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+The Project Gutenberg eBook of On the Origin of Species by Means of Natural Selection, by Charles Darwin
+
+This eBook is for the use of anyone anywhere in the United States and
+most other parts of the world 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. If you are not located in the United States, you
+will have to check the laws of the country where you are located before
+using this eBook.
+
+Title: On the Origin of Species by Means of Natural Selection or the
+Preservation of Favoured Races in the Struggle for Life. (2nd edition)
+
+Author: Charles Darwin
+
+Release Date: September 25, 2007 [eBook #22764]
+
+Language: English
+
+Character set encoding: UTF-8
+
+Produced by: Steven Gibbs, Keith Edkins and the Online Distributed Proofreading Team
+
+*** START OF THE PROJECT GUTENBERG EBOOK ORIGIN OF SPECIES ***
+
+
+
+
+Transcriber's note: A few typographical errors have been corrected: they
+are listed at the end of the text.
+
+       *       *       *       *       *
+
+
+ON THE
+
+ORIGIN OF SPECIES.
+
+       *       *       *       *       *
+
+"But with regard to the material world, we can at least go so far as
+this--we can perceive that events are brought about not by insulated
+interpositions of Divine power, exerted in each particular case, but by the
+establishment of general laws."
+
+WHEWELL: _Bridgewater Treatise_.
+
+"The only distinct meaning of the word 'natural' is _stated_, _fixed_, or
+_settled_; since what is natural as much requires and presupposes an
+intelligent agent to render it so, _i.e._ to effect it continually or at
+stated times, as what is supernatural or miraculous does to effect it for
+once."
+
+BUTLER: _Analogy of Revealed Religion_.
+
+"To conclude, therefore, let no man out of a weak conceit of sobriety, or
+an ill-applied moderation, think or maintain, that a man can search too far
+or be too well studied in the book of God's word, or in the book of God's
+works; divinity or philosophy; but rather let men endeavour an endless
+progress or proficience in both."
+
+BACON: _Advancement of Learning_.
+
+       *       *       *       *       *
+
+  _Down, Bromley, Kent,_
+      _October 1st, 1859._ (_1st Thousand_).
+
+       *       *       *       *       *
+
+
+ON
+
+THE ORIGIN OF SPECIES
+
+BY MEANS OF NATURAL SELECTION,
+
+OR THE
+
+PRESERVATION OF FAVOURED RACES IN THE STRUGGLE
+FOR LIFE.
+
+BY CHARLES DARWIN, M.A.,
+
+FELLOW OF THE ROYAL, GEOLOGICAL, LINNEAN, ETC., SOCIETIES;
+
+AUTHOR OF 'JOURNAL OF RESEARCHES DURING H. M. S. BEAGLE'S VOYAGE
+ROUND THE WORLD.'
+
+_FIFTH THOUSAND._
+
+LONDON:
+JOHN MURRAY, ALBEMARLE STREET.
+1860.
+
+_The right of Translation is reserved._
+
+       *       *       *       *       *
+
+LONDON: PRINTED BY W. CLOWES AND SONS, STAMFORD STREET,
+AND CHARING CROSS.
+
+       *       *       *       *       *
+
+
+{v}
+
+CONTENTS.
+
+       *       *       *       *       *
+
+INTRODUCTION
+
+Page 1
+
+CHAPTER I.
+
+VARIATION UNDER DOMESTICATION.
+
+Causes of Variability--Effects of Habit--Correlation of
+Growth--Inheritance--Character of Domestic Varieties--Difficulty of
+distinguishing between Varieties and Species--Origin of Domestic Varieties
+from one or more Species--Domestic Pigeons, their Differences and
+Origin--Principle of Selection anciently followed, its Effects--Methodical
+and Unconscious Selection--Unknown Origin of our Domestic
+Productions--Circumstances favourable to Man's power of Selection
+
+7-43
+
+CHAPTER II.
+
+VARIATION UNDER NATURE.
+
+Variability--Individual differences--Doubtful species--Wide ranging, much
+diffused, and common species vary most--Species of the larger genera in any
+country vary more than the species of the smaller genera--Many of the
+species of the larger genera resemble varieties in being very closely, but
+unequally, related to each other, and in having restricted ranges
+
+44-59
+
+{vi}
+
+CHAPTER III.
+
+STRUGGLE FOR EXISTENCE.
+
+Its bearing on natural selection--The term used in a wide
+sense--Geometrical powers of increase--Rapid increase of naturalised
+animals and plants--Nature of the checks to increase--Competition
+universal--Effects of climate--Protection from the number of
+individuals--Complex relations of all animals and plants throughout
+nature--Struggle for life most severe between individuals and varieties of
+the same species; often severe between species of the same genus--The
+relation of organism to organism the most important of all relations
+
+60-79
+
+CHAPTER IV.
+
+NATURAL SELECTION.
+
+Natural Selection--its power compared with man's selection--its power on
+characters of trifling importance--its power at all ages and on both
+sexes--Sexual Selection--On the generality of intercrosses between
+individuals of the same species--Circumstances favourable and unfavourable
+to Natural Selection, namely, intercrossing, isolation, number of
+individuals--Slow action--Extinction caused by Natural
+Selection--Divergence of Character, related to the diversity of inhabitants
+of any small area, and to naturalisation--Action of Natural Selection,
+through Divergence of Character and Extinction, on the descendants from a
+common parent--Explains the Grouping of all organic beings
+
+80-130
+
+CHAPTER V.
+
+LAWS OF VARIATION.
+
+Effects of external conditions--Use and disuse, combined with natural
+selection; organs of flight and of vision--Acclimatisation--Correlation of
+growth--Compensation and economy of growth--False correlations--Multiple,
+rudimentary, and lowly organised structures variable--Parts developed in an
+unusual manner are highly variable: specific characters more variable than
+generic: secondary sexual characters variable--Species of the same genus
+vary in an analogous manner--Reversions to long-lost characters--Summary
+
+131-170
+
+{vii}
+
+CHAPTER VI.
+
+DIFFICULTIES ON THEORY.
+
+Difficulties on the theory of descent with
+modification--Transitions--Absence or rarity of transitional
+varieties--Transitions in habits of life--Diversified habits in the same
+species--Species with habits widely different from those of their
+allies--Organs of extreme perfection--Means of transition--Cases of
+difficulty--Natura non facit saltum--Organs of small importance--Organs not
+in all cases absolutely perfect--The law of Unity of Type and of the
+Conditions of Existence embraced by the theory of Natural Selection
+
+171-206
+
+CHAPTER VII.
+
+INSTINCT.
+
+Instincts comparable with habits, but different in their origin--Instincts
+graduated--Aphides and ants--Instincts variable--Domestic instincts, their
+origin--Natural instincts of the cuckoo, ostrich, and parasitic
+bees--Slave-making ants--Hive-bee, its cell-making instinct--Difficulties
+on the theory of the Natural Selection of instincts--Neuter or sterile
+insects--Summary
+
+207-244
+
+CHAPTER VIII.
+
+HYBRIDISM.
+
+Distinction between the sterility of first crosses and of
+hybrids--Sterility various in degree, not universal, affected by close
+interbreeding, removed by domestication--Laws governing the sterility of
+hybrids--Sterility not a special endowment, but incidental on other
+differences--Causes of the sterility of first crosses and of
+hybrids--Parallelism between the effects of changed conditions of life and
+crossing--Fertility of varieties when crossed and of their mongrel
+offspring not universal--Hybrids and mongrels compared independently of
+their fertility--Summary
+
+245-278
+
+{viii}
+
+CHAPTER IX.
+
+ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
+
+On the absence of intermediate varieties at the present day--On the nature
+of extinct intermediate varieties; on their number--On the vast lapse of
+time, as inferred from the rate of deposition and of denudation--On the
+poorness of our palæontological collections--On the intermittence of
+geological formations--On the absence of intermediate varieties in any one
+formation--On the sudden appearance of groups of species--On their sudden
+appearance in the lowest known fossiliferous strata
+
+279-311
+
+CHAPTER X.
+
+ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
+
+On the slow and successive appearance of new species--On their different
+rates of change--Species once lost do not reappear--Groups of species
+follow the same general rules in their appearance and disappearance as do
+single species--On Extinction--On simultaneous changes in the forms of life
+throughout the world--On the affinities of extinct species to each other
+and to living species--On the state of development of ancient forms--On the
+succession of the same types within the same areas--Summary of preceding
+and present chapters
+
+312-345
+
+CHAPTER XI.
+
+GEOGRAPHICAL DISTRIBUTION.
+
+Present distribution cannot be accounted for by differences in physical
+conditions--Importance of barriers--Affinity of the productions of the same
+continent--Centres of creation--Means of dispersal, by changes of climate
+and of the level of the land, and by occasional means--Dispersal during the
+Glacial period co-extensive with the world
+
+346-382
+
+CHAPTER XII.
+
+GEOGRAPHICAL DISTRIBUTION--_continued_.
+
+Distribution of fresh-water productions--On the inhabitants of oceanic
+islands--Absence of Batrachians and of terrestrial Mammals--On the relation
+of the inhabitants of islands to those of the nearest mainland--On
+colonisation from the nearest source with subsequent modification--Summary
+of the last and present chapters
+
+383-410
+
+CHAPTER XIII.
+
+MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY
+ORGANS.
+
+CLASSIFICATION, groups subordinate to groups--Natural system--Rules and
+difficulties in classification, explained on the theory of descent with
+modification--Classification of varieties--Descent always used in
+classification--Analogical or adaptive characters--Affinities, general,
+complex and radiating--Extinction separates and defines groups--MORPHOLOGY,
+between members of the same class, between parts of the same
+individual--EMBRYOLOGY, laws of, explained by variations not supervening at
+an early age, and being inherited at a corresponding age--RUDIMENTARY
+ORGANS; their origin explained--Summary
+
+411-458
+
+CHAPTER XIV.
+
+RECAPITULATION AND CONCLUSION.
+
+Recapitulation of the difficulties on the theory of Natural
+Selection--Recapitulation of the general and special circumstances in its
+favour--Causes of the general belief in the immutability of species--How
+far the theory of natural selection may be extended--Effects of its
+adoption on the study of Natural history--Concluding remarks
+
+459-490
+
+       *       *       *       *       *
+
+
+{1}
+
+ON THE ORIGIN OF SPECIES.
+
+       *       *       *       *       *
+
+INTRODUCTION.
+
+When on board H.M.S. 'Beagle,' as naturalist, I was much struck with
+certain facts in the distribution of the inhabitants of South America, and
+in the geological relations of the present to the past inhabitants of that
+continent. These facts seemed to me to throw some light on the origin of
+species--that mystery of mysteries, as it has been called by one of our
+greatest philosophers. On my return home, it occurred to me, in 1837, that
+something might perhaps be made out on this question by patiently
+accumulating and reflecting on all sorts of facts which could possibly have
+any bearing on it. After five years' work I allowed myself to speculate on
+the subject, and drew up some short notes; these I enlarged in 1844 into a
+sketch of the conclusions, which then seemed to me probable: from that
+period to the present day I have steadily pursued the same object. I hope
+that I may be excused for entering on these personal details, as I give
+them to show that I have not been hasty in coming to a decision.
+
+My work is now nearly finished; but as it will take me two or three more
+years to complete it, and as my health is far from strong, I have been
+urged to publish this Abstract. I have more especially been induced to do
+this, as Mr. Wallace, who is now studying the {2} natural history of the
+Malay archipelago, has arrived at almost exactly the same general
+conclusions that I have on the origin of species. Last year he sent me a
+memoir on this subject, with a request that I would forward it to Sir
+Charles Lyell, who sent it to the Linnean Society, and it is published in
+the third volume of the Journal of that Society. Sir C. Lyell and Dr.
+Hooker, who both knew of my work--the latter having read my sketch of
+1844--honoured me by thinking it advisable to publish, with Mr. Wallace's
+excellent memoir, some brief extracts from my manuscripts.
+
+This Abstract, which I now publish, must necessarily be imperfect. I cannot
+here give references and authorities for my several statements; and I must
+trust to the reader reposing some confidence in my accuracy. No doubt
+errors will have crept in, though I hope I have always been cautious in
+trusting to good authorities alone. I can here give only the general
+conclusions at which I have arrived, with a few facts in illustration, but
+which, I hope, in most cases will suffice. No one can feel more sensible
+than I do of the necessity of hereafter publishing in detail all the facts,
+with references, on which my conclusions have been grounded; and I hope in
+a future work to do this. For I am well aware that scarcely a single point
+is discussed in this volume on which facts cannot be adduced, often
+apparently leading to conclusions directly opposite to those at which I
+have arrived. A fair result can be obtained only by fully stating and
+balancing the facts and arguments on both sides of each question; and this
+cannot possibly be here done.
+
+I much regret that want of space prevents my having the satisfaction of
+acknowledging the generous assistance which I have received from very many
+naturalists, some of them personally unknown to me. I cannot, however, {3}
+let this opportunity pass without expressing my deep obligations to Dr.
+Hooker, who for the last fifteen years has aided me in every possible way
+by his large stores of knowledge and his excellent judgment.
+
+In considering the Origin of Species, it is quite conceivable that a
+naturalist, reflecting on the mutual affinities of organic beings, on their
+embryological relations, their geographical distribution, geological
+succession, and other such facts, might come to the conclusion that each
+species had not been independently created, but had descended, like
+varieties, from other species. Nevertheless, such a conclusion, even if
+well founded, would be unsatisfactory, until it could be shown how the
+innumerable species inhabiting this world have been modified, so as to
+acquire that perfection of structure and coadaptation which most justly
+excites our admiration. Naturalists continually refer to external
+conditions, such as climate, food, &c., as the only possible cause of
+variation. In one very limited sense, as we shall hereafter see, this may
+be true; but it is preposterous to attribute to mere external conditions,
+the structure, for instance, of the woodpecker, with its feet, tail, beak,
+and tongue, so admirably adapted to catch insects under the bark of trees.
+In the case of the misseltoe, which draws its nourishment from certain
+trees, which has seeds that must be transported by certain birds, and which
+has flowers with separate sexes absolutely requiring the agency of certain
+insects to bring pollen from one flower to the other, it is equally
+preposterous to account for the structure of this parasite, with its
+relations to several distinct organic beings, by the effects of external
+conditions, or of habit, or of the volition of the plant itself.
+
+The author of the 'Vestiges of Creation' would, I presume, say that, after
+a certain unknown number of {4} generations, some bird had given birth to a
+woodpecker, and some plant to the missletoe, and that these had been
+produced perfect as we now see them; but this assumption seems to me to be
+no explanation, for it leaves the case of the coadaptations of organic
+beings to each other and to their physical conditions of life, untouched
+and unexplained.
+
+It is, therefore, of the highest importance to gain a clear insight into
+the means of modification and coadaptation. At the commencement of my
+observations it seemed to me probable that a careful study of domesticated
+animals and of cultivated plants would offer the best chance of making out
+this obscure problem. Nor have I been disappointed; in this and in all
+other perplexing cases I have invariably found that our knowledge,
+imperfect though it be, of variation under domestication, afforded the best
+and safest clue. I may venture to express my conviction of the high value
+of such studies, although they have been very commonly neglected by
+naturalists.
+
+From these considerations, I shall devote the first chapter of this
+Abstract to Variation under Domestication. We shall thus see that a large
+amount of hereditary modification is at least possible; and, what is
+equally or more important, we shall see how great is the power of man in
+accumulating by his Selection successive slight variations. I will then
+pass on to the variability of species in a state of nature; but I shall,
+unfortunately, be compelled to treat this subject far too briefly, as it
+can be treated properly only by giving long catalogues of facts. We shall,
+however, be enabled to discuss what circumstances are most favourable to
+variation. In the next chapter the Struggle for Existence amongst all
+organic beings throughout the world, which inevitably follows from the high
+geometrical ratio of their {5} increase, will be treated of. This is the
+doctrine of Malthus, applied to the whole animal and vegetable kingdoms. As
+many more individuals of each species are born than can possibly survive;
+and as, consequently, there is a frequently recurring struggle for
+existence, it follows that any being, if it vary however slightly in any
+manner profitable to itself, under the complex and sometimes varying
+conditions of life, will have a better chance of surviving, and thus be
+_naturally selected_. From the strong principle of inheritance, any
+selected variety will tend to propagate its new and modified form.
+
+This fundamental subject of Natural Selection will be treated at some
+length in the fourth chapter; and we shall then see how Natural Selection
+almost inevitably causes much Extinction of the less improved forms of
+life, and leads to what I have called Divergence of Character. In the next
+chapter I shall discuss the complex and little known laws of variation and
+of correlation of growth. In the four succeeding chapters, the most
+apparent and gravest difficulties on the theory will be given: namely,
+first, the difficulties of transitions, or in understanding how a simple
+being or a simple organ can be changed and perfected into a highly
+developed being or elaborately constructed organ; secondly, the subject of
+Instinct, or the mental powers of animals; thirdly, Hybridism, or the
+infertility of species and the fertility of varieties when intercrossed;
+and fourthly, the imperfection of the Geological Record. In the next
+chapter I shall consider the geological succession of organic beings
+throughout time; in the eleventh and twelfth, their geographical
+distribution throughout space; in the thirteenth, their classification or
+mutual affinities, both when mature and in an embryonic condition. In the
+last chapter I shall give a {6} brief recapitulation of the whole work, and
+a few concluding remarks.
+
+No one ought to feel surprise at much remaining as yet unexplained in
+regard to the origin of species and varieties, if he makes due allowance
+for our profound ignorance in regard to the mutual relations of all the
+beings which live around us. Who can explain why one species ranges widely
+and is very numerous, and why another allied species has a narrow range and
+is rare? Yet these relations are of the highest importance, for they
+determine the present welfare, and, as I believe, the future success and
+modification of every inhabitant of this world. Still less do we know of
+the mutual relations of the innumerable inhabitants of the world during the
+many past geological epochs in its history. Although much remains obscure,
+and will long remain obscure, I can entertain no doubt, after the most
+deliberate study and dispassionate judgment of which I am capable, that the
+view which most naturalists entertain, 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
+main but not exclusive means of modification.
+
+       *       *       *       *       *
+
+
+{7}
+
+CHAPTER I.
+
+VARIATION UNDER DOMESTICATION.
+
+    Causes of Variability--Effects of Habit--Correlation of
+    Growth--Inheritance--Character of Domestic Varieties--Difficulty of
+    distinguishing between Varieties and Species--Origin of Domestic
+    Varieties from one or more Species--Domestic Pigeons, their Differences
+    and Origin--Principle of Selection anciently followed, its
+    Effects--Methodical and Unconscious Selection--Unknown Origin of our
+    Domestic Productions--Circumstances favourable to Man's power of
+    Selection.
+
+When we look to the individuals of the same variety or sub-variety of our
+older cultivated plants and animals, one of the first points which strikes
+us, is, that they generally differ more from each other than do the
+individuals of any one species or variety in a state of nature. When we
+reflect on the vast diversity of the plants and animals which have been
+cultivated, and which have varied during all ages under the most different
+climates and treatment, I think we are driven to conclude that this great
+variability is simply due to our domestic productions having been raised
+under conditions of life not so uniform as, and somewhat different from,
+those to which the parent-species have been exposed under nature. There is
+also, I think, some probability in the view propounded by Andrew Knight,
+that this variability may be partly connected with excess of food. It seems
+pretty clear that organic beings must be exposed during several generations
+to the new conditions of life to cause any appreciable amount of variation;
+and that when the organisation has once begun to vary, it generally
+continues to vary for many generations. {8} No case is on record of a
+variable being ceasing to be variable under cultivation. Our oldest
+cultivated plants, such as wheat, still often yield new varieties: our
+oldest domesticated animals are still capable of rapid improvement or
+modification.
+
+It has been disputed at what period of life the causes of variability,
+whatever they may be, generally act; whether during the early or late
+period of development of the embryo, or at the instant of conception.
+Geoffroy St. Hilaire's experiments show that unnatural treatment of the
+embryo causes monstrosities; and monstrosities cannot be separated by any
+clear line of distinction from mere variations. But I am strongly inclined
+to suspect that the most frequent cause of variability may be attributed to
+the male and female reproductive elements having been affected prior to the
+act of conception. Several reasons make me believe in this; but the chief
+one is the remarkable effect which confinement or cultivation has on the
+function of the reproductive system; this system appearing to be far more
+susceptible than any other part of the organisation, to the action of any
+change in the conditions of life. Nothing is more easy than to tame an
+animal, and few things more difficult than to get it to breed freely under
+confinement, even in the many cases when the male and female unite. How
+many animals there are which will not breed, though living long under not
+very close confinement in their native country! This is generally
+attributed to vitiated instincts; but how many cultivated plants display
+the utmost vigour, and yet rarely or never seed! In some few such cases it
+has been discovered that very trifling changes, such as a little more or
+less water at some particular period of growth, will determine whether or
+not the plant sets a seed. I cannot here enter on the copious details which
+I have collected on {9} this curious subject; but to show how singular the
+laws are which determine the reproduction of animals under confinement, I
+may just mention that carnivorous animals, even from the tropics, breed in
+this country pretty freely under confinement, with the exception of the
+plantigrades or bear family; whereas carnivorous birds, with the rarest
+exceptions, hardly ever lay fertile eggs. Many exotic plants have pollen
+utterly worthless, in the same exact condition as in the most sterile
+hybrids. When, on the one hand, we see domesticated animals and plants,
+though often weak and sickly, yet breeding quite freely under confinement;
+and when, on the other hand, we see individuals, though taken young from a
+state of nature, perfectly tamed, long-lived, and healthy (of which I could
+give numerous instances), yet having their reproductive system so seriously
+affected by unperceived causes as to fail in acting, we need not be
+surprised at this system, when it does act under confinement, acting not
+quite regularly, and producing offspring not perfectly like their parents.
+
+Sterility has been said to be the bane of horticulture; but on this view we
+owe variability to the same cause which produces sterility; and variability
+is the source of all the choicest productions of the garden. I may add,
+that as some organisms will breed freely under the most unnatural
+conditions (for instance, the rabbit and ferret kept in hutches), showing
+that their reproductive system has not been thus affected; so will some
+animals and plants withstand domestication or cultivation, and vary very
+slightly--perhaps hardly more than in a state of nature.
+
+A long list could easily be given of "sporting plants;" by this term
+gardeners mean a single bud or offset, which suddenly assumes a new and
+sometimes very different character from that of the rest of the plant. {10}
+Such buds can be propagated by grafting, &c., and sometimes by seed. These
+"sports" are extremely rare under nature, but far from rare under
+cultivation; and in this case we see that the treatment of the parent has
+affected a bud or offset, and not the ovules or pollen. But it is the
+opinion of most physiologists that there is no essential difference between
+a bud and an ovule in their earliest stages of formation; so that, in fact,
+"sports" support my view, that variability may be largely attributed to the
+ovules or pollen, or to both, having been affected by the treatment of the
+parent prior to the act of conception. These cases anyhow show that
+variation is not necessarily connected, as some authors have supposed, with
+the act of generation.
+
+Seedlings from the same fruit, and the young of the same litter, sometimes
+differ considerably from each other, though both the young and the parents,
+as Müller has remarked, have apparently been exposed to exactly the same
+conditions of life; and this shows how unimportant the direct effects of
+the conditions of life are in comparison with the laws of reproduction, of
+growth, and of inheritance; for had the action of the conditions been
+direct, if any of the young had varied, all would probably have varied in
+the same manner. To judge how much, in the case of any variation, we should
+attribute to the direct action of heat, moisture, light, food, &c., is most
+difficult: my impression is, that with animals such agencies have produced
+very little direct effect, though apparently more in the case of plants.
+Under this point of view, Mr. Buckman's recent experiments on plants are
+extremely valuable. When all or nearly all the individuals exposed to
+certain conditions are affected in the same way, the change at first
+appears to be directly due to such conditions; but in some cases it can be
+shown that quite opposite conditions produce {11} similar changes of
+structure. Nevertheless some slight amount of change may, I think, be
+attributed to the direct action of the conditions of life--as, in some
+cases, increased size from amount of food, colour from particular kinds of
+food or from light, and perhaps the thickness of fur from climate.
+
+Habit also has a decided influence, as in the period of flowering with
+plants when transported from one climate to another. In animals it has a
+more marked effect; for instance, I find in the domestic duck that the
+bones of the wing weigh less and the bones of the leg more, in proportion
+to the whole skeleton, than do the same bones in the wild-duck; and I
+presume that this change may be safely attributed to the domestic duck
+flying much less, and walking more, than its wild parent. The great and
+inherited development of the udders in cows and goats in countries where
+they are habitually milked, in comparison with the state of these organs in
+other countries, is another instance of the effect of use. Not a single
+domestic animal can be named which has not in some country drooping ears;
+and the view suggested by some authors, that the drooping is due to the
+disuse of the muscles of the ear, from the animals not being much alarmed
+by danger, seems probable.
+
+There are many laws regulating variation, some few of which can be dimly
+seen, and will be hereafter briefly mentioned. I will here only allude to
+what may be called correlation of growth. Any change in the embryo or larva
+will almost certainly entail changes in the mature animal. In
+monstrosities, the correlations between quite distinct parts are very
+curious; and many instances are given in Isidore Geoffroy St. Hilaire's
+great work on this subject. Breeders believe that long limbs are almost
+always accompanied by an elongated head. Some instances of correlation are
+quite whimsical: thus {12} cats with blue eyes are invariably deaf; colour
+and constitutional peculiarities go together, of which many remarkable
+cases could be given amongst animals and plants. From the facts collected
+by Heusinger, it appears that white sheep and pigs are differently affected
+from coloured individuals by certain vegetable poisons. Hairless dogs have
+imperfect teeth: long-haired and coarse-haired animals are apt to have, as
+is asserted, long or many horns; pigeons with feathered feet have skin
+between their outer toes; pigeons with short beaks have small feet, and
+those with long beaks large feet. Hence, if man goes on selecting, and thus
+augmenting, any peculiarity, he will almost certainly unconsciously modify
+other parts of the structure, owing to the mysterious laws of the
+correlation of growth.
+
+The result of the various, quite unknown, or dimly seen laws of variation
+is infinitely complex and diversified. It is well worth while carefully to
+study the several treatises published on some of our old cultivated plants,
+as on the hyacinth, potato, even the dahlia, &c.; and it is really
+surprising to note the endless points in structure and constitution in
+which the varieties and sub-varieties differ slightly from each other. The
+whole organisation seems to have become plastic, and tends to depart in
+some small degree from that of the parental type.
+
+Any variation which is not inherited is unimportant for us. But the number
+and diversity of inheritable deviations of structure, both those of slight
+and those of considerable physiological importance, is endless. Dr. Prosper
+Lucas's treatise, in two large volumes, is the fullest and the best on this
+subject. No breeder doubts how strong is the tendency to inheritance: like
+produces like is his fundamental belief: doubts have been thrown on this
+principle by theoretical writers alone. When any deviation of structure
+often appears, and we see it in the {13} father and child, we cannot tell
+whether it may not be due to the same cause having acted on both; but when
+amongst individuals, apparently exposed to the same conditions, any very
+rare deviation, due to some extraordinary combination of circumstances,
+appears in the parent--say, once amongst several million individuals--and
+it reappears in the child, the mere doctrine of chances almost compels us
+to attribute its reappearance to inheritance. Every one must have heard of
+cases of albinism, prickly skin, hairy bodies, &c., appearing in several
+members of the same family. If strange and rare deviations of structure are
+truly inherited, less strange and commoner deviations may be freely
+admitted to be inheritable. Perhaps the correct way of viewing the whole
+subject, would be, to look at the inheritance of every character whatever
+as the rule, and non-inheritance as the anomaly.
+
+The laws governing inheritance are quite unknown; no one can say why a
+peculiarity in different individuals of the same species, or in individuals
+of different species, is sometimes inherited and sometimes not so; why the
+child often reverts in certain characters to its grandfather or grandmother
+or other more remote ancestor; why a peculiarity is often transmitted from
+one sex to both sexes, or to one sex alone, more commonly but not
+exclusively to the like sex. It is a fact of some little importance to us,
+that peculiarities appearing in the males of our domestic breeds are often
+transmitted either exclusively, or in a much greater degree, to males
+alone. A much more important rule, which I think may be trusted, is that,
+at whatever period of life a peculiarity first appears, it tends to appear
+in the offspring at a corresponding age, though sometimes earlier. In many
+cases this could not be otherwise: thus the inherited peculiarities in the
+horns of cattle could appear only in {14} the offspring when nearly mature;
+peculiarities in the silkworm are known to appear at the corresponding
+caterpillar or cocoon stage. But hereditary diseases and some other facts
+make me believe that the rule has a wider extension, and that when there is
+no apparent reason why a peculiarity should appear at any particular age,
+yet that it does tend to appear in the offspring at the same period at
+which it first appeared in the parent. I believe this rule to be of the
+highest importance in explaining the laws of embryology. These remarks are
+of course confined to the first _appearance_ of the peculiarity, and not to
+its primary cause, which may have acted on the ovules or male element; in
+nearly the same manner as in the crossed offspring from a short-horned cow
+by a long-horned bull, the greater length of horn, though appearing late in
+life, is clearly due to the male element.
+
+Having alluded to the subject of reversion, I may here refer to a statement
+often made by naturalists--namely, that our domestic varieties, when run
+wild, gradually but certainly revert in character to their aboriginal
+stocks. Hence it has been argued that no deductions can be drawn from
+domestic races to species in a state of nature. I have in vain endeavoured
+to discover on what decisive facts the above statement has so often and so
+boldly been made. There would be great difficulty in proving its truth: we
+may safely conclude that very many of the most strongly-marked domestic
+varieties could not possibly live in a wild state. In many cases we do not
+know what the aboriginal stock was, and so could not tell whether or not
+nearly perfect reversion had ensued. It would be quite necessary, in order
+to prevent the effects of intercrossing, that only a single variety should
+be turned loose in its new home. Nevertheless, as our varieties certainly
+do occasionally {15} revert in some of their characters to ancestral forms,
+it seems to me not improbable, that if we could succeed in naturalising, or
+were to cultivate, during many generations, the several races, for
+instance, of the cabbage, in very poor soil (in which case, however, some
+effect would have to be attributed to the direct action of the poor soil),
+that they would to a large extent, or even wholly, revert to the wild
+aboriginal stock. Whether or not the experiment would succeed, is not of
+great importance for our line of argument; for by the experiment itself the
+conditions of life are changed. If it could be shown that our domestic
+varieties manifested a strong tendency to reversion,--that is, to lose
+their acquired characters, whilst kept under the same conditions, and
+whilst kept in a considerable body, so that free intercrossing might check,
+by blending together, any slight deviations in their structure, in such
+case, I grant that we could deduce nothing from domestic varieties in
+regard to species. But there is not a shadow of evidence in favour of this
+view: to assert that we could not breed our cart and race-horses, long and
+short-horned cattle, and poultry of various breeds, and esculent
+vegetables, for an almost infinite number of generations, would be opposed
+to all experience. I may add, that when under nature the conditions of life
+do change, variations and reversions of character probably do occur; but
+natural selection, as will hereafter be explained, will determine how far
+the new characters thus arising shall be preserved.
+
+When we look to the hereditary varieties or races of our domestic animals
+and plants, and compare them with closely allied species, we generally
+perceive in each domestic race, as already remarked, less uniformity of
+character than in true species. Domestic races of the same species, also,
+often have a somewhat monstrous character; by which I mean, that, although
+differing {16} from each other, and from other species of the same genus,
+in several trifling respects, they often differ in an extreme degree in
+some one part, both when compared one with another, and more especially
+when compared with all the species in nature to which they are nearest
+allied. With these exceptions (and with that of the perfect fertility of
+varieties when crossed,--a subject hereafter to be discussed), domestic
+races of the same species differ from each other in the same manner as,
+only in most cases in a lesser degree than, do closely-allied species of
+the same genus in a state of nature. I think this must be admitted, when we
+find that there are hardly any domestic races, either amongst animals or
+plants, which have not been ranked by competent judges as mere varieties,
+and by other competent judges as the descendants of aboriginally distinct
+species. If any marked distinction existed between domestic races and
+species, this source of doubt could not so perpetually recur. It has often
+been stated that domestic races do not differ from each other in characters
+of generic value. I think it could be shown that this statement is hardly
+correct; but naturalists differ widely in determining what characters are
+of generic value; all such valuations being at present empirical. Moreover,
+on the view of the origin of genera which I shall presently give, we have
+no right to expect often to meet with generic differences in our
+domesticated productions.
+
+When we attempt to estimate the amount of structural difference between the
+domestic races of the same species, we are soon involved in doubt, from not
+knowing whether they have descended from one or several parent-species.
+This point, if it could be cleared up, would be interesting; if, for
+instance, it could be shown that the greyhound, bloodhound, terrier,
+spaniel, and bull-dog, which we all know propagate their kind so truly,
+were the {17} offspring of any single species, then such facts would have
+great weight in making us doubt about the immutability of the many very
+closely allied natural species--for instance, of the many foxes--inhabiting
+different quarters of the world. I do not believe, as we shall presently
+see, that the whole amount of difference between the several breeds of the
+dog has been produced under domestication; I believe that some small part
+of the difference is due to their being descended from distinct species. In
+the case of some other domesticated species, there is presumptive, or even
+strong evidence, that all the breeds have descended from a single wild
+stock.
+
+It has often been assumed that man has chosen for domestication animals and
+plants having an extraordinary inherent tendency to vary, and likewise to
+withstand diverse climates. I do not dispute that these capacities have
+added largely to the value of most of our domesticated productions; but how
+could a savage possibly know, when he first tamed an animal, whether it
+would vary in succeeding generations, and whether it would endure other
+climates? Has the little variability of the ass or guinea-fowl, or the
+small power of endurance of warmth by the reindeer, or of cold by the
+common camel, prevented their domestication? I cannot doubt that if other
+animals and plants, equal in number to our domesticated productions, and
+belonging to equally diverse classes and countries, were taken from a state
+of nature, and could be made to breed for an equal number of generations
+under domestication, they would vary on an average as largely as the parent
+species of our existing domesticated productions have varied.
+
+In the case of most of our anciently domesticated animals and plants, I do
+not think it is possible to come to any definite conclusion, whether they
+have descended from one or several wild species. The argument mainly relied
+on by those who believe in the multiple origin {18} of our domestic animals
+is, that we find in the most ancient records, more especially on the
+monuments of Egypt, much diversity in the breeds; and that some of the
+breeds closely resemble, perhaps are identical with, those still existing.
+Even if this latter fact were found more strictly and generally true than
+seems to me to be the case, what does it show, but that some of our breeds
+originated there, four or five thousand years ago? But Mr. Horner's
+researches have rendered it in some degree probable that man sufficiently
+civilized to have manufactured pottery existed in the valley of the Nile
+thirteen or fourteen thousand years ago; and who will pretend to say how
+long before these ancient periods, savages, like those of Tierra del Fuego
+or Australia, who possess a semi-domestic dog, may not have existed in
+Egypt?
+
+The whole subject must, I think, remain vague; nevertheless, I may, without
+here entering on any details, state that, from geographical and other
+considerations, I think it highly probable that our domestic dogs have
+descended from several wild species. Knowing, as we do, that savages are
+very fond of taming animals, it seems to me unlikely, in the case of the
+dog-genus, which is distributed in a wild state throughout the world, that
+since man first appeared one single species alone should have been
+domesticated. In regard to sheep and goats I can form no opinion. I should
+think, from facts communicated to me by Mr. Blyth, on the habits, voice,
+and constitution, &c., of the humped Indian cattle, that these had
+descended from a different aboriginal stock from our European cattle; and
+several competent judges believe that these latter have had more than one
+wild parent. With respect to horses, from reasons which I cannot give here,
+I am doubtfully inclined to believe, in opposition to several authors, that
+all the races have descended from one {19} wild stock. Mr. Blyth, whose
+opinion, from his large and varied stores of knowledge, I should value more
+than that of almost any one, thinks that all the breeds of poultry have
+proceeded from the common wild Indian fowl (Gallus bankiva). In regard to
+ducks and rabbits, the breeds of which differ considerably from each other
+in structure, I do not doubt that they have all descended from the common
+wild duck and rabbit.
+
+The doctrine of the origin of our several domestic races from several
+aboriginal stocks, has been carried to an absurd extreme by some authors.
+They believe that every race which breeds true, let the distinctive
+characters be ever so slight, has had its wild prototype. At this rate
+there must have existed at least a score of species of wild cattle, as many
+sheep, and several goats in Europe alone, and several even within Great
+Britain. One author believes that there formerly existed in Great Britain
+eleven wild species of sheep peculiar to it! When we bear in mind that
+Britain has now hardly one peculiar mammal, and France but few distinct
+from those of Germany and conversely, and so with Hungary, Spain, &c., but
+that each of these kingdoms possesses several peculiar breeds of cattle,
+sheep, &c., we must admit that many domestic breeds have originated in
+Europe; for whence could they have been derived, as these several countries
+do not possess a number of peculiar species as distinct parent-stocks? So
+it is in India. Even in the case of the domestic dogs of the whole world,
+which I fully admit have probably descended from several wild species, I
+cannot doubt that there has been an immense amount of inherited variation.
+Who can believe that animals closely resembling the Italian greyhound, the
+bloodhound, the bull-dog, or Blenheim spaniel, &c.--so unlike all wild
+Canidæ--ever existed freely in a state of nature? It has often been loosely
+said that all our races of dogs have {20} been produced by the crossing of
+a few aboriginal species; but by crossing we can only get forms in some
+degree intermediate between their parents; and if we account for our
+several domestic races by this process, we must admit the former existence
+of the most extreme forms, as the Italian greyhound, bloodhound, bull-dog,
+&c., in the wild state. Moreover, the possibility of making distinct races
+by crossing has been greatly exaggerated. There can be no doubt that a race
+may be modified by occasional crosses, if aided by the careful selection of
+those individual mongrels, which present any desired character; but that a
+race could be obtained nearly intermediate between two extremely different
+races or species, I can hardly believe. Sir J. Sebright expressly
+experimentised for this object, and failed. The offspring from the first
+cross between two pure breeds is tolerably and sometimes (as I have found
+with pigeons) extremely uniform, and everything seems simple enough; but
+when these mongrels are crossed one with another for several generations,
+hardly two of them will be alike, and then the extreme difficulty, or
+rather utter hopelessness, of the task becomes apparent. Certainly, a breed
+intermediate between _two very distinct_ breeds could not be got without
+extreme care and long-continued selection; nor can I find a single case on
+record of a permanent race having been thus formed.
+
+_On the Breeds of the Domestic Pigeon._--Believing that it is always best
+to study some special group, I have, after deliberation, taken up domestic
+pigeons. I have kept every breed which I could purchase or obtain, and have
+been most kindly favoured with skins from several quarters of the world,
+more especially by the Hon. W. Elliot from India, and by the Hon. C. Murray
+from Persia. Many treatises in different languages have been published on
+pigeons, and some of them are very important, as being of {21} considerable
+antiquity. I have associated with several eminent fanciers, and have been
+permitted to join two of the London Pigeon Clubs. The diversity of the
+breeds is something astonishing. Compare the English carrier and the
+short-faced tumbler, and see the wonderful difference in their beaks,
+entailing corresponding differences in their skulls. The carrier, more
+especially the male bird, is also remarkable from the wonderful development
+of the carunculated skin about the head, and this is accompanied by greatly
+elongated eyelids, very large external orifices to the nostrils, and a wide
+gape of mouth. The short-faced tumbler has a beak in outline almost like
+that of a finch; and the common tumbler has the singular inherited habit of
+flying at a great height in a compact flock, and tumbling in the air head
+over heels. The runt is a bird of great size, with long, massive beak and
+large feet; some of the sub-breeds of runts have very long necks, others
+very long wings and tails, others singularly short tails. The barb is
+allied to the carrier, but, instead of a very long beak, has a very short
+and very broad one. The pouter has a much elongated body, wings, and legs;
+and its enormously developed crop, which it glories in inflating, may well
+excite astonishment and even laughter. The turbit has a very short and
+conical beak, with a line of reversed feathers down the breast; and it has
+the habit of continually expanding slightly the upper part of the
+oesophagus. The Jacobin has the feathers so much reversed along the back of
+the neck that they form a hood, and it has, proportionally to its size,
+much elongated wing and tail feathers. The trumpeter and laugher, as their
+names express, utter a very different coo from the other breeds. The
+fantail has thirty or even forty tail feathers, instead of twelve or
+fourteen, the normal number in all members of the great pigeon family; and
+these feathers are kept expanded, and are {22} carried so erect that in
+good birds the head and tail touch; the oil-gland is quite aborted. Several
+other less distinct breeds might be specified.
+
+In the skeletons of the several breeds, the development of the bones of the
+face in length and breadth and curvature differs enormously. The shape, as
+well as the breadth and length of the ramus of the lower jaw, varies in a
+highly remarkable manner. The number of the caudal and sacral vertebræ
+vary; as does the number of the ribs, together with their relative breadth
+and the presence of processes. The size and shape of the apertures in the
+sternum are highly variable; so is the degree of divergence and relative
+size of the two arms of the furcula. The proportional width of the gape of
+mouth, the proportional length of the eyelids, of the orifice of the
+nostrils, of the tongue (not always in strict correlation with the length
+of beak), the size of the crop and of the upper part of the oesophagus; the
+development and abortion of the oil-gland; the number of the primary wing
+and caudal feathers; the relative length of wing and tail to each other and
+to the body; the relative length of leg and of the feet; the number of
+scutellæ on the toes, the development of skin between the toes, are all
+points of structure which are variable. The period at which the perfect
+plumage is acquired varies, as does the state of the down with which the
+nestling birds are clothed when hatched. The shape and size of the eggs
+vary. The manner of flight differs remarkably; as does in some breeds the
+voice and disposition. Lastly, in certain breeds, the males and females
+have come to differ to a slight degree from each other.
+
+Altogether at least a score of pigeons might be chosen, which if shown to
+an ornithologist, and he were told that they were wild birds, would
+certainly, I think, be ranked by him as well-defined species. Moreover, I
+do not believe that any ornithologist would place the {23} English carrier,
+the short-faced tumbler, the runt, the barb, pouter, and fantail in the
+same genus; more especially as in each of these breeds several
+truly-inherited sub-breeds, or species as he might have called them, could
+be shown him.
+
+Great as the differences are between the breeds of pigeons, I am fully
+convinced that the common opinion of naturalists is correct, namely, that
+all have descended from the rock-pigeon (Columba livia), including under
+this term several geographical races or sub-species, which differ from each
+other in the most trifling respects. As several of the reasons which have
+led me to this belief are in some degree applicable in other cases, I will
+here briefly give them. If the several breeds are not varieties, and have
+not proceeded from the rock-pigeon, they must have descended from at least
+seven or eight aboriginal stocks; for it is impossible to make the present
+domestic breeds by the crossing of any lesser number: how, for instance,
+could a pouter be produced by crossing two breeds unless one of the
+parent-stocks possessed the characteristic enormous crop? The supposed
+aboriginal stocks must all have been rock-pigeons, that is, not breeding or
+willingly perching on trees. But besides C. livia, with its geographical
+sub-species, only two or three other species of rock-pigeons are known; and
+these have not any of the characters of the domestic breeds. Hence the
+supposed aboriginal stocks must either still exist in the countries where
+they were originally domesticated, and yet be unknown to ornithologists;
+and this, considering their size, habits, and remarkable characters, seems
+very improbable; or they must have become extinct in the wild state. But
+birds breeding on precipices, and good fliers, are unlikely to be
+exterminated; and the common rock-pigeon, which has the same habits with
+the domestic breeds, has not been exterminated {24} even on several of the
+smaller British islets, or on the shores of the Mediterranean. Hence the
+supposed extermination of so many species having similar habits with the
+rock-pigeon seems to me a very rash assumption. Moreover, the several
+above-named domesticated breeds have been transported to all parts of the
+world, and, therefore, some of them must have been carried back again into
+their native country; but not one has ever become wild or feral, though the
+dovecot-pigeon, which is the rock-pigeon in a very slightly altered state,
+has become feral in several places. Again, all recent experience shows that
+it is most difficult to get any wild animal to breed freely under
+domestication; yet on the hypothesis of the multiple origin of our pigeons,
+it must be assumed that at least seven or eight species were so thoroughly
+domesticated in ancient times by half-civilized man, as to be quite
+prolific under confinement.
+
+An argument, as it seems to me, of great weight, and applicable in several
+other cases, is, that the above-specified breeds, though agreeing generally
+in constitution, habits, voice, colouring, and in most parts of their
+structure, with the wild rock-pigeon, yet are certainly highly abnormal in
+other parts of their structure; we may look in vain throughout the whole
+great family of Columbidæ for a beak like that of the English carrier, or
+that of the short-faced tumbler, or barb; for reversed feathers like those
+of the Jacobin; for a crop like that of the pouter; for tail-feathers like
+those of the fantail. Hence it must be assumed not only that half-civilized
+man succeeded in thoroughly domesticating several species, but that he
+intentionally or by chance picked out extraordinarily abnormal species; and
+further, that these very species have since all become extinct or unknown.
+So many strange contingencies seem to me improbable in the highest degree.
+{25}
+
+Some facts in regard to the colouring of pigeons well deserve
+consideration. The rock-pigeon is of a slaty-blue, and has a white rump
+(the Indian subspecies, C. intermedia of Strickland, having it bluish); the
+tail has a terminal dark bar, with the bases of the outer feathers
+externally edged with white; the wings have two black bars; some
+semi-domestic breeds and some apparently truly wild breeds have, besides
+the two black bars, the wings chequered with black. These several marks do
+not occur together in any other species of the whole family. Now, in every
+one of the domestic breeds, taking thoroughly well-bred birds, all the
+above marks, even to the white edging of the outer tail-feathers, sometimes
+concur perfectly developed. Moreover, when two birds belonging to two
+distinct breeds are crossed, neither of which is blue or has any of the
+above-specified marks, the mongrel offspring are very apt suddenly to
+acquire these characters; for instance, I crossed some uniformly white
+fantails with some uniformly black barbs, and they produced mottled brown
+and black birds; these I again crossed together, and one grandchild of the
+pure white fantail and pure black barb was of as beautiful a blue colour,
+with the white rump, double black wing-bar, and barred and white-edged
+tail-feathers, as any wild rock-pigeon! We can understand these facts, on
+the well-known principle of reversion to ancestral characters, if all the
+domestic breeds have descended from the rock-pigeon. But if we deny this,
+we must make one of the two following highly improbable suppositions.
+Either, firstly, that all the several imagined aboriginal stocks were
+coloured and marked like the rock-pigeon, although no other existing
+species is thus coloured and marked, so that in each separate breed there
+might be a tendency to revert to the very same colours and markings. Or,
+secondly, {26} that each breed, even the purest, has within a dozen or, at
+most, within a score of generations, been crossed by the rock-pigeon: I say
+within a dozen or twenty generations, for we know of no fact countenancing
+the belief that the child ever reverts to some one ancestor, removed by a
+greater number of generations. In a breed which has been crossed only once
+with some distinct breed, the tendency to reversion to any character
+derived from such cross will naturally become less and less, as in each
+succeeding generation there will be less of the foreign blood; but when
+there has been no cross with a distinct breed, and there is a tendency in
+both parents to revert to a character, which has been lost during some
+former generation, this tendency, for all that we can see to the contrary,
+may be transmitted undiminished for an indefinite number of generations.
+These two distinct cases are often confounded in treatises on inheritance.
+
+Lastly, the hybrids or mongrels from between all the domestic breeds of
+pigeons are perfectly fertile. I can state this from my own observations,
+purposely made, on the most distinct breeds. Now, it is difficult, perhaps
+impossible, to bring forward one case of the hybrid offspring of two
+animals _clearly distinct_ being themselves perfectly fertile. Some authors
+believe that long-continued domestication eliminates this strong tendency
+to sterility: from the history of the dog I think there is some probability
+in this hypothesis, if applied to species closely related together, though
+it is unsupported by a single experiment. But to extend the hypothesis so
+far as to suppose that species, aboriginally as distinct as carriers,
+tumblers, pouters, and fantails now are, should yield offspring perfectly
+fertile, _inter se_, seems to me rash in the extreme.
+
+From these several reasons, namely, the improbability of man having
+formerly got seven or eight supposed {27} species of pigeons to breed
+freely under domestication; these supposed species being quite unknown in a
+wild state, and their becoming nowhere feral; these species having very
+abnormal characters in certain respects, as compared with all other
+Columbidæ, though so like in most other respects to the rock-pigeon; the
+blue colour and various marks occasionally appearing in all the breeds,
+both when kept pure and when crossed; the mongrel offspring being perfectly
+fertile;--from these several reasons, taken together, I can feel no doubt
+that all our domestic breeds have descended from the Columba livia with its
+geographical sub-species.
+
+In favour of this view, I may add, firstly, that C. livia, or the
+rock-pigeon, has been found capable of domestication in Europe and in
+India; and that it agrees in habits and in a great number of points of
+structure with all the domestic breeds. Secondly, although an English
+carrier or short-faced tumbler differs immensely in certain characters from
+the rock-pigeon, yet by comparing the several sub-breeds of these
+varieties, more especially those brought from distant countries, we can
+make an almost perfect series between the extremes of structure. Thirdly,
+those characters which are mainly distinctive of each breed, for instance
+the wattle and length of beak of the carrier, the shortness of that of the
+tumbler, and the number of tail-feathers in the fantail, are in each breed
+eminently variable; and the explanation of this fact will be obvious when
+we come to treat of selection. Fourthly, pigeons have been watched, and
+tended with the utmost care, and loved by many people. They have been
+domesticated for thousands of years in several quarters of the world; the
+earliest known record of pigeons is in the fifth Ægyptian dynasty, about
+3000 B.C., as was pointed out to me by Professor Lepsius; but Mr. Birch
+informs me that pigeons are given in a bill {28} of fare in the previous
+dynasty. In the time of the Romans, as we hear from Pliny, immense prices
+were given for pigeons; "nay, they are come to this pass, that they can
+reckon up their pedigree and race." Pigeons were much valued by Akber Khan
+in India, about the year 1600; never less than 20,000 pigeons were taken
+with the court. "The monarchs of Iran and Turan sent him some very rare
+birds;" and, continues the courtly historian, "His Majesty by crossing the
+breeds, which method was never practised before, has improved them
+astonishingly." About this same period the Dutch were as eager about
+pigeons as were the old Romans. The paramount importance of these
+considerations in explaining the immense amount of variation which pigeons
+have undergone, will be obvious when we treat of Selection. We shall then,
+also, see how it is that the breeds so often have a somewhat monstrous
+character. It is also a most favourable circumstance for the production of
+distinct breeds, that male and female pigeons can be easily mated for life;
+and thus different breeds can be kept together in the same aviary.
+
+I have discussed the probable origin of domestic pigeons at some, yet quite
+insufficient, length; because when I first kept pigeons and watched the
+several kinds, knowing well how true they bred, I felt fully as much
+difficulty in believing that they could have descended from a common
+parent, as any naturalist could in coming to a similar conclusion in regard
+to the many species of finches, or other large groups of birds, in nature.
+One circumstance has struck me much; namely, that all the breeders of the
+various domestic animals and the cultivators of plants, with whom I have
+ever conversed, or whose treatises I have read, are firmly convinced that
+the several breeds to which each has attended, are descended from so many
+aboriginally distinct species. {29} Ask, as I have asked, a celebrated
+raiser of Hereford cattle, whether his cattle might not have descended from
+long-horns, and he will laugh you to scorn. I have never met a pigeon, or
+poultry, or duck, or rabbit fancier, who was not fully convinced that each
+main breed was descended from a distinct species. Van Mons, in his treatise
+on pears and apples, shows how utterly he disbelieves that the several
+sorts, for instance a Ribston-pippin or Codlin-apple, could ever have
+proceeded from the seeds of the same tree. Innumerable other examples could
+be given. The explanation, I think, is simple: from long-continued study
+they are strongly impressed with the differences between the several races;
+and though they well know that each race varies slightly, for they win
+their prizes by selecting such slight differences, yet they ignore all
+general arguments, and refuse to sum up in their minds slight differences
+accumulated during many successive generations. May not those naturalists
+who, knowing far less of the laws of inheritance than does the breeder, and
+knowing no more than he does of the intermediate links in the long lines of
+descent, yet admit that many of our domestic races have descended from the
+same parents--may they not learn a lesson of caution, when they deride the
+idea of species in a state of nature being lineal descendants of other
+species?
+
+_Selection._--Let us now briefly consider the steps by which domestic races
+have been produced, either from one or from several allied species. Some
+little effect may, perhaps, be attributed to the direct action of the
+external conditions of life, and some little to habit; but he would be a
+bold man who would account by such agencies for the differences of a dray
+and race horse, a greyhound and bloodhound, a carrier and tumbler pigeon.
+One of the most remarkable features in our domesticated races {30} is that
+we see in them adaptation, not indeed to the animal's or plant's own good,
+but to man's use or fancy. Some variations useful to him have probably
+arisen suddenly, or by one step; many botanists, for instance, believe that
+the fuller's teazle, with its hooks, which cannot be rivalled by any
+mechanical contrivance, is only a variety of the wild Dipsacus; and this
+amount of change may have suddenly arisen in a seedling. So it has probably
+been with the turnspit dog; and this is known to have been the case with
+the ancon sheep. But when we compare the dray-horse and race-horse, the
+dromedary and camel, the various breeds of sheep fitted either for
+cultivated land or mountain pasture, with the wool of one breed good for
+one purpose, and that of another breed for another purpose; when we compare
+the many breeds of dogs, each good for man in very different ways; when we
+compare the game-cock, so pertinacious in battle, with other breeds so
+little quarrelsome, with "everlasting layers" which never desire to sit,
+and with the bantam so small and elegant; when we compare the host of
+agricultural, culinary, orchard, and flower-garden races of plants, most
+useful to man at different seasons and for different purposes, or so
+beautiful in his eyes, we must, I think, look further than to mere
+variability. We cannot suppose that all the breeds were suddenly produced
+as perfect and as useful as we now see them; indeed, in several cases, we
+know that this has not been their history. The key is man's power of
+accumulative selection: nature gives successive variations; man adds them
+up in certain directions useful to him. In this sense he may be said to
+make for himself useful breeds.
+
+The great power of this principle of selection is not hypothetical. It is
+certain that several of our eminent breeders have, even within a single
+lifetime, modified to {31} a large extent some breeds of cattle and sheep.
+In order fully to realise what they have done, it is almost necessary to
+read several of the many treatises devoted to this subject, and to inspect
+the animals. Breeders habitually speak of an animal's organisation as
+something quite plastic, which they can model almost as they please. If I
+had space I could quote numerous passages to this effect from highly
+competent authorities. Youatt, who was probably better acquainted with the
+works of agriculturists than almost any other individual, and who was
+himself a very good judge of an animal, speaks of the principle of
+selection as "that which enables the agriculturist, not only to modify the
+character of his flock, but to change it altogether. It is the magician's
+wand, by means of which he may summon into life whatever form and mould he
+pleases." Lord Somerville, speaking of what breeders have done for sheep,
+says:--"It would seem as if they had chalked out upon a wall a form perfect
+in itself, and then had given it existence." That most skilful breeder, Sir
+John Sebright, used to say, with respect to pigeons, that "he would produce
+any given feather in three years, but it would take him six years to obtain
+head and beak." In Saxony the importance of the principle of selection in
+regard to merino sheep is so fully recognised, that men follow it as a
+trade: the sheep are placed on a table and are studied, like a picture by a
+connoisseur; this is done three times at intervals of months, and the sheep
+are each time marked and classed, so that the very best may ultimately be
+selected for breeding.
+
+What English breeders have actually effected is proved by the enormous
+prices given for animals with a good pedigree; and these have now been
+exported to almost every quarter of the world. The improvement is by no
+means generally due to crossing different breeds; {32} all the best
+breeders are strongly opposed to this practice, except sometimes amongst
+closely allied sub-breeds. And when a cross has been made, the closest
+selection is far more indispensable even than in ordinary cases. If
+selection consisted merely in separating some very distinct variety, and
+breeding from it, the principle would be so obvious as hardly to be worth
+notice; but its importance consists in the great effect produced by the
+accumulation in one direction, during successive generations, of
+differences absolutely inappreciable by an uneducated eye--differences
+which I for one have vainly attempted to appreciate. Not one man in a
+thousand has accuracy of eye and judgment sufficient to become an eminent
+breeder. If gifted with these qualities, and he studies his subject for
+years, and devotes his lifetime to it with indomitable perseverance, he
+will succeed, and may make great improvements; if he wants any of these
+qualities, he will assuredly fail. Few would readily believe in the natural
+capacity and years of practice requisite to become even a skilful
+pigeon-fancier.
+
+The same principles are followed by horticulturists; but the variations are
+here often more abrupt. No one supposes that our choicest productions have
+been produced by a single variation from the aboriginal stock. We have
+proofs that this is not so in some cases, in which exact records have been
+kept; thus, to give a very trifling instance, the steadily-increasing size
+of the common gooseberry may be quoted. We see an astonishing improvement
+in many florists' flowers, when the flowers of the present day are compared
+with drawings made only twenty or thirty years ago. When a race of plants
+is once pretty well established, the seed-raisers do not pick out the best
+plants, but merely go over their seed-beds, and pull up the "rogues," as
+they call the plants that deviate from the proper standard. With animals
+this {33} kind of selection is, in fact, also followed; for hardly any one
+is so careless as to allow his worst animals to breed.
+
+In regard to plants, there is another means of observing the accumulated
+effects of selection--namely, by comparing the diversity of flowers in the
+different varieties of the same species in the flower-garden; the diversity
+of leaves, pods, or tubers, or whatever part is valued, in the
+kitchen-garden, in comparison with the flowers of the same varieties; and
+the diversity of fruit of the same species in the orchard, in comparison
+with the leaves and flowers of the same set of varieties. See how different
+the leaves of the cabbage are, and how extremely alike the flowers; how
+unlike the flowers of the heartsease are, and how alike the leaves; how
+much the fruit of the different kinds of gooseberries differ in size,
+colour, shape, and hairiness, and yet the flowers present very slight
+differences. It is not that the varieties which differ largely in some one
+point do not differ at all in other points; this is hardly ever, perhaps
+never, the case. The laws of correlation of growth, the importance of which
+should never be overlooked, will ensure some differences; but, as a general
+rule, I cannot doubt that the continued selection of slight variations,
+either in the leaves, the flowers, or the fruit, will produce races
+differing from each other chiefly in these characters.
+
+It may be objected that the principle of selection has been reduced to
+methodical practice for scarcely more than three-quarters of a century; it
+has certainly been more attended to of late years, and many treatises have
+been published on the subject; and the result has been, in a corresponding
+degree, rapid and important. But it is very far from true that the
+principle is a modern discovery. I could give several references to the
+full acknowledgment of the importance of the principle in works of high
+antiquity. In rude and barbarous periods {34} of English history choice
+animals were often imported, and laws were passed to prevent their
+exportation: the destruction of horses under a certain size was ordered,
+and this may be compared to the "roguing" of plants by nurserymen. The
+principle of selection I find distinctly given in an ancient Chinese
+encyclopædia. Explicit rules are laid down by some of the Roman classical
+writers. From passages in Genesis, it is clear that the colour of domestic
+animals was at that early period attended to. Savages now sometimes cross
+their dogs with wild canine animals, to improve the breed, and they
+formerly did so, as is attested by passages in Pliny. The savages in South
+Africa match their draught cattle by colour, as do some of the Esquimaux
+their teams of dogs. Livingstone shows how much good domestic breeds are
+valued by the negroes of the interior of Africa who have not associated
+with Europeans. Some of these facts do not show actual selection, but they
+show that the breeding of domestic animals was carefully attended to in
+ancient times, and is now attended to by the lowest savages. It would,
+indeed, have been a strange fact, had attention not been paid to breeding,
+for the inheritance of good and bad qualities is so obvious.
+
+At the present time, eminent breeders try by methodical selection, with a
+distinct object in view, to make a new strain or sub-breed, superior to
+anything existing in the country. But, for our purpose, a kind of
+Selection, which may be called Unconscious, and which results from every
+one trying to possess and breed from the best individual animals, is more
+important. Thus, a man who intends keeping pointers naturally tries to get
+as good dogs as he can, and afterwards breeds from his own best dogs, but
+he has no wish or expectation of permanently altering the breed.
+Nevertheless I cannot doubt that this process, continued during centuries,
+{35} would improve and modify any breed, in the same way as Bakewell,
+Collins, &c., by this very same process, only carried on more methodically,
+did greatly modify, even during their own lifetimes, the forms and
+qualities of their cattle. Slow and insensible changes of this kind could
+never be recognised unless actual measurements or careful drawings of the
+breeds in question had been made long ago, which might serve for
+comparison. In some cases, however, unchanged, or but little changed
+individuals of the same breed may be found in less civilised districts,
+where the breed has been less improved. There is reason to believe that
+King Charles's spaniel has been unconsciously modified to a large extent
+since the time of that monarch. Some highly competent authorities are
+convinced that the setter is directly derived from the spaniel, and has
+probably been slowly altered from it. It is known that the English pointer
+has been greatly changed within the last century, and in this case the
+change has, it is believed, been chiefly effected by crosses with the
+fox-hound; but what concerns us is, that the change has been effected
+unconsciously and gradually, and yet so effectually, that, though the old
+Spanish pointer certainly came from Spain, Mr. Borrow has not seen, as I am
+informed by him, any native dog in Spain like our pointer.
+
+By a similar process of selection, and by careful training, the whole body
+of English racehorses have come to surpass in fleetness and size the parent
+Arab stock, so that the latter, by the regulations for the Goodwood Races,
+are favoured in the weights they carry. Lord Spencer and others have shown
+how the cattle of England have increased in weight and in early maturity,
+compared with the stock formerly kept in this country. By comparing the
+accounts given in old pigeon treatises of carriers and tumblers with these
+breeds as now existing in Britain, {36} India, and Persia, we can, I think,
+clearly trace the stages through which they have insensibly passed, and
+come to differ so greatly from the rock-pigeon.
+
+Youatt gives an excellent illustration of the effects of a course of
+selection, which may be considered as unconsciously followed, in so far
+that the breeders could never have expected or even have wished to have
+produced the result which ensued--namely, the production of two distinct
+strains. The two flocks of Leicester sheep kept by Mr. Buckley and Mr.
+Burgess, as Mr. Youatt remarks, "have been purely bred from the original
+stock of Mr. Bakewell for upwards of fifty years. 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 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."
+
+If there exist savages so barbarous as never to think of the inherited
+character of the offspring of their domestic animals, yet any one animal
+particularly useful to them, for any special purpose, would be carefully
+preserved during famines and other accidents, to which savages are so
+liable, and such choice animals would thus generally leave more offspring
+than the inferior ones; so that in this case there would be a kind of
+unconscious selection going on. We see the value set on animals even by the
+barbarians of Tierra del Fuego, by their killing and devouring their old
+women, in times of dearth, as of less value than their dogs.
+
+In plants the same gradual process of improvement, through the occasional
+preservation of the best individuals, whether or not sufficiently distinct
+to be ranked at their first appearance as distinct varieties, and whether
+{37} or not two or more species or races have become blended together by
+crossing, may plainly be recognised in the increased size and beauty which
+we now see in the varieties of the heartsease, rose, pelargonium, dahlia,
+and other plants, when compared with the older varieties or with their
+parent-stocks. No one would ever expect to get a first-rate heartsease or
+dahlia from the seed of a wild plant. No one would expect to raise a
+first-rate melting pear from the seed of the wild pear, though he might
+succeed from a poor seedling growing wild, if it had come from a
+garden-stock. The pear, though cultivated in classical times, appears, from
+Pliny's description, to have been a fruit of very inferior quality. I have
+seen great surprise expressed in horticultural works at the wonderful skill
+of gardeners, in having produced such splendid results from such poor
+materials; but the art, I cannot doubt, has been simple, and, as far as the
+final result is concerned, has been followed almost unconsciously. It has
+consisted in always cultivating the best known variety, sowing its seeds,
+and, when a slightly better variety has chanced to appear, selecting it,
+and so onwards. But the gardeners of the classical period, who cultivated
+the best pear they could procure, never thought what splendid fruit we
+should eat; though we owe our excellent fruit, in some small degree, to
+their having naturally chosen and preserved the best varieties they could
+anywhere find.
+
+A large amount of change in our cultivated plants, thus slowly and
+unconsciously accumulated, explains, as I believe, the well-known fact,
+that in a vast number of cases we cannot recognise, and therefore do not
+know, the wild parent-stocks of the plants which have been longest
+cultivated in our flower and kitchen gardens. If it has taken centuries or
+thousands of years to improve or modify most of our plants up to their
+present {38} standard of usefulness to man, we can understand how it is
+that neither Australia, the Cape of Good Hope, nor any other region
+inhabited by quite uncivilised man, has afforded us a single plant worth
+culture. It is not that these countries, so rich in species, do not by a
+strange chance possess the aboriginal stocks of any useful plants, but that
+the native plants have not been improved by continued selection up to a
+standard of perfection comparable with that given to the plants in
+countries anciently civilised.
+
+In regard to the domestic animals kept by uncivilised man, it should not be
+overlooked that they almost always have to struggle for their own food, at
+least during certain seasons. And in two countries very differently
+circumstanced, individuals of the same species, having slightly different
+constitutions or structure, would often succeed better in the one country
+than in the other; and thus by a process of "natural selection," as will
+hereafter be more fully explained, two sub-breeds might be formed. This,
+perhaps, partly explains what has been remarked by some authors, namely,
+that the varieties kept by savages have more of the character of species
+than the varieties kept in civilised countries.
+
+On the view here given of the all-important part which selection by man has
+played, it becomes at once obvious, how it is that our domestic races show
+adaptation in their structure or in their habits to man's wants or fancies.
+We can, I think, further understand the frequently abnormal character of
+our domestic races, and likewise their differences being so great in
+external characters and relatively so slight in internal parts or organs.
+Man can hardly select, or only with much difficulty, any deviation of
+structure excepting such as is externally visible; and indeed he rarely
+cares for what is internal. He can never act by selection, excepting on
+variations {39} which are first given to him in some slight degree by
+nature. No man would ever try to make a fantail, till he saw a pigeon with
+a tail developed in some slight degree in an unusual manner, or a pouter
+till he saw a pigeon with a crop of somewhat unusual size; and the more
+abnormal or unusual any character was when it first appeared, the more
+likely it would be to catch his attention. But to use such an expression as
+trying to make a fantail, is, I have no doubt, in most cases, utterly
+incorrect. The man who first selected a pigeon with a slightly larger tail,
+never dreamed what the descendants of that pigeon would become through
+long-continued, partly unconscious and partly methodical selection. Perhaps
+the parent bird of all fantails had only fourteen tail-feathers somewhat
+expanded, like the present Java fantail, or like individuals of other and
+distinct breeds, in which as many as seventeen tail-feathers have been
+counted. Perhaps the first pouter-pigeon did not inflate its crop much more
+than the turbit now does the upper part of its oesophagus,--a habit which
+is disregarded by all fanciers, as it is not one of the points of the
+breed.
+
+Nor let it be thought that some great deviation of structure would be
+necessary to catch the fancier's eye: he perceives extremely small
+differences, and it is in human nature to value any novelty, however
+slight, in one's own possession. Nor must the value which would formerly be
+set on any slight differences in the individuals of the same species, be
+judged of by the value which would now be set on them, after several breeds
+have once fairly been established. Many slight differences might, and
+indeed do now, arise amongst pigeons, which are rejected as faults or
+deviations from the standard of perfection of each breed. The common goose
+has not given rise to any marked varieties; hence the Thoulouse and the
+common breed, which differ only in colour, that {40} most fleeting of
+characters, have lately been exhibited as distinct at our poultry-shows.
+
+I think these views further explain what has sometimes been
+noticed--namely, that we know nothing about the origin or history of any of
+our domestic breeds. But, in fact, a breed, like a dialect of a language,
+can hardly be said to have had a definite origin. A man preserves and
+breeds from an individual with some slight deviation of structure, or takes
+more care than usual in matching his best animals and thus improves them,
+and the improved individuals slowly spread in the immediate neighbourhood.
+But as yet they will hardly have a distinct name, and from being only
+slightly valued, their history will be disregarded. When further improved
+by the same slow and gradual process, they will spread more widely, and
+will get recognised as something distinct and valuable, and will then
+probably first receive a provincial name. In semi-civilised countries, with
+little free communication, the spreading and knowledge of any new sub-breed
+will be a slow process. As soon as the points of value of the new sub-breed
+are once fully acknowledged, the principle, as I have called it, of
+unconscious selection will always tend,--perhaps more at one period than at
+another, as the breed rises or falls in fashion,--perhaps more in one
+district than in another, according to the state of civilization of the
+inhabitants,--slowly to add to the characteristic features of the breed,
+whatever they may be. But the chance will be infinitely small of any record
+having been preserved of such slow, varying, and insensible changes.
+
+I must now say a few words on the circumstances, favourable, or the
+reverse, to man's power of selection. A high degree of variability is
+obviously favourable, as freely giving the materials for selection to work
+on; not that mere individual differences are not amply {41} sufficient,
+with extreme care, to allow of the accumulation of a large amount of
+modification in almost any desired direction. But as variations manifestly
+useful or pleasing to man appear only occasionally, the chance of their
+appearance will be much increased by a large number of individuals being
+kept; and hence this comes to be of the highest importance to success. On
+this principle Marshall has remarked, with respect to the sheep of parts of
+Yorkshire, that "as they generally belong to poor people, and are mostly
+_in small lots_, they never can be improved." On the other hand,
+nurserymen, from raising large stocks of the same plants, are generally far
+more successful than amateurs in getting new and valuable varieties. The
+keeping of a large number of individuals of a species in any country
+requires that the species should be placed under favourable conditions of
+life, so as to breed freely in that country. When the individuals of any
+species are scanty, all the individuals, whatever their quality may be,
+will generally be allowed to breed, and this will effectually prevent
+selection. But probably the most important point of all, is, that the
+animal or plant should be so highly useful to man, or so much valued by
+him, that the closest attention should be paid to even the slightest
+deviation in the qualities or structure of each individual. Unless such
+attention be paid nothing can be effected. I have seen it gravely remarked,
+that it was most fortunate that the strawberry began to vary just when
+gardeners began to attend closely to this plant. No doubt the strawberry
+had always varied since it was cultivated, but the slight varieties had
+been neglected. As soon, however, as gardeners picked out individual plants
+with slightly larger, earlier, or better fruit, and raised seedlings from
+them, and again picked out the best seedlings and bred from them, then,
+there appeared (aided by some {42} crossing with distinct species) those
+many admirable varieties of the strawberry which have been raised during
+the last thirty or forty years.
+
+In the case of animals with separate sexes, facility in preventing crosses
+is an important element of success in the formation of new races,--at
+least, in a country which is already stocked with other races. In this
+respect enclosure of the land plays a part. Wandering savages or the
+inhabitants of open plains rarely possess more than one breed of the same
+species. Pigeons can be mated for life, and this is a great convenience to
+the fancier, for thus many races may be kept true, though mingled in the
+same aviary; and this circumstance must have largely favoured the
+improvement and formation of new breeds. Pigeons, I may add, can be
+propagated in great numbers and at a very quick rate, and inferior birds
+may be freely rejected, as when killed they serve for food. On the other
+hand, cats, from their nocturnal rambling habits, cannot be matched, and,
+although so much valued by women and children, we hardly ever see a
+distinct breed kept up; such breeds as we do sometimes see are almost
+always imported from some other country, often from islands. Although I do
+not doubt that some domestic animals vary less than others, yet the rarity
+or absence of distinct breeds of the cat, the donkey, peacock, goose, &c.,
+may be attributed in main part to selection not having been brought into
+play: in cats, from the difficulty in pairing them; in donkeys, from only a
+few being kept by poor people, and little attention paid to their breeding;
+in peacocks, from not being very easily reared and a large stock not kept;
+in geese, from being valuable only for two purposes, food and feathers, and
+more especially from no pleasure having been felt in the display of
+distinct breeds.
+
+To sum up on the origin of our Domestic Races of {43} animals and plants. I
+believe that the conditions of life, from their action on the reproductive
+system, are so far of the highest importance as causing variability. I do
+not believe that variability is an inherent and necessary contingency,
+under all circumstances, with all organic beings, as some authors have
+thought. The effects of variability are modified by various degrees of
+inheritance and of reversion. Variability is governed by many unknown laws,
+more especially by that of correlation of growth. Something may be
+attributed to the direct action of the conditions of life. Something must
+be attributed to use and disuse. The final result is thus rendered
+infinitely complex. In some cases, I do not doubt that the intercrossing of
+species, aboriginally distinct, has played an important part in the origin
+of our domestic productions. When in any country several domestic breeds
+have once been established, their occasional intercrossing, with the aid of
+selection, has, no doubt, largely aided in the formation of new sub-breeds;
+but the importance of the crossing of varieties has, I believe, been
+greatly exaggerated, both in regard to animals and to those plants which
+are propagated by seed. In plants which are temporarily propagated by
+cuttings, buds, &c., the importance of the crossing both of distinct
+species and of varieties is immense; for the cultivator here quite
+disregards the extreme variability both of hybrids and mongrels, and the
+frequent sterility of hybrids; but the cases of plants not propagated by
+seed are of little importance to us, for their endurance is only temporary.
+Over all these causes of Change I am convinced that the accumulative action
+of Selection, whether applied methodically and more quickly, or
+unconsciously and more slowly, but more efficiently, is by far the
+predominant Power.
+
+       *       *       *       *       *
+
+
+{44}
+
+CHAPTER II.
+
+VARIATION UNDER NATURE.
+
+    Variability--Individual differences--Doubtful species--Wide ranging,
+    much diffused, and common species vary most--Species of the larger
+    genera in any country vary more than the species of the smaller
+    genera--Many of the species of the larger genera resemble varieties in
+    being very closely, but unequally, related to each other, and in having
+    restricted ranges.
+
+Before applying the principles arrived at in the last chapter to organic
+beings in a state of nature, we must briefly discuss whether these latter
+are subject to any variation. To treat this subject at all properly, a long
+catalogue of dry facts should be given; but these I shall reserve for my
+future work. Nor shall I here discuss the various definitions which have
+been given of the term species. No one definition has as yet satisfied all
+naturalists; yet every naturalist knows vaguely what he means when he
+speaks of a species. Generally the term includes the unknown element of a
+distinct act of creation. The term "variety" is almost equally difficult to
+define; but here community of descent is almost universally implied, though
+it can rarely be proved. We have also what are called monstrosities; but
+they graduate into varieties. By a monstrosity I presume is meant some
+considerable deviation of structure in one part, either injurious to or not
+useful to the species, and not generally propagated. Some authors use the
+term "variation" in a technical sense, as implying a modification directly
+due to the physical conditions of life; and "variations" in this sense are
+supposed not to be inherited: but who can say that the dwarfed condition of
+shells in the brackish waters of the Baltic, or dwarfed {45} plants on
+Alpine summits, or the thicker fur of an animal from far northwards, would
+not in some cases be inherited for at least some few generations? and in
+this case I presume that the form would be called a variety.
+
+Again, we have many slight differences which may be called individual
+differences, such as are known frequently to appear in the offspring from
+the same parents, or which may be presumed to have thus arisen, from being
+frequently observed in the individuals of the same species inhabiting the
+same confined locality. No one supposes that all the individuals of the
+same species are cast in the very same mould. These individual differences
+are highly important for us, as they afford materials for natural selection
+to accumulate, in the same manner as man can accumulate in any given
+direction individual differences in his domesticated productions. These
+individual differences generally affect what naturalists consider
+unimportant parts; but I could show by a long catalogue of facts, that
+parts which must be called important, whether viewed under a physiological
+or classificatory point of view, sometimes vary in the individuals of the
+same species. I am convinced that the most experienced naturalist would be
+surprised at the number of the cases of variability, even in important
+parts of structure, which he could collect on good authority, as I have
+collected, during a course of years. It should be remembered that
+systematists are far from pleased at finding variability in important
+characters, and that there are not many men who will laboriously examine
+internal and important organs, and compare them in many specimens of the
+same species. I should never have expected that the branching of the main
+nerves close to the great central ganglion of an insect would have been
+variable in the same species; I should have expected that changes of this
+nature could have been effected only {46} by slow degrees: yet quite
+recently Mr. Lubbock has shown a degree of variability in these main nerves
+in Coccus, which may almost be compared to the irregular branching of the
+stem of a tree. This philosophical naturalist, I may add, has also quite
+recently shown that the muscles in the larvæ of certain insects are very
+far from uniform. Authors sometimes argue in a circle when they state that
+important organs never vary; for these same authors practically rank that
+character as important (as some few naturalists have honestly confessed)
+which does not vary; and, under this point of view, no instance of an
+important part varying will ever be found: but under any other point of
+view many instances assuredly can be given.
+
+There is one point connected with individual differences, which seems to me
+extremely perplexing: I refer to those genera which have sometimes been
+called "protean" or "polymorphic," in which the species present an
+inordinate amount of variation; and hardly two naturalists can agree which
+forms to rank as species and which as varieties. We may instance Rubus,
+Rosa, and Hieracium amongst plants, several genera of insects, and several
+genera of Brachiopod shells. In most polymorphic genera some of the species
+have fixed and definite characters. Genera which are polymorphic in one
+country seem to be, with some few exceptions, polymorphic in other
+countries, and likewise, judging from Brachiopod shells, at former periods
+of time. These facts seem to be very perplexing, for they seem to show that
+this kind of variability is independent of the conditions of life. I am
+inclined to suspect that we see in these polymorphic genera variations in
+points of structure which are of no service or disservice to the species,
+and which consequently have not been seized on and rendered definite by
+natural selection, as hereafter will be explained. {47}
+
+Those forms which possess in some considerable degree the character of
+species, but which are so closely similar to some other forms, or are so
+closely linked to them by intermediate gradations, that naturalists do not
+like to rank them as distinct species, are in several respects the most
+important for us. We have every reason to believe that many of these
+doubtful and closely-allied forms have permanently retained their
+characters in their own country for a long time; for as long, as far as we
+know, as have good and true species. Practically, when a naturalist can
+unite two forms together by others having intermediate characters, he
+treats the one as a variety of the other, ranking the most common, but
+sometimes the one first described, as the species, and the other as the
+variety. But cases of great difficulty, which I will not here enumerate,
+sometimes occur in deciding whether or not to rank one form as a variety of
+another, even when they are closely connected by intermediate links; nor
+will the commonly-assumed hybrid nature of the intermediate links always
+remove the difficulty. In very many cases, however, one form is ranked as a
+variety of another, not because the intermediate links have actually been
+found, but because analogy leads the observer to suppose either that they
+do now somewhere exist, or may formerly have existed; and here a wide door
+for the entry of doubt and conjecture is opened.
+
+Hence, in determining whether a form should be ranked as a species or a
+variety, the opinion of naturalists having sound judgment and wide
+experience seems the only guide to follow. We must, however, in many cases,
+decide by a majority of naturalists, for few well-marked and well-known
+varieties can be named which have not been ranked as species by at least
+some competent judges. {48}
+
+That varieties of this doubtful nature are far from uncommon cannot be
+disputed. Compare the several floras of Great Britain, of France or of the
+United States, drawn up by different botanists, and see what a surprising
+number of forms have been ranked by one botanist as good species, and by
+another as mere varieties. Mr. H. C. Watson, to whom I lie under deep
+obligation for assistance of all kinds, has marked for me 182 British
+plants, which are generally considered as varieties, but which have all
+been ranked by botanists as species; and in making this list he has omitted
+many trifling varieties, but which nevertheless have been ranked by some
+botanists as species, and he has entirely omitted several highly
+polymorphic genera. Under genera, including the most polymorphic forms, Mr.
+Babington gives 251 species, whereas Mr. Bentham gives only 112,--a
+difference of 139 doubtful forms! Amongst animals which unite for each
+birth, and which are highly locomotive, doubtful forms, ranked by one
+zoologist as a species and by another as a variety, can rarely be found
+within the same country, but are common in separated areas. How many of
+those birds and insects in North America and Europe, which differ very
+slightly from each other, have been ranked by one eminent naturalist as
+undoubted species, and by another as varieties, or, as they are often
+called, as geographical races! Many years ago, when comparing, and seeing
+others compare, the birds from the separate islands of the Galapagos
+Archipelago, both one with another, and with those from the American
+mainland, I was much struck how entirely vague and arbitrary is the
+distinction between species and varieties. On the islets of the little
+Madeira group there are many insects which are characterized as varieties
+in Mr. Wollaston's admirable work, but which it cannot {49} be doubted
+would be ranked as distinct species by many entomologists. Even Ireland has
+a few animals, now generally regarded as varieties, but which have been
+ranked as species by some zoologists. Several most experienced
+ornithologists consider our British red grouse as only a strongly-marked
+race of a Norwegian species, whereas the greater number rank it as an
+undoubted species peculiar to Great Britain. A wide distance between the
+homes of two doubtful forms leads many naturalists to rank both as distinct
+species; but what distance, it has been well asked, will suffice? if that
+between America and Europe is ample, will that between the Continent and
+the Azores, or Madeira, or the Canaries, or Ireland, be sufficient? It must
+be admitted that many forms, considered by highly-competent judges as
+varieties, have so perfectly the character of species that they are ranked
+by other highly-competent judges as good and true species. But to discuss
+whether they are rightly called species or varieties, before any definition
+of these terms has been generally accepted, is vainly to beat the air.
+
+Many of the cases of strongly-marked varieties or doubtful species well
+deserve consideration; for several interesting lines of argument, from
+geographical distribution, analogical variation, hybridism, &c., have been
+brought to bear on the attempt to determine their rank. I will here give
+only a single instance,--the well-known one of the primrose and cowslip, or
+Primula vulgaris and veris. These plants differ considerably in appearance;
+they have a different flavour, and emit a different odour; they flower at
+slightly different periods; they grow in somewhat different stations; they
+ascend mountains to different heights; they have different geographical
+ranges; and lastly, according to very numerous experiments made during
+several years by {50} that most careful observer Gärtner, they can be
+crossed only with much difficulty. We could hardly wish for better evidence
+of the two forms being specifically distinct. On the other hand, they are
+united by many intermediate links, and it is very doubtful whether these
+links are hybrids; and there is, as it seems to me, an overwhelming amount
+of experimental evidence, showing that they descend from common parents,
+and consequently must be ranked as varieties.
+
+Close investigation, in most cases, will bring naturalists to an agreement
+how to rank doubtful forms. Yet it must be confessed that it is in the
+best-known countries that we find the greatest number of forms of doubtful
+value. I have been struck with the fact, that if any animal or plant in a
+state of nature be highly useful to man, or from any cause closely attract
+his attention, varieties of it will almost universally be found recorded.
+These varieties, moreover, will be often ranked by some authors as species.
+Look at the common oak, how closely it has been studied; yet a German
+author makes more than a dozen species out of forms, which are very
+generally considered as varieties; and in this country the highest
+botanical authorities and practical men can be quoted to show that the
+sessile and pedunculated oaks are either good and distinct species or mere
+varieties.
+
+When a young naturalist commences the study of a group of organisms quite
+unknown to him, he is at first much perplexed to determine what differences
+to consider as specific, and what as varieties; for he knows nothing of the
+amount and kind of variation to which the group is subject; and this shows,
+at least, how very generally there is some variation. But if he confine his
+attention to one class within one country, he will soon make up his mind
+how to rank most of the doubtful forms. His {51} general tendency will be
+to make many species, for he will become impressed, just like the pigeon or
+poultry fancier before alluded to, with the amount of difference in the
+forms which he is continually studying; and he has little general knowledge
+of analogical variation in other groups and in other countries, by which to
+correct his first impressions. As he extends the range of his observations,
+he will meet with more cases of difficulty; for he will encounter a greater
+number of closely-allied forms. But if his observations be widely extended,
+he will in the end generally be enabled to make up his own mind which to
+call varieties and which species; but he will succeed in this at the
+expense of admitting much variation,--and the truth of this admission will
+often be disputed by other naturalists. When, moreover, he comes to study
+allied forms brought from countries not now continuous, in which case he
+can hardly hope to find the intermediate links between his doubtful forms,
+he will have to trust almost entirely to analogy, and his difficulties rise
+to a climax.
+
+Certainly no clear line of demarcation has as yet been drawn between
+species and sub-species--that is, the forms which in the opinion of some
+naturalists come very near to, but do not quite arrive at the rank of
+species; or, again, between sub-species and well-marked varieties, or
+between lesser varieties and individual differences. These differences
+blend into each other in an insensible series; and a series impresses the
+mind with the idea of an actual passage.
+
+Hence I look at individual differences, though of small interest to the
+systematist, as of high importance for us, as being the first step towards
+such slight varieties as are barely thought worth recording in works on
+natural history. And I look at varieties which are in any degree more
+distinct and permanent, as steps leading to more {52} strongly marked and
+more permanent varieties; and at these latter, as leading to sub-species,
+and to species. The passage from one stage of difference to another and
+higher stage may be, in some cases, due merely to the long-continued action
+of different physical conditions in two different regions; but I have not
+much faith in this view; and I attribute the passage of a variety, from a
+state in which it differs very slightly from its parent to one in which it
+differs more, to the action of natural selection in accumulating (as will
+hereafter be more fully explained) differences of structure in certain
+definite directions. Hence I believe a well-marked variety may be called an
+incipient species; but whether this belief be justifiable must be judged of
+by the general weight of the several facts and views given throughout this
+work.
+
+It need not be supposed that all varieties or incipient species necessarily
+attain the rank of species. They may whilst in this incipient state become
+extinct, or they may endure as varieties for very long periods, as has been
+shown to be the case by Mr. Wollaston with the varieties of certain fossil
+land-shells in Madeira. If a variety were to flourish so as to exceed in
+numbers the parent species, it would then rank as the species, and the
+species as the variety; or it might come to supplant and exterminate the
+parent species; or both might co-exist, and both rank as independent
+species. But we shall hereafter have to return to this subject.
+
+From these remarks it will be seen that I look at the term species, as one
+arbitrarily given for the sake of convenience to a set of individuals
+closely resembling each other, and that it does not essentially differ from
+the term variety, which is given to less distinct and more fluctuating
+forms. The term variety, again, in comparison with mere individual
+differences, is also applied arbitrarily, and for mere convenience' sake.
+{53}
+
+Guided by theoretical considerations, I thought that some interesting
+results might be obtained in regard to the nature and relations of the
+species which vary most, by tabulating all the varieties in several
+well-worked floras. At first this seemed a simple task; but Mr. H. C.
+Watson, to whom I am much indebted for valuable advice and assistance on
+this subject, soon convinced me that there were many difficulties, as did
+subsequently Dr. Hooker, even in stronger terms. I shall reserve for my
+future work the discussion of these difficulties, and the tables themselves
+of the proportional numbers of the varying species. Dr. Hooker permits me
+to add, that after having carefully read my manuscript, and examined the
+tables, he thinks that the following statements are fairly well
+established. The whole subject, however, treated as it necessarily here is
+with much brevity, is rather perplexing, and allusions cannot be avoided to
+the "struggle for existence," "divergence of character," and other
+questions, hereafter to be discussed.
+
+Alph. de Candolle and others have shown that plants which have very wide
+ranges generally present varieties; and this might have been expected, as
+they become exposed to diverse physical conditions, and as they come into
+competition (which, as we shall hereafter see, is a far more important
+circumstance) with different sets of organic beings. But my tables further
+show that, in any limited country, the species which are most common, that
+is abound most in individuals, and the species which are most widely
+diffused within their own country (and this is a different consideration
+from wide range, and to a certain extent from commonness), often give rise
+to varieties sufficiently well-marked to have been recorded in botanical
+works. Hence it is the most flourishing, or, as they may be called, the
+dominant species,--those {54} which range widely over the world, are the
+most diffused in their own country, and are the most numerous in
+individuals,--which oftenest produce well-marked varieties, or, as I
+consider them, incipient species. And this, perhaps, might have been
+anticipated; for, as varieties, in order to become in any degree permanent,
+necessarily have to struggle with the other inhabitants of the country, the
+species which are already dominant will be the most likely to yield
+offspring, which, though in some slight degree modified, still inherit
+those advantages that enabled their parents to become dominant over their
+compatriots.
+
+If the plants inhabiting a country and described in any Flora be divided
+into two equal masses, all those in the larger genera being placed on one
+side, and all those in the smaller genera on the other side, a somewhat
+larger number of the very common and much diffused or dominant species will
+be found on the side of the larger genera. This, again, might have been
+anticipated; for the mere fact of many species of the same genus inhabiting
+any country, shows that there is something in the organic or inorganic
+conditions of that country favourable to the genus; and, consequently, we
+might have expected to have found in the larger genera, or those including
+many species, a large proportional number of dominant species. But so many
+causes tend to obscure this result, that I am surprised that my tables show
+even a small majority on the side of the larger genera. I will here allude
+to only two causes of obscurity. Fresh-water and salt-loving plants have
+generally very wide ranges and are much diffused, but this seems to be
+connected with the nature of the stations inhabited by them, and has little
+or no relation to the size of the genera to which the species belong.
+Again, plants low in the scale of organisation are {55} generally much more
+widely diffused than plants higher in the scale; and here again there is no
+close relation to the size of the genera. The cause of lowly-organised
+plants ranging widely will be discussed in our chapter on geographical
+distribution.
+
+From looking at species as only strongly-marked and well-defined varieties,
+I was led to anticipate that the species of the larger genera in each
+country would oftener present varieties, than the species of the smaller
+genera; for wherever many closely related species (_i.e._ species of the
+same genus) have been formed, many varieties or incipient species ought, as
+a general rule, to be now forming. Where many large trees grow, we expect
+to find saplings. Where many species of a genus have been formed through
+variation, circumstances have been favourable for variation; and hence we
+might expect that the circumstances would generally be still favourable to
+variation. On the other hand, if we look at each species as a special act
+of creation, there is no apparent reason why more varieties should occur in
+a group having many species, than in one having few.
+
+To test the truth of this anticipation I have arranged the plants of twelve
+countries, and the coleopterous insects of two districts, into two nearly
+equal masses, the species of the larger genera on one side, and those of
+the smaller genera on the other side, and it has invariably proved to be
+the case that a larger proportion of the species on the side of the larger
+genera present varieties, than on the side of the smaller genera. Moreover,
+the species of the large genera which present any varieties, invariably
+present a larger average number of varieties than do the species of the
+small genera. Both these results follow when another division is made, and
+when all the smallest genera, with from only one to four species, are
+absolutely excluded from the tables. These {56} facts are of plain
+signification on the view that species are only strongly marked and
+permanent varieties; for wherever many species of the same genus have been
+formed, or where, if we may use the expression, the manufactory of species
+has been active, we ought generally to find the manufactory still in
+action, more especially as we have every reason to believe the process of
+manufacturing new species to be a slow one. And this certainly is the case,
+if varieties be looked at as incipient species; for my tables clearly show
+as a general rule that, wherever many species of a genus have been formed,
+the species of that genus present a number of varieties, that is of
+incipient species beyond the average. It is not that all large genera are
+now varying much, and are thus increasing in the number of their species,
+or that no small genera are now varying and increasing; for if this had
+been so, it would have been fatal to my theory; inasmuch as geology plainly
+tells us that small genera have in the lapse of time often increased
+greatly in size; and that large genera have often come to their maxima,
+declined, and disappeared. All that we want to show is, that where many
+species of a genus have been formed, on an average many are still forming;
+and this holds good.
+
+There are other relations between the species of large genera and their
+recorded varieties which deserve notice. We have seen that there is no
+infallible criterion by which to distinguish species and well-marked
+varieties; and in those cases in which intermediate links have not been
+found between doubtful forms, naturalists are compelled to come to a
+determination by the amount of difference between them, judging by analogy
+whether or not the amount suffices to raise one or both to the rank of
+species. Hence the amount of difference is one very important criterion in
+settling whether two forms {57} should be ranked as species or varieties.
+Now Fries has remarked in regard to plants, and Westwood in regard to
+insects, that in large genera the amount of difference between the species
+is often exceedingly small. I have endeavoured to test this numerically by
+averages, and, as far as my imperfect results go, they confirm the view. I
+have also consulted some sagacious and experienced observers, and, after
+deliberation, they concur in this view. In this respect, therefore, the
+species of the larger genera resemble varieties, more than do the species
+of the smaller genera. Or the case may be put in another way, and it may be
+said, that in the larger genera, in which a number of varieties or
+incipient species greater than the average are now manufacturing, many of
+the species already manufactured still to a certain extent resemble
+varieties, for they differ from each other by a less than usual amount of
+difference.
+
+Moreover, the species of the large genera are related to each other, in the
+same manner as the varieties of any one species are related to each other.
+No naturalist pretends that all the species of a genus are equally distinct
+from each other; they may generally be divided into sub-genera, or
+sections, or lesser groups. As Fries has well remarked, little groups of
+species are generally clustered like satellites around certain other
+species. And what are varieties but groups of forms, unequally related to
+each other, and clustered round certain forms--that is, round their
+parent-species? Undoubtedly there is one most important point of difference
+between varieties and species; namely, that the amount of difference
+between varieties, when compared with each other or with their
+parent-species, is much less than that between the species of the same
+genus. But when we come to discuss the principle, as I call it, of
+Divergence of Character, {58} we shall see how this may be explained, and
+how the lesser differences between varieties will tend to increase into the
+greater differences between species.
+
+There is one other point which seems to me worth notice. Varieties
+generally have much restricted ranges: this statement is indeed scarcely
+more than a truism, for if a variety were found to have a wider range than
+that of its supposed parent-species, their denominations ought to be
+reversed. But there is also reason to believe, that those species which are
+very closely allied to other species, and in so far resemble varieties,
+often have much restricted ranges. For instance, Mr. H. C. Watson has
+marked for me in the well-sifted London Catalogue of plants (4th edition)
+63 plants which are therein ranked as species, but which he considers as so
+closely allied to other species as to be of doubtful value: these 63
+reputed species range on an average over 6.9 of the provinces into which
+Mr. Watson has divided Great Britain. Now, in this same catalogue, 53
+acknowledged varieties are recorded, and these range over 7.7 provinces;
+whereas, the species to which these varieties belong range over 14.3
+provinces. So that the acknowledged varieties have very nearly the same
+restricted average range, as have those very closely allied forms, marked
+for me by Mr. Watson as doubtful species, but which are almost universally
+ranked by British botanists as good and true species.
+
+
+
+Finally, then, varieties have the same general characters as species, for
+they cannot be distinguished from species,--except, firstly, by the
+discovery of intermediate linking forms, and the occurrence of such links
+cannot affect the actual characters of the forms which they connect; and
+except, secondly by a certain amount of {59} difference, for two forms, if
+differing very little, are generally ranked as varieties, notwithstanding
+that intermediate linking forms have not been discovered; but the amount of
+difference considered necessary to give to two forms the rank of species is
+quite indefinite. In genera having more than the average number of species
+in any country, the species of these genera have more than the average
+number of varieties. In large genera the species are apt to be closely, but
+unequally allied together, forming little clusters round certain species.
+Species very closely allied to other species apparently have restricted
+ranges. In all these several respects the species of large genera present a
+strong analogy with varieties. And we can clearly understand these
+analogies, if species have once existed as varieties, and have thus
+originated: whereas, these analogies are utterly inexplicable if each
+species has been independently created.
+
+We have, also, seen that it is the most flourishing or dominant species of
+the larger genera which on an average vary most; and varieties, as we shall
+hereafter see, tend to become converted into new and distinct species. The
+larger genera thus tend to become larger; and throughout nature the forms
+of life which are now dominant tend to become still more dominant by
+leaving many modified and dominant descendants. But by steps hereafter to
+be explained, the larger genera also tend to break up into smaller genera.
+And thus, the forms of life throughout the universe become divided into
+groups subordinate to groups.
+
+       *       *       *       *       *
+
+
+{60}
+
+CHAPTER III.
+
+STRUGGLE FOR EXISTENCE.
+
+    Bears on natural selection--The term used in a wide sense--Geometrical
+    powers of increase--Rapid increase of naturalised animals and
+    plants--Nature of the checks to increase--Competition
+    universal--Effects of climate--Protection from the number of
+    individuals--Complex relations of all animals and plants throughout
+    nature--Struggle for life most severe between individuals and varieties
+    of the same species; often severe between species of the same
+    genus--The relation of organism to organism the most important of all
+    relations.
+
+Before entering on the subject of this chapter, I must make a few
+preliminary remarks, to show how the struggle for existence bears on
+Natural Selection. It has been seen in the last chapter that amongst
+organic beings in a state of nature there is some individual variability:
+indeed I am not aware that this has ever been disputed. It is immaterial
+for us whether a multitude of doubtful forms be called species or
+sub-species or varieties; what rank, for instance, the two or three hundred
+doubtful forms of British plants are entitled to hold, if the existence of
+any well-marked varieties be admitted. But the mere existence of individual
+variability and of some few well-marked varieties, though necessary as the
+foundation for the work, helps us but little in understanding how species
+arise in nature. How have all those exquisite adaptations of one part of
+the organisation to another part, and to the conditions of life, and of one
+distinct organic being to another being, been perfected? We see these
+beautiful co-adaptations most {61} plainly in the woodpecker and missletoe;
+and only a little less plainly in the humblest parasite which clings to the
+hairs of a quadruped or feathers of a bird; in the structure of the beetle
+which dives through the water; in the plumed seed which is wafted by the
+gentlest breeze; in short, we see beautiful adaptations everywhere and in
+every part of the organic world.
+
+Again, it may be asked, how is it that varieties, which I have called
+incipient species, become ultimately converted into good and distinct
+species, which in most cases obviously differ from each other far more than
+do the varieties of the same species? How do those groups of species, which
+constitute what are called distinct genera, and which differ from each
+other more than do the species of the same genus, arise? All these results,
+as we shall more fully see in the next chapter, follow from the struggle
+for life. Owing to this struggle for life, any variation, however slight,
+and from whatever cause proceeding, if it be in any degree profitable to an
+individual of any species, in its infinitely complex relations to other
+organic beings and to external nature, will tend to the preservation of
+that individual, and will generally be inherited by its offspring. The
+offspring, also, will thus have a better chance of surviving, for, of the
+many individuals of any species which are periodically born, but a small
+number can survive. I have called this principle, by which each slight
+variation, if useful, is preserved, by the term of Natural Selection, in
+order to mark its relation to man's power of selection. We have seen that
+man by selection can certainly produce great results, and can adapt organic
+beings to his own uses, through the accumulation of slight but useful
+variations, given to him by the hand of Nature. But Natural Selection, as
+we shall hereafter see, is a power incessantly ready for action, and is as
+{62} immeasurably superior to man's feeble efforts, as the works of Nature
+are to those of Art.
+
+We will now discuss in a little more detail the struggle for existence. In
+my future work this subject shall be treated, as it well deserves, at much
+greater length. The elder de Candolle and Lyell have largely and
+philosophically shown that all organic beings are exposed to severe
+competition. In regard to plants, no one has treated this subject with more
+spirit and ability than W. Herbert, Dean of Manchester, evidently the
+result of his great horticultural knowledge. Nothing is easier than to
+admit in words the truth of the universal struggle for life, or more
+difficult--at least I have found it so--than constantly to bear this
+conclusion in mind. Yet unless it be thoroughly engrained in the mind, I am
+convinced that the whole economy of nature, with every fact on
+distribution, rarity, abundance, extinction, and variation, will be dimly
+seen or quite misunderstood. We behold the face of nature bright with
+gladness, we often see superabundance of food; we do not see, or we forget
+that the birds which are idly singing round us mostly live on insects or
+seeds, and are thus constantly destroying life; or we forget how largely
+these songsters, or their eggs, or their nestlings, are destroyed by birds
+and beasts of prey; we do not always bear in mind, that though food may be
+now superabundant, it is not so at all seasons of each recurring year.
+
+I should premise that I use the term Struggle for Existence in a large and
+metaphorical sense, including dependence of one being on another, and
+including (which is more important) not only the life of the individual,
+but success in leaving progeny. Two canine animals in a time of dearth, may
+be truly said to struggle with each other which shall get food and live.
+But a plant on the edge of a desert is said to struggle {63} for life
+against the drought, though more properly it should be said to be dependent
+on the moisture. A plant which annually produces a thousand seeds, of which
+on an average only one comes to maturity, may be more truly said to
+struggle with the plants of the same and other kinds which already clothe
+the ground. The missletoe is dependent on the apple and a few other trees,
+but can only in a far-fetched sense be said to struggle with these trees,
+for if too many of these parasites grow on the same tree, it will languish
+and die. But several seedling missletoes, growing close together on the
+same branch, may more truly be said to struggle with each other. As the
+missletoe is disseminated by birds, its existence depends on birds; and it
+may metaphorically be said to struggle with other fruit-bearing plants, in
+order to tempt birds to devour and thus disseminate its seeds rather than
+those of other plants. In these several senses, which pass into each other,
+I use for convenience' sake the general term of struggle for existence.
+
+A struggle for existence inevitably follows from the high rate at which all
+organic beings tend to increase. Every being, which during its natural
+lifetime produces several eggs or seeds, must suffer destruction during
+some period of its life, and during some season or occasional year,
+otherwise, on the principle of geometrical increase, its numbers would
+quickly become so inordinately great that no country could support the
+product. Hence, as more individuals are produced than can possibly survive,
+there must in every case be a struggle for existence, either one individual
+with another of the same species, or with the individuals of distinct
+species, or with the physical conditions of life. It is the doctrine of
+Malthus applied with manifold force to the whole animal and vegetable
+kingdoms; for in this case there {64} can be no artificial increase of
+food, and no prudential restraint from marriage. Although some species may
+be now increasing, more or less rapidly, in numbers, all cannot do so, for
+the world would not hold them.
+
+There is no exception to the rule that every organic being naturally
+increases at so high a rate, that if not destroyed, the earth would soon be
+covered by the progeny of a single pair. Even slow-breeding man has doubled
+in twenty-five years, and at this rate, in a few thousand years, there
+would literally not be standing room for his progeny. Linnæus has
+calculated that if an annual plant produced only two seeds--and there is no
+plant so unproductive as this--and their seedlings next year produced two,
+and so on, then in twenty years there would be a million plants. The
+elephant is reckoned the slowest breeder of all known animals, and I have
+taken some pains to estimate its probable minimum rate of natural increase:
+it will be under the mark to assume that it breeds when thirty years old,
+and goes on breeding till ninety years old, bringing forth three pair of
+young in this interval; if this be so, at the end of the fifth century
+there would be alive fifteen million elephants, descended from the first
+pair.
+
+But we have better evidence on this subject than mere theoretical
+calculations, namely, the numerous recorded cases of the astonishingly
+rapid increase of various animals in a state of nature, when circumstances
+have been favourable to them during two or three following seasons. Still
+more striking is the evidence from our domestic animals of many kinds which
+have run wild in several parts of the world: if the statements of the rate
+of increase of slow-breeding cattle and horses in South America, and
+latterly in Australia, had not been well authenticated, they would have
+been incredible. So it is with plants: cases could be given of {65}
+introduced plants which have become common throughout whole islands in a
+period of less than ten years. Several of the plants, such as the cardoon
+and a tall thistle, now most numerous over the wide plains of La Plata,
+clothing square leagues of surface almost to the exclusion of all other
+plants, have been introduced from Europe; and there are plants which now
+range in India, as I hear from Dr. Falconer, from Cape Comorin to the
+Himalaya, which have been imported from America since its discovery. In
+such cases, and endless instances could be given, no one supposes that the
+fertility of these animals or plants has been suddenly and temporarily
+increased in any sensible degree. The obvious explanation is that the
+conditions of life have been very favourable, and that there has
+consequently been less destruction of the old and young, and that nearly
+all the young have been enabled to breed. In such cases the geometrical
+ratio of increase, the result of which never fails to be surprising, simply
+explains the extraordinarily rapid increase and wide diffusion of
+naturalised productions in their new homes.
+
+In a state of nature almost every plant produces seed, and amongst animals
+there are very few which do not annually pair. Hence we may confidently
+assert, that all plants and animals are tending to increase at a
+geometrical ratio, that all would most rapidly stock every station in which
+they could any how exist, and that the geometrical tendency to increase
+must be checked by destruction at some period of life. Our familiarity with
+the larger domestic animals tends, I think, to mislead us: we see no great
+destruction falling on them, and we forget that thousands are annually
+slaughtered for food, and that in a state of nature an equal number would
+have somehow to be disposed of.
+
+The only difference between organisms which annually {66} produce eggs or
+seeds by the thousand, and those which produce extremely few, is, that the
+slow-breeders would require a few more years to people, under favourable
+conditions, a whole district, let it be ever so large. The condor lays a
+couple of eggs and the ostrich a score, and yet in the same country the
+condor may be the more numerous of the two: the Fulmar petrel lays but one
+egg, yet it is believed to be the most numerous bird in the world. One fly
+deposits hundreds of eggs, and another, like the hippobosca, a single one;
+but this difference does not determine how many individuals of the two
+species can be supported in a district. A large number of eggs is of some
+importance to those species which depend on a rapidly fluctuating amount of
+food, for it allows them rapidly to increase in number. But the real
+importance of a large number of eggs or seeds is to make up for much
+destruction at some period of life; and this period in the great majority
+of cases is an early one. If an animal can in any way protect its own eggs
+or young, a small number may be produced, and yet the average stock be
+fully kept up; but if many eggs or young are destroyed, many must be
+produced, or the species will become extinct. It would suffice to keep up
+the full number of a tree, which lived on an average for a thousand years,
+if a single seed were produced once in a thousand years, supposing that
+this seed were never destroyed, and could be ensured to germinate in a
+fitting place. So that in all cases, the average number of any animal or
+plant depends only indirectly on the number of its eggs or seeds.
+
+In looking at Nature, it is most necessary to keep the foregoing
+considerations always in mind--never to forget that every single organic
+being around us may be said to be striving to the utmost to increase in
+numbers; that each lives by a struggle at some period of {67} its life;
+that heavy destruction inevitably falls either on the young or old, during
+each generation or at recurrent intervals. Lighten any check, mitigate the
+destruction ever so little, and the number of the species will almost
+instantaneously increase to any amount.
+
+The causes which check the natural tendency of each species to increase in
+number are most obscure. Look at the most vigorous species; by as much as
+it swarms in numbers, by so much will its tendency to increase be still
+further increased. We know not exactly what the checks are in even one
+single instance. Nor will this surprise any one who reflects how ignorant
+we are on this head, even in regard to mankind, so incomparably better
+known than any other animal. This subject has been ably treated by several
+authors, and I shall, in my future work, discuss some of the checks at
+considerable length, more especially in regard to the feral animals of
+South America. Here I will make only a few remarks, just to recall to the
+reader's mind some of the chief points. Eggs or very young animals seem
+generally to suffer most, but this is not invariably the case. With plants
+there is a vast destruction of seeds, but, from some observations which I
+have made, I believe that it is the seedlings which suffer most from
+germinating in ground already thickly stocked with other plants. Seedlings,
+also, are destroyed in vast numbers by various enemies; for instance, on a
+piece of ground three feet long and two wide, dug and cleared, and where
+there could be no choking from other plants, I marked all the seedlings of
+our native weeds as they came up, and out of the 357 no less than 295 were
+destroyed, chiefly by slugs and insects. If turf which has long been mown,
+and the case would be the same with turf closely browsed by quadrupeds, be
+let to grow, the more vigorous plants {68} gradually kill the less
+vigorous, though fully grown, plants: thus out of twenty species growing on
+a little plot of turf (three feet by four) nine species perished from the
+other species being allowed to grow up freely.
+
+The amount of food for each species of course gives the extreme limit to
+which each can increase; but very frequently it is not the obtaining food,
+but the serving as prey to other animals, which determines the average
+numbers of a species. Thus, there seems to be little doubt that the stock
+of partridges, grouse, and hares on any large estate depends chiefly on the
+destruction of vermin. If not one head of game were shot during the next
+twenty years in England, and, at the same time, if no vermin were
+destroyed, there would, in all probability, be less game than at present,
+although hundreds of thousands of game animals are now annually killed. On
+the other hand, in some cases, as with the elephant and rhinoceros, none
+are destroyed by beasts of prey: even the tiger in India most rarely dares
+to attack a young elephant protected by its dam.
+
+Climate plays an important part in determining the average numbers of a
+species, and periodical seasons of extreme cold or drought, I believe to be
+the most effective of all checks. I estimated that the winter of 1854-55
+destroyed four-fifths of the birds in my own grounds; and this is a
+tremendous destruction, when we remember that ten per cent, is an
+extraordinarily severe mortality from epidemics with man. The action of
+climate seems at first sight to be quite independent of the struggle for
+existence; but in so far as climate chiefly acts in reducing food, it
+brings on the most severe struggle between the individuals, whether of the
+same or of distinct species, which subsist on the same kind of food. Even
+when climate, for instance extreme cold, {69} acts directly, it will be the
+least vigorous, or those which have got least food through the advancing
+winter, which will suffer most. When we travel from south to north, or from
+a damp region to a dry, we invariably see some species gradually getting
+rarer and rarer, and finally disappearing; and the change of climate being
+conspicuous, we are tempted to attribute the whole effect to its direct
+action. But this is a false view: we forget that each species, even where
+it most abounds, is constantly suffering enormous destruction at some
+period of its life, from enemies or from competitors for the same place and
+food; and if these enemies or competitors be in the least degree favoured
+by any slight change of climate, they will increase in numbers, and, as
+each area is already fully stocked with inhabitants, the other species will
+decrease. When we travel southward and see a species decreasing in numbers,
+we may feel sure that the cause lies quite as much in other species being
+favoured, as in this one being hurt. So it is when we travel northward, but
+in a somewhat lesser degree, for the number of species of all kinds, and
+therefore of competitors, decreases northwards; hence in going northward,
+or in ascending a mountain, we far oftener meet with stunted forms, due to
+the _directly_ injurious action of climate, than we do in proceeding
+southwards or in descending a mountain. When we reach the Arctic regions,
+or snow-capped summits, or absolute deserts, the struggle for life is
+almost exclusively with the elements.
+
+That climate acts in main part indirectly by favouring other species, we
+may clearly see in the prodigious number of plants in our gardens which can
+perfectly well endure our climate, but which never become naturalised, for
+they cannot compete with our native plants nor resist destruction by our
+native animals. {70}
+
+When a species, owing to highly favourable circumstances, increases
+inordinately in numbers in a small tract, epidemics--at least, this seems
+generally to occur with our game animals--often ensue: and here we have a
+limiting check independent of the struggle for life. But even some of these
+so-called epidemics appear to be due to parasitic worms, which have from
+some cause, possibly in part through facility of diffusion amongst the
+crowded animals, been disproportionably favoured: and here comes in a sort
+of struggle between the parasite and its prey.
+
+On the other hand, in many cases, a large stock of individuals of the same
+species, relatively to the numbers of its enemies, is absolutely necessary
+for its preservation. Thus we can easily raise plenty of corn and
+rape-seed, &c., in our fields, because the seeds are in great excess
+compared with the number of birds which feed on them; nor can the birds,
+though having a superabundance of food at this one season, increase in
+number proportionally to the supply of seed, as their numbers are checked
+during winter: but any one who has tried, knows how troublesome it is to
+get seed from a few wheat or other such plants in a garden: I have in this
+case lost every single seed. This view of the necessity of a large stock of
+the same species for its preservation, explains, I believe, some singular
+facts in nature, such as that of very rare plants being sometimes extremely
+abundant in the few spots where they do occur; and that of some social
+plants being social, that is, abounding in individuals, even on the extreme
+confines of their range. For in such cases, we may believe, that a plant
+could exist only where the conditions of its life were so favourable that
+many could exist together, and thus save the species from utter
+destruction. I should add that the good effects of frequent intercrossing,
+and {71} the ill effects of close interbreeding, probably come into play in
+some of these cases; but on this intricate subject I will not here enlarge.
+
+Many cases are on record showing how complex and unexpected are the checks
+and relations between organic beings, which have to struggle together in
+the same country. I will give only a single instance, which, though a
+simple one, has interested me. In Staffordshire, on the estate of a
+relation, where I had ample means of investigation, there was a large and
+extremely barren heath, which had never been touched by the hand of man;
+but several hundred acres of exactly the same nature had been enclosed
+twenty-five years previously and planted with Scotch fir. The change in the
+native vegetation of the planted part of the heath was most remarkable,
+more than is generally seen in passing from one quite different soil to
+another: not only the proportional numbers of the heath-plants were wholly
+changed, but twelve species of plants (not counting grasses and carices)
+flourished in the plantations, which could not be found on the heath. The
+effect on the insects must have been still greater, for six insectivorous
+birds were very common in the plantations, which were not to be seen on the
+heath; and the heath was frequented by two or three distinct insectivorous
+birds. Here we see how potent has been the effect of the introduction of a
+single tree, nothing whatever else having been done, with the exception
+that the land had been enclosed, so that cattle could not enter. But how
+important an element enclosure is, I plainly saw near Farnham, in Surrey.
+Here there are extensive heaths, with a few clumps of old Scotch firs on
+the distant hill-tops: within the last ten years large spaces have been
+enclosed, and self-sown firs are now springing up in multitudes, so close
+together that all cannot live. {72} When I ascertained that these young
+trees had not been sown or planted, I was so much surprised at their
+numbers that I went to several points of view, whence I could examine
+hundreds of acres of the unenclosed heath, and literally I could not see a
+single Scotch fir, except the old planted clumps. But on looking closely
+between the stems of the heath, I found a multitude of seedlings and little
+trees, which had been perpetually browsed down by the cattle. In one square
+yard, at a point some hundred yards distant from one of the old clumps, I
+counted thirty-two little trees; and one of them, with twenty-six rings of
+growth, had during many years tried to raise its head above the stems of
+the heath, and had failed. No wonder that, as soon as the land was
+enclosed, it became thickly clothed with vigorously growing young firs. Yet
+the heath was so extremely barren and so extensive that no one would ever
+have imagined that cattle would have so closely and effectually searched it
+for food.
+
+Here we see that cattle absolutely determine the existence of the Scotch
+fir; but 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 number 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 birds. Hence, if certain insectivorous birds
+(whose numbers are probably regulated by hawks or beasts of prey) were to
+increase in Paraguay, the flies would decrease--then cattle and horses
+would became feral, and this would certainly greatly {73} alter (as indeed
+I have observed in parts of South America) the vegetation: this again would
+largely affect the insects; and this, as we just have seen in
+Staffordshire, the insectivorous birds, and so onwards in ever-increasing
+circles of complexity. We began this series by insectivorous birds, and we
+have ended with them, Not that in nature the relations can ever be as
+simple as this. Battle within battle must ever be recurring with varying
+success; and yet in the long-run the forces are so nicely balanced, that
+the face of nature remains uniform for long periods of time, though
+assuredly the merest trifle would often give the victory to one organic
+being over another. Nevertheless so profound is our ignorance, and so high
+our presumption, that we marvel when we hear of the extinction of an
+organic being; and as we do not see the cause, we invoke cataclysms to
+desolate the world, or invent laws on the duration of the forms of life!
+
+I am tempted to give one more instance showing how plants and animals, most
+remote in the scale of nature, are bound together by a web of complex
+relations. I shall hereafter have occasion to show that the exotic Lobelia
+fulgens, in this part of England, is never visited by insects, and
+consequently, from its peculiar structure, never can set a seed. Many of
+our orchidaceous plants absolutely require the visits of moths to remove
+their pollen-masses and thus to fertilise them. I have, also, reason to
+believe that humble-bees are indispensable to the fertilisation of the
+heartsease (Viola tricolor), for other bees do not visit this flower. From
+experiments which I have lately tried, I have found that the visits of bees
+are necessary for the fertilisation of some kinds of clover; but
+humble-bees alone visit the red clover (Trifolium pratense), as other bees
+cannot reach the nectar. Hence I have very little doubt, that if the {74}
+whole genus of humble-bees became extinct or very rare in England, the
+heartsease and red clover would become very rare, or wholly disappear. The
+number of humble-bees in any district depends in a great degree on the
+number of field-mice, which destroy their combs and nests; and Mr. H.
+Newman, who has long attended to the habits of humble-bees, believes that
+"more than two-thirds of them are thus destroyed all over England." Now the
+number of mice is largely dependent, as every one knows, on the number of
+cats; and Mr. Newman says, "Near villages and small towns I have found the
+nests of humble-bees more numerous than elsewhere, which I attribute to the
+number of cats that destroy the mice." Hence it is quite credible that the
+presence of a feline animal in large numbers in a district might determine,
+through the intervention first of mice and then of bees, the frequency of
+certain flowers in that district!
+
+In the case of every species, many different checks, acting at different
+periods of life, and during different seasons or years, probably come into
+play; some one check or some few being generally the most potent, but all
+concur in determining the average number or even the existence of the
+species. In some cases it can be shown that widely-different checks act on
+the same species in different districts. When we look at the plants and
+bushes clothing an entangled bank, we are tempted to attribute their
+proportional numbers and kinds to what we call chance. But how false a view
+is this! Every one has heard that when an American forest is cut down, a
+very different vegetation springs up; but it has been observed that ancient
+Indian ruins in the Southern United States, which must formerly have been
+cleared of trees, now display the same beautiful diversity and proportion
+of kinds as in the surrounding {75} virgin forests. What a struggle between
+the several kinds of trees must here have gone on during long centuries,
+each annually scattering its seeds by the thousand; what war between insect
+and insect--between insects, snails, and other animals with birds and
+beasts of prey--all striving to increase, and all feeding on each other or
+on the trees or their seeds and seedlings, or on the other plants which
+first clothed the ground and thus checked the growth of the trees! Throw up
+a handful of feathers, and all must fall to the ground according to
+definite laws; but how simple is this problem compared to the action and
+reaction of the innumerable plants and animals which have determined, in
+the course of centuries, the proportional numbers and kinds of trees now
+growing on the old Indian ruins!
+
+The dependency of one organic being on another, as of a parasite on its
+prey, lies generally between beings remote in the scale of nature. This is
+often the case with those which may strictly be said to struggle with each
+other for existence, as in the case of locusts and grass-feeding
+quadrupeds. But the struggle almost invariably will be most severe between
+the individuals of the same species, for they frequent the same districts,
+require the same food, and are exposed to the same dangers. In the case of
+varieties of the same species, the struggle will generally be almost
+equally severe, and we sometimes see the contest soon decided; for
+instance, if several varieties of wheat be sown together, and the mixed
+seed be resown, some of the varieties which best suit the soil or climate,
+or are naturally the most fertile, will beat the others and so yield more
+seed, and will consequently in a few years quite supplant the other
+varieties. To keep up a mixed stock of even such extremely close varieties
+as the variously {76} coloured sweet-peas, they must be each year harvested
+separately, and the seed then mixed in due proportion, otherwise the weaker
+kinds will steadily decrease in numbers and disappear. So again with the
+varieties of sheep: it has been asserted that certain mountain-varieties
+will starve out other mountain-varieties, so that they cannot be kept
+together. The same result has followed from keeping together different
+varieties of the medicinal leech. It may even be doubted whether the
+varieties of any one of our domestic plants or animals have so exactly the
+same strength, habits, and constitution, that the original proportions of a
+mixed stock could be kept up for half-a-dozen generations, if they were
+allowed to struggle together, like beings in a state of nature, and if the
+seed or young were not annually sorted.
+
+As species of the same genus have usually, though by no means invariably,
+some similarity in habits and constitution, and always in structure, the
+struggle will generally be more severe between species of the same genus,
+when they come into competition with each other, than between species of
+distinct genera. We see this in the recent extension over parts of the
+United States of one species of swallow having caused the decrease of
+another species. The recent increase of the missel-thrush in parts of
+Scotland has caused the decrease of the song-thrush. How frequently we hear
+of one species of rat taking the place of another species under the most
+different climates! In Russia the small Asiatic cockroach has everywhere
+driven before it its great congener. One species of charlock will supplant
+another, and so in other cases. We can dimly see why the competition should
+be most severe between allied forms, which fill nearly the same place in
+the economy of nature; {77} but probably in no one case could we precisely
+say why one species has been victorious over another in the great battle of
+life.
+
+A corollary of the highest importance may be deduced from the foregoing
+remarks, namely, that the structure of every organic being is related, in
+the most essential yet often hidden manner, to that of all other organic
+beings, with which it comes into competition for food or residence, or from
+which it has to escape, or on which it preys. This is obvious in the
+structure of the teeth and talons of the tiger; and in that of the legs and
+claws of the parasite which clings to the hair on the tiger's body. But in
+the beautifully plumed seed of the dandelion, and in the flattened and
+fringed legs of the water-beetle, the relation seems at first confined to
+the elements of air and water. Yet the advantage of plumed seeds no doubt
+stands in the closest relation to the land being already thickly clothed by
+other plants; so that the seeds may be widely distributed and fall on
+unoccupied ground. In the water-beetle, the structure of its legs, so well
+adapted for diving, allows it to compete with other aquatic insects, to
+hunt for its own prey, and to escape serving as prey to other animals.
+
+The store of nutriment laid up within the seeds of many plants seems at
+first sight to have no sort of relation to other plants. But from the
+strong growth of young plants produced from such seeds (as peas and beans),
+when sown in the midst of long grass, I suspect that the chief use of the
+nutriment in the seed is to favour the growth of the young seedling, whilst
+struggling with other plants growing vigorously all around.
+
+Look at a plant in the midst of its range, why does it not double or
+quadruple its numbers? We know {78} that it can perfectly well withstand a
+little more heat or cold, dampness or dryness, for elsewhere it ranges into
+slightly hotter or colder, damper or drier districts. In this case we can
+clearly see that if we wished in imagination to give the plant the power of
+increasing in number, we should have to give it some advantage over its
+competitors, or over the animals which preyed on it. On the confines of its
+geographical range, a change of constitution with respect to climate would
+clearly be an advantage to our plant; but we have reason to believe that
+only a few plants or animals range so far, that they are destroyed by the
+rigour of the climate alone. Not until we reach the extreme confines of
+life, in the Arctic regions or on the borders of an utter desert, will
+competition cease. The land may be extremely cold or dry, yet there will be
+competition between some few species, or between the individuals of the
+same species, for the warmest or dampest spots.
+
+Hence, also, we can see that when a plant or animal is placed in a new
+country amongst new competitors, though the climate may be exactly the same
+as in its former home, yet the conditions of its life will generally be
+changed in an essential manner. If we wished to increase its average
+numbers in its new home, we should have to modify it in a different way to
+what we should have done in its native country; for we should have to give
+it some advantage over a different set of competitors or enemies.
+
+It is good thus to try in our imagination to give any form some advantage
+over another. Probably in no single instance should we know what to do, so
+as to succeed. It will convince us of our ignorance on the mutual relations
+of all organic beings; a conviction as necessary, as it seems to be
+difficult to acquire. All that we can do, is to keep steadily in mind that
+each {79} organic being is striving to increase at a geometrical ratio;
+that each at some period of its life, during some season of the year,
+during each generation or at intervals, has to struggle for life, and to
+suffer great destruction. 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.
+
+       *       *       *       *       *
+
+
+{80}
+
+CHAPTER IV.
+
+NATURAL SELECTION.
+
+    Natural Selection--its power compared with man's selection--its power
+    on characters of trifling importance--its power at all ages and on both
+    sexes--Sexual Selection--On the generality of intercrosses between
+    individuals of the same species--Circumstances favourable and
+    unfavourable to Natural Selection, namely, intercrossing, isolation,
+    number of individuals--Slow action--Extinction caused by Natural
+    Selection--Divergence of Character, related to the diversity of
+    inhabitants of any small area, and to naturalisation--Action of Natural
+    Selection, through Divergence of Character and Extinction, on the
+    descendants from a common parent--Explains the Grouping of all organic
+    beings.
+
+How will the struggle for existence, discussed too briefly in the last
+chapter, act in regard to variation? Can the principle of selection, which
+we have seen is so potent in the hands of man, apply in nature? I think we
+shall see that it can act most effectually. Let it be borne in mind in what
+an endless number of strange peculiarities our domestic productions, and,
+in a lesser degree, those under nature, vary; and how strong the hereditary
+tendency is. Under domestication, it may be truly said that the whole
+organisation becomes in some degree plastic. Let it be borne in mind how
+infinitely complex and close-fitting are the mutual relations of all
+organic beings to each other and to their physical conditions of life. Can
+it, then, be thought improbable, seeing that variations useful to man have
+undoubtedly occurred, that other variations useful in some way to each
+being in the great and complex battle of life, should sometimes occur in
+the course of thousands of generations? If such do occur, can we doubt {81}
+(remembering that many more individuals are born than can possibly survive)
+that individuals having any advantage, however slight, over others, would
+have the best chance of surviving and of procreating their kind? On the
+other hand, we may feel sure that any variation in the least degree
+injurious would be rigidly destroyed. This preservation of favourable
+variations and the rejection of injurious variations, I call Natural
+Selection. Variations neither useful nor injurious would not be affected by
+natural selection, and would be left a fluctuating element, as perhaps we
+see in the species called polymorphic.
+
+We shall best understand the probable course of natural selection by taking
+the case of a country undergoing some physical change, for instance, of
+climate. The proportional numbers of its inhabitants would almost
+immediately undergo a change, and some species might become extinct. We may
+conclude, from what we have seen of the intimate and complex manner in
+which the inhabitants of each country are bound together, that any change
+in the numerical proportions of some of the inhabitants, independently of
+the change of climate itself, would seriously affect many of the others. If
+the country were open on its borders, new forms would certainly immigrate,
+and this also would seriously disturb the relations of some of the former
+inhabitants. Let it be remembered how powerful the influence of a single
+introduced tree or mammal has been shown to be. But in the case of an
+island, or of a country partly surrounded by barriers, into which new and
+better adapted forms could not freely enter, we should then have places in
+the economy of nature which would assuredly be better filled up, if some of
+the original inhabitants were in some manner modified; for, had the area
+been open to immigration, these same {82} places would have been seized on
+by intruders. In such case, every slight modification, which in the course
+of ages chanced to arise, and which in any way favoured the individuals of
+any of the species, by better adapting them to their altered conditions,
+would tend to be preserved; and natural selection would thus have free
+scope for the work of improvement.
+
+We have reason to believe, as stated in the first chapter, that a change in
+the conditions of life, by specially acting on the reproductive system,
+causes or increases variability; and in the foregoing case the conditions
+of life are supposed to have undergone a change, and this would manifestly
+be favourable to natural selection, by giving a better chance of profitable
+variations occurring; and unless profitable variations do occur, natural
+selection can do nothing. Not that, as I believe, any extreme amount of
+variability is necessary; as man can certainly produce great results by
+adding up in any given direction mere individual differences, so could
+Nature, but far more easily, from having incomparably longer time at her
+disposal. Nor do I believe that any great physical change, as of climate,
+or any unusual degree of isolation to check immigration, is actually
+necessary to produce new and unoccupied places for natural selection to
+fill up by modifying and improving some of the varying inhabitants. For as
+all the inhabitants of each country are struggling together with nicely
+balanced forces, extremely slight modifications in the structure or habits
+of one inhabitant would often give it an advantage over others; and still
+further modifications of the same kind would often still further increase
+the advantage. No country can be named in which all the native inhabitants
+are now so perfectly adapted to each other and to the physical conditions
+under which they live, that none of {83} them could anyhow be improved; for
+in all countries, the natives have been so far conquered by naturalised
+productions, that they have allowed foreigners to take firm possession of
+the land. And as foreigners have thus everywhere beaten some of the
+natives, we may safely conclude that the natives might have been modified
+with advantage, so as to have better resisted such intruders.
+
+As man can produce and certainly has produced a great result by his
+methodical and unconscious means of selection, what may not Nature effect?
+Man can act only on external and visible characters: Nature cares nothing
+for appearances, except in so far as they may be useful to any being. She
+can act on every internal organ, on every shade of constitutional
+difference, on the whole machinery of life. Man selects only for his own
+good; Nature only for that of the being which she tends. Every selected
+character is fully exercised by her; and the being is placed under
+well-suited conditions of life. Man keeps the natives of many climates in
+the same country; he seldom exercises each selected character in some
+peculiar and fitting manner; he feeds a long and a short beaked pigeon on
+the same food; he does not exercise a long-backed or long-legged quadruped
+in any peculiar manner; he exposes sheep with long and short wool to the
+same climate. He does not allow the most vigorous males to struggle for the
+females. He does not rigidly destroy all inferior animals, but protects
+during each varying season, as far as lies in his power, all his
+productions. He often begins his selection by some half-monstrous form; or
+at least by some modification prominent enough to catch his eye, or to be
+plainly useful to him. Under nature, the slightest difference of structure
+or constitution may well turn the nicely-balanced scale in the struggle for
+life, and so be {84} preserved. How fleeting are the wishes and efforts of
+man! how short his time! and consequently how poor will his products be,
+compared with those accumulated by Nature during whole geological periods.
+Can we wonder, then, that Nature's productions should be far "truer" in
+character than man's productions; that they should be infinitely better
+adapted to the most complex conditions of life, and should plainly bear the
+stamp of far higher workmanship?
+
+It may metaphorically be said that natural selection is daily and hourly
+scrutinising, throughout the world, every variation, even the slightest;
+rejecting that which is bad, preserving and adding up all that is good;
+silently and insensibly working, whenever and wherever opportunity offers,
+at the improvement of each organic being in relation to its organic and
+inorganic conditions of life. We see nothing of these slow changes in
+progress, until the hand of time has marked the long lapse of ages, and
+then so imperfect is our view into long past geological ages, that we only
+see that the forms of life are now different from what they formerly were.
+
+Although natural selection can act only through and for the good of each
+being, yet characters and structures, which we are apt to consider as of
+very trifling importance, may thus be acted on. When we see leaf-eating
+insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in
+winter, the red-grouse the colour of heather, and the black-grouse that of
+peaty earth, we must believe that these tints are of service to these birds
+and insects in preserving them from danger. Grouse, if not destroyed at
+some period of their lives, would increase in countless numbers; they are
+known to suffer largely from birds of prey; and hawks are guided by
+eyesight to their prey--so much so, that on {85} parts of the Continent
+persons are warned not to keep white pigeons, as being the most liable to
+destruction. Hence I can see no reason to doubt that natural selection
+might be most effective in giving the proper colour to each kind of grouse,
+and in keeping that colour, when once acquired, true and constant. Nor
+ought we to think that the occasional destruction of an animal of any
+particular colour would produce little effect: we should remember how
+essential it is in a flock of white sheep to destroy every lamb with the
+faintest trace of black. In plants the down on the fruit and the colour of
+the flesh are considered by botanists as characters of the most trifling
+importance: yet we hear from an excellent horticulturist, Downing, that in
+the United States smooth-skinned fruits suffer far more from a beetle, a
+curculio, than those with down; that purple plums suffer far more from a
+certain disease than yellow plums; whereas another disease attacks
+yellow-fleshed peaches far more than those with other coloured flesh. If,
+with all the aids of art, these slight differences make a great difference
+in cultivating the several varieties, assuredly, in a state of nature,
+where the trees would have to struggle with other trees and with a host of
+enemies, such differences would effectually settle which variety, whether a
+smooth or downy, a yellow or purple fleshed fruit, should succeed.
+
+In looking at many small points of difference between species, which, as
+far as our ignorance permits us to judge, seem quite unimportant, we must
+not forget that climate, food, &c., probably produce some slight and direct
+effect. It is, however, far more necessary to bear in mind that there are
+many unknown laws of correlation of growth, which, when one part of the
+organisation is modified through variation, and the modifications are
+accumulated by natural selection for {86} the good of the being, will cause
+other modifications, often of the most unexpected nature.
+
+As we see that those variations which under domestication appear at any
+particular period of life, tend to reappear in the offspring at the same
+period;--for instance, in the seeds of the many varieties of our culinary
+and agricultural plants; in the caterpillar and cocoon stages of the
+varieties of the silkworm; in the eggs of poultry, and in the colour of the
+down of their chickens; in the horns of our sheep and cattle when nearly
+adult;--so in a state of nature, natural selection will be enabled to act
+on and modify organic beings at any age, by the accumulation of variations
+profitable at that age, and by their inheritance at a corresponding age. If
+it profit a plant to have its seeds more and more widely disseminated by
+the wind, I can see no greater difficulty in this being effected through
+natural selection, than in the cotton-planter increasing and improving by
+selection the down in the pods on his cotton-trees. Natural selection may
+modify and adapt the larva of an insect to a score of contingencies, wholly
+different from those which concern the mature insect. These modifications
+will no doubt affect, through the laws of correlation, the structure of the
+adult; and probably in the case of those insects which live only for a few
+hours, and which never feed, a large part of their structure is merely the
+correlated result of successive changes in the structure of their larvæ.
+So, conversely, modifications in the adult will probably often affect the
+structure of the larva; but in all cases natural selection will ensure that
+modifications consequent on other modifications at a different period of
+life, shall not be in the least degree injurious: for if they became so,
+they would cause the extinction of the species.
+
+Natural selection will modify the structure of the {87} young in relation
+to the parent, and of the parent in relation to the young. In social
+animals it will adapt the structure of each individual for the benefit of
+the community; if each in consequence profits by the selected change. What
+natural selection cannot do, is to modify the structure of one species,
+without giving it any advantage, for the good of another species; and
+though statements to this effect may be found in works of natural history,
+I cannot find one case which will bear investigation. A structure used only
+once in an animal's whole life, if of high importance to it, might be
+modified to any extent by natural selection; for instance, the great jaws
+possessed by certain insects, used exclusively for opening the cocoon--or
+the hard tip to the beak of nestling birds, used for breaking the egg. It
+has been asserted, that of the best short-beaked tumbler-pigeons more
+perish in the egg than are able to get out of it; so that fanciers assist
+in the act of hatching. Now, if nature had to make the beak of a full-grown
+pigeon very short for the bird's own advantage, the process of modification
+would be very slow, and there would be simultaneously the most rigorous
+selection of the young birds within the egg, which had the most powerful
+and hardest beaks, for all with weak beaks would inevitably perish: or,
+more delicate and more easily broken shells might be selected, the
+thickness of the shell being known to vary like every other structure.
+
+
+
+_Sexual Selection._--Inasmuch as peculiarities often appear under
+domestication in one sex and become hereditarily attached to that sex, the
+same fact probably occurs under nature, and if so, natural selection will
+be able to modify one sex in its functional relations to the other sex, or
+in relation to wholly different habits of life in the two sexes, as is
+sometimes the case {88} with insects. And this leads me to say a few words
+on what I call Sexual Selection. This depends, not on a struggle for
+existence, but on a struggle between the males for possession of the
+females; the result is not death to the unsuccessful competitor, but few or
+no offspring. Sexual selection is, therefore, less rigorous than natural
+selection. Generally, the most vigorous males, those which are best fitted
+for their places in nature, will leave most progeny. But in many cases,
+victory depends not on general vigour, but on having special weapons,
+confined to the male sex. A hornless stag or spurless cock would have a
+poor chance of leaving offspring. Sexual selection by always allowing the
+victor to breed might surely give indomitable courage, length to the spur,
+and strength to the wing to strike in the spurred leg, as well as the
+brutal cock-fighter, who knows well that he can improve his breed by
+careful selection of the best cocks. How low in the scale of nature the law
+of battle descends, I know not; male alligators have been described as
+fighting, bellowing, and whirling round, like Indians in a war-dance, for
+the possession of the females; male salmons have been seen fighting all day
+long; male stag-beetles often bear wounds from the huge mandibles of other
+males. The war is, perhaps, severest between the males of polygamous
+animals, and these seem oftenest provided with special weapons. The males
+of carnivorous animals are already well armed; though to them and to
+others, special means of defence may be given through means of sexual
+selection, as the mane to the lion, the shoulder-pad to the boar, and the
+hooked jaw to the male salmon; for the shield may be as important for
+victory, as the sword or spear.
+
+Amongst birds, the contest is often of a more peaceful character. All those
+who have attended to the subject, {89} believe that there is the severest
+rivalry between the males of many species to attract by singing the
+females. The rock-thrush of Guiana, birds of Paradise, and some others,
+congregate; and successive males display their gorgeous plumage and perform
+strange antics before the females, which, standing by as spectators, at
+last choose the most attractive partner. Those who have closely attended to
+birds in confinement well know that they often take individual preferences
+and dislikes: thus Sir R. Heron has described how one pied peacock was
+eminently attractive to all his hen birds. It may appear childish to
+attribute any effect to such apparently weak means: I cannot here enter on
+the details necessary to support this view; but if man can in a short time
+give elegant carriage and beauty to his bantams, according to his standard
+of beauty, I can see no good reason to doubt that female birds, by
+selecting, during thousands of generations, the most melodious or beautiful
+males, according to their standard of beauty, might produce a marked
+effect. I strongly suspect that some well-known laws, with respect to the
+plumage of male and female birds, in comparison with the plumage of the
+young, can be explained on the view of plumage having been chiefly modified
+by sexual selection, acting when the birds have come to the breeding age or
+during the breeding season; the modifications thus produced being inherited
+at corresponding ages or seasons, either by the males alone, or by the
+males and females; but I have not space here to enter on this subject.
+
+Thus it is, as I believe, that when the males and females of any animal
+have the same general habits of life, but differ in structure, colour, or
+ornament, such differences have been mainly caused by sexual selection;
+that is, individual males have had, in successive generations, some slight
+advantage over other {90} males, in their weapons, means of defence, or
+charms; and have transmitted these advantages to their male offspring. Yet,
+I would not wish to attribute all such sexual differences to this agency:
+for we see peculiarities arising and becoming attached to the male sex in
+our domestic animals (as the wattle in male carriers, horn-like
+protuberances in the cocks of certain fowls, &c.), which we cannot believe
+to be either useful to the males in battle, or attractive to the females.
+We see analogous cases under nature, for instance, the tuft of hair on the
+breast of the turkey-cock, which can hardly be either useful or ornamental
+to this bird;--indeed, had the tuft appeared under domestication, it would
+have been called a monstrosity.
+
+
+
+_Illustrations of the action of Natural Selection._--In order to make it
+clear how, as I believe, natural selection acts, I must beg permission to
+give one or two imaginary illustrations. Let us take the case of a wolf,
+which preys on various animals, securing some by craft, some by strength,
+and some by fleetness; and let us suppose that the fleetest prey, a deer
+for instance, had from any change in the country increased in numbers, or
+that other prey had decreased in numbers, during that season of the year
+when the wolf is hardest pressed for food. I can under such circumstances
+see no reason to doubt that the swiftest and slimmest wolves would have the
+best chance of surviving, and so be preserved or selected,--provided always
+that they retained strength to master their prey at this or at some other
+period of the year, when they might be compelled to prey on other animals.
+I can see no more reason to doubt this, than that man can improve the
+fleetness of his greyhounds by careful and methodical selection, or by that
+unconscious selection which results from each man trying {91} to keep the
+best dogs without any thought of modifying the breed.
+
+Even without any change in the proportional numbers of the animals on which
+our wolf preyed, a cub might be born with an innate tendency to pursue
+certain kinds of prey. Nor can this be thought very improbable; for we
+often observe great differences in the natural tendencies of our domestic
+animals; one cat, for instance, taking to catch rats, another mice; one
+cat, according to Mr. St. John, bringing home winged game, another hares or
+rabbits, and another hunting on marshy ground and almost nightly catching
+woodcocks or snipes. The tendency to catch rats rather than mice is known
+to be inherited. Now, if any slight innate change of habit or of structure
+benefited an individual wolf, it would have the best chance of surviving
+and of leaving offspring. Some of its young would probably inherit the same
+habits or structure, and by the repetition of this process, a new variety
+might be formed which would either supplant or coexist with the parent form
+of wolf. Or, again, the wolves inhabiting a mountainous district, and those
+frequenting the lowlands, would naturally be forced to hunt different prey;
+and from the continued preservation of the individuals best fitted for the
+two sites, two varieties might slowly be formed. These varieties would
+cross and blend where they met; but to this subject of intercrossing we
+shall soon have to return. I may add, that, according to Mr. Pierce, there
+are two varieties of the wolf inhabiting the Catskill Mountains in the
+United States, one with a light greyhound-like form, which pursues deer,
+and the other more bulky, with shorter legs, which more frequently attacks
+the shepherd's flocks.
+
+Let us now take a more complex case. Certain plants excrete a sweet juice,
+apparently for the sake of eliminating something injurious from their sap:
+this is {92} effected by glands at the base of the stipules in some
+Leguminosæ, and at the back of the leaf of the common laurel. This juice,
+though small in quantity, is greedily sought by insects. Let us now suppose
+a little sweet juice or nectar to be excreted by the inner bases of the
+petals of a flower. In this case insects in seeking the nectar would get
+dusted with pollen, and would certainly often transport the pollen from one
+flower to the stigma of another flower. The flowers of two distinct
+individuals of the same species would thus get crossed; and the act of
+crossing, we have good reason to believe (as will hereafter be more fully
+alluded to), would produce very vigorous seedlings, which consequently
+would have the best chance of flourishing and surviving. Some of these
+seedlings would probably inherit the nectar-excreting power. Those
+individual flowers which had the largest glands or nectaries, and which
+excreted most nectar, would be oftenest visited by insects, and would be
+oftenest crossed; and so in the long-run would gain the upper hand. Those
+flowers, also, which had their stamens and pistils placed, in relation to
+the size and habits of the particular insects which visited them, so as to
+favour in any degree the transportal of their pollen from flower to flower,
+would likewise be favoured or selected. We might have taken the case of
+insects visiting flowers for the sake of collecting pollen instead of
+nectar; and as pollen is formed for the sole object of fertilisation, its
+destruction appears a simple loss to the plant; yet if a little pollen were
+carried, at first occasionally and then habitually, by the pollen-devouring
+insects from flower to flower, and a cross thus effected, although
+nine-tenths of the pollen were destroyed, it might still be a great gain to
+the plant; and those individuals which produced more and more pollen, and
+had larger and larger anthers, would be selected. {93}
+
+When our plant, by this process of the continued preservation or natural
+selection of more and more attractive flowers, had been rendered highly
+attractive to insects, they would, unintentionally on their part, regularly
+carry pollen from flower to flower; and that they can most effectually do
+this, I could easily show by many striking instances. I will give only
+one--not as a very striking case, but as likewise illustrating one step in
+the separation of the sexes of plants, presently to be alluded to. Some
+holly-trees bear only male flowers, which have four stamens producing a
+rather small quantity of pollen, and a rudimentary pistil; other
+holly-trees bear only female flowers; these have a full-sized pistil, and
+four stamens with shrivelled anthers, in which not a grain of pollen can be
+detected. Having found a female tree exactly sixty yards from a male tree,
+I put the stigmas of twenty flowers, taken from different branches, under
+the microscope, and on all, without exception, there were pollen-grains,
+and on some a profusion of pollen. As the wind had set for several days
+from the female to the male tree, the pollen could not thus have been
+carried. The weather had been cold and boisterous, and therefore not
+favourable to bees, nevertheless every female flower which I examined had
+been effectually fertilised by the bees, accidentally dusted with pollen,
+having flown from tree to tree in search of nectar. But to return to our
+imaginary case: as soon as the plant had been rendered so highly attractive
+to insects that pollen was regularly carried from flower to flower, another
+process might commence. No naturalist doubts the advantage of what has been
+called the "physiological division of labour;" hence we may believe that it
+would be advantageous to a plant to produce stamens alone in one flower or
+on one whole plant, and pistils alone in {94} another flower or on another
+plant. In plants under culture and placed under new conditions of life,
+sometimes the male organs and sometimes the female organs become more or
+less impotent; now if we suppose this to occur in ever so slight a degree
+under nature, then as pollen is already carried regularly from flower to
+flower, and as a more complete separation of the sexes of our plant would
+be advantageous on the principle of the division of labour, individuals
+with this tendency more and more increased, would be continually favoured
+or selected, until at last a complete separation of the sexes would be
+effected.
+
+Let us now turn to the nectar-feeding insects in our imaginary case: we may
+suppose the plant of which we have been slowly increasing the nectar by
+continued selection, to be a common plant; and that certain insects
+depended in main part on its nectar for food. I could give many facts,
+showing how anxious bees are to save time; for instance, their habit of
+cutting holes and sucking the nectar at the bases of certain flowers, which
+they can, with a very little more trouble, enter by the mouth. Bearing such
+facts in mind, I can see no reason to doubt that an accidental deviation in
+the size and form of the body, or in the curvature and length of the
+proboscis, &c., far too slight to be appreciated by us, might profit a bee
+or other insect, so that an individual so characterised would be able to
+obtain its food more quickly, and so have a better chance of living and
+leaving descendants. Its descendants would probably inherit a tendency to a
+similar slight deviation of structure. The tubes of the corollas of the
+common red and incarnate clovers (Trifolium pratense and incarnatum) do not
+on a hasty glance appear to differ in length; yet the hive-bee can easily
+suck the nectar out of the incarnate clover, but not out of the common red
+{95} clover, which is visited by humble-bees alone; so that whole fields of
+the red clover offer in vain an abundant supply of precious nectar to the
+hive-bee. Thus it might be a great advantage to the hive-bee to have a
+slightly longer or differently constructed proboscis. On the other hand, I
+have found by experiment that the fertility of clover depends on bees
+visiting and moving parts of the corolla, so as to push the pollen on to
+the stigmatic surface. Hence, again, if humble-bees were to become rare in
+any country, it might be a great advantage to the red clover to have a
+shorter or more deeply divided tube to its corolla, so that the hive-bee
+could visit its flowers. Thus I can understand how a flower and a bee might
+slowly become, either simultaneously or one after the other, modified and
+adapted in the most perfect manner to each other, by the continued
+preservation of individuals presenting mutual and slightly favourable
+deviations of structure.
+
+I am well aware that this doctrine of natural selection, exemplified in the
+above imaginary instances, is open to the same objections which were at
+first urged against Sir Charles Lyell's noble views on "the modern changes
+of the earth, as illustrative of geology;" but we now seldom hear the
+action, for instance, of the coast-waves, called a trifling and
+insignificant cause, when applied to the excavation of gigantic valleys or
+to the formation of the longest lines of inland cliffs. Natural selection
+can act only by the preservation and accumulation of infinitesimally small
+inherited modifications, each profitable to the preserved being; and as
+modern geology has almost banished such views as the excavation of a great
+valley by a single diluvial wave, so will natural selection, if it be a
+true principle, banish the belief of the continued creation of new organic
+{96} beings, or of any great and sudden modification in their structure.
+
+
+
+_On the Intercrossing of Individuals._--I must here introduce a short
+digression. In the case of animals and plants with separated sexes, it is
+of course obvious that two individuals must always (with the exception of
+the curious and not well-understood cases of parthenogenesis) unite for
+each birth; but in the case of hermaphrodites this is far from obvious.
+Nevertheless I am strongly inclined to believe that with all hermaphrodites
+two individuals, either occasionally or habitually, concur for the
+reproduction of their kind. This view was first suggested by Andrew Knight.
+We shall presently see its importance; but I must here treat the subject
+with extreme brevity, though I have the materials prepared for an ample
+discussion. All vertebrate animals, all insects, and some other large
+groups of animals, pair for each birth. Modern research has much diminished
+the number of supposed hermaphrodites, and of real hermaphrodites a large
+number pair; that is, two individuals regularly unite for reproduction,
+which is all that concerns us. But still there are many hermaphrodite
+animals which certainly do not habitually pair, and a vast majority of
+plants are hermaphrodites. What reason, it may be asked, is there for
+supposing in these cases that two individuals ever concur in reproduction?
+As it is impossible here to enter on details, I must trust to some general
+considerations alone.
+
+In the first place, I have collected so large a body of facts, showing, in
+accordance with the almost universal belief of breeders, that with animals
+and plants a cross between different varieties, or between individuals of
+the same variety but of another strain, gives vigour and {97} fertility to
+the offspring; and on the other hand, that _close_ interbreeding diminishes
+vigour and fertility; that these facts alone incline me to believe that it
+is a general law of nature (utterly ignorant though we be of the meaning of
+the law) that no organic being self-fertilises itself for an eternity of
+generations; but that a cross with another individual is
+occasionally--perhaps at very long intervals--indispensable.
+
+On the belief that this is a law of nature, we can, I think, understand
+several large classes of facts, such as the following, which on any other
+view are inexplicable. Every hybridizer knows how unfavourable exposure to
+wet is to the fertilisation of a flower, yet what a multitude of flowers
+have their anthers and stigmas fully exposed to the weather! but if an
+occasional cross be indispensable, the fullest freedom for the entrance of
+pollen from another individual will explain this state of exposure, more
+especially as the plant's own anthers and pistil generally stand so close
+together that self-fertilisation seems almost inevitable. Many flowers, on
+the other hand, have their organs of fructification closely enclosed, as in
+the great papilionaceous or pea-family; but in several, perhaps in all,
+such flowers, there is a very curious adaptation between the structure of
+the flower and the manner in which bees suck the nectar; for, in doing
+this, they either push the flower's own pollen on the stigma, or bring
+pollen from another flower. So necessary are the visits of bees to
+papilionaceous flowers, that I have found, by experiments published
+elsewhere, that their fertility is greatly diminished if these visits be
+prevented. Now, it is scarcely possible that bees should fly from flower to
+flower, and not carry pollen from one to the other, to the great good, as I
+believe, of the plant. Bees will act like a camel-hair pencil, and it is
+quite sufficient just to touch the anthers of {98} one flower and then the
+stigma of another with the same brush to ensure fertilisation; but it must
+not be supposed that bees would thus produce a multitude of hybrids between
+distinct species; for if you bring on the same brush a plant's own pollen
+and pollen from another species, the former will have such a prepotent
+effect, that it will invariably and completely destroy, as has been shown
+by Gärtner, any influence from the foreign pollen.
+
+When the stamens of a flower suddenly spring towards the pistil, or slowly
+move one after the other towards it, the contrivance seems adapted solely
+to ensure self-fertilisation; and no doubt it is useful for this end: but,
+the agency of insects is often required to cause the stamens to spring
+forward, as Kölreuter has shown to be the case with the barberry; and in
+this very genus, which seems to have a special contrivance for
+self-fertilisation, it is well known that if closely-allied forms or
+varieties are planted near each other, it is hardly possible to raise pure
+seedlings, so largely do they naturally cross. In many other cases, far
+from there being any aids for self-fertilisation, there are special
+contrivances, as I could show from the writings of C. C. Sprengel and from
+my own observations, which effectually prevent the stigma receiving pollen
+from its own flower: for instance, in Lobelia fulgens, there is a really
+beautiful and elaborate contrivance by which every one of the infinitely
+numerous pollen-granules are swept out of the conjoined anthers of each
+flower, before the stigma of that individual flower is ready to receive
+them; and as this flower is never visited, at least in my garden, by
+insects, it never sets a seed, though by placing pollen from one flower on
+the stigma of another, I raised plenty of seedlings; and whilst another
+species of Lobelia growing close by, which is visited by bees, seeds
+freely. In very many other cases, though there {99} be no special
+mechanical contrivance to prevent the stigma of a flower receiving its own
+pollen, yet, as C. C. Sprengel has shown, and as I can confirm, either the
+anthers burst before the stigma is ready for fertilisation, or the stigma
+is ready before the pollen of that flower is ready, so that these plants
+have in fact separated sexes, and must habitually be crossed. How strange
+are these facts! How strange that the pollen and stigmatic surface of the
+same flower, though placed so close together, as if for the very purpose of
+self-fertilisation, should in so many cases be mutually useless to each
+other! How simply are these facts explained on the view of an occasional
+cross with a distinct individual being advantageous or indispensable!
+
+If several varieties of the cabbage, radish, onion, and of some other
+plants, be allowed to seed near each other, a large majority, as I have
+found, of the seedlings thus raised will turn out mongrels: for instance, I
+raised 233 seedling cabbages from some plants of different varieties
+growing near each other, and of these only 78 were true to their kind, and
+some even of these were not perfectly true. Yet the pistil of each
+cabbage-flower is surrounded not only by its own six stamens, but by those
+of the many other flowers on the same plant. How, then, comes it that such
+a vast number of the seedlings are mongrelized? I suspect that it must
+arise from the pollen of a distinct _variety_ having a prepotent effect
+over a flower's own pollen; and that this is part of the general law of
+good being derived from the intercrossing of distinct individuals of the
+same species. When distinct _species_ are crossed the case is directly the
+reverse, for a plant's own pollen is always prepotent over foreign pollen;
+but to this subject we shall return in a future chapter.
+
+In the case of a gigantic tree covered with {100} innumerable flowers, it
+may be objected that pollen could seldom be carried from tree to tree, and
+at most only from flower to flower on the same tree, and that flowers on
+the same tree can be considered as distinct individuals only in a limited
+sense. I believe this objection to be valid, but that nature has largely
+provided against it by giving to trees a strong tendency to bear flowers
+with separated sexes. When the sexes are separated, although the male and
+female flowers may be produced on the same tree, we can see that pollen
+must be regularly carried from flower to flower; and this will give a
+better chance of pollen being occasionally carried from tree to tree. That
+trees belonging to all Orders have their sexes more often separated than
+other plants, I find to be the case in this country; and at my request Dr.
+Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the
+United States, and the result was as I anticipated. On the other hand, Dr.
+Hooker has recently informed me that he finds that the rule does not hold
+in Australia; and I have made these few remarks on the sexes of trees
+simply to call attention to the subject.
+
+Turning for a very brief space to animals: on the land there are some
+hermaphrodites, as land-mollusca and earth-worms; but these all pair. As
+yet I have not found a single case of a terrestrial animal which fertilises
+itself. We can understand this remarkable fact, which offers so strong a
+contrast with terrestrial plants, on the view of an occasional cross being
+indispensable, by considering the medium in which terrestrial animals live,
+and the nature of the fertilising element; for we know of no means,
+analogous to the action of insects and of the wind in the case of plants,
+by which an occasional cross could be effected with terrestrial animals
+without the concurrence of two individuals. Of aquatic animals, there are
+many self-fertilising hermaphrodites; but here {101} currents in the water
+offer an obvious means for an occasional cross. And, as in the case of
+flowers, I have as yet failed, after consultation with one of the highest
+authorities, namely, Professor Huxley, to discover a single case of an
+hermaphrodite animal with the organs of reproduction so perfectly enclosed
+within the body, that access from without and the occasional influence of a
+distinct individual can be shown to be physically impossible. Cirripedes
+long appeared to me to present a case of very great difficulty under this
+point of view; but I have been enabled, by a fortunate chance, elsewhere to
+prove that two individuals, though both are self-fertilising
+hermaphrodites, do sometimes cross.
+
+It must have struck most naturalists as a strange anomaly that, in the case
+of both animals and plants, species of the same family and even of the same
+genus, though agreeing closely with each other in almost their whole
+organisation, yet are not rarely, some of them hermaphrodites, and some of
+them unisexual. But if, in fact, all hermaphrodites do occasionally
+intercross with other individuals, the difference between hermaphrodites
+and unisexual species, as far as function is concerned, becomes very small.
+
+From these several considerations and from the many special facts which I
+have collected, but which I am not here able to give, I am strongly
+inclined to suspect that, both in the vegetable and animal kingdoms, an
+occasional intercross with a distinct individual is a law of nature. I am
+well aware that there are, on this view, many cases of difficulty, some of
+which I am trying to investigate. Finally then, we may conclude that in
+many organic beings, a cross between two individuals is an obvious
+necessity for each birth; in many others it occurs perhaps only at long
+intervals; but in none, as I suspect, can self-fertilisation go on for
+perpetuity. {102}
+
+
+
+_Circumstances favourable to Natural Selection._--This is an extremely
+intricate subject. A large amount of inheritable and diversified
+variability is favourable, but I believe mere individual differences
+suffice for the work. A large number of individuals, by giving a better
+chance for the appearance within any given period of profitable variations,
+will compensate for a lesser amount of variability in each individual, and
+is, I believe, an extremely important element of success. Though nature
+grants vast periods of time for the work of natural selection, she does not
+grant an indefinite period; for as all organic beings are striving, it may
+be said, to seize on each place in the economy of nature, if any one
+species does not become modified and improved in a corresponding degree
+with its competitors, it will soon be exterminated.
+
+In man's methodical selection, a breeder selects for some definite object,
+and free intercrossing will wholly stop his work. But when many men,
+without intending to alter the breed, have a nearly common standard of
+perfection, and all try to get and breed from the best animals, much
+improvement and modification surely but slowly follow from this unconscious
+process of selection, notwithstanding a large amount of crossing with
+inferior animals. Thus it will be in nature; for within a confined area,
+with some place in its polity not so perfectly occupied as might be,
+natural selection will always tend to preserve all the individuals varying
+in the right direction, though in different degrees, so as better to fill
+up the unoccupied place. But if the area be large, its several districts
+will almost certainly present different conditions of life; and then if
+natural selection be modifying and improving a species in the several
+districts, there will be intercrossing with the other individuals of the
+same species on the confines of each. And in {103} this case the effects of
+intercrossing can hardly be counterbalanced by natural selection always
+tending to modify all the individuals in each district in exactly the same
+manner to the conditions of each; for in a continuous area, the physical
+conditions at least will generally graduate away insensibly from one
+district to another. The intercrossing will most affect those animals which
+unite for each birth, which wander much, and which do not breed at a very
+quick rate. Hence in animals of this nature, for instance in birds,
+varieties will generally be confined to separated countries; and this I
+believe to be the case. In hermaphrodite organisms which cross only
+occasionally, and likewise in animals which unite for each birth, but which
+wander little and which can increase at a very rapid rate, a new and
+improved variety might be quickly formed on any one spot, and might there
+maintain itself in a body, so that whatever intercrossing took place would
+be chiefly between the individuals of the same new variety. A local variety
+when once thus formed might subsequently slowly spread to other districts.
+On the above principle, nurserymen always prefer getting seed from a large
+body of plants of the same variety, as the chance of intercrossing with
+other varieties is thus lessened.
+
+Even in the case of slow-breeding animals, which unite for each birth, we
+must not overrate the effects of intercrosses in retarding natural
+selection; for I can bring a considerable catalogue of facts, showing that
+within the same area, varieties of the same animal can long remain
+distinct, from haunting different stations, from breeding at slightly
+different seasons, or from varieties of the same kind preferring to pair
+together.
+
+Intercrossing plays a very important part in nature in keeping the
+individuals of the same species, or of the same variety, true and uniform
+in character. It will {104} obviously thus act far more efficiently with
+those animals which unite for each birth; but I have already attempted to
+show that we have reason to believe that occasional intercrosses take place
+with all animals and with all plants. Even if these take place only at long
+intervals, I am convinced that the young thus produced will gain so much in
+vigour and fertility over the offspring from long-continued
+self-fertilisation, that they will have a better chance of surviving and
+propagating their kind; and thus, in the long run, the influence of
+intercrosses, even at rare intervals, will be great. If there exist organic
+beings which never intercross, uniformity of character can be retained
+amongst them, as long as their conditions of life remain the same, only
+through the principle of inheritance, and through natural selection
+destroying any which depart from the proper type; but if their conditions
+of life change and they undergo modification, uniformity of character can
+be given to their modified offspring, solely by natural selection
+preserving the same favourable variations.
+
+Isolation, also, is an important element in the process of natural
+selection. In a confined or isolated area, if not very large, the organic
+and inorganic conditions of life will generally be in a great degree
+uniform; so that natural selection will tend to modify all the individuals
+of a varying species throughout the area in the same manner in relation to
+the same conditions. Intercrosses, also, with the individuals of the same
+species, which otherwise would have inhabited the surrounding and
+differently circumstanced districts, will be prevented. But isolation
+probably acts more efficiently in checking the immigration of better
+adapted organisms, after any physical change, such as of climate or
+elevation of the land, &c.; and thus new places in the natural economy of
+the country are left open for the old inhabitants to struggle for, and
+become adapted to, through {105} modifications in their structure and
+constitution. Lastly, isolation, by checking immigration and consequently
+competition, will give time for any new variety to be slowly improved; and
+this may sometimes be of importance in the production of new species. If,
+however, an isolated area be very small, either from being surrounded by
+barriers, or from having very peculiar physical conditions, the total
+number of the individuals supported on it will necessarily be very small;
+and fewness of individuals will greatly retard the production of new
+species through natural selection, by decreasing the chance of the
+appearance of favourable variations.
+
+If we turn to nature to test the truth of these remarks, and look at any
+small isolated area, such as an oceanic island, although the total number
+of the species inhabiting it, will be found to be small, as we shall see in
+our chapter on geographical distribution; yet of these species a very large
+proportion are endemic,--that is, have been produced there, and nowhere
+else. Hence an oceanic island at first sight seems to have been highly
+favourable for the production of new species. But we may thus greatly
+deceive ourselves, for to ascertain whether a small isolated area, or a
+large open area like a continent, has been most favourable for the
+production of new organic forms, we ought to make the comparison within
+equal times; and this we are incapable of doing.
+
+Although I do not doubt that isolation is of considerable importance in the
+production of new species, on the whole I am inclined to believe that
+largeness of area is of more importance, more especially in the production
+of species, which will prove capable of enduring for a long period, and of
+spreading widely. Throughout a great and open area, not only will there be
+a better chance of favourable variations arising from the large number of
+individuals of the same species {106} there supported, but the conditions
+of life are infinitely complex from the large number of already existing
+species; and if some of these many species become modified and improved,
+others will have to be improved in a corresponding degree or they will be
+exterminated. Each new form, also, as soon as it has been much improved,
+will be able to spread over the open and continuous area, and will thus
+come into competition with many others. Hence more new places will be
+formed, and the competition to fill them will be more severe, on a large
+than on a small and isolated area. Moreover, great areas, though now
+continuous, owing to oscillations of level, will often have recently
+existed in a broken condition, so that the good effects of isolation will
+generally, to a certain extent, have concurred. Finally, I conclude that,
+although small isolated areas probably have been in some respects highly
+favourable for the production of new species, yet that the course of
+modification will generally have been more rapid on large areas; and what
+is more important, that the new forms produced on large areas, which
+already have been victorious over many competitors, will be those that will
+spread most widely, will give rise to most new varieties and species, and
+will thus play an important part in the changing history of the organic
+world.
+
+We can, perhaps, on these views, understand some facts which will be again
+alluded to in our chapter on geographical distribution; for instance, that
+the productions of the smaller continent of Australia have formerly
+yielded, and apparently are now yielding, before those of the larger
+Europæo-Asiatic area. Thus, also, it is that continental productions have
+everywhere become so largely naturalised on islands. On a small island, the
+race for life will have been less severe, and there will have been less
+modification and less {107} extermination. Hence, perhaps, it comes that
+the flora of Madeira, according to Oswald Heer, resembles the extinct
+tertiary flora of Europe. All fresh-water basins, taken together, make a
+small area compared with that of the sea or of the land; and, consequently,
+the competition between fresh-water productions will have been less severe
+than elsewhere; new forms will have been more slowly formed, and old forms
+more slowly exterminated. And it is in fresh water that we find seven
+genera of Ganoid fishes, remnants of a once preponderant order: and in
+fresh water we find some of the most anomalous forms now known in the
+world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect
+to a certain extent orders now widely separated in the natural scale. These
+anomalous forms may almost be called living fossils; they have endured to
+the present day, from having inhabited a confined area, and from having
+thus been exposed to less severe competition.
+
+To sum up the circumstances favourable and unfavourable to natural
+selection, as far as the extreme intricacy of the subject permits. I
+conclude, looking to the future, that for terrestrial productions a large
+continental area, which will probably undergo many oscillations of level,
+and which consequently will exist for long periods in a broken condition,
+is the most favourable for the production of many new forms of life, likely
+to endure long and to spread widely. For the area first existed as a
+continent, and the inhabitants, at this period numerous in individuals and
+kinds, will have been subjected to very severe competition. When converted
+by subsidence into large separate islands, there will still exist many
+individuals of the same species on each island: intercrossing on the
+confines of the range of each species will thus be checked: after physical
+changes of any kind, immigration will be {108} prevented, so that new
+places in the polity of each island will have to be filled up by
+modifications of the old inhabitants; and time will be allowed for the
+varieties in each to become well modified and perfected. When, by renewed
+elevation, the islands shall be re-converted into a continental area, there
+will again be severe competition: the most favoured or improved varieties
+will be enabled to spread: there will be much extinction of the less
+improved forms, and the relative proportional numbers of the various
+inhabitants of the renewed continent will again be changed; and again there
+will be a fair field for natural selection to improve still further the
+inhabitants, and thus produce new species.
+
+That natural selection will always act with extreme slowness, I fully
+admit. Its action depends on there being places in the polity of nature,
+which can be better occupied by some of the inhabitants of the country
+undergoing modification of some kind. The existence of such places will
+often depend on physical changes, which are generally very slow, and on the
+immigration of better adapted forms having been checked. But the action of
+natural selection will probably still oftener depend on some of the
+inhabitants becoming slowly modified; the mutual relations of many of the
+other inhabitants being thus disturbed. Nothing can be effected, unless
+favourable variations occur, and variation itself is apparently always a
+very slow process. The process will often be greatly retarded by free
+intercrossing. Many will exclaim that these several causes are amply
+sufficient wholly to stop the action of natural selection. I do not believe
+so. On the other hand, I do believe that natural selection always acts very
+slowly, often only at long intervals of time, and generally on only a very
+few of the inhabitants of the same region at the same time. I further
+believe, that this very slow, {109} intermittent action of natural
+selection accords perfectly well with what geology tells us of the rate and
+manner at which the inhabitants of this world have changed.
+
+Slow though the process of selection may be, if feeble man can do much by
+his powers of artificial selection, I can see no limit to the amount of
+change, to the beauty and infinite complexity of the coadaptations between
+all organic beings, one with another and with their physical conditions of
+life, which may be effected in the long course of time by nature's power of
+selection.
+
+
+
+_Extinction._--This subject will be more fully discussed in our chapter on
+Geology; but it must be here alluded to from being intimately connected
+with natural selection. Natural selection acts solely through the
+preservation of variations in some way advantageous, which consequently
+endure. But as from the high geometrical ratio of increase of all organic
+beings, each area is already fully stocked with inhabitants, it follows
+that as each selected and favoured form increases in number, so will the
+less favoured forms decrease and become rare. Rarity, as geology tells us,
+is the precursor to extinction. We can, also, see that any form represented
+by few individuals will, during fluctuations in the seasons or in the
+number of its enemies, run a good chance of utter extinction. But we may go
+further than this; for as new forms are continually and slowly being
+produced, unless we believe that the number of specific forms goes on
+perpetually and almost indefinitely increasing, numbers inevitably must
+become extinct. That the number of specific forms has not indefinitely
+increased, geology shows us plainly; and indeed we can see reason why they
+should not have thus increased, for the number of places in the polity of
+nature is not indefinitely great,--not that we {110} have any means of
+knowing that any one region has as yet got its maximum of species. Probably
+no region is as yet fully stocked, for at the Cape of Good Hope, where more
+species of plants are crowded together than in any other quarter of the
+world, some foreign plants have become naturalised, without causing, as far
+as we know, the extinction of any natives.
+
+Furthermore, the species which are most numerous in individuals will have
+the best chance of producing within any given period favourable variations.
+We have evidence of this, in the facts given in the second chapter, showing
+that it is the common species which afford the greatest number of recorded
+varieties, or incipient species. Hence, rare species will be less quickly
+modified or improved within any given period, and they will consequently be
+beaten in the race for life by the modified descendants of the commoner
+species.
+
+From these several considerations I think it inevitably follows, that as
+new species in the course of time are formed through natural selection,
+others will become rarer and rarer, and finally extinct. The forms which
+stand in closest competition with those undergoing modification and
+improvement, will naturally suffer most. And we have seen in the chapter on
+the Struggle for Existence that it is the most closely-allied
+forms,--varieties of the same species, and species of the same genus or of
+related genera,--which, from having nearly the same structure,
+constitution, and habits, generally come into the severest competition with
+each other. Consequently, each new variety or species, during the progress
+of its formation, will generally press hardest on its nearest kindred, and
+tend to exterminate them. We see the same process of extermination amongst
+our domesticated productions, through the selection of improved forms by
+man. Many curious {111} instances could be given showing how quickly new
+breeds of cattle, sheep, and other animals, and varieties of flowers, take
+the place of older and inferior kinds. In Yorkshire, it is historically
+known that the ancient black cattle were displaced by the long-horns, and
+that these "were swept away by the short-horns" (I quote the words of an
+agricultural writer) "as if by some murderous pestilence."
+
+
+
+_Divergence of Character._--The principle, which I have designated by this
+term, is of high importance on my theory, and explains, as I believe,
+several important facts. In the first place, varieties, even
+strongly-marked ones, though having somewhat of the character of
+species--as is shown by the hopeless doubts in many cases how to rank
+them--yet certainly differ from each other far less than do good and
+distinct species. Nevertheless, according to my view, varieties are species
+in the process of formation, or are, as I have called them, incipient
+species. How, then, does the lesser difference between varieties become
+augmented into the greater difference between species? That this does
+habitually happen, we must infer from most of the innumerable species
+throughout nature presenting well-marked differences; whereas varieties,
+the supposed prototypes and parents of future well-marked species, present
+slight and ill-defined differences. Mere chance, as we may call it, might
+cause one variety to differ in some character from its parents, and the
+offspring of this variety again to differ from its parent in the very same
+character and in a greater degree; but this alone would never account for
+so habitual and large an amount of difference as that between varieties of
+the same species and species of the same genus.
+
+As has always been my practice, let us seek light on {112} this head from
+our domestic productions. We shall here find something analogous. A fancier
+is struck by a pigeon having a slightly shorter beak; another fancier is
+struck by a pigeon having a rather longer beak; and on the acknowledged
+principle that "fanciers do not and will not admire a medium standard, but
+like extremes," they both go on (as has actually occurred with
+tumbler-pigeons) choosing and breeding from birds with longer and longer
+beaks, or with shorter and shorter beaks. Again, we may suppose that at an
+early period one man preferred swifter horses; another stronger and more
+bulky horses. The early differences would be very slight; in the course of
+time, from the continued selection of swifter horses by some breeders, and
+of stronger ones by others, the differences would become greater, and would
+be noted as forming two sub-breeds; finally, after the lapse of centuries,
+the sub-breeds would become converted into two well-established and
+distinct breeds. As the differences slowly become greater, the inferior
+animals with intermediate characters, being neither very swift nor very
+strong, will have been neglected, and will have tended to disappear. Here,
+then, we see in man's productions the action of what may be called the
+principle of divergence, causing differences, at first barely appreciable,
+steadily to increase, and the breeds to diverge in character both from each
+other and from their common parent.
+
+But how, it may be asked, can any analogous principle apply in nature? I
+believe it can and does apply most efficiently, from the simple
+circumstance that the more diversified the descendants from any one species
+become in structure, constitution, and habits, by so much will they be
+better enabled to seize on many and widely diversified places in the polity
+of nature, and so be enabled to increase in numbers. {113}
+
+We can clearly see this in the case of animals with simple habits. Take the
+case of a carnivorous quadruped, of which the number that can be supported
+in any country has long ago arrived at its full average. If its natural
+powers of increase be allowed to act, it can succeed in increasing (the
+country not undergoing any change in its conditions) only by its varying
+descendants seizing on places at present occupied by other animals: some of
+them, for instance, being enabled to feed on new kinds of prey, either dead
+or alive; some inhabiting new stations, climbing trees, frequenting water,
+and some perhaps becoming less carnivorous. The more diversified in habits
+and structure the descendants of our carnivorous animal became, the more
+places they would be enabled to occupy. What applies to one animal will
+apply throughout all time to all animals--that is, if they vary--for
+otherwise natural selection can do nothing. So it will be with plants. It
+has been experimentally proved, that if a plot of ground be sown with one
+species of grass, and a similar plot be sown with several distinct genera
+of grasses, a greater number of plants and a greater weight of dry herbage
+can thus be raised. The same has been found to hold good when first one
+variety and then several mixed varieties of wheat have been sown on equal
+spaces of ground. Hence, if any one species of grass were to go on varying,
+and those varieties were continually selected which differed from each
+other in at all the same manner as distinct species and genera of grasses
+differ from each other, a greater number of individual plants of this
+species of grass, including its modified descendants, would succeed in
+living on the same piece of ground. And we well know that each species and
+each variety of grass is annually sowing almost countless seeds; and thus,
+as it may be said, is striving its utmost to increase its numbers. {114}
+Consequently, I cannot doubt that in the course of many thousands of
+generations, the most distinct varieties of any one species of grass would
+always have the best chance of succeeding and of increasing in numbers, and
+thus of supplanting the less distinct varieties; and varieties, when
+rendered very distinct from each other, take the rank of species.
+
+The truth of the principle, that the greatest amount of life can be
+supported by great diversification of structure, is seen under many natural
+circumstances. In an extremely small area, especially if freely open to
+immigration, and where the contest between individual and individual must
+be severe, we always find great diversity in its inhabitants. For instance,
+I found that a piece of turf, three feet by four in size, which had been
+exposed for many years to exactly the same conditions, supported twenty
+species of plants, and these belonged to eighteen genera and to eight
+orders, which shows how much these plants differed from each other. So it
+is with the plants and insects on small and uniform islets; and so in small
+ponds of fresh water. Farmers find that they can raise most food by a
+rotation of plants belonging to the most different orders: nature follows
+what may be called a simultaneous rotation. Most of the animals and plants
+which live close round any small piece of ground, could live on it
+(supposing it not to be in any way peculiar in its nature), and may be said
+to be striving to the utmost to live there; but, it is seen, that where
+they come into the closest competition with each other, the advantages of
+diversification of structure, with the accompanying differences of habit
+and constitution, determine that the inhabitants, which thus jostle each
+other most closely, shall, as a general rule, belong to what we call
+different genera and orders.
+
+The same principle is seen in the naturalisation of {115} plants through
+man's agency in foreign lands. It might have been expected that the plants
+which have succeeded in becoming naturalised in any land would generally
+have been closely allied to the indigenes; for these are commonly looked at
+as specially created and adapted for their own country. It might, also,
+perhaps have been expected that naturalised plants would have belonged to a
+few groups more especially adapted to certain stations in their new homes.
+But the case is very different; and Alph. De Candolle has well remarked in
+his great and admirable work, that floras gain by naturalisation,
+proportionally with the number of the native genera and species, far more
+in new genera than in new species. To give a single instance: in the last
+edition of Dr. Asa Gray's 'Manual of the Flora of the Northern United
+States,' 260 naturalised plants are enumerated, and these belong to 162
+genera. We thus see that these naturalised plants are of a highly
+diversified nature. They differ, moreover, to a large extent from the
+indigenes, for out of the 162 genera, no less than 100 genera are not there
+indigenous, and thus a large proportional addition is made to the genera of
+these States.
+
+By considering the nature of the plants or animals which have struggled
+successfully with the indigenes of any country, and have there become
+naturalised, we may gain some crude idea in what manner some of the natives
+would have to be modified, in order to gain an advantage over the other
+natives; and we may at least safely infer that diversification of
+structure, amounting to new generic differences, would be profitable to
+them.
+
+The advantage of diversification 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 individual body--a subject so well elucidated by Milne
+{116} Edwards. No physiologist doubts that a stomach adapted to digest
+vegetable matter alone, or flesh alone, draws most nutriment from these
+substances. So in the general economy of any land, the more widely and
+perfectly the animals and plants are diversified for different habits of
+life, so will a greater number of individuals be capable of there
+supporting themselves. A set of animals, with their organisation but little
+diversified, could hardly compete with a set more perfectly diversified in
+structure. It may be doubted, for instance, whether the Australian
+marsupials, which are divided into groups differing but little from each
+other, and feebly representing, as Mr. Waterhouse and others have remarked,
+our carnivorous, ruminant, and rodent mammals, could successfully compete
+with these well-pronounced orders. In the Australian mammals, we see the
+process of diversification in an early and incomplete stage of development.
+
+After the foregoing discussion, which ought to have been much amplified, we
+may, I think, assume that the modified descendants of any one species will
+succeed by so much the better as they become more diversified in structure,
+and are thus enabled to encroach on places occupied by other beings. Now
+let us see how this principle of benefit being derived from divergence of
+character, combined with the principles of natural selection and of
+extinction, will tend to act.
+
+The accompanying diagram will aid us in understanding this rather
+perplexing subject. Let A to L represent the species of a genus large in
+its own country; these species are supposed to resemble each other in
+unequal degrees, as is so generally the case in nature, and as is
+represented in the diagram by the letters standing at unequal distances. I
+have said a large genus, because we have seen in the second chapter, {117}
+that on an average more of the species of large genera vary than of small
+genera; and the varying species of the large genera present a greater
+number of varieties. We have, also, seen that the species, which are the
+commonest and the most widely-diffused, vary more than rare species with
+restricted ranges. Let (A) be a common, widely-diffused, and varying
+species, belonging to a genus large in its own country. The little fan of
+diverging dotted lines of unequal lengths proceeding from (A), may
+represent its varying offspring. The variations are supposed to be
+extremely slight, but of the most diversified nature; they are not supposed
+all to appear simultaneously, but often after long intervals of time; nor
+are they all supposed to endure for equal periods. Only those variations
+which are in some way profitable will be preserved or naturally selected.
+And here the importance of the principle of benefit being derived from
+divergence of character comes in; for this will generally lead to the most
+different or divergent variations (represented by the outer dotted lines)
+being preserved and accumulated by natural selection. When a dotted line
+reaches one of the horizontal lines, and is there marked by a small
+numbered letter, a sufficient amount of variation is supposed to have been
+accumulated to have formed a fairly well-marked variety, such as would be
+thought worthy of record in a systematic work.
+
+[Illustration]
+
+The intervals between the horizontal lines in the diagram, may represent
+each a thousand generations; but it would have been better if each had
+represented ten thousand generations. After a thousand generations, species
+(A) is supposed to have produced two fairly well-marked varieties, namely
+a^1 and m^1. These two varieties will generally continue to be exposed to
+the same conditions which made their parents variable, {118} and the
+tendency to variability is in itself hereditary, consequently they will
+tend to vary, and generally to vary in nearly the same manner as their
+parents varied. Moreover, these two varieties, being only slightly modified
+forms, will tend to inherit those advantages which made their parent (A)
+more numerous than most of the other inhabitants of the same country; they
+will likewise partake of those more general advantages which made the genus
+to which the parent-species belonged, a large genus in its own country. And
+these circumstances we know to be favourable to the production of new
+varieties.
+
+If, then, these two varieties be variable, the most divergent of their
+variations will generally be preserved during the next thousand
+generations. And after this interval, variety a^1 is supposed in the
+diagram to have produced variety a^2, which will, owing to the principle of
+divergence, differ more from (A) than did variety a^1. Variety m^1 is
+supposed to have produced two varieties, namely m^2 and s^2, differing from
+each other, and more considerably from their common parent (A). We may
+continue the process by similar steps for any length of time; some of the
+varieties, after each thousand generations, producing only a single
+variety, but in a more and more modified condition, some producing two or
+three varieties, and some failing to produce any. Thus the varieties or
+modified descendants, proceeding from the common parent (A), will generally
+go on increasing in number and diverging in character. In the diagram the
+process is represented up to the ten-thousandth generation, and under a
+condensed and simplified form up to the fourteen-thousandth generation.
+
+But I must here remark that I do not suppose that the process ever goes on
+so regularly as is represented in the diagram, though in itself made
+somewhat irregular. {119} I am far from thinking that the most divergent
+varieties will invariably prevail and multiply: a medium form may often
+long endure, and may or may not produce more than one modified descendant;
+for natural selection will always act according to the nature of the places
+which are either unoccupied or not perfectly occupied by other beings; and
+this will depend on infinitely complex relations. But as a general rule,
+the more diversified in structure the descendants from any one species can
+be rendered, the more places they will be enabled to seize on, and the more
+their modified progeny will be increased. In our diagram the line of
+succession is broken at regular intervals by small numbered letters marking
+the successive forms which have become sufficiently distinct to be recorded
+as varieties. But these breaks are imaginary, and might have been inserted
+anywhere, after intervals long enough to have allowed the accumulation of a
+considerable amount of divergent variation.
+
+As all the modified descendants from a common and widely-diffused species,
+belonging to a large genus, will tend to partake of the same advantages
+which made their parent successful in life, they will generally go on
+multiplying in number as well as diverging in character: this is
+represented in the diagram by the several divergent branches proceeding
+from (A). The modified offspring from the later and more highly improved
+branches in the lines of descent, will, it is probable, often take the
+place of, and so destroy, the earlier and less improved branches: this is
+represented in the diagram by some of the lower branches not reaching to
+the upper horizontal lines. In some cases I do not doubt that the process
+of modification will be confined to a single line of descent, and the
+number of the descendants will not be increased; although the amount {120}
+of divergent modification may have been increased in the successive
+generations. This case would be represented in the diagram, if all the
+lines proceeding from (A) were removed, excepting that from a^1 to a^{10}.
+In the same way, for instance, the English race-horse and English pointer
+have apparently both gone on slowly diverging in character from their
+original stocks, without either having given off any fresh branches or
+races.
+
+After ten thousand generations, species (A) is supposed to have produced
+three forms, a^{10}, f^{10}, and m^{10}, which, from having diverged in
+character during the successive generations, will have come to differ
+largely, but perhaps unequally, from each other and from their common
+parent. If we suppose the amount of change between each horizontal line in
+our diagram to be excessively small, these three forms may still be only
+well-marked varieties; or they may have arrived at the doubtful category of
+sub-species; but we have only to suppose the steps in the process of
+modification to be more numerous or greater in amount, to convert these
+three forms into well-defined species: thus the diagram illustrates the
+steps by which the small differences distinguishing varieties are increased
+into the larger differences distinguishing species. By continuing the same
+process for a greater number of generations (as shown in the diagram in a
+condensed and simplified manner), we get eight species, marked by the
+letters between a^{14} and m^{14}, all descended from (A). Thus, as I
+believe, species are multiplied and genera are formed.
+
+In a large genus it is probable that more than one species would vary. In
+the diagram I have assumed that a second species (I) has produced, by
+analogous steps, after ten thousand generations, either two well-marked
+varieties (w^{10} and z^{10}) or two species, according to the amount of
+change supposed to be represented {121} between the horizontal lines. After
+fourteen thousand generations, six new species, marked by the letters
+n^{14} to z^{14}, are supposed to have been produced. In each genus, the
+species, which are already extremely different in character, will generally
+tend to produce the greatest number of modified descendants; for these will
+have the best chance of filling new and widely different places in the
+polity of nature: hence in the diagram I have chosen the extreme species
+(A), and the nearly extreme species (I), as those which have largely
+varied, and have given rise to new varieties and species. The other nine
+species (marked by capital letters) of our original genus, may for a long
+period continue to transmit unaltered descendants; and this is shown in the
+diagram by the dotted lines not prolonged far upwards from want of space.
+
+But during the process of modification, represented in the diagram, another
+of our principles, namely that of extinction, will have played an important
+part. As in each fully stocked country natural selection necessarily acts
+by the selected form having some advantage in the struggle for life over
+other forms, there will be a constant tendency in the improved descendants
+of any one species to supplant and exterminate in each stage of descent
+their predecessors and their original parent. For it should be remembered
+that the competition will generally be most severe between those forms
+which are most nearly related to each other in habits, constitution, and
+structure. Hence all the intermediate forms between the earlier and later
+states, that is between the less and more improved state of a species, as
+well as the original parent-species itself, will generally tend to become
+extinct. So it probably will be with many whole collateral lines of
+descent, which will be conquered by later and improved lines of descent.
+If, however, the {122} modified offspring of a species get into some
+distinct country, or become quickly adapted to some quite new station, in
+which child and parent do not come into competition, both may continue to
+exist.
+
+If then our diagram be assumed to represent a considerable amount of
+modification, species (A) and all the earlier varieties will have become
+extinct, having been replaced by eight new species (a^{14} to m^{14}); and
+(I) will have been replaced by six (n^{14} to z^{14}) new species.
+
+But we may go further than this. The original species of our genus were
+supposed to resemble each other in unequal degrees, as is so generally the
+case in nature; species (A) being more nearly related to B, C, and D, than
+to the other species; and species (I) more to G, H, K, L, than to the
+others. These two species (A) and (I), were also supposed to be very common
+and widely diffused species, so that they must originally have had some
+advantage over most of the other species of the genus. Their modified
+descendants, fourteen in number at the fourteen-thousandth generation, will
+probably have inherited some of the same advantages: they have also been
+modified and improved in a diversified manner at each stage of descent, so
+as to have become adapted to many related places in the natural economy of
+their country. It seems, therefore, to me extremely probable that they will
+have taken the places of, and thus exterminated, not only their parents (A)
+and (I), but likewise some of the original species which were most nearly
+related to their parents. Hence very few of the original species will have
+transmitted offspring to the fourteen-thousandth generation. We may suppose
+that only one (F), of the two species which were least closely related to
+the other nine original species, has transmitted descendants to this late
+stage of descent. {123}
+
+The new species in our diagram descended from the original eleven species,
+will now be fifteen in number. Owing to the divergent tendency of natural
+selection, the extreme amount of difference in character between species
+a^{14} and z^{14} will be much greater than that between the most different
+of the original eleven species. The new species, moreover, will be allied
+to each other in a widely different manner. Of the eight descendants from
+(A) the three marked a^{14}, q^{14}, p^{14}, will be nearly related from
+having recently branched off from a^{10}; b^{14} and f^{14}, from having
+diverged at an earlier period from a^5, will be in some degree distinct
+from the three first-named species; and lastly, o^{14}, e^{14} and m^{14},
+will be nearly related one to the other, but from having diverged at the
+first commencement of the process of modification, will be widely different
+from the other five species, and may constitute a sub-genus or even a
+distinct genus.
+
+The six descendants from (I) will form two sub-genera or even genera. But
+as the original species (I) differed largely from (A), standing nearly at
+the extreme points of the original genus, the six descendants from (I)
+will, owing to inheritance alone, differ considerably from the eight
+descendants from (A); the two groups, moreover, are supposed to have gone
+on diverging in different directions. The intermediate species, also (and
+this is a very important consideration), which connected the original
+species (A) and (I), have all become, excepting (F), extinct, and have left
+no descendants. Hence the six new species descended from (I), and the eight
+descended from (A), will have to be ranked as very distinct genera, or even
+as distinct sub-families.
+
+Thus it is, as I believe, that two or more genera are produced by descent
+with modification, from two or more species of the same genus. And the two
+or {124} more parent-species are supposed to have descended from some one
+species of an earlier genus. In our diagram, this is indicated by the
+broken lines, beneath the capital letters, converging in sub-branches
+downwards towards a single point; this point representing a single species,
+the supposed single parent of our several new sub-genera and genera.
+
+It is worth while to reflect for a moment on the character of the new
+species F^{14}, which is supposed not to have diverged much in character,
+but to have retained the form of (F), either unaltered or altered only in a
+slight degree. In this case, its affinities to the other fourteen new
+species will be of a curious and circuitous nature. Having descended from a
+form which stood between the two parent-species (A) and (I), now supposed
+to be extinct and unknown, it will be in some degree intermediate in
+character between the two groups descended from these species. But as these
+two groups have gone on diverging in character from the type of their
+parents, the new species (F^{14}) will not be directly intermediate between
+them, but rather between types of the two groups; and every naturalist will
+be able to bring some such case before his mind.
+
+In the diagram, each horizontal line has hitherto been supposed to
+represent a thousand generations, but each may represent a million or
+hundred million generations, and likewise a section of the successive
+strata of the earth's crust including extinct remains. We shall, when we
+come to our chapter on Geology, have to refer again to this subject, and I
+think we shall then see that the diagram throws light on the affinities of
+extinct beings, which, though generally belonging to the same orders, or
+families, or genera, with those now living, yet are often, in some degree,
+intermediate in character between existing groups; and we can understand
+this fact, for {125} the extinct species lived at very ancient epochs when
+the branching lines of descent had diverged less.
+
+I see no reason to limit the process of modification, as now explained, to
+the formation of genera alone. If, in our diagram, we suppose the amount of
+change represented by each successive group of diverging dotted lines to be
+very great, the forms marked a^{14} to p^{14}, those marked b^{14} and
+f^{14}, and those marked o^{14} to m^{14}, will form three very distinct
+genera. We shall also have two very distinct genera descended from (I); and
+as these latter two genera, both from continued divergence of character and
+from inheritance from a different parent, will differ widely from the three
+genera descended from (A), the two little groups of genera will form two
+distinct families, or even orders, according to the amount of divergent
+modification supposed to be represented in the diagram. And the two new
+families, or orders, will have descended from two species of the original
+genus; and these two species are supposed to have descended from one
+species of a still more ancient and unknown genus.
+
+We have seen that in each country it is the species of the larger genera
+which oftenest present varieties or incipient species. This, indeed, might
+have been expected; for as natural selection acts through one form having
+some advantage over other forms in the struggle for existence, it will
+chiefly act on those which already have some advantage; and the largeness
+of any group shows that its species have inherited from a common ancestor
+some advantage in common. Hence, the struggle for the production of new and
+modified descendants, will mainly lie between the larger groups, which are
+all trying to increase in number. One large group will slowly conquer
+another large group, reduce its numbers, and thus lessen its chance of
+further variation and improvement. Within the same large {126} group, the
+later and more highly perfected sub-groups, from branching out and seizing
+on many new places in the polity of Nature, will constantly tend to
+supplant and destroy the earlier and less improved sub-groups. Small and
+broken groups and sub-groups will finally disappear. Looking to the future,
+we can predict that the groups of organic beings which are now large and
+triumphant, and which are least broken up, that is, which as yet have
+suffered least extinction, will for a long period continue to increase. But
+which groups will ultimately prevail, no man can predict; for we well know
+that many groups, formerly most extensively developed, have now become
+extinct. Looking still more remotely to the future, we may predict that,
+owing to the continued and steady increase of the larger groups, a
+multitude of smaller groups will become utterly extinct, and leave no
+modified descendants; and consequently that of the species living at any
+one period, extremely few will transmit descendants to a remote futurity. I
+shall have to return to this subject in the chapter on Classification, but
+I may add that on this view of extremely few of the more ancient species
+having transmitted descendants, and on the view of all the descendants of
+the same species making a class, we can understand how it is that there
+exist but very few classes in each main division of the animal and
+vegetable kingdoms. Although extremely few of the most ancient species may
+now have living and modified descendants, yet at the most remote geological
+period, the earth may have been as well peopled with many species of many
+genera, families, orders, and classes, as at the present day.
+
+
+
+_Summary of Chapter._--If during the long course of ages and under varying
+conditions of life, organic beings {127} vary at all in the several parts
+of their organisation, and I think this cannot be disputed; if there be,
+owing to the high geometrical ratio of increase of each species, a severe
+struggle for life at some age, season, or year, and this certainly cannot
+be disputed; then, considering the infinite complexity of the relations of
+all organic beings to each other and to their conditions of existence,
+causing an infinite diversity in structure, constitution, and habits, to be
+advantageous to them, I think it would be a most extraordinary fact if no
+variation ever had occurred useful to each being's own welfare, in the same
+manner as so many variations have occurred useful to man. But if variations
+useful to any organic being do occur, assuredly individuals thus
+characterised will have the best chance of being preserved in the struggle
+for life; and from the strong principle of inheritance they will tend to
+produce offspring similarly characterised. This principle of preservation,
+I have called, for the sake of brevity, Natural Selection; and it leads to
+the improvement of each creature in relation to its organic and inorganic
+conditions of life.
+
+Natural selection, on the principle of qualities being inherited at
+corresponding ages, can modify the egg, seed, or young, as easily as the
+adult. Amongst many animals, sexual selection will give its aid to ordinary
+selection, by assuring to the most vigorous and best adapted males the
+greatest number of offspring. Sexual selection will also give characters
+useful to the males alone, in their struggles with other males.
+
+Whether natural selection has really thus acted in nature, in modifying and
+adapting the various forms of life to their several conditions and
+stations, must be judged of by the general tenour and balance of evidence
+given in the following chapters. But we already see how it entails
+extinction; and how largely extinction {128} has acted in the world's
+history, geology plainly declares. Natural selection, also, leads to
+divergence of character; for more living beings can be supported on the
+same area the more they diverge in structure, habits, and constitution, of
+which we see proof by looking to the inhabitants of any small spot or to
+naturalised productions. Therefore during the modification of the
+descendants of any one species, and during the incessant struggle of all
+species to increase in numbers, the more diversified these descendants
+become, the better will be their chance of succeeding in the battle for
+life. Thus the small differences distinguishing varieties of the same
+species, steadily tend to increase till they come to equal the greater
+differences between species of the same genus, or even of distinct genera.
+
+We have seen that it is the common, the widely-diffused, and widely-ranging
+species, belonging to the larger genera, which vary most; and these tend to
+transmit to their modified offspring that superiority which now makes them
+dominant in their own countries. Natural selection, as has just been
+remarked, leads to divergence of character and to much extinction of the
+less improved and intermediate forms of life. On these principles, I
+believe, the nature of the affinities of all organic beings may be
+explained. It is a truly wonderful fact--the wonder of which we are apt to
+overlook from familiarity--that all animals and all plants throughout all
+time and space should be related to each other in group subordinate to
+group, in the manner which we everywhere behold--namely, varieties of the
+same species most closely related together, species of the same genus less
+closely and unequally related together, forming sections and sub-genera,
+species of distinct genera much less closely related, and genera related in
+different degrees, forming {129} sub-families, families, orders,
+sub-classes, and classes. The several subordinate groups in any class
+cannot be ranked in a single file, but seem rather to be clustered round
+points, and these round other points, and so on in almost endless cycles.
+On the view that each species has been independently created, I can see no
+explanation of this great fact in the classification of all organic beings;
+but, to the best of my judgment, it is explained through inheritance and
+the complex action of natural selection, entailing extinction and
+divergence of character, as we have seen illustrated in the diagram.
+
+The affinities of all the beings of the same class have sometimes been
+represented by a great tree. I believe this simile largely speaks the
+truth. The green and budding twigs may represent existing species; and
+those produced during each former year may represent the long succession of
+extinct species. At each period of growth all the growing twigs have tried
+to branch out on all sides, and to overtop and kill the surrounding twigs
+and branches, in the same manner as species and groups of species have
+tried to overmaster other species in the great battle for life. The limbs
+divided into great branches, and these into lesser and lesser branches,
+were themselves once, when the tree was small, budding twigs; and this
+connexion of the former and present buds by ramifying branches may well
+represent the classification of all extinct and living species in groups
+subordinate to groups. Of the many twigs which flourished when the tree was
+a mere bush, only two or three, now grown into great branches, yet survive
+and bear all the other branches; so with the species which lived during
+long-past geological periods, very few now have living and modified
+descendants. From the first growth of the tree, many a limb and branch has
+decayed and dropped off; and these lost branches of various {130} sizes may
+represent those whole orders, families, and genera which have now no living
+representatives, and which are known to us only from having been found in a
+fossil state. As we here and there see a thin straggling branch springing
+from a fork low down in a tree, and which by some chance has been favoured
+and is still alive on its summit, so we occasionally see an animal like the
+Ornithorhynchus or Lepidosiren, which in some small degree connects by its
+affinities two large branches of life, and which has apparently been saved
+from fatal competition by having inhabited a protected station. As buds
+give rise by growth to fresh buds, and these, if vigorous, branch out and
+overtop on all sides many a feebler branch, so by generation I believe it
+has been with the great Tree of Life, which fills with its dead and broken
+branches the crust of the earth, and covers the surface with its ever
+branching and beautiful ramifications.
+
+       *       *       *       *       *
+
+
+{131}
+
+CHAPTER V.
+
+LAWS OF VARIATION.
+
+    Effects of external conditions--Use and disuse, combined with natural
+    selection; organs of flight and of vision--Acclimatisation--Correlation
+    of growth--Compensation and economy of growth--False
+    correlations--Multiple, rudimentary, and lowly organised structures
+    variable--Parts developed in an unusual manner are highly variable:
+    specific characters more variable than generic: secondary sexual
+    characters variable--Species of the same genus vary in an analogous
+    manner--Reversions to long-lost characters--Summary.
+
+I have hitherto sometimes spoken as if the variations--so common and
+multiform in organic beings under domestication, and in a lesser degree in
+those in a state of nature--had been due to chance. This, of course, is a
+wholly incorrect expression, but it serves to acknowledge plainly our
+ignorance of the cause of each particular variation. Some authors believe
+it to be as much the function of the reproductive system to produce
+individual differences, or very slight deviations of structure, as to make
+the child like its parents. But the much greater variability, as well as
+the greater frequency of monstrosities, under domestication or cultivation,
+than under nature, leads me to believe that deviations of structure are in
+some way due to the nature of the conditions of life, to which the parents
+and their more remote ancestors have been exposed during several
+generations. I have remarked in the first chapter--but a long catalogue of
+facts which cannot be here given would be necessary to show the truth of
+the remark--that the reproductive system is eminently susceptible to
+changes in the conditions of life; and to {132} this system being
+functionally disturbed in the parents, I chiefly attribute the varying or
+plastic condition of the offspring. The male and female sexual elements
+seem to be affected before that union takes place which is to form a new
+being. In the case of "sporting" plants, the bud, which in its earliest
+condition does not apparently differ essentially from an ovule, is alone
+affected. But why, because the reproductive system is disturbed, this or
+that part should vary more or less, we are profoundly ignorant.
+Nevertheless, we can here and there dimly catch a faint ray of light, and
+we may feel sure that there must be some cause for each deviation of
+structure, however slight.
+
+How much direct effect difference of climate, food, &c., produces on any
+being is extremely doubtful. My impression is, that the effect is extremely
+small in the case of animals, but perhaps rather more in that of plants. We
+may, at least, safely conclude that such influences cannot have produced
+the many striking and complex co-adaptations of structure between one
+organic being and another, which we see everywhere throughout nature. Some
+little influence may be attributed to climate, food, &c.: thus, E. Forbes
+speaks confidently that shells at their southern limit, and when living in
+shallow water, are more brightly coloured than those of the same species
+further north or from greater depths. Gould believes that birds of the same
+species are more brightly coloured under a clear atmosphere, than when
+living on islands or near the coast. So with insects, Wollaston is
+convinced that residence near the sea affects their colours. Moquin-Tandon
+gives a list of plants which when growing near the sea-shore have their
+leaves in some degree fleshy, though not elsewhere fleshy. Several other
+such cases could be given.
+
+The fact of varieties of one species, when they range {133} into the zone
+of habitation of other species, often acquiring in a very slight degree
+some of the characters of such species, accords with our view that species
+of all kinds are only well-marked and permanent varieties. Thus the species
+of shells which are confined to tropical and shallow seas are generally
+brighter-coloured than those confined to cold and deeper seas. The birds
+which are confined to continents are, according to Mr. Gould,
+brighter-coloured than those of islands. The insect-species confined to
+sea-coasts, as every collector knows, are often brassy or lurid. Plants
+which live exclusively on the sea-side are very apt to have fleshy leaves.
+He who believes in the creation of each species, will have to say that this
+shell, for instance, was created with bright colours for a warm sea; but
+that this other shell became bright-coloured by variation when it ranged
+into warmer or shallower waters.
+
+When a variation is of the slightest use to a being, we cannot tell how
+much of it to attribute to the accumulative action of natural selection,
+and how much to the conditions of life. Thus, it is well known to furriers
+that animals of the same species have thicker and better fur the more
+severe the climate is under which they have lived; but who can tell how
+much of this difference may be due to the warmest-clad individuals having
+been favoured and preserved during many generations, and how much to the
+direct action of the severe climate? for it would appear that climate has
+some direct action on the hair of our domestic quadrupeds.
+
+Instances could be given of the same variety being produced under
+conditions of life as different as can well be conceived; and, on the other
+hand, of different varieties being produced from the same species under the
+same conditions. Such facts show how indirectly {134} the conditions of
+life act. Again, innumerable instances are known to every naturalist of
+species keeping true, or not varying at all, although living under the most
+opposite climates. Such considerations as these incline me to lay very
+little weight on the direct action of the conditions of life. Indirectly,
+as already remarked, they seem to play an important part in affecting the
+reproductive system, and in thus inducing variability; and natural
+selection will then accumulate all profitable variations, however slight,
+until they become plainly developed and appreciable by us.
+
+
+
+_Effects of Use and Disuse._--From the facts alluded to in the first
+chapter, I think there can be little doubt that use in our domestic animals
+strengthens and enlarges certain parts, and disuse diminishes them; and
+that such modifications are inherited. Under free nature, we can have no
+standard of comparison, by which to judge of the effects of long-continued
+use or disuse, for we know not the parent-forms; but many animals have
+structures which can be explained by the effects of disuse. As Professor
+Owen has remarked, there is no greater anomaly in nature than a bird that
+cannot fly; yet there are several in this state. The logger-headed duck of
+South America can only flap along the surface of the water, and has its
+wings in nearly the same condition as the domestic Aylesbury duck. As the
+larger ground-feeding birds seldom take flight except to escape danger, I
+believe that the nearly wingless condition of several birds, which now
+inhabit or have lately inhabited several oceanic islands, tenanted by no
+beast of prey, has been caused by disuse. The ostrich indeed inhabits
+continents and is exposed to danger from which it cannot escape by flight,
+but by kicking it can defend itself from enemies, as well as any of the
+smaller {135} quadrupeds. We may imagine that the early progenitor of the
+ostrich had habits like those of a bustard, and that as natural selection
+increased in successive generations the size and weight of its body, its
+legs were used more, and its wings less, until they became incapable of
+flight.
+
+Kirby has remarked (and I have observed the same fact) that the anterior
+tarsi, or feet, of many male dung-feeding beetles are very often broken
+off; he examined seventeen specimens in his own collection, and not one had
+even a relic left. In the Onites apelles the tarsi are so habitually lost,
+that the insect has been described as not having them. In some other genera
+they are present, but in a rudimentary condition. In the Ateuchus or sacred
+beetle of the Egyptians, they are totally deficient. There is not
+sufficient evidence to induce me to believe that mutilations are ever
+inherited; and I should prefer explaining the entire absence of the
+anterior tarsi in Ateuchus, and their rudimentary condition in some other
+genera, by the long-continued effects of disuse in their progenitors; for
+as the tarsi are almost always lost in many dung-feeding beetles, they must
+be lost early in life, and therefore cannot be much used by these insects.
+
+In some cases we might easily put down to disuse modifications of structure
+which are wholly, or mainly, due to natural selection. Mr. Wollaston has
+discovered the remarkable fact that 200 beetles, out of the 550 species
+inhabiting Madeira, are so far deficient in wings that they cannot fly; and
+that of the twenty-nine endemic genera, no less than twenty-three genera
+have all their species in this condition! Several facts, namely, that
+beetles in many parts of the world are frequently blown to sea and perish;
+that the beetles in Madeira, as observed by Mr. Wollaston, lie much
+concealed, {136} until the wind lulls and the sun shines; that the
+proportion of wingless beetles is larger on the exposed Desertas than in
+Madeira itself; and especially the extraordinary fact, so strongly insisted
+on by Mr. Wollaston, of the almost entire absence of certain large groups
+of beetles, elsewhere excessively numerous, and which groups have habits of
+life almost necessitating frequent flight;--these several considerations
+have made me believe that the wingless condition of so many Madeira beetles
+is mainly due to the action of natural selection, but combined probably
+with disuse. For during thousands of successive generations each individual
+beetle which flew least, either from its wings having been ever so little
+less perfectly developed or from indolent habit, will have had the best
+chance of surviving from not being blown out to sea; and, on the other
+hand, those beetles which most readily took to flight would oftenest have
+been blown to sea and thus have been destroyed.
+
+The insects in Madeira which are not ground-feeders, and which, as the
+flower-feeding coleoptera and lepidoptera, must habitually use their wings
+to gain their subsistence, have, as Mr. Wollaston suspects, their wings not
+at all reduced, but even enlarged. This is quite compatible with the action
+of natural selection. For when a new insect first arrived on the island,
+the tendency of natural selection to enlarge or to reduce the wings, would
+depend on whether a greater number of individuals were saved by
+successfully battling with the winds, or by giving up the attempt and
+rarely or never flying. As with mariners shipwrecked near a coast, it would
+have been better for the good swimmers if they had been able to swim still
+further, whereas it would have been better for the bad swimmers if they had
+not been able to swim at all and had stuck to the wreck. {137}
+
+The eyes of moles and of some burrowing rodents are rudimentary in size,
+and in some cases are quite covered up by skin and fur. This state of the
+eyes is probably due to gradual reduction from disuse, but aided perhaps by
+natural selection. In South America, a burrowing rodent, the tuco-tuco, or
+Ctenomys, is even more subterranean in its habits than the mole; and I was
+assured by a Spaniard, who had often caught them, that they were frequently
+blind; one which I kept alive was certainly in this condition, the cause,
+as appeared on dissection, having been inflammation of the nictitating
+membrane. As frequent inflammation of the eyes must be injurious to any
+animal, and as eyes are certainly not indispensable to animals with
+subterranean habits, a reduction in their size with the adhesion of the
+eyelids and growth of fur over them, might in such case be an advantage;
+and if so, natural selection would constantly aid the effects of disuse.
+
+It is well known that several animals, belonging to the most different
+classes, which inhabit the caves of Styria and of Kentucky, are blind. In
+some of the crabs the foot-stalk for the eye remains, though the eye is
+gone; the stand for the telescope is there, though the telescope with its
+glasses has been lost. As it is difficult to imagine that eyes, though
+useless, could be in any way injurious to animals living in darkness, I
+attribute their loss wholly to disuse. In one of the blind animals, namely,
+the cave-rat, the eyes are of immense size; and Professor Silliman thought
+that it regained, after living some days in the light, some slight power of
+vision. In the same manner as in Madeira the wings of some of the insects
+have been enlarged, and the wings of others have been reduced by natural
+selection aided by use and disuse, so in the case of the cave-rat natural
+selection seems to have struggled with the loss of light and {138} to have
+increased the size of the eyes; whereas with all the other inhabitants of
+the caves, disuse by itself seems to have done its work.
+
+It is difficult to imagine conditions of life more similar than deep
+limestone caverns under a nearly similar climate; so that on the common
+view of the blind animals having been separately created for the American
+and European caverns, close similarity in their organisation and affinities
+might have been expected; but, as Schiödte and others have remarked, this
+is not the case, and the cave-insects of the two continents are not more
+closely allied than might have been anticipated from the general
+resemblance of the other inhabitants of North America and Europe. On my
+view we must suppose that American animals, having ordinary powers of
+vision, slowly migrated by successive generations from the outer world into
+the deeper and deeper recesses of the Kentucky caves, as did European
+animals into the caves of Europe. We have some evidence of this gradation
+of habit; for, as Schiödte remarks, "animals not far remote from ordinary
+forms, prepare the transition from light to darkness. Next follow those
+that are constructed for twilight; and, last of all, those destined for
+total darkness." By the time that an animal had reached, after numberless
+generations, the deepest recesses, disuse will on this view have more or
+less perfectly obliterated its eyes, and natural selection will often have
+effected other changes, such as an increase in the length of the antennæ or
+palpi, as a compensation for blindness. Notwithstanding such modifications,
+we might expect still to see in the cave-animals of America, affinities to
+the other inhabitants of that continent, and in those of Europe, to the
+inhabitants of the European continent. And this is the case with some of
+the American cave-animals, as I hear from {139} Professor Dana; and some of
+the European cave-insects are very closely allied to those of the
+surrounding country. It would be most difficult to give any rational
+explanation of the affinities of the blind cave-animals to the other
+inhabitants of the two continents on the ordinary view of their independent
+creation. That several of the inhabitants of the caves of the Old and New
+Worlds should be closely related, we might expect from the well-known
+relationship of most of their other productions. Far from feeling any
+surprise that some of the cave-animals should be very anomalous, as Agassiz
+has remarked in regard to the blind fish, the Amblyopsis, and as is the
+case with the blind Proteus with reference to the reptiles of Europe, I am
+only surprised that more wrecks of ancient life have not been preserved,
+owing to the less severe competition to which the inhabitants of these dark
+abodes will probably have been exposed.
+
+
+
+_Acclimatisation._--Habit is hereditary with plants, as in the period of
+flowering, in the amount of rain requisite for seeds to germinate, in the
+time of sleep, &c., and this leads me to say a few words on
+acclimatisation. As it is extremely common for species of the same genus to
+inhabit very hot and very cold countries, and as I believe that all the
+species of the same genus have descended from a single parent, if this view
+be correct, acclimatisation must be readily effected during long-continued
+descent. It is notorious that each species is adapted to the climate of its
+own home: species from an arctic or even from a temperate region cannot
+endure a tropical climate, or conversely. So again, many succulent plants
+cannot endure a damp climate. But the degree of adaptation of species to
+the climates under which they live is often overrated. {140} We may infer
+this from our frequent inability to predict whether or not an imported
+plant will endure our climate, and from the number of plants and animals
+brought from warmer countries which here enjoy good health. We have reason
+to believe that species in a state of nature are limited in their ranges by
+the competition of other organic beings quite as much as, or more than, by
+adaptation to particular climates. But whether or not the adaptation be
+generally very close, we have evidence, in the case of some few plants, of
+their becoming, to a certain extent, naturally habituated to different
+temperatures, or becoming acclimatised: thus the pines and rhododendrons,
+raised from seed collected by Dr. Hooker from trees growing at different
+heights on the Himalaya, were found in this country to possess different
+constitutional powers of resisting cold. Mr. Thwaites informs me that he
+has observed similar facts in Ceylon, and analogous observations have been
+made by Mr. H. C. Watson on European species of plants brought from the
+Azores to England. In regard to animals, several authentic cases could be
+given of species within historical times having largely extended their
+range from warmer to cooler latitudes, and conversely; but we do not
+positively know that these animals were strictly adapted to their native
+climate, but in all ordinary cases we assume such to be the case; nor do we
+know that they have subsequently become acclimatised to their new homes.
+
+As I believe that our domestic animals were originally chosen by
+uncivilised man because they were useful and bred readily under
+confinement, and not because they were subsequently found capable of
+far-extended transportation, I think the common and extraordinary capacity
+in our domestic animals of not only withstanding the most different
+climates but of being perfectly {141} fertile (a far severer test) under
+them, may be used as an argument that a large proportion of other animals,
+now in a state of nature, could easily be brought to bear widely different
+climates. We must not, however, push the foregoing argument too far, on
+account of the probable origin of some of our domestic animals from several
+wild stocks: the blood, for instance, of a tropical and arctic wolf or wild
+dog may perhaps be mingled in our domestic breeds. The rat and mouse cannot
+be considered as domestic animals, but they have been transported by man to
+many parts of the world, and now have a far wider range than any other
+rodent, living free under the cold climate of Faroe in the north and of the
+Falklands in the south, and on many islands in the torrid zones. Hence I am
+inclined to look at adaptation to any special climate as a quality readily
+grafted on an innate wide flexibility of constitution, which is common to
+most animals. On this view, the capacity of enduring the most different
+climates by man himself and by his domestic animals, and such facts as that
+former species of the elephant and rhinoceros were capable of enduring a
+glacial climate, whereas the living species are now all tropical or
+sub-tropical in their habits, ought not to be looked at as anomalies, but
+merely as examples of a very common flexibility of constitution, brought,
+under peculiar circumstances, into play.
+
+How much of the acclimatisation of species to any peculiar climate is due
+to mere habit, and how much to the natural selection of varieties having
+different innate constitutions, and how much to both means combined, is a
+very obscure question. That habit or custom has some influence I must
+believe, both from analogy, and from the incessant advice given in
+agricultural works, even in the ancient Encyclopædias of China, to be very
+{142} cautious in transposing animals from one district to another; for it
+is not likely that man should have succeeded in selecting so many breeds
+and sub-breeds with constitutions specially fitted for their own districts:
+the result must, I think, be due to habit. On the other hand, I can see no
+reason to doubt that natural selection will continually tend to preserve
+those individuals which are born with constitutions best adapted to their
+native countries. In treatises on many kinds of cultivated plants, certain
+varieties are said to withstand certain climates better than others: this
+is very strikingly shown in works on fruit trees published in the United
+States, in which certain varieties are habitually recommended for the
+northern, and others for the southern States; and as most of these
+varieties are of recent origin, they cannot owe their constitutional
+differences to habit. The case of the Jerusalem artichoke, which is never
+propagated by seed, and of which consequently new varieties have not been
+produced, has even been advanced--for it is now as tender as ever it
+was--as proving that acclimatisation cannot be effected! The case, also, of
+the kidney-bean has been often cited for a similar purpose, and with much
+greater weight; but until some one will sow, during a score of generations,
+his kidney-beans so early that a very large proportion are destroyed by
+frost, and then collect seed from the few survivors, with care to prevent
+accidental crosses, and then again get seed from these seedlings, with the
+same precautions, the experiment cannot be said to have been even tried.
+Nor let it be supposed that no differences in the constitution of seedling
+kidney-beans ever appear, for an account has been published how much more
+hardy some seedlings appeared to be than others.
+
+On the whole, I think we may conclude that habit, {143} use, and disuse,
+have, in some cases, played a considerable part in the modification of the
+constitution, and of the structure of various organs; but that the effects
+of use and disuse have often been largely combined with, and sometimes
+overmastered by the natural selection of innate variations.
+
+
+
+_Correlation of Growth._--I mean by this expression that the whole
+organisation is so tied together during its growth and development, that
+when slight variations in any one part occur, and are accumulated through
+natural selection, other parts become modified. This is a very important
+subject, most imperfectly understood. The most obvious case is, that
+modifications accumulated solely for the good of the young or larva, will,
+it may safely be concluded, affect the structure of the adult; in the same
+manner as any malconformation affecting the early embryo, seriously affects
+the whole organisation of the adult. The several parts of the body which
+are homologous, and which, at an early embryonic period, are alike, seem
+liable to vary in an allied manner: we see this in the right and left sides
+of the body varying in the same manner; in the front and hind legs, and
+even in the jaws and limbs, varying together, for the lower jaw is believed
+to be homologous with the limbs. These tendencies, I do not doubt, may be
+mastered more or less completely by natural selection: thus a family of
+stags once existed with an antler only on one side; and if this had been of
+any great use to the breed it might probably have been rendered permanent
+by natural selection.
+
+Homologous parts, as has been remarked by some authors, tend to cohere;
+this is often seen in monstrous plants; and nothing is more common than the
+union of homologous parts in normal structures, as the union of {144} the
+petals of the corolla into a tube. Hard parts seem to affect the form of
+adjoining soft parts; it is believed by some authors that the diversity in
+the shape of the pelvis in birds causes the remarkable diversity in the
+shape of their kidneys. Others believe that the shape of the pelvis in the
+human mother influences by pressure the shape of the head of the child. In
+snakes, according to Schlegel, the shape of the body and the manner of
+swallowing determine the position of several of the most important viscera.
+
+The nature of the bond of correlation is very frequently quite obscure. M.
+Is. Geoffroy St. Hilaire has forcibly remarked, that certain
+malconformations very frequently, and that others rarely coexist, without
+our being able to assign any reason. What can be more singular than the
+relation between blue eyes and deafness in cats, and the tortoise-shell
+colour with the female sex; the feathered feet and skin between the outer
+toes in pigeons, and the presence of more or less down on the young birds
+when first hatched, with the future colour of their plumage; or, again, the
+relation between the hair and teeth in the naked Turkish dog, though here
+probably homology comes into play? With respect to this latter case of
+correlation, I think it can hardly be accidental, that if we pick out the
+two orders of mammalia which are most abnormal in their dermal covering,
+viz. Cetacea (whales) and Edentata (armadilloes, scaly anteaters, &c.),
+that these are likewise the most abnormal in their teeth.
+
+I know of no case better adapted to show the importance of the laws of
+correlation in modifying important structures, independently of utility
+and, therefore, of natural selection, than that of the difference between
+the outer and inner flowers in some Compositous and Umbelliferous plants.
+Every one knows the {145} difference in the ray and central florets of, for
+instance, the daisy, and this difference is often accompanied with the
+abortion of parts of the flower. But, in some Compositous plants, the seeds
+also differ in shape and sculpture; and even the ovary itself, with its
+accessory parts, differs, as has been described by Cassini. These
+differences have been attributed by some authors to pressure, and the shape
+of the seeds in the ray-florets in some Compositæ countenances this idea;
+but, in the case of the corolla of the Umbelliferæ, it is by no means, as
+Dr. Hooker informs me, in species with the densest heads that the inner and
+outer flowers most frequently differ. It might have been thought that the
+development of the ray-petals by drawing nourishment from certain other
+parts of the flower had caused their abortion; but in some Compositæ there
+is a difference in the seeds of the outer and inner florets without any
+difference in the corolla. Possibly, these several differences may be
+connected with some difference in the flow of nutriment towards the central
+and external flowers: we know, at least, that in irregular flowers, those
+nearest to the axis are oftenest subject to peloria, and become regular. I
+may add, as an instance of this, and of a striking case of correlation,
+that I have recently observed in some garden pelargoniums, that the central
+flower of the truss often loses the patches of darker colour in the two
+upper petals; and that when this occurs, the adherent nectary is quite
+aborted; when the colour is absent from only one of the two upper petals,
+the nectary is only much shortened.
+
+With respect to the difference in the corolla of the central and exterior
+flowers of a head or umbel, I do not feel at all sure that C. C. Sprengel's
+idea that the ray-florets serve to attract insects, whose agency is highly
+advantageous in the fertilisation of plants of {146} these two orders, is
+so far-fetched, as it may at first appear: and if it be advantageous,
+natural selection may have come into play. But in regard to the differences
+both in the internal and external structure of the seeds, which are not
+always correlated with any differences in the flowers, it seems impossible
+that they can be in any way advantageous to the plant: yet in the
+Umbelliferæ these differences are of such apparent importance--the seeds
+being in some cases, according to Tausch, orthospermous in the exterior
+flowers and coelospermous in the central flowers,--that the elder De
+Candolle founded his main divisions of the order on analogous differences.
+Hence we see that modifications of structure, viewed by systematists as of
+high value, may be wholly due to unknown laws of correlated growth, and
+without being, as far as we can see, of the slightest service to the
+species.
+
+We may often falsely attribute to correlation of growth, structures which
+are common to whole groups of species, and which in truth are simply due to
+inheritance; for an ancient progenitor may have acquired through natural
+selection some one modification in structure, and, after thousands of
+generations, some other and independent modification; and these two
+modifications, having been transmitted to a whole group of descendants with
+diverse habits, would naturally be thought to be correlated in some
+necessary manner. So, again, I do not doubt that some apparent
+correlations, occurring throughout whole orders, are entirely due to the
+manner alone in which natural selection can act. For instance, Alph. De
+Candolle has remarked that winged seeds are never found in fruits which do
+not open: I should explain the rule by the fact that seeds could not
+gradually become winged through natural selection, except in fruits which
+opened; so that the individual plants producing {147} seeds which were a
+little better fitted to be wafted further, might get an advantage over
+those producing seed less fitted for dispersal; and this process could not
+possibly go on in fruit which did not open.
+
+The elder Geoffroy and Goethe propounded, at about the same period, their
+law of compensation or balancement of growth; or, as Goethe expressed it,
+"in order to spend on one side, nature is forced to economise on the other
+side." I think this holds true to a certain extent with our domestic
+productions: if nourishment flows to one part or organ in excess, it rarely
+flows, at least in excess, to another part; thus it is difficult to get a
+cow to give much milk and to fatten readily. The same varieties of the
+cabbage do not yield abundant and nutritious foliage and a copious supply
+of oil-bearing seeds. When the seeds in our fruits become atrophied, the
+fruit itself gains largely in size and quality. In our poultry, a large
+tuft of feathers on the head is generally accompanied by a diminished comb,
+and a large beard by diminished wattles. With species in a state of nature
+it can hardly be maintained that the law is of universal application; but
+many good observers, more especially botanists, believe in its truth. I
+will not, however, here give any instances, for I see hardly any way of
+distinguishing between the effects, on the one hand, of a part being
+largely developed through natural selection and another and adjoining part
+being reduced by this same process or by disuse, and, on the other hand,
+the actual withdrawal of nutriment from one part owing to the excess of
+growth in another and adjoining part.
+
+I suspect, also, that some of the cases of compensation which have been
+advanced, and likewise some other facts, may be merged under a more general
+principle, namely, that natural selection is continually trying to
+economise in every part of the organisation. If under {148} changed
+conditions of life a structure before useful becomes less useful, any
+diminution, however slight, in its development, will be seized on by
+natural selection, for it will profit the individual not to have its
+nutriment wasted in building up an useless structure. I can thus only
+understand a fact with which I was much struck when examining cirripedes,
+and of which many other instances could be given: namely, that when a
+cirripede is parasitic within another and is thus protected, it loses more
+or less completely its own shell or carapace. This is the case with the
+male Ibla, and in a truly extraordinary manner with the Proteolepas: for
+the carapace in all other cirripedes consists of the three highly-important
+anterior segments of the head enormously developed, and furnished with
+great nerves and muscles; but in the parasitic and protected Proteolepas,
+the whole anterior part of the head is reduced to the merest rudiment
+attached to the bases of the prehensile antennæ. Now the saving of a large
+and complex structure, when rendered superfluous by the parasitic habits of
+the Proteolepas, though effected by slow steps, would be a decided
+advantage to each successive individual of the species; for in the struggle
+for life to which every animal is exposed, each individual Proteolepas
+would have a better chance of supporting itself, by less nutriment being
+wasted in developing a structure now become useless.
+
+Thus, as I believe, natural selection will always succeed in the long run
+in reducing and saving every part of the organisation, as soon as it is
+rendered superfluous, without by any means causing some other part to be
+largely developed in a corresponding degree. And, conversely, that natural
+selection may perfectly well succeed in largely developing any organ,
+without requiring as a necessary compensation the reduction of some
+adjoining part. {149}
+
+It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in
+varieties and in species, that when any part or organ is repeated many
+times in the structure of the same individual (as the vertebræ in snakes,
+and the stamens in polyandrous flowers) the number is variable; whereas the
+number of the same part or organ, when it occurs in lesser numbers, is
+constant. The same author and some botanists have further remarked that
+multiple parts are also very liable to variation in structure. Inasmuch as
+this "vegetative repetition," to use Prof. Owen's expression, seems to be a
+sign of low organisation, the foregoing remark seems connected with the
+very general opinion of naturalists, that beings low in the scale of nature
+are more variable than those which are higher. I presume that lowness in
+this case means that the several parts of the organisation have been but
+little specialised for particular functions; and as long as the same part
+has to perform diversified work, we can perhaps see why it should remain
+variable, that is, why natural selection should have preserved or rejected
+each little deviation of form less carefully than when the part has to
+serve for one special purpose alone. In the same way that a knife which has
+to cut all sorts of things may be of almost any shape; whilst a tool for
+some particular object had better be of some particular shape. Natural
+selection, it should never be forgotten, can act on each part of each
+being, solely through and for its advantage.
+
+Rudimentary parts, it has been stated by some authors, and I believe with
+truth, are apt to be highly variable. We shall have to recur to the general
+subject of rudimentary and aborted organs; and I will here only add that
+their variability seems to be owing to their uselessness, and therefore to
+natural selection having no power to check deviations in their structure.
+Thus {150} rudimentary parts are left to the free play of the various laws
+of growth, to the effects of long-continued disuse, and to the tendency to
+reversion.
+
+
+
+_A part developed in any species in an extraordinary degree or manner, in
+comparison with the same part in allied species, tends to be highly
+variable._--Several years ago I was much struck with a remark, nearly to
+the above effect, published by Mr. Waterhouse. I infer also from an
+observation made by Professor Owen, with respect to the length of the arms
+of the ourang-outang, that he has come to a nearly similar conclusion. It
+is hopeless to attempt to convince any one of the truth of this proposition
+without giving the long array of facts which I have collected, and which
+cannot possibly be here introduced. I can only state my conviction that it
+is a rule of high generality. I am aware of several causes of error, but I
+hope that I have made due allowance for them. It should be understood that
+the rule by no means applies to any part, however unusually developed,
+unless it be unusually developed in comparison with the same part in
+closely allied species. Thus, the bat's wing is a most abnormal structure
+in the class mammalia; but the rule would not here apply, because there is
+a whole group of bats having wings; it would apply only if some one species
+of bat had its wings developed in some remarkable manner in comparison with
+the other species of the same genus. The rule applies very strongly in the
+case of secondary sexual characters, when displayed in any unusual manner.
+The term, secondary sexual characters, used by Hunter, applies to
+characters which are attached to one sex, but are not directly connected
+with the act of reproduction. The rule applies to males and females; but as
+females more rarely offer remarkable secondary sexual characters, it
+applies {151} more rarely to them. The rule being so plainly applicable in
+the case of secondary sexual characters, may be due to the great
+variability of these characters, whether or not displayed in any unusual
+manner--of which fact I think there can be little doubt. But that our rule
+is not confined to secondary sexual characters is clearly shown in the case
+of hermaphrodite cirripedes; and I may here add, that I particularly
+attended to Mr. Waterhouse's remark, whilst investigating this Order, and I
+am fully convinced that the rule almost invariably holds good with
+cirripedes. I shall, in my future work, give a list of the more remarkable
+cases; I will here only briefly give one, as it illustrates the rule in its
+largest application. The opercular valves of sessile cirripedes (rock
+barnacles) are, in every sense of the word, very important structures, and
+they differ extremely little even in different genera; but in the several
+species of one genus, Pyrgoma, these valves present a marvellous amount of
+diversification: the homologous valves in the different species being
+sometimes wholly unlike in shape; and the amount of variation in the
+individuals of several of the species is so great, that it is no
+exaggeration to state that the varieties differ more from each other in the
+characters of these important valves than do other species of distinct
+genera.
+
+As birds within the same country vary in a remarkably small degree, I have
+particularly attended to them, and the rule seems to me certainly to hold
+good in this class. I cannot make out that it applies to plants, and this
+would seriously have shaken my belief in its truth, had not the great
+variability in plants made it particularly difficult to compare their
+relative degrees of variability.
+
+When we see any part or organ developed in a remarkable degree or manner in
+any species, the fair {152} presumption is that it is of high importance to
+that species; nevertheless the part in this case is eminently liable to
+variation. Why should this be so? On the view that each species has been
+independently created, with all its parts as we now see them, I can see no
+explanation. But on the view that groups of species have descended from
+other species, and have been modified through natural selection, I think we
+can obtain some light. In our domestic animals, if any part, or the whole
+animal, be neglected and no selection be applied, that part (for instance,
+the comb in the Dorking fowl) or the whole breed will cease to have a
+nearly uniform character. The breed will then be said to have degenerated.
+In rudimentary organs, and in those which have been but little specialised
+for any particular purpose, and perhaps in polymorphic groups, we see a
+nearly parallel natural case; for in such cases natural selection either
+has not or cannot come into full play, and thus the organisation is left in
+a fluctuating condition. But what here more especially concerns us is, that
+in our domestic animals those points, which at the present time are
+undergoing rapid change by continued selection, are also eminently liable
+to variation. Look at the breeds of the pigeon; see what a prodigious
+amount of difference there is in the beak of the different tumblers, in the
+beak and wattle of the different carriers, in the carriage and tail of our
+fantails, &c., these being the points now mainly attended to by English
+fanciers. Even in the sub-breeds, as in the short-faced tumbler, it is
+notoriously difficult to breed them nearly to perfection, and frequently
+individuals are born which depart widely from the standard. There may be
+truly said to be a constant struggle going on between, on the one hand, the
+tendency to reversion to a less modified state, as well as an innate
+tendency to further {153} variability of all kinds, and, on the other hand,
+the power of steady selection to keep the breed true. In the long run
+selection gains the day, and we do not expect to fail so far as to breed a
+bird as coarse as a common tumbler from a good short-faced strain. But as
+long as selection is rapidly going on, there may always be expected to be
+much variability in the structure undergoing modification. It further
+deserves notice that these variable characters, produced by man's
+selection, sometimes become attached, from causes quite unknown to us, more
+to one sex than to the other, generally to the male sex, as with the wattle
+of carriers and the enlarged crop of pouters.
+
+Now let us turn to nature. When a part has been developed in an
+extraordinary manner in any one species, compared with the other species of
+the same genus, we may conclude that this part has undergone an
+extraordinary amount of modification since the period when the species
+branched off from the common progenitor of the genus. This period will
+seldom be remote in any extreme degree, as species very rarely endure for
+more than one geological period. An extraordinary amount of modification
+implies an unusually large and long-continued amount of variability, which
+has continually been accumulated by natural selection for the benefit of
+the species. But as the variability of the extraordinarily-developed part
+or organ has been so great and long-continued within a period not
+excessively remote, we might, as a general rule, expect still to find more
+variability in such parts than in other parts of the organisation which
+have remained for a much longer period nearly constant. And this, I am
+convinced, is the case. That the struggle between natural selection on the
+one hand, and the tendency to reversion and variability on the other hand,
+will in the {154} course of time cease; and that the most abnormally
+developed organs may be made constant, I can see no reason to doubt. Hence
+when an organ, however abnormal it may be, has been transmitted in
+approximately the same condition to many modified descendants, as in the
+case of the wing of the bat, it must have existed, according to my theory,
+for an immense period in nearly the same state; and thus it comes to be no
+more variable than any other structure. It is only in those cases in which
+the modification has been comparatively recent and extraordinarily great
+that we ought to find the _generative variability_, as it may be called,
+still present in a high degree. For in this case the variability will
+seldom as yet have been fixed by the continued selection of the individuals
+varying in the required manner and degree, and by the continued rejection
+of those tending to revert to a former and less modified condition.
+
+The principle included in these remarks may be extended. It is notorious
+that specific characters are more variable than generic. To explain by a
+simple example what is meant. If some species in a large genus of plants
+had blue flowers and some had red, the colour would be only a specific
+character, and no one would be surprised at one of the blue species varying
+into red, or conversely; but if all the species had blue flowers, the
+colour would become a generic character, and its variation would be a more
+unusual circumstance. I have chosen this example because an explanation is
+not in this case applicable, which most naturalists would advance, namely,
+that specific characters are more variable than generic, because they are
+taken from parts of less physiological importance than those commonly used
+for classing genera. I believe this explanation is partly, yet only
+indirectly, true; I shall, however, have to {155} return to this subject in
+our chapter on Classification. It would be almost superfluous to adduce
+evidence in support of the above statement, that specific characters are
+more variable than generic; but I have repeatedly noticed in works on
+natural history, that when an author has remarked with surprise that some
+_important_ organ or part, which is generally very constant throughout
+large groups of species, has _differed_ considerably in closely-allied
+species, that it has, also, been _variable_ in the individuals of some of
+the species. And this fact shows that a character, which is generally of
+generic value, when it sinks in value and becomes only of specific value,
+often becomes variable, though its physiological importance may remain the
+same. Something of the same kind applies to monstrosities: at least Is.
+Geoffroy St. Hilaire seems to entertain no doubt, that the more an organ
+normally differs in the different species of the same group, the more
+subject it is to individual anomalies.
+
+On the ordinary view of each species having been independently created, why
+should that part of the structure, which differs from the same part in
+other independently-created species of the same genus, be more variable
+than those parts which are closely alike in the several species? I do not
+see that any explanation can be given. But on the view of species being
+only strongly marked and fixed varieties, we might surely expect to find
+them still often continuing to vary in those parts of their structure which
+have varied within a moderately recent period, and which have thus come to
+differ. Or to state the case in another manner:--the points in which all
+the species of a genus resemble each other, and in which they differ from
+the species of some other genus, are called generic characters; and these
+characters in common I attribute to {156} inheritance from a common
+progenitor, for it can rarely have happened that natural selection will
+have modified several species, fitted to more or less widely-different
+habits, in exactly the same manner: and as these so-called generic
+characters have been inherited from a remote period, since that period when
+the species first branched off from their common progenitor, and
+subsequently have not varied or come to differ in any degree, or only in a
+slight degree, it is not probable that they should vary at the present day.
+On the other hand, the points in which species differ from other species of
+the same genus, are called specific characters; and as these specific
+characters have varied and come to differ within the period of the
+branching off of the species from a common progenitor, it is probable that
+they should still often be in some degree variable,--at least more variable
+than those parts of the organisation which have for a very long period
+remained constant.
+
+In connexion with the present subject, I will make only two other remarks.
+I think it will be admitted, without my entering on details, that secondary
+sexual characters are very variable; I think it also will be admitted that
+species of the same group differ from each other more widely in their
+secondary sexual characters, than in other parts of their organisation;
+compare, for instance, the amount of difference between the males of
+gallinaceous birds, in which secondary sexual characters are strongly
+displayed, with the amount of difference between their females; and the
+truth of this proposition will be granted. The cause of the original
+variability of secondary sexual characters is not manifest; but we can see
+why these characters should not have been rendered as constant and uniform
+as other parts of the organisation; for secondary sexual characters have
+been accumulated by sexual selection, which {157} is less rigid in its
+action than ordinary selection, as it does not entail death, but only gives
+fewer offspring to the less favoured males. Whatever the cause may be of
+the variability of secondary sexual characters, as they are highly
+variable, sexual selection will have had a wide scope for action, and may
+thus readily have succeeded in giving to the species of the same group a
+greater amount of difference in their sexual characters, than in other
+parts of their structure.
+
+It is a remarkable fact, that the secondary sexual differences between the
+two sexes of the same species are generally displayed in the very same
+parts of the organisation in which the different species of the same genus
+differ from each other. Of this fact I will give in illustration two
+instances, the first which happen to stand on my list; and as the
+differences in these cases are of a very unusual nature, the relation can
+hardly be accidental. The same number of joints in the tarsi is a character
+generally common to very large groups of beetles, but in the Engidæ, as
+Westwood has remarked, the number varies greatly; and the number likewise
+differs in the two sexes of the same species: again in fossorial
+hymenoptera, the manner of neuration of the wings is a character of the
+highest importance, because common to large groups; but in certain genera
+the neuration differs in the different species, and likewise in the two
+sexes of the same species. This relation has a clear meaning on my view of
+the subject: I look at all the species of the same genus as having as
+certainly descended from the same progenitor, as have the two sexes of any
+one of the species. Consequently, whatever part of the structure of the
+common progenitor, or of its early descendants, became variable; variations
+of this part would, it is highly probable, be taken advantage of by natural
+and sexual selection, in order to fit {158} the several species to their
+several places in the economy of nature, and likewise to fit the two sexes
+of the same species to each other, or to fit the males and females to
+different habits of life, or the males to struggle with other males for the
+possession of the females.
+
+Finally, then, I conclude that the greater variability of specific
+characters, or those which distinguish species from species, than of
+generic characters, or those which the species possess in common;--that the
+frequent extreme variability of any part which is developed in a species in
+an extraordinary manner in comparison with the same part in its congeners;
+and the slight degree of variability in a part, however extraordinarily it
+may be developed, if it be common to a whole group of species;--that the
+great variability of secondary sexual characters, and the great amount of
+difference in these same characters between closely allied species;--that
+secondary sexual and ordinary specific differences are generally displayed
+in the same parts of the organisation,--are all principles closely
+connected together. All being mainly due to the species of the same group
+having descended from a common progenitor, from whom they have inherited
+much in common,--to parts which have recently and largely varied being more
+likely still to go on varying than parts which have long been inherited and
+have not varied,--to natural selection having more or less completely,
+according to the lapse of time, overmastered the tendency to reversion and
+to further variability,--to sexual selection being less rigid than ordinary
+selection,--and to variations in the same parts having been accumulated by
+natural and sexual selection, and having been thus adapted for secondary
+sexual, and for ordinary specific purposes. {159}
+
+
+
+_Distinct species present analogous variations; and a variety of one
+species often assumes some of the characters of an allied species, or
+reverts to some of the characters of an early progenitor._--These
+propositions will be most readily understood by looking to our domestic
+races. The most distinct breeds of pigeons, in countries most widely apart,
+present sub-varieties with reversed feathers on the head and feathers on
+the feet,--characters not possessed by the aboriginal rock-pigeon; these
+then are analogous variations in two or more distinct races. The frequent
+presence of fourteen or even sixteen tail-feathers in the pouter, may be
+considered as a variation representing the normal structure of another
+race, the fantail. I presume that no one will doubt that all such analogous
+variations are due to the several races of the pigeon having inherited from
+a common parent the same constitution and tendency to variation, when acted
+on by similar unknown influences. In the vegetable kingdom we have a case
+of analogous variation, in the enlarged stems, or roots as commonly called,
+of the Swedish turnip and Ruta baga, plants which several botanists rank as
+varieties produced by cultivation from a common parent: if this be not so,
+the case will then be one of analogous variation in two so-called distinct
+species; and to these a third may be added, namely, the common turnip.
+According to the ordinary view of each species having been independently
+created, we should have to attribute this similarity in the enlarged stems
+of these three plants, not to the _vera causa_ of community of descent, and
+a consequent tendency to vary in a like manner, but to three separate yet
+closely related acts of creation.
+
+With pigeons, however, we have another case, namely, the occasional
+appearance in all the breeds, of slaty-blue birds with two black bars on
+the wings, a white {160} rump, a bar at the end of the tail, with the outer
+feathers externally edged near their bases with white. As all these marks
+are characteristic of the parent rock-pigeon, I presume that no one will
+doubt that this is a case of reversion, and not of a new yet analogous
+variation appearing in the several breeds. We may I think confidently come
+to this conclusion, because, as we have seen, these coloured marks are
+eminently liable to appear in the crossed offspring of two distinct and
+differently coloured breeds; and in this case there is nothing in the
+external conditions of life to cause the reappearance of the slaty-blue,
+with the several marks, beyond the influence of the mere act of crossing on
+the laws of inheritance.
+
+No doubt it is a very surprising fact that characters should reappear after
+having been lost for many, perhaps for hundreds of generations. But when a
+breed has been crossed only once by some other breed, the offspring
+occasionally show a tendency to revert in character to the foreign breed
+for many generations--some say, for a dozen or even a score of generations.
+After twelve generations, the proportion of blood, to use a common
+expression, of any one ancestor, is only 1 in 2048; and yet, as we see, it
+is generally believed that a tendency to reversion is retained by this very
+small proportion of foreign blood. In a breed which has not been crossed,
+but in which _both_ parents have lost some character which their progenitor
+possessed, the tendency, whether strong or weak, to reproduce the lost
+character might be, as was formerly remarked, for all that we can see to
+the contrary, transmitted for almost any number of generations. When a
+character which has been lost in a breed, reappears after a great number of
+generations, the most probable hypothesis is, not that the offspring
+suddenly takes after an ancestor some hundred generations {161} distant,
+but that in each successive generation there has been a tendency to
+reproduce the character in question, which at last, under unknown
+favourable conditions, gains an ascendancy. For instance, it is probable
+that in each generation of the barb-pigeon, which produces most rarely a
+blue and black-barred bird, there has been a tendency in each generation in
+the plumage to assume this colour. This view is hypothetical, but could be
+supported by some facts; and I can see no more abstract improbability in a
+tendency to produce any character being inherited for an endless number of
+generations, than in quite useless or rudimentary organs being, as we all
+know them to be, thus inherited. Indeed, we may sometimes observe a mere
+tendency to produce a rudiment inherited: for instance, in the common
+snapdragon (Antirrhinum) a rudiment of a fifth stamen so often appears,
+that this plant must have an inherited tendency to produce it.
+
+As all the species of the same genus are supposed, on my theory, to have
+descended from a common parent, it might be expected that they would
+occasionally vary in an analogous manner; so that a variety of one species
+would resemble in some of its characters another species; this other
+species being on my view only a well-marked and permanent variety. But
+characters thus gained would probably be of an unimportant nature, for the
+presence of all important characters will be governed by natural selection,
+in accordance with the diverse habits of the species, and will not be left
+to the mutual action of the conditions of life and of a similar inherited
+constitution. It might further be expected that the species of the same
+genus would occasionally exhibit reversions to lost ancestral characters.
+As, however, we never know the exact character of the common ancestor of a
+group, we could not distinguish these two {162} cases: if, for instance, we
+did not know that the rock-pigeon was not feather-footed or turn-crowned,
+we could not have told, whether these characters in our domestic breeds
+were reversions or only analogous variations; but we might have inferred
+that the blueness was a case of reversion, from the number of the markings,
+which are correlated with the blue tint, and which it does not appear
+probable would all appear together from simple variation. More especially
+we might have inferred this, from the blue colour and marks so often
+appearing when distinct breeds of diverse colours are crossed. Hence,
+though under nature it must generally be left doubtful, what cases are
+reversions to an anciently existing character, and what are new but
+analogous variations, yet we ought, on my theory, sometimes to find the
+varying offspring of a species assuming characters (either from reversion
+or from analogous variation) which already occur in some other members of
+the same group. And this undoubtedly is the case in nature.
+
+A considerable part of the difficulty in recognising a variable species in
+our systematic works, is due to its varieties mocking, as it were, some of
+the other species of the same genus. A considerable catalogue, also, could
+be given of forms intermediate between two other forms, which themselves
+must be doubtfully ranked as either varieties or species; and this shows,
+unless all these forms be considered as independently created species, that
+the one in varying has assumed some of the characters of the other, so as
+to produce the intermediate form. But the best evidence is afforded by
+parts or organs of an important and uniform nature occasionally varying so
+as to acquire, in some degree, the character of the same part or organ in
+an allied species. I have collected a long list of such cases; but {163}
+here, as before, I lie under a great disadvantage in not being able to give
+them. I can only repeat that such cases certainly do occur, and seem to me
+very remarkable.
+
+I will, however, give one curious and complex case, not indeed as affecting
+any important character, but from occurring in several species of the same
+genus, partly under domestication and partly under nature. It is a case
+apparently of reversion. The ass not rarely has very distinct transverse
+bars on its legs, like those on the legs of the zebra: it has been asserted
+that these are plainest in the foal, and from inquiries which I have made,
+I believe this to be true. It has also been asserted that the stripe on
+each shoulder is sometimes double. The shoulder-stripe is certainly very
+variable in length and outline. A white ass, but _not_ an albino, has been
+described without either spinal or shoulder stripe; and these stripes are
+sometimes very obscure, or actually quite lost, in dark-coloured asses. The
+koulan of Pallas is said to have been seen with a double shoulder-stripe.
+The hemionus has no shoulder-stripe; but traces of it, as stated by Mr.
+Blyth and others, occasionally appear: and I have been informed by Colonel
+Poole that the foals of this species are generally striped on the legs, and
+faintly on the shoulder. The quagga, though so plainly barred like a zebra
+over the body, is without bars on the legs; but Dr. Gray has figured one
+specimen with very distinct zebra-like bars on the hocks.
+
+With respect to the horse, I have collected cases in England of the spinal
+stripe in horses of the most distinct breeds, and of _all_ colours;
+transverse bars on the legs are not rare in duns, mouse-duns, and in one
+instance in a chestnut: a faint shoulder-stripe may sometimes be seen in
+duns, and I have seen a trace in a {164} bay horse. My son made a careful
+examination and sketch for me of a dun Belgian cart-horse with a double
+stripe on each shoulder and with leg-stripes; and a man, whom I can
+implicitly trust, has examined for me a small dun Welch pony with _three_
+short parallel stripes on each shoulder.
+
+In the north-west part of India the Kattywar breed of horses is so
+generally striped, that, as I hear from Colonel Poole, who examined the
+breed for the Indian Government, a horse without stripes is not considered
+as purely-bred. The spine is always striped; the legs are generally barred;
+and the shoulder-stripe, which is sometimes double and sometimes treble, is
+common; the side of the face, moreover, is sometimes striped. The stripes
+are plainest in the foal; and sometimes quite disappear in old horses.
+Colonel Poole has seen both gray and bay Kattywar horses striped when first
+foaled. I have, also, reason to suspect, from information given me by Mr.
+W. W. Edwards, that with the English racehorse the spinal stripe is much
+commoner in the foal than in the full-grown animal. Without here entering
+on further details, I may state that I have collected cases of leg and
+shoulder stripes in horses of very different breeds, in various countries
+from Britain to Eastern China; and from Norway in the north to the Malay
+Archipelago in the south. In all parts of the world these stripes occur far
+oftenest in duns and mouse-duns; by the term dun a large range of colour is
+included, from one between brown and black to a close approach to
+cream-colour.
+
+I am aware that Colonel Hamilton Smith, who has written on this subject,
+believes that the several breeds of the horse have descended from several
+aboriginal species--one of which, the dun, was striped; and that the
+above-described appearances are all due to ancient {165} crosses with the
+dun stock. But I am not at all satisfied with this theory, and should be
+loth to apply it to breeds so distinct as the heavy Belgian cart-horse,
+Welch ponies, cobs, the lanky Kattywar race, &c., inhabiting the most
+distant parts of the world.
+
+Now let us turn to the effects of crossing the several species of the
+horse-genus. Rollin asserts, that the common mule from the ass and horse is
+particularly apt to have bars on its legs: according to Mr. Gosse, in
+certain parts of the United States about nine out of ten mules have striped
+legs. I once saw a mule with its legs so much striped that any one would at
+first have thought that it must have been the product of a zebra; and Mr.
+W. C. Martin, in his excellent treatise on the horse, has given a figure of
+a similar mule. In four coloured drawings, which I have seen, of hybrids
+between the ass and zebra, the legs were much more plainly barred than the
+rest of the body; and in one of them there was a double shoulder-stripe. In
+Lord Morton's famous hybrid from a chestnut mare and male quagga, the
+hybrid, and even the pure offspring subsequently produced from the mare by
+a black Arabian sire, were much more plainly barred across the legs than is
+even the pure quagga. Lastly, and this is another most remarkable case, a
+hybrid has been figured by Dr. Gray (and he informs me that he knows of a
+second case) from the ass and the hemionus; and this hybrid, though the ass
+seldom has stripes on his legs and the hemionus has none and has not even a
+shoulder-stripe, nevertheless had all four legs barred, and had three short
+shoulder-stripes, like those on the dun Welch pony, and even had some
+zebra-like stripes on the sides of its face. With respect to this last
+fact, I was so convinced that not even a stripe of colour appears from what
+would commonly be called an {166} accident, that I was led solely from the
+occurrence of the face-stripes on this hybrid from the ass and hemionus to
+ask Colonel Poole whether such face-stripes ever occur in the eminently
+striped Kattywar breed of horses, and was, as we have seen, answered in the
+affirmative.
+
+What now are we to say to these several facts? We see several very distinct
+species of the horse-genus becoming, by simple variation, striped on the
+legs like a zebra, or striped on the shoulders like an ass. In the horse we
+see this tendency strong whenever a dun tint appears--a tint which
+approaches to that of the general colouring of the other species of the
+genus. The appearance of the stripes is not accompanied by any change of
+form or by any other new character. We see this tendency to become striped
+most strongly displayed in hybrids from between several of the most
+distinct species. Now observe the case of the several breeds of pigeons:
+they are descended from a pigeon (including two or three sub-species or
+geographical races) of a bluish colour, with certain bars and other marks;
+and when any breed assumes by simple variation a bluish tint, these bars
+and other marks invariably reappear; but without any other change of form
+or character. When the oldest and truest breeds of various colours are
+crossed, we see a strong tendency for the blue tint and bars and marks to
+reappear in the mongrels. I have stated that the most probable hypothesis
+to account for the reappearance of very ancient characters, is--that there
+is a _tendency_ in the young of each successive generation to produce the
+long-lost character, and that this tendency, from unknown causes, sometimes
+prevails. And we have just seen that in several species of the horse-genus
+the stripes are either plainer or appear more commonly in the young than in
+the old. Call the breeds of pigeons, some of which have bred true for {167}
+centuries, species; and how exactly parallel is the case with that of the
+species of the horse-genus! For myself, I venture confidently to look back
+thousands on thousands of generations, and I see an animal striped like a
+zebra, but perhaps otherwise very differently constructed, the common
+parent of our domestic horse, whether or not it be descended from one or
+more wild stocks, of the ass, the hemionus, quagga, and zebra.
+
+He who believes that each equine species was independently created, will, I
+presume, assert that each species has been created with a tendency to vary,
+both under nature and under domestication, in this particular manner, so as
+often to become striped like other species of the genus; and that each has
+been created with a strong tendency, when crossed with species inhabiting
+distant quarters of the world, to produce hybrids resembling in their
+stripes, not their own parents, but other species of the genus. To admit
+this view is, as it seems to me, to reject a real for an unreal, or at
+least for an unknown, cause. It makes the works of God a mere mockery and
+deception; I would almost as soon believe with the old and ignorant
+cosmogonists, that fossil shells had never lived, but had been created in
+stone so as to mock the shells now living on the sea-shore.
+
+
+
+_Summary._--Our ignorance of the laws of variation is profound. Not in one
+case out of a hundred can we pretend to assign any reason why this or that
+part differs, more or less, from the same part in the parents. But whenever
+we have the means of instituting a comparison, the same laws appear to have
+acted in producing the lesser differences between varieties of the same
+species, and the greater differences between species of the same genus. The
+external conditions of life, as {168} climate and food, &c., seem to have
+induced some slight modifications. Habit in producing constitutional
+differences, and use in strengthening and disuse in weakening and
+diminishing organs, seem to have been more potent in their effects.
+Homologous parts tend to vary in the same way, and homologous parts tend to
+cohere. Modifications in hard parts and in external parts sometimes affect
+softer and internal parts. When one part is largely developed, perhaps it
+tends to draw nourishment from the adjoining parts; and every part of the
+structure which can be saved without detriment to the individual, will be
+saved. Changes of structure at an early age will generally affect parts
+subsequently developed; and there are very many other correlations of
+growth, the nature of which we are utterly unable to understand. Multiple
+parts are variable in number and in structure, perhaps arising from such
+parts not having been closely specialised to any particular function, so
+that their modifications have not been closely checked by natural
+selection. It is probably from this same cause that organic beings low in
+the scale of nature are more variable than those which have their whole
+organisation more specialised, and are higher in the scale. Rudimentary
+organs, from being useless, will be disregarded by natural selection, and
+hence probably are variable. Specific characters--that is, the characters
+which have come to differ since the several species of the same genus
+branched off from a common parent--are more variable than generic
+characters, or those which have long been inherited, and have not differed
+within this same period. In these remarks we have referred to special parts
+or organs being still variable, because they have recently varied and thus
+come to differ; but we have also seen in the second Chapter that the same
+principle applies to the whole individual; {169} for in a district where
+many species of any genus are found--that is, where there has been much
+former variation and differentiation, or where the manufactory of new
+specific forms has been actively at work--there, on an average, we now find
+most varieties or incipient species. Secondary sexual characters are highly
+variable, and such characters differ much in the species of the same group.
+Variability in the same parts of the organisation has generally been taken
+advantage of in giving secondary sexual differences to the sexes of the
+same species, and specific differences to the several species of the same
+genus. Any part or organ developed to an extraordinary size or in an
+extraordinary manner, in comparison with the same part or organ in the
+allied species, must have gone through an extraordinary amount of
+modification since the genus arose; and thus we can understand why it
+should often still be variable in a much higher degree than other parts;
+for variation is a long-continued and slow process, and natural selection
+will in such cases not as yet have had time to overcome the tendency to
+further variability and to reversion to a less modified state. But when a
+species with any extraordinarily-developed organ has become the parent of
+many modified descendants--which on my view must be a very slow process,
+requiring a long lapse of time--in this case, natural selection may readily
+have succeeded in giving a fixed character to the organ, in however
+extraordinary a manner it may be developed. Species inheriting nearly the
+same constitution from a common parent and exposed to similar influences
+will naturally tend to present analogous variations, and these same species
+may occasionally revert to some of the characters of their ancient
+progenitors. Although new and important modifications may not arise from
+reversion and analogous {170} variation, such modifications will add to the
+beautiful and harmonious diversity of nature.
+
+Whatever the cause may be of each slight difference in the offspring from
+their parents--and a cause for each must exist--it is the steady
+accumulation, through natural selection, of such differences, when
+beneficial to the individual, that gives rise to all the more important
+modifications of structure, by which the innumerable beings on the face of
+this earth are enabled to struggle with each other, and the best adapted to
+survive.
+
+       *       *       *       *       *
+
+
+{171}
+
+CHAPTER VI.
+
+DIFFICULTIES ON THEORY.
+
+    Difficulties on the theory of descent with
+    modification--Transitions--Absence or rarity of transitional
+    varieties--Transitions in habits of life--Diversified habits in the
+    same species--Species with habits widely different from those of their
+    allies--Organs of extreme perfection--Means of transition--Cases of
+    difficulty--Natura non facit saltum--Organs of small importance--Organs
+    not in all cases absolutely perfect--The law of Unity of Type and of
+    the Conditions of Existence embraced by the theory of Natural
+    Selection.
+
+Long before having arrived at this part of my work, a crowd of difficulties
+will have occurred to the reader. Some of them are so grave that to this
+day I can never reflect on them without being staggered; but, to the best
+of my judgment, the greater number are only apparent, and those that are
+real are not, I think, fatal to my theory.
+
+These difficulties and objections may be classed under the following
+heads:--Firstly, why, if species have descended from other species by
+insensibly fine gradations, do we not everywhere see innumerable
+transitional forms? Why is not all nature in confusion instead of the
+species being, as we see them, well defined?
+
+Secondly, is it possible that an animal having, for instance, the structure
+and habits of a bat, could have been formed by the modification of some
+animal with wholly different habits? Can we believe that natural selection
+could produce, on the one hand, organs of trifling importance, such as the
+tail of a giraffe, which serves as a fly-flapper, and, on the other hand,
+organs of {172} such wonderful structure, as the eye, of which we hardly as
+yet fully understand the inimitable perfection?
+
+Thirdly, can instincts be acquired and modified through natural selection?
+What shall we say to so marvellous an instinct as that which leads the bee
+to make cells, which has practically anticipated the discoveries of
+profound mathematicians?
+
+Fourthly, how can we account for species, when crossed, being sterile and
+producing sterile offspring, whereas, when varieties are crossed, their
+fertility is unimpaired?
+
+The two first heads shall be here discussed--Instinct and Hybridism in
+separate chapters.
+
+
+
+_On the absence or rarity of transitional varieties._--As natural selection
+acts solely by the preservation of profitable modifications, each new form
+will tend in a fully-stocked country to take the place of, and finally to
+exterminate, its own less improved parent or other less-favoured forms with
+which it comes into competition. Thus extinction and natural selection
+will, as we have seen, go hand in hand. Hence, if we look at each species
+as descended from some other unknown form, both the parent and all the
+transitional varieties will generally have been exterminated by the very
+process of formation and perfection of the new form.
+
+But, as by this theory innumerable transitional forms must have existed,
+why do we not find them embedded in countless numbers in the crust of the
+earth? It will be much more convenient to discuss this question in the
+chapter on the Imperfection of the geological record; and I will here only
+state that I believe the answer mainly lies in the record being
+incomparably less perfect than is generally supposed; the imperfection of
+the record being chiefly due to organic beings not inhabiting {173}
+profound depths of the sea, and to their remains being embedded and
+preserved to a future age only in masses of sediment sufficiently thick and
+extensive to withstand an enormous amount of future degradation; and such
+fossiliferous masses can be accumulated only where much sediment is
+deposited on the shallow bed of the sea, whilst it slowly subsides. These
+contingencies will concur only rarely, and after enormously long intervals.
+Whilst the bed of the sea is stationary or is rising, or when very little
+sediment is being deposited, there will be blanks in our geological
+history. The crust of the earth is a vast museum; but the natural
+collections have been made only at intervals of time immensely remote.
+
+But it may be urged that when several closely-allied species inhabit the
+same territory we surely ought to find at the present time many
+transitional forms. Let us take a simple case: in travelling from north to
+south over a continent, we generally meet at successive intervals with
+closely allied or representative species, evidently filling nearly the same
+place in the natural economy of the land. These representative species
+often meet and interlock; and as the one becomes rarer and rarer, the other
+becomes more and more frequent, till the one replaces the other. But if we
+compare these species where they intermingle, they are generally as
+absolutely distinct from each other in every detail of structure as are
+specimens taken from the metropolis inhabited by each. By my theory these
+allied species have descended from a common parent; and during the process
+of modification, each has become adapted to the conditions of life of its
+own region, and has supplanted and exterminated its original parent and all
+the transitional varieties between its past and present states. Hence we
+ought not to expect at the {174} present time to meet with numerous
+transitional varieties in each region, though they must have existed there,
+and may be embedded there in a fossil condition. But in the intermediate
+region, having intermediate conditions of life, why do we not now find
+closely-linking intermediate varieties? This difficulty for a long time
+quite confounded me. But I think it can be in large part explained.
+
+In the first place we should be extremely cautious in inferring, because an
+area is now continuous, that it has been continuous during a long period.
+Geology would lead us to believe that almost every continent has been
+broken up into islands even during the later tertiary periods; and in such
+islands distinct species might have been separately formed without the
+possibility of intermediate varieties existing in the intermediate zones.
+By changes in the form of the land and of climate, marine areas now
+continuous must often have existed within recent times in a far less
+continuous and uniform condition than at present. But I will pass over this
+way of escaping from the difficulty; for I believe that many perfectly
+defined species have been formed on strictly continuous areas; though I do
+not doubt that the formerly broken condition of areas now continuous has
+played an important part in the formation of new species, more especially
+with freely-crossing and wandering animals.
+
+In looking at species as they are now distributed over a wide area, we
+generally find them tolerably numerous over a large territory, then
+becoming somewhat abruptly rarer and rarer on the confines, and finally
+disappearing. Hence the neutral territory between two representative
+species is generally narrow in comparison with the territory proper to
+each. We see the same fact in ascending mountains, and sometimes {175} it
+is quite remarkable how abruptly, as Alph. de Candolle has observed, a
+common alpine species disappears. The same fact has been noticed by E.
+Forbes in sounding the depths of the sea with the dredge. To those who look
+at climate and the physical conditions of life as the all-important
+elements of distribution, these facts ought to cause surprise, as climate
+and height or depth graduate away insensibly. But when we bear in mind that
+almost every species, even in its metropolis, would increase immensely in
+numbers, were it not for other competing species; that nearly all either
+prey on or serve as prey for others; in short, that each organic being is
+either directly or indirectly related in the most important manner to other
+organic beings, we must see that the range of the inhabitants of any
+country by no means exclusively depends on insensibly changing physical
+conditions, but in large part on the presence of other species, on which it
+depends, or by which it is destroyed, or with which it comes into
+competition; and as these species are already defined objects (however they
+may have become so), not blending one into another by insensible
+gradations, the range of any one species, depending as it does on the range
+of others, will tend to be sharply defined. Moreover, each species on the
+confines of its range, where it exists in lessened numbers, will, during
+fluctuations in the number of its enemies or of its prey, or in the
+seasons, be extremely liable to utter extermination; and thus its
+geographical range will come to be still more sharply defined.
+
+If I am right in believing that allied or representative species, when
+inhabiting a continuous area, are generally so distributed that each has a
+wide range, with a comparatively narrow neutral territory between them, in
+which they become rather suddenly rarer and rarer; then, as varieties do
+not essentially differ from species, {176} the same rule will probably
+apply to both; and if we in imagination adapt a varying species to a very
+large area, we shall have to adapt two varieties to two large areas, and a
+third variety to a narrow intermediate zone. The intermediate variety,
+consequently, will exist in lesser numbers from inhabiting a narrow and
+lesser area; and practically, as far as I can make out, this rule holds
+good with varieties in a state of nature. I have met with striking
+instances of the rule in the case of varieties intermediate between
+well-marked varieties in the genus Balanus. And it would appear from
+information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that
+generally when varieties intermediate between two other forms occur, they
+are much rarer numerically than the forms which they connect. Now, if we
+may trust these facts and inferences, and therefore conclude that varieties
+linking two other varieties together have generally existed in lesser
+numbers than the forms which they connect, then, I think, we can understand
+why intermediate varieties should not endure for very long periods;--why as
+a general rule they should be exterminated and disappear, sooner than the
+forms which they originally linked together.
+
+For any form existing in lesser numbers would, as already remarked, run a
+greater chance of being exterminated than one existing in large numbers;
+and in this particular case the intermediate form would be eminently liable
+to the inroads of closely allied forms existing on both sides of it. But a
+far more important consideration, as I believe, is that, during the process
+of further modification, by which two varieties are supposed on my theory
+to be converted and perfected into two distinct species, the two which
+exist in larger numbers from inhabiting larger areas, will have a great
+advantage over the intermediate variety, which exists {177} in smaller
+numbers in a narrow and intermediate zone. For forms existing in larger
+numbers will always have a better chance, within any given period, of
+presenting further favourable variations for natural selection to seize on,
+than will the rarer forms which exist in lesser numbers. Hence, the more
+common forms, in the race for life, will tend to beat and supplant the less
+common forms, for these will be more slowly modified and improved. It is
+the same principle which, as I believe, accounts for the common species in
+each country, as shown in the second chapter, presenting on an average a
+greater number of well-marked varieties than do the rarer species. I may
+illustrate what I mean by supposing three varieties of sheep to be kept,
+one adapted to an extensive mountainous region; a second to a comparatively
+narrow, hilly tract; and a third to wide plains at the base; and that the
+inhabitants are all trying with equal steadiness and skill to improve their
+stocks by selection; the chances in this case will be strongly in favour of
+the great holders on the mountains or on the plains improving their breeds
+more quickly than the small holders on the intermediate narrow, hilly
+tract; and consequently the improved mountain or plain breed will soon take
+the place of the less improved hill breed; and thus the two breeds, which
+originally existed in greater numbers, will come into close contact with
+each other, without the interposition of the supplanted, intermediate
+hill-variety.
+
+To sum up, I believe that species come to be tolerably well-defined
+objects, and do not at any one period present an inextricable chaos of
+varying and intermediate links: firstly, because new varieties are very
+slowly formed, for variation is a very slow process, and natural selection
+can do nothing until favourable {178} variations chance to occur, and until
+a place in the natural polity of the country can be better filled by some
+modification of some one or more of its inhabitants. And such new places
+will depend on slow changes of climate, or on the occasional immigration of
+new inhabitants, and, probably, in a still more important degree, on some
+of the old inhabitants becoming slowly modified, with the new forms thus
+produced and the old ones acting and reacting on each other. So that, in
+any one region and at any one time, we ought only to see a few species
+presenting slight modifications of structure in some degree permanent; and
+this assuredly we do see.
+
+Secondly, areas now continuous must often have existed within the recent
+period in isolated portions, in which many forms, more especially amongst
+the classes which unite for each birth and wander much, may have separately
+been rendered sufficiently distinct to rank as representative species. In
+this case, intermediate varieties between the several representative
+species and their common parent, must formerly have existed in each broken
+portion of the land, but these links will have been supplanted and
+exterminated during the process of natural selection, so that they will no
+longer exist in a living state.
+
+Thirdly, when two or more varieties have been formed in different portions
+of a strictly continuous area, intermediate varieties will, it is probable,
+at first have been formed in the intermediate zones, but they will
+generally have had a short duration. For these intermediate varieties will,
+from reasons already assigned (namely from what we know of the actual
+distribution of closely allied or representative species, and likewise of
+acknowledged varieties), exist in the intermediate zones in lesser numbers
+than the varieties which they {179} tend to connect. From this cause alone
+the intermediate varieties will be liable to accidental extermination; and
+during the process of further modification through natural selection, they
+will almost certainly be beaten and supplanted by the forms which they
+connect; for these from existing in greater numbers will, in the aggregate,
+present more variation, and thus be further improved through natural
+selection and gain further advantages.
+
+Lastly, looking not to any one time, but to all time, if my theory be true,
+numberless intermediate varieties, linking most closely all the species of
+the same group together, must assuredly have existed; but the very process
+of natural selection constantly tends, as has been so often remarked, to
+exterminate the parent-forms and the intermediate links. Consequently
+evidence of their former existence could be found only amongst fossil
+remains, which are preserved, as we shall in a future chapter attempt to
+show, in an extremely imperfect and intermittent record.
+
+
+
+_On the origin and transitions of organic beings with peculiar habits and
+structure._--It has been asked by the opponents of such views as I hold,
+how, for instance, a land carnivorous animal could have been converted into
+one with aquatic habits; for how could the animal in its transitional state
+have subsisted? It would be easy to show that within the same group
+carnivorous animals exist having every intermediate grade between truly
+aquatic and strictly terrestrial habits; and as each exists by a struggle
+for life, it is clear that each is well adapted in its habits to its place
+in nature. Look at the Mustela vison of North America, which has webbed
+feet and which resembles an otter in its fur, short legs, and form of tail;
+during summer this animal {180} dives for and preys on fish, but during the
+long winter it leaves the frozen waters, and preys like other polecats on
+mice and land animals. If a different case had been taken, and it had been
+asked how an insectivorous quadruped could possibly have been converted
+into a flying bat, the question would have been far more difficult, and I
+could have given no answer. Yet I think such difficulties have very little
+weight.
+
+Here, as on other occasions, I lie under a heavy disadvantage, for out of
+the many striking cases which I have collected, I can give only one or two
+instances of transitional habits and structures in closely allied species
+of the same genus; and of diversified habits, either constant or
+occasional, in the same species. And it seems to me that nothing less than
+a long list of such cases is sufficient to lessen the difficulty in any
+particular case like that of the bat.
+
+Look at the family of squirrels; here we have the finest gradation from
+animals with their tails only slightly flattened, and from others, as Sir
+J. Richardson has remarked, with the posterior part of their bodies rather
+wide and with the skin on their flanks rather full, to the so-called flying
+squirrels; and flying squirrels have their limbs and even the base of the
+tail united by a broad expanse of skin, which serves as a parachute and
+allows them to glide through the air to an astonishing distance from tree
+to tree. We cannot doubt that each structure is of use to each kind of
+squirrel in its own country, by enabling it to escape birds or beasts of
+prey, or to collect food more quickly, or, as there is reason to believe,
+by lessening the danger from occasional falls. But it does not follow from
+this fact that the structure of each squirrel is the best that it is
+possible to conceive under all natural conditions. Let the climate and
+vegetation change, let other competing {181} rodents or new beasts of prey
+immigrate, or old ones become modified, and all analogy would lead us to
+believe that some at least of the squirrels would decrease in numbers or
+become exterminated, unless they also became modified and improved in
+structure in a corresponding manner. Therefore, I can see no difficulty,
+more especially under changing conditions of life, in the continued
+preservation of individuals with fuller and fuller flank-membranes, each
+modification being useful, each being propagated, until by the accumulated
+effects of this process of natural selection, a perfect so-called flying
+squirrel was produced.
+
+Now look at the Galeopithecus or flying lemur, which formerly was falsely
+ranked amongst bats. It has an extremely wide flank-membrane, stretching
+from the corners of the jaw to the tail, and including the limbs and the
+elongated fingers: the flank-membrane is, also, furnished with an extensor
+muscle. Although no graduated links of structure, fitted for gliding
+through the air, now connect the Galeopithecus with the other Lemuridæ, yet
+I see no difficulty in supposing that such links formerly existed, and that
+each had been formed by the same steps as in the case of the less perfectly
+gliding squirrels; and that each grade of structure was useful to its
+possessor. Nor can I see any insuperable difficulty in further believing it
+possible that the membrane-connected fingers and forearm of the
+Galeopithecus might be greatly lengthened by natural selection; and this,
+as far as the organs of flight are concerned, would convert it into a bat.
+In bats which have the wing-membrane extended from the top of the shoulder
+to the tail, including the hind-legs, we perhaps see traces of an apparatus
+originally constructed for gliding through the air rather than for flight.
+{182}
+
+If about a dozen genera of birds had become extinct or were unknown, who
+would have ventured to have surmised that birds might have existed which
+used their wings solely as flappers, like the logger-headed duck
+(Micropterus of Eyton); as fins in the water and front legs on the land,
+like the penguin; as sails, like the ostrich; and functionally for no
+purpose, like the Apteryx. Yet the structure of each of these birds is good
+for it, under the conditions of life to which it is exposed, for each has
+to live by a struggle; but it is not necessarily the best possible under
+all possible conditions. It must not be inferred from these remarks that
+any of the grades of wing-structure here alluded to, which perhaps may all
+have resulted from disuse, indicate the natural steps by which birds have
+acquired their perfect power of flight; but they serve, at least, to show
+what diversified means of transition are possible.
+
+Seeing that a few members of such water-breathing classes as the Crustacea
+and Mollusca are adapted to live on the land; and seeing that we have
+flying birds and mammals, flying insects of the most diversified types, and
+formerly had flying reptiles, it is conceivable that flying-fish, which now
+glide far through the air, slightly rising and turning by the aid of their
+fluttering fins, might have been modified into perfectly winged animals. If
+this had been effected, who would have ever imagined that in an early
+transitional state they had been inhabitants of the open ocean, and had
+used their incipient organs of flight exclusively, as far as we know, to
+escape being devoured by other fish?
+
+When we see any structure highly perfected for any particular habit, as the
+wings of a bird for flight, we should bear in mind that animals displaying
+early {183} transitional grades of the structure will seldom continue to
+exist to the present day, for they will have been supplanted by the very
+process of perfection through natural selection. Furthermore, we may
+conclude that transitional grades between structures fitted for very
+different habits of life will rarely have been developed at an early period
+in great numbers and under many subordinate forms. Thus, to return to our
+imaginary illustration of the flying-fish, it does not seem probable that
+fishes capable of true flight would have been developed under many
+subordinate forms, for taking prey of many kinds in many ways, on the land
+and in the water, until their organs of flight had come to a high stage of
+perfection, so as to have given them a decided advantage over other animals
+in the battle for life. Hence the chance of discovering species with
+transitional grades of structure in a fossil condition will always be less,
+from their having existed in lesser numbers, than in the case of species
+with fully developed structures.
+
+I will now give two or three instances of diversified and of changed habits
+in the individuals of the same species. When either case occurs, it would
+be easy for natural selection to fit the animal, by some modification of
+its structure, for its changed habits, or exclusively for one of its
+several different habits. But it is difficult to tell, and immaterial for
+us, whether habits generally change first and structure afterwards; or
+whether slight modifications of structure lead to changed habits; both
+probably often change almost simultaneously. Of cases of changed habits it
+will suffice merely to allude to that of the many British insects which now
+feed on exotic plants, or exclusively on artificial substances. Of
+diversified habits innumerable instances could be given: I have often
+watched a tyrant flycatcher (Saurophagus sulphuratus) in South America,
+hovering over one spot {184} and then proceeding to another, like a
+kestrel, and at other times standing stationary on the margin of water, and
+then dashing like a kingfisher at a fish. In our own country the larger
+titmouse (Parus major) may be seen climbing branches, almost like a
+creeper; it often, like a shrike, kills small birds by blows on the head;
+and I have many times seen and heard it hammering the seeds of the yew on a
+branch, and thus breaking them like a nuthatch. In North America the black
+bear was seen by Hearne swimming for hours with widely open mouth, thus
+catching, almost like a whale, insects in the water.
+
+As we sometimes see individuals of a species following habits widely
+different from those of their own species and of the other species of the
+same genus, we might expect, on my theory, that such individuals would
+occasionally have given rise to new species, having anomalous habits, and
+with their structure either slightly or considerably modified from that of
+their proper type. And such instances do occur in nature. Can a more
+striking instance of adaptation be given than that of a woodpecker for
+climbing trees and for seizing insects in the chinks of the bark? Yet in
+North America there are woodpeckers which feed largely on fruit, and others
+with elongated wings which chase insects on the wing; and on the plains of
+La Plata, where not a tree grows, there is a woodpecker, which in every
+essential part of its organisation, even in its colouring, in the harsh
+tone of its voice, and undulatory flight, told me plainly of its close
+blood-relationship to our common species; yet it is a woodpecker which
+never climbs a tree!
+
+Petrels are the most aërial and oceanic of birds, yet in the quiet Sounds
+of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its
+astonishing power of diving, its manner of swimming, and of flying when
+{185} unwillingly it takes flight, would be mistaken by any one for an auk
+or grebe; nevertheless, it is essentially a petrel, but with many parts of
+its organisation profoundly modified. On the other hand, the acutest
+observer by examining the dead body of the water-ouzel would never have
+suspected its sub-aquatic habits; yet this anomalous member of the strictly
+terrestrial thrush family wholly subsists by diving,--grasping the stones
+with its feet and using its wings under water.
+
+He who believes that each being has been created as we now see it, must
+occasionally have felt surprise when he has met with an animal having
+habits and structure not at all in agreement. What can be plainer than that
+the webbed feet of ducks and geese are formed for swimming? yet there are
+upland geese with webbed feet which rarely or never go near the water; and
+no one except Audubon has seen the frigate-bird, which has all its four
+toes webbed, alight on the surface of the sea. On the other hand grebes and
+coots are eminently aquatic, although their toes are only bordered by
+membrane. What seems plainer than that the long toes of grallatores are
+formed for walking over swamps and floating plants, yet the water-hen is
+nearly as aquatic as the coot; and the landrail nearly as terrestrial as
+the quail or partridge. In such cases, and many others could be given,
+habits have changed without a corresponding change of structure. The webbed
+feet of the upland goose may be said to have become rudimentary in
+function, though not in structure. In the frigate-bird, the deeply-scooped
+membrane between the toes shows that structure has begun to change.
+
+He who believes in separate and innumerable acts of creation will say, that
+in these cases it has pleased the Creator to cause a being of one type to
+take the place of one of another type; but this seems to me only {186}
+restating the fact in dignified language. He who believes in the struggle
+for existence and in the principle of natural selection, will acknowledge
+that every organic being is constantly endeavouring to increase in numbers;
+and that if any one being vary ever so little, either in habits or
+structure, and thus gain an advantage over some other inhabitant of the
+country, it will seize on the place of that inhabitant, however different
+it may be from its own place. Hence it will cause him no surprise that
+there should be geese and frigate-birds with webbed feet, living on the dry
+land or most rarely alighting on the water; that there should be long-toed
+corncrakes living in meadows instead of in swamps; that there should be
+woodpeckers where not a tree grows; that there should be diving thrushes,
+and petrels with the habits of auks.
+
+
+
+_Organs of extreme perfection and complication._--To suppose that the eye,
+with all its inimitable contrivances for adjusting the focus to different
+distances, for admitting different amounts of light, and for the correction
+of spherical and chromatic aberration, could have been formed by natural
+selection, seems, I freely confess, absurd in the highest possible degree.
+Yet reason tells me, that if numerous gradations from a perfect and complex
+eye to one very imperfect and simple, each grade being useful to its
+possessor, can be shown to exist; if further, the eye does vary ever so
+slightly, and the variations be inherited, which is certainly the case; and
+if any variation or modification in the organ be ever useful to an animal
+under changing conditions of life, then the difficulty of believing that a
+perfect and complex eye could be formed by natural selection, though
+insuperable by our imagination, can hardly be considered real. How a nerve
+comes to be sensitive to {187} light, hardly concerns us more than how life
+itself first originated; but I may remark that several facts make me
+suspect that any sensitive nerve may be rendered sensitive to light, and
+likewise to those coarser vibrations of the air which produce sound.
+
+In looking for the gradations by which an organ in any species has been
+perfected, we ought to look exclusively to its lineal ancestors; but this
+is scarcely ever possible, and we are forced in each case to look to
+species of the same group, that is to the collateral descendants from the
+same original parent-form, in order to see what gradations are possible,
+and for the chance of some gradations having been transmitted from the
+earlier stages of descent, in an unaltered or little altered condition.
+Amongst existing Vertebrata, we find but a small amount of gradation in the
+structure of the eye, and from fossil species we can learn nothing on this
+head. In this great class we should probably have to descend far beneath
+the lowest known fossiliferous stratum to discover the earlier stages, by
+which the eye has been perfected.
+
+In the Articulata we can commence a series with an optic nerve merely
+coated with pigment, and without any other mechanism; and from this low
+stage, numerous gradations of structure, branching off in two fundamentally
+different lines, can be shown to exist, until we reach a moderately high
+stage of perfection. In certain crustaceans, for instance, there is a
+double cornea, the inner one divided into facets, within each of which
+there is a lens-shaped swelling. In other crustaceans the transparent cones
+which are coated by pigment, and which properly act only by excluding
+lateral pencils of light, are convex at their upper ends and must act by
+convergence; and at their lower ends there seems to be an imperfect
+vitreous substance. {188} With these facts, here far too briefly and
+imperfectly given, which show that there is much graduated diversity in the
+eyes of living crustaceans, and bearing in mind how small the number of
+living animals is in proportion to those which have become extinct, I can
+see no very great difficulty (not more than in the case of many other
+structures) in believing that natural selection has converted the simple
+apparatus of an optic nerve merely coated with pigment and invested by
+transparent membrane, into an optical instrument as perfect as is possessed
+by any member of the great Articulate class.
+
+He who will go thus far, if he find on finishing this treatise that large
+bodies of facts, otherwise inexplicable, can be explained by the theory of
+descent, ought not to hesitate to go further, and to admit that a structure
+even as perfect as the eye of an eagle might be formed by natural
+selection, although in this case he does not know any of the transitional
+grades. His reason ought to conquer his imagination; though I have felt the
+difficulty far too keenly to be surprised at any degree of hesitation in
+extending the principle of natural selection to such startling lengths.
+
+It is scarcely possible to avoid comparing the eye to a telescope. We know
+that this instrument has been perfected by the long-continued efforts of
+the highest human intellects; and we naturally infer that the eye has been
+formed by a somewhat analogous process. But may not this inference be
+presumptuous? Have we any right to assume that the Creator works by
+intellectual powers like those of man? If we must compare the eye to an
+optical instrument, we ought in imagination to take a thick layer of
+transparent tissue, with a nerve sensitive to light beneath, and then
+suppose every part of this layer to be continually changing {189} slowly in
+density, so as to separate into layers of different densities and
+thicknesses, placed at different distances from each other, and with the
+surfaces of each layer slowly changing in form. Further we must suppose
+that there is a power always intently watching each slight accidental
+alteration in the transparent layers; and carefully selecting each
+alteration which, under varied circumstances, may in any way, or in any
+degree, tend to produce a distincter image. We must suppose each new state
+of the instrument to be multiplied by the million; and each to be preserved
+till a better be produced, and then the old ones to be destroyed. In living
+bodies, variation will cause the slight alterations, generation will
+multiply them almost infinitely, and natural selection will pick out with
+unerring skill each improvement. Let this process go on for millions on
+millions of years; and during each year on millions of individuals of many
+kinds; and may we not believe that a living optical instrument might thus
+be formed as superior to one of glass, as the works of the Creator are to
+those of man?
+
+If it could be demonstrated that any complex organ existed, which could not
+possibly have been formed by numerous, successive, slight modifications, my
+theory would absolutely break down. But I can find out no such case. No
+doubt many organs exist of which we do not know the transitional grades,
+more especially if we look to much-isolated species, round which, according
+to my theory, there has been much extinction. Or again, if we look to an
+organ common to all the members of a large class, for in this latter case
+the organ must have been first formed at an extremely remote period, since
+which all the many members of the class have been developed; and in order
+to discover the early transitional grades through which the organ has {190}
+passed, we should have to look to very ancient ancestral forms, long since
+become extinct.
+
+We should be extremely cautious in concluding that an organ could not have
+been formed by transitional gradations of some kind. Numerous cases could
+be given amongst the lower animals of the same organ performing at the same
+time wholly distinct functions; thus the alimentary canal respires,
+digests, and excretes in the larva of the dragon-fly and in the fish
+Cobites. In the Hydra, the animal may be turned inside out, and the
+exterior surface will then digest and the stomach respire. In such cases
+natural selection might easily specialise, if any advantage were thus
+gained, a part or organ, which had performed two functions, for one
+function alone, and thus wholly change its nature by insensible steps. Two
+distinct organs sometimes perform simultaneously the same function in the
+same individual; to give one instance, there are fish with gills or
+branchiæ that breathe the air dissolved in the water, at the same time that
+they breathe free air in their swimbladders, this latter organ having a
+ductus pneumaticus for its supply, and being divided by highly vascular
+partitions. In these cases one of the two organs might with ease be
+modified and perfected so as to perform all the work by itself, being aided
+during the process of modification by the other organ; and then this other
+organ might be modified for some other and quite distinct purpose, or be
+quite obliterated.
+
+The illustration of the swimbladder in fishes is a good one, because it
+shows us clearly the highly important fact that an organ originally
+constructed for one purpose, namely flotation, may be converted into one
+for a wholly different purpose, namely respiration. The swimbladder has,
+also, been worked in as an accessory to the auditory organs of certain
+fish, or, for I do not know {191} which view is now generally held, a part
+of the auditory apparatus has been worked in as a complement to the
+swimbladder. All physiologists admit that the swimbladder is homologous, or
+"ideally similar" in position and structure with the lungs of the higher
+vertebrate animals: hence there seems to me to be no great difficulty in
+believing that natural selection has actually converted a swimbladder into
+a lung, or organ used exclusively for respiration.
+
+I can, indeed, hardly doubt that all vertebrate animals having true lungs
+have descended by ordinary generation from an ancient prototype, of which
+we know nothing, furnished with a floating apparatus or swimbladder. We can
+thus, as I infer from Professor Owen's interesting description of these
+parts, understand the strange fact that every particle of food and drink
+which we swallow has to pass over the orifice of the trachea, with some
+risk of falling into the lungs, notwithstanding the beautiful contrivance
+by which the glottis is closed. In the higher Vertebrata the branchiæ have
+wholly disappeared--the slits on the sides of the neck and the loop-like
+course of the arteries still marking in the embryo their former position.
+But it is conceivable that the now utterly lost branchiæ might have been
+gradually worked in by natural selection for some quite distinct purpose:
+in the same manner as, on the view entertained by some naturalists that the
+branchiæ and dorsal scales of Annelids are homologous with the wings and
+wing-covers of insects, it is probable that organs which at a very ancient
+period served for respiration have been actually converted into organs of
+flight.
+
+In considering transitions of organs, it is so important to bear in mind
+the probability of conversion from one function to another, that I will
+give one more instance. Pedunculated cirripedes have two minute folds of
+skin, {192} called by me the ovigerous frena, which serve, through the
+means of a sticky secretion, to retain the eggs until they are hatched
+within the sack. These cirripedes have no branchiæ, the whole surface of
+the body and sack, including the small frena, serving for respiration. The
+Balanidæ or sessile cirripedes, on the other hand, have no ovigerous frena,
+the eggs lying loose at the bottom of the sack, in the well-enclosed shell;
+but they have large folded branchiæ. Now I think no one will dispute that
+the ovigerous frena in the one family are strictly homologous with the
+branchiæ of the other family; indeed, they graduate into each other.
+Therefore I do not doubt that little folds of skin, which originally served
+as ovigerous frena, but which, likewise, very slightly aided the act of
+respiration, have been gradually converted by natural selection into
+branchiæ, simply through an increase in their size and the obliteration of
+their adhesive glands. If all pedunculated cirripedes had become extinct,
+and they have already suffered far more extinction than have sessile
+cirripedes, who would ever have imagined that the branchiæ in this latter
+family had originally existed as organs for preventing the ova from being
+washed out of the sack?
+
+Although we must be extremely cautious in concluding that any organ could
+not possibly have been produced by successive transitional gradations, yet,
+undoubtedly, grave cases of difficulty occur, some of which will be
+discussed in my future work.
+
+One of the gravest is that of neuter insects, which are often very
+differently constructed from either the males or fertile females; but this
+case will be treated of in the next chapter. The electric organs of fishes
+offer another case of special difficulty; it is impossible to conceive by
+what steps these wondrous organs have been produced; but, as Owen and
+others have remarked, {193} their intimate structure closely resembles that
+of common muscle; and as it has lately been shown that Rays have an organ
+closely analogous to the electric apparatus, and yet do not, as Matteucci
+asserts, discharge any electricity, we must own that we are far too
+ignorant to argue that no transition of any kind is possible.
+
+The electric organs offer another and even more serious difficulty; for
+they occur in only about a dozen fishes, of which several are widely remote
+in their affinities. Generally when the same organ appears in several
+members of the same class, especially if in members having very different
+habits of life, we may attribute its presence to inheritance from a common
+ancestor; and its absence in some of the members to its loss through disuse
+or natural selection. But if the electric organs had been inherited from
+one ancient progenitor thus provided, we might have expected that all
+electric fishes would have been specially related to each other. Nor does
+geology at all lead to the belief that formerly most fishes had electric
+organs, which most of their modified descendants have lost. The presence of
+luminous organs in a few insects, belonging to different families and
+orders, offers a parallel case of difficulty. Other cases could be given;
+for instance in plants, the very curious contrivance of a mass of
+pollen-grains, borne on a foot-stalk with a sticky gland at the end, is the
+same in Orchis and Asclepias,--genera almost as remote as possible amongst
+flowering plants. In all these cases of two very distinct species furnished
+with apparently the same anomalous organ, it should be observed that,
+although the general appearance and function of the organ may be the same,
+yet some fundamental difference can generally be detected. I am inclined to
+believe that in nearly the same way as two men have sometimes independently
+hit on {194} the very same invention, so natural selection, working for the
+good of each being and taking advantage of analogous variations, has
+sometimes modified in very nearly the same manner two parts in two organic
+beings, which beings owe but little of their structure in common to
+inheritance from the same ancestor.
+
+Although in many cases it is most difficult to conjecture by what
+transitions organs could have arrived at their present state; yet,
+considering that the proportion of living and known forms to the extinct
+and unknown is very small, I have been astonished how rarely an organ can
+be named, towards which no transitional grade is known to lead. The truth
+of this remark is indeed shown by that old but somewhat exaggerated canon
+in natural history of "Natura non facit saltum." We meet with this
+admission in the writings of almost every experienced naturalist; or, as
+Milne Edwards has well expressed it, Nature is prodigal in variety, but
+niggard in innovation. Why, on the theory of Creation, should this be so?
+Why should all the parts and organs of many independent beings, each
+supposed to have been separately created for its proper place in nature, be
+so commonly linked together by graduated steps? Why should not Nature have
+taken a leap from structure to structure? On the theory of natural
+selection, we can clearly understand why she should not; for natural
+selection can act only by taking advantage of slight successive variations;
+she can never take a leap, but must advance by the shortest and slowest
+steps.
+
+
+
+_Organs of little apparent importance._--As natural selection acts by life
+and death,--by the preservation of individuals with any favourable
+variation, and by the destruction of those with any unfavourable deviation
+of structure,--I have sometimes felt much difficulty in {195} understanding
+the origin of simple parts, of which the importance does not seem
+sufficient to cause the preservation of successively varying individuals. I
+have sometimes felt as much difficulty, though of a very different kind, on
+this head, as in the case of an organ as perfect and complex as the eye.
+
+In the first place, we are much too ignorant in regard to the whole economy
+of any one organic being, to say what slight modifications would be of
+importance or not. In a former chapter I have given instances of most
+trifling characters, such as the down on fruit and the colour of its flesh,
+which, from determining the attacks of insects or from being correlated
+with constitutional differences, might assuredly be acted on by natural
+selection. The tail of the giraffe looks like an artificially constructed
+fly-flapper; and it seems at first incredible that this could have been
+adapted for its present purpose by successive slight modifications, each
+better and better, for so trifling an object as driving away flies; yet we
+should pause before being too positive even in this case, for we know that
+the distribution and existence of cattle and other animals in South America
+absolutely depends on their power of resisting the attacks of insects: so
+that individuals which could by any means defend themselves from these
+small enemies, would be able to range into new pastures and thus gain a
+great advantage. It is not that the larger quadrupeds are actually
+destroyed (except in some rare cases) by flies, but they are incessantly
+harassed and their strength reduced, so that they are more subject to
+disease, or not so well enabled in a coming dearth to search for food, or
+to escape from beasts of prey.
+
+Organs now of trifling importance have probably in some cases been of high
+importance to an early progenitor, and, after having been slowly perfected
+at a {196} former period, have been transmitted in nearly the same state,
+although now become of very slight use; and any actually injurious
+deviations in their structure will always have been checked by natural
+selection. Seeing how important an organ of locomotion the tail is in most
+aquatic animals, its general presence and use for many purposes in so many
+land animals, which in their lungs or modified swimbladders betray their
+aquatic origin, may perhaps be thus accounted for. A well-developed tail
+having been formed in an aquatic animal, it might subsequently come to be
+worked in for all sorts of purposes, as a fly-flapper, an organ of
+prehension, or as an aid in turning, as with the dog, though the aid must
+be slight, for the hare, with hardly any tail, can double quickly enough.
+
+In the second place, we may sometimes attribute importance to characters
+which are really of very little importance, and which have originated from
+quite secondary causes, independently of natural selection. We should
+remember that climate, food, &c., probably have some little direct
+influence on the organisation; that characters reappear from the law of
+reversion; that correlation of growth will have had a most important
+influence in modifying various structures; and finally, that sexual
+selection will often have largely modified the external characters of
+animals having a will, to give one male an advantage in fighting with
+another or in charming the females. Moreover when a modification of
+structure has primarily arisen from the above or other unknown causes, it
+may at first have been of no advantage to the species, but may subsequently
+have been taken advantage of by the descendants of the species under new
+conditions of life and with newly acquired habits.
+
+To give a few instances to illustrate these latter {197} remarks. If green
+woodpeckers alone had existed, and we did not know that there were many
+black and pied kinds, I dare say that we should have thought that the green
+colour was a beautiful adaptation to hide this tree-frequenting bird from
+its enemies; and consequently that it was a character of importance and
+might have been acquired through natural selection; as it is, I have no
+doubt that the colour is due to some quite distinct cause, probably to
+sexual selection. A trailing bamboo in the Malay Archipelago climbs the
+loftiest trees by the aid of exquisitely constructed hooks clustered around
+the ends of the branches, and this contrivance, no doubt, is of the highest
+service to the plant; but as we see nearly similar hooks on many trees
+which are not climbers, the hooks on the bamboo may have arisen from
+unknown laws of growth, and have been subsequently taken advantage of by
+the plant undergoing further modification and becoming a climber. The naked
+skin on the head of a vulture is generally looked at as a direct adaptation
+for wallowing in putridity; and so it may be, or it may possibly be due to
+the direct action of putrid matter; but we should be very cautious in
+drawing any such inference, when we see that the skin on the head of the
+clean-feeding male turkey is likewise naked. The sutures in the skulls of
+young mammals have been advanced as a beautiful adaptation for aiding
+parturition, and no doubt they facilitate, or may be indispensable for this
+act; but as sutures occur in the skulls of young birds and reptiles, which
+have only to escape from a broken egg, we may infer that this structure has
+arisen from the laws of growth, and has been taken advantage of in the
+parturition of the higher animals.
+
+We are profoundly ignorant of the causes producing slight and unimportant
+variations; and we are {198} immediately made conscious of this by
+reflecting on the differences in the breeds of our domesticated animals in
+different countries,--more especially in the less civilised countries where
+there has been but little artificial selection. Careful observers are
+convinced that a damp climate affects the growth of the hair, and that with
+the hair the horns are correlated. Mountain breeds always differ from
+lowland breeds; and a mountainous country would probably affect the hind
+limbs from exercising them more, and possibly even the form of the pelvis;
+and then by the law of homologous variation, the front limbs and even the
+head would probably be affected. The shape, also, of the pelvis might
+affect by pressure the shape of the head of the young in the womb. The
+laborious breathing necessary in high regions would, we have some reason to
+believe, increase the size of the chest; and again correlation would come
+into play. Animals kept by savages in different countries often have to
+struggle for their own subsistence, and would be exposed to a certain
+extent to natural selection, and individuals with slightly different
+constitutions would succeed best under different climates; and there is
+reason to believe that constitution and colour are correlated. A good
+observer, also, states that in cattle susceptibility to the attacks of
+flies is correlated with colour, as is the liability to be poisoned by
+certain plants; so that colour would be thus subjected to the action of
+natural selection. But we are far too ignorant to speculate on the relative
+importance of the several known and unknown laws of variation; and I have
+here alluded to them only to show that, if we are unable to account for the
+characteristic differences of our domestic breeds, which nevertheless we
+generally admit to have arisen through ordinary generation, we ought not to
+lay too much stress on our ignorance of the precise cause {199} of the
+slight analogous differences between species. I might have adduced for this
+same purpose the differences between the races of man, which are so
+strongly marked; I may add that some little light can apparently be thrown
+on the origin of these differences, chiefly through sexual selection of a
+particular kind, but without here entering on copious details my reasoning
+would appear frivolous.
+
+The foregoing remarks lead me to say a few words on the protest lately made
+by some naturalists, against the utilitarian doctrine that every detail of
+structure has been produced for the good of its possessor. They believe
+that very many structures have been created for beauty in the eyes of man,
+or for mere variety. This doctrine, if true, would be absolutely fatal to
+my theory. Yet I fully admit that many structures are of no direct use to
+their possessors. Physical conditions probably have had some little effect
+on structure, quite independently of any good thus gained. Correlation of
+growth has no doubt played a most important part, and a useful modification
+of one part will often have entailed on other parts diversified changes of
+no direct use. So again characters which formerly were useful, or which
+formerly had arisen from correlation of growth, or from other unknown
+cause, may reappear from the law of reversion, though now of no direct use.
+The effects of sexual selection, when displayed in beauty to charm the
+females, can be called useful only in rather a forced sense. But by far the
+most important consideration is that the chief part of the organisation of
+every being is simply due to inheritance; and consequently, though each
+being assuredly is well fitted for its place in nature, many structures now
+have no direct relation to the habits of life of each species. Thus, we can
+hardly believe that the webbed feet of the upland {200} goose or of the
+frigate-bird are of special use to these birds; we cannot believe that the
+same bones in the arm of the monkey, in the fore-leg of the horse, in the
+wing of the bat, and in the nipper of the seal, are of special use to these
+animals. We may safely attribute these structures to inheritance. But to
+the progenitor of the upland goose and of the frigate-bird, webbed feet no
+doubt were as useful as they now are to the most aquatic of existing birds.
+So we may believe that the progenitor of the seal had not a nipper, but a
+foot with five toes fitted for walking or grasping; and we may further
+venture to believe that the several bones in the limbs of the monkey,
+horse, and bat, which have been inherited from a common progenitor, were
+formerly of more special use to that progenitor, or its progenitors, than
+they now are to these animals having such widely diversified habits.
+Therefore we may infer that these several bones might have been acquired
+through natural selection, subjected formerly, as now, to the several laws
+of inheritance, reversion, correlation of growth, &c. Hence every detail of
+structure in every living creature (making some little allowance for the
+direct action of physical conditions) may be viewed, either as having been
+of special use to some ancestral form, or as being now of special use to
+the descendants of this form--either directly, or indirectly through the
+complex laws of growth.
+
+Natural selection cannot possibly produce any modification in any one
+species exclusively for the good of another species; though throughout
+nature one species incessantly takes advantage of, and profits by, the
+structure of another. But natural selection can and does often produce
+structures for the direct injury of other species, as we see in the fang of
+the adder, and in the ovipositor of the ichneumon, by which its eggs are
+{201} deposited in the living bodies of other insects. If it could be
+proved that any part of the structure of any one species had been formed
+for the exclusive good of another species, it would annihilate my theory,
+for such could not have been produced through natural selection. Although
+many statements may be found in works on natural history to this effect, I
+cannot find even one which seems to me of any weight. It is admitted that
+the rattlesnake has a poison-fang for its own defence and for the
+destruction of its prey; but some authors suppose that at the same time
+this snake is furnished with a rattle for its own injury, namely, to warn
+its prey to escape. I would almost as soon believe that the cat curls the
+end of its tail when preparing to spring, in order to warn the doomed
+mouse. But I have not space here to enter on this and other such cases.
+
+Natural selection will never produce in a being anything injurious to
+itself, for natural selection acts solely by and for the good of each. No
+organ will be formed, as Paley has remarked, for the purpose of causing
+pain or for doing an injury to its possessor. If a fair balance be struck
+between the good and evil caused by each part, each will be found on the
+whole advantageous. After the lapse of time, under changing conditions of
+life, if any part comes to be injurious, it will be modified; or if it be
+not so, the being will become extinct, as myriads have become extinct.
+
+Natural selection tends only to make each organic being as perfect as, or
+slightly more perfect than, the other inhabitants of the same country with
+which it has to struggle for existence. And we see that this is the degree
+of perfection attained under nature. The endemic productions of New
+Zealand, for instance, are perfect one compared with another; but they are
+now rapidly yielding before the advancing legions of plants {202} and
+animals introduced from Europe. Natural selection will not produce absolute
+perfection, nor do we always meet, as far as we can judge, with this high
+standard under nature. The correction for the aberration of light is said,
+on high authority, not to be perfect even in that most perfect organ, the
+eye. If our reason leads us to admire with enthusiasm a multitude of
+inimitable contrivances in nature, this same reason tells us, though we may
+easily err on both sides, that some other contrivances are less perfect.
+Can we consider the sting of the wasp or of the bee as perfect, which, when
+used against many attacking animals, cannot be withdrawn, owing to the
+backward serratures, and so inevitably causes the death of the insect by
+tearing out its viscera?
+
+If we look at the sting of the bee, as having originally existed in a
+remote progenitor as a boring and serrated instrument, like that in so many
+members of the same great order, and which has been modified but not
+perfected for its present purpose, with the poison originally adapted to
+cause galls subsequently intensified, we can perhaps understand how it is
+that the use of the sting should so often cause the insect's own death: for
+if on the whole the power of stinging be useful to the community, it will
+fulfil all the requirements of natural selection, though it may cause the
+death of some few members. If we admire the truly wonderful power of scent
+by which the males of many insects find their females, can we admire the
+production for this single purpose of thousands of drones, which are
+utterly useless to the community for any other end, and which are
+ultimately slaughtered by their industrious and sterile sisters? It may be
+difficult, but we ought to admire the savage instinctive hatred of the
+queen-bee, which urges her instantly to destroy the {203} young queens her
+daughters as soon as born, or to perish herself in the combat; for
+undoubtedly this is for the good of the community; and maternal love or
+maternal hatred, though the latter fortunately is most rare, is all the
+same to the inexorable principle of natural selection. If we admire the
+several ingenious contrivances, by which the flowers of the orchis and of
+many other plants are fertilised through insect agency, can we consider as
+equally perfect the elaboration by our fir-trees of dense clouds of pollen,
+in order that a few granules may be wafted by a chance breeze on to the
+ovules?
+
+
+
+_Summary of Chapter._--We have in this chapter discussed some of the
+difficulties and objections which may be urged against my theory. Many of
+them are very serious; but I think that in the discussion light has been
+thrown on several facts, which on the theory of independent acts of
+creation are utterly obscure. We have seen that species at any one period
+are not indefinitely variable, and are not linked together by a multitude
+of intermediate gradations, partly because the process of natural selection
+will always be very slow, and will act, at any one time, only on a very few
+forms; and partly because the very process of natural selection almost
+implies the continual supplanting and extinction of preceding and
+intermediate gradations. Closely allied species, now living on a continuous
+area, must often have been formed when the area was not continuous, and
+when the conditions of life did not insensibly graduate away from one part
+to another. When two varieties are formed in two districts of a continuous
+area, an intermediate variety will often be formed, fitted for an
+intermediate zone; but from reasons assigned, the intermediate variety will
+usually exist in lesser numbers than {204} the two forms which it connects;
+consequently the two latter, during the course of further modification,
+from existing in greater numbers, will have a great advantage over the less
+numerous intermediate variety, and will thus generally succeed in
+supplanting and exterminating it.
+
+We have seen in this chapter how cautious we should be in concluding that
+the most different habits of life could not graduate into each other; that
+a bat, for instance, could not have been formed by natural selection from
+an animal which at first could only glide through the air.
+
+We have seen that a species may under new conditions of life change its
+habits, or have diversified habits, with some habits very unlike those of
+its nearest congeners. Hence we can understand, bearing in mind that each
+organic being is trying to live wherever it can live, how it has arisen
+that there are upland geese with webbed feet, ground woodpeckers, diving
+thrushes, and petrels with the habits of auks.
+
+Although the belief that an organ so perfect as the eye could have been
+formed by natural selection, is more than enough to stagger any one; yet in
+the case of any organ, if we know of a long series of gradations in
+complexity, each good for its possessor, then, under changing conditions of
+life there is no logical impossibility in the acquirement of any
+conceivable degree of perfection through natural selection. In the cases in
+which we know of no intermediate or transitional states, we should be very
+cautious in concluding that none could have existed, for the homologies of
+many organs and their intermediate states show that wonderful metamorphoses
+in function are at least possible. For instance, a swim-bladder has
+apparently been converted into an air-breathing lung. The same organ having
+performed {205} simultaneously very different functions, and then having
+been specialised for one function; and two very distinct organs having
+performed at the same time the same function, the one having been perfected
+whilst aided by the other, must often have largely facilitated transitions.
+
+We are far too ignorant, in almost every case, to be enabled to assert that
+any part or organ is so unimportant for the welfare of a species, that
+modifications in its structure could not have been slowly accumulated by
+means of natural selection. But we may confidently believe that many
+modifications, wholly due to the laws of growth, and at first in no way
+advantageous to a species, have been subsequently taken advantage of by the
+still further modified descendants of this species. We may, also, believe
+that a part formerly of high importance has often been retained (as the
+tail of an aquatic animal by its terrestrial descendants), though it has
+become of such small importance that it could not, in its present state,
+have been acquired by natural selection,--a power which acts solely by the
+preservation of profitable variations in the struggle for life.
+
+Natural selection will produce nothing in one species for the exclusive
+good or injury of another; though it may well produce parts, organs, and
+excretions highly useful or even indispensable, or highly injurious to
+another species, but in all cases at the same time useful to the owner.
+Natural selection in each well-stocked country, must act chiefly through
+the competition of the inhabitants one with another, and consequently will
+produce perfection, or strength in the battle for life, only according to
+the standard of that country. Hence the inhabitants of one country,
+generally the smaller one, will often yield, as we see they do yield, to
+the inhabitants of another and generally larger country. For in {206} the
+larger country there will have existed more individuals, and more
+diversified forms, and the competition will have been severer, and thus the
+standard of perfection will have been rendered higher. Natural selection
+will not necessarily produce absolute perfection; nor, as far as we can
+judge by our limited faculties, can absolute perfection be everywhere
+found.
+
+On the theory of natural selection we can clearly understand the full
+meaning of that old canon in natural history, "Natura non facit saltum."
+This canon, if we look only to the present inhabitants of the world, is not
+strictly correct, but if we include all those of past times, it must by my
+theory be strictly true.
+
+It is generally acknowledged that all organic beings have been formed on
+two great laws--Unity of Type, and the Conditions of Existence. By unity of
+type is meant that fundamental agreement in structure, which we see in
+organic beings of the same class, and which is quite independent of their
+habits of life. On my theory, unity of type is explained by unity of
+descent. The expression of conditions of existence, so often insisted on by
+the illustrious Cuvier, is fully embraced by the principle of natural
+selection. For natural selection acts by either now adapting the varying
+parts of each being to its organic and inorganic conditions of life; or by
+having adapted them during long-past periods of time: the adaptations being
+aided in some cases by use and disuse, being slightly affected by the
+direct action of the external conditions of life, and being in all cases
+subjected to the several laws of growth. Hence, in fact, the law of the
+Conditions of Existence is the higher law; as it includes, through the
+inheritance of former adaptations, that of Unity of Type.
+
+       *       *       *       *       *
+
+
+{207}
+
+CHAPTER VII.
+
+INSTINCT.
+
+    Instincts comparable with habits, but different in their
+    origin--Instincts graduated--Aphides and ants--Instincts
+    variable--Domestic instincts, their origin--Natural instincts of the
+    cuckoo, ostrich, and parasitic bees--Slave-making-ants--Hive-bee, its
+    cell-making instinct--Difficulties on the theory of the Natural
+    Selection of instincts--Neuter or sterile insects--Summary.
+
+The subject of instinct might have been worked into the previous chapters;
+but I have thought that it would be more convenient to treat the subject
+separately, especially as so wonderful an instinct as that of the hive-bee
+making its cells will probably have occurred to many readers, as a
+difficulty sufficient to overthrow my whole theory. I must premise, that I
+have nothing to do with the origin of the primary mental powers, any more
+than I have with that of life itself. We are concerned only with the
+diversities of instinct and of the other mental qualities of animals within
+the same class.
+
+I will not attempt any definition of instinct. It would be easy to show
+that several distinct mental actions are commonly embraced by this term;
+but every one understands what is meant, when it is said that instinct
+impels the cuckoo to migrate and to lay her eggs in other birds' nests. An
+action, which we ourselves should require experience to enable us to
+perform, when performed by an animal, more especially by a very young one,
+without any experience, and when performed by many individuals in the same
+way, without their knowing for what purpose it is performed, is usually
+said to be instinctive. {208} But I could show that none of these
+characters of instinct are universal. A little dose, as Pierre Huber
+expresses it, of judgment or reason, often comes into play, even in animals
+very low in the scale of nature.
+
+Frederick Cuvier and several of the older metaphysicians have compared
+instinct with habit. This comparison gives, I think, a remarkably accurate
+notion of the frame of mind under which an instinctive action is performed,
+but not of its origin. How unconsciously many habitual actions are
+performed, indeed not rarely in direct opposition to our conscious will!
+yet they may be modified by the will or reason. Habits easily become
+associated with other habits, and with certain periods of time and states
+of the body. When once acquired, they often remain constant throughout
+life. Several other points of resemblance between instincts and habits
+could be pointed out. As in repeating a well-known song, so in instincts,
+one action follows another by a sort of rhythm; if a person be interrupted
+in a song, or in repeating anything by rote, he is generally forced to go
+back to recover the habitual train of thought: so P. Huber found it was
+with a caterpillar, which makes a very complicated hammock; for if he took
+a caterpillar which had completed its hammock up to, say, the sixth stage
+of construction, and put it into a hammock completed up only to the third
+stage, the caterpillar simply re-performed the fourth, fifth, and sixth
+stages of construction. If, however, a caterpillar were taken out of a
+hammock made up, for instance, to the third stage, and were put into one
+finished up to the sixth stage, so that much of its work, was already done
+for it, far from feeling the benefit of this, it was much embarrassed, and,
+in order to complete its hammock, seemed forced to start from the third
+stage, where it had left off, and thus tried to complete the already
+finished work. {209}
+
+If we suppose any habitual action to become inherited--and I think it can
+be shown that this does sometimes happen--then the resemblance between what
+originally was a habit and an instinct becomes so close as not to be
+distinguished. If Mozart, instead of playing the pianoforte at three years
+old with wonderfully little practice, had played a tune with no practice at
+all, he might truly be said to have done so instinctively. But it would be
+the most serious error to suppose that the greater number of instincts have
+been acquired by habit in one generation, and then transmitted by
+inheritance to succeeding generations. It can be clearly shown that the
+most wonderful instincts with which we are acquainted, namely, those of the
+hive-bee and of many ants, could not possibly have been thus acquired.
+
+It will be universally admitted that instincts are as important as
+corporeal structure for the welfare of each species, under its present
+conditions of life. Under changed conditions of life, it is at least
+possible that slight modifications of instinct might be profitable to a
+species; and if it can be shown that instincts do vary ever so little, then
+I can see no difficulty in natural selection preserving and continually
+accumulating variations of instinct to any extent that may be profitable.
+It is thus, as I believe, that all the most complex and wonderful instincts
+have originated. As modifications of corporeal structure arise from, and
+are increased by, use or habit, and are diminished or lost by disuse, so I
+do not doubt it has been with instincts. But I believe that the effects of
+habit are of quite subordinate importance to the effects of the natural
+selection of what may be called accidental variations of instincts;--that
+is of variations produced by the same unknown causes which produce slight
+deviations of bodily structure.
+
+No complex instinct can possibly be produced through {210} natural
+selection, except by the slow and gradual accumulation of numerous, slight,
+yet profitable, variations. Hence, as in the case of corporeal structures,
+we ought to find in nature, not the actual transitional gradations by which
+each complex instinct has been acquired--for these could be found only in
+the lineal ancestors of each species--but we ought to find in the
+collateral lines of descent some evidence of such gradations; or we ought
+at least to be able to show that gradations of some kind are possible; and
+this we certainly can do. I have been surprised to find, making allowance
+for the instincts of animals having been but little observed except in
+Europe and North America, and for no instinct being known amongst extinct
+species, how very generally gradations, leading to the most complex
+instincts, can be discovered. Changes of instinct may sometimes be
+facilitated by the same species having different instincts at different
+periods of life, or at different seasons of the year, or when placed under
+different circumstances &c.; in which case either one or the other instinct
+might be preserved by natural selection. And such instances of diversity of
+instinct in the same species can be shown to occur in nature.
+
+Again as in the case of corporeal structure, and conformably with my
+theory, the instinct of each species is good for itself, but has never, as
+far as we can judge, been produced for the exclusive good of others. One of
+the strongest instances of an animal apparently performing an action for
+the sole good of another, with which I am acquainted, is that of aphides
+voluntarily yielding their sweet excretion to ants: that they do so
+voluntarily, the following facts show. I removed all the ants from a group
+of about a dozen aphides on a dock-plant, and prevented their attendance
+during several hours. After this interval, I felt sure that the aphides
+{211} would want to excrete. I watched them for some time through a lens,
+but not one excreted; I then tickled and stroked them with a hair in the
+same manner, as well as I could, as the ants do with their antennæ; but not
+one excreted. Afterwards I allowed an ant to visit them, and it immediately
+seemed, by its eager way of running about, to be well aware what a rich
+flock it had discovered; it then began to play with its antennæ on the
+abdomen first of one aphis and then of another; and each aphis, as soon as
+it felt the antennæ, immediately lifted up its abdomen and excreted a
+limpid drop of sweet juice, which was eagerly devoured by the ant. Even the
+quite young aphides behaved in this manner, showing that the action was
+instinctive, and not the result of experience. But as the excretion is
+extremely viscid, it is probably a convenience to the aphides to have it
+removed; and therefore probably the aphides do not instinctively excrete
+for the sole good of the ants. Although I do not believe that any animal in
+the world performs an action for the exclusive good of another of a
+distinct species, yet each species tries to take advantage of the instincts
+of others, as each takes advantage of the weaker bodily structure of
+others. So again, in some few cases, certain instincts cannot be considered
+as absolutely perfect; but as details on this and other such points are not
+indispensable, they may be here passed over.
+
+As some degree of variation in instincts under a state of nature, and the
+inheritance of such variations, are indispensable for the action of natural
+selection, as many instances as possible ought to be here given; but want
+of space prevents me. I can only assert, that instincts certainly do
+vary--for instance, the migratory instinct, both in extent and direction,
+and in its total loss. So it is with the nests of birds, which vary partly
+{212} in dependence on the situations chosen, and on the nature and
+temperature of the country inhabited, but often from causes wholly unknown
+to us: Audubon has given several remarkable cases of differences in the
+nests of the same species in the northern and southern United States. Fear
+of any particular enemy is certainly an instinctive quality, as may be seen
+in nestling birds, though it is strengthened by experience, and by the
+sight of fear of the same enemy in other animals. But fear of man is slowly
+acquired, as I have elsewhere shown, by various animals inhabiting desert
+islands; and we may see an instance of this, even in England, in the
+greater wildness of all our large birds than of our small birds; for the
+large birds have been most persecuted by man. We may safely attribute the
+greater wildness of our large birds to this cause; for in uninhabited
+islands large birds are not more fearful than small; and the magpie, so
+wary in England, is tame in Norway, as is the hooded crow in Egypt.
+
+That the general disposition of individuals of the same species, born in a
+state of nature, is extremely diversified, can be shown by a multitude of
+facts. Several cases also, could be given, of occasional and strange habits
+in certain species, which might, if advantageous to the species, give rise,
+through natural selection, to quite new instincts. But I am well aware that
+these general statements, without facts given in detail, can produce but a
+feeble effect on the reader's mind. I can only repeat my assurance, that I
+do not speak without good evidence.
+
+The possibility, or even probability, of inherited variations of instinct
+in a state of nature will be strengthened by briefly considering a few
+cases under domestication. We shall thus also be enabled to see the
+respective parts which habit and the selection of {213} so-called
+accidental variations have played in modifying the mental qualities of our
+domestic animals. A number of curious and authentic instances could be
+given of the inheritance of all shades of disposition and tastes, and
+likewise of the oddest tricks, associated with certain frames of mind or
+periods of time. But let us look to the familiar case of the several breeds
+of dogs: it cannot be doubted that young pointers (I have myself seen a
+striking instance) will sometimes point and even back other dogs the very
+first time that they are taken out; retrieving is certainly in some degree
+inherited by retrievers; and a tendency to run round, instead of at, a
+flock of sheep, by shepherd-dogs. I cannot see that these actions,
+performed without experience by the young, and in nearly the same manner by
+each individual, performed with eager delight by each breed, and without
+the end being known,--for the young pointer can no more know that he points
+to aid his master, than the white butterfly knows why she lays her eggs on
+the leaf of the cabbage,--I cannot see that these actions differ
+essentially from true instincts. If we were to see one kind of wolf, when
+young and without any training, as soon as it scented its prey, stand
+motionless like a statue, and then slowly crawl forward with a peculiar
+gait; and another kind of wolf rushing round, instead of at, a herd of
+deer, and driving them to a distant point, we should assuredly call these
+actions instinctive. Domestic instincts, as they may be called, are
+certainly far less fixed or invariable than natural instincts; but they
+have been acted on by far less rigorous selection, and have been
+transmitted for an incomparably shorter period, under less fixed conditions
+of life.
+
+How strongly these domestic instincts, habits, and dispositions are
+inherited, and how curiously they become mingled, is well shown when
+different breeds of dogs are {214} crossed. Thus it is known that a cross
+with a bull-dog has affected for many generations the courage and obstinacy
+of greyhounds; and a cross with a greyhound has given to a whole family of
+shepherd-dogs a tendency to hunt hares. These domestic instincts, when thus
+tested by crossing, resemble natural instincts, which in a like manner
+become curiously blended together, and for a long period exhibit traces of
+the instincts of either parent: for example, Le Roy describes a dog, whose
+great-grandfather was a wolf, and this dog showed a trace of its wild
+parentage only in one way, by not coming in a straight line to his master
+when called.
+
+Domestic instincts are sometimes spoken of as actions which have become
+inherited solely from long-continued and compulsory habit, but this, I
+think, is not true. No one would ever have thought of teaching, or probably
+could have taught, the tumbler-pigeon to tumble,--an action which, as I
+have witnessed, is performed by young birds, that have never seen a pigeon
+tumble. We may believe that some one pigeon showed a slight tendency to
+this strange habit, and that the long-continued selection of the best
+individuals in successive generations made tumblers what they now are; and
+near Glasgow there are house-tumblers, as I hear from Mr. Brent, which
+cannot fly eighteen inches high without going head over heels. It may be
+doubted whether any one would have thought of training a dog to point, had
+not some one dog naturally shown a tendency in this line; and this is known
+occasionally to happen, as I once saw in a pure terrier: the act of
+pointing is probably, as many have thought, only the exaggerated pause of
+an animal preparing to spring on its prey. When the first tendency to point
+was once displayed, methodical selection and the inherited effects of
+compulsory training in each successive generation would soon complete the
+{215} work; and unconscious selection is still at work, as each man tries
+to procure, without intending to improve the breed, dogs which will stand
+and hunt best. On the other hand, habit alone in some cases has sufficed;
+no animal is more difficult to tame than the young of the wild rabbit;
+scarcely any animal is tamer than the young of the tame rabbit; but I do
+not suppose that domestic rabbits have ever been selected for tameness; and
+I presume that we must attribute the whole of the inherited change from
+extreme wildness to extreme tameness, simply to habit and long-continued
+close confinement.
+
+Natural instincts are lost under domestication: a remarkable instance of
+this is seen in those breeds of fowls which very rarely or never become
+"broody," that is, never wish to sit on their eggs. Familiarity alone
+prevents our seeing how universally and largely the minds of our domestic
+animals have been modified by domestication. It is scarcely possible to
+doubt that the love of man has become instinctive in the dog. All wolves,
+foxes, jackals, and species of the cat genus, when kept tame, are most
+eager to attack poultry, sheep, and pigs; and this tendency has been found
+incurable in dogs which have been brought home as puppies from countries,
+such as Tierra del Fuego and Australia, where the savages do not keep these
+domestic animals. How rarely, on the other hand, do our civilised dogs,
+even when quite young, require to be taught not to attack poultry, sheep,
+and pigs! No doubt they occasionally do make an attack, and are then
+beaten; and if not cured, they are destroyed; so that habit, with some
+degree of selection, has probably concurred in civilising by inheritance
+our dogs. On the other hand, young chickens have lost, wholly by habit,
+that fear of the dog and cat which no doubt was originally instinctive in
+them, in the same way as it is so plainly instinctive in {216} young
+pheasants, though reared under a hen. It is not that chickens have lost all
+fear, but fear only of dogs and cats, for if the hen gives the
+danger-chuckle, they will run (more especially young turkeys) from under
+her, and conceal themselves in the surrounding grass or thickets; and this
+is evidently done for the instinctive purpose of allowing, as we see in
+wild ground-birds, their mother to fly away. But this instinct retained by
+our chickens has become useless under domestication, for the mother-hen has
+almost lost by disuse the power of flight.
+
+Hence, we may conclude, that domestic instincts have been acquired and
+natural instincts have been lost partly by habit, and partly by man
+selecting and accumulating during successive generations, peculiar mental
+habits and actions, which at first appeared from what we must in our
+ignorance call an accident. In some cases compulsory habit alone has
+sufficed to produce such inherited mental changes; in other cases
+compulsory habit has done nothing, and all has been the result of
+selection, pursued both methodically and unconsciously; but in most cases,
+probably, habit and selection have acted together.
+
+We shall, perhaps, best understand how instincts in a state of nature have
+become modified by selection, by considering a few cases. I will select
+only three, out of the several which I shall have to discuss in my future
+work,--namely, the instinct which leads the cuckoo to lay her eggs in other
+birds' nests; the slave-making instinct of certain ants; and the
+comb-making power of the hive-bee; these two latter instincts have
+generally, and most justly, been ranked by naturalists as the most
+wonderful of all known instincts.
+
+It is now commonly admitted that the more immediate and final cause of the
+cuckoo's instinct is, that {217} she lays her eggs, not daily, but at
+intervals of two or three days; so that, if she were to make her own nest
+and sit on her own eggs, those first laid would have to be left for some
+time unincubated, or there would be eggs and young birds of different ages
+in the same nest. If this were the case, the process of laying and hatching
+might be inconveniently long, more especially as she has to migrate at a
+very early period; and the first hatched young would probably have to be
+fed by the male alone. But the American cuckoo is in this predicament; for
+she makes her own nest and has eggs and young successively hatched, all at
+the same time. It has been asserted that the American cuckoo occasionally
+lays her eggs in other birds' nests; but I hear on the high authority of
+Dr. Brewer, that this is a mistake. Nevertheless, I could give several
+instances of various birds which have been known occasionally to lay their
+eggs in other birds' nests. Now let us suppose that the ancient progenitor
+of our European cuckoo had the habits of the American cuckoo; but that
+occasionally she laid an egg in another bird's nest. If the old bird
+profited by this occasional habit, or if the young were made more vigorous
+by advantage having been taken of the mistaken maternal instinct of another
+bird, than by their own mother's care, encumbered as she can hardly fail to
+be by having eggs and young of different ages at the same time; then the
+old birds or the fostered young would gain an advantage. And analogy would
+lead me to believe, that the young thus reared would be apt to follow by
+inheritance the occasional and aberrant habit of their mother, and in their
+turn would be apt to lay their eggs in other birds' nests, and thus be
+successful in rearing their young. By a continued process of this nature, I
+believe that the strange instinct of our cuckoo could be, and has been,
+{218} generated. I may add that, according to Dr. Gray and to some other
+observers, the European cuckoo has not utterly lost all maternal love and
+care for her own offspring.
+
+The occasional habit of birds laying their eggs in other birds' nests,
+either of the same or of a distinct species, is not very uncommon with the
+Gallinaceæ; and this perhaps explains the origin of a singular instinct in
+the allied group of ostriches. For several hen ostriches, at least in the
+case of the American species, unite and lay first a few eggs in one nest
+and then in another; and these are hatched by the males. This instinct may
+probably be accounted for by the fact of the hens laying a large number of
+eggs; but, as in the case of the cuckoo, at intervals of two or three days.
+This instinct, however, of the American ostrich has not as yet been
+perfected; for a surprising number of eggs lie strewed over the plains, so
+that in one day's hunting I picked up no less than twenty lost and wasted
+eggs.
+
+Many bees are parasitic, and always lay their eggs in the nests of bees of
+other kinds. This case is more remarkable than that of the cuckoo; for
+these bees have not only their instincts but their structure modified in
+accordance with their parasitic habits; for they do not possess the
+pollen-collecting apparatus which would be necessary if they had to store
+food for their own young. Some species, likewise, of Sphegidæ (wasp-like
+insects) are parasitic on other species; and M. Fabre has lately shown good
+reason for believing that although the Tachytes nigra generally makes its
+own burrow and stores it with paralysed prey for its own larvæ to feed on,
+yet that when this insect finds a burrow already made and stored by another
+sphex, it takes advantage of the prize, and becomes for the occasion
+parasitic. In this case, as with the supposed case of the cuckoo, I can
+{219} see no difficulty in natural selection making an occasional habit
+permanent, if of advantage to the species, and if the insect whose nest and
+stored food are thus feloniously appropriated, be not thus exterminated.
+
+
+
+_Slave-making instinct._--This remarkable instinct was first discovered in
+the Formica (Polyerges) rufescens by Pierre Huber, a better observer even
+than his celebrated father. This ant is absolutely dependent on its slaves;
+without their aid, the species would certainly become extinct in a single
+year. The males and fertile females do no work. The workers or sterile
+females, though most energetic and courageous in capturing slaves, do no
+other work. They are incapable of making their own nests, or of feeding
+their own larvæ. When the old nest is found inconvenient, and they have to
+migrate, it is the slaves which determine the migration, and actually carry
+their masters in their jaws. So utterly helpless are the masters, that when
+Huber shut up thirty of them without a slave, but with plenty of the food
+which they like best, and with their larvae and pupæ to stimulate them to
+work, they did nothing; they could not even feed themselves, and many
+perished of hunger. Huber then introduced a single slave (F. fusca), and
+she instantly set to work, fed and saved the survivors; made some cells and
+tended the larvæ, and put all to rights. What can be more extraordinary
+than these well-ascertained facts? If we had not known of any other
+slave-making ant, it would have been hopeless to have speculated how so
+wonderful an instinct could have been perfected.
+
+Another species, Formica sanguinea, was likewise first discovered by P.
+Huber to be a slave-making ant. This species is found in the southern parts
+of England, and its habits have been attended to by Mr. F. Smith, of {220}
+the British Museum, to whom I am much indebted for information on this and
+other subjects. Although fully trusting to the statements of Huber and Mr.
+Smith, I tried to approach the subject in a sceptical frame of mind, as any
+one may well be excused for doubting the truth of so extraordinary and
+odious an instinct as that of making slaves. Hence I will give the
+observations which I have myself made, in some little detail. I opened
+fourteen nests of F. sanguinea, and found a few slaves in all. Males and
+fertile females of the slave-species (F. fusca) are found only in their own
+proper communities, and have never been observed in the nests of F.
+sanguinea. The slaves are black and not above half the size of their red
+masters, so that the contrast in their appearance is very great. When the
+nest is slightly disturbed, the slaves occasionally come out, and like
+their masters are much agitated and defend the nest: when the nest is much
+disturbed and the larvæ and pupæ are exposed, the slaves work energetically
+with their masters in carrying them away to a place of safety. Hence, it is
+clear, that the slaves feel quite at home. During the months of June and
+July, on three successive years, I have watched for many hours several
+nests in Surrey and Sussex, and never saw a slave either leave or enter a
+nest. As, during these months, the slaves are very few in number, I thought
+that they might behave differently when more numerous; but Mr. Smith
+informs me that he has watched the nests at various hours during May, June
+and August, both in Surrey and Hampshire, and has never seen the slaves,
+through present in large numbers in August, either leave or enter the nest.
+Hence he considers them as strictly household slaves. The masters, on the
+other hand, may be constantly seen bringing in materials for the nest, and
+food of all kinds. During the present year, however, in the month {221} of
+July, I came across a community with an unusually large stock of slaves,
+and I observed a few slaves mingled with their masters leaving the nest,
+and marching along the same road to a tall Scotch-fir-tree, twenty-five
+yards distant, which they ascended together, probably in search of aphides
+or cocci. According to Huber, who had ample opportunities for observation,
+in Switzerland the slaves habitually work with their masters in making the
+nest, and they alone open and close the doors in the morning and evening;
+and, as Huber expressly states, their principal office is to search for
+aphides. This difference in the usual habits of the masters and slaves in
+the two countries, probably depends merely on the slaves being captured in
+greater numbers in Switzerland than in England.
+
+One day I fortunately witnessed a migration of F. sanguinea from one nest
+to another, and it was a most interesting spectacle to behold the masters
+carefully carrying (instead of being carried by, as in the case of F.
+rufescens) their slaves in their jaws. Another day my attention was struck
+by about a score of the slave-makers haunting the same spot, and evidently
+not in search of food; they approached and were vigorously repulsed by an
+independent community of the slave-species (F. fusca); sometimes as many as
+three of these ants clinging to the legs of the slave-making F. sanguinea.
+The latter ruthlessly killed their small opponents, and carried their dead
+bodies as food to their nest, twenty-nine yards distant; but they were
+prevented from getting any pupæ to rear as slaves. I then dug up a small
+parcel of the pupæ of F. fusca from another nest, and put them down on a
+bare spot near the place of combat; they were eagerly seized, and carried
+off by the tyrants, who perhaps fancied that, after all, they had been
+victorious in their late combat. {222}
+
+At the same time I laid on the same place a small parcel of the pupæ of
+another species, F. flava, with a few of these little yellow ants still
+clinging to the fragments of the nest. This species is sometimes, though
+rarely, made into slaves, as has been described by Mr. Smith. Although so
+small a species, it is very courageous, and I have seen it ferociously
+attack other ants. In one instance I found to my surprise an independent
+community of F. flava under a stone beneath a nest of the slave-making F.
+sanguinea; and when I had accidentally disturbed both nests, the little
+ants attacked their big neighbours with surprising courage. Now I was
+curious to ascertain whether F. sanguinea could distinguish the pupæ of F.
+fusca, which they habitually make into slaves, from those of the little and
+furious F. flava, which they rarely capture, and it was evident that they
+did at once distinguish them: for we have seen that they eagerly and
+instantly seized the pupæ of F. fusca, whereas they were much terrified
+when they came across the pupæ, or even the earth from the nest of F.
+flava, and quickly ran away; but in about a quarter of an hour, shortly
+after all the little yellow ants had crawled away, they took heart and
+carried off the pupæ.
+
+One evening I visited another community of F. sanguinea, and found a number
+of these ants returning home and entering their nests, carrying the dead
+bodies of F. fusca (showing that it was not a migration) and numerous pupæ.
+I traced a long file of ants burthened with booty, for about forty yards,
+to a very thick clump of heath, whence I saw the last individual of F.
+sanguinea emerge, carrying a pupa; but I was not able to find the desolated
+nest in the thick heath. The nest, however, must have been close at hand,
+for two or three individuals of F. fusca were rushing about in the greatest
+{223} agitation, and one was perched motionless with its own pupa in its
+mouth on the top of a spray of heath, an image of despair, over its ravaged
+home.
+
+Such are the facts, though they did not need confirmation by me, in regard
+to the wonderful instinct of making slaves. Let it be observed what a
+contrast the instinctive habits of F. sanguinea present with those of the
+continental F. rufescens. The latter does not build its own nest, does not
+determine its own migrations, does not collect food for itself or its
+young, and cannot even feed itself: it is absolutely dependent on its
+numerous slaves. Formica sanguinea, on the other hand, possesses much fewer
+slaves, and in the early part of the summer extremely few: the masters
+determine when and where a new nest shall be formed, and when they migrate,
+the masters carry the slaves. Both in Switzerland and England the slaves
+seem to have the exclusive care of the larvæ, and the masters alone go on
+slave-making expeditions. In Switzerland the slaves and masters work
+together, making and bringing materials for the nest: both, but chiefly the
+slaves, tend, and milk as it may be called, their aphides; and thus both
+collect food for the community. In England the masters alone usually leave
+the nest to collect building materials and food for themselves, their
+slaves and larvæ. So that the masters in this country receive much less
+service from their slaves than they do in Switzerland.
+
+By what steps the instinct of F. sanguinea originated I will not pretend to
+conjecture. But as ants, which are not slave-makers, will, as I have seen,
+carry off pupæ of other species, if scattered near their nests, it is
+possible that such pupæ originally stored as food might become developed;
+and the foreign ants thus unintentionally reared would then follow their
+proper instincts, and do {224} what work they could. If their presence
+proved useful to the species which had seized them--if it were more
+advantageous to this species to capture workers than to procreate them--the
+habit of collecting pupae originally for food might by natural selection be
+strengthened and rendered permanent for the very different purpose of
+raising slaves. When the instinct was once acquired, if carried out to a
+much less extent even than in our British F. sanguinea, which, as we have
+seen, is less aided by its slaves than the same species in Switzerland, I
+can see no difficulty in natural selection increasing and modifying the
+instinct--always supposing each modification to be of use to the
+species--until an ant was formed as abjectly dependent on its slaves as is
+the Formica rufescens.
+
+
+
+_Cell-making instinct of the Hive-Bee._--I will not here enter on minute
+details on this subject, but will merely give an outline of the conclusions
+at which I have arrived. He must be a dull man who can examine the
+exquisite structure of a comb, so beautifully adapted to its end, without
+enthusiastic admiration. We hear from mathematicians that bees have
+practically solved a recondite problem, and have made their cells of the
+proper shape to hold the greatest possible amount of honey, with the least
+possible consumption of precious wax in their construction. It has been
+remarked that a skilful workman, with fitting tools and measures, would
+find it very difficult to make cells of wax of the true form, though this
+is perfectly effected by a crowd of bees working in a dark hive. Grant
+whatever instincts you please, and it seems at first quite inconceivable
+how they can make all the necessary angles and planes, or even perceive
+when they are correctly made. But the difficulty is not {225} nearly so
+great as it at first appears: all this beautiful work can be shown, I
+think, to follow from a few very simple instincts.
+
+I was led to investigate this subject by Mr. Waterhouse, who has shown that
+the form of the cell stands in close relation to the presence of adjoining
+cells; and the following view may, perhaps, be considered only as a
+modification of his theory. Let us look to the great principle of
+gradation, and see whether Nature does not reveal to us her method of work.
+At one end of a short series we have humble-bees, which use their old
+cocoons to hold honey, sometimes adding to them short tubes of wax, and
+likewise making separate and very irregular rounded cells of wax. At the
+other end of the series we have the cells of the hive-bee, placed in a
+double layer: each cell, as is well known, is an hexagonal prism, with the
+basal edges of its six sides bevelled so as to fit on to a pyramid, formed
+of three rhombs. These rhombs have certain angles, and the three which form
+the pyramidal base of a single cell on one side of the comb, enter into the
+composition of the bases of three adjoining cells on the opposite side. In
+the series between the extreme perfection of the cells of the hive-bee and
+the simplicity of those of the humble-bee, we have the cells of the Mexican
+Melipona domestica, carefully described and figured by Pierre Huber. The
+Melipona itself is intermediate in structure between the hive and humble
+bee, but more nearly related to the latter: it forms a nearly regular waxen
+comb of cylindrical cells, in which the young are hatched, and, in
+addition, some large cells of wax for holding honey. These latter cells are
+nearly spherical and of nearly equal sizes, and are aggregated into an
+irregular mass. But the important point to notice, is that these cells are
+always made at that degree of nearness to each other, that they would have
+{226} intersected or broken into each other, if the spheres had been
+completed; but this is never permitted, the bees building perfectly flat
+walls of wax between the spheres which thus tend to intersect. Hence each
+cell consists of an outer spherical portion and of two, three, or more
+perfectly flat surfaces, according as the cell adjoins two, three, or more
+other cells. When one cell comes into contact with three other cells,
+which, from the spheres being nearly of the same size, is very frequently
+and necessarily the case, the three flat surfaces are united into a
+pyramid; and this pyramid, as Huber has remarked, is manifestly a gross
+imitation of the three-sided pyramidal bases of the cell of the hive-bee.
+As in the cells of the hive-bee, so here, the three plane surfaces in any
+one cell necessarily enter into the construction of three adjoining cells.
+It is obvious that the Melipona saves wax by this manner of building; for
+the flat walls between the adjoining cells are not double, but are of the
+same thickness as the outer spherical portions, and yet each flat portion
+forms a part of two cells.
+
+Reflecting on this case, it occurred to me that if the Melipona had made
+its spheres at some given distance from each other, and had made them of
+equal sizes and had arranged them symmetrically in a double layer, the
+resulting structure would probably have been as perfect as the comb of the
+hive-bee. Accordingly I wrote to Professor Miller, of Cambridge, and this
+geometer has kindly read over the following statement, drawn up from his
+information, and tells me that it is strictly correct:--
+
+If a number of equal spheres be described with their centres placed in two
+parallel layers; with the centre of each sphere at the distance of radius ×
+[root]2, or radius × 1.41421 (or at some lesser distance), from the centres
+of the six surrounding spheres in the same {227} layer; and at the same
+distance from the centres of the adjoining spheres in the other and
+parallel layer; then, if planes of intersection between the several spheres
+in both layers be formed, there will result a double layer of hexagonal
+prisms united together by pyramidal bases formed of three rhombs; and the
+rhombs and the sides of the hexagonal prisms will have every angle
+identically the same with the best measurements which have been made of the
+cells of the hive-bee.
+
+Hence we may safely conclude that if we could slightly modify the instincts
+already possessed by the Melipona, and in themselves not very wonderful,
+this bee would make a structure as wonderfully perfect as that of the
+hive-bee. We must suppose the Melipona to make her cells truly spherical,
+and of equal sizes; and this would not be very surprising, seeing that she
+already does so to a certain extent, and seeing what perfectly cylindrical
+burrows in wood many insects can make, apparently by turning round on a
+fixed point. We must suppose the Melipona to arrange her cells in level
+layers, as she already does her cylindrical cells; and we must further
+suppose, and this is the greatest difficulty, that she can somehow judge
+accurately at what distance to stand from her fellow-labourers when several
+are making their spheres; but she is already so far enabled to judge of
+distance, that she always describes her spheres so as to intersect largely;
+and then she unites the points of intersection by perfectly flat surfaces.
+We have further to suppose, but this is no difficulty, that after hexagonal
+prisms have been formed by the intersection of adjoining spheres in the
+same layer, she can prolong the hexagon to any length requisite to hold the
+stock of honey; in the same way as the rude humble-bee adds cylinders of
+wax to the circular mouths of her old cocoons. By such {228} modifications
+of instincts in themselves not very wonderful,--hardly more wonderful than
+those which guide a bird to make its nest,--I believe that the hive-bee has
+acquired, through natural selection, her inimitable architectural powers.
+
+But this theory can be tested by experiment. Following the example of Mr.
+Tegetmeier, I separated two combs, and put between them a long, thick,
+square strip of wax: the bees instantly began to excavate minute circular
+pits in it; and as they deepened these little pits, they made them wider
+and wider until they were converted into shallow basins, appearing to the
+eye perfectly true or parts of a sphere, and of about the diameter of a
+cell. It was most interesting to me to observe that wherever several bees
+had begun to excavate these basins near together, they had begun their work
+at such a distance from each other, that by the time the basins had
+acquired the above stated width (_i.e._ about the width of an ordinary
+cell), and were in depth about one sixth of the diameter of the sphere of
+which they formed a part, the rims of the basins intersected or broke into
+each other. As soon as this occurred, the bees ceased to excavate, and
+began to build up flat walls of wax on the lines of intersection between
+the basins, so that each hexagonal prism was built upon the scalloped edge
+of a smooth basin, instead of on the straight edges of a three-sided
+pyramid as in the case of ordinary cells.
+
+I then put into the hive, instead of a thick, square piece of wax, a thin
+and narrow, knife-edged ridge, coloured with vermilion. The bees instantly
+began on both sides to excavate little basins near to each other, in the
+same way as before; but the ridge of wax was so thin, that the bottoms of
+the basins, if they had been excavated to the same depth as in the former
+{229} experiment, would have broken into each other from the opposite
+sides. The bees, however, did not suffer this to happen, and they stopped
+their excavations in due time; so that the basins, as soon as they had been
+a little deepened, came to have flat bottoms; and these flat bottoms,
+formed by thin little plates of the vermilion wax having been left
+ungnawed, were situated, as far as the eye could judge, exactly along the
+planes of imaginary intersection between the basins on the opposite sides
+of the ridge of wax. In parts, only little bits, in other parts, large
+portions of a rhombic plate had been left between the opposed basins, but
+the work, from the unnatural state of things, had not been neatly
+performed. The bees must have worked at very nearly the same rate on the
+opposite sides of the ridge of vermilion wax, as they circularly gnawed
+away and deepened the basins on both sides, in order to have succeeded in
+thus leaving flat plates between the basins, by stopping work along the
+intermediate planes or planes of intersection.
+
+Considering how flexible thin wax is, I do not see that there is any
+difficulty in the bees, whilst at work on the two sides of a strip of wax,
+perceiving when they have gnawed the wax away to the proper thinness, and
+then stopping their work. In ordinary combs it has appeared to me that the
+bees do not always succeed in working at exactly the same rate from the
+opposite sides; for I have noticed half-completed rhombs at the base of a
+just-commenced cell, which were slightly concave on one side, where I
+suppose that the bees had excavated too quickly, and convex on the opposed
+side, where the bees had worked less quickly. In one well-marked instance,
+I put the comb back into the hive, and allowed the bees to go on working
+for a short time, and again examined the cell, and I found that the rhombic
+{230} plate had been completed, and had become _perfectly flat_: it was
+absolutely impossible, from the extreme thinness of the little rhombic
+plate, that they could have effected this by gnawing away the convex side;
+and I suspect that the bees in such cases stand in the opposed cells and
+push and bend the ductile and warm wax (which as I have tried is easily
+done) into its proper intermediate plane, and thus flatten it.
+
+From the experiment of the ridge of vermilion wax, we can clearly see that
+if the bees were to build for themselves a thin wall of wax, they could
+make their cells of the proper shape, by standing at the proper distance
+from each other, by excavating at the same rate, and by endeavouring to
+make equal spherical hollows, but never allowing the spheres to break into
+each other. Now bees, as may be clearly seen by examining the edge of a
+growing comb, do make a rough, circumferential wall or rim all round the
+comb; and they gnaw into this from the opposite sides, always working
+circularly as they deepen each cell. They do not make the whole three-sided
+pyramidal base of any one cell at the same time, but only the one rhombic
+plate which stands on the extreme growing margin, or the two plates, as the
+case may be; and they never complete the upper edges of the rhombic plates,
+until the hexagonal walls are commenced. Some of these statements differ
+from those made by the justly celebrated elder Huber, but I am convinced of
+their accuracy; and if I had space, I could show that they are conformable
+with my theory.
+
+Huber's statement that the very first cell is excavated out of a little
+parallel-sided wall of wax, is not, as far as I have seen, strictly
+correct; the first commencement having always been a little hood of wax;
+but I will not here enter on these details. We see how important {231} a
+part excavation plays in the construction of the cells; but it would be a
+great error to suppose that the bees cannot build up a rough wall of wax in
+the proper position--that is, along the plane of intersection between two
+adjoining spheres. I have several specimens showing clearly that they can
+do this. Even in the rude circumferential rim or wall of wax round a
+growing comb, flexures may sometimes be observed, corresponding in position
+to the planes of the rhombic basal plates of future cells. But the rough
+wall of wax has in every case to be finished off, by being largely gnawed
+away on both sides. The manner in which the bees build is curious; they
+always make the first rough wall from ten to twenty times thicker than the
+excessively thin finished wall of the cell, which will ultimately be left.
+We shall understand how they work, by supposing masons first to pile up a
+broad ridge of cement, and then to begin cutting it away equally on both
+sides near the ground, till a smooth, very thin wall is left in the middle;
+the masons always piling up the cut-away cement, and adding fresh cement,
+on the summit of the ridge. We shall thus have a thin wall steadily growing
+upward; but always crowned by a gigantic coping. From all the cells, both
+those just commenced and those completed, being thus crowned by a strong
+coping of wax, the bees can cluster and crawl over the comb without
+injuring the delicate hexagonal walls, which are only about one
+four-hundredth of an inch in thickness; the plates of the pyramidal basis
+being about twice as thick. By this singular manner of building, strength
+is continually given to the comb, with the utmost ultimate economy of wax.
+
+It seems at first to add to the difficulty of understanding how the cells
+are made, that a multitude of bees all work together; one bee after working
+a short time at one cell going to another, so that, as Huber has stated,
+{232} a score of individuals work even at the commencement of the first
+cell. I was able practically to show this fact, by covering the edges of
+the hexagonal walls of a single cell, or the extreme margin of the
+circumferential rim of a growing comb, with an extremely thin layer of
+melted vermilion wax; and I invariably found that the colour was most
+delicately diffused by the bees--as delicately as a painter could have done
+with his brush--by atoms of the coloured wax having been taken from the
+spot on which it had been placed, and worked into the growing edges of the
+cells all round. The work of construction seems to be a sort of balance
+struck between many bees, all instinctively standing at the same relative
+distance from each other, all trying to sweep equal spheres, and then
+building up, or leaving ungnawed, the planes of intersection between these
+spheres. It was really curious to note in cases of difficulty, as when two
+pieces of comb met at an angle, how often the bees would pull down and
+rebuild in different ways the same cell, sometimes recurring to a shape
+which they had at first rejected.
+
+When bees have a place on which they can stand in their proper positions
+for working,--for instance, on a slip of wood, placed directly under the
+middle of a comb growing downwards so that the comb has to be built over
+one face of the slip--in this case the bees can lay the foundations of one
+wall of a new hexagon, in its strictly proper place, projecting beyond the
+other completed cells. It suffices that the bees should be enabled to stand
+at their proper relative distances from each other and from the walls of
+the last completed cells, and then, by striking imaginary spheres, they can
+build up a wall intermediate between two adjoining spheres; but, as far as
+I have seen, they never gnaw away and finish off the angles of a cell till
+a large part both of that cell and of {233} the adjoining cells has been
+built. This capacity in bees of laying down under certain circumstances a
+rough wall in its proper place between two just-commenced cells, is
+important, as it bears on a fact, which seems at first quite subversive of
+the foregoing theory; namely, that the cells on the extreme margin of
+wasp-combs are sometimes strictly hexagonal; but I have not space here to
+enter on this subject. Nor does there seem to me any great difficulty in a
+single insect (as in the case of a queen-wasp) making hexagonal cells, if
+she work alternately on the inside and outside of two or three cells
+commenced at the same time, always standing at the proper relative distance
+from the parts of the cells just begun, sweeping spheres or cylinders, and
+building up intermediate planes. It is even conceivable that an insect
+might, by fixing on a point at which to commence a cell, and then moving
+outside, first to one point, and then to five other points, at the proper
+relative distances from the central point and from each other, strike the
+planes of intersection, and so make an isolated hexagon: but I am not aware
+that any such case has been observed; nor would any good be derived from a
+single hexagon being built, as in its construction more materials would be
+required than for a cylinder.
+
+As natural selection acts only by the accumulation of slight modifications
+of structure or instinct, each profitable to the individual under its
+conditions of life, it may reasonably be asked, how a long and graduated
+succession of modified architectural instincts, all tending towards the
+present perfect plan of construction, could have profited the progenitors
+of the hive-bee? I think the answer is not difficult: it is known that bees
+are often hard pressed to get sufficient nectar; and I am informed by Mr.
+Tegetmeier that it has been experimentally found that no less than from
+twelve to fifteen pounds of dry sugar {234} are consumed by a hive of bees
+for the secretion of each pound of wax; to that a prodigious quantity of
+fluid nectar must be collected and consumed by the bees in a hive for the
+secretion of the wax necessary for the construction of their combs.
+Moreover, many bees have to remain idle for many days during the process of
+secretion. A large store of honey is indispensable to support a large stock
+of bees during the winter; and the security of the hive is known mainly to
+depend on a large number of bees being supported. Hence the saving of wax
+by largely saving honey must be a most important element of success in any
+family of bees. Of course the success of any species of bee may be
+dependent on the number of its parasites or other enemies, or on quite
+distinct causes, and so be altogether independent of the quantity of honey
+which the bees could collect. But let us suppose that this latter
+circumstance determined, as it probably often does determine, the numbers
+of a humble-bee which could exist in a country; and let us further suppose
+that the community lived throughout the winter, and consequently required a
+store of honey: there can in this case be no doubt that it would be an
+advantage to our humble-bee, if a slight modification of her instinct led
+her to make her waxen cells near together, so as to intersect a little; for
+a wall in common even to two adjoining cells, would save some little wax.
+Hence it would continually be more and more advantageous to our humble-bee,
+if she were to make her cells more and more regular, nearer together, and
+aggregated into a mass, like the cells of the Melipona; for in this case a
+large part of the bounding surface of each cell would serve to bound other
+cells, and much wax would be saved. Again, from the same cause, it would be
+advantageous to the Melipona, if she were to make her cells closer
+together, and more regular in every way {235} than at present; for then, as
+we have seen, the spherical surfaces would wholly disappear, and would all
+be replaced by plane surfaces; and the Melipona would make a comb as
+perfect as that of the hive-bee. Beyond this stage of perfection in
+architecture, natural selection could not lead; for the comb of the
+hive-bee, as far as we can see, is absolutely perfect in economising wax.
+
+Thus, as I believe, the most wonderful of all known instincts, that of the
+hive-bee, can be explained by natural selection having taken advantage of
+numerous, successive, slight modifications of simpler instincts; natural
+selection having by slow degrees, more and more perfectly, led the bees to
+sweep equal spheres at a given distance from each other in a double layer,
+and to build up and excavate the wax along the planes of intersection. The
+bees, of course, no more knowing that they swept their spheres at one
+particular distance from each other, than they know what are the several
+angles of the hexagonal prisms and of the basal rhombic plates. The motive
+power of the process of natural selection having been economy of wax; that
+individual swarm which wasted least honey in the secretion of wax, having
+succeeded best, and having transmitted by inheritance its newly acquired
+economical instinct to new swarms, which in their turn will have had the
+best chance of succeeding in the struggle for existence.
+
+
+
+No doubt many instincts of very difficult explanation could be opposed to
+the theory of natural selection,--cases, in which we cannot see how an
+instinct could possibly have originated; cases, in which no intermediate
+gradations are known to exist; cases of instinct of apparently such
+trifling importance, that they could {236} hardly have been acted on by
+natural selection; cases of instincts almost identically the same in
+animals so remote in the scale of nature, that we cannot account for their
+similarity by inheritance from a common parent, and must therefore believe
+that they have been acquired by independent acts of natural selection. I
+will not here enter on these several cases, but will confine myself to one
+special difficulty, which at first appeared to me insuperable, and actually
+fatal to my whole theory. I allude to the neuters or sterile females in
+insect-communities: for these neuters often differ widely in instinct and
+in structure from both the males and fertile females, and yet, from being
+sterile, they cannot propagate their kind.
+
+The subject well deserves to be discussed at great length, but I will here
+take only a single case, that of working or sterile ants. How the workers
+have been rendered sterile is a difficulty; but not much greater than that
+of any other striking modification of structure; for it can be shown that
+some insects and other articulate animals in a state of nature occasionally
+become sterile; and if such insects had been social, and it had been
+profitable to the community that a number should have been annually born
+capable of work, but incapable of procreation, I can see no very great
+difficulty in this being effected by natural selection. But I must pass
+over this preliminary difficulty. The great difficulty lies in the working
+ants differing widely from both the males and the fertile females in
+structure, as in the shape of the thorax and in being destitute of wings
+and sometimes of eyes, and in instinct. As far as instinct alone is
+concerned, the prodigious difference in this respect between the workers
+and the perfect females, would have been far better exemplified by the
+hive-bee. If a working ant or other neuter insect had been an animal {237}
+in the ordinary state, I should have unhesitatingly assumed that all its
+characters had been slowly acquired through natural selection; namely, by
+an individual having been born with some slight profitable modification of
+structure, this being inherited by its offspring, which again varied and
+were again selected, and so onwards. But with the working ant we have an
+insect differing greatly from its parents, yet absolutely sterile; so that
+it could never have transmitted successively acquired modifications of
+structure or instinct to its progeny. It may well be asked how is it
+possible to reconcile this case with the theory of natural selection?
+
+First, let it be remembered that we have innumerable instances, both in our
+domestic productions and in those in a state of nature, of all sorts of
+differences of structure which have become correlated to certain ages, and
+to either sex. We have differences correlated not only to one sex, but to
+that short period alone when the reproductive system is active, as in the
+nuptial plumage of many birds, and in the hooked jaws of the male salmon.
+We have even slight differences in the horns of different breeds of cattle
+in relation to an artificially imperfect state of the male sex; for oxen of
+certain breeds have longer horns than in other breeds, in comparison with
+the horns of the bulls or cows of these same breeds. Hence I can see no
+real difficulty in any character having become correlated with the sterile
+condition of certain members of insect-communities: the difficulty lies in
+understanding how such correlated modifications of structure could have
+been slowly accumulated by natural selection.
+
+This difficulty, though appearing insuperable, is lessened, or, as I
+believe, disappears, when it is remembered that selection may be applied to
+the family, as well as to the individual, and may thus gain the {238}
+desired end. Thus, a well-flavoured vegetable is cooked, and the individual
+is destroyed; but the horticulturist sows seeds of the same stock, and
+confidently expects to get nearly the same variety: breeders of cattle wish
+the flesh and fat to be well marbled together; the animal has been
+slaughtered, but the breeder goes with confidence to the same family. I
+have such faith in the powers of selection, that I do not doubt that a
+breed of cattle, always yielding oxen with extraordinarily long horns,
+could be slowly formed by carefully watching which individual bulls and
+cows, when matched, produced oxen with the longest horns; and yet no one ox
+could ever have propagated its kind. Thus I believe it has been with social
+insects: a slight modification of structure, or instinct, correlated with
+the sterile condition of certain members of the community, has been
+advantageous to the community: consequently the fertile males and females
+of the same community flourished, and transmitted to their fertile
+offspring a tendency to produce sterile members having the same
+modification. And I believe that this process has been repeated, until that
+prodigious amount of difference between the fertile and sterile females of
+the same species has been produced, which we see in many social insects.
+
+But we have not as yet touched on the climax of the difficulty; namely, the
+fact that the neuters of several ants differ, not only from the fertile
+females and males, but from each other, sometimes to an almost incredible
+degree, and are thus divided into two or even three castes. The castes,
+moreover, do not generally graduate into each other, but are perfectly well
+defined; being as distinct from each other, as are any two species of the
+same genus, or rather as any two genera of the same family. Thus in Eciton,
+there are working and soldier neuters, with jaws and instincts
+extraordinarily {239} different: in Cryptocerus, the workers of one caste
+alone carry a wonderful sort of shield on their heads, the use of which is
+quite unknown: in the Mexican Myrmecocystus, the workers of one caste never
+leave the nest; they are fed by the workers of another caste, and they have
+an enormously developed abdomen which secretes a sort of honey, supplying
+the place of that excreted by the aphides, or the domestic cattle as they
+may be called, which our European ants guard or imprison.
+
+It will indeed be thought that I have an overweening confidence in the
+principle of natural selection, when I do not admit that such wonderful and
+well-established facts at once annihilate my theory. In the simpler case of
+neuter insects all of one caste or of the same kind, which have been
+rendered by natural selection, as I believe to be quite possible, different
+from the fertile males and females,--in this case, we may safely conclude
+from the analogy of ordinary variations, that each successive, slight,
+profitable modification did not probably at first appear in all the
+individual neuters in the same nest, but in a few alone; and that by the
+long-continued selection of the fertile parents which produced most neuters
+with the profitable modification, all the neuters ultimately came to have
+the desired character. On this view we ought occasionally to find
+neuter-insects of the same species, in the same nest, presenting gradations
+of structure; and this we do find, even often, considering how few
+neuter-insects out of Europe have been carefully examined. Mr. F. Smith has
+shown how surprisingly the neuters of several British ants differ from each
+other in size and sometimes in colour; and that the extreme forms can
+sometimes be perfectly linked together by individuals taken out of the same
+nest: I have myself compared perfect gradations of this kind. It often
+happens that the larger or the smaller sized workers {240} are the most
+numerous; or that both large and small are numerous, with those of an
+intermediate size scanty in numbers. Formica flava has larger and smaller
+workers, with some of intermediate size; and, in this species, as Mr. F.
+Smith has observed, the larger workers have simple eyes (ocelli), which
+though small can be plainly distinguished, whereas the smaller workers have
+their ocelli rudimentary. Having carefully dissected several specimens of
+these workers, I can affirm that the eyes are far more rudimentary in the
+smaller workers than can be accounted for merely by their proportionally
+lesser size; and I fully believe, though I dare not assert so positively,
+that the workers of intermediate size have their ocelli in an exactly
+intermediate condition. So that we here have two bodies of sterile workers
+in the same nest, differing not only in size, but in their organs of
+vision, yet connected by some few members in an intermediate condition. I
+may digress by adding, that if the smaller workers had been the most useful
+to the community, and those males and females had been continually
+selected, which produced more and more of the smaller workers, until all
+the workers had come to be in this condition; we should then have had a
+species of ant with neuters very nearly in the same condition with those of
+Myrmica. For the workers of Myrmica have not even rudiments of ocelli,
+though the male and female ants of this genus have well-developed ocelli.
+
+I may give one other case: so confidently did I expect to find gradations
+in important points of structure between the different castes of neuters in
+the same species, that I gladly availed myself of Mr. F. Smith's offer of
+numerous specimens from the same nest of the driver ant (Anomma) of West
+Africa. The reader will perhaps best appreciate the amount of difference in
+these {241} workers, by my giving not the actual measurements, but a
+strictly accurate illustration: the difference was the same as if we were
+to see a set of workmen building a house of whom many were five feet four
+inches high, and many sixteen feet high; but we must suppose that the
+larger workmen had heads four instead of three times as big as those of the
+smaller men, and jaws nearly five times as big. The jaws, moreover, of the
+working ants of the several sizes differed wonderfully in shape, and in the
+form and number of the teeth. But the important fact for us is, that though
+the workers can be grouped into castes of different sizes, yet they
+graduate insensibly into each other, as does the widely-different structure
+of their jaws. I speak confidently on this latter point, as Mr. Lubbock
+made drawings for me with the camera lucida of the jaws which I had
+dissected from the workers of the several sizes.
+
+With these facts before me, I believe that natural selection, by acting on
+the fertile parents, could form a species which should regularly produce
+neuters, either all of large size with one form of jaw, or all of small
+size with jaws having a widely different structure; or lastly, and this is
+our climax of difficulty, one set of workers of one size and structure, and
+simultaneously another set of workers of a different size and structure;--a
+graduated series having been first formed, as in the case of the driver
+ant, and then the extreme forms, from being the most useful to the
+community, having been produced in greater and greater numbers through the
+natural selection of the parents which generated them; until none with an
+intermediate structure were produced.
+
+Thus, as I believe, the wonderful fact of two distinctly defined castes of
+sterile workers existing in the same nest, both widely different from each
+other and from {242} their parents, has originated. We can see how useful
+their production may have been to a social community of insects, on the
+same principle that the division of labour is useful to civilised man. As
+ants work by inherited instincts and by inherited organs or tools, and not
+by acquired knowledge and manufactured instruments, a perfect division of
+labour could be effected with them only by the workers being sterile; for
+had they been fertile, they would have intercrossed, and their instincts
+and structure would have become blended. And nature has, as I believe,
+effected this admirable division of labour in the communities of ants, by
+the means of natural selection. But I am bound to confess, that, with all
+my faith in this principle, I should never have anticipated that natural
+selection could have been efficient in so high a degree, had not the case
+of these neuter insects convinced me of the fact. I have, therefore,
+discussed this case, at some little but wholly insufficient length, in
+order to show the power of natural selection, and likewise because this is
+by far the most serious special difficulty, which my theory has
+encountered. The case, also, is very interesting, as it proves that with
+animals, as with plants, any amount of modification in structure can be
+effected by the accumulation of numerous, slight, and as we must call them
+accidental, variations, which are in any manner profitable, without
+exercise or habit having come into play. For no amount of exercise, or
+habit, or volition, in the utterly sterile members of a community could
+possibly affect the structure or instincts of the fertile members, which
+alone leave descendants. I am surprised that no one has advanced this
+demonstrative case of neuter insects, against the well-known doctrine of
+Lamarck.
+
+
+
+_Summary._--I have endeavoured briefly in this chapter {243} to show that
+the mental qualities of our domestic animals vary, and that the variations
+are inherited. Still more briefly I have attempted to show that instincts
+vary slightly in a state of nature. No one will dispute that instincts are
+of the highest importance to each animal. Therefore I can see no
+difficulty, under changing conditions of life, in natural selection
+accumulating slight modifications of instinct to any extent, in any useful
+direction. In some cases habit or use and disuse have probably come into
+play. I do not pretend that the facts given in this chapter strengthen in
+any great degree my theory; but none of the cases of difficulty, to the
+best of my judgment, annihilate it. On the other hand, the fact that
+instincts are not always absolutely perfect and are liable to
+mistakes;--that no instinct has been produced for the exclusive good of
+other animals, but that each animal takes advantage of the instincts of
+others;--that the canon in natural history, of "Natura non facit saltum,"
+is applicable to instincts as well as to corporeal structure, and is
+plainly explicable on the foregoing views, but is otherwise
+inexplicable,--all tend to corroborate the theory of natural selection.
+
+This theory is, also, strengthened by some few other facts in regard to
+instincts; as by that common case of closely allied, but certainly
+distinct, species, when inhabiting distant parts of the world and living
+under considerably different conditions of life, yet often retaining nearly
+the same instincts. For instance, we can understand on the principle of
+inheritance, how it is that the thrush of South America lines its nest with
+mud, in the same peculiar manner as does our British thrush: how it is that
+the male wrens (Troglodytes) of North America, build "cock-nests," to roost
+in, like the males of our distinct Kitty-wrens,--a habit wholly unlike that
+of {244} any other known bird. Finally, it may not be a logical deduction,
+but to my imagination it is far more satisfactory to look at such instincts
+as the young cuckoo ejecting its foster-brothers,--ants making slaves,--the
+larvae of ichneumonidæ feeding within the live bodies of caterpillars,--not
+as specially endowed or created instincts, but as small consequences of one
+general law, leading to the advancement of all organic beings, namely,
+multiply, vary, let the strongest live and the weakest die.
+
+       *       *       *       *       *
+
+
+{245}
+
+CHAPTER VIII.
+
+HYBRIDISM.
+
+    Distinction between the sterility of first crosses and of
+    hybrids--Sterility various in degree, not universal, affected by close
+    interbreeding, removed by domestication--Laws governing the sterility
+    of hybrids--Sterility not a special endowment, but incidental on other
+    differences--Causes of the sterility of first crosses and of
+    hybrids--Parallelism between the effects of changed conditions of life
+    and crossing--Fertility of varieties when crossed and of their mongrel
+    offspring not universal--Hybrids and mongrels compared independently of
+    their fertility--Summary.
+
+The view generally entertained by naturalists is that species, when
+intercrossed, have been specially endowed with the quality of sterility, in
+order to prevent the confusion of all organic forms. This view certainly
+seems at first probable, for species within the same country could hardly
+have kept distinct had they been capable of crossing freely. The importance
+of the fact that hybrids are very generally sterile, has, I think, been
+much underrated by some late writers. On the theory of natural selection
+the case is especially important, inasmuch as the sterility of hybrids
+could not possibly be of any advantage to them, and therefore could not
+have been acquired by the continued preservation of successive profitable
+degrees of sterility. I hope, however, to be able to show that sterility is
+not a specially acquired or endowed quality, but is incidental on other
+acquired differences.
+
+In treating this subject, two classes of facts, to a large extent
+fundamentally different, have generally been confounded together; namely,
+the sterility of two species {246} when first crossed, and the sterility of
+the hybrids produced from them.
+
+Pure species have of course their organs of reproduction in a perfect
+condition, yet when intercrossed they produce either few or no offspring.
+Hybrids, on the other hand, have their reproductive organs functionally
+impotent, as may be clearly seen in the state of the male element in both
+plants and animals; though the organs themselves are perfect in structure,
+as far as the microscope reveals. In the first case the two sexual elements
+which go to form the embryo are perfect; in the second case they are either
+not at all developed, or are imperfectly developed. This distinction is
+important, when the cause of the sterility, which is common to the two
+cases, has to be considered. The distinction has probably been slurred
+over, owing to the sterility in both cases being looked on as a special
+endowment, beyond the province of our reasoning powers.
+
+The fertility of varieties, that is of the forms known or believed to have
+descended from common parents, when intercrossed, and likewise the
+fertility of their mongrel offspring, is, on my theory, of equal importance
+with the sterility of species; for it seems to make a broad and clear
+distinction between varieties and species.
+
+First, for the sterility of species when crossed and of their hybrid
+offspring. It is impossible to study the several memoirs and works of those
+two conscientious and admirable observers, Kölreuter and Gärtner, who
+almost devoted their lives to this subject, without being deeply impressed
+with the high generality of some degree of sterility. Kölreuter makes the
+rule universal; but then he cuts the knot, for in ten cases in which he
+found two forms, considered by most authors as distinct species, quite
+fertile together, he unhesitatingly ranks {247} them as varieties. Gärtner,
+also, makes the rule equally universal; and he disputes the entire
+fertility of Kölreuter's ten cases. But in these and in many other cases,
+Gärtner is obliged carefully to count the seeds, in order to show that
+there is any degree of sterility. He always compares the maximum number of
+seeds produced by two species when crossed and by their hybrid offspring,
+with the average number produced by both pure parent-species in a state of
+nature. But a serious cause of error seems to me to be here introduced: a
+plant to be hybridised must be castrated, and, what is often more
+important, must be secluded in order to prevent pollen being brought to it
+by insects from other plants. Nearly all the plants experimentised on by
+Gärtner were potted, and apparently were kept in a chamber in his house.
+That these processes are often injurious to the fertility of a plant cannot
+be doubted; for Gärtner gives in his table about a score of cases of plants
+which he castrated, and artificially fertilised with their own pollen, and
+(excluding all cases such as the Leguminosæ, in which there is an
+acknowledged difficulty in the manipulation) half of these twenty plants
+had their fertility in some degree impaired. Moreover, as Gärtner during
+several years repeatedly crossed the primrose and cowslip, which we have
+such good reason to believe to be varieties, and only once or twice
+succeeded in getting fertile seed; as he found the common red and blue
+pimpernels (Anagallis arvensis and coerulea), which the best botanists rank
+as varieties, absolutely sterile together; and as he came to the same
+conclusion in several other analogous cases; it seems to me that we may
+well be permitted to doubt whether many other species are really so
+sterile, when intercrossed, as Gärtner believes. {248}
+
+It is certain, on the one hand, that the sterility of various species when
+crossed is so different in degree and graduates away so insensibly, and, on
+the other hand, that the fertility of pure species is so easily affected by
+various circumstances, that for all practical purposes it is most difficult
+to say where perfect fertility ends and sterility begins. I think no better
+evidence of this can be required than that the two most experienced
+observers who have ever lived, namely, Kölreuter and Gärtner, should have
+arrived at diametrically opposite conclusions in regard to the very same
+species. It is also most instructive to compare--but I have not space here
+to enter on details--the evidence advanced by our best botanists on the
+question whether certain doubtful forms should be ranked as species or
+varieties, with the evidence from fertility adduced by different
+hybridisers, or by the same author, from experiments made during different
+years. It can thus be shown that neither sterility nor fertility affords
+any clear distinction between species and varieties; but that the evidence
+from this source graduates away, and is doubtful in the same degree as is
+the evidence derived from other constitutional and structural differences.
+
+In regard to the sterility of hybrids in successive generations; though
+Gärtner was enabled to rear some hybrids, carefully guarding them from a
+cross with either pure parent, for six or seven, and in one case for ten
+generations, yet he asserts positively that their fertility never
+increased, but generally greatly decreased. I do not doubt that this is
+usually the case, and that the fertility often suddenly decreases in the
+first few generations. Nevertheless I believe that in all these experiments
+the fertility has been diminished by an independent cause, namely, from
+close interbreeding. I have collected so large a body of facts, showing
+{249} that close interbreeding lessens fertility, and, on the other hand,
+that an occasional cross with a distinct individual or variety increases
+fertility, that I cannot doubt the correctness of this almost universal
+belief amongst breeders. Hybrids are seldom raised by experimentalists in
+great numbers; and as the parent-species, or other allied hybrids,
+generally grow in the same garden, the visits of insects must be carefully
+prevented during the flowering season: hence hybrids will generally be
+fertilised during each generation by their own individual pollen; and I am
+convinced that this would be injurious to their fertility, already lessened
+by their hybrid origin. I am strengthened in this conviction by a
+remarkable statement repeatedly made by Gärtner, namely, that if even the
+less fertile hybrids be artificially fertilised with hybrid pollen of the
+same kind, their fertility, notwithstanding the frequent ill effects of
+manipulation, sometimes decidedly increases, and goes on increasing. Now,
+in artificial fertilisation pollen is as often taken by chance (as I know
+from my own experience) from the anthers of another flower, as from the
+anthers of the flower itself which is to be fertilised; so that a cross
+between two flowers, though probably on the same plant, would be thus
+effected. Moreover, whenever complicated experiments are in progress, so
+careful an observer as Gärtner would have castrated his hybrids, and this
+would have insured in each generation a cross with a pollen from a distinct
+flower, either from the same plant or from another plant of the same hybrid
+nature. And thus, the strange fact of the increase of fertility in the
+successive generations of _artificially fertilised_ hybrids may, I believe,
+be accounted for by close interbreeding having been avoided.
+
+Now let us turn to the results arrived at by the third most experienced
+hybridiser, namely, the Hon. and {250} Rev. W. Herbert. He is as emphatic
+in his conclusion that some hybrids are perfectly fertile--as fertile as
+the pure parent-species--as are Kölreuter and Gärtner that some degree of
+sterility between distinct species is a universal law of nature. He
+experimentised on some of the very same species as did Gärtner. The
+difference in their results may, I think, be in part accounted for by
+Herbert's great horticultural skill, and by his having hothouses at his
+command. Of his many important statements I will here give only a single
+one as an example, namely, that "every ovule in a pod of Crinum capense
+fertilised by C. revolutum produced a plant, which (he says) I never saw to
+occur in a case of its natural fecundation." So that we here have perfect,
+or even more than commonly perfect, fertility in a first cross between two
+distinct species.
+
+This case of the Crinum leads me to refer to a most singular fact, namely,
+that there are individual plants of certain species of Lobelia and of some
+other genera, which can be far more easily fertilised by the pollen of
+another and distinct species, than by their own pollen; and all the
+individuals of nearly all the species of Hippeastrum seem to be in this
+predicament. For these plants have been found to yield seed to the pollen
+of a distinct species, though quite sterile with their own pollen,
+notwithstanding that their own pollen was found to be perfectly good, for
+it fertilised distinct species. So that certain individual plants and all
+the individuals of certain species can actually be hybridised much more
+readily than they can be self-fertilised! For instance, a bulb of
+Hippeastrum aulicum produced four flowers; three were fertilised by Herbert
+with their own pollen, and the fourth was subsequently fertilised by the
+pollen of a compound hybrid descended from three other and distinct {251}
+species: the result was that "the ovaries of the three first flowers soon
+ceased to grow, and after a few days perished entirely, whereas the pod
+impregnated by the pollen of the hybrid made vigorous growth and rapid
+progress to maturity, and bore good seed, which vegetated freely." In a
+letter to me, in 1839, Mr. Herbert told me that he had then tried the
+experiment during five years, and he continued to try it during several
+subsequent years, and always with the same result. This result has, also,
+been confirmed by other observers in the case of Hippeastrum with its
+sub-genera, and in the case of some other genera, as Lobelia, Passiflora
+and Verbascum. Although the plants in these experiments appeared perfectly
+healthy, and although both the ovules and pollen of the same flower were
+perfectly good with respect to other species, yet as they were functionally
+imperfect in their mutual self-action, we must infer that the plants were
+in an unnatural state. Nevertheless these facts show on what slight and
+mysterious causes the lesser or greater fertility of species when crossed,
+in comparison with the same species when self-fertilised, sometimes
+depends.
+
+The practical experiments of horticulturists, though not made with
+scientific precision, deserve some notice. It is notorious in how
+complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
+Petunia, Rhododendron, &c., have been crossed, yet many of these hybrids
+seed freely. For instance, Herbert asserts that a hybrid from Calceolaria
+integrifolia and plantaginea, species most widely dissimilar in general
+habit, "reproduced itself as perfectly as if it had been a natural species
+from the mountains of Chile." I have taken some pains to ascertain the
+degree of fertility of some of the complex crosses of Rhododendrons, and I
+am assured that many of them {252} are perfectly fertile. Mr. C. Noble, for
+instance, informs me that he raises stocks for grafting from a hybrid
+between Rhod. Ponticum and Catawbiense, and that this hybrid "seeds as
+freely as it is possible to imagine." Had hybrids, when fairly treated,
+gone on decreasing in fertility in each successive generation, as Gärtner
+believes to be the case, the fact would have been notorious to nurserymen.
+Horticulturists raise large beds of the same hybrids, and such alone are
+fairly treated, for by insect agency the several individuals of the same
+hybrid variety are allowed to freely cross with each other, and the
+injurious influence of close interbreeding is thus prevented. Any one may
+readily convince himself of the efficiency of insect-agency by examining
+the flowers of the more sterile kinds of hybrid rhododendrons, which
+produce no pollen, for he will find on their stigmas plenty of pollen
+brought from other flowers.
+
+In regard to animals, much fewer experiments have been carefully tried than
+with plants. If our systematic arrangements can be trusted, that is if the
+genera of animals are as distinct from each other, as are the genera of
+plants, then we may infer that animals more widely separated in the scale
+of nature can be more easily crossed than in the case of plants; but the
+hybrids themselves are, I think, more sterile. I doubt whether any case of
+a perfectly fertile hybrid animal can be considered as thoroughly well
+authenticated. It should, however, be borne in mind that, owing to few
+animals breeding freely under confinement, few experiments have been fairly
+tried: for instance, the canary-bird has been crossed with nine other
+finches, but as not one of these nine species breeds freely in confinement,
+we have no right to expect that the first crosses between them and the
+canary, or that their hybrids, {253} should be perfectly fertile. Again,
+with respect to the fertility in successive generations of the more fertile
+hybrid animals, I hardly know of an instance in which two families of the
+same hybrid have been raised at the same time from different parents, so as
+to avoid the ill effects of close interbreeding. On the contrary, brothers
+and sisters have usually been crossed in each successive generation, in
+opposition to the constantly repeated admonition of every breeder. And in
+this case, it is not at all surprising that the inherent sterility in the
+hybrids should have gone on increasing. If we were to act thus, and pair
+brothers and sisters in the case of any pure animal, which from any cause
+had the least tendency to sterility, the breed would assuredly be lost in a
+very few generations.
+
+Although I do not know of any thoroughly well-authenticated cases of
+perfectly fertile hybrid animals, I have some reason to believe that the
+hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus
+with P. torquatus and with P. versicolor are perfectly fertile. There is no
+doubt that these three pheasants, namely, the common, the true ring-necked,
+and the Japan, intercross, and are becoming blended together in the woods
+of several parts of England. The hybrids from the common and Chinese geese
+(A. cygnoides), species which are so different that they are generally
+ranked in distinct genera, have often bred in this country with either pure
+parent, and in one single instance they have bred _inter se_. This was
+effected by Mr. Eyton, who raised two hybrids from the same parents but
+from different hatches; and from these two birds he raised no less than
+eight hybrids (grandchildren of the pure geese) from one nest. In India,
+however, these cross-bred geese must be far more fertile; for I am assured
+by two eminently capable judges, namely {254} Mr. Blyth and Capt. Hutton,
+that whole flocks of these crossed geese are kept in various parts of the
+country; and as they are kept for profit, where neither pure parent-species
+exists, they must certainly be highly fertile.
+
+A doctrine which originated with Pallas, has been largely accepted by
+modern naturalists; namely, that most of our domestic animals have
+descended from two or more wild species, since commingled by intercrossing.
+On this view, the aboriginal species must either at first have produced
+quite fertile hybrids, or the hybrids must have become in subsequent
+generations quite fertile under domestication. This latter alternative
+seems to me the most probable, and I am inclined to believe in its truth,
+although it rests on no direct evidence. I believe, for instance, that our
+dogs have descended from several wild stocks; yet, with perhaps the
+exception of certain indigenous domestic dogs of South America, all are
+quite fertile together; and analogy makes me greatly doubt, whether the
+several aboriginal species would at first have freely bred together and
+have produced quite fertile hybrids. So again there is reason to believe
+that our European and the humped Indian cattle are quite fertile together;
+but from facts communicated to me by Mr. Blyth, I think they must be
+considered as distinct species. On this view of the origin of many of our
+domestic animals, we must either give up the belief of the almost universal
+sterility of distinct species of animals when crossed; or we must look at
+sterility, not as an indelible characteristic, but as one capable of being
+removed by domestication.
+
+Finally, looking to all the ascertained facts on the intercrossing of
+plants and animals, it may be concluded that some degree of sterility, both
+in first crosses {255} and in hybrids, is an extremely general result; but
+that it cannot, under our present state of knowledge, be considered as
+absolutely universal.
+
+
+
+_Laws governing the Sterility of first Crosses and of Hybrids._--We will
+now consider a little more in detail the circumstances and rules governing
+the sterility of first crosses and of hybrids. Our chief object will be to
+see whether or not the rules indicate that species have specially been
+endowed with this quality, in order to prevent their crossing and blending
+together in utter confusion. The following rules and conclusions are
+chiefly drawn up from Gärtner's admirable work on the hybridisation of
+plants. I have taken much pains to ascertain how far the rules apply to
+animals, and considering how scanty our knowledge is in regard to hybrid
+animals, I have been surprised to find how generally the same rules apply
+to both kingdoms.
+
+It has been already remarked, that the degree of fertility, both of first
+crosses and of hybrids, graduates from zero to perfect fertility. It is
+surprising in how many curious ways this gradation can be shown to exist;
+but only the barest outline of the facts can here be given. When pollen
+from a plant of one family is placed on the stigma of a plant of a distinct
+family, it exerts no more influence than so much inorganic dust. From this
+absolute zero of fertility, the pollen of different species of the same
+genus applied to the stigma of some one species, yields a perfect gradation
+in the number of seeds produced, up to nearly complete or even quite
+complete fertility; and, as we have seen, in certain abnormal cases, even
+to an excess of fertility, beyond that which the plant's own pollen will
+produce. So in hybrids themselves, there are some which never have
+produced, and probably never would produce, even {256} with the pollen of
+either pure parent, a single fertile seed: but in some of these cases a
+first trace of fertility may be detected, by the pollen of one of the pure
+parent-species causing the flower of the hybrid to wither earlier than it
+otherwise would have done; and the early withering of the flower is well
+known to be a sign of incipient fertilisation. From this extreme degree of
+sterility we have self-fertilised hybrids producing a greater and greater
+number of seeds up to perfect fertility.
+
+Hybrids from two species which are very difficult to cross, and which
+rarely produce any offspring, are generally very sterile; but the
+parallelism between the difficulty of making a first cross, and the
+sterility of the hybrids thus produced--two classes of facts which are
+generally confounded together--is by no means strict. There are many cases,
+in which two pure species can be united with unusual facility, and produce
+numerous hybrid-offspring, yet these hybrids are remarkably sterile. On the
+other hand, there are species which can be crossed very rarely, or with
+extreme difficulty, but the hybrids, when at last produced, are very
+fertile. Even within the limits of the same genus, for instance in
+Dianthus, these two opposite cases occur.
+
+The fertility, both of first crosses and of hybrids, is more easily
+affected by unfavourable conditions, than is the fertility of pure species.
+But the degree of fertility is likewise innately variable; for it is not
+always the same when the same two species are crossed under the same
+circumstances, but depends in part upon the constitution of the individuals
+which happen to have been chosen for the experiment. So it is with hybrids,
+for their degree of fertility is often found to differ greatly in the
+several individuals raised from seed out of the same capsule and exposed to
+exactly the same conditions. {257}
+
+By the term systematic affinity is meant, the resemblance between species
+in structure and in constitution, more especially in the structure of parts
+which are of high physiological importance and which differ little in the
+allied species. Now the fertility of first crosses between species, and of
+the hybrids produced from them, is largely governed by their systematic
+affinity. This is clearly shown by hybrids never having been raised between
+species ranked by systematists in distinct families; and on the other hand,
+by very closely allied species generally uniting with facility. But the
+correspondence between systematic affinity and the facility of crossing is
+by no means strict. A multitude of cases could be given of very closely
+allied species which will not unite, or only with extreme difficulty; and
+on the other hand of very distinct species which unite with the utmost
+facility. In the same family there may be a genus, as Dianthus, in which
+very many species can most readily be crossed; and another genus, as
+Silene, in which the most persevering efforts have failed to produce
+between extremely close species a single hybrid. Even within the limits of
+the same genus, we meet with this same difference; for instance, the many
+species of Nicotiana have been more largely crossed than the species of
+almost any other genus; but Gärtner found that N. acuminata, which is not a
+particularly distinct species, obstinately failed to fertilise, or to be
+fertilised by, no less than eight other species of Nicotiana. Very many
+analogous facts could be given.
+
+No one has been able to point out what kind, or what amount, of difference
+in any recognisable character is sufficient to prevent two species
+crossing. It can be shown that plants most widely different in habit and
+general appearance, and having strongly marked {258} differences in every
+part of the flower, even in the pollen, in the fruit, and in the
+cotyledons, can be crossed. Annual and perennial plants, deciduous and
+evergreen trees, plants inhabiting different stations and fitted for
+extremely different climates, can often be crossed with ease.
+
+By a reciprocal cross between two species, I mean the case, for instance,
+of a stallion-horse being first crossed with a female-ass, and then a
+male-ass with a mare: these two species may then be said to have been
+reciprocally crossed. There is often the widest possible difference in the
+facility of making reciprocal crosses. Such cases are highly important, for
+they prove that the capacity in any two species to cross is often
+completely independent of their systematic affinity, or of any recognisable
+difference in their whole organisation. On the other hand, these cases
+clearly show that the capacity for crossing is connected with
+constitutional differences imperceptible by us, and confined to the
+reproductive system. This difference in the result of reciprocal crosses
+between the same two species was long ago observed by Kölreuter. To give an
+instance: Mirabilis jalapa can easily be fertilised by the pollen of M.
+longiflora, and the hybrids thus produced are sufficiently fertile; but
+Kölreuter tried more than two hundred times, during eight following years,
+to fertilise reciprocally M. longiflora with the pollen of M. jalapa, and
+utterly failed. Several other equally striking cases could be given. Thuret
+has observed the same fact with certain sea-weeds or Fuci. Gärtner,
+moreover, found that this difference of facility in making reciprocal
+crosses is extremely common in a lesser degree. He has observed it even
+between forms so closely related (as Matthiola annua and glabra) that many
+botanists rank them only as varieties. It is also a remarkable fact, that
+hybrids raised from reciprocal crosses, though {259} of course compounded
+of the very same two species, the one species having first been used as the
+father and then as the mother, generally differ in fertility in a small,
+and occasionally in a high degree.
+
+Several other singular rules could be given from Gärtner: for instance,
+some species have a remarkable power of crossing with other species; other
+species of the same genus have a remarkable power of impressing their
+likeness on their hybrid offspring; but these two powers do not at all
+necessarily go together. There are certain hybrids which instead of having,
+as is usual, an intermediate character between their two parents, always
+closely resemble one of them; and such hybrids, though externally so like
+one of their pure parent-species, are with rare exceptions extremely
+sterile. So again amongst hybrids which are usually intermediate in
+structure between their parents, exceptional and abnormal individuals
+sometimes are born, which closely resemble one of their pure parents; and
+these hybrids are almost always utterly sterile, even when the other
+hybrids raised from seed from the same capsule have a considerable degree
+of fertility. These facts show how completely fertility in the hybrid is
+independent of its external resemblance to either pure parent.
+
+Considering the several rules now given, which govern the fertility of
+first crosses and of hybrids, we see that when forms, which must be
+considered as good and distinct species, are united, their fertility
+graduates from zero to perfect fertility, or even to fertility under
+certain conditions in excess. That their fertility, besides being eminently
+susceptible to favourable and unfavourable conditions, is innately
+variable. That it is by no means always the same in degree in the first
+cross and in the hybrids produced {260} from this cross. That the fertility
+of hybrids is not related to the degree in which they resemble in external
+appearance either parent. And lastly, that the facility of making a first
+cross between any two species is not always governed by their systematic
+affinity or degree of resemblance to each other. This latter statement is
+clearly proved by reciprocal crosses between the same two species, for
+according as the one species or the other is used as the father or the
+mother, there is generally some difference, and occasionally the widest
+possible difference, in the facility of effecting an union. The hybrids,
+moreover, produced from reciprocal crosses often differ in fertility.
+
+Now do these complex and singular rules indicate that species have been
+endowed with sterility simply to prevent their becoming confounded in
+nature? I think not. For why should the sterility be so extremely different
+in degree, when various species are crossed, all of which we must suppose
+it would be equally important to keep from blending together? Why should
+the degree of sterility be innately variable in the individuals of the same
+species? Why should some species cross with facility, and yet produce very
+sterile hybrids; and other species cross with extreme difficulty, and yet
+produce fairly fertile hybrids? Why should there often be so great a
+difference in the result of a reciprocal cross between the same two
+species? Why, it may even be asked, has the production of hybrids been
+permitted? to grant to species the special power of producing hybrids, and
+then to stop their further propagation by different degrees of sterility,
+not strictly related to the facility of the first union between their
+parents, seems to be a strange arrangement.
+
+The foregoing rules and facts, on the other hand, {261} appear to me
+clearly to indicate that the sterility both of first crosses and of hybrids
+is simply incidental or dependent on unknown differences, chiefly in the
+reproductive systems, of the species which are crossed. The differences
+being of so peculiar and limited a nature, that, in reciprocal crosses
+between two species the male sexual element of the one will often freely
+act on the female sexual element of the other, but not in a reversed
+direction. It will be advisable to explain a little more fully by an
+example what I mean by sterility being incidental on other differences, and
+not a specially endowed quality. As the capacity of one plant to be grafted
+or budded on another is so entirely unimportant for its welfare in a state
+of nature, I presume that no one will suppose that this capacity is a
+_specially_ endowed quality, but will admit that it is incidental on
+differences in the laws of growth of the two plants. We can sometimes see
+the reason why one tree will not take on another, from differences in their
+rate of growth, in the hardness of their wood, in the period of the flow or
+nature of their sap, &c.; but in a multitude of cases we can assign no
+reason whatever. Great diversity in the size of two plants, one being woody
+and the other herbaceous, one being evergreen and the other deciduous, and
+adaptation to widely different climates, does not always prevent the two
+grafting together. As in hybridisation, so with grafting, the capacity is
+limited by systematic affinity, for no one has been able to graft trees
+together belonging to quite distinct families; and, on the other hand,
+closely allied species, and varieties of the same species, can usually, but
+not invariably, be grafted with ease. But this capacity, as in
+hybridisation, is by no means absolutely governed by systematic affinity.
+Although many distinct genera within the same family have been grafted
+{262} together, in other cases species of the same genus will not take on
+each other. The pear can be grafted far more readily on the quince, which
+is ranked as a distinct genus, than on the apple, which is a member of the
+same genus. Even different varieties of the pear take with different
+degrees of facility on the quince; so do different varieties of the apricot
+and peach on certain varieties of the plum.
+
+As Gärtner found that there was sometimes an innate difference in different
+_individuals_ of the same two species in crossing; so Sagaret believes this
+to be the case with different individuals of the same two species in being
+grafted together. As in reciprocal crosses, the facility of effecting an
+union is often very far from equal, so it sometimes is in grafting; the
+common gooseberry, for instance, cannot be grafted on the currant, whereas
+the currant will take, though with difficulty, on the gooseberry.
+
+We have seen that the sterility of hybrids, which have their reproductive
+organs in an imperfect condition, is a very different case from the
+difficulty of uniting two pure species, which have their reproductive
+organs perfect; yet these two distinct cases run to a certain extent
+parallel. Something analogous occurs in grafting; for Thouin found that
+three species of Robinia, which seeded freely on their own roots, and which
+could be grafted with no great difficulty on another species, when thus
+grafted were rendered barren. On the other hand, certain species of Sorbus,
+when grafted on other species, yielded twice as much fruit as when on their
+own roots. We are reminded by this latter fact of the extraordinary case of
+Hippeastrum, Lobelia, &c., which seeded much more freely when fertilised
+with the pollen of distinct species, than when self-fertilised with their
+own pollen. {263}
+
+We thus see, that although there is a clear and fundamental difference
+between the mere adhesion of grafted stocks, and the union of the male and
+female elements in the act of reproduction, yet that there is a rude degree
+of parallelism in the results of grafting and of crossing distinct species.
+And as we must look at the curious and complex laws governing the facility
+with which trees can be grafted on each other as incidental on unknown
+differences in their vegetative systems, so I believe that the still more
+complex laws governing the facility of first crosses, are incidental on
+unknown differences, chiefly in their reproductive systems. These
+differences, in both cases, follow to a certain extent, as might have been
+expected, systematic affinity, by which every kind of resemblance and
+dissimilarity between organic beings is attempted to be expressed. The
+facts by no means seem to me to indicate that the greater or lesser
+difficulty of either grafting or crossing together various species has been
+a special endowment; although in the case of crossing, the difficulty is as
+important for the endurance and stability of specific forms, as in the case
+of grafting it is unimportant for their welfare.
+
+
+
+_Causes of the Sterility of first Crosses and of Hybrids._--We may now look
+a little closer at the probable causes of the sterility of first crosses
+and of hybrids. These two cases are fundamentally different, for, as just
+remarked, in the union of two pure species the male and female sexual
+elements are perfect, whereas in hybrids they are imperfect. Even in first
+crosses, the greater or lesser difficulty in effecting a union apparently
+depends on several distinct causes. There must sometimes be a physical
+impossibility in the male element reaching the ovule, as would be the case
+with a plant {264} having a pistil too long for the pollen-tubes to reach
+the ovarium. It has also been observed that when pollen of one species is
+placed on the stigma of a distantly allied species, though the pollen-tubes
+protrude, they do not penetrate the stigmatic surface. Again, the male
+element may reach the female element, but be incapable of causing an embryo
+to be developed, as seems to have been the case with some of Thuret's
+experiments on Fuci. No explanation can be given of these facts, any more
+than why certain trees cannot be grafted on others. Lastly, an embryo may
+be developed, and then perish at an early period. This latter alternative
+has not been sufficiently attended to; but I believe, from observations
+communicated to me by Mr. Hewitt, who has had great experience in
+hybridising gallinaceous birds, that the early death of the embryo is a
+very frequent cause of sterility in first crosses. I was at first very
+unwilling to believe in this view; as hybrids, when once born, are
+generally healthy and long-lived, as we see in the case of the common mule.
+Hybrids, however, are differently circumstanced before and after birth:
+when born and living in a country where their two parents can live, they
+are generally placed under suitable conditions of life. But a hybrid
+partakes of only half of the nature and constitution of its mother, and
+therefore before birth, as long as it is nourished within its mother's womb
+or within the egg or seed produced by the mother, it may be exposed to
+conditions in some degree unsuitable, and consequently be liable to perish
+at an early period; more especially as all very young beings seem eminently
+sensitive to injurious or unnatural conditions of life.
+
+In regard to the sterility of hybrids, in which the sexual elements are
+imperfectly developed, the case is {265} very different. I have more than
+once alluded to a large body of facts, which I have collected, showing that
+when animals and plants are removed from their natural conditions, they are
+extremely liable to have their reproductive systems seriously affected.
+This, in fact, is the great bar to the domestication of animals. Between
+the sterility thus superinduced and that of hybrids, there are many points
+of similarity. In both cases the sterility is independent of general
+health, and is often accompanied by excess of size or great luxuriance. In
+both cases, the sterility occurs in various degrees; in both, the male
+element is the most liable to be affected; but sometimes the female more
+than the male. In both, the tendency goes to a certain extent with
+systematic affinity, for whole groups of animals and plants are rendered
+impotent by the same unnatural conditions; and whole groups of species tend
+to produce sterile hybrids. On the other hand, one species in a group will
+sometimes resist great changes of conditions with unimpaired fertility; and
+certain species in a group will produce unusually fertile hybrids. No one
+can tell, till he tries, whether any particular animal will breed under
+confinement or any exotic plant seed freely under culture; nor can he tell,
+till he tries, whether any two species of a genus will produce more or less
+sterile hybrids. Lastly, when organic beings are placed during several
+generations under conditions not natural to them, they are extremely liable
+to vary, which is due, as I believe, to their reproductive systems having
+been specially affected, though in a lesser degree than when sterility
+ensues. So it is with hybrids, for hybrids in successive generations are
+eminently liable to vary, as every experimentalist has observed.
+
+Thus we see that when organic beings are placed under new and unnatural
+conditions, and when hybrids {266} are produced by the unnatural crossing
+of two species, the reproductive system, independently of the general state
+of health, is affected by sterility in a very similar manner. In the one
+case, the conditions of life have been disturbed, though often in so slight
+a degree as to be inappreciable by us; in the other case, or that of
+hybrids, the external conditions have remained the same, but the
+organisation has been disturbed by two different structures and
+constitutions having been blended into one. For it is scarcely possible
+that two organisations should be compounded into one, without some
+disturbance occurring in the development, or periodical action, or mutual
+relation of the different parts and organs one to another, or to the
+conditions of life. When hybrids are able to breed _inter se_, they
+transmit to their offspring from generation to generation the same
+compounded organisation, and hence we need not be surprised that their
+sterility, though in some degree variable, rarely diminishes.
+
+It must, however, be confessed that we cannot understand, excepting on
+vague hypotheses, several facts with respect to the sterility of hybrids;
+for instance, the unequal fertility of hybrids produced from reciprocal
+crosses; or the increased sterility in those hybrids which occasionally and
+exceptionally resemble closely either pure parent. Nor do I pretend that
+the foregoing remarks go to the root of the matter: no explanation is
+offered why an organism, when placed under unnatural conditions, is
+rendered sterile. All that I have attempted to show, is that in two cases,
+in some respects allied, sterility is the common result,--in the one case
+from the conditions of life having been disturbed, in the other case from
+the organisation having been disturbed by two organisations having been
+compounded into one.
+
+It may seem fanciful, but I suspect that a similar {267} parallelism
+extends to an allied yet very different class of facts. It is an old and
+almost universal belief, founded, I think, on a considerable body of
+evidence, that slight changes in the conditions of life are beneficial to
+all living things. We see this acted on by farmers and gardeners in their
+frequent exchanges of seed, tubers, &c., from one soil or climate to
+another, and back again. During the convalescence of animals, we plainly
+see that great benefit is derived from almost any change in the habits of
+life. Again, both with plants and animals, there is abundant evidence, that
+a cross between very distinct individuals of the same species, that is
+between members of different strains or sub-breeds, gives vigour and
+fertility to the offspring. I believe, indeed, from the facts alluded to in
+our fourth chapter, that a certain amount of crossing is indispensable even
+with hermaphrodites; and that close interbreeding continued during several
+generations between the nearest relations, especially if these be kept
+under the same conditions of life, always induces weakness and sterility in
+the progeny.
+
+Hence it seems that, on the one hand, slight changes in the conditions of
+life benefit all organic beings, and on the other hand, that slight
+crosses, that is crosses between the males and females of the same species
+which have varied and become slightly different, give vigour and fertility
+to the offspring. But we have seen that greater changes, or changes of a
+particular nature, often render organic beings in some degree sterile; and
+that greater crosses, that is crosses between males and females which have
+become widely or specifically different, produce hybrids which are
+generally sterile in some degree. I cannot persuade myself that this
+parallelism is an accident or an illusion. Both series of facts seem to be
+connected together by some {268} common but unknown bond, which is
+essentially related to the principle of life.
+
+
+
+_Fertility of Varieties when crossed, and of their Mongrel offspring._--It
+may be urged, as a most forcible argument, that there must be some
+essential distinction between species and varieties, and that there must be
+some error in all the foregoing remarks, inasmuch as varieties, however
+much they may differ from each other in external appearance, cross with
+perfect facility, and yield perfectly fertile offspring. I fully admit that
+this is almost invariably the case. But if we look to varieties produced
+under nature, we are immediately involved in hopeless difficulties; for if
+two hitherto reputed varieties be found in any degree sterile together,
+they are at once ranked by most naturalists as species. For instance, the
+blue and red pimpernel, the primrose and cowslip, which are considered by
+many of our best botanists as varieties, are said by Gärtner not to be
+quite fertile when crossed, and he consequently ranks them as undoubted
+species. If we thus argue in a circle, the fertility of all varieties
+produced under nature will assuredly have to be granted.
+
+If we turn to varieties, produced, or supposed to have been produced, under
+domestication, we are still involved in doubt. For when it is stated, for
+instance, that the German Spitz dog unites more easily than other dogs with
+foxes, or that certain South American indigenous domestic dogs do not
+readily cross with European dogs, the explanation which will occur to every
+one, and probably the true one, is that these dogs have descended from
+several aboriginally distinct species. Nevertheless the perfect fertility
+of so many domestic varieties, differing widely from each other in
+appearance, for instance of the pigeon or of the cabbage, is {269} a
+remarkable fact; more especially when we reflect how many species there
+are, which, though resembling each other most closely, are utterly sterile
+when intercrossed. Several considerations, however, render the fertility of
+domestic varieties less remarkable than at first appears. It can, in the
+first place, be clearly shown that mere external dissimilarity between two
+species does not determine their greater or lesser degree of sterility when
+crossed; and we may apply the same rule to domestic varieties. In the
+second place, some eminent naturalists believe that a long course of
+domestication tends to eliminate sterility in the successive generations of
+hybrids which were at first only slightly sterile; and if this be so, we
+surely ought not to expect to find sterility both appearing and
+disappearing under nearly the same conditions of life. Lastly, and this
+seems to me by far the most important consideration, new races of animals
+and plants are produced under domestication by man's methodical and
+unconscious power of selection, for his own use and pleasure: he neither
+wishes to select, nor could select, slight differences in the reproductive
+system, or other constitutional differences correlated with the
+reproductive system. He supplies his several varieties with the same food;
+treats them in nearly the same manner, and does not wish to alter their
+general habits of life. Nature acts uniformly and slowly during vast
+periods of time on the whole organisation, in any way which may be for each
+creature's own good; and thus she may, either directly, or more probably
+indirectly, through correlation, modify the reproductive system in the
+several descendants from any one species. Seeing this difference in the
+process of selection, as carried on by man and nature, we need not be
+surprised at some difference in the result.
+
+I have as yet spoken as if the varieties of the same {270} species were
+invariably fertile when intercrossed. But it seems to me impossible to
+resist the evidence of the existence of a certain amount of sterility in
+the few following cases, which I will briefly abstract. The evidence is at
+least as good as that from which we believe in the sterility of a multitude
+of species. The evidence is, also, derived from hostile witnesses, who in
+all other cases consider fertility and sterility as safe criterions of
+specific distinction. Gärtner kept during several years a dwarf kind of
+maize with yellow seeds, and a tall variety with red seeds, growing near
+each other in his garden; and although these plants have separated sexes,
+they never naturally crossed. He then fertilised thirteen flowers of the
+one with the pollen of the other; but only a single head produced any seed,
+and this one head produced only five grains. Manipulation in this case
+could not have been injurious, as the plants have separated sexes. No one,
+I believe, has suspected that these varieties of maize are distinct
+species; and it is important to notice that the hybrid plants thus raised
+were themselves _perfectly_ fertile; so that even Gärtner did not venture
+to consider the two varieties as specifically distinct.
+
+Girou de Buzareingues crossed three varieties of gourd, which like the
+maize has separated sexes, and he asserts that their mutual fertilisation
+is by so much the less easy as their differences are greater. How far these
+experiments may be trusted, I know not; but the forms experimentised on,
+are ranked by Sagaret, who mainly founds his classification by the test of
+infertility, as varieties.
+
+The following case is far more remarkable, and seems at first quite
+incredible; but it is the result of an astonishing number of experiments
+made during many years on nine species of Verbascum, by so good an observer
+{271} and so hostile a witness, as Gärtner: namely, that yellow and white
+varieties of the same species of Verbascum when intercrossed produce less
+seed, than do either coloured varieties when fertilised with pollen from
+their own coloured flowers. Moreover, he asserts that when yellow and white
+varieties of one species are crossed with yellow and white varieties of a
+_distinct_ species, more seed is produced by the crosses between the
+similarly coloured flowers, than between those which are differently
+coloured. Yet these varieties of Verbascum present no other difference
+besides the mere colour of the flower; and one variety can sometimes be
+raised from the seed of the other.
+
+From observations which I have made on certain varieties of hollyhock, I am
+inclined to suspect that they present analogous facts.
+
+Kölreuter, whose accuracy has been confirmed by every subsequent observer,
+has proved the remarkable fact, that one variety of the common tobacco is
+more fertile, when crossed with a widely distinct species, than are the
+other varieties. He experimentised on five forms, which are commonly
+reputed to be varieties, and which he tested by the severest trial, namely,
+by reciprocal crosses, and he found their mongrel offspring perfectly
+fertile. But one of these five varieties, when used either as father or
+mother, and crossed with the Nicotiana glutinosa, always yielded hybrids
+not so sterile as those which were produced from the four other varieties
+when crossed with N. glutinosa. Hence the reproductive system of this one
+variety must have been in some manner and in some degree modified.
+
+From these facts; from the great difficulty of ascertaining the infertility
+of varieties in a state of nature, for a supposed variety if infertile in
+any degree would generally be ranked as species; from man selecting only
+{272} external characters in the production of the most distinct domestic
+varieties, and from not wishing or being able to produce recondite and
+functional differences in the reproductive system; from these several
+considerations and facts, I do not think that the very general fertility of
+varieties can be proved to be of universal occurrence, or to form a
+fundamental distinction between varieties and species. The general
+fertility of varieties does not seem to me sufficient to overthrow the view
+which I have taken with respect to the very general, but not invariable,
+sterility of first crosses and of hybrids, namely, that it is not a special
+endowment, but is incidental on slowly acquired modifications, more
+especially in the reproductive systems of the forms which are crossed.
+
+
+
+_Hybrids and Mongrels compared, independently of their
+fertility._--Independently of the question of fertility, the offspring of
+species when crossed and of varieties when crossed may be compared in
+several other respects. Gärtner, whose strong wish was to draw a marked
+line of distinction between species and varieties, could find very few and,
+as it seems to me, quite unimportant differences between the so-called
+hybrid offspring of species, and the so-called mongrel offspring of
+varieties. And, on the other hand, they agree most closely in very many
+important respects.
+
+I shall here discuss this subject with extreme brevity. The most important
+distinction is, that in the first generation mongrels are more variable
+than hybrids; but Gärtner admits that hybrids from species which have long
+been cultivated are often variable in the first generation; and I have
+myself seen striking instances of this fact. Gärtner further admits that
+hybrids between very closely allied species are more variable {273} than
+those from very distinct species; and this shows that the difference in the
+degree of variability graduates away. When mongrels and the more fertile
+hybrids are propagated for several generations an extreme amount of
+variability in their offspring is notorious; but some few cases both of
+hybrids and mongrels long retaining uniformity of character could be given.
+The variability, however, in the successive generations of mongrels is,
+perhaps, greater than in hybrids.
+
+This greater variability of mongrels than of hybrids does not seem to me at
+all surprising. For the parents of mongrels are varieties, and mostly
+domestic varieties (very few experiments having been tried on natural
+varieties), and this implies in most cases that there has been recent
+variability; and therefore we might expect that such variability would
+often continue and be superadded to that arising from the mere act of
+crossing. The slight degree of variability in hybrids from the first cross
+or in the first generation, in contrast with their extreme variability in
+the succeeding generations, is a curious fact and deserves attention. For
+it bears on and corroborates the view which I have taken on the cause of
+ordinary variability; namely, that it is due to the reproductive system
+being eminently sensitive to any change in the conditions of life, being
+thus often rendered either impotent or at least incapable of its proper
+function of producing offspring identical with the parent-form. Now hybrids
+in the first generation are descended from species (excluding those long
+cultivated) which have not had their reproductive systems in any way
+affected, and they are not variable; but hybrids themselves have their
+reproductive systems seriously affected, and their descendants are highly
+variable.
+
+But to return to our comparison of mongrels and {274} hybrids: Gärtner
+states that mongrels are more liable than hybrids to revert to either
+parent-form; but this, if it be true, is certainly only a difference in
+degree. Gärtner further insists that when any two species, although most
+closely allied to each other, are crossed with a third species, the hybrids
+are widely different from each other; whereas if two very distinct
+varieties of one species are crossed with another species, the hybrids do
+not differ much. But this conclusion, as far as I can make out, is founded
+on a single experiment; and seems directly opposed to the results of
+several experiments made by Kölreuter.
+
+These alone are the unimportant differences, which Gärtner is able to point
+out, between hybrid and mongrel plants. On the other hand, the resemblance
+in mongrels and in hybrids to their respective parents, more especially in
+hybrids produced from nearly related species, follows according to Gärtner
+the same laws. When two species are crossed, one has sometimes a prepotent
+power of impressing its likeness on the hybrid; and so I believe it to be
+with varieties of plants. With animals one variety certainly often has this
+prepotent power over another variety. Hybrid plants produced from a
+reciprocal cross, generally resemble each other closely; and so it is with
+mongrels from a reciprocal cross. Both hybrids and mongrels can be reduced
+to either pure parent-form, by repeated crosses in successive generations
+with either parent.
+
+These several remarks are apparently applicable to animals; but the subject
+is here excessively complicated, partly owing to the existence of secondary
+sexual characters; but more especially owing to prepotency in transmitting
+likeness running more strongly in one sex than in the other, both when one
+species is crossed with another, and when, one variety is crossed with
+{275} another variety. For instance, I think those authors are right, who
+maintain that the ass has a prepotent power over the horse, so that both
+the mule and the hinny more resemble the ass than the horse; but that the
+prepotency runs more strongly in the male-ass than in the female, so that
+the mule, which is the offspring of the male-ass and mare, is more like an
+ass, than is the hinny, which is the offspring of the female-ass and
+stallion.
+
+Much stress has been laid by some authors on the supposed fact, that
+mongrel animals alone are born closely like one of their parents; but it
+can be shown that this does sometimes occur with hybrids; yet I grant much
+less frequently with hybrids than with mongrels. Looking to the cases which
+I have collected of cross-bred animals closely resembling one parent, the
+resemblances seem chiefly confined to characters almost monstrous in their
+nature, and which have suddenly appeared--such as albinism, melanism,
+deficiency of tail or horns, or additional fingers and toes; and do not
+relate to characters which have been slowly acquired by selection.
+Consequently, sudden reversions to the perfect character of either parent
+would be more likely to occur with mongrels, which are descended from
+varieties often suddenly produced and semi-monstrous in character, than
+with hybrids, which are descended from species slowly and naturally
+produced. On the whole I entirely agree with Dr. Prosper Lucas, who, after
+arranging an enormous body of facts with respect to animals, comes to the
+conclusion, that the laws of resemblance of the child to its parents are
+the same, whether the two parents differ much or little from each other,
+namely in the union of individuals of the same variety, or of different
+varieties, or of distinct species.
+
+Laying aside the question of fertility and sterility, {276} in all other
+respects there seems to be a general and close similarity in the offspring
+of crossed species, and of crossed varieties. If we look at species as
+having been specially created, and at varieties as having been produced by
+secondary laws, this similarity would be an astonishing fact. But it
+harmonises perfectly with the view that there is no essential distinction
+between species and varieties.
+
+
+
+_Summary of Chapter._--First crosses between forms sufficiently distinct to
+be ranked as species, and their hybrids, are very generally, but not
+universally, sterile. The sterility is of all degrees, and is often so
+slight that the two most careful experimentalists who have ever lived, have
+come to diametrically opposite conclusions in ranking forms by this test.
+The sterility is innately variable in individuals of the same species, and
+is eminently susceptible of favourable and unfavourable conditions. The
+degree of sterility does not strictly follow systematic affinity, but is
+governed by several curious and complex laws. It is generally different,
+and sometimes widely different, in reciprocal crosses between the same two
+species. It is not always equal in degree in a first cross and in the
+hybrid produced from this cross.
+
+In the same manner as in grafting trees, the capacity of one species or
+variety to take on another, is incidental on generally unknown differences
+in their vegetative systems, so in crossing, the greater or less facility
+of one species to unite with another, is incidental on unknown differences
+in their reproductive systems. There is no more reason to think that
+species have been specially endowed with various degrees of sterility to
+prevent them crossing and blending in nature, than to think that trees have
+been specially endowed with various and {277} somewhat analogous degrees of
+difficulty in being grafted together in order to prevent them becoming
+inarched in our forests.
+
+The sterility of first crosses between pure species, which have their
+reproductive systems perfect, seems to depend on several circumstances; in
+some cases largely on the early death of the embryo. The sterility of
+hybrids, which have their reproductive systems imperfect, and which have
+had this system and their whole organisation disturbed by being compounded
+of two distinct species, seems closely allied to that sterility which so
+frequently affects pure species, when their natural conditions of life have
+been disturbed. This view is supported by a parallelism of another
+kind;--namely, that the crossing of forms only slightly different is
+favourable to the vigour and fertility of their offspring; and that slight
+changes in the conditions of life are apparently favourable to the vigour
+and fertility of all organic beings. It is not surprising that the degree
+of difficulty in uniting two species, and the degree of sterility of their
+hybrid-offspring should generally correspond, though due to distinct
+causes; for both depend on the amount of difference of some kind between
+the species which are crossed. Nor is it surprising that the facility of
+effecting a first cross, the fertility of the hybrids produced from it, and
+the capacity of being grafted together--though this latter capacity
+evidently depends on widely different circumstances--should all run, to a
+certain extent, parallel with the systematic affinity of the forms which
+are subjected to experiment; for systematic affinity attempts to express
+all kinds of resemblance between all species.
+
+First crosses between forms known to be varieties, or sufficiently alike to
+be considered as varieties, and their mongrel offspring, are very
+generally, but not quite {278} universally, fertile. Nor is this nearly
+general and perfect fertility surprising, when we remember how liable we
+are to argue in a circle with respect to varieties in a state of nature;
+and when we remember that the greater number of varieties have been
+produced under domestication by the selection of mere external differences,
+and not of differences in the reproductive system. In all other respects,
+excluding fertility, there is a close general resemblance between hybrids
+and mongrels. Finally, then, the facts briefly given in this chapter do not
+seem to me opposed to, but even rather to support the view, that there is
+no fundamental distinction between species and varieties.
+
+       *       *       *       *       *
+
+
+{279}
+
+CHAPTER IX.
+
+ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
+
+    On the absence of intermediate varieties at the present day--On the
+    nature of extinct intermediate varieties; on their number--On the vast
+    lapse of time, as inferred from the rate of deposition and of
+    denudation--On the poorness of our palæontological collections--On the
+    intermittence of geological formations--On the absence of intermediate
+    varieties in any one formation--On the sudden appearance of groups of
+    species--On their sudden appearance in the lowest known fossiliferous
+    strata.
+
+In the sixth chapter I enumerated the chief objections which might be
+justly urged against the views maintained in this volume. Most of them have
+now been discussed. One, namely the distinctness of specific forms, and
+their not being blended together by innumerable transitional links, is a
+very obvious difficulty. I assigned reasons why such links do not commonly
+occur at the present day, under the circumstances apparently most
+favourable for their presence, namely on an extensive and continuous area
+with graduated physical conditions. I endeavoured to show, that the life of
+each species depends in a more important manner on the presence of other
+already defined organic forms, than on climate; and, therefore, that the
+really governing conditions of life do not graduate away quite insensibly
+like heat or moisture. I endeavoured, also, to show that intermediate
+varieties, from existing in lesser numbers than the forms which they
+connect, will generally be beaten out and exterminated during the course of
+further modification and improvement. The main cause, however, of
+innumerable intermediate links not now occurring everywhere throughout
+nature {280} depends on the very process of natural selection, through
+which new varieties continually take the places of and exterminate their
+parent-forms. But just in proportion as this process of extermination has
+acted on an enormous scale, so must the number of intermediate varieties,
+which have formerly existed on the earth, be truly enormous. Why then is
+not every geological formation and every stratum full of such intermediate
+links? Geology assuredly does not reveal any such finely graduated organic
+chain; and this, perhaps, is the most obvious and gravest objection which
+can be urged against my theory. The explanation lies, as I believe, in the
+extreme imperfection of the geological record.
+
+In the first place it should always be borne in mind what sort of
+intermediate forms must, on my theory, have formerly existed. I have found
+it difficult, when looking at any two species, to avoid picturing to
+myself, forms _directly_ intermediate between them. But this is a wholly
+false view; we should always look for forms intermediate between each
+species and a common but unknown progenitor; and the progenitor will
+generally have differed in some respects from all its modified descendants.
+To give a simple illustration: the fantail and pouter pigeons have both
+descended from the rock-pigeon; if we possessed all the intermediate
+varieties which have ever existed, we should have an extremely close series
+between both and the rock-pigeon; but we should have no varieties directly
+intermediate between the fantail and pouter; none, for instance, combining
+a tail somewhat expanded with a crop somewhat enlarged, the characteristic
+features of these two breeds. These two breeds, moreover, have become so
+much modified, that if we had no historical or indirect evidence regarding
+their origin, it would not have been possible to have {281} determined from
+a mere comparison of their structure with that of the rock-pigeon, whether
+they had descended from this species or from some other allied species,
+such as C. oenas.
+
+So with natural species, if we look to forms very distinct, for instance to
+the horse and tapir, we have no reason to suppose that links ever existed
+directly intermediate between them, but between each and an unknown common
+parent. The common parent will have had in its whole organisation much
+general resemblance to the tapir and to the horse; but in some points of
+structure may have differed considerably from both, even perhaps more than
+they differ from each other. Hence in all such cases, we should be unable
+to recognise the parent-form of any two or more species, even if we closely
+compared the structure of the parent with that of its modified descendants,
+unless at the same time we had a nearly perfect chain of the intermediate
+links.
+
+It is just possible by my theory, that one of two living forms might have
+descended from the other; for instance, a horse from a tapir; and in this
+case _direct_ intermediate links will have existed between them. But such a
+case would imply that one form had remained for a very long period
+unaltered, whilst its descendants had undergone a vast amount of change;
+and the principle of competition between organism and organism, between
+child and parent, will render this a very rare event; for in all cases the
+new and improved forms of life tend to supplant the old and unimproved
+forms.
+
+By the theory of natural selection all living species have been connected
+with the parent-species of each genus, by differences not greater than we
+see between the varieties of the same species at the present {282} day; and
+these parent-species, now generally extinct, have in their turn been
+similarly connected with more ancient species; and so on backwards, always
+converging to the common ancestor of each great class. So that the number
+of intermediate and transitional links, between all living and extinct
+species, must have been inconceivably great. But assuredly, if this theory
+be true, such have lived upon this earth.
+
+
+
+_On the lapse of Time._--Independently of our not finding fossil remains of
+such infinitely numerous connecting links, it may be objected, that time
+will not have sufficed for so great an amount of organic change, all
+changes having been effected very slowly through natural selection. It is
+hardly possible for me even to recall to the reader, who may not be a
+practical geologist, the facts leading the mind feebly to comprehend the
+lapse of time. He who can read Sir Charles Lyell's grand work on the
+Principles of Geology, which the future historian will recognise as having
+produced a revolution in natural science, yet does not admit how
+incomprehensively vast have been the past periods of time, may at once
+close this volume. Not that it suffices to study the Principles of Geology,
+or to read special treatises by different observers on separate formations,
+and to mark how each author attempts to give an inadequate idea of the
+duration of each formation or even each stratum. A man must for years
+examine for himself great piles of superimposed strata, and watch the sea
+at work grinding down old rocks and making fresh sediment, before he can
+hope to comprehend anything of the lapse of time, the monuments of which we
+see around us.
+
+It is good to wander along lines of sea-coast, when formed of moderately
+hard rocks, and mark the {283} process of degradation. The tides in most
+cases reach the cliffs only for a short time twice a day, and the waves eat
+into them only when they are charged with sand or pebbles; for there is
+good evidence that pure water can effect little or nothing in wearing away
+rock. At last the base of the cliff is undermined, huge fragments fall
+down, and these remaining fixed, have to be worn away, atom by atom, until
+reduced in size they can be rolled about by the waves, and then are more
+quickly ground into pebbles, sand, or mud. But how often do we see along
+the bases of retreating cliffs rounded boulders, all thickly clothed by
+marine productions, showing how little they are abraded and how seldom they
+are rolled about! Moreover, if we follow for a few miles any line of rocky
+cliff, which is undergoing degradation, we find that it is only here and
+there, along a short length or round a promontory, that the cliffs are at
+the present time suffering. The appearance of the surface and the
+vegetation show that elsewhere years have elapsed since the waters washed
+their base.
+
+He who most closely studies the action of the sea on our shores, will, I
+believe, be most deeply impressed with the slowness with which rocky coasts
+are worn away. The observations on this head by Hugh Miller, and by that
+excellent observer Mr. Smith of Jordan Hill, are most impressive. With the
+mind thus impressed, let any one examine beds of conglomerate many thousand
+feet in thickness, which, though probably formed at a quicker rate than
+many other deposits, yet, from being formed of worn and rounded pebbles,
+each of which bears the stamp of time, are good to show how slowly the mass
+has been accumulated. In the Cordillera I estimated one pile of
+conglomerate at ten thousand feet in thickness. Let the {284} observer
+remember Lyell's profound remark that the thickness and extent of
+sedimentary formations are the result and measure of the degradation which
+the earth's crust has elsewhere suffered. And what an amount of degradation
+is implied by the sedimentary deposits of many countries! Professor Ramsay
+has given me the maximum thickness, in most cases from actual measurement,
+in a few cases from estimate, of each formation in different parts of Great
+Britain; and this is the result:--
+
+                                                     Feet.
+  Palæozoic strata (not including igneous beds)      57,154
+  Secondary strata                                   13,190
+  Tertiary strata                                     2,240
+
+--making altogether 72,584 feet; that is, very nearly thirteen and
+three-quarters British miles. Some of the formations, which are represented
+in England by thin beds, are thousands of feet in thickness on the
+Continent. Moreover, between each successive formation, we have, in the
+opinion of most geologists, enormously long blank periods. So that the
+lofty pile of sedimentary rocks in Britain, gives but an inadequate idea of
+the time which has elapsed during their accumulation; yet what time this
+must have consumed! Good observers have estimated that sediment is
+deposited by the great Mississippi river at the rate of only 600 feet in a
+hundred thousand years. This estimate has no pretension to strict
+exactness; yet, considering over what wide spaces very fine sediment is
+transported by the currents of the sea, the process of accumulation in any
+one area must be extremely slow.
+
+But the amount of denudation which the strata have in many places suffered,
+independently of the rate of accumulation of the degraded matter, probably
+offers the best evidence of the lapse of time. I remember {285} having been
+much struck with the evidence of denudation, when viewing volcanic islands,
+which have been worn by the waves and pared all round into perpendicular
+cliffs of one or two thousand feet in height; for the gentle slope of the
+lava-streams, due to their formerly liquid state, showed at a glance how
+far the hard, rocky beds had once extended into the open ocean. The same
+story is still more plainly told by faults,--those great cracks along which
+the strata have been upheaved on one side, or thrown down on the other, to
+the height or depth of thousands of feet; for since the crust cracked, the
+surface of the land has been so completely planed down by the action of the
+sea, that no trace of these vast dislocations is externally visible.
+
+The Craven fault, for instance, extends for upwards of 30 miles, and along
+this line the vertical displacement of the strata has varied from 600 to
+3000 feet. Prof. Ramsay has published an account of a downthrow in Anglesea
+of 2300 feet; and he informs me that he fully believes there is one in
+Merionethshire of 12,000 feet; yet in these cases there is nothing on the
+surface to show such prodigious movements; the pile of rocks on the one or
+other side having been smoothly swept away. The consideration of these
+facts impresses my mind almost in the same manner as does the vain
+endeavour to grapple with the idea of eternity.
+
+I am tempted to give one other case, the well-known one of the denudation
+of the Weald. Though it must be admitted that the denudation of the Weald
+has been a mere trifle, in comparison with that which has removed masses of
+our palæozoic strata, in parts ten thousand feet in thickness, as shown in
+Prof. Ramsay's masterly memoir on this subject: yet it is an admirable
+lesson to stand on the intermediate hilly country and look on the one hand
+at the North Downs, and {286} on the other hand at the South Downs; for,
+remembering that at no great distance to the west the northern and southern
+escarpments meet and close, one can safely picture to oneself the great
+dome of rocks which must have covered up the Weald within so limited a
+period as since the latter part of the Chalk formation. The distance from
+the northern to the southern Downs is about 22 miles, and the thickness of
+the several formations is on an average about 1100 feet, as I am informed
+by Prof. Ramsay. But if, as some geologists suppose, a range of older rocks
+underlies the Weald, on the flanks of which the overlying sedimentary
+deposits might have accumulated in thinner masses than elsewhere, the above
+estimate would be erroneous; but this source of doubt probably would not
+greatly affect the estimate as applied to the western extremity of the
+district. If, then, we knew the rate at which the sea commonly wears away a
+line of cliff of any given height, we could measure the time requisite to
+have denuded the Weald. This, of course cannot be done; but we may, in
+order to form some crude notion on the subject, assume that the sea would
+eat into cliffs 500 feet in height at the rate of one inch in a century.
+This will at first appear much too small an allowance; but it is the same
+as if we were to assume a cliff one yard in height to be eaten back along a
+whole line of coast at the rate of one yard in nearly every twenty-two
+years. I doubt whether any rock, even as soft as chalk, would yield at this
+rate excepting on the most exposed coasts; though no doubt the degradation
+of a lofty cliff would be more rapid from the breakage of the fallen
+fragments. On the other hand, I do not believe that any line of coast, ten
+or twenty miles in length, ever suffers degradation at the same time along
+its whole indented length; and we {287} must remember that almost all
+strata contain harder layers or nodules, which from long resisting
+attrition form a breakwater at the base. We may at least confidently
+believe that no rocky coast 500 feet in height commonly yields at the rate
+of a foot per century; for this would be the same in amount as a cliff one
+yard in height retreating twelve yards in twenty-two years; and no one, I
+think, who has carefully observed the shape of old fallen fragments at the
+base of cliffs, will admit any near approach to such rapid wearing away.
+Hence, under ordinary circumstances, I should infer that for a cliff 500
+feet in height, a denudation of one inch per century for the whole length
+would be a sufficient allowance. At this rate, on the above data, the
+denudation of the Weald must have required 306,662,400 years; or say three
+hundred million years. But perhaps it would be safer to allow two or three
+inches per century, and this would reduce the number of years to one
+hundred and fifty or one hundred million years.
+
+The action of fresh water on the gently inclined Wealden district, when
+upraised, could hardly have been great, but it would somewhat reduce the
+above estimate. On the other hand, during oscillations of level, which we
+know this area has undergone, the surface may have existed for millions of
+years as land, and thus have escaped the action of the sea: when deeply
+submerged for perhaps equally long periods, it would, likewise, have
+escaped the action of the coast-waves. So that it is not improbable that a
+longer period than 300 million years has elapsed since the latter part of
+the Secondary period.
+
+I have made these few remarks because it is highly important for us to gain
+some notion, however imperfect, of the lapse of years. During each of these
+years, {288} over the whole world, the land and the water has been peopled
+by hosts of living forms. What an infinite number of generations, which the
+mind cannot grasp, must have succeeded each other in the long roll of
+years! Now turn to our richest geological museums, and what a paltry
+display we behold!
+
+
+
+_On the poorness of our Palæontological collections._--That our
+palæontological collections are very imperfect, is admitted by every one.
+The remark of that admirable palæontologist, the late Edward Forbes, should
+not be forgotten, namely, that numbers of our fossil species are known and
+named from single and often broken specimens, or from a few specimens
+collected on some one spot. Only a small portion of the surface of the
+earth has been geologically explored, and no part with sufficient care, as
+the important discoveries made every year in Europe prove. No organism
+wholly soft can be preserved. Shells and bones will decay and disappear
+when left on the bottom of the sea, where sediment is not accumulating. I
+believe we are continually taking a most erroneous view, when we tacitly
+admit to ourselves that sediment is being deposited over nearly the whole
+bed of the sea, at a rate sufficiently quick to embed and preserve fossil
+remains. Throughout an enormously large proportion of the ocean, the bright
+blue tint of the water bespeaks its purity. The many cases on record of a
+formation conformably covered, after an enormous interval of time, by
+another and later formation, without the underlying bed having suffered in
+the interval any wear and tear, seem explicable only on the view of the
+bottom of the sea not rarely lying for ages in an unaltered condition. The
+remains which do become embedded, if in sand or gravel, will when the beds
+are upraised generally be dissolved {289} by the percolation of rain-water.
+I suspect that but few of the very many animals which live on the beach
+between high and low watermark are preserved. For instance, the several
+species of the Chthamalinæ (a subfamily of sessile cirripedes) coat the
+rocks all over the world in infinite numbers: they are all strictly
+littoral, with the exception of a single Mediterranean species, which
+inhabits deep water and has been found fossil in Sicily, whereas not one
+other species has hitherto been found in any tertiary formation: yet it is
+now known that the genus Chthamalus existed during the chalk period. The
+molluscan genus Chiton offers a partially analogous case.
+
+With respect to the terrestrial productions which lived during the
+Secondary and Palæozoic periods, it is superfluous to state that our
+evidence from fossil remains is fragmentary in an extreme degree. For
+instance, not a land shell is known belonging to either of these vast
+periods, with the exception of one species discovered by Sir C. Lyell and
+Dr. Dawson in the carboniferous strata of North America, of which shell
+several specimens have now been collected. In regard to mammiferous
+remains, a single glance at the historical table published in the
+Supplement to Lyell's Manual, will bring home the truth, how accidental and
+rare is their preservation, far better than pages of detail. Nor is their
+rarity surprising, when we remember how large a proportion of the bones of
+tertiary mammals have been discovered either in caves or in lacustrine
+deposits; and that not a cave or true lacustrine bed is known belonging to
+the age of our secondary or palæozoic formations.
+
+But the imperfection in the geological record mainly results from another
+and more important cause than any of the foregoing; namely, from the
+several formations {290} being separated from each other by wide intervals
+of time. When we see the formations tabulated in written works, or when we
+follow them in nature, it is difficult to avoid believing that they are
+closely consecutive. But we know, for instance, from Sir R. Murchison's
+great work on Russia, what wide gaps there are in that country between the
+superimposed formations; so it is in North America, and in many other parts
+of the world. The most skilful geologist, if his attention had been
+exclusively confined to these large territories, would never have suspected
+that during the periods which were blank and barren in his own country,
+great piles of sediment, charged with new and peculiar forms of life, had
+elsewhere been accumulated. And if in each separate territory, hardly any
+idea can be formed of the length of time which has elapsed between the
+consecutive formations, we may infer that this could nowhere be
+ascertained. The frequent and great changes in the mineralogical
+composition of consecutive formations, generally implying great changes in
+the geography of the surrounding lands, whence the sediment has been
+derived, accords with the belief of vast intervals of time having elapsed
+between each formation.
+
+But we can, I think, see why the geological formations of each region are
+almost invariably intermittent; that is, have not followed each other in
+close sequence. Scarcely any fact struck me more when examining many
+hundred miles of the South American coasts, which have been upraised
+several hundred feet within the recent period, than the absence of any
+recent deposits sufficiently extensive to last for even a short geological
+period. Along the whole west coast, which is inhabited by a peculiar marine
+fauna, tertiary beds are so poorly developed, that no record of several
+{291} successive and peculiar marine faunas will probably be preserved to a
+distant age. A little reflection will explain why along the rising coast of
+the western side of South America, no extensive formations with recent or
+tertiary remains can anywhere be found, though the supply of sediment must
+for ages have been great, from the enormous degradation of the coast-rocks
+and from muddy streams entering the sea. The explanation, no doubt, is,
+that the littoral and sub-littoral deposits are continually worn away, as
+soon as they are brought up by the slow and gradual rising of the land
+within the grinding action of the coast-waves.
+
+We may, I think, safely conclude that sediment must be accumulated in
+extremely thick, solid, or extensive masses, in order to withstand the
+incessant action of the waves, when first upraised and during subsequent
+oscillations of level. Such thick and extensive accumulations of sediment
+may be formed in two ways; either, in profound depths of the sea, in which
+case, judging from the researches of E. Forbes, we may conclude that the
+bottom will be inhabited by extremely few animals, and the mass when
+upraised will give a most imperfect record of the forms of life which then
+existed; or, sediment may be accumulated to any thickness and extent over a
+shallow bottom, if it continue slowly to subside. In this latter case, as
+long as the rate of subsidence and supply of sediment nearly balance each
+other, the sea will remain shallow and favourable for life, and thus a
+fossiliferous formation thick enough, when upraised, to resist any amount
+of degradation, may be formed.
+
+I am convinced that all our ancient formations, which are rich in fossils,
+have thus been formed during subsidence. Since publishing my views on this
+subject in 1845, I have watched the progress of {292} Geology, and have
+been surprised to note how author after author, in treating of this or that
+great formation, has come to the conclusion that it was accumulated during
+subsidence. I may add, that the only ancient tertiary formation on the west
+coast of South America, which has been bulky enough to resist such
+degradation as it has as yet suffered, but which will hardly last to a
+distant geological age, was certainly deposited during a downward
+oscillation of level, and thus gained considerable thickness.
+
+All geological facts tell us plainly that each area has undergone numerous
+slow oscillations of level, and apparently these oscillations have affected
+wide spaces. Consequently formations rich in fossils and sufficiently thick
+and extensive to resist subsequent degradation, may have been formed over
+wide spaces during periods of subsidence, but only where the supply of
+sediment was sufficient to keep the sea shallow and to embed and preserve
+the remains before they had time to decay. On the other hand, as long as
+the bed of the sea remained stationary, _thick_ deposits could not have
+been accumulated in the shallow parts, which are the most favourable to
+life. Still less could this have happened during the alternate periods of
+elevation; or, to speak more accurately, the beds which were then
+accumulated will have been destroyed by being upraised and brought within
+the limits of the coast-action.
+
+Thus the geological record will almost necessarily be rendered
+intermittent. I feel much confidence in the truth of these views, for they
+are in strict accordance with the general principles inculcated by Sir C.
+Lyell; and E. Forbes subsequently but independently arrived at a similar
+conclusion.
+
+One remark is here worth a passing notice. During periods of elevation the
+area of the land and of the {293} adjoining shoal parts of the sea will be
+increased, and new stations will often be formed;--all circumstances most
+favourable, as previously explained, for the formation of new varieties and
+species; but during such periods there will generally be a blank in the
+geological record. On the other hand, during subsidence, the inhabited area
+and number of inhabitants will decrease (excepting the productions on the
+shores of a continent when first broken up into an archipelago), and
+consequently during subsidence, though there will be much extinction, fewer
+new varieties or species will be formed; and it is during these very
+periods of subsidence, that our great deposits rich in fossils have been
+accumulated. Nature may almost be said to have guarded against the frequent
+discovery of her transitional or linking forms.
+
+From the foregoing considerations it cannot be doubted that the geological
+record, viewed as a whole, is extremely imperfect; but if we confine our
+attention to any one formation, it becomes more difficult to understand,
+why we do not therein find closely graduated varieties between the allied
+species which lived at its commencement and at its close. Some cases are on
+record of the same species presenting distinct varieties in the upper and
+lower parts of the same formation, but, as they are rare, they may be here
+passed over. Although each formation has indisputably required a vast
+number of years for its deposition, I can see several reasons why each
+should not include a graduated series of links between the species which
+then lived; but I can by no means pretend to assign due proportional weight
+to the following considerations.
+
+Although each formation may mark a very long lapse of years, each perhaps
+is short compared with the period requisite to change one species into
+another. I am {294} aware that two palæontologists, whose opinions are
+worthy of much deference, namely Bronn and Woodward, have concluded that
+the average duration of each formation is twice or thrice as long as the
+average duration of specific forms. But insuperable difficulties, as it
+seems to me, prevent us coming to any just conclusion on this head. When we
+see a species first appearing in the middle of any formation, it would be
+rash in the extreme to infer that it had not elsewhere previously existed.
+So again when we find a species disappearing before the uppermost layers
+have been deposited, it would be equally rash to suppose that it then
+became wholly extinct. We forget how small the area of Europe is compared
+with the rest of the world; nor have the several stages of the same
+formation throughout Europe been correlated with perfect accuracy.
+
+With marine animals of all kinds, we may safely infer a large amount of
+migration during climatal and other changes; and when we see a species
+first appearing in any formation, the probability is that it only then
+first immigrated into that area. It is well known, for instance, that
+several species appeared somewhat earlier in the palæozoic beds of North
+America than in those of Europe; time having apparently been required for
+their migration from the American to the European seas. In examining the
+latest deposits of various quarters of the world, it has everywhere been
+noted, that some few still existing species are common in the deposit, but
+have become extinct in the immediately surrounding sea; or, conversely,
+that some are now abundant in the neighbouring sea, but are rare or absent
+in this particular deposit. It is an excellent lesson to reflect on the
+ascertained amount of migration of the inhabitants of Europe during the
+Glacial period, which forms only a part of one whole geological period;
+{295} and likewise to reflect on the great changes of level, on the
+inordinately great change of climate, on the prodigious lapse of time, all
+included within this same glacial period. Yet it may be doubted whether in
+any quarter of the world, sedimentary deposits, _including fossil remains_,
+have gone on accumulating within the same area during the whole of this
+period. It is not, for instance, probable that sediment was deposited
+during the whole of the glacial period near the mouth of the Mississippi,
+within that limit of depth at which marine animals can flourish; for we
+know what vast geographical changes occurred in other parts of America
+during this space of time. When such beds as were deposited in shallow
+water near the mouth of the Mississippi during some part of the glacial
+period shall have been upraised, organic remains will probably first appear
+and disappear at different levels, owing to the migration of species and to
+geographical changes. And in the distant future, a geologist examining
+these beds, might be tempted to conclude that the average duration of life
+of the embedded fossils had been less than that of the glacial period,
+instead of having been really far greater, that is extending from before
+the glacial epoch to the present day.
+
+In order to get a perfect gradation between two forms in the upper and
+lower parts of the same formation, the deposit must have gone on
+accumulating for a very long period, in order to have given sufficient time
+for the slow process of variation; hence the deposit will generally have to
+be a very thick one; and the species undergoing modification will have had
+to live on the same area throughout this whole time. But we have seen that
+a thick fossiliferous formation can only be accumulated during a period of
+subsidence; and to keep the depth approximately the same, which is
+necessary in {296} order to enable the same species to live on the same
+space, the supply of sediment must nearly have counterbalanced the amount
+of subsidence. But this same movement of subsidence will often tend to sink
+the area whence the sediment is derived, and thus diminish the supply
+whilst the downward movement continues. In fact, this nearly exact
+balancing between the supply of sediment and the amount of subsidence is
+probably a rare contingency; for it has been observed by more than one
+palæontologist, that very thick deposits are usually barren of organic
+remains, except near their upper or lower limits.
+
+It would seem that each separate formation, like the whole pile of
+formations in any country, has generally been intermittent in its
+accumulation. When we see, as is so often the case, a formation composed of
+beds of different mineralogical composition, we may reasonably suspect that
+the process of deposition has been much interrupted, as a change in the
+currents of the sea and a supply of sediment of a different nature will
+generally have been due to geographical changes requiring much time. Nor
+will the closest inspection of a formation give any idea of the time which
+its deposition has consumed. Many instances could be given of beds only a
+few feet in thickness, representing formations, elsewhere thousands of feet
+in thickness, and which must have required an enormous period for their
+accumulation; yet no one ignorant of this fact would have suspected the
+vast lapse of time represented by the thinner formation. Many cases could
+be given of the lower beds of a formation having been upraised, denuded,
+submerged, and then re-covered by the upper beds of the same
+formation,--facts, showing what wide, yet easily overlooked, intervals have
+occurred in its accumulation. In other cases we have the plainest evidence
+{297} in great fossilised trees, still standing upright as they grew, of
+many long intervals of time and changes of level during the process of
+deposition, which would never even have been suspected, had not the trees
+chanced to have been preserved: thus Messrs. Lyell and Dawson found
+carboniferous beds 1400 feet thick in Nova Scotia, with ancient
+root-bearing strata, one above the other, at no less than sixty-eight
+different levels. Hence, when the same species occur at the bottom, middle,
+and top of a formation, the probability is that they have not lived on the
+same spot during the whole period of deposition, but have disappeared and
+reappeared, perhaps many times, during the same geological period. So that
+if such species were to undergo a considerable amount of modification
+during any one geological period, a section would not probably include all
+the fine intermediate gradations which must on my theory have existed
+between them, but abrupt, though perhaps very slight, changes of form.
+
+It is all-important to remember that naturalists have no golden rule by
+which to distinguish species and varieties; they grant some little
+variability to each species, but when they meet with a somewhat greater
+amount of difference between any two forms, they rank both as species,
+unless they are enabled to connect them together by close intermediate
+gradations. And this from the reasons just assigned we can seldom hope to
+effect in any one geological section. Supposing B and C to be two species,
+and a third, A, to be found in an underlying bed; even if A were strictly
+intermediate between B and C, it would simply be ranked as a third and
+distinct species, unless at the same time it could be most closely
+connected with either one or both forms by intermediate varieties. Nor
+should it be forgotten, as before explained, that A might be the actual
+progenitor {298} of B and C, and yet might not at all necessarily be
+strictly intermediate between them in all points of structure. So that we
+might obtain the parent-species and its several modified descendants from
+the lower and upper beds of a formation, and unless we obtained numerous
+transitional gradations, we should not recognise their relationship, and
+should consequently be compelled to rank them all as distinct species.
+
+It is notorious on what excessively slight differences many palæontologists
+have founded their species; and they do this the more readily if the
+specimens come from different sub-stages of the same formation. Some
+experienced conchologists are now sinking many of the very fine species of
+D'Orbigny and others into the rank of varieties; and on this view we do
+find the kind of evidence of change which on my theory we ought to find.
+Moreover, if we look to rather wider intervals, namely, to distinct but
+consecutive stages of the same great formation, we find that the embedded
+fossils, though almost universally ranked as specifically different, yet
+are far more closely allied to each other than are the species found in
+more widely separated formations; but to this subject I shall have to
+return in the following chapter.
+
+One other consideration is worth notice: with animals and plants that can
+propagate rapidly and are not highly locomotive, there is reason to
+suspect, as we have formerly seen, that their varieties are generally at
+first local; and that such local varieties do not spread widely and
+supplant their parent-forms until they have been modified and perfected in
+some considerable degree. According to this view, the chance of discovering
+in a formation in any one country all the early stages of transition
+between any two forms, is small, for the successive changes are supposed to
+have been local or {299} confined to some one spot. Most marine animals
+have a wide range; and we have seen that with plants it is those which have
+the widest range, that oftenest present varieties; so that with shells and
+other marine animals, it is probably those which have had the widest range,
+far exceeding the limits of the known geological formations of Europe,
+which have oftenest given rise, first to local varieties and ultimately to
+new species; and this again would greatly lessen the chance of our being
+able to trace the stages of transition in any one geological formation.
+
+It should not be forgotten, that at the present day, with perfect specimens
+for examination, two forms can seldom be connected by intermediate
+varieties and thus proved to be the same species, until many specimens have
+been collected from many places; and in the case of fossil species this
+could rarely be effected by palæontologists. We shall, perhaps, best
+perceive the improbability of our being enabled to connect species by
+numerous, fine, intermediate, fossil links, by asking ourselves whether,
+for instance, geologists at some future period will be able to prove, that
+our different breeds of cattle, sheep, horses, and dogs have descended from
+a single stock or from several aboriginal stocks; or, again, whether
+certain sea-shells inhabiting the shores of North America, which are ranked
+by some conchologists as distinct species from their European
+representatives, and by other conchologists as only varieties, are really
+varieties or are, as it is called, specifically distinct. This could be
+effected only by the future geologist discovering in a fossil state
+numerous intermediate gradations; and such success seems to me improbable
+in the highest degree.
+
+Geological research, though it has added numerous species to existing and
+extinct genera, and has made the {300} intervals between some few groups
+less wide than they otherwise would have been, yet has done scarcely
+anything in breaking down the distinction between species, by connecting
+them together by numerous, fine, intermediate varieties; and this not
+having been effected, is probably the gravest and most obvious of all the
+many objections which may be urged against my views. Hence it will be worth
+while to sum up the foregoing remarks, under an imaginary illustration. The
+Malay Archipelago is of about the size of Europe from the North Cape to the
+Mediterranean, and from Britain to Russia; and therefore equals all the
+geological formations which have been examined with any accuracy, excepting
+those of the United States of America. I fully agree with Mr.
+Godwin-Austen, that the present condition of the Malay Archipelago, with
+its numerous large islands separated by wide and shallow seas, probably
+represents the former state of Europe, whilst most of our formations were
+accumulating. The Malay Archipelago is one of the richest regions of the
+whole world in organic beings; yet if all the species were to be collected
+which have ever lived there, how imperfectly would they represent the
+natural history of the world!
+
+But we have every reason to believe that the terrestrial productions of the
+archipelago would be preserved in an excessively imperfect manner in the
+formations which we suppose to be there accumulating. I suspect that not
+many of the strictly littoral animals, or of those which lived on naked
+submarine rocks, would be embedded; and those embedded in gravel or sand,
+would not endure to a distant epoch. Wherever sediment did not accumulate
+on the bed of the sea, or where it did not accumulate at a sufficient rate
+to protect organic bodies from decay, no remains could be preserved.
+
+I believe that fossiliferous formations could be formed {301} in the
+archipelago, of thickness sufficient to last to an age as distant in
+futurity as the secondary formations lie in the past, only during periods
+of subsidence. These periods of subsidence would be separated from each
+other by enormous intervals, during which the area would be either
+stationary or rising; whilst rising, each fossiliferous formation would be
+destroyed, almost as soon as accumulated, by the incessant coast-action, as
+we now see on the shores of South America. During the periods of subsidence
+there would probably be much extinction of life; during the periods of
+elevation, there would be much variation, but the geological record would
+then be least perfect.
+
+It may be doubted whether the duration of any one great period of
+subsidence over the whole or part of the archipelago, together with a
+contemporaneous accumulation of sediment, would _exceed_ the average
+duration of the same specific forms; and these contingencies are
+indispensable for the preservation of all the transitional gradations
+between any two or more species. If such gradations were not fully
+preserved, transitional varieties would merely appear as so many distinct
+species. It is, also, probable that each great period of subsidence would
+be interrupted by oscillations of level, and that slight climatal changes
+would intervene during such lengthy periods; and in these cases the
+inhabitants of the archipelago would have to migrate, and no closely
+consecutive record of their modifications could be preserved in any one
+formation.
+
+Very many of the marine inhabitants of the archipelago now range thousands
+of miles beyond its confines; and analogy leads me to believe that it would
+be chiefly these far-ranging species which would oftenest produce new
+varieties; and the varieties would at first generally be local or confined
+to one place, but if possessed {302} of any decided advantage, or when
+further modified and improved, they would slowly spread and supplant their
+parent-forms. When such varieties returned to their ancient homes, as they
+would differ from their former state, in a nearly uniform, though perhaps
+extremely slight degree, they would, according to the principles followed
+by many palæontologists, be ranked as new and distinct species.
+
+If then, there be some degree of truth in these remarks, we have no right
+to expect to find in our geological formations, an infinite number of those
+fine transitional forms, which on my theory assuredly have connected all
+the past and present species of the same group into one long and branching
+chain of life. We ought only to look for a few links, some more closely,
+some more distantly related to each other; and these links, let them be
+ever so close, if found in different stages of the same formation, would,
+by most palæontologists, be ranked as distinct species. But I do not
+pretend that I should ever have suspected how poor a record of the
+mutations of life, the best preserved geological section presented, had not
+the difficulty of our not discovering innumerable transitional links
+between the species which appeared at the commencement and close of each
+formation, pressed so hardly on my theory.
+
+
+
+_On the sudden appearance of whole groups of Allied Species._--The abrupt
+manner in which whole groups of species suddenly appear in certain
+formations, has been urged by several palæontologists--for instance, by
+Agassiz, Pictet, and by none more forcibly than by Professor Sedgwick--as a
+fatal objection to the belief in the transmutation of species. If numerous
+species, belonging to the same genera or families, have really {303}
+started into life all at once, the fact would be fatal to the theory of
+descent with slow modification through natural selection. For the
+development of a group of forms, all of which have descended from some one
+progenitor, must have been an extremely slow process; and the progenitors
+must have lived long ages before their modified descendants. But we
+continually over-rate the perfection of the geological record, and falsely
+infer, because certain genera or families have not been found beneath a
+certain stage, that they did not exist before that stage. We continually
+forget how large the world is, compared with the area over which our
+geological formations have been carefully examined; we forget that groups
+of species may elsewhere have long existed and have slowly multiplied
+before they invaded the ancient archipelagoes of Europe and of the United
+States. We do not make due allowance for the enormous intervals of time,
+which have probably elapsed between our consecutive formations,--longer
+perhaps in most cases than the time required for the accumulation of each
+formation. These intervals will have given time for the multiplication of
+species from some one or some few parent-forms; and in the succeeding
+formation such species will appear as if suddenly created.
+
+I may here recall a remark formerly made, namely that it might require a
+long succession of ages to adapt an organism to some new and peculiar line
+of life, for instance to fly through the air; but that when this had been
+effected, and a few species had thus acquired a great advantage over other
+organisms, a comparatively short time would be necessary to produce many
+divergent forms, which would be able to spread rapidly and widely
+throughout the world.
+
+I will now give a few examples to illustrate these {304} remarks, and to
+show how liable we are to error in supposing that whole groups of species
+have suddenly been produced. I may recall the well-known fact that in
+geological treatises, published not many years ago, the great class of
+mammals was always spoken of as having abruptly come in at the commencement
+of the tertiary series. And now one of the richest known accumulations of
+fossil mammals, for its thickness, belongs to the middle of the secondary
+series; and one true mammal has been discovered in the new red sandstone at
+nearly the commencement of this great series. Cuvier used to urge that no
+monkey occurred in any tertiary stratum; but now extinct species have been
+discovered in India, South America, and in Europe even as far back as the
+eocene stage. Had it not been for the rare accident of the preservation of
+footsteps in the new red sandstone of the United States, who would have
+ventured to suppose that, besides reptiles, no less than at least thirty
+kinds of birds, some of gigantic size, existed during that period? Not a
+fragment of bone has been discovered in these beds. Notwithstanding that
+the number of joints shown in the fossil impressions correspond with the
+number in the several toes of living birds' feet, some authors doubt
+whether the animals which left the impressions were really birds. Until
+quite recently these authors might have maintained, and some have
+maintained, that the whole class of birds came suddenly into existence
+during an early tertiary period; but now we know, on the authority of
+Professor Owen (as may be seen in Lyell's 'Manual'), that a bird certainly
+lived during the deposition of the upper greensand.
+
+I may give another instance, which from having passed under my own eyes has
+much struck me. In a memoir on Fossil Sessile Cirripedes, I have stated
+that, from the {305} number of existing and extinct tertiary species; from
+the extraordinary abundance of the individuals of many species all over the
+world, from the Arctic regions to the equator, inhabiting various zones of
+depths from the upper tidal limits to 50 fathoms; from the perfect manner
+in which specimens are preserved in the oldest tertiary beds; from the ease
+with which even a fragment of a valve can be recognised; from all these
+circumstances, I inferred that had sessile cirripedes existed during the
+secondary periods, they would certainly have been preserved and discovered;
+and as not one species had then been discovered in beds of this age, I
+concluded that this great group had been suddenly developed at the
+commencement of the tertiary series. This was a sore trouble to me, adding
+as I thought one more instance of the abrupt appearance of a great group of
+species. But my work had hardly been published, when a skilful
+palæontologist, M. Bosquet, sent me a drawing of a perfect specimen of an
+unmistakeable sessile cirripede, which he had himself extracted from the
+chalk of Belgium. And, as if to make the case as striking as possible, this
+sessile cirripede was a Chthamalus, a very common, large, and ubiquitous
+genus, of which not one specimen has as yet been found even in any tertiary
+stratum. Hence we now positively know that sessile cirripedes existed
+during the secondary period; and these cirripedes might have been the
+progenitors of our many tertiary and existing species.
+
+The case most frequently insisted on by palæontologists of the apparently
+sudden appearance of a whole group of species, is that of the teleostean
+fishes, low down in the Chalk period. This group includes the large
+majority of existing species. Lately, Professor Pictet has carried their
+existence one sub-stage further back; and some palæontologists believe that
+certain {306} much older fishes, of which the affinities are as yet
+imperfectly known, are really teleostean. Assuming, however, that the whole
+of them did appear, as Agassiz believes, at the commencement of the chalk
+formation, the fact would certainly be highly remarkable; but I cannot see
+that it would be an insuperable difficulty on my theory, unless it could
+likewise be shown that the species of this group appeared suddenly and
+simultaneously throughout the world at this same period. It is almost
+superfluous to remark that hardly any fossil-fish are known from south of
+the equator; and by running through Pictet's Palæontology it will be seen
+that very few species are known from several formations in Europe. Some few
+families of fish now have a confined range; the teleostean fish might
+formerly have had a similarly confined range, and after having been largely
+developed in some one sea, might have spread widely. Nor have we any right
+to suppose that the seas of the world have always been so freely open from
+south to north as they are at present. Even at this day, if the Malay
+Archipelago were converted into land, the tropical parts of the Indian
+Ocean would form a large and perfectly enclosed basin, in which any great
+group of marine animals might be multiplied; and here they would remain
+confined, until some of the species became adapted to a cooler climate, and
+were enabled to double the southern capes of Africa or Australia, and thus
+reach other and distant seas.
+
+From these and similar considerations, but chiefly from our ignorance of
+the geology of other countries beyond the confines of Europe and the United
+States; and from the revolution in our palæontological ideas on many
+points, which the discoveries of even the last dozen years have effected,
+it seems to me to be about as rash in us to dogmatize on the succession of
+organic {307} beings throughout the world, as it would be for a naturalist
+to land for five minutes on some one barren point in Australia, and then to
+discuss the number and range of its productions.
+
+
+
+_On the sudden appearance of groups of Allied Species in the lowest known
+fossiliferous strata._--There is another and allied difficulty, which is
+much graver. I allude to the manner in which numbers of species of the same
+group, suddenly appear in the lowest known fossiliferous rocks. Most of the
+arguments which have convinced me that all the existing species of the same
+group have descended from one progenitor, apply with nearly equal force to
+the earliest known species. For instance, I cannot doubt that all the
+Silurian trilobites have descended from some one crustacean, which must
+have lived long before the Silurian age, and which probably differed
+greatly from any known animal. Some of the most ancient Silurian animals,
+as the Nautilus, Lingula, &c., do not differ much from living species; and
+it cannot on my theory be supposed, that these old species were the
+progenitors of all the species of the orders to which they belong, for they
+do not present characters in any degree intermediate between them. If,
+moreover, they had been the progenitors of these orders, they would almost
+certainly have been long ago supplanted and exterminated by their numerous
+and improved descendants.
+
+Consequently, if my theory be true, it is indisputable that before the
+lowest Silurian stratum was deposited, long periods elapsed, as long as, or
+probably far longer than, the whole interval from the Silurian age to the
+present day; and that during these vast, yet quite unknown, periods of
+time, the world swarmed with living creatures. {308}
+
+To the question why we do not find records of these vast primordial
+periods, I can give no satisfactory answer. Several of the most eminent
+geologists, with Sir E. Murchison at their head, are convinced that we see
+in the organic remains of the lowest Silurian stratum the dawn of life on
+this planet. Other highly competent judges, as Lyell and the late E.
+Forbes, dispute this conclusion. We should not forget that only a small
+portion of the world is known with accuracy. M. Barrande has lately added
+another and lower stage to the Silurian system, abounding with new and
+peculiar species. Traces of life have been detected in the Longmynd beds,
+beneath Barrande's so-called primordial zone. The presence of phosphatic
+nodules and bituminous matter in some of the lowest azoic rocks, probably
+indicates the former existence of life at these periods. But the difficulty
+of understanding the absence of vast piles of fossiliferous strata, which
+on my theory no doubt were somewhere accumulated before the Silurian epoch,
+is very great. If these most ancient beds had been wholly worn away by
+denudation, or obliterated by metamorphic action, we ought to find only
+small remnants of the formations next succeeding them in age, and these
+ought to be very generally in a metamorphosed condition. But the
+descriptions which we now possess of the Silurian deposits over immense
+territories in Russia and in North America, do not support the view, that
+the older a formation is, the more it has always suffered the extremity of
+denudation and metamorphism.
+
+The case at present must remain inexplicable; and may be truly urged as a
+valid argument against the views here entertained. To show that it may
+hereafter receive some explanation, I will give the following hypothesis.
+From the nature of the organic remains which {309} do not appear to have
+inhabited profound depths, in the several formations of Europe and of the
+United States; and from the amount of sediment, miles in thickness, of
+which the formations are composed, we may infer that from first to last
+large islands or tracts of land, whence the sediment was derived, occurred
+in the neighbourhood of the existing continents of Europe and North
+America. But we do not know what was the state of things in the intervals
+between the successive formations; whether Europe and the United States
+during these intervals existed as dry land, or as a submarine surface near
+land, on which sediment was not deposited, or as the bed of an open and
+unfathomable sea.
+
+Looking to the existing oceans, which are thrice as extensive as the land,
+we see them studded with many islands; but not one oceanic island is as yet
+known to afford even a remnant of any palæozoic or secondary formation.
+Hence we may perhaps infer, that during the palæozoic and secondary
+periods, neither continents nor continental islands existed where our
+oceans now extend; for had they existed there, palæozoic and secondary
+formations would in all probability have been accumulated from sediment
+derived from their wear and tear; and would have been at least partially
+upheaved by the oscillations of level, which we may fairly conclude must
+have intervened during these enormously long periods. If then we may infer
+anything from these facts, we may infer that where our oceans now extend,
+oceans have extended from the remotest period of which we have any record;
+and on the other hand, that where continents now exist, large tracts of
+land have existed, subjected no doubt to great oscillations of level, since
+the earliest silurian period. The coloured map appended to my volume on
+Coral Reefs, led me to conclude that the great oceans are still mainly
+areas of {310} subsidence, the great archipelagoes still areas of
+oscillations of level, and the continents areas of elevation. But have we
+any right to assume that things have thus remained from the beginning of
+this world? Our continents seem to have been formed by a preponderance,
+during many oscillations of level, of the force of elevation; but may not
+the areas of preponderant movement have changed in the lapse of ages? At a
+period immeasurably antecedent to the silurian epoch, continents may have
+existed where oceans are now spread out; and clear and open oceans may have
+existed where our continents now stand. Nor should we be justified in
+assuming that if, for instance, the bed of the Pacific Ocean were now
+converted into a continent, we should there find formations older than the
+silurian strata, supposing such to have been formerly deposited; for it
+might well happen that strata which had subsided some miles nearer to the
+centre of the earth, and which had been pressed on by an enormous weight of
+superincumbent water, might have undergone far more metamorphic action than
+strata which have always remained nearer to the surface. The immense areas
+in some parts of the world, for instance in South America, of bare
+metamorphic rocks, which must have been heated under great pressure, have
+always seemed to me to require some special explanation; and we may perhaps
+believe that we see in these large areas, the many formations long anterior
+to the silurian epoch in a completely metamorphosed condition.
+
+
+
+The several difficulties here discussed, namely our not finding in the
+successive formations infinitely numerous transitional links between the
+many species which now exist or have existed; the sudden manner {311} in
+which whole groups of species appear in our European formations; the almost
+entire absence, as at present known, of fossiliferous formations beneath
+the Silurian strata, are all undoubtedly of the gravest nature. We see this
+in the plainest manner by the fact that all the most eminent
+palæontologists, namely Cuvier, Agassiz, Barrande, Falconer, E. Forbes,
+&c., and all our greatest geologists, as Lyell, Murchison, Sedgwick, &c.,
+have unanimously, often vehemently, maintained the immutability of species.
+But I have reason to believe that one great authority, Sir Charles Lyell,
+from further reflexion entertains grave doubts on this subject. I feel how
+rash it is to differ from these authorities, to whom, with others, we owe
+all our knowledge. Those who think the natural geological record in any
+degree perfect, and who do not attach much weight to the facts and
+arguments of other kinds given in this volume, will undoubtedly at once
+reject my theory. For my part, following out Lyell's metaphor, I look at
+the natural geological record, as a history of the world imperfectly kept,
+and written in a changing dialect; of this history we possess the last
+volume alone, relating only to two or three countries. Of this volume, only
+here and there a short chapter has been preserved; and of each page, only
+here and there a few lines. Each word of the slowly-changing language, in
+which the history is supposed to be written, being more or less different
+in the interrupted succession of chapters, may represent the apparently
+abruptly changed forms of life, entombed in our consecutive, but widely
+separated, formations. On this view, the difficulties above discussed are
+greatly diminished, or even disappear.
+
+       *       *       *       *       *
+
+
+{312}
+
+CHAPTER X.
+
+ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
+
+    On the slow and successive appearance of new species--On their
+    different rates of change--Species once lost do not reappear--Groups of
+    species follow the same general rules in their appearance and
+    disappearance as do single species--On Extinction--On simultaneous
+    changes in the forms of life throughout the world--On the affinities of
+    extinct species to each other and to living species--On the state of
+    development of ancient forms--On the succession of the same types
+    within the same areas--Summary of preceding and present chapters.
+
+Let us now see whether the several facts and rules relating to the
+geological succession of organic beings, better accord with the common view
+of the immutability of species, or with that of their slow and gradual
+modification, through descent and natural selection.
+
+New species have appeared very slowly, one after another, both on the land
+and in the waters. Lyell has shown that it is hardly possible to resist the
+evidence on this head in the case of the several tertiary stages; and every
+year tends to fill up the blanks between them, and to make the percentage
+system of lost and new forms more gradual. In some of the most recent beds,
+though undoubtedly of high antiquity if measured by years, only one or two
+species are lost forms, and only one or two are new forms, having here
+appeared for the first time, either locally, or, as far as we know, on the
+face of the earth. If we may trust the observations of Philippi in Sicily,
+the successive changes in the marine inhabitants of that island have been
+many and most gradual. The secondary formations are more broken; but, as
+Bronn has remarked, neither the appearance {313} nor disappearance of their
+many now extinct species has been simultaneous in each separate formation.
+
+Species of different genera and classes have not changed at the same rate,
+or in the same degree. In the oldest tertiary beds a few living shells may
+still be found in the midst of a multitude of extinct forms. Falconer has
+given a striking instance of a similar fact, in an existing crocodile
+associated with many strange and lost mammals and reptiles in the
+sub-Himalayan deposits. The Silurian Lingula differs but little from the
+living species of this genus; whereas most of the other Silurian Molluscs
+and all the Crustaceans have changed greatly. The productions of the land
+seem to change at a quicker rate than those of the sea, of which a striking
+instance has lately been observed in Switzerland. There is some reason to
+believe that organisms, considered high in the scale of nature, change more
+quickly than those that are low: though there are exceptions to this rule.
+The amount of organic change, as Pictet has remarked, does not strictly
+correspond with the succession of our geological formations; so that
+between each two consecutive formations, the forms of life have seldom
+changed in exactly the same degree. Yet if we compare any but the most
+closely related formations, all the species will be found to have undergone
+some change. When a species has once disappeared from the face of the
+earth, we have reason to believe that the same identical form never
+reappears. The strongest apparent exception to this latter rule, is that of
+the so-called "colonies" of M. Barrande, which intrude for a period in the
+midst of an older formation, and then allow the pre-existing fauna to
+reappear; but Lyell's explanation, namely, that it is a case of temporary
+migration from a distinct geographical province, seems to me satisfactory.
+{314}
+
+These several facts accord well with my theory. I believe in no fixed law
+of development, causing all the inhabitants of a country to change
+abruptly, or simultaneously, or to an equal degree. The process of
+modification must be extremely slow. The variability of each species is
+quite independent of that of all others. Whether such variability be taken
+advantage of by natural selection, and whether the variations be
+accumulated to a greater or lesser amount, thus causing a greater or lesser
+amount of modification in the varying species, depends on many complex
+contingencies,--on the variability being of a beneficial nature, on the
+power of intercrossing, on the rate of breeding, on the slowly changing
+physical conditions of the country, and more especially on the nature of
+the other inhabitants with which the varying species comes into
+competition. Hence it is by no means surprising that one species should
+retain the same identical form much longer than others; or, if changing,
+that it should change less. We see the same fact in geographical
+distribution; for instance, in the land-shells and coleopterous insects of
+Madeira having come to differ considerably from their nearest allies on the
+continent of Europe, whereas the marine shells and birds have remained
+unaltered. We can perhaps understand the apparently quicker rate of change
+in terrestrial and in more highly organised productions compared with
+marine and lower productions, by the more complex relations of the higher
+beings to their organic and inorganic conditions of life, as explained in a
+former chapter. When many of the inhabitants of a country have become
+modified and improved, we can understand, on the principle of competition,
+and on that of the many all-important relations of organism to organism,
+that any form which does not become in some degree modified and improved,
+{315} will be liable to be exterminated. Hence we can see why all the
+species in the same region do at last, if we look to wide enough intervals
+of time, become modified; for those which do not change will become
+extinct.
+
+In members of the same class the average amount of change, during long and
+equal periods of time, may, perhaps, be nearly the same; but as the
+accumulation of long-enduring fossiliferous formations depends on great
+masses of sediment having been deposited on areas whilst subsiding, our
+formations have been almost necessarily accumulated at wide and irregularly
+intermittent intervals; consequently the amount of organic change exhibited
+by the fossils embedded in consecutive formations is not equal. Each
+formation, on this view, does not mark a new and complete act of creation,
+but only an occasional scene, taken almost at hazard, in a slowly changing
+drama.
+
+We can clearly understand why a species when once lost should never
+reappear, even if the very same conditions of life, organic and inorganic,
+should recur. For though the offspring of one species might be adapted (and
+no doubt this has occurred in innumerable instances) to fill the exact
+place of another species in the economy of nature, and thus supplant it;
+yet the two forms--the old and the new--would not be identically the same;
+for both would almost certainly inherit different characters from their
+distinct progenitors. For instance, it is just possible, if our
+fantail-pigeons were all destroyed, that fanciers, by striving during long
+ages for the same object, might make a new breed hardly distinguishable
+from our present fantail; but if the parent rock-pigeon were also
+destroyed, and in nature we have every reason to believe that the
+parent-form will generally be supplanted and exterminated by its improved
+offspring, it is quite {316} incredible that a fantail, identical with the
+existing breed, could be raised from any other species of pigeon, or even
+from the other well-established races of the domestic pigeon, for the
+newly-formed fantail would be almost sure to inherit from its new
+progenitor some slight characteristic differences.
+
+Groups of species, that is, genera and families, follow the same general
+rules in their appearance and disappearance as do single species, changing
+more or less quickly, and in a greater or lesser degree. A group does not
+reappear after it has once disappeared; or its existence, as long as it
+lasts, is continuous. I am aware that there are some apparent exceptions to
+this rule, but the exceptions are surprisingly few, so few that E. Forbes,
+Pictet, and Woodward (though all strongly opposed to such views as I
+maintain) admit its truth; and the rule strictly accords with my theory.
+For as all the species of the same group have descended from some one
+species, it is clear that as long as any species of the group have appeared
+in the long succession of ages, so long must its members have continuously
+existed, in order to have generated either new and modified or the same old
+and unmodified forms. Species of the genus Lingula, for instance, must have
+continuously existed by an unbroken succession of generations, from the
+lowest Silurian stratum to the present day.
+
+We have seen in the last chapter that the species of a group sometimes
+falsely appear to have come in abruptly; and I have attempted to give an
+explanation of this fact, which if true would have been fatal to my views.
+But such cases are certainly exceptional; the general rule being a gradual
+increase in number, till the group reaches its maximum, and then, sooner or
+later, it gradually decreases. If the number of the species of a genus, or
+the number of {317} the genera of a family, be represented by a vertical
+line of varying thickness, crossing the successive geological formations in
+which the species are found, the line will sometimes falsely appear to
+begin at its lower end, not in a sharp point, but abruptly; it then
+gradually thickens upwards, sometimes keeping for a space of equal
+thickness, and ultimately thins out in the upper beds, marking the decrease
+and final extinction of the species. This gradual increase in number of the
+species of a group is strictly conformable with my theory; as the species
+of the same genus, and the genera of the same family, can increase only
+slowly and progressively; for the process of modification and the
+production of a number of allied forms must be slow and gradual,--one
+species giving rise first to two or three varieties, these being slowly
+converted into species, which in their turn produce by equally slow steps
+other species, and so on, like the branching of a great tree from a single
+stem, till the group becomes large.
+
+
+
+_On Extinction._--We have as yet spoken only incidentally of the
+disappearance of species and of groups of species. On the theory of natural
+selection the extinction of old forms and the production of new and
+improved forms are intimately connected together. The old notion of all the
+inhabitants of the earth having been swept away at successive periods by
+catastrophes, is very generally given up, even by those geologists, as Elie
+de Beaumont, Murchison, Barrande, &c., whose general views would naturally
+lead them to this conclusion. On the contrary, we have every reason to
+believe, from the study of the tertiary formations, that species and groups
+of species gradually disappear, one after another, first from one spot,
+then from another, and finally from the world. Both single species and
+whole {318} groups of species last for very unequal periods; some groups,
+as we have seen, having endured from the earliest known dawn of life to the
+present day; some having disappeared before the close of the palæozoic
+period. No fixed law seems to determine the length of time during which any
+single species or any single genus endures. There is reason to believe that
+the complete extinction of the species of a group is generally a slower
+process than their production: if the appearance and disappearance of a
+group of species be represented, as before, by a vertical line of varying
+thickness, the line is found to taper more gradually at its upper end,
+which marks the progress of extermination, than at its lower end, which
+marks the first appearance and increase in numbers of the species. In some
+cases, however, the extermination of whole groups of beings, as of
+ammonites towards the close of the secondary period, has been wonderfully
+sudden.
+
+The whole subject of the extinction of species has been involved in the
+most gratuitous mystery. Some authors have even supposed that as the
+individual has a definite length of life, so have species a definite
+duration. No one I think can have marvelled more at the extinction of
+species, than I have done. When I found in La Plata the tooth of a horse
+embedded with the remains of Mastodon, Megatherium, Toxodon, and other
+extinct monsters, which all co-existed with still living shells at a very
+late geological period, I was filled with astonishment; for seeing that the
+horse, since its introduction by the Spaniards into South America, has run
+wild over the whole country and has increased in numbers at an unparalleled
+rate, I asked myself what could so recently have exterminated the former
+horse under conditions of life apparently so favourable. But how utterly
+groundless was my astonishment! {319} Professor Owen soon perceived that
+the tooth, though so like that of the existing horse, belonged to an
+extinct species. Had this horse been still living, but in some degree rare,
+no naturalist would have felt the least surprise at its rarity; for rarity
+is the attribute of a vast number of species of all classes, in all
+countries. If we ask ourselves why this or that species is rare, we answer
+that something is unfavourable in its conditions of life; but what that
+something is, we can hardly ever tell. On the supposition of the fossil
+horse still existing as a rare species, we might have felt certain from the
+analogy of all other mammals, even of the slow-breeding elephant, and from
+the history of the naturalisation of the domestic horse in South America,
+that under more favourable conditions it would in a very few years have
+stocked the whole continent. But we could not have told what the
+unfavourable conditions were which checked its increase, whether some one
+or several contingencies, and at what period of the horse's life, and in
+what degree, they severally acted. If the conditions had gone on, however
+slowly, becoming less and less favourable, we assuredly should not have
+perceived the fact, yet the fossil horse would certainly have become rarer
+and rarer, and finally extinct;--its place being seized on by some more
+successful competitor.
+
+It is most difficult always to remember that the increase of every living
+being is constantly being checked by unperceived injurious agencies; and
+that these same unperceived agencies are amply sufficient to cause rarity,
+and finally extinction. We see in many cases in the more recent tertiary
+formations, that rarity precedes extinction; and we know that this has been
+the progress of events with those animals which have been exterminated,
+either locally or wholly, through {320} man's agency. I may repeat what I
+published in 1845, namely, that to admit that species generally become rare
+before they become extinct--to feel no surprise at the rarity of a species,
+and yet to marvel greatly when it ceases to exist, is much the same as to
+admit that sickness in the individual is the forerunner of death--to feel
+no surprise at sickness, but when the sick man dies, to wonder and to
+suspect that he died by some unknown deed of violence.
+
+The theory of natural selection is grounded on the belief that each new
+variety, and ultimately each new species, is produced and maintained by
+having some advantage over those with which it comes into competition; and
+the consequent extinction of less-favoured forms almost inevitably follows.
+It is the same with our domestic productions: when a new and slightly
+improved variety has been raised, it at first supplants the less improved
+varieties in the same neighbourhood; when much improved it is transported
+far and near, like our short-horn cattle, and takes the place of other
+breeds in other countries. Thus the appearance of new forms and the
+disappearance of old forms, both natural and artificial, are bound
+together. In certain flourishing groups, the number of new specific forms
+which have been produced within a given time is probably greater than that
+of the old specific forms which have been exterminated; but we know that
+the number of species has not gone on indefinitely increasing, at least
+during the later geological periods, so that looking to later times we may
+believe that the production of new forms has caused the extinction of about
+the same number of old forms.
+
+The competition will generally be most severe, as formerly explained and
+illustrated by examples, between the forms which are most like each other
+in all respects. {321} Hence the improved and modified descendants of a
+species will generally cause the extermination of the parent-species; and
+if many new forms have been developed from any one species, the nearest
+allies of that species, _i.e._ the species of the same genus, will be the
+most liable to extermination. Thus, as I believe, a number of new species
+descended from one species, that is a new genus, comes to supplant an old
+genus, belonging to the same family. But it must often have happened that a
+new species belonging to some one group will have seized on the place
+occupied by a species belonging to a distinct group, and thus caused its
+extermination; and if many allied forms be developed from the successful
+intruder, many will have to yield their places; and it will generally be
+allied forms, which will suffer from some inherited inferiority in common.
+But whether it be species belonging to the same or to a distinct class,
+which yield their places to other species which have been modified and
+improved, a few of the sufferers may often long be preserved, from being
+fitted to some peculiar line of life, or from inhabiting some distant and
+isolated station, where they have escaped severe competition. For instance,
+a single species of Trigonia, a great genus of shells in the secondary
+formations, survives in the Australian seas; and a few members of the great
+and almost extinct group of Ganoid fishes still inhabit our fresh waters.
+Therefore the utter extinction of a group is generally, as we have seen, a
+slower process than its production.
+
+With respect to the apparently sudden extermination of whole families or
+orders, as of Trilobites at the close of the palæozoic period and of
+Ammonites at the close of the secondary period, we must remember what has
+been already said on the probable wide intervals of time {322} between our
+consecutive formations; and in these intervals there may have been much
+slow extermination. Moreover, when by sudden immigration or by unusually
+rapid development, many species of a new group have taken possession of a
+new area, they will have exterminated in a correspondingly rapid manner
+many of the old inhabitants; and the forms which thus yield their places
+will commonly be allied, for they will partake of some inferiority in
+common.
+
+Thus, as it seems to me, the manner in which single species and whole
+groups of species become extinct, accords well with the theory of natural
+selection. We need not marvel at extinction; if we must marvel, let it be
+at our presumption in imagining for a moment that we understand the many
+complex contingencies, on which the existence of each species depends. If
+we forget for an instant, that each species tends to increase inordinately,
+and that some check is always in action, yet seldom perceived by us, the
+whole economy of nature will be utterly obscured. Whenever we can precisely
+say why this species is more abundant in individuals than that; why this
+species and not another can be naturalised in a given country; then, and
+not till then, we may justly feel surprise why we cannot account for the
+extinction of this particular species or group of species.
+
+
+
+_On the Forms of Life changing almost simultaneously throughout the
+World._--Scarcely any palæontological discovery is more striking than the
+fact, that the forms of life change almost simultaneously throughout the
+world. Thus our European Chalk formation can be recognised in many distant
+parts of the world, under the most different climates, where not a fragment
+of the mineral chalk itself can be found; namely, in North {323} America,
+in equatorial South America, in Tierra del Fuego, at the Cape of Good Hope,
+and in the peninsula of India. For at these distant points, the organic
+remains in certain beds present an unmistakeable degree of resemblance to
+those of the Chalk. It is not that the same species are met with; for in
+some cases not one species is identically the same, but they belong to the
+same families, genera, and sections of genera, and sometimes are similarly
+characterised in such trifling points as mere superficial sculpture.
+Moreover other forms, which are not found in the Chalk of Europe, but which
+occur in the formations either above or below, are similarly absent at
+these distant points of the world. In the several successive palæozoic
+formations of Russia, Western Europe and North America, a similar
+parallelism in the forms of life has been observed by several authors: so
+it is, according to Lyell, with the several European and North American
+tertiary deposits. Even if the few fossil species which are common to the
+Old and New Worlds be kept wholly out of view, the general parallelism in
+the successive forms of life, in the stages of the widely separated
+palæozoic and tertiary periods, would still be manifest, and the several
+formations could be easily correlated.
+
+These observations, however, relate to the marine inhabitants of distant
+parts of the world: we have not sufficient data to judge whether the
+productions of the land and of fresh water change at distant points in the
+same parallel manner. We may doubt whether they have thus changed: if the
+Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe
+from La Plata, without any information in regard to their geological
+position, no one would have suspected that they had co-existed with still
+living sea-shells; but as these anomalous monsters co-existed with the
+{324} Mastodon and Horse, it might at least have been inferred that they
+had lived during one of the later tertiary stages.
+
+When the marine forms of life are spoken of as having changed
+simultaneously throughout the world, it must not be supposed that this
+expression relates to the same thousandth or hundred-thousandth year, or
+even that it has a very strict geological sense; for if all the marine
+animals which live at the present day in Europe, and all those that lived
+in Europe during the pleistocene period (an enormously remote period as
+measured by years, including the whole glacial epoch), were to be compared
+with those now living in South America or in Australia, the most skilful
+naturalist would hardly be able to say whether the existing or the
+pleistocene inhabitants of Europe resembled most closely those of the
+southern hemisphere. So, again, several highly competent observers believe
+that the existing productions of the United States are more closely related
+to those which lived in Europe during certain later tertiary stages, than
+to those which now live here; and if this be so, it is evident that
+fossiliferous beds deposited at the present day on the shores of North
+America would hereafter be liable to be classed with somewhat older
+European beds. Nevertheless, looking to a remotely future epoch, there can,
+I think, be little doubt that all the more modern _marine_ formations,
+namely, the upper pliocene, the pleistocene and strictly modern beds, of
+Europe, North and South America, and Australia, from containing fossil
+remains in some degree allied, and from not including those forms which are
+only found in the older underlying deposits, would be correctly ranked as
+simultaneous in a geological sense.
+
+The fact of the forms of life changing simultaneously, in the above large
+sense, at distant parts of the world, has greatly struck those admirable
+observers, MM. {325} de Verneuil and d'Archiac. After referring to the
+parallelism of the palæozoic forms of life in various parts of Europe, they
+add, "If struck by this strange sequence, we turn our attention to North
+America, and there discover a series of analogous phenomena, it will appear
+certain that all these modifications of species, their extinction, and the
+introduction of new ones, cannot be owing to mere changes in marine
+currents or other causes more or less local and temporary, but depend on
+general laws which govern the whole animal kingdom." M. Barrande has made
+forcible remarks to precisely the same effect. It is, indeed, quite futile
+to look to changes of currents, climate, or other physical conditions, as
+the cause of these great mutations in the forms of life throughout the
+world, under the most different climates. We must, as Barrande has
+remarked, look to some special law. We shall see this more clearly when we
+treat of the present distribution of organic beings, and find how slight is
+the relation between the physical conditions of various countries, and the
+nature of their inhabitants.
+
+This great fact of the parallel succession of the forms of life throughout
+the world, is explicable on the theory of natural selection. New species
+are formed by new varieties arising, which have some advantage over older
+forms; and those forms, which are already dominant, or have some advantage
+over the other forms in their own country, would naturally oftenest give
+rise to new varieties or incipient species; for these latter must be
+victorious in a still higher degree in order to be preserved and to
+survive. We have distinct evidence on this head, in the plants which are
+dominant, that is, which are commonest in their own homes, and are most
+widely diffused, having produced the greatest number of new varieties. It
+is also natural that the {326} dominant, varying, and far-spreading
+species, which already have invaded to a certain extent the territories of
+other species, should be those which would have the best chance of
+spreading still further, and of giving rise in new countries to new
+varieties and species. The process of diffusion may often be very slow,
+being dependent on climatal and geographical changes, or on strange
+accidents, but in the long run the dominant forms will generally succeed in
+spreading. The diffusion would, it is probable, be slower with the
+terrestrial inhabitants of distinct continents than with the marine
+inhabitants of the continuous sea. We might therefore expect to find, as we
+apparently do find, a less strict degree of parallel succession in the
+productions of the land than of the sea.
+
+Dominant species spreading from any region might encounter still more
+dominant species, and then their triumphant course, or even their
+existence, would cease. We know not at all precisely what are all the
+conditions most favourable for the multiplication of new and dominant
+species; but we can, I think, clearly see that a number of individuals,
+from giving a better chance of the appearance of favourable variations, and
+that severe competition with many already existing forms, would be highly
+favourable, as would be the power of spreading into new territories. A
+certain amount of isolation, recurring at long intervals of time, would
+probably be also favourable, as before explained. One quarter of the world
+may have been most favourable for the production of new and dominant
+species on the land, and another for those in the waters of the sea. If two
+great regions had been for a long period favourably circumstanced in an
+equal degree, whenever their inhabitants met, the battle would be prolonged
+and severe; and some from one birthplace and some from the other might be
+victorious. But in the course of time, the {327} forms dominant in the
+highest degree, wherever produced, would tend everywhere to prevail. As
+they prevailed, they would cause the extinction of other and inferior
+forms; and as these inferior forms would be allied in groups by
+inheritance, whole groups would tend slowly to disappear; though here and
+there a single member might long be enabled to survive.
+
+Thus, as it seems to me, the parallel, and, taken in a large sense,
+simultaneous, succession of the same forms of life throughout the world,
+accords well with the principle of new species having been formed by
+dominant species spreading widely and varying; the new species thus
+produced being themselves dominant owing to inheritance, and to having
+already had some advantage over their parents or over other species; these
+again spreading, varying, and producing new species. The forms which are
+beaten and which yield their places to the new and victorious forms, will
+generally be allied in groups, from inheriting some inferiority in common;
+and therefore as new and improved groups spread throughout the world, old
+groups will disappear from the world; and the succession of forms in both
+ways will everywhere tend to correspond.
+
+There is one other remark connected with this subject worth making. I have
+given my reasons for believing that all our greater fossiliferous
+formations were deposited during periods of subsidence; and that blank
+intervals of vast duration occurred during the periods when the bed of the
+sea was either stationary or rising, and likewise when sediment was not
+thrown down quickly enough to embed and preserve organic remains. During
+these long and blank intervals I suppose that the inhabitants of each
+region underwent a considerable amount of modification and extinction, and
+that there was much migration from {328} other parts of the world. As we
+have reason to believe that large areas are affected by the same movement,
+it is probable that strictly contemporaneous formations have often been
+accumulated over very wide spaces in the same quarter of the world; but we
+are far from having any right to conclude that this has invariably been the
+case, and that large areas have invariably been affected by the same
+movements. When two formations have been deposited in two regions during
+nearly, but not exactly the same period, we should find in both, from the
+causes explained in the foregoing paragraphs, the same general succession
+in the forms of life; but the species would not exactly correspond; for
+there will have been a little more time in the one region than in the other
+for modification, extinction, and immigration.
+
+I suspect that cases of this nature occur in Europe. Mr. Prestwich, in his
+admirable Memoirs on the eocene deposits of England and France, is able to
+draw a close general parallelism between the successive stages in the two
+countries; but when he compares certain stages in England with those in
+France, although he finds in both a curious accordance in the numbers of
+the species belonging to the same genera, yet the species themselves differ
+in a manner very difficult to account for, considering the proximity of the
+two areas,--unless, indeed, it be assumed that an isthmus separated two
+seas inhabited by distinct, but contemporaneous, faunas. Lyell has made
+similar observations on some of the later tertiary formations. Barrande,
+also, shows that there is a striking general parallelism in the successive
+Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a
+surprising amount of difference in the species. If the several formations
+in these regions have not been deposited during the same exact {329}
+periods,--a formation in one region often corresponding with a blank
+interval in the other,--and if in both regions the species have gone on
+slowly changing during the accumulation of the several formations and
+during the long intervals of time between them; in this case, the several
+formations in the two regions could be arranged in the same order, in
+accordance with the general succession of the form of life, and the order
+would falsely appear to be strictly parallel; nevertheless the species
+would not all be the same in the apparently corresponding stages in the two
+regions.
+
+
+
+_On the Affinities of extinct Species to each other, and to living
+forms._--Let us now look to the mutual affinities of extinct and living
+species. They all fall into one grand natural system; and this fact is at
+once explained on the principle of descent. The more ancient any form is,
+the more, as a general rule, it differs from living forms. But, as Buckland
+long ago remarked, all fossils can be classed either in still existing
+groups, or between them. That the extinct forms of life help to fill up the
+wide intervals between existing genera, families, and orders, cannot be
+disputed. For if we confine our attention either to the living or to the
+extinct alone, the series is far less perfect than if we combine both into
+one general system. With respect to the Vertebrata, whole pages could be
+filled with striking illustrations from our great palaeontologist, Owen,
+showing how extinct animals fall in between existing groups. Cuvier ranked
+the Ruminants and Pachyderms, as the two most distinct orders of mammals;
+but Owen has discovered so many fossil links, that he has had to alter the
+whole classification of these two orders; and has placed certain pachyderms
+in the same sub-order with ruminants: for example, he dissolves by fine
+gradations the apparently {330} wide difference between the pig and the
+camel. In regard to the Invertebrata, Barrande, and a higher authority
+could not be named, asserts that he is every day taught that Palaeozoic
+animals, though belonging to the same orders, families, or genera with
+those living at the present day, were not at this early epoch limited in
+such distinct groups as they now are.
+
+Some writers have objected to any extinct species or group of species being
+considered as intermediate between living species or groups. If by this
+term it is meant that an extinct form is directly intermediate in all its
+characters between two living forms, the objection is probably valid. But I
+apprehend that in a perfectly natural classification many fossil species
+would have to stand between living species, and some extinct genera between
+living genera, even between genera belonging to distinct families. The most
+common case, especially with respect to very distinct groups, such as fish
+and reptiles, seems to be, that supposing them to be distinguished at the
+present day from each other by a dozen characters, the ancient members of
+the same two groups would be distinguished by a somewhat lesser number of
+characters, so that the two groups, though formerly quite distinct, at that
+period made some small approach to each other.
+
+It is a common belief that the more ancient a form is, by so much the more
+it tends to connect by some of its characters groups now widely separated
+from each other. This remark no doubt must be restricted to those groups
+which have undergone much change in the course of geological ages; and it
+would be difficult to prove the truth of the proposition, for every now and
+then even a living animal, as the Lepidosiren, is discovered having
+affinities directed towards very distinct groups. Yet if we compare the
+older Reptiles and {331} Batrachians, the older Fish, the older
+Cephalopods, and the eocene Mammals, with the more recent members of the
+same classes, we must admit that there is some truth in the remark.
+
+Let us see how far these several facts and inferences accord with the
+theory of descent with modification. As the subject is somewhat complex, I
+must request the reader to turn to the diagram in the fourth chapter. We
+may suppose that the numbered letters represent genera, and the dotted
+lines diverging from them the species in each genus. The diagram is much
+too simple, too few genera and too few species being given, but this is
+unimportant for us. The horizontal lines may represent successive
+geological formations, and all the forms beneath the uppermost line may be
+considered as extinct. The three existing genera, a^{14}, q^{14}, p^{14},
+will form a small family; b^{14} and f^{14} a closely allied family or
+sub-family; and o^{14}, e^{14}, m^{14}, a third family. These three
+families, together with the many extinct genera on the several lines of
+descent diverging from the parent-form (A), will form an order; for all
+will have inherited something in common from their ancient and common
+progenitor. On the principle of the continued tendency to divergence of
+character, which was formerly illustrated by this diagram, the more recent
+any form is, the more it will generally differ from its ancient progenitor.
+Hence we can understand the rule that the most ancient fossils differ most
+from existing forms. We must not, however, assume that divergence of
+character is a necessary contingency; it depends solely on the descendants
+from a species being thus enabled to seize on many and different places in
+the economy of nature. Therefore it is quite possible, as we have seen in
+the case of some Silurian forms, that a species might go on being slightly
+modified in relation to its slightly altered conditions of {332} life, and
+yet retain throughout a vast period the same general characteristics. This
+is represented in the diagram by the letter F^{14}.
+
+All the many forms, extinct and recent, descended from (A), make, as before
+remarked, one order; and this order, from the continued effects of
+extinction and divergence of character, has become divided into several
+sub-families and families, some of which are supposed to have perished at
+different periods, and some to have endured to the present day.
+
+By looking at the diagram we can see that if many of the extinct forms,
+supposed to be embedded in the successive formations, were discovered at
+several points low down in the series, the three existing families on the
+uppermost line would be rendered less distinct from each other. If, for
+instance, the genera a^1, a^5, a^{10}, f^8, m^3, m^6, m^9, were
+disinterred, these three families would be so closely linked together that
+they probably would have to be united into one great family, in nearly the
+same manner as has occurred with ruminants and pachyderms. Yet he who
+objected to call the extinct genera, which thus linked the living genera of
+three families together, intermediate in character, would be justified, as
+they are intermediate, not directly, but only by a long and circuitous
+course through many widely different forms. If many extinct forms were to
+be discovered above one of the middle horizontal lines or geological
+formations --for instance, above No. VI.--but none from beneath this line,
+then only the two families on the left hand (namely, a^{14}, &c., and
+b^{14}, &c.) would have to be united into one family; and the two other
+families (namely, a^{14} to f^{14} now including five genera, and o^{14} to
+m^{14}) would yet remain distinct. These two families, however, would be
+less distinct from each other than they were before the discovery of the
+fossils. If, for instance, we suppose the existing genera of the two
+families to differ from each {333} other by a dozen characters, in this
+case the genera, at the early period marked VI., would differ by a lesser
+number of characters; for at this early stage of descent they have not
+diverged in character from the common progenitor of the order, nearly so
+much as they subsequently diverged. Thus it comes that ancient and extinct
+genera are often in some slight degree intermediate in character between
+their modified descendants, or between their collateral relations.
+
+In nature the case will be far more complicated than is represented in the
+diagram; for the groups will have been more numerous, they will have
+endured for extremely unequal lengths of time, and will have been modified
+in various degrees. As we possess only the last volume of the geological
+record, and that in a very broken condition, we have no right to expect,
+except in very rare cases, to fill up wide intervals in the natural system,
+and thus unite distinct families or orders. All that we have a right to
+expect, is that those groups, which have within known geological periods
+undergone much modification, should in the older formations make some
+slight approach to each other; so that the older members should differ less
+from each other in some of their characters than do the existing members of
+the same groups; and this by the concurrent evidence of our best
+palæontologists seems frequently to be the case.
+
+Thus, on the theory of descent with modification, the main facts with
+respect to the mutual affinities of the extinct forms of life to each other
+and to living forms, seem to me explained in a satisfactory manner. And
+they are wholly inexplicable on any other view.
+
+On this same theory, it is evident that the fauna of any great period in
+the earth's history will be intermediate in general character between that
+which preceded and that which succeeded it. Thus, the species which lived
+at the sixth great stage of descent in the {334} diagram are the modified
+offspring of those which lived at the fifth stage, and are the parents of
+those which became still more modified at the seventh stage; hence they
+could hardly fail to be nearly intermediate in character between the forms
+of life above and below. We must, however, allow for the entire extinction
+of some preceding forms, and in any one region for the immigration of new
+forms from other regions, and for a large amount of modification, during
+the long and blank intervals between the successive formations. Subject to
+these allowances, the fauna of each geological period undoubtedly is
+intermediate in character, between the preceding and succeeding faunas. I
+need give only one instance, namely, the manner in which the fossils of the
+Devonian system, when this system was first discovered, were at once
+recognised by palæontologists as intermediate in character between those of
+the overlying carboniferous, and underlying Silurian system. But each fauna
+is not necessarily exactly intermediate, as unequal intervals of time have
+elapsed between consecutive formations.
+
+It is no real objection to the truth of the statement, that the fauna of
+each period as a whole is nearly intermediate in character between the
+preceding and succeeding faunas, that certain genera offer exceptions to
+the rule. For instance, mastodons and elephants, when arranged by Dr.
+Falconer in two series, first according to their mutual affinities and then
+according to their periods of existence, do not accord in arrangement. The
+species extreme in character are not the oldest, or the most recent; nor
+are those which are intermediate in character, intermediate in age. But
+supposing for an instant, in this and other such cases, that the record of
+the first appearance and disappearance of the species was perfect, we have
+no reason to believe that forms successively produced necessarily endure
+for {335} corresponding lengths of time: a very ancient form might
+occasionally last much longer than a form elsewhere subsequently produced,
+especially in the case of terrestrial productions inhabiting separated
+districts. To compare small things with great: if the principal living and
+extinct races of the domestic pigeon were arranged as well as they could be
+in serial affinity, this arrangement would not closely accord with the
+order in time of their production, and still less with the order of their
+disappearance; for the parent rock-pigeon now lives; and many varieties
+between the rock-pigeon and the carrier have become extinct; and carriers
+which are extreme in the important character of length of beak originated
+earlier than short-beaked tumblers, which are at the opposite end of the
+series in this same respect.
+
+Closely connected with the statement, that the organic remains from an
+intermediate formation are in some degree intermediate in character, is the
+fact, insisted on by all palæontologists, that fossils from two consecutive
+formations are far more closely related to each other, than are the fossils
+from two remote formations. Pictet gives as a well-known instance, the
+general resemblance of the organic remains from the several stages of the
+Chalk formation, though the species are distinct in each stage. This fact
+alone, from its generality, seems to have shaken Professor Pictet in his
+firm belief in the immutability of species. He who is acquainted with the
+distribution of existing species over the globe, will not attempt to
+account for the close resemblance of the distinct species in closely
+consecutive formations, by the physical conditions of the ancient areas
+having remained nearly the same. Let it be remembered that the forms of
+life, at least those inhabiting the sea, have changed almost simultaneously
+throughout the world, and therefore under the most different climates and
+conditions. Consider the {336} prodigious vicissitudes of climate during
+the pleistocene period, which includes the whole glacial period, and note
+how little the specific forms of the inhabitants of the sea have been
+affected.
+
+On the theory of descent, the full meaning of the fact of fossil remains
+from closely consecutive formations, though ranked as distinct species,
+being closely related, is obvious. As the accumulation of each formation
+has often been interrupted, and as long blank intervals have intervened
+between successive formations, we ought not to expect to find, as I
+attempted to show in the last chapter, in any one or two formations all the
+intermediate varieties between the species which appeared at the
+commencement and close of these periods; but we ought to find after
+intervals, very long as measured by years, but only moderately long as
+measured geologically, closely allied forms, or, as they have been called
+by some authors, representative species; and these we assuredly do find. We
+find, in short, such evidence of the slow and scarcely sensible mutation of
+specific forms, as we have a just right to expect to find.
+
+
+
+_On the state of Development of Ancient Forms._--There has been much
+discussion whether recent forms are more highly developed than ancient. I
+will not here enter on this subject, for naturalists have not as yet
+defined to each other's satisfaction what is meant by high and low forms.
+The best definition probably is, that the higher forms have their organs
+more distinctly specialised for different functions; and as such division
+of physiological labour seems to be an advantage to each being, natural
+selection will constantly tend in so far to make the later and more
+modified forms higher than their early progenitors, or than the slightly
+modified descendants of such progenitors. In a more general sense the {337}
+more recent forms must, on my theory, be higher than the more ancient; for
+each new species is formed by having had some advantage in the struggle for
+life over other and preceding forms. If under a nearly similar climate, the
+eocene inhabitants of one quarter of the world were put into competition
+with the existing inhabitants of the same or some other quarter, the eocene
+fauna or flora would certainly be beaten and exterminated; as would a
+secondary fauna by an eocene, and a palæozoic fauna by a secondary fauna. I
+do not doubt that this process of improvement has affected in a marked and
+sensible manner the organisation of the more recent and victorious forms of
+life, in comparison with the ancient and beaten forms; but I can see no way
+of testing this sort of progress. Crustaceans, for instance, not the
+highest in their own class, may have beaten the highest molluscs. From the
+extraordinary manner in which European productions have recently spread
+over New Zealand, and have seized on places which must have been previously
+occupied, we may believe, if all the animals and plants of Great Britain
+were set free in New Zealand, that in the course of time a multitude of
+British forms would become thoroughly naturalized there, and would
+exterminate many of the natives. On the other hand, from what we see now
+occurring in New Zealand, and from hardly a single inhabitant of the
+southern hemisphere having become wild in any part of Europe, we may doubt,
+if all the productions of New Zealand were set free in Great Britain,
+whether any considerable number would be enabled to seize on places now
+occupied by our native plants and animals. Under this point of view, the
+productions of Great Britain may be said to be higher than those of New
+Zealand. Yet the most skilful naturalist from an examination of the {338}
+species of the two countries could not have foreseen this result.
+
+Agassiz insists that ancient animals resemble to a certain extent the
+embryos of recent animals of the same classes; or that the geological
+succession of extinct forms is in some degree parallel to the embryological
+development of recent forms. I must follow Pictet and Huxley in thinking
+that the truth of this doctrine is very far from proved. Yet I fully expect
+to see it hereafter confirmed, at least in regard to subordinate groups,
+which have branched off from each other within comparatively recent times.
+For this doctrine of Agassiz accords well with the theory of natural
+selection. In a future chapter I shall attempt to show that the adult
+differs from its embryo, owing to variations supervening at a not early
+age, and being inherited at a corresponding age. This process, whilst it
+leaves the embryo almost unaltered, continually adds, in the course of
+successive generations, more and more difference to the adult.
+
+Thus the embryo comes to be left as a sort of picture, preserved by nature,
+of the ancient and less modified condition of each animal. This view may be
+true, and yet it may never be capable of full proof. Seeing, for instance,
+that the oldest known mammals, reptiles, and fish strictly belong to their
+own proper classes, though some of these old forms are in a slight degree
+less distinct from each other than are the typical members of the same
+groups at the present day, it would be vain to look for animals having the
+common embryological character of the Vertebrata, until beds far beneath
+the lowest Silurian strata are discovered--a discovery of which the chance
+is very small.
+
+
+
+_On the Succession of the same Types within the same {339} areas, during
+the later tertiary periods._--Mr. Clift many years ago showed that the
+fossil mammals from the Australian caves were closely allied to the living
+marsupials of that continent. In South America, a similar relationship is
+manifest, even to an uneducated eye, in the gigantic pieces of armour like
+those of the armadillo, found in several parts of La Plata; and Professor
+Owen has shown in the most striking manner that most of the fossil mammals,
+buried there in such numbers, are related to South American types. This
+relationship is even more clearly seen in the wonderful collection of
+fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so
+much impressed with these facts that I strongly insisted, in 1839 and 1845,
+on this "law of the succession of types,"--on "this wonderful relationship
+in the same continent between the dead and the living." Professor Owen has
+subsequently extended the same generalisation to the mammals of the Old
+World. We see the same law in this author's restorations of the extinct and
+gigantic birds of New Zealand. We see it also in the birds of the caves of
+Brazil. Mr. Woodward has shown that the same law holds good with
+sea-shells, but from the wide distribution of most genera of molluscs, it
+is not well displayed by them. Other cases could be added, as the relation
+between the extinct and living land-shells of Madeira; and between the
+extinct and living brackish-water shells of the Aralo-Caspian Sea.
+
+Now what does this remarkable law of the succession of the same types
+within the same areas mean? He would be a bold man, who after comparing the
+present climate of Australia and of parts of South America under the same
+latitude, would attempt to account, on the one hand, by dissimilar physical
+conditions for the dissimilarity of the inhabitants of these two
+continents, {340} and, on the other hand, by similarity of conditions, for
+the uniformity of the same types in each during the later tertiary periods.
+Nor can it be pretended that it is an immutable law that marsupials should
+have been chiefly or solely produced in Australia; or that Edentata and
+other American types should have been solely produced in South America. For
+we know that Europe in ancient times was peopled by numerous marsupials;
+and I have shown in the publications above alluded to, that in America the
+law of distribution of terrestrial mammals was formerly different from what
+it now is. North America formerly partook strongly of the present character
+of the southern half of the continent; and the southern half was formerly
+more closely allied, than it is at present, to the northern half. In a
+similar manner we know from Falconer and Cautley's discoveries, that
+northern India was formerly more closely related in its mammals to Africa
+than it is at the present time. Analogous facts could be given in relation
+to the distribution of marine animals.
+
+On the theory of descent with modification, the great law of the long
+enduring, but not immutable, succession of the same types within the same
+areas, is at once explained; for the inhabitants of each quarter of the
+world will obviously tend to leave in that quarter, during the next
+succeeding period of time, closely allied though in some degree modified
+descendants. If the inhabitants of one continent formerly differed greatly
+from those of another continent, so will their modified descendants still
+differ in nearly the same manner and degree. But after very long intervals
+of time and after great geographical changes, permitting much
+inter-migration, the feebler will yield to the more dominant forms, and
+there will be nothing immutable in the laws of past and present
+distribution. {341}
+
+It may be asked in ridicule, whether I suppose that the megatherium and
+other allied huge monsters have left behind them in South America, the
+sloth, armadillo, and anteater, as their degenerate descendants. This
+cannot for an instant be admitted. These huge animals have become wholly
+extinct, and have left no progeny. But in the caves of Brazil, there are
+many extinct species which are closely allied in size and in other
+characters to the species still living in South America; and some of these
+fossils may be the actual progenitors of living species. It must not be
+forgotten that, on my theory, all the species of the same genus have
+descended from some one species; so that if six genera, each having eight
+species, be found in one geological formation, and in the next succeeding
+formation there be six other allied or representative genera with the same
+number of species, then we may conclude that only one species of each of
+the six older genera has left modified descendants, constituting the six
+new genera. The other seven species of the old genera have all died out and
+have left no progeny. Or, which would probably be a far commoner case, two
+or three species of two or three alone of the six older genera will have
+been the parents of the six new genera; the other old species and the other
+whole old genera having become utterly extinct. In failing orders, with the
+genera and species decreasing in numbers, as apparently is the case of the
+Edentata of South America, still fewer genera and species will have left
+modified blood-descendants.
+
+
+
+_Summary of the preceding and present Chapters._--I have attempted to show
+that the geological record is extremely imperfect; that only a small
+portion of the globe has been geologically explored with care; that {342}
+only certain classes of organic beings have been largely preserved in a
+fossil state; that the number both of specimens and of species, preserved
+in our museums, is absolutely as nothing compared with the incalculable
+number of generations which must have passed away even during a single
+formation; that, owing to subsidence being necessary for the accumulation
+of fossiliferous deposits thick enough to resist future degradation,
+enormous intervals of time have elapsed between the successive formations;
+that there has probably been more extinction during the periods of
+subsidence, and more variation during the periods of elevation, and during
+the latter the record will have been least perfectly kept; that each single
+formation has not been continuously deposited; that the duration of each
+formation is, perhaps, short compared with the average duration of specific
+forms; that migration has played an important part in the first appearance
+of new forms in any one area and formation; that widely ranging species are
+those which have varied most, and have oftenest given rise to new species;
+and that varieties have at first often been local. All these causes taken
+conjointly, must have tended to make the geological record extremely
+imperfect, and will to a large extent explain why we do not find
+interminable varieties, connecting together all the extinct and existing
+forms of life by the finest graduated steps.
+
+He who rejects these views on the nature of the geological record, will
+rightly reject my whole theory. For he may ask in vain where are the
+numberless transitional links which must formerly have connected the
+closely allied or representative species, found in the several stages of
+the same great formation. He may disbelieve in the enormous intervals of
+time which have elapsed between our consecutive formations; he {343} may
+overlook how important a part migration must have played, when the
+formations of any one great region alone, as that of Europe, are
+considered; he may urge the apparent, but often falsely apparent, sudden
+coming in of whole groups of species. He may ask where are the remains of
+those infinitely numerous organisms which must have existed long before the
+first bed of the Silurian system was deposited: I can answer this latter
+question only hypothetically, by saying that as far as we can see, where
+our oceans now extend they have for an enormous period extended, and where
+our oscillating continents now stand they have stood ever since the
+Silurian epoch; but that long before that period, the world may have
+presented a wholly different aspect; and that the older continents, formed
+of formations older than any known to us, may now all be in a metamorphosed
+condition, or may lie buried under the ocean.
+
+Passing from these difficulties, all the other great leading facts in
+palæontology seem to me simply to follow on the theory of descent with
+modification through natural selection. We can thus understand how it is
+that new species come in slowly and successively; how species of different
+classes do not necessarily change together, or at the same rate, or in the
+same degree; yet in the long run that all undergo modification to some
+extent. The extinction of old forms is the almost inevitable consequence of
+the production of new forms. We can understand why when a species has once
+disappeared it never reappears. Groups of species increase in numbers
+slowly, and endure for unequal periods of time; for the process of
+modification is necessarily slow, and depends on many complex
+contingencies. The dominant species of the larger dominant groups tend to
+leave many modified {344} descendants, and thus new sub-groups and groups
+are formed. As these are formed, the species of the less vigorous groups,
+from their inferiority inherited from a common progenitor, tend to become
+extinct together, and to leave no modified offspring on the face of the
+earth. But the utter extinction of a whole group of species may often be a
+very slow process, from the survival of a few descendants, lingering in
+protected and isolated situations. When a group has once wholly
+disappeared, it does not reappear; for the link of generation has been
+broken.
+
+We can understand how the spreading of the dominant forms of life, which
+are those that oftenest vary, will in the long run tend to people the world
+with allied, but modified, descendants; and these will generally succeed in
+taking the places of those groups of species which are their inferiors in
+the struggle for existence. Hence, after long intervals of time, the
+productions of the world will appear to have changed simultaneously.
+
+We can understand how it is that all the forms of life, ancient and recent,
+make together one grand system; for all are connected by generation. We can
+understand, from the continued tendency to divergence of character, why the
+more ancient a form is, the more it generally differs from those now
+living. Why ancient and extinct forms often tend to fill up gaps between
+existing forms, sometimes blending two groups previously classed as
+distinct into one; but more commonly only bringing them a little closer
+together. The more ancient a form is, the more often, apparently, it
+displays characters in some degree intermediate between groups now
+distinct; for the more ancient a form is, the more nearly it will be
+related to, and consequently resemble, the common progenitor of groups,
+since {345} become widely divergent. Extinct forms are seldom directly
+intermediate between existing forms; but are intermediate only by a long
+and circuitous course through many extinct and very different forms. We can
+clearly see why the organic remains of closely consecutive formations are
+more closely allied to each other, than are those of remote formations; for
+the forms are more closely linked together by generation: we can clearly
+see why the remains of an intermediate formation are intermediate in
+character.
+
+The inhabitants of each successive period in the world's history have
+beaten their predecessors in the race for life, and are, in so far, higher
+in the scale of nature; and this may account for that vague yet ill-defined
+sentiment, felt by many palæontologists, that organisation on the whole has
+progressed. If it should hereafter be proved that ancient animals resemble
+to a certain extent the embryos of more recent animals of the same class,
+the fact will be intelligible. The succession of the same types of
+structure within the same areas during the later geological periods ceases
+to be mysterious, and is simply explained by inheritance.
+
+If then the geological record be as imperfect as I believe it to be, and it
+may at least be asserted that the record cannot be proved to be much more
+perfect, the main objections to the theory of natural selection are greatly
+diminished or disappear. On the other hand, all the chief laws of
+palæontology plainly proclaim, as it seems to me, that species have been
+produced by ordinary generation: old forms having been supplanted by new
+and improved forms of life, produced by the laws of variation still acting
+round us, and preserved by Natural Selection.
+
+       *       *       *       *       *
+
+
+{346}
+
+CHAPTER XI.
+
+GEOGRAPHICAL DISTRIBUTION.
+
+    Present distribution cannot be accounted for by differences in physical
+    conditions--Importance of barriers--Affinity of the productions of the
+    same continent--Centres of creation--Means of dispersal, by changes of
+    climate and of the level of the land, and by occasional
+    means--Dispersal during the Glacial period co-extensive with the world.
+
+In considering the distribution of organic beings over the face of the
+globe, the first great fact which strikes us is, that neither the
+similarity nor the dissimilarity of the inhabitants of various regions can
+be accounted for by their climatal and other physical conditions. Of late,
+almost every author who has studied the subject has come to this
+conclusion. The case of America alone would almost suffice to prove its
+truth: for if we exclude the northern parts where the circumpolar land is
+almost continuous, all authors agree that one of the most fundamental
+divisions in geographical distribution is that between the New and Old
+Worlds; yet if we travel over the vast American continent, from the central
+parts of the United States to its extreme southern point, we meet with the
+most diversified conditions; the most humid districts, arid deserts, lofty
+mountains, grassy plains, forests, marshes, lakes, and great rivers, under
+almost every temperature. There is hardly a climate or condition in the Old
+World which cannot be paralleled in the New--at least as closely as the
+same species generally require; for it is a most rare case to find a group
+of organisms confined to any small spot, having conditions peculiar in only
+a slight {347} degree; for instance, small areas in the Old World could be
+pointed out hotter than any in the New World, yet these are not inhabited
+by a peculiar fauna or flora. Notwithstanding this parallelism in the
+conditions of the Old and New Worlds, how widely different are their living
+productions!
+
+In the southern hemisphere, if we compare large tracts of land in
+Australia, South Africa, and western South America, between latitudes 25°
+and 35°, we shall find parts extremely similar in all their conditions, yet
+it would not be possible to point out three faunas and floras more utterly
+dissimilar. Or again we may compare the productions of South America south
+of lat. 35° with those north of 25°, which consequently inhabit a
+considerably different climate, and they will be found incomparably more
+closely related to each other, than they are to the productions of
+Australia or Africa under nearly the same climate. Analogous facts could be
+given with respect to the inhabitants of the sea.
+
+A second great fact which strikes us in our general review is, that
+barriers of any kind, or obstacles to free migration, are related in a
+close and important manner to the differences between the productions of
+various regions. We see this in the great difference of nearly all the
+terrestrial productions of the New and Old Worlds, excepting in the
+northern parts, where the land almost joins, and where, under a slightly
+different climate, there might have been free migration for the northern
+temperate forms, as there now is for the strictly arctic productions. We
+see the same fact in the great difference between the inhabitants of
+Australia, Africa, and South America under the same latitude: for these
+countries are almost as much isolated from each other as is possible. On
+each continent, also, we see the same fact; for on the opposite sides of
+{348} lofty and continuous mountain-ranges, and of great deserts, and
+sometimes even of large rivers, we find different productions; though as
+mountain-chains, deserts, &c., are not as impassable, or likely to have
+endured so long as the oceans separating continents, the differences are
+very inferior in degree to those characteristic of distinct continents.
+
+Turning to the sea, we find the same law. No two marine faunas are more
+distinct, with hardly a fish, shell, or crab in common, than those of the
+eastern and western shores of South and Central America; yet these great
+faunas are separated only by the narrow, but impassable, isthmus of Panama.
+Westward of the shores of America, a wide space of open ocean extends, with
+not an island as a halting-place for emigrants; here we have a barrier of
+another kind, and as soon as this is passed we meet in the eastern islands
+of the Pacific, with another and totally distinct fauna. So that here three
+marine faunas range far northward and southward, in parallel lines not far
+from each other, under corresponding climates; but from being separated
+from each other by impassable barriers, either of land or open sea, they
+are wholly distinct. On the other hand, proceeding still further westward
+from the eastern islands of the tropical parts of the Pacific, we encounter
+no impassable barriers, and we have innumerable islands as halting-places,
+or continuous coasts, until after travelling over a hemisphere we come to
+the shores of Africa; and over this vast space we meet with no well-defined
+and distinct marine faunas. Although hardly one shell, crab or fish is
+common to the above-named three approximate faunas of Eastern and Western
+America and the eastern Pacific islands, yet many fish range from the
+Pacific into the Indian Ocean, and many shells are common to the eastern
+islands of the Pacific {349} and the eastern shores of Africa, on almost
+exactly opposite meridians of longitude.
+
+A third great fact, partly included in the foregoing statements, is the
+affinity of the productions of the same continent or sea, though the
+species themselves are distinct at different points and stations. It is a
+law of the widest generality, and every continent offers innumerable
+instances. Nevertheless the naturalist in travelling, for instance, from
+north to south never fails to be struck by the manner in which successive
+groups of beings, specifically distinct, yet clearly related, replace each
+other. He hears from closely allied, yet distinct kinds of birds, notes
+nearly similar, and sees their nests similarly constructed, but not quite
+alike, with eggs coloured in nearly the same manner. The plains near the
+Straits of Magellan are inhabited by one species of Rhea (American
+ostrich), and northward the plains of La Plata by another species of the
+same genus; and not by a true ostrich or emu, like those found in Africa
+and Australia under the same latitude. On these same plains of La Plata, we
+see the agouti and bizcacha, animals having nearly the same habits as our
+hares and rabbits and belonging to the same order of Rodents, but they
+plainly display an American type of structure. We ascend the lofty peaks of
+the Cordillera and we find an alpine species of bizcacha; we look to the
+waters, and we do not find the beaver or musk-rat, but the coypu and
+capybara, rodents of the American type. Innumerable other instances could
+be given. If we look to the islands off the American shore, however much
+they may differ in geological structure, the inhabitants, though they may
+be all peculiar species, are essentially American. We may look back to past
+ages, as shown in the last chapter, and we find American types then
+prevalent on {350} the American continent and in the American seas. We see
+in these facts some deep organic bond, prevailing throughout space and
+time, over the same areas of land and water, and independent of their
+physical conditions. The naturalist must feel little curiosity, who is not
+led to inquire what this bond is.
+
+This bond, on my theory, is simply inheritance, that cause which alone, as
+far as we positively know, produces organisms quite like, or, as we see in
+the case of varieties, nearly like each other. The dissimilarity of the
+inhabitants of different regions may be attributed to modification through
+natural selection, and in a quite subordinate degree to the direct
+influence of different physical conditions. The degree of dissimilarity
+will depend on the migration of the more dominant forms of life from one
+region into another having been effected with more or less ease, at periods
+more or less remote;--on the nature and number of the former
+immigrants;--and on their action and reaction, in their mutual struggles
+for life;--the relation of organism to organism being, as I have already
+often remarked, the most important of all relations. Thus the high
+importance of barriers comes into play by checking migration; as does time
+for the slow process of modification through natural selection.
+Widely-ranging species, abounding in individuals, which have already
+triumphed over many competitors in their own widely-extended homes will
+have the best chance of seizing on new places, when they spread into new
+countries. In their new homes they will be exposed to new conditions, and
+will frequently undergo further modification and improvement; and thus they
+will become still further victorious, and will produce groups of modified
+descendants. On this principle of inheritance with modification, we can
+understand how it is that sections of genera, whole genera, {351} and even
+families are confined to the same areas, as is so commonly and notoriously
+the case.
+
+I believe, as was remarked in the last chapter, in no law of necessary
+development. As the variability of each species is an independent property,
+and will be taken advantage of by natural selection, only so far as it
+profits the individual in its complex struggle for life, so the degree of
+modification in different species will be no uniform quantity. If, for
+instance, a number of species, which stand in direct competition with each
+other, migrate in a body into a new and afterwards isolated country, they
+will be little liable to modification; for neither migration nor isolation
+in themselves can do anything. These principles come into play only by
+bringing organisms into new relations with each other, and in a lesser
+degree with the surrounding physical conditions. As we have seen in the
+last chapter that some forms have retained nearly the same character from
+an enormously remote geological period, so certain species have migrated
+over vast spaces, and have not become greatly modified.
+
+On these views, it is obvious, that the several species of the same genus,
+though inhabiting the most distant quarters of the world, must originally
+have proceeded from the same source, as they have descended from the same
+progenitor. In the case of those species, which have undergone during whole
+geological periods but little modification, there is not much difficulty in
+believing that they may have migrated from the same region; for during the
+vast geographical and climatal changes which will have supervened since
+ancient times, almost any amount of migration is possible. But in many
+other cases, in which we have reason to believe that the species of a genus
+have been produced within comparatively recent times, there is great
+difficulty on this head. It {352} is also obvious that the individuals of
+the same species, though now inhabiting distant and isolated regions, must
+have proceeded from one spot, where their parents were first produced: for,
+as explained in the last chapter, it is incredible that individuals
+identically the same should ever have been produced through natural
+selection from parents specifically distinct.
+
+We are thus brought to the question which has been largely discussed by
+naturalists, namely, whether species have been created at one or more
+points of the earth's surface. Undoubtedly there are very many cases of
+extreme difficulty, in understanding how the same species could possibly
+have migrated from some one point to the several distant and isolated
+points, where now found. Nevertheless the simplicity of the view that each
+species was first produced within a single region captivates the mind. He
+who rejects it, rejects the _vera causa_ of ordinary generation with
+subsequent migration, and calls in the agency of a miracle. It is
+universally admitted, that in most cases the area inhabited by a species is
+continuous; and when a plant or animal inhabits two points so distant from
+each other, or with an interval of such a nature, that the space could not
+be easily passed over by migration, the fact is given as something
+remarkable and exceptional. The capacity of migrating across the sea is
+more distinctly limited in terrestrial mammals, than perhaps in any other
+organic beings; and, accordingly, we find no inexplicable cases of the same
+mammal inhabiting distant points of the world. No geologist will feel any
+difficulty in such cases as Great Britain having been formerly united to
+Europe, and consequently possessing the same quadrupeds. But if the same
+species can be produced at two separate points, why do we not find a single
+mammal common to Europe and {353} Australia or South America? The
+conditions of life are nearly the same, so that a multitude of European
+animals and plants have become naturalised in America and Australia; and
+some of the aboriginal plants are identically the same at these distant
+points of the northern and southern hemispheres? The answer, as I believe,
+is, that mammals have not been able to migrate, whereas some plants, from
+their varied means of dispersal, have migrated across the vast and broken
+interspace. The great and striking influence which barriers of every kind
+have had on distribution, is intelligible only on the view that the great
+majority of species have been produced on one side alone, and have not been
+able to migrate to the other side. Some few families, many sub-families,
+very many genera, and a still greater number of sections of genera are
+confined to a single region; and it has been observed by several
+naturalists, that the most natural genera, or those genera in which the
+species are most closely related to each other, are generally local, or
+confined to one area. What a strange anomaly it would be, if, when coming
+one step lower in the series, to the individuals of the same species, a
+directly opposite rule prevailed; and species were not local, but had been
+produced in two or more distinct areas!
+
+Hence it seems to me, as it has to many other naturalists, that the view of
+each species having been produced in one area alone, and having
+subsequently migrated from that area as far as its powers of migration and
+subsistence under past and present conditions permitted, is the most
+probable. Undoubtedly many cases occur, in which we cannot explain how the
+same species could have passed from one point to the other. But the
+geographical and climatal changes, which have certainly occurred within
+recent geological times, must have interrupted or rendered discontinuous
+the {354} formerly continuous range of many species. So that we are reduced
+to consider whether the exceptions to continuity of range are so numerous
+and of so grave a nature, that we ought to give up the belief, rendered
+probable by general considerations, that each species has been produced
+within one area, and has migrated thence as far as it could. It would be
+hopelessly tedious to discuss all the exceptional cases of the same
+species, now living at distant and separated points; nor do I for a moment
+pretend that any explanation could be offered of many such cases. But after
+some preliminary remarks, I will discuss a few of the most striking classes
+of facts; namely, the existence of the same species on the summits of
+distant mountain-ranges, and at distant points in the arctic and antarctic
+regions; and secondly (in the following chapter), the wide distribution of
+freshwater productions; and thirdly, the occurrence of the same terrestrial
+species on islands and on the mainland, though separated by hundreds of
+miles of open sea. If the existence of the same species at distant and
+isolated points of the earth's surface, can in many instances be explained
+on the view of each species having migrated from a single birthplace; then,
+considering our ignorance with respect to former climatal and geographical
+changes and various occasional means of transport, the belief that this has
+been the universal law, seems to me incomparably the safest.
+
+In discussing this subject, we shall be enabled at the same time to
+consider a point equally important for us, namely, whether the several
+distinct species of a genus, which on my theory have all descended from a
+common progenitor, can have migrated (undergoing modification during some
+part of their migration) from the area inhabited by their progenitor. If it
+can be shown to be almost invariably the case, that a region, of which
+{355} most of its inhabitants are closely related to, or belong to the same
+genera with the species of a second region, has probably received at some
+former period immigrants from this other region, my theory will be
+strengthened; for we can clearly understand, on the principle of
+modification, why the inhabitants of a region should be related to those of
+another region, whence it has been stocked. A volcanic island, for
+instance, upheaved and formed at the distance of a few hundreds of miles
+from a continent, would probably receive from it in the course of time a
+few colonists, and their descendants, though modified, would still be
+plainly related by inheritance to the inhabitants of the continent. Cases
+of this nature are common, and are, as we shall hereafter more fully see,
+inexplicable on the theory of independent creation. This view of the
+relation of species in one region to those in another, does not differ much
+(by substituting the word variety for species) from that lately advanced in
+an ingenious paper by Mr. Wallace, in which he concludes, that "every
+species has come into existence coincident both in space and time with a
+pre-existing closely allied species." And I now know from correspondence,
+that this coincidence he attributes to generation with modification.
+
+The previous remarks on "single and multiple centres of creation" do not
+directly bear on another allied question,--namely whether all the
+individuals of the same species have descended from a single pair, or
+single hermaphrodite, or whether, as some authors suppose, from many
+individuals simultaneously created. With those organic beings which never
+intercross (if such exist), the species, on my theory, must have descended
+from a succession of improved varieties, which will never have blended with
+other individuals or varieties, but will have supplanted each other; so
+that, at each {356} successive stage of modification and improvement, all
+the individuals of each variety will have descended from a single parent.
+But in the majority of cases, namely, with all organisms which habitually
+unite for each birth, or which often intercross, I believe that during the
+slow process of modification the individuals of the species will have been
+kept nearly uniform by intercrossing; so that many individuals will have
+gone on simultaneously changing, and the whole amount of modification will
+not have been due, at each stage, to descent from a single parent. To
+illustrate what I mean: our English racehorses differ slightly from the
+horses of every other breed; but they do not owe their difference and
+superiority to descent from any single pair, but to continued care in
+selecting and training many individuals during many generations.
+
+Before discussing the three classes of facts, which I have selected as
+presenting the greatest amount of difficulty on the theory of "single
+centres of creation," I must say a few words on the means of dispersal.
+
+
+
+_Means of Dispersal._--Sir C. Lyell and other authors have ably treated
+this subject. I can give here only the briefest abstract of the more
+important facts. Change of climate must have had a powerful influence on
+migration: a region when its climate was different may have been a high
+road for migration, but now be impassable; I shall, however, presently have
+to discuss this branch of the subject in some detail. Changes of level in
+the land must also have been highly influential: a narrow isthmus now
+separates two marine faunas; submerge it, or let it formerly have been
+submerged, and the two faunas will now blend or may formerly have blended:
+where the sea now extends, land may at a former period have connected
+islands or {357} possibly even continents together, and thus have allowed
+terrestrial productions to pass from one to the other. No geologist will
+dispute that great mutations of level have occurred within the period of
+existing organisms. Edward Forbes insisted that all the islands in the
+Atlantic must recently have been connected with Europe or Africa, and
+Europe likewise with America. Other authors have thus hypothetically
+bridged over every ocean, and have united almost every island to some
+mainland. If indeed the arguments used by Forbes are to be trusted, it must
+be admitted that scarcely a single island exists which has not recently
+been united to some continent. This view cuts the Gordian knot of the
+dispersal of the same species to the most distant points, and removes many
+a difficulty: but to the best of my judgment we are not authorized in
+admitting such enormous geographical changes within the period of existing
+species. It seems to me that we have abundant evidence of great
+oscillations of level in our continents; but not of such vast changes in
+their position and extension, as to have united them within the recent
+period to each other and to the several intervening oceanic islands. I
+freely admit the former existence of many islands, now buried beneath the
+sea, which may have served as halting places for plants and for many
+animals during their migration. In the coral-producing oceans such sunken
+islands are now marked, as I believe, by rings of coral or atolls standing
+over them. Whenever it is fully admitted, as I believe it will some day be,
+that each species has proceeded from a single birthplace, and when in the
+course of time we know something definite about the means of distribution,
+we shall be enabled to speculate with security on the former extension of
+the land. But I do not believe that it will ever be proved that within the
+{358} recent period continents which are now quite separate, have been
+continuously, or almost continuously, united with each other, and with the
+many existing oceanic islands. Several facts in distribution,--such as the
+great difference in the marine faunas on the opposite sides of almost every
+continent,--the close relation of the tertiary inhabitants of several lands
+and even seas to their present inhabitants,--a certain degree of relation
+(as we shall hereafter see) between the distribution of mammals and the
+depth of the sea,--these and other such facts seem to me opposed to the
+admission of such prodigious geographical revolutions within the recent
+period, as are necessitated on the view advanced by Forbes and admitted by
+his many followers. The nature and relative proportions of the inhabitants
+of oceanic islands likewise seem to me opposed to the belief of their
+former continuity with continents. Nor does their almost universally
+volcanic composition favour the admission that they are the wrecks of
+sunken continents;--if they had originally existed as mountain-ranges on
+the land, some at least of the islands would have been formed, like other
+mountain-summits, of granite, metamorphic schists, old fossiliferous or
+other such rocks, instead of consisting of mere piles of volcanic matter.
+
+I must now say a few words on what are called accidental means, but which
+more properly might be called occasional means of distribution. I shall
+here confine myself to plants. In botanical works, this or that plant is
+stated to be ill adapted for wide dissemination; but for transport across
+the sea, the greater or less facilities may be said to be almost wholly
+unknown. Until I tried, with Mr. Berkeley's aid, a few experiments, it was
+not even known how far seeds could resist the injurious action of
+sea-water. To my surprise I found that {359} out of 87 kinds, 64 germinated
+after an immersion of 28 days, and a few survived an immersion of 137 days.
+For convenience' sake I chiefly tried small seeds, without the capsule or
+fruit; and as all of these sank in a few days, they could not be floated
+across wide spaces of the sea, whether or not they were injured by the
+salt-water. Afterwards I tried some larger fruits, capsules, &c., and some
+of these floated for a long time. It is well known what a difference there
+is in the buoyancy of green and seasoned timber; and it occurred to me that
+floods might wash down plants or branches, and that these might be dried on
+the banks, and then by a fresh rise in the stream be washed into the sea.
+Hence I was led to dry stems and branches of 94 plants with ripe fruit, and
+to place them on sea-water. The majority sank quickly, but some which
+whilst green floated for a very short time, when dried floated much longer;
+for instance, ripe hazel-nuts sank immediately, but when dried they floated
+for 90 days, and afterwards when planted they germinated; an asparagus
+plant with ripe berries floated for 23 days, when dried it floated for 85
+days, and the seeds afterwards germinated; the ripe seeds of Helosciadium
+sank in two days, when dried they floated for above 90 days, and afterwards
+germinated. Altogether out of the 94 dried plants, 18 floated for above 28
+days, and some of the 18 floated for a very much longer period. So that as
+64/87 seeds germinated after an immersion of 28 days; and as 18/94 plants
+with ripe fruit (but not all the same species as in the foregoing
+experiment) floated, after being dried, for above 28 days, as far as we may
+infer anything from these scanty facts, we may conclude that the seeds of
+14/100 plants of any country might be floated by sea-currents during 28
+days, and would retain their power of germination. In Johnston's Physical
+Atlas, the average {360} rate of the several Atlantic currents is 33 miles
+per diem (some currents running at the rate of 60 miles per diem); on this
+average, the seeds of 14/100 plants belonging to one country might be
+floated across 924 miles of sea to another country; and when stranded, if
+blown to a favourable spot by an inland gale, they would germinate.
+
+Subsequently to my experiments, M. Martens tried similar ones, but in a
+much better manner, for he placed the seeds in a box in the actual sea, so
+that they were alternately wet and exposed to the air like really floating
+plants. He tried 98 seeds, mostly different from mine; but he chose many
+large fruits and likewise seeds from plants which live near the sea; and
+this would have favoured the average length of their flotation and of their
+resistance to the injurious action of the salt-water. On the other hand he
+did not previously dry the plants or branches with the fruit; and this, as
+we have seen, would have caused some of them to have floated much longer.
+The result was that 18/98 of his seeds floated for 42 days, and were then
+capable of germination. But I do not doubt that plants exposed to the waves
+would float for a less time than those protected from violent movement as
+in our experiments. Therefore it would perhaps be safer to assume that the
+seeds of about 10/100 plants of a flora, after having been dried, could be
+floated across a space of sea 900 miles in width, and would then germinate.
+The fact of the larger fruits often floating longer than the small, is
+interesting; as plants with large seeds or fruit could hardly be
+transported by any other means; and Alph. de Candolle has shown that such
+plants generally have restricted ranges.
+
+But seeds may be occasionally transported in another manner. Drift timber
+is thrown up on most islands, {361} even on those in the midst of the
+widest oceans; and the natives of the coral-islands in the Pacific, procure
+stones for their tools, solely from the roots of drifted trees, these
+stones being a valuable royal tax. I find on examination, that when
+irregularly shaped stones are embedded in the roots of trees, small parcels
+of earth are very frequently enclosed in their interstices and behind
+them,--so perfectly that not a particle could be washed away in the longest
+transport: out of one small portion of earth thus _completely_ enclosed by
+wood in an oak about 50 years old, three dicotyledonous plants germinated:
+I am certain of the accuracy of this observation. Again, I can show that
+the carcasses of birds, when floating on the sea, sometimes escape being
+immediately devoured; and seeds of many kinds in the crops of floating
+birds long retain their vitality: peas and vetches, for instance, are
+killed by even a few days' immersion in sea-water; but some taken out of
+the crop of a pigeon, which had floated on artificial salt-water for 30
+days, to my surprise nearly all germinated.
+
+Living birds can hardly fail to be highly effective agents in the
+transportation of seeds. I could give many facts showing how frequently
+birds of many kinds are blown by gales to vast distances across the ocean.
+We may I think safely assume that under such circumstances their rate of
+flight would often be 35 miles an hour; and some authors have given a far
+higher estimate. I have never seen an instance of nutritious seeds passing
+through the intestines of a bird; but hard seeds of fruit pass uninjured
+through even the digestive organs of a turkey. In the course of two months,
+I picked up in my garden 12 kinds of seeds, out of the excrement of small
+birds, and these seemed perfect, and some of them, which I tried,
+germinated. {362} But the following fact is more important: the crops of
+birds do not secrete gastric juice, and do not in the least injure, as I
+know by trial, the germination of seeds; now after a bird has found and
+devoured a large supply of food, it is positively asserted that all the
+grains do not pass into the gizzard for 12 or even 18 hours. A bird in this
+interval might easily be blown to the distance of 500 miles, and hawks are
+known to look out for tired birds, and the contents of their torn crops
+might thus readily get scattered. Mr. Brent informs me that a friend of his
+had to give up flying carrier-pigeons from France to England, as the hawks
+on the English coast destroyed so many on their arrival. Some hawks and
+owls bolt their prey whole, and after an interval of from twelve to twenty
+hours, disgorge pellets, which, as I know from experiments made in the
+Zoological Gardens, include seeds capable of germination. Some seeds of the
+oat, wheat, millet, canary, hemp, clover, and beet germinated after having
+been from twelve to twenty-one hours in the stomachs of different birds of
+prey; and two seeds of beet grew after having been thus retained for two
+days and fourteen hours. Freshwater fish, I find, eat seeds of many land
+and water plants: fish are frequently devoured by birds, and thus the seeds
+might be transported from place to place. I forced many kinds of seeds into
+the stomachs of dead fish, and then gave their bodies to fishing-eagles,
+storks, and pelicans; these birds after an interval of many hours, either
+rejected the seeds in pellets or passed them in their excrement; and
+several of these seeds retained their power of germination. Certain seeds,
+however, were always killed by this process.
+
+Although the beaks and feet of birds are generally quite clean, I can show
+that earth sometimes adheres to them: in one instance I removed twenty-two
+grains {363} of dry argillaceous earth from one foot of a partridge, and in
+this earth there was a pebble quite as large as the seed of a vetch. Thus
+seeds might occasionally be transported to great distances; for many facts
+could be given showing that soil almost everywhere is charged with seeds.
+Reflect for a moment on the millions of quails which annually cross the
+Mediterranean; and can we doubt that the earth adhering to their feet would
+sometimes include a few minute seeds? But I shall presently have to recur
+to this subject.
+
+As icebergs are known to be sometimes loaded with earth and stones, and
+have even carried brushwood, bones, and the nest of a land-bird, I can
+hardly doubt that they must occasionally have transported seeds from one
+part to another of the arctic and antarctic regions, as suggested by Lyell;
+and during the Glacial period from one part of the now temperate regions to
+another. In the Azores, from the large number of the species of plants
+common to Europe, in comparison with the plants of other oceanic islands
+nearer to the mainland, and (as remarked by Mr. H. C. Watson) from the
+somewhat northern character of the flora in comparison with the latitude, I
+suspected that these islands had been partly stocked by ice-borne seeds,
+during the Glacial epoch. At my request Sir C. Lyell wrote to M. Hartung to
+inquire whether he had observed erratic boulders on these islands, and he
+answered that he had found large fragments of granite and other rocks,
+which do not occur in the archipelago. Hence we may safely infer that
+icebergs formerly landed their rocky burthens on the shores of these
+mid-ocean islands, and it is at least possible that they may have brought
+thither the seeds of northern plants.
+
+Considering that the several above means of transport, and that several
+other means, which without {364} doubt remain to be discovered, have been
+in action year after year, for centuries and tens of thousands of years, it
+would I think be a marvellous fact if many plants had not thus become
+widely transported. These means of transport are sometimes called
+accidental, but this is not strictly correct: the currents of the sea are
+not accidental, nor is the direction of prevalent gales of wind. It should
+be observed that scarcely any means of transport would carry seeds for very
+great distances; for seeds do not retain their vitality when exposed for a
+great length of time to the action of sea-water; nor could they be long
+carried in the crops or intestines of birds. These means, however, would
+suffice for occasional transport across tracts of sea some hundred miles in
+breadth, or from island to island, or from a continent to a neighbouring
+island, but not from one distant continent to another. The floras of
+distant continents would not by such means become mingled in any great
+degree; but would remain as distinct as we now see them to be. The
+currents, from their course, would never bring seeds from North America to
+Britain, though they might and do bring seeds from the West Indies to our
+western shores, where, if not killed by so long an immersion in salt-water,
+they could not endure our climate. Almost every year, one or two land-birds
+are blown across the whole Atlantic Ocean, from North America to the
+western shores of Ireland and England; but seeds could be transported by
+these wanderers only by one means, namely, in dirt sticking to their feet,
+which is in itself a rare accident. Even in this case, how small would the
+chance be of a seed falling on favourable soil, and coming to maturity! But
+it would be a great error to argue that because a well-stocked island, like
+Great Britain, has not, as far as is known {365} (and it would be very
+difficult to prove this), received within the last few centuries, through
+occasional means of transport, immigrants from Europe or any other
+continent, that a poorly-stocked island, though standing more remote from
+the mainland, would not receive colonists by similar means. I do not doubt
+that out of twenty seeds or animals transported to an island, even if far
+less well-stocked than Britain, scarcely more than one would be so well
+fitted to its new home, as to become naturalised. But this, as it seems to
+me, is no valid argument against what would be effected by occasional means
+of transport, during the long lapse of geological time, whilst an island
+was being upheaved and formed, and before it had become fully stocked with
+inhabitants. On almost bare land, with few or no destructive insects or
+birds living there, nearly every seed, which chanced to arrive, if fitted
+for the climate, would be sure to germinate and survive.
+
+
+
+_Dispersal during the Glacial period._--The identity of many plants and
+animals, on mountain-summits, separated from each other by hundreds of
+miles of lowlands, where the Alpine species could not possibly exist, is
+one of the most striking cases known of the same species living at distant
+points, without the apparent possibility of their having migrated from one
+to the other. It is indeed a remarkable fact to see so many of the same
+plants living on the snowy regions of the Alps or Pyrenees, and in the
+extreme northern parts of Europe; but it is far more remarkable, that the
+plants on the White Mountains, in the United States of America, are all the
+same with those of Labrador, and nearly all the same, as we hear from Asa
+Gray, with those on the loftiest mountains of Europe. Even as long ago as
+1747, such facts led Gmelin to conclude that the {366} same species must
+have been independently created at several distinct points; and we might
+have remained in this same belief, had not Agassiz and others called vivid
+attention to the Glacial period, which, as we shall immediately see,
+affords a simple explanation of these facts. We have evidence of almost
+every conceivable kind, organic and inorganic, that within a very recent
+geological period, central Europe and North America suffered under an
+Arctic climate. The ruins of a house burnt by fire do not tell their tale
+more plainly, than do the mountains of Scotland and Wales, with their
+scored flanks, polished surfaces, and perched boulders, of the icy streams
+with which their valleys were lately filled. So greatly has the climate of
+Europe changed, that in Northern Italy, gigantic moraines, left by old
+glaciers, are now clothed by the vine and maize. Throughout a large part of
+the United States, erratic boulders, and rocks scored by drifted icebergs
+and coast-ice, plainly reveal a former cold period.
+
+The former influence of the glacial climate on the distribution of the
+inhabitants of Europe, as explained with remarkable clearness by Edward
+Forbes, is substantially as follows. But we shall follow the changes more
+readily, by supposing a new glacial period to come slowly on, and then pass
+away, as formerly occurred. As the cold came on, and as each more southern
+zone became fitted for arctic beings and ill-fitted for their former more
+temperate inhabitants, the latter would be supplanted and arctic
+productions would take their places. The inhabitants of the more temperate
+regions would at the same time travel southward, unless they were stopped
+by barriers, in which case they would perish. The mountains would become
+covered with snow and ice, and their former Alpine inhabitants would
+descend to the plains. By the time that the cold had reached {367} its
+maximum, we should have a uniform arctic fauna and flora, covering the
+central parts of Europe, as far south as the Alps and Pyrenees, and even
+stretching into Spain. The now temperate regions of the United States would
+likewise be covered by arctic plants and animals, and these would be nearly
+the same with those of Europe; for the present circumpolar inhabitants,
+which we suppose to have everywhere travelled southward, are remarkably
+uniform round the world. We may suppose that the Glacial period came on a
+little earlier or later in North America than in Europe, so will the
+southern migration there have been a little earlier or later; but this will
+make no difference in the final result.
+
+As the warmth returned, the arctic forms would retreat northward, closely
+followed up in their retreat by the productions of the more temperate
+regions. And as the snow melted from the bases of the mountains, the arctic
+forms would seize on the cleared and thawed ground, always ascending higher
+and higher, as the warmth increased, whilst their brethren were pursuing
+their northern journey. Hence, when the warmth had fully returned, the same
+arctic species, which had lately lived in a body together on the lowlands
+of the Old and New Worlds, would be left isolated on distant
+mountain-summits (having been exterminated on all lesser heights) and in
+the arctic regions of both hemispheres.
+
+Thus we can understand the identity of many plants at points so immensely
+remote as on the mountains of the United States and of Europe. We can thus
+also understand the fact that the Alpine plants of each mountain-range are
+more especially related to the arctic forms living due north or nearly due
+north of them: for the migration as the cold came on, and the re-migration
+on the returning warmth, will generally {368} have been due south and
+north. The Alpine plants, for example, of Scotland, as remarked by Mr.
+H. C. Watson, and those of the Pyrenees, as remarked by Ramond, are more
+especially allied to the plants of northern Scandinavia; those of the
+United States to Labrador; those of the mountains of Siberia to the arctic
+regions of that country. These views, grounded as they are on the perfectly
+well-ascertained occurrence of a former Glacial period, seem to me to
+explain in so satisfactory a manner the present distribution of the Alpine
+and Arctic productions of Europe and America, that when in other regions we
+find the same species on distant mountain-summits, we may almost conclude
+without other evidence, that a colder climate permitted their former
+migration across the low intervening tracts, since become too warm for
+their existence.
+
+If the climate, since the Glacial period, has ever been in any degree
+warmer than at present (as some geologists in the United States believe to
+have been the case, chiefly from the distribution of the fossil Gnathodon),
+then the arctic and temperate productions will at a very late period have
+marched a little further north, and subsequently have retreated to their
+present homes; but I have met with no satisfactory evidence with respect to
+this intercalated slightly warmer period, since the Glacial period.
+
+The arctic forms, during their long southern migration and re-migration
+northward, will have been exposed to nearly the same climate, and, as is
+especially to be noticed, they will have kept in a body together;
+consequently their mutual relations will not have been much disturbed, and,
+in accordance with the principles inculcated in this volume, they will not
+have been liable to much modification. But with our Alpine productions,
+left isolated from the moment of the returning warmth, {369} first at the
+bases and ultimately on the summits of the mountains, the case will have
+been somewhat different; for it is not likely that all the same arctic
+species will have been left on mountain ranges distant from each other, and
+have survived there ever since; they will, also, in all probability have
+become mingled with ancient Alpine species, which must have existed on the
+mountains before the commencement of the Glacial epoch, and which during
+its coldest period will have been temporarily driven down to the plains;
+they will, also, have been exposed to somewhat different climatal
+influences. Their mutual relations will thus have been in some degree
+disturbed; consequently they will have been liable to modification; and
+this we find has been the case; for if we compare the present Alpine plants
+and animals of the several great European mountain-ranges, though very many
+of the species are identically the same, some present varieties, some are
+ranked as doubtful forms, and some few are distinct yet closely allied or
+representative species.
+
+In illustrating what, as I believe, actually took place during the Glacial
+period, I assumed that at its commencement the arctic productions were as
+uniform round the polar regions as they are at the present day. But the
+foregoing remarks on distribution apply not only to strictly arctic forms,
+but also to many sub-arctic and to some few northern temperate forms, for
+some of these are the same on the lower mountains and on the plains of
+North America and Europe; and it may be reasonably asked how I account for
+the necessary degree of uniformity of the sub-arctic and northern temperate
+forms round the world, at the commencement of the Glacial period. At the
+present day, the sub-arctic and northern temperate productions of the Old
+and New Worlds are separated from each other by the {370} Atlantic Ocean
+and by the extreme northern part of the Pacific. During the Glacial period,
+when the inhabitants of the Old and New Worlds lived further southwards
+than at present, they must have been still more completely separated by
+wider spaces of ocean. I believe the above difficulty may be surmounted by
+looking to still earlier changes of climate of an opposite nature. We have
+good reason to believe that during the newer Pliocene period, before the
+Glacial epoch, and whilst the majority of the inhabitants of the world were
+specifically the same as now, the climate was warmer than at the present
+day. Hence we may suppose that the organisms now living under the climate
+of latitude 60°, during the Pliocene period lived further north under the
+Polar Circle, in latitude 66°-67°; and that the strictly arctic productions
+then lived on the broken land still nearer to the pole. Now if we look at a
+globe, we shall see that under the Polar Circle there is almost continuous
+land from western Europe, through Siberia, to eastern America. And to this
+continuity of the circumpolar land, and to the consequent freedom for
+intermigration under a more favourable climate, I attribute the necessary
+amount of uniformity in the sub-arctic and northern temperate productions
+of the Old and New Worlds, at a period anterior to the Glacial epoch.
+
+Believing, from reasons before alluded to, that our continents have long
+remained in nearly the same relative position, though subjected to large,
+but partial oscillations of level, I am strongly inclined to extend the
+above view, and to infer that during some earlier and still warmer period,
+such as the older Pliocene period, a large number of the same plants and
+animals inhabited the almost continuous circumpolar land; and that these
+plants and animals, both in the Old and {371} New Worlds, began slowly to
+migrate southwards as the climate became less warm, long before the
+commencement of the Glacial period. We now see, as I believe, their
+descendants, mostly in a modified condition, in the central parts of Europe
+and the United States. On this view we can understand the relationship,
+with very little identity, between the productions of North America and
+Europe,--a relationship which is most remarkable, considering the distance
+of the two areas, and their separation by the Atlantic Ocean. We can
+further understand the singular fact remarked on by several observers, that
+the productions of Europe and America during the later tertiary stages were
+more closely related to each other than they are at the present time; for
+during these warmer periods the northern parts of the Old and New Worlds
+will have been almost continuously united by land, serving as a bridge,
+since rendered impassable by cold, for the intermigration of their
+inhabitants.
+
+During the slowly decreasing warmth of the Pliocene period, as soon as the
+species in common, which inhabited the New and Old Worlds, migrated south
+of the Polar Circle, they must have been completely cut off from each
+other. This separation, as far as the more temperate productions are
+concerned, took place long ages ago. And as the plants and animals migrated
+southward, they will have become mingled in the one great region with the
+native American productions, and have had to compete with them; and in the
+other great region, with those of the Old World. Consequently we have here
+everything favourable for much modification,--for far more modification
+than with the Alpine productions, left isolated, within a much more recent
+period, on the several mountain-ranges and on the arctic lands of the two
+Worlds. Hence it has come, that when we compare {372} the now living
+productions of the temperate regions of the New and Old Worlds, we find
+very few identical species (though Asa Gray has lately shown that more
+plants are identical than was formerly supposed), but we find in every
+great class many forms, which some naturalists rank as geographical races,
+and others as distinct species; and a host of closely allied or
+representative forms which are ranked by all naturalists as specifically
+distinct.
+
+As on the land, so in the waters of the sea, a slow southern migration of a
+marine fauna, which during the Pliocene or even a somewhat earlier period,
+was nearly uniform along the continuous shores of the Polar Circle, will
+account, on the theory of modification, for many closely allied forms now
+living in areas completely sundered. Thus, I think, we can understand the
+presence of many existing and tertiary representative forms on the eastern
+and western shores of temperate North America; and the still more striking
+case of many closely allied crustaceans (as described in Dana's admirable
+work), of some fish and other marine animals, in the Mediterranean and in
+the seas of Japan,--areas now separated by a continent and by nearly a
+hemisphere of equatorial ocean.
+
+These cases of relationship, without identity, of the inhabitants of seas
+now disjoined, and likewise of the past and present inhabitants of the
+temperate lands of North America and Europe, are inexplicable on the theory
+of creation. We cannot say that they have been created alike, in
+correspondence with the nearly similar physical conditions of the areas;
+for if we compare, for instance, certain parts of South America with the
+southern continents of the Old World, we see countries closely
+corresponding in all their physical conditions, but with their inhabitants
+utterly dissimilar. {373}
+
+But we must return to our more immediate subject, the Glacial period. I am
+convinced that Forbes's view may be largely extended. In Europe we have the
+plainest evidence of the cold period, from the western shores of Britain to
+the Oural range, and southward to the Pyrenees. We may infer from the
+frozen mammals and nature of the mountain vegetation, that Siberia was
+similarly affected. Along the Himalaya, at points 900 miles apart, glaciers
+have left the marks of their former low descent; and in Sikkim, Dr. Hooker
+saw maize growing on gigantic ancient moraines. South of the equator, we
+have some direct evidence of former glacial action in New Zealand; and the
+same plants, found on widely separated mountains in that island, tell the
+same story. If one account which has been published can be trusted, we have
+direct evidence of glacial action in the south-eastern corner of Australia.
+
+Looking to America; in the northern half, ice-borne fragments of rock have
+been observed on the eastern side as far south as lat. 36°-37°, and on the
+shores of the Pacific, where the climate is now so different, as far south
+as lat. 46°; erratic boulders have, also, been noticed on the Rocky
+Mountains. In the Cordillera of Equatorial South America, glaciers once
+extended far below their present level. In central Chili I was astonished
+at the structure of a vast mound of detritus, about 800 feet in height,
+crossing a valley of the Andes; and this I now feel convinced was a
+gigantic moraine, left far below any existing glacier. Further south on
+both sides of the continent, from lat. 41° to the southernmost extremity,
+we have the clearest evidence of former glacial action, in huge boulders
+transported far from their parent source.
+
+We do not know that the Glacial epoch was strictly simultaneous at these
+several far distant points on {374} opposite sides of the world. But we
+have good evidence in almost every case, that the epoch was included within
+the latest geological period. We have, also, excellent evidence, that it
+endured for an enormous time, as measured by years, at each point. The cold
+may have come on, or have ceased, earlier at one point of the globe than at
+another, but seeing that it endured for long at each, and that it was
+contemporaneous in a geological sense, it seems to me probable that it was,
+during a part at least of the period, actually simultaneous throughout the
+world. Without some distinct evidence to the contrary, we may at least
+admit as probable that the glacial action was simultaneous on the eastern
+and western sides of North America, in the Cordillera under the equator and
+under the warmer temperate zones, and on both sides of the southern
+extremity of the continent. If this be admitted, it is difficult to avoid
+believing that the temperature of the whole world was at this period
+simultaneously cooler. But it would suffice for my purpose, if the
+temperature was at the same time lower along certain broad belts of
+longitude.
+
+On this view of the whole world, or at least of broad longitudinal belts,
+having been simultaneously colder from pole to pole, much light can be
+thrown on the present distribution of identical and allied species. In
+America, Dr. Hooker has shown that between forty and fifty of the flowering
+plants of Tierra del Fuego, forming no inconsiderable part of its scanty
+flora, are common to Europe, enormously remote as these two points are; and
+there are many closely allied species. On the lofty mountains of equatorial
+America a host of peculiar species belonging to European genera occur. On
+the highest mountains of Brazil, some few European genera were found by
+Gardner, which do not exist in the wide {375} intervening hot countries. So
+on the Silla of Caraccas the illustrious Humboldt long ago found species
+belonging to genera characteristic of the Cordillera. On the mountains of
+Abyssinia, several European forms and some few representatives of the
+peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope a
+very few European species, believed not to have been introduced by man, and
+on the mountains, some few representative European forms are found, which
+have not been discovered in the intertropical parts of Africa. On the
+Himalaya, and on the isolated mountain-ranges of the peninsula of India, on
+the heights of Ceylon, and on the volcanic cones of Java, many plants
+occur, either identically the same or representing each other, and at the
+same time representing plants of Europe, not found in the intervening hot
+lowlands. A list of the genera collected on the loftier peaks of Java
+raises a picture of a collection made on a hill in Europe! Still more
+striking is the fact that southern Australian forms are clearly represented
+by plants growing on the summits of the mountains of Borneo. Some of these
+Australian forms, as I hear from Dr. Hooker, extend along the heights of
+the peninsula of Malacca, and are thinly scattered, on the one hand over
+India and on the other as far north as Japan.
+
+On the southern mountains of Australia, Dr. F. Müller has discovered
+several European species; other species, not introduced by man, occur on
+the lowlands; and a long list can be given, as I am informed by Dr. Hooker,
+of European genera, found in Australia, but not in the intermediate torrid
+regions. In the admirable 'Introduction to the Flora of New Zealand,' by
+Dr. Hooker, analogous and striking facts are given in regard to the plants
+of that large island. Hence we see that throughout the world, the plants
+growing on the {376} more lofty mountains, and on the temperate lowlands of
+the northern and southern hemispheres, are sometimes identically the same;
+but they are much oftener specifically distinct, though related to each
+other in a most remarkable manner.
+
+This brief abstract applies to plants alone: some strictly analogous facts
+could be given on the distribution of terrestrial animals. In marine
+productions, similar cases occur; as an example, I may quote a remark by
+the highest authority, Prof. Dana, that "it is certainly a wonderful fact
+that New Zealand should have a closer resemblance in its Crustacea to Great
+Britain, its antipode, than to any other part of the world." Sir J.
+Richardson, also, speaks of the reappearance on the shores of New Zealand,
+Tasmania, &c., of northern forms of fish. Dr. Hooker informs me that
+twenty-five species of Algæ are common to New Zealand and to Europe, but
+have not been found in the intermediate tropical seas.
+
+It should be observed that the northern species and forms found in the
+southern parts of the southern hemisphere, and on the mountain-ranges of
+the intertropical regions, are not arctic, but belong to the northern
+temperate zones. As Mr. H. C. Watson has recently remarked, "In receding
+from polar towards equatorial latitudes, the Alpine or mountain floras
+really become less and less arctic." Many of the forms living on the
+mountains of the warmer regions of the earth and in the southern hemisphere
+are of doubtful value, being ranked by some naturalists as specifically
+distinct, by others as varieties; but some are certainly identical, and
+many, though closely related to northern forms, must be ranked as distinct
+species.
+
+Now let us see what light can be thrown on the foregoing facts, on the
+belief, supported as it is by a large {377} body of geological evidence,
+that the whole world, or a large part of it, was during the Glacial period
+simultaneously much colder than at present. The Glacial period, as measured
+by years, must have been very long; and when we remember over what vast
+spaces some naturalised plants and animals have spread within a few
+centuries, this period will have been ample for any amount of migration. As
+the cold came slowly on, all the tropical plants and other productions will
+have retreated from both sides towards the equator, followed in the rear by
+the temperate productions, and these by the arctic; but with the latter we
+are not now concerned. The tropical plants probably suffered much
+extinction; how much no one can say; perhaps formerly the tropics supported
+as many species as we see at the present day crowded together at the Cape
+of Good Hope, and in parts of temperate Australia. As we know that many
+tropical plants and animals can withstand a considerable amount of cold,
+many might have escaped extermination during a moderate fall of
+temperature, more especially by escaping into the lowest, most protected,
+and warmest districts. But the great fact to bear in mind is, that all
+tropical productions will have suffered to a certain extent. On the other
+hand, the temperate productions, after migrating nearer to the equator,
+though they will have been placed under somewhat new conditions, will have
+suffered less. And it is certain that many temperate plants, if protected
+from the inroads of competitors, can withstand a much warmer climate than
+their own. Hence, it seems to me possible, bearing in mind that the
+tropical productions were in a suffering state and could not have presented
+a firm front against intruders, that a certain number of the more vigorous
+and dominant temperate forms might have penetrated the native ranks and
+have reached or {378} even crossed the equator. The invasion would, of
+course, have been greatly favoured by high land, and perhaps by a dry
+climate; for Dr. Falconer informs me that it is the damp with the heat of
+the tropics which is so destructive to perennial plants from a temperate
+climate. On the other hand, the most humid and hottest districts will have
+afforded an asylum to the tropical natives. The mountain-ranges north-west
+of the Himalaya, and the long line of the Cordillera, seem to have afforded
+two great lines of invasion: and it is a striking fact, lately communicated
+to me by Dr. Hooker, that all the flowering plants, about forty-six in
+number, common to Tierra del Fuego and to Europe still exist in North
+America, which must have lain on the line of march. But I do not doubt that
+some temperate productions entered and crossed even the _lowlands_ of the
+tropics at the period when the cold was most intense,--when arctic forms
+had migrated some twenty-five degrees of latitude from their native country
+and covered the land at the foot of the Pyrenees. At this period of extreme
+cold, I believe that the climate under the equator at the level of the sea
+was about the same with that now felt there at the height of six or seven
+thousand feet. During this the coldest period, I suppose that large spaces
+of the tropical lowlands were clothed with a mingled tropical and temperate
+vegetation, like that now growing with strange luxuriance at the base of
+the Himalaya, as graphically described by Hooker.
+
+Thus, as I believe, a considerable number of plants, a few terrestrial
+animals, and some marine productions, migrated during the Glacial period
+from the northern and southern temperate zones into the intertropical
+regions, and some even crossed the equator. As the warmth returned, these
+temperate forms would naturally ascend the higher mountains, being
+exterminated on the {379} lowlands; those which had not reached the equator
+would re-migrate northward or southward towards their former homes; but the
+forms, chiefly northern, which had crossed the equator, would travel still
+further from their homes into the more temperate latitudes of the opposite
+hemisphere. Although we have reason to believe from geological evidence
+that the whole body of arctic shells underwent scarcely any modification
+during their long southern migration and re-migration northward, the case
+may have been wholly different with those intruding forms which settled
+themselves on the intertropical mountains, and in the southern hemisphere.
+These being surrounded by strangers will have had to compete with many new
+forms of life; and it is probable that selected modifications in their
+structure, habits, and constitutions will have profited them. Thus many of
+these wanderers, though still plainly related by inheritance to their
+brethren of the northern or southern hemispheres, now exist in their new
+homes as well-marked varieties or as distinct species.
+
+It is a remarkable fact, strongly insisted on by Hooker in regard to
+America, and by Alph. de Candolle in regard to Australia, that many more
+identical plants and allied forms have apparently migrated from the north
+to the south, than in a reversed direction. We see, however, a few southern
+vegetable forms on the mountains of Borneo and Abyssinia. I suspect that
+this preponderant migration from north to south is due to the greater
+extent of land in the north, and to the northern forms having existed in
+their own homes in greater numbers, and having consequently been advanced
+through natural selection and competition to a higher stage of perfection
+or dominating power, than the southern forms. And thus, when they became
+commingled during the Glacial period, the northern forms {380} were enabled
+to beat the less powerful southern forms. Just in the same manner as we see
+at the present day, that very many European productions cover the ground in
+La Plata, and in a lesser degree in Australia, and have to a certain extent
+beaten the natives; whereas extremely few southern forms have become
+naturalised in any part of Europe, though hides, wool, and other objects
+likely to carry seeds have been largely imported into Europe during the
+last two or three centuries from La Plata, and during the last thirty or
+forty years from Australia. Something of the same kind must have occurred
+on the intertropical mountains: no doubt before the Glacial period they
+were stocked with endemic Alpine forms; but these have almost everywhere
+largely yielded to the more dominant forms, generated in the larger areas
+and more efficient workshops of the north. In many islands the native
+productions are nearly equalled or even outnumbered by the naturalised; and
+if the natives have not been actually exterminated, their numbers have been
+greatly reduced, and this is the first stage towards extinction. A mountain
+is an island on the land; and the intertropical mountains before the
+Glacial period must have been completely isolated; and I believe that the
+productions of these islands on the land yielded to those produced within
+the larger areas of the north, just in the same way as the productions of
+real islands have everywhere lately yielded to continental forms,
+naturalised by man's agency.
+
+I am far from supposing that all difficulties are removed on the view here
+given in regard to the range and affinities of the allied species which
+live in the northern and southern temperate zones and on the mountains of
+the intertropical regions. Very many difficulties remain to be solved. I do
+not pretend to {381} indicate the exact lines and means of migration, or
+the reason why certain species and not others have migrated; why certain
+species have been modified and have given rise to new groups of forms, and
+others have remained unaltered. We cannot hope to explain such facts, until
+we can say why one species and not another becomes naturalised by man's
+agency in a foreign land; why one ranges twice or thrice as far, and is
+twice or thrice as common, as another species within their own homes.
+
+I have said that many difficulties remain to be solved: some of the most
+remarkable are stated with admirable clearness by Dr. Hooker in his
+botanical works on the antarctic regions. These cannot be here discussed. I
+will only say that as far as regards the occurrence of identical species at
+points so enormously remote as Kerguelen Land, New Zealand, and Fuegia, I
+believe that towards the close of the Glacial period, icebergs, as
+suggested by Lyell, have been largely concerned in their dispersal. But the
+existence of several quite distinct species, belonging to genera
+exclusively confined to the south, at these and other distant points of the
+southern hemisphere, is, on my theory of descent with modification, a far
+more remarkable case of difficulty. For some of these species are so
+distinct, that we cannot suppose that there has been time since the
+commencement of the Glacial period for their migration, and for their
+subsequent modification to the necessary degree. The facts seem to me to
+indicate that peculiar and very distinct species have migrated in radiating
+lines from some common centre; and I am inclined to look in the southern,
+as in the northern hemisphere, to a former and warmer period, before the
+commencement of the Glacial period, when the antarctic lands, now covered
+with ice, supported a highly peculiar {382} and isolated flora. I suspect
+that before this flora was exterminated by the Glacial epoch, a few forms
+were widely dispersed to various points of the southern hemisphere by
+occasional means of transport, and by the aid, as halting-places, of
+existing and now sunken islands: By these means, as I believe, the southern
+shores of America, Australia, New Zealand, have become slightly tinted by
+the same peculiar forms of vegetable life.
+
+Sir C. Lyell in a striking passage has speculated, in language almost
+identical with mine, on the effects of great alternations of climate on
+geographical distribution. I believe that the world has recently felt one
+of his great cycles of change; and that on this view, combined with
+modification through natural selection, a multitude of facts in the present
+distribution both of the same and of allied forms of life can be explained.
+The living waters may be said to have flowed during one short period from
+the north and from the south, and to have crossed at the equator; but to
+have flowed with greater force from the north so as to have freely
+inundated the south. As the tide leaves its drift in horizontal lines,
+though rising higher on the shores where the tide rises highest, so have
+the living waters left their living drift on our mountain-summits, in a
+line gently rising from the arctic lowlands to a great height under the
+equator. The various beings thus left stranded may be compared with savage
+races of man, driven up and surviving in the mountain-fastnesses of almost
+every land, which serve as a record, full of interest to us, of the former
+inhabitants of the surrounding lowlands.
+
+       *       *       *       *       *
+
+
+{383}
+
+CHAPTER XII.
+
+GEOGRAPHICAL DISTRIBUTION--_continued_.
+
+    Distribution of fresh-water productions--On the inhabitants of oceanic
+    islands--Absence of Batrachians and of terrestrial Mammals--On the
+    relation of the inhabitants of islands to those of the nearest
+    mainland--On colonisation from the nearest source with subsequent
+    modification--Summary of the last and present chapters.
+
+As lakes and river-systems are separated from each other by barriers of
+land, it might have been thought that fresh-water productions would not
+have ranged widely within the same country, and as the sea is apparently a
+still more impassable barrier, that they never would have extended to
+distant countries. But the case is exactly the reverse. Not only have many
+fresh-water species, belonging to quite different classes, an enormous
+range, but allied species prevail in a remarkable manner throughout the
+world. I well remember, when first collecting in the fresh waters of
+Brazil, feeling much surprise at the similarity of the fresh-water insects,
+shells, &c., and at the dissimilarity of the surrounding terrestrial
+beings, compared with those of Britain.
+
+But this power in fresh-water productions of ranging widely, though so
+unexpected, can, I think, in most cases be explained by their having become
+fitted, in a manner highly useful to them, for short and frequent
+migrations from pond to pond, or from stream to stream; and liability to
+wide dispersal would follow from this capacity as an almost necessary
+consequence. We can here consider only a few cases. In regard to {384}
+fish, I believe that the same species never occur in the fresh waters of
+distant continents. But on the same continent the species often range
+widely and almost capriciously; for two river-systems will have some fish
+in common and some different. A few facts seem to favour the possibility of
+their occasional transport by accidental means; like that of the live fish
+not rarely dropped by whirlwinds in India, and the vitality of their ova
+when removed from the water. But I am inclined to attribute the dispersal
+of fresh-water fish mainly to slight changes within the recent period in
+the level of the land, having caused rivers to flow into each other.
+Instances, also, could be given of this having occurred during floods,
+without any change of level. We have evidence in the loess of the Rhine of
+considerable changes of level in the land within a very recent geological
+period, and when the surface was peopled by existing land and fresh-water
+shells. The wide difference of the fish on opposite sides of continuous
+mountain-ranges, which from an early period must have parted river-systems
+and completely prevented their inosculation, seems to lead to this same
+conclusion. With respect to allied fresh-water fish occurring at very
+distant points of the world, no doubt there are many cases which cannot at
+present be explained: but some fresh-water fish belong to very ancient
+forms, and in such cases there will have been ample time for great
+geographical changes, and consequently time and means for much migration.
+In the second place, salt-water fish can with care be slowly accustomed to
+live in fresh water; and, according to Valenciennes, there is hardly a
+single group of fishes confined exclusively to fresh water, so that we may
+imagine that a marine member of a fresh-water group might travel far along
+the shores of the sea, and {385} subsequently become modified and adapted
+to the fresh waters of a distant land.
+
+Some species of fresh-water shells have a very wide range, and allied
+species, which, on my theory, are descended from a common parent and must
+have proceeded from a single source, prevail throughout the world. Their
+distribution at first perplexed me much, as their ova are not likely to be
+transported by birds, and they are immediately killed by sea-water, as are
+the adults. I could not even understand how some naturalised species have
+rapidly spread throughout the same country. But two facts, which I have
+observed--and no doubt many others remain to be observed--throw some light
+on this subject. When a duck suddenly emerges from a pond covered with
+duck-weed, I have twice seen these little plants adhering to its back; and
+it has happened to me, in removing a little duckweed from one aquarium to
+another, that I have quite unintentionally stocked the one with fresh-water
+shells from the other. But another agency is perhaps more effectual: I
+suspended a duck's feet, which might represent those of a bird sleeping in
+a natural pond, in an aquarium, where many ova of fresh-water shells were
+hatching; and I found that numbers of the extremely minute and just-hatched
+shells crawled on the feet, and clung to them so firmly that when taken out
+of the water they could not be jarred off, though at a somewhat more
+advanced age they would voluntarily drop off. These just hatched molluscs,
+though aquatic in their nature, survived on the duck's feet, in damp air,
+from twelve to twenty hours; and in this length of time a duck or heron
+might fly at least six or seven hundred miles, and would be sure to alight
+on a pool or rivulet, if blown across sea to an oceanic island or to any
+other distant point. Sir Charles Lyell also {386} informs me that a Dyticus
+has been caught with an Ancylus (a fresh-water shell like a limpet) firmly
+adhering to it; and a water-beetle of the same family, a Colymbetes, once
+flew on board the 'Beagle,' when forty-five miles distant from the nearest
+land: how much farther it might have flown with a favouring gale no one can
+tell.
+
+With respect to plants, it has long been known what enormous ranges many
+fresh-water and even marsh-species have, both over continents and to the
+most remote oceanic islands. This is strikingly shown, as remarked by Alph.
+de Candolle, in large groups of terrestrial plants, which have only a very
+few aquatic members; for these latter seem immediately to acquire, as if in
+consequence, a very wide range. I think favourable means of dispersal
+explain this fact. I have before mentioned that earth occasionally, though
+rarely, adheres in some quantity to the feet and beaks of birds. Wading
+birds, which frequent the muddy edges of ponds, if suddenly flushed, would
+be the most likely to have muddy feet. Birds of this order I can show are
+the greatest wanderers, and are occasionally found on the most remote and
+barren islands in the open ocean; they would not be likely to alight on the
+surface of the sea, so that the dirt would not be washed off their feet;
+when making land, they would be sure to fly to their natural fresh-water
+haunts. I do not believe that botanists are aware how charged the mud of
+ponds is with seeds: I have tried several little experiments, but will here
+give only the most striking case: I took in February three table-spoonfuls
+of mud from three different points, beneath water, on the edge of a little
+pond; this mud when dry weighed only 6¾ ounces; I kept it covered up in my
+study for six months, pulling up and counting each plant as it grew; the
+plants were {387} of many kinds, and were altogether 537 in number; and yet
+the viscid mud was all contained in a breakfast cup! Considering these
+facts, I think it would be an inexplicable circumstance if water-birds did
+not transport the seeds of fresh-water plants to vast distances, and if
+consequently the range of these plants was not very great. The same agency
+may have come into play with the eggs of some of the smaller fresh-water
+animals.
+
+Other and unknown agencies probably have also played a part. I have stated
+that fresh-water fish eat some kinds of seeds, though they reject many
+other kinds after having swallowed them; even small fish swallow seeds of
+moderate size, as of the yellow water-lily and Potamogeton. Herons and
+other birds, century after century, have gone on daily devouring fish; they
+then take flight and go to other waters, or are blown across the sea; and
+we have seen that seeds retain their power of germination, when rejected in
+pellets or in excrement, many hours afterwards. When I saw the great size
+of the seeds of that fine water-lily, the Nelumbium, and remembered Alph.
+de Candolle's remarks on this plant, I thought that its distribution must
+remain quite inexplicable; but Audubon states that he found the seeds of
+the great southern water-lily (probably, according to Dr. Hooker, the
+Nelumbium luteum) in a heron's stomach; although I do not know the fact,
+yet analogy makes me believe that a heron flying to another pond and
+getting a hearty meal of fish, would probably reject from its stomach a
+pellet containing the seeds of the Nelumbium undigested; or the seeds might
+be dropped by the bird whilst feeding its young, in the same way as fish
+are known sometimes to be dropped.
+
+In considering these several means of distribution, {388} it should be
+remembered that when a pond or stream is first formed, for instance, on a
+rising islet, it will be unoccupied; and a single seed or egg will have a
+good chance of succeeding. Although there will always be a struggle for
+life between the individuals of the species, however few, already occupying
+any pond, yet as the number of kinds is small, compared with those on the
+land, the competition will probably be less severe between aquatic than
+between terrestrial species; consequently an intruder from the waters of a
+foreign country, would have a better chance of seizing on a place, than in
+the case of terrestrial colonists. We should, also, remember that some,
+perhaps many, freshwater productions are low in the scale of nature, and
+that we have reason to believe that such low beings change or become
+modified less quickly than the high; and this will give longer time than
+the average for the migration of the same aquatic species. We should not
+forget the probability of many species having formerly ranged as
+continuously as fresh-water productions ever can range, over immense areas,
+and having subsequently become extinct in intermediate regions. But the
+wide distribution of fresh-water plants and of the lower animals, whether
+retaining the same identical form or in some degree modified, I believe
+mainly depends on the wide dispersal of their seeds and eggs by animals,
+more especially by fresh-water birds, which have large powers of flight,
+and naturally travel from one to another and often distant piece of water.
+Nature, like a careful gardener, thus takes her seeds from a bed of a
+particular nature, and drops them in another equally well fitted for them.
+
+
+
+_On the Inhabitants of Oceanic Islands._--We now come to the last of the
+three classes of facts, which I {389} have selected as presenting the
+greatest amount of difficulty, on the view that all the individuals both of
+the same and of allied species have descended from a single parent; and
+therefore have all proceeded from a common birthplace, notwithstanding that
+in the course of time they have come to inhabit distant points of the
+globe. I have already stated that I cannot honestly admit Forbes's view on
+continental extensions, which, if legitimately followed out, would lead to
+the belief that within the recent period all existing islands have been
+nearly or quite joined to some continent. This view would remove many
+difficulties, but it would not, I think, explain all the facts in regard to
+insular productions. In the following remarks I shall not confine myself to
+the mere question of dispersal; but shall consider some other facts, which
+bear on the truth of the two theories of independent creation and of
+descent with modification.
+
+The species of all kinds which inhabit oceanic islands are few in number
+compared with those on equal continental areas: Alph. de Candolle admits
+this for plants, and Wollaston for insects. If we look to the large size
+and varied stations of New Zealand, extending over 780 miles of latitude,
+and compare its flowering plants, only 750 in number, with those on an
+equal area at the Cape of Good Hope or in Australia, we must, I think,
+admit that something quite independently of any difference in physical
+conditions has caused so great a difference in number. Even the uniform
+county of Cambridge has 847 plants, and the little island of Anglesea 764,
+but a few ferns and a few introduced plants are included in these numbers,
+and the comparison in some other respects is not quite fair. We have
+evidence that the barren island of Ascension aboriginally possessed under
+half-a-dozen flowering plants; {390} yet many have become naturalised on
+it, as they have on New Zealand and on every other oceanic island which can
+be named. In St. Helena there is reason to believe that the naturalised
+plants and animals have nearly or quite exterminated many native
+productions. He who admits the doctrine of the creation of each separate
+species, will have to admit, that a sufficient number of the best adapted
+plants and animals have not been created on oceanic islands; for man has
+unintentionally stocked them from various sources far more fully and
+perfectly than has nature.
+
+Although in oceanic islands the number of kinds of inhabitants is scanty,
+the proportion of endemic species (_i.e._ those found nowhere else in the
+world) is often extremely large. If we compare, for instance, the number of
+the endemic land-shells in Madeira, or of the endemic birds in the
+Galapagos Archipelago, with the number found on any continent, and then
+compare the area of the islands with that of the continent, we shall see
+that this is true. This fact might have been expected on my theory, for, as
+already explained, species occasionally arriving after long intervals in a
+new and isolated district, and having to compete with new associates, will
+be eminently liable to modification, and will often produce groups of
+modified descendants. But it by no means follows, that, because in an
+island nearly all the species of one class are peculiar, those of another
+class, or of another section of the same class, are peculiar; and this
+difference seems to depend partly on the species which do not become
+modified having immigrated with facility and in a body, so that their
+mutual relations have not been much disturbed; and partly on the frequent
+arrival of unmodified immigrants from the mother-country, and the
+consequent intercrossing with them. With respect to the effects of this
+intercrossing, {391} it should be remembered that the offspring of such
+crosses would almost certainly gain in vigour; so that even an occasional
+cross would produce more effect than might at first have been anticipated.
+To give a few examples: in the Galapagos Islands nearly every land-bird,
+but only two out of the eleven marine birds, are peculiar; and it is
+obvious that marine birds could arrive at these islands more easily than
+land-birds. Bermuda, on the other hand, which lies at about the same
+distance from North America as the Galapagos Islands do from South America,
+and which has a very peculiar soil, does not possess one endemic land-bird;
+and we know from Mr. J. M. Jones's admirable account of Bermuda, that very
+many North American birds, during their great annual migrations, visit
+either periodically or occasionally this island. Madeira does not possess
+one peculiar bird, and many European and African birds are almost every
+year blown there, as I am informed by Mr. E. V. Harcourt. So that these two
+islands of Bermuda and Madeira have been stocked by birds, which for long
+ages have struggled together in their former homes, and have become
+mutually adapted to each other; and when settled in their new homes, each
+kind will have been kept by the others to their proper places and habits,
+and will consequently have been little liable to modification. Any tendency
+to modification will, also, have been checked by intercrossing with the
+unmodified immigrants from the mother-country. Madeira, again, is inhabited
+by a wonderful number of peculiar land-shells, whereas not one species of
+sea-shell is confined to its shores: now, though we do not know how
+sea-shells are dispersed, yet we can see that their eggs or larvae, perhaps
+attached to seaweed or floating timber, or to the feet of wading-birds,
+might be transported far more easily than {392} land-shells, across three
+or four hundred miles of open sea. The different orders of insects in
+Madeira apparently present analogous facts.
+
+Oceanic islands are sometimes deficient in certain classes, and their
+places are apparently occupied by the other inhabitants; in the Galapagos
+Islands reptiles, and in New Zealand gigantic wingless birds, take the
+place of mammals. In the plants of the Galapagos Islands, Dr. Hooker has
+shown that the proportional numbers of the different orders are very
+different from what they are elsewhere. Such cases are generally accounted
+for by the physical conditions of the islands; but this explanation seems
+to me not a little doubtful. Facility of immigration, I believe, has been
+at least as important as the nature of the conditions.
+
+Many remarkable little facts could be given with respect to the inhabitants
+of remote islands. For instance, in certain islands not tenanted by
+mammals, some of the endemic plants have beautifully hooked seeds; yet few
+relations are more striking than the adaptation of hooked seeds for
+transportal by the wool and fur of quadrupeds. This case presents no
+difficulty on my view, for a hooked seed might be transported to an island
+by some other means; and the plant then becoming slightly modified, but
+still retaining its hooked seeds, would form an endemic species, having as
+useless an appendage as any rudimentary organ,--for instance, as the
+shrivelled wings under the soldered elytra of many insular beetles. Again,
+islands often possess trees or bushes belonging to orders which elsewhere
+include only herbaceous species; now trees, as Alph. de Candolle has shown,
+generally have, whatever the cause may be, confined ranges. Hence trees
+would be little likely to reach distant oceanic islands; and an herbaceous
+plant, though it would have no chance of {393} successfully competing in
+stature with a fully developed tree, when established on an island and
+having to compete with herbaceous plants alone, might readily gain an
+advantage by growing taller and taller and overtopping the other plants. If
+so, natural selection would often tend to add to the stature of herbaceous
+plants when growing on an oceanic island, to whatever order they belonged,
+and thus convert them first into bushes and ultimately into trees.
+
+With respect to the absence of whole orders on oceanic islands, Bory St.
+Vincent long ago remarked that Batrachians (frogs, toads, newts) have never
+been found on any of the many islands with which the great oceans are
+studded. I have taken pains to verify this assertion, and I have found it
+strictly true. I have, however, been assured that a frog exists on the
+mountains of the great island of New Zealand; but I suspect that this
+exception (if the information be correct) may be explained through glacial
+agency. This general absence of frogs, toads, and newts on so many oceanic
+islands cannot be accounted for by their physical conditions; indeed it
+seems that islands are peculiarly well fitted for these animals; for frogs
+have been introduced into Madeira, the Azores, and Mauritius, and have
+multiplied so as to become a nuisance. But as these animals and their spawn
+are known to be immediately killed by sea-water, on my view we can see that
+there would be great difficulty in their transportal across the sea, and
+therefore why they do not exist on any oceanic island. But why, on the
+theory of creation, they should not have been created there, it would be
+very difficult to explain.
+
+Mammals offer another and similar case. I have carefully searched the
+oldest voyages, but have not finished my search; as yet I have not found a
+single {394} instance, free from doubt, of a terrestrial mammal (excluding
+domesticated animals kept by the natives) inhabiting an island situated
+above 300 miles from a continent or great continental island; and many
+islands situated at a much less distance are equally barren. The Falkland
+Islands, which are inhabited by a wolf-like fox, come nearest to an
+exception; but this group cannot be considered as oceanic, as it lies on a
+bank connected with the mainland; moreover, icebergs formerly brought
+boulders to its western shores, and they may have formerly transported
+foxes, as so frequently now happens in the arctic regions. Yet it cannot be
+said that small islands will not support small mammals, for they occur in
+many parts of the world on very small islands, if close to a continent; and
+hardly an island can be named on which our smaller quadrupeds have not
+become naturalised and greatly multiplied. It cannot be said, on the
+ordinary view of creation, that there has not been time for the creation of
+mammals; many volcanic islands are sufficiently ancient, as shown by the
+stupendous degradation which they have suffered and by their tertiary
+strata: there has also been time for the production of endemic species
+belonging to other classes; and on continents it is thought that mammals
+appear and disappear at a quicker rate than other and lower animals. Though
+terrestrial mammals do not occur on oceanic islands, aërial mammals do
+occur on almost every island. New Zealand possesses two bats found nowhere
+else in the world: Norfolk Island, the Viti Archipelago, the Bonin Islands,
+the Caroline and Marianne Archipelagoes, and Mauritius, all possess their
+peculiar bats. Why, it may be asked, has the supposed creative force
+produced bats and no other mammals on remote islands? On my view this
+question can easily be answered; for no {395} terrestrial mammal can be
+transported across a wide space of sea, but bats can fly across. Bats have
+been seen wandering by day far over the Atlantic Ocean; and two North
+American species either regularly or occasionally visit Bermuda, at the
+distance of 600 miles from the mainland. I hear from Mr. Tomes, who has
+specially studied this family, that many of the same species have enormous
+ranges, and are found on continents and on far distant islands. Hence we
+have only to suppose that such wandering species have been modified through
+natural selection in their new homes in relation to their new position, and
+we can understand the presence of endemic bats on islands, with the absence
+of all terrestrial mammals.
+
+Besides the absence of terrestrial mammals in relation to the remoteness of
+islands from continents, there is also a relation, to a certain extent
+independent of distance, between the depth of the sea separating an island
+from the neighbouring mainland, and the presence in both of the same
+mammiferous species or of allied species in a more or less modified
+condition. Mr. Windsor Earl has made some striking observations on this
+head in regard to the great Malay Archipelago, which is traversed near
+Celebes by a space of deep ocean; and this space separates two widely
+distinct mammalian faunas. On either side the islands are situated on
+moderately deep submarine banks, and they are inhabited by closely allied
+or identical quadrupeds. No doubt some few anomalies occur in this great
+archipelago, and there is much difficulty in forming a judgment in some
+cases owing to the probable naturalisation of certain mammals through man's
+agency; but we shall soon have much light thrown on the natural history of
+this archipelago by the admirable zeal and researches of Mr. Wallace. I
+have not as yet had time to {396} follow up this subject in all other
+quarters of the world; but as far as I have gone, the relation generally
+holds good. We see Britain separated by a shallow channel from Europe, and
+the mammals are the same on both sides; we meet with analogous facts on
+many islands separated by similar channels from Australia. The West Indian
+Islands stand on a deeply submerged bank, nearly 1000 fathoms in depth, and
+here we find American forms, but the species and even the genera are
+distinct. As the amount of modification in all cases depends to a certain
+degree on the lapse of time, and as during changes of level it is obvious
+that islands separated by shallow channels are more likely to have been
+continuously united within a recent period to the mainland than islands
+separated by deeper channels, we can understand the frequent relation
+between the depth of the sea and the degree of affinity of the mammalian
+inhabitants of islands with those of a neighbouring continent,--an
+inexplicable relation on the view of independent acts of creation.
+
+All the foregoing remarks on the inhabitants of oceanic islands,--namely,
+the scarcity of kinds--the richness in endemic forms in particular classes
+or sections of classes,--the absence of whole groups, as of batrachians,
+and of terrestrial mammals notwithstanding the presence of aërial
+bats,--the singular proportions of certain orders of plants,--herbaceous
+forms having been developed into trees, &c.,--seem to me to accord better
+with the view of occasional means of transport having been largely
+efficient in the long course of time, than with the view of all our oceanic
+islands having been formerly connected by continuous land with the nearest
+continent; for on this latter view the migration would probably have been
+more complete; and if modification be admitted, all the forms of life would
+have been more {397} equally modified, in accordance with the paramount
+importance of the relation of organism to organism.
+
+I do not deny that there are many and grave difficulties in understanding
+how several of the inhabitants of the more remote islands, whether still
+retaining the same specific form or modified since their arrival, could
+have reached their present homes. But the probability of many islands
+having existed as halting-places, of which not a wreck now remains, must
+not be overlooked. I will here give a single instance of one of the cases
+of difficulty. Almost all oceanic islands, even the most isolated and
+smallest, are inhabited by land-shells, generally by endemic species, but
+sometimes by species found elsewhere. Dr. Aug. A. Gould has given several
+interesting cases in regard to the land-shells of the islands of the
+Pacific. Now it is notorious that land-shells are very easily killed by
+salt; their eggs, at least such as I have tried, sink in sea-water and are
+killed by it. Yet there must be, on my view, some unknown, but highly
+efficient means for their transportal. Would the just-hatched young
+occasionally crawl on and adhere to the feet of birds roosting on the
+ground, and thus get transported? It occurred to me that land-shells, when
+hybernating and having a membranous diaphragm over the mouth of the shell,
+might be floated in chinks of drifted timber across moderately wide arms of
+the sea. And I found that several species did in this state withstand
+uninjured an immersion in sea-water during seven days: one of these shells
+was the Helix pomatia, and after it had again hybernated I put it in
+sea-water for twenty days, and it perfectly recovered. As this species has
+a thick calcareous operculum, I removed it, and when it had formed a new
+membranous one, I immersed it for fourteen days in sea-water, and it
+recovered and crawled away: but more experiments are wanted on this head.
+{398}
+
+The most striking and important fact for us in regard to the inhabitants of
+islands, is their affinity to those of the nearest mainland, without being
+actually the same species. Numerous instances could be given of this fact.
+I will give only one, that of the Galapagos Archipelago, situated under the
+equator, between 500 and 600 miles from the shores of South America. Here
+almost every product of the land and water bears the unmistakeable stamp of
+the American continent. There are twenty-six land-birds, and twenty-five of
+these are ranked by Mr. Gould as distinct species, supposed to have been
+created here; yet the close affinity of most of these birds to American
+species in every character, in their habits, gestures, and tones of voice,
+was manifest. So it is with the other animals, and with nearly all the
+plants, as shown by Dr. Hooker in his admirable memoir on the Flora of this
+archipelago. The naturalist, looking at the inhabitants of these volcanic
+islands in the Pacific, distant several hundred miles from the continent,
+yet feels that he is standing on American land. Why should this be so? why
+should the species which are supposed to have been created in the Galapagos
+Archipelago, and nowhere else, bear so plain a stamp of affinity to those
+created in America? There is nothing in the conditions of life, in the
+geological nature of the islands, in their height or climate, or in the
+proportions in which the several classes are associated together, which
+resembles closely the conditions of the South American coast: in fact there
+is a considerable dissimilarity in all these respects. On the other hand,
+there is a considerable degree of resemblance in the volcanic nature of the
+soil, in climate, height, and size of the islands, between the Galapagos
+and Cape de Verde Archipelagos: but what an entire and absolute difference
+in their inhabitants! The inhabitants of the Cape de Verde Islands are
+related to {399} those of Africa, like those of the Galapagos to America. I
+believe this grand fact can receive no sort of explanation on the ordinary
+view of independent creation; whereas on the view here maintained, it is
+obvious that the Galapagos Islands would be likely to receive colonists,
+whether by occasional means of transport or by formerly continuous land,
+from America; and the Cape de Verde Islands from Africa; and that such
+colonists would be liable to modification;--the principle of inheritance
+still betraying their original birthplace.
+
+Many analogous facts could be given: indeed it is an almost universal rule
+that the endemic productions of islands are related to those of the nearest
+continent, or of other near islands. The exceptions are few, and most of
+them can be explained. Thus the plants of Kerguelen Land, though standing
+nearer to Africa than to America, are related, and that very closely, as we
+know from Dr. Hooker's account, to those of America: but on the view that
+this island has been mainly stocked by seeds brought with earth and stones
+on icebergs, drifted by the prevailing currents, this anomaly disappears.
+New Zealand in its endemic plants is much more closely related to
+Australia, the nearest mainland, than to any other region: and this is what
+might have been expected; but it is also plainly related to South America,
+which, although the next nearest continent, is so enormously remote, that
+the fact becomes an anomaly. But this difficulty almost disappears on the
+view that both New Zealand, South America, and other southern lands were
+long ago partially stocked from a nearly intermediate though distant point,
+namely from the antarctic islands, when they were clothed with vegetation,
+before the commencement of the Glacial period. The affinity, which, though
+feeble, I am assured by Dr. Hooker is real, between the flora of the
+south-western corner of Australia and of the Cape of Good {400} Hope, is a
+far more remarkable case, and is at present inexplicable: but this affinity
+is confined to the plants, and will, I do not doubt, be some day explained.
+
+The law which causes the inhabitants of an archipelago, though specifically
+distinct, to be closely allied to those of the nearest continent, we
+sometimes see displayed on a small scale, yet in a most interesting manner,
+within the limits of the same archipelago. Thus the several islands of the
+Galapagos Archipelago are tenanted, as I have elsewhere shown, in a quite
+marvellous manner, by very closely related species; so that the inhabitants
+of each separate island, though mostly distinct, are related in an
+incomparably closer degree to each other than to the inhabitants of any
+other part of the world. And this is just what might have been expected on
+my view, for the islands are situated so near each other that they would
+almost certainly receive immigrants from the same original source, or from
+each other. But this dissimilarity between the endemic inhabitants of the
+islands may be used as an argument against my views; for it may be asked,
+how has it happened in the several islands situated within sight of each
+other, having the same geological nature, the same height, climate, &c.,
+that many of the immigrants should have been differently modified, though
+only in a small degree. This long appeared to me a great difficulty: but it
+arises in chief part from the deeply-seated error of considering the
+physical conditions of a country as the most important for its inhabitants;
+whereas it cannot, I think, be disputed that the nature of the other
+inhabitants, with which each has to compete, is as least as important, and
+generally a far more important element of success. Now if we look to those
+inhabitants of the Galapagos Archipelago which are found in other parts of
+the world (laying on one side for the moment the {401} endemic species,
+which cannot be here fairly included, as we are considering how they have
+come to be modified since their arrival), we find a considerable amount of
+difference in the several islands. This difference might indeed have been
+expected on the view of the islands having been stocked by occasional means
+of transport--a seed, for instance, of one plant having been brought to one
+island, and that of another plant to another island. Hence when in former
+times an immigrant settled on any one or more of the islands, or when it
+subsequently spread from one island to another, it would undoubtedly be
+exposed to different conditions of life in the different islands, for it
+would have to compete with different sets of organisms: a plant for
+instance, would find the best-fitted ground more perfectly occupied by
+distinct plants in one island than in another, and it would be exposed to
+the attacks of somewhat different enemies. If then it varied, natural
+selection would probably favour different varieties in the different
+islands. Some species, however, might spread and yet retain the same
+character throughout the group, just as we see on continents some species
+spreading widely and remaining the same.
+
+The really surprising fact in this case of the Galapagos Archipelago, and
+in a lesser degree in some analogous instances, is that the new species
+formed in the separate islands have not quickly spread to the other
+islands. But the islands, though in sight of each other, are separated by
+deep arms of the sea, in most cases wider than the British Channel, and
+there is no reason to suppose that they have at any former period been
+continuously united. The currents of the sea are rapid and sweep across the
+archipelago, and gales of wind are extraordinarily rare; so that the
+islands are far more effectually separated from each other than they appear
+to be on a map. Nevertheless a good many {402} species, both those found in
+other parts of the world and those confined to the archipelago, are common
+to the several islands, and we may infer from certain facts that these have
+probably spread from some one island to the others. But we often take, I
+think, an erroneous view of the probability of closely-allied species
+invading each other's territory, when put into free intercommunication.
+Undoubtedly if one species has any advantage whatever over another, it will
+in a very brief time wholly or in part supplant it; but if both are equally
+well fitted for their own places in nature, both probably will hold their
+own places and keep separate for almost any length of time. Being familiar
+with the fact that many species, naturalised through man's agency, have
+spread with astonishing rapidity over new countries, we are apt to infer
+that most species would thus spread; but we should remember that the forms
+which become naturalised in new countries are not generally closely allied
+to the aboriginal inhabitants, but are very distinct species, belonging in
+a large proportion of cases, as shown by Alph. de Candolle, to distinct
+genera. In the Galapagos Archipelago, many even of the birds, though so
+well adapted for flying from island to island, are distinct on each; thus
+there are three closely-allied species of mocking-thrush, each confined to
+its own island. Now let us suppose the mocking-thrush of Chatham Island to
+be blown to Charles Island, which has its own mocking-thrush: why should it
+succeed in establishing itself there? We may safely infer that Charles
+Island is well stocked with its own species, for annually more eggs are
+laid there than can possibly be reared; and we may infer that the
+mocking-thrush peculiar to Charles Island is at least as well fitted for
+its home as is the species peculiar to Chatham Island. Sir C. Lyell and Mr.
+Wollaston have communicated to me a remarkable fact bearing on this {403}
+subject; namely, that Madeira and the adjoining islet of Porto Santo
+possess many distinct but representative land-shells, some of which live in
+crevices of stone; and although large quantities of stone are annually
+transported from Porto Santo to Madeira, yet this latter island has not
+become colonised by the Porto Santo species: nevertheless both islands have
+been colonised by some European land-shells, which no doubt had some
+advantage over the indigenous species. From these considerations I think we
+need not greatly marvel at the endemic and representative species, which
+inhabit the several islands of the Galapagos Archipelago, not having
+universally spread from island to island. In many other instances, as in
+the several districts of the same continent, pre-occupation has probably
+played an important part in checking the commingling of species under the
+same conditions of life. Thus, the south-east and south-west corners of
+Australia have nearly the same physical conditions, and are united by
+continuous land, yet they are inhabited by a vast number of distinct
+mammals, birds, and plants.
+
+The principle which determines the general character of the fauna and flora
+of oceanic islands, namely, that the inhabitants, when not identically the
+same, yet are plainly related to the inhabitants of that region whence
+colonists could most readily have been derived,--the colonists having been
+subsequently modified and better fitted to their new homes,--is of the
+widest application throughout nature. We see this on every mountain, in
+every lake and marsh. For Alpine species, excepting in so far as the same
+forms, chiefly of plants, have spread widely throughout the world during
+the recent Glacial epoch, are related to those of the surrounding
+lowlands;--thus we have in South America, Alpine humming-birds, Alpine
+rodents, Alpine plants, {404} &c., all of strictly American forms, and it
+is obvious that a mountain, as it became slowly upheaved, would naturally
+be colonised from the surrounding lowlands. So it is with the inhabitants
+of lakes and marshes, excepting in so far as great facility of transport
+has given the same general forms to the whole world. We see this same
+principle in the blind animals inhabiting the caves of America and of
+Europe. Other analogous facts could be given. And it will, I believe, be
+universally found to be true, that wherever in two regions, let them be
+ever so distant, many closely-allied or representative species occur, there
+will likewise be found some identical species, showing, in accordance with
+the foregoing view, that at some former period there has been
+intercommunication or migration between the two regions. And wherever many
+closely-allied species occur, there will be found many forms which some
+naturalists rank as distinct species, and some as varieties; these doubtful
+forms showing us the steps in the process of modification.
+
+This relation between the power and extent of migration of a species,
+either at the present time or at some former period under different
+physical conditions, and the existence at remote points of the world of
+other species allied to it, is shown in another and more general way. Mr.
+Gould remarked to me long ago, that in those genera of birds which range
+over the world, many of the species have very wide ranges. I can hardly
+doubt that this rule is generally true, though it would be difficult to
+prove it. Amongst mammals, we see it strikingly displayed in Bats, and in a
+lesser degree in the Felidæ and Canidæ. We see it, if we compare the
+distribution of butterflies and beetles. So it is with most fresh-water
+productions, in which so many genera range over the world, and many
+individual species have {405} enormous ranges. It is not meant that in
+world-ranging genera all the species have a wide range, or even that they
+have on an _average_ a wide range; but only that some of the species range
+very widely; for the facility with which widely-ranging species vary and
+give rise to new forms will largely determine their average range. For
+instance, two varieties of the same species inhabit America and Europe, and
+the species thus has an immense range; but, if the variation had been a
+little greater, the two varieties would have been ranked as distinct
+species, and the common range would have been greatly reduced. Still less
+is it meant, that a species which apparently has the capacity of crossing
+barriers and ranging widely, as in the case of certain powerfully-winged
+birds, will necessarily range widely; for we should never forget that to
+range widely implies not only the power of crossing barriers, but the more
+important power of being victorious in distant lands in the struggle for
+life with foreign associates. But on the view of all the species of a genus
+having descended from a single parent, though now distributed to the most
+remote points of the world, we ought to find, and I believe as a general
+rule we do find, that some at least of the species range very widely; for
+it is necessary that the unmodified parent should range widely, undergoing
+modification during its diffusion, and should place itself under diverse
+conditions favourable for the conversion of its offspring, firstly into new
+varieties and ultimately into new species.
+
+In considering the wide distribution of certain genera, we should bear in
+mind that some are extremely ancient, and must have branched off from a
+common parent at a remote epoch; so that in such cases there will have been
+ample time for great climatal and geographical changes and for accidents of
+transport; and consequently for the migration of some of the species into
+all {406} quarters of the world, where they may have become slightly
+modified in relation to their new conditions. There is, also, some reason
+to believe from geological evidence that organisms low in the scale within
+each great class, generally change at a slower rate than the higher forms;
+and consequently the lower forms will have had a better chance of ranging
+widely and of still retaining the same specific character. This fact,
+together with the seeds and eggs of many low forms being very minute and
+better fitted for distant transportation, probably accounts for a law which
+has long been observed, and which has lately been admirably discussed by
+Alph. de Candolle in regard to plants, namely, that the lower any group of
+organisms is, the more widely it is apt to range.
+
+The relations just discussed,--namely, low and slowly-changing organisms
+ranging more widely than the high,--some of the species of widely-ranging
+genera themselves ranging widely,--such facts, as alpine, lacustrine, and
+marsh productions being related (with the exceptions before specified) to
+those on the surrounding low lands and dry lands, though these stations are
+so different,--the very close relation of the distinct species which
+inhabit the islets of the same archipelago,--and especially the striking
+relation of the inhabitants of each whole archipelago or island to those of
+the nearest mainland,--are, I think, utterly inexplicable on the ordinary
+view of the independent creation of each species, but are explicable on the
+view of colonisation from the nearest or readiest source, together with the
+subsequent modification and better adaptation of the colonists to their new
+homes.
+
+
+
+_Summary of last and present Chapters._--In these chapters I have
+endeavoured to show, that if we make due allowance for our ignorance of the
+full effects of all {407} the changes of climate and of the level of the
+land, which have certainly occurred within the recent period, and of other
+similar changes which may have occurred within the same period; if we
+remember how profoundly ignorant we are with respect to the many and
+curious means of occasional transport,--a subject which has hardly ever
+been properly experimentised on; if we bear in mind how often a species may
+have ranged continuously over a wide area, and then have become extinct in
+the intermediate tracts, I think the difficulties in believing that all the
+individuals of the same species, wherever located, have descended from the
+same parents, are not insuperable. And we are led to this conclusion, which
+has been arrived at by many naturalists under the designation of single
+centres of creation, by some general considerations, more especially from
+the importance of barriers and from the analogical distribution of
+sub-genera, genera, and families.
+
+With respect to the distinct species of the same genus, which on my theory
+must have spread from one parent-source; if we make the same allowances as
+before for our ignorance, and remember that some forms of life change most
+slowly, enormous periods of time being thus granted for their migration, I
+do not think that the difficulties are insuperable; though they often are
+in this case, and in that of the individuals of the same species, extremely
+great.
+
+As exemplifying the effects of climatal changes on distribution, I have
+attempted to show how important has been the influence of the modern
+Glacial period, which I am fully convinced simultaneously affected the
+whole world, or at least great meridional belts. As showing how diversified
+are the means of occasional transport, I have discussed at some little
+length the means of dispersal of fresh-water productions. {408}
+
+If the difficulties be not insuperable in admitting that in the long course
+of time the individuals of the same species, and likewise of allied
+species, have proceeded from some one source; then I think all the grand
+leading facts of geographical distribution are explicable on the theory of
+migration (generally of the more dominant forms of life), together with
+subsequent modification and the multiplication of new forms. We can thus
+understand the high importance of barriers, whether of land or water, which
+separate our several zoological and botanical provinces. We can thus
+understand the localisation of sub-genera, genera, and families; and how it
+is that under different latitudes, for instance in South America, the
+inhabitants of the plains and mountains, of the forests, marshes, and
+deserts, are in so mysterious a manner linked together by affinity, and are
+likewise linked to the extinct beings which formerly inhabited the same
+continent. Bearing in mind that the mutual relation of organism to organism
+is of the highest importance, we can see why two areas having nearly the
+same physical conditions should often be inhabited by very different forms
+of life; for according to the length of time which has elapsed since new
+inhabitants entered one region; according to the nature of the
+communication which allowed certain forms and not others to enter, either
+in greater or lesser numbers; according or not, as those which entered
+happened to come in more or less direct competition with each other and
+with the aborigines; and according as the immigrants were capable of
+varying more or less rapidly, there would ensue in different regions,
+independently of their physical conditions, infinitely diversified
+conditions of life,--there would be an almost endless amount of organic
+action and reaction,--and we should find, as we do find, some groups of
+beings greatly, and some only slightly modified,--some {409} developed in
+great force, some existing in scanty numbers--in the different great
+geographical provinces of the world.
+
+On these same principles, we can understand, as I have endeavoured to show,
+why oceanic islands should have few inhabitants, but of these a great
+number should be endemic or peculiar; and why, in relation to the means of
+migration, one group of beings, even within the same class, should have all
+its species endemic, and another group should have all its species common
+to other quarters of the world. We can see why whole groups of organisms,
+as batrachians and terrestrial mammals, should be absent from oceanic
+islands, whilst the most isolated islands possess their own peculiar
+species of aërial mammals or bats. We can see why there should be some
+relation between the presence of mammals, in a more or less modified
+condition, and the depth of the sea between an island and the mainland. We
+can clearly see why all the inhabitants of an archipelago, though
+specifically distinct on the several islets, should be closely related to
+each other, and likewise be related, but less closely, to those of the
+nearest continent or other source whence immigrants were probably derived.
+We can see why in two areas, however distant from each other, there should
+be a correlation, in the presence of identical species, of varieties, of
+doubtful species, and of distinct but representative species.
+
+As the late Edward Forbes often insisted, there is a striking parallelism
+in the laws of life throughout time and space: the laws governing the
+succession of forms in past times being nearly the same with those
+governing at the present time the differences in different areas. We see
+this in many facts. The endurance of each species and group of species is
+continuous in time; for the exceptions to the rule are so few, that they
+may {410} fairly be attributed to our not having as yet discovered in an
+intermediate deposit the forms which are therein absent, but which occur
+above and below: so in space, it certainly is the general rule that the
+area inhabited by a single species, or by a group of species, is
+continuous; and the exceptions, which are not rare, may, as I have
+attempted to show, be accounted for by migration at some former period
+under different conditions or by occasional means of transport, and by the
+species having become extinct in the intermediate tracts. Both in time and
+space, species and groups of species have their points of maximum
+development. Groups of species, belonging either to a certain period of
+time, or to a certain area, are often characterised by trifling characters
+in common, as of sculpture or colour. In looking to the long succession of
+ages, as in now looking to distant provinces throughout the world, we find
+that some organisms differ little, whilst others belonging to a different
+class, or to a different order, or even only to a different family of the
+same order, differ greatly. In both time and space the lower members of
+each class generally change less than the higher; but there are in both
+cases marked exceptions to the rule. On my theory these several relations
+throughout time and space are intelligible; for whether we look to the
+forms of life which have changed during successive ages within the same
+quarter of the world, or to those which have changed after having migrated
+into distant quarters, in both cases the forms within each class have been
+connected by the same bond of ordinary generation; and the more nearly any
+two forms are related in blood, the nearer they will generally stand to
+each other in time and space; in both cases the laws of variation have been
+the same, and modifications have been accumulated by the same power of
+natural selection.
+
+       *       *       *       *       *
+
+
+{411}
+
+CHAPTER XIII.
+
+MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY
+ORGANS.
+
+    CLASSIFICATION, groups subordinate to groups--Natural system--Rules and
+    difficulties in classification, explained on the theory of descent with
+    modification--Classification of varieties--Descent always used in
+    classification--Analogical or adaptive characters--Affinities, general,
+    complex and radiating--Extinction separates and defines
+    groups--MORPHOLOGY, between members of the same class, between parts of
+    the same individual--EMBRYOLOGY, laws of, explained by variations not
+    supervening at an early age, and being inherited at a corresponding
+    age--RUDIMENTARY ORGANS; their origin explained--Summary.
+
+From the first dawn of life, all organic beings are found to resemble each
+other in descending degrees, so that they can be classed in groups under
+groups. This classification is evidently not arbitrary like the grouping of
+the stars in constellations. The existence of groups would have been of
+simple signification, if one group had been exclusively fitted to inhabit
+the land, and another the water; one to feed on flesh, another on vegetable
+matter, and so on; but the case is widely different in nature; for it is
+notorious how commonly members of even the same sub-group have different
+habits. In our second and fourth chapters, on Variation and on Natural
+Selection, I have attempted to show that it is the widely ranging, the much
+diffused and common, that is the dominant species belonging to the larger
+genera, which vary most. The varieties, or incipient species, thus produced
+ultimately become converted, as I believe, into new and distinct species;
+and these, on the principle of inheritance, tend to produce other new and
+dominant {412} species. Consequently the groups which are now large, and
+which generally include many dominant species, tend to go on increasing
+indefinitely in size. I further attempted to show that from the varying
+descendants of each species trying to occupy as many and as different
+places as possible in the economy of nature, there is a constant tendency
+in their characters to diverge. This conclusion was supported by looking at
+the great diversity of the forms of life which, in any small area, come
+into the closest competition, and by looking to certain facts in
+naturalisation.
+
+I attempted also to show that there is a constant tendency in the forms
+which are increasing in number and diverging in character, to supplant and
+exterminate the less divergent, the less improved, and preceding forms. I
+request the reader to turn to the diagram illustrating the action, as
+formerly explained, of these several principles; and he will see that the
+inevitable result is that the modified descendants proceeding from one
+progenitor become broken up into groups subordinate to groups. In the
+diagram each letter on the uppermost line may represent a genus including
+several species; and all the genera on this line form together one class,
+for all have descended from one ancient but unseen parent, and,
+consequently, have inherited something in common. But the three genera on
+the left hand have, on this same principle, much in common, and form a
+sub-family, distinct from that including the next two genera on the right
+hand, which diverged from a common parent at the fifth stage of descent.
+These five genera have also much, though less, in common; and they form a
+family distinct from that including the three genera still further to the
+right hand, which diverged at a still earlier period. And all these genera,
+descended from (A), form an order distinct from the {413} genera descended
+from (I). So that we here have many species descended from a single
+progenitor grouped into genera; and the genera are included in, or
+subordinate to, sub-families, families, and orders, all united into one
+class. Thus, the grand fact in natural history of the subordination of
+group under group, which, from its familiarity, does not always
+sufficiently strike us, is in my judgment explained.
+
+Naturalists try to arrange the species, genera, and families in each class,
+on what is called the Natural System. But what is meant by this system?
+Some authors look at it merely as a scheme for arranging together those
+living objects which are most alike, and for separating those which are
+most unlike; or as an artificial means for enunciating, as briefly as
+possible, general propositions,--that is, by one sentence to give the
+characters common, for instance, to all mammals, by another those common to
+all carnivora, by another those common to the dog-genus, and then by adding
+a single sentence, a full description is given of each kind of dog. The
+ingenuity and utility of this system are indisputable. But many naturalists
+think that something more is meant by the Natural System; they believe that
+it reveals the plan of the Creator; but unless it be specified whether
+order in time or space, or what else is meant by the plan of the Creator,
+it seems to me that nothing is thus added to our knowledge. Such
+expressions as that famous one of Linnæus, and which we often meet with in
+a more or less concealed form, that the characters do not make the genus,
+but that the genus gives the characters, seem to imply that something more
+is included in our classification, than mere resemblance. I believe that
+something more is included; and that propinquity of descent,--the only
+known cause of the similarity of organic beings,--is the bond, hidden as it
+is by various degrees of {414} modification, which is partially revealed to
+us by our classifications.
+
+Let us now consider the rules followed in classification, and the
+difficulties which are encountered on the view that classification either
+gives some unknown plan of creation, or is simply a scheme for enunciating
+general propositions and of placing together the forms most like each
+other. It might have been thought (and was in ancient times thought) that
+those parts of the structure which determined the habits of life, and the
+general place of each being in the economy of nature, would be of very high
+importance in classification. Nothing can be more false. No one regards the
+external similarity of a mouse to a shrew, of a dugong to a whale, of a
+whale to a fish, as of any importance. These resemblances, though so
+intimately connected with the whole life of the being, are ranked as merely
+"adaptive or analogical characters;" but to the consideration of these
+resemblances we shall have to recur. It may even be given as a general
+rule, that the less any part of the organisation is concerned with special
+habits, the more important it becomes for classification. As an instance:
+Owen, in speaking of the dugong, says, "The generative organs being those
+which are most remotely related to the habits and food of an animal, I have
+always regarded as affording very clear indications of its true affinities.
+We are least likely in the modifications of these organs to mistake a
+merely adaptive for an essential character." So with plants, how remarkable
+it is that the organs of vegetation, on which their whole life depends, are
+of little signification, excepting in the first main divisions; whereas the
+organs of reproduction, with their product the seed, are of paramount
+importance!
+
+We must not, therefore, in classifying, trust to resemblances in parts of
+the organisation, however important {415} they may be for the welfare of
+the being in relation to the outer world. Perhaps from this cause it has
+partly arisen, that almost all naturalists lay the greatest stress on
+resemblances in organs of high vital or physiological importance. No doubt
+this view of the classificatory importance of organs which are important is
+generally, but by no means always, true. But their importance for
+classification, I believe, depends on their greater constancy throughout
+large groups of species; and this constancy depends on such organs having
+generally been subjected to less change in the adaptation of the species to
+their conditions of life. That the mere physiological importance of an
+organ does not determine its classificatory value, is almost shown by the
+one fact, that in allied groups, in which the same organ, as we have every
+reason to suppose, has nearly the same physiological value, its
+classificatory value is widely different. No naturalist can have worked at
+any group without being struck with this fact; and it has been fully
+acknowledged in the writings of almost every author. It will suffice to
+quote the highest authority, Robert Brown, who in speaking of certain
+organs in the Proteaceæ, says their generic importance, "like that of all
+their parts, not only in this but, as I apprehend, in every natural family,
+is very unequal, and in some cases seems to be entirely lost." Again in
+another work he says, the genera of the Connaraceæ "differ in having one or
+more ovaria, in the existence or absence of albumen, in the imbricate or
+valvular æstivation. Any one of these characters singly is frequently of
+more than generic importance, though here even when all taken together they
+appear insufficient to separate Cnestis from Connarus." To give an example
+amongst insects, in one great division of the Hymenoptera, the antennæ, as
+Westwood has remarked, are most constant in structure; {416} in another
+division they differ much, and the differences are of quite subordinate
+value in classification; yet no one probably will say that the antennae in
+these two divisions of the same order are of unequal physiological
+importance. Any number of instances could be given of the varying
+importance for classification of the same important organ within the same
+group of beings.
+
+Again, no one will say that rudimentary or atrophied organs are of high
+physiological or vital importance; yet, undoubtedly, organs in this
+condition are often of high value in classification. No one will dispute
+that the rudimentary teeth in the upper jaws of young ruminants, and
+certain rudimentary bones of the leg, are highly serviceable in exhibiting
+the close affinity between Ruminants and Pachyderms. Robert Brown has
+strongly insisted on the fact that the rudimentary florets are of the
+highest importance in the classification of the Grasses.
+
+Numerous instances could be given of characters derived from parts which
+must be considered of very trifling physiological importance, but which are
+universally admitted as highly serviceable in the definition of whole
+groups. For instance, whether or not there is an open passage from the
+nostrils to the mouth, the only character, according to Owen, which
+absolutely distinguishes fishes and reptiles--the inflection of the angle
+of the jaws in Marsupials--the manner in which the wings of insects are
+folded--mere colour in certain Algæ--mere pubescence on parts of the flower
+in grasses--the nature of the dermal covering, as hair or feathers, in the
+Vertebrata. If the Ornithorhynchus had been covered with feathers instead
+of hair, this external and trifling character would, I think, have been
+considered by naturalists as important an aid in determining the degree of
+affinity of this strange creature to {417} birds and reptiles, as an
+approach in structure in any one internal and important organ.
+
+The importance, for classification, of trifling characters, mainly depends
+on their being correlated with several other characters of more or less
+importance. The value indeed of an aggregate of characters is very evident
+in natural history. Hence, as has often been remarked, a species may depart
+from its allies in several characters, both of high physiological
+importance and of almost universal prevalence, and yet leave us in no doubt
+where it should be ranked. Hence, also, it has been found, that a
+classification founded on any single character, however important that may
+be, has always failed; for no part of the organisation is universally
+constant. The importance of an aggregate of characters, even when none are
+important, alone explains, I think, that saying of Linnæus, that the
+characters do not give the genus, but the genus gives the characters; for
+this saying seems founded on an appreciation of many trifling points of
+resemblance, too slight to be defined. Certain plants, belonging to the
+Malpighiaceæ, bear perfect and degraded flowers; in the latter, as A. de
+Jussieu has remarked, "the greater number of the characters proper to the
+species, to the genus, to the family, to the class, disappear, and thus
+laugh at our classification." But when Aspicarpa produced in France, during
+several years, only degraded flowers, departing so wonderfully in a number
+of the most important points of structure from the proper type of the
+order, yet M. Richard sagaciously saw, as Jussieu observes, that this genus
+should still be retained amongst the Malpighiaceæ. This case seems to me
+well to illustrate the spirit with which our classifications are sometimes
+necessarily founded.
+
+Practically when naturalists are at work, they do {418} not trouble
+themselves about the physiological value of the characters which they use
+in defining a group, or in allocating any particular species. If they find
+a character nearly uniform, and common to a great number of forms, and not
+common to others, they use it as one of high value; if common to some
+lesser number, they use it as of subordinate value. This principle has been
+broadly confessed by some naturalists to be the true one; and by none more
+clearly than by that excellent botanist, Aug. St. Hilaire. If certain
+characters are always found correlated with others, though no apparent bond
+of connexion can be discovered between them, especial value is set on them.
+As in most groups of animals, important organs, such as those for
+propelling the blood, or for aërating it, or those for propagating the
+race, are found nearly uniform, they are considered as highly serviceable
+in classification; but in some groups of animals all these, the most
+important vital organs, are found to offer characters of quite subordinate
+value.
+
+We can see why characters derived from the embryo should be of equal
+importance with those derived from the adult, for our classifications of
+course include all ages of each species. But it is by no means obvious, on
+the ordinary view, why the structure of the embryo should be more important
+for this purpose than that of the adult, which alone plays its full part in
+the economy of nature. Yet it has been strongly urged by those great
+naturalists, Milne Edwards and Agassiz, that embryonic characters are the
+most important of any in the classification of animals; and this doctrine
+has very generally been admitted as true. The same fact holds good with
+flowering plants, of which the two main divisions have been founded on
+characters derived from the embryo,--on the number and position of the
+{419} embryonic leaves or cotyledons, and on the mode of development of the
+plumule and radicle. In our discussion on embryology, we shall see why such
+characters are so valuable, on the view of classification tacitly including
+the idea of descent.
+
+Our classifications are often plainly influenced by chains of affinities.
+Nothing can be easier than to define a number of characters common to all
+birds; but in the case of crustaceans, such definition has hitherto been
+found impossible. There are crustaceans at the opposite ends of the series,
+which have hardly a character in common; yet the species at both ends, from
+being plainly allied to others, and these to others, and so onwards, can be
+recognised as unequivocally belonging to this, and to no other class of the
+Articulata.
+
+Geographical distribution has often been used, though perhaps not quite
+logically, in classification, more especially in very large groups of
+closely allied forms. Temminck insists on the utility or even necessity of
+this practice in certain groups of birds; and it has been followed by
+several entomologists and botanists.
+
+Finally, with respect to the comparative value of the various groups of
+species, such as orders, sub-orders, families, sub-families, and genera,
+they seem to be, at least at present, almost arbitrary. Several of the best
+botanists, such as Mr. Bentham and others, have strongly insisted on their
+arbitrary value. Instances could be given amongst plants and insects, of a
+group of forms, first ranked by practised naturalists as only a genus, and
+then raised to the rank of a sub-family or family; and this has been done,
+not because further research has detected important structural differences,
+at first overlooked, but because numerous allied species, with slightly
+different grades of difference, have been subsequently discovered. {420}
+
+All the foregoing rules and aids and difficulties in classification are
+explained, if I do not greatly deceive myself, on the view that the natural
+system is founded on descent with modification; that the characters which
+naturalists consider as showing true affinity between any two or more
+species, are those which have been inherited from a common parent, and, in
+so far, all true classification is genealogical; that community of descent
+is the hidden bond which naturalists have been unconsciously seeking, and
+not some unknown plan of creation, or the enunciation of general
+propositions, and the mere putting together and separating objects more or
+less alike.
+
+But I must explain my meaning more fully. I believe that the _arrangement_
+of the groups within each class, in due subordination and relation to the
+other groups, must be strictly genealogical in order to be natural; but
+that the _amount_ of difference in the several branches or groups, though
+allied in the same degree in blood to their common progenitor, may differ
+greatly, being due to the different degrees of modification which they have
+undergone; and this is expressed by the forms being ranked under different
+genera, families, sections, or orders. The reader will best understand what
+is meant, if he will take the trouble of referring to the diagram in the
+fourth chapter. We will suppose the letters A to L to represent allied
+genera, which lived during the Silurian epoch, and these have descended
+from a species which existed at an unknown anterior period. Species of
+three of these genera (A, F, and I) have transmitted modified descendants
+to the present day, represented by the fifteen genera (a^{14} to z^{14}) on
+the uppermost horizontal line. Now all these modified descendants from a
+single species, are represented as related in blood or descent to the same
+{421} degree; they may metaphorically be called cousins to the same
+millionth degree; yet they differ widely and in different degrees from each
+other. The forms descended from A, now broken up into two or three
+families, constitute a distinct order from those descended from I, also
+broken up into two families. Nor can the existing species, descended from
+A, be ranked in the same genus with the parent A; or those from I, with the
+parent I. But the existing genus F^{14} may be supposed to have been but
+slightly modified; and it will then rank with the parent-genus F; just as
+some few still living organic beings belong to Silurian genera. So that the
+amount or value of the differences between organic beings all related to
+each other in the same degree in blood, has come to be widely different.
+Nevertheless their genealogical _arrangement_ remains strictly true, not
+only at the present time, but at each successive period of descent. All the
+modified descendants from A will have inherited something in common from
+their common parent, as will all the descendants from I; so will it be with
+each subordinate branch of descendants, at each successive period. If,
+however, we choose to suppose that any of the descendants of A or of I have
+been so much modified as to have more or less completely lost traces of
+their parentage, in this case, their places in a natural classification
+will have been more or less completely lost,--as sometimes seems to have
+occurred with existing organisms. All the descendants of the genus F, along
+its whole line of descent, are supposed to have been but little modified,
+and they yet form a single genus. But this genus, though much isolated,
+will still occupy its proper intermediate position; for F originally was
+intermediate in character between A and I, and the several genera descended
+from these two genera will {422} have inherited to a certain extent their
+characters. This natural arrangement is shown, as far as is possible on
+paper, in the diagram, but in much too simple a manner. If a branching
+diagram had not been used, and only the names of the groups had been
+written in a linear series, it would have been still less possible to have
+given a natural arrangement; and it is notoriously not possible to
+represent in a series, on a flat surface, the affinities which we discover
+in nature amongst the beings of the same group. Thus, on the view which I
+hold, the natural system is genealogical in its arrangement, like a
+pedigree; but the degrees of modification which the different groups have
+undergone, have to be expressed by ranking them under different so-called
+genera, sub-families, families, sections, orders, and classes.
+
+It may be worth while to illustrate this view of classification, by taking
+the case of languages. If we possessed a perfect pedigree of mankind, a
+genealogical arrangement of the races of man would afford the best
+classification of the various languages now spoken throughout the world;
+and if all extinct languages, and all intermediate and slowly changing
+dialects, had to be included, such an arrangement would, I think, be the
+only possible one. Yet it might be that some very ancient language had
+altered little, and had given rise to few new languages, whilst others
+(owing to the spreading and subsequent isolation and states of civilisation
+of the several races, descended from a common race) had altered much, and
+had given rise to many new languages and dialects. The various degrees of
+difference in the languages from the same stock, would have to be expressed
+by groups subordinate to groups; but the proper or even only possible
+arrangement would still be genealogical; and this would be strictly
+natural, as {423} it would connect together all languages, extinct and
+modern, by the closest affinities, and would give the filiation and origin
+of each tongue.
+
+In confirmation of this view, let us glance at the classification of
+varieties, which are believed or known to have descended from one species.
+These are grouped under species, with sub-varieties under varieties; and
+with our domestic productions, several other grades of difference are
+requisite, as we have seen with pigeons. The origin of the existence of
+groups subordinate to groups, is the same with varieties as with species,
+namely, closeness of descent with various degrees of modification. Nearly
+the same rules are followed in classifying varieties, as with species.
+Authors have insisted on the necessity of classing varieties on a natural
+instead of an artificial system; we are cautioned, for instance, not to
+class two varieties of the pine-apple together, merely because their fruit,
+though the most important part, happens to be nearly identical; no one puts
+the swedish and common turnips together, though the esculent and thickened
+stems are so similar. Whatever part is found to be most constant, is used
+in classing varieties: thus the great agriculturist Marshall says the horns
+are very useful for this purpose with cattle, because they are less
+variable than the shape or colour of the body, &c.; whereas with sheep the
+horns are much less serviceable, because less constant. In classing
+varieties, I apprehend if we had a real pedigree, a genealogical
+classification would be universally preferred; and it has been attempted by
+some authors. For we might feel sure, whether there had been more or less
+modification, the principle of inheritance would keep the forms together
+which were allied in the greatest number of points. In tumbler pigeons,
+though some sub-varieties differ from the others {424} in the important
+character of having a longer beak, yet all are kept together from having
+the common habit of tumbling; but the short-faced breed has nearly or quite
+lost this habit; nevertheless, without any reasoning or thinking on the
+subject, these tumblers are kept in the same group, because allied in blood
+and alike in some other respects. If it could be proved that the Hottentot
+had descended from the Negro, I think he would be classed under the Negro
+group, however much he might differ in colour and other important
+characters from negroes.
+
+With species in a state of nature, every naturalist has in fact brought
+descent into his classification; for he includes in his lowest grade, or
+that of a species, the two sexes; and how enormously these sometimes differ
+in the most important characters, is known to every naturalist: scarcely a
+single fact can be predicated in common of the males and hermaphrodites of
+certain cirripedes, when adult, and yet no one dreams of separating them.
+The naturalist includes as one species the several larval stages of the
+same individual, however much they may differ from each other and from the
+adult; as he likewise includes the so-called alternate generations of
+Steenstrup, which can only in a technical sense be considered as the same
+individual. He includes monsters; he includes varieties, not solely because
+they closely resemble the parent-form, but because they are descended from
+it. He who believes that the cowslip is descended from the primrose, or
+conversely, ranks them together as a single species, and gives a single
+definition. As soon as three Orchidean forms (Monochanthus, Myanthus, and
+Catasetum), which had previously been ranked as three distinct genera, were
+known to be sometimes produced on the same spike, they were immediately
+included as a single species. {425}
+
+As descent has universally been used in classing together the individuals
+of the same species, though the males and females and larvæ are sometimes
+extremely different; and as it has been used in classing varieties which
+have undergone a certain, and sometimes a considerable amount of
+modification, may not this same element of descent have been unconsciously
+used in grouping species under genera, and genera under higher groups,
+though in these cases the modification has been greater in degree, and has
+taken a longer time to complete? I believe it has thus been unconsciously
+used; and only thus can I understand the several rules and guides which
+have been followed by our best systematists. We have no written pedigrees;
+we have to make out community of descent by resemblances of any kind.
+Therefore we choose those characters which, as far as we can judge, are the
+least likely to have been modified in relation to the conditions of life to
+which each species has been recently exposed. Rudimentary structures on
+this view are as good as, or even sometimes better than, other parts of the
+organisation. We care not how trifling a character may be--let it be the
+mere inflection of the angle of the jaw, the manner in which an insect's
+wing is folded, whether the skin be covered by hair or feathers--if it
+prevail throughout many and different species, especially those having very
+different habits of life, it assumes high value; for we can account for its
+presence in so many forms with such different habits, only by its
+inheritance from a common parent. We may err in this respect in regard to
+single points of structure, but when several characters, let them be ever
+so trifling, occur together throughout a large group of beings having
+different habits, we may feel almost sure, on the theory of descent, that
+these characters have been inherited from a common ancestor. {426} And we
+know that such correlated or aggregated characters have especial value in
+classification.
+
+We can understand why a species or a group of species may depart, in
+several of its most important characteristics, from its allies, and yet be
+safely classed with them. This may be safely done, and is often done, as
+long as a sufficient number of characters, let them be ever so unimportant,
+betrays the hidden bond of community of descent. Let two forms have not a
+single character in common, yet if these extreme forms are connected
+together by a chain of intermediate groups, we may at once infer their
+community of descent, and we put them all into the same class. As we find
+organs of high physiological importance--those which serve to preserve life
+under the most diverse conditions of existence--are generally the most
+constant, we attach especial value to them; but if these same organs, in
+another group or section of a group, are found to differ much, we at once
+value them less in our classification. We shall hereafter, I think, clearly
+see why embryological characters are of such high classificatory
+importance. Geographical distribution may sometimes be brought usefully
+into play in classing large and widely-distributed genera, because all the
+species of the same genus, inhabiting any distinct and isolated region,
+have in all probability descended from the same parents.
+
+We can understand, on these views, the very important distinction between
+real affinities and analogical or adaptive resemblances. Lamarck first
+called attention to this distinction, and he has been ably followed by
+Macleay and others. The resemblance, in the shape of the body and in the
+fin-like anterior limbs, between the dugong, which is a pachydermatous
+animal, and the whale, and between both these mammals and fishes, is
+analogical. Amongst insects there are innumerable {427} instances: thus
+Linnæus, misled by external appearances, actually classed an homopterous
+insect as a moth. We see something of the same kind even in our domestic
+varieties, as in the thickened stems of the common and swedish turnip. The
+resemblance of the greyhound and racehorse is hardly more fanciful than the
+analogies which have been drawn by some authors between very distinct
+animals. On my view of characters being of real importance for
+classification, only in so far as they reveal descent, we can clearly
+understand why analogical or adaptive character, although of the utmost
+importance to the welfare of the being, are almost valueless to the
+systematist. For animals, belonging to two most distinct lines of descent,
+may readily become adapted to similar conditions, and thus assume a close
+external resemblance; but such resemblances will not reveal--will rather
+tend to conceal their blood-relationship to their proper lines of descent.
+We can also understand the apparent paradox, that the very same characters
+are analogical when one class or order is compared with another, but give
+true affinities when the members of the same class or order are compared
+one with another: thus the shape of the body and fin-like limbs are only
+analogical when whales are compared with fishes, being adaptations in both
+classes for swimming through the water; but the shape of the body and
+fin-like limbs serve as characters exhibiting true affinity between the
+several members of the whale family; for these cetaceans agree in so many
+characters, great and small, that we cannot doubt that they have inherited
+their general shape of body and structure of limbs from a common ancestor.
+So it is with fishes.
+
+As members of distinct classes have often been adapted by successive slight
+modifications to live under nearly similar circumstances,--to inhabit for
+instance {428} the three elements of land, air, and water,--we can perhaps
+understand how it is that a numerical parallelism has sometimes been
+observed between the sub-groups in distinct classes. A naturalist, struck
+by a parallelism of this nature in any one class, by arbitrarily raising or
+sinking the value of the groups in other classes (and all our experience
+shows that this valuation has hitherto been arbitrary), could easily extend
+the parallelism over a wide range; and thus the septenary, quinary,
+quaternary, and ternary classifications have probably arisen.
+
+As the modified descendants of dominant species, belonging to the larger
+genera, tend to inherit the advantages, which made the groups to which they
+belong large and their parents dominant, they are almost sure to spread
+widely, and to seize on more and more places in the economy of nature. The
+larger and more dominant groups thus tend to go on increasing in size; and
+they consequently supplant many smaller and feebler groups. Thus we can
+account for the fact that all organisms, recent and extinct, are included
+under a few great orders, under still fewer classes, and all in one great
+natural system. As showing how few the higher groups are in number, and how
+widely spread they are throughout the world, the fact is striking, that the
+discovery of Australia has not added a single insect belonging to a new
+class; and that in the vegetable kingdom, as I learn from Dr. Hooker, it
+has added only two or three orders of small size.
+
+In the chapter on geological succession I attempted to show, on the
+principle of each group having generally diverged much in character during
+the long-continued process of modification, how it is that the more ancient
+forms of life often present characters in some slight degree intermediate
+between existing groups. A few {429} old and intermediate parent-forms
+having occasionally transmitted to the present day descendants but little
+modified, will give to us our so-called osculant or aberrant groups. The
+more aberrant any form is, the greater must be the number of connecting
+forms which on my theory have been exterminated and utterly lost. And we
+have some evidence of aberrant forms having suffered severely from
+extinction, for they are generally represented by extremely few species;
+and such species as do occur are generally very distinct from each other,
+which again implies extinction. The genera Ornithorhynchus and Lepidosiren,
+for example, would not have been less aberrant had each been represented by
+a dozen species instead of by a single one; but such richness in species,
+as I find after some investigation, does not commonly fall to the lot of
+aberrant genera. We can, I think, account for this fact only by looking at
+aberrant forms as failing groups conquered by more successful competitors,
+with a few members preserved by some unusual coincidence of favourable
+circumstances.
+
+Mr. Waterhouse has remarked that, when a member belonging to one group of
+animals exhibits an affinity to a quite distinct group, this affinity in
+most cases is general and not special: thus, according to Mr. Waterhouse,
+of all Rodents, the bizcacha is most nearly related to Marsupials; but in
+the points in which it approaches this order, its relations are general,
+and not to any one marsupial species more than to another. As the points of
+affinity of the bizcacha to Marsupials are believed to be real and not
+merely adaptive, they are due on my theory to inheritance in common.
+Therefore we must suppose either that all Rodents, including the bizcacha,
+branched off from some very ancient Marsupial, which will have had a
+character in some degree intermediate with respect to all existing
+Marsupials; or {430} that both Rodents and Marsupials branched off from a
+common progenitor, and that both groups have since undergone much
+modification in divergent directions. On either view we may suppose that
+the bizcacha has retained, by inheritance, more of the character of its
+ancient progenitor than have other Rodents; and therefore it will not be
+specially related to any one existing Marsupial, but indirectly to all or
+nearly all Marsupials, from having partially retained the character of
+their common progenitor, or of an early member of the group. On the other
+hand, of all Marsupials, as Mr. Waterhouse has remarked, the phascolomys
+resembles most nearly, not any one species, but the general order of
+Rodents. In this case, however, it may be strongly suspected that the
+resemblance is only analogical, owing to the phascolomys having become
+adapted to habits like those of a Rodent. The elder De Candolle has made
+nearly similar observations on the general nature of the affinities of
+distinct orders of plants.
+
+On the principle of the multiplication and gradual divergence in character
+of the species descended from a common parent, together with their
+retention by inheritance of some characters in common, we can understand
+the excessively complex and radiating affinities by which all the members
+of the same family or higher group are connected together. For the common
+parent of a whole family of species, now broken up by extinction into
+distinct groups and sub-groups, will have transmitted some of its
+characters, modified in various ways and degrees, to all; and the several
+species will consequently be related to each other by circuitous lines of
+affinity of various lengths (as may be seen in the diagram so often
+referred to), mounting up through many predecessors. As it is difficult to
+show the blood-relationship between the numerous kindred {431} of any
+ancient and noble family, even by the aid of a genealogical tree, and
+almost impossible to do this without this aid, we can understand the
+extraordinary difficulty which naturalists have experienced in describing,
+without the aid of a diagram, the various affinities which they perceive
+between the many living and extinct members of the same great natural
+class.
+
+Extinction, as we have seen in the fourth chapter, has played an important
+part in defining and widening the intervals between the several groups in
+each class. We may thus account even for the distinctness of whole classes
+from each other--for instance, of birds from all other vertebrate
+animals--by the belief that many ancient forms of life have been utterly
+lost, through which the early progenitors of birds were formerly connected
+with the early progenitors of the other vertebrate classes. There has been
+less entire extinction of the forms of life which once connected fishes
+with batrachians. There has been still less in some other classes, as in
+that of the Crustacea, for here the most wonderfully diverse forms are
+still tied together by a long, but broken, chain of affinities. Extinction
+has only separated groups: it has by no means made them; for if every form
+which has ever lived on this earth were suddenly to reappear, though it
+would be quite impossible to give definitions by which each group could be
+distinguished from other groups, as all would blend together by steps as
+fine as those between the finest existing varieties, nevertheless a natural
+classification, or at least a natural arrangement, would be possible. We
+shall see this by turning to the diagram: the letters, A to L, may
+represent eleven Silurian genera, some of which have produced large groups
+of modified descendants. Every intermediate link between these eleven
+genera and their primordial parent, and every {432} intermediate link in
+each branch and sub-branch of their descendants, may be supposed to be
+still alive; and the links to be as fine as those between the finest
+varieties. In this case it would be quite impossible to give any definition
+by which the several members of the several groups could be distinguished
+from their more immediate parents; or these parents from their ancient and
+unknown progenitor. Yet the natural arrangement in the diagram would still
+hold good; and, on the principle of inheritance, all the forms descended
+from A, or from I, would have something in common. In a tree we can specify
+this or that branch, though at the actual fork the two unite and blend
+together. We could not, as I have said, define the several groups; but we
+could pick out types, or forms, representing most of the characters of each
+group, whether large or small, and thus give a general idea of the value of
+the differences between them. This is what we should be driven to, if we
+were ever to succeed in collecting all the forms in any class which have
+lived throughout all time and space. We shall certainly never succeed in
+making so perfect a collection: nevertheless, in certain classes, we are
+tending in this direction; and Milne Edwards has lately insisted, in an
+able paper, on the high importance of looking to types, whether or not we
+can separate and define the groups to which such types belong.
+
+Finally, we have seen that natural selection, which results from the
+struggle for existence, and which almost inevitably induces extinction and
+divergence of character in the many descendants from one dominant
+parent-species, explains that great and universal feature in the affinities
+of all organic beings, namely, their subordination in group under group. We
+use the element of descent in classing the individuals of both sexes and of
+all ages, although having few characters in common, {433} under one
+species; we use descent in classing acknowledged varieties, however
+different they may be from their parent; and I believe this element of
+descent is the hidden bond of connexion which naturalists have sought under
+the term of the Natural System. On this idea of the natural system being,
+in so far as it has been perfected, genealogical in its arrangement, with
+the grades of difference between the descendants from a common parent,
+expressed by the terms genera, families, orders, &c., we can understand the
+rules which we are compelled to follow in our classification. We can
+understand why we value certain resemblances far more than others; why we
+are permitted to use rudimentary and useless organs, or others of trifling
+physiological importance; why, in comparing one group with a distinct
+group, we summarily reject analogical or adaptive characters, and yet use
+these same characters within the limits of the same group. We can clearly
+see how it is that all living and extinct forms can be grouped together in
+one great system; and how the several members of each class are connected
+together by the most complex and radiating lines of affinities. We shall
+never, probably, disentangle the inextricable web of affinities between the
+members of any one class; but when we have a distinct object in view, and
+do not look to some unknown plan of creation, we may hope to make sure but
+slow progress.
+
+
+
+_Morphology._--We have seen that the members of the same class,
+independently of their habits of life, resemble each other in the general
+plan of their organisation. This resemblance is often expressed by the term
+"unity of type;" or by saying that the several parts and organs in the
+different species of the class are homologous. The whole subject is
+included under {434} the general name of Morphology. This is the most
+interesting department of natural history, and may be said to be its very
+soul. What can be more curious than that the hand of a man, formed for
+grasping, that of a mole for digging, the leg of the horse, the paddle of
+the porpoise, and the wing of the bat, should all be constructed on the
+same pattern, and should include similar bones, in the same relative
+positions? Geoffroy St. Hilaire has insisted strongly on the high
+importance of relative connexion in homologous organs: the parts may change
+to almost any extent in form and size, and yet they always remain connected
+together in the same order. We never find, for instance, the bones of the
+arm and forearm, or of the thigh and leg, transposed. Hence the same names
+can be given to the homologous bones in widely different animals. We see
+the same great law in the construction of the mouths of insects: what can
+be more different than the immensely long spiral proboscis of a
+sphinx-moth, the curious folded one of a bee or bug, and the great jaws of
+a beetle?--yet all these organs, serving for such different purposes, are
+formed by infinitely numerous modifications of an upper lip, mandibles, and
+two pairs of maxillæ. Analogous laws govern the construction of the mouths
+and limbs of crustaceans. So it is with the flowers of plants.
+
+Nothing can be more hopeless than to attempt to explain this similarity of
+pattern in members of the same class, by utility or by the doctrine of
+final causes. The hopelessness of the attempt has been expressly admitted
+by Owen in his most interesting work on the 'Nature of Limbs.' On the
+ordinary view of the independent creation of each being, we can only say
+that so it is;--that it has so pleased the Creator to construct each animal
+and plant.
+
+The explanation is manifest on the theory of the {435} natural selection of
+successive slight modifications,--each modification being profitable in
+some way to the modified form, but often affecting by correlation of growth
+other parts of the organisation. In changes of this nature, there will be
+little or no tendency to modify the original pattern, or to transpose
+parts. The bones of a limb might be shortened and widened to any extent,
+and become gradually enveloped in thick membrane, so as to serve as a fin;
+or a webbed foot might have all its bones, or certain bones, lengthened to
+any extent, and the membrane connecting them increased to any extent, so as
+to serve as a wing: yet in all this great amount of modification there will
+be no tendency to alter the framework of bones or the relative connexion of
+the several parts. If we suppose that the ancient progenitor, the archetype
+as it may be called, of all mammals, had its limbs constructed on the
+existing general pattern, for whatever purpose they served, we can at once
+perceive the plain signification of the homologous construction of the
+limbs throughout the whole class. So with the mouths of insects, we have
+only to suppose that their common progenitor had an upper lip, mandibles,
+and two pair of maxillæ, these parts being perhaps very simple in form; and
+then natural selection, acting on some originally created form, will
+account for the infinite diversity in structure and function of the mouths
+of insects. Nevertheless, it is conceivable that the general pattern of an
+organ might become so much obscured as to be finally lost, by the atrophy
+and ultimately by the complete abortion of certain parts, by the soldering
+together of other parts, and by the doubling or multiplication of
+others,--variations which we know to be within the limits of possibility.
+In the paddles of the extinct gigantic sea-lizards, and in the mouths of
+certain suctorial crustaceans, the {436} general pattern seems to have been
+thus to a certain extent obscured.
+
+There is another and equally curious branch of the present subject; namely,
+the comparison not of the same part in different members of a class, but of
+the different parts or organs in the same individual. Most physiologists
+believe that the bones of the skull are homologous with--that is correspond
+in number and in relative connexion with--the elemental parts of a certain
+number of vertebræ. The anterior and posterior limbs in each member of the
+vertebrate and articulate classes are plainly homologous. We see the same
+law in comparing the wonderfully complex jaws and legs in crustaceans. It
+is familiar to almost every one, that in a flower the relative position of
+the sepals, petals, stamens, and pistils, as well as their intimate
+structure, are intelligible on the view that they consist of metamorphosed
+leaves, arranged in a spire. In monstrous plants, we often get direct
+evidence of the possibility of one organ being transformed into another;
+and we can actually see in embryonic crustaceans and in many other animals,
+and in flowers, that organs, which when mature become extremely different,
+are at an early stage of growth exactly alike.
+
+How inexplicable are these facts on the ordinary view of creation! Why
+should the brain be enclosed in a box composed of such numerous and such
+extraordinary shaped pieces of bone? As Owen has remarked, the benefit
+derived from the yielding of the separate pieces in the act of parturition
+of mammals, will by no means explain the same construction in the skulls of
+birds. Why should similar bones have been created in the formation of the
+wing and leg of a bat, used as they are for such totally different
+purposes? Why should one crustacean, which has an extremely complex {437}
+mouth formed of many parts, consequently always have fewer legs; or
+conversely, those with many legs have simpler mouths? Why should the
+sepals, petals, stamens, and pistils in any individual flower, though
+fitted for such widely different purposes, be all constructed on the same
+pattern?
+
+On the theory of natural selection, we can satisfactorily answer these
+questions. In the vertebrata, we see a series of internal vertebræ bearing
+certain processes and appendages; in the articulata, we see the body
+divided into a series of segments, bearing external appendages; and in
+flowering plants, we see a series of successive spiral whorls of leaves. An
+indefinite repetition of the same part or organ is the common
+characteristic (as Owen has observed) of all low or little-modified forms;
+therefore we may readily believe that the unknown progenitor of the
+vertebrata possessed many vertebræ; the unknown progenitor of the
+articulata, many segments; and the unknown progenitor of flowering plants,
+many spiral whorls of leaves. We have formerly seen that parts many times
+repeated are eminently liable to vary in number and structure; consequently
+it is quite probable that natural selection, during a long-continued course
+of modification, should have seized on a certain number of the primordially
+similar elements, many times repeated, and have adapted them to the most
+diverse purposes. And as the whole amount of modification will have been
+effected by slight successive steps, we need not wonder at discovering in
+such parts or organs, a certain degree of fundamental resemblance, retained
+by the strong principle of inheritance.
+
+In the great class of molluscs, though we can homologise the parts of one
+species with those of other and distinct species, we can indicate but few
+serial homologies; that is, we are seldom enabled to say that one {438}
+part or organ is homologous with another in the same individual. And we can
+understand this fact; for in molluscs, even in the lowest members of the
+class, we do not find nearly so much indefinite repetition of any one part,
+as we find in the other great classes of the animal and vegetable kingdoms.
+
+Naturalists frequently speak of the skull as formed of metamorphosed
+vertebræ: the jaws of crabs as metamorphosed legs; the stamens and pistils
+of flowers as metamorphosed leaves; but it would in these cases probably be
+more correct, as Professor Huxley has remarked, to speak of both skull and
+vertebræ, both jaws and legs, &c.,--as having been metamorphosed, not one
+from the other, but from some common element. Naturalists, however, use
+such language only in a metaphorical sense: they are far from meaning that
+during a long course of descent, primordial organs of any kind--vertebræ in
+the one case and legs in the other--have actually been modified into skulls
+or jaws. Yet so strong is the appearance of a modification of this nature
+having occurred, that naturalists can hardly avoid employing language
+having this plain signification. On my view these terms may be used
+literally; and the wonderful fact of the jaws, for instance, of a crab
+retaining numerous characters, which they would probably have retained
+through inheritance, if they had really been metamorphosed during a long
+course of descent from true legs, or from some simple appendage, is
+explained.
+
+
+
+_Embryology._--It has already been casually remarked that certain organs in
+the individual, which when mature become widely different and serve for
+different purposes, are in the embryo exactly alike. The embryos, also, of
+distinct animals within the same class are often strikingly similar: a
+better proof of this cannot be given, than a {439} circumstance mentioned
+by Agassiz, namely, that having forgotten to ticket the embryo of some
+vertebrate animal, he cannot now tell whether it be that of a mammal, bird,
+or reptile. The vermiform larvæ of moths, flies, beetles, &c., resemble
+each other much more closely than do the mature insects; but in the case of
+larvæ, the embryos are active, and have been adapted for special lines of
+life. A trace of the law of embryonic resemblance, sometimes lasts till a
+rather late age: thus birds of the same genus, and of closely allied
+genera, often resemble each other in their first and second plumage; as we
+see in the spotted feathers in the thrush group. In the cat tribe, most of
+the species are striped or spotted in lines; and stripes can be plainly
+distinguished in the whelp of the lion. We occasionally though rarely see
+something of this kind in plants: thus the embryonic leaves of the ulex or
+furze, and the first leaves of the phyllodineous acaceas, are pinnate or
+divided like the ordinary leaves of the leguminosæ.
+
+The points of structure, in which the embryos of widely different animals
+of the same class resemble each other, often have no direct relation to
+their conditions of existence. We cannot, for instance, suppose that in the
+embryos of the vertebrata the peculiar loop-like course of the arteries
+near the branchial slits are related to similar conditions,--in the young
+mammal which is nourished in the womb of its mother, in the egg of the bird
+which is hatched in a nest, and in the spawn of a frog under water. We have
+no more reason to believe in such a relation, than we have to believe that
+the same bones in the hand of a man, wing of a bat, and fin of a porpoise,
+are related to similar conditions of life. No one will suppose that the
+stripes on the whelp of a lion, or the spots on the young blackbird, {440}
+are of any use to these animals, or are related to the conditions to which
+they are exposed.
+
+The case, however, is different when an animal during any part of its
+embryonic career is active, and has to provide for itself. The period of
+activity may come on earlier or later in life; but whenever it comes on,
+the adaptation of the larva to its conditions of life is just as perfect
+and as beautiful as in the adult animal. From such special adaptations, the
+similarity of the larvæ or active embryos of allied animals is sometimes
+much obscured; and cases could be given of the larvæ of two species, or of
+two groups of species, differing quite as much, or even more, from each
+other than do their adult parents. In most cases, however, the larvæ,
+though active, still obey, more or less closely, the law of common
+embryonic resemblance. Cirripedes afford a good instance of this: even the
+illustrious Cuvier did not perceive that a barnacle was, as it certainly
+is, a crustacean; but a glance at the larva shows this to be the case in an
+unmistakeable manner. So again the two main divisions of cirripedes, the
+pedunculated and sessile, which differ widely in external appearance, have
+larvæ in all their stages barely distinguishable.
+
+The embryo in the course of development generally rises in organisation: I
+use this expression, though I am aware that it is hardly possible to define
+clearly what is meant by the organisation being higher or lower. But no one
+probably will dispute that the butterfly is higher than the caterpillar. In
+some cases, however, the mature animal is generally considered as lower in
+the scale than the larva, as with certain parasitic crustaceans. To refer
+once again to cirripedes: the larvæ in the first stage have three pairs of
+legs, a very simple single eye, and a probosciformed mouth, with which they
+feed largely, for they increase much in {441} size. In the second stage,
+answering to the chrysalis stage of butterflies, they have six pairs of
+beautifully constructed natatory legs, a pair of magnificent compound eyes,
+and extremely complex antennæ; but they have a closed and imperfect mouth,
+and cannot feed: their function at this stage is, to search by their
+well-developed organs of sense, and to reach by their active powers of
+swimming, a proper place on which to become attached and to undergo their
+final metamorphosis. When this is completed they are fixed for life: their
+legs are now converted into prehensile organs; they again obtain a
+well-constructed mouth; but they have no antennæ, and their two eyes are
+now reconverted into a minute, single, and very simple eye-spot. In this
+last and complete state, cirripedes may be considered as either more highly
+or more lowly organised than they were in the larval condition. But in some
+genera the larvæ become developed either into hermaphrodites having the
+ordinary structure, or into what I have called complemental males: and in
+the latter, the development has assuredly been retrograde; for the male is
+a mere sack, which lives for a short time, and is destitute of mouth,
+stomach, or other organ of importance, excepting for reproduction.
+
+We are so much accustomed to see differences in structure between the
+embryo and the adult, and likewise a close similarity in the embryos of
+widely different animals within the same class, that we might be led to
+look at these facts as necessarily contingent in some manner on growth. But
+there is no obvious reason why, for instance, the wing of a bat, or the fin
+of a porpoise, should not have been sketched out with all the parts in
+proper proportion, as soon as any structure became visible in the embryo.
+And in some whole groups of animals and in certain members of other groups,
+the embryo does not at any period differ widely from the {442} adult: thus
+Owen has remarked in regard to cuttle-fish, "there is no metamorphosis; the
+cephalopodic character is manifested long before the parts of the embryo
+are completed;" and again in spiders, "there is nothing worthy to be called
+a metamorphosis." The larvæ of insects, whether adapted to the most diverse
+and active habits, or quite inactive, being fed by their parents or placed
+in the midst of proper nutriment, yet nearly all pass through a similar
+worm-like stage of development; but in some few cases, as in that of Aphis,
+if we look to the admirable drawings by Professor Huxley of the development
+of this insect, we see no trace of the vermiform stage.
+
+How, then, can we explain these several facts in embryology,--namely the
+very general, but not universal difference in structure between the embryo
+and the adult;--of parts in the same individual embryo, which ultimately
+become very unlike and serve for diverse purposes, being at this early
+period of growth alike;--of embryos of different species within the same
+class, generally, but not universally, resembling each other;--of the
+structure of the embryo not being closely related to its conditions of
+existence, except when the embryo becomes at any period of life active and
+has to provide for itself;--of the embryo apparently having sometimes a
+higher organisation than the mature animal, into which it is developed? I
+believe that all these facts can be explained, as follows, on the view of
+descent with modification.
+
+It is commonly assumed, perhaps from monstrosities often affecting the
+embryos at a very early period, that slight variations necessarily appear
+at an equally early period. But we have little evidence on this
+head--indeed the evidence rather points the other way; for it is notorious
+that breeders of cattle, horses, and various {443} fancy animals, cannot
+positively tell, until some time after the animal has been born, what its
+merits or form will ultimately turn out. We see this plainly in our own
+children; we cannot always tell whether the child will be tall or short, or
+what its precise features will be. The question is not, at what period of
+life any variation has been caused, but at what period it is fully
+displayed. The cause may have acted, and I believe generally has acted,
+even before the embryo is formed; and the variation may be due to the male
+and female sexual elements having been affected by the conditions to which
+either parent, or their ancestors, have been exposed. Nevertheless an
+effect thus caused at a very early period, even before the formation of the
+embryo, may appear late in life; as when an hereditary disease, which
+appears in old age alone, has been communicated to the offspring from the
+reproductive element of one parent. Or again, as when the horns of
+cross-bred cattle have been affected by the shape of the horns of either
+parent. For the welfare of a very young animal, as long as it remains in
+its mother's womb, or in the egg, or as long as it is nourished and
+protected by its parent, it must be quite unimportant whether most of its
+characters are fully acquired a little earlier or later in life. It would
+not signify, for instance, to a bird which obtained its food best by having
+a long beak, whether or not it assumed a beak of this particular length, as
+long as it was fed by its parents. Hence, I conclude, that it is quite
+possible, that each of the many successive modifications, by which each
+species has acquired its present structure, may have supervened at a not
+very early period of life; and some direct evidence from our domestic
+animals supports this view. But in other cases it is quite possible that
+each successive modification, or {444} most of them, may have appeared at
+an extremely early period.
+
+I have stated in the first chapter, that there is some evidence to render
+it probable, that at whatever age any variation first appears in the
+parent, it tends to reappear at a corresponding age in the offspring.
+Certain variations can only appear at corresponding ages, for instance,
+peculiarities in the caterpillar, cocoon, or imago states of the silk-moth;
+or, again, in the horns of almost full-grown cattle. But further than this,
+variations which, for all that we can see, might have appeared earlier or
+later in life, tend to appear at a corresponding age in the offspring and
+parent. I am far from meaning that this is invariably the case; and I could
+give a good many cases of variations (taking the word in the largest sense)
+which have supervened at an earlier age in the child than in the parent.
+
+These two principles, if their truth be admitted, will, I believe, explain
+all the above specified leading facts in embryology. But first let us look
+at a few analogous cases in domestic varieties. Some authors who have
+written on Dogs, maintain that the greyhound and bulldog, though appearing
+so different, are really varieties most closely allied, and have probably
+descended from the same wild stock; hence I was curious to see how far
+their puppies differed from each other: I was told by breeders that they
+differed just as much as their parents, and this, judging by the eye,
+seemed almost to be the case; but on actually measuring the old dogs and
+their six-days old puppies, I found that the puppies had not nearly
+acquired their full amount of proportional difference. So, again, I was
+told that the foals of cart and race-horses differed as much as the
+full-grown animals; and this surprised me greatly, as I think it probable
+that the difference between these two breeds has been wholly {445} caused
+by selection under domestication; but having had careful measurements made
+of the dam and of a three-days old colt of a race and heavy cart-horse, I
+find that the colts have by no means acquired their full amount of
+proportional difference.
+
+As the evidence appears to me conclusive, that the several domestic breeds
+of Pigeon have descended from one wild species, I compared young pigeons of
+various breeds, within twelve hours after being hatched; I carefully
+measured the proportions (but will not here give details) of the beak,
+width of mouth, length of nostril and of eyelid, size of feet and length of
+leg, in the wild stock, in pouters, fantails, runts, barbs, dragons,
+carriers, and tumblers. Now some of these birds, when mature, differ so
+extraordinarily in length and form of beak, that they would, I cannot
+doubt, be ranked in distinct genera, had they been natural productions. But
+when the nestling birds of these several breeds were placed in a row,
+though most of them could be distinguished from each other, yet their
+proportional differences in the above specified several points were
+incomparably less than in the full-grown birds. Some characteristic points
+of difference--for instance, that of the width of mouth--could hardly be
+detected in the young. But there was one remarkable exception to this rule,
+for the young of the short-faced tumbler differed from the young of the
+wild rock-pigeon and of the other breeds, in all its proportions, almost
+exactly as much as in the adult state.
+
+The two principles above given seem to me to explain these facts in regard
+to the later embryonic stages of our domestic varieties. Fanciers select
+their horses, dogs, and pigeons, for breeding, when they are nearly grown
+up: they are indifferent whether the desired qualities and structures have
+been acquired earlier or {446} later in life, if the full-grown animal
+possesses them. And the cases just given, more especially that of pigeons,
+seem to show that the characteristic differences which give value to each
+breed, and which have been accumulated by man's selection, have not
+generally first appeared at an early period of life, and have been
+inherited by the offspring at a corresponding not early period. But the
+case of the short-faced tumbler, which when twelve hours old had acquired
+its proper proportions, proves that this is not the universal rule; for
+here the characteristic differences must either have appeared at an earlier
+period than usual, or, if not so, the differences must have been inherited,
+not at the corresponding, but at an earlier age.
+
+Now let us apply these facts and the above two principles--which latter,
+though not proved true, can be shown to be in some degree probable--to
+species in a state of nature. Let us take a genus of birds, descended on my
+theory from some one parent-species, and of which the several new species
+have become modified through natural selection in accordance with their
+diverse habits. Then, from the many slight successive steps of variation
+having supervened at a rather late age, and having been inherited at a
+corresponding age, the young of the new species of our supposed genus will
+manifestly tend to resemble each other much more closely than do the
+adults, just as we have seen in the case of pigeons. We may extend this
+view to whole families or even classes. The fore-limbs, for instance, which
+served as legs in the parent-species, may have become, by a long course of
+modification, adapted in one descendant to act as hands, in another as
+paddles, in another as wings; and on the above two principles--namely of
+each successive modification supervening at a rather late age, and being
+inherited at a {447} corresponding late age--the fore-limbs in the embryos
+of the several descendants of the parent-species will still resemble each
+other closely, for they will not have been modified. But in each of our new
+species, the embryonic fore-limbs will differ greatly from the fore-limbs
+in the mature animal; the limbs in the latter having undergone much
+modification at a rather late period of life, and having thus been
+converted into hands, or paddles, or wings. Whatever influence
+long-continued exercise or use on the one hand, and disuse on the other,
+may have in modifying an organ, such influence will mainly affect the
+mature animal, which has come to its full powers of activity and has to
+gain its own living; and the effects thus produced will be inherited at a
+corresponding mature age. Whereas the young will remain unmodified, or be
+modified in a lesser degree, by the effects of use and disuse.
+
+In certain cases the successive steps of variation might supervene, from
+causes of which we are wholly ignorant, at a very early period of life, or
+each step might be inherited at an earlier period than that at which it
+first appeared. In either case (as with the short-faced tumbler) the young
+or embryo would closely resemble the mature parent-form. We have seen that
+this is the rule of development in certain whole groups of animals, as with
+cuttle-fish and spiders, and with a few members of the great class of
+insects, as with Aphis. With respect to the final cause of the young in
+these cases not undergoing any metamorphosis, or closely resembling their
+parents from their earliest age, we can see that this would result from the
+two following contingencies: firstly, from the young, during a course of
+modification carried on for many generations, having to provide for their
+own wants at a very early stage {448} of development, and secondly, from
+their following exactly the same habits of life with their parents; for in
+this case, it would be indispensable for the existence of the species, that
+the child should be modified at a very early age in the same manner with
+its parents, in accordance with their similar habits. Some further
+explanation, however, of the embryo not undergoing any metamorphosis is
+perhaps requisite. If, on the other hand, it profited the young to follow
+habits of life in any degree different from those of their parent, and
+consequently to be constructed in a slightly different manner, then, on the
+principle of inheritance at corresponding ages, the active young or larvæ
+might easily be rendered by natural selection different to any conceivable
+extent from their parents. Such differences might, also, become correlated
+with successive stages of development; so that the larvæ, in the first
+stage, might differ greatly from the larvæ in the second stage, as we have
+seen to be the case with cirripedes. The adult might become fitted for
+sites or habits, in which organs of locomotion or of the senses, &c., would
+be useless; and in this case the final metamorphosis would be said to be
+retrograde.
+
+As all the organic beings, extinct and recent, which have ever lived on
+this earth have to be classed together, and as all have been connected by
+the finest gradations, the best, or indeed, if our collections were nearly
+perfect, the only possible arrangement, would be genealogical. Descent
+being on my view the hidden bond of connexion which naturalists have been
+seeking under the term of the natural system. On this view we can
+understand how it is that, in the eyes of most naturalists, the structure
+of the embryo is even more important for classification than that of the
+adult. For the embryo is the animal in its less modified state; {449} and
+in so far it reveals the structure of its progenitor. In two groups of
+animals, however much they may at present differ from each other in
+structure and habits, if they pass through the same or similar embryonic
+stages, we may feel assured that they have both descended from the same or
+nearly similar parents, and are therefore in that degree closely related.
+Thus, community in embryonic structure reveals community of descent. It
+will reveal this community of descent, however much the structure of the
+adult may have been modified and obscured; we have seen, for instance, that
+cirripedes can at once be recognised by their larvæ as belonging to the
+great class of crustaceans. As the embryonic state of each species and
+group of species partially shows us the structure of their less modified
+ancient progenitors, we can clearly see why ancient and extinct forms of
+life should resemble the embryos of their descendants,--our existing
+species. Agassiz believes this to be a law of nature; but I am bound to
+confess that I only hope to see the law hereafter proved true. It can be
+proved true in those cases alone in which the ancient state, now supposed
+to be represented in existing embryos, has not been obliterated, either by
+the successive variations in a long course of modification having
+supervened at a very early age, or by the variations having been inherited
+at an earlier period than that at which they first appeared. It should also
+be borne in mind, that the supposed law of resemblance of ancient forms of
+life to the embryonic stages of recent forms, may be true, but yet, owing
+to the geological record not extending far enough back in time, may remain
+for a long period, or for ever, incapable of demonstration.
+
+Thus, as it seems to me, the leading facts in embryology, which are second
+in importance to none in natural history, are explained on the principle of
+slight {450} modifications not appearing, in the many descendants from some
+one ancient progenitor, at a very early period in the life of each, though
+perhaps caused at the earliest, and being inherited at a corresponding not
+early period. Embryology rises greatly in interest, when we thus look at
+the embryo as a picture, more or less obscured, of the common parent-form
+of each great class of animals.
+
+
+
+_Rudimentary, atrophied, or aborted Organs._--Organs or parts in this
+strange condition, bearing the stamp of inutility, are extremely common
+throughout nature. For instance, rudimentary mammæ are very general in the
+males of mammals: I presume that the "bastard-wing" in birds may be safely
+considered as a digit in a rudimentary state: in very many snakes one lobe
+of the lungs is rudimentary; in other snakes there are rudiments of the
+pelvis and hind limbs. Some of the cases of rudimentary organs are
+extremely curious; for instance, the presence of teeth in foetal whales,
+which when grown up have not a tooth in their heads; and the presence of
+teeth, which never cut through the gums, in the upper jaws of our unborn
+calves. It has even been stated on good authority that rudiments of teeth
+can be detected in the beaks of certain embryonic birds. Nothing can be
+plainer than that wings are formed for flight, yet in how many insects do
+we see wings so reduced in size as to be utterly incapable of flight, and
+not rarely lying under wing-cases, firmly soldered together!
+
+The meaning of rudimentary organs is often quite unmistakeable: for
+instance there are beetles of the same genus (and even of the same species)
+resembling each other most closely in all respects, one of which will have
+full-sized wings, and another mere rudiments of membrane; and here it is
+impossible to doubt, that the {451} rudiments represent wings. Rudimentary
+organs sometimes retain their potentiality, and are merely not developed:
+this seems to be the case with the mammæ of male mammals, for many
+instances are on record of these organs having become well developed in
+full-grown males, and having secreted milk. So again there are normally
+four developed and two rudimentary teats in the udders of the genus Bos,
+but in our domestic cows the two sometimes become developed and give milk.
+In plants of the same species the petals sometimes occur as mere rudiments,
+and sometimes in a well-developed state. In plants with separated sexes,
+the male flowers often have a rudiment of a pistil; and Kölreuter found
+that by crossing such male plants with an hermaphrodite species, the
+rudiment of the pistil in the hybrid offspring was much increased in size;
+and this shows that the rudiment and the perfect pistil are essentially
+alike in nature.
+
+An organ serving for two purposes, may become rudimentary or utterly
+aborted for one, even the more important purpose; and remain perfectly
+efficient for the other. Thus in plants, the office of the pistil is to
+allow the pollen-tubes to reach the ovules protected in the ovarium at its
+base. The pistil consists of a stigma supported on the style; but in some
+Compositæ, the male florets, which of course cannot be fecundated, have a
+pistil, which is in a rudimentary state, for it is not crowned with a
+stigma; but the style remains well developed, and is clothed with hairs as
+in other compositæ, for the purpose of brushing the pollen out of the
+surrounding anthers. Again, an organ may become rudimentary for its proper
+purpose, and be used for a distinct object: in certain fish the
+swim-bladder seems to be nearly rudimentary for its proper function of
+giving buoyancy, but has become converted into a {452} nascent breathing
+organ or lung. Other similar instances could be given.
+
+Organs, however little developed, if of use, should not be called
+rudimentary; they cannot properly be said to be in an atrophied condition;
+they may be called nascent, and may hereafter be developed to any extent by
+natural selection. Rudimentary organs, on the other hand, are essentially
+useless, as teeth which never cut through the gums; in a still less
+developed condition, they would be of still less use. They cannot,
+therefore, under their present condition, have been formed by natural
+selection, which acts solely by the preservation of useful modifications;
+they have been retained, as we shall see, by inheritance, and relate to a
+former condition of their possessor. It is difficult to know what are
+nascent organs; looking to the future, we cannot of course tell how any
+part will be developed, and whether it is now nascent; looking to the past,
+creatures with an organ in a nascent condition will generally have been
+supplanted and exterminated by their successors with the organ in a more
+perfect and developed condition. The wing of the penguin is of high
+service, and acts as a fin; it may, therefore, represent the nascent state
+of the wings of birds; not that I believe this to be the case, it is more
+probably a reduced organ, modified for a new function: the wing of the
+Apteryx is useless, and is truly rudimentary. The mammary glands of the
+Ornithorhynchus may, perhaps, be considered, in comparison with the udder
+of a cow, as in a nascent state. The ovigerous frena of certain cirripedes,
+which are only slightly developed and which have ceased to give attachment
+to the ova, are nascent branchiæ.
+
+Rudimentary organs in the individuals of the same species are very liable
+to vary in degree of development {453} and in other respects. Moreover, in
+closely allied species, the degree to which the same organ has been
+rendered rudimentary occasionally differs much. This latter fact is well
+exemplified in the state of the wings of the female moths in certain
+groups. Rudimentary organs may be utterly aborted; and this implies, that
+we find in an animal or plant no trace of an organ, which analogy would
+lead us to expect to find, and which is occasionally found in monstrous
+individuals of the species. Thus in the snapdragon (antirrhinum) we
+generally do not find a rudiment of a fifth stamen; but this may sometimes
+be seen. In tracing the homologies of the same part in different members of
+a class, nothing is more common, or more necessary, than the use and
+discovery of rudiments. This is well shown in the drawings given by Owen of
+the bones of the leg of the horse, ox, and rhinoceros.
+
+It is an important fact that rudimentary organs, such as teeth in the upper
+jaws of whales and ruminants, can often be detected in the embryo, but
+afterwards wholly disappear. It is also, I believe, a universal rule, that
+a rudimentary part or organ is of greater size relatively to the adjoining
+parts in the embryo, than in the adult; so that the organ at this early age
+is less rudimentary, or even cannot be said to be in any degree
+rudimentary. Hence, also, a rudimentary organ in the adult is often said to
+have retained its embryonic condition.
+
+I have now given the leading facts with respect to rudimentary organs. In
+reflecting on them, every one must be struck with astonishment: for the
+same reasoning power which tells us plainly that most parts and organs are
+exquisitely adapted for certain purposes, tells us with equal plainness
+that these rudimentary or atrophied organs, are imperfect and useless. In
+works {454} on natural history rudimentary organs are generally said to
+have been created "for the sake of symmetry," or in order "to complete the
+scheme of nature;" but this seems to me no explanation, merely a
+re-statement of the fact. Would it be thought sufficient to say that
+because planets revolve in elliptic courses round the sun, satellites
+follow the same course round the planets, for the sake of symmetry, and to
+complete the scheme of nature? An eminent physiologist accounts for the
+presence of rudimentary organs, by supposing that they serve to excrete
+matter in excess, or injurious to the system; but can we suppose that the
+minute papilla, which often represents the pistil in male flowers, and
+which is formed merely of cellular tissue, can thus act? Can we suppose
+that the formation of rudimentary teeth, which are subsequently absorbed,
+can be of any service to the rapidly growing embryonic calf by the
+excretion of precious phosphate of lime? When a man's fingers have been
+amputated, imperfect nails sometimes appear on the stumps: I could as soon
+believe that these vestiges of nails have appeared, not from unknown laws
+of growth, but in order to excrete horny matter, as that the rudimentary
+nails on the fin of the manatee were formed for this purpose.
+
+On my view of descent with modification, the origin of rudimentary organs
+is simple. We have plenty of cases of rudimentary organs in our domestic
+productions,--as the stump of a tail in tailless breeds,--the vestige of an
+ear in earless breeds,--the reappearance of minute dangling horns in
+hornless breeds of cattle, more especially, according to Youatt, in young
+animals,--and the state of the whole flower in the cauliflower. We often
+see rudiments of various parts in monsters. But I doubt whether any of
+these cases throw light on the origin of rudimentary organs in a state of
+nature, {455} further than by showing that rudiments can be produced; for I
+doubt whether species under nature ever undergo abrupt changes. I believe
+that disuse has been the main agency; that it has led in successive
+generations to the gradual reduction of various organs, until they have
+become rudimentary,--as in the case of the eyes of animals inhabiting dark
+caverns, and of the wings of birds inhabiting oceanic islands, which have
+seldom been forced to take flight, and have ultimately lost the power of
+flying. Again, an organ useful under certain conditions, might become
+injurious under others, as with the wings of beetles living on small and
+exposed islands; and in this case natural selection would continue slowly
+to reduce the organ, until it was rendered harmless and rudimentary.
+
+Any change in function, which can be effected by insensibly small steps, is
+within the power of natural selection; so that an organ rendered, during
+changed habits of life, useless or injurious for one purpose, might be
+modified and used for another purpose. Or an organ might be retained for
+one alone of its former functions. An organ, when rendered useless, may
+well be variable, for its variations cannot be checked by natural
+selection. At whatever period of life disuse or selection reduces an organ,
+and this will generally be when the being has come to maturity and to its
+full powers of action, the principle of inheritance at corresponding ages
+will reproduce the organ in its reduced state at the same age, and
+consequently will seldom affect or reduce it in the embryo. Thus we can
+understand the greater relative size of rudimentary organs in the embryo,
+and their lesser relative size in the adult. But if each step of the
+process of reduction were to be inherited, not at the corresponding age,
+but at an extremely early period of life (as we have good {456} reason to
+believe to be possible), the rudimentary part would tend to be wholly lost,
+and we should have a case of complete abortion. The principle, also, of
+economy, explained in a former chapter, by which the materials forming any
+part or structure, if not useful to the possessor, will be saved as far as
+is possible, will probably often come into play; and this will tend to
+cause the entire obliteration of a rudimentary organ.
+
+As the presence of rudimentary organs is thus due to the tendency in every
+part of the organisation, which has long existed, to be inherited--we can
+understand, on the genealogical view of classification, how it is that
+systematists have found rudimentary parts as useful as, or even sometimes
+more useful than, parts of high physiological importance. Rudimentary
+organs may be compared with the letters in a word, still retained in the
+spelling, but become useless in the pronunciation, but which serve as a
+clue in seeking for its derivation. On the view of descent with
+modification, we may conclude that the existence of organs in a
+rudimentary, imperfect, and useless condition, or quite aborted, far from
+presenting a strange difficulty, as they assuredly do on the ordinary
+doctrine of creation, might even have been anticipated, and can be
+accounted for by the laws of inheritance.
+
+
+
+_Summary._--In this chapter I have attempted to show, that the
+subordination of group to group in all organisms throughout all time; that
+the nature of the relationship, by which all living and extinct beings are
+united by complex, radiating, and circuitous lines of affinities into one
+grand system; the rules followed and the difficulties encountered by
+naturalists in their classifications; the value set upon characters, if
+constant and prevalent, whether of high vital importance, or of the most
+trifling {457} importance, or, as in rudimentary organs, of no importance;
+the wide opposition in value between analogical or adaptive characters, and
+characters of true affinity; and other such rules;--all naturally follow on
+the view of the common parentage of those forms which are considered by
+naturalists as allied, together with their modification through natural
+selection, with its contingencies of extinction and divergence of
+character. In considering this view of classification, it should be borne
+in mind that the element of descent has been universally used in ranking
+together the sexes, ages, and acknowledged varieties of the same species,
+however different they may be in structure. If we extend the use of this
+element of descent,--the only certainly known cause of similarity in
+organic beings,--we shall understand what is meant by the natural system:
+it is genealogical in its attempted arrangement, with the grades of
+acquired difference marked by the terms varieties, species, genera,
+families, orders, and classes.
+
+On this same view of descent with modification, all the great facts in
+Morphology become intelligible,--whether we look to the same pattern
+displayed in the homologous organs, to whatever purpose applied, of the
+different species of a class; or to the homologous parts constructed on the
+same pattern in each individual animal and plant.
+
+On the principle of successive slight variations, not necessarily or
+generally supervening at a very early period of life, and being inherited
+at a corresponding period, we can understand the great leading facts in
+Embryology; namely, the resemblance in an individual embryo of the
+homologous parts, which when matured will become widely different from each
+other in structure and function; and the resemblance in different species
+of a class of the homologous parts or {458} organs, though fitted in the
+adult members for purposes as different as possible. Larvæ are active
+embryos, which have become specially modified in relation to their habits
+of life, through the principle of modifications being inherited at
+corresponding ages. On this same principle--and bearing in mind, that when
+organs are reduced in size, either from disuse or selection, it will
+generally be at that period of life when the being has to provide for its
+own wants, and bearing in mind how strong is the principle of
+inheritance--the occurrence of rudimentary organs and their final abortion,
+present to us no inexplicable difficulties; on the contrary, their presence
+might have been even anticipated. The importance of embryological
+characters and of rudimentary organs in classification is intelligible, on
+the view that an arrangement is only so far natural as it is genealogical.
+
+Finally, the several classes of facts which have been considered in this
+chapter, seem to me to proclaim so plainly, that the innumerable species,
+genera, and families of organic beings, with which this world is peopled,
+have all descended, each within its own class or group, from common
+parents, and have all been modified in the course of descent, that I should
+without hesitation adopt this view, even if it were unsupported by other
+facts or arguments.
+
+       *       *       *       *       *
+
+
+{459}
+
+CHAPTER XIV.
+
+RECAPITULATION AND CONCLUSION.
+
+    Recapitulation of the difficulties on the theory of Natural
+    Selection--Recapitulation of the general and special circumstances in
+    its favour--Causes of the general belief in the immutability of
+    species--How far the theory of natural selection may be
+    extended--Effects of its adoption on the study of Natural
+    history--Concluding remarks.
+
+As this whole volume is one long argument, it may be convenient to the
+reader to have the leading facts and inferences briefly recapitulated.
+
+That many and serious objections may be advanced against the theory of
+descent with modification through natural selection, I do not deny. I have
+endeavoured to give to them their full force. Nothing at first can appear
+more difficult to believe than that the more complex organs and instincts
+should have been perfected, not by means superior to, though analogous
+with, human reason, but by the accumulation of innumerable slight
+variations, each good for the individual possessor. Nevertheless, this
+difficulty, though appearing to our imagination insuperably great, cannot
+be considered real if we admit the following propositions, namely,--that
+gradations in the perfection of any organ or instinct which we may
+consider, either do now exist or could have existed, each good of its
+kind,--that all organs and instincts are, in ever so slight a degree,
+variable,--and, lastly, that there is a struggle for existence leading to
+the preservation of each profitable deviation of structure or instinct. The
+truth of these propositions cannot, I think, be disputed. {460}
+
+It is, no doubt, extremely difficult even to conjecture by what gradations
+many structures have been perfected, more especially amongst broken and
+failing groups of organic beings; but we see so many strange gradations in
+nature, that we ought to be extremely cautious in saying that any organ or
+instinct, or any whole being, could not have arrived at its present state
+by many graduated steps. There are, it must be admitted, cases of special
+difficulty on the theory of natural selection; and one of the most curious
+of these is the existence of two or three defined castes of workers or
+sterile females in the same community of ants; but I have attempted to show
+how this difficulty can be mastered.
+
+With respect to the almost universal sterility of species when first
+crossed, which forms so remarkable a contrast with the almost universal
+fertility of varieties when crossed, I must refer the reader to the
+recapitulation of the facts given at the end of the eighth chapter, which
+seem to me conclusively to show that this sterility is no more a special
+endowment than is the incapacity of two trees to be grafted together; but
+that it is incidental on constitutional differences in the reproductive
+systems of the intercrossed species. We see the truth of this conclusion in
+the vast difference in the result, when the same two species are crossed
+reciprocally; that is, when one species is first used as the father and
+then as the mother.
+
+The fertility of varieties when intercrossed and of their mongrel offspring
+cannot be considered as universal; nor is their very general fertility
+surprising when we remember that it is not likely that either their
+constitutions or their reproductive systems should have been profoundly
+modified. Moreover, most of the varieties which have been experimentised on
+have been {461} produced under domestication; and as domestication (I do
+not mean mere confinement) apparently tends to eliminate sterility, we
+ought not to expect it also to produce sterility.
+
+The sterility of hybrids is a very different case from that of first
+crosses, for their reproductive organs are more or less functionally
+impotent; whereas in first crosses the organs on both sides are in a
+perfect condition. As we continually see that organisms of all kinds are
+rendered in some degree sterile from their constitutions having been
+disturbed by slightly different and new conditions of life, we need not
+feel surprise at hybrids being in some degree sterile, for their
+constitutions can hardly fail to have been disturbed from being compounded
+of two distinct organisations. This parallelism is supported by another
+parallel, but directly opposite, class of facts; namely, that the vigour
+and fertility of all organic beings are increased by slight changes in
+their conditions of life, and that the offspring of slightly modified forms
+or varieties acquire from being crossed increased vigour and fertility. So
+that, on the one hand, considerable changes in the conditions of life and
+crosses between greatly modified forms, lessen fertility; and on the other
+hand, lesser changes in the conditions of life and crosses between less
+modified forms, increase fertility.
+
+Turning to geographical distribution, the difficulties encountered on the
+theory of descent with modification are grave enough. All the individuals
+of the same species, and all the species of the same genus, or even higher
+group, must have descended from common parents; and therefore, in however
+distant and isolated parts of the world they are now found, they must in
+the course of successive generations have passed from some one part to the
+others. We are often wholly unable {462} even to conjecture how this could
+have been effected. Yet, as we have reason to believe that some species
+have retained the same specific form for very long periods, enormously long
+as measured by years, too much stress ought not to be laid on the
+occasional wide diffusion of the same species; for during very long periods
+of time there will always have been a good chance for wide migration by
+many means. A broken or interrupted range may often be accounted for by the
+extinction of the species in the intermediate regions. It cannot be denied
+that we are as yet very ignorant of the full extent of the various climatal
+and geographical changes which have affected the earth during modern
+periods; and such changes will obviously have greatly facilitated
+migration. As an example, I have attempted to show how potent has been the
+influence of the Glacial period on the distribution both of the same and of
+representative species throughout the world. We are as yet profoundly
+ignorant of the many occasional means of transport. With respect to
+distinct species of the same genus inhabiting very distant and isolated
+regions, as the process of modification has necessarily been slow, all the
+means of migration will have been possible during a very long period; and
+consequently the difficulty of the wide diffusion of species of the same
+genus is in some degree lessened.
+
+As on the theory of natural selection an interminable number of
+intermediate forms must have existed, linking together all the species in
+each group by gradations as fine as our present varieties, it may be asked,
+Why do we not see these linking forms all around us? Why are not all
+organic beings blended together in an inextricable chaos? With respect to
+existing forms, we should remember that we have no right to expect
+(excepting in rare cases) to discover _directly_ connecting {463} links
+between them, but only between each and some extinct and supplanted form.
+Even on a wide area, which has during a long period remained continuous,
+and of which the climate and other conditions of life change insensibly in
+going from a district occupied by one species into another district
+occupied by a closely allied species, we have no just right to expect often
+to find intermediate varieties in the intermediate zone. For we have reason
+to believe that only a few species are undergoing change at any one period;
+and all changes are slowly effected. I have also shown that the
+intermediate varieties which will at first probably exist in the
+intermediate zones, will be liable to be supplanted by the allied forms on
+either hand; and the latter, from existing in greater numbers, will
+generally be modified and improved at a quicker rate than the intermediate
+varieties, which exist in lesser numbers; so that the intermediate
+varieties will, in the long run, be supplanted and exterminated.
+
+On this doctrine of the extermination of an infinitude of connecting links,
+between the living and extinct inhabitants of the world, and at each
+successive period between the extinct and still older species, why is not
+every geological formation charged with such links? Why does not every
+collection of fossil remains afford plain evidence of the gradation and
+mutation of the forms of life? We meet with no such evidence, and this is
+the most obvious and forcible of the many objections which may be urged
+against my theory. Why, again, do whole groups of allied species appear,
+though certainly they often falsely appear, to have come in suddenly on the
+several geological stages? Why do we not find great piles of strata beneath
+the Silurian system, stored with the remains of the progenitors of the
+Silurian groups of fossils? For certainly on my theory such {464} strata
+must somewhere have been deposited at these ancient and utterly unknown
+epochs in the world's history.
+
+I can answer these questions and grave objections only on the supposition
+that the geological record is far more imperfect than most geologists
+believe. It cannot be objected that there has not been time sufficient for
+any amount of organic change; for the lapse of time has been so great as to
+be utterly inappreciable by the human intellect. The number of specimens in
+all our museums is absolutely as nothing compared with the countless
+generations of countless species which certainly have existed. We should
+not be able to recognise a species as the parent of any one or more species
+if we were to examine them ever so closely, unless we likewise possessed
+many of the intermediate links between their past or parent and present
+states; and these many links we could hardly ever expect to discover, owing
+to the imperfection of the geological record. Numerous existing doubtful
+forms could be named which are probably varieties; but who will pretend
+that in future ages so many fossil links will be discovered, that
+naturalists will be able to decide, on the common view, whether or not
+these doubtful forms are varieties? As long as most of the links between
+any two species are unknown, if any one link or intermediate variety be
+discovered, it will simply be classed as another and distinct species. Only
+a small portion of the world has been geologically explored. Only organic
+beings of certain classes can be preserved in a fossil condition, at least
+in any great number. Widely ranging species vary most, and varieties are
+often at first local,--both causes rendering the discovery of intermediate
+links less likely. Local varieties will not spread into other and distant
+regions until they are considerably modified and {465} improved; and when
+they do spread, if discovered in a geological formation, they will appear
+as if suddenly created there, and will be simply classed as new species.
+Most formations have been intermittent in their accumulation; and their
+duration, I am inclined to believe, has been shorter than the average
+duration of specific forms. Successive formations are separated from each
+other by enormous blank intervals of time; for fossiliferous formations,
+thick enough to resist future degradation, can be accumulated only where
+much sediment is deposited on the subsiding bed of the sea. During the
+alternate periods of elevation and of stationary level the record will be
+blank. During these latter periods there will probably be more variability
+in the forms of life; during periods of subsidence, more extinction.
+
+With respect to the absence of fossiliferous formations beneath the lowest
+Silurian strata, I can only recur to the hypothesis given in the ninth
+chapter. That the geological record is imperfect all will admit; but that
+it is imperfect to the degree which I require, few will be inclined to
+admit. If we look to long enough intervals of time, geology plainly
+declares that all species have changed; and they have changed in the manner
+which my theory requires, for they have changed slowly and in a graduated
+manner. We clearly see this in the fossil remains from consecutive
+formations invariably being much more closely related to each other, than
+are the fossils from formations distant from each other in time.
+
+Such is the sum of the several chief objections and difficulties which may
+justly be urged against my theory; and I have now briefly recapitulated the
+answers and explanations which can be given to them. I have felt these
+difficulties far too heavily during many years to {466} doubt their weight.
+But it deserves especial notice that the more important objections relate
+to questions on which we are confessedly ignorant; nor do we know how
+ignorant we are. We do not know all the possible transitional gradations
+between the simplest and the most perfect organs; it cannot be pretended
+that we know all the varied means of Distribution during the long lapse of
+years, or that we know how imperfect the Geological Record is. Grave as
+these several difficulties are, in my judgment they do not overthrow the
+theory of descent from a few created forms with subsequent modification.
+
+
+
+Now let us turn to the other side of the argument. Under domestication we
+see much variability. This seems to be mainly due to the reproductive
+system being eminently susceptible to changes in the conditions of life; so
+that this system, when not rendered impotent, fails to reproduce offspring
+exactly like the parent-form. Variability is governed by many complex
+laws,--by correlation of growth, by use and disuse, and by the direct
+action of the physical conditions of life. There is much difficulty in
+ascertaining how much modification our domestic productions have undergone;
+but we may safely infer that the amount has been large, and that
+modifications can be inherited for long periods. As long as the conditions
+of life remain the same, we have reason to believe that a modification,
+which has already been inherited for many generations, may continue to be
+inherited for an almost infinite number of generations. On the other hand
+we have evidence that variability, when it has once come into play, does
+not wholly cease; for new varieties are still occasionally produced by our
+most anciently domesticated productions. {467}
+
+Man does not actually produce variability; he only unintentionally exposes
+organic beings to new conditions of life, and then nature acts on the
+organisation, and causes variability. But man can and does select the
+variations given to him by nature, and thus accumulate them in any desired
+manner. He thus adapts animals and plants for his own benefit or pleasure.
+He may do this methodically, or he may do it unconsciously by preserving
+the individuals most useful to him at the time, without any thought of
+altering the breed. It is certain that he can largely influence the
+character of a breed by selecting, in each successive generation,
+individual differences so slight as to be quite inappreciable by an
+uneducated eye. This process of selection has been the great agency in the
+production of the most distinct and useful domestic breeds. That many of
+the breeds produced by man have to a large extent the character of natural
+species, is shown by the inextricable doubts whether very many of them are
+varieties or aboriginal species.
+
+There is no obvious reason why the principles which have acted so
+efficiently under domestication should not have acted under nature. In the
+preservation of favoured individuals and races, during the
+constantly-recurrent Struggle for Existence, we see the most powerful and
+ever-acting means of selection. The struggle for existence inevitably
+follows from the high geometrical ratio of increase which is common to all
+organic beings. This high rate of increase is proved by calculation,--by
+the rapid increase of many animals and plants during a succession of
+peculiar seasons, or when naturalised in a new country. More individuals
+are born than can possibly survive. A grain in the balance will determine
+which individual shall live and which shall die,--which variety or species
+shall increase in number, and which {468} shall decrease, or finally become
+extinct. As the individuals of the same species come in all respects into
+the closest competition with each other, the struggle will generally be
+most severe between them; it will be almost equally severe between the
+varieties of the same species, and next in severity between the species of
+the same genus. But the struggle will often be very severe between beings
+most remote in the scale of nature. The slightest advantage in one being,
+at any age or during any season, over those with which it comes into
+competition, or better adaptation in however slight a degree to the
+surrounding physical conditions, will turn the balance.
+
+With animals having separated sexes there will in most cases be a struggle
+between the males for possession of the females. The most vigorous
+individuals, or those which have most successfully struggled with their
+conditions of life, will generally leave most progeny. But success will
+often depend on having special weapons or means of defence, or on the
+charms of the males; and the slightest advantage will lead to victory.
+
+As geology plainly proclaims that each land has undergone great physical
+changes, we might have expected that organic beings would have varied under
+nature, in the same way as they generally have varied under the changed
+conditions of domestication. And if there be any variability under nature,
+it would be an unaccountable fact if natural selection had not come into
+play. It has often been asserted, but the assertion is quite incapable of
+proof, that the amount of variation under nature is a strictly limited
+quantity. Man, though acting on external characters alone and often
+capriciously, can produce within a short period a great result by adding up
+mere individual differences in his domestic productions; and every one
+admits that there are at least individual differences in species under
+{469} nature. But, besides such differences, all naturalists have admitted
+the existence of varieties, which they think sufficiently distinct to be
+worthy of record in systematic works. No one can draw any clear distinction
+between individual differences and slight varieties; or between more
+plainly marked varieties and sub-species, and species. Let it be observed
+how naturalists differ in the rank which they assign to the many
+representative forms in Europe and North America.
+
+If then we have under nature variability and a powerful agent always ready
+to act and select, why should we doubt that variations in any way useful to
+beings, under their excessively complex relations of life, would be
+preserved, accumulated, and inherited? Why, if man can by patience select
+variations most useful to himself, should nature fail in selecting
+variations useful, under changing conditions of life, to her living
+products? What limit can be put to this power, acting during long ages and
+rigidly scrutinising the whole constitution, structure, and habits of each
+creature,--favouring the good and rejecting the bad? I can see no limit to
+this power, in slowly and beautifully adapting each form to the most
+complex relations of life. The theory of natural selection, even if we
+looked no further than this, seems to me to be in itself probable. I have
+already recapitulated, as fairly as I could, the opposed difficulties and
+objections: now let us turn to the special facts and arguments in favour of
+the theory.
+
+On the view that species are only strongly marked and permanent varieties,
+and that each species first existed as a variety, we can see why it is that
+no line of demarcation can be drawn between species, commonly supposed to
+have been produced by special acts of creation, and varieties which are
+acknowledged to have been produced by secondary laws. On this same {470}
+view we can understand how it is that in each region where many species of
+a genus have been produced, and where they now flourish, these same species
+should present many varieties; for where the manufactory of species has
+been active, we might expect, as a general rule, to find it still in
+action; and this is the case if varieties be incipient species. Moreover,
+the species of the larger genera, which afford the greater number of
+varieties or incipient species, retain to a certain degree the character of
+varieties; for they differ from each other by a less amount of difference
+than do the species of smaller genera. The closely allied species also of
+the larger genera apparently have restricted ranges, and in their
+affinities they are clustered in little groups round other species--in
+which respects they resemble varieties. These are strange relations on the
+view of each species having been independently created, but are
+intelligible if all species first existed as varieties.
+
+As each species tends by its geometrical ratio of reproduction to increase
+inordinately in number; and as the modified descendants of each species
+will be enabled to increase by so much the more as they become diversified
+in habits and structure, so as to be enabled to seize on many and widely
+different places in the economy of nature, there will be a constant
+tendency in natural selection to preserve the most divergent offspring of
+any one species. Hence during a long-continued course of modification, the
+slight differences, characteristic of varieties of the same species, tend
+to be augmented into the greater differences characteristic of species of
+the same genus. New and improved varieties will inevitably supplant and
+exterminate the older, less improved and intermediate varieties; and thus
+species are rendered to a large extent defined and distinct objects.
+Dominant species belonging to the {471} larger groups tend to give birth to
+new and dominant forms; so that each large group tends to become still
+larger, and at the same time more divergent in character. But as all groups
+cannot thus succeed in increasing in size, for the world would not hold
+them, the more dominant groups beat the less dominant. This tendency in the
+large groups to go on increasing in size and diverging in character,
+together with the almost inevitable contingency of much extinction,
+explains the arrangement of all the forms of life, in groups subordinate to
+groups, all within a few great classes, which we now see everywhere around
+us, and which has prevailed throughout all time. This grand fact of the
+grouping of all organic beings seems to me utterly inexplicable on the
+theory of creation.
+
+As natural selection acts solely by accumulating slight, successive,
+favourable variations, it can produce no great or sudden modification; it
+can act only by very short and slow steps. Hence the canon of "Natura non
+facit saltum," which every fresh addition to our knowledge tends to make
+truer, is on this theory simply intelligible. We can plainly see why nature
+is prodigal in variety, though niggard in innovation. But why this should
+be a law of nature if each species has been independently created, no man
+can explain.
+
+Many other facts are, as it seems to me, explicable on this theory. How
+strange it is that a bird, under the form of woodpecker, should have been
+created to prey on insects on the ground; that upland geese, which never or
+rarely swim, should have been created with webbed feet; that a thrush
+should have been created to dive and feed on sub-aquatic insects; and that
+a petrel should have been created with habits and structure fitting it for
+the life of an auk or grebe! and so on in endless other cases. But on the
+view of each {472} species constantly trying to increase in number, with
+natural selection always ready to adapt the slowly varying descendants of
+each to any unoccupied or ill-occupied place in nature, these facts cease
+to be strange, or perhaps might even have been anticipated.
+
+As natural selection acts by competition, it adapts the inhabitants of each
+country only in relation to the degree of perfection of their associates;
+so that we need feel no surprise at the inhabitants of any one country,
+although on the ordinary view supposed to have been specially created and
+adapted for that country, being beaten and supplanted by the naturalised
+productions from another land. Nor ought we to marvel if all the
+contrivances in nature be not, as far as we can judge, absolutely perfect;
+and if some of them be abhorrent to our ideas of fitness. We need not
+marvel at the sting of the bee causing the bee's own death; at drones being
+produced in such vast numbers for one single act, with the great majority
+slaughtered by their sterile sisters; at the astonishing waste of pollen by
+our fir-trees; at the instinctive hatred of the queen bee for her own
+fertile daughters; at ichneumonidæ feeding within the live bodies of
+caterpillars; and at other such cases. The wonder indeed is, on the theory
+of natural selection, that more cases of the want of absolute perfection
+have not been observed.
+
+The complex and little known laws governing variation are the same, as far
+as we can see, with the laws which have governed the production of
+so-called specific forms. In both cases physical conditions seem to have
+produced but little direct effect; yet when varieties enter any zone, they
+occasionally assume some of the characters of the species proper to that
+zone. In both varieties and species, use and disuse seem to have produced
+some effect; for it is difficult to resist this {473} conclusion when we
+look, for instance, at the logger-headed duck, which has wings incapable of
+flight, in nearly the same condition as in the domestic duck; or when we
+look at the burrowing tucutucu, which is occasionally blind, and then at
+certain moles, which are habitually blind and have their eyes covered with
+skin; or when we look at the blind animals inhabiting the dark caves of
+America and Europe. In both varieties and species correlation of growth
+seems to have played a most important part, so that when one part has been
+modified other parts are necessarily modified. In both varieties and
+species reversions to long-lost characters occur. How inexplicable on the
+theory of creation is the occasional appearance of stripes on the shoulder
+and legs of the several species of the horse-genus and in their hybrids!
+How simply is this fact explained if we believe that these species have
+descended from a striped progenitor, in the same manner as the several
+domestic breeds of pigeon have descended from the blue and barred
+rock-pigeon!
+
+On the ordinary view of each species having been independently created, why
+should the specific characters, or those by which the species of the same
+genus differ from each other, be more variable than the generic characters
+in which they all agree? Why, for instance, should the colour of a flower
+be more likely to vary in any one species of a genus, if the other species,
+supposed to have been created independently, have differently coloured
+flowers, than if all the species of the genus have the same coloured
+flowers? If species are only well-marked varieties, of which the characters
+have become in a high degree permanent, we can understand this fact; for
+they have already varied since they branched off from a common progenitor
+in certain characters, by which they have come to be specifically distinct
+from each other; {474} and therefore these same characters would be more
+likely still to be variable than the generic characters which have been
+inherited without change for an enormous period. It is inexplicable on the
+theory of creation why a part developed in a very unusual manner in any one
+species of a genus, and therefore, as we may naturally infer, of great
+importance to the species, should be eminently liable to variation; but, on
+my view, this part has undergone, since the several species branched off
+from a common progenitor, an unusual amount of variability and
+modification, and therefore we might expect this part generally to be still
+variable. But a part may be developed in the most unusual manner, like the
+wing of a bat, and yet not be more variable than any other structure, if
+the part be common to many subordinate forms, that is, if it has been
+inherited for a very long period; for in this case it will have been
+rendered constant by long-continued natural selection.
+
+Glancing at instincts, marvellous as some are, they offer no greater
+difficulty than does corporeal structure on the theory of the natural
+selection of successive, slight, but profitable modifications. We can thus
+understand why nature moves by graduated steps in endowing different
+animals of the same class with their several instincts. I have attempted to
+show how much light the principle of gradation throws on the admirable
+architectural powers of the hive-bee. Habit no doubt sometimes comes into
+play in modifying instincts; but it certainly is not indispensable, as we
+see, in the case of neuter insects, which leave no progeny to inherit the
+effects of long-continued habit. On the view of all the species of the same
+genus having descended from a common parent, and having inherited much in
+common, we can understand how it is that allied species, when placed under
+considerably different conditions of life, {475} yet should follow nearly
+the same instincts; why the thrush of South America, for instance, lines
+her nest with mud like our British species. On the view of instincts having
+been slowly acquired through natural selection we need not marvel at some
+instincts being apparently not perfect and liable to mistakes, and at many
+instincts causing other animals to suffer.
+
+If species be only well-marked and permanent varieties, we can at once see
+why their crossed offspring should follow the same complex laws in their
+degrees and kinds of resemblance to their parents,--in being absorbed into
+each other by successive crosses, and in other such points,--as do the
+crossed offspring of acknowledged varieties. On the other hand, these would
+be strange facts if species have been independently created, and varieties
+have been produced by secondary laws.
+
+If we admit that the geological record is imperfect in an extreme degree,
+then such facts as the record gives, support the theory of descent with
+modification. New species have come on the stage slowly and at successive
+intervals; and the amount of change, after equal intervals of time, is
+widely different in different groups. The extinction of species and of
+whole groups of species, which has played so conspicuous a part in the
+history of the organic world, almost inevitably follows on the principle of
+natural selection; for old forms will be supplanted by new and improved
+forms. Neither single species nor groups of species reappear when the chain
+of ordinary generation has once been broken. The gradual diffusion of
+dominant forms, with the slow modification of their descendants, causes the
+forms of life, after long intervals of time, to appear as if they had
+changed simultaneously throughout the world. The fact of the fossil remains
+of each formation being in some degree intermediate in character between
+the {476} fossils in the formations above and below, is simply explained by
+their intermediate position in the chain of descent. The grand fact that
+all extinct organic beings belong to the same system with recent beings,
+falling either into the same or into intermediate groups, follows from the
+living and the extinct being the offspring of common parents. As the groups
+which have descended from an ancient progenitor have generally diverged in
+character, the progenitor with its early descendants will often be
+intermediate in character in comparison with its later descendants; and
+thus we can see why the more ancient a fossil is, the oftener it stands in
+some degree intermediate between existing and allied groups. Recent forms
+are generally looked at as being, in some vague sense, higher than ancient
+and extinct forms; and they are in so far higher as the later and more
+improved forms have conquered the older and less improved organic beings in
+the struggle for life. Lastly, the law of the long endurance of allied
+forms on the same continent,--of marsupials in Australia, of edentata in
+America, and other such cases,--is intelligible, for within a confined
+country, the recent and the extinct will naturally be allied by descent.
+
+Looking to geographical distribution, if we admit that there has been
+during the long course of ages much migration from one part of the world to
+another, owing to former climatal and geographical changes and to the many
+occasional and unknown means of dispersal, then we can understand, on the
+theory of descent with modification, most of the great leading facts in
+Distribution. We can see why there should be so striking a parallelism in
+the distribution of organic beings throughout space, and in their
+geological succession throughout time; for in both cases the beings have
+been connected by the bond of ordinary generation, and the means of {477}
+modification have been the same. We see the full meaning of the wonderful
+fact, which must have struck every traveller, namely, that on the same
+continent, under the most diverse conditions, under heat and cold, on
+mountain and lowland, on deserts and marshes, most of the inhabitants
+within each great class are plainly related; for they will generally be
+descendants of the same progenitors and early colonists. On this same
+principle of former migration, combined in most cases with modification, we
+can understand, by the aid of the Glacial period, the identity of some few
+plants, and the close alliance of many others, on the most distant
+mountains, under the most different climates; and likewise the close
+alliance of some of the inhabitants of the sea in the northern and southern
+temperate zones, though separated by the whole intertropical ocean.
+Although two areas may present the same physical conditions of life, we
+need feel no surprise at their inhabitants being widely different, if they
+have been for a long period completely separated from each other; for as
+the relation of organism to organism is the most important of all
+relations, and as the two areas will have received colonists from some
+third source or from each other, at various periods and in different
+proportions, the course of modification in the two areas will inevitably be
+different.
+
+On this view of migration, with subsequent modification, we can see why
+oceanic islands should be inhabited by few species, but of these, that many
+should be peculiar. We can clearly see why those animals which cannot cross
+wide spaces of ocean, as frogs and terrestrial mammals, should not inhabit
+oceanic islands; and why, on the other hand, new and peculiar species of
+bats, which can traverse the ocean, should so often be found on islands far
+distant from any continent. Such facts {478} as the presence of peculiar
+species of bats, and the absence of all other mammals, on oceanic islands,
+are utterly inexplicable on the theory of independent acts of creation.
+
+The existence of closely allied or representative species in any two areas,
+implies, on the theory of descent with modification, that the same parents
+formerly inhabited both areas; and we almost invariably find that wherever
+many closely allied species inhabit two areas, some identical species
+common to both still exist. Wherever many closely allied yet distinct
+species occur, many doubtful forms and varieties of the same species
+likewise occur. It is a rule of high generality that the inhabitants of
+each area are related to the inhabitants of the nearest source whence
+immigrants might have been derived. We see this in nearly all the plants
+and animals of the Galapagos archipelago, of Juan Fernandez, and of the
+other American islands being related in the most striking manner to the
+plants and animals of the neighbouring American mainland; and those of the
+Cape de Verde archipelago and other African islands to the African
+mainland. It must be admitted that these facts receive no explanation on
+the theory of creation.
+
+The fact, as we have seen, that all past and present organic beings
+constitute one grand natural system, with group subordinate to group, and
+with extinct groups often falling in between recent groups, is intelligible
+on the theory of natural selection with its contingencies of extinction and
+divergence of character. On these same principles we see how it is, that
+the mutual affinities of the species and genera within each class are so
+complex and circuitous. We see why certain characters are far more
+serviceable than others for classification;--why adaptive characters,
+though of paramount importance to the being, are of hardly any {479}
+importance in classification; why characters derived from rudimentary
+parts, though of no service to the being, are often of high classificatory
+value; and why embryological characters are the most valuable of all. The
+real affinities of all organic beings are due to inheritance or community
+of descent. The natural system is a genealogical arrangement, in which we
+have to discover the lines of descent by the most permanent characters,
+however slight their vital importance may be.
+
+The framework of bones being the same in the hand of a man, wing of a bat,
+fin of the porpoise, and leg of the horse,--the same number of vertebræ
+forming the neck of the giraffe and of the elephant,--and innumerable other
+such facts, at once explain themselves on the theory of descent with slow
+and slight successive modifications. The similarity of pattern in the wing
+and leg of a bat, though used for such different purpose,--in the jaws and
+legs of a crab,--in the petals, stamens, and pistils of a flower, is
+likewise intelligible on the view of the gradual modification of parts or
+organs, which were alike in the early progenitor of each class. On the
+principle of successive variations not always supervening at an early age,
+and being inherited at a corresponding not early period of life, we can
+clearly see why the embryos of mammals, birds, reptiles, and fishes should
+be so closely alike, and should be so unlike the adult forms. We may cease
+marvelling at the embryo of an air-breathing mammal or bird having
+branchial slits and arteries running in loops, like those in a fish which
+has to breathe the air dissolved in water, by the aid of well-developed
+branchiæ.
+
+Disuse, aided sometimes by natural selection, will often tend to reduce an
+organ, when it has become useless by changed habits or under changed
+conditions {480} of life; and we can clearly understand on this view the
+meaning of rudimentary organs. But disuse and selection will generally act
+on each creature, when it has come to maturity and has to play its full
+part in the struggle for existence, and will thus have little power of
+acting on an organ during early life; hence the organ will not be much
+reduced or rendered rudimentary at this early age. The calf, for instance,
+has inherited teeth, which never cut through the gums of the upper jaw,
+from an early progenitor having well-developed teeth; and we may believe,
+that the teeth in the mature animal were reduced, during successive
+generations, by disuse or by the tongue and palate having been better
+fitted by natural selection to browse without their aid; whereas in the
+calf, the teeth have been left untouched by selection or disuse, and on the
+principle of inheritance at corresponding ages have been inherited from a
+remote period to the present day. On the view of each organic being and
+each separate organ having been specially created, how utterly inexplicable
+it is that parts, like the teeth in the embryonic calf or like the
+shrivelled wings under the soldered wing-covers of some beetles, should
+thus so frequently bear the plain stamp of inutility! Nature may be said to
+have taken pains to reveal, by rudimentary organs and by homologous
+structures, her scheme of modification, which it seems that we wilfully
+will not understand.
+
+
+
+I have now recapitulated the chief facts and considerations which have
+thoroughly convinced me that species have been modified, during a long
+course of descent, by the preservation or the natural selection of many
+successive slight favourable variations. I cannot believe that a false
+theory would explain, as it seems to me that the theory of natural
+selection does explain, {481} the several large classes of facts above
+specified. I see no good reason why the views given in this volume should
+shock the religious feelings of any one. A celebrated author and divine has
+written to me that "he has gradually learnt to see that it is just as noble
+a conception of the Deity to believe that He created a few original forms
+capable of self-development into other and needful forms, as to believe
+that He required a fresh act of creation to supply the voids caused by the
+action of His laws."
+
+Why, it may be asked, have all the most eminent living naturalists and
+geologists rejected this view of the mutability of species? It cannot be
+asserted that organic beings in a state of nature are subject to no
+variation; it cannot be proved that the amount of variation in the course
+of long ages is a limited quantity; no clear distinction has been, or can
+be, drawn between species and well-marked varieties. It cannot be
+maintained that species when intercrossed are invariably sterile, and
+varieties invariably fertile; or that sterility is a special endowment and
+sign of creation. The belief that species were immutable productions was
+almost unavoidable as long as the history of the world was thought to be of
+short duration; and now that we have acquired some idea of the lapse of
+time, we are too apt to assume, without proof, that the geological record
+is so perfect that it would have afforded us plain evidence of the mutation
+of species, if they had undergone mutation.
+
+But the chief cause of our natural unwillingness to admit that one species
+has given birth to other and distinct species, is that we are always slow
+in admitting any great change of which we do not see the intermediate
+steps. The difficulty is the same as that felt by so many geologists, when
+Lyell first insisted that long {482} lines of inland cliffs had been
+formed, and great valleys excavated, by the slow action of the coast-waves.
+The mind cannot possibly grasp the full meaning of the term of a hundred
+million years; it cannot add up and perceive the full effects of many
+slight variations, accumulated during an almost infinite number of
+generations.
+
+Although I am fully convinced of the truth of the views given in this
+volume under the form of an abstract, I by no means expect to convince
+experienced naturalists whose minds are stocked with a multitude of facts
+all viewed, during a long course of years, from a point of view directly
+opposite to mine. It is so easy to hide our ignorance under such
+expressions as the "plan of creation," "unity of design," &c., and to think
+that we give an explanation when we only restate a fact. Any one whose
+disposition leads him to attach more weight to unexplained difficulties
+than to the explanation of a certain number of facts will certainly reject
+my theory. A few naturalists, endowed with much flexibility of mind, and
+who have already begun to doubt on the immutability of species, may be
+influenced by this volume; but I look with confidence to the future, to
+young and rising naturalists, who will be able to view both sides of the
+question with impartiality. Whoever is led to believe that species are
+mutable will do good service by conscientiously expressing his conviction;
+for only thus can the load of prejudice by which this subject is
+overwhelmed be removed.
+
+Several eminent naturalists have of late published their belief that a
+multitude of reputed species in each genus are not real species; but that
+other species are real, that is, have been independently created. This
+seems to me a strange conclusion to arrive at. They admit that a multitude
+of forms, which till lately {483} they themselves thought were special
+creations, and which are still thus looked at by the majority of
+naturalists, and which consequently have every external characteristic
+feature of true species,--they admit that these have been produced by
+variation, but they refuse to extend the same view to other and very
+slightly different forms. Nevertheless they do not pretend that they can
+define, or even conjecture, which are the created forms of life, and which
+are those produced by secondary laws. They admit variation as a _vera
+causa_ in one case, they arbitrarily reject it in another, without
+assigning any distinction in the two cases. The day will come when this
+will be given as a curious illustration of the blindness of preconceived
+opinion. These authors seem no more startled at a miraculous act of
+creation than at an ordinary birth. But do they really believe that at
+innumerable periods in the earth's history certain elemental atoms have
+been commanded suddenly to flash into living tissues? Do they believe that
+at each supposed act of creation one individual or many were produced? Were
+all the infinitely numerous kinds of animals and plants created as eggs or
+seed, or as full grown? and in the case of mammals, were they created
+bearing the false marks of nourishment from the mother's womb? Although
+naturalists very properly demand a full explanation of every difficulty
+from those who believe in the mutability of species, on their own side they
+ignore the whole subject of the first appearance of species in what they
+consider reverent silence.
+
+It may be asked how far I extend the doctrine of the modification of
+species. The question is difficult to answer, because the more distinct the
+forms are which we may consider, by so much the arguments fall away in
+force. But some arguments of the greatest weight {484} extend very far. All
+the members of whole classes can be connected together by chains of
+affinities, and all can be classified on the same principle, in groups
+subordinate to groups. Fossil remains sometimes tend to fill up very wide
+intervals between existing orders. Organs in a rudimentary condition
+plainly show that an early progenitor had the organ in a fully developed
+state; and this in some instances necessarily implies an enormous amount of
+modification in the descendants. Throughout whole classes various
+structures are formed on the same pattern, and at an embryonic age the
+species closely resemble each other. Therefore I cannot doubt that the
+theory of descent with modification embraces all the members of the same
+class. I believe that animals have descended from at most only four or five
+progenitors, and plants from an equal or lesser number.
+
+Analogy would lead me one step further, namely, to the belief that all
+animals and plants have descended from some one prototype. But analogy may
+be a deceitful guide. Nevertheless all living things have much in common,
+in their chemical composition, their germinal vesicles, their cellular
+structure, and their laws of growth and reproduction. We see this even in
+so trifling a circumstance as that the same poison often similarly affects
+plants and animals; or that the poison secreted by the gall-fly produces
+monstrous growths on the wild rose or oak-tree. Therefore I should infer
+from analogy that probably all the organic beings which have ever lived on
+this earth have descended from some one primordial form, into which life
+was first breathed by the Creator.
+
+
+
+When the views advanced by me in this volume, and by Mr. Wallace in the
+Linnean Journal, or when analogous views on the origin of species are
+generally {485} admitted, we can dimly foresee that there will be a
+considerable revolution in natural history. Systematists will be able to
+pursue their labours as at present; but they will not be incessantly
+haunted by the shadowy doubt whether this or that form be in essence a
+species. This I feel sure, and I speak after experience, will be no slight
+relief. The endless disputes whether or not some fifty species of British
+brambles are true species will cease. Systematists will have only to decide
+(not that this will be easy) whether any form be sufficiently constant and
+distinct from other forms, to be capable of definition; and if definable,
+whether the differences be sufficiently important to deserve a specific
+name. This latter point will become a far more essential consideration than
+it is at present; for differences, however slight, between any two forms,
+if not blended by intermediate gradations, are looked at by most
+naturalists as sufficient to raise both forms to the rank of species.
+Hereafter we shall be compelled to acknowledge that the only distinction
+between species and well-marked varieties is, that the latter are known, or
+believed, to be connected at the present day by intermediate gradations,
+whereas species were formerly thus connected. Hence, without rejecting the
+consideration of the present existence of intermediate gradations between
+any two forms, we shall be led to weigh more carefully and to value higher
+the actual amount of difference between them. It is quite possible that
+forms now generally acknowledged to be merely varieties may hereafter be
+thought worthy of specific names, as with the primrose and cowslip; and in
+this case scientific and common language will come into accordance. In
+short, we shall have to treat species in the same manner as those
+naturalists treat genera, who admit that genera are merely artificial
+combinations {486} made for convenience. This may not be a cheering
+prospect; but we shall at least be freed from the vain search for the
+undiscovered and undiscoverable essence of the term species.
+
+The other and more general departments of natural history will rise greatly
+in interest. The terms used by naturalists of affinity, relationship,
+community of type, paternity, morphology, adaptive characters, rudimentary
+and aborted organs, &c., will cease to be metaphorical, and will have a
+plain signification. When we no longer look at an organic being as a savage
+looks at a ship, as at something wholly beyond his comprehension; when we
+regard every production of nature as one which has had a history; when we
+contemplate every complex structure and instinct as the summing up of many
+contrivances, each useful to the possessor, nearly in the same way as when
+we look at any great mechanical invention as the summing up of the labour,
+the experience, the reason, and even the blunders of numerous workmen; when
+we thus view each organic being, how far more interesting, I speak from
+experience, will the study of natural history become!
+
+A grand and almost untrodden field of inquiry will be opened, on the causes
+and laws of variation, on correlation of growth, on the effects of use and
+disuse, on the direct action of external conditions, and so forth. The
+study of domestic productions will rise immensely in value. A new variety
+raised by man will be a more important and interesting subject for study
+than one more species added to the infinitude of already recorded species.
+Our classifications will come to be, as far as they can be so made,
+genealogies; and will then truly give what may be called the plan of
+creation. The rules for classifying will no doubt become simpler when we
+have a definite object in view. We possess no {487} pedigrees or armorial
+bearings; and we have to discover and trace the many diverging lines of
+descent in our natural genealogies, by characters of any kind which have
+long been inherited. Rudimentary organs will speak infallibly with respect
+to the nature of long-lost structures. Species and groups of species, which
+are called aberrant, and which may fancifully be called living fossils,
+will aid us in forming a picture of the ancient forms of life. Embryology
+will reveal to us the structure, in some degree obscured, of the prototypes
+of each great class.
+
+When we can feel assured that all the individuals of the same species, and
+all the closely allied species of most genera, have within a not very
+remote period descended from one parent, and have migrated from some one
+birthplace; and when we better know the many means of migration, then, by
+the light which geology now throws, and will continue to throw, on former
+changes of climate and of the level of the land, we shall surely be enabled
+to trace in an admirable manner the former migrations of the inhabitants of
+the whole world. Even at present, by comparing the differences of the
+inhabitants of the sea on the opposite sides of a continent, and the nature
+of the various inhabitants of that continent in relation to their apparent
+means of immigration, some light can be thrown on ancient geography.
+
+The noble science of Geology loses glory from the extreme imperfection of
+the record. The crust of the earth with its embedded remains must not be
+looked at as a well-filled museum, but as a poor collection made at hazard
+and at rare intervals. The accumulation of each great fossiliferous
+formation will be recognised as having depended on an unusual concurrence
+of circumstances, and the blank intervals between the successive stages as
+having been of vast duration. But we shall {488} be able to gauge with some
+security the duration of these intervals by a comparison of the preceding
+and succeeding organic forms. We must be cautious in attempting to
+correlate as strictly contemporaneous two formations, which include few
+identical species, by the general succession of their forms of life. As
+species are produced and exterminated by slowly acting and still existing
+causes, and not by miraculous acts of creation and by catastrophes; and as
+the most important of all causes of organic change is one which is almost
+independent of altered and perhaps suddenly altered physical conditions,
+namely, the mutual relation of organism to organism,--the improvement of
+one being entailing the improvement or the extermination of others; it
+follows, that the amount of organic change in the fossils of consecutive
+formations probably serves as a fair measure of the lapse of actual time. A
+number of species, however, keeping in a body might remain for a long
+period unchanged, whilst within this same period, several of these species,
+by migrating into new countries and coming into competition with foreign
+associates, might become modified; so that we must not overrate the
+accuracy of organic change as a measure of time. During early periods of
+the earth's history, when the forms of life were probably fewer and
+simpler, the rate of change was probably slower; and at the first dawn of
+life, when very few forms of the simplest structure existed, the rate of
+change may have been slow in an extreme degree. The whole history of the
+world, as at present known, although of a length quite incomprehensible by
+us, will hereafter be recognised as a mere fragment of time, compared with
+the ages which have elapsed since the first creature, the progenitor of
+innumerable extinct and living descendants, was created.
+
+In the distant future I see open fields for far more {489} important
+researches. Psychology will be based on a new foundation, that of the
+necessary acquirement of each mental power and capacity by gradation. Light
+will be thrown on the origin of man and his history.
+
+Authors of the highest eminence seem to be fully satisfied with the view
+that each species has been independently created. To my mind it accords
+better with what we know of the laws impressed on matter by the Creator,
+that the production and extinction of the past and present inhabitants of
+the world should have been due to secondary causes, like those determining
+the birth and death of the individual. When I view all beings not as
+special creations, but as the lineal descendants of some few beings which
+lived long before the first bed of the Silurian system was deposited, they
+seem to me to become ennobled. Judging from the past, we may safely infer
+that not one living species will transmit its unaltered likeness to a
+distant futurity. And of the species now living very few will transmit
+progeny of any kind to a far distant futurity; for the manner in which all
+organic beings are grouped, shows that the greater number of species of
+each genus, and all the species of many genera, have left no descendants,
+but have become utterly extinct. We can so far take a prophetic glance into
+futurity as to foretel that it will be the common and widely-spread
+species, belonging to the larger and dominant groups, which will ultimately
+prevail and procreate new and dominant species. As all the living forms of
+life are the lineal descendants of those which lived long before the
+Silurian epoch, we may feel certain that the ordinary succession by
+generation has never once been broken, and that no cataclysm has desolated
+the whole world. Hence we may look with some confidence to a secure future
+of equally inappreciable length. And as natural selection works {490}
+solely by and for the good of each being, all corporeal and mental
+endowments will tend to progress towards perfection.
+
+It is interesting to contemplate an entangled bank, clothed with many
+plants of many kinds, with birds singing on the bushes, with various
+insects flitting about, and with worms crawling through the damp earth, and
+to reflect that these elaborately constructed forms, so different from each
+other, and dependent on each other in so complex a manner, have all been
+produced by laws acting around us. These laws, taken in the largest sense,
+being Growth with Reproduction; Inheritance which is almost implied by
+reproduction; Variability from the indirect and direct action of the
+external conditions of life, and from use and disuse; a Ratio of Increase
+so high as to lead to a Struggle for Life, and as a consequence to Natural
+Selection, entailing Divergence of Character and the Extinction of
+less-improved forms. Thus, from the war of nature, from famine and death,
+the most exalted object which we are capable of conceiving, namely, the
+production of the higher animals, directly follows. There is grandeur in
+this view of life, with its several powers, having been originally breathed
+by the Creator into a few forms or into one; and that, whilst this planet
+has gone cycling on according to the fixed law of gravity, from so simple a
+beginning endless forms most beautiful and most wonderful have been, and
+are being, evolved.
+
+       *       *       *       *       *
+
+
+{491}
+
+INDEX.
+
+          A.
+
+  Aberrant groups, 429.
+  Abyssinia, plants of, 375.
+  Acclimatisation, 139.
+  Affinities of extinct species, 329.
+  ---- of organic beings, 411.
+  Agassiz on Amblyopsis, 139.
+  ---- on groups of species suddenly appearing, 302, 305.
+  ---- on embryological succession, 338.
+  ---- on the glacial period, 366.
+  ---- on embryological characters, 418.
+  ---- on the embryos of vertebrata, 439.
+  ---- on parallelism of embryological development and geological
+      succession, 449.
+  Algæ of New Zealand, 376.
+  Alligators, males, fighting, 88.
+  Amblyopsis, blind fish, 139.
+  America, North, productions allied to those of Europe, 371.
+  --------, boulders and glaciers of, 373.
+  ----, South, no modern formations on west coast, 290.
+  Ammonites, sudden extinction of, 321.
+  Anagallis, sterility of, 247.
+  Analogy of variations, 159.
+  Ancylus, 386.
+  Animals, not domesticated from being variable, 17.
+  ----, domestic, descended from several stocks, 19.
+  --------, acclimatisation of, 141.
+  ---- of Australia, 116.
+  ---- with thicker fur in cold climates, 133.
+  ----, blind, in caves, 137.
+  ----, extinct, of Australia, 339.
+  Anomma, 240.
+  Antarctic islands, ancient flora of, 399.
+  Antirrhinum, 161.
+  Ants attending aphides, 210.
+  ----, slave-making instinct, 219.
+  ----, neuter, structure of, 236.
+  Aphides, attended by ants, 210.
+  Aphis, development of, 442.
+  Apteryx, 182.
+  Arab horses, 35.
+  Aralo-Caspian Sea, 339.
+  Archaic, M. de, on the succession of species, 325.
+  Artichoke, Jerusalem, 142.
+  Ascension, plants of, 389.
+  Asclepias, pollen of, 193.
+  Asparagus, 359.
+  Aspicarpa, 417.
+  Asses, striped, 163.
+  Ateuchus, 135.
+  Audubon on habits of frigate-bird, 185.
+  ---- on variation in birds'-nests, 212.
+  ---- on heron eating seeds, 387.
+  Australia, animals of, 116.
+  ----. dogs of, 215.
+  ----, extinct animals of, 339.
+  ----, European plants in, 375.
+  Azara on flies destroying cattle, 72.
+  Azores, flora of, 363.
+
+          B.
+
+  Babington, Mr., on British plants, 48.
+  Balancement of growth, 147.
+  Bamboo with hooks, 197.
+  Barberry, flowers of, 98.
+  Barrande, M., on Silurian colonies, 313.
+  ---- on the succession of species, 325.
+  ---- on parallelism of palæozoic formations, 328.
+  ---- on affinities of ancient species, 330.
+  Barriers, importance of, 347.
+  Batrachians on islands, 393.
+  Bats, how structure acquired, 180.
+  ----, distribution of, 394.
+  Bear, catching water-insects, 184.
+  Bee, sting of, 202.
+  ----, queen, killing rivals, 202.
+  Bees fertilising flowers, 73.
+  ----, hive, not sucking the red clover, 95.
+  {492}
+  --------, cell-making instinct, 224.
+  ----, humble, cells of, 225.
+  ----, parasitic, 218.
+  Beetles, wingless, in Madeira, 135.
+  ---- with deficient tarsi, 135.
+  Bentham, Mr., on British plants, 48.
+  ----, on classification, 419.
+  Berkeley, Mr., on seeds in salt-water, 358.
+  Bermuda, birds of, 391.
+  Birds acquiring fear, 212.
+  ---- annually cross the Atlantic, 364.
+  ----, colour of, on continents, 132.
+  ----, footsteps and remains of, in secondary rocks, 304.
+  ----, fossil, in caves of Brazil, 339.
+  ---- of Madeira, Bermuda, and Galapagos, 391.
+  ----, song of males, 89.
+  ---- transporting seeds, 361.
+  ----, waders, 385.
+  ----, wingless, 134, 182.
+  ----, with traces of embryonic teeth, 450.
+  Bizcacha, 349.
+  ----, affinities of, 429.
+  Bladder for swimming in fish, 190.
+  Blindness of cave animals, 137.
+  Blyth, Mr., on distinctness of Indian cattle, 18.
+  ----, on striped Hemionus, 163.
+  ----, on crossed geese, 254.
+  Boar, shoulder-pad of, 88.
+  Borrow, Mr., on the Spanish pointer, 35.
+  Bory St. Vincent on Batrachians, 393.
+  Bosquet, M., on fossil Chthamalus, 305.
+  Boulders, erratic, on the Azores, 363.
+  Branchiæ, 190.
+  Brent, Mr., on house-tumblers, 214.
+  ----, on hawks killing pigeons, 362.
+  Brewer, Dr., on American cuckoo, 217.
+  Britain, mammals of, 396.
+  Bronn on duration of specific forms, 294.
+  Brown, Robert, on classification, 415.
+  Buckman on variation in plants, 10.
+  Buzareingues on sterility of varieties, 270.
+
+          C.
+
+  Cabbage, varieties of, crossed, 99.
+  Calceolaria, 251.
+  Canary-birds, sterility of hybrids, 252.
+  Cape de Verde islands, 398.
+  Cape of Good Hope, plants of, 110, 375.
+  Carrier-pigeons killed by hawks, 362.
+  Cassini on flowers of compositæ, 145.
+  Catasetum, 424.
+  Cats, with blue eyes, deaf, 12.
+  ----, variation in habits of, 91.
+  ---- curling tail when going to spring, 201.
+  Cattle destroying fir-trees, 72.
+  ---- destroyed by flies in La Plata, 72.
+  ----, breeds of, locally extinct, 111.
+  ----, fertility of Indian and European breeds, 254.
+  Cave, inhabitants of, blind, 137.
+  Centres of creation, 352.
+  Cephalopodæ, development of, 442.
+  Cervulus, 253.
+  Cetacea, teeth and hair, 144.
+  Ceylon, plants of, 375.
+  Chalk formation, 322.
+  Characters, divergence of, 111.
+  ----, sexual, variable, 156.
+  ----, adaptive or analogical, 426.
+  Charlock, 76.
+  Checks to increase, 67.
+  ---- ----, mutual, 71.
+  Chickens, instinctive tameness of, 216.
+  Chthamalinæ, 289.
+  Chthamalus, cretacean species of, 305.
+  Circumstances favourable to selection of domestic products, 40.
+  ---- ---- to natural selection, 102.
+  Cirripedes capable of crossing, 101.
+  ----, carapace aborted, 148.
+  ----, their ovigerous frena, 192.
+  ----, fossil, 304.
+  ----, larvæ of, 440.
+  Classification, 413.
+  Clift, Mr., on the succession of types, 339.
+  Climate, effects of, in checking increase of beings, 68.
+  ----, adaptation of, to organisms, 139.
+  {493}
+  Cobites, intestine of, 190.
+  Cockroach, 76.
+  Collections, palæontological, poor, 288.
+  Colour, influenced by climate, 132.
+  ----, in relation to attacks by flies, 198.
+  Columba livia, parent of domestic pigeons, 23.
+  Colymbetes, 386.
+  Compensation of growth, 147.
+  Compositæ, outer and inner florets of, 144.
+  ----, male flowers of, 451.
+  Conclusion, general, 480.
+  Conditions, slight changes in, favourable to fertility, 267.
+  Coot, 185.
+  Coral-islands, seeds drifted to, 361.
+  ---- reefs, indicating movements of earth, 310.
+  Corn-crake, 186.
+  Correlation of growth in domestic productions, 11.
+  ---- of growth, 143, 198.
+  Cowslip, 49.
+  Creation, single centres of, 352.
+  Crinum, 250.
+  Crosses, reciprocal, 258.
+  Crossing of domestic animals, importance in altering breeds, 20.
+  ----, advantages of, 96.
+  ---- unfavourable to selection, 102.
+  Crustacea of New Zealand, 376.
+  Crustacean, blind, 137.
+  Cryptocerus, 239.
+  Ctenomys, blind, 137.
+  Cuckoo, instinct of, 216.
+  Currants, grafts of, 262.
+  Currents of sea, rate of, 360.
+  Cuvier on conditions of existence, 206.
+  ---- on fossil monkeys, 304.
+  ----, Fred., on instinct, 208.
+
+          D.
+
+  Dana, Prof., on blind cave-animals, 139.
+  ----, on relations of crustaceans of Japan, 372.
+  ----, on crustaceans of New Zealand, 376.
+  De Candolle on struggle for existence, 62.
+  ---- on umbelliferæ, 146.
+  ---- on general affinities, 430.
+  ----, Alph., on low plants, widely dispersed, 406.
+  ----, ----, on widely-ranging plants being variable, 53.
+  ----, ----, on naturalisation, 115.
+  ----, ----, on winged seeds, 146.
+  ----, ----, on Alpine species suddenly becoming rare, 175.
+  ----, ----, on distribution of plants with large seeds, 360.
+  ----, ----, on vegetation of Australia, 379.
+  ----, ----, on fresh-water plants, 386.
+  ----, ----, on insular plants, 389.
+  Degradation of coast-rocks, 282.
+  Denudation, rate of, 285.
+  ---- of oldest rocks, 308.
+  Development of ancient forms, 336.
+  Devonian system, 334.
+  Dianthus, fertility of crosses, 256.
+  Dirt on feet of birds, 362.
+  Dispersal, means of, 356.
+  ---- during glacial period, 365.
+  Distribution, geographical, 346.
+  ----, means of, 356.
+  Disuse, effects of, under nature, 134.
+  Divergence of character, 111.
+  Division, physiological, of labour, 115.
+  Dogs, hairless, with imperfect teeth, 12.
+  ---- descended from several wild stocks, 18.
+  ----, domestic instincts of, 213.
+  ----, inherited civilisation of, 215.
+  ----, fertility of breeds together, 254.
+  ----, ---- of crosses, 268.
+  ----, proportions of, when young, 444.
+  Domestication, variation under, 7.
+  Downing, Mr., on fruit-trees in America, 85.
+  Downs, North and South, 286.
+  Dragon-flies, intestines of, 190.
+  Drift-timber, 360.
+  Driver-ant, 240.
+  Drones killed by other bees, 202.
+  Duck, domestic, wings of, reduced, 11.
+  ----, logger-headed, 182.
+  {494}
+  Duckweed, 385.
+  Dugong, affinities of, 414.
+  Dung-beetles with deficient tarsi, 135.
+  Dyticus, 386.
+
+          E.
+
+  Earl, Mr. W., on the Malay Archipelago, 395.
+  Ears, drooping, in domestic animals, 11.
+  ----, rudimentary, 454.
+  Earth, seeds in roots of trees, 361.
+  Eciton, 238.
+  Economy of organisation, 147.
+  Edentata, teeth and hair, 144.
+  ----, fossil species of, 339.
+  Edwards, Milne, on physiological divisions of labour, 115.
+  ----, on gradations of structure, 194.
+  ----, on embryonical characters, 418.
+  Eggs, young birds escaping from, 87.
+  Electric organs, 192.
+  Elephant, rate of increase, 64.
+  ---- of glacial period, 141.
+  Embryology, 438.
+  Existence, struggle for, 60.
+  ----, conditions of, 206.
+  Extinction, as bearing on natural selection, 109.
+  ---- of domestic varieties, 111,
+  ----, 317.
+  Eye, structure of, 187.
+  ----, correction for aberration, 202.
+  Eyes reduced in moles, 137.
+
+          F.
+
+  Fabre, M. on parasitic sphex, 218.
+  Falconer, Dr., on naturalisation of plants in India, 65.
+  ---- on fossil crocodile, 313.
+  ---- on elephants and mastodons, 334.
+  ---- and Cautley on mammals of sub-Himalayan beds, 340.
+  Falkland Island, wolf of, 394.
+  Faults, 285.
+  Faunas, marine, 348.
+  Fear, instinctive, in birds, 212.
+  Feet of bird, young molluscs adhering to, 385.
+  Fertility of hybrids, 249.
+  ---- from slight changes in conditions, 267.
+  ---- of crossed varieties, 268.
+  Fir-trees destroyed by cattle, 72.
+  ---- ----, pollen of, 203.
+  Fish, flying, 182.
+  ----, teleostean, sudden appearance of, 305.
+  ---- eating seeds, 362, 387.
+  ----, fresh-water, distribution of, 384.
+  Fishes, ganoid, now confined to fresh water, 107.
+  ----, electric organs of, 192.
+  ----, ganoid, living in fresh water, 321.
+  ---- of southern hemisphere, 376.
+  Flight, powers of, how acquired, 182.
+  Flowers, structure of, in relation to crossing, 97.
+  ---- of compositæ and umbelliferæ, 144.
+  Forbes, E., on colours of shells, 132.
+  ---- on abrupt range of shells in depth, 175.
+  ---- on poorness of palæontological collections, 288.
+  ---- on continuous succession of genera, 316.
+  ---- on continental extensions, 357.
+  ---- on distribution during glacial period, 366.
+  ---- on parallelism in time and space, 409.
+  Forests, changes in, in America, 74.
+  Formation, Devonian, 334.
+  Formations, thickness of, in Britain, 284.
+  ----, intermittent, 290.
+  Formica rufescens, 219.
+  ---- sanguinea, 219.
+  ---- flava, neuter of, 240.
+  Frena, ovigerous, of cirripedes, 192.
+  Fresh-water productions, dispersal of, 383.
+  Fries on species in large genera being closely allied to other species,
+      57.
+  Frigate-bird, 185.
+  Frogs on islands, 393.
+  Fruit-trees, gradual improvement of, 37.
+  ---- ---- in United States, 85.
+  ---- ----, varieties of, acclimatised in United States, 142.
+  {495}
+  Fuci, crossed, 258.
+  Fur, thicker in cold climates, 133.
+  Furze, 439.
+
+          G.
+
+  Galapagos Archipelago, birds of, 390.
+  ----, productions of, 398, 400.
+  Galeopithecus, 181.
+  Game, increase of, checked by vermin, 68.
+  Gärtner on sterility of hybrids, 247, 255.
+  ----, on reciprocal crosses, 258.
+  ----, on crossed maize and verbascum, 270.
+  ----, on comparison of hybrids and mongrels, 272.
+  Geese, fertility when crossed, 253.
+  ----, upland, 185.
+  Genealogy important in classification, 425.
+  Geoffroy St. Hilaire on balancement, 147.
+  ---- ---- on homologous organs, 434.
+  ---- ----, Isidore, on variability of repeated parts, 149.
+  ---- ----, on correlation in monstrosities, 11.
+  ---- ----, on correlation, 144.
+  ---- ----, on variable parts being often monstrous, 155.
+  Geographical distribution, 346.
+  Geography, ancient, 487.
+  Geology, future progress of, 487.
+  ----, imperfection of the record, 279.
+  Giraffe, tail of, 195.
+  Glacial period, 365.
+  Gmelin on distribution, 365.
+  Gnathodon, fossil, 368.
+  Godwin-Austen, Mr., on the Malay Archipelago, 300.
+  Goethe on compensation of growth, 147.
+  Gooseberry, grafts of, 262.
+  Gould, Dr. A., on land-shells, 397.
+  ----, Mr., on colours of birds, 132.
+  ----, on birds of the Galapagos, 398.
+  ----, on distribution of genera of birds, 404.
+  Gourds, crossed, 270.
+  Grafts, capacity of, 261.
+  Grasses, varieties of, 113.
+  Gray, Dr. Asa, on trees of United States, 100.
+  ----, on naturalised plants in the United States, 115.
+  ----, on rarity of intermediate varieties, 176.
+  ----, on Alpine plants, 365.
+  ----, Dr. J. E., on striped mule, 165.
+  Grebe, 185.
+  Groups, aberrant, 429.
+  Grouse, colours of, 84.
+  ----, red, a doubtful species, 49.
+  Growth, compensation of, 147.
+  ----, correlation of, in domestic products, 11.
+  ----, correlation of, 143.
+
+          H.
+
+  Habit, effect of, under domestication, 11.
+  ----, effect of, under nature, 134.
+  ----, diversified, of same species, 183.
+  Hair and teeth, correlated, 144.
+  Harcourt, Mr. E. V., on the birds of Madeira, 391.
+  Hartung, M. on boulders in the Azores, 363.
+  Hazel-nuts, 359.
+  Hearne on habits of bears, 184.
+  Heath, changes in vegetation, 72.
+  Heer, O., on plants of Madeira, 107.
+  Helix pomatia, 397.
+  Helosciadium, 359.
+  Hemionus, striped, 163.
+  Herbert, W., on struggle for existence, 62.
+  ----, on sterility of hybrids, 249.
+  Hermaphrodites crossing, 96.
+  Heron eating seed, 387.
+  Heron, Sir R., on peacocks, 89.
+  Heusinger on white animals not poisoned by certain plants, 12.
+  Hewitt, Mr., on sterility of first crosses, 264.
+  Himalaya, glaciers of, 373.
+  ----, plants of, 375.
+  Hippeastrum, 250.
+  Holly-trees, sexes of, 93.
+  Hollyhock, varieties of, crossed, 271.
+  Hooker, Dr., on trees of New Zealand, 100.
+  {496}
+  ----, on acclimatisation of Himalayan trees, 140.
+  ----, on flowers of umbelliferæ, 145.
+  ----, on glaciers of Himalaya, 373.
+  ----, on algæ of New Zealand, 376.
+  ----, on vegetation at the base of the Himalaya, 378.
+  ----, on plants of Tierra del Fuego, 374, 378.
+  ----, on Australian plants, 375, 399.
+  ----, on relations of flora of South America, 379.
+  ----, on flora of the Antarctic lands, 381, 399.
+  ----, on the plants of the Galapagos, 392, 398.
+  Hooks on bamboos, 197.
+  ---- to seeds on islands, 392.
+  Horner, Mr., on the antiquity of Egyptians, 18.
+  Horns, rudimentary, 454.
+  Horse, fossil, in La Plata, 318.
+  Horses destroyed by flies in La Plata, 72.
+  ----, striped, 163.
+  ----, proportions of, when young, 444.
+  Horticulturists, selection applied by, 32.
+  Huber on cells of bees, 230.
+  ----, P., on reason blended with instinct, 208.
+  ----, on habitual nature of instincts, 208.
+  ----, on slave-making ants, 219.
+  ----, on Melipona domestica, 225.
+  Humble-bees, cells of, 225.
+  Hunter, J., on secondary sexual characters, 150.
+  Hutton, Captain, on crossed geese, 254.
+  Huxley, Prof., on structure of hermaphrodites, 101.
+  ----, on embryological succession, 338.
+  ----, on homologous organs, 438.
+  ----, on the development of aphis, 442.
+  Hybrids and mongrels compared, 272.
+  Hybridism, 245.
+  Hydra, structure of, 190.
+
+          I.
+
+  Ibla, 148.
+  Icebergs transporting seeds, 363.
+  Increase, rate of, 63.
+  Individuals, numbers favourable to selection, 102.
+  ----, many, whether simultaneously created, 355.
+  Inheritance, laws of, 12.
+  ---- at corresponding ages, 14, 86.
+  Insects, colour of, fitted for habitations, 84.
+  ----, sea-side, colours of, 132.
+  ----, blind, in caves, 138.
+  ----, luminous, 193.
+  ----, neuter, 236.
+  Instinct, 207.
+  Instincts, domestic, 213.
+  Intercrossing, advantages of, 96.
+  Islands, oceanic, 388.
+  Isolation favourable to selection, 104.
+
+          J.
+
+  Japan, productions of, 372.
+  Java, plants of, 375.
+  Jones, Mr. J. M., on the birds of Bermuda, 391.
+  Jussieu on classification, 417.
+
+          K.
+
+  Kentucky, caves of, 137.
+  Kerguelen-land, flora of, 381, 399.
+  Kidney-bean, acclimatisation of, 142.
+  Kidneys of birds, 144.
+  Kirby on tarsi deficient in beetles, 135.
+  Knight, Andrew, on cause of variation, 7.
+  Kölreuter on the barberry, 98.
+  ---- on sterility of hybrids, 246.
+  ---- on reciprocal crosses, 258.
+  ---- on crossed varieties of nicotiana, 271.
+  ---- on crossing male and hermaphrodite flowers, 451.
+
+          L.
+
+  Lamarck on adaptive characters, 426.
+  Land-shells, distribution of, 397.
+  ---- of Madeira, naturalised, 403.
+  Languages, classification of, 422.
+  Lapse, great, of time, 282.
+  {497}
+  Larvæ, 440.
+  Laurel, nectar secreted by the leaves,
+  Laws of variation, 131.
+  Leech, varieties of, 76.
+  Leguminosæ, nectar secreted by glands, 92.
+  Lepidosiren, 107, 330.
+  Life, struggle for, 60.
+  Lingula, Silurian, 307.
+  Linnæus, aphorism of, 413.
+  Lion, mane of, 88.
+  ----, young of, striped, 439.
+  Lobelia fulgens, 73, 98.
+  Lobelia, sterility of crosses, 250.
+  Loess of the Rhine, 384.
+  Lowness of structure connected with variability, 149.
+  Lowness, related to wide distribution, 406.
+  Lubbock, Mr., on the nerves of coccus, 46.
+  Lucas, Dr. P., on inheritance, 12.
+  ----, on resemblance of child to parent, 275.
+  Lund and Clausen on fossils of Brazil, 339.
+  Lyell, Sir C, on the struggle for existence, 62.
+  ----, on modern changes of the earth, 95.
+  ----, on measure of denudation, 284.
+  ----, on a carboniferous land-shell, 289.
+  ----, on strata beneath Silurian system, 308.
+  ----, on the imperfection of the geological record, 311.
+  ----, on the appearance of species, 312.
+  ----, on Barrande's colonies, 313.
+  ----, on tertiary formations of Europe and North America, 323.
+  ----, on parallelism of tertiary formations, 328.
+  ----, on transport of seeds by icebergs, 363.
+  ----, on great alternations of climate, 382.
+  ----, on the distribution of fresh-water shells, 385.
+  ----, on land-shells of Madeira, 402.
+  Lyell and Dawson on fossilized trees in Nova Scotia, 297.
+
+          M.
+
+  Macleay on analogical characters, 426.
+  Madeira, plants of, 107.
+  ----, beetles of, wingless, 135.
+  ----, fossil land-shells of, 339.
+  ----, birds of, 390.
+  Magpie tame in Norway, 212.
+  Maize, crossed, 270.
+  Malay Archipelago compared with Europe, 300.
+  ----, mammals of, 395.
+  Malpighiaceæ, 417.
+  Mammæ, rudimentary, 451.
+  Mammals, fossil, in secondary formation, 304.
+  ----, insular, 394.
+  Man, origin of races of, 199.
+  Manatee, rudimentary nails of, 454.
+  Marsupials of Australia, 116.
+  ----, fossil species of, 339.
+  Martens, M., experiment on seeds, 360.
+  Martin, Mr. W. C., on striped mules, 165.
+  Matteucci on the electric organs of rays, 193.
+  Matthiola, reciprocal crosses of, 258.
+  Means of dispersal, 356.
+  Melipona domestica, 225.
+  Metamorphism of oldest rocks, 308.
+  Mice destroying bees, 74.
+  ----, acclimatisation of, 141.
+  Migration, bears on first appearance of fossils, 297.
+  Miller, Prof., on the cells of bees, 226.
+  Mirabilis, crosses of, 258.
+  Missel-thrush, 76.
+  Misseltoe, complex relations of, 3.
+  Mississippi, rate of deposition at mouth, 284.
+  Mocking-thrush of the Galapagos, 402.
+  Modification of species, how far applicable, 483.
+  Moles, blind, 137.
+  Mongrels, fertility and sterility of, 268.
+  ---- and hybrids compared, 272.
+  {498}
+  Monkeys, fossil, 304.
+  Monocanthus, 424.
+  Mons, Van, on the origin of fruit-trees, 29.
+  Moquin-Tandon on sea-side plants, 132.
+  Morphology, 433.
+  Mozart, musical powers of, 209.
+  Mud, seeds in, 386.
+  Mules, striped, 165.
+  Müller, Dr. F., on Alpine Australian plants, 375.
+  Murchison, Sir R., on the formations of Russia, 290.
+  ----, on azoic formations, 308.
+  ----, on extinction, 317.
+  Mustela vison, 179.
+  Myanthus, 424.
+  Myrmecocystus, 239.
+  Myrmica, eyes of, 240.
+
+          N.
+
+  Nails, rudimentary, 454.
+  Natural history, future progress of, 485.
+  ---- selection, 80.
+  ---- system, 413.
+  Naturalisation of forms distinct from the indigenous species, 115.
+  ---- in New Zealand, 201.
+  Nautilus, Silurian, 307.
+  Nectar of plants, 92.
+  Nectaries, how formed, 92.
+  Nelumbium luteum, 387.
+  Nests, variation in, 211.
+  Neuter insects, 236.
+  Newman, Mr., on humble-bees, 74.
+  New Zealand, productions of, not perfect, 201.
+  ----, naturalised products of, 337.
+  ----, fossil birds of, 339.
+  ----, glacial action in, 373.
+  ----, crustaceans of, 376.
+  ----, algæ of, 376.
+  ----, number of plants of, 389.
+  ----, flora of, 399.
+  Nicotiana, crossed varieties of, 271.
+  ----, certain species very sterile, 257.
+  Noble, Mr., on fertility of Rhododendron, 252.
+  Nodules, phosphatic, in azoic rocks, 308.
+
+          O.
+
+  Oak, varieties of, 50.
+  Onites apelles, 135.
+  Orchis, pollen of, 193.
+  Organs of extreme perfection, 186.
+  ----, electric, of fishes, 192.
+  ---- of little importance, 194.
+  ----, homologous, 434.
+  ----, rudiments of, and nascent, 450.
+  Ornithorhynchus, 107, 416.
+  Ostrich not capable of flight, 134.
+  ----, habit of laying eggs together, 218.
+  ----, American, two species of, 349.
+  Otter, habits of, how acquired, 179.
+  Ouzel, water, 185.
+  Owen, Prof., on birds not flying, 134.
+  ----, on vegetative repetition, 149.
+  ----, on variable length of arms in ourang-outang, 150.
+  ----, on the swim-bladder of fishes, 191.
+  ----, on electric organs, 192.
+  ----, on fossil horse of La Plata, 319.
+  ----, on relations of ruminants and pachyderms, 329.
+  ----, on fossil birds of New Zealand, 339.
+  ----, on succession of types, 339.
+  ----, on affinities of the dugong, 414.
+  ----, on homologous organs, 434.
+  ----, on the metamorphosis of cephalopods and spiders, 442.
+
+          P.
+
+  Pacific Ocean, faunas of, 348.
+  Paley on no organ formed to give pain, 201.
+  Pallas on the fertility of the wild stocks of domestic animals, 254.
+  Paraguay, cattle destroyed by flies, 72.
+  Parasites, 217.
+  Partridge, dirt on feet, 363.
+  Parts greatly developed, variable, 150.
+  ----, degrees of utility of, 201.
+  Parus major, 184.
+  Passiflora, 251.
+  Peaches in United States, 85.
+  Pear, grafts of, 262.
+  {499}
+  Pelargonium, flowers of, 145.
+  ----, sterility of, 251.
+  Pelvis of women, 144.
+  Peloria, 145.
+  Period, glacial, 365.
+  Petrels, habits of, 184.
+  Phasianus, fertility of hybrids, 253.
+  Pheasant, young, wild, 216.
+  Philippi on tertiary species in Sicily, 312.
+  Pictet, Prof., on groups of species suddenly appearing, 302, 305.
+  ----, on rate of organic change, 313.
+  ----, on continuous succession of genera, 316.
+  ----, on close alliance of fossils in consecutive formations, 335.
+  ----, on embryological succession, 338.
+  Pierce, Mr., on varieties of wolves, 91.
+  Pigeons with feathered feet and skin between toes, 12.
+  ----, breeds described, and origin of, 20.
+  ----, breeds of, how produced, 39, 42.
+  ----, tumbler, not being able to get out of egg, 87.
+  ----, reverting to blue colour, 160.
+  ----, instinct of tumbling, 214.
+  ----, carriers, killed by hawks, 362.
+  ----, young of, 445.
+  Pistil, rudimentary, 451.
+  Plants, poisonous, not affecting certain coloured animals, 12.
+  ----, selection applied to, 32.
+  ----, gradual improvement of, 37.
+  ---- not improved in barbarous countries, 38.
+  ---- destroyed by insects, 67.
+  ----, in midst of range, have to struggle with other plants, 77.
+  ----, nectar of, 92.
+  ----, fleshy, on sea-shores, 132.
+  ----, fresh-water, distribution of, 386.
+  ----, low in scale, widely distributed, 406.
+  Plumage, laws of change in sexes of birds, 89.
+  Plums in the United States, 85.
+  Pointer dog, origin of, 35.
+  ----, habits of, 213.
+  Poison not affecting certain coloured animals, 12.
+  ----, similar effect of, on animals and plants, 484.
+  Pollen of fir-trees, 203.
+  Poole, Col., on striped hemionus, 163.
+  Potamogeton, 387.
+  Prestwich, Mr., on English and French eocene formations, 328.
+  Primrose, 49.
+  ----, sterility of, 247.
+  Primula, varieties of, 49.
+  Proteolepas, 148.
+  Proteus, 139.
+  Psychology, future progress of, 489.
+
+          Q.
+
+  Quagga, striped, 165.
+  Quince, grafts of, 262.
+
+          R.
+
+  Rabbit, disposition of young, 215.
+  Races, domestic, characters of, 16.
+  Race-horses, Arab, 35.
+  ----, English, 356.
+  Ramond on plants of Pyrenees, 368.
+  Ramsay, Prof., on thickness of the British formations, 284.
+  ----, on faults, 285.
+  Ratio of increase, 63.
+  Rats, supplanting each other, 76.
+  ----, acclimatisation of, 141.
+  ----, blind in cave, 137.
+  Rattle-snake, 201.
+  Reason and instinct, 208.
+  Recapitulation, general, 459.
+  Reciprocity of crosses, 258.
+  Record, geological, imperfect, 279.
+  Rengger on flies destroying cattle, 72.
+  Reproduction, rate of, 63.
+  Resemblance to parents in mongrels and hybrids, 273.
+  Reversion, law of inheritance, 14.
+  ---- in pigeons to blue colour, 160.
+  Rhododendron, sterility of, 251.
+  Richard, Prof., on Aspicarpa, 417.
+  Richardson, Sir J., on structure of squirrels, 180.
+  ----, on fishes of the southern hemisphere, 376.
+  Robinia, grafts of, 262.
+  {500}
+  Rodents, blind, 137.
+  Rudimentary organs, 450.
+  Rudiments important for classification, 416.
+
+          S.
+
+  Sagaret on grafts, 262.
+  Salmons, males fighting, and hooked jaws of, 88.
+  Salt-water, how far injurious to seeds, 358.
+  Saurophagus sulphuratus, 183.
+  Schiödte on blind insects, 138.
+  Schlegel on snakes, 144.
+  Sea-water, how far injurious to seeds, 358.
+  Sebright, Sir J., on crossed animals, 20.
+  ----, on selection of pigeons, 31.
+  Sedgwick, Prof., on groups of species suddenly appearing, 302.
+  Seedlings destroyed by insects, 67.
+  Seeds, nutriment in, 77.
+  ----, winged, 146.
+  ----, power of resisting salt-water, 358.
+  ---- in crops and intestines of birds, 361.
+  ---- eaten by fish, 362, 387.
+  ---- in mud, 386.
+  ----, hooked, on islands, 392.
+  Selection of domestic products, 29.
+  ----, principle not of recent origin, 33.
+  ----, unconscious, 34.
+  ----, natural, 80.
+  ----, sexual, 87.
+  ----, natural, circumstances favourable to, 102.
+  Sexes, relations of, 87.
+  Sexual characters variable, 156.
+  ---- selection, 87.
+  Sheep, Merino, their selection, 31.
+  ----, two sub-breeds unintentionally produced, 36.
+  ----, mountain, varieties of, 76.
+  Shells, colours of, 132.
+  ----, littoral, seldom embedded, 288.
+  ----, fresh-water, dispersal of, 385
+  ---- of Madeira, 391.
+  ----, land, distribution of, 397.
+  Silene, fertility of crosses, 257.
+  Silliman, Prof., on blind rat, 137.
+  Skulls of young mammals, 197, 436.
+  Slave-making instinct, 219.
+  Smith, Col. Hamilton, on striped horses, 164.
+  ----, Mr. Fred., on slave-making ants, 219.
+  ----, on neuter ants, 239.
+  ----, Mr., of Jordan Hill, on the degradation of coast-rocks, 283.
+  Snap-dragon, 161.
+  Somerville, Lord, on selection of sheep, 31.
+  Sorbus, grafts of, 262.
+  Spaniel, King Charles's breed, 35.
+  Species, polymorphic, 46.
+  ----, common, variable, 53.
+  ---- in large genera variable, 54.
+  ----, groups of, suddenly appearing, 302, 307.
+  ---- beneath Silurian formations, 307.
+  ---- successively appearing, 312.
+  ---- changing simultaneously throughout the world, 322.
+  Spencer, Lord, on increase in size of cattle, 35.
+  Sphex, parasitic, 218.
+  Spiders, development of, 442.
+  Spitz-dog crossed with fox, 268.
+  Sports in plants, 9.
+  Sprengel, C. C, on crossing, 98.
+  ----, on ray-florets, 145.
+  Squirrels, gradations in structure, 180.
+  Staffordshire, heath, changes in, 71.
+  Stag-beetles, fighting, 88.
+  Sterility from changed conditions of life, 9.
+  ---- of hybrids, 246.
+  ---- ----, laws of, 255.
+  ---- ----, causes of, 263.
+  ---- from unfavourable conditions, 265.
+  ---- of certain varieties, 269.
+  St. Helena, productions of, 390.
+  St. Hilaire, Aug., on classification, 418.
+  St. John, Mr., on habits of cats, 91.
+  Sting of bee, 202.
+  Stocks, aboriginal, of domestic animals, 18.
+  Strata, thickness of, in Britain, 284.
+  Stripes on horses, 163.
+  {501}
+  Structure, degrees of utility of, 201.
+  Struggle for existence, 60.
+  Succession, geological, 312.
+  Succession of types in same areas, 338.
+  Swallow, one species supplanting another, 76.
+  Swim-bladder, 190.
+  System, natural, 413.
+
+          T.
+
+  Tail of giraffe, 195.
+  ---- of aquatic animals, 196.
+  ----, rudimentary, 454.
+  Tarsi deficient, 135.
+  Tausch on umbelliferous flowers, 146.
+  Teeth and hair correlated, 144.
+  ----, embryonic, traces of, in birds, 450.
+  ----, rudimentary, in embryonic calf, 450, 480.
+  Tegetmeier, Mr., on cells of bees, 228, 233.
+  Temminck on distribution aiding classification, 419.
+  Thouin on grafts, 262.
+  Thrush, aquatic species of, 185.
+  ----, mocking, of the Galapagos, 402.
+  ----, young of, spotted, 439.
+  ----, nest of, 243.
+  Thuret, M., on crossed fuci, 258.
+  Thwaites, Mr., on acclimatisation, 140.
+  Tierra del Fuego, dogs of, 215.
+  ----, plants of, 374, 378.
+  Timber-drift, 360.
+  Time, lapse of, 282.
+  Titmouse, 184.
+  Toads on islands, 393.
+  Tobacco, crossed varieties of, 271.
+  Tomes, Mr., on the distribution of bats, 395.
+  Transitions in varieties rare, 172.
+  Trees on islands belong to peculiar orders, 392.
+  ---- with separated sexes, 99.
+  Trifolium pratense, 73, 94.
+  ---- incarnatum, 94.
+  Trigonia, 321.
+  Trilobites, 307.
+  ----, sudden extinction of, 321.
+  Troglodytes, 243.
+  Tucutucu, blind, 137.
+  Tumbler pigeons, habits of, hereditary, 214.
+  ----, young of, 446.
+  Turkey-cock, brush of hair on breast, 90.
+  Turkey, naked skin on head, 197.
+  ----, young, wild, 216.
+  Turnip and cabbage, analogous variations of, 159.
+  Type, unity of, 206.
+  Types, succession of, in same areas, 339.
+
+          U.
+
+  Udders enlarged by use, 11.
+  ----, rudimentary, 451.
+  Ulex, young leaves of, 439.
+  Umbelliferæ, outer and inner florets of, 144.
+  Unity of type, 206.
+  Use, effects of, under domestication, 11.
+  ----, effects of, in a state of nature, 134.
+  Utility, how far important in the construction of each part, 199.
+
+          V.
+
+  Valenciennes on fresh-water fish, 384.
+  Variability of mongrels and hybrids, 274.
+  Variation under domestication, 7.
+  ---- caused by reproductive system being affected by conditions of life,
+      8.
+  ---- under nature, 44.
+  ----, laws of, 131.
+  Variations appear at corresponding ages, 14, 86.
+  ----, analogous in distinct species, 159.
+  Varieties, natural, 44.
+  ----, struggle between, 75.
+  ----, domestic, extinction of, 111.
+  ----, transitional, rarity of, 172.
+  ----, when crossed, fertile, 268.
+  ----, when crossed, sterile, 269.
+  ----, classification of, 423.
+  Verbascum, sterility of, 251.
+  ----, varieties of, crossed, 271.
+  Verneuil, M. de, on the succession of species, 325.
+  Viola tricolor, 73.
+  {502}
+  Volcanic islands, denudation of, 285.
+  Vulture, naked skin on head, 197.
+
+          W.
+
+  Wading-birds, 386.
+  Wallace, Mr., on origin of species, 2.
+  ----, on law of geographical distribution, 355.
+  ----, on the Malay Archipelago, 395.
+  Wasp, sting of, 202.
+  Water, fresh, productions of, 383.
+  Water-hen, 185.
+  Waterhouse, Mr., on Australian marsupials, 116.
+  ----, on greatly developed parts being variable, 150.
+  ----, on the cells of bees, 225.
+  ----, on general affinities, 429.
+  Water-ouzel, 185.
+  Watson, Mr. H. C, on range of varieties of British plants, 58.
+  ----, on acclimatisation, 140.
+  ----, on flora of Azores, 363.
+  ----, on Alpine plants, 368, 376.
+  ----, on rarity of intermediate varieties, 176.
+  Weald, denudation of, 285.
+  Web of feet in water-birds, 185.
+  West Indian islands, mammals of, 396.
+  Westwood on species in large genera being closely allied to others, 57.
+  ---- on the tarsi of Engidæ, 157.
+  ---- on the antennæ of hymenopterous insects, 415.
+  Wheat, varieties of, 113.
+  White Mountains, flora of, 365.
+  Wings, reduction of size, 134.
+  ---- of insects homologous with branchiæ, 191.
+  ----, rudimentary, in insects, 450.
+  Wolf crossed with dog, 214.
+  ---- of Falkland Isles, 394.
+  Wollaston, Mr., on varieties of insects, 48.
+  ----, on fossil varieties of land-shells in Madeira, 52.
+  ----, on colours of insects on sea-shore, 132.
+  ----, on wingless beetles, 135.
+  ----, on rarity of intermediate varieties, 176.
+  ----, on insular insects, 389.
+  ----, on land-shells of Madeira, naturalised, 402.
+  Wolves, varieties of, 90.
+  Woodpecker, habits of, 184.
+  ----, green colour of, 197.
+  Woodward, Mr., on the duration of specific forms, 294.
+  ----, on the continuous succession of genera, 316.
+  ----, on the succession of types, 339.
+  World, species changing simultaneously throughout, 322.
+  Wrens, nest of, 243.
+
+          Y.
+
+  Youatt, Mr., on selection, 31.
+  ----, on sub-breeds of sheep, 36.
+  ----, on rudimentary horns in young cattle, 454.
+
+          Z.
+
+  Zebra, stripes on, 163.
+
+THE END.
+
+       *       *       *       *       *
+
+LONDON: PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET, AND CHARING
+CROSS.
+
+       *       *       *       *       *
+
+
+Corrections made to printed original.
+
+p. 133. "the slightest use to a being": 'slighest' in original.
+
+p. 193. "as Matteucci asserts": 'Matteucei' in original (the index
+correctly has Matteucci).
+
+p. 201. "deposited in the living bodies of other insects": 'depo-sisted'
+(across page break) in original.
+
+p. 315. "the newly-formed fantail": 'faintail' in original.
+
+p. 398. "the volcanic nature of the soil": 'volanic' in original.
+
+p. 403. "Madeira and the adjoining islet": 'Maderia' in original; and so in
+"from Porto Santo to Madeira".
+
+p. 442. "the same individual embryo": 'indivividual' in original.
+
+p. 458. "innumerable species, genera, and families": 'inumerable' in
+original.
+
+p. 490. "Inheritance which is almost implied by reproduction":
+'Inheritrnce' in original.
+
+
+
+
+*** END OF THE PROJECT GUTENBERG EBOOK ORIGIN OF SPECIES ***
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+<div style='text-align:center; font-size:1.2em; font-weight:bold;'>The Project Gutenberg eBook of On the Origin of Species by Means of Natural Selection, by Charles Darwin</div>
+<div style='display:block; margin:1em 0'>
+This eBook is for the use of anyone anywhere in the United States and
+most other parts of the world 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 <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you
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+country where you are located before using this eBook.
+</div>
+<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: On the Origin of Species by Means of Natural Selection
+or the Preservation of Favoured Races in the Struggle for Life. (2nd edition)</div>
+<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Charles Darwin</div>
+<div style='display:block;margin:1em 0'>Release Date: September 25, 2007 [eBook #22764]</div>
+<div style='display:block;margin:1em 0'>Language: English</div>
+<div style='display:block;margin:1em 0'>Character set encoding: UTF-8</div>
+<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Steven Gibbs, Keith Edkins and the Online Distributed Proofreading Team</div>
+<div style='margin-top:2em;margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK ORIGIN OF SPECIES ***</div>
+
+<h4>There are several editions of this ebook in the Project Gutenberg collection. Various characteristics of each ebook are listed to aid in selecting the preferred file.<br />Click on any of the filenumbers below to quickly view each ebook.
+</h4>
+
+
+<table summary="" style="margin-right: auto; margin-left: auto" cellpadding="4" border="3">
+
+<tr><td>
+ <b><a href="http://www.gutenberg.org/files/1228/1228-h/1228-h.htm">
+1228</a> </b> </td><td>1859, First Edition
+</td></tr>
+
+<tr><td>
+ <b><a href="http://www.gutenberg.org/files/22764/22764-h/22764-h.htm">
+22764</a></b></td><td>1860, Second Edition
+</td></tr>
+
+<tr><td>
+ <b><a href="http://www.gutenberg.org/files/2009/2009-h/2009-h.htm">
+2009</a></b> </td><td>1872, Sixth Edition, considered the definitive edition.
+</td></tr>
+
+</table>
+
+
+<table border="0" cellpadding="10" style="background-color: #ccccff;">
+<tr>
+<td style="width:25%; vertical-align:top">
+Transcriber's note:
+</td>
+<td>
+A few typographical errors have been corrected. They
+appear in the text <span class="correction" title="explanation will pop up">like this</span>, and the
+explanation will appear when the mouse pointer is moved over the marked
+passage.
+</td>
+</tr>
+</table>
+
+<h3>ON THE</h3>
+
+<h1>ORIGIN OF SPECIES.</h1>
+
+<hr />
+
+  <p>"But with regard to the material world, we can at least go so far as
+  this&mdash;we can perceive that events are brought about not by insulated
+  interpositions of Divine power, exerted in each particular case, but by
+  the establishment of general laws."</p>
+
+  <p class="author"><span class="sc">Whewell</span>: <i>Bridgewater Treatise</i>.</p>
+
+  <p>"The only distinct meaning of the word 'natural' is <i>stated</i>,
+  <i>fixed</i>, or <i>settled</i>; since what is natural as much requires
+  and presupposes an intelligent agent to render it so, <i>i.e.</i> to
+  effect it continually or at stated times, as what is supernatural or
+  miraculous does to effect it for once."</p>
+
+  <p class="author"><span class="sc">Butler</span>: <i>Analogy of Revealed Religion</i>.</p>
+
+  <p>"To conclude, therefore, let no man out of a weak conceit of sobriety,
+  or an ill-applied moderation, think or maintain, that a man can search
+  too far or be too well studied in the book of God's word, or in the book
+  of God's works; divinity or philosophy; but rather let men endeavour an
+  endless progress or proficience in both."</p>
+
+  <p class="author"><span class="sc">Bacon</span>: <i>Advancement of Learning</i>.</p>
+
+<hr />
+
+  <div class="poem">
+    <div class="stanza">
+      <p><i>Down, Bromley, Kent,</i></p>
+      <p class="i4"><i>October 1st, 1859.</i> (<i>1st Thousand</i>).</p>
+    </div>
+  </div>
+<hr />
+
+<h3>ON</h3>
+
+<h1>THE ORIGIN OF SPECIES</h1>
+
+<h2>BY MEANS OF NATURAL SELECTION,</h2>
+
+<p class="cenhead">OR THE</p>
+
+<h3>PRESERVATION OF FAVOURED RACES IN THE STRUGGLE<br />
+FOR LIFE.</h3>
+
+<h2><span class="sc">By</span> CHARLES DARWIN, M.A.,</h2>
+
+<p class="cenhead">FELLOW OF THE ROYAL, GEOLOGICAL, LINNEAN, ETC., SOCIETIES;</p>
+
+<p class="cenhead">AUTHOR OF 'JOURNAL OF RESEARCHES DURING H. M. S. BEAGLE'S VOYAGE<br />
+ROUND THE WORLD.'</p>
+
+<h3><i>FIFTH THOUSAND.</i></h3>
+
+<h3>LONDON:<br />
+JOHN MURRAY, ALBEMARLE STREET.<br />
+1860.</h3>
+
+<p class="cenhead"><i>The right of Translation is reserved.</i></p>
+
+<hr />
+
+<p class="cenhead">LONDON: PRINTED BY W. CLOWES AND SONS, STAMFORD STREET,<br />
+AND CHARING CROSS.</p>
+
+<hr />
+
+<p><!-- Page v --><span class="pagenum"><a name="pagev"></a>[v]</span></p>
+
+<h2>CONTENTS.</h2>
+
+<hr />
+
+  <p><span class="sc">Introduction</span></p>
+
+  <p class="author">Page <a href="#page1">1</a></p>
+
+<h3>CHAPTER I.</h3>
+
+<p class="cenhead"><span class="sc">Variation under Domestication.</span></p>
+
+  <p>Causes of Variability&mdash;Effects of Habit&mdash;Correlation of
+  Growth&mdash;Inheritance&mdash;Character of Domestic
+  Varieties&mdash;Difficulty of distinguishing between Varieties and
+  Species&mdash;Origin of Domestic Varieties from one or more
+  Species&mdash;Domestic Pigeons, their Differences and
+  Origin&mdash;Principle of Selection anciently followed, its
+  Effects&mdash;Methodical and Unconscious Selection&mdash;Unknown Origin
+  of our Domestic Productions&mdash;Circumstances favourable to Man's power
+  of Selection</p>
+
+  <p class="author"><a href="#page7">7</a>-<a href="#page43">43</a></p>
+
+<h3>CHAPTER II.</h3>
+
+<p class="cenhead"><span class="sc">Variation under Nature.</span></p>
+
+  <p>Variability&mdash;Individual differences&mdash;Doubtful
+  species&mdash;Wide ranging, much diffused, and common species vary
+  most&mdash;Species of the larger genera in any country vary more than the
+  species of the smaller genera&mdash;Many of the species of the larger
+  genera resemble varieties in being very closely, but unequally, related
+  to each other, and in having restricted ranges</p>
+
+  <p class="author"><a href="#page44">44</a>-<a href="#page59">59</a></p>
+
+<p><!-- Page vi --><span class="pagenum"><a name="pagevi"></a>[vi]</span></p>
+
+<h3>CHAPTER III.</h3>
+
+<p class="cenhead"><span class="sc">Struggle for Existence.</span></p>
+
+  <p>Its bearing on natural selection&mdash;The term used in a wide
+  sense&mdash;Geometrical powers of increase&mdash;Rapid increase of
+  naturalised animals and plants&mdash;Nature of the checks to
+  increase&mdash;Competition universal&mdash;Effects of
+  climate&mdash;Protection from the number of individuals&mdash;Complex
+  relations of all animals and plants throughout nature&mdash;Struggle for
+  life most severe between individuals and varieties of the same species;
+  often severe between species of the same genus&mdash;The relation of
+  organism to organism the most important of all relations</p>
+
+  <p class="author"><a href="#page60">60</a>-<a href="#page79">79</a></p>
+
+<h3>CHAPTER IV.</h3>
+
+<p class="cenhead"><span class="sc">Natural Selection.</span></p>
+
+  <p>Natural Selection&mdash;its power compared with man's
+  selection&mdash;its power on characters of trifling importance&mdash;its
+  power at all ages and on both sexes&mdash;Sexual Selection&mdash;On the
+  generality of intercrosses between individuals of the same
+  species&mdash;Circumstances favourable and unfavourable to Natural
+  Selection, namely, intercrossing, isolation, number of
+  individuals&mdash;Slow action&mdash;Extinction caused by Natural
+  Selection&mdash;Divergence of Character, related to the diversity of
+  inhabitants of any small area, and to naturalisation&mdash;Action of
+  Natural Selection, through Divergence of Character and Extinction, on the
+  descendants from a common parent&mdash;Explains the Grouping of all
+  organic beings</p>
+
+  <p class="author"><a href="#page80">80</a>-<a href="#page130">130</a></p>
+
+<h3>CHAPTER V.</h3>
+
+<p class="cenhead"><span class="sc">Laws of Variation.</span></p>
+
+  <p>Effects of external conditions&mdash;Use and disuse, combined with
+  natural selection; organs of flight and of
+  vision&mdash;Acclimatisation&mdash;Correlation of
+  growth&mdash;Compensation and economy of growth&mdash;False
+  correlations&mdash;Multiple, rudimentary, and lowly organised structures
+  variable&mdash;Parts developed in an unusual manner are highly variable:
+  specific characters more variable than generic: secondary sexual
+  characters variable&mdash;Species of the same genus vary in an analogous
+  manner&mdash;Reversions to long-lost characters&mdash;Summary</p>
+
+  <p class="author"><a href="#page131">131</a>-<a href="#page170">170</a></p>
+
+<p><!-- Page vii --><span class="pagenum"><a name="pagevii"></a>[vii]</span></p>
+
+<h3>CHAPTER VI.</h3>
+
+<p class="cenhead"><span class="sc">Difficulties on Theory.</span></p>
+
+  <p>Difficulties on the theory of descent with
+  modification&mdash;Transitions&mdash;Absence or rarity of transitional
+  varieties&mdash;Transitions in habits of life&mdash;Diversified habits in
+  the same species&mdash;Species with habits widely different from those of
+  their allies&mdash;Organs of extreme perfection&mdash;Means of
+  transition&mdash;Cases of difficulty&mdash;Natura non facit
+  saltum&mdash;Organs of small importance&mdash;Organs not in all cases
+  absolutely perfect&mdash;The law of Unity of Type and of the Conditions
+  of Existence embraced by the theory of Natural Selection</p>
+
+  <p class="author"><a href="#page171">171</a>-<a href="#page206">206</a></p>
+
+<h3>CHAPTER VII.</h3>
+
+<p class="cenhead"><span class="sc">Instinct.</span></p>
+
+  <p>Instincts comparable with habits, but different in their
+  origin&mdash;Instincts graduated&mdash;Aphides and ants&mdash;Instincts
+  variable&mdash;Domestic instincts, their origin&mdash;Natural instincts
+  of the cuckoo, ostrich, and parasitic bees&mdash;Slave-making
+  ants&mdash;Hive-bee, its cell-making instinct&mdash;Difficulties on the
+  theory of the Natural Selection of instincts&mdash;Neuter or sterile
+  insects&mdash;Summary</p>
+
+  <p class="author"><a href="#page207">207</a>-<a href="#page244">244</a></p>
+
+<h3>CHAPTER VIII.</h3>
+
+<p class="cenhead"><span class="sc">Hybridism.</span></p>
+
+  <p>Distinction between the sterility of first crosses and of
+  hybrids&mdash;Sterility various in degree, not universal, affected by
+  close interbreeding, removed by domestication&mdash;Laws governing the
+  sterility of hybrids&mdash;Sterility not a special endowment, but
+  incidental on other differences&mdash;Causes of the sterility of first
+  crosses and of hybrids&mdash;Parallelism between the effects of changed
+  conditions of life and crossing&mdash;Fertility of varieties when crossed
+  and of their mongrel offspring not universal&mdash;Hybrids and mongrels
+  compared independently of their fertility&mdash;Summary</p>
+
+  <p class="author"><a href="#page245">245</a>-<a href="#page278">278</a></p>
+
+<p><!-- Page viii --><span class="pagenum"><a name="pageviii"></a>[viii]</span></p>
+
+<h3>CHAPTER IX.</h3>
+
+<p class="cenhead"><span class="sc">On the Imperfection of the Geological Record.</span></p>
+
+  <p>On the absence of intermediate varieties at the present day&mdash;On
+  the nature of extinct intermediate varieties; on their number&mdash;On
+  the vast lapse of time, as inferred from the rate of deposition and of
+  denudation&mdash;On the poorness of our palæontological
+  collections&mdash;On the intermittence of geological formations&mdash;On
+  the absence of intermediate varieties in any one formation&mdash;On the
+  sudden appearance of groups of species&mdash;On their sudden appearance
+  in the lowest known fossiliferous strata</p>
+
+  <p class="author"><a href="#page279">279</a>-<a href="#page311">311</a></p>
+
+<h3>CHAPTER X.</h3>
+
+<p class="cenhead"><span class="sc">On the Geological Succession of Organic Beings.</span></p>
+
+  <p>On the slow and successive appearance of new species&mdash;On their
+  different rates of change&mdash;Species once lost do not
+  reappear&mdash;Groups of species follow the same general rules in their
+  appearance and disappearance as do single species&mdash;On
+  Extinction&mdash;On simultaneous changes in the forms of life throughout
+  the world&mdash;On the affinities of extinct species to each other and to
+  living species&mdash;On the state of development of ancient
+  forms&mdash;On the succession of the same types within the same
+  areas&mdash;Summary of preceding and present chapters</p>
+
+  <p class="author"><a href="#page312">312</a>-<a href="#page345">345</a></p>
+
+<h3>CHAPTER XI.</h3>
+
+<p class="cenhead"><span class="sc">Geographical Distribution.</span></p>
+
+  <p>Present distribution cannot be accounted for by differences in
+  physical conditions&mdash;Importance of barriers&mdash;Affinity of the
+  productions of the same continent&mdash;Centres of creation&mdash;Means
+  of dispersal, by changes of climate and of the level of the land, and by
+  occasional means&mdash;Dispersal during the Glacial period co-extensive
+  with the world</p>
+
+  <p class="author"><a href="#page346">346</a>-<a href="#page382">382</a></p>
+
+<h3>CHAPTER XII.</h3>
+
+<p class="cenhead"><span class="sc">Geographical Distribution</span>&mdash;<i>continued</i>.</p>
+
+  <p>Distribution of fresh-water productions&mdash;On the inhabitants of
+  oceanic islands&mdash;Absence of Batrachians and of terrestrial
+  Mammals&mdash;On the relation of the inhabitants of islands to those of
+  the nearest mainland&mdash;On colonisation from the nearest source with
+  subsequent modification&mdash;Summary of the last and present
+  chapters</p>
+
+  <p class="author"><a href="#page383">383</a>-<a href="#page410">410</a></p>
+
+<h3>CHAPTER XIII.</h3>
+
+<p class="cenhead"><span class="sc">Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.</span></p>
+
+  <p><span class="sc">Classification</span>, groups subordinate to
+  groups&mdash;Natural system&mdash;Rules and difficulties in
+  classification, explained on the theory of descent with
+  modification&mdash;Classification of varieties&mdash;Descent always used
+  in classification&mdash;Analogical or adaptive
+  characters&mdash;Affinities, general, complex and
+  radiating&mdash;Extinction separates and defines groups&mdash;<span
+  class="sc">Morphology</span>, between members of the same class, between
+  parts of the same individual&mdash;<span class="sc">Embryology</span>,
+  laws of, explained by variations not supervening at an early age, and
+  being inherited at a corresponding age&mdash;<span class="sc">Rudimentary
+  organs</span>; their origin explained&mdash;Summary</p>
+
+  <p class="author"><a href="#page411">411</a>-<a href="#page458">458</a></p>
+
+<h3>CHAPTER XIV.</h3>
+
+<p class="cenhead"><span class="sc">Recapitulation and Conclusion.</span></p>
+
+  <p>Recapitulation of the difficulties on the theory of Natural
+  Selection&mdash;Recapitulation of the general and special circumstances
+  in its favour&mdash;Causes of the general belief in the immutability of
+  species&mdash;How far the theory of natural selection may be
+  extended&mdash;Effects of its adoption on the study of Natural
+  history&mdash;Concluding remarks</p>
+
+  <p class="author"><a href="#page459">459</a>-<a href="#page490">490</a></p>
+
+<hr />
+
+<p><!-- Page 1 --><span class="pagenum"><a name="page1"></a>[1]</span></p>
+
+<h1>ON THE ORIGIN OF SPECIES.</h1>
+
+<hr />
+
+<h3>INTRODUCTION.</h3>
+
+  <p>When on board H.M.S. 'Beagle,' as naturalist, I was much struck with
+  certain facts in the distribution of the inhabitants of South America,
+  and in the geological relations of the present to the past inhabitants of
+  that continent. These facts seemed to me to throw some light on the
+  origin of species&mdash;that mystery of mysteries, as it has been called
+  by one of our greatest philosophers. On my return home, it occurred to
+  me, in 1837, that something might perhaps be made out on this question by
+  patiently accumulating and reflecting on all sorts of facts which could
+  possibly have any bearing on it. After five years' work I allowed myself
+  to speculate on the subject, and drew up some short notes; these I
+  enlarged in 1844 into a sketch of the conclusions, which then seemed to
+  me probable: from that period to the present day I have steadily pursued
+  the same object. I hope that I may be excused for entering on these
+  personal details, as I give them to show that I have not been hasty in
+  coming to a decision.</p>
+
+  <p>My work is now nearly finished; but as it will take me two or three
+  more years to complete it, and as my health is far from strong, I have
+  been urged to publish this Abstract. I have more especially been induced
+  to do this, as Mr. Wallace, who is now studying the <!-- Page 2 --><span
+  class="pagenum"><a name="page2"></a>[2]</span>natural history of the
+  Malay archipelago, has arrived at almost exactly the same general
+  conclusions that I have on the origin of species. Last year he sent me a
+  memoir on this subject, with a request that I would forward it to Sir
+  Charles Lyell, who sent it to the Linnean Society, and it is published in
+  the third volume of the Journal of that Society. Sir C. Lyell and Dr.
+  Hooker, who both knew of my work&mdash;the latter having read my sketch
+  of 1844&mdash;honoured me by thinking it advisable to publish, with Mr.
+  Wallace's excellent memoir, some brief extracts from my manuscripts.</p>
+
+  <p>This Abstract, which I now publish, must necessarily be imperfect. I
+  cannot here give references and authorities for my several statements;
+  and I must trust to the reader reposing some confidence in my accuracy.
+  No doubt errors will have crept in, though I hope I have always been
+  cautious in trusting to good authorities alone. I can here give only the
+  general conclusions at which I have arrived, with a few facts in
+  illustration, but which, I hope, in most cases will suffice. No one can
+  feel more sensible than I do of the necessity of hereafter publishing in
+  detail all the facts, with references, on which my conclusions have been
+  grounded; and I hope in a future work to do this. For I am well aware
+  that scarcely a single point is discussed in this volume on which facts
+  cannot be adduced, often apparently leading to conclusions directly
+  opposite to those at which I have arrived. A fair result can be obtained
+  only by fully stating and balancing the facts and arguments on both sides
+  of each question; and this cannot possibly be here done.</p>
+
+  <p>I much regret that want of space prevents my having the satisfaction
+  of acknowledging the generous assistance which I have received from very
+  many naturalists, some of them personally unknown to me. I cannot,
+  however, <!-- Page 3 --><span class="pagenum"><a
+  name="page3"></a>[3]</span>let this opportunity pass without expressing
+  my deep obligations to Dr. Hooker, who for the last fifteen years has
+  aided me in every possible way by his large stores of knowledge and his
+  excellent judgment.</p>
+
+  <p>In considering the Origin of Species, it is quite conceivable that a
+  naturalist, reflecting on the mutual affinities of organic beings, on
+  their embryological relations, their geographical distribution,
+  geological succession, and other such facts, might come to the conclusion
+  that each species had not been independently created, but had descended,
+  like varieties, from other species. Nevertheless, such a conclusion, even
+  if well founded, would be unsatisfactory, until it could be shown how the
+  innumerable species inhabiting this world have been modified, so as to
+  acquire that perfection of structure and coadaptation which most justly
+  excites our admiration. Naturalists continually refer to external
+  conditions, such as climate, food, &amp;c., as the only possible cause of
+  variation. In one very limited sense, as we shall hereafter see, this may
+  be true; but it is preposterous to attribute to mere external conditions,
+  the structure, for instance, of the woodpecker, with its feet, tail,
+  beak, and tongue, so admirably adapted to catch insects under the bark of
+  trees. In the case of the misseltoe, which draws its nourishment from
+  certain trees, which has seeds that must be transported by certain birds,
+  and which has flowers with separate sexes absolutely requiring the agency
+  of certain insects to bring pollen from one flower to the other, it is
+  equally preposterous to account for the structure of this parasite, with
+  its relations to several distinct organic beings, by the effects of
+  external conditions, or of habit, or of the volition of the plant
+  itself.</p>
+
+  <p>The author of the 'Vestiges of Creation' would, I presume, say that,
+  after a certain unknown number of <!-- Page 4 --><span class="pagenum"><a
+  name="page4"></a>[4]</span>generations, some bird had given birth to a
+  woodpecker, and some plant to the missletoe, and that these had been
+  produced perfect as we now see them; but this assumption seems to me to
+  be no explanation, for it leaves the case of the coadaptations of organic
+  beings to each other and to their physical conditions of life, untouched
+  and unexplained.</p>
+
+  <p>It is, therefore, of the highest importance to gain a clear insight
+  into the means of modification and coadaptation. At the commencement of
+  my observations it seemed to me probable that a careful study of
+  domesticated animals and of cultivated plants would offer the best chance
+  of making out this obscure problem. Nor have I been disappointed; in this
+  and in all other perplexing cases I have invariably found that our
+  knowledge, imperfect though it be, of variation under domestication,
+  afforded the best and safest clue. I may venture to express my conviction
+  of the high value of such studies, although they have been very commonly
+  neglected by naturalists.</p>
+
+  <p>From these considerations, I shall devote the first chapter of this
+  Abstract to Variation under Domestication. We shall thus see that a large
+  amount of hereditary modification is at least possible; and, what is
+  equally or more important, we shall see how great is the power of man in
+  accumulating by his Selection successive slight variations. I will then
+  pass on to the variability of species in a state of nature; but I shall,
+  unfortunately, be compelled to treat this subject far too briefly, as it
+  can be treated properly only by giving long catalogues of facts. We
+  shall, however, be enabled to discuss what circumstances are most
+  favourable to variation. In the next chapter the Struggle for Existence
+  amongst all organic beings throughout the world, which inevitably follows
+  from the high geometrical ratio of their <!-- Page 5 --><span
+  class="pagenum"><a name="page5"></a>[5]</span>increase, will be treated
+  of. This is the doctrine of Malthus, applied to the whole animal and
+  vegetable kingdoms. As many more individuals of each species are born
+  than can possibly survive; and as, consequently, there is a frequently
+  recurring struggle for existence, it follows that any being, if it vary
+  however slightly in any manner profitable to itself, under the complex
+  and sometimes varying conditions of life, will have a better chance of
+  surviving, and thus be <i>naturally selected</i>. From the strong
+  principle of inheritance, any selected variety will tend to propagate its
+  new and modified form.</p>
+
+  <p>This fundamental subject of Natural Selection will be treated at some
+  length in the fourth chapter; and we shall then see how Natural Selection
+  almost inevitably causes much Extinction of the less improved forms of
+  life, and leads to what I have called Divergence of Character. In the
+  next chapter I shall discuss the complex and little known laws of
+  variation and of correlation of growth. In the four succeeding chapters,
+  the most apparent and gravest difficulties on the theory will be given:
+  namely, first, the difficulties of transitions, or in understanding how a
+  simple being or a simple organ can be changed and perfected into a highly
+  developed being or elaborately constructed organ; secondly, the subject
+  of Instinct, or the mental powers of animals; thirdly, Hybridism, or the
+  infertility of species and the fertility of varieties when intercrossed;
+  and fourthly, the imperfection of the Geological Record. In the next
+  chapter I shall consider the geological succession of organic beings
+  throughout time; in the eleventh and twelfth, their geographical
+  distribution throughout space; in the thirteenth, their classification or
+  mutual affinities, both when mature and in an embryonic condition. In the
+  last chapter I shall give a <!-- Page 6 --><span class="pagenum"><a
+  name="page6"></a>[6]</span>brief recapitulation of the whole work, and a
+  few concluding remarks.</p>
+
+  <p>No one ought to feel surprise at much remaining as yet unexplained in
+  regard to the origin of species and varieties, if he makes due allowance
+  for our profound ignorance in regard to the mutual relations of all the
+  beings which live around us. Who can explain why one species ranges
+  widely and is very numerous, and why another allied species has a narrow
+  range and is rare? Yet these relations are of the highest importance, for
+  they determine the present welfare, and, as I believe, the future success
+  and modification of every inhabitant of this world. Still less do we know
+  of the mutual relations of the innumerable inhabitants of the world
+  during the many past geological epochs in its history. Although much
+  remains obscure, and will long remain obscure, I can entertain no doubt,
+  after the most deliberate study and dispassionate judgment of which I am
+  capable, that the view which most naturalists entertain, and which I
+  formerly entertained&mdash;namely, that each species has been
+  independently created&mdash;is erroneous. I am fully convinced that
+  species are not immutable; but that those belonging to what are called
+  the same genera are lineal descendants of some other and generally
+  extinct species, in the same manner as the acknowledged varieties of any
+  one species are the descendants of that species. Furthermore, I am
+  convinced that Natural Selection has been the main but not exclusive
+  means of modification.</p>
+
+<hr />
+
+<p><!-- Page 7 --><span class="pagenum"><a name="page7"></a>[7]</span></p>
+
+<h3>CHAPTER I.</h3>
+
+<p class="cenhead"><span class="sc">Variation under Domestication.</span></p>
+
+<blockquote class="b1n">
+
+  <p>Causes of Variability&mdash;Effects of Habit&mdash;Correlation of
+  Growth&mdash;Inheritance&mdash;Character of Domestic
+  Varieties&mdash;Difficulty of distinguishing between Varieties and
+  Species&mdash;Origin of Domestic Varieties from one or more
+  Species&mdash;Domestic Pigeons, their Differences and
+  Origin&mdash;Principle of Selection anciently followed, its
+  Effects&mdash;Methodical and Unconscious Selection&mdash;Unknown Origin
+  of our Domestic Productions&mdash;Circumstances favourable to Man's power
+  of Selection.</p>
+
+</blockquote>
+
+  <p>When we look to the individuals of the same variety or sub-variety of
+  our older cultivated plants and animals, one of the first points which
+  strikes us, is, that they generally differ more from each other than do
+  the individuals of any one species or variety in a state of nature. When
+  we reflect on the vast diversity of the plants and animals which have
+  been cultivated, and which have varied during all ages under the most
+  different climates and treatment, I think we are driven to conclude that
+  this great variability is simply due to our domestic productions having
+  been raised under conditions of life not so uniform as, and somewhat
+  different from, those to which the parent-species have been exposed under
+  nature. There is also, I think, some probability in the view propounded
+  by Andrew Knight, that this variability may be partly connected with
+  excess of food. It seems pretty clear that organic beings must be exposed
+  during several generations to the new conditions of life to cause any
+  appreciable amount of variation; and that when the organisation has once
+  begun to vary, it generally continues to vary for many generations. <!--
+  Page 8 --><span class="pagenum"><a name="page8"></a>[8]</span>No case is
+  on record of a variable being ceasing to be variable under cultivation.
+  Our oldest cultivated plants, such as wheat, still often yield new
+  varieties: our oldest domesticated animals are still capable of rapid
+  improvement or modification.</p>
+
+  <p>It has been disputed at what period of life the causes of variability,
+  whatever they may be, generally act; whether during the early or late
+  period of development of the embryo, or at the instant of conception.
+  Geoffroy St. Hilaire's experiments show that unnatural treatment of the
+  embryo causes monstrosities; and monstrosities cannot be separated by any
+  clear line of distinction from mere variations. But I am strongly
+  inclined to suspect that the most frequent cause of variability may be
+  attributed to the male and female reproductive elements having been
+  affected prior to the act of conception. Several reasons make me believe
+  in this; but the chief one is the remarkable effect which confinement or
+  cultivation has on the function of the reproductive system; this system
+  appearing to be far more susceptible than any other part of the
+  organisation, to the action of any change in the conditions of life.
+  Nothing is more easy than to tame an animal, and few things more
+  difficult than to get it to breed freely under confinement, even in the
+  many cases when the male and female unite. How many animals there are
+  which will not breed, though living long under not very close confinement
+  in their native country! This is generally attributed to vitiated
+  instincts; but how many cultivated plants display the utmost vigour, and
+  yet rarely or never seed! In some few such cases it has been discovered
+  that very trifling changes, such as a little more or less water at some
+  particular period of growth, will determine whether or not the plant sets
+  a seed. I cannot here enter on the copious details which I have collected
+  on <!-- Page 9 --><span class="pagenum"><a
+  name="page9"></a>[9]</span>this curious subject; but to show how singular
+  the laws are which determine the reproduction of animals under
+  confinement, I may just mention that carnivorous animals, even from the
+  tropics, breed in this country pretty freely under confinement, with the
+  exception of the plantigrades or bear family; whereas carnivorous birds,
+  with the rarest exceptions, hardly ever lay fertile eggs. Many exotic
+  plants have pollen utterly worthless, in the same exact condition as in
+  the most sterile hybrids. When, on the one hand, we see domesticated
+  animals and plants, though often weak and sickly, yet breeding quite
+  freely under confinement; and when, on the other hand, we see
+  individuals, though taken young from a state of nature, perfectly tamed,
+  long-lived, and healthy (of which I could give numerous instances), yet
+  having their reproductive system so seriously affected by unperceived
+  causes as to fail in acting, we need not be surprised at this system,
+  when it does act under confinement, acting not quite regularly, and
+  producing offspring not perfectly like their parents.</p>
+
+  <p>Sterility has been said to be the bane of horticulture; but on this
+  view we owe variability to the same cause which produces sterility; and
+  variability is the source of all the choicest productions of the garden.
+  I may add, that as some organisms will breed freely under the most
+  unnatural conditions (for instance, the rabbit and ferret kept in
+  hutches), showing that their reproductive system has not been thus
+  affected; so will some animals and plants withstand domestication or
+  cultivation, and vary very slightly&mdash;perhaps hardly more than in a
+  state of nature.</p>
+
+  <p>A long list could easily be given of "sporting plants;" by this term
+  gardeners mean a single bud or offset, which suddenly assumes a new and
+  sometimes very different character from that of the rest of the plant.
+  <!-- Page 10 --><span class="pagenum"><a
+  name="page10"></a>[10]</span>Such buds can be propagated by grafting,
+  &amp;c., and sometimes by seed. These "sports" are extremely rare under
+  nature, but far from rare under cultivation; and in this case we see that
+  the treatment of the parent has affected a bud or offset, and not the
+  ovules or pollen. But it is the opinion of most physiologists that there
+  is no essential difference between a bud and an ovule in their earliest
+  stages of formation; so that, in fact, "sports" support my view, that
+  variability may be largely attributed to the ovules or pollen, or to
+  both, having been affected by the treatment of the parent prior to the
+  act of conception. These cases anyhow show that variation is not
+  necessarily connected, as some authors have supposed, with the act of
+  generation.</p>
+
+  <p>Seedlings from the same fruit, and the young of the same litter,
+  sometimes differ considerably from each other, though both the young and
+  the parents, as Müller has remarked, have apparently been exposed to
+  exactly the same conditions of life; and this shows how unimportant the
+  direct effects of the conditions of life are in comparison with the laws
+  of reproduction, of growth, and of inheritance; for had the action of the
+  conditions been direct, if any of the young had varied, all would
+  probably have varied in the same manner. To judge how much, in the case
+  of any variation, we should attribute to the direct action of heat,
+  moisture, light, food, &amp;c., is most difficult: my impression is, that
+  with animals such agencies have produced very little direct effect,
+  though apparently more in the case of plants. Under this point of view,
+  Mr. Buckman's recent experiments on plants are extremely valuable. When
+  all or nearly all the individuals exposed to certain conditions are
+  affected in the same way, the change at first appears to be directly due
+  to such conditions; but in some cases it can be shown that quite opposite
+  conditions produce <!-- Page 11 --><span class="pagenum"><a
+  name="page11"></a>[11]</span>similar changes of structure. Nevertheless
+  some slight amount of change may, I think, be attributed to the direct
+  action of the conditions of life&mdash;as, in some cases, increased size
+  from amount of food, colour from particular kinds of food or from light,
+  and perhaps the thickness of fur from climate.</p>
+
+  <p>Habit also has a decided influence, as in the period of flowering with
+  plants when transported from one climate to another. In animals it has a
+  more marked effect; for instance, I find in the domestic duck that the
+  bones of the wing weigh less and the bones of the leg more, in proportion
+  to the whole skeleton, than do the same bones in the wild-duck; and I
+  presume that this change may be safely attributed to the domestic duck
+  flying much less, and walking more, than its wild parent. The great and
+  inherited development of the udders in cows and goats in countries where
+  they are habitually milked, in comparison with the state of these organs
+  in other countries, is another instance of the effect of use. Not a
+  single domestic animal can be named which has not in some country
+  drooping ears; and the view suggested by some authors, that the drooping
+  is due to the disuse of the muscles of the ear, from the animals not
+  being much alarmed by danger, seems probable.</p>
+
+  <p>There are many laws regulating variation, some few of which can be
+  dimly seen, and will be hereafter briefly mentioned. I will here only
+  allude to what may be called correlation of growth. Any change in the
+  embryo or larva will almost certainly entail changes in the mature
+  animal. In monstrosities, the correlations between quite distinct parts
+  are very curious; and many instances are given in Isidore Geoffroy St.
+  Hilaire's great work on this subject. Breeders believe that long limbs
+  are almost always accompanied by an elongated head. Some instances of
+  correlation are quite whimsical: thus <!-- Page 12 --><span
+  class="pagenum"><a name="page12"></a>[12]</span>cats with blue eyes are
+  invariably deaf; colour and constitutional peculiarities go together, of
+  which many remarkable cases could be given amongst animals and plants.
+  From the facts collected by Heusinger, it appears that white sheep and
+  pigs are differently affected from coloured individuals by certain
+  vegetable poisons. Hairless dogs have imperfect teeth: long-haired and
+  coarse-haired animals are apt to have, as is asserted, long or many
+  horns; pigeons with feathered feet have skin between their outer toes;
+  pigeons with short beaks have small feet, and those with long beaks large
+  feet. Hence, if man goes on selecting, and thus augmenting, any
+  peculiarity, he will almost certainly unconsciously modify other parts of
+  the structure, owing to the mysterious laws of the correlation of
+  growth.</p>
+
+  <p>The result of the various, quite unknown, or dimly seen laws of
+  variation is infinitely complex and diversified. It is well worth while
+  carefully to study the several treatises published on some of our old
+  cultivated plants, as on the hyacinth, potato, even the dahlia, &amp;c.;
+  and it is really surprising to note the endless points in structure and
+  constitution in which the varieties and sub-varieties differ slightly
+  from each other. The whole organisation seems to have become plastic, and
+  tends to depart in some small degree from that of the parental type.</p>
+
+  <p>Any variation which is not inherited is unimportant for us. But the
+  number and diversity of inheritable deviations of structure, both those
+  of slight and those of considerable physiological importance, is endless.
+  Dr. Prosper Lucas's treatise, in two large volumes, is the fullest and
+  the best on this subject. No breeder doubts how strong is the tendency to
+  inheritance: like produces like is his fundamental belief: doubts have
+  been thrown on this principle by theoretical writers alone. When any
+  deviation of structure often appears, and we see it in the <!-- Page 13
+  --><span class="pagenum"><a name="page13"></a>[13]</span>father and
+  child, we cannot tell whether it may not be due to the same cause having
+  acted on both; but when amongst individuals, apparently exposed to the
+  same conditions, any very rare deviation, due to some extraordinary
+  combination of circumstances, appears in the parent&mdash;say, once
+  amongst several million individuals&mdash;and it reappears in the child,
+  the mere doctrine of chances almost compels us to attribute its
+  reappearance to inheritance. Every one must have heard of cases of
+  albinism, prickly skin, hairy bodies, &amp;c., appearing in several
+  members of the same family. If strange and rare deviations of structure
+  are truly inherited, less strange and commoner deviations may be freely
+  admitted to be inheritable. Perhaps the correct way of viewing the whole
+  subject, would be, to look at the inheritance of every character whatever
+  as the rule, and non-inheritance as the anomaly.</p>
+
+  <p>The laws governing inheritance are quite unknown; no one can say why a
+  peculiarity in different individuals of the same species, or in
+  individuals of different species, is sometimes inherited and sometimes
+  not so; why the child often reverts in certain characters to its
+  grandfather or grandmother or other more remote ancestor; why a
+  peculiarity is often transmitted from one sex to both sexes, or to one
+  sex alone, more commonly but not exclusively to the like sex. It is a
+  fact of some little importance to us, that peculiarities appearing in the
+  males of our domestic breeds are often transmitted either exclusively, or
+  in a much greater degree, to males alone. A much more important rule,
+  which I think may be trusted, is that, at whatever period of life a
+  peculiarity first appears, it tends to appear in the offspring at a
+  corresponding age, though sometimes earlier. In many cases this could not
+  be otherwise: thus the inherited peculiarities in the horns of cattle
+  could appear only in <!-- Page 14 --><span class="pagenum"><a
+  name="page14"></a>[14]</span>the offspring when nearly mature;
+  peculiarities in the silkworm are known to appear at the corresponding
+  caterpillar or cocoon stage. But hereditary diseases and some other facts
+  make me believe that the rule has a wider extension, and that when there
+  is no apparent reason why a peculiarity should appear at any particular
+  age, yet that it does tend to appear in the offspring at the same period
+  at which it first appeared in the parent. I believe this rule to be of
+  the highest importance in explaining the laws of embryology. These
+  remarks are of course confined to the first <i>appearance</i> of the
+  peculiarity, and not to its primary cause, which may have acted on the
+  ovules or male element; in nearly the same manner as in the crossed
+  offspring from a short-horned cow by a long-horned bull, the greater
+  length of horn, though appearing late in life, is clearly due to the male
+  element.</p>
+
+  <p>Having alluded to the subject of reversion, I may here refer to a
+  statement often made by naturalists&mdash;namely, that our domestic
+  varieties, when run wild, gradually but certainly revert in character to
+  their aboriginal stocks. Hence it has been argued that no deductions can
+  be drawn from domestic races to species in a state of nature. I have in
+  vain endeavoured to discover on what decisive facts the above statement
+  has so often and so boldly been made. There would be great difficulty in
+  proving its truth: we may safely conclude that very many of the most
+  strongly-marked domestic varieties could not possibly live in a wild
+  state. In many cases we do not know what the aboriginal stock was, and so
+  could not tell whether or not nearly perfect reversion had ensued. It
+  would be quite necessary, in order to prevent the effects of
+  intercrossing, that only a single variety should be turned loose in its
+  new home. Nevertheless, as our varieties certainly do occasionally <!--
+  Page 15 --><span class="pagenum"><a name="page15"></a>[15]</span>revert
+  in some of their characters to ancestral forms, it seems to me not
+  improbable, that if we could succeed in naturalising, or were to
+  cultivate, during many generations, the several races, for instance, of
+  the cabbage, in very poor soil (in which case, however, some effect would
+  have to be attributed to the direct action of the poor soil), that they
+  would to a large extent, or even wholly, revert to the wild aboriginal
+  stock. Whether or not the experiment would succeed, is not of great
+  importance for our line of argument; for by the experiment itself the
+  conditions of life are changed. If it could be shown that our domestic
+  varieties manifested a strong tendency to reversion,&mdash;that is, to
+  lose their acquired characters, whilst kept under the same conditions,
+  and whilst kept in a considerable body, so that free intercrossing might
+  check, by blending together, any slight deviations in their structure, in
+  such case, I grant that we could deduce nothing from domestic varieties
+  in regard to species. But there is not a shadow of evidence in favour of
+  this view: to assert that we could not breed our cart and race-horses,
+  long and short-horned cattle, and poultry of various breeds, and esculent
+  vegetables, for an almost infinite number of generations, would be
+  opposed to all experience. I may add, that when under nature the
+  conditions of life do change, variations and reversions of character
+  probably do occur; but natural selection, as will hereafter be explained,
+  will determine how far the new characters thus arising shall be
+  preserved.</p>
+
+  <p>When we look to the hereditary varieties or races of our domestic
+  animals and plants, and compare them with closely allied species, we
+  generally perceive in each domestic race, as already remarked, less
+  uniformity of character than in true species. Domestic races of the same
+  species, also, often have a somewhat monstrous character; by which I
+  mean, that, although differing <!-- Page 16 --><span class="pagenum"><a
+  name="page16"></a>[16]</span>from each other, and from other species of
+  the same genus, in several trifling respects, they often differ in an
+  extreme degree in some one part, both when compared one with another, and
+  more especially when compared with all the species in nature to which
+  they are nearest allied. With these exceptions (and with that of the
+  perfect fertility of varieties when crossed,&mdash;a subject hereafter to
+  be discussed), domestic races of the same species differ from each other
+  in the same manner as, only in most cases in a lesser degree than, do
+  closely-allied species of the same genus in a state of nature. I think
+  this must be admitted, when we find that there are hardly any domestic
+  races, either amongst animals or plants, which have not been ranked by
+  competent judges as mere varieties, and by other competent judges as the
+  descendants of aboriginally distinct species. If any marked distinction
+  existed between domestic races and species, this source of doubt could
+  not so perpetually recur. It has often been stated that domestic races do
+  not differ from each other in characters of generic value. I think it
+  could be shown that this statement is hardly correct; but naturalists
+  differ widely in determining what characters are of generic value; all
+  such valuations being at present empirical. Moreover, on the view of the
+  origin of genera which I shall presently give, we have no right to expect
+  often to meet with generic differences in our domesticated
+  productions.</p>
+
+  <p>When we attempt to estimate the amount of structural difference
+  between the domestic races of the same species, we are soon involved in
+  doubt, from not knowing whether they have descended from one or several
+  parent-species. This point, if it could be cleared up, would be
+  interesting; if, for instance, it could be shown that the greyhound,
+  bloodhound, terrier, spaniel, and bull-dog, which we all know propagate
+  their kind so truly, were the <!-- Page 17 --><span class="pagenum"><a
+  name="page17"></a>[17]</span>offspring of any single species, then such
+  facts would have great weight in making us doubt about the immutability
+  of the many very closely allied natural species&mdash;for instance, of
+  the many foxes&mdash;inhabiting different quarters of the world. I do not
+  believe, as we shall presently see, that the whole amount of difference
+  between the several breeds of the dog has been produced under
+  domestication; I believe that some small part of the difference is due to
+  their being descended from distinct species. In the case of some other
+  domesticated species, there is presumptive, or even strong evidence, that
+  all the breeds have descended from a single wild stock.</p>
+
+  <p>It has often been assumed that man has chosen for domestication
+  animals and plants having an extraordinary inherent tendency to vary, and
+  likewise to withstand diverse climates. I do not dispute that these
+  capacities have added largely to the value of most of our domesticated
+  productions; but how could a savage possibly know, when he first tamed an
+  animal, whether it would vary in succeeding generations, and whether it
+  would endure other climates? Has the little variability of the ass or
+  guinea-fowl, or the small power of endurance of warmth by the reindeer,
+  or of cold by the common camel, prevented their domestication? I cannot
+  doubt that if other animals and plants, equal in number to our
+  domesticated productions, and belonging to equally diverse classes and
+  countries, were taken from a state of nature, and could be made to breed
+  for an equal number of generations under domestication, they would vary
+  on an average as largely as the parent species of our existing
+  domesticated productions have varied.</p>
+
+  <p>In the case of most of our anciently domesticated animals and plants,
+  I do not think it is possible to come to any definite conclusion, whether
+  they have descended from one or several wild species. The argument mainly
+  relied on by those who believe in the multiple origin <!-- Page 18
+  --><span class="pagenum"><a name="page18"></a>[18]</span>of our domestic
+  animals is, that we find in the most ancient records, more especially on
+  the monuments of Egypt, much diversity in the breeds; and that some of
+  the breeds closely resemble, perhaps are identical with, those still
+  existing. Even if this latter fact were found more strictly and generally
+  true than seems to me to be the case, what does it show, but that some of
+  our breeds originated there, four or five thousand years ago? But Mr.
+  Horner's researches have rendered it in some degree probable that man
+  sufficiently civilized to have manufactured pottery existed in the valley
+  of the Nile thirteen or fourteen thousand years ago; and who will pretend
+  to say how long before these ancient periods, savages, like those of
+  Tierra del Fuego or Australia, who possess a semi-domestic dog, may not
+  have existed in Egypt?</p>
+
+  <p>The whole subject must, I think, remain vague; nevertheless, I may,
+  without here entering on any details, state that, from geographical and
+  other considerations, I think it highly probable that our domestic dogs
+  have descended from several wild species. Knowing, as we do, that savages
+  are very fond of taming animals, it seems to me unlikely, in the case of
+  the dog-genus, which is distributed in a wild state throughout the world,
+  that since man first appeared one single species alone should have been
+  domesticated. In regard to sheep and goats I can form no opinion. I
+  should think, from facts communicated to me by Mr. Blyth, on the habits,
+  voice, and constitution, &amp;c., of the humped Indian cattle, that these
+  had descended from a different aboriginal stock from our European cattle;
+  and several competent judges believe that these latter have had more than
+  one wild parent. With respect to horses, from reasons which I cannot give
+  here, I am doubtfully inclined to believe, in opposition to several
+  authors, that all the races have descended from one <!-- Page 19 --><span
+  class="pagenum"><a name="page19"></a>[19]</span>wild stock. Mr. Blyth,
+  whose opinion, from his large and varied stores of knowledge, I should
+  value more than that of almost any one, thinks that all the breeds of
+  poultry have proceeded from the common wild Indian fowl (Gallus bankiva).
+  In regard to ducks and rabbits, the breeds of which differ considerably
+  from each other in structure, I do not doubt that they have all descended
+  from the common wild duck and rabbit.</p>
+
+  <p>The doctrine of the origin of our several domestic races from several
+  aboriginal stocks, has been carried to an absurd extreme by some authors.
+  They believe that every race which breeds true, let the distinctive
+  characters be ever so slight, has had its wild prototype. At this rate
+  there must have existed at least a score of species of wild cattle, as
+  many sheep, and several goats in Europe alone, and several even within
+  Great Britain. One author believes that there formerly existed in Great
+  Britain eleven wild species of sheep peculiar to it! When we bear in mind
+  that Britain has now hardly one peculiar mammal, and France but few
+  distinct from those of Germany and conversely, and so with Hungary,
+  Spain, &amp;c., but that each of these kingdoms possesses several
+  peculiar breeds of cattle, sheep, &amp;c., we must admit that many
+  domestic breeds have originated in Europe; for whence could they have
+  been derived, as these several countries do not possess a number of
+  peculiar species as distinct parent-stocks? So it is in India. Even in
+  the case of the domestic dogs of the whole world, which I fully admit
+  have probably descended from several wild species, I cannot doubt that
+  there has been an immense amount of inherited variation. Who can believe
+  that animals closely resembling the Italian greyhound, the bloodhound,
+  the bull-dog, or Blenheim spaniel, &amp;c.&mdash;so unlike all wild
+  Canidæ&mdash;ever existed freely in a state of nature? It has often been
+  loosely said that all our races of dogs have <!-- Page 20 --><span
+  class="pagenum"><a name="page20"></a>[20]</span>been produced by the
+  crossing of a few aboriginal species; but by crossing we can only get
+  forms in some degree intermediate between their parents; and if we
+  account for our several domestic races by this process, we must admit the
+  former existence of the most extreme forms, as the Italian greyhound,
+  bloodhound, bull-dog, &amp;c., in the wild state. Moreover, the
+  possibility of making distinct races by crossing has been greatly
+  exaggerated. There can be no doubt that a race may be modified by
+  occasional crosses, if aided by the careful selection of those individual
+  mongrels, which present any desired character; but that a race could be
+  obtained nearly intermediate between two extremely different races or
+  species, I can hardly believe. Sir J. Sebright expressly experimentised
+  for this object, and failed. The offspring from the first cross between
+  two pure breeds is tolerably and sometimes (as I have found with pigeons)
+  extremely uniform, and everything seems simple enough; but when these
+  mongrels are crossed one with another for several generations, hardly two
+  of them will be alike, and then the extreme difficulty, or rather utter
+  hopelessness, of the task becomes apparent. Certainly, a breed
+  intermediate between <i>two very distinct</i> breeds could not be got
+  without extreme care and long-continued selection; nor can I find a
+  single case on record of a permanent race having been thus formed.</p>
+
+  <p><i>On the Breeds of the Domestic Pigeon.</i>&mdash;Believing that it
+  is always best to study some special group, I have, after deliberation,
+  taken up domestic pigeons. I have kept every breed which I could purchase
+  or obtain, and have been most kindly favoured with skins from several
+  quarters of the world, more especially by the Hon. W. Elliot from India,
+  and by the Hon. C. Murray from Persia. Many treatises in different
+  languages have been published on pigeons, and some of them are very
+  important, as being of <!-- Page 21 --><span class="pagenum"><a
+  name="page21"></a>[21]</span>considerable antiquity. I have associated
+  with several eminent fanciers, and have been permitted to join two of the
+  London Pigeon Clubs. The diversity of the breeds is something
+  astonishing. Compare the English carrier and the short-faced tumbler, and
+  see the wonderful difference in their beaks, entailing corresponding
+  differences in their skulls. The carrier, more especially the male bird,
+  is also remarkable from the wonderful development of the carunculated
+  skin about the head, and this is accompanied by greatly elongated
+  eyelids, very large external orifices to the nostrils, and a wide gape of
+  mouth. The short-faced tumbler has a beak in outline almost like that of
+  a finch; and the common tumbler has the singular inherited habit of
+  flying at a great height in a compact flock, and tumbling in the air head
+  over heels. The runt is a bird of great size, with long, massive beak and
+  large feet; some of the sub-breeds of runts have very long necks, others
+  very long wings and tails, others singularly short tails. The barb is
+  allied to the carrier, but, instead of a very long beak, has a very short
+  and very broad one. The pouter has a much elongated body, wings, and
+  legs; and its enormously developed crop, which it glories in inflating,
+  may well excite astonishment and even laughter. The turbit has a very
+  short and conical beak, with a line of reversed feathers down the breast;
+  and it has the habit of continually expanding slightly the upper part of
+  the &oelig;sophagus. The Jacobin has the feathers so much reversed along
+  the back of the neck that they form a hood, and it has, proportionally to
+  its size, much elongated wing and tail feathers. The trumpeter and
+  laugher, as their names express, utter a very different coo from the
+  other breeds. The fantail has thirty or even forty tail feathers, instead
+  of twelve or fourteen, the normal number in all members of the great
+  pigeon family; and these feathers are kept expanded, and are <!-- Page 22
+  --><span class="pagenum"><a name="page22"></a>[22]</span>carried so erect
+  that in good birds the head and tail touch; the oil-gland is quite
+  aborted. Several other less distinct breeds might be specified.</p>
+
+  <p>In the skeletons of the several breeds, the development of the bones
+  of the face in length and breadth and curvature differs enormously. The
+  shape, as well as the breadth and length of the ramus of the lower jaw,
+  varies in a highly remarkable manner. The number of the caudal and sacral
+  vertebræ vary; as does the number of the ribs, together with their
+  relative breadth and the presence of processes. The size and shape of the
+  apertures in the sternum are highly variable; so is the degree of
+  divergence and relative size of the two arms of the furcula. The
+  proportional width of the gape of mouth, the proportional length of the
+  eyelids, of the orifice of the nostrils, of the tongue (not always in
+  strict correlation with the length of beak), the size of the crop and of
+  the upper part of the &oelig;sophagus; the development and abortion of
+  the oil-gland; the number of the primary wing and caudal feathers; the
+  relative length of wing and tail to each other and to the body; the
+  relative length of leg and of the feet; the number of scutellæ on the
+  toes, the development of skin between the toes, are all points of
+  structure which are variable. The period at which the perfect plumage is
+  acquired varies, as does the state of the down with which the nestling
+  birds are clothed when hatched. The shape and size of the eggs vary. The
+  manner of flight differs remarkably; as does in some breeds the voice and
+  disposition. Lastly, in certain breeds, the males and females have come
+  to differ to a slight degree from each other.</p>
+
+  <p>Altogether at least a score of pigeons might be chosen, which if shown
+  to an ornithologist, and he were told that they were wild birds, would
+  certainly, I think, be ranked by him as well-defined species. Moreover, I
+  do not believe that any ornithologist would place the <!-- Page 23
+  --><span class="pagenum"><a name="page23"></a>[23]</span>English carrier,
+  the short-faced tumbler, the runt, the barb, pouter, and fantail in the
+  same genus; more especially as in each of these breeds several
+  truly-inherited sub-breeds, or species as he might have called them,
+  could be shown him.</p>
+
+  <p>Great as the differences are between the breeds of pigeons, I am fully
+  convinced that the common opinion of naturalists is correct, namely, that
+  all have descended from the rock-pigeon (Columba livia), including under
+  this term several geographical races or sub-species, which differ from
+  each other in the most trifling respects. As several of the reasons which
+  have led me to this belief are in some degree applicable in other cases,
+  I will here briefly give them. If the several breeds are not varieties,
+  and have not proceeded from the rock-pigeon, they must have descended
+  from at least seven or eight aboriginal stocks; for it is impossible to
+  make the present domestic breeds by the crossing of any lesser number:
+  how, for instance, could a pouter be produced by crossing two breeds
+  unless one of the parent-stocks possessed the characteristic enormous
+  crop? The supposed aboriginal stocks must all have been rock-pigeons,
+  that is, not breeding or willingly perching on trees. But besides C.
+  livia, with its geographical sub-species, only two or three other species
+  of rock-pigeons are known; and these have not any of the characters of
+  the domestic breeds. Hence the supposed aboriginal stocks must either
+  still exist in the countries where they were originally domesticated, and
+  yet be unknown to ornithologists; and this, considering their size,
+  habits, and remarkable characters, seems very improbable; or they must
+  have become extinct in the wild state. But birds breeding on precipices,
+  and good fliers, are unlikely to be exterminated; and the common
+  rock-pigeon, which has the same habits with the domestic breeds, has not
+  been exterminated <!-- Page 24 --><span class="pagenum"><a
+  name="page24"></a>[24]</span>even on several of the smaller British
+  islets, or on the shores of the Mediterranean. Hence the supposed
+  extermination of so many species having similar habits with the
+  rock-pigeon seems to me a very rash assumption. Moreover, the several
+  above-named domesticated breeds have been transported to all parts of the
+  world, and, therefore, some of them must have been carried back again
+  into their native country; but not one has ever become wild or feral,
+  though the dovecot-pigeon, which is the rock-pigeon in a very slightly
+  altered state, has become feral in several places. Again, all recent
+  experience shows that it is most difficult to get any wild animal to
+  breed freely under domestication; yet on the hypothesis of the multiple
+  origin of our pigeons, it must be assumed that at least seven or eight
+  species were so thoroughly domesticated in ancient times by
+  half-civilized man, as to be quite prolific under confinement.</p>
+
+  <p>An argument, as it seems to me, of great weight, and applicable in
+  several other cases, is, that the above-specified breeds, though agreeing
+  generally in constitution, habits, voice, colouring, and in most parts of
+  their structure, with the wild rock-pigeon, yet are certainly highly
+  abnormal in other parts of their structure; we may look in vain
+  throughout the whole great family of Columbidæ for a beak like that of
+  the English carrier, or that of the short-faced tumbler, or barb; for
+  reversed feathers like those of the Jacobin; for a crop like that of the
+  pouter; for tail-feathers like those of the fantail. Hence it must be
+  assumed not only that half-civilized man succeeded in thoroughly
+  domesticating several species, but that he intentionally or by chance
+  picked out extraordinarily abnormal species; and further, that these very
+  species have since all become extinct or unknown. So many strange
+  contingencies seem to me improbable in the highest degree. <!-- Page 25
+  --><span class="pagenum"><a name="page25"></a>[25]</span></p>
+
+  <p>Some facts in regard to the colouring of pigeons well deserve
+  consideration. The rock-pigeon is of a slaty-blue, and has a white rump
+  (the Indian subspecies, C. intermedia of Strickland, having it bluish);
+  the tail has a terminal dark bar, with the bases of the outer feathers
+  externally edged with white; the wings have two black bars; some
+  semi-domestic breeds and some apparently truly wild breeds have, besides
+  the two black bars, the wings chequered with black. These several marks
+  do not occur together in any other species of the whole family. Now, in
+  every one of the domestic breeds, taking thoroughly well-bred birds, all
+  the above marks, even to the white edging of the outer tail-feathers,
+  sometimes concur perfectly developed. Moreover, when two birds belonging
+  to two distinct breeds are crossed, neither of which is blue or has any
+  of the above-specified marks, the mongrel offspring are very apt suddenly
+  to acquire these characters; for instance, I crossed some uniformly white
+  fantails with some uniformly black barbs, and they produced mottled brown
+  and black birds; these I again crossed together, and one grandchild of
+  the pure white fantail and pure black barb was of as beautiful a blue
+  colour, with the white rump, double black wing-bar, and barred and
+  white-edged tail-feathers, as any wild rock-pigeon! We can understand
+  these facts, on the well-known principle of reversion to ancestral
+  characters, if all the domestic breeds have descended from the
+  rock-pigeon. But if we deny this, we must make one of the two following
+  highly improbable suppositions. Either, firstly, that all the several
+  imagined aboriginal stocks were coloured and marked like the rock-pigeon,
+  although no other existing species is thus coloured and marked, so that
+  in each separate breed there might be a tendency to revert to the very
+  same colours and markings. Or, secondly, <!-- Page 26 --><span
+  class="pagenum"><a name="page26"></a>[26]</span>that each breed, even the
+  purest, has within a dozen or, at most, within a score of generations,
+  been crossed by the rock-pigeon: I say within a dozen or twenty
+  generations, for we know of no fact countenancing the belief that the
+  child ever reverts to some one ancestor, removed by a greater number of
+  generations. In a breed which has been crossed only once with some
+  distinct breed, the tendency to reversion to any character derived from
+  such cross will naturally become less and less, as in each succeeding
+  generation there will be less of the foreign blood; but when there has
+  been no cross with a distinct breed, and there is a tendency in both
+  parents to revert to a character, which has been lost during some former
+  generation, this tendency, for all that we can see to the contrary, may
+  be transmitted undiminished for an indefinite number of generations.
+  These two distinct cases are often confounded in treatises on
+  inheritance.</p>
+
+  <p>Lastly, the hybrids or mongrels from between all the domestic breeds
+  of pigeons are perfectly fertile. I can state this from my own
+  observations, purposely made, on the most distinct breeds. Now, it is
+  difficult, perhaps impossible, to bring forward one case of the hybrid
+  offspring of two animals <i>clearly distinct</i> being themselves
+  perfectly fertile. Some authors believe that long-continued domestication
+  eliminates this strong tendency to sterility: from the history of the dog
+  I think there is some probability in this hypothesis, if applied to
+  species closely related together, though it is unsupported by a single
+  experiment. But to extend the hypothesis so far as to suppose that
+  species, aboriginally as distinct as carriers, tumblers, pouters, and
+  fantails now are, should yield offspring perfectly fertile, <i>inter
+  se</i>, seems to me rash in the extreme.</p>
+
+  <p>From these several reasons, namely, the improbability of man having
+  formerly got seven or eight supposed <!-- Page 27 --><span
+  class="pagenum"><a name="page27"></a>[27]</span>species of pigeons to
+  breed freely under domestication; these supposed species being quite
+  unknown in a wild state, and their becoming nowhere feral; these species
+  having very abnormal characters in certain respects, as compared with all
+  other Columbidæ, though so like in most other respects to the
+  rock-pigeon; the blue colour and various marks occasionally appearing in
+  all the breeds, both when kept pure and when crossed; the mongrel
+  offspring being perfectly fertile;&mdash;from these several reasons,
+  taken together, I can feel no doubt that all our domestic breeds have
+  descended from the Columba livia with its geographical sub-species.</p>
+
+  <p>In favour of this view, I may add, firstly, that C. livia, or the
+  rock-pigeon, has been found capable of domestication in Europe and in
+  India; and that it agrees in habits and in a great number of points of
+  structure with all the domestic breeds. Secondly, although an English
+  carrier or short-faced tumbler differs immensely in certain characters
+  from the rock-pigeon, yet by comparing the several sub-breeds of these
+  varieties, more especially those brought from distant countries, we can
+  make an almost perfect series between the extremes of structure. Thirdly,
+  those characters which are mainly distinctive of each breed, for instance
+  the wattle and length of beak of the carrier, the shortness of that of
+  the tumbler, and the number of tail-feathers in the fantail, are in each
+  breed eminently variable; and the explanation of this fact will be
+  obvious when we come to treat of selection. Fourthly, pigeons have been
+  watched, and tended with the utmost care, and loved by many people. They
+  have been domesticated for thousands of years in several quarters of the
+  world; the earliest known record of pigeons is in the fifth Ægyptian
+  dynasty, about 3000 B.C., as was pointed out to me by Professor Lepsius;
+  but Mr. Birch informs me that pigeons are given in a bill <!-- Page 28
+  --><span class="pagenum"><a name="page28"></a>[28]</span>of fare in the
+  previous dynasty. In the time of the Romans, as we hear from Pliny,
+  immense prices were given for pigeons; "nay, they are come to this pass,
+  that they can reckon up their pedigree and race." Pigeons were much
+  valued by Akber Khan in India, about the year 1600; never less than
+  20,000 pigeons were taken with the court. "The monarchs of Iran and Turan
+  sent him some very rare birds;" and, continues the courtly historian,
+  "His Majesty by crossing the breeds, which method was never practised
+  before, has improved them astonishingly." About this same period the
+  Dutch were as eager about pigeons as were the old Romans. The paramount
+  importance of these considerations in explaining the immense amount of
+  variation which pigeons have undergone, will be obvious when we treat of
+  Selection. We shall then, also, see how it is that the breeds so often
+  have a somewhat monstrous character. It is also a most favourable
+  circumstance for the production of distinct breeds, that male and female
+  pigeons can be easily mated for life; and thus different breeds can be
+  kept together in the same aviary.</p>
+
+  <p>I have discussed the probable origin of domestic pigeons at some, yet
+  quite insufficient, length; because when I first kept pigeons and watched
+  the several kinds, knowing well how true they bred, I felt fully as much
+  difficulty in believing that they could have descended from a common
+  parent, as any naturalist could in coming to a similar conclusion in
+  regard to the many species of finches, or other large groups of birds, in
+  nature. One circumstance has struck me much; namely, that all the
+  breeders of the various domestic animals and the cultivators of plants,
+  with whom I have ever conversed, or whose treatises I have read, are
+  firmly convinced that the several breeds to which each has attended, are
+  descended from so many aboriginally distinct species. <!-- Page 29
+  --><span class="pagenum"><a name="page29"></a>[29]</span>Ask, as I have
+  asked, a celebrated raiser of Hereford cattle, whether his cattle might
+  not have descended from long-horns, and he will laugh you to scorn. I
+  have never met a pigeon, or poultry, or duck, or rabbit fancier, who was
+  not fully convinced that each main breed was descended from a distinct
+  species. Van Mons, in his treatise on pears and apples, shows how utterly
+  he disbelieves that the several sorts, for instance a Ribston-pippin or
+  Codlin-apple, could ever have proceeded from the seeds of the same tree.
+  Innumerable other examples could be given. The explanation, I think, is
+  simple: from long-continued study they are strongly impressed with the
+  differences between the several races; and though they well know that
+  each race varies slightly, for they win their prizes by selecting such
+  slight differences, yet they ignore all general arguments, and refuse to
+  sum up in their minds slight differences accumulated during many
+  successive generations. May not those naturalists who, knowing far less
+  of the laws of inheritance than does the breeder, and knowing no more
+  than he does of the intermediate links in the long lines of descent, yet
+  admit that many of our domestic races have descended from the same
+  parents&mdash;may they not learn a lesson of caution, when they deride
+  the idea of species in a state of nature being lineal descendants of
+  other species?</p>
+
+  <p><i>Selection.</i>&mdash;Let us now briefly consider the steps by which
+  domestic races have been produced, either from one or from several allied
+  species. Some little effect may, perhaps, be attributed to the direct
+  action of the external conditions of life, and some little to habit; but
+  he would be a bold man who would account by such agencies for the
+  differences of a dray and race horse, a greyhound and bloodhound, a
+  carrier and tumbler pigeon. One of the most remarkable features in our
+  domesticated races <!-- Page 30 --><span class="pagenum"><a
+  name="page30"></a>[30]</span>is that we see in them adaptation, not
+  indeed to the animal's or plant's own good, but to man's use or fancy.
+  Some variations useful to him have probably arisen suddenly, or by one
+  step; many botanists, for instance, believe that the fuller's teazle,
+  with its hooks, which cannot be rivalled by any mechanical contrivance,
+  is only a variety of the wild Dipsacus; and this amount of change may
+  have suddenly arisen in a seedling. So it has probably been with the
+  turnspit dog; and this is known to have been the case with the ancon
+  sheep. But when we compare the dray-horse and race-horse, the dromedary
+  and camel, the various breeds of sheep fitted either for cultivated land
+  or mountain pasture, with the wool of one breed good for one purpose, and
+  that of another breed for another purpose; when we compare the many
+  breeds of dogs, each good for man in very different ways; when we compare
+  the game-cock, so pertinacious in battle, with other breeds so little
+  quarrelsome, with "everlasting layers" which never desire to sit, and
+  with the bantam so small and elegant; when we compare the host of
+  agricultural, culinary, orchard, and flower-garden races of plants, most
+  useful to man at different seasons and for different purposes, or so
+  beautiful in his eyes, we must, I think, look further than to mere
+  variability. We cannot suppose that all the breeds were suddenly produced
+  as perfect and as useful as we now see them; indeed, in several cases, we
+  know that this has not been their history. The key is man's power of
+  accumulative selection: nature gives successive variations; man adds them
+  up in certain directions useful to him. In this sense he may be said to
+  make for himself useful breeds.</p>
+
+  <p>The great power of this principle of selection is not hypothetical. It
+  is certain that several of our eminent breeders have, even within a
+  single lifetime, modified to <!-- Page 31 --><span class="pagenum"><a
+  name="page31"></a>[31]</span>a large extent some breeds of cattle and
+  sheep. In order fully to realise what they have done, it is almost
+  necessary to read several of the many treatises devoted to this subject,
+  and to inspect the animals. Breeders habitually speak of an animal's
+  organisation as something quite plastic, which they can model almost as
+  they please. If I had space I could quote numerous passages to this
+  effect from highly competent authorities. Youatt, who was probably better
+  acquainted with the works of agriculturists than almost any other
+  individual, and who was himself a very good judge of an animal, speaks of
+  the principle of selection as "that which enables the agriculturist, not
+  only to modify the character of his flock, but to change it altogether.
+  It is the magician's wand, by means of which he may summon into life
+  whatever form and mould he pleases." Lord Somerville, speaking of what
+  breeders have done for sheep, says:&mdash;"It would seem as if they had
+  chalked out upon a wall a form perfect in itself, and then had given it
+  existence." That most skilful breeder, Sir John Sebright, used to say,
+  with respect to pigeons, that "he would produce any given feather in
+  three years, but it would take him six years to obtain head and beak." In
+  Saxony the importance of the principle of selection in regard to merino
+  sheep is so fully recognised, that men follow it as a trade: the sheep
+  are placed on a table and are studied, like a picture by a connoisseur;
+  this is done three times at intervals of months, and the sheep are each
+  time marked and classed, so that the very best may ultimately be selected
+  for breeding.</p>
+
+  <p>What English breeders have actually effected is proved by the enormous
+  prices given for animals with a good pedigree; and these have now been
+  exported to almost every quarter of the world. The improvement is by no
+  means generally due to crossing different breeds; <!-- Page 32 --><span
+  class="pagenum"><a name="page32"></a>[32]</span>all the best breeders are
+  strongly opposed to this practice, except sometimes amongst closely
+  allied sub-breeds. And when a cross has been made, the closest selection
+  is far more indispensable even than in ordinary cases. If selection
+  consisted merely in separating some very distinct variety, and breeding
+  from it, the principle would be so obvious as hardly to be worth notice;
+  but its importance consists in the great effect produced by the
+  accumulation in one direction, during successive generations, of
+  differences absolutely inappreciable by an uneducated
+  eye&mdash;differences which I for one have vainly attempted to
+  appreciate. Not one man in a thousand has accuracy of eye and judgment
+  sufficient to become an eminent breeder. If gifted with these qualities,
+  and he studies his subject for years, and devotes his lifetime to it with
+  indomitable perseverance, he will succeed, and may make great
+  improvements; if he wants any of these qualities, he will assuredly fail.
+  Few would readily believe in the natural capacity and years of practice
+  requisite to become even a skilful pigeon-fancier.</p>
+
+  <p>The same principles are followed by horticulturists; but the
+  variations are here often more abrupt. No one supposes that our choicest
+  productions have been produced by a single variation from the aboriginal
+  stock. We have proofs that this is not so in some cases, in which exact
+  records have been kept; thus, to give a very trifling instance, the
+  steadily-increasing size of the common gooseberry may be quoted. We see
+  an astonishing improvement in many florists' flowers, when the flowers of
+  the present day are compared with drawings made only twenty or thirty
+  years ago. When a race of plants is once pretty well established, the
+  seed-raisers do not pick out the best plants, but merely go over their
+  seed-beds, and pull up the "rogues," as they call the plants that deviate
+  from the proper standard. With animals this <!-- Page 33 --><span
+  class="pagenum"><a name="page33"></a>[33]</span>kind of selection is, in
+  fact, also followed; for hardly any one is so careless as to allow his
+  worst animals to breed.</p>
+
+  <p>In regard to plants, there is another means of observing the
+  accumulated effects of selection&mdash;namely, by comparing the diversity
+  of flowers in the different varieties of the same species in the
+  flower-garden; the diversity of leaves, pods, or tubers, or whatever part
+  is valued, in the kitchen-garden, in comparison with the flowers of the
+  same varieties; and the diversity of fruit of the same species in the
+  orchard, in comparison with the leaves and flowers of the same set of
+  varieties. See how different the leaves of the cabbage are, and how
+  extremely alike the flowers; how unlike the flowers of the heartsease
+  are, and how alike the leaves; how much the fruit of the different kinds
+  of gooseberries differ in size, colour, shape, and hairiness, and yet the
+  flowers present very slight differences. It is not that the varieties
+  which differ largely in some one point do not differ at all in other
+  points; this is hardly ever, perhaps never, the case. The laws of
+  correlation of growth, the importance of which should never be
+  overlooked, will ensure some differences; but, as a general rule, I
+  cannot doubt that the continued selection of slight variations, either in
+  the leaves, the flowers, or the fruit, will produce races differing from
+  each other chiefly in these characters.</p>
+
+  <p>It may be objected that the principle of selection has been reduced to
+  methodical practice for scarcely more than three-quarters of a century;
+  it has certainly been more attended to of late years, and many treatises
+  have been published on the subject; and the result has been, in a
+  corresponding degree, rapid and important. But it is very far from true
+  that the principle is a modern discovery. I could give several references
+  to the full acknowledgment of the importance of the principle in works of
+  high antiquity. In rude and barbarous periods <!-- Page 34 --><span
+  class="pagenum"><a name="page34"></a>[34]</span>of English history choice
+  animals were often imported, and laws were passed to prevent their
+  exportation: the destruction of horses under a certain size was ordered,
+  and this may be compared to the "roguing" of plants by nurserymen. The
+  principle of selection I find distinctly given in an ancient Chinese
+  encyclopædia. Explicit rules are laid down by some of the Roman classical
+  writers. From passages in Genesis, it is clear that the colour of
+  domestic animals was at that early period attended to. Savages now
+  sometimes cross their dogs with wild canine animals, to improve the
+  breed, and they formerly did so, as is attested by passages in Pliny. The
+  savages in South Africa match their draught cattle by colour, as do some
+  of the Esquimaux their teams of dogs. Livingstone shows how much good
+  domestic breeds are valued by the negroes of the interior of Africa who
+  have not associated with Europeans. Some of these facts do not show
+  actual selection, but they show that the breeding of domestic animals was
+  carefully attended to in ancient times, and is now attended to by the
+  lowest savages. It would, indeed, have been a strange fact, had attention
+  not been paid to breeding, for the inheritance of good and bad qualities
+  is so obvious.</p>
+
+  <p>At the present time, eminent breeders try by methodical selection,
+  with a distinct object in view, to make a new strain or sub-breed,
+  superior to anything existing in the country. But, for our purpose, a
+  kind of Selection, which may be called Unconscious, and which results
+  from every one trying to possess and breed from the best individual
+  animals, is more important. Thus, a man who intends keeping pointers
+  naturally tries to get as good dogs as he can, and afterwards breeds from
+  his own best dogs, but he has no wish or expectation of permanently
+  altering the breed. Nevertheless I cannot doubt that this process,
+  continued during centuries, <!-- Page 35 --><span class="pagenum"><a
+  name="page35"></a>[35]</span>would improve and modify any breed, in the
+  same way as Bakewell, Collins, &amp;c., by this very same process, only
+  carried on more methodically, did greatly modify, even during their own
+  lifetimes, the forms and qualities of their cattle. Slow and insensible
+  changes of this kind could never be recognised unless actual measurements
+  or careful drawings of the breeds in question had been made long ago,
+  which might serve for comparison. In some cases, however, unchanged, or
+  but little changed individuals of the same breed may be found in less
+  civilised districts, where the breed has been less improved. There is
+  reason to believe that King Charles's spaniel has been unconsciously
+  modified to a large extent since the time of that monarch. Some highly
+  competent authorities are convinced that the setter is directly derived
+  from the spaniel, and has probably been slowly altered from it. It is
+  known that the English pointer has been greatly changed within the last
+  century, and in this case the change has, it is believed, been chiefly
+  effected by crosses with the fox-hound; but what concerns us is, that the
+  change has been effected unconsciously and gradually, and yet so
+  effectually, that, though the old Spanish pointer certainly came from
+  Spain, Mr. Borrow has not seen, as I am informed by him, any native dog
+  in Spain like our pointer.</p>
+
+  <p>By a similar process of selection, and by careful training, the whole
+  body of English racehorses have come to surpass in fleetness and size the
+  parent Arab stock, so that the latter, by the regulations for the
+  Goodwood Races, are favoured in the weights they carry. Lord Spencer and
+  others have shown how the cattle of England have increased in weight and
+  in early maturity, compared with the stock formerly kept in this country.
+  By comparing the accounts given in old pigeon treatises of carriers and
+  tumblers with these breeds as now existing in Britain, <!-- Page 36
+  --><span class="pagenum"><a name="page36"></a>[36]</span>India, and
+  Persia, we can, I think, clearly trace the stages through which they have
+  insensibly passed, and come to differ so greatly from the
+  rock-pigeon.</p>
+
+  <p>Youatt gives an excellent illustration of the effects of a course of
+  selection, which may be considered as unconsciously followed, in so far
+  that the breeders could never have expected or even have wished to have
+  produced the result which ensued&mdash;namely, the production of two
+  distinct strains. The two flocks of Leicester sheep kept by Mr. Buckley
+  and Mr. Burgess, as Mr. Youatt remarks, "have been purely bred from the
+  original stock of Mr. Bakewell for upwards of fifty years. 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 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."</p>
+
+  <p>If there exist savages so barbarous as never to think of the inherited
+  character of the offspring of their domestic animals, yet any one animal
+  particularly useful to them, for any special purpose, would be carefully
+  preserved during famines and other accidents, to which savages are so
+  liable, and such choice animals would thus generally leave more offspring
+  than the inferior ones; so that in this case there would be a kind of
+  unconscious selection going on. We see the value set on animals even by
+  the barbarians of Tierra del Fuego, by their killing and devouring their
+  old women, in times of dearth, as of less value than their dogs.</p>
+
+  <p>In plants the same gradual process of improvement, through the
+  occasional preservation of the best individuals, whether or not
+  sufficiently distinct to be ranked at their first appearance as distinct
+  varieties, and whether <!-- Page 37 --><span class="pagenum"><a
+  name="page37"></a>[37]</span>or not two or more species or races have
+  become blended together by crossing, may plainly be recognised in the
+  increased size and beauty which we now see in the varieties of the
+  heartsease, rose, pelargonium, dahlia, and other plants, when compared
+  with the older varieties or with their parent-stocks. No one would ever
+  expect to get a first-rate heartsease or dahlia from the seed of a wild
+  plant. No one would expect to raise a first-rate melting pear from the
+  seed of the wild pear, though he might succeed from a poor seedling
+  growing wild, if it had come from a garden-stock. The pear, though
+  cultivated in classical times, appears, from Pliny's description, to have
+  been a fruit of very inferior quality. I have seen great surprise
+  expressed in horticultural works at the wonderful skill of gardeners, in
+  having produced such splendid results from such poor materials; but the
+  art, I cannot doubt, has been simple, and, as far as the final result is
+  concerned, has been followed almost unconsciously. It has consisted in
+  always cultivating the best known variety, sowing its seeds, and, when a
+  slightly better variety has chanced to appear, selecting it, and so
+  onwards. But the gardeners of the classical period, who cultivated the
+  best pear they could procure, never thought what splendid fruit we should
+  eat; though we owe our excellent fruit, in some small degree, to their
+  having naturally chosen and preserved the best varieties they could
+  anywhere find.</p>
+
+  <p>A large amount of change in our cultivated plants, thus slowly and
+  unconsciously accumulated, explains, as I believe, the well-known fact,
+  that in a vast number of cases we cannot recognise, and therefore do not
+  know, the wild parent-stocks of the plants which have been longest
+  cultivated in our flower and kitchen gardens. If it has taken centuries
+  or thousands of years to improve or modify most of our plants up to their
+  present <!-- Page 38 --><span class="pagenum"><a
+  name="page38"></a>[38]</span>standard of usefulness to man, we can
+  understand how it is that neither Australia, the Cape of Good Hope, nor
+  any other region inhabited by quite uncivilised man, has afforded us a
+  single plant worth culture. It is not that these countries, so rich in
+  species, do not by a strange chance possess the aboriginal stocks of any
+  useful plants, but that the native plants have not been improved by
+  continued selection up to a standard of perfection comparable with that
+  given to the plants in countries anciently civilised.</p>
+
+  <p>In regard to the domestic animals kept by uncivilised man, it should
+  not be overlooked that they almost always have to struggle for their own
+  food, at least during certain seasons. And in two countries very
+  differently circumstanced, individuals of the same species, having
+  slightly different constitutions or structure, would often succeed better
+  in the one country than in the other; and thus by a process of "natural
+  selection," as will hereafter be more fully explained, two sub-breeds
+  might be formed. This, perhaps, partly explains what has been remarked by
+  some authors, namely, that the varieties kept by savages have more of the
+  character of species than the varieties kept in civilised countries.</p>
+
+  <p>On the view here given of the all-important part which selection by
+  man has played, it becomes at once obvious, how it is that our domestic
+  races show adaptation in their structure or in their habits to man's
+  wants or fancies. We can, I think, further understand the frequently
+  abnormal character of our domestic races, and likewise their differences
+  being so great in external characters and relatively so slight in
+  internal parts or organs. Man can hardly select, or only with much
+  difficulty, any deviation of structure excepting such as is externally
+  visible; and indeed he rarely cares for what is internal. He can never
+  act by selection, excepting on variations <!-- Page 39 --><span
+  class="pagenum"><a name="page39"></a>[39]</span>which are first given to
+  him in some slight degree by nature. No man would ever try to make a
+  fantail, till he saw a pigeon with a tail developed in some slight degree
+  in an unusual manner, or a pouter till he saw a pigeon with a crop of
+  somewhat unusual size; and the more abnormal or unusual any character was
+  when it first appeared, the more likely it would be to catch his
+  attention. But to use such an expression as trying to make a fantail, is,
+  I have no doubt, in most cases, utterly incorrect. The man who first
+  selected a pigeon with a slightly larger tail, never dreamed what the
+  descendants of that pigeon would become through long-continued, partly
+  unconscious and partly methodical selection. Perhaps the parent bird of
+  all fantails had only fourteen tail-feathers somewhat expanded, like the
+  present Java fantail, or like individuals of other and distinct breeds,
+  in which as many as seventeen tail-feathers have been counted. Perhaps
+  the first pouter-pigeon did not inflate its crop much more than the
+  turbit now does the upper part of its &oelig;sophagus,&mdash;a habit
+  which is disregarded by all fanciers, as it is not one of the points of
+  the breed.</p>
+
+  <p>Nor let it be thought that some great deviation of structure would be
+  necessary to catch the fancier's eye: he perceives extremely small
+  differences, and it is in human nature to value any novelty, however
+  slight, in one's own possession. Nor must the value which would formerly
+  be set on any slight differences in the individuals of the same species,
+  be judged of by the value which would now be set on them, after several
+  breeds have once fairly been established. Many slight differences might,
+  and indeed do now, arise amongst pigeons, which are rejected as faults or
+  deviations from the standard of perfection of each breed. The common
+  goose has not given rise to any marked varieties; hence the Thoulouse and
+  the common breed, which differ only in colour, that <!-- Page 40 --><span
+  class="pagenum"><a name="page40"></a>[40]</span>most fleeting of
+  characters, have lately been exhibited as distinct at our
+  poultry-shows.</p>
+
+  <p>I think these views further explain what has sometimes been
+  noticed&mdash;namely, that we know nothing about the origin or history of
+  any of our domestic breeds. But, in fact, a breed, like a dialect of a
+  language, can hardly be said to have had a definite origin. A man
+  preserves and breeds from an individual with some slight deviation of
+  structure, or takes more care than usual in matching his best animals and
+  thus improves them, and the improved individuals slowly spread in the
+  immediate neighbourhood. But as yet they will hardly have a distinct
+  name, and from being only slightly valued, their history will be
+  disregarded. When further improved by the same slow and gradual process,
+  they will spread more widely, and will get recognised as something
+  distinct and valuable, and will then probably first receive a provincial
+  name. In semi-civilised countries, with little free communication, the
+  spreading and knowledge of any new sub-breed will be a slow process. As
+  soon as the points of value of the new sub-breed are once fully
+  acknowledged, the principle, as I have called it, of unconscious
+  selection will always tend,&mdash;perhaps more at one period than at
+  another, as the breed rises or falls in fashion,&mdash;perhaps more in
+  one district than in another, according to the state of civilization of
+  the inhabitants,&mdash;slowly to add to the characteristic features of
+  the breed, whatever they may be. But the chance will be infinitely small
+  of any record having been preserved of such slow, varying, and insensible
+  changes.</p>
+
+  <p>I must now say a few words on the circumstances, favourable, or the
+  reverse, to man's power of selection. A high degree of variability is
+  obviously favourable, as freely giving the materials for selection to
+  work on; not that mere individual differences are not amply <!-- Page 41
+  --><span class="pagenum"><a name="page41"></a>[41]</span>sufficient, with
+  extreme care, to allow of the accumulation of a large amount of
+  modification in almost any desired direction. But as variations
+  manifestly useful or pleasing to man appear only occasionally, the chance
+  of their appearance will be much increased by a large number of
+  individuals being kept; and hence this comes to be of the highest
+  importance to success. On this principle Marshall has remarked, with
+  respect to the sheep of parts of Yorkshire, that "as they generally
+  belong to poor people, and are mostly <i>in small lots</i>, they never
+  can be improved." On the other hand, nurserymen, from raising large
+  stocks of the same plants, are generally far more successful than
+  amateurs in getting new and valuable varieties. The keeping of a large
+  number of individuals of a species in any country requires that the
+  species should be placed under favourable conditions of life, so as to
+  breed freely in that country. When the individuals of any species are
+  scanty, all the individuals, whatever their quality may be, will
+  generally be allowed to breed, and this will effectually prevent
+  selection. But probably the most important point of all, is, that the
+  animal or plant should be so highly useful to man, or so much valued by
+  him, that the closest attention should be paid to even the slightest
+  deviation in the qualities or structure of each individual. Unless such
+  attention be paid nothing can be effected. I have seen it gravely
+  remarked, that it was most fortunate that the strawberry began to vary
+  just when gardeners began to attend closely to this plant. No doubt the
+  strawberry had always varied since it was cultivated, but the slight
+  varieties had been neglected. As soon, however, as gardeners picked out
+  individual plants with slightly larger, earlier, or better fruit, and
+  raised seedlings from them, and again picked out the best seedlings and
+  bred from them, then, there appeared (aided by some <!-- Page 42 --><span
+  class="pagenum"><a name="page42"></a>[42]</span>crossing with distinct
+  species) those many admirable varieties of the strawberry which have been
+  raised during the last thirty or forty years.</p>
+
+  <p>In the case of animals with separate sexes, facility in preventing
+  crosses is an important element of success in the formation of new
+  races,&mdash;at least, in a country which is already stocked with other
+  races. In this respect enclosure of the land plays a part. Wandering
+  savages or the inhabitants of open plains rarely possess more than one
+  breed of the same species. Pigeons can be mated for life, and this is a
+  great convenience to the fancier, for thus many races may be kept true,
+  though mingled in the same aviary; and this circumstance must have
+  largely favoured the improvement and formation of new breeds. Pigeons, I
+  may add, can be propagated in great numbers and at a very quick rate, and
+  inferior birds may be freely rejected, as when killed they serve for
+  food. On the other hand, cats, from their nocturnal rambling habits,
+  cannot be matched, and, although so much valued by women and children, we
+  hardly ever see a distinct breed kept up; such breeds as we do sometimes
+  see are almost always imported from some other country, often from
+  islands. Although I do not doubt that some domestic animals vary less
+  than others, yet the rarity or absence of distinct breeds of the cat, the
+  donkey, peacock, goose, &amp;c., may be attributed in main part to
+  selection not having been brought into play: in cats, from the difficulty
+  in pairing them; in donkeys, from only a few being kept by poor people,
+  and little attention paid to their breeding; in peacocks, from not being
+  very easily reared and a large stock not kept; in geese, from being
+  valuable only for two purposes, food and feathers, and more especially
+  from no pleasure having been felt in the display of distinct breeds.</p>
+
+  <p>To sum up on the origin of our Domestic Races of <!-- Page 43 --><span
+  class="pagenum"><a name="page43"></a>[43]</span>animals and plants. I
+  believe that the conditions of life, from their action on the
+  reproductive system, are so far of the highest importance as causing
+  variability. I do not believe that variability is an inherent and
+  necessary contingency, under all circumstances, with all organic beings,
+  as some authors have thought. The effects of variability are modified by
+  various degrees of inheritance and of reversion. Variability is governed
+  by many unknown laws, more especially by that of correlation of growth.
+  Something may be attributed to the direct action of the conditions of
+  life. Something must be attributed to use and disuse. The final result is
+  thus rendered infinitely complex. In some cases, I do not doubt that the
+  intercrossing of species, aboriginally distinct, has played an important
+  part in the origin of our domestic productions. When in any country
+  several domestic breeds have once been established, their occasional
+  intercrossing, with the aid of selection, has, no doubt, largely aided in
+  the formation of new sub-breeds; but the importance of the crossing of
+  varieties has, I believe, been greatly exaggerated, both in regard to
+  animals and to those plants which are propagated by seed. In plants which
+  are temporarily propagated by cuttings, buds, &amp;c., the importance of
+  the crossing both of distinct species and of varieties is immense; for
+  the cultivator here quite disregards the extreme variability both of
+  hybrids and mongrels, and the frequent sterility of hybrids; but the
+  cases of plants not propagated by seed are of little importance to us,
+  for their endurance is only temporary. Over all these causes of Change I
+  am convinced that the accumulative action of Selection, whether applied
+  methodically and more quickly, or unconsciously and more slowly, but more
+  efficiently, is by far the predominant Power.</p>
+
+<hr />
+
+<p><!-- Page 44 --><span class="pagenum"><a name="page44"></a>[44]</span></p>
+
+<h3>CHAPTER II.</h3>
+
+<p class="cenhead"><span class="sc">Variation under Nature.</span></p>
+
+<blockquote class="b1n">
+
+  <p>Variability&mdash;Individual differences&mdash;Doubtful
+  species&mdash;Wide ranging, much diffused, and common species vary
+  most&mdash;Species of the larger genera in any country vary more than the
+  species of the smaller genera&mdash;Many of the species of the larger
+  genera resemble varieties in being very closely, but unequally, related
+  to each other, and in having restricted ranges.</p>
+
+</blockquote>
+
+  <p>Before applying the principles arrived at in the last chapter to
+  organic beings in a state of nature, we must briefly discuss whether
+  these latter are subject to any variation. To treat this subject at all
+  properly, a long catalogue of dry facts should be given; but these I
+  shall reserve for my future work. Nor shall I here discuss the various
+  definitions which have been given of the term species. No one definition
+  has as yet satisfied all naturalists; yet every naturalist knows vaguely
+  what he means when he speaks of a species. Generally the term includes
+  the unknown element of a distinct act of creation. The term "variety" is
+  almost equally difficult to define; but here community of descent is
+  almost universally implied, though it can rarely be proved. We have also
+  what are called monstrosities; but they graduate into varieties. By a
+  monstrosity I presume is meant some considerable deviation of structure
+  in one part, either injurious to or not useful to the species, and not
+  generally propagated. Some authors use the term "variation" in a
+  technical sense, as implying a modification directly due to the physical
+  conditions of life; and "variations" in this sense are supposed not to be
+  inherited: but who can say that the dwarfed condition of shells in the
+  brackish waters of the Baltic, or dwarfed <!-- Page 45 --><span
+  class="pagenum"><a name="page45"></a>[45]</span>plants on Alpine summits,
+  or the thicker fur of an animal from far northwards, would not in some
+  cases be inherited for at least some few generations? and in this case I
+  presume that the form would be called a variety.</p>
+
+  <p>Again, we have many slight differences which may be called individual
+  differences, such as are known frequently to appear in the offspring from
+  the same parents, or which may be presumed to have thus arisen, from
+  being frequently observed in the individuals of the same species
+  inhabiting the same confined locality. No one supposes that all the
+  individuals of the same species are cast in the very same mould. These
+  individual differences are highly important for us, as they afford
+  materials for natural selection to accumulate, in the same manner as man
+  can accumulate in any given direction individual differences in his
+  domesticated productions. These individual differences generally affect
+  what naturalists consider unimportant parts; but I could show by a long
+  catalogue of facts, that parts which must be called important, whether
+  viewed under a physiological or classificatory point of view, sometimes
+  vary in the individuals of the same species. I am convinced that the most
+  experienced naturalist would be surprised at the number of the cases of
+  variability, even in important parts of structure, which he could collect
+  on good authority, as I have collected, during a course of years. It
+  should be remembered that systematists are far from pleased at finding
+  variability in important characters, and that there are not many men who
+  will laboriously examine internal and important organs, and compare them
+  in many specimens of the same species. I should never have expected that
+  the branching of the main nerves close to the great central ganglion of
+  an insect would have been variable in the same species; I should have
+  expected that changes of this nature could have been effected only <!--
+  Page 46 --><span class="pagenum"><a name="page46"></a>[46]</span>by slow
+  degrees: yet quite recently Mr. Lubbock has shown a degree of variability
+  in these main nerves in Coccus, which may almost be compared to the
+  irregular branching of the stem of a tree. This philosophical naturalist,
+  I may add, has also quite recently shown that the muscles in the larvæ of
+  certain insects are very far from uniform. Authors sometimes argue in a
+  circle when they state that important organs never vary; for these same
+  authors practically rank that character as important (as some few
+  naturalists have honestly confessed) which does not vary; and, under this
+  point of view, no instance of an important part varying will ever be
+  found: but under any other point of view many instances assuredly can be
+  given.</p>
+
+  <p>There is one point connected with individual differences, which seems
+  to me extremely perplexing: I refer to those genera which have sometimes
+  been called "protean" or "polymorphic," in which the species present an
+  inordinate amount of variation; and hardly two naturalists can agree
+  which forms to rank as species and which as varieties. We may instance
+  Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and
+  several genera of Brachiopod shells. In most polymorphic genera some of
+  the species have fixed and definite characters. Genera which are
+  polymorphic in one country seem to be, with some few exceptions,
+  polymorphic in other countries, and likewise, judging from Brachiopod
+  shells, at former periods of time. These facts seem to be very
+  perplexing, for they seem to show that this kind of variability is
+  independent of the conditions of life. I am inclined to suspect that we
+  see in these polymorphic genera variations in points of structure which
+  are of no service or disservice to the species, and which consequently
+  have not been seized on and rendered definite by natural selection, as
+  hereafter will be explained. <!-- Page 47 --><span class="pagenum"><a
+  name="page47"></a>[47]</span></p>
+
+  <p>Those forms which possess in some considerable degree the character of
+  species, but which are so closely similar to some other forms, or are so
+  closely linked to them by intermediate gradations, that naturalists do
+  not like to rank them as distinct species, are in several respects the
+  most important for us. We have every reason to believe that many of these
+  doubtful and closely-allied forms have permanently retained their
+  characters in their own country for a long time; for as long, as far as
+  we know, as have good and true species. Practically, when a naturalist
+  can unite two forms together by others having intermediate characters, he
+  treats the one as a variety of the other, ranking the most common, but
+  sometimes the one first described, as the species, and the other as the
+  variety. But cases of great difficulty, which I will not here enumerate,
+  sometimes occur in deciding whether or not to rank one form as a variety
+  of another, even when they are closely connected by intermediate links;
+  nor will the commonly-assumed hybrid nature of the intermediate links
+  always remove the difficulty. In very many cases, however, one form is
+  ranked as a variety of another, not because the intermediate links have
+  actually been found, but because analogy leads the observer to suppose
+  either that they do now somewhere exist, or may formerly have existed;
+  and here a wide door for the entry of doubt and conjecture is opened.</p>
+
+  <p>Hence, in determining whether a form should be ranked as a species or
+  a variety, the opinion of naturalists having sound judgment and wide
+  experience seems the only guide to follow. We must, however, in many
+  cases, decide by a majority of naturalists, for few well-marked and
+  well-known varieties can be named which have not been ranked as species
+  by at least some competent judges. <!-- Page 48 --><span
+  class="pagenum"><a name="page48"></a>[48]</span></p>
+
+  <p>That varieties of this doubtful nature are far from uncommon cannot be
+  disputed. Compare the several floras of Great Britain, of France or of
+  the United States, drawn up by different botanists, and see what a
+  surprising number of forms have been ranked by one botanist as good
+  species, and by another as mere varieties. Mr. H.&nbsp;C. Watson, to whom I
+  lie under deep obligation for assistance of all kinds, has marked for me
+  182 British plants, which are generally considered as varieties, but
+  which have all been ranked by botanists as species; and in making this
+  list he has omitted many trifling varieties, but which nevertheless have
+  been ranked by some botanists as species, and he has entirely omitted
+  several highly polymorphic genera. Under genera, including the most
+  polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham
+  gives only 112,&mdash;a difference of 139 doubtful forms! Amongst animals
+  which unite for each birth, and which are highly locomotive, doubtful
+  forms, ranked by one zoologist as a species and by another as a variety,
+  can rarely be found within the same country, but are common in separated
+  areas. How many of those birds and insects in North America and Europe,
+  which differ very slightly from each other, have been ranked by one
+  eminent naturalist as undoubted species, and by another as varieties, or,
+  as they are often called, as geographical races! Many years ago, when
+  comparing, and seeing others compare, the birds from the separate islands
+  of the Galapagos Archipelago, both one with another, and with those from
+  the American mainland, I was much struck how entirely vague and arbitrary
+  is the distinction between species and varieties. On the islets of the
+  little Madeira group there are many insects which are characterized as
+  varieties in Mr. Wollaston's admirable work, but which it cannot <!--
+  Page 49 --><span class="pagenum"><a name="page49"></a>[49]</span>be
+  doubted would be ranked as distinct species by many entomologists. Even
+  Ireland has a few animals, now generally regarded as varieties, but which
+  have been ranked as species by some zoologists. Several most experienced
+  ornithologists consider our British red grouse as only a strongly-marked
+  race of a Norwegian species, whereas the greater number rank it as an
+  undoubted species peculiar to Great Britain. A wide distance between the
+  homes of two doubtful forms leads many naturalists to rank both as
+  distinct species; but what distance, it has been well asked, will
+  suffice? if that between America and Europe is ample, will that between
+  the Continent and the Azores, or Madeira, or the Canaries, or Ireland, be
+  sufficient? It must be admitted that many forms, considered by
+  highly-competent judges as varieties, have so perfectly the character of
+  species that they are ranked by other highly-competent judges as good and
+  true species. But to discuss whether they are rightly called species or
+  varieties, before any definition of these terms has been generally
+  accepted, is vainly to beat the air.</p>
+
+  <p>Many of the cases of strongly-marked varieties or doubtful species
+  well deserve consideration; for several interesting lines of argument,
+  from geographical distribution, analogical variation, hybridism, &amp;c.,
+  have been brought to bear on the attempt to determine their rank. I will
+  here give only a single instance,&mdash;the well-known one of the
+  primrose and cowslip, or Primula vulgaris and veris. These plants differ
+  considerably in appearance; they have a different flavour, and emit a
+  different odour; they flower at slightly different periods; they grow in
+  somewhat different stations; they ascend mountains to different heights;
+  they have different geographical ranges; and lastly, according to very
+  numerous experiments made during several years by <!-- Page 50 --><span
+  class="pagenum"><a name="page50"></a>[50]</span>that most careful
+  observer Gärtner, they can be crossed only with much difficulty. We could
+  hardly wish for better evidence of the two forms being specifically
+  distinct. On the other hand, they are united by many intermediate links,
+  and it is very doubtful whether these links are hybrids; and there is, as
+  it seems to me, an overwhelming amount of experimental evidence, showing
+  that they descend from common parents, and consequently must be ranked as
+  varieties.</p>
+
+  <p>Close investigation, in most cases, will bring naturalists to an
+  agreement how to rank doubtful forms. Yet it must be confessed that it is
+  in the best-known countries that we find the greatest number of forms of
+  doubtful value. I have been struck with the fact, that if any animal or
+  plant in a state of nature be highly useful to man, or from any cause
+  closely attract his attention, varieties of it will almost universally be
+  found recorded. These varieties, moreover, will be often ranked by some
+  authors as species. Look at the common oak, how closely it has been
+  studied; yet a German author makes more than a dozen species out of
+  forms, which are very generally considered as varieties; and in this
+  country the highest botanical authorities and practical men can be quoted
+  to show that the sessile and pedunculated oaks are either good and
+  distinct species or mere varieties.</p>
+
+  <p>When a young naturalist commences the study of a group of organisms
+  quite unknown to him, he is at first much perplexed to determine what
+  differences to consider as specific, and what as varieties; for he knows
+  nothing of the amount and kind of variation to which the group is
+  subject; and this shows, at least, how very generally there is some
+  variation. But if he confine his attention to one class within one
+  country, he will soon make up his mind how to rank most of the doubtful
+  forms. His <!-- Page 51 --><span class="pagenum"><a
+  name="page51"></a>[51]</span>general tendency will be to make many
+  species, for he will become impressed, just like the pigeon or poultry
+  fancier before alluded to, with the amount of difference in the forms
+  which he is continually studying; and he has little general knowledge of
+  analogical variation in other groups and in other countries, by which to
+  correct his first impressions. As he extends the range of his
+  observations, he will meet with more cases of difficulty; for he will
+  encounter a greater number of closely-allied forms. But if his
+  observations be widely extended, he will in the end generally be enabled
+  to make up his own mind which to call varieties and which species; but he
+  will succeed in this at the expense of admitting much
+  variation,&mdash;and the truth of this admission will often be disputed
+  by other naturalists. When, moreover, he comes to study allied forms
+  brought from countries not now continuous, in which case he can hardly
+  hope to find the intermediate links between his doubtful forms, he will
+  have to trust almost entirely to analogy, and his difficulties rise to a
+  climax.</p>
+
+  <p>Certainly no clear line of demarcation has as yet been drawn between
+  species and sub-species&mdash;that is, the forms which in the opinion of
+  some naturalists come very near to, but do not quite arrive at the rank
+  of species; or, again, between sub-species and well-marked varieties, or
+  between lesser varieties and individual differences. These differences
+  blend into each other in an insensible series; and a series impresses the
+  mind with the idea of an actual passage.</p>
+
+  <p>Hence I look at individual differences, though of small interest to
+  the systematist, as of high importance for us, as being the first step
+  towards such slight varieties as are barely thought worth recording in
+  works on natural history. And I look at varieties which are in any degree
+  more distinct and permanent, as steps leading to more <!-- Page 52
+  --><span class="pagenum"><a name="page52"></a>[52]</span>strongly marked
+  and more permanent varieties; and at these latter, as leading to
+  sub-species, and to species. The passage from one stage of difference to
+  another and higher stage may be, in some cases, due merely to the
+  long-continued action of different physical conditions in two different
+  regions; but I have not much faith in this view; and I attribute the
+  passage of a variety, from a state in which it differs very slightly from
+  its parent to one in which it differs more, to the action of natural
+  selection in accumulating (as will hereafter be more fully explained)
+  differences of structure in certain definite directions. Hence I believe
+  a well-marked variety may be called an incipient species; but whether
+  this belief be justifiable must be judged of by the general weight of the
+  several facts and views given throughout this work.</p>
+
+  <p>It need not be supposed that all varieties or incipient species
+  necessarily attain the rank of species. They may whilst in this incipient
+  state become extinct, or they may endure as varieties for very long
+  periods, as has been shown to be the case by Mr. Wollaston with the
+  varieties of certain fossil land-shells in Madeira. If a variety were to
+  flourish so as to exceed in numbers the parent species, it would then
+  rank as the species, and the species as the variety; or it might come to
+  supplant and exterminate the parent species; or both might co-exist, and
+  both rank as independent species. But we shall hereafter have to return
+  to this subject.</p>
+
+  <p>From these remarks it will be seen that I look at the term species, as
+  one arbitrarily given for the sake of convenience to a set of individuals
+  closely resembling each other, and that it does not essentially differ
+  from the term variety, which is given to less distinct and more
+  fluctuating forms. The term variety, again, in comparison with mere
+  individual differences, is also applied arbitrarily, and for mere
+  convenience' sake. <!-- Page 53 --><span class="pagenum"><a
+  name="page53"></a>[53]</span></p>
+
+  <p>Guided by theoretical considerations, I thought that some interesting
+  results might be obtained in regard to the nature and relations of the
+  species which vary most, by tabulating all the varieties in several
+  well-worked floras. At first this seemed a simple task; but Mr. H. C.
+  Watson, to whom I am much indebted for valuable advice and assistance on
+  this subject, soon convinced me that there were many difficulties, as did
+  subsequently Dr. Hooker, even in stronger terms. I shall reserve for my
+  future work the discussion of these difficulties, and the tables
+  themselves of the proportional numbers of the varying species. Dr. Hooker
+  permits me to add, that after having carefully read my manuscript, and
+  examined the tables, he thinks that the following statements are fairly
+  well established. The whole subject, however, treated as it necessarily
+  here is with much brevity, is rather perplexing, and allusions cannot be
+  avoided to the "struggle for existence," "divergence of character," and
+  other questions, hereafter to be discussed.</p>
+
+  <p>Alph. de Candolle and others have shown that plants which have very
+  wide ranges generally present varieties; and this might have been
+  expected, as they become exposed to diverse physical conditions, and as
+  they come into competition (which, as we shall hereafter see, is a far
+  more important circumstance) with different sets of organic beings. But
+  my tables further show that, in any limited country, the species which
+  are most common, that is abound most in individuals, and the species
+  which are most widely diffused within their own country (and this is a
+  different consideration from wide range, and to a certain extent from
+  commonness), often give rise to varieties sufficiently well-marked to
+  have been recorded in botanical works. Hence it is the most flourishing,
+  or, as they may be called, the dominant species,&mdash;those <!-- Page 54
+  --><span class="pagenum"><a name="page54"></a>[54]</span>which range
+  widely over the world, are the most diffused in their own country, and
+  are the most numerous in individuals,&mdash;which oftenest produce
+  well-marked varieties, or, as I consider them, incipient species. And
+  this, perhaps, might have been anticipated; for, as varieties, in order
+  to become in any degree permanent, necessarily have to struggle with the
+  other inhabitants of the country, the species which are already dominant
+  will be the most likely to yield offspring, which, though in some slight
+  degree modified, still inherit those advantages that enabled their
+  parents to become dominant over their compatriots.</p>
+
+  <p>If the plants inhabiting a country and described in any Flora be
+  divided into two equal masses, all those in the larger genera being
+  placed on one side, and all those in the smaller genera on the other
+  side, a somewhat larger number of the very common and much diffused or
+  dominant species will be found on the side of the larger genera. This,
+  again, might have been anticipated; for the mere fact of many species of
+  the same genus inhabiting any country, shows that there is something in
+  the organic or inorganic conditions of that country favourable to the
+  genus; and, consequently, we might have expected to have found in the
+  larger genera, or those including many species, a large proportional
+  number of dominant species. But so many causes tend to obscure this
+  result, that I am surprised that my tables show even a small majority on
+  the side of the larger genera. I will here allude to only two causes of
+  obscurity. Fresh-water and salt-loving plants have generally very wide
+  ranges and are much diffused, but this seems to be connected with the
+  nature of the stations inhabited by them, and has little or no relation
+  to the size of the genera to which the species belong. Again, plants low
+  in the scale of organisation are <!-- Page 55 --><span class="pagenum"><a
+  name="page55"></a>[55]</span>generally much more widely diffused than
+  plants higher in the scale; and here again there is no close relation to
+  the size of the genera. The cause of lowly-organised plants ranging
+  widely will be discussed in our chapter on geographical distribution.</p>
+
+  <p>From looking at species as only strongly-marked and well-defined
+  varieties, I was led to anticipate that the species of the larger genera
+  in each country would oftener present varieties, than the species of the
+  smaller genera; for wherever many closely related species (<i>i.e.</i>
+  species of the same genus) have been formed, many varieties or incipient
+  species ought, as a general rule, to be now forming. Where many large
+  trees grow, we expect to find saplings. Where many species of a genus
+  have been formed through variation, circumstances have been favourable
+  for variation; and hence we might expect that the circumstances would
+  generally be still favourable to variation. On the other hand, if we look
+  at each species as a special act of creation, there is no apparent reason
+  why more varieties should occur in a group having many species, than in
+  one having few.</p>
+
+  <p>To test the truth of this anticipation I have arranged the plants of
+  twelve countries, and the coleopterous insects of two districts, into two
+  nearly equal masses, the species of the larger genera on one side, and
+  those of the smaller genera on the other side, and it has invariably
+  proved to be the case that a larger proportion of the species on the side
+  of the larger genera present varieties, than on the side of the smaller
+  genera. Moreover, the species of the large genera which present any
+  varieties, invariably present a larger average number of varieties than
+  do the species of the small genera. Both these results follow when
+  another division is made, and when all the smallest genera, with from
+  only one to four species, are absolutely excluded from the tables. These
+  <!-- Page 56 --><span class="pagenum"><a
+  name="page56"></a>[56]</span>facts are of plain signification on the view
+  that species are only strongly marked and permanent varieties; for
+  wherever many species of the same genus have been formed, or where, if we
+  may use the expression, the manufactory of species has been active, we
+  ought generally to find the manufactory still in action, more especially
+  as we have every reason to believe the process of manufacturing new
+  species to be a slow one. And this certainly is the case, if varieties be
+  looked at as incipient species; for my tables clearly show as a general
+  rule that, wherever many species of a genus have been formed, the species
+  of that genus present a number of varieties, that is of incipient species
+  beyond the average. It is not that all large genera are now varying much,
+  and are thus increasing in the number of their species, or that no small
+  genera are now varying and increasing; for if this had been so, it would
+  have been fatal to my theory; inasmuch as geology plainly tells us that
+  small genera have in the lapse of time often increased greatly in size;
+  and that large genera have often come to their maxima, declined, and
+  disappeared. All that we want to show is, that where many species of a
+  genus have been formed, on an average many are still forming; and this
+  holds good.</p>
+
+  <p>There are other relations between the species of large genera and
+  their recorded varieties which deserve notice. We have seen that there is
+  no infallible criterion by which to distinguish species and well-marked
+  varieties; and in those cases in which intermediate links have not been
+  found between doubtful forms, naturalists are compelled to come to a
+  determination by the amount of difference between them, judging by
+  analogy whether or not the amount suffices to raise one or both to the
+  rank of species. Hence the amount of difference is one very important
+  criterion in settling whether two forms <!-- Page 57 --><span
+  class="pagenum"><a name="page57"></a>[57]</span>should be ranked as
+  species or varieties. Now Fries has remarked in regard to plants, and
+  Westwood in regard to insects, that in large genera the amount of
+  difference between the species is often exceedingly small. I have
+  endeavoured to test this numerically by averages, and, as far as my
+  imperfect results go, they confirm the view. I have also consulted some
+  sagacious and experienced observers, and, after deliberation, they concur
+  in this view. In this respect, therefore, the species of the larger
+  genera resemble varieties, more than do the species of the smaller
+  genera. Or the case may be put in another way, and it may be said, that
+  in the larger genera, in which a number of varieties or incipient species
+  greater than the average are now manufacturing, many of the species
+  already manufactured still to a certain extent resemble varieties, for
+  they differ from each other by a less than usual amount of
+  difference.</p>
+
+  <p>Moreover, the species of the large genera are related to each other,
+  in the same manner as the varieties of any one species are related to
+  each other. No naturalist pretends that all the species of a genus are
+  equally distinct from each other; they may generally be divided into
+  sub-genera, or sections, or lesser groups. As Fries has well remarked,
+  little groups of species are generally clustered like satellites around
+  certain other species. And what are varieties but groups of forms,
+  unequally related to each other, and clustered round certain
+  forms&mdash;that is, round their parent-species? Undoubtedly there is one
+  most important point of difference between varieties and species; namely,
+  that the amount of difference between varieties, when compared with each
+  other or with their parent-species, is much less than that between the
+  species of the same genus. But when we come to discuss the principle, as
+  I call it, of Divergence of Character, <!-- Page 58 --><span
+  class="pagenum"><a name="page58"></a>[58]</span>we shall see how this may
+  be explained, and how the lesser differences between varieties will tend
+  to increase into the greater differences between species.</p>
+
+  <p>There is one other point which seems to me worth notice. Varieties
+  generally have much restricted ranges: this statement is indeed scarcely
+  more than a truism, for if a variety were found to have a wider range
+  than that of its supposed parent-species, their denominations ought to be
+  reversed. But there is also reason to believe, that those species which
+  are very closely allied to other species, and in so far resemble
+  varieties, often have much restricted ranges. For instance, Mr. H.&nbsp;C.
+  Watson has marked for me in the well-sifted London Catalogue of plants
+  (4th edition) 63 plants which are therein ranked as species, but which he
+  considers as so closely allied to other species as to be of doubtful
+  value: these 63 reputed species range on an average over 6.9 of the
+  provinces into which Mr. Watson has divided Great Britain. Now, in this
+  same catalogue, 53 acknowledged varieties are recorded, and these range
+  over 7.7 provinces; whereas, the species to which these varieties belong
+  range over 14.3 provinces. So that the acknowledged varieties have very
+  nearly the same restricted average range, as have those very closely
+  allied forms, marked for me by Mr. Watson as doubtful species, but which
+  are almost universally ranked by British botanists as good and true
+  species.</p>
+
+  <p>&nbsp;</p>
+
+  <p>Finally, then, varieties have the same general characters as species,
+  for they cannot be distinguished from species,&mdash;except, firstly, by
+  the discovery of intermediate linking forms, and the occurrence of such
+  links cannot affect the actual characters of the forms which they
+  connect; and except, secondly by a certain amount of <!-- Page 59
+  --><span class="pagenum"><a name="page59"></a>[59]</span>difference, for
+  two forms, if differing very little, are generally ranked as varieties,
+  notwithstanding that intermediate linking forms have not been discovered;
+  but the amount of difference considered necessary to give to two forms
+  the rank of species is quite indefinite. In genera having more than the
+  average number of species in any country, the species of these genera
+  have more than the average number of varieties. In large genera the
+  species are apt to be closely, but unequally allied together, forming
+  little clusters round certain species. Species very closely allied to
+  other species apparently have restricted ranges. In all these several
+  respects the species of large genera present a strong analogy with
+  varieties. And we can clearly understand these analogies, if species have
+  once existed as varieties, and have thus originated: whereas, these
+  analogies are utterly inexplicable if each species has been independently
+  created.</p>
+
+  <p>We have, also, seen that it is the most flourishing or dominant
+  species of the larger genera which on an average vary most; and
+  varieties, as we shall hereafter see, tend to become converted into new
+  and distinct species. The larger genera thus tend to become larger; and
+  throughout nature the forms of life which are now dominant tend to become
+  still more dominant by leaving many modified and dominant descendants.
+  But by steps hereafter to be explained, the larger genera also tend to
+  break up into smaller genera. And thus, the forms of life throughout the
+  universe become divided into groups subordinate to groups.</p>
+
+<hr />
+
+<p><!-- Page 60 --><span class="pagenum"><a name="page60"></a>[60]</span></p>
+
+<h3>CHAPTER III.</h3>
+
+<p class="cenhead"><span class="sc">Struggle for Existence</span>.</p>
+
+<blockquote class="b1n">
+
+  <p>Bears on natural selection&mdash;The term used in a wide
+  sense&mdash;Geometrical powers of increase&mdash;Rapid increase of
+  naturalised animals and plants&mdash;Nature of the checks to
+  increase&mdash;Competition universal&mdash;Effects of
+  climate&mdash;Protection from the number of individuals&mdash;Complex
+  relations of all animals and plants throughout nature&mdash;Struggle for
+  life most severe between individuals and varieties of the same species;
+  often severe between species of the same genus&mdash;The relation of
+  organism to organism the most important of all relations.</p>
+
+</blockquote>
+
+  <p>Before entering on the subject of this chapter, I must make a few
+  preliminary remarks, to show how the struggle for existence bears on
+  Natural Selection. It has been seen in the last chapter that amongst
+  organic beings in a state of nature there is some individual variability:
+  indeed I am not aware that this has ever been disputed. It is immaterial
+  for us whether a multitude of doubtful forms be called species or
+  sub-species or varieties; what rank, for instance, the two or three
+  hundred doubtful forms of British plants are entitled to hold, if the
+  existence of any well-marked varieties be admitted. But the mere
+  existence of individual variability and of some few well-marked
+  varieties, though necessary as the foundation for the work, helps us but
+  little in understanding how species arise in nature. How have all those
+  exquisite adaptations of one part of the organisation to another part,
+  and to the conditions of life, and of one distinct organic being to
+  another being, been perfected? We see these beautiful co-adaptations most
+  <!-- Page 61 --><span class="pagenum"><a
+  name="page61"></a>[61]</span>plainly in the woodpecker and missletoe; and
+  only a little less plainly in the humblest parasite which clings to the
+  hairs of a quadruped or feathers of a bird; in the structure of the
+  beetle which dives through the water; in the plumed seed which is wafted
+  by the gentlest breeze; in short, we see beautiful adaptations everywhere
+  and in every part of the organic world.</p>
+
+  <p>Again, it may be asked, how is it that varieties, which I have called
+  incipient species, become ultimately converted into good and distinct
+  species, which in most cases obviously differ from each other far more
+  than do the varieties of the same species? How do those groups of
+  species, which constitute what are called distinct genera, and which
+  differ from each other more than do the species of the same genus, arise?
+  All these results, as we shall more fully see in the next chapter, follow
+  from the struggle for life. Owing to this struggle for life, any
+  variation, however slight, and from whatever cause proceeding, if it be
+  in any degree profitable to an individual of any species, in its
+  infinitely complex relations to other organic beings and to external
+  nature, will tend to the preservation of that individual, and will
+  generally be inherited by its offspring. The offspring, also, will thus
+  have a better chance of surviving, for, of the many individuals of any
+  species which are periodically born, but a small number can survive. I
+  have called this principle, by which each slight variation, if useful, is
+  preserved, by the term of Natural Selection, in order to mark its
+  relation to man's power of selection. We have seen that man by selection
+  can certainly produce great results, and can adapt organic beings to his
+  own uses, through the accumulation of slight but useful variations, given
+  to him by the hand of Nature. But Natural Selection, as we shall
+  hereafter see, is a power incessantly ready for action, and is as <!--
+  Page 62 --><span class="pagenum"><a
+  name="page62"></a>[62]</span>immeasurably superior to man's feeble
+  efforts, as the works of Nature are to those of Art.</p>
+
+  <p>We will now discuss in a little more detail the struggle for
+  existence. In my future work this subject shall be treated, as it well
+  deserves, at much greater length. The elder de Candolle and Lyell have
+  largely and philosophically shown that all organic beings are exposed to
+  severe competition. In regard to plants, no one has treated this subject
+  with more spirit and ability than W. Herbert, Dean of Manchester,
+  evidently the result of his great horticultural knowledge. Nothing is
+  easier than to admit in words the truth of the universal struggle for
+  life, or more difficult&mdash;at least I have found it so&mdash;than
+  constantly to bear this conclusion in mind. Yet unless it be thoroughly
+  engrained in the mind, I am convinced that the whole economy of nature,
+  with every fact on distribution, rarity, abundance, extinction, and
+  variation, will be dimly seen or quite misunderstood. We behold the face
+  of nature bright with gladness, we often see superabundance of food; we
+  do not see, or we forget that the birds which are idly singing round us
+  mostly live on insects or seeds, and are thus constantly destroying life;
+  or we forget how largely these songsters, or their eggs, or their
+  nestlings, are destroyed by birds and beasts of prey; we do not always
+  bear in mind, that though food may be now superabundant, it is not so at
+  all seasons of each recurring year.</p>
+
+  <p>I should premise that I use the term Struggle for Existence in a large
+  and metaphorical sense, including dependence of one being on another, and
+  including (which is more important) not only the life of the individual,
+  but success in leaving progeny. Two canine animals in a time of dearth,
+  may be truly said to struggle with each other which shall get food and
+  live. But a plant on the edge of a desert is said to struggle <!-- Page
+  63 --><span class="pagenum"><a name="page63"></a>[63]</span>for life
+  against the drought, though more properly it should be said to be
+  dependent on the moisture. A plant which annually produces a thousand
+  seeds, of which on an average only one comes to maturity, may be more
+  truly said to struggle with the plants of the same and other kinds which
+  already clothe the ground. The missletoe is dependent on the apple and a
+  few other trees, but can only in a far-fetched sense be said to struggle
+  with these trees, for if too many of these parasites grow on the same
+  tree, it will languish and die. But several seedling missletoes, growing
+  close together on the same branch, may more truly be said to struggle
+  with each other. As the missletoe is disseminated by birds, its existence
+  depends on birds; and it may metaphorically be said to struggle with
+  other fruit-bearing plants, in order to tempt birds to devour and thus
+  disseminate its seeds rather than those of other plants. In these several
+  senses, which pass into each other, I use for convenience' sake the
+  general term of struggle for existence.</p>
+
+  <p>A struggle for existence inevitably follows from the high rate at
+  which all organic beings tend to increase. Every being, which during its
+  natural lifetime produces several eggs or seeds, must suffer destruction
+  during some period of its life, and during some season or occasional
+  year, otherwise, on the principle of geometrical increase, its numbers
+  would quickly become so inordinately great that no country could support
+  the product. Hence, as more individuals are produced than can possibly
+  survive, there must in every case be a struggle for existence, either one
+  individual with another of the same species, or with the individuals of
+  distinct species, or with the physical conditions of life. It is the
+  doctrine of Malthus applied with manifold force to the whole animal and
+  vegetable kingdoms; for in this case there <!-- Page 64 --><span
+  class="pagenum"><a name="page64"></a>[64]</span>can be no artificial
+  increase of food, and no prudential restraint from marriage. Although
+  some species may be now increasing, more or less rapidly, in numbers, all
+  cannot do so, for the world would not hold them.</p>
+
+  <p>There is no exception to the rule that every organic being naturally
+  increases at so high a rate, that if not destroyed, the earth would soon
+  be covered by the progeny of a single pair. Even slow-breeding man has
+  doubled in twenty-five years, and at this rate, in a few thousand years,
+  there would literally not be standing room for his progeny. Linnæus has
+  calculated that if an annual plant produced only two seeds&mdash;and
+  there is no plant so unproductive as this&mdash;and their seedlings next
+  year produced two, and so on, then in twenty years there would be a
+  million plants. The elephant is reckoned the slowest breeder of all known
+  animals, and I have taken some pains to estimate its probable minimum
+  rate of natural increase: it will be under the mark to assume that it
+  breeds when thirty years old, and goes on breeding till ninety years old,
+  bringing forth three pair of young in this interval; if this be so, at
+  the end of the fifth century there would be alive fifteen million
+  elephants, descended from the first pair.</p>
+
+  <p>But we have better evidence on this subject than mere theoretical
+  calculations, namely, the numerous recorded cases of the astonishingly
+  rapid increase of various animals in a state of nature, when
+  circumstances have been favourable to them during two or three following
+  seasons. Still more striking is the evidence from our domestic animals of
+  many kinds which have run wild in several parts of the world: if the
+  statements of the rate of increase of slow-breeding cattle and horses in
+  South America, and latterly in Australia, had not been well
+  authenticated, they would have been incredible. So it is with plants:
+  cases could be given of <!-- Page 65 --><span class="pagenum"><a
+  name="page65"></a>[65]</span>introduced plants which have become common
+  throughout whole islands in a period of less than ten years. Several of
+  the plants, such as the cardoon and a tall thistle, now most numerous
+  over the wide plains of La Plata, clothing square leagues of surface
+  almost to the exclusion of all other plants, have been introduced from
+  Europe; and there are plants which now range in India, as I hear from Dr.
+  Falconer, from Cape Comorin to the Himalaya, which have been imported
+  from America since its discovery. In such cases, and endless instances
+  could be given, no one supposes that the fertility of these animals or
+  plants has been suddenly and temporarily increased in any sensible
+  degree. The obvious explanation is that the conditions of life have been
+  very favourable, and that there has consequently been less destruction of
+  the old and young, and that nearly all the young have been enabled to
+  breed. In such cases the geometrical ratio of increase, the result of
+  which never fails to be surprising, simply explains the extraordinarily
+  rapid increase and wide diffusion of naturalised productions in their new
+  homes.</p>
+
+  <p>In a state of nature almost every plant produces seed, and amongst
+  animals there are very few which do not annually pair. Hence we may
+  confidently assert, that all plants and animals are tending to increase
+  at a geometrical ratio, that all would most rapidly stock every station
+  in which they could any how exist, and that the geometrical tendency to
+  increase must be checked by destruction at some period of life. Our
+  familiarity with the larger domestic animals tends, I think, to mislead
+  us: we see no great destruction falling on them, and we forget that
+  thousands are annually slaughtered for food, and that in a state of
+  nature an equal number would have somehow to be disposed of.</p>
+
+  <p>The only difference between organisms which annually <!-- Page 66
+  --><span class="pagenum"><a name="page66"></a>[66]</span>produce eggs or
+  seeds by the thousand, and those which produce extremely few, is, that
+  the slow-breeders would require a few more years to people, under
+  favourable conditions, a whole district, let it be ever so large. The
+  condor lays a couple of eggs and the ostrich a score, and yet in the same
+  country the condor may be the more numerous of the two: the Fulmar petrel
+  lays but one egg, yet it is believed to be the most numerous bird in the
+  world. One fly deposits hundreds of eggs, and another, like the
+  hippobosca, a single one; but this difference does not determine how many
+  individuals of the two species can be supported in a district. A large
+  number of eggs is of some importance to those species which depend on a
+  rapidly fluctuating amount of food, for it allows them rapidly to
+  increase in number. But the real importance of a large number of eggs or
+  seeds is to make up for much destruction at some period of life; and this
+  period in the great majority of cases is an early one. If an animal can
+  in any way protect its own eggs or young, a small number may be produced,
+  and yet the average stock be fully kept up; but if many eggs or young are
+  destroyed, many must be produced, or the species will become extinct. It
+  would suffice to keep up the full number of a tree, which lived on an
+  average for a thousand years, if a single seed were produced once in a
+  thousand years, supposing that this seed were never destroyed, and could
+  be ensured to germinate in a fitting place. So that in all cases, the
+  average number of any animal or plant depends only indirectly on the
+  number of its eggs or seeds.</p>
+
+  <p>In looking at Nature, it is most necessary to keep the foregoing
+  considerations always in mind&mdash;never to forget that every single
+  organic being around us may be said to be striving to the utmost to
+  increase in numbers; that each lives by a struggle at some period of <!--
+  Page 67 --><span class="pagenum"><a name="page67"></a>[67]</span>its
+  life; that heavy destruction inevitably falls either on the young or old,
+  during each generation or at recurrent intervals. Lighten any check,
+  mitigate the destruction ever so little, and the number of the species
+  will almost instantaneously increase to any amount.</p>
+
+  <p>The causes which check the natural tendency of each species to
+  increase in number are most obscure. Look at the most vigorous species;
+  by as much as it swarms in numbers, by so much will its tendency to
+  increase be still further increased. We know not exactly what the checks
+  are in even one single instance. Nor will this surprise any one who
+  reflects how ignorant we are on this head, even in regard to mankind, so
+  incomparably better known than any other animal. This subject has been
+  ably treated by several authors, and I shall, in my future work, discuss
+  some of the checks at considerable length, more especially in regard to
+  the feral animals of South America. Here I will make only a few remarks,
+  just to recall to the reader's mind some of the chief points. Eggs or
+  very young animals seem generally to suffer most, but this is not
+  invariably the case. With plants there is a vast destruction of seeds,
+  but, from some observations which I have made, I believe that it is the
+  seedlings which suffer most from germinating in ground already thickly
+  stocked with other plants. Seedlings, also, are destroyed in vast numbers
+  by various enemies; for instance, on a piece of ground three feet long
+  and two wide, dug and cleared, and where there could be no choking from
+  other plants, I marked all the seedlings of our native weeds as they came
+  up, and out of the 357 no less than 295 were destroyed, chiefly by slugs
+  and insects. If turf which has long been mown, and the case would be the
+  same with turf closely browsed by quadrupeds, be let to grow, the more
+  vigorous plants <!-- Page 68 --><span class="pagenum"><a
+  name="page68"></a>[68]</span>gradually kill the less vigorous, though
+  fully grown, plants: thus out of twenty species growing on a little plot
+  of turf (three feet by four) nine species perished from the other species
+  being allowed to grow up freely.</p>
+
+  <p>The amount of food for each species of course gives the extreme limit
+  to which each can increase; but very frequently it is not the obtaining
+  food, but the serving as prey to other animals, which determines the
+  average numbers of a species. Thus, there seems to be little doubt that
+  the stock of partridges, grouse, and hares on any large estate depends
+  chiefly on the destruction of vermin. If not one head of game were shot
+  during the next twenty years in England, and, at the same time, if no
+  vermin were destroyed, there would, in all probability, be less game than
+  at present, although hundreds of thousands of game animals are now
+  annually killed. On the other hand, in some cases, as with the elephant
+  and rhinoceros, none are destroyed by beasts of prey: even the tiger in
+  India most rarely dares to attack a young elephant protected by its
+  dam.</p>
+
+  <p>Climate plays an important part in determining the average numbers of
+  a species, and periodical seasons of extreme cold or drought, I believe
+  to be the most effective of all checks. I estimated that the winter of
+  1854-55 destroyed four-fifths of the birds in my own grounds; and this is
+  a tremendous destruction, when we remember that ten per cent, is an
+  extraordinarily severe mortality from epidemics with man. The action of
+  climate seems at first sight to be quite independent of the struggle for
+  existence; but in so far as climate chiefly acts in reducing food, it
+  brings on the most severe struggle between the individuals, whether of
+  the same or of distinct species, which subsist on the same kind of food.
+  Even when climate, for instance extreme cold, <!-- Page 69 --><span
+  class="pagenum"><a name="page69"></a>[69]</span>acts directly, it will be
+  the least vigorous, or those which have got least food through the
+  advancing winter, which will suffer most. When we travel from south to
+  north, or from a damp region to a dry, we invariably see some species
+  gradually getting rarer and rarer, and finally disappearing; and the
+  change of climate being conspicuous, we are tempted to attribute the
+  whole effect to its direct action. But this is a false view: we forget
+  that each species, even where it most abounds, is constantly suffering
+  enormous destruction at some period of its life, from enemies or from
+  competitors for the same place and food; and if these enemies or
+  competitors be in the least degree favoured by any slight change of
+  climate, they will increase in numbers, and, as each area is already
+  fully stocked with inhabitants, the other species will decrease. When we
+  travel southward and see a species decreasing in numbers, we may feel
+  sure that the cause lies quite as much in other species being favoured,
+  as in this one being hurt. So it is when we travel northward, but in a
+  somewhat lesser degree, for the number of species of all kinds, and
+  therefore of competitors, decreases northwards; hence in going northward,
+  or in ascending a mountain, we far oftener meet with stunted forms, due
+  to the <i>directly</i> injurious action of climate, than we do in
+  proceeding southwards or in descending a mountain. When we reach the
+  Arctic regions, or snow-capped summits, or absolute deserts, the struggle
+  for life is almost exclusively with the elements.</p>
+
+  <p>That climate acts in main part indirectly by favouring other species,
+  we may clearly see in the prodigious number of plants in our gardens
+  which can perfectly well endure our climate, but which never become
+  naturalised, for they cannot compete with our native plants nor resist
+  destruction by our native animals. <!-- Page 70 --><span
+  class="pagenum"><a name="page70"></a>[70]</span></p>
+
+  <p>When a species, owing to highly favourable circumstances, increases
+  inordinately in numbers in a small tract, epidemics&mdash;at least, this
+  seems generally to occur with our game animals&mdash;often ensue: and
+  here we have a limiting check independent of the struggle for life. But
+  even some of these so-called epidemics appear to be due to parasitic
+  worms, which have from some cause, possibly in part through facility of
+  diffusion amongst the crowded animals, been disproportionably favoured:
+  and here comes in a sort of struggle between the parasite and its
+  prey.</p>
+
+  <p>On the other hand, in many cases, a large stock of individuals of the
+  same species, relatively to the numbers of its enemies, is absolutely
+  necessary for its preservation. Thus we can easily raise plenty of corn
+  and rape-seed, &amp;c., in our fields, because the seeds are in great
+  excess compared with the number of birds which feed on them; nor can the
+  birds, though having a superabundance of food at this one season,
+  increase in number proportionally to the supply of seed, as their numbers
+  are checked during winter: but any one who has tried, knows how
+  troublesome it is to get seed from a few wheat or other such plants in a
+  garden: I have in this case lost every single seed. This view of the
+  necessity of a large stock of the same species for its preservation,
+  explains, I believe, some singular facts in nature, such as that of very
+  rare plants being sometimes extremely abundant in the few spots where
+  they do occur; and that of some social plants being social, that is,
+  abounding in individuals, even on the extreme confines of their range.
+  For in such cases, we may believe, that a plant could exist only where
+  the conditions of its life were so favourable that many could exist
+  together, and thus save the species from utter destruction. I should add
+  that the good effects of frequent intercrossing, and <!-- Page 71
+  --><span class="pagenum"><a name="page71"></a>[71]</span>the ill effects
+  of close interbreeding, probably come into play in some of these cases;
+  but on this intricate subject I will not here enlarge.</p>
+
+  <p>Many cases are on record showing how complex and unexpected are the
+  checks and relations between organic beings, which have to struggle
+  together in the same country. I will give only a single instance, which,
+  though a simple one, has interested me. In Staffordshire, on the estate
+  of a relation, where I had ample means of investigation, there was a
+  large and extremely barren heath, which had never been touched by the
+  hand of man; but several hundred acres of exactly the same nature had
+  been enclosed twenty-five years previously and planted with Scotch fir.
+  The change in the native vegetation of the planted part of the heath was
+  most remarkable, more than is generally seen in passing from one quite
+  different soil to another: not only the proportional numbers of the
+  heath-plants were wholly changed, but twelve species of plants (not
+  counting grasses and carices) flourished in the plantations, which could
+  not be found on the heath. The effect on the insects must have been still
+  greater, for six insectivorous birds were very common in the plantations,
+  which were not to be seen on the heath; and the heath was frequented by
+  two or three distinct insectivorous birds. Here we see how potent has
+  been the effect of the introduction of a single tree, nothing whatever
+  else having been done, with the exception that the land had been
+  enclosed, so that cattle could not enter. But how important an element
+  enclosure is, I plainly saw near Farnham, in Surrey. Here there are
+  extensive heaths, with a few clumps of old Scotch firs on the distant
+  hill-tops: within the last ten years large spaces have been enclosed, and
+  self-sown firs are now springing up in multitudes, so close together that
+  all cannot live. <!-- Page 72 --><span class="pagenum"><a
+  name="page72"></a>[72]</span>When I ascertained that these young trees
+  had not been sown or planted, I was so much surprised at their numbers
+  that I went to several points of view, whence I could examine hundreds of
+  acres of the unenclosed heath, and literally I could not see a single
+  Scotch fir, except the old planted clumps. But on looking closely between
+  the stems of the heath, I found a multitude of seedlings and little
+  trees, which had been perpetually browsed down by the cattle. In one
+  square yard, at a point some hundred yards distant from one of the old
+  clumps, I counted thirty-two little trees; and one of them, with
+  twenty-six rings of growth, had during many years tried to raise its head
+  above the stems of the heath, and had failed. No wonder that, as soon as
+  the land was enclosed, it became thickly clothed with vigorously growing
+  young firs. Yet the heath was so extremely barren and so extensive that
+  no one would ever have imagined that cattle would have so closely and
+  effectually searched it for food.</p>
+
+  <p>Here we see that cattle absolutely determine the existence of the
+  Scotch fir; but 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 number 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 birds. Hence, if certain
+  insectivorous birds (whose numbers are probably regulated by hawks or
+  beasts of prey) were to increase in Paraguay, the flies would
+  decrease&mdash;then cattle and horses would became feral, and this would
+  certainly greatly <!-- Page 73 --><span class="pagenum"><a
+  name="page73"></a>[73]</span>alter (as indeed I have observed in parts of
+  South America) the vegetation: this again would largely affect the
+  insects; and this, as we just have seen in Staffordshire, the
+  insectivorous birds, and so onwards in ever-increasing circles of
+  complexity. We began this series by insectivorous birds, and we have
+  ended with them. Not that in nature the relations can ever be as simple
+  as this. Battle within battle must ever be recurring with varying
+  success; and yet in the long-run the forces are so nicely balanced, that
+  the face of nature remains uniform for long periods of time, though
+  assuredly the merest trifle would often give the victory to one organic
+  being over another. Nevertheless so profound is our ignorance, and so
+  high our presumption, that we marvel when we hear of the extinction of an
+  organic being; and as we do not see the cause, we invoke cataclysms to
+  desolate the world, or invent laws on the duration of the forms of
+  life!</p>
+
+  <p>I am tempted to give one more instance showing how plants and animals,
+  most remote in the scale of nature, are bound together by a web of
+  complex relations. I shall hereafter have occasion to show that the
+  exotic Lobelia fulgens, in this part of England, is never visited by
+  insects, and consequently, from its peculiar structure, never can set a
+  seed. Many of our orchidaceous plants absolutely require the visits of
+  moths to remove their pollen-masses and thus to fertilise them. I have,
+  also, reason to believe that humble-bees are indispensable to the
+  fertilisation of the heartsease (Viola tricolor), for other bees do not
+  visit this flower. From experiments which I have lately tried, I have
+  found that the visits of bees are necessary for the fertilisation of some
+  kinds of clover; but humble-bees alone visit the red clover (Trifolium
+  pratense), as other bees cannot reach the nectar. Hence I have very
+  little doubt, that if the <!-- Page 74 --><span class="pagenum"><a
+  name="page74"></a>[74]</span>whole genus of humble-bees became extinct or
+  very rare in England, the heartsease and red clover would become very
+  rare, or wholly disappear. The number of humble-bees in any district
+  depends in a great degree on the number of field-mice, which destroy
+  their combs and nests; and Mr. H. Newman, who has long attended to the
+  habits of humble-bees, believes that "more than two-thirds of them are
+  thus destroyed all over England." Now the number of mice is largely
+  dependent, as every one knows, on the number of cats; and Mr. Newman
+  says, "Near villages and small towns I have found the nests of
+  humble-bees more numerous than elsewhere, which I attribute to the number
+  of cats that destroy the mice." Hence it is quite credible that the
+  presence of a feline animal in large numbers in a district might
+  determine, through the intervention first of mice and then of bees, the
+  frequency of certain flowers in that district!</p>
+
+  <p>In the case of every species, many different checks, acting at
+  different periods of life, and during different seasons or years,
+  probably come into play; some one check or some few being generally the
+  most potent, but all concur in determining the average number or even the
+  existence of the species. In some cases it can be shown that
+  widely-different checks act on the same species in different districts.
+  When we look at the plants and bushes clothing an entangled bank, we are
+  tempted to attribute their proportional numbers and kinds to what we call
+  chance. But how false a view is this! Every one has heard that when an
+  American forest is cut down, a very different vegetation springs up; but
+  it has been observed that ancient Indian ruins in the Southern United
+  States, which must formerly have been cleared of trees, now display the
+  same beautiful diversity and proportion of kinds as in the surrounding
+  <!-- Page 75 --><span class="pagenum"><a
+  name="page75"></a>[75]</span>virgin forests. What a struggle between the
+  several kinds of trees must here have gone on during long centuries, each
+  annually scattering its seeds by the thousand; what war between insect
+  and insect&mdash;between insects, snails, and other animals with birds
+  and beasts of prey&mdash;all striving to increase, and all feeding on
+  each other or on the trees or their seeds and seedlings, or on the other
+  plants which first clothed the ground and thus checked the growth of the
+  trees! Throw up a handful of feathers, and all must fall to the ground
+  according to definite laws; but how simple is this problem compared to
+  the action and reaction of the innumerable plants and animals which have
+  determined, in the course of centuries, the proportional numbers and
+  kinds of trees now growing on the old Indian ruins!</p>
+
+  <p>The dependency of one organic being on another, as of a parasite on
+  its prey, lies generally between beings remote in the scale of nature.
+  This is often the case with those which may strictly be said to struggle
+  with each other for existence, as in the case of locusts and
+  grass-feeding quadrupeds. But the struggle almost invariably will be most
+  severe between the individuals of the same species, for they frequent the
+  same districts, require the same food, and are exposed to the same
+  dangers. In the case of varieties of the same species, the struggle will
+  generally be almost equally severe, and we sometimes see the contest soon
+  decided; for instance, if several varieties of wheat be sown together,
+  and the mixed seed be resown, some of the varieties which best suit the
+  soil or climate, or are naturally the most fertile, will beat the others
+  and so yield more seed, and will consequently in a few years quite
+  supplant the other varieties. To keep up a mixed stock of even such
+  extremely close varieties as the variously <!-- Page 76 --><span
+  class="pagenum"><a name="page76"></a>[76]</span>coloured sweet-peas, they
+  must be each year harvested separately, and the seed then mixed in due
+  proportion, otherwise the weaker kinds will steadily decrease in numbers
+  and disappear. So again with the varieties of sheep: it has been asserted
+  that certain mountain-varieties will starve out other mountain-varieties,
+  so that they cannot be kept together. The same result has followed from
+  keeping together different varieties of the medicinal leech. It may even
+  be doubted whether the varieties of any one of our domestic plants or
+  animals have so exactly the same strength, habits, and constitution, that
+  the original proportions of a mixed stock could be kept up for
+  half-a-dozen generations, if they were allowed to struggle together, like
+  beings in a state of nature, and if the seed or young were not annually
+  sorted.</p>
+
+  <p>As species of the same genus have usually, though by no means
+  invariably, some similarity in habits and constitution, and always in
+  structure, the struggle will generally be more severe between species of
+  the same genus, when they come into competition with each other, than
+  between species of distinct genera. We see this in the recent extension
+  over parts of the United States of one species of swallow having caused
+  the decrease of another species. The recent increase of the missel-thrush
+  in parts of Scotland has caused the decrease of the song-thrush. How
+  frequently we hear of one species of rat taking the place of another
+  species under the most different climates! In Russia the small Asiatic
+  cockroach has everywhere driven before it its great congener. One species
+  of charlock will supplant another, and so in other cases. We can dimly
+  see why the competition should be most severe between allied forms, which
+  fill nearly the same place in the economy of nature; <!-- Page 77
+  --><span class="pagenum"><a name="page77"></a>[77]</span>but probably in
+  no one case could we precisely say why one species has been victorious
+  over another in the great battle of life.</p>
+
+  <p>A corollary of the highest importance may be deduced from the
+  foregoing remarks, namely, that the structure of every organic being is
+  related, in the most essential yet often hidden manner, to that of all
+  other organic beings, with which it comes into competition for food or
+  residence, or from which it has to escape, or on which it preys. This is
+  obvious in the structure of the teeth and talons of the tiger; and in
+  that of the legs and claws of the parasite which clings to the hair on
+  the tiger's body. But in the beautifully plumed seed of the dandelion,
+  and in the flattened and fringed legs of the water-beetle, the relation
+  seems at first confined to the elements of air and water. Yet the
+  advantage of plumed seeds no doubt stands in the closest relation to the
+  land being already thickly clothed by other plants; so that the seeds may
+  be widely distributed and fall on unoccupied ground. In the water-beetle,
+  the structure of its legs, so well adapted for diving, allows it to
+  compete with other aquatic insects, to hunt for its own prey, and to
+  escape serving as prey to other animals.</p>
+
+  <p>The store of nutriment laid up within the seeds of many plants seems
+  at first sight to have no sort of relation to other plants. But from the
+  strong growth of young plants produced from such seeds (as peas and
+  beans), when sown in the midst of long grass, I suspect that the chief
+  use of the nutriment in the seed is to favour the growth of the young
+  seedling, whilst struggling with other plants growing vigorously all
+  around.</p>
+
+  <p>Look at a plant in the midst of its range, why does it not double or
+  quadruple its numbers? We know <!-- Page 78 --><span class="pagenum"><a
+  name="page78"></a>[78]</span>that it can perfectly well withstand a
+  little more heat or cold, dampness or dryness, for elsewhere it ranges
+  into slightly hotter or colder, damper or drier districts. In this case
+  we can clearly see that if we wished in imagination to give the plant the
+  power of increasing in number, we should have to give it some advantage
+  over its competitors, or over the animals which preyed on it. On the
+  confines of its geographical range, a change of constitution with respect
+  to climate would clearly be an advantage to our plant; but we have reason
+  to believe that only a few plants or animals range so far, that they are
+  destroyed by the rigour of the climate alone. Not until we reach the
+  extreme confines of life, in the Arctic regions or on the borders of an
+  utter desert, will competition cease. The land may be extremely cold or
+  dry, yet there will be competition between some few species, or between
+  the individuals of the same species, for the warmest or dampest
+  spots.</p>
+
+  <p>Hence, also, we can see that when a plant or animal is placed in a new
+  country amongst new competitors, though the climate may be exactly the
+  same as in its former home, yet the conditions of its life will generally
+  be changed in an essential manner. If we wished to increase its average
+  numbers in its new home, we should have to modify it in a different way
+  to what we should have done in its native country; for we should have to
+  give it some advantage over a different set of competitors or
+  enemies.</p>
+
+  <p>It is good thus to try in our imagination to give any form some
+  advantage over another. Probably in no single instance should we know
+  what to do, so as to succeed. It will convince us of our ignorance on the
+  mutual relations of all organic beings; a conviction as necessary, as it
+  seems to be difficult to acquire. All that we can do, is to keep steadily
+  in mind that each <!-- Page 79 --><span class="pagenum"><a
+  name="page79"></a>[79]</span>organic being is striving to increase at a
+  geometrical ratio; that each at some period of its life, during some
+  season of the year, during each generation or at intervals, has to
+  struggle for life, and to suffer great destruction. When we reflect on
+  this struggle, we may console ourselves with the full belief, that the
+  war of nature is not incessant, that no fear is felt, that death is
+  generally prompt, and that the vigorous, the healthy, and the happy
+  survive and multiply.</p>
+
+<hr />
+
+<p><!-- Page 80 --><span class="pagenum"><a name="page80"></a>[80]</span></p>
+
+<h3>CHAPTER IV.</h3>
+
+<p class="cenhead"><span class="sc">Natural Selection.</span></p>
+
+<blockquote class="b1n">
+
+  <p>Natural Selection&mdash;its power compared with man's
+  selection&mdash;its power on characters of trifling importance&mdash;its
+  power at all ages and on both sexes&mdash;Sexual Selection&mdash;On the
+  generality of intercrosses between individuals of the same
+  species&mdash;Circumstances favourable and unfavourable to Natural
+  Selection, namely, intercrossing, isolation, number of
+  individuals&mdash;Slow action&mdash;Extinction caused by Natural
+  Selection&mdash;Divergence of Character, related to the diversity of
+  inhabitants of any small area, and to naturalisation&mdash;Action of
+  Natural Selection, through Divergence of Character and Extinction, on the
+  descendants from a common parent&mdash;Explains the Grouping of all
+  organic beings.</p>
+
+</blockquote>
+
+  <p>How will the struggle for existence, discussed too briefly in the last
+  chapter, act in regard to variation? Can the principle of selection,
+  which we have seen is so potent in the hands of man, apply in nature? I
+  think we shall see that it can act most effectually. Let it be borne in
+  mind in what an endless number of strange peculiarities our domestic
+  productions, and, in a lesser degree, those under nature, vary; and how
+  strong the hereditary tendency is. Under domestication, it may be truly
+  said that the whole organisation becomes in some degree plastic. Let it
+  be borne in mind how infinitely complex and close-fitting are the mutual
+  relations of all organic beings to each other and to their physical
+  conditions of life. Can it, then, be thought improbable, seeing that
+  variations useful to man have undoubtedly occurred, that other variations
+  useful in some way to each being in the great and complex battle of life,
+  should sometimes occur in the course of thousands of generations? If such
+  do occur, can we doubt <!-- Page 81 --><span class="pagenum"><a
+  name="page81"></a>[81]</span>(remembering that many more individuals are
+  born than can possibly survive) that individuals having any advantage,
+  however slight, over others, would have the best chance of surviving and
+  of procreating their kind? On the other hand, we may feel sure that any
+  variation in the least degree injurious would be rigidly destroyed. This
+  preservation of favourable variations and the rejection of injurious
+  variations, I call Natural Selection. Variations neither useful nor
+  injurious would not be affected by natural selection, and would be left a
+  fluctuating element, as perhaps we see in the species called
+  polymorphic.</p>
+
+  <p>We shall best understand the probable course of natural selection by
+  taking the case of a country undergoing some physical change, for
+  instance, of climate. The proportional numbers of its inhabitants would
+  almost immediately undergo a change, and some species might become
+  extinct. We may conclude, from what we have seen of the intimate and
+  complex manner in which the inhabitants of each country are bound
+  together, that any change in the numerical proportions of some of the
+  inhabitants, independently of the change of climate itself, would
+  seriously affect many of the others. If the country were open on its
+  borders, new forms would certainly immigrate, and this also would
+  seriously disturb the relations of some of the former inhabitants. Let it
+  be remembered how powerful the influence of a single introduced tree or
+  mammal has been shown to be. But in the case of an island, or of a
+  country partly surrounded by barriers, into which new and better adapted
+  forms could not freely enter, we should then have places in the economy
+  of nature which would assuredly be better filled up, if some of the
+  original inhabitants were in some manner modified; for, had the area been
+  open to immigration, these same <!-- Page 82 --><span class="pagenum"><a
+  name="page82"></a>[82]</span>places would have been seized on by
+  intruders. In such case, every slight modification, which in the course
+  of ages chanced to arise, and which in any way favoured the individuals
+  of any of the species, by better adapting them to their altered
+  conditions, would tend to be preserved; and natural selection would thus
+  have free scope for the work of improvement.</p>
+
+  <p>We have reason to believe, as stated in the first chapter, that a
+  change in the conditions of life, by specially acting on the reproductive
+  system, causes or increases variability; and in the foregoing case the
+  conditions of life are supposed to have undergone a change, and this
+  would manifestly be favourable to natural selection, by giving a better
+  chance of profitable variations occurring; and unless profitable
+  variations do occur, natural selection can do nothing. Not that, as I
+  believe, any extreme amount of variability is necessary; as man can
+  certainly produce great results by adding up in any given direction mere
+  individual differences, so could Nature, but far more easily, from having
+  incomparably longer time at her disposal. Nor do I believe that any great
+  physical change, as of climate, or any unusual degree of isolation to
+  check immigration, is actually necessary to produce new and unoccupied
+  places for natural selection to fill up by modifying and improving some
+  of the varying inhabitants. For as all the inhabitants of each country
+  are struggling together with nicely balanced forces, extremely slight
+  modifications in the structure or habits of one inhabitant would often
+  give it an advantage over others; and still further modifications of the
+  same kind would often still further increase the advantage. No country
+  can be named in which all the native inhabitants are now so perfectly
+  adapted to each other and to the physical conditions under which they
+  live, that none of <!-- Page 83 --><span class="pagenum"><a
+  name="page83"></a>[83]</span>them could anyhow be improved; for in all
+  countries, the natives have been so far conquered by naturalised
+  productions, that they have allowed foreigners to take firm possession of
+  the land. And as foreigners have thus everywhere beaten some of the
+  natives, we may safely conclude that the natives might have been modified
+  with advantage, so as to have better resisted such intruders.</p>
+
+  <p>As man can produce and certainly has produced a great result by his
+  methodical and unconscious means of selection, what may not Nature
+  effect? Man can act only on external and visible characters: Nature cares
+  nothing for appearances, except in so far as they may be useful to any
+  being. She can act on every internal organ, on every shade of
+  constitutional difference, on the whole machinery of life. Man selects
+  only for his own good; Nature only for that of the being which she tends.
+  Every selected character is fully exercised by her; and the being is
+  placed under well-suited conditions of life. Man keeps the natives of
+  many climates in the same country; he seldom exercises each selected
+  character in some peculiar and fitting manner; he feeds a long and a
+  short beaked pigeon on the same food; he does not exercise a long-backed
+  or long-legged quadruped in any peculiar manner; he exposes sheep with
+  long and short wool to the same climate. He does not allow the most
+  vigorous males to struggle for the females. He does not rigidly destroy
+  all inferior animals, but protects during each varying season, as far as
+  lies in his power, all his productions. He often begins his selection by
+  some half-monstrous form; or at least by some modification prominent
+  enough to catch his eye, or to be plainly useful to him. Under nature,
+  the slightest difference of structure or constitution may well turn the
+  nicely-balanced scale in the struggle for life, and so be <!-- Page 84
+  --><span class="pagenum"><a name="page84"></a>[84]</span>preserved. How
+  fleeting are the wishes and efforts of man! how short his time! and
+  consequently how poor will his products be, compared with those
+  accumulated by Nature during whole geological periods. Can we wonder,
+  then, that Nature's productions should be far "truer" in character than
+  man's productions; that they should be infinitely better adapted to the
+  most complex conditions of life, and should plainly bear the stamp of far
+  higher workmanship?</p>
+
+  <p>It may metaphorically be said that natural selection is daily and
+  hourly scrutinising, throughout the world, every variation, even the
+  slightest; rejecting that which is bad, preserving and adding up all that
+  is good; silently and insensibly working, whenever and wherever
+  opportunity offers, at the improvement of each organic being in relation
+  to its organic and inorganic conditions of life. We see nothing of these
+  slow changes in progress, until the hand of time has marked the long
+  lapse of ages, and then so imperfect is our view into long past
+  geological ages, that we only see that the forms of life are now
+  different from what they formerly were.</p>
+
+  <p>Although natural selection can act only through and for the good of
+  each being, yet characters and structures, which we are apt to consider
+  as of very trifling importance, may thus be acted on. When we see
+  leaf-eating insects green, and bark-feeders mottled-grey; the alpine
+  ptarmigan white in winter, the red-grouse the colour of heather, and the
+  black-grouse that of peaty earth, we must believe that these tints are of
+  service to these birds and insects in preserving them from danger.
+  Grouse, if not destroyed at some period of their lives, would increase in
+  countless numbers; they are known to suffer largely from birds of prey;
+  and hawks are guided by eyesight to their prey&mdash;so much so, that on
+  <!-- Page 85 --><span class="pagenum"><a
+  name="page85"></a>[85]</span>parts of the Continent persons are warned
+  not to keep white pigeons, as being the most liable to destruction. Hence
+  I can see no reason to doubt that natural selection might be most
+  effective in giving the proper colour to each kind of grouse, and in
+  keeping that colour, when once acquired, true and constant. Nor ought we
+  to think that the occasional destruction of an animal of any particular
+  colour would produce little effect: we should remember how essential it
+  is in a flock of white sheep to destroy every lamb with the faintest
+  trace of black. In plants the down on the fruit and the colour of the
+  flesh are considered by botanists as characters of the most trifling
+  importance: yet we hear from an excellent horticulturist, Downing, that
+  in the United States smooth-skinned fruits suffer far more from a beetle,
+  a curculio, than those with down; that purple plums suffer far more from
+  a certain disease than yellow plums; whereas another disease attacks
+  yellow-fleshed peaches far more than those with other coloured flesh. If,
+  with all the aids of art, these slight differences make a great
+  difference in cultivating the several varieties, assuredly, in a state of
+  nature, where the trees would have to struggle with other trees and with
+  a host of enemies, such differences would effectually settle which
+  variety, whether a smooth or downy, a yellow or purple fleshed fruit,
+  should succeed.</p>
+
+  <p>In looking at many small points of difference between species, which,
+  as far as our ignorance permits us to judge, seem quite unimportant, we
+  must not forget that climate, food, &amp;c., probably produce some slight
+  and direct effect. It is, however, far more necessary to bear in mind
+  that there are many unknown laws of correlation of growth, which, when
+  one part of the organisation is modified through variation, and the
+  modifications are accumulated by natural selection for <!-- Page 86
+  --><span class="pagenum"><a name="page86"></a>[86]</span>the good of the
+  being, will cause other modifications, often of the most unexpected
+  nature.</p>
+
+  <p>As we see that those variations which under domestication appear at
+  any particular period of life, tend to reappear in the offspring at the
+  same period;&mdash;for instance, in the seeds of the many varieties of
+  our culinary and agricultural plants; in the caterpillar and cocoon
+  stages of the varieties of the silkworm; in the eggs of poultry, and in
+  the colour of the down of their chickens; in the horns of our sheep and
+  cattle when nearly adult;&mdash;so in a state of nature, natural
+  selection will be enabled to act on and modify organic beings at any age,
+  by the accumulation of variations profitable at that age, and by their
+  inheritance at a corresponding age. If it profit a plant to have its
+  seeds more and more widely disseminated by the wind, I can see no greater
+  difficulty in this being effected through natural selection, than in the
+  cotton-planter increasing and improving by selection the down in the pods
+  on his cotton-trees. Natural selection may modify and adapt the larva of
+  an insect to a score of contingencies, wholly different from those which
+  concern the mature insect. These modifications will no doubt affect,
+  through the laws of correlation, the structure of the adult; and probably
+  in the case of those insects which live only for a few hours, and which
+  never feed, a large part of their structure is merely the correlated
+  result of successive changes in the structure of their larvæ. So,
+  conversely, modifications in the adult will probably often affect the
+  structure of the larva; but in all cases natural selection will ensure
+  that modifications consequent on other modifications at a different
+  period of life, shall not be in the least degree injurious: for if they
+  became so, they would cause the extinction of the species.</p>
+
+  <p>Natural selection will modify the structure of the <!-- Page 87
+  --><span class="pagenum"><a name="page87"></a>[87]</span>young in
+  relation to the parent, and of the parent in relation to the young. In
+  social animals it will adapt the structure of each individual for the
+  benefit of the community; if each in consequence profits by the selected
+  change. What natural selection cannot do, is to modify the structure of
+  one species, without giving it any advantage, for the good of another
+  species; and though statements to this effect may be found in works of
+  natural history, I cannot find one case which will bear investigation. A
+  structure used only once in an animal's whole life, if of high importance
+  to it, might be modified to any extent by natural selection; for
+  instance, the great jaws possessed by certain insects, used exclusively
+  for opening the cocoon&mdash;or the hard tip to the beak of nestling
+  birds, used for breaking the egg. It has been asserted, that of the best
+  short-beaked tumbler-pigeons more perish in the egg than are able to get
+  out of it; so that fanciers assist in the act of hatching. Now, if nature
+  had to make the beak of a full-grown pigeon very short for the bird's own
+  advantage, the process of modification would be very slow, and there
+  would be simultaneously the most rigorous selection of the young birds
+  within the egg, which had the most powerful and hardest beaks, for all
+  with weak beaks would inevitably perish: or, more delicate and more
+  easily broken shells might be selected, the thickness of the shell being
+  known to vary like every other structure.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Sexual Selection.</i>&mdash;Inasmuch as peculiarities often appear
+  under domestication in one sex and become hereditarily attached to that
+  sex, the same fact probably occurs under nature, and if so, natural
+  selection will be able to modify one sex in its functional relations to
+  the other sex, or in relation to wholly different habits of life in the
+  two sexes, as is sometimes the case <!-- Page 88 --><span
+  class="pagenum"><a name="page88"></a>[88]</span>with insects. And this
+  leads me to say a few words on what I call Sexual Selection. This
+  depends, not on a struggle for existence, but on a struggle between the
+  males for possession of the females; the result is not death to the
+  unsuccessful competitor, but few or no offspring. Sexual selection is,
+  therefore, less rigorous than natural selection. Generally, the most
+  vigorous males, those which are best fitted for their places in nature,
+  will leave most progeny. But in many cases, victory depends not on
+  general vigour, but on having special weapons, confined to the male sex.
+  A hornless stag or spurless cock would have a poor chance of leaving
+  offspring. Sexual selection by always allowing the victor to breed might
+  surely give indomitable courage, length to the spur, and strength to the
+  wing to strike in the spurred leg, as well as the brutal cock-fighter,
+  who knows well that he can improve his breed by careful selection of the
+  best cocks. How low in the scale of nature the law of battle descends, I
+  know not; male alligators have been described as fighting, bellowing, and
+  whirling round, like Indians in a war-dance, for the possession of the
+  females; male salmons have been seen fighting all day long; male
+  stag-beetles often bear wounds from the huge mandibles of other males.
+  The war is, perhaps, severest between the males of polygamous animals,
+  and these seem oftenest provided with special weapons. The males of
+  carnivorous animals are already well armed; though to them and to others,
+  special means of defence may be given through means of sexual selection,
+  as the mane to the lion, the shoulder-pad to the boar, and the hooked jaw
+  to the male salmon; for the shield may be as important for victory, as
+  the sword or spear.</p>
+
+  <p>Amongst birds, the contest is often of a more peaceful character. All
+  those who have attended to the subject, <!-- Page 89 --><span
+  class="pagenum"><a name="page89"></a>[89]</span>believe that there is the
+  severest rivalry between the males of many species to attract by singing
+  the females. The rock-thrush of Guiana, birds of Paradise, and some
+  others, congregate; and successive males display their gorgeous plumage
+  and perform strange antics before the females, which, standing by as
+  spectators, at last choose the most attractive partner. Those who have
+  closely attended to birds in confinement well know that they often take
+  individual preferences and dislikes: thus Sir R. Heron has described how
+  one pied peacock was eminently attractive to all his hen birds. It may
+  appear childish to attribute any effect to such apparently weak means: I
+  cannot here enter on the details necessary to support this view; but if
+  man can in a short time give elegant carriage and beauty to his bantams,
+  according to his standard of beauty, I can see no good reason to doubt
+  that female birds, by selecting, during thousands of generations, the
+  most melodious or beautiful males, according to their standard of beauty,
+  might produce a marked effect. I strongly suspect that some well-known
+  laws, with respect to the plumage of male and female birds, in comparison
+  with the plumage of the young, can be explained on the view of plumage
+  having been chiefly modified by sexual selection, acting when the birds
+  have come to the breeding age or during the breeding season; the
+  modifications thus produced being inherited at corresponding ages or
+  seasons, either by the males alone, or by the males and females; but I
+  have not space here to enter on this subject.</p>
+
+  <p>Thus it is, as I believe, that when the males and females of any
+  animal have the same general habits of life, but differ in structure,
+  colour, or ornament, such differences have been mainly caused by sexual
+  selection; that is, individual males have had, in successive generations,
+  some slight advantage over other <!-- Page 90 --><span class="pagenum"><a
+  name="page90"></a>[90]</span>males, in their weapons, means of defence,
+  or charms; and have transmitted these advantages to their male offspring.
+  Yet, I would not wish to attribute all such sexual differences to this
+  agency: for we see peculiarities arising and becoming attached to the
+  male sex in our domestic animals (as the wattle in male carriers,
+  horn-like protuberances in the cocks of certain fowls, &amp;c.), which we
+  cannot believe to be either useful to the males in battle, or attractive
+  to the females. We see analogous cases under nature, for instance, the
+  tuft of hair on the breast of the turkey-cock, which can hardly be either
+  useful or ornamental to this bird;&mdash;indeed, had the tuft appeared
+  under domestication, it would have been called a monstrosity.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Illustrations of the action of Natural Selection.</i>&mdash;In
+  order to make it clear how, as I believe, natural selection acts, I must
+  beg permission to give one or two imaginary illustrations. Let us take
+  the case of a wolf, which preys on various animals, securing some by
+  craft, some by strength, and some by fleetness; and let us suppose that
+  the fleetest prey, a deer for instance, had from any change in the
+  country increased in numbers, or that other prey had decreased in
+  numbers, during that season of the year when the wolf is hardest pressed
+  for food. I can under such circumstances see no reason to doubt that the
+  swiftest and slimmest wolves would have the best chance of surviving, and
+  so be preserved or selected,&mdash;provided always that they retained
+  strength to master their prey at this or at some other period of the
+  year, when they might be compelled to prey on other animals. I can see no
+  more reason to doubt this, than that man can improve the fleetness of his
+  greyhounds by careful and methodical selection, or by that unconscious
+  selection which results from each man trying <!-- Page 91 --><span
+  class="pagenum"><a name="page91"></a>[91]</span>to keep the best dogs
+  without any thought of modifying the breed.</p>
+
+  <p>Even without any change in the proportional numbers of the animals on
+  which our wolf preyed, a cub might be born with an innate tendency to
+  pursue certain kinds of prey. Nor can this be thought very improbable;
+  for we often observe great differences in the natural tendencies of our
+  domestic animals; one cat, for instance, taking to catch rats, another
+  mice; one cat, according to Mr. St. John, bringing home winged game,
+  another hares or rabbits, and another hunting on marshy ground and almost
+  nightly catching woodcocks or snipes. The tendency to catch rats rather
+  than mice is known to be inherited. Now, if any slight innate change of
+  habit or of structure benefited an individual wolf, it would have the
+  best chance of surviving and of leaving offspring. Some of its young
+  would probably inherit the same habits or structure, and by the
+  repetition of this process, a new variety might be formed which would
+  either supplant or coexist with the parent form of wolf. Or, again, the
+  wolves inhabiting a mountainous district, and those frequenting the
+  lowlands, would naturally be forced to hunt different prey; and from the
+  continued preservation of the individuals best fitted for the two sites,
+  two varieties might slowly be formed. These varieties would cross and
+  blend where they met; but to this subject of intercrossing we shall soon
+  have to return. I may add, that, according to Mr. Pierce, there are two
+  varieties of the wolf inhabiting the Catskill Mountains in the United
+  States, one with a light greyhound-like form, which pursues deer, and the
+  other more bulky, with shorter legs, which more frequently attacks the
+  shepherd's flocks.</p>
+
+  <p>Let us now take a more complex case. Certain plants excrete a sweet
+  juice, apparently for the sake of eliminating something injurious from
+  their sap: this is <!-- Page 92 --><span class="pagenum"><a
+  name="page92"></a>[92]</span>effected by glands at the base of the
+  stipules in some Leguminosæ, and at the back of the leaf of the common
+  laurel. This juice, though small in quantity, is greedily sought by
+  insects. Let us now suppose a little sweet juice or nectar to be excreted
+  by the inner bases of the petals of a flower. In this case insects in
+  seeking the nectar would get dusted with pollen, and would certainly
+  often transport the pollen from one flower to the stigma of another
+  flower. The flowers of two distinct individuals of the same species would
+  thus get crossed; and the act of crossing, we have good reason to believe
+  (as will hereafter be more fully alluded to), would produce very vigorous
+  seedlings, which consequently would have the best chance of flourishing
+  and surviving. Some of these seedlings would probably inherit the
+  nectar-excreting power. Those individual flowers which had the largest
+  glands or nectaries, and which excreted most nectar, would be oftenest
+  visited by insects, and would be oftenest crossed; and so in the long-run
+  would gain the upper hand. Those flowers, also, which had their stamens
+  and pistils placed, in relation to the size and habits of the particular
+  insects which visited them, so as to favour in any degree the transportal
+  of their pollen from flower to flower, would likewise be favoured or
+  selected. We might have taken the case of insects visiting flowers for
+  the sake of collecting pollen instead of nectar; and as pollen is formed
+  for the sole object of fertilisation, its destruction appears a simple
+  loss to the plant; yet if a little pollen were carried, at first
+  occasionally and then habitually, by the pollen-devouring insects from
+  flower to flower, and a cross thus effected, although nine-tenths of the
+  pollen were destroyed, it might still be a great gain to the plant; and
+  those individuals which produced more and more pollen, and had larger and
+  larger anthers, would be selected. <!-- Page 93 --><span
+  class="pagenum"><a name="page93"></a>[93]</span></p>
+
+  <p>When our plant, by this process of the continued preservation or
+  natural selection of more and more attractive flowers, had been rendered
+  highly attractive to insects, they would, unintentionally on their part,
+  regularly carry pollen from flower to flower; and that they can most
+  effectually do this, I could easily show by many striking instances. I
+  will give only one&mdash;not as a very striking case, but as likewise
+  illustrating one step in the separation of the sexes of plants, presently
+  to be alluded to. Some holly-trees bear only male flowers, which have
+  four stamens producing a rather small quantity of pollen, and a
+  rudimentary pistil; other holly-trees bear only female flowers; these
+  have a full-sized pistil, and four stamens with shrivelled anthers, in
+  which not a grain of pollen can be detected. Having found a female tree
+  exactly sixty yards from a male tree, I put the stigmas of twenty
+  flowers, taken from different branches, under the microscope, and on all,
+  without exception, there were pollen-grains, and on some a profusion of
+  pollen. As the wind had set for several days from the female to the male
+  tree, the pollen could not thus have been carried. The weather had been
+  cold and boisterous, and therefore not favourable to bees, nevertheless
+  every female flower which I examined had been effectually fertilised by
+  the bees, accidentally dusted with pollen, having flown from tree to tree
+  in search of nectar. But to return to our imaginary case: as soon as the
+  plant had been rendered so highly attractive to insects that pollen was
+  regularly carried from flower to flower, another process might commence.
+  No naturalist doubts the advantage of what has been called the
+  "physiological division of labour;" hence we may believe that it would be
+  advantageous to a plant to produce stamens alone in one flower or on one
+  whole plant, and pistils alone in <!-- Page 94 --><span
+  class="pagenum"><a name="page94"></a>[94]</span>another flower or on
+  another plant. In plants under culture and placed under new conditions of
+  life, sometimes the male organs and sometimes the female organs become
+  more or less impotent; now if we suppose this to occur in ever so slight
+  a degree under nature, then as pollen is already carried regularly from
+  flower to flower, and as a more complete separation of the sexes of our
+  plant would be advantageous on the principle of the division of labour,
+  individuals with this tendency more and more increased, would be
+  continually favoured or selected, until at last a complete separation of
+  the sexes would be effected.</p>
+
+  <p>Let us now turn to the nectar-feeding insects in our imaginary case:
+  we may suppose the plant of which we have been slowly increasing the
+  nectar by continued selection, to be a common plant; and that certain
+  insects depended in main part on its nectar for food. I could give many
+  facts, showing how anxious bees are to save time; for instance, their
+  habit of cutting holes and sucking the nectar at the bases of certain
+  flowers, which they can, with a very little more trouble, enter by the
+  mouth. Bearing such facts in mind, I can see no reason to doubt that an
+  accidental deviation in the size and form of the body, or in the
+  curvature and length of the proboscis, &amp;c., far too slight to be
+  appreciated by us, might profit a bee or other insect, so that an
+  individual so characterised would be able to obtain its food more
+  quickly, and so have a better chance of living and leaving descendants.
+  Its descendants would probably inherit a tendency to a similar slight
+  deviation of structure. The tubes of the corollas of the common red and
+  incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty
+  glance appear to differ in length; yet the hive-bee can easily suck the
+  nectar out of the incarnate clover, but not out of the common red <!--
+  Page 95 --><span class="pagenum"><a name="page95"></a>[95]</span>clover,
+  which is visited by humble-bees alone; so that whole fields of the red
+  clover offer in vain an abundant supply of precious nectar to the
+  hive-bee. Thus it might be a great advantage to the hive-bee to have a
+  slightly longer or differently constructed proboscis. On the other hand,
+  I have found by experiment that the fertility of clover depends on bees
+  visiting and moving parts of the corolla, so as to push the pollen on to
+  the stigmatic surface. Hence, again, if humble-bees were to become rare
+  in any country, it might be a great advantage to the red clover to have a
+  shorter or more deeply divided tube to its corolla, so that the hive-bee
+  could visit its flowers. Thus I can understand how a flower and a bee
+  might slowly become, either simultaneously or one after the other,
+  modified and adapted in the most perfect manner to each other, by the
+  continued preservation of individuals presenting mutual and slightly
+  favourable deviations of structure.</p>
+
+  <p>I am well aware that this doctrine of natural selection, exemplified
+  in the above imaginary instances, is open to the same objections which
+  were at first urged against Sir Charles Lyell's noble views on "the
+  modern changes of the earth, as illustrative of geology;" but we now
+  seldom hear the action, for instance, of the coast-waves, called a
+  trifling and insignificant cause, when applied to the excavation of
+  gigantic valleys or to the formation of the longest lines of inland
+  cliffs. Natural selection can act only by the preservation and
+  accumulation of infinitesimally small inherited modifications, each
+  profitable to the preserved being; and as modern geology has almost
+  banished such views as the excavation of a great valley by a single
+  diluvial wave, so will natural selection, if it be a true principle,
+  banish the belief of the continued creation of new organic <!-- Page 96
+  --><span class="pagenum"><a name="page96"></a>[96]</span>beings, or of
+  any great and sudden modification in their structure.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the Intercrossing of Individuals.</i>&mdash;I must here
+  introduce a short digression. In the case of animals and plants with
+  separated sexes, it is of course obvious that two individuals must always
+  (with the exception of the curious and not well-understood cases of
+  parthenogenesis) unite for each birth; but in the case of hermaphrodites
+  this is far from obvious. Nevertheless I am strongly inclined to believe
+  that with all hermaphrodites two individuals, either occasionally or
+  habitually, concur for the reproduction of their kind. This view was
+  first suggested by Andrew Knight. We shall presently see its importance;
+  but I must here treat the subject with extreme brevity, though I have the
+  materials prepared for an ample discussion. All vertebrate animals, all
+  insects, and some other large groups of animals, pair for each birth.
+  Modern research has much diminished the number of supposed
+  hermaphrodites, and of real hermaphrodites a large number pair; that is,
+  two individuals regularly unite for reproduction, which is all that
+  concerns us. But still there are many hermaphrodite animals which
+  certainly do not habitually pair, and a vast majority of plants are
+  hermaphrodites. What reason, it may be asked, is there for supposing in
+  these cases that two individuals ever concur in reproduction? As it is
+  impossible here to enter on details, I must trust to some general
+  considerations alone.</p>
+
+  <p>In the first place, I have collected so large a body of facts,
+  showing, in accordance with the almost universal belief of breeders, that
+  with animals and plants a cross between different varieties, or between
+  individuals of the same variety but of another strain, gives vigour and
+  <!-- Page 97 --><span class="pagenum"><a
+  name="page97"></a>[97]</span>fertility to the offspring; and on the other
+  hand, that <i>close</i> interbreeding diminishes vigour and fertility;
+  that these facts alone incline me to believe that it is a general law of
+  nature (utterly ignorant though we be of the meaning of the law) that no
+  organic being self-fertilises itself for an eternity of generations; but
+  that a cross with another individual is occasionally&mdash;perhaps at
+  very long intervals&mdash;indispensable.</p>
+
+  <p>On the belief that this is a law of nature, we can, I think,
+  understand several large classes of facts, such as the following, which
+  on any other view are inexplicable. Every hybridizer knows how
+  unfavourable exposure to wet is to the fertilisation of a flower, yet
+  what a multitude of flowers have their anthers and stigmas fully exposed
+  to the weather! but if an occasional cross be indispensable, the fullest
+  freedom for the entrance of pollen from another individual will explain
+  this state of exposure, more especially as the plant's own anthers and
+  pistil generally stand so close together that self-fertilisation seems
+  almost inevitable. Many flowers, on the other hand, have their organs of
+  fructification closely enclosed, as in the great papilionaceous or
+  pea-family; but in several, perhaps in all, such flowers, there is a very
+  curious adaptation between the structure of the flower and the manner in
+  which bees suck the nectar; for, in doing this, they either push the
+  flower's own pollen on the stigma, or bring pollen from another flower.
+  So necessary are the visits of bees to papilionaceous flowers, that I
+  have found, by experiments published elsewhere, that their fertility is
+  greatly diminished if these visits be prevented. Now, it is scarcely
+  possible that bees should fly from flower to flower, and not carry pollen
+  from one to the other, to the great good, as I believe, of the plant.
+  Bees will act like a camel-hair pencil, and it is quite sufficient just
+  to touch the anthers of <!-- Page 98 --><span class="pagenum"><a
+  name="page98"></a>[98]</span>one flower and then the stigma of another
+  with the same brush to ensure fertilisation; but it must not be supposed
+  that bees would thus produce a multitude of hybrids between distinct
+  species; for if you bring on the same brush a plant's own pollen and
+  pollen from another species, the former will have such a prepotent
+  effect, that it will invariably and completely destroy, as has been shown
+  by Gärtner, any influence from the foreign pollen.</p>
+
+  <p>When the stamens of a flower suddenly spring towards the pistil, or
+  slowly move one after the other towards it, the contrivance seems adapted
+  solely to ensure self-fertilisation; and no doubt it is useful for this
+  end: but, the agency of insects is often required to cause the stamens to
+  spring forward, as Kölreuter has shown to be the case with the barberry;
+  and in this very genus, which seems to have a special contrivance for
+  self-fertilisation, it is well known that if closely-allied forms or
+  varieties are planted near each other, it is hardly possible to raise
+  pure seedlings, so largely do they naturally cross. In many other cases,
+  far from there being any aids for self-fertilisation, there are special
+  contrivances, as I could show from the writings of C.&nbsp;C. Sprengel and
+  from my own observations, which effectually prevent the stigma receiving
+  pollen from its own flower: for instance, in Lobelia fulgens, there is a
+  really beautiful and elaborate contrivance by which every one of the
+  infinitely numerous pollen-granules are swept out of the conjoined
+  anthers of each flower, before the stigma of that individual flower is
+  ready to receive them; and as this flower is never visited, at least in
+  my garden, by insects, it never sets a seed, though by placing pollen
+  from one flower on the stigma of another, I raised plenty of seedlings;
+  and whilst another species of Lobelia growing close by, which is visited
+  by bees, seeds freely. In very many other cases, though there <!-- Page
+  99 --><span class="pagenum"><a name="page99"></a>[99]</span>be no special
+  mechanical contrivance to prevent the stigma of a flower receiving its
+  own pollen, yet, as C.&nbsp;C. Sprengel has shown, and as I can confirm,
+  either the anthers burst before the stigma is ready for fertilisation, or
+  the stigma is ready before the pollen of that flower is ready, so that
+  these plants have in fact separated sexes, and must habitually be
+  crossed. How strange are these facts! How strange that the pollen and
+  stigmatic surface of the same flower, though placed so close together, as
+  if for the very purpose of self-fertilisation, should in so many cases be
+  mutually useless to each other! How simply are these facts explained on
+  the view of an occasional cross with a distinct individual being
+  advantageous or indispensable!</p>
+
+  <p>If several varieties of the cabbage, radish, onion, and of some other
+  plants, be allowed to seed near each other, a large majority, as I have
+  found, of the seedlings thus raised will turn out mongrels: for instance,
+  I raised 233 seedling cabbages from some plants of different varieties
+  growing near each other, and of these only 78 were true to their kind,
+  and some even of these were not perfectly true. Yet the pistil of each
+  cabbage-flower is surrounded not only by its own six stamens, but by
+  those of the many other flowers on the same plant. How, then, comes it
+  that such a vast number of the seedlings are mongrelized? I suspect that
+  it must arise from the pollen of a distinct <i>variety</i> having a
+  prepotent effect over a flower's own pollen; and that this is part of the
+  general law of good being derived from the intercrossing of distinct
+  individuals of the same species. When distinct <i>species</i> are crossed
+  the case is directly the reverse, for a plant's own pollen is always
+  prepotent over foreign pollen; but to this subject we shall return in a
+  future chapter.</p>
+
+  <p>In the case of a gigantic tree covered with <!-- Page 100 --><span
+  class="pagenum"><a name="page100"></a>[100]</span>innumerable flowers, it
+  may be objected that pollen could seldom be carried from tree to tree,
+  and at most only from flower to flower on the same tree, and that flowers
+  on the same tree can be considered as distinct individuals only in a
+  limited sense. I believe this objection to be valid, but that nature has
+  largely provided against it by giving to trees a strong tendency to bear
+  flowers with separated sexes. When the sexes are separated, although the
+  male and female flowers may be produced on the same tree, we can see that
+  pollen must be regularly carried from flower to flower; and this will
+  give a better chance of pollen being occasionally carried from tree to
+  tree. That trees belonging to all Orders have their sexes more often
+  separated than other plants, I find to be the case in this country; and
+  at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa
+  Gray those of the United States, and the result was as I anticipated. On
+  the other hand, Dr. Hooker has recently informed me that he finds that
+  the rule does not hold in Australia; and I have made these few remarks on
+  the sexes of trees simply to call attention to the subject.</p>
+
+  <p>Turning for a very brief space to animals: on the land there are some
+  hermaphrodites, as land-mollusca and earth-worms; but these all pair. As
+  yet I have not found a single case of a terrestrial animal which
+  fertilises itself. We can understand this remarkable fact, which offers
+  so strong a contrast with terrestrial plants, on the view of an
+  occasional cross being indispensable, by considering the medium in which
+  terrestrial animals live, and the nature of the fertilising element; for
+  we know of no means, analogous to the action of insects and of the wind
+  in the case of plants, by which an occasional cross could be effected
+  with terrestrial animals without the concurrence of two individuals. Of
+  aquatic animals, there are many self-fertilising hermaphrodites; but here
+  <!-- Page 101 --><span class="pagenum"><a
+  name="page101"></a>[101]</span>currents in the water offer an obvious
+  means for an occasional cross. And, as in the case of flowers, I have as
+  yet failed, after consultation with one of the highest authorities,
+  namely, Professor Huxley, to discover a single case of an hermaphrodite
+  animal with the organs of reproduction so perfectly enclosed within the
+  body, that access from without and the occasional influence of a distinct
+  individual can be shown to be physically impossible. Cirripedes long
+  appeared to me to present a case of very great difficulty under this
+  point of view; but I have been enabled, by a fortunate chance, elsewhere
+  to prove that two individuals, though both are self-fertilising
+  hermaphrodites, do sometimes cross.</p>
+
+  <p>It must have struck most naturalists as a strange anomaly that, in the
+  case of both animals and plants, species of the same family and even of
+  the same genus, though agreeing closely with each other in almost their
+  whole organisation, yet are not rarely, some of them hermaphrodites, and
+  some of them unisexual. But if, in fact, all hermaphrodites do
+  occasionally intercross with other individuals, the difference between
+  hermaphrodites and unisexual species, as far as function is concerned,
+  becomes very small.</p>
+
+  <p>From these several considerations and from the many special facts
+  which I have collected, but which I am not here able to give, I am
+  strongly inclined to suspect that, both in the vegetable and animal
+  kingdoms, an occasional intercross with a distinct individual is a law of
+  nature. I am well aware that there are, on this view, many cases of
+  difficulty, some of which I am trying to investigate. Finally then, we
+  may conclude that in many organic beings, a cross between two individuals
+  is an obvious necessity for each birth; in many others it occurs perhaps
+  only at long intervals; but in none, as I suspect, can self-fertilisation
+  go on for perpetuity. <!-- Page 102 --><span class="pagenum"><a
+  name="page102"></a>[102]</span></p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Circumstances favourable to Natural Selection.</i>&mdash;This is an
+  extremely intricate subject. A large amount of inheritable and
+  diversified variability is favourable, but I believe mere individual
+  differences suffice for the work. A large number of individuals, by
+  giving a better chance for the appearance within any given period of
+  profitable variations, will compensate for a lesser amount of variability
+  in each individual, and is, I believe, an extremely important element of
+  success. Though nature grants vast periods of time for the work of
+  natural selection, she does not grant an indefinite period; for as all
+  organic beings are striving, it may be said, to seize on each place in
+  the economy of nature, if any one species does not become modified and
+  improved in a corresponding degree with its competitors, it will soon be
+  exterminated.</p>
+
+  <p>In man's methodical selection, a breeder selects for some definite
+  object, and free intercrossing will wholly stop his work. But when many
+  men, without intending to alter the breed, have a nearly common standard
+  of perfection, and all try to get and breed from the best animals, much
+  improvement and modification surely but slowly follow from this
+  unconscious process of selection, notwithstanding a large amount of
+  crossing with inferior animals. Thus it will be in nature; for within a
+  confined area, with some place in its polity not so perfectly occupied as
+  might be, natural selection will always tend to preserve all the
+  individuals varying in the right direction, though in different degrees,
+  so as better to fill up the unoccupied place. But if the area be large,
+  its several districts will almost certainly present different conditions
+  of life; and then if natural selection be modifying and improving a
+  species in the several districts, there will be intercrossing with the
+  other individuals of the same species on the confines of each. And in
+  <!-- Page 103 --><span class="pagenum"><a
+  name="page103"></a>[103]</span>this case the effects of intercrossing can
+  hardly be counterbalanced by natural selection always tending to modify
+  all the individuals in each district in exactly the same manner to the
+  conditions of each; for in a continuous area, the physical conditions at
+  least will generally graduate away insensibly from one district to
+  another. The intercrossing will most affect those animals which unite for
+  each birth, which wander much, and which do not breed at a very quick
+  rate. Hence in animals of this nature, for instance in birds, varieties
+  will generally be confined to separated countries; and this I believe to
+  be the case. In hermaphrodite organisms which cross only occasionally,
+  and likewise in animals which unite for each birth, but which wander
+  little and which can increase at a very rapid rate, a new and improved
+  variety might be quickly formed on any one spot, and might there maintain
+  itself in a body, so that whatever intercrossing took place would be
+  chiefly between the individuals of the same new variety. A local variety
+  when once thus formed might subsequently slowly spread to other
+  districts. On the above principle, nurserymen always prefer getting seed
+  from a large body of plants of the same variety, as the chance of
+  intercrossing with other varieties is thus lessened.</p>
+
+  <p>Even in the case of slow-breeding animals, which unite for each birth,
+  we must not overrate the effects of intercrosses in retarding natural
+  selection; for I can bring a considerable catalogue of facts, showing
+  that within the same area, varieties of the same animal can long remain
+  distinct, from haunting different stations, from breeding at slightly
+  different seasons, or from varieties of the same kind preferring to pair
+  together.</p>
+
+  <p>Intercrossing plays a very important part in nature in keeping the
+  individuals of the same species, or of the same variety, true and uniform
+  in character. It will <!-- Page 104 --><span class="pagenum"><a
+  name="page104"></a>[104]</span>obviously thus act far more efficiently
+  with those animals which unite for each birth; but I have already
+  attempted to show that we have reason to believe that occasional
+  intercrosses take place with all animals and with all plants. Even if
+  these take place only at long intervals, I am convinced that the young
+  thus produced will gain so much in vigour and fertility over the
+  offspring from long-continued self-fertilisation, that they will have a
+  better chance of surviving and propagating their kind; and thus, in the
+  long run, the influence of intercrosses, even at rare intervals, will be
+  great. If there exist organic beings which never intercross, uniformity
+  of character can be retained amongst them, as long as their conditions of
+  life remain the same, only through the principle of inheritance, and
+  through natural selection destroying any which depart from the proper
+  type; but if their conditions of life change and they undergo
+  modification, uniformity of character can be given to their modified
+  offspring, solely by natural selection preserving the same favourable
+  variations.</p>
+
+  <p>Isolation, also, is an important element in the process of natural
+  selection. In a confined or isolated area, if not very large, the organic
+  and inorganic conditions of life will generally be in a great degree
+  uniform; so that natural selection will tend to modify all the
+  individuals of a varying species throughout the area in the same manner
+  in relation to the same conditions. Intercrosses, also, with the
+  individuals of the same species, which otherwise would have inhabited the
+  surrounding and differently circumstanced districts, will be prevented.
+  But isolation probably acts more efficiently in checking the immigration
+  of better adapted organisms, after any physical change, such as of
+  climate or elevation of the land, &amp;c.; and thus new places in the
+  natural economy of the country are left open for the old inhabitants to
+  struggle for, and become adapted to, through <!-- Page 105 --><span
+  class="pagenum"><a name="page105"></a>[105]</span>modifications in their
+  structure and constitution. Lastly, isolation, by checking immigration
+  and consequently competition, will give time for any new variety to be
+  slowly improved; and this may sometimes be of importance in the
+  production of new species. If, however, an isolated area be very small,
+  either from being surrounded by barriers, or from having very peculiar
+  physical conditions, the total number of the individuals supported on it
+  will necessarily be very small; and fewness of individuals will greatly
+  retard the production of new species through natural selection, by
+  decreasing the chance of the appearance of favourable variations.</p>
+
+  <p>If we turn to nature to test the truth of these remarks, and look at
+  any small isolated area, such as an oceanic island, although the total
+  number of the species inhabiting it, will be found to be small, as we
+  shall see in our chapter on geographical distribution; yet of these
+  species a very large proportion are endemic,&mdash;that is, have been
+  produced there, and nowhere else. Hence an oceanic island at first sight
+  seems to have been highly favourable for the production of new species.
+  But we may thus greatly deceive ourselves, for to ascertain whether a
+  small isolated area, or a large open area like a continent, has been most
+  favourable for the production of new organic forms, we ought to make the
+  comparison within equal times; and this we are incapable of doing.</p>
+
+  <p>Although I do not doubt that isolation is of considerable importance
+  in the production of new species, on the whole I am inclined to believe
+  that largeness of area is of more importance, more especially in the
+  production of species, which will prove capable of enduring for a long
+  period, and of spreading widely. Throughout a great and open area, not
+  only will there be a better chance of favourable variations arising from
+  the large number of individuals of the same species <!-- Page 106
+  --><span class="pagenum"><a name="page106"></a>[106]</span>there
+  supported, but the conditions of life are infinitely complex from the
+  large number of already existing species; and if some of these many
+  species become modified and improved, others will have to be improved in
+  a corresponding degree or they will be exterminated. Each new form, also,
+  as soon as it has been much improved, will be able to spread over the
+  open and continuous area, and will thus come into competition with many
+  others. Hence more new places will be formed, and the competition to fill
+  them will be more severe, on a large than on a small and isolated area.
+  Moreover, great areas, though now continuous, owing to oscillations of
+  level, will often have recently existed in a broken condition, so that
+  the good effects of isolation will generally, to a certain extent, have
+  concurred. Finally, I conclude that, although small isolated areas
+  probably have been in some respects highly favourable for the production
+  of new species, yet that the course of modification will generally have
+  been more rapid on large areas; and what is more important, that the new
+  forms produced on large areas, which already have been victorious over
+  many competitors, will be those that will spread most widely, will give
+  rise to most new varieties and species, and will thus play an important
+  part in the changing history of the organic world.</p>
+
+  <p>We can, perhaps, on these views, understand some facts which will be
+  again alluded to in our chapter on geographical distribution; for
+  instance, that the productions of the smaller continent of Australia have
+  formerly yielded, and apparently are now yielding, before those of the
+  larger Europæo-Asiatic area. Thus, also, it is that continental
+  productions have everywhere become so largely naturalised on islands. On
+  a small island, the race for life will have been less severe, and there
+  will have been less modification and less <!-- Page 107 --><span
+  class="pagenum"><a name="page107"></a>[107]</span>extermination. Hence,
+  perhaps, it comes that the flora of Madeira, according to Oswald Heer,
+  resembles the extinct tertiary flora of Europe. All fresh-water basins,
+  taken together, make a small area compared with that of the sea or of the
+  land; and, consequently, the competition between fresh-water productions
+  will have been less severe than elsewhere; new forms will have been more
+  slowly formed, and old forms more slowly exterminated. And it is in fresh
+  water that we find seven genera of Ganoid fishes, remnants of a once
+  preponderant order: and in fresh water we find some of the most anomalous
+  forms now known in the world, as the Ornithorhynchus and Lepidosiren,
+  which, like fossils, connect to a certain extent orders now widely
+  separated in the natural scale. These anomalous forms may almost be
+  called living fossils; they have endured to the present day, from having
+  inhabited a confined area, and from having thus been exposed to less
+  severe competition.</p>
+
+  <p>To sum up the circumstances favourable and unfavourable to natural
+  selection, as far as the extreme intricacy of the subject permits. I
+  conclude, looking to the future, that for terrestrial productions a large
+  continental area, which will probably undergo many oscillations of level,
+  and which consequently will exist for long periods in a broken condition,
+  is the most favourable for the production of many new forms of life,
+  likely to endure long and to spread widely. For the area first existed as
+  a continent, and the inhabitants, at this period numerous in individuals
+  and kinds, will have been subjected to very severe competition. When
+  converted by subsidence into large separate islands, there will still
+  exist many individuals of the same species on each island: intercrossing
+  on the confines of the range of each species will thus be checked: after
+  physical changes of any kind, immigration will be <!-- Page 108 --><span
+  class="pagenum"><a name="page108"></a>[108]</span>prevented, so that new
+  places in the polity of each island will have to be filled up by
+  modifications of the old inhabitants; and time will be allowed for the
+  varieties in each to become well modified and perfected. When, by renewed
+  elevation, the islands shall be re-converted into a continental area,
+  there will again be severe competition: the most favoured or improved
+  varieties will be enabled to spread: there will be much extinction of the
+  less improved forms, and the relative proportional numbers of the various
+  inhabitants of the renewed continent will again be changed; and again
+  there will be a fair field for natural selection to improve still further
+  the inhabitants, and thus produce new species.</p>
+
+  <p>That natural selection will always act with extreme slowness, I fully
+  admit. Its action depends on there being places in the polity of nature,
+  which can be better occupied by some of the inhabitants of the country
+  undergoing modification of some kind. The existence of such places will
+  often depend on physical changes, which are generally very slow, and on
+  the immigration of better adapted forms having been checked. But the
+  action of natural selection will probably still oftener depend on some of
+  the inhabitants becoming slowly modified; the mutual relations of many of
+  the other inhabitants being thus disturbed. Nothing can be effected,
+  unless favourable variations occur, and variation itself is apparently
+  always a very slow process. The process will often be greatly retarded by
+  free intercrossing. Many will exclaim that these several causes are amply
+  sufficient wholly to stop the action of natural selection. I do not
+  believe so. On the other hand, I do believe that natural selection always
+  acts very slowly, often only at long intervals of time, and generally on
+  only a very few of the inhabitants of the same region at the same time. I
+  further believe, that this very slow, <!-- Page 109 --><span
+  class="pagenum"><a name="page109"></a>[109]</span>intermittent action of
+  natural selection accords perfectly well with what geology tells us of
+  the rate and manner at which the inhabitants of this world have
+  changed.</p>
+
+  <p>Slow though the process of selection may be, if feeble man can do much
+  by his powers of artificial selection, I can see no limit to the amount
+  of change, to the beauty and infinite complexity of the coadaptations
+  between all organic beings, one with another and with their physical
+  conditions of life, which may be effected in the long course of time by
+  nature's power of selection.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Extinction.</i>&mdash;This subject will be more fully discussed in
+  our chapter on Geology; but it must be here alluded to from being
+  intimately connected with natural selection. Natural selection acts
+  solely through the preservation of variations in some way advantageous,
+  which consequently endure. But as from the high geometrical ratio of
+  increase of all organic beings, each area is already fully stocked with
+  inhabitants, it follows that as each selected and favoured form increases
+  in number, so will the less favoured forms decrease and become rare.
+  Rarity, as geology tells us, is the precursor to extinction. We can,
+  also, see that any form represented by few individuals will, during
+  fluctuations in the seasons or in the number of its enemies, run a good
+  chance of utter extinction. But we may go further than this; for as new
+  forms are continually and slowly being produced, unless we believe that
+  the number of specific forms goes on perpetually and almost indefinitely
+  increasing, numbers inevitably must become extinct. That the number of
+  specific forms has not indefinitely increased, geology shows us plainly;
+  and indeed we can see reason why they should not have thus increased, for
+  the number of places in the polity of nature is not indefinitely
+  great,&mdash;not that we <!-- Page 110 --><span class="pagenum"><a
+  name="page110"></a>[110]</span>have any means of knowing that any one
+  region has as yet got its maximum of species. Probably no region is as
+  yet fully stocked, for at the Cape of Good Hope, where more species of
+  plants are crowded together than in any other quarter of the world, some
+  foreign plants have become naturalised, without causing, as far as we
+  know, the extinction of any natives.</p>
+
+  <p>Furthermore, the species which are most numerous in individuals will
+  have the best chance of producing within any given period favourable
+  variations. We have evidence of this, in the facts given in the second
+  chapter, showing that it is the common species which afford the greatest
+  number of recorded varieties, or incipient species. Hence, rare species
+  will be less quickly modified or improved within any given period, and
+  they will consequently be beaten in the race for life by the modified
+  descendants of the commoner species.</p>
+
+  <p>From these several considerations I think it inevitably follows, that
+  as new species in the course of time are formed through natural
+  selection, others will become rarer and rarer, and finally extinct. The
+  forms which stand in closest competition with those undergoing
+  modification and improvement, will naturally suffer most. And we have
+  seen in the chapter on the Struggle for Existence that it is the most
+  closely-allied forms,&mdash;varieties of the same species, and species of
+  the same genus or of related genera,&mdash;which, from having nearly the
+  same structure, constitution, and habits, generally come into the
+  severest competition with each other. Consequently, each new variety or
+  species, during the progress of its formation, will generally press
+  hardest on its nearest kindred, and tend to exterminate them. We see the
+  same process of extermination amongst our domesticated productions,
+  through the selection of improved forms by man. Many curious <!-- Page
+  111 --><span class="pagenum"><a name="page111"></a>[111]</span>instances
+  could be given showing how quickly new breeds of cattle, sheep, and other
+  animals, and varieties of flowers, take the place of older and inferior
+  kinds. In Yorkshire, it is historically known that the ancient black
+  cattle were displaced by the long-horns, and that these "were swept away
+  by the short-horns" (I quote the words of an agricultural writer) "as if
+  by some murderous pestilence."</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Divergence of Character.</i>&mdash;The principle, which I have
+  designated by this term, is of high importance on my theory, and
+  explains, as I believe, several important facts. In the first place,
+  varieties, even strongly-marked ones, though having somewhat of the
+  character of species&mdash;as is shown by the hopeless doubts in many
+  cases how to rank them&mdash;yet certainly differ from each other far
+  less than do good and distinct species. Nevertheless, according to my
+  view, varieties are species in the process of formation, or are, as I
+  have called them, incipient species. How, then, does the lesser
+  difference between varieties become augmented into the greater difference
+  between species? That this does habitually happen, we must infer from
+  most of the innumerable species throughout nature presenting well-marked
+  differences; whereas varieties, the supposed prototypes and parents of
+  future well-marked species, present slight and ill-defined differences.
+  Mere chance, as we may call it, might cause one variety to differ in some
+  character from its parents, and the offspring of this variety again to
+  differ from its parent in the very same character and in a greater
+  degree; but this alone would never account for so habitual and large an
+  amount of difference as that between varieties of the same species and
+  species of the same genus.</p>
+
+  <p>As has always been my practice, let us seek light on <!-- Page 112
+  --><span class="pagenum"><a name="page112"></a>[112]</span>this head from
+  our domestic productions. We shall here find something analogous. A
+  fancier is struck by a pigeon having a slightly shorter beak; another
+  fancier is struck by a pigeon having a rather longer beak; and on the
+  acknowledged principle that "fanciers do not and will not admire a medium
+  standard, but like extremes," they both go on (as has actually occurred
+  with tumbler-pigeons) choosing and breeding from birds with longer and
+  longer beaks, or with shorter and shorter beaks. Again, we may suppose
+  that at an early period one man preferred swifter horses; another
+  stronger and more bulky horses. The early differences would be very
+  slight; in the course of time, from the continued selection of swifter
+  horses by some breeders, and of stronger ones by others, the differences
+  would become greater, and would be noted as forming two sub-breeds;
+  finally, after the lapse of centuries, the sub-breeds would become
+  converted into two well-established and distinct breeds. As the
+  differences slowly become greater, the inferior animals with intermediate
+  characters, being neither very swift nor very strong, will have been
+  neglected, and will have tended to disappear. Here, then, we see in man's
+  productions the action of what may be called the principle of divergence,
+  causing differences, at first barely appreciable, steadily to increase,
+  and the breeds to diverge in character both from each other and from
+  their common parent.</p>
+
+  <p>But how, it may be asked, can any analogous principle apply in nature?
+  I believe it can and does apply most efficiently, from the simple
+  circumstance that the more diversified the descendants from any one
+  species become in structure, constitution, and habits, by so much will
+  they be better enabled to seize on many and widely diversified places in
+  the polity of nature, and so be enabled to increase in numbers. <!-- Page
+  113 --><span class="pagenum"><a name="page113"></a>[113]</span></p>
+
+  <p>We can clearly see this in the case of animals with simple habits.
+  Take the case of a carnivorous quadruped, of which the number that can be
+  supported in any country has long ago arrived at its full average. If its
+  natural powers of increase be allowed to act, it can succeed in
+  increasing (the country not undergoing any change in its conditions) only
+  by its varying descendants seizing on places at present occupied by other
+  animals: some of them, for instance, being enabled to feed on new kinds
+  of prey, either dead or alive; some inhabiting new stations, climbing
+  trees, frequenting water, and some perhaps becoming less carnivorous. The
+  more diversified in habits and structure the descendants of our
+  carnivorous animal became, the more places they would be enabled to
+  occupy. What applies to one animal will apply throughout all time to all
+  animals&mdash;that is, if they vary&mdash;for otherwise natural selection
+  can do nothing. So it will be with plants. It has been experimentally
+  proved, that if a plot of ground be sown with one species of grass, and a
+  similar plot be sown with several distinct genera of grasses, a greater
+  number of plants and a greater weight of dry herbage can thus be raised.
+  The same has been found to hold good when first one variety and then
+  several mixed varieties of wheat have been sown on equal spaces of
+  ground. Hence, if any one species of grass were to go on varying, and
+  those varieties were continually selected which differed from each other
+  in at all the same manner as distinct species and genera of grasses
+  differ from each other, a greater number of individual plants of this
+  species of grass, including its modified descendants, would succeed in
+  living on the same piece of ground. And we well know that each species
+  and each variety of grass is annually sowing almost countless seeds; and
+  thus, as it may be said, is striving its utmost to increase its numbers.
+  <!-- Page 114 --><span class="pagenum"><a
+  name="page114"></a>[114]</span>Consequently, I cannot doubt that in the
+  course of many thousands of generations, the most distinct varieties of
+  any one species of grass would always have the best chance of succeeding
+  and of increasing in numbers, and thus of supplanting the less distinct
+  varieties; and varieties, when rendered very distinct from each other,
+  take the rank of species.</p>
+
+  <p>The truth of the principle, that the greatest amount of life can be
+  supported by great diversification of structure, is seen under many
+  natural circumstances. In an extremely small area, especially if freely
+  open to immigration, and where the contest between individual and
+  individual must be severe, we always find great diversity in its
+  inhabitants. For instance, I found that a piece of turf, three feet by
+  four in size, which had been exposed for many years to exactly the same
+  conditions, supported twenty species of plants, and these belonged to
+  eighteen genera and to eight orders, which shows how much these plants
+  differed from each other. So it is with the plants and insects on small
+  and uniform islets; and so in small ponds of fresh water. Farmers find
+  that they can raise most food by a rotation of plants belonging to the
+  most different orders: nature follows what may be called a simultaneous
+  rotation. Most of the animals and plants which live close round any small
+  piece of ground, could live on it (supposing it not to be in any way
+  peculiar in its nature), and may be said to be striving to the utmost to
+  live there; but, it is seen, that where they come into the closest
+  competition with each other, the advantages of diversification of
+  structure, with the accompanying differences of habit and constitution,
+  determine that the inhabitants, which thus jostle each other most
+  closely, shall, as a general rule, belong to what we call different
+  genera and orders.</p>
+
+  <p>The same principle is seen in the naturalisation of <!-- Page 115
+  --><span class="pagenum"><a name="page115"></a>[115]</span>plants through
+  man's agency in foreign lands. It might have been expected that the
+  plants which have succeeded in becoming naturalised in any land would
+  generally have been closely allied to the indigenes; for these are
+  commonly looked at as specially created and adapted for their own
+  country. It might, also, perhaps have been expected that naturalised
+  plants would have belonged to a few groups more especially adapted to
+  certain stations in their new homes. But the case is very different; and
+  Alph. De Candolle has well remarked in his great and admirable work, that
+  floras gain by naturalisation, proportionally with the number of the
+  native genera and species, far more in new genera than in new species. To
+  give a single instance: in the last edition of Dr. Asa Gray's 'Manual of
+  the Flora of the Northern United States,' 260 naturalised plants are
+  enumerated, and these belong to 162 genera. We thus see that these
+  naturalised plants are of a highly diversified nature. They differ,
+  moreover, to a large extent from the indigenes, for out of the 162
+  genera, no less than 100 genera are not there indigenous, and thus a
+  large proportional addition is made to the genera of these States.</p>
+
+  <p>By considering the nature of the plants or animals which have
+  struggled successfully with the indigenes of any country, and have there
+  become naturalised, we may gain some crude idea in what manner some of
+  the natives would have to be modified, in order to gain an advantage over
+  the other natives; and we may at least safely infer that diversification
+  of structure, amounting to new generic differences, would be profitable
+  to them.</p>
+
+  <p>The advantage of diversification 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 individual body&mdash;a subject so well elucidated
+  by Milne <!-- Page 116 --><span class="pagenum"><a
+  name="page116"></a>[116]</span>Edwards. No physiologist doubts that a
+  stomach adapted to digest vegetable matter alone, or flesh alone, draws
+  most nutriment from these substances. So in the general economy of any
+  land, the more widely and perfectly the animals and plants are
+  diversified for different habits of life, so will a greater number of
+  individuals be capable of there supporting themselves. A set of animals,
+  with their organisation but little diversified, could hardly compete with
+  a set more perfectly diversified in structure. It may be doubted, for
+  instance, whether the Australian marsupials, which are divided into
+  groups differing but little from each other, and feebly representing, as
+  Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and
+  rodent mammals, could successfully compete with these well-pronounced
+  orders. In the Australian mammals, we see the process of diversification
+  in an early and incomplete stage of development.</p>
+
+  <p>After the foregoing discussion, which ought to have been much
+  amplified, we may, I think, assume that the modified descendants of any
+  one species will succeed by so much the better as they become more
+  diversified in structure, and are thus enabled to encroach on places
+  occupied by other beings. Now let us see how this principle of benefit
+  being derived from divergence of character, combined with the principles
+  of natural selection and of extinction, will tend to act.</p>
+
+  <p>The accompanying diagram will aid us in understanding this rather
+  perplexing subject. Let A to L represent the species of a genus large in
+  its own country; these species are supposed to resemble each other in
+  unequal degrees, as is so generally the case in nature, and as is
+  represented in the diagram by the letters standing at unequal distances.
+  I have said a large genus, because we have seen in the second chapter,
+  <!-- Page 117 --><span class="pagenum"><a
+  name="page117"></a>[117]</span>that on an average more of the species of
+  large genera vary than of small genera; and the varying species of the
+  large genera present a greater number of varieties. We have, also, seen
+  that the species, which are the commonest and the most widely-diffused,
+  vary more than rare species with restricted ranges. Let (A) be a common,
+  widely-diffused, and varying species, belonging to a genus large in its
+  own country. The little fan of diverging dotted lines of unequal lengths
+  proceeding from (A), may represent its varying offspring. The variations
+  are supposed to be extremely slight, but of the most diversified nature;
+  they are not supposed all to appear simultaneously, but often after long
+  intervals of time; nor are they all supposed to endure for equal periods.
+  Only those variations which are in some way profitable will be preserved
+  or naturally selected. And here the importance of the principle of
+  benefit being derived from divergence of character comes in; for this
+  will generally lead to the most different or divergent variations
+  (represented by the outer dotted lines) being preserved and accumulated
+  by natural selection. When a dotted line reaches one of the horizontal
+  lines, and is there marked by a small numbered letter, a sufficient
+  amount of variation is supposed to have been accumulated to have formed a
+  fairly well-marked variety, such as would be thought worthy of record in
+  a systematic work.</p>
+
+  <div class="figcenter" style="width:90%;">
+      <a href="images/fp117.png"><img style="width:100%" src="images/fp117.png"
+      alt="Facing page 117." title="Facing page 117." /></a>
+  </div>
+  <p>The intervals between the horizontal lines in the diagram, may
+  represent each a thousand generations; but it would have been better if
+  each had represented ten thousand generations. After a thousand
+  generations, species (A) is supposed to have produced two fairly
+  well-marked varieties, namely <i>a</i><sup>1</sup> and
+  <i>m</i><sup>1</sup>. These two varieties will generally continue to be
+  exposed to the same conditions which made their parents variable, <!--
+  Page 118 --><span class="pagenum"><a name="page118"></a>[118]</span>and
+  the tendency to variability is in itself hereditary, consequently they
+  will tend to vary, and generally to vary in nearly the same manner as
+  their parents varied. Moreover, these two varieties, being only slightly
+  modified forms, will tend to inherit those advantages which made their
+  parent (A) more numerous than most of the other inhabitants of the same
+  country; they will likewise partake of those more general advantages
+  which made the genus to which the parent-species belonged, a large genus
+  in its own country. And these circumstances we know to be favourable to
+  the production of new varieties.</p>
+
+  <p>If, then, these two varieties be variable, the most divergent of their
+  variations will generally be preserved during the next thousand
+  generations. And after this interval, variety <i>a</i><sup>1</sup> is
+  supposed in the diagram to have produced variety <i>a</i><sup>2</sup>,
+  which will, owing to the principle of divergence, differ more from (A)
+  than did variety <i>a</i><sup>1</sup>. Variety <i>m</i><sup>1</sup> is
+  supposed to have produced two varieties, namely <i>m</i><sup>2</sup> and
+  <i>s</i><sup>2</sup>, differing from each other, and more considerably
+  from their common parent (A). We may continue the process by similar
+  steps for any length of time; some of the varieties, after each thousand
+  generations, producing only a single variety, but in a more and more
+  modified condition, some producing two or three varieties, and some
+  failing to produce any. Thus the varieties or modified descendants,
+  proceeding from the common parent (A), will generally go on increasing in
+  number and diverging in character. In the diagram the process is
+  represented up to the ten-thousandth generation, and under a condensed
+  and simplified form up to the fourteen-thousandth generation.</p>
+
+  <p>But I must here remark that I do not suppose that the process ever
+  goes on so regularly as is represented in the diagram, though in itself
+  made somewhat irregular. <!-- Page 119 --><span class="pagenum"><a
+  name="page119"></a>[119]</span>I am far from thinking that the most
+  divergent varieties will invariably prevail and multiply: a medium form
+  may often long endure, and may or may not produce more than one modified
+  descendant; for natural selection will always act according to the nature
+  of the places which are either unoccupied or not perfectly occupied by
+  other beings; and this will depend on infinitely complex relations. But
+  as a general rule, the more diversified in structure the descendants from
+  any one species can be rendered, the more places they will be enabled to
+  seize on, and the more their modified progeny will be increased. In our
+  diagram the line of succession is broken at regular intervals by small
+  numbered letters marking the successive forms which have become
+  sufficiently distinct to be recorded as varieties. But these breaks are
+  imaginary, and might have been inserted anywhere, after intervals long
+  enough to have allowed the accumulation of a considerable amount of
+  divergent variation.</p>
+
+  <p>As all the modified descendants from a common and widely-diffused
+  species, belonging to a large genus, will tend to partake of the same
+  advantages which made their parent successful in life, they will
+  generally go on multiplying in number as well as diverging in character:
+  this is represented in the diagram by the several divergent branches
+  proceeding from (A). The modified offspring from the later and more
+  highly improved branches in the lines of descent, will, it is probable,
+  often take the place of, and so destroy, the earlier and less improved
+  branches: this is represented in the diagram by some of the lower
+  branches not reaching to the upper horizontal lines. In some cases I do
+  not doubt that the process of modification will be confined to a single
+  line of descent, and the number of the descendants will not be increased;
+  although the amount <!-- Page 120 --><span class="pagenum"><a
+  name="page120"></a>[120]</span>of divergent modification may have been
+  increased in the successive generations. This case would be represented
+  in the diagram, if all the lines proceeding from (A) were removed,
+  excepting that from <i>a</i><sup>1</sup> to <i>a</i><sup>10</sup>. In the
+  same way, for instance, the English race-horse and English pointer have
+  apparently both gone on slowly diverging in character from their original
+  stocks, without either having given off any fresh branches or races.</p>
+
+  <p>After ten thousand generations, species (A) is supposed to have
+  produced three forms, <i>a</i><sup>10</sup>, <i>f</i><sup>10</sup>, and
+  <i>m</i><sup>10</sup>, which, from having diverged in character during
+  the successive generations, will have come to differ largely, but perhaps
+  unequally, from each other and from their common parent. If we suppose
+  the amount of change between each horizontal line in our diagram to be
+  excessively small, these three forms may still be only well-marked
+  varieties; or they may have arrived at the doubtful category of
+  sub-species; but we have only to suppose the steps in the process of
+  modification to be more numerous or greater in amount, to convert these
+  three forms into well-defined species: thus the diagram illustrates the
+  steps by which the small differences distinguishing varieties are
+  increased into the larger differences distinguishing species. By
+  continuing the same process for a greater number of generations (as shown
+  in the diagram in a condensed and simplified manner), we get eight
+  species, marked by the letters between <i>a</i><sup>14</sup> and
+  <i>m</i><sup>14</sup>, all descended from (A). Thus, as I believe,
+  species are multiplied and genera are formed.</p>
+
+  <p>In a large genus it is probable that more than one species would vary.
+  In the diagram I have assumed that a second species (I) has produced, by
+  analogous steps, after ten thousand generations, either two well-marked
+  varieties (<i>w</i><sup>10</sup> and <i>z</i><sup>10</sup>) or two
+  species, according to the amount of change supposed to be represented
+  <!-- Page 121 --><span class="pagenum"><a
+  name="page121"></a>[121]</span>between the horizontal lines. After
+  fourteen thousand generations, six new species, marked by the letters
+  <i>n</i><sup>14</sup> to <i>z</i><sup>14</sup>, are supposed to have been
+  produced. In each genus, the species, which are already extremely
+  different in character, will generally tend to produce the greatest
+  number of modified descendants; for these will have the best chance of
+  filling new and widely different places in the polity of nature: hence in
+  the diagram I have chosen the extreme species (A), and the nearly extreme
+  species (I), as those which have largely varied, and have given rise to
+  new varieties and species. The other nine species (marked by capital
+  letters) of our original genus, may for a long period continue to
+  transmit unaltered descendants; and this is shown in the diagram by the
+  dotted lines not prolonged far upwards from want of space.</p>
+
+  <p>But during the process of modification, represented in the diagram,
+  another of our principles, namely that of extinction, will have played an
+  important part. As in each fully stocked country natural selection
+  necessarily acts by the selected form having some advantage in the
+  struggle for life over other forms, there will be a constant tendency in
+  the improved descendants of any one species to supplant and exterminate
+  in each stage of descent their predecessors and their original parent.
+  For it should be remembered that the competition will generally be most
+  severe between those forms which are most nearly related to each other in
+  habits, constitution, and structure. Hence all the intermediate forms
+  between the earlier and later states, that is between the less and more
+  improved state of a species, as well as the original parent-species
+  itself, will generally tend to become extinct. So it probably will be
+  with many whole collateral lines of descent, which will be conquered by
+  later and improved lines of descent. If, however, the <!-- Page 122
+  --><span class="pagenum"><a name="page122"></a>[122]</span>modified
+  offspring of a species get into some distinct country, or become quickly
+  adapted to some quite new station, in which child and parent do not come
+  into competition, both may continue to exist.</p>
+
+  <p>If then our diagram be assumed to represent a considerable amount of
+  modification, species (A) and all the earlier varieties will have become
+  extinct, having been replaced by eight new species (<i>a</i><sup>14</sup>
+  to <i>m</i><sup>14</sup>); and (I) will have been replaced by six
+  (<i>n</i><sup>14</sup> to <i>z</i><sup>14</sup>) new species.</p>
+
+  <p>But we may go further than this. The original species of our genus
+  were supposed to resemble each other in unequal degrees, as is so
+  generally the case in nature; species (A) being more nearly related to B,
+  C, and D, than to the other species; and species (I) more to G, H, K, L,
+  than to the others. These two species (A) and (I), were also supposed to
+  be very common and widely diffused species, so that they must originally
+  have had some advantage over most of the other species of the genus.
+  Their modified descendants, fourteen in number at the fourteen-thousandth
+  generation, will probably have inherited some of the same advantages:
+  they have also been modified and improved in a diversified manner at each
+  stage of descent, so as to have become adapted to many related places in
+  the natural economy of their country. It seems, therefore, to me
+  extremely probable that they will have taken the places of, and thus
+  exterminated, not only their parents (A) and (I), but likewise some of
+  the original species which were most nearly related to their parents.
+  Hence very few of the original species will have transmitted offspring to
+  the fourteen-thousandth generation. We may suppose that only one (F), of
+  the two species which were least closely related to the other nine
+  original species, has transmitted descendants to this late stage of
+  descent. <!-- Page 123 --><span class="pagenum"><a
+  name="page123"></a>[123]</span></p>
+
+  <p>The new species in our diagram descended from the original eleven
+  species, will now be fifteen in number. Owing to the divergent tendency
+  of natural selection, the extreme amount of difference in character
+  between species <i>a</i><sup>14</sup> and <i>z</i><sup>14</sup> will be
+  much greater than that between the most different of the original eleven
+  species. The new species, moreover, will be allied to each other in a
+  widely different manner. Of the eight descendants from (A) the three
+  marked <i>a</i><sup>14</sup>, <i>q</i><sup>14</sup>,
+  <i>p</i><sup>14</sup>, will be nearly related from having recently
+  branched off from <i>a</i><sup>10</sup>; <i>b</i><sup>14</sup> and
+  <i>f</i><sup>14</sup>, from having diverged at an earlier period from
+  <i>a</i><sup>5</sup>, will be in some degree distinct from the three
+  first-named species; and lastly, <i>o</i><sup>14</sup>,
+  <i>e</i><sup>14</sup> and <i>m</i><sup>14</sup>, will be nearly related
+  one to the other, but from having diverged at the first commencement of
+  the process of modification, will be widely different from the other five
+  species, and may constitute a sub-genus or even a distinct genus.</p>
+
+  <p>The six descendants from (I) will form two sub-genera or even genera.
+  But as the original species (I) differed largely from (A), standing
+  nearly at the extreme points of the original genus, the six descendants
+  from (I) will, owing to inheritance alone, differ considerably from the
+  eight descendants from (A); the two groups, moreover, are supposed to
+  have gone on diverging in different directions. The intermediate species,
+  also (and this is a very important consideration), which connected the
+  original species (A) and (I), have all become, excepting (F), extinct,
+  and have left no descendants. Hence the six new species descended from
+  (I), and the eight descended from (A), will have to be ranked as very
+  distinct genera, or even as distinct sub-families.</p>
+
+  <p>Thus it is, as I believe, that two or more genera are produced by
+  descent with modification, from two or more species of the same genus.
+  And the two or <!-- Page 124 --><span class="pagenum"><a
+  name="page124"></a>[124]</span>more parent-species are supposed to have
+  descended from some one species of an earlier genus. In our diagram, this
+  is indicated by the broken lines, beneath the capital letters, converging
+  in sub-branches downwards towards a single point; this point representing
+  a single species, the supposed single parent of our several new
+  sub-genera and genera.</p>
+
+  <p>It is worth while to reflect for a moment on the character of the new
+  species <span class="scac">F</span><sup>14</sup>, which is supposed not
+  to have diverged much in character, but to have retained the form of (F),
+  either unaltered or altered only in a slight degree. In this case, its
+  affinities to the other fourteen new species will be of a curious and
+  circuitous nature. Having descended from a form which stood between the
+  two parent-species (A) and (I), now supposed to be extinct and unknown,
+  it will be in some degree intermediate in character between the two
+  groups descended from these species. But as these two groups have gone on
+  diverging in character from the type of their parents, the new species
+  (<span class="scac">F</span><sup>14</sup>) will not be directly
+  intermediate between them, but rather between types of the two groups;
+  and every naturalist will be able to bring some such case before his
+  mind.</p>
+
+  <p>In the diagram, each horizontal line has hitherto been supposed to
+  represent a thousand generations, but each may represent a million or
+  hundred million generations, and likewise a section of the successive
+  strata of the earth's crust including extinct remains. We shall, when we
+  come to our chapter on Geology, have to refer again to this subject, and
+  I think we shall then see that the diagram throws light on the affinities
+  of extinct beings, which, though generally belonging to the same orders,
+  or families, or genera, with those now living, yet are often, in some
+  degree, intermediate in character between existing groups; and we can
+  understand this fact, for <!-- Page 125 --><span class="pagenum"><a
+  name="page125"></a>[125]</span>the extinct species lived at very ancient
+  epochs when the branching lines of descent had diverged less.</p>
+
+  <p>I see no reason to limit the process of modification, as now
+  explained, to the formation of genera alone. If, in our diagram, we
+  suppose the amount of change represented by each successive group of
+  diverging dotted lines to be very great, the forms marked
+  <i>a</i><sup>14</sup> to <i>p</i><sup>14</sup>, those marked
+  <i>b</i><sup>14</sup> and <i>f</i><sup>14</sup>, and those marked
+  <i>o</i><sup>14</sup> to <i>m</i><sup>14</sup>, will form three very
+  distinct genera. We shall also have two very distinct genera descended
+  from (I); and as these latter two genera, both from continued divergence
+  of character and from inheritance from a different parent, will differ
+  widely from the three genera descended from (A), the two little groups of
+  genera will form two distinct families, or even orders, according to the
+  amount of divergent modification supposed to be represented in the
+  diagram. And the two new families, or orders, will have descended from
+  two species of the original genus; and these two species are supposed to
+  have descended from one species of a still more ancient and unknown
+  genus.</p>
+
+  <p>We have seen that in each country it is the species of the larger
+  genera which oftenest present varieties or incipient species. This,
+  indeed, might have been expected; for as natural selection acts through
+  one form having some advantage over other forms in the struggle for
+  existence, it will chiefly act on those which already have some
+  advantage; and the largeness of any group shows that its species have
+  inherited from a common ancestor some advantage in common. Hence, the
+  struggle for the production of new and modified descendants, will mainly
+  lie between the larger groups, which are all trying to increase in
+  number. One large group will slowly conquer another large group, reduce
+  its numbers, and thus lessen its chance of further variation and
+  improvement. Within the same large <!-- Page 126 --><span
+  class="pagenum"><a name="page126"></a>[126]</span>group, the later and
+  more highly perfected sub-groups, from branching out and seizing on many
+  new places in the polity of Nature, will constantly tend to supplant and
+  destroy the earlier and less improved sub-groups. Small and broken groups
+  and sub-groups will finally disappear. Looking to the future, we can
+  predict that the groups of organic beings which are now large and
+  triumphant, and which are least broken up, that is, which as yet have
+  suffered least extinction, will for a long period continue to increase.
+  But which groups will ultimately prevail, no man can predict; for we well
+  know that many groups, formerly most extensively developed, have now
+  become extinct. Looking still more remotely to the future, we may predict
+  that, owing to the continued and steady increase of the larger groups, a
+  multitude of smaller groups will become utterly extinct, and leave no
+  modified descendants; and consequently that of the species living at any
+  one period, extremely few will transmit descendants to a remote futurity.
+  I shall have to return to this subject in the chapter on Classification,
+  but I may add that on this view of extremely few of the more ancient
+  species having transmitted descendants, and on the view of all the
+  descendants of the same species making a class, we can understand how it
+  is that there exist but very few classes in each main division of the
+  animal and vegetable kingdoms. Although extremely few of the most ancient
+  species may now have living and modified descendants, yet at the most
+  remote geological period, the earth may have been as well peopled with
+  many species of many genera, families, orders, and classes, as at the
+  present day.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Summary of Chapter.</i>&mdash;If during the long course of ages and
+  under varying conditions of life, organic beings <!-- Page 127 --><span
+  class="pagenum"><a name="page127"></a>[127]</span>vary at all in the
+  several parts of their organisation, and I think this cannot be disputed;
+  if there be, owing to the high geometrical ratio of increase of each
+  species, a severe struggle for life at some age, season, or year, and
+  this certainly cannot be disputed; then, considering the infinite
+  complexity of the relations of all organic beings to each other and to
+  their conditions of existence, causing an infinite diversity in
+  structure, constitution, and habits, to be advantageous to them, I think
+  it would be a most extraordinary fact if no variation ever had occurred
+  useful to each being's own welfare, in the same manner as so many
+  variations have occurred useful to man. But if variations useful to any
+  organic being do occur, assuredly individuals thus characterised will
+  have the best chance of being preserved in the struggle for life; and
+  from the strong principle of inheritance they will tend to produce
+  offspring similarly characterised. This principle of preservation, I have
+  called, for the sake of brevity, Natural Selection; and it leads to the
+  improvement of each creature in relation to its organic and inorganic
+  conditions of life.</p>
+
+  <p>Natural selection, on the principle of qualities being inherited at
+  corresponding ages, can modify the egg, seed, or young, as easily as the
+  adult. Amongst many animals, sexual selection will give its aid to
+  ordinary selection, by assuring to the most vigorous and best adapted
+  males the greatest number of offspring. Sexual selection will also give
+  characters useful to the males alone, in their struggles with other
+  males.</p>
+
+  <p>Whether natural selection has really thus acted in nature, in
+  modifying and adapting the various forms of life to their several
+  conditions and stations, must be judged of by the general tenour and
+  balance of evidence given in the following chapters. But we already see
+  how it entails extinction; and how largely extinction <!-- Page 128
+  --><span class="pagenum"><a name="page128"></a>[128]</span>has acted in
+  the world's history, geology plainly declares. Natural selection, also,
+  leads to divergence of character; for more living beings can be supported
+  on the same area the more they diverge in structure, habits, and
+  constitution, of which we see proof by looking to the inhabitants of any
+  small spot or to naturalised productions. Therefore during the
+  modification of the descendants of any one species, and during the
+  incessant struggle of all species to increase in numbers, the more
+  diversified these descendants become, the better will be their chance of
+  succeeding in the battle for life. Thus the small differences
+  distinguishing varieties of the same species, steadily tend to increase
+  till they come to equal the greater differences between species of the
+  same genus, or even of distinct genera.</p>
+
+  <p>We have seen that it is the common, the widely-diffused, and
+  widely-ranging species, belonging to the larger genera, which vary most;
+  and these tend to transmit to their modified offspring that superiority
+  which now makes them dominant in their own countries. Natural selection,
+  as has just been remarked, leads to divergence of character and to much
+  extinction of the less improved and intermediate forms of life. On these
+  principles, I believe, the nature of the affinities of all organic beings
+  may be explained. It is a truly wonderful fact&mdash;the wonder of which
+  we are apt to overlook from familiarity&mdash;that all animals and all
+  plants throughout all time and space should be related to each other in
+  group subordinate to group, in the manner which we everywhere
+  behold&mdash;namely, varieties of the same species most closely related
+  together, species of the same genus less closely and unequally related
+  together, forming sections and sub-genera, species of distinct genera
+  much less closely related, and genera related in different degrees,
+  forming <!-- Page 129 --><span class="pagenum"><a
+  name="page129"></a>[129]</span>sub-families, families, orders,
+  sub-classes, and classes. The several subordinate groups in any class
+  cannot be ranked in a single file, but seem rather to be clustered round
+  points, and these round other points, and so on in almost endless cycles.
+  On the view that each species has been independently created, I can see
+  no explanation of this great fact in the classification of all organic
+  beings; but, to the best of my judgment, it is explained through
+  inheritance and the complex action of natural selection, entailing
+  extinction and divergence of character, as we have seen illustrated in
+  the diagram.</p>
+
+  <p>The affinities of all the beings of the same class have sometimes been
+  represented by a great tree. I believe this simile largely speaks the
+  truth. The green and budding twigs may represent existing species; and
+  those produced during each former year may represent the long succession
+  of extinct species. At each period of growth all the growing twigs have
+  tried to branch out on all sides, and to overtop and kill the surrounding
+  twigs and branches, in the same manner as species and groups of species
+  have tried to overmaster other species in the great battle for life. The
+  limbs divided into great branches, and these into lesser and lesser
+  branches, were themselves once, when the tree was small, budding twigs;
+  and this connexion of the former and present buds by ramifying branches
+  may well represent the classification of all extinct and living species
+  in groups subordinate to groups. Of the many twigs which flourished when
+  the tree was a mere bush, only two or three, now grown into great
+  branches, yet survive and bear all the other branches; so with the
+  species which lived during long-past geological periods, very few now
+  have living and modified descendants. From the first growth of the tree,
+  many a limb and branch has decayed and dropped off; and these lost
+  branches of various <!-- Page 130 --><span class="pagenum"><a
+  name="page130"></a>[130]</span>sizes may represent those whole orders,
+  families, and genera which have now no living representatives, and which
+  are known to us only from having been found in a fossil state. As we here
+  and there see a thin straggling branch springing from a fork low down in
+  a tree, and which by some chance has been favoured and is still alive on
+  its summit, so we occasionally see an animal like the Ornithorhynchus or
+  Lepidosiren, which in some small degree connects by its affinities two
+  large branches of life, and which has apparently been saved from fatal
+  competition by having inhabited a protected station. As buds give rise by
+  growth to fresh buds, and these, if vigorous, branch out and overtop on
+  all sides many a feebler branch, so by generation I believe it has been
+  with the great Tree of Life, which fills with its dead and broken
+  branches the crust of the earth, and covers the surface with its ever
+  branching and beautiful ramifications.</p>
+
+<hr />
+
+<p><!-- Page 131 --><span class="pagenum"><a name="page131"></a>[131]</span></p>
+
+<h3>CHAPTER V.</h3>
+
+<p class="cenhead"><span class="sc">Laws of Variation</span>.</p>
+
+<blockquote class="b1n">
+
+  <p>Effects of external conditions&mdash;Use and disuse, combined with
+  natural selection; organs of flight and of
+  vision&mdash;Acclimatisation&mdash;Correlation of
+  growth&mdash;Compensation and economy of growth&mdash;False
+  correlations&mdash;Multiple, rudimentary, and lowly organised structures
+  variable&mdash;Parts developed in an unusual manner are highly variable:
+  specific characters more variable than generic: secondary sexual
+  characters variable&mdash;Species of the same genus vary in an analogous
+  manner&mdash;Reversions to long-lost characters&mdash;Summary.</p>
+
+</blockquote>
+
+  <p>I have hitherto sometimes spoken as if the variations&mdash;so common
+  and multiform in organic beings under domestication, and in a lesser
+  degree in those in a state of nature&mdash;had been due to chance. This,
+  of course, is a wholly incorrect expression, but it serves to acknowledge
+  plainly our ignorance of the cause of each particular variation. Some
+  authors believe it to be as much the function of the reproductive system
+  to produce individual differences, or very slight deviations of
+  structure, as to make the child like its parents. But the much greater
+  variability, as well as the greater frequency of monstrosities, under
+  domestication or cultivation, than under nature, leads me to believe that
+  deviations of structure are in some way due to the nature of the
+  conditions of life, to which the parents and their more remote ancestors
+  have been exposed during several generations. I have remarked in the
+  first chapter&mdash;but a long catalogue of facts which cannot be here
+  given would be necessary to show the truth of the remark&mdash;that the
+  reproductive system is eminently susceptible to changes in the conditions
+  of life; and to <!-- Page 132 --><span class="pagenum"><a
+  name="page132"></a>[132]</span>this system being functionally disturbed
+  in the parents, I chiefly attribute the varying or plastic condition of
+  the offspring. The male and female sexual elements seem to be affected
+  before that union takes place which is to form a new being. In the case
+  of "sporting" plants, the bud, which in its earliest condition does not
+  apparently differ essentially from an ovule, is alone affected. But why,
+  because the reproductive system is disturbed, this or that part should
+  vary more or less, we are profoundly ignorant. Nevertheless, we can here
+  and there dimly catch a faint ray of light, and we may feel sure that
+  there must be some cause for each deviation of structure, however
+  slight.</p>
+
+  <p>How much direct effect difference of climate, food, &amp;c., produces
+  on any being is extremely doubtful. My impression is, that the effect is
+  extremely small in the case of animals, but perhaps rather more in that
+  of plants. We may, at least, safely conclude that such influences cannot
+  have produced the many striking and complex co-adaptations of structure
+  between one organic being and another, which we see everywhere throughout
+  nature. Some little influence may be attributed to climate, food,
+  &amp;c.: thus, E. Forbes speaks confidently that shells at their southern
+  limit, and when living in shallow water, are more brightly coloured than
+  those of the same species further north or from greater depths. Gould
+  believes that birds of the same species are more brightly coloured under
+  a clear atmosphere, than when living on islands or near the coast. So
+  with insects, Wollaston is convinced that residence near the sea affects
+  their colours. Moquin-Tandon gives a list of plants which when growing
+  near the sea-shore have their leaves in some degree fleshy, though not
+  elsewhere fleshy. Several other such cases could be given.</p>
+
+  <p>The fact of varieties of one species, when they range <!-- Page 133
+  --><span class="pagenum"><a name="page133"></a>[133]</span>into the zone
+  of habitation of other species, often acquiring in a very slight degree
+  some of the characters of such species, accords with our view that
+  species of all kinds are only well-marked and permanent varieties. Thus
+  the species of shells which are confined to tropical and shallow seas are
+  generally brighter-coloured than those confined to cold and deeper seas.
+  The birds which are confined to continents are, according to Mr. Gould,
+  brighter-coloured than those of islands. The insect-species confined to
+  sea-coasts, as every collector knows, are often brassy or lurid. Plants
+  which live exclusively on the sea-side are very apt to have fleshy
+  leaves. He who believes in the creation of each species, will have to say
+  that this shell, for instance, was created with bright colours for a warm
+  sea; but that this other shell became bright-coloured by variation when
+  it ranged into warmer or shallower waters.</p>
+
+  <p>When a variation is of the <span class="correction" title="text reads `slighest'"
+  >slightest</span> use to a being, we cannot tell how much of it to
+  attribute to the accumulative action of natural selection, and how much
+  to the conditions of life. Thus, it is well known to furriers that
+  animals of the same species have thicker and better fur the more severe
+  the climate is under which they have lived; but who can tell how much of
+  this difference may be due to the warmest-clad individuals having been
+  favoured and preserved during many generations, and how much to the
+  direct action of the severe climate? for it would appear that climate has
+  some direct action on the hair of our domestic quadrupeds.</p>
+
+  <p>Instances could be given of the same variety being produced under
+  conditions of life as different as can well be conceived; and, on the
+  other hand, of different varieties being produced from the same species
+  under the same conditions. Such facts show how indirectly <!-- Page 134
+  --><span class="pagenum"><a name="page134"></a>[134]</span>the conditions
+  of life act. Again, innumerable instances are known to every naturalist
+  of species keeping true, or not varying at all, although living under the
+  most opposite climates. Such considerations as these incline me to lay
+  very little weight on the direct action of the conditions of life.
+  Indirectly, as already remarked, they seem to play an important part in
+  affecting the reproductive system, and in thus inducing variability; and
+  natural selection will then accumulate all profitable variations, however
+  slight, until they become plainly developed and appreciable by us.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Effects of Use and Disuse.</i>&mdash;From the facts alluded to in
+  the first chapter, I think there can be little doubt that use in our
+  domestic animals strengthens and enlarges certain parts, and disuse
+  diminishes them; and that such modifications are inherited. Under free
+  nature, we can have no standard of comparison, by which to judge of the
+  effects of long-continued use or disuse, for we know not the
+  parent-forms; but many animals have structures which can be explained by
+  the effects of disuse. As Professor Owen has remarked, there is no
+  greater anomaly in nature than a bird that cannot fly; yet there are
+  several in this state. The logger-headed duck of South America can only
+  flap along the surface of the water, and has its wings in nearly the same
+  condition as the domestic Aylesbury duck. As the larger ground-feeding
+  birds seldom take flight except to escape danger, I believe that the
+  nearly wingless condition of several birds, which now inhabit or have
+  lately inhabited several oceanic islands, tenanted by no beast of prey,
+  has been caused by disuse. The ostrich indeed inhabits continents and is
+  exposed to danger from which it cannot escape by flight, but by kicking
+  it can defend itself from enemies, as well as any of the smaller <!--
+  Page 135 --><span class="pagenum"><a
+  name="page135"></a>[135]</span>quadrupeds. We may imagine that the early
+  progenitor of the ostrich had habits like those of a bustard, and that as
+  natural selection increased in successive generations the size and weight
+  of its body, its legs were used more, and its wings less, until they
+  became incapable of flight.</p>
+
+  <p>Kirby has remarked (and I have observed the same fact) that the
+  anterior tarsi, or feet, of many male dung-feeding beetles are very often
+  broken off; he examined seventeen specimens in his own collection, and
+  not one had even a relic left. In the Onites apelles the tarsi are so
+  habitually lost, that the insect has been described as not having them.
+  In some other genera they are present, but in a rudimentary condition. In
+  the Ateuchus or sacred beetle of the Egyptians, they are totally
+  deficient. There is not sufficient evidence to induce me to believe that
+  mutilations are ever inherited; and I should prefer explaining the entire
+  absence of the anterior tarsi in Ateuchus, and their rudimentary
+  condition in some other genera, by the long-continued effects of disuse
+  in their progenitors; for as the tarsi are almost always lost in many
+  dung-feeding beetles, they must be lost early in life, and therefore
+  cannot be much used by these insects.</p>
+
+  <p>In some cases we might easily put down to disuse modifications of
+  structure which are wholly, or mainly, due to natural selection. Mr.
+  Wollaston has discovered the remarkable fact that 200 beetles, out of the
+  550 species inhabiting Madeira, are so far deficient in wings that they
+  cannot fly; and that of the twenty-nine endemic genera, no less than
+  twenty-three genera have all their species in this condition! Several
+  facts, namely, that beetles in many parts of the world are frequently
+  blown to sea and perish; that the beetles in Madeira, as observed by Mr.
+  Wollaston, lie much concealed, <!-- Page 136 --><span class="pagenum"><a
+  name="page136"></a>[136]</span>until the wind lulls and the sun shines;
+  that the proportion of wingless beetles is larger on the exposed Desertas
+  than in Madeira itself; and especially the extraordinary fact, so
+  strongly insisted on by Mr. Wollaston, of the almost entire absence of
+  certain large groups of beetles, elsewhere excessively numerous, and
+  which groups have habits of life almost necessitating frequent
+  flight;&mdash;these several considerations have made me believe that the
+  wingless condition of so many Madeira beetles is mainly due to the action
+  of natural selection, but combined probably with disuse. For during
+  thousands of successive generations each individual beetle which flew
+  least, either from its wings having been ever so little less perfectly
+  developed or from indolent habit, will have had the best chance of
+  surviving from not being blown out to sea; and, on the other hand, those
+  beetles which most readily took to flight would oftenest have been blown
+  to sea and thus have been destroyed.</p>
+
+  <p>The insects in Madeira which are not ground-feeders, and which, as the
+  flower-feeding coleoptera and lepidoptera, must habitually use their
+  wings to gain their subsistence, have, as Mr. Wollaston suspects, their
+  wings not at all reduced, but even enlarged. This is quite compatible
+  with the action of natural selection. For when a new insect first arrived
+  on the island, the tendency of natural selection to enlarge or to reduce
+  the wings, would depend on whether a greater number of individuals were
+  saved by successfully battling with the winds, or by giving up the
+  attempt and rarely or never flying. As with mariners shipwrecked near a
+  coast, it would have been better for the good swimmers if they had been
+  able to swim still further, whereas it would have been better for the bad
+  swimmers if they had not been able to swim at all and had stuck to the
+  wreck. <!-- Page 137 --><span class="pagenum"><a
+  name="page137"></a>[137]</span></p>
+
+  <p>The eyes of moles and of some burrowing rodents are rudimentary in
+  size, and in some cases are quite covered up by skin and fur. This state
+  of the eyes is probably due to gradual reduction from disuse, but aided
+  perhaps by natural selection. In South America, a burrowing rodent, the
+  tuco-tuco, or Ctenomys, is even more subterranean in its habits than the
+  mole; and I was assured by a Spaniard, who had often caught them, that
+  they were frequently blind; one which I kept alive was certainly in this
+  condition, the cause, as appeared on dissection, having been inflammation
+  of the nictitating membrane. As frequent inflammation of the eyes must be
+  injurious to any animal, and as eyes are certainly not indispensable to
+  animals with subterranean habits, a reduction in their size with the
+  adhesion of the eyelids and growth of fur over them, might in such case
+  be an advantage; and if so, natural selection would constantly aid the
+  effects of disuse.</p>
+
+  <p>It is well known that several animals, belonging to the most different
+  classes, which inhabit the caves of Styria and of Kentucky, are blind. In
+  some of the crabs the foot-stalk for the eye remains, though the eye is
+  gone; the stand for the telescope is there, though the telescope with its
+  glasses has been lost. As it is difficult to imagine that eyes, though
+  useless, could be in any way injurious to animals living in darkness, I
+  attribute their loss wholly to disuse. In one of the blind animals,
+  namely, the cave-rat, the eyes are of immense size; and Professor
+  Silliman thought that it regained, after living some days in the light,
+  some slight power of vision. In the same manner as in Madeira the wings
+  of some of the insects have been enlarged, and the wings of others have
+  been reduced by natural selection aided by use and disuse, so in the case
+  of the cave-rat natural selection seems to have struggled with the loss
+  of light and <!-- Page 138 --><span class="pagenum"><a
+  name="page138"></a>[138]</span>to have increased the size of the eyes;
+  whereas with all the other inhabitants of the caves, disuse by itself
+  seems to have done its work.</p>
+
+  <p>It is difficult to imagine conditions of life more similar than deep
+  limestone caverns under a nearly similar climate; so that on the common
+  view of the blind animals having been separately created for the American
+  and European caverns, close similarity in their organisation and
+  affinities might have been expected; but, as Schiödte and others have
+  remarked, this is not the case, and the cave-insects of the two
+  continents are not more closely allied than might have been anticipated
+  from the general resemblance of the other inhabitants of North America
+  and Europe. On my view we must suppose that American animals, having
+  ordinary powers of vision, slowly migrated by successive generations from
+  the outer world into the deeper and deeper recesses of the Kentucky
+  caves, as did European animals into the caves of Europe. We have some
+  evidence of this gradation of habit; for, as Schiödte remarks, "animals
+  not far remote from ordinary forms, prepare the transition from light to
+  darkness. Next follow those that are constructed for twilight; and, last
+  of all, those destined for total darkness." By the time that an animal
+  had reached, after numberless generations, the deepest recesses, disuse
+  will on this view have more or less perfectly obliterated its eyes, and
+  natural selection will often have effected other changes, such as an
+  increase in the length of the antennæ or palpi, as a compensation for
+  blindness. Notwithstanding such modifications, we might expect still to
+  see in the cave-animals of America, affinities to the other inhabitants
+  of that continent, and in those of Europe, to the inhabitants of the
+  European continent. And this is the case with some of the American
+  cave-animals, as I hear from <!-- Page 139 --><span class="pagenum"><a
+  name="page139"></a>[139]</span>Professor Dana; and some of the European
+  cave-insects are very closely allied to those of the surrounding country.
+  It would be most difficult to give any rational explanation of the
+  affinities of the blind cave-animals to the other inhabitants of the two
+  continents on the ordinary view of their independent creation. That
+  several of the inhabitants of the caves of the Old and New Worlds should
+  be closely related, we might expect from the well-known relationship of
+  most of their other productions. Far from feeling any surprise that some
+  of the cave-animals should be very anomalous, as Agassiz has remarked in
+  regard to the blind fish, the Amblyopsis, and as is the case with the
+  blind Proteus with reference to the reptiles of Europe, I am only
+  surprised that more wrecks of ancient life have not been preserved, owing
+  to the less severe competition to which the inhabitants of these dark
+  abodes will probably have been exposed.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Acclimatisation.</i>&mdash;Habit is hereditary with plants, as in
+  the period of flowering, in the amount of rain requisite for seeds to
+  germinate, in the time of sleep, &amp;c., and this leads me to say a few
+  words on acclimatisation. As it is extremely common for species of the
+  same genus to inhabit very hot and very cold countries, and as I believe
+  that all the species of the same genus have descended from a single
+  parent, if this view be correct, acclimatisation must be readily effected
+  during long-continued descent. It is notorious that each species is
+  adapted to the climate of its own home: species from an arctic or even
+  from a temperate region cannot endure a tropical climate, or conversely.
+  So again, many succulent plants cannot endure a damp climate. But the
+  degree of adaptation of species to the climates under which they live is
+  often overrated. <!-- Page 140 --><span class="pagenum"><a
+  name="page140"></a>[140]</span>We may infer this from our frequent
+  inability to predict whether or not an imported plant will endure our
+  climate, and from the number of plants and animals brought from warmer
+  countries which here enjoy good health. We have reason to believe that
+  species in a state of nature are limited in their ranges by the
+  competition of other organic beings quite as much as, or more than, by
+  adaptation to particular climates. But whether or not the adaptation be
+  generally very close, we have evidence, in the case of some few plants,
+  of their becoming, to a certain extent, naturally habituated to different
+  temperatures, or becoming acclimatised: thus the pines and rhododendrons,
+  raised from seed collected by Dr. Hooker from trees growing at different
+  heights on the Himalaya, were found in this country to possess different
+  constitutional powers of resisting cold. Mr. Thwaites informs me that he
+  has observed similar facts in Ceylon, and analogous observations have
+  been made by Mr. H.&nbsp;C. Watson on European species of plants brought from
+  the Azores to England. In regard to animals, several authentic cases
+  could be given of species within historical times having largely extended
+  their range from warmer to cooler latitudes, and conversely; but we do
+  not positively know that these animals were strictly adapted to their
+  native climate, but in all ordinary cases we assume such to be the case;
+  nor do we know that they have subsequently become acclimatised to their
+  new homes.</p>
+
+  <p>As I believe that our domestic animals were originally chosen by
+  uncivilised man because they were useful and bred readily under
+  confinement, and not because they were subsequently found capable of
+  far-extended transportation, I think the common and extraordinary
+  capacity in our domestic animals of not only withstanding the most
+  different climates but of being perfectly <!-- Page 141 --><span
+  class="pagenum"><a name="page141"></a>[141]</span>fertile (a far severer
+  test) under them, may be used as an argument that a large proportion of
+  other animals, now in a state of nature, could easily be brought to bear
+  widely different climates. We must not, however, push the foregoing
+  argument too far, on account of the probable origin of some of our
+  domestic animals from several wild stocks: the blood, for instance, of a
+  tropical and arctic wolf or wild dog may perhaps be mingled in our
+  domestic breeds. The rat and mouse cannot be considered as domestic
+  animals, but they have been transported by man to many parts of the
+  world, and now have a far wider range than any other rodent, living free
+  under the cold climate of Faroe in the north and of the Falklands in the
+  south, and on many islands in the torrid zones. Hence I am inclined to
+  look at adaptation to any special climate as a quality readily grafted on
+  an innate wide flexibility of constitution, which is common to most
+  animals. On this view, the capacity of enduring the most different
+  climates by man himself and by his domestic animals, and such facts as
+  that former species of the elephant and rhinoceros were capable of
+  enduring a glacial climate, whereas the living species are now all
+  tropical or sub-tropical in their habits, ought not to be looked at as
+  anomalies, but merely as examples of a very common flexibility of
+  constitution, brought, under peculiar circumstances, into play.</p>
+
+  <p>How much of the acclimatisation of species to any peculiar climate is
+  due to mere habit, and how much to the natural selection of varieties
+  having different innate constitutions, and how much to both means
+  combined, is a very obscure question. That habit or custom has some
+  influence I must believe, both from analogy, and from the incessant
+  advice given in agricultural works, even in the ancient Encyclopædias of
+  China, to be very <!-- Page 142 --><span class="pagenum"><a
+  name="page142"></a>[142]</span>cautious in transposing animals from one
+  district to another; for it is not likely that man should have succeeded
+  in selecting so many breeds and sub-breeds with constitutions specially
+  fitted for their own districts: the result must, I think, be due to
+  habit. On the other hand, I can see no reason to doubt that natural
+  selection will continually tend to preserve those individuals which are
+  born with constitutions best adapted to their native countries. In
+  treatises on many kinds of cultivated plants, certain varieties are said
+  to withstand certain climates better than others: this is very strikingly
+  shown in works on fruit trees published in the United States, in which
+  certain varieties are habitually recommended for the northern, and others
+  for the southern States; and as most of these varieties are of recent
+  origin, they cannot owe their constitutional differences to habit. The
+  case of the Jerusalem artichoke, which is never propagated by seed, and
+  of which consequently new varieties have not been produced, has even been
+  advanced&mdash;for it is now as tender as ever it was&mdash;as proving
+  that acclimatisation cannot be effected! The case, also, of the
+  kidney-bean has been often cited for a similar purpose, and with much
+  greater weight; but until some one will sow, during a score of
+  generations, his kidney-beans so early that a very large proportion are
+  destroyed by frost, and then collect seed from the few survivors, with
+  care to prevent accidental crosses, and then again get seed from these
+  seedlings, with the same precautions, the experiment cannot be said to
+  have been even tried. Nor let it be supposed that no differences in the
+  constitution of seedling kidney-beans ever appear, for an account has
+  been published how much more hardy some seedlings appeared to be than
+  others.</p>
+
+  <p>On the whole, I think we may conclude that habit, <!-- Page 143
+  --><span class="pagenum"><a name="page143"></a>[143]</span>use, and
+  disuse, have, in some cases, played a considerable part in the
+  modification of the constitution, and of the structure of various organs;
+  but that the effects of use and disuse have often been largely combined
+  with, and sometimes overmastered by the natural selection of innate
+  variations.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Correlation of Growth.</i>&mdash;I mean by this expression that the
+  whole organisation is so tied together during its growth and development,
+  that when slight variations in any one part occur, and are accumulated
+  through natural selection, other parts become modified. This is a very
+  important subject, most imperfectly understood. The most obvious case is,
+  that modifications accumulated solely for the good of the young or larva,
+  will, it may safely be concluded, affect the structure of the adult; in
+  the same manner as any malconformation affecting the early embryo,
+  seriously affects the whole organisation of the adult. The several parts
+  of the body which are homologous, and which, at an early embryonic
+  period, are alike, seem liable to vary in an allied manner: we see this
+  in the right and left sides of the body varying in the same manner; in
+  the front and hind legs, and even in the jaws and limbs, varying
+  together, for the lower jaw is believed to be homologous with the limbs.
+  These tendencies, I do not doubt, may be mastered more or less completely
+  by natural selection: thus a family of stags once existed with an antler
+  only on one side; and if this had been of any great use to the breed it
+  might probably have been rendered permanent by natural selection.</p>
+
+  <p>Homologous parts, as has been remarked by some authors, tend to
+  cohere; this is often seen in monstrous plants; and nothing is more
+  common than the union of homologous parts in normal structures, as the
+  union of <!-- Page 144 --><span class="pagenum"><a
+  name="page144"></a>[144]</span>the petals of the corolla into a tube.
+  Hard parts seem to affect the form of adjoining soft parts; it is
+  believed by some authors that the diversity in the shape of the pelvis in
+  birds causes the remarkable diversity in the shape of their kidneys.
+  Others believe that the shape of the pelvis in the human mother
+  influences by pressure the shape of the head of the child. In snakes,
+  according to Schlegel, the shape of the body and the manner of swallowing
+  determine the position of several of the most important viscera.</p>
+
+  <p>The nature of the bond of correlation is very frequently quite
+  obscure. M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain
+  malconformations very frequently, and that others rarely coexist, without
+  our being able to assign any reason. What can be more singular than the
+  relation between blue eyes and deafness in cats, and the tortoise-shell
+  colour with the female sex; the feathered feet and skin between the outer
+  toes in pigeons, and the presence of more or less down on the young birds
+  when first hatched, with the future colour of their plumage; or, again,
+  the relation between the hair and teeth in the naked Turkish dog, though
+  here probably homology comes into play? With respect to this latter case
+  of correlation, I think it can hardly be accidental, that if we pick out
+  the two orders of mammalia which are most abnormal in their dermal
+  covering, viz. Cetacea (whales) and Edentata (armadilloes, scaly
+  anteaters, &amp;c.), that these are likewise the most abnormal in their
+  teeth.</p>
+
+  <p>I know of no case better adapted to show the importance of the laws of
+  correlation in modifying important structures, independently of utility
+  and, therefore, of natural selection, than that of the difference between
+  the outer and inner flowers in some Compositous and Umbelliferous plants.
+  Every one knows the <!-- Page 145 --><span class="pagenum"><a
+  name="page145"></a>[145]</span>difference in the ray and central florets
+  of, for instance, the daisy, and this difference is often accompanied
+  with the abortion of parts of the flower. But, in some Compositous
+  plants, the seeds also differ in shape and sculpture; and even the ovary
+  itself, with its accessory parts, differs, as has been described by
+  Cassini. These differences have been attributed by some authors to
+  pressure, and the shape of the seeds in the ray-florets in some Compositæ
+  countenances this idea; but, in the case of the corolla of the
+  Umbelliferæ, it is by no means, as Dr. Hooker informs me, in species with
+  the densest heads that the inner and outer flowers most frequently
+  differ. It might have been thought that the development of the ray-petals
+  by drawing nourishment from certain other parts of the flower had caused
+  their abortion; but in some Compositæ there is a difference in the seeds
+  of the outer and inner florets without any difference in the corolla.
+  Possibly, these several differences may be connected with some difference
+  in the flow of nutriment towards the central and external flowers: we
+  know, at least, that in irregular flowers, those nearest to the axis are
+  oftenest subject to peloria, and become regular. I may add, as an
+  instance of this, and of a striking case of correlation, that I have
+  recently observed in some garden pelargoniums, that the central flower of
+  the truss often loses the patches of darker colour in the two upper
+  petals; and that when this occurs, the adherent nectary is quite aborted;
+  when the colour is absent from only one of the two upper petals, the
+  nectary is only much shortened.</p>
+
+  <p>With respect to the difference in the corolla of the central and
+  exterior flowers of a head or umbel, I do not feel at all sure that C.&nbsp;C.
+  Sprengel's idea that the ray-florets serve to attract insects, whose
+  agency is highly advantageous in the fertilisation of plants of <!-- Page
+  146 --><span class="pagenum"><a name="page146"></a>[146]</span>these two
+  orders, is so far-fetched, as it may at first appear: and if it be
+  advantageous, natural selection may have come into play. But in regard to
+  the differences both in the internal and external structure of the seeds,
+  which are not always correlated with any differences in the flowers, it
+  seems impossible that they can be in any way advantageous to the plant:
+  yet in the Umbelliferæ these differences are of such apparent
+  importance&mdash;the seeds being in some cases, according to Tausch,
+  orthospermous in the exterior flowers and c&oelig;lospermous in the
+  central flowers,&mdash;that the elder De Candolle founded his main
+  divisions of the order on analogous differences. Hence we see that
+  modifications of structure, viewed by systematists as of high value, may
+  be wholly due to unknown laws of correlated growth, and without being, as
+  far as we can see, of the slightest service to the species.</p>
+
+  <p>We may often falsely attribute to correlation of growth, structures
+  which are common to whole groups of species, and which in truth are
+  simply due to inheritance; for an ancient progenitor may have acquired
+  through natural selection some one modification in structure, and, after
+  thousands of generations, some other and independent modification; and
+  these two modifications, having been transmitted to a whole group of
+  descendants with diverse habits, would naturally be thought to be
+  correlated in some necessary manner. So, again, I do not doubt that some
+  apparent correlations, occurring throughout whole orders, are entirely
+  due to the manner alone in which natural selection can act. For instance,
+  Alph. De Candolle has remarked that winged seeds are never found in
+  fruits which do not open: I should explain the rule by the fact that
+  seeds could not gradually become winged through natural selection, except
+  in fruits which opened; so that the individual plants producing <!-- Page
+  147 --><span class="pagenum"><a name="page147"></a>[147]</span>seeds
+  which were a little better fitted to be wafted further, might get an
+  advantage over those producing seed less fitted for dispersal; and this
+  process could not possibly go on in fruit which did not open.</p>
+
+  <p>The elder Geoffroy and Goethe propounded, at about the same period,
+  their law of compensation or balancement of growth; or, as Goethe
+  expressed it, "in order to spend on one side, nature is forced to
+  economise on the other side." I think this holds true to a certain extent
+  with our domestic productions: if nourishment flows to one part or organ
+  in excess, it rarely flows, at least in excess, to another part; thus it
+  is difficult to get a cow to give much milk and to fatten readily. The
+  same varieties of the cabbage do not yield abundant and nutritious
+  foliage and a copious supply of oil-bearing seeds. When the seeds in our
+  fruits become atrophied, the fruit itself gains largely in size and
+  quality. In our poultry, a large tuft of feathers on the head is
+  generally accompanied by a diminished comb, and a large beard by
+  diminished wattles. With species in a state of nature it can hardly be
+  maintained that the law is of universal application; but many good
+  observers, more especially botanists, believe in its truth. I will not,
+  however, here give any instances, for I see hardly any way of
+  distinguishing between the effects, on the one hand, of a part being
+  largely developed through natural selection and another and adjoining
+  part being reduced by this same process or by disuse, and, on the other
+  hand, the actual withdrawal of nutriment from one part owing to the
+  excess of growth in another and adjoining part.</p>
+
+  <p>I suspect, also, that some of the cases of compensation which have
+  been advanced, and likewise some other facts, may be merged under a more
+  general principle, namely, that natural selection is continually trying
+  to economise in every part of the organisation. If under <!-- Page 148
+  --><span class="pagenum"><a name="page148"></a>[148]</span>changed
+  conditions of life a structure before useful becomes less useful, any
+  diminution, however slight, in its development, will be seized on by
+  natural selection, for it will profit the individual not to have its
+  nutriment wasted in building up an useless structure. I can thus only
+  understand a fact with which I was much struck when examining cirripedes,
+  and of which many other instances could be given: namely, that when a
+  cirripede is parasitic within another and is thus protected, it loses
+  more or less completely its own shell or carapace. This is the case with
+  the male Ibla, and in a truly extraordinary manner with the Proteolepas:
+  for the carapace in all other cirripedes consists of the three
+  highly-important anterior segments of the head enormously developed, and
+  furnished with great nerves and muscles; but in the parasitic and
+  protected Proteolepas, the whole anterior part of the head is reduced to
+  the merest rudiment attached to the bases of the prehensile antennæ. Now
+  the saving of a large and complex structure, when rendered superfluous by
+  the parasitic habits of the Proteolepas, though effected by slow steps,
+  would be a decided advantage to each successive individual of the
+  species; for in the struggle for life to which every animal is exposed,
+  each individual Proteolepas would have a better chance of supporting
+  itself, by less nutriment being wasted in developing a structure now
+  become useless.</p>
+
+  <p>Thus, as I believe, natural selection will always succeed in the long
+  run in reducing and saving every part of the organisation, as soon as it
+  is rendered superfluous, without by any means causing some other part to
+  be largely developed in a corresponding degree. And, conversely, that
+  natural selection may perfectly well succeed in largely developing any
+  organ, without requiring as a necessary compensation the reduction of
+  some adjoining part. <!-- Page 149 --><span class="pagenum"><a
+  name="page149"></a>[149]</span></p>
+
+  <p>It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both
+  in varieties and in species, that when any part or organ is repeated many
+  times in the structure of the same individual (as the vertebræ in snakes,
+  and the stamens in polyandrous flowers) the number is variable; whereas
+  the number of the same part or organ, when it occurs in lesser numbers,
+  is constant. The same author and some botanists have further remarked
+  that multiple parts are also very liable to variation in structure.
+  Inasmuch as this "vegetative repetition," to use Prof. Owen's expression,
+  seems to be a sign of low organisation, the foregoing remark seems
+  connected with the very general opinion of naturalists, that beings low
+  in the scale of nature are more variable than those which are higher. I
+  presume that lowness in this case means that the several parts of the
+  organisation have been but little specialised for particular functions;
+  and as long as the same part has to perform diversified work, we can
+  perhaps see why it should remain variable, that is, why natural selection
+  should have preserved or rejected each little deviation of form less
+  carefully than when the part has to serve for one special purpose alone.
+  In the same way that a knife which has to cut all sorts of things may be
+  of almost any shape; whilst a tool for some particular object had better
+  be of some particular shape. Natural selection, it should never be
+  forgotten, can act on each part of each being, solely through and for its
+  advantage.</p>
+
+  <p>Rudimentary parts, it has been stated by some authors, and I believe
+  with truth, are apt to be highly variable. We shall have to recur to the
+  general subject of rudimentary and aborted organs; and I will here only
+  add that their variability seems to be owing to their uselessness, and
+  therefore to natural selection having no power to check deviations in
+  their structure. Thus <!-- Page 150 --><span class="pagenum"><a
+  name="page150"></a>[150]</span>rudimentary parts are left to the free
+  play of the various laws of growth, to the effects of long-continued
+  disuse, and to the tendency to reversion.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>A part developed in any species in an extraordinary degree or
+  manner, in comparison with the same part in allied species, tends to be
+  highly variable.</i>&mdash;Several years ago I was much struck with a
+  remark, nearly to the above effect, published by Mr. Waterhouse. I infer
+  also from an observation made by Professor Owen, with respect to the
+  length of the arms of the ourang-outang, that he has come to a nearly
+  similar conclusion. It is hopeless to attempt to convince any one of the
+  truth of this proposition without giving the long array of facts which I
+  have collected, and which cannot possibly be here introduced. I can only
+  state my conviction that it is a rule of high generality. I am aware of
+  several causes of error, but I hope that I have made due allowance for
+  them. It should be understood that the rule by no means applies to any
+  part, however unusually developed, unless it be unusually developed in
+  comparison with the same part in closely allied species. Thus, the bat's
+  wing is a most abnormal structure in the class mammalia; but the rule
+  would not here apply, because there is a whole group of bats having
+  wings; it would apply only if some one species of bat had its wings
+  developed in some remarkable manner in comparison with the other species
+  of the same genus. The rule applies very strongly in the case of
+  secondary sexual characters, when displayed in any unusual manner. The
+  term, secondary sexual characters, used by Hunter, applies to characters
+  which are attached to one sex, but are not directly connected with the
+  act of reproduction. The rule applies to males and females; but as
+  females more rarely offer remarkable secondary sexual characters, it
+  applies <!-- Page 151 --><span class="pagenum"><a
+  name="page151"></a>[151]</span>more rarely to them. The rule being so
+  plainly applicable in the case of secondary sexual characters, may be due
+  to the great variability of these characters, whether or not displayed in
+  any unusual manner&mdash;of which fact I think there can be little doubt.
+  But that our rule is not confined to secondary sexual characters is
+  clearly shown in the case of hermaphrodite cirripedes; and I may here
+  add, that I particularly attended to Mr. Waterhouse's remark, whilst
+  investigating this Order, and I am fully convinced that the rule almost
+  invariably holds good with cirripedes. I shall, in my future work, give a
+  list of the more remarkable cases; I will here only briefly give one, as
+  it illustrates the rule in its largest application. The opercular valves
+  of sessile cirripedes (rock barnacles) are, in every sense of the word,
+  very important structures, and they differ extremely little even in
+  different genera; but in the several species of one genus, Pyrgoma, these
+  valves present a marvellous amount of diversification: the homologous
+  valves in the different species being sometimes wholly unlike in shape;
+  and the amount of variation in the individuals of several of the species
+  is so great, that it is no exaggeration to state that the varieties
+  differ more from each other in the characters of these important valves
+  than do other species of distinct genera.</p>
+
+  <p>As birds within the same country vary in a remarkably small degree, I
+  have particularly attended to them, and the rule seems to me certainly to
+  hold good in this class. I cannot make out that it applies to plants, and
+  this would seriously have shaken my belief in its truth, had not the
+  great variability in plants made it particularly difficult to compare
+  their relative degrees of variability.</p>
+
+  <p>When we see any part or organ developed in a remarkable degree or
+  manner in any species, the fair <!-- Page 152 --><span class="pagenum"><a
+  name="page152"></a>[152]</span>presumption is that it is of high
+  importance to that species; nevertheless the part in this case is
+  eminently liable to variation. Why should this be so? On the view that
+  each species has been independently created, with all its parts as we now
+  see them, I can see no explanation. But on the view that groups of
+  species have descended from other species, and have been modified through
+  natural selection, I think we can obtain some light. In our domestic
+  animals, if any part, or the whole animal, be neglected and no selection
+  be applied, that part (for instance, the comb in the Dorking fowl) or the
+  whole breed will cease to have a nearly uniform character. The breed will
+  then be said to have degenerated. In rudimentary organs, and in those
+  which have been but little specialised for any particular purpose, and
+  perhaps in polymorphic groups, we see a nearly parallel natural case; for
+  in such cases natural selection either has not or cannot come into full
+  play, and thus the organisation is left in a fluctuating condition. But
+  what here more especially concerns us is, that in our domestic animals
+  those points, which at the present time are undergoing rapid change by
+  continued selection, are also eminently liable to variation. Look at the
+  breeds of the pigeon; see what a prodigious amount of difference there is
+  in the beak of the different tumblers, in the beak and wattle of the
+  different carriers, in the carriage and tail of our fantails, &amp;c.,
+  these being the points now mainly attended to by English fanciers. Even
+  in the sub-breeds, as in the short-faced tumbler, it is notoriously
+  difficult to breed them nearly to perfection, and frequently individuals
+  are born which depart widely from the standard. There may be truly said
+  to be a constant struggle going on between, on the one hand, the tendency
+  to reversion to a less modified state, as well as an innate tendency to
+  further <!-- Page 153 --><span class="pagenum"><a
+  name="page153"></a>[153]</span>variability of all kinds, and, on the
+  other hand, the power of steady selection to keep the breed true. In the
+  long run selection gains the day, and we do not expect to fail so far as
+  to breed a bird as coarse as a common tumbler from a good short-faced
+  strain. But as long as selection is rapidly going on, there may always be
+  expected to be much variability in the structure undergoing modification.
+  It further deserves notice that these variable characters, produced by
+  man's selection, sometimes become attached, from causes quite unknown to
+  us, more to one sex than to the other, generally to the male sex, as with
+  the wattle of carriers and the enlarged crop of pouters.</p>
+
+  <p>Now let us turn to nature. When a part has been developed in an
+  extraordinary manner in any one species, compared with the other species
+  of the same genus, we may conclude that this part has undergone an
+  extraordinary amount of modification since the period when the species
+  branched off from the common progenitor of the genus. This period will
+  seldom be remote in any extreme degree, as species very rarely endure for
+  more than one geological period. An extraordinary amount of modification
+  implies an unusually large and long-continued amount of variability,
+  which has continually been accumulated by natural selection for the
+  benefit of the species. But as the variability of the
+  extraordinarily-developed part or organ has been so great and
+  long-continued within a period not excessively remote, we might, as a
+  general rule, expect still to find more variability in such parts than in
+  other parts of the organisation which have remained for a much longer
+  period nearly constant. And this, I am convinced, is the case. That the
+  struggle between natural selection on the one hand, and the tendency to
+  reversion and variability on the other hand, will in the <!-- Page 154
+  --><span class="pagenum"><a name="page154"></a>[154]</span>course of time
+  cease; and that the most abnormally developed organs may be made
+  constant, I can see no reason to doubt. Hence when an organ, however
+  abnormal it may be, has been transmitted in approximately the same
+  condition to many modified descendants, as in the case of the wing of the
+  bat, it must have existed, according to my theory, for an immense period
+  in nearly the same state; and thus it comes to be no more variable than
+  any other structure. It is only in those cases in which the modification
+  has been comparatively recent and extraordinarily great that we ought to
+  find the <i>generative variability</i>, as it may be called, still
+  present in a high degree. For in this case the variability will seldom as
+  yet have been fixed by the continued selection of the individuals varying
+  in the required manner and degree, and by the continued rejection of
+  those tending to revert to a former and less modified condition.</p>
+
+  <p>The principle included in these remarks may be extended. It is
+  notorious that specific characters are more variable than generic. To
+  explain by a simple example what is meant. If some species in a large
+  genus of plants had blue flowers and some had red, the colour would be
+  only a specific character, and no one would be surprised at one of the
+  blue species varying into red, or conversely; but if all the species had
+  blue flowers, the colour would become a generic character, and its
+  variation would be a more unusual circumstance. I have chosen this
+  example because an explanation is not in this case applicable, which most
+  naturalists would advance, namely, that specific characters are more
+  variable than generic, because they are taken from parts of less
+  physiological importance than those commonly used for classing genera. I
+  believe this explanation is partly, yet only indirectly, true; I shall,
+  however, have to <!-- Page 155 --><span class="pagenum"><a
+  name="page155"></a>[155]</span>return to this subject in our chapter on
+  Classification. It would be almost superfluous to adduce evidence in
+  support of the above statement, that specific characters are more
+  variable than generic; but I have repeatedly noticed in works on natural
+  history, that when an author has remarked with surprise that some
+  <i>important</i> organ or part, which is generally very constant
+  throughout large groups of species, has <i>differed</i> considerably in
+  closely-allied species, that it has, also, been <i>variable</i> in the
+  individuals of some of the species. And this fact shows that a character,
+  which is generally of generic value, when it sinks in value and becomes
+  only of specific value, often becomes variable, though its physiological
+  importance may remain the same. Something of the same kind applies to
+  monstrosities: at least Is. Geoffroy St. Hilaire seems to entertain no
+  doubt, that the more an organ normally differs in the different species
+  of the same group, the more subject it is to individual anomalies.</p>
+
+  <p>On the ordinary view of each species having been independently
+  created, why should that part of the structure, which differs from the
+  same part in other independently-created species of the same genus, be
+  more variable than those parts which are closely alike in the several
+  species? I do not see that any explanation can be given. But on the view
+  of species being only strongly marked and fixed varieties, we might
+  surely expect to find them still often continuing to vary in those parts
+  of their structure which have varied within a moderately recent period,
+  and which have thus come to differ. Or to state the case in another
+  manner:&mdash;the points in which all the species of a genus resemble
+  each other, and in which they differ from the species of some other
+  genus, are called generic characters; and these characters in common I
+  attribute to <!-- Page 156 --><span class="pagenum"><a
+  name="page156"></a>[156]</span>inheritance from a common progenitor, for
+  it can rarely have happened that natural selection will have modified
+  several species, fitted to more or less widely-different habits, in
+  exactly the same manner: and as these so-called generic characters have
+  been inherited from a remote period, since that period when the species
+  first branched off from their common progenitor, and subsequently have
+  not varied or come to differ in any degree, or only in a slight degree,
+  it is not probable that they should vary at the present day. On the other
+  hand, the points in which species differ from other species of the same
+  genus, are called specific characters; and as these specific characters
+  have varied and come to differ within the period of the branching off of
+  the species from a common progenitor, it is probable that they should
+  still often be in some degree variable,&mdash;at least more variable than
+  those parts of the organisation which have for a very long period
+  remained constant.</p>
+
+  <p>In connexion with the present subject, I will make only two other
+  remarks. I think it will be admitted, without my entering on details,
+  that secondary sexual characters are very variable; I think it also will
+  be admitted that species of the same group differ from each other more
+  widely in their secondary sexual characters, than in other parts of their
+  organisation; compare, for instance, the amount of difference between the
+  males of gallinaceous birds, in which secondary sexual characters are
+  strongly displayed, with the amount of difference between their females;
+  and the truth of this proposition will be granted. The cause of the
+  original variability of secondary sexual characters is not manifest; but
+  we can see why these characters should not have been rendered as constant
+  and uniform as other parts of the organisation; for secondary sexual
+  characters have been accumulated by sexual selection, which <!-- Page 157
+  --><span class="pagenum"><a name="page157"></a>[157]</span>is less rigid
+  in its action than ordinary selection, as it does not entail death, but
+  only gives fewer offspring to the less favoured males. Whatever the cause
+  may be of the variability of secondary sexual characters, as they are
+  highly variable, sexual selection will have had a wide scope for action,
+  and may thus readily have succeeded in giving to the species of the same
+  group a greater amount of difference in their sexual characters, than in
+  other parts of their structure.</p>
+
+  <p>It is a remarkable fact, that the secondary sexual differences between
+  the two sexes of the same species are generally displayed in the very
+  same parts of the organisation in which the different species of the same
+  genus differ from each other. Of this fact I will give in illustration
+  two instances, the first which happen to stand on my list; and as the
+  differences in these cases are of a very unusual nature, the relation can
+  hardly be accidental. The same number of joints in the tarsi is a
+  character generally common to very large groups of beetles, but in the
+  Engidæ, as Westwood has remarked, the number varies greatly; and the
+  number likewise differs in the two sexes of the same species: again in
+  fossorial hymenoptera, the manner of neuration of the wings is a
+  character of the highest importance, because common to large groups; but
+  in certain genera the neuration differs in the different species, and
+  likewise in the two sexes of the same species. This relation has a clear
+  meaning on my view of the subject: I look at all the species of the same
+  genus as having as certainly descended from the same progenitor, as have
+  the two sexes of any one of the species. Consequently, whatever part of
+  the structure of the common progenitor, or of its early descendants,
+  became variable; variations of this part would, it is highly probable, be
+  taken advantage of by natural and sexual selection, in order to fit <!--
+  Page 158 --><span class="pagenum"><a name="page158"></a>[158]</span>the
+  several species to their several places in the economy of nature, and
+  likewise to fit the two sexes of the same species to each other, or to
+  fit the males and females to different habits of life, or the males to
+  struggle with other males for the possession of the females.</p>
+
+  <p>Finally, then, I conclude that the greater variability of specific
+  characters, or those which distinguish species from species, than of
+  generic characters, or those which the species possess in
+  common;&mdash;that the frequent extreme variability of any part which is
+  developed in a species in an extraordinary manner in comparison with the
+  same part in its congeners; and the slight degree of variability in a
+  part, however extraordinarily it may be developed, if it be common to a
+  whole group of species;&mdash;that the great variability of secondary
+  sexual characters, and the great amount of difference in these same
+  characters between closely allied species;&mdash;that secondary sexual
+  and ordinary specific differences are generally displayed in the same
+  parts of the organisation,&mdash;are all principles closely connected
+  together. All being mainly due to the species of the same group having
+  descended from a common progenitor, from whom they have inherited much in
+  common,&mdash;to parts which have recently and largely varied being more
+  likely still to go on varying than parts which have long been inherited
+  and have not varied,&mdash;to natural selection having more or less
+  completely, according to the lapse of time, overmastered the tendency to
+  reversion and to further variability,&mdash;to sexual selection being
+  less rigid than ordinary selection,&mdash;and to variations in the same
+  parts having been accumulated by natural and sexual selection, and having
+  been thus adapted for secondary sexual, and for ordinary specific
+  purposes. <!-- Page 159 --><span class="pagenum"><a
+  name="page159"></a>[159]</span></p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Distinct species present analogous variations; and a variety of one
+  species often assumes some of the characters of an allied species, or
+  reverts to some of the characters of an early progenitor.</i>&mdash;These
+  propositions will be most readily understood by looking to our domestic
+  races. The most distinct breeds of pigeons, in countries most widely
+  apart, present sub-varieties with reversed feathers on the head and
+  feathers on the feet,&mdash;characters not possessed by the aboriginal
+  rock-pigeon; these then are analogous variations in two or more distinct
+  races. The frequent presence of fourteen or even sixteen tail-feathers in
+  the pouter, may be considered as a variation representing the normal
+  structure of another race, the fantail. I presume that no one will doubt
+  that all such analogous variations are due to the several races of the
+  pigeon having inherited from a common parent the same constitution and
+  tendency to variation, when acted on by similar unknown influences. In
+  the vegetable kingdom we have a case of analogous variation, in the
+  enlarged stems, or roots as commonly called, of the Swedish turnip and
+  Ruta baga, plants which several botanists rank as varieties produced by
+  cultivation from a common parent: if this be not so, the case will then
+  be one of analogous variation in two so-called distinct species; and to
+  these a third may be added, namely, the common turnip. According to the
+  ordinary view of each species having been independently created, we
+  should have to attribute this similarity in the enlarged stems of these
+  three plants, not to the <i>vera causa</i> of community of descent, and a
+  consequent tendency to vary in a like manner, but to three separate yet
+  closely related acts of creation.</p>
+
+  <p>With pigeons, however, we have another case, namely, the occasional
+  appearance in all the breeds, of slaty-blue birds with two black bars on
+  the wings, a white <!-- Page 160 --><span class="pagenum"><a
+  name="page160"></a>[160]</span>rump, a bar at the end of the tail, with
+  the outer feathers externally edged near their bases with white. As all
+  these marks are characteristic of the parent rock-pigeon, I presume that
+  no one will doubt that this is a case of reversion, and not of a new yet
+  analogous variation appearing in the several breeds. We may I think
+  confidently come to this conclusion, because, as we have seen, these
+  coloured marks are eminently liable to appear in the crossed offspring of
+  two distinct and differently coloured breeds; and in this case there is
+  nothing in the external conditions of life to cause the reappearance of
+  the slaty-blue, with the several marks, beyond the influence of the mere
+  act of crossing on the laws of inheritance.</p>
+
+  <p>No doubt it is a very surprising fact that characters should reappear
+  after having been lost for many, perhaps for hundreds of generations. But
+  when a breed has been crossed only once by some other breed, the
+  offspring occasionally show a tendency to revert in character to the
+  foreign breed for many generations&mdash;some say, for a dozen or even a
+  score of generations. After twelve generations, the proportion of blood,
+  to use a common expression, of any one ancestor, is only 1 in 2048; and
+  yet, as we see, it is generally believed that a tendency to reversion is
+  retained by this very small proportion of foreign blood. In a breed which
+  has not been crossed, but in which <i>both</i> parents have lost some
+  character which their progenitor possessed, the tendency, whether strong
+  or weak, to reproduce the lost character might be, as was formerly
+  remarked, for all that we can see to the contrary, transmitted for almost
+  any number of generations. When a character which has been lost in a
+  breed, reappears after a great number of generations, the most probable
+  hypothesis is, not that the offspring suddenly takes after an ancestor
+  some hundred generations <!-- Page 161 --><span class="pagenum"><a
+  name="page161"></a>[161]</span>distant, but that in each successive
+  generation there has been a tendency to reproduce the character in
+  question, which at last, under unknown favourable conditions, gains an
+  ascendancy. For instance, it is probable that in each generation of the
+  barb-pigeon, which produces most rarely a blue and black-barred bird,
+  there has been a tendency in each generation in the plumage to assume
+  this colour. This view is hypothetical, but could be supported by some
+  facts; and I can see no more abstract improbability in a tendency to
+  produce any character being inherited for an endless number of
+  generations, than in quite useless or rudimentary organs being, as we all
+  know them to be, thus inherited. Indeed, we may sometimes observe a mere
+  tendency to produce a rudiment inherited: for instance, in the common
+  snapdragon (Antirrhinum) a rudiment of a fifth stamen so often appears,
+  that this plant must have an inherited tendency to produce it.</p>
+
+  <p>As all the species of the same genus are supposed, on my theory, to
+  have descended from a common parent, it might be expected that they would
+  occasionally vary in an analogous manner; so that a variety of one
+  species would resemble in some of its characters another species; this
+  other species being on my view only a well-marked and permanent variety.
+  But characters thus gained would probably be of an unimportant nature,
+  for the presence of all important characters will be governed by natural
+  selection, in accordance with the diverse habits of the species, and will
+  not be left to the mutual action of the conditions of life and of a
+  similar inherited constitution. It might further be expected that the
+  species of the same genus would occasionally exhibit reversions to lost
+  ancestral characters. As, however, we never know the exact character of
+  the common ancestor of a group, we could not distinguish these two <!--
+  Page 162 --><span class="pagenum"><a
+  name="page162"></a>[162]</span>cases: if, for instance, we did not know
+  that the rock-pigeon was not feather-footed or turn-crowned, we could not
+  have told, whether these characters in our domestic breeds were
+  reversions or only analogous variations; but we might have inferred that
+  the blueness was a case of reversion, from the number of the markings,
+  which are correlated with the blue tint, and which it does not appear
+  probable would all appear together from simple variation. More especially
+  we might have inferred this, from the blue colour and marks so often
+  appearing when distinct breeds of diverse colours are crossed. Hence,
+  though under nature it must generally be left doubtful, what cases are
+  reversions to an anciently existing character, and what are new but
+  analogous variations, yet we ought, on my theory, sometimes to find the
+  varying offspring of a species assuming characters (either from reversion
+  or from analogous variation) which already occur in some other members of
+  the same group. And this undoubtedly is the case in nature.</p>
+
+  <p>A considerable part of the difficulty in recognising a variable
+  species in our systematic works, is due to its varieties mocking, as it
+  were, some of the other species of the same genus. A considerable
+  catalogue, also, could be given of forms intermediate between two other
+  forms, which themselves must be doubtfully ranked as either varieties or
+  species; and this shows, unless all these forms be considered as
+  independently created species, that the one in varying has assumed some
+  of the characters of the other, so as to produce the intermediate form.
+  But the best evidence is afforded by parts or organs of an important and
+  uniform nature occasionally varying so as to acquire, in some degree, the
+  character of the same part or organ in an allied species. I have
+  collected a long list of such cases; but <!-- Page 163 --><span
+  class="pagenum"><a name="page163"></a>[163]</span>here, as before, I lie
+  under a great disadvantage in not being able to give them. I can only
+  repeat that such cases certainly do occur, and seem to me very
+  remarkable.</p>
+
+  <p>I will, however, give one curious and complex case, not indeed as
+  affecting any important character, but from occurring in several species
+  of the same genus, partly under domestication and partly under nature. It
+  is a case apparently of reversion. The ass not rarely has very distinct
+  transverse bars on its legs, like those on the legs of the zebra: it has
+  been asserted that these are plainest in the foal, and from inquiries
+  which I have made, I believe this to be true. It has also been asserted
+  that the stripe on each shoulder is sometimes double. The shoulder-stripe
+  is certainly very variable in length and outline. A white ass, but
+  <i>not</i> an albino, has been described without either spinal or
+  shoulder stripe; and these stripes are sometimes very obscure, or
+  actually quite lost, in dark-coloured asses. The koulan of Pallas is said
+  to have been seen with a double shoulder-stripe. The hemionus has no
+  shoulder-stripe; but traces of it, as stated by Mr. Blyth and others,
+  occasionally appear: and I have been informed by Colonel Poole that the
+  foals of this species are generally striped on the legs, and faintly on
+  the shoulder. The quagga, though so plainly barred like a zebra over the
+  body, is without bars on the legs; but Dr. Gray has figured one specimen
+  with very distinct zebra-like bars on the hocks.</p>
+
+  <p>With respect to the horse, I have collected cases in England of the
+  spinal stripe in horses of the most distinct breeds, and of <i>all</i>
+  colours; transverse bars on the legs are not rare in duns, mouse-duns,
+  and in one instance in a chestnut: a faint shoulder-stripe may sometimes
+  be seen in duns, and I have seen a trace in a <!-- Page 164 --><span
+  class="pagenum"><a name="page164"></a>[164]</span>bay horse. My son made
+  a careful examination and sketch for me of a dun Belgian cart-horse with
+  a double stripe on each shoulder and with leg-stripes; and a man, whom I
+  can implicitly trust, has examined for me a small dun Welch pony with
+  <i>three</i> short parallel stripes on each shoulder.</p>
+
+  <p>In the north-west part of India the Kattywar breed of horses is so
+  generally striped, that, as I hear from Colonel Poole, who examined the
+  breed for the Indian Government, a horse without stripes is not
+  considered as purely-bred. The spine is always striped; the legs are
+  generally barred; and the shoulder-stripe, which is sometimes double and
+  sometimes treble, is common; the side of the face, moreover, is sometimes
+  striped. The stripes are plainest in the foal; and sometimes quite
+  disappear in old horses. Colonel Poole has seen both gray and bay
+  Kattywar horses striped when first foaled. I have, also, reason to
+  suspect, from information given me by Mr. W.&nbsp;W. Edwards, that with the
+  English racehorse the spinal stripe is much commoner in the foal than in
+  the full-grown animal. Without here entering on further details, I may
+  state that I have collected cases of leg and shoulder stripes in horses
+  of very different breeds, in various countries from Britain to Eastern
+  China; and from Norway in the north to the Malay Archipelago in the
+  south. In all parts of the world these stripes occur far oftenest in duns
+  and mouse-duns; by the term dun a large range of colour is included, from
+  one between brown and black to a close approach to cream-colour.</p>
+
+  <p>I am aware that Colonel Hamilton Smith, who has written on this
+  subject, believes that the several breeds of the horse have descended
+  from several aboriginal species&mdash;one of which, the dun, was striped;
+  and that the above-described appearances are all due to ancient <!-- Page
+  165 --><span class="pagenum"><a name="page165"></a>[165]</span>crosses
+  with the dun stock. But I am not at all satisfied with this theory, and
+  should be loth to apply it to breeds so distinct as the heavy Belgian
+  cart-horse, Welch ponies, cobs, the lanky Kattywar race, &amp;c.,
+  inhabiting the most distant parts of the world.</p>
+
+  <p>Now let us turn to the effects of crossing the several species of the
+  horse-genus. Rollin asserts, that the common mule from the ass and horse
+  is particularly apt to have bars on its legs: according to Mr. Gosse, in
+  certain parts of the United States about nine out of ten mules have
+  striped legs. I once saw a mule with its legs so much striped that any
+  one would at first have thought that it must have been the product of a
+  zebra; and Mr. W.&nbsp;C. Martin, in his excellent treatise on the horse, has
+  given a figure of a similar mule. In four coloured drawings, which I have
+  seen, of hybrids between the ass and zebra, the legs were much more
+  plainly barred than the rest of the body; and in one of them there was a
+  double shoulder-stripe. In Lord Morton's famous hybrid from a chestnut
+  mare and male quagga, the hybrid, and even the pure offspring
+  subsequently produced from the mare by a black Arabian sire, were much
+  more plainly barred across the legs than is even the pure quagga. Lastly,
+  and this is another most remarkable case, a hybrid has been figured by
+  Dr. Gray (and he informs me that he knows of a second case) from the ass
+  and the hemionus; and this hybrid, though the ass seldom has stripes on
+  his legs and the hemionus has none and has not even a shoulder-stripe,
+  nevertheless had all four legs barred, and had three short
+  shoulder-stripes, like those on the dun Welch pony, and even had some
+  zebra-like stripes on the sides of its face. With respect to this last
+  fact, I was so convinced that not even a stripe of colour appears from
+  what would commonly be called an <!-- Page 166 --><span
+  class="pagenum"><a name="page166"></a>[166]</span>accident, that I was
+  led solely from the occurrence of the face-stripes on this hybrid from
+  the ass and hemionus to ask Colonel Poole whether such face-stripes ever
+  occur in the eminently striped Kattywar breed of horses, and was, as we
+  have seen, answered in the affirmative.</p>
+
+  <p>What now are we to say to these several facts? We see several very
+  distinct species of the horse-genus becoming, by simple variation,
+  striped on the legs like a zebra, or striped on the shoulders like an
+  ass. In the horse we see this tendency strong whenever a dun tint
+  appears&mdash;a tint which approaches to that of the general colouring of
+  the other species of the genus. The appearance of the stripes is not
+  accompanied by any change of form or by any other new character. We see
+  this tendency to become striped most strongly displayed in hybrids from
+  between several of the most distinct species. Now observe the case of the
+  several breeds of pigeons: they are descended from a pigeon (including
+  two or three sub-species or geographical races) of a bluish colour, with
+  certain bars and other marks; and when any breed assumes by simple
+  variation a bluish tint, these bars and other marks invariably reappear;
+  but without any other change of form or character. When the oldest and
+  truest breeds of various colours are crossed, we see a strong tendency
+  for the blue tint and bars and marks to reappear in the mongrels. I have
+  stated that the most probable hypothesis to account for the reappearance
+  of very ancient characters, is&mdash;that there is a <i>tendency</i> in
+  the young of each successive generation to produce the long-lost
+  character, and that this tendency, from unknown causes, sometimes
+  prevails. And we have just seen that in several species of the
+  horse-genus the stripes are either plainer or appear more commonly in the
+  young than in the old. Call the breeds of pigeons, some of which have
+  bred true for <!-- Page 167 --><span class="pagenum"><a
+  name="page167"></a>[167]</span>centuries, species; and how exactly
+  parallel is the case with that of the species of the horse-genus! For
+  myself, I venture confidently to look back thousands on thousands of
+  generations, and I see an animal striped like a zebra, but perhaps
+  otherwise very differently constructed, the common parent of our domestic
+  horse, whether or not it be descended from one or more wild stocks, of
+  the ass, the hemionus, quagga, and zebra.</p>
+
+  <p>He who believes that each equine species was independently created,
+  will, I presume, assert that each species has been created with a
+  tendency to vary, both under nature and under domestication, in this
+  particular manner, so as often to become striped like other species of
+  the genus; and that each has been created with a strong tendency, when
+  crossed with species inhabiting distant quarters of the world, to produce
+  hybrids resembling in their stripes, not their own parents, but other
+  species of the genus. To admit this view is, as it seems to me, to reject
+  a real for an unreal, or at least for an unknown, cause. It makes the
+  works of God a mere mockery and deception; I would almost as soon believe
+  with the old and ignorant cosmogonists, that fossil shells had never
+  lived, but had been created in stone so as to mock the shells now living
+  on the sea-shore.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Summary.</i>&mdash;Our ignorance of the laws of variation is
+  profound. Not in one case out of a hundred can we pretend to assign any
+  reason why this or that part differs, more or less, from the same part in
+  the parents. But whenever we have the means of instituting a comparison,
+  the same laws appear to have acted in producing the lesser differences
+  between varieties of the same species, and the greater differences
+  between species of the same genus. The external conditions of life, as
+  <!-- Page 168 --><span class="pagenum"><a
+  name="page168"></a>[168]</span>climate and food, &amp;c., seem to have
+  induced some slight modifications. Habit in producing constitutional
+  differences, and use in strengthening and disuse in weakening and
+  diminishing organs, seem to have been more potent in their effects.
+  Homologous parts tend to vary in the same way, and homologous parts tend
+  to cohere. Modifications in hard parts and in external parts sometimes
+  affect softer and internal parts. When one part is largely developed,
+  perhaps it tends to draw nourishment from the adjoining parts; and every
+  part of the structure which can be saved without detriment to the
+  individual, will be saved. Changes of structure at an early age will
+  generally affect parts subsequently developed; and there are very many
+  other correlations of growth, the nature of which we are utterly unable
+  to understand. Multiple parts are variable in number and in structure,
+  perhaps arising from such parts not having been closely specialised to
+  any particular function, so that their modifications have not been
+  closely checked by natural selection. It is probably from this same cause
+  that organic beings low in the scale of nature are more variable than
+  those which have their whole organisation more specialised, and are
+  higher in the scale. Rudimentary organs, from being useless, will be
+  disregarded by natural selection, and hence probably are variable.
+  Specific characters&mdash;that is, the characters which have come to
+  differ since the several species of the same genus branched off from a
+  common parent&mdash;are more variable than generic characters, or those
+  which have long been inherited, and have not differed within this same
+  period. In these remarks we have referred to special parts or organs
+  being still variable, because they have recently varied and thus come to
+  differ; but we have also seen in the second Chapter that the same
+  principle applies to the whole individual; <!-- Page 169 --><span
+  class="pagenum"><a name="page169"></a>[169]</span>for in a district where
+  many species of any genus are found&mdash;that is, where there has been
+  much former variation and differentiation, or where the manufactory of
+  new specific forms has been actively at work&mdash;there, on an average,
+  we now find most varieties or incipient species. Secondary sexual
+  characters are highly variable, and such characters differ much in the
+  species of the same group. Variability in the same parts of the
+  organisation has generally been taken advantage of in giving secondary
+  sexual differences to the sexes of the same species, and specific
+  differences to the several species of the same genus. Any part or organ
+  developed to an extraordinary size or in an extraordinary manner, in
+  comparison with the same part or organ in the allied species, must have
+  gone through an extraordinary amount of modification since the genus
+  arose; and thus we can understand why it should often still be variable
+  in a much higher degree than other parts; for variation is a
+  long-continued and slow process, and natural selection will in such cases
+  not as yet have had time to overcome the tendency to further variability
+  and to reversion to a less modified state. But when a species with any
+  extraordinarily-developed organ has become the parent of many modified
+  descendants&mdash;which on my view must be a very slow process, requiring
+  a long lapse of time&mdash;in this case, natural selection may readily
+  have succeeded in giving a fixed character to the organ, in however
+  extraordinary a manner it may be developed. Species inheriting nearly the
+  same constitution from a common parent and exposed to similar influences
+  will naturally tend to present analogous variations, and these same
+  species may occasionally revert to some of the characters of their
+  ancient progenitors. Although new and important modifications may not
+  arise from reversion and analogous <!-- Page 170 --><span
+  class="pagenum"><a name="page170"></a>[170]</span>variation, such
+  modifications will add to the beautiful and harmonious diversity of
+  nature.</p>
+
+  <p>Whatever the cause may be of each slight difference in the offspring
+  from their parents&mdash;and a cause for each must exist&mdash;it is the
+  steady accumulation, through natural selection, of such differences, when
+  beneficial to the individual, that gives rise to all the more important
+  modifications of structure, by which the innumerable beings on the face
+  of this earth are enabled to struggle with each other, and the best
+  adapted to survive.</p>
+
+<hr />
+
+<p><!-- Page 171 --><span class="pagenum"><a name="page171"></a>[171]</span></p>
+
+<h3>CHAPTER VI.</h3>
+
+<p class="cenhead"><span class="sc">Difficulties on Theory.</span></p>
+
+<blockquote class="b1n">
+
+  <p>Difficulties on the theory of descent with
+  modification&mdash;Transitions&mdash;Absence or rarity of transitional
+  varieties&mdash;Transitions in habits of life&mdash;Diversified habits in
+  the same species&mdash;Species with habits widely different from those of
+  their allies&mdash;Organs of extreme perfection&mdash;Means of
+  transition&mdash;Cases of difficulty&mdash;Natura non facit
+  saltum&mdash;Organs of small importance&mdash;Organs not in all cases
+  absolutely perfect&mdash;The law of Unity of Type and of the Conditions
+  of Existence embraced by the theory of Natural Selection.</p>
+
+</blockquote>
+
+  <p>Long before having arrived at this part of my work, a crowd of
+  difficulties will have occurred to the reader. Some of them are so grave
+  that to this day I can never reflect on them without being staggered;
+  but, to the best of my judgment, the greater number are only apparent,
+  and those that are real are not, I think, fatal to my theory.</p>
+
+  <p>These difficulties and objections may be classed under the following
+  heads:&mdash;Firstly, why, if species have descended from other species
+  by insensibly fine gradations, do we not everywhere see innumerable
+  transitional forms? Why is not all nature in confusion instead of the
+  species being, as we see them, well defined?</p>
+
+  <p>Secondly, is it possible that an animal having, for instance, the
+  structure and habits of a bat, could have been formed by the modification
+  of some animal with wholly different habits? Can we believe that natural
+  selection could produce, on the one hand, organs of trifling importance,
+  such as the tail of a giraffe, which serves as a fly-flapper, and, on
+  the other hand, organs of <!-- Page 172 --><span class="pagenum"><a
+  name="page172"></a>[172]</span>such wonderful structure, as the eye, of
+  which we hardly as yet fully understand the inimitable perfection?</p>
+
+  <p>Thirdly, can instincts be acquired and modified through natural
+  selection? What shall we say to so marvellous an instinct as that which
+  leads the bee to make cells, which has practically anticipated the
+  discoveries of profound mathematicians?</p>
+
+  <p>Fourthly, how can we account for species, when crossed, being sterile
+  and producing sterile offspring, whereas, when varieties are crossed,
+  their fertility is unimpaired?</p>
+
+  <p>The two first heads shall be here discussed&mdash;Instinct and
+  Hybridism in separate chapters.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the absence or rarity of transitional varieties.</i>&mdash;As
+  natural selection acts solely by the preservation of profitable
+  modifications, each new form will tend in a fully-stocked country to take
+  the place of, and finally to exterminate, its own less improved parent or
+  other less-favoured forms with which it comes into competition. Thus
+  extinction and natural selection will, as we have seen, go hand in hand.
+  Hence, if we look at each species as descended from some other unknown
+  form, both the parent and all the transitional varieties will generally
+  have been exterminated by the very process of formation and perfection of
+  the new form.</p>
+
+  <p>But, as by this theory innumerable transitional forms must have
+  existed, why do we not find them embedded in countless numbers in the
+  crust of the earth? It will be much more convenient to discuss this
+  question in the chapter on the Imperfection of the geological record; and
+  I will here only state that I believe the answer mainly lies in the
+  record being incomparably less perfect than is generally supposed; the
+  imperfection of the record being chiefly due to organic beings not
+  inhabiting <!-- Page 173 --><span class="pagenum"><a
+  name="page173"></a>[173]</span>profound depths of the sea, and to their
+  remains being embedded and preserved to a future age only in masses of
+  sediment sufficiently thick and extensive to withstand an enormous amount
+  of future degradation; and such fossiliferous masses can be accumulated
+  only where much sediment is deposited on the shallow bed of the sea,
+  whilst it slowly subsides. These contingencies will concur only rarely,
+  and after enormously long intervals. Whilst the bed of the sea is
+  stationary or is rising, or when very little sediment is being deposited,
+  there will be blanks in our geological history. The crust of the earth is
+  a vast museum; but the natural collections have been made only at
+  intervals of time immensely remote.</p>
+
+  <p>But it may be urged that when several closely-allied species inhabit
+  the same territory we surely ought to find at the present time many
+  transitional forms. Let us take a simple case: in travelling from north
+  to south over a continent, we generally meet at successive intervals with
+  closely allied or representative species, evidently filling nearly the
+  same place in the natural economy of the land. These representative
+  species often meet and interlock; and as the one becomes rarer and rarer,
+  the other becomes more and more frequent, till the one replaces the
+  other. But if we compare these species where they intermingle, they are
+  generally as absolutely distinct from each other in every detail of
+  structure as are specimens taken from the metropolis inhabited by each.
+  By my theory these allied species have descended from a common parent;
+  and during the process of modification, each has become adapted to the
+  conditions of life of its own region, and has supplanted and exterminated
+  its original parent and all the transitional varieties between its past
+  and present states. Hence we ought not to expect at the <!-- Page 174
+  --><span class="pagenum"><a name="page174"></a>[174]</span>present time
+  to meet with numerous transitional varieties in each region, though they
+  must have existed there, and may be embedded there in a fossil condition.
+  But in the intermediate region, having intermediate conditions of life,
+  why do we not now find closely-linking intermediate varieties? This
+  difficulty for a long time quite confounded me. But I think it can be in
+  large part explained.</p>
+
+  <p>In the first place we should be extremely cautious in inferring,
+  because an area is now continuous, that it has been continuous during a
+  long period. Geology would lead us to believe that almost every continent
+  has been broken up into islands even during the later tertiary periods;
+  and in such islands distinct species might have been separately formed
+  without the possibility of intermediate varieties existing in the
+  intermediate zones. By changes in the form of the land and of climate,
+  marine areas now continuous must often have existed within recent times
+  in a far less continuous and uniform condition than at present. But I
+  will pass over this way of escaping from the difficulty; for I believe
+  that many perfectly defined species have been formed on strictly
+  continuous areas; though I do not doubt that the formerly broken
+  condition of areas now continuous has played an important part in the
+  formation of new species, more especially with freely-crossing and
+  wandering animals.</p>
+
+  <p>In looking at species as they are now distributed over a wide area, we
+  generally find them tolerably numerous over a large territory, then
+  becoming somewhat abruptly rarer and rarer on the confines, and finally
+  disappearing. Hence the neutral territory between two representative
+  species is generally narrow in comparison with the territory proper to
+  each. We see the same fact in ascending mountains, and sometimes <!--
+  Page 175 --><span class="pagenum"><a name="page175"></a>[175]</span>it is
+  quite remarkable how abruptly, as Alph. de Candolle has observed, a
+  common alpine species disappears. The same fact has been noticed by E.
+  Forbes in sounding the depths of the sea with the dredge. To those who
+  look at climate and the physical conditions of life as the all-important
+  elements of distribution, these facts ought to cause surprise, as climate
+  and height or depth graduate away insensibly. But when we bear in mind
+  that almost every species, even in its metropolis, would increase
+  immensely in numbers, were it not for other competing species; that
+  nearly all either prey on or serve as prey for others; in short, that
+  each organic being is either directly or indirectly related in the most
+  important manner to other organic beings, we must see that the range of
+  the inhabitants of any country by no means exclusively depends on
+  insensibly changing physical conditions, but in large part on the
+  presence of other species, on which it depends, or by which it is
+  destroyed, or with which it comes into competition; and as these species
+  are already defined objects (however they may have become so), not
+  blending one into another by insensible gradations, the range of any one
+  species, depending as it does on the range of others, will tend to be
+  sharply defined. Moreover, each species on the confines of its range,
+  where it exists in lessened numbers, will, during fluctuations in the
+  number of its enemies or of its prey, or in the seasons, be extremely
+  liable to utter extermination; and thus its geographical range will come
+  to be still more sharply defined.</p>
+
+  <p>If I am right in believing that allied or representative species, when
+  inhabiting a continuous area, are generally so distributed that each has
+  a wide range, with a comparatively narrow neutral territory between them,
+  in which they become rather suddenly rarer and rarer; then, as varieties
+  do not essentially differ from species, <!-- Page 176 --><span
+  class="pagenum"><a name="page176"></a>[176]</span>the same rule will
+  probably apply to both; and if we in imagination adapt a varying species
+  to a very large area, we shall have to adapt two varieties to two large
+  areas, and a third variety to a narrow intermediate zone. The
+  intermediate variety, consequently, will exist in lesser numbers from
+  inhabiting a narrow and lesser area; and practically, as far as I can
+  make out, this rule holds good with varieties in a state of nature. I
+  have met with striking instances of the rule in the case of varieties
+  intermediate between well-marked varieties in the genus Balanus. And it
+  would appear from information given me by Mr. Watson, Dr. Asa Gray, and
+  Mr. Wollaston, that generally when varieties intermediate between two
+  other forms occur, they are much rarer numerically than the forms which
+  they connect. Now, if we may trust these facts and inferences, and
+  therefore conclude that varieties linking two other varieties together
+  have generally existed in lesser numbers than the forms which they
+  connect, then, I think, we can understand why intermediate varieties
+  should not endure for very long periods;&mdash;why as a general rule they
+  should be exterminated and disappear, sooner than the forms which they
+  originally linked together.</p>
+
+  <p>For any form existing in lesser numbers would, as already remarked,
+  run a greater chance of being exterminated than one existing in large
+  numbers; and in this particular case the intermediate form would be
+  eminently liable to the inroads of closely allied forms existing on both
+  sides of it. But a far more important consideration, as I believe, is
+  that, during the process of further modification, by which two varieties
+  are supposed on my theory to be converted and perfected into two distinct
+  species, the two which exist in larger numbers from inhabiting larger
+  areas, will have a great advantage over the intermediate variety, which
+  exists <!-- Page 177 --><span class="pagenum"><a
+  name="page177"></a>[177]</span>in smaller numbers in a narrow and
+  intermediate zone. For forms existing in larger numbers will always have
+  a better chance, within any given period, of presenting further
+  favourable variations for natural selection to seize on, than will the
+  rarer forms which exist in lesser numbers. Hence, the more common forms,
+  in the race for life, will tend to beat and supplant the less common
+  forms, for these will be more slowly modified and improved. It is the
+  same principle which, as I believe, accounts for the common species in
+  each country, as shown in the second chapter, presenting on an average a
+  greater number of well-marked varieties than do the rarer species. I may
+  illustrate what I mean by supposing three varieties of sheep to be kept,
+  one adapted to an extensive mountainous region; a second to a
+  comparatively narrow, hilly tract; and a third to wide plains at the
+  base; and that the inhabitants are all trying with equal steadiness and
+  skill to improve their stocks by selection; the chances in this case will
+  be strongly in favour of the great holders on the mountains or on the
+  plains improving their breeds more quickly than the small holders on the
+  intermediate narrow, hilly tract; and consequently the improved mountain
+  or plain breed will soon take the place of the less improved hill breed;
+  and thus the two breeds, which originally existed in greater numbers,
+  will come into close contact with each other, without the interposition
+  of the supplanted, intermediate hill-variety.</p>
+
+  <p>To sum up, I believe that species come to be tolerably well-defined
+  objects, and do not at any one period present an inextricable chaos of
+  varying and intermediate links: firstly, because new varieties are very
+  slowly formed, for variation is a very slow process, and natural
+  selection can do nothing until favourable <!-- Page 178 --><span
+  class="pagenum"><a name="page178"></a>[178]</span>variations chance to
+  occur, and until a place in the natural polity of the country can be
+  better filled by some modification of some one or more of its
+  inhabitants. And such new places will depend on slow changes of climate,
+  or on the occasional immigration of new inhabitants, and, probably, in a
+  still more important degree, on some of the old inhabitants becoming
+  slowly modified, with the new forms thus produced and the old ones acting
+  and reacting on each other. So that, in any one region and at any one
+  time, we ought only to see a few species presenting slight modifications
+  of structure in some degree permanent; and this assuredly we do see.</p>
+
+  <p>Secondly, areas now continuous must often have existed within the
+  recent period in isolated portions, in which many forms, more especially
+  amongst the classes which unite for each birth and wander much, may have
+  separately been rendered sufficiently distinct to rank as representative
+  species. In this case, intermediate varieties between the several
+  representative species and their common parent, must formerly have
+  existed in each broken portion of the land, but these links will have
+  been supplanted and exterminated during the process of natural selection,
+  so that they will no longer exist in a living state.</p>
+
+  <p>Thirdly, when two or more varieties have been formed in different
+  portions of a strictly continuous area, intermediate varieties will, it
+  is probable, at first have been formed in the intermediate zones, but
+  they will generally have had a short duration. For these intermediate
+  varieties will, from reasons already assigned (namely from what we know
+  of the actual distribution of closely allied or representative species,
+  and likewise of acknowledged varieties), exist in the intermediate zones
+  in lesser numbers than the varieties which they <!-- Page 179 --><span
+  class="pagenum"><a name="page179"></a>[179]</span>tend to connect. From
+  this cause alone the intermediate varieties will be liable to accidental
+  extermination; and during the process of further modification through
+  natural selection, they will almost certainly be beaten and supplanted by
+  the forms which they connect; for these from existing in greater numbers
+  will, in the aggregate, present more variation, and thus be further
+  improved through natural selection and gain further advantages.</p>
+
+  <p>Lastly, looking not to any one time, but to all time, if my theory be
+  true, numberless intermediate varieties, linking most closely all the
+  species of the same group together, must assuredly have existed; but the
+  very process of natural selection constantly tends, as has been so often
+  remarked, to exterminate the parent-forms and the intermediate links.
+  Consequently evidence of their former existence could be found only
+  amongst fossil remains, which are preserved, as we shall in a future
+  chapter attempt to show, in an extremely imperfect and intermittent
+  record.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the origin and transitions of organic beings with peculiar
+  habits and structure.</i>&mdash;It has been asked by the opponents of
+  such views as I hold, how, for instance, a land carnivorous animal could
+  have been converted into one with aquatic habits; for how could the
+  animal in its transitional state have subsisted? It would be easy to show
+  that within the same group carnivorous animals exist having every
+  intermediate grade between truly aquatic and strictly terrestrial habits;
+  and as each exists by a struggle for life, it is clear that each is well
+  adapted in its habits to its place in nature. Look at the Mustela vison
+  of North America, which has webbed feet and which resembles an otter in
+  its fur, short legs, and form of tail; during summer this animal <!--
+  Page 180 --><span class="pagenum"><a name="page180"></a>[180]</span>dives
+  for and preys on fish, but during the long winter it leaves the frozen
+  waters, and preys like other polecats on mice and land animals. If a
+  different case had been taken, and it had been asked how an insectivorous
+  quadruped could possibly have been converted into a flying bat, the
+  question would have been far more difficult, and I could have given no
+  answer. Yet I think such difficulties have very little weight.</p>
+
+  <p>Here, as on other occasions, I lie under a heavy disadvantage, for out
+  of the many striking cases which I have collected, I can give only one or
+  two instances of transitional habits and structures in closely allied
+  species of the same genus; and of diversified habits, either constant or
+  occasional, in the same species. And it seems to me that nothing less
+  than a long list of such cases is sufficient to lessen the difficulty in
+  any particular case like that of the bat.</p>
+
+  <p>Look at the family of squirrels; here we have the finest gradation
+  from animals with their tails only slightly flattened, and from others,
+  as Sir J. Richardson has remarked, with the posterior part of their
+  bodies rather wide and with the skin on their flanks rather full, to the
+  so-called flying squirrels; and flying squirrels have their limbs and
+  even the base of the tail united by a broad expanse of skin, which serves
+  as a parachute and allows them to glide through the air to an astonishing
+  distance from tree to tree. We cannot doubt that each structure is of use
+  to each kind of squirrel in its own country, by enabling it to escape
+  birds or beasts of prey, or to collect food more quickly, or, as there is
+  reason to believe, by lessening the danger from occasional falls. But it
+  does not follow from this fact that the structure of each squirrel is the
+  best that it is possible to conceive under all natural conditions. Let
+  the climate and vegetation change, let other competing <!-- Page 181
+  --><span class="pagenum"><a name="page181"></a>[181]</span>rodents or new
+  beasts of prey immigrate, or old ones become modified, and all analogy
+  would lead us to believe that some at least of the squirrels would
+  decrease in numbers or become exterminated, unless they also became
+  modified and improved in structure in a corresponding manner. Therefore,
+  I can see no difficulty, more especially under changing conditions of
+  life, in the continued preservation of individuals with fuller and fuller
+  flank-membranes, each modification being useful, each being propagated,
+  until by the accumulated effects of this process of natural selection, a
+  perfect so-called flying squirrel was produced.</p>
+
+  <p>Now look at the Galeopithecus or flying lemur, which formerly was
+  falsely ranked amongst bats. It has an extremely wide flank-membrane,
+  stretching from the corners of the jaw to the tail, and including the
+  limbs and the elongated fingers: the flank-membrane is, also, furnished
+  with an extensor muscle. Although no graduated links of structure, fitted
+  for gliding through the air, now connect the Galeopithecus with the other
+  Lemuridæ, yet I see no difficulty in supposing that such links formerly
+  existed, and that each had been formed by the same steps as in the case
+  of the less perfectly gliding squirrels; and that each grade of structure
+  was useful to its possessor. Nor can I see any insuperable difficulty in
+  further believing it possible that the membrane-connected fingers and
+  forearm of the Galeopithecus might be greatly lengthened by natural
+  selection; and this, as far as the organs of flight are concerned, would
+  convert it into a bat. In bats which have the wing-membrane extended from
+  the top of the shoulder to the tail, including the hind-legs, we perhaps
+  see traces of an apparatus originally constructed for gliding through the
+  air rather than for flight. <!-- Page 182 --><span class="pagenum"><a
+  name="page182"></a>[182]</span></p>
+
+  <p>If about a dozen genera of birds had become extinct or were unknown,
+  who would have ventured to have surmised that birds might have existed
+  which used their wings solely as flappers, like the logger-headed duck
+  (Micropterus of Eyton); as fins in the water and front legs on the land,
+  like the penguin; as sails, like the ostrich; and functionally for no
+  purpose, like the Apteryx. Yet the structure of each of these birds is
+  good for it, under the conditions of life to which it is exposed, for
+  each has to live by a struggle; but it is not necessarily the best
+  possible under all possible conditions. It must not be inferred from
+  these remarks that any of the grades of wing-structure here alluded to,
+  which perhaps may all have resulted from disuse, indicate the natural
+  steps by which birds have acquired their perfect power of flight; but
+  they serve, at least, to show what diversified means of transition are
+  possible.</p>
+
+  <p>Seeing that a few members of such water-breathing classes as the
+  Crustacea and Mollusca are adapted to live on the land; and seeing that
+  we have flying birds and mammals, flying insects of the most diversified
+  types, and formerly had flying reptiles, it is conceivable that
+  flying-fish, which now glide far through the air, slightly rising and
+  turning by the aid of their fluttering fins, might have been modified
+  into perfectly winged animals. If this had been effected, who would have
+  ever imagined that in an early transitional state they had been
+  inhabitants of the open ocean, and had used their incipient organs of
+  flight exclusively, as far as we know, to escape being devoured by other
+  fish?</p>
+
+  <p>When we see any structure highly perfected for any particular habit,
+  as the wings of a bird for flight, we should bear in mind that animals
+  displaying early <!-- Page 183 --><span class="pagenum"><a
+  name="page183"></a>[183]</span>transitional grades of the structure will
+  seldom continue to exist to the present day, for they will have been
+  supplanted by the very process of perfection through natural selection.
+  Furthermore, we may conclude that transitional grades between structures
+  fitted for very different habits of life will rarely have been developed
+  at an early period in great numbers and under many subordinate forms.
+  Thus, to return to our imaginary illustration of the flying-fish, it does
+  not seem probable that fishes capable of true flight would have been
+  developed under many subordinate forms, for taking prey of many kinds in
+  many ways, on the land and in the water, until their organs of flight had
+  come to a high stage of perfection, so as to have given them a decided
+  advantage over other animals in the battle for life. Hence the chance of
+  discovering species with transitional grades of structure in a fossil
+  condition will always be less, from their having existed in lesser
+  numbers, than in the case of species with fully developed structures.</p>
+
+  <p>I will now give two or three instances of diversified and of changed
+  habits in the individuals of the same species. When either case occurs,
+  it would be easy for natural selection to fit the animal, by some
+  modification of its structure, for its changed habits, or exclusively for
+  one of its several different habits. But it is difficult to tell, and
+  immaterial for us, whether habits generally change first and structure
+  afterwards; or whether slight modifications of structure lead to changed
+  habits; both probably often change almost simultaneously. Of cases of
+  changed habits it will suffice merely to allude to that of the many
+  British insects which now feed on exotic plants, or exclusively on
+  artificial substances. Of diversified habits innumerable instances could
+  be given: I have often watched a tyrant flycatcher (Saurophagus
+  sulphuratus) in South America, hovering over one spot <!-- Page 184
+  --><span class="pagenum"><a name="page184"></a>[184]</span>and then
+  proceeding to another, like a kestrel, and at other times standing
+  stationary on the margin of water, and then dashing like a kingfisher at
+  a fish. In our own country the larger titmouse (Parus major) may be seen
+  climbing branches, almost like a creeper; it often, like a shrike, kills
+  small birds by blows on the head; and I have many times seen and heard it
+  hammering the seeds of the yew on a branch, and thus breaking them like a
+  nuthatch. In North America the black bear was seen by Hearne swimming for
+  hours with widely open mouth, thus catching, almost like a whale, insects
+  in the water.</p>
+
+  <p>As we sometimes see individuals of a species following habits widely
+  different from those of their own species and of the other species of the
+  same genus, we might expect, on my theory, that such individuals would
+  occasionally have given rise to new species, having anomalous habits, and
+  with their structure either slightly or considerably modified from that
+  of their proper type. And such instances do occur in nature. Can a more
+  striking instance of adaptation be given than that of a woodpecker for
+  climbing trees and for seizing insects in the chinks of the bark? Yet in
+  North America there are woodpeckers which feed largely on fruit, and
+  others with elongated wings which chase insects on the wing; and on the
+  plains of La Plata, where not a tree grows, there is a woodpecker, which
+  in every essential part of its organisation, even in its colouring, in
+  the harsh tone of its voice, and undulatory flight, told me plainly of
+  its close blood-relationship to our common species; yet it is a
+  woodpecker which never climbs a tree!</p>
+
+  <p>Petrels are the most aërial and oceanic of birds, yet in the quiet
+  Sounds of Tierra del Fuego, the Puffinuria berardi, in its general
+  habits, in its astonishing power of diving, its manner of swimming, and
+  of flying when <!-- Page 185 --><span class="pagenum"><a
+  name="page185"></a>[185]</span>unwillingly it takes flight, would be
+  mistaken by any one for an auk or grebe; nevertheless, it is essentially
+  a petrel, but with many parts of its organisation profoundly modified. On
+  the other hand, the acutest observer by examining the dead body of the
+  water-ouzel would never have suspected its sub-aquatic habits; yet this
+  anomalous member of the strictly terrestrial thrush family wholly
+  subsists by diving,&mdash;grasping the stones with its feet and using its
+  wings under water.</p>
+
+  <p>He who believes that each being has been created as we now see it,
+  must occasionally have felt surprise when he has met with an animal
+  having habits and structure not at all in agreement. What can be plainer
+  than that the webbed feet of ducks and geese are formed for swimming? yet
+  there are upland geese with webbed feet which rarely or never go near the
+  water; and no one except Audubon has seen the frigate-bird, which has all
+  its four toes webbed, alight on the surface of the sea. On the other hand
+  grebes and coots are eminently aquatic, although their toes are only
+  bordered by membrane. What seems plainer than that the long toes of
+  grallatores are formed for walking over swamps and floating plants, yet
+  the water-hen is nearly as aquatic as the coot; and the landrail nearly
+  as terrestrial as the quail or partridge. In such cases, and many others
+  could be given, habits have changed without a corresponding change of
+  structure. The webbed feet of the upland goose may be said to have become
+  rudimentary in function, though not in structure. In the frigate-bird,
+  the deeply-scooped membrane between the toes shows that structure has
+  begun to change.</p>
+
+  <p>He who believes in separate and innumerable acts of creation will say,
+  that in these cases it has pleased the Creator to cause a being of one
+  type to take the place of one of another type; but this seems to me only
+  <!-- Page 186 --><span class="pagenum"><a
+  name="page186"></a>[186]</span>restating the fact in dignified language.
+  He who believes in the struggle for existence and in the principle of
+  natural selection, will acknowledge that every organic being is
+  constantly endeavouring to increase in numbers; and that if any one being
+  vary ever so little, either in habits or structure, and thus gain an
+  advantage over some other inhabitant of the country, it will seize on the
+  place of that inhabitant, however different it may be from its own place.
+  Hence it will cause him no surprise that there should be geese and
+  frigate-birds with webbed feet, living on the dry land or most rarely
+  alighting on the water; that there should be long-toed corncrakes living
+  in meadows instead of in swamps; that there should be woodpeckers where
+  not a tree grows; that there should be diving thrushes, and petrels with
+  the habits of auks.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Organs of extreme perfection and complication.</i>&mdash;To suppose
+  that the eye, with all its inimitable contrivances for adjusting the
+  focus to different distances, for admitting different amounts of light,
+  and for the correction of spherical and chromatic aberration, could have
+  been formed by natural selection, seems, I freely confess, absurd in the
+  highest possible degree. Yet reason tells me, that if numerous gradations
+  from a perfect and complex eye to one very imperfect and simple, each
+  grade being useful to its possessor, can be shown to exist; if further,
+  the eye does vary ever so slightly, and the variations be inherited,
+  which is certainly the case; and if any variation or modification in the
+  organ be ever useful to an animal under changing conditions of life, then
+  the difficulty of believing that a perfect and complex eye could be
+  formed by natural selection, though insuperable by our imagination, can
+  hardly be considered real. How a nerve comes to be sensitive to <!-- Page
+  187 --><span class="pagenum"><a name="page187"></a>[187]</span>light,
+  hardly concerns us more than how life itself first originated; but I may
+  remark that several facts make me suspect that any sensitive nerve may be
+  rendered sensitive to light, and likewise to those coarser vibrations of
+  the air which produce sound.</p>
+
+  <p>In looking for the gradations by which an organ in any species has
+  been perfected, we ought to look exclusively to its lineal ancestors; but
+  this is scarcely ever possible, and we are forced in each case to look to
+  species of the same group, that is to the collateral descendants from the
+  same original parent-form, in order to see what gradations are possible,
+  and for the chance of some gradations having been transmitted from the
+  earlier stages of descent, in an unaltered or little altered condition.
+  Amongst existing Vertebrata, we find but a small amount of gradation in
+  the structure of the eye, and from fossil species we can learn nothing on
+  this head. In this great class we should probably have to descend far
+  beneath the lowest known fossiliferous stratum to discover the earlier
+  stages, by which the eye has been perfected.</p>
+
+  <p>In the Articulata we can commence a series with an optic nerve merely
+  coated with pigment, and without any other mechanism; and from this low
+  stage, numerous gradations of structure, branching off in two
+  fundamentally different lines, can be shown to exist, until we reach a
+  moderately high stage of perfection. In certain crustaceans, for
+  instance, there is a double cornea, the inner one divided into facets,
+  within each of which there is a lens-shaped swelling. In other
+  crustaceans the transparent cones which are coated by pigment, and which
+  properly act only by excluding lateral pencils of light, are convex at
+  their upper ends and must act by convergence; and at their lower ends
+  there seems to be an imperfect vitreous substance. <!-- Page 188 --><span
+  class="pagenum"><a name="page188"></a>[188]</span>With these facts, here
+  far too briefly and imperfectly given, which show that there is much
+  graduated diversity in the eyes of living crustaceans, and bearing in
+  mind how small the number of living animals is in proportion to those
+  which have become extinct, I can see no very great difficulty (not more
+  than in the case of many other structures) in believing that natural
+  selection has converted the simple apparatus of an optic nerve merely
+  coated with pigment and invested by transparent membrane, into an optical
+  instrument as perfect as is possessed by any member of the great
+  Articulate class.</p>
+
+  <p>He who will go thus far, if he find on finishing this treatise that
+  large bodies of facts, otherwise inexplicable, can be explained by the
+  theory of descent, ought not to hesitate to go further, and to admit that
+  a structure even as perfect as the eye of an eagle might be formed by
+  natural selection, although in this case he does not know any of the
+  transitional grades. His reason ought to conquer his imagination; though
+  I have felt the difficulty far too keenly to be surprised at any degree
+  of hesitation in extending the principle of natural selection to such
+  startling lengths.</p>
+
+  <p>It is scarcely possible to avoid comparing the eye to a telescope. We
+  know that this instrument has been perfected by the long-continued
+  efforts of the highest human intellects; and we naturally infer that the
+  eye has been formed by a somewhat analogous process. But may not this
+  inference be presumptuous? Have we any right to assume that the Creator
+  works by intellectual powers like those of man? If we must compare the
+  eye to an optical instrument, we ought in imagination to take a thick
+  layer of transparent tissue, with a nerve sensitive to light beneath, and
+  then suppose every part of this layer to be continually changing <!--
+  Page 189 --><span class="pagenum"><a
+  name="page189"></a>[189]</span>slowly in density, so as to separate into
+  layers of different densities and thicknesses, placed at different
+  distances from each other, and with the surfaces of each layer slowly
+  changing in form. Further we must suppose that there is a power always
+  intently watching each slight accidental alteration in the transparent
+  layers; and carefully selecting each alteration which, under varied
+  circumstances, may in any way, or in any degree, tend to produce a
+  distincter image. We must suppose each new state of the instrument to be
+  multiplied by the million; and each to be preserved till a better be
+  produced, and then the old ones to be destroyed. In living bodies,
+  variation will cause the slight alterations, generation will multiply
+  them almost infinitely, and natural selection will pick out with unerring
+  skill each improvement. Let this process go on for millions on millions
+  of years; and during each year on millions of individuals of many kinds;
+  and may we not believe that a living optical instrument might thus be
+  formed as superior to one of glass, as the works of the Creator are to
+  those of man?</p>
+
+  <p>If it could be demonstrated that any complex organ existed, which
+  could not possibly have been formed by numerous, successive, slight
+  modifications, my theory would absolutely break down. But I can find out
+  no such case. No doubt many organs exist of which we do not know the
+  transitional grades, more especially if we look to much-isolated species,
+  round which, according to my theory, there has been much extinction. Or
+  again, if we look to an organ common to all the members of a large class,
+  for in this latter case the organ must have been first formed at an
+  extremely remote period, since which all the many members of the class
+  have been developed; and in order to discover the early transitional
+  grades through which the organ has <!-- Page 190 --><span
+  class="pagenum"><a name="page190"></a>[190]</span>passed, we should have
+  to look to very ancient ancestral forms, long since become extinct.</p>
+
+  <p>We should be extremely cautious in concluding that an organ could not
+  have been formed by transitional gradations of some kind. Numerous cases
+  could be given amongst the lower animals of the same organ performing at
+  the same time wholly distinct functions; thus the alimentary canal
+  respires, digests, and excretes in the larva of the dragon-fly and in the
+  fish Cobites. In the Hydra, the animal may be turned inside out, and the
+  exterior surface will then digest and the stomach respire. In such cases
+  natural selection might easily specialise, if any advantage were thus
+  gained, a part or organ, which had performed two functions, for one
+  function alone, and thus wholly change its nature by insensible steps.
+  Two distinct organs sometimes perform simultaneously the same function in
+  the same individual; to give one instance, there are fish with gills or
+  branchiæ that breathe the air dissolved in the water, at the same time
+  that they breathe free air in their swimbladders, this latter organ
+  having a ductus pneumaticus for its supply, and being divided by highly
+  vascular partitions. In these cases one of the two organs might with ease
+  be modified and perfected so as to perform all the work by itself, being
+  aided during the process of modification by the other organ; and then
+  this other organ might be modified for some other and quite distinct
+  purpose, or be quite obliterated.</p>
+
+  <p>The illustration of the swimbladder in fishes is a good one, because
+  it shows us clearly the highly important fact that an organ originally
+  constructed for one purpose, namely flotation, may be converted into one
+  for a wholly different purpose, namely respiration. The swimbladder has,
+  also, been worked in as an accessory to the auditory organs of certain
+  fish, or, for I do not know <!-- Page 191 --><span class="pagenum"><a
+  name="page191"></a>[191]</span>which view is now generally held, a part
+  of the auditory apparatus has been worked in as a complement to the
+  swimbladder. All physiologists admit that the swimbladder is homologous,
+  or "ideally similar" in position and structure with the lungs of the
+  higher vertebrate animals: hence there seems to me to be no great
+  difficulty in believing that natural selection has actually converted a
+  swimbladder into a lung, or organ used exclusively for respiration.</p>
+
+  <p>I can, indeed, hardly doubt that all vertebrate animals having true
+  lungs have descended by ordinary generation from an ancient prototype, of
+  which we know nothing, furnished with a floating apparatus or
+  swimbladder. We can thus, as I infer from Professor Owen's interesting
+  description of these parts, understand the strange fact that every
+  particle of food and drink which we swallow has to pass over the orifice
+  of the trachea, with some risk of falling into the lungs, notwithstanding
+  the beautiful contrivance by which the glottis is closed. In the higher
+  Vertebrata the branchiæ have wholly disappeared&mdash;the slits on the
+  sides of the neck and the loop-like course of the arteries still marking
+  in the embryo their former position. But it is conceivable that the now
+  utterly lost branchiæ might have been gradually worked in by natural
+  selection for some quite distinct purpose: in the same manner as, on the
+  view entertained by some naturalists that the branchiæ and dorsal scales
+  of Annelids are homologous with the wings and wing-covers of insects, it
+  is probable that organs which at a very ancient period served for
+  respiration have been actually converted into organs of flight.</p>
+
+  <p>In considering transitions of organs, it is so important to bear in
+  mind the probability of conversion from one function to another, that I
+  will give one more instance. Pedunculated cirripedes have two minute
+  folds of skin, <!-- Page 192 --><span class="pagenum"><a
+  name="page192"></a>[192]</span>called by me the ovigerous frena, which
+  serve, through the means of a sticky secretion, to retain the eggs until
+  they are hatched within the sack. These cirripedes have no branchiæ, the
+  whole surface of the body and sack, including the small frena, serving
+  for respiration. The Balanidæ or sessile cirripedes, on the other hand,
+  have no ovigerous frena, the eggs lying loose at the bottom of the sack,
+  in the well-enclosed shell; but they have large folded branchiæ. Now I
+  think no one will dispute that the ovigerous frena in the one family are
+  strictly homologous with the branchiæ of the other family; indeed, they
+  graduate into each other. Therefore I do not doubt that little folds of
+  skin, which originally served as ovigerous frena, but which, likewise,
+  very slightly aided the act of respiration, have been gradually converted
+  by natural selection into branchiæ, simply through an increase in their
+  size and the obliteration of their adhesive glands. If all pedunculated
+  cirripedes had become extinct, and they have already suffered far more
+  extinction than have sessile cirripedes, who would ever have imagined
+  that the branchiæ in this latter family had originally existed as organs
+  for preventing the ova from being washed out of the sack?</p>
+
+  <p>Although we must be extremely cautious in concluding that any organ
+  could not possibly have been produced by successive transitional
+  gradations, yet, undoubtedly, grave cases of difficulty occur, some of
+  which will be discussed in my future work.</p>
+
+  <p>One of the gravest is that of neuter insects, which are often very
+  differently constructed from either the males or fertile females; but
+  this case will be treated of in the next chapter. The electric organs of
+  fishes offer another case of special difficulty; it is impossible to
+  conceive by what steps these wondrous organs have been produced; but, as
+  Owen and others have remarked, <!-- Page 193 --><span class="pagenum"><a
+  name="page193"></a>[193]</span>their intimate structure closely resembles
+  that of common muscle; and as it has lately been shown that Rays have an
+  organ closely analogous to the electric apparatus, and yet do not, as
+  <span class="correction" title="text reads `Matteucei' - index has Matteucci"
+  >Matteucci</span> asserts, discharge any electricity, we must own that we
+  are far too ignorant to argue that no transition of any kind is
+  possible.</p>
+
+  <p>The electric organs offer another and even more serious difficulty;
+  for they occur in only about a dozen fishes, of which several are widely
+  remote in their affinities. Generally when the same organ appears in
+  several members of the same class, especially if in members having very
+  different habits of life, we may attribute its presence to inheritance
+  from a common ancestor; and its absence in some of the members to its
+  loss through disuse or natural selection. But if the electric organs had
+  been inherited from one ancient progenitor thus provided, we might have
+  expected that all electric fishes would have been specially related to
+  each other. Nor does geology at all lead to the belief that formerly most
+  fishes had electric organs, which most of their modified descendants have
+  lost. The presence of luminous organs in a few insects, belonging to
+  different families and orders, offers a parallel case of difficulty.
+  Other cases could be given; for instance in plants, the very curious
+  contrivance of a mass of pollen-grains, borne on a foot-stalk with a
+  sticky gland at the end, is the same in Orchis and
+  Asclepias,&mdash;genera almost as remote as possible amongst flowering
+  plants. In all these cases of two very distinct species furnished with
+  apparently the same anomalous organ, it should be observed that, although
+  the general appearance and function of the organ may be the same, yet
+  some fundamental difference can generally be detected. I am inclined to
+  believe that in nearly the same way as two men have sometimes
+  independently hit on <!-- Page 194 --><span class="pagenum"><a
+  name="page194"></a>[194]</span>the very same invention, so natural
+  selection, working for the good of each being and taking advantage of
+  analogous variations, has sometimes modified in very nearly the same
+  manner two parts in two organic beings, which beings owe but little of
+  their structure in common to inheritance from the same ancestor.</p>
+
+  <p>Although in many cases it is most difficult to conjecture by what
+  transitions organs could have arrived at their present state; yet,
+  considering that the proportion of living and known forms to the extinct
+  and unknown is very small, I have been astonished how rarely an organ can
+  be named, towards which no transitional grade is known to lead. The truth
+  of this remark is indeed shown by that old but somewhat exaggerated canon
+  in natural history of "Natura non facit saltum." We meet with this
+  admission in the writings of almost every experienced naturalist; or, as
+  Milne Edwards has well expressed it, Nature is prodigal in variety, but
+  niggard in innovation. Why, on the theory of Creation, should this be so?
+  Why should all the parts and organs of many independent beings, each
+  supposed to have been separately created for its proper place in nature,
+  be so commonly linked together by graduated steps? Why should not Nature
+  have taken a leap from structure to structure? On the theory of natural
+  selection, we can clearly understand why she should not; for natural
+  selection can act only by taking advantage of slight successive
+  variations; she can never take a leap, but must advance by the shortest
+  and slowest steps.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Organs of little apparent importance.</i>&mdash;As natural
+  selection acts by life and death,&mdash;by the preservation of
+  individuals with any favourable variation, and by the destruction of
+  those with any unfavourable deviation of structure,&mdash;I have
+  sometimes felt much difficulty in <!-- Page 195 --><span
+  class="pagenum"><a name="page195"></a>[195]</span>understanding the
+  origin of simple parts, of which the importance does not seem sufficient
+  to cause the preservation of successively varying individuals. I have
+  sometimes felt as much difficulty, though of a very different kind, on
+  this head, as in the case of an organ as perfect and complex as the
+  eye.</p>
+
+  <p>In the first place, we are much too ignorant in regard to the whole
+  economy of any one organic being, to say what slight modifications would
+  be of importance or not. In a former chapter I have given instances of
+  most trifling characters, such as the down on fruit and the colour of its
+  flesh, which, from determining the attacks of insects or from being
+  correlated with constitutional differences, might assuredly be acted on
+  by natural selection. The tail of the giraffe looks like an artificially
+  constructed fly-flapper; and it seems at first incredible that this could
+  have been adapted for its present purpose by successive slight
+  modifications, each better and better, for so trifling an object as
+  driving away flies; yet we should pause before being too positive even in
+  this case, for we know that the distribution and existence of cattle and
+  other animals in South America absolutely depends on their power of
+  resisting the attacks of insects: so that individuals which could by any
+  means defend themselves from these small enemies, would be able to range
+  into new pastures and thus gain a great advantage. It is not that the
+  larger quadrupeds are actually destroyed (except in some rare cases) by
+  flies, but they are incessantly harassed and their strength reduced, so
+  that they are more subject to disease, or not so well enabled in a coming
+  dearth to search for food, or to escape from beasts of prey.</p>
+
+  <p>Organs now of trifling importance have probably in some cases been of
+  high importance to an early progenitor, and, after having been slowly
+  perfected at a <!-- Page 196 --><span class="pagenum"><a
+  name="page196"></a>[196]</span>former period, have been transmitted in
+  nearly the same state, although now become of very slight use; and any
+  actually injurious deviations in their structure will always have been
+  checked by natural selection. Seeing how important an organ of locomotion
+  the tail is in most aquatic animals, its general presence and use for
+  many purposes in so many land animals, which in their lungs or modified
+  swimbladders betray their aquatic origin, may perhaps be thus accounted
+  for. A well-developed tail having been formed in an aquatic animal, it
+  might subsequently come to be worked in for all sorts of purposes, as a
+  fly-flapper, an organ of prehension, or as an aid in turning, as with the
+  dog, though the aid must be slight, for the hare, with hardly any tail,
+  can double quickly enough.</p>
+
+  <p>In the second place, we may sometimes attribute importance to
+  characters which are really of very little importance, and which have
+  originated from quite secondary causes, independently of natural
+  selection. We should remember that climate, food, &amp;c., probably have
+  some little direct influence on the organisation; that characters
+  reappear from the law of reversion; that correlation of growth will have
+  had a most important influence in modifying various structures; and
+  finally, that sexual selection will often have largely modified the
+  external characters of animals having a will, to give one male an
+  advantage in fighting with another or in charming the females. Moreover
+  when a modification of structure has primarily arisen from the above or
+  other unknown causes, it may at first have been of no advantage to the
+  species, but may subsequently have been taken advantage of by the
+  descendants of the species under new conditions of life and with newly
+  acquired habits.</p>
+
+  <p>To give a few instances to illustrate these latter <!-- Page 197
+  --><span class="pagenum"><a name="page197"></a>[197]</span>remarks. If
+  green woodpeckers alone had existed, and we did not know that there were
+  many black and pied kinds, I dare say that we should have thought that
+  the green colour was a beautiful adaptation to hide this tree-frequenting
+  bird from its enemies; and consequently that it was a character of
+  importance and might have been acquired through natural selection; as it
+  is, I have no doubt that the colour is due to some quite distinct cause,
+  probably to sexual selection. A trailing bamboo in the Malay Archipelago
+  climbs the loftiest trees by the aid of exquisitely constructed hooks
+  clustered around the ends of the branches, and this contrivance, no
+  doubt, is of the highest service to the plant; but as we see nearly
+  similar hooks on many trees which are not climbers, the hooks on the
+  bamboo may have arisen from unknown laws of growth, and have been
+  subsequently taken advantage of by the plant undergoing further
+  modification and becoming a climber. The naked skin on the head of a
+  vulture is generally looked at as a direct adaptation for wallowing in
+  putridity; and so it may be, or it may possibly be due to the direct
+  action of putrid matter; but we should be very cautious in drawing any
+  such inference, when we see that the skin on the head of the
+  clean-feeding male turkey is likewise naked. The sutures in the skulls of
+  young mammals have been advanced as a beautiful adaptation for aiding
+  parturition, and no doubt they facilitate, or may be indispensable for
+  this act; but as sutures occur in the skulls of young birds and reptiles,
+  which have only to escape from a broken egg, we may infer that this
+  structure has arisen from the laws of growth, and has been taken
+  advantage of in the parturition of the higher animals.</p>
+
+  <p>We are profoundly ignorant of the causes producing slight and
+  unimportant variations; and we are <!-- Page 198 --><span
+  class="pagenum"><a name="page198"></a>[198]</span>immediately made
+  conscious of this by reflecting on the differences in the breeds of our
+  domesticated animals in different countries,&mdash;more especially in the
+  less civilised countries where there has been but little artificial
+  selection. Careful observers are convinced that a damp climate affects
+  the growth of the hair, and that with the hair the horns are correlated.
+  Mountain breeds always differ from lowland breeds; and a mountainous
+  country would probably affect the hind limbs from exercising them more,
+  and possibly even the form of the pelvis; and then by the law of
+  homologous variation, the front limbs and even the head would probably be
+  affected. The shape, also, of the pelvis might affect by pressure the
+  shape of the head of the young in the womb. The laborious breathing
+  necessary in high regions would, we have some reason to believe, increase
+  the size of the chest; and again correlation would come into play.
+  Animals kept by savages in different countries often have to struggle for
+  their own subsistence, and would be exposed to a certain extent to
+  natural selection, and individuals with slightly different constitutions
+  would succeed best under different climates; and there is reason to
+  believe that constitution and colour are correlated. A good observer,
+  also, states that in cattle susceptibility to the attacks of flies is
+  correlated with colour, as is the liability to be poisoned by certain
+  plants; so that colour would be thus subjected to the action of natural
+  selection. But we are far too ignorant to speculate on the relative
+  importance of the several known and unknown laws of variation; and I have
+  here alluded to them only to show that, if we are unable to account for
+  the characteristic differences of our domestic breeds, which nevertheless
+  we generally admit to have arisen through ordinary generation, we ought
+  not to lay too much stress on our ignorance of the precise cause <!--
+  Page 199 --><span class="pagenum"><a name="page199"></a>[199]</span>of
+  the slight analogous differences between species. I might have adduced
+  for this same purpose the differences between the races of man, which are
+  so strongly marked; I may add that some little light can apparently be
+  thrown on the origin of these differences, chiefly through sexual
+  selection of a particular kind, but without here entering on copious
+  details my reasoning would appear frivolous.</p>
+
+  <p>The foregoing remarks lead me to say a few words on the protest lately
+  made by some naturalists, against the utilitarian doctrine that every
+  detail of structure has been produced for the good of its possessor. They
+  believe that very many structures have been created for beauty in the
+  eyes of man, or for mere variety. This doctrine, if true, would be
+  absolutely fatal to my theory. Yet I fully admit that many structures are
+  of no direct use to their possessors. Physical conditions probably have
+  had some little effect on structure, quite independently of any good thus
+  gained. Correlation of growth has no doubt played a most important part,
+  and a useful modification of one part will often have entailed on other
+  parts diversified changes of no direct use. So again characters which
+  formerly were useful, or which formerly had arisen from correlation of
+  growth, or from other unknown cause, may reappear from the law of
+  reversion, though now of no direct use. The effects of sexual selection,
+  when displayed in beauty to charm the females, can be called useful only
+  in rather a forced sense. But by far the most important consideration is
+  that the chief part of the organisation of every being is simply due to
+  inheritance; and consequently, though each being assuredly is well fitted
+  for its place in nature, many structures now have no direct relation to
+  the habits of life of each species. Thus, we can hardly believe that the
+  webbed feet of the upland <!-- Page 200 --><span class="pagenum"><a
+  name="page200"></a>[200]</span>goose or of the frigate-bird are of
+  special use to these birds; we cannot believe that the same bones in the
+  arm of the monkey, in the fore-leg of the horse, in the wing of the bat,
+  and in the nipper of the seal, are of special use to these animals. We
+  may safely attribute these structures to inheritance. But to the
+  progenitor of the upland goose and of the frigate-bird, webbed feet no
+  doubt were as useful as they now are to the most aquatic of existing
+  birds. So we may believe that the progenitor of the seal had not a
+  nipper, but a foot with five toes fitted for walking or grasping; and we
+  may further venture to believe that the several bones in the limbs of the
+  monkey, horse, and bat, which have been inherited from a common
+  progenitor, were formerly of more special use to that progenitor, or its
+  progenitors, than they now are to these animals having such widely
+  diversified habits. Therefore we may infer that these several bones might
+  have been acquired through natural selection, subjected formerly, as now,
+  to the several laws of inheritance, reversion, correlation of growth,
+  &amp;c. Hence every detail of structure in every living creature (making
+  some little allowance for the direct action of physical conditions) may
+  be viewed, either as having been of special use to some ancestral form,
+  or as being now of special use to the descendants of this
+  form&mdash;either directly, or indirectly through the complex laws of
+  growth.</p>
+
+  <p>Natural selection cannot possibly produce any modification in any one
+  species exclusively for the good of another species; though throughout
+  nature one species incessantly takes advantage of, and profits by, the
+  structure of another. But natural selection can and does often produce
+  structures for the direct injury of other species, as we see in the fang
+  of the adder, and in the ovipositor of the ichneumon, by which its eggs
+  are <!-- Page 201 --><span class="pagenum"><a
+  name="page201"></a>[201]</span><span class="correction" title="text reads `depo-sisted' across page break"
+  >deposited</span> in the living bodies of other insects. If it could be
+  proved that any part of the structure of any one species had been formed
+  for the exclusive good of another species, it would annihilate my theory,
+  for such could not have been produced through natural selection. Although
+  many statements may be found in works on natural history to this effect,
+  I cannot find even one which seems to me of any weight. It is admitted
+  that the rattlesnake has a poison-fang for its own defence and for the
+  destruction of its prey; but some authors suppose that at the same time
+  this snake is furnished with a rattle for its own injury, namely, to warn
+  its prey to escape. I would almost as soon believe that the cat curls the
+  end of its tail when preparing to spring, in order to warn the doomed
+  mouse. But I have not space here to enter on this and other such
+  cases.</p>
+
+  <p>Natural selection will never produce in a being anything injurious to
+  itself, for natural selection acts solely by and for the good of each. No
+  organ will be formed, as Paley has remarked, for the purpose of causing
+  pain or for doing an injury to its possessor. If a fair balance be struck
+  between the good and evil caused by each part, each will be found on the
+  whole advantageous. After the lapse of time, under changing conditions of
+  life, if any part comes to be injurious, it will be modified; or if it be
+  not so, the being will become extinct, as myriads have become
+  extinct.</p>
+
+  <p>Natural selection tends only to make each organic being as perfect as,
+  or slightly more perfect than, the other inhabitants of the same country
+  with which it has to struggle for existence. And we see that this is the
+  degree of perfection attained under nature. The endemic productions of
+  New Zealand, for instance, are perfect one compared with another; but
+  they are now rapidly yielding before the advancing legions of plants <!--
+  Page 202 --><span class="pagenum"><a name="page202"></a>[202]</span>and
+  animals introduced from Europe. Natural selection will not produce
+  absolute perfection, nor do we always meet, as far as we can judge, with
+  this high standard under nature. The correction for the aberration of
+  light is said, on high authority, not to be perfect even in that most
+  perfect organ, the eye. If our reason leads us to admire with enthusiasm
+  a multitude of inimitable contrivances in nature, this same reason tells
+  us, though we may easily err on both sides, that some other contrivances
+  are less perfect. Can we consider the sting of the wasp or of the bee as
+  perfect, which, when used against many attacking animals, cannot be
+  withdrawn, owing to the backward serratures, and so inevitably causes the
+  death of the insect by tearing out its viscera?</p>
+
+  <p>If we look at the sting of the bee, as having originally existed in a
+  remote progenitor as a boring and serrated instrument, like that in so
+  many members of the same great order, and which has been modified but not
+  perfected for its present purpose, with the poison originally adapted to
+  cause galls subsequently intensified, we can perhaps understand how it is
+  that the use of the sting should so often cause the insect's own death:
+  for if on the whole the power of stinging be useful to the community, it
+  will fulfil all the requirements of natural selection, though it may
+  cause the death of some few members. If we admire the truly wonderful
+  power of scent by which the males of many insects find their females, can
+  we admire the production for this single purpose of thousands of drones,
+  which are utterly useless to the community for any other end, and which
+  are ultimately slaughtered by their industrious and sterile sisters? It
+  may be difficult, but we ought to admire the savage instinctive hatred of
+  the queen-bee, which urges her instantly to destroy the <!-- Page 203
+  --><span class="pagenum"><a name="page203"></a>[203]</span>young queens
+  her daughters as soon as born, or to perish herself in the combat; for
+  undoubtedly this is for the good of the community; and maternal love or
+  maternal hatred, though the latter fortunately is most rare, is all the
+  same to the inexorable principle of natural selection. If we admire the
+  several ingenious contrivances, by which the flowers of the orchis and of
+  many other plants are fertilised through insect agency, can we consider
+  as equally perfect the elaboration by our fir-trees of dense clouds of
+  pollen, in order that a few granules may be wafted by a chance breeze on
+  to the ovules?</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Summary of Chapter.</i>&mdash;We have in this chapter discussed
+  some of the difficulties and objections which may be urged against my
+  theory. Many of them are very serious; but I think that in the discussion
+  light has been thrown on several facts, which on the theory of
+  independent acts of creation are utterly obscure. We have seen that
+  species at any one period are not indefinitely variable, and are not
+  linked together by a multitude of intermediate gradations, partly because
+  the process of natural selection will always be very slow, and will act,
+  at any one time, only on a very few forms; and partly because the very
+  process of natural selection almost implies the continual supplanting and
+  extinction of preceding and intermediate gradations. Closely allied
+  species, now living on a continuous area, must often have been formed
+  when the area was not continuous, and when the conditions of life did not
+  insensibly graduate away from one part to another. When two varieties are
+  formed in two districts of a continuous area, an intermediate variety
+  will often be formed, fitted for an intermediate zone; but from reasons
+  assigned, the intermediate variety will usually exist in lesser numbers
+  than <!-- Page 204 --><span class="pagenum"><a
+  name="page204"></a>[204]</span>the two forms which it connects;
+  consequently the two latter, during the course of further modification,
+  from existing in greater numbers, will have a great advantage over the
+  less numerous intermediate variety, and will thus generally succeed in
+  supplanting and exterminating it.</p>
+
+  <p>We have seen in this chapter how cautious we should be in concluding
+  that the most different habits of life could not graduate into each
+  other; that a bat, for instance, could not have been formed by natural
+  selection from an animal which at first could only glide through the
+  air.</p>
+
+  <p>We have seen that a species may under new conditions of life change
+  its habits, or have diversified habits, with some habits very unlike
+  those of its nearest congeners. Hence we can understand, bearing in mind
+  that each organic being is trying to live wherever it can live, how it
+  has arisen that there are upland geese with webbed feet, ground
+  woodpeckers, diving thrushes, and petrels with the habits of auks.</p>
+
+  <p>Although the belief that an organ so perfect as the eye could have
+  been formed by natural selection, is more than enough to stagger any one;
+  yet in the case of any organ, if we know of a long series of gradations
+  in complexity, each good for its possessor, then, under changing
+  conditions of life there is no logical impossibility in the acquirement
+  of any conceivable degree of perfection through natural selection. In the
+  cases in which we know of no intermediate or transitional states, we
+  should be very cautious in concluding that none could have existed, for
+  the homologies of many organs and their intermediate states show that
+  wonderful metamorphoses in function are at least possible. For instance,
+  a swim-bladder has apparently been converted into an air-breathing lung.
+  The same organ having performed <!-- Page 205 --><span class="pagenum"><a
+  name="page205"></a>[205]</span>simultaneously very different functions,
+  and then having been specialised for one function; and two very distinct
+  organs having performed at the same time the same function, the one
+  having been perfected whilst aided by the other, must often have largely
+  facilitated transitions.</p>
+
+  <p>We are far too ignorant, in almost every case, to be enabled to assert
+  that any part or organ is so unimportant for the welfare of a species,
+  that modifications in its structure could not have been slowly
+  accumulated by means of natural selection. But we may confidently believe
+  that many modifications, wholly due to the laws of growth, and at first
+  in no way advantageous to a species, have been subsequently taken
+  advantage of by the still further modified descendants of this species.
+  We may, also, believe that a part formerly of high importance has often
+  been retained (as the tail of an aquatic animal by its terrestrial
+  descendants), though it has become of such small importance that it could
+  not, in its present state, have been acquired by natural
+  selection,&mdash;a power which acts solely by the preservation of
+  profitable variations in the struggle for life.</p>
+
+  <p>Natural selection will produce nothing in one species for the
+  exclusive good or injury of another; though it may well produce parts,
+  organs, and excretions highly useful or even indispensable, or highly
+  injurious to another species, but in all cases at the same time useful to
+  the owner. Natural selection in each well-stocked country, must act
+  chiefly through the competition of the inhabitants one with another, and
+  consequently will produce perfection, or strength in the battle for life,
+  only according to the standard of that country. Hence the inhabitants of
+  one country, generally the smaller one, will often yield, as we see they
+  do yield, to the inhabitants of another and generally larger country. For
+  in <!-- Page 206 --><span class="pagenum"><a
+  name="page206"></a>[206]</span>the larger country there will have existed
+  more individuals, and more diversified forms, and the competition will
+  have been severer, and thus the standard of perfection will have been
+  rendered higher. Natural selection will not necessarily produce absolute
+  perfection; nor, as far as we can judge by our limited faculties, can
+  absolute perfection be everywhere found.</p>
+
+  <p>On the theory of natural selection we can clearly understand the full
+  meaning of that old canon in natural history, "Natura non facit saltum."
+  This canon, if we look only to the present inhabitants of the world, is
+  not strictly correct, but if we include all those of past times, it must
+  by my theory be strictly true.</p>
+
+  <p>It is generally acknowledged that all organic beings have been formed
+  on two great laws&mdash;Unity of Type, and the Conditions of Existence.
+  By unity of type is meant that fundamental agreement in structure, which
+  we see in organic beings of the same class, and which is quite
+  independent of their habits of life. On my theory, unity of type is
+  explained by unity of descent. The expression of conditions of existence,
+  so often insisted on by the illustrious Cuvier, is fully embraced by the
+  principle of natural selection. For natural selection acts by either now
+  adapting the varying parts of each being to its organic and inorganic
+  conditions of life; or by having adapted them during long-past periods of
+  time: the adaptations being aided in some cases by use and disuse, being
+  slightly affected by the direct action of the external conditions of
+  life, and being in all cases subjected to the several laws of growth.
+  Hence, in fact, the law of the Conditions of Existence is the higher law;
+  as it includes, through the inheritance of former adaptations, that of
+  Unity of Type.</p>
+
+<hr />
+
+<p><!-- Page 207 --><span class="pagenum"><a name="page207"></a>[207]</span></p>
+
+<h3>CHAPTER VII.</h3>
+
+<p class="cenhead"><span class="sc">Instinct.</span></p>
+
+<blockquote class="b1n">
+
+  <p>Instincts comparable with habits, but different in their
+  origin&mdash;Instincts graduated&mdash;Aphides and ants&mdash;Instincts
+  variable&mdash;Domestic instincts, their origin&mdash;Natural instincts
+  of the cuckoo, ostrich, and parasitic
+  bees&mdash;Slave-making-ants&mdash;Hive-bee, its cell-making
+  instinct&mdash;Difficulties on the theory of the Natural Selection of
+  instincts&mdash;Neuter or sterile insects&mdash;Summary.</p>
+
+</blockquote>
+
+  <p>The subject of instinct might have been worked into the previous
+  chapters; but I have thought that it would be more convenient to treat
+  the subject separately, especially as so wonderful an instinct as that of
+  the hive-bee making its cells will probably have occurred to many
+  readers, as a difficulty sufficient to overthrow my whole theory. I must
+  premise, that I have nothing to do with the origin of the primary mental
+  powers, any more than I have with that of life itself. We are concerned
+  only with the diversities of instinct and of the other mental qualities
+  of animals within the same class.</p>
+
+  <p>I will not attempt any definition of instinct. It would be easy to
+  show that several distinct mental actions are commonly embraced by this
+  term; but every one understands what is meant, when it is said that
+  instinct impels the cuckoo to migrate and to lay her eggs in other birds'
+  nests. An action, which we ourselves should require experience to enable
+  us to perform, when performed by an animal, more especially by a very
+  young one, without any experience, and when performed by many individuals
+  in the same way, without their knowing for what purpose it is performed,
+  is usually said to be instinctive. <!-- Page 208 --><span
+  class="pagenum"><a name="page208"></a>[208]</span>But I could show that
+  none of these characters of instinct are universal. A little dose, as
+  Pierre Huber expresses it, of judgment or reason, often comes into play,
+  even in animals very low in the scale of nature.</p>
+
+  <p>Frederick Cuvier and several of the older metaphysicians have compared
+  instinct with habit. This comparison gives, I think, a remarkably
+  accurate notion of the frame of mind under which an instinctive action is
+  performed, but not of its origin. How unconsciously many habitual actions
+  are performed, indeed not rarely in direct opposition to our conscious
+  will! yet they may be modified by the will or reason. Habits easily
+  become associated with other habits, and with certain periods of time and
+  states of the body. When once acquired, they often remain constant
+  throughout life. Several other points of resemblance between instincts
+  and habits could be pointed out. As in repeating a well-known song, so in
+  instincts, one action follows another by a sort of rhythm; if a person be
+  interrupted in a song, or in repeating anything by rote, he is generally
+  forced to go back to recover the habitual train of thought: so P. Huber
+  found it was with a caterpillar, which makes a very complicated hammock;
+  for if he took a caterpillar which had completed its hammock up to, say,
+  the sixth stage of construction, and put it into a hammock completed up
+  only to the third stage, the caterpillar simply re-performed the fourth,
+  fifth, and sixth stages of construction. If, however, a caterpillar were
+  taken out of a hammock made up, for instance, to the third stage, and
+  were put into one finished up to the sixth stage, so that much of its
+  work, was already done for it, far from feeling the benefit of this, it
+  was much embarrassed, and, in order to complete its hammock, seemed
+  forced to start from the third stage, where it had left off, and thus
+  tried to complete the already finished work. <!-- Page 209 --><span
+  class="pagenum"><a name="page209"></a>[209]</span></p>
+
+  <p>If we suppose any habitual action to become inherited&mdash;and I
+  think it can be shown that this does sometimes happen&mdash;then the
+  resemblance between what originally was a habit and an instinct becomes
+  so close as not to be distinguished. If Mozart, instead of playing the
+  pianoforte at three years old with wonderfully little practice, had
+  played a tune with no practice at all, he might truly be said to have
+  done so instinctively. But it would be the most serious error to suppose
+  that the greater number of instincts have been acquired by habit in one
+  generation, and then transmitted by inheritance to succeeding
+  generations. It can be clearly shown that the most wonderful instincts
+  with which we are acquainted, namely, those of the hive-bee and of many
+  ants, could not possibly have been thus acquired.</p>
+
+  <p>It will be universally admitted that instincts are as important as
+  corporeal structure for the welfare of each species, under its present
+  conditions of life. Under changed conditions of life, it is at least
+  possible that slight modifications of instinct might be profitable to a
+  species; and if it can be shown that instincts do vary ever so little,
+  then I can see no difficulty in natural selection preserving and
+  continually accumulating variations of instinct to any extent that may be
+  profitable. It is thus, as I believe, that all the most complex and
+  wonderful instincts have originated. As modifications of corporeal
+  structure arise from, and are increased by, use or habit, and are
+  diminished or lost by disuse, so I do not doubt it has been with
+  instincts. But I believe that the effects of habit are of quite
+  subordinate importance to the effects of the natural selection of what
+  may be called accidental variations of instincts;&mdash;that is of
+  variations produced by the same unknown causes which produce slight
+  deviations of bodily structure.</p>
+
+  <p>No complex instinct can possibly be produced through <!-- Page 210
+  --><span class="pagenum"><a name="page210"></a>[210]</span>natural
+  selection, except by the slow and gradual accumulation of numerous,
+  slight, yet profitable, variations. Hence, as in the case of corporeal
+  structures, we ought to find in nature, not the actual transitional
+  gradations by which each complex instinct has been acquired&mdash;for
+  these could be found only in the lineal ancestors of each
+  species&mdash;but we ought to find in the collateral lines of descent
+  some evidence of such gradations; or we ought at least to be able to show
+  that gradations of some kind are possible; and this we certainly can do.
+  I have been surprised to find, making allowance for the instincts of
+  animals having been but little observed except in Europe and North
+  America, and for no instinct being known amongst extinct species, how
+  very generally gradations, leading to the most complex instincts, can be
+  discovered. Changes of instinct may sometimes be facilitated by the same
+  species having different instincts at different periods of life, or at
+  different seasons of the year, or when placed under different
+  circumstances &amp;c.; in which case either one or the other instinct
+  might be preserved by natural selection. And such instances of diversity
+  of instinct in the same species can be shown to occur in nature.</p>
+
+  <p>Again as in the case of corporeal structure, and conformably with my
+  theory, the instinct of each species is good for itself, but has never,
+  as far as we can judge, been produced for the exclusive good of others.
+  One of the strongest instances of an animal apparently performing an
+  action for the sole good of another, with which I am acquainted, is that
+  of aphides voluntarily yielding their sweet excretion to ants: that they
+  do so voluntarily, the following facts show. I removed all the ants from
+  a group of about a dozen aphides on a dock-plant, and prevented their
+  attendance during several hours. After this interval, I felt sure that
+  the aphides <!-- Page 211 --><span class="pagenum"><a
+  name="page211"></a>[211]</span>would want to excrete. I watched them for
+  some time through a lens, but not one excreted; I then tickled and
+  stroked them with a hair in the same manner, as well as I could, as the
+  ants do with their antennæ; but not one excreted. Afterwards I allowed an
+  ant to visit them, and it immediately seemed, by its eager way of running
+  about, to be well aware what a rich flock it had discovered; it then
+  began to play with its antennæ on the abdomen first of one aphis and then
+  of another; and each aphis, as soon as it felt the antennæ, immediately
+  lifted up its abdomen and excreted a limpid drop of sweet juice, which
+  was eagerly devoured by the ant. Even the quite young aphides behaved in
+  this manner, showing that the action was instinctive, and not the result
+  of experience. But as the excretion is extremely viscid, it is probably a
+  convenience to the aphides to have it removed; and therefore probably the
+  aphides do not instinctively excrete for the sole good of the ants.
+  Although I do not believe that any animal in the world performs an action
+  for the exclusive good of another of a distinct species, yet each species
+  tries to take advantage of the instincts of others, as each takes
+  advantage of the weaker bodily structure of others. So again, in some few
+  cases, certain instincts cannot be considered as absolutely perfect; but
+  as details on this and other such points are not indispensable, they may
+  be here passed over.</p>
+
+  <p>As some degree of variation in instincts under a state of nature, and
+  the inheritance of such variations, are indispensable for the action of
+  natural selection, as many instances as possible ought to be here given;
+  but want of space prevents me. I can only assert, that instincts
+  certainly do vary&mdash;for instance, the migratory instinct, both in
+  extent and direction, and in its total loss. So it is with the nests of
+  birds, which vary partly <!-- Page 212 --><span class="pagenum"><a
+  name="page212"></a>[212]</span>in dependence on the situations chosen,
+  and on the nature and temperature of the country inhabited, but often
+  from causes wholly unknown to us: Audubon has given several remarkable
+  cases of differences in the nests of the same species in the northern and
+  southern United States. Fear of any particular enemy is certainly an
+  instinctive quality, as may be seen in nestling birds, though it is
+  strengthened by experience, and by the sight of fear of the same enemy in
+  other animals. But fear of man is slowly acquired, as I have elsewhere
+  shown, by various animals inhabiting desert islands; and we may see an
+  instance of this, even in England, in the greater wildness of all our
+  large birds than of our small birds; for the large birds have been most
+  persecuted by man. We may safely attribute the greater wildness of our
+  large birds to this cause; for in uninhabited islands large birds are not
+  more fearful than small; and the magpie, so wary in England, is tame in
+  Norway, as is the hooded crow in Egypt.</p>
+
+  <p>That the general disposition of individuals of the same species, born
+  in a state of nature, is extremely diversified, can be shown by a
+  multitude of facts. Several cases also, could be given, of occasional and
+  strange habits in certain species, which might, if advantageous to the
+  species, give rise, through natural selection, to quite new instincts.
+  But I am well aware that these general statements, without facts given in
+  detail, can produce but a feeble effect on the reader's mind. I can only
+  repeat my assurance, that I do not speak without good evidence.</p>
+
+  <p>The possibility, or even probability, of inherited variations of
+  instinct in a state of nature will be strengthened by briefly considering
+  a few cases under domestication. We shall thus also be enabled to see the
+  respective parts which habit and the selection of <!-- Page 213 --><span
+  class="pagenum"><a name="page213"></a>[213]</span>so-called accidental
+  variations have played in modifying the mental qualities of our domestic
+  animals. A number of curious and authentic instances could be given of
+  the inheritance of all shades of disposition and tastes, and likewise of
+  the oddest tricks, associated with certain frames of mind or periods of
+  time. But let us look to the familiar case of the several breeds of dogs:
+  it cannot be doubted that young pointers (I have myself seen a striking
+  instance) will sometimes point and even back other dogs the very first
+  time that they are taken out; retrieving is certainly in some degree
+  inherited by retrievers; and a tendency to run round, instead of at, a
+  flock of sheep, by shepherd-dogs. I cannot see that these actions,
+  performed without experience by the young, and in nearly the same manner
+  by each individual, performed with eager delight by each breed, and
+  without the end being known,&mdash;for the young pointer can no more know
+  that he points to aid his master, than the white butterfly knows why she
+  lays her eggs on the leaf of the cabbage,&mdash;I cannot see that these
+  actions differ essentially from true instincts. If we were to see one
+  kind of wolf, when young and without any training, as soon as it scented
+  its prey, stand motionless like a statue, and then slowly crawl forward
+  with a peculiar gait; and another kind of wolf rushing round, instead of
+  at, a herd of deer, and driving them to a distant point, we should
+  assuredly call these actions instinctive. Domestic instincts, as they may
+  be called, are certainly far less fixed or invariable than natural
+  instincts; but they have been acted on by far less rigorous selection,
+  and have been transmitted for an incomparably shorter period, under less
+  fixed conditions of life.</p>
+
+  <p>How strongly these domestic instincts, habits, and dispositions are
+  inherited, and how curiously they become mingled, is well shown when
+  different breeds of dogs are <!-- Page 214 --><span class="pagenum"><a
+  name="page214"></a>[214]</span>crossed. Thus it is known that a cross
+  with a bull-dog has affected for many generations the courage and
+  obstinacy of greyhounds; and a cross with a greyhound has given to a
+  whole family of shepherd-dogs a tendency to hunt hares. These domestic
+  instincts, when thus tested by crossing, resemble natural instincts,
+  which in a like manner become curiously blended together, and for a long
+  period exhibit traces of the instincts of either parent: for example, Le
+  Roy describes a dog, whose great-grandfather was a wolf, and this dog
+  showed a trace of its wild parentage only in one way, by not coming in a
+  straight line to his master when called.</p>
+
+  <p>Domestic instincts are sometimes spoken of as actions which have
+  become inherited solely from long-continued and compulsory habit, but
+  this, I think, is not true. No one would ever have thought of teaching,
+  or probably could have taught, the tumbler-pigeon to tumble,&mdash;an
+  action which, as I have witnessed, is performed by young birds, that have
+  never seen a pigeon tumble. We may believe that some one pigeon showed a
+  slight tendency to this strange habit, and that the long-continued
+  selection of the best individuals in successive generations made tumblers
+  what they now are; and near Glasgow there are house-tumblers, as I hear
+  from Mr. Brent, which cannot fly eighteen inches high without going head
+  over heels. It may be doubted whether any one would have thought of
+  training a dog to point, had not some one dog naturally shown a tendency
+  in this line; and this is known occasionally to happen, as I once saw in
+  a pure terrier: the act of pointing is probably, as many have thought,
+  only the exaggerated pause of an animal preparing to spring on its prey.
+  When the first tendency to point was once displayed, methodical selection
+  and the inherited effects of compulsory training in each successive
+  generation would soon complete the <!-- Page 215 --><span
+  class="pagenum"><a name="page215"></a>[215]</span>work; and unconscious
+  selection is still at work, as each man tries to procure, without
+  intending to improve the breed, dogs which will stand and hunt best. On
+  the other hand, habit alone in some cases has sufficed; no animal is more
+  difficult to tame than the young of the wild rabbit; scarcely any animal
+  is tamer than the young of the tame rabbit; but I do not suppose that
+  domestic rabbits have ever been selected for tameness; and I presume that
+  we must attribute the whole of the inherited change from extreme wildness
+  to extreme tameness, simply to habit and long-continued close
+  confinement.</p>
+
+  <p>Natural instincts are lost under domestication: a remarkable instance
+  of this is seen in those breeds of fowls which very rarely or never
+  become "broody," that is, never wish to sit on their eggs. Familiarity
+  alone prevents our seeing how universally and largely the minds of our
+  domestic animals have been modified by domestication. It is scarcely
+  possible to doubt that the love of man has become instinctive in the dog.
+  All wolves, foxes, jackals, and species of the cat genus, when kept tame,
+  are most eager to attack poultry, sheep, and pigs; and this tendency has
+  been found incurable in dogs which have been brought home as puppies from
+  countries, such as Tierra del Fuego and Australia, where the savages do
+  not keep these domestic animals. How rarely, on the other hand, do our
+  civilised dogs, even when quite young, require to be taught not to attack
+  poultry, sheep, and pigs! No doubt they occasionally do make an attack,
+  and are then beaten; and if not cured, they are destroyed; so that habit,
+  with some degree of selection, has probably concurred in civilising by
+  inheritance our dogs. On the other hand, young chickens have lost, wholly
+  by habit, that fear of the dog and cat which no doubt was originally
+  instinctive in them, in the same way as it is so plainly instinctive in
+  <!-- Page 216 --><span class="pagenum"><a
+  name="page216"></a>[216]</span>young pheasants, though reared under a
+  hen. It is not that chickens have lost all fear, but fear only of dogs
+  and cats, for if the hen gives the danger-chuckle, they will run (more
+  especially young turkeys) from under her, and conceal themselves in the
+  surrounding grass or thickets; and this is evidently done for the
+  instinctive purpose of allowing, as we see in wild ground-birds, their
+  mother to fly away. But this instinct retained by our chickens has become
+  useless under domestication, for the mother-hen has almost lost by disuse
+  the power of flight.</p>
+
+  <p>Hence, we may conclude, that domestic instincts have been acquired and
+  natural instincts have been lost partly by habit, and partly by man
+  selecting and accumulating during successive generations, peculiar mental
+  habits and actions, which at first appeared from what we must in our
+  ignorance call an accident. In some cases compulsory habit alone has
+  sufficed to produce such inherited mental changes; in other cases
+  compulsory habit has done nothing, and all has been the result of
+  selection, pursued both methodically and unconsciously; but in most
+  cases, probably, habit and selection have acted together.</p>
+
+  <p>We shall, perhaps, best understand how instincts in a state of nature
+  have become modified by selection, by considering a few cases. I will
+  select only three, out of the several which I shall have to discuss in my
+  future work,&mdash;namely, the instinct which leads the cuckoo to lay her
+  eggs in other birds' nests; the slave-making instinct of certain ants;
+  and the comb-making power of the hive-bee; these two latter instincts
+  have generally, and most justly, been ranked by naturalists as the most
+  wonderful of all known instincts.</p>
+
+  <p>It is now commonly admitted that the more immediate and final cause of
+  the cuckoo's instinct is, that <!-- Page 217 --><span class="pagenum"><a
+  name="page217"></a>[217]</span>she lays her eggs, not daily, but at
+  intervals of two or three days; so that, if she were to make her own nest
+  and sit on her own eggs, those first laid would have to be left for some
+  time unincubated, or there would be eggs and young birds of different
+  ages in the same nest. If this were the case, the process of laying and
+  hatching might be inconveniently long, more especially as she has to
+  migrate at a very early period; and the first hatched young would
+  probably have to be fed by the male alone. But the American cuckoo is in
+  this predicament; for she makes her own nest and has eggs and young
+  successively hatched, all at the same time. It has been asserted that the
+  American cuckoo occasionally lays her eggs in other birds' nests; but I
+  hear on the high authority of Dr. Brewer, that this is a mistake.
+  Nevertheless, I could give several instances of various birds which have
+  been known occasionally to lay their eggs in other birds' nests. Now let
+  us suppose that the ancient progenitor of our European cuckoo had the
+  habits of the American cuckoo; but that occasionally she laid an egg in
+  another bird's nest. If the old bird profited by this occasional habit,
+  or if the young were made more vigorous by advantage having been taken of
+  the mistaken maternal instinct of another bird, than by their own
+  mother's care, encumbered as she can hardly fail to be by having eggs and
+  young of different ages at the same time; then the old birds or the
+  fostered young would gain an advantage. And analogy would lead me to
+  believe, that the young thus reared would be apt to follow by inheritance
+  the occasional and aberrant habit of their mother, and in their turn
+  would be apt to lay their eggs in other birds' nests, and thus be
+  successful in rearing their young. By a continued process of this nature,
+  I believe that the strange instinct of our cuckoo could be, and has been,
+  <!-- Page 218 --><span class="pagenum"><a
+  name="page218"></a>[218]</span>generated. I may add that, according to
+  Dr. Gray and to some other observers, the European cuckoo has not utterly
+  lost all maternal love and care for her own offspring.</p>
+
+  <p>The occasional habit of birds laying their eggs in other birds' nests,
+  either of the same or of a distinct species, is not very uncommon with
+  the Gallinaceæ; and this perhaps explains the origin of a singular
+  instinct in the allied group of ostriches. For several hen ostriches, at
+  least in the case of the American species, unite and lay first a few eggs
+  in one nest and then in another; and these are hatched by the males. This
+  instinct may probably be accounted for by the fact of the hens laying a
+  large number of eggs; but, as in the case of the cuckoo, at intervals of
+  two or three days. This instinct, however, of the American ostrich has
+  not as yet been perfected; for a surprising number of eggs lie strewed
+  over the plains, so that in one day's hunting I picked up no less than
+  twenty lost and wasted eggs.</p>
+
+  <p>Many bees are parasitic, and always lay their eggs in the nests of
+  bees of other kinds. This case is more remarkable than that of the
+  cuckoo; for these bees have not only their instincts but their structure
+  modified in accordance with their parasitic habits; for they do not
+  possess the pollen-collecting apparatus which would be necessary if they
+  had to store food for their own young. Some species, likewise, of
+  Sphegidæ (wasp-like insects) are parasitic on other species; and M. Fabre
+  has lately shown good reason for believing that although the Tachytes
+  nigra generally makes its own burrow and stores it with paralysed prey
+  for its own larvæ to feed on, yet that when this insect finds a burrow
+  already made and stored by another sphex, it takes advantage of the
+  prize, and becomes for the occasion parasitic. In this case, as with the
+  supposed case of the cuckoo, I can <!-- Page 219 --><span
+  class="pagenum"><a name="page219"></a>[219]</span>see no difficulty in
+  natural selection making an occasional habit permanent, if of advantage
+  to the species, and if the insect whose nest and stored food are thus
+  feloniously appropriated, be not thus exterminated.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Slave-making instinct.</i>&mdash;This remarkable instinct was first
+  discovered in the Formica (Polyerges) rufescens by Pierre Huber, a better
+  observer even than his celebrated father. This ant is absolutely
+  dependent on its slaves; without their aid, the species would certainly
+  become extinct in a single year. The males and fertile females do no
+  work. The workers or sterile females, though most energetic and
+  courageous in capturing slaves, do no other work. They are incapable of
+  making their own nests, or of feeding their own larvæ. When the old nest
+  is found inconvenient, and they have to migrate, it is the slaves which
+  determine the migration, and actually carry their masters in their jaws.
+  So utterly helpless are the masters, that when Huber shut up thirty of
+  them without a slave, but with plenty of the food which they like best,
+  and with their larvae and pupæ to stimulate them to work, they did
+  nothing; they could not even feed themselves, and many perished of
+  hunger. Huber then introduced a single slave (F. fusca), and she
+  instantly set to work, fed and saved the survivors; made some cells and
+  tended the larvæ, and put all to rights. What can be more extraordinary
+  than these well-ascertained facts? If we had not known of any other
+  slave-making ant, it would have been hopeless to have speculated how so
+  wonderful an instinct could have been perfected.</p>
+
+  <p>Another species, Formica sanguinea, was likewise first discovered by
+  P. Huber to be a slave-making ant. This species is found in the southern
+  parts of England, and its habits have been attended to by Mr. F. Smith,
+  of <!-- Page 220 --><span class="pagenum"><a
+  name="page220"></a>[220]</span>the British Museum, to whom I am much
+  indebted for information on this and other subjects. Although fully
+  trusting to the statements of Huber and Mr. Smith, I tried to approach
+  the subject in a sceptical frame of mind, as any one may well be excused
+  for doubting the truth of so extraordinary and odious an instinct as that
+  of making slaves. Hence I will give the observations which I have myself
+  made, in some little detail. I opened fourteen nests of F. sanguinea, and
+  found a few slaves in all. Males and fertile females of the slave-species
+  (F. fusca) are found only in their own proper communities, and have never
+  been observed in the nests of F. sanguinea. The slaves are black and not
+  above half the size of their red masters, so that the contrast in their
+  appearance is very great. When the nest is slightly disturbed, the slaves
+  occasionally come out, and like their masters are much agitated and
+  defend the nest: when the nest is much disturbed and the larvæ and pupæ
+  are exposed, the slaves work energetically with their masters in carrying
+  them away to a place of safety. Hence, it is clear, that the slaves feel
+  quite at home. During the months of June and July, on three successive
+  years, I have watched for many hours several nests in Surrey and Sussex,
+  and never saw a slave either leave or enter a nest. As, during these
+  months, the slaves are very few in number, I thought that they might
+  behave differently when more numerous; but Mr. Smith informs me that he
+  has watched the nests at various hours during May, June and August, both
+  in Surrey and Hampshire, and has never seen the slaves, through present
+  in large numbers in August, either leave or enter the nest. Hence he
+  considers them as strictly household slaves. The masters, on the other
+  hand, may be constantly seen bringing in materials for the nest, and food
+  of all kinds. During the present year, however, in the month <!-- Page
+  221 --><span class="pagenum"><a name="page221"></a>[221]</span>of July, I
+  came across a community with an unusually large stock of slaves, and I
+  observed a few slaves mingled with their masters leaving the nest, and
+  marching along the same road to a tall Scotch-fir-tree, twenty-five yards
+  distant, which they ascended together, probably in search of aphides or
+  cocci. According to Huber, who had ample opportunities for observation,
+  in Switzerland the slaves habitually work with their masters in making
+  the nest, and they alone open and close the doors in the morning and
+  evening; and, as Huber expressly states, their principal office is to
+  search for aphides. This difference in the usual habits of the masters
+  and slaves in the two countries, probably depends merely on the slaves
+  being captured in greater numbers in Switzerland than in England.</p>
+
+  <p>One day I fortunately witnessed a migration of F. sanguinea from one
+  nest to another, and it was a most interesting spectacle to behold the
+  masters carefully carrying (instead of being carried by, as in the case
+  of F. rufescens) their slaves in their jaws. Another day my attention was
+  struck by about a score of the slave-makers haunting the same spot, and
+  evidently not in search of food; they approached and were vigorously
+  repulsed by an independent community of the slave-species (F. fusca);
+  sometimes as many as three of these ants clinging to the legs of the
+  slave-making F. sanguinea. The latter ruthlessly killed their small
+  opponents, and carried their dead bodies as food to their nest,
+  twenty-nine yards distant; but they were prevented from getting any pupæ
+  to rear as slaves. I then dug up a small parcel of the pupæ of F. fusca
+  from another nest, and put them down on a bare spot near the place of
+  combat; they were eagerly seized, and carried off by the tyrants, who
+  perhaps fancied that, after all, they had been victorious in their late
+  combat. <!-- Page 222 --><span class="pagenum"><a
+  name="page222"></a>[222]</span></p>
+
+  <p>At the same time I laid on the same place a small parcel of the pupæ
+  of another species, F. flava, with a few of these little yellow ants
+  still clinging to the fragments of the nest. This species is sometimes,
+  though rarely, made into slaves, as has been described by Mr. Smith.
+  Although so small a species, it is very courageous, and I have seen it
+  ferociously attack other ants. In one instance I found to my surprise an
+  independent community of F. flava under a stone beneath a nest of the
+  slave-making F. sanguinea; and when I had accidentally disturbed both
+  nests, the little ants attacked their big neighbours with surprising
+  courage. Now I was curious to ascertain whether F. sanguinea could
+  distinguish the pupæ of F. fusca, which they habitually make into slaves,
+  from those of the little and furious F. flava, which they rarely capture,
+  and it was evident that they did at once distinguish them: for we have
+  seen that they eagerly and instantly seized the pupæ of F. fusca, whereas
+  they were much terrified when they came across the pupæ, or even the
+  earth from the nest of F. flava, and quickly ran away; but in about a
+  quarter of an hour, shortly after all the little yellow ants had crawled
+  away, they took heart and carried off the pupæ.</p>
+
+  <p>One evening I visited another community of F. sanguinea, and found a
+  number of these ants returning home and entering their nests, carrying
+  the dead bodies of F. fusca (showing that it was not a migration) and
+  numerous pupæ. I traced a long file of ants burthened with booty, for
+  about forty yards, to a very thick clump of heath, whence I saw the last
+  individual of F. sanguinea emerge, carrying a pupa; but I was not able to
+  find the desolated nest in the thick heath. The nest, however, must have
+  been close at hand, for two or three individuals of F. fusca were rushing
+  about in the greatest <!-- Page 223 --><span class="pagenum"><a
+  name="page223"></a>[223]</span>agitation, and one was perched motionless
+  with its own pupa in its mouth on the top of a spray of heath, an image
+  of despair, over its ravaged home.</p>
+
+  <p>Such are the facts, though they did not need confirmation by me, in
+  regard to the wonderful instinct of making slaves. Let it be observed
+  what a contrast the instinctive habits of F. sanguinea present with those
+  of the continental F. rufescens. The latter does not build its own nest,
+  does not determine its own migrations, does not collect food for itself
+  or its young, and cannot even feed itself: it is absolutely dependent on
+  its numerous slaves. Formica sanguinea, on the other hand, possesses much
+  fewer slaves, and in the early part of the summer extremely few: the
+  masters determine when and where a new nest shall be formed, and when
+  they migrate, the masters carry the slaves. Both in Switzerland and
+  England the slaves seem to have the exclusive care of the larvæ, and the
+  masters alone go on slave-making expeditions. In Switzerland the slaves
+  and masters work together, making and bringing materials for the nest:
+  both, but chiefly the slaves, tend, and milk as it may be called, their
+  aphides; and thus both collect food for the community. In England the
+  masters alone usually leave the nest to collect building materials and
+  food for themselves, their slaves and larvæ. So that the masters in this
+  country receive much less service from their slaves than they do in
+  Switzerland.</p>
+
+  <p>By what steps the instinct of F. sanguinea originated I will not
+  pretend to conjecture. But as ants, which are not slave-makers, will, as
+  I have seen, carry off pupæ of other species, if scattered near their
+  nests, it is possible that such pupæ originally stored as food might
+  become developed; and the foreign ants thus unintentionally reared would
+  then follow their proper instincts, and do <!-- Page 224 --><span
+  class="pagenum"><a name="page224"></a>[224]</span>what work they could.
+  If their presence proved useful to the species which had seized
+  them&mdash;if it were more advantageous to this species to capture
+  workers than to procreate them&mdash;the habit of collecting pupae
+  originally for food might by natural selection be strengthened and
+  rendered permanent for the very different purpose of raising slaves. When
+  the instinct was once acquired, if carried out to a much less extent even
+  than in our British F. sanguinea, which, as we have seen, is less aided
+  by its slaves than the same species in Switzerland, I can see no
+  difficulty in natural selection increasing and modifying the
+  instinct&mdash;always supposing each modification to be of use to the
+  species&mdash;until an ant was formed as abjectly dependent on its slaves
+  as is the Formica rufescens.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Cell-making instinct of the Hive-Bee.</i>&mdash;I will not here
+  enter on minute details on this subject, but will merely give an outline
+  of the conclusions at which I have arrived. He must be a dull man who can
+  examine the exquisite structure of a comb, so beautifully adapted to its
+  end, without enthusiastic admiration. We hear from mathematicians that
+  bees have practically solved a recondite problem, and have made their
+  cells of the proper shape to hold the greatest possible amount of honey,
+  with the least possible consumption of precious wax in their
+  construction. It has been remarked that a skilful workman, with fitting
+  tools and measures, would find it very difficult to make cells of wax of
+  the true form, though this is perfectly effected by a crowd of bees
+  working in a dark hive. Grant whatever instincts you please, and it seems
+  at first quite inconceivable how they can make all the necessary angles
+  and planes, or even perceive when they are correctly made. But the
+  difficulty is not <!-- Page 225 --><span class="pagenum"><a
+  name="page225"></a>[225]</span>nearly so great as it at first appears:
+  all this beautiful work can be shown, I think, to follow from a few very
+  simple instincts.</p>
+
+  <p>I was led to investigate this subject by Mr. Waterhouse, who has shown
+  that the form of the cell stands in close relation to the presence of
+  adjoining cells; and the following view may, perhaps, be considered only
+  as a modification of his theory. Let us look to the great principle of
+  gradation, and see whether Nature does not reveal to us her method of
+  work. At one end of a short series we have humble-bees, which use their
+  old cocoons to hold honey, sometimes adding to them short tubes of wax,
+  and likewise making separate and very irregular rounded cells of wax. At
+  the other end of the series we have the cells of the hive-bee, placed in
+  a double layer: each cell, as is well known, is an hexagonal prism, with
+  the basal edges of its six sides bevelled so as to fit on to a pyramid,
+  formed of three rhombs. These rhombs have certain angles, and the three
+  which form the pyramidal base of a single cell on one side of the comb,
+  enter into the composition of the bases of three adjoining cells on the
+  opposite side. In the series between the extreme perfection of the cells
+  of the hive-bee and the simplicity of those of the humble-bee, we have
+  the cells of the Mexican Melipona domestica, carefully described and
+  figured by Pierre Huber. The Melipona itself is intermediate in structure
+  between the hive and humble bee, but more nearly related to the latter:
+  it forms a nearly regular waxen comb of cylindrical cells, in which the
+  young are hatched, and, in addition, some large cells of wax for holding
+  honey. These latter cells are nearly spherical and of nearly equal sizes,
+  and are aggregated into an irregular mass. But the important point to
+  notice, is that these cells are always made at that degree of nearness to
+  each other, that they would have <!-- Page 226 --><span
+  class="pagenum"><a name="page226"></a>[226]</span>intersected or broken
+  into each other, if the spheres had been completed; but this is never
+  permitted, the bees building perfectly flat walls of wax between the
+  spheres which thus tend to intersect. Hence each cell consists of an
+  outer spherical portion and of two, three, or more perfectly flat
+  surfaces, according as the cell adjoins two, three, or more other cells.
+  When one cell comes into contact with three other cells, which, from the
+  spheres being nearly of the same size, is very frequently and necessarily
+  the case, the three flat surfaces are united into a pyramid; and this
+  pyramid, as Huber has remarked, is manifestly a gross imitation of the
+  three-sided pyramidal bases of the cell of the hive-bee. As in the cells
+  of the hive-bee, so here, the three plane surfaces in any one cell
+  necessarily enter into the construction of three adjoining cells. It is
+  obvious that the Melipona saves wax by this manner of building; for the
+  flat walls between the adjoining cells are not double, but are of the
+  same thickness as the outer spherical portions, and yet each flat portion
+  forms a part of two cells.</p>
+
+  <p>Reflecting on this case, it occurred to me that if the Melipona had
+  made its spheres at some given distance from each other, and had made
+  them of equal sizes and had arranged them symmetrically in a double
+  layer, the resulting structure would probably have been as perfect as the
+  comb of the hive-bee. Accordingly I wrote to Professor Miller, of
+  Cambridge, and this geometer has kindly read over the following
+  statement, drawn up from his information, and tells me that it is
+  strictly correct:&mdash;</p>
+
+  <p>If a number of equal spheres be described with their centres placed in
+  two parallel layers; with the centre of each sphere at the distance of
+  radius × &radic;2, or radius × 1.41421 (or at some lesser distance), from
+  the centres of the six surrounding spheres in the same <!-- Page 227
+  --><span class="pagenum"><a name="page227"></a>[227]</span>layer; and at
+  the same distance from the centres of the adjoining spheres in the other
+  and parallel layer; then, if planes of intersection between the several
+  spheres in both layers be formed, there will result a double layer of
+  hexagonal prisms united together by pyramidal bases formed of three
+  rhombs; and the rhombs and the sides of the hexagonal prisms will have
+  every angle identically the same with the best measurements which have
+  been made of the cells of the hive-bee.</p>
+
+  <p>Hence we may safely conclude that if we could slightly modify the
+  instincts already possessed by the Melipona, and in themselves not very
+  wonderful, this bee would make a structure as wonderfully perfect as that
+  of the hive-bee. We must suppose the Melipona to make her cells truly
+  spherical, and of equal sizes; and this would not be very surprising,
+  seeing that she already does so to a certain extent, and seeing what
+  perfectly cylindrical burrows in wood many insects can make, apparently
+  by turning round on a fixed point. We must suppose the Melipona to
+  arrange her cells in level layers, as she already does her cylindrical
+  cells; and we must further suppose, and this is the greatest difficulty,
+  that she can somehow judge accurately at what distance to stand from her
+  fellow-labourers when several are making their spheres; but she is
+  already so far enabled to judge of distance, that she always describes
+  her spheres so as to intersect largely; and then she unites the points of
+  intersection by perfectly flat surfaces. We have further to suppose, but
+  this is no difficulty, that after hexagonal prisms have been formed by
+  the intersection of adjoining spheres in the same layer, she can prolong
+  the hexagon to any length requisite to hold the stock of honey; in the
+  same way as the rude humble-bee adds cylinders of wax to the circular
+  mouths of her old cocoons. By such <!-- Page 228 --><span
+  class="pagenum"><a name="page228"></a>[228]</span>modifications of
+  instincts in themselves not very wonderful,&mdash;hardly more wonderful
+  than those which guide a bird to make its nest,&mdash;I believe that the
+  hive-bee has acquired, through natural selection, her inimitable
+  architectural powers.</p>
+
+  <p>But this theory can be tested by experiment. Following the example of
+  Mr. Tegetmeier, I separated two combs, and put between them a long,
+  thick, square strip of wax: the bees instantly began to excavate minute
+  circular pits in it; and as they deepened these little pits, they made
+  them wider and wider until they were converted into shallow basins,
+  appearing to the eye perfectly true or parts of a sphere, and of about
+  the diameter of a cell. It was most interesting to me to observe that
+  wherever several bees had begun to excavate these basins near together,
+  they had begun their work at such a distance from each other, that by the
+  time the basins had acquired the above stated width (<i>i.e.</i> about
+  the width of an ordinary cell), and were in depth about one sixth of the
+  diameter of the sphere of which they formed a part, the rims of the
+  basins intersected or broke into each other. As soon as this occurred,
+  the bees ceased to excavate, and began to build up flat walls of wax on
+  the lines of intersection between the basins, so that each hexagonal
+  prism was built upon the scalloped edge of a smooth basin, instead of on
+  the straight edges of a three-sided pyramid as in the case of ordinary
+  cells.</p>
+
+  <p>I then put into the hive, instead of a thick, square piece of wax, a
+  thin and narrow, knife-edged ridge, coloured with vermilion. The bees
+  instantly began on both sides to excavate little basins near to each
+  other, in the same way as before; but the ridge of wax was so thin, that
+  the bottoms of the basins, if they had been excavated to the same depth
+  as in the former <!-- Page 229 --><span class="pagenum"><a
+  name="page229"></a>[229]</span>experiment, would have broken into each
+  other from the opposite sides. The bees, however, did not suffer this to
+  happen, and they stopped their excavations in due time; so that the
+  basins, as soon as they had been a little deepened, came to have flat
+  bottoms; and these flat bottoms, formed by thin little plates of the
+  vermilion wax having been left ungnawed, were situated, as far as the eye
+  could judge, exactly along the planes of imaginary intersection between
+  the basins on the opposite sides of the ridge of wax. In parts, only
+  little bits, in other parts, large portions of a rhombic plate had been
+  left between the opposed basins, but the work, from the unnatural state
+  of things, had not been neatly performed. The bees must have worked at
+  very nearly the same rate on the opposite sides of the ridge of vermilion
+  wax, as they circularly gnawed away and deepened the basins on both
+  sides, in order to have succeeded in thus leaving flat plates between the
+  basins, by stopping work along the intermediate planes or planes of
+  intersection.</p>
+
+  <p>Considering how flexible thin wax is, I do not see that there is any
+  difficulty in the bees, whilst at work on the two sides of a strip of
+  wax, perceiving when they have gnawed the wax away to the proper
+  thinness, and then stopping their work. In ordinary combs it has appeared
+  to me that the bees do not always succeed in working at exactly the same
+  rate from the opposite sides; for I have noticed half-completed rhombs at
+  the base of a just-commenced cell, which were slightly concave on one
+  side, where I suppose that the bees had excavated too quickly, and convex
+  on the opposed side, where the bees had worked less quickly. In one
+  well-marked instance, I put the comb back into the hive, and allowed the
+  bees to go on working for a short time, and again examined the cell, and
+  I found that the rhombic <!-- Page 230 --><span class="pagenum"><a
+  name="page230"></a>[230]</span>plate had been completed, and had become
+  <i>perfectly flat</i>: it was absolutely impossible, from the extreme
+  thinness of the little rhombic plate, that they could have effected this
+  by gnawing away the convex side; and I suspect that the bees in such
+  cases stand in the opposed cells and push and bend the ductile and warm
+  wax (which as I have tried is easily done) into its proper intermediate
+  plane, and thus flatten it.</p>
+
+  <p>From the experiment of the ridge of vermilion wax, we can clearly see
+  that if the bees were to build for themselves a thin wall of wax, they
+  could make their cells of the proper shape, by standing at the proper
+  distance from each other, by excavating at the same rate, and by
+  endeavouring to make equal spherical hollows, but never allowing the
+  spheres to break into each other. Now bees, as may be clearly seen by
+  examining the edge of a growing comb, do make a rough, circumferential
+  wall or rim all round the comb; and they gnaw into this from the opposite
+  sides, always working circularly as they deepen each cell. They do not
+  make the whole three-sided pyramidal base of any one cell at the same
+  time, but only the one rhombic plate which stands on the extreme growing
+  margin, or the two plates, as the case may be; and they never complete
+  the upper edges of the rhombic plates, until the hexagonal walls are
+  commenced. Some of these statements differ from those made by the justly
+  celebrated elder Huber, but I am convinced of their accuracy; and if I
+  had space, I could show that they are conformable with my theory.</p>
+
+  <p>Huber's statement that the very first cell is excavated out of a
+  little parallel-sided wall of wax, is not, as far as I have seen,
+  strictly correct; the first commencement having always been a little hood
+  of wax; but I will not here enter on these details. We see how important
+  <!-- Page 231 --><span class="pagenum"><a
+  name="page231"></a>[231]</span>a part excavation plays in the
+  construction of the cells; but it would be a great error to suppose that
+  the bees cannot build up a rough wall of wax in the proper
+  position&mdash;that is, along the plane of intersection between two
+  adjoining spheres. I have several specimens showing clearly that they can
+  do this. Even in the rude circumferential rim or wall of wax round a
+  growing comb, flexures may sometimes be observed, corresponding in
+  position to the planes of the rhombic basal plates of future cells. But
+  the rough wall of wax has in every case to be finished off, by being
+  largely gnawed away on both sides. The manner in which the bees build is
+  curious; they always make the first rough wall from ten to twenty times
+  thicker than the excessively thin finished wall of the cell, which will
+  ultimately be left. We shall understand how they work, by supposing
+  masons first to pile up a broad ridge of cement, and then to begin
+  cutting it away equally on both sides near the ground, till a smooth,
+  very thin wall is left in the middle; the masons always piling up the
+  cut-away cement, and adding fresh cement, on the summit of the ridge. We
+  shall thus have a thin wall steadily growing upward; but always crowned
+  by a gigantic coping. From all the cells, both those just commenced and
+  those completed, being thus crowned by a strong coping of wax, the bees
+  can cluster and crawl over the comb without injuring the delicate
+  hexagonal walls, which are only about one four-hundredth of an inch in
+  thickness; the plates of the pyramidal basis being about twice as thick.
+  By this singular manner of building, strength is continually given to the
+  comb, with the utmost ultimate economy of wax.</p>
+
+  <p>It seems at first to add to the difficulty of understanding how the
+  cells are made, that a multitude of bees all work together; one bee after
+  working a short time at one cell going to another, so that, as Huber has
+  stated, <!-- Page 232 --><span class="pagenum"><a
+  name="page232"></a>[232]</span>a score of individuals work even at the
+  commencement of the first cell. I was able practically to show this fact,
+  by covering the edges of the hexagonal walls of a single cell, or the
+  extreme margin of the circumferential rim of a growing comb, with an
+  extremely thin layer of melted vermilion wax; and I invariably found that
+  the colour was most delicately diffused by the bees&mdash;as delicately
+  as a painter could have done with his brush&mdash;by atoms of the
+  coloured wax having been taken from the spot on which it had been placed,
+  and worked into the growing edges of the cells all round. The work of
+  construction seems to be a sort of balance struck between many bees, all
+  instinctively standing at the same relative distance from each other, all
+  trying to sweep equal spheres, and then building up, or leaving ungnawed,
+  the planes of intersection between these spheres. It was really curious
+  to note in cases of difficulty, as when two pieces of comb met at an
+  angle, how often the bees would pull down and rebuild in different ways
+  the same cell, sometimes recurring to a shape which they had at first
+  rejected.</p>
+
+  <p>When bees have a place on which they can stand in their proper
+  positions for working,&mdash;for instance, on a slip of wood, placed
+  directly under the middle of a comb growing downwards so that the comb
+  has to be built over one face of the slip&mdash;in this case the bees can
+  lay the foundations of one wall of a new hexagon, in its strictly proper
+  place, projecting beyond the other completed cells. It suffices that the
+  bees should be enabled to stand at their proper relative distances from
+  each other and from the walls of the last completed cells, and then, by
+  striking imaginary spheres, they can build up a wall intermediate between
+  two adjoining spheres; but, as far as I have seen, they never gnaw away
+  and finish off the angles of a cell till a large part both of that cell
+  and of <!-- Page 233 --><span class="pagenum"><a
+  name="page233"></a>[233]</span>the adjoining cells has been built. This
+  capacity in bees of laying down under certain circumstances a rough wall
+  in its proper place between two just-commenced cells, is important, as it
+  bears on a fact, which seems at first quite subversive of the foregoing
+  theory; namely, that the cells on the extreme margin of wasp-combs are
+  sometimes strictly hexagonal; but I have not space here to enter on this
+  subject. Nor does there seem to me any great difficulty in a single
+  insect (as in the case of a queen-wasp) making hexagonal cells, if she
+  work alternately on the inside and outside of two or three cells
+  commenced at the same time, always standing at the proper relative
+  distance from the parts of the cells just begun, sweeping spheres or
+  cylinders, and building up intermediate planes. It is even conceivable
+  that an insect might, by fixing on a point at which to commence a cell,
+  and then moving outside, first to one point, and then to five other
+  points, at the proper relative distances from the central point and from
+  each other, strike the planes of intersection, and so make an isolated
+  hexagon: but I am not aware that any such case has been observed; nor
+  would any good be derived from a single hexagon being built, as in its
+  construction more materials would be required than for a cylinder.</p>
+
+  <p>As natural selection acts only by the accumulation of slight
+  modifications of structure or instinct, each profitable to the individual
+  under its conditions of life, it may reasonably be asked, how a long and
+  graduated succession of modified architectural instincts, all tending
+  towards the present perfect plan of construction, could have profited the
+  progenitors of the hive-bee? I think the answer is not difficult: it is
+  known that bees are often hard pressed to get sufficient nectar; and I am
+  informed by Mr. Tegetmeier that it has been experimentally found that no
+  less than from twelve to fifteen pounds of dry sugar <!-- Page 234
+  --><span class="pagenum"><a name="page234"></a>[234]</span>are consumed
+  by a hive of bees for the secretion of each pound of wax; to that a
+  prodigious quantity of fluid nectar must be collected and consumed by the
+  bees in a hive for the secretion of the wax necessary for the
+  construction of their combs. Moreover, many bees have to remain idle for
+  many days during the process of secretion. A large store of honey is
+  indispensable to support a large stock of bees during the winter; and the
+  security of the hive is known mainly to depend on a large number of bees
+  being supported. Hence the saving of wax by largely saving honey must be
+  a most important element of success in any family of bees. Of course the
+  success of any species of bee may be dependent on the number of its
+  parasites or other enemies, or on quite distinct causes, and so be
+  altogether independent of the quantity of honey which the bees could
+  collect. But let us suppose that this latter circumstance determined, as
+  it probably often does determine, the numbers of a humble-bee which could
+  exist in a country; and let us further suppose that the community lived
+  throughout the winter, and consequently required a store of honey: there
+  can in this case be no doubt that it would be an advantage to our
+  humble-bee, if a slight modification of her instinct led her to make her
+  waxen cells near together, so as to intersect a little; for a wall in
+  common even to two adjoining cells, would save some little wax. Hence it
+  would continually be more and more advantageous to our humble-bee, if she
+  were to make her cells more and more regular, nearer together, and
+  aggregated into a mass, like the cells of the Melipona; for in this case
+  a large part of the bounding surface of each cell would serve to bound
+  other cells, and much wax would be saved. Again, from the same cause, it
+  would be advantageous to the Melipona, if she were to make her cells
+  closer together, and more regular in every way <!-- Page 235 --><span
+  class="pagenum"><a name="page235"></a>[235]</span>than at present; for
+  then, as we have seen, the spherical surfaces would wholly disappear, and
+  would all be replaced by plane surfaces; and the Melipona would make a
+  comb as perfect as that of the hive-bee. Beyond this stage of perfection
+  in architecture, natural selection could not lead; for the comb of the
+  hive-bee, as far as we can see, is absolutely perfect in economising
+  wax.</p>
+
+  <p>Thus, as I believe, the most wonderful of all known instincts, that of
+  the hive-bee, can be explained by natural selection having taken
+  advantage of numerous, successive, slight modifications of simpler
+  instincts; natural selection having by slow degrees, more and more
+  perfectly, led the bees to sweep equal spheres at a given distance from
+  each other in a double layer, and to build up and excavate the wax along
+  the planes of intersection. The bees, of course, no more knowing that
+  they swept their spheres at one particular distance from each other, than
+  they know what are the several angles of the hexagonal prisms and of the
+  basal rhombic plates. The motive power of the process of natural
+  selection having been economy of wax; that individual swarm which wasted
+  least honey in the secretion of wax, having succeeded best, and having
+  transmitted by inheritance its newly acquired economical instinct to new
+  swarms, which in their turn will have had the best chance of succeeding
+  in the struggle for existence.</p>
+
+  <p>&nbsp;</p>
+
+  <p>No doubt many instincts of very difficult explanation could be opposed
+  to the theory of natural selection,&mdash;cases, in which we cannot see
+  how an instinct could possibly have originated; cases, in which no
+  intermediate gradations are known to exist; cases of instinct of
+  apparently such trifling importance, that they could <!-- Page 236
+  --><span class="pagenum"><a name="page236"></a>[236]</span>hardly have
+  been acted on by natural selection; cases of instincts almost identically
+  the same in animals so remote in the scale of nature, that we cannot
+  account for their similarity by inheritance from a common parent, and
+  must therefore believe that they have been acquired by independent acts
+  of natural selection. I will not here enter on these several cases, but
+  will confine myself to one special difficulty, which at first appeared to
+  me insuperable, and actually fatal to my whole theory. I allude to the
+  neuters or sterile females in insect-communities: for these neuters often
+  differ widely in instinct and in structure from both the males and
+  fertile females, and yet, from being sterile, they cannot propagate their
+  kind.</p>
+
+  <p>The subject well deserves to be discussed at great length, but I will
+  here take only a single case, that of working or sterile ants. How the
+  workers have been rendered sterile is a difficulty; but not much greater
+  than that of any other striking modification of structure; for it can be
+  shown that some insects and other articulate animals in a state of nature
+  occasionally become sterile; and if such insects had been social, and it
+  had been profitable to the community that a number should have been
+  annually born capable of work, but incapable of procreation, I can see no
+  very great difficulty in this being effected by natural selection. But I
+  must pass over this preliminary difficulty. The great difficulty lies in
+  the working ants differing widely from both the males and the fertile
+  females in structure, as in the shape of the thorax and in being
+  destitute of wings and sometimes of eyes, and in instinct. As far as
+  instinct alone is concerned, the prodigious difference in this respect
+  between the workers and the perfect females, would have been far better
+  exemplified by the hive-bee. If a working ant or other neuter insect had
+  been an animal <!-- Page 237 --><span class="pagenum"><a
+  name="page237"></a>[237]</span>in the ordinary state, I should have
+  unhesitatingly assumed that all its characters had been slowly acquired
+  through natural selection; namely, by an individual having been born with
+  some slight profitable modification of structure, this being inherited by
+  its offspring, which again varied and were again selected, and so
+  onwards. But with the working ant we have an insect differing greatly
+  from its parents, yet absolutely sterile; so that it could never have
+  transmitted successively acquired modifications of structure or instinct
+  to its progeny. It may well be asked how is it possible to reconcile this
+  case with the theory of natural selection?</p>
+
+  <p>First, let it be remembered that we have innumerable instances, both
+  in our domestic productions and in those in a state of nature, of all
+  sorts of differences of structure which have become correlated to certain
+  ages, and to either sex. We have differences correlated not only to one
+  sex, but to that short period alone when the reproductive system is
+  active, as in the nuptial plumage of many birds, and in the hooked jaws
+  of the male salmon. We have even slight differences in the horns of
+  different breeds of cattle in relation to an artificially imperfect state
+  of the male sex; for oxen of certain breeds have longer horns than in
+  other breeds, in comparison with the horns of the bulls or cows of these
+  same breeds. Hence I can see no real difficulty in any character having
+  become correlated with the sterile condition of certain members of
+  insect-communities: the difficulty lies in understanding how such
+  correlated modifications of structure could have been slowly accumulated
+  by natural selection.</p>
+
+  <p>This difficulty, though appearing insuperable, is lessened, or, as I
+  believe, disappears, when it is remembered that selection may be applied
+  to the family, as well as to the individual, and may thus gain the <!--
+  Page 238 --><span class="pagenum"><a
+  name="page238"></a>[238]</span>desired end. Thus, a well-flavoured
+  vegetable is cooked, and the individual is destroyed; but the
+  horticulturist sows seeds of the same stock, and confidently expects to
+  get nearly the same variety: breeders of cattle wish the flesh and fat to
+  be well marbled together; the animal has been slaughtered, but the
+  breeder goes with confidence to the same family. I have such faith in the
+  powers of selection, that I do not doubt that a breed of cattle, always
+  yielding oxen with extraordinarily long horns, could be slowly formed by
+  carefully watching which individual bulls and cows, when matched,
+  produced oxen with the longest horns; and yet no one ox could ever have
+  propagated its kind. Thus I believe it has been with social insects: a
+  slight modification of structure, or instinct, correlated with the
+  sterile condition of certain members of the community, has been
+  advantageous to the community: consequently the fertile males and females
+  of the same community flourished, and transmitted to their fertile
+  offspring a tendency to produce sterile members having the same
+  modification. And I believe that this process has been repeated, until
+  that prodigious amount of difference between the fertile and sterile
+  females of the same species has been produced, which we see in many
+  social insects.</p>
+
+  <p>But we have not as yet touched on the climax of the difficulty;
+  namely, the fact that the neuters of several ants differ, not only from
+  the fertile females and males, but from each other, sometimes to an
+  almost incredible degree, and are thus divided into two or even three
+  castes. The castes, moreover, do not generally graduate into each other,
+  but are perfectly well defined; being as distinct from each other, as are
+  any two species of the same genus, or rather as any two genera of the
+  same family. Thus in Eciton, there are working and soldier neuters, with
+  jaws and instincts extraordinarily <!-- Page 239 --><span
+  class="pagenum"><a name="page239"></a>[239]</span>different: in
+  Cryptocerus, the workers of one caste alone carry a wonderful sort of
+  shield on their heads, the use of which is quite unknown: in the Mexican
+  Myrmecocystus, the workers of one caste never leave the nest; they are
+  fed by the workers of another caste, and they have an enormously
+  developed abdomen which secretes a sort of honey, supplying the place of
+  that excreted by the aphides, or the domestic cattle as they may be
+  called, which our European ants guard or imprison.</p>
+
+  <p>It will indeed be thought that I have an overweening confidence in the
+  principle of natural selection, when I do not admit that such wonderful
+  and well-established facts at once annihilate my theory. In the simpler
+  case of neuter insects all of one caste or of the same kind, which have
+  been rendered by natural selection, as I believe to be quite possible,
+  different from the fertile males and females,&mdash;in this case, we may
+  safely conclude from the analogy of ordinary variations, that each
+  successive, slight, profitable modification did not probably at first
+  appear in all the individual neuters in the same nest, but in a few
+  alone; and that by the long-continued selection of the fertile parents
+  which produced most neuters with the profitable modification, all the
+  neuters ultimately came to have the desired character. On this view we
+  ought occasionally to find neuter-insects of the same species, in the
+  same nest, presenting gradations of structure; and this we do find, even
+  often, considering how few neuter-insects out of Europe have been
+  carefully examined. Mr. F. Smith has shown how surprisingly the neuters
+  of several British ants differ from each other in size and sometimes in
+  colour; and that the extreme forms can sometimes be perfectly linked
+  together by individuals taken out of the same nest: I have myself
+  compared perfect gradations of this kind. It often happens that the
+  larger or the smaller sized workers <!-- Page 240 --><span
+  class="pagenum"><a name="page240"></a>[240]</span>are the most numerous;
+  or that both large and small are numerous, with those of an intermediate
+  size scanty in numbers. Formica flava has larger and smaller workers,
+  with some of intermediate size; and, in this species, as Mr. F. Smith has
+  observed, the larger workers have simple eyes (ocelli), which though
+  small can be plainly distinguished, whereas the smaller workers have
+  their ocelli rudimentary. Having carefully dissected several specimens of
+  these workers, I can affirm that the eyes are far more rudimentary in the
+  smaller workers than can be accounted for merely by their proportionally
+  lesser size; and I fully believe, though I dare not assert so positively,
+  that the workers of intermediate size have their ocelli in an exactly
+  intermediate condition. So that we here have two bodies of sterile
+  workers in the same nest, differing not only in size, but in their organs
+  of vision, yet connected by some few members in an intermediate
+  condition. I may digress by adding, that if the smaller workers had been
+  the most useful to the community, and those males and females had been
+  continually selected, which produced more and more of the smaller
+  workers, until all the workers had come to be in this condition; we
+  should then have had a species of ant with neuters very nearly in the
+  same condition with those of Myrmica. For the workers of Myrmica have not
+  even rudiments of ocelli, though the male and female ants of this genus
+  have well-developed ocelli.</p>
+
+  <p>I may give one other case: so confidently did I expect to find
+  gradations in important points of structure between the different castes
+  of neuters in the same species, that I gladly availed myself of Mr. F.
+  Smith's offer of numerous specimens from the same nest of the driver ant
+  (Anomma) of West Africa. The reader will perhaps best appreciate the
+  amount of difference in these <!-- Page 241 --><span class="pagenum"><a
+  name="page241"></a>[241]</span>workers, by my giving not the actual
+  measurements, but a strictly accurate illustration: the difference was
+  the same as if we were to see a set of workmen building a house of whom
+  many were five feet four inches high, and many sixteen feet high; but we
+  must suppose that the larger workmen had heads four instead of three
+  times as big as those of the smaller men, and jaws nearly five times as
+  big. The jaws, moreover, of the working ants of the several sizes
+  differed wonderfully in shape, and in the form and number of the teeth.
+  But the important fact for us is, that though the workers can be grouped
+  into castes of different sizes, yet they graduate insensibly into each
+  other, as does the widely-different structure of their jaws. I speak
+  confidently on this latter point, as Mr. Lubbock made drawings for me
+  with the camera lucida of the jaws which I had dissected from the workers
+  of the several sizes.</p>
+
+  <p>With these facts before me, I believe that natural selection, by
+  acting on the fertile parents, could form a species which should
+  regularly produce neuters, either all of large size with one form of jaw,
+  or all of small size with jaws having a widely different structure; or
+  lastly, and this is our climax of difficulty, one set of workers of one
+  size and structure, and simultaneously another set of workers of a
+  different size and structure;&mdash;a graduated series having been first
+  formed, as in the case of the driver ant, and then the extreme forms,
+  from being the most useful to the community, having been produced in
+  greater and greater numbers through the natural selection of the parents
+  which generated them; until none with an intermediate structure were
+  produced.</p>
+
+  <p>Thus, as I believe, the wonderful fact of two distinctly defined
+  castes of sterile workers existing in the same nest, both widely
+  different from each other and from <!-- Page 242 --><span
+  class="pagenum"><a name="page242"></a>[242]</span>their parents, has
+  originated. We can see how useful their production may have been to a
+  social community of insects, on the same principle that the division of
+  labour is useful to civilised man. As ants work by inherited instincts
+  and by inherited organs or tools, and not by acquired knowledge and
+  manufactured instruments, a perfect division of labour could be effected
+  with them only by the workers being sterile; for had they been fertile,
+  they would have intercrossed, and their instincts and structure would
+  have become blended. And nature has, as I believe, effected this
+  admirable division of labour in the communities of ants, by the means of
+  natural selection. But I am bound to confess, that, with all my faith in
+  this principle, I should never have anticipated that natural selection
+  could have been efficient in so high a degree, had not the case of these
+  neuter insects convinced me of the fact. I have, therefore, discussed
+  this case, at some little but wholly insufficient length, in order to
+  show the power of natural selection, and likewise because this is by far
+  the most serious special difficulty, which my theory has encountered. The
+  case, also, is very interesting, as it proves that with animals, as with
+  plants, any amount of modification in structure can be effected by the
+  accumulation of numerous, slight, and as we must call them accidental,
+  variations, which are in any manner profitable, without exercise or habit
+  having come into play. For no amount of exercise, or habit, or volition,
+  in the utterly sterile members of a community could possibly affect the
+  structure or instincts of the fertile members, which alone leave
+  descendants. I am surprised that no one has advanced this demonstrative
+  case of neuter insects, against the well-known doctrine of Lamarck.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Summary.</i>&mdash;I have endeavoured briefly in this chapter <!--
+  Page 243 --><span class="pagenum"><a name="page243"></a>[243]</span>to
+  show that the mental qualities of our domestic animals vary, and that the
+  variations are inherited. Still more briefly I have attempted to show
+  that instincts vary slightly in a state of nature. No one will dispute
+  that instincts are of the highest importance to each animal. Therefore I
+  can see no difficulty, under changing conditions of life, in natural
+  selection accumulating slight modifications of instinct to any extent, in
+  any useful direction. In some cases habit or use and disuse have probably
+  come into play. I do not pretend that the facts given in this chapter
+  strengthen in any great degree my theory; but none of the cases of
+  difficulty, to the best of my judgment, annihilate it. On the other hand,
+  the fact that instincts are not always absolutely perfect and are liable
+  to mistakes;&mdash;that no instinct has been produced for the exclusive
+  good of other animals, but that each animal takes advantage of the
+  instincts of others;&mdash;that the canon in natural history, of "Natura
+  non facit saltum," is applicable to instincts as well as to corporeal
+  structure, and is plainly explicable on the foregoing views, but is
+  otherwise inexplicable,&mdash;all tend to corroborate the theory of
+  natural selection.</p>
+
+  <p>This theory is, also, strengthened by some few other facts in regard
+  to instincts; as by that common case of closely allied, but certainly
+  distinct, species, when inhabiting distant parts of the world and living
+  under considerably different conditions of life, yet often retaining
+  nearly the same instincts. For instance, we can understand on the
+  principle of inheritance, how it is that the thrush of South America
+  lines its nest with mud, in the same peculiar manner as does our British
+  thrush: how it is that the male wrens (Troglodytes) of North America,
+  build "cock-nests," to roost in, like the males of our distinct
+  Kitty-wrens,&mdash;a habit wholly unlike that of <!-- Page 244 --><span
+  class="pagenum"><a name="page244"></a>[244]</span>any other known bird.
+  Finally, it may not be a logical deduction, but to my imagination it is
+  far more satisfactory to look at such instincts as the young cuckoo
+  ejecting its foster-brothers,&mdash;ants making slaves,&mdash;the larvae
+  of ichneumonidæ feeding within the live bodies of caterpillars,&mdash;not
+  as specially endowed or created instincts, but as small consequences of
+  one general law, leading to the advancement of all organic beings,
+  namely, multiply, vary, let the strongest live and the weakest die.</p>
+
+<hr />
+
+<p><!-- Page 245 --><span class="pagenum"><a name="page245"></a>[245]</span></p>
+
+<h3>CHAPTER VIII.</h3>
+
+<p class="cenhead"><span class="sc">Hybridism.</span></p>
+
+<blockquote class="b1n">
+
+  <p>Distinction between the sterility of first crosses and of
+  hybrids&mdash;Sterility various in degree, not universal, affected by
+  close interbreeding, removed by domestication&mdash;Laws governing the
+  sterility of hybrids&mdash;Sterility not a special endowment, but
+  incidental on other differences&mdash;Causes of the sterility of first
+  crosses and of hybrids&mdash;Parallelism between the effects of changed
+  conditions of life and crossing&mdash;Fertility of varieties when crossed
+  and of their mongrel offspring not universal&mdash;Hybrids and mongrels
+  compared independently of their fertility&mdash;Summary.</p>
+
+</blockquote>
+
+  <p>The view generally entertained by naturalists is that species, when
+  intercrossed, have been specially endowed with the quality of sterility,
+  in order to prevent the confusion of all organic forms. This view
+  certainly seems at first probable, for species within the same country
+  could hardly have kept distinct had they been capable of crossing freely.
+  The importance of the fact that hybrids are very generally sterile, has,
+  I think, been much underrated by some late writers. On the theory of
+  natural selection the case is especially important, inasmuch as the
+  sterility of hybrids could not possibly be of any advantage to them, and
+  therefore could not have been acquired by the continued preservation of
+  successive profitable degrees of sterility. I hope, however, to be able
+  to show that sterility is not a specially acquired or endowed quality,
+  but is incidental on other acquired differences.</p>
+
+  <p>In treating this subject, two classes of facts, to a large extent
+  fundamentally different, have generally been confounded together; namely,
+  the sterility of two species <!-- Page 246 --><span class="pagenum"><a
+  name="page246"></a>[246]</span>when first crossed, and the sterility of
+  the hybrids produced from them.</p>
+
+  <p>Pure species have of course their organs of reproduction in a perfect
+  condition, yet when intercrossed they produce either few or no offspring.
+  Hybrids, on the other hand, have their reproductive organs functionally
+  impotent, as may be clearly seen in the state of the male element in both
+  plants and animals; though the organs themselves are perfect in
+  structure, as far as the microscope reveals. In the first case the two
+  sexual elements which go to form the embryo are perfect; in the second
+  case they are either not at all developed, or are imperfectly developed.
+  This distinction is important, when the cause of the sterility, which is
+  common to the two cases, has to be considered. The distinction has
+  probably been slurred over, owing to the sterility in both cases being
+  looked on as a special endowment, beyond the province of our reasoning
+  powers.</p>
+
+  <p>The fertility of varieties, that is of the forms known or believed to
+  have descended from common parents, when intercrossed, and likewise the
+  fertility of their mongrel offspring, is, on my theory, of equal
+  importance with the sterility of species; for it seems to make a broad
+  and clear distinction between varieties and species.</p>
+
+  <p>First, for the sterility of species when crossed and of their hybrid
+  offspring. It is impossible to study the several memoirs and works of
+  those two conscientious and admirable observers, Kölreuter and Gärtner,
+  who almost devoted their lives to this subject, without being deeply
+  impressed with the high generality of some degree of sterility. Kölreuter
+  makes the rule universal; but then he cuts the knot, for in ten cases in
+  which he found two forms, considered by most authors as distinct species,
+  quite fertile together, he unhesitatingly ranks <!-- Page 247 --><span
+  class="pagenum"><a name="page247"></a>[247]</span>them as varieties.
+  Gärtner, also, makes the rule equally universal; and he disputes the
+  entire fertility of Kölreuter's ten cases. But in these and in many other
+  cases, Gärtner is obliged carefully to count the seeds, in order to show
+  that there is any degree of sterility. He always compares the maximum
+  number of seeds produced by two species when crossed and by their hybrid
+  offspring, with the average number produced by both pure parent-species
+  in a state of nature. But a serious cause of error seems to me to be here
+  introduced: a plant to be hybridised must be castrated, and, what is
+  often more important, must be secluded in order to prevent pollen being
+  brought to it by insects from other plants. Nearly all the plants
+  experimentised on by Gärtner were potted, and apparently were kept in a
+  chamber in his house. That these processes are often injurious to the
+  fertility of a plant cannot be doubted; for Gärtner gives in his table
+  about a score of cases of plants which he castrated, and artificially
+  fertilised with their own pollen, and (excluding all cases such as the
+  Leguminosæ, in which there is an acknowledged difficulty in the
+  manipulation) half of these twenty plants had their fertility in some
+  degree impaired. Moreover, as Gärtner during several years repeatedly
+  crossed the primrose and cowslip, which we have such good reason to
+  believe to be varieties, and only once or twice succeeded in getting
+  fertile seed; as he found the common red and blue pimpernels (Anagallis
+  arvensis and c&oelig;rulea), which the best botanists rank as varieties,
+  absolutely sterile together; and as he came to the same conclusion in
+  several other analogous cases; it seems to me that we may well be
+  permitted to doubt whether many other species are really so sterile, when
+  intercrossed, as Gärtner believes. <!-- Page 248 --><span
+  class="pagenum"><a name="page248"></a>[248]</span></p>
+
+  <p>It is certain, on the one hand, that the sterility of various species
+  when crossed is so different in degree and graduates away so insensibly,
+  and, on the other hand, that the fertility of pure species is so easily
+  affected by various circumstances, that for all practical purposes it is
+  most difficult to say where perfect fertility ends and sterility begins.
+  I think no better evidence of this can be required than that the two most
+  experienced observers who have ever lived, namely, Kölreuter and Gärtner,
+  should have arrived at diametrically opposite conclusions in regard to
+  the very same species. It is also most instructive to compare&mdash;but I
+  have not space here to enter on details&mdash;the evidence advanced by
+  our best botanists on the question whether certain doubtful forms should
+  be ranked as species or varieties, with the evidence from fertility
+  adduced by different hybridisers, or by the same author, from experiments
+  made during different years. It can thus be shown that neither sterility
+  nor fertility affords any clear distinction between species and
+  varieties; but that the evidence from this source graduates away, and is
+  doubtful in the same degree as is the evidence derived from other
+  constitutional and structural differences.</p>
+
+  <p>In regard to the sterility of hybrids in successive generations;
+  though Gärtner was enabled to rear some hybrids, carefully guarding them
+  from a cross with either pure parent, for six or seven, and in one case
+  for ten generations, yet he asserts positively that their fertility never
+  increased, but generally greatly decreased. I do not doubt that this is
+  usually the case, and that the fertility often suddenly decreases in the
+  first few generations. Nevertheless I believe that in all these
+  experiments the fertility has been diminished by an independent cause,
+  namely, from close interbreeding. I have collected so large a body of
+  facts, showing <!-- Page 249 --><span class="pagenum"><a
+  name="page249"></a>[249]</span>that close interbreeding lessens
+  fertility, and, on the other hand, that an occasional cross with a
+  distinct individual or variety increases fertility, that I cannot doubt
+  the correctness of this almost universal belief amongst breeders. Hybrids
+  are seldom raised by experimentalists in great numbers; and as the
+  parent-species, or other allied hybrids, generally grow in the same
+  garden, the visits of insects must be carefully prevented during the
+  flowering season: hence hybrids will generally be fertilised during each
+  generation by their own individual pollen; and I am convinced that this
+  would be injurious to their fertility, already lessened by their hybrid
+  origin. I am strengthened in this conviction by a remarkable statement
+  repeatedly made by Gärtner, namely, that if even the less fertile hybrids
+  be artificially fertilised with hybrid pollen of the same kind, their
+  fertility, notwithstanding the frequent ill effects of manipulation,
+  sometimes decidedly increases, and goes on increasing. Now, in artificial
+  fertilisation pollen is as often taken by chance (as I know from my own
+  experience) from the anthers of another flower, as from the anthers of
+  the flower itself which is to be fertilised; so that a cross between two
+  flowers, though probably on the same plant, would be thus effected.
+  Moreover, whenever complicated experiments are in progress, so careful an
+  observer as Gärtner would have castrated his hybrids, and this would have
+  insured in each generation a cross with a pollen from a distinct flower,
+  either from the same plant or from another plant of the same hybrid
+  nature. And thus, the strange fact of the increase of fertility in the
+  successive generations of <i>artificially fertilised</i> hybrids may, I
+  believe, be accounted for by close interbreeding having been avoided.</p>
+
+  <p>Now let us turn to the results arrived at by the third most
+  experienced hybridiser, namely, the Hon. and <!-- Page 250 --><span
+  class="pagenum"><a name="page250"></a>[250]</span>Rev. W. Herbert. He is
+  as emphatic in his conclusion that some hybrids are perfectly
+  fertile&mdash;as fertile as the pure parent-species&mdash;as are
+  Kölreuter and Gärtner that some degree of sterility between distinct
+  species is a universal law of nature. He experimentised on some of the
+  very same species as did Gärtner. The difference in their results may, I
+  think, be in part accounted for by Herbert's great horticultural skill,
+  and by his having hothouses at his command. Of his many important
+  statements I will here give only a single one as an example, namely, that
+  "every ovule in a pod of Crinum capense fertilised by C. revolutum
+  produced a plant, which (he says) I never saw to occur in a case of its
+  natural fecundation." So that we here have perfect, or even more than
+  commonly perfect, fertility in a first cross between two distinct
+  species.</p>
+
+  <p>This case of the Crinum leads me to refer to a most singular fact,
+  namely, that there are individual plants of certain species of Lobelia
+  and of some other genera, which can be far more easily fertilised by the
+  pollen of another and distinct species, than by their own pollen; and all
+  the individuals of nearly all the species of Hippeastrum seem to be in
+  this predicament. For these plants have been found to yield seed to the
+  pollen of a distinct species, though quite sterile with their own pollen,
+  notwithstanding that their own pollen was found to be perfectly good, for
+  it fertilised distinct species. So that certain individual plants and all
+  the individuals of certain species can actually be hybridised much more
+  readily than they can be self-fertilised! For instance, a bulb of
+  Hippeastrum aulicum produced four flowers; three were fertilised by
+  Herbert with their own pollen, and the fourth was subsequently fertilised
+  by the pollen of a compound hybrid descended from three other and
+  distinct <!-- Page 251 --><span class="pagenum"><a
+  name="page251"></a>[251]</span>species: the result was that "the ovaries
+  of the three first flowers soon ceased to grow, and after a few days
+  perished entirely, whereas the pod impregnated by the pollen of the
+  hybrid made vigorous growth and rapid progress to maturity, and bore good
+  seed, which vegetated freely." In a letter to me, in 1839, Mr. Herbert
+  told me that he had then tried the experiment during five years, and he
+  continued to try it during several subsequent years, and always with the
+  same result. This result has, also, been confirmed by other observers in
+  the case of Hippeastrum with its sub-genera, and in the case of some
+  other genera, as Lobelia, Passiflora and Verbascum. Although the plants
+  in these experiments appeared perfectly healthy, and although both the
+  ovules and pollen of the same flower were perfectly good with respect to
+  other species, yet as they were functionally imperfect in their mutual
+  self-action, we must infer that the plants were in an unnatural state.
+  Nevertheless these facts show on what slight and mysterious causes the
+  lesser or greater fertility of species when crossed, in comparison with
+  the same species when self-fertilised, sometimes depends.</p>
+
+  <p>The practical experiments of horticulturists, though not made with
+  scientific precision, deserve some notice. It is notorious in how
+  complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
+  Petunia, Rhododendron, &amp;c., have been crossed, yet many of these
+  hybrids seed freely. For instance, Herbert asserts that a hybrid from
+  Calceolaria integrifolia and plantaginea, species most widely dissimilar
+  in general habit, "reproduced itself as perfectly as if it had been a
+  natural species from the mountains of Chile." I have taken some pains to
+  ascertain the degree of fertility of some of the complex crosses of
+  Rhododendrons, and I am assured that many of them <!-- Page 252 --><span
+  class="pagenum"><a name="page252"></a>[252]</span>are perfectly fertile.
+  Mr. C. Noble, for instance, informs me that he raises stocks for grafting
+  from a hybrid between Rhod. Ponticum and Catawbiense, and that this
+  hybrid "seeds as freely as it is possible to imagine." Had hybrids, when
+  fairly treated, gone on decreasing in fertility in each successive
+  generation, as Gärtner believes to be the case, the fact would have been
+  notorious to nurserymen. Horticulturists raise large beds of the same
+  hybrids, and such alone are fairly treated, for by insect agency the
+  several individuals of the same hybrid variety are allowed to freely
+  cross with each other, and the injurious influence of close interbreeding
+  is thus prevented. Any one may readily convince himself of the efficiency
+  of insect-agency by examining the flowers of the more sterile kinds of
+  hybrid rhododendrons, which produce no pollen, for he will find on their
+  stigmas plenty of pollen brought from other flowers.</p>
+
+  <p>In regard to animals, much fewer experiments have been carefully tried
+  than with plants. If our systematic arrangements can be trusted, that is
+  if the genera of animals are as distinct from each other, as are the
+  genera of plants, then we may infer that animals more widely separated in
+  the scale of nature can be more easily crossed than in the case of
+  plants; but the hybrids themselves are, I think, more sterile. I doubt
+  whether any case of a perfectly fertile hybrid animal can be considered
+  as thoroughly well authenticated. It should, however, be borne in mind
+  that, owing to few animals breeding freely under confinement, few
+  experiments have been fairly tried: for instance, the canary-bird has
+  been crossed with nine other finches, but as not one of these nine
+  species breeds freely in confinement, we have no right to expect that the
+  first crosses between them and the canary, or that their hybrids, <!--
+  Page 253 --><span class="pagenum"><a
+  name="page253"></a>[253]</span>should be perfectly fertile. Again, with
+  respect to the fertility in successive generations of the more fertile
+  hybrid animals, I hardly know of an instance in which two families of the
+  same hybrid have been raised at the same time from different parents, so
+  as to avoid the ill effects of close interbreeding. On the contrary,
+  brothers and sisters have usually been crossed in each successive
+  generation, in opposition to the constantly repeated admonition of every
+  breeder. And in this case, it is not at all surprising that the inherent
+  sterility in the hybrids should have gone on increasing. If we were to
+  act thus, and pair brothers and sisters in the case of any pure animal,
+  which from any cause had the least tendency to sterility, the breed would
+  assuredly be lost in a very few generations.</p>
+
+  <p>Although I do not know of any thoroughly well-authenticated cases of
+  perfectly fertile hybrid animals, I have some reason to believe that the
+  hybrids from Cervulus vaginalis and Reevesii, and from Phasianus
+  colchicus with P. torquatus and with P. versicolor are perfectly fertile.
+  There is no doubt that these three pheasants, namely, the common, the
+  true ring-necked, and the Japan, intercross, and are becoming blended
+  together in the woods of several parts of England. The hybrids from the
+  common and Chinese geese (A. cygnoides), species which are so different
+  that they are generally ranked in distinct genera, have often bred in
+  this country with either pure parent, and in one single instance they
+  have bred <i>inter se</i>. This was effected by Mr. Eyton, who raised two
+  hybrids from the same parents but from different hatches; and from these
+  two birds he raised no less than eight hybrids (grandchildren of the pure
+  geese) from one nest. In India, however, these cross-bred geese must be
+  far more fertile; for I am assured by two eminently capable judges,
+  namely <!-- Page 254 --><span class="pagenum"><a
+  name="page254"></a>[254]</span>Mr. Blyth and Capt. Hutton, that whole
+  flocks of these crossed geese are kept in various parts of the country;
+  and as they are kept for profit, where neither pure parent-species
+  exists, they must certainly be highly fertile.</p>
+
+  <p>A doctrine which originated with Pallas, has been largely accepted by
+  modern naturalists; namely, that most of our domestic animals have
+  descended from two or more wild species, since commingled by
+  intercrossing. On this view, the aboriginal species must either at first
+  have produced quite fertile hybrids, or the hybrids must have become in
+  subsequent generations quite fertile under domestication. This latter
+  alternative seems to me the most probable, and I am inclined to believe
+  in its truth, although it rests on no direct evidence. I believe, for
+  instance, that our dogs have descended from several wild stocks; yet,
+  with perhaps the exception of certain indigenous domestic dogs of South
+  America, all are quite fertile together; and analogy makes me greatly
+  doubt, whether the several aboriginal species would at first have freely
+  bred together and have produced quite fertile hybrids. So again there is
+  reason to believe that our European and the humped Indian cattle are
+  quite fertile together; but from facts communicated to me by Mr. Blyth, I
+  think they must be considered as distinct species. On this view of the
+  origin of many of our domestic animals, we must either give up the belief
+  of the almost universal sterility of distinct species of animals when
+  crossed; or we must look at sterility, not as an indelible
+  characteristic, but as one capable of being removed by domestication.</p>
+
+  <p>Finally, looking to all the ascertained facts on the intercrossing of
+  plants and animals, it may be concluded that some degree of sterility,
+  both in first crosses <!-- Page 255 --><span class="pagenum"><a
+  name="page255"></a>[255]</span>and in hybrids, is an extremely general
+  result; but that it cannot, under our present state of knowledge, be
+  considered as absolutely universal.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Laws governing the Sterility of first Crosses and of
+  Hybrids.</i>&mdash;We will now consider a little more in detail the
+  circumstances and rules governing the sterility of first crosses and of
+  hybrids. Our chief object will be to see whether or not the rules
+  indicate that species have specially been endowed with this quality, in
+  order to prevent their crossing and blending together in utter confusion.
+  The following rules and conclusions are chiefly drawn up from Gärtner's
+  admirable work on the hybridisation of plants. I have taken much pains to
+  ascertain how far the rules apply to animals, and considering how scanty
+  our knowledge is in regard to hybrid animals, I have been surprised to
+  find how generally the same rules apply to both kingdoms.</p>
+
+  <p>It has been already remarked, that the degree of fertility, both of
+  first crosses and of hybrids, graduates from zero to perfect fertility.
+  It is surprising in how many curious ways this gradation can be shown to
+  exist; but only the barest outline of the facts can here be given. When
+  pollen from a plant of one family is placed on the stigma of a plant of a
+  distinct family, it exerts no more influence than so much inorganic dust.
+  From this absolute zero of fertility, the pollen of different species of
+  the same genus applied to the stigma of some one species, yields a
+  perfect gradation in the number of seeds produced, up to nearly complete
+  or even quite complete fertility; and, as we have seen, in certain
+  abnormal cases, even to an excess of fertility, beyond that which the
+  plant's own pollen will produce. So in hybrids themselves, there are some
+  which never have produced, and probably never would produce, even <!--
+  Page 256 --><span class="pagenum"><a name="page256"></a>[256]</span>with
+  the pollen of either pure parent, a single fertile seed: but in some of
+  these cases a first trace of fertility may be detected, by the pollen of
+  one of the pure parent-species causing the flower of the hybrid to wither
+  earlier than it otherwise would have done; and the early withering of the
+  flower is well known to be a sign of incipient fertilisation. From this
+  extreme degree of sterility we have self-fertilised hybrids producing a
+  greater and greater number of seeds up to perfect fertility.</p>
+
+  <p>Hybrids from two species which are very difficult to cross, and which
+  rarely produce any offspring, are generally very sterile; but the
+  parallelism between the difficulty of making a first cross, and the
+  sterility of the hybrids thus produced&mdash;two classes of facts which
+  are generally confounded together&mdash;is by no means strict. There are
+  many cases, in which two pure species can be united with unusual
+  facility, and produce numerous hybrid-offspring, yet these hybrids are
+  remarkably sterile. On the other hand, there are species which can be
+  crossed very rarely, or with extreme difficulty, but the hybrids, when at
+  last produced, are very fertile. Even within the limits of the same
+  genus, for instance in Dianthus, these two opposite cases occur.</p>
+
+  <p>The fertility, both of first crosses and of hybrids, is more easily
+  affected by unfavourable conditions, than is the fertility of pure
+  species. But the degree of fertility is likewise innately variable; for
+  it is not always the same when the same two species are crossed under the
+  same circumstances, but depends in part upon the constitution of the
+  individuals which happen to have been chosen for the experiment. So it is
+  with hybrids, for their degree of fertility is often found to differ
+  greatly in the several individuals raised from seed out of the same
+  capsule and exposed to exactly the same conditions. <!-- Page 257
+  --><span class="pagenum"><a name="page257"></a>[257]</span></p>
+
+  <p>By the term systematic affinity is meant, the resemblance between
+  species in structure and in constitution, more especially in the
+  structure of parts which are of high physiological importance and which
+  differ little in the allied species. Now the fertility of first crosses
+  between species, and of the hybrids produced from them, is largely
+  governed by their systematic affinity. This is clearly shown by hybrids
+  never having been raised between species ranked by systematists in
+  distinct families; and on the other hand, by very closely allied species
+  generally uniting with facility. But the correspondence between
+  systematic affinity and the facility of crossing is by no means strict. A
+  multitude of cases could be given of very closely allied species which
+  will not unite, or only with extreme difficulty; and on the other hand of
+  very distinct species which unite with the utmost facility. In the same
+  family there may be a genus, as Dianthus, in which very many species can
+  most readily be crossed; and another genus, as Silene, in which the most
+  persevering efforts have failed to produce between extremely close
+  species a single hybrid. Even within the limits of the same genus, we
+  meet with this same difference; for instance, the many species of
+  Nicotiana have been more largely crossed than the species of almost any
+  other genus; but Gärtner found that N. acuminata, which is not a
+  particularly distinct species, obstinately failed to fertilise, or to be
+  fertilised by, no less than eight other species of Nicotiana. Very many
+  analogous facts could be given.</p>
+
+  <p>No one has been able to point out what kind, or what amount, of
+  difference in any recognisable character is sufficient to prevent two
+  species crossing. It can be shown that plants most widely different in
+  habit and general appearance, and having strongly marked <!-- Page 258
+  --><span class="pagenum"><a name="page258"></a>[258]</span>differences in
+  every part of the flower, even in the pollen, in the fruit, and in the
+  cotyledons, can be crossed. Annual and perennial plants, deciduous and
+  evergreen trees, plants inhabiting different stations and fitted for
+  extremely different climates, can often be crossed with ease.</p>
+
+  <p>By a reciprocal cross between two species, I mean the case, for
+  instance, of a stallion-horse being first crossed with a female-ass, and
+  then a male-ass with a mare: these two species may then be said to have
+  been reciprocally crossed. There is often the widest possible difference
+  in the facility of making reciprocal crosses. Such cases are highly
+  important, for they prove that the capacity in any two species to cross
+  is often completely independent of their systematic affinity, or of any
+  recognisable difference in their whole organisation. On the other hand,
+  these cases clearly show that the capacity for crossing is connected with
+  constitutional differences imperceptible by us, and confined to the
+  reproductive system. This difference in the result of reciprocal crosses
+  between the same two species was long ago observed by Kölreuter. To give
+  an instance: Mirabilis jalapa can easily be fertilised by the pollen of
+  M. longiflora, and the hybrids thus produced are sufficiently fertile;
+  but Kölreuter tried more than two hundred times, during eight following
+  years, to fertilise reciprocally M. longiflora with the pollen of M.
+  jalapa, and utterly failed. Several other equally striking cases could be
+  given. Thuret has observed the same fact with certain sea-weeds or Fuci.
+  Gärtner, moreover, found that this difference of facility in making
+  reciprocal crosses is extremely common in a lesser degree. He has
+  observed it even between forms so closely related (as Matthiola annua and
+  glabra) that many botanists rank them only as varieties. It is also a
+  remarkable fact, that hybrids raised from reciprocal crosses, though <!--
+  Page 259 --><span class="pagenum"><a name="page259"></a>[259]</span>of
+  course compounded of the very same two species, the one species having
+  first been used as the father and then as the mother, generally differ in
+  fertility in a small, and occasionally in a high degree.</p>
+
+  <p>Several other singular rules could be given from Gärtner: for
+  instance, some species have a remarkable power of crossing with other
+  species; other species of the same genus have a remarkable power of
+  impressing their likeness on their hybrid offspring; but these two powers
+  do not at all necessarily go together. There are certain hybrids which
+  instead of having, as is usual, an intermediate character between their
+  two parents, always closely resemble one of them; and such hybrids,
+  though externally so like one of their pure parent-species, are with rare
+  exceptions extremely sterile. So again amongst hybrids which are usually
+  intermediate in structure between their parents, exceptional and abnormal
+  individuals sometimes are born, which closely resemble one of their pure
+  parents; and these hybrids are almost always utterly sterile, even when
+  the other hybrids raised from seed from the same capsule have a
+  considerable degree of fertility. These facts show how completely
+  fertility in the hybrid is independent of its external resemblance to
+  either pure parent.</p>
+
+  <p>Considering the several rules now given, which govern the fertility of
+  first crosses and of hybrids, we see that when forms, which must be
+  considered as good and distinct species, are united, their fertility
+  graduates from zero to perfect fertility, or even to fertility under
+  certain conditions in excess. That their fertility, besides being
+  eminently susceptible to favourable and unfavourable conditions, is
+  innately variable. That it is by no means always the same in degree in
+  the first cross and in the hybrids produced <!-- Page 260 --><span
+  class="pagenum"><a name="page260"></a>[260]</span>from this cross. That
+  the fertility of hybrids is not related to the degree in which they
+  resemble in external appearance either parent. And lastly, that the
+  facility of making a first cross between any two species is not always
+  governed by their systematic affinity or degree of resemblance to each
+  other. This latter statement is clearly proved by reciprocal crosses
+  between the same two species, for according as the one species or the
+  other is used as the father or the mother, there is generally some
+  difference, and occasionally the widest possible difference, in the
+  facility of effecting an union. The hybrids, moreover, produced from
+  reciprocal crosses often differ in fertility.</p>
+
+  <p>Now do these complex and singular rules indicate that species have
+  been endowed with sterility simply to prevent their becoming confounded
+  in nature? I think not. For why should the sterility be so extremely
+  different in degree, when various species are crossed, all of which we
+  must suppose it would be equally important to keep from blending
+  together? Why should the degree of sterility be innately variable in the
+  individuals of the same species? Why should some species cross with
+  facility, and yet produce very sterile hybrids; and other species cross
+  with extreme difficulty, and yet produce fairly fertile hybrids? Why
+  should there often be so great a difference in the result of a reciprocal
+  cross between the same two species? Why, it may even be asked, has the
+  production of hybrids been permitted? to grant to species the special
+  power of producing hybrids, and then to stop their further propagation by
+  different degrees of sterility, not strictly related to the facility of
+  the first union between their parents, seems to be a strange
+  arrangement.</p>
+
+  <p>The foregoing rules and facts, on the other hand, <!-- Page 261
+  --><span class="pagenum"><a name="page261"></a>[261]</span>appear to me
+  clearly to indicate that the sterility both of first crosses and of
+  hybrids is simply incidental or dependent on unknown differences, chiefly
+  in the reproductive systems, of the species which are crossed. The
+  differences being of so peculiar and limited a nature, that, in
+  reciprocal crosses between two species the male sexual element of the one
+  will often freely act on the female sexual element of the other, but not
+  in a reversed direction. It will be advisable to explain a little more
+  fully by an example what I mean by sterility being incidental on other
+  differences, and not a specially endowed quality. As the capacity of one
+  plant to be grafted or budded on another is so entirely unimportant for
+  its welfare in a state of nature, I presume that no one will suppose that
+  this capacity is a <i>specially</i> endowed quality, but will admit that
+  it is incidental on differences in the laws of growth of the two plants.
+  We can sometimes see the reason why one tree will not take on another,
+  from differences in their rate of growth, in the hardness of their wood,
+  in the period of the flow or nature of their sap, &amp;c.; but in a
+  multitude of cases we can assign no reason whatever. Great diversity in
+  the size of two plants, one being woody and the other herbaceous, one
+  being evergreen and the other deciduous, and adaptation to widely
+  different climates, does not always prevent the two grafting together. As
+  in hybridisation, so with grafting, the capacity is limited by systematic
+  affinity, for no one has been able to graft trees together belonging to
+  quite distinct families; and, on the other hand, closely allied species,
+  and varieties of the same species, can usually, but not invariably, be
+  grafted with ease. But this capacity, as in hybridisation, is by no means
+  absolutely governed by systematic affinity. Although many distinct genera
+  within the same family have been grafted <!-- Page 262 --><span
+  class="pagenum"><a name="page262"></a>[262]</span>together, in other
+  cases species of the same genus will not take on each other. The pear can
+  be grafted far more readily on the quince, which is ranked as a distinct
+  genus, than on the apple, which is a member of the same genus. Even
+  different varieties of the pear take with different degrees of facility
+  on the quince; so do different varieties of the apricot and peach on
+  certain varieties of the plum.</p>
+
+  <p>As Gärtner found that there was sometimes an innate difference in
+  different <i>individuals</i> of the same two species in crossing; so
+  Sagaret believes this to be the case with different individuals of the
+  same two species in being grafted together. As in reciprocal crosses, the
+  facility of effecting an union is often very far from equal, so it
+  sometimes is in grafting; the common gooseberry, for instance, cannot be
+  grafted on the currant, whereas the currant will take, though with
+  difficulty, on the gooseberry.</p>
+
+  <p>We have seen that the sterility of hybrids, which have their
+  reproductive organs in an imperfect condition, is a very different case
+  from the difficulty of uniting two pure species, which have their
+  reproductive organs perfect; yet these two distinct cases run to a
+  certain extent parallel. Something analogous occurs in grafting; for
+  Thouin found that three species of Robinia, which seeded freely on their
+  own roots, and which could be grafted with no great difficulty on another
+  species, when thus grafted were rendered barren. On the other hand,
+  certain species of Sorbus, when grafted on other species, yielded twice
+  as much fruit as when on their own roots. We are reminded by this latter
+  fact of the extraordinary case of Hippeastrum, Lobelia, &amp;c., which
+  seeded much more freely when fertilised with the pollen of distinct
+  species, than when self-fertilised with their own pollen. <!-- Page 263
+  --><span class="pagenum"><a name="page263"></a>[263]</span></p>
+
+  <p>We thus see, that although there is a clear and fundamental difference
+  between the mere adhesion of grafted stocks, and the union of the male
+  and female elements in the act of reproduction, yet that there is a rude
+  degree of parallelism in the results of grafting and of crossing distinct
+  species. And as we must look at the curious and complex laws governing
+  the facility with which trees can be grafted on each other as incidental
+  on unknown differences in their vegetative systems, so I believe that the
+  still more complex laws governing the facility of first crosses, are
+  incidental on unknown differences, chiefly in their reproductive systems.
+  These differences, in both cases, follow to a certain extent, as might
+  have been expected, systematic affinity, by which every kind of
+  resemblance and dissimilarity between organic beings is attempted to be
+  expressed. The facts by no means seem to me to indicate that the greater
+  or lesser difficulty of either grafting or crossing together various
+  species has been a special endowment; although in the case of crossing,
+  the difficulty is as important for the endurance and stability of
+  specific forms, as in the case of grafting it is unimportant for their
+  welfare.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Causes of the Sterility of first Crosses and of
+  Hybrids.</i>&mdash;We may now look a little closer at the probable causes
+  of the sterility of first crosses and of hybrids. These two cases are
+  fundamentally different, for, as just remarked, in the union of two pure
+  species the male and female sexual elements are perfect, whereas in
+  hybrids they are imperfect. Even in first crosses, the greater or lesser
+  difficulty in effecting a union apparently depends on several distinct
+  causes. There must sometimes be a physical impossibility in the male
+  element reaching the ovule, as would be the case with a plant <!-- Page
+  264 --><span class="pagenum"><a name="page264"></a>[264]</span>having a
+  pistil too long for the pollen-tubes to reach the ovarium. It has also
+  been observed that when pollen of one species is placed on the stigma of
+  a distantly allied species, though the pollen-tubes protrude, they do not
+  penetrate the stigmatic surface. Again, the male element may reach the
+  female element, but be incapable of causing an embryo to be developed, as
+  seems to have been the case with some of Thuret's experiments on Fuci. No
+  explanation can be given of these facts, any more than why certain trees
+  cannot be grafted on others. Lastly, an embryo may be developed, and then
+  perish at an early period. This latter alternative has not been
+  sufficiently attended to; but I believe, from observations communicated
+  to me by Mr. Hewitt, who has had great experience in hybridising
+  gallinaceous birds, that the early death of the embryo is a very frequent
+  cause of sterility in first crosses. I was at first very unwilling to
+  believe in this view; as hybrids, when once born, are generally healthy
+  and long-lived, as we see in the case of the common mule. Hybrids,
+  however, are differently circumstanced before and after birth: when born
+  and living in a country where their two parents can live, they are
+  generally placed under suitable conditions of life. But a hybrid partakes
+  of only half of the nature and constitution of its mother, and therefore
+  before birth, as long as it is nourished within its mother's womb or
+  within the egg or seed produced by the mother, it may be exposed to
+  conditions in some degree unsuitable, and consequently be liable to
+  perish at an early period; more especially as all very young beings seem
+  eminently sensitive to injurious or unnatural conditions of life.</p>
+
+  <p>In regard to the sterility of hybrids, in which the sexual elements
+  are imperfectly developed, the case is <!-- Page 265 --><span
+  class="pagenum"><a name="page265"></a>[265]</span>very different. I have
+  more than once alluded to a large body of facts, which I have collected,
+  showing that when animals and plants are removed from their natural
+  conditions, they are extremely liable to have their reproductive systems
+  seriously affected. This, in fact, is the great bar to the domestication
+  of animals. Between the sterility thus superinduced and that of hybrids,
+  there are many points of similarity. In both cases the sterility is
+  independent of general health, and is often accompanied by excess of size
+  or great luxuriance. In both cases, the sterility occurs in various
+  degrees; in both, the male element is the most liable to be affected; but
+  sometimes the female more than the male. In both, the tendency goes to a
+  certain extent with systematic affinity, for whole groups of animals and
+  plants are rendered impotent by the same unnatural conditions; and whole
+  groups of species tend to produce sterile hybrids. On the other hand, one
+  species in a group will sometimes resist great changes of conditions with
+  unimpaired fertility; and certain species in a group will produce
+  unusually fertile hybrids. No one can tell, till he tries, whether any
+  particular animal will breed under confinement or any exotic plant seed
+  freely under culture; nor can he tell, till he tries, whether any two
+  species of a genus will produce more or less sterile hybrids. Lastly,
+  when organic beings are placed during several generations under
+  conditions not natural to them, they are extremely liable to vary, which
+  is due, as I believe, to their reproductive systems having been specially
+  affected, though in a lesser degree than when sterility ensues. So it is
+  with hybrids, for hybrids in successive generations are eminently liable
+  to vary, as every experimentalist has observed.</p>
+
+  <p>Thus we see that when organic beings are placed under new and
+  unnatural conditions, and when hybrids <!-- Page 266 --><span
+  class="pagenum"><a name="page266"></a>[266]</span>are produced by the
+  unnatural crossing of two species, the reproductive system, independently
+  of the general state of health, is affected by sterility in a very
+  similar manner. In the one case, the conditions of life have been
+  disturbed, though often in so slight a degree as to be inappreciable by
+  us; in the other case, or that of hybrids, the external conditions have
+  remained the same, but the organisation has been disturbed by two
+  different structures and constitutions having been blended into one. For
+  it is scarcely possible that two organisations should be compounded into
+  one, without some disturbance occurring in the development, or periodical
+  action, or mutual relation of the different parts and organs one to
+  another, or to the conditions of life. When hybrids are able to breed
+  <i>inter se</i>, they transmit to their offspring from generation to
+  generation the same compounded organisation, and hence we need not be
+  surprised that their sterility, though in some degree variable, rarely
+  diminishes.</p>
+
+  <p>It must, however, be confessed that we cannot understand, excepting on
+  vague hypotheses, several facts with respect to the sterility of hybrids;
+  for instance, the unequal fertility of hybrids produced from reciprocal
+  crosses; or the increased sterility in those hybrids which occasionally
+  and exceptionally resemble closely either pure parent. Nor do I pretend
+  that the foregoing remarks go to the root of the matter: no explanation
+  is offered why an organism, when placed under unnatural conditions, is
+  rendered sterile. All that I have attempted to show, is that in two
+  cases, in some respects allied, sterility is the common result,&mdash;in
+  the one case from the conditions of life having been disturbed, in the
+  other case from the organisation having been disturbed by two
+  organisations having been compounded into one.</p>
+
+  <p>It may seem fanciful, but I suspect that a similar <!-- Page 267
+  --><span class="pagenum"><a name="page267"></a>[267]</span>parallelism
+  extends to an allied yet very different class of facts. It is an old and
+  almost universal belief, founded, I think, on a considerable body of
+  evidence, that slight changes in the conditions of life are beneficial to
+  all living things. We see this acted on by farmers and gardeners in their
+  frequent exchanges of seed, tubers, &amp;c., from one soil or climate to
+  another, and back again. During the convalescence of animals, we plainly
+  see that great benefit is derived from almost any change in the habits of
+  life. Again, both with plants and animals, there is abundant evidence,
+  that a cross between very distinct individuals of the same species, that
+  is between members of different strains or sub-breeds, gives vigour and
+  fertility to the offspring. I believe, indeed, from the facts alluded to
+  in our fourth chapter, that a certain amount of crossing is indispensable
+  even with hermaphrodites; and that close interbreeding continued during
+  several generations between the nearest relations, especially if these be
+  kept under the same conditions of life, always induces weakness and
+  sterility in the progeny.</p>
+
+  <p>Hence it seems that, on the one hand, slight changes in the conditions
+  of life benefit all organic beings, and on the other hand, that slight
+  crosses, that is crosses between the males and females of the same
+  species which have varied and become slightly different, give vigour and
+  fertility to the offspring. But we have seen that greater changes, or
+  changes of a particular nature, often render organic beings in some
+  degree sterile; and that greater crosses, that is crosses between males
+  and females which have become widely or specifically different, produce
+  hybrids which are generally sterile in some degree. I cannot persuade
+  myself that this parallelism is an accident or an illusion. Both series
+  of facts seem to be connected together by some <!-- Page 268 --><span
+  class="pagenum"><a name="page268"></a>[268]</span>common but unknown
+  bond, which is essentially related to the principle of life.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Fertility of Varieties when crossed, and of their Mongrel
+  offspring.</i>&mdash;It may be urged, as a most forcible argument, that
+  there must be some essential distinction between species and varieties,
+  and that there must be some error in all the foregoing remarks, inasmuch
+  as varieties, however much they may differ from each other in external
+  appearance, cross with perfect facility, and yield perfectly fertile
+  offspring. I fully admit that this is almost invariably the case. But if
+  we look to varieties produced under nature, we are immediately involved
+  in hopeless difficulties; for if two hitherto reputed varieties be found
+  in any degree sterile together, they are at once ranked by most
+  naturalists as species. For instance, the blue and red pimpernel, the
+  primrose and cowslip, which are considered by many of our best botanists
+  as varieties, are said by Gärtner not to be quite fertile when crossed,
+  and he consequently ranks them as undoubted species. If we thus argue in
+  a circle, the fertility of all varieties produced under nature will
+  assuredly have to be granted.</p>
+
+  <p>If we turn to varieties, produced, or supposed to have been produced,
+  under domestication, we are still involved in doubt. For when it is
+  stated, for instance, that the German Spitz dog unites more easily than
+  other dogs with foxes, or that certain South American indigenous domestic
+  dogs do not readily cross with European dogs, the explanation which will
+  occur to every one, and probably the true one, is that these dogs have
+  descended from several aboriginally distinct species. Nevertheless the
+  perfect fertility of so many domestic varieties, differing widely from
+  each other in appearance, for instance of the pigeon or of the cabbage,
+  is <!-- Page 269 --><span class="pagenum"><a
+  name="page269"></a>[269]</span>a remarkable fact; more especially when we
+  reflect how many species there are, which, though resembling each other
+  most closely, are utterly sterile when intercrossed. Several
+  considerations, however, render the fertility of domestic varieties less
+  remarkable than at first appears. It can, in the first place, be clearly
+  shown that mere external dissimilarity between two species does not
+  determine their greater or lesser degree of sterility when crossed; and
+  we may apply the same rule to domestic varieties. In the second place,
+  some eminent naturalists believe that a long course of domestication
+  tends to eliminate sterility in the successive generations of hybrids
+  which were at first only slightly sterile; and if this be so, we surely
+  ought not to expect to find sterility both appearing and disappearing
+  under nearly the same conditions of life. Lastly, and this seems to me by
+  far the most important consideration, new races of animals and plants are
+  produced under domestication by man's methodical and unconscious power of
+  selection, for his own use and pleasure: he neither wishes to select, nor
+  could select, slight differences in the reproductive system, or other
+  constitutional differences correlated with the reproductive system. He
+  supplies his several varieties with the same food; treats them in nearly
+  the same manner, and does not wish to alter their general habits of life.
+  Nature acts uniformly and slowly during vast periods of time on the whole
+  organisation, in any way which may be for each creature's own good; and
+  thus she may, either directly, or more probably indirectly, through
+  correlation, modify the reproductive system in the several descendants
+  from any one species. Seeing this difference in the process of selection,
+  as carried on by man and nature, we need not be surprised at some
+  difference in the result.</p>
+
+  <p>I have as yet spoken as if the varieties of the same <!-- Page 270
+  --><span class="pagenum"><a name="page270"></a>[270]</span>species were
+  invariably fertile when intercrossed. But it seems to me impossible to
+  resist the evidence of the existence of a certain amount of sterility in
+  the few following cases, which I will briefly abstract. The evidence is
+  at least as good as that from which we believe in the sterility of a
+  multitude of species. The evidence is, also, derived from hostile
+  witnesses, who in all other cases consider fertility and sterility as
+  safe criterions of specific distinction. Gärtner kept during several
+  years a dwarf kind of maize with yellow seeds, and a tall variety with
+  red seeds, growing near each other in his garden; and although these
+  plants have separated sexes, they never naturally crossed. He then
+  fertilised thirteen flowers of the one with the pollen of the other; but
+  only a single head produced any seed, and this one head produced only
+  five grains. Manipulation in this case could not have been injurious, as
+  the plants have separated sexes. No one, I believe, has suspected that
+  these varieties of maize are distinct species; and it is important to
+  notice that the hybrid plants thus raised were themselves
+  <i>perfectly</i> fertile; so that even Gärtner did not venture to
+  consider the two varieties as specifically distinct.</p>
+
+  <p>Girou de Buzareingues crossed three varieties of gourd, which like the
+  maize has separated sexes, and he asserts that their mutual fertilisation
+  is by so much the less easy as their differences are greater. How far
+  these experiments may be trusted, I know not; but the forms
+  experimentised on, are ranked by Sagaret, who mainly founds his
+  classification by the test of infertility, as varieties.</p>
+
+  <p>The following case is far more remarkable, and seems at first quite
+  incredible; but it is the result of an astonishing number of experiments
+  made during many years on nine species of Verbascum, by so good an
+  observer <!-- Page 271 --><span class="pagenum"><a
+  name="page271"></a>[271]</span>and so hostile a witness, as Gärtner:
+  namely, that yellow and white varieties of the same species of Verbascum
+  when intercrossed produce less seed, than do either coloured varieties
+  when fertilised with pollen from their own coloured flowers. Moreover, he
+  asserts that when yellow and white varieties of one species are crossed
+  with yellow and white varieties of a <i>distinct</i> species, more seed
+  is produced by the crosses between the similarly coloured flowers, than
+  between those which are differently coloured. Yet these varieties of
+  Verbascum present no other difference besides the mere colour of the
+  flower; and one variety can sometimes be raised from the seed of the
+  other.</p>
+
+  <p>From observations which I have made on certain varieties of hollyhock,
+  I am inclined to suspect that they present analogous facts.</p>
+
+  <p>Kölreuter, whose accuracy has been confirmed by every subsequent
+  observer, has proved the remarkable fact, that one variety of the common
+  tobacco is more fertile, when crossed with a widely distinct species,
+  than are the other varieties. He experimentised on five forms, which are
+  commonly reputed to be varieties, and which he tested by the severest
+  trial, namely, by reciprocal crosses, and he found their mongrel
+  offspring perfectly fertile. But one of these five varieties, when used
+  either as father or mother, and crossed with the Nicotiana glutinosa,
+  always yielded hybrids not so sterile as those which were produced from
+  the four other varieties when crossed with N. glutinosa. Hence the
+  reproductive system of this one variety must have been in some manner and
+  in some degree modified.</p>
+
+  <p>From these facts; from the great difficulty of ascertaining the
+  infertility of varieties in a state of nature, for a supposed variety if
+  infertile in any degree would generally be ranked as species; from man
+  selecting only <!-- Page 272 --><span class="pagenum"><a
+  name="page272"></a>[272]</span>external characters in the production of
+  the most distinct domestic varieties, and from not wishing or being able
+  to produce recondite and functional differences in the reproductive
+  system; from these several considerations and facts, I do not think that
+  the very general fertility of varieties can be proved to be of universal
+  occurrence, or to form a fundamental distinction between varieties and
+  species. The general fertility of varieties does not seem to me
+  sufficient to overthrow the view which I have taken with respect to the
+  very general, but not invariable, sterility of first crosses and of
+  hybrids, namely, that it is not a special endowment, but is incidental on
+  slowly acquired modifications, more especially in the reproductive
+  systems of the forms which are crossed.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Hybrids and Mongrels compared, independently of their
+  fertility.</i>&mdash;Independently of the question of fertility, the
+  offspring of species when crossed and of varieties when crossed may be
+  compared in several other respects. Gärtner, whose strong wish was to
+  draw a marked line of distinction between species and varieties, could
+  find very few and, as it seems to me, quite unimportant differences
+  between the so-called hybrid offspring of species, and the so-called
+  mongrel offspring of varieties. And, on the other hand, they agree most
+  closely in very many important respects.</p>
+
+  <p>I shall here discuss this subject with extreme brevity. The most
+  important distinction is, that in the first generation mongrels are more
+  variable than hybrids; but Gärtner admits that hybrids from species which
+  have long been cultivated are often variable in the first generation; and
+  I have myself seen striking instances of this fact. Gärtner further
+  admits that hybrids between very closely allied species are more variable
+  <!-- Page 273 --><span class="pagenum"><a
+  name="page273"></a>[273]</span>than those from very distinct species; and
+  this shows that the difference in the degree of variability graduates
+  away. When mongrels and the more fertile hybrids are propagated for
+  several generations an extreme amount of variability in their offspring
+  is notorious; but some few cases both of hybrids and mongrels long
+  retaining uniformity of character could be given. The variability,
+  however, in the successive generations of mongrels is, perhaps, greater
+  than in hybrids.</p>
+
+  <p>This greater variability of mongrels than of hybrids does not seem to
+  me at all surprising. For the parents of mongrels are varieties, and
+  mostly domestic varieties (very few experiments having been tried on
+  natural varieties), and this implies in most cases that there has been
+  recent variability; and therefore we might expect that such variability
+  would often continue and be superadded to that arising from the mere act
+  of crossing. The slight degree of variability in hybrids from the first
+  cross or in the first generation, in contrast with their extreme
+  variability in the succeeding generations, is a curious fact and deserves
+  attention. For it bears on and corroborates the view which I have taken
+  on the cause of ordinary variability; namely, that it is due to the
+  reproductive system being eminently sensitive to any change in the
+  conditions of life, being thus often rendered either impotent or at least
+  incapable of its proper function of producing offspring identical with
+  the parent-form. Now hybrids in the first generation are descended from
+  species (excluding those long cultivated) which have not had their
+  reproductive systems in any way affected, and they are not variable; but
+  hybrids themselves have their reproductive systems seriously affected,
+  and their descendants are highly variable.</p>
+
+  <p>But to return to our comparison of mongrels and <!-- Page 274 --><span
+  class="pagenum"><a name="page274"></a>[274]</span>hybrids: Gärtner states
+  that mongrels are more liable than hybrids to revert to either
+  parent-form; but this, if it be true, is certainly only a difference in
+  degree. Gärtner further insists that when any two species, although most
+  closely allied to each other, are crossed with a third species, the
+  hybrids are widely different from each other; whereas if two very
+  distinct varieties of one species are crossed with another species, the
+  hybrids do not differ much. But this conclusion, as far as I can make
+  out, is founded on a single experiment; and seems directly opposed to the
+  results of several experiments made by Kölreuter.</p>
+
+  <p>These alone are the unimportant differences, which Gärtner is able to
+  point out, between hybrid and mongrel plants. On the other hand, the
+  resemblance in mongrels and in hybrids to their respective parents, more
+  especially in hybrids produced from nearly related species, follows
+  according to Gärtner the same laws. When two species are crossed, one has
+  sometimes a prepotent power of impressing its likeness on the hybrid; and
+  so I believe it to be with varieties of plants. With animals one variety
+  certainly often has this prepotent power over another variety. Hybrid
+  plants produced from a reciprocal cross, generally resemble each other
+  closely; and so it is with mongrels from a reciprocal cross. Both hybrids
+  and mongrels can be reduced to either pure parent-form, by repeated
+  crosses in successive generations with either parent.</p>
+
+  <p>These several remarks are apparently applicable to animals; but the
+  subject is here excessively complicated, partly owing to the existence of
+  secondary sexual characters; but more especially owing to prepotency in
+  transmitting likeness running more strongly in one sex than in the other,
+  both when one species is crossed with another, and when, one variety is
+  crossed with <!-- Page 275 --><span class="pagenum"><a
+  name="page275"></a>[275]</span>another variety. For instance, I think
+  those authors are right, who maintain that the ass has a prepotent power
+  over the horse, so that both the mule and the hinny more resemble the ass
+  than the horse; but that the prepotency runs more strongly in the
+  male-ass than in the female, so that the mule, which is the offspring of
+  the male-ass and mare, is more like an ass, than is the hinny, which is
+  the offspring of the female-ass and stallion.</p>
+
+  <p>Much stress has been laid by some authors on the supposed fact, that
+  mongrel animals alone are born closely like one of their parents; but it
+  can be shown that this does sometimes occur with hybrids; yet I grant
+  much less frequently with hybrids than with mongrels. Looking to the
+  cases which I have collected of cross-bred animals closely resembling one
+  parent, the resemblances seem chiefly confined to characters almost
+  monstrous in their nature, and which have suddenly appeared&mdash;such as
+  albinism, melanism, deficiency of tail or horns, or additional fingers
+  and toes; and do not relate to characters which have been slowly acquired
+  by selection. Consequently, sudden reversions to the perfect character of
+  either parent would be more likely to occur with mongrels, which are
+  descended from varieties often suddenly produced and semi-monstrous in
+  character, than with hybrids, which are descended from species slowly and
+  naturally produced. On the whole I entirely agree with Dr. Prosper Lucas,
+  who, after arranging an enormous body of facts with respect to animals,
+  comes to the conclusion, that the laws of resemblance of the child to its
+  parents are the same, whether the two parents differ much or little from
+  each other, namely in the union of individuals of the same variety, or of
+  different varieties, or of distinct species.</p>
+
+  <p>Laying aside the question of fertility and sterility, <!-- Page 276
+  --><span class="pagenum"><a name="page276"></a>[276]</span>in all other
+  respects there seems to be a general and close similarity in the
+  offspring of crossed species, and of crossed varieties. If we look at
+  species as having been specially created, and at varieties as having been
+  produced by secondary laws, this similarity would be an astonishing fact.
+  But it harmonises perfectly with the view that there is no essential
+  distinction between species and varieties.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Summary of Chapter.</i>&mdash;First crosses between forms
+  sufficiently distinct to be ranked as species, and their hybrids, are
+  very generally, but not universally, sterile. The sterility is of all
+  degrees, and is often so slight that the two most careful
+  experimentalists who have ever lived, have come to diametrically opposite
+  conclusions in ranking forms by this test. The sterility is innately
+  variable in individuals of the same species, and is eminently susceptible
+  of favourable and unfavourable conditions. The degree of sterility does
+  not strictly follow systematic affinity, but is governed by several
+  curious and complex laws. It is generally different, and sometimes widely
+  different, in reciprocal crosses between the same two species. It is not
+  always equal in degree in a first cross and in the hybrid produced from
+  this cross.</p>
+
+  <p>In the same manner as in grafting trees, the capacity of one species
+  or variety to take on another, is incidental on generally unknown
+  differences in their vegetative systems, so in crossing, the greater or
+  less facility of one species to unite with another, is incidental on
+  unknown differences in their reproductive systems. There is no more
+  reason to think that species have been specially endowed with various
+  degrees of sterility to prevent them crossing and blending in nature,
+  than to think that trees have been specially endowed with various and
+  <!-- Page 277 --><span class="pagenum"><a
+  name="page277"></a>[277]</span>somewhat analogous degrees of difficulty
+  in being grafted together in order to prevent them becoming inarched in
+  our forests.</p>
+
+  <p>The sterility of first crosses between pure species, which have their
+  reproductive systems perfect, seems to depend on several circumstances;
+  in some cases largely on the early death of the embryo. The sterility of
+  hybrids, which have their reproductive systems imperfect, and which have
+  had this system and their whole organisation disturbed by being
+  compounded of two distinct species, seems closely allied to that
+  sterility which so frequently affects pure species, when their natural
+  conditions of life have been disturbed. This view is supported by a
+  parallelism of another kind;&mdash;namely, that the crossing of forms
+  only slightly different is favourable to the vigour and fertility of
+  their offspring; and that slight changes in the conditions of life are
+  apparently favourable to the vigour and fertility of all organic beings.
+  It is not surprising that the degree of difficulty in uniting two
+  species, and the degree of sterility of their hybrid-offspring should
+  generally correspond, though due to distinct causes; for both depend on
+  the amount of difference of some kind between the species which are
+  crossed. Nor is it surprising that the facility of effecting a first
+  cross, the fertility of the hybrids produced from it, and the capacity of
+  being grafted together&mdash;though this latter capacity evidently
+  depends on widely different circumstances&mdash;should all run, to a
+  certain extent, parallel with the systematic affinity of the forms which
+  are subjected to experiment; for systematic affinity attempts to express
+  all kinds of resemblance between all species.</p>
+
+  <p>First crosses between forms known to be varieties, or sufficiently
+  alike to be considered as varieties, and their mongrel offspring, are
+  very generally, but not quite <!-- Page 278 --><span class="pagenum"><a
+  name="page278"></a>[278]</span>universally, fertile. Nor is this nearly
+  general and perfect fertility surprising, when we remember how liable we
+  are to argue in a circle with respect to varieties in a state of nature;
+  and when we remember that the greater number of varieties have been
+  produced under domestication by the selection of mere external
+  differences, and not of differences in the reproductive system. In all
+  other respects, excluding fertility, there is a close general resemblance
+  between hybrids and mongrels. Finally, then, the facts briefly given in
+  this chapter do not seem to me opposed to, but even rather to support the
+  view, that there is no fundamental distinction between species and
+  varieties.</p>
+
+<hr />
+
+<p><!-- Page 279 --><span class="pagenum"><a name="page279"></a>[279]</span></p>
+
+<h3>CHAPTER IX.</h3>
+
+<p class="cenhead"><span class="sc">On the Imperfection of the Geological Record.</span></p>
+
+<blockquote class="b1n">
+
+  <p>On the absence of intermediate varieties at the present day&mdash;On
+  the nature of extinct intermediate varieties; on their number&mdash;On
+  the vast lapse of time, as inferred from the rate of deposition and of
+  denudation&mdash;On the poorness of our palæontological
+  collections&mdash;On the intermittence of geological formations&mdash;On
+  the absence of intermediate varieties in any one formation&mdash;On the
+  sudden appearance of groups of species&mdash;On their sudden appearance
+  in the lowest known fossiliferous strata.</p>
+
+</blockquote>
+
+  <p>In the sixth chapter I enumerated the chief objections which might be
+  justly urged against the views maintained in this volume. Most of them
+  have now been discussed. One, namely the distinctness of specific forms,
+  and their not being blended together by innumerable transitional links,
+  is a very obvious difficulty. I assigned reasons why such links do not
+  commonly occur at the present day, under the circumstances apparently
+  most favourable for their presence, namely on an extensive and continuous
+  area with graduated physical conditions. I endeavoured to show, that the
+  life of each species depends in a more important manner on the presence
+  of other already defined organic forms, than on climate; and, therefore,
+  that the really governing conditions of life do not graduate away quite
+  insensibly like heat or moisture. I endeavoured, also, to show that
+  intermediate varieties, from existing in lesser numbers than the forms
+  which they connect, will generally be beaten out and exterminated during
+  the course of further modification and improvement. The main cause,
+  however, of innumerable intermediate links not now occurring everywhere
+  throughout nature <!-- Page 280 --><span class="pagenum"><a
+  name="page280"></a>[280]</span>depends on the very process of natural
+  selection, through which new varieties continually take the places of and
+  exterminate their parent-forms. But just in proportion as this process of
+  extermination has acted on an enormous scale, so must the number of
+  intermediate varieties, which have formerly existed on the earth, be
+  truly enormous. Why then is not every geological formation and every
+  stratum full of such intermediate links? Geology assuredly does not
+  reveal any such finely graduated organic chain; and this, perhaps, is the
+  most obvious and gravest objection which can be urged against my theory.
+  The explanation lies, as I believe, in the extreme imperfection of the
+  geological record.</p>
+
+  <p>In the first place it should always be borne in mind what sort of
+  intermediate forms must, on my theory, have formerly existed. I have
+  found it difficult, when looking at any two species, to avoid picturing
+  to myself, forms <i>directly</i> intermediate between them. But this is a
+  wholly false view; we should always look for forms intermediate between
+  each species and a common but unknown progenitor; and the progenitor will
+  generally have differed in some respects from all its modified
+  descendants. To give a simple illustration: the fantail and pouter
+  pigeons have both descended from the rock-pigeon; if we possessed all the
+  intermediate varieties which have ever existed, we should have an
+  extremely close series between both and the rock-pigeon; but we should
+  have no varieties directly intermediate between the fantail and pouter;
+  none, for instance, combining a tail somewhat expanded with a crop
+  somewhat enlarged, the characteristic features of these two breeds. These
+  two breeds, moreover, have become so much modified, that if we had no
+  historical or indirect evidence regarding their origin, it would not have
+  been possible to have <!-- Page 281 --><span class="pagenum"><a
+  name="page281"></a>[281]</span>determined from a mere comparison of their
+  structure with that of the rock-pigeon, whether they had descended from
+  this species or from some other allied species, such as C. oenas.</p>
+
+  <p>So with natural species, if we look to forms very distinct, for
+  instance to the horse and tapir, we have no reason to suppose that links
+  ever existed directly intermediate between them, but between each and an
+  unknown common parent. The common parent will have had in its whole
+  organisation much general resemblance to the tapir and to the horse; but
+  in some points of structure may have differed considerably from both,
+  even perhaps more than they differ from each other. Hence in all such
+  cases, we should be unable to recognise the parent-form of any two or
+  more species, even if we closely compared the structure of the parent
+  with that of its modified descendants, unless at the same time we had a
+  nearly perfect chain of the intermediate links.</p>
+
+  <p>It is just possible by my theory, that one of two living forms might
+  have descended from the other; for instance, a horse from a tapir; and in
+  this case <i>direct</i> intermediate links will have existed between
+  them. But such a case would imply that one form had remained for a very
+  long period unaltered, whilst its descendants had undergone a vast amount
+  of change; and the principle of competition between organism and
+  organism, between child and parent, will render this a very rare event;
+  for in all cases the new and improved forms of life tend to supplant the
+  old and unimproved forms.</p>
+
+  <p>By the theory of natural selection all living species have been
+  connected with the parent-species of each genus, by differences not
+  greater than we see between the varieties of the same species at the
+  present <!-- Page 282 --><span class="pagenum"><a
+  name="page282"></a>[282]</span>day; and these parent-species, now
+  generally extinct, have in their turn been similarly connected with more
+  ancient species; and so on backwards, always converging to the common
+  ancestor of each great class. So that the number of intermediate and
+  transitional links, between all living and extinct species, must have
+  been inconceivably great. But assuredly, if this theory be true, such
+  have lived upon this earth.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the lapse of Time.</i>&mdash;Independently of our not finding
+  fossil remains of such infinitely numerous connecting links, it may be
+  objected, that time will not have sufficed for so great an amount of
+  organic change, all changes having been effected very slowly through
+  natural selection. It is hardly possible for me even to recall to the
+  reader, who may not be a practical geologist, the facts leading the mind
+  feebly to comprehend the lapse of time. He who can read Sir Charles
+  Lyell's grand work on the Principles of Geology, which the future
+  historian will recognise as having produced a revolution in natural
+  science, yet does not admit how incomprehensively vast have been the past
+  periods of time, may at once close this volume. Not that it suffices to
+  study the Principles of Geology, or to read special treatises by
+  different observers on separate formations, and to mark how each author
+  attempts to give an inadequate idea of the duration of each formation or
+  even each stratum. A man must for years examine for himself great piles
+  of superimposed strata, and watch the sea at work grinding down old rocks
+  and making fresh sediment, before he can hope to comprehend anything of
+  the lapse of time, the monuments of which we see around us.</p>
+
+  <p>It is good to wander along lines of sea-coast, when formed of
+  moderately hard rocks, and mark the <!-- Page 283 --><span
+  class="pagenum"><a name="page283"></a>[283]</span>process of degradation.
+  The tides in most cases reach the cliffs only for a short time twice a
+  day, and the waves eat into them only when they are charged with sand or
+  pebbles; for there is good evidence that pure water can effect little or
+  nothing in wearing away rock. At last the base of the cliff is
+  undermined, huge fragments fall down, and these remaining fixed, have to
+  be worn away, atom by atom, until reduced in size they can be rolled
+  about by the waves, and then are more quickly ground into pebbles, sand,
+  or mud. But how often do we see along the bases of retreating cliffs
+  rounded boulders, all thickly clothed by marine productions, showing how
+  little they are abraded and how seldom they are rolled about! Moreover,
+  if we follow for a few miles any line of rocky cliff, which is undergoing
+  degradation, we find that it is only here and there, along a short length
+  or round a promontory, that the cliffs are at the present time suffering.
+  The appearance of the surface and the vegetation show that elsewhere
+  years have elapsed since the waters washed their base.</p>
+
+  <p>He who most closely studies the action of the sea on our shores, will,
+  I believe, be most deeply impressed with the slowness with which rocky
+  coasts are worn away. The observations on this head by Hugh Miller, and
+  by that excellent observer Mr. Smith of Jordan Hill, are most impressive.
+  With the mind thus impressed, let any one examine beds of conglomerate
+  many thousand feet in thickness, which, though probably formed at a
+  quicker rate than many other deposits, yet, from being formed of worn and
+  rounded pebbles, each of which bears the stamp of time, are good to show
+  how slowly the mass has been accumulated. In the Cordillera I estimated
+  one pile of conglomerate at ten thousand feet in thickness. Let the <!--
+  Page 284 --><span class="pagenum"><a
+  name="page284"></a>[284]</span>observer remember Lyell's profound remark
+  that the thickness and extent of sedimentary formations are the result
+  and measure of the degradation which the earth's crust has elsewhere
+  suffered. And what an amount of degradation is implied by the sedimentary
+  deposits of many countries! Professor Ramsay has given me the maximum
+  thickness, in most cases from actual measurement, in a few cases from
+  estimate, of each formation in different parts of Great Britain; and this
+  is the result:&mdash;</p>
+
+
+<table width="43%" class="nobctr" summary="thickness of strata" title="thickness of strata">
+  <tr>
+    <td class="spacsingle" style="text-align:right" colspan="2">
+      <p>Feet.</p>
+    </td>
+  </tr>
+
+  <tr>
+    <td class="spacsingle" style="text-align:left; width:85%">
+      <p>Palæozoic strata (not including igneous beds)</p>
+    </td>
+    <td class="spacsingle" style="text-align:right; width:14%">
+      <p>57,154</p>
+    </td>
+  </tr>
+
+  <tr>
+    <td class="spacsingle" style="text-align:left">
+      <p>Secondary strata</p>
+    </td>
+    <td class="spacsingle" style="text-align:right">
+      <p>13,190</p>
+    </td>
+  </tr>
+
+  <tr>
+    <td class="spacsingle" style="text-align:left">
+      <p>Tertiary strata</p>
+    </td>
+    <td class="spacsingle" style="text-align:right">
+      <p>2,240</p>
+    </td>
+  </tr>
+</table>
+
+  <p>&mdash;making altogether 72,584 feet; that is, very nearly thirteen
+  and three-quarters British miles. Some of the formations, which are
+  represented in England by thin beds, are thousands of feet in thickness
+  on the Continent. Moreover, between each successive formation, we have,
+  in the opinion of most geologists, enormously long blank periods. So that
+  the lofty pile of sedimentary rocks in Britain, gives but an inadequate
+  idea of the time which has elapsed during their accumulation; yet what
+  time this must have consumed! Good observers have estimated that sediment
+  is deposited by the great Mississippi river at the rate of only 600 feet
+  in a hundred thousand years. This estimate has no pretension to strict
+  exactness; yet, considering over what wide spaces very fine sediment is
+  transported by the currents of the sea, the process of accumulation in
+  any one area must be extremely slow.</p>
+
+  <p>But the amount of denudation which the strata have in many places
+  suffered, independently of the rate of accumulation of the degraded
+  matter, probably offers the best evidence of the lapse of time. I
+  remember <!-- Page 285 --><span class="pagenum"><a
+  name="page285"></a>[285]</span>having been much struck with the evidence
+  of denudation, when viewing volcanic islands, which have been worn by the
+  waves and pared all round into perpendicular cliffs of one or two
+  thousand feet in height; for the gentle slope of the lava-streams, due to
+  their formerly liquid state, showed at a glance how far the hard, rocky
+  beds had once extended into the open ocean. The same story is still more
+  plainly told by faults,&mdash;those great cracks along which the strata
+  have been upheaved on one side, or thrown down on the other, to the
+  height or depth of thousands of feet; for since the crust cracked, the
+  surface of the land has been so completely planed down by the action of
+  the sea, that no trace of these vast dislocations is externally
+  visible.</p>
+
+  <p>The Craven fault, for instance, extends for upwards of 30 miles, and
+  along this line the vertical displacement of the strata has varied from
+  600 to 3000 feet. Prof. Ramsay has published an account of a downthrow in
+  Anglesea of 2300 feet; and he informs me that he fully believes there is
+  one in Merionethshire of 12,000 feet; yet in these cases there is nothing
+  on the surface to show such prodigious movements; the pile of rocks on
+  the one or other side having been smoothly swept away. The consideration
+  of these facts impresses my mind almost in the same manner as does the
+  vain endeavour to grapple with the idea of eternity.</p>
+
+  <p>I am tempted to give one other case, the well-known one of the
+  denudation of the Weald. Though it must be admitted that the denudation
+  of the Weald has been a mere trifle, in comparison with that which has
+  removed masses of our palæozoic strata, in parts ten thousand feet in
+  thickness, as shown in Prof. Ramsay's masterly memoir on this subject:
+  yet it is an admirable lesson to stand on the intermediate hilly country
+  and look on the one hand at the North Downs, and <!-- Page 286 --><span
+  class="pagenum"><a name="page286"></a>[286]</span>on the other hand at
+  the South Downs; for, remembering that at no great distance to the west
+  the northern and southern escarpments meet and close, one can safely
+  picture to oneself the great dome of rocks which must have covered up the
+  Weald within so limited a period as since the latter part of the Chalk
+  formation. The distance from the northern to the southern Downs is about
+  22 miles, and the thickness of the several formations is on an average
+  about 1100 feet, as I am informed by Prof. Ramsay. But if, as some
+  geologists suppose, a range of older rocks underlies the Weald, on the
+  flanks of which the overlying sedimentary deposits might have accumulated
+  in thinner masses than elsewhere, the above estimate would be erroneous;
+  but this source of doubt probably would not greatly affect the estimate
+  as applied to the western extremity of the district. If, then, we knew
+  the rate at which the sea commonly wears away a line of cliff of any
+  given height, we could measure the time requisite to have denuded the
+  Weald. This, of course cannot be done; but we may, in order to form some
+  crude notion on the subject, assume that the sea would eat into cliffs
+  500 feet in height at the rate of one inch in a century. This will at
+  first appear much too small an allowance; but it is the same as if we
+  were to assume a cliff one yard in height to be eaten back along a whole
+  line of coast at the rate of one yard in nearly every twenty-two years. I
+  doubt whether any rock, even as soft as chalk, would yield at this rate
+  excepting on the most exposed coasts; though no doubt the degradation of
+  a lofty cliff would be more rapid from the breakage of the fallen
+  fragments. On the other hand, I do not believe that any line of coast,
+  ten or twenty miles in length, ever suffers degradation at the same time
+  along its whole indented length; and we <!-- Page 287 --><span
+  class="pagenum"><a name="page287"></a>[287]</span>must remember that
+  almost all strata contain harder layers or nodules, which from long
+  resisting attrition form a breakwater at the base. We may at least
+  confidently believe that no rocky coast 500 feet in height commonly
+  yields at the rate of a foot per century; for this would be the same in
+  amount as a cliff one yard in height retreating twelve yards in
+  twenty-two years; and no one, I think, who has carefully observed the
+  shape of old fallen fragments at the base of cliffs, will admit any near
+  approach to such rapid wearing away. Hence, under ordinary circumstances,
+  I should infer that for a cliff 500 feet in height, a denudation of one
+  inch per century for the whole length would be a sufficient allowance. At
+  this rate, on the above data, the denudation of the Weald must have
+  required 306,662,400 years; or say three hundred million years. But
+  perhaps it would be safer to allow two or three inches per century, and
+  this would reduce the number of years to one hundred and fifty or one
+  hundred million years.</p>
+
+  <p>The action of fresh water on the gently inclined Wealden district,
+  when upraised, could hardly have been great, but it would somewhat reduce
+  the above estimate. On the other hand, during oscillations of level,
+  which we know this area has undergone, the surface may have existed for
+  millions of years as land, and thus have escaped the action of the sea:
+  when deeply submerged for perhaps equally long periods, it would,
+  likewise, have escaped the action of the coast-waves. So that it is not
+  improbable that a longer period than 300 million years has elapsed since
+  the latter part of the Secondary period.</p>
+
+  <p>I have made these few remarks because it is highly important for us to
+  gain some notion, however imperfect, of the lapse of years. During each
+  of these years, <!-- Page 288 --><span class="pagenum"><a
+  name="page288"></a>[288]</span>over the whole world, the land and the
+  water has been peopled by hosts of living forms. What an infinite number
+  of generations, which the mind cannot grasp, must have succeeded each
+  other in the long roll of years! Now turn to our richest geological
+  museums, and what a paltry display we behold!</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the poorness of our Palæontological collections.</i>&mdash;That
+  our palæontological collections are very imperfect, is admitted by every
+  one. The remark of that admirable palæontologist, the late Edward Forbes,
+  should not be forgotten, namely, that numbers of our fossil species are
+  known and named from single and often broken specimens, or from a few
+  specimens collected on some one spot. Only a small portion of the surface
+  of the earth has been geologically explored, and no part with sufficient
+  care, as the important discoveries made every year in Europe prove. No
+  organism wholly soft can be preserved. Shells and bones will decay and
+  disappear when left on the bottom of the sea, where sediment is not
+  accumulating. I believe we are continually taking a most erroneous view,
+  when we tacitly admit to ourselves that sediment is being deposited over
+  nearly the whole bed of the sea, at a rate sufficiently quick to embed
+  and preserve fossil remains. Throughout an enormously large proportion of
+  the ocean, the bright blue tint of the water bespeaks its purity. The
+  many cases on record of a formation conformably covered, after an
+  enormous interval of time, by another and later formation, without the
+  underlying bed having suffered in the interval any wear and tear, seem
+  explicable only on the view of the bottom of the sea not rarely lying for
+  ages in an unaltered condition. The remains which do become embedded, if
+  in sand or gravel, will when the beds are upraised generally be dissolved
+  <!-- Page 289 --><span class="pagenum"><a
+  name="page289"></a>[289]</span>by the percolation of rain-water. I
+  suspect that but few of the very many animals which live on the beach
+  between high and low watermark are preserved. For instance, the several
+  species of the Chthamalinæ (a subfamily of sessile cirripedes) coat the
+  rocks all over the world in infinite numbers: they are all strictly
+  littoral, with the exception of a single Mediterranean species, which
+  inhabits deep water and has been found fossil in Sicily, whereas not one
+  other species has hitherto been found in any tertiary formation: yet it
+  is now known that the genus Chthamalus existed during the chalk period.
+  The molluscan genus Chiton offers a partially analogous case.</p>
+
+  <p>With respect to the terrestrial productions which lived during the
+  Secondary and Palæozoic periods, it is superfluous to state that our
+  evidence from fossil remains is fragmentary in an extreme degree. For
+  instance, not a land shell is known belonging to either of these vast
+  periods, with the exception of one species discovered by Sir C. Lyell and
+  Dr. Dawson in the carboniferous strata of North America, of which shell
+  several specimens have now been collected. In regard to mammiferous
+  remains, a single glance at the historical table published in the
+  Supplement to Lyell's Manual, will bring home the truth, how accidental
+  and rare is their preservation, far better than pages of detail. Nor is
+  their rarity surprising, when we remember how large a proportion of the
+  bones of tertiary mammals have been discovered either in caves or in
+  lacustrine deposits; and that not a cave or true lacustrine bed is known
+  belonging to the age of our secondary or palæozoic formations.</p>
+
+  <p>But the imperfection in the geological record mainly results from
+  another and more important cause than any of the foregoing; namely, from
+  the several formations <!-- Page 290 --><span class="pagenum"><a
+  name="page290"></a>[290]</span>being separated from each other by wide
+  intervals of time. When we see the formations tabulated in written works,
+  or when we follow them in nature, it is difficult to avoid believing that
+  they are closely consecutive. But we know, for instance, from Sir R.
+  Murchison's great work on Russia, what wide gaps there are in that
+  country between the superimposed formations; so it is in North America,
+  and in many other parts of the world. The most skilful geologist, if his
+  attention had been exclusively confined to these large territories, would
+  never have suspected that during the periods which were blank and barren
+  in his own country, great piles of sediment, charged with new and
+  peculiar forms of life, had elsewhere been accumulated. And if in each
+  separate territory, hardly any idea can be formed of the length of time
+  which has elapsed between the consecutive formations, we may infer that
+  this could nowhere be ascertained. The frequent and great changes in the
+  mineralogical composition of consecutive formations, generally implying
+  great changes in the geography of the surrounding lands, whence the
+  sediment has been derived, accords with the belief of vast intervals of
+  time having elapsed between each formation.</p>
+
+  <p>But we can, I think, see why the geological formations of each region
+  are almost invariably intermittent; that is, have not followed each other
+  in close sequence. Scarcely any fact struck me more when examining many
+  hundred miles of the South American coasts, which have been upraised
+  several hundred feet within the recent period, than the absence of any
+  recent deposits sufficiently extensive to last for even a short
+  geological period. Along the whole west coast, which is inhabited by a
+  peculiar marine fauna, tertiary beds are so poorly developed, that no
+  record of several <!-- Page 291 --><span class="pagenum"><a
+  name="page291"></a>[291]</span>successive and peculiar marine faunas will
+  probably be preserved to a distant age. A little reflection will explain
+  why along the rising coast of the western side of South America, no
+  extensive formations with recent or tertiary remains can anywhere be
+  found, though the supply of sediment must for ages have been great, from
+  the enormous degradation of the coast-rocks and from muddy streams
+  entering the sea. The explanation, no doubt, is, that the littoral and
+  sub-littoral deposits are continually worn away, as soon as they are
+  brought up by the slow and gradual rising of the land within the grinding
+  action of the coast-waves.</p>
+
+  <p>We may, I think, safely conclude that sediment must be accumulated in
+  extremely thick, solid, or extensive masses, in order to withstand the
+  incessant action of the waves, when first upraised and during subsequent
+  oscillations of level. Such thick and extensive accumulations of sediment
+  may be formed in two ways; either, in profound depths of the sea, in
+  which case, judging from the researches of E. Forbes, we may conclude
+  that the bottom will be inhabited by extremely few animals, and the mass
+  when upraised will give a most imperfect record of the forms of life
+  which then existed; or, sediment may be accumulated to any thickness and
+  extent over a shallow bottom, if it continue slowly to subside. In this
+  latter case, as long as the rate of subsidence and supply of sediment
+  nearly balance each other, the sea will remain shallow and favourable for
+  life, and thus a fossiliferous formation thick enough, when upraised, to
+  resist any amount of degradation, may be formed.</p>
+
+  <p>I am convinced that all our ancient formations, which are rich in
+  fossils, have thus been formed during subsidence. Since publishing my
+  views on this subject in 1845, I have watched the progress of <!-- Page
+  292 --><span class="pagenum"><a name="page292"></a>[292]</span>Geology,
+  and have been surprised to note how author after author, in treating of
+  this or that great formation, has come to the conclusion that it was
+  accumulated during subsidence. I may add, that the only ancient tertiary
+  formation on the west coast of South America, which has been bulky enough
+  to resist such degradation as it has as yet suffered, but which will
+  hardly last to a distant geological age, was certainly deposited during a
+  downward oscillation of level, and thus gained considerable
+  thickness.</p>
+
+  <p>All geological facts tell us plainly that each area has undergone
+  numerous slow oscillations of level, and apparently these oscillations
+  have affected wide spaces. Consequently formations rich in fossils and
+  sufficiently thick and extensive to resist subsequent degradation, may
+  have been formed over wide spaces during periods of subsidence, but only
+  where the supply of sediment was sufficient to keep the sea shallow and
+  to embed and preserve the remains before they had time to decay. On the
+  other hand, as long as the bed of the sea remained stationary,
+  <i>thick</i> deposits could not have been accumulated in the shallow
+  parts, which are the most favourable to life. Still less could this have
+  happened during the alternate periods of elevation; or, to speak more
+  accurately, the beds which were then accumulated will have been destroyed
+  by being upraised and brought within the limits of the coast-action.</p>
+
+  <p>Thus the geological record will almost necessarily be rendered
+  intermittent. I feel much confidence in the truth of these views, for
+  they are in strict accordance with the general principles inculcated by
+  Sir C. Lyell; and E. Forbes subsequently but independently arrived at a
+  similar conclusion.</p>
+
+  <p>One remark is here worth a passing notice. During periods of elevation
+  the area of the land and of the <!-- Page 293 --><span class="pagenum"><a
+  name="page293"></a>[293]</span>adjoining shoal parts of the sea will be
+  increased, and new stations will often be formed;&mdash;all circumstances
+  most favourable, as previously explained, for the formation of new
+  varieties and species; but during such periods there will generally be a
+  blank in the geological record. On the other hand, during subsidence, the
+  inhabited area and number of inhabitants will decrease (excepting the
+  productions on the shores of a continent when first broken up into an
+  archipelago), and consequently during subsidence, though there will be
+  much extinction, fewer new varieties or species will be formed; and it is
+  during these very periods of subsidence, that our great deposits rich in
+  fossils have been accumulated. Nature may almost be said to have guarded
+  against the frequent discovery of her transitional or linking forms.</p>
+
+  <p>From the foregoing considerations it cannot be doubted that the
+  geological record, viewed as a whole, is extremely imperfect; but if we
+  confine our attention to any one formation, it becomes more difficult to
+  understand, why we do not therein find closely graduated varieties
+  between the allied species which lived at its commencement and at its
+  close. Some cases are on record of the same species presenting distinct
+  varieties in the upper and lower parts of the same formation, but, as
+  they are rare, they may be here passed over. Although each formation has
+  indisputably required a vast number of years for its deposition, I can
+  see several reasons why each should not include a graduated series of
+  links between the species which then lived; but I can by no means pretend
+  to assign due proportional weight to the following considerations.</p>
+
+  <p>Although each formation may mark a very long lapse of years, each
+  perhaps is short compared with the period requisite to change one species
+  into another. I am <!-- Page 294 --><span class="pagenum"><a
+  name="page294"></a>[294]</span>aware that two palæontologists, whose
+  opinions are worthy of much deference, namely Bronn and Woodward, have
+  concluded that the average duration of each formation is twice or thrice
+  as long as the average duration of specific forms. But insuperable
+  difficulties, as it seems to me, prevent us coming to any just conclusion
+  on this head. When we see a species first appearing in the middle of any
+  formation, it would be rash in the extreme to infer that it had not
+  elsewhere previously existed. So again when we find a species
+  disappearing before the uppermost layers have been deposited, it would be
+  equally rash to suppose that it then became wholly extinct. We forget how
+  small the area of Europe is compared with the rest of the world; nor have
+  the several stages of the same formation throughout Europe been
+  correlated with perfect accuracy.</p>
+
+  <p>With marine animals of all kinds, we may safely infer a large amount
+  of migration during climatal and other changes; and when we see a species
+  first appearing in any formation, the probability is that it only then
+  first immigrated into that area. It is well known, for instance, that
+  several species appeared somewhat earlier in the palæozoic beds of North
+  America than in those of Europe; time having apparently been required for
+  their migration from the American to the European seas. In examining the
+  latest deposits of various quarters of the world, it has everywhere been
+  noted, that some few still existing species are common in the deposit,
+  but have become extinct in the immediately surrounding sea; or,
+  conversely, that some are now abundant in the neighbouring sea, but are
+  rare or absent in this particular deposit. It is an excellent lesson to
+  reflect on the ascertained amount of migration of the inhabitants of
+  Europe during the Glacial period, which forms only a part of one whole
+  geological period; <!-- Page 295 --><span class="pagenum"><a
+  name="page295"></a>[295]</span>and likewise to reflect on the great
+  changes of level, on the inordinately great change of climate, on the
+  prodigious lapse of time, all included within this same glacial period.
+  Yet it may be doubted whether in any quarter of the world, sedimentary
+  deposits, <i>including fossil remains</i>, have gone on accumulating
+  within the same area during the whole of this period. It is not, for
+  instance, probable that sediment was deposited during the whole of the
+  glacial period near the mouth of the Mississippi, within that limit of
+  depth at which marine animals can flourish; for we know what vast
+  geographical changes occurred in other parts of America during this space
+  of time. When such beds as were deposited in shallow water near the mouth
+  of the Mississippi during some part of the glacial period shall have been
+  upraised, organic remains will probably first appear and disappear at
+  different levels, owing to the migration of species and to geographical
+  changes. And in the distant future, a geologist examining these beds,
+  might be tempted to conclude that the average duration of life of the
+  embedded fossils had been less than that of the glacial period, instead
+  of having been really far greater, that is extending from before the
+  glacial epoch to the present day.</p>
+
+  <p>In order to get a perfect gradation between two forms in the upper and
+  lower parts of the same formation, the deposit must have gone on
+  accumulating for a very long period, in order to have given sufficient
+  time for the slow process of variation; hence the deposit will generally
+  have to be a very thick one; and the species undergoing modification will
+  have had to live on the same area throughout this whole time. But we have
+  seen that a thick fossiliferous formation can only be accumulated during
+  a period of subsidence; and to keep the depth approximately the same,
+  which is necessary in <!-- Page 296 --><span class="pagenum"><a
+  name="page296"></a>[296]</span>order to enable the same species to live
+  on the same space, the supply of sediment must nearly have
+  counterbalanced the amount of subsidence. But this same movement of
+  subsidence will often tend to sink the area whence the sediment is
+  derived, and thus diminish the supply whilst the downward movement
+  continues. In fact, this nearly exact balancing between the supply of
+  sediment and the amount of subsidence is probably a rare contingency; for
+  it has been observed by more than one palæontologist, that very thick
+  deposits are usually barren of organic remains, except near their upper
+  or lower limits.</p>
+
+  <p>It would seem that each separate formation, like the whole pile of
+  formations in any country, has generally been intermittent in its
+  accumulation. When we see, as is so often the case, a formation composed
+  of beds of different mineralogical composition, we may reasonably suspect
+  that the process of deposition has been much interrupted, as a change in
+  the currents of the sea and a supply of sediment of a different nature
+  will generally have been due to geographical changes requiring much time.
+  Nor will the closest inspection of a formation give any idea of the time
+  which its deposition has consumed. Many instances could be given of beds
+  only a few feet in thickness, representing formations, elsewhere
+  thousands of feet in thickness, and which must have required an enormous
+  period for their accumulation; yet no one ignorant of this fact would
+  have suspected the vast lapse of time represented by the thinner
+  formation. Many cases could be given of the lower beds of a formation
+  having been upraised, denuded, submerged, and then re-covered by the
+  upper beds of the same formation,&mdash;facts, showing what wide, yet
+  easily overlooked, intervals have occurred in its accumulation. In other
+  cases we have the plainest evidence <!-- Page 297 --><span
+  class="pagenum"><a name="page297"></a>[297]</span>in great fossilised
+  trees, still standing upright as they grew, of many long intervals of
+  time and changes of level during the process of deposition, which would
+  never even have been suspected, had not the trees chanced to have been
+  preserved: thus Messrs. Lyell and Dawson found carboniferous beds 1400
+  feet thick in Nova Scotia, with ancient root-bearing strata, one above
+  the other, at no less than sixty-eight different levels. Hence, when the
+  same species occur at the bottom, middle, and top of a formation, the
+  probability is that they have not lived on the same spot during the whole
+  period of deposition, but have disappeared and reappeared, perhaps many
+  times, during the same geological period. So that if such species were to
+  undergo a considerable amount of modification during any one geological
+  period, a section would not probably include all the fine intermediate
+  gradations which must on my theory have existed between them, but abrupt,
+  though perhaps very slight, changes of form.</p>
+
+  <p>It is all-important to remember that naturalists have no golden rule
+  by which to distinguish species and varieties; they grant some little
+  variability to each species, but when they meet with a somewhat greater
+  amount of difference between any two forms, they rank both as species,
+  unless they are enabled to connect them together by close intermediate
+  gradations. And this from the reasons just assigned we can seldom hope to
+  effect in any one geological section. Supposing B and C to be two
+  species, and a third, A, to be found in an underlying bed; even if A were
+  strictly intermediate between B and C, it would simply be ranked as a
+  third and distinct species, unless at the same time it could be most
+  closely connected with either one or both forms by intermediate
+  varieties. Nor should it be forgotten, as before explained, that A might
+  be the actual progenitor <!-- Page 298 --><span class="pagenum"><a
+  name="page298"></a>[298]</span>of B and C, and yet might not at all
+  necessarily be strictly intermediate between them in all points of
+  structure. So that we might obtain the parent-species and its several
+  modified descendants from the lower and upper beds of a formation, and
+  unless we obtained numerous transitional gradations, we should not
+  recognise their relationship, and should consequently be compelled to
+  rank them all as distinct species.</p>
+
+  <p>It is notorious on what excessively slight differences many
+  palæontologists have founded their species; and they do this the more
+  readily if the specimens come from different sub-stages of the same
+  formation. Some experienced conchologists are now sinking many of the
+  very fine species of D'Orbigny and others into the rank of varieties; and
+  on this view we do find the kind of evidence of change which on my theory
+  we ought to find. Moreover, if we look to rather wider intervals, namely,
+  to distinct but consecutive stages of the same great formation, we find
+  that the embedded fossils, though almost universally ranked as
+  specifically different, yet are far more closely allied to each other
+  than are the species found in more widely separated formations; but to
+  this subject I shall have to return in the following chapter.</p>
+
+  <p>One other consideration is worth notice: with animals and plants that
+  can propagate rapidly and are not highly locomotive, there is reason to
+  suspect, as we have formerly seen, that their varieties are generally at
+  first local; and that such local varieties do not spread widely and
+  supplant their parent-forms until they have been modified and perfected
+  in some considerable degree. According to this view, the chance of
+  discovering in a formation in any one country all the early stages of
+  transition between any two forms, is small, for the successive changes
+  are supposed to have been local or <!-- Page 299 --><span
+  class="pagenum"><a name="page299"></a>[299]</span>confined to some one
+  spot. Most marine animals have a wide range; and we have seen that with
+  plants it is those which have the widest range, that oftenest present
+  varieties; so that with shells and other marine animals, it is probably
+  those which have had the widest range, far exceeding the limits of the
+  known geological formations of Europe, which have oftenest given rise,
+  first to local varieties and ultimately to new species; and this again
+  would greatly lessen the chance of our being able to trace the stages of
+  transition in any one geological formation.</p>
+
+  <p>It should not be forgotten, that at the present day, with perfect
+  specimens for examination, two forms can seldom be connected by
+  intermediate varieties and thus proved to be the same species, until many
+  specimens have been collected from many places; and in the case of fossil
+  species this could rarely be effected by palæontologists. We shall,
+  perhaps, best perceive the improbability of our being enabled to connect
+  species by numerous, fine, intermediate, fossil links, by asking
+  ourselves whether, for instance, geologists at some future period will be
+  able to prove, that our different breeds of cattle, sheep, horses, and
+  dogs have descended from a single stock or from several aboriginal
+  stocks; or, again, whether certain sea-shells inhabiting the shores of
+  North America, which are ranked by some conchologists as distinct species
+  from their European representatives, and by other conchologists as only
+  varieties, are really varieties or are, as it is called, specifically
+  distinct. This could be effected only by the future geologist discovering
+  in a fossil state numerous intermediate gradations; and such success
+  seems to me improbable in the highest degree.</p>
+
+  <p>Geological research, though it has added numerous species to existing
+  and extinct genera, and has made the <!-- Page 300 --><span
+  class="pagenum"><a name="page300"></a>[300]</span>intervals between some
+  few groups less wide than they otherwise would have been, yet has done
+  scarcely anything in breaking down the distinction between species, by
+  connecting them together by numerous, fine, intermediate varieties; and
+  this not having been effected, is probably the gravest and most obvious
+  of all the many objections which may be urged against my views. Hence it
+  will be worth while to sum up the foregoing remarks, under an imaginary
+  illustration. The Malay Archipelago is of about the size of Europe from
+  the North Cape to the Mediterranean, and from Britain to Russia; and
+  therefore equals all the geological formations which have been examined
+  with any accuracy, excepting those of the United States of America. I
+  fully agree with Mr. Godwin-Austen, that the present condition of the
+  Malay Archipelago, with its numerous large islands separated by wide and
+  shallow seas, probably represents the former state of Europe, whilst most
+  of our formations were accumulating. The Malay Archipelago is one of the
+  richest regions of the whole world in organic beings; yet if all the
+  species were to be collected which have ever lived there, how imperfectly
+  would they represent the natural history of the world!</p>
+
+  <p>But we have every reason to believe that the terrestrial productions
+  of the archipelago would be preserved in an excessively imperfect manner
+  in the formations which we suppose to be there accumulating. I suspect
+  that not many of the strictly littoral animals, or of those which lived
+  on naked submarine rocks, would be embedded; and those embedded in gravel
+  or sand, would not endure to a distant epoch. Wherever sediment did not
+  accumulate on the bed of the sea, or where it did not accumulate at a
+  sufficient rate to protect organic bodies from decay, no remains could be
+  preserved.</p>
+
+  <p>I believe that fossiliferous formations could be formed <!-- Page 301
+  --><span class="pagenum"><a name="page301"></a>[301]</span>in the
+  archipelago, of thickness sufficient to last to an age as distant in
+  futurity as the secondary formations lie in the past, only during periods
+  of subsidence. These periods of subsidence would be separated from each
+  other by enormous intervals, during which the area would be either
+  stationary or rising; whilst rising, each fossiliferous formation would
+  be destroyed, almost as soon as accumulated, by the incessant
+  coast-action, as we now see on the shores of South America. During the
+  periods of subsidence there would probably be much extinction of life;
+  during the periods of elevation, there would be much variation, but the
+  geological record would then be least perfect.</p>
+
+  <p>It may be doubted whether the duration of any one great period of
+  subsidence over the whole or part of the archipelago, together with a
+  contemporaneous accumulation of sediment, would <i>exceed</i> the average
+  duration of the same specific forms; and these contingencies are
+  indispensable for the preservation of all the transitional gradations
+  between any two or more species. If such gradations were not fully
+  preserved, transitional varieties would merely appear as so many distinct
+  species. It is, also, probable that each great period of subsidence would
+  be interrupted by oscillations of level, and that slight climatal changes
+  would intervene during such lengthy periods; and in these cases the
+  inhabitants of the archipelago would have to migrate, and no closely
+  consecutive record of their modifications could be preserved in any one
+  formation.</p>
+
+  <p>Very many of the marine inhabitants of the archipelago now range
+  thousands of miles beyond its confines; and analogy leads me to believe
+  that it would be chiefly these far-ranging species which would oftenest
+  produce new varieties; and the varieties would at first generally be
+  local or confined to one place, but if possessed <!-- Page 302 --><span
+  class="pagenum"><a name="page302"></a>[302]</span>of any decided
+  advantage, or when further modified and improved, they would slowly
+  spread and supplant their parent-forms. When such varieties returned to
+  their ancient homes, as they would differ from their former state, in a
+  nearly uniform, though perhaps extremely slight degree, they would,
+  according to the principles followed by many palæontologists, be ranked
+  as new and distinct species.</p>
+
+  <p>If then, there be some degree of truth in these remarks, we have no
+  right to expect to find in our geological formations, an infinite number
+  of those fine transitional forms, which on my theory assuredly have
+  connected all the past and present species of the same group into one
+  long and branching chain of life. We ought only to look for a few links,
+  some more closely, some more distantly related to each other; and these
+  links, let them be ever so close, if found in different stages of the
+  same formation, would, by most palæontologists, be ranked as distinct
+  species. But I do not pretend that I should ever have suspected how poor
+  a record of the mutations of life, the best preserved geological section
+  presented, had not the difficulty of our not discovering innumerable
+  transitional links between the species which appeared at the commencement
+  and close of each formation, pressed so hardly on my theory.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the sudden appearance of whole groups of Allied
+  Species.</i>&mdash;The abrupt manner in which whole groups of species
+  suddenly appear in certain formations, has been urged by several
+  palæontologists&mdash;for instance, by Agassiz, Pictet, and by none more
+  forcibly than by Professor Sedgwick&mdash;as a fatal objection to the
+  belief in the transmutation of species. If numerous species, belonging to
+  the same genera or families, have really <!-- Page 303 --><span
+  class="pagenum"><a name="page303"></a>[303]</span>started into life all
+  at once, the fact would be fatal to the theory of descent with slow
+  modification through natural selection. For the development of a group of
+  forms, all of which have descended from some one progenitor, must have
+  been an extremely slow process; and the progenitors must have lived long
+  ages before their modified descendants. But we continually over-rate the
+  perfection of the geological record, and falsely infer, because certain
+  genera or families have not been found beneath a certain stage, that they
+  did not exist before that stage. We continually forget how large the
+  world is, compared with the area over which our geological formations
+  have been carefully examined; we forget that groups of species may
+  elsewhere have long existed and have slowly multiplied before they
+  invaded the ancient archipelagoes of Europe and of the United States. We
+  do not make due allowance for the enormous intervals of time, which have
+  probably elapsed between our consecutive formations,&mdash;longer perhaps
+  in most cases than the time required for the accumulation of each
+  formation. These intervals will have given time for the multiplication of
+  species from some one or some few parent-forms; and in the succeeding
+  formation such species will appear as if suddenly created.</p>
+
+  <p>I may here recall a remark formerly made, namely that it might require
+  a long succession of ages to adapt an organism to some new and peculiar
+  line of life, for instance to fly through the air; but that when this had
+  been effected, and a few species had thus acquired a great advantage over
+  other organisms, a comparatively short time would be necessary to produce
+  many divergent forms, which would be able to spread rapidly and widely
+  throughout the world.</p>
+
+  <p>I will now give a few examples to illustrate these <!-- Page 304
+  --><span class="pagenum"><a name="page304"></a>[304]</span>remarks, and
+  to show how liable we are to error in supposing that whole groups of
+  species have suddenly been produced. I may recall the well-known fact
+  that in geological treatises, published not many years ago, the great
+  class of mammals was always spoken of as having abruptly come in at the
+  commencement of the tertiary series. And now one of the richest known
+  accumulations of fossil mammals, for its thickness, belongs to the middle
+  of the secondary series; and one true mammal has been discovered in the
+  new red sandstone at nearly the commencement of this great series. Cuvier
+  used to urge that no monkey occurred in any tertiary stratum; but now
+  extinct species have been discovered in India, South America, and in
+  Europe even as far back as the eocene stage. Had it not been for the rare
+  accident of the preservation of footsteps in the new red sandstone of the
+  United States, who would have ventured to suppose that, besides reptiles,
+  no less than at least thirty kinds of birds, some of gigantic size,
+  existed during that period? Not a fragment of bone has been discovered in
+  these beds. Notwithstanding that the number of joints shown in the fossil
+  impressions correspond with the number in the several toes of living
+  birds' feet, some authors doubt whether the animals which left the
+  impressions were really birds. Until quite recently these authors might
+  have maintained, and some have maintained, that the whole class of birds
+  came suddenly into existence during an early tertiary period; but now we
+  know, on the authority of Professor Owen (as may be seen in Lyell's
+  'Manual'), that a bird certainly lived during the deposition of the upper
+  greensand.</p>
+
+  <p>I may give another instance, which from having passed under my own
+  eyes has much struck me. In a memoir on Fossil Sessile Cirripedes, I have
+  stated that, from the <!-- Page 305 --><span class="pagenum"><a
+  name="page305"></a>[305]</span>number of existing and extinct tertiary
+  species; from the extraordinary abundance of the individuals of many
+  species all over the world, from the Arctic regions to the equator,
+  inhabiting various zones of depths from the upper tidal limits to 50
+  fathoms; from the perfect manner in which specimens are preserved in the
+  oldest tertiary beds; from the ease with which even a fragment of a valve
+  can be recognised; from all these circumstances, I inferred that had
+  sessile cirripedes existed during the secondary periods, they would
+  certainly have been preserved and discovered; and as not one species had
+  then been discovered in beds of this age, I concluded that this great
+  group had been suddenly developed at the commencement of the tertiary
+  series. This was a sore trouble to me, adding as I thought one more
+  instance of the abrupt appearance of a great group of species. But my
+  work had hardly been published, when a skilful palæontologist, M.
+  Bosquet, sent me a drawing of a perfect specimen of an unmistakeable
+  sessile cirripede, which he had himself extracted from the chalk of
+  Belgium. And, as if to make the case as striking as possible, this
+  sessile cirripede was a Chthamalus, a very common, large, and ubiquitous
+  genus, of which not one specimen has as yet been found even in any
+  tertiary stratum. Hence we now positively know that sessile cirripedes
+  existed during the secondary period; and these cirripedes might have been
+  the progenitors of our many tertiary and existing species.</p>
+
+  <p>The case most frequently insisted on by palæontologists of the
+  apparently sudden appearance of a whole group of species, is that of the
+  teleostean fishes, low down in the Chalk period. This group includes the
+  large majority of existing species. Lately, Professor Pictet has carried
+  their existence one sub-stage further back; and some palæontologists
+  believe that certain <!-- Page 306 --><span class="pagenum"><a
+  name="page306"></a>[306]</span>much older fishes, of which the affinities
+  are as yet imperfectly known, are really teleostean. Assuming, however,
+  that the whole of them did appear, as Agassiz believes, at the
+  commencement of the chalk formation, the fact would certainly be highly
+  remarkable; but I cannot see that it would be an insuperable difficulty
+  on my theory, unless it could likewise be shown that the species of this
+  group appeared suddenly and simultaneously throughout the world at this
+  same period. It is almost superfluous to remark that hardly any
+  fossil-fish are known from south of the equator; and by running through
+  Pictet's Palæontology it will be seen that very few species are known
+  from several formations in Europe. Some few families of fish now have a
+  confined range; the teleostean fish might formerly have had a similarly
+  confined range, and after having been largely developed in some one sea,
+  might have spread widely. Nor have we any right to suppose that the seas
+  of the world have always been so freely open from south to north as they
+  are at present. Even at this day, if the Malay Archipelago were converted
+  into land, the tropical parts of the Indian Ocean would form a large and
+  perfectly enclosed basin, in which any great group of marine animals
+  might be multiplied; and here they would remain confined, until some of
+  the species became adapted to a cooler climate, and were enabled to
+  double the southern capes of Africa or Australia, and thus reach other
+  and distant seas.</p>
+
+  <p>From these and similar considerations, but chiefly from our ignorance
+  of the geology of other countries beyond the confines of Europe and the
+  United States; and from the revolution in our palæontological ideas on
+  many points, which the discoveries of even the last dozen years have
+  effected, it seems to me to be about as rash in us to dogmatize on the
+  succession of organic <!-- Page 307 --><span class="pagenum"><a
+  name="page307"></a>[307]</span>beings throughout the world, as it would
+  be for a naturalist to land for five minutes on some one barren point in
+  Australia, and then to discuss the number and range of its
+  productions.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the sudden appearance of groups of Allied Species in the lowest
+  known fossiliferous strata.</i>&mdash;There is another and allied
+  difficulty, which is much graver. I allude to the manner in which numbers
+  of species of the same group, suddenly appear in the lowest known
+  fossiliferous rocks. Most of the arguments which have convinced me that
+  all the existing species of the same group have descended from one
+  progenitor, apply with nearly equal force to the earliest known species.
+  For instance, I cannot doubt that all the Silurian trilobites have
+  descended from some one crustacean, which must have lived long before the
+  Silurian age, and which probably differed greatly from any known animal.
+  Some of the most ancient Silurian animals, as the Nautilus, Lingula,
+  &amp;c., do not differ much from living species; and it cannot on my
+  theory be supposed, that these old species were the progenitors of all
+  the species of the orders to which they belong, for they do not present
+  characters in any degree intermediate between them. If, moreover, they
+  had been the progenitors of these orders, they would almost certainly
+  have been long ago supplanted and exterminated by their numerous and
+  improved descendants.</p>
+
+  <p>Consequently, if my theory be true, it is indisputable that before the
+  lowest Silurian stratum was deposited, long periods elapsed, as long as,
+  or probably far longer than, the whole interval from the Silurian age to
+  the present day; and that during these vast, yet quite unknown, periods
+  of time, the world swarmed with living creatures. <!-- Page 308 --><span
+  class="pagenum"><a name="page308"></a>[308]</span></p>
+
+  <p>To the question why we do not find records of these vast primordial
+  periods, I can give no satisfactory answer. Several of the most eminent
+  geologists, with Sir E. Murchison at their head, are convinced that we
+  see in the organic remains of the lowest Silurian stratum the dawn of
+  life on this planet. Other highly competent judges, as Lyell and the late
+  E. Forbes, dispute this conclusion. We should not forget that only a
+  small portion of the world is known with accuracy. M. Barrande has lately
+  added another and lower stage to the Silurian system, abounding with new
+  and peculiar species. Traces of life have been detected in the Longmynd
+  beds, beneath Barrande's so-called primordial zone. The presence of
+  phosphatic nodules and bituminous matter in some of the lowest azoic
+  rocks, probably indicates the former existence of life at these periods.
+  But the difficulty of understanding the absence of vast piles of
+  fossiliferous strata, which on my theory no doubt were somewhere
+  accumulated before the Silurian epoch, is very great. If these most
+  ancient beds had been wholly worn away by denudation, or obliterated by
+  metamorphic action, we ought to find only small remnants of the
+  formations next succeeding them in age, and these ought to be very
+  generally in a metamorphosed condition. But the descriptions which we now
+  possess of the Silurian deposits over immense territories in Russia and
+  in North America, do not support the view, that the older a formation is,
+  the more it has always suffered the extremity of denudation and
+  metamorphism.</p>
+
+  <p>The case at present must remain inexplicable; and may be truly urged
+  as a valid argument against the views here entertained. To show that it
+  may hereafter receive some explanation, I will give the following
+  hypothesis. From the nature of the organic remains which <!-- Page 309
+  --><span class="pagenum"><a name="page309"></a>[309]</span>do not appear
+  to have inhabited profound depths, in the several formations of Europe
+  and of the United States; and from the amount of sediment, miles in
+  thickness, of which the formations are composed, we may infer that from
+  first to last large islands or tracts of land, whence the sediment was
+  derived, occurred in the neighbourhood of the existing continents of
+  Europe and North America. But we do not know what was the state of things
+  in the intervals between the successive formations; whether Europe and
+  the United States during these intervals existed as dry land, or as a
+  submarine surface near land, on which sediment was not deposited, or as
+  the bed of an open and unfathomable sea.</p>
+
+  <p>Looking to the existing oceans, which are thrice as extensive as the
+  land, we see them studded with many islands; but not one oceanic island
+  is as yet known to afford even a remnant of any palæozoic or secondary
+  formation. Hence we may perhaps infer, that during the palæozoic and
+  secondary periods, neither continents nor continental islands existed
+  where our oceans now extend; for had they existed there, palæozoic and
+  secondary formations would in all probability have been accumulated from
+  sediment derived from their wear and tear; and would have been at least
+  partially upheaved by the oscillations of level, which we may fairly
+  conclude must have intervened during these enormously long periods. If
+  then we may infer anything from these facts, we may infer that where our
+  oceans now extend, oceans have extended from the remotest period of which
+  we have any record; and on the other hand, that where continents now
+  exist, large tracts of land have existed, subjected no doubt to great
+  oscillations of level, since the earliest silurian period. The coloured
+  map appended to my volume on Coral Reefs, led me to conclude that the
+  great oceans are still mainly areas of <!-- Page 310 --><span
+  class="pagenum"><a name="page310"></a>[310]</span>subsidence, the great
+  archipelagoes still areas of oscillations of level, and the continents
+  areas of elevation. But have we any right to assume that things have thus
+  remained from the beginning of this world? Our continents seem to have
+  been formed by a preponderance, during many oscillations of level, of the
+  force of elevation; but may not the areas of preponderant movement have
+  changed in the lapse of ages? At a period immeasurably antecedent to the
+  silurian epoch, continents may have existed where oceans are now spread
+  out; and clear and open oceans may have existed where our continents now
+  stand. Nor should we be justified in assuming that if, for instance, the
+  bed of the Pacific Ocean were now converted into a continent, we should
+  there find formations older than the silurian strata, supposing such to
+  have been formerly deposited; for it might well happen that strata which
+  had subsided some miles nearer to the centre of the earth, and which had
+  been pressed on by an enormous weight of superincumbent water, might have
+  undergone far more metamorphic action than strata which have always
+  remained nearer to the surface. The immense areas in some parts of the
+  world, for instance in South America, of bare metamorphic rocks, which
+  must have been heated under great pressure, have always seemed to me to
+  require some special explanation; and we may perhaps believe that we see
+  in these large areas, the many formations long anterior to the silurian
+  epoch in a completely metamorphosed condition.</p>
+
+  <p>&nbsp;</p>
+
+  <p>The several difficulties here discussed, namely our not finding in the
+  successive formations infinitely numerous transitional links between the
+  many species which now exist or have existed; the sudden manner <!-- Page
+  311 --><span class="pagenum"><a name="page311"></a>[311]</span>in which
+  whole groups of species appear in our European formations; the almost
+  entire absence, as at present known, of fossiliferous formations beneath
+  the Silurian strata, are all undoubtedly of the gravest nature. We see
+  this in the plainest manner by the fact that all the most eminent
+  palæontologists, namely Cuvier, Agassiz, Barrande, Falconer, E. Forbes,
+  &amp;c., and all our greatest geologists, as Lyell, Murchison, Sedgwick,
+  &amp;c., have unanimously, often vehemently, maintained the immutability
+  of species. But I have reason to believe that one great authority, Sir
+  Charles Lyell, from further reflexion entertains grave doubts on this
+  subject. I feel how rash it is to differ from these authorities, to whom,
+  with others, we owe all our knowledge. Those who think the natural
+  geological record in any degree perfect, and who do not attach much
+  weight to the facts and arguments of other kinds given in this volume,
+  will undoubtedly at once reject my theory. For my part, following out
+  Lyell's metaphor, I look at the natural geological record, as a history
+  of the world imperfectly kept, and written in a changing dialect; of this
+  history we possess the last volume alone, relating only to two or three
+  countries. Of this volume, only here and there a short chapter has been
+  preserved; and of each page, only here and there a few lines. Each word
+  of the slowly-changing language, in which the history is supposed to be
+  written, being more or less different in the interrupted succession of
+  chapters, may represent the apparently abruptly changed forms of life,
+  entombed in our consecutive, but widely separated, formations. On this
+  view, the difficulties above discussed are greatly diminished, or even
+  disappear.</p>
+
+<hr />
+
+<p><!-- Page 312 --><span class="pagenum"><a name="page312"></a>[312]</span></p>
+
+<h3>CHAPTER X.</h3>
+
+<p class="cenhead"><span class="sc">On the Geological Succession of Organic Beings.</span></p>
+
+<blockquote class="b1n">
+
+  <p>On the slow and successive appearance of new species&mdash;On their
+  different rates of change&mdash;Species once lost do not
+  reappear&mdash;Groups of species follow the same general rules in their
+  appearance and disappearance as do single species&mdash;On
+  Extinction&mdash;On simultaneous changes in the forms of life throughout
+  the world&mdash;On the affinities of extinct species to each other and to
+  living species&mdash;On the state of development of ancient
+  forms&mdash;On the succession of the same types within the same
+  areas&mdash;Summary of preceding and present chapters.</p>
+
+</blockquote>
+
+  <p>Let us now see whether the several facts and rules relating to the
+  geological succession of organic beings, better accord with the common
+  view of the immutability of species, or with that of their slow and
+  gradual modification, through descent and natural selection.</p>
+
+  <p>New species have appeared very slowly, one after another, both on the
+  land and in the waters. Lyell has shown that it is hardly possible to
+  resist the evidence on this head in the case of the several tertiary
+  stages; and every year tends to fill up the blanks between them, and to
+  make the percentage system of lost and new forms more gradual. In some of
+  the most recent beds, though undoubtedly of high antiquity if measured by
+  years, only one or two species are lost forms, and only one or two are
+  new forms, having here appeared for the first time, either locally, or,
+  as far as we know, on the face of the earth. If we may trust the
+  observations of Philippi in Sicily, the successive changes in the marine
+  inhabitants of that island have been many and most gradual. The secondary
+  formations are more broken; but, as Bronn has remarked, neither the
+  appearance <!-- Page 313 --><span class="pagenum"><a
+  name="page313"></a>[313]</span>nor disappearance of their many now
+  extinct species has been simultaneous in each separate formation.</p>
+
+  <p>Species of different genera and classes have not changed at the same
+  rate, or in the same degree. In the oldest tertiary beds a few living
+  shells may still be found in the midst of a multitude of extinct forms.
+  Falconer has given a striking instance of a similar fact, in an existing
+  crocodile associated with many strange and lost mammals and reptiles in
+  the sub-Himalayan deposits. The Silurian Lingula differs but little from
+  the living species of this genus; whereas most of the other Silurian
+  Molluscs and all the Crustaceans have changed greatly. The productions of
+  the land seem to change at a quicker rate than those of the sea, of which
+  a striking instance has lately been observed in Switzerland. There is
+  some reason to believe that organisms, considered high in the scale of
+  nature, change more quickly than those that are low: though there are
+  exceptions to this rule. The amount of organic change, as Pictet has
+  remarked, does not strictly correspond with the succession of our
+  geological formations; so that between each two consecutive formations,
+  the forms of life have seldom changed in exactly the same degree. Yet if
+  we compare any but the most closely related formations, all the species
+  will be found to have undergone some change. When a species has once
+  disappeared from the face of the earth, we have reason to believe that
+  the same identical form never reappears. The strongest apparent exception
+  to this latter rule, is that of the so-called "colonies" of M. Barrande,
+  which intrude for a period in the midst of an older formation, and then
+  allow the pre-existing fauna to reappear; but Lyell's explanation,
+  namely, that it is a case of temporary migration from a distinct
+  geographical province, seems to me satisfactory. <!-- Page 314 --><span
+  class="pagenum"><a name="page314"></a>[314]</span></p>
+
+  <p>These several facts accord well with my theory. I believe in no fixed
+  law of development, causing all the inhabitants of a country to change
+  abruptly, or simultaneously, or to an equal degree. The process of
+  modification must be extremely slow. The variability of each species is
+  quite independent of that of all others. Whether such variability be
+  taken advantage of by natural selection, and whether the variations be
+  accumulated to a greater or lesser amount, thus causing a greater or
+  lesser amount of modification in the varying species, depends on many
+  complex contingencies,&mdash;on the variability being of a beneficial
+  nature, on the power of intercrossing, on the rate of breeding, on the
+  slowly changing physical conditions of the country, and more especially
+  on the nature of the other inhabitants with which the varying species
+  comes into competition. Hence it is by no means surprising that one
+  species should retain the same identical form much longer than others;
+  or, if changing, that it should change less. We see the same fact in
+  geographical distribution; for instance, in the land-shells and
+  coleopterous insects of Madeira having come to differ considerably from
+  their nearest allies on the continent of Europe, whereas the marine
+  shells and birds have remained unaltered. We can perhaps understand the
+  apparently quicker rate of change in terrestrial and in more highly
+  organised productions compared with marine and lower productions, by the
+  more complex relations of the higher beings to their organic and
+  inorganic conditions of life, as explained in a former chapter. When many
+  of the inhabitants of a country have become modified and improved, we can
+  understand, on the principle of competition, and on that of the many
+  all-important relations of organism to organism, that any form which does
+  not become in some degree modified and improved, <!-- Page 315 --><span
+  class="pagenum"><a name="page315"></a>[315]</span>will be liable to be
+  exterminated. Hence we can see why all the species in the same region do
+  at last, if we look to wide enough intervals of time, become modified;
+  for those which do not change will become extinct.</p>
+
+  <p>In members of the same class the average amount of change, during long
+  and equal periods of time, may, perhaps, be nearly the same; but as the
+  accumulation of long-enduring fossiliferous formations depends on great
+  masses of sediment having been deposited on areas whilst subsiding, our
+  formations have been almost necessarily accumulated at wide and
+  irregularly intermittent intervals; consequently the amount of organic
+  change exhibited by the fossils embedded in consecutive formations is not
+  equal. Each formation, on this view, does not mark a new and complete act
+  of creation, but only an occasional scene, taken almost at hazard, in a
+  slowly changing drama.</p>
+
+  <p>We can clearly understand why a species when once lost should never
+  reappear, even if the very same conditions of life, organic and
+  inorganic, should recur. For though the offspring of one species might be
+  adapted (and no doubt this has occurred in innumerable instances) to fill
+  the exact place of another species in the economy of nature, and thus
+  supplant it; yet the two forms&mdash;the old and the new&mdash;would not
+  be identically the same; for both would almost certainly inherit
+  different characters from their distinct progenitors. For instance, it is
+  just possible, if our fantail-pigeons were all destroyed, that fanciers,
+  by striving during long ages for the same object, might make a new breed
+  hardly distinguishable from our present fantail; but if the parent
+  rock-pigeon were also destroyed, and in nature we have every reason to
+  believe that the parent-form will generally be supplanted and
+  exterminated by its improved offspring, it is quite <!-- Page 316
+  --><span class="pagenum"><a name="page316"></a>[316]</span>incredible
+  that a fantail, identical with the existing breed, could be raised from
+  any other species of pigeon, or even from the other well-established
+  races of the domestic pigeon, for the newly-formed <span
+  class="correction" title="text reads `faintail'">fantail</span> would be
+  almost sure to inherit from its new progenitor some slight characteristic
+  differences.</p>
+
+  <p>Groups of species, that is, genera and families, follow the same
+  general rules in their appearance and disappearance as do single species,
+  changing more or less quickly, and in a greater or lesser degree. A group
+  does not reappear after it has once disappeared; or its existence, as
+  long as it lasts, is continuous. I am aware that there are some apparent
+  exceptions to this rule, but the exceptions are surprisingly few, so few
+  that E. Forbes, Pictet, and Woodward (though all strongly opposed to such
+  views as I maintain) admit its truth; and the rule strictly accords with
+  my theory. For as all the species of the same group have descended from
+  some one species, it is clear that as long as any species of the group
+  have appeared in the long succession of ages, so long must its members
+  have continuously existed, in order to have generated either new and
+  modified or the same old and unmodified forms. Species of the genus
+  Lingula, for instance, must have continuously existed by an unbroken
+  succession of generations, from the lowest Silurian stratum to the
+  present day.</p>
+
+  <p>We have seen in the last chapter that the species of a group sometimes
+  falsely appear to have come in abruptly; and I have attempted to give an
+  explanation of this fact, which if true would have been fatal to my
+  views. But such cases are certainly exceptional; the general rule being a
+  gradual increase in number, till the group reaches its maximum, and then,
+  sooner or later, it gradually decreases. If the number of the species of
+  a genus, or the number of <!-- Page 317 --><span class="pagenum"><a
+  name="page317"></a>[317]</span>the genera of a family, be represented by
+  a vertical line of varying thickness, crossing the successive geological
+  formations in which the species are found, the line will sometimes
+  falsely appear to begin at its lower end, not in a sharp point, but
+  abruptly; it then gradually thickens upwards, sometimes keeping for a
+  space of equal thickness, and ultimately thins out in the upper beds,
+  marking the decrease and final extinction of the species. This gradual
+  increase in number of the species of a group is strictly conformable with
+  my theory; as the species of the same genus, and the genera of the same
+  family, can increase only slowly and progressively; for the process of
+  modification and the production of a number of allied forms must be slow
+  and gradual,&mdash;one species giving rise first to two or three
+  varieties, these being slowly converted into species, which in their turn
+  produce by equally slow steps other species, and so on, like the
+  branching of a great tree from a single stem, till the group becomes
+  large.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On Extinction.</i>&mdash;We have as yet spoken only incidentally of
+  the disappearance of species and of groups of species. On the theory of
+  natural selection the extinction of old forms and the production of new
+  and improved forms are intimately connected together. The old notion of
+  all the inhabitants of the earth having been swept away at successive
+  periods by catastrophes, is very generally given up, even by those
+  geologists, as Elie de Beaumont, Murchison, Barrande, &amp;c., whose
+  general views would naturally lead them to this conclusion. On the
+  contrary, we have every reason to believe, from the study of the tertiary
+  formations, that species and groups of species gradually disappear, one
+  after another, first from one spot, then from another, and finally from
+  the world. Both single species and whole <!-- Page 318 --><span
+  class="pagenum"><a name="page318"></a>[318]</span>groups of species last
+  for very unequal periods; some groups, as we have seen, having endured
+  from the earliest known dawn of life to the present day; some having
+  disappeared before the close of the palæozoic period. No fixed law seems
+  to determine the length of time during which any single species or any
+  single genus endures. There is reason to believe that the complete
+  extinction of the species of a group is generally a slower process than
+  their production: if the appearance and disappearance of a group of
+  species be represented, as before, by a vertical line of varying
+  thickness, the line is found to taper more gradually at its upper end,
+  which marks the progress of extermination, than at its lower end, which
+  marks the first appearance and increase in numbers of the species. In
+  some cases, however, the extermination of whole groups of beings, as of
+  ammonites towards the close of the secondary period, has been wonderfully
+  sudden.</p>
+
+  <p>The whole subject of the extinction of species has been involved in
+  the most gratuitous mystery. Some authors have even supposed that as the
+  individual has a definite length of life, so have species a definite
+  duration. No one I think can have marvelled more at the extinction of
+  species, than I have done. When I found in La Plata the tooth of a horse
+  embedded with the remains of Mastodon, Megatherium, Toxodon, and other
+  extinct monsters, which all co-existed with still living shells at a very
+  late geological period, I was filled with astonishment; for seeing that
+  the horse, since its introduction by the Spaniards into South America,
+  has run wild over the whole country and has increased in numbers at an
+  unparalleled rate, I asked myself what could so recently have
+  exterminated the former horse under conditions of life apparently so
+  favourable. But how utterly groundless was my astonishment! <!-- Page 319
+  --><span class="pagenum"><a name="page319"></a>[319]</span>Professor Owen
+  soon perceived that the tooth, though so like that of the existing horse,
+  belonged to an extinct species. Had this horse been still living, but in
+  some degree rare, no naturalist would have felt the least surprise at its
+  rarity; for rarity is the attribute of a vast number of species of all
+  classes, in all countries. If we ask ourselves why this or that species
+  is rare, we answer that something is unfavourable in its conditions of
+  life; but what that something is, we can hardly ever tell. On the
+  supposition of the fossil horse still existing as a rare species, we
+  might have felt certain from the analogy of all other mammals, even of
+  the slow-breeding elephant, and from the history of the naturalisation of
+  the domestic horse in South America, that under more favourable
+  conditions it would in a very few years have stocked the whole continent.
+  But we could not have told what the unfavourable conditions were which
+  checked its increase, whether some one or several contingencies, and at
+  what period of the horse's life, and in what degree, they severally
+  acted. If the conditions had gone on, however slowly, becoming less and
+  less favourable, we assuredly should not have perceived the fact, yet the
+  fossil horse would certainly have become rarer and rarer, and finally
+  extinct;&mdash;its place being seized on by some more successful
+  competitor.</p>
+
+  <p>It is most difficult always to remember that the increase of every
+  living being is constantly being checked by unperceived injurious
+  agencies; and that these same unperceived agencies are amply sufficient
+  to cause rarity, and finally extinction. We see in many cases in the more
+  recent tertiary formations, that rarity precedes extinction; and we know
+  that this has been the progress of events with those animals which have
+  been exterminated, either locally or wholly, through <!-- Page 320
+  --><span class="pagenum"><a name="page320"></a>[320]</span>man's agency.
+  I may repeat what I published in 1845, namely, that to admit that species
+  generally become rare before they become extinct&mdash;to feel no
+  surprise at the rarity of a species, and yet to marvel greatly when it
+  ceases to exist, is much the same as to admit that sickness in the
+  individual is the forerunner of death&mdash;to feel no surprise at
+  sickness, but when the sick man dies, to wonder and to suspect that he
+  died by some unknown deed of violence.</p>
+
+  <p>The theory of natural selection is grounded on the belief that each
+  new variety, and ultimately each new species, is produced and maintained
+  by having some advantage over those with which it comes into competition;
+  and the consequent extinction of less-favoured forms almost inevitably
+  follows. It is the same with our domestic productions: when a new and
+  slightly improved variety has been raised, it at first supplants the less
+  improved varieties in the same neighbourhood; when much improved it is
+  transported far and near, like our short-horn cattle, and takes the place
+  of other breeds in other countries. Thus the appearance of new forms and
+  the disappearance of old forms, both natural and artificial, are bound
+  together. In certain flourishing groups, the number of new specific forms
+  which have been produced within a given time is probably greater than
+  that of the old specific forms which have been exterminated; but we know
+  that the number of species has not gone on indefinitely increasing, at
+  least during the later geological periods, so that looking to later times
+  we may believe that the production of new forms has caused the extinction
+  of about the same number of old forms.</p>
+
+  <p>The competition will generally be most severe, as formerly explained
+  and illustrated by examples, between the forms which are most like each
+  other in all respects. <!-- Page 321 --><span class="pagenum"><a
+  name="page321"></a>[321]</span>Hence the improved and modified
+  descendants of a species will generally cause the extermination of the
+  parent-species; and if many new forms have been developed from any one
+  species, the nearest allies of that species, <i>i.e.</i> the species of
+  the same genus, will be the most liable to extermination. Thus, as I
+  believe, a number of new species descended from one species, that is a
+  new genus, comes to supplant an old genus, belonging to the same family.
+  But it must often have happened that a new species belonging to some one
+  group will have seized on the place occupied by a species belonging to a
+  distinct group, and thus caused its extermination; and if many allied
+  forms be developed from the successful intruder, many will have to yield
+  their places; and it will generally be allied forms, which will suffer
+  from some inherited inferiority in common. But whether it be species
+  belonging to the same or to a distinct class, which yield their places to
+  other species which have been modified and improved, a few of the
+  sufferers may often long be preserved, from being fitted to some peculiar
+  line of life, or from inhabiting some distant and isolated station, where
+  they have escaped severe competition. For instance, a single species of
+  Trigonia, a great genus of shells in the secondary formations, survives
+  in the Australian seas; and a few members of the great and almost extinct
+  group of Ganoid fishes still inhabit our fresh waters. Therefore the
+  utter extinction of a group is generally, as we have seen, a slower
+  process than its production.</p>
+
+  <p>With respect to the apparently sudden extermination of whole families
+  or orders, as of Trilobites at the close of the palæozoic period and of
+  Ammonites at the close of the secondary period, we must remember what has
+  been already said on the probable wide intervals of time <!-- Page 322
+  --><span class="pagenum"><a name="page322"></a>[322]</span>between our
+  consecutive formations; and in these intervals there may have been much
+  slow extermination. Moreover, when by sudden immigration or by unusually
+  rapid development, many species of a new group have taken possession of a
+  new area, they will have exterminated in a correspondingly rapid manner
+  many of the old inhabitants; and the forms which thus yield their places
+  will commonly be allied, for they will partake of some inferiority in
+  common.</p>
+
+  <p>Thus, as it seems to me, the manner in which single species and whole
+  groups of species become extinct, accords well with the theory of natural
+  selection. We need not marvel at extinction; if we must marvel, let it be
+  at our presumption in imagining for a moment that we understand the many
+  complex contingencies, on which the existence of each species depends. If
+  we forget for an instant, that each species tends to increase
+  inordinately, and that some check is always in action, yet seldom
+  perceived by us, the whole economy of nature will be utterly obscured.
+  Whenever we can precisely say why this species is more abundant in
+  individuals than that; why this species and not another can be
+  naturalised in a given country; then, and not till then, we may justly
+  feel surprise why we cannot account for the extinction of this particular
+  species or group of species.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the Forms of Life changing almost simultaneously throughout the
+  World.</i>&mdash;Scarcely any palæontological discovery is more striking
+  than the fact, that the forms of life change almost simultaneously
+  throughout the world. Thus our European Chalk formation can be recognised
+  in many distant parts of the world, under the most different climates,
+  where not a fragment of the mineral chalk itself can be found; namely, in
+  North <!-- Page 323 --><span class="pagenum"><a
+  name="page323"></a>[323]</span>America, in equatorial South America, in
+  Tierra del Fuego, at the Cape of Good Hope, and in the peninsula of
+  India. For at these distant points, the organic remains in certain beds
+  present an unmistakeable degree of resemblance to those of the Chalk. It
+  is not that the same species are met with; for in some cases not one
+  species is identically the same, but they belong to the same families,
+  genera, and sections of genera, and sometimes are similarly characterised
+  in such trifling points as mere superficial sculpture. Moreover other
+  forms, which are not found in the Chalk of Europe, but which occur in the
+  formations either above or below, are similarly absent at these distant
+  points of the world. In the several successive palæozoic formations of
+  Russia, Western Europe and North America, a similar parallelism in the
+  forms of life has been observed by several authors: so it is, according
+  to Lyell, with the several European and North American tertiary deposits.
+  Even if the few fossil species which are common to the Old and New Worlds
+  be kept wholly out of view, the general parallelism in the successive
+  forms of life, in the stages of the widely separated palæozoic and
+  tertiary periods, would still be manifest, and the several formations
+  could be easily correlated.</p>
+
+  <p>These observations, however, relate to the marine inhabitants of
+  distant parts of the world: we have not sufficient data to judge whether
+  the productions of the land and of fresh water change at distant points
+  in the same parallel manner. We may doubt whether they have thus changed:
+  if the Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought
+  to Europe from La Plata, without any information in regard to their
+  geological position, no one would have suspected that they had co-existed
+  with still living sea-shells; but as these anomalous monsters co-existed
+  with the <!-- Page 324 --><span class="pagenum"><a
+  name="page324"></a>[324]</span>Mastodon and Horse, it might at least have
+  been inferred that they had lived during one of the later tertiary
+  stages.</p>
+
+  <p>When the marine forms of life are spoken of as having changed
+  simultaneously throughout the world, it must not be supposed that this
+  expression relates to the same thousandth or hundred-thousandth year, or
+  even that it has a very strict geological sense; for if all the marine
+  animals which live at the present day in Europe, and all those that lived
+  in Europe during the pleistocene period (an enormously remote period as
+  measured by years, including the whole glacial epoch), were to be
+  compared with those now living in South America or in Australia, the most
+  skilful naturalist would hardly be able to say whether the existing or
+  the pleistocene inhabitants of Europe resembled most closely those of the
+  southern hemisphere. So, again, several highly competent observers
+  believe that the existing productions of the United States are more
+  closely related to those which lived in Europe during certain later
+  tertiary stages, than to those which now live here; and if this be so, it
+  is evident that fossiliferous beds deposited at the present day on the
+  shores of North America would hereafter be liable to be classed with
+  somewhat older European beds. Nevertheless, looking to a remotely future
+  epoch, there can, I think, be little doubt that all the more modern
+  <i>marine</i> formations, namely, the upper pliocene, the pleistocene and
+  strictly modern beds, of Europe, North and South America, and Australia,
+  from containing fossil remains in some degree allied, and from not
+  including those forms which are only found in the older underlying
+  deposits, would be correctly ranked as simultaneous in a geological
+  sense.</p>
+
+  <p>The fact of the forms of life changing simultaneously, in the above
+  large sense, at distant parts of the world, has greatly struck those
+  admirable observers, MM. <!-- Page 325 --><span class="pagenum"><a
+  name="page325"></a>[325]</span>de Verneuil and d'Archiac. After referring
+  to the parallelism of the palæozoic forms of life in various parts of
+  Europe, they add, "If struck by this strange sequence, we turn our
+  attention to North America, and there discover a series of analogous
+  phenomena, it will appear certain that all these modifications of
+  species, their extinction, and the introduction of new ones, cannot be
+  owing to mere changes in marine currents or other causes more or less
+  local and temporary, but depend on general laws which govern the whole
+  animal kingdom." M. Barrande has made forcible remarks to precisely the
+  same effect. It is, indeed, quite futile to look to changes of currents,
+  climate, or other physical conditions, as the cause of these great
+  mutations in the forms of life throughout the world, under the most
+  different climates. We must, as Barrande has remarked, look to some
+  special law. We shall see this more clearly when we treat of the present
+  distribution of organic beings, and find how slight is the relation
+  between the physical conditions of various countries, and the nature of
+  their inhabitants.</p>
+
+  <p>This great fact of the parallel succession of the forms of life
+  throughout the world, is explicable on the theory of natural selection.
+  New species are formed by new varieties arising, which have some
+  advantage over older forms; and those forms, which are already dominant,
+  or have some advantage over the other forms in their own country, would
+  naturally oftenest give rise to new varieties or incipient species; for
+  these latter must be victorious in a still higher degree in order to be
+  preserved and to survive. We have distinct evidence on this head, in the
+  plants which are dominant, that is, which are commonest in their own
+  homes, and are most widely diffused, having produced the greatest number
+  of new varieties. It is also natural that the <!-- Page 326 --><span
+  class="pagenum"><a name="page326"></a>[326]</span>dominant, varying, and
+  far-spreading species, which already have invaded to a certain extent the
+  territories of other species, should be those which would have the best
+  chance of spreading still further, and of giving rise in new countries to
+  new varieties and species. The process of diffusion may often be very
+  slow, being dependent on climatal and geographical changes, or on strange
+  accidents, but in the long run the dominant forms will generally succeed
+  in spreading. The diffusion would, it is probable, be slower with the
+  terrestrial inhabitants of distinct continents than with the marine
+  inhabitants of the continuous sea. We might therefore expect to find, as
+  we apparently do find, a less strict degree of parallel succession in the
+  productions of the land than of the sea.</p>
+
+  <p>Dominant species spreading from any region might encounter still more
+  dominant species, and then their triumphant course, or even their
+  existence, would cease. We know not at all precisely what are all the
+  conditions most favourable for the multiplication of new and dominant
+  species; but we can, I think, clearly see that a number of individuals,
+  from giving a better chance of the appearance of favourable variations,
+  and that severe competition with many already existing forms, would be
+  highly favourable, as would be the power of spreading into new
+  territories. A certain amount of isolation, recurring at long intervals
+  of time, would probably be also favourable, as before explained. One
+  quarter of the world may have been most favourable for the production of
+  new and dominant species on the land, and another for those in the waters
+  of the sea. If two great regions had been for a long period favourably
+  circumstanced in an equal degree, whenever their inhabitants met, the
+  battle would be prolonged and severe; and some from one birthplace and
+  some from the other might be victorious. But in the course of time, the
+  <!-- Page 327 --><span class="pagenum"><a
+  name="page327"></a>[327]</span>forms dominant in the highest degree,
+  wherever produced, would tend everywhere to prevail. As they prevailed,
+  they would cause the extinction of other and inferior forms; and as these
+  inferior forms would be allied in groups by inheritance, whole groups
+  would tend slowly to disappear; though here and there a single member
+  might long be enabled to survive.</p>
+
+  <p>Thus, as it seems to me, the parallel, and, taken in a large sense,
+  simultaneous, succession of the same forms of life throughout the world,
+  accords well with the principle of new species having been formed by
+  dominant species spreading widely and varying; the new species thus
+  produced being themselves dominant owing to inheritance, and to having
+  already had some advantage over their parents or over other species;
+  these again spreading, varying, and producing new species. The forms
+  which are beaten and which yield their places to the new and victorious
+  forms, will generally be allied in groups, from inheriting some
+  inferiority in common; and therefore as new and improved groups spread
+  throughout the world, old groups will disappear from the world; and the
+  succession of forms in both ways will everywhere tend to correspond.</p>
+
+  <p>There is one other remark connected with this subject worth making. I
+  have given my reasons for believing that all our greater fossiliferous
+  formations were deposited during periods of subsidence; and that blank
+  intervals of vast duration occurred during the periods when the bed of
+  the sea was either stationary or rising, and likewise when sediment was
+  not thrown down quickly enough to embed and preserve organic remains.
+  During these long and blank intervals I suppose that the inhabitants of
+  each region underwent a considerable amount of modification and
+  extinction, and that there was much migration from <!-- Page 328 --><span
+  class="pagenum"><a name="page328"></a>[328]</span>other parts of the
+  world. As we have reason to believe that large areas are affected by the
+  same movement, it is probable that strictly contemporaneous formations
+  have often been accumulated over very wide spaces in the same quarter of
+  the world; but we are far from having any right to conclude that this has
+  invariably been the case, and that large areas have invariably been
+  affected by the same movements. When two formations have been deposited
+  in two regions during nearly, but not exactly the same period, we should
+  find in both, from the causes explained in the foregoing paragraphs, the
+  same general succession in the forms of life; but the species would not
+  exactly correspond; for there will have been a little more time in the
+  one region than in the other for modification, extinction, and
+  immigration.</p>
+
+  <p>I suspect that cases of this nature occur in Europe. Mr. Prestwich, in
+  his admirable Memoirs on the eocene deposits of England and France, is
+  able to draw a close general parallelism between the successive stages in
+  the two countries; but when he compares certain stages in England with
+  those in France, although he finds in both a curious accordance in the
+  numbers of the species belonging to the same genera, yet the species
+  themselves differ in a manner very difficult to account for, considering
+  the proximity of the two areas,&mdash;unless, indeed, it be assumed that
+  an isthmus separated two seas inhabited by distinct, but contemporaneous,
+  faunas. Lyell has made similar observations on some of the later tertiary
+  formations. Barrande, also, shows that there is a striking general
+  parallelism in the successive Silurian deposits of Bohemia and
+  Scandinavia; nevertheless he finds a surprising amount of difference in
+  the species. If the several formations in these regions have not been
+  deposited during the same exact <!-- Page 329 --><span class="pagenum"><a
+  name="page329"></a>[329]</span>periods,&mdash;a formation in one region
+  often corresponding with a blank interval in the other,&mdash;and if in
+  both regions the species have gone on slowly changing during the
+  accumulation of the several formations and during the long intervals of
+  time between them; in this case, the several formations in the two
+  regions could be arranged in the same order, in accordance with the
+  general succession of the form of life, and the order would falsely
+  appear to be strictly parallel; nevertheless the species would not all be
+  the same in the apparently corresponding stages in the two regions.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the Affinities of extinct Species to each other, and to living
+  forms.</i>&mdash;Let us now look to the mutual affinities of extinct and
+  living species. They all fall into one grand natural system; and this
+  fact is at once explained on the principle of descent. The more ancient
+  any form is, the more, as a general rule, it differs from living forms.
+  But, as Buckland long ago remarked, all fossils can be classed either in
+  still existing groups, or between them. That the extinct forms of life
+  help to fill up the wide intervals between existing genera, families, and
+  orders, cannot be disputed. For if we confine our attention either to the
+  living or to the extinct alone, the series is far less perfect than if we
+  combine both into one general system. With respect to the Vertebrata,
+  whole pages could be filled with striking illustrations from our great
+  palaeontologist, Owen, showing how extinct animals fall in between
+  existing groups. Cuvier ranked the Ruminants and Pachyderms, as the two
+  most distinct orders of mammals; but Owen has discovered so many fossil
+  links, that he has had to alter the whole classification of these two
+  orders; and has placed certain pachyderms in the same sub-order with
+  ruminants: for example, he dissolves by fine gradations the apparently
+  <!-- Page 330 --><span class="pagenum"><a
+  name="page330"></a>[330]</span>wide difference between the pig and the
+  camel. In regard to the Invertebrata, Barrande, and a higher authority
+  could not be named, asserts that he is every day taught that Palaeozoic
+  animals, though belonging to the same orders, families, or genera with
+  those living at the present day, were not at this early epoch limited in
+  such distinct groups as they now are.</p>
+
+  <p>Some writers have objected to any extinct species or group of species
+  being considered as intermediate between living species or groups. If by
+  this term it is meant that an extinct form is directly intermediate in
+  all its characters between two living forms, the objection is probably
+  valid. But I apprehend that in a perfectly natural classification many
+  fossil species would have to stand between living species, and some
+  extinct genera between living genera, even between genera belonging to
+  distinct families. The most common case, especially with respect to very
+  distinct groups, such as fish and reptiles, seems to be, that supposing
+  them to be distinguished at the present day from each other by a dozen
+  characters, the ancient members of the same two groups would be
+  distinguished by a somewhat lesser number of characters, so that the two
+  groups, though formerly quite distinct, at that period made some small
+  approach to each other.</p>
+
+  <p>It is a common belief that the more ancient a form is, by so much the
+  more it tends to connect by some of its characters groups now widely
+  separated from each other. This remark no doubt must be restricted to
+  those groups which have undergone much change in the course of geological
+  ages; and it would be difficult to prove the truth of the proposition,
+  for every now and then even a living animal, as the Lepidosiren, is
+  discovered having affinities directed towards very distinct groups. Yet
+  if we compare the older Reptiles and <!-- Page 331 --><span
+  class="pagenum"><a name="page331"></a>[331]</span>Batrachians, the older
+  Fish, the older Cephalopods, and the eocene Mammals, with the more recent
+  members of the same classes, we must admit that there is some truth in
+  the remark.</p>
+
+  <p>Let us see how far these several facts and inferences accord with the
+  theory of descent with modification. As the subject is somewhat complex,
+  I must request the reader to turn to the diagram in the fourth chapter.
+  We may suppose that the numbered letters represent genera, and the dotted
+  lines diverging from them the species in each genus. The diagram is much
+  too simple, too few genera and too few species being given, but this is
+  unimportant for us. The horizontal lines may represent successive
+  geological formations, and all the forms beneath the uppermost line may
+  be considered as extinct. The three existing genera,
+  <i>a</i><sup>14</sup>, <i>q</i><sup>14</sup>, <i>p</i><sup>14</sup>, will
+  form a small family; <i>b</i><sup>14</sup> and <i>f</i><sup>14</sup> a
+  closely allied family or sub-family; and <i>o</i><sup>14</sup>,
+  <i>e</i><sup>14</sup>, <i>m</i><sup>14</sup>, a third family. These three
+  families, together with the many extinct genera on the several lines of
+  descent diverging from the parent-form (A), will form an order; for all
+  will have inherited something in common from their ancient and common
+  progenitor. On the principle of the continued tendency to divergence of
+  character, which was formerly illustrated by this diagram, the more
+  recent any form is, the more it will generally differ from its ancient
+  progenitor. Hence we can understand the rule that the most ancient
+  fossils differ most from existing forms. We must not, however, assume
+  that divergence of character is a necessary contingency; it depends
+  solely on the descendants from a species being thus enabled to seize on
+  many and different places in the economy of nature. Therefore it is quite
+  possible, as we have seen in the case of some Silurian forms, that a
+  species might go on being slightly modified in relation to its slightly
+  altered conditions of <!-- Page 332 --><span class="pagenum"><a
+  name="page332"></a>[332]</span>life, and yet retain throughout a vast
+  period the same general characteristics. This is represented in the
+  diagram by the letter <span class="scac">F</span><sup>14</sup>.</p>
+
+  <p>All the many forms, extinct and recent, descended from (A), make, as
+  before remarked, one order; and this order, from the continued effects of
+  extinction and divergence of character, has become divided into several
+  sub-families and families, some of which are supposed to have perished at
+  different periods, and some to have endured to the present day.</p>
+
+  <p>By looking at the diagram we can see that if many of the extinct
+  forms, supposed to be embedded in the successive formations, were
+  discovered at several points low down in the series, the three existing
+  families on the uppermost line would be rendered less distinct from each
+  other. If, for instance, the genera <i>a</i><sup>1</sup>,
+  <i>a</i><sup>5</sup>, <i>a</i><sup>10</sup>, <i>f</i><sup>8</sup>,
+  <i>m</i><sup>3</sup>, <i>m</i><sup>6</sup>, <i>m</i><sup>9</sup>, were
+  disinterred, these three families would be so closely linked together
+  that they probably would have to be united into one great family, in
+  nearly the same manner as has occurred with ruminants and pachyderms. Yet
+  he who objected to call the extinct genera, which thus linked the living
+  genera of three families together, intermediate in character, would be
+  justified, as they are intermediate, not directly, but only by a long and
+  circuitous course through many widely different forms. If many extinct
+  forms were to be discovered above one of the middle horizontal lines or
+  geological formations &mdash;for instance, above No. VI.&mdash;but none
+  from beneath this line, then only the two families on the left hand
+  (namely, <i>a</i><sup>14</sup>, &amp;c., and
+  <i>b</i><sup>14</sup>, &amp;c.) would have to be united into one family;
+  and the two other families (namely, <i>a</i><sup>14</sup> to
+  <i>f</i><sup>14</sup> now including five genera, and
+  <i>o</i><sup>14</sup> to <i>m</i><sup>14</sup>) would yet remain
+  distinct. These two families, however, would be less distinct from each
+  other than they were before the discovery of the fossils. If, for
+  instance, we suppose the existing genera of the two families to differ
+  from each <!-- Page 333 --><span class="pagenum"><a
+  name="page333"></a>[333]</span>other by a dozen characters, in this case
+  the genera, at the early period marked VI., would differ by a lesser
+  number of characters; for at this early stage of descent they have not
+  diverged in character from the common progenitor of the order, nearly so
+  much as they subsequently diverged. Thus it comes that ancient and
+  extinct genera are often in some slight degree intermediate in character
+  between their modified descendants, or between their collateral
+  relations.</p>
+
+  <p>In nature the case will be far more complicated than is represented in
+  the diagram; for the groups will have been more numerous, they will have
+  endured for extremely unequal lengths of time, and will have been
+  modified in various degrees. As we possess only the last volume of the
+  geological record, and that in a very broken condition, we have no right
+  to expect, except in very rare cases, to fill up wide intervals in the
+  natural system, and thus unite distinct families or orders. All that we
+  have a right to expect, is that those groups, which have within known
+  geological periods undergone much modification, should in the older
+  formations make some slight approach to each other; so that the older
+  members should differ less from each other in some of their characters
+  than do the existing members of the same groups; and this by the
+  concurrent evidence of our best palæontologists seems frequently to be
+  the case.</p>
+
+  <p>Thus, on the theory of descent with modification, the main facts with
+  respect to the mutual affinities of the extinct forms of life to each
+  other and to living forms, seem to me explained in a satisfactory manner.
+  And they are wholly inexplicable on any other view.</p>
+
+  <p>On this same theory, it is evident that the fauna of any great period
+  in the earth's history will be intermediate in general character between
+  that which preceded and that which succeeded it. Thus, the species which
+  lived at the sixth great stage of descent in the <!-- Page 334 --><span
+  class="pagenum"><a name="page334"></a>[334]</span>diagram are the
+  modified offspring of those which lived at the fifth stage, and are the
+  parents of those which became still more modified at the seventh stage;
+  hence they could hardly fail to be nearly intermediate in character
+  between the forms of life above and below. We must, however, allow for
+  the entire extinction of some preceding forms, and in any one region for
+  the immigration of new forms from other regions, and for a large amount
+  of modification, during the long and blank intervals between the
+  successive formations. Subject to these allowances, the fauna of each
+  geological period undoubtedly is intermediate in character, between the
+  preceding and succeeding faunas. I need give only one instance, namely,
+  the manner in which the fossils of the Devonian system, when this system
+  was first discovered, were at once recognised by palæontologists as
+  intermediate in character between those of the overlying carboniferous,
+  and underlying Silurian system. But each fauna is not necessarily exactly
+  intermediate, as unequal intervals of time have elapsed between
+  consecutive formations.</p>
+
+  <p>It is no real objection to the truth of the statement, that the fauna
+  of each period as a whole is nearly intermediate in character between the
+  preceding and succeeding faunas, that certain genera offer exceptions to
+  the rule. For instance, mastodons and elephants, when arranged by Dr.
+  Falconer in two series, first according to their mutual affinities and
+  then according to their periods of existence, do not accord in
+  arrangement. The species extreme in character are not the oldest, or the
+  most recent; nor are those which are intermediate in character,
+  intermediate in age. But supposing for an instant, in this and other such
+  cases, that the record of the first appearance and disappearance of the
+  species was perfect, we have no reason to believe that forms successively
+  produced necessarily endure for <!-- Page 335 --><span class="pagenum"><a
+  name="page335"></a>[335]</span>corresponding lengths of time: a very
+  ancient form might occasionally last much longer than a form elsewhere
+  subsequently produced, especially in the case of terrestrial productions
+  inhabiting separated districts. To compare small things with great: if
+  the principal living and extinct races of the domestic pigeon were
+  arranged as well as they could be in serial affinity, this arrangement
+  would not closely accord with the order in time of their production, and
+  still less with the order of their disappearance; for the parent
+  rock-pigeon now lives; and many varieties between the rock-pigeon and the
+  carrier have become extinct; and carriers which are extreme in the
+  important character of length of beak originated earlier than
+  short-beaked tumblers, which are at the opposite end of the series in
+  this same respect.</p>
+
+  <p>Closely connected with the statement, that the organic remains from an
+  intermediate formation are in some degree intermediate in character, is
+  the fact, insisted on by all palæontologists, that fossils from two
+  consecutive formations are far more closely related to each other, than
+  are the fossils from two remote formations. Pictet gives as a well-known
+  instance, the general resemblance of the organic remains from the several
+  stages of the Chalk formation, though the species are distinct in each
+  stage. This fact alone, from its generality, seems to have shaken
+  Professor Pictet in his firm belief in the immutability of species. He
+  who is acquainted with the distribution of existing species over the
+  globe, will not attempt to account for the close resemblance of the
+  distinct species in closely consecutive formations, by the physical
+  conditions of the ancient areas having remained nearly the same. Let it
+  be remembered that the forms of life, at least those inhabiting the sea,
+  have changed almost simultaneously throughout the world, and therefore
+  under the most different climates and conditions. Consider the <!-- Page
+  336 --><span class="pagenum"><a name="page336"></a>[336]</span>prodigious
+  vicissitudes of climate during the pleistocene period, which includes the
+  whole glacial period, and note how little the specific forms of the
+  inhabitants of the sea have been affected.</p>
+
+  <p>On the theory of descent, the full meaning of the fact of fossil
+  remains from closely consecutive formations, though ranked as distinct
+  species, being closely related, is obvious. As the accumulation of each
+  formation has often been interrupted, and as long blank intervals have
+  intervened between successive formations, we ought not to expect to find,
+  as I attempted to show in the last chapter, in any one or two formations
+  all the intermediate varieties between the species which appeared at the
+  commencement and close of these periods; but we ought to find after
+  intervals, very long as measured by years, but only moderately long as
+  measured geologically, closely allied forms, or, as they have been called
+  by some authors, representative species; and these we assuredly do find.
+  We find, in short, such evidence of the slow and scarcely sensible
+  mutation of specific forms, as we have a just right to expect to
+  find.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the state of Development of Ancient Forms.</i>&mdash;There has
+  been much discussion whether recent forms are more highly developed than
+  ancient. I will not here enter on this subject, for naturalists have not
+  as yet defined to each other's satisfaction what is meant by high and low
+  forms. The best definition probably is, that the higher forms have their
+  organs more distinctly specialised for different functions; and as such
+  division of physiological labour seems to be an advantage to each being,
+  natural selection will constantly tend in so far to make the later and
+  more modified forms higher than their early progenitors, or than the
+  slightly modified descendants of such progenitors. In a more general
+  sense the <!-- Page 337 --><span class="pagenum"><a
+  name="page337"></a>[337]</span>more recent forms must, on my theory, be
+  higher than the more ancient; for each new species is formed by having
+  had some advantage in the struggle for life over other and preceding
+  forms. If under a nearly similar climate, the eocene inhabitants of one
+  quarter of the world were put into competition with the existing
+  inhabitants of the same or some other quarter, the eocene fauna or flora
+  would certainly be beaten and exterminated; as would a secondary fauna by
+  an eocene, and a palæozoic fauna by a secondary fauna. I do not doubt
+  that this process of improvement has affected in a marked and sensible
+  manner the organisation of the more recent and victorious forms of life,
+  in comparison with the ancient and beaten forms; but I can see no way of
+  testing this sort of progress. Crustaceans, for instance, not the highest
+  in their own class, may have beaten the highest molluscs. From the
+  extraordinary manner in which European productions have recently spread
+  over New Zealand, and have seized on places which must have been
+  previously occupied, we may believe, if all the animals and plants of
+  Great Britain were set free in New Zealand, that in the course of time a
+  multitude of British forms would become thoroughly naturalized there, and
+  would exterminate many of the natives. On the other hand, from what we
+  see now occurring in New Zealand, and from hardly a single inhabitant of
+  the southern hemisphere having become wild in any part of Europe, we may
+  doubt, if all the productions of New Zealand were set free in Great
+  Britain, whether any considerable number would be enabled to seize on
+  places now occupied by our native plants and animals. Under this point of
+  view, the productions of Great Britain may be said to be higher than
+  those of New Zealand. Yet the most skilful naturalist from an examination
+  of the <!-- Page 338 --><span class="pagenum"><a
+  name="page338"></a>[338]</span>species of the two countries could not
+  have foreseen this result.</p>
+
+  <p>Agassiz insists that ancient animals resemble to a certain extent the
+  embryos of recent animals of the same classes; or that the geological
+  succession of extinct forms is in some degree parallel to the
+  embryological development of recent forms. I must follow Pictet and
+  Huxley in thinking that the truth of this doctrine is very far from
+  proved. Yet I fully expect to see it hereafter confirmed, at least in
+  regard to subordinate groups, which have branched off from each other
+  within comparatively recent times. For this doctrine of Agassiz accords
+  well with the theory of natural selection. In a future chapter I shall
+  attempt to show that the adult differs from its embryo, owing to
+  variations supervening at a not early age, and being inherited at a
+  corresponding age. This process, whilst it leaves the embryo almost
+  unaltered, continually adds, in the course of successive generations,
+  more and more difference to the adult.</p>
+
+  <p>Thus the embryo comes to be left as a sort of picture, preserved by
+  nature, of the ancient and less modified condition of each animal. This
+  view may be true, and yet it may never be capable of full proof. Seeing,
+  for instance, that the oldest known mammals, reptiles, and fish strictly
+  belong to their own proper classes, though some of these old forms are in
+  a slight degree less distinct from each other than are the typical
+  members of the same groups at the present day, it would be vain to look
+  for animals having the common embryological character of the Vertebrata,
+  until beds far beneath the lowest Silurian strata are discovered&mdash;a
+  discovery of which the chance is very small.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the Succession of the same Types within the same <!-- Page 339
+  --><span class="pagenum"><a name="page339"></a>[339]</span>areas, during
+  the later tertiary periods.</i>&mdash;Mr. Clift many years ago showed
+  that the fossil mammals from the Australian caves were closely allied to
+  the living marsupials of that continent. In South America, a similar
+  relationship is manifest, even to an uneducated eye, in the gigantic
+  pieces of armour like those of the armadillo, found in several parts of
+  La Plata; and Professor Owen has shown in the most striking manner that
+  most of the fossil mammals, buried there in such numbers, are related to
+  South American types. This relationship is even more clearly seen in the
+  wonderful collection of fossil bones made by MM. Lund and Clausen in the
+  caves of Brazil. I was so much impressed with these facts that I strongly
+  insisted, in 1839 and 1845, on this "law of the succession of
+  types,"&mdash;on "this wonderful relationship in the same continent
+  between the dead and the living." Professor Owen has subsequently
+  extended the same generalisation to the mammals of the Old World. We see
+  the same law in this author's restorations of the extinct and gigantic
+  birds of New Zealand. We see it also in the birds of the caves of Brazil.
+  Mr. Woodward has shown that the same law holds good with sea-shells, but
+  from the wide distribution of most genera of molluscs, it is not well
+  displayed by them. Other cases could be added, as the relation between
+  the extinct and living land-shells of Madeira; and between the extinct
+  and living brackish-water shells of the Aralo-Caspian Sea.</p>
+
+  <p>Now what does this remarkable law of the succession of the same types
+  within the same areas mean? He would be a bold man, who after comparing
+  the present climate of Australia and of parts of South America under the
+  same latitude, would attempt to account, on the one hand, by dissimilar
+  physical conditions for the dissimilarity of the inhabitants of these two
+  continents, <!-- Page 340 --><span class="pagenum"><a
+  name="page340"></a>[340]</span>and, on the other hand, by similarity of
+  conditions, for the uniformity of the same types in each during the later
+  tertiary periods. Nor can it be pretended that it is an immutable law
+  that marsupials should have been chiefly or solely produced in Australia;
+  or that Edentata and other American types should have been solely
+  produced in South America. For we know that Europe in ancient times was
+  peopled by numerous marsupials; and I have shown in the publications
+  above alluded to, that in America the law of distribution of terrestrial
+  mammals was formerly different from what it now is. North America
+  formerly partook strongly of the present character of the southern half
+  of the continent; and the southern half was formerly more closely allied,
+  than it is at present, to the northern half. In a similar manner we know
+  from Falconer and Cautley's discoveries, that northern India was formerly
+  more closely related in its mammals to Africa than it is at the present
+  time. Analogous facts could be given in relation to the distribution of
+  marine animals.</p>
+
+  <p>On the theory of descent with modification, the great law of the long
+  enduring, but not immutable, succession of the same types within the same
+  areas, is at once explained; for the inhabitants of each quarter of the
+  world will obviously tend to leave in that quarter, during the next
+  succeeding period of time, closely allied though in some degree modified
+  descendants. If the inhabitants of one continent formerly differed
+  greatly from those of another continent, so will their modified
+  descendants still differ in nearly the same manner and degree. But after
+  very long intervals of time and after great geographical changes,
+  permitting much inter-migration, the feebler will yield to the more
+  dominant forms, and there will be nothing immutable in the laws of past
+  and present distribution. <!-- Page 341 --><span class="pagenum"><a
+  name="page341"></a>[341]</span></p>
+
+  <p>It may be asked in ridicule, whether I suppose that the megatherium
+  and other allied huge monsters have left behind them in South America,
+  the sloth, armadillo, and anteater, as their degenerate descendants. This
+  cannot for an instant be admitted. These huge animals have become wholly
+  extinct, and have left no progeny. But in the caves of Brazil, there are
+  many extinct species which are closely allied in size and in other
+  characters to the species still living in South America; and some of
+  these fossils may be the actual progenitors of living species. It must
+  not be forgotten that, on my theory, all the species of the same genus
+  have descended from some one species; so that if six genera, each having
+  eight species, be found in one geological formation, and in the next
+  succeeding formation there be six other allied or representative genera
+  with the same number of species, then we may conclude that only one
+  species of each of the six older genera has left modified descendants,
+  constituting the six new genera. The other seven species of the old
+  genera have all died out and have left no progeny. Or, which would
+  probably be a far commoner case, two or three species of two or three
+  alone of the six older genera will have been the parents of the six new
+  genera; the other old species and the other whole old genera having
+  become utterly extinct. In failing orders, with the genera and species
+  decreasing in numbers, as apparently is the case of the Edentata of South
+  America, still fewer genera and species will have left modified
+  blood-descendants.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Summary of the preceding and present Chapters.</i>&mdash;I have
+  attempted to show that the geological record is extremely imperfect; that
+  only a small portion of the globe has been geologically explored with
+  care; that <!-- Page 342 --><span class="pagenum"><a
+  name="page342"></a>[342]</span>only certain classes of organic beings
+  have been largely preserved in a fossil state; that the number both of
+  specimens and of species, preserved in our museums, is absolutely as
+  nothing compared with the incalculable number of generations which must
+  have passed away even during a single formation; that, owing to
+  subsidence being necessary for the accumulation of fossiliferous deposits
+  thick enough to resist future degradation, enormous intervals of time
+  have elapsed between the successive formations; that there has probably
+  been more extinction during the periods of subsidence, and more variation
+  during the periods of elevation, and during the latter the record will
+  have been least perfectly kept; that each single formation has not been
+  continuously deposited; that the duration of each formation is, perhaps,
+  short compared with the average duration of specific forms; that
+  migration has played an important part in the first appearance of new
+  forms in any one area and formation; that widely ranging species are
+  those which have varied most, and have oftenest given rise to new
+  species; and that varieties have at first often been local. All these
+  causes taken conjointly, must have tended to make the geological record
+  extremely imperfect, and will to a large extent explain why we do not
+  find interminable varieties, connecting together all the extinct and
+  existing forms of life by the finest graduated steps.</p>
+
+  <p>He who rejects these views on the nature of the geological record,
+  will rightly reject my whole theory. For he may ask in vain where are the
+  numberless transitional links which must formerly have connected the
+  closely allied or representative species, found in the several stages of
+  the same great formation. He may disbelieve in the enormous intervals of
+  time which have elapsed between our consecutive formations; he <!-- Page
+  343 --><span class="pagenum"><a name="page343"></a>[343]</span>may
+  overlook how important a part migration must have played, when the
+  formations of any one great region alone, as that of Europe, are
+  considered; he may urge the apparent, but often falsely apparent, sudden
+  coming in of whole groups of species. He may ask where are the remains of
+  those infinitely numerous organisms which must have existed long before
+  the first bed of the Silurian system was deposited: I can answer this
+  latter question only hypothetically, by saying that as far as we can see,
+  where our oceans now extend they have for an enormous period extended,
+  and where our oscillating continents now stand they have stood ever since
+  the Silurian epoch; but that long before that period, the world may have
+  presented a wholly different aspect; and that the older continents,
+  formed of formations older than any known to us, may now all be in a
+  metamorphosed condition, or may lie buried under the ocean.</p>
+
+  <p>Passing from these difficulties, all the other great leading facts in
+  palæontology seem to me simply to follow on the theory of descent with
+  modification through natural selection. We can thus understand how it is
+  that new species come in slowly and successively; how species of
+  different classes do not necessarily change together, or at the same
+  rate, or in the same degree; yet in the long run that all undergo
+  modification to some extent. The extinction of old forms is the almost
+  inevitable consequence of the production of new forms. We can understand
+  why when a species has once disappeared it never reappears. Groups of
+  species increase in numbers slowly, and endure for unequal periods of
+  time; for the process of modification is necessarily slow, and depends on
+  many complex contingencies. The dominant species of the larger dominant
+  groups tend to leave many modified <!-- Page 344 --><span
+  class="pagenum"><a name="page344"></a>[344]</span>descendants, and thus
+  new sub-groups and groups are formed. As these are formed, the species of
+  the less vigorous groups, from their inferiority inherited from a common
+  progenitor, tend to become extinct together, and to leave no modified
+  offspring on the face of the earth. But the utter extinction of a whole
+  group of species may often be a very slow process, from the survival of a
+  few descendants, lingering in protected and isolated situations. When a
+  group has once wholly disappeared, it does not reappear; for the link of
+  generation has been broken.</p>
+
+  <p>We can understand how the spreading of the dominant forms of life,
+  which are those that oftenest vary, will in the long run tend to people
+  the world with allied, but modified, descendants; and these will
+  generally succeed in taking the places of those groups of species which
+  are their inferiors in the struggle for existence. Hence, after long
+  intervals of time, the productions of the world will appear to have
+  changed simultaneously.</p>
+
+  <p>We can understand how it is that all the forms of life, ancient and
+  recent, make together one grand system; for all are connected by
+  generation. We can understand, from the continued tendency to divergence
+  of character, why the more ancient a form is, the more it generally
+  differs from those now living. Why ancient and extinct forms often tend
+  to fill up gaps between existing forms, sometimes blending two groups
+  previously classed as distinct into one; but more commonly only bringing
+  them a little closer together. The more ancient a form is, the more
+  often, apparently, it displays characters in some degree intermediate
+  between groups now distinct; for the more ancient a form is, the more
+  nearly it will be related to, and consequently resemble, the common
+  progenitor of groups, since <!-- Page 345 --><span class="pagenum"><a
+  name="page345"></a>[345]</span>become widely divergent. Extinct forms are
+  seldom directly intermediate between existing forms; but are intermediate
+  only by a long and circuitous course through many extinct and very
+  different forms. We can clearly see why the organic remains of closely
+  consecutive formations are more closely allied to each other, than are
+  those of remote formations; for the forms are more closely linked
+  together by generation: we can clearly see why the remains of an
+  intermediate formation are intermediate in character.</p>
+
+  <p>The inhabitants of each successive period in the world's history have
+  beaten their predecessors in the race for life, and are, in so far,
+  higher in the scale of nature; and this may account for that vague yet
+  ill-defined sentiment, felt by many palæontologists, that organisation on
+  the whole has progressed. If it should hereafter be proved that ancient
+  animals resemble to a certain extent the embryos of more recent animals
+  of the same class, the fact will be intelligible. The succession of the
+  same types of structure within the same areas during the later geological
+  periods ceases to be mysterious, and is simply explained by
+  inheritance.</p>
+
+  <p>If then the geological record be as imperfect as I believe it to be,
+  and it may at least be asserted that the record cannot be proved to be
+  much more perfect, the main objections to the theory of natural selection
+  are greatly diminished or disappear. On the other hand, all the chief
+  laws of palæontology plainly proclaim, as it seems to me, that species
+  have been produced by ordinary generation: old forms having been
+  supplanted by new and improved forms of life, produced by the laws of
+  variation still acting round us, and preserved by Natural Selection.</p>
+
+<hr />
+
+<p><!-- Page 346 --><span class="pagenum"><a name="page346"></a>[346]</span></p>
+
+<h3>CHAPTER XI.</h3>
+
+<p class="cenhead"><span class="sc">Geographical Distribution.</span></p>
+
+<blockquote class="b1n">
+
+  <p>Present distribution cannot be accounted for by differences in
+  physical conditions&mdash;Importance of barriers&mdash;Affinity of the
+  productions of the same continent&mdash;Centres of creation&mdash;Means
+  of dispersal, by changes of climate and of the level of the land, and by
+  occasional means&mdash;Dispersal during the Glacial period co-extensive
+  with the world.</p>
+
+</blockquote>
+
+  <p>In considering the distribution of organic beings over the face of the
+  globe, the first great fact which strikes us is, that neither the
+  similarity nor the dissimilarity of the inhabitants of various regions
+  can be accounted for by their climatal and other physical conditions. Of
+  late, almost every author who has studied the subject has come to this
+  conclusion. The case of America alone would almost suffice to prove its
+  truth: for if we exclude the northern parts where the circumpolar land is
+  almost continuous, all authors agree that one of the most fundamental
+  divisions in geographical distribution is that between the New and Old
+  Worlds; yet if we travel over the vast American continent, from the
+  central parts of the United States to its extreme southern point, we meet
+  with the most diversified conditions; the most humid districts, arid
+  deserts, lofty mountains, grassy plains, forests, marshes, lakes, and
+  great rivers, under almost every temperature. There is hardly a climate
+  or condition in the Old World which cannot be paralleled in the
+  New&mdash;at least as closely as the same species generally require; for
+  it is a most rare case to find a group of organisms confined to any small
+  spot, having conditions peculiar in only a slight <!-- Page 347 --><span
+  class="pagenum"><a name="page347"></a>[347]</span>degree; for instance,
+  small areas in the Old World could be pointed out hotter than any in the
+  New World, yet these are not inhabited by a peculiar fauna or flora.
+  Notwithstanding this parallelism in the conditions of the Old and New
+  Worlds, how widely different are their living productions!</p>
+
+  <p>In the southern hemisphere, if we compare large tracts of land in
+  Australia, South Africa, and western South America, between latitudes 25°
+  and 35°, we shall find parts extremely similar in all their conditions,
+  yet it would not be possible to point out three faunas and floras more
+  utterly dissimilar. Or again we may compare the productions of South
+  America south of lat. 35° with those north of 25°, which consequently
+  inhabit a considerably different climate, and they will be found
+  incomparably more closely related to each other, than they are to the
+  productions of Australia or Africa under nearly the same climate.
+  Analogous facts could be given with respect to the inhabitants of the
+  sea.</p>
+
+  <p>A second great fact which strikes us in our general review is, that
+  barriers of any kind, or obstacles to free migration, are related in a
+  close and important manner to the differences between the productions of
+  various regions. We see this in the great difference of nearly all the
+  terrestrial productions of the New and Old Worlds, excepting in the
+  northern parts, where the land almost joins, and where, under a slightly
+  different climate, there might have been free migration for the northern
+  temperate forms, as there now is for the strictly arctic productions. We
+  see the same fact in the great difference between the inhabitants of
+  Australia, Africa, and South America under the same latitude: for these
+  countries are almost as much isolated from each other as is possible. On
+  each continent, also, we see the same fact; for on the opposite sides of
+  <!-- Page 348 --><span class="pagenum"><a
+  name="page348"></a>[348]</span>lofty and continuous mountain-ranges, and
+  of great deserts, and sometimes even of large rivers, we find different
+  productions; though as mountain-chains, deserts, &amp;c., are not as
+  impassable, or likely to have endured so long as the oceans separating
+  continents, the differences are very inferior in degree to those
+  characteristic of distinct continents.</p>
+
+  <p>Turning to the sea, we find the same law. No two marine faunas are
+  more distinct, with hardly a fish, shell, or crab in common, than those
+  of the eastern and western shores of South and Central America; yet these
+  great faunas are separated only by the narrow, but impassable, isthmus of
+  Panama. Westward of the shores of America, a wide space of open ocean
+  extends, with not an island as a halting-place for emigrants; here we
+  have a barrier of another kind, and as soon as this is passed we meet in
+  the eastern islands of the Pacific, with another and totally distinct
+  fauna. So that here three marine faunas range far northward and
+  southward, in parallel lines not far from each other, under corresponding
+  climates; but from being separated from each other by impassable
+  barriers, either of land or open sea, they are wholly distinct. On the
+  other hand, proceeding still further westward from the eastern islands of
+  the tropical parts of the Pacific, we encounter no impassable barriers,
+  and we have innumerable islands as halting-places, or continuous coasts,
+  until after travelling over a hemisphere we come to the shores of Africa;
+  and over this vast space we meet with no well-defined and distinct marine
+  faunas. Although hardly one shell, crab or fish is common to the
+  above-named three approximate faunas of Eastern and Western America and
+  the eastern Pacific islands, yet many fish range from the Pacific into
+  the Indian Ocean, and many shells are common to the eastern islands of
+  the Pacific <!-- Page 349 --><span class="pagenum"><a
+  name="page349"></a>[349]</span>and the eastern shores of Africa, on
+  almost exactly opposite meridians of longitude.</p>
+
+  <p>A third great fact, partly included in the foregoing statements, is
+  the affinity of the productions of the same continent or sea, though the
+  species themselves are distinct at different points and stations. It is a
+  law of the widest generality, and every continent offers innumerable
+  instances. Nevertheless the naturalist in travelling, for instance, from
+  north to south never fails to be struck by the manner in which successive
+  groups of beings, specifically distinct, yet clearly related, replace
+  each other. He hears from closely allied, yet distinct kinds of birds,
+  notes nearly similar, and sees their nests similarly constructed, but not
+  quite alike, with eggs coloured in nearly the same manner. The plains
+  near the Straits of Magellan are inhabited by one species of Rhea
+  (American ostrich), and northward the plains of La Plata by another
+  species of the same genus; and not by a true ostrich or emu, like those
+  found in Africa and Australia under the same latitude. On these same
+  plains of La Plata, we see the agouti and bizcacha, animals having nearly
+  the same habits as our hares and rabbits and belonging to the same order
+  of Rodents, but they plainly display an American type of structure. We
+  ascend the lofty peaks of the Cordillera and we find an alpine species of
+  bizcacha; we look to the waters, and we do not find the beaver or
+  musk-rat, but the coypu and capybara, rodents of the American type.
+  Innumerable other instances could be given. If we look to the islands off
+  the American shore, however much they may differ in geological structure,
+  the inhabitants, though they may be all peculiar species, are essentially
+  American. We may look back to past ages, as shown in the last chapter,
+  and we find American types then prevalent on <!-- Page 350 --><span
+  class="pagenum"><a name="page350"></a>[350]</span>the American continent
+  and in the American seas. We see in these facts some deep organic bond,
+  prevailing throughout space and time, over the same areas of land and
+  water, and independent of their physical conditions. The naturalist must
+  feel little curiosity, who is not led to inquire what this bond is.</p>
+
+  <p>This bond, on my theory, is simply inheritance, that cause which
+  alone, as far as we positively know, produces organisms quite like, or,
+  as we see in the case of varieties, nearly like each other. The
+  dissimilarity of the inhabitants of different regions may be attributed
+  to modification through natural selection, and in a quite subordinate
+  degree to the direct influence of different physical conditions. The
+  degree of dissimilarity will depend on the migration of the more dominant
+  forms of life from one region into another having been effected with more
+  or less ease, at periods more or less remote;&mdash;on the nature and
+  number of the former immigrants;&mdash;and on their action and reaction,
+  in their mutual struggles for life;&mdash;the relation of organism to
+  organism being, as I have already often remarked, the most important of
+  all relations. Thus the high importance of barriers comes into play by
+  checking migration; as does time for the slow process of modification
+  through natural selection. Widely-ranging species, abounding in
+  individuals, which have already triumphed over many competitors in their
+  own widely-extended homes will have the best chance of seizing on new
+  places, when they spread into new countries. In their new homes they will
+  be exposed to new conditions, and will frequently undergo further
+  modification and improvement; and thus they will become still further
+  victorious, and will produce groups of modified descendants. On this
+  principle of inheritance with modification, we can understand how it is
+  that sections of genera, whole genera, <!-- Page 351 --><span
+  class="pagenum"><a name="page351"></a>[351]</span>and even families are
+  confined to the same areas, as is so commonly and notoriously the
+  case.</p>
+
+  <p>I believe, as was remarked in the last chapter, in no law of necessary
+  development. As the variability of each species is an independent
+  property, and will be taken advantage of by natural selection, only so
+  far as it profits the individual in its complex struggle for life, so the
+  degree of modification in different species will be no uniform quantity.
+  If, for instance, a number of species, which stand in direct competition
+  with each other, migrate in a body into a new and afterwards isolated
+  country, they will be little liable to modification; for neither
+  migration nor isolation in themselves can do anything. These principles
+  come into play only by bringing organisms into new relations with each
+  other, and in a lesser degree with the surrounding physical conditions.
+  As we have seen in the last chapter that some forms have retained nearly
+  the same character from an enormously remote geological period, so
+  certain species have migrated over vast spaces, and have not become
+  greatly modified.</p>
+
+  <p>On these views, it is obvious, that the several species of the same
+  genus, though inhabiting the most distant quarters of the world, must
+  originally have proceeded from the same source, as they have descended
+  from the same progenitor. In the case of those species, which have
+  undergone during whole geological periods but little modification, there
+  is not much difficulty in believing that they may have migrated from the
+  same region; for during the vast geographical and climatal changes which
+  will have supervened since ancient times, almost any amount of migration
+  is possible. But in many other cases, in which we have reason to believe
+  that the species of a genus have been produced within comparatively
+  recent times, there is great difficulty on this head. It <!-- Page 352
+  --><span class="pagenum"><a name="page352"></a>[352]</span>is also
+  obvious that the individuals of the same species, though now inhabiting
+  distant and isolated regions, must have proceeded from one spot, where
+  their parents were first produced: for, as explained in the last chapter,
+  it is incredible that individuals identically the same should ever have
+  been produced through natural selection from parents specifically
+  distinct.</p>
+
+  <p>We are thus brought to the question which has been largely discussed
+  by naturalists, namely, whether species have been created at one or more
+  points of the earth's surface. Undoubtedly there are very many cases of
+  extreme difficulty, in understanding how the same species could possibly
+  have migrated from some one point to the several distant and isolated
+  points, where now found. Nevertheless the simplicity of the view that
+  each species was first produced within a single region captivates the
+  mind. He who rejects it, rejects the <i>vera causa</i> of ordinary
+  generation with subsequent migration, and calls in the agency of a
+  miracle. It is universally admitted, that in most cases the area
+  inhabited by a species is continuous; and when a plant or animal inhabits
+  two points so distant from each other, or with an interval of such a
+  nature, that the space could not be easily passed over by migration, the
+  fact is given as something remarkable and exceptional. The capacity of
+  migrating across the sea is more distinctly limited in terrestrial
+  mammals, than perhaps in any other organic beings; and, accordingly, we
+  find no inexplicable cases of the same mammal inhabiting distant points
+  of the world. No geologist will feel any difficulty in such cases as
+  Great Britain having been formerly united to Europe, and consequently
+  possessing the same quadrupeds. But if the same species can be produced
+  at two separate points, why do we not find a single mammal common to
+  Europe and <!-- Page 353 --><span class="pagenum"><a
+  name="page353"></a>[353]</span>Australia or South America? The conditions
+  of life are nearly the same, so that a multitude of European animals and
+  plants have become naturalised in America and Australia; and some of the
+  aboriginal plants are identically the same at these distant points of the
+  northern and southern hemispheres? The answer, as I believe, is, that
+  mammals have not been able to migrate, whereas some plants, from their
+  varied means of dispersal, have migrated across the vast and broken
+  interspace. The great and striking influence which barriers of every kind
+  have had on distribution, is intelligible only on the view that the great
+  majority of species have been produced on one side alone, and have not
+  been able to migrate to the other side. Some few families, many
+  sub-families, very many genera, and a still greater number of sections of
+  genera are confined to a single region; and it has been observed by
+  several naturalists, that the most natural genera, or those genera in
+  which the species are most closely related to each other, are generally
+  local, or confined to one area. What a strange anomaly it would be, if,
+  when coming one step lower in the series, to the individuals of the same
+  species, a directly opposite rule prevailed; and species were not local,
+  but had been produced in two or more distinct areas!</p>
+
+  <p>Hence it seems to me, as it has to many other naturalists, that the
+  view of each species having been produced in one area alone, and having
+  subsequently migrated from that area as far as its powers of migration
+  and subsistence under past and present conditions permitted, is the most
+  probable. Undoubtedly many cases occur, in which we cannot explain how
+  the same species could have passed from one point to the other. But the
+  geographical and climatal changes, which have certainly occurred within
+  recent geological times, must have interrupted or rendered discontinuous
+  the <!-- Page 354 --><span class="pagenum"><a
+  name="page354"></a>[354]</span>formerly continuous range of many species.
+  So that we are reduced to consider whether the exceptions to continuity
+  of range are so numerous and of so grave a nature, that we ought to give
+  up the belief, rendered probable by general considerations, that each
+  species has been produced within one area, and has migrated thence as far
+  as it could. It would be hopelessly tedious to discuss all the
+  exceptional cases of the same species, now living at distant and
+  separated points; nor do I for a moment pretend that any explanation
+  could be offered of many such cases. But after some preliminary remarks,
+  I will discuss a few of the most striking classes of facts; namely, the
+  existence of the same species on the summits of distant mountain-ranges,
+  and at distant points in the arctic and antarctic regions; and secondly
+  (in the following chapter), the wide distribution of freshwater
+  productions; and thirdly, the occurrence of the same terrestrial species
+  on islands and on the mainland, though separated by hundreds of miles of
+  open sea. If the existence of the same species at distant and isolated
+  points of the earth's surface, can in many instances be explained on the
+  view of each species having migrated from a single birthplace; then,
+  considering our ignorance with respect to former climatal and
+  geographical changes and various occasional means of transport, the
+  belief that this has been the universal law, seems to me incomparably the
+  safest.</p>
+
+  <p>In discussing this subject, we shall be enabled at the same time to
+  consider a point equally important for us, namely, whether the several
+  distinct species of a genus, which on my theory have all descended from a
+  common progenitor, can have migrated (undergoing modification during some
+  part of their migration) from the area inhabited by their progenitor. If
+  it can be shown to be almost invariably the case, that a region, of which
+  <!-- Page 355 --><span class="pagenum"><a
+  name="page355"></a>[355]</span>most of its inhabitants are closely
+  related to, or belong to the same genera with the species of a second
+  region, has probably received at some former period immigrants from this
+  other region, my theory will be strengthened; for we can clearly
+  understand, on the principle of modification, why the inhabitants of a
+  region should be related to those of another region, whence it has been
+  stocked. A volcanic island, for instance, upheaved and formed at the
+  distance of a few hundreds of miles from a continent, would probably
+  receive from it in the course of time a few colonists, and their
+  descendants, though modified, would still be plainly related by
+  inheritance to the inhabitants of the continent. Cases of this nature are
+  common, and are, as we shall hereafter more fully see, inexplicable on
+  the theory of independent creation. This view of the relation of species
+  in one region to those in another, does not differ much (by substituting
+  the word variety for species) from that lately advanced in an ingenious
+  paper by Mr. Wallace, in which he concludes, that "every species has come
+  into existence coincident both in space and time with a pre-existing
+  closely allied species." And I now know from correspondence, that this
+  coincidence he attributes to generation with modification.</p>
+
+  <p>The previous remarks on "single and multiple centres of creation" do
+  not directly bear on another allied question,&mdash;namely whether all
+  the individuals of the same species have descended from a single pair, or
+  single hermaphrodite, or whether, as some authors suppose, from many
+  individuals simultaneously created. With those organic beings which never
+  intercross (if such exist), the species, on my theory, must have
+  descended from a succession of improved varieties, which will never have
+  blended with other individuals or varieties, but will have supplanted
+  each other; so that, at each <!-- Page 356 --><span class="pagenum"><a
+  name="page356"></a>[356]</span>successive stage of modification and
+  improvement, all the individuals of each variety will have descended from
+  a single parent. But in the majority of cases, namely, with all organisms
+  which habitually unite for each birth, or which often intercross, I
+  believe that during the slow process of modification the individuals of
+  the species will have been kept nearly uniform by intercrossing; so that
+  many individuals will have gone on simultaneously changing, and the whole
+  amount of modification will not have been due, at each stage, to descent
+  from a single parent. To illustrate what I mean: our English racehorses
+  differ slightly from the horses of every other breed; but they do not owe
+  their difference and superiority to descent from any single pair, but to
+  continued care in selecting and training many individuals during many
+  generations.</p>
+
+  <p>Before discussing the three classes of facts, which I have selected as
+  presenting the greatest amount of difficulty on the theory of "single
+  centres of creation," I must say a few words on the means of
+  dispersal.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Means of Dispersal.</i>&mdash;Sir C. Lyell and other authors have
+  ably treated this subject. I can give here only the briefest abstract of
+  the more important facts. Change of climate must have had a powerful
+  influence on migration: a region when its climate was different may have
+  been a high road for migration, but now be impassable; I shall, however,
+  presently have to discuss this branch of the subject in some detail.
+  Changes of level in the land must also have been highly influential: a
+  narrow isthmus now separates two marine faunas; submerge it, or let it
+  formerly have been submerged, and the two faunas will now blend or may
+  formerly have blended: where the sea now extends, land may at a former
+  period have connected islands or <!-- Page 357 --><span
+  class="pagenum"><a name="page357"></a>[357]</span>possibly even
+  continents together, and thus have allowed terrestrial productions to
+  pass from one to the other. No geologist will dispute that great
+  mutations of level have occurred within the period of existing organisms.
+  Edward Forbes insisted that all the islands in the Atlantic must recently
+  have been connected with Europe or Africa, and Europe likewise with
+  America. Other authors have thus hypothetically bridged over every ocean,
+  and have united almost every island to some mainland. If indeed the
+  arguments used by Forbes are to be trusted, it must be admitted that
+  scarcely a single island exists which has not recently been united to
+  some continent. This view cuts the Gordian knot of the dispersal of the
+  same species to the most distant points, and removes many a difficulty:
+  but to the best of my judgment we are not authorized in admitting such
+  enormous geographical changes within the period of existing species. It
+  seems to me that we have abundant evidence of great oscillations of level
+  in our continents; but not of such vast changes in their position and
+  extension, as to have united them within the recent period to each other
+  and to the several intervening oceanic islands. I freely admit the former
+  existence of many islands, now buried beneath the sea, which may have
+  served as halting places for plants and for many animals during their
+  migration. In the coral-producing oceans such sunken islands are now
+  marked, as I believe, by rings of coral or atolls standing over them.
+  Whenever it is fully admitted, as I believe it will some day be, that
+  each species has proceeded from a single birthplace, and when in the
+  course of time we know something definite about the means of
+  distribution, we shall be enabled to speculate with security on the
+  former extension of the land. But I do not believe that it will ever be
+  proved that within the <!-- Page 358 --><span class="pagenum"><a
+  name="page358"></a>[358]</span>recent period continents which are now
+  quite separate, have been continuously, or almost continuously, united
+  with each other, and with the many existing oceanic islands. Several
+  facts in distribution,&mdash;such as the great difference in the marine
+  faunas on the opposite sides of almost every continent,&mdash;the close
+  relation of the tertiary inhabitants of several lands and even seas to
+  their present inhabitants,&mdash;a certain degree of relation (as we
+  shall hereafter see) between the distribution of mammals and the depth of
+  the sea,&mdash;these and other such facts seem to me opposed to the
+  admission of such prodigious geographical revolutions within the recent
+  period, as are necessitated on the view advanced by Forbes and admitted
+  by his many followers. The nature and relative proportions of the
+  inhabitants of oceanic islands likewise seem to me opposed to the belief
+  of their former continuity with continents. Nor does their almost
+  universally volcanic composition favour the admission that they are the
+  wrecks of sunken continents;&mdash;if they had originally existed as
+  mountain-ranges on the land, some at least of the islands would have been
+  formed, like other mountain-summits, of granite, metamorphic schists, old
+  fossiliferous or other such rocks, instead of consisting of mere piles of
+  volcanic matter.</p>
+
+  <p>I must now say a few words on what are called accidental means, but
+  which more properly might be called occasional means of distribution. I
+  shall here confine myself to plants. In botanical works, this or that
+  plant is stated to be ill adapted for wide dissemination; but for
+  transport across the sea, the greater or less facilities may be said to
+  be almost wholly unknown. Until I tried, with Mr. Berkeley's aid, a few
+  experiments, it was not even known how far seeds could resist the
+  injurious action of sea-water. To my surprise I found that <!-- Page 359
+  --><span class="pagenum"><a name="page359"></a>[359]</span>out of 87
+  kinds, 64 germinated after an immersion of 28 days, and a few survived an
+  immersion of 137 days. For convenience' sake I chiefly tried small seeds,
+  without the capsule or fruit; and as all of these sank in a few days,
+  they could not be floated across wide spaces of the sea, whether or not
+  they were injured by the salt-water. Afterwards I tried some larger
+  fruits, capsules, &amp;c., and some of these floated for a long time. It
+  is well known what a difference there is in the buoyancy of green and
+  seasoned timber; and it occurred to me that floods might wash down plants
+  or branches, and that these might be dried on the banks, and then by a
+  fresh rise in the stream be washed into the sea. Hence I was led to dry
+  stems and branches of 94 plants with ripe fruit, and to place them on
+  sea-water. The majority sank quickly, but some which whilst green floated
+  for a very short time, when dried floated much longer; for instance, ripe
+  hazel-nuts sank immediately, but when dried they floated for 90 days, and
+  afterwards when planted they germinated; an asparagus plant with ripe
+  berries floated for 23 days, when dried it floated for 85 days, and the
+  seeds afterwards germinated; the ripe seeds of Helosciadium sank in two
+  days, when dried they floated for above 90 days, and afterwards
+  germinated. Altogether out of the 94 dried plants, 18 floated for above
+  28 days, and some of the 18 floated for a very much longer period. So
+  that as 64/87 seeds germinated after an immersion of 28 days; and as
+  18/94 plants with ripe fruit (but not all the same species as in the
+  foregoing experiment) floated, after being dried, for above 28 days, as
+  far as we may infer anything from these scanty facts, we may conclude
+  that the seeds of 14/100 plants of any country might be floated by
+  sea-currents during 28 days, and would retain their power of germination.
+  In Johnston's Physical Atlas, the average <!-- Page 360 --><span
+  class="pagenum"><a name="page360"></a>[360]</span>rate of the several
+  Atlantic currents is 33 miles per diem (some currents running at the rate
+  of 60 miles per diem); on this average, the seeds of 14/100 plants
+  belonging to one country might be floated across 924 miles of sea to
+  another country; and when stranded, if blown to a favourable spot by an
+  inland gale, they would germinate.</p>
+
+  <p>Subsequently to my experiments, M. Martens tried similar ones, but in
+  a much better manner, for he placed the seeds in a box in the actual sea,
+  so that they were alternately wet and exposed to the air like really
+  floating plants. He tried 98 seeds, mostly different from mine; but he
+  chose many large fruits and likewise seeds from plants which live near
+  the sea; and this would have favoured the average length of their
+  flotation and of their resistance to the injurious action of the
+  salt-water. On the other hand he did not previously dry the plants or
+  branches with the fruit; and this, as we have seen, would have caused
+  some of them to have floated much longer. The result was that 18/98 of
+  his seeds floated for 42 days, and were then capable of germination. But
+  I do not doubt that plants exposed to the waves would float for a less
+  time than those protected from violent movement as in our experiments.
+  Therefore it would perhaps be safer to assume that the seeds of about
+  10/100 plants of a flora, after having been dried, could be floated
+  across a space of sea 900 miles in width, and would then germinate. The
+  fact of the larger fruits often floating longer than the small, is
+  interesting; as plants with large seeds or fruit could hardly be
+  transported by any other means; and Alph. de Candolle has shown that such
+  plants generally have restricted ranges.</p>
+
+  <p>But seeds may be occasionally transported in another manner. Drift
+  timber is thrown up on most islands, <!-- Page 361 --><span
+  class="pagenum"><a name="page361"></a>[361]</span>even on those in the
+  midst of the widest oceans; and the natives of the coral-islands in the
+  Pacific, procure stones for their tools, solely from the roots of drifted
+  trees, these stones being a valuable royal tax. I find on examination,
+  that when irregularly shaped stones are embedded in the roots of trees,
+  small parcels of earth are very frequently enclosed in their interstices
+  and behind them,&mdash;so perfectly that not a particle could be washed
+  away in the longest transport: out of one small portion of earth thus
+  <i>completely</i> enclosed by wood in an oak about 50 years old, three
+  dicotyledonous plants germinated: I am certain of the accuracy of this
+  observation. Again, I can show that the carcasses of birds, when floating
+  on the sea, sometimes escape being immediately devoured; and seeds of
+  many kinds in the crops of floating birds long retain their vitality:
+  peas and vetches, for instance, are killed by even a few days' immersion
+  in sea-water; but some taken out of the crop of a pigeon, which had
+  floated on artificial salt-water for 30 days, to my surprise nearly all
+  germinated.</p>
+
+  <p>Living birds can hardly fail to be highly effective agents in the
+  transportation of seeds. I could give many facts showing how frequently
+  birds of many kinds are blown by gales to vast distances across the
+  ocean. We may I think safely assume that under such circumstances their
+  rate of flight would often be 35 miles an hour; and some authors have
+  given a far higher estimate. I have never seen an instance of nutritious
+  seeds passing through the intestines of a bird; but hard seeds of fruit
+  pass uninjured through even the digestive organs of a turkey. In the
+  course of two months, I picked up in my garden 12 kinds of seeds, out of
+  the excrement of small birds, and these seemed perfect, and some of them,
+  which I tried, germinated. <!-- Page 362 --><span class="pagenum"><a
+  name="page362"></a>[362]</span>But the following fact is more important:
+  the crops of birds do not secrete gastric juice, and do not in the least
+  injure, as I know by trial, the germination of seeds; now after a bird
+  has found and devoured a large supply of food, it is positively asserted
+  that all the grains do not pass into the gizzard for 12 or even 18 hours.
+  A bird in this interval might easily be blown to the distance of 500
+  miles, and hawks are known to look out for tired birds, and the contents
+  of their torn crops might thus readily get scattered. Mr. Brent informs
+  me that a friend of his had to give up flying carrier-pigeons from France
+  to England, as the hawks on the English coast destroyed so many on their
+  arrival. Some hawks and owls bolt their prey whole, and after an interval
+  of from twelve to twenty hours, disgorge pellets, which, as I know from
+  experiments made in the Zoological Gardens, include seeds capable of
+  germination. Some seeds of the oat, wheat, millet, canary, hemp, clover,
+  and beet germinated after having been from twelve to twenty-one hours in
+  the stomachs of different birds of prey; and two seeds of beet grew after
+  having been thus retained for two days and fourteen hours. Freshwater
+  fish, I find, eat seeds of many land and water plants: fish are
+  frequently devoured by birds, and thus the seeds might be transported
+  from place to place. I forced many kinds of seeds into the stomachs of
+  dead fish, and then gave their bodies to fishing-eagles, storks, and
+  pelicans; these birds after an interval of many hours, either rejected
+  the seeds in pellets or passed them in their excrement; and several of
+  these seeds retained their power of germination. Certain seeds, however,
+  were always killed by this process.</p>
+
+  <p>Although the beaks and feet of birds are generally quite clean, I can
+  show that earth sometimes adheres to them: in one instance I removed
+  twenty-two grains <!-- Page 363 --><span class="pagenum"><a
+  name="page363"></a>[363]</span>of dry argillaceous earth from one foot of
+  a partridge, and in this earth there was a pebble quite as large as the
+  seed of a vetch. Thus seeds might occasionally be transported to great
+  distances; for many facts could be given showing that soil almost
+  everywhere is charged with seeds. Reflect for a moment on the millions of
+  quails which annually cross the Mediterranean; and can we doubt that the
+  earth adhering to their feet would sometimes include a few minute seeds?
+  But I shall presently have to recur to this subject.</p>
+
+  <p>As icebergs are known to be sometimes loaded with earth and stones,
+  and have even carried brushwood, bones, and the nest of a land-bird, I
+  can hardly doubt that they must occasionally have transported seeds from
+  one part to another of the arctic and antarctic regions, as suggested by
+  Lyell; and during the Glacial period from one part of the now temperate
+  regions to another. In the Azores, from the large number of the species
+  of plants common to Europe, in comparison with the plants of other
+  oceanic islands nearer to the mainland, and (as remarked by Mr. H.&nbsp;C.
+  Watson) from the somewhat northern character of the flora in comparison
+  with the latitude, I suspected that these islands had been partly stocked
+  by ice-borne seeds, during the Glacial epoch. At my request Sir C. Lyell
+  wrote to M. Hartung to inquire whether he had observed erratic boulders
+  on these islands, and he answered that he had found large fragments of
+  granite and other rocks, which do not occur in the archipelago. Hence we
+  may safely infer that icebergs formerly landed their rocky burthens on
+  the shores of these mid-ocean islands, and it is at least possible that
+  they may have brought thither the seeds of northern plants.</p>
+
+  <p>Considering that the several above means of transport, and that
+  several other means, which without <!-- Page 364 --><span
+  class="pagenum"><a name="page364"></a>[364]</span>doubt remain to be
+  discovered, have been in action year after year, for centuries and tens
+  of thousands of years, it would I think be a marvellous fact if many
+  plants had not thus become widely transported. These means of transport
+  are sometimes called accidental, but this is not strictly correct: the
+  currents of the sea are not accidental, nor is the direction of prevalent
+  gales of wind. It should be observed that scarcely any means of transport
+  would carry seeds for very great distances; for seeds do not retain their
+  vitality when exposed for a great length of time to the action of
+  sea-water; nor could they be long carried in the crops or intestines of
+  birds. These means, however, would suffice for occasional transport
+  across tracts of sea some hundred miles in breadth, or from island to
+  island, or from a continent to a neighbouring island, but not from one
+  distant continent to another. The floras of distant continents would not
+  by such means become mingled in any great degree; but would remain as
+  distinct as we now see them to be. The currents, from their course, would
+  never bring seeds from North America to Britain, though they might and do
+  bring seeds from the West Indies to our western shores, where, if not
+  killed by so long an immersion in salt-water, they could not endure our
+  climate. Almost every year, one or two land-birds are blown across the
+  whole Atlantic Ocean, from North America to the western shores of Ireland
+  and England; but seeds could be transported by these wanderers only by
+  one means, namely, in dirt sticking to their feet, which is in itself a
+  rare accident. Even in this case, how small would the chance be of a seed
+  falling on favourable soil, and coming to maturity! But it would be a
+  great error to argue that because a well-stocked island, like Great
+  Britain, has not, as far as is known <!-- Page 365 --><span
+  class="pagenum"><a name="page365"></a>[365]</span>(and it would be very
+  difficult to prove this), received within the last few centuries, through
+  occasional means of transport, immigrants from Europe or any other
+  continent, that a poorly-stocked island, though standing more remote from
+  the mainland, would not receive colonists by similar means. I do not
+  doubt that out of twenty seeds or animals transported to an island, even
+  if far less well-stocked than Britain, scarcely more than one would be so
+  well fitted to its new home, as to become naturalised. But this, as it
+  seems to me, is no valid argument against what would be effected by
+  occasional means of transport, during the long lapse of geological time,
+  whilst an island was being upheaved and formed, and before it had become
+  fully stocked with inhabitants. On almost bare land, with few or no
+  destructive insects or birds living there, nearly every seed, which
+  chanced to arrive, if fitted for the climate, would be sure to germinate
+  and survive.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Dispersal during the Glacial period.</i>&mdash;The identity of many
+  plants and animals, on mountain-summits, separated from each other by
+  hundreds of miles of lowlands, where the Alpine species could not
+  possibly exist, is one of the most striking cases known of the same
+  species living at distant points, without the apparent possibility of
+  their having migrated from one to the other. It is indeed a remarkable
+  fact to see so many of the same plants living on the snowy regions of the
+  Alps or Pyrenees, and in the extreme northern parts of Europe; but it is
+  far more remarkable, that the plants on the White Mountains, in the
+  United States of America, are all the same with those of Labrador, and
+  nearly all the same, as we hear from Asa Gray, with those on the loftiest
+  mountains of Europe. Even as long ago as 1747, such facts led Gmelin to
+  conclude that the <!-- Page 366 --><span class="pagenum"><a
+  name="page366"></a>[366]</span>same species must have been independently
+  created at several distinct points; and we might have remained in this
+  same belief, had not Agassiz and others called vivid attention to the
+  Glacial period, which, as we shall immediately see, affords a simple
+  explanation of these facts. We have evidence of almost every conceivable
+  kind, organic and inorganic, that within a very recent geological period,
+  central Europe and North America suffered under an Arctic climate. The
+  ruins of a house burnt by fire do not tell their tale more plainly, than
+  do the mountains of Scotland and Wales, with their scored flanks,
+  polished surfaces, and perched boulders, of the icy streams with which
+  their valleys were lately filled. So greatly has the climate of Europe
+  changed, that in Northern Italy, gigantic moraines, left by old glaciers,
+  are now clothed by the vine and maize. Throughout a large part of the
+  United States, erratic boulders, and rocks scored by drifted icebergs and
+  coast-ice, plainly reveal a former cold period.</p>
+
+  <p>The former influence of the glacial climate on the distribution of the
+  inhabitants of Europe, as explained with remarkable clearness by Edward
+  Forbes, is substantially as follows. But we shall follow the changes more
+  readily, by supposing a new glacial period to come slowly on, and then
+  pass away, as formerly occurred. As the cold came on, and as each more
+  southern zone became fitted for arctic beings and ill-fitted for their
+  former more temperate inhabitants, the latter would be supplanted and
+  arctic productions would take their places. The inhabitants of the more
+  temperate regions would at the same time travel southward, unless they
+  were stopped by barriers, in which case they would perish. The mountains
+  would become covered with snow and ice, and their former Alpine
+  inhabitants would descend to the plains. By the time that the cold had
+  reached <!-- Page 367 --><span class="pagenum"><a
+  name="page367"></a>[367]</span>its maximum, we should have a uniform
+  arctic fauna and flora, covering the central parts of Europe, as far
+  south as the Alps and Pyrenees, and even stretching into Spain. The now
+  temperate regions of the United States would likewise be covered by
+  arctic plants and animals, and these would be nearly the same with those
+  of Europe; for the present circumpolar inhabitants, which we suppose to
+  have everywhere travelled southward, are remarkably uniform round the
+  world. We may suppose that the Glacial period came on a little earlier or
+  later in North America than in Europe, so will the southern migration
+  there have been a little earlier or later; but this will make no
+  difference in the final result.</p>
+
+  <p>As the warmth returned, the arctic forms would retreat northward,
+  closely followed up in their retreat by the productions of the more
+  temperate regions. And as the snow melted from the bases of the
+  mountains, the arctic forms would seize on the cleared and thawed ground,
+  always ascending higher and higher, as the warmth increased, whilst their
+  brethren were pursuing their northern journey. Hence, when the warmth had
+  fully returned, the same arctic species, which had lately lived in a body
+  together on the lowlands of the Old and New Worlds, would be left
+  isolated on distant mountain-summits (having been exterminated on all
+  lesser heights) and in the arctic regions of both hemispheres.</p>
+
+  <p>Thus we can understand the identity of many plants at points so
+  immensely remote as on the mountains of the United States and of Europe.
+  We can thus also understand the fact that the Alpine plants of each
+  mountain-range are more especially related to the arctic forms living due
+  north or nearly due north of them: for the migration as the cold came on,
+  and the re-migration on the returning warmth, will generally <!-- Page
+  368 --><span class="pagenum"><a name="page368"></a>[368]</span>have been
+  due south and north. The Alpine plants, for example, of Scotland, as
+  remarked by Mr. H.&nbsp;C. Watson, and those of the Pyrenees, as remarked by
+  Ramond, are more especially allied to the plants of northern Scandinavia;
+  those of the United States to Labrador; those of the mountains of Siberia
+  to the arctic regions of that country. These views, grounded as they are
+  on the perfectly well-ascertained occurrence of a former Glacial period,
+  seem to me to explain in so satisfactory a manner the present
+  distribution of the Alpine and Arctic productions of Europe and America,
+  that when in other regions we find the same species on distant
+  mountain-summits, we may almost conclude without other evidence, that a
+  colder climate permitted their former migration across the low
+  intervening tracts, since become too warm for their existence.</p>
+
+  <p>If the climate, since the Glacial period, has ever been in any degree
+  warmer than at present (as some geologists in the United States believe
+  to have been the case, chiefly from the distribution of the fossil
+  Gnathodon), then the arctic and temperate productions will at a very late
+  period have marched a little further north, and subsequently have
+  retreated to their present homes; but I have met with no satisfactory
+  evidence with respect to this intercalated slightly warmer period, since
+  the Glacial period.</p>
+
+  <p>The arctic forms, during their long southern migration and
+  re-migration northward, will have been exposed to nearly the same
+  climate, and, as is especially to be noticed, they will have kept in a
+  body together; consequently their mutual relations will not have been
+  much disturbed, and, in accordance with the principles inculcated in this
+  volume, they will not have been liable to much modification. But with our
+  Alpine productions, left isolated from the moment of the returning
+  warmth, <!-- Page 369 --><span class="pagenum"><a
+  name="page369"></a>[369]</span>first at the bases and ultimately on the
+  summits of the mountains, the case will have been somewhat different; for
+  it is not likely that all the same arctic species will have been left on
+  mountain ranges distant from each other, and have survived there ever
+  since; they will, also, in all probability have become mingled with
+  ancient Alpine species, which must have existed on the mountains before
+  the commencement of the Glacial epoch, and which during its coldest
+  period will have been temporarily driven down to the plains; they will,
+  also, have been exposed to somewhat different climatal influences. Their
+  mutual relations will thus have been in some degree disturbed;
+  consequently they will have been liable to modification; and this we find
+  has been the case; for if we compare the present Alpine plants and
+  animals of the several great European mountain-ranges, though very many
+  of the species are identically the same, some present varieties, some are
+  ranked as doubtful forms, and some few are distinct yet closely allied or
+  representative species.</p>
+
+  <p>In illustrating what, as I believe, actually took place during the
+  Glacial period, I assumed that at its commencement the arctic productions
+  were as uniform round the polar regions as they are at the present day.
+  But the foregoing remarks on distribution apply not only to strictly
+  arctic forms, but also to many sub-arctic and to some few northern
+  temperate forms, for some of these are the same on the lower mountains
+  and on the plains of North America and Europe; and it may be reasonably
+  asked how I account for the necessary degree of uniformity of the
+  sub-arctic and northern temperate forms round the world, at the
+  commencement of the Glacial period. At the present day, the sub-arctic
+  and northern temperate productions of the Old and New Worlds are
+  separated from each other by the <!-- Page 370 --><span
+  class="pagenum"><a name="page370"></a>[370]</span>Atlantic Ocean and by
+  the extreme northern part of the Pacific. During the Glacial period, when
+  the inhabitants of the Old and New Worlds lived further southwards than
+  at present, they must have been still more completely separated by wider
+  spaces of ocean. I believe the above difficulty may be surmounted by
+  looking to still earlier changes of climate of an opposite nature. We
+  have good reason to believe that during the newer Pliocene period, before
+  the Glacial epoch, and whilst the majority of the inhabitants of the
+  world were specifically the same as now, the climate was warmer than at
+  the present day. Hence we may suppose that the organisms now living under
+  the climate of latitude 60°, during the Pliocene period lived further
+  north under the Polar Circle, in latitude 66°-67°; and that the strictly
+  arctic productions then lived on the broken land still nearer to the
+  pole. Now if we look at a globe, we shall see that under the Polar Circle
+  there is almost continuous land from western Europe, through Siberia, to
+  eastern America. And to this continuity of the circumpolar land, and to
+  the consequent freedom for intermigration under a more favourable
+  climate, I attribute the necessary amount of uniformity in the sub-arctic
+  and northern temperate productions of the Old and New Worlds, at a period
+  anterior to the Glacial epoch.</p>
+
+  <p>Believing, from reasons before alluded to, that our continents have
+  long remained in nearly the same relative position, though subjected to
+  large, but partial oscillations of level, I am strongly inclined to
+  extend the above view, and to infer that during some earlier and still
+  warmer period, such as the older Pliocene period, a large number of the
+  same plants and animals inhabited the almost continuous circumpolar land;
+  and that these plants and animals, both in the Old and <!-- Page 371
+  --><span class="pagenum"><a name="page371"></a>[371]</span>New Worlds,
+  began slowly to migrate southwards as the climate became less warm, long
+  before the commencement of the Glacial period. We now see, as I believe,
+  their descendants, mostly in a modified condition, in the central parts
+  of Europe and the United States. On this view we can understand the
+  relationship, with very little identity, between the productions of North
+  America and Europe,&mdash;a relationship which is most remarkable,
+  considering the distance of the two areas, and their separation by the
+  Atlantic Ocean. We can further understand the singular fact remarked on
+  by several observers, that the productions of Europe and America during
+  the later tertiary stages were more closely related to each other than
+  they are at the present time; for during these warmer periods the
+  northern parts of the Old and New Worlds will have been almost
+  continuously united by land, serving as a bridge, since rendered
+  impassable by cold, for the intermigration of their inhabitants.</p>
+
+  <p>During the slowly decreasing warmth of the Pliocene period, as soon as
+  the species in common, which inhabited the New and Old Worlds, migrated
+  south of the Polar Circle, they must have been completely cut off from
+  each other. This separation, as far as the more temperate productions are
+  concerned, took place long ages ago. And as the plants and animals
+  migrated southward, they will have become mingled in the one great region
+  with the native American productions, and have had to compete with them;
+  and in the other great region, with those of the Old World. Consequently
+  we have here everything favourable for much modification,&mdash;for far
+  more modification than with the Alpine productions, left isolated, within
+  a much more recent period, on the several mountain-ranges and on the
+  arctic lands of the two Worlds. Hence it has come, that when we compare
+  <!-- Page 372 --><span class="pagenum"><a
+  name="page372"></a>[372]</span>the now living productions of the
+  temperate regions of the New and Old Worlds, we find very few identical
+  species (though Asa Gray has lately shown that more plants are identical
+  than was formerly supposed), but we find in every great class many forms,
+  which some naturalists rank as geographical races, and others as distinct
+  species; and a host of closely allied or representative forms which are
+  ranked by all naturalists as specifically distinct.</p>
+
+  <p>As on the land, so in the waters of the sea, a slow southern migration
+  of a marine fauna, which during the Pliocene or even a somewhat earlier
+  period, was nearly uniform along the continuous shores of the Polar
+  Circle, will account, on the theory of modification, for many closely
+  allied forms now living in areas completely sundered. Thus, I think, we
+  can understand the presence of many existing and tertiary representative
+  forms on the eastern and western shores of temperate North America; and
+  the still more striking case of many closely allied crustaceans (as
+  described in Dana's admirable work), of some fish and other marine
+  animals, in the Mediterranean and in the seas of Japan,&mdash;areas now
+  separated by a continent and by nearly a hemisphere of equatorial
+  ocean.</p>
+
+  <p>These cases of relationship, without identity, of the inhabitants of
+  seas now disjoined, and likewise of the past and present inhabitants of
+  the temperate lands of North America and Europe, are inexplicable on the
+  theory of creation. We cannot say that they have been created alike, in
+  correspondence with the nearly similar physical conditions of the areas;
+  for if we compare, for instance, certain parts of South America with the
+  southern continents of the Old World, we see countries closely
+  corresponding in all their physical conditions, but with their
+  inhabitants utterly dissimilar. <!-- Page 373 --><span class="pagenum"><a
+  name="page373"></a>[373]</span></p>
+
+  <p>But we must return to our more immediate subject, the Glacial period.
+  I am convinced that Forbes's view may be largely extended. In Europe we
+  have the plainest evidence of the cold period, from the western shores of
+  Britain to the Oural range, and southward to the Pyrenees. We may infer
+  from the frozen mammals and nature of the mountain vegetation, that
+  Siberia was similarly affected. Along the Himalaya, at points 900 miles
+  apart, glaciers have left the marks of their former low descent; and in
+  Sikkim, Dr. Hooker saw maize growing on gigantic ancient moraines. South
+  of the equator, we have some direct evidence of former glacial action in
+  New Zealand; and the same plants, found on widely separated mountains in
+  that island, tell the same story. If one account which has been published
+  can be trusted, we have direct evidence of glacial action in the
+  south-eastern corner of Australia.</p>
+
+  <p>Looking to America; in the northern half, ice-borne fragments of rock
+  have been observed on the eastern side as far south as lat. 36°-37°, and
+  on the shores of the Pacific, where the climate is now so different, as
+  far south as lat. 46°; erratic boulders have, also, been noticed on the
+  Rocky Mountains. In the Cordillera of Equatorial South America, glaciers
+  once extended far below their present level. In central Chili I was
+  astonished at the structure of a vast mound of detritus, about 800 feet
+  in height, crossing a valley of the Andes; and this I now feel convinced
+  was a gigantic moraine, left far below any existing glacier. Further
+  south on both sides of the continent, from lat. 41° to the southernmost
+  extremity, we have the clearest evidence of former glacial action, in
+  huge boulders transported far from their parent source.</p>
+
+  <p>We do not know that the Glacial epoch was strictly simultaneous at
+  these several far distant points on <!-- Page 374 --><span
+  class="pagenum"><a name="page374"></a>[374]</span>opposite sides of the
+  world. But we have good evidence in almost every case, that the epoch was
+  included within the latest geological period. We have, also, excellent
+  evidence, that it endured for an enormous time, as measured by years, at
+  each point. The cold may have come on, or have ceased, earlier at one
+  point of the globe than at another, but seeing that it endured for long
+  at each, and that it was contemporaneous in a geological sense, it seems
+  to me probable that it was, during a part at least of the period,
+  actually simultaneous throughout the world. Without some distinct
+  evidence to the contrary, we may at least admit as probable that the
+  glacial action was simultaneous on the eastern and western sides of North
+  America, in the Cordillera under the equator and under the warmer
+  temperate zones, and on both sides of the southern extremity of the
+  continent. If this be admitted, it is difficult to avoid believing that
+  the temperature of the whole world was at this period simultaneously
+  cooler. But it would suffice for my purpose, if the temperature was at
+  the same time lower along certain broad belts of longitude.</p>
+
+  <p>On this view of the whole world, or at least of broad longitudinal
+  belts, having been simultaneously colder from pole to pole, much light
+  can be thrown on the present distribution of identical and allied
+  species. In America, Dr. Hooker has shown that between forty and fifty of
+  the flowering plants of Tierra del Fuego, forming no inconsiderable part
+  of its scanty flora, are common to Europe, enormously remote as these two
+  points are; and there are many closely allied species. On the lofty
+  mountains of equatorial America a host of peculiar species belonging to
+  European genera occur. On the highest mountains of Brazil, some few
+  European genera were found by Gardner, which do not exist in the wide
+  <!-- Page 375 --><span class="pagenum"><a
+  name="page375"></a>[375]</span>intervening hot countries. So on the Silla
+  of Caraccas the illustrious Humboldt long ago found species belonging to
+  genera characteristic of the Cordillera. On the mountains of Abyssinia,
+  several European forms and some few representatives of the peculiar flora
+  of the Cape of Good Hope occur. At the Cape of Good Hope a very few
+  European species, believed not to have been introduced by man, and on the
+  mountains, some few representative European forms are found, which have
+  not been discovered in the intertropical parts of Africa. On the
+  Himalaya, and on the isolated mountain-ranges of the peninsula of India,
+  on the heights of Ceylon, and on the volcanic cones of Java, many plants
+  occur, either identically the same or representing each other, and at the
+  same time representing plants of Europe, not found in the intervening hot
+  lowlands. A list of the genera collected on the loftier peaks of Java
+  raises a picture of a collection made on a hill in Europe! Still more
+  striking is the fact that southern Australian forms are clearly
+  represented by plants growing on the summits of the mountains of Borneo.
+  Some of these Australian forms, as I hear from Dr. Hooker, extend along
+  the heights of the peninsula of Malacca, and are thinly scattered, on the
+  one hand over India and on the other as far north as Japan.</p>
+
+  <p>On the southern mountains of Australia, Dr. F. Müller has discovered
+  several European species; other species, not introduced by man, occur on
+  the lowlands; and a long list can be given, as I am informed by Dr.
+  Hooker, of European genera, found in Australia, but not in the
+  intermediate torrid regions. In the admirable 'Introduction to the Flora
+  of New Zealand,' by Dr. Hooker, analogous and striking facts are given in
+  regard to the plants of that large island. Hence we see that throughout
+  the world, the plants growing on the <!-- Page 376 --><span
+  class="pagenum"><a name="page376"></a>[376]</span>more lofty mountains,
+  and on the temperate lowlands of the northern and southern hemispheres,
+  are sometimes identically the same; but they are much oftener
+  specifically distinct, though related to each other in a most remarkable
+  manner.</p>
+
+  <p>This brief abstract applies to plants alone: some strictly analogous
+  facts could be given on the distribution of terrestrial animals. In
+  marine productions, similar cases occur; as an example, I may quote a
+  remark by the highest authority, Prof. Dana, that "it is certainly a
+  wonderful fact that New Zealand should have a closer resemblance in its
+  Crustacea to Great Britain, its antipode, than to any other part of the
+  world." Sir J. Richardson, also, speaks of the reappearance on the shores
+  of New Zealand, Tasmania, &amp;c., of northern forms of fish. Dr. Hooker
+  informs me that twenty-five species of Algæ are common to New Zealand and
+  to Europe, but have not been found in the intermediate tropical seas.</p>
+
+  <p>It should be observed that the northern species and forms found in the
+  southern parts of the southern hemisphere, and on the mountain-ranges of
+  the intertropical regions, are not arctic, but belong to the northern
+  temperate zones. As Mr. H.&nbsp;C. Watson has recently remarked, "In receding
+  from polar towards equatorial latitudes, the Alpine or mountain floras
+  really become less and less arctic." Many of the forms living on the
+  mountains of the warmer regions of the earth and in the southern
+  hemisphere are of doubtful value, being ranked by some naturalists as
+  specifically distinct, by others as varieties; but some are certainly
+  identical, and many, though closely related to northern forms, must be
+  ranked as distinct species.</p>
+
+  <p>Now let us see what light can be thrown on the foregoing facts, on the
+  belief, supported as it is by a large <!-- Page 377 --><span
+  class="pagenum"><a name="page377"></a>[377]</span>body of geological
+  evidence, that the whole world, or a large part of it, was during the
+  Glacial period simultaneously much colder than at present. The Glacial
+  period, as measured by years, must have been very long; and when we
+  remember over what vast spaces some naturalised plants and animals have
+  spread within a few centuries, this period will have been ample for any
+  amount of migration. As the cold came slowly on, all the tropical plants
+  and other productions will have retreated from both sides towards the
+  equator, followed in the rear by the temperate productions, and these by
+  the arctic; but with the latter we are not now concerned. The tropical
+  plants probably suffered much extinction; how much no one can say;
+  perhaps formerly the tropics supported as many species as we see at the
+  present day crowded together at the Cape of Good Hope, and in parts of
+  temperate Australia. As we know that many tropical plants and animals can
+  withstand a considerable amount of cold, many might have escaped
+  extermination during a moderate fall of temperature, more especially by
+  escaping into the lowest, most protected, and warmest districts. But the
+  great fact to bear in mind is, that all tropical productions will have
+  suffered to a certain extent. On the other hand, the temperate
+  productions, after migrating nearer to the equator, though they will have
+  been placed under somewhat new conditions, will have suffered less. And
+  it is certain that many temperate plants, if protected from the inroads
+  of competitors, can withstand a much warmer climate than their own.
+  Hence, it seems to me possible, bearing in mind that the tropical
+  productions were in a suffering state and could not have presented a firm
+  front against intruders, that a certain number of the more vigorous and
+  dominant temperate forms might have penetrated the native ranks and have
+  reached or <!-- Page 378 --><span class="pagenum"><a
+  name="page378"></a>[378]</span>even crossed the equator. The invasion
+  would, of course, have been greatly favoured by high land, and perhaps by
+  a dry climate; for Dr. Falconer informs me that it is the damp with the
+  heat of the tropics which is so destructive to perennial plants from a
+  temperate climate. On the other hand, the most humid and hottest
+  districts will have afforded an asylum to the tropical natives. The
+  mountain-ranges north-west of the Himalaya, and the long line of the
+  Cordillera, seem to have afforded two great lines of invasion: and it is
+  a striking fact, lately communicated to me by Dr. Hooker, that all the
+  flowering plants, about forty-six in number, common to Tierra del Fuego
+  and to Europe still exist in North America, which must have lain on the
+  line of march. But I do not doubt that some temperate productions entered
+  and crossed even the <i>lowlands</i> of the tropics at the period when
+  the cold was most intense,&mdash;when arctic forms had migrated some
+  twenty-five degrees of latitude from their native country and covered the
+  land at the foot of the Pyrenees. At this period of extreme cold, I
+  believe that the climate under the equator at the level of the sea was
+  about the same with that now felt there at the height of six or seven
+  thousand feet. During this the coldest period, I suppose that large
+  spaces of the tropical lowlands were clothed with a mingled tropical and
+  temperate vegetation, like that now growing with strange luxuriance at
+  the base of the Himalaya, as graphically described by Hooker.</p>
+
+  <p>Thus, as I believe, a considerable number of plants, a few terrestrial
+  animals, and some marine productions, migrated during the Glacial period
+  from the northern and southern temperate zones into the intertropical
+  regions, and some even crossed the equator. As the warmth returned, these
+  temperate forms would naturally ascend the higher mountains, being
+  exterminated on the <!-- Page 379 --><span class="pagenum"><a
+  name="page379"></a>[379]</span>lowlands; those which had not reached the
+  equator would re-migrate northward or southward towards their former
+  homes; but the forms, chiefly northern, which had crossed the equator,
+  would travel still further from their homes into the more temperate
+  latitudes of the opposite hemisphere. Although we have reason to believe
+  from geological evidence that the whole body of arctic shells underwent
+  scarcely any modification during their long southern migration and
+  re-migration northward, the case may have been wholly different with
+  those intruding forms which settled themselves on the intertropical
+  mountains, and in the southern hemisphere. These being surrounded by
+  strangers will have had to compete with many new forms of life; and it is
+  probable that selected modifications in their structure, habits, and
+  constitutions will have profited them. Thus many of these wanderers,
+  though still plainly related by inheritance to their brethren of the
+  northern or southern hemispheres, now exist in their new homes as
+  well-marked varieties or as distinct species.</p>
+
+  <p>It is a remarkable fact, strongly insisted on by Hooker in regard to
+  America, and by Alph. de Candolle in regard to Australia, that many more
+  identical plants and allied forms have apparently migrated from the north
+  to the south, than in a reversed direction. We see, however, a few
+  southern vegetable forms on the mountains of Borneo and Abyssinia. I
+  suspect that this preponderant migration from north to south is due to
+  the greater extent of land in the north, and to the northern forms having
+  existed in their own homes in greater numbers, and having consequently
+  been advanced through natural selection and competition to a higher stage
+  of perfection or dominating power, than the southern forms. And thus,
+  when they became commingled during the Glacial period, the northern forms
+  <!-- Page 380 --><span class="pagenum"><a
+  name="page380"></a>[380]</span>were enabled to beat the less powerful
+  southern forms. Just in the same manner as we see at the present day,
+  that very many European productions cover the ground in La Plata, and in
+  a lesser degree in Australia, and have to a certain extent beaten the
+  natives; whereas extremely few southern forms have become naturalised in
+  any part of Europe, though hides, wool, and other objects likely to carry
+  seeds have been largely imported into Europe during the last two or three
+  centuries from La Plata, and during the last thirty or forty years from
+  Australia. Something of the same kind must have occurred on the
+  intertropical mountains: no doubt before the Glacial period they were
+  stocked with endemic Alpine forms; but these have almost everywhere
+  largely yielded to the more dominant forms, generated in the larger areas
+  and more efficient workshops of the north. In many islands the native
+  productions are nearly equalled or even outnumbered by the naturalised;
+  and if the natives have not been actually exterminated, their numbers
+  have been greatly reduced, and this is the first stage towards
+  extinction. A mountain is an island on the land; and the intertropical
+  mountains before the Glacial period must have been completely isolated;
+  and I believe that the productions of these islands on the land yielded
+  to those produced within the larger areas of the north, just in the same
+  way as the productions of real islands have everywhere lately yielded to
+  continental forms, naturalised by man's agency.</p>
+
+  <p>I am far from supposing that all difficulties are removed on the view
+  here given in regard to the range and affinities of the allied species
+  which live in the northern and southern temperate zones and on the
+  mountains of the intertropical regions. Very many difficulties remain to
+  be solved. I do not pretend to <!-- Page 381 --><span class="pagenum"><a
+  name="page381"></a>[381]</span>indicate the exact lines and means of
+  migration, or the reason why certain species and not others have
+  migrated; why certain species have been modified and have given rise to
+  new groups of forms, and others have remained unaltered. We cannot hope
+  to explain such facts, until we can say why one species and not another
+  becomes naturalised by man's agency in a foreign land; why one ranges
+  twice or thrice as far, and is twice or thrice as common, as another
+  species within their own homes.</p>
+
+  <p>I have said that many difficulties remain to be solved: some of the
+  most remarkable are stated with admirable clearness by Dr. Hooker in his
+  botanical works on the antarctic regions. These cannot be here discussed.
+  I will only say that as far as regards the occurrence of identical
+  species at points so enormously remote as Kerguelen Land, New Zealand,
+  and Fuegia, I believe that towards the close of the Glacial period,
+  icebergs, as suggested by Lyell, have been largely concerned in their
+  dispersal. But the existence of several quite distinct species, belonging
+  to genera exclusively confined to the south, at these and other distant
+  points of the southern hemisphere, is, on my theory of descent with
+  modification, a far more remarkable case of difficulty. For some of these
+  species are so distinct, that we cannot suppose that there has been time
+  since the commencement of the Glacial period for their migration, and for
+  their subsequent modification to the necessary degree. The facts seem to
+  me to indicate that peculiar and very distinct species have migrated in
+  radiating lines from some common centre; and I am inclined to look in the
+  southern, as in the northern hemisphere, to a former and warmer period,
+  before the commencement of the Glacial period, when the antarctic lands,
+  now covered with ice, supported a highly peculiar <!-- Page 382 --><span
+  class="pagenum"><a name="page382"></a>[382]</span>and isolated flora. I
+  suspect that before this flora was exterminated by the Glacial epoch, a
+  few forms were widely dispersed to various points of the southern
+  hemisphere by occasional means of transport, and by the aid, as
+  halting-places, of existing and now sunken islands: By these means, as I
+  believe, the southern shores of America, Australia, New Zealand, have
+  become slightly tinted by the same peculiar forms of vegetable life.</p>
+
+  <p>Sir C. Lyell in a striking passage has speculated, in language almost
+  identical with mine, on the effects of great alternations of climate on
+  geographical distribution. I believe that the world has recently felt one
+  of his great cycles of change; and that on this view, combined with
+  modification through natural selection, a multitude of facts in the
+  present distribution both of the same and of allied forms of life can be
+  explained. The living waters may be said to have flowed during one short
+  period from the north and from the south, and to have crossed at the
+  equator; but to have flowed with greater force from the north so as to
+  have freely inundated the south. As the tide leaves its drift in
+  horizontal lines, though rising higher on the shores where the tide rises
+  highest, so have the living waters left their living drift on our
+  mountain-summits, in a line gently rising from the arctic lowlands to a
+  great height under the equator. The various beings thus left stranded may
+  be compared with savage races of man, driven up and surviving in the
+  mountain-fastnesses of almost every land, which serve as a record, full
+  of interest to us, of the former inhabitants of the surrounding
+  lowlands.</p>
+
+<hr />
+
+<p><!-- Page 383 --><span class="pagenum"><a name="page383"></a>[383]</span></p>
+
+<h3>CHAPTER XII.</h3>
+
+<p class="cenhead"><span class="sc">Geographical Distribution</span>&mdash;<i>continued</i>.</p>
+
+<blockquote class="b1n">
+
+  <p>Distribution of fresh-water productions&mdash;On the inhabitants of
+  oceanic islands&mdash;Absence of Batrachians and of terrestrial
+  Mammals&mdash;On the relation of the inhabitants of islands to those of
+  the nearest mainland&mdash;On colonisation from the nearest source with
+  subsequent modification&mdash;Summary of the last and present
+  chapters.</p>
+
+</blockquote>
+
+  <p>As lakes and river-systems are separated from each other by barriers
+  of land, it might have been thought that fresh-water productions would
+  not have ranged widely within the same country, and as the sea is
+  apparently a still more impassable barrier, that they never would have
+  extended to distant countries. But the case is exactly the reverse. Not
+  only have many fresh-water species, belonging to quite different classes,
+  an enormous range, but allied species prevail in a remarkable manner
+  throughout the world. I well remember, when first collecting in the fresh
+  waters of Brazil, feeling much surprise at the similarity of the
+  fresh-water insects, shells, &amp;c., and at the dissimilarity of the
+  surrounding terrestrial beings, compared with those of Britain.</p>
+
+  <p>But this power in fresh-water productions of ranging widely, though so
+  unexpected, can, I think, in most cases be explained by their having
+  become fitted, in a manner highly useful to them, for short and frequent
+  migrations from pond to pond, or from stream to stream; and liability to
+  wide dispersal would follow from this capacity as an almost necessary
+  consequence. We can here consider only a few cases. In regard to <!--
+  Page 384 --><span class="pagenum"><a name="page384"></a>[384]</span>fish,
+  I believe that the same species never occur in the fresh waters of
+  distant continents. But on the same continent the species often range
+  widely and almost capriciously; for two river-systems will have some fish
+  in common and some different. A few facts seem to favour the possibility
+  of their occasional transport by accidental means; like that of the live
+  fish not rarely dropped by whirlwinds in India, and the vitality of their
+  ova when removed from the water. But I am inclined to attribute the
+  dispersal of fresh-water fish mainly to slight changes within the recent
+  period in the level of the land, having caused rivers to flow into each
+  other. Instances, also, could be given of this having occurred during
+  floods, without any change of level. We have evidence in the loess of the
+  Rhine of considerable changes of level in the land within a very recent
+  geological period, and when the surface was peopled by existing land and
+  fresh-water shells. The wide difference of the fish on opposite sides of
+  continuous mountain-ranges, which from an early period must have parted
+  river-systems and completely prevented their inosculation, seems to lead
+  to this same conclusion. With respect to allied fresh-water fish
+  occurring at very distant points of the world, no doubt there are many
+  cases which cannot at present be explained: but some fresh-water fish
+  belong to very ancient forms, and in such cases there will have been
+  ample time for great geographical changes, and consequently time and
+  means for much migration. In the second place, salt-water fish can with
+  care be slowly accustomed to live in fresh water; and, according to
+  Valenciennes, there is hardly a single group of fishes confined
+  exclusively to fresh water, so that we may imagine that a marine member
+  of a fresh-water group might travel far along the shores of the sea, and
+  <!-- Page 385 --><span class="pagenum"><a
+  name="page385"></a>[385]</span>subsequently become modified and adapted
+  to the fresh waters of a distant land.</p>
+
+  <p>Some species of fresh-water shells have a very wide range, and allied
+  species, which, on my theory, are descended from a common parent and must
+  have proceeded from a single source, prevail throughout the world. Their
+  distribution at first perplexed me much, as their ova are not likely to
+  be transported by birds, and they are immediately killed by sea-water, as
+  are the adults. I could not even understand how some naturalised species
+  have rapidly spread throughout the same country. But two facts, which I
+  have observed&mdash;and no doubt many others remain to be
+  observed&mdash;throw some light on this subject. When a duck suddenly
+  emerges from a pond covered with duck-weed, I have twice seen these
+  little plants adhering to its back; and it has happened to me, in
+  removing a little duckweed from one aquarium to another, that I have
+  quite unintentionally stocked the one with fresh-water shells from the
+  other. But another agency is perhaps more effectual: I suspended a duck's
+  feet, which might represent those of a bird sleeping in a natural pond,
+  in an aquarium, where many ova of fresh-water shells were hatching; and I
+  found that numbers of the extremely minute and just-hatched shells
+  crawled on the feet, and clung to them so firmly that when taken out of
+  the water they could not be jarred off, though at a somewhat more
+  advanced age they would voluntarily drop off. These just hatched
+  molluscs, though aquatic in their nature, survived on the duck's feet, in
+  damp air, from twelve to twenty hours; and in this length of time a duck
+  or heron might fly at least six or seven hundred miles, and would be sure
+  to alight on a pool or rivulet, if blown across sea to an oceanic island
+  or to any other distant point. Sir Charles Lyell also <!-- Page 386
+  --><span class="pagenum"><a name="page386"></a>[386]</span>informs me
+  that a Dyticus has been caught with an Ancylus (a fresh-water shell like
+  a limpet) firmly adhering to it; and a water-beetle of the same family, a
+  Colymbetes, once flew on board the 'Beagle,' when forty-five miles
+  distant from the nearest land: how much farther it might have flown with
+  a favouring gale no one can tell.</p>
+
+  <p>With respect to plants, it has long been known what enormous ranges
+  many fresh-water and even marsh-species have, both over continents and to
+  the most remote oceanic islands. This is strikingly shown, as remarked by
+  Alph. de Candolle, in large groups of terrestrial plants, which have only
+  a very few aquatic members; for these latter seem immediately to acquire,
+  as if in consequence, a very wide range. I think favourable means of
+  dispersal explain this fact. I have before mentioned that earth
+  occasionally, though rarely, adheres in some quantity to the feet and
+  beaks of birds. Wading birds, which frequent the muddy edges of ponds, if
+  suddenly flushed, would be the most likely to have muddy feet. Birds of
+  this order I can show are the greatest wanderers, and are occasionally
+  found on the most remote and barren islands in the open ocean; they would
+  not be likely to alight on the surface of the sea, so that the dirt would
+  not be washed off their feet; when making land, they would be sure to fly
+  to their natural fresh-water haunts. I do not believe that botanists are
+  aware how charged the mud of ponds is with seeds: I have tried several
+  little experiments, but will here give only the most striking case: I
+  took in February three table-spoonfuls of mud from three different
+  points, beneath water, on the edge of a little pond; this mud when dry
+  weighed only 6¾ ounces; I kept it covered up in my study for six months,
+  pulling up and counting each plant as it grew; the plants were <!-- Page
+  387 --><span class="pagenum"><a name="page387"></a>[387]</span>of many
+  kinds, and were altogether 537 in number; and yet the viscid mud was all
+  contained in a breakfast cup! Considering these facts, I think it would
+  be an inexplicable circumstance if water-birds did not transport the
+  seeds of fresh-water plants to vast distances, and if consequently the
+  range of these plants was not very great. The same agency may have come
+  into play with the eggs of some of the smaller fresh-water animals.</p>
+
+  <p>Other and unknown agencies probably have also played a part. I have
+  stated that fresh-water fish eat some kinds of seeds, though they reject
+  many other kinds after having swallowed them; even small fish swallow
+  seeds of moderate size, as of the yellow water-lily and Potamogeton.
+  Herons and other birds, century after century, have gone on daily
+  devouring fish; they then take flight and go to other waters, or are
+  blown across the sea; and we have seen that seeds retain their power of
+  germination, when rejected in pellets or in excrement, many hours
+  afterwards. When I saw the great size of the seeds of that fine
+  water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks on
+  this plant, I thought that its distribution must remain quite
+  inexplicable; but Audubon states that he found the seeds of the great
+  southern water-lily (probably, according to Dr. Hooker, the Nelumbium
+  luteum) in a heron's stomach; although I do not know the fact, yet
+  analogy makes me believe that a heron flying to another pond and getting
+  a hearty meal of fish, would probably reject from its stomach a pellet
+  containing the seeds of the Nelumbium undigested; or the seeds might be
+  dropped by the bird whilst feeding its young, in the same way as fish are
+  known sometimes to be dropped.</p>
+
+  <p>In considering these several means of distribution, <!-- Page 388
+  --><span class="pagenum"><a name="page388"></a>[388]</span>it should be
+  remembered that when a pond or stream is first formed, for instance, on a
+  rising islet, it will be unoccupied; and a single seed or egg will have a
+  good chance of succeeding. Although there will always be a struggle for
+  life between the individuals of the species, however few, already
+  occupying any pond, yet as the number of kinds is small, compared with
+  those on the land, the competition will probably be less severe between
+  aquatic than between terrestrial species; consequently an intruder from
+  the waters of a foreign country, would have a better chance of seizing on
+  a place, than in the case of terrestrial colonists. We should, also,
+  remember that some, perhaps many, freshwater productions are low in the
+  scale of nature, and that we have reason to believe that such low beings
+  change or become modified less quickly than the high; and this will give
+  longer time than the average for the migration of the same aquatic
+  species. We should not forget the probability of many species having
+  formerly ranged as continuously as fresh-water productions ever can
+  range, over immense areas, and having subsequently become extinct in
+  intermediate regions. But the wide distribution of fresh-water plants and
+  of the lower animals, whether retaining the same identical form or in
+  some degree modified, I believe mainly depends on the wide dispersal of
+  their seeds and eggs by animals, more especially by fresh-water birds,
+  which have large powers of flight, and naturally travel from one to
+  another and often distant piece of water. Nature, like a careful
+  gardener, thus takes her seeds from a bed of a particular nature, and
+  drops them in another equally well fitted for them.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>On the Inhabitants of Oceanic Islands.</i>&mdash;We now come to the
+  last of the three classes of facts, which I <!-- Page 389 --><span
+  class="pagenum"><a name="page389"></a>[389]</span>have selected as
+  presenting the greatest amount of difficulty, on the view that all the
+  individuals both of the same and of allied species have descended from a
+  single parent; and therefore have all proceeded from a common birthplace,
+  notwithstanding that in the course of time they have come to inhabit
+  distant points of the globe. I have already stated that I cannot honestly
+  admit Forbes's view on continental extensions, which, if legitimately
+  followed out, would lead to the belief that within the recent period all
+  existing islands have been nearly or quite joined to some continent. This
+  view would remove many difficulties, but it would not, I think, explain
+  all the facts in regard to insular productions. In the following remarks
+  I shall not confine myself to the mere question of dispersal; but shall
+  consider some other facts, which bear on the truth of the two theories of
+  independent creation and of descent with modification.</p>
+
+  <p>The species of all kinds which inhabit oceanic islands are few in
+  number compared with those on equal continental areas: Alph. de Candolle
+  admits this for plants, and Wollaston for insects. If we look to the
+  large size and varied stations of New Zealand, extending over 780 miles
+  of latitude, and compare its flowering plants, only 750 in number, with
+  those on an equal area at the Cape of Good Hope or in Australia, we must,
+  I think, admit that something quite independently of any difference in
+  physical conditions has caused so great a difference in number. Even the
+  uniform county of Cambridge has 847 plants, and the little island of
+  Anglesea 764, but a few ferns and a few introduced plants are included in
+  these numbers, and the comparison in some other respects is not quite
+  fair. We have evidence that the barren island of Ascension aboriginally
+  possessed under half-a-dozen flowering plants; <!-- Page 390 --><span
+  class="pagenum"><a name="page390"></a>[390]</span>yet many have become
+  naturalised on it, as they have on New Zealand and on every other oceanic
+  island which can be named. In St. Helena there is reason to believe that
+  the naturalised plants and animals have nearly or quite exterminated many
+  native productions. He who admits the doctrine of the creation of each
+  separate species, will have to admit, that a sufficient number of the
+  best adapted plants and animals have not been created on oceanic islands;
+  for man has unintentionally stocked them from various sources far more
+  fully and perfectly than has nature.</p>
+
+  <p>Although in oceanic islands the number of kinds of inhabitants is
+  scanty, the proportion of endemic species (<i>i.e.</i> those found
+  nowhere else in the world) is often extremely large. If we compare, for
+  instance, the number of the endemic land-shells in Madeira, or of the
+  endemic birds in the Galapagos Archipelago, with the number found on any
+  continent, and then compare the area of the islands with that of the
+  continent, we shall see that this is true. This fact might have been
+  expected on my theory, for, as already explained, species occasionally
+  arriving after long intervals in a new and isolated district, and having
+  to compete with new associates, will be eminently liable to modification,
+  and will often produce groups of modified descendants. But it by no means
+  follows, that, because in an island nearly all the species of one class
+  are peculiar, those of another class, or of another section of the same
+  class, are peculiar; and this difference seems to depend partly on the
+  species which do not become modified having immigrated with facility and
+  in a body, so that their mutual relations have not been much disturbed;
+  and partly on the frequent arrival of unmodified immigrants from the
+  mother-country, and the consequent intercrossing with them. With respect
+  to the effects of this intercrossing, <!-- Page 391 --><span
+  class="pagenum"><a name="page391"></a>[391]</span>it should be remembered
+  that the offspring of such crosses would almost certainly gain in vigour;
+  so that even an occasional cross would produce more effect than might at
+  first have been anticipated. To give a few examples: in the Galapagos
+  Islands nearly every land-bird, but only two out of the eleven marine
+  birds, are peculiar; and it is obvious that marine birds could arrive at
+  these islands more easily than land-birds. Bermuda, on the other hand,
+  which lies at about the same distance from North America as the Galapagos
+  Islands do from South America, and which has a very peculiar soil, does
+  not possess one endemic land-bird; and we know from Mr. J.&nbsp;M. Jones's
+  admirable account of Bermuda, that very many North American birds, during
+  their great annual migrations, visit either periodically or occasionally
+  this island. Madeira does not possess one peculiar bird, and many
+  European and African birds are almost every year blown there, as I am
+  informed by Mr. E.&nbsp;V. Harcourt. So that these two islands of Bermuda and
+  Madeira have been stocked by birds, which for long ages have struggled
+  together in their former homes, and have become mutually adapted to each
+  other; and when settled in their new homes, each kind will have been kept
+  by the others to their proper places and habits, and will consequently
+  have been little liable to modification. Any tendency to modification
+  will, also, have been checked by intercrossing with the unmodified
+  immigrants from the mother-country. Madeira, again, is inhabited by a
+  wonderful number of peculiar land-shells, whereas not one species of
+  sea-shell is confined to its shores: now, though we do not know how
+  sea-shells are dispersed, yet we can see that their eggs or larvae,
+  perhaps attached to seaweed or floating timber, or to the feet of
+  wading-birds, might be transported far more easily than <!-- Page 392
+  --><span class="pagenum"><a name="page392"></a>[392]</span>land-shells,
+  across three or four hundred miles of open sea. The different orders of
+  insects in Madeira apparently present analogous facts.</p>
+
+  <p>Oceanic islands are sometimes deficient in certain classes, and their
+  places are apparently occupied by the other inhabitants; in the Galapagos
+  Islands reptiles, and in New Zealand gigantic wingless birds, take the
+  place of mammals. In the plants of the Galapagos Islands, Dr. Hooker has
+  shown that the proportional numbers of the different orders are very
+  different from what they are elsewhere. Such cases are generally
+  accounted for by the physical conditions of the islands; but this
+  explanation seems to me not a little doubtful. Facility of immigration, I
+  believe, has been at least as important as the nature of the
+  conditions.</p>
+
+  <p>Many remarkable little facts could be given with respect to the
+  inhabitants of remote islands. For instance, in certain islands not
+  tenanted by mammals, some of the endemic plants have beautifully hooked
+  seeds; yet few relations are more striking than the adaptation of hooked
+  seeds for transportal by the wool and fur of quadrupeds. This case
+  presents no difficulty on my view, for a hooked seed might be transported
+  to an island by some other means; and the plant then becoming slightly
+  modified, but still retaining its hooked seeds, would form an endemic
+  species, having as useless an appendage as any rudimentary
+  organ,&mdash;for instance, as the shrivelled wings under the soldered
+  elytra of many insular beetles. Again, islands often possess trees or
+  bushes belonging to orders which elsewhere include only herbaceous
+  species; now trees, as Alph. de Candolle has shown, generally have,
+  whatever the cause may be, confined ranges. Hence trees would be little
+  likely to reach distant oceanic islands; and an herbaceous plant, though
+  it would have no chance of <!-- Page 393 --><span class="pagenum"><a
+  name="page393"></a>[393]</span>successfully competing in stature with a
+  fully developed tree, when established on an island and having to compete
+  with herbaceous plants alone, might readily gain an advantage by growing
+  taller and taller and overtopping the other plants. If so, natural
+  selection would often tend to add to the stature of herbaceous plants
+  when growing on an oceanic island, to whatever order they belonged, and
+  thus convert them first into bushes and ultimately into trees.</p>
+
+  <p>With respect to the absence of whole orders on oceanic islands, Bory
+  St. Vincent long ago remarked that Batrachians (frogs, toads, newts) have
+  never been found on any of the many islands with which the great oceans
+  are studded. I have taken pains to verify this assertion, and I have
+  found it strictly true. I have, however, been assured that a frog exists
+  on the mountains of the great island of New Zealand; but I suspect that
+  this exception (if the information be correct) may be explained through
+  glacial agency. This general absence of frogs, toads, and newts on so
+  many oceanic islands cannot be accounted for by their physical
+  conditions; indeed it seems that islands are peculiarly well fitted for
+  these animals; for frogs have been introduced into Madeira, the Azores,
+  and Mauritius, and have multiplied so as to become a nuisance. But as
+  these animals and their spawn are known to be immediately killed by
+  sea-water, on my view we can see that there would be great difficulty in
+  their transportal across the sea, and therefore why they do not exist on
+  any oceanic island. But why, on the theory of creation, they should not
+  have been created there, it would be very difficult to explain.</p>
+
+  <p>Mammals offer another and similar case. I have carefully searched the
+  oldest voyages, but have not finished my search; as yet I have not found
+  a single <!-- Page 394 --><span class="pagenum"><a
+  name="page394"></a>[394]</span>instance, free from doubt, of a
+  terrestrial mammal (excluding domesticated animals kept by the natives)
+  inhabiting an island situated above 300 miles from a continent or great
+  continental island; and many islands situated at a much less distance are
+  equally barren. The Falkland Islands, which are inhabited by a wolf-like
+  fox, come nearest to an exception; but this group cannot be considered as
+  oceanic, as it lies on a bank connected with the mainland; moreover,
+  icebergs formerly brought boulders to its western shores, and they may
+  have formerly transported foxes, as so frequently now happens in the
+  arctic regions. Yet it cannot be said that small islands will not support
+  small mammals, for they occur in many parts of the world on very small
+  islands, if close to a continent; and hardly an island can be named on
+  which our smaller quadrupeds have not become naturalised and greatly
+  multiplied. It cannot be said, on the ordinary view of creation, that
+  there has not been time for the creation of mammals; many volcanic
+  islands are sufficiently ancient, as shown by the stupendous degradation
+  which they have suffered and by their tertiary strata: there has also
+  been time for the production of endemic species belonging to other
+  classes; and on continents it is thought that mammals appear and
+  disappear at a quicker rate than other and lower animals. Though
+  terrestrial mammals do not occur on oceanic islands, aërial mammals do
+  occur on almost every island. New Zealand possesses two bats found
+  nowhere else in the world: Norfolk Island, the Viti Archipelago, the
+  Bonin Islands, the Caroline and Marianne Archipelagoes, and Mauritius,
+  all possess their peculiar bats. Why, it may be asked, has the supposed
+  creative force produced bats and no other mammals on remote islands? On
+  my view this question can easily be answered; for no <!-- Page 395
+  --><span class="pagenum"><a name="page395"></a>[395]</span>terrestrial
+  mammal can be transported across a wide space of sea, but bats can fly
+  across. Bats have been seen wandering by day far over the Atlantic Ocean;
+  and two North American species either regularly or occasionally visit
+  Bermuda, at the distance of 600 miles from the mainland. I hear from Mr.
+  Tomes, who has specially studied this family, that many of the same
+  species have enormous ranges, and are found on continents and on far
+  distant islands. Hence we have only to suppose that such wandering
+  species have been modified through natural selection in their new homes
+  in relation to their new position, and we can understand the presence of
+  endemic bats on islands, with the absence of all terrestrial mammals.</p>
+
+  <p>Besides the absence of terrestrial mammals in relation to the
+  remoteness of islands from continents, there is also a relation, to a
+  certain extent independent of distance, between the depth of the sea
+  separating an island from the neighbouring mainland, and the presence in
+  both of the same mammiferous species or of allied species in a more or
+  less modified condition. Mr. Windsor Earl has made some striking
+  observations on this head in regard to the great Malay Archipelago, which
+  is traversed near Celebes by a space of deep ocean; and this space
+  separates two widely distinct mammalian faunas. On either side the
+  islands are situated on moderately deep submarine banks, and they are
+  inhabited by closely allied or identical quadrupeds. No doubt some few
+  anomalies occur in this great archipelago, and there is much difficulty
+  in forming a judgment in some cases owing to the probable naturalisation
+  of certain mammals through man's agency; but we shall soon have much
+  light thrown on the natural history of this archipelago by the admirable
+  zeal and researches of Mr. Wallace. I have not as yet had time to <!--
+  Page 396 --><span class="pagenum"><a
+  name="page396"></a>[396]</span>follow up this subject in all other
+  quarters of the world; but as far as I have gone, the relation generally
+  holds good. We see Britain separated by a shallow channel from Europe,
+  and the mammals are the same on both sides; we meet with analogous facts
+  on many islands separated by similar channels from Australia. The West
+  Indian Islands stand on a deeply submerged bank, nearly 1000 fathoms in
+  depth, and here we find American forms, but the species and even the
+  genera are distinct. As the amount of modification in all cases depends
+  to a certain degree on the lapse of time, and as during changes of level
+  it is obvious that islands separated by shallow channels are more likely
+  to have been continuously united within a recent period to the mainland
+  than islands separated by deeper channels, we can understand the frequent
+  relation between the depth of the sea and the degree of affinity of the
+  mammalian inhabitants of islands with those of a neighbouring
+  continent,&mdash;an inexplicable relation on the view of independent acts
+  of creation.</p>
+
+  <p>All the foregoing remarks on the inhabitants of oceanic
+  islands,&mdash;namely, the scarcity of kinds&mdash;the richness in
+  endemic forms in particular classes or sections of classes,&mdash;the
+  absence of whole groups, as of batrachians, and of terrestrial mammals
+  notwithstanding the presence of aërial bats,&mdash;the singular
+  proportions of certain orders of plants,&mdash;herbaceous forms having
+  been developed into trees, &amp;c.,&mdash;seem to me to accord better
+  with the view of occasional means of transport having been largely
+  efficient in the long course of time, than with the view of all our
+  oceanic islands having been formerly connected by continuous land with
+  the nearest continent; for on this latter view the migration would
+  probably have been more complete; and if modification be admitted, all
+  the forms of life would have been more <!-- Page 397 --><span
+  class="pagenum"><a name="page397"></a>[397]</span>equally modified, in
+  accordance with the paramount importance of the relation of organism to
+  organism.</p>
+
+  <p>I do not deny that there are many and grave difficulties in
+  understanding how several of the inhabitants of the more remote islands,
+  whether still retaining the same specific form or modified since their
+  arrival, could have reached their present homes. But the probability of
+  many islands having existed as halting-places, of which not a wreck now
+  remains, must not be overlooked. I will here give a single instance of
+  one of the cases of difficulty. Almost all oceanic islands, even the most
+  isolated and smallest, are inhabited by land-shells, generally by endemic
+  species, but sometimes by species found elsewhere. Dr. Aug. A. Gould has
+  given several interesting cases in regard to the land-shells of the
+  islands of the Pacific. Now it is notorious that land-shells are very
+  easily killed by salt; their eggs, at least such as I have tried, sink in
+  sea-water and are killed by it. Yet there must be, on my view, some
+  unknown, but highly efficient means for their transportal. Would the
+  just-hatched young occasionally crawl on and adhere to the feet of birds
+  roosting on the ground, and thus get transported? It occurred to me that
+  land-shells, when hybernating and having a membranous diaphragm over the
+  mouth of the shell, might be floated in chinks of drifted timber across
+  moderately wide arms of the sea. And I found that several species did in
+  this state withstand uninjured an immersion in sea-water during seven
+  days: one of these shells was the Helix pomatia, and after it had again
+  hybernated I put it in sea-water for twenty days, and it perfectly
+  recovered. As this species has a thick calcareous operculum, I removed
+  it, and when it had formed a new membranous one, I immersed it for
+  fourteen days in sea-water, and it recovered and crawled away: but more
+  experiments are wanted on this head. <!-- Page 398 --><span
+  class="pagenum"><a name="page398"></a>[398]</span></p>
+
+  <p>The most striking and important fact for us in regard to the
+  inhabitants of islands, is their affinity to those of the nearest
+  mainland, without being actually the same species. Numerous instances
+  could be given of this fact. I will give only one, that of the Galapagos
+  Archipelago, situated under the equator, between 500 and 600 miles from
+  the shores of South America. Here almost every product of the land and
+  water bears the unmistakeable stamp of the American continent. There are
+  twenty-six land-birds, and twenty-five of these are ranked by Mr. Gould
+  as distinct species, supposed to have been created here; yet the close
+  affinity of most of these birds to American species in every character,
+  in their habits, gestures, and tones of voice, was manifest. So it is
+  with the other animals, and with nearly all the plants, as shown by Dr.
+  Hooker in his admirable memoir on the Flora of this archipelago. The
+  naturalist, looking at the inhabitants of these volcanic islands in the
+  Pacific, distant several hundred miles from the continent, yet feels that
+  he is standing on American land. Why should this be so? why should the
+  species which are supposed to have been created in the Galapagos
+  Archipelago, and nowhere else, bear so plain a stamp of affinity to those
+  created in America? There is nothing in the conditions of life, in the
+  geological nature of the islands, in their height or climate, or in the
+  proportions in which the several classes are associated together, which
+  resembles closely the conditions of the South American coast: in fact
+  there is a considerable dissimilarity in all these respects. On the other
+  hand, there is a considerable degree of resemblance in the <span
+  class="correction" title="text reads `volanic'">volcanic</span> nature of
+  the soil, in climate, height, and size of the islands, between the
+  Galapagos and Cape de Verde Archipelagos: but what an entire and absolute
+  difference in their inhabitants! The inhabitants of the Cape de Verde
+  Islands are related to <!-- Page 399 --><span class="pagenum"><a
+  name="page399"></a>[399]</span>those of Africa, like those of the
+  Galapagos to America. I believe this grand fact can receive no sort of
+  explanation on the ordinary view of independent creation; whereas on the
+  view here maintained, it is obvious that the Galapagos Islands would be
+  likely to receive colonists, whether by occasional means of transport or
+  by formerly continuous land, from America; and the Cape de Verde Islands
+  from Africa; and that such colonists would be liable to
+  modification;&mdash;the principle of inheritance still betraying their
+  original birthplace.</p>
+
+  <p>Many analogous facts could be given: indeed it is an almost universal
+  rule that the endemic productions of islands are related to those of the
+  nearest continent, or of other near islands. The exceptions are few, and
+  most of them can be explained. Thus the plants of Kerguelen Land, though
+  standing nearer to Africa than to America, are related, and that very
+  closely, as we know from Dr. Hooker's account, to those of America: but
+  on the view that this island has been mainly stocked by seeds brought
+  with earth and stones on icebergs, drifted by the prevailing currents,
+  this anomaly disappears. New Zealand in its endemic plants is much more
+  closely related to Australia, the nearest mainland, than to any other
+  region: and this is what might have been expected; but it is also plainly
+  related to South America, which, although the next nearest continent, is
+  so enormously remote, that the fact becomes an anomaly. But this
+  difficulty almost disappears on the view that both New Zealand, South
+  America, and other southern lands were long ago partially stocked from a
+  nearly intermediate though distant point, namely from the antarctic
+  islands, when they were clothed with vegetation, before the commencement
+  of the Glacial period. The affinity, which, though feeble, I am assured
+  by Dr. Hooker is real, between the flora of the south-western corner of
+  Australia and of the Cape of Good <!-- Page 400 --><span
+  class="pagenum"><a name="page400"></a>[400]</span>Hope, is a far more
+  remarkable case, and is at present inexplicable: but this affinity is
+  confined to the plants, and will, I do not doubt, be some day
+  explained.</p>
+
+  <p>The law which causes the inhabitants of an archipelago, though
+  specifically distinct, to be closely allied to those of the nearest
+  continent, we sometimes see displayed on a small scale, yet in a most
+  interesting manner, within the limits of the same archipelago. Thus the
+  several islands of the Galapagos Archipelago are tenanted, as I have
+  elsewhere shown, in a quite marvellous manner, by very closely related
+  species; so that the inhabitants of each separate island, though mostly
+  distinct, are related in an incomparably closer degree to each other than
+  to the inhabitants of any other part of the world. And this is just what
+  might have been expected on my view, for the islands are situated so near
+  each other that they would almost certainly receive immigrants from the
+  same original source, or from each other. But this dissimilarity between
+  the endemic inhabitants of the islands may be used as an argument against
+  my views; for it may be asked, how has it happened in the several islands
+  situated within sight of each other, having the same geological nature,
+  the same height, climate, &amp;c., that many of the immigrants should
+  have been differently modified, though only in a small degree. This long
+  appeared to me a great difficulty: but it arises in chief part from the
+  deeply-seated error of considering the physical conditions of a country
+  as the most important for its inhabitants; whereas it cannot, I think, be
+  disputed that the nature of the other inhabitants, with which each has to
+  compete, is as least as important, and generally a far more important
+  element of success. Now if we look to those inhabitants of the Galapagos
+  Archipelago which are found in other parts of the world (laying on one
+  side for the moment the <!-- Page 401 --><span class="pagenum"><a
+  name="page401"></a>[401]</span>endemic species, which cannot be here
+  fairly included, as we are considering how they have come to be modified
+  since their arrival), we find a considerable amount of difference in the
+  several islands. This difference might indeed have been expected on the
+  view of the islands having been stocked by occasional means of
+  transport&mdash;a seed, for instance, of one plant having been brought to
+  one island, and that of another plant to another island. Hence when in
+  former times an immigrant settled on any one or more of the islands, or
+  when it subsequently spread from one island to another, it would
+  undoubtedly be exposed to different conditions of life in the different
+  islands, for it would have to compete with different sets of organisms: a
+  plant for instance, would find the best-fitted ground more perfectly
+  occupied by distinct plants in one island than in another, and it would
+  be exposed to the attacks of somewhat different enemies. If then it
+  varied, natural selection would probably favour different varieties in
+  the different islands. Some species, however, might spread and yet retain
+  the same character throughout the group, just as we see on continents
+  some species spreading widely and remaining the same.</p>
+
+  <p>The really surprising fact in this case of the Galapagos Archipelago,
+  and in a lesser degree in some analogous instances, is that the new
+  species formed in the separate islands have not quickly spread to the
+  other islands. But the islands, though in sight of each other, are
+  separated by deep arms of the sea, in most cases wider than the British
+  Channel, and there is no reason to suppose that they have at any former
+  period been continuously united. The currents of the sea are rapid and
+  sweep across the archipelago, and gales of wind are extraordinarily rare;
+  so that the islands are far more effectually separated from each other
+  than they appear to be on a map. Nevertheless a good many <!-- Page 402
+  --><span class="pagenum"><a name="page402"></a>[402]</span>species, both
+  those found in other parts of the world and those confined to the
+  archipelago, are common to the several islands, and we may infer from
+  certain facts that these have probably spread from some one island to the
+  others. But we often take, I think, an erroneous view of the probability
+  of closely-allied species invading each other's territory, when put into
+  free intercommunication. Undoubtedly if one species has any advantage
+  whatever over another, it will in a very brief time wholly or in part
+  supplant it; but if both are equally well fitted for their own places in
+  nature, both probably will hold their own places and keep separate for
+  almost any length of time. Being familiar with the fact that many
+  species, naturalised through man's agency, have spread with astonishing
+  rapidity over new countries, we are apt to infer that most species would
+  thus spread; but we should remember that the forms which become
+  naturalised in new countries are not generally closely allied to the
+  aboriginal inhabitants, but are very distinct species, belonging in a
+  large proportion of cases, as shown by Alph. de Candolle, to distinct
+  genera. In the Galapagos Archipelago, many even of the birds, though so
+  well adapted for flying from island to island, are distinct on each; thus
+  there are three closely-allied species of mocking-thrush, each confined
+  to its own island. Now let us suppose the mocking-thrush of Chatham
+  Island to be blown to Charles Island, which has its own mocking-thrush:
+  why should it succeed in establishing itself there? We may safely infer
+  that Charles Island is well stocked with its own species, for annually
+  more eggs are laid there than can possibly be reared; and we may infer
+  that the mocking-thrush peculiar to Charles Island is at least as well
+  fitted for its home as is the species peculiar to Chatham Island. Sir C.
+  Lyell and Mr. Wollaston have communicated to me a remarkable fact bearing
+  on this <!-- Page 403 --><span class="pagenum"><a
+  name="page403"></a>[403]</span>subject; namely, that <span
+  class="correction" title="text reads `Maderia'">Madeira</span> and the
+  adjoining islet of Porto Santo possess many distinct but representative
+  land-shells, some of which live in crevices of stone; and although large
+  quantities of stone are annually transported from Porto Santo to <span
+  class="correction" title="text reads `Maderia'">Madeira</span>, yet this
+  latter island has not become colonised by the Porto Santo species:
+  nevertheless both islands have been colonised by some European
+  land-shells, which no doubt had some advantage over the indigenous
+  species. From these considerations I think we need not greatly marvel at
+  the endemic and representative species, which inhabit the several islands
+  of the Galapagos Archipelago, not having universally spread from island
+  to island. In many other instances, as in the several districts of the
+  same continent, pre-occupation has probably played an important part in
+  checking the commingling of species under the same conditions of life.
+  Thus, the south-east and south-west corners of Australia have nearly the
+  same physical conditions, and are united by continuous land, yet they are
+  inhabited by a vast number of distinct mammals, birds, and plants.</p>
+
+  <p>The principle which determines the general character of the fauna and
+  flora of oceanic islands, namely, that the inhabitants, when not
+  identically the same, yet are plainly related to the inhabitants of that
+  region whence colonists could most readily have been derived,&mdash;the
+  colonists having been subsequently modified and better fitted to their
+  new homes,&mdash;is of the widest application throughout nature. We see
+  this on every mountain, in every lake and marsh. For Alpine species,
+  excepting in so far as the same forms, chiefly of plants, have spread
+  widely throughout the world during the recent Glacial epoch, are related
+  to those of the surrounding lowlands;&mdash;thus we have in South
+  America, Alpine humming-birds, Alpine rodents, Alpine plants, <!-- Page
+  404 --><span class="pagenum"><a name="page404"></a>[404]</span>&amp;c.,
+  all of strictly American forms, and it is obvious that a mountain, as it
+  became slowly upheaved, would naturally be colonised from the surrounding
+  lowlands. So it is with the inhabitants of lakes and marshes, excepting
+  in so far as great facility of transport has given the same general forms
+  to the whole world. We see this same principle in the blind animals
+  inhabiting the caves of America and of Europe. Other analogous facts
+  could be given. And it will, I believe, be universally found to be true,
+  that wherever in two regions, let them be ever so distant, many
+  closely-allied or representative species occur, there will likewise be
+  found some identical species, showing, in accordance with the foregoing
+  view, that at some former period there has been intercommunication or
+  migration between the two regions. And wherever many closely-allied
+  species occur, there will be found many forms which some naturalists rank
+  as distinct species, and some as varieties; these doubtful forms showing
+  us the steps in the process of modification.</p>
+
+  <p>This relation between the power and extent of migration of a species,
+  either at the present time or at some former period under different
+  physical conditions, and the existence at remote points of the world of
+  other species allied to it, is shown in another and more general way. Mr.
+  Gould remarked to me long ago, that in those genera of birds which range
+  over the world, many of the species have very wide ranges. I can hardly
+  doubt that this rule is generally true, though it would be difficult to
+  prove it. Amongst mammals, we see it strikingly displayed in Bats, and in
+  a lesser degree in the Felidæ and Canidæ. We see it, if we compare the
+  distribution of butterflies and beetles. So it is with most fresh-water
+  productions, in which so many genera range over the world, and many
+  individual species have <!-- Page 405 --><span class="pagenum"><a
+  name="page405"></a>[405]</span>enormous ranges. It is not meant that in
+  world-ranging genera all the species have a wide range, or even that they
+  have on an <i>average</i> a wide range; but only that some of the species
+  range very widely; for the facility with which widely-ranging species
+  vary and give rise to new forms will largely determine their average
+  range. For instance, two varieties of the same species inhabit America
+  and Europe, and the species thus has an immense range; but, if the
+  variation had been a little greater, the two varieties would have been
+  ranked as distinct species, and the common range would have been greatly
+  reduced. Still less is it meant, that a species which apparently has the
+  capacity of crossing barriers and ranging widely, as in the case of
+  certain powerfully-winged birds, will necessarily range widely; for we
+  should never forget that to range widely implies not only the power of
+  crossing barriers, but the more important power of being victorious in
+  distant lands in the struggle for life with foreign associates. But on
+  the view of all the species of a genus having descended from a single
+  parent, though now distributed to the most remote points of the world, we
+  ought to find, and I believe as a general rule we do find, that some at
+  least of the species range very widely; for it is necessary that the
+  unmodified parent should range widely, undergoing modification during its
+  diffusion, and should place itself under diverse conditions favourable
+  for the conversion of its offspring, firstly into new varieties and
+  ultimately into new species.</p>
+
+  <p>In considering the wide distribution of certain genera, we should bear
+  in mind that some are extremely ancient, and must have branched off from
+  a common parent at a remote epoch; so that in such cases there will have
+  been ample time for great climatal and geographical changes and for
+  accidents of transport; and consequently for the migration of some of the
+  species into all <!-- Page 406 --><span class="pagenum"><a
+  name="page406"></a>[406]</span>quarters of the world, where they may have
+  become slightly modified in relation to their new conditions. There is,
+  also, some reason to believe from geological evidence that organisms low
+  in the scale within each great class, generally change at a slower rate
+  than the higher forms; and consequently the lower forms will have had a
+  better chance of ranging widely and of still retaining the same specific
+  character. This fact, together with the seeds and eggs of many low forms
+  being very minute and better fitted for distant transportation, probably
+  accounts for a law which has long been observed, and which has lately
+  been admirably discussed by Alph. de Candolle in regard to plants,
+  namely, that the lower any group of organisms is, the more widely it is
+  apt to range.</p>
+
+  <p>The relations just discussed,&mdash;namely, low and slowly-changing
+  organisms ranging more widely than the high,&mdash;some of the species of
+  widely-ranging genera themselves ranging widely,&mdash;such facts, as
+  alpine, lacustrine, and marsh productions being related (with the
+  exceptions before specified) to those on the surrounding low lands and
+  dry lands, though these stations are so different,&mdash;the very close
+  relation of the distinct species which inhabit the islets of the same
+  archipelago,&mdash;and especially the striking relation of the
+  inhabitants of each whole archipelago or island to those of the nearest
+  mainland,&mdash;are, I think, utterly inexplicable on the ordinary view
+  of the independent creation of each species, but are explicable on the
+  view of colonisation from the nearest or readiest source, together with
+  the subsequent modification and better adaptation of the colonists to
+  their new homes.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Summary of last and present Chapters.</i>&mdash;In these chapters I
+  have endeavoured to show, that if we make due allowance for our ignorance
+  of the full effects of all <!-- Page 407 --><span class="pagenum"><a
+  name="page407"></a>[407]</span>the changes of climate and of the level of
+  the land, which have certainly occurred within the recent period, and of
+  other similar changes which may have occurred within the same period; if
+  we remember how profoundly ignorant we are with respect to the many and
+  curious means of occasional transport,&mdash;a subject which has hardly
+  ever been properly experimentised on; if we bear in mind how often a
+  species may have ranged continuously over a wide area, and then have
+  become extinct in the intermediate tracts, I think the difficulties in
+  believing that all the individuals of the same species, wherever located,
+  have descended from the same parents, are not insuperable. And we are led
+  to this conclusion, which has been arrived at by many naturalists under
+  the designation of single centres of creation, by some general
+  considerations, more especially from the importance of barriers and from
+  the analogical distribution of sub-genera, genera, and families.</p>
+
+  <p>With respect to the distinct species of the same genus, which on my
+  theory must have spread from one parent-source; if we make the same
+  allowances as before for our ignorance, and remember that some forms of
+  life change most slowly, enormous periods of time being thus granted for
+  their migration, I do not think that the difficulties are insuperable;
+  though they often are in this case, and in that of the individuals of the
+  same species, extremely great.</p>
+
+  <p>As exemplifying the effects of climatal changes on distribution, I
+  have attempted to show how important has been the influence of the modern
+  Glacial period, which I am fully convinced simultaneously affected the
+  whole world, or at least great meridional belts. As showing how
+  diversified are the means of occasional transport, I have discussed at
+  some little length the means of dispersal of fresh-water productions.
+  <!-- Page 408 --><span class="pagenum"><a
+  name="page408"></a>[408]</span></p>
+
+  <p>If the difficulties be not insuperable in admitting that in the long
+  course of time the individuals of the same species, and likewise of
+  allied species, have proceeded from some one source; then I think all the
+  grand leading facts of geographical distribution are explicable on the
+  theory of migration (generally of the more dominant forms of life),
+  together with subsequent modification and the multiplication of new
+  forms. We can thus understand the high importance of barriers, whether of
+  land or water, which separate our several zoological and botanical
+  provinces. We can thus understand the localisation of sub-genera, genera,
+  and families; and how it is that under different latitudes, for instance
+  in South America, the inhabitants of the plains and mountains, of the
+  forests, marshes, and deserts, are in so mysterious a manner linked
+  together by affinity, and are likewise linked to the extinct beings which
+  formerly inhabited the same continent. Bearing in mind that the mutual
+  relation of organism to organism is of the highest importance, we can see
+  why two areas having nearly the same physical conditions should often be
+  inhabited by very different forms of life; for according to the length of
+  time which has elapsed since new inhabitants entered one region;
+  according to the nature of the communication which allowed certain forms
+  and not others to enter, either in greater or lesser numbers; according
+  or not, as those which entered happened to come in more or less direct
+  competition with each other and with the aborigines; and according as the
+  immigrants were capable of varying more or less rapidly, there would
+  ensue in different regions, independently of their physical conditions,
+  infinitely diversified conditions of life,&mdash;there would be an almost
+  endless amount of organic action and reaction,&mdash;and we should find,
+  as we do find, some groups of beings greatly, and some only slightly
+  modified,&mdash;some <!-- Page 409 --><span class="pagenum"><a
+  name="page409"></a>[409]</span>developed in great force, some existing in
+  scanty numbers&mdash;in the different great geographical provinces of the
+  world.</p>
+
+  <p>On these same principles, we can understand, as I have endeavoured to
+  show, why oceanic islands should have few inhabitants, but of these a
+  great number should be endemic or peculiar; and why, in relation to the
+  means of migration, one group of beings, even within the same class,
+  should have all its species endemic, and another group should have all
+  its species common to other quarters of the world. We can see why whole
+  groups of organisms, as batrachians and terrestrial mammals, should be
+  absent from oceanic islands, whilst the most isolated islands possess
+  their own peculiar species of aërial mammals or bats. We can see why
+  there should be some relation between the presence of mammals, in a more
+  or less modified condition, and the depth of the sea between an island
+  and the mainland. We can clearly see why all the inhabitants of an
+  archipelago, though specifically distinct on the several islets, should
+  be closely related to each other, and likewise be related, but less
+  closely, to those of the nearest continent or other source whence
+  immigrants were probably derived. We can see why in two areas, however
+  distant from each other, there should be a correlation, in the presence
+  of identical species, of varieties, of doubtful species, and of distinct
+  but representative species.</p>
+
+  <p>As the late Edward Forbes often insisted, there is a striking
+  parallelism in the laws of life throughout time and space: the laws
+  governing the succession of forms in past times being nearly the same
+  with those governing at the present time the differences in different
+  areas. We see this in many facts. The endurance of each species and group
+  of species is continuous in time; for the exceptions to the rule are so
+  few, that they may <!-- Page 410 --><span class="pagenum"><a
+  name="page410"></a>[410]</span>fairly be attributed to our not having as
+  yet discovered in an intermediate deposit the forms which are therein
+  absent, but which occur above and below: so in space, it certainly is the
+  general rule that the area inhabited by a single species, or by a group
+  of species, is continuous; and the exceptions, which are not rare, may,
+  as I have attempted to show, be accounted for by migration at some former
+  period under different conditions or by occasional means of transport,
+  and by the species having become extinct in the intermediate tracts. Both
+  in time and space, species and groups of species have their points of
+  maximum development. Groups of species, belonging either to a certain
+  period of time, or to a certain area, are often characterised by trifling
+  characters in common, as of sculpture or colour. In looking to the long
+  succession of ages, as in now looking to distant provinces throughout the
+  world, we find that some organisms differ little, whilst others belonging
+  to a different class, or to a different order, or even only to a
+  different family of the same order, differ greatly. In both time and
+  space the lower members of each class generally change less than the
+  higher; but there are in both cases marked exceptions to the rule. On my
+  theory these several relations throughout time and space are
+  intelligible; for whether we look to the forms of life which have changed
+  during successive ages within the same quarter of the world, or to those
+  which have changed after having migrated into distant quarters, in both
+  cases the forms within each class have been connected by the same bond of
+  ordinary generation; and the more nearly any two forms are related in
+  blood, the nearer they will generally stand to each other in time and
+  space; in both cases the laws of variation have been the same, and
+  modifications have been accumulated by the same power of natural
+  selection.</p>
+
+<hr />
+
+<p><!-- Page 411 --><span class="pagenum"><a name="page411"></a>[411]</span></p>
+
+<h3>CHAPTER XIII.</h3>
+
+<p class="cenhead"><span class="sc">Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.</span></p>
+
+<blockquote class="b1n">
+
+  <p><span class="sc">Classification</span>, groups subordinate to
+  groups&mdash;Natural system&mdash;Rules and difficulties in
+  classification, explained on the theory of descent with
+  modification&mdash;Classification of varieties&mdash;Descent always used
+  in classification&mdash;Analogical or adaptive
+  characters&mdash;Affinities, general, complex and
+  radiating&mdash;Extinction separates and defines groups&mdash;<span
+  class="sc">Morphology</span>, between members of the same class, between
+  parts of the same individual&mdash;<span class="sc">Embryology</span>,
+  laws of, explained by variations not supervening at an early age, and
+  being inherited at a corresponding age&mdash;<span class="sc">Rudimentary
+  organs</span>; their origin explained&mdash;Summary.</p>
+
+</blockquote>
+
+  <p>From the first dawn of life, all organic beings are found to resemble
+  each other in descending degrees, so that they can be classed in groups
+  under groups. This classification is evidently not arbitrary like the
+  grouping of the stars in constellations. The existence of groups would
+  have been of simple signification, if one group had been exclusively
+  fitted to inhabit the land, and another the water; one to feed on flesh,
+  another on vegetable matter, and so on; but the case is widely different
+  in nature; for it is notorious how commonly members of even the same
+  sub-group have different habits. In our second and fourth chapters, on
+  Variation and on Natural Selection, I have attempted to show that it is
+  the widely ranging, the much diffused and common, that is the dominant
+  species belonging to the larger genera, which vary most. The varieties,
+  or incipient species, thus produced ultimately become converted, as I
+  believe, into new and distinct species; and these, on the principle of
+  inheritance, tend to produce other new and dominant <!-- Page 412
+  --><span class="pagenum"><a name="page412"></a>[412]</span>species.
+  Consequently the groups which are now large, and which generally include
+  many dominant species, tend to go on increasing indefinitely in size. I
+  further attempted to show that from the varying descendants of each
+  species trying to occupy as many and as different places as possible in
+  the economy of nature, there is a constant tendency in their characters
+  to diverge. This conclusion was supported by looking at the great
+  diversity of the forms of life which, in any small area, come into the
+  closest competition, and by looking to certain facts in
+  naturalisation.</p>
+
+  <p>I attempted also to show that there is a constant tendency in the
+  forms which are increasing in number and diverging in character, to
+  supplant and exterminate the less divergent, the less improved, and
+  preceding forms. I request the reader to turn to the diagram illustrating
+  the action, as formerly explained, of these several principles; and he
+  will see that the inevitable result is that the modified descendants
+  proceeding from one progenitor become broken up into groups subordinate
+  to groups. In the diagram each letter on the uppermost line may represent
+  a genus including several species; and all the genera on this line form
+  together one class, for all have descended from one ancient but unseen
+  parent, and, consequently, have inherited something in common. But the
+  three genera on the left hand have, on this same principle, much in
+  common, and form a sub-family, distinct from that including the next two
+  genera on the right hand, which diverged from a common parent at the
+  fifth stage of descent. These five genera have also much, though less, in
+  common; and they form a family distinct from that including the three
+  genera still further to the right hand, which diverged at a still earlier
+  period. And all these genera, descended from (A), form an order distinct
+  from the <!-- Page 413 --><span class="pagenum"><a
+  name="page413"></a>[413]</span>genera descended from (I). So that we here
+  have many species descended from a single progenitor grouped into genera;
+  and the genera are included in, or subordinate to, sub-families,
+  families, and orders, all united into one class. Thus, the grand fact in
+  natural history of the subordination of group under group, which, from
+  its familiarity, does not always sufficiently strike us, is in my
+  judgment explained.</p>
+
+  <p>Naturalists try to arrange the species, genera, and families in each
+  class, on what is called the Natural System. But what is meant by this
+  system? Some authors look at it merely as a scheme for arranging together
+  those living objects which are most alike, and for separating those which
+  are most unlike; or as an artificial means for enunciating, as briefly as
+  possible, general propositions,&mdash;that is, by one sentence to give
+  the characters common, for instance, to all mammals, by another those
+  common to all carnivora, by another those common to the dog-genus, and
+  then by adding a single sentence, a full description is given of each
+  kind of dog. The ingenuity and utility of this system are indisputable.
+  But many naturalists think that something more is meant by the Natural
+  System; they believe that it reveals the plan of the Creator; but unless
+  it be specified whether order in time or space, or what else is meant by
+  the plan of the Creator, it seems to me that nothing is thus added to our
+  knowledge. Such expressions as that famous one of Linnæus, and which we
+  often meet with in a more or less concealed form, that the characters do
+  not make the genus, but that the genus gives the characters, seem to
+  imply that something more is included in our classification, than mere
+  resemblance. I believe that something more is included; and that
+  propinquity of descent,&mdash;the only known cause of the similarity of
+  organic beings,&mdash;is the bond, hidden as it is by various degrees of
+  <!-- Page 414 --><span class="pagenum"><a
+  name="page414"></a>[414]</span>modification, which is partially revealed
+  to us by our classifications.</p>
+
+  <p>Let us now consider the rules followed in classification, and the
+  difficulties which are encountered on the view that classification either
+  gives some unknown plan of creation, or is simply a scheme for
+  enunciating general propositions and of placing together the forms most
+  like each other. It might have been thought (and was in ancient times
+  thought) that those parts of the structure which determined the habits of
+  life, and the general place of each being in the economy of nature, would
+  be of very high importance in classification. Nothing can be more false.
+  No one regards the external similarity of a mouse to a shrew, of a dugong
+  to a whale, of a whale to a fish, as of any importance. These
+  resemblances, though so intimately connected with the whole life of the
+  being, are ranked as merely "adaptive or analogical characters;" but to
+  the consideration of these resemblances we shall have to recur. It may
+  even be given as a general rule, that the less any part of the
+  organisation is concerned with special habits, the more important it
+  becomes for classification. As an instance: Owen, in speaking of the
+  dugong, says, "The generative organs being those which are most remotely
+  related to the habits and food of an animal, I have always regarded as
+  affording very clear indications of its true affinities. We are least
+  likely in the modifications of these organs to mistake a merely adaptive
+  for an essential character." So with plants, how remarkable it is that
+  the organs of vegetation, on which their whole life depends, are of
+  little signification, excepting in the first main divisions; whereas the
+  organs of reproduction, with their product the seed, are of paramount
+  importance!</p>
+
+  <p>We must not, therefore, in classifying, trust to resemblances in parts
+  of the organisation, however important <!-- Page 415 --><span
+  class="pagenum"><a name="page415"></a>[415]</span>they may be for the
+  welfare of the being in relation to the outer world. Perhaps from this
+  cause it has partly arisen, that almost all naturalists lay the greatest
+  stress on resemblances in organs of high vital or physiological
+  importance. No doubt this view of the classificatory importance of organs
+  which are important is generally, but by no means always, true. But their
+  importance for classification, I believe, depends on their greater
+  constancy throughout large groups of species; and this constancy depends
+  on such organs having generally been subjected to less change in the
+  adaptation of the species to their conditions of life. That the mere
+  physiological importance of an organ does not determine its
+  classificatory value, is almost shown by the one fact, that in allied
+  groups, in which the same organ, as we have every reason to suppose, has
+  nearly the same physiological value, its classificatory value is widely
+  different. No naturalist can have worked at any group without being
+  struck with this fact; and it has been fully acknowledged in the writings
+  of almost every author. It will suffice to quote the highest authority,
+  Robert Brown, who in speaking of certain organs in the Proteaceæ, says
+  their generic importance, "like that of all their parts, not only in this
+  but, as I apprehend, in every natural family, is very unequal, and in
+  some cases seems to be entirely lost." Again in another work he says, the
+  genera of the Connaraceæ "differ in having one or more ovaria, in the
+  existence or absence of albumen, in the imbricate or valvular æstivation.
+  Any one of these characters singly is frequently of more than generic
+  importance, though here even when all taken together they appear
+  insufficient to separate Cnestis from Connarus." To give an example
+  amongst insects, in one great division of the Hymenoptera, the antennæ,
+  as Westwood has remarked, are most constant in structure; <!-- Page 416
+  --><span class="pagenum"><a name="page416"></a>[416]</span>in another
+  division they differ much, and the differences are of quite subordinate
+  value in classification; yet no one probably will say that the antennae
+  in these two divisions of the same order are of unequal physiological
+  importance. Any number of instances could be given of the varying
+  importance for classification of the same important organ within the same
+  group of beings.</p>
+
+  <p>Again, no one will say that rudimentary or atrophied organs are of
+  high physiological or vital importance; yet, undoubtedly, organs in this
+  condition are often of high value in classification. No one will dispute
+  that the rudimentary teeth in the upper jaws of young ruminants, and
+  certain rudimentary bones of the leg, are highly serviceable in
+  exhibiting the close affinity between Ruminants and Pachyderms. Robert
+  Brown has strongly insisted on the fact that the rudimentary florets are
+  of the highest importance in the classification of the Grasses.</p>
+
+  <p>Numerous instances could be given of characters derived from parts
+  which must be considered of very trifling physiological importance, but
+  which are universally admitted as highly serviceable in the definition of
+  whole groups. For instance, whether or not there is an open passage from
+  the nostrils to the mouth, the only character, according to Owen, which
+  absolutely distinguishes fishes and reptiles&mdash;the inflection of the
+  angle of the jaws in Marsupials&mdash;the manner in which the wings of
+  insects are folded&mdash;mere colour in certain Algæ&mdash;mere
+  pubescence on parts of the flower in grasses&mdash;the nature of the
+  dermal covering, as hair or feathers, in the Vertebrata. If the
+  Ornithorhynchus had been covered with feathers instead of hair, this
+  external and trifling character would, I think, have been considered by
+  naturalists as important an aid in determining the degree of affinity of
+  this strange creature to <!-- Page 417 --><span class="pagenum"><a
+  name="page417"></a>[417]</span>birds and reptiles, as an approach in
+  structure in any one internal and important organ.</p>
+
+  <p>The importance, for classification, of trifling characters, mainly
+  depends on their being correlated with several other characters of more
+  or less importance. The value indeed of an aggregate of characters is
+  very evident in natural history. Hence, as has often been remarked, a
+  species may depart from its allies in several characters, both of high
+  physiological importance and of almost universal prevalence, and yet
+  leave us in no doubt where it should be ranked. Hence, also, it has been
+  found, that a classification founded on any single character, however
+  important that may be, has always failed; for no part of the organisation
+  is universally constant. The importance of an aggregate of characters,
+  even when none are important, alone explains, I think, that saying of
+  Linnæus, that the characters do not give the genus, but the genus gives
+  the characters; for this saying seems founded on an appreciation of many
+  trifling points of resemblance, too slight to be defined. Certain plants,
+  belonging to the Malpighiaceæ, bear perfect and degraded flowers; in the
+  latter, as A. de Jussieu has remarked, "the greater number of the
+  characters proper to the species, to the genus, to the family, to the
+  class, disappear, and thus laugh at our classification." But when
+  Aspicarpa produced in France, during several years, only degraded
+  flowers, departing so wonderfully in a number of the most important
+  points of structure from the proper type of the order, yet M. Richard
+  sagaciously saw, as Jussieu observes, that this genus should still be
+  retained amongst the Malpighiaceæ. This case seems to me well to
+  illustrate the spirit with which our classifications are sometimes
+  necessarily founded.</p>
+
+  <p>Practically when naturalists are at work, they do <!-- Page 418
+  --><span class="pagenum"><a name="page418"></a>[418]</span>not trouble
+  themselves about the physiological value of the characters which they use
+  in defining a group, or in allocating any particular species. If they
+  find a character nearly uniform, and common to a great number of forms,
+  and not common to others, they use it as one of high value; if common to
+  some lesser number, they use it as of subordinate value. This principle
+  has been broadly confessed by some naturalists to be the true one; and by
+  none more clearly than by that excellent botanist, Aug. St. Hilaire. If
+  certain characters are always found correlated with others, though no
+  apparent bond of connexion can be discovered between them, especial value
+  is set on them. As in most groups of animals, important organs, such as
+  those for propelling the blood, or for aërating it, or those for
+  propagating the race, are found nearly uniform, they are considered as
+  highly serviceable in classification; but in some groups of animals all
+  these, the most important vital organs, are found to offer characters of
+  quite subordinate value.</p>
+
+  <p>We can see why characters derived from the embryo should be of equal
+  importance with those derived from the adult, for our classifications of
+  course include all ages of each species. But it is by no means obvious,
+  on the ordinary view, why the structure of the embryo should be more
+  important for this purpose than that of the adult, which alone plays its
+  full part in the economy of nature. Yet it has been strongly urged by
+  those great naturalists, Milne Edwards and Agassiz, that embryonic
+  characters are the most important of any in the classification of
+  animals; and this doctrine has very generally been admitted as true. The
+  same fact holds good with flowering plants, of which the two main
+  divisions have been founded on characters derived from the
+  embryo,&mdash;on the number and position of the <!-- Page 419 --><span
+  class="pagenum"><a name="page419"></a>[419]</span>embryonic leaves or
+  cotyledons, and on the mode of development of the plumule and radicle. In
+  our discussion on embryology, we shall see why such characters are so
+  valuable, on the view of classification tacitly including the idea of
+  descent.</p>
+
+  <p>Our classifications are often plainly influenced by chains of
+  affinities. Nothing can be easier than to define a number of characters
+  common to all birds; but in the case of crustaceans, such definition has
+  hitherto been found impossible. There are crustaceans at the opposite
+  ends of the series, which have hardly a character in common; yet the
+  species at both ends, from being plainly allied to others, and these to
+  others, and so onwards, can be recognised as unequivocally belonging to
+  this, and to no other class of the Articulata.</p>
+
+  <p>Geographical distribution has often been used, though perhaps not
+  quite logically, in classification, more especially in very large groups
+  of closely allied forms. Temminck insists on the utility or even
+  necessity of this practice in certain groups of birds; and it has been
+  followed by several entomologists and botanists.</p>
+
+  <p>Finally, with respect to the comparative value of the various groups
+  of species, such as orders, sub-orders, families, sub-families, and
+  genera, they seem to be, at least at present, almost arbitrary. Several
+  of the best botanists, such as Mr. Bentham and others, have strongly
+  insisted on their arbitrary value. Instances could be given amongst
+  plants and insects, of a group of forms, first ranked by practised
+  naturalists as only a genus, and then raised to the rank of a sub-family
+  or family; and this has been done, not because further research has
+  detected important structural differences, at first overlooked, but
+  because numerous allied species, with slightly different grades of
+  difference, have been subsequently discovered. <!-- Page 420 --><span
+  class="pagenum"><a name="page420"></a>[420]</span></p>
+
+  <p>All the foregoing rules and aids and difficulties in classification
+  are explained, if I do not greatly deceive myself, on the view that the
+  natural system is founded on descent with modification; that the
+  characters which naturalists consider as showing true affinity between
+  any two or more species, are those which have been inherited from a
+  common parent, and, in so far, all true classification is genealogical;
+  that community of descent is the hidden bond which naturalists have been
+  unconsciously seeking, and not some unknown plan of creation, or the
+  enunciation of general propositions, and the mere putting together and
+  separating objects more or less alike.</p>
+
+  <p>But I must explain my meaning more fully. I believe that the
+  <i>arrangement</i> of the groups within each class, in due subordination
+  and relation to the other groups, must be strictly genealogical in order
+  to be natural; but that the <i>amount</i> of difference in the several
+  branches or groups, though allied in the same degree in blood to their
+  common progenitor, may differ greatly, being due to the different degrees
+  of modification which they have undergone; and this is expressed by the
+  forms being ranked under different genera, families, sections, or orders.
+  The reader will best understand what is meant, if he will take the
+  trouble of referring to the diagram in the fourth chapter. We will
+  suppose the letters A to L to represent allied genera, which lived during
+  the Silurian epoch, and these have descended from a species which existed
+  at an unknown anterior period. Species of three of these genera (A, F,
+  and I) have transmitted modified descendants to the present day,
+  represented by the fifteen genera (<i>a</i><sup>14</sup> to
+  <i>z</i><sup>14</sup>) on the uppermost horizontal line. Now all these
+  modified descendants from a single species, are represented as related in
+  blood or descent to the same <!-- Page 421 --><span class="pagenum"><a
+  name="page421"></a>[421]</span>degree; they may metaphorically be called
+  cousins to the same millionth degree; yet they differ widely and in
+  different degrees from each other. The forms descended from A, now broken
+  up into two or three families, constitute a distinct order from those
+  descended from I, also broken up into two families. Nor can the existing
+  species, descended from A, be ranked in the same genus with the parent A;
+  or those from I, with the parent I. But the existing genus <span
+  class="scac">F</span><sup>14</sup> may be supposed to have been but
+  slightly modified; and it will then rank with the parent-genus F; just as
+  some few still living organic beings belong to Silurian genera. So that
+  the amount or value of the differences between organic beings all related
+  to each other in the same degree in blood, has come to be widely
+  different. Nevertheless their genealogical <i>arrangement</i> remains
+  strictly true, not only at the present time, but at each successive
+  period of descent. All the modified descendants from A will have
+  inherited something in common from their common parent, as will all the
+  descendants from I; so will it be with each subordinate branch of
+  descendants, at each successive period. If, however, we choose to suppose
+  that any of the descendants of A or of I have been so much modified as to
+  have more or less completely lost traces of their parentage, in this
+  case, their places in a natural classification will have been more or
+  less completely lost,&mdash;as sometimes seems to have occurred with
+  existing organisms. All the descendants of the genus F, along its whole
+  line of descent, are supposed to have been but little modified, and they
+  yet form a single genus. But this genus, though much isolated, will still
+  occupy its proper intermediate position; for F originally was
+  intermediate in character between A and I, and the several genera
+  descended from these two genera will <!-- Page 422 --><span
+  class="pagenum"><a name="page422"></a>[422]</span>have inherited to a
+  certain extent their characters. This natural arrangement is shown, as
+  far as is possible on paper, in the diagram, but in much too simple a
+  manner. If a branching diagram had not been used, and only the names of
+  the groups had been written in a linear series, it would have been still
+  less possible to have given a natural arrangement; and it is notoriously
+  not possible to represent in a series, on a flat surface, the affinities
+  which we discover in nature amongst the beings of the same group. Thus,
+  on the view which I hold, the natural system is genealogical in its
+  arrangement, like a pedigree; but the degrees of modification which the
+  different groups have undergone, have to be expressed by ranking them
+  under different so-called genera, sub-families, families, sections,
+  orders, and classes.</p>
+
+  <p>It may be worth while to illustrate this view of classification, by
+  taking the case of languages. If we possessed a perfect pedigree of
+  mankind, a genealogical arrangement of the races of man would afford the
+  best classification of the various languages now spoken throughout the
+  world; and if all extinct languages, and all intermediate and slowly
+  changing dialects, had to be included, such an arrangement would, I
+  think, be the only possible one. Yet it might be that some very ancient
+  language had altered little, and had given rise to few new languages,
+  whilst others (owing to the spreading and subsequent isolation and states
+  of civilisation of the several races, descended from a common race) had
+  altered much, and had given rise to many new languages and dialects. The
+  various degrees of difference in the languages from the same stock, would
+  have to be expressed by groups subordinate to groups; but the proper or
+  even only possible arrangement would still be genealogical; and this
+  would be strictly natural, as <!-- Page 423 --><span class="pagenum"><a
+  name="page423"></a>[423]</span>it would connect together all languages,
+  extinct and modern, by the closest affinities, and would give the
+  filiation and origin of each tongue.</p>
+
+  <p>In confirmation of this view, let us glance at the classification of
+  varieties, which are believed or known to have descended from one
+  species. These are grouped under species, with sub-varieties under
+  varieties; and with our domestic productions, several other grades of
+  difference are requisite, as we have seen with pigeons. The origin of the
+  existence of groups subordinate to groups, is the same with varieties as
+  with species, namely, closeness of descent with various degrees of
+  modification. Nearly the same rules are followed in classifying
+  varieties, as with species. Authors have insisted on the necessity of
+  classing varieties on a natural instead of an artificial system; we are
+  cautioned, for instance, not to class two varieties of the pine-apple
+  together, merely because their fruit, though the most important part,
+  happens to be nearly identical; no one puts the swedish and common
+  turnips together, though the esculent and thickened stems are so similar.
+  Whatever part is found to be most constant, is used in classing
+  varieties: thus the great agriculturist Marshall says the horns are very
+  useful for this purpose with cattle, because they are less variable than
+  the shape or colour of the body, &amp;c.; whereas with sheep the horns
+  are much less serviceable, because less constant. In classing varieties,
+  I apprehend if we had a real pedigree, a genealogical classification
+  would be universally preferred; and it has been attempted by some
+  authors. For we might feel sure, whether there had been more or less
+  modification, the principle of inheritance would keep the forms together
+  which were allied in the greatest number of points. In tumbler pigeons,
+  though some sub-varieties differ from the others <!-- Page 424 --><span
+  class="pagenum"><a name="page424"></a>[424]</span>in the important
+  character of having a longer beak, yet all are kept together from having
+  the common habit of tumbling; but the short-faced breed has nearly or
+  quite lost this habit; nevertheless, without any reasoning or thinking on
+  the subject, these tumblers are kept in the same group, because allied in
+  blood and alike in some other respects. If it could be proved that the
+  Hottentot had descended from the Negro, I think he would be classed under
+  the Negro group, however much he might differ in colour and other
+  important characters from negroes.</p>
+
+  <p>With species in a state of nature, every naturalist has in fact
+  brought descent into his classification; for he includes in his lowest
+  grade, or that of a species, the two sexes; and how enormously these
+  sometimes differ in the most important characters, is known to every
+  naturalist: scarcely a single fact can be predicated in common of the
+  males and hermaphrodites of certain cirripedes, when adult, and yet no
+  one dreams of separating them. The naturalist includes as one species the
+  several larval stages of the same individual, however much they may
+  differ from each other and from the adult; as he likewise includes the
+  so-called alternate generations of Steenstrup, which can only in a
+  technical sense be considered as the same individual. He includes
+  monsters; he includes varieties, not solely because they closely resemble
+  the parent-form, but because they are descended from it. He who believes
+  that the cowslip is descended from the primrose, or conversely, ranks
+  them together as a single species, and gives a single definition. As soon
+  as three Orchidean forms (Monochanthus, Myanthus, and Catasetum), which
+  had previously been ranked as three distinct genera, were known to be
+  sometimes produced on the same spike, they were immediately included as a
+  single species. <!-- Page 425 --><span class="pagenum"><a
+  name="page425"></a>[425]</span></p>
+
+  <p>As descent has universally been used in classing together the
+  individuals of the same species, though the males and females and larvæ
+  are sometimes extremely different; and as it has been used in classing
+  varieties which have undergone a certain, and sometimes a considerable
+  amount of modification, may not this same element of descent have been
+  unconsciously used in grouping species under genera, and genera under
+  higher groups, though in these cases the modification has been greater in
+  degree, and has taken a longer time to complete? I believe it has thus
+  been unconsciously used; and only thus can I understand the several rules
+  and guides which have been followed by our best systematists. We have no
+  written pedigrees; we have to make out community of descent by
+  resemblances of any kind. Therefore we choose those characters which, as
+  far as we can judge, are the least likely to have been modified in
+  relation to the conditions of life to which each species has been
+  recently exposed. Rudimentary structures on this view are as good as, or
+  even sometimes better than, other parts of the organisation. We care not
+  how trifling a character may be&mdash;let it be the mere inflection of
+  the angle of the jaw, the manner in which an insect's wing is folded,
+  whether the skin be covered by hair or feathers&mdash;if it prevail
+  throughout many and different species, especially those having very
+  different habits of life, it assumes high value; for we can account for
+  its presence in so many forms with such different habits, only by its
+  inheritance from a common parent. We may err in this respect in regard to
+  single points of structure, but when several characters, let them be ever
+  so trifling, occur together throughout a large group of beings having
+  different habits, we may feel almost sure, on the theory of descent, that
+  these characters have been inherited from a common ancestor. <!-- Page
+  426 --><span class="pagenum"><a name="page426"></a>[426]</span>And we
+  know that such correlated or aggregated characters have especial value in
+  classification.</p>
+
+  <p>We can understand why a species or a group of species may depart, in
+  several of its most important characteristics, from its allies, and yet
+  be safely classed with them. This may be safely done, and is often done,
+  as long as a sufficient number of characters, let them be ever so
+  unimportant, betrays the hidden bond of community of descent. Let two
+  forms have not a single character in common, yet if these extreme forms
+  are connected together by a chain of intermediate groups, we may at once
+  infer their community of descent, and we put them all into the same
+  class. As we find organs of high physiological importance&mdash;those
+  which serve to preserve life under the most diverse conditions of
+  existence&mdash;are generally the most constant, we attach especial value
+  to them; but if these same organs, in another group or section of a
+  group, are found to differ much, we at once value them less in our
+  classification. We shall hereafter, I think, clearly see why
+  embryological characters are of such high classificatory importance.
+  Geographical distribution may sometimes be brought usefully into play in
+  classing large and widely-distributed genera, because all the species of
+  the same genus, inhabiting any distinct and isolated region, have in all
+  probability descended from the same parents.</p>
+
+  <p>We can understand, on these views, the very important distinction
+  between real affinities and analogical or adaptive resemblances. Lamarck
+  first called attention to this distinction, and he has been ably followed
+  by Macleay and others. The resemblance, in the shape of the body and in
+  the fin-like anterior limbs, between the dugong, which is a
+  pachydermatous animal, and the whale, and between both these mammals and
+  fishes, is analogical. Amongst insects there are innumerable <!-- Page
+  427 --><span class="pagenum"><a name="page427"></a>[427]</span>instances:
+  thus Linnæus, misled by external appearances, actually classed an
+  homopterous insect as a moth. We see something of the same kind even in
+  our domestic varieties, as in the thickened stems of the common and
+  swedish turnip. The resemblance of the greyhound and racehorse is hardly
+  more fanciful than the analogies which have been drawn by some authors
+  between very distinct animals. On my view of characters being of real
+  importance for classification, only in so far as they reveal descent, we
+  can clearly understand why analogical or adaptive character, although of
+  the utmost importance to the welfare of the being, are almost valueless
+  to the systematist. For animals, belonging to two most distinct lines of
+  descent, may readily become adapted to similar conditions, and thus
+  assume a close external resemblance; but such resemblances will not
+  reveal&mdash;will rather tend to conceal their blood-relationship to
+  their proper lines of descent. We can also understand the apparent
+  paradox, that the very same characters are analogical when one class or
+  order is compared with another, but give true affinities when the members
+  of the same class or order are compared one with another: thus the shape
+  of the body and fin-like limbs are only analogical when whales are
+  compared with fishes, being adaptations in both classes for swimming
+  through the water; but the shape of the body and fin-like limbs serve as
+  characters exhibiting true affinity between the several members of the
+  whale family; for these cetaceans agree in so many characters, great and
+  small, that we cannot doubt that they have inherited their general shape
+  of body and structure of limbs from a common ancestor. So it is with
+  fishes.</p>
+
+  <p>As members of distinct classes have often been adapted by successive
+  slight modifications to live under nearly similar circumstances,&mdash;to
+  inhabit for instance <!-- Page 428 --><span class="pagenum"><a
+  name="page428"></a>[428]</span>the three elements of land, air, and
+  water,&mdash;we can perhaps understand how it is that a numerical
+  parallelism has sometimes been observed between the sub-groups in
+  distinct classes. A naturalist, struck by a parallelism of this nature in
+  any one class, by arbitrarily raising or sinking the value of the groups
+  in other classes (and all our experience shows that this valuation has
+  hitherto been arbitrary), could easily extend the parallelism over a wide
+  range; and thus the septenary, quinary, quaternary, and ternary
+  classifications have probably arisen.</p>
+
+  <p>As the modified descendants of dominant species, belonging to the
+  larger genera, tend to inherit the advantages, which made the groups to
+  which they belong large and their parents dominant, they are almost sure
+  to spread widely, and to seize on more and more places in the economy of
+  nature. The larger and more dominant groups thus tend to go on increasing
+  in size; and they consequently supplant many smaller and feebler groups.
+  Thus we can account for the fact that all organisms, recent and extinct,
+  are included under a few great orders, under still fewer classes, and all
+  in one great natural system. As showing how few the higher groups are in
+  number, and how widely spread they are throughout the world, the fact is
+  striking, that the discovery of Australia has not added a single insect
+  belonging to a new class; and that in the vegetable kingdom, as I learn
+  from Dr. Hooker, it has added only two or three orders of small size.</p>
+
+  <p>In the chapter on geological succession I attempted to show, on the
+  principle of each group having generally diverged much in character
+  during the long-continued process of modification, how it is that the
+  more ancient forms of life often present characters in some slight degree
+  intermediate between existing groups. A few <!-- Page 429 --><span
+  class="pagenum"><a name="page429"></a>[429]</span>old and intermediate
+  parent-forms having occasionally transmitted to the present day
+  descendants but little modified, will give to us our so-called osculant
+  or aberrant groups. The more aberrant any form is, the greater must be
+  the number of connecting forms which on my theory have been exterminated
+  and utterly lost. And we have some evidence of aberrant forms having
+  suffered severely from extinction, for they are generally represented by
+  extremely few species; and such species as do occur are generally very
+  distinct from each other, which again implies extinction. The genera
+  Ornithorhynchus and Lepidosiren, for example, would not have been less
+  aberrant had each been represented by a dozen species instead of by a
+  single one; but such richness in species, as I find after some
+  investigation, does not commonly fall to the lot of aberrant genera. We
+  can, I think, account for this fact only by looking at aberrant forms as
+  failing groups conquered by more successful competitors, with a few
+  members preserved by some unusual coincidence of favourable
+  circumstances.</p>
+
+  <p>Mr. Waterhouse has remarked that, when a member belonging to one group
+  of animals exhibits an affinity to a quite distinct group, this affinity
+  in most cases is general and not special: thus, according to Mr.
+  Waterhouse, of all Rodents, the bizcacha is most nearly related to
+  Marsupials; but in the points in which it approaches this order, its
+  relations are general, and not to any one marsupial species more than to
+  another. As the points of affinity of the bizcacha to Marsupials are
+  believed to be real and not merely adaptive, they are due on my theory to
+  inheritance in common. Therefore we must suppose either that all Rodents,
+  including the bizcacha, branched off from some very ancient Marsupial,
+  which will have had a character in some degree intermediate with respect
+  to all existing Marsupials; or <!-- Page 430 --><span class="pagenum"><a
+  name="page430"></a>[430]</span>that both Rodents and Marsupials branched
+  off from a common progenitor, and that both groups have since undergone
+  much modification in divergent directions. On either view we may suppose
+  that the bizcacha has retained, by inheritance, more of the character of
+  its ancient progenitor than have other Rodents; and therefore it will not
+  be specially related to any one existing Marsupial, but indirectly to all
+  or nearly all Marsupials, from having partially retained the character of
+  their common progenitor, or of an early member of the group. On the other
+  hand, of all Marsupials, as Mr. Waterhouse has remarked, the phascolomys
+  resembles most nearly, not any one species, but the general order of
+  Rodents. In this case, however, it may be strongly suspected that the
+  resemblance is only analogical, owing to the phascolomys having become
+  adapted to habits like those of a Rodent. The elder De Candolle has made
+  nearly similar observations on the general nature of the affinities of
+  distinct orders of plants.</p>
+
+  <p>On the principle of the multiplication and gradual divergence in
+  character of the species descended from a common parent, together with
+  their retention by inheritance of some characters in common, we can
+  understand the excessively complex and radiating affinities by which all
+  the members of the same family or higher group are connected together.
+  For the common parent of a whole family of species, now broken up by
+  extinction into distinct groups and sub-groups, will have transmitted
+  some of its characters, modified in various ways and degrees, to all; and
+  the several species will consequently be related to each other by
+  circuitous lines of affinity of various lengths (as may be seen in the
+  diagram so often referred to), mounting up through many predecessors. As
+  it is difficult to show the blood-relationship between the numerous
+  kindred <!-- Page 431 --><span class="pagenum"><a
+  name="page431"></a>[431]</span>of any ancient and noble family, even by
+  the aid of a genealogical tree, and almost impossible to do this without
+  this aid, we can understand the extraordinary difficulty which
+  naturalists have experienced in describing, without the aid of a diagram,
+  the various affinities which they perceive between the many living and
+  extinct members of the same great natural class.</p>
+
+  <p>Extinction, as we have seen in the fourth chapter, has played an
+  important part in defining and widening the intervals between the several
+  groups in each class. We may thus account even for the distinctness of
+  whole classes from each other&mdash;for instance, of birds from all other
+  vertebrate animals&mdash;by the belief that many ancient forms of life
+  have been utterly lost, through which the early progenitors of birds were
+  formerly connected with the early progenitors of the other vertebrate
+  classes. There has been less entire extinction of the forms of life which
+  once connected fishes with batrachians. There has been still less in some
+  other classes, as in that of the Crustacea, for here the most wonderfully
+  diverse forms are still tied together by a long, but broken, chain of
+  affinities. Extinction has only separated groups: it has by no means made
+  them; for if every form which has ever lived on this earth were suddenly
+  to reappear, though it would be quite impossible to give definitions by
+  which each group could be distinguished from other groups, as all would
+  blend together by steps as fine as those between the finest existing
+  varieties, nevertheless a natural classification, or at least a natural
+  arrangement, would be possible. We shall see this by turning to the
+  diagram: the letters, A to L, may represent eleven Silurian genera, some
+  of which have produced large groups of modified descendants. Every
+  intermediate link between these eleven genera and their primordial
+  parent, and every <!-- Page 432 --><span class="pagenum"><a
+  name="page432"></a>[432]</span>intermediate link in each branch and
+  sub-branch of their descendants, may be supposed to be still alive; and
+  the links to be as fine as those between the finest varieties. In this
+  case it would be quite impossible to give any definition by which the
+  several members of the several groups could be distinguished from their
+  more immediate parents; or these parents from their ancient and unknown
+  progenitor. Yet the natural arrangement in the diagram would still hold
+  good; and, on the principle of inheritance, all the forms descended from
+  A, or from I, would have something in common. In a tree we can specify
+  this or that branch, though at the actual fork the two unite and blend
+  together. We could not, as I have said, define the several groups; but we
+  could pick out types, or forms, representing most of the characters of
+  each group, whether large or small, and thus give a general idea of the
+  value of the differences between them. This is what we should be driven
+  to, if we were ever to succeed in collecting all the forms in any class
+  which have lived throughout all time and space. We shall certainly never
+  succeed in making so perfect a collection: nevertheless, in certain
+  classes, we are tending in this direction; and Milne Edwards has lately
+  insisted, in an able paper, on the high importance of looking to types,
+  whether or not we can separate and define the groups to which such types
+  belong.</p>
+
+  <p>Finally, we have seen that natural selection, which results from the
+  struggle for existence, and which almost inevitably induces extinction
+  and divergence of character in the many descendants from one dominant
+  parent-species, explains that great and universal feature in the
+  affinities of all organic beings, namely, their subordination in group
+  under group. We use the element of descent in classing the individuals of
+  both sexes and of all ages, although having few characters in common,
+  <!-- Page 433 --><span class="pagenum"><a
+  name="page433"></a>[433]</span>under one species; we use descent in
+  classing acknowledged varieties, however different they may be from their
+  parent; and I believe this element of descent is the hidden bond of
+  connexion which naturalists have sought under the term of the Natural
+  System. On this idea of the natural system being, in so far as it has
+  been perfected, genealogical in its arrangement, with the grades of
+  difference between the descendants from a common parent, expressed by the
+  terms genera, families, orders, &amp;c., we can understand the rules
+  which we are compelled to follow in our classification. We can understand
+  why we value certain resemblances far more than others; why we are
+  permitted to use rudimentary and useless organs, or others of trifling
+  physiological importance; why, in comparing one group with a distinct
+  group, we summarily reject analogical or adaptive characters, and yet use
+  these same characters within the limits of the same group. We can clearly
+  see how it is that all living and extinct forms can be grouped together
+  in one great system; and how the several members of each class are
+  connected together by the most complex and radiating lines of affinities.
+  We shall never, probably, disentangle the inextricable web of affinities
+  between the members of any one class; but when we have a distinct object
+  in view, and do not look to some unknown plan of creation, we may hope to
+  make sure but slow progress.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Morphology.</i>&mdash;We have seen that the members of the same
+  class, independently of their habits of life, resemble each other in the
+  general plan of their organisation. This resemblance is often expressed
+  by the term "unity of type;" or by saying that the several parts and
+  organs in the different species of the class are homologous. The whole
+  subject is included under <!-- Page 434 --><span class="pagenum"><a
+  name="page434"></a>[434]</span>the general name of Morphology. This is
+  the most interesting department of natural history, and may be said to be
+  its very soul. What can be more curious than that the hand of a man,
+  formed for grasping, that of a mole for digging, the leg of the horse,
+  the paddle of the porpoise, and the wing of the bat, should all be
+  constructed on the same pattern, and should include similar bones, in the
+  same relative positions? Geoffroy St. Hilaire has insisted strongly on
+  the high importance of relative connexion in homologous organs: the parts
+  may change to almost any extent in form and size, and yet they always
+  remain connected together in the same order. We never find, for instance,
+  the bones of the arm and forearm, or of the thigh and leg, transposed.
+  Hence the same names can be given to the homologous bones in widely
+  different animals. We see the same great law in the construction of the
+  mouths of insects: what can be more different than the immensely long
+  spiral proboscis of a sphinx-moth, the curious folded one of a bee or
+  bug, and the great jaws of a beetle?&mdash;yet all these organs, serving
+  for such different purposes, are formed by infinitely numerous
+  modifications of an upper lip, mandibles, and two pairs of maxillæ.
+  Analogous laws govern the construction of the mouths and limbs of
+  crustaceans. So it is with the flowers of plants.</p>
+
+  <p>Nothing can be more hopeless than to attempt to explain this
+  similarity of pattern in members of the same class, by utility or by the
+  doctrine of final causes. The hopelessness of the attempt has been
+  expressly admitted by Owen in his most interesting work on the 'Nature of
+  Limbs.' On the ordinary view of the independent creation of each being,
+  we can only say that so it is;&mdash;that it has so pleased the Creator
+  to construct each animal and plant.</p>
+
+  <p>The explanation is manifest on the theory of the <!-- Page 435
+  --><span class="pagenum"><a name="page435"></a>[435]</span>natural
+  selection of successive slight modifications,&mdash;each modification
+  being profitable in some way to the modified form, but often affecting by
+  correlation of growth other parts of the organisation. In changes of this
+  nature, there will be little or no tendency to modify the original
+  pattern, or to transpose parts. The bones of a limb might be shortened
+  and widened to any extent, and become gradually enveloped in thick
+  membrane, so as to serve as a fin; or a webbed foot might have all its
+  bones, or certain bones, lengthened to any extent, and the membrane
+  connecting them increased to any extent, so as to serve as a wing: yet in
+  all this great amount of modification there will be no tendency to alter
+  the framework of bones or the relative connexion of the several parts. If
+  we suppose that the ancient progenitor, the archetype as it may be
+  called, of all mammals, had its limbs constructed on the existing general
+  pattern, for whatever purpose they served, we can at once perceive the
+  plain signification of the homologous construction of the limbs
+  throughout the whole class. So with the mouths of insects, we have only
+  to suppose that their common progenitor had an upper lip, mandibles, and
+  two pair of maxillæ, these parts being perhaps very simple in form; and
+  then natural selection, acting on some originally created form, will
+  account for the infinite diversity in structure and function of the
+  mouths of insects. Nevertheless, it is conceivable that the general
+  pattern of an organ might become so much obscured as to be finally lost,
+  by the atrophy and ultimately by the complete abortion of certain parts,
+  by the soldering together of other parts, and by the doubling or
+  multiplication of others,&mdash;variations which we know to be within the
+  limits of possibility. In the paddles of the extinct gigantic
+  sea-lizards, and in the mouths of certain suctorial crustaceans, the <!--
+  Page 436 --><span class="pagenum"><a
+  name="page436"></a>[436]</span>general pattern seems to have been thus to
+  a certain extent obscured.</p>
+
+  <p>There is another and equally curious branch of the present subject;
+  namely, the comparison not of the same part in different members of a
+  class, but of the different parts or organs in the same individual. Most
+  physiologists believe that the bones of the skull are homologous
+  with&mdash;that is correspond in number and in relative connexion
+  with&mdash;the elemental parts of a certain number of vertebræ. The
+  anterior and posterior limbs in each member of the vertebrate and
+  articulate classes are plainly homologous. We see the same law in
+  comparing the wonderfully complex jaws and legs in crustaceans. It is
+  familiar to almost every one, that in a flower the relative position of
+  the sepals, petals, stamens, and pistils, as well as their intimate
+  structure, are intelligible on the view that they consist of
+  metamorphosed leaves, arranged in a spire. In monstrous plants, we often
+  get direct evidence of the possibility of one organ being transformed
+  into another; and we can actually see in embryonic crustaceans and in
+  many other animals, and in flowers, that organs, which when mature become
+  extremely different, are at an early stage of growth exactly alike.</p>
+
+  <p>How inexplicable are these facts on the ordinary view of creation! Why
+  should the brain be enclosed in a box composed of such numerous and such
+  extraordinary shaped pieces of bone? As Owen has remarked, the benefit
+  derived from the yielding of the separate pieces in the act of
+  parturition of mammals, will by no means explain the same construction in
+  the skulls of birds. Why should similar bones have been created in the
+  formation of the wing and leg of a bat, used as they are for such totally
+  different purposes? Why should one crustacean, which has an extremely
+  complex <!-- Page 437 --><span class="pagenum"><a
+  name="page437"></a>[437]</span>mouth formed of many parts, consequently
+  always have fewer legs; or conversely, those with many legs have simpler
+  mouths? Why should the sepals, petals, stamens, and pistils in any
+  individual flower, though fitted for such widely different purposes, be
+  all constructed on the same pattern?</p>
+
+  <p>On the theory of natural selection, we can satisfactorily answer these
+  questions. In the vertebrata, we see a series of internal vertebræ
+  bearing certain processes and appendages; in the articulata, we see the
+  body divided into a series of segments, bearing external appendages; and
+  in flowering plants, we see a series of successive spiral whorls of
+  leaves. An indefinite repetition of the same part or organ is the common
+  characteristic (as Owen has observed) of all low or little-modified
+  forms; therefore we may readily believe that the unknown progenitor of
+  the vertebrata possessed many vertebræ; the unknown progenitor of the
+  articulata, many segments; and the unknown progenitor of flowering
+  plants, many spiral whorls of leaves. We have formerly seen that parts
+  many times repeated are eminently liable to vary in number and structure;
+  consequently it is quite probable that natural selection, during a
+  long-continued course of modification, should have seized on a certain
+  number of the primordially similar elements, many times repeated, and
+  have adapted them to the most diverse purposes. And as the whole amount
+  of modification will have been effected by slight successive steps, we
+  need not wonder at discovering in such parts or organs, a certain degree
+  of fundamental resemblance, retained by the strong principle of
+  inheritance.</p>
+
+  <p>In the great class of molluscs, though we can homologise the parts of
+  one species with those of other and distinct species, we can indicate but
+  few serial homologies; that is, we are seldom enabled to say that one
+  <!-- Page 438 --><span class="pagenum"><a
+  name="page438"></a>[438]</span>part or organ is homologous with another
+  in the same individual. And we can understand this fact; for in molluscs,
+  even in the lowest members of the class, we do not find nearly so much
+  indefinite repetition of any one part, as we find in the other great
+  classes of the animal and vegetable kingdoms.</p>
+
+  <p>Naturalists frequently speak of the skull as formed of metamorphosed
+  vertebræ: the jaws of crabs as metamorphosed legs; the stamens and
+  pistils of flowers as metamorphosed leaves; but it would in these cases
+  probably be more correct, as Professor Huxley has remarked, to speak of
+  both skull and vertebræ, both jaws and legs, &amp;c.,&mdash;as having
+  been metamorphosed, not one from the other, but from some common element.
+  Naturalists, however, use such language only in a metaphorical sense:
+  they are far from meaning that during a long course of descent,
+  primordial organs of any kind&mdash;vertebræ in the one case and legs in
+  the other&mdash;have actually been modified into skulls or jaws. Yet so
+  strong is the appearance of a modification of this nature having
+  occurred, that naturalists can hardly avoid employing language having
+  this plain signification. On my view these terms may be used literally;
+  and the wonderful fact of the jaws, for instance, of a crab retaining
+  numerous characters, which they would probably have retained through
+  inheritance, if they had really been metamorphosed during a long course
+  of descent from true legs, or from some simple appendage, is
+  explained.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Embryology.</i>&mdash;It has already been casually remarked that
+  certain organs in the individual, which when mature become widely
+  different and serve for different purposes, are in the embryo exactly
+  alike. The embryos, also, of distinct animals within the same class are
+  often strikingly similar: a better proof of this cannot be given, than a
+  <!-- Page 439 --><span class="pagenum"><a
+  name="page439"></a>[439]</span>circumstance mentioned by Agassiz, namely,
+  that having forgotten to ticket the embryo of some vertebrate animal, he
+  cannot now tell whether it be that of a mammal, bird, or reptile. The
+  vermiform larvæ of moths, flies, beetles, &amp;c., resemble each other
+  much more closely than do the mature insects; but in the case of larvæ,
+  the embryos are active, and have been adapted for special lines of life.
+  A trace of the law of embryonic resemblance, sometimes lasts till a
+  rather late age: thus birds of the same genus, and of closely allied
+  genera, often resemble each other in their first and second plumage; as
+  we see in the spotted feathers in the thrush group. In the cat tribe,
+  most of the species are striped or spotted in lines; and stripes can be
+  plainly distinguished in the whelp of the lion. We occasionally though
+  rarely see something of this kind in plants: thus the embryonic leaves of
+  the ulex or furze, and the first leaves of the phyllodineous acaceas, are
+  pinnate or divided like the ordinary leaves of the leguminosæ.</p>
+
+  <p>The points of structure, in which the embryos of widely different
+  animals of the same class resemble each other, often have no direct
+  relation to their conditions of existence. We cannot, for instance,
+  suppose that in the embryos of the vertebrata the peculiar loop-like
+  course of the arteries near the branchial slits are related to similar
+  conditions,&mdash;in the young mammal which is nourished in the womb of
+  its mother, in the egg of the bird which is hatched in a nest, and in the
+  spawn of a frog under water. We have no more reason to believe in such a
+  relation, than we have to believe that the same bones in the hand of a
+  man, wing of a bat, and fin of a porpoise, are related to similar
+  conditions of life. No one will suppose that the stripes on the whelp of
+  a lion, or the spots on the young blackbird, <!-- Page 440 --><span
+  class="pagenum"><a name="page440"></a>[440]</span>are of any use to these
+  animals, or are related to the conditions to which they are exposed.</p>
+
+  <p>The case, however, is different when an animal during any part of its
+  embryonic career is active, and has to provide for itself. The period of
+  activity may come on earlier or later in life; but whenever it comes on,
+  the adaptation of the larva to its conditions of life is just as perfect
+  and as beautiful as in the adult animal. From such special adaptations,
+  the similarity of the larvæ or active embryos of allied animals is
+  sometimes much obscured; and cases could be given of the larvæ of two
+  species, or of two groups of species, differing quite as much, or even
+  more, from each other than do their adult parents. In most cases,
+  however, the larvæ, though active, still obey, more or less closely, the
+  law of common embryonic resemblance. Cirripedes afford a good instance of
+  this: even the illustrious Cuvier did not perceive that a barnacle was,
+  as it certainly is, a crustacean; but a glance at the larva shows this to
+  be the case in an unmistakeable manner. So again the two main divisions
+  of cirripedes, the pedunculated and sessile, which differ widely in
+  external appearance, have larvæ in all their stages barely
+  distinguishable.</p>
+
+  <p>The embryo in the course of development generally rises in
+  organisation: I use this expression, though I am aware that it is hardly
+  possible to define clearly what is meant by the organisation being higher
+  or lower. But no one probably will dispute that the butterfly is higher
+  than the caterpillar. In some cases, however, the mature animal is
+  generally considered as lower in the scale than the larva, as with
+  certain parasitic crustaceans. To refer once again to cirripedes: the
+  larvæ in the first stage have three pairs of legs, a very simple single
+  eye, and a probosciformed mouth, with which they feed largely, for they
+  increase much in <!-- Page 441 --><span class="pagenum"><a
+  name="page441"></a>[441]</span>size. In the second stage, answering to
+  the chrysalis stage of butterflies, they have six pairs of beautifully
+  constructed natatory legs, a pair of magnificent compound eyes, and
+  extremely complex antennæ; but they have a closed and imperfect mouth,
+  and cannot feed: their function at this stage is, to search by their
+  well-developed organs of sense, and to reach by their active powers of
+  swimming, a proper place on which to become attached and to undergo their
+  final metamorphosis. When this is completed they are fixed for life:
+  their legs are now converted into prehensile organs; they again obtain a
+  well-constructed mouth; but they have no antennæ, and their two eyes are
+  now reconverted into a minute, single, and very simple eye-spot. In this
+  last and complete state, cirripedes may be considered as either more
+  highly or more lowly organised than they were in the larval condition.
+  But in some genera the larvæ become developed either into hermaphrodites
+  having the ordinary structure, or into what I have called complemental
+  males: and in the latter, the development has assuredly been retrograde;
+  for the male is a mere sack, which lives for a short time, and is
+  destitute of mouth, stomach, or other organ of importance, excepting for
+  reproduction.</p>
+
+  <p>We are so much accustomed to see differences in structure between the
+  embryo and the adult, and likewise a close similarity in the embryos of
+  widely different animals within the same class, that we might be led to
+  look at these facts as necessarily contingent in some manner on growth.
+  But there is no obvious reason why, for instance, the wing of a bat, or
+  the fin of a porpoise, should not have been sketched out with all the
+  parts in proper proportion, as soon as any structure became visible in
+  the embryo. And in some whole groups of animals and in certain members of
+  other groups, the embryo does not at any period differ widely from the
+  <!-- Page 442 --><span class="pagenum"><a
+  name="page442"></a>[442]</span>adult: thus Owen has remarked in regard to
+  cuttle-fish, "there is no metamorphosis; the cephalopodic character is
+  manifested long before the parts of the embryo are completed;" and again
+  in spiders, "there is nothing worthy to be called a metamorphosis." The
+  larvæ of insects, whether adapted to the most diverse and active habits,
+  or quite inactive, being fed by their parents or placed in the midst of
+  proper nutriment, yet nearly all pass through a similar worm-like stage
+  of development; but in some few cases, as in that of Aphis, if we look to
+  the admirable drawings by Professor Huxley of the development of this
+  insect, we see no trace of the vermiform stage.</p>
+
+  <p>How, then, can we explain these several facts in
+  embryology,&mdash;namely the very general, but not universal difference
+  in structure between the embryo and the adult;&mdash;of parts in the same
+  <span class="correction" title="text reads `indivividual'"
+  >individual</span> embryo, which ultimately become very unlike and serve
+  for diverse purposes, being at this early period of growth
+  alike;&mdash;of embryos of different species within the same class,
+  generally, but not universally, resembling each other;&mdash;of the
+  structure of the embryo not being closely related to its conditions of
+  existence, except when the embryo becomes at any period of life active
+  and has to provide for itself;&mdash;of the embryo apparently having
+  sometimes a higher organisation than the mature animal, into which it is
+  developed? I believe that all these facts can be explained, as follows,
+  on the view of descent with modification.</p>
+
+  <p>It is commonly assumed, perhaps from monstrosities often affecting the
+  embryos at a very early period, that slight variations necessarily appear
+  at an equally early period. But we have little evidence on this
+  head&mdash;indeed the evidence rather points the other way; for it is
+  notorious that breeders of cattle, horses, and various <!-- Page 443
+  --><span class="pagenum"><a name="page443"></a>[443]</span>fancy animals,
+  cannot positively tell, until some time after the animal has been born,
+  what its merits or form will ultimately turn out. We see this plainly in
+  our own children; we cannot always tell whether the child will be tall or
+  short, or what its precise features will be. The question is not, at what
+  period of life any variation has been caused, but at what period it is
+  fully displayed. The cause may have acted, and I believe generally has
+  acted, even before the embryo is formed; and the variation may be due to
+  the male and female sexual elements having been affected by the
+  conditions to which either parent, or their ancestors, have been exposed.
+  Nevertheless an effect thus caused at a very early period, even before
+  the formation of the embryo, may appear late in life; as when an
+  hereditary disease, which appears in old age alone, has been communicated
+  to the offspring from the reproductive element of one parent. Or again,
+  as when the horns of cross-bred cattle have been affected by the shape of
+  the horns of either parent. For the welfare of a very young animal, as
+  long as it remains in its mother's womb, or in the egg, or as long as it
+  is nourished and protected by its parent, it must be quite unimportant
+  whether most of its characters are fully acquired a little earlier or
+  later in life. It would not signify, for instance, to a bird which
+  obtained its food best by having a long beak, whether or not it assumed a
+  beak of this particular length, as long as it was fed by its parents.
+  Hence, I conclude, that it is quite possible, that each of the many
+  successive modifications, by which each species has acquired its present
+  structure, may have supervened at a not very early period of life; and
+  some direct evidence from our domestic animals supports this view. But in
+  other cases it is quite possible that each successive modification, or
+  <!-- Page 444 --><span class="pagenum"><a
+  name="page444"></a>[444]</span>most of them, may have appeared at an
+  extremely early period.</p>
+
+  <p>I have stated in the first chapter, that there is some evidence to
+  render it probable, that at whatever age any variation first appears in
+  the parent, it tends to reappear at a corresponding age in the offspring.
+  Certain variations can only appear at corresponding ages, for instance,
+  peculiarities in the caterpillar, cocoon, or imago states of the
+  silk-moth; or, again, in the horns of almost full-grown cattle. But
+  further than this, variations which, for all that we can see, might have
+  appeared earlier or later in life, tend to appear at a corresponding age
+  in the offspring and parent. I am far from meaning that this is
+  invariably the case; and I could give a good many cases of variations
+  (taking the word in the largest sense) which have supervened at an
+  earlier age in the child than in the parent.</p>
+
+  <p>These two principles, if their truth be admitted, will, I believe,
+  explain all the above specified leading facts in embryology. But first
+  let us look at a few analogous cases in domestic varieties. Some authors
+  who have written on Dogs, maintain that the greyhound and bulldog, though
+  appearing so different, are really varieties most closely allied, and
+  have probably descended from the same wild stock; hence I was curious to
+  see how far their puppies differed from each other: I was told by
+  breeders that they differed just as much as their parents, and this,
+  judging by the eye, seemed almost to be the case; but on actually
+  measuring the old dogs and their six-days old puppies, I found that the
+  puppies had not nearly acquired their full amount of proportional
+  difference. So, again, I was told that the foals of cart and race-horses
+  differed as much as the full-grown animals; and this surprised me
+  greatly, as I think it probable that the difference between these two
+  breeds has been wholly <!-- Page 445 --><span class="pagenum"><a
+  name="page445"></a>[445]</span>caused by selection under domestication;
+  but having had careful measurements made of the dam and of a three-days
+  old colt of a race and heavy cart-horse, I find that the colts have by no
+  means acquired their full amount of proportional difference.</p>
+
+  <p>As the evidence appears to me conclusive, that the several domestic
+  breeds of Pigeon have descended from one wild species, I compared young
+  pigeons of various breeds, within twelve hours after being hatched; I
+  carefully measured the proportions (but will not here give details) of
+  the beak, width of mouth, length of nostril and of eyelid, size of feet
+  and length of leg, in the wild stock, in pouters, fantails, runts, barbs,
+  dragons, carriers, and tumblers. Now some of these birds, when mature,
+  differ so extraordinarily in length and form of beak, that they would, I
+  cannot doubt, be ranked in distinct genera, had they been natural
+  productions. But when the nestling birds of these several breeds were
+  placed in a row, though most of them could be distinguished from each
+  other, yet their proportional differences in the above specified several
+  points were incomparably less than in the full-grown birds. Some
+  characteristic points of difference&mdash;for instance, that of the width
+  of mouth&mdash;could hardly be detected in the young. But there was one
+  remarkable exception to this rule, for the young of the short-faced
+  tumbler differed from the young of the wild rock-pigeon and of the other
+  breeds, in all its proportions, almost exactly as much as in the adult
+  state.</p>
+
+  <p>The two principles above given seem to me to explain these facts in
+  regard to the later embryonic stages of our domestic varieties. Fanciers
+  select their horses, dogs, and pigeons, for breeding, when they are
+  nearly grown up: they are indifferent whether the desired qualities and
+  structures have been acquired earlier or <!-- Page 446 --><span
+  class="pagenum"><a name="page446"></a>[446]</span>later in life, if the
+  full-grown animal possesses them. And the cases just given, more
+  especially that of pigeons, seem to show that the characteristic
+  differences which give value to each breed, and which have been
+  accumulated by man's selection, have not generally first appeared at an
+  early period of life, and have been inherited by the offspring at a
+  corresponding not early period. But the case of the short-faced tumbler,
+  which when twelve hours old had acquired its proper proportions, proves
+  that this is not the universal rule; for here the characteristic
+  differences must either have appeared at an earlier period than usual,
+  or, if not so, the differences must have been inherited, not at the
+  corresponding, but at an earlier age.</p>
+
+  <p>Now let us apply these facts and the above two principles&mdash;which
+  latter, though not proved true, can be shown to be in some degree
+  probable&mdash;to species in a state of nature. Let us take a genus of
+  birds, descended on my theory from some one parent-species, and of which
+  the several new species have become modified through natural selection in
+  accordance with their diverse habits. Then, from the many slight
+  successive steps of variation having supervened at a rather late age, and
+  having been inherited at a corresponding age, the young of the new
+  species of our supposed genus will manifestly tend to resemble each other
+  much more closely than do the adults, just as we have seen in the case of
+  pigeons. We may extend this view to whole families or even classes. The
+  fore-limbs, for instance, which served as legs in the parent-species, may
+  have become, by a long course of modification, adapted in one descendant
+  to act as hands, in another as paddles, in another as wings; and on the
+  above two principles&mdash;namely of each successive modification
+  supervening at a rather late age, and being inherited at a <!-- Page 447
+  --><span class="pagenum"><a name="page447"></a>[447]</span>corresponding
+  late age&mdash;the fore-limbs in the embryos of the several descendants
+  of the parent-species will still resemble each other closely, for they
+  will not have been modified. But in each of our new species, the
+  embryonic fore-limbs will differ greatly from the fore-limbs in the
+  mature animal; the limbs in the latter having undergone much modification
+  at a rather late period of life, and having thus been converted into
+  hands, or paddles, or wings. Whatever influence long-continued exercise
+  or use on the one hand, and disuse on the other, may have in modifying an
+  organ, such influence will mainly affect the mature animal, which has
+  come to its full powers of activity and has to gain its own living; and
+  the effects thus produced will be inherited at a corresponding mature
+  age. Whereas the young will remain unmodified, or be modified in a lesser
+  degree, by the effects of use and disuse.</p>
+
+  <p>In certain cases the successive steps of variation might supervene,
+  from causes of which we are wholly ignorant, at a very early period of
+  life, or each step might be inherited at an earlier period than that at
+  which it first appeared. In either case (as with the short-faced tumbler)
+  the young or embryo would closely resemble the mature parent-form. We
+  have seen that this is the rule of development in certain whole groups of
+  animals, as with cuttle-fish and spiders, and with a few members of the
+  great class of insects, as with Aphis. With respect to the final cause of
+  the young in these cases not undergoing any metamorphosis, or closely
+  resembling their parents from their earliest age, we can see that this
+  would result from the two following contingencies: firstly, from the
+  young, during a course of modification carried on for many generations,
+  having to provide for their own wants at a very early stage <!-- Page 448
+  --><span class="pagenum"><a name="page448"></a>[448]</span>of
+  development, and secondly, from their following exactly the same habits
+  of life with their parents; for in this case, it would be indispensable
+  for the existence of the species, that the child should be modified at a
+  very early age in the same manner with its parents, in accordance with
+  their similar habits. Some further explanation, however, of the embryo
+  not undergoing any metamorphosis is perhaps requisite. If, on the other
+  hand, it profited the young to follow habits of life in any degree
+  different from those of their parent, and consequently to be constructed
+  in a slightly different manner, then, on the principle of inheritance at
+  corresponding ages, the active young or larvæ might easily be rendered by
+  natural selection different to any conceivable extent from their parents.
+  Such differences might, also, become correlated with successive stages of
+  development; so that the larvæ, in the first stage, might differ greatly
+  from the larvæ in the second stage, as we have seen to be the case with
+  cirripedes. The adult might become fitted for sites or habits, in which
+  organs of locomotion or of the senses, &amp;c., would be useless; and in
+  this case the final metamorphosis would be said to be retrograde.</p>
+
+  <p>As all the organic beings, extinct and recent, which have ever lived
+  on this earth have to be classed together, and as all have been connected
+  by the finest gradations, the best, or indeed, if our collections were
+  nearly perfect, the only possible arrangement, would be genealogical.
+  Descent being on my view the hidden bond of connexion which naturalists
+  have been seeking under the term of the natural system. On this view we
+  can understand how it is that, in the eyes of most naturalists, the
+  structure of the embryo is even more important for classification than
+  that of the adult. For the embryo is the animal in its less modified
+  state; <!-- Page 449 --><span class="pagenum"><a
+  name="page449"></a>[449]</span>and in so far it reveals the structure of
+  its progenitor. In two groups of animals, however much they may at
+  present differ from each other in structure and habits, if they pass
+  through the same or similar embryonic stages, we may feel assured that
+  they have both descended from the same or nearly similar parents, and are
+  therefore in that degree closely related. Thus, community in embryonic
+  structure reveals community of descent. It will reveal this community of
+  descent, however much the structure of the adult may have been modified
+  and obscured; we have seen, for instance, that cirripedes can at once be
+  recognised by their larvæ as belonging to the great class of crustaceans.
+  As the embryonic state of each species and group of species partially
+  shows us the structure of their less modified ancient progenitors, we can
+  clearly see why ancient and extinct forms of life should resemble the
+  embryos of their descendants,&mdash;our existing species. Agassiz
+  believes this to be a law of nature; but I am bound to confess that I
+  only hope to see the law hereafter proved true. It can be proved true in
+  those cases alone in which the ancient state, now supposed to be
+  represented in existing embryos, has not been obliterated, either by the
+  successive variations in a long course of modification having supervened
+  at a very early age, or by the variations having been inherited at an
+  earlier period than that at which they first appeared. It should also be
+  borne in mind, that the supposed law of resemblance of ancient forms of
+  life to the embryonic stages of recent forms, may be true, but yet, owing
+  to the geological record not extending far enough back in time, may
+  remain for a long period, or for ever, incapable of demonstration.</p>
+
+  <p>Thus, as it seems to me, the leading facts in embryology, which are
+  second in importance to none in natural history, are explained on the
+  principle of slight <!-- Page 450 --><span class="pagenum"><a
+  name="page450"></a>[450]</span>modifications not appearing, in the many
+  descendants from some one ancient progenitor, at a very early period in
+  the life of each, though perhaps caused at the earliest, and being
+  inherited at a corresponding not early period. Embryology rises greatly
+  in interest, when we thus look at the embryo as a picture, more or less
+  obscured, of the common parent-form of each great class of animals.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Rudimentary, atrophied, or aborted Organs.</i>&mdash;Organs or
+  parts in this strange condition, bearing the stamp of inutility, are
+  extremely common throughout nature. For instance, rudimentary mammæ are
+  very general in the males of mammals: I presume that the "bastard-wing"
+  in birds may be safely considered as a digit in a rudimentary state: in
+  very many snakes one lobe of the lungs is rudimentary; in other snakes
+  there are rudiments of the pelvis and hind limbs. Some of the cases of
+  rudimentary organs are extremely curious; for instance, the presence of
+  teeth in f&oelig;tal whales, which when grown up have not a tooth in
+  their heads; and the presence of teeth, which never cut through the gums,
+  in the upper jaws of our unborn calves. It has even been stated on good
+  authority that rudiments of teeth can be detected in the beaks of certain
+  embryonic birds. Nothing can be plainer than that wings are formed for
+  flight, yet in how many insects do we see wings so reduced in size as to
+  be utterly incapable of flight, and not rarely lying under wing-cases,
+  firmly soldered together!</p>
+
+  <p>The meaning of rudimentary organs is often quite unmistakeable: for
+  instance there are beetles of the same genus (and even of the same
+  species) resembling each other most closely in all respects, one of which
+  will have full-sized wings, and another mere rudiments of membrane; and
+  here it is impossible to doubt, that the <!-- Page 451 --><span
+  class="pagenum"><a name="page451"></a>[451]</span>rudiments represent
+  wings. Rudimentary organs sometimes retain their potentiality, and are
+  merely not developed: this seems to be the case with the mammæ of male
+  mammals, for many instances are on record of these organs having become
+  well developed in full-grown males, and having secreted milk. So again
+  there are normally four developed and two rudimentary teats in the udders
+  of the genus Bos, but in our domestic cows the two sometimes become
+  developed and give milk. In plants of the same species the petals
+  sometimes occur as mere rudiments, and sometimes in a well-developed
+  state. In plants with separated sexes, the male flowers often have a
+  rudiment of a pistil; and Kölreuter found that by crossing such male
+  plants with an hermaphrodite species, the rudiment of the pistil in the
+  hybrid offspring was much increased in size; and this shows that the
+  rudiment and the perfect pistil are essentially alike in nature.</p>
+
+  <p>An organ serving for two purposes, may become rudimentary or utterly
+  aborted for one, even the more important purpose; and remain perfectly
+  efficient for the other. Thus in plants, the office of the pistil is to
+  allow the pollen-tubes to reach the ovules protected in the ovarium at
+  its base. The pistil consists of a stigma supported on the style; but in
+  some Compositæ, the male florets, which of course cannot be fecundated,
+  have a pistil, which is in a rudimentary state, for it is not crowned
+  with a stigma; but the style remains well developed, and is clothed with
+  hairs as in other compositæ, for the purpose of brushing the pollen out
+  of the surrounding anthers. Again, an organ may become rudimentary for
+  its proper purpose, and be used for a distinct object: in certain fish
+  the swim-bladder seems to be nearly rudimentary for its proper function
+  of giving buoyancy, but has become converted into a <!-- Page 452
+  --><span class="pagenum"><a name="page452"></a>[452]</span>nascent
+  breathing organ or lung. Other similar instances could be given.</p>
+
+  <p>Organs, however little developed, if of use, should not be called
+  rudimentary; they cannot properly be said to be in an atrophied
+  condition; they may be called nascent, and may hereafter be developed to
+  any extent by natural selection. Rudimentary organs, on the other hand,
+  are essentially useless, as teeth which never cut through the gums; in a
+  still less developed condition, they would be of still less use. They
+  cannot, therefore, under their present condition, have been formed by
+  natural selection, which acts solely by the preservation of useful
+  modifications; they have been retained, as we shall see, by inheritance,
+  and relate to a former condition of their possessor. It is difficult to
+  know what are nascent organs; looking to the future, we cannot of course
+  tell how any part will be developed, and whether it is now nascent;
+  looking to the past, creatures with an organ in a nascent condition will
+  generally have been supplanted and exterminated by their successors with
+  the organ in a more perfect and developed condition. The wing of the
+  penguin is of high service, and acts as a fin; it may, therefore,
+  represent the nascent state of the wings of birds; not that I believe
+  this to be the case, it is more probably a reduced organ, modified for a
+  new function: the wing of the Apteryx is useless, and is truly
+  rudimentary. The mammary glands of the Ornithorhynchus may, perhaps, be
+  considered, in comparison with the udder of a cow, as in a nascent state.
+  The ovigerous frena of certain cirripedes, which are only slightly
+  developed and which have ceased to give attachment to the ova, are
+  nascent branchiæ.</p>
+
+  <p>Rudimentary organs in the individuals of the same species are very
+  liable to vary in degree of development <!-- Page 453 --><span
+  class="pagenum"><a name="page453"></a>[453]</span>and in other respects.
+  Moreover, in closely allied species, the degree to which the same organ
+  has been rendered rudimentary occasionally differs much. This latter fact
+  is well exemplified in the state of the wings of the female moths in
+  certain groups. Rudimentary organs may be utterly aborted; and this
+  implies, that we find in an animal or plant no trace of an organ, which
+  analogy would lead us to expect to find, and which is occasionally found
+  in monstrous individuals of the species. Thus in the snapdragon
+  (antirrhinum) we generally do not find a rudiment of a fifth stamen; but
+  this may sometimes be seen. In tracing the homologies of the same part in
+  different members of a class, nothing is more common, or more necessary,
+  than the use and discovery of rudiments. This is well shown in the
+  drawings given by Owen of the bones of the leg of the horse, ox, and
+  rhinoceros.</p>
+
+  <p>It is an important fact that rudimentary organs, such as teeth in the
+  upper jaws of whales and ruminants, can often be detected in the embryo,
+  but afterwards wholly disappear. It is also, I believe, a universal rule,
+  that a rudimentary part or organ is of greater size relatively to the
+  adjoining parts in the embryo, than in the adult; so that the organ at
+  this early age is less rudimentary, or even cannot be said to be in any
+  degree rudimentary. Hence, also, a rudimentary organ in the adult is
+  often said to have retained its embryonic condition.</p>
+
+  <p>I have now given the leading facts with respect to rudimentary organs.
+  In reflecting on them, every one must be struck with astonishment: for
+  the same reasoning power which tells us plainly that most parts and
+  organs are exquisitely adapted for certain purposes, tells us with equal
+  plainness that these rudimentary or atrophied organs, are imperfect and
+  useless. In works <!-- Page 454 --><span class="pagenum"><a
+  name="page454"></a>[454]</span>on natural history rudimentary organs are
+  generally said to have been created "for the sake of symmetry," or in
+  order "to complete the scheme of nature;" but this seems to me no
+  explanation, merely a re-statement of the fact. Would it be thought
+  sufficient to say that because planets revolve in elliptic courses round
+  the sun, satellites follow the same course round the planets, for the
+  sake of symmetry, and to complete the scheme of nature? An eminent
+  physiologist accounts for the presence of rudimentary organs, by
+  supposing that they serve to excrete matter in excess, or injurious to
+  the system; but can we suppose that the minute papilla, which often
+  represents the pistil in male flowers, and which is formed merely of
+  cellular tissue, can thus act? Can we suppose that the formation of
+  rudimentary teeth, which are subsequently absorbed, can be of any service
+  to the rapidly growing embryonic calf by the excretion of precious
+  phosphate of lime? When a man's fingers have been amputated, imperfect
+  nails sometimes appear on the stumps: I could as soon believe that these
+  vestiges of nails have appeared, not from unknown laws of growth, but in
+  order to excrete horny matter, as that the rudimentary nails on the fin
+  of the manatee were formed for this purpose.</p>
+
+  <p>On my view of descent with modification, the origin of rudimentary
+  organs is simple. We have plenty of cases of rudimentary organs in our
+  domestic productions,&mdash;as the stump of a tail in tailless
+  breeds,&mdash;the vestige of an ear in earless breeds,&mdash;the
+  reappearance of minute dangling horns in hornless breeds of cattle, more
+  especially, according to Youatt, in young animals,&mdash;and the state of
+  the whole flower in the cauliflower. We often see rudiments of various
+  parts in monsters. But I doubt whether any of these cases throw light on
+  the origin of rudimentary organs in a state of nature, <!-- Page 455
+  --><span class="pagenum"><a name="page455"></a>[455]</span>further than
+  by showing that rudiments can be produced; for I doubt whether species
+  under nature ever undergo abrupt changes. I believe that disuse has been
+  the main agency; that it has led in successive generations to the gradual
+  reduction of various organs, until they have become rudimentary,&mdash;as
+  in the case of the eyes of animals inhabiting dark caverns, and of the
+  wings of birds inhabiting oceanic islands, which have seldom been forced
+  to take flight, and have ultimately lost the power of flying. Again, an
+  organ useful under certain conditions, might become injurious under
+  others, as with the wings of beetles living on small and exposed islands;
+  and in this case natural selection would continue slowly to reduce the
+  organ, until it was rendered harmless and rudimentary.</p>
+
+  <p>Any change in function, which can be effected by insensibly small
+  steps, is within the power of natural selection; so that an organ
+  rendered, during changed habits of life, useless or injurious for one
+  purpose, might be modified and used for another purpose. Or an organ
+  might be retained for one alone of its former functions. An organ, when
+  rendered useless, may well be variable, for its variations cannot be
+  checked by natural selection. At whatever period of life disuse or
+  selection reduces an organ, and this will generally be when the being has
+  come to maturity and to its full powers of action, the principle of
+  inheritance at corresponding ages will reproduce the organ in its reduced
+  state at the same age, and consequently will seldom affect or reduce it
+  in the embryo. Thus we can understand the greater relative size of
+  rudimentary organs in the embryo, and their lesser relative size in the
+  adult. But if each step of the process of reduction were to be inherited,
+  not at the corresponding age, but at an extremely early period of life
+  (as we have good <!-- Page 456 --><span class="pagenum"><a
+  name="page456"></a>[456]</span>reason to believe to be possible), the
+  rudimentary part would tend to be wholly lost, and we should have a case
+  of complete abortion. The principle, also, of economy, explained in a
+  former chapter, by which the materials forming any part or structure, if
+  not useful to the possessor, will be saved as far as is possible, will
+  probably often come into play; and this will tend to cause the entire
+  obliteration of a rudimentary organ.</p>
+
+  <p>As the presence of rudimentary organs is thus due to the tendency in
+  every part of the organisation, which has long existed, to be
+  inherited&mdash;we can understand, on the genealogical view of
+  classification, how it is that systematists have found rudimentary parts
+  as useful as, or even sometimes more useful than, parts of high
+  physiological importance. Rudimentary organs may be compared with the
+  letters in a word, still retained in the spelling, but become useless in
+  the pronunciation, but which serve as a clue in seeking for its
+  derivation. On the view of descent with modification, we may conclude
+  that the existence of organs in a rudimentary, imperfect, and useless
+  condition, or quite aborted, far from presenting a strange difficulty, as
+  they assuredly do on the ordinary doctrine of creation, might even have
+  been anticipated, and can be accounted for by the laws of
+  inheritance.</p>
+
+  <p>&nbsp;</p>
+
+  <p><i>Summary.</i>&mdash;In this chapter I have attempted to show, that
+  the subordination of group to group in all organisms throughout all time;
+  that the nature of the relationship, by which all living and extinct
+  beings are united by complex, radiating, and circuitous lines of
+  affinities into one grand system; the rules followed and the difficulties
+  encountered by naturalists in their classifications; the value set upon
+  characters, if constant and prevalent, whether of high vital importance,
+  or of the most trifling <!-- Page 457 --><span class="pagenum"><a
+  name="page457"></a>[457]</span>importance, or, as in rudimentary organs,
+  of no importance; the wide opposition in value between analogical or
+  adaptive characters, and characters of true affinity; and other such
+  rules;&mdash;all naturally follow on the view of the common parentage of
+  those forms which are considered by naturalists as allied, together with
+  their modification through natural selection, with its contingencies of
+  extinction and divergence of character. In considering this view of
+  classification, it should be borne in mind that the element of descent
+  has been universally used in ranking together the sexes, ages, and
+  acknowledged varieties of the same species, however different they may be
+  in structure. If we extend the use of this element of descent,&mdash;the
+  only certainly known cause of similarity in organic beings,&mdash;we
+  shall understand what is meant by the natural system: it is genealogical
+  in its attempted arrangement, with the grades of acquired difference
+  marked by the terms varieties, species, genera, families, orders, and
+  classes.</p>
+
+  <p>On this same view of descent with modification, all the great facts in
+  Morphology become intelligible,&mdash;whether we look to the same pattern
+  displayed in the homologous organs, to whatever purpose applied, of the
+  different species of a class; or to the homologous parts constructed on
+  the same pattern in each individual animal and plant.</p>
+
+  <p>On the principle of successive slight variations, not necessarily or
+  generally supervening at a very early period of life, and being inherited
+  at a corresponding period, we can understand the great leading facts in
+  Embryology; namely, the resemblance in an individual embryo of the
+  homologous parts, which when matured will become widely different from
+  each other in structure and function; and the resemblance in different
+  species of a class of the homologous parts or <!-- Page 458 --><span
+  class="pagenum"><a name="page458"></a>[458]</span>organs, though fitted
+  in the adult members for purposes as different as possible. Larvæ are
+  active embryos, which have become specially modified in relation to their
+  habits of life, through the principle of modifications being inherited at
+  corresponding ages. On this same principle&mdash;and bearing in mind,
+  that when organs are reduced in size, either from disuse or selection, it
+  will generally be at that period of life when the being has to provide
+  for its own wants, and bearing in mind how strong is the principle of
+  inheritance&mdash;the occurrence of rudimentary organs and their final
+  abortion, present to us no inexplicable difficulties; on the contrary,
+  their presence might have been even anticipated. The importance of
+  embryological characters and of rudimentary organs in classification is
+  intelligible, on the view that an arrangement is only so far natural as
+  it is genealogical.</p>
+
+  <p>Finally, the several classes of facts which have been considered in
+  this chapter, seem to me to proclaim so plainly, that the <span
+  class="correction" title="text reads `inumerable'">innumerable</span>
+  species, genera, and families of organic beings, with which this world is
+  peopled, have all descended, each within its own class or group, from
+  common parents, and have all been modified in the course of descent, that
+  I should without hesitation adopt this view, even if it were unsupported
+  by other facts or arguments.</p>
+
+<hr />
+
+<p><!-- Page 459 --><span class="pagenum"><a name="page459"></a>[459]</span></p>
+
+<h3>CHAPTER XIV.</h3>
+
+<p class="cenhead"><span class="sc">Recapitulation and Conclusion.</span></p>
+
+<blockquote class="b1n">
+
+  <p>Recapitulation of the difficulties on the theory of Natural
+  Selection&mdash;Recapitulation of the general and special circumstances
+  in its favour&mdash;Causes of the general belief in the immutability of
+  species&mdash;How far the theory of natural selection may be
+  extended&mdash;Effects of its adoption on the study of Natural
+  history&mdash;Concluding remarks.</p>
+
+</blockquote>
+
+  <p>As this whole volume is one long argument, it may be convenient to the
+  reader to have the leading facts and inferences briefly
+  recapitulated.</p>
+
+  <p>That many and serious objections may be advanced against the theory of
+  descent with modification through natural selection, I do not deny. I
+  have endeavoured to give to them their full force. Nothing at first can
+  appear more difficult to believe than that the more complex organs and
+  instincts should have been perfected, not by means superior to, though
+  analogous with, human reason, but by the accumulation of innumerable
+  slight variations, each good for the individual possessor. Nevertheless,
+  this difficulty, though appearing to our imagination insuperably great,
+  cannot be considered real if we admit the following propositions,
+  namely,&mdash;that gradations in the perfection of any organ or instinct
+  which we may consider, either do now exist or could have existed, each
+  good of its kind,&mdash;that all organs and instincts are, in ever so
+  slight a degree, variable,&mdash;and, lastly, that there is a struggle
+  for existence leading to the preservation of each profitable deviation of
+  structure or instinct. The truth of these propositions cannot, I think,
+  be disputed. <!-- Page 460 --><span class="pagenum"><a
+  name="page460"></a>[460]</span></p>
+
+  <p>It is, no doubt, extremely difficult even to conjecture by what
+  gradations many structures have been perfected, more especially amongst
+  broken and failing groups of organic beings; but we see so many strange
+  gradations in nature, that we ought to be extremely cautious in saying
+  that any organ or instinct, or any whole being, could not have arrived at
+  its present state by many graduated steps. There are, it must be
+  admitted, cases of special difficulty on the theory of natural selection;
+  and one of the most curious of these is the existence of two or three
+  defined castes of workers or sterile females in the same community of
+  ants; but I have attempted to show how this difficulty can be
+  mastered.</p>
+
+  <p>With respect to the almost universal sterility of species when first
+  crossed, which forms so remarkable a contrast with the almost universal
+  fertility of varieties when crossed, I must refer the reader to the
+  recapitulation of the facts given at the end of the eighth chapter, which
+  seem to me conclusively to show that this sterility is no more a special
+  endowment than is the incapacity of two trees to be grafted together; but
+  that it is incidental on constitutional differences in the reproductive
+  systems of the intercrossed species. We see the truth of this conclusion
+  in the vast difference in the result, when the same two species are
+  crossed reciprocally; that is, when one species is first used as the
+  father and then as the mother.</p>
+
+  <p>The fertility of varieties when intercrossed and of their mongrel
+  offspring cannot be considered as universal; nor is their very general
+  fertility surprising when we remember that it is not likely that either
+  their constitutions or their reproductive systems should have been
+  profoundly modified. Moreover, most of the varieties which have been
+  experimentised on have been <!-- Page 461 --><span class="pagenum"><a
+  name="page461"></a>[461]</span>produced under domestication; and as
+  domestication (I do not mean mere confinement) apparently tends to
+  eliminate sterility, we ought not to expect it also to produce
+  sterility.</p>
+
+  <p>The sterility of hybrids is a very different case from that of first
+  crosses, for their reproductive organs are more or less functionally
+  impotent; whereas in first crosses the organs on both sides are in a
+  perfect condition. As we continually see that organisms of all kinds are
+  rendered in some degree sterile from their constitutions having been
+  disturbed by slightly different and new conditions of life, we need not
+  feel surprise at hybrids being in some degree sterile, for their
+  constitutions can hardly fail to have been disturbed from being
+  compounded of two distinct organisations. This parallelism is supported
+  by another parallel, but directly opposite, class of facts; namely, that
+  the vigour and fertility of all organic beings are increased by slight
+  changes in their conditions of life, and that the offspring of slightly
+  modified forms or varieties acquire from being crossed increased vigour
+  and fertility. So that, on the one hand, considerable changes in the
+  conditions of life and crosses between greatly modified forms, lessen
+  fertility; and on the other hand, lesser changes in the conditions of
+  life and crosses between less modified forms, increase fertility.</p>
+
+  <p>Turning to geographical distribution, the difficulties encountered on
+  the theory of descent with modification are grave enough. All the
+  individuals of the same species, and all the species of the same genus,
+  or even higher group, must have descended from common parents; and
+  therefore, in however distant and isolated parts of the world they are
+  now found, they must in the course of successive generations have passed
+  from some one part to the others. We are often wholly unable <!-- Page
+  462 --><span class="pagenum"><a name="page462"></a>[462]</span>even to
+  conjecture how this could have been effected. Yet, as we have reason to
+  believe that some species have retained the same specific form for very
+  long periods, enormously long as measured by years, too much stress ought
+  not to be laid on the occasional wide diffusion of the same species; for
+  during very long periods of time there will always have been a good
+  chance for wide migration by many means. A broken or interrupted range
+  may often be accounted for by the extinction of the species in the
+  intermediate regions. It cannot be denied that we are as yet very
+  ignorant of the full extent of the various climatal and geographical
+  changes which have affected the earth during modern periods; and such
+  changes will obviously have greatly facilitated migration. As an example,
+  I have attempted to show how potent has been the influence of the Glacial
+  period on the distribution both of the same and of representative species
+  throughout the world. We are as yet profoundly ignorant of the many
+  occasional means of transport. With respect to distinct species of the
+  same genus inhabiting very distant and isolated regions, as the process
+  of modification has necessarily been slow, all the means of migration
+  will have been possible during a very long period; and consequently the
+  difficulty of the wide diffusion of species of the same genus is in some
+  degree lessened.</p>
+
+  <p>As on the theory of natural selection an interminable number of
+  intermediate forms must have existed, linking together all the species in
+  each group by gradations as fine as our present varieties, it may be
+  asked, Why do we not see these linking forms all around us? Why are not
+  all organic beings blended together in an inextricable chaos? With
+  respect to existing forms, we should remember that we have no right to
+  expect (excepting in rare cases) to discover <i>directly</i> connecting
+  <!-- Page 463 --><span class="pagenum"><a
+  name="page463"></a>[463]</span>links between them, but only between each
+  and some extinct and supplanted form. Even on a wide area, which has
+  during a long period remained continuous, and of which the climate and
+  other conditions of life change insensibly in going from a district
+  occupied by one species into another district occupied by a closely
+  allied species, we have no just right to expect often to find
+  intermediate varieties in the intermediate zone. For we have reason to
+  believe that only a few species are undergoing change at any one period;
+  and all changes are slowly effected. I have also shown that the
+  intermediate varieties which will at first probably exist in the
+  intermediate zones, will be liable to be supplanted by the allied forms
+  on either hand; and the latter, from existing in greater numbers, will
+  generally be modified and improved at a quicker rate than the
+  intermediate varieties, which exist in lesser numbers; so that the
+  intermediate varieties will, in the long run, be supplanted and
+  exterminated.</p>
+
+  <p>On this doctrine of the extermination of an infinitude of connecting
+  links, between the living and extinct inhabitants of the world, and at
+  each successive period between the extinct and still older species, why
+  is not every geological formation charged with such links? Why does not
+  every collection of fossil remains afford plain evidence of the gradation
+  and mutation of the forms of life? We meet with no such evidence, and
+  this is the most obvious and forcible of the many objections which may be
+  urged against my theory. Why, again, do whole groups of allied species
+  appear, though certainly they often falsely appear, to have come in
+  suddenly on the several geological stages? Why do we not find great piles
+  of strata beneath the Silurian system, stored with the remains of the
+  progenitors of the Silurian groups of fossils? For certainly on my theory
+  such <!-- Page 464 --><span class="pagenum"><a
+  name="page464"></a>[464]</span>strata must somewhere have been deposited
+  at these ancient and utterly unknown epochs in the world's history.</p>
+
+  <p>I can answer these questions and grave objections only on the
+  supposition that the geological record is far more imperfect than most
+  geologists believe. It cannot be objected that there has not been time
+  sufficient for any amount of organic change; for the lapse of time has
+  been so great as to be utterly inappreciable by the human intellect. The
+  number of specimens in all our museums is absolutely as nothing compared
+  with the countless generations of countless species which certainly have
+  existed. We should not be able to recognise a species as the parent of
+  any one or more species if we were to examine them ever so closely,
+  unless we likewise possessed many of the intermediate links between their
+  past or parent and present states; and these many links we could hardly
+  ever expect to discover, owing to the imperfection of the geological
+  record. Numerous existing doubtful forms could be named which are
+  probably varieties; but who will pretend that in future ages so many
+  fossil links will be discovered, that naturalists will be able to decide,
+  on the common view, whether or not these doubtful forms are varieties? As
+  long as most of the links between any two species are unknown, if any one
+  link or intermediate variety be discovered, it will simply be classed as
+  another and distinct species. Only a small portion of the world has been
+  geologically explored. Only organic beings of certain classes can be
+  preserved in a fossil condition, at least in any great number. Widely
+  ranging species vary most, and varieties are often at first
+  local,&mdash;both causes rendering the discovery of intermediate links
+  less likely. Local varieties will not spread into other and distant
+  regions until they are considerably modified and <!-- Page 465 --><span
+  class="pagenum"><a name="page465"></a>[465]</span>improved; and when they
+  do spread, if discovered in a geological formation, they will appear as
+  if suddenly created there, and will be simply classed as new species.
+  Most formations have been intermittent in their accumulation; and their
+  duration, I am inclined to believe, has been shorter than the average
+  duration of specific forms. Successive formations are separated from each
+  other by enormous blank intervals of time; for fossiliferous formations,
+  thick enough to resist future degradation, can be accumulated only where
+  much sediment is deposited on the subsiding bed of the sea. During the
+  alternate periods of elevation and of stationary level the record will be
+  blank. During these latter periods there will probably be more
+  variability in the forms of life; during periods of subsidence, more
+  extinction.</p>
+
+  <p>With respect to the absence of fossiliferous formations beneath the
+  lowest Silurian strata, I can only recur to the hypothesis given in the
+  ninth chapter. That the geological record is imperfect all will admit;
+  but that it is imperfect to the degree which I require, few will be
+  inclined to admit. If we look to long enough intervals of time, geology
+  plainly declares that all species have changed; and they have changed in
+  the manner which my theory requires, for they have changed slowly and in
+  a graduated manner. We clearly see this in the fossil remains from
+  consecutive formations invariably being much more closely related to each
+  other, than are the fossils from formations distant from each other in
+  time.</p>
+
+  <p>Such is the sum of the several chief objections and difficulties which
+  may justly be urged against my theory; and I have now briefly
+  recapitulated the answers and explanations which can be given to them. I
+  have felt these difficulties far too heavily during many years to <!--
+  Page 466 --><span class="pagenum"><a name="page466"></a>[466]</span>doubt
+  their weight. But it deserves especial notice that the more important
+  objections relate to questions on which we are confessedly ignorant; nor
+  do we know how ignorant we are. We do not know all the possible
+  transitional gradations between the simplest and the most perfect organs;
+  it cannot be pretended that we know all the varied means of Distribution
+  during the long lapse of years, or that we know how imperfect the
+  Geological Record is. Grave as these several difficulties are, in my
+  judgment they do not overthrow the theory of descent from a few created
+  forms with subsequent modification.</p>
+
+  <p>&nbsp;</p>
+
+  <p>Now let us turn to the other side of the argument. Under domestication
+  we see much variability. This seems to be mainly due to the reproductive
+  system being eminently susceptible to changes in the conditions of life;
+  so that this system, when not rendered impotent, fails to reproduce
+  offspring exactly like the parent-form. Variability is governed by many
+  complex laws,&mdash;by correlation of growth, by use and disuse, and by
+  the direct action of the physical conditions of life. There is much
+  difficulty in ascertaining how much modification our domestic productions
+  have undergone; but we may safely infer that the amount has been large,
+  and that modifications can be inherited for long periods. As long as the
+  conditions of life remain the same, we have reason to believe that a
+  modification, which has already been inherited for many generations, may
+  continue to be inherited for an almost infinite number of generations. On
+  the other hand we have evidence that variability, when it has once come
+  into play, does not wholly cease; for new varieties are still
+  occasionally produced by our most anciently domesticated productions.
+  <!-- Page 467 --><span class="pagenum"><a
+  name="page467"></a>[467]</span></p>
+
+  <p>Man does not actually produce variability; he only unintentionally
+  exposes organic beings to new conditions of life, and then nature acts on
+  the organisation, and causes variability. But man can and does select the
+  variations given to him by nature, and thus accumulate them in any
+  desired manner. He thus adapts animals and plants for his own benefit or
+  pleasure. He may do this methodically, or he may do it unconsciously by
+  preserving the individuals most useful to him at the time, without any
+  thought of altering the breed. It is certain that he can largely
+  influence the character of a breed by selecting, in each successive
+  generation, individual differences so slight as to be quite inappreciable
+  by an uneducated eye. This process of selection has been the great agency
+  in the production of the most distinct and useful domestic breeds. That
+  many of the breeds produced by man have to a large extent the character
+  of natural species, is shown by the inextricable doubts whether very many
+  of them are varieties or aboriginal species.</p>
+
+  <p>There is no obvious reason why the principles which have acted so
+  efficiently under domestication should not have acted under nature. In
+  the preservation of favoured individuals and races, during the
+  constantly-recurrent Struggle for Existence, we see the most powerful and
+  ever-acting means of selection. The struggle for existence inevitably
+  follows from the high geometrical ratio of increase which is common to
+  all organic beings. This high rate of increase is proved by
+  calculation,&mdash;by the rapid increase of many animals and plants
+  during a succession of peculiar seasons, or when naturalised in a new
+  country. More individuals are born than can possibly survive. A grain in
+  the balance will determine which individual shall live and which shall
+  die,&mdash;which variety or species shall increase in number, and which
+  <!-- Page 468 --><span class="pagenum"><a
+  name="page468"></a>[468]</span>shall decrease, or finally become extinct.
+  As the individuals of the same species come in all respects into the
+  closest competition with each other, the struggle will generally be most
+  severe between them; it will be almost equally severe between the
+  varieties of the same species, and next in severity between the species
+  of the same genus. But the struggle will often be very severe between
+  beings most remote in the scale of nature. The slightest advantage in one
+  being, at any age or during any season, over those with which it comes
+  into competition, or better adaptation in however slight a degree to the
+  surrounding physical conditions, will turn the balance.</p>
+
+  <p>With animals having separated sexes there will in most cases be a
+  struggle between the males for possession of the females. The most
+  vigorous individuals, or those which have most successfully struggled
+  with their conditions of life, will generally leave most progeny. But
+  success will often depend on having special weapons or means of defence,
+  or on the charms of the males; and the slightest advantage will lead to
+  victory.</p>
+
+  <p>As geology plainly proclaims that each land has undergone great
+  physical changes, we might have expected that organic beings would have
+  varied under nature, in the same way as they generally have varied under
+  the changed conditions of domestication. And if there be any variability
+  under nature, it would be an unaccountable fact if natural selection had
+  not come into play. It has often been asserted, but the assertion is
+  quite incapable of proof, that the amount of variation under nature is a
+  strictly limited quantity. Man, though acting on external characters
+  alone and often capriciously, can produce within a short period a great
+  result by adding up mere individual differences in his domestic
+  productions; and every one admits that there are at least individual
+  differences in species under <!-- Page 469 --><span class="pagenum"><a
+  name="page469"></a>[469]</span>nature. But, besides such differences, all
+  naturalists have admitted the existence of varieties, which they think
+  sufficiently distinct to be worthy of record in systematic works. No one
+  can draw any clear distinction between individual differences and slight
+  varieties; or between more plainly marked varieties and sub-species, and
+  species. Let it be observed how naturalists differ in the rank which they
+  assign to the many representative forms in Europe and North America.</p>
+
+  <p>If then we have under nature variability and a powerful agent always
+  ready to act and select, why should we doubt that variations in any way
+  useful to beings, under their excessively complex relations of life,
+  would be preserved, accumulated, and inherited? Why, if man can by
+  patience select variations most useful to himself, should nature fail in
+  selecting variations useful, under changing conditions of life, to her
+  living products? What limit can be put to this power, acting during long
+  ages and rigidly scrutinising the whole constitution, structure, and
+  habits of each creature,&mdash;favouring the good and rejecting the bad?
+  I can see no limit to this power, in slowly and beautifully adapting each
+  form to the most complex relations of life. The theory of natural
+  selection, even if we looked no further than this, seems to me to be in
+  itself probable. I have already recapitulated, as fairly as I could, the
+  opposed difficulties and objections: now let us turn to the special facts
+  and arguments in favour of the theory.</p>
+
+  <p>On the view that species are only strongly marked and permanent
+  varieties, and that each species first existed as a variety, we can see
+  why it is that no line of demarcation can be drawn between species,
+  commonly supposed to have been produced by special acts of creation, and
+  varieties which are acknowledged to have been produced by secondary laws.
+  On this same <!-- Page 470 --><span class="pagenum"><a
+  name="page470"></a>[470]</span>view we can understand how it is that in
+  each region where many species of a genus have been produced, and where
+  they now flourish, these same species should present many varieties; for
+  where the manufactory of species has been active, we might expect, as a
+  general rule, to find it still in action; and this is the case if
+  varieties be incipient species. Moreover, the species of the larger
+  genera, which afford the greater number of varieties or incipient
+  species, retain to a certain degree the character of varieties; for they
+  differ from each other by a less amount of difference than do the species
+  of smaller genera. The closely allied species also of the larger genera
+  apparently have restricted ranges, and in their affinities they are
+  clustered in little groups round other species&mdash;in which respects
+  they resemble varieties. These are strange relations on the view of each
+  species having been independently created, but are intelligible if all
+  species first existed as varieties.</p>
+
+  <p>As each species tends by its geometrical ratio of reproduction to
+  increase inordinately in number; and as the modified descendants of each
+  species will be enabled to increase by so much the more as they become
+  diversified in habits and structure, so as to be enabled to seize on many
+  and widely different places in the economy of nature, there will be a
+  constant tendency in natural selection to preserve the most divergent
+  offspring of any one species. Hence during a long-continued course of
+  modification, the slight differences, characteristic of varieties of the
+  same species, tend to be augmented into the greater differences
+  characteristic of species of the same genus. New and improved varieties
+  will inevitably supplant and exterminate the older, less improved and
+  intermediate varieties; and thus species are rendered to a large extent
+  defined and distinct objects. Dominant species belonging to the <!-- Page
+  471 --><span class="pagenum"><a name="page471"></a>[471]</span>larger
+  groups tend to give birth to new and dominant forms; so that each large
+  group tends to become still larger, and at the same time more divergent
+  in character. But as all groups cannot thus succeed in increasing in
+  size, for the world would not hold them, the more dominant groups beat
+  the less dominant. This tendency in the large groups to go on increasing
+  in size and diverging in character, together with the almost inevitable
+  contingency of much extinction, explains the arrangement of all the forms
+  of life, in groups subordinate to groups, all within a few great classes,
+  which we now see everywhere around us, and which has prevailed throughout
+  all time. This grand fact of the grouping of all organic beings seems to
+  me utterly inexplicable on the theory of creation.</p>
+
+  <p>As natural selection acts solely by accumulating slight, successive,
+  favourable variations, it can produce no great or sudden modification; it
+  can act only by very short and slow steps. Hence the canon of "Natura non
+  facit saltum," which every fresh addition to our knowledge tends to make
+  truer, is on this theory simply intelligible. We can plainly see why
+  nature is prodigal in variety, though niggard in innovation. But why this
+  should be a law of nature if each species has been independently created,
+  no man can explain.</p>
+
+  <p>Many other facts are, as it seems to me, explicable on this theory.
+  How strange it is that a bird, under the form of woodpecker, should have
+  been created to prey on insects on the ground; that upland geese, which
+  never or rarely swim, should have been created with webbed feet; that a
+  thrush should have been created to dive and feed on sub-aquatic insects;
+  and that a petrel should have been created with habits and structure
+  fitting it for the life of an auk or grebe! and so on in endless other
+  cases. But on the view of each <!-- Page 472 --><span class="pagenum"><a
+  name="page472"></a>[472]</span>species constantly trying to increase in
+  number, with natural selection always ready to adapt the slowly varying
+  descendants of each to any unoccupied or ill-occupied place in nature,
+  these facts cease to be strange, or perhaps might even have been
+  anticipated.</p>
+
+  <p>As natural selection acts by competition, it adapts the inhabitants of
+  each country only in relation to the degree of perfection of their
+  associates; so that we need feel no surprise at the inhabitants of any
+  one country, although on the ordinary view supposed to have been
+  specially created and adapted for that country, being beaten and
+  supplanted by the naturalised productions from another land. Nor ought we
+  to marvel if all the contrivances in nature be not, as far as we can
+  judge, absolutely perfect; and if some of them be abhorrent to our ideas
+  of fitness. We need not marvel at the sting of the bee causing the bee's
+  own death; at drones being produced in such vast numbers for one single
+  act, with the great majority slaughtered by their sterile sisters; at the
+  astonishing waste of pollen by our fir-trees; at the instinctive hatred
+  of the queen bee for her own fertile daughters; at ichneumonidæ feeding
+  within the live bodies of caterpillars; and at other such cases. The
+  wonder indeed is, on the theory of natural selection, that more cases of
+  the want of absolute perfection have not been observed.</p>
+
+  <p>The complex and little known laws governing variation are the same, as
+  far as we can see, with the laws which have governed the production of
+  so-called specific forms. In both cases physical conditions seem to have
+  produced but little direct effect; yet when varieties enter any zone,
+  they occasionally assume some of the characters of the species proper to
+  that zone. In both varieties and species, use and disuse seem to have
+  produced some effect; for it is difficult to resist this <!-- Page 473
+  --><span class="pagenum"><a name="page473"></a>[473]</span>conclusion
+  when we look, for instance, at the logger-headed duck, which has wings
+  incapable of flight, in nearly the same condition as in the domestic
+  duck; or when we look at the burrowing tucutucu, which is occasionally
+  blind, and then at certain moles, which are habitually blind and have
+  their eyes covered with skin; or when we look at the blind animals
+  inhabiting the dark caves of America and Europe. In both varieties and
+  species correlation of growth seems to have played a most important part,
+  so that when one part has been modified other parts are necessarily
+  modified. In both varieties and species reversions to long-lost
+  characters occur. How inexplicable on the theory of creation is the
+  occasional appearance of stripes on the shoulder and legs of the several
+  species of the horse-genus and in their hybrids! How simply is this fact
+  explained if we believe that these species have descended from a striped
+  progenitor, in the same manner as the several domestic breeds of pigeon
+  have descended from the blue and barred rock-pigeon!</p>
+
+  <p>On the ordinary view of each species having been independently
+  created, why should the specific characters, or those by which the
+  species of the same genus differ from each other, be more variable than
+  the generic characters in which they all agree? Why, for instance, should
+  the colour of a flower be more likely to vary in any one species of a
+  genus, if the other species, supposed to have been created independently,
+  have differently coloured flowers, than if all the species of the genus
+  have the same coloured flowers? If species are only well-marked
+  varieties, of which the characters have become in a high degree
+  permanent, we can understand this fact; for they have already varied
+  since they branched off from a common progenitor in certain characters,
+  by which they have come to be specifically distinct from each other; <!--
+  Page 474 --><span class="pagenum"><a name="page474"></a>[474]</span>and
+  therefore these same characters would be more likely still to be variable
+  than the generic characters which have been inherited without change for
+  an enormous period. It is inexplicable on the theory of creation why a
+  part developed in a very unusual manner in any one species of a genus,
+  and therefore, as we may naturally infer, of great importance to the
+  species, should be eminently liable to variation; but, on my view, this
+  part has undergone, since the several species branched off from a common
+  progenitor, an unusual amount of variability and modification, and
+  therefore we might expect this part generally to be still variable. But a
+  part may be developed in the most unusual manner, like the wing of a bat,
+  and yet not be more variable than any other structure, if the part be
+  common to many subordinate forms, that is, if it has been inherited for a
+  very long period; for in this case it will have been rendered constant by
+  long-continued natural selection.</p>
+
+  <p>Glancing at instincts, marvellous as some are, they offer no greater
+  difficulty than does corporeal structure on the theory of the natural
+  selection of successive, slight, but profitable modifications. We can
+  thus understand why nature moves by graduated steps in endowing different
+  animals of the same class with their several instincts. I have attempted
+  to show how much light the principle of gradation throws on the admirable
+  architectural powers of the hive-bee. Habit no doubt sometimes comes into
+  play in modifying instincts; but it certainly is not indispensable, as we
+  see, in the case of neuter insects, which leave no progeny to inherit the
+  effects of long-continued habit. On the view of all the species of the
+  same genus having descended from a common parent, and having inherited
+  much in common, we can understand how it is that allied species, when
+  placed under considerably different conditions of life, <!-- Page 475
+  --><span class="pagenum"><a name="page475"></a>[475]</span>yet should
+  follow nearly the same instincts; why the thrush of South America, for
+  instance, lines her nest with mud like our British species. On the view
+  of instincts having been slowly acquired through natural selection we
+  need not marvel at some instincts being apparently not perfect and liable
+  to mistakes, and at many instincts causing other animals to suffer.</p>
+
+  <p>If species be only well-marked and permanent varieties, we can at once
+  see why their crossed offspring should follow the same complex laws in
+  their degrees and kinds of resemblance to their parents,&mdash;in being
+  absorbed into each other by successive crosses, and in other such
+  points,&mdash;as do the crossed offspring of acknowledged varieties. On
+  the other hand, these would be strange facts if species have been
+  independently created, and varieties have been produced by secondary
+  laws.</p>
+
+  <p>If we admit that the geological record is imperfect in an extreme
+  degree, then such facts as the record gives, support the theory of
+  descent with modification. New species have come on the stage slowly and
+  at successive intervals; and the amount of change, after equal intervals
+  of time, is widely different in different groups. The extinction of
+  species and of whole groups of species, which has played so conspicuous a
+  part in the history of the organic world, almost inevitably follows on
+  the principle of natural selection; for old forms will be supplanted by
+  new and improved forms. Neither single species nor groups of species
+  reappear when the chain of ordinary generation has once been broken. The
+  gradual diffusion of dominant forms, with the slow modification of their
+  descendants, causes the forms of life, after long intervals of time, to
+  appear as if they had changed simultaneously throughout the world. The
+  fact of the fossil remains of each formation being in some degree
+  intermediate in character between the <!-- Page 476 --><span
+  class="pagenum"><a name="page476"></a>[476]</span>fossils in the
+  formations above and below, is simply explained by their intermediate
+  position in the chain of descent. The grand fact that all extinct organic
+  beings belong to the same system with recent beings, falling either into
+  the same or into intermediate groups, follows from the living and the
+  extinct being the offspring of common parents. As the groups which have
+  descended from an ancient progenitor have generally diverged in
+  character, the progenitor with its early descendants will often be
+  intermediate in character in comparison with its later descendants; and
+  thus we can see why the more ancient a fossil is, the oftener it stands
+  in some degree intermediate between existing and allied groups. Recent
+  forms are generally looked at as being, in some vague sense, higher than
+  ancient and extinct forms; and they are in so far higher as the later and
+  more improved forms have conquered the older and less improved organic
+  beings in the struggle for life. Lastly, the law of the long endurance of
+  allied forms on the same continent,&mdash;of marsupials in Australia, of
+  edentata in America, and other such cases,&mdash;is intelligible, for
+  within a confined country, the recent and the extinct will naturally be
+  allied by descent.</p>
+
+  <p>Looking to geographical distribution, if we admit that there has been
+  during the long course of ages much migration from one part of the world
+  to another, owing to former climatal and geographical changes and to the
+  many occasional and unknown means of dispersal, then we can understand,
+  on the theory of descent with modification, most of the great leading
+  facts in Distribution. We can see why there should be so striking a
+  parallelism in the distribution of organic beings throughout space, and
+  in their geological succession throughout time; for in both cases the
+  beings have been connected by the bond of ordinary generation, and the
+  means of <!-- Page 477 --><span class="pagenum"><a
+  name="page477"></a>[477]</span>modification have been the same. We see
+  the full meaning of the wonderful fact, which must have struck every
+  traveller, namely, that on the same continent, under the most diverse
+  conditions, under heat and cold, on mountain and lowland, on deserts and
+  marshes, most of the inhabitants within each great class are plainly
+  related; for they will generally be descendants of the same progenitors
+  and early colonists. On this same principle of former migration, combined
+  in most cases with modification, we can understand, by the aid of the
+  Glacial period, the identity of some few plants, and the close alliance
+  of many others, on the most distant mountains, under the most different
+  climates; and likewise the close alliance of some of the inhabitants of
+  the sea in the northern and southern temperate zones, though separated by
+  the whole intertropical ocean. Although two areas may present the same
+  physical conditions of life, we need feel no surprise at their
+  inhabitants being widely different, if they have been for a long period
+  completely separated from each other; for as the relation of organism to
+  organism is the most important of all relations, and as the two areas
+  will have received colonists from some third source or from each other,
+  at various periods and in different proportions, the course of
+  modification in the two areas will inevitably be different.</p>
+
+  <p>On this view of migration, with subsequent modification, we can see
+  why oceanic islands should be inhabited by few species, but of these,
+  that many should be peculiar. We can clearly see why those animals which
+  cannot cross wide spaces of ocean, as frogs and terrestrial mammals,
+  should not inhabit oceanic islands; and why, on the other hand, new and
+  peculiar species of bats, which can traverse the ocean, should so often
+  be found on islands far distant from any continent. Such facts <!-- Page
+  478 --><span class="pagenum"><a name="page478"></a>[478]</span>as the
+  presence of peculiar species of bats, and the absence of all other
+  mammals, on oceanic islands, are utterly inexplicable on the theory of
+  independent acts of creation.</p>
+
+  <p>The existence of closely allied or representative species in any two
+  areas, implies, on the theory of descent with modification, that the same
+  parents formerly inhabited both areas; and we almost invariably find that
+  wherever many closely allied species inhabit two areas, some identical
+  species common to both still exist. Wherever many closely allied yet
+  distinct species occur, many doubtful forms and varieties of the same
+  species likewise occur. It is a rule of high generality that the
+  inhabitants of each area are related to the inhabitants of the nearest
+  source whence immigrants might have been derived. We see this in nearly
+  all the plants and animals of the Galapagos archipelago, of Juan
+  Fernandez, and of the other American islands being related in the most
+  striking manner to the plants and animals of the neighbouring American
+  mainland; and those of the Cape de Verde archipelago and other African
+  islands to the African mainland. It must be admitted that these facts
+  receive no explanation on the theory of creation.</p>
+
+  <p>The fact, as we have seen, that all past and present organic beings
+  constitute one grand natural system, with group subordinate to group, and
+  with extinct groups often falling in between recent groups, is
+  intelligible on the theory of natural selection with its contingencies of
+  extinction and divergence of character. On these same principles we see
+  how it is, that the mutual affinities of the species and genera within
+  each class are so complex and circuitous. We see why certain characters
+  are far more serviceable than others for classification;&mdash;why
+  adaptive characters, though of paramount importance to the being, are of
+  hardly any <!-- Page 479 --><span class="pagenum"><a
+  name="page479"></a>[479]</span>importance in classification; why
+  characters derived from rudimentary parts, though of no service to the
+  being, are often of high classificatory value; and why embryological
+  characters are the most valuable of all. The real affinities of all
+  organic beings are due to inheritance or community of descent. The
+  natural system is a genealogical arrangement, in which we have to
+  discover the lines of descent by the most permanent characters, however
+  slight their vital importance may be.</p>
+
+  <p>The framework of bones being the same in the hand of a man, wing of a
+  bat, fin of the porpoise, and leg of the horse,&mdash;the same number of
+  vertebræ forming the neck of the giraffe and of the elephant,&mdash;and
+  innumerable other such facts, at once explain themselves on the theory of
+  descent with slow and slight successive modifications. The similarity of
+  pattern in the wing and leg of a bat, though used for such different
+  purpose,&mdash;in the jaws and legs of a crab,&mdash;in the petals,
+  stamens, and pistils of a flower, is likewise intelligible on the view of
+  the gradual modification of parts or organs, which were alike in the
+  early progenitor of each class. On the principle of successive variations
+  not always supervening at an early age, and being inherited at a
+  corresponding not early period of life, we can clearly see why the
+  embryos of mammals, birds, reptiles, and fishes should be so closely
+  alike, and should be so unlike the adult forms. We may cease marvelling
+  at the embryo of an air-breathing mammal or bird having branchial slits
+  and arteries running in loops, like those in a fish which has to breathe
+  the air dissolved in water, by the aid of well-developed branchiæ.</p>
+
+  <p>Disuse, aided sometimes by natural selection, will often tend to
+  reduce an organ, when it has become useless by changed habits or under
+  changed conditions <!-- Page 480 --><span class="pagenum"><a
+  name="page480"></a>[480]</span>of life; and we can clearly understand on
+  this view the meaning of rudimentary organs. But disuse and selection
+  will generally act on each creature, when it has come to maturity and has
+  to play its full part in the struggle for existence, and will thus have
+  little power of acting on an organ during early life; hence the organ
+  will not be much reduced or rendered rudimentary at this early age. The
+  calf, for instance, has inherited teeth, which never cut through the gums
+  of the upper jaw, from an early progenitor having well-developed teeth;
+  and we may believe, that the teeth in the mature animal were reduced,
+  during successive generations, by disuse or by the tongue and palate
+  having been better fitted by natural selection to browse without their
+  aid; whereas in the calf, the teeth have been left untouched by selection
+  or disuse, and on the principle of inheritance at corresponding ages have
+  been inherited from a remote period to the present day. On the view of
+  each organic being and each separate organ having been specially created,
+  how utterly inexplicable it is that parts, like the teeth in the
+  embryonic calf or like the shrivelled wings under the soldered
+  wing-covers of some beetles, should thus so frequently bear the plain
+  stamp of inutility! Nature may be said to have taken pains to reveal, by
+  rudimentary organs and by homologous structures, her scheme of
+  modification, which it seems that we wilfully will not understand.</p>
+
+  <p>&nbsp;</p>
+
+  <p>I have now recapitulated the chief facts and considerations which have
+  thoroughly convinced me that species have been modified, during a long
+  course of descent, by the preservation or the natural selection of many
+  successive slight favourable variations. I cannot believe that a false
+  theory would explain, as it seems to me that the theory of natural
+  selection does explain, <!-- Page 481 --><span class="pagenum"><a
+  name="page481"></a>[481]</span>the several large classes of facts above
+  specified. I see no good reason why the views given in this volume should
+  shock the religious feelings of any one. A celebrated author and divine
+  has written to me that "he has gradually learnt to see that it is just as
+  noble a conception of the Deity to believe that He created a few original
+  forms capable of self-development into other and needful forms, as to
+  believe that He required a fresh act of creation to supply the voids
+  caused by the action of His laws."</p>
+
+  <p>Why, it may be asked, have all the most eminent living naturalists and
+  geologists rejected this view of the mutability of species? It cannot be
+  asserted that organic beings in a state of nature are subject to no
+  variation; it cannot be proved that the amount of variation in the course
+  of long ages is a limited quantity; no clear distinction has been, or can
+  be, drawn between species and well-marked varieties. It cannot be
+  maintained that species when intercrossed are invariably sterile, and
+  varieties invariably fertile; or that sterility is a special endowment
+  and sign of creation. The belief that species were immutable productions
+  was almost unavoidable as long as the history of the world was thought to
+  be of short duration; and now that we have acquired some idea of the
+  lapse of time, we are too apt to assume, without proof, that the
+  geological record is so perfect that it would have afforded us plain
+  evidence of the mutation of species, if they had undergone mutation.</p>
+
+  <p>But the chief cause of our natural unwillingness to admit that one
+  species has given birth to other and distinct species, is that we are
+  always slow in admitting any great change of which we do not see the
+  intermediate steps. The difficulty is the same as that felt by so many
+  geologists, when Lyell first insisted that long <!-- Page 482 --><span
+  class="pagenum"><a name="page482"></a>[482]</span>lines of inland cliffs
+  had been formed, and great valleys excavated, by the slow action of the
+  coast-waves. The mind cannot possibly grasp the full meaning of the term
+  of a hundred million years; it cannot add up and perceive the full
+  effects of many slight variations, accumulated during an almost infinite
+  number of generations.</p>
+
+  <p>Although I am fully convinced of the truth of the views given in this
+  volume under the form of an abstract, I by no means expect to convince
+  experienced naturalists whose minds are stocked with a multitude of facts
+  all viewed, during a long course of years, from a point of view directly
+  opposite to mine. It is so easy to hide our ignorance under such
+  expressions as the "plan of creation," "unity of design," &amp;c., and to
+  think that we give an explanation when we only restate a fact. Any one
+  whose disposition leads him to attach more weight to unexplained
+  difficulties than to the explanation of a certain number of facts will
+  certainly reject my theory. A few naturalists, endowed with much
+  flexibility of mind, and who have already begun to doubt on the
+  immutability of species, may be influenced by this volume; but I look
+  with confidence to the future, to young and rising naturalists, who will
+  be able to view both sides of the question with impartiality. Whoever is
+  led to believe that species are mutable will do good service by
+  conscientiously expressing his conviction; for only thus can the load of
+  prejudice by which this subject is overwhelmed be removed.</p>
+
+  <p>Several eminent naturalists have of late published their belief that a
+  multitude of reputed species in each genus are not real species; but that
+  other species are real, that is, have been independently created. This
+  seems to me a strange conclusion to arrive at. They admit that a
+  multitude of forms, which till lately <!-- Page 483 --><span
+  class="pagenum"><a name="page483"></a>[483]</span>they themselves thought
+  were special creations, and which are still thus looked at by the
+  majority of naturalists, and which consequently have every external
+  characteristic feature of true species,&mdash;they admit that these have
+  been produced by variation, but they refuse to extend the same view to
+  other and very slightly different forms. Nevertheless they do not pretend
+  that they can define, or even conjecture, which are the created forms of
+  life, and which are those produced by secondary laws. They admit
+  variation as a <i>vera causa</i> in one case, they arbitrarily reject it
+  in another, without assigning any distinction in the two cases. The day
+  will come when this will be given as a curious illustration of the
+  blindness of preconceived opinion. These authors seem no more startled at
+  a miraculous act of creation than at an ordinary birth. But do they
+  really believe that at innumerable periods in the earth's history certain
+  elemental atoms have been commanded suddenly to flash into living
+  tissues? Do they believe that at each supposed act of creation one
+  individual or many were produced? Were all the infinitely numerous kinds
+  of animals and plants created as eggs or seed, or as full grown? and in
+  the case of mammals, were they created bearing the false marks of
+  nourishment from the mother's womb? Although naturalists very properly
+  demand a full explanation of every difficulty from those who believe in
+  the mutability of species, on their own side they ignore the whole
+  subject of the first appearance of species in what they consider reverent
+  silence.</p>
+
+  <p>It may be asked how far I extend the doctrine of the modification of
+  species. The question is difficult to answer, because the more distinct
+  the forms are which we may consider, by so much the arguments fall away
+  in force. But some arguments of the greatest weight <!-- Page 484
+  --><span class="pagenum"><a name="page484"></a>[484]</span>extend very
+  far. All the members of whole classes can be connected together by chains
+  of affinities, and all can be classified on the same principle, in groups
+  subordinate to groups. Fossil remains sometimes tend to fill up very wide
+  intervals between existing orders. Organs in a rudimentary condition
+  plainly show that an early progenitor had the organ in a fully developed
+  state; and this in some instances necessarily implies an enormous amount
+  of modification in the descendants. Throughout whole classes various
+  structures are formed on the same pattern, and at an embryonic age the
+  species closely resemble each other. Therefore I cannot doubt that the
+  theory of descent with modification embraces all the members of the same
+  class. I believe that animals have descended from at most only four or
+  five progenitors, and plants from an equal or lesser number.</p>
+
+  <p>Analogy would lead me one step further, namely, to the belief that all
+  animals and plants have descended from some one prototype. But analogy
+  may be a deceitful guide. Nevertheless all living things have much in
+  common, in their chemical composition, their germinal vesicles, their
+  cellular structure, and their laws of growth and reproduction. We see
+  this even in so trifling a circumstance as that the same poison often
+  similarly affects plants and animals; or that the poison secreted by the
+  gall-fly produces monstrous growths on the wild rose or oak-tree.
+  Therefore I should infer from analogy that probably all the organic
+  beings which have ever lived on this earth have descended from some one
+  primordial form, into which life was first breathed by the Creator.</p>
+
+  <p>&nbsp;</p>
+
+  <p>When the views advanced by me in this volume, and by Mr. Wallace in
+  the Linnean Journal, or when analogous views on the origin of species are
+  generally <!-- Page 485 --><span class="pagenum"><a
+  name="page485"></a>[485]</span>admitted, we can dimly foresee that there
+  will be a considerable revolution in natural history. Systematists will
+  be able to pursue their labours as at present; but they will not be
+  incessantly haunted by the shadowy doubt whether this or that form be in
+  essence a species. This I feel sure, and I speak after experience, will
+  be no slight relief. The endless disputes whether or not some fifty
+  species of British brambles are true species will cease. Systematists
+  will have only to decide (not that this will be easy) whether any form be
+  sufficiently constant and distinct from other forms, to be capable of
+  definition; and if definable, whether the differences be sufficiently
+  important to deserve a specific name. This latter point will become a far
+  more essential consideration than it is at present; for differences,
+  however slight, between any two forms, if not blended by intermediate
+  gradations, are looked at by most naturalists as sufficient to raise both
+  forms to the rank of species. Hereafter we shall be compelled to
+  acknowledge that the only distinction between species and well-marked
+  varieties is, that the latter are known, or believed, to be connected at
+  the present day by intermediate gradations, whereas species were formerly
+  thus connected. Hence, without rejecting the consideration of the present
+  existence of intermediate gradations between any two forms, we shall be
+  led to weigh more carefully and to value higher the actual amount of
+  difference between them. It is quite possible that forms now generally
+  acknowledged to be merely varieties may hereafter be thought worthy of
+  specific names, as with the primrose and cowslip; and in this case
+  scientific and common language will come into accordance. In short, we
+  shall have to treat species in the same manner as those naturalists treat
+  genera, who admit that genera are merely artificial combinations <!--
+  Page 486 --><span class="pagenum"><a name="page486"></a>[486]</span>made
+  for convenience. This may not be a cheering prospect; but we shall at
+  least be freed from the vain search for the undiscovered and
+  undiscoverable essence of the term species.</p>
+
+  <p>The other and more general departments of natural history will rise
+  greatly in interest. The terms used by naturalists of affinity,
+  relationship, community of type, paternity, morphology, adaptive
+  characters, rudimentary and aborted organs, &amp;c., will cease to be
+  metaphorical, and will have a plain signification. When we no longer look
+  at an organic being as a savage looks at a ship, as at something wholly
+  beyond his comprehension; when we regard every production of nature as
+  one which has had a history; when we contemplate every complex structure
+  and instinct as the summing up of many contrivances, each useful to the
+  possessor, nearly in the same way as when we look at any great mechanical
+  invention as the summing up of the labour, the experience, the reason,
+  and even the blunders of numerous workmen; when we thus view each organic
+  being, how far more interesting, I speak from experience, will the study
+  of natural history become!</p>
+
+  <p>A grand and almost untrodden field of inquiry will be opened, on the
+  causes and laws of variation, on correlation of growth, on the effects of
+  use and disuse, on the direct action of external conditions, and so
+  forth. The study of domestic productions will rise immensely in value. A
+  new variety raised by man will be a more important and interesting
+  subject for study than one more species added to the infinitude of
+  already recorded species. Our classifications will come to be, as far as
+  they can be so made, genealogies; and will then truly give what may be
+  called the plan of creation. The rules for classifying will no doubt
+  become simpler when we have a definite object in view. We possess no <!--
+  Page 487 --><span class="pagenum"><a
+  name="page487"></a>[487]</span>pedigrees or armorial bearings; and we
+  have to discover and trace the many diverging lines of descent in our
+  natural genealogies, by characters of any kind which have long been
+  inherited. Rudimentary organs will speak infallibly with respect to the
+  nature of long-lost structures. Species and groups of species, which are
+  called aberrant, and which may fancifully be called living fossils, will
+  aid us in forming a picture of the ancient forms of life. Embryology will
+  reveal to us the structure, in some degree obscured, of the prototypes of
+  each great class.</p>
+
+  <p>When we can feel assured that all the individuals of the same species,
+  and all the closely allied species of most genera, have within a not very
+  remote period descended from one parent, and have migrated from some one
+  birthplace; and when we better know the many means of migration, then, by
+  the light which geology now throws, and will continue to throw, on former
+  changes of climate and of the level of the land, we shall surely be
+  enabled to trace in an admirable manner the former migrations of the
+  inhabitants of the whole world. Even at present, by comparing the
+  differences of the inhabitants of the sea on the opposite sides of a
+  continent, and the nature of the various inhabitants of that continent in
+  relation to their apparent means of immigration, some light can be thrown
+  on ancient geography.</p>
+
+  <p>The noble science of Geology loses glory from the extreme imperfection
+  of the record. The crust of the earth with its embedded remains must not
+  be looked at as a well-filled museum, but as a poor collection made at
+  hazard and at rare intervals. The accumulation of each great
+  fossiliferous formation will be recognised as having depended on an
+  unusual concurrence of circumstances, and the blank intervals between the
+  successive stages as having been of vast duration. But we shall <!-- Page
+  488 --><span class="pagenum"><a name="page488"></a>[488]</span>be able to
+  gauge with some security the duration of these intervals by a comparison
+  of the preceding and succeeding organic forms. We must be cautious in
+  attempting to correlate as strictly contemporaneous two formations, which
+  include few identical species, by the general succession of their forms
+  of life. As species are produced and exterminated by slowly acting and
+  still existing causes, and not by miraculous acts of creation and by
+  catastrophes; and as the most important of all causes of organic change
+  is one which is almost independent of altered and perhaps suddenly
+  altered physical conditions, namely, the mutual relation of organism to
+  organism,&mdash;the improvement of one being entailing the improvement or
+  the extermination of others; it follows, that the amount of organic
+  change in the fossils of consecutive formations probably serves as a fair
+  measure of the lapse of actual time. A number of species, however,
+  keeping in a body might remain for a long period unchanged, whilst within
+  this same period, several of these species, by migrating into new
+  countries and coming into competition with foreign associates, might
+  become modified; so that we must not overrate the accuracy of organic
+  change as a measure of time. During early periods of the earth's history,
+  when the forms of life were probably fewer and simpler, the rate of
+  change was probably slower; and at the first dawn of life, when very few
+  forms of the simplest structure existed, the rate of change may have been
+  slow in an extreme degree. The whole history of the world, as at present
+  known, although of a length quite incomprehensible by us, will hereafter
+  be recognised as a mere fragment of time, compared with the ages which
+  have elapsed since the first creature, the progenitor of innumerable
+  extinct and living descendants, was created.</p>
+
+  <p>In the distant future I see open fields for far more <!-- Page 489
+  --><span class="pagenum"><a name="page489"></a>[489]</span>important
+  researches. Psychology will be based on a new foundation, that of the
+  necessary acquirement of each mental power and capacity by gradation.
+  Light will be thrown on the origin of man and his history.</p>
+
+  <p>Authors of the highest eminence seem to be fully satisfied with the
+  view that each species has been independently created. To my mind it
+  accords better with what we know of the laws impressed on matter by the
+  Creator, that the production and extinction of the past and present
+  inhabitants of the world should have been due to secondary causes, like
+  those determining the birth and death of the individual. When I view all
+  beings not as special creations, but as the lineal descendants of some
+  few beings which lived long before the first bed of the Silurian system
+  was deposited, they seem to me to become ennobled. Judging from the past,
+  we may safely infer that not one living species will transmit its
+  unaltered likeness to a distant futurity. And of the species now living
+  very few will transmit progeny of any kind to a far distant futurity; for
+  the manner in which all organic beings are grouped, shows that the
+  greater number of species of each genus, and all the species of many
+  genera, have left no descendants, but have become utterly extinct. We can
+  so far take a prophetic glance into futurity as to foretel that it will
+  be the common and widely-spread species, belonging to the larger and
+  dominant groups, which will ultimately prevail and procreate new and
+  dominant species. As all the living forms of life are the lineal
+  descendants of those which lived long before the Silurian epoch, we may
+  feel certain that the ordinary succession by generation has never once
+  been broken, and that no cataclysm has desolated the whole world. Hence
+  we may look with some confidence to a secure future of equally
+  inappreciable length. And as natural selection works <!-- Page 490
+  --><span class="pagenum"><a name="page490"></a>[490]</span>solely by and
+  for the good of each being, all corporeal and mental endowments will tend
+  to progress towards perfection.</p>
+
+  <p>It is interesting to contemplate an entangled bank, clothed with many
+  plants of many kinds, with birds singing on the bushes, with various
+  insects flitting about, and with worms crawling through the damp earth,
+  and to reflect that these elaborately constructed forms, so different
+  from each other, and dependent on each other in so complex a manner, have
+  all been produced by laws acting around us. These laws, taken in the
+  largest sense, being Growth with Reproduction; <span class="correction"
+  title="text reads `Inheritrnce'">Inheritance</span> which is almost
+  implied by reproduction; Variability from the indirect and direct action
+  of the external conditions of life, and from use and disuse; a Ratio of
+  Increase so high as to lead to a Struggle for Life, and as a consequence
+  to Natural Selection, entailing Divergence of Character and the
+  Extinction of less-improved forms. Thus, from the war of nature, from
+  famine and death, the most exalted object which we are capable of
+  conceiving, namely, the production of the higher animals, directly
+  follows. There is grandeur in this view of life, with its several powers,
+  having been originally breathed by the Creator into a few forms or into
+  one; and that, whilst this planet has gone cycling on according to the
+  fixed law of gravity, from so simple a beginning endless forms most
+  beautiful and most wonderful have been, and are being, evolved.</p>
+
+<hr />
+
+<p><!-- Page 491 --><span class="pagenum"><a name="page491"></a>[491]</span></p>
+
+<h2>INDEX.</h2>
+
+  <div class="poem">
+    <div class="stanza">
+      <p class="i8"><b>A.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Aberrant groups, <a href="#page429">429</a>.</p>
+      <p>Abyssinia, plants of, <a href="#page375">375</a>.</p>
+      <p>Acclimatisation, <a href="#page139">139</a>.</p>
+      <p>Affinities of extinct species, <a href="#page329">329</a>.</p>
+      <p>&mdash;&mdash; of organic beings, <a href="#page411">411</a>.</p>
+      <p>Agassiz on Amblyopsis, <a href="#page139">139</a>.</p>
+      <p>&mdash;&mdash; on groups of species suddenly appearing, <a href="#page302">302</a>, <a href="#page305">305</a>.</p>
+      <p>&mdash;&mdash; on embryological succession, <a href="#page338">338</a>.</p>
+      <p>&mdash;&mdash; on the glacial period, <a href="#page366">366</a>.</p>
+      <p>&mdash;&mdash; on embryological characters, <a href="#page418">418</a>.</p>
+      <p>&mdash;&mdash; on the embryos of vertebrata, <a href="#page439">439</a>.</p>
+      <p>&mdash;&mdash; on parallelism of embryological development and geological succession, <a href="#page449">449</a>.</p>
+      <p>Algæ of New Zealand, <a href="#page376">376</a>.</p>
+      <p>Alligators, males, fighting, <a href="#page88">88</a>.</p>
+      <p>Amblyopsis, blind fish, <a href="#page139">139</a>.</p>
+      <p>America, North, productions allied to those of Europe, <a href="#page371">371</a>.</p>
+      <p>&mdash;&mdash;&mdash;&mdash;, boulders and glaciers of, <a href="#page373">373</a>.</p>
+      <p>&mdash;&mdash;, South, no modern formations on west coast, <a href="#page290">290</a>.</p>
+      <p>Ammonites, sudden extinction of, <a href="#page321">321</a>.</p>
+      <p>Anagallis, sterility of, <a href="#page247">247</a>.</p>
+      <p>Analogy of variations, <a href="#page159">159</a>.</p>
+      <p>Ancylus, <a href="#page386">386</a>.</p>
+      <p>Animals, not domesticated from being variable, <a href="#page17">17</a>.</p>
+      <p>&mdash;&mdash;, domestic, descended from several stocks, <a href="#page19">19</a>.</p>
+      <p>&mdash;&mdash;&mdash;&mdash;, acclimatisation of, <a href="#page141">141</a>.</p>
+      <p>&mdash;&mdash; of Australia, <a href="#page116">116</a>.</p>
+      <p>&mdash;&mdash; with thicker fur in cold climates, <a href="#page133">133</a>.</p>
+      <p>&mdash;&mdash;, blind, in caves, <a href="#page137">137</a>.</p>
+      <p>&mdash;&mdash;, extinct, of Australia, <a href="#page339">339</a>.</p>
+      <p>Anomma, <a href="#page240">240</a>.</p>
+      <p>Antarctic islands, ancient flora of, <a href="#page399">399</a>.</p>
+      <p>Antirrhinum, <a href="#page161">161</a>.</p>
+      <p>Ants attending aphides, <a href="#page210">210</a>.</p>
+      <p>&mdash;&mdash;, slave-making instinct, <a href="#page219">219</a>.</p>
+      <p>&mdash;&mdash;, neuter, structure of, <a href="#page236">236</a>.</p>
+      <p>Aphides, attended by ants, <a href="#page210">210</a>.</p>
+      <p>Aphis, development of, <a href="#page442">442</a>.</p>
+      <p>Apteryx, <a href="#page182">182</a>.</p>
+      <p>Arab horses, <a href="#page35">35</a>.</p>
+      <p>Aralo-Caspian Sea, <a href="#page339">339</a>.</p>
+      <p>Archaic, M. de, on the succession of species, <a href="#page325">325</a>.</p>
+      <p>Artichoke, Jerusalem, <a href="#page142">142</a>.</p>
+      <p>Ascension, plants of, <a href="#page389">389</a>.</p>
+      <p>Asclepias, pollen of, <a href="#page193">193</a>.</p>
+      <p>Asparagus, <a href="#page359">359</a>.</p>
+      <p>Aspicarpa, <a href="#page417">417</a>.</p>
+      <p>Asses, striped, <a href="#page163">163</a>.</p>
+      <p>Ateuchus, <a href="#page135">135</a>.</p>
+      <p>Audubon on habits of frigate-bird, <a href="#page185">185</a>.</p>
+      <p>&mdash;&mdash; on variation in birds'-nests, <a href="#page212">212</a>.</p>
+      <p>&mdash;&mdash; on heron eating seeds, <a href="#page387">387</a>.</p>
+      <p>Australia, animals of, <a href="#page116">116</a>.</p>
+      <p>&mdash;&mdash;. dogs of, <a href="#page215">215</a>.</p>
+      <p>&mdash;&mdash;, extinct animals of, <a href="#page339">339</a>.</p>
+      <p>&mdash;&mdash;, European plants in, <a href="#page375">375</a>.</p>
+      <p>Azara on flies destroying cattle, <a href="#page72">72</a>.</p>
+      <p>Azores, flora of, <a href="#page363">363</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>B.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Babington, Mr., on British plants, <a href="#page48">48</a>.</p>
+      <p>Balancement of growth, <a href="#page147">147</a>.</p>
+      <p>Bamboo with hooks, <a href="#page197">197</a>.</p>
+      <p>Barberry, flowers of, <a href="#page98">98</a>.</p>
+      <p>Barrande, M., on Silurian colonies, <a href="#page313">313</a>.</p>
+      <p>&mdash;&mdash; on the succession of species, <a href="#page325">325</a>.</p>
+      <p>&mdash;&mdash; on parallelism of palæozoic formations, <a href="#page328">328</a>.</p>
+      <p>&mdash;&mdash; on affinities of ancient species, <a href="#page330">330</a>.</p>
+      <p>Barriers, importance of, <a href="#page347">347</a>.</p>
+      <p>Batrachians on islands, <a href="#page393">393</a>.</p>
+      <p>Bats, how structure acquired, <a href="#page180">180</a>.</p>
+      <p>&mdash;&mdash;, distribution of, <a href="#page394">394</a>.</p>
+      <p>Bear, catching water-insects, <a href="#page184">184</a>.</p>
+      <p>Bee, sting of, <a href="#page202">202</a>.</p>
+      <p>&mdash;&mdash;, queen, killing rivals, <a href="#page202">202</a>.</p>
+      <p>Bees fertilising flowers, <a href="#page73">73</a>.</p>
+      <p>&mdash;&mdash;, hive, not sucking the red clover, <a href="#page95">95</a>.</p>
+<!-- Page 492 --><span class="pagenum"><a name="page492"></a>[492]</span>
+      <p>&mdash;&mdash;&mdash;&mdash;, cell-making instinct, <a href="#page224">224</a>.</p>
+      <p>&mdash;&mdash;, humble, cells of, <a href="#page225">225</a>.</p>
+      <p>&mdash;&mdash;, parasitic, <a href="#page218">218</a>.</p>
+      <p>Beetles, wingless, in Madeira, <a href="#page135">135</a>.</p>
+      <p>&mdash;&mdash; with deficient tarsi, <a href="#page135">135</a>.</p>
+      <p>Bentham, Mr., on British plants, <a href="#page48">48</a>.</p>
+      <p>&mdash;&mdash;, on classification, <a href="#page419">419</a>.</p>
+      <p>Berkeley, Mr., on seeds in salt-water, <a href="#page358">358</a>.</p>
+      <p>Bermuda, birds of, <a href="#page391">391</a>.</p>
+      <p>Birds acquiring fear, <a href="#page212">212</a>.</p>
+      <p>&mdash;&mdash; annually cross the Atlantic, <a href="#page364">364</a>.</p>
+      <p>&mdash;&mdash;, colour of, on continents, <a href="#page132">132</a>.</p>
+      <p>&mdash;&mdash;, footsteps and remains of, in secondary rocks, <a href="#page304">304</a>.</p>
+      <p>&mdash;&mdash;, fossil, in caves of Brazil, <a href="#page339">339</a>.</p>
+      <p>&mdash;&mdash; of Madeira, Bermuda, and Galapagos, <a href="#page391">391</a>.</p>
+      <p>&mdash;&mdash;, song of males, <a href="#page89">89</a>.</p>
+      <p>&mdash;&mdash; transporting seeds, <a href="#page361">361</a>.</p>
+      <p>&mdash;&mdash;, waders, <a href="#page385">385</a>.</p>
+      <p>&mdash;&mdash;, wingless, <a href="#page134">134</a>, <a href="#page182">182</a>.</p>
+      <p>&mdash;&mdash;, with traces of embryonic teeth, <a href="#page450">450</a>.</p>
+      <p>Bizcacha, <a href="#page349">349</a>.</p>
+      <p>&mdash;&mdash;, affinities of, <a href="#page429">429</a>.</p>
+      <p>Bladder for swimming in fish, <a href="#page190">190</a>.</p>
+      <p>Blindness of cave animals, <a href="#page137">137</a>.</p>
+      <p>Blyth, Mr., on distinctness of Indian cattle, <a href="#page18">18</a>.</p>
+      <p>&mdash;&mdash;, on striped Hemionus, <a href="#page163">163</a>.</p>
+      <p>&mdash;&mdash;, on crossed geese, <a href="#page254">254</a>.</p>
+      <p>Boar, shoulder-pad of, <a href="#page88">88</a>.</p>
+      <p>Borrow, Mr., on the Spanish pointer, <a href="#page35">35</a>.</p>
+      <p>Bory St. Vincent on Batrachians, <a href="#page393">393</a>.</p>
+      <p>Bosquet, M., on fossil Chthamalus, <a href="#page305">305</a>.</p>
+      <p>Boulders, erratic, on the Azores, <a href="#page363">363</a>.</p>
+      <p>Branchiæ, <a href="#page190">190</a>.</p>
+      <p>Brent, Mr., on house-tumblers, <a href="#page214">214</a>.</p>
+      <p>&mdash;&mdash;, on hawks killing pigeons, <a href="#page362">362</a>.</p>
+      <p>Brewer, Dr., on American cuckoo, <a href="#page217">217</a>.</p>
+      <p>Britain, mammals of, <a href="#page396">396</a>.</p>
+      <p>Bronn on duration of specific forms, <a href="#page294">294</a>.</p>
+      <p>Brown, Robert, on classification, <a href="#page415">415</a>.</p>
+      <p>Buckman on variation in plants, <a href="#page10">10</a>.</p>
+      <p>Buzareingues on sterility of varieties, <a href="#page270">270</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>C.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Cabbage, varieties of, crossed, <a href="#page99">99</a>.</p>
+      <p>Calceolaria, <a href="#page251">251</a>.</p>
+      <p>Canary-birds, sterility of hybrids, <a href="#page252">252</a>.</p>
+      <p>Cape de Verde islands, <a href="#page398">398</a>.</p>
+      <p>Cape of Good Hope, plants of, <a href="#page110">110</a>, <a href="#page375">375</a>.</p>
+      <p>Carrier-pigeons killed by hawks, <a href="#page362">362</a>.</p>
+      <p>Cassini on flowers of compositæ, <a href="#page145">145</a>.</p>
+      <p>Catasetum, <a href="#page424">424</a>.</p>
+      <p>Cats, with blue eyes, deaf, <a href="#page12">12</a>.</p>
+      <p>&mdash;&mdash;, variation in habits of, <a href="#page91">91</a>.</p>
+      <p>&mdash;&mdash; curling tail when going to spring, <a href="#page201">201</a>.</p>
+      <p>Cattle destroying fir-trees, <a href="#page72">72</a>.</p>
+      <p>&mdash;&mdash; destroyed by flies in La Plata, <a href="#page72">72</a>.</p>
+      <p>&mdash;&mdash;, breeds of, locally extinct, <a href="#page111">111</a>.</p>
+      <p>&mdash;&mdash;, fertility of Indian and European breeds, <a href="#page254">254</a>.</p>
+      <p>Cave, inhabitants of, blind, <a href="#page137">137</a>.</p>
+      <p>Centres of creation, <a href="#page352">352</a>.</p>
+      <p>Cephalopodæ, development of, <a href="#page442">442</a>.</p>
+      <p>Cervulus, <a href="#page253">253</a>.</p>
+      <p>Cetacea, teeth and hair, <a href="#page144">144</a>.</p>
+      <p>Ceylon, plants of, <a href="#page375">375</a>.</p>
+      <p>Chalk formation, <a href="#page322">322</a>.</p>
+      <p>Characters, divergence of, <a href="#page111">111</a>.</p>
+      <p>&mdash;&mdash;, sexual, variable, <a href="#page156">156</a>.</p>
+      <p>&mdash;&mdash;, adaptive or analogical, <a href="#page426">426</a>.</p>
+      <p>Charlock, <a href="#page76">76</a>.</p>
+      <p>Checks to increase, <a href="#page67">67</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash;, mutual, <a href="#page71">71</a>.</p>
+      <p>Chickens, instinctive tameness of, <a href="#page216">216</a>.</p>
+      <p>Chthamalinæ, <a href="#page289">289</a>.</p>
+      <p>Chthamalus, cretacean species of, <a href="#page305">305</a>.</p>
+      <p>Circumstances favourable to selection of domestic products, <a href="#page40">40</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash; to natural selection, <a href="#page102">102</a>.</p>
+      <p>Cirripedes capable of crossing, <a href="#page101">101</a>.</p>
+      <p>&mdash;&mdash;, carapace aborted, <a href="#page148">148</a>.</p>
+      <p>&mdash;&mdash;, their ovigerous frena, <a href="#page192">192</a>.</p>
+      <p>&mdash;&mdash;, fossil, <a href="#page304">304</a>.</p>
+      <p>&mdash;&mdash;, larvæ of, <a href="#page440">440</a>.</p>
+      <p>Classification, <a href="#page413">413</a>.</p>
+      <p>Clift, Mr., on the succession of types, <a href="#page339">339</a>.</p>
+      <p>Climate, effects of, in checking increase of beings, <a href="#page68">68</a>.</p>
+      <p>&mdash;&mdash;, adaptation of, to organisms, <a href="#page139">139</a>.</p>
+<!-- Page 493 --><span class="pagenum"><a name="page493"></a>[493]</span>
+      <p>Cobites, intestine of, <a href="#page190">190</a>.</p>
+      <p>Cockroach, <a href="#page76">76</a>.</p>
+      <p>Collections, palæontological, poor, <a href="#page288">288</a>.</p>
+      <p>Colour, influenced by climate, <a href="#page132">132</a>.</p>
+      <p>&mdash;&mdash;, in relation to attacks by flies, <a href="#page198">198</a>.</p>
+      <p>Columba livia, parent of domestic pigeons, <a href="#page23">23</a>.</p>
+      <p>Colymbetes, <a href="#page386">386</a>.</p>
+      <p>Compensation of growth, <a href="#page147">147</a>.</p>
+      <p>Compositæ, outer and inner florets of, <a href="#page144">144</a>.</p>
+      <p>&mdash;&mdash;, male flowers of, <a href="#page451">451</a>.</p>
+      <p>Conclusion, general, <a href="#page480">480</a>.</p>
+      <p>Conditions, slight changes in, favourable to fertility, <a href="#page267">267</a>.</p>
+      <p>Coot, <a href="#page185">185</a>.</p>
+      <p>Coral-islands, seeds drifted to, <a href="#page361">361</a>.</p>
+      <p>&mdash;&mdash; reefs, indicating movements of earth, <a href="#page310">310</a>.</p>
+      <p>Corn-crake, <a href="#page186">186</a>.</p>
+      <p>Correlation of growth in domestic productions, <a href="#page11">11</a>.</p>
+      <p>&mdash;&mdash; of growth, <a href="#page143">143</a>, <a href="#page198">198</a>.</p>
+      <p>Cowslip, <a href="#page49">49</a>.</p>
+      <p>Creation, single centres of, <a href="#page352">352</a>.</p>
+      <p>Crinum, <a href="#page250">250</a>.</p>
+      <p>Crosses, reciprocal, <a href="#page258">258</a>.</p>
+      <p>Crossing of domestic animals, importance in altering breeds, <a href="#page20">20</a>.</p>
+      <p>&mdash;&mdash;, advantages of, <a href="#page96">96</a>.</p>
+      <p>&mdash;&mdash; unfavourable to selection, <a href="#page102">102</a>.</p>
+      <p>Crustacea of New Zealand, <a href="#page376">376</a>.</p>
+      <p>Crustacean, blind, <a href="#page137">137</a>.</p>
+      <p>Cryptocerus, <a href="#page239">239</a>.</p>
+      <p>Ctenomys, blind, <a href="#page137">137</a>.</p>
+      <p>Cuckoo, instinct of, <a href="#page216">216</a>.</p>
+      <p>Currants, grafts of, <a href="#page262">262</a>.</p>
+      <p>Currents of sea, rate of, <a href="#page360">360</a>.</p>
+      <p>Cuvier on conditions of existence, <a href="#page206">206</a>.</p>
+      <p>&mdash;&mdash; on fossil monkeys, <a href="#page304">304</a>.</p>
+      <p>&mdash;&mdash;, Fred., on instinct, <a href="#page208">208</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>D.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Dana, Prof., on blind cave-animals, <a href="#page139">139</a>.</p>
+      <p>&mdash;&mdash;, on relations of crustaceans of Japan, <a href="#page372">372</a>.</p>
+      <p>&mdash;&mdash;, on crustaceans of New Zealand, <a href="#page376">376</a>.</p>
+      <p>De Candolle on struggle for existence, <a href="#page62">62</a>.</p>
+      <p>&mdash;&mdash; on umbelliferæ, <a href="#page146">146</a>.</p>
+      <p>&mdash;&mdash; on general affinities, <a href="#page430">430</a>.</p>
+      <p>&mdash;&mdash;, Alph., on low plants, widely dispersed, <a href="#page406">406</a>.</p>
+      <p>&mdash;&mdash;, &mdash;&mdash;, on widely-ranging plants being variable, <a href="#page53">53</a>.</p>
+      <p>&mdash;&mdash;, &mdash;&mdash;, on naturalisation, <a href="#page115">115</a>.</p>
+      <p>&mdash;&mdash;, &mdash;&mdash;, on winged seeds, <a href="#page146">146</a>.</p>
+      <p>&mdash;&mdash;, &mdash;&mdash;, on Alpine species suddenly becoming rare, <a href="#page175">175</a>.</p>
+      <p>&mdash;&mdash;, &mdash;&mdash;, on distribution of plants with large seeds, <a href="#page360">360</a>.</p>
+      <p>&mdash;&mdash;, &mdash;&mdash;, on vegetation of Australia, <a href="#page379">379</a>.</p>
+      <p>&mdash;&mdash;, &mdash;&mdash;, on fresh-water plants, <a href="#page386">386</a>.</p>
+      <p>&mdash;&mdash;, &mdash;&mdash;, on insular plants, <a href="#page389">389</a>.</p>
+      <p>Degradation of coast-rocks, <a href="#page282">282</a>.</p>
+      <p>Denudation, rate of, <a href="#page285">285</a>.</p>
+      <p>&mdash;&mdash; of oldest rocks, <a href="#page308">308</a>.</p>
+      <p>Development of ancient forms, <a href="#page336">336</a>.</p>
+      <p>Devonian system, <a href="#page334">334</a>.</p>
+      <p>Dianthus, fertility of crosses, <a href="#page256">256</a>.</p>
+      <p>Dirt on feet of birds, <a href="#page362">362</a>.</p>
+      <p>Dispersal, means of, <a href="#page356">356</a>.</p>
+      <p>&mdash;&mdash; during glacial period, <a href="#page365">365</a>.</p>
+      <p>Distribution, geographical, <a href="#page346">346</a>.</p>
+      <p>&mdash;&mdash;, means of, <a href="#page356">356</a>.</p>
+      <p>Disuse, effects of, under nature, <a href="#page134">134</a>.</p>
+      <p>Divergence of character, <a href="#page111">111</a>.</p>
+      <p>Division, physiological, of labour, <a href="#page115">115</a>.</p>
+      <p>Dogs, hairless, with imperfect teeth, <a href="#page12">12</a>.</p>
+      <p>&mdash;&mdash; descended from several wild stocks, <a href="#page18">18</a>.</p>
+      <p>&mdash;&mdash;, domestic instincts of, <a href="#page213">213</a>.</p>
+      <p>&mdash;&mdash;, inherited civilisation of, <a href="#page215">215</a>.</p>
+      <p>&mdash;&mdash;, fertility of breeds together, <a href="#page254">254</a>.</p>
+      <p>&mdash;&mdash;, &mdash;&mdash; of crosses, <a href="#page268">268</a>.</p>
+      <p>&mdash;&mdash;, proportions of, when young, <a href="#page444">444</a>.</p>
+      <p>Domestication, variation under, <a href="#page7">7</a>.</p>
+      <p>Downing, Mr., on fruit-trees in America, <a href="#page85">85</a>.</p>
+      <p>Downs, North and South, <a href="#page286">286</a>.</p>
+      <p>Dragon-flies, intestines of, <a href="#page190">190</a>.</p>
+      <p>Drift-timber, <a href="#page360">360</a>.</p>
+      <p>Driver-ant, <a href="#page240">240</a>.</p>
+      <p>Drones killed by other bees, <a href="#page202">202</a>.</p>
+      <p>Duck, domestic, wings of, reduced, <a href="#page11">11</a>.</p>
+      <p>&mdash;&mdash;, logger-headed, <a href="#page182">182</a>.</p>
+<!-- Page 494 --><span class="pagenum"><a name="page494"></a>[494]</span>
+      <p>Duckweed, <a href="#page385">385</a>.</p>
+      <p>Dugong, affinities of, <a href="#page414">414</a>.</p>
+      <p>Dung-beetles with deficient tarsi, <a href="#page135">135</a>.</p>
+      <p>Dyticus, <a href="#page386">386</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>E.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Earl, Mr. W., on the Malay Archipelago, <a href="#page395">395</a>.</p>
+      <p>Ears, drooping, in domestic animals, <a href="#page11">11</a>.</p>
+      <p>&mdash;&mdash;, rudimentary, <a href="#page454">454</a>.</p>
+      <p>Earth, seeds in roots of trees, <a href="#page361">361</a>.</p>
+      <p>Eciton, <a href="#page238">238</a>.</p>
+      <p>Economy of organisation, <a href="#page147">147</a>.</p>
+      <p>Edentata, teeth and hair, <a href="#page144">144</a>.</p>
+      <p>&mdash;&mdash;, fossil species of, <a href="#page339">339</a>.</p>
+      <p>Edwards, Milne, on physiological divisions of labour, <a href="#page115">115</a>.</p>
+      <p>&mdash;&mdash;, on gradations of structure, <a href="#page194">194</a>.</p>
+      <p>&mdash;&mdash;, on embryonical characters, <a href="#page418">418</a>.</p>
+      <p>Eggs, young birds escaping from, <a href="#page87">87</a>.</p>
+      <p>Electric organs, <a href="#page192">192</a>.</p>
+      <p>Elephant, rate of increase, <a href="#page64">64</a>.</p>
+      <p>&mdash;&mdash; of glacial period, <a href="#page141">141</a>.</p>
+      <p>Embryology, <a href="#page438">438</a>.</p>
+      <p>Existence, struggle for, <a href="#page60">60</a>.</p>
+      <p>&mdash;&mdash;, conditions of, <a href="#page206">206</a>.</p>
+      <p>Extinction, as bearing on natural selection, <a href="#page109">109</a>.</p>
+      <p>&mdash;&mdash; of domestic varieties, <a href="#page111">111</a>,</p>
+      <p>&mdash;&mdash;, <a href="#page317">317</a>.</p>
+      <p>Eye, structure of, <a href="#page187">187</a>.</p>
+      <p>&mdash;&mdash;, correction for aberration, <a href="#page202">202</a>.</p>
+      <p>Eyes reduced in moles, <a href="#page137">137</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>F.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Fabre, M. on parasitic sphex, <a href="#page218">218</a>.</p>
+      <p>Falconer, Dr., on naturalisation of plants in India, <a href="#page65">65</a>.</p>
+      <p>&mdash;&mdash; on fossil crocodile, <a href="#page313">313</a>.</p>
+      <p>&mdash;&mdash; on elephants and mastodons, <a href="#page334">334</a>.</p>
+      <p>&mdash;&mdash; and Cautley on mammals of sub-Himalayan beds, <a href="#page340">340</a>.</p>
+      <p>Falkland Island, wolf of, <a href="#page394">394</a>.</p>
+      <p>Faults, <a href="#page285">285</a>.</p>
+      <p>Faunas, marine, <a href="#page348">348</a>.</p>
+      <p>Fear, instinctive, in birds, <a href="#page212">212</a>.</p>
+      <p>Feet of bird, young molluscs adhering to, <a href="#page385">385</a>.</p>
+      <p>Fertility of hybrids, <a href="#page249">249</a>.</p>
+      <p>&mdash;&mdash; from slight changes in conditions, <a href="#page267">267</a>.</p>
+      <p>&mdash;&mdash; of crossed varieties, <a href="#page268">268</a>.</p>
+      <p>Fir-trees destroyed by cattle, <a href="#page72">72</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash;, pollen of, <a href="#page203">203</a>.</p>
+      <p>Fish, flying, <a href="#page182">182</a>.</p>
+      <p>&mdash;&mdash;, teleostean, sudden appearance of, <a href="#page305">305</a>.</p>
+      <p>&mdash;&mdash; eating seeds, <a href="#page362">362</a>, <a href="#page387">387</a>.</p>
+      <p>&mdash;&mdash;, fresh-water, distribution of, <a href="#page384">384</a>.</p>
+      <p>Fishes, ganoid, now confined to fresh water, <a href="#page107">107</a>.</p>
+      <p>&mdash;&mdash;, electric organs of, <a href="#page192">192</a>.</p>
+      <p>&mdash;&mdash;, ganoid, living in fresh water, <a href="#page321">321</a>.</p>
+      <p>&mdash;&mdash; of southern hemisphere, <a href="#page376">376</a>.</p>
+      <p>Flight, powers of, how acquired, <a href="#page182">182</a>.</p>
+      <p>Flowers, structure of, in relation to crossing, <a href="#page97">97</a>.</p>
+      <p>&mdash;&mdash; of compositæ and umbelliferæ, <a href="#page144">144</a>.</p>
+      <p>Forbes, E., on colours of shells, <a href="#page132">132</a>.</p>
+      <p>&mdash;&mdash; on abrupt range of shells in depth, <a href="#page175">175</a>.</p>
+      <p>&mdash;&mdash; on poorness of palæontological collections, <a href="#page288">288</a>.</p>
+      <p>&mdash;&mdash; on continuous succession of genera, <a href="#page316">316</a>.</p>
+      <p>&mdash;&mdash; on continental extensions, <a href="#page357">357</a>.</p>
+      <p>&mdash;&mdash; on distribution during glacial period, <a href="#page366">366</a>.</p>
+      <p>&mdash;&mdash; on parallelism in time and space, <a href="#page409">409</a>.</p>
+      <p>Forests, changes in, in America, <a href="#page74">74</a>.</p>
+      <p>Formation, Devonian, <a href="#page334">334</a>.</p>
+      <p>Formations, thickness of, in Britain, <a href="#page284">284</a>.</p>
+      <p>&mdash;&mdash;, intermittent, <a href="#page290">290</a>.</p>
+      <p>Formica rufescens, <a href="#page219">219</a>.</p>
+      <p>&mdash;&mdash; sanguinea, <a href="#page219">219</a>.</p>
+      <p>&mdash;&mdash; flava, neuter of, <a href="#page240">240</a>.</p>
+      <p>Frena, ovigerous, of cirripedes, <a href="#page192">192</a>.</p>
+      <p>Fresh-water productions, dispersal of, <a href="#page383">383</a>.</p>
+      <p>Fries on species in large genera being closely allied to other species, <a href="#page57">57</a>.</p>
+      <p>Frigate-bird, <a href="#page185">185</a>.</p>
+      <p>Frogs on islands, <a href="#page393">393</a>.</p>
+      <p>Fruit-trees, gradual improvement of, <a href="#page37">37</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash; in United States, <a href="#page85">85</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash;, varieties of, acclimatised in United States, <a href="#page142">142</a>.</p>
+<!-- Page 495 --><span class="pagenum"><a name="page495"></a>[495]</span>
+      <p>Fuci, crossed, <a href="#page258">258</a>.</p>
+      <p>Fur, thicker in cold climates, <a href="#page133">133</a>.</p>
+      <p>Furze, <a href="#page439">439</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>G.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Galapagos Archipelago, birds of, <a href="#page390">390</a>.</p>
+      <p>&mdash;&mdash;, productions of, <a href="#page398">398</a>, <a href="#page400">400</a>.</p>
+      <p>Galeopithecus, <a href="#page181">181</a>.</p>
+      <p>Game, increase of, checked by vermin, <a href="#page68">68</a>.</p>
+      <p>Gärtner on sterility of hybrids, <a href="#page247">247</a>, <a href="#page255">255</a>.</p>
+      <p>&mdash;&mdash;, on reciprocal crosses, <a href="#page258">258</a>.</p>
+      <p>&mdash;&mdash;, on crossed maize and verbascum, <a href="#page270">270</a>.</p>
+      <p>&mdash;&mdash;, on comparison of hybrids and mongrels, <a href="#page272">272</a>.</p>
+      <p>Geese, fertility when crossed, <a href="#page253">253</a>.</p>
+      <p>&mdash;&mdash;, upland, <a href="#page185">185</a>.</p>
+      <p>Genealogy important in classification, <a href="#page425">425</a>.</p>
+      <p>Geoffroy St. Hilaire on balancement, <a href="#page147">147</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash; on homologous organs, <a href="#page434">434</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash;, Isidore, on variability of repeated parts, <a href="#page149">149</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash;, on correlation in monstrosities, <a href="#page11">11</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash;, on correlation, <a href="#page144">144</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash;, on variable parts being often monstrous, <a href="#page155">155</a>.</p>
+      <p>Geographical distribution, <a href="#page346">346</a>.</p>
+      <p>Geography, ancient, <a href="#page487">487</a>.</p>
+      <p>Geology, future progress of, <a href="#page487">487</a>.</p>
+      <p>&mdash;&mdash;, imperfection of the record, <a href="#page279">279</a>.</p>
+      <p>Giraffe, tail of, <a href="#page195">195</a>.</p>
+      <p>Glacial period, <a href="#page365">365</a>.</p>
+      <p>Gmelin on distribution, <a href="#page365">365</a>.</p>
+      <p>Gnathodon, fossil, <a href="#page368">368</a>.</p>
+      <p>Godwin-Austen, Mr., on the Malay Archipelago, <a href="#page300">300</a>.</p>
+      <p>Goethe on compensation of growth, <a href="#page147">147</a>.</p>
+      <p>Gooseberry, grafts of, <a href="#page262">262</a>.</p>
+      <p>Gould, Dr. A., on land-shells, <a href="#page397">397</a>.</p>
+      <p>&mdash;&mdash;, Mr., on colours of birds, <a href="#page132">132</a>.</p>
+      <p>&mdash;&mdash;, on birds of the Galapagos, <a href="#page398">398</a>.</p>
+      <p>&mdash;&mdash;, on distribution of genera of birds, <a href="#page404">404</a>.</p>
+      <p>Gourds, crossed, <a href="#page270">270</a>.</p>
+      <p>Grafts, capacity of, <a href="#page261">261</a>.</p>
+      <p>Grasses, varieties of, <a href="#page113">113</a>.</p>
+      <p>Gray, Dr. Asa, on trees of United States, <a href="#page100">100</a>.</p>
+      <p>&mdash;&mdash;, on naturalised plants in the United States, <a href="#page115">115</a>.</p>
+      <p>&mdash;&mdash;, on rarity of intermediate varieties, <a href="#page176">176</a>.</p>
+      <p>&mdash;&mdash;, on Alpine plants, <a href="#page365">365</a>.</p>
+      <p>&mdash;&mdash;, Dr. J. E., on striped mule, <a href="#page165">165</a>.</p>
+      <p>Grebe, <a href="#page185">185</a>.</p>
+      <p>Groups, aberrant, <a href="#page429">429</a>.</p>
+      <p>Grouse, colours of, <a href="#page84">84</a>.</p>
+      <p>&mdash;&mdash;, red, a doubtful species, <a href="#page49">49</a>.</p>
+      <p>Growth, compensation of, <a href="#page147">147</a>.</p>
+      <p>&mdash;&mdash;, correlation of, in domestic products, <a href="#page11">11</a>.</p>
+      <p>&mdash;&mdash;, correlation of, <a href="#page143">143</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>H.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Habit, effect of, under domestication, <a href="#page11">11</a>.</p>
+      <p>&mdash;&mdash;, effect of, under nature, <a href="#page134">134</a>.</p>
+      <p>&mdash;&mdash;, diversified, of same species, <a href="#page183">183</a>.</p>
+      <p>Hair and teeth, correlated, <a href="#page144">144</a>.</p>
+      <p>Harcourt, Mr. E. V., on the birds of Madeira, <a href="#page391">391</a>.</p>
+      <p>Hartung, M. on boulders in the Azores, <a href="#page363">363</a>.</p>
+      <p>Hazel-nuts, <a href="#page359">359</a>.</p>
+      <p>Hearne on habits of bears, <a href="#page184">184</a>.</p>
+      <p>Heath, changes in vegetation, <a href="#page72">72</a>.</p>
+      <p>Heer, O., on plants of Madeira, <a href="#page107">107</a>.</p>
+      <p>Helix pomatia, <a href="#page397">397</a>.</p>
+      <p>Helosciadium, <a href="#page359">359</a>.</p>
+      <p>Hemionus, striped, <a href="#page163">163</a>.</p>
+      <p>Herbert, W., on struggle for existence, <a href="#page62">62</a>.</p>
+      <p>&mdash;&mdash;, on sterility of hybrids, <a href="#page249">249</a>.</p>
+      <p>Hermaphrodites crossing, <a href="#page96">96</a>.</p>
+      <p>Heron eating seed, <a href="#page387">387</a>.</p>
+      <p>Heron, Sir R., on peacocks, <a href="#page89">89</a>.</p>
+      <p>Heusinger on white animals not poisoned by certain plants, <a href="#page12">12</a>.</p>
+      <p>Hewitt, Mr., on sterility of first crosses, <a href="#page264">264</a>.</p>
+      <p>Himalaya, glaciers of, <a href="#page373">373</a>.</p>
+      <p>&mdash;&mdash;, plants of, <a href="#page375">375</a>.</p>
+      <p>Hippeastrum, <a href="#page250">250</a>.</p>
+      <p>Holly-trees, sexes of, <a href="#page93">93</a>.</p>
+      <p>Hollyhock, varieties of, crossed, <a href="#page271">271</a>.</p>
+      <p>Hooker, Dr., on trees of New Zealand, <a href="#page100">100</a>.</p>
+<!-- Page 496 --><span class="pagenum"><a name="page496"></a>[496]</span>
+      <p>&mdash;&mdash;, on acclimatisation of Himalayan trees, <a href="#page140">140</a>.</p>
+      <p>&mdash;&mdash;, on flowers of umbelliferæ, <a href="#page145">145</a>.</p>
+      <p>&mdash;&mdash;, on glaciers of Himalaya, <a href="#page373">373</a>.</p>
+      <p>&mdash;&mdash;, on algæ of New Zealand, <a href="#page376">376</a>.</p>
+      <p>&mdash;&mdash;, on vegetation at the base of the Himalaya, <a href="#page378">378</a>.</p>
+      <p>&mdash;&mdash;, on plants of Tierra del Fuego, <a href="#page374">374</a>, <a href="#page378">378</a>.</p>
+      <p>&mdash;&mdash;, on Australian plants, <a href="#page375">375</a>, <a href="#page399">399</a>.</p>
+      <p>&mdash;&mdash;, on relations of flora of South America, <a href="#page379">379</a>.</p>
+      <p>&mdash;&mdash;, on flora of the Antarctic lands, <a href="#page381">381</a>, <a href="#page399">399</a>.</p>
+      <p>&mdash;&mdash;, on the plants of the Galapagos, <a href="#page392">392</a>, <a href="#page398">398</a>.</p>
+      <p>Hooks on bamboos, <a href="#page197">197</a>.</p>
+      <p>&mdash;&mdash; to seeds on islands, <a href="#page392">392</a>.</p>
+      <p>Horner, Mr., on the antiquity of Egyptians, <a href="#page18">18</a>.</p>
+      <p>Horns, rudimentary, <a href="#page454">454</a>.</p>
+      <p>Horse, fossil, in La Plata, <a href="#page318">318</a>.</p>
+      <p>Horses destroyed by flies in La Plata, <a href="#page72">72</a>.</p>
+      <p>&mdash;&mdash;, striped, <a href="#page163">163</a>.</p>
+      <p>&mdash;&mdash;, proportions of, when young, <a href="#page444">444</a>.</p>
+      <p>Horticulturists, selection applied by, <a href="#page32">32</a>.</p>
+      <p>Huber on cells of bees, <a href="#page230">230</a>.</p>
+      <p>&mdash;&mdash;, P., on reason blended with instinct, <a href="#page208">208</a>.</p>
+      <p>&mdash;&mdash;, on habitual nature of instincts, <a href="#page208">208</a>.</p>
+      <p>&mdash;&mdash;, on slave-making ants, <a href="#page219">219</a>.</p>
+      <p>&mdash;&mdash;, on Melipona domestica, <a href="#page225">225</a>.</p>
+      <p>Humble-bees, cells of, <a href="#page225">225</a>.</p>
+      <p>Hunter, J., on secondary sexual characters, <a href="#page150">150</a>.</p>
+      <p>Hutton, Captain, on crossed geese, <a href="#page254">254</a>.</p>
+      <p>Huxley, Prof., on structure of hermaphrodites, <a href="#page101">101</a>.</p>
+      <p>&mdash;&mdash;, on embryological succession, <a href="#page338">338</a>.</p>
+      <p>&mdash;&mdash;, on homologous organs, <a href="#page438">438</a>.</p>
+      <p>&mdash;&mdash;, on the development of aphis, <a href="#page442">442</a>.</p>
+      <p>Hybrids and mongrels compared, <a href="#page272">272</a>.</p>
+      <p>Hybridism, <a href="#page245">245</a>.</p>
+      <p>Hydra, structure of, <a href="#page190">190</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>I.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Ibla, <a href="#page148">148</a>.</p>
+      <p>Icebergs transporting seeds, <a href="#page363">363</a>.</p>
+      <p>Increase, rate of, <a href="#page63">63</a>.</p>
+      <p>Individuals, numbers favourable to selection, <a href="#page102">102</a>.</p>
+      <p>&mdash;&mdash;, many, whether simultaneously created, <a href="#page355">355</a>.</p>
+      <p>Inheritance, laws of, <a href="#page12">12</a>.</p>
+      <p>&mdash;&mdash; at corresponding ages, <a href="#page14">14</a>, <a href="#page86">86</a>.</p>
+      <p>Insects, colour of, fitted for habitations, <a href="#page84">84</a>.</p>
+      <p>&mdash;&mdash;, sea-side, colours of, <a href="#page132">132</a>.</p>
+      <p>&mdash;&mdash;, blind, in caves, <a href="#page138">138</a>.</p>
+      <p>&mdash;&mdash;, luminous, <a href="#page193">193</a>.</p>
+      <p>&mdash;&mdash;, neuter, <a href="#page236">236</a>.</p>
+      <p>Instinct, <a href="#page207">207</a>.</p>
+      <p>Instincts, domestic, <a href="#page213">213</a>.</p>
+      <p>Intercrossing, advantages of, <a href="#page96">96</a>.</p>
+      <p>Islands, oceanic, <a href="#page388">388</a>.</p>
+      <p>Isolation favourable to selection, <a href="#page104">104</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>J.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Japan, productions of, <a href="#page372">372</a>.</p>
+      <p>Java, plants of, <a href="#page375">375</a>.</p>
+      <p>Jones, Mr. J. M., on the birds of Bermuda, <a href="#page391">391</a>.</p>
+      <p>Jussieu on classification, <a href="#page417">417</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>K.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Kentucky, caves of, <a href="#page137">137</a>.</p>
+      <p>Kerguelen-land, flora of, <a href="#page381">381</a>, <a href="#page399">399</a>.</p>
+      <p>Kidney-bean, acclimatisation of, <a href="#page142">142</a>.</p>
+      <p>Kidneys of birds, <a href="#page144">144</a>.</p>
+      <p>Kirby on tarsi deficient in beetles, <a href="#page135">135</a>.</p>
+      <p>Knight, Andrew, on cause of variation, <a href="#page7">7</a>.</p>
+      <p>Kölreuter on the barberry, <a href="#page98">98</a>.</p>
+      <p>&mdash;&mdash; on sterility of hybrids, <a href="#page246">246</a>.</p>
+      <p>&mdash;&mdash; on reciprocal crosses, <a href="#page258">258</a>.</p>
+      <p>&mdash;&mdash; on crossed varieties of nicotiana, <a href="#page271">271</a>.</p>
+      <p>&mdash;&mdash; on crossing male and hermaphrodite flowers, <a href="#page451">451</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>L.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Lamarck on adaptive characters, <a href="#page426">426</a>.</p>
+      <p>Land-shells, distribution of, <a href="#page397">397</a>.</p>
+      <p>&mdash;&mdash; of Madeira, naturalised, <a href="#page403">403</a>.</p>
+      <p>Languages, classification of, <a href="#page422">422</a>.</p>
+      <p>Lapse, great, of time, <a href="#page282">282</a>.</p>
+<!-- Page 497 --><span class="pagenum"><a name="page497"></a>[497]</span>
+      <p>Larvæ, <a href="#page440">440</a>.</p>
+      <p>Laurel, nectar secreted by the leaves,</p>
+      <p>Laws of variation, <a href="#page131">131</a>.</p>
+      <p>Leech, varieties of, <a href="#page76">76</a>.</p>
+      <p>Leguminosæ, nectar secreted by glands, <a href="#page92">92</a>.</p>
+      <p>Lepidosiren, <a href="#page107">107</a>, <a href="#page330">330</a>.</p>
+      <p>Life, struggle for, <a href="#page60">60</a>.</p>
+      <p>Lingula, Silurian, <a href="#page307">307</a>.</p>
+      <p>Linnæus, aphorism of, <a href="#page413">413</a>.</p>
+      <p>Lion, mane of, <a href="#page88">88</a>.</p>
+      <p>&mdash;&mdash;, young of, striped, <a href="#page439">439</a>.</p>
+      <p>Lobelia fulgens, <a href="#page73">73</a>, <a href="#page98">98</a>.</p>
+      <p>Lobelia, sterility of crosses, <a href="#page250">250</a>.</p>
+      <p>Loess of the Rhine, <a href="#page384">384</a>.</p>
+      <p>Lowness of structure connected with variability, <a href="#page149">149</a>.</p>
+      <p>Lowness, related to wide distribution, <a href="#page406">406</a>.</p>
+      <p>Lubbock, Mr., on the nerves of coccus, <a href="#page46">46</a>.</p>
+      <p>Lucas, Dr. P., on inheritance, <a href="#page12">12</a>.</p>
+      <p>&mdash;&mdash;, on resemblance of child to parent, <a href="#page275">275</a>.</p>
+      <p>Lund and Clausen on fossils of Brazil, <a href="#page339">339</a>.</p>
+      <p>Lyell, Sir C, on the struggle for existence, <a href="#page62">62</a>.</p>
+      <p>&mdash;&mdash;, on modern changes of the earth, <a href="#page95">95</a>.</p>
+      <p>&mdash;&mdash;, on measure of denudation, <a href="#page284">284</a>.</p>
+      <p>&mdash;&mdash;, on a carboniferous land-shell, <a href="#page289">289</a>.</p>
+      <p>&mdash;&mdash;, on strata beneath Silurian system, <a href="#page308">308</a>.</p>
+      <p>&mdash;&mdash;, on the imperfection of the geological record, <a href="#page311">311</a>.</p>
+      <p>&mdash;&mdash;, on the appearance of species, <a href="#page312">312</a>.</p>
+      <p>&mdash;&mdash;, on Barrande's colonies, <a href="#page313">313</a>.</p>
+      <p>&mdash;&mdash;, on tertiary formations of Europe and North America, <a href="#page323">323</a>.</p>
+      <p>&mdash;&mdash;, on parallelism of tertiary formations, <a href="#page328">328</a>.</p>
+      <p>&mdash;&mdash;, on transport of seeds by icebergs, <a href="#page363">363</a>.</p>
+      <p>&mdash;&mdash;, on great alternations of climate, <a href="#page382">382</a>.</p>
+      <p>&mdash;&mdash;, on the distribution of fresh-water shells, <a href="#page385">385</a>.</p>
+      <p>&mdash;&mdash;, on land-shells of Madeira, <a href="#page402">402</a>.</p>
+      <p>Lyell and Dawson on fossilized trees in Nova Scotia, <a href="#page297">297</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>M.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Macleay on analogical characters, <a href="#page426">426</a>.</p>
+      <p>Madeira, plants of, <a href="#page107">107</a>.</p>
+      <p>&mdash;&mdash;, beetles of, wingless, <a href="#page135">135</a>.</p>
+      <p>&mdash;&mdash;, fossil land-shells of, <a href="#page339">339</a>.</p>
+      <p>&mdash;&mdash;, birds of, <a href="#page390">390</a>.</p>
+      <p>Magpie tame in Norway, <a href="#page212">212</a>.</p>
+      <p>Maize, crossed, <a href="#page270">270</a>.</p>
+      <p>Malay Archipelago compared with Europe, <a href="#page300">300</a>.</p>
+      <p>&mdash;&mdash;, mammals of, <a href="#page395">395</a>.</p>
+      <p>Malpighiaceæ, <a href="#page417">417</a>.</p>
+      <p>Mammæ, rudimentary, <a href="#page451">451</a>.</p>
+      <p>Mammals, fossil, in secondary formation, <a href="#page304">304</a>.</p>
+      <p>&mdash;&mdash;, insular, <a href="#page394">394</a>.</p>
+      <p>Man, origin of races of, <a href="#page199">199</a>.</p>
+      <p>Manatee, rudimentary nails of, <a href="#page454">454</a>.</p>
+      <p>Marsupials of Australia, <a href="#page116">116</a>.</p>
+      <p>&mdash;&mdash;, fossil species of, <a href="#page339">339</a>.</p>
+      <p>Martens, M., experiment on seeds, <a href="#page360">360</a>.</p>
+      <p>Martin, Mr. W. C., on striped mules, <a href="#page165">165</a>.</p>
+      <p>Matteucci on the electric organs of rays, <a href="#page193">193</a>.</p>
+      <p>Matthiola, reciprocal crosses of, <a href="#page258">258</a>.</p>
+      <p>Means of dispersal, <a href="#page356">356</a>.</p>
+      <p>Melipona domestica, <a href="#page225">225</a>.</p>
+      <p>Metamorphism of oldest rocks, <a href="#page308">308</a>.</p>
+      <p>Mice destroying bees, <a href="#page74">74</a>.</p>
+      <p>&mdash;&mdash;, acclimatisation of, <a href="#page141">141</a>.</p>
+      <p>Migration, bears on first appearance of fossils, <a href="#page297">297</a>.</p>
+      <p>Miller, Prof., on the cells of bees, <a href="#page226">226</a>.</p>
+      <p>Mirabilis, crosses of, <a href="#page258">258</a>.</p>
+      <p>Missel-thrush, <a href="#page76">76</a>.</p>
+      <p>Misseltoe, complex relations of, <a href="#page3">3</a>.</p>
+      <p>Mississippi, rate of deposition at mouth, <a href="#page284">284</a>.</p>
+      <p>Mocking-thrush of the Galapagos, <a href="#page402">402</a>.</p>
+      <p>Modification of species, how far applicable, <a href="#page483">483</a>.</p>
+      <p>Moles, blind, <a href="#page137">137</a>.</p>
+      <p>Mongrels, fertility and sterility of, <a href="#page268">268</a>.</p>
+      <p>&mdash;&mdash; and hybrids compared, <a href="#page272">272</a>.</p>
+<!-- Page 498 --><span class="pagenum"><a name="page498"></a>[498]</span>
+      <p>Monkeys, fossil, <a href="#page304">304</a>.</p>
+      <p>Monocanthus, <a href="#page424">424</a>.</p>
+      <p>Mons, Van, on the origin of fruit-trees, <a href="#page29">29</a>.</p>
+      <p>Moquin-Tandon on sea-side plants, <a href="#page132">132</a>.</p>
+      <p>Morphology, <a href="#page433">433</a>.</p>
+      <p>Mozart, musical powers of, <a href="#page209">209</a>.</p>
+      <p>Mud, seeds in, <a href="#page386">386</a>.</p>
+      <p>Mules, striped, <a href="#page165">165</a>.</p>
+      <p>Müller, Dr. F., on Alpine Australian plants, <a href="#page375">375</a>.</p>
+      <p>Murchison, Sir R., on the formations of Russia, <a href="#page290">290</a>.</p>
+      <p>&mdash;&mdash;, on azoic formations, <a href="#page308">308</a>.</p>
+      <p>&mdash;&mdash;, on extinction, <a href="#page317">317</a>.</p>
+      <p>Mustela vison, <a href="#page179">179</a>.</p>
+      <p>Myanthus, <a href="#page424">424</a>.</p>
+      <p>Myrmecocystus, <a href="#page239">239</a>.</p>
+      <p>Myrmica, eyes of, <a href="#page240">240</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>N.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Nails, rudimentary, <a href="#page454">454</a>.</p>
+      <p>Natural history, future progress of, <a href="#page485">485</a>.</p>
+      <p>&mdash;&mdash; selection, <a href="#page80">80</a>.</p>
+      <p>&mdash;&mdash; system, <a href="#page413">413</a>.</p>
+      <p>Naturalisation of forms distinct from the indigenous species, <a href="#page115">115</a>.</p>
+      <p>&mdash;&mdash; in New Zealand, <a href="#page201">201</a>.</p>
+      <p>Nautilus, Silurian, <a href="#page307">307</a>.</p>
+      <p>Nectar of plants, <a href="#page92">92</a>.</p>
+      <p>Nectaries, how formed, <a href="#page92">92</a>.</p>
+      <p>Nelumbium luteum, <a href="#page387">387</a>.</p>
+      <p>Nests, variation in, <a href="#page211">211</a>.</p>
+      <p>Neuter insects, <a href="#page236">236</a>.</p>
+      <p>Newman, Mr., on humble-bees, <a href="#page74">74</a>.</p>
+      <p>New Zealand, productions of, not perfect, <a href="#page201">201</a>.</p>
+      <p>&mdash;&mdash;, naturalised products of, <a href="#page337">337</a>.</p>
+      <p>&mdash;&mdash;, fossil birds of, <a href="#page339">339</a>.</p>
+      <p>&mdash;&mdash;, glacial action in, <a href="#page373">373</a>.</p>
+      <p>&mdash;&mdash;, crustaceans of, <a href="#page376">376</a>.</p>
+      <p>&mdash;&mdash;, algæ of, <a href="#page376">376</a>.</p>
+      <p>&mdash;&mdash;, number of plants of, <a href="#page389">389</a>.</p>
+      <p>&mdash;&mdash;, flora of, <a href="#page399">399</a>.</p>
+      <p>Nicotiana, crossed varieties of, <a href="#page271">271</a>.</p>
+      <p>&mdash;&mdash;, certain species very sterile, <a href="#page257">257</a>.</p>
+      <p>Noble, Mr., on fertility of Rhododendron, <a href="#page252">252</a>.</p>
+      <p>Nodules, phosphatic, in azoic rocks, <a href="#page308">308</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>O.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Oak, varieties of, <a href="#page50">50</a>.</p>
+      <p>Onites apelles, <a href="#page135">135</a>.</p>
+      <p>Orchis, pollen of, <a href="#page193">193</a>.</p>
+      <p>Organs of extreme perfection, <a href="#page186">186</a>.</p>
+      <p>&mdash;&mdash;, electric, of fishes, <a href="#page192">192</a>.</p>
+      <p>&mdash;&mdash; of little importance, <a href="#page194">194</a>.</p>
+      <p>&mdash;&mdash;, homologous, <a href="#page434">434</a>.</p>
+      <p>&mdash;&mdash;, rudiments of, and nascent, <a href="#page450">450</a>.</p>
+      <p>Ornithorhynchus, <a href="#page107">107</a>, <a href="#page416">416</a>.</p>
+      <p>Ostrich not capable of flight, <a href="#page134">134</a>.</p>
+      <p>&mdash;&mdash;, habit of laying eggs together, <a href="#page218">218</a>.</p>
+      <p>&mdash;&mdash;, American, two species of, <a href="#page349">349</a>.</p>
+      <p>Otter, habits of, how acquired, <a href="#page179">179</a>.</p>
+      <p>Ouzel, water, <a href="#page185">185</a>.</p>
+      <p>Owen, Prof., on birds not flying, <a href="#page134">134</a>.</p>
+      <p>&mdash;&mdash;, on vegetative repetition, <a href="#page149">149</a>.</p>
+      <p>&mdash;&mdash;, on variable length of arms in ourang-outang, <a href="#page150">150</a>.</p>
+      <p>&mdash;&mdash;, on the swim-bladder of fishes, <a href="#page191">191</a>.</p>
+      <p>&mdash;&mdash;, on electric organs, <a href="#page192">192</a>.</p>
+      <p>&mdash;&mdash;, on fossil horse of La Plata, <a href="#page319">319</a>.</p>
+      <p>&mdash;&mdash;, on relations of ruminants and pachyderms, <a href="#page329">329</a>.</p>
+      <p>&mdash;&mdash;, on fossil birds of New Zealand, <a href="#page339">339</a>.</p>
+      <p>&mdash;&mdash;, on succession of types, <a href="#page339">339</a>.</p>
+      <p>&mdash;&mdash;, on affinities of the dugong, <a href="#page414">414</a>.</p>
+      <p>&mdash;&mdash;, on homologous organs, <a href="#page434">434</a>.</p>
+      <p>&mdash;&mdash;, on the metamorphosis of cephalopods and spiders, <a href="#page442">442</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>P.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Pacific Ocean, faunas of, <a href="#page348">348</a>.</p>
+      <p>Paley on no organ formed to give pain, <a href="#page201">201</a>.</p>
+      <p>Pallas on the fertility of the wild stocks of domestic animals, <a href="#page254">254</a>.</p>
+      <p>Paraguay, cattle destroyed by flies, <a href="#page72">72</a>.</p>
+      <p>Parasites, <a href="#page217">217</a>.</p>
+      <p>Partridge, dirt on feet, <a href="#page363">363</a>.</p>
+      <p>Parts greatly developed, variable, <a href="#page150">150</a>.</p>
+      <p>&mdash;&mdash;, degrees of utility of, <a href="#page201">201</a>.</p>
+      <p>Parus major, <a href="#page184">184</a>.</p>
+      <p>Passiflora, <a href="#page251">251</a>.</p>
+      <p>Peaches in United States, <a href="#page85">85</a>.</p>
+      <p>Pear, grafts of, <a href="#page262">262</a>.</p>
+<!-- Page 499 --><span class="pagenum"><a name="page499"></a>[499]</span>
+      <p>Pelargonium, flowers of, <a href="#page145">145</a>.</p>
+      <p>&mdash;&mdash;, sterility of, <a href="#page251">251</a>.</p>
+      <p>Pelvis of women, <a href="#page144">144</a>.</p>
+      <p>Peloria, <a href="#page145">145</a>.</p>
+      <p>Period, glacial, <a href="#page365">365</a>.</p>
+      <p>Petrels, habits of, <a href="#page184">184</a>.</p>
+      <p>Phasianus, fertility of hybrids, <a href="#page253">253</a>.</p>
+      <p>Pheasant, young, wild, <a href="#page216">216</a>.</p>
+      <p>Philippi on tertiary species in Sicily, <a href="#page312">312</a>.</p>
+      <p>Pictet, Prof., on groups of species suddenly appearing, <a href="#page302">302</a>, <a href="#page305">305</a>.</p>
+      <p>&mdash;&mdash;, on rate of organic change, <a href="#page313">313</a>.</p>
+      <p>&mdash;&mdash;, on continuous succession of genera, <a href="#page316">316</a>.</p>
+      <p>&mdash;&mdash;, on close alliance of fossils in consecutive formations, <a href="#page335">335</a>.</p>
+      <p>&mdash;&mdash;, on embryological succession, <a href="#page338">338</a>.</p>
+      <p>Pierce, Mr., on varieties of wolves, <a href="#page91">91</a>.</p>
+      <p>Pigeons with feathered feet and skin between toes, <a href="#page12">12</a>.</p>
+      <p>&mdash;&mdash;, breeds described, and origin of, <a href="#page20">20</a>.</p>
+      <p>&mdash;&mdash;, breeds of, how produced, <a href="#page39">39</a>, <a href="#page42">42</a>.</p>
+      <p>&mdash;&mdash;, tumbler, not being able to get out of egg, <a href="#page87">87</a>.</p>
+      <p>&mdash;&mdash;, reverting to blue colour, <a href="#page160">160</a>.</p>
+      <p>&mdash;&mdash;, instinct of tumbling, <a href="#page214">214</a>.</p>
+      <p>&mdash;&mdash;, carriers, killed by hawks, <a href="#page362">362</a>.</p>
+      <p>&mdash;&mdash;, young of, <a href="#page445">445</a>.</p>
+      <p>Pistil, rudimentary, <a href="#page451">451</a>.</p>
+      <p>Plants, poisonous, not affecting certain coloured animals, <a href="#page12">12</a>.</p>
+      <p>&mdash;&mdash;, selection applied to, <a href="#page32">32</a>.</p>
+      <p>&mdash;&mdash;, gradual improvement of, <a href="#page37">37</a>.</p>
+      <p>&mdash;&mdash; not improved in barbarous countries, <a href="#page38">38</a>.</p>
+      <p>&mdash;&mdash; destroyed by insects, <a href="#page67">67</a>.</p>
+      <p>&mdash;&mdash;, in midst of range, have to struggle with other plants, <a href="#page77">77</a>.</p>
+      <p>&mdash;&mdash;, nectar of, <a href="#page92">92</a>.</p>
+      <p>&mdash;&mdash;, fleshy, on sea-shores, <a href="#page132">132</a>.</p>
+      <p>&mdash;&mdash;, fresh-water, distribution of, <a href="#page386">386</a>.</p>
+      <p>&mdash;&mdash;, low in scale, widely distributed, <a href="#page406">406</a>.</p>
+      <p>Plumage, laws of change in sexes of birds, <a href="#page89">89</a>.</p>
+      <p>Plums in the United States, <a href="#page85">85</a>.</p>
+      <p>Pointer dog, origin of, <a href="#page35">35</a>.</p>
+      <p>&mdash;&mdash;, habits of, <a href="#page213">213</a>.</p>
+      <p>Poison not affecting certain coloured animals, <a href="#page12">12</a>.</p>
+      <p>&mdash;&mdash;, similar effect of, on animals and plants, <a href="#page484">484</a>.</p>
+      <p>Pollen of fir-trees, <a href="#page203">203</a>.</p>
+      <p>Poole, Col., on striped hemionus, <a href="#page163">163</a>.</p>
+      <p>Potamogeton, <a href="#page387">387</a>.</p>
+      <p>Prestwich, Mr., on English and French eocene formations, <a href="#page328">328</a>.</p>
+      <p>Primrose, <a href="#page49">49</a>.</p>
+      <p>&mdash;&mdash;, sterility of, <a href="#page247">247</a>.</p>
+      <p>Primula, varieties of, <a href="#page49">49</a>.</p>
+      <p>Proteolepas, <a href="#page148">148</a>.</p>
+      <p>Proteus, <a href="#page139">139</a>.</p>
+      <p>Psychology, future progress of, <a href="#page489">489</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>Q.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Quagga, striped, <a href="#page165">165</a>.</p>
+      <p>Quince, grafts of, <a href="#page262">262</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>R.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Rabbit, disposition of young, <a href="#page215">215</a>.</p>
+      <p>Races, domestic, characters of, <a href="#page16">16</a>.</p>
+      <p>Race-horses, Arab, <a href="#page35">35</a>.</p>
+      <p>&mdash;&mdash;, English, <a href="#page356">356</a>.</p>
+      <p>Ramond on plants of Pyrenees, <a href="#page368">368</a>.</p>
+      <p>Ramsay, Prof., on thickness of the British formations, <a href="#page284">284</a>.</p>
+      <p>&mdash;&mdash;, on faults, <a href="#page285">285</a>.</p>
+      <p>Ratio of increase, <a href="#page63">63</a>.</p>
+      <p>Rats, supplanting each other, <a href="#page76">76</a>.</p>
+      <p>&mdash;&mdash;, acclimatisation of, <a href="#page141">141</a>.</p>
+      <p>&mdash;&mdash;, blind in cave, <a href="#page137">137</a>.</p>
+      <p>Rattle-snake, <a href="#page201">201</a>.</p>
+      <p>Reason and instinct, <a href="#page208">208</a>.</p>
+      <p>Recapitulation, general, <a href="#page459">459</a>.</p>
+      <p>Reciprocity of crosses, <a href="#page258">258</a>.</p>
+      <p>Record, geological, imperfect, <a href="#page279">279</a>.</p>
+      <p>Rengger on flies destroying cattle, <a href="#page72">72</a>.</p>
+      <p>Reproduction, rate of, <a href="#page63">63</a>.</p>
+      <p>Resemblance to parents in mongrels and hybrids, <a href="#page273">273</a>.</p>
+      <p>Reversion, law of inheritance, <a href="#page14">14</a>.</p>
+      <p>&mdash;&mdash; in pigeons to blue colour, <a href="#page160">160</a>.</p>
+      <p>Rhododendron, sterility of, <a href="#page251">251</a>.</p>
+      <p>Richard, Prof., on Aspicarpa, <a href="#page417">417</a>.</p>
+      <p>Richardson, Sir J., on structure of squirrels, <a href="#page180">180</a>.</p>
+      <p>&mdash;&mdash;, on fishes of the southern hemisphere, <a href="#page376">376</a>.</p>
+      <p>Robinia, grafts of, <a href="#page262">262</a>.</p>
+<!-- Page 500 --><span class="pagenum"><a name="page500"></a>[500]</span>
+      <p>Rodents, blind, <a href="#page137">137</a>.</p>
+      <p>Rudimentary organs, <a href="#page450">450</a>.</p>
+      <p>Rudiments important for classification, <a href="#page416">416</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>S.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Sagaret on grafts, <a href="#page262">262</a>.</p>
+      <p>Salmons, males fighting, and hooked jaws of, <a href="#page88">88</a>.</p>
+      <p>Salt-water, how far injurious to seeds, <a href="#page358">358</a>.</p>
+      <p>Saurophagus sulphuratus, <a href="#page183">183</a>.</p>
+      <p>Schiödte on blind insects, <a href="#page138">138</a>.</p>
+      <p>Schlegel on snakes, <a href="#page144">144</a>.</p>
+      <p>Sea-water, how far injurious to seeds, <a href="#page358">358</a>.</p>
+      <p>Sebright, Sir J., on crossed animals, <a href="#page20">20</a>.</p>
+      <p>&mdash;&mdash;, on selection of pigeons, <a href="#page31">31</a>.</p>
+      <p>Sedgwick, Prof., on groups of species suddenly appearing, <a href="#page302">302</a>.</p>
+      <p>Seedlings destroyed by insects, <a href="#page67">67</a>.</p>
+      <p>Seeds, nutriment in, <a href="#page77">77</a>.</p>
+      <p>&mdash;&mdash;, winged, <a href="#page146">146</a>.</p>
+      <p>&mdash;&mdash;, power of resisting salt-water, <a href="#page358">358</a>.</p>
+      <p>&mdash;&mdash; in crops and intestines of birds, <a href="#page361">361</a>.</p>
+      <p>&mdash;&mdash; eaten by fish, <a href="#page362">362</a>, <a href="#page387">387</a>.</p>
+      <p>&mdash;&mdash; in mud, <a href="#page386">386</a>.</p>
+      <p>&mdash;&mdash;, hooked, on islands, <a href="#page392">392</a>.</p>
+      <p>Selection of domestic products, <a href="#page29">29</a>.</p>
+      <p>&mdash;&mdash;, principle not of recent origin, <a href="#page33">33</a>.</p>
+      <p>&mdash;&mdash;, unconscious, <a href="#page34">34</a>.</p>
+      <p>&mdash;&mdash;, natural, <a href="#page80">80</a>.</p>
+      <p>&mdash;&mdash;, sexual, <a href="#page87">87</a>.</p>
+      <p>&mdash;&mdash;, natural, circumstances favourable to, <a href="#page102">102</a>.</p>
+      <p>Sexes, relations of, <a href="#page87">87</a>.</p>
+      <p>Sexual characters variable, <a href="#page156">156</a>.</p>
+      <p>&mdash;&mdash; selection, <a href="#page87">87</a>.</p>
+      <p>Sheep, Merino, their selection, <a href="#page31">31</a>.</p>
+      <p>&mdash;&mdash;, two sub-breeds unintentionally produced, <a href="#page36">36</a>.</p>
+      <p>&mdash;&mdash;, mountain, varieties of, <a href="#page76">76</a>.</p>
+      <p>Shells, colours of, <a href="#page132">132</a>.</p>
+      <p>&mdash;&mdash;, littoral, seldom embedded, <a href="#page288">288</a>.</p>
+      <p>&mdash;&mdash;, fresh-water, dispersal of, <a href="#page385">385</a></p>
+      <p>&mdash;&mdash; of Madeira, <a href="#page391">391</a>.</p>
+      <p>&mdash;&mdash;, land, distribution of, <a href="#page397">397</a>.</p>
+      <p>Silene, fertility of crosses, <a href="#page257">257</a>.</p>
+      <p>Silliman, Prof., on blind rat, <a href="#page137">137</a>.</p>
+      <p>Skulls of young mammals, <a href="#page197">197</a>, <a href="#page436">436</a>.</p>
+      <p>Slave-making instinct, <a href="#page219">219</a>.</p>
+      <p>Smith, Col. Hamilton, on striped horses, <a href="#page164">164</a>.</p>
+      <p>&mdash;&mdash;, Mr. Fred., on slave-making ants, <a href="#page219">219</a>.</p>
+      <p>&mdash;&mdash;, on neuter ants, <a href="#page239">239</a>.</p>
+      <p>&mdash;&mdash;, Mr., of Jordan Hill, on the degradation of coast-rocks, <a href="#page283">283</a>.</p>
+      <p>Snap-dragon, <a href="#page161">161</a>.</p>
+      <p>Somerville, Lord, on selection of sheep, <a href="#page31">31</a>.</p>
+      <p>Sorbus, grafts of, <a href="#page262">262</a>.</p>
+      <p>Spaniel, King Charles's breed, <a href="#page35">35</a>.</p>
+      <p>Species, polymorphic, <a href="#page46">46</a>.</p>
+      <p>&mdash;&mdash;, common, variable, <a href="#page53">53</a>.</p>
+      <p>&mdash;&mdash; in large genera variable, <a href="#page54">54</a>.</p>
+      <p>&mdash;&mdash;, groups of, suddenly appearing, <a href="#page302">302</a>, <a href="#page307">307</a>.</p>
+      <p>&mdash;&mdash; beneath Silurian formations, <a href="#page307">307</a>.</p>
+      <p>&mdash;&mdash; successively appearing, <a href="#page312">312</a>.</p>
+      <p>&mdash;&mdash; changing simultaneously throughout the world, <a href="#page322">322</a>.</p>
+      <p>Spencer, Lord, on increase in size of cattle, <a href="#page35">35</a>.</p>
+      <p>Sphex, parasitic, <a href="#page218">218</a>.</p>
+      <p>Spiders, development of, <a href="#page442">442</a>.</p>
+      <p>Spitz-dog crossed with fox, <a href="#page268">268</a>.</p>
+      <p>Sports in plants, <a href="#page9">9</a>.</p>
+      <p>Sprengel, C. C, on crossing, <a href="#page98">98</a>.</p>
+      <p>&mdash;&mdash;, on ray-florets, <a href="#page145">145</a>.</p>
+      <p>Squirrels, gradations in structure, <a href="#page180">180</a>.</p>
+      <p>Staffordshire, heath, changes in, <a href="#page71">71</a>.</p>
+      <p>Stag-beetles, fighting, <a href="#page88">88</a>.</p>
+      <p>Sterility from changed conditions of life, <a href="#page9">9</a>.</p>
+      <p>&mdash;&mdash; of hybrids, <a href="#page246">246</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash;, laws of, <a href="#page255">255</a>.</p>
+      <p>&mdash;&mdash; &mdash;&mdash;, causes of, <a href="#page263">263</a>.</p>
+      <p>&mdash;&mdash; from unfavourable conditions, <a href="#page265">265</a>.</p>
+      <p>&mdash;&mdash; of certain varieties, <a href="#page269">269</a>.</p>
+      <p>St. Helena, productions of, <a href="#page390">390</a>.</p>
+      <p>St. Hilaire, Aug., on classification, <a href="#page418">418</a>.</p>
+      <p>St. John, Mr., on habits of cats, <a href="#page91">91</a>.</p>
+      <p>Sting of bee, <a href="#page202">202</a>.</p>
+      <p>Stocks, aboriginal, of domestic animals, <a href="#page18">18</a>.</p>
+      <p>Strata, thickness of, in Britain, <a href="#page284">284</a>.</p>
+      <p>Stripes on horses, <a href="#page163">163</a>.</p>
+<!-- Page 501 --><span class="pagenum"><a name="page501"></a>[501]</span>
+      <p>Structure, degrees of utility of, <a href="#page201">201</a>.</p>
+      <p>Struggle for existence, <a href="#page60">60</a>.</p>
+      <p>Succession, geological, <a href="#page312">312</a>.</p>
+      <p>Succession of types in same areas, <a href="#page338">338</a>.</p>
+      <p>Swallow, one species supplanting another, <a href="#page76">76</a>.</p>
+      <p>Swim-bladder, <a href="#page190">190</a>.</p>
+      <p>System, natural, <a href="#page413">413</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>T.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Tail of giraffe, <a href="#page195">195</a>.</p>
+      <p>&mdash;&mdash; of aquatic animals, <a href="#page196">196</a>.</p>
+      <p>&mdash;&mdash;, rudimentary, <a href="#page454">454</a>.</p>
+      <p>Tarsi deficient, <a href="#page135">135</a>.</p>
+      <p>Tausch on umbelliferous flowers, <a href="#page146">146</a>.</p>
+      <p>Teeth and hair correlated, <a href="#page144">144</a>.</p>
+      <p>&mdash;&mdash;, embryonic, traces of, in birds, <a href="#page450">450</a>.</p>
+      <p>&mdash;&mdash;, rudimentary, in embryonic calf, <a href="#page450">450</a>, <a href="#page480">480</a>.</p>
+      <p>Tegetmeier, Mr., on cells of bees, <a href="#page228">228</a>, <a href="#page233">233</a>.</p>
+      <p>Temminck on distribution aiding classification, <a href="#page419">419</a>.</p>
+      <p>Thouin on grafts, <a href="#page262">262</a>.</p>
+      <p>Thrush, aquatic species of, <a href="#page185">185</a>.</p>
+      <p>&mdash;&mdash;, mocking, of the Galapagos, <a href="#page402">402</a>.</p>
+      <p>&mdash;&mdash;, young of, spotted, <a href="#page439">439</a>.</p>
+      <p>&mdash;&mdash;, nest of, <a href="#page243">243</a>.</p>
+      <p>Thuret, M., on crossed fuci, <a href="#page258">258</a>.</p>
+      <p>Thwaites, Mr., on acclimatisation, <a href="#page140">140</a>.</p>
+      <p>Tierra del Fuego, dogs of, <a href="#page215">215</a>.</p>
+      <p>&mdash;&mdash;, plants of, <a href="#page374">374</a>, <a href="#page378">378</a>.</p>
+      <p>Timber-drift, <a href="#page360">360</a>.</p>
+      <p>Time, lapse of, <a href="#page282">282</a>.</p>
+      <p>Titmouse, <a href="#page184">184</a>.</p>
+      <p>Toads on islands, <a href="#page393">393</a>.</p>
+      <p>Tobacco, crossed varieties of, <a href="#page271">271</a>.</p>
+      <p>Tomes, Mr., on the distribution of bats, <a href="#page395">395</a>.</p>
+      <p>Transitions in varieties rare, <a href="#page172">172</a>.</p>
+      <p>Trees on islands belong to peculiar orders, <a href="#page392">392</a>.</p>
+      <p>&mdash;&mdash; with separated sexes, <a href="#page99">99</a>.</p>
+      <p>Trifolium pratense, <a href="#page73">73</a>, <a href="#page94">94</a>.</p>
+      <p>&mdash;&mdash; incarnatum, <a href="#page94">94</a>.</p>
+      <p>Trigonia, <a href="#page321">321</a>.</p>
+      <p>Trilobites, <a href="#page307">307</a>.</p>
+      <p>&mdash;&mdash;, sudden extinction of, <a href="#page321">321</a>.</p>
+      <p>Troglodytes, <a href="#page243">243</a>.</p>
+      <p>Tucutucu, blind, <a href="#page137">137</a>.</p>
+      <p>Tumbler pigeons, habits of, hereditary, <a href="#page214">214</a>.</p>
+      <p>&mdash;&mdash;, young of, <a href="#page446">446</a>.</p>
+      <p>Turkey-cock, brush of hair on breast, <a href="#page90">90</a>.</p>
+      <p>Turkey, naked skin on head, <a href="#page197">197</a>.</p>
+      <p>&mdash;&mdash;, young, wild, <a href="#page216">216</a>.</p>
+      <p>Turnip and cabbage, analogous variations of, <a href="#page159">159</a>.</p>
+      <p>Type, unity of, <a href="#page206">206</a>.</p>
+      <p>Types, succession of, in same areas, <a href="#page339">339</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>U.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Udders enlarged by use, <a href="#page11">11</a>.</p>
+      <p>&mdash;&mdash;, rudimentary, <a href="#page451">451</a>.</p>
+      <p>Ulex, young leaves of, <a href="#page439">439</a>.</p>
+      <p>Umbelliferæ, outer and inner florets of, <a href="#page144">144</a>.</p>
+      <p>Unity of type, <a href="#page206">206</a>.</p>
+      <p>Use, effects of, under domestication, <a href="#page11">11</a>.</p>
+      <p>&mdash;&mdash;, effects of, in a state of nature, <a href="#page134">134</a>.</p>
+      <p>Utility, how far important in the construction of each part, <a href="#page199">199</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>V.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Valenciennes on fresh-water fish, <a href="#page384">384</a>.</p>
+      <p>Variability of mongrels and hybrids, <a href="#page274">274</a>.</p>
+      <p>Variation under domestication, <a href="#page7">7</a>.</p>
+      <p>&mdash;&mdash; caused by reproductive system being affected by conditions of life, <a href="#page8">8</a>.</p>
+      <p>&mdash;&mdash; under nature, <a href="#page44">44</a>.</p>
+      <p>&mdash;&mdash;, laws of, <a href="#page131">131</a>.</p>
+      <p>Variations appear at corresponding ages, <a href="#page14">14</a>, <a href="#page86">86</a>.</p>
+      <p>&mdash;&mdash;, analogous in distinct species, <a href="#page159">159</a>.</p>
+      <p>Varieties, natural, <a href="#page44">44</a>.</p>
+      <p>&mdash;&mdash;, struggle between, <a href="#page75">75</a>.</p>
+      <p>&mdash;&mdash;, domestic, extinction of, <a href="#page111">111</a>.</p>
+      <p>&mdash;&mdash;, transitional, rarity of, <a href="#page172">172</a>.</p>
+      <p>&mdash;&mdash;, when crossed, fertile, <a href="#page268">268</a>.</p>
+      <p>&mdash;&mdash;, when crossed, sterile, <a href="#page269">269</a>.</p>
+      <p>&mdash;&mdash;, classification of, <a href="#page423">423</a>.</p>
+      <p>Verbascum, sterility of, <a href="#page251">251</a>.</p>
+      <p>&mdash;&mdash;, varieties of, crossed, <a href="#page271">271</a>.</p>
+      <p>Verneuil, M. de, on the succession of species, <a href="#page325">325</a>.</p>
+      <p>Viola tricolor, <a href="#page73">73</a>.</p>
+<!-- Page 502 --><span class="pagenum"><a name="page502"></a>[502]</span>
+      <p>Volcanic islands, denudation of, <a href="#page285">285</a>.</p>
+      <p>Vulture, naked skin on head, <a href="#page197">197</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>W.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Wading-birds, <a href="#page386">386</a>.</p>
+      <p>Wallace, Mr., on origin of species, <a href="#page2">2</a>.</p>
+      <p>&mdash;&mdash;, on law of geographical distribution, <a href="#page355">355</a>.</p>
+      <p>&mdash;&mdash;, on the Malay Archipelago, <a href="#page395">395</a>.</p>
+      <p>Wasp, sting of, <a href="#page202">202</a>.</p>
+      <p>Water, fresh, productions of, <a href="#page383">383</a>.</p>
+      <p>Water-hen, <a href="#page185">185</a>.</p>
+      <p>Waterhouse, Mr., on Australian marsupials, <a href="#page116">116</a>.</p>
+      <p>&mdash;&mdash;, on greatly developed parts being variable, <a href="#page150">150</a>.</p>
+      <p>&mdash;&mdash;, on the cells of bees, <a href="#page225">225</a>.</p>
+      <p>&mdash;&mdash;, on general affinities, <a href="#page429">429</a>.</p>
+      <p>Water-ouzel, <a href="#page185">185</a>.</p>
+      <p>Watson, Mr. H. C, on range of varieties of British plants, <a href="#page58">58</a>.</p>
+      <p>&mdash;&mdash;, on acclimatisation, <a href="#page140">140</a>.</p>
+      <p>&mdash;&mdash;, on flora of Azores, <a href="#page363">363</a>.</p>
+      <p>&mdash;&mdash;, on Alpine plants, <a href="#page368">368</a>, <a href="#page376">376</a>.</p>
+      <p>&mdash;&mdash;, on rarity of intermediate varieties, <a href="#page176">176</a>.</p>
+      <p>Weald, denudation of, <a href="#page285">285</a>.</p>
+      <p>Web of feet in water-birds, <a href="#page185">185</a>.</p>
+      <p>West Indian islands, mammals of, <a href="#page396">396</a>.</p>
+      <p>Westwood on species in large genera being closely allied to others, <a href="#page57">57</a>.</p>
+      <p>&mdash;&mdash; on the tarsi of Engidæ, <a href="#page157">157</a>.</p>
+      <p>&mdash;&mdash; on the antennæ of hymenopterous insects, <a href="#page415">415</a>.</p>
+      <p>Wheat, varieties of, <a href="#page113">113</a>.</p>
+      <p>White Mountains, flora of, <a href="#page365">365</a>.</p>
+      <p>Wings, reduction of size, <a href="#page134">134</a>.</p>
+      <p>&mdash;&mdash; of insects homologous with branchiæ, <a href="#page191">191</a>.</p>
+      <p>&mdash;&mdash;, rudimentary, in insects, <a href="#page450">450</a>.</p>
+      <p>Wolf crossed with dog, <a href="#page214">214</a>.</p>
+      <p>&mdash;&mdash; of Falkland Isles, <a href="#page394">394</a>.</p>
+      <p>Wollaston, Mr., on varieties of insects, <a href="#page48">48</a>.</p>
+      <p>&mdash;&mdash;, on fossil varieties of land-shells in Madeira, <a href="#page52">52</a>.</p>
+      <p>&mdash;&mdash;, on colours of insects on sea-shore, <a href="#page132">132</a>.</p>
+      <p>&mdash;&mdash;, on wingless beetles, <a href="#page135">135</a>.</p>
+      <p>&mdash;&mdash;, on rarity of intermediate varieties, <a href="#page176">176</a>.</p>
+      <p>&mdash;&mdash;, on insular insects, <a href="#page389">389</a>.</p>
+      <p>&mdash;&mdash;, on land-shells of Madeira, naturalised, <a href="#page402">402</a>.</p>
+      <p>Wolves, varieties of, <a href="#page90">90</a>.</p>
+      <p>Woodpecker, habits of, <a href="#page184">184</a>.</p>
+      <p>&mdash;&mdash;, green colour of, <a href="#page197">197</a>.</p>
+      <p>Woodward, Mr., on the duration of specific forms, <a href="#page294">294</a>.</p>
+      <p>&mdash;&mdash;, on the continuous succession of genera, <a href="#page316">316</a>.</p>
+      <p>&mdash;&mdash;, on the succession of types, <a href="#page339">339</a>.</p>
+      <p>World, species changing simultaneously throughout, <a href="#page322">322</a>.</p>
+      <p>Wrens, nest of, <a href="#page243">243</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>Y.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Youatt, Mr., on selection, <a href="#page31">31</a>.</p>
+      <p>&mdash;&mdash;, on sub-breeds of sheep, <a href="#page36">36</a>.</p>
+      <p>&mdash;&mdash;, on rudimentary horns in young cattle, <a href="#page454">454</a>.</p>
+    </div>
+
+    <div class="stanza">
+      <p class="i8"><b>Z.</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Zebra, stripes on, <a href="#page163">163</a>.</p>
+    </div>
+  </div>
+<h3>THE END.</h3>
+
+<hr />
+
+<p class="cenhead"><span class="scac">LONDON: PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET, AND CHARING CROSS.</span></p>
+
+<div style='display:block;margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK ORIGIN OF SPECIES ***</div>
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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #22764 (https://www.gutenberg.org/ebooks/22764)
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+The Project Gutenberg EBook of On the Origin of Species by Means of
+Natural Selection, by Charles Darwin
+
+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: On the Origin of Species by Means of Natural Selection
+       or the Preservation of Favoured Races in the Struggle for
+       Life. (2nd edition)
+
+Author: Charles Darwin
+
+Release Date: September 25, 2007 [EBook #22764]
+
+Language: English
+
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ORIGIN OF SPECIES ***
+
+
+
+
+Produced by Steven Gibbs, Keith Edkins and the Online
+Distributed Proofreading Team at http://www.pgdp.net
+
+
+
+
+
+Transcriber's note: A few typographical errors have been corrected: they
+are listed at the end of the text.
+
+       *       *       *       *       *
+
+
+ON THE
+
+ORIGIN OF SPECIES.
+
+       *       *       *       *       *
+
+"But with regard to the material world, we can at least go so far as
+this--we can perceive that events are brought about not by insulated
+interpositions of Divine power, exerted in each particular case, but by the
+establishment of general laws."
+
+WHEWELL: _Bridgewater Treatise_.
+
+"The only distinct meaning of the word 'natural' is _stated_, _fixed_, or
+_settled_; since what is natural as much requires and presupposes an
+intelligent agent to render it so, _i.e._ to effect it continually or at
+stated times, as what is supernatural or miraculous does to effect it for
+once."
+
+BUTLER: _Analogy of Revealed Religion_.
+
+"To conclude, therefore, let no man out of a weak conceit of sobriety, or
+an ill-applied moderation, think or maintain, that a man can search too far
+or be too well studied in the book of God's word, or in the book of God's
+works; divinity or philosophy; but rather let men endeavour an endless
+progress or proficience in both."
+
+BACON: _Advancement of Learning_.
+
+       *       *       *       *       *
+
+  _Down, Bromley, Kent,_
+      _October 1st, 1859._ (_1st Thousand_).
+
+       *       *       *       *       *
+
+
+ON
+
+THE ORIGIN OF SPECIES
+
+BY MEANS OF NATURAL SELECTION,
+
+OR THE
+
+PRESERVATION OF FAVOURED RACES IN THE STRUGGLE
+FOR LIFE.
+
+BY CHARLES DARWIN, M.A.,
+
+FELLOW OF THE ROYAL, GEOLOGICAL, LINNEAN, ETC., SOCIETIES;
+
+AUTHOR OF 'JOURNAL OF RESEARCHES DURING H. M. S. BEAGLE'S VOYAGE
+ROUND THE WORLD.'
+
+_FIFTH THOUSAND._
+
+LONDON:
+JOHN MURRAY, ALBEMARLE STREET.
+1860.
+
+_The right of Translation is reserved._
+
+       *       *       *       *       *
+
+LONDON: PRINTED BY W. CLOWES AND SONS, STAMFORD STREET,
+AND CHARING CROSS.
+
+       *       *       *       *       *
+
+
+{v}
+
+CONTENTS.
+
+       *       *       *       *       *
+
+INTRODUCTION
+
+Page 1
+
+CHAPTER I.
+
+VARIATION UNDER DOMESTICATION.
+
+Causes of Variability--Effects of Habit--Correlation of
+Growth--Inheritance--Character of Domestic Varieties--Difficulty of
+distinguishing between Varieties and Species--Origin of Domestic Varieties
+from one or more Species--Domestic Pigeons, their Differences and
+Origin--Principle of Selection anciently followed, its Effects--Methodical
+and Unconscious Selection--Unknown Origin of our Domestic
+Productions--Circumstances favourable to Man's power of Selection
+
+7-43
+
+CHAPTER II.
+
+VARIATION UNDER NATURE.
+
+Variability--Individual differences--Doubtful species--Wide ranging, much
+diffused, and common species vary most--Species of the larger genera in any
+country vary more than the species of the smaller genera--Many of the
+species of the larger genera resemble varieties in being very closely, but
+unequally, related to each other, and in having restricted ranges
+
+44-59
+
+{vi}
+
+CHAPTER III.
+
+STRUGGLE FOR EXISTENCE.
+
+Its bearing on natural selection--The term used in a wide
+sense--Geometrical powers of increase--Rapid increase of naturalised
+animals and plants--Nature of the checks to increase--Competition
+universal--Effects of climate--Protection from the number of
+individuals--Complex relations of all animals and plants throughout
+nature--Struggle for life most severe between individuals and varieties of
+the same species; often severe between species of the same genus--The
+relation of organism to organism the most important of all relations
+
+60-79
+
+CHAPTER IV.
+
+NATURAL SELECTION.
+
+Natural Selection--its power compared with man's selection--its power on
+characters of trifling importance--its power at all ages and on both
+sexes--Sexual Selection--On the generality of intercrosses between
+individuals of the same species--Circumstances favourable and unfavourable
+to Natural Selection, namely, intercrossing, isolation, number of
+individuals--Slow action--Extinction caused by Natural
+Selection--Divergence of Character, related to the diversity of inhabitants
+of any small area, and to naturalisation--Action of Natural Selection,
+through Divergence of Character and Extinction, on the descendants from a
+common parent--Explains the Grouping of all organic beings
+
+80-130
+
+CHAPTER V.
+
+LAWS OF VARIATION.
+
+Effects of external conditions--Use and disuse, combined with natural
+selection; organs of flight and of vision--Acclimatisation--Correlation of
+growth--Compensation and economy of growth--False correlations--Multiple,
+rudimentary, and lowly organised structures variable--Parts developed in an
+unusual manner are highly variable: specific characters more variable than
+generic: secondary sexual characters variable--Species of the same genus
+vary in an analogous manner--Reversions to long-lost characters--Summary
+
+131-170
+
+{vii}
+
+CHAPTER VI.
+
+DIFFICULTIES ON THEORY.
+
+Difficulties on the theory of descent with
+modification--Transitions--Absence or rarity of transitional
+varieties--Transitions in habits of life--Diversified habits in the same
+species--Species with habits widely different from those of their
+allies--Organs of extreme perfection--Means of transition--Cases of
+difficulty--Natura non facit saltum--Organs of small importance--Organs not
+in all cases absolutely perfect--The law of Unity of Type and of the
+Conditions of Existence embraced by the theory of Natural Selection
+
+171-206
+
+CHAPTER VII.
+
+INSTINCT.
+
+Instincts comparable with habits, but different in their origin--Instincts
+graduated--Aphides and ants--Instincts variable--Domestic instincts, their
+origin--Natural instincts of the cuckoo, ostrich, and parasitic
+bees--Slave-making ants--Hive-bee, its cell-making instinct--Difficulties
+on the theory of the Natural Selection of instincts--Neuter or sterile
+insects--Summary
+
+207-244
+
+CHAPTER VIII.
+
+HYBRIDISM.
+
+Distinction between the sterility of first crosses and of
+hybrids--Sterility various in degree, not universal, affected by close
+interbreeding, removed by domestication--Laws governing the sterility of
+hybrids--Sterility not a special endowment, but incidental on other
+differences--Causes of the sterility of first crosses and of
+hybrids--Parallelism between the effects of changed conditions of life and
+crossing--Fertility of varieties when crossed and of their mongrel
+offspring not universal--Hybrids and mongrels compared independently of
+their fertility--Summary
+
+245-278
+
+{viii}
+
+CHAPTER IX.
+
+ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
+
+On the absence of intermediate varieties at the present day--On the nature
+of extinct intermediate varieties; on their number--On the vast lapse of
+time, as inferred from the rate of deposition and of denudation--On the
+poorness of our palæontological collections--On the intermittence of
+geological formations--On the absence of intermediate varieties in any one
+formation--On the sudden appearance of groups of species--On their sudden
+appearance in the lowest known fossiliferous strata
+
+279-311
+
+CHAPTER X.
+
+ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
+
+On the slow and successive appearance of new species--On their different
+rates of change--Species once lost do not reappear--Groups of species
+follow the same general rules in their appearance and disappearance as do
+single species--On Extinction--On simultaneous changes in the forms of life
+throughout the world--On the affinities of extinct species to each other
+and to living species--On the state of development of ancient forms--On the
+succession of the same types within the same areas--Summary of preceding
+and present chapters
+
+312-345
+
+CHAPTER XI.
+
+GEOGRAPHICAL DISTRIBUTION.
+
+Present distribution cannot be accounted for by differences in physical
+conditions--Importance of barriers--Affinity of the productions of the same
+continent--Centres of creation--Means of dispersal, by changes of climate
+and of the level of the land, and by occasional means--Dispersal during the
+Glacial period co-extensive with the world
+
+346-382
+
+CHAPTER XII.
+
+GEOGRAPHICAL DISTRIBUTION--_continued_.
+
+Distribution of fresh-water productions--On the inhabitants of oceanic
+islands--Absence of Batrachians and of terrestrial Mammals--On the relation
+of the inhabitants of islands to those of the nearest mainland--On
+colonisation from the nearest source with subsequent modification--Summary
+of the last and present chapters
+
+383-410
+
+CHAPTER XIII.
+
+MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY
+ORGANS.
+
+CLASSIFICATION, groups subordinate to groups--Natural system--Rules and
+difficulties in classification, explained on the theory of descent with
+modification--Classification of varieties--Descent always used in
+classification--Analogical or adaptive characters--Affinities, general,
+complex and radiating--Extinction separates and defines groups--MORPHOLOGY,
+between members of the same class, between parts of the same
+individual--EMBRYOLOGY, laws of, explained by variations not supervening at
+an early age, and being inherited at a corresponding age--RUDIMENTARY
+ORGANS; their origin explained--Summary
+
+411-458
+
+CHAPTER XIV.
+
+RECAPITULATION AND CONCLUSION.
+
+Recapitulation of the difficulties on the theory of Natural
+Selection--Recapitulation of the general and special circumstances in its
+favour--Causes of the general belief in the immutability of species--How
+far the theory of natural selection may be extended--Effects of its
+adoption on the study of Natural history--Concluding remarks
+
+459-490
+
+       *       *       *       *       *
+
+
+{1}
+
+ON THE ORIGIN OF SPECIES.
+
+       *       *       *       *       *
+
+INTRODUCTION.
+
+When on board H.M.S. 'Beagle,' as naturalist, I was much struck with
+certain facts in the distribution of the inhabitants of South America, and
+in the geological relations of the present to the past inhabitants of that
+continent. These facts seemed to me to throw some light on the origin of
+species--that mystery of mysteries, as it has been called by one of our
+greatest philosophers. On my return home, it occurred to me, in 1837, that
+something might perhaps be made out on this question by patiently
+accumulating and reflecting on all sorts of facts which could possibly have
+any bearing on it. After five years' work I allowed myself to speculate on
+the subject, and drew up some short notes; these I enlarged in 1844 into a
+sketch of the conclusions, which then seemed to me probable: from that
+period to the present day I have steadily pursued the same object. I hope
+that I may be excused for entering on these personal details, as I give
+them to show that I have not been hasty in coming to a decision.
+
+My work is now nearly finished; but as it will take me two or three more
+years to complete it, and as my health is far from strong, I have been
+urged to publish this Abstract. I have more especially been induced to do
+this, as Mr. Wallace, who is now studying the {2} natural history of the
+Malay archipelago, has arrived at almost exactly the same general
+conclusions that I have on the origin of species. Last year he sent me a
+memoir on this subject, with a request that I would forward it to Sir
+Charles Lyell, who sent it to the Linnean Society, and it is published in
+the third volume of the Journal of that Society. Sir C. Lyell and Dr.
+Hooker, who both knew of my work--the latter having read my sketch of
+1844--honoured me by thinking it advisable to publish, with Mr. Wallace's
+excellent memoir, some brief extracts from my manuscripts.
+
+This Abstract, which I now publish, must necessarily be imperfect. I cannot
+here give references and authorities for my several statements; and I must
+trust to the reader reposing some confidence in my accuracy. No doubt
+errors will have crept in, though I hope I have always been cautious in
+trusting to good authorities alone. I can here give only the general
+conclusions at which I have arrived, with a few facts in illustration, but
+which, I hope, in most cases will suffice. No one can feel more sensible
+than I do of the necessity of hereafter publishing in detail all the facts,
+with references, on which my conclusions have been grounded; and I hope in
+a future work to do this. For I am well aware that scarcely a single point
+is discussed in this volume on which facts cannot be adduced, often
+apparently leading to conclusions directly opposite to those at which I
+have arrived. A fair result can be obtained only by fully stating and
+balancing the facts and arguments on both sides of each question; and this
+cannot possibly be here done.
+
+I much regret that want of space prevents my having the satisfaction of
+acknowledging the generous assistance which I have received from very many
+naturalists, some of them personally unknown to me. I cannot, however, {3}
+let this opportunity pass without expressing my deep obligations to Dr.
+Hooker, who for the last fifteen years has aided me in every possible way
+by his large stores of knowledge and his excellent judgment.
+
+In considering the Origin of Species, it is quite conceivable that a
+naturalist, reflecting on the mutual affinities of organic beings, on their
+embryological relations, their geographical distribution, geological
+succession, and other such facts, might come to the conclusion that each
+species had not been independently created, but had descended, like
+varieties, from other species. Nevertheless, such a conclusion, even if
+well founded, would be unsatisfactory, until it could be shown how the
+innumerable species inhabiting this world have been modified, so as to
+acquire that perfection of structure and coadaptation which most justly
+excites our admiration. Naturalists continually refer to external
+conditions, such as climate, food, &c., as the only possible cause of
+variation. In one very limited sense, as we shall hereafter see, this may
+be true; but it is preposterous to attribute to mere external conditions,
+the structure, for instance, of the woodpecker, with its feet, tail, beak,
+and tongue, so admirably adapted to catch insects under the bark of trees.
+In the case of the misseltoe, which draws its nourishment from certain
+trees, which has seeds that must be transported by certain birds, and which
+has flowers with separate sexes absolutely requiring the agency of certain
+insects to bring pollen from one flower to the other, it is equally
+preposterous to account for the structure of this parasite, with its
+relations to several distinct organic beings, by the effects of external
+conditions, or of habit, or of the volition of the plant itself.
+
+The author of the 'Vestiges of Creation' would, I presume, say that, after
+a certain unknown number of {4} generations, some bird had given birth to a
+woodpecker, and some plant to the missletoe, and that these had been
+produced perfect as we now see them; but this assumption seems to me to be
+no explanation, for it leaves the case of the coadaptations of organic
+beings to each other and to their physical conditions of life, untouched
+and unexplained.
+
+It is, therefore, of the highest importance to gain a clear insight into
+the means of modification and coadaptation. At the commencement of my
+observations it seemed to me probable that a careful study of domesticated
+animals and of cultivated plants would offer the best chance of making out
+this obscure problem. Nor have I been disappointed; in this and in all
+other perplexing cases I have invariably found that our knowledge,
+imperfect though it be, of variation under domestication, afforded the best
+and safest clue. I may venture to express my conviction of the high value
+of such studies, although they have been very commonly neglected by
+naturalists.
+
+From these considerations, I shall devote the first chapter of this
+Abstract to Variation under Domestication. We shall thus see that a large
+amount of hereditary modification is at least possible; and, what is
+equally or more important, we shall see how great is the power of man in
+accumulating by his Selection successive slight variations. I will then
+pass on to the variability of species in a state of nature; but I shall,
+unfortunately, be compelled to treat this subject far too briefly, as it
+can be treated properly only by giving long catalogues of facts. We shall,
+however, be enabled to discuss what circumstances are most favourable to
+variation. In the next chapter the Struggle for Existence amongst all
+organic beings throughout the world, which inevitably follows from the high
+geometrical ratio of their {5} increase, will be treated of. This is the
+doctrine of Malthus, applied to the whole animal and vegetable kingdoms. As
+many more individuals of each species are born than can possibly survive;
+and as, consequently, there is a frequently recurring struggle for
+existence, it follows that any being, if it vary however slightly in any
+manner profitable to itself, under the complex and sometimes varying
+conditions of life, will have a better chance of surviving, and thus be
+_naturally selected_. From the strong principle of inheritance, any
+selected variety will tend to propagate its new and modified form.
+
+This fundamental subject of Natural Selection will be treated at some
+length in the fourth chapter; and we shall then see how Natural Selection
+almost inevitably causes much Extinction of the less improved forms of
+life, and leads to what I have called Divergence of Character. In the next
+chapter I shall discuss the complex and little known laws of variation and
+of correlation of growth. In the four succeeding chapters, the most
+apparent and gravest difficulties on the theory will be given: namely,
+first, the difficulties of transitions, or in understanding how a simple
+being or a simple organ can be changed and perfected into a highly
+developed being or elaborately constructed organ; secondly, the subject of
+Instinct, or the mental powers of animals; thirdly, Hybridism, or the
+infertility of species and the fertility of varieties when intercrossed;
+and fourthly, the imperfection of the Geological Record. In the next
+chapter I shall consider the geological succession of organic beings
+throughout time; in the eleventh and twelfth, their geographical
+distribution throughout space; in the thirteenth, their classification or
+mutual affinities, both when mature and in an embryonic condition. In the
+last chapter I shall give a {6} brief recapitulation of the whole work, and
+a few concluding remarks.
+
+No one ought to feel surprise at much remaining as yet unexplained in
+regard to the origin of species and varieties, if he makes due allowance
+for our profound ignorance in regard to the mutual relations of all the
+beings which live around us. Who can explain why one species ranges widely
+and is very numerous, and why another allied species has a narrow range and
+is rare? Yet these relations are of the highest importance, for they
+determine the present welfare, and, as I believe, the future success and
+modification of every inhabitant of this world. Still less do we know of
+the mutual relations of the innumerable inhabitants of the world during the
+many past geological epochs in its history. Although much remains obscure,
+and will long remain obscure, I can entertain no doubt, after the most
+deliberate study and dispassionate judgment of which I am capable, that the
+view which most naturalists entertain, 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
+main but not exclusive means of modification.
+
+       *       *       *       *       *
+
+
+{7}
+
+CHAPTER I.
+
+VARIATION UNDER DOMESTICATION.
+
+    Causes of Variability--Effects of Habit--Correlation of
+    Growth--Inheritance--Character of Domestic Varieties--Difficulty of
+    distinguishing between Varieties and Species--Origin of Domestic
+    Varieties from one or more Species--Domestic Pigeons, their Differences
+    and Origin--Principle of Selection anciently followed, its
+    Effects--Methodical and Unconscious Selection--Unknown Origin of our
+    Domestic Productions--Circumstances favourable to Man's power of
+    Selection.
+
+When we look to the individuals of the same variety or sub-variety of our
+older cultivated plants and animals, one of the first points which strikes
+us, is, that they generally differ more from each other than do the
+individuals of any one species or variety in a state of nature. When we
+reflect on the vast diversity of the plants and animals which have been
+cultivated, and which have varied during all ages under the most different
+climates and treatment, I think we are driven to conclude that this great
+variability is simply due to our domestic productions having been raised
+under conditions of life not so uniform as, and somewhat different from,
+those to which the parent-species have been exposed under nature. There is
+also, I think, some probability in the view propounded by Andrew Knight,
+that this variability may be partly connected with excess of food. It seems
+pretty clear that organic beings must be exposed during several generations
+to the new conditions of life to cause any appreciable amount of variation;
+and that when the organisation has once begun to vary, it generally
+continues to vary for many generations. {8} No case is on record of a
+variable being ceasing to be variable under cultivation. Our oldest
+cultivated plants, such as wheat, still often yield new varieties: our
+oldest domesticated animals are still capable of rapid improvement or
+modification.
+
+It has been disputed at what period of life the causes of variability,
+whatever they may be, generally act; whether during the early or late
+period of development of the embryo, or at the instant of conception.
+Geoffroy St. Hilaire's experiments show that unnatural treatment of the
+embryo causes monstrosities; and monstrosities cannot be separated by any
+clear line of distinction from mere variations. But I am strongly inclined
+to suspect that the most frequent cause of variability may be attributed to
+the male and female reproductive elements having been affected prior to the
+act of conception. Several reasons make me believe in this; but the chief
+one is the remarkable effect which confinement or cultivation has on the
+function of the reproductive system; this system appearing to be far more
+susceptible than any other part of the organisation, to the action of any
+change in the conditions of life. Nothing is more easy than to tame an
+animal, and few things more difficult than to get it to breed freely under
+confinement, even in the many cases when the male and female unite. How
+many animals there are which will not breed, though living long under not
+very close confinement in their native country! This is generally
+attributed to vitiated instincts; but how many cultivated plants display
+the utmost vigour, and yet rarely or never seed! In some few such cases it
+has been discovered that very trifling changes, such as a little more or
+less water at some particular period of growth, will determine whether or
+not the plant sets a seed. I cannot here enter on the copious details which
+I have collected on {9} this curious subject; but to show how singular the
+laws are which determine the reproduction of animals under confinement, I
+may just mention that carnivorous animals, even from the tropics, breed in
+this country pretty freely under confinement, with the exception of the
+plantigrades or bear family; whereas carnivorous birds, with the rarest
+exceptions, hardly ever lay fertile eggs. Many exotic plants have pollen
+utterly worthless, in the same exact condition as in the most sterile
+hybrids. When, on the one hand, we see domesticated animals and plants,
+though often weak and sickly, yet breeding quite freely under confinement;
+and when, on the other hand, we see individuals, though taken young from a
+state of nature, perfectly tamed, long-lived, and healthy (of which I could
+give numerous instances), yet having their reproductive system so seriously
+affected by unperceived causes as to fail in acting, we need not be
+surprised at this system, when it does act under confinement, acting not
+quite regularly, and producing offspring not perfectly like their parents.
+
+Sterility has been said to be the bane of horticulture; but on this view we
+owe variability to the same cause which produces sterility; and variability
+is the source of all the choicest productions of the garden. I may add,
+that as some organisms will breed freely under the most unnatural
+conditions (for instance, the rabbit and ferret kept in hutches), showing
+that their reproductive system has not been thus affected; so will some
+animals and plants withstand domestication or cultivation, and vary very
+slightly--perhaps hardly more than in a state of nature.
+
+A long list could easily be given of "sporting plants;" by this term
+gardeners mean a single bud or offset, which suddenly assumes a new and
+sometimes very different character from that of the rest of the plant. {10}
+Such buds can be propagated by grafting, &c., and sometimes by seed. These
+"sports" are extremely rare under nature, but far from rare under
+cultivation; and in this case we see that the treatment of the parent has
+affected a bud or offset, and not the ovules or pollen. But it is the
+opinion of most physiologists that there is no essential difference between
+a bud and an ovule in their earliest stages of formation; so that, in fact,
+"sports" support my view, that variability may be largely attributed to the
+ovules or pollen, or to both, having been affected by the treatment of the
+parent prior to the act of conception. These cases anyhow show that
+variation is not necessarily connected, as some authors have supposed, with
+the act of generation.
+
+Seedlings from the same fruit, and the young of the same litter, sometimes
+differ considerably from each other, though both the young and the parents,
+as Müller has remarked, have apparently been exposed to exactly the same
+conditions of life; and this shows how unimportant the direct effects of
+the conditions of life are in comparison with the laws of reproduction, of
+growth, and of inheritance; for had the action of the conditions been
+direct, if any of the young had varied, all would probably have varied in
+the same manner. To judge how much, in the case of any variation, we should
+attribute to the direct action of heat, moisture, light, food, &c., is most
+difficult: my impression is, that with animals such agencies have produced
+very little direct effect, though apparently more in the case of plants.
+Under this point of view, Mr. Buckman's recent experiments on plants are
+extremely valuable. When all or nearly all the individuals exposed to
+certain conditions are affected in the same way, the change at first
+appears to be directly due to such conditions; but in some cases it can be
+shown that quite opposite conditions produce {11} similar changes of
+structure. Nevertheless some slight amount of change may, I think, be
+attributed to the direct action of the conditions of life--as, in some
+cases, increased size from amount of food, colour from particular kinds of
+food or from light, and perhaps the thickness of fur from climate.
+
+Habit also has a decided influence, as in the period of flowering with
+plants when transported from one climate to another. In animals it has a
+more marked effect; for instance, I find in the domestic duck that the
+bones of the wing weigh less and the bones of the leg more, in proportion
+to the whole skeleton, than do the same bones in the wild-duck; and I
+presume that this change may be safely attributed to the domestic duck
+flying much less, and walking more, than its wild parent. The great and
+inherited development of the udders in cows and goats in countries where
+they are habitually milked, in comparison with the state of these organs in
+other countries, is another instance of the effect of use. Not a single
+domestic animal can be named which has not in some country drooping ears;
+and the view suggested by some authors, that the drooping is due to the
+disuse of the muscles of the ear, from the animals not being much alarmed
+by danger, seems probable.
+
+There are many laws regulating variation, some few of which can be dimly
+seen, and will be hereafter briefly mentioned. I will here only allude to
+what may be called correlation of growth. Any change in the embryo or larva
+will almost certainly entail changes in the mature animal. In
+monstrosities, the correlations between quite distinct parts are very
+curious; and many instances are given in Isidore Geoffroy St. Hilaire's
+great work on this subject. Breeders believe that long limbs are almost
+always accompanied by an elongated head. Some instances of correlation are
+quite whimsical: thus {12} cats with blue eyes are invariably deaf; colour
+and constitutional peculiarities go together, of which many remarkable
+cases could be given amongst animals and plants. From the facts collected
+by Heusinger, it appears that white sheep and pigs are differently affected
+from coloured individuals by certain vegetable poisons. Hairless dogs have
+imperfect teeth: long-haired and coarse-haired animals are apt to have, as
+is asserted, long or many horns; pigeons with feathered feet have skin
+between their outer toes; pigeons with short beaks have small feet, and
+those with long beaks large feet. Hence, if man goes on selecting, and thus
+augmenting, any peculiarity, he will almost certainly unconsciously modify
+other parts of the structure, owing to the mysterious laws of the
+correlation of growth.
+
+The result of the various, quite unknown, or dimly seen laws of variation
+is infinitely complex and diversified. It is well worth while carefully to
+study the several treatises published on some of our old cultivated plants,
+as on the hyacinth, potato, even the dahlia, &c.; and it is really
+surprising to note the endless points in structure and constitution in
+which the varieties and sub-varieties differ slightly from each other. The
+whole organisation seems to have become plastic, and tends to depart in
+some small degree from that of the parental type.
+
+Any variation which is not inherited is unimportant for us. But the number
+and diversity of inheritable deviations of structure, both those of slight
+and those of considerable physiological importance, is endless. Dr. Prosper
+Lucas's treatise, in two large volumes, is the fullest and the best on this
+subject. No breeder doubts how strong is the tendency to inheritance: like
+produces like is his fundamental belief: doubts have been thrown on this
+principle by theoretical writers alone. When any deviation of structure
+often appears, and we see it in the {13} father and child, we cannot tell
+whether it may not be due to the same cause having acted on both; but when
+amongst individuals, apparently exposed to the same conditions, any very
+rare deviation, due to some extraordinary combination of circumstances,
+appears in the parent--say, once amongst several million individuals--and
+it reappears in the child, the mere doctrine of chances almost compels us
+to attribute its reappearance to inheritance. Every one must have heard of
+cases of albinism, prickly skin, hairy bodies, &c., appearing in several
+members of the same family. If strange and rare deviations of structure are
+truly inherited, less strange and commoner deviations may be freely
+admitted to be inheritable. Perhaps the correct way of viewing the whole
+subject, would be, to look at the inheritance of every character whatever
+as the rule, and non-inheritance as the anomaly.
+
+The laws governing inheritance are quite unknown; no one can say why a
+peculiarity in different individuals of the same species, or in individuals
+of different species, is sometimes inherited and sometimes not so; why the
+child often reverts in certain characters to its grandfather or grandmother
+or other more remote ancestor; why a peculiarity is often transmitted from
+one sex to both sexes, or to one sex alone, more commonly but not
+exclusively to the like sex. It is a fact of some little importance to us,
+that peculiarities appearing in the males of our domestic breeds are often
+transmitted either exclusively, or in a much greater degree, to males
+alone. A much more important rule, which I think may be trusted, is that,
+at whatever period of life a peculiarity first appears, it tends to appear
+in the offspring at a corresponding age, though sometimes earlier. In many
+cases this could not be otherwise: thus the inherited peculiarities in the
+horns of cattle could appear only in {14} the offspring when nearly mature;
+peculiarities in the silkworm are known to appear at the corresponding
+caterpillar or cocoon stage. But hereditary diseases and some other facts
+make me believe that the rule has a wider extension, and that when there is
+no apparent reason why a peculiarity should appear at any particular age,
+yet that it does tend to appear in the offspring at the same period at
+which it first appeared in the parent. I believe this rule to be of the
+highest importance in explaining the laws of embryology. These remarks are
+of course confined to the first _appearance_ of the peculiarity, and not to
+its primary cause, which may have acted on the ovules or male element; in
+nearly the same manner as in the crossed offspring from a short-horned cow
+by a long-horned bull, the greater length of horn, though appearing late in
+life, is clearly due to the male element.
+
+Having alluded to the subject of reversion, I may here refer to a statement
+often made by naturalists--namely, that our domestic varieties, when run
+wild, gradually but certainly revert in character to their aboriginal
+stocks. Hence it has been argued that no deductions can be drawn from
+domestic races to species in a state of nature. I have in vain endeavoured
+to discover on what decisive facts the above statement has so often and so
+boldly been made. There would be great difficulty in proving its truth: we
+may safely conclude that very many of the most strongly-marked domestic
+varieties could not possibly live in a wild state. In many cases we do not
+know what the aboriginal stock was, and so could not tell whether or not
+nearly perfect reversion had ensued. It would be quite necessary, in order
+to prevent the effects of intercrossing, that only a single variety should
+be turned loose in its new home. Nevertheless, as our varieties certainly
+do occasionally {15} revert in some of their characters to ancestral forms,
+it seems to me not improbable, that if we could succeed in naturalising, or
+were to cultivate, during many generations, the several races, for
+instance, of the cabbage, in very poor soil (in which case, however, some
+effect would have to be attributed to the direct action of the poor soil),
+that they would to a large extent, or even wholly, revert to the wild
+aboriginal stock. Whether or not the experiment would succeed, is not of
+great importance for our line of argument; for by the experiment itself the
+conditions of life are changed. If it could be shown that our domestic
+varieties manifested a strong tendency to reversion,--that is, to lose
+their acquired characters, whilst kept under the same conditions, and
+whilst kept in a considerable body, so that free intercrossing might check,
+by blending together, any slight deviations in their structure, in such
+case, I grant that we could deduce nothing from domestic varieties in
+regard to species. But there is not a shadow of evidence in favour of this
+view: to assert that we could not breed our cart and race-horses, long and
+short-horned cattle, and poultry of various breeds, and esculent
+vegetables, for an almost infinite number of generations, would be opposed
+to all experience. I may add, that when under nature the conditions of life
+do change, variations and reversions of character probably do occur; but
+natural selection, as will hereafter be explained, will determine how far
+the new characters thus arising shall be preserved.
+
+When we look to the hereditary varieties or races of our domestic animals
+and plants, and compare them with closely allied species, we generally
+perceive in each domestic race, as already remarked, less uniformity of
+character than in true species. Domestic races of the same species, also,
+often have a somewhat monstrous character; by which I mean, that, although
+differing {16} from each other, and from other species of the same genus,
+in several trifling respects, they often differ in an extreme degree in
+some one part, both when compared one with another, and more especially
+when compared with all the species in nature to which they are nearest
+allied. With these exceptions (and with that of the perfect fertility of
+varieties when crossed,--a subject hereafter to be discussed), domestic
+races of the same species differ from each other in the same manner as,
+only in most cases in a lesser degree than, do closely-allied species of
+the same genus in a state of nature. I think this must be admitted, when we
+find that there are hardly any domestic races, either amongst animals or
+plants, which have not been ranked by competent judges as mere varieties,
+and by other competent judges as the descendants of aboriginally distinct
+species. If any marked distinction existed between domestic races and
+species, this source of doubt could not so perpetually recur. It has often
+been stated that domestic races do not differ from each other in characters
+of generic value. I think it could be shown that this statement is hardly
+correct; but naturalists differ widely in determining what characters are
+of generic value; all such valuations being at present empirical. Moreover,
+on the view of the origin of genera which I shall presently give, we have
+no right to expect often to meet with generic differences in our
+domesticated productions.
+
+When we attempt to estimate the amount of structural difference between the
+domestic races of the same species, we are soon involved in doubt, from not
+knowing whether they have descended from one or several parent-species.
+This point, if it could be cleared up, would be interesting; if, for
+instance, it could be shown that the greyhound, bloodhound, terrier,
+spaniel, and bull-dog, which we all know propagate their kind so truly,
+were the {17} offspring of any single species, then such facts would have
+great weight in making us doubt about the immutability of the many very
+closely allied natural species--for instance, of the many foxes--inhabiting
+different quarters of the world. I do not believe, as we shall presently
+see, that the whole amount of difference between the several breeds of the
+dog has been produced under domestication; I believe that some small part
+of the difference is due to their being descended from distinct species. In
+the case of some other domesticated species, there is presumptive, or even
+strong evidence, that all the breeds have descended from a single wild
+stock.
+
+It has often been assumed that man has chosen for domestication animals and
+plants having an extraordinary inherent tendency to vary, and likewise to
+withstand diverse climates. I do not dispute that these capacities have
+added largely to the value of most of our domesticated productions; but how
+could a savage possibly know, when he first tamed an animal, whether it
+would vary in succeeding generations, and whether it would endure other
+climates? Has the little variability of the ass or guinea-fowl, or the
+small power of endurance of warmth by the reindeer, or of cold by the
+common camel, prevented their domestication? I cannot doubt that if other
+animals and plants, equal in number to our domesticated productions, and
+belonging to equally diverse classes and countries, were taken from a state
+of nature, and could be made to breed for an equal number of generations
+under domestication, they would vary on an average as largely as the parent
+species of our existing domesticated productions have varied.
+
+In the case of most of our anciently domesticated animals and plants, I do
+not think it is possible to come to any definite conclusion, whether they
+have descended from one or several wild species. The argument mainly relied
+on by those who believe in the multiple origin {18} of our domestic animals
+is, that we find in the most ancient records, more especially on the
+monuments of Egypt, much diversity in the breeds; and that some of the
+breeds closely resemble, perhaps are identical with, those still existing.
+Even if this latter fact were found more strictly and generally true than
+seems to me to be the case, what does it show, but that some of our breeds
+originated there, four or five thousand years ago? But Mr. Horner's
+researches have rendered it in some degree probable that man sufficiently
+civilized to have manufactured pottery existed in the valley of the Nile
+thirteen or fourteen thousand years ago; and who will pretend to say how
+long before these ancient periods, savages, like those of Tierra del Fuego
+or Australia, who possess a semi-domestic dog, may not have existed in
+Egypt?
+
+The whole subject must, I think, remain vague; nevertheless, I may, without
+here entering on any details, state that, from geographical and other
+considerations, I think it highly probable that our domestic dogs have
+descended from several wild species. Knowing, as we do, that savages are
+very fond of taming animals, it seems to me unlikely, in the case of the
+dog-genus, which is distributed in a wild state throughout the world, that
+since man first appeared one single species alone should have been
+domesticated. In regard to sheep and goats I can form no opinion. I should
+think, from facts communicated to me by Mr. Blyth, on the habits, voice,
+and constitution, &c., of the humped Indian cattle, that these had
+descended from a different aboriginal stock from our European cattle; and
+several competent judges believe that these latter have had more than one
+wild parent. With respect to horses, from reasons which I cannot give here,
+I am doubtfully inclined to believe, in opposition to several authors, that
+all the races have descended from one {19} wild stock. Mr. Blyth, whose
+opinion, from his large and varied stores of knowledge, I should value more
+than that of almost any one, thinks that all the breeds of poultry have
+proceeded from the common wild Indian fowl (Gallus bankiva). In regard to
+ducks and rabbits, the breeds of which differ considerably from each other
+in structure, I do not doubt that they have all descended from the common
+wild duck and rabbit.
+
+The doctrine of the origin of our several domestic races from several
+aboriginal stocks, has been carried to an absurd extreme by some authors.
+They believe that every race which breeds true, let the distinctive
+characters be ever so slight, has had its wild prototype. At this rate
+there must have existed at least a score of species of wild cattle, as many
+sheep, and several goats in Europe alone, and several even within Great
+Britain. One author believes that there formerly existed in Great Britain
+eleven wild species of sheep peculiar to it! When we bear in mind that
+Britain has now hardly one peculiar mammal, and France but few distinct
+from those of Germany and conversely, and so with Hungary, Spain, &c., but
+that each of these kingdoms possesses several peculiar breeds of cattle,
+sheep, &c., we must admit that many domestic breeds have originated in
+Europe; for whence could they have been derived, as these several countries
+do not possess a number of peculiar species as distinct parent-stocks? So
+it is in India. Even in the case of the domestic dogs of the whole world,
+which I fully admit have probably descended from several wild species, I
+cannot doubt that there has been an immense amount of inherited variation.
+Who can believe that animals closely resembling the Italian greyhound, the
+bloodhound, the bull-dog, or Blenheim spaniel, &c.--so unlike all wild
+Canidæ--ever existed freely in a state of nature? It has often been loosely
+said that all our races of dogs have {20} been produced by the crossing of
+a few aboriginal species; but by crossing we can only get forms in some
+degree intermediate between their parents; and if we account for our
+several domestic races by this process, we must admit the former existence
+of the most extreme forms, as the Italian greyhound, bloodhound, bull-dog,
+&c., in the wild state. Moreover, the possibility of making distinct races
+by crossing has been greatly exaggerated. There can be no doubt that a race
+may be modified by occasional crosses, if aided by the careful selection of
+those individual mongrels, which present any desired character; but that a
+race could be obtained nearly intermediate between two extremely different
+races or species, I can hardly believe. Sir J. Sebright expressly
+experimentised for this object, and failed. The offspring from the first
+cross between two pure breeds is tolerably and sometimes (as I have found
+with pigeons) extremely uniform, and everything seems simple enough; but
+when these mongrels are crossed one with another for several generations,
+hardly two of them will be alike, and then the extreme difficulty, or
+rather utter hopelessness, of the task becomes apparent. Certainly, a breed
+intermediate between _two very distinct_ breeds could not be got without
+extreme care and long-continued selection; nor can I find a single case on
+record of a permanent race having been thus formed.
+
+_On the Breeds of the Domestic Pigeon._--Believing that it is always best
+to study some special group, I have, after deliberation, taken up domestic
+pigeons. I have kept every breed which I could purchase or obtain, and have
+been most kindly favoured with skins from several quarters of the world,
+more especially by the Hon. W. Elliot from India, and by the Hon. C. Murray
+from Persia. Many treatises in different languages have been published on
+pigeons, and some of them are very important, as being of {21} considerable
+antiquity. I have associated with several eminent fanciers, and have been
+permitted to join two of the London Pigeon Clubs. The diversity of the
+breeds is something astonishing. Compare the English carrier and the
+short-faced tumbler, and see the wonderful difference in their beaks,
+entailing corresponding differences in their skulls. The carrier, more
+especially the male bird, is also remarkable from the wonderful development
+of the carunculated skin about the head, and this is accompanied by greatly
+elongated eyelids, very large external orifices to the nostrils, and a wide
+gape of mouth. The short-faced tumbler has a beak in outline almost like
+that of a finch; and the common tumbler has the singular inherited habit of
+flying at a great height in a compact flock, and tumbling in the air head
+over heels. The runt is a bird of great size, with long, massive beak and
+large feet; some of the sub-breeds of runts have very long necks, others
+very long wings and tails, others singularly short tails. The barb is
+allied to the carrier, but, instead of a very long beak, has a very short
+and very broad one. The pouter has a much elongated body, wings, and legs;
+and its enormously developed crop, which it glories in inflating, may well
+excite astonishment and even laughter. The turbit has a very short and
+conical beak, with a line of reversed feathers down the breast; and it has
+the habit of continually expanding slightly the upper part of the
+oesophagus. The Jacobin has the feathers so much reversed along the back of
+the neck that they form a hood, and it has, proportionally to its size,
+much elongated wing and tail feathers. The trumpeter and laugher, as their
+names express, utter a very different coo from the other breeds. The
+fantail has thirty or even forty tail feathers, instead of twelve or
+fourteen, the normal number in all members of the great pigeon family; and
+these feathers are kept expanded, and are {22} carried so erect that in
+good birds the head and tail touch; the oil-gland is quite aborted. Several
+other less distinct breeds might be specified.
+
+In the skeletons of the several breeds, the development of the bones of the
+face in length and breadth and curvature differs enormously. The shape, as
+well as the breadth and length of the ramus of the lower jaw, varies in a
+highly remarkable manner. The number of the caudal and sacral vertebræ
+vary; as does the number of the ribs, together with their relative breadth
+and the presence of processes. The size and shape of the apertures in the
+sternum are highly variable; so is the degree of divergence and relative
+size of the two arms of the furcula. The proportional width of the gape of
+mouth, the proportional length of the eyelids, of the orifice of the
+nostrils, of the tongue (not always in strict correlation with the length
+of beak), the size of the crop and of the upper part of the oesophagus; the
+development and abortion of the oil-gland; the number of the primary wing
+and caudal feathers; the relative length of wing and tail to each other and
+to the body; the relative length of leg and of the feet; the number of
+scutellæ on the toes, the development of skin between the toes, are all
+points of structure which are variable. The period at which the perfect
+plumage is acquired varies, as does the state of the down with which the
+nestling birds are clothed when hatched. The shape and size of the eggs
+vary. The manner of flight differs remarkably; as does in some breeds the
+voice and disposition. Lastly, in certain breeds, the males and females
+have come to differ to a slight degree from each other.
+
+Altogether at least a score of pigeons might be chosen, which if shown to
+an ornithologist, and he were told that they were wild birds, would
+certainly, I think, be ranked by him as well-defined species. Moreover, I
+do not believe that any ornithologist would place the {23} English carrier,
+the short-faced tumbler, the runt, the barb, pouter, and fantail in the
+same genus; more especially as in each of these breeds several
+truly-inherited sub-breeds, or species as he might have called them, could
+be shown him.
+
+Great as the differences are between the breeds of pigeons, I am fully
+convinced that the common opinion of naturalists is correct, namely, that
+all have descended from the rock-pigeon (Columba livia), including under
+this term several geographical races or sub-species, which differ from each
+other in the most trifling respects. As several of the reasons which have
+led me to this belief are in some degree applicable in other cases, I will
+here briefly give them. If the several breeds are not varieties, and have
+not proceeded from the rock-pigeon, they must have descended from at least
+seven or eight aboriginal stocks; for it is impossible to make the present
+domestic breeds by the crossing of any lesser number: how, for instance,
+could a pouter be produced by crossing two breeds unless one of the
+parent-stocks possessed the characteristic enormous crop? The supposed
+aboriginal stocks must all have been rock-pigeons, that is, not breeding or
+willingly perching on trees. But besides C. livia, with its geographical
+sub-species, only two or three other species of rock-pigeons are known; and
+these have not any of the characters of the domestic breeds. Hence the
+supposed aboriginal stocks must either still exist in the countries where
+they were originally domesticated, and yet be unknown to ornithologists;
+and this, considering their size, habits, and remarkable characters, seems
+very improbable; or they must have become extinct in the wild state. But
+birds breeding on precipices, and good fliers, are unlikely to be
+exterminated; and the common rock-pigeon, which has the same habits with
+the domestic breeds, has not been exterminated {24} even on several of the
+smaller British islets, or on the shores of the Mediterranean. Hence the
+supposed extermination of so many species having similar habits with the
+rock-pigeon seems to me a very rash assumption. Moreover, the several
+above-named domesticated breeds have been transported to all parts of the
+world, and, therefore, some of them must have been carried back again into
+their native country; but not one has ever become wild or feral, though the
+dovecot-pigeon, which is the rock-pigeon in a very slightly altered state,
+has become feral in several places. Again, all recent experience shows that
+it is most difficult to get any wild animal to breed freely under
+domestication; yet on the hypothesis of the multiple origin of our pigeons,
+it must be assumed that at least seven or eight species were so thoroughly
+domesticated in ancient times by half-civilized man, as to be quite
+prolific under confinement.
+
+An argument, as it seems to me, of great weight, and applicable in several
+other cases, is, that the above-specified breeds, though agreeing generally
+in constitution, habits, voice, colouring, and in most parts of their
+structure, with the wild rock-pigeon, yet are certainly highly abnormal in
+other parts of their structure; we may look in vain throughout the whole
+great family of Columbidæ for a beak like that of the English carrier, or
+that of the short-faced tumbler, or barb; for reversed feathers like those
+of the Jacobin; for a crop like that of the pouter; for tail-feathers like
+those of the fantail. Hence it must be assumed not only that half-civilized
+man succeeded in thoroughly domesticating several species, but that he
+intentionally or by chance picked out extraordinarily abnormal species; and
+further, that these very species have since all become extinct or unknown.
+So many strange contingencies seem to me improbable in the highest degree.
+{25}
+
+Some facts in regard to the colouring of pigeons well deserve
+consideration. The rock-pigeon is of a slaty-blue, and has a white rump
+(the Indian subspecies, C. intermedia of Strickland, having it bluish); the
+tail has a terminal dark bar, with the bases of the outer feathers
+externally edged with white; the wings have two black bars; some
+semi-domestic breeds and some apparently truly wild breeds have, besides
+the two black bars, the wings chequered with black. These several marks do
+not occur together in any other species of the whole family. Now, in every
+one of the domestic breeds, taking thoroughly well-bred birds, all the
+above marks, even to the white edging of the outer tail-feathers, sometimes
+concur perfectly developed. Moreover, when two birds belonging to two
+distinct breeds are crossed, neither of which is blue or has any of the
+above-specified marks, the mongrel offspring are very apt suddenly to
+acquire these characters; for instance, I crossed some uniformly white
+fantails with some uniformly black barbs, and they produced mottled brown
+and black birds; these I again crossed together, and one grandchild of the
+pure white fantail and pure black barb was of as beautiful a blue colour,
+with the white rump, double black wing-bar, and barred and white-edged
+tail-feathers, as any wild rock-pigeon! We can understand these facts, on
+the well-known principle of reversion to ancestral characters, if all the
+domestic breeds have descended from the rock-pigeon. But if we deny this,
+we must make one of the two following highly improbable suppositions.
+Either, firstly, that all the several imagined aboriginal stocks were
+coloured and marked like the rock-pigeon, although no other existing
+species is thus coloured and marked, so that in each separate breed there
+might be a tendency to revert to the very same colours and markings. Or,
+secondly, {26} that each breed, even the purest, has within a dozen or, at
+most, within a score of generations, been crossed by the rock-pigeon: I say
+within a dozen or twenty generations, for we know of no fact countenancing
+the belief that the child ever reverts to some one ancestor, removed by a
+greater number of generations. In a breed which has been crossed only once
+with some distinct breed, the tendency to reversion to any character
+derived from such cross will naturally become less and less, as in each
+succeeding generation there will be less of the foreign blood; but when
+there has been no cross with a distinct breed, and there is a tendency in
+both parents to revert to a character, which has been lost during some
+former generation, this tendency, for all that we can see to the contrary,
+may be transmitted undiminished for an indefinite number of generations.
+These two distinct cases are often confounded in treatises on inheritance.
+
+Lastly, the hybrids or mongrels from between all the domestic breeds of
+pigeons are perfectly fertile. I can state this from my own observations,
+purposely made, on the most distinct breeds. Now, it is difficult, perhaps
+impossible, to bring forward one case of the hybrid offspring of two
+animals _clearly distinct_ being themselves perfectly fertile. Some authors
+believe that long-continued domestication eliminates this strong tendency
+to sterility: from the history of the dog I think there is some probability
+in this hypothesis, if applied to species closely related together, though
+it is unsupported by a single experiment. But to extend the hypothesis so
+far as to suppose that species, aboriginally as distinct as carriers,
+tumblers, pouters, and fantails now are, should yield offspring perfectly
+fertile, _inter se_, seems to me rash in the extreme.
+
+From these several reasons, namely, the improbability of man having
+formerly got seven or eight supposed {27} species of pigeons to breed
+freely under domestication; these supposed species being quite unknown in a
+wild state, and their becoming nowhere feral; these species having very
+abnormal characters in certain respects, as compared with all other
+Columbidæ, though so like in most other respects to the rock-pigeon; the
+blue colour and various marks occasionally appearing in all the breeds,
+both when kept pure and when crossed; the mongrel offspring being perfectly
+fertile;--from these several reasons, taken together, I can feel no doubt
+that all our domestic breeds have descended from the Columba livia with its
+geographical sub-species.
+
+In favour of this view, I may add, firstly, that C. livia, or the
+rock-pigeon, has been found capable of domestication in Europe and in
+India; and that it agrees in habits and in a great number of points of
+structure with all the domestic breeds. Secondly, although an English
+carrier or short-faced tumbler differs immensely in certain characters from
+the rock-pigeon, yet by comparing the several sub-breeds of these
+varieties, more especially those brought from distant countries, we can
+make an almost perfect series between the extremes of structure. Thirdly,
+those characters which are mainly distinctive of each breed, for instance
+the wattle and length of beak of the carrier, the shortness of that of the
+tumbler, and the number of tail-feathers in the fantail, are in each breed
+eminently variable; and the explanation of this fact will be obvious when
+we come to treat of selection. Fourthly, pigeons have been watched, and
+tended with the utmost care, and loved by many people. They have been
+domesticated for thousands of years in several quarters of the world; the
+earliest known record of pigeons is in the fifth Ægyptian dynasty, about
+3000 B.C., as was pointed out to me by Professor Lepsius; but Mr. Birch
+informs me that pigeons are given in a bill {28} of fare in the previous
+dynasty. In the time of the Romans, as we hear from Pliny, immense prices
+were given for pigeons; "nay, they are come to this pass, that they can
+reckon up their pedigree and race." Pigeons were much valued by Akber Khan
+in India, about the year 1600; never less than 20,000 pigeons were taken
+with the court. "The monarchs of Iran and Turan sent him some very rare
+birds;" and, continues the courtly historian, "His Majesty by crossing the
+breeds, which method was never practised before, has improved them
+astonishingly." About this same period the Dutch were as eager about
+pigeons as were the old Romans. The paramount importance of these
+considerations in explaining the immense amount of variation which pigeons
+have undergone, will be obvious when we treat of Selection. We shall then,
+also, see how it is that the breeds so often have a somewhat monstrous
+character. It is also a most favourable circumstance for the production of
+distinct breeds, that male and female pigeons can be easily mated for life;
+and thus different breeds can be kept together in the same aviary.
+
+I have discussed the probable origin of domestic pigeons at some, yet quite
+insufficient, length; because when I first kept pigeons and watched the
+several kinds, knowing well how true they bred, I felt fully as much
+difficulty in believing that they could have descended from a common
+parent, as any naturalist could in coming to a similar conclusion in regard
+to the many species of finches, or other large groups of birds, in nature.
+One circumstance has struck me much; namely, that all the breeders of the
+various domestic animals and the cultivators of plants, with whom I have
+ever conversed, or whose treatises I have read, are firmly convinced that
+the several breeds to which each has attended, are descended from so many
+aboriginally distinct species. {29} Ask, as I have asked, a celebrated
+raiser of Hereford cattle, whether his cattle might not have descended from
+long-horns, and he will laugh you to scorn. I have never met a pigeon, or
+poultry, or duck, or rabbit fancier, who was not fully convinced that each
+main breed was descended from a distinct species. Van Mons, in his treatise
+on pears and apples, shows how utterly he disbelieves that the several
+sorts, for instance a Ribston-pippin or Codlin-apple, could ever have
+proceeded from the seeds of the same tree. Innumerable other examples could
+be given. The explanation, I think, is simple: from long-continued study
+they are strongly impressed with the differences between the several races;
+and though they well know that each race varies slightly, for they win
+their prizes by selecting such slight differences, yet they ignore all
+general arguments, and refuse to sum up in their minds slight differences
+accumulated during many successive generations. May not those naturalists
+who, knowing far less of the laws of inheritance than does the breeder, and
+knowing no more than he does of the intermediate links in the long lines of
+descent, yet admit that many of our domestic races have descended from the
+same parents--may they not learn a lesson of caution, when they deride the
+idea of species in a state of nature being lineal descendants of other
+species?
+
+_Selection._--Let us now briefly consider the steps by which domestic races
+have been produced, either from one or from several allied species. Some
+little effect may, perhaps, be attributed to the direct action of the
+external conditions of life, and some little to habit; but he would be a
+bold man who would account by such agencies for the differences of a dray
+and race horse, a greyhound and bloodhound, a carrier and tumbler pigeon.
+One of the most remarkable features in our domesticated races {30} is that
+we see in them adaptation, not indeed to the animal's or plant's own good,
+but to man's use or fancy. Some variations useful to him have probably
+arisen suddenly, or by one step; many botanists, for instance, believe that
+the fuller's teazle, with its hooks, which cannot be rivalled by any
+mechanical contrivance, is only a variety of the wild Dipsacus; and this
+amount of change may have suddenly arisen in a seedling. So it has probably
+been with the turnspit dog; and this is known to have been the case with
+the ancon sheep. But when we compare the dray-horse and race-horse, the
+dromedary and camel, the various breeds of sheep fitted either for
+cultivated land or mountain pasture, with the wool of one breed good for
+one purpose, and that of another breed for another purpose; when we compare
+the many breeds of dogs, each good for man in very different ways; when we
+compare the game-cock, so pertinacious in battle, with other breeds so
+little quarrelsome, with "everlasting layers" which never desire to sit,
+and with the bantam so small and elegant; when we compare the host of
+agricultural, culinary, orchard, and flower-garden races of plants, most
+useful to man at different seasons and for different purposes, or so
+beautiful in his eyes, we must, I think, look further than to mere
+variability. We cannot suppose that all the breeds were suddenly produced
+as perfect and as useful as we now see them; indeed, in several cases, we
+know that this has not been their history. The key is man's power of
+accumulative selection: nature gives successive variations; man adds them
+up in certain directions useful to him. In this sense he may be said to
+make for himself useful breeds.
+
+The great power of this principle of selection is not hypothetical. It is
+certain that several of our eminent breeders have, even within a single
+lifetime, modified to {31} a large extent some breeds of cattle and sheep.
+In order fully to realise what they have done, it is almost necessary to
+read several of the many treatises devoted to this subject, and to inspect
+the animals. Breeders habitually speak of an animal's organisation as
+something quite plastic, which they can model almost as they please. If I
+had space I could quote numerous passages to this effect from highly
+competent authorities. Youatt, who was probably better acquainted with the
+works of agriculturists than almost any other individual, and who was
+himself a very good judge of an animal, speaks of the principle of
+selection as "that which enables the agriculturist, not only to modify the
+character of his flock, but to change it altogether. It is the magician's
+wand, by means of which he may summon into life whatever form and mould he
+pleases." Lord Somerville, speaking of what breeders have done for sheep,
+says:--"It would seem as if they had chalked out upon a wall a form perfect
+in itself, and then had given it existence." That most skilful breeder, Sir
+John Sebright, used to say, with respect to pigeons, that "he would produce
+any given feather in three years, but it would take him six years to obtain
+head and beak." In Saxony the importance of the principle of selection in
+regard to merino sheep is so fully recognised, that men follow it as a
+trade: the sheep are placed on a table and are studied, like a picture by a
+connoisseur; this is done three times at intervals of months, and the sheep
+are each time marked and classed, so that the very best may ultimately be
+selected for breeding.
+
+What English breeders have actually effected is proved by the enormous
+prices given for animals with a good pedigree; and these have now been
+exported to almost every quarter of the world. The improvement is by no
+means generally due to crossing different breeds; {32} all the best
+breeders are strongly opposed to this practice, except sometimes amongst
+closely allied sub-breeds. And when a cross has been made, the closest
+selection is far more indispensable even than in ordinary cases. If
+selection consisted merely in separating some very distinct variety, and
+breeding from it, the principle would be so obvious as hardly to be worth
+notice; but its importance consists in the great effect produced by the
+accumulation in one direction, during successive generations, of
+differences absolutely inappreciable by an uneducated eye--differences
+which I for one have vainly attempted to appreciate. Not one man in a
+thousand has accuracy of eye and judgment sufficient to become an eminent
+breeder. If gifted with these qualities, and he studies his subject for
+years, and devotes his lifetime to it with indomitable perseverance, he
+will succeed, and may make great improvements; if he wants any of these
+qualities, he will assuredly fail. Few would readily believe in the natural
+capacity and years of practice requisite to become even a skilful
+pigeon-fancier.
+
+The same principles are followed by horticulturists; but the variations are
+here often more abrupt. No one supposes that our choicest productions have
+been produced by a single variation from the aboriginal stock. We have
+proofs that this is not so in some cases, in which exact records have been
+kept; thus, to give a very trifling instance, the steadily-increasing size
+of the common gooseberry may be quoted. We see an astonishing improvement
+in many florists' flowers, when the flowers of the present day are compared
+with drawings made only twenty or thirty years ago. When a race of plants
+is once pretty well established, the seed-raisers do not pick out the best
+plants, but merely go over their seed-beds, and pull up the "rogues," as
+they call the plants that deviate from the proper standard. With animals
+this {33} kind of selection is, in fact, also followed; for hardly any one
+is so careless as to allow his worst animals to breed.
+
+In regard to plants, there is another means of observing the accumulated
+effects of selection--namely, by comparing the diversity of flowers in the
+different varieties of the same species in the flower-garden; the diversity
+of leaves, pods, or tubers, or whatever part is valued, in the
+kitchen-garden, in comparison with the flowers of the same varieties; and
+the diversity of fruit of the same species in the orchard, in comparison
+with the leaves and flowers of the same set of varieties. See how different
+the leaves of the cabbage are, and how extremely alike the flowers; how
+unlike the flowers of the heartsease are, and how alike the leaves; how
+much the fruit of the different kinds of gooseberries differ in size,
+colour, shape, and hairiness, and yet the flowers present very slight
+differences. It is not that the varieties which differ largely in some one
+point do not differ at all in other points; this is hardly ever, perhaps
+never, the case. The laws of correlation of growth, the importance of which
+should never be overlooked, will ensure some differences; but, as a general
+rule, I cannot doubt that the continued selection of slight variations,
+either in the leaves, the flowers, or the fruit, will produce races
+differing from each other chiefly in these characters.
+
+It may be objected that the principle of selection has been reduced to
+methodical practice for scarcely more than three-quarters of a century; it
+has certainly been more attended to of late years, and many treatises have
+been published on the subject; and the result has been, in a corresponding
+degree, rapid and important. But it is very far from true that the
+principle is a modern discovery. I could give several references to the
+full acknowledgment of the importance of the principle in works of high
+antiquity. In rude and barbarous periods {34} of English history choice
+animals were often imported, and laws were passed to prevent their
+exportation: the destruction of horses under a certain size was ordered,
+and this may be compared to the "roguing" of plants by nurserymen. The
+principle of selection I find distinctly given in an ancient Chinese
+encyclopædia. Explicit rules are laid down by some of the Roman classical
+writers. From passages in Genesis, it is clear that the colour of domestic
+animals was at that early period attended to. Savages now sometimes cross
+their dogs with wild canine animals, to improve the breed, and they
+formerly did so, as is attested by passages in Pliny. The savages in South
+Africa match their draught cattle by colour, as do some of the Esquimaux
+their teams of dogs. Livingstone shows how much good domestic breeds are
+valued by the negroes of the interior of Africa who have not associated
+with Europeans. Some of these facts do not show actual selection, but they
+show that the breeding of domestic animals was carefully attended to in
+ancient times, and is now attended to by the lowest savages. It would,
+indeed, have been a strange fact, had attention not been paid to breeding,
+for the inheritance of good and bad qualities is so obvious.
+
+At the present time, eminent breeders try by methodical selection, with a
+distinct object in view, to make a new strain or sub-breed, superior to
+anything existing in the country. But, for our purpose, a kind of
+Selection, which may be called Unconscious, and which results from every
+one trying to possess and breed from the best individual animals, is more
+important. Thus, a man who intends keeping pointers naturally tries to get
+as good dogs as he can, and afterwards breeds from his own best dogs, but
+he has no wish or expectation of permanently altering the breed.
+Nevertheless I cannot doubt that this process, continued during centuries,
+{35} would improve and modify any breed, in the same way as Bakewell,
+Collins, &c., by this very same process, only carried on more methodically,
+did greatly modify, even during their own lifetimes, the forms and
+qualities of their cattle. Slow and insensible changes of this kind could
+never be recognised unless actual measurements or careful drawings of the
+breeds in question had been made long ago, which might serve for
+comparison. In some cases, however, unchanged, or but little changed
+individuals of the same breed may be found in less civilised districts,
+where the breed has been less improved. There is reason to believe that
+King Charles's spaniel has been unconsciously modified to a large extent
+since the time of that monarch. Some highly competent authorities are
+convinced that the setter is directly derived from the spaniel, and has
+probably been slowly altered from it. It is known that the English pointer
+has been greatly changed within the last century, and in this case the
+change has, it is believed, been chiefly effected by crosses with the
+fox-hound; but what concerns us is, that the change has been effected
+unconsciously and gradually, and yet so effectually, that, though the old
+Spanish pointer certainly came from Spain, Mr. Borrow has not seen, as I am
+informed by him, any native dog in Spain like our pointer.
+
+By a similar process of selection, and by careful training, the whole body
+of English racehorses have come to surpass in fleetness and size the parent
+Arab stock, so that the latter, by the regulations for the Goodwood Races,
+are favoured in the weights they carry. Lord Spencer and others have shown
+how the cattle of England have increased in weight and in early maturity,
+compared with the stock formerly kept in this country. By comparing the
+accounts given in old pigeon treatises of carriers and tumblers with these
+breeds as now existing in Britain, {36} India, and Persia, we can, I think,
+clearly trace the stages through which they have insensibly passed, and
+come to differ so greatly from the rock-pigeon.
+
+Youatt gives an excellent illustration of the effects of a course of
+selection, which may be considered as unconsciously followed, in so far
+that the breeders could never have expected or even have wished to have
+produced the result which ensued--namely, the production of two distinct
+strains. The two flocks of Leicester sheep kept by Mr. Buckley and Mr.
+Burgess, as Mr. Youatt remarks, "have been purely bred from the original
+stock of Mr. Bakewell for upwards of fifty years. 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 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."
+
+If there exist savages so barbarous as never to think of the inherited
+character of the offspring of their domestic animals, yet any one animal
+particularly useful to them, for any special purpose, would be carefully
+preserved during famines and other accidents, to which savages are so
+liable, and such choice animals would thus generally leave more offspring
+than the inferior ones; so that in this case there would be a kind of
+unconscious selection going on. We see the value set on animals even by the
+barbarians of Tierra del Fuego, by their killing and devouring their old
+women, in times of dearth, as of less value than their dogs.
+
+In plants the same gradual process of improvement, through the occasional
+preservation of the best individuals, whether or not sufficiently distinct
+to be ranked at their first appearance as distinct varieties, and whether
+{37} or not two or more species or races have become blended together by
+crossing, may plainly be recognised in the increased size and beauty which
+we now see in the varieties of the heartsease, rose, pelargonium, dahlia,
+and other plants, when compared with the older varieties or with their
+parent-stocks. No one would ever expect to get a first-rate heartsease or
+dahlia from the seed of a wild plant. No one would expect to raise a
+first-rate melting pear from the seed of the wild pear, though he might
+succeed from a poor seedling growing wild, if it had come from a
+garden-stock. The pear, though cultivated in classical times, appears, from
+Pliny's description, to have been a fruit of very inferior quality. I have
+seen great surprise expressed in horticultural works at the wonderful skill
+of gardeners, in having produced such splendid results from such poor
+materials; but the art, I cannot doubt, has been simple, and, as far as the
+final result is concerned, has been followed almost unconsciously. It has
+consisted in always cultivating the best known variety, sowing its seeds,
+and, when a slightly better variety has chanced to appear, selecting it,
+and so onwards. But the gardeners of the classical period, who cultivated
+the best pear they could procure, never thought what splendid fruit we
+should eat; though we owe our excellent fruit, in some small degree, to
+their having naturally chosen and preserved the best varieties they could
+anywhere find.
+
+A large amount of change in our cultivated plants, thus slowly and
+unconsciously accumulated, explains, as I believe, the well-known fact,
+that in a vast number of cases we cannot recognise, and therefore do not
+know, the wild parent-stocks of the plants which have been longest
+cultivated in our flower and kitchen gardens. If it has taken centuries or
+thousands of years to improve or modify most of our plants up to their
+present {38} standard of usefulness to man, we can understand how it is
+that neither Australia, the Cape of Good Hope, nor any other region
+inhabited by quite uncivilised man, has afforded us a single plant worth
+culture. It is not that these countries, so rich in species, do not by a
+strange chance possess the aboriginal stocks of any useful plants, but that
+the native plants have not been improved by continued selection up to a
+standard of perfection comparable with that given to the plants in
+countries anciently civilised.
+
+In regard to the domestic animals kept by uncivilised man, it should not be
+overlooked that they almost always have to struggle for their own food, at
+least during certain seasons. And in two countries very differently
+circumstanced, individuals of the same species, having slightly different
+constitutions or structure, would often succeed better in the one country
+than in the other; and thus by a process of "natural selection," as will
+hereafter be more fully explained, two sub-breeds might be formed. This,
+perhaps, partly explains what has been remarked by some authors, namely,
+that the varieties kept by savages have more of the character of species
+than the varieties kept in civilised countries.
+
+On the view here given of the all-important part which selection by man has
+played, it becomes at once obvious, how it is that our domestic races show
+adaptation in their structure or in their habits to man's wants or fancies.
+We can, I think, further understand the frequently abnormal character of
+our domestic races, and likewise their differences being so great in
+external characters and relatively so slight in internal parts or organs.
+Man can hardly select, or only with much difficulty, any deviation of
+structure excepting such as is externally visible; and indeed he rarely
+cares for what is internal. He can never act by selection, excepting on
+variations {39} which are first given to him in some slight degree by
+nature. No man would ever try to make a fantail, till he saw a pigeon with
+a tail developed in some slight degree in an unusual manner, or a pouter
+till he saw a pigeon with a crop of somewhat unusual size; and the more
+abnormal or unusual any character was when it first appeared, the more
+likely it would be to catch his attention. But to use such an expression as
+trying to make a fantail, is, I have no doubt, in most cases, utterly
+incorrect. The man who first selected a pigeon with a slightly larger tail,
+never dreamed what the descendants of that pigeon would become through
+long-continued, partly unconscious and partly methodical selection. Perhaps
+the parent bird of all fantails had only fourteen tail-feathers somewhat
+expanded, like the present Java fantail, or like individuals of other and
+distinct breeds, in which as many as seventeen tail-feathers have been
+counted. Perhaps the first pouter-pigeon did not inflate its crop much more
+than the turbit now does the upper part of its oesophagus,--a habit which
+is disregarded by all fanciers, as it is not one of the points of the
+breed.
+
+Nor let it be thought that some great deviation of structure would be
+necessary to catch the fancier's eye: he perceives extremely small
+differences, and it is in human nature to value any novelty, however
+slight, in one's own possession. Nor must the value which would formerly be
+set on any slight differences in the individuals of the same species, be
+judged of by the value which would now be set on them, after several breeds
+have once fairly been established. Many slight differences might, and
+indeed do now, arise amongst pigeons, which are rejected as faults or
+deviations from the standard of perfection of each breed. The common goose
+has not given rise to any marked varieties; hence the Thoulouse and the
+common breed, which differ only in colour, that {40} most fleeting of
+characters, have lately been exhibited as distinct at our poultry-shows.
+
+I think these views further explain what has sometimes been
+noticed--namely, that we know nothing about the origin or history of any of
+our domestic breeds. But, in fact, a breed, like a dialect of a language,
+can hardly be said to have had a definite origin. A man preserves and
+breeds from an individual with some slight deviation of structure, or takes
+more care than usual in matching his best animals and thus improves them,
+and the improved individuals slowly spread in the immediate neighbourhood.
+But as yet they will hardly have a distinct name, and from being only
+slightly valued, their history will be disregarded. When further improved
+by the same slow and gradual process, they will spread more widely, and
+will get recognised as something distinct and valuable, and will then
+probably first receive a provincial name. In semi-civilised countries, with
+little free communication, the spreading and knowledge of any new sub-breed
+will be a slow process. As soon as the points of value of the new sub-breed
+are once fully acknowledged, the principle, as I have called it, of
+unconscious selection will always tend,--perhaps more at one period than at
+another, as the breed rises or falls in fashion,--perhaps more in one
+district than in another, according to the state of civilization of the
+inhabitants,--slowly to add to the characteristic features of the breed,
+whatever they may be. But the chance will be infinitely small of any record
+having been preserved of such slow, varying, and insensible changes.
+
+I must now say a few words on the circumstances, favourable, or the
+reverse, to man's power of selection. A high degree of variability is
+obviously favourable, as freely giving the materials for selection to work
+on; not that mere individual differences are not amply {41} sufficient,
+with extreme care, to allow of the accumulation of a large amount of
+modification in almost any desired direction. But as variations manifestly
+useful or pleasing to man appear only occasionally, the chance of their
+appearance will be much increased by a large number of individuals being
+kept; and hence this comes to be of the highest importance to success. On
+this principle Marshall has remarked, with respect to the sheep of parts of
+Yorkshire, that "as they generally belong to poor people, and are mostly
+_in small lots_, they never can be improved." On the other hand,
+nurserymen, from raising large stocks of the same plants, are generally far
+more successful than amateurs in getting new and valuable varieties. The
+keeping of a large number of individuals of a species in any country
+requires that the species should be placed under favourable conditions of
+life, so as to breed freely in that country. When the individuals of any
+species are scanty, all the individuals, whatever their quality may be,
+will generally be allowed to breed, and this will effectually prevent
+selection. But probably the most important point of all, is, that the
+animal or plant should be so highly useful to man, or so much valued by
+him, that the closest attention should be paid to even the slightest
+deviation in the qualities or structure of each individual. Unless such
+attention be paid nothing can be effected. I have seen it gravely remarked,
+that it was most fortunate that the strawberry began to vary just when
+gardeners began to attend closely to this plant. No doubt the strawberry
+had always varied since it was cultivated, but the slight varieties had
+been neglected. As soon, however, as gardeners picked out individual plants
+with slightly larger, earlier, or better fruit, and raised seedlings from
+them, and again picked out the best seedlings and bred from them, then,
+there appeared (aided by some {42} crossing with distinct species) those
+many admirable varieties of the strawberry which have been raised during
+the last thirty or forty years.
+
+In the case of animals with separate sexes, facility in preventing crosses
+is an important element of success in the formation of new races,--at
+least, in a country which is already stocked with other races. In this
+respect enclosure of the land plays a part. Wandering savages or the
+inhabitants of open plains rarely possess more than one breed of the same
+species. Pigeons can be mated for life, and this is a great convenience to
+the fancier, for thus many races may be kept true, though mingled in the
+same aviary; and this circumstance must have largely favoured the
+improvement and formation of new breeds. Pigeons, I may add, can be
+propagated in great numbers and at a very quick rate, and inferior birds
+may be freely rejected, as when killed they serve for food. On the other
+hand, cats, from their nocturnal rambling habits, cannot be matched, and,
+although so much valued by women and children, we hardly ever see a
+distinct breed kept up; such breeds as we do sometimes see are almost
+always imported from some other country, often from islands. Although I do
+not doubt that some domestic animals vary less than others, yet the rarity
+or absence of distinct breeds of the cat, the donkey, peacock, goose, &c.,
+may be attributed in main part to selection not having been brought into
+play: in cats, from the difficulty in pairing them; in donkeys, from only a
+few being kept by poor people, and little attention paid to their breeding;
+in peacocks, from not being very easily reared and a large stock not kept;
+in geese, from being valuable only for two purposes, food and feathers, and
+more especially from no pleasure having been felt in the display of
+distinct breeds.
+
+To sum up on the origin of our Domestic Races of {43} animals and plants. I
+believe that the conditions of life, from their action on the reproductive
+system, are so far of the highest importance as causing variability. I do
+not believe that variability is an inherent and necessary contingency,
+under all circumstances, with all organic beings, as some authors have
+thought. The effects of variability are modified by various degrees of
+inheritance and of reversion. Variability is governed by many unknown laws,
+more especially by that of correlation of growth. Something may be
+attributed to the direct action of the conditions of life. Something must
+be attributed to use and disuse. The final result is thus rendered
+infinitely complex. In some cases, I do not doubt that the intercrossing of
+species, aboriginally distinct, has played an important part in the origin
+of our domestic productions. When in any country several domestic breeds
+have once been established, their occasional intercrossing, with the aid of
+selection, has, no doubt, largely aided in the formation of new sub-breeds;
+but the importance of the crossing of varieties has, I believe, been
+greatly exaggerated, both in regard to animals and to those plants which
+are propagated by seed. In plants which are temporarily propagated by
+cuttings, buds, &c., the importance of the crossing both of distinct
+species and of varieties is immense; for the cultivator here quite
+disregards the extreme variability both of hybrids and mongrels, and the
+frequent sterility of hybrids; but the cases of plants not propagated by
+seed are of little importance to us, for their endurance is only temporary.
+Over all these causes of Change I am convinced that the accumulative action
+of Selection, whether applied methodically and more quickly, or
+unconsciously and more slowly, but more efficiently, is by far the
+predominant Power.
+
+       *       *       *       *       *
+
+
+{44}
+
+CHAPTER II.
+
+VARIATION UNDER NATURE.
+
+    Variability--Individual differences--Doubtful species--Wide ranging,
+    much diffused, and common species vary most--Species of the larger
+    genera in any country vary more than the species of the smaller
+    genera--Many of the species of the larger genera resemble varieties in
+    being very closely, but unequally, related to each other, and in having
+    restricted ranges.
+
+Before applying the principles arrived at in the last chapter to organic
+beings in a state of nature, we must briefly discuss whether these latter
+are subject to any variation. To treat this subject at all properly, a long
+catalogue of dry facts should be given; but these I shall reserve for my
+future work. Nor shall I here discuss the various definitions which have
+been given of the term species. No one definition has as yet satisfied all
+naturalists; yet every naturalist knows vaguely what he means when he
+speaks of a species. Generally the term includes the unknown element of a
+distinct act of creation. The term "variety" is almost equally difficult to
+define; but here community of descent is almost universally implied, though
+it can rarely be proved. We have also what are called monstrosities; but
+they graduate into varieties. By a monstrosity I presume is meant some
+considerable deviation of structure in one part, either injurious to or not
+useful to the species, and not generally propagated. Some authors use the
+term "variation" in a technical sense, as implying a modification directly
+due to the physical conditions of life; and "variations" in this sense are
+supposed not to be inherited: but who can say that the dwarfed condition of
+shells in the brackish waters of the Baltic, or dwarfed {45} plants on
+Alpine summits, or the thicker fur of an animal from far northwards, would
+not in some cases be inherited for at least some few generations? and in
+this case I presume that the form would be called a variety.
+
+Again, we have many slight differences which may be called individual
+differences, such as are known frequently to appear in the offspring from
+the same parents, or which may be presumed to have thus arisen, from being
+frequently observed in the individuals of the same species inhabiting the
+same confined locality. No one supposes that all the individuals of the
+same species are cast in the very same mould. These individual differences
+are highly important for us, as they afford materials for natural selection
+to accumulate, in the same manner as man can accumulate in any given
+direction individual differences in his domesticated productions. These
+individual differences generally affect what naturalists consider
+unimportant parts; but I could show by a long catalogue of facts, that
+parts which must be called important, whether viewed under a physiological
+or classificatory point of view, sometimes vary in the individuals of the
+same species. I am convinced that the most experienced naturalist would be
+surprised at the number of the cases of variability, even in important
+parts of structure, which he could collect on good authority, as I have
+collected, during a course of years. It should be remembered that
+systematists are far from pleased at finding variability in important
+characters, and that there are not many men who will laboriously examine
+internal and important organs, and compare them in many specimens of the
+same species. I should never have expected that the branching of the main
+nerves close to the great central ganglion of an insect would have been
+variable in the same species; I should have expected that changes of this
+nature could have been effected only {46} by slow degrees: yet quite
+recently Mr. Lubbock has shown a degree of variability in these main nerves
+in Coccus, which may almost be compared to the irregular branching of the
+stem of a tree. This philosophical naturalist, I may add, has also quite
+recently shown that the muscles in the larvæ of certain insects are very
+far from uniform. Authors sometimes argue in a circle when they state that
+important organs never vary; for these same authors practically rank that
+character as important (as some few naturalists have honestly confessed)
+which does not vary; and, under this point of view, no instance of an
+important part varying will ever be found: but under any other point of
+view many instances assuredly can be given.
+
+There is one point connected with individual differences, which seems to me
+extremely perplexing: I refer to those genera which have sometimes been
+called "protean" or "polymorphic," in which the species present an
+inordinate amount of variation; and hardly two naturalists can agree which
+forms to rank as species and which as varieties. We may instance Rubus,
+Rosa, and Hieracium amongst plants, several genera of insects, and several
+genera of Brachiopod shells. In most polymorphic genera some of the species
+have fixed and definite characters. Genera which are polymorphic in one
+country seem to be, with some few exceptions, polymorphic in other
+countries, and likewise, judging from Brachiopod shells, at former periods
+of time. These facts seem to be very perplexing, for they seem to show that
+this kind of variability is independent of the conditions of life. I am
+inclined to suspect that we see in these polymorphic genera variations in
+points of structure which are of no service or disservice to the species,
+and which consequently have not been seized on and rendered definite by
+natural selection, as hereafter will be explained. {47}
+
+Those forms which possess in some considerable degree the character of
+species, but which are so closely similar to some other forms, or are so
+closely linked to them by intermediate gradations, that naturalists do not
+like to rank them as distinct species, are in several respects the most
+important for us. We have every reason to believe that many of these
+doubtful and closely-allied forms have permanently retained their
+characters in their own country for a long time; for as long, as far as we
+know, as have good and true species. Practically, when a naturalist can
+unite two forms together by others having intermediate characters, he
+treats the one as a variety of the other, ranking the most common, but
+sometimes the one first described, as the species, and the other as the
+variety. But cases of great difficulty, which I will not here enumerate,
+sometimes occur in deciding whether or not to rank one form as a variety of
+another, even when they are closely connected by intermediate links; nor
+will the commonly-assumed hybrid nature of the intermediate links always
+remove the difficulty. In very many cases, however, one form is ranked as a
+variety of another, not because the intermediate links have actually been
+found, but because analogy leads the observer to suppose either that they
+do now somewhere exist, or may formerly have existed; and here a wide door
+for the entry of doubt and conjecture is opened.
+
+Hence, in determining whether a form should be ranked as a species or a
+variety, the opinion of naturalists having sound judgment and wide
+experience seems the only guide to follow. We must, however, in many cases,
+decide by a majority of naturalists, for few well-marked and well-known
+varieties can be named which have not been ranked as species by at least
+some competent judges. {48}
+
+That varieties of this doubtful nature are far from uncommon cannot be
+disputed. Compare the several floras of Great Britain, of France or of the
+United States, drawn up by different botanists, and see what a surprising
+number of forms have been ranked by one botanist as good species, and by
+another as mere varieties. Mr. H. C. Watson, to whom I lie under deep
+obligation for assistance of all kinds, has marked for me 182 British
+plants, which are generally considered as varieties, but which have all
+been ranked by botanists as species; and in making this list he has omitted
+many trifling varieties, but which nevertheless have been ranked by some
+botanists as species, and he has entirely omitted several highly
+polymorphic genera. Under genera, including the most polymorphic forms, Mr.
+Babington gives 251 species, whereas Mr. Bentham gives only 112,--a
+difference of 139 doubtful forms! Amongst animals which unite for each
+birth, and which are highly locomotive, doubtful forms, ranked by one
+zoologist as a species and by another as a variety, can rarely be found
+within the same country, but are common in separated areas. How many of
+those birds and insects in North America and Europe, which differ very
+slightly from each other, have been ranked by one eminent naturalist as
+undoubted species, and by another as varieties, or, as they are often
+called, as geographical races! Many years ago, when comparing, and seeing
+others compare, the birds from the separate islands of the Galapagos
+Archipelago, both one with another, and with those from the American
+mainland, I was much struck how entirely vague and arbitrary is the
+distinction between species and varieties. On the islets of the little
+Madeira group there are many insects which are characterized as varieties
+in Mr. Wollaston's admirable work, but which it cannot {49} be doubted
+would be ranked as distinct species by many entomologists. Even Ireland has
+a few animals, now generally regarded as varieties, but which have been
+ranked as species by some zoologists. Several most experienced
+ornithologists consider our British red grouse as only a strongly-marked
+race of a Norwegian species, whereas the greater number rank it as an
+undoubted species peculiar to Great Britain. A wide distance between the
+homes of two doubtful forms leads many naturalists to rank both as distinct
+species; but what distance, it has been well asked, will suffice? if that
+between America and Europe is ample, will that between the Continent and
+the Azores, or Madeira, or the Canaries, or Ireland, be sufficient? It must
+be admitted that many forms, considered by highly-competent judges as
+varieties, have so perfectly the character of species that they are ranked
+by other highly-competent judges as good and true species. But to discuss
+whether they are rightly called species or varieties, before any definition
+of these terms has been generally accepted, is vainly to beat the air.
+
+Many of the cases of strongly-marked varieties or doubtful species well
+deserve consideration; for several interesting lines of argument, from
+geographical distribution, analogical variation, hybridism, &c., have been
+brought to bear on the attempt to determine their rank. I will here give
+only a single instance,--the well-known one of the primrose and cowslip, or
+Primula vulgaris and veris. These plants differ considerably in appearance;
+they have a different flavour, and emit a different odour; they flower at
+slightly different periods; they grow in somewhat different stations; they
+ascend mountains to different heights; they have different geographical
+ranges; and lastly, according to very numerous experiments made during
+several years by {50} that most careful observer Gärtner, they can be
+crossed only with much difficulty. We could hardly wish for better evidence
+of the two forms being specifically distinct. On the other hand, they are
+united by many intermediate links, and it is very doubtful whether these
+links are hybrids; and there is, as it seems to me, an overwhelming amount
+of experimental evidence, showing that they descend from common parents,
+and consequently must be ranked as varieties.
+
+Close investigation, in most cases, will bring naturalists to an agreement
+how to rank doubtful forms. Yet it must be confessed that it is in the
+best-known countries that we find the greatest number of forms of doubtful
+value. I have been struck with the fact, that if any animal or plant in a
+state of nature be highly useful to man, or from any cause closely attract
+his attention, varieties of it will almost universally be found recorded.
+These varieties, moreover, will be often ranked by some authors as species.
+Look at the common oak, how closely it has been studied; yet a German
+author makes more than a dozen species out of forms, which are very
+generally considered as varieties; and in this country the highest
+botanical authorities and practical men can be quoted to show that the
+sessile and pedunculated oaks are either good and distinct species or mere
+varieties.
+
+When a young naturalist commences the study of a group of organisms quite
+unknown to him, he is at first much perplexed to determine what differences
+to consider as specific, and what as varieties; for he knows nothing of the
+amount and kind of variation to which the group is subject; and this shows,
+at least, how very generally there is some variation. But if he confine his
+attention to one class within one country, he will soon make up his mind
+how to rank most of the doubtful forms. His {51} general tendency will be
+to make many species, for he will become impressed, just like the pigeon or
+poultry fancier before alluded to, with the amount of difference in the
+forms which he is continually studying; and he has little general knowledge
+of analogical variation in other groups and in other countries, by which to
+correct his first impressions. As he extends the range of his observations,
+he will meet with more cases of difficulty; for he will encounter a greater
+number of closely-allied forms. But if his observations be widely extended,
+he will in the end generally be enabled to make up his own mind which to
+call varieties and which species; but he will succeed in this at the
+expense of admitting much variation,--and the truth of this admission will
+often be disputed by other naturalists. When, moreover, he comes to study
+allied forms brought from countries not now continuous, in which case he
+can hardly hope to find the intermediate links between his doubtful forms,
+he will have to trust almost entirely to analogy, and his difficulties rise
+to a climax.
+
+Certainly no clear line of demarcation has as yet been drawn between
+species and sub-species--that is, the forms which in the opinion of some
+naturalists come very near to, but do not quite arrive at the rank of
+species; or, again, between sub-species and well-marked varieties, or
+between lesser varieties and individual differences. These differences
+blend into each other in an insensible series; and a series impresses the
+mind with the idea of an actual passage.
+
+Hence I look at individual differences, though of small interest to the
+systematist, as of high importance for us, as being the first step towards
+such slight varieties as are barely thought worth recording in works on
+natural history. And I look at varieties which are in any degree more
+distinct and permanent, as steps leading to more {52} strongly marked and
+more permanent varieties; and at these latter, as leading to sub-species,
+and to species. The passage from one stage of difference to another and
+higher stage may be, in some cases, due merely to the long-continued action
+of different physical conditions in two different regions; but I have not
+much faith in this view; and I attribute the passage of a variety, from a
+state in which it differs very slightly from its parent to one in which it
+differs more, to the action of natural selection in accumulating (as will
+hereafter be more fully explained) differences of structure in certain
+definite directions. Hence I believe a well-marked variety may be called an
+incipient species; but whether this belief be justifiable must be judged of
+by the general weight of the several facts and views given throughout this
+work.
+
+It need not be supposed that all varieties or incipient species necessarily
+attain the rank of species. They may whilst in this incipient state become
+extinct, or they may endure as varieties for very long periods, as has been
+shown to be the case by Mr. Wollaston with the varieties of certain fossil
+land-shells in Madeira. If a variety were to flourish so as to exceed in
+numbers the parent species, it would then rank as the species, and the
+species as the variety; or it might come to supplant and exterminate the
+parent species; or both might co-exist, and both rank as independent
+species. But we shall hereafter have to return to this subject.
+
+From these remarks it will be seen that I look at the term species, as one
+arbitrarily given for the sake of convenience to a set of individuals
+closely resembling each other, and that it does not essentially differ from
+the term variety, which is given to less distinct and more fluctuating
+forms. The term variety, again, in comparison with mere individual
+differences, is also applied arbitrarily, and for mere convenience' sake.
+{53}
+
+Guided by theoretical considerations, I thought that some interesting
+results might be obtained in regard to the nature and relations of the
+species which vary most, by tabulating all the varieties in several
+well-worked floras. At first this seemed a simple task; but Mr. H. C.
+Watson, to whom I am much indebted for valuable advice and assistance on
+this subject, soon convinced me that there were many difficulties, as did
+subsequently Dr. Hooker, even in stronger terms. I shall reserve for my
+future work the discussion of these difficulties, and the tables themselves
+of the proportional numbers of the varying species. Dr. Hooker permits me
+to add, that after having carefully read my manuscript, and examined the
+tables, he thinks that the following statements are fairly well
+established. The whole subject, however, treated as it necessarily here is
+with much brevity, is rather perplexing, and allusions cannot be avoided to
+the "struggle for existence," "divergence of character," and other
+questions, hereafter to be discussed.
+
+Alph. de Candolle and others have shown that plants which have very wide
+ranges generally present varieties; and this might have been expected, as
+they become exposed to diverse physical conditions, and as they come into
+competition (which, as we shall hereafter see, is a far more important
+circumstance) with different sets of organic beings. But my tables further
+show that, in any limited country, the species which are most common, that
+is abound most in individuals, and the species which are most widely
+diffused within their own country (and this is a different consideration
+from wide range, and to a certain extent from commonness), often give rise
+to varieties sufficiently well-marked to have been recorded in botanical
+works. Hence it is the most flourishing, or, as they may be called, the
+dominant species,--those {54} which range widely over the world, are the
+most diffused in their own country, and are the most numerous in
+individuals,--which oftenest produce well-marked varieties, or, as I
+consider them, incipient species. And this, perhaps, might have been
+anticipated; for, as varieties, in order to become in any degree permanent,
+necessarily have to struggle with the other inhabitants of the country, the
+species which are already dominant will be the most likely to yield
+offspring, which, though in some slight degree modified, still inherit
+those advantages that enabled their parents to become dominant over their
+compatriots.
+
+If the plants inhabiting a country and described in any Flora be divided
+into two equal masses, all those in the larger genera being placed on one
+side, and all those in the smaller genera on the other side, a somewhat
+larger number of the very common and much diffused or dominant species will
+be found on the side of the larger genera. This, again, might have been
+anticipated; for the mere fact of many species of the same genus inhabiting
+any country, shows that there is something in the organic or inorganic
+conditions of that country favourable to the genus; and, consequently, we
+might have expected to have found in the larger genera, or those including
+many species, a large proportional number of dominant species. But so many
+causes tend to obscure this result, that I am surprised that my tables show
+even a small majority on the side of the larger genera. I will here allude
+to only two causes of obscurity. Fresh-water and salt-loving plants have
+generally very wide ranges and are much diffused, but this seems to be
+connected with the nature of the stations inhabited by them, and has little
+or no relation to the size of the genera to which the species belong.
+Again, plants low in the scale of organisation are {55} generally much more
+widely diffused than plants higher in the scale; and here again there is no
+close relation to the size of the genera. The cause of lowly-organised
+plants ranging widely will be discussed in our chapter on geographical
+distribution.
+
+From looking at species as only strongly-marked and well-defined varieties,
+I was led to anticipate that the species of the larger genera in each
+country would oftener present varieties, than the species of the smaller
+genera; for wherever many closely related species (_i.e._ species of the
+same genus) have been formed, many varieties or incipient species ought, as
+a general rule, to be now forming. Where many large trees grow, we expect
+to find saplings. Where many species of a genus have been formed through
+variation, circumstances have been favourable for variation; and hence we
+might expect that the circumstances would generally be still favourable to
+variation. On the other hand, if we look at each species as a special act
+of creation, there is no apparent reason why more varieties should occur in
+a group having many species, than in one having few.
+
+To test the truth of this anticipation I have arranged the plants of twelve
+countries, and the coleopterous insects of two districts, into two nearly
+equal masses, the species of the larger genera on one side, and those of
+the smaller genera on the other side, and it has invariably proved to be
+the case that a larger proportion of the species on the side of the larger
+genera present varieties, than on the side of the smaller genera. Moreover,
+the species of the large genera which present any varieties, invariably
+present a larger average number of varieties than do the species of the
+small genera. Both these results follow when another division is made, and
+when all the smallest genera, with from only one to four species, are
+absolutely excluded from the tables. These {56} facts are of plain
+signification on the view that species are only strongly marked and
+permanent varieties; for wherever many species of the same genus have been
+formed, or where, if we may use the expression, the manufactory of species
+has been active, we ought generally to find the manufactory still in
+action, more especially as we have every reason to believe the process of
+manufacturing new species to be a slow one. And this certainly is the case,
+if varieties be looked at as incipient species; for my tables clearly show
+as a general rule that, wherever many species of a genus have been formed,
+the species of that genus present a number of varieties, that is of
+incipient species beyond the average. It is not that all large genera are
+now varying much, and are thus increasing in the number of their species,
+or that no small genera are now varying and increasing; for if this had
+been so, it would have been fatal to my theory; inasmuch as geology plainly
+tells us that small genera have in the lapse of time often increased
+greatly in size; and that large genera have often come to their maxima,
+declined, and disappeared. All that we want to show is, that where many
+species of a genus have been formed, on an average many are still forming;
+and this holds good.
+
+There are other relations between the species of large genera and their
+recorded varieties which deserve notice. We have seen that there is no
+infallible criterion by which to distinguish species and well-marked
+varieties; and in those cases in which intermediate links have not been
+found between doubtful forms, naturalists are compelled to come to a
+determination by the amount of difference between them, judging by analogy
+whether or not the amount suffices to raise one or both to the rank of
+species. Hence the amount of difference is one very important criterion in
+settling whether two forms {57} should be ranked as species or varieties.
+Now Fries has remarked in regard to plants, and Westwood in regard to
+insects, that in large genera the amount of difference between the species
+is often exceedingly small. I have endeavoured to test this numerically by
+averages, and, as far as my imperfect results go, they confirm the view. I
+have also consulted some sagacious and experienced observers, and, after
+deliberation, they concur in this view. In this respect, therefore, the
+species of the larger genera resemble varieties, more than do the species
+of the smaller genera. Or the case may be put in another way, and it may be
+said, that in the larger genera, in which a number of varieties or
+incipient species greater than the average are now manufacturing, many of
+the species already manufactured still to a certain extent resemble
+varieties, for they differ from each other by a less than usual amount of
+difference.
+
+Moreover, the species of the large genera are related to each other, in the
+same manner as the varieties of any one species are related to each other.
+No naturalist pretends that all the species of a genus are equally distinct
+from each other; they may generally be divided into sub-genera, or
+sections, or lesser groups. As Fries has well remarked, little groups of
+species are generally clustered like satellites around certain other
+species. And what are varieties but groups of forms, unequally related to
+each other, and clustered round certain forms--that is, round their
+parent-species? Undoubtedly there is one most important point of difference
+between varieties and species; namely, that the amount of difference
+between varieties, when compared with each other or with their
+parent-species, is much less than that between the species of the same
+genus. But when we come to discuss the principle, as I call it, of
+Divergence of Character, {58} we shall see how this may be explained, and
+how the lesser differences between varieties will tend to increase into the
+greater differences between species.
+
+There is one other point which seems to me worth notice. Varieties
+generally have much restricted ranges: this statement is indeed scarcely
+more than a truism, for if a variety were found to have a wider range than
+that of its supposed parent-species, their denominations ought to be
+reversed. But there is also reason to believe, that those species which are
+very closely allied to other species, and in so far resemble varieties,
+often have much restricted ranges. For instance, Mr. H. C. Watson has
+marked for me in the well-sifted London Catalogue of plants (4th edition)
+63 plants which are therein ranked as species, but which he considers as so
+closely allied to other species as to be of doubtful value: these 63
+reputed species range on an average over 6.9 of the provinces into which
+Mr. Watson has divided Great Britain. Now, in this same catalogue, 53
+acknowledged varieties are recorded, and these range over 7.7 provinces;
+whereas, the species to which these varieties belong range over 14.3
+provinces. So that the acknowledged varieties have very nearly the same
+restricted average range, as have those very closely allied forms, marked
+for me by Mr. Watson as doubtful species, but which are almost universally
+ranked by British botanists as good and true species.
+
+
+
+Finally, then, varieties have the same general characters as species, for
+they cannot be distinguished from species,--except, firstly, by the
+discovery of intermediate linking forms, and the occurrence of such links
+cannot affect the actual characters of the forms which they connect; and
+except, secondly by a certain amount of {59} difference, for two forms, if
+differing very little, are generally ranked as varieties, notwithstanding
+that intermediate linking forms have not been discovered; but the amount of
+difference considered necessary to give to two forms the rank of species is
+quite indefinite. In genera having more than the average number of species
+in any country, the species of these genera have more than the average
+number of varieties. In large genera the species are apt to be closely, but
+unequally allied together, forming little clusters round certain species.
+Species very closely allied to other species apparently have restricted
+ranges. In all these several respects the species of large genera present a
+strong analogy with varieties. And we can clearly understand these
+analogies, if species have once existed as varieties, and have thus
+originated: whereas, these analogies are utterly inexplicable if each
+species has been independently created.
+
+We have, also, seen that it is the most flourishing or dominant species of
+the larger genera which on an average vary most; and varieties, as we shall
+hereafter see, tend to become converted into new and distinct species. The
+larger genera thus tend to become larger; and throughout nature the forms
+of life which are now dominant tend to become still more dominant by
+leaving many modified and dominant descendants. But by steps hereafter to
+be explained, the larger genera also tend to break up into smaller genera.
+And thus, the forms of life throughout the universe become divided into
+groups subordinate to groups.
+
+       *       *       *       *       *
+
+
+{60}
+
+CHAPTER III.
+
+STRUGGLE FOR EXISTENCE.
+
+    Bears on natural selection--The term used in a wide sense--Geometrical
+    powers of increase--Rapid increase of naturalised animals and
+    plants--Nature of the checks to increase--Competition
+    universal--Effects of climate--Protection from the number of
+    individuals--Complex relations of all animals and plants throughout
+    nature--Struggle for life most severe between individuals and varieties
+    of the same species; often severe between species of the same
+    genus--The relation of organism to organism the most important of all
+    relations.
+
+Before entering on the subject of this chapter, I must make a few
+preliminary remarks, to show how the struggle for existence bears on
+Natural Selection. It has been seen in the last chapter that amongst
+organic beings in a state of nature there is some individual variability:
+indeed I am not aware that this has ever been disputed. It is immaterial
+for us whether a multitude of doubtful forms be called species or
+sub-species or varieties; what rank, for instance, the two or three hundred
+doubtful forms of British plants are entitled to hold, if the existence of
+any well-marked varieties be admitted. But the mere existence of individual
+variability and of some few well-marked varieties, though necessary as the
+foundation for the work, helps us but little in understanding how species
+arise in nature. How have all those exquisite adaptations of one part of
+the organisation to another part, and to the conditions of life, and of one
+distinct organic being to another being, been perfected? We see these
+beautiful co-adaptations most {61} plainly in the woodpecker and missletoe;
+and only a little less plainly in the humblest parasite which clings to the
+hairs of a quadruped or feathers of a bird; in the structure of the beetle
+which dives through the water; in the plumed seed which is wafted by the
+gentlest breeze; in short, we see beautiful adaptations everywhere and in
+every part of the organic world.
+
+Again, it may be asked, how is it that varieties, which I have called
+incipient species, become ultimately converted into good and distinct
+species, which in most cases obviously differ from each other far more than
+do the varieties of the same species? How do those groups of species, which
+constitute what are called distinct genera, and which differ from each
+other more than do the species of the same genus, arise? All these results,
+as we shall more fully see in the next chapter, follow from the struggle
+for life. Owing to this struggle for life, any variation, however slight,
+and from whatever cause proceeding, if it be in any degree profitable to an
+individual of any species, in its infinitely complex relations to other
+organic beings and to external nature, will tend to the preservation of
+that individual, and will generally be inherited by its offspring. The
+offspring, also, will thus have a better chance of surviving, for, of the
+many individuals of any species which are periodically born, but a small
+number can survive. I have called this principle, by which each slight
+variation, if useful, is preserved, by the term of Natural Selection, in
+order to mark its relation to man's power of selection. We have seen that
+man by selection can certainly produce great results, and can adapt organic
+beings to his own uses, through the accumulation of slight but useful
+variations, given to him by the hand of Nature. But Natural Selection, as
+we shall hereafter see, is a power incessantly ready for action, and is as
+{62} immeasurably superior to man's feeble efforts, as the works of Nature
+are to those of Art.
+
+We will now discuss in a little more detail the struggle for existence. In
+my future work this subject shall be treated, as it well deserves, at much
+greater length. The elder de Candolle and Lyell have largely and
+philosophically shown that all organic beings are exposed to severe
+competition. In regard to plants, no one has treated this subject with more
+spirit and ability than W. Herbert, Dean of Manchester, evidently the
+result of his great horticultural knowledge. Nothing is easier than to
+admit in words the truth of the universal struggle for life, or more
+difficult--at least I have found it so--than constantly to bear this
+conclusion in mind. Yet unless it be thoroughly engrained in the mind, I am
+convinced that the whole economy of nature, with every fact on
+distribution, rarity, abundance, extinction, and variation, will be dimly
+seen or quite misunderstood. We behold the face of nature bright with
+gladness, we often see superabundance of food; we do not see, or we forget
+that the birds which are idly singing round us mostly live on insects or
+seeds, and are thus constantly destroying life; or we forget how largely
+these songsters, or their eggs, or their nestlings, are destroyed by birds
+and beasts of prey; we do not always bear in mind, that though food may be
+now superabundant, it is not so at all seasons of each recurring year.
+
+I should premise that I use the term Struggle for Existence in a large and
+metaphorical sense, including dependence of one being on another, and
+including (which is more important) not only the life of the individual,
+but success in leaving progeny. Two canine animals in a time of dearth, may
+be truly said to struggle with each other which shall get food and live.
+But a plant on the edge of a desert is said to struggle {63} for life
+against the drought, though more properly it should be said to be dependent
+on the moisture. A plant which annually produces a thousand seeds, of which
+on an average only one comes to maturity, may be more truly said to
+struggle with the plants of the same and other kinds which already clothe
+the ground. The missletoe is dependent on the apple and a few other trees,
+but can only in a far-fetched sense be said to struggle with these trees,
+for if too many of these parasites grow on the same tree, it will languish
+and die. But several seedling missletoes, growing close together on the
+same branch, may more truly be said to struggle with each other. As the
+missletoe is disseminated by birds, its existence depends on birds; and it
+may metaphorically be said to struggle with other fruit-bearing plants, in
+order to tempt birds to devour and thus disseminate its seeds rather than
+those of other plants. In these several senses, which pass into each other,
+I use for convenience' sake the general term of struggle for existence.
+
+A struggle for existence inevitably follows from the high rate at which all
+organic beings tend to increase. Every being, which during its natural
+lifetime produces several eggs or seeds, must suffer destruction during
+some period of its life, and during some season or occasional year,
+otherwise, on the principle of geometrical increase, its numbers would
+quickly become so inordinately great that no country could support the
+product. Hence, as more individuals are produced than can possibly survive,
+there must in every case be a struggle for existence, either one individual
+with another of the same species, or with the individuals of distinct
+species, or with the physical conditions of life. It is the doctrine of
+Malthus applied with manifold force to the whole animal and vegetable
+kingdoms; for in this case there {64} can be no artificial increase of
+food, and no prudential restraint from marriage. Although some species may
+be now increasing, more or less rapidly, in numbers, all cannot do so, for
+the world would not hold them.
+
+There is no exception to the rule that every organic being naturally
+increases at so high a rate, that if not destroyed, the earth would soon be
+covered by the progeny of a single pair. Even slow-breeding man has doubled
+in twenty-five years, and at this rate, in a few thousand years, there
+would literally not be standing room for his progeny. Linnæus has
+calculated that if an annual plant produced only two seeds--and there is no
+plant so unproductive as this--and their seedlings next year produced two,
+and so on, then in twenty years there would be a million plants. The
+elephant is reckoned the slowest breeder of all known animals, and I have
+taken some pains to estimate its probable minimum rate of natural increase:
+it will be under the mark to assume that it breeds when thirty years old,
+and goes on breeding till ninety years old, bringing forth three pair of
+young in this interval; if this be so, at the end of the fifth century
+there would be alive fifteen million elephants, descended from the first
+pair.
+
+But we have better evidence on this subject than mere theoretical
+calculations, namely, the numerous recorded cases of the astonishingly
+rapid increase of various animals in a state of nature, when circumstances
+have been favourable to them during two or three following seasons. Still
+more striking is the evidence from our domestic animals of many kinds which
+have run wild in several parts of the world: if the statements of the rate
+of increase of slow-breeding cattle and horses in South America, and
+latterly in Australia, had not been well authenticated, they would have
+been incredible. So it is with plants: cases could be given of {65}
+introduced plants which have become common throughout whole islands in a
+period of less than ten years. Several of the plants, such as the cardoon
+and a tall thistle, now most numerous over the wide plains of La Plata,
+clothing square leagues of surface almost to the exclusion of all other
+plants, have been introduced from Europe; and there are plants which now
+range in India, as I hear from Dr. Falconer, from Cape Comorin to the
+Himalaya, which have been imported from America since its discovery. In
+such cases, and endless instances could be given, no one supposes that the
+fertility of these animals or plants has been suddenly and temporarily
+increased in any sensible degree. The obvious explanation is that the
+conditions of life have been very favourable, and that there has
+consequently been less destruction of the old and young, and that nearly
+all the young have been enabled to breed. In such cases the geometrical
+ratio of increase, the result of which never fails to be surprising, simply
+explains the extraordinarily rapid increase and wide diffusion of
+naturalised productions in their new homes.
+
+In a state of nature almost every plant produces seed, and amongst animals
+there are very few which do not annually pair. Hence we may confidently
+assert, that all plants and animals are tending to increase at a
+geometrical ratio, that all would most rapidly stock every station in which
+they could any how exist, and that the geometrical tendency to increase
+must be checked by destruction at some period of life. Our familiarity with
+the larger domestic animals tends, I think, to mislead us: we see no great
+destruction falling on them, and we forget that thousands are annually
+slaughtered for food, and that in a state of nature an equal number would
+have somehow to be disposed of.
+
+The only difference between organisms which annually {66} produce eggs or
+seeds by the thousand, and those which produce extremely few, is, that the
+slow-breeders would require a few more years to people, under favourable
+conditions, a whole district, let it be ever so large. The condor lays a
+couple of eggs and the ostrich a score, and yet in the same country the
+condor may be the more numerous of the two: the Fulmar petrel lays but one
+egg, yet it is believed to be the most numerous bird in the world. One fly
+deposits hundreds of eggs, and another, like the hippobosca, a single one;
+but this difference does not determine how many individuals of the two
+species can be supported in a district. A large number of eggs is of some
+importance to those species which depend on a rapidly fluctuating amount of
+food, for it allows them rapidly to increase in number. But the real
+importance of a large number of eggs or seeds is to make up for much
+destruction at some period of life; and this period in the great majority
+of cases is an early one. If an animal can in any way protect its own eggs
+or young, a small number may be produced, and yet the average stock be
+fully kept up; but if many eggs or young are destroyed, many must be
+produced, or the species will become extinct. It would suffice to keep up
+the full number of a tree, which lived on an average for a thousand years,
+if a single seed were produced once in a thousand years, supposing that
+this seed were never destroyed, and could be ensured to germinate in a
+fitting place. So that in all cases, the average number of any animal or
+plant depends only indirectly on the number of its eggs or seeds.
+
+In looking at Nature, it is most necessary to keep the foregoing
+considerations always in mind--never to forget that every single organic
+being around us may be said to be striving to the utmost to increase in
+numbers; that each lives by a struggle at some period of {67} its life;
+that heavy destruction inevitably falls either on the young or old, during
+each generation or at recurrent intervals. Lighten any check, mitigate the
+destruction ever so little, and the number of the species will almost
+instantaneously increase to any amount.
+
+The causes which check the natural tendency of each species to increase in
+number are most obscure. Look at the most vigorous species; by as much as
+it swarms in numbers, by so much will its tendency to increase be still
+further increased. We know not exactly what the checks are in even one
+single instance. Nor will this surprise any one who reflects how ignorant
+we are on this head, even in regard to mankind, so incomparably better
+known than any other animal. This subject has been ably treated by several
+authors, and I shall, in my future work, discuss some of the checks at
+considerable length, more especially in regard to the feral animals of
+South America. Here I will make only a few remarks, just to recall to the
+reader's mind some of the chief points. Eggs or very young animals seem
+generally to suffer most, but this is not invariably the case. With plants
+there is a vast destruction of seeds, but, from some observations which I
+have made, I believe that it is the seedlings which suffer most from
+germinating in ground already thickly stocked with other plants. Seedlings,
+also, are destroyed in vast numbers by various enemies; for instance, on a
+piece of ground three feet long and two wide, dug and cleared, and where
+there could be no choking from other plants, I marked all the seedlings of
+our native weeds as they came up, and out of the 357 no less than 295 were
+destroyed, chiefly by slugs and insects. If turf which has long been mown,
+and the case would be the same with turf closely browsed by quadrupeds, be
+let to grow, the more vigorous plants {68} gradually kill the less
+vigorous, though fully grown, plants: thus out of twenty species growing on
+a little plot of turf (three feet by four) nine species perished from the
+other species being allowed to grow up freely.
+
+The amount of food for each species of course gives the extreme limit to
+which each can increase; but very frequently it is not the obtaining food,
+but the serving as prey to other animals, which determines the average
+numbers of a species. Thus, there seems to be little doubt that the stock
+of partridges, grouse, and hares on any large estate depends chiefly on the
+destruction of vermin. If not one head of game were shot during the next
+twenty years in England, and, at the same time, if no vermin were
+destroyed, there would, in all probability, be less game than at present,
+although hundreds of thousands of game animals are now annually killed. On
+the other hand, in some cases, as with the elephant and rhinoceros, none
+are destroyed by beasts of prey: even the tiger in India most rarely dares
+to attack a young elephant protected by its dam.
+
+Climate plays an important part in determining the average numbers of a
+species, and periodical seasons of extreme cold or drought, I believe to be
+the most effective of all checks. I estimated that the winter of 1854-55
+destroyed four-fifths of the birds in my own grounds; and this is a
+tremendous destruction, when we remember that ten per cent, is an
+extraordinarily severe mortality from epidemics with man. The action of
+climate seems at first sight to be quite independent of the struggle for
+existence; but in so far as climate chiefly acts in reducing food, it
+brings on the most severe struggle between the individuals, whether of the
+same or of distinct species, which subsist on the same kind of food. Even
+when climate, for instance extreme cold, {69} acts directly, it will be the
+least vigorous, or those which have got least food through the advancing
+winter, which will suffer most. When we travel from south to north, or from
+a damp region to a dry, we invariably see some species gradually getting
+rarer and rarer, and finally disappearing; and the change of climate being
+conspicuous, we are tempted to attribute the whole effect to its direct
+action. But this is a false view: we forget that each species, even where
+it most abounds, is constantly suffering enormous destruction at some
+period of its life, from enemies or from competitors for the same place and
+food; and if these enemies or competitors be in the least degree favoured
+by any slight change of climate, they will increase in numbers, and, as
+each area is already fully stocked with inhabitants, the other species will
+decrease. When we travel southward and see a species decreasing in numbers,
+we may feel sure that the cause lies quite as much in other species being
+favoured, as in this one being hurt. So it is when we travel northward, but
+in a somewhat lesser degree, for the number of species of all kinds, and
+therefore of competitors, decreases northwards; hence in going northward,
+or in ascending a mountain, we far oftener meet with stunted forms, due to
+the _directly_ injurious action of climate, than we do in proceeding
+southwards or in descending a mountain. When we reach the Arctic regions,
+or snow-capped summits, or absolute deserts, the struggle for life is
+almost exclusively with the elements.
+
+That climate acts in main part indirectly by favouring other species, we
+may clearly see in the prodigious number of plants in our gardens which can
+perfectly well endure our climate, but which never become naturalised, for
+they cannot compete with our native plants nor resist destruction by our
+native animals. {70}
+
+When a species, owing to highly favourable circumstances, increases
+inordinately in numbers in a small tract, epidemics--at least, this seems
+generally to occur with our game animals--often ensue: and here we have a
+limiting check independent of the struggle for life. But even some of these
+so-called epidemics appear to be due to parasitic worms, which have from
+some cause, possibly in part through facility of diffusion amongst the
+crowded animals, been disproportionably favoured: and here comes in a sort
+of struggle between the parasite and its prey.
+
+On the other hand, in many cases, a large stock of individuals of the same
+species, relatively to the numbers of its enemies, is absolutely necessary
+for its preservation. Thus we can easily raise plenty of corn and
+rape-seed, &c., in our fields, because the seeds are in great excess
+compared with the number of birds which feed on them; nor can the birds,
+though having a superabundance of food at this one season, increase in
+number proportionally to the supply of seed, as their numbers are checked
+during winter: but any one who has tried, knows how troublesome it is to
+get seed from a few wheat or other such plants in a garden: I have in this
+case lost every single seed. This view of the necessity of a large stock of
+the same species for its preservation, explains, I believe, some singular
+facts in nature, such as that of very rare plants being sometimes extremely
+abundant in the few spots where they do occur; and that of some social
+plants being social, that is, abounding in individuals, even on the extreme
+confines of their range. For in such cases, we may believe, that a plant
+could exist only where the conditions of its life were so favourable that
+many could exist together, and thus save the species from utter
+destruction. I should add that the good effects of frequent intercrossing,
+and {71} the ill effects of close interbreeding, probably come into play in
+some of these cases; but on this intricate subject I will not here enlarge.
+
+Many cases are on record showing how complex and unexpected are the checks
+and relations between organic beings, which have to struggle together in
+the same country. I will give only a single instance, which, though a
+simple one, has interested me. In Staffordshire, on the estate of a
+relation, where I had ample means of investigation, there was a large and
+extremely barren heath, which had never been touched by the hand of man;
+but several hundred acres of exactly the same nature had been enclosed
+twenty-five years previously and planted with Scotch fir. The change in the
+native vegetation of the planted part of the heath was most remarkable,
+more than is generally seen in passing from one quite different soil to
+another: not only the proportional numbers of the heath-plants were wholly
+changed, but twelve species of plants (not counting grasses and carices)
+flourished in the plantations, which could not be found on the heath. The
+effect on the insects must have been still greater, for six insectivorous
+birds were very common in the plantations, which were not to be seen on the
+heath; and the heath was frequented by two or three distinct insectivorous
+birds. Here we see how potent has been the effect of the introduction of a
+single tree, nothing whatever else having been done, with the exception
+that the land had been enclosed, so that cattle could not enter. But how
+important an element enclosure is, I plainly saw near Farnham, in Surrey.
+Here there are extensive heaths, with a few clumps of old Scotch firs on
+the distant hill-tops: within the last ten years large spaces have been
+enclosed, and self-sown firs are now springing up in multitudes, so close
+together that all cannot live. {72} When I ascertained that these young
+trees had not been sown or planted, I was so much surprised at their
+numbers that I went to several points of view, whence I could examine
+hundreds of acres of the unenclosed heath, and literally I could not see a
+single Scotch fir, except the old planted clumps. But on looking closely
+between the stems of the heath, I found a multitude of seedlings and little
+trees, which had been perpetually browsed down by the cattle. In one square
+yard, at a point some hundred yards distant from one of the old clumps, I
+counted thirty-two little trees; and one of them, with twenty-six rings of
+growth, had during many years tried to raise its head above the stems of
+the heath, and had failed. No wonder that, as soon as the land was
+enclosed, it became thickly clothed with vigorously growing young firs. Yet
+the heath was so extremely barren and so extensive that no one would ever
+have imagined that cattle would have so closely and effectually searched it
+for food.
+
+Here we see that cattle absolutely determine the existence of the Scotch
+fir; but 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 number 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 birds. Hence, if certain insectivorous birds
+(whose numbers are probably regulated by hawks or beasts of prey) were to
+increase in Paraguay, the flies would decrease--then cattle and horses
+would became feral, and this would certainly greatly {73} alter (as indeed
+I have observed in parts of South America) the vegetation: this again would
+largely affect the insects; and this, as we just have seen in
+Staffordshire, the insectivorous birds, and so onwards in ever-increasing
+circles of complexity. We began this series by insectivorous birds, and we
+have ended with them, Not that in nature the relations can ever be as
+simple as this. Battle within battle must ever be recurring with varying
+success; and yet in the long-run the forces are so nicely balanced, that
+the face of nature remains uniform for long periods of time, though
+assuredly the merest trifle would often give the victory to one organic
+being over another. Nevertheless so profound is our ignorance, and so high
+our presumption, that we marvel when we hear of the extinction of an
+organic being; and as we do not see the cause, we invoke cataclysms to
+desolate the world, or invent laws on the duration of the forms of life!
+
+I am tempted to give one more instance showing how plants and animals, most
+remote in the scale of nature, are bound together by a web of complex
+relations. I shall hereafter have occasion to show that the exotic Lobelia
+fulgens, in this part of England, is never visited by insects, and
+consequently, from its peculiar structure, never can set a seed. Many of
+our orchidaceous plants absolutely require the visits of moths to remove
+their pollen-masses and thus to fertilise them. I have, also, reason to
+believe that humble-bees are indispensable to the fertilisation of the
+heartsease (Viola tricolor), for other bees do not visit this flower. From
+experiments which I have lately tried, I have found that the visits of bees
+are necessary for the fertilisation of some kinds of clover; but
+humble-bees alone visit the red clover (Trifolium pratense), as other bees
+cannot reach the nectar. Hence I have very little doubt, that if the {74}
+whole genus of humble-bees became extinct or very rare in England, the
+heartsease and red clover would become very rare, or wholly disappear. The
+number of humble-bees in any district depends in a great degree on the
+number of field-mice, which destroy their combs and nests; and Mr. H.
+Newman, who has long attended to the habits of humble-bees, believes that
+"more than two-thirds of them are thus destroyed all over England." Now the
+number of mice is largely dependent, as every one knows, on the number of
+cats; and Mr. Newman says, "Near villages and small towns I have found the
+nests of humble-bees more numerous than elsewhere, which I attribute to the
+number of cats that destroy the mice." Hence it is quite credible that the
+presence of a feline animal in large numbers in a district might determine,
+through the intervention first of mice and then of bees, the frequency of
+certain flowers in that district!
+
+In the case of every species, many different checks, acting at different
+periods of life, and during different seasons or years, probably come into
+play; some one check or some few being generally the most potent, but all
+concur in determining the average number or even the existence of the
+species. In some cases it can be shown that widely-different checks act on
+the same species in different districts. When we look at the plants and
+bushes clothing an entangled bank, we are tempted to attribute their
+proportional numbers and kinds to what we call chance. But how false a view
+is this! Every one has heard that when an American forest is cut down, a
+very different vegetation springs up; but it has been observed that ancient
+Indian ruins in the Southern United States, which must formerly have been
+cleared of trees, now display the same beautiful diversity and proportion
+of kinds as in the surrounding {75} virgin forests. What a struggle between
+the several kinds of trees must here have gone on during long centuries,
+each annually scattering its seeds by the thousand; what war between insect
+and insect--between insects, snails, and other animals with birds and
+beasts of prey--all striving to increase, and all feeding on each other or
+on the trees or their seeds and seedlings, or on the other plants which
+first clothed the ground and thus checked the growth of the trees! Throw up
+a handful of feathers, and all must fall to the ground according to
+definite laws; but how simple is this problem compared to the action and
+reaction of the innumerable plants and animals which have determined, in
+the course of centuries, the proportional numbers and kinds of trees now
+growing on the old Indian ruins!
+
+The dependency of one organic being on another, as of a parasite on its
+prey, lies generally between beings remote in the scale of nature. This is
+often the case with those which may strictly be said to struggle with each
+other for existence, as in the case of locusts and grass-feeding
+quadrupeds. But the struggle almost invariably will be most severe between
+the individuals of the same species, for they frequent the same districts,
+require the same food, and are exposed to the same dangers. In the case of
+varieties of the same species, the struggle will generally be almost
+equally severe, and we sometimes see the contest soon decided; for
+instance, if several varieties of wheat be sown together, and the mixed
+seed be resown, some of the varieties which best suit the soil or climate,
+or are naturally the most fertile, will beat the others and so yield more
+seed, and will consequently in a few years quite supplant the other
+varieties. To keep up a mixed stock of even such extremely close varieties
+as the variously {76} coloured sweet-peas, they must be each year harvested
+separately, and the seed then mixed in due proportion, otherwise the weaker
+kinds will steadily decrease in numbers and disappear. So again with the
+varieties of sheep: it has been asserted that certain mountain-varieties
+will starve out other mountain-varieties, so that they cannot be kept
+together. The same result has followed from keeping together different
+varieties of the medicinal leech. It may even be doubted whether the
+varieties of any one of our domestic plants or animals have so exactly the
+same strength, habits, and constitution, that the original proportions of a
+mixed stock could be kept up for half-a-dozen generations, if they were
+allowed to struggle together, like beings in a state of nature, and if the
+seed or young were not annually sorted.
+
+As species of the same genus have usually, though by no means invariably,
+some similarity in habits and constitution, and always in structure, the
+struggle will generally be more severe between species of the same genus,
+when they come into competition with each other, than between species of
+distinct genera. We see this in the recent extension over parts of the
+United States of one species of swallow having caused the decrease of
+another species. The recent increase of the missel-thrush in parts of
+Scotland has caused the decrease of the song-thrush. How frequently we hear
+of one species of rat taking the place of another species under the most
+different climates! In Russia the small Asiatic cockroach has everywhere
+driven before it its great congener. One species of charlock will supplant
+another, and so in other cases. We can dimly see why the competition should
+be most severe between allied forms, which fill nearly the same place in
+the economy of nature; {77} but probably in no one case could we precisely
+say why one species has been victorious over another in the great battle of
+life.
+
+A corollary of the highest importance may be deduced from the foregoing
+remarks, namely, that the structure of every organic being is related, in
+the most essential yet often hidden manner, to that of all other organic
+beings, with which it comes into competition for food or residence, or from
+which it has to escape, or on which it preys. This is obvious in the
+structure of the teeth and talons of the tiger; and in that of the legs and
+claws of the parasite which clings to the hair on the tiger's body. But in
+the beautifully plumed seed of the dandelion, and in the flattened and
+fringed legs of the water-beetle, the relation seems at first confined to
+the elements of air and water. Yet the advantage of plumed seeds no doubt
+stands in the closest relation to the land being already thickly clothed by
+other plants; so that the seeds may be widely distributed and fall on
+unoccupied ground. In the water-beetle, the structure of its legs, so well
+adapted for diving, allows it to compete with other aquatic insects, to
+hunt for its own prey, and to escape serving as prey to other animals.
+
+The store of nutriment laid up within the seeds of many plants seems at
+first sight to have no sort of relation to other plants. But from the
+strong growth of young plants produced from such seeds (as peas and beans),
+when sown in the midst of long grass, I suspect that the chief use of the
+nutriment in the seed is to favour the growth of the young seedling, whilst
+struggling with other plants growing vigorously all around.
+
+Look at a plant in the midst of its range, why does it not double or
+quadruple its numbers? We know {78} that it can perfectly well withstand a
+little more heat or cold, dampness or dryness, for elsewhere it ranges into
+slightly hotter or colder, damper or drier districts. In this case we can
+clearly see that if we wished in imagination to give the plant the power of
+increasing in number, we should have to give it some advantage over its
+competitors, or over the animals which preyed on it. On the confines of its
+geographical range, a change of constitution with respect to climate would
+clearly be an advantage to our plant; but we have reason to believe that
+only a few plants or animals range so far, that they are destroyed by the
+rigour of the climate alone. Not until we reach the extreme confines of
+life, in the Arctic regions or on the borders of an utter desert, will
+competition cease. The land may be extremely cold or dry, yet there will be
+competition between some few species, or between the individuals of the
+same species, for the warmest or dampest spots.
+
+Hence, also, we can see that when a plant or animal is placed in a new
+country amongst new competitors, though the climate may be exactly the same
+as in its former home, yet the conditions of its life will generally be
+changed in an essential manner. If we wished to increase its average
+numbers in its new home, we should have to modify it in a different way to
+what we should have done in its native country; for we should have to give
+it some advantage over a different set of competitors or enemies.
+
+It is good thus to try in our imagination to give any form some advantage
+over another. Probably in no single instance should we know what to do, so
+as to succeed. It will convince us of our ignorance on the mutual relations
+of all organic beings; a conviction as necessary, as it seems to be
+difficult to acquire. All that we can do, is to keep steadily in mind that
+each {79} organic being is striving to increase at a geometrical ratio;
+that each at some period of its life, during some season of the year,
+during each generation or at intervals, has to struggle for life, and to
+suffer great destruction. 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.
+
+       *       *       *       *       *
+
+
+{80}
+
+CHAPTER IV.
+
+NATURAL SELECTION.
+
+    Natural Selection--its power compared with man's selection--its power
+    on characters of trifling importance--its power at all ages and on both
+    sexes--Sexual Selection--On the generality of intercrosses between
+    individuals of the same species--Circumstances favourable and
+    unfavourable to Natural Selection, namely, intercrossing, isolation,
+    number of individuals--Slow action--Extinction caused by Natural
+    Selection--Divergence of Character, related to the diversity of
+    inhabitants of any small area, and to naturalisation--Action of Natural
+    Selection, through Divergence of Character and Extinction, on the
+    descendants from a common parent--Explains the Grouping of all organic
+    beings.
+
+How will the struggle for existence, discussed too briefly in the last
+chapter, act in regard to variation? Can the principle of selection, which
+we have seen is so potent in the hands of man, apply in nature? I think we
+shall see that it can act most effectually. Let it be borne in mind in what
+an endless number of strange peculiarities our domestic productions, and,
+in a lesser degree, those under nature, vary; and how strong the hereditary
+tendency is. Under domestication, it may be truly said that the whole
+organisation becomes in some degree plastic. Let it be borne in mind how
+infinitely complex and close-fitting are the mutual relations of all
+organic beings to each other and to their physical conditions of life. Can
+it, then, be thought improbable, seeing that variations useful to man have
+undoubtedly occurred, that other variations useful in some way to each
+being in the great and complex battle of life, should sometimes occur in
+the course of thousands of generations? If such do occur, can we doubt {81}
+(remembering that many more individuals are born than can possibly survive)
+that individuals having any advantage, however slight, over others, would
+have the best chance of surviving and of procreating their kind? On the
+other hand, we may feel sure that any variation in the least degree
+injurious would be rigidly destroyed. This preservation of favourable
+variations and the rejection of injurious variations, I call Natural
+Selection. Variations neither useful nor injurious would not be affected by
+natural selection, and would be left a fluctuating element, as perhaps we
+see in the species called polymorphic.
+
+We shall best understand the probable course of natural selection by taking
+the case of a country undergoing some physical change, for instance, of
+climate. The proportional numbers of its inhabitants would almost
+immediately undergo a change, and some species might become extinct. We may
+conclude, from what we have seen of the intimate and complex manner in
+which the inhabitants of each country are bound together, that any change
+in the numerical proportions of some of the inhabitants, independently of
+the change of climate itself, would seriously affect many of the others. If
+the country were open on its borders, new forms would certainly immigrate,
+and this also would seriously disturb the relations of some of the former
+inhabitants. Let it be remembered how powerful the influence of a single
+introduced tree or mammal has been shown to be. But in the case of an
+island, or of a country partly surrounded by barriers, into which new and
+better adapted forms could not freely enter, we should then have places in
+the economy of nature which would assuredly be better filled up, if some of
+the original inhabitants were in some manner modified; for, had the area
+been open to immigration, these same {82} places would have been seized on
+by intruders. In such case, every slight modification, which in the course
+of ages chanced to arise, and which in any way favoured the individuals of
+any of the species, by better adapting them to their altered conditions,
+would tend to be preserved; and natural selection would thus have free
+scope for the work of improvement.
+
+We have reason to believe, as stated in the first chapter, that a change in
+the conditions of life, by specially acting on the reproductive system,
+causes or increases variability; and in the foregoing case the conditions
+of life are supposed to have undergone a change, and this would manifestly
+be favourable to natural selection, by giving a better chance of profitable
+variations occurring; and unless profitable variations do occur, natural
+selection can do nothing. Not that, as I believe, any extreme amount of
+variability is necessary; as man can certainly produce great results by
+adding up in any given direction mere individual differences, so could
+Nature, but far more easily, from having incomparably longer time at her
+disposal. Nor do I believe that any great physical change, as of climate,
+or any unusual degree of isolation to check immigration, is actually
+necessary to produce new and unoccupied places for natural selection to
+fill up by modifying and improving some of the varying inhabitants. For as
+all the inhabitants of each country are struggling together with nicely
+balanced forces, extremely slight modifications in the structure or habits
+of one inhabitant would often give it an advantage over others; and still
+further modifications of the same kind would often still further increase
+the advantage. No country can be named in which all the native inhabitants
+are now so perfectly adapted to each other and to the physical conditions
+under which they live, that none of {83} them could anyhow be improved; for
+in all countries, the natives have been so far conquered by naturalised
+productions, that they have allowed foreigners to take firm possession of
+the land. And as foreigners have thus everywhere beaten some of the
+natives, we may safely conclude that the natives might have been modified
+with advantage, so as to have better resisted such intruders.
+
+As man can produce and certainly has produced a great result by his
+methodical and unconscious means of selection, what may not Nature effect?
+Man can act only on external and visible characters: Nature cares nothing
+for appearances, except in so far as they may be useful to any being. She
+can act on every internal organ, on every shade of constitutional
+difference, on the whole machinery of life. Man selects only for his own
+good; Nature only for that of the being which she tends. Every selected
+character is fully exercised by her; and the being is placed under
+well-suited conditions of life. Man keeps the natives of many climates in
+the same country; he seldom exercises each selected character in some
+peculiar and fitting manner; he feeds a long and a short beaked pigeon on
+the same food; he does not exercise a long-backed or long-legged quadruped
+in any peculiar manner; he exposes sheep with long and short wool to the
+same climate. He does not allow the most vigorous males to struggle for the
+females. He does not rigidly destroy all inferior animals, but protects
+during each varying season, as far as lies in his power, all his
+productions. He often begins his selection by some half-monstrous form; or
+at least by some modification prominent enough to catch his eye, or to be
+plainly useful to him. Under nature, the slightest difference of structure
+or constitution may well turn the nicely-balanced scale in the struggle for
+life, and so be {84} preserved. How fleeting are the wishes and efforts of
+man! how short his time! and consequently how poor will his products be,
+compared with those accumulated by Nature during whole geological periods.
+Can we wonder, then, that Nature's productions should be far "truer" in
+character than man's productions; that they should be infinitely better
+adapted to the most complex conditions of life, and should plainly bear the
+stamp of far higher workmanship?
+
+It may metaphorically be said that natural selection is daily and hourly
+scrutinising, throughout the world, every variation, even the slightest;
+rejecting that which is bad, preserving and adding up all that is good;
+silently and insensibly working, whenever and wherever opportunity offers,
+at the improvement of each organic being in relation to its organic and
+inorganic conditions of life. We see nothing of these slow changes in
+progress, until the hand of time has marked the long lapse of ages, and
+then so imperfect is our view into long past geological ages, that we only
+see that the forms of life are now different from what they formerly were.
+
+Although natural selection can act only through and for the good of each
+being, yet characters and structures, which we are apt to consider as of
+very trifling importance, may thus be acted on. When we see leaf-eating
+insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in
+winter, the red-grouse the colour of heather, and the black-grouse that of
+peaty earth, we must believe that these tints are of service to these birds
+and insects in preserving them from danger. Grouse, if not destroyed at
+some period of their lives, would increase in countless numbers; they are
+known to suffer largely from birds of prey; and hawks are guided by
+eyesight to their prey--so much so, that on {85} parts of the Continent
+persons are warned not to keep white pigeons, as being the most liable to
+destruction. Hence I can see no reason to doubt that natural selection
+might be most effective in giving the proper colour to each kind of grouse,
+and in keeping that colour, when once acquired, true and constant. Nor
+ought we to think that the occasional destruction of an animal of any
+particular colour would produce little effect: we should remember how
+essential it is in a flock of white sheep to destroy every lamb with the
+faintest trace of black. In plants the down on the fruit and the colour of
+the flesh are considered by botanists as characters of the most trifling
+importance: yet we hear from an excellent horticulturist, Downing, that in
+the United States smooth-skinned fruits suffer far more from a beetle, a
+curculio, than those with down; that purple plums suffer far more from a
+certain disease than yellow plums; whereas another disease attacks
+yellow-fleshed peaches far more than those with other coloured flesh. If,
+with all the aids of art, these slight differences make a great difference
+in cultivating the several varieties, assuredly, in a state of nature,
+where the trees would have to struggle with other trees and with a host of
+enemies, such differences would effectually settle which variety, whether a
+smooth or downy, a yellow or purple fleshed fruit, should succeed.
+
+In looking at many small points of difference between species, which, as
+far as our ignorance permits us to judge, seem quite unimportant, we must
+not forget that climate, food, &c., probably produce some slight and direct
+effect. It is, however, far more necessary to bear in mind that there are
+many unknown laws of correlation of growth, which, when one part of the
+organisation is modified through variation, and the modifications are
+accumulated by natural selection for {86} the good of the being, will cause
+other modifications, often of the most unexpected nature.
+
+As we see that those variations which under domestication appear at any
+particular period of life, tend to reappear in the offspring at the same
+period;--for instance, in the seeds of the many varieties of our culinary
+and agricultural plants; in the caterpillar and cocoon stages of the
+varieties of the silkworm; in the eggs of poultry, and in the colour of the
+down of their chickens; in the horns of our sheep and cattle when nearly
+adult;--so in a state of nature, natural selection will be enabled to act
+on and modify organic beings at any age, by the accumulation of variations
+profitable at that age, and by their inheritance at a corresponding age. If
+it profit a plant to have its seeds more and more widely disseminated by
+the wind, I can see no greater difficulty in this being effected through
+natural selection, than in the cotton-planter increasing and improving by
+selection the down in the pods on his cotton-trees. Natural selection may
+modify and adapt the larva of an insect to a score of contingencies, wholly
+different from those which concern the mature insect. These modifications
+will no doubt affect, through the laws of correlation, the structure of the
+adult; and probably in the case of those insects which live only for a few
+hours, and which never feed, a large part of their structure is merely the
+correlated result of successive changes in the structure of their larvæ.
+So, conversely, modifications in the adult will probably often affect the
+structure of the larva; but in all cases natural selection will ensure that
+modifications consequent on other modifications at a different period of
+life, shall not be in the least degree injurious: for if they became so,
+they would cause the extinction of the species.
+
+Natural selection will modify the structure of the {87} young in relation
+to the parent, and of the parent in relation to the young. In social
+animals it will adapt the structure of each individual for the benefit of
+the community; if each in consequence profits by the selected change. What
+natural selection cannot do, is to modify the structure of one species,
+without giving it any advantage, for the good of another species; and
+though statements to this effect may be found in works of natural history,
+I cannot find one case which will bear investigation. A structure used only
+once in an animal's whole life, if of high importance to it, might be
+modified to any extent by natural selection; for instance, the great jaws
+possessed by certain insects, used exclusively for opening the cocoon--or
+the hard tip to the beak of nestling birds, used for breaking the egg. It
+has been asserted, that of the best short-beaked tumbler-pigeons more
+perish in the egg than are able to get out of it; so that fanciers assist
+in the act of hatching. Now, if nature had to make the beak of a full-grown
+pigeon very short for the bird's own advantage, the process of modification
+would be very slow, and there would be simultaneously the most rigorous
+selection of the young birds within the egg, which had the most powerful
+and hardest beaks, for all with weak beaks would inevitably perish: or,
+more delicate and more easily broken shells might be selected, the
+thickness of the shell being known to vary like every other structure.
+
+
+
+_Sexual Selection._--Inasmuch as peculiarities often appear under
+domestication in one sex and become hereditarily attached to that sex, the
+same fact probably occurs under nature, and if so, natural selection will
+be able to modify one sex in its functional relations to the other sex, or
+in relation to wholly different habits of life in the two sexes, as is
+sometimes the case {88} with insects. And this leads me to say a few words
+on what I call Sexual Selection. This depends, not on a struggle for
+existence, but on a struggle between the males for possession of the
+females; the result is not death to the unsuccessful competitor, but few or
+no offspring. Sexual selection is, therefore, less rigorous than natural
+selection. Generally, the most vigorous males, those which are best fitted
+for their places in nature, will leave most progeny. But in many cases,
+victory depends not on general vigour, but on having special weapons,
+confined to the male sex. A hornless stag or spurless cock would have a
+poor chance of leaving offspring. Sexual selection by always allowing the
+victor to breed might surely give indomitable courage, length to the spur,
+and strength to the wing to strike in the spurred leg, as well as the
+brutal cock-fighter, who knows well that he can improve his breed by
+careful selection of the best cocks. How low in the scale of nature the law
+of battle descends, I know not; male alligators have been described as
+fighting, bellowing, and whirling round, like Indians in a war-dance, for
+the possession of the females; male salmons have been seen fighting all day
+long; male stag-beetles often bear wounds from the huge mandibles of other
+males. The war is, perhaps, severest between the males of polygamous
+animals, and these seem oftenest provided with special weapons. The males
+of carnivorous animals are already well armed; though to them and to
+others, special means of defence may be given through means of sexual
+selection, as the mane to the lion, the shoulder-pad to the boar, and the
+hooked jaw to the male salmon; for the shield may be as important for
+victory, as the sword or spear.
+
+Amongst birds, the contest is often of a more peaceful character. All those
+who have attended to the subject, {89} believe that there is the severest
+rivalry between the males of many species to attract by singing the
+females. The rock-thrush of Guiana, birds of Paradise, and some others,
+congregate; and successive males display their gorgeous plumage and perform
+strange antics before the females, which, standing by as spectators, at
+last choose the most attractive partner. Those who have closely attended to
+birds in confinement well know that they often take individual preferences
+and dislikes: thus Sir R. Heron has described how one pied peacock was
+eminently attractive to all his hen birds. It may appear childish to
+attribute any effect to such apparently weak means: I cannot here enter on
+the details necessary to support this view; but if man can in a short time
+give elegant carriage and beauty to his bantams, according to his standard
+of beauty, I can see no good reason to doubt that female birds, by
+selecting, during thousands of generations, the most melodious or beautiful
+males, according to their standard of beauty, might produce a marked
+effect. I strongly suspect that some well-known laws, with respect to the
+plumage of male and female birds, in comparison with the plumage of the
+young, can be explained on the view of plumage having been chiefly modified
+by sexual selection, acting when the birds have come to the breeding age or
+during the breeding season; the modifications thus produced being inherited
+at corresponding ages or seasons, either by the males alone, or by the
+males and females; but I have not space here to enter on this subject.
+
+Thus it is, as I believe, that when the males and females of any animal
+have the same general habits of life, but differ in structure, colour, or
+ornament, such differences have been mainly caused by sexual selection;
+that is, individual males have had, in successive generations, some slight
+advantage over other {90} males, in their weapons, means of defence, or
+charms; and have transmitted these advantages to their male offspring. Yet,
+I would not wish to attribute all such sexual differences to this agency:
+for we see peculiarities arising and becoming attached to the male sex in
+our domestic animals (as the wattle in male carriers, horn-like
+protuberances in the cocks of certain fowls, &c.), which we cannot believe
+to be either useful to the males in battle, or attractive to the females.
+We see analogous cases under nature, for instance, the tuft of hair on the
+breast of the turkey-cock, which can hardly be either useful or ornamental
+to this bird;--indeed, had the tuft appeared under domestication, it would
+have been called a monstrosity.
+
+
+
+_Illustrations of the action of Natural Selection._--In order to make it
+clear how, as I believe, natural selection acts, I must beg permission to
+give one or two imaginary illustrations. Let us take the case of a wolf,
+which preys on various animals, securing some by craft, some by strength,
+and some by fleetness; and let us suppose that the fleetest prey, a deer
+for instance, had from any change in the country increased in numbers, or
+that other prey had decreased in numbers, during that season of the year
+when the wolf is hardest pressed for food. I can under such circumstances
+see no reason to doubt that the swiftest and slimmest wolves would have the
+best chance of surviving, and so be preserved or selected,--provided always
+that they retained strength to master their prey at this or at some other
+period of the year, when they might be compelled to prey on other animals.
+I can see no more reason to doubt this, than that man can improve the
+fleetness of his greyhounds by careful and methodical selection, or by that
+unconscious selection which results from each man trying {91} to keep the
+best dogs without any thought of modifying the breed.
+
+Even without any change in the proportional numbers of the animals on which
+our wolf preyed, a cub might be born with an innate tendency to pursue
+certain kinds of prey. Nor can this be thought very improbable; for we
+often observe great differences in the natural tendencies of our domestic
+animals; one cat, for instance, taking to catch rats, another mice; one
+cat, according to Mr. St. John, bringing home winged game, another hares or
+rabbits, and another hunting on marshy ground and almost nightly catching
+woodcocks or snipes. The tendency to catch rats rather than mice is known
+to be inherited. Now, if any slight innate change of habit or of structure
+benefited an individual wolf, it would have the best chance of surviving
+and of leaving offspring. Some of its young would probably inherit the same
+habits or structure, and by the repetition of this process, a new variety
+might be formed which would either supplant or coexist with the parent form
+of wolf. Or, again, the wolves inhabiting a mountainous district, and those
+frequenting the lowlands, would naturally be forced to hunt different prey;
+and from the continued preservation of the individuals best fitted for the
+two sites, two varieties might slowly be formed. These varieties would
+cross and blend where they met; but to this subject of intercrossing we
+shall soon have to return. I may add, that, according to Mr. Pierce, there
+are two varieties of the wolf inhabiting the Catskill Mountains in the
+United States, one with a light greyhound-like form, which pursues deer,
+and the other more bulky, with shorter legs, which more frequently attacks
+the shepherd's flocks.
+
+Let us now take a more complex case. Certain plants excrete a sweet juice,
+apparently for the sake of eliminating something injurious from their sap:
+this is {92} effected by glands at the base of the stipules in some
+Leguminosæ, and at the back of the leaf of the common laurel. This juice,
+though small in quantity, is greedily sought by insects. Let us now suppose
+a little sweet juice or nectar to be excreted by the inner bases of the
+petals of a flower. In this case insects in seeking the nectar would get
+dusted with pollen, and would certainly often transport the pollen from one
+flower to the stigma of another flower. The flowers of two distinct
+individuals of the same species would thus get crossed; and the act of
+crossing, we have good reason to believe (as will hereafter be more fully
+alluded to), would produce very vigorous seedlings, which consequently
+would have the best chance of flourishing and surviving. Some of these
+seedlings would probably inherit the nectar-excreting power. Those
+individual flowers which had the largest glands or nectaries, and which
+excreted most nectar, would be oftenest visited by insects, and would be
+oftenest crossed; and so in the long-run would gain the upper hand. Those
+flowers, also, which had their stamens and pistils placed, in relation to
+the size and habits of the particular insects which visited them, so as to
+favour in any degree the transportal of their pollen from flower to flower,
+would likewise be favoured or selected. We might have taken the case of
+insects visiting flowers for the sake of collecting pollen instead of
+nectar; and as pollen is formed for the sole object of fertilisation, its
+destruction appears a simple loss to the plant; yet if a little pollen were
+carried, at first occasionally and then habitually, by the pollen-devouring
+insects from flower to flower, and a cross thus effected, although
+nine-tenths of the pollen were destroyed, it might still be a great gain to
+the plant; and those individuals which produced more and more pollen, and
+had larger and larger anthers, would be selected. {93}
+
+When our plant, by this process of the continued preservation or natural
+selection of more and more attractive flowers, had been rendered highly
+attractive to insects, they would, unintentionally on their part, regularly
+carry pollen from flower to flower; and that they can most effectually do
+this, I could easily show by many striking instances. I will give only
+one--not as a very striking case, but as likewise illustrating one step in
+the separation of the sexes of plants, presently to be alluded to. Some
+holly-trees bear only male flowers, which have four stamens producing a
+rather small quantity of pollen, and a rudimentary pistil; other
+holly-trees bear only female flowers; these have a full-sized pistil, and
+four stamens with shrivelled anthers, in which not a grain of pollen can be
+detected. Having found a female tree exactly sixty yards from a male tree,
+I put the stigmas of twenty flowers, taken from different branches, under
+the microscope, and on all, without exception, there were pollen-grains,
+and on some a profusion of pollen. As the wind had set for several days
+from the female to the male tree, the pollen could not thus have been
+carried. The weather had been cold and boisterous, and therefore not
+favourable to bees, nevertheless every female flower which I examined had
+been effectually fertilised by the bees, accidentally dusted with pollen,
+having flown from tree to tree in search of nectar. But to return to our
+imaginary case: as soon as the plant had been rendered so highly attractive
+to insects that pollen was regularly carried from flower to flower, another
+process might commence. No naturalist doubts the advantage of what has been
+called the "physiological division of labour;" hence we may believe that it
+would be advantageous to a plant to produce stamens alone in one flower or
+on one whole plant, and pistils alone in {94} another flower or on another
+plant. In plants under culture and placed under new conditions of life,
+sometimes the male organs and sometimes the female organs become more or
+less impotent; now if we suppose this to occur in ever so slight a degree
+under nature, then as pollen is already carried regularly from flower to
+flower, and as a more complete separation of the sexes of our plant would
+be advantageous on the principle of the division of labour, individuals
+with this tendency more and more increased, would be continually favoured
+or selected, until at last a complete separation of the sexes would be
+effected.
+
+Let us now turn to the nectar-feeding insects in our imaginary case: we may
+suppose the plant of which we have been slowly increasing the nectar by
+continued selection, to be a common plant; and that certain insects
+depended in main part on its nectar for food. I could give many facts,
+showing how anxious bees are to save time; for instance, their habit of
+cutting holes and sucking the nectar at the bases of certain flowers, which
+they can, with a very little more trouble, enter by the mouth. Bearing such
+facts in mind, I can see no reason to doubt that an accidental deviation in
+the size and form of the body, or in the curvature and length of the
+proboscis, &c., far too slight to be appreciated by us, might profit a bee
+or other insect, so that an individual so characterised would be able to
+obtain its food more quickly, and so have a better chance of living and
+leaving descendants. Its descendants would probably inherit a tendency to a
+similar slight deviation of structure. The tubes of the corollas of the
+common red and incarnate clovers (Trifolium pratense and incarnatum) do not
+on a hasty glance appear to differ in length; yet the hive-bee can easily
+suck the nectar out of the incarnate clover, but not out of the common red
+{95} clover, which is visited by humble-bees alone; so that whole fields of
+the red clover offer in vain an abundant supply of precious nectar to the
+hive-bee. Thus it might be a great advantage to the hive-bee to have a
+slightly longer or differently constructed proboscis. On the other hand, I
+have found by experiment that the fertility of clover depends on bees
+visiting and moving parts of the corolla, so as to push the pollen on to
+the stigmatic surface. Hence, again, if humble-bees were to become rare in
+any country, it might be a great advantage to the red clover to have a
+shorter or more deeply divided tube to its corolla, so that the hive-bee
+could visit its flowers. Thus I can understand how a flower and a bee might
+slowly become, either simultaneously or one after the other, modified and
+adapted in the most perfect manner to each other, by the continued
+preservation of individuals presenting mutual and slightly favourable
+deviations of structure.
+
+I am well aware that this doctrine of natural selection, exemplified in the
+above imaginary instances, is open to the same objections which were at
+first urged against Sir Charles Lyell's noble views on "the modern changes
+of the earth, as illustrative of geology;" but we now seldom hear the
+action, for instance, of the coast-waves, called a trifling and
+insignificant cause, when applied to the excavation of gigantic valleys or
+to the formation of the longest lines of inland cliffs. Natural selection
+can act only by the preservation and accumulation of infinitesimally small
+inherited modifications, each profitable to the preserved being; and as
+modern geology has almost banished such views as the excavation of a great
+valley by a single diluvial wave, so will natural selection, if it be a
+true principle, banish the belief of the continued creation of new organic
+{96} beings, or of any great and sudden modification in their structure.
+
+
+
+_On the Intercrossing of Individuals._--I must here introduce a short
+digression. In the case of animals and plants with separated sexes, it is
+of course obvious that two individuals must always (with the exception of
+the curious and not well-understood cases of parthenogenesis) unite for
+each birth; but in the case of hermaphrodites this is far from obvious.
+Nevertheless I am strongly inclined to believe that with all hermaphrodites
+two individuals, either occasionally or habitually, concur for the
+reproduction of their kind. This view was first suggested by Andrew Knight.
+We shall presently see its importance; but I must here treat the subject
+with extreme brevity, though I have the materials prepared for an ample
+discussion. All vertebrate animals, all insects, and some other large
+groups of animals, pair for each birth. Modern research has much diminished
+the number of supposed hermaphrodites, and of real hermaphrodites a large
+number pair; that is, two individuals regularly unite for reproduction,
+which is all that concerns us. But still there are many hermaphrodite
+animals which certainly do not habitually pair, and a vast majority of
+plants are hermaphrodites. What reason, it may be asked, is there for
+supposing in these cases that two individuals ever concur in reproduction?
+As it is impossible here to enter on details, I must trust to some general
+considerations alone.
+
+In the first place, I have collected so large a body of facts, showing, in
+accordance with the almost universal belief of breeders, that with animals
+and plants a cross between different varieties, or between individuals of
+the same variety but of another strain, gives vigour and {97} fertility to
+the offspring; and on the other hand, that _close_ interbreeding diminishes
+vigour and fertility; that these facts alone incline me to believe that it
+is a general law of nature (utterly ignorant though we be of the meaning of
+the law) that no organic being self-fertilises itself for an eternity of
+generations; but that a cross with another individual is
+occasionally--perhaps at very long intervals--indispensable.
+
+On the belief that this is a law of nature, we can, I think, understand
+several large classes of facts, such as the following, which on any other
+view are inexplicable. Every hybridizer knows how unfavourable exposure to
+wet is to the fertilisation of a flower, yet what a multitude of flowers
+have their anthers and stigmas fully exposed to the weather! but if an
+occasional cross be indispensable, the fullest freedom for the entrance of
+pollen from another individual will explain this state of exposure, more
+especially as the plant's own anthers and pistil generally stand so close
+together that self-fertilisation seems almost inevitable. Many flowers, on
+the other hand, have their organs of fructification closely enclosed, as in
+the great papilionaceous or pea-family; but in several, perhaps in all,
+such flowers, there is a very curious adaptation between the structure of
+the flower and the manner in which bees suck the nectar; for, in doing
+this, they either push the flower's own pollen on the stigma, or bring
+pollen from another flower. So necessary are the visits of bees to
+papilionaceous flowers, that I have found, by experiments published
+elsewhere, that their fertility is greatly diminished if these visits be
+prevented. Now, it is scarcely possible that bees should fly from flower to
+flower, and not carry pollen from one to the other, to the great good, as I
+believe, of the plant. Bees will act like a camel-hair pencil, and it is
+quite sufficient just to touch the anthers of {98} one flower and then the
+stigma of another with the same brush to ensure fertilisation; but it must
+not be supposed that bees would thus produce a multitude of hybrids between
+distinct species; for if you bring on the same brush a plant's own pollen
+and pollen from another species, the former will have such a prepotent
+effect, that it will invariably and completely destroy, as has been shown
+by Gärtner, any influence from the foreign pollen.
+
+When the stamens of a flower suddenly spring towards the pistil, or slowly
+move one after the other towards it, the contrivance seems adapted solely
+to ensure self-fertilisation; and no doubt it is useful for this end: but,
+the agency of insects is often required to cause the stamens to spring
+forward, as Kölreuter has shown to be the case with the barberry; and in
+this very genus, which seems to have a special contrivance for
+self-fertilisation, it is well known that if closely-allied forms or
+varieties are planted near each other, it is hardly possible to raise pure
+seedlings, so largely do they naturally cross. In many other cases, far
+from there being any aids for self-fertilisation, there are special
+contrivances, as I could show from the writings of C. C. Sprengel and from
+my own observations, which effectually prevent the stigma receiving pollen
+from its own flower: for instance, in Lobelia fulgens, there is a really
+beautiful and elaborate contrivance by which every one of the infinitely
+numerous pollen-granules are swept out of the conjoined anthers of each
+flower, before the stigma of that individual flower is ready to receive
+them; and as this flower is never visited, at least in my garden, by
+insects, it never sets a seed, though by placing pollen from one flower on
+the stigma of another, I raised plenty of seedlings; and whilst another
+species of Lobelia growing close by, which is visited by bees, seeds
+freely. In very many other cases, though there {99} be no special
+mechanical contrivance to prevent the stigma of a flower receiving its own
+pollen, yet, as C. C. Sprengel has shown, and as I can confirm, either the
+anthers burst before the stigma is ready for fertilisation, or the stigma
+is ready before the pollen of that flower is ready, so that these plants
+have in fact separated sexes, and must habitually be crossed. How strange
+are these facts! How strange that the pollen and stigmatic surface of the
+same flower, though placed so close together, as if for the very purpose of
+self-fertilisation, should in so many cases be mutually useless to each
+other! How simply are these facts explained on the view of an occasional
+cross with a distinct individual being advantageous or indispensable!
+
+If several varieties of the cabbage, radish, onion, and of some other
+plants, be allowed to seed near each other, a large majority, as I have
+found, of the seedlings thus raised will turn out mongrels: for instance, I
+raised 233 seedling cabbages from some plants of different varieties
+growing near each other, and of these only 78 were true to their kind, and
+some even of these were not perfectly true. Yet the pistil of each
+cabbage-flower is surrounded not only by its own six stamens, but by those
+of the many other flowers on the same plant. How, then, comes it that such
+a vast number of the seedlings are mongrelized? I suspect that it must
+arise from the pollen of a distinct _variety_ having a prepotent effect
+over a flower's own pollen; and that this is part of the general law of
+good being derived from the intercrossing of distinct individuals of the
+same species. When distinct _species_ are crossed the case is directly the
+reverse, for a plant's own pollen is always prepotent over foreign pollen;
+but to this subject we shall return in a future chapter.
+
+In the case of a gigantic tree covered with {100} innumerable flowers, it
+may be objected that pollen could seldom be carried from tree to tree, and
+at most only from flower to flower on the same tree, and that flowers on
+the same tree can be considered as distinct individuals only in a limited
+sense. I believe this objection to be valid, but that nature has largely
+provided against it by giving to trees a strong tendency to bear flowers
+with separated sexes. When the sexes are separated, although the male and
+female flowers may be produced on the same tree, we can see that pollen
+must be regularly carried from flower to flower; and this will give a
+better chance of pollen being occasionally carried from tree to tree. That
+trees belonging to all Orders have their sexes more often separated than
+other plants, I find to be the case in this country; and at my request Dr.
+Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the
+United States, and the result was as I anticipated. On the other hand, Dr.
+Hooker has recently informed me that he finds that the rule does not hold
+in Australia; and I have made these few remarks on the sexes of trees
+simply to call attention to the subject.
+
+Turning for a very brief space to animals: on the land there are some
+hermaphrodites, as land-mollusca and earth-worms; but these all pair. As
+yet I have not found a single case of a terrestrial animal which fertilises
+itself. We can understand this remarkable fact, which offers so strong a
+contrast with terrestrial plants, on the view of an occasional cross being
+indispensable, by considering the medium in which terrestrial animals live,
+and the nature of the fertilising element; for we know of no means,
+analogous to the action of insects and of the wind in the case of plants,
+by which an occasional cross could be effected with terrestrial animals
+without the concurrence of two individuals. Of aquatic animals, there are
+many self-fertilising hermaphrodites; but here {101} currents in the water
+offer an obvious means for an occasional cross. And, as in the case of
+flowers, I have as yet failed, after consultation with one of the highest
+authorities, namely, Professor Huxley, to discover a single case of an
+hermaphrodite animal with the organs of reproduction so perfectly enclosed
+within the body, that access from without and the occasional influence of a
+distinct individual can be shown to be physically impossible. Cirripedes
+long appeared to me to present a case of very great difficulty under this
+point of view; but I have been enabled, by a fortunate chance, elsewhere to
+prove that two individuals, though both are self-fertilising
+hermaphrodites, do sometimes cross.
+
+It must have struck most naturalists as a strange anomaly that, in the case
+of both animals and plants, species of the same family and even of the same
+genus, though agreeing closely with each other in almost their whole
+organisation, yet are not rarely, some of them hermaphrodites, and some of
+them unisexual. But if, in fact, all hermaphrodites do occasionally
+intercross with other individuals, the difference between hermaphrodites
+and unisexual species, as far as function is concerned, becomes very small.
+
+From these several considerations and from the many special facts which I
+have collected, but which I am not here able to give, I am strongly
+inclined to suspect that, both in the vegetable and animal kingdoms, an
+occasional intercross with a distinct individual is a law of nature. I am
+well aware that there are, on this view, many cases of difficulty, some of
+which I am trying to investigate. Finally then, we may conclude that in
+many organic beings, a cross between two individuals is an obvious
+necessity for each birth; in many others it occurs perhaps only at long
+intervals; but in none, as I suspect, can self-fertilisation go on for
+perpetuity. {102}
+
+
+
+_Circumstances favourable to Natural Selection._--This is an extremely
+intricate subject. A large amount of inheritable and diversified
+variability is favourable, but I believe mere individual differences
+suffice for the work. A large number of individuals, by giving a better
+chance for the appearance within any given period of profitable variations,
+will compensate for a lesser amount of variability in each individual, and
+is, I believe, an extremely important element of success. Though nature
+grants vast periods of time for the work of natural selection, she does not
+grant an indefinite period; for as all organic beings are striving, it may
+be said, to seize on each place in the economy of nature, if any one
+species does not become modified and improved in a corresponding degree
+with its competitors, it will soon be exterminated.
+
+In man's methodical selection, a breeder selects for some definite object,
+and free intercrossing will wholly stop his work. But when many men,
+without intending to alter the breed, have a nearly common standard of
+perfection, and all try to get and breed from the best animals, much
+improvement and modification surely but slowly follow from this unconscious
+process of selection, notwithstanding a large amount of crossing with
+inferior animals. Thus it will be in nature; for within a confined area,
+with some place in its polity not so perfectly occupied as might be,
+natural selection will always tend to preserve all the individuals varying
+in the right direction, though in different degrees, so as better to fill
+up the unoccupied place. But if the area be large, its several districts
+will almost certainly present different conditions of life; and then if
+natural selection be modifying and improving a species in the several
+districts, there will be intercrossing with the other individuals of the
+same species on the confines of each. And in {103} this case the effects of
+intercrossing can hardly be counterbalanced by natural selection always
+tending to modify all the individuals in each district in exactly the same
+manner to the conditions of each; for in a continuous area, the physical
+conditions at least will generally graduate away insensibly from one
+district to another. The intercrossing will most affect those animals which
+unite for each birth, which wander much, and which do not breed at a very
+quick rate. Hence in animals of this nature, for instance in birds,
+varieties will generally be confined to separated countries; and this I
+believe to be the case. In hermaphrodite organisms which cross only
+occasionally, and likewise in animals which unite for each birth, but which
+wander little and which can increase at a very rapid rate, a new and
+improved variety might be quickly formed on any one spot, and might there
+maintain itself in a body, so that whatever intercrossing took place would
+be chiefly between the individuals of the same new variety. A local variety
+when once thus formed might subsequently slowly spread to other districts.
+On the above principle, nurserymen always prefer getting seed from a large
+body of plants of the same variety, as the chance of intercrossing with
+other varieties is thus lessened.
+
+Even in the case of slow-breeding animals, which unite for each birth, we
+must not overrate the effects of intercrosses in retarding natural
+selection; for I can bring a considerable catalogue of facts, showing that
+within the same area, varieties of the same animal can long remain
+distinct, from haunting different stations, from breeding at slightly
+different seasons, or from varieties of the same kind preferring to pair
+together.
+
+Intercrossing plays a very important part in nature in keeping the
+individuals of the same species, or of the same variety, true and uniform
+in character. It will {104} obviously thus act far more efficiently with
+those animals which unite for each birth; but I have already attempted to
+show that we have reason to believe that occasional intercrosses take place
+with all animals and with all plants. Even if these take place only at long
+intervals, I am convinced that the young thus produced will gain so much in
+vigour and fertility over the offspring from long-continued
+self-fertilisation, that they will have a better chance of surviving and
+propagating their kind; and thus, in the long run, the influence of
+intercrosses, even at rare intervals, will be great. If there exist organic
+beings which never intercross, uniformity of character can be retained
+amongst them, as long as their conditions of life remain the same, only
+through the principle of inheritance, and through natural selection
+destroying any which depart from the proper type; but if their conditions
+of life change and they undergo modification, uniformity of character can
+be given to their modified offspring, solely by natural selection
+preserving the same favourable variations.
+
+Isolation, also, is an important element in the process of natural
+selection. In a confined or isolated area, if not very large, the organic
+and inorganic conditions of life will generally be in a great degree
+uniform; so that natural selection will tend to modify all the individuals
+of a varying species throughout the area in the same manner in relation to
+the same conditions. Intercrosses, also, with the individuals of the same
+species, which otherwise would have inhabited the surrounding and
+differently circumstanced districts, will be prevented. But isolation
+probably acts more efficiently in checking the immigration of better
+adapted organisms, after any physical change, such as of climate or
+elevation of the land, &c.; and thus new places in the natural economy of
+the country are left open for the old inhabitants to struggle for, and
+become adapted to, through {105} modifications in their structure and
+constitution. Lastly, isolation, by checking immigration and consequently
+competition, will give time for any new variety to be slowly improved; and
+this may sometimes be of importance in the production of new species. If,
+however, an isolated area be very small, either from being surrounded by
+barriers, or from having very peculiar physical conditions, the total
+number of the individuals supported on it will necessarily be very small;
+and fewness of individuals will greatly retard the production of new
+species through natural selection, by decreasing the chance of the
+appearance of favourable variations.
+
+If we turn to nature to test the truth of these remarks, and look at any
+small isolated area, such as an oceanic island, although the total number
+of the species inhabiting it, will be found to be small, as we shall see in
+our chapter on geographical distribution; yet of these species a very large
+proportion are endemic,--that is, have been produced there, and nowhere
+else. Hence an oceanic island at first sight seems to have been highly
+favourable for the production of new species. But we may thus greatly
+deceive ourselves, for to ascertain whether a small isolated area, or a
+large open area like a continent, has been most favourable for the
+production of new organic forms, we ought to make the comparison within
+equal times; and this we are incapable of doing.
+
+Although I do not doubt that isolation is of considerable importance in the
+production of new species, on the whole I am inclined to believe that
+largeness of area is of more importance, more especially in the production
+of species, which will prove capable of enduring for a long period, and of
+spreading widely. Throughout a great and open area, not only will there be
+a better chance of favourable variations arising from the large number of
+individuals of the same species {106} there supported, but the conditions
+of life are infinitely complex from the large number of already existing
+species; and if some of these many species become modified and improved,
+others will have to be improved in a corresponding degree or they will be
+exterminated. Each new form, also, as soon as it has been much improved,
+will be able to spread over the open and continuous area, and will thus
+come into competition with many others. Hence more new places will be
+formed, and the competition to fill them will be more severe, on a large
+than on a small and isolated area. Moreover, great areas, though now
+continuous, owing to oscillations of level, will often have recently
+existed in a broken condition, so that the good effects of isolation will
+generally, to a certain extent, have concurred. Finally, I conclude that,
+although small isolated areas probably have been in some respects highly
+favourable for the production of new species, yet that the course of
+modification will generally have been more rapid on large areas; and what
+is more important, that the new forms produced on large areas, which
+already have been victorious over many competitors, will be those that will
+spread most widely, will give rise to most new varieties and species, and
+will thus play an important part in the changing history of the organic
+world.
+
+We can, perhaps, on these views, understand some facts which will be again
+alluded to in our chapter on geographical distribution; for instance, that
+the productions of the smaller continent of Australia have formerly
+yielded, and apparently are now yielding, before those of the larger
+Europæo-Asiatic area. Thus, also, it is that continental productions have
+everywhere become so largely naturalised on islands. On a small island, the
+race for life will have been less severe, and there will have been less
+modification and less {107} extermination. Hence, perhaps, it comes that
+the flora of Madeira, according to Oswald Heer, resembles the extinct
+tertiary flora of Europe. All fresh-water basins, taken together, make a
+small area compared with that of the sea or of the land; and, consequently,
+the competition between fresh-water productions will have been less severe
+than elsewhere; new forms will have been more slowly formed, and old forms
+more slowly exterminated. And it is in fresh water that we find seven
+genera of Ganoid fishes, remnants of a once preponderant order: and in
+fresh water we find some of the most anomalous forms now known in the
+world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect
+to a certain extent orders now widely separated in the natural scale. These
+anomalous forms may almost be called living fossils; they have endured to
+the present day, from having inhabited a confined area, and from having
+thus been exposed to less severe competition.
+
+To sum up the circumstances favourable and unfavourable to natural
+selection, as far as the extreme intricacy of the subject permits. I
+conclude, looking to the future, that for terrestrial productions a large
+continental area, which will probably undergo many oscillations of level,
+and which consequently will exist for long periods in a broken condition,
+is the most favourable for the production of many new forms of life, likely
+to endure long and to spread widely. For the area first existed as a
+continent, and the inhabitants, at this period numerous in individuals and
+kinds, will have been subjected to very severe competition. When converted
+by subsidence into large separate islands, there will still exist many
+individuals of the same species on each island: intercrossing on the
+confines of the range of each species will thus be checked: after physical
+changes of any kind, immigration will be {108} prevented, so that new
+places in the polity of each island will have to be filled up by
+modifications of the old inhabitants; and time will be allowed for the
+varieties in each to become well modified and perfected. When, by renewed
+elevation, the islands shall be re-converted into a continental area, there
+will again be severe competition: the most favoured or improved varieties
+will be enabled to spread: there will be much extinction of the less
+improved forms, and the relative proportional numbers of the various
+inhabitants of the renewed continent will again be changed; and again there
+will be a fair field for natural selection to improve still further the
+inhabitants, and thus produce new species.
+
+That natural selection will always act with extreme slowness, I fully
+admit. Its action depends on there being places in the polity of nature,
+which can be better occupied by some of the inhabitants of the country
+undergoing modification of some kind. The existence of such places will
+often depend on physical changes, which are generally very slow, and on the
+immigration of better adapted forms having been checked. But the action of
+natural selection will probably still oftener depend on some of the
+inhabitants becoming slowly modified; the mutual relations of many of the
+other inhabitants being thus disturbed. Nothing can be effected, unless
+favourable variations occur, and variation itself is apparently always a
+very slow process. The process will often be greatly retarded by free
+intercrossing. Many will exclaim that these several causes are amply
+sufficient wholly to stop the action of natural selection. I do not believe
+so. On the other hand, I do believe that natural selection always acts very
+slowly, often only at long intervals of time, and generally on only a very
+few of the inhabitants of the same region at the same time. I further
+believe, that this very slow, {109} intermittent action of natural
+selection accords perfectly well with what geology tells us of the rate and
+manner at which the inhabitants of this world have changed.
+
+Slow though the process of selection may be, if feeble man can do much by
+his powers of artificial selection, I can see no limit to the amount of
+change, to the beauty and infinite complexity of the coadaptations between
+all organic beings, one with another and with their physical conditions of
+life, which may be effected in the long course of time by nature's power of
+selection.
+
+
+
+_Extinction._--This subject will be more fully discussed in our chapter on
+Geology; but it must be here alluded to from being intimately connected
+with natural selection. Natural selection acts solely through the
+preservation of variations in some way advantageous, which consequently
+endure. But as from the high geometrical ratio of increase of all organic
+beings, each area is already fully stocked with inhabitants, it follows
+that as each selected and favoured form increases in number, so will the
+less favoured forms decrease and become rare. Rarity, as geology tells us,
+is the precursor to extinction. We can, also, see that any form represented
+by few individuals will, during fluctuations in the seasons or in the
+number of its enemies, run a good chance of utter extinction. But we may go
+further than this; for as new forms are continually and slowly being
+produced, unless we believe that the number of specific forms goes on
+perpetually and almost indefinitely increasing, numbers inevitably must
+become extinct. That the number of specific forms has not indefinitely
+increased, geology shows us plainly; and indeed we can see reason why they
+should not have thus increased, for the number of places in the polity of
+nature is not indefinitely great,--not that we {110} have any means of
+knowing that any one region has as yet got its maximum of species. Probably
+no region is as yet fully stocked, for at the Cape of Good Hope, where more
+species of plants are crowded together than in any other quarter of the
+world, some foreign plants have become naturalised, without causing, as far
+as we know, the extinction of any natives.
+
+Furthermore, the species which are most numerous in individuals will have
+the best chance of producing within any given period favourable variations.
+We have evidence of this, in the facts given in the second chapter, showing
+that it is the common species which afford the greatest number of recorded
+varieties, or incipient species. Hence, rare species will be less quickly
+modified or improved within any given period, and they will consequently be
+beaten in the race for life by the modified descendants of the commoner
+species.
+
+From these several considerations I think it inevitably follows, that as
+new species in the course of time are formed through natural selection,
+others will become rarer and rarer, and finally extinct. The forms which
+stand in closest competition with those undergoing modification and
+improvement, will naturally suffer most. And we have seen in the chapter on
+the Struggle for Existence that it is the most closely-allied
+forms,--varieties of the same species, and species of the same genus or of
+related genera,--which, from having nearly the same structure,
+constitution, and habits, generally come into the severest competition with
+each other. Consequently, each new variety or species, during the progress
+of its formation, will generally press hardest on its nearest kindred, and
+tend to exterminate them. We see the same process of extermination amongst
+our domesticated productions, through the selection of improved forms by
+man. Many curious {111} instances could be given showing how quickly new
+breeds of cattle, sheep, and other animals, and varieties of flowers, take
+the place of older and inferior kinds. In Yorkshire, it is historically
+known that the ancient black cattle were displaced by the long-horns, and
+that these "were swept away by the short-horns" (I quote the words of an
+agricultural writer) "as if by some murderous pestilence."
+
+
+
+_Divergence of Character._--The principle, which I have designated by this
+term, is of high importance on my theory, and explains, as I believe,
+several important facts. In the first place, varieties, even
+strongly-marked ones, though having somewhat of the character of
+species--as is shown by the hopeless doubts in many cases how to rank
+them--yet certainly differ from each other far less than do good and
+distinct species. Nevertheless, according to my view, varieties are species
+in the process of formation, or are, as I have called them, incipient
+species. How, then, does the lesser difference between varieties become
+augmented into the greater difference between species? That this does
+habitually happen, we must infer from most of the innumerable species
+throughout nature presenting well-marked differences; whereas varieties,
+the supposed prototypes and parents of future well-marked species, present
+slight and ill-defined differences. Mere chance, as we may call it, might
+cause one variety to differ in some character from its parents, and the
+offspring of this variety again to differ from its parent in the very same
+character and in a greater degree; but this alone would never account for
+so habitual and large an amount of difference as that between varieties of
+the same species and species of the same genus.
+
+As has always been my practice, let us seek light on {112} this head from
+our domestic productions. We shall here find something analogous. A fancier
+is struck by a pigeon having a slightly shorter beak; another fancier is
+struck by a pigeon having a rather longer beak; and on the acknowledged
+principle that "fanciers do not and will not admire a medium standard, but
+like extremes," they both go on (as has actually occurred with
+tumbler-pigeons) choosing and breeding from birds with longer and longer
+beaks, or with shorter and shorter beaks. Again, we may suppose that at an
+early period one man preferred swifter horses; another stronger and more
+bulky horses. The early differences would be very slight; in the course of
+time, from the continued selection of swifter horses by some breeders, and
+of stronger ones by others, the differences would become greater, and would
+be noted as forming two sub-breeds; finally, after the lapse of centuries,
+the sub-breeds would become converted into two well-established and
+distinct breeds. As the differences slowly become greater, the inferior
+animals with intermediate characters, being neither very swift nor very
+strong, will have been neglected, and will have tended to disappear. Here,
+then, we see in man's productions the action of what may be called the
+principle of divergence, causing differences, at first barely appreciable,
+steadily to increase, and the breeds to diverge in character both from each
+other and from their common parent.
+
+But how, it may be asked, can any analogous principle apply in nature? I
+believe it can and does apply most efficiently, from the simple
+circumstance that the more diversified the descendants from any one species
+become in structure, constitution, and habits, by so much will they be
+better enabled to seize on many and widely diversified places in the polity
+of nature, and so be enabled to increase in numbers. {113}
+
+We can clearly see this in the case of animals with simple habits. Take the
+case of a carnivorous quadruped, of which the number that can be supported
+in any country has long ago arrived at its full average. If its natural
+powers of increase be allowed to act, it can succeed in increasing (the
+country not undergoing any change in its conditions) only by its varying
+descendants seizing on places at present occupied by other animals: some of
+them, for instance, being enabled to feed on new kinds of prey, either dead
+or alive; some inhabiting new stations, climbing trees, frequenting water,
+and some perhaps becoming less carnivorous. The more diversified in habits
+and structure the descendants of our carnivorous animal became, the more
+places they would be enabled to occupy. What applies to one animal will
+apply throughout all time to all animals--that is, if they vary--for
+otherwise natural selection can do nothing. So it will be with plants. It
+has been experimentally proved, that if a plot of ground be sown with one
+species of grass, and a similar plot be sown with several distinct genera
+of grasses, a greater number of plants and a greater weight of dry herbage
+can thus be raised. The same has been found to hold good when first one
+variety and then several mixed varieties of wheat have been sown on equal
+spaces of ground. Hence, if any one species of grass were to go on varying,
+and those varieties were continually selected which differed from each
+other in at all the same manner as distinct species and genera of grasses
+differ from each other, a greater number of individual plants of this
+species of grass, including its modified descendants, would succeed in
+living on the same piece of ground. And we well know that each species and
+each variety of grass is annually sowing almost countless seeds; and thus,
+as it may be said, is striving its utmost to increase its numbers. {114}
+Consequently, I cannot doubt that in the course of many thousands of
+generations, the most distinct varieties of any one species of grass would
+always have the best chance of succeeding and of increasing in numbers, and
+thus of supplanting the less distinct varieties; and varieties, when
+rendered very distinct from each other, take the rank of species.
+
+The truth of the principle, that the greatest amount of life can be
+supported by great diversification of structure, is seen under many natural
+circumstances. In an extremely small area, especially if freely open to
+immigration, and where the contest between individual and individual must
+be severe, we always find great diversity in its inhabitants. For instance,
+I found that a piece of turf, three feet by four in size, which had been
+exposed for many years to exactly the same conditions, supported twenty
+species of plants, and these belonged to eighteen genera and to eight
+orders, which shows how much these plants differed from each other. So it
+is with the plants and insects on small and uniform islets; and so in small
+ponds of fresh water. Farmers find that they can raise most food by a
+rotation of plants belonging to the most different orders: nature follows
+what may be called a simultaneous rotation. Most of the animals and plants
+which live close round any small piece of ground, could live on it
+(supposing it not to be in any way peculiar in its nature), and may be said
+to be striving to the utmost to live there; but, it is seen, that where
+they come into the closest competition with each other, the advantages of
+diversification of structure, with the accompanying differences of habit
+and constitution, determine that the inhabitants, which thus jostle each
+other most closely, shall, as a general rule, belong to what we call
+different genera and orders.
+
+The same principle is seen in the naturalisation of {115} plants through
+man's agency in foreign lands. It might have been expected that the plants
+which have succeeded in becoming naturalised in any land would generally
+have been closely allied to the indigenes; for these are commonly looked at
+as specially created and adapted for their own country. It might, also,
+perhaps have been expected that naturalised plants would have belonged to a
+few groups more especially adapted to certain stations in their new homes.
+But the case is very different; and Alph. De Candolle has well remarked in
+his great and admirable work, that floras gain by naturalisation,
+proportionally with the number of the native genera and species, far more
+in new genera than in new species. To give a single instance: in the last
+edition of Dr. Asa Gray's 'Manual of the Flora of the Northern United
+States,' 260 naturalised plants are enumerated, and these belong to 162
+genera. We thus see that these naturalised plants are of a highly
+diversified nature. They differ, moreover, to a large extent from the
+indigenes, for out of the 162 genera, no less than 100 genera are not there
+indigenous, and thus a large proportional addition is made to the genera of
+these States.
+
+By considering the nature of the plants or animals which have struggled
+successfully with the indigenes of any country, and have there become
+naturalised, we may gain some crude idea in what manner some of the natives
+would have to be modified, in order to gain an advantage over the other
+natives; and we may at least safely infer that diversification of
+structure, amounting to new generic differences, would be profitable to
+them.
+
+The advantage of diversification 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 individual body--a subject so well elucidated by Milne
+{116} Edwards. No physiologist doubts that a stomach adapted to digest
+vegetable matter alone, or flesh alone, draws most nutriment from these
+substances. So in the general economy of any land, the more widely and
+perfectly the animals and plants are diversified for different habits of
+life, so will a greater number of individuals be capable of there
+supporting themselves. A set of animals, with their organisation but little
+diversified, could hardly compete with a set more perfectly diversified in
+structure. It may be doubted, for instance, whether the Australian
+marsupials, which are divided into groups differing but little from each
+other, and feebly representing, as Mr. Waterhouse and others have remarked,
+our carnivorous, ruminant, and rodent mammals, could successfully compete
+with these well-pronounced orders. In the Australian mammals, we see the
+process of diversification in an early and incomplete stage of development.
+
+After the foregoing discussion, which ought to have been much amplified, we
+may, I think, assume that the modified descendants of any one species will
+succeed by so much the better as they become more diversified in structure,
+and are thus enabled to encroach on places occupied by other beings. Now
+let us see how this principle of benefit being derived from divergence of
+character, combined with the principles of natural selection and of
+extinction, will tend to act.
+
+The accompanying diagram will aid us in understanding this rather
+perplexing subject. Let A to L represent the species of a genus large in
+its own country; these species are supposed to resemble each other in
+unequal degrees, as is so generally the case in nature, and as is
+represented in the diagram by the letters standing at unequal distances. I
+have said a large genus, because we have seen in the second chapter, {117}
+that on an average more of the species of large genera vary than of small
+genera; and the varying species of the large genera present a greater
+number of varieties. We have, also, seen that the species, which are the
+commonest and the most widely-diffused, vary more than rare species with
+restricted ranges. Let (A) be a common, widely-diffused, and varying
+species, belonging to a genus large in its own country. The little fan of
+diverging dotted lines of unequal lengths proceeding from (A), may
+represent its varying offspring. The variations are supposed to be
+extremely slight, but of the most diversified nature; they are not supposed
+all to appear simultaneously, but often after long intervals of time; nor
+are they all supposed to endure for equal periods. Only those variations
+which are in some way profitable will be preserved or naturally selected.
+And here the importance of the principle of benefit being derived from
+divergence of character comes in; for this will generally lead to the most
+different or divergent variations (represented by the outer dotted lines)
+being preserved and accumulated by natural selection. When a dotted line
+reaches one of the horizontal lines, and is there marked by a small
+numbered letter, a sufficient amount of variation is supposed to have been
+accumulated to have formed a fairly well-marked variety, such as would be
+thought worthy of record in a systematic work.
+
+[Illustration]
+
+The intervals between the horizontal lines in the diagram, may represent
+each a thousand generations; but it would have been better if each had
+represented ten thousand generations. After a thousand generations, species
+(A) is supposed to have produced two fairly well-marked varieties, namely
+a^1 and m^1. These two varieties will generally continue to be exposed to
+the same conditions which made their parents variable, {118} and the
+tendency to variability is in itself hereditary, consequently they will
+tend to vary, and generally to vary in nearly the same manner as their
+parents varied. Moreover, these two varieties, being only slightly modified
+forms, will tend to inherit those advantages which made their parent (A)
+more numerous than most of the other inhabitants of the same country; they
+will likewise partake of those more general advantages which made the genus
+to which the parent-species belonged, a large genus in its own country. And
+these circumstances we know to be favourable to the production of new
+varieties.
+
+If, then, these two varieties be variable, the most divergent of their
+variations will generally be preserved during the next thousand
+generations. And after this interval, variety a^1 is supposed in the
+diagram to have produced variety a^2, which will, owing to the principle of
+divergence, differ more from (A) than did variety a^1. Variety m^1 is
+supposed to have produced two varieties, namely m^2 and s^2, differing from
+each other, and more considerably from their common parent (A). We may
+continue the process by similar steps for any length of time; some of the
+varieties, after each thousand generations, producing only a single
+variety, but in a more and more modified condition, some producing two or
+three varieties, and some failing to produce any. Thus the varieties or
+modified descendants, proceeding from the common parent (A), will generally
+go on increasing in number and diverging in character. In the diagram the
+process is represented up to the ten-thousandth generation, and under a
+condensed and simplified form up to the fourteen-thousandth generation.
+
+But I must here remark that I do not suppose that the process ever goes on
+so regularly as is represented in the diagram, though in itself made
+somewhat irregular. {119} I am far from thinking that the most divergent
+varieties will invariably prevail and multiply: a medium form may often
+long endure, and may or may not produce more than one modified descendant;
+for natural selection will always act according to the nature of the places
+which are either unoccupied or not perfectly occupied by other beings; and
+this will depend on infinitely complex relations. But as a general rule,
+the more diversified in structure the descendants from any one species can
+be rendered, the more places they will be enabled to seize on, and the more
+their modified progeny will be increased. In our diagram the line of
+succession is broken at regular intervals by small numbered letters marking
+the successive forms which have become sufficiently distinct to be recorded
+as varieties. But these breaks are imaginary, and might have been inserted
+anywhere, after intervals long enough to have allowed the accumulation of a
+considerable amount of divergent variation.
+
+As all the modified descendants from a common and widely-diffused species,
+belonging to a large genus, will tend to partake of the same advantages
+which made their parent successful in life, they will generally go on
+multiplying in number as well as diverging in character: this is
+represented in the diagram by the several divergent branches proceeding
+from (A). The modified offspring from the later and more highly improved
+branches in the lines of descent, will, it is probable, often take the
+place of, and so destroy, the earlier and less improved branches: this is
+represented in the diagram by some of the lower branches not reaching to
+the upper horizontal lines. In some cases I do not doubt that the process
+of modification will be confined to a single line of descent, and the
+number of the descendants will not be increased; although the amount {120}
+of divergent modification may have been increased in the successive
+generations. This case would be represented in the diagram, if all the
+lines proceeding from (A) were removed, excepting that from a^1 to a^{10}.
+In the same way, for instance, the English race-horse and English pointer
+have apparently both gone on slowly diverging in character from their
+original stocks, without either having given off any fresh branches or
+races.
+
+After ten thousand generations, species (A) is supposed to have produced
+three forms, a^{10}, f^{10}, and m^{10}, which, from having diverged in
+character during the successive generations, will have come to differ
+largely, but perhaps unequally, from each other and from their common
+parent. If we suppose the amount of change between each horizontal line in
+our diagram to be excessively small, these three forms may still be only
+well-marked varieties; or they may have arrived at the doubtful category of
+sub-species; but we have only to suppose the steps in the process of
+modification to be more numerous or greater in amount, to convert these
+three forms into well-defined species: thus the diagram illustrates the
+steps by which the small differences distinguishing varieties are increased
+into the larger differences distinguishing species. By continuing the same
+process for a greater number of generations (as shown in the diagram in a
+condensed and simplified manner), we get eight species, marked by the
+letters between a^{14} and m^{14}, all descended from (A). Thus, as I
+believe, species are multiplied and genera are formed.
+
+In a large genus it is probable that more than one species would vary. In
+the diagram I have assumed that a second species (I) has produced, by
+analogous steps, after ten thousand generations, either two well-marked
+varieties (w^{10} and z^{10}) or two species, according to the amount of
+change supposed to be represented {121} between the horizontal lines. After
+fourteen thousand generations, six new species, marked by the letters
+n^{14} to z^{14}, are supposed to have been produced. In each genus, the
+species, which are already extremely different in character, will generally
+tend to produce the greatest number of modified descendants; for these will
+have the best chance of filling new and widely different places in the
+polity of nature: hence in the diagram I have chosen the extreme species
+(A), and the nearly extreme species (I), as those which have largely
+varied, and have given rise to new varieties and species. The other nine
+species (marked by capital letters) of our original genus, may for a long
+period continue to transmit unaltered descendants; and this is shown in the
+diagram by the dotted lines not prolonged far upwards from want of space.
+
+But during the process of modification, represented in the diagram, another
+of our principles, namely that of extinction, will have played an important
+part. As in each fully stocked country natural selection necessarily acts
+by the selected form having some advantage in the struggle for life over
+other forms, there will be a constant tendency in the improved descendants
+of any one species to supplant and exterminate in each stage of descent
+their predecessors and their original parent. For it should be remembered
+that the competition will generally be most severe between those forms
+which are most nearly related to each other in habits, constitution, and
+structure. Hence all the intermediate forms between the earlier and later
+states, that is between the less and more improved state of a species, as
+well as the original parent-species itself, will generally tend to become
+extinct. So it probably will be with many whole collateral lines of
+descent, which will be conquered by later and improved lines of descent.
+If, however, the {122} modified offspring of a species get into some
+distinct country, or become quickly adapted to some quite new station, in
+which child and parent do not come into competition, both may continue to
+exist.
+
+If then our diagram be assumed to represent a considerable amount of
+modification, species (A) and all the earlier varieties will have become
+extinct, having been replaced by eight new species (a^{14} to m^{14}); and
+(I) will have been replaced by six (n^{14} to z^{14}) new species.
+
+But we may go further than this. The original species of our genus were
+supposed to resemble each other in unequal degrees, as is so generally the
+case in nature; species (A) being more nearly related to B, C, and D, than
+to the other species; and species (I) more to G, H, K, L, than to the
+others. These two species (A) and (I), were also supposed to be very common
+and widely diffused species, so that they must originally have had some
+advantage over most of the other species of the genus. Their modified
+descendants, fourteen in number at the fourteen-thousandth generation, will
+probably have inherited some of the same advantages: they have also been
+modified and improved in a diversified manner at each stage of descent, so
+as to have become adapted to many related places in the natural economy of
+their country. It seems, therefore, to me extremely probable that they will
+have taken the places of, and thus exterminated, not only their parents (A)
+and (I), but likewise some of the original species which were most nearly
+related to their parents. Hence very few of the original species will have
+transmitted offspring to the fourteen-thousandth generation. We may suppose
+that only one (F), of the two species which were least closely related to
+the other nine original species, has transmitted descendants to this late
+stage of descent. {123}
+
+The new species in our diagram descended from the original eleven species,
+will now be fifteen in number. Owing to the divergent tendency of natural
+selection, the extreme amount of difference in character between species
+a^{14} and z^{14} will be much greater than that between the most different
+of the original eleven species. The new species, moreover, will be allied
+to each other in a widely different manner. Of the eight descendants from
+(A) the three marked a^{14}, q^{14}, p^{14}, will be nearly related from
+having recently branched off from a^{10}; b^{14} and f^{14}, from having
+diverged at an earlier period from a^5, will be in some degree distinct
+from the three first-named species; and lastly, o^{14}, e^{14} and m^{14},
+will be nearly related one to the other, but from having diverged at the
+first commencement of the process of modification, will be widely different
+from the other five species, and may constitute a sub-genus or even a
+distinct genus.
+
+The six descendants from (I) will form two sub-genera or even genera. But
+as the original species (I) differed largely from (A), standing nearly at
+the extreme points of the original genus, the six descendants from (I)
+will, owing to inheritance alone, differ considerably from the eight
+descendants from (A); the two groups, moreover, are supposed to have gone
+on diverging in different directions. The intermediate species, also (and
+this is a very important consideration), which connected the original
+species (A) and (I), have all become, excepting (F), extinct, and have left
+no descendants. Hence the six new species descended from (I), and the eight
+descended from (A), will have to be ranked as very distinct genera, or even
+as distinct sub-families.
+
+Thus it is, as I believe, that two or more genera are produced by descent
+with modification, from two or more species of the same genus. And the two
+or {124} more parent-species are supposed to have descended from some one
+species of an earlier genus. In our diagram, this is indicated by the
+broken lines, beneath the capital letters, converging in sub-branches
+downwards towards a single point; this point representing a single species,
+the supposed single parent of our several new sub-genera and genera.
+
+It is worth while to reflect for a moment on the character of the new
+species F^{14}, which is supposed not to have diverged much in character,
+but to have retained the form of (F), either unaltered or altered only in a
+slight degree. In this case, its affinities to the other fourteen new
+species will be of a curious and circuitous nature. Having descended from a
+form which stood between the two parent-species (A) and (I), now supposed
+to be extinct and unknown, it will be in some degree intermediate in
+character between the two groups descended from these species. But as these
+two groups have gone on diverging in character from the type of their
+parents, the new species (F^{14}) will not be directly intermediate between
+them, but rather between types of the two groups; and every naturalist will
+be able to bring some such case before his mind.
+
+In the diagram, each horizontal line has hitherto been supposed to
+represent a thousand generations, but each may represent a million or
+hundred million generations, and likewise a section of the successive
+strata of the earth's crust including extinct remains. We shall, when we
+come to our chapter on Geology, have to refer again to this subject, and I
+think we shall then see that the diagram throws light on the affinities of
+extinct beings, which, though generally belonging to the same orders, or
+families, or genera, with those now living, yet are often, in some degree,
+intermediate in character between existing groups; and we can understand
+this fact, for {125} the extinct species lived at very ancient epochs when
+the branching lines of descent had diverged less.
+
+I see no reason to limit the process of modification, as now explained, to
+the formation of genera alone. If, in our diagram, we suppose the amount of
+change represented by each successive group of diverging dotted lines to be
+very great, the forms marked a^{14} to p^{14}, those marked b^{14} and
+f^{14}, and those marked o^{14} to m^{14}, will form three very distinct
+genera. We shall also have two very distinct genera descended from (I); and
+as these latter two genera, both from continued divergence of character and
+from inheritance from a different parent, will differ widely from the three
+genera descended from (A), the two little groups of genera will form two
+distinct families, or even orders, according to the amount of divergent
+modification supposed to be represented in the diagram. And the two new
+families, or orders, will have descended from two species of the original
+genus; and these two species are supposed to have descended from one
+species of a still more ancient and unknown genus.
+
+We have seen that in each country it is the species of the larger genera
+which oftenest present varieties or incipient species. This, indeed, might
+have been expected; for as natural selection acts through one form having
+some advantage over other forms in the struggle for existence, it will
+chiefly act on those which already have some advantage; and the largeness
+of any group shows that its species have inherited from a common ancestor
+some advantage in common. Hence, the struggle for the production of new and
+modified descendants, will mainly lie between the larger groups, which are
+all trying to increase in number. One large group will slowly conquer
+another large group, reduce its numbers, and thus lessen its chance of
+further variation and improvement. Within the same large {126} group, the
+later and more highly perfected sub-groups, from branching out and seizing
+on many new places in the polity of Nature, will constantly tend to
+supplant and destroy the earlier and less improved sub-groups. Small and
+broken groups and sub-groups will finally disappear. Looking to the future,
+we can predict that the groups of organic beings which are now large and
+triumphant, and which are least broken up, that is, which as yet have
+suffered least extinction, will for a long period continue to increase. But
+which groups will ultimately prevail, no man can predict; for we well know
+that many groups, formerly most extensively developed, have now become
+extinct. Looking still more remotely to the future, we may predict that,
+owing to the continued and steady increase of the larger groups, a
+multitude of smaller groups will become utterly extinct, and leave no
+modified descendants; and consequently that of the species living at any
+one period, extremely few will transmit descendants to a remote futurity. I
+shall have to return to this subject in the chapter on Classification, but
+I may add that on this view of extremely few of the more ancient species
+having transmitted descendants, and on the view of all the descendants of
+the same species making a class, we can understand how it is that there
+exist but very few classes in each main division of the animal and
+vegetable kingdoms. Although extremely few of the most ancient species may
+now have living and modified descendants, yet at the most remote geological
+period, the earth may have been as well peopled with many species of many
+genera, families, orders, and classes, as at the present day.
+
+
+
+_Summary of Chapter._--If during the long course of ages and under varying
+conditions of life, organic beings {127} vary at all in the several parts
+of their organisation, and I think this cannot be disputed; if there be,
+owing to the high geometrical ratio of increase of each species, a severe
+struggle for life at some age, season, or year, and this certainly cannot
+be disputed; then, considering the infinite complexity of the relations of
+all organic beings to each other and to their conditions of existence,
+causing an infinite diversity in structure, constitution, and habits, to be
+advantageous to them, I think it would be a most extraordinary fact if no
+variation ever had occurred useful to each being's own welfare, in the same
+manner as so many variations have occurred useful to man. But if variations
+useful to any organic being do occur, assuredly individuals thus
+characterised will have the best chance of being preserved in the struggle
+for life; and from the strong principle of inheritance they will tend to
+produce offspring similarly characterised. This principle of preservation,
+I have called, for the sake of brevity, Natural Selection; and it leads to
+the improvement of each creature in relation to its organic and inorganic
+conditions of life.
+
+Natural selection, on the principle of qualities being inherited at
+corresponding ages, can modify the egg, seed, or young, as easily as the
+adult. Amongst many animals, sexual selection will give its aid to ordinary
+selection, by assuring to the most vigorous and best adapted males the
+greatest number of offspring. Sexual selection will also give characters
+useful to the males alone, in their struggles with other males.
+
+Whether natural selection has really thus acted in nature, in modifying and
+adapting the various forms of life to their several conditions and
+stations, must be judged of by the general tenour and balance of evidence
+given in the following chapters. But we already see how it entails
+extinction; and how largely extinction {128} has acted in the world's
+history, geology plainly declares. Natural selection, also, leads to
+divergence of character; for more living beings can be supported on the
+same area the more they diverge in structure, habits, and constitution, of
+which we see proof by looking to the inhabitants of any small spot or to
+naturalised productions. Therefore during the modification of the
+descendants of any one species, and during the incessant struggle of all
+species to increase in numbers, the more diversified these descendants
+become, the better will be their chance of succeeding in the battle for
+life. Thus the small differences distinguishing varieties of the same
+species, steadily tend to increase till they come to equal the greater
+differences between species of the same genus, or even of distinct genera.
+
+We have seen that it is the common, the widely-diffused, and widely-ranging
+species, belonging to the larger genera, which vary most; and these tend to
+transmit to their modified offspring that superiority which now makes them
+dominant in their own countries. Natural selection, as has just been
+remarked, leads to divergence of character and to much extinction of the
+less improved and intermediate forms of life. On these principles, I
+believe, the nature of the affinities of all organic beings may be
+explained. It is a truly wonderful fact--the wonder of which we are apt to
+overlook from familiarity--that all animals and all plants throughout all
+time and space should be related to each other in group subordinate to
+group, in the manner which we everywhere behold--namely, varieties of the
+same species most closely related together, species of the same genus less
+closely and unequally related together, forming sections and sub-genera,
+species of distinct genera much less closely related, and genera related in
+different degrees, forming {129} sub-families, families, orders,
+sub-classes, and classes. The several subordinate groups in any class
+cannot be ranked in a single file, but seem rather to be clustered round
+points, and these round other points, and so on in almost endless cycles.
+On the view that each species has been independently created, I can see no
+explanation of this great fact in the classification of all organic beings;
+but, to the best of my judgment, it is explained through inheritance and
+the complex action of natural selection, entailing extinction and
+divergence of character, as we have seen illustrated in the diagram.
+
+The affinities of all the beings of the same class have sometimes been
+represented by a great tree. I believe this simile largely speaks the
+truth. The green and budding twigs may represent existing species; and
+those produced during each former year may represent the long succession of
+extinct species. At each period of growth all the growing twigs have tried
+to branch out on all sides, and to overtop and kill the surrounding twigs
+and branches, in the same manner as species and groups of species have
+tried to overmaster other species in the great battle for life. The limbs
+divided into great branches, and these into lesser and lesser branches,
+were themselves once, when the tree was small, budding twigs; and this
+connexion of the former and present buds by ramifying branches may well
+represent the classification of all extinct and living species in groups
+subordinate to groups. Of the many twigs which flourished when the tree was
+a mere bush, only two or three, now grown into great branches, yet survive
+and bear all the other branches; so with the species which lived during
+long-past geological periods, very few now have living and modified
+descendants. From the first growth of the tree, many a limb and branch has
+decayed and dropped off; and these lost branches of various {130} sizes may
+represent those whole orders, families, and genera which have now no living
+representatives, and which are known to us only from having been found in a
+fossil state. As we here and there see a thin straggling branch springing
+from a fork low down in a tree, and which by some chance has been favoured
+and is still alive on its summit, so we occasionally see an animal like the
+Ornithorhynchus or Lepidosiren, which in some small degree connects by its
+affinities two large branches of life, and which has apparently been saved
+from fatal competition by having inhabited a protected station. As buds
+give rise by growth to fresh buds, and these, if vigorous, branch out and
+overtop on all sides many a feebler branch, so by generation I believe it
+has been with the great Tree of Life, which fills with its dead and broken
+branches the crust of the earth, and covers the surface with its ever
+branching and beautiful ramifications.
+
+       *       *       *       *       *
+
+
+{131}
+
+CHAPTER V.
+
+LAWS OF VARIATION.
+
+    Effects of external conditions--Use and disuse, combined with natural
+    selection; organs of flight and of vision--Acclimatisation--Correlation
+    of growth--Compensation and economy of growth--False
+    correlations--Multiple, rudimentary, and lowly organised structures
+    variable--Parts developed in an unusual manner are highly variable:
+    specific characters more variable than generic: secondary sexual
+    characters variable--Species of the same genus vary in an analogous
+    manner--Reversions to long-lost characters--Summary.
+
+I have hitherto sometimes spoken as if the variations--so common and
+multiform in organic beings under domestication, and in a lesser degree in
+those in a state of nature--had been due to chance. This, of course, is a
+wholly incorrect expression, but it serves to acknowledge plainly our
+ignorance of the cause of each particular variation. Some authors believe
+it to be as much the function of the reproductive system to produce
+individual differences, or very slight deviations of structure, as to make
+the child like its parents. But the much greater variability, as well as
+the greater frequency of monstrosities, under domestication or cultivation,
+than under nature, leads me to believe that deviations of structure are in
+some way due to the nature of the conditions of life, to which the parents
+and their more remote ancestors have been exposed during several
+generations. I have remarked in the first chapter--but a long catalogue of
+facts which cannot be here given would be necessary to show the truth of
+the remark--that the reproductive system is eminently susceptible to
+changes in the conditions of life; and to {132} this system being
+functionally disturbed in the parents, I chiefly attribute the varying or
+plastic condition of the offspring. The male and female sexual elements
+seem to be affected before that union takes place which is to form a new
+being. In the case of "sporting" plants, the bud, which in its earliest
+condition does not apparently differ essentially from an ovule, is alone
+affected. But why, because the reproductive system is disturbed, this or
+that part should vary more or less, we are profoundly ignorant.
+Nevertheless, we can here and there dimly catch a faint ray of light, and
+we may feel sure that there must be some cause for each deviation of
+structure, however slight.
+
+How much direct effect difference of climate, food, &c., produces on any
+being is extremely doubtful. My impression is, that the effect is extremely
+small in the case of animals, but perhaps rather more in that of plants. We
+may, at least, safely conclude that such influences cannot have produced
+the many striking and complex co-adaptations of structure between one
+organic being and another, which we see everywhere throughout nature. Some
+little influence may be attributed to climate, food, &c.: thus, E. Forbes
+speaks confidently that shells at their southern limit, and when living in
+shallow water, are more brightly coloured than those of the same species
+further north or from greater depths. Gould believes that birds of the same
+species are more brightly coloured under a clear atmosphere, than when
+living on islands or near the coast. So with insects, Wollaston is
+convinced that residence near the sea affects their colours. Moquin-Tandon
+gives a list of plants which when growing near the sea-shore have their
+leaves in some degree fleshy, though not elsewhere fleshy. Several other
+such cases could be given.
+
+The fact of varieties of one species, when they range {133} into the zone
+of habitation of other species, often acquiring in a very slight degree
+some of the characters of such species, accords with our view that species
+of all kinds are only well-marked and permanent varieties. Thus the species
+of shells which are confined to tropical and shallow seas are generally
+brighter-coloured than those confined to cold and deeper seas. The birds
+which are confined to continents are, according to Mr. Gould,
+brighter-coloured than those of islands. The insect-species confined to
+sea-coasts, as every collector knows, are often brassy or lurid. Plants
+which live exclusively on the sea-side are very apt to have fleshy leaves.
+He who believes in the creation of each species, will have to say that this
+shell, for instance, was created with bright colours for a warm sea; but
+that this other shell became bright-coloured by variation when it ranged
+into warmer or shallower waters.
+
+When a variation is of the slightest use to a being, we cannot tell how
+much of it to attribute to the accumulative action of natural selection,
+and how much to the conditions of life. Thus, it is well known to furriers
+that animals of the same species have thicker and better fur the more
+severe the climate is under which they have lived; but who can tell how
+much of this difference may be due to the warmest-clad individuals having
+been favoured and preserved during many generations, and how much to the
+direct action of the severe climate? for it would appear that climate has
+some direct action on the hair of our domestic quadrupeds.
+
+Instances could be given of the same variety being produced under
+conditions of life as different as can well be conceived; and, on the other
+hand, of different varieties being produced from the same species under the
+same conditions. Such facts show how indirectly {134} the conditions of
+life act. Again, innumerable instances are known to every naturalist of
+species keeping true, or not varying at all, although living under the most
+opposite climates. Such considerations as these incline me to lay very
+little weight on the direct action of the conditions of life. Indirectly,
+as already remarked, they seem to play an important part in affecting the
+reproductive system, and in thus inducing variability; and natural
+selection will then accumulate all profitable variations, however slight,
+until they become plainly developed and appreciable by us.
+
+
+
+_Effects of Use and Disuse._--From the facts alluded to in the first
+chapter, I think there can be little doubt that use in our domestic animals
+strengthens and enlarges certain parts, and disuse diminishes them; and
+that such modifications are inherited. Under free nature, we can have no
+standard of comparison, by which to judge of the effects of long-continued
+use or disuse, for we know not the parent-forms; but many animals have
+structures which can be explained by the effects of disuse. As Professor
+Owen has remarked, there is no greater anomaly in nature than a bird that
+cannot fly; yet there are several in this state. The logger-headed duck of
+South America can only flap along the surface of the water, and has its
+wings in nearly the same condition as the domestic Aylesbury duck. As the
+larger ground-feeding birds seldom take flight except to escape danger, I
+believe that the nearly wingless condition of several birds, which now
+inhabit or have lately inhabited several oceanic islands, tenanted by no
+beast of prey, has been caused by disuse. The ostrich indeed inhabits
+continents and is exposed to danger from which it cannot escape by flight,
+but by kicking it can defend itself from enemies, as well as any of the
+smaller {135} quadrupeds. We may imagine that the early progenitor of the
+ostrich had habits like those of a bustard, and that as natural selection
+increased in successive generations the size and weight of its body, its
+legs were used more, and its wings less, until they became incapable of
+flight.
+
+Kirby has remarked (and I have observed the same fact) that the anterior
+tarsi, or feet, of many male dung-feeding beetles are very often broken
+off; he examined seventeen specimens in his own collection, and not one had
+even a relic left. In the Onites apelles the tarsi are so habitually lost,
+that the insect has been described as not having them. In some other genera
+they are present, but in a rudimentary condition. In the Ateuchus or sacred
+beetle of the Egyptians, they are totally deficient. There is not
+sufficient evidence to induce me to believe that mutilations are ever
+inherited; and I should prefer explaining the entire absence of the
+anterior tarsi in Ateuchus, and their rudimentary condition in some other
+genera, by the long-continued effects of disuse in their progenitors; for
+as the tarsi are almost always lost in many dung-feeding beetles, they must
+be lost early in life, and therefore cannot be much used by these insects.
+
+In some cases we might easily put down to disuse modifications of structure
+which are wholly, or mainly, due to natural selection. Mr. Wollaston has
+discovered the remarkable fact that 200 beetles, out of the 550 species
+inhabiting Madeira, are so far deficient in wings that they cannot fly; and
+that of the twenty-nine endemic genera, no less than twenty-three genera
+have all their species in this condition! Several facts, namely, that
+beetles in many parts of the world are frequently blown to sea and perish;
+that the beetles in Madeira, as observed by Mr. Wollaston, lie much
+concealed, {136} until the wind lulls and the sun shines; that the
+proportion of wingless beetles is larger on the exposed Desertas than in
+Madeira itself; and especially the extraordinary fact, so strongly insisted
+on by Mr. Wollaston, of the almost entire absence of certain large groups
+of beetles, elsewhere excessively numerous, and which groups have habits of
+life almost necessitating frequent flight;--these several considerations
+have made me believe that the wingless condition of so many Madeira beetles
+is mainly due to the action of natural selection, but combined probably
+with disuse. For during thousands of successive generations each individual
+beetle which flew least, either from its wings having been ever so little
+less perfectly developed or from indolent habit, will have had the best
+chance of surviving from not being blown out to sea; and, on the other
+hand, those beetles which most readily took to flight would oftenest have
+been blown to sea and thus have been destroyed.
+
+The insects in Madeira which are not ground-feeders, and which, as the
+flower-feeding coleoptera and lepidoptera, must habitually use their wings
+to gain their subsistence, have, as Mr. Wollaston suspects, their wings not
+at all reduced, but even enlarged. This is quite compatible with the action
+of natural selection. For when a new insect first arrived on the island,
+the tendency of natural selection to enlarge or to reduce the wings, would
+depend on whether a greater number of individuals were saved by
+successfully battling with the winds, or by giving up the attempt and
+rarely or never flying. As with mariners shipwrecked near a coast, it would
+have been better for the good swimmers if they had been able to swim still
+further, whereas it would have been better for the bad swimmers if they had
+not been able to swim at all and had stuck to the wreck. {137}
+
+The eyes of moles and of some burrowing rodents are rudimentary in size,
+and in some cases are quite covered up by skin and fur. This state of the
+eyes is probably due to gradual reduction from disuse, but aided perhaps by
+natural selection. In South America, a burrowing rodent, the tuco-tuco, or
+Ctenomys, is even more subterranean in its habits than the mole; and I was
+assured by a Spaniard, who had often caught them, that they were frequently
+blind; one which I kept alive was certainly in this condition, the cause,
+as appeared on dissection, having been inflammation of the nictitating
+membrane. As frequent inflammation of the eyes must be injurious to any
+animal, and as eyes are certainly not indispensable to animals with
+subterranean habits, a reduction in their size with the adhesion of the
+eyelids and growth of fur over them, might in such case be an advantage;
+and if so, natural selection would constantly aid the effects of disuse.
+
+It is well known that several animals, belonging to the most different
+classes, which inhabit the caves of Styria and of Kentucky, are blind. In
+some of the crabs the foot-stalk for the eye remains, though the eye is
+gone; the stand for the telescope is there, though the telescope with its
+glasses has been lost. As it is difficult to imagine that eyes, though
+useless, could be in any way injurious to animals living in darkness, I
+attribute their loss wholly to disuse. In one of the blind animals, namely,
+the cave-rat, the eyes are of immense size; and Professor Silliman thought
+that it regained, after living some days in the light, some slight power of
+vision. In the same manner as in Madeira the wings of some of the insects
+have been enlarged, and the wings of others have been reduced by natural
+selection aided by use and disuse, so in the case of the cave-rat natural
+selection seems to have struggled with the loss of light and {138} to have
+increased the size of the eyes; whereas with all the other inhabitants of
+the caves, disuse by itself seems to have done its work.
+
+It is difficult to imagine conditions of life more similar than deep
+limestone caverns under a nearly similar climate; so that on the common
+view of the blind animals having been separately created for the American
+and European caverns, close similarity in their organisation and affinities
+might have been expected; but, as Schiödte and others have remarked, this
+is not the case, and the cave-insects of the two continents are not more
+closely allied than might have been anticipated from the general
+resemblance of the other inhabitants of North America and Europe. On my
+view we must suppose that American animals, having ordinary powers of
+vision, slowly migrated by successive generations from the outer world into
+the deeper and deeper recesses of the Kentucky caves, as did European
+animals into the caves of Europe. We have some evidence of this gradation
+of habit; for, as Schiödte remarks, "animals not far remote from ordinary
+forms, prepare the transition from light to darkness. Next follow those
+that are constructed for twilight; and, last of all, those destined for
+total darkness." By the time that an animal had reached, after numberless
+generations, the deepest recesses, disuse will on this view have more or
+less perfectly obliterated its eyes, and natural selection will often have
+effected other changes, such as an increase in the length of the antennæ or
+palpi, as a compensation for blindness. Notwithstanding such modifications,
+we might expect still to see in the cave-animals of America, affinities to
+the other inhabitants of that continent, and in those of Europe, to the
+inhabitants of the European continent. And this is the case with some of
+the American cave-animals, as I hear from {139} Professor Dana; and some of
+the European cave-insects are very closely allied to those of the
+surrounding country. It would be most difficult to give any rational
+explanation of the affinities of the blind cave-animals to the other
+inhabitants of the two continents on the ordinary view of their independent
+creation. That several of the inhabitants of the caves of the Old and New
+Worlds should be closely related, we might expect from the well-known
+relationship of most of their other productions. Far from feeling any
+surprise that some of the cave-animals should be very anomalous, as Agassiz
+has remarked in regard to the blind fish, the Amblyopsis, and as is the
+case with the blind Proteus with reference to the reptiles of Europe, I am
+only surprised that more wrecks of ancient life have not been preserved,
+owing to the less severe competition to which the inhabitants of these dark
+abodes will probably have been exposed.
+
+
+
+_Acclimatisation._--Habit is hereditary with plants, as in the period of
+flowering, in the amount of rain requisite for seeds to germinate, in the
+time of sleep, &c., and this leads me to say a few words on
+acclimatisation. As it is extremely common for species of the same genus to
+inhabit very hot and very cold countries, and as I believe that all the
+species of the same genus have descended from a single parent, if this view
+be correct, acclimatisation must be readily effected during long-continued
+descent. It is notorious that each species is adapted to the climate of its
+own home: species from an arctic or even from a temperate region cannot
+endure a tropical climate, or conversely. So again, many succulent plants
+cannot endure a damp climate. But the degree of adaptation of species to
+the climates under which they live is often overrated. {140} We may infer
+this from our frequent inability to predict whether or not an imported
+plant will endure our climate, and from the number of plants and animals
+brought from warmer countries which here enjoy good health. We have reason
+to believe that species in a state of nature are limited in their ranges by
+the competition of other organic beings quite as much as, or more than, by
+adaptation to particular climates. But whether or not the adaptation be
+generally very close, we have evidence, in the case of some few plants, of
+their becoming, to a certain extent, naturally habituated to different
+temperatures, or becoming acclimatised: thus the pines and rhododendrons,
+raised from seed collected by Dr. Hooker from trees growing at different
+heights on the Himalaya, were found in this country to possess different
+constitutional powers of resisting cold. Mr. Thwaites informs me that he
+has observed similar facts in Ceylon, and analogous observations have been
+made by Mr. H. C. Watson on European species of plants brought from the
+Azores to England. In regard to animals, several authentic cases could be
+given of species within historical times having largely extended their
+range from warmer to cooler latitudes, and conversely; but we do not
+positively know that these animals were strictly adapted to their native
+climate, but in all ordinary cases we assume such to be the case; nor do we
+know that they have subsequently become acclimatised to their new homes.
+
+As I believe that our domestic animals were originally chosen by
+uncivilised man because they were useful and bred readily under
+confinement, and not because they were subsequently found capable of
+far-extended transportation, I think the common and extraordinary capacity
+in our domestic animals of not only withstanding the most different
+climates but of being perfectly {141} fertile (a far severer test) under
+them, may be used as an argument that a large proportion of other animals,
+now in a state of nature, could easily be brought to bear widely different
+climates. We must not, however, push the foregoing argument too far, on
+account of the probable origin of some of our domestic animals from several
+wild stocks: the blood, for instance, of a tropical and arctic wolf or wild
+dog may perhaps be mingled in our domestic breeds. The rat and mouse cannot
+be considered as domestic animals, but they have been transported by man to
+many parts of the world, and now have a far wider range than any other
+rodent, living free under the cold climate of Faroe in the north and of the
+Falklands in the south, and on many islands in the torrid zones. Hence I am
+inclined to look at adaptation to any special climate as a quality readily
+grafted on an innate wide flexibility of constitution, which is common to
+most animals. On this view, the capacity of enduring the most different
+climates by man himself and by his domestic animals, and such facts as that
+former species of the elephant and rhinoceros were capable of enduring a
+glacial climate, whereas the living species are now all tropical or
+sub-tropical in their habits, ought not to be looked at as anomalies, but
+merely as examples of a very common flexibility of constitution, brought,
+under peculiar circumstances, into play.
+
+How much of the acclimatisation of species to any peculiar climate is due
+to mere habit, and how much to the natural selection of varieties having
+different innate constitutions, and how much to both means combined, is a
+very obscure question. That habit or custom has some influence I must
+believe, both from analogy, and from the incessant advice given in
+agricultural works, even in the ancient Encyclopædias of China, to be very
+{142} cautious in transposing animals from one district to another; for it
+is not likely that man should have succeeded in selecting so many breeds
+and sub-breeds with constitutions specially fitted for their own districts:
+the result must, I think, be due to habit. On the other hand, I can see no
+reason to doubt that natural selection will continually tend to preserve
+those individuals which are born with constitutions best adapted to their
+native countries. In treatises on many kinds of cultivated plants, certain
+varieties are said to withstand certain climates better than others: this
+is very strikingly shown in works on fruit trees published in the United
+States, in which certain varieties are habitually recommended for the
+northern, and others for the southern States; and as most of these
+varieties are of recent origin, they cannot owe their constitutional
+differences to habit. The case of the Jerusalem artichoke, which is never
+propagated by seed, and of which consequently new varieties have not been
+produced, has even been advanced--for it is now as tender as ever it
+was--as proving that acclimatisation cannot be effected! The case, also, of
+the kidney-bean has been often cited for a similar purpose, and with much
+greater weight; but until some one will sow, during a score of generations,
+his kidney-beans so early that a very large proportion are destroyed by
+frost, and then collect seed from the few survivors, with care to prevent
+accidental crosses, and then again get seed from these seedlings, with the
+same precautions, the experiment cannot be said to have been even tried.
+Nor let it be supposed that no differences in the constitution of seedling
+kidney-beans ever appear, for an account has been published how much more
+hardy some seedlings appeared to be than others.
+
+On the whole, I think we may conclude that habit, {143} use, and disuse,
+have, in some cases, played a considerable part in the modification of the
+constitution, and of the structure of various organs; but that the effects
+of use and disuse have often been largely combined with, and sometimes
+overmastered by the natural selection of innate variations.
+
+
+
+_Correlation of Growth._--I mean by this expression that the whole
+organisation is so tied together during its growth and development, that
+when slight variations in any one part occur, and are accumulated through
+natural selection, other parts become modified. This is a very important
+subject, most imperfectly understood. The most obvious case is, that
+modifications accumulated solely for the good of the young or larva, will,
+it may safely be concluded, affect the structure of the adult; in the same
+manner as any malconformation affecting the early embryo, seriously affects
+the whole organisation of the adult. The several parts of the body which
+are homologous, and which, at an early embryonic period, are alike, seem
+liable to vary in an allied manner: we see this in the right and left sides
+of the body varying in the same manner; in the front and hind legs, and
+even in the jaws and limbs, varying together, for the lower jaw is believed
+to be homologous with the limbs. These tendencies, I do not doubt, may be
+mastered more or less completely by natural selection: thus a family of
+stags once existed with an antler only on one side; and if this had been of
+any great use to the breed it might probably have been rendered permanent
+by natural selection.
+
+Homologous parts, as has been remarked by some authors, tend to cohere;
+this is often seen in monstrous plants; and nothing is more common than the
+union of homologous parts in normal structures, as the union of {144} the
+petals of the corolla into a tube. Hard parts seem to affect the form of
+adjoining soft parts; it is believed by some authors that the diversity in
+the shape of the pelvis in birds causes the remarkable diversity in the
+shape of their kidneys. Others believe that the shape of the pelvis in the
+human mother influences by pressure the shape of the head of the child. In
+snakes, according to Schlegel, the shape of the body and the manner of
+swallowing determine the position of several of the most important viscera.
+
+The nature of the bond of correlation is very frequently quite obscure. M.
+Is. Geoffroy St. Hilaire has forcibly remarked, that certain
+malconformations very frequently, and that others rarely coexist, without
+our being able to assign any reason. What can be more singular than the
+relation between blue eyes and deafness in cats, and the tortoise-shell
+colour with the female sex; the feathered feet and skin between the outer
+toes in pigeons, and the presence of more or less down on the young birds
+when first hatched, with the future colour of their plumage; or, again, the
+relation between the hair and teeth in the naked Turkish dog, though here
+probably homology comes into play? With respect to this latter case of
+correlation, I think it can hardly be accidental, that if we pick out the
+two orders of mammalia which are most abnormal in their dermal covering,
+viz. Cetacea (whales) and Edentata (armadilloes, scaly anteaters, &c.),
+that these are likewise the most abnormal in their teeth.
+
+I know of no case better adapted to show the importance of the laws of
+correlation in modifying important structures, independently of utility
+and, therefore, of natural selection, than that of the difference between
+the outer and inner flowers in some Compositous and Umbelliferous plants.
+Every one knows the {145} difference in the ray and central florets of, for
+instance, the daisy, and this difference is often accompanied with the
+abortion of parts of the flower. But, in some Compositous plants, the seeds
+also differ in shape and sculpture; and even the ovary itself, with its
+accessory parts, differs, as has been described by Cassini. These
+differences have been attributed by some authors to pressure, and the shape
+of the seeds in the ray-florets in some Compositæ countenances this idea;
+but, in the case of the corolla of the Umbelliferæ, it is by no means, as
+Dr. Hooker informs me, in species with the densest heads that the inner and
+outer flowers most frequently differ. It might have been thought that the
+development of the ray-petals by drawing nourishment from certain other
+parts of the flower had caused their abortion; but in some Compositæ there
+is a difference in the seeds of the outer and inner florets without any
+difference in the corolla. Possibly, these several differences may be
+connected with some difference in the flow of nutriment towards the central
+and external flowers: we know, at least, that in irregular flowers, those
+nearest to the axis are oftenest subject to peloria, and become regular. I
+may add, as an instance of this, and of a striking case of correlation,
+that I have recently observed in some garden pelargoniums, that the central
+flower of the truss often loses the patches of darker colour in the two
+upper petals; and that when this occurs, the adherent nectary is quite
+aborted; when the colour is absent from only one of the two upper petals,
+the nectary is only much shortened.
+
+With respect to the difference in the corolla of the central and exterior
+flowers of a head or umbel, I do not feel at all sure that C. C. Sprengel's
+idea that the ray-florets serve to attract insects, whose agency is highly
+advantageous in the fertilisation of plants of {146} these two orders, is
+so far-fetched, as it may at first appear: and if it be advantageous,
+natural selection may have come into play. But in regard to the differences
+both in the internal and external structure of the seeds, which are not
+always correlated with any differences in the flowers, it seems impossible
+that they can be in any way advantageous to the plant: yet in the
+Umbelliferæ these differences are of such apparent importance--the seeds
+being in some cases, according to Tausch, orthospermous in the exterior
+flowers and coelospermous in the central flowers,--that the elder De
+Candolle founded his main divisions of the order on analogous differences.
+Hence we see that modifications of structure, viewed by systematists as of
+high value, may be wholly due to unknown laws of correlated growth, and
+without being, as far as we can see, of the slightest service to the
+species.
+
+We may often falsely attribute to correlation of growth, structures which
+are common to whole groups of species, and which in truth are simply due to
+inheritance; for an ancient progenitor may have acquired through natural
+selection some one modification in structure, and, after thousands of
+generations, some other and independent modification; and these two
+modifications, having been transmitted to a whole group of descendants with
+diverse habits, would naturally be thought to be correlated in some
+necessary manner. So, again, I do not doubt that some apparent
+correlations, occurring throughout whole orders, are entirely due to the
+manner alone in which natural selection can act. For instance, Alph. De
+Candolle has remarked that winged seeds are never found in fruits which do
+not open: I should explain the rule by the fact that seeds could not
+gradually become winged through natural selection, except in fruits which
+opened; so that the individual plants producing {147} seeds which were a
+little better fitted to be wafted further, might get an advantage over
+those producing seed less fitted for dispersal; and this process could not
+possibly go on in fruit which did not open.
+
+The elder Geoffroy and Goethe propounded, at about the same period, their
+law of compensation or balancement of growth; or, as Goethe expressed it,
+"in order to spend on one side, nature is forced to economise on the other
+side." I think this holds true to a certain extent with our domestic
+productions: if nourishment flows to one part or organ in excess, it rarely
+flows, at least in excess, to another part; thus it is difficult to get a
+cow to give much milk and to fatten readily. The same varieties of the
+cabbage do not yield abundant and nutritious foliage and a copious supply
+of oil-bearing seeds. When the seeds in our fruits become atrophied, the
+fruit itself gains largely in size and quality. In our poultry, a large
+tuft of feathers on the head is generally accompanied by a diminished comb,
+and a large beard by diminished wattles. With species in a state of nature
+it can hardly be maintained that the law is of universal application; but
+many good observers, more especially botanists, believe in its truth. I
+will not, however, here give any instances, for I see hardly any way of
+distinguishing between the effects, on the one hand, of a part being
+largely developed through natural selection and another and adjoining part
+being reduced by this same process or by disuse, and, on the other hand,
+the actual withdrawal of nutriment from one part owing to the excess of
+growth in another and adjoining part.
+
+I suspect, also, that some of the cases of compensation which have been
+advanced, and likewise some other facts, may be merged under a more general
+principle, namely, that natural selection is continually trying to
+economise in every part of the organisation. If under {148} changed
+conditions of life a structure before useful becomes less useful, any
+diminution, however slight, in its development, will be seized on by
+natural selection, for it will profit the individual not to have its
+nutriment wasted in building up an useless structure. I can thus only
+understand a fact with which I was much struck when examining cirripedes,
+and of which many other instances could be given: namely, that when a
+cirripede is parasitic within another and is thus protected, it loses more
+or less completely its own shell or carapace. This is the case with the
+male Ibla, and in a truly extraordinary manner with the Proteolepas: for
+the carapace in all other cirripedes consists of the three highly-important
+anterior segments of the head enormously developed, and furnished with
+great nerves and muscles; but in the parasitic and protected Proteolepas,
+the whole anterior part of the head is reduced to the merest rudiment
+attached to the bases of the prehensile antennæ. Now the saving of a large
+and complex structure, when rendered superfluous by the parasitic habits of
+the Proteolepas, though effected by slow steps, would be a decided
+advantage to each successive individual of the species; for in the struggle
+for life to which every animal is exposed, each individual Proteolepas
+would have a better chance of supporting itself, by less nutriment being
+wasted in developing a structure now become useless.
+
+Thus, as I believe, natural selection will always succeed in the long run
+in reducing and saving every part of the organisation, as soon as it is
+rendered superfluous, without by any means causing some other part to be
+largely developed in a corresponding degree. And, conversely, that natural
+selection may perfectly well succeed in largely developing any organ,
+without requiring as a necessary compensation the reduction of some
+adjoining part. {149}
+
+It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in
+varieties and in species, that when any part or organ is repeated many
+times in the structure of the same individual (as the vertebræ in snakes,
+and the stamens in polyandrous flowers) the number is variable; whereas the
+number of the same part or organ, when it occurs in lesser numbers, is
+constant. The same author and some botanists have further remarked that
+multiple parts are also very liable to variation in structure. Inasmuch as
+this "vegetative repetition," to use Prof. Owen's expression, seems to be a
+sign of low organisation, the foregoing remark seems connected with the
+very general opinion of naturalists, that beings low in the scale of nature
+are more variable than those which are higher. I presume that lowness in
+this case means that the several parts of the organisation have been but
+little specialised for particular functions; and as long as the same part
+has to perform diversified work, we can perhaps see why it should remain
+variable, that is, why natural selection should have preserved or rejected
+each little deviation of form less carefully than when the part has to
+serve for one special purpose alone. In the same way that a knife which has
+to cut all sorts of things may be of almost any shape; whilst a tool for
+some particular object had better be of some particular shape. Natural
+selection, it should never be forgotten, can act on each part of each
+being, solely through and for its advantage.
+
+Rudimentary parts, it has been stated by some authors, and I believe with
+truth, are apt to be highly variable. We shall have to recur to the general
+subject of rudimentary and aborted organs; and I will here only add that
+their variability seems to be owing to their uselessness, and therefore to
+natural selection having no power to check deviations in their structure.
+Thus {150} rudimentary parts are left to the free play of the various laws
+of growth, to the effects of long-continued disuse, and to the tendency to
+reversion.
+
+
+
+_A part developed in any species in an extraordinary degree or manner, in
+comparison with the same part in allied species, tends to be highly
+variable._--Several years ago I was much struck with a remark, nearly to
+the above effect, published by Mr. Waterhouse. I infer also from an
+observation made by Professor Owen, with respect to the length of the arms
+of the ourang-outang, that he has come to a nearly similar conclusion. It
+is hopeless to attempt to convince any one of the truth of this proposition
+without giving the long array of facts which I have collected, and which
+cannot possibly be here introduced. I can only state my conviction that it
+is a rule of high generality. I am aware of several causes of error, but I
+hope that I have made due allowance for them. It should be understood that
+the rule by no means applies to any part, however unusually developed,
+unless it be unusually developed in comparison with the same part in
+closely allied species. Thus, the bat's wing is a most abnormal structure
+in the class mammalia; but the rule would not here apply, because there is
+a whole group of bats having wings; it would apply only if some one species
+of bat had its wings developed in some remarkable manner in comparison with
+the other species of the same genus. The rule applies very strongly in the
+case of secondary sexual characters, when displayed in any unusual manner.
+The term, secondary sexual characters, used by Hunter, applies to
+characters which are attached to one sex, but are not directly connected
+with the act of reproduction. The rule applies to males and females; but as
+females more rarely offer remarkable secondary sexual characters, it
+applies {151} more rarely to them. The rule being so plainly applicable in
+the case of secondary sexual characters, may be due to the great
+variability of these characters, whether or not displayed in any unusual
+manner--of which fact I think there can be little doubt. But that our rule
+is not confined to secondary sexual characters is clearly shown in the case
+of hermaphrodite cirripedes; and I may here add, that I particularly
+attended to Mr. Waterhouse's remark, whilst investigating this Order, and I
+am fully convinced that the rule almost invariably holds good with
+cirripedes. I shall, in my future work, give a list of the more remarkable
+cases; I will here only briefly give one, as it illustrates the rule in its
+largest application. The opercular valves of sessile cirripedes (rock
+barnacles) are, in every sense of the word, very important structures, and
+they differ extremely little even in different genera; but in the several
+species of one genus, Pyrgoma, these valves present a marvellous amount of
+diversification: the homologous valves in the different species being
+sometimes wholly unlike in shape; and the amount of variation in the
+individuals of several of the species is so great, that it is no
+exaggeration to state that the varieties differ more from each other in the
+characters of these important valves than do other species of distinct
+genera.
+
+As birds within the same country vary in a remarkably small degree, I have
+particularly attended to them, and the rule seems to me certainly to hold
+good in this class. I cannot make out that it applies to plants, and this
+would seriously have shaken my belief in its truth, had not the great
+variability in plants made it particularly difficult to compare their
+relative degrees of variability.
+
+When we see any part or organ developed in a remarkable degree or manner in
+any species, the fair {152} presumption is that it is of high importance to
+that species; nevertheless the part in this case is eminently liable to
+variation. Why should this be so? On the view that each species has been
+independently created, with all its parts as we now see them, I can see no
+explanation. But on the view that groups of species have descended from
+other species, and have been modified through natural selection, I think we
+can obtain some light. In our domestic animals, if any part, or the whole
+animal, be neglected and no selection be applied, that part (for instance,
+the comb in the Dorking fowl) or the whole breed will cease to have a
+nearly uniform character. The breed will then be said to have degenerated.
+In rudimentary organs, and in those which have been but little specialised
+for any particular purpose, and perhaps in polymorphic groups, we see a
+nearly parallel natural case; for in such cases natural selection either
+has not or cannot come into full play, and thus the organisation is left in
+a fluctuating condition. But what here more especially concerns us is, that
+in our domestic animals those points, which at the present time are
+undergoing rapid change by continued selection, are also eminently liable
+to variation. Look at the breeds of the pigeon; see what a prodigious
+amount of difference there is in the beak of the different tumblers, in the
+beak and wattle of the different carriers, in the carriage and tail of our
+fantails, &c., these being the points now mainly attended to by English
+fanciers. Even in the sub-breeds, as in the short-faced tumbler, it is
+notoriously difficult to breed them nearly to perfection, and frequently
+individuals are born which depart widely from the standard. There may be
+truly said to be a constant struggle going on between, on the one hand, the
+tendency to reversion to a less modified state, as well as an innate
+tendency to further {153} variability of all kinds, and, on the other hand,
+the power of steady selection to keep the breed true. In the long run
+selection gains the day, and we do not expect to fail so far as to breed a
+bird as coarse as a common tumbler from a good short-faced strain. But as
+long as selection is rapidly going on, there may always be expected to be
+much variability in the structure undergoing modification. It further
+deserves notice that these variable characters, produced by man's
+selection, sometimes become attached, from causes quite unknown to us, more
+to one sex than to the other, generally to the male sex, as with the wattle
+of carriers and the enlarged crop of pouters.
+
+Now let us turn to nature. When a part has been developed in an
+extraordinary manner in any one species, compared with the other species of
+the same genus, we may conclude that this part has undergone an
+extraordinary amount of modification since the period when the species
+branched off from the common progenitor of the genus. This period will
+seldom be remote in any extreme degree, as species very rarely endure for
+more than one geological period. An extraordinary amount of modification
+implies an unusually large and long-continued amount of variability, which
+has continually been accumulated by natural selection for the benefit of
+the species. But as the variability of the extraordinarily-developed part
+or organ has been so great and long-continued within a period not
+excessively remote, we might, as a general rule, expect still to find more
+variability in such parts than in other parts of the organisation which
+have remained for a much longer period nearly constant. And this, I am
+convinced, is the case. That the struggle between natural selection on the
+one hand, and the tendency to reversion and variability on the other hand,
+will in the {154} course of time cease; and that the most abnormally
+developed organs may be made constant, I can see no reason to doubt. Hence
+when an organ, however abnormal it may be, has been transmitted in
+approximately the same condition to many modified descendants, as in the
+case of the wing of the bat, it must have existed, according to my theory,
+for an immense period in nearly the same state; and thus it comes to be no
+more variable than any other structure. It is only in those cases in which
+the modification has been comparatively recent and extraordinarily great
+that we ought to find the _generative variability_, as it may be called,
+still present in a high degree. For in this case the variability will
+seldom as yet have been fixed by the continued selection of the individuals
+varying in the required manner and degree, and by the continued rejection
+of those tending to revert to a former and less modified condition.
+
+The principle included in these remarks may be extended. It is notorious
+that specific characters are more variable than generic. To explain by a
+simple example what is meant. If some species in a large genus of plants
+had blue flowers and some had red, the colour would be only a specific
+character, and no one would be surprised at one of the blue species varying
+into red, or conversely; but if all the species had blue flowers, the
+colour would become a generic character, and its variation would be a more
+unusual circumstance. I have chosen this example because an explanation is
+not in this case applicable, which most naturalists would advance, namely,
+that specific characters are more variable than generic, because they are
+taken from parts of less physiological importance than those commonly used
+for classing genera. I believe this explanation is partly, yet only
+indirectly, true; I shall, however, have to {155} return to this subject in
+our chapter on Classification. It would be almost superfluous to adduce
+evidence in support of the above statement, that specific characters are
+more variable than generic; but I have repeatedly noticed in works on
+natural history, that when an author has remarked with surprise that some
+_important_ organ or part, which is generally very constant throughout
+large groups of species, has _differed_ considerably in closely-allied
+species, that it has, also, been _variable_ in the individuals of some of
+the species. And this fact shows that a character, which is generally of
+generic value, when it sinks in value and becomes only of specific value,
+often becomes variable, though its physiological importance may remain the
+same. Something of the same kind applies to monstrosities: at least Is.
+Geoffroy St. Hilaire seems to entertain no doubt, that the more an organ
+normally differs in the different species of the same group, the more
+subject it is to individual anomalies.
+
+On the ordinary view of each species having been independently created, why
+should that part of the structure, which differs from the same part in
+other independently-created species of the same genus, be more variable
+than those parts which are closely alike in the several species? I do not
+see that any explanation can be given. But on the view of species being
+only strongly marked and fixed varieties, we might surely expect to find
+them still often continuing to vary in those parts of their structure which
+have varied within a moderately recent period, and which have thus come to
+differ. Or to state the case in another manner:--the points in which all
+the species of a genus resemble each other, and in which they differ from
+the species of some other genus, are called generic characters; and these
+characters in common I attribute to {156} inheritance from a common
+progenitor, for it can rarely have happened that natural selection will
+have modified several species, fitted to more or less widely-different
+habits, in exactly the same manner: and as these so-called generic
+characters have been inherited from a remote period, since that period when
+the species first branched off from their common progenitor, and
+subsequently have not varied or come to differ in any degree, or only in a
+slight degree, it is not probable that they should vary at the present day.
+On the other hand, the points in which species differ from other species of
+the same genus, are called specific characters; and as these specific
+characters have varied and come to differ within the period of the
+branching off of the species from a common progenitor, it is probable that
+they should still often be in some degree variable,--at least more variable
+than those parts of the organisation which have for a very long period
+remained constant.
+
+In connexion with the present subject, I will make only two other remarks.
+I think it will be admitted, without my entering on details, that secondary
+sexual characters are very variable; I think it also will be admitted that
+species of the same group differ from each other more widely in their
+secondary sexual characters, than in other parts of their organisation;
+compare, for instance, the amount of difference between the males of
+gallinaceous birds, in which secondary sexual characters are strongly
+displayed, with the amount of difference between their females; and the
+truth of this proposition will be granted. The cause of the original
+variability of secondary sexual characters is not manifest; but we can see
+why these characters should not have been rendered as constant and uniform
+as other parts of the organisation; for secondary sexual characters have
+been accumulated by sexual selection, which {157} is less rigid in its
+action than ordinary selection, as it does not entail death, but only gives
+fewer offspring to the less favoured males. Whatever the cause may be of
+the variability of secondary sexual characters, as they are highly
+variable, sexual selection will have had a wide scope for action, and may
+thus readily have succeeded in giving to the species of the same group a
+greater amount of difference in their sexual characters, than in other
+parts of their structure.
+
+It is a remarkable fact, that the secondary sexual differences between the
+two sexes of the same species are generally displayed in the very same
+parts of the organisation in which the different species of the same genus
+differ from each other. Of this fact I will give in illustration two
+instances, the first which happen to stand on my list; and as the
+differences in these cases are of a very unusual nature, the relation can
+hardly be accidental. The same number of joints in the tarsi is a character
+generally common to very large groups of beetles, but in the Engidæ, as
+Westwood has remarked, the number varies greatly; and the number likewise
+differs in the two sexes of the same species: again in fossorial
+hymenoptera, the manner of neuration of the wings is a character of the
+highest importance, because common to large groups; but in certain genera
+the neuration differs in the different species, and likewise in the two
+sexes of the same species. This relation has a clear meaning on my view of
+the subject: I look at all the species of the same genus as having as
+certainly descended from the same progenitor, as have the two sexes of any
+one of the species. Consequently, whatever part of the structure of the
+common progenitor, or of its early descendants, became variable; variations
+of this part would, it is highly probable, be taken advantage of by natural
+and sexual selection, in order to fit {158} the several species to their
+several places in the economy of nature, and likewise to fit the two sexes
+of the same species to each other, or to fit the males and females to
+different habits of life, or the males to struggle with other males for the
+possession of the females.
+
+Finally, then, I conclude that the greater variability of specific
+characters, or those which distinguish species from species, than of
+generic characters, or those which the species possess in common;--that the
+frequent extreme variability of any part which is developed in a species in
+an extraordinary manner in comparison with the same part in its congeners;
+and the slight degree of variability in a part, however extraordinarily it
+may be developed, if it be common to a whole group of species;--that the
+great variability of secondary sexual characters, and the great amount of
+difference in these same characters between closely allied species;--that
+secondary sexual and ordinary specific differences are generally displayed
+in the same parts of the organisation,--are all principles closely
+connected together. All being mainly due to the species of the same group
+having descended from a common progenitor, from whom they have inherited
+much in common,--to parts which have recently and largely varied being more
+likely still to go on varying than parts which have long been inherited and
+have not varied,--to natural selection having more or less completely,
+according to the lapse of time, overmastered the tendency to reversion and
+to further variability,--to sexual selection being less rigid than ordinary
+selection,--and to variations in the same parts having been accumulated by
+natural and sexual selection, and having been thus adapted for secondary
+sexual, and for ordinary specific purposes. {159}
+
+
+
+_Distinct species present analogous variations; and a variety of one
+species often assumes some of the characters of an allied species, or
+reverts to some of the characters of an early progenitor._--These
+propositions will be most readily understood by looking to our domestic
+races. The most distinct breeds of pigeons, in countries most widely apart,
+present sub-varieties with reversed feathers on the head and feathers on
+the feet,--characters not possessed by the aboriginal rock-pigeon; these
+then are analogous variations in two or more distinct races. The frequent
+presence of fourteen or even sixteen tail-feathers in the pouter, may be
+considered as a variation representing the normal structure of another
+race, the fantail. I presume that no one will doubt that all such analogous
+variations are due to the several races of the pigeon having inherited from
+a common parent the same constitution and tendency to variation, when acted
+on by similar unknown influences. In the vegetable kingdom we have a case
+of analogous variation, in the enlarged stems, or roots as commonly called,
+of the Swedish turnip and Ruta baga, plants which several botanists rank as
+varieties produced by cultivation from a common parent: if this be not so,
+the case will then be one of analogous variation in two so-called distinct
+species; and to these a third may be added, namely, the common turnip.
+According to the ordinary view of each species having been independently
+created, we should have to attribute this similarity in the enlarged stems
+of these three plants, not to the _vera causa_ of community of descent, and
+a consequent tendency to vary in a like manner, but to three separate yet
+closely related acts of creation.
+
+With pigeons, however, we have another case, namely, the occasional
+appearance in all the breeds, of slaty-blue birds with two black bars on
+the wings, a white {160} rump, a bar at the end of the tail, with the outer
+feathers externally edged near their bases with white. As all these marks
+are characteristic of the parent rock-pigeon, I presume that no one will
+doubt that this is a case of reversion, and not of a new yet analogous
+variation appearing in the several breeds. We may I think confidently come
+to this conclusion, because, as we have seen, these coloured marks are
+eminently liable to appear in the crossed offspring of two distinct and
+differently coloured breeds; and in this case there is nothing in the
+external conditions of life to cause the reappearance of the slaty-blue,
+with the several marks, beyond the influence of the mere act of crossing on
+the laws of inheritance.
+
+No doubt it is a very surprising fact that characters should reappear after
+having been lost for many, perhaps for hundreds of generations. But when a
+breed has been crossed only once by some other breed, the offspring
+occasionally show a tendency to revert in character to the foreign breed
+for many generations--some say, for a dozen or even a score of generations.
+After twelve generations, the proportion of blood, to use a common
+expression, of any one ancestor, is only 1 in 2048; and yet, as we see, it
+is generally believed that a tendency to reversion is retained by this very
+small proportion of foreign blood. In a breed which has not been crossed,
+but in which _both_ parents have lost some character which their progenitor
+possessed, the tendency, whether strong or weak, to reproduce the lost
+character might be, as was formerly remarked, for all that we can see to
+the contrary, transmitted for almost any number of generations. When a
+character which has been lost in a breed, reappears after a great number of
+generations, the most probable hypothesis is, not that the offspring
+suddenly takes after an ancestor some hundred generations {161} distant,
+but that in each successive generation there has been a tendency to
+reproduce the character in question, which at last, under unknown
+favourable conditions, gains an ascendancy. For instance, it is probable
+that in each generation of the barb-pigeon, which produces most rarely a
+blue and black-barred bird, there has been a tendency in each generation in
+the plumage to assume this colour. This view is hypothetical, but could be
+supported by some facts; and I can see no more abstract improbability in a
+tendency to produce any character being inherited for an endless number of
+generations, than in quite useless or rudimentary organs being, as we all
+know them to be, thus inherited. Indeed, we may sometimes observe a mere
+tendency to produce a rudiment inherited: for instance, in the common
+snapdragon (Antirrhinum) a rudiment of a fifth stamen so often appears,
+that this plant must have an inherited tendency to produce it.
+
+As all the species of the same genus are supposed, on my theory, to have
+descended from a common parent, it might be expected that they would
+occasionally vary in an analogous manner; so that a variety of one species
+would resemble in some of its characters another species; this other
+species being on my view only a well-marked and permanent variety. But
+characters thus gained would probably be of an unimportant nature, for the
+presence of all important characters will be governed by natural selection,
+in accordance with the diverse habits of the species, and will not be left
+to the mutual action of the conditions of life and of a similar inherited
+constitution. It might further be expected that the species of the same
+genus would occasionally exhibit reversions to lost ancestral characters.
+As, however, we never know the exact character of the common ancestor of a
+group, we could not distinguish these two {162} cases: if, for instance, we
+did not know that the rock-pigeon was not feather-footed or turn-crowned,
+we could not have told, whether these characters in our domestic breeds
+were reversions or only analogous variations; but we might have inferred
+that the blueness was a case of reversion, from the number of the markings,
+which are correlated with the blue tint, and which it does not appear
+probable would all appear together from simple variation. More especially
+we might have inferred this, from the blue colour and marks so often
+appearing when distinct breeds of diverse colours are crossed. Hence,
+though under nature it must generally be left doubtful, what cases are
+reversions to an anciently existing character, and what are new but
+analogous variations, yet we ought, on my theory, sometimes to find the
+varying offspring of a species assuming characters (either from reversion
+or from analogous variation) which already occur in some other members of
+the same group. And this undoubtedly is the case in nature.
+
+A considerable part of the difficulty in recognising a variable species in
+our systematic works, is due to its varieties mocking, as it were, some of
+the other species of the same genus. A considerable catalogue, also, could
+be given of forms intermediate between two other forms, which themselves
+must be doubtfully ranked as either varieties or species; and this shows,
+unless all these forms be considered as independently created species, that
+the one in varying has assumed some of the characters of the other, so as
+to produce the intermediate form. But the best evidence is afforded by
+parts or organs of an important and uniform nature occasionally varying so
+as to acquire, in some degree, the character of the same part or organ in
+an allied species. I have collected a long list of such cases; but {163}
+here, as before, I lie under a great disadvantage in not being able to give
+them. I can only repeat that such cases certainly do occur, and seem to me
+very remarkable.
+
+I will, however, give one curious and complex case, not indeed as affecting
+any important character, but from occurring in several species of the same
+genus, partly under domestication and partly under nature. It is a case
+apparently of reversion. The ass not rarely has very distinct transverse
+bars on its legs, like those on the legs of the zebra: it has been asserted
+that these are plainest in the foal, and from inquiries which I have made,
+I believe this to be true. It has also been asserted that the stripe on
+each shoulder is sometimes double. The shoulder-stripe is certainly very
+variable in length and outline. A white ass, but _not_ an albino, has been
+described without either spinal or shoulder stripe; and these stripes are
+sometimes very obscure, or actually quite lost, in dark-coloured asses. The
+koulan of Pallas is said to have been seen with a double shoulder-stripe.
+The hemionus has no shoulder-stripe; but traces of it, as stated by Mr.
+Blyth and others, occasionally appear: and I have been informed by Colonel
+Poole that the foals of this species are generally striped on the legs, and
+faintly on the shoulder. The quagga, though so plainly barred like a zebra
+over the body, is without bars on the legs; but Dr. Gray has figured one
+specimen with very distinct zebra-like bars on the hocks.
+
+With respect to the horse, I have collected cases in England of the spinal
+stripe in horses of the most distinct breeds, and of _all_ colours;
+transverse bars on the legs are not rare in duns, mouse-duns, and in one
+instance in a chestnut: a faint shoulder-stripe may sometimes be seen in
+duns, and I have seen a trace in a {164} bay horse. My son made a careful
+examination and sketch for me of a dun Belgian cart-horse with a double
+stripe on each shoulder and with leg-stripes; and a man, whom I can
+implicitly trust, has examined for me a small dun Welch pony with _three_
+short parallel stripes on each shoulder.
+
+In the north-west part of India the Kattywar breed of horses is so
+generally striped, that, as I hear from Colonel Poole, who examined the
+breed for the Indian Government, a horse without stripes is not considered
+as purely-bred. The spine is always striped; the legs are generally barred;
+and the shoulder-stripe, which is sometimes double and sometimes treble, is
+common; the side of the face, moreover, is sometimes striped. The stripes
+are plainest in the foal; and sometimes quite disappear in old horses.
+Colonel Poole has seen both gray and bay Kattywar horses striped when first
+foaled. I have, also, reason to suspect, from information given me by Mr.
+W. W. Edwards, that with the English racehorse the spinal stripe is much
+commoner in the foal than in the full-grown animal. Without here entering
+on further details, I may state that I have collected cases of leg and
+shoulder stripes in horses of very different breeds, in various countries
+from Britain to Eastern China; and from Norway in the north to the Malay
+Archipelago in the south. In all parts of the world these stripes occur far
+oftenest in duns and mouse-duns; by the term dun a large range of colour is
+included, from one between brown and black to a close approach to
+cream-colour.
+
+I am aware that Colonel Hamilton Smith, who has written on this subject,
+believes that the several breeds of the horse have descended from several
+aboriginal species--one of which, the dun, was striped; and that the
+above-described appearances are all due to ancient {165} crosses with the
+dun stock. But I am not at all satisfied with this theory, and should be
+loth to apply it to breeds so distinct as the heavy Belgian cart-horse,
+Welch ponies, cobs, the lanky Kattywar race, &c., inhabiting the most
+distant parts of the world.
+
+Now let us turn to the effects of crossing the several species of the
+horse-genus. Rollin asserts, that the common mule from the ass and horse is
+particularly apt to have bars on its legs: according to Mr. Gosse, in
+certain parts of the United States about nine out of ten mules have striped
+legs. I once saw a mule with its legs so much striped that any one would at
+first have thought that it must have been the product of a zebra; and Mr.
+W. C. Martin, in his excellent treatise on the horse, has given a figure of
+a similar mule. In four coloured drawings, which I have seen, of hybrids
+between the ass and zebra, the legs were much more plainly barred than the
+rest of the body; and in one of them there was a double shoulder-stripe. In
+Lord Morton's famous hybrid from a chestnut mare and male quagga, the
+hybrid, and even the pure offspring subsequently produced from the mare by
+a black Arabian sire, were much more plainly barred across the legs than is
+even the pure quagga. Lastly, and this is another most remarkable case, a
+hybrid has been figured by Dr. Gray (and he informs me that he knows of a
+second case) from the ass and the hemionus; and this hybrid, though the ass
+seldom has stripes on his legs and the hemionus has none and has not even a
+shoulder-stripe, nevertheless had all four legs barred, and had three short
+shoulder-stripes, like those on the dun Welch pony, and even had some
+zebra-like stripes on the sides of its face. With respect to this last
+fact, I was so convinced that not even a stripe of colour appears from what
+would commonly be called an {166} accident, that I was led solely from the
+occurrence of the face-stripes on this hybrid from the ass and hemionus to
+ask Colonel Poole whether such face-stripes ever occur in the eminently
+striped Kattywar breed of horses, and was, as we have seen, answered in the
+affirmative.
+
+What now are we to say to these several facts? We see several very distinct
+species of the horse-genus becoming, by simple variation, striped on the
+legs like a zebra, or striped on the shoulders like an ass. In the horse we
+see this tendency strong whenever a dun tint appears--a tint which
+approaches to that of the general colouring of the other species of the
+genus. The appearance of the stripes is not accompanied by any change of
+form or by any other new character. We see this tendency to become striped
+most strongly displayed in hybrids from between several of the most
+distinct species. Now observe the case of the several breeds of pigeons:
+they are descended from a pigeon (including two or three sub-species or
+geographical races) of a bluish colour, with certain bars and other marks;
+and when any breed assumes by simple variation a bluish tint, these bars
+and other marks invariably reappear; but without any other change of form
+or character. When the oldest and truest breeds of various colours are
+crossed, we see a strong tendency for the blue tint and bars and marks to
+reappear in the mongrels. I have stated that the most probable hypothesis
+to account for the reappearance of very ancient characters, is--that there
+is a _tendency_ in the young of each successive generation to produce the
+long-lost character, and that this tendency, from unknown causes, sometimes
+prevails. And we have just seen that in several species of the horse-genus
+the stripes are either plainer or appear more commonly in the young than in
+the old. Call the breeds of pigeons, some of which have bred true for {167}
+centuries, species; and how exactly parallel is the case with that of the
+species of the horse-genus! For myself, I venture confidently to look back
+thousands on thousands of generations, and I see an animal striped like a
+zebra, but perhaps otherwise very differently constructed, the common
+parent of our domestic horse, whether or not it be descended from one or
+more wild stocks, of the ass, the hemionus, quagga, and zebra.
+
+He who believes that each equine species was independently created, will, I
+presume, assert that each species has been created with a tendency to vary,
+both under nature and under domestication, in this particular manner, so as
+often to become striped like other species of the genus; and that each has
+been created with a strong tendency, when crossed with species inhabiting
+distant quarters of the world, to produce hybrids resembling in their
+stripes, not their own parents, but other species of the genus. To admit
+this view is, as it seems to me, to reject a real for an unreal, or at
+least for an unknown, cause. It makes the works of God a mere mockery and
+deception; I would almost as soon believe with the old and ignorant
+cosmogonists, that fossil shells had never lived, but had been created in
+stone so as to mock the shells now living on the sea-shore.
+
+
+
+_Summary._--Our ignorance of the laws of variation is profound. Not in one
+case out of a hundred can we pretend to assign any reason why this or that
+part differs, more or less, from the same part in the parents. But whenever
+we have the means of instituting a comparison, the same laws appear to have
+acted in producing the lesser differences between varieties of the same
+species, and the greater differences between species of the same genus. The
+external conditions of life, as {168} climate and food, &c., seem to have
+induced some slight modifications. Habit in producing constitutional
+differences, and use in strengthening and disuse in weakening and
+diminishing organs, seem to have been more potent in their effects.
+Homologous parts tend to vary in the same way, and homologous parts tend to
+cohere. Modifications in hard parts and in external parts sometimes affect
+softer and internal parts. When one part is largely developed, perhaps it
+tends to draw nourishment from the adjoining parts; and every part of the
+structure which can be saved without detriment to the individual, will be
+saved. Changes of structure at an early age will generally affect parts
+subsequently developed; and there are very many other correlations of
+growth, the nature of which we are utterly unable to understand. Multiple
+parts are variable in number and in structure, perhaps arising from such
+parts not having been closely specialised to any particular function, so
+that their modifications have not been closely checked by natural
+selection. It is probably from this same cause that organic beings low in
+the scale of nature are more variable than those which have their whole
+organisation more specialised, and are higher in the scale. Rudimentary
+organs, from being useless, will be disregarded by natural selection, and
+hence probably are variable. Specific characters--that is, the characters
+which have come to differ since the several species of the same genus
+branched off from a common parent--are more variable than generic
+characters, or those which have long been inherited, and have not differed
+within this same period. In these remarks we have referred to special parts
+or organs being still variable, because they have recently varied and thus
+come to differ; but we have also seen in the second Chapter that the same
+principle applies to the whole individual; {169} for in a district where
+many species of any genus are found--that is, where there has been much
+former variation and differentiation, or where the manufactory of new
+specific forms has been actively at work--there, on an average, we now find
+most varieties or incipient species. Secondary sexual characters are highly
+variable, and such characters differ much in the species of the same group.
+Variability in the same parts of the organisation has generally been taken
+advantage of in giving secondary sexual differences to the sexes of the
+same species, and specific differences to the several species of the same
+genus. Any part or organ developed to an extraordinary size or in an
+extraordinary manner, in comparison with the same part or organ in the
+allied species, must have gone through an extraordinary amount of
+modification since the genus arose; and thus we can understand why it
+should often still be variable in a much higher degree than other parts;
+for variation is a long-continued and slow process, and natural selection
+will in such cases not as yet have had time to overcome the tendency to
+further variability and to reversion to a less modified state. But when a
+species with any extraordinarily-developed organ has become the parent of
+many modified descendants--which on my view must be a very slow process,
+requiring a long lapse of time--in this case, natural selection may readily
+have succeeded in giving a fixed character to the organ, in however
+extraordinary a manner it may be developed. Species inheriting nearly the
+same constitution from a common parent and exposed to similar influences
+will naturally tend to present analogous variations, and these same species
+may occasionally revert to some of the characters of their ancient
+progenitors. Although new and important modifications may not arise from
+reversion and analogous {170} variation, such modifications will add to the
+beautiful and harmonious diversity of nature.
+
+Whatever the cause may be of each slight difference in the offspring from
+their parents--and a cause for each must exist--it is the steady
+accumulation, through natural selection, of such differences, when
+beneficial to the individual, that gives rise to all the more important
+modifications of structure, by which the innumerable beings on the face of
+this earth are enabled to struggle with each other, and the best adapted to
+survive.
+
+       *       *       *       *       *
+
+
+{171}
+
+CHAPTER VI.
+
+DIFFICULTIES ON THEORY.
+
+    Difficulties on the theory of descent with
+    modification--Transitions--Absence or rarity of transitional
+    varieties--Transitions in habits of life--Diversified habits in the
+    same species--Species with habits widely different from those of their
+    allies--Organs of extreme perfection--Means of transition--Cases of
+    difficulty--Natura non facit saltum--Organs of small importance--Organs
+    not in all cases absolutely perfect--The law of Unity of Type and of
+    the Conditions of Existence embraced by the theory of Natural
+    Selection.
+
+Long before having arrived at this part of my work, a crowd of difficulties
+will have occurred to the reader. Some of them are so grave that to this
+day I can never reflect on them without being staggered; but, to the best
+of my judgment, the greater number are only apparent, and those that are
+real are not, I think, fatal to my theory.
+
+These difficulties and objections may be classed under the following
+heads:--Firstly, why, if species have descended from other species by
+insensibly fine gradations, do we not everywhere see innumerable
+transitional forms? Why is not all nature in confusion instead of the
+species being, as we see them, well defined?
+
+Secondly, is it possible that an animal having, for instance, the structure
+and habits of a bat, could have been formed by the modification of some
+animal with wholly different habits? Can we believe that natural selection
+could produce, on the one hand, organs of trifling importance, such as the
+tail of a giraffe, which serves as a fly-flapper, and, on the other hand,
+organs of {172} such wonderful structure, as the eye, of which we hardly as
+yet fully understand the inimitable perfection?
+
+Thirdly, can instincts be acquired and modified through natural selection?
+What shall we say to so marvellous an instinct as that which leads the bee
+to make cells, which has practically anticipated the discoveries of
+profound mathematicians?
+
+Fourthly, how can we account for species, when crossed, being sterile and
+producing sterile offspring, whereas, when varieties are crossed, their
+fertility is unimpaired?
+
+The two first heads shall be here discussed--Instinct and Hybridism in
+separate chapters.
+
+
+
+_On the absence or rarity of transitional varieties._--As natural selection
+acts solely by the preservation of profitable modifications, each new form
+will tend in a fully-stocked country to take the place of, and finally to
+exterminate, its own less improved parent or other less-favoured forms with
+which it comes into competition. Thus extinction and natural selection
+will, as we have seen, go hand in hand. Hence, if we look at each species
+as descended from some other unknown form, both the parent and all the
+transitional varieties will generally have been exterminated by the very
+process of formation and perfection of the new form.
+
+But, as by this theory innumerable transitional forms must have existed,
+why do we not find them embedded in countless numbers in the crust of the
+earth? It will be much more convenient to discuss this question in the
+chapter on the Imperfection of the geological record; and I will here only
+state that I believe the answer mainly lies in the record being
+incomparably less perfect than is generally supposed; the imperfection of
+the record being chiefly due to organic beings not inhabiting {173}
+profound depths of the sea, and to their remains being embedded and
+preserved to a future age only in masses of sediment sufficiently thick and
+extensive to withstand an enormous amount of future degradation; and such
+fossiliferous masses can be accumulated only where much sediment is
+deposited on the shallow bed of the sea, whilst it slowly subsides. These
+contingencies will concur only rarely, and after enormously long intervals.
+Whilst the bed of the sea is stationary or is rising, or when very little
+sediment is being deposited, there will be blanks in our geological
+history. The crust of the earth is a vast museum; but the natural
+collections have been made only at intervals of time immensely remote.
+
+But it may be urged that when several closely-allied species inhabit the
+same territory we surely ought to find at the present time many
+transitional forms. Let us take a simple case: in travelling from north to
+south over a continent, we generally meet at successive intervals with
+closely allied or representative species, evidently filling nearly the same
+place in the natural economy of the land. These representative species
+often meet and interlock; and as the one becomes rarer and rarer, the other
+becomes more and more frequent, till the one replaces the other. But if we
+compare these species where they intermingle, they are generally as
+absolutely distinct from each other in every detail of structure as are
+specimens taken from the metropolis inhabited by each. By my theory these
+allied species have descended from a common parent; and during the process
+of modification, each has become adapted to the conditions of life of its
+own region, and has supplanted and exterminated its original parent and all
+the transitional varieties between its past and present states. Hence we
+ought not to expect at the {174} present time to meet with numerous
+transitional varieties in each region, though they must have existed there,
+and may be embedded there in a fossil condition. But in the intermediate
+region, having intermediate conditions of life, why do we not now find
+closely-linking intermediate varieties? This difficulty for a long time
+quite confounded me. But I think it can be in large part explained.
+
+In the first place we should be extremely cautious in inferring, because an
+area is now continuous, that it has been continuous during a long period.
+Geology would lead us to believe that almost every continent has been
+broken up into islands even during the later tertiary periods; and in such
+islands distinct species might have been separately formed without the
+possibility of intermediate varieties existing in the intermediate zones.
+By changes in the form of the land and of climate, marine areas now
+continuous must often have existed within recent times in a far less
+continuous and uniform condition than at present. But I will pass over this
+way of escaping from the difficulty; for I believe that many perfectly
+defined species have been formed on strictly continuous areas; though I do
+not doubt that the formerly broken condition of areas now continuous has
+played an important part in the formation of new species, more especially
+with freely-crossing and wandering animals.
+
+In looking at species as they are now distributed over a wide area, we
+generally find them tolerably numerous over a large territory, then
+becoming somewhat abruptly rarer and rarer on the confines, and finally
+disappearing. Hence the neutral territory between two representative
+species is generally narrow in comparison with the territory proper to
+each. We see the same fact in ascending mountains, and sometimes {175} it
+is quite remarkable how abruptly, as Alph. de Candolle has observed, a
+common alpine species disappears. The same fact has been noticed by E.
+Forbes in sounding the depths of the sea with the dredge. To those who look
+at climate and the physical conditions of life as the all-important
+elements of distribution, these facts ought to cause surprise, as climate
+and height or depth graduate away insensibly. But when we bear in mind that
+almost every species, even in its metropolis, would increase immensely in
+numbers, were it not for other competing species; that nearly all either
+prey on or serve as prey for others; in short, that each organic being is
+either directly or indirectly related in the most important manner to other
+organic beings, we must see that the range of the inhabitants of any
+country by no means exclusively depends on insensibly changing physical
+conditions, but in large part on the presence of other species, on which it
+depends, or by which it is destroyed, or with which it comes into
+competition; and as these species are already defined objects (however they
+may have become so), not blending one into another by insensible
+gradations, the range of any one species, depending as it does on the range
+of others, will tend to be sharply defined. Moreover, each species on the
+confines of its range, where it exists in lessened numbers, will, during
+fluctuations in the number of its enemies or of its prey, or in the
+seasons, be extremely liable to utter extermination; and thus its
+geographical range will come to be still more sharply defined.
+
+If I am right in believing that allied or representative species, when
+inhabiting a continuous area, are generally so distributed that each has a
+wide range, with a comparatively narrow neutral territory between them, in
+which they become rather suddenly rarer and rarer; then, as varieties do
+not essentially differ from species, {176} the same rule will probably
+apply to both; and if we in imagination adapt a varying species to a very
+large area, we shall have to adapt two varieties to two large areas, and a
+third variety to a narrow intermediate zone. The intermediate variety,
+consequently, will exist in lesser numbers from inhabiting a narrow and
+lesser area; and practically, as far as I can make out, this rule holds
+good with varieties in a state of nature. I have met with striking
+instances of the rule in the case of varieties intermediate between
+well-marked varieties in the genus Balanus. And it would appear from
+information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that
+generally when varieties intermediate between two other forms occur, they
+are much rarer numerically than the forms which they connect. Now, if we
+may trust these facts and inferences, and therefore conclude that varieties
+linking two other varieties together have generally existed in lesser
+numbers than the forms which they connect, then, I think, we can understand
+why intermediate varieties should not endure for very long periods;--why as
+a general rule they should be exterminated and disappear, sooner than the
+forms which they originally linked together.
+
+For any form existing in lesser numbers would, as already remarked, run a
+greater chance of being exterminated than one existing in large numbers;
+and in this particular case the intermediate form would be eminently liable
+to the inroads of closely allied forms existing on both sides of it. But a
+far more important consideration, as I believe, is that, during the process
+of further modification, by which two varieties are supposed on my theory
+to be converted and perfected into two distinct species, the two which
+exist in larger numbers from inhabiting larger areas, will have a great
+advantage over the intermediate variety, which exists {177} in smaller
+numbers in a narrow and intermediate zone. For forms existing in larger
+numbers will always have a better chance, within any given period, of
+presenting further favourable variations for natural selection to seize on,
+than will the rarer forms which exist in lesser numbers. Hence, the more
+common forms, in the race for life, will tend to beat and supplant the less
+common forms, for these will be more slowly modified and improved. It is
+the same principle which, as I believe, accounts for the common species in
+each country, as shown in the second chapter, presenting on an average a
+greater number of well-marked varieties than do the rarer species. I may
+illustrate what I mean by supposing three varieties of sheep to be kept,
+one adapted to an extensive mountainous region; a second to a comparatively
+narrow, hilly tract; and a third to wide plains at the base; and that the
+inhabitants are all trying with equal steadiness and skill to improve their
+stocks by selection; the chances in this case will be strongly in favour of
+the great holders on the mountains or on the plains improving their breeds
+more quickly than the small holders on the intermediate narrow, hilly
+tract; and consequently the improved mountain or plain breed will soon take
+the place of the less improved hill breed; and thus the two breeds, which
+originally existed in greater numbers, will come into close contact with
+each other, without the interposition of the supplanted, intermediate
+hill-variety.
+
+To sum up, I believe that species come to be tolerably well-defined
+objects, and do not at any one period present an inextricable chaos of
+varying and intermediate links: firstly, because new varieties are very
+slowly formed, for variation is a very slow process, and natural selection
+can do nothing until favourable {178} variations chance to occur, and until
+a place in the natural polity of the country can be better filled by some
+modification of some one or more of its inhabitants. And such new places
+will depend on slow changes of climate, or on the occasional immigration of
+new inhabitants, and, probably, in a still more important degree, on some
+of the old inhabitants becoming slowly modified, with the new forms thus
+produced and the old ones acting and reacting on each other. So that, in
+any one region and at any one time, we ought only to see a few species
+presenting slight modifications of structure in some degree permanent; and
+this assuredly we do see.
+
+Secondly, areas now continuous must often have existed within the recent
+period in isolated portions, in which many forms, more especially amongst
+the classes which unite for each birth and wander much, may have separately
+been rendered sufficiently distinct to rank as representative species. In
+this case, intermediate varieties between the several representative
+species and their common parent, must formerly have existed in each broken
+portion of the land, but these links will have been supplanted and
+exterminated during the process of natural selection, so that they will no
+longer exist in a living state.
+
+Thirdly, when two or more varieties have been formed in different portions
+of a strictly continuous area, intermediate varieties will, it is probable,
+at first have been formed in the intermediate zones, but they will
+generally have had a short duration. For these intermediate varieties will,
+from reasons already assigned (namely from what we know of the actual
+distribution of closely allied or representative species, and likewise of
+acknowledged varieties), exist in the intermediate zones in lesser numbers
+than the varieties which they {179} tend to connect. From this cause alone
+the intermediate varieties will be liable to accidental extermination; and
+during the process of further modification through natural selection, they
+will almost certainly be beaten and supplanted by the forms which they
+connect; for these from existing in greater numbers will, in the aggregate,
+present more variation, and thus be further improved through natural
+selection and gain further advantages.
+
+Lastly, looking not to any one time, but to all time, if my theory be true,
+numberless intermediate varieties, linking most closely all the species of
+the same group together, must assuredly have existed; but the very process
+of natural selection constantly tends, as has been so often remarked, to
+exterminate the parent-forms and the intermediate links. Consequently
+evidence of their former existence could be found only amongst fossil
+remains, which are preserved, as we shall in a future chapter attempt to
+show, in an extremely imperfect and intermittent record.
+
+
+
+_On the origin and transitions of organic beings with peculiar habits and
+structure._--It has been asked by the opponents of such views as I hold,
+how, for instance, a land carnivorous animal could have been converted into
+one with aquatic habits; for how could the animal in its transitional state
+have subsisted? It would be easy to show that within the same group
+carnivorous animals exist having every intermediate grade between truly
+aquatic and strictly terrestrial habits; and as each exists by a struggle
+for life, it is clear that each is well adapted in its habits to its place
+in nature. Look at the Mustela vison of North America, which has webbed
+feet and which resembles an otter in its fur, short legs, and form of tail;
+during summer this animal {180} dives for and preys on fish, but during the
+long winter it leaves the frozen waters, and preys like other polecats on
+mice and land animals. If a different case had been taken, and it had been
+asked how an insectivorous quadruped could possibly have been converted
+into a flying bat, the question would have been far more difficult, and I
+could have given no answer. Yet I think such difficulties have very little
+weight.
+
+Here, as on other occasions, I lie under a heavy disadvantage, for out of
+the many striking cases which I have collected, I can give only one or two
+instances of transitional habits and structures in closely allied species
+of the same genus; and of diversified habits, either constant or
+occasional, in the same species. And it seems to me that nothing less than
+a long list of such cases is sufficient to lessen the difficulty in any
+particular case like that of the bat.
+
+Look at the family of squirrels; here we have the finest gradation from
+animals with their tails only slightly flattened, and from others, as Sir
+J. Richardson has remarked, with the posterior part of their bodies rather
+wide and with the skin on their flanks rather full, to the so-called flying
+squirrels; and flying squirrels have their limbs and even the base of the
+tail united by a broad expanse of skin, which serves as a parachute and
+allows them to glide through the air to an astonishing distance from tree
+to tree. We cannot doubt that each structure is of use to each kind of
+squirrel in its own country, by enabling it to escape birds or beasts of
+prey, or to collect food more quickly, or, as there is reason to believe,
+by lessening the danger from occasional falls. But it does not follow from
+this fact that the structure of each squirrel is the best that it is
+possible to conceive under all natural conditions. Let the climate and
+vegetation change, let other competing {181} rodents or new beasts of prey
+immigrate, or old ones become modified, and all analogy would lead us to
+believe that some at least of the squirrels would decrease in numbers or
+become exterminated, unless they also became modified and improved in
+structure in a corresponding manner. Therefore, I can see no difficulty,
+more especially under changing conditions of life, in the continued
+preservation of individuals with fuller and fuller flank-membranes, each
+modification being useful, each being propagated, until by the accumulated
+effects of this process of natural selection, a perfect so-called flying
+squirrel was produced.
+
+Now look at the Galeopithecus or flying lemur, which formerly was falsely
+ranked amongst bats. It has an extremely wide flank-membrane, stretching
+from the corners of the jaw to the tail, and including the limbs and the
+elongated fingers: the flank-membrane is, also, furnished with an extensor
+muscle. Although no graduated links of structure, fitted for gliding
+through the air, now connect the Galeopithecus with the other Lemuridæ, yet
+I see no difficulty in supposing that such links formerly existed, and that
+each had been formed by the same steps as in the case of the less perfectly
+gliding squirrels; and that each grade of structure was useful to its
+possessor. Nor can I see any insuperable difficulty in further believing it
+possible that the membrane-connected fingers and forearm of the
+Galeopithecus might be greatly lengthened by natural selection; and this,
+as far as the organs of flight are concerned, would convert it into a bat.
+In bats which have the wing-membrane extended from the top of the shoulder
+to the tail, including the hind-legs, we perhaps see traces of an apparatus
+originally constructed for gliding through the air rather than for flight.
+{182}
+
+If about a dozen genera of birds had become extinct or were unknown, who
+would have ventured to have surmised that birds might have existed which
+used their wings solely as flappers, like the logger-headed duck
+(Micropterus of Eyton); as fins in the water and front legs on the land,
+like the penguin; as sails, like the ostrich; and functionally for no
+purpose, like the Apteryx. Yet the structure of each of these birds is good
+for it, under the conditions of life to which it is exposed, for each has
+to live by a struggle; but it is not necessarily the best possible under
+all possible conditions. It must not be inferred from these remarks that
+any of the grades of wing-structure here alluded to, which perhaps may all
+have resulted from disuse, indicate the natural steps by which birds have
+acquired their perfect power of flight; but they serve, at least, to show
+what diversified means of transition are possible.
+
+Seeing that a few members of such water-breathing classes as the Crustacea
+and Mollusca are adapted to live on the land; and seeing that we have
+flying birds and mammals, flying insects of the most diversified types, and
+formerly had flying reptiles, it is conceivable that flying-fish, which now
+glide far through the air, slightly rising and turning by the aid of their
+fluttering fins, might have been modified into perfectly winged animals. If
+this had been effected, who would have ever imagined that in an early
+transitional state they had been inhabitants of the open ocean, and had
+used their incipient organs of flight exclusively, as far as we know, to
+escape being devoured by other fish?
+
+When we see any structure highly perfected for any particular habit, as the
+wings of a bird for flight, we should bear in mind that animals displaying
+early {183} transitional grades of the structure will seldom continue to
+exist to the present day, for they will have been supplanted by the very
+process of perfection through natural selection. Furthermore, we may
+conclude that transitional grades between structures fitted for very
+different habits of life will rarely have been developed at an early period
+in great numbers and under many subordinate forms. Thus, to return to our
+imaginary illustration of the flying-fish, it does not seem probable that
+fishes capable of true flight would have been developed under many
+subordinate forms, for taking prey of many kinds in many ways, on the land
+and in the water, until their organs of flight had come to a high stage of
+perfection, so as to have given them a decided advantage over other animals
+in the battle for life. Hence the chance of discovering species with
+transitional grades of structure in a fossil condition will always be less,
+from their having existed in lesser numbers, than in the case of species
+with fully developed structures.
+
+I will now give two or three instances of diversified and of changed habits
+in the individuals of the same species. When either case occurs, it would
+be easy for natural selection to fit the animal, by some modification of
+its structure, for its changed habits, or exclusively for one of its
+several different habits. But it is difficult to tell, and immaterial for
+us, whether habits generally change first and structure afterwards; or
+whether slight modifications of structure lead to changed habits; both
+probably often change almost simultaneously. Of cases of changed habits it
+will suffice merely to allude to that of the many British insects which now
+feed on exotic plants, or exclusively on artificial substances. Of
+diversified habits innumerable instances could be given: I have often
+watched a tyrant flycatcher (Saurophagus sulphuratus) in South America,
+hovering over one spot {184} and then proceeding to another, like a
+kestrel, and at other times standing stationary on the margin of water, and
+then dashing like a kingfisher at a fish. In our own country the larger
+titmouse (Parus major) may be seen climbing branches, almost like a
+creeper; it often, like a shrike, kills small birds by blows on the head;
+and I have many times seen and heard it hammering the seeds of the yew on a
+branch, and thus breaking them like a nuthatch. In North America the black
+bear was seen by Hearne swimming for hours with widely open mouth, thus
+catching, almost like a whale, insects in the water.
+
+As we sometimes see individuals of a species following habits widely
+different from those of their own species and of the other species of the
+same genus, we might expect, on my theory, that such individuals would
+occasionally have given rise to new species, having anomalous habits, and
+with their structure either slightly or considerably modified from that of
+their proper type. And such instances do occur in nature. Can a more
+striking instance of adaptation be given than that of a woodpecker for
+climbing trees and for seizing insects in the chinks of the bark? Yet in
+North America there are woodpeckers which feed largely on fruit, and others
+with elongated wings which chase insects on the wing; and on the plains of
+La Plata, where not a tree grows, there is a woodpecker, which in every
+essential part of its organisation, even in its colouring, in the harsh
+tone of its voice, and undulatory flight, told me plainly of its close
+blood-relationship to our common species; yet it is a woodpecker which
+never climbs a tree!
+
+Petrels are the most aërial and oceanic of birds, yet in the quiet Sounds
+of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its
+astonishing power of diving, its manner of swimming, and of flying when
+{185} unwillingly it takes flight, would be mistaken by any one for an auk
+or grebe; nevertheless, it is essentially a petrel, but with many parts of
+its organisation profoundly modified. On the other hand, the acutest
+observer by examining the dead body of the water-ouzel would never have
+suspected its sub-aquatic habits; yet this anomalous member of the strictly
+terrestrial thrush family wholly subsists by diving,--grasping the stones
+with its feet and using its wings under water.
+
+He who believes that each being has been created as we now see it, must
+occasionally have felt surprise when he has met with an animal having
+habits and structure not at all in agreement. What can be plainer than that
+the webbed feet of ducks and geese are formed for swimming? yet there are
+upland geese with webbed feet which rarely or never go near the water; and
+no one except Audubon has seen the frigate-bird, which has all its four
+toes webbed, alight on the surface of the sea. On the other hand grebes and
+coots are eminently aquatic, although their toes are only bordered by
+membrane. What seems plainer than that the long toes of grallatores are
+formed for walking over swamps and floating plants, yet the water-hen is
+nearly as aquatic as the coot; and the landrail nearly as terrestrial as
+the quail or partridge. In such cases, and many others could be given,
+habits have changed without a corresponding change of structure. The webbed
+feet of the upland goose may be said to have become rudimentary in
+function, though not in structure. In the frigate-bird, the deeply-scooped
+membrane between the toes shows that structure has begun to change.
+
+He who believes in separate and innumerable acts of creation will say, that
+in these cases it has pleased the Creator to cause a being of one type to
+take the place of one of another type; but this seems to me only {186}
+restating the fact in dignified language. He who believes in the struggle
+for existence and in the principle of natural selection, will acknowledge
+that every organic being is constantly endeavouring to increase in numbers;
+and that if any one being vary ever so little, either in habits or
+structure, and thus gain an advantage over some other inhabitant of the
+country, it will seize on the place of that inhabitant, however different
+it may be from its own place. Hence it will cause him no surprise that
+there should be geese and frigate-birds with webbed feet, living on the dry
+land or most rarely alighting on the water; that there should be long-toed
+corncrakes living in meadows instead of in swamps; that there should be
+woodpeckers where not a tree grows; that there should be diving thrushes,
+and petrels with the habits of auks.
+
+
+
+_Organs of extreme perfection and complication._--To suppose that the eye,
+with all its inimitable contrivances for adjusting the focus to different
+distances, for admitting different amounts of light, and for the correction
+of spherical and chromatic aberration, could have been formed by natural
+selection, seems, I freely confess, absurd in the highest possible degree.
+Yet reason tells me, that if numerous gradations from a perfect and complex
+eye to one very imperfect and simple, each grade being useful to its
+possessor, can be shown to exist; if further, the eye does vary ever so
+slightly, and the variations be inherited, which is certainly the case; and
+if any variation or modification in the organ be ever useful to an animal
+under changing conditions of life, then the difficulty of believing that a
+perfect and complex eye could be formed by natural selection, though
+insuperable by our imagination, can hardly be considered real. How a nerve
+comes to be sensitive to {187} light, hardly concerns us more than how life
+itself first originated; but I may remark that several facts make me
+suspect that any sensitive nerve may be rendered sensitive to light, and
+likewise to those coarser vibrations of the air which produce sound.
+
+In looking for the gradations by which an organ in any species has been
+perfected, we ought to look exclusively to its lineal ancestors; but this
+is scarcely ever possible, and we are forced in each case to look to
+species of the same group, that is to the collateral descendants from the
+same original parent-form, in order to see what gradations are possible,
+and for the chance of some gradations having been transmitted from the
+earlier stages of descent, in an unaltered or little altered condition.
+Amongst existing Vertebrata, we find but a small amount of gradation in the
+structure of the eye, and from fossil species we can learn nothing on this
+head. In this great class we should probably have to descend far beneath
+the lowest known fossiliferous stratum to discover the earlier stages, by
+which the eye has been perfected.
+
+In the Articulata we can commence a series with an optic nerve merely
+coated with pigment, and without any other mechanism; and from this low
+stage, numerous gradations of structure, branching off in two fundamentally
+different lines, can be shown to exist, until we reach a moderately high
+stage of perfection. In certain crustaceans, for instance, there is a
+double cornea, the inner one divided into facets, within each of which
+there is a lens-shaped swelling. In other crustaceans the transparent cones
+which are coated by pigment, and which properly act only by excluding
+lateral pencils of light, are convex at their upper ends and must act by
+convergence; and at their lower ends there seems to be an imperfect
+vitreous substance. {188} With these facts, here far too briefly and
+imperfectly given, which show that there is much graduated diversity in the
+eyes of living crustaceans, and bearing in mind how small the number of
+living animals is in proportion to those which have become extinct, I can
+see no very great difficulty (not more than in the case of many other
+structures) in believing that natural selection has converted the simple
+apparatus of an optic nerve merely coated with pigment and invested by
+transparent membrane, into an optical instrument as perfect as is possessed
+by any member of the great Articulate class.
+
+He who will go thus far, if he find on finishing this treatise that large
+bodies of facts, otherwise inexplicable, can be explained by the theory of
+descent, ought not to hesitate to go further, and to admit that a structure
+even as perfect as the eye of an eagle might be formed by natural
+selection, although in this case he does not know any of the transitional
+grades. His reason ought to conquer his imagination; though I have felt the
+difficulty far too keenly to be surprised at any degree of hesitation in
+extending the principle of natural selection to such startling lengths.
+
+It is scarcely possible to avoid comparing the eye to a telescope. We know
+that this instrument has been perfected by the long-continued efforts of
+the highest human intellects; and we naturally infer that the eye has been
+formed by a somewhat analogous process. But may not this inference be
+presumptuous? Have we any right to assume that the Creator works by
+intellectual powers like those of man? If we must compare the eye to an
+optical instrument, we ought in imagination to take a thick layer of
+transparent tissue, with a nerve sensitive to light beneath, and then
+suppose every part of this layer to be continually changing {189} slowly in
+density, so as to separate into layers of different densities and
+thicknesses, placed at different distances from each other, and with the
+surfaces of each layer slowly changing in form. Further we must suppose
+that there is a power always intently watching each slight accidental
+alteration in the transparent layers; and carefully selecting each
+alteration which, under varied circumstances, may in any way, or in any
+degree, tend to produce a distincter image. We must suppose each new state
+of the instrument to be multiplied by the million; and each to be preserved
+till a better be produced, and then the old ones to be destroyed. In living
+bodies, variation will cause the slight alterations, generation will
+multiply them almost infinitely, and natural selection will pick out with
+unerring skill each improvement. Let this process go on for millions on
+millions of years; and during each year on millions of individuals of many
+kinds; and may we not believe that a living optical instrument might thus
+be formed as superior to one of glass, as the works of the Creator are to
+those of man?
+
+If it could be demonstrated that any complex organ existed, which could not
+possibly have been formed by numerous, successive, slight modifications, my
+theory would absolutely break down. But I can find out no such case. No
+doubt many organs exist of which we do not know the transitional grades,
+more especially if we look to much-isolated species, round which, according
+to my theory, there has been much extinction. Or again, if we look to an
+organ common to all the members of a large class, for in this latter case
+the organ must have been first formed at an extremely remote period, since
+which all the many members of the class have been developed; and in order
+to discover the early transitional grades through which the organ has {190}
+passed, we should have to look to very ancient ancestral forms, long since
+become extinct.
+
+We should be extremely cautious in concluding that an organ could not have
+been formed by transitional gradations of some kind. Numerous cases could
+be given amongst the lower animals of the same organ performing at the same
+time wholly distinct functions; thus the alimentary canal respires,
+digests, and excretes in the larva of the dragon-fly and in the fish
+Cobites. In the Hydra, the animal may be turned inside out, and the
+exterior surface will then digest and the stomach respire. In such cases
+natural selection might easily specialise, if any advantage were thus
+gained, a part or organ, which had performed two functions, for one
+function alone, and thus wholly change its nature by insensible steps. Two
+distinct organs sometimes perform simultaneously the same function in the
+same individual; to give one instance, there are fish with gills or
+branchiæ that breathe the air dissolved in the water, at the same time that
+they breathe free air in their swimbladders, this latter organ having a
+ductus pneumaticus for its supply, and being divided by highly vascular
+partitions. In these cases one of the two organs might with ease be
+modified and perfected so as to perform all the work by itself, being aided
+during the process of modification by the other organ; and then this other
+organ might be modified for some other and quite distinct purpose, or be
+quite obliterated.
+
+The illustration of the swimbladder in fishes is a good one, because it
+shows us clearly the highly important fact that an organ originally
+constructed for one purpose, namely flotation, may be converted into one
+for a wholly different purpose, namely respiration. The swimbladder has,
+also, been worked in as an accessory to the auditory organs of certain
+fish, or, for I do not know {191} which view is now generally held, a part
+of the auditory apparatus has been worked in as a complement to the
+swimbladder. All physiologists admit that the swimbladder is homologous, or
+"ideally similar" in position and structure with the lungs of the higher
+vertebrate animals: hence there seems to me to be no great difficulty in
+believing that natural selection has actually converted a swimbladder into
+a lung, or organ used exclusively for respiration.
+
+I can, indeed, hardly doubt that all vertebrate animals having true lungs
+have descended by ordinary generation from an ancient prototype, of which
+we know nothing, furnished with a floating apparatus or swimbladder. We can
+thus, as I infer from Professor Owen's interesting description of these
+parts, understand the strange fact that every particle of food and drink
+which we swallow has to pass over the orifice of the trachea, with some
+risk of falling into the lungs, notwithstanding the beautiful contrivance
+by which the glottis is closed. In the higher Vertebrata the branchiæ have
+wholly disappeared--the slits on the sides of the neck and the loop-like
+course of the arteries still marking in the embryo their former position.
+But it is conceivable that the now utterly lost branchiæ might have been
+gradually worked in by natural selection for some quite distinct purpose:
+in the same manner as, on the view entertained by some naturalists that the
+branchiæ and dorsal scales of Annelids are homologous with the wings and
+wing-covers of insects, it is probable that organs which at a very ancient
+period served for respiration have been actually converted into organs of
+flight.
+
+In considering transitions of organs, it is so important to bear in mind
+the probability of conversion from one function to another, that I will
+give one more instance. Pedunculated cirripedes have two minute folds of
+skin, {192} called by me the ovigerous frena, which serve, through the
+means of a sticky secretion, to retain the eggs until they are hatched
+within the sack. These cirripedes have no branchiæ, the whole surface of
+the body and sack, including the small frena, serving for respiration. The
+Balanidæ or sessile cirripedes, on the other hand, have no ovigerous frena,
+the eggs lying loose at the bottom of the sack, in the well-enclosed shell;
+but they have large folded branchiæ. Now I think no one will dispute that
+the ovigerous frena in the one family are strictly homologous with the
+branchiæ of the other family; indeed, they graduate into each other.
+Therefore I do not doubt that little folds of skin, which originally served
+as ovigerous frena, but which, likewise, very slightly aided the act of
+respiration, have been gradually converted by natural selection into
+branchiæ, simply through an increase in their size and the obliteration of
+their adhesive glands. If all pedunculated cirripedes had become extinct,
+and they have already suffered far more extinction than have sessile
+cirripedes, who would ever have imagined that the branchiæ in this latter
+family had originally existed as organs for preventing the ova from being
+washed out of the sack?
+
+Although we must be extremely cautious in concluding that any organ could
+not possibly have been produced by successive transitional gradations, yet,
+undoubtedly, grave cases of difficulty occur, some of which will be
+discussed in my future work.
+
+One of the gravest is that of neuter insects, which are often very
+differently constructed from either the males or fertile females; but this
+case will be treated of in the next chapter. The electric organs of fishes
+offer another case of special difficulty; it is impossible to conceive by
+what steps these wondrous organs have been produced; but, as Owen and
+others have remarked, {193} their intimate structure closely resembles that
+of common muscle; and as it has lately been shown that Rays have an organ
+closely analogous to the electric apparatus, and yet do not, as Matteucci
+asserts, discharge any electricity, we must own that we are far too
+ignorant to argue that no transition of any kind is possible.
+
+The electric organs offer another and even more serious difficulty; for
+they occur in only about a dozen fishes, of which several are widely remote
+in their affinities. Generally when the same organ appears in several
+members of the same class, especially if in members having very different
+habits of life, we may attribute its presence to inheritance from a common
+ancestor; and its absence in some of the members to its loss through disuse
+or natural selection. But if the electric organs had been inherited from
+one ancient progenitor thus provided, we might have expected that all
+electric fishes would have been specially related to each other. Nor does
+geology at all lead to the belief that formerly most fishes had electric
+organs, which most of their modified descendants have lost. The presence of
+luminous organs in a few insects, belonging to different families and
+orders, offers a parallel case of difficulty. Other cases could be given;
+for instance in plants, the very curious contrivance of a mass of
+pollen-grains, borne on a foot-stalk with a sticky gland at the end, is the
+same in Orchis and Asclepias,--genera almost as remote as possible amongst
+flowering plants. In all these cases of two very distinct species furnished
+with apparently the same anomalous organ, it should be observed that,
+although the general appearance and function of the organ may be the same,
+yet some fundamental difference can generally be detected. I am inclined to
+believe that in nearly the same way as two men have sometimes independently
+hit on {194} the very same invention, so natural selection, working for the
+good of each being and taking advantage of analogous variations, has
+sometimes modified in very nearly the same manner two parts in two organic
+beings, which beings owe but little of their structure in common to
+inheritance from the same ancestor.
+
+Although in many cases it is most difficult to conjecture by what
+transitions organs could have arrived at their present state; yet,
+considering that the proportion of living and known forms to the extinct
+and unknown is very small, I have been astonished how rarely an organ can
+be named, towards which no transitional grade is known to lead. The truth
+of this remark is indeed shown by that old but somewhat exaggerated canon
+in natural history of "Natura non facit saltum." We meet with this
+admission in the writings of almost every experienced naturalist; or, as
+Milne Edwards has well expressed it, Nature is prodigal in variety, but
+niggard in innovation. Why, on the theory of Creation, should this be so?
+Why should all the parts and organs of many independent beings, each
+supposed to have been separately created for its proper place in nature, be
+so commonly linked together by graduated steps? Why should not Nature have
+taken a leap from structure to structure? On the theory of natural
+selection, we can clearly understand why she should not; for natural
+selection can act only by taking advantage of slight successive variations;
+she can never take a leap, but must advance by the shortest and slowest
+steps.
+
+
+
+_Organs of little apparent importance._--As natural selection acts by life
+and death,--by the preservation of individuals with any favourable
+variation, and by the destruction of those with any unfavourable deviation
+of structure,--I have sometimes felt much difficulty in {195} understanding
+the origin of simple parts, of which the importance does not seem
+sufficient to cause the preservation of successively varying individuals. I
+have sometimes felt as much difficulty, though of a very different kind, on
+this head, as in the case of an organ as perfect and complex as the eye.
+
+In the first place, we are much too ignorant in regard to the whole economy
+of any one organic being, to say what slight modifications would be of
+importance or not. In a former chapter I have given instances of most
+trifling characters, such as the down on fruit and the colour of its flesh,
+which, from determining the attacks of insects or from being correlated
+with constitutional differences, might assuredly be acted on by natural
+selection. The tail of the giraffe looks like an artificially constructed
+fly-flapper; and it seems at first incredible that this could have been
+adapted for its present purpose by successive slight modifications, each
+better and better, for so trifling an object as driving away flies; yet we
+should pause before being too positive even in this case, for we know that
+the distribution and existence of cattle and other animals in South America
+absolutely depends on their power of resisting the attacks of insects: so
+that individuals which could by any means defend themselves from these
+small enemies, would be able to range into new pastures and thus gain a
+great advantage. It is not that the larger quadrupeds are actually
+destroyed (except in some rare cases) by flies, but they are incessantly
+harassed and their strength reduced, so that they are more subject to
+disease, or not so well enabled in a coming dearth to search for food, or
+to escape from beasts of prey.
+
+Organs now of trifling importance have probably in some cases been of high
+importance to an early progenitor, and, after having been slowly perfected
+at a {196} former period, have been transmitted in nearly the same state,
+although now become of very slight use; and any actually injurious
+deviations in their structure will always have been checked by natural
+selection. Seeing how important an organ of locomotion the tail is in most
+aquatic animals, its general presence and use for many purposes in so many
+land animals, which in their lungs or modified swimbladders betray their
+aquatic origin, may perhaps be thus accounted for. A well-developed tail
+having been formed in an aquatic animal, it might subsequently come to be
+worked in for all sorts of purposes, as a fly-flapper, an organ of
+prehension, or as an aid in turning, as with the dog, though the aid must
+be slight, for the hare, with hardly any tail, can double quickly enough.
+
+In the second place, we may sometimes attribute importance to characters
+which are really of very little importance, and which have originated from
+quite secondary causes, independently of natural selection. We should
+remember that climate, food, &c., probably have some little direct
+influence on the organisation; that characters reappear from the law of
+reversion; that correlation of growth will have had a most important
+influence in modifying various structures; and finally, that sexual
+selection will often have largely modified the external characters of
+animals having a will, to give one male an advantage in fighting with
+another or in charming the females. Moreover when a modification of
+structure has primarily arisen from the above or other unknown causes, it
+may at first have been of no advantage to the species, but may subsequently
+have been taken advantage of by the descendants of the species under new
+conditions of life and with newly acquired habits.
+
+To give a few instances to illustrate these latter {197} remarks. If green
+woodpeckers alone had existed, and we did not know that there were many
+black and pied kinds, I dare say that we should have thought that the green
+colour was a beautiful adaptation to hide this tree-frequenting bird from
+its enemies; and consequently that it was a character of importance and
+might have been acquired through natural selection; as it is, I have no
+doubt that the colour is due to some quite distinct cause, probably to
+sexual selection. A trailing bamboo in the Malay Archipelago climbs the
+loftiest trees by the aid of exquisitely constructed hooks clustered around
+the ends of the branches, and this contrivance, no doubt, is of the highest
+service to the plant; but as we see nearly similar hooks on many trees
+which are not climbers, the hooks on the bamboo may have arisen from
+unknown laws of growth, and have been subsequently taken advantage of by
+the plant undergoing further modification and becoming a climber. The naked
+skin on the head of a vulture is generally looked at as a direct adaptation
+for wallowing in putridity; and so it may be, or it may possibly be due to
+the direct action of putrid matter; but we should be very cautious in
+drawing any such inference, when we see that the skin on the head of the
+clean-feeding male turkey is likewise naked. The sutures in the skulls of
+young mammals have been advanced as a beautiful adaptation for aiding
+parturition, and no doubt they facilitate, or may be indispensable for this
+act; but as sutures occur in the skulls of young birds and reptiles, which
+have only to escape from a broken egg, we may infer that this structure has
+arisen from the laws of growth, and has been taken advantage of in the
+parturition of the higher animals.
+
+We are profoundly ignorant of the causes producing slight and unimportant
+variations; and we are {198} immediately made conscious of this by
+reflecting on the differences in the breeds of our domesticated animals in
+different countries,--more especially in the less civilised countries where
+there has been but little artificial selection. Careful observers are
+convinced that a damp climate affects the growth of the hair, and that with
+the hair the horns are correlated. Mountain breeds always differ from
+lowland breeds; and a mountainous country would probably affect the hind
+limbs from exercising them more, and possibly even the form of the pelvis;
+and then by the law of homologous variation, the front limbs and even the
+head would probably be affected. The shape, also, of the pelvis might
+affect by pressure the shape of the head of the young in the womb. The
+laborious breathing necessary in high regions would, we have some reason to
+believe, increase the size of the chest; and again correlation would come
+into play. Animals kept by savages in different countries often have to
+struggle for their own subsistence, and would be exposed to a certain
+extent to natural selection, and individuals with slightly different
+constitutions would succeed best under different climates; and there is
+reason to believe that constitution and colour are correlated. A good
+observer, also, states that in cattle susceptibility to the attacks of
+flies is correlated with colour, as is the liability to be poisoned by
+certain plants; so that colour would be thus subjected to the action of
+natural selection. But we are far too ignorant to speculate on the relative
+importance of the several known and unknown laws of variation; and I have
+here alluded to them only to show that, if we are unable to account for the
+characteristic differences of our domestic breeds, which nevertheless we
+generally admit to have arisen through ordinary generation, we ought not to
+lay too much stress on our ignorance of the precise cause {199} of the
+slight analogous differences between species. I might have adduced for this
+same purpose the differences between the races of man, which are so
+strongly marked; I may add that some little light can apparently be thrown
+on the origin of these differences, chiefly through sexual selection of a
+particular kind, but without here entering on copious details my reasoning
+would appear frivolous.
+
+The foregoing remarks lead me to say a few words on the protest lately made
+by some naturalists, against the utilitarian doctrine that every detail of
+structure has been produced for the good of its possessor. They believe
+that very many structures have been created for beauty in the eyes of man,
+or for mere variety. This doctrine, if true, would be absolutely fatal to
+my theory. Yet I fully admit that many structures are of no direct use to
+their possessors. Physical conditions probably have had some little effect
+on structure, quite independently of any good thus gained. Correlation of
+growth has no doubt played a most important part, and a useful modification
+of one part will often have entailed on other parts diversified changes of
+no direct use. So again characters which formerly were useful, or which
+formerly had arisen from correlation of growth, or from other unknown
+cause, may reappear from the law of reversion, though now of no direct use.
+The effects of sexual selection, when displayed in beauty to charm the
+females, can be called useful only in rather a forced sense. But by far the
+most important consideration is that the chief part of the organisation of
+every being is simply due to inheritance; and consequently, though each
+being assuredly is well fitted for its place in nature, many structures now
+have no direct relation to the habits of life of each species. Thus, we can
+hardly believe that the webbed feet of the upland {200} goose or of the
+frigate-bird are of special use to these birds; we cannot believe that the
+same bones in the arm of the monkey, in the fore-leg of the horse, in the
+wing of the bat, and in the nipper of the seal, are of special use to these
+animals. We may safely attribute these structures to inheritance. But to
+the progenitor of the upland goose and of the frigate-bird, webbed feet no
+doubt were as useful as they now are to the most aquatic of existing birds.
+So we may believe that the progenitor of the seal had not a nipper, but a
+foot with five toes fitted for walking or grasping; and we may further
+venture to believe that the several bones in the limbs of the monkey,
+horse, and bat, which have been inherited from a common progenitor, were
+formerly of more special use to that progenitor, or its progenitors, than
+they now are to these animals having such widely diversified habits.
+Therefore we may infer that these several bones might have been acquired
+through natural selection, subjected formerly, as now, to the several laws
+of inheritance, reversion, correlation of growth, &c. Hence every detail of
+structure in every living creature (making some little allowance for the
+direct action of physical conditions) may be viewed, either as having been
+of special use to some ancestral form, or as being now of special use to
+the descendants of this form--either directly, or indirectly through the
+complex laws of growth.
+
+Natural selection cannot possibly produce any modification in any one
+species exclusively for the good of another species; though throughout
+nature one species incessantly takes advantage of, and profits by, the
+structure of another. But natural selection can and does often produce
+structures for the direct injury of other species, as we see in the fang of
+the adder, and in the ovipositor of the ichneumon, by which its eggs are
+{201} deposited in the living bodies of other insects. If it could be
+proved that any part of the structure of any one species had been formed
+for the exclusive good of another species, it would annihilate my theory,
+for such could not have been produced through natural selection. Although
+many statements may be found in works on natural history to this effect, I
+cannot find even one which seems to me of any weight. It is admitted that
+the rattlesnake has a poison-fang for its own defence and for the
+destruction of its prey; but some authors suppose that at the same time
+this snake is furnished with a rattle for its own injury, namely, to warn
+its prey to escape. I would almost as soon believe that the cat curls the
+end of its tail when preparing to spring, in order to warn the doomed
+mouse. But I have not space here to enter on this and other such cases.
+
+Natural selection will never produce in a being anything injurious to
+itself, for natural selection acts solely by and for the good of each. No
+organ will be formed, as Paley has remarked, for the purpose of causing
+pain or for doing an injury to its possessor. If a fair balance be struck
+between the good and evil caused by each part, each will be found on the
+whole advantageous. After the lapse of time, under changing conditions of
+life, if any part comes to be injurious, it will be modified; or if it be
+not so, the being will become extinct, as myriads have become extinct.
+
+Natural selection tends only to make each organic being as perfect as, or
+slightly more perfect than, the other inhabitants of the same country with
+which it has to struggle for existence. And we see that this is the degree
+of perfection attained under nature. The endemic productions of New
+Zealand, for instance, are perfect one compared with another; but they are
+now rapidly yielding before the advancing legions of plants {202} and
+animals introduced from Europe. Natural selection will not produce absolute
+perfection, nor do we always meet, as far as we can judge, with this high
+standard under nature. The correction for the aberration of light is said,
+on high authority, not to be perfect even in that most perfect organ, the
+eye. If our reason leads us to admire with enthusiasm a multitude of
+inimitable contrivances in nature, this same reason tells us, though we may
+easily err on both sides, that some other contrivances are less perfect.
+Can we consider the sting of the wasp or of the bee as perfect, which, when
+used against many attacking animals, cannot be withdrawn, owing to the
+backward serratures, and so inevitably causes the death of the insect by
+tearing out its viscera?
+
+If we look at the sting of the bee, as having originally existed in a
+remote progenitor as a boring and serrated instrument, like that in so many
+members of the same great order, and which has been modified but not
+perfected for its present purpose, with the poison originally adapted to
+cause galls subsequently intensified, we can perhaps understand how it is
+that the use of the sting should so often cause the insect's own death: for
+if on the whole the power of stinging be useful to the community, it will
+fulfil all the requirements of natural selection, though it may cause the
+death of some few members. If we admire the truly wonderful power of scent
+by which the males of many insects find their females, can we admire the
+production for this single purpose of thousands of drones, which are
+utterly useless to the community for any other end, and which are
+ultimately slaughtered by their industrious and sterile sisters? It may be
+difficult, but we ought to admire the savage instinctive hatred of the
+queen-bee, which urges her instantly to destroy the {203} young queens her
+daughters as soon as born, or to perish herself in the combat; for
+undoubtedly this is for the good of the community; and maternal love or
+maternal hatred, though the latter fortunately is most rare, is all the
+same to the inexorable principle of natural selection. If we admire the
+several ingenious contrivances, by which the flowers of the orchis and of
+many other plants are fertilised through insect agency, can we consider as
+equally perfect the elaboration by our fir-trees of dense clouds of pollen,
+in order that a few granules may be wafted by a chance breeze on to the
+ovules?
+
+
+
+_Summary of Chapter._--We have in this chapter discussed some of the
+difficulties and objections which may be urged against my theory. Many of
+them are very serious; but I think that in the discussion light has been
+thrown on several facts, which on the theory of independent acts of
+creation are utterly obscure. We have seen that species at any one period
+are not indefinitely variable, and are not linked together by a multitude
+of intermediate gradations, partly because the process of natural selection
+will always be very slow, and will act, at any one time, only on a very few
+forms; and partly because the very process of natural selection almost
+implies the continual supplanting and extinction of preceding and
+intermediate gradations. Closely allied species, now living on a continuous
+area, must often have been formed when the area was not continuous, and
+when the conditions of life did not insensibly graduate away from one part
+to another. When two varieties are formed in two districts of a continuous
+area, an intermediate variety will often be formed, fitted for an
+intermediate zone; but from reasons assigned, the intermediate variety will
+usually exist in lesser numbers than {204} the two forms which it connects;
+consequently the two latter, during the course of further modification,
+from existing in greater numbers, will have a great advantage over the less
+numerous intermediate variety, and will thus generally succeed in
+supplanting and exterminating it.
+
+We have seen in this chapter how cautious we should be in concluding that
+the most different habits of life could not graduate into each other; that
+a bat, for instance, could not have been formed by natural selection from
+an animal which at first could only glide through the air.
+
+We have seen that a species may under new conditions of life change its
+habits, or have diversified habits, with some habits very unlike those of
+its nearest congeners. Hence we can understand, bearing in mind that each
+organic being is trying to live wherever it can live, how it has arisen
+that there are upland geese with webbed feet, ground woodpeckers, diving
+thrushes, and petrels with the habits of auks.
+
+Although the belief that an organ so perfect as the eye could have been
+formed by natural selection, is more than enough to stagger any one; yet in
+the case of any organ, if we know of a long series of gradations in
+complexity, each good for its possessor, then, under changing conditions of
+life there is no logical impossibility in the acquirement of any
+conceivable degree of perfection through natural selection. In the cases in
+which we know of no intermediate or transitional states, we should be very
+cautious in concluding that none could have existed, for the homologies of
+many organs and their intermediate states show that wonderful metamorphoses
+in function are at least possible. For instance, a swim-bladder has
+apparently been converted into an air-breathing lung. The same organ having
+performed {205} simultaneously very different functions, and then having
+been specialised for one function; and two very distinct organs having
+performed at the same time the same function, the one having been perfected
+whilst aided by the other, must often have largely facilitated transitions.
+
+We are far too ignorant, in almost every case, to be enabled to assert that
+any part or organ is so unimportant for the welfare of a species, that
+modifications in its structure could not have been slowly accumulated by
+means of natural selection. But we may confidently believe that many
+modifications, wholly due to the laws of growth, and at first in no way
+advantageous to a species, have been subsequently taken advantage of by the
+still further modified descendants of this species. We may, also, believe
+that a part formerly of high importance has often been retained (as the
+tail of an aquatic animal by its terrestrial descendants), though it has
+become of such small importance that it could not, in its present state,
+have been acquired by natural selection,--a power which acts solely by the
+preservation of profitable variations in the struggle for life.
+
+Natural selection will produce nothing in one species for the exclusive
+good or injury of another; though it may well produce parts, organs, and
+excretions highly useful or even indispensable, or highly injurious to
+another species, but in all cases at the same time useful to the owner.
+Natural selection in each well-stocked country, must act chiefly through
+the competition of the inhabitants one with another, and consequently will
+produce perfection, or strength in the battle for life, only according to
+the standard of that country. Hence the inhabitants of one country,
+generally the smaller one, will often yield, as we see they do yield, to
+the inhabitants of another and generally larger country. For in {206} the
+larger country there will have existed more individuals, and more
+diversified forms, and the competition will have been severer, and thus the
+standard of perfection will have been rendered higher. Natural selection
+will not necessarily produce absolute perfection; nor, as far as we can
+judge by our limited faculties, can absolute perfection be everywhere
+found.
+
+On the theory of natural selection we can clearly understand the full
+meaning of that old canon in natural history, "Natura non facit saltum."
+This canon, if we look only to the present inhabitants of the world, is not
+strictly correct, but if we include all those of past times, it must by my
+theory be strictly true.
+
+It is generally acknowledged that all organic beings have been formed on
+two great laws--Unity of Type, and the Conditions of Existence. By unity of
+type is meant that fundamental agreement in structure, which we see in
+organic beings of the same class, and which is quite independent of their
+habits of life. On my theory, unity of type is explained by unity of
+descent. The expression of conditions of existence, so often insisted on by
+the illustrious Cuvier, is fully embraced by the principle of natural
+selection. For natural selection acts by either now adapting the varying
+parts of each being to its organic and inorganic conditions of life; or by
+having adapted them during long-past periods of time: the adaptations being
+aided in some cases by use and disuse, being slightly affected by the
+direct action of the external conditions of life, and being in all cases
+subjected to the several laws of growth. Hence, in fact, the law of the
+Conditions of Existence is the higher law; as it includes, through the
+inheritance of former adaptations, that of Unity of Type.
+
+       *       *       *       *       *
+
+
+{207}
+
+CHAPTER VII.
+
+INSTINCT.
+
+    Instincts comparable with habits, but different in their
+    origin--Instincts graduated--Aphides and ants--Instincts
+    variable--Domestic instincts, their origin--Natural instincts of the
+    cuckoo, ostrich, and parasitic bees--Slave-making-ants--Hive-bee, its
+    cell-making instinct--Difficulties on the theory of the Natural
+    Selection of instincts--Neuter or sterile insects--Summary.
+
+The subject of instinct might have been worked into the previous chapters;
+but I have thought that it would be more convenient to treat the subject
+separately, especially as so wonderful an instinct as that of the hive-bee
+making its cells will probably have occurred to many readers, as a
+difficulty sufficient to overthrow my whole theory. I must premise, that I
+have nothing to do with the origin of the primary mental powers, any more
+than I have with that of life itself. We are concerned only with the
+diversities of instinct and of the other mental qualities of animals within
+the same class.
+
+I will not attempt any definition of instinct. It would be easy to show
+that several distinct mental actions are commonly embraced by this term;
+but every one understands what is meant, when it is said that instinct
+impels the cuckoo to migrate and to lay her eggs in other birds' nests. An
+action, which we ourselves should require experience to enable us to
+perform, when performed by an animal, more especially by a very young one,
+without any experience, and when performed by many individuals in the same
+way, without their knowing for what purpose it is performed, is usually
+said to be instinctive. {208} But I could show that none of these
+characters of instinct are universal. A little dose, as Pierre Huber
+expresses it, of judgment or reason, often comes into play, even in animals
+very low in the scale of nature.
+
+Frederick Cuvier and several of the older metaphysicians have compared
+instinct with habit. This comparison gives, I think, a remarkably accurate
+notion of the frame of mind under which an instinctive action is performed,
+but not of its origin. How unconsciously many habitual actions are
+performed, indeed not rarely in direct opposition to our conscious will!
+yet they may be modified by the will or reason. Habits easily become
+associated with other habits, and with certain periods of time and states
+of the body. When once acquired, they often remain constant throughout
+life. Several other points of resemblance between instincts and habits
+could be pointed out. As in repeating a well-known song, so in instincts,
+one action follows another by a sort of rhythm; if a person be interrupted
+in a song, or in repeating anything by rote, he is generally forced to go
+back to recover the habitual train of thought: so P. Huber found it was
+with a caterpillar, which makes a very complicated hammock; for if he took
+a caterpillar which had completed its hammock up to, say, the sixth stage
+of construction, and put it into a hammock completed up only to the third
+stage, the caterpillar simply re-performed the fourth, fifth, and sixth
+stages of construction. If, however, a caterpillar were taken out of a
+hammock made up, for instance, to the third stage, and were put into one
+finished up to the sixth stage, so that much of its work, was already done
+for it, far from feeling the benefit of this, it was much embarrassed, and,
+in order to complete its hammock, seemed forced to start from the third
+stage, where it had left off, and thus tried to complete the already
+finished work. {209}
+
+If we suppose any habitual action to become inherited--and I think it can
+be shown that this does sometimes happen--then the resemblance between what
+originally was a habit and an instinct becomes so close as not to be
+distinguished. If Mozart, instead of playing the pianoforte at three years
+old with wonderfully little practice, had played a tune with no practice at
+all, he might truly be said to have done so instinctively. But it would be
+the most serious error to suppose that the greater number of instincts have
+been acquired by habit in one generation, and then transmitted by
+inheritance to succeeding generations. It can be clearly shown that the
+most wonderful instincts with which we are acquainted, namely, those of the
+hive-bee and of many ants, could not possibly have been thus acquired.
+
+It will be universally admitted that instincts are as important as
+corporeal structure for the welfare of each species, under its present
+conditions of life. Under changed conditions of life, it is at least
+possible that slight modifications of instinct might be profitable to a
+species; and if it can be shown that instincts do vary ever so little, then
+I can see no difficulty in natural selection preserving and continually
+accumulating variations of instinct to any extent that may be profitable.
+It is thus, as I believe, that all the most complex and wonderful instincts
+have originated. As modifications of corporeal structure arise from, and
+are increased by, use or habit, and are diminished or lost by disuse, so I
+do not doubt it has been with instincts. But I believe that the effects of
+habit are of quite subordinate importance to the effects of the natural
+selection of what may be called accidental variations of instincts;--that
+is of variations produced by the same unknown causes which produce slight
+deviations of bodily structure.
+
+No complex instinct can possibly be produced through {210} natural
+selection, except by the slow and gradual accumulation of numerous, slight,
+yet profitable, variations. Hence, as in the case of corporeal structures,
+we ought to find in nature, not the actual transitional gradations by which
+each complex instinct has been acquired--for these could be found only in
+the lineal ancestors of each species--but we ought to find in the
+collateral lines of descent some evidence of such gradations; or we ought
+at least to be able to show that gradations of some kind are possible; and
+this we certainly can do. I have been surprised to find, making allowance
+for the instincts of animals having been but little observed except in
+Europe and North America, and for no instinct being known amongst extinct
+species, how very generally gradations, leading to the most complex
+instincts, can be discovered. Changes of instinct may sometimes be
+facilitated by the same species having different instincts at different
+periods of life, or at different seasons of the year, or when placed under
+different circumstances &c.; in which case either one or the other instinct
+might be preserved by natural selection. And such instances of diversity of
+instinct in the same species can be shown to occur in nature.
+
+Again as in the case of corporeal structure, and conformably with my
+theory, the instinct of each species is good for itself, but has never, as
+far as we can judge, been produced for the exclusive good of others. One of
+the strongest instances of an animal apparently performing an action for
+the sole good of another, with which I am acquainted, is that of aphides
+voluntarily yielding their sweet excretion to ants: that they do so
+voluntarily, the following facts show. I removed all the ants from a group
+of about a dozen aphides on a dock-plant, and prevented their attendance
+during several hours. After this interval, I felt sure that the aphides
+{211} would want to excrete. I watched them for some time through a lens,
+but not one excreted; I then tickled and stroked them with a hair in the
+same manner, as well as I could, as the ants do with their antennæ; but not
+one excreted. Afterwards I allowed an ant to visit them, and it immediately
+seemed, by its eager way of running about, to be well aware what a rich
+flock it had discovered; it then began to play with its antennæ on the
+abdomen first of one aphis and then of another; and each aphis, as soon as
+it felt the antennæ, immediately lifted up its abdomen and excreted a
+limpid drop of sweet juice, which was eagerly devoured by the ant. Even the
+quite young aphides behaved in this manner, showing that the action was
+instinctive, and not the result of experience. But as the excretion is
+extremely viscid, it is probably a convenience to the aphides to have it
+removed; and therefore probably the aphides do not instinctively excrete
+for the sole good of the ants. Although I do not believe that any animal in
+the world performs an action for the exclusive good of another of a
+distinct species, yet each species tries to take advantage of the instincts
+of others, as each takes advantage of the weaker bodily structure of
+others. So again, in some few cases, certain instincts cannot be considered
+as absolutely perfect; but as details on this and other such points are not
+indispensable, they may be here passed over.
+
+As some degree of variation in instincts under a state of nature, and the
+inheritance of such variations, are indispensable for the action of natural
+selection, as many instances as possible ought to be here given; but want
+of space prevents me. I can only assert, that instincts certainly do
+vary--for instance, the migratory instinct, both in extent and direction,
+and in its total loss. So it is with the nests of birds, which vary partly
+{212} in dependence on the situations chosen, and on the nature and
+temperature of the country inhabited, but often from causes wholly unknown
+to us: Audubon has given several remarkable cases of differences in the
+nests of the same species in the northern and southern United States. Fear
+of any particular enemy is certainly an instinctive quality, as may be seen
+in nestling birds, though it is strengthened by experience, and by the
+sight of fear of the same enemy in other animals. But fear of man is slowly
+acquired, as I have elsewhere shown, by various animals inhabiting desert
+islands; and we may see an instance of this, even in England, in the
+greater wildness of all our large birds than of our small birds; for the
+large birds have been most persecuted by man. We may safely attribute the
+greater wildness of our large birds to this cause; for in uninhabited
+islands large birds are not more fearful than small; and the magpie, so
+wary in England, is tame in Norway, as is the hooded crow in Egypt.
+
+That the general disposition of individuals of the same species, born in a
+state of nature, is extremely diversified, can be shown by a multitude of
+facts. Several cases also, could be given, of occasional and strange habits
+in certain species, which might, if advantageous to the species, give rise,
+through natural selection, to quite new instincts. But I am well aware that
+these general statements, without facts given in detail, can produce but a
+feeble effect on the reader's mind. I can only repeat my assurance, that I
+do not speak without good evidence.
+
+The possibility, or even probability, of inherited variations of instinct
+in a state of nature will be strengthened by briefly considering a few
+cases under domestication. We shall thus also be enabled to see the
+respective parts which habit and the selection of {213} so-called
+accidental variations have played in modifying the mental qualities of our
+domestic animals. A number of curious and authentic instances could be
+given of the inheritance of all shades of disposition and tastes, and
+likewise of the oddest tricks, associated with certain frames of mind or
+periods of time. But let us look to the familiar case of the several breeds
+of dogs: it cannot be doubted that young pointers (I have myself seen a
+striking instance) will sometimes point and even back other dogs the very
+first time that they are taken out; retrieving is certainly in some degree
+inherited by retrievers; and a tendency to run round, instead of at, a
+flock of sheep, by shepherd-dogs. I cannot see that these actions,
+performed without experience by the young, and in nearly the same manner by
+each individual, performed with eager delight by each breed, and without
+the end being known,--for the young pointer can no more know that he points
+to aid his master, than the white butterfly knows why she lays her eggs on
+the leaf of the cabbage,--I cannot see that these actions differ
+essentially from true instincts. If we were to see one kind of wolf, when
+young and without any training, as soon as it scented its prey, stand
+motionless like a statue, and then slowly crawl forward with a peculiar
+gait; and another kind of wolf rushing round, instead of at, a herd of
+deer, and driving them to a distant point, we should assuredly call these
+actions instinctive. Domestic instincts, as they may be called, are
+certainly far less fixed or invariable than natural instincts; but they
+have been acted on by far less rigorous selection, and have been
+transmitted for an incomparably shorter period, under less fixed conditions
+of life.
+
+How strongly these domestic instincts, habits, and dispositions are
+inherited, and how curiously they become mingled, is well shown when
+different breeds of dogs are {214} crossed. Thus it is known that a cross
+with a bull-dog has affected for many generations the courage and obstinacy
+of greyhounds; and a cross with a greyhound has given to a whole family of
+shepherd-dogs a tendency to hunt hares. These domestic instincts, when thus
+tested by crossing, resemble natural instincts, which in a like manner
+become curiously blended together, and for a long period exhibit traces of
+the instincts of either parent: for example, Le Roy describes a dog, whose
+great-grandfather was a wolf, and this dog showed a trace of its wild
+parentage only in one way, by not coming in a straight line to his master
+when called.
+
+Domestic instincts are sometimes spoken of as actions which have become
+inherited solely from long-continued and compulsory habit, but this, I
+think, is not true. No one would ever have thought of teaching, or probably
+could have taught, the tumbler-pigeon to tumble,--an action which, as I
+have witnessed, is performed by young birds, that have never seen a pigeon
+tumble. We may believe that some one pigeon showed a slight tendency to
+this strange habit, and that the long-continued selection of the best
+individuals in successive generations made tumblers what they now are; and
+near Glasgow there are house-tumblers, as I hear from Mr. Brent, which
+cannot fly eighteen inches high without going head over heels. It may be
+doubted whether any one would have thought of training a dog to point, had
+not some one dog naturally shown a tendency in this line; and this is known
+occasionally to happen, as I once saw in a pure terrier: the act of
+pointing is probably, as many have thought, only the exaggerated pause of
+an animal preparing to spring on its prey. When the first tendency to point
+was once displayed, methodical selection and the inherited effects of
+compulsory training in each successive generation would soon complete the
+{215} work; and unconscious selection is still at work, as each man tries
+to procure, without intending to improve the breed, dogs which will stand
+and hunt best. On the other hand, habit alone in some cases has sufficed;
+no animal is more difficult to tame than the young of the wild rabbit;
+scarcely any animal is tamer than the young of the tame rabbit; but I do
+not suppose that domestic rabbits have ever been selected for tameness; and
+I presume that we must attribute the whole of the inherited change from
+extreme wildness to extreme tameness, simply to habit and long-continued
+close confinement.
+
+Natural instincts are lost under domestication: a remarkable instance of
+this is seen in those breeds of fowls which very rarely or never become
+"broody," that is, never wish to sit on their eggs. Familiarity alone
+prevents our seeing how universally and largely the minds of our domestic
+animals have been modified by domestication. It is scarcely possible to
+doubt that the love of man has become instinctive in the dog. All wolves,
+foxes, jackals, and species of the cat genus, when kept tame, are most
+eager to attack poultry, sheep, and pigs; and this tendency has been found
+incurable in dogs which have been brought home as puppies from countries,
+such as Tierra del Fuego and Australia, where the savages do not keep these
+domestic animals. How rarely, on the other hand, do our civilised dogs,
+even when quite young, require to be taught not to attack poultry, sheep,
+and pigs! No doubt they occasionally do make an attack, and are then
+beaten; and if not cured, they are destroyed; so that habit, with some
+degree of selection, has probably concurred in civilising by inheritance
+our dogs. On the other hand, young chickens have lost, wholly by habit,
+that fear of the dog and cat which no doubt was originally instinctive in
+them, in the same way as it is so plainly instinctive in {216} young
+pheasants, though reared under a hen. It is not that chickens have lost all
+fear, but fear only of dogs and cats, for if the hen gives the
+danger-chuckle, they will run (more especially young turkeys) from under
+her, and conceal themselves in the surrounding grass or thickets; and this
+is evidently done for the instinctive purpose of allowing, as we see in
+wild ground-birds, their mother to fly away. But this instinct retained by
+our chickens has become useless under domestication, for the mother-hen has
+almost lost by disuse the power of flight.
+
+Hence, we may conclude, that domestic instincts have been acquired and
+natural instincts have been lost partly by habit, and partly by man
+selecting and accumulating during successive generations, peculiar mental
+habits and actions, which at first appeared from what we must in our
+ignorance call an accident. In some cases compulsory habit alone has
+sufficed to produce such inherited mental changes; in other cases
+compulsory habit has done nothing, and all has been the result of
+selection, pursued both methodically and unconsciously; but in most cases,
+probably, habit and selection have acted together.
+
+We shall, perhaps, best understand how instincts in a state of nature have
+become modified by selection, by considering a few cases. I will select
+only three, out of the several which I shall have to discuss in my future
+work,--namely, the instinct which leads the cuckoo to lay her eggs in other
+birds' nests; the slave-making instinct of certain ants; and the
+comb-making power of the hive-bee; these two latter instincts have
+generally, and most justly, been ranked by naturalists as the most
+wonderful of all known instincts.
+
+It is now commonly admitted that the more immediate and final cause of the
+cuckoo's instinct is, that {217} she lays her eggs, not daily, but at
+intervals of two or three days; so that, if she were to make her own nest
+and sit on her own eggs, those first laid would have to be left for some
+time unincubated, or there would be eggs and young birds of different ages
+in the same nest. If this were the case, the process of laying and hatching
+might be inconveniently long, more especially as she has to migrate at a
+very early period; and the first hatched young would probably have to be
+fed by the male alone. But the American cuckoo is in this predicament; for
+she makes her own nest and has eggs and young successively hatched, all at
+the same time. It has been asserted that the American cuckoo occasionally
+lays her eggs in other birds' nests; but I hear on the high authority of
+Dr. Brewer, that this is a mistake. Nevertheless, I could give several
+instances of various birds which have been known occasionally to lay their
+eggs in other birds' nests. Now let us suppose that the ancient progenitor
+of our European cuckoo had the habits of the American cuckoo; but that
+occasionally she laid an egg in another bird's nest. If the old bird
+profited by this occasional habit, or if the young were made more vigorous
+by advantage having been taken of the mistaken maternal instinct of another
+bird, than by their own mother's care, encumbered as she can hardly fail to
+be by having eggs and young of different ages at the same time; then the
+old birds or the fostered young would gain an advantage. And analogy would
+lead me to believe, that the young thus reared would be apt to follow by
+inheritance the occasional and aberrant habit of their mother, and in their
+turn would be apt to lay their eggs in other birds' nests, and thus be
+successful in rearing their young. By a continued process of this nature, I
+believe that the strange instinct of our cuckoo could be, and has been,
+{218} generated. I may add that, according to Dr. Gray and to some other
+observers, the European cuckoo has not utterly lost all maternal love and
+care for her own offspring.
+
+The occasional habit of birds laying their eggs in other birds' nests,
+either of the same or of a distinct species, is not very uncommon with the
+Gallinaceæ; and this perhaps explains the origin of a singular instinct in
+the allied group of ostriches. For several hen ostriches, at least in the
+case of the American species, unite and lay first a few eggs in one nest
+and then in another; and these are hatched by the males. This instinct may
+probably be accounted for by the fact of the hens laying a large number of
+eggs; but, as in the case of the cuckoo, at intervals of two or three days.
+This instinct, however, of the American ostrich has not as yet been
+perfected; for a surprising number of eggs lie strewed over the plains, so
+that in one day's hunting I picked up no less than twenty lost and wasted
+eggs.
+
+Many bees are parasitic, and always lay their eggs in the nests of bees of
+other kinds. This case is more remarkable than that of the cuckoo; for
+these bees have not only their instincts but their structure modified in
+accordance with their parasitic habits; for they do not possess the
+pollen-collecting apparatus which would be necessary if they had to store
+food for their own young. Some species, likewise, of Sphegidæ (wasp-like
+insects) are parasitic on other species; and M. Fabre has lately shown good
+reason for believing that although the Tachytes nigra generally makes its
+own burrow and stores it with paralysed prey for its own larvæ to feed on,
+yet that when this insect finds a burrow already made and stored by another
+sphex, it takes advantage of the prize, and becomes for the occasion
+parasitic. In this case, as with the supposed case of the cuckoo, I can
+{219} see no difficulty in natural selection making an occasional habit
+permanent, if of advantage to the species, and if the insect whose nest and
+stored food are thus feloniously appropriated, be not thus exterminated.
+
+
+
+_Slave-making instinct._--This remarkable instinct was first discovered in
+the Formica (Polyerges) rufescens by Pierre Huber, a better observer even
+than his celebrated father. This ant is absolutely dependent on its slaves;
+without their aid, the species would certainly become extinct in a single
+year. The males and fertile females do no work. The workers or sterile
+females, though most energetic and courageous in capturing slaves, do no
+other work. They are incapable of making their own nests, or of feeding
+their own larvæ. When the old nest is found inconvenient, and they have to
+migrate, it is the slaves which determine the migration, and actually carry
+their masters in their jaws. So utterly helpless are the masters, that when
+Huber shut up thirty of them without a slave, but with plenty of the food
+which they like best, and with their larvae and pupæ to stimulate them to
+work, they did nothing; they could not even feed themselves, and many
+perished of hunger. Huber then introduced a single slave (F. fusca), and
+she instantly set to work, fed and saved the survivors; made some cells and
+tended the larvæ, and put all to rights. What can be more extraordinary
+than these well-ascertained facts? If we had not known of any other
+slave-making ant, it would have been hopeless to have speculated how so
+wonderful an instinct could have been perfected.
+
+Another species, Formica sanguinea, was likewise first discovered by P.
+Huber to be a slave-making ant. This species is found in the southern parts
+of England, and its habits have been attended to by Mr. F. Smith, of {220}
+the British Museum, to whom I am much indebted for information on this and
+other subjects. Although fully trusting to the statements of Huber and Mr.
+Smith, I tried to approach the subject in a sceptical frame of mind, as any
+one may well be excused for doubting the truth of so extraordinary and
+odious an instinct as that of making slaves. Hence I will give the
+observations which I have myself made, in some little detail. I opened
+fourteen nests of F. sanguinea, and found a few slaves in all. Males and
+fertile females of the slave-species (F. fusca) are found only in their own
+proper communities, and have never been observed in the nests of F.
+sanguinea. The slaves are black and not above half the size of their red
+masters, so that the contrast in their appearance is very great. When the
+nest is slightly disturbed, the slaves occasionally come out, and like
+their masters are much agitated and defend the nest: when the nest is much
+disturbed and the larvæ and pupæ are exposed, the slaves work energetically
+with their masters in carrying them away to a place of safety. Hence, it is
+clear, that the slaves feel quite at home. During the months of June and
+July, on three successive years, I have watched for many hours several
+nests in Surrey and Sussex, and never saw a slave either leave or enter a
+nest. As, during these months, the slaves are very few in number, I thought
+that they might behave differently when more numerous; but Mr. Smith
+informs me that he has watched the nests at various hours during May, June
+and August, both in Surrey and Hampshire, and has never seen the slaves,
+through present in large numbers in August, either leave or enter the nest.
+Hence he considers them as strictly household slaves. The masters, on the
+other hand, may be constantly seen bringing in materials for the nest, and
+food of all kinds. During the present year, however, in the month {221} of
+July, I came across a community with an unusually large stock of slaves,
+and I observed a few slaves mingled with their masters leaving the nest,
+and marching along the same road to a tall Scotch-fir-tree, twenty-five
+yards distant, which they ascended together, probably in search of aphides
+or cocci. According to Huber, who had ample opportunities for observation,
+in Switzerland the slaves habitually work with their masters in making the
+nest, and they alone open and close the doors in the morning and evening;
+and, as Huber expressly states, their principal office is to search for
+aphides. This difference in the usual habits of the masters and slaves in
+the two countries, probably depends merely on the slaves being captured in
+greater numbers in Switzerland than in England.
+
+One day I fortunately witnessed a migration of F. sanguinea from one nest
+to another, and it was a most interesting spectacle to behold the masters
+carefully carrying (instead of being carried by, as in the case of F.
+rufescens) their slaves in their jaws. Another day my attention was struck
+by about a score of the slave-makers haunting the same spot, and evidently
+not in search of food; they approached and were vigorously repulsed by an
+independent community of the slave-species (F. fusca); sometimes as many as
+three of these ants clinging to the legs of the slave-making F. sanguinea.
+The latter ruthlessly killed their small opponents, and carried their dead
+bodies as food to their nest, twenty-nine yards distant; but they were
+prevented from getting any pupæ to rear as slaves. I then dug up a small
+parcel of the pupæ of F. fusca from another nest, and put them down on a
+bare spot near the place of combat; they were eagerly seized, and carried
+off by the tyrants, who perhaps fancied that, after all, they had been
+victorious in their late combat. {222}
+
+At the same time I laid on the same place a small parcel of the pupæ of
+another species, F. flava, with a few of these little yellow ants still
+clinging to the fragments of the nest. This species is sometimes, though
+rarely, made into slaves, as has been described by Mr. Smith. Although so
+small a species, it is very courageous, and I have seen it ferociously
+attack other ants. In one instance I found to my surprise an independent
+community of F. flava under a stone beneath a nest of the slave-making F.
+sanguinea; and when I had accidentally disturbed both nests, the little
+ants attacked their big neighbours with surprising courage. Now I was
+curious to ascertain whether F. sanguinea could distinguish the pupæ of F.
+fusca, which they habitually make into slaves, from those of the little and
+furious F. flava, which they rarely capture, and it was evident that they
+did at once distinguish them: for we have seen that they eagerly and
+instantly seized the pupæ of F. fusca, whereas they were much terrified
+when they came across the pupæ, or even the earth from the nest of F.
+flava, and quickly ran away; but in about a quarter of an hour, shortly
+after all the little yellow ants had crawled away, they took heart and
+carried off the pupæ.
+
+One evening I visited another community of F. sanguinea, and found a number
+of these ants returning home and entering their nests, carrying the dead
+bodies of F. fusca (showing that it was not a migration) and numerous pupæ.
+I traced a long file of ants burthened with booty, for about forty yards,
+to a very thick clump of heath, whence I saw the last individual of F.
+sanguinea emerge, carrying a pupa; but I was not able to find the desolated
+nest in the thick heath. The nest, however, must have been close at hand,
+for two or three individuals of F. fusca were rushing about in the greatest
+{223} agitation, and one was perched motionless with its own pupa in its
+mouth on the top of a spray of heath, an image of despair, over its ravaged
+home.
+
+Such are the facts, though they did not need confirmation by me, in regard
+to the wonderful instinct of making slaves. Let it be observed what a
+contrast the instinctive habits of F. sanguinea present with those of the
+continental F. rufescens. The latter does not build its own nest, does not
+determine its own migrations, does not collect food for itself or its
+young, and cannot even feed itself: it is absolutely dependent on its
+numerous slaves. Formica sanguinea, on the other hand, possesses much fewer
+slaves, and in the early part of the summer extremely few: the masters
+determine when and where a new nest shall be formed, and when they migrate,
+the masters carry the slaves. Both in Switzerland and England the slaves
+seem to have the exclusive care of the larvæ, and the masters alone go on
+slave-making expeditions. In Switzerland the slaves and masters work
+together, making and bringing materials for the nest: both, but chiefly the
+slaves, tend, and milk as it may be called, their aphides; and thus both
+collect food for the community. In England the masters alone usually leave
+the nest to collect building materials and food for themselves, their
+slaves and larvæ. So that the masters in this country receive much less
+service from their slaves than they do in Switzerland.
+
+By what steps the instinct of F. sanguinea originated I will not pretend to
+conjecture. But as ants, which are not slave-makers, will, as I have seen,
+carry off pupæ of other species, if scattered near their nests, it is
+possible that such pupæ originally stored as food might become developed;
+and the foreign ants thus unintentionally reared would then follow their
+proper instincts, and do {224} what work they could. If their presence
+proved useful to the species which had seized them--if it were more
+advantageous to this species to capture workers than to procreate them--the
+habit of collecting pupae originally for food might by natural selection be
+strengthened and rendered permanent for the very different purpose of
+raising slaves. When the instinct was once acquired, if carried out to a
+much less extent even than in our British F. sanguinea, which, as we have
+seen, is less aided by its slaves than the same species in Switzerland, I
+can see no difficulty in natural selection increasing and modifying the
+instinct--always supposing each modification to be of use to the
+species--until an ant was formed as abjectly dependent on its slaves as is
+the Formica rufescens.
+
+
+
+_Cell-making instinct of the Hive-Bee._--I will not here enter on minute
+details on this subject, but will merely give an outline of the conclusions
+at which I have arrived. He must be a dull man who can examine the
+exquisite structure of a comb, so beautifully adapted to its end, without
+enthusiastic admiration. We hear from mathematicians that bees have
+practically solved a recondite problem, and have made their cells of the
+proper shape to hold the greatest possible amount of honey, with the least
+possible consumption of precious wax in their construction. It has been
+remarked that a skilful workman, with fitting tools and measures, would
+find it very difficult to make cells of wax of the true form, though this
+is perfectly effected by a crowd of bees working in a dark hive. Grant
+whatever instincts you please, and it seems at first quite inconceivable
+how they can make all the necessary angles and planes, or even perceive
+when they are correctly made. But the difficulty is not {225} nearly so
+great as it at first appears: all this beautiful work can be shown, I
+think, to follow from a few very simple instincts.
+
+I was led to investigate this subject by Mr. Waterhouse, who has shown that
+the form of the cell stands in close relation to the presence of adjoining
+cells; and the following view may, perhaps, be considered only as a
+modification of his theory. Let us look to the great principle of
+gradation, and see whether Nature does not reveal to us her method of work.
+At one end of a short series we have humble-bees, which use their old
+cocoons to hold honey, sometimes adding to them short tubes of wax, and
+likewise making separate and very irregular rounded cells of wax. At the
+other end of the series we have the cells of the hive-bee, placed in a
+double layer: each cell, as is well known, is an hexagonal prism, with the
+basal edges of its six sides bevelled so as to fit on to a pyramid, formed
+of three rhombs. These rhombs have certain angles, and the three which form
+the pyramidal base of a single cell on one side of the comb, enter into the
+composition of the bases of three adjoining cells on the opposite side. In
+the series between the extreme perfection of the cells of the hive-bee and
+the simplicity of those of the humble-bee, we have the cells of the Mexican
+Melipona domestica, carefully described and figured by Pierre Huber. The
+Melipona itself is intermediate in structure between the hive and humble
+bee, but more nearly related to the latter: it forms a nearly regular waxen
+comb of cylindrical cells, in which the young are hatched, and, in
+addition, some large cells of wax for holding honey. These latter cells are
+nearly spherical and of nearly equal sizes, and are aggregated into an
+irregular mass. But the important point to notice, is that these cells are
+always made at that degree of nearness to each other, that they would have
+{226} intersected or broken into each other, if the spheres had been
+completed; but this is never permitted, the bees building perfectly flat
+walls of wax between the spheres which thus tend to intersect. Hence each
+cell consists of an outer spherical portion and of two, three, or more
+perfectly flat surfaces, according as the cell adjoins two, three, or more
+other cells. When one cell comes into contact with three other cells,
+which, from the spheres being nearly of the same size, is very frequently
+and necessarily the case, the three flat surfaces are united into a
+pyramid; and this pyramid, as Huber has remarked, is manifestly a gross
+imitation of the three-sided pyramidal bases of the cell of the hive-bee.
+As in the cells of the hive-bee, so here, the three plane surfaces in any
+one cell necessarily enter into the construction of three adjoining cells.
+It is obvious that the Melipona saves wax by this manner of building; for
+the flat walls between the adjoining cells are not double, but are of the
+same thickness as the outer spherical portions, and yet each flat portion
+forms a part of two cells.
+
+Reflecting on this case, it occurred to me that if the Melipona had made
+its spheres at some given distance from each other, and had made them of
+equal sizes and had arranged them symmetrically in a double layer, the
+resulting structure would probably have been as perfect as the comb of the
+hive-bee. Accordingly I wrote to Professor Miller, of Cambridge, and this
+geometer has kindly read over the following statement, drawn up from his
+information, and tells me that it is strictly correct:--
+
+If a number of equal spheres be described with their centres placed in two
+parallel layers; with the centre of each sphere at the distance of radius ×
+[root]2, or radius × 1.41421 (or at some lesser distance), from the centres
+of the six surrounding spheres in the same {227} layer; and at the same
+distance from the centres of the adjoining spheres in the other and
+parallel layer; then, if planes of intersection between the several spheres
+in both layers be formed, there will result a double layer of hexagonal
+prisms united together by pyramidal bases formed of three rhombs; and the
+rhombs and the sides of the hexagonal prisms will have every angle
+identically the same with the best measurements which have been made of the
+cells of the hive-bee.
+
+Hence we may safely conclude that if we could slightly modify the instincts
+already possessed by the Melipona, and in themselves not very wonderful,
+this bee would make a structure as wonderfully perfect as that of the
+hive-bee. We must suppose the Melipona to make her cells truly spherical,
+and of equal sizes; and this would not be very surprising, seeing that she
+already does so to a certain extent, and seeing what perfectly cylindrical
+burrows in wood many insects can make, apparently by turning round on a
+fixed point. We must suppose the Melipona to arrange her cells in level
+layers, as she already does her cylindrical cells; and we must further
+suppose, and this is the greatest difficulty, that she can somehow judge
+accurately at what distance to stand from her fellow-labourers when several
+are making their spheres; but she is already so far enabled to judge of
+distance, that she always describes her spheres so as to intersect largely;
+and then she unites the points of intersection by perfectly flat surfaces.
+We have further to suppose, but this is no difficulty, that after hexagonal
+prisms have been formed by the intersection of adjoining spheres in the
+same layer, she can prolong the hexagon to any length requisite to hold the
+stock of honey; in the same way as the rude humble-bee adds cylinders of
+wax to the circular mouths of her old cocoons. By such {228} modifications
+of instincts in themselves not very wonderful,--hardly more wonderful than
+those which guide a bird to make its nest,--I believe that the hive-bee has
+acquired, through natural selection, her inimitable architectural powers.
+
+But this theory can be tested by experiment. Following the example of Mr.
+Tegetmeier, I separated two combs, and put between them a long, thick,
+square strip of wax: the bees instantly began to excavate minute circular
+pits in it; and as they deepened these little pits, they made them wider
+and wider until they were converted into shallow basins, appearing to the
+eye perfectly true or parts of a sphere, and of about the diameter of a
+cell. It was most interesting to me to observe that wherever several bees
+had begun to excavate these basins near together, they had begun their work
+at such a distance from each other, that by the time the basins had
+acquired the above stated width (_i.e._ about the width of an ordinary
+cell), and were in depth about one sixth of the diameter of the sphere of
+which they formed a part, the rims of the basins intersected or broke into
+each other. As soon as this occurred, the bees ceased to excavate, and
+began to build up flat walls of wax on the lines of intersection between
+the basins, so that each hexagonal prism was built upon the scalloped edge
+of a smooth basin, instead of on the straight edges of a three-sided
+pyramid as in the case of ordinary cells.
+
+I then put into the hive, instead of a thick, square piece of wax, a thin
+and narrow, knife-edged ridge, coloured with vermilion. The bees instantly
+began on both sides to excavate little basins near to each other, in the
+same way as before; but the ridge of wax was so thin, that the bottoms of
+the basins, if they had been excavated to the same depth as in the former
+{229} experiment, would have broken into each other from the opposite
+sides. The bees, however, did not suffer this to happen, and they stopped
+their excavations in due time; so that the basins, as soon as they had been
+a little deepened, came to have flat bottoms; and these flat bottoms,
+formed by thin little plates of the vermilion wax having been left
+ungnawed, were situated, as far as the eye could judge, exactly along the
+planes of imaginary intersection between the basins on the opposite sides
+of the ridge of wax. In parts, only little bits, in other parts, large
+portions of a rhombic plate had been left between the opposed basins, but
+the work, from the unnatural state of things, had not been neatly
+performed. The bees must have worked at very nearly the same rate on the
+opposite sides of the ridge of vermilion wax, as they circularly gnawed
+away and deepened the basins on both sides, in order to have succeeded in
+thus leaving flat plates between the basins, by stopping work along the
+intermediate planes or planes of intersection.
+
+Considering how flexible thin wax is, I do not see that there is any
+difficulty in the bees, whilst at work on the two sides of a strip of wax,
+perceiving when they have gnawed the wax away to the proper thinness, and
+then stopping their work. In ordinary combs it has appeared to me that the
+bees do not always succeed in working at exactly the same rate from the
+opposite sides; for I have noticed half-completed rhombs at the base of a
+just-commenced cell, which were slightly concave on one side, where I
+suppose that the bees had excavated too quickly, and convex on the opposed
+side, where the bees had worked less quickly. In one well-marked instance,
+I put the comb back into the hive, and allowed the bees to go on working
+for a short time, and again examined the cell, and I found that the rhombic
+{230} plate had been completed, and had become _perfectly flat_: it was
+absolutely impossible, from the extreme thinness of the little rhombic
+plate, that they could have effected this by gnawing away the convex side;
+and I suspect that the bees in such cases stand in the opposed cells and
+push and bend the ductile and warm wax (which as I have tried is easily
+done) into its proper intermediate plane, and thus flatten it.
+
+From the experiment of the ridge of vermilion wax, we can clearly see that
+if the bees were to build for themselves a thin wall of wax, they could
+make their cells of the proper shape, by standing at the proper distance
+from each other, by excavating at the same rate, and by endeavouring to
+make equal spherical hollows, but never allowing the spheres to break into
+each other. Now bees, as may be clearly seen by examining the edge of a
+growing comb, do make a rough, circumferential wall or rim all round the
+comb; and they gnaw into this from the opposite sides, always working
+circularly as they deepen each cell. They do not make the whole three-sided
+pyramidal base of any one cell at the same time, but only the one rhombic
+plate which stands on the extreme growing margin, or the two plates, as the
+case may be; and they never complete the upper edges of the rhombic plates,
+until the hexagonal walls are commenced. Some of these statements differ
+from those made by the justly celebrated elder Huber, but I am convinced of
+their accuracy; and if I had space, I could show that they are conformable
+with my theory.
+
+Huber's statement that the very first cell is excavated out of a little
+parallel-sided wall of wax, is not, as far as I have seen, strictly
+correct; the first commencement having always been a little hood of wax;
+but I will not here enter on these details. We see how important {231} a
+part excavation plays in the construction of the cells; but it would be a
+great error to suppose that the bees cannot build up a rough wall of wax in
+the proper position--that is, along the plane of intersection between two
+adjoining spheres. I have several specimens showing clearly that they can
+do this. Even in the rude circumferential rim or wall of wax round a
+growing comb, flexures may sometimes be observed, corresponding in position
+to the planes of the rhombic basal plates of future cells. But the rough
+wall of wax has in every case to be finished off, by being largely gnawed
+away on both sides. The manner in which the bees build is curious; they
+always make the first rough wall from ten to twenty times thicker than the
+excessively thin finished wall of the cell, which will ultimately be left.
+We shall understand how they work, by supposing masons first to pile up a
+broad ridge of cement, and then to begin cutting it away equally on both
+sides near the ground, till a smooth, very thin wall is left in the middle;
+the masons always piling up the cut-away cement, and adding fresh cement,
+on the summit of the ridge. We shall thus have a thin wall steadily growing
+upward; but always crowned by a gigantic coping. From all the cells, both
+those just commenced and those completed, being thus crowned by a strong
+coping of wax, the bees can cluster and crawl over the comb without
+injuring the delicate hexagonal walls, which are only about one
+four-hundredth of an inch in thickness; the plates of the pyramidal basis
+being about twice as thick. By this singular manner of building, strength
+is continually given to the comb, with the utmost ultimate economy of wax.
+
+It seems at first to add to the difficulty of understanding how the cells
+are made, that a multitude of bees all work together; one bee after working
+a short time at one cell going to another, so that, as Huber has stated,
+{232} a score of individuals work even at the commencement of the first
+cell. I was able practically to show this fact, by covering the edges of
+the hexagonal walls of a single cell, or the extreme margin of the
+circumferential rim of a growing comb, with an extremely thin layer of
+melted vermilion wax; and I invariably found that the colour was most
+delicately diffused by the bees--as delicately as a painter could have done
+with his brush--by atoms of the coloured wax having been taken from the
+spot on which it had been placed, and worked into the growing edges of the
+cells all round. The work of construction seems to be a sort of balance
+struck between many bees, all instinctively standing at the same relative
+distance from each other, all trying to sweep equal spheres, and then
+building up, or leaving ungnawed, the planes of intersection between these
+spheres. It was really curious to note in cases of difficulty, as when two
+pieces of comb met at an angle, how often the bees would pull down and
+rebuild in different ways the same cell, sometimes recurring to a shape
+which they had at first rejected.
+
+When bees have a place on which they can stand in their proper positions
+for working,--for instance, on a slip of wood, placed directly under the
+middle of a comb growing downwards so that the comb has to be built over
+one face of the slip--in this case the bees can lay the foundations of one
+wall of a new hexagon, in its strictly proper place, projecting beyond the
+other completed cells. It suffices that the bees should be enabled to stand
+at their proper relative distances from each other and from the walls of
+the last completed cells, and then, by striking imaginary spheres, they can
+build up a wall intermediate between two adjoining spheres; but, as far as
+I have seen, they never gnaw away and finish off the angles of a cell till
+a large part both of that cell and of {233} the adjoining cells has been
+built. This capacity in bees of laying down under certain circumstances a
+rough wall in its proper place between two just-commenced cells, is
+important, as it bears on a fact, which seems at first quite subversive of
+the foregoing theory; namely, that the cells on the extreme margin of
+wasp-combs are sometimes strictly hexagonal; but I have not space here to
+enter on this subject. Nor does there seem to me any great difficulty in a
+single insect (as in the case of a queen-wasp) making hexagonal cells, if
+she work alternately on the inside and outside of two or three cells
+commenced at the same time, always standing at the proper relative distance
+from the parts of the cells just begun, sweeping spheres or cylinders, and
+building up intermediate planes. It is even conceivable that an insect
+might, by fixing on a point at which to commence a cell, and then moving
+outside, first to one point, and then to five other points, at the proper
+relative distances from the central point and from each other, strike the
+planes of intersection, and so make an isolated hexagon: but I am not aware
+that any such case has been observed; nor would any good be derived from a
+single hexagon being built, as in its construction more materials would be
+required than for a cylinder.
+
+As natural selection acts only by the accumulation of slight modifications
+of structure or instinct, each profitable to the individual under its
+conditions of life, it may reasonably be asked, how a long and graduated
+succession of modified architectural instincts, all tending towards the
+present perfect plan of construction, could have profited the progenitors
+of the hive-bee? I think the answer is not difficult: it is known that bees
+are often hard pressed to get sufficient nectar; and I am informed by Mr.
+Tegetmeier that it has been experimentally found that no less than from
+twelve to fifteen pounds of dry sugar {234} are consumed by a hive of bees
+for the secretion of each pound of wax; to that a prodigious quantity of
+fluid nectar must be collected and consumed by the bees in a hive for the
+secretion of the wax necessary for the construction of their combs.
+Moreover, many bees have to remain idle for many days during the process of
+secretion. A large store of honey is indispensable to support a large stock
+of bees during the winter; and the security of the hive is known mainly to
+depend on a large number of bees being supported. Hence the saving of wax
+by largely saving honey must be a most important element of success in any
+family of bees. Of course the success of any species of bee may be
+dependent on the number of its parasites or other enemies, or on quite
+distinct causes, and so be altogether independent of the quantity of honey
+which the bees could collect. But let us suppose that this latter
+circumstance determined, as it probably often does determine, the numbers
+of a humble-bee which could exist in a country; and let us further suppose
+that the community lived throughout the winter, and consequently required a
+store of honey: there can in this case be no doubt that it would be an
+advantage to our humble-bee, if a slight modification of her instinct led
+her to make her waxen cells near together, so as to intersect a little; for
+a wall in common even to two adjoining cells, would save some little wax.
+Hence it would continually be more and more advantageous to our humble-bee,
+if she were to make her cells more and more regular, nearer together, and
+aggregated into a mass, like the cells of the Melipona; for in this case a
+large part of the bounding surface of each cell would serve to bound other
+cells, and much wax would be saved. Again, from the same cause, it would be
+advantageous to the Melipona, if she were to make her cells closer
+together, and more regular in every way {235} than at present; for then, as
+we have seen, the spherical surfaces would wholly disappear, and would all
+be replaced by plane surfaces; and the Melipona would make a comb as
+perfect as that of the hive-bee. Beyond this stage of perfection in
+architecture, natural selection could not lead; for the comb of the
+hive-bee, as far as we can see, is absolutely perfect in economising wax.
+
+Thus, as I believe, the most wonderful of all known instincts, that of the
+hive-bee, can be explained by natural selection having taken advantage of
+numerous, successive, slight modifications of simpler instincts; natural
+selection having by slow degrees, more and more perfectly, led the bees to
+sweep equal spheres at a given distance from each other in a double layer,
+and to build up and excavate the wax along the planes of intersection. The
+bees, of course, no more knowing that they swept their spheres at one
+particular distance from each other, than they know what are the several
+angles of the hexagonal prisms and of the basal rhombic plates. The motive
+power of the process of natural selection having been economy of wax; that
+individual swarm which wasted least honey in the secretion of wax, having
+succeeded best, and having transmitted by inheritance its newly acquired
+economical instinct to new swarms, which in their turn will have had the
+best chance of succeeding in the struggle for existence.
+
+
+
+No doubt many instincts of very difficult explanation could be opposed to
+the theory of natural selection,--cases, in which we cannot see how an
+instinct could possibly have originated; cases, in which no intermediate
+gradations are known to exist; cases of instinct of apparently such
+trifling importance, that they could {236} hardly have been acted on by
+natural selection; cases of instincts almost identically the same in
+animals so remote in the scale of nature, that we cannot account for their
+similarity by inheritance from a common parent, and must therefore believe
+that they have been acquired by independent acts of natural selection. I
+will not here enter on these several cases, but will confine myself to one
+special difficulty, which at first appeared to me insuperable, and actually
+fatal to my whole theory. I allude to the neuters or sterile females in
+insect-communities: for these neuters often differ widely in instinct and
+in structure from both the males and fertile females, and yet, from being
+sterile, they cannot propagate their kind.
+
+The subject well deserves to be discussed at great length, but I will here
+take only a single case, that of working or sterile ants. How the workers
+have been rendered sterile is a difficulty; but not much greater than that
+of any other striking modification of structure; for it can be shown that
+some insects and other articulate animals in a state of nature occasionally
+become sterile; and if such insects had been social, and it had been
+profitable to the community that a number should have been annually born
+capable of work, but incapable of procreation, I can see no very great
+difficulty in this being effected by natural selection. But I must pass
+over this preliminary difficulty. The great difficulty lies in the working
+ants differing widely from both the males and the fertile females in
+structure, as in the shape of the thorax and in being destitute of wings
+and sometimes of eyes, and in instinct. As far as instinct alone is
+concerned, the prodigious difference in this respect between the workers
+and the perfect females, would have been far better exemplified by the
+hive-bee. If a working ant or other neuter insect had been an animal {237}
+in the ordinary state, I should have unhesitatingly assumed that all its
+characters had been slowly acquired through natural selection; namely, by
+an individual having been born with some slight profitable modification of
+structure, this being inherited by its offspring, which again varied and
+were again selected, and so onwards. But with the working ant we have an
+insect differing greatly from its parents, yet absolutely sterile; so that
+it could never have transmitted successively acquired modifications of
+structure or instinct to its progeny. It may well be asked how is it
+possible to reconcile this case with the theory of natural selection?
+
+First, let it be remembered that we have innumerable instances, both in our
+domestic productions and in those in a state of nature, of all sorts of
+differences of structure which have become correlated to certain ages, and
+to either sex. We have differences correlated not only to one sex, but to
+that short period alone when the reproductive system is active, as in the
+nuptial plumage of many birds, and in the hooked jaws of the male salmon.
+We have even slight differences in the horns of different breeds of cattle
+in relation to an artificially imperfect state of the male sex; for oxen of
+certain breeds have longer horns than in other breeds, in comparison with
+the horns of the bulls or cows of these same breeds. Hence I can see no
+real difficulty in any character having become correlated with the sterile
+condition of certain members of insect-communities: the difficulty lies in
+understanding how such correlated modifications of structure could have
+been slowly accumulated by natural selection.
+
+This difficulty, though appearing insuperable, is lessened, or, as I
+believe, disappears, when it is remembered that selection may be applied to
+the family, as well as to the individual, and may thus gain the {238}
+desired end. Thus, a well-flavoured vegetable is cooked, and the individual
+is destroyed; but the horticulturist sows seeds of the same stock, and
+confidently expects to get nearly the same variety: breeders of cattle wish
+the flesh and fat to be well marbled together; the animal has been
+slaughtered, but the breeder goes with confidence to the same family. I
+have such faith in the powers of selection, that I do not doubt that a
+breed of cattle, always yielding oxen with extraordinarily long horns,
+could be slowly formed by carefully watching which individual bulls and
+cows, when matched, produced oxen with the longest horns; and yet no one ox
+could ever have propagated its kind. Thus I believe it has been with social
+insects: a slight modification of structure, or instinct, correlated with
+the sterile condition of certain members of the community, has been
+advantageous to the community: consequently the fertile males and females
+of the same community flourished, and transmitted to their fertile
+offspring a tendency to produce sterile members having the same
+modification. And I believe that this process has been repeated, until that
+prodigious amount of difference between the fertile and sterile females of
+the same species has been produced, which we see in many social insects.
+
+But we have not as yet touched on the climax of the difficulty; namely, the
+fact that the neuters of several ants differ, not only from the fertile
+females and males, but from each other, sometimes to an almost incredible
+degree, and are thus divided into two or even three castes. The castes,
+moreover, do not generally graduate into each other, but are perfectly well
+defined; being as distinct from each other, as are any two species of the
+same genus, or rather as any two genera of the same family. Thus in Eciton,
+there are working and soldier neuters, with jaws and instincts
+extraordinarily {239} different: in Cryptocerus, the workers of one caste
+alone carry a wonderful sort of shield on their heads, the use of which is
+quite unknown: in the Mexican Myrmecocystus, the workers of one caste never
+leave the nest; they are fed by the workers of another caste, and they have
+an enormously developed abdomen which secretes a sort of honey, supplying
+the place of that excreted by the aphides, or the domestic cattle as they
+may be called, which our European ants guard or imprison.
+
+It will indeed be thought that I have an overweening confidence in the
+principle of natural selection, when I do not admit that such wonderful and
+well-established facts at once annihilate my theory. In the simpler case of
+neuter insects all of one caste or of the same kind, which have been
+rendered by natural selection, as I believe to be quite possible, different
+from the fertile males and females,--in this case, we may safely conclude
+from the analogy of ordinary variations, that each successive, slight,
+profitable modification did not probably at first appear in all the
+individual neuters in the same nest, but in a few alone; and that by the
+long-continued selection of the fertile parents which produced most neuters
+with the profitable modification, all the neuters ultimately came to have
+the desired character. On this view we ought occasionally to find
+neuter-insects of the same species, in the same nest, presenting gradations
+of structure; and this we do find, even often, considering how few
+neuter-insects out of Europe have been carefully examined. Mr. F. Smith has
+shown how surprisingly the neuters of several British ants differ from each
+other in size and sometimes in colour; and that the extreme forms can
+sometimes be perfectly linked together by individuals taken out of the same
+nest: I have myself compared perfect gradations of this kind. It often
+happens that the larger or the smaller sized workers {240} are the most
+numerous; or that both large and small are numerous, with those of an
+intermediate size scanty in numbers. Formica flava has larger and smaller
+workers, with some of intermediate size; and, in this species, as Mr. F.
+Smith has observed, the larger workers have simple eyes (ocelli), which
+though small can be plainly distinguished, whereas the smaller workers have
+their ocelli rudimentary. Having carefully dissected several specimens of
+these workers, I can affirm that the eyes are far more rudimentary in the
+smaller workers than can be accounted for merely by their proportionally
+lesser size; and I fully believe, though I dare not assert so positively,
+that the workers of intermediate size have their ocelli in an exactly
+intermediate condition. So that we here have two bodies of sterile workers
+in the same nest, differing not only in size, but in their organs of
+vision, yet connected by some few members in an intermediate condition. I
+may digress by adding, that if the smaller workers had been the most useful
+to the community, and those males and females had been continually
+selected, which produced more and more of the smaller workers, until all
+the workers had come to be in this condition; we should then have had a
+species of ant with neuters very nearly in the same condition with those of
+Myrmica. For the workers of Myrmica have not even rudiments of ocelli,
+though the male and female ants of this genus have well-developed ocelli.
+
+I may give one other case: so confidently did I expect to find gradations
+in important points of structure between the different castes of neuters in
+the same species, that I gladly availed myself of Mr. F. Smith's offer of
+numerous specimens from the same nest of the driver ant (Anomma) of West
+Africa. The reader will perhaps best appreciate the amount of difference in
+these {241} workers, by my giving not the actual measurements, but a
+strictly accurate illustration: the difference was the same as if we were
+to see a set of workmen building a house of whom many were five feet four
+inches high, and many sixteen feet high; but we must suppose that the
+larger workmen had heads four instead of three times as big as those of the
+smaller men, and jaws nearly five times as big. The jaws, moreover, of the
+working ants of the several sizes differed wonderfully in shape, and in the
+form and number of the teeth. But the important fact for us is, that though
+the workers can be grouped into castes of different sizes, yet they
+graduate insensibly into each other, as does the widely-different structure
+of their jaws. I speak confidently on this latter point, as Mr. Lubbock
+made drawings for me with the camera lucida of the jaws which I had
+dissected from the workers of the several sizes.
+
+With these facts before me, I believe that natural selection, by acting on
+the fertile parents, could form a species which should regularly produce
+neuters, either all of large size with one form of jaw, or all of small
+size with jaws having a widely different structure; or lastly, and this is
+our climax of difficulty, one set of workers of one size and structure, and
+simultaneously another set of workers of a different size and structure;--a
+graduated series having been first formed, as in the case of the driver
+ant, and then the extreme forms, from being the most useful to the
+community, having been produced in greater and greater numbers through the
+natural selection of the parents which generated them; until none with an
+intermediate structure were produced.
+
+Thus, as I believe, the wonderful fact of two distinctly defined castes of
+sterile workers existing in the same nest, both widely different from each
+other and from {242} their parents, has originated. We can see how useful
+their production may have been to a social community of insects, on the
+same principle that the division of labour is useful to civilised man. As
+ants work by inherited instincts and by inherited organs or tools, and not
+by acquired knowledge and manufactured instruments, a perfect division of
+labour could be effected with them only by the workers being sterile; for
+had they been fertile, they would have intercrossed, and their instincts
+and structure would have become blended. And nature has, as I believe,
+effected this admirable division of labour in the communities of ants, by
+the means of natural selection. But I am bound to confess, that, with all
+my faith in this principle, I should never have anticipated that natural
+selection could have been efficient in so high a degree, had not the case
+of these neuter insects convinced me of the fact. I have, therefore,
+discussed this case, at some little but wholly insufficient length, in
+order to show the power of natural selection, and likewise because this is
+by far the most serious special difficulty, which my theory has
+encountered. The case, also, is very interesting, as it proves that with
+animals, as with plants, any amount of modification in structure can be
+effected by the accumulation of numerous, slight, and as we must call them
+accidental, variations, which are in any manner profitable, without
+exercise or habit having come into play. For no amount of exercise, or
+habit, or volition, in the utterly sterile members of a community could
+possibly affect the structure or instincts of the fertile members, which
+alone leave descendants. I am surprised that no one has advanced this
+demonstrative case of neuter insects, against the well-known doctrine of
+Lamarck.
+
+
+
+_Summary._--I have endeavoured briefly in this chapter {243} to show that
+the mental qualities of our domestic animals vary, and that the variations
+are inherited. Still more briefly I have attempted to show that instincts
+vary slightly in a state of nature. No one will dispute that instincts are
+of the highest importance to each animal. Therefore I can see no
+difficulty, under changing conditions of life, in natural selection
+accumulating slight modifications of instinct to any extent, in any useful
+direction. In some cases habit or use and disuse have probably come into
+play. I do not pretend that the facts given in this chapter strengthen in
+any great degree my theory; but none of the cases of difficulty, to the
+best of my judgment, annihilate it. On the other hand, the fact that
+instincts are not always absolutely perfect and are liable to
+mistakes;--that no instinct has been produced for the exclusive good of
+other animals, but that each animal takes advantage of the instincts of
+others;--that the canon in natural history, of "Natura non facit saltum,"
+is applicable to instincts as well as to corporeal structure, and is
+plainly explicable on the foregoing views, but is otherwise
+inexplicable,--all tend to corroborate the theory of natural selection.
+
+This theory is, also, strengthened by some few other facts in regard to
+instincts; as by that common case of closely allied, but certainly
+distinct, species, when inhabiting distant parts of the world and living
+under considerably different conditions of life, yet often retaining nearly
+the same instincts. For instance, we can understand on the principle of
+inheritance, how it is that the thrush of South America lines its nest with
+mud, in the same peculiar manner as does our British thrush: how it is that
+the male wrens (Troglodytes) of North America, build "cock-nests," to roost
+in, like the males of our distinct Kitty-wrens,--a habit wholly unlike that
+of {244} any other known bird. Finally, it may not be a logical deduction,
+but to my imagination it is far more satisfactory to look at such instincts
+as the young cuckoo ejecting its foster-brothers,--ants making slaves,--the
+larvae of ichneumonidæ feeding within the live bodies of caterpillars,--not
+as specially endowed or created instincts, but as small consequences of one
+general law, leading to the advancement of all organic beings, namely,
+multiply, vary, let the strongest live and the weakest die.
+
+       *       *       *       *       *
+
+
+{245}
+
+CHAPTER VIII.
+
+HYBRIDISM.
+
+    Distinction between the sterility of first crosses and of
+    hybrids--Sterility various in degree, not universal, affected by close
+    interbreeding, removed by domestication--Laws governing the sterility
+    of hybrids--Sterility not a special endowment, but incidental on other
+    differences--Causes of the sterility of first crosses and of
+    hybrids--Parallelism between the effects of changed conditions of life
+    and crossing--Fertility of varieties when crossed and of their mongrel
+    offspring not universal--Hybrids and mongrels compared independently of
+    their fertility--Summary.
+
+The view generally entertained by naturalists is that species, when
+intercrossed, have been specially endowed with the quality of sterility, in
+order to prevent the confusion of all organic forms. This view certainly
+seems at first probable, for species within the same country could hardly
+have kept distinct had they been capable of crossing freely. The importance
+of the fact that hybrids are very generally sterile, has, I think, been
+much underrated by some late writers. On the theory of natural selection
+the case is especially important, inasmuch as the sterility of hybrids
+could not possibly be of any advantage to them, and therefore could not
+have been acquired by the continued preservation of successive profitable
+degrees of sterility. I hope, however, to be able to show that sterility is
+not a specially acquired or endowed quality, but is incidental on other
+acquired differences.
+
+In treating this subject, two classes of facts, to a large extent
+fundamentally different, have generally been confounded together; namely,
+the sterility of two species {246} when first crossed, and the sterility of
+the hybrids produced from them.
+
+Pure species have of course their organs of reproduction in a perfect
+condition, yet when intercrossed they produce either few or no offspring.
+Hybrids, on the other hand, have their reproductive organs functionally
+impotent, as may be clearly seen in the state of the male element in both
+plants and animals; though the organs themselves are perfect in structure,
+as far as the microscope reveals. In the first case the two sexual elements
+which go to form the embryo are perfect; in the second case they are either
+not at all developed, or are imperfectly developed. This distinction is
+important, when the cause of the sterility, which is common to the two
+cases, has to be considered. The distinction has probably been slurred
+over, owing to the sterility in both cases being looked on as a special
+endowment, beyond the province of our reasoning powers.
+
+The fertility of varieties, that is of the forms known or believed to have
+descended from common parents, when intercrossed, and likewise the
+fertility of their mongrel offspring, is, on my theory, of equal importance
+with the sterility of species; for it seems to make a broad and clear
+distinction between varieties and species.
+
+First, for the sterility of species when crossed and of their hybrid
+offspring. It is impossible to study the several memoirs and works of those
+two conscientious and admirable observers, Kölreuter and Gärtner, who
+almost devoted their lives to this subject, without being deeply impressed
+with the high generality of some degree of sterility. Kölreuter makes the
+rule universal; but then he cuts the knot, for in ten cases in which he
+found two forms, considered by most authors as distinct species, quite
+fertile together, he unhesitatingly ranks {247} them as varieties. Gärtner,
+also, makes the rule equally universal; and he disputes the entire
+fertility of Kölreuter's ten cases. But in these and in many other cases,
+Gärtner is obliged carefully to count the seeds, in order to show that
+there is any degree of sterility. He always compares the maximum number of
+seeds produced by two species when crossed and by their hybrid offspring,
+with the average number produced by both pure parent-species in a state of
+nature. But a serious cause of error seems to me to be here introduced: a
+plant to be hybridised must be castrated, and, what is often more
+important, must be secluded in order to prevent pollen being brought to it
+by insects from other plants. Nearly all the plants experimentised on by
+Gärtner were potted, and apparently were kept in a chamber in his house.
+That these processes are often injurious to the fertility of a plant cannot
+be doubted; for Gärtner gives in his table about a score of cases of plants
+which he castrated, and artificially fertilised with their own pollen, and
+(excluding all cases such as the Leguminosæ, in which there is an
+acknowledged difficulty in the manipulation) half of these twenty plants
+had their fertility in some degree impaired. Moreover, as Gärtner during
+several years repeatedly crossed the primrose and cowslip, which we have
+such good reason to believe to be varieties, and only once or twice
+succeeded in getting fertile seed; as he found the common red and blue
+pimpernels (Anagallis arvensis and coerulea), which the best botanists rank
+as varieties, absolutely sterile together; and as he came to the same
+conclusion in several other analogous cases; it seems to me that we may
+well be permitted to doubt whether many other species are really so
+sterile, when intercrossed, as Gärtner believes. {248}
+
+It is certain, on the one hand, that the sterility of various species when
+crossed is so different in degree and graduates away so insensibly, and, on
+the other hand, that the fertility of pure species is so easily affected by
+various circumstances, that for all practical purposes it is most difficult
+to say where perfect fertility ends and sterility begins. I think no better
+evidence of this can be required than that the two most experienced
+observers who have ever lived, namely, Kölreuter and Gärtner, should have
+arrived at diametrically opposite conclusions in regard to the very same
+species. It is also most instructive to compare--but I have not space here
+to enter on details--the evidence advanced by our best botanists on the
+question whether certain doubtful forms should be ranked as species or
+varieties, with the evidence from fertility adduced by different
+hybridisers, or by the same author, from experiments made during different
+years. It can thus be shown that neither sterility nor fertility affords
+any clear distinction between species and varieties; but that the evidence
+from this source graduates away, and is doubtful in the same degree as is
+the evidence derived from other constitutional and structural differences.
+
+In regard to the sterility of hybrids in successive generations; though
+Gärtner was enabled to rear some hybrids, carefully guarding them from a
+cross with either pure parent, for six or seven, and in one case for ten
+generations, yet he asserts positively that their fertility never
+increased, but generally greatly decreased. I do not doubt that this is
+usually the case, and that the fertility often suddenly decreases in the
+first few generations. Nevertheless I believe that in all these experiments
+the fertility has been diminished by an independent cause, namely, from
+close interbreeding. I have collected so large a body of facts, showing
+{249} that close interbreeding lessens fertility, and, on the other hand,
+that an occasional cross with a distinct individual or variety increases
+fertility, that I cannot doubt the correctness of this almost universal
+belief amongst breeders. Hybrids are seldom raised by experimentalists in
+great numbers; and as the parent-species, or other allied hybrids,
+generally grow in the same garden, the visits of insects must be carefully
+prevented during the flowering season: hence hybrids will generally be
+fertilised during each generation by their own individual pollen; and I am
+convinced that this would be injurious to their fertility, already lessened
+by their hybrid origin. I am strengthened in this conviction by a
+remarkable statement repeatedly made by Gärtner, namely, that if even the
+less fertile hybrids be artificially fertilised with hybrid pollen of the
+same kind, their fertility, notwithstanding the frequent ill effects of
+manipulation, sometimes decidedly increases, and goes on increasing. Now,
+in artificial fertilisation pollen is as often taken by chance (as I know
+from my own experience) from the anthers of another flower, as from the
+anthers of the flower itself which is to be fertilised; so that a cross
+between two flowers, though probably on the same plant, would be thus
+effected. Moreover, whenever complicated experiments are in progress, so
+careful an observer as Gärtner would have castrated his hybrids, and this
+would have insured in each generation a cross with a pollen from a distinct
+flower, either from the same plant or from another plant of the same hybrid
+nature. And thus, the strange fact of the increase of fertility in the
+successive generations of _artificially fertilised_ hybrids may, I believe,
+be accounted for by close interbreeding having been avoided.
+
+Now let us turn to the results arrived at by the third most experienced
+hybridiser, namely, the Hon. and {250} Rev. W. Herbert. He is as emphatic
+in his conclusion that some hybrids are perfectly fertile--as fertile as
+the pure parent-species--as are Kölreuter and Gärtner that some degree of
+sterility between distinct species is a universal law of nature. He
+experimentised on some of the very same species as did Gärtner. The
+difference in their results may, I think, be in part accounted for by
+Herbert's great horticultural skill, and by his having hothouses at his
+command. Of his many important statements I will here give only a single
+one as an example, namely, that "every ovule in a pod of Crinum capense
+fertilised by C. revolutum produced a plant, which (he says) I never saw to
+occur in a case of its natural fecundation." So that we here have perfect,
+or even more than commonly perfect, fertility in a first cross between two
+distinct species.
+
+This case of the Crinum leads me to refer to a most singular fact, namely,
+that there are individual plants of certain species of Lobelia and of some
+other genera, which can be far more easily fertilised by the pollen of
+another and distinct species, than by their own pollen; and all the
+individuals of nearly all the species of Hippeastrum seem to be in this
+predicament. For these plants have been found to yield seed to the pollen
+of a distinct species, though quite sterile with their own pollen,
+notwithstanding that their own pollen was found to be perfectly good, for
+it fertilised distinct species. So that certain individual plants and all
+the individuals of certain species can actually be hybridised much more
+readily than they can be self-fertilised! For instance, a bulb of
+Hippeastrum aulicum produced four flowers; three were fertilised by Herbert
+with their own pollen, and the fourth was subsequently fertilised by the
+pollen of a compound hybrid descended from three other and distinct {251}
+species: the result was that "the ovaries of the three first flowers soon
+ceased to grow, and after a few days perished entirely, whereas the pod
+impregnated by the pollen of the hybrid made vigorous growth and rapid
+progress to maturity, and bore good seed, which vegetated freely." In a
+letter to me, in 1839, Mr. Herbert told me that he had then tried the
+experiment during five years, and he continued to try it during several
+subsequent years, and always with the same result. This result has, also,
+been confirmed by other observers in the case of Hippeastrum with its
+sub-genera, and in the case of some other genera, as Lobelia, Passiflora
+and Verbascum. Although the plants in these experiments appeared perfectly
+healthy, and although both the ovules and pollen of the same flower were
+perfectly good with respect to other species, yet as they were functionally
+imperfect in their mutual self-action, we must infer that the plants were
+in an unnatural state. Nevertheless these facts show on what slight and
+mysterious causes the lesser or greater fertility of species when crossed,
+in comparison with the same species when self-fertilised, sometimes
+depends.
+
+The practical experiments of horticulturists, though not made with
+scientific precision, deserve some notice. It is notorious in how
+complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
+Petunia, Rhododendron, &c., have been crossed, yet many of these hybrids
+seed freely. For instance, Herbert asserts that a hybrid from Calceolaria
+integrifolia and plantaginea, species most widely dissimilar in general
+habit, "reproduced itself as perfectly as if it had been a natural species
+from the mountains of Chile." I have taken some pains to ascertain the
+degree of fertility of some of the complex crosses of Rhododendrons, and I
+am assured that many of them {252} are perfectly fertile. Mr. C. Noble, for
+instance, informs me that he raises stocks for grafting from a hybrid
+between Rhod. Ponticum and Catawbiense, and that this hybrid "seeds as
+freely as it is possible to imagine." Had hybrids, when fairly treated,
+gone on decreasing in fertility in each successive generation, as Gärtner
+believes to be the case, the fact would have been notorious to nurserymen.
+Horticulturists raise large beds of the same hybrids, and such alone are
+fairly treated, for by insect agency the several individuals of the same
+hybrid variety are allowed to freely cross with each other, and the
+injurious influence of close interbreeding is thus prevented. Any one may
+readily convince himself of the efficiency of insect-agency by examining
+the flowers of the more sterile kinds of hybrid rhododendrons, which
+produce no pollen, for he will find on their stigmas plenty of pollen
+brought from other flowers.
+
+In regard to animals, much fewer experiments have been carefully tried than
+with plants. If our systematic arrangements can be trusted, that is if the
+genera of animals are as distinct from each other, as are the genera of
+plants, then we may infer that animals more widely separated in the scale
+of nature can be more easily crossed than in the case of plants; but the
+hybrids themselves are, I think, more sterile. I doubt whether any case of
+a perfectly fertile hybrid animal can be considered as thoroughly well
+authenticated. It should, however, be borne in mind that, owing to few
+animals breeding freely under confinement, few experiments have been fairly
+tried: for instance, the canary-bird has been crossed with nine other
+finches, but as not one of these nine species breeds freely in confinement,
+we have no right to expect that the first crosses between them and the
+canary, or that their hybrids, {253} should be perfectly fertile. Again,
+with respect to the fertility in successive generations of the more fertile
+hybrid animals, I hardly know of an instance in which two families of the
+same hybrid have been raised at the same time from different parents, so as
+to avoid the ill effects of close interbreeding. On the contrary, brothers
+and sisters have usually been crossed in each successive generation, in
+opposition to the constantly repeated admonition of every breeder. And in
+this case, it is not at all surprising that the inherent sterility in the
+hybrids should have gone on increasing. If we were to act thus, and pair
+brothers and sisters in the case of any pure animal, which from any cause
+had the least tendency to sterility, the breed would assuredly be lost in a
+very few generations.
+
+Although I do not know of any thoroughly well-authenticated cases of
+perfectly fertile hybrid animals, I have some reason to believe that the
+hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus
+with P. torquatus and with P. versicolor are perfectly fertile. There is no
+doubt that these three pheasants, namely, the common, the true ring-necked,
+and the Japan, intercross, and are becoming blended together in the woods
+of several parts of England. The hybrids from the common and Chinese geese
+(A. cygnoides), species which are so different that they are generally
+ranked in distinct genera, have often bred in this country with either pure
+parent, and in one single instance they have bred _inter se_. This was
+effected by Mr. Eyton, who raised two hybrids from the same parents but
+from different hatches; and from these two birds he raised no less than
+eight hybrids (grandchildren of the pure geese) from one nest. In India,
+however, these cross-bred geese must be far more fertile; for I am assured
+by two eminently capable judges, namely {254} Mr. Blyth and Capt. Hutton,
+that whole flocks of these crossed geese are kept in various parts of the
+country; and as they are kept for profit, where neither pure parent-species
+exists, they must certainly be highly fertile.
+
+A doctrine which originated with Pallas, has been largely accepted by
+modern naturalists; namely, that most of our domestic animals have
+descended from two or more wild species, since commingled by intercrossing.
+On this view, the aboriginal species must either at first have produced
+quite fertile hybrids, or the hybrids must have become in subsequent
+generations quite fertile under domestication. This latter alternative
+seems to me the most probable, and I am inclined to believe in its truth,
+although it rests on no direct evidence. I believe, for instance, that our
+dogs have descended from several wild stocks; yet, with perhaps the
+exception of certain indigenous domestic dogs of South America, all are
+quite fertile together; and analogy makes me greatly doubt, whether the
+several aboriginal species would at first have freely bred together and
+have produced quite fertile hybrids. So again there is reason to believe
+that our European and the humped Indian cattle are quite fertile together;
+but from facts communicated to me by Mr. Blyth, I think they must be
+considered as distinct species. On this view of the origin of many of our
+domestic animals, we must either give up the belief of the almost universal
+sterility of distinct species of animals when crossed; or we must look at
+sterility, not as an indelible characteristic, but as one capable of being
+removed by domestication.
+
+Finally, looking to all the ascertained facts on the intercrossing of
+plants and animals, it may be concluded that some degree of sterility, both
+in first crosses {255} and in hybrids, is an extremely general result; but
+that it cannot, under our present state of knowledge, be considered as
+absolutely universal.
+
+
+
+_Laws governing the Sterility of first Crosses and of Hybrids._--We will
+now consider a little more in detail the circumstances and rules governing
+the sterility of first crosses and of hybrids. Our chief object will be to
+see whether or not the rules indicate that species have specially been
+endowed with this quality, in order to prevent their crossing and blending
+together in utter confusion. The following rules and conclusions are
+chiefly drawn up from Gärtner's admirable work on the hybridisation of
+plants. I have taken much pains to ascertain how far the rules apply to
+animals, and considering how scanty our knowledge is in regard to hybrid
+animals, I have been surprised to find how generally the same rules apply
+to both kingdoms.
+
+It has been already remarked, that the degree of fertility, both of first
+crosses and of hybrids, graduates from zero to perfect fertility. It is
+surprising in how many curious ways this gradation can be shown to exist;
+but only the barest outline of the facts can here be given. When pollen
+from a plant of one family is placed on the stigma of a plant of a distinct
+family, it exerts no more influence than so much inorganic dust. From this
+absolute zero of fertility, the pollen of different species of the same
+genus applied to the stigma of some one species, yields a perfect gradation
+in the number of seeds produced, up to nearly complete or even quite
+complete fertility; and, as we have seen, in certain abnormal cases, even
+to an excess of fertility, beyond that which the plant's own pollen will
+produce. So in hybrids themselves, there are some which never have
+produced, and probably never would produce, even {256} with the pollen of
+either pure parent, a single fertile seed: but in some of these cases a
+first trace of fertility may be detected, by the pollen of one of the pure
+parent-species causing the flower of the hybrid to wither earlier than it
+otherwise would have done; and the early withering of the flower is well
+known to be a sign of incipient fertilisation. From this extreme degree of
+sterility we have self-fertilised hybrids producing a greater and greater
+number of seeds up to perfect fertility.
+
+Hybrids from two species which are very difficult to cross, and which
+rarely produce any offspring, are generally very sterile; but the
+parallelism between the difficulty of making a first cross, and the
+sterility of the hybrids thus produced--two classes of facts which are
+generally confounded together--is by no means strict. There are many cases,
+in which two pure species can be united with unusual facility, and produce
+numerous hybrid-offspring, yet these hybrids are remarkably sterile. On the
+other hand, there are species which can be crossed very rarely, or with
+extreme difficulty, but the hybrids, when at last produced, are very
+fertile. Even within the limits of the same genus, for instance in
+Dianthus, these two opposite cases occur.
+
+The fertility, both of first crosses and of hybrids, is more easily
+affected by unfavourable conditions, than is the fertility of pure species.
+But the degree of fertility is likewise innately variable; for it is not
+always the same when the same two species are crossed under the same
+circumstances, but depends in part upon the constitution of the individuals
+which happen to have been chosen for the experiment. So it is with hybrids,
+for their degree of fertility is often found to differ greatly in the
+several individuals raised from seed out of the same capsule and exposed to
+exactly the same conditions. {257}
+
+By the term systematic affinity is meant, the resemblance between species
+in structure and in constitution, more especially in the structure of parts
+which are of high physiological importance and which differ little in the
+allied species. Now the fertility of first crosses between species, and of
+the hybrids produced from them, is largely governed by their systematic
+affinity. This is clearly shown by hybrids never having been raised between
+species ranked by systematists in distinct families; and on the other hand,
+by very closely allied species generally uniting with facility. But the
+correspondence between systematic affinity and the facility of crossing is
+by no means strict. A multitude of cases could be given of very closely
+allied species which will not unite, or only with extreme difficulty; and
+on the other hand of very distinct species which unite with the utmost
+facility. In the same family there may be a genus, as Dianthus, in which
+very many species can most readily be crossed; and another genus, as
+Silene, in which the most persevering efforts have failed to produce
+between extremely close species a single hybrid. Even within the limits of
+the same genus, we meet with this same difference; for instance, the many
+species of Nicotiana have been more largely crossed than the species of
+almost any other genus; but Gärtner found that N. acuminata, which is not a
+particularly distinct species, obstinately failed to fertilise, or to be
+fertilised by, no less than eight other species of Nicotiana. Very many
+analogous facts could be given.
+
+No one has been able to point out what kind, or what amount, of difference
+in any recognisable character is sufficient to prevent two species
+crossing. It can be shown that plants most widely different in habit and
+general appearance, and having strongly marked {258} differences in every
+part of the flower, even in the pollen, in the fruit, and in the
+cotyledons, can be crossed. Annual and perennial plants, deciduous and
+evergreen trees, plants inhabiting different stations and fitted for
+extremely different climates, can often be crossed with ease.
+
+By a reciprocal cross between two species, I mean the case, for instance,
+of a stallion-horse being first crossed with a female-ass, and then a
+male-ass with a mare: these two species may then be said to have been
+reciprocally crossed. There is often the widest possible difference in the
+facility of making reciprocal crosses. Such cases are highly important, for
+they prove that the capacity in any two species to cross is often
+completely independent of their systematic affinity, or of any recognisable
+difference in their whole organisation. On the other hand, these cases
+clearly show that the capacity for crossing is connected with
+constitutional differences imperceptible by us, and confined to the
+reproductive system. This difference in the result of reciprocal crosses
+between the same two species was long ago observed by Kölreuter. To give an
+instance: Mirabilis jalapa can easily be fertilised by the pollen of M.
+longiflora, and the hybrids thus produced are sufficiently fertile; but
+Kölreuter tried more than two hundred times, during eight following years,
+to fertilise reciprocally M. longiflora with the pollen of M. jalapa, and
+utterly failed. Several other equally striking cases could be given. Thuret
+has observed the same fact with certain sea-weeds or Fuci. Gärtner,
+moreover, found that this difference of facility in making reciprocal
+crosses is extremely common in a lesser degree. He has observed it even
+between forms so closely related (as Matthiola annua and glabra) that many
+botanists rank them only as varieties. It is also a remarkable fact, that
+hybrids raised from reciprocal crosses, though {259} of course compounded
+of the very same two species, the one species having first been used as the
+father and then as the mother, generally differ in fertility in a small,
+and occasionally in a high degree.
+
+Several other singular rules could be given from Gärtner: for instance,
+some species have a remarkable power of crossing with other species; other
+species of the same genus have a remarkable power of impressing their
+likeness on their hybrid offspring; but these two powers do not at all
+necessarily go together. There are certain hybrids which instead of having,
+as is usual, an intermediate character between their two parents, always
+closely resemble one of them; and such hybrids, though externally so like
+one of their pure parent-species, are with rare exceptions extremely
+sterile. So again amongst hybrids which are usually intermediate in
+structure between their parents, exceptional and abnormal individuals
+sometimes are born, which closely resemble one of their pure parents; and
+these hybrids are almost always utterly sterile, even when the other
+hybrids raised from seed from the same capsule have a considerable degree
+of fertility. These facts show how completely fertility in the hybrid is
+independent of its external resemblance to either pure parent.
+
+Considering the several rules now given, which govern the fertility of
+first crosses and of hybrids, we see that when forms, which must be
+considered as good and distinct species, are united, their fertility
+graduates from zero to perfect fertility, or even to fertility under
+certain conditions in excess. That their fertility, besides being eminently
+susceptible to favourable and unfavourable conditions, is innately
+variable. That it is by no means always the same in degree in the first
+cross and in the hybrids produced {260} from this cross. That the fertility
+of hybrids is not related to the degree in which they resemble in external
+appearance either parent. And lastly, that the facility of making a first
+cross between any two species is not always governed by their systematic
+affinity or degree of resemblance to each other. This latter statement is
+clearly proved by reciprocal crosses between the same two species, for
+according as the one species or the other is used as the father or the
+mother, there is generally some difference, and occasionally the widest
+possible difference, in the facility of effecting an union. The hybrids,
+moreover, produced from reciprocal crosses often differ in fertility.
+
+Now do these complex and singular rules indicate that species have been
+endowed with sterility simply to prevent their becoming confounded in
+nature? I think not. For why should the sterility be so extremely different
+in degree, when various species are crossed, all of which we must suppose
+it would be equally important to keep from blending together? Why should
+the degree of sterility be innately variable in the individuals of the same
+species? Why should some species cross with facility, and yet produce very
+sterile hybrids; and other species cross with extreme difficulty, and yet
+produce fairly fertile hybrids? Why should there often be so great a
+difference in the result of a reciprocal cross between the same two
+species? Why, it may even be asked, has the production of hybrids been
+permitted? to grant to species the special power of producing hybrids, and
+then to stop their further propagation by different degrees of sterility,
+not strictly related to the facility of the first union between their
+parents, seems to be a strange arrangement.
+
+The foregoing rules and facts, on the other hand, {261} appear to me
+clearly to indicate that the sterility both of first crosses and of hybrids
+is simply incidental or dependent on unknown differences, chiefly in the
+reproductive systems, of the species which are crossed. The differences
+being of so peculiar and limited a nature, that, in reciprocal crosses
+between two species the male sexual element of the one will often freely
+act on the female sexual element of the other, but not in a reversed
+direction. It will be advisable to explain a little more fully by an
+example what I mean by sterility being incidental on other differences, and
+not a specially endowed quality. As the capacity of one plant to be grafted
+or budded on another is so entirely unimportant for its welfare in a state
+of nature, I presume that no one will suppose that this capacity is a
+_specially_ endowed quality, but will admit that it is incidental on
+differences in the laws of growth of the two plants. We can sometimes see
+the reason why one tree will not take on another, from differences in their
+rate of growth, in the hardness of their wood, in the period of the flow or
+nature of their sap, &c.; but in a multitude of cases we can assign no
+reason whatever. Great diversity in the size of two plants, one being woody
+and the other herbaceous, one being evergreen and the other deciduous, and
+adaptation to widely different climates, does not always prevent the two
+grafting together. As in hybridisation, so with grafting, the capacity is
+limited by systematic affinity, for no one has been able to graft trees
+together belonging to quite distinct families; and, on the other hand,
+closely allied species, and varieties of the same species, can usually, but
+not invariably, be grafted with ease. But this capacity, as in
+hybridisation, is by no means absolutely governed by systematic affinity.
+Although many distinct genera within the same family have been grafted
+{262} together, in other cases species of the same genus will not take on
+each other. The pear can be grafted far more readily on the quince, which
+is ranked as a distinct genus, than on the apple, which is a member of the
+same genus. Even different varieties of the pear take with different
+degrees of facility on the quince; so do different varieties of the apricot
+and peach on certain varieties of the plum.
+
+As Gärtner found that there was sometimes an innate difference in different
+_individuals_ of the same two species in crossing; so Sagaret believes this
+to be the case with different individuals of the same two species in being
+grafted together. As in reciprocal crosses, the facility of effecting an
+union is often very far from equal, so it sometimes is in grafting; the
+common gooseberry, for instance, cannot be grafted on the currant, whereas
+the currant will take, though with difficulty, on the gooseberry.
+
+We have seen that the sterility of hybrids, which have their reproductive
+organs in an imperfect condition, is a very different case from the
+difficulty of uniting two pure species, which have their reproductive
+organs perfect; yet these two distinct cases run to a certain extent
+parallel. Something analogous occurs in grafting; for Thouin found that
+three species of Robinia, which seeded freely on their own roots, and which
+could be grafted with no great difficulty on another species, when thus
+grafted were rendered barren. On the other hand, certain species of Sorbus,
+when grafted on other species, yielded twice as much fruit as when on their
+own roots. We are reminded by this latter fact of the extraordinary case of
+Hippeastrum, Lobelia, &c., which seeded much more freely when fertilised
+with the pollen of distinct species, than when self-fertilised with their
+own pollen. {263}
+
+We thus see, that although there is a clear and fundamental difference
+between the mere adhesion of grafted stocks, and the union of the male and
+female elements in the act of reproduction, yet that there is a rude degree
+of parallelism in the results of grafting and of crossing distinct species.
+And as we must look at the curious and complex laws governing the facility
+with which trees can be grafted on each other as incidental on unknown
+differences in their vegetative systems, so I believe that the still more
+complex laws governing the facility of first crosses, are incidental on
+unknown differences, chiefly in their reproductive systems. These
+differences, in both cases, follow to a certain extent, as might have been
+expected, systematic affinity, by which every kind of resemblance and
+dissimilarity between organic beings is attempted to be expressed. The
+facts by no means seem to me to indicate that the greater or lesser
+difficulty of either grafting or crossing together various species has been
+a special endowment; although in the case of crossing, the difficulty is as
+important for the endurance and stability of specific forms, as in the case
+of grafting it is unimportant for their welfare.
+
+
+
+_Causes of the Sterility of first Crosses and of Hybrids._--We may now look
+a little closer at the probable causes of the sterility of first crosses
+and of hybrids. These two cases are fundamentally different, for, as just
+remarked, in the union of two pure species the male and female sexual
+elements are perfect, whereas in hybrids they are imperfect. Even in first
+crosses, the greater or lesser difficulty in effecting a union apparently
+depends on several distinct causes. There must sometimes be a physical
+impossibility in the male element reaching the ovule, as would be the case
+with a plant {264} having a pistil too long for the pollen-tubes to reach
+the ovarium. It has also been observed that when pollen of one species is
+placed on the stigma of a distantly allied species, though the pollen-tubes
+protrude, they do not penetrate the stigmatic surface. Again, the male
+element may reach the female element, but be incapable of causing an embryo
+to be developed, as seems to have been the case with some of Thuret's
+experiments on Fuci. No explanation can be given of these facts, any more
+than why certain trees cannot be grafted on others. Lastly, an embryo may
+be developed, and then perish at an early period. This latter alternative
+has not been sufficiently attended to; but I believe, from observations
+communicated to me by Mr. Hewitt, who has had great experience in
+hybridising gallinaceous birds, that the early death of the embryo is a
+very frequent cause of sterility in first crosses. I was at first very
+unwilling to believe in this view; as hybrids, when once born, are
+generally healthy and long-lived, as we see in the case of the common mule.
+Hybrids, however, are differently circumstanced before and after birth:
+when born and living in a country where their two parents can live, they
+are generally placed under suitable conditions of life. But a hybrid
+partakes of only half of the nature and constitution of its mother, and
+therefore before birth, as long as it is nourished within its mother's womb
+or within the egg or seed produced by the mother, it may be exposed to
+conditions in some degree unsuitable, and consequently be liable to perish
+at an early period; more especially as all very young beings seem eminently
+sensitive to injurious or unnatural conditions of life.
+
+In regard to the sterility of hybrids, in which the sexual elements are
+imperfectly developed, the case is {265} very different. I have more than
+once alluded to a large body of facts, which I have collected, showing that
+when animals and plants are removed from their natural conditions, they are
+extremely liable to have their reproductive systems seriously affected.
+This, in fact, is the great bar to the domestication of animals. Between
+the sterility thus superinduced and that of hybrids, there are many points
+of similarity. In both cases the sterility is independent of general
+health, and is often accompanied by excess of size or great luxuriance. In
+both cases, the sterility occurs in various degrees; in both, the male
+element is the most liable to be affected; but sometimes the female more
+than the male. In both, the tendency goes to a certain extent with
+systematic affinity, for whole groups of animals and plants are rendered
+impotent by the same unnatural conditions; and whole groups of species tend
+to produce sterile hybrids. On the other hand, one species in a group will
+sometimes resist great changes of conditions with unimpaired fertility; and
+certain species in a group will produce unusually fertile hybrids. No one
+can tell, till he tries, whether any particular animal will breed under
+confinement or any exotic plant seed freely under culture; nor can he tell,
+till he tries, whether any two species of a genus will produce more or less
+sterile hybrids. Lastly, when organic beings are placed during several
+generations under conditions not natural to them, they are extremely liable
+to vary, which is due, as I believe, to their reproductive systems having
+been specially affected, though in a lesser degree than when sterility
+ensues. So it is with hybrids, for hybrids in successive generations are
+eminently liable to vary, as every experimentalist has observed.
+
+Thus we see that when organic beings are placed under new and unnatural
+conditions, and when hybrids {266} are produced by the unnatural crossing
+of two species, the reproductive system, independently of the general state
+of health, is affected by sterility in a very similar manner. In the one
+case, the conditions of life have been disturbed, though often in so slight
+a degree as to be inappreciable by us; in the other case, or that of
+hybrids, the external conditions have remained the same, but the
+organisation has been disturbed by two different structures and
+constitutions having been blended into one. For it is scarcely possible
+that two organisations should be compounded into one, without some
+disturbance occurring in the development, or periodical action, or mutual
+relation of the different parts and organs one to another, or to the
+conditions of life. When hybrids are able to breed _inter se_, they
+transmit to their offspring from generation to generation the same
+compounded organisation, and hence we need not be surprised that their
+sterility, though in some degree variable, rarely diminishes.
+
+It must, however, be confessed that we cannot understand, excepting on
+vague hypotheses, several facts with respect to the sterility of hybrids;
+for instance, the unequal fertility of hybrids produced from reciprocal
+crosses; or the increased sterility in those hybrids which occasionally and
+exceptionally resemble closely either pure parent. Nor do I pretend that
+the foregoing remarks go to the root of the matter: no explanation is
+offered why an organism, when placed under unnatural conditions, is
+rendered sterile. All that I have attempted to show, is that in two cases,
+in some respects allied, sterility is the common result,--in the one case
+from the conditions of life having been disturbed, in the other case from
+the organisation having been disturbed by two organisations having been
+compounded into one.
+
+It may seem fanciful, but I suspect that a similar {267} parallelism
+extends to an allied yet very different class of facts. It is an old and
+almost universal belief, founded, I think, on a considerable body of
+evidence, that slight changes in the conditions of life are beneficial to
+all living things. We see this acted on by farmers and gardeners in their
+frequent exchanges of seed, tubers, &c., from one soil or climate to
+another, and back again. During the convalescence of animals, we plainly
+see that great benefit is derived from almost any change in the habits of
+life. Again, both with plants and animals, there is abundant evidence, that
+a cross between very distinct individuals of the same species, that is
+between members of different strains or sub-breeds, gives vigour and
+fertility to the offspring. I believe, indeed, from the facts alluded to in
+our fourth chapter, that a certain amount of crossing is indispensable even
+with hermaphrodites; and that close interbreeding continued during several
+generations between the nearest relations, especially if these be kept
+under the same conditions of life, always induces weakness and sterility in
+the progeny.
+
+Hence it seems that, on the one hand, slight changes in the conditions of
+life benefit all organic beings, and on the other hand, that slight
+crosses, that is crosses between the males and females of the same species
+which have varied and become slightly different, give vigour and fertility
+to the offspring. But we have seen that greater changes, or changes of a
+particular nature, often render organic beings in some degree sterile; and
+that greater crosses, that is crosses between males and females which have
+become widely or specifically different, produce hybrids which are
+generally sterile in some degree. I cannot persuade myself that this
+parallelism is an accident or an illusion. Both series of facts seem to be
+connected together by some {268} common but unknown bond, which is
+essentially related to the principle of life.
+
+
+
+_Fertility of Varieties when crossed, and of their Mongrel offspring._--It
+may be urged, as a most forcible argument, that there must be some
+essential distinction between species and varieties, and that there must be
+some error in all the foregoing remarks, inasmuch as varieties, however
+much they may differ from each other in external appearance, cross with
+perfect facility, and yield perfectly fertile offspring. I fully admit that
+this is almost invariably the case. But if we look to varieties produced
+under nature, we are immediately involved in hopeless difficulties; for if
+two hitherto reputed varieties be found in any degree sterile together,
+they are at once ranked by most naturalists as species. For instance, the
+blue and red pimpernel, the primrose and cowslip, which are considered by
+many of our best botanists as varieties, are said by Gärtner not to be
+quite fertile when crossed, and he consequently ranks them as undoubted
+species. If we thus argue in a circle, the fertility of all varieties
+produced under nature will assuredly have to be granted.
+
+If we turn to varieties, produced, or supposed to have been produced, under
+domestication, we are still involved in doubt. For when it is stated, for
+instance, that the German Spitz dog unites more easily than other dogs with
+foxes, or that certain South American indigenous domestic dogs do not
+readily cross with European dogs, the explanation which will occur to every
+one, and probably the true one, is that these dogs have descended from
+several aboriginally distinct species. Nevertheless the perfect fertility
+of so many domestic varieties, differing widely from each other in
+appearance, for instance of the pigeon or of the cabbage, is {269} a
+remarkable fact; more especially when we reflect how many species there
+are, which, though resembling each other most closely, are utterly sterile
+when intercrossed. Several considerations, however, render the fertility of
+domestic varieties less remarkable than at first appears. It can, in the
+first place, be clearly shown that mere external dissimilarity between two
+species does not determine their greater or lesser degree of sterility when
+crossed; and we may apply the same rule to domestic varieties. In the
+second place, some eminent naturalists believe that a long course of
+domestication tends to eliminate sterility in the successive generations of
+hybrids which were at first only slightly sterile; and if this be so, we
+surely ought not to expect to find sterility both appearing and
+disappearing under nearly the same conditions of life. Lastly, and this
+seems to me by far the most important consideration, new races of animals
+and plants are produced under domestication by man's methodical and
+unconscious power of selection, for his own use and pleasure: he neither
+wishes to select, nor could select, slight differences in the reproductive
+system, or other constitutional differences correlated with the
+reproductive system. He supplies his several varieties with the same food;
+treats them in nearly the same manner, and does not wish to alter their
+general habits of life. Nature acts uniformly and slowly during vast
+periods of time on the whole organisation, in any way which may be for each
+creature's own good; and thus she may, either directly, or more probably
+indirectly, through correlation, modify the reproductive system in the
+several descendants from any one species. Seeing this difference in the
+process of selection, as carried on by man and nature, we need not be
+surprised at some difference in the result.
+
+I have as yet spoken as if the varieties of the same {270} species were
+invariably fertile when intercrossed. But it seems to me impossible to
+resist the evidence of the existence of a certain amount of sterility in
+the few following cases, which I will briefly abstract. The evidence is at
+least as good as that from which we believe in the sterility of a multitude
+of species. The evidence is, also, derived from hostile witnesses, who in
+all other cases consider fertility and sterility as safe criterions of
+specific distinction. Gärtner kept during several years a dwarf kind of
+maize with yellow seeds, and a tall variety with red seeds, growing near
+each other in his garden; and although these plants have separated sexes,
+they never naturally crossed. He then fertilised thirteen flowers of the
+one with the pollen of the other; but only a single head produced any seed,
+and this one head produced only five grains. Manipulation in this case
+could not have been injurious, as the plants have separated sexes. No one,
+I believe, has suspected that these varieties of maize are distinct
+species; and it is important to notice that the hybrid plants thus raised
+were themselves _perfectly_ fertile; so that even Gärtner did not venture
+to consider the two varieties as specifically distinct.
+
+Girou de Buzareingues crossed three varieties of gourd, which like the
+maize has separated sexes, and he asserts that their mutual fertilisation
+is by so much the less easy as their differences are greater. How far these
+experiments may be trusted, I know not; but the forms experimentised on,
+are ranked by Sagaret, who mainly founds his classification by the test of
+infertility, as varieties.
+
+The following case is far more remarkable, and seems at first quite
+incredible; but it is the result of an astonishing number of experiments
+made during many years on nine species of Verbascum, by so good an observer
+{271} and so hostile a witness, as Gärtner: namely, that yellow and white
+varieties of the same species of Verbascum when intercrossed produce less
+seed, than do either coloured varieties when fertilised with pollen from
+their own coloured flowers. Moreover, he asserts that when yellow and white
+varieties of one species are crossed with yellow and white varieties of a
+_distinct_ species, more seed is produced by the crosses between the
+similarly coloured flowers, than between those which are differently
+coloured. Yet these varieties of Verbascum present no other difference
+besides the mere colour of the flower; and one variety can sometimes be
+raised from the seed of the other.
+
+From observations which I have made on certain varieties of hollyhock, I am
+inclined to suspect that they present analogous facts.
+
+Kölreuter, whose accuracy has been confirmed by every subsequent observer,
+has proved the remarkable fact, that one variety of the common tobacco is
+more fertile, when crossed with a widely distinct species, than are the
+other varieties. He experimentised on five forms, which are commonly
+reputed to be varieties, and which he tested by the severest trial, namely,
+by reciprocal crosses, and he found their mongrel offspring perfectly
+fertile. But one of these five varieties, when used either as father or
+mother, and crossed with the Nicotiana glutinosa, always yielded hybrids
+not so sterile as those which were produced from the four other varieties
+when crossed with N. glutinosa. Hence the reproductive system of this one
+variety must have been in some manner and in some degree modified.
+
+From these facts; from the great difficulty of ascertaining the infertility
+of varieties in a state of nature, for a supposed variety if infertile in
+any degree would generally be ranked as species; from man selecting only
+{272} external characters in the production of the most distinct domestic
+varieties, and from not wishing or being able to produce recondite and
+functional differences in the reproductive system; from these several
+considerations and facts, I do not think that the very general fertility of
+varieties can be proved to be of universal occurrence, or to form a
+fundamental distinction between varieties and species. The general
+fertility of varieties does not seem to me sufficient to overthrow the view
+which I have taken with respect to the very general, but not invariable,
+sterility of first crosses and of hybrids, namely, that it is not a special
+endowment, but is incidental on slowly acquired modifications, more
+especially in the reproductive systems of the forms which are crossed.
+
+
+
+_Hybrids and Mongrels compared, independently of their
+fertility._--Independently of the question of fertility, the offspring of
+species when crossed and of varieties when crossed may be compared in
+several other respects. Gärtner, whose strong wish was to draw a marked
+line of distinction between species and varieties, could find very few and,
+as it seems to me, quite unimportant differences between the so-called
+hybrid offspring of species, and the so-called mongrel offspring of
+varieties. And, on the other hand, they agree most closely in very many
+important respects.
+
+I shall here discuss this subject with extreme brevity. The most important
+distinction is, that in the first generation mongrels are more variable
+than hybrids; but Gärtner admits that hybrids from species which have long
+been cultivated are often variable in the first generation; and I have
+myself seen striking instances of this fact. Gärtner further admits that
+hybrids between very closely allied species are more variable {273} than
+those from very distinct species; and this shows that the difference in the
+degree of variability graduates away. When mongrels and the more fertile
+hybrids are propagated for several generations an extreme amount of
+variability in their offspring is notorious; but some few cases both of
+hybrids and mongrels long retaining uniformity of character could be given.
+The variability, however, in the successive generations of mongrels is,
+perhaps, greater than in hybrids.
+
+This greater variability of mongrels than of hybrids does not seem to me at
+all surprising. For the parents of mongrels are varieties, and mostly
+domestic varieties (very few experiments having been tried on natural
+varieties), and this implies in most cases that there has been recent
+variability; and therefore we might expect that such variability would
+often continue and be superadded to that arising from the mere act of
+crossing. The slight degree of variability in hybrids from the first cross
+or in the first generation, in contrast with their extreme variability in
+the succeeding generations, is a curious fact and deserves attention. For
+it bears on and corroborates the view which I have taken on the cause of
+ordinary variability; namely, that it is due to the reproductive system
+being eminently sensitive to any change in the conditions of life, being
+thus often rendered either impotent or at least incapable of its proper
+function of producing offspring identical with the parent-form. Now hybrids
+in the first generation are descended from species (excluding those long
+cultivated) which have not had their reproductive systems in any way
+affected, and they are not variable; but hybrids themselves have their
+reproductive systems seriously affected, and their descendants are highly
+variable.
+
+But to return to our comparison of mongrels and {274} hybrids: Gärtner
+states that mongrels are more liable than hybrids to revert to either
+parent-form; but this, if it be true, is certainly only a difference in
+degree. Gärtner further insists that when any two species, although most
+closely allied to each other, are crossed with a third species, the hybrids
+are widely different from each other; whereas if two very distinct
+varieties of one species are crossed with another species, the hybrids do
+not differ much. But this conclusion, as far as I can make out, is founded
+on a single experiment; and seems directly opposed to the results of
+several experiments made by Kölreuter.
+
+These alone are the unimportant differences, which Gärtner is able to point
+out, between hybrid and mongrel plants. On the other hand, the resemblance
+in mongrels and in hybrids to their respective parents, more especially in
+hybrids produced from nearly related species, follows according to Gärtner
+the same laws. When two species are crossed, one has sometimes a prepotent
+power of impressing its likeness on the hybrid; and so I believe it to be
+with varieties of plants. With animals one variety certainly often has this
+prepotent power over another variety. Hybrid plants produced from a
+reciprocal cross, generally resemble each other closely; and so it is with
+mongrels from a reciprocal cross. Both hybrids and mongrels can be reduced
+to either pure parent-form, by repeated crosses in successive generations
+with either parent.
+
+These several remarks are apparently applicable to animals; but the subject
+is here excessively complicated, partly owing to the existence of secondary
+sexual characters; but more especially owing to prepotency in transmitting
+likeness running more strongly in one sex than in the other, both when one
+species is crossed with another, and when, one variety is crossed with
+{275} another variety. For instance, I think those authors are right, who
+maintain that the ass has a prepotent power over the horse, so that both
+the mule and the hinny more resemble the ass than the horse; but that the
+prepotency runs more strongly in the male-ass than in the female, so that
+the mule, which is the offspring of the male-ass and mare, is more like an
+ass, than is the hinny, which is the offspring of the female-ass and
+stallion.
+
+Much stress has been laid by some authors on the supposed fact, that
+mongrel animals alone are born closely like one of their parents; but it
+can be shown that this does sometimes occur with hybrids; yet I grant much
+less frequently with hybrids than with mongrels. Looking to the cases which
+I have collected of cross-bred animals closely resembling one parent, the
+resemblances seem chiefly confined to characters almost monstrous in their
+nature, and which have suddenly appeared--such as albinism, melanism,
+deficiency of tail or horns, or additional fingers and toes; and do not
+relate to characters which have been slowly acquired by selection.
+Consequently, sudden reversions to the perfect character of either parent
+would be more likely to occur with mongrels, which are descended from
+varieties often suddenly produced and semi-monstrous in character, than
+with hybrids, which are descended from species slowly and naturally
+produced. On the whole I entirely agree with Dr. Prosper Lucas, who, after
+arranging an enormous body of facts with respect to animals, comes to the
+conclusion, that the laws of resemblance of the child to its parents are
+the same, whether the two parents differ much or little from each other,
+namely in the union of individuals of the same variety, or of different
+varieties, or of distinct species.
+
+Laying aside the question of fertility and sterility, {276} in all other
+respects there seems to be a general and close similarity in the offspring
+of crossed species, and of crossed varieties. If we look at species as
+having been specially created, and at varieties as having been produced by
+secondary laws, this similarity would be an astonishing fact. But it
+harmonises perfectly with the view that there is no essential distinction
+between species and varieties.
+
+
+
+_Summary of Chapter._--First crosses between forms sufficiently distinct to
+be ranked as species, and their hybrids, are very generally, but not
+universally, sterile. The sterility is of all degrees, and is often so
+slight that the two most careful experimentalists who have ever lived, have
+come to diametrically opposite conclusions in ranking forms by this test.
+The sterility is innately variable in individuals of the same species, and
+is eminently susceptible of favourable and unfavourable conditions. The
+degree of sterility does not strictly follow systematic affinity, but is
+governed by several curious and complex laws. It is generally different,
+and sometimes widely different, in reciprocal crosses between the same two
+species. It is not always equal in degree in a first cross and in the
+hybrid produced from this cross.
+
+In the same manner as in grafting trees, the capacity of one species or
+variety to take on another, is incidental on generally unknown differences
+in their vegetative systems, so in crossing, the greater or less facility
+of one species to unite with another, is incidental on unknown differences
+in their reproductive systems. There is no more reason to think that
+species have been specially endowed with various degrees of sterility to
+prevent them crossing and blending in nature, than to think that trees have
+been specially endowed with various and {277} somewhat analogous degrees of
+difficulty in being grafted together in order to prevent them becoming
+inarched in our forests.
+
+The sterility of first crosses between pure species, which have their
+reproductive systems perfect, seems to depend on several circumstances; in
+some cases largely on the early death of the embryo. The sterility of
+hybrids, which have their reproductive systems imperfect, and which have
+had this system and their whole organisation disturbed by being compounded
+of two distinct species, seems closely allied to that sterility which so
+frequently affects pure species, when their natural conditions of life have
+been disturbed. This view is supported by a parallelism of another
+kind;--namely, that the crossing of forms only slightly different is
+favourable to the vigour and fertility of their offspring; and that slight
+changes in the conditions of life are apparently favourable to the vigour
+and fertility of all organic beings. It is not surprising that the degree
+of difficulty in uniting two species, and the degree of sterility of their
+hybrid-offspring should generally correspond, though due to distinct
+causes; for both depend on the amount of difference of some kind between
+the species which are crossed. Nor is it surprising that the facility of
+effecting a first cross, the fertility of the hybrids produced from it, and
+the capacity of being grafted together--though this latter capacity
+evidently depends on widely different circumstances--should all run, to a
+certain extent, parallel with the systematic affinity of the forms which
+are subjected to experiment; for systematic affinity attempts to express
+all kinds of resemblance between all species.
+
+First crosses between forms known to be varieties, or sufficiently alike to
+be considered as varieties, and their mongrel offspring, are very
+generally, but not quite {278} universally, fertile. Nor is this nearly
+general and perfect fertility surprising, when we remember how liable we
+are to argue in a circle with respect to varieties in a state of nature;
+and when we remember that the greater number of varieties have been
+produced under domestication by the selection of mere external differences,
+and not of differences in the reproductive system. In all other respects,
+excluding fertility, there is a close general resemblance between hybrids
+and mongrels. Finally, then, the facts briefly given in this chapter do not
+seem to me opposed to, but even rather to support the view, that there is
+no fundamental distinction between species and varieties.
+
+       *       *       *       *       *
+
+
+{279}
+
+CHAPTER IX.
+
+ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
+
+    On the absence of intermediate varieties at the present day--On the
+    nature of extinct intermediate varieties; on their number--On the vast
+    lapse of time, as inferred from the rate of deposition and of
+    denudation--On the poorness of our palæontological collections--On the
+    intermittence of geological formations--On the absence of intermediate
+    varieties in any one formation--On the sudden appearance of groups of
+    species--On their sudden appearance in the lowest known fossiliferous
+    strata.
+
+In the sixth chapter I enumerated the chief objections which might be
+justly urged against the views maintained in this volume. Most of them have
+now been discussed. One, namely the distinctness of specific forms, and
+their not being blended together by innumerable transitional links, is a
+very obvious difficulty. I assigned reasons why such links do not commonly
+occur at the present day, under the circumstances apparently most
+favourable for their presence, namely on an extensive and continuous area
+with graduated physical conditions. I endeavoured to show, that the life of
+each species depends in a more important manner on the presence of other
+already defined organic forms, than on climate; and, therefore, that the
+really governing conditions of life do not graduate away quite insensibly
+like heat or moisture. I endeavoured, also, to show that intermediate
+varieties, from existing in lesser numbers than the forms which they
+connect, will generally be beaten out and exterminated during the course of
+further modification and improvement. The main cause, however, of
+innumerable intermediate links not now occurring everywhere throughout
+nature {280} depends on the very process of natural selection, through
+which new varieties continually take the places of and exterminate their
+parent-forms. But just in proportion as this process of extermination has
+acted on an enormous scale, so must the number of intermediate varieties,
+which have formerly existed on the earth, be truly enormous. Why then is
+not every geological formation and every stratum full of such intermediate
+links? Geology assuredly does not reveal any such finely graduated organic
+chain; and this, perhaps, is the most obvious and gravest objection which
+can be urged against my theory. The explanation lies, as I believe, in the
+extreme imperfection of the geological record.
+
+In the first place it should always be borne in mind what sort of
+intermediate forms must, on my theory, have formerly existed. I have found
+it difficult, when looking at any two species, to avoid picturing to
+myself, forms _directly_ intermediate between them. But this is a wholly
+false view; we should always look for forms intermediate between each
+species and a common but unknown progenitor; and the progenitor will
+generally have differed in some respects from all its modified descendants.
+To give a simple illustration: the fantail and pouter pigeons have both
+descended from the rock-pigeon; if we possessed all the intermediate
+varieties which have ever existed, we should have an extremely close series
+between both and the rock-pigeon; but we should have no varieties directly
+intermediate between the fantail and pouter; none, for instance, combining
+a tail somewhat expanded with a crop somewhat enlarged, the characteristic
+features of these two breeds. These two breeds, moreover, have become so
+much modified, that if we had no historical or indirect evidence regarding
+their origin, it would not have been possible to have {281} determined from
+a mere comparison of their structure with that of the rock-pigeon, whether
+they had descended from this species or from some other allied species,
+such as C. oenas.
+
+So with natural species, if we look to forms very distinct, for instance to
+the horse and tapir, we have no reason to suppose that links ever existed
+directly intermediate between them, but between each and an unknown common
+parent. The common parent will have had in its whole organisation much
+general resemblance to the tapir and to the horse; but in some points of
+structure may have differed considerably from both, even perhaps more than
+they differ from each other. Hence in all such cases, we should be unable
+to recognise the parent-form of any two or more species, even if we closely
+compared the structure of the parent with that of its modified descendants,
+unless at the same time we had a nearly perfect chain of the intermediate
+links.
+
+It is just possible by my theory, that one of two living forms might have
+descended from the other; for instance, a horse from a tapir; and in this
+case _direct_ intermediate links will have existed between them. But such a
+case would imply that one form had remained for a very long period
+unaltered, whilst its descendants had undergone a vast amount of change;
+and the principle of competition between organism and organism, between
+child and parent, will render this a very rare event; for in all cases the
+new and improved forms of life tend to supplant the old and unimproved
+forms.
+
+By the theory of natural selection all living species have been connected
+with the parent-species of each genus, by differences not greater than we
+see between the varieties of the same species at the present {282} day; and
+these parent-species, now generally extinct, have in their turn been
+similarly connected with more ancient species; and so on backwards, always
+converging to the common ancestor of each great class. So that the number
+of intermediate and transitional links, between all living and extinct
+species, must have been inconceivably great. But assuredly, if this theory
+be true, such have lived upon this earth.
+
+
+
+_On the lapse of Time._--Independently of our not finding fossil remains of
+such infinitely numerous connecting links, it may be objected, that time
+will not have sufficed for so great an amount of organic change, all
+changes having been effected very slowly through natural selection. It is
+hardly possible for me even to recall to the reader, who may not be a
+practical geologist, the facts leading the mind feebly to comprehend the
+lapse of time. He who can read Sir Charles Lyell's grand work on the
+Principles of Geology, which the future historian will recognise as having
+produced a revolution in natural science, yet does not admit how
+incomprehensively vast have been the past periods of time, may at once
+close this volume. Not that it suffices to study the Principles of Geology,
+or to read special treatises by different observers on separate formations,
+and to mark how each author attempts to give an inadequate idea of the
+duration of each formation or even each stratum. A man must for years
+examine for himself great piles of superimposed strata, and watch the sea
+at work grinding down old rocks and making fresh sediment, before he can
+hope to comprehend anything of the lapse of time, the monuments of which we
+see around us.
+
+It is good to wander along lines of sea-coast, when formed of moderately
+hard rocks, and mark the {283} process of degradation. The tides in most
+cases reach the cliffs only for a short time twice a day, and the waves eat
+into them only when they are charged with sand or pebbles; for there is
+good evidence that pure water can effect little or nothing in wearing away
+rock. At last the base of the cliff is undermined, huge fragments fall
+down, and these remaining fixed, have to be worn away, atom by atom, until
+reduced in size they can be rolled about by the waves, and then are more
+quickly ground into pebbles, sand, or mud. But how often do we see along
+the bases of retreating cliffs rounded boulders, all thickly clothed by
+marine productions, showing how little they are abraded and how seldom they
+are rolled about! Moreover, if we follow for a few miles any line of rocky
+cliff, which is undergoing degradation, we find that it is only here and
+there, along a short length or round a promontory, that the cliffs are at
+the present time suffering. The appearance of the surface and the
+vegetation show that elsewhere years have elapsed since the waters washed
+their base.
+
+He who most closely studies the action of the sea on our shores, will, I
+believe, be most deeply impressed with the slowness with which rocky coasts
+are worn away. The observations on this head by Hugh Miller, and by that
+excellent observer Mr. Smith of Jordan Hill, are most impressive. With the
+mind thus impressed, let any one examine beds of conglomerate many thousand
+feet in thickness, which, though probably formed at a quicker rate than
+many other deposits, yet, from being formed of worn and rounded pebbles,
+each of which bears the stamp of time, are good to show how slowly the mass
+has been accumulated. In the Cordillera I estimated one pile of
+conglomerate at ten thousand feet in thickness. Let the {284} observer
+remember Lyell's profound remark that the thickness and extent of
+sedimentary formations are the result and measure of the degradation which
+the earth's crust has elsewhere suffered. And what an amount of degradation
+is implied by the sedimentary deposits of many countries! Professor Ramsay
+has given me the maximum thickness, in most cases from actual measurement,
+in a few cases from estimate, of each formation in different parts of Great
+Britain; and this is the result:--
+
+                                                     Feet.
+  Palæozoic strata (not including igneous beds)      57,154
+  Secondary strata                                   13,190
+  Tertiary strata                                     2,240
+
+--making altogether 72,584 feet; that is, very nearly thirteen and
+three-quarters British miles. Some of the formations, which are represented
+in England by thin beds, are thousands of feet in thickness on the
+Continent. Moreover, between each successive formation, we have, in the
+opinion of most geologists, enormously long blank periods. So that the
+lofty pile of sedimentary rocks in Britain, gives but an inadequate idea of
+the time which has elapsed during their accumulation; yet what time this
+must have consumed! Good observers have estimated that sediment is
+deposited by the great Mississippi river at the rate of only 600 feet in a
+hundred thousand years. This estimate has no pretension to strict
+exactness; yet, considering over what wide spaces very fine sediment is
+transported by the currents of the sea, the process of accumulation in any
+one area must be extremely slow.
+
+But the amount of denudation which the strata have in many places suffered,
+independently of the rate of accumulation of the degraded matter, probably
+offers the best evidence of the lapse of time. I remember {285} having been
+much struck with the evidence of denudation, when viewing volcanic islands,
+which have been worn by the waves and pared all round into perpendicular
+cliffs of one or two thousand feet in height; for the gentle slope of the
+lava-streams, due to their formerly liquid state, showed at a glance how
+far the hard, rocky beds had once extended into the open ocean. The same
+story is still more plainly told by faults,--those great cracks along which
+the strata have been upheaved on one side, or thrown down on the other, to
+the height or depth of thousands of feet; for since the crust cracked, the
+surface of the land has been so completely planed down by the action of the
+sea, that no trace of these vast dislocations is externally visible.
+
+The Craven fault, for instance, extends for upwards of 30 miles, and along
+this line the vertical displacement of the strata has varied from 600 to
+3000 feet. Prof. Ramsay has published an account of a downthrow in Anglesea
+of 2300 feet; and he informs me that he fully believes there is one in
+Merionethshire of 12,000 feet; yet in these cases there is nothing on the
+surface to show such prodigious movements; the pile of rocks on the one or
+other side having been smoothly swept away. The consideration of these
+facts impresses my mind almost in the same manner as does the vain
+endeavour to grapple with the idea of eternity.
+
+I am tempted to give one other case, the well-known one of the denudation
+of the Weald. Though it must be admitted that the denudation of the Weald
+has been a mere trifle, in comparison with that which has removed masses of
+our palæozoic strata, in parts ten thousand feet in thickness, as shown in
+Prof. Ramsay's masterly memoir on this subject: yet it is an admirable
+lesson to stand on the intermediate hilly country and look on the one hand
+at the North Downs, and {286} on the other hand at the South Downs; for,
+remembering that at no great distance to the west the northern and southern
+escarpments meet and close, one can safely picture to oneself the great
+dome of rocks which must have covered up the Weald within so limited a
+period as since the latter part of the Chalk formation. The distance from
+the northern to the southern Downs is about 22 miles, and the thickness of
+the several formations is on an average about 1100 feet, as I am informed
+by Prof. Ramsay. But if, as some geologists suppose, a range of older rocks
+underlies the Weald, on the flanks of which the overlying sedimentary
+deposits might have accumulated in thinner masses than elsewhere, the above
+estimate would be erroneous; but this source of doubt probably would not
+greatly affect the estimate as applied to the western extremity of the
+district. If, then, we knew the rate at which the sea commonly wears away a
+line of cliff of any given height, we could measure the time requisite to
+have denuded the Weald. This, of course cannot be done; but we may, in
+order to form some crude notion on the subject, assume that the sea would
+eat into cliffs 500 feet in height at the rate of one inch in a century.
+This will at first appear much too small an allowance; but it is the same
+as if we were to assume a cliff one yard in height to be eaten back along a
+whole line of coast at the rate of one yard in nearly every twenty-two
+years. I doubt whether any rock, even as soft as chalk, would yield at this
+rate excepting on the most exposed coasts; though no doubt the degradation
+of a lofty cliff would be more rapid from the breakage of the fallen
+fragments. On the other hand, I do not believe that any line of coast, ten
+or twenty miles in length, ever suffers degradation at the same time along
+its whole indented length; and we {287} must remember that almost all
+strata contain harder layers or nodules, which from long resisting
+attrition form a breakwater at the base. We may at least confidently
+believe that no rocky coast 500 feet in height commonly yields at the rate
+of a foot per century; for this would be the same in amount as a cliff one
+yard in height retreating twelve yards in twenty-two years; and no one, I
+think, who has carefully observed the shape of old fallen fragments at the
+base of cliffs, will admit any near approach to such rapid wearing away.
+Hence, under ordinary circumstances, I should infer that for a cliff 500
+feet in height, a denudation of one inch per century for the whole length
+would be a sufficient allowance. At this rate, on the above data, the
+denudation of the Weald must have required 306,662,400 years; or say three
+hundred million years. But perhaps it would be safer to allow two or three
+inches per century, and this would reduce the number of years to one
+hundred and fifty or one hundred million years.
+
+The action of fresh water on the gently inclined Wealden district, when
+upraised, could hardly have been great, but it would somewhat reduce the
+above estimate. On the other hand, during oscillations of level, which we
+know this area has undergone, the surface may have existed for millions of
+years as land, and thus have escaped the action of the sea: when deeply
+submerged for perhaps equally long periods, it would, likewise, have
+escaped the action of the coast-waves. So that it is not improbable that a
+longer period than 300 million years has elapsed since the latter part of
+the Secondary period.
+
+I have made these few remarks because it is highly important for us to gain
+some notion, however imperfect, of the lapse of years. During each of these
+years, {288} over the whole world, the land and the water has been peopled
+by hosts of living forms. What an infinite number of generations, which the
+mind cannot grasp, must have succeeded each other in the long roll of
+years! Now turn to our richest geological museums, and what a paltry
+display we behold!
+
+
+
+_On the poorness of our Palæontological collections._--That our
+palæontological collections are very imperfect, is admitted by every one.
+The remark of that admirable palæontologist, the late Edward Forbes, should
+not be forgotten, namely, that numbers of our fossil species are known and
+named from single and often broken specimens, or from a few specimens
+collected on some one spot. Only a small portion of the surface of the
+earth has been geologically explored, and no part with sufficient care, as
+the important discoveries made every year in Europe prove. No organism
+wholly soft can be preserved. Shells and bones will decay and disappear
+when left on the bottom of the sea, where sediment is not accumulating. I
+believe we are continually taking a most erroneous view, when we tacitly
+admit to ourselves that sediment is being deposited over nearly the whole
+bed of the sea, at a rate sufficiently quick to embed and preserve fossil
+remains. Throughout an enormously large proportion of the ocean, the bright
+blue tint of the water bespeaks its purity. The many cases on record of a
+formation conformably covered, after an enormous interval of time, by
+another and later formation, without the underlying bed having suffered in
+the interval any wear and tear, seem explicable only on the view of the
+bottom of the sea not rarely lying for ages in an unaltered condition. The
+remains which do become embedded, if in sand or gravel, will when the beds
+are upraised generally be dissolved {289} by the percolation of rain-water.
+I suspect that but few of the very many animals which live on the beach
+between high and low watermark are preserved. For instance, the several
+species of the Chthamalinæ (a subfamily of sessile cirripedes) coat the
+rocks all over the world in infinite numbers: they are all strictly
+littoral, with the exception of a single Mediterranean species, which
+inhabits deep water and has been found fossil in Sicily, whereas not one
+other species has hitherto been found in any tertiary formation: yet it is
+now known that the genus Chthamalus existed during the chalk period. The
+molluscan genus Chiton offers a partially analogous case.
+
+With respect to the terrestrial productions which lived during the
+Secondary and Palæozoic periods, it is superfluous to state that our
+evidence from fossil remains is fragmentary in an extreme degree. For
+instance, not a land shell is known belonging to either of these vast
+periods, with the exception of one species discovered by Sir C. Lyell and
+Dr. Dawson in the carboniferous strata of North America, of which shell
+several specimens have now been collected. In regard to mammiferous
+remains, a single glance at the historical table published in the
+Supplement to Lyell's Manual, will bring home the truth, how accidental and
+rare is their preservation, far better than pages of detail. Nor is their
+rarity surprising, when we remember how large a proportion of the bones of
+tertiary mammals have been discovered either in caves or in lacustrine
+deposits; and that not a cave or true lacustrine bed is known belonging to
+the age of our secondary or palæozoic formations.
+
+But the imperfection in the geological record mainly results from another
+and more important cause than any of the foregoing; namely, from the
+several formations {290} being separated from each other by wide intervals
+of time. When we see the formations tabulated in written works, or when we
+follow them in nature, it is difficult to avoid believing that they are
+closely consecutive. But we know, for instance, from Sir R. Murchison's
+great work on Russia, what wide gaps there are in that country between the
+superimposed formations; so it is in North America, and in many other parts
+of the world. The most skilful geologist, if his attention had been
+exclusively confined to these large territories, would never have suspected
+that during the periods which were blank and barren in his own country,
+great piles of sediment, charged with new and peculiar forms of life, had
+elsewhere been accumulated. And if in each separate territory, hardly any
+idea can be formed of the length of time which has elapsed between the
+consecutive formations, we may infer that this could nowhere be
+ascertained. The frequent and great changes in the mineralogical
+composition of consecutive formations, generally implying great changes in
+the geography of the surrounding lands, whence the sediment has been
+derived, accords with the belief of vast intervals of time having elapsed
+between each formation.
+
+But we can, I think, see why the geological formations of each region are
+almost invariably intermittent; that is, have not followed each other in
+close sequence. Scarcely any fact struck me more when examining many
+hundred miles of the South American coasts, which have been upraised
+several hundred feet within the recent period, than the absence of any
+recent deposits sufficiently extensive to last for even a short geological
+period. Along the whole west coast, which is inhabited by a peculiar marine
+fauna, tertiary beds are so poorly developed, that no record of several
+{291} successive and peculiar marine faunas will probably be preserved to a
+distant age. A little reflection will explain why along the rising coast of
+the western side of South America, no extensive formations with recent or
+tertiary remains can anywhere be found, though the supply of sediment must
+for ages have been great, from the enormous degradation of the coast-rocks
+and from muddy streams entering the sea. The explanation, no doubt, is,
+that the littoral and sub-littoral deposits are continually worn away, as
+soon as they are brought up by the slow and gradual rising of the land
+within the grinding action of the coast-waves.
+
+We may, I think, safely conclude that sediment must be accumulated in
+extremely thick, solid, or extensive masses, in order to withstand the
+incessant action of the waves, when first upraised and during subsequent
+oscillations of level. Such thick and extensive accumulations of sediment
+may be formed in two ways; either, in profound depths of the sea, in which
+case, judging from the researches of E. Forbes, we may conclude that the
+bottom will be inhabited by extremely few animals, and the mass when
+upraised will give a most imperfect record of the forms of life which then
+existed; or, sediment may be accumulated to any thickness and extent over a
+shallow bottom, if it continue slowly to subside. In this latter case, as
+long as the rate of subsidence and supply of sediment nearly balance each
+other, the sea will remain shallow and favourable for life, and thus a
+fossiliferous formation thick enough, when upraised, to resist any amount
+of degradation, may be formed.
+
+I am convinced that all our ancient formations, which are rich in fossils,
+have thus been formed during subsidence. Since publishing my views on this
+subject in 1845, I have watched the progress of {292} Geology, and have
+been surprised to note how author after author, in treating of this or that
+great formation, has come to the conclusion that it was accumulated during
+subsidence. I may add, that the only ancient tertiary formation on the west
+coast of South America, which has been bulky enough to resist such
+degradation as it has as yet suffered, but which will hardly last to a
+distant geological age, was certainly deposited during a downward
+oscillation of level, and thus gained considerable thickness.
+
+All geological facts tell us plainly that each area has undergone numerous
+slow oscillations of level, and apparently these oscillations have affected
+wide spaces. Consequently formations rich in fossils and sufficiently thick
+and extensive to resist subsequent degradation, may have been formed over
+wide spaces during periods of subsidence, but only where the supply of
+sediment was sufficient to keep the sea shallow and to embed and preserve
+the remains before they had time to decay. On the other hand, as long as
+the bed of the sea remained stationary, _thick_ deposits could not have
+been accumulated in the shallow parts, which are the most favourable to
+life. Still less could this have happened during the alternate periods of
+elevation; or, to speak more accurately, the beds which were then
+accumulated will have been destroyed by being upraised and brought within
+the limits of the coast-action.
+
+Thus the geological record will almost necessarily be rendered
+intermittent. I feel much confidence in the truth of these views, for they
+are in strict accordance with the general principles inculcated by Sir C.
+Lyell; and E. Forbes subsequently but independently arrived at a similar
+conclusion.
+
+One remark is here worth a passing notice. During periods of elevation the
+area of the land and of the {293} adjoining shoal parts of the sea will be
+increased, and new stations will often be formed;--all circumstances most
+favourable, as previously explained, for the formation of new varieties and
+species; but during such periods there will generally be a blank in the
+geological record. On the other hand, during subsidence, the inhabited area
+and number of inhabitants will decrease (excepting the productions on the
+shores of a continent when first broken up into an archipelago), and
+consequently during subsidence, though there will be much extinction, fewer
+new varieties or species will be formed; and it is during these very
+periods of subsidence, that our great deposits rich in fossils have been
+accumulated. Nature may almost be said to have guarded against the frequent
+discovery of her transitional or linking forms.
+
+From the foregoing considerations it cannot be doubted that the geological
+record, viewed as a whole, is extremely imperfect; but if we confine our
+attention to any one formation, it becomes more difficult to understand,
+why we do not therein find closely graduated varieties between the allied
+species which lived at its commencement and at its close. Some cases are on
+record of the same species presenting distinct varieties in the upper and
+lower parts of the same formation, but, as they are rare, they may be here
+passed over. Although each formation has indisputably required a vast
+number of years for its deposition, I can see several reasons why each
+should not include a graduated series of links between the species which
+then lived; but I can by no means pretend to assign due proportional weight
+to the following considerations.
+
+Although each formation may mark a very long lapse of years, each perhaps
+is short compared with the period requisite to change one species into
+another. I am {294} aware that two palæontologists, whose opinions are
+worthy of much deference, namely Bronn and Woodward, have concluded that
+the average duration of each formation is twice or thrice as long as the
+average duration of specific forms. But insuperable difficulties, as it
+seems to me, prevent us coming to any just conclusion on this head. When we
+see a species first appearing in the middle of any formation, it would be
+rash in the extreme to infer that it had not elsewhere previously existed.
+So again when we find a species disappearing before the uppermost layers
+have been deposited, it would be equally rash to suppose that it then
+became wholly extinct. We forget how small the area of Europe is compared
+with the rest of the world; nor have the several stages of the same
+formation throughout Europe been correlated with perfect accuracy.
+
+With marine animals of all kinds, we may safely infer a large amount of
+migration during climatal and other changes; and when we see a species
+first appearing in any formation, the probability is that it only then
+first immigrated into that area. It is well known, for instance, that
+several species appeared somewhat earlier in the palæozoic beds of North
+America than in those of Europe; time having apparently been required for
+their migration from the American to the European seas. In examining the
+latest deposits of various quarters of the world, it has everywhere been
+noted, that some few still existing species are common in the deposit, but
+have become extinct in the immediately surrounding sea; or, conversely,
+that some are now abundant in the neighbouring sea, but are rare or absent
+in this particular deposit. It is an excellent lesson to reflect on the
+ascertained amount of migration of the inhabitants of Europe during the
+Glacial period, which forms only a part of one whole geological period;
+{295} and likewise to reflect on the great changes of level, on the
+inordinately great change of climate, on the prodigious lapse of time, all
+included within this same glacial period. Yet it may be doubted whether in
+any quarter of the world, sedimentary deposits, _including fossil remains_,
+have gone on accumulating within the same area during the whole of this
+period. It is not, for instance, probable that sediment was deposited
+during the whole of the glacial period near the mouth of the Mississippi,
+within that limit of depth at which marine animals can flourish; for we
+know what vast geographical changes occurred in other parts of America
+during this space of time. When such beds as were deposited in shallow
+water near the mouth of the Mississippi during some part of the glacial
+period shall have been upraised, organic remains will probably first appear
+and disappear at different levels, owing to the migration of species and to
+geographical changes. And in the distant future, a geologist examining
+these beds, might be tempted to conclude that the average duration of life
+of the embedded fossils had been less than that of the glacial period,
+instead of having been really far greater, that is extending from before
+the glacial epoch to the present day.
+
+In order to get a perfect gradation between two forms in the upper and
+lower parts of the same formation, the deposit must have gone on
+accumulating for a very long period, in order to have given sufficient time
+for the slow process of variation; hence the deposit will generally have to
+be a very thick one; and the species undergoing modification will have had
+to live on the same area throughout this whole time. But we have seen that
+a thick fossiliferous formation can only be accumulated during a period of
+subsidence; and to keep the depth approximately the same, which is
+necessary in {296} order to enable the same species to live on the same
+space, the supply of sediment must nearly have counterbalanced the amount
+of subsidence. But this same movement of subsidence will often tend to sink
+the area whence the sediment is derived, and thus diminish the supply
+whilst the downward movement continues. In fact, this nearly exact
+balancing between the supply of sediment and the amount of subsidence is
+probably a rare contingency; for it has been observed by more than one
+palæontologist, that very thick deposits are usually barren of organic
+remains, except near their upper or lower limits.
+
+It would seem that each separate formation, like the whole pile of
+formations in any country, has generally been intermittent in its
+accumulation. When we see, as is so often the case, a formation composed of
+beds of different mineralogical composition, we may reasonably suspect that
+the process of deposition has been much interrupted, as a change in the
+currents of the sea and a supply of sediment of a different nature will
+generally have been due to geographical changes requiring much time. Nor
+will the closest inspection of a formation give any idea of the time which
+its deposition has consumed. Many instances could be given of beds only a
+few feet in thickness, representing formations, elsewhere thousands of feet
+in thickness, and which must have required an enormous period for their
+accumulation; yet no one ignorant of this fact would have suspected the
+vast lapse of time represented by the thinner formation. Many cases could
+be given of the lower beds of a formation having been upraised, denuded,
+submerged, and then re-covered by the upper beds of the same
+formation,--facts, showing what wide, yet easily overlooked, intervals have
+occurred in its accumulation. In other cases we have the plainest evidence
+{297} in great fossilised trees, still standing upright as they grew, of
+many long intervals of time and changes of level during the process of
+deposition, which would never even have been suspected, had not the trees
+chanced to have been preserved: thus Messrs. Lyell and Dawson found
+carboniferous beds 1400 feet thick in Nova Scotia, with ancient
+root-bearing strata, one above the other, at no less than sixty-eight
+different levels. Hence, when the same species occur at the bottom, middle,
+and top of a formation, the probability is that they have not lived on the
+same spot during the whole period of deposition, but have disappeared and
+reappeared, perhaps many times, during the same geological period. So that
+if such species were to undergo a considerable amount of modification
+during any one geological period, a section would not probably include all
+the fine intermediate gradations which must on my theory have existed
+between them, but abrupt, though perhaps very slight, changes of form.
+
+It is all-important to remember that naturalists have no golden rule by
+which to distinguish species and varieties; they grant some little
+variability to each species, but when they meet with a somewhat greater
+amount of difference between any two forms, they rank both as species,
+unless they are enabled to connect them together by close intermediate
+gradations. And this from the reasons just assigned we can seldom hope to
+effect in any one geological section. Supposing B and C to be two species,
+and a third, A, to be found in an underlying bed; even if A were strictly
+intermediate between B and C, it would simply be ranked as a third and
+distinct species, unless at the same time it could be most closely
+connected with either one or both forms by intermediate varieties. Nor
+should it be forgotten, as before explained, that A might be the actual
+progenitor {298} of B and C, and yet might not at all necessarily be
+strictly intermediate between them in all points of structure. So that we
+might obtain the parent-species and its several modified descendants from
+the lower and upper beds of a formation, and unless we obtained numerous
+transitional gradations, we should not recognise their relationship, and
+should consequently be compelled to rank them all as distinct species.
+
+It is notorious on what excessively slight differences many palæontologists
+have founded their species; and they do this the more readily if the
+specimens come from different sub-stages of the same formation. Some
+experienced conchologists are now sinking many of the very fine species of
+D'Orbigny and others into the rank of varieties; and on this view we do
+find the kind of evidence of change which on my theory we ought to find.
+Moreover, if we look to rather wider intervals, namely, to distinct but
+consecutive stages of the same great formation, we find that the embedded
+fossils, though almost universally ranked as specifically different, yet
+are far more closely allied to each other than are the species found in
+more widely separated formations; but to this subject I shall have to
+return in the following chapter.
+
+One other consideration is worth notice: with animals and plants that can
+propagate rapidly and are not highly locomotive, there is reason to
+suspect, as we have formerly seen, that their varieties are generally at
+first local; and that such local varieties do not spread widely and
+supplant their parent-forms until they have been modified and perfected in
+some considerable degree. According to this view, the chance of discovering
+in a formation in any one country all the early stages of transition
+between any two forms, is small, for the successive changes are supposed to
+have been local or {299} confined to some one spot. Most marine animals
+have a wide range; and we have seen that with plants it is those which have
+the widest range, that oftenest present varieties; so that with shells and
+other marine animals, it is probably those which have had the widest range,
+far exceeding the limits of the known geological formations of Europe,
+which have oftenest given rise, first to local varieties and ultimately to
+new species; and this again would greatly lessen the chance of our being
+able to trace the stages of transition in any one geological formation.
+
+It should not be forgotten, that at the present day, with perfect specimens
+for examination, two forms can seldom be connected by intermediate
+varieties and thus proved to be the same species, until many specimens have
+been collected from many places; and in the case of fossil species this
+could rarely be effected by palæontologists. We shall, perhaps, best
+perceive the improbability of our being enabled to connect species by
+numerous, fine, intermediate, fossil links, by asking ourselves whether,
+for instance, geologists at some future period will be able to prove, that
+our different breeds of cattle, sheep, horses, and dogs have descended from
+a single stock or from several aboriginal stocks; or, again, whether
+certain sea-shells inhabiting the shores of North America, which are ranked
+by some conchologists as distinct species from their European
+representatives, and by other conchologists as only varieties, are really
+varieties or are, as it is called, specifically distinct. This could be
+effected only by the future geologist discovering in a fossil state
+numerous intermediate gradations; and such success seems to me improbable
+in the highest degree.
+
+Geological research, though it has added numerous species to existing and
+extinct genera, and has made the {300} intervals between some few groups
+less wide than they otherwise would have been, yet has done scarcely
+anything in breaking down the distinction between species, by connecting
+them together by numerous, fine, intermediate varieties; and this not
+having been effected, is probably the gravest and most obvious of all the
+many objections which may be urged against my views. Hence it will be worth
+while to sum up the foregoing remarks, under an imaginary illustration. The
+Malay Archipelago is of about the size of Europe from the North Cape to the
+Mediterranean, and from Britain to Russia; and therefore equals all the
+geological formations which have been examined with any accuracy, excepting
+those of the United States of America. I fully agree with Mr.
+Godwin-Austen, that the present condition of the Malay Archipelago, with
+its numerous large islands separated by wide and shallow seas, probably
+represents the former state of Europe, whilst most of our formations were
+accumulating. The Malay Archipelago is one of the richest regions of the
+whole world in organic beings; yet if all the species were to be collected
+which have ever lived there, how imperfectly would they represent the
+natural history of the world!
+
+But we have every reason to believe that the terrestrial productions of the
+archipelago would be preserved in an excessively imperfect manner in the
+formations which we suppose to be there accumulating. I suspect that not
+many of the strictly littoral animals, or of those which lived on naked
+submarine rocks, would be embedded; and those embedded in gravel or sand,
+would not endure to a distant epoch. Wherever sediment did not accumulate
+on the bed of the sea, or where it did not accumulate at a sufficient rate
+to protect organic bodies from decay, no remains could be preserved.
+
+I believe that fossiliferous formations could be formed {301} in the
+archipelago, of thickness sufficient to last to an age as distant in
+futurity as the secondary formations lie in the past, only during periods
+of subsidence. These periods of subsidence would be separated from each
+other by enormous intervals, during which the area would be either
+stationary or rising; whilst rising, each fossiliferous formation would be
+destroyed, almost as soon as accumulated, by the incessant coast-action, as
+we now see on the shores of South America. During the periods of subsidence
+there would probably be much extinction of life; during the periods of
+elevation, there would be much variation, but the geological record would
+then be least perfect.
+
+It may be doubted whether the duration of any one great period of
+subsidence over the whole or part of the archipelago, together with a
+contemporaneous accumulation of sediment, would _exceed_ the average
+duration of the same specific forms; and these contingencies are
+indispensable for the preservation of all the transitional gradations
+between any two or more species. If such gradations were not fully
+preserved, transitional varieties would merely appear as so many distinct
+species. It is, also, probable that each great period of subsidence would
+be interrupted by oscillations of level, and that slight climatal changes
+would intervene during such lengthy periods; and in these cases the
+inhabitants of the archipelago would have to migrate, and no closely
+consecutive record of their modifications could be preserved in any one
+formation.
+
+Very many of the marine inhabitants of the archipelago now range thousands
+of miles beyond its confines; and analogy leads me to believe that it would
+be chiefly these far-ranging species which would oftenest produce new
+varieties; and the varieties would at first generally be local or confined
+to one place, but if possessed {302} of any decided advantage, or when
+further modified and improved, they would slowly spread and supplant their
+parent-forms. When such varieties returned to their ancient homes, as they
+would differ from their former state, in a nearly uniform, though perhaps
+extremely slight degree, they would, according to the principles followed
+by many palæontologists, be ranked as new and distinct species.
+
+If then, there be some degree of truth in these remarks, we have no right
+to expect to find in our geological formations, an infinite number of those
+fine transitional forms, which on my theory assuredly have connected all
+the past and present species of the same group into one long and branching
+chain of life. We ought only to look for a few links, some more closely,
+some more distantly related to each other; and these links, let them be
+ever so close, if found in different stages of the same formation, would,
+by most palæontologists, be ranked as distinct species. But I do not
+pretend that I should ever have suspected how poor a record of the
+mutations of life, the best preserved geological section presented, had not
+the difficulty of our not discovering innumerable transitional links
+between the species which appeared at the commencement and close of each
+formation, pressed so hardly on my theory.
+
+
+
+_On the sudden appearance of whole groups of Allied Species._--The abrupt
+manner in which whole groups of species suddenly appear in certain
+formations, has been urged by several palæontologists--for instance, by
+Agassiz, Pictet, and by none more forcibly than by Professor Sedgwick--as a
+fatal objection to the belief in the transmutation of species. If numerous
+species, belonging to the same genera or families, have really {303}
+started into life all at once, the fact would be fatal to the theory of
+descent with slow modification through natural selection. For the
+development of a group of forms, all of which have descended from some one
+progenitor, must have been an extremely slow process; and the progenitors
+must have lived long ages before their modified descendants. But we
+continually over-rate the perfection of the geological record, and falsely
+infer, because certain genera or families have not been found beneath a
+certain stage, that they did not exist before that stage. We continually
+forget how large the world is, compared with the area over which our
+geological formations have been carefully examined; we forget that groups
+of species may elsewhere have long existed and have slowly multiplied
+before they invaded the ancient archipelagoes of Europe and of the United
+States. We do not make due allowance for the enormous intervals of time,
+which have probably elapsed between our consecutive formations,--longer
+perhaps in most cases than the time required for the accumulation of each
+formation. These intervals will have given time for the multiplication of
+species from some one or some few parent-forms; and in the succeeding
+formation such species will appear as if suddenly created.
+
+I may here recall a remark formerly made, namely that it might require a
+long succession of ages to adapt an organism to some new and peculiar line
+of life, for instance to fly through the air; but that when this had been
+effected, and a few species had thus acquired a great advantage over other
+organisms, a comparatively short time would be necessary to produce many
+divergent forms, which would be able to spread rapidly and widely
+throughout the world.
+
+I will now give a few examples to illustrate these {304} remarks, and to
+show how liable we are to error in supposing that whole groups of species
+have suddenly been produced. I may recall the well-known fact that in
+geological treatises, published not many years ago, the great class of
+mammals was always spoken of as having abruptly come in at the commencement
+of the tertiary series. And now one of the richest known accumulations of
+fossil mammals, for its thickness, belongs to the middle of the secondary
+series; and one true mammal has been discovered in the new red sandstone at
+nearly the commencement of this great series. Cuvier used to urge that no
+monkey occurred in any tertiary stratum; but now extinct species have been
+discovered in India, South America, and in Europe even as far back as the
+eocene stage. Had it not been for the rare accident of the preservation of
+footsteps in the new red sandstone of the United States, who would have
+ventured to suppose that, besides reptiles, no less than at least thirty
+kinds of birds, some of gigantic size, existed during that period? Not a
+fragment of bone has been discovered in these beds. Notwithstanding that
+the number of joints shown in the fossil impressions correspond with the
+number in the several toes of living birds' feet, some authors doubt
+whether the animals which left the impressions were really birds. Until
+quite recently these authors might have maintained, and some have
+maintained, that the whole class of birds came suddenly into existence
+during an early tertiary period; but now we know, on the authority of
+Professor Owen (as may be seen in Lyell's 'Manual'), that a bird certainly
+lived during the deposition of the upper greensand.
+
+I may give another instance, which from having passed under my own eyes has
+much struck me. In a memoir on Fossil Sessile Cirripedes, I have stated
+that, from the {305} number of existing and extinct tertiary species; from
+the extraordinary abundance of the individuals of many species all over the
+world, from the Arctic regions to the equator, inhabiting various zones of
+depths from the upper tidal limits to 50 fathoms; from the perfect manner
+in which specimens are preserved in the oldest tertiary beds; from the ease
+with which even a fragment of a valve can be recognised; from all these
+circumstances, I inferred that had sessile cirripedes existed during the
+secondary periods, they would certainly have been preserved and discovered;
+and as not one species had then been discovered in beds of this age, I
+concluded that this great group had been suddenly developed at the
+commencement of the tertiary series. This was a sore trouble to me, adding
+as I thought one more instance of the abrupt appearance of a great group of
+species. But my work had hardly been published, when a skilful
+palæontologist, M. Bosquet, sent me a drawing of a perfect specimen of an
+unmistakeable sessile cirripede, which he had himself extracted from the
+chalk of Belgium. And, as if to make the case as striking as possible, this
+sessile cirripede was a Chthamalus, a very common, large, and ubiquitous
+genus, of which not one specimen has as yet been found even in any tertiary
+stratum. Hence we now positively know that sessile cirripedes existed
+during the secondary period; and these cirripedes might have been the
+progenitors of our many tertiary and existing species.
+
+The case most frequently insisted on by palæontologists of the apparently
+sudden appearance of a whole group of species, is that of the teleostean
+fishes, low down in the Chalk period. This group includes the large
+majority of existing species. Lately, Professor Pictet has carried their
+existence one sub-stage further back; and some palæontologists believe that
+certain {306} much older fishes, of which the affinities are as yet
+imperfectly known, are really teleostean. Assuming, however, that the whole
+of them did appear, as Agassiz believes, at the commencement of the chalk
+formation, the fact would certainly be highly remarkable; but I cannot see
+that it would be an insuperable difficulty on my theory, unless it could
+likewise be shown that the species of this group appeared suddenly and
+simultaneously throughout the world at this same period. It is almost
+superfluous to remark that hardly any fossil-fish are known from south of
+the equator; and by running through Pictet's Palæontology it will be seen
+that very few species are known from several formations in Europe. Some few
+families of fish now have a confined range; the teleostean fish might
+formerly have had a similarly confined range, and after having been largely
+developed in some one sea, might have spread widely. Nor have we any right
+to suppose that the seas of the world have always been so freely open from
+south to north as they are at present. Even at this day, if the Malay
+Archipelago were converted into land, the tropical parts of the Indian
+Ocean would form a large and perfectly enclosed basin, in which any great
+group of marine animals might be multiplied; and here they would remain
+confined, until some of the species became adapted to a cooler climate, and
+were enabled to double the southern capes of Africa or Australia, and thus
+reach other and distant seas.
+
+From these and similar considerations, but chiefly from our ignorance of
+the geology of other countries beyond the confines of Europe and the United
+States; and from the revolution in our palæontological ideas on many
+points, which the discoveries of even the last dozen years have effected,
+it seems to me to be about as rash in us to dogmatize on the succession of
+organic {307} beings throughout the world, as it would be for a naturalist
+to land for five minutes on some one barren point in Australia, and then to
+discuss the number and range of its productions.
+
+
+
+_On the sudden appearance of groups of Allied Species in the lowest known
+fossiliferous strata._--There is another and allied difficulty, which is
+much graver. I allude to the manner in which numbers of species of the same
+group, suddenly appear in the lowest known fossiliferous rocks. Most of the
+arguments which have convinced me that all the existing species of the same
+group have descended from one progenitor, apply with nearly equal force to
+the earliest known species. For instance, I cannot doubt that all the
+Silurian trilobites have descended from some one crustacean, which must
+have lived long before the Silurian age, and which probably differed
+greatly from any known animal. Some of the most ancient Silurian animals,
+as the Nautilus, Lingula, &c., do not differ much from living species; and
+it cannot on my theory be supposed, that these old species were the
+progenitors of all the species of the orders to which they belong, for they
+do not present characters in any degree intermediate between them. If,
+moreover, they had been the progenitors of these orders, they would almost
+certainly have been long ago supplanted and exterminated by their numerous
+and improved descendants.
+
+Consequently, if my theory be true, it is indisputable that before the
+lowest Silurian stratum was deposited, long periods elapsed, as long as, or
+probably far longer than, the whole interval from the Silurian age to the
+present day; and that during these vast, yet quite unknown, periods of
+time, the world swarmed with living creatures. {308}
+
+To the question why we do not find records of these vast primordial
+periods, I can give no satisfactory answer. Several of the most eminent
+geologists, with Sir E. Murchison at their head, are convinced that we see
+in the organic remains of the lowest Silurian stratum the dawn of life on
+this planet. Other highly competent judges, as Lyell and the late E.
+Forbes, dispute this conclusion. We should not forget that only a small
+portion of the world is known with accuracy. M. Barrande has lately added
+another and lower stage to the Silurian system, abounding with new and
+peculiar species. Traces of life have been detected in the Longmynd beds,
+beneath Barrande's so-called primordial zone. The presence of phosphatic
+nodules and bituminous matter in some of the lowest azoic rocks, probably
+indicates the former existence of life at these periods. But the difficulty
+of understanding the absence of vast piles of fossiliferous strata, which
+on my theory no doubt were somewhere accumulated before the Silurian epoch,
+is very great. If these most ancient beds had been wholly worn away by
+denudation, or obliterated by metamorphic action, we ought to find only
+small remnants of the formations next succeeding them in age, and these
+ought to be very generally in a metamorphosed condition. But the
+descriptions which we now possess of the Silurian deposits over immense
+territories in Russia and in North America, do not support the view, that
+the older a formation is, the more it has always suffered the extremity of
+denudation and metamorphism.
+
+The case at present must remain inexplicable; and may be truly urged as a
+valid argument against the views here entertained. To show that it may
+hereafter receive some explanation, I will give the following hypothesis.
+From the nature of the organic remains which {309} do not appear to have
+inhabited profound depths, in the several formations of Europe and of the
+United States; and from the amount of sediment, miles in thickness, of
+which the formations are composed, we may infer that from first to last
+large islands or tracts of land, whence the sediment was derived, occurred
+in the neighbourhood of the existing continents of Europe and North
+America. But we do not know what was the state of things in the intervals
+between the successive formations; whether Europe and the United States
+during these intervals existed as dry land, or as a submarine surface near
+land, on which sediment was not deposited, or as the bed of an open and
+unfathomable sea.
+
+Looking to the existing oceans, which are thrice as extensive as the land,
+we see them studded with many islands; but not one oceanic island is as yet
+known to afford even a remnant of any palæozoic or secondary formation.
+Hence we may perhaps infer, that during the palæozoic and secondary
+periods, neither continents nor continental islands existed where our
+oceans now extend; for had they existed there, palæozoic and secondary
+formations would in all probability have been accumulated from sediment
+derived from their wear and tear; and would have been at least partially
+upheaved by the oscillations of level, which we may fairly conclude must
+have intervened during these enormously long periods. If then we may infer
+anything from these facts, we may infer that where our oceans now extend,
+oceans have extended from the remotest period of which we have any record;
+and on the other hand, that where continents now exist, large tracts of
+land have existed, subjected no doubt to great oscillations of level, since
+the earliest silurian period. The coloured map appended to my volume on
+Coral Reefs, led me to conclude that the great oceans are still mainly
+areas of {310} subsidence, the great archipelagoes still areas of
+oscillations of level, and the continents areas of elevation. But have we
+any right to assume that things have thus remained from the beginning of
+this world? Our continents seem to have been formed by a preponderance,
+during many oscillations of level, of the force of elevation; but may not
+the areas of preponderant movement have changed in the lapse of ages? At a
+period immeasurably antecedent to the silurian epoch, continents may have
+existed where oceans are now spread out; and clear and open oceans may have
+existed where our continents now stand. Nor should we be justified in
+assuming that if, for instance, the bed of the Pacific Ocean were now
+converted into a continent, we should there find formations older than the
+silurian strata, supposing such to have been formerly deposited; for it
+might well happen that strata which had subsided some miles nearer to the
+centre of the earth, and which had been pressed on by an enormous weight of
+superincumbent water, might have undergone far more metamorphic action than
+strata which have always remained nearer to the surface. The immense areas
+in some parts of the world, for instance in South America, of bare
+metamorphic rocks, which must have been heated under great pressure, have
+always seemed to me to require some special explanation; and we may perhaps
+believe that we see in these large areas, the many formations long anterior
+to the silurian epoch in a completely metamorphosed condition.
+
+
+
+The several difficulties here discussed, namely our not finding in the
+successive formations infinitely numerous transitional links between the
+many species which now exist or have existed; the sudden manner {311} in
+which whole groups of species appear in our European formations; the almost
+entire absence, as at present known, of fossiliferous formations beneath
+the Silurian strata, are all undoubtedly of the gravest nature. We see this
+in the plainest manner by the fact that all the most eminent
+palæontologists, namely Cuvier, Agassiz, Barrande, Falconer, E. Forbes,
+&c., and all our greatest geologists, as Lyell, Murchison, Sedgwick, &c.,
+have unanimously, often vehemently, maintained the immutability of species.
+But I have reason to believe that one great authority, Sir Charles Lyell,
+from further reflexion entertains grave doubts on this subject. I feel how
+rash it is to differ from these authorities, to whom, with others, we owe
+all our knowledge. Those who think the natural geological record in any
+degree perfect, and who do not attach much weight to the facts and
+arguments of other kinds given in this volume, will undoubtedly at once
+reject my theory. For my part, following out Lyell's metaphor, I look at
+the natural geological record, as a history of the world imperfectly kept,
+and written in a changing dialect; of this history we possess the last
+volume alone, relating only to two or three countries. Of this volume, only
+here and there a short chapter has been preserved; and of each page, only
+here and there a few lines. Each word of the slowly-changing language, in
+which the history is supposed to be written, being more or less different
+in the interrupted succession of chapters, may represent the apparently
+abruptly changed forms of life, entombed in our consecutive, but widely
+separated, formations. On this view, the difficulties above discussed are
+greatly diminished, or even disappear.
+
+       *       *       *       *       *
+
+
+{312}
+
+CHAPTER X.
+
+ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
+
+    On the slow and successive appearance of new species--On their
+    different rates of change--Species once lost do not reappear--Groups of
+    species follow the same general rules in their appearance and
+    disappearance as do single species--On Extinction--On simultaneous
+    changes in the forms of life throughout the world--On the affinities of
+    extinct species to each other and to living species--On the state of
+    development of ancient forms--On the succession of the same types
+    within the same areas--Summary of preceding and present chapters.
+
+Let us now see whether the several facts and rules relating to the
+geological succession of organic beings, better accord with the common view
+of the immutability of species, or with that of their slow and gradual
+modification, through descent and natural selection.
+
+New species have appeared very slowly, one after another, both on the land
+and in the waters. Lyell has shown that it is hardly possible to resist the
+evidence on this head in the case of the several tertiary stages; and every
+year tends to fill up the blanks between them, and to make the percentage
+system of lost and new forms more gradual. In some of the most recent beds,
+though undoubtedly of high antiquity if measured by years, only one or two
+species are lost forms, and only one or two are new forms, having here
+appeared for the first time, either locally, or, as far as we know, on the
+face of the earth. If we may trust the observations of Philippi in Sicily,
+the successive changes in the marine inhabitants of that island have been
+many and most gradual. The secondary formations are more broken; but, as
+Bronn has remarked, neither the appearance {313} nor disappearance of their
+many now extinct species has been simultaneous in each separate formation.
+
+Species of different genera and classes have not changed at the same rate,
+or in the same degree. In the oldest tertiary beds a few living shells may
+still be found in the midst of a multitude of extinct forms. Falconer has
+given a striking instance of a similar fact, in an existing crocodile
+associated with many strange and lost mammals and reptiles in the
+sub-Himalayan deposits. The Silurian Lingula differs but little from the
+living species of this genus; whereas most of the other Silurian Molluscs
+and all the Crustaceans have changed greatly. The productions of the land
+seem to change at a quicker rate than those of the sea, of which a striking
+instance has lately been observed in Switzerland. There is some reason to
+believe that organisms, considered high in the scale of nature, change more
+quickly than those that are low: though there are exceptions to this rule.
+The amount of organic change, as Pictet has remarked, does not strictly
+correspond with the succession of our geological formations; so that
+between each two consecutive formations, the forms of life have seldom
+changed in exactly the same degree. Yet if we compare any but the most
+closely related formations, all the species will be found to have undergone
+some change. When a species has once disappeared from the face of the
+earth, we have reason to believe that the same identical form never
+reappears. The strongest apparent exception to this latter rule, is that of
+the so-called "colonies" of M. Barrande, which intrude for a period in the
+midst of an older formation, and then allow the pre-existing fauna to
+reappear; but Lyell's explanation, namely, that it is a case of temporary
+migration from a distinct geographical province, seems to me satisfactory.
+{314}
+
+These several facts accord well with my theory. I believe in no fixed law
+of development, causing all the inhabitants of a country to change
+abruptly, or simultaneously, or to an equal degree. The process of
+modification must be extremely slow. The variability of each species is
+quite independent of that of all others. Whether such variability be taken
+advantage of by natural selection, and whether the variations be
+accumulated to a greater or lesser amount, thus causing a greater or lesser
+amount of modification in the varying species, depends on many complex
+contingencies,--on the variability being of a beneficial nature, on the
+power of intercrossing, on the rate of breeding, on the slowly changing
+physical conditions of the country, and more especially on the nature of
+the other inhabitants with which the varying species comes into
+competition. Hence it is by no means surprising that one species should
+retain the same identical form much longer than others; or, if changing,
+that it should change less. We see the same fact in geographical
+distribution; for instance, in the land-shells and coleopterous insects of
+Madeira having come to differ considerably from their nearest allies on the
+continent of Europe, whereas the marine shells and birds have remained
+unaltered. We can perhaps understand the apparently quicker rate of change
+in terrestrial and in more highly organised productions compared with
+marine and lower productions, by the more complex relations of the higher
+beings to their organic and inorganic conditions of life, as explained in a
+former chapter. When many of the inhabitants of a country have become
+modified and improved, we can understand, on the principle of competition,
+and on that of the many all-important relations of organism to organism,
+that any form which does not become in some degree modified and improved,
+{315} will be liable to be exterminated. Hence we can see why all the
+species in the same region do at last, if we look to wide enough intervals
+of time, become modified; for those which do not change will become
+extinct.
+
+In members of the same class the average amount of change, during long and
+equal periods of time, may, perhaps, be nearly the same; but as the
+accumulation of long-enduring fossiliferous formations depends on great
+masses of sediment having been deposited on areas whilst subsiding, our
+formations have been almost necessarily accumulated at wide and irregularly
+intermittent intervals; consequently the amount of organic change exhibited
+by the fossils embedded in consecutive formations is not equal. Each
+formation, on this view, does not mark a new and complete act of creation,
+but only an occasional scene, taken almost at hazard, in a slowly changing
+drama.
+
+We can clearly understand why a species when once lost should never
+reappear, even if the very same conditions of life, organic and inorganic,
+should recur. For though the offspring of one species might be adapted (and
+no doubt this has occurred in innumerable instances) to fill the exact
+place of another species in the economy of nature, and thus supplant it;
+yet the two forms--the old and the new--would not be identically the same;
+for both would almost certainly inherit different characters from their
+distinct progenitors. For instance, it is just possible, if our
+fantail-pigeons were all destroyed, that fanciers, by striving during long
+ages for the same object, might make a new breed hardly distinguishable
+from our present fantail; but if the parent rock-pigeon were also
+destroyed, and in nature we have every reason to believe that the
+parent-form will generally be supplanted and exterminated by its improved
+offspring, it is quite {316} incredible that a fantail, identical with the
+existing breed, could be raised from any other species of pigeon, or even
+from the other well-established races of the domestic pigeon, for the
+newly-formed fantail would be almost sure to inherit from its new
+progenitor some slight characteristic differences.
+
+Groups of species, that is, genera and families, follow the same general
+rules in their appearance and disappearance as do single species, changing
+more or less quickly, and in a greater or lesser degree. A group does not
+reappear after it has once disappeared; or its existence, as long as it
+lasts, is continuous. I am aware that there are some apparent exceptions to
+this rule, but the exceptions are surprisingly few, so few that E. Forbes,
+Pictet, and Woodward (though all strongly opposed to such views as I
+maintain) admit its truth; and the rule strictly accords with my theory.
+For as all the species of the same group have descended from some one
+species, it is clear that as long as any species of the group have appeared
+in the long succession of ages, so long must its members have continuously
+existed, in order to have generated either new and modified or the same old
+and unmodified forms. Species of the genus Lingula, for instance, must have
+continuously existed by an unbroken succession of generations, from the
+lowest Silurian stratum to the present day.
+
+We have seen in the last chapter that the species of a group sometimes
+falsely appear to have come in abruptly; and I have attempted to give an
+explanation of this fact, which if true would have been fatal to my views.
+But such cases are certainly exceptional; the general rule being a gradual
+increase in number, till the group reaches its maximum, and then, sooner or
+later, it gradually decreases. If the number of the species of a genus, or
+the number of {317} the genera of a family, be represented by a vertical
+line of varying thickness, crossing the successive geological formations in
+which the species are found, the line will sometimes falsely appear to
+begin at its lower end, not in a sharp point, but abruptly; it then
+gradually thickens upwards, sometimes keeping for a space of equal
+thickness, and ultimately thins out in the upper beds, marking the decrease
+and final extinction of the species. This gradual increase in number of the
+species of a group is strictly conformable with my theory; as the species
+of the same genus, and the genera of the same family, can increase only
+slowly and progressively; for the process of modification and the
+production of a number of allied forms must be slow and gradual,--one
+species giving rise first to two or three varieties, these being slowly
+converted into species, which in their turn produce by equally slow steps
+other species, and so on, like the branching of a great tree from a single
+stem, till the group becomes large.
+
+
+
+_On Extinction._--We have as yet spoken only incidentally of the
+disappearance of species and of groups of species. On the theory of natural
+selection the extinction of old forms and the production of new and
+improved forms are intimately connected together. The old notion of all the
+inhabitants of the earth having been swept away at successive periods by
+catastrophes, is very generally given up, even by those geologists, as Elie
+de Beaumont, Murchison, Barrande, &c., whose general views would naturally
+lead them to this conclusion. On the contrary, we have every reason to
+believe, from the study of the tertiary formations, that species and groups
+of species gradually disappear, one after another, first from one spot,
+then from another, and finally from the world. Both single species and
+whole {318} groups of species last for very unequal periods; some groups,
+as we have seen, having endured from the earliest known dawn of life to the
+present day; some having disappeared before the close of the palæozoic
+period. No fixed law seems to determine the length of time during which any
+single species or any single genus endures. There is reason to believe that
+the complete extinction of the species of a group is generally a slower
+process than their production: if the appearance and disappearance of a
+group of species be represented, as before, by a vertical line of varying
+thickness, the line is found to taper more gradually at its upper end,
+which marks the progress of extermination, than at its lower end, which
+marks the first appearance and increase in numbers of the species. In some
+cases, however, the extermination of whole groups of beings, as of
+ammonites towards the close of the secondary period, has been wonderfully
+sudden.
+
+The whole subject of the extinction of species has been involved in the
+most gratuitous mystery. Some authors have even supposed that as the
+individual has a definite length of life, so have species a definite
+duration. No one I think can have marvelled more at the extinction of
+species, than I have done. When I found in La Plata the tooth of a horse
+embedded with the remains of Mastodon, Megatherium, Toxodon, and other
+extinct monsters, which all co-existed with still living shells at a very
+late geological period, I was filled with astonishment; for seeing that the
+horse, since its introduction by the Spaniards into South America, has run
+wild over the whole country and has increased in numbers at an unparalleled
+rate, I asked myself what could so recently have exterminated the former
+horse under conditions of life apparently so favourable. But how utterly
+groundless was my astonishment! {319} Professor Owen soon perceived that
+the tooth, though so like that of the existing horse, belonged to an
+extinct species. Had this horse been still living, but in some degree rare,
+no naturalist would have felt the least surprise at its rarity; for rarity
+is the attribute of a vast number of species of all classes, in all
+countries. If we ask ourselves why this or that species is rare, we answer
+that something is unfavourable in its conditions of life; but what that
+something is, we can hardly ever tell. On the supposition of the fossil
+horse still existing as a rare species, we might have felt certain from the
+analogy of all other mammals, even of the slow-breeding elephant, and from
+the history of the naturalisation of the domestic horse in South America,
+that under more favourable conditions it would in a very few years have
+stocked the whole continent. But we could not have told what the
+unfavourable conditions were which checked its increase, whether some one
+or several contingencies, and at what period of the horse's life, and in
+what degree, they severally acted. If the conditions had gone on, however
+slowly, becoming less and less favourable, we assuredly should not have
+perceived the fact, yet the fossil horse would certainly have become rarer
+and rarer, and finally extinct;--its place being seized on by some more
+successful competitor.
+
+It is most difficult always to remember that the increase of every living
+being is constantly being checked by unperceived injurious agencies; and
+that these same unperceived agencies are amply sufficient to cause rarity,
+and finally extinction. We see in many cases in the more recent tertiary
+formations, that rarity precedes extinction; and we know that this has been
+the progress of events with those animals which have been exterminated,
+either locally or wholly, through {320} man's agency. I may repeat what I
+published in 1845, namely, that to admit that species generally become rare
+before they become extinct--to feel no surprise at the rarity of a species,
+and yet to marvel greatly when it ceases to exist, is much the same as to
+admit that sickness in the individual is the forerunner of death--to feel
+no surprise at sickness, but when the sick man dies, to wonder and to
+suspect that he died by some unknown deed of violence.
+
+The theory of natural selection is grounded on the belief that each new
+variety, and ultimately each new species, is produced and maintained by
+having some advantage over those with which it comes into competition; and
+the consequent extinction of less-favoured forms almost inevitably follows.
+It is the same with our domestic productions: when a new and slightly
+improved variety has been raised, it at first supplants the less improved
+varieties in the same neighbourhood; when much improved it is transported
+far and near, like our short-horn cattle, and takes the place of other
+breeds in other countries. Thus the appearance of new forms and the
+disappearance of old forms, both natural and artificial, are bound
+together. In certain flourishing groups, the number of new specific forms
+which have been produced within a given time is probably greater than that
+of the old specific forms which have been exterminated; but we know that
+the number of species has not gone on indefinitely increasing, at least
+during the later geological periods, so that looking to later times we may
+believe that the production of new forms has caused the extinction of about
+the same number of old forms.
+
+The competition will generally be most severe, as formerly explained and
+illustrated by examples, between the forms which are most like each other
+in all respects. {321} Hence the improved and modified descendants of a
+species will generally cause the extermination of the parent-species; and
+if many new forms have been developed from any one species, the nearest
+allies of that species, _i.e._ the species of the same genus, will be the
+most liable to extermination. Thus, as I believe, a number of new species
+descended from one species, that is a new genus, comes to supplant an old
+genus, belonging to the same family. But it must often have happened that a
+new species belonging to some one group will have seized on the place
+occupied by a species belonging to a distinct group, and thus caused its
+extermination; and if many allied forms be developed from the successful
+intruder, many will have to yield their places; and it will generally be
+allied forms, which will suffer from some inherited inferiority in common.
+But whether it be species belonging to the same or to a distinct class,
+which yield their places to other species which have been modified and
+improved, a few of the sufferers may often long be preserved, from being
+fitted to some peculiar line of life, or from inhabiting some distant and
+isolated station, where they have escaped severe competition. For instance,
+a single species of Trigonia, a great genus of shells in the secondary
+formations, survives in the Australian seas; and a few members of the great
+and almost extinct group of Ganoid fishes still inhabit our fresh waters.
+Therefore the utter extinction of a group is generally, as we have seen, a
+slower process than its production.
+
+With respect to the apparently sudden extermination of whole families or
+orders, as of Trilobites at the close of the palæozoic period and of
+Ammonites at the close of the secondary period, we must remember what has
+been already said on the probable wide intervals of time {322} between our
+consecutive formations; and in these intervals there may have been much
+slow extermination. Moreover, when by sudden immigration or by unusually
+rapid development, many species of a new group have taken possession of a
+new area, they will have exterminated in a correspondingly rapid manner
+many of the old inhabitants; and the forms which thus yield their places
+will commonly be allied, for they will partake of some inferiority in
+common.
+
+Thus, as it seems to me, the manner in which single species and whole
+groups of species become extinct, accords well with the theory of natural
+selection. We need not marvel at extinction; if we must marvel, let it be
+at our presumption in imagining for a moment that we understand the many
+complex contingencies, on which the existence of each species depends. If
+we forget for an instant, that each species tends to increase inordinately,
+and that some check is always in action, yet seldom perceived by us, the
+whole economy of nature will be utterly obscured. Whenever we can precisely
+say why this species is more abundant in individuals than that; why this
+species and not another can be naturalised in a given country; then, and
+not till then, we may justly feel surprise why we cannot account for the
+extinction of this particular species or group of species.
+
+
+
+_On the Forms of Life changing almost simultaneously throughout the
+World._--Scarcely any palæontological discovery is more striking than the
+fact, that the forms of life change almost simultaneously throughout the
+world. Thus our European Chalk formation can be recognised in many distant
+parts of the world, under the most different climates, where not a fragment
+of the mineral chalk itself can be found; namely, in North {323} America,
+in equatorial South America, in Tierra del Fuego, at the Cape of Good Hope,
+and in the peninsula of India. For at these distant points, the organic
+remains in certain beds present an unmistakeable degree of resemblance to
+those of the Chalk. It is not that the same species are met with; for in
+some cases not one species is identically the same, but they belong to the
+same families, genera, and sections of genera, and sometimes are similarly
+characterised in such trifling points as mere superficial sculpture.
+Moreover other forms, which are not found in the Chalk of Europe, but which
+occur in the formations either above or below, are similarly absent at
+these distant points of the world. In the several successive palæozoic
+formations of Russia, Western Europe and North America, a similar
+parallelism in the forms of life has been observed by several authors: so
+it is, according to Lyell, with the several European and North American
+tertiary deposits. Even if the few fossil species which are common to the
+Old and New Worlds be kept wholly out of view, the general parallelism in
+the successive forms of life, in the stages of the widely separated
+palæozoic and tertiary periods, would still be manifest, and the several
+formations could be easily correlated.
+
+These observations, however, relate to the marine inhabitants of distant
+parts of the world: we have not sufficient data to judge whether the
+productions of the land and of fresh water change at distant points in the
+same parallel manner. We may doubt whether they have thus changed: if the
+Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe
+from La Plata, without any information in regard to their geological
+position, no one would have suspected that they had co-existed with still
+living sea-shells; but as these anomalous monsters co-existed with the
+{324} Mastodon and Horse, it might at least have been inferred that they
+had lived during one of the later tertiary stages.
+
+When the marine forms of life are spoken of as having changed
+simultaneously throughout the world, it must not be supposed that this
+expression relates to the same thousandth or hundred-thousandth year, or
+even that it has a very strict geological sense; for if all the marine
+animals which live at the present day in Europe, and all those that lived
+in Europe during the pleistocene period (an enormously remote period as
+measured by years, including the whole glacial epoch), were to be compared
+with those now living in South America or in Australia, the most skilful
+naturalist would hardly be able to say whether the existing or the
+pleistocene inhabitants of Europe resembled most closely those of the
+southern hemisphere. So, again, several highly competent observers believe
+that the existing productions of the United States are more closely related
+to those which lived in Europe during certain later tertiary stages, than
+to those which now live here; and if this be so, it is evident that
+fossiliferous beds deposited at the present day on the shores of North
+America would hereafter be liable to be classed with somewhat older
+European beds. Nevertheless, looking to a remotely future epoch, there can,
+I think, be little doubt that all the more modern _marine_ formations,
+namely, the upper pliocene, the pleistocene and strictly modern beds, of
+Europe, North and South America, and Australia, from containing fossil
+remains in some degree allied, and from not including those forms which are
+only found in the older underlying deposits, would be correctly ranked as
+simultaneous in a geological sense.
+
+The fact of the forms of life changing simultaneously, in the above large
+sense, at distant parts of the world, has greatly struck those admirable
+observers, MM. {325} de Verneuil and d'Archiac. After referring to the
+parallelism of the palæozoic forms of life in various parts of Europe, they
+add, "If struck by this strange sequence, we turn our attention to North
+America, and there discover a series of analogous phenomena, it will appear
+certain that all these modifications of species, their extinction, and the
+introduction of new ones, cannot be owing to mere changes in marine
+currents or other causes more or less local and temporary, but depend on
+general laws which govern the whole animal kingdom." M. Barrande has made
+forcible remarks to precisely the same effect. It is, indeed, quite futile
+to look to changes of currents, climate, or other physical conditions, as
+the cause of these great mutations in the forms of life throughout the
+world, under the most different climates. We must, as Barrande has
+remarked, look to some special law. We shall see this more clearly when we
+treat of the present distribution of organic beings, and find how slight is
+the relation between the physical conditions of various countries, and the
+nature of their inhabitants.
+
+This great fact of the parallel succession of the forms of life throughout
+the world, is explicable on the theory of natural selection. New species
+are formed by new varieties arising, which have some advantage over older
+forms; and those forms, which are already dominant, or have some advantage
+over the other forms in their own country, would naturally oftenest give
+rise to new varieties or incipient species; for these latter must be
+victorious in a still higher degree in order to be preserved and to
+survive. We have distinct evidence on this head, in the plants which are
+dominant, that is, which are commonest in their own homes, and are most
+widely diffused, having produced the greatest number of new varieties. It
+is also natural that the {326} dominant, varying, and far-spreading
+species, which already have invaded to a certain extent the territories of
+other species, should be those which would have the best chance of
+spreading still further, and of giving rise in new countries to new
+varieties and species. The process of diffusion may often be very slow,
+being dependent on climatal and geographical changes, or on strange
+accidents, but in the long run the dominant forms will generally succeed in
+spreading. The diffusion would, it is probable, be slower with the
+terrestrial inhabitants of distinct continents than with the marine
+inhabitants of the continuous sea. We might therefore expect to find, as we
+apparently do find, a less strict degree of parallel succession in the
+productions of the land than of the sea.
+
+Dominant species spreading from any region might encounter still more
+dominant species, and then their triumphant course, or even their
+existence, would cease. We know not at all precisely what are all the
+conditions most favourable for the multiplication of new and dominant
+species; but we can, I think, clearly see that a number of individuals,
+from giving a better chance of the appearance of favourable variations, and
+that severe competition with many already existing forms, would be highly
+favourable, as would be the power of spreading into new territories. A
+certain amount of isolation, recurring at long intervals of time, would
+probably be also favourable, as before explained. One quarter of the world
+may have been most favourable for the production of new and dominant
+species on the land, and another for those in the waters of the sea. If two
+great regions had been for a long period favourably circumstanced in an
+equal degree, whenever their inhabitants met, the battle would be prolonged
+and severe; and some from one birthplace and some from the other might be
+victorious. But in the course of time, the {327} forms dominant in the
+highest degree, wherever produced, would tend everywhere to prevail. As
+they prevailed, they would cause the extinction of other and inferior
+forms; and as these inferior forms would be allied in groups by
+inheritance, whole groups would tend slowly to disappear; though here and
+there a single member might long be enabled to survive.
+
+Thus, as it seems to me, the parallel, and, taken in a large sense,
+simultaneous, succession of the same forms of life throughout the world,
+accords well with the principle of new species having been formed by
+dominant species spreading widely and varying; the new species thus
+produced being themselves dominant owing to inheritance, and to having
+already had some advantage over their parents or over other species; these
+again spreading, varying, and producing new species. The forms which are
+beaten and which yield their places to the new and victorious forms, will
+generally be allied in groups, from inheriting some inferiority in common;
+and therefore as new and improved groups spread throughout the world, old
+groups will disappear from the world; and the succession of forms in both
+ways will everywhere tend to correspond.
+
+There is one other remark connected with this subject worth making. I have
+given my reasons for believing that all our greater fossiliferous
+formations were deposited during periods of subsidence; and that blank
+intervals of vast duration occurred during the periods when the bed of the
+sea was either stationary or rising, and likewise when sediment was not
+thrown down quickly enough to embed and preserve organic remains. During
+these long and blank intervals I suppose that the inhabitants of each
+region underwent a considerable amount of modification and extinction, and
+that there was much migration from {328} other parts of the world. As we
+have reason to believe that large areas are affected by the same movement,
+it is probable that strictly contemporaneous formations have often been
+accumulated over very wide spaces in the same quarter of the world; but we
+are far from having any right to conclude that this has invariably been the
+case, and that large areas have invariably been affected by the same
+movements. When two formations have been deposited in two regions during
+nearly, but not exactly the same period, we should find in both, from the
+causes explained in the foregoing paragraphs, the same general succession
+in the forms of life; but the species would not exactly correspond; for
+there will have been a little more time in the one region than in the other
+for modification, extinction, and immigration.
+
+I suspect that cases of this nature occur in Europe. Mr. Prestwich, in his
+admirable Memoirs on the eocene deposits of England and France, is able to
+draw a close general parallelism between the successive stages in the two
+countries; but when he compares certain stages in England with those in
+France, although he finds in both a curious accordance in the numbers of
+the species belonging to the same genera, yet the species themselves differ
+in a manner very difficult to account for, considering the proximity of the
+two areas,--unless, indeed, it be assumed that an isthmus separated two
+seas inhabited by distinct, but contemporaneous, faunas. Lyell has made
+similar observations on some of the later tertiary formations. Barrande,
+also, shows that there is a striking general parallelism in the successive
+Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a
+surprising amount of difference in the species. If the several formations
+in these regions have not been deposited during the same exact {329}
+periods,--a formation in one region often corresponding with a blank
+interval in the other,--and if in both regions the species have gone on
+slowly changing during the accumulation of the several formations and
+during the long intervals of time between them; in this case, the several
+formations in the two regions could be arranged in the same order, in
+accordance with the general succession of the form of life, and the order
+would falsely appear to be strictly parallel; nevertheless the species
+would not all be the same in the apparently corresponding stages in the two
+regions.
+
+
+
+_On the Affinities of extinct Species to each other, and to living
+forms._--Let us now look to the mutual affinities of extinct and living
+species. They all fall into one grand natural system; and this fact is at
+once explained on the principle of descent. The more ancient any form is,
+the more, as a general rule, it differs from living forms. But, as Buckland
+long ago remarked, all fossils can be classed either in still existing
+groups, or between them. That the extinct forms of life help to fill up the
+wide intervals between existing genera, families, and orders, cannot be
+disputed. For if we confine our attention either to the living or to the
+extinct alone, the series is far less perfect than if we combine both into
+one general system. With respect to the Vertebrata, whole pages could be
+filled with striking illustrations from our great palaeontologist, Owen,
+showing how extinct animals fall in between existing groups. Cuvier ranked
+the Ruminants and Pachyderms, as the two most distinct orders of mammals;
+but Owen has discovered so many fossil links, that he has had to alter the
+whole classification of these two orders; and has placed certain pachyderms
+in the same sub-order with ruminants: for example, he dissolves by fine
+gradations the apparently {330} wide difference between the pig and the
+camel. In regard to the Invertebrata, Barrande, and a higher authority
+could not be named, asserts that he is every day taught that Palaeozoic
+animals, though belonging to the same orders, families, or genera with
+those living at the present day, were not at this early epoch limited in
+such distinct groups as they now are.
+
+Some writers have objected to any extinct species or group of species being
+considered as intermediate between living species or groups. If by this
+term it is meant that an extinct form is directly intermediate in all its
+characters between two living forms, the objection is probably valid. But I
+apprehend that in a perfectly natural classification many fossil species
+would have to stand between living species, and some extinct genera between
+living genera, even between genera belonging to distinct families. The most
+common case, especially with respect to very distinct groups, such as fish
+and reptiles, seems to be, that supposing them to be distinguished at the
+present day from each other by a dozen characters, the ancient members of
+the same two groups would be distinguished by a somewhat lesser number of
+characters, so that the two groups, though formerly quite distinct, at that
+period made some small approach to each other.
+
+It is a common belief that the more ancient a form is, by so much the more
+it tends to connect by some of its characters groups now widely separated
+from each other. This remark no doubt must be restricted to those groups
+which have undergone much change in the course of geological ages; and it
+would be difficult to prove the truth of the proposition, for every now and
+then even a living animal, as the Lepidosiren, is discovered having
+affinities directed towards very distinct groups. Yet if we compare the
+older Reptiles and {331} Batrachians, the older Fish, the older
+Cephalopods, and the eocene Mammals, with the more recent members of the
+same classes, we must admit that there is some truth in the remark.
+
+Let us see how far these several facts and inferences accord with the
+theory of descent with modification. As the subject is somewhat complex, I
+must request the reader to turn to the diagram in the fourth chapter. We
+may suppose that the numbered letters represent genera, and the dotted
+lines diverging from them the species in each genus. The diagram is much
+too simple, too few genera and too few species being given, but this is
+unimportant for us. The horizontal lines may represent successive
+geological formations, and all the forms beneath the uppermost line may be
+considered as extinct. The three existing genera, a^{14}, q^{14}, p^{14},
+will form a small family; b^{14} and f^{14} a closely allied family or
+sub-family; and o^{14}, e^{14}, m^{14}, a third family. These three
+families, together with the many extinct genera on the several lines of
+descent diverging from the parent-form (A), will form an order; for all
+will have inherited something in common from their ancient and common
+progenitor. On the principle of the continued tendency to divergence of
+character, which was formerly illustrated by this diagram, the more recent
+any form is, the more it will generally differ from its ancient progenitor.
+Hence we can understand the rule that the most ancient fossils differ most
+from existing forms. We must not, however, assume that divergence of
+character is a necessary contingency; it depends solely on the descendants
+from a species being thus enabled to seize on many and different places in
+the economy of nature. Therefore it is quite possible, as we have seen in
+the case of some Silurian forms, that a species might go on being slightly
+modified in relation to its slightly altered conditions of {332} life, and
+yet retain throughout a vast period the same general characteristics. This
+is represented in the diagram by the letter F^{14}.
+
+All the many forms, extinct and recent, descended from (A), make, as before
+remarked, one order; and this order, from the continued effects of
+extinction and divergence of character, has become divided into several
+sub-families and families, some of which are supposed to have perished at
+different periods, and some to have endured to the present day.
+
+By looking at the diagram we can see that if many of the extinct forms,
+supposed to be embedded in the successive formations, were discovered at
+several points low down in the series, the three existing families on the
+uppermost line would be rendered less distinct from each other. If, for
+instance, the genera a^1, a^5, a^{10}, f^8, m^3, m^6, m^9, were
+disinterred, these three families would be so closely linked together that
+they probably would have to be united into one great family, in nearly the
+same manner as has occurred with ruminants and pachyderms. Yet he who
+objected to call the extinct genera, which thus linked the living genera of
+three families together, intermediate in character, would be justified, as
+they are intermediate, not directly, but only by a long and circuitous
+course through many widely different forms. If many extinct forms were to
+be discovered above one of the middle horizontal lines or geological
+formations --for instance, above No. VI.--but none from beneath this line,
+then only the two families on the left hand (namely, a^{14}, &c., and
+b^{14}, &c.) would have to be united into one family; and the two other
+families (namely, a^{14} to f^{14} now including five genera, and o^{14} to
+m^{14}) would yet remain distinct. These two families, however, would be
+less distinct from each other than they were before the discovery of the
+fossils. If, for instance, we suppose the existing genera of the two
+families to differ from each {333} other by a dozen characters, in this
+case the genera, at the early period marked VI., would differ by a lesser
+number of characters; for at this early stage of descent they have not
+diverged in character from the common progenitor of the order, nearly so
+much as they subsequently diverged. Thus it comes that ancient and extinct
+genera are often in some slight degree intermediate in character between
+their modified descendants, or between their collateral relations.
+
+In nature the case will be far more complicated than is represented in the
+diagram; for the groups will have been more numerous, they will have
+endured for extremely unequal lengths of time, and will have been modified
+in various degrees. As we possess only the last volume of the geological
+record, and that in a very broken condition, we have no right to expect,
+except in very rare cases, to fill up wide intervals in the natural system,
+and thus unite distinct families or orders. All that we have a right to
+expect, is that those groups, which have within known geological periods
+undergone much modification, should in the older formations make some
+slight approach to each other; so that the older members should differ less
+from each other in some of their characters than do the existing members of
+the same groups; and this by the concurrent evidence of our best
+palæontologists seems frequently to be the case.
+
+Thus, on the theory of descent with modification, the main facts with
+respect to the mutual affinities of the extinct forms of life to each other
+and to living forms, seem to me explained in a satisfactory manner. And
+they are wholly inexplicable on any other view.
+
+On this same theory, it is evident that the fauna of any great period in
+the earth's history will be intermediate in general character between that
+which preceded and that which succeeded it. Thus, the species which lived
+at the sixth great stage of descent in the {334} diagram are the modified
+offspring of those which lived at the fifth stage, and are the parents of
+those which became still more modified at the seventh stage; hence they
+could hardly fail to be nearly intermediate in character between the forms
+of life above and below. We must, however, allow for the entire extinction
+of some preceding forms, and in any one region for the immigration of new
+forms from other regions, and for a large amount of modification, during
+the long and blank intervals between the successive formations. Subject to
+these allowances, the fauna of each geological period undoubtedly is
+intermediate in character, between the preceding and succeeding faunas. I
+need give only one instance, namely, the manner in which the fossils of the
+Devonian system, when this system was first discovered, were at once
+recognised by palæontologists as intermediate in character between those of
+the overlying carboniferous, and underlying Silurian system. But each fauna
+is not necessarily exactly intermediate, as unequal intervals of time have
+elapsed between consecutive formations.
+
+It is no real objection to the truth of the statement, that the fauna of
+each period as a whole is nearly intermediate in character between the
+preceding and succeeding faunas, that certain genera offer exceptions to
+the rule. For instance, mastodons and elephants, when arranged by Dr.
+Falconer in two series, first according to their mutual affinities and then
+according to their periods of existence, do not accord in arrangement. The
+species extreme in character are not the oldest, or the most recent; nor
+are those which are intermediate in character, intermediate in age. But
+supposing for an instant, in this and other such cases, that the record of
+the first appearance and disappearance of the species was perfect, we have
+no reason to believe that forms successively produced necessarily endure
+for {335} corresponding lengths of time: a very ancient form might
+occasionally last much longer than a form elsewhere subsequently produced,
+especially in the case of terrestrial productions inhabiting separated
+districts. To compare small things with great: if the principal living and
+extinct races of the domestic pigeon were arranged as well as they could be
+in serial affinity, this arrangement would not closely accord with the
+order in time of their production, and still less with the order of their
+disappearance; for the parent rock-pigeon now lives; and many varieties
+between the rock-pigeon and the carrier have become extinct; and carriers
+which are extreme in the important character of length of beak originated
+earlier than short-beaked tumblers, which are at the opposite end of the
+series in this same respect.
+
+Closely connected with the statement, that the organic remains from an
+intermediate formation are in some degree intermediate in character, is the
+fact, insisted on by all palæontologists, that fossils from two consecutive
+formations are far more closely related to each other, than are the fossils
+from two remote formations. Pictet gives as a well-known instance, the
+general resemblance of the organic remains from the several stages of the
+Chalk formation, though the species are distinct in each stage. This fact
+alone, from its generality, seems to have shaken Professor Pictet in his
+firm belief in the immutability of species. He who is acquainted with the
+distribution of existing species over the globe, will not attempt to
+account for the close resemblance of the distinct species in closely
+consecutive formations, by the physical conditions of the ancient areas
+having remained nearly the same. Let it be remembered that the forms of
+life, at least those inhabiting the sea, have changed almost simultaneously
+throughout the world, and therefore under the most different climates and
+conditions. Consider the {336} prodigious vicissitudes of climate during
+the pleistocene period, which includes the whole glacial period, and note
+how little the specific forms of the inhabitants of the sea have been
+affected.
+
+On the theory of descent, the full meaning of the fact of fossil remains
+from closely consecutive formations, though ranked as distinct species,
+being closely related, is obvious. As the accumulation of each formation
+has often been interrupted, and as long blank intervals have intervened
+between successive formations, we ought not to expect to find, as I
+attempted to show in the last chapter, in any one or two formations all the
+intermediate varieties between the species which appeared at the
+commencement and close of these periods; but we ought to find after
+intervals, very long as measured by years, but only moderately long as
+measured geologically, closely allied forms, or, as they have been called
+by some authors, representative species; and these we assuredly do find. We
+find, in short, such evidence of the slow and scarcely sensible mutation of
+specific forms, as we have a just right to expect to find.
+
+
+
+_On the state of Development of Ancient Forms._--There has been much
+discussion whether recent forms are more highly developed than ancient. I
+will not here enter on this subject, for naturalists have not as yet
+defined to each other's satisfaction what is meant by high and low forms.
+The best definition probably is, that the higher forms have their organs
+more distinctly specialised for different functions; and as such division
+of physiological labour seems to be an advantage to each being, natural
+selection will constantly tend in so far to make the later and more
+modified forms higher than their early progenitors, or than the slightly
+modified descendants of such progenitors. In a more general sense the {337}
+more recent forms must, on my theory, be higher than the more ancient; for
+each new species is formed by having had some advantage in the struggle for
+life over other and preceding forms. If under a nearly similar climate, the
+eocene inhabitants of one quarter of the world were put into competition
+with the existing inhabitants of the same or some other quarter, the eocene
+fauna or flora would certainly be beaten and exterminated; as would a
+secondary fauna by an eocene, and a palæozoic fauna by a secondary fauna. I
+do not doubt that this process of improvement has affected in a marked and
+sensible manner the organisation of the more recent and victorious forms of
+life, in comparison with the ancient and beaten forms; but I can see no way
+of testing this sort of progress. Crustaceans, for instance, not the
+highest in their own class, may have beaten the highest molluscs. From the
+extraordinary manner in which European productions have recently spread
+over New Zealand, and have seized on places which must have been previously
+occupied, we may believe, if all the animals and plants of Great Britain
+were set free in New Zealand, that in the course of time a multitude of
+British forms would become thoroughly naturalized there, and would
+exterminate many of the natives. On the other hand, from what we see now
+occurring in New Zealand, and from hardly a single inhabitant of the
+southern hemisphere having become wild in any part of Europe, we may doubt,
+if all the productions of New Zealand were set free in Great Britain,
+whether any considerable number would be enabled to seize on places now
+occupied by our native plants and animals. Under this point of view, the
+productions of Great Britain may be said to be higher than those of New
+Zealand. Yet the most skilful naturalist from an examination of the {338}
+species of the two countries could not have foreseen this result.
+
+Agassiz insists that ancient animals resemble to a certain extent the
+embryos of recent animals of the same classes; or that the geological
+succession of extinct forms is in some degree parallel to the embryological
+development of recent forms. I must follow Pictet and Huxley in thinking
+that the truth of this doctrine is very far from proved. Yet I fully expect
+to see it hereafter confirmed, at least in regard to subordinate groups,
+which have branched off from each other within comparatively recent times.
+For this doctrine of Agassiz accords well with the theory of natural
+selection. In a future chapter I shall attempt to show that the adult
+differs from its embryo, owing to variations supervening at a not early
+age, and being inherited at a corresponding age. This process, whilst it
+leaves the embryo almost unaltered, continually adds, in the course of
+successive generations, more and more difference to the adult.
+
+Thus the embryo comes to be left as a sort of picture, preserved by nature,
+of the ancient and less modified condition of each animal. This view may be
+true, and yet it may never be capable of full proof. Seeing, for instance,
+that the oldest known mammals, reptiles, and fish strictly belong to their
+own proper classes, though some of these old forms are in a slight degree
+less distinct from each other than are the typical members of the same
+groups at the present day, it would be vain to look for animals having the
+common embryological character of the Vertebrata, until beds far beneath
+the lowest Silurian strata are discovered--a discovery of which the chance
+is very small.
+
+
+
+_On the Succession of the same Types within the same {339} areas, during
+the later tertiary periods._--Mr. Clift many years ago showed that the
+fossil mammals from the Australian caves were closely allied to the living
+marsupials of that continent. In South America, a similar relationship is
+manifest, even to an uneducated eye, in the gigantic pieces of armour like
+those of the armadillo, found in several parts of La Plata; and Professor
+Owen has shown in the most striking manner that most of the fossil mammals,
+buried there in such numbers, are related to South American types. This
+relationship is even more clearly seen in the wonderful collection of
+fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so
+much impressed with these facts that I strongly insisted, in 1839 and 1845,
+on this "law of the succession of types,"--on "this wonderful relationship
+in the same continent between the dead and the living." Professor Owen has
+subsequently extended the same generalisation to the mammals of the Old
+World. We see the same law in this author's restorations of the extinct and
+gigantic birds of New Zealand. We see it also in the birds of the caves of
+Brazil. Mr. Woodward has shown that the same law holds good with
+sea-shells, but from the wide distribution of most genera of molluscs, it
+is not well displayed by them. Other cases could be added, as the relation
+between the extinct and living land-shells of Madeira; and between the
+extinct and living brackish-water shells of the Aralo-Caspian Sea.
+
+Now what does this remarkable law of the succession of the same types
+within the same areas mean? He would be a bold man, who after comparing the
+present climate of Australia and of parts of South America under the same
+latitude, would attempt to account, on the one hand, by dissimilar physical
+conditions for the dissimilarity of the inhabitants of these two
+continents, {340} and, on the other hand, by similarity of conditions, for
+the uniformity of the same types in each during the later tertiary periods.
+Nor can it be pretended that it is an immutable law that marsupials should
+have been chiefly or solely produced in Australia; or that Edentata and
+other American types should have been solely produced in South America. For
+we know that Europe in ancient times was peopled by numerous marsupials;
+and I have shown in the publications above alluded to, that in America the
+law of distribution of terrestrial mammals was formerly different from what
+it now is. North America formerly partook strongly of the present character
+of the southern half of the continent; and the southern half was formerly
+more closely allied, than it is at present, to the northern half. In a
+similar manner we know from Falconer and Cautley's discoveries, that
+northern India was formerly more closely related in its mammals to Africa
+than it is at the present time. Analogous facts could be given in relation
+to the distribution of marine animals.
+
+On the theory of descent with modification, the great law of the long
+enduring, but not immutable, succession of the same types within the same
+areas, is at once explained; for the inhabitants of each quarter of the
+world will obviously tend to leave in that quarter, during the next
+succeeding period of time, closely allied though in some degree modified
+descendants. If the inhabitants of one continent formerly differed greatly
+from those of another continent, so will their modified descendants still
+differ in nearly the same manner and degree. But after very long intervals
+of time and after great geographical changes, permitting much
+inter-migration, the feebler will yield to the more dominant forms, and
+there will be nothing immutable in the laws of past and present
+distribution. {341}
+
+It may be asked in ridicule, whether I suppose that the megatherium and
+other allied huge monsters have left behind them in South America, the
+sloth, armadillo, and anteater, as their degenerate descendants. This
+cannot for an instant be admitted. These huge animals have become wholly
+extinct, and have left no progeny. But in the caves of Brazil, there are
+many extinct species which are closely allied in size and in other
+characters to the species still living in South America; and some of these
+fossils may be the actual progenitors of living species. It must not be
+forgotten that, on my theory, all the species of the same genus have
+descended from some one species; so that if six genera, each having eight
+species, be found in one geological formation, and in the next succeeding
+formation there be six other allied or representative genera with the same
+number of species, then we may conclude that only one species of each of
+the six older genera has left modified descendants, constituting the six
+new genera. The other seven species of the old genera have all died out and
+have left no progeny. Or, which would probably be a far commoner case, two
+or three species of two or three alone of the six older genera will have
+been the parents of the six new genera; the other old species and the other
+whole old genera having become utterly extinct. In failing orders, with the
+genera and species decreasing in numbers, as apparently is the case of the
+Edentata of South America, still fewer genera and species will have left
+modified blood-descendants.
+
+
+
+_Summary of the preceding and present Chapters._--I have attempted to show
+that the geological record is extremely imperfect; that only a small
+portion of the globe has been geologically explored with care; that {342}
+only certain classes of organic beings have been largely preserved in a
+fossil state; that the number both of specimens and of species, preserved
+in our museums, is absolutely as nothing compared with the incalculable
+number of generations which must have passed away even during a single
+formation; that, owing to subsidence being necessary for the accumulation
+of fossiliferous deposits thick enough to resist future degradation,
+enormous intervals of time have elapsed between the successive formations;
+that there has probably been more extinction during the periods of
+subsidence, and more variation during the periods of elevation, and during
+the latter the record will have been least perfectly kept; that each single
+formation has not been continuously deposited; that the duration of each
+formation is, perhaps, short compared with the average duration of specific
+forms; that migration has played an important part in the first appearance
+of new forms in any one area and formation; that widely ranging species are
+those which have varied most, and have oftenest given rise to new species;
+and that varieties have at first often been local. All these causes taken
+conjointly, must have tended to make the geological record extremely
+imperfect, and will to a large extent explain why we do not find
+interminable varieties, connecting together all the extinct and existing
+forms of life by the finest graduated steps.
+
+He who rejects these views on the nature of the geological record, will
+rightly reject my whole theory. For he may ask in vain where are the
+numberless transitional links which must formerly have connected the
+closely allied or representative species, found in the several stages of
+the same great formation. He may disbelieve in the enormous intervals of
+time which have elapsed between our consecutive formations; he {343} may
+overlook how important a part migration must have played, when the
+formations of any one great region alone, as that of Europe, are
+considered; he may urge the apparent, but often falsely apparent, sudden
+coming in of whole groups of species. He may ask where are the remains of
+those infinitely numerous organisms which must have existed long before the
+first bed of the Silurian system was deposited: I can answer this latter
+question only hypothetically, by saying that as far as we can see, where
+our oceans now extend they have for an enormous period extended, and where
+our oscillating continents now stand they have stood ever since the
+Silurian epoch; but that long before that period, the world may have
+presented a wholly different aspect; and that the older continents, formed
+of formations older than any known to us, may now all be in a metamorphosed
+condition, or may lie buried under the ocean.
+
+Passing from these difficulties, all the other great leading facts in
+palæontology seem to me simply to follow on the theory of descent with
+modification through natural selection. We can thus understand how it is
+that new species come in slowly and successively; how species of different
+classes do not necessarily change together, or at the same rate, or in the
+same degree; yet in the long run that all undergo modification to some
+extent. The extinction of old forms is the almost inevitable consequence of
+the production of new forms. We can understand why when a species has once
+disappeared it never reappears. Groups of species increase in numbers
+slowly, and endure for unequal periods of time; for the process of
+modification is necessarily slow, and depends on many complex
+contingencies. The dominant species of the larger dominant groups tend to
+leave many modified {344} descendants, and thus new sub-groups and groups
+are formed. As these are formed, the species of the less vigorous groups,
+from their inferiority inherited from a common progenitor, tend to become
+extinct together, and to leave no modified offspring on the face of the
+earth. But the utter extinction of a whole group of species may often be a
+very slow process, from the survival of a few descendants, lingering in
+protected and isolated situations. When a group has once wholly
+disappeared, it does not reappear; for the link of generation has been
+broken.
+
+We can understand how the spreading of the dominant forms of life, which
+are those that oftenest vary, will in the long run tend to people the world
+with allied, but modified, descendants; and these will generally succeed in
+taking the places of those groups of species which are their inferiors in
+the struggle for existence. Hence, after long intervals of time, the
+productions of the world will appear to have changed simultaneously.
+
+We can understand how it is that all the forms of life, ancient and recent,
+make together one grand system; for all are connected by generation. We can
+understand, from the continued tendency to divergence of character, why the
+more ancient a form is, the more it generally differs from those now
+living. Why ancient and extinct forms often tend to fill up gaps between
+existing forms, sometimes blending two groups previously classed as
+distinct into one; but more commonly only bringing them a little closer
+together. The more ancient a form is, the more often, apparently, it
+displays characters in some degree intermediate between groups now
+distinct; for the more ancient a form is, the more nearly it will be
+related to, and consequently resemble, the common progenitor of groups,
+since {345} become widely divergent. Extinct forms are seldom directly
+intermediate between existing forms; but are intermediate only by a long
+and circuitous course through many extinct and very different forms. We can
+clearly see why the organic remains of closely consecutive formations are
+more closely allied to each other, than are those of remote formations; for
+the forms are more closely linked together by generation: we can clearly
+see why the remains of an intermediate formation are intermediate in
+character.
+
+The inhabitants of each successive period in the world's history have
+beaten their predecessors in the race for life, and are, in so far, higher
+in the scale of nature; and this may account for that vague yet ill-defined
+sentiment, felt by many palæontologists, that organisation on the whole has
+progressed. If it should hereafter be proved that ancient animals resemble
+to a certain extent the embryos of more recent animals of the same class,
+the fact will be intelligible. The succession of the same types of
+structure within the same areas during the later geological periods ceases
+to be mysterious, and is simply explained by inheritance.
+
+If then the geological record be as imperfect as I believe it to be, and it
+may at least be asserted that the record cannot be proved to be much more
+perfect, the main objections to the theory of natural selection are greatly
+diminished or disappear. On the other hand, all the chief laws of
+palæontology plainly proclaim, as it seems to me, that species have been
+produced by ordinary generation: old forms having been supplanted by new
+and improved forms of life, produced by the laws of variation still acting
+round us, and preserved by Natural Selection.
+
+       *       *       *       *       *
+
+
+{346}
+
+CHAPTER XI.
+
+GEOGRAPHICAL DISTRIBUTION.
+
+    Present distribution cannot be accounted for by differences in physical
+    conditions--Importance of barriers--Affinity of the productions of the
+    same continent--Centres of creation--Means of dispersal, by changes of
+    climate and of the level of the land, and by occasional
+    means--Dispersal during the Glacial period co-extensive with the world.
+
+In considering the distribution of organic beings over the face of the
+globe, the first great fact which strikes us is, that neither the
+similarity nor the dissimilarity of the inhabitants of various regions can
+be accounted for by their climatal and other physical conditions. Of late,
+almost every author who has studied the subject has come to this
+conclusion. The case of America alone would almost suffice to prove its
+truth: for if we exclude the northern parts where the circumpolar land is
+almost continuous, all authors agree that one of the most fundamental
+divisions in geographical distribution is that between the New and Old
+Worlds; yet if we travel over the vast American continent, from the central
+parts of the United States to its extreme southern point, we meet with the
+most diversified conditions; the most humid districts, arid deserts, lofty
+mountains, grassy plains, forests, marshes, lakes, and great rivers, under
+almost every temperature. There is hardly a climate or condition in the Old
+World which cannot be paralleled in the New--at least as closely as the
+same species generally require; for it is a most rare case to find a group
+of organisms confined to any small spot, having conditions peculiar in only
+a slight {347} degree; for instance, small areas in the Old World could be
+pointed out hotter than any in the New World, yet these are not inhabited
+by a peculiar fauna or flora. Notwithstanding this parallelism in the
+conditions of the Old and New Worlds, how widely different are their living
+productions!
+
+In the southern hemisphere, if we compare large tracts of land in
+Australia, South Africa, and western South America, between latitudes 25°
+and 35°, we shall find parts extremely similar in all their conditions, yet
+it would not be possible to point out three faunas and floras more utterly
+dissimilar. Or again we may compare the productions of South America south
+of lat. 35° with those north of 25°, which consequently inhabit a
+considerably different climate, and they will be found incomparably more
+closely related to each other, than they are to the productions of
+Australia or Africa under nearly the same climate. Analogous facts could be
+given with respect to the inhabitants of the sea.
+
+A second great fact which strikes us in our general review is, that
+barriers of any kind, or obstacles to free migration, are related in a
+close and important manner to the differences between the productions of
+various regions. We see this in the great difference of nearly all the
+terrestrial productions of the New and Old Worlds, excepting in the
+northern parts, where the land almost joins, and where, under a slightly
+different climate, there might have been free migration for the northern
+temperate forms, as there now is for the strictly arctic productions. We
+see the same fact in the great difference between the inhabitants of
+Australia, Africa, and South America under the same latitude: for these
+countries are almost as much isolated from each other as is possible. On
+each continent, also, we see the same fact; for on the opposite sides of
+{348} lofty and continuous mountain-ranges, and of great deserts, and
+sometimes even of large rivers, we find different productions; though as
+mountain-chains, deserts, &c., are not as impassable, or likely to have
+endured so long as the oceans separating continents, the differences are
+very inferior in degree to those characteristic of distinct continents.
+
+Turning to the sea, we find the same law. No two marine faunas are more
+distinct, with hardly a fish, shell, or crab in common, than those of the
+eastern and western shores of South and Central America; yet these great
+faunas are separated only by the narrow, but impassable, isthmus of Panama.
+Westward of the shores of America, a wide space of open ocean extends, with
+not an island as a halting-place for emigrants; here we have a barrier of
+another kind, and as soon as this is passed we meet in the eastern islands
+of the Pacific, with another and totally distinct fauna. So that here three
+marine faunas range far northward and southward, in parallel lines not far
+from each other, under corresponding climates; but from being separated
+from each other by impassable barriers, either of land or open sea, they
+are wholly distinct. On the other hand, proceeding still further westward
+from the eastern islands of the tropical parts of the Pacific, we encounter
+no impassable barriers, and we have innumerable islands as halting-places,
+or continuous coasts, until after travelling over a hemisphere we come to
+the shores of Africa; and over this vast space we meet with no well-defined
+and distinct marine faunas. Although hardly one shell, crab or fish is
+common to the above-named three approximate faunas of Eastern and Western
+America and the eastern Pacific islands, yet many fish range from the
+Pacific into the Indian Ocean, and many shells are common to the eastern
+islands of the Pacific {349} and the eastern shores of Africa, on almost
+exactly opposite meridians of longitude.
+
+A third great fact, partly included in the foregoing statements, is the
+affinity of the productions of the same continent or sea, though the
+species themselves are distinct at different points and stations. It is a
+law of the widest generality, and every continent offers innumerable
+instances. Nevertheless the naturalist in travelling, for instance, from
+north to south never fails to be struck by the manner in which successive
+groups of beings, specifically distinct, yet clearly related, replace each
+other. He hears from closely allied, yet distinct kinds of birds, notes
+nearly similar, and sees their nests similarly constructed, but not quite
+alike, with eggs coloured in nearly the same manner. The plains near the
+Straits of Magellan are inhabited by one species of Rhea (American
+ostrich), and northward the plains of La Plata by another species of the
+same genus; and not by a true ostrich or emu, like those found in Africa
+and Australia under the same latitude. On these same plains of La Plata, we
+see the agouti and bizcacha, animals having nearly the same habits as our
+hares and rabbits and belonging to the same order of Rodents, but they
+plainly display an American type of structure. We ascend the lofty peaks of
+the Cordillera and we find an alpine species of bizcacha; we look to the
+waters, and we do not find the beaver or musk-rat, but the coypu and
+capybara, rodents of the American type. Innumerable other instances could
+be given. If we look to the islands off the American shore, however much
+they may differ in geological structure, the inhabitants, though they may
+be all peculiar species, are essentially American. We may look back to past
+ages, as shown in the last chapter, and we find American types then
+prevalent on {350} the American continent and in the American seas. We see
+in these facts some deep organic bond, prevailing throughout space and
+time, over the same areas of land and water, and independent of their
+physical conditions. The naturalist must feel little curiosity, who is not
+led to inquire what this bond is.
+
+This bond, on my theory, is simply inheritance, that cause which alone, as
+far as we positively know, produces organisms quite like, or, as we see in
+the case of varieties, nearly like each other. The dissimilarity of the
+inhabitants of different regions may be attributed to modification through
+natural selection, and in a quite subordinate degree to the direct
+influence of different physical conditions. The degree of dissimilarity
+will depend on the migration of the more dominant forms of life from one
+region into another having been effected with more or less ease, at periods
+more or less remote;--on the nature and number of the former
+immigrants;--and on their action and reaction, in their mutual struggles
+for life;--the relation of organism to organism being, as I have already
+often remarked, the most important of all relations. Thus the high
+importance of barriers comes into play by checking migration; as does time
+for the slow process of modification through natural selection.
+Widely-ranging species, abounding in individuals, which have already
+triumphed over many competitors in their own widely-extended homes will
+have the best chance of seizing on new places, when they spread into new
+countries. In their new homes they will be exposed to new conditions, and
+will frequently undergo further modification and improvement; and thus they
+will become still further victorious, and will produce groups of modified
+descendants. On this principle of inheritance with modification, we can
+understand how it is that sections of genera, whole genera, {351} and even
+families are confined to the same areas, as is so commonly and notoriously
+the case.
+
+I believe, as was remarked in the last chapter, in no law of necessary
+development. As the variability of each species is an independent property,
+and will be taken advantage of by natural selection, only so far as it
+profits the individual in its complex struggle for life, so the degree of
+modification in different species will be no uniform quantity. If, for
+instance, a number of species, which stand in direct competition with each
+other, migrate in a body into a new and afterwards isolated country, they
+will be little liable to modification; for neither migration nor isolation
+in themselves can do anything. These principles come into play only by
+bringing organisms into new relations with each other, and in a lesser
+degree with the surrounding physical conditions. As we have seen in the
+last chapter that some forms have retained nearly the same character from
+an enormously remote geological period, so certain species have migrated
+over vast spaces, and have not become greatly modified.
+
+On these views, it is obvious, that the several species of the same genus,
+though inhabiting the most distant quarters of the world, must originally
+have proceeded from the same source, as they have descended from the same
+progenitor. In the case of those species, which have undergone during whole
+geological periods but little modification, there is not much difficulty in
+believing that they may have migrated from the same region; for during the
+vast geographical and climatal changes which will have supervened since
+ancient times, almost any amount of migration is possible. But in many
+other cases, in which we have reason to believe that the species of a genus
+have been produced within comparatively recent times, there is great
+difficulty on this head. It {352} is also obvious that the individuals of
+the same species, though now inhabiting distant and isolated regions, must
+have proceeded from one spot, where their parents were first produced: for,
+as explained in the last chapter, it is incredible that individuals
+identically the same should ever have been produced through natural
+selection from parents specifically distinct.
+
+We are thus brought to the question which has been largely discussed by
+naturalists, namely, whether species have been created at one or more
+points of the earth's surface. Undoubtedly there are very many cases of
+extreme difficulty, in understanding how the same species could possibly
+have migrated from some one point to the several distant and isolated
+points, where now found. Nevertheless the simplicity of the view that each
+species was first produced within a single region captivates the mind. He
+who rejects it, rejects the _vera causa_ of ordinary generation with
+subsequent migration, and calls in the agency of a miracle. It is
+universally admitted, that in most cases the area inhabited by a species is
+continuous; and when a plant or animal inhabits two points so distant from
+each other, or with an interval of such a nature, that the space could not
+be easily passed over by migration, the fact is given as something
+remarkable and exceptional. The capacity of migrating across the sea is
+more distinctly limited in terrestrial mammals, than perhaps in any other
+organic beings; and, accordingly, we find no inexplicable cases of the same
+mammal inhabiting distant points of the world. No geologist will feel any
+difficulty in such cases as Great Britain having been formerly united to
+Europe, and consequently possessing the same quadrupeds. But if the same
+species can be produced at two separate points, why do we not find a single
+mammal common to Europe and {353} Australia or South America? The
+conditions of life are nearly the same, so that a multitude of European
+animals and plants have become naturalised in America and Australia; and
+some of the aboriginal plants are identically the same at these distant
+points of the northern and southern hemispheres? The answer, as I believe,
+is, that mammals have not been able to migrate, whereas some plants, from
+their varied means of dispersal, have migrated across the vast and broken
+interspace. The great and striking influence which barriers of every kind
+have had on distribution, is intelligible only on the view that the great
+majority of species have been produced on one side alone, and have not been
+able to migrate to the other side. Some few families, many sub-families,
+very many genera, and a still greater number of sections of genera are
+confined to a single region; and it has been observed by several
+naturalists, that the most natural genera, or those genera in which the
+species are most closely related to each other, are generally local, or
+confined to one area. What a strange anomaly it would be, if, when coming
+one step lower in the series, to the individuals of the same species, a
+directly opposite rule prevailed; and species were not local, but had been
+produced in two or more distinct areas!
+
+Hence it seems to me, as it has to many other naturalists, that the view of
+each species having been produced in one area alone, and having
+subsequently migrated from that area as far as its powers of migration and
+subsistence under past and present conditions permitted, is the most
+probable. Undoubtedly many cases occur, in which we cannot explain how the
+same species could have passed from one point to the other. But the
+geographical and climatal changes, which have certainly occurred within
+recent geological times, must have interrupted or rendered discontinuous
+the {354} formerly continuous range of many species. So that we are reduced
+to consider whether the exceptions to continuity of range are so numerous
+and of so grave a nature, that we ought to give up the belief, rendered
+probable by general considerations, that each species has been produced
+within one area, and has migrated thence as far as it could. It would be
+hopelessly tedious to discuss all the exceptional cases of the same
+species, now living at distant and separated points; nor do I for a moment
+pretend that any explanation could be offered of many such cases. But after
+some preliminary remarks, I will discuss a few of the most striking classes
+of facts; namely, the existence of the same species on the summits of
+distant mountain-ranges, and at distant points in the arctic and antarctic
+regions; and secondly (in the following chapter), the wide distribution of
+freshwater productions; and thirdly, the occurrence of the same terrestrial
+species on islands and on the mainland, though separated by hundreds of
+miles of open sea. If the existence of the same species at distant and
+isolated points of the earth's surface, can in many instances be explained
+on the view of each species having migrated from a single birthplace; then,
+considering our ignorance with respect to former climatal and geographical
+changes and various occasional means of transport, the belief that this has
+been the universal law, seems to me incomparably the safest.
+
+In discussing this subject, we shall be enabled at the same time to
+consider a point equally important for us, namely, whether the several
+distinct species of a genus, which on my theory have all descended from a
+common progenitor, can have migrated (undergoing modification during some
+part of their migration) from the area inhabited by their progenitor. If it
+can be shown to be almost invariably the case, that a region, of which
+{355} most of its inhabitants are closely related to, or belong to the same
+genera with the species of a second region, has probably received at some
+former period immigrants from this other region, my theory will be
+strengthened; for we can clearly understand, on the principle of
+modification, why the inhabitants of a region should be related to those of
+another region, whence it has been stocked. A volcanic island, for
+instance, upheaved and formed at the distance of a few hundreds of miles
+from a continent, would probably receive from it in the course of time a
+few colonists, and their descendants, though modified, would still be
+plainly related by inheritance to the inhabitants of the continent. Cases
+of this nature are common, and are, as we shall hereafter more fully see,
+inexplicable on the theory of independent creation. This view of the
+relation of species in one region to those in another, does not differ much
+(by substituting the word variety for species) from that lately advanced in
+an ingenious paper by Mr. Wallace, in which he concludes, that "every
+species has come into existence coincident both in space and time with a
+pre-existing closely allied species." And I now know from correspondence,
+that this coincidence he attributes to generation with modification.
+
+The previous remarks on "single and multiple centres of creation" do not
+directly bear on another allied question,--namely whether all the
+individuals of the same species have descended from a single pair, or
+single hermaphrodite, or whether, as some authors suppose, from many
+individuals simultaneously created. With those organic beings which never
+intercross (if such exist), the species, on my theory, must have descended
+from a succession of improved varieties, which will never have blended with
+other individuals or varieties, but will have supplanted each other; so
+that, at each {356} successive stage of modification and improvement, all
+the individuals of each variety will have descended from a single parent.
+But in the majority of cases, namely, with all organisms which habitually
+unite for each birth, or which often intercross, I believe that during the
+slow process of modification the individuals of the species will have been
+kept nearly uniform by intercrossing; so that many individuals will have
+gone on simultaneously changing, and the whole amount of modification will
+not have been due, at each stage, to descent from a single parent. To
+illustrate what I mean: our English racehorses differ slightly from the
+horses of every other breed; but they do not owe their difference and
+superiority to descent from any single pair, but to continued care in
+selecting and training many individuals during many generations.
+
+Before discussing the three classes of facts, which I have selected as
+presenting the greatest amount of difficulty on the theory of "single
+centres of creation," I must say a few words on the means of dispersal.
+
+
+
+_Means of Dispersal._--Sir C. Lyell and other authors have ably treated
+this subject. I can give here only the briefest abstract of the more
+important facts. Change of climate must have had a powerful influence on
+migration: a region when its climate was different may have been a high
+road for migration, but now be impassable; I shall, however, presently have
+to discuss this branch of the subject in some detail. Changes of level in
+the land must also have been highly influential: a narrow isthmus now
+separates two marine faunas; submerge it, or let it formerly have been
+submerged, and the two faunas will now blend or may formerly have blended:
+where the sea now extends, land may at a former period have connected
+islands or {357} possibly even continents together, and thus have allowed
+terrestrial productions to pass from one to the other. No geologist will
+dispute that great mutations of level have occurred within the period of
+existing organisms. Edward Forbes insisted that all the islands in the
+Atlantic must recently have been connected with Europe or Africa, and
+Europe likewise with America. Other authors have thus hypothetically
+bridged over every ocean, and have united almost every island to some
+mainland. If indeed the arguments used by Forbes are to be trusted, it must
+be admitted that scarcely a single island exists which has not recently
+been united to some continent. This view cuts the Gordian knot of the
+dispersal of the same species to the most distant points, and removes many
+a difficulty: but to the best of my judgment we are not authorized in
+admitting such enormous geographical changes within the period of existing
+species. It seems to me that we have abundant evidence of great
+oscillations of level in our continents; but not of such vast changes in
+their position and extension, as to have united them within the recent
+period to each other and to the several intervening oceanic islands. I
+freely admit the former existence of many islands, now buried beneath the
+sea, which may have served as halting places for plants and for many
+animals during their migration. In the coral-producing oceans such sunken
+islands are now marked, as I believe, by rings of coral or atolls standing
+over them. Whenever it is fully admitted, as I believe it will some day be,
+that each species has proceeded from a single birthplace, and when in the
+course of time we know something definite about the means of distribution,
+we shall be enabled to speculate with security on the former extension of
+the land. But I do not believe that it will ever be proved that within the
+{358} recent period continents which are now quite separate, have been
+continuously, or almost continuously, united with each other, and with the
+many existing oceanic islands. Several facts in distribution,--such as the
+great difference in the marine faunas on the opposite sides of almost every
+continent,--the close relation of the tertiary inhabitants of several lands
+and even seas to their present inhabitants,--a certain degree of relation
+(as we shall hereafter see) between the distribution of mammals and the
+depth of the sea,--these and other such facts seem to me opposed to the
+admission of such prodigious geographical revolutions within the recent
+period, as are necessitated on the view advanced by Forbes and admitted by
+his many followers. The nature and relative proportions of the inhabitants
+of oceanic islands likewise seem to me opposed to the belief of their
+former continuity with continents. Nor does their almost universally
+volcanic composition favour the admission that they are the wrecks of
+sunken continents;--if they had originally existed as mountain-ranges on
+the land, some at least of the islands would have been formed, like other
+mountain-summits, of granite, metamorphic schists, old fossiliferous or
+other such rocks, instead of consisting of mere piles of volcanic matter.
+
+I must now say a few words on what are called accidental means, but which
+more properly might be called occasional means of distribution. I shall
+here confine myself to plants. In botanical works, this or that plant is
+stated to be ill adapted for wide dissemination; but for transport across
+the sea, the greater or less facilities may be said to be almost wholly
+unknown. Until I tried, with Mr. Berkeley's aid, a few experiments, it was
+not even known how far seeds could resist the injurious action of
+sea-water. To my surprise I found that {359} out of 87 kinds, 64 germinated
+after an immersion of 28 days, and a few survived an immersion of 137 days.
+For convenience' sake I chiefly tried small seeds, without the capsule or
+fruit; and as all of these sank in a few days, they could not be floated
+across wide spaces of the sea, whether or not they were injured by the
+salt-water. Afterwards I tried some larger fruits, capsules, &c., and some
+of these floated for a long time. It is well known what a difference there
+is in the buoyancy of green and seasoned timber; and it occurred to me that
+floods might wash down plants or branches, and that these might be dried on
+the banks, and then by a fresh rise in the stream be washed into the sea.
+Hence I was led to dry stems and branches of 94 plants with ripe fruit, and
+to place them on sea-water. The majority sank quickly, but some which
+whilst green floated for a very short time, when dried floated much longer;
+for instance, ripe hazel-nuts sank immediately, but when dried they floated
+for 90 days, and afterwards when planted they germinated; an asparagus
+plant with ripe berries floated for 23 days, when dried it floated for 85
+days, and the seeds afterwards germinated; the ripe seeds of Helosciadium
+sank in two days, when dried they floated for above 90 days, and afterwards
+germinated. Altogether out of the 94 dried plants, 18 floated for above 28
+days, and some of the 18 floated for a very much longer period. So that as
+64/87 seeds germinated after an immersion of 28 days; and as 18/94 plants
+with ripe fruit (but not all the same species as in the foregoing
+experiment) floated, after being dried, for above 28 days, as far as we may
+infer anything from these scanty facts, we may conclude that the seeds of
+14/100 plants of any country might be floated by sea-currents during 28
+days, and would retain their power of germination. In Johnston's Physical
+Atlas, the average {360} rate of the several Atlantic currents is 33 miles
+per diem (some currents running at the rate of 60 miles per diem); on this
+average, the seeds of 14/100 plants belonging to one country might be
+floated across 924 miles of sea to another country; and when stranded, if
+blown to a favourable spot by an inland gale, they would germinate.
+
+Subsequently to my experiments, M. Martens tried similar ones, but in a
+much better manner, for he placed the seeds in a box in the actual sea, so
+that they were alternately wet and exposed to the air like really floating
+plants. He tried 98 seeds, mostly different from mine; but he chose many
+large fruits and likewise seeds from plants which live near the sea; and
+this would have favoured the average length of their flotation and of their
+resistance to the injurious action of the salt-water. On the other hand he
+did not previously dry the plants or branches with the fruit; and this, as
+we have seen, would have caused some of them to have floated much longer.
+The result was that 18/98 of his seeds floated for 42 days, and were then
+capable of germination. But I do not doubt that plants exposed to the waves
+would float for a less time than those protected from violent movement as
+in our experiments. Therefore it would perhaps be safer to assume that the
+seeds of about 10/100 plants of a flora, after having been dried, could be
+floated across a space of sea 900 miles in width, and would then germinate.
+The fact of the larger fruits often floating longer than the small, is
+interesting; as plants with large seeds or fruit could hardly be
+transported by any other means; and Alph. de Candolle has shown that such
+plants generally have restricted ranges.
+
+But seeds may be occasionally transported in another manner. Drift timber
+is thrown up on most islands, {361} even on those in the midst of the
+widest oceans; and the natives of the coral-islands in the Pacific, procure
+stones for their tools, solely from the roots of drifted trees, these
+stones being a valuable royal tax. I find on examination, that when
+irregularly shaped stones are embedded in the roots of trees, small parcels
+of earth are very frequently enclosed in their interstices and behind
+them,--so perfectly that not a particle could be washed away in the longest
+transport: out of one small portion of earth thus _completely_ enclosed by
+wood in an oak about 50 years old, three dicotyledonous plants germinated:
+I am certain of the accuracy of this observation. Again, I can show that
+the carcasses of birds, when floating on the sea, sometimes escape being
+immediately devoured; and seeds of many kinds in the crops of floating
+birds long retain their vitality: peas and vetches, for instance, are
+killed by even a few days' immersion in sea-water; but some taken out of
+the crop of a pigeon, which had floated on artificial salt-water for 30
+days, to my surprise nearly all germinated.
+
+Living birds can hardly fail to be highly effective agents in the
+transportation of seeds. I could give many facts showing how frequently
+birds of many kinds are blown by gales to vast distances across the ocean.
+We may I think safely assume that under such circumstances their rate of
+flight would often be 35 miles an hour; and some authors have given a far
+higher estimate. I have never seen an instance of nutritious seeds passing
+through the intestines of a bird; but hard seeds of fruit pass uninjured
+through even the digestive organs of a turkey. In the course of two months,
+I picked up in my garden 12 kinds of seeds, out of the excrement of small
+birds, and these seemed perfect, and some of them, which I tried,
+germinated. {362} But the following fact is more important: the crops of
+birds do not secrete gastric juice, and do not in the least injure, as I
+know by trial, the germination of seeds; now after a bird has found and
+devoured a large supply of food, it is positively asserted that all the
+grains do not pass into the gizzard for 12 or even 18 hours. A bird in this
+interval might easily be blown to the distance of 500 miles, and hawks are
+known to look out for tired birds, and the contents of their torn crops
+might thus readily get scattered. Mr. Brent informs me that a friend of his
+had to give up flying carrier-pigeons from France to England, as the hawks
+on the English coast destroyed so many on their arrival. Some hawks and
+owls bolt their prey whole, and after an interval of from twelve to twenty
+hours, disgorge pellets, which, as I know from experiments made in the
+Zoological Gardens, include seeds capable of germination. Some seeds of the
+oat, wheat, millet, canary, hemp, clover, and beet germinated after having
+been from twelve to twenty-one hours in the stomachs of different birds of
+prey; and two seeds of beet grew after having been thus retained for two
+days and fourteen hours. Freshwater fish, I find, eat seeds of many land
+and water plants: fish are frequently devoured by birds, and thus the seeds
+might be transported from place to place. I forced many kinds of seeds into
+the stomachs of dead fish, and then gave their bodies to fishing-eagles,
+storks, and pelicans; these birds after an interval of many hours, either
+rejected the seeds in pellets or passed them in their excrement; and
+several of these seeds retained their power of germination. Certain seeds,
+however, were always killed by this process.
+
+Although the beaks and feet of birds are generally quite clean, I can show
+that earth sometimes adheres to them: in one instance I removed twenty-two
+grains {363} of dry argillaceous earth from one foot of a partridge, and in
+this earth there was a pebble quite as large as the seed of a vetch. Thus
+seeds might occasionally be transported to great distances; for many facts
+could be given showing that soil almost everywhere is charged with seeds.
+Reflect for a moment on the millions of quails which annually cross the
+Mediterranean; and can we doubt that the earth adhering to their feet would
+sometimes include a few minute seeds? But I shall presently have to recur
+to this subject.
+
+As icebergs are known to be sometimes loaded with earth and stones, and
+have even carried brushwood, bones, and the nest of a land-bird, I can
+hardly doubt that they must occasionally have transported seeds from one
+part to another of the arctic and antarctic regions, as suggested by Lyell;
+and during the Glacial period from one part of the now temperate regions to
+another. In the Azores, from the large number of the species of plants
+common to Europe, in comparison with the plants of other oceanic islands
+nearer to the mainland, and (as remarked by Mr. H. C. Watson) from the
+somewhat northern character of the flora in comparison with the latitude, I
+suspected that these islands had been partly stocked by ice-borne seeds,
+during the Glacial epoch. At my request Sir C. Lyell wrote to M. Hartung to
+inquire whether he had observed erratic boulders on these islands, and he
+answered that he had found large fragments of granite and other rocks,
+which do not occur in the archipelago. Hence we may safely infer that
+icebergs formerly landed their rocky burthens on the shores of these
+mid-ocean islands, and it is at least possible that they may have brought
+thither the seeds of northern plants.
+
+Considering that the several above means of transport, and that several
+other means, which without {364} doubt remain to be discovered, have been
+in action year after year, for centuries and tens of thousands of years, it
+would I think be a marvellous fact if many plants had not thus become
+widely transported. These means of transport are sometimes called
+accidental, but this is not strictly correct: the currents of the sea are
+not accidental, nor is the direction of prevalent gales of wind. It should
+be observed that scarcely any means of transport would carry seeds for very
+great distances; for seeds do not retain their vitality when exposed for a
+great length of time to the action of sea-water; nor could they be long
+carried in the crops or intestines of birds. These means, however, would
+suffice for occasional transport across tracts of sea some hundred miles in
+breadth, or from island to island, or from a continent to a neighbouring
+island, but not from one distant continent to another. The floras of
+distant continents would not by such means become mingled in any great
+degree; but would remain as distinct as we now see them to be. The
+currents, from their course, would never bring seeds from North America to
+Britain, though they might and do bring seeds from the West Indies to our
+western shores, where, if not killed by so long an immersion in salt-water,
+they could not endure our climate. Almost every year, one or two land-birds
+are blown across the whole Atlantic Ocean, from North America to the
+western shores of Ireland and England; but seeds could be transported by
+these wanderers only by one means, namely, in dirt sticking to their feet,
+which is in itself a rare accident. Even in this case, how small would the
+chance be of a seed falling on favourable soil, and coming to maturity! But
+it would be a great error to argue that because a well-stocked island, like
+Great Britain, has not, as far as is known {365} (and it would be very
+difficult to prove this), received within the last few centuries, through
+occasional means of transport, immigrants from Europe or any other
+continent, that a poorly-stocked island, though standing more remote from
+the mainland, would not receive colonists by similar means. I do not doubt
+that out of twenty seeds or animals transported to an island, even if far
+less well-stocked than Britain, scarcely more than one would be so well
+fitted to its new home, as to become naturalised. But this, as it seems to
+me, is no valid argument against what would be effected by occasional means
+of transport, during the long lapse of geological time, whilst an island
+was being upheaved and formed, and before it had become fully stocked with
+inhabitants. On almost bare land, with few or no destructive insects or
+birds living there, nearly every seed, which chanced to arrive, if fitted
+for the climate, would be sure to germinate and survive.
+
+
+
+_Dispersal during the Glacial period._--The identity of many plants and
+animals, on mountain-summits, separated from each other by hundreds of
+miles of lowlands, where the Alpine species could not possibly exist, is
+one of the most striking cases known of the same species living at distant
+points, without the apparent possibility of their having migrated from one
+to the other. It is indeed a remarkable fact to see so many of the same
+plants living on the snowy regions of the Alps or Pyrenees, and in the
+extreme northern parts of Europe; but it is far more remarkable, that the
+plants on the White Mountains, in the United States of America, are all the
+same with those of Labrador, and nearly all the same, as we hear from Asa
+Gray, with those on the loftiest mountains of Europe. Even as long ago as
+1747, such facts led Gmelin to conclude that the {366} same species must
+have been independently created at several distinct points; and we might
+have remained in this same belief, had not Agassiz and others called vivid
+attention to the Glacial period, which, as we shall immediately see,
+affords a simple explanation of these facts. We have evidence of almost
+every conceivable kind, organic and inorganic, that within a very recent
+geological period, central Europe and North America suffered under an
+Arctic climate. The ruins of a house burnt by fire do not tell their tale
+more plainly, than do the mountains of Scotland and Wales, with their
+scored flanks, polished surfaces, and perched boulders, of the icy streams
+with which their valleys were lately filled. So greatly has the climate of
+Europe changed, that in Northern Italy, gigantic moraines, left by old
+glaciers, are now clothed by the vine and maize. Throughout a large part of
+the United States, erratic boulders, and rocks scored by drifted icebergs
+and coast-ice, plainly reveal a former cold period.
+
+The former influence of the glacial climate on the distribution of the
+inhabitants of Europe, as explained with remarkable clearness by Edward
+Forbes, is substantially as follows. But we shall follow the changes more
+readily, by supposing a new glacial period to come slowly on, and then pass
+away, as formerly occurred. As the cold came on, and as each more southern
+zone became fitted for arctic beings and ill-fitted for their former more
+temperate inhabitants, the latter would be supplanted and arctic
+productions would take their places. The inhabitants of the more temperate
+regions would at the same time travel southward, unless they were stopped
+by barriers, in which case they would perish. The mountains would become
+covered with snow and ice, and their former Alpine inhabitants would
+descend to the plains. By the time that the cold had reached {367} its
+maximum, we should have a uniform arctic fauna and flora, covering the
+central parts of Europe, as far south as the Alps and Pyrenees, and even
+stretching into Spain. The now temperate regions of the United States would
+likewise be covered by arctic plants and animals, and these would be nearly
+the same with those of Europe; for the present circumpolar inhabitants,
+which we suppose to have everywhere travelled southward, are remarkably
+uniform round the world. We may suppose that the Glacial period came on a
+little earlier or later in North America than in Europe, so will the
+southern migration there have been a little earlier or later; but this will
+make no difference in the final result.
+
+As the warmth returned, the arctic forms would retreat northward, closely
+followed up in their retreat by the productions of the more temperate
+regions. And as the snow melted from the bases of the mountains, the arctic
+forms would seize on the cleared and thawed ground, always ascending higher
+and higher, as the warmth increased, whilst their brethren were pursuing
+their northern journey. Hence, when the warmth had fully returned, the same
+arctic species, which had lately lived in a body together on the lowlands
+of the Old and New Worlds, would be left isolated on distant
+mountain-summits (having been exterminated on all lesser heights) and in
+the arctic regions of both hemispheres.
+
+Thus we can understand the identity of many plants at points so immensely
+remote as on the mountains of the United States and of Europe. We can thus
+also understand the fact that the Alpine plants of each mountain-range are
+more especially related to the arctic forms living due north or nearly due
+north of them: for the migration as the cold came on, and the re-migration
+on the returning warmth, will generally {368} have been due south and
+north. The Alpine plants, for example, of Scotland, as remarked by Mr.
+H. C. Watson, and those of the Pyrenees, as remarked by Ramond, are more
+especially allied to the plants of northern Scandinavia; those of the
+United States to Labrador; those of the mountains of Siberia to the arctic
+regions of that country. These views, grounded as they are on the perfectly
+well-ascertained occurrence of a former Glacial period, seem to me to
+explain in so satisfactory a manner the present distribution of the Alpine
+and Arctic productions of Europe and America, that when in other regions we
+find the same species on distant mountain-summits, we may almost conclude
+without other evidence, that a colder climate permitted their former
+migration across the low intervening tracts, since become too warm for
+their existence.
+
+If the climate, since the Glacial period, has ever been in any degree
+warmer than at present (as some geologists in the United States believe to
+have been the case, chiefly from the distribution of the fossil Gnathodon),
+then the arctic and temperate productions will at a very late period have
+marched a little further north, and subsequently have retreated to their
+present homes; but I have met with no satisfactory evidence with respect to
+this intercalated slightly warmer period, since the Glacial period.
+
+The arctic forms, during their long southern migration and re-migration
+northward, will have been exposed to nearly the same climate, and, as is
+especially to be noticed, they will have kept in a body together;
+consequently their mutual relations will not have been much disturbed, and,
+in accordance with the principles inculcated in this volume, they will not
+have been liable to much modification. But with our Alpine productions,
+left isolated from the moment of the returning warmth, {369} first at the
+bases and ultimately on the summits of the mountains, the case will have
+been somewhat different; for it is not likely that all the same arctic
+species will have been left on mountain ranges distant from each other, and
+have survived there ever since; they will, also, in all probability have
+become mingled with ancient Alpine species, which must have existed on the
+mountains before the commencement of the Glacial epoch, and which during
+its coldest period will have been temporarily driven down to the plains;
+they will, also, have been exposed to somewhat different climatal
+influences. Their mutual relations will thus have been in some degree
+disturbed; consequently they will have been liable to modification; and
+this we find has been the case; for if we compare the present Alpine plants
+and animals of the several great European mountain-ranges, though very many
+of the species are identically the same, some present varieties, some are
+ranked as doubtful forms, and some few are distinct yet closely allied or
+representative species.
+
+In illustrating what, as I believe, actually took place during the Glacial
+period, I assumed that at its commencement the arctic productions were as
+uniform round the polar regions as they are at the present day. But the
+foregoing remarks on distribution apply not only to strictly arctic forms,
+but also to many sub-arctic and to some few northern temperate forms, for
+some of these are the same on the lower mountains and on the plains of
+North America and Europe; and it may be reasonably asked how I account for
+the necessary degree of uniformity of the sub-arctic and northern temperate
+forms round the world, at the commencement of the Glacial period. At the
+present day, the sub-arctic and northern temperate productions of the Old
+and New Worlds are separated from each other by the {370} Atlantic Ocean
+and by the extreme northern part of the Pacific. During the Glacial period,
+when the inhabitants of the Old and New Worlds lived further southwards
+than at present, they must have been still more completely separated by
+wider spaces of ocean. I believe the above difficulty may be surmounted by
+looking to still earlier changes of climate of an opposite nature. We have
+good reason to believe that during the newer Pliocene period, before the
+Glacial epoch, and whilst the majority of the inhabitants of the world were
+specifically the same as now, the climate was warmer than at the present
+day. Hence we may suppose that the organisms now living under the climate
+of latitude 60°, during the Pliocene period lived further north under the
+Polar Circle, in latitude 66°-67°; and that the strictly arctic productions
+then lived on the broken land still nearer to the pole. Now if we look at a
+globe, we shall see that under the Polar Circle there is almost continuous
+land from western Europe, through Siberia, to eastern America. And to this
+continuity of the circumpolar land, and to the consequent freedom for
+intermigration under a more favourable climate, I attribute the necessary
+amount of uniformity in the sub-arctic and northern temperate productions
+of the Old and New Worlds, at a period anterior to the Glacial epoch.
+
+Believing, from reasons before alluded to, that our continents have long
+remained in nearly the same relative position, though subjected to large,
+but partial oscillations of level, I am strongly inclined to extend the
+above view, and to infer that during some earlier and still warmer period,
+such as the older Pliocene period, a large number of the same plants and
+animals inhabited the almost continuous circumpolar land; and that these
+plants and animals, both in the Old and {371} New Worlds, began slowly to
+migrate southwards as the climate became less warm, long before the
+commencement of the Glacial period. We now see, as I believe, their
+descendants, mostly in a modified condition, in the central parts of Europe
+and the United States. On this view we can understand the relationship,
+with very little identity, between the productions of North America and
+Europe,--a relationship which is most remarkable, considering the distance
+of the two areas, and their separation by the Atlantic Ocean. We can
+further understand the singular fact remarked on by several observers, that
+the productions of Europe and America during the later tertiary stages were
+more closely related to each other than they are at the present time; for
+during these warmer periods the northern parts of the Old and New Worlds
+will have been almost continuously united by land, serving as a bridge,
+since rendered impassable by cold, for the intermigration of their
+inhabitants.
+
+During the slowly decreasing warmth of the Pliocene period, as soon as the
+species in common, which inhabited the New and Old Worlds, migrated south
+of the Polar Circle, they must have been completely cut off from each
+other. This separation, as far as the more temperate productions are
+concerned, took place long ages ago. And as the plants and animals migrated
+southward, they will have become mingled in the one great region with the
+native American productions, and have had to compete with them; and in the
+other great region, with those of the Old World. Consequently we have here
+everything favourable for much modification,--for far more modification
+than with the Alpine productions, left isolated, within a much more recent
+period, on the several mountain-ranges and on the arctic lands of the two
+Worlds. Hence it has come, that when we compare {372} the now living
+productions of the temperate regions of the New and Old Worlds, we find
+very few identical species (though Asa Gray has lately shown that more
+plants are identical than was formerly supposed), but we find in every
+great class many forms, which some naturalists rank as geographical races,
+and others as distinct species; and a host of closely allied or
+representative forms which are ranked by all naturalists as specifically
+distinct.
+
+As on the land, so in the waters of the sea, a slow southern migration of a
+marine fauna, which during the Pliocene or even a somewhat earlier period,
+was nearly uniform along the continuous shores of the Polar Circle, will
+account, on the theory of modification, for many closely allied forms now
+living in areas completely sundered. Thus, I think, we can understand the
+presence of many existing and tertiary representative forms on the eastern
+and western shores of temperate North America; and the still more striking
+case of many closely allied crustaceans (as described in Dana's admirable
+work), of some fish and other marine animals, in the Mediterranean and in
+the seas of Japan,--areas now separated by a continent and by nearly a
+hemisphere of equatorial ocean.
+
+These cases of relationship, without identity, of the inhabitants of seas
+now disjoined, and likewise of the past and present inhabitants of the
+temperate lands of North America and Europe, are inexplicable on the theory
+of creation. We cannot say that they have been created alike, in
+correspondence with the nearly similar physical conditions of the areas;
+for if we compare, for instance, certain parts of South America with the
+southern continents of the Old World, we see countries closely
+corresponding in all their physical conditions, but with their inhabitants
+utterly dissimilar. {373}
+
+But we must return to our more immediate subject, the Glacial period. I am
+convinced that Forbes's view may be largely extended. In Europe we have the
+plainest evidence of the cold period, from the western shores of Britain to
+the Oural range, and southward to the Pyrenees. We may infer from the
+frozen mammals and nature of the mountain vegetation, that Siberia was
+similarly affected. Along the Himalaya, at points 900 miles apart, glaciers
+have left the marks of their former low descent; and in Sikkim, Dr. Hooker
+saw maize growing on gigantic ancient moraines. South of the equator, we
+have some direct evidence of former glacial action in New Zealand; and the
+same plants, found on widely separated mountains in that island, tell the
+same story. If one account which has been published can be trusted, we have
+direct evidence of glacial action in the south-eastern corner of Australia.
+
+Looking to America; in the northern half, ice-borne fragments of rock have
+been observed on the eastern side as far south as lat. 36°-37°, and on the
+shores of the Pacific, where the climate is now so different, as far south
+as lat. 46°; erratic boulders have, also, been noticed on the Rocky
+Mountains. In the Cordillera of Equatorial South America, glaciers once
+extended far below their present level. In central Chili I was astonished
+at the structure of a vast mound of detritus, about 800 feet in height,
+crossing a valley of the Andes; and this I now feel convinced was a
+gigantic moraine, left far below any existing glacier. Further south on
+both sides of the continent, from lat. 41° to the southernmost extremity,
+we have the clearest evidence of former glacial action, in huge boulders
+transported far from their parent source.
+
+We do not know that the Glacial epoch was strictly simultaneous at these
+several far distant points on {374} opposite sides of the world. But we
+have good evidence in almost every case, that the epoch was included within
+the latest geological period. We have, also, excellent evidence, that it
+endured for an enormous time, as measured by years, at each point. The cold
+may have come on, or have ceased, earlier at one point of the globe than at
+another, but seeing that it endured for long at each, and that it was
+contemporaneous in a geological sense, it seems to me probable that it was,
+during a part at least of the period, actually simultaneous throughout the
+world. Without some distinct evidence to the contrary, we may at least
+admit as probable that the glacial action was simultaneous on the eastern
+and western sides of North America, in the Cordillera under the equator and
+under the warmer temperate zones, and on both sides of the southern
+extremity of the continent. If this be admitted, it is difficult to avoid
+believing that the temperature of the whole world was at this period
+simultaneously cooler. But it would suffice for my purpose, if the
+temperature was at the same time lower along certain broad belts of
+longitude.
+
+On this view of the whole world, or at least of broad longitudinal belts,
+having been simultaneously colder from pole to pole, much light can be
+thrown on the present distribution of identical and allied species. In
+America, Dr. Hooker has shown that between forty and fifty of the flowering
+plants of Tierra del Fuego, forming no inconsiderable part of its scanty
+flora, are common to Europe, enormously remote as these two points are; and
+there are many closely allied species. On the lofty mountains of equatorial
+America a host of peculiar species belonging to European genera occur. On
+the highest mountains of Brazil, some few European genera were found by
+Gardner, which do not exist in the wide {375} intervening hot countries. So
+on the Silla of Caraccas the illustrious Humboldt long ago found species
+belonging to genera characteristic of the Cordillera. On the mountains of
+Abyssinia, several European forms and some few representatives of the
+peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope a
+very few European species, believed not to have been introduced by man, and
+on the mountains, some few representative European forms are found, which
+have not been discovered in the intertropical parts of Africa. On the
+Himalaya, and on the isolated mountain-ranges of the peninsula of India, on
+the heights of Ceylon, and on the volcanic cones of Java, many plants
+occur, either identically the same or representing each other, and at the
+same time representing plants of Europe, not found in the intervening hot
+lowlands. A list of the genera collected on the loftier peaks of Java
+raises a picture of a collection made on a hill in Europe! Still more
+striking is the fact that southern Australian forms are clearly represented
+by plants growing on the summits of the mountains of Borneo. Some of these
+Australian forms, as I hear from Dr. Hooker, extend along the heights of
+the peninsula of Malacca, and are thinly scattered, on the one hand over
+India and on the other as far north as Japan.
+
+On the southern mountains of Australia, Dr. F. Müller has discovered
+several European species; other species, not introduced by man, occur on
+the lowlands; and a long list can be given, as I am informed by Dr. Hooker,
+of European genera, found in Australia, but not in the intermediate torrid
+regions. In the admirable 'Introduction to the Flora of New Zealand,' by
+Dr. Hooker, analogous and striking facts are given in regard to the plants
+of that large island. Hence we see that throughout the world, the plants
+growing on the {376} more lofty mountains, and on the temperate lowlands of
+the northern and southern hemispheres, are sometimes identically the same;
+but they are much oftener specifically distinct, though related to each
+other in a most remarkable manner.
+
+This brief abstract applies to plants alone: some strictly analogous facts
+could be given on the distribution of terrestrial animals. In marine
+productions, similar cases occur; as an example, I may quote a remark by
+the highest authority, Prof. Dana, that "it is certainly a wonderful fact
+that New Zealand should have a closer resemblance in its Crustacea to Great
+Britain, its antipode, than to any other part of the world." Sir J.
+Richardson, also, speaks of the reappearance on the shores of New Zealand,
+Tasmania, &c., of northern forms of fish. Dr. Hooker informs me that
+twenty-five species of Algæ are common to New Zealand and to Europe, but
+have not been found in the intermediate tropical seas.
+
+It should be observed that the northern species and forms found in the
+southern parts of the southern hemisphere, and on the mountain-ranges of
+the intertropical regions, are not arctic, but belong to the northern
+temperate zones. As Mr. H. C. Watson has recently remarked, "In receding
+from polar towards equatorial latitudes, the Alpine or mountain floras
+really become less and less arctic." Many of the forms living on the
+mountains of the warmer regions of the earth and in the southern hemisphere
+are of doubtful value, being ranked by some naturalists as specifically
+distinct, by others as varieties; but some are certainly identical, and
+many, though closely related to northern forms, must be ranked as distinct
+species.
+
+Now let us see what light can be thrown on the foregoing facts, on the
+belief, supported as it is by a large {377} body of geological evidence,
+that the whole world, or a large part of it, was during the Glacial period
+simultaneously much colder than at present. The Glacial period, as measured
+by years, must have been very long; and when we remember over what vast
+spaces some naturalised plants and animals have spread within a few
+centuries, this period will have been ample for any amount of migration. As
+the cold came slowly on, all the tropical plants and other productions will
+have retreated from both sides towards the equator, followed in the rear by
+the temperate productions, and these by the arctic; but with the latter we
+are not now concerned. The tropical plants probably suffered much
+extinction; how much no one can say; perhaps formerly the tropics supported
+as many species as we see at the present day crowded together at the Cape
+of Good Hope, and in parts of temperate Australia. As we know that many
+tropical plants and animals can withstand a considerable amount of cold,
+many might have escaped extermination during a moderate fall of
+temperature, more especially by escaping into the lowest, most protected,
+and warmest districts. But the great fact to bear in mind is, that all
+tropical productions will have suffered to a certain extent. On the other
+hand, the temperate productions, after migrating nearer to the equator,
+though they will have been placed under somewhat new conditions, will have
+suffered less. And it is certain that many temperate plants, if protected
+from the inroads of competitors, can withstand a much warmer climate than
+their own. Hence, it seems to me possible, bearing in mind that the
+tropical productions were in a suffering state and could not have presented
+a firm front against intruders, that a certain number of the more vigorous
+and dominant temperate forms might have penetrated the native ranks and
+have reached or {378} even crossed the equator. The invasion would, of
+course, have been greatly favoured by high land, and perhaps by a dry
+climate; for Dr. Falconer informs me that it is the damp with the heat of
+the tropics which is so destructive to perennial plants from a temperate
+climate. On the other hand, the most humid and hottest districts will have
+afforded an asylum to the tropical natives. The mountain-ranges north-west
+of the Himalaya, and the long line of the Cordillera, seem to have afforded
+two great lines of invasion: and it is a striking fact, lately communicated
+to me by Dr. Hooker, that all the flowering plants, about forty-six in
+number, common to Tierra del Fuego and to Europe still exist in North
+America, which must have lain on the line of march. But I do not doubt that
+some temperate productions entered and crossed even the _lowlands_ of the
+tropics at the period when the cold was most intense,--when arctic forms
+had migrated some twenty-five degrees of latitude from their native country
+and covered the land at the foot of the Pyrenees. At this period of extreme
+cold, I believe that the climate under the equator at the level of the sea
+was about the same with that now felt there at the height of six or seven
+thousand feet. During this the coldest period, I suppose that large spaces
+of the tropical lowlands were clothed with a mingled tropical and temperate
+vegetation, like that now growing with strange luxuriance at the base of
+the Himalaya, as graphically described by Hooker.
+
+Thus, as I believe, a considerable number of plants, a few terrestrial
+animals, and some marine productions, migrated during the Glacial period
+from the northern and southern temperate zones into the intertropical
+regions, and some even crossed the equator. As the warmth returned, these
+temperate forms would naturally ascend the higher mountains, being
+exterminated on the {379} lowlands; those which had not reached the equator
+would re-migrate northward or southward towards their former homes; but the
+forms, chiefly northern, which had crossed the equator, would travel still
+further from their homes into the more temperate latitudes of the opposite
+hemisphere. Although we have reason to believe from geological evidence
+that the whole body of arctic shells underwent scarcely any modification
+during their long southern migration and re-migration northward, the case
+may have been wholly different with those intruding forms which settled
+themselves on the intertropical mountains, and in the southern hemisphere.
+These being surrounded by strangers will have had to compete with many new
+forms of life; and it is probable that selected modifications in their
+structure, habits, and constitutions will have profited them. Thus many of
+these wanderers, though still plainly related by inheritance to their
+brethren of the northern or southern hemispheres, now exist in their new
+homes as well-marked varieties or as distinct species.
+
+It is a remarkable fact, strongly insisted on by Hooker in regard to
+America, and by Alph. de Candolle in regard to Australia, that many more
+identical plants and allied forms have apparently migrated from the north
+to the south, than in a reversed direction. We see, however, a few southern
+vegetable forms on the mountains of Borneo and Abyssinia. I suspect that
+this preponderant migration from north to south is due to the greater
+extent of land in the north, and to the northern forms having existed in
+their own homes in greater numbers, and having consequently been advanced
+through natural selection and competition to a higher stage of perfection
+or dominating power, than the southern forms. And thus, when they became
+commingled during the Glacial period, the northern forms {380} were enabled
+to beat the less powerful southern forms. Just in the same manner as we see
+at the present day, that very many European productions cover the ground in
+La Plata, and in a lesser degree in Australia, and have to a certain extent
+beaten the natives; whereas extremely few southern forms have become
+naturalised in any part of Europe, though hides, wool, and other objects
+likely to carry seeds have been largely imported into Europe during the
+last two or three centuries from La Plata, and during the last thirty or
+forty years from Australia. Something of the same kind must have occurred
+on the intertropical mountains: no doubt before the Glacial period they
+were stocked with endemic Alpine forms; but these have almost everywhere
+largely yielded to the more dominant forms, generated in the larger areas
+and more efficient workshops of the north. In many islands the native
+productions are nearly equalled or even outnumbered by the naturalised; and
+if the natives have not been actually exterminated, their numbers have been
+greatly reduced, and this is the first stage towards extinction. A mountain
+is an island on the land; and the intertropical mountains before the
+Glacial period must have been completely isolated; and I believe that the
+productions of these islands on the land yielded to those produced within
+the larger areas of the north, just in the same way as the productions of
+real islands have everywhere lately yielded to continental forms,
+naturalised by man's agency.
+
+I am far from supposing that all difficulties are removed on the view here
+given in regard to the range and affinities of the allied species which
+live in the northern and southern temperate zones and on the mountains of
+the intertropical regions. Very many difficulties remain to be solved. I do
+not pretend to {381} indicate the exact lines and means of migration, or
+the reason why certain species and not others have migrated; why certain
+species have been modified and have given rise to new groups of forms, and
+others have remained unaltered. We cannot hope to explain such facts, until
+we can say why one species and not another becomes naturalised by man's
+agency in a foreign land; why one ranges twice or thrice as far, and is
+twice or thrice as common, as another species within their own homes.
+
+I have said that many difficulties remain to be solved: some of the most
+remarkable are stated with admirable clearness by Dr. Hooker in his
+botanical works on the antarctic regions. These cannot be here discussed. I
+will only say that as far as regards the occurrence of identical species at
+points so enormously remote as Kerguelen Land, New Zealand, and Fuegia, I
+believe that towards the close of the Glacial period, icebergs, as
+suggested by Lyell, have been largely concerned in their dispersal. But the
+existence of several quite distinct species, belonging to genera
+exclusively confined to the south, at these and other distant points of the
+southern hemisphere, is, on my theory of descent with modification, a far
+more remarkable case of difficulty. For some of these species are so
+distinct, that we cannot suppose that there has been time since the
+commencement of the Glacial period for their migration, and for their
+subsequent modification to the necessary degree. The facts seem to me to
+indicate that peculiar and very distinct species have migrated in radiating
+lines from some common centre; and I am inclined to look in the southern,
+as in the northern hemisphere, to a former and warmer period, before the
+commencement of the Glacial period, when the antarctic lands, now covered
+with ice, supported a highly peculiar {382} and isolated flora. I suspect
+that before this flora was exterminated by the Glacial epoch, a few forms
+were widely dispersed to various points of the southern hemisphere by
+occasional means of transport, and by the aid, as halting-places, of
+existing and now sunken islands: By these means, as I believe, the southern
+shores of America, Australia, New Zealand, have become slightly tinted by
+the same peculiar forms of vegetable life.
+
+Sir C. Lyell in a striking passage has speculated, in language almost
+identical with mine, on the effects of great alternations of climate on
+geographical distribution. I believe that the world has recently felt one
+of his great cycles of change; and that on this view, combined with
+modification through natural selection, a multitude of facts in the present
+distribution both of the same and of allied forms of life can be explained.
+The living waters may be said to have flowed during one short period from
+the north and from the south, and to have crossed at the equator; but to
+have flowed with greater force from the north so as to have freely
+inundated the south. As the tide leaves its drift in horizontal lines,
+though rising higher on the shores where the tide rises highest, so have
+the living waters left their living drift on our mountain-summits, in a
+line gently rising from the arctic lowlands to a great height under the
+equator. The various beings thus left stranded may be compared with savage
+races of man, driven up and surviving in the mountain-fastnesses of almost
+every land, which serve as a record, full of interest to us, of the former
+inhabitants of the surrounding lowlands.
+
+       *       *       *       *       *
+
+
+{383}
+
+CHAPTER XII.
+
+GEOGRAPHICAL DISTRIBUTION--_continued_.
+
+    Distribution of fresh-water productions--On the inhabitants of oceanic
+    islands--Absence of Batrachians and of terrestrial Mammals--On the
+    relation of the inhabitants of islands to those of the nearest
+    mainland--On colonisation from the nearest source with subsequent
+    modification--Summary of the last and present chapters.
+
+As lakes and river-systems are separated from each other by barriers of
+land, it might have been thought that fresh-water productions would not
+have ranged widely within the same country, and as the sea is apparently a
+still more impassable barrier, that they never would have extended to
+distant countries. But the case is exactly the reverse. Not only have many
+fresh-water species, belonging to quite different classes, an enormous
+range, but allied species prevail in a remarkable manner throughout the
+world. I well remember, when first collecting in the fresh waters of
+Brazil, feeling much surprise at the similarity of the fresh-water insects,
+shells, &c., and at the dissimilarity of the surrounding terrestrial
+beings, compared with those of Britain.
+
+But this power in fresh-water productions of ranging widely, though so
+unexpected, can, I think, in most cases be explained by their having become
+fitted, in a manner highly useful to them, for short and frequent
+migrations from pond to pond, or from stream to stream; and liability to
+wide dispersal would follow from this capacity as an almost necessary
+consequence. We can here consider only a few cases. In regard to {384}
+fish, I believe that the same species never occur in the fresh waters of
+distant continents. But on the same continent the species often range
+widely and almost capriciously; for two river-systems will have some fish
+in common and some different. A few facts seem to favour the possibility of
+their occasional transport by accidental means; like that of the live fish
+not rarely dropped by whirlwinds in India, and the vitality of their ova
+when removed from the water. But I am inclined to attribute the dispersal
+of fresh-water fish mainly to slight changes within the recent period in
+the level of the land, having caused rivers to flow into each other.
+Instances, also, could be given of this having occurred during floods,
+without any change of level. We have evidence in the loess of the Rhine of
+considerable changes of level in the land within a very recent geological
+period, and when the surface was peopled by existing land and fresh-water
+shells. The wide difference of the fish on opposite sides of continuous
+mountain-ranges, which from an early period must have parted river-systems
+and completely prevented their inosculation, seems to lead to this same
+conclusion. With respect to allied fresh-water fish occurring at very
+distant points of the world, no doubt there are many cases which cannot at
+present be explained: but some fresh-water fish belong to very ancient
+forms, and in such cases there will have been ample time for great
+geographical changes, and consequently time and means for much migration.
+In the second place, salt-water fish can with care be slowly accustomed to
+live in fresh water; and, according to Valenciennes, there is hardly a
+single group of fishes confined exclusively to fresh water, so that we may
+imagine that a marine member of a fresh-water group might travel far along
+the shores of the sea, and {385} subsequently become modified and adapted
+to the fresh waters of a distant land.
+
+Some species of fresh-water shells have a very wide range, and allied
+species, which, on my theory, are descended from a common parent and must
+have proceeded from a single source, prevail throughout the world. Their
+distribution at first perplexed me much, as their ova are not likely to be
+transported by birds, and they are immediately killed by sea-water, as are
+the adults. I could not even understand how some naturalised species have
+rapidly spread throughout the same country. But two facts, which I have
+observed--and no doubt many others remain to be observed--throw some light
+on this subject. When a duck suddenly emerges from a pond covered with
+duck-weed, I have twice seen these little plants adhering to its back; and
+it has happened to me, in removing a little duckweed from one aquarium to
+another, that I have quite unintentionally stocked the one with fresh-water
+shells from the other. But another agency is perhaps more effectual: I
+suspended a duck's feet, which might represent those of a bird sleeping in
+a natural pond, in an aquarium, where many ova of fresh-water shells were
+hatching; and I found that numbers of the extremely minute and just-hatched
+shells crawled on the feet, and clung to them so firmly that when taken out
+of the water they could not be jarred off, though at a somewhat more
+advanced age they would voluntarily drop off. These just hatched molluscs,
+though aquatic in their nature, survived on the duck's feet, in damp air,
+from twelve to twenty hours; and in this length of time a duck or heron
+might fly at least six or seven hundred miles, and would be sure to alight
+on a pool or rivulet, if blown across sea to an oceanic island or to any
+other distant point. Sir Charles Lyell also {386} informs me that a Dyticus
+has been caught with an Ancylus (a fresh-water shell like a limpet) firmly
+adhering to it; and a water-beetle of the same family, a Colymbetes, once
+flew on board the 'Beagle,' when forty-five miles distant from the nearest
+land: how much farther it might have flown with a favouring gale no one can
+tell.
+
+With respect to plants, it has long been known what enormous ranges many
+fresh-water and even marsh-species have, both over continents and to the
+most remote oceanic islands. This is strikingly shown, as remarked by Alph.
+de Candolle, in large groups of terrestrial plants, which have only a very
+few aquatic members; for these latter seem immediately to acquire, as if in
+consequence, a very wide range. I think favourable means of dispersal
+explain this fact. I have before mentioned that earth occasionally, though
+rarely, adheres in some quantity to the feet and beaks of birds. Wading
+birds, which frequent the muddy edges of ponds, if suddenly flushed, would
+be the most likely to have muddy feet. Birds of this order I can show are
+the greatest wanderers, and are occasionally found on the most remote and
+barren islands in the open ocean; they would not be likely to alight on the
+surface of the sea, so that the dirt would not be washed off their feet;
+when making land, they would be sure to fly to their natural fresh-water
+haunts. I do not believe that botanists are aware how charged the mud of
+ponds is with seeds: I have tried several little experiments, but will here
+give only the most striking case: I took in February three table-spoonfuls
+of mud from three different points, beneath water, on the edge of a little
+pond; this mud when dry weighed only 6¾ ounces; I kept it covered up in my
+study for six months, pulling up and counting each plant as it grew; the
+plants were {387} of many kinds, and were altogether 537 in number; and yet
+the viscid mud was all contained in a breakfast cup! Considering these
+facts, I think it would be an inexplicable circumstance if water-birds did
+not transport the seeds of fresh-water plants to vast distances, and if
+consequently the range of these plants was not very great. The same agency
+may have come into play with the eggs of some of the smaller fresh-water
+animals.
+
+Other and unknown agencies probably have also played a part. I have stated
+that fresh-water fish eat some kinds of seeds, though they reject many
+other kinds after having swallowed them; even small fish swallow seeds of
+moderate size, as of the yellow water-lily and Potamogeton. Herons and
+other birds, century after century, have gone on daily devouring fish; they
+then take flight and go to other waters, or are blown across the sea; and
+we have seen that seeds retain their power of germination, when rejected in
+pellets or in excrement, many hours afterwards. When I saw the great size
+of the seeds of that fine water-lily, the Nelumbium, and remembered Alph.
+de Candolle's remarks on this plant, I thought that its distribution must
+remain quite inexplicable; but Audubon states that he found the seeds of
+the great southern water-lily (probably, according to Dr. Hooker, the
+Nelumbium luteum) in a heron's stomach; although I do not know the fact,
+yet analogy makes me believe that a heron flying to another pond and
+getting a hearty meal of fish, would probably reject from its stomach a
+pellet containing the seeds of the Nelumbium undigested; or the seeds might
+be dropped by the bird whilst feeding its young, in the same way as fish
+are known sometimes to be dropped.
+
+In considering these several means of distribution, {388} it should be
+remembered that when a pond or stream is first formed, for instance, on a
+rising islet, it will be unoccupied; and a single seed or egg will have a
+good chance of succeeding. Although there will always be a struggle for
+life between the individuals of the species, however few, already occupying
+any pond, yet as the number of kinds is small, compared with those on the
+land, the competition will probably be less severe between aquatic than
+between terrestrial species; consequently an intruder from the waters of a
+foreign country, would have a better chance of seizing on a place, than in
+the case of terrestrial colonists. We should, also, remember that some,
+perhaps many, freshwater productions are low in the scale of nature, and
+that we have reason to believe that such low beings change or become
+modified less quickly than the high; and this will give longer time than
+the average for the migration of the same aquatic species. We should not
+forget the probability of many species having formerly ranged as
+continuously as fresh-water productions ever can range, over immense areas,
+and having subsequently become extinct in intermediate regions. But the
+wide distribution of fresh-water plants and of the lower animals, whether
+retaining the same identical form or in some degree modified, I believe
+mainly depends on the wide dispersal of their seeds and eggs by animals,
+more especially by fresh-water birds, which have large powers of flight,
+and naturally travel from one to another and often distant piece of water.
+Nature, like a careful gardener, thus takes her seeds from a bed of a
+particular nature, and drops them in another equally well fitted for them.
+
+
+
+_On the Inhabitants of Oceanic Islands._--We now come to the last of the
+three classes of facts, which I {389} have selected as presenting the
+greatest amount of difficulty, on the view that all the individuals both of
+the same and of allied species have descended from a single parent; and
+therefore have all proceeded from a common birthplace, notwithstanding that
+in the course of time they have come to inhabit distant points of the
+globe. I have already stated that I cannot honestly admit Forbes's view on
+continental extensions, which, if legitimately followed out, would lead to
+the belief that within the recent period all existing islands have been
+nearly or quite joined to some continent. This view would remove many
+difficulties, but it would not, I think, explain all the facts in regard to
+insular productions. In the following remarks I shall not confine myself to
+the mere question of dispersal; but shall consider some other facts, which
+bear on the truth of the two theories of independent creation and of
+descent with modification.
+
+The species of all kinds which inhabit oceanic islands are few in number
+compared with those on equal continental areas: Alph. de Candolle admits
+this for plants, and Wollaston for insects. If we look to the large size
+and varied stations of New Zealand, extending over 780 miles of latitude,
+and compare its flowering plants, only 750 in number, with those on an
+equal area at the Cape of Good Hope or in Australia, we must, I think,
+admit that something quite independently of any difference in physical
+conditions has caused so great a difference in number. Even the uniform
+county of Cambridge has 847 plants, and the little island of Anglesea 764,
+but a few ferns and a few introduced plants are included in these numbers,
+and the comparison in some other respects is not quite fair. We have
+evidence that the barren island of Ascension aboriginally possessed under
+half-a-dozen flowering plants; {390} yet many have become naturalised on
+it, as they have on New Zealand and on every other oceanic island which can
+be named. In St. Helena there is reason to believe that the naturalised
+plants and animals have nearly or quite exterminated many native
+productions. He who admits the doctrine of the creation of each separate
+species, will have to admit, that a sufficient number of the best adapted
+plants and animals have not been created on oceanic islands; for man has
+unintentionally stocked them from various sources far more fully and
+perfectly than has nature.
+
+Although in oceanic islands the number of kinds of inhabitants is scanty,
+the proportion of endemic species (_i.e._ those found nowhere else in the
+world) is often extremely large. If we compare, for instance, the number of
+the endemic land-shells in Madeira, or of the endemic birds in the
+Galapagos Archipelago, with the number found on any continent, and then
+compare the area of the islands with that of the continent, we shall see
+that this is true. This fact might have been expected on my theory, for, as
+already explained, species occasionally arriving after long intervals in a
+new and isolated district, and having to compete with new associates, will
+be eminently liable to modification, and will often produce groups of
+modified descendants. But it by no means follows, that, because in an
+island nearly all the species of one class are peculiar, those of another
+class, or of another section of the same class, are peculiar; and this
+difference seems to depend partly on the species which do not become
+modified having immigrated with facility and in a body, so that their
+mutual relations have not been much disturbed; and partly on the frequent
+arrival of unmodified immigrants from the mother-country, and the
+consequent intercrossing with them. With respect to the effects of this
+intercrossing, {391} it should be remembered that the offspring of such
+crosses would almost certainly gain in vigour; so that even an occasional
+cross would produce more effect than might at first have been anticipated.
+To give a few examples: in the Galapagos Islands nearly every land-bird,
+but only two out of the eleven marine birds, are peculiar; and it is
+obvious that marine birds could arrive at these islands more easily than
+land-birds. Bermuda, on the other hand, which lies at about the same
+distance from North America as the Galapagos Islands do from South America,
+and which has a very peculiar soil, does not possess one endemic land-bird;
+and we know from Mr. J. M. Jones's admirable account of Bermuda, that very
+many North American birds, during their great annual migrations, visit
+either periodically or occasionally this island. Madeira does not possess
+one peculiar bird, and many European and African birds are almost every
+year blown there, as I am informed by Mr. E. V. Harcourt. So that these two
+islands of Bermuda and Madeira have been stocked by birds, which for long
+ages have struggled together in their former homes, and have become
+mutually adapted to each other; and when settled in their new homes, each
+kind will have been kept by the others to their proper places and habits,
+and will consequently have been little liable to modification. Any tendency
+to modification will, also, have been checked by intercrossing with the
+unmodified immigrants from the mother-country. Madeira, again, is inhabited
+by a wonderful number of peculiar land-shells, whereas not one species of
+sea-shell is confined to its shores: now, though we do not know how
+sea-shells are dispersed, yet we can see that their eggs or larvae, perhaps
+attached to seaweed or floating timber, or to the feet of wading-birds,
+might be transported far more easily than {392} land-shells, across three
+or four hundred miles of open sea. The different orders of insects in
+Madeira apparently present analogous facts.
+
+Oceanic islands are sometimes deficient in certain classes, and their
+places are apparently occupied by the other inhabitants; in the Galapagos
+Islands reptiles, and in New Zealand gigantic wingless birds, take the
+place of mammals. In the plants of the Galapagos Islands, Dr. Hooker has
+shown that the proportional numbers of the different orders are very
+different from what they are elsewhere. Such cases are generally accounted
+for by the physical conditions of the islands; but this explanation seems
+to me not a little doubtful. Facility of immigration, I believe, has been
+at least as important as the nature of the conditions.
+
+Many remarkable little facts could be given with respect to the inhabitants
+of remote islands. For instance, in certain islands not tenanted by
+mammals, some of the endemic plants have beautifully hooked seeds; yet few
+relations are more striking than the adaptation of hooked seeds for
+transportal by the wool and fur of quadrupeds. This case presents no
+difficulty on my view, for a hooked seed might be transported to an island
+by some other means; and the plant then becoming slightly modified, but
+still retaining its hooked seeds, would form an endemic species, having as
+useless an appendage as any rudimentary organ,--for instance, as the
+shrivelled wings under the soldered elytra of many insular beetles. Again,
+islands often possess trees or bushes belonging to orders which elsewhere
+include only herbaceous species; now trees, as Alph. de Candolle has shown,
+generally have, whatever the cause may be, confined ranges. Hence trees
+would be little likely to reach distant oceanic islands; and an herbaceous
+plant, though it would have no chance of {393} successfully competing in
+stature with a fully developed tree, when established on an island and
+having to compete with herbaceous plants alone, might readily gain an
+advantage by growing taller and taller and overtopping the other plants. If
+so, natural selection would often tend to add to the stature of herbaceous
+plants when growing on an oceanic island, to whatever order they belonged,
+and thus convert them first into bushes and ultimately into trees.
+
+With respect to the absence of whole orders on oceanic islands, Bory St.
+Vincent long ago remarked that Batrachians (frogs, toads, newts) have never
+been found on any of the many islands with which the great oceans are
+studded. I have taken pains to verify this assertion, and I have found it
+strictly true. I have, however, been assured that a frog exists on the
+mountains of the great island of New Zealand; but I suspect that this
+exception (if the information be correct) may be explained through glacial
+agency. This general absence of frogs, toads, and newts on so many oceanic
+islands cannot be accounted for by their physical conditions; indeed it
+seems that islands are peculiarly well fitted for these animals; for frogs
+have been introduced into Madeira, the Azores, and Mauritius, and have
+multiplied so as to become a nuisance. But as these animals and their spawn
+are known to be immediately killed by sea-water, on my view we can see that
+there would be great difficulty in their transportal across the sea, and
+therefore why they do not exist on any oceanic island. But why, on the
+theory of creation, they should not have been created there, it would be
+very difficult to explain.
+
+Mammals offer another and similar case. I have carefully searched the
+oldest voyages, but have not finished my search; as yet I have not found a
+single {394} instance, free from doubt, of a terrestrial mammal (excluding
+domesticated animals kept by the natives) inhabiting an island situated
+above 300 miles from a continent or great continental island; and many
+islands situated at a much less distance are equally barren. The Falkland
+Islands, which are inhabited by a wolf-like fox, come nearest to an
+exception; but this group cannot be considered as oceanic, as it lies on a
+bank connected with the mainland; moreover, icebergs formerly brought
+boulders to its western shores, and they may have formerly transported
+foxes, as so frequently now happens in the arctic regions. Yet it cannot be
+said that small islands will not support small mammals, for they occur in
+many parts of the world on very small islands, if close to a continent; and
+hardly an island can be named on which our smaller quadrupeds have not
+become naturalised and greatly multiplied. It cannot be said, on the
+ordinary view of creation, that there has not been time for the creation of
+mammals; many volcanic islands are sufficiently ancient, as shown by the
+stupendous degradation which they have suffered and by their tertiary
+strata: there has also been time for the production of endemic species
+belonging to other classes; and on continents it is thought that mammals
+appear and disappear at a quicker rate than other and lower animals. Though
+terrestrial mammals do not occur on oceanic islands, aërial mammals do
+occur on almost every island. New Zealand possesses two bats found nowhere
+else in the world: Norfolk Island, the Viti Archipelago, the Bonin Islands,
+the Caroline and Marianne Archipelagoes, and Mauritius, all possess their
+peculiar bats. Why, it may be asked, has the supposed creative force
+produced bats and no other mammals on remote islands? On my view this
+question can easily be answered; for no {395} terrestrial mammal can be
+transported across a wide space of sea, but bats can fly across. Bats have
+been seen wandering by day far over the Atlantic Ocean; and two North
+American species either regularly or occasionally visit Bermuda, at the
+distance of 600 miles from the mainland. I hear from Mr. Tomes, who has
+specially studied this family, that many of the same species have enormous
+ranges, and are found on continents and on far distant islands. Hence we
+have only to suppose that such wandering species have been modified through
+natural selection in their new homes in relation to their new position, and
+we can understand the presence of endemic bats on islands, with the absence
+of all terrestrial mammals.
+
+Besides the absence of terrestrial mammals in relation to the remoteness of
+islands from continents, there is also a relation, to a certain extent
+independent of distance, between the depth of the sea separating an island
+from the neighbouring mainland, and the presence in both of the same
+mammiferous species or of allied species in a more or less modified
+condition. Mr. Windsor Earl has made some striking observations on this
+head in regard to the great Malay Archipelago, which is traversed near
+Celebes by a space of deep ocean; and this space separates two widely
+distinct mammalian faunas. On either side the islands are situated on
+moderately deep submarine banks, and they are inhabited by closely allied
+or identical quadrupeds. No doubt some few anomalies occur in this great
+archipelago, and there is much difficulty in forming a judgment in some
+cases owing to the probable naturalisation of certain mammals through man's
+agency; but we shall soon have much light thrown on the natural history of
+this archipelago by the admirable zeal and researches of Mr. Wallace. I
+have not as yet had time to {396} follow up this subject in all other
+quarters of the world; but as far as I have gone, the relation generally
+holds good. We see Britain separated by a shallow channel from Europe, and
+the mammals are the same on both sides; we meet with analogous facts on
+many islands separated by similar channels from Australia. The West Indian
+Islands stand on a deeply submerged bank, nearly 1000 fathoms in depth, and
+here we find American forms, but the species and even the genera are
+distinct. As the amount of modification in all cases depends to a certain
+degree on the lapse of time, and as during changes of level it is obvious
+that islands separated by shallow channels are more likely to have been
+continuously united within a recent period to the mainland than islands
+separated by deeper channels, we can understand the frequent relation
+between the depth of the sea and the degree of affinity of the mammalian
+inhabitants of islands with those of a neighbouring continent,--an
+inexplicable relation on the view of independent acts of creation.
+
+All the foregoing remarks on the inhabitants of oceanic islands,--namely,
+the scarcity of kinds--the richness in endemic forms in particular classes
+or sections of classes,--the absence of whole groups, as of batrachians,
+and of terrestrial mammals notwithstanding the presence of aërial
+bats,--the singular proportions of certain orders of plants,--herbaceous
+forms having been developed into trees, &c.,--seem to me to accord better
+with the view of occasional means of transport having been largely
+efficient in the long course of time, than with the view of all our oceanic
+islands having been formerly connected by continuous land with the nearest
+continent; for on this latter view the migration would probably have been
+more complete; and if modification be admitted, all the forms of life would
+have been more {397} equally modified, in accordance with the paramount
+importance of the relation of organism to organism.
+
+I do not deny that there are many and grave difficulties in understanding
+how several of the inhabitants of the more remote islands, whether still
+retaining the same specific form or modified since their arrival, could
+have reached their present homes. But the probability of many islands
+having existed as halting-places, of which not a wreck now remains, must
+not be overlooked. I will here give a single instance of one of the cases
+of difficulty. Almost all oceanic islands, even the most isolated and
+smallest, are inhabited by land-shells, generally by endemic species, but
+sometimes by species found elsewhere. Dr. Aug. A. Gould has given several
+interesting cases in regard to the land-shells of the islands of the
+Pacific. Now it is notorious that land-shells are very easily killed by
+salt; their eggs, at least such as I have tried, sink in sea-water and are
+killed by it. Yet there must be, on my view, some unknown, but highly
+efficient means for their transportal. Would the just-hatched young
+occasionally crawl on and adhere to the feet of birds roosting on the
+ground, and thus get transported? It occurred to me that land-shells, when
+hybernating and having a membranous diaphragm over the mouth of the shell,
+might be floated in chinks of drifted timber across moderately wide arms of
+the sea. And I found that several species did in this state withstand
+uninjured an immersion in sea-water during seven days: one of these shells
+was the Helix pomatia, and after it had again hybernated I put it in
+sea-water for twenty days, and it perfectly recovered. As this species has
+a thick calcareous operculum, I removed it, and when it had formed a new
+membranous one, I immersed it for fourteen days in sea-water, and it
+recovered and crawled away: but more experiments are wanted on this head.
+{398}
+
+The most striking and important fact for us in regard to the inhabitants of
+islands, is their affinity to those of the nearest mainland, without being
+actually the same species. Numerous instances could be given of this fact.
+I will give only one, that of the Galapagos Archipelago, situated under the
+equator, between 500 and 600 miles from the shores of South America. Here
+almost every product of the land and water bears the unmistakeable stamp of
+the American continent. There are twenty-six land-birds, and twenty-five of
+these are ranked by Mr. Gould as distinct species, supposed to have been
+created here; yet the close affinity of most of these birds to American
+species in every character, in their habits, gestures, and tones of voice,
+was manifest. So it is with the other animals, and with nearly all the
+plants, as shown by Dr. Hooker in his admirable memoir on the Flora of this
+archipelago. The naturalist, looking at the inhabitants of these volcanic
+islands in the Pacific, distant several hundred miles from the continent,
+yet feels that he is standing on American land. Why should this be so? why
+should the species which are supposed to have been created in the Galapagos
+Archipelago, and nowhere else, bear so plain a stamp of affinity to those
+created in America? There is nothing in the conditions of life, in the
+geological nature of the islands, in their height or climate, or in the
+proportions in which the several classes are associated together, which
+resembles closely the conditions of the South American coast: in fact there
+is a considerable dissimilarity in all these respects. On the other hand,
+there is a considerable degree of resemblance in the volcanic nature of the
+soil, in climate, height, and size of the islands, between the Galapagos
+and Cape de Verde Archipelagos: but what an entire and absolute difference
+in their inhabitants! The inhabitants of the Cape de Verde Islands are
+related to {399} those of Africa, like those of the Galapagos to America. I
+believe this grand fact can receive no sort of explanation on the ordinary
+view of independent creation; whereas on the view here maintained, it is
+obvious that the Galapagos Islands would be likely to receive colonists,
+whether by occasional means of transport or by formerly continuous land,
+from America; and the Cape de Verde Islands from Africa; and that such
+colonists would be liable to modification;--the principle of inheritance
+still betraying their original birthplace.
+
+Many analogous facts could be given: indeed it is an almost universal rule
+that the endemic productions of islands are related to those of the nearest
+continent, or of other near islands. The exceptions are few, and most of
+them can be explained. Thus the plants of Kerguelen Land, though standing
+nearer to Africa than to America, are related, and that very closely, as we
+know from Dr. Hooker's account, to those of America: but on the view that
+this island has been mainly stocked by seeds brought with earth and stones
+on icebergs, drifted by the prevailing currents, this anomaly disappears.
+New Zealand in its endemic plants is much more closely related to
+Australia, the nearest mainland, than to any other region: and this is what
+might have been expected; but it is also plainly related to South America,
+which, although the next nearest continent, is so enormously remote, that
+the fact becomes an anomaly. But this difficulty almost disappears on the
+view that both New Zealand, South America, and other southern lands were
+long ago partially stocked from a nearly intermediate though distant point,
+namely from the antarctic islands, when they were clothed with vegetation,
+before the commencement of the Glacial period. The affinity, which, though
+feeble, I am assured by Dr. Hooker is real, between the flora of the
+south-western corner of Australia and of the Cape of Good {400} Hope, is a
+far more remarkable case, and is at present inexplicable: but this affinity
+is confined to the plants, and will, I do not doubt, be some day explained.
+
+The law which causes the inhabitants of an archipelago, though specifically
+distinct, to be closely allied to those of the nearest continent, we
+sometimes see displayed on a small scale, yet in a most interesting manner,
+within the limits of the same archipelago. Thus the several islands of the
+Galapagos Archipelago are tenanted, as I have elsewhere shown, in a quite
+marvellous manner, by very closely related species; so that the inhabitants
+of each separate island, though mostly distinct, are related in an
+incomparably closer degree to each other than to the inhabitants of any
+other part of the world. And this is just what might have been expected on
+my view, for the islands are situated so near each other that they would
+almost certainly receive immigrants from the same original source, or from
+each other. But this dissimilarity between the endemic inhabitants of the
+islands may be used as an argument against my views; for it may be asked,
+how has it happened in the several islands situated within sight of each
+other, having the same geological nature, the same height, climate, &c.,
+that many of the immigrants should have been differently modified, though
+only in a small degree. This long appeared to me a great difficulty: but it
+arises in chief part from the deeply-seated error of considering the
+physical conditions of a country as the most important for its inhabitants;
+whereas it cannot, I think, be disputed that the nature of the other
+inhabitants, with which each has to compete, is as least as important, and
+generally a far more important element of success. Now if we look to those
+inhabitants of the Galapagos Archipelago which are found in other parts of
+the world (laying on one side for the moment the {401} endemic species,
+which cannot be here fairly included, as we are considering how they have
+come to be modified since their arrival), we find a considerable amount of
+difference in the several islands. This difference might indeed have been
+expected on the view of the islands having been stocked by occasional means
+of transport--a seed, for instance, of one plant having been brought to one
+island, and that of another plant to another island. Hence when in former
+times an immigrant settled on any one or more of the islands, or when it
+subsequently spread from one island to another, it would undoubtedly be
+exposed to different conditions of life in the different islands, for it
+would have to compete with different sets of organisms: a plant for
+instance, would find the best-fitted ground more perfectly occupied by
+distinct plants in one island than in another, and it would be exposed to
+the attacks of somewhat different enemies. If then it varied, natural
+selection would probably favour different varieties in the different
+islands. Some species, however, might spread and yet retain the same
+character throughout the group, just as we see on continents some species
+spreading widely and remaining the same.
+
+The really surprising fact in this case of the Galapagos Archipelago, and
+in a lesser degree in some analogous instances, is that the new species
+formed in the separate islands have not quickly spread to the other
+islands. But the islands, though in sight of each other, are separated by
+deep arms of the sea, in most cases wider than the British Channel, and
+there is no reason to suppose that they have at any former period been
+continuously united. The currents of the sea are rapid and sweep across the
+archipelago, and gales of wind are extraordinarily rare; so that the
+islands are far more effectually separated from each other than they appear
+to be on a map. Nevertheless a good many {402} species, both those found in
+other parts of the world and those confined to the archipelago, are common
+to the several islands, and we may infer from certain facts that these have
+probably spread from some one island to the others. But we often take, I
+think, an erroneous view of the probability of closely-allied species
+invading each other's territory, when put into free intercommunication.
+Undoubtedly if one species has any advantage whatever over another, it will
+in a very brief time wholly or in part supplant it; but if both are equally
+well fitted for their own places in nature, both probably will hold their
+own places and keep separate for almost any length of time. Being familiar
+with the fact that many species, naturalised through man's agency, have
+spread with astonishing rapidity over new countries, we are apt to infer
+that most species would thus spread; but we should remember that the forms
+which become naturalised in new countries are not generally closely allied
+to the aboriginal inhabitants, but are very distinct species, belonging in
+a large proportion of cases, as shown by Alph. de Candolle, to distinct
+genera. In the Galapagos Archipelago, many even of the birds, though so
+well adapted for flying from island to island, are distinct on each; thus
+there are three closely-allied species of mocking-thrush, each confined to
+its own island. Now let us suppose the mocking-thrush of Chatham Island to
+be blown to Charles Island, which has its own mocking-thrush: why should it
+succeed in establishing itself there? We may safely infer that Charles
+Island is well stocked with its own species, for annually more eggs are
+laid there than can possibly be reared; and we may infer that the
+mocking-thrush peculiar to Charles Island is at least as well fitted for
+its home as is the species peculiar to Chatham Island. Sir C. Lyell and Mr.
+Wollaston have communicated to me a remarkable fact bearing on this {403}
+subject; namely, that Madeira and the adjoining islet of Porto Santo
+possess many distinct but representative land-shells, some of which live in
+crevices of stone; and although large quantities of stone are annually
+transported from Porto Santo to Madeira, yet this latter island has not
+become colonised by the Porto Santo species: nevertheless both islands have
+been colonised by some European land-shells, which no doubt had some
+advantage over the indigenous species. From these considerations I think we
+need not greatly marvel at the endemic and representative species, which
+inhabit the several islands of the Galapagos Archipelago, not having
+universally spread from island to island. In many other instances, as in
+the several districts of the same continent, pre-occupation has probably
+played an important part in checking the commingling of species under the
+same conditions of life. Thus, the south-east and south-west corners of
+Australia have nearly the same physical conditions, and are united by
+continuous land, yet they are inhabited by a vast number of distinct
+mammals, birds, and plants.
+
+The principle which determines the general character of the fauna and flora
+of oceanic islands, namely, that the inhabitants, when not identically the
+same, yet are plainly related to the inhabitants of that region whence
+colonists could most readily have been derived,--the colonists having been
+subsequently modified and better fitted to their new homes,--is of the
+widest application throughout nature. We see this on every mountain, in
+every lake and marsh. For Alpine species, excepting in so far as the same
+forms, chiefly of plants, have spread widely throughout the world during
+the recent Glacial epoch, are related to those of the surrounding
+lowlands;--thus we have in South America, Alpine humming-birds, Alpine
+rodents, Alpine plants, {404} &c., all of strictly American forms, and it
+is obvious that a mountain, as it became slowly upheaved, would naturally
+be colonised from the surrounding lowlands. So it is with the inhabitants
+of lakes and marshes, excepting in so far as great facility of transport
+has given the same general forms to the whole world. We see this same
+principle in the blind animals inhabiting the caves of America and of
+Europe. Other analogous facts could be given. And it will, I believe, be
+universally found to be true, that wherever in two regions, let them be
+ever so distant, many closely-allied or representative species occur, there
+will likewise be found some identical species, showing, in accordance with
+the foregoing view, that at some former period there has been
+intercommunication or migration between the two regions. And wherever many
+closely-allied species occur, there will be found many forms which some
+naturalists rank as distinct species, and some as varieties; these doubtful
+forms showing us the steps in the process of modification.
+
+This relation between the power and extent of migration of a species,
+either at the present time or at some former period under different
+physical conditions, and the existence at remote points of the world of
+other species allied to it, is shown in another and more general way. Mr.
+Gould remarked to me long ago, that in those genera of birds which range
+over the world, many of the species have very wide ranges. I can hardly
+doubt that this rule is generally true, though it would be difficult to
+prove it. Amongst mammals, we see it strikingly displayed in Bats, and in a
+lesser degree in the Felidæ and Canidæ. We see it, if we compare the
+distribution of butterflies and beetles. So it is with most fresh-water
+productions, in which so many genera range over the world, and many
+individual species have {405} enormous ranges. It is not meant that in
+world-ranging genera all the species have a wide range, or even that they
+have on an _average_ a wide range; but only that some of the species range
+very widely; for the facility with which widely-ranging species vary and
+give rise to new forms will largely determine their average range. For
+instance, two varieties of the same species inhabit America and Europe, and
+the species thus has an immense range; but, if the variation had been a
+little greater, the two varieties would have been ranked as distinct
+species, and the common range would have been greatly reduced. Still less
+is it meant, that a species which apparently has the capacity of crossing
+barriers and ranging widely, as in the case of certain powerfully-winged
+birds, will necessarily range widely; for we should never forget that to
+range widely implies not only the power of crossing barriers, but the more
+important power of being victorious in distant lands in the struggle for
+life with foreign associates. But on the view of all the species of a genus
+having descended from a single parent, though now distributed to the most
+remote points of the world, we ought to find, and I believe as a general
+rule we do find, that some at least of the species range very widely; for
+it is necessary that the unmodified parent should range widely, undergoing
+modification during its diffusion, and should place itself under diverse
+conditions favourable for the conversion of its offspring, firstly into new
+varieties and ultimately into new species.
+
+In considering the wide distribution of certain genera, we should bear in
+mind that some are extremely ancient, and must have branched off from a
+common parent at a remote epoch; so that in such cases there will have been
+ample time for great climatal and geographical changes and for accidents of
+transport; and consequently for the migration of some of the species into
+all {406} quarters of the world, where they may have become slightly
+modified in relation to their new conditions. There is, also, some reason
+to believe from geological evidence that organisms low in the scale within
+each great class, generally change at a slower rate than the higher forms;
+and consequently the lower forms will have had a better chance of ranging
+widely and of still retaining the same specific character. This fact,
+together with the seeds and eggs of many low forms being very minute and
+better fitted for distant transportation, probably accounts for a law which
+has long been observed, and which has lately been admirably discussed by
+Alph. de Candolle in regard to plants, namely, that the lower any group of
+organisms is, the more widely it is apt to range.
+
+The relations just discussed,--namely, low and slowly-changing organisms
+ranging more widely than the high,--some of the species of widely-ranging
+genera themselves ranging widely,--such facts, as alpine, lacustrine, and
+marsh productions being related (with the exceptions before specified) to
+those on the surrounding low lands and dry lands, though these stations are
+so different,--the very close relation of the distinct species which
+inhabit the islets of the same archipelago,--and especially the striking
+relation of the inhabitants of each whole archipelago or island to those of
+the nearest mainland,--are, I think, utterly inexplicable on the ordinary
+view of the independent creation of each species, but are explicable on the
+view of colonisation from the nearest or readiest source, together with the
+subsequent modification and better adaptation of the colonists to their new
+homes.
+
+
+
+_Summary of last and present Chapters._--In these chapters I have
+endeavoured to show, that if we make due allowance for our ignorance of the
+full effects of all {407} the changes of climate and of the level of the
+land, which have certainly occurred within the recent period, and of other
+similar changes which may have occurred within the same period; if we
+remember how profoundly ignorant we are with respect to the many and
+curious means of occasional transport,--a subject which has hardly ever
+been properly experimentised on; if we bear in mind how often a species may
+have ranged continuously over a wide area, and then have become extinct in
+the intermediate tracts, I think the difficulties in believing that all the
+individuals of the same species, wherever located, have descended from the
+same parents, are not insuperable. And we are led to this conclusion, which
+has been arrived at by many naturalists under the designation of single
+centres of creation, by some general considerations, more especially from
+the importance of barriers and from the analogical distribution of
+sub-genera, genera, and families.
+
+With respect to the distinct species of the same genus, which on my theory
+must have spread from one parent-source; if we make the same allowances as
+before for our ignorance, and remember that some forms of life change most
+slowly, enormous periods of time being thus granted for their migration, I
+do not think that the difficulties are insuperable; though they often are
+in this case, and in that of the individuals of the same species, extremely
+great.
+
+As exemplifying the effects of climatal changes on distribution, I have
+attempted to show how important has been the influence of the modern
+Glacial period, which I am fully convinced simultaneously affected the
+whole world, or at least great meridional belts. As showing how diversified
+are the means of occasional transport, I have discussed at some little
+length the means of dispersal of fresh-water productions. {408}
+
+If the difficulties be not insuperable in admitting that in the long course
+of time the individuals of the same species, and likewise of allied
+species, have proceeded from some one source; then I think all the grand
+leading facts of geographical distribution are explicable on the theory of
+migration (generally of the more dominant forms of life), together with
+subsequent modification and the multiplication of new forms. We can thus
+understand the high importance of barriers, whether of land or water, which
+separate our several zoological and botanical provinces. We can thus
+understand the localisation of sub-genera, genera, and families; and how it
+is that under different latitudes, for instance in South America, the
+inhabitants of the plains and mountains, of the forests, marshes, and
+deserts, are in so mysterious a manner linked together by affinity, and are
+likewise linked to the extinct beings which formerly inhabited the same
+continent. Bearing in mind that the mutual relation of organism to organism
+is of the highest importance, we can see why two areas having nearly the
+same physical conditions should often be inhabited by very different forms
+of life; for according to the length of time which has elapsed since new
+inhabitants entered one region; according to the nature of the
+communication which allowed certain forms and not others to enter, either
+in greater or lesser numbers; according or not, as those which entered
+happened to come in more or less direct competition with each other and
+with the aborigines; and according as the immigrants were capable of
+varying more or less rapidly, there would ensue in different regions,
+independently of their physical conditions, infinitely diversified
+conditions of life,--there would be an almost endless amount of organic
+action and reaction,--and we should find, as we do find, some groups of
+beings greatly, and some only slightly modified,--some {409} developed in
+great force, some existing in scanty numbers--in the different great
+geographical provinces of the world.
+
+On these same principles, we can understand, as I have endeavoured to show,
+why oceanic islands should have few inhabitants, but of these a great
+number should be endemic or peculiar; and why, in relation to the means of
+migration, one group of beings, even within the same class, should have all
+its species endemic, and another group should have all its species common
+to other quarters of the world. We can see why whole groups of organisms,
+as batrachians and terrestrial mammals, should be absent from oceanic
+islands, whilst the most isolated islands possess their own peculiar
+species of aërial mammals or bats. We can see why there should be some
+relation between the presence of mammals, in a more or less modified
+condition, and the depth of the sea between an island and the mainland. We
+can clearly see why all the inhabitants of an archipelago, though
+specifically distinct on the several islets, should be closely related to
+each other, and likewise be related, but less closely, to those of the
+nearest continent or other source whence immigrants were probably derived.
+We can see why in two areas, however distant from each other, there should
+be a correlation, in the presence of identical species, of varieties, of
+doubtful species, and of distinct but representative species.
+
+As the late Edward Forbes often insisted, there is a striking parallelism
+in the laws of life throughout time and space: the laws governing the
+succession of forms in past times being nearly the same with those
+governing at the present time the differences in different areas. We see
+this in many facts. The endurance of each species and group of species is
+continuous in time; for the exceptions to the rule are so few, that they
+may {410} fairly be attributed to our not having as yet discovered in an
+intermediate deposit the forms which are therein absent, but which occur
+above and below: so in space, it certainly is the general rule that the
+area inhabited by a single species, or by a group of species, is
+continuous; and the exceptions, which are not rare, may, as I have
+attempted to show, be accounted for by migration at some former period
+under different conditions or by occasional means of transport, and by the
+species having become extinct in the intermediate tracts. Both in time and
+space, species and groups of species have their points of maximum
+development. Groups of species, belonging either to a certain period of
+time, or to a certain area, are often characterised by trifling characters
+in common, as of sculpture or colour. In looking to the long succession of
+ages, as in now looking to distant provinces throughout the world, we find
+that some organisms differ little, whilst others belonging to a different
+class, or to a different order, or even only to a different family of the
+same order, differ greatly. In both time and space the lower members of
+each class generally change less than the higher; but there are in both
+cases marked exceptions to the rule. On my theory these several relations
+throughout time and space are intelligible; for whether we look to the
+forms of life which have changed during successive ages within the same
+quarter of the world, or to those which have changed after having migrated
+into distant quarters, in both cases the forms within each class have been
+connected by the same bond of ordinary generation; and the more nearly any
+two forms are related in blood, the nearer they will generally stand to
+each other in time and space; in both cases the laws of variation have been
+the same, and modifications have been accumulated by the same power of
+natural selection.
+
+       *       *       *       *       *
+
+
+{411}
+
+CHAPTER XIII.
+
+MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY
+ORGANS.
+
+    CLASSIFICATION, groups subordinate to groups--Natural system--Rules and
+    difficulties in classification, explained on the theory of descent with
+    modification--Classification of varieties--Descent always used in
+    classification--Analogical or adaptive characters--Affinities, general,
+    complex and radiating--Extinction separates and defines
+    groups--MORPHOLOGY, between members of the same class, between parts of
+    the same individual--EMBRYOLOGY, laws of, explained by variations not
+    supervening at an early age, and being inherited at a corresponding
+    age--RUDIMENTARY ORGANS; their origin explained--Summary.
+
+From the first dawn of life, all organic beings are found to resemble each
+other in descending degrees, so that they can be classed in groups under
+groups. This classification is evidently not arbitrary like the grouping of
+the stars in constellations. The existence of groups would have been of
+simple signification, if one group had been exclusively fitted to inhabit
+the land, and another the water; one to feed on flesh, another on vegetable
+matter, and so on; but the case is widely different in nature; for it is
+notorious how commonly members of even the same sub-group have different
+habits. In our second and fourth chapters, on Variation and on Natural
+Selection, I have attempted to show that it is the widely ranging, the much
+diffused and common, that is the dominant species belonging to the larger
+genera, which vary most. The varieties, or incipient species, thus produced
+ultimately become converted, as I believe, into new and distinct species;
+and these, on the principle of inheritance, tend to produce other new and
+dominant {412} species. Consequently the groups which are now large, and
+which generally include many dominant species, tend to go on increasing
+indefinitely in size. I further attempted to show that from the varying
+descendants of each species trying to occupy as many and as different
+places as possible in the economy of nature, there is a constant tendency
+in their characters to diverge. This conclusion was supported by looking at
+the great diversity of the forms of life which, in any small area, come
+into the closest competition, and by looking to certain facts in
+naturalisation.
+
+I attempted also to show that there is a constant tendency in the forms
+which are increasing in number and diverging in character, to supplant and
+exterminate the less divergent, the less improved, and preceding forms. I
+request the reader to turn to the diagram illustrating the action, as
+formerly explained, of these several principles; and he will see that the
+inevitable result is that the modified descendants proceeding from one
+progenitor become broken up into groups subordinate to groups. In the
+diagram each letter on the uppermost line may represent a genus including
+several species; and all the genera on this line form together one class,
+for all have descended from one ancient but unseen parent, and,
+consequently, have inherited something in common. But the three genera on
+the left hand have, on this same principle, much in common, and form a
+sub-family, distinct from that including the next two genera on the right
+hand, which diverged from a common parent at the fifth stage of descent.
+These five genera have also much, though less, in common; and they form a
+family distinct from that including the three genera still further to the
+right hand, which diverged at a still earlier period. And all these genera,
+descended from (A), form an order distinct from the {413} genera descended
+from (I). So that we here have many species descended from a single
+progenitor grouped into genera; and the genera are included in, or
+subordinate to, sub-families, families, and orders, all united into one
+class. Thus, the grand fact in natural history of the subordination of
+group under group, which, from its familiarity, does not always
+sufficiently strike us, is in my judgment explained.
+
+Naturalists try to arrange the species, genera, and families in each class,
+on what is called the Natural System. But what is meant by this system?
+Some authors look at it merely as a scheme for arranging together those
+living objects which are most alike, and for separating those which are
+most unlike; or as an artificial means for enunciating, as briefly as
+possible, general propositions,--that is, by one sentence to give the
+characters common, for instance, to all mammals, by another those common to
+all carnivora, by another those common to the dog-genus, and then by adding
+a single sentence, a full description is given of each kind of dog. The
+ingenuity and utility of this system are indisputable. But many naturalists
+think that something more is meant by the Natural System; they believe that
+it reveals the plan of the Creator; but unless it be specified whether
+order in time or space, or what else is meant by the plan of the Creator,
+it seems to me that nothing is thus added to our knowledge. Such
+expressions as that famous one of Linnæus, and which we often meet with in
+a more or less concealed form, that the characters do not make the genus,
+but that the genus gives the characters, seem to imply that something more
+is included in our classification, than mere resemblance. I believe that
+something more is included; and that propinquity of descent,--the only
+known cause of the similarity of organic beings,--is the bond, hidden as it
+is by various degrees of {414} modification, which is partially revealed to
+us by our classifications.
+
+Let us now consider the rules followed in classification, and the
+difficulties which are encountered on the view that classification either
+gives some unknown plan of creation, or is simply a scheme for enunciating
+general propositions and of placing together the forms most like each
+other. It might have been thought (and was in ancient times thought) that
+those parts of the structure which determined the habits of life, and the
+general place of each being in the economy of nature, would be of very high
+importance in classification. Nothing can be more false. No one regards the
+external similarity of a mouse to a shrew, of a dugong to a whale, of a
+whale to a fish, as of any importance. These resemblances, though so
+intimately connected with the whole life of the being, are ranked as merely
+"adaptive or analogical characters;" but to the consideration of these
+resemblances we shall have to recur. It may even be given as a general
+rule, that the less any part of the organisation is concerned with special
+habits, the more important it becomes for classification. As an instance:
+Owen, in speaking of the dugong, says, "The generative organs being those
+which are most remotely related to the habits and food of an animal, I have
+always regarded as affording very clear indications of its true affinities.
+We are least likely in the modifications of these organs to mistake a
+merely adaptive for an essential character." So with plants, how remarkable
+it is that the organs of vegetation, on which their whole life depends, are
+of little signification, excepting in the first main divisions; whereas the
+organs of reproduction, with their product the seed, are of paramount
+importance!
+
+We must not, therefore, in classifying, trust to resemblances in parts of
+the organisation, however important {415} they may be for the welfare of
+the being in relation to the outer world. Perhaps from this cause it has
+partly arisen, that almost all naturalists lay the greatest stress on
+resemblances in organs of high vital or physiological importance. No doubt
+this view of the classificatory importance of organs which are important is
+generally, but by no means always, true. But their importance for
+classification, I believe, depends on their greater constancy throughout
+large groups of species; and this constancy depends on such organs having
+generally been subjected to less change in the adaptation of the species to
+their conditions of life. That the mere physiological importance of an
+organ does not determine its classificatory value, is almost shown by the
+one fact, that in allied groups, in which the same organ, as we have every
+reason to suppose, has nearly the same physiological value, its
+classificatory value is widely different. No naturalist can have worked at
+any group without being struck with this fact; and it has been fully
+acknowledged in the writings of almost every author. It will suffice to
+quote the highest authority, Robert Brown, who in speaking of certain
+organs in the Proteaceæ, says their generic importance, "like that of all
+their parts, not only in this but, as I apprehend, in every natural family,
+is very unequal, and in some cases seems to be entirely lost." Again in
+another work he says, the genera of the Connaraceæ "differ in having one or
+more ovaria, in the existence or absence of albumen, in the imbricate or
+valvular æstivation. Any one of these characters singly is frequently of
+more than generic importance, though here even when all taken together they
+appear insufficient to separate Cnestis from Connarus." To give an example
+amongst insects, in one great division of the Hymenoptera, the antennæ, as
+Westwood has remarked, are most constant in structure; {416} in another
+division they differ much, and the differences are of quite subordinate
+value in classification; yet no one probably will say that the antennae in
+these two divisions of the same order are of unequal physiological
+importance. Any number of instances could be given of the varying
+importance for classification of the same important organ within the same
+group of beings.
+
+Again, no one will say that rudimentary or atrophied organs are of high
+physiological or vital importance; yet, undoubtedly, organs in this
+condition are often of high value in classification. No one will dispute
+that the rudimentary teeth in the upper jaws of young ruminants, and
+certain rudimentary bones of the leg, are highly serviceable in exhibiting
+the close affinity between Ruminants and Pachyderms. Robert Brown has
+strongly insisted on the fact that the rudimentary florets are of the
+highest importance in the classification of the Grasses.
+
+Numerous instances could be given of characters derived from parts which
+must be considered of very trifling physiological importance, but which are
+universally admitted as highly serviceable in the definition of whole
+groups. For instance, whether or not there is an open passage from the
+nostrils to the mouth, the only character, according to Owen, which
+absolutely distinguishes fishes and reptiles--the inflection of the angle
+of the jaws in Marsupials--the manner in which the wings of insects are
+folded--mere colour in certain Algæ--mere pubescence on parts of the flower
+in grasses--the nature of the dermal covering, as hair or feathers, in the
+Vertebrata. If the Ornithorhynchus had been covered with feathers instead
+of hair, this external and trifling character would, I think, have been
+considered by naturalists as important an aid in determining the degree of
+affinity of this strange creature to {417} birds and reptiles, as an
+approach in structure in any one internal and important organ.
+
+The importance, for classification, of trifling characters, mainly depends
+on their being correlated with several other characters of more or less
+importance. The value indeed of an aggregate of characters is very evident
+in natural history. Hence, as has often been remarked, a species may depart
+from its allies in several characters, both of high physiological
+importance and of almost universal prevalence, and yet leave us in no doubt
+where it should be ranked. Hence, also, it has been found, that a
+classification founded on any single character, however important that may
+be, has always failed; for no part of the organisation is universally
+constant. The importance of an aggregate of characters, even when none are
+important, alone explains, I think, that saying of Linnæus, that the
+characters do not give the genus, but the genus gives the characters; for
+this saying seems founded on an appreciation of many trifling points of
+resemblance, too slight to be defined. Certain plants, belonging to the
+Malpighiaceæ, bear perfect and degraded flowers; in the latter, as A. de
+Jussieu has remarked, "the greater number of the characters proper to the
+species, to the genus, to the family, to the class, disappear, and thus
+laugh at our classification." But when Aspicarpa produced in France, during
+several years, only degraded flowers, departing so wonderfully in a number
+of the most important points of structure from the proper type of the
+order, yet M. Richard sagaciously saw, as Jussieu observes, that this genus
+should still be retained amongst the Malpighiaceæ. This case seems to me
+well to illustrate the spirit with which our classifications are sometimes
+necessarily founded.
+
+Practically when naturalists are at work, they do {418} not trouble
+themselves about the physiological value of the characters which they use
+in defining a group, or in allocating any particular species. If they find
+a character nearly uniform, and common to a great number of forms, and not
+common to others, they use it as one of high value; if common to some
+lesser number, they use it as of subordinate value. This principle has been
+broadly confessed by some naturalists to be the true one; and by none more
+clearly than by that excellent botanist, Aug. St. Hilaire. If certain
+characters are always found correlated with others, though no apparent bond
+of connexion can be discovered between them, especial value is set on them.
+As in most groups of animals, important organs, such as those for
+propelling the blood, or for aërating it, or those for propagating the
+race, are found nearly uniform, they are considered as highly serviceable
+in classification; but in some groups of animals all these, the most
+important vital organs, are found to offer characters of quite subordinate
+value.
+
+We can see why characters derived from the embryo should be of equal
+importance with those derived from the adult, for our classifications of
+course include all ages of each species. But it is by no means obvious, on
+the ordinary view, why the structure of the embryo should be more important
+for this purpose than that of the adult, which alone plays its full part in
+the economy of nature. Yet it has been strongly urged by those great
+naturalists, Milne Edwards and Agassiz, that embryonic characters are the
+most important of any in the classification of animals; and this doctrine
+has very generally been admitted as true. The same fact holds good with
+flowering plants, of which the two main divisions have been founded on
+characters derived from the embryo,--on the number and position of the
+{419} embryonic leaves or cotyledons, and on the mode of development of the
+plumule and radicle. In our discussion on embryology, we shall see why such
+characters are so valuable, on the view of classification tacitly including
+the idea of descent.
+
+Our classifications are often plainly influenced by chains of affinities.
+Nothing can be easier than to define a number of characters common to all
+birds; but in the case of crustaceans, such definition has hitherto been
+found impossible. There are crustaceans at the opposite ends of the series,
+which have hardly a character in common; yet the species at both ends, from
+being plainly allied to others, and these to others, and so onwards, can be
+recognised as unequivocally belonging to this, and to no other class of the
+Articulata.
+
+Geographical distribution has often been used, though perhaps not quite
+logically, in classification, more especially in very large groups of
+closely allied forms. Temminck insists on the utility or even necessity of
+this practice in certain groups of birds; and it has been followed by
+several entomologists and botanists.
+
+Finally, with respect to the comparative value of the various groups of
+species, such as orders, sub-orders, families, sub-families, and genera,
+they seem to be, at least at present, almost arbitrary. Several of the best
+botanists, such as Mr. Bentham and others, have strongly insisted on their
+arbitrary value. Instances could be given amongst plants and insects, of a
+group of forms, first ranked by practised naturalists as only a genus, and
+then raised to the rank of a sub-family or family; and this has been done,
+not because further research has detected important structural differences,
+at first overlooked, but because numerous allied species, with slightly
+different grades of difference, have been subsequently discovered. {420}
+
+All the foregoing rules and aids and difficulties in classification are
+explained, if I do not greatly deceive myself, on the view that the natural
+system is founded on descent with modification; that the characters which
+naturalists consider as showing true affinity between any two or more
+species, are those which have been inherited from a common parent, and, in
+so far, all true classification is genealogical; that community of descent
+is the hidden bond which naturalists have been unconsciously seeking, and
+not some unknown plan of creation, or the enunciation of general
+propositions, and the mere putting together and separating objects more or
+less alike.
+
+But I must explain my meaning more fully. I believe that the _arrangement_
+of the groups within each class, in due subordination and relation to the
+other groups, must be strictly genealogical in order to be natural; but
+that the _amount_ of difference in the several branches or groups, though
+allied in the same degree in blood to their common progenitor, may differ
+greatly, being due to the different degrees of modification which they have
+undergone; and this is expressed by the forms being ranked under different
+genera, families, sections, or orders. The reader will best understand what
+is meant, if he will take the trouble of referring to the diagram in the
+fourth chapter. We will suppose the letters A to L to represent allied
+genera, which lived during the Silurian epoch, and these have descended
+from a species which existed at an unknown anterior period. Species of
+three of these genera (A, F, and I) have transmitted modified descendants
+to the present day, represented by the fifteen genera (a^{14} to z^{14}) on
+the uppermost horizontal line. Now all these modified descendants from a
+single species, are represented as related in blood or descent to the same
+{421} degree; they may metaphorically be called cousins to the same
+millionth degree; yet they differ widely and in different degrees from each
+other. The forms descended from A, now broken up into two or three
+families, constitute a distinct order from those descended from I, also
+broken up into two families. Nor can the existing species, descended from
+A, be ranked in the same genus with the parent A; or those from I, with the
+parent I. But the existing genus F^{14} may be supposed to have been but
+slightly modified; and it will then rank with the parent-genus F; just as
+some few still living organic beings belong to Silurian genera. So that the
+amount or value of the differences between organic beings all related to
+each other in the same degree in blood, has come to be widely different.
+Nevertheless their genealogical _arrangement_ remains strictly true, not
+only at the present time, but at each successive period of descent. All the
+modified descendants from A will have inherited something in common from
+their common parent, as will all the descendants from I; so will it be with
+each subordinate branch of descendants, at each successive period. If,
+however, we choose to suppose that any of the descendants of A or of I have
+been so much modified as to have more or less completely lost traces of
+their parentage, in this case, their places in a natural classification
+will have been more or less completely lost,--as sometimes seems to have
+occurred with existing organisms. All the descendants of the genus F, along
+its whole line of descent, are supposed to have been but little modified,
+and they yet form a single genus. But this genus, though much isolated,
+will still occupy its proper intermediate position; for F originally was
+intermediate in character between A and I, and the several genera descended
+from these two genera will {422} have inherited to a certain extent their
+characters. This natural arrangement is shown, as far as is possible on
+paper, in the diagram, but in much too simple a manner. If a branching
+diagram had not been used, and only the names of the groups had been
+written in a linear series, it would have been still less possible to have
+given a natural arrangement; and it is notoriously not possible to
+represent in a series, on a flat surface, the affinities which we discover
+in nature amongst the beings of the same group. Thus, on the view which I
+hold, the natural system is genealogical in its arrangement, like a
+pedigree; but the degrees of modification which the different groups have
+undergone, have to be expressed by ranking them under different so-called
+genera, sub-families, families, sections, orders, and classes.
+
+It may be worth while to illustrate this view of classification, by taking
+the case of languages. If we possessed a perfect pedigree of mankind, a
+genealogical arrangement of the races of man would afford the best
+classification of the various languages now spoken throughout the world;
+and if all extinct languages, and all intermediate and slowly changing
+dialects, had to be included, such an arrangement would, I think, be the
+only possible one. Yet it might be that some very ancient language had
+altered little, and had given rise to few new languages, whilst others
+(owing to the spreading and subsequent isolation and states of civilisation
+of the several races, descended from a common race) had altered much, and
+had given rise to many new languages and dialects. The various degrees of
+difference in the languages from the same stock, would have to be expressed
+by groups subordinate to groups; but the proper or even only possible
+arrangement would still be genealogical; and this would be strictly
+natural, as {423} it would connect together all languages, extinct and
+modern, by the closest affinities, and would give the filiation and origin
+of each tongue.
+
+In confirmation of this view, let us glance at the classification of
+varieties, which are believed or known to have descended from one species.
+These are grouped under species, with sub-varieties under varieties; and
+with our domestic productions, several other grades of difference are
+requisite, as we have seen with pigeons. The origin of the existence of
+groups subordinate to groups, is the same with varieties as with species,
+namely, closeness of descent with various degrees of modification. Nearly
+the same rules are followed in classifying varieties, as with species.
+Authors have insisted on the necessity of classing varieties on a natural
+instead of an artificial system; we are cautioned, for instance, not to
+class two varieties of the pine-apple together, merely because their fruit,
+though the most important part, happens to be nearly identical; no one puts
+the swedish and common turnips together, though the esculent and thickened
+stems are so similar. Whatever part is found to be most constant, is used
+in classing varieties: thus the great agriculturist Marshall says the horns
+are very useful for this purpose with cattle, because they are less
+variable than the shape or colour of the body, &c.; whereas with sheep the
+horns are much less serviceable, because less constant. In classing
+varieties, I apprehend if we had a real pedigree, a genealogical
+classification would be universally preferred; and it has been attempted by
+some authors. For we might feel sure, whether there had been more or less
+modification, the principle of inheritance would keep the forms together
+which were allied in the greatest number of points. In tumbler pigeons,
+though some sub-varieties differ from the others {424} in the important
+character of having a longer beak, yet all are kept together from having
+the common habit of tumbling; but the short-faced breed has nearly or quite
+lost this habit; nevertheless, without any reasoning or thinking on the
+subject, these tumblers are kept in the same group, because allied in blood
+and alike in some other respects. If it could be proved that the Hottentot
+had descended from the Negro, I think he would be classed under the Negro
+group, however much he might differ in colour and other important
+characters from negroes.
+
+With species in a state of nature, every naturalist has in fact brought
+descent into his classification; for he includes in his lowest grade, or
+that of a species, the two sexes; and how enormously these sometimes differ
+in the most important characters, is known to every naturalist: scarcely a
+single fact can be predicated in common of the males and hermaphrodites of
+certain cirripedes, when adult, and yet no one dreams of separating them.
+The naturalist includes as one species the several larval stages of the
+same individual, however much they may differ from each other and from the
+adult; as he likewise includes the so-called alternate generations of
+Steenstrup, which can only in a technical sense be considered as the same
+individual. He includes monsters; he includes varieties, not solely because
+they closely resemble the parent-form, but because they are descended from
+it. He who believes that the cowslip is descended from the primrose, or
+conversely, ranks them together as a single species, and gives a single
+definition. As soon as three Orchidean forms (Monochanthus, Myanthus, and
+Catasetum), which had previously been ranked as three distinct genera, were
+known to be sometimes produced on the same spike, they were immediately
+included as a single species. {425}
+
+As descent has universally been used in classing together the individuals
+of the same species, though the males and females and larvæ are sometimes
+extremely different; and as it has been used in classing varieties which
+have undergone a certain, and sometimes a considerable amount of
+modification, may not this same element of descent have been unconsciously
+used in grouping species under genera, and genera under higher groups,
+though in these cases the modification has been greater in degree, and has
+taken a longer time to complete? I believe it has thus been unconsciously
+used; and only thus can I understand the several rules and guides which
+have been followed by our best systematists. We have no written pedigrees;
+we have to make out community of descent by resemblances of any kind.
+Therefore we choose those characters which, as far as we can judge, are the
+least likely to have been modified in relation to the conditions of life to
+which each species has been recently exposed. Rudimentary structures on
+this view are as good as, or even sometimes better than, other parts of the
+organisation. We care not how trifling a character may be--let it be the
+mere inflection of the angle of the jaw, the manner in which an insect's
+wing is folded, whether the skin be covered by hair or feathers--if it
+prevail throughout many and different species, especially those having very
+different habits of life, it assumes high value; for we can account for its
+presence in so many forms with such different habits, only by its
+inheritance from a common parent. We may err in this respect in regard to
+single points of structure, but when several characters, let them be ever
+so trifling, occur together throughout a large group of beings having
+different habits, we may feel almost sure, on the theory of descent, that
+these characters have been inherited from a common ancestor. {426} And we
+know that such correlated or aggregated characters have especial value in
+classification.
+
+We can understand why a species or a group of species may depart, in
+several of its most important characteristics, from its allies, and yet be
+safely classed with them. This may be safely done, and is often done, as
+long as a sufficient number of characters, let them be ever so unimportant,
+betrays the hidden bond of community of descent. Let two forms have not a
+single character in common, yet if these extreme forms are connected
+together by a chain of intermediate groups, we may at once infer their
+community of descent, and we put them all into the same class. As we find
+organs of high physiological importance--those which serve to preserve life
+under the most diverse conditions of existence--are generally the most
+constant, we attach especial value to them; but if these same organs, in
+another group or section of a group, are found to differ much, we at once
+value them less in our classification. We shall hereafter, I think, clearly
+see why embryological characters are of such high classificatory
+importance. Geographical distribution may sometimes be brought usefully
+into play in classing large and widely-distributed genera, because all the
+species of the same genus, inhabiting any distinct and isolated region,
+have in all probability descended from the same parents.
+
+We can understand, on these views, the very important distinction between
+real affinities and analogical or adaptive resemblances. Lamarck first
+called attention to this distinction, and he has been ably followed by
+Macleay and others. The resemblance, in the shape of the body and in the
+fin-like anterior limbs, between the dugong, which is a pachydermatous
+animal, and the whale, and between both these mammals and fishes, is
+analogical. Amongst insects there are innumerable {427} instances: thus
+Linnæus, misled by external appearances, actually classed an homopterous
+insect as a moth. We see something of the same kind even in our domestic
+varieties, as in the thickened stems of the common and swedish turnip. The
+resemblance of the greyhound and racehorse is hardly more fanciful than the
+analogies which have been drawn by some authors between very distinct
+animals. On my view of characters being of real importance for
+classification, only in so far as they reveal descent, we can clearly
+understand why analogical or adaptive character, although of the utmost
+importance to the welfare of the being, are almost valueless to the
+systematist. For animals, belonging to two most distinct lines of descent,
+may readily become adapted to similar conditions, and thus assume a close
+external resemblance; but such resemblances will not reveal--will rather
+tend to conceal their blood-relationship to their proper lines of descent.
+We can also understand the apparent paradox, that the very same characters
+are analogical when one class or order is compared with another, but give
+true affinities when the members of the same class or order are compared
+one with another: thus the shape of the body and fin-like limbs are only
+analogical when whales are compared with fishes, being adaptations in both
+classes for swimming through the water; but the shape of the body and
+fin-like limbs serve as characters exhibiting true affinity between the
+several members of the whale family; for these cetaceans agree in so many
+characters, great and small, that we cannot doubt that they have inherited
+their general shape of body and structure of limbs from a common ancestor.
+So it is with fishes.
+
+As members of distinct classes have often been adapted by successive slight
+modifications to live under nearly similar circumstances,--to inhabit for
+instance {428} the three elements of land, air, and water,--we can perhaps
+understand how it is that a numerical parallelism has sometimes been
+observed between the sub-groups in distinct classes. A naturalist, struck
+by a parallelism of this nature in any one class, by arbitrarily raising or
+sinking the value of the groups in other classes (and all our experience
+shows that this valuation has hitherto been arbitrary), could easily extend
+the parallelism over a wide range; and thus the septenary, quinary,
+quaternary, and ternary classifications have probably arisen.
+
+As the modified descendants of dominant species, belonging to the larger
+genera, tend to inherit the advantages, which made the groups to which they
+belong large and their parents dominant, they are almost sure to spread
+widely, and to seize on more and more places in the economy of nature. The
+larger and more dominant groups thus tend to go on increasing in size; and
+they consequently supplant many smaller and feebler groups. Thus we can
+account for the fact that all organisms, recent and extinct, are included
+under a few great orders, under still fewer classes, and all in one great
+natural system. As showing how few the higher groups are in number, and how
+widely spread they are throughout the world, the fact is striking, that the
+discovery of Australia has not added a single insect belonging to a new
+class; and that in the vegetable kingdom, as I learn from Dr. Hooker, it
+has added only two or three orders of small size.
+
+In the chapter on geological succession I attempted to show, on the
+principle of each group having generally diverged much in character during
+the long-continued process of modification, how it is that the more ancient
+forms of life often present characters in some slight degree intermediate
+between existing groups. A few {429} old and intermediate parent-forms
+having occasionally transmitted to the present day descendants but little
+modified, will give to us our so-called osculant or aberrant groups. The
+more aberrant any form is, the greater must be the number of connecting
+forms which on my theory have been exterminated and utterly lost. And we
+have some evidence of aberrant forms having suffered severely from
+extinction, for they are generally represented by extremely few species;
+and such species as do occur are generally very distinct from each other,
+which again implies extinction. The genera Ornithorhynchus and Lepidosiren,
+for example, would not have been less aberrant had each been represented by
+a dozen species instead of by a single one; but such richness in species,
+as I find after some investigation, does not commonly fall to the lot of
+aberrant genera. We can, I think, account for this fact only by looking at
+aberrant forms as failing groups conquered by more successful competitors,
+with a few members preserved by some unusual coincidence of favourable
+circumstances.
+
+Mr. Waterhouse has remarked that, when a member belonging to one group of
+animals exhibits an affinity to a quite distinct group, this affinity in
+most cases is general and not special: thus, according to Mr. Waterhouse,
+of all Rodents, the bizcacha is most nearly related to Marsupials; but in
+the points in which it approaches this order, its relations are general,
+and not to any one marsupial species more than to another. As the points of
+affinity of the bizcacha to Marsupials are believed to be real and not
+merely adaptive, they are due on my theory to inheritance in common.
+Therefore we must suppose either that all Rodents, including the bizcacha,
+branched off from some very ancient Marsupial, which will have had a
+character in some degree intermediate with respect to all existing
+Marsupials; or {430} that both Rodents and Marsupials branched off from a
+common progenitor, and that both groups have since undergone much
+modification in divergent directions. On either view we may suppose that
+the bizcacha has retained, by inheritance, more of the character of its
+ancient progenitor than have other Rodents; and therefore it will not be
+specially related to any one existing Marsupial, but indirectly to all or
+nearly all Marsupials, from having partially retained the character of
+their common progenitor, or of an early member of the group. On the other
+hand, of all Marsupials, as Mr. Waterhouse has remarked, the phascolomys
+resembles most nearly, not any one species, but the general order of
+Rodents. In this case, however, it may be strongly suspected that the
+resemblance is only analogical, owing to the phascolomys having become
+adapted to habits like those of a Rodent. The elder De Candolle has made
+nearly similar observations on the general nature of the affinities of
+distinct orders of plants.
+
+On the principle of the multiplication and gradual divergence in character
+of the species descended from a common parent, together with their
+retention by inheritance of some characters in common, we can understand
+the excessively complex and radiating affinities by which all the members
+of the same family or higher group are connected together. For the common
+parent of a whole family of species, now broken up by extinction into
+distinct groups and sub-groups, will have transmitted some of its
+characters, modified in various ways and degrees, to all; and the several
+species will consequently be related to each other by circuitous lines of
+affinity of various lengths (as may be seen in the diagram so often
+referred to), mounting up through many predecessors. As it is difficult to
+show the blood-relationship between the numerous kindred {431} of any
+ancient and noble family, even by the aid of a genealogical tree, and
+almost impossible to do this without this aid, we can understand the
+extraordinary difficulty which naturalists have experienced in describing,
+without the aid of a diagram, the various affinities which they perceive
+between the many living and extinct members of the same great natural
+class.
+
+Extinction, as we have seen in the fourth chapter, has played an important
+part in defining and widening the intervals between the several groups in
+each class. We may thus account even for the distinctness of whole classes
+from each other--for instance, of birds from all other vertebrate
+animals--by the belief that many ancient forms of life have been utterly
+lost, through which the early progenitors of birds were formerly connected
+with the early progenitors of the other vertebrate classes. There has been
+less entire extinction of the forms of life which once connected fishes
+with batrachians. There has been still less in some other classes, as in
+that of the Crustacea, for here the most wonderfully diverse forms are
+still tied together by a long, but broken, chain of affinities. Extinction
+has only separated groups: it has by no means made them; for if every form
+which has ever lived on this earth were suddenly to reappear, though it
+would be quite impossible to give definitions by which each group could be
+distinguished from other groups, as all would blend together by steps as
+fine as those between the finest existing varieties, nevertheless a natural
+classification, or at least a natural arrangement, would be possible. We
+shall see this by turning to the diagram: the letters, A to L, may
+represent eleven Silurian genera, some of which have produced large groups
+of modified descendants. Every intermediate link between these eleven
+genera and their primordial parent, and every {432} intermediate link in
+each branch and sub-branch of their descendants, may be supposed to be
+still alive; and the links to be as fine as those between the finest
+varieties. In this case it would be quite impossible to give any definition
+by which the several members of the several groups could be distinguished
+from their more immediate parents; or these parents from their ancient and
+unknown progenitor. Yet the natural arrangement in the diagram would still
+hold good; and, on the principle of inheritance, all the forms descended
+from A, or from I, would have something in common. In a tree we can specify
+this or that branch, though at the actual fork the two unite and blend
+together. We could not, as I have said, define the several groups; but we
+could pick out types, or forms, representing most of the characters of each
+group, whether large or small, and thus give a general idea of the value of
+the differences between them. This is what we should be driven to, if we
+were ever to succeed in collecting all the forms in any class which have
+lived throughout all time and space. We shall certainly never succeed in
+making so perfect a collection: nevertheless, in certain classes, we are
+tending in this direction; and Milne Edwards has lately insisted, in an
+able paper, on the high importance of looking to types, whether or not we
+can separate and define the groups to which such types belong.
+
+Finally, we have seen that natural selection, which results from the
+struggle for existence, and which almost inevitably induces extinction and
+divergence of character in the many descendants from one dominant
+parent-species, explains that great and universal feature in the affinities
+of all organic beings, namely, their subordination in group under group. We
+use the element of descent in classing the individuals of both sexes and of
+all ages, although having few characters in common, {433} under one
+species; we use descent in classing acknowledged varieties, however
+different they may be from their parent; and I believe this element of
+descent is the hidden bond of connexion which naturalists have sought under
+the term of the Natural System. On this idea of the natural system being,
+in so far as it has been perfected, genealogical in its arrangement, with
+the grades of difference between the descendants from a common parent,
+expressed by the terms genera, families, orders, &c., we can understand the
+rules which we are compelled to follow in our classification. We can
+understand why we value certain resemblances far more than others; why we
+are permitted to use rudimentary and useless organs, or others of trifling
+physiological importance; why, in comparing one group with a distinct
+group, we summarily reject analogical or adaptive characters, and yet use
+these same characters within the limits of the same group. We can clearly
+see how it is that all living and extinct forms can be grouped together in
+one great system; and how the several members of each class are connected
+together by the most complex and radiating lines of affinities. We shall
+never, probably, disentangle the inextricable web of affinities between the
+members of any one class; but when we have a distinct object in view, and
+do not look to some unknown plan of creation, we may hope to make sure but
+slow progress.
+
+
+
+_Morphology._--We have seen that the members of the same class,
+independently of their habits of life, resemble each other in the general
+plan of their organisation. This resemblance is often expressed by the term
+"unity of type;" or by saying that the several parts and organs in the
+different species of the class are homologous. The whole subject is
+included under {434} the general name of Morphology. This is the most
+interesting department of natural history, and may be said to be its very
+soul. What can be more curious than that the hand of a man, formed for
+grasping, that of a mole for digging, the leg of the horse, the paddle of
+the porpoise, and the wing of the bat, should all be constructed on the
+same pattern, and should include similar bones, in the same relative
+positions? Geoffroy St. Hilaire has insisted strongly on the high
+importance of relative connexion in homologous organs: the parts may change
+to almost any extent in form and size, and yet they always remain connected
+together in the same order. We never find, for instance, the bones of the
+arm and forearm, or of the thigh and leg, transposed. Hence the same names
+can be given to the homologous bones in widely different animals. We see
+the same great law in the construction of the mouths of insects: what can
+be more different than the immensely long spiral proboscis of a
+sphinx-moth, the curious folded one of a bee or bug, and the great jaws of
+a beetle?--yet all these organs, serving for such different purposes, are
+formed by infinitely numerous modifications of an upper lip, mandibles, and
+two pairs of maxillæ. Analogous laws govern the construction of the mouths
+and limbs of crustaceans. So it is with the flowers of plants.
+
+Nothing can be more hopeless than to attempt to explain this similarity of
+pattern in members of the same class, by utility or by the doctrine of
+final causes. The hopelessness of the attempt has been expressly admitted
+by Owen in his most interesting work on the 'Nature of Limbs.' On the
+ordinary view of the independent creation of each being, we can only say
+that so it is;--that it has so pleased the Creator to construct each animal
+and plant.
+
+The explanation is manifest on the theory of the {435} natural selection of
+successive slight modifications,--each modification being profitable in
+some way to the modified form, but often affecting by correlation of growth
+other parts of the organisation. In changes of this nature, there will be
+little or no tendency to modify the original pattern, or to transpose
+parts. The bones of a limb might be shortened and widened to any extent,
+and become gradually enveloped in thick membrane, so as to serve as a fin;
+or a webbed foot might have all its bones, or certain bones, lengthened to
+any extent, and the membrane connecting them increased to any extent, so as
+to serve as a wing: yet in all this great amount of modification there will
+be no tendency to alter the framework of bones or the relative connexion of
+the several parts. If we suppose that the ancient progenitor, the archetype
+as it may be called, of all mammals, had its limbs constructed on the
+existing general pattern, for whatever purpose they served, we can at once
+perceive the plain signification of the homologous construction of the
+limbs throughout the whole class. So with the mouths of insects, we have
+only to suppose that their common progenitor had an upper lip, mandibles,
+and two pair of maxillæ, these parts being perhaps very simple in form; and
+then natural selection, acting on some originally created form, will
+account for the infinite diversity in structure and function of the mouths
+of insects. Nevertheless, it is conceivable that the general pattern of an
+organ might become so much obscured as to be finally lost, by the atrophy
+and ultimately by the complete abortion of certain parts, by the soldering
+together of other parts, and by the doubling or multiplication of
+others,--variations which we know to be within the limits of possibility.
+In the paddles of the extinct gigantic sea-lizards, and in the mouths of
+certain suctorial crustaceans, the {436} general pattern seems to have been
+thus to a certain extent obscured.
+
+There is another and equally curious branch of the present subject; namely,
+the comparison not of the same part in different members of a class, but of
+the different parts or organs in the same individual. Most physiologists
+believe that the bones of the skull are homologous with--that is correspond
+in number and in relative connexion with--the elemental parts of a certain
+number of vertebræ. The anterior and posterior limbs in each member of the
+vertebrate and articulate classes are plainly homologous. We see the same
+law in comparing the wonderfully complex jaws and legs in crustaceans. It
+is familiar to almost every one, that in a flower the relative position of
+the sepals, petals, stamens, and pistils, as well as their intimate
+structure, are intelligible on the view that they consist of metamorphosed
+leaves, arranged in a spire. In monstrous plants, we often get direct
+evidence of the possibility of one organ being transformed into another;
+and we can actually see in embryonic crustaceans and in many other animals,
+and in flowers, that organs, which when mature become extremely different,
+are at an early stage of growth exactly alike.
+
+How inexplicable are these facts on the ordinary view of creation! Why
+should the brain be enclosed in a box composed of such numerous and such
+extraordinary shaped pieces of bone? As Owen has remarked, the benefit
+derived from the yielding of the separate pieces in the act of parturition
+of mammals, will by no means explain the same construction in the skulls of
+birds. Why should similar bones have been created in the formation of the
+wing and leg of a bat, used as they are for such totally different
+purposes? Why should one crustacean, which has an extremely complex {437}
+mouth formed of many parts, consequently always have fewer legs; or
+conversely, those with many legs have simpler mouths? Why should the
+sepals, petals, stamens, and pistils in any individual flower, though
+fitted for such widely different purposes, be all constructed on the same
+pattern?
+
+On the theory of natural selection, we can satisfactorily answer these
+questions. In the vertebrata, we see a series of internal vertebræ bearing
+certain processes and appendages; in the articulata, we see the body
+divided into a series of segments, bearing external appendages; and in
+flowering plants, we see a series of successive spiral whorls of leaves. An
+indefinite repetition of the same part or organ is the common
+characteristic (as Owen has observed) of all low or little-modified forms;
+therefore we may readily believe that the unknown progenitor of the
+vertebrata possessed many vertebræ; the unknown progenitor of the
+articulata, many segments; and the unknown progenitor of flowering plants,
+many spiral whorls of leaves. We have formerly seen that parts many times
+repeated are eminently liable to vary in number and structure; consequently
+it is quite probable that natural selection, during a long-continued course
+of modification, should have seized on a certain number of the primordially
+similar elements, many times repeated, and have adapted them to the most
+diverse purposes. And as the whole amount of modification will have been
+effected by slight successive steps, we need not wonder at discovering in
+such parts or organs, a certain degree of fundamental resemblance, retained
+by the strong principle of inheritance.
+
+In the great class of molluscs, though we can homologise the parts of one
+species with those of other and distinct species, we can indicate but few
+serial homologies; that is, we are seldom enabled to say that one {438}
+part or organ is homologous with another in the same individual. And we can
+understand this fact; for in molluscs, even in the lowest members of the
+class, we do not find nearly so much indefinite repetition of any one part,
+as we find in the other great classes of the animal and vegetable kingdoms.
+
+Naturalists frequently speak of the skull as formed of metamorphosed
+vertebræ: the jaws of crabs as metamorphosed legs; the stamens and pistils
+of flowers as metamorphosed leaves; but it would in these cases probably be
+more correct, as Professor Huxley has remarked, to speak of both skull and
+vertebræ, both jaws and legs, &c.,--as having been metamorphosed, not one
+from the other, but from some common element. Naturalists, however, use
+such language only in a metaphorical sense: they are far from meaning that
+during a long course of descent, primordial organs of any kind--vertebræ in
+the one case and legs in the other--have actually been modified into skulls
+or jaws. Yet so strong is the appearance of a modification of this nature
+having occurred, that naturalists can hardly avoid employing language
+having this plain signification. On my view these terms may be used
+literally; and the wonderful fact of the jaws, for instance, of a crab
+retaining numerous characters, which they would probably have retained
+through inheritance, if they had really been metamorphosed during a long
+course of descent from true legs, or from some simple appendage, is
+explained.
+
+
+
+_Embryology._--It has already been casually remarked that certain organs in
+the individual, which when mature become widely different and serve for
+different purposes, are in the embryo exactly alike. The embryos, also, of
+distinct animals within the same class are often strikingly similar: a
+better proof of this cannot be given, than a {439} circumstance mentioned
+by Agassiz, namely, that having forgotten to ticket the embryo of some
+vertebrate animal, he cannot now tell whether it be that of a mammal, bird,
+or reptile. The vermiform larvæ of moths, flies, beetles, &c., resemble
+each other much more closely than do the mature insects; but in the case of
+larvæ, the embryos are active, and have been adapted for special lines of
+life. A trace of the law of embryonic resemblance, sometimes lasts till a
+rather late age: thus birds of the same genus, and of closely allied
+genera, often resemble each other in their first and second plumage; as we
+see in the spotted feathers in the thrush group. In the cat tribe, most of
+the species are striped or spotted in lines; and stripes can be plainly
+distinguished in the whelp of the lion. We occasionally though rarely see
+something of this kind in plants: thus the embryonic leaves of the ulex or
+furze, and the first leaves of the phyllodineous acaceas, are pinnate or
+divided like the ordinary leaves of the leguminosæ.
+
+The points of structure, in which the embryos of widely different animals
+of the same class resemble each other, often have no direct relation to
+their conditions of existence. We cannot, for instance, suppose that in the
+embryos of the vertebrata the peculiar loop-like course of the arteries
+near the branchial slits are related to similar conditions,--in the young
+mammal which is nourished in the womb of its mother, in the egg of the bird
+which is hatched in a nest, and in the spawn of a frog under water. We have
+no more reason to believe in such a relation, than we have to believe that
+the same bones in the hand of a man, wing of a bat, and fin of a porpoise,
+are related to similar conditions of life. No one will suppose that the
+stripes on the whelp of a lion, or the spots on the young blackbird, {440}
+are of any use to these animals, or are related to the conditions to which
+they are exposed.
+
+The case, however, is different when an animal during any part of its
+embryonic career is active, and has to provide for itself. The period of
+activity may come on earlier or later in life; but whenever it comes on,
+the adaptation of the larva to its conditions of life is just as perfect
+and as beautiful as in the adult animal. From such special adaptations, the
+similarity of the larvæ or active embryos of allied animals is sometimes
+much obscured; and cases could be given of the larvæ of two species, or of
+two groups of species, differing quite as much, or even more, from each
+other than do their adult parents. In most cases, however, the larvæ,
+though active, still obey, more or less closely, the law of common
+embryonic resemblance. Cirripedes afford a good instance of this: even the
+illustrious Cuvier did not perceive that a barnacle was, as it certainly
+is, a crustacean; but a glance at the larva shows this to be the case in an
+unmistakeable manner. So again the two main divisions of cirripedes, the
+pedunculated and sessile, which differ widely in external appearance, have
+larvæ in all their stages barely distinguishable.
+
+The embryo in the course of development generally rises in organisation: I
+use this expression, though I am aware that it is hardly possible to define
+clearly what is meant by the organisation being higher or lower. But no one
+probably will dispute that the butterfly is higher than the caterpillar. In
+some cases, however, the mature animal is generally considered as lower in
+the scale than the larva, as with certain parasitic crustaceans. To refer
+once again to cirripedes: the larvæ in the first stage have three pairs of
+legs, a very simple single eye, and a probosciformed mouth, with which they
+feed largely, for they increase much in {441} size. In the second stage,
+answering to the chrysalis stage of butterflies, they have six pairs of
+beautifully constructed natatory legs, a pair of magnificent compound eyes,
+and extremely complex antennæ; but they have a closed and imperfect mouth,
+and cannot feed: their function at this stage is, to search by their
+well-developed organs of sense, and to reach by their active powers of
+swimming, a proper place on which to become attached and to undergo their
+final metamorphosis. When this is completed they are fixed for life: their
+legs are now converted into prehensile organs; they again obtain a
+well-constructed mouth; but they have no antennæ, and their two eyes are
+now reconverted into a minute, single, and very simple eye-spot. In this
+last and complete state, cirripedes may be considered as either more highly
+or more lowly organised than they were in the larval condition. But in some
+genera the larvæ become developed either into hermaphrodites having the
+ordinary structure, or into what I have called complemental males: and in
+the latter, the development has assuredly been retrograde; for the male is
+a mere sack, which lives for a short time, and is destitute of mouth,
+stomach, or other organ of importance, excepting for reproduction.
+
+We are so much accustomed to see differences in structure between the
+embryo and the adult, and likewise a close similarity in the embryos of
+widely different animals within the same class, that we might be led to
+look at these facts as necessarily contingent in some manner on growth. But
+there is no obvious reason why, for instance, the wing of a bat, or the fin
+of a porpoise, should not have been sketched out with all the parts in
+proper proportion, as soon as any structure became visible in the embryo.
+And in some whole groups of animals and in certain members of other groups,
+the embryo does not at any period differ widely from the {442} adult: thus
+Owen has remarked in regard to cuttle-fish, "there is no metamorphosis; the
+cephalopodic character is manifested long before the parts of the embryo
+are completed;" and again in spiders, "there is nothing worthy to be called
+a metamorphosis." The larvæ of insects, whether adapted to the most diverse
+and active habits, or quite inactive, being fed by their parents or placed
+in the midst of proper nutriment, yet nearly all pass through a similar
+worm-like stage of development; but in some few cases, as in that of Aphis,
+if we look to the admirable drawings by Professor Huxley of the development
+of this insect, we see no trace of the vermiform stage.
+
+How, then, can we explain these several facts in embryology,--namely the
+very general, but not universal difference in structure between the embryo
+and the adult;--of parts in the same individual embryo, which ultimately
+become very unlike and serve for diverse purposes, being at this early
+period of growth alike;--of embryos of different species within the same
+class, generally, but not universally, resembling each other;--of the
+structure of the embryo not being closely related to its conditions of
+existence, except when the embryo becomes at any period of life active and
+has to provide for itself;--of the embryo apparently having sometimes a
+higher organisation than the mature animal, into which it is developed? I
+believe that all these facts can be explained, as follows, on the view of
+descent with modification.
+
+It is commonly assumed, perhaps from monstrosities often affecting the
+embryos at a very early period, that slight variations necessarily appear
+at an equally early period. But we have little evidence on this
+head--indeed the evidence rather points the other way; for it is notorious
+that breeders of cattle, horses, and various {443} fancy animals, cannot
+positively tell, until some time after the animal has been born, what its
+merits or form will ultimately turn out. We see this plainly in our own
+children; we cannot always tell whether the child will be tall or short, or
+what its precise features will be. The question is not, at what period of
+life any variation has been caused, but at what period it is fully
+displayed. The cause may have acted, and I believe generally has acted,
+even before the embryo is formed; and the variation may be due to the male
+and female sexual elements having been affected by the conditions to which
+either parent, or their ancestors, have been exposed. Nevertheless an
+effect thus caused at a very early period, even before the formation of the
+embryo, may appear late in life; as when an hereditary disease, which
+appears in old age alone, has been communicated to the offspring from the
+reproductive element of one parent. Or again, as when the horns of
+cross-bred cattle have been affected by the shape of the horns of either
+parent. For the welfare of a very young animal, as long as it remains in
+its mother's womb, or in the egg, or as long as it is nourished and
+protected by its parent, it must be quite unimportant whether most of its
+characters are fully acquired a little earlier or later in life. It would
+not signify, for instance, to a bird which obtained its food best by having
+a long beak, whether or not it assumed a beak of this particular length, as
+long as it was fed by its parents. Hence, I conclude, that it is quite
+possible, that each of the many successive modifications, by which each
+species has acquired its present structure, may have supervened at a not
+very early period of life; and some direct evidence from our domestic
+animals supports this view. But in other cases it is quite possible that
+each successive modification, or {444} most of them, may have appeared at
+an extremely early period.
+
+I have stated in the first chapter, that there is some evidence to render
+it probable, that at whatever age any variation first appears in the
+parent, it tends to reappear at a corresponding age in the offspring.
+Certain variations can only appear at corresponding ages, for instance,
+peculiarities in the caterpillar, cocoon, or imago states of the silk-moth;
+or, again, in the horns of almost full-grown cattle. But further than this,
+variations which, for all that we can see, might have appeared earlier or
+later in life, tend to appear at a corresponding age in the offspring and
+parent. I am far from meaning that this is invariably the case; and I could
+give a good many cases of variations (taking the word in the largest sense)
+which have supervened at an earlier age in the child than in the parent.
+
+These two principles, if their truth be admitted, will, I believe, explain
+all the above specified leading facts in embryology. But first let us look
+at a few analogous cases in domestic varieties. Some authors who have
+written on Dogs, maintain that the greyhound and bulldog, though appearing
+so different, are really varieties most closely allied, and have probably
+descended from the same wild stock; hence I was curious to see how far
+their puppies differed from each other: I was told by breeders that they
+differed just as much as their parents, and this, judging by the eye,
+seemed almost to be the case; but on actually measuring the old dogs and
+their six-days old puppies, I found that the puppies had not nearly
+acquired their full amount of proportional difference. So, again, I was
+told that the foals of cart and race-horses differed as much as the
+full-grown animals; and this surprised me greatly, as I think it probable
+that the difference between these two breeds has been wholly {445} caused
+by selection under domestication; but having had careful measurements made
+of the dam and of a three-days old colt of a race and heavy cart-horse, I
+find that the colts have by no means acquired their full amount of
+proportional difference.
+
+As the evidence appears to me conclusive, that the several domestic breeds
+of Pigeon have descended from one wild species, I compared young pigeons of
+various breeds, within twelve hours after being hatched; I carefully
+measured the proportions (but will not here give details) of the beak,
+width of mouth, length of nostril and of eyelid, size of feet and length of
+leg, in the wild stock, in pouters, fantails, runts, barbs, dragons,
+carriers, and tumblers. Now some of these birds, when mature, differ so
+extraordinarily in length and form of beak, that they would, I cannot
+doubt, be ranked in distinct genera, had they been natural productions. But
+when the nestling birds of these several breeds were placed in a row,
+though most of them could be distinguished from each other, yet their
+proportional differences in the above specified several points were
+incomparably less than in the full-grown birds. Some characteristic points
+of difference--for instance, that of the width of mouth--could hardly be
+detected in the young. But there was one remarkable exception to this rule,
+for the young of the short-faced tumbler differed from the young of the
+wild rock-pigeon and of the other breeds, in all its proportions, almost
+exactly as much as in the adult state.
+
+The two principles above given seem to me to explain these facts in regard
+to the later embryonic stages of our domestic varieties. Fanciers select
+their horses, dogs, and pigeons, for breeding, when they are nearly grown
+up: they are indifferent whether the desired qualities and structures have
+been acquired earlier or {446} later in life, if the full-grown animal
+possesses them. And the cases just given, more especially that of pigeons,
+seem to show that the characteristic differences which give value to each
+breed, and which have been accumulated by man's selection, have not
+generally first appeared at an early period of life, and have been
+inherited by the offspring at a corresponding not early period. But the
+case of the short-faced tumbler, which when twelve hours old had acquired
+its proper proportions, proves that this is not the universal rule; for
+here the characteristic differences must either have appeared at an earlier
+period than usual, or, if not so, the differences must have been inherited,
+not at the corresponding, but at an earlier age.
+
+Now let us apply these facts and the above two principles--which latter,
+though not proved true, can be shown to be in some degree probable--to
+species in a state of nature. Let us take a genus of birds, descended on my
+theory from some one parent-species, and of which the several new species
+have become modified through natural selection in accordance with their
+diverse habits. Then, from the many slight successive steps of variation
+having supervened at a rather late age, and having been inherited at a
+corresponding age, the young of the new species of our supposed genus will
+manifestly tend to resemble each other much more closely than do the
+adults, just as we have seen in the case of pigeons. We may extend this
+view to whole families or even classes. The fore-limbs, for instance, which
+served as legs in the parent-species, may have become, by a long course of
+modification, adapted in one descendant to act as hands, in another as
+paddles, in another as wings; and on the above two principles--namely of
+each successive modification supervening at a rather late age, and being
+inherited at a {447} corresponding late age--the fore-limbs in the embryos
+of the several descendants of the parent-species will still resemble each
+other closely, for they will not have been modified. But in each of our new
+species, the embryonic fore-limbs will differ greatly from the fore-limbs
+in the mature animal; the limbs in the latter having undergone much
+modification at a rather late period of life, and having thus been
+converted into hands, or paddles, or wings. Whatever influence
+long-continued exercise or use on the one hand, and disuse on the other,
+may have in modifying an organ, such influence will mainly affect the
+mature animal, which has come to its full powers of activity and has to
+gain its own living; and the effects thus produced will be inherited at a
+corresponding mature age. Whereas the young will remain unmodified, or be
+modified in a lesser degree, by the effects of use and disuse.
+
+In certain cases the successive steps of variation might supervene, from
+causes of which we are wholly ignorant, at a very early period of life, or
+each step might be inherited at an earlier period than that at which it
+first appeared. In either case (as with the short-faced tumbler) the young
+or embryo would closely resemble the mature parent-form. We have seen that
+this is the rule of development in certain whole groups of animals, as with
+cuttle-fish and spiders, and with a few members of the great class of
+insects, as with Aphis. With respect to the final cause of the young in
+these cases not undergoing any metamorphosis, or closely resembling their
+parents from their earliest age, we can see that this would result from the
+two following contingencies: firstly, from the young, during a course of
+modification carried on for many generations, having to provide for their
+own wants at a very early stage {448} of development, and secondly, from
+their following exactly the same habits of life with their parents; for in
+this case, it would be indispensable for the existence of the species, that
+the child should be modified at a very early age in the same manner with
+its parents, in accordance with their similar habits. Some further
+explanation, however, of the embryo not undergoing any metamorphosis is
+perhaps requisite. If, on the other hand, it profited the young to follow
+habits of life in any degree different from those of their parent, and
+consequently to be constructed in a slightly different manner, then, on the
+principle of inheritance at corresponding ages, the active young or larvæ
+might easily be rendered by natural selection different to any conceivable
+extent from their parents. Such differences might, also, become correlated
+with successive stages of development; so that the larvæ, in the first
+stage, might differ greatly from the larvæ in the second stage, as we have
+seen to be the case with cirripedes. The adult might become fitted for
+sites or habits, in which organs of locomotion or of the senses, &c., would
+be useless; and in this case the final metamorphosis would be said to be
+retrograde.
+
+As all the organic beings, extinct and recent, which have ever lived on
+this earth have to be classed together, and as all have been connected by
+the finest gradations, the best, or indeed, if our collections were nearly
+perfect, the only possible arrangement, would be genealogical. Descent
+being on my view the hidden bond of connexion which naturalists have been
+seeking under the term of the natural system. On this view we can
+understand how it is that, in the eyes of most naturalists, the structure
+of the embryo is even more important for classification than that of the
+adult. For the embryo is the animal in its less modified state; {449} and
+in so far it reveals the structure of its progenitor. In two groups of
+animals, however much they may at present differ from each other in
+structure and habits, if they pass through the same or similar embryonic
+stages, we may feel assured that they have both descended from the same or
+nearly similar parents, and are therefore in that degree closely related.
+Thus, community in embryonic structure reveals community of descent. It
+will reveal this community of descent, however much the structure of the
+adult may have been modified and obscured; we have seen, for instance, that
+cirripedes can at once be recognised by their larvæ as belonging to the
+great class of crustaceans. As the embryonic state of each species and
+group of species partially shows us the structure of their less modified
+ancient progenitors, we can clearly see why ancient and extinct forms of
+life should resemble the embryos of their descendants,--our existing
+species. Agassiz believes this to be a law of nature; but I am bound to
+confess that I only hope to see the law hereafter proved true. It can be
+proved true in those cases alone in which the ancient state, now supposed
+to be represented in existing embryos, has not been obliterated, either by
+the successive variations in a long course of modification having
+supervened at a very early age, or by the variations having been inherited
+at an earlier period than that at which they first appeared. It should also
+be borne in mind, that the supposed law of resemblance of ancient forms of
+life to the embryonic stages of recent forms, may be true, but yet, owing
+to the geological record not extending far enough back in time, may remain
+for a long period, or for ever, incapable of demonstration.
+
+Thus, as it seems to me, the leading facts in embryology, which are second
+in importance to none in natural history, are explained on the principle of
+slight {450} modifications not appearing, in the many descendants from some
+one ancient progenitor, at a very early period in the life of each, though
+perhaps caused at the earliest, and being inherited at a corresponding not
+early period. Embryology rises greatly in interest, when we thus look at
+the embryo as a picture, more or less obscured, of the common parent-form
+of each great class of animals.
+
+
+
+_Rudimentary, atrophied, or aborted Organs._--Organs or parts in this
+strange condition, bearing the stamp of inutility, are extremely common
+throughout nature. For instance, rudimentary mammæ are very general in the
+males of mammals: I presume that the "bastard-wing" in birds may be safely
+considered as a digit in a rudimentary state: in very many snakes one lobe
+of the lungs is rudimentary; in other snakes there are rudiments of the
+pelvis and hind limbs. Some of the cases of rudimentary organs are
+extremely curious; for instance, the presence of teeth in foetal whales,
+which when grown up have not a tooth in their heads; and the presence of
+teeth, which never cut through the gums, in the upper jaws of our unborn
+calves. It has even been stated on good authority that rudiments of teeth
+can be detected in the beaks of certain embryonic birds. Nothing can be
+plainer than that wings are formed for flight, yet in how many insects do
+we see wings so reduced in size as to be utterly incapable of flight, and
+not rarely lying under wing-cases, firmly soldered together!
+
+The meaning of rudimentary organs is often quite unmistakeable: for
+instance there are beetles of the same genus (and even of the same species)
+resembling each other most closely in all respects, one of which will have
+full-sized wings, and another mere rudiments of membrane; and here it is
+impossible to doubt, that the {451} rudiments represent wings. Rudimentary
+organs sometimes retain their potentiality, and are merely not developed:
+this seems to be the case with the mammæ of male mammals, for many
+instances are on record of these organs having become well developed in
+full-grown males, and having secreted milk. So again there are normally
+four developed and two rudimentary teats in the udders of the genus Bos,
+but in our domestic cows the two sometimes become developed and give milk.
+In plants of the same species the petals sometimes occur as mere rudiments,
+and sometimes in a well-developed state. In plants with separated sexes,
+the male flowers often have a rudiment of a pistil; and Kölreuter found
+that by crossing such male plants with an hermaphrodite species, the
+rudiment of the pistil in the hybrid offspring was much increased in size;
+and this shows that the rudiment and the perfect pistil are essentially
+alike in nature.
+
+An organ serving for two purposes, may become rudimentary or utterly
+aborted for one, even the more important purpose; and remain perfectly
+efficient for the other. Thus in plants, the office of the pistil is to
+allow the pollen-tubes to reach the ovules protected in the ovarium at its
+base. The pistil consists of a stigma supported on the style; but in some
+Compositæ, the male florets, which of course cannot be fecundated, have a
+pistil, which is in a rudimentary state, for it is not crowned with a
+stigma; but the style remains well developed, and is clothed with hairs as
+in other compositæ, for the purpose of brushing the pollen out of the
+surrounding anthers. Again, an organ may become rudimentary for its proper
+purpose, and be used for a distinct object: in certain fish the
+swim-bladder seems to be nearly rudimentary for its proper function of
+giving buoyancy, but has become converted into a {452} nascent breathing
+organ or lung. Other similar instances could be given.
+
+Organs, however little developed, if of use, should not be called
+rudimentary; they cannot properly be said to be in an atrophied condition;
+they may be called nascent, and may hereafter be developed to any extent by
+natural selection. Rudimentary organs, on the other hand, are essentially
+useless, as teeth which never cut through the gums; in a still less
+developed condition, they would be of still less use. They cannot,
+therefore, under their present condition, have been formed by natural
+selection, which acts solely by the preservation of useful modifications;
+they have been retained, as we shall see, by inheritance, and relate to a
+former condition of their possessor. It is difficult to know what are
+nascent organs; looking to the future, we cannot of course tell how any
+part will be developed, and whether it is now nascent; looking to the past,
+creatures with an organ in a nascent condition will generally have been
+supplanted and exterminated by their successors with the organ in a more
+perfect and developed condition. The wing of the penguin is of high
+service, and acts as a fin; it may, therefore, represent the nascent state
+of the wings of birds; not that I believe this to be the case, it is more
+probably a reduced organ, modified for a new function: the wing of the
+Apteryx is useless, and is truly rudimentary. The mammary glands of the
+Ornithorhynchus may, perhaps, be considered, in comparison with the udder
+of a cow, as in a nascent state. The ovigerous frena of certain cirripedes,
+which are only slightly developed and which have ceased to give attachment
+to the ova, are nascent branchiæ.
+
+Rudimentary organs in the individuals of the same species are very liable
+to vary in degree of development {453} and in other respects. Moreover, in
+closely allied species, the degree to which the same organ has been
+rendered rudimentary occasionally differs much. This latter fact is well
+exemplified in the state of the wings of the female moths in certain
+groups. Rudimentary organs may be utterly aborted; and this implies, that
+we find in an animal or plant no trace of an organ, which analogy would
+lead us to expect to find, and which is occasionally found in monstrous
+individuals of the species. Thus in the snapdragon (antirrhinum) we
+generally do not find a rudiment of a fifth stamen; but this may sometimes
+be seen. In tracing the homologies of the same part in different members of
+a class, nothing is more common, or more necessary, than the use and
+discovery of rudiments. This is well shown in the drawings given by Owen of
+the bones of the leg of the horse, ox, and rhinoceros.
+
+It is an important fact that rudimentary organs, such as teeth in the upper
+jaws of whales and ruminants, can often be detected in the embryo, but
+afterwards wholly disappear. It is also, I believe, a universal rule, that
+a rudimentary part or organ is of greater size relatively to the adjoining
+parts in the embryo, than in the adult; so that the organ at this early age
+is less rudimentary, or even cannot be said to be in any degree
+rudimentary. Hence, also, a rudimentary organ in the adult is often said to
+have retained its embryonic condition.
+
+I have now given the leading facts with respect to rudimentary organs. In
+reflecting on them, every one must be struck with astonishment: for the
+same reasoning power which tells us plainly that most parts and organs are
+exquisitely adapted for certain purposes, tells us with equal plainness
+that these rudimentary or atrophied organs, are imperfect and useless. In
+works {454} on natural history rudimentary organs are generally said to
+have been created "for the sake of symmetry," or in order "to complete the
+scheme of nature;" but this seems to me no explanation, merely a
+re-statement of the fact. Would it be thought sufficient to say that
+because planets revolve in elliptic courses round the sun, satellites
+follow the same course round the planets, for the sake of symmetry, and to
+complete the scheme of nature? An eminent physiologist accounts for the
+presence of rudimentary organs, by supposing that they serve to excrete
+matter in excess, or injurious to the system; but can we suppose that the
+minute papilla, which often represents the pistil in male flowers, and
+which is formed merely of cellular tissue, can thus act? Can we suppose
+that the formation of rudimentary teeth, which are subsequently absorbed,
+can be of any service to the rapidly growing embryonic calf by the
+excretion of precious phosphate of lime? When a man's fingers have been
+amputated, imperfect nails sometimes appear on the stumps: I could as soon
+believe that these vestiges of nails have appeared, not from unknown laws
+of growth, but in order to excrete horny matter, as that the rudimentary
+nails on the fin of the manatee were formed for this purpose.
+
+On my view of descent with modification, the origin of rudimentary organs
+is simple. We have plenty of cases of rudimentary organs in our domestic
+productions,--as the stump of a tail in tailless breeds,--the vestige of an
+ear in earless breeds,--the reappearance of minute dangling horns in
+hornless breeds of cattle, more especially, according to Youatt, in young
+animals,--and the state of the whole flower in the cauliflower. We often
+see rudiments of various parts in monsters. But I doubt whether any of
+these cases throw light on the origin of rudimentary organs in a state of
+nature, {455} further than by showing that rudiments can be produced; for I
+doubt whether species under nature ever undergo abrupt changes. I believe
+that disuse has been the main agency; that it has led in successive
+generations to the gradual reduction of various organs, until they have
+become rudimentary,--as in the case of the eyes of animals inhabiting dark
+caverns, and of the wings of birds inhabiting oceanic islands, which have
+seldom been forced to take flight, and have ultimately lost the power of
+flying. Again, an organ useful under certain conditions, might become
+injurious under others, as with the wings of beetles living on small and
+exposed islands; and in this case natural selection would continue slowly
+to reduce the organ, until it was rendered harmless and rudimentary.
+
+Any change in function, which can be effected by insensibly small steps, is
+within the power of natural selection; so that an organ rendered, during
+changed habits of life, useless or injurious for one purpose, might be
+modified and used for another purpose. Or an organ might be retained for
+one alone of its former functions. An organ, when rendered useless, may
+well be variable, for its variations cannot be checked by natural
+selection. At whatever period of life disuse or selection reduces an organ,
+and this will generally be when the being has come to maturity and to its
+full powers of action, the principle of inheritance at corresponding ages
+will reproduce the organ in its reduced state at the same age, and
+consequently will seldom affect or reduce it in the embryo. Thus we can
+understand the greater relative size of rudimentary organs in the embryo,
+and their lesser relative size in the adult. But if each step of the
+process of reduction were to be inherited, not at the corresponding age,
+but at an extremely early period of life (as we have good {456} reason to
+believe to be possible), the rudimentary part would tend to be wholly lost,
+and we should have a case of complete abortion. The principle, also, of
+economy, explained in a former chapter, by which the materials forming any
+part or structure, if not useful to the possessor, will be saved as far as
+is possible, will probably often come into play; and this will tend to
+cause the entire obliteration of a rudimentary organ.
+
+As the presence of rudimentary organs is thus due to the tendency in every
+part of the organisation, which has long existed, to be inherited--we can
+understand, on the genealogical view of classification, how it is that
+systematists have found rudimentary parts as useful as, or even sometimes
+more useful than, parts of high physiological importance. Rudimentary
+organs may be compared with the letters in a word, still retained in the
+spelling, but become useless in the pronunciation, but which serve as a
+clue in seeking for its derivation. On the view of descent with
+modification, we may conclude that the existence of organs in a
+rudimentary, imperfect, and useless condition, or quite aborted, far from
+presenting a strange difficulty, as they assuredly do on the ordinary
+doctrine of creation, might even have been anticipated, and can be
+accounted for by the laws of inheritance.
+
+
+
+_Summary._--In this chapter I have attempted to show, that the
+subordination of group to group in all organisms throughout all time; that
+the nature of the relationship, by which all living and extinct beings are
+united by complex, radiating, and circuitous lines of affinities into one
+grand system; the rules followed and the difficulties encountered by
+naturalists in their classifications; the value set upon characters, if
+constant and prevalent, whether of high vital importance, or of the most
+trifling {457} importance, or, as in rudimentary organs, of no importance;
+the wide opposition in value between analogical or adaptive characters, and
+characters of true affinity; and other such rules;--all naturally follow on
+the view of the common parentage of those forms which are considered by
+naturalists as allied, together with their modification through natural
+selection, with its contingencies of extinction and divergence of
+character. In considering this view of classification, it should be borne
+in mind that the element of descent has been universally used in ranking
+together the sexes, ages, and acknowledged varieties of the same species,
+however different they may be in structure. If we extend the use of this
+element of descent,--the only certainly known cause of similarity in
+organic beings,--we shall understand what is meant by the natural system:
+it is genealogical in its attempted arrangement, with the grades of
+acquired difference marked by the terms varieties, species, genera,
+families, orders, and classes.
+
+On this same view of descent with modification, all the great facts in
+Morphology become intelligible,--whether we look to the same pattern
+displayed in the homologous organs, to whatever purpose applied, of the
+different species of a class; or to the homologous parts constructed on the
+same pattern in each individual animal and plant.
+
+On the principle of successive slight variations, not necessarily or
+generally supervening at a very early period of life, and being inherited
+at a corresponding period, we can understand the great leading facts in
+Embryology; namely, the resemblance in an individual embryo of the
+homologous parts, which when matured will become widely different from each
+other in structure and function; and the resemblance in different species
+of a class of the homologous parts or {458} organs, though fitted in the
+adult members for purposes as different as possible. Larvæ are active
+embryos, which have become specially modified in relation to their habits
+of life, through the principle of modifications being inherited at
+corresponding ages. On this same principle--and bearing in mind, that when
+organs are reduced in size, either from disuse or selection, it will
+generally be at that period of life when the being has to provide for its
+own wants, and bearing in mind how strong is the principle of
+inheritance--the occurrence of rudimentary organs and their final abortion,
+present to us no inexplicable difficulties; on the contrary, their presence
+might have been even anticipated. The importance of embryological
+characters and of rudimentary organs in classification is intelligible, on
+the view that an arrangement is only so far natural as it is genealogical.
+
+Finally, the several classes of facts which have been considered in this
+chapter, seem to me to proclaim so plainly, that the innumerable species,
+genera, and families of organic beings, with which this world is peopled,
+have all descended, each within its own class or group, from common
+parents, and have all been modified in the course of descent, that I should
+without hesitation adopt this view, even if it were unsupported by other
+facts or arguments.
+
+       *       *       *       *       *
+
+
+{459}
+
+CHAPTER XIV.
+
+RECAPITULATION AND CONCLUSION.
+
+    Recapitulation of the difficulties on the theory of Natural
+    Selection--Recapitulation of the general and special circumstances in
+    its favour--Causes of the general belief in the immutability of
+    species--How far the theory of natural selection may be
+    extended--Effects of its adoption on the study of Natural
+    history--Concluding remarks.
+
+As this whole volume is one long argument, it may be convenient to the
+reader to have the leading facts and inferences briefly recapitulated.
+
+That many and serious objections may be advanced against the theory of
+descent with modification through natural selection, I do not deny. I have
+endeavoured to give to them their full force. Nothing at first can appear
+more difficult to believe than that the more complex organs and instincts
+should have been perfected, not by means superior to, though analogous
+with, human reason, but by the accumulation of innumerable slight
+variations, each good for the individual possessor. Nevertheless, this
+difficulty, though appearing to our imagination insuperably great, cannot
+be considered real if we admit the following propositions, namely,--that
+gradations in the perfection of any organ or instinct which we may
+consider, either do now exist or could have existed, each good of its
+kind,--that all organs and instincts are, in ever so slight a degree,
+variable,--and, lastly, that there is a struggle for existence leading to
+the preservation of each profitable deviation of structure or instinct. The
+truth of these propositions cannot, I think, be disputed. {460}
+
+It is, no doubt, extremely difficult even to conjecture by what gradations
+many structures have been perfected, more especially amongst broken and
+failing groups of organic beings; but we see so many strange gradations in
+nature, that we ought to be extremely cautious in saying that any organ or
+instinct, or any whole being, could not have arrived at its present state
+by many graduated steps. There are, it must be admitted, cases of special
+difficulty on the theory of natural selection; and one of the most curious
+of these is the existence of two or three defined castes of workers or
+sterile females in the same community of ants; but I have attempted to show
+how this difficulty can be mastered.
+
+With respect to the almost universal sterility of species when first
+crossed, which forms so remarkable a contrast with the almost universal
+fertility of varieties when crossed, I must refer the reader to the
+recapitulation of the facts given at the end of the eighth chapter, which
+seem to me conclusively to show that this sterility is no more a special
+endowment than is the incapacity of two trees to be grafted together; but
+that it is incidental on constitutional differences in the reproductive
+systems of the intercrossed species. We see the truth of this conclusion in
+the vast difference in the result, when the same two species are crossed
+reciprocally; that is, when one species is first used as the father and
+then as the mother.
+
+The fertility of varieties when intercrossed and of their mongrel offspring
+cannot be considered as universal; nor is their very general fertility
+surprising when we remember that it is not likely that either their
+constitutions or their reproductive systems should have been profoundly
+modified. Moreover, most of the varieties which have been experimentised on
+have been {461} produced under domestication; and as domestication (I do
+not mean mere confinement) apparently tends to eliminate sterility, we
+ought not to expect it also to produce sterility.
+
+The sterility of hybrids is a very different case from that of first
+crosses, for their reproductive organs are more or less functionally
+impotent; whereas in first crosses the organs on both sides are in a
+perfect condition. As we continually see that organisms of all kinds are
+rendered in some degree sterile from their constitutions having been
+disturbed by slightly different and new conditions of life, we need not
+feel surprise at hybrids being in some degree sterile, for their
+constitutions can hardly fail to have been disturbed from being compounded
+of two distinct organisations. This parallelism is supported by another
+parallel, but directly opposite, class of facts; namely, that the vigour
+and fertility of all organic beings are increased by slight changes in
+their conditions of life, and that the offspring of slightly modified forms
+or varieties acquire from being crossed increased vigour and fertility. So
+that, on the one hand, considerable changes in the conditions of life and
+crosses between greatly modified forms, lessen fertility; and on the other
+hand, lesser changes in the conditions of life and crosses between less
+modified forms, increase fertility.
+
+Turning to geographical distribution, the difficulties encountered on the
+theory of descent with modification are grave enough. All the individuals
+of the same species, and all the species of the same genus, or even higher
+group, must have descended from common parents; and therefore, in however
+distant and isolated parts of the world they are now found, they must in
+the course of successive generations have passed from some one part to the
+others. We are often wholly unable {462} even to conjecture how this could
+have been effected. Yet, as we have reason to believe that some species
+have retained the same specific form for very long periods, enormously long
+as measured by years, too much stress ought not to be laid on the
+occasional wide diffusion of the same species; for during very long periods
+of time there will always have been a good chance for wide migration by
+many means. A broken or interrupted range may often be accounted for by the
+extinction of the species in the intermediate regions. It cannot be denied
+that we are as yet very ignorant of the full extent of the various climatal
+and geographical changes which have affected the earth during modern
+periods; and such changes will obviously have greatly facilitated
+migration. As an example, I have attempted to show how potent has been the
+influence of the Glacial period on the distribution both of the same and of
+representative species throughout the world. We are as yet profoundly
+ignorant of the many occasional means of transport. With respect to
+distinct species of the same genus inhabiting very distant and isolated
+regions, as the process of modification has necessarily been slow, all the
+means of migration will have been possible during a very long period; and
+consequently the difficulty of the wide diffusion of species of the same
+genus is in some degree lessened.
+
+As on the theory of natural selection an interminable number of
+intermediate forms must have existed, linking together all the species in
+each group by gradations as fine as our present varieties, it may be asked,
+Why do we not see these linking forms all around us? Why are not all
+organic beings blended together in an inextricable chaos? With respect to
+existing forms, we should remember that we have no right to expect
+(excepting in rare cases) to discover _directly_ connecting {463} links
+between them, but only between each and some extinct and supplanted form.
+Even on a wide area, which has during a long period remained continuous,
+and of which the climate and other conditions of life change insensibly in
+going from a district occupied by one species into another district
+occupied by a closely allied species, we have no just right to expect often
+to find intermediate varieties in the intermediate zone. For we have reason
+to believe that only a few species are undergoing change at any one period;
+and all changes are slowly effected. I have also shown that the
+intermediate varieties which will at first probably exist in the
+intermediate zones, will be liable to be supplanted by the allied forms on
+either hand; and the latter, from existing in greater numbers, will
+generally be modified and improved at a quicker rate than the intermediate
+varieties, which exist in lesser numbers; so that the intermediate
+varieties will, in the long run, be supplanted and exterminated.
+
+On this doctrine of the extermination of an infinitude of connecting links,
+between the living and extinct inhabitants of the world, and at each
+successive period between the extinct and still older species, why is not
+every geological formation charged with such links? Why does not every
+collection of fossil remains afford plain evidence of the gradation and
+mutation of the forms of life? We meet with no such evidence, and this is
+the most obvious and forcible of the many objections which may be urged
+against my theory. Why, again, do whole groups of allied species appear,
+though certainly they often falsely appear, to have come in suddenly on the
+several geological stages? Why do we not find great piles of strata beneath
+the Silurian system, stored with the remains of the progenitors of the
+Silurian groups of fossils? For certainly on my theory such {464} strata
+must somewhere have been deposited at these ancient and utterly unknown
+epochs in the world's history.
+
+I can answer these questions and grave objections only on the supposition
+that the geological record is far more imperfect than most geologists
+believe. It cannot be objected that there has not been time sufficient for
+any amount of organic change; for the lapse of time has been so great as to
+be utterly inappreciable by the human intellect. The number of specimens in
+all our museums is absolutely as nothing compared with the countless
+generations of countless species which certainly have existed. We should
+not be able to recognise a species as the parent of any one or more species
+if we were to examine them ever so closely, unless we likewise possessed
+many of the intermediate links between their past or parent and present
+states; and these many links we could hardly ever expect to discover, owing
+to the imperfection of the geological record. Numerous existing doubtful
+forms could be named which are probably varieties; but who will pretend
+that in future ages so many fossil links will be discovered, that
+naturalists will be able to decide, on the common view, whether or not
+these doubtful forms are varieties? As long as most of the links between
+any two species are unknown, if any one link or intermediate variety be
+discovered, it will simply be classed as another and distinct species. Only
+a small portion of the world has been geologically explored. Only organic
+beings of certain classes can be preserved in a fossil condition, at least
+in any great number. Widely ranging species vary most, and varieties are
+often at first local,--both causes rendering the discovery of intermediate
+links less likely. Local varieties will not spread into other and distant
+regions until they are considerably modified and {465} improved; and when
+they do spread, if discovered in a geological formation, they will appear
+as if suddenly created there, and will be simply classed as new species.
+Most formations have been intermittent in their accumulation; and their
+duration, I am inclined to believe, has been shorter than the average
+duration of specific forms. Successive formations are separated from each
+other by enormous blank intervals of time; for fossiliferous formations,
+thick enough to resist future degradation, can be accumulated only where
+much sediment is deposited on the subsiding bed of the sea. During the
+alternate periods of elevation and of stationary level the record will be
+blank. During these latter periods there will probably be more variability
+in the forms of life; during periods of subsidence, more extinction.
+
+With respect to the absence of fossiliferous formations beneath the lowest
+Silurian strata, I can only recur to the hypothesis given in the ninth
+chapter. That the geological record is imperfect all will admit; but that
+it is imperfect to the degree which I require, few will be inclined to
+admit. If we look to long enough intervals of time, geology plainly
+declares that all species have changed; and they have changed in the manner
+which my theory requires, for they have changed slowly and in a graduated
+manner. We clearly see this in the fossil remains from consecutive
+formations invariably being much more closely related to each other, than
+are the fossils from formations distant from each other in time.
+
+Such is the sum of the several chief objections and difficulties which may
+justly be urged against my theory; and I have now briefly recapitulated the
+answers and explanations which can be given to them. I have felt these
+difficulties far too heavily during many years to {466} doubt their weight.
+But it deserves especial notice that the more important objections relate
+to questions on which we are confessedly ignorant; nor do we know how
+ignorant we are. We do not know all the possible transitional gradations
+between the simplest and the most perfect organs; it cannot be pretended
+that we know all the varied means of Distribution during the long lapse of
+years, or that we know how imperfect the Geological Record is. Grave as
+these several difficulties are, in my judgment they do not overthrow the
+theory of descent from a few created forms with subsequent modification.
+
+
+
+Now let us turn to the other side of the argument. Under domestication we
+see much variability. This seems to be mainly due to the reproductive
+system being eminently susceptible to changes in the conditions of life; so
+that this system, when not rendered impotent, fails to reproduce offspring
+exactly like the parent-form. Variability is governed by many complex
+laws,--by correlation of growth, by use and disuse, and by the direct
+action of the physical conditions of life. There is much difficulty in
+ascertaining how much modification our domestic productions have undergone;
+but we may safely infer that the amount has been large, and that
+modifications can be inherited for long periods. As long as the conditions
+of life remain the same, we have reason to believe that a modification,
+which has already been inherited for many generations, may continue to be
+inherited for an almost infinite number of generations. On the other hand
+we have evidence that variability, when it has once come into play, does
+not wholly cease; for new varieties are still occasionally produced by our
+most anciently domesticated productions. {467}
+
+Man does not actually produce variability; he only unintentionally exposes
+organic beings to new conditions of life, and then nature acts on the
+organisation, and causes variability. But man can and does select the
+variations given to him by nature, and thus accumulate them in any desired
+manner. He thus adapts animals and plants for his own benefit or pleasure.
+He may do this methodically, or he may do it unconsciously by preserving
+the individuals most useful to him at the time, without any thought of
+altering the breed. It is certain that he can largely influence the
+character of a breed by selecting, in each successive generation,
+individual differences so slight as to be quite inappreciable by an
+uneducated eye. This process of selection has been the great agency in the
+production of the most distinct and useful domestic breeds. That many of
+the breeds produced by man have to a large extent the character of natural
+species, is shown by the inextricable doubts whether very many of them are
+varieties or aboriginal species.
+
+There is no obvious reason why the principles which have acted so
+efficiently under domestication should not have acted under nature. In the
+preservation of favoured individuals and races, during the
+constantly-recurrent Struggle for Existence, we see the most powerful and
+ever-acting means of selection. The struggle for existence inevitably
+follows from the high geometrical ratio of increase which is common to all
+organic beings. This high rate of increase is proved by calculation,--by
+the rapid increase of many animals and plants during a succession of
+peculiar seasons, or when naturalised in a new country. More individuals
+are born than can possibly survive. A grain in the balance will determine
+which individual shall live and which shall die,--which variety or species
+shall increase in number, and which {468} shall decrease, or finally become
+extinct. As the individuals of the same species come in all respects into
+the closest competition with each other, the struggle will generally be
+most severe between them; it will be almost equally severe between the
+varieties of the same species, and next in severity between the species of
+the same genus. But the struggle will often be very severe between beings
+most remote in the scale of nature. The slightest advantage in one being,
+at any age or during any season, over those with which it comes into
+competition, or better adaptation in however slight a degree to the
+surrounding physical conditions, will turn the balance.
+
+With animals having separated sexes there will in most cases be a struggle
+between the males for possession of the females. The most vigorous
+individuals, or those which have most successfully struggled with their
+conditions of life, will generally leave most progeny. But success will
+often depend on having special weapons or means of defence, or on the
+charms of the males; and the slightest advantage will lead to victory.
+
+As geology plainly proclaims that each land has undergone great physical
+changes, we might have expected that organic beings would have varied under
+nature, in the same way as they generally have varied under the changed
+conditions of domestication. And if there be any variability under nature,
+it would be an unaccountable fact if natural selection had not come into
+play. It has often been asserted, but the assertion is quite incapable of
+proof, that the amount of variation under nature is a strictly limited
+quantity. Man, though acting on external characters alone and often
+capriciously, can produce within a short period a great result by adding up
+mere individual differences in his domestic productions; and every one
+admits that there are at least individual differences in species under
+{469} nature. But, besides such differences, all naturalists have admitted
+the existence of varieties, which they think sufficiently distinct to be
+worthy of record in systematic works. No one can draw any clear distinction
+between individual differences and slight varieties; or between more
+plainly marked varieties and sub-species, and species. Let it be observed
+how naturalists differ in the rank which they assign to the many
+representative forms in Europe and North America.
+
+If then we have under nature variability and a powerful agent always ready
+to act and select, why should we doubt that variations in any way useful to
+beings, under their excessively complex relations of life, would be
+preserved, accumulated, and inherited? Why, if man can by patience select
+variations most useful to himself, should nature fail in selecting
+variations useful, under changing conditions of life, to her living
+products? What limit can be put to this power, acting during long ages and
+rigidly scrutinising the whole constitution, structure, and habits of each
+creature,--favouring the good and rejecting the bad? I can see no limit to
+this power, in slowly and beautifully adapting each form to the most
+complex relations of life. The theory of natural selection, even if we
+looked no further than this, seems to me to be in itself probable. I have
+already recapitulated, as fairly as I could, the opposed difficulties and
+objections: now let us turn to the special facts and arguments in favour of
+the theory.
+
+On the view that species are only strongly marked and permanent varieties,
+and that each species first existed as a variety, we can see why it is that
+no line of demarcation can be drawn between species, commonly supposed to
+have been produced by special acts of creation, and varieties which are
+acknowledged to have been produced by secondary laws. On this same {470}
+view we can understand how it is that in each region where many species of
+a genus have been produced, and where they now flourish, these same species
+should present many varieties; for where the manufactory of species has
+been active, we might expect, as a general rule, to find it still in
+action; and this is the case if varieties be incipient species. Moreover,
+the species of the larger genera, which afford the greater number of
+varieties or incipient species, retain to a certain degree the character of
+varieties; for they differ from each other by a less amount of difference
+than do the species of smaller genera. The closely allied species also of
+the larger genera apparently have restricted ranges, and in their
+affinities they are clustered in little groups round other species--in
+which respects they resemble varieties. These are strange relations on the
+view of each species having been independently created, but are
+intelligible if all species first existed as varieties.
+
+As each species tends by its geometrical ratio of reproduction to increase
+inordinately in number; and as the modified descendants of each species
+will be enabled to increase by so much the more as they become diversified
+in habits and structure, so as to be enabled to seize on many and widely
+different places in the economy of nature, there will be a constant
+tendency in natural selection to preserve the most divergent offspring of
+any one species. Hence during a long-continued course of modification, the
+slight differences, characteristic of varieties of the same species, tend
+to be augmented into the greater differences characteristic of species of
+the same genus. New and improved varieties will inevitably supplant and
+exterminate the older, less improved and intermediate varieties; and thus
+species are rendered to a large extent defined and distinct objects.
+Dominant species belonging to the {471} larger groups tend to give birth to
+new and dominant forms; so that each large group tends to become still
+larger, and at the same time more divergent in character. But as all groups
+cannot thus succeed in increasing in size, for the world would not hold
+them, the more dominant groups beat the less dominant. This tendency in the
+large groups to go on increasing in size and diverging in character,
+together with the almost inevitable contingency of much extinction,
+explains the arrangement of all the forms of life, in groups subordinate to
+groups, all within a few great classes, which we now see everywhere around
+us, and which has prevailed throughout all time. This grand fact of the
+grouping of all organic beings seems to me utterly inexplicable on the
+theory of creation.
+
+As natural selection acts solely by accumulating slight, successive,
+favourable variations, it can produce no great or sudden modification; it
+can act only by very short and slow steps. Hence the canon of "Natura non
+facit saltum," which every fresh addition to our knowledge tends to make
+truer, is on this theory simply intelligible. We can plainly see why nature
+is prodigal in variety, though niggard in innovation. But why this should
+be a law of nature if each species has been independently created, no man
+can explain.
+
+Many other facts are, as it seems to me, explicable on this theory. How
+strange it is that a bird, under the form of woodpecker, should have been
+created to prey on insects on the ground; that upland geese, which never or
+rarely swim, should have been created with webbed feet; that a thrush
+should have been created to dive and feed on sub-aquatic insects; and that
+a petrel should have been created with habits and structure fitting it for
+the life of an auk or grebe! and so on in endless other cases. But on the
+view of each {472} species constantly trying to increase in number, with
+natural selection always ready to adapt the slowly varying descendants of
+each to any unoccupied or ill-occupied place in nature, these facts cease
+to be strange, or perhaps might even have been anticipated.
+
+As natural selection acts by competition, it adapts the inhabitants of each
+country only in relation to the degree of perfection of their associates;
+so that we need feel no surprise at the inhabitants of any one country,
+although on the ordinary view supposed to have been specially created and
+adapted for that country, being beaten and supplanted by the naturalised
+productions from another land. Nor ought we to marvel if all the
+contrivances in nature be not, as far as we can judge, absolutely perfect;
+and if some of them be abhorrent to our ideas of fitness. We need not
+marvel at the sting of the bee causing the bee's own death; at drones being
+produced in such vast numbers for one single act, with the great majority
+slaughtered by their sterile sisters; at the astonishing waste of pollen by
+our fir-trees; at the instinctive hatred of the queen bee for her own
+fertile daughters; at ichneumonidæ feeding within the live bodies of
+caterpillars; and at other such cases. The wonder indeed is, on the theory
+of natural selection, that more cases of the want of absolute perfection
+have not been observed.
+
+The complex and little known laws governing variation are the same, as far
+as we can see, with the laws which have governed the production of
+so-called specific forms. In both cases physical conditions seem to have
+produced but little direct effect; yet when varieties enter any zone, they
+occasionally assume some of the characters of the species proper to that
+zone. In both varieties and species, use and disuse seem to have produced
+some effect; for it is difficult to resist this {473} conclusion when we
+look, for instance, at the logger-headed duck, which has wings incapable of
+flight, in nearly the same condition as in the domestic duck; or when we
+look at the burrowing tucutucu, which is occasionally blind, and then at
+certain moles, which are habitually blind and have their eyes covered with
+skin; or when we look at the blind animals inhabiting the dark caves of
+America and Europe. In both varieties and species correlation of growth
+seems to have played a most important part, so that when one part has been
+modified other parts are necessarily modified. In both varieties and
+species reversions to long-lost characters occur. How inexplicable on the
+theory of creation is the occasional appearance of stripes on the shoulder
+and legs of the several species of the horse-genus and in their hybrids!
+How simply is this fact explained if we believe that these species have
+descended from a striped progenitor, in the same manner as the several
+domestic breeds of pigeon have descended from the blue and barred
+rock-pigeon!
+
+On the ordinary view of each species having been independently created, why
+should the specific characters, or those by which the species of the same
+genus differ from each other, be more variable than the generic characters
+in which they all agree? Why, for instance, should the colour of a flower
+be more likely to vary in any one species of a genus, if the other species,
+supposed to have been created independently, have differently coloured
+flowers, than if all the species of the genus have the same coloured
+flowers? If species are only well-marked varieties, of which the characters
+have become in a high degree permanent, we can understand this fact; for
+they have already varied since they branched off from a common progenitor
+in certain characters, by which they have come to be specifically distinct
+from each other; {474} and therefore these same characters would be more
+likely still to be variable than the generic characters which have been
+inherited without change for an enormous period. It is inexplicable on the
+theory of creation why a part developed in a very unusual manner in any one
+species of a genus, and therefore, as we may naturally infer, of great
+importance to the species, should be eminently liable to variation; but, on
+my view, this part has undergone, since the several species branched off
+from a common progenitor, an unusual amount of variability and
+modification, and therefore we might expect this part generally to be still
+variable. But a part may be developed in the most unusual manner, like the
+wing of a bat, and yet not be more variable than any other structure, if
+the part be common to many subordinate forms, that is, if it has been
+inherited for a very long period; for in this case it will have been
+rendered constant by long-continued natural selection.
+
+Glancing at instincts, marvellous as some are, they offer no greater
+difficulty than does corporeal structure on the theory of the natural
+selection of successive, slight, but profitable modifications. We can thus
+understand why nature moves by graduated steps in endowing different
+animals of the same class with their several instincts. I have attempted to
+show how much light the principle of gradation throws on the admirable
+architectural powers of the hive-bee. Habit no doubt sometimes comes into
+play in modifying instincts; but it certainly is not indispensable, as we
+see, in the case of neuter insects, which leave no progeny to inherit the
+effects of long-continued habit. On the view of all the species of the same
+genus having descended from a common parent, and having inherited much in
+common, we can understand how it is that allied species, when placed under
+considerably different conditions of life, {475} yet should follow nearly
+the same instincts; why the thrush of South America, for instance, lines
+her nest with mud like our British species. On the view of instincts having
+been slowly acquired through natural selection we need not marvel at some
+instincts being apparently not perfect and liable to mistakes, and at many
+instincts causing other animals to suffer.
+
+If species be only well-marked and permanent varieties, we can at once see
+why their crossed offspring should follow the same complex laws in their
+degrees and kinds of resemblance to their parents,--in being absorbed into
+each other by successive crosses, and in other such points,--as do the
+crossed offspring of acknowledged varieties. On the other hand, these would
+be strange facts if species have been independently created, and varieties
+have been produced by secondary laws.
+
+If we admit that the geological record is imperfect in an extreme degree,
+then such facts as the record gives, support the theory of descent with
+modification. New species have come on the stage slowly and at successive
+intervals; and the amount of change, after equal intervals of time, is
+widely different in different groups. The extinction of species and of
+whole groups of species, which has played so conspicuous a part in the
+history of the organic world, almost inevitably follows on the principle of
+natural selection; for old forms will be supplanted by new and improved
+forms. Neither single species nor groups of species reappear when the chain
+of ordinary generation has once been broken. The gradual diffusion of
+dominant forms, with the slow modification of their descendants, causes the
+forms of life, after long intervals of time, to appear as if they had
+changed simultaneously throughout the world. The fact of the fossil remains
+of each formation being in some degree intermediate in character between
+the {476} fossils in the formations above and below, is simply explained by
+their intermediate position in the chain of descent. The grand fact that
+all extinct organic beings belong to the same system with recent beings,
+falling either into the same or into intermediate groups, follows from the
+living and the extinct being the offspring of common parents. As the groups
+which have descended from an ancient progenitor have generally diverged in
+character, the progenitor with its early descendants will often be
+intermediate in character in comparison with its later descendants; and
+thus we can see why the more ancient a fossil is, the oftener it stands in
+some degree intermediate between existing and allied groups. Recent forms
+are generally looked at as being, in some vague sense, higher than ancient
+and extinct forms; and they are in so far higher as the later and more
+improved forms have conquered the older and less improved organic beings in
+the struggle for life. Lastly, the law of the long endurance of allied
+forms on the same continent,--of marsupials in Australia, of edentata in
+America, and other such cases,--is intelligible, for within a confined
+country, the recent and the extinct will naturally be allied by descent.
+
+Looking to geographical distribution, if we admit that there has been
+during the long course of ages much migration from one part of the world to
+another, owing to former climatal and geographical changes and to the many
+occasional and unknown means of dispersal, then we can understand, on the
+theory of descent with modification, most of the great leading facts in
+Distribution. We can see why there should be so striking a parallelism in
+the distribution of organic beings throughout space, and in their
+geological succession throughout time; for in both cases the beings have
+been connected by the bond of ordinary generation, and the means of {477}
+modification have been the same. We see the full meaning of the wonderful
+fact, which must have struck every traveller, namely, that on the same
+continent, under the most diverse conditions, under heat and cold, on
+mountain and lowland, on deserts and marshes, most of the inhabitants
+within each great class are plainly related; for they will generally be
+descendants of the same progenitors and early colonists. On this same
+principle of former migration, combined in most cases with modification, we
+can understand, by the aid of the Glacial period, the identity of some few
+plants, and the close alliance of many others, on the most distant
+mountains, under the most different climates; and likewise the close
+alliance of some of the inhabitants of the sea in the northern and southern
+temperate zones, though separated by the whole intertropical ocean.
+Although two areas may present the same physical conditions of life, we
+need feel no surprise at their inhabitants being widely different, if they
+have been for a long period completely separated from each other; for as
+the relation of organism to organism is the most important of all
+relations, and as the two areas will have received colonists from some
+third source or from each other, at various periods and in different
+proportions, the course of modification in the two areas will inevitably be
+different.
+
+On this view of migration, with subsequent modification, we can see why
+oceanic islands should be inhabited by few species, but of these, that many
+should be peculiar. We can clearly see why those animals which cannot cross
+wide spaces of ocean, as frogs and terrestrial mammals, should not inhabit
+oceanic islands; and why, on the other hand, new and peculiar species of
+bats, which can traverse the ocean, should so often be found on islands far
+distant from any continent. Such facts {478} as the presence of peculiar
+species of bats, and the absence of all other mammals, on oceanic islands,
+are utterly inexplicable on the theory of independent acts of creation.
+
+The existence of closely allied or representative species in any two areas,
+implies, on the theory of descent with modification, that the same parents
+formerly inhabited both areas; and we almost invariably find that wherever
+many closely allied species inhabit two areas, some identical species
+common to both still exist. Wherever many closely allied yet distinct
+species occur, many doubtful forms and varieties of the same species
+likewise occur. It is a rule of high generality that the inhabitants of
+each area are related to the inhabitants of the nearest source whence
+immigrants might have been derived. We see this in nearly all the plants
+and animals of the Galapagos archipelago, of Juan Fernandez, and of the
+other American islands being related in the most striking manner to the
+plants and animals of the neighbouring American mainland; and those of the
+Cape de Verde archipelago and other African islands to the African
+mainland. It must be admitted that these facts receive no explanation on
+the theory of creation.
+
+The fact, as we have seen, that all past and present organic beings
+constitute one grand natural system, with group subordinate to group, and
+with extinct groups often falling in between recent groups, is intelligible
+on the theory of natural selection with its contingencies of extinction and
+divergence of character. On these same principles we see how it is, that
+the mutual affinities of the species and genera within each class are so
+complex and circuitous. We see why certain characters are far more
+serviceable than others for classification;--why adaptive characters,
+though of paramount importance to the being, are of hardly any {479}
+importance in classification; why characters derived from rudimentary
+parts, though of no service to the being, are often of high classificatory
+value; and why embryological characters are the most valuable of all. The
+real affinities of all organic beings are due to inheritance or community
+of descent. The natural system is a genealogical arrangement, in which we
+have to discover the lines of descent by the most permanent characters,
+however slight their vital importance may be.
+
+The framework of bones being the same in the hand of a man, wing of a bat,
+fin of the porpoise, and leg of the horse,--the same number of vertebræ
+forming the neck of the giraffe and of the elephant,--and innumerable other
+such facts, at once explain themselves on the theory of descent with slow
+and slight successive modifications. The similarity of pattern in the wing
+and leg of a bat, though used for such different purpose,--in the jaws and
+legs of a crab,--in the petals, stamens, and pistils of a flower, is
+likewise intelligible on the view of the gradual modification of parts or
+organs, which were alike in the early progenitor of each class. On the
+principle of successive variations not always supervening at an early age,
+and being inherited at a corresponding not early period of life, we can
+clearly see why the embryos of mammals, birds, reptiles, and fishes should
+be so closely alike, and should be so unlike the adult forms. We may cease
+marvelling at the embryo of an air-breathing mammal or bird having
+branchial slits and arteries running in loops, like those in a fish which
+has to breathe the air dissolved in water, by the aid of well-developed
+branchiæ.
+
+Disuse, aided sometimes by natural selection, will often tend to reduce an
+organ, when it has become useless by changed habits or under changed
+conditions {480} of life; and we can clearly understand on this view the
+meaning of rudimentary organs. But disuse and selection will generally act
+on each creature, when it has come to maturity and has to play its full
+part in the struggle for existence, and will thus have little power of
+acting on an organ during early life; hence the organ will not be much
+reduced or rendered rudimentary at this early age. The calf, for instance,
+has inherited teeth, which never cut through the gums of the upper jaw,
+from an early progenitor having well-developed teeth; and we may believe,
+that the teeth in the mature animal were reduced, during successive
+generations, by disuse or by the tongue and palate having been better
+fitted by natural selection to browse without their aid; whereas in the
+calf, the teeth have been left untouched by selection or disuse, and on the
+principle of inheritance at corresponding ages have been inherited from a
+remote period to the present day. On the view of each organic being and
+each separate organ having been specially created, how utterly inexplicable
+it is that parts, like the teeth in the embryonic calf or like the
+shrivelled wings under the soldered wing-covers of some beetles, should
+thus so frequently bear the plain stamp of inutility! Nature may be said to
+have taken pains to reveal, by rudimentary organs and by homologous
+structures, her scheme of modification, which it seems that we wilfully
+will not understand.
+
+
+
+I have now recapitulated the chief facts and considerations which have
+thoroughly convinced me that species have been modified, during a long
+course of descent, by the preservation or the natural selection of many
+successive slight favourable variations. I cannot believe that a false
+theory would explain, as it seems to me that the theory of natural
+selection does explain, {481} the several large classes of facts above
+specified. I see no good reason why the views given in this volume should
+shock the religious feelings of any one. A celebrated author and divine has
+written to me that "he has gradually learnt to see that it is just as noble
+a conception of the Deity to believe that He created a few original forms
+capable of self-development into other and needful forms, as to believe
+that He required a fresh act of creation to supply the voids caused by the
+action of His laws."
+
+Why, it may be asked, have all the most eminent living naturalists and
+geologists rejected this view of the mutability of species? It cannot be
+asserted that organic beings in a state of nature are subject to no
+variation; it cannot be proved that the amount of variation in the course
+of long ages is a limited quantity; no clear distinction has been, or can
+be, drawn between species and well-marked varieties. It cannot be
+maintained that species when intercrossed are invariably sterile, and
+varieties invariably fertile; or that sterility is a special endowment and
+sign of creation. The belief that species were immutable productions was
+almost unavoidable as long as the history of the world was thought to be of
+short duration; and now that we have acquired some idea of the lapse of
+time, we are too apt to assume, without proof, that the geological record
+is so perfect that it would have afforded us plain evidence of the mutation
+of species, if they had undergone mutation.
+
+But the chief cause of our natural unwillingness to admit that one species
+has given birth to other and distinct species, is that we are always slow
+in admitting any great change of which we do not see the intermediate
+steps. The difficulty is the same as that felt by so many geologists, when
+Lyell first insisted that long {482} lines of inland cliffs had been
+formed, and great valleys excavated, by the slow action of the coast-waves.
+The mind cannot possibly grasp the full meaning of the term of a hundred
+million years; it cannot add up and perceive the full effects of many
+slight variations, accumulated during an almost infinite number of
+generations.
+
+Although I am fully convinced of the truth of the views given in this
+volume under the form of an abstract, I by no means expect to convince
+experienced naturalists whose minds are stocked with a multitude of facts
+all viewed, during a long course of years, from a point of view directly
+opposite to mine. It is so easy to hide our ignorance under such
+expressions as the "plan of creation," "unity of design," &c., and to think
+that we give an explanation when we only restate a fact. Any one whose
+disposition leads him to attach more weight to unexplained difficulties
+than to the explanation of a certain number of facts will certainly reject
+my theory. A few naturalists, endowed with much flexibility of mind, and
+who have already begun to doubt on the immutability of species, may be
+influenced by this volume; but I look with confidence to the future, to
+young and rising naturalists, who will be able to view both sides of the
+question with impartiality. Whoever is led to believe that species are
+mutable will do good service by conscientiously expressing his conviction;
+for only thus can the load of prejudice by which this subject is
+overwhelmed be removed.
+
+Several eminent naturalists have of late published their belief that a
+multitude of reputed species in each genus are not real species; but that
+other species are real, that is, have been independently created. This
+seems to me a strange conclusion to arrive at. They admit that a multitude
+of forms, which till lately {483} they themselves thought were special
+creations, and which are still thus looked at by the majority of
+naturalists, and which consequently have every external characteristic
+feature of true species,--they admit that these have been produced by
+variation, but they refuse to extend the same view to other and very
+slightly different forms. Nevertheless they do not pretend that they can
+define, or even conjecture, which are the created forms of life, and which
+are those produced by secondary laws. They admit variation as a _vera
+causa_ in one case, they arbitrarily reject it in another, without
+assigning any distinction in the two cases. The day will come when this
+will be given as a curious illustration of the blindness of preconceived
+opinion. These authors seem no more startled at a miraculous act of
+creation than at an ordinary birth. But do they really believe that at
+innumerable periods in the earth's history certain elemental atoms have
+been commanded suddenly to flash into living tissues? Do they believe that
+at each supposed act of creation one individual or many were produced? Were
+all the infinitely numerous kinds of animals and plants created as eggs or
+seed, or as full grown? and in the case of mammals, were they created
+bearing the false marks of nourishment from the mother's womb? Although
+naturalists very properly demand a full explanation of every difficulty
+from those who believe in the mutability of species, on their own side they
+ignore the whole subject of the first appearance of species in what they
+consider reverent silence.
+
+It may be asked how far I extend the doctrine of the modification of
+species. The question is difficult to answer, because the more distinct the
+forms are which we may consider, by so much the arguments fall away in
+force. But some arguments of the greatest weight {484} extend very far. All
+the members of whole classes can be connected together by chains of
+affinities, and all can be classified on the same principle, in groups
+subordinate to groups. Fossil remains sometimes tend to fill up very wide
+intervals between existing orders. Organs in a rudimentary condition
+plainly show that an early progenitor had the organ in a fully developed
+state; and this in some instances necessarily implies an enormous amount of
+modification in the descendants. Throughout whole classes various
+structures are formed on the same pattern, and at an embryonic age the
+species closely resemble each other. Therefore I cannot doubt that the
+theory of descent with modification embraces all the members of the same
+class. I believe that animals have descended from at most only four or five
+progenitors, and plants from an equal or lesser number.
+
+Analogy would lead me one step further, namely, to the belief that all
+animals and plants have descended from some one prototype. But analogy may
+be a deceitful guide. Nevertheless all living things have much in common,
+in their chemical composition, their germinal vesicles, their cellular
+structure, and their laws of growth and reproduction. We see this even in
+so trifling a circumstance as that the same poison often similarly affects
+plants and animals; or that the poison secreted by the gall-fly produces
+monstrous growths on the wild rose or oak-tree. Therefore I should infer
+from analogy that probably all the organic beings which have ever lived on
+this earth have descended from some one primordial form, into which life
+was first breathed by the Creator.
+
+
+
+When the views advanced by me in this volume, and by Mr. Wallace in the
+Linnean Journal, or when analogous views on the origin of species are
+generally {485} admitted, we can dimly foresee that there will be a
+considerable revolution in natural history. Systematists will be able to
+pursue their labours as at present; but they will not be incessantly
+haunted by the shadowy doubt whether this or that form be in essence a
+species. This I feel sure, and I speak after experience, will be no slight
+relief. The endless disputes whether or not some fifty species of British
+brambles are true species will cease. Systematists will have only to decide
+(not that this will be easy) whether any form be sufficiently constant and
+distinct from other forms, to be capable of definition; and if definable,
+whether the differences be sufficiently important to deserve a specific
+name. This latter point will become a far more essential consideration than
+it is at present; for differences, however slight, between any two forms,
+if not blended by intermediate gradations, are looked at by most
+naturalists as sufficient to raise both forms to the rank of species.
+Hereafter we shall be compelled to acknowledge that the only distinction
+between species and well-marked varieties is, that the latter are known, or
+believed, to be connected at the present day by intermediate gradations,
+whereas species were formerly thus connected. Hence, without rejecting the
+consideration of the present existence of intermediate gradations between
+any two forms, we shall be led to weigh more carefully and to value higher
+the actual amount of difference between them. It is quite possible that
+forms now generally acknowledged to be merely varieties may hereafter be
+thought worthy of specific names, as with the primrose and cowslip; and in
+this case scientific and common language will come into accordance. In
+short, we shall have to treat species in the same manner as those
+naturalists treat genera, who admit that genera are merely artificial
+combinations {486} made for convenience. This may not be a cheering
+prospect; but we shall at least be freed from the vain search for the
+undiscovered and undiscoverable essence of the term species.
+
+The other and more general departments of natural history will rise greatly
+in interest. The terms used by naturalists of affinity, relationship,
+community of type, paternity, morphology, adaptive characters, rudimentary
+and aborted organs, &c., will cease to be metaphorical, and will have a
+plain signification. When we no longer look at an organic being as a savage
+looks at a ship, as at something wholly beyond his comprehension; when we
+regard every production of nature as one which has had a history; when we
+contemplate every complex structure and instinct as the summing up of many
+contrivances, each useful to the possessor, nearly in the same way as when
+we look at any great mechanical invention as the summing up of the labour,
+the experience, the reason, and even the blunders of numerous workmen; when
+we thus view each organic being, how far more interesting, I speak from
+experience, will the study of natural history become!
+
+A grand and almost untrodden field of inquiry will be opened, on the causes
+and laws of variation, on correlation of growth, on the effects of use and
+disuse, on the direct action of external conditions, and so forth. The
+study of domestic productions will rise immensely in value. A new variety
+raised by man will be a more important and interesting subject for study
+than one more species added to the infinitude of already recorded species.
+Our classifications will come to be, as far as they can be so made,
+genealogies; and will then truly give what may be called the plan of
+creation. The rules for classifying will no doubt become simpler when we
+have a definite object in view. We possess no {487} pedigrees or armorial
+bearings; and we have to discover and trace the many diverging lines of
+descent in our natural genealogies, by characters of any kind which have
+long been inherited. Rudimentary organs will speak infallibly with respect
+to the nature of long-lost structures. Species and groups of species, which
+are called aberrant, and which may fancifully be called living fossils,
+will aid us in forming a picture of the ancient forms of life. Embryology
+will reveal to us the structure, in some degree obscured, of the prototypes
+of each great class.
+
+When we can feel assured that all the individuals of the same species, and
+all the closely allied species of most genera, have within a not very
+remote period descended from one parent, and have migrated from some one
+birthplace; and when we better know the many means of migration, then, by
+the light which geology now throws, and will continue to throw, on former
+changes of climate and of the level of the land, we shall surely be enabled
+to trace in an admirable manner the former migrations of the inhabitants of
+the whole world. Even at present, by comparing the differences of the
+inhabitants of the sea on the opposite sides of a continent, and the nature
+of the various inhabitants of that continent in relation to their apparent
+means of immigration, some light can be thrown on ancient geography.
+
+The noble science of Geology loses glory from the extreme imperfection of
+the record. The crust of the earth with its embedded remains must not be
+looked at as a well-filled museum, but as a poor collection made at hazard
+and at rare intervals. The accumulation of each great fossiliferous
+formation will be recognised as having depended on an unusual concurrence
+of circumstances, and the blank intervals between the successive stages as
+having been of vast duration. But we shall {488} be able to gauge with some
+security the duration of these intervals by a comparison of the preceding
+and succeeding organic forms. We must be cautious in attempting to
+correlate as strictly contemporaneous two formations, which include few
+identical species, by the general succession of their forms of life. As
+species are produced and exterminated by slowly acting and still existing
+causes, and not by miraculous acts of creation and by catastrophes; and as
+the most important of all causes of organic change is one which is almost
+independent of altered and perhaps suddenly altered physical conditions,
+namely, the mutual relation of organism to organism,--the improvement of
+one being entailing the improvement or the extermination of others; it
+follows, that the amount of organic change in the fossils of consecutive
+formations probably serves as a fair measure of the lapse of actual time. A
+number of species, however, keeping in a body might remain for a long
+period unchanged, whilst within this same period, several of these species,
+by migrating into new countries and coming into competition with foreign
+associates, might become modified; so that we must not overrate the
+accuracy of organic change as a measure of time. During early periods of
+the earth's history, when the forms of life were probably fewer and
+simpler, the rate of change was probably slower; and at the first dawn of
+life, when very few forms of the simplest structure existed, the rate of
+change may have been slow in an extreme degree. The whole history of the
+world, as at present known, although of a length quite incomprehensible by
+us, will hereafter be recognised as a mere fragment of time, compared with
+the ages which have elapsed since the first creature, the progenitor of
+innumerable extinct and living descendants, was created.
+
+In the distant future I see open fields for far more {489} important
+researches. Psychology will be based on a new foundation, that of the
+necessary acquirement of each mental power and capacity by gradation. Light
+will be thrown on the origin of man and his history.
+
+Authors of the highest eminence seem to be fully satisfied with the view
+that each species has been independently created. To my mind it accords
+better with what we know of the laws impressed on matter by the Creator,
+that the production and extinction of the past and present inhabitants of
+the world should have been due to secondary causes, like those determining
+the birth and death of the individual. When I view all beings not as
+special creations, but as the lineal descendants of some few beings which
+lived long before the first bed of the Silurian system was deposited, they
+seem to me to become ennobled. Judging from the past, we may safely infer
+that not one living species will transmit its unaltered likeness to a
+distant futurity. And of the species now living very few will transmit
+progeny of any kind to a far distant futurity; for the manner in which all
+organic beings are grouped, shows that the greater number of species of
+each genus, and all the species of many genera, have left no descendants,
+but have become utterly extinct. We can so far take a prophetic glance into
+futurity as to foretel that it will be the common and widely-spread
+species, belonging to the larger and dominant groups, which will ultimately
+prevail and procreate new and dominant species. As all the living forms of
+life are the lineal descendants of those which lived long before the
+Silurian epoch, we may feel certain that the ordinary succession by
+generation has never once been broken, and that no cataclysm has desolated
+the whole world. Hence we may look with some confidence to a secure future
+of equally inappreciable length. And as natural selection works {490}
+solely by and for the good of each being, all corporeal and mental
+endowments will tend to progress towards perfection.
+
+It is interesting to contemplate an entangled bank, clothed with many
+plants of many kinds, with birds singing on the bushes, with various
+insects flitting about, and with worms crawling through the damp earth, and
+to reflect that these elaborately constructed forms, so different from each
+other, and dependent on each other in so complex a manner, have all been
+produced by laws acting around us. These laws, taken in the largest sense,
+being Growth with Reproduction; Inheritance which is almost implied by
+reproduction; Variability from the indirect and direct action of the
+external conditions of life, and from use and disuse; a Ratio of Increase
+so high as to lead to a Struggle for Life, and as a consequence to Natural
+Selection, entailing Divergence of Character and the Extinction of
+less-improved forms. Thus, from the war of nature, from famine and death,
+the most exalted object which we are capable of conceiving, namely, the
+production of the higher animals, directly follows. There is grandeur in
+this view of life, with its several powers, having been originally breathed
+by the Creator into a few forms or into one; and that, whilst this planet
+has gone cycling on according to the fixed law of gravity, from so simple a
+beginning endless forms most beautiful and most wonderful have been, and
+are being, evolved.
+
+       *       *       *       *       *
+
+
+{491}
+
+INDEX.
+
+          A.
+
+  Aberrant groups, 429.
+  Abyssinia, plants of, 375.
+  Acclimatisation, 139.
+  Affinities of extinct species, 329.
+  ---- of organic beings, 411.
+  Agassiz on Amblyopsis, 139.
+  ---- on groups of species suddenly appearing, 302, 305.
+  ---- on embryological succession, 338.
+  ---- on the glacial period, 366.
+  ---- on embryological characters, 418.
+  ---- on the embryos of vertebrata, 439.
+  ---- on parallelism of embryological development and geological
+      succession, 449.
+  Algæ of New Zealand, 376.
+  Alligators, males, fighting, 88.
+  Amblyopsis, blind fish, 139.
+  America, North, productions allied to those of Europe, 371.
+  --------, boulders and glaciers of, 373.
+  ----, South, no modern formations on west coast, 290.
+  Ammonites, sudden extinction of, 321.
+  Anagallis, sterility of, 247.
+  Analogy of variations, 159.
+  Ancylus, 386.
+  Animals, not domesticated from being variable, 17.
+  ----, domestic, descended from several stocks, 19.
+  --------, acclimatisation of, 141.
+  ---- of Australia, 116.
+  ---- with thicker fur in cold climates, 133.
+  ----, blind, in caves, 137.
+  ----, extinct, of Australia, 339.
+  Anomma, 240.
+  Antarctic islands, ancient flora of, 399.
+  Antirrhinum, 161.
+  Ants attending aphides, 210.
+  ----, slave-making instinct, 219.
+  ----, neuter, structure of, 236.
+  Aphides, attended by ants, 210.
+  Aphis, development of, 442.
+  Apteryx, 182.
+  Arab horses, 35.
+  Aralo-Caspian Sea, 339.
+  Archaic, M. de, on the succession of species, 325.
+  Artichoke, Jerusalem, 142.
+  Ascension, plants of, 389.
+  Asclepias, pollen of, 193.
+  Asparagus, 359.
+  Aspicarpa, 417.
+  Asses, striped, 163.
+  Ateuchus, 135.
+  Audubon on habits of frigate-bird, 185.
+  ---- on variation in birds'-nests, 212.
+  ---- on heron eating seeds, 387.
+  Australia, animals of, 116.
+  ----. dogs of, 215.
+  ----, extinct animals of, 339.
+  ----, European plants in, 375.
+  Azara on flies destroying cattle, 72.
+  Azores, flora of, 363.
+
+          B.
+
+  Babington, Mr., on British plants, 48.
+  Balancement of growth, 147.
+  Bamboo with hooks, 197.
+  Barberry, flowers of, 98.
+  Barrande, M., on Silurian colonies, 313.
+  ---- on the succession of species, 325.
+  ---- on parallelism of palæozoic formations, 328.
+  ---- on affinities of ancient species, 330.
+  Barriers, importance of, 347.
+  Batrachians on islands, 393.
+  Bats, how structure acquired, 180.
+  ----, distribution of, 394.
+  Bear, catching water-insects, 184.
+  Bee, sting of, 202.
+  ----, queen, killing rivals, 202.
+  Bees fertilising flowers, 73.
+  ----, hive, not sucking the red clover, 95.
+  {492}
+  --------, cell-making instinct, 224.
+  ----, humble, cells of, 225.
+  ----, parasitic, 218.
+  Beetles, wingless, in Madeira, 135.
+  ---- with deficient tarsi, 135.
+  Bentham, Mr., on British plants, 48.
+  ----, on classification, 419.
+  Berkeley, Mr., on seeds in salt-water, 358.
+  Bermuda, birds of, 391.
+  Birds acquiring fear, 212.
+  ---- annually cross the Atlantic, 364.
+  ----, colour of, on continents, 132.
+  ----, footsteps and remains of, in secondary rocks, 304.
+  ----, fossil, in caves of Brazil, 339.
+  ---- of Madeira, Bermuda, and Galapagos, 391.
+  ----, song of males, 89.
+  ---- transporting seeds, 361.
+  ----, waders, 385.
+  ----, wingless, 134, 182.
+  ----, with traces of embryonic teeth, 450.
+  Bizcacha, 349.
+  ----, affinities of, 429.
+  Bladder for swimming in fish, 190.
+  Blindness of cave animals, 137.
+  Blyth, Mr., on distinctness of Indian cattle, 18.
+  ----, on striped Hemionus, 163.
+  ----, on crossed geese, 254.
+  Boar, shoulder-pad of, 88.
+  Borrow, Mr., on the Spanish pointer, 35.
+  Bory St. Vincent on Batrachians, 393.
+  Bosquet, M., on fossil Chthamalus, 305.
+  Boulders, erratic, on the Azores, 363.
+  Branchiæ, 190.
+  Brent, Mr., on house-tumblers, 214.
+  ----, on hawks killing pigeons, 362.
+  Brewer, Dr., on American cuckoo, 217.
+  Britain, mammals of, 396.
+  Bronn on duration of specific forms, 294.
+  Brown, Robert, on classification, 415.
+  Buckman on variation in plants, 10.
+  Buzareingues on sterility of varieties, 270.
+
+          C.
+
+  Cabbage, varieties of, crossed, 99.
+  Calceolaria, 251.
+  Canary-birds, sterility of hybrids, 252.
+  Cape de Verde islands, 398.
+  Cape of Good Hope, plants of, 110, 375.
+  Carrier-pigeons killed by hawks, 362.
+  Cassini on flowers of compositæ, 145.
+  Catasetum, 424.
+  Cats, with blue eyes, deaf, 12.
+  ----, variation in habits of, 91.
+  ---- curling tail when going to spring, 201.
+  Cattle destroying fir-trees, 72.
+  ---- destroyed by flies in La Plata, 72.
+  ----, breeds of, locally extinct, 111.
+  ----, fertility of Indian and European breeds, 254.
+  Cave, inhabitants of, blind, 137.
+  Centres of creation, 352.
+  Cephalopodæ, development of, 442.
+  Cervulus, 253.
+  Cetacea, teeth and hair, 144.
+  Ceylon, plants of, 375.
+  Chalk formation, 322.
+  Characters, divergence of, 111.
+  ----, sexual, variable, 156.
+  ----, adaptive or analogical, 426.
+  Charlock, 76.
+  Checks to increase, 67.
+  ---- ----, mutual, 71.
+  Chickens, instinctive tameness of, 216.
+  Chthamalinæ, 289.
+  Chthamalus, cretacean species of, 305.
+  Circumstances favourable to selection of domestic products, 40.
+  ---- ---- to natural selection, 102.
+  Cirripedes capable of crossing, 101.
+  ----, carapace aborted, 148.
+  ----, their ovigerous frena, 192.
+  ----, fossil, 304.
+  ----, larvæ of, 440.
+  Classification, 413.
+  Clift, Mr., on the succession of types, 339.
+  Climate, effects of, in checking increase of beings, 68.
+  ----, adaptation of, to organisms, 139.
+  {493}
+  Cobites, intestine of, 190.
+  Cockroach, 76.
+  Collections, palæontological, poor, 288.
+  Colour, influenced by climate, 132.
+  ----, in relation to attacks by flies, 198.
+  Columba livia, parent of domestic pigeons, 23.
+  Colymbetes, 386.
+  Compensation of growth, 147.
+  Compositæ, outer and inner florets of, 144.
+  ----, male flowers of, 451.
+  Conclusion, general, 480.
+  Conditions, slight changes in, favourable to fertility, 267.
+  Coot, 185.
+  Coral-islands, seeds drifted to, 361.
+  ---- reefs, indicating movements of earth, 310.
+  Corn-crake, 186.
+  Correlation of growth in domestic productions, 11.
+  ---- of growth, 143, 198.
+  Cowslip, 49.
+  Creation, single centres of, 352.
+  Crinum, 250.
+  Crosses, reciprocal, 258.
+  Crossing of domestic animals, importance in altering breeds, 20.
+  ----, advantages of, 96.
+  ---- unfavourable to selection, 102.
+  Crustacea of New Zealand, 376.
+  Crustacean, blind, 137.
+  Cryptocerus, 239.
+  Ctenomys, blind, 137.
+  Cuckoo, instinct of, 216.
+  Currants, grafts of, 262.
+  Currents of sea, rate of, 360.
+  Cuvier on conditions of existence, 206.
+  ---- on fossil monkeys, 304.
+  ----, Fred., on instinct, 208.
+
+          D.
+
+  Dana, Prof., on blind cave-animals, 139.
+  ----, on relations of crustaceans of Japan, 372.
+  ----, on crustaceans of New Zealand, 376.
+  De Candolle on struggle for existence, 62.
+  ---- on umbelliferæ, 146.
+  ---- on general affinities, 430.
+  ----, Alph., on low plants, widely dispersed, 406.
+  ----, ----, on widely-ranging plants being variable, 53.
+  ----, ----, on naturalisation, 115.
+  ----, ----, on winged seeds, 146.
+  ----, ----, on Alpine species suddenly becoming rare, 175.
+  ----, ----, on distribution of plants with large seeds, 360.
+  ----, ----, on vegetation of Australia, 379.
+  ----, ----, on fresh-water plants, 386.
+  ----, ----, on insular plants, 389.
+  Degradation of coast-rocks, 282.
+  Denudation, rate of, 285.
+  ---- of oldest rocks, 308.
+  Development of ancient forms, 336.
+  Devonian system, 334.
+  Dianthus, fertility of crosses, 256.
+  Dirt on feet of birds, 362.
+  Dispersal, means of, 356.
+  ---- during glacial period, 365.
+  Distribution, geographical, 346.
+  ----, means of, 356.
+  Disuse, effects of, under nature, 134.
+  Divergence of character, 111.
+  Division, physiological, of labour, 115.
+  Dogs, hairless, with imperfect teeth, 12.
+  ---- descended from several wild stocks, 18.
+  ----, domestic instincts of, 213.
+  ----, inherited civilisation of, 215.
+  ----, fertility of breeds together, 254.
+  ----, ---- of crosses, 268.
+  ----, proportions of, when young, 444.
+  Domestication, variation under, 7.
+  Downing, Mr., on fruit-trees in America, 85.
+  Downs, North and South, 286.
+  Dragon-flies, intestines of, 190.
+  Drift-timber, 360.
+  Driver-ant, 240.
+  Drones killed by other bees, 202.
+  Duck, domestic, wings of, reduced, 11.
+  ----, logger-headed, 182.
+  {494}
+  Duckweed, 385.
+  Dugong, affinities of, 414.
+  Dung-beetles with deficient tarsi, 135.
+  Dyticus, 386.
+
+          E.
+
+  Earl, Mr. W., on the Malay Archipelago, 395.
+  Ears, drooping, in domestic animals, 11.
+  ----, rudimentary, 454.
+  Earth, seeds in roots of trees, 361.
+  Eciton, 238.
+  Economy of organisation, 147.
+  Edentata, teeth and hair, 144.
+  ----, fossil species of, 339.
+  Edwards, Milne, on physiological divisions of labour, 115.
+  ----, on gradations of structure, 194.
+  ----, on embryonical characters, 418.
+  Eggs, young birds escaping from, 87.
+  Electric organs, 192.
+  Elephant, rate of increase, 64.
+  ---- of glacial period, 141.
+  Embryology, 438.
+  Existence, struggle for, 60.
+  ----, conditions of, 206.
+  Extinction, as bearing on natural selection, 109.
+  ---- of domestic varieties, 111,
+  ----, 317.
+  Eye, structure of, 187.
+  ----, correction for aberration, 202.
+  Eyes reduced in moles, 137.
+
+          F.
+
+  Fabre, M. on parasitic sphex, 218.
+  Falconer, Dr., on naturalisation of plants in India, 65.
+  ---- on fossil crocodile, 313.
+  ---- on elephants and mastodons, 334.
+  ---- and Cautley on mammals of sub-Himalayan beds, 340.
+  Falkland Island, wolf of, 394.
+  Faults, 285.
+  Faunas, marine, 348.
+  Fear, instinctive, in birds, 212.
+  Feet of bird, young molluscs adhering to, 385.
+  Fertility of hybrids, 249.
+  ---- from slight changes in conditions, 267.
+  ---- of crossed varieties, 268.
+  Fir-trees destroyed by cattle, 72.
+  ---- ----, pollen of, 203.
+  Fish, flying, 182.
+  ----, teleostean, sudden appearance of, 305.
+  ---- eating seeds, 362, 387.
+  ----, fresh-water, distribution of, 384.
+  Fishes, ganoid, now confined to fresh water, 107.
+  ----, electric organs of, 192.
+  ----, ganoid, living in fresh water, 321.
+  ---- of southern hemisphere, 376.
+  Flight, powers of, how acquired, 182.
+  Flowers, structure of, in relation to crossing, 97.
+  ---- of compositæ and umbelliferæ, 144.
+  Forbes, E., on colours of shells, 132.
+  ---- on abrupt range of shells in depth, 175.
+  ---- on poorness of palæontological collections, 288.
+  ---- on continuous succession of genera, 316.
+  ---- on continental extensions, 357.
+  ---- on distribution during glacial period, 366.
+  ---- on parallelism in time and space, 409.
+  Forests, changes in, in America, 74.
+  Formation, Devonian, 334.
+  Formations, thickness of, in Britain, 284.
+  ----, intermittent, 290.
+  Formica rufescens, 219.
+  ---- sanguinea, 219.
+  ---- flava, neuter of, 240.
+  Frena, ovigerous, of cirripedes, 192.
+  Fresh-water productions, dispersal of, 383.
+  Fries on species in large genera being closely allied to other species,
+      57.
+  Frigate-bird, 185.
+  Frogs on islands, 393.
+  Fruit-trees, gradual improvement of, 37.
+  ---- ---- in United States, 85.
+  ---- ----, varieties of, acclimatised in United States, 142.
+  {495}
+  Fuci, crossed, 258.
+  Fur, thicker in cold climates, 133.
+  Furze, 439.
+
+          G.
+
+  Galapagos Archipelago, birds of, 390.
+  ----, productions of, 398, 400.
+  Galeopithecus, 181.
+  Game, increase of, checked by vermin, 68.
+  Gärtner on sterility of hybrids, 247, 255.
+  ----, on reciprocal crosses, 258.
+  ----, on crossed maize and verbascum, 270.
+  ----, on comparison of hybrids and mongrels, 272.
+  Geese, fertility when crossed, 253.
+  ----, upland, 185.
+  Genealogy important in classification, 425.
+  Geoffroy St. Hilaire on balancement, 147.
+  ---- ---- on homologous organs, 434.
+  ---- ----, Isidore, on variability of repeated parts, 149.
+  ---- ----, on correlation in monstrosities, 11.
+  ---- ----, on correlation, 144.
+  ---- ----, on variable parts being often monstrous, 155.
+  Geographical distribution, 346.
+  Geography, ancient, 487.
+  Geology, future progress of, 487.
+  ----, imperfection of the record, 279.
+  Giraffe, tail of, 195.
+  Glacial period, 365.
+  Gmelin on distribution, 365.
+  Gnathodon, fossil, 368.
+  Godwin-Austen, Mr., on the Malay Archipelago, 300.
+  Goethe on compensation of growth, 147.
+  Gooseberry, grafts of, 262.
+  Gould, Dr. A., on land-shells, 397.
+  ----, Mr., on colours of birds, 132.
+  ----, on birds of the Galapagos, 398.
+  ----, on distribution of genera of birds, 404.
+  Gourds, crossed, 270.
+  Grafts, capacity of, 261.
+  Grasses, varieties of, 113.
+  Gray, Dr. Asa, on trees of United States, 100.
+  ----, on naturalised plants in the United States, 115.
+  ----, on rarity of intermediate varieties, 176.
+  ----, on Alpine plants, 365.
+  ----, Dr. J. E., on striped mule, 165.
+  Grebe, 185.
+  Groups, aberrant, 429.
+  Grouse, colours of, 84.
+  ----, red, a doubtful species, 49.
+  Growth, compensation of, 147.
+  ----, correlation of, in domestic products, 11.
+  ----, correlation of, 143.
+
+          H.
+
+  Habit, effect of, under domestication, 11.
+  ----, effect of, under nature, 134.
+  ----, diversified, of same species, 183.
+  Hair and teeth, correlated, 144.
+  Harcourt, Mr. E. V., on the birds of Madeira, 391.
+  Hartung, M. on boulders in the Azores, 363.
+  Hazel-nuts, 359.
+  Hearne on habits of bears, 184.
+  Heath, changes in vegetation, 72.
+  Heer, O., on plants of Madeira, 107.
+  Helix pomatia, 397.
+  Helosciadium, 359.
+  Hemionus, striped, 163.
+  Herbert, W., on struggle for existence, 62.
+  ----, on sterility of hybrids, 249.
+  Hermaphrodites crossing, 96.
+  Heron eating seed, 387.
+  Heron, Sir R., on peacocks, 89.
+  Heusinger on white animals not poisoned by certain plants, 12.
+  Hewitt, Mr., on sterility of first crosses, 264.
+  Himalaya, glaciers of, 373.
+  ----, plants of, 375.
+  Hippeastrum, 250.
+  Holly-trees, sexes of, 93.
+  Hollyhock, varieties of, crossed, 271.
+  Hooker, Dr., on trees of New Zealand, 100.
+  {496}
+  ----, on acclimatisation of Himalayan trees, 140.
+  ----, on flowers of umbelliferæ, 145.
+  ----, on glaciers of Himalaya, 373.
+  ----, on algæ of New Zealand, 376.
+  ----, on vegetation at the base of the Himalaya, 378.
+  ----, on plants of Tierra del Fuego, 374, 378.
+  ----, on Australian plants, 375, 399.
+  ----, on relations of flora of South America, 379.
+  ----, on flora of the Antarctic lands, 381, 399.
+  ----, on the plants of the Galapagos, 392, 398.
+  Hooks on bamboos, 197.
+  ---- to seeds on islands, 392.
+  Horner, Mr., on the antiquity of Egyptians, 18.
+  Horns, rudimentary, 454.
+  Horse, fossil, in La Plata, 318.
+  Horses destroyed by flies in La Plata, 72.
+  ----, striped, 163.
+  ----, proportions of, when young, 444.
+  Horticulturists, selection applied by, 32.
+  Huber on cells of bees, 230.
+  ----, P., on reason blended with instinct, 208.
+  ----, on habitual nature of instincts, 208.
+  ----, on slave-making ants, 219.
+  ----, on Melipona domestica, 225.
+  Humble-bees, cells of, 225.
+  Hunter, J., on secondary sexual characters, 150.
+  Hutton, Captain, on crossed geese, 254.
+  Huxley, Prof., on structure of hermaphrodites, 101.
+  ----, on embryological succession, 338.
+  ----, on homologous organs, 438.
+  ----, on the development of aphis, 442.
+  Hybrids and mongrels compared, 272.
+  Hybridism, 245.
+  Hydra, structure of, 190.
+
+          I.
+
+  Ibla, 148.
+  Icebergs transporting seeds, 363.
+  Increase, rate of, 63.
+  Individuals, numbers favourable to selection, 102.
+  ----, many, whether simultaneously created, 355.
+  Inheritance, laws of, 12.
+  ---- at corresponding ages, 14, 86.
+  Insects, colour of, fitted for habitations, 84.
+  ----, sea-side, colours of, 132.
+  ----, blind, in caves, 138.
+  ----, luminous, 193.
+  ----, neuter, 236.
+  Instinct, 207.
+  Instincts, domestic, 213.
+  Intercrossing, advantages of, 96.
+  Islands, oceanic, 388.
+  Isolation favourable to selection, 104.
+
+          J.
+
+  Japan, productions of, 372.
+  Java, plants of, 375.
+  Jones, Mr. J. M., on the birds of Bermuda, 391.
+  Jussieu on classification, 417.
+
+          K.
+
+  Kentucky, caves of, 137.
+  Kerguelen-land, flora of, 381, 399.
+  Kidney-bean, acclimatisation of, 142.
+  Kidneys of birds, 144.
+  Kirby on tarsi deficient in beetles, 135.
+  Knight, Andrew, on cause of variation, 7.
+  Kölreuter on the barberry, 98.
+  ---- on sterility of hybrids, 246.
+  ---- on reciprocal crosses, 258.
+  ---- on crossed varieties of nicotiana, 271.
+  ---- on crossing male and hermaphrodite flowers, 451.
+
+          L.
+
+  Lamarck on adaptive characters, 426.
+  Land-shells, distribution of, 397.
+  ---- of Madeira, naturalised, 403.
+  Languages, classification of, 422.
+  Lapse, great, of time, 282.
+  {497}
+  Larvæ, 440.
+  Laurel, nectar secreted by the leaves,
+  Laws of variation, 131.
+  Leech, varieties of, 76.
+  Leguminosæ, nectar secreted by glands, 92.
+  Lepidosiren, 107, 330.
+  Life, struggle for, 60.
+  Lingula, Silurian, 307.
+  Linnæus, aphorism of, 413.
+  Lion, mane of, 88.
+  ----, young of, striped, 439.
+  Lobelia fulgens, 73, 98.
+  Lobelia, sterility of crosses, 250.
+  Loess of the Rhine, 384.
+  Lowness of structure connected with variability, 149.
+  Lowness, related to wide distribution, 406.
+  Lubbock, Mr., on the nerves of coccus, 46.
+  Lucas, Dr. P., on inheritance, 12.
+  ----, on resemblance of child to parent, 275.
+  Lund and Clausen on fossils of Brazil, 339.
+  Lyell, Sir C, on the struggle for existence, 62.
+  ----, on modern changes of the earth, 95.
+  ----, on measure of denudation, 284.
+  ----, on a carboniferous land-shell, 289.
+  ----, on strata beneath Silurian system, 308.
+  ----, on the imperfection of the geological record, 311.
+  ----, on the appearance of species, 312.
+  ----, on Barrande's colonies, 313.
+  ----, on tertiary formations of Europe and North America, 323.
+  ----, on parallelism of tertiary formations, 328.
+  ----, on transport of seeds by icebergs, 363.
+  ----, on great alternations of climate, 382.
+  ----, on the distribution of fresh-water shells, 385.
+  ----, on land-shells of Madeira, 402.
+  Lyell and Dawson on fossilized trees in Nova Scotia, 297.
+
+          M.
+
+  Macleay on analogical characters, 426.
+  Madeira, plants of, 107.
+  ----, beetles of, wingless, 135.
+  ----, fossil land-shells of, 339.
+  ----, birds of, 390.
+  Magpie tame in Norway, 212.
+  Maize, crossed, 270.
+  Malay Archipelago compared with Europe, 300.
+  ----, mammals of, 395.
+  Malpighiaceæ, 417.
+  Mammæ, rudimentary, 451.
+  Mammals, fossil, in secondary formation, 304.
+  ----, insular, 394.
+  Man, origin of races of, 199.
+  Manatee, rudimentary nails of, 454.
+  Marsupials of Australia, 116.
+  ----, fossil species of, 339.
+  Martens, M., experiment on seeds, 360.
+  Martin, Mr. W. C., on striped mules, 165.
+  Matteucci on the electric organs of rays, 193.
+  Matthiola, reciprocal crosses of, 258.
+  Means of dispersal, 356.
+  Melipona domestica, 225.
+  Metamorphism of oldest rocks, 308.
+  Mice destroying bees, 74.
+  ----, acclimatisation of, 141.
+  Migration, bears on first appearance of fossils, 297.
+  Miller, Prof., on the cells of bees, 226.
+  Mirabilis, crosses of, 258.
+  Missel-thrush, 76.
+  Misseltoe, complex relations of, 3.
+  Mississippi, rate of deposition at mouth, 284.
+  Mocking-thrush of the Galapagos, 402.
+  Modification of species, how far applicable, 483.
+  Moles, blind, 137.
+  Mongrels, fertility and sterility of, 268.
+  ---- and hybrids compared, 272.
+  {498}
+  Monkeys, fossil, 304.
+  Monocanthus, 424.
+  Mons, Van, on the origin of fruit-trees, 29.
+  Moquin-Tandon on sea-side plants, 132.
+  Morphology, 433.
+  Mozart, musical powers of, 209.
+  Mud, seeds in, 386.
+  Mules, striped, 165.
+  Müller, Dr. F., on Alpine Australian plants, 375.
+  Murchison, Sir R., on the formations of Russia, 290.
+  ----, on azoic formations, 308.
+  ----, on extinction, 317.
+  Mustela vison, 179.
+  Myanthus, 424.
+  Myrmecocystus, 239.
+  Myrmica, eyes of, 240.
+
+          N.
+
+  Nails, rudimentary, 454.
+  Natural history, future progress of, 485.
+  ---- selection, 80.
+  ---- system, 413.
+  Naturalisation of forms distinct from the indigenous species, 115.
+  ---- in New Zealand, 201.
+  Nautilus, Silurian, 307.
+  Nectar of plants, 92.
+  Nectaries, how formed, 92.
+  Nelumbium luteum, 387.
+  Nests, variation in, 211.
+  Neuter insects, 236.
+  Newman, Mr., on humble-bees, 74.
+  New Zealand, productions of, not perfect, 201.
+  ----, naturalised products of, 337.
+  ----, fossil birds of, 339.
+  ----, glacial action in, 373.
+  ----, crustaceans of, 376.
+  ----, algæ of, 376.
+  ----, number of plants of, 389.
+  ----, flora of, 399.
+  Nicotiana, crossed varieties of, 271.
+  ----, certain species very sterile, 257.
+  Noble, Mr., on fertility of Rhododendron, 252.
+  Nodules, phosphatic, in azoic rocks, 308.
+
+          O.
+
+  Oak, varieties of, 50.
+  Onites apelles, 135.
+  Orchis, pollen of, 193.
+  Organs of extreme perfection, 186.
+  ----, electric, of fishes, 192.
+  ---- of little importance, 194.
+  ----, homologous, 434.
+  ----, rudiments of, and nascent, 450.
+  Ornithorhynchus, 107, 416.
+  Ostrich not capable of flight, 134.
+  ----, habit of laying eggs together, 218.
+  ----, American, two species of, 349.
+  Otter, habits of, how acquired, 179.
+  Ouzel, water, 185.
+  Owen, Prof., on birds not flying, 134.
+  ----, on vegetative repetition, 149.
+  ----, on variable length of arms in ourang-outang, 150.
+  ----, on the swim-bladder of fishes, 191.
+  ----, on electric organs, 192.
+  ----, on fossil horse of La Plata, 319.
+  ----, on relations of ruminants and pachyderms, 329.
+  ----, on fossil birds of New Zealand, 339.
+  ----, on succession of types, 339.
+  ----, on affinities of the dugong, 414.
+  ----, on homologous organs, 434.
+  ----, on the metamorphosis of cephalopods and spiders, 442.
+
+          P.
+
+  Pacific Ocean, faunas of, 348.
+  Paley on no organ formed to give pain, 201.
+  Pallas on the fertility of the wild stocks of domestic animals, 254.
+  Paraguay, cattle destroyed by flies, 72.
+  Parasites, 217.
+  Partridge, dirt on feet, 363.
+  Parts greatly developed, variable, 150.
+  ----, degrees of utility of, 201.
+  Parus major, 184.
+  Passiflora, 251.
+  Peaches in United States, 85.
+  Pear, grafts of, 262.
+  {499}
+  Pelargonium, flowers of, 145.
+  ----, sterility of, 251.
+  Pelvis of women, 144.
+  Peloria, 145.
+  Period, glacial, 365.
+  Petrels, habits of, 184.
+  Phasianus, fertility of hybrids, 253.
+  Pheasant, young, wild, 216.
+  Philippi on tertiary species in Sicily, 312.
+  Pictet, Prof., on groups of species suddenly appearing, 302, 305.
+  ----, on rate of organic change, 313.
+  ----, on continuous succession of genera, 316.
+  ----, on close alliance of fossils in consecutive formations, 335.
+  ----, on embryological succession, 338.
+  Pierce, Mr., on varieties of wolves, 91.
+  Pigeons with feathered feet and skin between toes, 12.
+  ----, breeds described, and origin of, 20.
+  ----, breeds of, how produced, 39, 42.
+  ----, tumbler, not being able to get out of egg, 87.
+  ----, reverting to blue colour, 160.
+  ----, instinct of tumbling, 214.
+  ----, carriers, killed by hawks, 362.
+  ----, young of, 445.
+  Pistil, rudimentary, 451.
+  Plants, poisonous, not affecting certain coloured animals, 12.
+  ----, selection applied to, 32.
+  ----, gradual improvement of, 37.
+  ---- not improved in barbarous countries, 38.
+  ---- destroyed by insects, 67.
+  ----, in midst of range, have to struggle with other plants, 77.
+  ----, nectar of, 92.
+  ----, fleshy, on sea-shores, 132.
+  ----, fresh-water, distribution of, 386.
+  ----, low in scale, widely distributed, 406.
+  Plumage, laws of change in sexes of birds, 89.
+  Plums in the United States, 85.
+  Pointer dog, origin of, 35.
+  ----, habits of, 213.
+  Poison not affecting certain coloured animals, 12.
+  ----, similar effect of, on animals and plants, 484.
+  Pollen of fir-trees, 203.
+  Poole, Col., on striped hemionus, 163.
+  Potamogeton, 387.
+  Prestwich, Mr., on English and French eocene formations, 328.
+  Primrose, 49.
+  ----, sterility of, 247.
+  Primula, varieties of, 49.
+  Proteolepas, 148.
+  Proteus, 139.
+  Psychology, future progress of, 489.
+
+          Q.
+
+  Quagga, striped, 165.
+  Quince, grafts of, 262.
+
+          R.
+
+  Rabbit, disposition of young, 215.
+  Races, domestic, characters of, 16.
+  Race-horses, Arab, 35.
+  ----, English, 356.
+  Ramond on plants of Pyrenees, 368.
+  Ramsay, Prof., on thickness of the British formations, 284.
+  ----, on faults, 285.
+  Ratio of increase, 63.
+  Rats, supplanting each other, 76.
+  ----, acclimatisation of, 141.
+  ----, blind in cave, 137.
+  Rattle-snake, 201.
+  Reason and instinct, 208.
+  Recapitulation, general, 459.
+  Reciprocity of crosses, 258.
+  Record, geological, imperfect, 279.
+  Rengger on flies destroying cattle, 72.
+  Reproduction, rate of, 63.
+  Resemblance to parents in mongrels and hybrids, 273.
+  Reversion, law of inheritance, 14.
+  ---- in pigeons to blue colour, 160.
+  Rhododendron, sterility of, 251.
+  Richard, Prof., on Aspicarpa, 417.
+  Richardson, Sir J., on structure of squirrels, 180.
+  ----, on fishes of the southern hemisphere, 376.
+  Robinia, grafts of, 262.
+  {500}
+  Rodents, blind, 137.
+  Rudimentary organs, 450.
+  Rudiments important for classification, 416.
+
+          S.
+
+  Sagaret on grafts, 262.
+  Salmons, males fighting, and hooked jaws of, 88.
+  Salt-water, how far injurious to seeds, 358.
+  Saurophagus sulphuratus, 183.
+  Schiödte on blind insects, 138.
+  Schlegel on snakes, 144.
+  Sea-water, how far injurious to seeds, 358.
+  Sebright, Sir J., on crossed animals, 20.
+  ----, on selection of pigeons, 31.
+  Sedgwick, Prof., on groups of species suddenly appearing, 302.
+  Seedlings destroyed by insects, 67.
+  Seeds, nutriment in, 77.
+  ----, winged, 146.
+  ----, power of resisting salt-water, 358.
+  ---- in crops and intestines of birds, 361.
+  ---- eaten by fish, 362, 387.
+  ---- in mud, 386.
+  ----, hooked, on islands, 392.
+  Selection of domestic products, 29.
+  ----, principle not of recent origin, 33.
+  ----, unconscious, 34.
+  ----, natural, 80.
+  ----, sexual, 87.
+  ----, natural, circumstances favourable to, 102.
+  Sexes, relations of, 87.
+  Sexual characters variable, 156.
+  ---- selection, 87.
+  Sheep, Merino, their selection, 31.
+  ----, two sub-breeds unintentionally produced, 36.
+  ----, mountain, varieties of, 76.
+  Shells, colours of, 132.
+  ----, littoral, seldom embedded, 288.
+  ----, fresh-water, dispersal of, 385
+  ---- of Madeira, 391.
+  ----, land, distribution of, 397.
+  Silene, fertility of crosses, 257.
+  Silliman, Prof., on blind rat, 137.
+  Skulls of young mammals, 197, 436.
+  Slave-making instinct, 219.
+  Smith, Col. Hamilton, on striped horses, 164.
+  ----, Mr. Fred., on slave-making ants, 219.
+  ----, on neuter ants, 239.
+  ----, Mr., of Jordan Hill, on the degradation of coast-rocks, 283.
+  Snap-dragon, 161.
+  Somerville, Lord, on selection of sheep, 31.
+  Sorbus, grafts of, 262.
+  Spaniel, King Charles's breed, 35.
+  Species, polymorphic, 46.
+  ----, common, variable, 53.
+  ---- in large genera variable, 54.
+  ----, groups of, suddenly appearing, 302, 307.
+  ---- beneath Silurian formations, 307.
+  ---- successively appearing, 312.
+  ---- changing simultaneously throughout the world, 322.
+  Spencer, Lord, on increase in size of cattle, 35.
+  Sphex, parasitic, 218.
+  Spiders, development of, 442.
+  Spitz-dog crossed with fox, 268.
+  Sports in plants, 9.
+  Sprengel, C. C, on crossing, 98.
+  ----, on ray-florets, 145.
+  Squirrels, gradations in structure, 180.
+  Staffordshire, heath, changes in, 71.
+  Stag-beetles, fighting, 88.
+  Sterility from changed conditions of life, 9.
+  ---- of hybrids, 246.
+  ---- ----, laws of, 255.
+  ---- ----, causes of, 263.
+  ---- from unfavourable conditions, 265.
+  ---- of certain varieties, 269.
+  St. Helena, productions of, 390.
+  St. Hilaire, Aug., on classification, 418.
+  St. John, Mr., on habits of cats, 91.
+  Sting of bee, 202.
+  Stocks, aboriginal, of domestic animals, 18.
+  Strata, thickness of, in Britain, 284.
+  Stripes on horses, 163.
+  {501}
+  Structure, degrees of utility of, 201.
+  Struggle for existence, 60.
+  Succession, geological, 312.
+  Succession of types in same areas, 338.
+  Swallow, one species supplanting another, 76.
+  Swim-bladder, 190.
+  System, natural, 413.
+
+          T.
+
+  Tail of giraffe, 195.
+  ---- of aquatic animals, 196.
+  ----, rudimentary, 454.
+  Tarsi deficient, 135.
+  Tausch on umbelliferous flowers, 146.
+  Teeth and hair correlated, 144.
+  ----, embryonic, traces of, in birds, 450.
+  ----, rudimentary, in embryonic calf, 450, 480.
+  Tegetmeier, Mr., on cells of bees, 228, 233.
+  Temminck on distribution aiding classification, 419.
+  Thouin on grafts, 262.
+  Thrush, aquatic species of, 185.
+  ----, mocking, of the Galapagos, 402.
+  ----, young of, spotted, 439.
+  ----, nest of, 243.
+  Thuret, M., on crossed fuci, 258.
+  Thwaites, Mr., on acclimatisation, 140.
+  Tierra del Fuego, dogs of, 215.
+  ----, plants of, 374, 378.
+  Timber-drift, 360.
+  Time, lapse of, 282.
+  Titmouse, 184.
+  Toads on islands, 393.
+  Tobacco, crossed varieties of, 271.
+  Tomes, Mr., on the distribution of bats, 395.
+  Transitions in varieties rare, 172.
+  Trees on islands belong to peculiar orders, 392.
+  ---- with separated sexes, 99.
+  Trifolium pratense, 73, 94.
+  ---- incarnatum, 94.
+  Trigonia, 321.
+  Trilobites, 307.
+  ----, sudden extinction of, 321.
+  Troglodytes, 243.
+  Tucutucu, blind, 137.
+  Tumbler pigeons, habits of, hereditary, 214.
+  ----, young of, 446.
+  Turkey-cock, brush of hair on breast, 90.
+  Turkey, naked skin on head, 197.
+  ----, young, wild, 216.
+  Turnip and cabbage, analogous variations of, 159.
+  Type, unity of, 206.
+  Types, succession of, in same areas, 339.
+
+          U.
+
+  Udders enlarged by use, 11.
+  ----, rudimentary, 451.
+  Ulex, young leaves of, 439.
+  Umbelliferæ, outer and inner florets of, 144.
+  Unity of type, 206.
+  Use, effects of, under domestication, 11.
+  ----, effects of, in a state of nature, 134.
+  Utility, how far important in the construction of each part, 199.
+
+          V.
+
+  Valenciennes on fresh-water fish, 384.
+  Variability of mongrels and hybrids, 274.
+  Variation under domestication, 7.
+  ---- caused by reproductive system being affected by conditions of life,
+      8.
+  ---- under nature, 44.
+  ----, laws of, 131.
+  Variations appear at corresponding ages, 14, 86.
+  ----, analogous in distinct species, 159.
+  Varieties, natural, 44.
+  ----, struggle between, 75.
+  ----, domestic, extinction of, 111.
+  ----, transitional, rarity of, 172.
+  ----, when crossed, fertile, 268.
+  ----, when crossed, sterile, 269.
+  ----, classification of, 423.
+  Verbascum, sterility of, 251.
+  ----, varieties of, crossed, 271.
+  Verneuil, M. de, on the succession of species, 325.
+  Viola tricolor, 73.
+  {502}
+  Volcanic islands, denudation of, 285.
+  Vulture, naked skin on head, 197.
+
+          W.
+
+  Wading-birds, 386.
+  Wallace, Mr., on origin of species, 2.
+  ----, on law of geographical distribution, 355.
+  ----, on the Malay Archipelago, 395.
+  Wasp, sting of, 202.
+  Water, fresh, productions of, 383.
+  Water-hen, 185.
+  Waterhouse, Mr., on Australian marsupials, 116.
+  ----, on greatly developed parts being variable, 150.
+  ----, on the cells of bees, 225.
+  ----, on general affinities, 429.
+  Water-ouzel, 185.
+  Watson, Mr. H. C, on range of varieties of British plants, 58.
+  ----, on acclimatisation, 140.
+  ----, on flora of Azores, 363.
+  ----, on Alpine plants, 368, 376.
+  ----, on rarity of intermediate varieties, 176.
+  Weald, denudation of, 285.
+  Web of feet in water-birds, 185.
+  West Indian islands, mammals of, 396.
+  Westwood on species in large genera being closely allied to others, 57.
+  ---- on the tarsi of Engidæ, 157.
+  ---- on the antennæ of hymenopterous insects, 415.
+  Wheat, varieties of, 113.
+  White Mountains, flora of, 365.
+  Wings, reduction of size, 134.
+  ---- of insects homologous with branchiæ, 191.
+  ----, rudimentary, in insects, 450.
+  Wolf crossed with dog, 214.
+  ---- of Falkland Isles, 394.
+  Wollaston, Mr., on varieties of insects, 48.
+  ----, on fossil varieties of land-shells in Madeira, 52.
+  ----, on colours of insects on sea-shore, 132.
+  ----, on wingless beetles, 135.
+  ----, on rarity of intermediate varieties, 176.
+  ----, on insular insects, 389.
+  ----, on land-shells of Madeira, naturalised, 402.
+  Wolves, varieties of, 90.
+  Woodpecker, habits of, 184.
+  ----, green colour of, 197.
+  Woodward, Mr., on the duration of specific forms, 294.
+  ----, on the continuous succession of genera, 316.
+  ----, on the succession of types, 339.
+  World, species changing simultaneously throughout, 322.
+  Wrens, nest of, 243.
+
+          Y.
+
+  Youatt, Mr., on selection, 31.
+  ----, on sub-breeds of sheep, 36.
+  ----, on rudimentary horns in young cattle, 454.
+
+          Z.
+
+  Zebra, stripes on, 163.
+
+THE END.
+
+       *       *       *       *       *
+
+LONDON: PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET, AND CHARING
+CROSS.
+
+       *       *       *       *       *
+
+
+Corrections made to printed original.
+
+p. 133. "the slightest use to a being": 'slighest' in original.
+
+p. 193. "as Matteucci asserts": 'Matteucei' in original (the index
+correctly has Matteucci).
+
+p. 201. "deposited in the living bodies of other insects": 'depo-sisted'
+(across page break) in original.
+
+p. 315. "the newly-formed fantail": 'faintail' in original.
+
+p. 398. "the volcanic nature of the soil": 'volanic' in original.
+
+p. 403. "Madeira and the adjoining islet": 'Maderia' in original; and so in
+"from Porto Santo to Madeira".
+
+p. 442. "the same individual embryo": 'indivividual' in original.
+
+p. 458. "innumerable species, genera, and families": 'inumerable' in
+original.
+
+p. 490. "Inheritance which is almost implied by reproduction":
+'Inheritrnce' in original.
+
+
+
+
+
+
+End of the Project Gutenberg EBook of On the Origin of Species by Means of
+Natural Selection, by Charles Darwin
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