summaryrefslogtreecommitdiff
path: root/old/14558.txt
diff options
context:
space:
mode:
Diffstat (limited to 'old/14558.txt')
-rw-r--r--old/14558.txt19888
1 files changed, 19888 insertions, 0 deletions
diff --git a/old/14558.txt b/old/14558.txt
new file mode 100644
index 0000000..08de2e5
--- /dev/null
+++ b/old/14558.txt
@@ -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
+
+*** END OF THIS PROJECT GUTENBERG EBOOK DARWINISM (1889) ***
+
+***** This file should be named 14558.txt or 14558.zip *****
+This and all associated files of various formats will be found in:
+ https://www.gutenberg.org/1/4/5/5/14558/
+
+Produced by StevenGibbs and the Online Distributed Proofreading Team
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions means that no
+one owns a United States copyright in these works, so the Foundation
+(and you!) can copy and distribute it in the United States without
+permission and without paying copyright royalties. Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark. Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission. If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy. You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research. They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks. Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1. General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A. By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement. If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B. "Project Gutenberg" is a registered trademark. It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement. There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement. See
+paragraph 1.C below. There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works. See paragraph 1.E below.
+
+1.C. The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works. Nearly all the individual works in the
+collection are in the public domain in the United States. If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed. Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work. You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D. The copyright laws of the place where you are located also govern
+what you can do with this work. Copyright laws in most countries are in
+a constant state of change. If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work. The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E. Unless you have removed all references to Project Gutenberg:
+
+1.E.1. The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+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
+
+1.E.2. If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges. If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3. If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder. Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5. Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6. You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form. However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form. Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7. Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8. You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+ the use of Project Gutenberg-tm works calculated using the method
+ you already use to calculate your applicable taxes. The fee is
+ owed to the owner of the Project Gutenberg-tm trademark, but he
+ has agreed to donate royalties under this paragraph to the
+ Project Gutenberg Literary Archive Foundation. Royalty payments
+ must be paid within 60 days following each date on which you
+ prepare (or are legally required to prepare) your periodic tax
+ returns. Royalty payments should be clearly marked as such and
+ sent to the Project Gutenberg Literary Archive Foundation at the
+ address specified in Section 4, "Information about donations to
+ the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+ you in writing (or by e-mail) within 30 days of receipt that s/he
+ does not agree to the terms of the full Project Gutenberg-tm
+ License. You must require such a user to return or
+ destroy all copies of the works possessed in a physical medium
+ and discontinue all use of and all access to other copies of
+ Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+ money paid for a work or a replacement copy, if a defect in the
+ electronic work is discovered and reported to you within 90 days
+ of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+ distribution of Project Gutenberg-tm works.
+
+1.E.9. If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark. Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1. Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection. Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3. YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from. If you
+received the work on a physical medium, you must return the medium with
+your written explanation. The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund. If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund. If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4. Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5. Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law. The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section 2. Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers. It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need, is critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come. In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3. Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service. The Foundation's EIN or federal tax identification
+number is 64-6221541. Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising. Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations. Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org. Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+ Dr. Gregory B. Newby
+ Chief Executive and Director
+ gbnewby@pglaf.org
+
+
+Section 4. Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment. Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States. Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements. We do not solicit donations in locations
+where we have not received written confirmation of compliance. To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States. U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses. Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations. To donate, please visit: https://pglaf.org/donate
+
+
+Section 5. General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone. For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included. Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+
+Most people start at our Web site which has the main PG search facility:
+
+ https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
generated by cgit v1.2.3 (git 2.25.1) at 2026-05-22 23:28:07 +0000