise, because in almost every case we find that
+within a comparatively short distance from the shores of all existing
+continents, the ocean floor sinks rapidly to a depth of from 2000 to
+3000 fathoms, and maintains a similar depth, generally speaking, over a
+large portion of the oceanic areas. In order, therefore, that any area
+of continental extent be upraised from the great oceans, there must be a
+subsidence of a land area five or six times as great, unless it can be
+shown that an extensive elevation of the ocean floor up to and far
+above the surface could occur without an equivalent depression
+elsewhere. The fact that the waters of the ocean are sufficient to cover
+the whole globe to a depth of two miles, is alone sufficient to indicate
+that the great ocean basins are permanent features of the earth's
+surface, since any process of alternation of these with the land areas
+would have been almost certain to result again and again in the total
+disappearance of large portions, if not of all, of the dry land of the
+globe. But the continuity of terrestrial life since the Devonian and
+Carboniferous periods, and the existence of very similar forms in the
+corresponding deposits of every continent--as well as the occurrence of
+sedimentary rocks, indicating the proximity of land at the time of their
+deposit, over a large portion of the surface of all the continents, and
+in every geological period--assure us that no such disappearance has
+ever occurred.
+
+
+_Oceanic and Continental Areas._
+
+When we speak of the permanence of oceanic and continental areas as one
+of the established facts of modern research, we do not mean that
+existing continents and oceans have always maintained the exact areas
+and outlines that they now present, but merely, that while all of them
+have been undergoing changes in outline and extent from age to age, they
+have yet maintained substantially the same positions, and have never
+actually changed places with each other. There are, moreover, certain
+physical and biological facts which enable us to mark out these areas
+with some confidence.
+
+We have seen that there are a large number of islands which may be
+classed as oceanic, because they have never formed parts of continents,
+but have originated in mid-ocean, and have derived their forms of life
+by migration across the sea. Their peculiarities are seen to be very
+marked in comparison with those islands which there is good reason to
+believe are really fragments of more extensive land areas, and are hence
+termed "continental." These continental islands consist in every case of
+a variety of stratified rocks of various ages, thus corresponding
+closely with the usual structure of continents; although many of the
+islands are small like Jersey or the Shetland Islands, or far from
+continental land like the Falkland Islands or New Zealand. They all
+contain indigenous mammalia or batrachia, and generally a much greater
+variety of birds, reptiles, insects, and plants, than do the oceanic
+islands. From these various characteristics we conclude that they have
+all once formed parts of continents, or at all events of much larger
+land areas, and have become isolated, either by subsidence of the
+intervening land or by the effects of long-continued marine denudation.
+
+Now, if we trace the thousand-fathom line around all our existing
+continents we find that, with only two exceptions, every island which
+can be classed as "continental" falls within this line, while all that
+lie beyond it have the undoubted characteristics of "oceanic" islands.
+We, therefore, conclude that the thousand-fathom line marks out,
+approximately, the "continental area,"--that is, the limits within which
+continental development and change throughout known geological time have
+gone on. There may, of course, have been some extensions of land beyond
+this limit, while some areas within it may always have been ocean; but
+so far as we have any direct evidence, this line may be taken to mark
+out, approximately, the most probable boundary between the "continental
+area," which has always consisted of land and shallow sea in varying
+proportions, and the great oceanic basins, within the limits of which
+volcanic activity has been building up numerous islands, but whose
+profound depths have apparently undergone little change.
+
+
+_Madagascar and New Zealand._
+
+The two exceptions just referred to are Madagascar and New Zealand, and
+all the evidence goes to show that in these cases the land connection
+with the nearest continental area was very remote in time. The
+extraordinary isolation of the productions of Madagascar--almost all the
+most characteristic forms of mammalia, birds, and reptiles of Africa
+being absent from it--renders it certain that it must have been
+separated from that continent very early in the Tertiary, if not as far
+back as the latter part of the Secondary period; and this extreme
+antiquity is indicated by a depth of considerably more than a thousand
+fathoms in the Mozambique Channel, though this deep portion is less than
+a hundred miles wide between the Comoro Islands and the mainland.[166]
+Madagascar is the only island on the globe with a fairly rich mammalian
+fauna which is separated from a continent by a depth greater than a
+thousand fathoms; and no other island presents so many peculiarities in
+these animals, or has preserved so many lowly organised and archaic
+forms. The exceptional character of its productions agrees exactly with
+its exceptional isolation by means of a very deep arm of the sea.
+
+New Zealand possesses no known mammals and only a single species of
+batrachian; but its geological structure is perfectly continental. There
+is also much evidence that it does possess one mammal, although no
+specimens have been yet obtained.[167] Its reptiles and birds are highly
+peculiar and more numerous than in any truly oceanic island. Now the sea
+which directly separates New Zealand from Australia is more than 2000
+fathoms deep, but in a north-west direction there is an extensive bank
+under 1000 fathoms, extending to and including Lord Howe's Island, while
+north of this are other banks of the same depth, approaching towards a
+submarine extension of Queensland on the one hand, and New Caledonia on
+the other, and altogether suggestive of a land union with Australia at
+some very remote period. Now the peculiar relations of the New Zealand
+fauna and flora with those of Australia and of the tropical Pacific
+Islands to the northward indicate such a connection, probably during the
+Cretaceous period; and here, again, we have the exceptional depth of the
+dividing sea and the form of the ocean bottom according well with the
+altogether exceptional isolation of New Zealand, an isolation which has
+been held by some naturalists to be great enough to justify its claim to
+be one of the primary Zoological Regions.
+
+
+_The Teachings of the Thousand-Fathom Line._
+
+If now we accept the annexed map as showing us approximately how far
+beyond their present limits our continents may have extended during any
+portion of the Tertiary and Secondary periods, we shall obtain a
+foundation of inestimable value for our inquiries into those migrations
+of animals and plants during past ages which have resulted in their
+present peculiarities of distribution. We see, for instance, that the
+South American and African continents have always been separated by
+nearly as wide an ocean as at present, and that whatever similarities
+there may be in their productions must be due to the similar forms
+having been derived from a common origin in one of the great northern
+continents. The radical difference between the higher forms of life of
+the two continents accords perfectly with their permanent separation. If
+there had been any direct connection between them during Tertiary times,
+we should hardly have found the deep-seated differences between the
+Quadrumana of the two regions--no family even being common to both; nor
+the peculiar Insectivora of the one continent, and the equally peculiar
+Edentata of the other. The very numerous families of birds quite
+peculiar to one or other of these continents, many of which, by their
+structural isolation and varied development of generic and specific
+forms, indicate a high antiquity, equally suggest that there has been no
+near approach to a land connection during the same epoch.
+
+Looking to the two great northern continents, we see indications of a
+possible connection between them both in the North Atlantic and the
+North Pacific oceans; and when we remember that from middle Tertiary
+times backward--so far as we know continuously to the earliest
+Palaeozoic epoch--a temperate and equable climate, with abundant woody
+vegetation, prevailed up to and within the arctic circle, we see what
+facilities may have been afforded for migration from one continent to
+the other, sometimes between America and Europe, sometimes between
+America and Asia. Admitting these highly probable connections, no
+bridging of the Atlantic in more southern latitudes (of which there is
+not a particle of evidence) will have been necessary to account for all
+the intermigration that has occurred between the two continents. If, on
+the other hand, we remember how long must have been the route, and how
+diverse must always have been the conditions between the more northern
+and the more southern portions of the American and Euro-Asiatic
+continents, we shall not be surprised that many widespread forms in
+either continent have not crossed into the other; and that while the
+skunks (Mephitis), the pouched rats (Saccomyidae), and the turkeys
+(Meleagris) are confined to America, the pigs and the hedgehogs, the
+true flycatchers and the pheasants are found only in the Euro-Asiatic
+continent. But, just as there have been periods which facilitated
+intermigration between America and the Old World, there have almost
+certainly been periods, perhaps of long duration even geologically, when
+these continents have been separated by seas as wide as, or even wider
+than, those of the present day; and thus may be explained such curious
+anomalies as the origination of the camel-tribe in America, and its
+entrance into Asia in comparatively recent Tertiary times, while the
+introduction of oxen and bears into America from the Euro-Asiatic
+continent appears to have been equally recent.[168]
+
+We shall find on examination that this view of the general permanence of
+the oceanic and continental areas, with constant minor fluctuations of
+land and sea over the whole extent of the latter, enables us to
+understand, and offer a rational explanation of, most of the difficult
+problems of geographical distribution; and further, that our power of
+doing this is in direct proportion to our acquaintance with the
+distribution of fossil forms of life during the Tertiary period. We
+must, also, take due note of many other facts of almost equal importance
+for a due appreciation of the problems presented for solution, the most
+essential being, the various powers of dispersal possessed by the
+different groups of animals and plants, the geological antiquity of the
+species and genera, and the width and depth of the seas which separate
+the countries they, inhabit. A few illustrations will now be given of
+the way in which these branches of knowledge enable us to deal with the
+difficulties and anomalies that present themselves.
+
+
+_The Distribution of Marsupials._
+
+This singular and lowly organised type of mammals constitutes almost the
+sole representative of the class in Australia and New Guinea, while it
+is entirely unknown in Asia, Africa, or Europe. It reappears in America,
+where several species of opossums are found; and it was long thought
+necessary to postulate a direct southern connection of these distant
+countries, in order to account for this curious fact of distribution.
+When, however, we look to what is known of the geological history of the
+marsupials the difficulty vanishes. In the Upper Eocene deposits of
+Western Europe the remains of several animals closely allied to the
+American opossums have been found; and as, at this period, a very mild
+climate prevailed far up into the arctic regions, there is no difficulty
+in supposing that the ancestors of the group entered America from Europe
+or Northern Asia during early Tertiary times.
+
+But we must go much further back for the origin of the Australian
+marsupials. All the chief types of the higher mammalia were in existence
+in the Eocene, if not in the preceding Cretaceous period, and as we find
+none of these in Australia, that country must have been finally
+separated from the Asiatic continent during the Secondary or Mesozoic
+period. Now during that period, in the Upper and the Lower Oolite and in
+the still older Trias, the jaw-bones of numerous small mammalia have
+been found, forming eight distinct genera, which are believed to have
+been either marsupials or some allied lowly forms. In North America
+also, in beds of the Jurassic and Triassic formations, the remains of an
+equally great variety of these small mammalia have been discovered; and
+from the examination of more than sixty specimens, belonging to at least
+six distinct genera, Professor Marsh is of opinion that they represent a
+generalised type, from which the more specialised marsupials and
+insectivora were developed.
+
+From the fact that very similar mammals occur both in Europe and America
+at corresponding periods, and in beds which represent a long succession
+of geological time, and that during the whole of this time no fragments
+of any higher forms have been discovered, it seems probable that both
+the northern continents (or the larger portion of their area) were then
+inhabited by no other mammalia than these, with perhaps other equally
+low types. It was, probably, not later than the Jurassic age when some
+of these primitive marsupials were able to enter Australia, where they
+have since remained almost completely isolated; and, being free from
+the competition of higher forms, they have developed into the great
+variety of types we now behold there. These occupy the place, and have
+to some extent acquired the form and structure of distinct orders of the
+higher mammals--the rodents, the insectivora, and the carnivora,--while
+still preserving the essential characteristics and lowly organisation of
+the marsupials. At a much later period--probably in late Tertiary
+times--the ancestors of the various species of rats and mice which now
+abound in Australia, and which, with the aerial bats, constitute its
+only forms of placental mammals, entered the country from some of the
+adjacent islands. For this purpose a land connection was not necessary,
+as these small creatures might easily be conveyed among the branches or
+in the crevices of trees uprooted by floods and carried down to the sea,
+and then floated to a shore many miles distant. That no actual land
+connection with, or very close approximation to, an Asiatic island has
+occurred in recent times, is sufficiently proved by the fact that no
+squirrel, pig, civet, or other widespread mammal of the Eastern
+hemisphere has been able to reach the Australian continent.
+
+
+_The Distribution of Tapirs._
+
+These curious animals form one of the puzzles of geographical
+distribution, being now confined to two very remote regions of the
+globe--the Malay Peninsula and adjacent islands of Sumatra and Borneo,
+inhabited by one species, and tropical America, where there are three or
+four species, ranging from Brazil to Ecuador and Guatemala. If we
+considered these living forms only, we should be obliged to speculate on
+enormous changes of land and sea in order that these tropical animals
+might have passed from one country to the other. But geological
+discoveries have rendered all such hypothetical changes unnecessary.
+During Miocene and Pliocene times tapirs abounded over the whole of
+Europe and Asia, their remains having been found in the tertiary
+deposits of France, India, Burmah, and China. In both North and South
+America fossil remains of tapirs occur only in caves and deposits of
+Post-Pliocene age, showing that they are comparatively recent immigrants
+into that continent. They perhaps entered by the route of Kamchatka and
+Alaska, where the climate, even now so much milder and more equable than
+on the north-east of America, might have been warm enough in late
+Pliocene times to have allowed the migration of these animals. In Asia
+they were driven southwards by the competition of numerous higher and
+more powerful forms, but have found a last resting-place in the swampy
+forests of the Malay region.
+
+
+_What these Facts Prove._
+
+Now these two cases, of the marsupials and the tapirs, are in the
+highest degree instructive, because they show us that, without any
+hypothetical bridging of deep oceans, and with only such changes of sea
+and land as are indicated by the extent of the comparatively shallow
+seas surrounding and connecting the existing continents, we are able to
+account for the anomaly of allied forms occurring only in remote and
+widely separated areas. These examples really constitute crucial tests,
+because, of all classes of animals, mammalia are least able to surmount
+physical barriers. They are obviously unable to pass over wide arms of
+the sea, while the necessity for constant supplies of food and water
+renders sandy deserts or snow-clad plains equally impassable. Then,
+again, the peculiar kinds of food on which alone many of them can
+subsist, and their liability to the attacks of other animals, put a
+further check upon their migrations. In these respects almost all other
+organisms have great advantages over mammals. Birds can often fly long
+distances, and can thus cross arms of the sea, deserts, or mountain
+ranges; insects not only fly, but are frequently carried great distances
+by gales of wind, as shown by the numerous cases of their visits to
+ships hundreds of miles from land. Reptiles, though slow of movement,
+have advantages in their greater capacity for enduring hunger or thirst,
+their power of resisting cold or drought in a state of torpidity, and
+they have also some facilities for migration across the sea by means of
+their eggs, which may be conveyed in crevices of timber or among masses
+of floating vegetable matter. And when we come to the vegetable kingdom,
+the means of transport are at their maximum, numbers of seeds having
+special adaptations for being carried by mammalia or birds, and for
+floating in the water, or through the air, while many are so small and
+so light that there is practically no limit to the distances they may be
+carried by gales and hurricanes.
+
+We may, therefore, feel quite certain that the means of distribution
+that have enabled the larger mammalia to reach the most remote regions
+from a common starting-point, will be at least as efficacious, and
+usually far more efficacious, with all other land animals and plants;
+and if in every case the existing distribution of this class can be
+explained on the theory of oceanic and continental permanence, with the
+limited changes of sea and land already referred to, no valid objections
+can be taken against this theory founded on anomalies of distribution in
+other orders. Yet nothing is more common than for students of this or
+that group to assort that the theory of oceanic permanence is quite
+inconsistent with the distribution of its various species and genera.
+Because a few Indian genera and closely allied species of birds are
+found in Madagascar, a land termed "Lemuria" has been supposed to have
+united the two countries during a comparatively recent geological epoch;
+while the similarity of fossil plants and reptiles, from the Permian and
+Miocene formations of India and South Africa, has been adduced as
+further evidence of this connection. But there are also genera of
+snakes, of insects, and of plants, common to Madagascar and South
+America only, which have been held to necessitate a direct land
+connection between these countries. These views evidently refute
+themselves, because any such land connections must have led to a far
+greater similarity in the productions of the several countries than
+actually exists, and would besides render altogether inexplicable the
+absence of all the chief types of African and Indian mammalia from
+Madagascar, and its marvellous individuality in every department of the
+organic world.[169]
+
+
+_Powers of Dispersal as illustrated by Insular Organisms._
+
+Having arrived at the conclusion that our existing oceans have remained
+practically unaltered throughout the Tertiary and Secondary periods of
+geology, and that the distribution of the mammalia is such as might
+have been brought about by their known powers of dispersal, and by such
+changes of land and sea as have probably or certainly occurred, we are,
+of course, restricted to similar causes to explain the much wider and
+sometimes more eccentric distribution of other classes of animals and of
+plants. In doing so, we have to rely partly on direct evidence of
+dispersal, afforded by the land organisms that have been observed far
+out at sea, or which have taken refuge on ships, as well as by the
+periodical visitants to remote islands; but very largely on indirect
+evidence, afforded by the frequent presence of certain groups on remote
+oceanic islands, which some ancestral forms must, therefore, have
+reached by transmission across the ocean from distant lands.
+
+
+_Birds._
+
+These vary much in their powers of flight, and their capability of
+traversing wide seas and oceans. Many swimming and wading birds can
+continue long on the wing, fly swiftly, and have, besides, the power of
+resting safely on the surface of the water. These would hardly be
+limited by any width of ocean, except for the need of food; and many of
+them, as the gulls, petrels, and divers, find abundance of food on the
+surface of the sea itself. These groups have a wide distribution
+_across_ the oceans; while waders--especially plovers, sandpipers,
+snipes, and herons--are equally cosmopolitan, travelling _along_ the
+coasts of all the continents, and across the narrow seas which separate
+them. Many of these birds seem unaffected by climate, and as the
+organisms on which they feed are equally abundant on arctic, temperate,
+and tropical shores, there is hardly any limit to the range even of some
+of the species.
+
+Land-birds are much more restricted in their range, owing to their
+usually limited powers of flight, their inability to rest on the surface
+of the sea or to obtain food from it, and their greater specialisation,
+which renders them less able to maintain themselves in the new countries
+they may occasionally reach. Many of them are adapted to live only in
+woods, or in marshes, or in deserts; they need particular kinds of food
+or a limited range of temperature; and they are adapted to cope only
+with the special enemies or the particular group of competitors among
+which they have been developed. Such birds as these may pass again and
+again to a new country, but are never able to establish themselves in
+it; and it is this organic barrier, as it is termed, rather than any
+physical barrier, which, in many cases, determines the presence of a
+species in one area and its absence from another. We must always
+remember, therefore, that, although the presence of a species in a
+remote oceanic island clearly proves that its ancestors must at one time
+have found their way there, the absence of a species does not prove the
+contrary, since it also may have reached the island, but have been
+unable to maintain itself, owing to the inorganic or organic conditions
+not being suitable to it. This general principle applies to all classes
+of organisms, and there are many striking illustrations of it. In the
+Azores there are eighteen species of land-birds which are permanent
+residents, but there are also several others which reach the islands
+almost every year after great storms, but have never been able to
+establish themselves. In Bermuda the facts are still more striking,
+since there are only ten species of resident birds, while no less than
+twenty other species of land-birds and more than a hundred species of
+waders and aquatics are frequent visitors, often in great numbers, but
+are never able to establish themselves. On the same principle we account
+for the fact that, of the many continental insects and birds that have
+been let loose, or have escaped from confinement, in this country,
+hardly one has been able to maintain itself, and the same phenomenon is
+still more striking in the case of plants. Of the thousands of hardy
+plants which grow easily in our gardens, very few have ever run wild,
+and when the experiment is purposely tried it invariably fails. Thus A.
+de Candolle informs us that several botanists of Paris, Geneva, and
+especially of Montpellier, have sown the seeds of many hundreds of
+species of exotic hardy plants, in what appeared to be the most
+favourable situations, but that in hardly a single case has any one of
+them become naturalised.[170] Still more, then, in plants than in
+animals the absence of a species does not prove that it has never
+reached the locality, but merely that it has not been able to maintain
+itself in competition with the native productions. In other cases, as
+we have seen, facts of an exactly opposite nature occur. The rat, the
+pig, and the rabbit, the water-cress, the clover, and many other plants,
+when introduced into New Zealand, nourish exceedingly, and even
+exterminate their native competitors; so that in these cases we may feel
+sure that the species in question did not exist in New Zealand simply
+because they had been unable to reach that country by their natural
+means of dispersal. I will now give a few cases, in addition to those
+recorded in my previous works, of birds and insects which have been
+observed far from any land.
+
+
+_Birds and Insects at Sea._
+
+Captain D. Fullarton of the ship _Timaru_ recorded in his log the
+occurrence of a great number of small land-birds about the ship on 15th
+March 1886, when in Lat. 48 deg. 31' N., Long. 8 deg. 16' W. He says: "A great
+many small land-birds about us; put about sixty into a coop, evidently
+tired out." And two days later, 17th March, "Over fifty of the birds
+cooped on 15th died, though fed. Sparrows, finches, water-wagtails, two
+small birds, name unknown, one kind like a linnet, and a large bird like
+a starling. In all there have been on board over seventy birds, besides
+some that hovered about us for some time and then fell into the sea
+exhausted." Easterly winds and severe weather were experienced at the
+time.[171] The spot where this remarkable flight of birds was met with
+is about 160 miles due west of Brest, and this is the least distance the
+birds must have been carried. It is interesting to note that the
+position of the ship is nearly in the line from the English and French
+coasts to the Azores, where, after great storms, so many bird stragglers
+arrive annually. These birds were probably blown out to sea during their
+spring migration along the south coast of England to Wales and Ireland.
+During the autumnal migration, however, great flocks of
+birds--especially starlings, thrushes, and fieldfares--have been
+observed every year flying out to sea from the west coast of Ireland,
+almost the whole of which must perish. At the Nash Lighthouse, in the
+Bristol Channel on the coast of Glamorganshire, an enormous number of
+small birds were observed on 3d September, including nightjars,
+buntings, white-throats, willow-wrens, cuckoos, house-sparrows, robins,
+wheatears, and blackbirds. These had probably crossed from
+Somersetshire, and had they been caught by a storm the larger portion of
+them must have been blown out to sea.[172]
+
+These facts enable us to account sufficiently well for the birds of
+oceanic islands, the number and variety of which are seen to be
+proportionate to their facilities for reaching the island and
+maintaining themselves in it. Thus, though more birds yearly reach
+Bermuda than the Azores, the number of residents in the latter islands
+is much larger, due to the greater extent of the islands, their number,
+and their more varied surface. In the Galapagos the land-birds are still
+more numerous, due in part to their larger area and greater proximity to
+the continent, but chiefly to the absence of storms, so that the birds
+which originally reached the islands have remained long isolated and
+have developed into many closely allied species adapted to the special
+conditions. All the species of the Galapagos but one are peculiar to the
+islands, while the Azores possess only one peculiar species, and Bermuda
+none--a fact which is clearly due to the continual immigration of fresh
+individuals keeping up the purity of the breed by intercrossing. In the
+Sandwich Islands, which are extremely isolated, being more than 2000
+miles from any continent or large island, we have a condition of things
+similar to what prevails in the Galapagos, the land-birds, eighteen in
+number, being all peculiar, and belonging, except one, to peculiar
+genera. These birds have probably all descended from three or four
+original types which reached the islands at some remote period, probably
+by means of intervening islets that have since disappeared. In St.
+Helena we have a degree of permanent isolation which has prevented any
+land-birds from reaching the island; for although its distance from the
+continent, 1100 miles, is not so great as in the case of the Sandwich
+Islands, it is situated in an ocean almost entirely destitute of small
+islands, while its position within the tropics renders it free from
+violent storms. Neither is there, on the nearest part of the coast of
+Africa, a perpetual stream of migrating birds like that which supplies
+the innumerable stragglers which every year reach Bermuda and the
+Azores.
+
+
+_Insects._
+
+Winged insects have been mainly dispersed in the same way as birds, by
+their power of flight, aided by violent or long-continued winds. Being
+so small, and of such low specific gravity, they are occasionally
+carried to still greater distances; and thus no islands, however remote,
+are altogether without them. The eggs of insects, being often deposited
+in borings or in crevices of timber, may have been conveyed long
+distances by floating trees, as may the larvae of those species which
+feed on wood. Several cases have been published of insects coming on
+board ships at great distances from land; and Darwin records having
+caught a large grasshopper when the ship was 370 miles from the coast of
+Africa, whence the insect had probably come.
+
+In the _Entomologists' Monthly Magazine_ for June 1885, Mr. MacLachlan
+has recorded the occurrence of a swarm of moths in the Atlantic ocean,
+from the log of the ship _Pleione_. The vessel was homeward bound from
+New Zealand, and in Lat. 6 deg. 47' N., Long. 32 deg. 50' W., hundreds of moths
+appeared about the ship, settling in numbers on the spars and rigging.
+The wind for four days previously had been very light from north,
+north-west, or north-east, and sometimes calm. The north-east trade wind
+occasionally extends to the ship's position at that time of year. The
+captain adds that "frequently, in that part of the ocean, he has had
+moths and butterflies come on board." The position is 960 miles
+south-west of the Cape Verde Islands, and about 440 north-east of the
+South American coast. The specimen preserved is Deiopeia pulchella, a
+very common species in dry localities in the Eastern tropics, and rarely
+found in Britain, but, Mr. MacLachlan thinks, not found in South
+America. They must have come, therefore, from the Cape Verde Islands, or
+from some parts of the African coast, and must have traversed about a
+thousand miles of ocean with the assistance, no doubt, of a strong
+north-east trade wind for a great part of the distance. In the British
+Museum collection there is a specimen of the same moth caught at sea
+during the voyage of the _Rattlesnake_, in Lat. 6 deg. N., Long. 22-1/2 deg.
+W., being between the former position and Sierra Leone, thus rendering
+it probable that the moths came from that part of the African coast, in
+which case the swarm encountered by the _Pleione_ must have travelled
+more than 1200 miles.
+
+A similar case was recorded by Mr. F.A. Lucas in the American periodical
+_Science_ of 8th April 1887. He states that in 1870 he met with numerous
+moths of many species while at sea in the South Atlantic (Lat. 25 deg. S.,
+Long. 24 deg. W.), about 1000 miles from the coast of Brazil. As this
+position is just beyond the south-east trades, the insects may have been
+brought from the land by a westerly gale. In the _Zoologist_ (1864, p.
+8920) is the record of a small longicorn beetle which flew on board a
+ship 500 miles off the west coast of Africa. Numerous other cases are
+recorded of insects at less distances from land, and, taken in
+connection with those already given, they are sufficient to show that
+great numbers must be continually carried out to sea, and that
+occasionally they are able to reach enormous distances. But the
+reproductive powers of insects are so great that all we require, in
+order to stock a remote island, is that some few specimens shall reach
+it even once in a century, or once in a thousand years.
+
+
+_Insects at great Altitudes._
+
+Equally important is the proof we possess that insects are often carried
+to great altitudes by upward currents of air. Humboldt noticed them up
+to heights of 15,000 and 18,000 feet in South America, and Mr. Albert
+Mueller has collected many interesting cases of the same character in
+Europe.[173] A moth (Plusia gamma) has been found on the summit of Mont
+Blanc; small hymenoptera and moths have been seen on the Pyrenees at a
+height of 11,000 feet, while numerous flies and beetles, some of
+considerable size, have been caught on the glaciers and snow-fields of
+various parts of the Alps. Upward currents of air, whirlwinds and
+tornadoes, occur in all parts of the world, and large numbers of insects
+are thus carried up into the higher regions of the atmosphere, where
+they are liable to be caught by strong winds, and thus conveyed enormous
+distances over seas or continents. With such powerful means of
+dispersal the distribution of insects over the entire globe, and their
+presence in the most remote oceanic islands, offer no difficulties.
+
+
+_The Dispersal of Plants._
+
+The dispersal of seeds is effected in a greater variety of ways than are
+available in the case of any animals. Some fruits or seed-vessels, and
+some seeds, will float for many weeks, and after immersion in salt water
+for that period the seeds will often germinate. Extreme cases are the
+double cocoa-nut of the Seychelles, which has been found on the coast of
+Sumatra, about 3000 miles distant; the fruits of the Sapindus saponaria
+(soap-berry), which has been brought to Bermuda by the Gulf Stream from
+the West Indies, and has grown after a journey in the sea of about 1500
+miles; and the West Indian bean, Entada scandens, which reached the
+Azores from the West Indies, a distance of full 3000 miles, and
+afterwards germinated at Kew. By these means we can account for the
+similarity in the shore flora of the Malay Archipelago and most of the
+islands of the Pacific; and from an examination of the fruits and seeds,
+collected among drift during the voyage of the _Challenger_, Mr. Hemsley
+has compiled a list of 121 species which are probably widely dispersed
+by this means.
+
+A still larger number of species owe their dispersal to birds in several
+distinct ways. An immense number of fruits in all parts of the world are
+devoured by birds, and have been attractively coloured (as we have
+seen), in order to be so devoured, because the seeds pass through the
+birds' bodies and germinate where they fall. We have seen how frequently
+birds are forced by gales of wind across a wide expanse of ocean, and
+thus seeds must be occasionally carried. It is a very suggestive fact,
+that all the trees and shrubs in the Azores bear berries or small fruits
+which are eaten by birds; while all those which bear larger fruits, or
+are eaten chiefly by mammals--such as oaks, beeches, hazels, crabs,
+etc.--are entirely wanting. Game-birds and waders often have portions of
+mud attached to their feet, and Mr. Darwin has proved by experiment that
+such mud frequently contains seeds. One partridge had such a quantity of
+mud attached to its foot as to contain seeds from which eighty-two
+plants germinated; this proves that a very small portion of mud may
+serve to convey seeds, and such an occurrence repeated even at long
+intervals may greatly aid in stocking remote islands with vegetation.
+Many seeds also adhere to the feathers of birds, and thus, again, may be
+conveyed as far as birds are ever carried. Dr. Guppy found a small hard
+seed in the gizzard of a Cape Petrel, taken about 550 miles east of
+Tristan da Cunha.
+
+
+_Dispersal of Seeds by the Wind._
+
+In the preceding cases we have been able to obtain direct evidence of
+transportal; but although we know that many seeds are specially adapted
+to be dispersed by the wind, we cannot obtain direct proof that they are
+so carried for hundreds or thousands of miles across the sea, owing to
+the difficulty of detecting single objects which are so small and
+inconspicuous. It is probable, however, that the wind as an agent of
+dispersal is really more effective than any of those we have hitherto
+considered, because a very large number of plants have seeds which are
+very small and light, and are often of such a form as to facilitate
+aerial carriage for enormous distances. It is evident that such seeds
+are especially liable to be transported by violent winds, because they
+become ripe in autumn at the time when storms are most prevalent, while
+they either lie upon the surface of the ground, or are disposed in dry
+capsules on the plant ready to be blown away. If inorganic particles
+comparable in weight, size, or form with such seeds are carried for
+great distances, we may be sure that seeds will also be occasionally
+carried in the same way. It will, therefore, be necessary to give a few
+examples of wind-carriage of small objects.
+
+On 27th July 1875 a remarkable shower of small pieces of hay occurred at
+Monkstown, near Dublin. They appeared floating slowly down from a great
+height, as if falling from a dark cloud which hung overhead. The pieces
+picked up were wet, and varied from single blades of grass to tufts
+weighing one or two ounces. A similar shower occurred a few days earlier
+in Denbighshire, and was observed to travel in a direction contrary to
+that of the wind in the lower atmosphere.[174] There is no evidence of
+the distance from which the hay was brought, but as it had been carried
+to a great height, it was in a position to be conveyed to almost any
+distance by a violent wind, had such occurred at the time.
+
+
+_Mineral Matter carried by the Wind._
+
+The numerous cases of sand and volcanic dust being carried enormous
+distances through the atmosphere sufficiently prove the importance of
+wind as a carrier of solid matter, but unfortunately the matter
+collected has not been hitherto examined with a view to determine the
+maximum size and weight of the particles. A few facts, however, have
+been kindly furnished me by Professor Judd, F.R.S. Some dust which fell
+at Genoa on 15th October 1885, and was believed to have been brought
+from the African desert, consisted of quartz, hornblende, and other
+minerals, and contained particles having a diameter of 1/500 inch, each
+weighing 1/200,000 grain. This dust had probably travelled over 600
+miles. In the dust from Krakatoa, which fell at Batavia, about 100 miles
+distant, during the great eruption, there are many solid particles even
+larger than those mentioned above. Some of this dust was given me by
+Professor Judd, and I found in it several ovoid particles of a much
+larger size, being 1/50 inch long, and 1/70 wide and deep. The dust from
+the same eruption, which fell on board the ship _Arabella_, 970 miles
+from the volcano, also contained solid particles 1/500 inch diameter.
+Mr. John Murray of the _Challenger_ Expedition writes to me that he
+finds in the deep sea deposits 500 and even 700 miles west of the coast
+of Africa, rounded particles of quartz, having a diameter of 1/250 inch,
+and similar particles are found at equally great distances from the
+south-west coasts of Australia; and he considers these to be atmospheric
+dust carried to that distance by the wind. Taking the sp. gr. of quartz
+at 2.6, these particles would weigh about 1/25,000 grain each. These
+interesting facts can, however, by no means be taken as indicating the
+extreme limits of the power of wind in carrying solid particles. During
+the Krakatoa eruption no gale of special violence occurred, and the
+region is one of comparative calms. The grains of quartz found by Mr.
+Murray more nearly indicate the limit, but the very small portions of
+matter brought up by the dredge, as compared with the enormous areas of
+sea-bottom, over which the atmospheric dust must have been scattered,
+render it in the highest degree improbable that the maximum limit either
+of size of particles, or of distance from land has been reached.
+
+Let us, however, assume that the quartz grains, found by Mr. Murray in
+the deep-sea ooze 700 miles from land, give us the extreme limit of the
+power of the atmosphere as a carrier of solid particles, and let us
+compare with these the weights of some seeds. From a small collection of
+the seeds of thirty species of herbaceous plants sent me from Kew, those
+in the above table were selected, and small portions of eight of them
+carefully weighed in a chemical balance.[175] By counting these portions
+I was able to estimate the number of seeds weighing one grain. The three
+very minute species, whose numbers are marked with an asterisk (*), were
+estimated by the comparison of their sizes with those of the smaller
+weighed seeds.
+
+
+No|    Species.         |Approximate |      Approximate  |  Remarks.
+  |                     |No. of Seeds|      Dimensions.  |
+  |                     |In one Grain|                   |
+  |                     |            | in.    in.    in. |
+ 1|Draba verna          |  1,800     |1/60 x 1/90 x 1/160|Oval, flat.
+ 2|Hypericum perforatum |    520     |      1/30 x 1/80  |Cylindrical.
+ 3|Astilbe rivularis    |  4,500     |      1/50 x 1/100 |Elongate, flat, tailed,
+  |                     |            |                   |   wavy.
+ 4|Saxifraga coriophylla|    750     |      1/40 x 1/75  |Surface rough, adhere
+  |                     |            |                   |   to the dry capsules.
+ 5|Oenothera rosea      |    640     |  1/40 x 1/80      |Ovate.
+ 6|Hypericum hirsutum   |    700     |      1/30 x 1/100 |Cylindrical, rough.
+ 7|Mimulus luteus       |  2,900     |      1/60 x 1/100 |Oval, minute.
+ 8|Penthorum sedoides   |  8,000*    |      1/70 x 1/150 |Flattened, very minute.
+ 9|Sagina procumbens    | 12,000*    |          1/120    |Sub-triangular, flat.
+10|Orchis maculata      | 15,000*    |           ---     |Margined, flat,
+  |                     |            |                   | very minute.
+11|Gentiana purpurea    |     35     |           1/25    |Wavy, rough, with this
+  |                     |            |                   |   coriaceous margins.
+12|Silene alpina        |   ---      |           1/30    |Flat, with fringed
+  |                     |            |                   |   margins.
+13|Adenophora communis  |   ---      |       1/20 x 1/40 |Very thin, wavy, light.
+  |Quartz grains        | 25,000     |           1/250   |Deep sea ... 700 miles.
+  |Do.                  |200,000     |           1/500   |Genoa ... 600 miles.
+
+
+If now we compare the seeds with the quartz grains, we find that
+several are from twice to three times the weight of the grains found by
+Mr. Murray, and others five times, eight times, and fifteen times as
+heavy; but they are proportionately very much larger, and, being usually
+irregular in shape or compressed, they expose a very much larger surface
+to the air. The surface is often rough, and several have dilated margins
+or tailed appendages, increasing friction and rendering the uniform rate
+of falling through still air immensely less than in the case of the
+smooth, rounded, solid quartz grains. With these advantages it is a
+moderate estimate that seeds ten times the weight of the quartz grains
+could be carried quite as far through the air by a violent gale and
+under the most favourable conditions. These limits will include five of
+the seeds here given, as well as hundreds of others which do not exceed
+them in weight; and to these we may add some larger seeds which have
+other favourable characteristics, as is the case with numbers 11-13,
+which, though very much larger than the rest, are so formed as in all
+probability to be still more easily carried great distances by a gale of
+wind. It appears, therefore, to be absolutely certain that every
+autumnal gale capable of conveying solid mineral particles to great
+distances, must also carry numbers of small seeds at least as far; and
+if this is so, the wind alone will form one of the most effective agents
+in the dispersal of plants.
+
+Hitherto this mode of conveyance, as applying to the transmission of
+seeds for great distances across the ocean, has been rejected by
+botanists, for two reasons. In the first place, there is said to be no
+direct evidence of such conveyance; and, secondly, the peculiar plants
+of remote oceanic islands do not appear to have seeds specially adapted
+for aerial transmission. I will consider briefly each of these
+objections.
+
+
+_Objection to the Theory of Wind-Dispersal._
+
+To obtain direct evidence of the transmission of such minute and
+perishable objects, which do not exist in great quantities, and are
+probably carried to the greatest distances but rarely and as single
+specimens, is extremely difficult. A bird or insect can be seen if it
+comes on board ship, but who would ever detect the seeds of Mimulus or
+Orchis even if a score of them fell on a ship's deck? Yet if but one
+such seed per century were carried to an oceanic island, that island
+might become rapidly overrun by the plant, if the conditions were
+favourable to its growth and reproduction. It is further objected that
+search has been made for such seeds, and they have not been found.
+Professor Kerner of Innsbruck examined the snow on the surface of
+glaciers, and assiduously collected all the seeds he could find, and
+these were all of plants which grew in the adjacent mountains or in the
+same district. In like manner, the plants growing on moraines were found
+to be those of the adjacent mountains, plateaux, or lowlands. Hence he
+concluded that the prevalent opinion that seeds may be carried through
+the air for very great distances "is not supported by fact."[176] The
+opinion is certainly not supported by Kerner's facts, but neither is it
+opposed by them. It is obvious that the seeds that would be carried by
+the wind to moraines or to the surface of glaciers would be, first and
+in the greatest abundance, those of the immediately surrounding
+district; then, very much more rarely, those from more remote mountains;
+and lastly, in extreme rarity, those from distant countries or
+altogether distinct mountain ranges. Let us suppose the first to be so
+abundant that a single seed could be found by industrious search on each
+square yard of the surface of the glacier; the second so scarce that
+only one could possibly be found in a hundred yards square; while to
+find one of the third class it would be necessary exhaustively to
+examine a square mile of surface. Should we expect that _one_ ever to be
+found, and should the fact that it could not be found be taken as a
+proof that it was not there? Besides, a glacier is altogether in a bad
+position to receive such remote wanderers, since it is generally
+surrounded by lofty mountains, often range behind range, which would
+intercept the few air-borne seeds that might have been carried from a
+distant land. The conditions in an oceanic island, on the other hand,
+are the most favourable, since the land, especially if high, will
+intercept objects carried by the wind, and will thus cause more of the
+solid matter to fall on it than on an equal area of ocean. We know that
+winds at sea often blow violently for days together, and the rate of
+motion is indicated by the fact that 72 miles an hour was the average
+velocity of the wind observed during twelve hours at the Ben Nevis
+observatory, while the velocity sometimes rises to 120 miles an hour. A
+twelve hours' gale might, therefore, carry light seeds a thousand miles
+as easily and certainly as it could carry quartz-grains of much greater
+specific gravity, rotundity, and smoothness, 500 or even 100 miles; and
+it is difficult even to imagine a sufficient reason why they should not
+be so carried--perhaps very rarely and under exceptionally favourable
+conditions,--but this is all that is required.
+
+As regards the second objection, it has been observed that orchideae,
+which have often exceedingly small and light seeds, are remarkably
+absent from oceanic islands. This, however, may be very largely due to
+their extreme specialisation and dependence on insect agency for their
+fertilisation; while the fact that they do occur in such very remote
+islands as the Azores, Tahiti, and the Sandwich Islands, proves that
+they must have once reached these localities either by the agency of
+birds or by transmission through the air; and the facts I have given
+above render the latter mode at least as probable as the former. Sir
+Joseph Hooker remarks on the composite plant of Kerguelen Island (Cotula
+plumosa) being found also on Lord Auckland and MacQuarrie Islands, and
+yet having no pappus, while other species of the genus possess it. This
+is certainly remarkable, and proves that the plant must have, or once
+have had, some other means of dispersal across wide oceans.[177] One of
+the most widely dispersed species in the whole world (Sonchus oleraceus)
+possesses pappus, as do four out of five of the species which are common
+to Europe and New Zealand, all of which have a very wide distribution.
+The same author remarks on the limited area occupied by most species of
+Compositae, notwithstanding their facilities for dispersal by means of
+their feathered seeds; but it has been already shown that limitations
+of area are almost always due to the competition of allied forms,
+facilities for dispersal being only one of many factors in determining
+the wide range of species. It is, however, a specially important factor
+in the case of the inhabitants of remote oceanic islands, since, whether
+they are peculiar species or not, they or their remote ancestors must at
+some time or other have reached their present position by natural means.
+
+I have already shown elsewhere, that the flora of the Azores strikingly
+supports the view of the species having been introduced by aerial
+transmission only, that is, by the agency of birds and the wind, because
+all plants that could not possibly have been carried by these means are
+absent.[178] In the same way we may account for the extreme rarity of
+Leguminosae in all oceanic islands. Mr. Hemsley, in his Report on
+Insular Floras, says that they "are wanting in a large number of oceanic
+islands where there is no true littoral flora," as St. Helena, Juan
+Fernandez, and all the islands of the South Atlantic and South Indian
+Oceans. Even in the tropical islands, such as Mauritius and Bourbon,
+there are no endemic species, and very few in the Galapagos and the
+remoter Pacific Islands. All these facts are quite in accordance with
+the absence of facilities for transmission through the air, either by
+birds or the wind, owing to the comparatively large size and weight of
+the seeds; and an additional proof is thus afforded of the extreme
+rarity of the successful floating of seeds for great distances across
+the ocean.[179]
+
+
+_Explanation of North Temperate Plants in the Southern Hemisphere._
+
+If we now admit that many seeds which are either minute in size, of thin
+texture or wavy form, or so fringed or margined as to afford a good hold
+to the air, are capable of being carried for many hundreds of miles by
+exceptionally violent and long-continued gales of wind, we shall not
+only be better able to account for the floras of some of the remotest
+oceanic islands, but shall also find in the fact a sufficient
+explanation of the wide diffusion of many genera, and even species, of
+arctic and north temperate plants in the southern hemisphere or on the
+summits of tropical mountains. Nearly fifty of the flowering plants of
+Tierra-del-Fuego are found also in North America or Europe, but in no
+intermediate country; while fifty-eight species are common to New
+Zealand and Northern Europe; thirty-eight to Australia, Northern Europe,
+and Asia; and no less than seventy-seven common to New Zealand,
+Australia, and South America.[180] On lofty mountains far removed from
+each other, identical or closely allied plants often occur. Thus the
+fine Primula imperialis of a single mountain peak in Java has been found
+(or a closely allied species) in the Himalayas; and many other plants of
+the high mountains of Java, Ceylon, and North India are either identical
+or closely allied forms. So, in Africa, some species, found on the
+summits of the Cameroons and Fernando Po in West Africa, are closely
+allied to species in the Abyssinian highlands and in Temperate Europe;
+while other Abyssinian and Cameroons species have recently been found on
+the mountains of Madagascar. Some peculiar Australian forms have been
+found represented on the summit of Kini Balu in Borneo. Again, on the
+summit of the Organ mountains in Brazil there are species allied to
+those of the Andes, but not found in the intervening lowlands.
+
+
+_No Proof of Recent Lower Temperature in the Tropics._
+
+Now all these facts, and numerous others of like character, were
+supposed by Mr. Darwin to be due to a lowering of temperature during
+glacial epochs, which allowed these temperate forms to migrate across
+the intervening tropical lowlands. But any such change within the epoch
+of existing species is almost inconceivable. In the first place, it
+would necessitate the extinction of much of the tropical flora (and with
+it of the insect life), because without such extinction alpine
+herbaceous plants could certainly never spread over tropical forest
+lowlands; and, in the next place, there is not a particle of direct
+evidence that any such lowering of temperature in inter-tropical
+lowlands ever took place. The only alleged evidence of the kind is that
+adduced by the late Professor Agassiz and Mr. Hartt; but I am informed
+by my friend, Mr. J.C. Branner (now State Geologist of Arkansas, U.S.),
+who succeeded Mr. Hartt, and spent several years completing the
+geological survey of Brazil, that the supposed moraines and glaciated
+granite rocks near Rio Janeiro and elsewhere, as well as the so-called
+boulder-clay of the same region, are entirely explicable as the results
+of sub-aerial denudation and weathering, and that there is no proof
+whatever of glaciation in any part of Brazil.
+
+
+_Lower Temperature not needed to Explain the Facts._
+
+But any such vast physical change as that suggested by Darwin, involving
+as it does such tremendous issues as regards its effects on the tropical
+fauna and flora of the whole world, is really quite uncalled for,
+because the facts to be explained are of the same essential nature as
+those presented by remote oceanic islands, between which and the nearest
+continents no temperate land connection is postulated. In proportion to
+their limited area and extreme isolation, the Azores, St. Helena, the
+Galapagos, and the Sandwich Islands, each possess a fairly rich--the
+last a very rich--indigenous flora; and the means which sufficed to
+stock them with a great variety of plants would probably suffice to
+transmit others from mountain-top to mountain-top in various parts of
+the globe. In the case of the Azores, we have large numbers of species
+identical with those of Europe, and others closely allied, forming an
+exactly parallel case to the species found on the various mountain
+summits which have been referred to. The distances from Madagascar to
+the South African mountains and to Kilimandjaro, and from the latter to
+Abyssinia, are no greater than from Spain to the Azores, while there are
+other equatorial mountains forming stepping-stones at about an equal
+distance to the Cameroons. Between Java and the Himalayas we have the
+lofty mountains of Sumatra and of North-western Burma, forming steps at
+about the same distance apart; while between Kini Balu and the
+Australian Alps we have the unexplored snow mountains of New Guinea,
+the Bellenden Ker mountains in Queensland, and the New England and Blue
+Mountains of New South Wales. Between Brazil and Bolivia the distances
+are no greater; while the unbroken range of mountains from Arctic
+America to Tierra-del-Fuego offers the greatest facilities for
+transmission, the partial gap between the lofty peak of Chiriqui and the
+high Andes of New Grenada being far less than from Spain to the Azores.
+Thus, whatever means have sufficed for stocking oceanic islands must
+have been to some extent effective in transmitting northern forms from
+mountain to mountain, across the equator, to the southern hemisphere;
+while for this latter form of dispersal there are special facilities, in
+the abundance of fresh and unoccupied surfaces always occurring in
+mountain regions, owing to avalanches, torrents, mountain-slides, and
+rock-falls, thus affording stations on which air-borne seeds may
+germinate and find a temporary home till driven out by the inroads of
+the indigenous vegetation. These temporary stations may be at much lower
+altitudes than the original habitat of the species, if other conditions
+are favourable. Alpine plants often descend into the valleys on glacial
+moraines, while some arctic species grow equally well on mountain
+summits and on the seashore. The distances above referred to between the
+loftier mountains may thus be greatly reduced by the occurrence of
+suitable conditions at lower altitudes, and the facilities for
+transmission by means of aerial currents proportionally increased.[181]
+
+
+_Facts Explained by the Wind-Carriage of Seeds._
+
+But if we altogether reject aerial transmission of seeds for great
+distances, except by the agency of birds, it will be difficult, if not
+impossible, to account for the presence of so many identical species of
+plants on remote mountain summits, or for that "continuous current of
+vegetation" described by Sir Joseph Hooker as having apparently long
+existed from the northern to the southern hemisphere. It may be admitted
+that we can, possibly, account for the greater portion of the floras of
+remote oceanic islands by the agency of birds alone; because, when blown
+out to sea land-birds must reach some island or perish, and all which
+come within sight of an island will struggle to reach it as their only
+refuge. But, with mountain summits the case is altogether different,
+because, being surrounded by land instead of by sea, no bird would need
+to fly, or to be carried by the wind, for several hundred miles at a
+stretch to another mountain summit, but would find a refuge in the
+surrounding uplands, ridges, valleys, or plains. As a rule the birds
+that frequent lofty mountain tops are peculiar species, allied to those
+of the surrounding district; and there is no indication whatever of the
+passage of birds from one remote mountain to another in any way
+comparable with the flights of birds which are known to reach the Azores
+annually, or even with the few regular migrants from Australia to New
+Zealand. It is almost impossible to conceive that the seeds of the
+Himalayan primula should have been thus carried to Java; but, by means
+of gales of wind, and intermediate stations from fifty to a few hundred
+miles apart, where the seeds might vegetate for a year or two and
+produce fresh seed to be again carried on in the same manner, the
+transmission might, after many failures, be at last effected.
+
+A very important consideration is the vastly larger scale on which
+wind-carriage of seeds must act, as compared with bird-carriage. It can
+only be a few birds which carry seeds attached to their feathers or
+feet. A very small proportion of these would carry the seeds of Alpine
+plants; while an almost infinitesimal fraction of these latter would
+convey the few seeds attached to them safely to an oceanic island or
+remote mountain. But winds, in the form of whirlwinds or tornadoes,
+gales or hurricanes, are perpetually at work over large areas of land
+and sea. Insects and light particles of matter are often carried up to
+the tops of high mountains; and, from the very nature and origin of
+winds, they usually consist of ascending or descending currents, the
+former capable of suspending such small and light objects as are many
+seeds long enough for them to be carried enormous distances. For each
+single seed carried away by external attachment to the feet or feathers
+of a bird, countless millions are probably carried away by violent
+winds; and the chance of conveyance to a great distance and in a
+definite direction must be many times greater by the latter mode than
+by the former.[182] We have seen that inorganic particles of much
+greater specific gravity than seeds, and nearly as heavy as the smallest
+kinds, are carried to great distances through the air, and we can
+therefore hardly doubt that some seeds are carried as far. The direct
+agency of the wind, as a supplement to bird-transport, will help to
+explain the presence in oceanic islands of plants growing in dry or
+rocky places whose small seeds are not likely to become attached to
+birds; while it seems to be the only effective agency possible in the
+dispersal of those species of alpine or sub-alpine plants found on the
+summits of distant mountains, or still more widely separated in the
+temperate zones of the northern and southern hemispheres.
+
+
+_Concluding Remarks._
+
+On the general principles that have been now laid down, it will be found
+that all the chief facts of the geographical distribution of animals and
+plants can be sufficiently understood. There will, of course, be many
+cases of difficulty and some seeming anomalies, but these can usually be
+seen to depend on our ignorance of some of the essential factors of the
+problem. Either we do not know the distribution of the group in recent
+geological times, or we are still ignorant of the special methods by
+which the organisms are able to cross the sea. The latter difficulty
+applies especially to the lizard tribe, which are found in almost all
+the tropical oceanic islands; but the particular mode in which they are
+able to traverse a wide expanse of ocean, which is a perfect barrier to
+batrachia and almost so to snakes, has not yet been discovered. Lizards
+are found in all the larger Pacific Islands as far as Tahiti, while
+snakes do not extend beyond the Fiji Islands; and the latter are also
+absent from Mauritius and Bourbon, where lizards of seven or eight
+species abound. Naturalists resident in the Pacific Islands would make a
+valuable contribution to our science by studying the life-history of the
+native lizards, and endeavouring to ascertain the special facilities
+they possess for crossing over wide spaces of ocean.
+
+FOOTNOTES:
+
+[Footnote 163: See A. Agassiz, _Three Cruises of the Blake_ (Cambridge,
+Mass., 1888), vol. i. p. 127, footnote.]
+
+[Footnote 164: Even the extremely fine Mississippi mud is nowhere found
+beyond a hundred miles from the mouths of the river in the Gulf of
+Mexico (A. Agassiz, _Three Cruises of the Blake_, vol. i. p. 128).]
+
+[Footnote 165: I have given a full summary of the evidence for the
+permanence of oceanic and continental areas in my _Island Life_, chap.
+vi.]
+
+[Footnote 166: For a full account of the peculiarities of the Madagascar
+fauna, see my _Island Life_, chap. xix.]
+
+[Footnote 167: See _Island Life_, p. 446, and the whole of chaps. xxi.
+xxii. More recent soundings have shown that the Map at p. 443, as well
+as that of the Madagascar group at p. 387, are erroneous, the ocean
+around Norfolk Island and in the Straits of Mozambique being more than
+1000 fathoms deep. The general argument is, however, unaffected.]
+
+[Footnote 168: For some details of these migrations, see the author's
+_Geographical Distribution of Animals_, vol. i. p. 140; also Heilprin's
+_Geographical and Geological Distribution of Animals_.]
+
+[Footnote 169: For a full discussion of this question, see _Island
+Life_, pp. 390-420.]
+
+[Footnote 170: _Geographie Botanique_, p. 798.]
+
+[Footnote 171: _Nature_, 1st April 1886.]
+
+[Footnote 172: Report of the Brit. Assoc. Committee on Migration of
+Birds during 1886.]
+
+[Footnote 173: _Trans. Ent. Soc._, 1871, p. 184.]
+
+[Footnote 174: _Nature_ (1875), vol. xii. pp. 279, 298.]
+
+[Footnote 175: I am indebted to Professor R. Meldola of the Finsbury
+Technical Institute, and Rev. T.D. Titmas of Charterhouse for furnishing
+me with the weights required.]
+
+[Footnote 176: See _Nature_, vol. vi. p. 164, for a summary of Kerner's
+paper.]
+
+[Footnote 177: It seems quite possible that the absence of pappus in
+this case is a recent adaptation, and that it has been brought about by
+causes similar to those which have reduced or aborted the wings of
+insects in oceanic islands. For when a plant has once reached one of the
+storm-swept islands of the southern ocean, the pappus will be injurious
+for the same reason that the wings of insects are injurious, since it
+will lead to the seeds being blown out to sea and destroyed. The seeds
+which are heaviest and have least pappus will have the best chance of
+falling on the ground and remaining there to germinate, and this process
+of selection might rapidly lead to the entire disappearance of the
+pappus.]
+
+[Footnote 178: See _Island Life_, p. 251.]
+
+[Footnote 179: Mr. Hemsley suggests that it is not so much the
+difficulty of transmission by floating, as the bad conditions the seeds
+are usually exposed to when they reach land. Many, even if they
+germinate, are destroyed by the waves, as Burchell noticed at St.
+Helena; while even a flat and sheltered shore would be an unsuitable
+position for many inland plants. Air-borne seeds, on the other hand, may
+be carried far inland, and so scattered that some of them are likely to
+reach suitable stations.]
+
+[Footnote 180: For fuller particulars, see Sir J. Hooker's _Introduction
+to Floras of New Zealand and Australia_, and a summary in my _Island
+Life_, chaps. xxii. xxiii.]
+
+[Footnote 181: For a fuller discussion of this subject, see my _Island
+Life_, chap. xxiii.]
+
+[Footnote 182: A very remarkable case of wind conveyance of seeds on a
+large scale is described in a letter from Mr. Thomas Hanbury to his
+brother, the late Daniel Hanbury, which has been kindly communicated to
+me by Mr. Hemsley of Kew. The letter is dated "Shanghai, 1st May 1856,"
+and the passage referred to is as follows:--
+
+
+"For the past three days we have had very warm weather for this time of
+year, in fact almost as warm as the middle of summer. Last evening the
+wind suddenly changed round to the north and blew all night with
+considerable violence, making a great change in the atmosphere.
+
+"This morning, myriads of small white particles are floating about in
+the air; there is not a single cloud and no mist, yet the sun is quite
+obscured by this substance, and it looks like a white fog in England. I
+enclose thee a sample, thinking it may interest. It is evidently a
+vegetable production; I think, apparently, some kind of seed."
+
+
+Mr. Hemsley adds, that this substance proves to be the plumose seeds of
+a poplar or willow. In order to produce the effects described--_quite
+obscuring the sun like a white fog_,--the seeds must have filled the air
+to a very great height; and they must have been brought from some
+district where there were extensive tracts covered with the tree which
+produced them.]
+
+
+
+
+CHAPTER XIII
+
+THE GEOLOGICAL EVIDENCES OF EVOLUTION
+
+
+    What we may expect--The number of known species of extinct
+    animals--Causes of the imperfection of the geological
+    record--Geological evidences of
+    evolution--Shells--Crocodiles--The rhinoceros tribe--The
+    pedigree of the horse tribe--Development of deer's horns--Brain
+    development--Local relations of fossil and living animals--Cause
+    of extinction of large animals--Indications of general progress
+    in plants and animals--The progressive development of
+    plants--Possible cause of sudden late appearance of
+    exogens--Geological distribution of insects--Geological
+    succession of vertebrata--Concluding remarks.
+
+
+
+The theory of evolution in the organic world necessarily implies that
+the forms of animals and plants have, broadly speaking, progressed from
+a more generalised to a more specialised structure, and from simpler to
+more complex forms. We know, however, that this progression has been by
+no means regular, but has been accompanied by repeated degradation and
+degeneration; while extinction on an enormous scale has again and again
+stopped all progress in certain directions, and has often compelled a
+fresh start in development from some comparatively low and imperfect
+type.
+
+The enormous extension of geological research in recent times has made
+us acquainted with a vast number of extinct organisms, so vast that in
+some important groups--such as the mollusca--the fossil are more
+numerous than the living species; while in the mammalia they are not
+much less numerous, the preponderance of living species being chiefly in
+the smaller and in the arboreal forms which have not been so well
+preserved as the members of the larger groups. With such a wealth of
+material to illustrate the successive stages through which animals have
+passed, it will naturally be expected that we should find important
+evidence of evolution. We should hope to learn the steps by which some
+isolated forms have been connected with their nearest allies, and in
+many cases to have the gaps filled up which now separate genus from
+genus, or species from species. In some cases these expectations are
+fulfilled, but in many other cases we seek in vain for evidence of the
+kind we desire; and this absence of evidence with such an apparent
+wealth of material is held by many persons to throw doubt on the theory
+of evolution itself. They urge, with much appearance of reason, that all
+the arguments we have hitherto adduced fall short of demonstration, and
+that the crucial test consists in being able to show, in a great number
+of cases, those connecting links which we say must have existed. Many of
+the gaps that still remain are so vast that it seems incredible to these
+writers that they could ever have been filled up by a close succession
+of species, since these must have spread over so many ages, and have
+existed in such numbers, that it seems impossible to account for their
+total absence from deposits in which great numbers of species belonging
+to other groups are preserved and have been discovered. In order to
+appreciate the force, or weakness, of these objections, we must inquire
+into the character and completeness of that record of the past life of
+the earth which geology has unfolded, and ascertain the nature and
+amount of the evidence which, under actual conditions, we may expect to
+find.
+
+
+_The Number of known Species of Extinct Animals._
+
+When we state that the known fossil mollusca are considerably more
+numerous than those which now live on the earth, it appears at first
+sight that our knowledge is very complete, but this is far from being
+the case. The species have been continually changing throughout
+geological time, and at each period have probably been as numerous as
+they are now. If we divide the fossiliferous strata into twelve great
+divisions--the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias,
+Trias, Permian, Carboniferous, Devonian, Silurian, and Cambrian,--we
+find not only that each has a very distinct and characteristic molluscan
+fauna, but that the different subdivisions often present a widely
+different series of species; so that although a certain number of
+species are common to two or more of the great divisions, the totality
+of the species that have lived upon the earth must be very much more
+than twelve times--perhaps even thirty or forty times--the number now
+living. In like manner, although the species of fossil mammals now
+recognised by more or less fragmentary fossil remains may not be much
+less numerous than the living species, yet the duration of existence of
+these was comparatively so short that they were almost completely
+changed, perhaps six or seven times, during the Tertiary period; and
+this is certainly only a fragment of the geological time during which
+mammalia existed on the globe.
+
+There is also reason to believe that the higher animals were much more
+abundant in species during past geological epochs than now, owing to the
+greater equability of the climate which rendered even the arctic regions
+as habitable as the temperate zones are in our time.
+
+The same equable climate would probably cause a more uniform
+distribution of moisture, and render what are now desert regions capable
+of supporting abundance of animal life. This is indicated by the number
+and variety of the species of large animals that have been found fossil
+in very limited areas which they evidently inhabited at one period. M.
+Albert Gaudry found, in the deposits of a mountain stream at Pikermi in
+Greece, an abundance of large mammalia such as are nowhere to be found
+living together at the present time. Among them were two species of
+Mastodon, two different rhinoceroses, a gigantic wild boar, a camel and
+a giraffe larger than those now living, several monkeys, carnivora
+ranging from martens and civets to lions and hyaenas of the largest
+size, numerous antelopes of at least five distinct genera, and besides
+these many forms altogether extinct. Such were the great herds of
+Hipparion, an ancestral form of horse; the Helladotherium, a huge animal
+bigger than the giraffe; the Ancylotherium, one of the Edentata; the
+huge Dinotherium; the Aceratherium, allied to the rhinoceros; and the
+monstrous Chalicotherium, allied to the swine and ruminants, but as
+large as a rhinoceros; and to prey upon these, the great Machairodus or
+sabre-toothed tiger. And all these remains were found in a space 300
+paces long by 60 paces broad, many of the species existing in enormous
+quantities.
+
+The Pikermi fossils belong to the Upper Miocene formation, but an
+equally rich deposit of Upper Eocene age has been discovered in
+South-Western France at Quercy, where M. Filhol has determined the
+presence of no less than forty-two species of beasts of prey alone.
+Equally remarkable are the various discoveries of mammalian fossils in
+North America, especially in the old lake bottoms now forming what are
+called the "bad lands" of Dakota and Nebraska, belonging to the Miocene
+period. Here are found an enormous assemblage of remains, often perfect
+skeletons, of herbivora and carnivora, as varied and interesting as
+those from the localities already referred to in Europe; but altogether
+distinct, and far exceeding, in number and variety of species of the
+larger animals, the whole existing fauna of North America. Very similar
+phenomena occur in South America and in Australia, leading us to the
+conclusion that the earth at the present time is impoverished as regards
+the larger animals, and that at each successive period of Tertiary time,
+at all events, it contained a far greater number of species than now
+inhabit it. The very richness and abundance of the remains which we find
+in limited areas, serve to convince us how imperfect and fragmentary
+must be our knowledge of the earth's fauna at any one past epoch; since
+we cannot believe that all, or nearly all, of the animals which
+inhabited any district were entombed in a single lake, or overwhelmed by
+the floods of a single river.
+
+But the spots where such rich deposits occur are exceedingly few and far
+between when compared with the vast areas of continental land, and we
+have every reason to believe that in past ages, as now, numbers of
+curious species were rare or local, the commoner and more abundant
+species giving a very imperfect idea of the existing series of animal
+forms. Yet more important, as showing the imperfection of our knowledge,
+is the enormous lapse of time between the several formations in which we
+find organic remains in any abundance, so vast that in many cases we
+find ourselves almost in a new world, all the species and most of the
+genera of the higher animals having undergone a complete change.
+
+
+_Causes of the Imperfection of the Geological Record._
+
+These facts are quite in accordance with the conclusions of geologists
+as to the necessary imperfection of the geological record, since it
+requires the concurrence of a number of favourable conditions to
+preserve any adequate representation of the life of a given epoch. In
+the first place, the animals to be preserved must not die a natural
+death by disease, or old age, or by being the prey of other animals, but
+must be destroyed by some accident which shall lead to their being
+embedded in the soil. They must be either carried away by floods, sink
+into bogs or quicksands, or be enveloped in the mud or ashes of a
+volcanic eruption; and when thus embedded they must remain undisturbed
+amid all the future changes of the earth's surface.
+
+But the chances against this are enormous, because denudation is always
+going on, and the rocks we now find at the earth's surface are only a
+small fragment of those which were originally laid down. The
+alternations of marine and freshwater deposits, and the frequent
+unconformability of strata with those which overlie them, tell us
+plainly of repeated elevations and depressions of the surface, and of
+denudation on an enormous scale. Almost every mountain range, with its
+peaks, ridges, and valleys, is but the remnant of some vast plateau
+eaten away by sub-aerial agencies; every range of sea-cliffs tell us of
+long slopes of land destroyed by the waves; while almost all the older
+rocks which now form the surface of the earth have been once covered
+with newer deposits which have long since disappeared. Nowhere are the
+evidences of this denudation more apparent than in North and South
+America, where granitic or metamorphic rocks cover an area hardly less
+than that of all Europe. The same rocks are largely developed in Central
+Africa and Eastern Asia; while, besides those portions that appear
+exposed on the surface, areas of unknown extent are buried under strata
+which rest on them uncomformably, and could not, therefore, constitute
+the original capping under which the whole of these rocks must once have
+been deeply buried; because granite can only be formed, and metamorphism
+can only go on, deep down in the crust of the earth. What an
+overwhelming idea does this give us of the destruction of whole piles
+of rock, miles in thickness and covering areas comparable with those of
+continents; and how great must have been the loss of the innumerable
+fossil forms which those rocks contained! In view of such destruction we
+are forced to conclude that our palaeontological collections, rich
+though they may appear, are really but small and random samples, giving
+no adequate idea of the mighty series of organism which have lived upon
+the earth.[183]
+
+Admitting, however, the extreme imperfection of the geological record as
+a whole, it may be urged that certain limited portions of it are fairly
+complete--as, for example, the various Miocene deposits of India,
+Europe, and North America,--and that in these we ought to find many
+examples of species and genera linked together by intermediate forms. It
+may be replied that in several cases this really occurs; and the reason
+why it does not occur more often is, that the theory of evolution
+requires that distinct genera should be linked together, not by a direct
+passage, but by the descent of both from a common ancestor, which may
+have lived in some much earlier age the record of which is either
+wanting or very incomplete. An illustration given by Mr. Darwin will
+make this more clear to those who have not studied the subject. The
+fantail and pouter pigeons are two very distinct and unlike breeds,
+which we yet know to have been both derived from the common wild
+rock-pigeon. Now, if we had every variety of living pigeon before us, or
+even all those which have lived during the present century, we should
+find no intermediate types between these two--none combining in any
+degree the characters of the pouter with that of the fantail. Neither
+should we ever find such an intermediate form, even had there been
+preserved a specimen of every breed of pigeon since the ancestral
+rock-pigeon was first tamed by man--a period of probably several
+thousand years. We thus see that a complete passage from one very
+distinct species to another could not be expected even had we a complete
+record of the life of any one period. What we require is a complete
+record of all the species that have existed since the two forms began
+to diverge from their common ancestor, and this the known imperfection
+of the record renders it almost impossible that we should ever attain.
+All that we have a right to expect is, that, as we multiply the fossil
+forms in any group, the gaps that at first existed in that group shall
+become less wide and less numerous; and also that, in some cases, a
+tolerably direct series shall be found, by which the more specialised
+forms of the present day shall be connected with more generalised
+ancestral types. We might also expect that when a country is now
+characterised by special groups of animals, the fossil forms that
+immediately preceded them shall, for the most part, belong to the same
+groups; and further, that, comparing the more ancient with the more
+modern types, we should find indications of progression, the earlier
+forms being, on the whole, lower in organisation, and less specialised
+in structure than the later. Now evidence of evolution of these varied
+kinds is what we do find, and almost every fresh discovery adds to their
+number and cogency. In order, therefore, to show that the testimony
+given by geology is entirely in favour of the theory of descent with
+modification, some of the more striking of the facts will now be given.
+
+
+_Geological Evidences of Evolution._
+
+In an article in _Nature_ (vol. xiv. p. 275), Professor Judd calls
+attention to some recent discoveries in the Hungarian plains, of fossil
+lacustrine shells, and their careful study by Dr. Neumayr and M. Paul of
+the Austrian Geological Survey. The beds in which they occur have
+accumulated to the thickness of 2000 feet, containing throughout
+abundance of fossils, and divisible into eight zones, each of which
+exhibits a well-marked and characteristic fauna. Professor Judd then
+describes the bearing of these discoveries as follows--
+
+
+    "The group of shells which affords the most interesting evidence
+    of the origin of new forms through descent with modification is
+    that of the genus Vivipara or Paludina, which occurs in
+    prodigious abundance throughout the whole series of freshwater
+    strata. We shall not, of course, attempt in this place to enter
+    into any details concerning the forty distinct _forms_ of this
+    genus (Dr. Neumayr very properly hesitates to call them all
+    _species_), which are named and described in this monograph,
+    and between which, as the authors show, so many connecting
+    links, clearly illustrating the derivation of the newer from the
+    older types, have been detected. On the minds of those who
+    carefully examine the admirably engraved figures given in the
+    plates accompanying this valuable memoir, or still better, the
+    very large series of specimens from among which the subjects of
+    these figures are selected, and which are now in the museum of
+    the Reichsanstalt of Vienna, but little doubt will, we suspect,
+    remain that the authors have fully made out their case, and have
+    demonstrated that, beyond all controversy, the series with
+    highly complicated ornamentation were variously derived by
+    descent--the lines of which are in most cases perfectly clear
+    and obvious--from the simple and unornamented Vivipara
+    achatinoides of the Congerien-Schichten (the lower division of
+    the series of strata). It is interesting to notice that a large
+    portion of these unquestionably derived forms depart so widely
+    from the type of the genus Vivipara, that they have been
+    separated on so high an authority as that of Sandberger, as a
+    new genus, under the name of Tulotoma. And hence we are led to
+    the conclusion that a vast number of forms, certainly exhibiting
+    specific distinctions, and according to some naturalists,
+    differences even entitled to be regarded of generic value, have
+    all a common ancestry."
+
+
+It is, as Professor Judd remarks, owing to the exceptionally favourable
+circumstances of a long-continued and unbroken series of deposits being
+formed under physical conditions either identical or very slowly
+changing, that we owe so complete a record of the process of organic
+change. Usually, some disturbing elements, such as a sudden change of
+physical conditions, or the immigration of new sets of forms from other
+areas and the consequent retreat or partial extinction of the older
+fauna, interferes with the continuity of organic development, and
+produces those puzzling discordances so generally met with in geological
+formations of marine origin. While a case of the kind now described
+affords evidence of the origin of species complete and conclusive,
+though on a necessarily very limited scale, the very rarity of the
+conditions which are essential to such completeness serves to explain
+why it is that in most cases the direct evidence of evolution is not to
+be obtained.
+
+Another illustration of the filling up of gaps between existing groups
+is afforded by Professor Huxley's researches on fossil crocodiles. The
+gap between the existing crocodiles and the lizards is very wide, but as
+we go back in geological time we meet with fossil forms which are to
+some extent intermediate and form a connected series. The three living
+genera--Crocodilus, Alligator, and Gavialis--are found in the Eocene
+formation, and allied forms of another genus, Holops, in the Chalk. From
+the Chalk backward to the Lias another group of genera occurs, having
+anatomical characteristics intermediate between the living crocodiles
+and the most ancient forms. These, forming two genera Belodon and
+Stagonolepis, are found in a still older formation, the Trias. They have
+characters resembling some lizards, especially the remarkable Hatteria
+of New Zealand, and have also some resemblances to the
+Dinosaurians--reptiles which in some respects approach birds.
+Considering how comparatively few are the remains of this group of
+animals, the evidence which it affords of progressive development is
+remarkably clear.[184]
+
+Among the higher animals the rhinoceros, the horse, and the deer afford
+good evidence of advance in organisation and of the filling up of the
+gaps which separate the living forms from their nearest allies. The
+earliest ancestral forms of the rhinoceroses occur in the Middle Eocene
+of the United States, and were to some extent intermediate between the
+rhinoceros and tapir families, having like the latter four toes to the
+front feet, and three to those behind. These are followed in the Upper
+Eocene by the genus Amynodon, in which the skull assumes more distinctly
+the rhinocerotic type. Following this in the Lower Miocene we have the
+Aceratherium, like the last in its feet, but still more decidedly a
+rhinoceros in its general structure. From this there are two diverging
+lines--one in the Old World, the other in the New. In the former, to
+which the Aceratherium is supposed to have migrated in early Miocene
+times, when a mild climate and luxuriant vegetation prevailed far within
+the arctic circle, it gave rise to the Ceratorhinus and the various
+horned rhinoceroses of late Tertiary times and of those now living. In
+America a number of large hornless rhinoceroses were developed--they
+are found in the Upper Miocene, Pliocene, and Post-Pliocene
+formations--and then became extinct. The true rhinoceroses have three
+toes on all the feet.[185]
+
+
+_The Pedigree of the Horse Tribe._
+
+Yet more remarkable is the evidence afforded by the ancestral forms of
+the horse tribe which have been discovered in the American tertiaries.
+The family Equidae, comprising the living horse, asses, and zebras,
+differ widely from all other mammals in the peculiar structure of the
+feet, all of which terminate in a single large toe forming the hoof.
+They have forty teeth, the molars being formed of hard and soft material
+in crescentic folds, so as to be a powerful agent in grinding up hard
+grasses and other vegetable food. The former peculiarities depend upon
+modifications of the skeleton, which have been thus described by
+Professor Huxley:--
+
+
+    "Let us turn in the first place to the fore-limb. In most
+    quadrupeds, as in ourselves, the fore-arm contains distinct
+    bones, called the radius and the ulna. The corresponding region
+    in the horse seems at first to possess but one bone. Careful
+    observation, however, enables us to distinguish in this bone a
+    part which clearly answers to the upper end of the ulna. This is
+    closely united with the chief mass of the bone which represents
+    the radius, and runs out into a slender shaft, which may be
+    traced for some distance downwards upon the back of the radius,
+    and then in most cases thins out and vanishes. It takes still
+    more trouble to make sure of what is nevertheless the fact, that
+    a small part of the lower end of the bone of a horse's fore-arm,
+    which is only distinct in a very young foal, is really the lower
+    extremity of the ulna.
+
+    "What is commonly called the knee of a horse is its wrist. The
+    'cannon bone' answers to the middle bone of the five metacarpal
+    bones which support the palm of the hand in ourselves. The
+    pastern, coronary, and coffin bones of veterinarians answer to
+    the joints of our middle fingers, while the hoof is simply a
+    greatly enlarged and thickened nail. But if what lies below the
+    horse's 'knee' thus corresponds to the middle finger in
+    ourselves, what has become of the four other fingers or digits?
+    We find in the places of the second and fourth digits only two
+    slender splintlike bones, about two-thirds as long as the cannon
+    bone, which gradually taper to their lower ends and bear no
+    finger joints, or, as they are termed, phalanges. Sometimes,
+    small bony or gristly nodules are to be found at the bases of
+    these two metacarpal splints, and it is probable that these
+    represent rudiments of the first and fifth toes. Thus, the part
+    of the horse's skeleton which corresponds with that of the human
+    hand, contains one overgrown middle digit, and at least two
+    imperfect lateral digits; and these answer, respectively, to the
+    third, the second, and the fourth fingers in man.
+
+    "Corresponding modifications are found in the hind limb. In
+    ourselves, and in most quadrupeds, the leg contains two distinct
+    bones, a large bone, the tibia, and a smaller and more slender
+    bone, the fibula. But, in the horse, the fibula seems, at first,
+    to be reduced to its upper end; a short slender bone united with
+    the tibia, and ending in a point below, occupying its place.
+    Examination of the lower end of a young foal's shin-bone,
+    however, shows a distinct portion of osseous matter which is the
+    lower end of the fibula; so that the, apparently single, lower
+    end of the shin-bone is really made up of the coalesced ends of
+    the tibia and fibula, just as the, apparently single, lower end
+    of the fore-arm bone is composed of the coalesced radius and
+    ulna.
+
+    "The heel of the horse is the part commonly known as the hock.
+    The hinder cannon bone answers to the middle metatarsal bone of
+    the human foot, the pastern, coronary, and coffin bones, to the
+    middle toe bones; the hind hoof to the nail; as in the forefoot.
+    And, as in the forefoot, there are merely two splints to
+    represent the second and the fourth toes. Sometimes a rudiment
+    of a fifth toe appears to be traceable.
+
+    "The teeth of a horse are not less peculiar than its limbs. The
+    living engine, like all others, must be well stoked if it is to
+    do its work; and the horse, if it is to make good its wear and
+    tear, and to exert the enormous amount of force required for its
+    propulsion, must be well and rapidly fed. To this end, good
+    cutting instruments and powerful and lasting crushers are
+    needful. Accordingly, the twelve cutting teeth of a horse are
+    close-set and concentrated in the forepart of its mouth, like so
+    many adzes or chisels. The grinders or molars are large, and
+    have an extremely complicated structure, being composed of a
+    number of different substances of unequal hardness. The
+    consequence of this is that they wear away at different rates;
+    and, hence, the surface of each grinder is always as uneven as
+    that of a good millstone."[186]
+
+
+We thus see that the Equidae differ very widely in structure from most
+other mammals. Assuming the truth of the theory of evolution, we should
+expect to find traces among extinct animals of the steps by which this
+great modification has been effected; and we do really find traces of
+these steps, imperfectly among European fossils, but far more completely
+among those of America.
+
+It is a singular fact that, although no horse inhabited America when
+discovered by Europeans, yet abundance of remains of extinct horses have
+been found both in North and South America in Post-Tertiary and Upper
+Pliocene deposits; and from these an almost continuous series of
+modified forms can be traced in the Tertiary formation, till we reach,
+at the very base of the series, a primitive form so unlike our perfected
+animal, that, had we not the intermediate links, few persons would
+believe that the one was the ancestor of the other. The tracing out of
+this marvellous history we owe chiefly to Professor Marsh of Yale
+College, who has himself discovered no less than thirty species of
+fossil Equidae; and we will allow him to tell the story of the
+development of the horse from a humble progenitor in his own words.
+
+
+    "The oldest representative of the horse at present known is the
+    diminutive Eohippus from the Lower Eocene. Several species have
+    been found, all about the size of a fox. Like most of the early
+    mammals, these ungulates had forty-four teeth, the molars with
+    short crowns and quite distinct in form from the premolars. The
+    ulna and fibula were entire and distinct, and there were four
+    well-developed toes and a rudiment of another on the forefeet,
+    and three toes behind. In the structure of the feet and teeth,
+    the Eohippus unmistakably indicates that the direct ancestral
+    line to the modern horse has already separated from the other
+    perissodactyles, or odd-toed ungulates.
+
+    "In the next higher division of the Eocene another genus,
+    Orohippus, makes its appearance, replacing Eohippus, and showing
+    a greater, though still distant, resemblance to the equine type.
+    The rudimentary first digit of the forefoot has disappeared, and
+    the last premolar has gone over to the molar series. Orohippus
+    was but little larger than Eohippus, and in most other respects
+    very similar. Several species have been found, but none occur
+    later than the Upper Eocene.
+
+    "Near the base of the Miocene, we find a third closely allied
+    genus, Mesohippus, which is about as large as a sheep, and one
+    stage nearer the horse. There are only three toes and a
+    rudimentary splint on the forefeet, and three toes behind. Two
+    of the premolar teeth are quite like the molars. The ulna is no
+    longer distinct or the fibula entire, and other characters show
+    clearly that the transition is advancing.
+
+    "In the Upper Miocene Mesohippus is not found, but in its place
+    a fourth form, Miohippus, continues the line. This genus is near
+    the Anchitherium of Europe, but presents several important
+    differences. The three toes in each foot are more nearly of a
+    size, and a rudiment of the fifth metacarpal bone is retained.
+    All the known species of this genus are larger than those of
+    Mesohippus, and none of them pass above the Miocene formation.
+
+    "The genus Protohippus of the Lower Pliocene is yet more equine,
+    and some of its species equalled the ass in size. There are
+    still three toes on each foot, but only the middle one,
+    corresponding to the single toe of the horse, comes to the
+    ground. This genus resembles most nearly the Hipparion of
+    Europe.
+
+    "In the Pliocene we have the last stage of the series before
+    reaching the horse, in the genus Pliohippus, which has lost the
+    small hooflets, and in other respects is very equine. Only in
+    the Upper Pliocene does the true Equus appear and complete the
+    genealogy of the horse, which in the Post-Tertiary roamed over
+    the whole of North and South America, and soon after became
+    extinct. This occurred long before the discovery of the
+    continent by Europeans, and no satisfactory reason for the
+    extinction has yet been given. Besides the characters I have
+    mentioned, there are many others in the skeleton, skull, teeth,
+    and brain of the forty or more intermediate species, which show
+    that the transition from the Eocene Eohippus to the modern Equus
+    has taken place in the order indicated"[187] (see Fig. 33).
+
+
+[Illustration: FIG. 33.--Geological development of the horse tribe
+(Eohippus since discovered).]
+
+Well may Professor Huxley say that this is demonstrative evidence of
+evolution; the doctrine resting upon exactly as secure a foundation as
+did the Copernican theory of the motions of the heavenly bodies at the
+time of its promulgation. Both have the same basis--the coincidence of
+the observed facts with the theoretical requirements.
+
+
+_Development of Deer's Horns._
+
+Another clear and unmistakable proof of evolution is afforded by one of
+the highest and latest developed tribes of mammals--the true deer. These
+differ from all other ruminants in possessing solid deciduous horns
+which are always more or less branched. They first appear in the Middle
+Miocene formation, and continue down to our time; and their development
+has been carefully traced by Professor Boyd Dawkins, who thus summarises
+his results:--
+
+
+    "In the middle stage of the Miocene the cervine antler consists
+    merely of a simple forked crown (as in Cervus dicroceros), which
+    increases in size in the Upper Miocene, although it still
+    remains small and erect, like that of the roe. In Cervus
+    Matheroni it measures 11.4 inches, and throws off not more than
+    four tines, all small. The deer living in Auvergne in the
+    succeeding or Pliocene age, present us with another stage in the
+    history of antler development. There, for the first time, we see
+    antlers of the Axis and Rusa type, larger and longer, and more
+    branching than any antlers were before, and possessing three or
+    more well-developed tines. Deer of this type abounded in
+    Pliocene Europe. They belong to the Oriental division of the
+    Cervidae, and their presence in Europe confirms the evidence of
+    the flora, brought forward by the Comte de Saporta, that the
+    Pliocene climate was warm. They have probably disappeared from
+    Europe in consequence of the lowering of the temperature in the
+    Pleistocene age, while their descendants have found a congenial
+    home in the warmer regions of Eastern Asia.
+
+    "In the latest stage of the Pliocene--the Upper Pliocene of the
+    Val d'Arno--the Cervus dicranios of Nesti presents us with
+    antlers much smaller than those of the Irish elk, but very
+    complicated in their branching. This animal survived into the
+    succeeding age, and is found in the pre-glacial forest bed of
+    Norfolk, being described by Dr. Falconer under the name of
+    Sedgwick's deer. The Irish elk, moose, stag, reindeer, and
+    fallow deer appear in Europe in the Pleistocene age, all with
+    highly complicated antlers in the adult, and the first
+    possessing the largest antlers yet known. Of these the Irish elk
+    disappeared in the Prehistoric age, after having lived in
+    countless herds in Ireland, while the rest have lived on into
+    our own times in Euro-Asia, and, with the exception of the last,
+    also in North America.
+
+    "From this survey it is obvious that the cervine antlers have
+    increased in size and complexity from the Mid-Miocene to the
+    Pleistocene age, and that their successive changes are analogous
+    to those which are observed in the development of antlers in the
+    living deer, which begin with a simple point, and increase in
+    number of tines till their limit of growth be reached. In other
+    words, the development of antlers indicated at successive and
+    widely-separated pages of the geological record is the same as
+    that observed in the history of a single living species. It is
+    also obvious that the progressive diminution of size and
+    complexity in the antlers, from the present time back into the
+    early Tertiary age, shows that we are approaching the zero of
+    antler development in the Mid-Miocene. No trace of any
+    antler-bearing ruminant has been met with in the lower Miocenes,
+    either of Europe or the United States."[188]
+
+
+
+_Progressive Brain-Development._
+
+The three illustrations now given sufficiently prove that, whenever the
+geological record approaches to completeness, we have evidence of the
+progressive change of species in definite directions, and from less
+developed to more developed types--exactly such a change as we may
+expect to find if the evolution theory be the true one. Many other
+illustrations of a similar change could be given, but the animal groups
+in which they occur being less familiar, the details would be less
+interesting, and perhaps hardly intelligible. There is, however, one
+very remarkable proof of development that must be briefly noticed--that
+afforded by the steady increase in the size of the brain. This may be
+best stated in the words of Professor Marsh:--
+
+
+    "The real progress of mammalian life in America, from the
+    beginning of the Tertiary to the present, is well illustrated by
+    the brain-growth, in which we have the key to many other
+    changes. The earliest known Tertiary mammals all had very small
+    brains, and in some forms this organ was proportionally less
+    than in certain reptiles. There was a gradual increase in the
+    size of the brain during this period, and it is interesting to
+    find that this growth was mainly confined to the cerebral
+    hemispheres, or higher portion of the brain. In most groups of
+    mammals the brain has gradually become more convoluted, and thus
+    increased in quality as well as quantity. In some also the
+    cerebellum and olfactory lobes, the lower parts of the brain,
+    have even diminished in size. In the long struggle for existence
+    during Tertiary time the big brains won, then as now; and the
+    increasing power thus gained rendered useless many structures
+    inherited from primitive ancestors, but no longer adapted to new
+    conditions."
+
+
+This remarkable proof of development in the organ of the mental
+faculties, forms a fitting climax to the evidence already adduced of the
+progressive evolution of the general structure of the body, as
+illustrated by the bony skeleton. We now pass on to another class of
+facts equally suggestive of evolution.
+
+
+_The Local Relations of Fossil and Living Animals._
+
+If all existing animals have been produced from ancestral forms--mostly
+extinct--under the law of variation and natural selection, we may expect
+to find in most cases a close relation between the living forms of each
+country and those which inhabited it in the immediately preceding epoch.
+But if species have originated in some quite different way, either by
+any kind of special creation, or by sudden advances of organisation in
+the offspring of preceding types, such close relationship would not be
+found; and facts of this kind become, therefore, to some extent a test
+of evolution under natural selection or some other law of gradual
+change. Of course the relationship will not appear when extensive
+migration has occurred, by which the inhabitants of one region have been
+able to take possession of another region, and destroy or drive out its
+original inhabitants, as has sometimes happened. But such cases are
+comparatively rare, except where great changes of climate are known to
+have occurred; and we usually do find a remarkable continuity between
+the existing fauna and flora of a country and those of the immediately
+preceding age. A few of the more remarkable of these cases will now be
+briefly noticed.
+
+The mammalian fauna of Australia consists, as is well known, wholly of
+the lowest forms--the Marsupials and Monotremata--except only a few
+species of mice. This is accounted for by the complete isolation of the
+country from the Asiatic continent during the whole period of the
+development of the higher animals. At some earlier epoch the ancestral
+marsupials, which abounded both in Europe and North America in the
+middle of the Secondary period, entered the country, and have since
+remained there, free from the competition of higher forms, and have
+undergone a special development in accordance with the peculiar
+conditions of a limited area. While in the large continents higher forms
+of mammalia have been developed, which have almost or wholly
+exterminated the less perfect marsupials, in Australia these latter have
+become modified into such varied forms as the leaping kangaroos, the
+burrowing wombats, the arboreal phalangers, the insectivorous
+bandicoots, and the carnivorous Dasyuridae or native cats, culminating
+in the Thylacinus or "tiger-wolf" of Tasmania--animals as unlike each
+other as our sheep, rabbits, squirrels, and dogs, but all retaining the
+characteristic features of the marsupial type.
+
+Now in the caves and late Tertiary or Post-Tertiary deposits of
+Australia the remains of many extinct mammalia have been found, but all
+are marsupials. There are many kangaroos, some larger than any living
+species, and others more allied to the tree-kangaroos of New Guinea; a
+large wombat as large as a tapir; the Diprotodon, a thick-limbed
+kangaroo the size of a rhinoceros or small elephant; and a quite
+different animal, the Nototherium, nearly as large. The carnivorous
+Thylacinus of Tasmania is also found fossil; and a huge phalanger,
+Thylacoleo, the size of a lion, believed by Professor Owen and by
+Professor Oscar Schmidt to have been equally carnivorous and
+destructive.[189] Besides these, there are many other species more
+resembling the living forms both in size and structure, of which they
+may be, in some cases, the direct ancestors. Two species of extinct
+Echidna, belonging to the very low Monotremata, have also been found in
+New South Wales.
+
+Next to Australia, South America possesses the most remarkable
+assemblage of peculiar mammals, in its numerous Edentata--the sloths,
+ant-eaters, and armadillos; its rodents, such as the cavies and
+chinchillas; its marsupial opossums, and its quadrumana of the family
+Cebidae. Remains of extinct species of all these have been found in the
+caves of Brazil, of Post-Pliocene age; while in the earlier Pliocene
+deposits of the pampas many distinct genera of these groups have been
+found, some of gigantic size and extraordinary form. There are
+armadillos of many types, some being as large as elephants; gigantic
+sloths of the genera Megatherium, Megalonyx, Mylodon, Lestodon, and many
+others; rodents belonging to the American families Cavidae and
+Chinchillidae; and ungulates allied to the llama; besides many other
+extinct forms of intermediate types or of uncertain affinities.[190] The
+extinct Moas of New Zealand--huge wingless birds allied to the living
+Apteryx--illustrate the same general law.
+
+The examples now quoted, besides illustrating and enforcing the general
+fact of evolution, throw some light on the usual character of the
+modification and progression of animal forms. In the cases where the
+geological record is tolerably complete, we find a continuous
+development of some kind--either in complexity of ornamentation, as in
+the fossil Paludinas of the Hungarian lake-basins; in size and in the
+specialisation of the feet and teeth, as in the American fossil horses;
+or in the increased development of the branching horns, as in the true
+deer. In each of these cases specialisation and adaptation to the
+conditions of the environment appear to have reached their limits, and
+any change of these conditions, especially if it be at all rapid or
+accompanied by the competition of less developed but more adaptable
+forms, is liable to cause the extinction of the most highly developed
+groups. Such we know was the case with the horse tribe in America, which
+totally disappeared in that continent at an epoch so recent that we
+cannot be sure that the disappearance was not witnessed, perhaps caused,
+by man; while even in the Eastern hemisphere it is the smaller
+species--the asses and the zebras--that have persisted, while the larger
+and more highly developed true horses have almost, if not quite,
+disappeared in a state of nature. So we find, both in Australia and
+South America, that in a quite recent period many of the largest and
+most specialised forms have become extinct, while only the smaller types
+have survived to our day; and a similar fact is to be observed in many
+of the earlier geological epochs, a group progressing and reaching a
+maximum of size or complexity and then dying out, or leaving at most but
+few and pigmy representatives.
+
+
+_Cause of Extinction of Large Animals._
+
+Now there are several reasons for the repeated extinction of large
+rather than of small animals. In the first place, animals of great bulk
+require a proportionate supply of food, and any adverse change of
+conditions would affect them more seriously than it would smaller
+animals. In the next place, the extreme specialisation of many of these
+large animals would render it less easy for them to be modified in any
+new direction suited to changed conditions. Still more important,
+perhaps, is the fact that very large animals always increase slowly as
+compared with small ones--the elephant producing a single young one
+every three years, while a rabbit may have a litter of seven or eight
+young two or three times a year. Now the probability of favourable
+variations will be in direct proportion to the population of the
+species, and as the smaller animals are not only many hundred times more
+numerous than the largest, but also increase perhaps a hundred times as
+rapidly, they are able to become quickly modified by variation and
+natural selection in harmony with changed conditions, while the large
+and bulky species, being unable to vary quickly enough, are obliged to
+succumb in the struggle for existence. As Professor Marsh well observes:
+"In every vigorous primitive type which was destined to survive many
+geological changes, there seems to have been a tendency to throw off
+lateral branches, which became highly specialised and soon died out,
+because they were unable to adapt themselves to new conditions." And he
+goes on to show how the whole narrow path of the persistent Suilline
+type, throughout the entire series of the American tertiaries, is
+strewed with the remains of such ambitious offshoots, many of them
+attaining the size of a rhinoceros; "while the typical pig, with an
+obstinacy never lost, has held on in spite of catastrophes and
+evolution, and still lives in America to-day."
+
+
+_Indications of General Progression in Plants and Animals._
+
+One of the most powerful arguments formerly adduced against evolution
+was, that geology afforded no evidence of the gradual development of
+organic forms, but that whole tribes and classes appeared suddenly at
+definite epochs, and often in great variety and exhibiting a very
+perfect organisation. The mammalia, for example, were long thought to
+have first appeared in Tertiary times, where they are represented in
+some of the earlier deposits by all the great divisions of the class
+fully developed--carnivora, rodents, insectivora, marsupials, and even
+the perissodactyle and artiodactyle divisions of the ungulata--as
+clearly defined as at the present day. The discovery in 1818 of a single
+lower jaw in the Stonesfield Slate of Oxfordshire hardly threw doubt on
+the generalisation, since either its mammalian character was denied, or
+the geological position of the strata, in which it was found, was held
+to have been erroneously determined. But since then, at intervals of
+many years, other remains of mammalia have been discovered in the
+Secondary strata, ranging from the Upper Oolite to the Upper Trias both
+in Europe and the United States, and one even (Tritylodon) in the Trias
+of South Africa. All these are either marsupials, or of some still lower
+type of mammalia; but they consist of many distinct forms classed in
+about twenty genera. Nevertheless, a great gap still exists between
+these mammals and those of the Tertiary strata, since no mammal of any
+kind has been found in any part of the Cretaceous formation, although in
+several of its subdivisions abundance of land plants, freshwater shells,
+and air-breathing reptiles have been discovered. So with fishes. In the
+last century none had been obtained lower than the Carboniferous
+formation; thirty years later they were found to be very abundant in the
+Devonian rocks, and later still they were discovered in the Upper Ludlow
+and Lower Ludlow beds of the Silurian formation.
+
+We thus see that such sudden appearances are deceptive, and are, in
+fact, only what we ought to expect from the known imperfection of the
+geological record. The conditions favourable to the fossilisation of any
+group of animals occur comparatively rarely, and only in very limited
+areas; while the conditions essential for their permanent preservation
+in the rocks, amid all the destruction caused by denudation or
+metamorphism, are still more exceptional. And when they are thus
+preserved to our day, the particular part of the rocks in which they lie
+hidden may not be on the surface but buried down deep under other
+strata, and may thus, except in the case of mineral-bearing deposits, be
+altogether out of our reach. Then, again, how large a proportion of the
+earth consists of wild and uncivilised regions in which no exploration
+of the rocks has been yet made, so that whether we shall find the
+fossilised remains of any particular group of animals which lived during
+a limited period of the earth's history, and in a limited area, depends
+upon at least a fivefold combination of chances. Now, if we take each of
+these chances separately as only ten to one against us (and some are
+certainly more than this), then the actual chance against our finding
+the fossil remains, say of any one order of mammalia, or of land plants,
+at any particular geological horizon, will be about a hundred thousand
+to one.
+
+It may be said, if the chances are so great, how is it that we find such
+immense numbers of fossil species exceeding in number, in some groups,
+all those that are now living? But this is exactly what we should
+expect, because the number of species of organisms that have ever lived
+upon the earth, since the earliest geological times, will probably be
+many hundred times greater than those now existing of which we have any
+knowledge; and hence the enormous gaps and chasms in the geological
+record of extinct forms is not to be wondered at. Yet, notwithstanding
+these chasms in our knowledge, if evolution is true, there ought to have
+been, on the whole, progression in all the chief types of life. The
+higher and more specialised forms should have come into existence later
+than the lower and more generalised forms; and however fragmentary the
+portions we possess of the whole tree of life upon the earth, they ought
+to show us broadly that such a progressive evolution has taken place. We
+have seen that in some special groups, already referred to, such a
+progression is clearly visible, and we will now cast a hasty glance over
+the entire series of fossil forms, in order to see if a similar
+progression is manifested by them as a whole.
+
+
+_The Progressive Development of Plants._
+
+Ever since fossil plants have been collected and studied, the broad fact
+has been apparent that the early plants--those of the Coal
+formation--were mainly cryptogamous, while in the Tertiary deposits the
+higher flowering plants prevailed. In the intermediate secondary epoch
+the gymnosperms--cycads and coniferae--formed a prominent part of the
+vegetation, and as these have usually been held to be a kind of
+transition form between the flowerless and flowering plants, the
+geological succession has always, broadly speaking, been in accordance
+with the theory of evolution. Beyond this, however, the facts were very
+puzzling. The highest cryptogams--ferns, lycopods, and
+equisetaceae--appeared suddenly, and in immense profusion in the Coal
+formation, at which period they attained a development they have never
+since surpassed or even equalled; while the highest plants--the
+dicotyledonous and monocotyledonous angiosperms--which now form the bulk
+of the vegetation of the world, and exhibit the most wonderful
+modifications of form and structure, were almost unknown till the
+Tertiary period, when they suddenly appeared in full development, and,
+for the most part, under the same generic forms as now exist.
+
+During the latter half of the present century, however, great additions
+have been made to our knowledge of fossil plants; and although there
+are still indications of vast gaps in our knowledge, due, no doubt, to
+the very exceptional conditions required for the preservation of plant
+remains, we now possess evidence of a more continuous development of the
+various types of vegetation. According to Mr. Lester F. Ward, between
+8000 and 9000 species of fossil plants have been described or indicated;
+and, owing to the careful study of the nervation of leaves, a large
+number of these are referable to their proper orders or genera, and
+therefore give us some notion--which, though very imperfect, is probably
+accurate in its main outlines--of the progressive development of
+vegetation on the earth.[191] The following is a summary of the facts as
+given by Mr. Ward:--
+
+The lowest forms of vegetable life--the cellular plants--have been found
+in Lower Silurian deposits in the form of three species of marine algae;
+and in the whole Silurian formation fifty species have been recognised.
+We cannot for a moment suppose, however, that this indicates the first
+appearance of vegetable life upon the earth, for in these same Lower
+Silurian beds the more highly organised vascular cryptogams appear in
+the form of rhizocarps--plants allied to Marsilea and Azolla,--and a
+very little higher, ferns, lycopods, and even conifers appear. We have
+indications, however, of a still more ancient vegetation, in the
+carbonaceous shales and thick beds of graphite far down in the Middle
+Laurentian, since there is no other known agency than the vegetable cell
+by means of which carbon can be extracted from the atmosphere and fixed
+in the solid state. These great beds of graphite, therefore, imply the
+existence of abundance of vegetable life at the very commencement of the
+era of which we have any geological record.[192]
+
+Ferns, as already stated, begin in the Middle Silurian formation with
+the Eopteris Morrieri. In the Devonian, we have 79 species, in the
+Carboniferous 627, and in the Permian 186 species; after which fossil
+ferns diminish greatly, though they are found in every formation; and
+the fact that fully 3000 living species are known, while the richest
+portion of the Tertiary in fossil plants--the Miocene--- has only
+produced 87 species, will serve to indicate the extreme imperfection of
+the geological record.
+
+The Equisetaceae (horsetails) which also first appear in the Silurian and
+reach their maximum development in the Coal formation, are, in all
+succeeding formations, far less numerous than ferns, and only thirty
+living species are known. Lycopodiaceae, though still more abundant in
+the Coal formation, are very rarely found in any succeeding deposit,
+though the living species are tolerably numerous, about 500 having been
+described. As we cannot suppose them to have really diminished and then
+increased again in this extraordinary manner, we have another indication
+of the exceptional nature of plant preservation and the extreme and
+erratic character of the imperfection of the record.
+
+Passing now to the next higher division of plants--the gymnosperms--we
+find Coniferae appearing in the Upper Silurian, becoming tolerably
+abundant in the Devonian, and reaching a maximum in the Carboniferous,
+from which formation more than 300 species are known, equal to the
+number recorded as now living. They occur in all succeeding formations,
+being abundant in the Oolite, and excessively so in the Miocene, from
+which 250 species have been described. The allied family of gymnosperms,
+the Cycadaceae, first appear in the Carboniferous era, but very
+scantily; are most abundant in the Oolite, from which formation 116
+species are known, and then steadily diminish to the Tertiary, although
+there are seventy-five living species.
+
+We now come to the true flowering plants, and we first meet with
+monocotyledons in the Carboniferous and Permian formations. The
+character of these fossils was long disputed, but is now believed to be
+well established; and the sub-class continues to be present in small
+numbers in all succeeding deposits, becoming rather plentiful in the
+Upper Cretaceous, and very abundant in the Eocene and Miocene. In the
+latter formation 272 species have been discovered; but the 116 species
+in the Eocene form a larger proportion of the total vegetation of the
+period.
+
+True dicotyledons appear very much later, in the Cretaceous period, and
+only in its upper division, if we except a single species from the
+Urgonian beds of Greenland. The remarkable thing is that we here find
+the sub-class fully developed and in great luxuriance of types, all the
+three divisions--Apetalae, Polypetalae, and Gamopetalae--being
+represented, with a total of no less than 770 species. Among them are
+such familiar forms as the poplar, the birch, the beech, the sycamore,
+and the oak; as well as the fig, the true laurel, the sassafras, the
+persimmon, the maple, the walnut, the magnolia, and even the apple and
+the plum tribes. Passing on to the Tertiary period the numbers increase,
+till they reach their maximum in the Miocene, where more than 2000
+species of dicotyledons have been discovered. Among these the
+proportionate number of the higher gamopetalae has slightly increased,
+but is considerably less than at the present day.
+
+
+_Possible Cause of sudden late Appearance of Exogens._
+
+The sudden appearance of fully developed exogenous flowering plants in
+the Cretaceous period is very analogous to the equally sudden appearance
+of all the chief types of placental mammalia in the Eocene; and in both
+cases we must feel sure that this suddenness is only apparent, due to
+unknown conditions which have prevented their preservation (or their
+discovery) in earlier formations. The case of the dicotyledonous plants
+is in some respects the most extraordinary, because in the earlier
+Mesozoic formations we appear to have a fair representation of the flora
+of the period, including such varied forms as ferns, equisetums, cycads,
+conifers, and monocotyledons. The only hint at an explanation of this
+anomaly has been given by Mr. Ball, who supposes that all these groups
+inhabited the lowlands, where there was not only excessive heat and
+moisture, but also a superabundance of carbonic acid in the
+atmosphere--conditions under which these groups had been developed, but
+which were prejudicial to the dicotyledons. These latter are supposed to
+have originated on the high table-lands and mountain ranges, in a rarer
+and drier atmosphere in which the quantity of carbonic acid gas was much
+less; and any deposits formed in lake beds at high altitudes and at such
+a remote epoch have been destroyed by denudation, and hence we have no
+record of their existence.[193]
+
+During a few weeks spent recently in the Rocky Mountains, I was struck
+by the great scarcity of monocotyledons and ferns in comparison with
+dicotyledons--a scarcity due apparently to the dryness and rarity of the
+atmosphere favouring the higher groups. If we compare Coulter's _Rocky
+Mountain Botany_ with Gray's _Botany of the Northern (East) United
+States_, we have two areas which differ chiefly in the points of
+altitude and atmospheric moisture. Unfortunately, in neither of these
+works are the species consecutively numbered; but by taking the pages
+occupied by the two divisions of dicotyledons on the one hand,
+monocotyledons and ferns on the other, we can obtain a good
+approximation. In this way we find that in the flora of the
+North-Eastern States the monocotyledons and ferns are to the
+dicotyledons in the proportion of 45 to 100; in the Rocky Mountains they
+are in the proportion of only 34 to 100; while if we take an exclusively
+Alpine flora, as given by Mr. Ball, there are not one-fifth as many
+monocotyledons as dicotyledons. These facts show that even at the
+present day elevated plateaux and mountains are more favourable to
+dicotyledons than to monocotyledons, and we may, therefore, well suppose
+that the former originated within such elevated areas, and were for long
+ages confined to them. It is interesting to note that their richest
+early remains have been found in the central regions of the North
+American continent, where they now, proportionally, most abound, and
+where the conditions of altitude and a dry atmosphere were probably
+present at a very early period.
+
+[Illustration: FIG. 34.--Diagram illustrating the Geological
+Distribution of Plants.]
+
+The diagram (Fig. 34), slightly modified from one given by Mr. Ward,
+will illustrate our present knowledge of the development of the
+vegetable kingdom in geological time. The shaded vertical bands exhibit
+the proportions of the fossil forms actually discovered, while the
+outline extensions are intended to show what we may fairly presume to
+have been the approximate periods of origin, and progressive increase of
+the number of species, of the chief divisions of the vegetable kingdom.
+These seem to accord fairly well with their respective grades of
+development, and thus offer no obstacle to the acceptance of the belief
+in their progressive evolution.
+
+
+_Geological Distribution of Insects._
+
+The marvellous development of insects into such an endless variety of
+forms, their extreme specialisation, and their adaptation to almost
+every possible condition of life, would almost necessarily imply an
+extreme antiquity. Owing, however, to their small size, their lightness,
+and their usually aerial habits, no class of animals has been so
+scantily preserved in the rocks; and it is only recently that the whole
+of the scattered material relating to fossil insects and their allies
+have been brought together by Mr. Samuel H. Scudder of Boston, and we
+have thus learned their bearing on the theory of evolution.[194]
+
+The most striking fact which presents itself on a glance at the
+distribution of fossil insects, is the completeness of the
+representation of all the chief types far back in the Secondary period,
+at which time many of the existing families appear to have been
+perfectly differentiated. Thus in the Lias we find dragonflies
+"apparently as highly specialised as to-day, no less than four tribes
+being present." Of beetles we have undoubted Curculionidae from the Lias
+and Trias; Chrysomelidae in the same deposits; Cerambycidae in the
+Oolites; Scarabaeidae in the Lias; Buprestidae in the Trias; Elateridae,
+Trogositidae, and Nitidulidae in the Lias; Staphylinidae in the English
+Purbecks; while Hydrophilidae, Gyrinidae, and Carabidae occur in the
+Lias. All these forms are well represented, but there are many other
+families doubtfully identified in equally ancient rocks. Diptera of the
+families Empidae, Asilidae, and Tipulidae have been found as far back as
+the Lias. Of Lepidoptera, Sphingidae and Tineidae have been found in
+the Oolite; while ants, representing the highly specialised Hymenoptera,
+have occurred in the Purbeck and Lias.
+
+This remarkable identity of the families of very ancient with those of
+existing insects is quite comparable with the apparently sudden
+appearance of existing genera of trees in the Cretaceous epoch. In both
+cases we feel certain that we must go very much farther back in order to
+find the ancestral forms from which they were developed, and that at any
+moment some fresh discovery may revolutionise our ideas as to the
+antiquity of certain groups. Such a discovery was made while Mr.
+Scudder's work was passing through the press. Up to that date all the
+existing orders of true insects appeared to have originated in the
+Trias, the alleged moth and beetle of the Coal formation having been
+incorrectly determined. But now, undoubted remains of beetles have been
+found in the Coal measures of Silesia, thus supporting the
+interpretation of the borings in carboniferous trees as having been made
+by insects of this order, and carrying back this highly specialised form
+of insect life well into Palaeozoic times. Such a discovery renders all
+speculation as to the origin of true insects premature, because we may
+feel sure that all the other orders of insects, except perhaps
+hymenoptera and lepidoptera, were contemporaneous with the highly
+specialised beetles.
+
+The less highly organised terrestrial arthropoda--the Arachnida and
+Myriapoda--are, as might be expected, much more ancient. A fossil spider
+has been found in the Carboniferous, and scorpions in the Upper Silurian
+rocks of Scotland, Sweden, and the United States. Myriapoda have been
+found abundantly in the Carboniferous and Devonian formations; but all
+are of extinct orders, exhibiting a more generalised structure than
+living forms.
+
+Much more extraordinary, however, is the presence in the Palaeozoic
+formations of ancestral forms of true insects, termed by Mr. Scudder
+Palaeodictyoptera. They consist of generalised cockroaches and
+walking-stick insects (Orthopteroidea); ancient mayflies and allied
+forms, of which there are six families and more than thirty genera
+(Neuropteroidea); three genera of Hemipteroidea resembling various
+Homoptera and Hemiptera, mostly from the Carboniferous formation, a few
+from the Devonian, and one ancestral cockroach (Palaeoblattina) from
+the Middle Silurian sandstone of France. If this occurrence of a true
+hexapod insect from the Middle Silurian be really established, taken in
+connection with the well-defined Coleoptera from the Carboniferous, the
+origin of the entire group of terrestrial arthropoda is necessarily
+thrown back into the Cambrian epoch, if not earlier. And this cannot be
+considered improbable in view of the highly differentiated land
+plants--ferns, equisetums, and lycopods--in the Middle or Lower
+Silurian, and even a conifer (Cordaites Robbii) in the Upper Silurian;
+while the beds of graphite in the Laurentian were probably formed from
+terrestrial vegetation.
+
+On the whole, then, we may affirm that, although the geological record
+of the insect life of the earth is exceptionally imperfect, it yet
+decidedly supports the evolution hypothesis. The most specialised order,
+Lepidoptera, is the most recent, only dating back to the Oolite; the
+Hymenoptera, Diptera, and Homoptera go as far as the Lias; while the
+Orthoptera and Neuroptera extend to the Trias. The recent discovery of
+Coleoptera in the Carboniferous shows, however, that the preceding
+limits are not absolute, and will probably soon be overpassed. Only the
+more generalised ancestral forms of winged insects have been traced back
+to Silurian time, and along with them the less highly organised
+scorpions; facts which serve to show us the extreme imperfection of our
+knowledge, and indicate possibilities of a world of terrestrial life in
+the remotest Palaeozoic times.
+
+
+_Geological Succession of Vertebrata._
+
+The lowest forms of vertebrates are the fishes, and these appear first
+in the geological record in the Upper Silurian formation. The most
+ancient known fish is a Pteraspis, one of the bucklered ganoids or
+plated fishes--by no means a very low type--allied to the sturgeon
+(Accipenser) and alligator-gar (Lepidosteus), but, as a group, now
+nearly extinct. Almost equally ancient are the sharks, which under
+various forms still abound in our seas. We cannot suppose these to be
+nearly the earliest fishes, especially as the two lowest orders, now
+represented by the Amphioxus or lancelet and the lampreys, have not yet
+been found fossil. The ganoids were greatly developed in the Devonian
+era, and continued till the Cretaceous, when they gave way to the true
+osseous fishes, which had first appeared in the Jurassic period, and
+have continued to increase till the present day. This much later
+appearance of the higher osseous fishes is quite in accordance with
+evolution, although some of the very lowest forms, the lancelet and the
+lampreys, together with the archaic ceratodus, have survived to our
+time.
+
+The Amphibia, represented by the extinct labyrinthodons, appear first in
+the Carboniferous rocks, and these peculiar forms became extinct early
+in the Secondary period. The labyrinthodons were, however, highly
+specialised, and do not at all indicate the origin of the class, which
+may be as ancient as the lower forms of fishes. Hardly any recognisable
+remains of our existing groups--the frogs, toads, and salamanders--are
+found before the Tertiary period, a fact which indicates the extreme
+imperfection of the record as regards this class of animals.
+
+True reptiles have not been found till we reach the Permian where
+Prohatteria and Proterosaurus occur, the former closely allied to the
+lizard-like Sphenodon of New Zealand, the latter having its nearest
+allies in the same group of reptiles--Rhyncocephala, other forms of
+which occur in the Trias. In this last-named formation the earliest
+crocodiles--Phytosaurus (Belodon) and Stagonolepis occur, as well as the
+earliest tortoises--Chelytherium, Proganochelys, and Psephoderma.[195]
+Fossil serpents have been first found in the Cretaceous formation, but
+the conditions for the preservation of these forms have evidently been
+unfavourable, and the record is correspondingly incomplete. The marine
+Plesiosauri and Ichthyosauri, the flying Pterodactyles, the terrestrial
+Iguanodon of Europe, and the huge Atlantosaurus of Colorado--the largest
+land animal that has ever lived upon the earth[196]--all belong to
+special developments of the reptilian type which flourished during the
+Secondary epoch, and then became extinct.
+
+Birds are among the rarest of fossils, due, no doubt, to their aerial
+habits removing them from the ordinary dangers of flood, bog, or ice
+which overwhelm mammals and reptiles, and also to their small specific
+gravity which keeps them floating on the surface of water till devoured.
+Their remains were long confined to Tertiary deposits, where many living
+genera and a few extinct forms have been found. The only birds yet known
+from the older rocks are the toothed birds (Odontornithes) of the
+Cretaceous beds of the United States, belonging to two distinct families
+and many genera; a penguin-like form (Enaliornis) from the Upper
+Greensand of Cambridge; and the well-known long-tailed Archaeopteryx
+from the Upper Oolite of Bavaria. The record is thus imperfect and
+fragmentary in the extreme; but it yet shows us, in the few birds
+discovered in the older rocks, more primitive and generalised types,
+while the Tertiary birds had already become specialised like those
+living, and had lost both the teeth and the long vertebral tail, which
+indicate reptilian affinities in the earlier Mammalia have been found,
+as already stated, as far back as the Trias formation, in Europe in the
+United States and in South Africa, all being very small, and belonging
+either to the Marsupial order, or to some still lower and more
+generalised type, out of which both Marsupials and Insectivora were
+developed. Other allied forms have been found in the Lower and Upper
+Oolite both of Europe and the United States. But there is then a great
+gap in the whole Cretaceous formation, from which no mammal has been
+obtained, although both in the Wealden and the Upper Chalk in Europe,
+and in the Upper Cretaceous deposits of the United States an abundant
+and well-preserved terrestrial flora has been discovered. Why no mammals
+have left their remains here it is impossible to say. We can only
+suppose that the limited areas in which land plants have been so
+abundantly preserved, did not present the conditions which are needed
+for the fossilisation and preservation of mammalian remains.
+
+When we come to the Tertiary formation, we find mammals in abundance;
+but a wonderful change has taken place. The obscure early types have
+disappeared, and we discover in their place a whole series of forms
+belonging to existing orders, and even sometimes to existing families.
+Thus, in the Eocene we have remains of the opossum family; bats
+apparently belonging to living genera; rodents allied to the South
+American cavies and to dormice and squirrels; hoofed animals belonging
+to the odd-toed and even-toed groups; and ancestral forms of cats,
+civets, dogs, with a number of more generalised forms of carnivora.
+Besides these there are whales, lemurs, and many strange ancestral forms
+of proboscidea.[197]
+
+The great diversity of forms and structures at so remote an epoch would
+require for their development an amount of time, which, judging by the
+changes that have occurred in other groups, would carry us back far into
+the Mesozoic period. In order to understand why we have no record of
+these changes in any part of the world, we must fall back upon some such
+supposition as we made in the case of the dicotyledonous plants.
+Perhaps, indeed, the two cases are really connected, and the upland
+regions of the primeval world, which saw the development of our higher
+vegetation, may have also afforded the theatre for the gradual
+development of the varied mammalian types which surprise us by their
+sudden appearance in Tertiary times.
+
+[Illustration: GEOLOGICAL DISTRIBUTION OF MAMMALIA.]
+
+Notwithstanding these irregularities and gaps in the record, the
+accompanying table, summarising our actual knowledge of the geological
+distribution of the five classes of vertebrata, exhibits a steady
+progression from lower to higher types, excepting only the deficiency in
+the bird record which is easily explained. The comparative perfection of
+type in which each of these classes first appears, renders it certain
+that the origin of each and all of them must be sought much farther back
+than any records which have yet been discovered. The researches of
+palaeontologists and embryologists indicate a reptilian origin for birds
+and mammals, while reptiles and amphibia arose, perhaps independently,
+from fishes.
+
+
+_Concluding Remarks._
+
+The brief review we have now taken of the more suggestive facts
+presented by the geological succession of organic forms, is sufficient
+to show that most, if not all, of the supposed difficulties which it
+presents in the way of evolution, are due either to imperfections in the
+geological record itself, or to our still very incomplete knowledge of
+what is really recorded in the earth's crust. We learn, however, that
+just as discovery progresses, gaps are filled up and difficulties
+disappear; while, in the case of many individual groups, we have already
+obtained all the evidence of progressive development that can reasonably
+be expected. We conclude, therefore, that the geological difficulty has
+now disappeared; and that this noble science, when properly understood,
+affords clear and weighty evidence of evolution.
+
+FOOTNOTES:
+
+[Footnote 183: The reader who desires to understand this subject more
+fully, should study chap. x. of the _Origin of Species_, and chap. xiv.
+of Sir Charles Lyell's _Principles of Geology_.]
+
+[Footnote 184: On "Stagonolepis Robertsoni and on the Evolution of the
+Crocodilia," in _Q.J. of Geological Society_, 1875; and abstract in
+_Nature_, vol. xii. p. 38.]
+
+[Footnote 185: From a paper by Messrs. Scott and Osborne, "On the Origin
+and Development of the Rhinoceros Group," read before the British
+Association in 1883.]
+
+[Footnote 186: American Addresses, pp. 73-76.]
+
+[Footnote 187: Lecture on the Introduction and Succession of Vertebrate
+Life in America, _Nature_, vol. xvi. p. 471.]
+
+[Footnote 188: _Nature_, vol. xxv. p. 84.]
+
+[Footnote 189: See _The Mammalia in their Relation to Primeval Times_,
+p. 102.]
+
+[Footnote 190: For a brief enumeration and description of these fossils,
+see the author's _Geographical Distribution of Animals_, vol. i. p.
+146.]
+
+[Footnote 191: Sketch of Palaeobotany in Fifth Annual Report of U.S.
+Geological Survey, 1883-84, pp. 363-452, with diagrams. Sir J. William
+Dawson, speaking of the value of leaves for the determination of fossil
+plants, says: "In my own experience I have often found determinations of
+the leaves of trees confirmed by the discovery of their fruits or of the
+structure of their stems. Thus, in the rich cretaceous plant-beds of the
+Dunvegan series, we have beech-nuts associated in the same bed with
+leaves referred to _Fagus_. In the Laramie beds I determined many years
+ago nuts of the _Trapa_ or water-chestnut, and subsequently Lesquereux
+found in beds in the United States leaves which he referred to the same
+genus. Later, I found in collections made on the Red Deer River of
+Canada my fruits and Lesquereux's leaves on the same slab. The presence
+of trees of the genera _Carya_ and _Juglans_ in the same formation was
+inferred from their leaves, and specimens have since been obtained of
+silicified wood with the microscopic structure of the modern butternut.
+Still we are willing to admit that determinations from leaves alone are
+liable to doubt."--_The Geological History of Plants_, p. 196.]
+
+[Footnote 192: Sir J. William Dawson's _Geological History of Plants_,
+p. 18.]
+
+[Footnote 193: "On the Origin of the Flora of the European Alps," _Proc.
+of Roy. Geog. Society_, vol. i. (1879), pp. 564-588.]
+
+[Footnote 194: Systematic Review of our Present Knowledge of Fossil
+Insects, including Myriapods and Arachnids (_Bull. of U.S. Geol.
+Survey_, No. 31, Washington, 1886).]
+
+[Footnote 195: For the facts as to the early appearance of the above
+named groups of reptiles I am indebted to Mr. E. Lydekker of the
+Geological Department of the Natural History Museum.]
+
+[Footnote 196: According to Professor Marsh this creature was 50 or 60
+feet long, and when erect, at least 30 feet in height. It fed upon the
+foliage of the mountain forests of the Cretaceous epoch, the remains of
+which are preserved with it.]
+
+[Footnote 197: For fuller details, see the author's _Geographical
+Distribution of Animals_, and Heilprin's _Geographical and Geological
+Distribution of Animals_.]
+
+
+
+
+CHAPTER XIV
+
+FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
+
+
+    Fundamental difficulties and objections--Mr. Herbert Spencer's
+    factors of organic evolution--Disuse and effects of withdrawal
+    of natural selection--Supposed effects of disuse among wild
+    animals--Difficulty as to co-adaptation of parts by variation
+    and selection--Direct action of the environment--The American
+    school of evolutionists--Origin of the feet of the
+    ungulates--Supposed action of animal intelligence--Semper on the
+    direct influence of the environment--Professor Geddes's theory
+    of variation in plants--Objections to the theory--On the origin
+    of spines--Variation and selection overpower the effects of use
+    and disuse--Supposed action of the environment in imitating
+    variations--Weismann's theory of heredity--The cause of
+    variation--The non-heredity of acquired characters--The theory
+    of instinct--Concluding remarks.
+
+
+
+Having now set forth and illustrated at some length the most important
+of the applications of the development hypothesis in the explanation of
+the broader and more generally interesting phenomena presented by the
+organic world, we propose to discuss some of the more fundamental
+problems and difficulties which have recently been adduced by eminent
+naturalists. It is the more necessary to do this, because there is now a
+tendency to minimise the action of natural selection in the production
+of organic forms, and to set up in its place certain fundamental
+principles of variation or laws of growth, which it is urged are the
+real originators of the several lines of development, and of most of the
+variety of form and structure in the vegetable and animal kingdoms.
+These views have, moreover, been seized upon by popular writers to throw
+doubt and discredit on the whole theory of evolution, and especially on
+Darwin's presentation of that theory, to the bewilderment of the general
+public, who are quite unable to decide how far the new views, even if
+well established, tend to subvert the Darwinian theory, or whether they
+are really more than subsidiary parts of it, and quite powerless without
+it to produce any effect whatever.
+
+The writers whose special views we now propose to consider are: (1) Mr.
+Herbert Spencer, on modification of structures arising from modification
+of functions, as set forth in his _Factors of Organic Evolution_. (2)
+Dr. E.D. Cope, who advocates similar views in detail, in his work
+entitled _The Origin of the Fittest_, and may be considered the head of
+a school of American naturalists who minimise the agency of natural
+selection. (3) Dr. Karl Semper, who has especially studied the direct
+influence of the environment in the whole animal kingdom, and has set
+forth his views in a volume on _The Natural Conditions of Existence as
+they Affect Animal Life_. (4) Mr. Patrick Geddes, who urges that
+fundamental laws of growth, and the antagonism of vegetative and
+reproductive forces, account for much that has been imputed to natural
+selection.
+
+We will now endeavour to ascertain what are the more important facts and
+arguments adduced by each of the above writers, and how far they offer a
+substitute for the action of natural selection; having done which, a
+brief account will be given of the views of Dr. Aug. Weismann, whose
+theory of heredity will, if established, strike at the very root of the
+arguments of the first three of the writers above referred to.
+
+
+_Mr. Herbert Spencer's Factors of Organic Evolution._
+
+Mr. Spencer, while fully recognising the importance and wide range of
+the principle of natural selection, thinks that sufficient weight has
+not been given to the effects of use and disuse as a factor in
+evolution, or to the direct action of the environment in determining or
+modifying organic structures. As examples of the former class of
+actions, he adduces the decreased size of the jaws in the civilised
+races of mankind, the inheritance of nervous disease produced by
+overwork, the great and inherited development of the udders in cows and
+goats, and the shortened legs, jaws, and snout in improved races of
+pigs--the two latter examples being quoted from Mr. Darwin,--and other
+cases of like nature. As examples of the latter, Mr. Darwin is again
+quoted as admitting that there are many cases in which the action of
+similar conditions appears to have produced corresponding changes in
+different species; and we have a very elaborate discussion of the direct
+action of the medium in modifying the protoplasm of simple organisms, so
+as to bring about the difference between the outer surface and the inner
+part that characterises the cells or other units of which they are
+formed.
+
+Now, although this essay did little more than bring together facts which
+had been already adduced by Mr. Darwin or by Mr. Spencer himself, and
+lay stress upon their importance, its publication in a popular review
+was immediately seized upon as "an avowed and definite declaration
+against some of the leading ideas on which the Mechanical Philosophy
+depends," and as being "fatal to the adequacy of the Mechanical
+Philosophy as any explanation of organic evolution,"[198]--an expression
+of opinion which would be repudiated by every Darwinian. For, even
+admitting the interpretation which Mr. Spencer puts on the facts he
+adduces, they are all included in the causes which Darwin himself
+recognised as having acted in bringing about the infinitude of forms in
+the organic world. In the concluding chapter of the _Origin of Species_
+he says: "I have now recapitulated the facts and considerations which
+have thoroughly convinced me that species have been modified during a
+long course of descent. This has been effected chiefly through the
+natural selection of numerous successive, slight, favourable variations;
+aided in an important manner by the inherited effects of the use and
+disuse of parts; and in an unimportant manner--that is, in relation to
+adaptive structures whether past or present, by the direct action of
+external conditions, and by variations which seem to us, in our
+ignorance, to arise spontaneously." This passage, summarising Darwin's
+whole inquiry, and explaining his final point of view, shows how very
+inaccurate may be the popular notion, as expressed by the Duke of
+Argyll, of any supposed additions to the causes of change of species as
+recognised by Darwin.
+
+But, as we shall see presently, there is now much reason to believe
+that the supposed inheritance of acquired modifications--that is, of the
+effects of use and disuse, or of the direct influence of the
+environment--is not a fact; and if so, the very foundation is taken away
+from the whole class of objections on which so much stress is now laid.
+It therefore becomes important to inquire whether the facts adduced by
+Darwin, Spencer, and others, do really necessitate such inheritance, or
+whether any other interpretation of them is possible. I believe there is
+such an interpretation; and we will first consider the cases of disuse
+on which Mr. Spencer lays most stress.
+
+The cases Mr. Spencer adduces as demonstrating the effects of disuse in
+diminishing the size and strength of organs are, the diminished size of
+the jaws in the races of civilised men, and the diminution of the
+muscles used in closing the jaws in the case of pet-dogs fed for
+generations on soft food. He argues that the minute reduction in any one
+generation could not possibly have been useful, and, therefore, not the
+subject of natural selection; and against the theory of correlation of
+the diminished jaw with increased brain in man, he urges that there are
+cases of large brain development, accompanied by jaws above the average
+size. Against the theory of economy of nutrition in the case of the
+pet-dogs, he places the abundant food of these animals which would
+render such economy needless.
+
+But neither he nor Mr. Darwin has considered the effects of the
+withdrawal of the action of natural selection in keeping up the parts in
+question to their full dimensions, which, of itself, seems to me quite
+adequate to produce the results observed. Recurring to the evidence,
+adduced in Chapter III, of the constant variation occurring in all parts
+of the organism, while selection is constantly acting on these
+variations in eliminating all that fall below the best working standard,
+and preserving only those that are fully up to it; and, remembering
+further, that, of the whole number of the increase produced annually,
+only a small percentage of the best adapted can be preserved, we shall
+see that every useful organ will be kept up nearly to its higher limit
+of size and efficiency. Now Mr. Galton has proved experimentally that,
+when any part has thus been increased (or diminished) by selection,
+there is in the offspring a strong tendency to revert to a mean or
+average size, which tends to check further increase. And this mean
+appears to be, not the mean of the actual existing individuals but a
+lower mean, or that from which they had been recently raised by
+selection.[199] He calls this the law of "Regression towards
+Mediocrity," and it has been proved by experiments with vegetables and
+by observations on mankind. This regression, in every generation, takes
+place even when both parents have been selected for their high
+development of the organ in question; but when there is no such
+selection, and crosses are allowed among individuals of every grade of
+development, the deterioration will be very rapid; and after a time not
+only will the average size of the part be greatly reduced, but the
+instances of full development will become very rare. Thus what Weismann
+terms "panmixia," or free intercrossing, will co-operate with Galton's
+law of "regression towards mediocrity," and the result will be that,
+whenever selection ceases to act on any part or organ which has
+heretofore been kept up to a maximum of size and efficiency, the organ
+in question will rapidly decrease till it reaches a mean value
+considerably below the mean of the progeny that has usually been
+produced each year, and very greatly below the mean of that portion
+which has survived annually; and this will take place by the general law
+of heredity, and quite irrespective of any _use_ or _disuse_ of the part
+in question. Now, no observations have been adduced by Mr. Spencer or
+others, showing that the average amount of change supposed to be due to
+_disuse_ is greater than that due to the law of regression towards
+mediocrity; while even if it were somewhat greater, we can see many
+possible contributory causes to its production. In the case of civilised
+man's diminished jaw, there may well be some correlation between the jaw
+and the brain, seeing that increased mental activity would lead to the
+withdrawal of blood and of nervous energy from adjacent parts, and might
+thus lead to diminished growth of those parts in the individual. And in
+the case of pet-dogs, the selection of small or short-headed individuals
+would imply the unconscious selection of those with less massive
+temporal muscles, and thus lead to the concomitant reduction of those
+muscles. The amount of reduction observed by Darwin in the wing-bones of
+domestic ducks and poultry, and in the hind legs of tame rabbits, is
+very small, and is certainly no greater than the above causes will well
+account for; while so many of the external characters of all our
+domestic animals have been subject to long-continued artificial
+selection, and we are so ignorant of the possible correlations of
+different parts, that the phenomena presented by them seem sufficiently
+explained without recurrence to the assumption that any changes in the
+individual, due to disuse, are inherited by the offspring.
+
+
+_Supposed Effects of Disuse among Wild Animals._
+
+It may be urged, however, that among wild animals we have many undoubted
+results of disuse much more pronounced than those among domestic kinds,
+results which cannot be explained by the causes already adduced. Such
+are the reduced size of the wings of many birds on oceanic islands; the
+abortion of the eyes in many cave animals, and in some which live
+underground; and the loss of the hind limbs in whales and in some
+lizards. These cases differ greatly in the amount of the reduction of
+parts which has taken place, and may be due to different causes. It is
+remarkable that in some of the birds of oceanic islands the reduction is
+little if at all greater than in domestic birds, as in the water-hen of
+Tristan d'Acunha. Now if the reduction of wing were due to the
+hereditary effects of disuse, we should expect a very much greater
+effect in a bird inhabiting an oceanic island than in a domestic bird,
+where the disuse has been in action for an indefinitely shorter period.
+In the case of many other birds, however--as some of the New Zealand
+rails and the extinct dodo of Mauritius--the wings have been reduced to
+a much more rudimentary condition, though it is still obvious that they
+were once organs of flight; and in these cases we certainly require some
+other causes than those which have reduced the wings of our domestic
+fowls. One such cause may have been of the same nature as that which has
+been so efficient in reducing the wings of the insects of oceanic
+islands--the destruction of those which, during the occasional use of
+their wings, were carried out to sea. This form of natural selection may
+well have acted in the case of birds whose powers of flight were
+already somewhat reduced, and to whom, there being no enemies to escape
+from, their use was only a source of danger. We may thus, perhaps,
+account for the fact that many of these birds retain small but useless
+wings with which they never fly; for, the wings having been reduced to
+this functionless condition, no power could reduce them further except
+correlation of growth or economy of nutrition, causes which only rarely
+come into play.
+
+The complete loss of eyes in some cave animals may, perhaps, be
+explained in a somewhat similar way. Whenever, owing to the total
+darkness, they became useless, they might also become injurious, on
+account of their delicacy of organisation and liability to accidents and
+disease; in which case natural selection would begin to act to reduce,
+and finally abort them; and this explains why, in some cases, the
+rudimentary eye remains, although completely covered by a protective
+outer skin. Whales, like moas and cassowaries, carry us back to a remote
+past, of whose conditions we know too little for safe speculation. We
+are quite ignorant of the ancestral forms of either of these groups, and
+are therefore without the materials needful for determining the steps by
+which the change took place, or the causes which brought it about.[200]
+
+On a review of the various examples that have been given by Mr. Darwin
+and others of organs that have been reduced or aborted, there seems too
+much diversity in the results for all to be due to so direct and uniform
+a cause as the individual effects of disuse accumulated by heredity. For
+if that were the only or chief efficient cause, and a cause capable of
+producing a decided effect during the comparatively short period of the
+existence of animals in a state of domestication, we should expect to
+find that, in wild species, all unused parts or organs had been reduced
+to the smallest rudiments, or had wholly disappeared. Instead of this we
+find various grades of reduction, indicating the probable result of
+several distinct causes, sometimes acting separately, sometimes in
+combination, such as those we have already pointed out.
+
+And if we find no positive evidence of _disuse_, acting by its direct
+effect on the individual, being transmitted to the offspring, still less
+can we find such evidence in the case of the _use_ of organs. For here
+the very fact of _use_, in a wild state, implies _utility_, and utility
+is the constant subject for the action of natural selection; while among
+domestic animals those parts which are exceptionally used are so used in
+the service of man, and have thus become the subjects of artificial
+selection. Thus "the great and inherited development of the udders in
+cows and goats," quoted by Spencer from Darwin, really affords no proof
+of inheritance of the increase due to use, because, from the earliest
+period of the domestication of these animals, abundant milk-production
+has been highly esteemed, and has thus been the subject of selection;
+while there are no cases among wild animals that may not be better
+explained by variation and natural selection.
+
+
+_Difficulty as to Co-adaptation of Parts by Variation and Selection._
+
+Mr. Spencer again brings forward this difficulty, as he did in his
+_Principles of Biology_ twenty-five years ago, and urges that all the
+adjustments of bones, muscles, blood-vessels, and nerves which would be
+required during, for example, the development of the neck and fore-limbs
+of the giraffe, could not have been effected by "simultaneous fortunate
+spontaneous variations." But this difficulty is fully disposed of by the
+facts of simultaneous variation adduced in our third chapter, and has
+also been specially considered in Chapter VI, p. 127. The best answer to
+this objection may, perhaps, be found in the fact that the very thing
+said to be impossible by variation and natural selection has been again
+and again effected by variation and artificial selection. During the
+process of formation of such breeds as the greyhound or the bulldog, of
+the race-horse and carthorse, of the fantail pigeon or the otter-sheep,
+many co-ordinate adjustments have been produced; and no difficulty has
+occurred, whether the change has been effected by a single variation--as
+in the last case named--or by slow steps, as in all the others. It seems
+to be forgotten that most animals have such a surplus of vitality and
+strength for all the ordinary occasions of life that any slight
+superiority in one part can be at once utilised; while the moment any
+want of balance occurs, variations in the insufficiently developed parts
+will be selected to bring back the harmony of the whole organisation.
+The fact that, in all domestic animals, variations do occur, rendering
+them swifter or stronger, larger or smaller, stouter or slenderer, and
+that such variations can be separately selected and accumulated for
+man's purposes, is sufficient to render it certain that similar or even
+greater changes may be effected by natural selection, which, as Darwin
+well remarks, "acts on every internal organ, on every shade of
+constitututional difference, on the whole machinery of life." The
+difficulty as to co-adaptation of parts by variation and natural
+selection appears to me, therefore, to be a wholly imaginary difficulty
+which has no place whatever in the operations of nature.
+
+
+_Direct Action of the Environment._
+
+Mr. Spencer's last objection to the wide scope given by Darwinians to
+the agency of natural selection is, that organisms are acted upon by the
+environment, which produces in them definite changes, and that these
+changes in the individual are transmitted by inheritance, and thus
+become increased in successive generations. That such changes are
+produced in the individual there is ample evidence, but that they are
+inherited independently of any form of selection or of reversion is
+exceedingly doubtful, and Darwin nowhere expresses himself as satisfied
+with the evidence. The two very strongest cases he mentions are the
+twenty-nine species of American trees which all differed in a
+corresponding way from their nearest European allies; and the American
+maize which became changed after three generations in Europe. But in the
+case of the trees the differences alleged may be partly due to
+correlation with constitutional peculiarities dependent on climate,
+especially as regards the deeper tint of the fading leaves and the
+smaller size of the buds and seeds in America than in Europe; while the
+less deeply toothed or serrated leaves in the American species are, in
+our present complete ignorance of the causes and uses of serration,
+quite as likely to be due to some form of adaptation as to any direct
+action of the climate. Again, we are not told how many of the allied
+species do not vary in this particular manner, and this is certainly an
+important factor in any conclusion we may form on the question.
+
+In the case of the maize it appears that one of the more remarkable and
+highly selected American varieties was cultivated in Germany, and in
+three years nearly all resemblance to the original parent was lost; and
+in the sixth year it closely resembled a common European variety, but
+was of somewhat more vigorous growth. In this case no selection appears
+to have been practised, and the effects may have been due to that
+"reversion to mediocrity" which invariably occurs, and is more
+especially marked in the case of varieties which have been rapidly
+produced by artificial selection. It may be considered as a partial
+reversion to the wild or unimproved stock; and the same thing would
+probably have occurred, though perhaps less rapidly, in America itself.
+As this is stated by Darwin to be the most remarkable case known to him
+"of the direct and prompt action of climate on a plant," we must
+conclude that such direct effects have not been proved to be accumulated
+by inheritance, independently of reversion or selection.
+
+The remaining part of Mr. Spencer's essay is devoted to a consideration
+of the hypothetical action of the environment on the lower organisms
+which consist of simple cells or formless masses of protoplasm; and he
+shows with great elaboration that the outer and inner parts of these
+are necessarily subject to different conditions; and that the outer
+actions of air or water lead to the formation of integuments, and
+sometimes to other definite modifications of the surface, whence arise
+permanent differences of structure. Although in these cases also it is
+very difficult to determine how much is due to direct modification by
+external agencies transmitted and accumulated by inheritance, and how
+much to spontaneous variations accumulated by natural selection, the
+probabilities in favour of the former mode of action are here greater,
+because there is no differentiation of nutritive and reproductive cells
+in these simple organisms; and it can be readily seen that any change
+produced in the latter will almost certainly affect the next
+generation.[201] We are thus carried back almost to the origin of life,
+and can only vaguely speculate on what took place under conditions of
+which we know so little.
+
+
+_The American School of Evolutionists._
+
+The tentative views of Mr. Spencer which we have just discussed, are
+carried much further, and attempts have been made to work them out in
+great detail, by many American naturalists, whose best representative is
+Dr. E.D. Cope of Philadelphia.[202] This school endeavours to explain
+all the chief modifications of form in the animal kingdom by fundamental
+laws of growth and the inherited effects of use and effort, returning,
+in fact, to the teachings of Lamarck as being at least equally important
+with those of Darwin.
+
+The following extract will serve to show the high position claimed by
+this school as original discoverers, and as having made important
+additions to the theory of evolution:
+
+"Wallace and Darwin have propounded as the cause of modification in
+descent their law of natural selection. This law has been epitomised by
+Spencer as the 'survival of the fittest.' This neat expression no doubt
+covers the case, but it leaves the origin of the fittest entirely
+untouched. Darwin assumes a 'tendency to variation' in nature, and it is
+plainly necessary to do this, in order that materials for the exercise
+of a selection should exist. Darwin and Wallace's law is then only
+restrictive, directive, conservative, or destructive of something
+already created. I propose, then, to seek for the originative laws by
+which these subjects are furnished; in other words, for the causes of
+the origin of the fittest."[203]
+
+Mr. Cope lays great stress on the existence of a special developmental
+force termed "bathmism" or growth-force, which acts by means of
+retardation and acceleration "without any reference to fitness at all;"
+that "instead of being controlled by fitness it is the controller of
+fitness." He argues that "all the characteristics of generalised groups
+from genera up (excepting, perhaps, families) have been evolved under
+the law of acceleration and retardation," combined with some
+intervention of natural selection; and that specific characters, or
+species, have been evolved by natural selection with some assistance
+from the higher law. He, therefore, makes species and genera two
+absolutely distinct things, the latter not developed out of the former;
+generic characters and specific characters are, in his opinion,
+fundamentally different, and have had different origins, and whole
+groups of species have been simultaneously modified, so as to belong to
+another genus; whence he thinks it "highly probable that the same
+specific form has existed through a succession of genera, and perhaps in
+different epochs of geologic time."
+
+Useful characters, he concludes, have been produced by the special
+location of growth-force by use; useless ones have been produced by
+location of growth-force without the influence of use. Another element
+which determines the direction of growth-force, and which precedes use,
+is effort; and "it is thought that effort becomes incorporated into the
+metaphysical acquisitions of the parent, and is inherited with other
+metaphysical qualities by the young, which, during the period of growth,
+is much more susceptible to modifying influences, and is likely to
+exhibit structural change in consequence."[204]
+
+From these few examples of their teachings, it is clear that these
+American evolutionists have departed very widely from the views of Mr.
+Darwin, and in place of the well-established causes and admitted laws to
+which he appeals have introduced theoretical conceptions which have not
+yet been tested by experiments or facts, as well as metaphysical
+conceptions which are incapable of proof. And when they come to
+illustrate these views by an appeal to palaeontology or morphology, we
+find that a far simpler and more complete explanation of the facts is
+afforded by the established principles of variation and natural
+selection. The confidence with which these new ideas are enunciated, and
+the repeated assertion that without them Darwinism is powerless to
+explain the origin of organic forms, renders it necessary to bestow a
+little more time on the explanations they give us of well-known
+phenomena with which, they assert, other theories are incompetent to
+grapple.
+
+As examples of use producing structural change, Mr. Cope adduces the
+hooked and toothed beaks of the falcons and the butcher-birds, and he
+argues that the fact of these birds belonging to widely different groups
+proves that similarity of use has produced a similar structural result.
+But no attempt is made to show any direct causal connection between the
+use of a bill to cut or tear flesh and the development of a tooth on the
+mandible. Such use might conceivably strengthen the bill or increase its
+size, but not cause a special tooth-like outgrowth which was not present
+in the ancestral thrush-like forms of the butcher-bird. On the other
+hand, it is clear that any variations of the bill tending towards a hook
+or tooth would give the possessor some advantage in seizing and tearing
+its prey, and would thus be preserved and increased by natural
+selection. Again, Mr. Cope urges the effects of a supposed "law of polar
+or centrifugal growth" to counteract a tendency to unsymmetrical growth,
+where one side of the body is used more than the other. But the
+undoubted hurtfulness of want of symmetry in many important actions or
+functions would rapidly eliminate any such tendency. When, however, it
+has become useful, as in the case of the single enlarged claw of many
+Crustacea, it has been preserved by natural selection.
+
+
+_Origin of the Feet of the Ungulates._
+
+Perhaps the most original and suggestive of Mr. Cope's applications of
+the theory of use and effort in modifying structure are, his chapters
+"On the Origin of the Foot-Structure of the Ungulates;" and that "On the
+Effect of Impacts and Strains on the Feet of Mammalia;" and they will
+serve also to show the comparative merits of this theory and that of
+natural selection in explaining a difficult case of modification,
+especially as it is an explanation claimed as new and original when
+first enunciated in 1881. Let us, then, see how he deals with the
+problem.
+
+The remarkable progressive change of a four or five-toed ancestor into
+the one-toed horse, and the equally remarkable division of the whole
+group of ungulate animals into the odd-toed and even-toed divisions, Mr.
+Cope attempts to explain by the effects of impact and use among animals
+which frequented hard or swampy ground respectively. On hard ground, it
+is urged, the long middle toe would be most used and subjected to the
+greatest strains, and would therefore acquire both strength and
+development. It would then be still more exclusively used, and the extra
+nourishment required by it would be drawn from the adjacent less-used
+toes, which would accordingly diminish in size, till, after a long
+series of changes, the records of which are so well preserved in the
+American tertiary rocks, the true one-toed horse was developed. In soft
+or swampy ground, on the other hand, the tendency would be to spread out
+the foot so that there were two toes on each side. The two middle toes
+would thus be most used and most subject to strains, and would,
+therefore, increase at the expense of the lateral toes. There would be,
+no doubt, an advantage in these two functional toes being of equal size,
+so as to prevent twisting of the foot while walking; and variations
+tending to bring this about would be advantageous, and would therefore
+be preserved. Thus, by a parallel series of changes in another
+direction, adapted to a distinct set of conditions, we should arrive at
+the symmetrical divided hoofs of our deer and cattle. The fact that
+sheep and goats are specially mountain and rock-loving animals may be
+explained by their being a later modification, since the divided hoof
+once formed is evidently well adapted to secure a firm footing on rugged
+and precipitous ground, although it could hardly have been first
+developed in such localities. Mr. Cope thus concludes: "Certain it is
+that the length of the bones in the feet of the ungulate orders has a
+direct relation to the dryness of the ground they inhabit, and the
+possibility of speed which their habit permits them or necessarily
+imposes on them."[205]
+
+If there is any truth in the explanation here briefly summarised, it
+must entirely depend on the fact of individual modifications thus
+produced being hereditary, and we yet await the proof of this. In the
+meantime it is clear that the very same results could have been brought
+about by variation and natural selection. For the toes, like all other
+organs, vary in size and proportions, and in their degree of union or
+separation; and if in one group of animals it was beneficial to have the
+middle toe larger and longer, and in another set to have the two middle
+toes of the same size, nothing can be more certain than that these
+particular modifications would be continuously preserved, and the very
+results we see ultimately produced.
+
+The oft-repeated objections that the cause of variations is unknown,
+that there must be something to determine variations in the right
+direction; that "natural selection includes no actively progressive
+principle, but must wait for the development of variation, and then,
+after securing the survival of the best, wait again for the best to
+project its own variations for selection," we have already sufficiently
+answered by showing that variation--in abundant or typical species--is
+always present in ample amount; that it exists in all parts and organs;
+that these vary, for the most part, independently, so that any required
+combination of variations can be secured; and finally, that all
+variation is necessarily either in excess or defect of the mean
+condition, and that, consequently, the right or favourable variations
+are so frequently present that the unerring power of natural selection
+never wants materials to work upon.
+
+_Supposed Action of Animal Intelligence._
+
+The following passage briefly summarises Mr. Cope's position:
+"Intelligence is a conservative principle, and will always direct effort
+and use into lines which will be beneficial to its possessor. Here we
+have the source of the fittest, _i.e._ addition of parts by increase and
+location of growth-force, directed by the influence of various kinds of
+compulsion in the lower, and intelligent option among higher animals.
+Thus intelligent choice, taking advantage of the successive evolution of
+physical conditions, may be regarded as the _originator of the fittest_,
+while natural selection is the tribunal to which all results of
+accelerated growth are submitted. This preserves or destroys them, and
+determines the new points of departure on which accelerated growth shall
+build."[206]
+
+This notion of "intelligence"--the intelligence of the animal
+itself--determining its own variation, is so evidently a very partial
+theory, inapplicable to the whole vegetable kingdom, and almost so to
+all the lower forms of animals, amongst which, nevertheless, there is
+the very same adaptation and co-ordination of parts and functions as
+among the highest, that it is strange to see it put forward with such
+confidence as necessary for the completion of Darwin's theory. If "the
+various kinds of compulsion"--by which are apparently meant the laws of
+variation, growth, and reproduction, the struggle for existence, and the
+actions necessary to preserve life under the conditions of the animal's
+environment--are sufficient to have developed the varied forms of the
+lower animals and of plants, we can see no reason why the same
+"compulsion" should not have carried on the development of the higher
+animals also. The action of this "intelligent option" is altogether
+unproved; while the acknowledgment that natural selection is the
+tribunal which either preserves or destroys the variations submitted to
+it, seems quite inconsistent with the statement that intelligent choice
+is the "orginator of the fittest," since whatever is really "the
+fittest" can never be destroyed by natural selection, which is but
+another name for the survival of the fittest. If "the fittest" is always
+definitely produced by some other power, then natural selection is not
+wanted. If, on the other hand, both fit and unfit are produced, and
+natural selection decides between them, that is pure Darwinism, and Mr.
+Cope's theories have added nothing to it.
+
+[Illustration: FIG. 35.--Transformation of Artemia salina to A.
+Milhausenii; 1, tail-lobe of A. salina, and its transition through
+2,3,4,5, to 6, into that of A. Milhausenii; 7, post-abdomen of A.
+salina; 8, post-abdomen of a form bred in brackish water; 9, gill of A.
+Milhausenii; 10, gill of A. salina. (From Schmankewitsch.)]
+
+
+_Semper on the Direct Influence of the Environment._
+
+Another eminent naturalist, Professor Karl Semper of Wuerzburg, also
+adopts the view of the direct transforming power of the environment, and
+has brought together an immense body of interesting facts showing the
+influence of food, of light, of temperature, of still water and moving
+water, of the atmosphere and its currents, of gravitation, and of other
+organisms, in modifying the forms and other characteristics of
+animals.[207] He believes that these various influences produce a direct
+and important effect, and that this effect is accumulated by
+inheritance; yet he acknowledges that we have no direct evidence of
+this, and there is hardly a single case adduced in the book which is not
+equally well explained by adaptation, brought about by the survival of
+beneficial variations. Perhaps the most remarkable case he has brought
+forward is that of the transformation of species of crustaceans by a
+change in the saltness of the water (see Fig. 35). Artemia salina lives
+in brackish water, while A. Milhausenii inhabits water which is much
+salter. They differ greatly in the form of the tail-lobes, and in the
+presence or absence of spines upon the tail, and had always been
+considered perfectly distinct species. Yet either was transformed into
+the other in a few generations, during which the saltness of the water
+was gradually altered. Yet more, A. salina was gradually accustomed to
+fresher water, and in the course of a few generations, when the water
+had become perfectly fresh, the species was changed into Branchipus
+stagnalis, which had always been considered to belong to a different
+genus on account of differences in the form of the antennae and of the
+posterior segments of the body (see Fig. 36). This certainly appears to
+be a proof of change of conditions producing a change of form
+independently of selection, and of that change of form, while remaining
+under the same conditions, being inherited. Yet there is this
+peculiarity in the case, that there is a chemical change in the water,
+and that this water permeates the whole body, and must be absorbed by
+the tissues, and thus affect the ova and even the reproductive
+elements, and in this way may profoundly modify the whole organisation.
+Why and how the external effects are limited to special details of the
+structure we do not know; but it does not seem as if any far-reaching
+conclusions as to the cumulative effect of external conditions on the
+higher terrestrial animals and plants, can be drawn from such an
+exceptional phenomenon. It seems rather analogous to those effects of
+external influences on the very lowest organisms in which the vegetative
+and reproductive organs are hardly differentiated, in which case such
+effects are doubtless inherited.[208]
+
+[Illustration: FIG. 36. _a._ Branchipus stagnalis. _b._ Artemia salina.]
+
+
+_Professor Geddes's Theory of Variation in Plants._
+
+In a paper read before the Edinburgh Botanical Society in 1886 Mr.
+Patrick Geddes laid down the outlines of a fundamental theory of plant
+variation, which he has further extended in the article "Variation and
+Selection" in the _Encydopaedia Britannica_, and in a paper read before
+the Linnaean Society but not yet published.
+
+A theory of variation should deal alike with the origin of specific
+distinctions and with those vaster differences which characterise the
+larger groups, and he thinks it should answer such questions as--How an
+axis comes to be arrested to form a flower? how the various forms of
+inflorescence were evolved? how did perigynous or epigynous flowers
+arise from hypogynous flowers? and many others equally fundamental.
+Natural selection acting upon numerous accidental variations will not,
+he urges, account for such general facts as these, which must depend on
+some constant law of variation. This law he believes to be the
+well-known antagonism of vegetative and reproductive growth acting
+throughout the whole course of plant development; and he uses it to
+explain many of the most characteristic features of the structure of
+flowers and fruits.
+
+Commencing with the origin of the flower, which all botanists agree in
+regarding as a shortened branch, he explains this shortening as an
+inevitable physiological fact, since the cost of the development of the
+reproductive elements is so great as necessarily to check vegetative
+growth. In the same manner the shortening of the inflorescence from
+raceme to spike or umbel, and thence to the capitulum or dense
+flower-head of the composite plants is brought about. This shortening,
+carried still further, produces the flattened leaf-like receptacle of
+Dorstenia, and further still the deeply hollowed fruity receptacle of
+the fig.
+
+The flower itself undergoes a parallel modification due to a similar
+cause. It is formed by a series of modified leaves arranged round a
+shortened axis. In its earlier stages the number of these modified
+leaves is indefinite, as in many Ranunculaceae; and the axis itself is
+not greatly shortened, as in Myosurus. The first advance is to a
+definite number of parts and a permanently shortened axis, in the
+arrangement termed hypogynous, in which all the whorls are quite
+distinct from each other. In the next stage there is a further
+shortening of the central axis, leaving the outer portion as a ring on
+which the petals are inserted, producing the arrangement termed
+perigynous. A still further advance is made by the contraction of the
+axis, so as to leave the central part forming the ovary quite below the
+flower, which is then termed epigynous.
+
+These several modifications are said to be parallel and definite, and to
+be determined by the continuous checking of vegetation by reproduction
+along what is an absolute groove of progressive change. This being the
+case, the importance of natural selection is greatly diminished. Instead
+of selecting and accumulating spontaneous indefinite variations, its
+function is to retard them after the stage of maximum utility has been
+independently reached. The same simple conception is said to unlock
+innumerable problems of vegetable morphology, large and small alike. It
+explains the inevitable development of gymnosperm into angiosperm by the
+checked vegetative growth of the ovule-bearing leaf or carpel; while
+such minor adaptations as the splitting fruit of the geranium or the
+cupped stigma of the pansy, can be no longer looked upon as achievements
+of natural selection, but must be regarded as naturally traceable to
+the vegetative checking of their respective types of leaf organ. Again,
+a detailed examination of spiny plants practically excludes the
+hypothesis of mammalian selection altogether, and shows spines to arise
+as an expression of the diminishing vegetativeness--in fact, the ebbing
+vitality of a species.[209]
+
+
+_Objections to the Theory._
+
+The theory here sketched out is enticing, and at first sight seems
+calculated to throw much light on the history of plant development; but
+on further consideration, it seems wanting in definiteness, while it is
+beset with difficulties at every step. Take first the shortening of the
+raceme into the umbel and the capitulum, said to be caused by arrest of
+vegetative growth, due to the antagonism of reproduction. If this were
+the whole explanation of the phenomenon, we should expect the quantity
+of seed to increase as this vegetative growth diminished, since the seed
+is the product of the reproductive energy of the plant, and its quantity
+the best measure of that energy. But is this the case? The ranunculus
+has comparatively few seeds, and the flowers are not numerous; while in
+the same order the larkspur and the columbine have far more seeds as
+well as more flowers, but there is no shortening of the raceme or
+diminution of the foliage, although the flowers are large and complex.
+So, the extremely shortened and compressed flower-heads of the
+compositae produce comparatively few seeds--one only to each flower;
+while the foxglove, with its long spike of showy flowers, produces an
+enormous number.
+
+Again, if the shortening of the central axis in the successive stages of
+hypogynous, perigynous, and epigynous flowers were an indication of
+preponderant reproduction and diminished vegetation, we should find
+everywhere some clear indications of this fact. The plants with
+hypogynous flowers should, as a rule, have less seed and more vigorous
+and abundant foliage than those at the other extreme with epigynous
+flowers. But the hypogynous poppies, pinks, and St. John's worts have
+abundance of seed and rather scanty foliage; while the epigynous
+dogwoods and honeysuckles have few seeds and abundant foliage. If,
+instead of the number of the seeds, we take the size of the fruit as an
+indication of reproductive energy, we find this at a maximum in the
+gourd family, yet their rapid and luxuriant growth shows no diminution
+of vegetative power. So that the statement that plant modifications
+proceed "along an absolute groove of progressive change" is contradicted
+by innumerable facts indicating advance and regression, improvement or
+degradation, according as the ever-changing environment renders one form
+more advantageous than the other. As one instance I may mention the
+Anonaceae or custard-apple tribe, which are certainly an advance from
+the Ranunculaceae; yet in the genus Polyalthea the fruit consists of a
+number of separate carpels, each borne on a long stalk, as if reverting
+to the primitive stalked carpellary leaves.
+
+
+_On the Origin of Spines._
+
+But perhaps the most extraordinary application of the theory is that
+which considers spines to be an indication of the "ebbing vitality of a
+species," and which excludes "mammalian selection altogether." If this
+were true, spines should occur mainly in feeble, rare, and dying-out
+species, instead of which we have the hawthorn, one of our most vigorous
+shrubs or trees, with abundant vitality and an extensive range over the
+whole Palaearctic region, showing that it is really a dominant species.
+In North America the numerous thorny species of Crataegus are equally
+vigorous, as are the false acacia (Robinia) and the honey-locust
+(Gleditschia). Neither have the numerous species of very spiny Acacias
+been noticed to be rarer or less vigorous than the unarmed kinds.
+
+On the other point--that spines are not due to mammalian selection--we
+are able to adduce what must be considered direct and conclusive
+evidence. For if spines, admittedly produced by aborted branches,
+petioles, or peduncles, are due solely or mainly to diminished
+vegetativeness or ebbing vitality, they ought to occur in all countries
+alike, or at all events in all whose similar conditions tend to check
+vegetation; whereas, if they are, solely or mainly, developed as a
+protection against the attacks of herbivorous mammals, they ought to be
+most abundant where these are plentiful, and rare or absent where
+indigenous mammalia are wanting. Oceanic islands, as compared with
+continents, would thus furnish a crucial test of the two theories; and
+Mr. Hemsley of Kew, who has specially studied insular floras, has given
+me some valuable information on this point. He says: "There are no spiny
+or prickly plants in the indigenous element of the St. Helena flora. The
+relatively rich flora of the Sandwich Isles is not absolutely without a
+prickly plant, but almost so. All the endemic genera are unarmed, and
+the endemic species of almost every other genus. Even such genera as
+Zanthoxylon, Acacia, Xylosoma, Lycium, and Solanum, of which there are
+many armed species in other countries, are only represented by unarmed
+species. The two endemic Rubi have the prickles reduced to the setaceous
+condition, and the two palms are unarmed.
+
+"The flora of the Galapagos includes a number of prickly plants, among
+them several cacti (these have not been investigated and may be American
+species), but I do not think one of the known endemic species of any
+family is prickly or spiny.
+
+"Spiny and prickly plants are also rare in New Zealand, but there are
+the formidably armed species of wild Spaniard (Aciphylla), one species
+of Rubus, the pungent-leaved Epacrideae and a few others."
+
+Mr. J.G. Baker of Kew, who has specially studied the flora of Mauritius
+and the adjacent islands, also writes me on this point. He says: "Taking
+Mauritius alone, I do not call to mind a single species that is a
+spinose endemic tree or shrub. If you take the whole group of islands
+(Mauritius, Bourbon, Seychelles, and Rodriguez), there will be about a
+dozen species, but then nine of these are palms. Leaving out palms, the
+trees and shrubs of that part of the world are exceptionally
+non-spinose."
+
+These are certainly remarkable facts, and quite inexplicable on the
+theory of spines being caused solely by checked vegetative growth, due
+to weakness of constitution or to an arid soil and climate. For the
+Galapagos and many parts of the Sandwich Islands are very arid, as is a
+considerable part of the North Island of New Zealand. Yet in our own
+moist climate and with our very limited number of trees and shrubs we
+have about eighteen spiny or prickly species, more, apparently, than in
+the whole endemic floras of the Mauritius, Sandwich Islands, and
+Galapagos, though these are all especially rich in shrubby and arboreal
+species. In New Zealand the prickly Rubus is a leafless trailing plant,
+and its prickles are probably a protection against the large snails of
+the country, several of which have shells from two to three and a half
+inches long.[210] The "wild Spaniards" are very spiny herbaceous
+Umbelliferae, and may have gained their spines to preserve them from
+being trodden down or eaten by the Moas, which, for countless ages, took
+the place of mammals in New Zealand. The exact use or meaning of the
+spines in palms is more doubtful, though they are, no doubt, protective
+against some animals; but it is certainly an extraordinary fact that in
+the entire flora of the Mauritius, so largely consisting of trees and
+shrubs, not a single endemic species should be thorny or spiny.
+
+If now we consider that every continental flora produces a considerable
+proportion of spiny and thorny species, and that these rise to a maximum
+in South Africa, where herbivorous mammalia were (before the settlement
+of the country), perhaps, more abundant and varied than in any other
+part of the world; while another district, remarkable for well-armed
+vegetation, is Chile, where the camel-like vicugnas, llamas, and
+alpacas, and an abundance of large rodents wage perpetual war against
+shrubby vegetation, we shall see the full significance of the almost
+total absence of thorny and spiny plants in the chief oceanic islands;
+and so far from "excluding the hypothesis of mammalian selection
+altogether," we shall find in this hypothesis the only satisfactory
+explanation of the facts.
+
+From the brief consideration of Professor Geddes's theory now given, we
+conclude that, although the antagonism between vegetative and
+reproductive growth is a real agency, and must be taken account of in
+our endeavour to explain many of the fundamental facts in the structure
+and form of plants, yet it is so overpowered and directed at every step
+by the natural selection of favourable variations, that the results of
+its exclusive and unmodified action are nowhere to be found in nature.
+It may be allowed to rank as one of those "laws of growth," of which so
+many have now been indicated, and which were always recognised by Darwin
+as underlying all variation; but unless we bear in mind that its action
+must always be subordinated to natural selection, and that it is
+continually checked, or diverted, or even reversed by the necessity of
+adaptation to the environment, we shall be liable to fall into such
+glaring errors as the imputing to "ebbing vitality" alone such a
+widespread phenomenon as the occurrence of spines and thorns, while
+ignoring altogether the influence of the organic environment in their
+production.[211]
+
+The sketch now given of the chief attempts that have been made to prove
+that either the direct action of the environment or certain fundamental
+laws of variation are independent causes of modification of species,
+shows us that their authors have, in every case, failed to establish
+their contention. Any direct action of the environment, or any
+characters acquired by use or disuse, can have no effect whatever upon
+the race unless they are inherited; and that they are inherited in any
+case, except when they directly affect the reproductive cells, has not
+been proved. On the other hand, as we shall presently show, there is
+much reason for believing that such acquired characters are in their
+nature non-heritable.
+
+
+_Variation and Selection Overpower the Effects of Use and Disuse._
+
+But there is another objection to this theory arising from the very
+nature of the effects produced. In each generation the effects of use or
+disuse, or of effort, will certainly be very small, while of this small
+effect it is not maintained that the whole will be always inherited by
+the next generation. How small the effect is we have no means of
+determining, except in the case of disuse, which Mr. Darwin investigated
+carefully. He found that in twelve fancy breeds of pigeons, which are
+often kept in aviaries, or if free fly but little, the sternum had been
+reduced by about one-seventh or one-eighth of its entire length, and
+that of the scapula about one-ninth. In domestic ducks the weight of the
+wing-bones in proportion to that of the whole skeleton had decreased
+about one-tenth. In domestic rabbits the bones of the legs were found to
+have increased in weight in due proportion to the increased weight of
+the body, but those of the hind legs were rather less in proportion to
+those of the fore legs than in the wild animal, a difference which may
+be imputed to their being less used in rapid motion. The pigeons,
+therefore, afford the greatest amount of reduction by
+disuse--one-seventh of the length of the sternum. But the pigeon has
+certainly been domesticated four or five thousand years; and if the
+reduction of the wings by disuse has only been going on for the last
+thousand years, the amount of reduction in each generation would be
+absolutely imperceptible, and quite within the limits of the reduction
+due to the absence of selection, as already explained. But, as we have
+seen in Chapter III, the fortuitous variation of every part or organ
+usually amounts to one-tenth, and often to one-sixth of the average
+dimensions--that is, the fortuitous variation in one generation among a
+limited number of the individuals of a species is as great as the
+cumulative effects of disuse in a thousand generations! If we assume
+that the effects of use or of effort in the individual are equal to the
+effects of disuse, or even ten or a hundred times greater, they will
+even then not equal, in each generation, the amount of the fortuitous
+variations of the same part. If it be urged that the effects of use
+would modify all the individuals of a species, while the fortuitous
+variations to the amount named only apply to a portion of them, it may
+be replied, that that portion is sufficiently large to afford ample
+materials for selection, since it often equals the numbers that can
+annually survive; while the recurrence in each successive generation of
+a like amount of variation would render possible such a rapid adjustment
+to new conditions that the effects of use or disuse would be as nothing
+in comparison. It follows, that even admitting the modifying effects of
+the environment, and that such modifications are inherited, they would
+yet be entirely swamped by the greater effects of fortuitous variation,
+and the far more rapid cumulative results of the selection of such
+variations.
+
+
+_Supposed Action of the Environment in Initiating Variations._
+
+It is, however, urged that the reaction of the environment initiates
+variations, which without it would never arise; such, for instance, as
+the origin of horns through the pressures and irritations caused by
+butting, or otherwise using the head as a weapon or for defence.
+Admitting, for the sake of argument, that this is so, all the evidence
+we possess shows that, from the very first appearance of the rudiment of
+such an organ, it would vary to a greater extent than the amount of
+growth directly produced by use; and these variations would be subject
+to selection, and would thus modify the organ in ways which use alone
+would never bring about. We have seen that this has been the case with
+the branching antlers of the stag, which have been modified by
+selection, so as to become useful in other ways than as a mere weapon;
+and the same has almost certainly been the case with the variously
+curved and twisted horns of antelopes. In like manner, every conceivable
+rudiment would, from its first appearance, be subject to the law of
+variation and selection, to which, thenceforth, the direct effect of the
+environment would be altogether subordinate.
+
+A very similar mode of reasoning will apply to the other branch of the
+subject--the initiation of structures and organs by the action of the
+fundamental laws of growth. Admitting that such laws have determined
+some of the main divisions of the animal and vegetable kingdom, have
+originated certain important organs, and have been the fundamental cause
+of certain lines of development, yet at every step of the process these
+laws must have acted in entire subordination to the law of natural
+selection. No modification thus initiated could have advanced a single
+step, unless it were, on the whole, a useful modification; while its
+entire future course would be necessarily subject to the laws of
+variation and selection, by which it would be sometimes checked,
+sometimes hastened on, sometimes diverted to one purpose, sometimes to
+another, according as the needs of the organism, under the special
+conditions of its existence, required such modification. We need not
+deny that such laws and influences may have acted in the manner
+suggested, but what we do deny is that they could possibly escape from
+the ever-present and all-powerful modifying effects of variation and
+natural selection.[212]
+
+
+_Weismann's Theory of Heredity._
+
+Professor August Weismann has put forth a new theory of heredity founded
+upon the "continuity of the germ-plasm," one of the logical consequences
+of which is, that acquired characters of whatever kind are not
+transmitted from parent to offspring. As this is a matter of vital
+importance to the theory of natural selection, and as, if well founded,
+it strikes away the foundations of most of the theories discussed in the
+present chapter, a brief outline of Weismann's views must be attempted,
+although it is very difficult to make them intelligible to persons
+unfamiliar with the main facts of modern embryology.[213]
+
+The problem is thus stated by Weismann: "How is it that in the case of
+all higher animals and plants a single cell is able to separate itself
+from amongst the millions of most various kinds of which an organism is
+composed, and by division and complicated differentiation to reconstruct
+a new individual with marvellous likeness, unchanged in many cases even
+throughout whole geological periods?" Darwin attempted to solve the
+problem by his theory of "Pangenesis," which supposed that every
+individual cell in the body gave off gemmules or germs capable of
+reproducing themselves, and that portions of these germs of each of the
+almost infinite number of cells permeate the whole body and become
+collected in the generative cells, and are thus able to reproduce the
+whole organism. This theory is felt to be so ponderously complex and
+difficult that it has met with no general acceptance among
+physiologists.
+
+The fact that the germ-cells _do_ reproduce with wonderful accuracy not
+only the general characters of the species, but many of the individual
+characteristics of the parents or more remote ancestors, and that this
+process is continued from generation to generation, can be accounted
+for, Weismann thinks, only on two suppositions which are physiologically
+possible. Either the substance of the parent germ-cell, after passing
+through a cycle of changes required for the construction of a new
+individual, possesses the capability of producing anew germ-cells
+identical with those from which that individual was developed, or _the
+new germ-cells arise, as far as their essential and characteristic
+substance is concerned, not at all out of the body of the individual,
+but direct from the parent germ-cell_. This latter view Weismann holds
+to be the correct one, and, on this theory, heredity depends on the fact
+that a substance of special molecular composition passes over from one
+generation to another. This is the "germ-plasm," the power of which to
+develop itself into a perfect organism depends on the extraordinary
+complication of its minutest structure. At every new birth a portion of
+the specific germ-plasm, which the parent egg-cell contains, is not used
+up in producing the offspring, but is reserved unchanged to produce the
+germ-cells of the following generation. Thus the germ-cells--so far as
+regards their essential part the germ-plasm--are not a product of the
+body itself, but are related to one another in the same way as are a
+series of generations of unicellular organisms derived from one another
+by a continuous course of simple division. Thus the question of heredity
+is reduced to one of growth. A minute portion of the very same
+germ-plasm from which, first the germ-cell, and then the whole organism
+of the parent, were developed, becomes the starting-point of the growth
+of the child.
+
+
+_The Cause of Variation._
+
+But if this were all, the offspring would reproduce the parent exactly,
+in every detail of form and structure; and here we see the importance of
+sex, for each new germ grows out of the united germ-plasms of two
+parents, whence arises a mingling of their characters in the offspring.
+This occurs in each generation; hence every individual is a complex
+result reproducing in ever-varying degrees the diverse characteristics
+of his two parents, four grandparents, eight great-grandparents, and
+other more remote ancestors; and that ever-present individual variation
+arises which furnishes the material for natural selection to act upon.
+Diversity of sex becomes, therefore, of primary importance as _the cause
+of variation_. Where asexual generation prevails, the characteristics of
+the individual alone are reproduced, and there are thus no means of
+effecting the change of form or structure required by changed conditions
+of existence. Under such changed conditions a complex organism, if only
+asexually propagated, would become extinct. But when a complex organism
+is sexually propagated, there is an ever-present cause of change which,
+though slight in any one generation, is cumulative, and under the
+influence of selection is sufficient to keep up the harmony between the
+organism and its slowly changing environment.[214]
+
+
+_The Non-Heredity of Acquired Characters._
+
+Certain observations on the embryology of the lower animals are held to
+afford direct proof of this theory of heredity, but they are too
+technical to be made clear to ordinary readers. A logical result of the
+theory is the impossibility of the transmission of acquired characters,
+since the molecular structure of the germ-plasm is already determined
+within the embryo; and Weismann holds that there are no facts which
+really prove that acquired characters can be inherited, although their
+inheritance has, by most writers, been considered so probable as hardly
+to stand in need of direct proof.
+
+We have already shown, in the earlier part of this chapter, that many
+instances of change, imputed to the inheritance of acquired variations,
+are really cases of selection; while the very fact that _use_ implies
+_usefulness_ renders it almost impossible to eliminate the action of
+selection in a state of nature. As regards mutilations, it is generally
+admitted that they are not hereditary, and there is ample evidence on
+this point. When it was the fashion to dock horses' tails, it was not
+found that horses were born with short tails; nor are Chinese women born
+with distorted feet; nor are any of the numerous forms of racial
+mutilation in man, which have in some cases been carried on for hundreds
+of generations, inherited. Nevertheless, a few cases of apparent
+inheritance of mutilations have been recorded,[215] and these, if
+trustworthy, are difficulties in the way of the theory. The undoubted
+inheritance of disease is hardly a difficulty, because the
+predisposition to disease is a congenital, not an acquired character,
+and as such would be the subject of inheritance. The often-quoted case
+of a disease induced by mutilation being inherited (Brown-Sequard's
+epileptic guinea-pigs) has been discussed by Professor Weismann, and
+shown to be not conclusive. The mutilation itself--a section of certain
+nerves--was never inherited, but the resulting epilepsy, or a general
+state of weakness, deformity, or sores, was sometimes inherited. It is,
+however, possible that the mere injury introduced and encouraged the
+growth of certain microbes, which, spreading through the organism,
+sometimes reached the germ-cells, and thus transmitted a diseased
+condition to the offspring. Such a transference of microbes is believed
+to occur in syphilis and tuberculosis, and has been ascertained to occur
+in the case of the muscardine silkworm disease.[216]
+
+
+_The Theory of Instinct._
+
+The theory now briefly outlined cannot be said to be proved, but it
+commends itself to many physiologists as being inherently probable, and
+as furnishing a good working hypothesis till displaced by a better. We
+cannot, therefore, accept any arguments against the agency of natural
+selection which are based upon the opposite and equally unproved theory
+that acquired characters are inherited; and as this applies to the whole
+school of what may be termed Neo-Lamarckians, their speculations cease
+to have any weight.
+
+The same remark applies to the popular theory of instincts as being
+inherited habits; though Darwin gave very little weight to this, but
+derived almost all instincts from spontaneous useful variations which,
+like other spontaneous variations, are of course inherited. At first
+sight it appears as if the acquired habits of our trained
+dogs--pointers, retrievers, etc.--are certainly inherited; but this need
+not be the case, because there must be some structural or psychical
+peculiarities, such as modifications in the attachments of muscles,
+increased delicacy of smell or sight, or peculiar likes and dislikes,
+which are inherited; and from these, peculiar habits follow as a natural
+consequence, or are easily acquired. Now, as selection has been
+constantly at work in improving all our domestic animals, we have
+unconsciously modified the structure, while preserving only those
+animals which best served our purpose in their peculiar faculties,
+instincts, or habits.
+
+Much of the mystery of instinct arises from the persistent refusal to
+recognise the agency of imitation, memory, observation, and reason as
+often forming part of it. Yet there is ample evidence that such agency
+must be taken into account. Both Wilson and Leroy state that young birds
+build inferior nests to old ones, and the latter author observes that
+the best nests are made by birds whose young remain longest in the nest.
+So, migration is now well ascertained to be effected by means of vision,
+long flights being made on bright moonlight nights when the birds fly
+very high, while on cloudy nights they fly low, and then often lose
+their way. Thousands annually fly out to sea and perish, showing that
+the instinct to migrate is imperfect, and is not a good substitute for
+reason and observation.
+
+Again, much of the perfection of instinct is due to the extreme severity
+of the selection during its development, any failure involving
+destruction. The chick which cannot break the eggshell, the caterpillar
+that fails to suspend itself properly or to spin a safe cocoon, the bees
+that lose their way or that fail to store honey, inevitably perish. So
+the birds that fail to feed and protect their young, or the butterflies
+that lay their eggs on the wrong food-plant, leave no offspring, and the
+race with imperfect instincts perishes. Now, during the long and very
+slow course of development of each organism, this rigid selection at
+every step of progress has led to the preservation of every detail of
+structure, faculty, or habit that has been necessary for the
+preservation of the race, and has thus gradually built up the various
+instincts which seem so marvellous to us, but which can yet be shown to
+be in many cases still imperfect. Here, as everywhere else in nature, we
+find comparative, not absolute perfection, with every gradation from
+what is clearly due to imitation or reason up to what seems to us
+perfect instinct--that in which a complex action is performed without
+any previous experience or instruction.[217]
+
+
+_Concluding Remarks._
+
+Having now passed in review the more important of the recent objections
+to, or criticisms of, the theory of natural selection, we have arrived
+at the conclusion that in no one case have the writers in question been
+able materially to diminish its importance, or to show that any of the
+laws or forces to which they appeal can act otherwise than in strict
+subordination to it. The direct action of the environment as set forth
+by Mr. Herbert Spencer, Dr. Cope, and Dr. Karl Semper, even if we admit
+that its effects on the individual are transmitted by inheritance, are
+so small in comparison with the amount of spontaneous variation of every
+part of the organism that they must be quite overshadowed by the latter.
+And if such direct action may, in some cases, have initiated certain
+organs or outgrowths, these must from their very first beginnings have
+been subject to variation and natural selection, and their further
+development have been almost wholly due to these ever-present and
+powerful causes. The same remark applies to the views of Professor
+Geddes on the laws of growth which have determined certain essential
+features in the morphology of plants and animals. The attempt to
+substitute these laws for those of variation and natural selection has
+failed in cases where we can apply a definite test, as in that of the
+origin of spines on trees and shrubs; while the extreme diversity of
+vegetable structure and form among the plants of the same country and of
+the same natural order, of itself affords a proof of the preponderating
+influence of variation and natural selection in keeping the many diverse
+forms in harmony with the highly complex and ever-changing environment.
+
+Lastly, we have seen that Professor Weismann's theory of the continuity
+of the germ-plasm and the consequent non-heredity of acquired
+characters, while in perfect harmony with all the well-ascertained facts
+of heredity and development, adds greatly to the importance of natural
+selection as the one invariable and ever-present factor in all organic
+change, and that which can alone have produced the temporary fixity
+combined with the secular modification of species. While admitting, as
+Darwin always admitted, the co-operation of the fundamental laws of
+growth and variation, of correlation and heredity, in determining the
+direction of lines of variation or in the initiation of peculiar organs,
+we find that variation and natural selection are ever-present agencies,
+which take possession, as it were, of every minute change originated by
+these fundamental causes, check or favour their further development, or
+modify them in countless varied ways according to the varying needs of
+the organism. Whatever other causes have been at work, Natural Selection
+is supreme, to an extent which even Darwin himself hesitated to claim
+for it. The more we study it the more we are convinced of its
+overpowering importance, and the more confidently we claim, in Darwin's
+own words, that it "has been the most important, but not the exclusive,
+means of modification."
+
+FOOTNOTES:
+
+[Footnote 198: See the Duke of Argyll's letter in _Nature_, vol. xxxiv.
+p. 336.]
+
+[Footnote 199: _Journal of the Anthropological Institute,_ vol. xv. pp.
+246-260.]
+
+[Footnote 200: The idea of the non-heredity of acquired variations was
+suggested by the summary of Professor Weismann's views, in _Nature_,
+referred to later on. But since this chapter was written I have, through
+the kindness of Mr. E.B. Poulton, seen some of the proofs of the
+forthcoming translation of Weismann's Essays on Heredity, in which he
+sets forth an explanation very similar to that here given. On the
+difficult question of the almost entire disappearance of organs, as in
+the limbs of snakes and of some lizards, he adduces "a certain form of
+correlation, which Roux calls 'the struggle of the parts in the
+organism,'" as playing an important part. Atrophy following disuse is
+nearly always attended by the corresponding increase of other organs:
+blind animals possess more developed organs of touch, hearing, and
+smell; the loss of power in the wings is accompanied by increased
+strength of the legs, etc. Now as these latter characters, being useful,
+will be selected, it is easy to understand that a congenital increase of
+these will be accompanied by a corresponding congenital diminution of
+the unused organ; and in cases where the means of nutrition are
+deficient, every diminution of these useless parts will be a gain to the
+whole organism, and thus their complete disappearance will, in some
+cases, be brought about directly by natural selection. This corresponds
+with what we know of these rudimentary organs.
+
+It must, however, be pointed out that the non-heredity of acquired
+characters was maintained by Mr. Francis Galton more than twelve years
+ago, on theoretical considerations almost identical with those urged by
+Professor Weismann; while the insufficiency of the evidence for their
+hereditary transmission was shown, by similar arguments to those used
+above and in the work of Professor Weismann already referred to (see "A
+Theory of Heredity," in _Journ. Anthrop. Instit._, vol. v. pp.
+343-345).]
+
+[Footnote 201: This explanation is derived from Weismann's Theory of the
+Continuity of the Germ-Plasm as summarised in _Nature_.]
+
+[Footnote 202: See a collection of his essays under the title, _The
+Origin of the Fittest: Essays on Evolution_, D. Appleton and Co. New
+York. 1887.]
+
+[Footnote 203: _Origin of the Fittest_, p. 174.]
+
+[Footnote 204: _Ibid._ p. 29. It may be here noted that Darwin found
+these theories unintelligible. In a letter to Professor E.T. Morse in
+1877, he writes: "There is one point which I regret you did not make
+clear in your Address, namely, what is the meaning and importance of
+Professors Cope and Hyatt's views on acceleration and retardation? I
+have endeavoured, and given up in despair, the attempt to grasp their
+meaning" (_Life and Letters_, vol. iii. p. 233).]
+
+[Footnote 205: _Origin of the Fittest_, p. 374.]
+
+[Footnote 206: _Origin of the Fittest_, p. 40.]
+
+[Footnote 207: _The Natural Conditions of Existence as they Affect
+Animal Life._ London, 1883.]
+
+[Footnote 208: In Dr. Weismann's essay on "Heredity," already referred
+to, he considers it not improbable that changes in organisms produced by
+climatic influences may be inherited, because, as these changes do not
+affect the external parts of an organism only, but often, as in the case
+of warmth or moisture permeate the whole structure, they may possibly
+modify the germ-plasm itself, and thus induce variations in the next
+generation. In this way, he thinks, may possibly be explained the
+climatic varieties of certain butterflies, and some other changes which
+seem to be effected by change of climate in a few generations.]
+
+[Footnote 209: This brief indication of Professor Geddes's views is
+taken from the article "Variation and Selection" in the _Encyclopedia
+Britannica_, and a paper "On the Nature and Causes of Variation in
+Plants" in _Trans. and Proc. of the Edinburgh Botanical Society_, 1886;
+and is, for the most part, expressed in his own words.]
+
+[Footnote 210: Placostylis bovinus, 31/2 inches long; Paryphanta Busbyi, 3
+in. diam.; P. Hochstetteri, 23/4 in. diam.]
+
+[Footnote 211: The general arguments and objections here set forth will
+apply with equal force to Professor G. Henslow's theory of the origin of
+the various forms and structures of flowers as due to "the responsive
+actions of the protoplasm in consequence of the irritations set up by
+the weights, pressures, thrusts, tensions, etc., of the insect visitors"
+(_The Origin of Floral Structures through Insect and other Agencies_, p.
+340). On the assumption that acquired characters are inherited, such
+irritations may have had something to do with the initiation of
+variations and with the production of certain details of structure, but
+they are clearly incompetent to have brought about the more important
+structural and functional modifications of flowers. Such are, the
+various adjustments of length and position of the stamens to bring the
+pollen to the insect and from the insect to the stigma; the various
+motions of stamens and styles at the right time and the right direction;
+the physiological adjustments bringing about fertility or sterility in
+heterostyled plants; the traps, springs, and complex movements of
+various parts of orchids; and innumerable other remarkable phenomena.
+
+For the explanation of these we have no resource but variation and
+selection, to the effects of which, acting alternately with regression
+or degradation as above explained (p. 328) must be imputed the
+development of the countless floral structures we now behold. Even the
+primitive flowers, whose initiation may, perhaps, have been caused, or
+rendered possible, by the irritation set up by insects' visits, must,
+from their very origin, have been modified, in accordance with the
+supreme law of utility, by means of variation and survival of the
+fittest.]
+
+[Footnote 212: In an essay on "The Duration of Life," forming part of
+the translation of Dr. Weismann's papers already referred to, the author
+still further extends the sphere of natural selection by showing that
+the average duration of life in each species has been determined by it.
+A certain length of life is essential in order that the species may
+produce offspring sufficient to ensure its continuance under the most
+unfavourable conditions; and it is shown that the remarkable
+inequalities of longevity in different species and groups may be thus
+accounted for. Yet more, the occurrence of death in the higher
+organisms, in place of the continued survival of the unicellular
+organisms however much they may increase by subdivision, may be traced
+to the same great law of utility for the race and survival of the
+fittest. The whole essay is of exceeding interest, and will repay a
+careful perusal. A similar idea occurred to the present writer about
+twenty years back, and was briefly noted down at the time, but
+subsequently forgotten.]
+
+[Footnote 213: The outline here given is derived from two articles in
+_Nature_, vol. xxxiii. p. 154, and vol. xxxiv. p. 629, in which
+Weismann's papers are summarised and partly translated.]
+
+[Footnote 214: There are many indications that this explanation of the
+cause of variation is the true one. Mr. E.B. Poulton suggests one, in
+the fact that parthenogenetic reproduction only occurs in isolated
+species, not in groups of related species; as this shows that
+parthenogenesis cannot lead to the evolution of new forms. Again, in
+parthenogenetic females the complete apparatus for fertilisation remains
+unreduced; but if these varied as do sexually produced animals, the
+organs referred to, being unused, would become rudimentary.
+
+Even more important is the significance of the "polar bodies," as
+explained by Weismann in one of his _Essays_; since, if his
+interpretation of them be correct, variability is a necessary
+consequence of sexual generation.]
+
+[Footnote 215: Darwin's _Animals and Plants_, vol. ii. pp. 23, 24.]
+
+[Footnote 216: In his essay on "Heredity," Dr. Weismann discusses many
+other cases of supposed inheritance of acquired characters, and shows
+that they can all be explained in other ways. Shortsightedness among
+civilised nations, for example, is due partly to the absence of
+selection and consequent regression towards a mean, and partly to its
+individual production by constant reading.]
+
+[Footnote 217: Weismann explains instinct on similar lines, and gives
+many interesting illustrations (see _Essays on Heredity_). He holds
+"that all instinct is entirely due to the operation of natural
+selection, and has its foundation, not upon inherited experiences, but
+upon variations of the germ." Many interesting and difficult cases of
+instinct are discussed by Darwin in Chapter VIII of the _Origin of
+Species_, which should be read in connection with the above remarks.
+
+Since this chapter was written my attention has been directed to Mr.
+Francis Galton's _Theory of Heredity_ (already referred to at p. 417)
+which was published thirteen years ago as an alternative for Darwin's
+theory of pangenesis.
+
+Mr. Galton's theory, although it attracted little attention, appears to
+me to be substantially the same as that of Professor Weismann. Galton's
+"stirp" is Weismann's "germ-plasm." Galton supposes the sexual elements
+in the offspring to be directly formed from the residue of the _stirp_
+not used up in the development of the body of the parent--Weismann's
+"continuity of the germ-plasm." Galton also draws many of the same
+conclusions from his theory. He maintains that characters acquired by
+the individual as the result of external influences cannot be inherited,
+unless such influences act directly on the reproductive
+elements--instancing the possible heredity of alcoholism, because the
+alcohol permeates the tissues and may reach the sexual elements. He
+discusses the supposed heredity of effects produced by use or disuse,
+and explains them much in the same manner as does Weismann. Galton is an
+anthropologist, and applies the theory, mainly, to explain the
+peculiarities of hereditary transmission in man, many of which
+peculiarities he discusses and elucidates. Weismann is a biologist, and
+is mostly concerned with the application of the theory to explain
+variation and instinct, and to the further development of the theory of
+evolution. He has worked it out more thoroughly, and has adduced
+embryological evidence in its support; but the views of both writers are
+substantially the same, and their theories were arrived at quite
+independently. The names of Galton and Weismann should therefore be
+associated as discoverers of what may be considered (if finally
+established) the most important contribution to the evolution theory
+since the appearance of the _Origin of Species_.]
+
+
+
+
+CHAPTER XV
+
+DARWINISM APPLIED TO MAN
+
+
+    General identity of human and animal structure--Rudiments and
+    variations showing relation of man to other mammals--The
+    embryonic development of man and other mammalia--Diseases common
+    to man and the lower animals--The animals most nearly allied to
+    man--The brains of man and apes--External differences of man and
+    apes--Summary of the animal characteristics of man--The
+    geological antiquity of man--The probable birthplace of man--The
+    origin of the moral and intellectual nature of man--The argument
+    from continuity--The origin of the mathematical faculty--The
+    origin of the musical and artistic faculties--Independent proof
+    that these faculties have not been developed by natural
+    selection--The interpretation of the facts--Concluding remarks.
+
+
+
+Our review of modern Darwinism might fitly have terminated with the
+preceding chapter; but the immense interest that attaches to the origin
+of the human race, and the amount of misconception which prevails
+regarding the essential teachings of Darwin's theory on this question,
+as well as regarding my own special views upon it, induce me to devote a
+final chapter to its discussion.
+
+To any one who considers the structure of man's body, even in the most
+superficial manner, it must be evident that it is the body of an animal,
+differing greatly, it is true, from the bodies of all other animals, but
+agreeing with them in all essential features. The bony structure of man
+classes him as a vertebrate; the mode of suckling his young classes him
+as a mammal; his blood, his muscles, and his nerves, the structure of
+his heart with its veins and arteries, his lungs and his whole
+respiratory and circulatory systems, all closely correspond to those of
+other mammals, and are often almost identical with them. He possesses
+the same number of limbs terminating in the same number of digits as
+belong fundamentally to the mammalian class. His senses are identical
+with theirs, and his organs of sense are the same in number and occupy
+the same relative position. Every detail of structure which is common to
+the mammalia as a class is found also in man, while he only differs from
+them in such ways and degrees as the various species or groups of
+mammals differ from each other. If, then, we have good reason to believe
+that every existing group of mammalia has descended from some common
+ancestral form--as we saw to be so completely demonstrated in the case
+of the horse tribe,--and that each family, each order, and even the
+whole class must similarly have descended from some much more ancient
+and more generalised type, it would be in the highest degree
+improbable--so improbable as to be almost inconceivable--that man,
+agreeing with them so closely in every detail of his structure, should
+have had some quite distinct mode of origin. Let us, then, see what
+other evidence bears upon the question, and whether it is sufficient to
+convert the probability of his animal origin into a practical certainty.
+
+
+_Rudiments and Variations as Indicating the Relation of Man to other
+Mammals._
+
+All the higher animals present rudiments of organs which, though useless
+to them, are useful in some allied group, and are believed to have
+descended from a common ancestor in which they were useful. Thus there
+are in ruminants rudiments of incisor teeth which, in some species,
+never cut through the gums; many lizards have external rudimentary legs;
+while many birds, as the Apteryx, have quite rudimentary wings. Now man
+possesses similar rudiments, sometimes constantly, sometimes only
+occasionally present, which serve intimately to connect his bodily
+structure with that of the lower animals. Many animals, for example,
+have a special muscle for moving or twitching the skin. In man there are
+remnants of this in certain parts of the body, especially in the
+forehead, enabling us to raise our eyebrows; but some persons have it in
+other parts. A few persons are able to move the whole scalp so as to
+throw off any object placed on the head, and this property has been
+proved, in one case, to be inherited. In the outer fold of the ear there
+is sometimes a projecting point, corresponding in position to the
+pointed ear of many animals, and believed to be a rudiment of it. In the
+alimentary canal there is a rudiment--the vermiform appendage of the
+caecum--which is not only useless, but is sometimes a cause of disease
+and death in man; yet in many vegetable feeding animals it is very long,
+and even in the orang-utan it is of considerable length and convoluted.
+So, man possesses rudimentary bones of a tail concealed beneath the
+skin, and, in some rare cases, this forms a minute external tail.
+
+The variability of every part of man's structure is very great, and many
+of these variations tend to approximate towards the structure of other
+animals. The courses of the arteries are eminently variable, so that for
+surgical purposes it has been necessary to determine the probable
+proportion of each variation. The muscles are so variable that in fifty
+cases the muscles of the foot were found to be not strictly alike in any
+two, and in some the deviations were considerable; while in thirty-six
+subjects Mr. J. Wood observed no fewer than 558 muscular variations. The
+same author states that in a single male subject there were no fewer
+than seven muscular variations, all of which plainly represented muscles
+proper to various kinds of apes. The muscles of the hands and
+arms--parts which are so eminently characteristic of man--are extremely
+liable to vary, so as to resemble the corresponding muscles of the lower
+animals. That such variations are due to reversion to a former state of
+existence Mr. Darwin thinks highly probable, and he adds: "It is quite
+incredible that a man should, through mere accident, abnormally resemble
+certain apes in no less than seven of his muscles, if there had been no
+genetic connection between them. On the other hand, if man is descended
+from some ape-like creature, no valid reason can be assigned why certain
+muscles should not suddenly reappear after an interval of many thousand
+generations, in the same manner as, with horses, asses, and mules, dark
+coloured stripes suddenly reappear on the legs and shoulders, after an
+interval of hundreds, or more probably of thousands of
+generations."[218]
+
+
+_The Embryonic Development of Man and other Mammalia._
+
+The progressive development of any vertebrate from the ovum or minute
+embryonic egg affords one of the most marvellous chapters in Natural
+History. We see the contents of the ovum undergoing numerous definite
+changes, its interior dividing and subdividing till it consists of a
+mass of cells, then a groove appears marking out the median line or
+vertebral column of the future animal, and thereafter are slowly
+developed the various essential organs of the body. After describing in
+some detail what takes place in the case of the ovum of the dog,
+Professor Huxley continues: "The history of the development of any other
+vertebrate animal, lizard, snake, frog, or fish tells the same story.
+There is always to begin with, an egg having the same essential
+structure as that of the dog; the yelk of that egg undergoes division or
+segmentation, as it is called, the ultimate products of that
+segmentation constitute the building materials for the body of the young
+animal; and this is built up round a primitive groove, in the floor of
+which a notochord is developed. Furthermore, there is a period in which
+the young of all these animals resemble one another, not merely in
+outward form, but in all essentials of structure, so closely, that the
+differences between them are inconsiderable, while in their subsequent
+course they diverge more and more widely from one another. And it is a
+general law that the more closely any animals resemble one another in
+adult structure, the larger and the more intimately do their embryos
+resemble one another; so that, for example, the embryos of a snake and
+of a lizard remain like one another longer than do those of a snake and
+a bird; and the embryos of a dog and of a cat remain like one another
+for a far longer period than do those of a dog and a bird, or of a dog
+and an opossum, or even than those of a dog and a monkey."[219]
+
+We thus see that the study of development affords a test of affinity in
+animals that are externally very much unlike each other; and we
+naturally ask how this applies to man. Is he developed in a different
+way from other mammals, as we should certainly expect if he has had a
+distinct and altogether different origin? "The reply," says Professor
+Huxley, "is not doubtful for a moment. Without question, the mode of
+origin and the early stages of the development of man are identical with
+those of the animals immediately below him in the scale." And again he
+tells us: "It is very long before the body of the young human being can
+be readily discriminated from that of the young puppy; but at a
+tolerably early period the two become distinguishable by the different
+forms of their adjuncts, the yelk-sac and the allantois;" and after
+describing these differences he continues: "But exactly in those
+respects in which the developing man differs from the dog, he resembles
+the ape.... So that it is only quite in the latter stages of development
+that the young human being presents marked differences from the young
+ape, while the latter departs as much from the dog in its development as
+the man does. Startling as this last assertion may appear to be, it is
+demonstrably true, and it alone appears to me sufficient to place beyond
+all doubt the structural unity of man with the rest of the animal world,
+and more particularly and closely with the apes."[220]
+
+A few of the curious details in which man passes through stages common
+to the lower animals may be mentioned. At one stage the os coccyx
+projects like a true tail, extending considerably beyond the rudimentary
+legs. In the seventh month the convolutions of the brain resemble those
+of an adult baboon. The great toe, so characteristic of man, forming the
+fulcrum which most assists him in standing erect, in an early stage of
+the embryo is much shorter than the other toes, and instead of being
+parallel with them, projects at an angle from the side of the foot, thus
+corresponding with its permanent condition in the quadrumana. Numerous
+other examples might be quoted, all illustrating the same general law.
+
+
+_Diseases Common to Man and the Lower Animals._
+
+Though the fact is so well known, it is certainly one of profound
+significance that many animal diseases can be communicated to man, since
+it shows similarity, if not identity, in the minute structure of the
+tissues, the nature of the blood, the nerves, and the brain. Such
+diseases as hydrophobia, variola, the glanders, cholera, herpes, etc.,
+can be transmitted from animals to man or the reverse; while monkeys are
+liable to many of the same non-contagious diseases as we are. Rengger,
+who carefully observed the common monkey (Cebus Azarae) in Paraguay,
+found it liable to catarrh, with the usual symptoms, terminating
+sometimes in consumption. These monkeys also suffered from apoplexy,
+inflammation of the bowels, and cataract in the eye. Medicines produced
+the same effect upon them as upon us. Many kinds of monkeys have a
+strong taste for tea, coffee, spirits, and even tobacco. These facts
+show the similarity of the nerves of taste in monkeys and in ourselves,
+and that their whole nervous system is affected in a similar way. Even
+the parasites, both external and internal, that affect man are not
+altogether peculiar to him, but belong to the same families or genera as
+those which infest animals, and in one case, scabies, even the same
+species.[221] These curious facts seem quite inconsistent with the idea
+that man's bodily structure and nature are altogether distinct from
+those of animals, and have had a different origin; while the facts are
+just what we should expect if he has been produced by descent with
+modification from some common ancestor.
+
+
+_The Animals most nearly Allied to Man._
+
+By universal consent we see in the monkey tribe a caricature of
+humanity. Their faces, their hands, their actions and expressions
+present ludicrous resemblances to our own. But there is one group of
+this great tribe in which this resemblance is greatest, and they have
+hence been called the anthropoid or man-like apes. These are few in
+number, and inhabit only the equatorial regions of Africa and Asia,
+countries where the climate is most uniform, the forests densest, and
+the supply of fruit abundant throughout the year. These animals are now
+comparatively well known, consisting of the orang-utan of Borneo and
+Sumatra, the chimpanzee and the gorilla of West Africa, and the group of
+gibbons or long-armed apes, consisting of many species and inhabiting
+South-Eastern Asia and the larger Malay Islands. These last are far
+less like man than the other three, one or other of which has at various
+times been claimed to be the most man-like of the apes and our nearest
+relations in the animal kingdom. The question of the degree of
+resemblance of these animals to ourselves is one of great interest,
+leading, as it does, to some important conclusions as to our origin and
+geological antiquity, and we will therefore briefly consider it.
+
+If we compare the skeletons of the orang or chimpanzee with that of man,
+we find them to be a kind of distorted copy, every bone corresponding
+(with very few exceptions), but altered somewhat in size, proportions,
+and position. So great is this resemblance that it led Professor Owen to
+remark: "I cannot shut my eyes to the significance of that all-pervading
+similitude of structure--every tooth, every bone, strictly
+homologous--which makes the determination of the difference between
+_Homo_ and _Pithecus_ the anatomist's difficulty."
+
+The actual differences in the skeletons of these apes and that of
+man--that is, differences dependent on the presence or absence of
+certain bones, and not on their form or position--have been enumerated
+by Mr. Mivart as follows:--(1) In the breast-bone consisting of but two
+bones, man agrees with the gibbons; the chimpanzee and gorilla having
+this part consisting of seven bones in a single series, while in the
+orang they are arranged in a double series of ten bones. (2) The normal
+number of the ribs in the orang and some gibbons is twelve pairs, as in
+man, while in the chimpanzee and gorilla there are thirteen pairs. (3)
+The orang and the gibbons also agree with man in having five lumbar
+vertebrae, while in the gorilla and the chimpanzee there are but four,
+and sometimes only three. (4) The gorilla and chimpanzee agree with man
+in having eight small bones in the wrist, while the orang and the
+gibbons, as well as all other monkeys, have nine.[222]
+
+The differences in the form, size, and attachments of the various bones,
+muscles, and other organs of these apes and man are very numerous and
+exceedingly complex, sometimes one species, sometimes another agreeing
+most nearly with ourselves, thus presenting a tangled web of affinities
+which it is very difficult to unravel. Estimated by the skeleton alone,
+the chimpanzee and gorilla seem nearer to man than the orang, which last
+is also inferior as presenting certain aberrations in the muscles. In
+the form of the ear the gorilla is more human than any other ape, while
+in the tongue the orang is the more man-like. In the stomach and liver
+the gibbons approach nearest to man, then come the orang and chimpanzee,
+while the gorilla has a degraded liver more resembling that of the lower
+monkeys and baboons.
+
+
+_The Brains of Man and Apes._
+
+We come now to that part of his organisation in which man is so much
+higher than all the lower animals--the brain; and here, Mr. Mivart
+informs us, the orang stands highest in rank. The height of the orang's
+cerebrum in front is greater in proportion than in either the chimpanzee
+or the gorilla. "On comparing the brain of man with the brains of the
+orang, chimpanzee, and baboon, we find a successive decrease in the
+frontal lobe, and a successive and very great increase in the relative
+size of the occipital lobe. Concomitantly with this increase and
+decrease, certain folds of brain substance, called 'bridging
+convolutions,' which in man are conspicuously interposed between the
+parietal and occipital lobes, seem as utterly to disappear in the
+chimpanzee, as they do in the baboon. In the orang, however, though much
+reduced, they are still to be distinguished.... The actual and absolute
+mass of the brain is, however, slightly greater in the chimpanzee than
+in the orang, as is the relative vertical extent of the middle part of
+the cerebrum, although, as already stated, the frontal portion is higher
+in the orang; while, according to M. Gratiolet, the gorilla is not only
+inferior to the orang in cerebral development, but even to his smaller
+African congener, the chimpanzee."[223]
+
+On the whole, then, we find that no one of the great apes can be
+positively asserted to be nearest to man in structure. Each of them
+approaches him in certain characteristics, while in others it is widely
+removed, giving the idea, so consonant with the theory of evolution as
+developed by Darwin, that all are derived from a common ancestor, from
+which the existing anthropoid apes as well as man have diverged. When,
+however, we turn from the details of anatomy to peculiarities of
+external form and motions, we find that, in a variety of characters, all
+these apes resemble each other and differ from man, so that we may
+fairly say that, while they have diverged somewhat from each other, they
+have diverged much more widely from ourselves. Let us briefly enumerate
+some of these differences.
+
+
+_External Differences of Man and Apes._
+
+All apes have large canine teeth, while in man these are no longer than
+the adjacent incisors or premolars, the whole forming a perfectly even
+series. In apes the arms are proportionately much longer than in man,
+while the thighs are much shorter. No ape stands really erect, a posture
+which is natural in man. The thumb is proportionately larger in man, and
+more perfectly opposable than in that of any ape. The foot of man
+differs largely from that of all apes, in the horizontal sole, the
+projecting heel, the short toes, and the powerful great toe firmly
+attached parallel to the other toes; all perfectly adapted for
+maintaining the erect posture, and for free motion without any aid from
+the arms or hands. In apes the foot is formed almost exactly like our
+hand, with a large thumb-like great toe quite free from the other toes,
+and so articulated as to be opposable to them; forming with the long
+finger-like toes a perfect grasping hand. The sole cannot be placed
+horizontally on the ground; but when standing on a level surface the
+animal rests on the outer edge of the foot with the finger and
+thumb-like toes partly closed, while the hands are placed on the ground
+resting on the knuckles. The illustration on the next page (Fig. 37)
+shows, fairly well, the peculiarities of the hands and feet of the
+chimpanzee, and their marked differences, both in form and use, from
+those of man.
+
+The four limbs, with the peculiarly formed feet and hands, are those of
+arboreal animals which only occasionally and awkwardly move on level
+ground. The arms are used in progression equally with the feet, and the
+hands are only adapted for uses similar to those of our hands when the
+animal is at rest, and then but clumsily. Lastly, the apes are all hairy
+animals, like the majority of other mammals, man alone having a smooth
+and almost naked skin. These numerous and striking differences, even
+more than those of the skeleton and internal anatomy, point to an
+enormously remote epoch when the race that was ultimately to develop
+into man diverged from that other stock which continued the animal type
+and ultimately produced the existing varieties of anthropoid apes.
+
+[Illustration: FIG. 37.--Chimpanzee (Troglodytes niger).]
+
+
+_Summary of the Animal Characteristics of Man._
+
+The facts now very briefly summarised amount almost to a demonstration
+that man, in his bodily structure, has been derived from the lower
+animals, of which he is the culminating development. In his possession
+of rudimentary structures which are functional in some of the mammalia;
+in the numerous variations of his muscles and other organs agreeing with
+characters which are constant in some apes; in his embryonic
+development, absolutely identical in character with that of mammalia in
+general, and closely resembling in its details that of the higher
+quadrumana; in the diseases which he has in common with other mammalia;
+and in the wonderful approximation of his skeleton to those of one or
+other of the anthropoid apes, we have an amount of evidence in this
+direction which it seems impossible to explain away. And this evidence
+will appear more forcible if we consider for a moment what the rejection
+of it implies. For the only alternative supposition is, that man has
+been specially created--that is to say, has been produced in some quite
+different way from other animals and altogether independently of them.
+But in that case the rudimentary structures, the animal-like variations,
+the identical course of development, and all the other animal
+characteristics he possesses are deceptive, and inevitably lead us, as
+thinking beings making use of the reason which is our noblest and most
+distinctive feature, into gross error.
+
+We cannot believe, however, that a careful study of the facts of nature
+leads to conclusions directly opposed to the truth; and, as we seek in
+vain, in our physical structure and the course of its development, for
+any indication of an origin independent of the rest of the animal world,
+we are compelled to reject the idea of "special creation" for man, as
+being entirely unsupported by facts as well as in the highest degree
+improbable.
+
+
+_The Geological Antiquity of Man._
+
+The evidence we now possess of the exact nature of the resemblance of
+man to the various species of anthropoid apes, shows us that he has
+little special affinity for any one rather than another species, while
+he differs from them all in several important characters in which they
+agree with each other. The conclusion to be drawn from these facts is,
+that his points of affinity connect him with the whole group, while his
+special peculiarities equally separate him from the whole group, and
+that he must, therefore, have diverged from the common ancestral form
+before the existing types of anthropoid apes had diverged from each
+other. Now, this divergence almost certainly took place as early as the
+Miocene period, because in the Upper Miocene deposits of Western Europe
+remains of two species of ape have been found allied to the gibbons, one
+of them, Dryopithecus, nearly as large as a man, and believed by M.
+Lartet to have approached man in its dentition more than the existing
+apes. We seem hardly, therefore, to have reached, in the Upper Miocene,
+the epoch of the common ancestor of man and the anthropoids.
+
+The evidence of the antiquity of man himself is also scanty, and takes
+us but very little way back into the past. We have clear proof of his
+existence in Europe in the latter stages of the glacial epoch, with many
+indications of his presence in interglacial or even pre-glacial times;
+while both the actual remains and the works of man found in the
+auriferous gravels of California deep under lava-flows of Pliocene age,
+show that he existed in the New World at least as early as in the
+Old.[224] These earliest remains of man have been received with doubt,
+and even with ridicule, as if there were some extreme improbability in
+them. But, in point of fact, the wonder is that human remains have not
+been found more frequently in pre-glacial deposits. Referring to the
+most ancient fossil remains found in Europe--the Engis and Neanderthal
+crania,--Professor Huxley makes the following weighty remark: "In
+conclusion, I may say, that the fossil remains of Man hitherto
+discovered do not seem to me to take us appreciably nearer to that lower
+pithecoid form, by the modification of which he has, probably, become
+what he is." The Californian remains and works of art, above referred
+to, give no indication of a specially low form of man; and it remains an
+unsolved problem why no traces of the long line of man's ancestors, back
+to the remote period when he first branched off from the pithecoid type,
+have yet been discovered.
+
+It has been objected by some writers--notably by Professor Boyd
+Dawkins--that man did not probably exist in Pliocene times, because
+almost all the known mammalia of that epoch are distinct species from
+those now living on the earth, and that the same changes of the
+environment which led to the modification of other mammalian species
+would also have led to a change in man. But this argument overlooks the
+fact that man differs essentially from all other mammals in this
+respect, that whereas any important adaptation to new conditions can be
+effected in them only by a change in bodily structure, man is able to
+adapt himself to much greater changes of conditions by a mental
+development leading him to the use of fire, of tools, of clothing, of
+improved dwellings, of nets and snares, and of agriculture. By the help
+of these, without any change whatever in his bodily structure, he has
+been able to spread over and occupy the whole earth; to dwell securely
+in forest, plain, or mountain; to inhabit alike the burning desert or
+the arctic wastes; to cope with every kind of wild beast, and to provide
+himself with food in districts where, as an animal trusting to nature's
+unaided productions, he would have starved.[225]
+
+It follows, therefore, that from the time when the ancestral man first
+walked erect, with hands freed from any active part in locomotion, and
+when his brain-power became sufficient to cause him to use his hands in
+making weapons and tools, houses and clothing, to use fire for cooking,
+and to plant seeds or roots to supply himself with stores of food, the
+power of natural selection would cease to act in producing modifications
+of his body, but would continuously advance his mind through the
+development of its organ, the brain. Hence man may have become truly
+man--the species, Homo sapiens--even in the Miocene period; and while
+all other mammals were becoming modified from age to age under the
+influence of ever-changing physical and biological conditions, he would
+be advancing mainly in intelligence, but perhaps also in stature, and by
+that advance alone would be able to maintain himself as the master of
+all other animals and as the most widespread occupier of the earth. It
+is quite in accordance with this view that we find the most pronounced
+distinction between man and the anthropoid apes in the size and
+complexity of his brain. Thus, Professor Huxley tells us that "it may be
+doubted whether a healthy human adult brain ever weighed less than 31
+or 32 ounces, or that the heaviest gorilla brain has exceeded 20
+ounces," although "a full-grown gorilla is probably pretty nearly twice
+as heavy as a Bosjes man, or as many an European woman."[226] The
+average human brain, however, weighs 48 or 49 ounces, and if we take the
+average ape brain at only 2 ounces less than the very largest gorilla's
+brain, or 18 ounces, we shall see better the enormous increase which has
+taken place in the brain of man since the time when he branched off from
+the apes; and this increase will be still greater if we consider that
+the brains of apes, like those of all other mammals, have also increased
+from earlier to later geological times.
+
+If these various considerations are taken into account, we must conclude
+that the essential features of man's structure as compared with that of
+apes--his erect posture and free hands--were acquired at a comparatively
+early period, and were, in fact, the characteristics which gave him his
+superiority over other mammals, and started him on the line of
+development which has led to his conquest of the world. But during this
+long and steady development of brain and intellect, mankind must have
+continuously increased in numbers and in the area which they
+occupied--they must have formed what Darwin terms a "dominant race." For
+had they been few in numbers and confined to a limited area, they could
+hardly have successfully struggled against the numerous fierce carnivora
+of that period, and against those adverse influences which led to the
+extinction of so many more powerful animals. A large population spread
+over an extensive area is also needed to supply an adequate number of
+brain variations for man's progressive improvement. But this large
+population and long-continued development in a single line of advance
+renders it the more difficult to account for the complete absence of
+human or pre-human remains in all those deposits which have furnished,
+in such rich abundance, the remains of other land animals. It is true
+that the remains of apes are also very rare, and we may well suppose
+that the superior intelligence of man led him to avoid that extensive
+destruction by flood or in morass which seems to have often overwhelmed
+other animals. Yet, when we consider that, even in our own day, men are
+not unfrequently overwhelmed by volcanic eruptions, as in Java and
+Japan, or carried away in vast numbers by floods, as in Bengal and
+China, it seems impossible but that ample remains of Miocene and
+Pliocene man do exist buried in the most recent layers of the earth's
+crust, and that more extended research or some fortunate discovery will
+some day bring them to light.
+
+
+_The Probable Birthplace of Man._
+
+It has usually been considered that the ancestral form of man originated
+in the tropics, where vegetation is most abundant and the climate most
+equable. But there are some important objections to this view. The
+anthropoid apes, as well as most of the monkey tribe, are essentially
+arboreal in their structure, whereas the great distinctive character of
+man is his special adaptation to terrestrial locomotion. We can hardly
+suppose, therefore, that he originated in a forest region, where fruits
+to be obtained by climbing are the chief vegetable food. It is more
+probable that he began his existence on the open plains or high plateaux
+of the temperate or sub-tropical zone, where the seeds of indigenous
+cereals and numerous herbivora, rodents, and game-birds, with fishes and
+molluscs in the lakes, rivers, and seas supplied him with an abundance
+of varied food. In such a region he would develop skill as a hunter,
+trapper, or fisherman, and later as a herdsman and cultivator,--a
+succession of which we find indications in the palaeolithic and
+neolithic races of Europe.
+
+In seeking to determine the particular areas in which his earliest
+traces are likely to be found, we are restricted to some portion of the
+Eastern hemisphere, where alone the anthropoid apes exist, or have
+apparently ever existed.
+
+There is good reason to believe, also, that Africa must be excluded,
+because it is known to have been separated from the northern continent
+in early tertiary times, and to have acquired its existing fauna of the
+higher mammalia by a later union with that continent after the
+separation from it of Madagascar, an island which has preserved for us a
+sample, as it were, of the early African mammalian fauna, from which not
+only the anthropoid apes, but all the higher quadrumana are
+absent.[227] There remains only the great Euro-Asiatic continent; and
+its enormous plateaux, extending from Persia right across Tibet and
+Siberia to Manchuria, afford an area, some part or other of which
+probably offered suitable conditions, in late Miocene or early Pliocene
+times, for the development of ancestral man.
+
+It is in this area that we still find that type of mankind--the
+Mongolian--which retains a colour of the skin midway between the black
+or brown-black of the negro, and the ruddy or olive-white of the
+Caucasian types, a colour which still prevails over all Northern Asia,
+over the American continents, and over much of Polynesia. From this
+primary tint arose, under the influence of varied conditions, and
+probably in correlation with constitutional changes adapted to peculiar
+climates, the varied tints which still exist among mankind. If the
+reasoning by which this conclusion is reached be sound, and all the
+earlier stages of man's development from an animal form occurred in the
+area now indicated, we can better understand how it is that we have as
+yet met with no traces of the missing links, or even of man's existence
+during late tertiary times, because no part of the world is so entirely
+unexplored by the geologist as this very region. The area in question is
+sufficiently extensive and varied to admit of primeval man having
+attained to a considerable population, and having developed his full
+human characteristics, both physical and mental, before there was any
+need for him to migrate beyond its limits. One of his earliest important
+migrations was probably into Africa, where, spreading westward, he
+became modified in colour and hair in correlation with physiological
+changes adapting him to the climate of the equatorial lowlands.
+Spreading north-westward into Europe the moist and cool climate led to a
+modification of an opposite character, and thus may have arisen the
+three great human types which still exist. Somewhat later, probably, he
+spread eastward into North-West America and soon scattered himself over
+the whole continent; and all this may well have occurred in early or
+middle Pliocene times. Thereafter, at very long intervals, successive
+waves of migration carried him into every part of the habitable world,
+and by conquest and intermixture led ultimately to that puzzling
+gradation of types which the ethnologist in vain seeks to unravel.
+
+
+_The Origin of the Moral and Intellectual Nature of Man._
+
+From the foregoing discussion it will be seen that I fully accept Mr.
+Darwin's conclusion as to the essential identity of man's bodily
+structure with that of the higher mammalia, and his descent from some
+ancestral form common to man and the anthropoid apes. The evidence of
+such descent appears to me to be overwhelming and conclusive. Again, as
+to the cause and method of such descent and modification, we may admit,
+at all events provisionally, that the laws of variation and natural
+selection, acting through the struggle for existence and the continual
+need of more perfect adaptation to the physical and biological
+environments, may have brought about, first that perfection of bodily
+structure in which he is so far above all other animals, and in
+co-ordination with it the larger and more developed brain, by means of
+which he has been able to utilise that structure in the more and more
+complete subjection of the whole animal and vegetable kingdoms to his
+service.
+
+But this is only the beginning of Mr. Darwin's work, since he goes on to
+discuss the moral nature and mental faculties of man, and derives these
+too by gradual modification and development from the lower animals.
+Although, perhaps, nowhere distinctly formulated, his whole argument
+tends to the conclusion that man's entire nature and all his faculties,
+whether moral, intellectual, or spiritual, have been derived from their
+rudiments in the lower animals, in the same manner and by the action of
+the same general laws as his physical structure has been derived. As
+this conclusion appears to me not to be supported by adequate evidence,
+and to be directly opposed to many well-ascertained facts, I propose to
+devote a brief space to its discussion.
+
+
+_The Argument from Continuity._
+
+Mr. Darwin's mode of argument consists in showing that the rudiments of
+most, if not of all, the mental and moral faculties of man can be
+detected in some animals. The manifestations of intelligence, amounting
+in some cases to distinct acts of reasoning, in many animals, are
+adduced as exhibiting in a much less degree the intelligence and reason
+of man. Instances of curiosity, imitation, attention, wonder, and memory
+are given; while examples are also adduced which may be interpreted as
+proving that animals exhibit kindness to their fellows, or manifest
+pride, contempt, and shame. Some are said to have the rudiments of
+language, because they utter several different sounds, each of which has
+a definite meaning to their fellows or to their young; others the
+rudiments of arithmetic, because they seem to count and remember up to
+three, four, or even five. A sense of beauty is imputed to them on
+account of their own bright colours or the use of coloured objects in
+their nests; while dogs, cats, and horses are said to have imagination,
+because they appear to be disturbed by dreams. Even some distant
+approach to the rudiments of religion is said to be found in the deep
+love and complete submission of a dog to his master.[228]
+
+Turning from animals to man, it is shown that in the lowest savages many
+of these faculties are very little advanced from the condition in which
+they appear in the higher animals; while others, although fairly well
+exhibited, are yet greatly inferior to the point of development they
+have reached in civilised races. In particular, the moral sense is said
+to have been developed from the social instincts of savages, and to
+depend mainly on the enduring discomfort produced by any action which
+excites the general disapproval of the tribe. Thus, every act of an
+individual which is believed to be contrary to the interests of the
+tribe, excites its unvarying disapprobation and is held to be immoral;
+while every act, on the other hand, which is, as a rule, beneficial to
+the tribe, is warmly and constantly approved, and is thus considered to
+be right or moral. From the mental struggle, when an act that would
+benefit self is injurious to the tribe, there arises conscience; and
+thus the social instincts are the foundation of the moral sense and of
+the fundamental principles of morality.[229]
+
+The question of the origin and nature of the moral sense and of
+conscience is far too vast and complex to be discussed here, and a
+reference to it has been introduced only to complete the sketch of Mr.
+Darwin's view of the continuity and gradual development of all human
+faculties from the lower animals up to savages, and from savage up to
+civilised man. The point to which I wish specially to call attention is,
+that to prove continuity and the progressive development of the
+intellectual and moral faculties from animals to man, is not the same as
+proving that these faculties have been developed by natural selection;
+and this last is what Mr. Darwin has hardly attempted, although to
+support his theory it was absolutely essential to prove it. Because
+man's physical structure has been developed from an animal form by
+natural selection, it does not necessarily follow that his mental
+nature, even though developed _pari passu_ with it, has been developed
+by the same causes only. To illustrate by a physical analogy. Upheaval
+and depression of land, combined with sub-aerial denudation by wind and
+frost, rain and rivers, and marine denudation on coastlines, were long
+thought to account for all the modelling of the earth's surface not
+directly due to volcanic action; and in the early editions of Lyell's
+_Principles of Geology_ these are the sole causes appealed to. But when
+the action of glaciers was studied and the recent occurrence of a
+glacial epoch demonstrated as a fact, many phenomena--such as moraines
+and other gravel deposits, boulder clay, erratic boulders, grooved and
+rounded rocks, and Alpine lake basins--were seen to be due to this
+altogether distinct cause. There was no breach of continuity, no sudden
+catastrophe; the cold period came on and passed away in the most gradual
+manner, and its effects often passed insensibly into those produced by
+denudation or upheaval; yet none the less a new agency appeared at a
+definite time, and new effects were produced which, though continuous
+with preceding effects, were not due to the same causes. It is not,
+therefore, to be assumed, without proof or against independent evidence,
+that the later stages of an apparently continuous development are
+necessarily due to the same causes only as the earlier stages. Applying
+this argument to the case of man's intellectual and moral nature, I
+propose to show that certain definite portions of it could not have been
+developed by variation and natural selection alone, and that, therefore,
+some other influence, law, or agency is required to account for them.
+If this can be clearly shown for any one or more of the special
+faculties of intellectual man, we shall be justified in assuming that
+the same unknown cause or power may have had a much wider influence, and
+may have profoundly influenced the whole course of his development.
+
+
+_The Origin of the Mathematical Faculty._
+
+We have ample evidence that, in all the lower races of man, what may be
+termed the mathematical faculty is, either absent, or, if present, quite
+unexercised. The Bushmen and the Brazilian Wood-Indians are said not to
+count beyond two. Many Australian tribes only have words for one and
+two, which are combined to make three, four, five, or six, beyond which
+they do not count. The Damaras of South Africa only count to three; and
+Mr. Galton gives a curious description of how one of them was hopelessly
+puzzled when he had sold two sheep for two sticks of tobacco each, and
+received four sticks in payment. He could only find out that he was
+correctly paid by taking two sticks and then giving one sheep, then
+receiving two sticks more and giving the other sheep. Even the
+comparatively intellectual Zulus can only count up to ten by using the
+hands and fingers. The Ahts of North-West America count in nearly the
+same manner, and most of the tribes of South America are no further
+advanced.[230] The Kaffirs have great herds of cattle, and if one is
+lost they miss it immediately, but this is not by counting, but by
+noticing the absence of one they know; just as in a large family or a
+school a boy is missed without going through the process of counting.
+Somewhat higher races, as the Esquimaux, can count up to twenty by using
+the hands and the feet; and other races get even further than this by
+saying "one man" for twenty, "two men" for forty, and so on, equivalent
+to our rural mode of reckoning by scores. From the fact that so many of
+the existing savage races can only count to four or five, Sir John
+Lubbock thinks it improbable that our earliest ancestors could have
+counted as high as ten.[231]
+
+When we turn to the more civilised races, we find the use of numbers
+and the art of counting greatly extended. Even the Tongas of the South
+Sea islands are said to have been able to count as high as 100,000. But
+mere counting does not imply either the possession or the use of
+anything that can be really called the mathematical faculty, the
+exercise of which in any broad sense has only been possible since the
+introduction of the decimal notation. The Greeks, the Romans, the
+Egyptians, the Jews, and the Chinese had all such cumbrous systems, that
+anything like a science of arithmetic, beyond very simple operations,
+was impossible; and the Roman system, by which the year 1888 would be
+written MDCCCLXXXVIII, was that in common use in Europe down to the
+fourteenth or fifteenth centuries, and even much later in some places.
+Algebra, which was invented by the Hindoos, from whom also came the
+decimal notation, was not introduced into Europe till the thirteenth
+century, although the Greeks had some acquaintance with it; and it
+reached Western Europe from Italy only in the sixteenth century.[232] It
+was, no doubt, owing to the absence of a sound system of numeration that
+the mathematical talent of the Greeks was directed chiefly to geometry,
+in which science Euclid, Archimedes, and others made such brilliant
+discoveries. It is, however, during the last three centuries only that
+the civilised world appears to have become conscious of the possession
+of a marvellous faculty which, when supplied with the necessary tools in
+the decimal notation, the elements of algebra and geometry, and the
+power of rapidly communicating discoveries and ideas by the art of
+printing, has developed to an extent, the full grandeur of which can be
+appreciated only by those who have devoted some time (even if
+unsuccessfully) to the study.
+
+The facts now set forth as to the almost total absence of mathematical
+faculty in savages and its wonderful development in quite recent times,
+are exceedingly suggestive, and in regard to them we are limited to two
+possible theories. Either prehistoric and savage man did not possess
+this faculty at all (or only in its merest rudiments); or they did
+possess it, but had neither the means nor the incitements for its
+exercise. In the former case we have to ask by what means has this
+faculty been so rapidly developed in all civilised races, many of which
+a few centuries back were, in this respect, almost savages themselves;
+while in the latter case the difficulty is still greater, for we have to
+assume the existence of a faculty which had never been used either by
+the supposed possessors of it or by their ancestors.
+
+Let us take, then, the least difficult supposition--that savages
+possessed only the mere rudiments of the faculty, such as their ability
+to count, sometimes up to ten, but with an utter inability to perform
+the very simplest processes of arithmetic or of geometry--and inquire
+how this rudimentary faculty became rapidly developed into that of a
+Newton, a La Place, a Gauss, or a Cayley. We will admit that there is
+every possible gradation between these extremes, and that there has been
+perfect continuity in the development of the faculty; but we ask, What
+motive power caused its development?
+
+It must be remembered we are here dealing solely with the capability of
+the Darwinian theory to account for the origin of the _mind_, as well as
+it accounts for the origin of the _body_ of man, and we must, therefore,
+recall the essential features of that theory. These are, the
+preservation of useful variations in the struggle for life; that no
+creature can be improved beyond its necessities for the time being; that
+the law acts by life and death, and by the survival of the fittest. We
+have to ask, therefore, what relation the successive stages of
+improvement of the mathematical faculty had to the life or death of its
+possessors; to the struggles of tribe with tribe, or nation with nation;
+or to the ultimate survival of one race and the extinction of another.
+If it cannot possibly have had any such effects, then it cannot have
+been produced by natural selection.
+
+It is evident that in the struggles of savage man with the elements and
+with wild beasts, or of tribe with tribe, this faculty can have had no
+influence. It had nothing to do with the early migrations of man, or
+with the conquest and extermination of weaker by more powerful peoples.
+The Greeks did not successfully resist the Persian invaders by any aid
+from their few mathematicians, but by military training, patriotism, and
+self-sacrifice. The barbarous conquerors of the East, Timurlane and
+Gengkhis Khan, did not owe their success to any superiority of intellect
+or of mathematical faculty in themselves or their followers. Even if the
+great conquests of the Romans were, in part, due to their systematic
+military organisation, and to their skill in making roads and
+encampments, which may, perhaps, be imputed to some exercise of the
+mathematical faculty, that did not prevent them from being conquered in
+turn by barbarians, in whom it was almost entirely absent. And if we
+take the most civilised peoples of the ancient world--the Hindoos, the
+Arabs, the Greeks, and the Romans, all of whom had some amount of
+mathematical talent--we find that it is not these, but the descendants
+of the barbarians of those days--the Celts, the Teutons, and the
+Slavs--who have proved themselves the fittest to survive in the great
+struggle of races, although we cannot trace their steadily growing
+success during past centuries either to the possession of any
+exceptional mathematical faculty or to its exercise. They have indeed
+proved themselves, to-day, to be possessed of a marvellous endowment of
+the mathematical faculty; but their success at home and abroad, as
+colonists or as conquerors, as individuals or as nations, can in no way
+be traced to this faculty, since they were almost the last who devoted
+themselves to its exercise. We conclude, then, that the present gigantic
+development of the mathematical faculty is wholly unexplained by the
+theory of natural selection, and must be due to some altogether distinct
+cause.
+
+
+_The Origin of the Musical and Artistic Faculties._
+
+These distinctively human faculties follow very closely the lines of the
+mathematical faculty in their progressive development, and serve to
+enforce the same argument. Among the lower savages music, as we
+understand it, hardly exists, though they all delight in rude musical
+sounds, as of drums, tom-toms, or gongs; and they also sing in
+monotonous chants. Almost exactly as they advance in general intellect,
+and in the arts of social life, their appreciation of music appears to
+rise in proportion; and we find among them rude stringed instruments and
+whistles, till, in Java, we have regular bands of skilled performers
+probably the successors of Hindoo musicians of the age before the
+Mahometan conquest. The Egyptians are believed to have been the earliest
+musicians, and from them the Jews and the Greeks, no doubt, derived
+their knowledge of the art; but it seems to be admitted that neither the
+latter nor the Romans knew anything of harmony or of the essential
+features of modern music.[233] Till the fifteenth century little
+progress appears to have been made in the science or the practice of
+music; but since that era it has advanced with marvellous rapidity, its
+progress being curiously parallel with that of mathematics, inasmuch as
+great musical geniuses appeared suddenly among different nations, equal
+in their possession of this special faculty to any that have since
+arisen.
+
+As with the mathematical, so with the musical faculty, it is impossible
+to trace any connection between its possession and survival in the
+struggle for existence. It seems to have arisen as a _result_ of social
+and intellectual advancement, not as a _cause_; and there is some
+evidence that it is latent in the lower races, since under European
+training native military bands have been formed in many parts of the
+world, which have been able to perform creditably the best modern music.
+
+The artistic faculty has run a somewhat different course, though
+analogous to that of the faculties already discussed. Most savages
+exhibit some rudiments of it, either in drawing or carving human or
+animal figures; but, almost without exception, these figures are rude
+and such as would be executed by the ordinary inartistic child. In fact,
+modern savages are, in this respect hardly equal to those prehistoric
+men who represented the mammoth and the reindeer on pieces of horn or
+bone. With any advance in the arts of social life, we have a
+corresponding advance in artistic skill and taste, rising very high in
+the art of Japan and India, but culminating in the marvellous sculpture
+of the best period of Grecian history. In the Middle Ages art was
+chiefly manifested in ecclesiastical architecture and the illumination
+of manuscripts, but from the thirteenth to the fifteenth centuries
+pictorial art revived in Italy and attained to a degree of perfection
+which has never been surpassed. This revival was followed closely by the
+schools of Germany, the Netherlands, Spain, France, and England, showing
+that the true artistic faculty belonged to no one nation, but was fairly
+distributed among the various European races.
+
+These several developments of the artistic faculty, whether manifested
+in sculpture, painting, or architecture, are evidently outgrowths of the
+human intellect which have no immediate influence on the survival of
+individuals or of tribes, or on the success of nations in their
+struggles for supremacy or for existence. The glorious art of Greece did
+not prevent the nation from falling under the sway of the less advanced
+Roman; while we ourselves, among whom art was the latest to arise, have
+taken the lead in the colonisation of the world, thus proving our mixed
+race to be the fittest to survive.
+
+
+_Independent Proof that the Mathematical, Musical, and Artistic
+Faculties have not been Developed under the Law of Natural Selection._
+
+The law of Natural Selection or the survival of the fittest is, as its
+name implies, a rigid law, which acts by the life or death of the
+individuals submitted to its action. From its very nature it can act
+only on useful or hurtful characteristics, eliminating the latter and
+keeping up the former to a fairly general level of efficiency. Hence it
+necessarily follows that the characters developed by its means will be
+present in all the individuals of a species, and, though varying, will
+not vary very widely from a common standard. The amount of variation we
+found, in our third chapter, to be about one-fifth or one-sixth of the
+mean value--that is, if the mean value were taken at 100, the variations
+would reach from 80 to 120, or somewhat more, if very large numbers were
+compared. In accordance with this law we find, that all those characters
+in man which were certainly essential to him during his early stages of
+development, exist in all savages with some approach to equality. In the
+speed of running, in bodily strength, in skill with weapons, in
+acuteness of vision, or in power of following a trail, all are fairly
+proficient, and the differences of endowment do not probably exceed the
+limits of variation in animals above referred to. So, in animal instinct
+or intelligence, we find the same general level of development. Every
+wren makes a fairly good nest like its fellows; every fox has an average
+amount of the sagacity of its race; while all the higher birds and
+mammals have the necessary affections and instincts needful for the
+protection and bringing-up of their offspring.
+
+But in those specially developed faculties of civilised man which we
+have been considering, the case is very different. They exist only in a
+small proportion of individuals, while the difference of capacity
+between these favoured individuals and the average of mankind is
+enormous. Taking first the mathematical faculty, probably fewer than one
+in a hundred really possess it, the great bulk of the population having
+no natural ability for the study, or feeling the slightest interest in
+it.[234] And if we attempt to measure the amount of variation in the
+faculty itself between a first-class mathematician and the ordinary run
+of people who find any kind of calculation confusing and altogether
+devoid of interest, it is probable that the former could not be
+estimated at less than a hundred times the latter, and perhaps a
+thousand times would more nearly measure the difference between them.
+
+The artistic faculty appears to agree pretty closely with the
+mathematical in its frequency. The boys and girls who, going beyond the
+mere conventional designs of children, draw what they _see_, not what
+they _know_ to be the shape of things; who naturally sketch in
+perspective, because it is thus they see objects; who see, and represent
+in their sketches, the light and shade as well as the mere outlines of
+objects; and who can draw recognisable sketches of every one they know,
+are certainly very few compared with those who are totally incapable of
+anything of the kind. From some inquiries I have made in schools, and
+from my own observation, I believe that those who are endowed with this
+natural artistic talent do not exceed, even if they come up to, one per
+cent of the whole population.
+
+The variations in the amount of artistic faculty are certainly very
+great, even if we do not take the extremes. The gradations of power
+between the ordinary man or woman "who does not draw," and whose
+attempts at representing any object, animate or inanimate, would be
+laughable, and the average good artist who, with a few bold strokes, can
+produce a recognisable and even effective sketch of a landscape, a
+street, or an animal, are very numerous; and we can hardly measure the
+difference between them at less than fifty or a hundred fold.
+
+The musical faculty is undoubtedly, in its lower forms, less uncommon
+than either of the preceding, but it still differs essentially from the
+necessary or useful faculties in that it is almost entirely wanting in
+one-half even of civilised men. For every person who draws, as it were
+instinctively, there are probably five or ten who sing or play without
+having been taught and from mere innate love and perception of melody
+and harmony.[235] On the other hand, there are probably about as many
+who seem absolutely deficient in musical perception, who take little
+pleasure in it, who cannot perceive discords or remember tunes, and who
+could not learn to sing or play with any amount of study. The
+gradations, too, are here quite as great as in mathematics or pictorial
+art, and the special faculty of the great musical composer must be
+reckoned many hundreds or perhaps thousands of times greater than that
+of the ordinary "unmusical" person above referred to.
+
+It appears then, that, both on account of the limited number of persons
+gifted with the mathematical, the artistic, or the musical faculty, as
+well as from the enormous variations in its development, these mental
+powers differ widely from those which are essential to man, and are, for
+the most part, common to him and the lower animals; and that they could
+not, therefore, possibly have been developed in him by means of the law
+of natural selection.
+
+       *       *       *       *       *
+
+We have thus shown, by two distinct lines of argument, that faculties
+are developed in civilised man which, both in their mode of origin,
+their function, and their variations, are altogether distinct from those
+other characters and faculties which are essential to him, and which
+have been brought to their actual state of efficiency by the necessities
+of his existence. And besides the three which have been specially
+referred to, there are others which evidently belong to the same class.
+Such is the metaphysical faculty, which enables us to form abstract
+conceptions of a kind the most remote from all practical applications,
+to discuss the ultimate causes of things, the nature and qualities of
+matter, motion, and force, of space and time, of cause and effect, of
+will and conscience. Speculations on these abstract and difficult
+questions are impossible to savages, who seem to have no mental faculty
+enabling them to grasp the essential ideas or conceptions; yet whenever
+any race attains to civilisation, and comprises a body of people who,
+whether as priests or philosophers, are relieved from the necessity of
+labour or of taking an active part in war or government, the
+metaphysical faculty appears to spring suddenly into existence,
+although, like the other faculties we have referred to, it is always
+confined to a very limited proportion of the population.
+
+In the same class we may place the peculiar faculty of wit and humour,
+an altogether natural gift whose development appears to be parallel with
+that of the other exceptional faculties. Like them, it is almost unknown
+among savages, but appears more or less frequently as civilisation
+advances and the interests of life become more numerous and more
+complex. Like them, too, it is altogether removed from utility in the
+struggle for life, and appears sporadically in a very small percentage
+of the population; the majority being, as is well known, totally unable
+to say a witty thing or make a pun even to save their lives.[236]
+
+
+_The Interpretation of the Facts._
+
+The facts now set forth prove the existence of a number of mental
+faculties which either do not exist at all or exist in a very
+rudimentary condition in savages, but appear almost suddenly and in
+perfect development in the higher civilised races. These same faculties
+are further distinguished by their sporadic character, being well
+developed only in a very small proportion of the community; and by the
+enormous amount of variation in their development, the higher
+manifestations of them being many times--perhaps a hundred or a thousand
+times--stronger than the lower. Each of these characteristics is totally
+inconsistent with any action of the law of natural selection in the
+production of the faculties referred to; and the facts, taken in their
+entirety, compel us to recognise some origin for them wholly distinct
+from that which has served to account for the animal
+characteristics--whether bodily or mental--of man.
+
+The special faculties we have been discussing clearly point to the
+existence in man of something which he has not derived from his animal
+progenitors--something which we may best refer to as being of a
+spiritual essence or nature, capable of progressive development under
+favourable conditions. On the hypothesis of this spiritual nature,
+superadded to the animal nature of man, we are able to understand much
+that is otherwise mysterious or unintelligible in regard to him,
+especially the enormous influence of ideas, principles, and beliefs over
+his whole life and actions. Thus alone we can understand the constancy
+of the martyr, the unselfishness of the philanthropist, the devotion of
+the patriot, the enthusiasm of the artist, and the resolute and
+persevering search of the scientific worker after nature's secrets. Thus
+we may perceive that the love of truth, the delight in beauty, the
+passion for justice, and the thrill of exultation with which we hear of
+any act of courageous self-sacrifice, are the workings within us of a
+higher nature which has not been developed by means of the struggle for
+material existence.
+
+It will, no doubt, be urged that the admitted continuity of man's
+progress from the brute does not admit of the introduction of new
+causes, and that we have no evidence of the sudden change of nature
+which such introduction would bring about. The fallacy as to new causes
+involving any breach of continuity, or any sudden or abrupt change, in
+the effects, has already been shown; but we will further point out that
+there are at least three stages in the development of the organic world
+when some new cause or power must necessarily have come into action.
+
+The first stage is the change from inorganic to organic, when the
+earliest vegetable cell, or the living protoplasm out of which it arose,
+first appeared. This is often imputed to a mere increase of complexity
+of chemical compounds; but increase of complexity, with consequent
+instability, even if we admit that it may have produced protoplasm as a
+chemical compound, could certainly not have produced _living_
+protoplasm--protoplasm which has the power of growth and of
+reproduction, and of that continuous process of development which has
+resulted in the marvellous variety and complex organisation of the whole
+vegetable kingdom. There is in all this something quite beyond and
+apart from chemical changes, however complex; and it has been well said
+that the first vegetable cell was a new thing in the world, possessing
+altogether new powers--that of extracting and fixing carbon from the
+carbon-dioxide of the atmosphere, that of indefinite reproduction, and,
+still more marvellous, the power of variation and of reproducing those
+variations till endless complications of structure and varieties of form
+have been the result. Here, then, we have indications of a new power at
+work, which we may term _vitality_, since it gives to certain forms of
+matter all those characters and properties which constitute Life.
+
+The next stage is still more marvellous, still more completely beyond
+all possibility of explanation by matter, its laws and forces. It is the
+introduction of sensation or consciousness, constituting the fundamental
+distinction between the animal and vegetable kingdoms. Here all idea of
+mere complication of structure producing the result is out of the
+question. We feel it to be altogether preposterous to assume that at a
+certain stage of complexity of atomic constitution, and as a necessary
+result of that complexity alone, an _ego_ should start into existence, a
+thing that _feels_, that is _conscious_ of its own existence. Here we
+have the certainty that something new has arisen, a being whose nascent
+consciousness has gone on increasing in power and definiteness till it
+has culminated in the higher animals. No verbal explanation or attempt
+at explanation--such as the statement that life is the result of the
+molecular forces of the protoplasm, or that the whole existing organic
+universe from the amaeba up to man was latent in the fire-mist from
+which the solar system was developed--can afford any mental
+satisfaction, or help us in any way to a solution of the mystery.
+
+The third stage is, as we have seen, the existence in man of a number of
+his most characteristic and noblest faculties, those which raise him
+furthest above the brutes and open up possibilities of almost indefinite
+advancement. These faculties could not possibly have been developed by
+means of the same laws which have determined the progressive development
+of the organic world in general, and also of man's physical
+organism.[237]
+
+These three distinct stages of progress from the inorganic world of
+matter and motion up to man, point clearly to an unseen universe--to a
+world of spirit, to which the world of matter is altogether subordinate.
+To this spiritual world we may refer the marvellously complex forces
+which we know as gravitation, cohesion, chemical force, radiant force,
+and electricity, without which the material universe could not exist for
+a moment in its present form, and perhaps not at all, since without
+these forces, and perhaps others which may be termed atomic, it is
+doubtful whether matter itself could have any existence. And still more
+surely can we refer to it those progressive manifestations of Life in
+the vegetable, the animal, and man--which we may classify as
+unconscious, conscious, and intellectual life,--and which probably
+depend upon different degrees of spiritual influx. I have already shown
+that this involves no necessary infraction of the law of continuity in
+physical or mental evolution; whence it follows that any difficulty we
+may find in discriminating the inorganic from the organic, the lower
+vegetable from the lower animal organisms, or the higher animals from
+the lowest types of man, has no bearing at all upon the question. This
+is to be decided by showing that a change in essential nature (due,
+probably, to causes of a higher order than those of the material
+universe) took place at the several stages of progress which I have
+indicated; a change which may be none the less real because absolutely
+imperceptible at its point of origin, as is the change that takes place
+in the curve in which a body is moving when the application of some new
+force causes the curve to be slightly altered.
+
+
+_Concluding Remarks._
+
+Those who admit my interpretation of the evidence now adduced--strictly
+scientific evidence in its appeal to facts which are clearly what ought
+_not_ to be on the materialistic theory--will be able to accept the
+spiritual nature of man, as not in any way inconsistent with the theory
+of evolution, but as dependent on those fundamental laws and causes
+which furnish the very materials for evolution to work with. They will
+also be relieved from the crushing mental burthen imposed upon those
+who--maintaining that we, in common with the rest of nature, are but
+products of the blind eternal forces of the universe, and believing also
+that the time must come when the sun will lose his heat and all life on
+the earth necessarily cease--have to contemplate a not very distant
+future in which all this glorious earth--which for untold millions of
+years has been slowly developing forms of life and beauty to culminate
+at last in man--shall be as if it had never existed; who are compelled
+to suppose that all the slow growths of our race struggling towards a
+higher life, all the agony of martyrs, all the groans of victims, all
+the evil and misery and undeserved suffering of the ages, all the
+struggles for freedom, all the efforts towards justice, all the
+aspirations for virtue and the wellbeing of humanity, shall absolutely
+vanish, and, "like the baseless fabric of a vision, leave not a wrack
+behind."
+
+As contrasted with this hopeless and soul-deadening belief, we, who
+accept the existence of a spiritual world, can look upon the universe as
+a grand consistent whole adapted in all its parts to the development of
+spiritual beings capable of indefinite life and perfectibility. To us,
+the whole purpose, the only _raison d'etre_ of the world--with all its
+complexities of physical structure, with its grand geological progress,
+the slow evolution of the vegetable and animal kingdoms, and the
+ultimate appearance of man--was the development of the human spirit in
+association with the human body. From the fact that the spirit of
+man--the man himself--_is_ so developed, we may well believe that this
+is the only, or at least the best, way for its development; and we may
+even see in what is usually termed "evil" on the earth, one of the most
+efficient means of its growth. For we know that the noblest faculties of
+man are strengthened and perfected by struggle and effort; it is by
+unceasing warfare against physical evils and in the midst of difficulty
+and danger that energy, courage, self-reliance, and industry have become
+the common qualities of the northern races; it is by the battle with
+moral evil in all its hydra-headed forms, that the still nobler
+qualities of justice and mercy and humanity and self-sacrifice have been
+steadily increasing in the world. Beings thus trained and strengthened
+by their surroundings, and possessing latent faculties capable of such
+noble development, are surely destined for a higher and more permanent
+existence; and we may confidently believe with our greatest living
+poet--
+
+
+    That life is not as idle ore,
+
+    But iron dug from central gloom,
+        And heated hot with burning fears,
+        And dipt in baths of hissing tears,
+    And batter'd with the shocks of doom
+
+    To shape and use.
+
+
+We thus find that the Darwinian theory, even when carried out to its
+extreme logical conclusion, not only does not oppose, but lends a
+decided support to, a belief in the spiritual nature of man. It shows us
+how man's body may have been developed from that of a lower animal form
+under the law of natural selection; but it also teaches us that we
+possess intellectual and moral faculties which could not have been so
+developed, but must have had another origin; and for this origin we can
+only find an adequate cause in the unseen universe of Spirit.
+
+FOOTNOTES:
+
+[Footnote 218: _Descent of Man_, pp. 41-43; also pp. 13-15.]
+
+[Footnote 219: _Man's Place in Nature_, p. 64.]
+
+[Footnote 220: _Man's Place in Nature_, p. 67. See Figs. of Embryos of
+Man and Dog in Darwin's _Descent of Man_, p. 10.]
+
+[Footnote 221: _The Descent of Man_, pp. 7, 8.]
+
+[Footnote 222: _Man and Apes._ By St. George Mivart, F.R.S., 1873. It is
+an interesting fact (for which I am indebted to Mr. E.B. Poulton) that
+the human embryo possesses the extra rib and wrist-bone referred to
+above in (2) and (4) as occurring in some of the apes.]
+
+[Footnote 223: _Man and Apes_, pp. 138, 144.]
+
+[Footnote 224: For a sketch of the evidence of Man's Antiquity in
+America, see _The Nineteenth Century_ for November 1887.]
+
+[Footnote 225: This subject was first discussed in an article in the
+_Anthropological Review_, May 1864, and republished in my _Contributions
+to Natural Selection_, chap, ix, in 1870.]
+
+[Footnote 226: _Man's Place in Nature_, p. 102.]
+
+[Footnote 227: For a full discussion of this question, see the author's
+_Geographical Distribution of Animals_, vol. i. p. 285.]
+
+[Footnote 228: For a full discussion of all these points, see _Descent
+of Man_, chap. iii.]
+
+[Footnote 229: _Descent of Man_, chap. iv.]
+
+[Footnote 230: Lubbock's _Origin of Civilisation_, fourth edition, pp.
+434-440; Tylor's _Primitive Culture_, chap. vii.]
+
+[Footnote 231: It has been recently stated that some of these facts are
+erroneous, and that some Australians can keep accurate reckoning up to
+100, or more, when required. But this does not alter the general fact
+that many low races, including the Australians, have no words for high
+numbers and never require to use them. If they are now, with a little
+practice, able to count much higher, this indicates the possession of a
+faculty which could not have been developed under the law of utility
+only, since the absence of words for such high numbers shows that they
+were neither used nor required.]
+
+[Footnote 232: Article Arithmetic in _Eng. Cyc. of Arts and Sciences_.]
+
+[Footnote 233: See "History of Music," in _Eng. Cyc._, Science and Arts
+Division.]
+
+[Footnote 234: This is the estimate furnished me by two mathematical
+masters in one of our great public schools of the proportion of boys who
+have any special taste or capacity for mathematical studies. Many more,
+of course, can be drilled into a fair knowledge of elementary
+mathematics, but only this small proportion possess the natural faculty
+which renders it possible for them ever to rank high as mathematicians,
+to take any pleasure in it, or to do any original mathematical work.]
+
+[Footnote 235: I am informed, however, by a music master in a large
+school that only about one per cent have real or decided musical talent,
+corresponding curiously with the estimate of the mathematicians.]
+
+[Footnote 236: In the latter part of his essay on Heredity (pp. 91-93 of
+the volume of _Essays_), Dr. Weismann refers to this question of the
+origin of "talents" in man, and, like myself, comes to the conclusion
+that they could not be developed under the law of natural selection. He
+says: "It may be objected that, in man, in addition to the instincts
+inherent in every individual, special individual predispositions are
+also found, of such a nature that it is impossible they can have arisen
+by individual variations of the germ-plasm. On the other hand, these
+predispositions--which we call talents--cannot have arisen through
+natural selection, because life is in no way dependent on their
+presence, and there seems to be no way of explaining their origin except
+by an assumption of the summation of the skill attained by exercise in
+the course of each single life. In this case, therefore, we seem at
+first sight to be compelled to accept the transmission of acquired
+characters." Weismann then goes on to show that the facts do not support
+this view; that the mathematical, musical, or artistic faculties often
+appear suddenly in a family whose other members and ancestors were in no
+way distinguished; and that even when hereditary in families, the talent
+often appears at its maximum at the commencement or in the middle of the
+series, not increasing to the end, as it should do if it depended in any
+way on the transmission of acquired skill. Gauss was not the son of a
+mathematician, nor Handel of a musician, nor Titian of a painter, and
+there is no proof of any special talent in the ancestors of these men of
+genius, who at once developed the most marvellous pre-eminence in their
+respective talents. And after showing that such great men only appear at
+certain stages of human development, and that two or more of the special
+talents are not unfrequently combined in one individual, he concludes
+thus--
+
+
+    "Upon this subject I only wish to add that, in my opinion,
+    talents do not appear to depend upon the improvement of any
+    special mental quality by continued practice, but they are the
+    expression, and to a certain extent the bye-product, of the
+    human mind, which is so highly developed in all directions."
+
+
+It will, I think, be admitted that this view hardly accounts for the
+existence of the highly peculiar human faculties in question.]
+
+[Footnote 237: For an earlier discussion of this subject, with some
+wider applications, see the author's _Contributions to the Theory of
+Natural Selection_, chap. x.]
+
+
+
+
+INDEX
+
+
+
+=A=
+
+Abbott, Dr. C.C., instability of habits of birds, 76
+  on American water-thrushes (Seiurus), 117
+  Mr., drawings of caterpillars and their food plants, 203
+
+Accessory plumes, development and display of, 293
+
+Acclimatisation, 94
+
+Achatinellidae, Gulick on variations in, 147
+
+Acquired characters, non-heredity of, 440
+
+Acraeidae, mimicry of, 247
+
+Adaptation to conditions at various periods of life, 112
+
+Adolias dirtea, sexual diversity of, 271
+
+Aegeriidae, mimicry by, 240
+
+Agaristidae, mimicry of, 246
+
+Agassiz, on species, 5
+  on North American weeds, 15.
+
+Agelaeus phoeniceus, diagram showing variations of, 56;
+  proportionate numbers which vary, 64
+
+Albatross, courtship of great, 287
+
+Allen, Mr. Grant, on forms of leaves, 133
+  on degradation of wind-fertilised from insect-fertilised flowers,
+  325 (note)
+  on insects and flowers, 332
+  on production of colour through the agency of the colour sense, 334
+  Mr. J.A., on the variability of birds, 50
+
+Allen, Mr. J.A.,  on colour as influenced by climate, 228
+
+Alluring coloration, 210
+
+American school of evolutionists, 420
+
+Anemone nemorosa, variability of, 78
+
+Animal coloration, a theory of, 288
+  general laws of, 296
+  intelligence, supposed action of, 425
+  characteristics of man, 454
+
+Animals, the struggle among, 18
+  wild, their enjoyment of life, 39
+  usually die painless deaths, 38
+  constitutional variation of, 94
+  uses of colours of, 134
+  supposed effects of disuse in wild, 415
+  most allied to man, 450
+
+Antelopes, recognition marks of, 219
+
+Anthrocera filipendula inedible, 235
+
+Apples, variations of, 87
+
+Arctic animals, supposed causes of white colour of, 191
+
+Argyll, Duke of, on goose reared by a golden eagle, 75
+
+Artemia salina and A. milhausenii, 426
+
+Asclepias curassavica, spread of, 28
+
+Asses running wild in Quito, 28
+
+Attractive fruits, 306
+
+Australia, spread of the Cape-weed in, 29
+  fossil and recent mammals of, 392
+
+Azara, on cause of horses and cattle not running wild in Paraguay, 19
+
+Azores, flora of, supports aerial transmission of seeds, 368
+
+
+=B=
+
+Baker, Mr. J.G., on rarity of spiny plants in Mauritius, 432
+
+Ball, Mr., on cause of late appearance of exogens, 400
+
+Barber, Mrs., on variable colouring of pupae of Papilio nireus, 197
+  on protective colours of African sun-birds, 200
+
+Barbs, 91
+
+Barriers, importance of, in questions of distribution, 341
+
+Bates, Mr. H.W., on varieties of butterflies, 44
+  on inedibility of Heliconidae, 234
+  on a conspicuous caterpillar, 236
+  on mimicry, 240, 243, 249
+
+Bathmism or growth-force, Cope on, 421
+
+Beddard, Mr. F.E., variations of earthworms, 67
+  on plumes of bird of paradise, 292
+
+Beech trees, aggressive in Denmark, 21
+
+Beetle and wasp (figs.), 259
+
+Beetle, fossil in coal measures of Silesia, 404
+
+Beginnings of important organs, 128
+
+Belt, Mr., on leaf-like locust, 203
+  on birds avoiding Heliconidae, 234
+
+Belt's frog, 266
+
+Birds, rate of increase of, 25
+  how destroyed, 26
+  variation among, 49
+  variation of markings of, 52
+  variation of wings and tails of, 53
+  diagram showing variation of tarsus and toes, 60
+  use of structural peculiarities of, 135
+  eggs, coloration of, 212
+  recognition marks of, 222
+  and butterflies, white in tropical islands, 230
+  sometimes seize inedible butterflies, 255
+  mimicry among, 263
+
+Birds, sexual coloration of, 275
+  cause of dull colour of female, 277
+  choice of female not known to be determined by colour, etc., 285
+  decorative plumage of, 285
+  antics of unornamented, 287
+  which fertilise flowers, 319
+  colours of, not dependent on the colours of flowers, 336
+  no proof of aesthetic tastes in, 336
+  dispersal of, 355
+  and insects at sea, 357
+  of oceanic islands, 358
+  carrying seeds on their feet, 361
+  ancestral forms of, 407
+
+Birthplace, probable, of man, 459
+
+Bombyx regia, protective form of larva of, 210
+
+Boyd Dawkins, on development of deer's horns, 389
+  on origin of man, 456
+
+Brady, Mr. George, on protective colouring of starfishes, 209
+
+Brain development, progressive, 390
+
+Brains of man and apes, 452
+
+Branner, Mr. J.C., on supposed proofs of glaciation in Brazil, 370
+
+Brazil, supposed proof of glaciation in, 370
+
+Brewer, Professor W.H., on want of symmetry in colours of animals, 217
+
+Bromelia, animals inhabiting leaves of, 118
+
+Bronn, Professor, on supposed uselessness of variations of ears
+  and tails, 136
+
+Butler, Mr. A.G., on inedibility of conspicuous caterpillars, 237
+
+Butterflies, varieties of, 44
+  small, of Isle of Man, 106
+  special protective colouring of, 206
+  recognition by, 226
+  inedibility of some, 234
+  mimicry among, 240, 249
+  colour development of, 274
+  sexual coloration of, 271
+
+
+=C=
+
+Caddis-fly larvae inhabiting bromelia leaves, 118
+
+Callophis, harmless mimicking poisonous species, 262
+
+Candolle, Alp. de, on variation in oaks, 77
+  on variability of Papaver bracteatum, 79
+
+Cardinalis virginianus, diagram showing proportionate numbers
+  which vary, 65;
+  variations of, 58
+
+Carpenter, Dr. W.B., on variation in the Foraminifera, 43
+
+Carriers, 91
+
+Caterpillars, resemblance of, to their food plants, 203-205
+  inedible, 236
+
+Cattle, how they prevent the growth of trees, 18
+  increase of, in St. Domingo, Mexico, and the pampas, 27
+
+Ceylon, spread of Lantana mixta in, 29
+
+Chaffinch, change of habit of, in New Zealand, 76
+
+Chambers, Robert, on origin of species, 3
+
+Chance rarely determines survival, 123
+
+Change of conditions, utility of, 326
+
+Characters, non-adaptive, 131
+  transferred from useless to useful class, 132
+
+Charaxes psaphon persecuted by a bird, 235
+
+Chile, numerous red tubular flowers in, 320
+
+Chimpanzee, figure of, 454
+
+Clark, Mr. Edwin, on cause of absence of forests on the pampas, 23
+  on the struggle for life in the South American valleys, 24
+
+Cleistogamous flowers, 322
+
+Close interbreeding, supposed evil results of, 326
+
+Clover, white, spread of, in New Zealand, 28
+
+Co-adaptation of parts by variation, no real difficulty, 418
+
+Cobra, use of hood of, 262
+
+Coccinella mimicked by grasshopper, (figure), 260
+
+Collingwood, Mr., on butterflies recognising their kind, 226
+
+Coloration, alluring, 210
+  of birds' eggs, 212
+  a theory of animal, 288
+
+Colour correlated with sterility, 169
+  correlated with constitutional peculiarities, 170
+  in nature, the problem to be solved, 188
+  constancy, in animals indicates utility, 189
+  and environment, 190
+  general theories of animal, 193
+  animal, supposed causes of, 193
+  obscure, of many tropical animals, 194
+  produced by surrounding objects, 195
+  adaptations, local, 199
+  for recognition, 217
+  of wild animals not quite symmetrical, 217 (note)
+  as influenced by locality or climate, 228
+  development in butterflies, 274
+  more variable than habits, 278
+  and nerve distribution, 290
+  and tegumentary appendages, 291
+  of flowers, 308
+  change of, in flowers when fertilised, 317
+  in nature, concluding remarks on, 299, 333
+  of fruits, 304
+  of flowers growing together contrasted, 318
+
+Complexity of flowers due to alternate adaptation to insect
+  and self-fertilisation, 328
+
+Composite, a, widely dispersed without pappus, 367
+
+Confinement, affecting fertility, 154
+
+Continental and oceanic areas, 346
+
+Continents and oceans cannot have changed places, 345
+  possible connections between, 349
+
+Continuity does not prove identity of origin, 463
+
+Cope, Dr. E.D., on non-adaptive characters, 131
+  on fundamental laws of growth, 420
+  on bathmism or growth-force, 421
+  on use producing structural change, 422
+  on law of centrifugal growth, 422
+  on origin of the feet of ungulates, 423
+  on action of animal intelligence, 425
+
+Correlations in pigeons, horses, etc., 140
+
+Corvus frugilegus, 2
+  corone, 2
+
+Coursers, figures of secondary quills, 224
+
+Cowslip, two forms of, 157
+
+Crab, sexual diversity of colour of, 269
+
+Cretaceous period, dicotyledons of, 400
+
+Crisp, Dr., on variations of gall bladder and alimentary canal, 69
+
+Crosses, a cause of variation, 99
+  reciprocal, 155
+
+Cross-fertilisation, modes of securing, 310
+  difference in, 155
+
+Crossing and changed conditions,
+  parallelism of, 166
+
+Cruciferae, variations of structure in, 80
+
+Cuckoo, eggs of, 216
+
+Cuckoos mimick hawks, 263
+
+Cultivated plants, origin of useful, 97
+
+Curculionidae mimicked by various insects (figs.), 260
+
+Curves of variation, 64
+
+
+=D=
+
+Dana, Professor, on the permanence of continents, 342
+
+Danaidae little attacked by mites, 235
+  mimicry of, 246
+
+Darwin, change of opinion effected by, 8
+  the Newton of Natural History, 9
+  his view of his own work, 10
+  on the enemies of plants, 16
+  on fir-trees destroyed by cattle, 17
+  on change of plants and animals caused by planting, 18
+  on absence of wild cattle in Paraguay, 19
+  on cats and red clover, 20
+  on variety of plants in old turf, 35
+  on the beneficent action of the struggle for existence, 40
+  on variability of wild geraniums, 79
+  on variability of common species, 80
+     his non-recognition of extreme variability of wild species, 82
+  on races of domestic pigeon, 90
+  on constitutional variation in plants, 95
+  on unconscious selection, 96
+  on a case of divergence, 105
+  on advantage of diversification of structure in inhabitants
+     of one region, 110
+  on species of plants in turf, 110
+  on isolation, 119
+  on origin of mammary glands, 129
+  on eyes of flatfish, 129
+  on origin of the eye, 130
+  on useless characters, 131
+  on use of ears and tails, 136
+  on disappearance of sports, 140
+  on tendency to vary in one direction, 141
+  on rare perpetuation of sports, 142
+  on utility of specific characters, 142 (note)
+  on importance of biological environment, 148
+  on variable fertility of plants, 155
+  on fertile hybrids among plants, 164
+
+Darwin, on correlation of sterility and colour, 169
+  on selective association, 172
+  on infertility and natural selection, 174
+  on cause of infertility of hybrids, 185
+  on white tail of rabbit, 218
+  on conspicuous caterpillars, 236
+  on sexual selection in insects, 274
+  on decorative plumage of male birds, 285
+  on development of ocelli, 290
+  on value of cross-fertilisation, 309
+  on limits  to  utility  of  intercrossing, 326
+  on flowers due to insects, 332
+  on oceanic islands, 342
+  on effects of disuse in domestic animals, 415, 435
+  on direct action of environment, 419
+  on unintelligibility of theory of retardation and acceleration,
+     421 (note)
+  on origin of man's moral nature, 461
+  Mr. George, on intermarriages of British aristocracy, 326
+
+Darwinian theory, statement of, 10
+  not opposed to spiritual nature of man, 478
+Dawkins, Professor Boyd, on development of deer's horns, 389
+  on recent origin of man, 456
+Dawson, Sir W., on determination of fossil plants by leaves, 398 (note)
+
+Death of wild animals usually painless, 38
+
+De Candolle, definition of species, 1
+  on difficulty of naturalising plants, 15
+  on war between plants, 16
+  on origin of useful cultivated plants, 97
+
+Deer's horns, development of, 389
+
+Degeneration, 121
+
+Delboeuf's law of variation, 141
+
+Dendraeca coronata, variation of wing-feathers of, 51
+
+Denmark, struggle between trees in, 20
+
+Denudation, evidences of, 379
+
+Desert animals, colour of, 192
+
+Deserts, effect of goats and camels in destroying vegetation in, 17
+
+Development and display of accessory plumes, 293
+
+Diadema anomala, 271
+  misippus, great diversity of sexes in, 271
+
+Diaphora mendica mimics Spilosoma menthrasti, 249
+
+Difficulties in the facts of fertilisation of flowers, 325
+
+Dimorphism and trimorphism, 156
+
+Dippers, probable origin of, 116
+
+Disease and markings, 290
+
+Diseases common to man and animals, 449
+
+Display of decorative plumage, 287
+
+Distribution of organisms should be explained by theory of descent, 338
+  conditions which have determined the, 341
+  of marsupials, 350
+  of tapirs, 352
+
+Disuse, effects of, among wild animals, 415
+  no proof that the effects of, are inherited, 417
+
+Divergence of character, 105-109
+  leads to maximum of forms of life in each area, 109
+
+Diversity of fauna and flora with geographical proximity, 339
+
+Dixon, Mr. C, changed habits of chaffinch in New Zealand, 76
+
+Dogs, origin of, 88
+  varieties of, 89
+
+Dolichonyx oryzivorus, diagram showing variations of, 55
+
+Domestic animals, varieties of, 88
+
+Draba verna, varieties of, 77
+
+Dress of men not determined by female choice, 286
+
+Dust from Krakatoa, size of particles of, 363
+
+
+=E=
+
+Eastern butterflies, variation of, 45
+
+Eaton, Rev. A.E., on Kerguelen insects, 106
+
+Edwards, Mr. W.H., on dark forms of Papilio turnus, 248
+
+Eggs protectively coloured, 214, 215
+  theory of varied colours of, 216
+
+Elaps mimicked by harmless snakes, 261
+
+Embryonic development of man and other mammalia, 448
+
+Ennis, Mr. John, on willows driving out watercresses from
+  rivers of New Zealand, 24
+
+Entomostraca, in bromelia leaves, 118
+
+Environment never identical for two species, 149
+  direct action of, 418
+  direct influence of, 426
+  as initiator of variations, 436
+  action of, overpowered by natural selection, 437
+
+Ethical aspect of the struggle for existence, 36
+
+Euchelia jacobeae inedible, 235
+
+Everett, Mr. A., on a caterpillar resembling moss, 205
+
+Evidence of evolution that may be expected among fossil forms, 380
+
+Evolutionists, American school of, 420
+
+Exogens, possible cause of sudden late appearance of, 400
+
+External differences of man and apes, 453
+
+Extinct animals, number of species of, 376
+
+Extinction of large animals, cause of, 394
+
+Eye, origin of, 130
+
+Eyes, explanation of loss of in cave animals, 416
+
+
+=F=
+
+Facts of natural selection, summary of, 122
+
+Falcons illustrating divergence, 108
+  and butcher birds, hooked and toothed beaks of, 422
+
+Fantails, 91
+
+Female birds, why often dull coloured, 277
+
+Female birds, what their choice of mates is determined by, 286
+  butterflies, why dull coloured, 272
+  brighter than male bird, 281
+  choice a doubtful agent in selection, 283
+  preference neutralised by natural selection, 294
+
+Fertility of domestic animals, 154
+
+Flatfish, eyes of, 129
+
+Flesh-fly, enormous increase of, 25
+
+Floral structure, great differences of, in allied genera and species, 329
+
+Flowers, variations of, 88
+  colours of, 308
+  with sham nectaries, 317
+  changing colour when fertilised, 317
+  adapted to bees or to butterflies, 318
+  contrasted colours of, at same season and locality, 318
+  fertilisation of, by birds, 319
+  self-fertilisation of, 321
+  once insect-fertilised now self-fertile, 323
+  how the struggle for existence acts among, 328
+  repeatedly modified during whole Tertiary period, 331
+  the product of insect agency, 332
+
+Forbes, Mr. H.O., on protective colour of a pigeon, 200
+  on spider imitating birds' dropping, 211
+
+Fossil shells, complete series of transitional forms of, 381
+  crocodiles afford evidence of evolution, 383
+  horses in America, 386
+  and living animals, local relations of, 391
+
+Fowl, early domestication of, 97
+
+Frill-back, Indian, 93
+
+Frog inhabiting bromelia leaves, 118
+
+Fruits, use of characters of, 133
+  colours of, 304
+  edible or attractive, 306
+  poisonous, 307
+
+Fulica atra, protectively coloured eggs of, 215
+
+Fulmar petrel, abundance of, 30
+
+
+=G=
+
+Gallinaceae, ornamental plumes of, 292
+
+Galton, Mr. F., diagrams of variability used by, 74
+  on markings of zebra, 220 (note)
+  on regression towards  mediocrity, 414
+  theory of heredity by, 443 (note)
+  on imperfect counting of the Damaras, 464
+
+Gaudry on extinct animals at Pikermi, 377
+
+Gay, Mons. T., on variations of structure in Cruciferae, 80
+
+Gazella soemmerringi (figure), 219
+
+Gazelles, recognition marks of, 218
+
+Geddes, Professor, on variation in plants, 428
+  objection to theory of, 430
+
+Geikie, Dr. Archibald, on formation of marine stratified rocks, 344
+
+Geoffroy St. Hilaire, on species, 6
+
+Geological evidences of evolution, 376, 381
+  record, causes of imperfection of, 379
+  distribution of insects, 403
+  antiquity of man, 455
+
+Ghost-moth, colours of, 270
+
+Glaciation, no proofs of, in Brazil, 370
+
+Glow-worm, light a warning of inedibility, 287
+
+Gomphia oleaefolia, variability of, 79
+
+Goose eating flesh, 75
+
+Gosse, Mr. P.H., on variation in the sea-anemones, 43
+  on sea-anemone and bullhead, 265
+
+Gould, Mr., on colours of coast and inland birds, 228
+
+Grant Allen, on forms of leaves, 133
+  on insects and flowers, 332
+
+Graphite in Laurentian implies abundant plant life, 398
+
+Gray, Dr. Asa, on naturalised plants in the United States, 110
+  Dr. J.E., on variation of skulls of mammalia, 71
+
+Great fertility not essential to rapid increase, 30
+
+Great powers of increase of animals, 27
+
+Green colour of birds in tropical forests, 192
+
+Grouse, red, recent divergence of, 106
+
+Gulick, Rev. J.T., on variation of land-shells, 43
+  on isolation and variation, 147, 150
+  on divergent evolution, 148
+
+
+=H=
+
+Habits  of animals, variability of, 74
+
+Hairy caterpillars inedible, 237
+
+Hanbury, Mr. Thomas, on a remarkable case of wind
+  conveyance of seed, 373 (note)
+
+Hansten-Blangsted, on succession of trees in Denmark, 21
+
+Harvest mice, prehensile tails of young, 136
+
+Hawkweed, species and varieties of British, 77
+
+Hector, Sir James, use of horns of deer, 137
+
+Heliconidae, warning colours of, 234
+  mimicry of, 240
+
+Helix nemoralis, varieties of, 43
+  hortensis, varieties of, 43
+
+Hemsley, Mr., on rarity of spines in oceanic islands, 432
+
+Henslow, Professor G., on vigour of self-fertilised plants, 323
+  on wind-fertilised as degradations from insect-fertilised flowers, 324
+  on origin of forms and structures of flowers, 434 (note)
+
+Herbert, Dean, on species, 6
+  on plant hybrids, 164
+
+Herbivora, recognition marks of, 218
+
+Heredity, 11
+  Weismann's theory of, 437
+
+Herschel, Sir John, on species, 3
+
+Hooker, Sir Joseph, on attempts at naturalising Australian
+  plants in New Zealand, 16
+
+Home, Mr. C, on inedibility of an Indian locust, 267
+
+Horns of deer, uses of, 136
+
+Horse tribe, pedigree of, 384
+  ancestral forms of, 386
+
+Humming-birds, recognition marks of, 226
+
+Huth, Mr., on close interbreeding, 160
+
+Huxley, Professor on the struggle for existence, 37
+  on fossil crocodiles, 383
+  on anatomical peculiarities of the horse tribe, 384
+  on development of vertebrates, 448
+  on early man, 456
+  on brains of man and the gorilla, 457
+
+Hybridity, remarks on facts of, 166
+  summary on, 184
+
+Hybrids, infertility of, supposed test of distinct species, 152
+  fertility of, 159
+  fertile among animals, 162
+  between sheep and goat, 162
+  fertile between distinct species of moths, 163
+  fertile among plants, 163
+
+Hymenopus bicornis, resembling flower, 212
+
+
+=I=
+
+Icterus Baltimore, diagram showing proportionate numbers which
+vary, 63
+
+Imitative resemblances, how produced, 205
+
+Increase of organisms in a geometrical ratio, 25
+
+Inedible fruits rarely coloured, 308
+
+Insect and self-fertilisation, alternation of, in flowers, 328
+
+Insect-fertilisation, facts relating to, 316
+
+Insects, coloured for recognition, 226
+  warning colours of, 233
+  sexual coloration of, 269
+  importance of dull colours to female, 272
+  visiting one kind of flower at a time, 318
+  and flowers, the most brilliant not found together, 335
+
+Insects, no proof of love of colour by, 336
+  and birds at sea, 357
+  in mid-ocean, 359
+  at great altitudes, 360
+  geological distribution of, 403
+  ancestral in Silurian, 405
+  fossil support evolution, 405
+
+Instability of useless characters, 138
+
+Instinct, the theory of, 441
+
+Insular organisms illustrate powers of dispersal, 354
+
+Interbreeding, close, injurious effects of, 160
+  supposed evil results of close, 326
+
+Intercrossing, swamping effects of, 142
+  not necessarily useful, 325
+
+Intermediate forms, why not found, 380
+
+Islands, all oceanic are volcanic or coralline, 342
+
+Isle of Man, small butterflies of, 106
+
+Isolation, the importance of, 119
+  to prevent intercrossing, 144
+  by variations of habits, etc., 145
+  Rev. J.G. Gulick on, 147
+  when ineffective, 150
+
+Ituna Ilione and Thyridia megisto, figures of wings of, 251
+
+
+=J=
+
+Jacobin, 93
+
+Jenyns, Rev. L., on internal variations of mammalia, 69
+
+Jordan, Mons. A., on varieties of Draba verna, 77
+
+Judd, Professor, on dust fallen at Genoa, 363
+  on Hungarian fossil lacustrine shells, 381
+
+
+=K=
+
+Kerguelen Island, wingless insects of, 106
+
+Kerivoula picta, protective colour of, 201
+
+Kerner, Professor, on use of external characters of plants, 133
+  on seeds found on glaciers, 366
+
+Kingfishers illustrating divergence of character, 109
+
+
+=L=
+
+Lacerta muralis, diagram of variation of, 47
+
+Lagopus scoticus, origin of, 107
+
+Lamarck, on origin of species, 3
+
+Land debris deposited near coasts, 343
+  and ocean, diagram showing comparative height and depth of, 345
+
+Large animals, cause of extinction of, 394
+
+Larvae of moths, variability of, 46
+
+Laughers, Frill-backs, Nuns, Spots, and Swallows, 93
+
+Law of relation of colour and nest, 278, 279
+
+Laws of animal coloration, 296
+
+Lawson Tait, on uses of tails, 136
+
+Leaf-butterflies, 207
+
+Leguminosae, rare in oceanic islands, 368
+
+Lemuria, an unsound hypothesis, 354
+
+Lepidoptera, variation of, 44
+
+Leyden Museum, diagram showing variability of birds in, 61
+
+Life, Weismann on duration of, 437 (note)
+
+Limenitis misippus mimics Danais archippus, 248
+  ursula mimics Papilio philenor, 248
+
+Linnaeus, on rapid increase of the flesh-fly, 25
+
+Livingstone, his sensations when seized by a lion, 38
+
+Lizards, variation among, 46
+  diagram of variation of, 48
+  sexual colours of, 281
+
+Local colour adaptations, 199
+
+Locusts with warning colours inedible, 267
+
+Longicorns mimic Malacoderms, 257
+
+Low, Mr., on effects of close interbreeding, 160
+
+Low, Mr., on fertile crosses between sheep and goat, 162
+  on selective association, 172
+
+Low forms of life, continued existence of, explained, 114
+  forms, persistence of, 121
+  temperature of tropics not needed to explain plant dispersal, 370
+
+Lower types, extinction of, among the higher animals, 114
+
+Lubbock, Sir John, on forms of leaves, 133
+  on imperfect counting of early man, 464
+
+Lyell, Sir Charles, on variation of species, 4
+  on the shifting of continents, 342
+
+
+=M=
+
+Madagascar and New Zealand, 347
+
+Madeira, wingless beetles of, 105
+
+Maize, origin of, 98
+
+Male rivalry, a real cause of selection, 283
+
+Males of many animals fights together, 282
+
+Malm, on eyes of flatfish, 129
+
+Mammalia, variation of, 65
+  sexual colours of, 281, 282
+  afford crucial tests of theories of distribution, 353
+  early forms of, 407
+  geological distribution of, 408
+
+Mammary glands, supposed origin of, 129
+
+Man, summary of animal characteristics of, 454
+  geological antiquity of, 455
+  early remains of, in California, 456
+  probably as old as the Miocene, 457
+  probable birthplace of, 459
+  origin of moral and intellectual nature of, 461
+  possesses mental qualities not derived exclusively
+    from his animal progenitors, 474
+
+Man's body that of an animal, 444
+  development similar to that of animals, 449
+  structure compared with that of the anthropoid apes, 451
+
+Mania typica refused by lizards, 238
+
+Mantidae resembling flowers, 212
+
+Marcgravia nepenthoides fertilised by birds (woodcut), 320
+
+Marine animals, protective resemblance among, 208
+  with warning colours, 266
+
+Marsh, Mr., on destructiveness to vegetation of goats and camels, 17
+  Professor O., on the development of the horse tribe, 386
+  on brain development of Tertiary mammals, 391
+  on specialised forms dying out, 395
+
+Marsupials, distribution of, 350
+
+Mathematical faculty, the origin of the, 464
+  how developed, 466
+  not developed by law of natural selection, 469
+
+Mathematics, late development of, 465
+
+Meldola, Professor R., on variable protective colouring, 196
+  on mimicry among British moths, 249 (note)
+  on an extension of the theory of mimicry, 255 (note)
+
+Melons, variations of, 87
+
+Methona psidii and Leptalis orise (figs.), 241
+
+Meyer, Dr. A.B., on mimicry of snakes, 262
+
+Milne Edwards, on variation of lizards, 46
+
+Mimicking birds deceive naturalists, 264
+  butterfly, figure of, 241
+
+Mimicry, 239
+  how it has been produced, 242
+  among protected genera, 249
+  extension of, 255
+  in various orders of insects, 257
+  among vertebrata, 261
+  among birds, 263
+  objections to theory of, 264
+
+Mineral particles carried by wind, 363
+
+Miocene fossils of North America, 378
+
+Missing links, character of, 380
+
+Mivart, Dr. St. George, on variation of ribs and dorsal vertebrae, 69
+  on supposed useless characters, 138 (note)
+  on resemblance of man and apes, 451
+
+Modifications for special purposes, 113
+
+Mongrels, sterility of, 165
+
+Monkeys affected by medicines as are men, 450
+
+Monocotyledons degradations from dicotyledons, 325 (note)
+  scarcity of, in Rocky Mountains, 401
+  scarcity of, in Alpine flora, 401
+
+Moral nature of man, origin of, 461
+
+Morse, Professor E.T., on protective colouring of marine mollusca, 209
+
+Moseley, Professor, on protective resemblance among marine
+animals, 208
+  on courtship of Great Albatross, 287
+
+Moths, protected groups of, 235
+
+Mountains, remote, with identical plants, 369
+
+Mueller, Dr. Fritz, on inhabitants of bromelia leaves, 118
+  on butterfly, deceived by its mimic, 245
+  his explanation of mimicry among protected genera, 252
+  Dr. Hermann, on variability of Myosurus minimus, 78
+
+Murray, Mr. John, on bulk of land and ocean, 344
+  on quartz particles on ocean floor, 363
+  Rev. R.P., variation in the neuration of butterflies' wings, 45
+
+Musical and artistic faculties, origin of, 467
+
+Myosurns minimus, variability of, 78
+
+
+=N=
+
+Natural selection with changed and unchanged conditions, 103
+  and sterility, 173
+  overpowers effects of use and disuse, 435
+  the most important agency in modifying species, 444
+
+Naturalist deceived by a mimicking insect, 259
+  by mimicking birds, 264
+
+Naudin, M., on varieties of melons, 87
+
+Nectarinea amethystina, protective colouring of, 201
+
+Nestor notabilis, variation of habits of, 75
+
+Nests of birds influence the colour of females, 278
+
+New species, conditions favourable to origin of, 115
+
+Newton, Professor A., on fertile hybrid ducks, 162
+
+New Zealand, European plants in, 15
+  spread of white clover in, 28
+  effects of introduced plants in, 29
+  native rat and fly exterminated by European species, 34
+  many plants of, incapable of self-fertilisation, 321
+  fauna of, 348
+  few spiny plants in, 433
+
+Nocturnal animals, colours of, 193
+
+Non-adaptive characters, instability of, 138
+
+Normandy pigs, fleshy appendages to jaws of, 139
+
+North America, Miocene fossils of, 378
+
+Northern plants in southern hemisphere, 368
+
+Nostus Borbonicus, variability of, 80
+
+Number of individuals which vary, 62
+
+Nutmeg, how dispersed, 307
+
+Nuts, not meant to be eaten, 305
+
+
+=O=
+
+Oaks, great variability of, 78
+
+Objections to Darwin's theory, 126
+
+Ocean floor, deposits on, 343
+
+Oceanic animals, colours of, 193
+  and continental areas, 346
+  islands have no mammals or batrachia, 342
+
+Oceans, the permanence of, 341
+
+Oedicnemus, figures of wings of, 223
+
+Opthalmis lincea and Artaxa simulans (figs.), 247
+
+Orang-utans, variations of skull of, 69
+
+Orchideae, why scarce on oceanic islands, 367
+
+Orchis pyramidalis, mode of fertilisation of, 314
+  figures illustrating fertilisation of, 315
+
+Organic development, three stages of, involving new cause or power, 474
+  world, the development of, implies a spiritual world, 476
+
+Organisation, advance of, by natural selection, 120
+  degradation in, 121
+
+Origin of species, objections, 7
+  of accessory plumes, 291
+
+Orioles mimicking honey-suckers, 263
+
+Ornamental plumes and vitality, 293
+
+
+=P=
+
+Pachyrhynchi subjects of mimicry, 261
+
+Pampas, effects of drought in, 23
+
+Papaver bracteatum, variability of, 79
+
+Papilio, use of forked tentacle of larva of, 210
+  protected groups of, 235
+  mimicry of, 247
+
+Paraguay, absence of wild cattle and horses, 19
+
+Parnassia palustris, sham nectaries of, 317
+
+Parrot, change of habits of New Zealand, 75
+
+Parus, species of, illustrate divergence, of character, 107
+
+Passenger-pigeon, account of its breeding-places and numbers, 31
+
+Pelagic animals, colours of, 193
+
+Phasmidae, resemblance of, to sticks and leaves, 203
+
+Physiological selection, 180
+
+Pickard-Cambridge, Rev. O., on sexual selection, 296 (note)
+
+Pieridae, sexual diversity among, 271
+
+Pigeons, varieties of, 89
+  domestic, derived from wild rock-pigeons, 90
+  curious correlations in, 140
+  white eggs of, protective, 213
+
+Pigs, great increase of, in South America and New Zealand, 28
+
+Pikermi, extinct animals of, 377
+
+Pipits as illustrating divergence, 108
+
+Planorbidae, variations of, 44
+
+Plants, the enemies of, 16
+  variability of, 76
+  constitutional variation of, 94
+  colour relations of, 302
+  true mimicry rare in, 303
+  exotic rarely naturalised in Europe, 356
+  dispersal of, 361
+  northern, in southern hemisphere, 368
+  identical on summits of remote mountains, 369
+  progressive development of, 397
+  geological development of (diagram), 402
+
+Plovers, recognition marks of (figure), 221
+
+Plumes, origin of accessory, 291
+  muscular relation of ornamental, 292
+
+Poisonous fruits, 307
+
+Porto Santo, rabbits of, 326
+
+Poulton, Mr. E.B., on variable colours of larvae and pupae, 196, 198
+  on concealments of insects by resemblance to environment, 202
+  on protective form of Notodonta ziczac, 210
+  on inedibility of conspicuous larvae, 237
+
+Pouters, 90
+
+Primulaceae, variations of structure in, 79
+
+Problem, the, before Darwin, 6
+
+Problems in variation and heredity, 410
+
+Progression in plants and animals, 395
+
+Protection by terrifying enemies, 209
+
+Protective colouring, variable, 195
+  of white-headed fruit-pigeon, 200
+  of African sun-birds, 200
+  of Kerivoula picta, 201
+  of sloths, 201
+  of larva of Sphinx ligustri, 202
+  of stick and leaf insects, 203
+  of caterpillars, 203, 205
+  of butterflies, 206
+
+Ptilopus cinctus, protective colour of, 200
+
+Pugnacity of birds with accessory plumes, 294
+
+
+=R=
+
+Rabbit, use of white tail of, 218
+
+Rapid increase of plants, 28
+
+Raspail, M., on variability in a grass, 80
+
+Rat, black, spread of, 34
+
+Rattlesnake, use of rattle of, 262
+
+Raven, why black in arctic regions, 191
+
+Reciprocal crosses, 155
+
+Recognition marks of herbivora, 218
+  of birds, 222
+  of tropical forest birds, 224
+  of insects, 226
+
+Reproductive functions, susceptibility of, 153
+
+Reptiles, geological distribution of, 406
+
+Rhinoceroses, evidence of evolution afforded by fossil, 383
+
+Rocks, all stratified formed in shallow water, 344
+
+Rocky Mountains, scarcity of monocotyledons in, 401
+
+Rodents, prevent woody vegetation in the pampas, 23
+
+Romanes, Professor G.J., on useless characters, 131, 139
+  on meaningless peculiarities of structure, 140
+  on supposed absence of simultaneous variations, 142
+  on physiological selection, 180
+
+Rook and crow, 2
+
+Roses, Mr. Baker on varieties of, 77
+
+Rubus, Bentham and Babington on species and varieties of, 77
+
+Rudiments and variations in man, 446
+
+Runts, 91
+
+Rutaceae, variation of structure in, 79
+
+
+=S=
+
+St. Helena, destruction of forests by goats, 17
+
+St. Hilaire, M. Aug., variability of Gomphia oleaefolia, 79
+
+Saxicola, divergence of character in species of, 108
+  recognition marks of, 222
+
+Scientific opinion before Darwin, 4
+
+Scolopax, figures of tails of, 225
+
+Scudder, Mr. S.H., on inedibility of Danais archippus, 238
+  on fossil insects, 403
+
+Seebohm, Mr., on swamping effects of intercrossing, 143
+
+Seeds, how dispersed, 306
+  how protected, 307
+  floating great distances, 361
+  dispersal of, by wind, 362
+  weight and dimensions of, 364
+  importance of wind-carriage of, 372
+  remarkable case of wind-carriage of, 373
+
+Seiurus carolinensis, diagram of variation, 67
+  sp., habits of, 117
+
+Selection, artificial, 84
+  by man, circumstances favourable to, 96
+  unconscious, 96
+
+Selective association, isolation by, 171
+
+Self-fertilisation of flowers, 321
+
+Semper, Professor, on casting hairs of reptiles and Crustacea, 137 (note)
+  on direct influence of environment, 426
+
+Sesiidae, mimicry by, 240
+
+Sex colour and nests of birds, 277
+
+Sex, colours characteristic of, 269
+
+Sexual colours of insects, probable causes of, 273
+  of birds, 275
+  characters due to natural selection, 283
+  diversity the cause of variation, 439
+
+Sexual selection and colour, 274
+  by struggles of males, 282
+  neutralised by natural selection, 294-296
+  restricted to male struggles, 296
+
+Shetland Islands, variety of ghost-moth in, 270
+
+Shrews and field-mice, internal variations of, 69
+
+Shrikes, recognition marks of, 222
+
+Similarity of forms of life not due to similarity of conditions, 339
+
+Singing of male birds, use of, 284
+
+Skull of wolf, diagram of variations of, 70
+  of Ursus labiatus, diagram of variations of, 72
+  of Sus cristatus, diagram of variations of, 73
+
+Skunk an illustration of warning colour, 233
+
+Slack, Baron von, on protective markings of sloths, 201
+
+Sloth, protective colour and marking of, 201
+
+Snakes, mimicry of poisonous, 261
+
+Snipe, tails of two species (figs.), 225
+
+Sounds and odours peculiar to male,
+  how useful, 284
+  produced by peculiar feathers, 284
+
+South America, fossil and recent mammals of, 393
+
+Species, definition of, 1, 2
+  origin of, 2, 6
+  Lyell on, 4
+  Agassiz on, 5
+  transmutation of, 6
+  Geoffroy St. Hilaire on, 6
+  Dean Herbert on, 6
+  Professor Grant on, 6
+  Von Buch on, 6
+  allied, found in distinct areas, 36
+
+Species, which vary little, 80
+  closely allied inhabit distinct areas, 111
+  vigour and fertility of, how kept up, 327
+
+Spencer, Mr. Herbert, on factors of organic evolution, 411
+  on effects of disuse, 413
+  on difficulty as to co-adaptation of parts, 417
+  on direct action of environment, 418
+
+Sphingidae, protective attitudes of larvae, 210
+
+Sphinx ligustri, general resemblance of larva to food plant, 202
+
+Spider, alluring coloration of, 211
+
+Spines, on origin of, 431
+  rarity of, in oceanic islands, 432
+
+Spiny plants abundant in South Africa and Chile, 433
+
+Spots a primitive ornamentation of animals, 289
+
+Sprengel on flowers and insects, 309
+
+Staphylinidae, protective habit of, 210
+
+Sterility of mongrels, 165
+  correlated with colour, etc., 168
+  and natural selection, 173
+  of hybrids produced by natural selection, 179
+
+Struggle for existence, 14
+  among plants, 15
+  for life, illustrations of, 18
+  for existence on the pampas, 22
+  for life between closely allied forms most severe, 33
+  for existence, ethics of, 36
+  how it acts among flowers, 328
+
+Summary of facts of colouring for protection and recognition, 227
+
+Survival of the fittest, 11, 122, 123
+
+Swainson, definition of species, 2
+
+Swamping effects of intercrossing, 142
+
+Sweden, destruction of grass by larvae of moths in, 17
+
+Swinhoe, Mr., on protective colouring of a bat, 201
+
+Symmetry, bilateral in colours of animals needful for recognition, 217
+
+
+=T=
+
+Tails used as respirators, 136
+
+Tapirs, distribution of, 352
+
+Tegetmeier, Mr., on feeding habits of pigeons and fowls, 75
+  on sparrows and crocuses, 75
+  on curious correlations in pigeons, 140
+
+Tegumentary appendages and colour, 291
+
+Thousand-fathom line divides oceanic from continental islands, 347
+  the teachings of, 348
+  map showing, 349
+
+Thwaites, Mr., on spread of Lantana mixta in Ceylon, 30
+
+Tiger, use of stripes of, 199
+
+Titmice as illustrating divergence, 107
+
+Transformation of species of crustacea, 427
+
+Transmutationists, the early, 3
+
+Travers, Mr. W.L., on effects of introduced plants in New Zealand, 29
+
+Trees, great variety of, in many forests, 36
+
+Trimen, Mr., on butterfly deceived by its mimic, 245
+  on mimicry, 247
+
+Tropical animals, why brilliantly coloured, 299
+
+Tropics, no proof of lower temperature of, 369
+
+Tropidorhynchi mimicked by orioles, 263
+
+Trumpeter, 93
+
+Tumblers, 91
+
+Turbits and owls, 91
+
+Tylor, Mr. A., on _Coloration in Animals and Plants_, 285
+
+
+=U=
+
+Ungulates, origin of feet of, 423
+
+Use and disuse, effects of, overpowered by natural selection, 435
+
+Useless characters, 131
+  not specific, 132
+
+Useless specific characters, no proof of existence of, 141
+
+Utriculariae inhabiting bromelias, 118
+
+
+=V=
+
+Vanessa callirhoe, small variety in Porto Santo, 106
+
+Variability of the lower animals, 42
+  of the Foraminifera, 43
+  of sea-anemones, 43
+  of land mollusca, 43
+  of insects, 44
+  of lizards, 46
+  of birds, 49
+  of primary wing-feathers, 51
+  of wings and tail, 53
+  of Dolichonyx oryzivorus, 55
+  of Agelaeus phoeniceus, 56
+  of Cardinalis virginianus, 58
+  of tarsus and toes, 60
+  of birds in Leyden Museum, 61
+  of Sciurus carolinensis, 67
+  of skulls of wolf, 70
+  of skulls of a bear, 72
+  of skulls of Sus cristatus, 73
+  of plants, 76
+  of oaks, 77
+
+Variation, Lyell on, 4
+  in internal organs, 66
+  the facts of, 83
+  proofs of generality of, 85
+  of vegetables and fruits, 86
+  of apples and melons, 87
+  under domestication accords with that under nature, 100
+  coincident not necessary, 127
+  and heredity, problems of, 410
+  Professor Geddes's theory of, 428
+  the cause of, 439
+
+Variations of flowers, 88
+  of domestic animals, 88
+  of domestic pigeons, 89
+  conditions favourable to production of, 98
+  beneficial, 143
+
+Varieties, importance of, 41
+  of same species adapted to self or to insect-fertilisation, 330
+
+Vegetables, variation of, 86
+
+Vegetation and reproduction, antagonism of, 428
+
+Vertebrata, mimicry among, 261
+  geological succession of, 405
+
+_Vestiges of Creation_, 3
+
+Viola odorata, 2
+  canina, 2
+
+Violets, as illustrating species, 2
+
+Von Buch on species, 6
+
+
+=W=
+
+Wallace, Dr. Alexander, on absence of choice by female moths, 275
+
+Ward, Mr. Lester F., on progressive development of plants, 398
+
+Warning coloration, 232
+
+Warning colours of marine animals, 265
+
+Wasps and bees, mimicry of, 258
+  poisonous with warning colours, 287
+
+Water-cress, chokes rivers in New Zealand, 24
+  driven out by willows, 24
+
+Water-ouzels, probable origin of, 116
+
+Weale, Mr. Mansel, on protective colours of butterflies, 206
+
+Weeds of United States, 15
+
+Weir, Mr. Jenner, on deceptive resemblance of a caterpillar to
+a twig, 204
+  on inedibility of conspicuous caterpillars, 236
+  on birds disregarding inedible larvae, 254
+
+Weismann on progressive adaptation of colours of larva, 206
+  on non-heredity of acquired characters, 440
+  and Galton's theories of heredity almost identical, 443 (note)
+  on origin of the mathematical faculty, 472 (note)
+
+Weismann's theory of heredity, 437
+
+Westwood, Professor, on variation of insects, 44
+  deceived by a mimicking cricket, 259
+
+White coloration of insular birds and butterflies, 230
+
+Whymper, Mr., his sensations when falling on the Matterhorn, 38
+
+Willows, species and varieties of British, 77
+
+Wilson, Alexander, his account of the passenger-pigeon in North
+  America, 31
+
+Wind-carriage of seeds explains many facts of plant distribution,
+  371
+
+Wind-dispersal of seeds, objections to, 365
+
+Wind-fertilised degraded from insect-fertilised flowers, 324
+
+Wings of stone-curlews (figure), 223 why small but useless are
+  retained, 416
+
+Wit and humour, origin of faculties of, 472
+
+Wollaston, Mr. T.W., on variation of beetles, 44 on small
+butterfly in Porto Santo, 106
+
+Wolves, varieties of in Catskill Mountains, 105
+
+Wood, Mr. J., on muscular variations, 447
+
+Mr. T.W., on variable colouring of pupae of cabbage butterflies,
+  197
+
+Woodward, Dr. S.P., on variation of mollusca, 43
+
+
+=Y=
+
+Youatt, on breeds of sheep, 97
+
+Young animals often spotted, 289
+
+
+=Z=
+
+Zebra, markings for recognition and protection, 220 (note)
+
+
+
+THE END
+
+
+
+
+
+End of Project Gutenberg's Darwinism (1889), by Alfred Russel Wallace
+
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