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THE HARVARD CLASSICS

EDITED BY CHARLES W ELIOT LL D

THE ORIGIN OF SPECIES

BY CHARLES DARWIN

WITH INTRODUCTIONS, NOTES AND ILLUSTRATIONS

'DR ELIOT'S FIVE-FOOT SHELF OF BOOKS"

\?^*>-^^f

P F COLLIER & SON NEW YORK

Copyright, 1909 By p. F. Collier & Son

"But with regard to the material world, we can at least go so far as this — we can perceive tliat events are brought about not by insulated interpositions of Divine power, exerted in each particular case, but by the estab- lishment of general laws."

VViiEWELL: Bridgewnter 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 con- ceit 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, Beckenham, Kent,

First Edition, 'November 2^th, 1859. Sixth Edition, Jamuary, 18112.

A— HC XI

CONTENTS

PAGE

EDITOR'S INTRODUCTION ....0.5

AN HISTORICAL SKETCH

Of the Progress of Opinion on the Origin of Species 9

INTRODUCTION 21

CHAPTER I Variation under Domestication 25

CHAPTER II Variation under Nature 58

CHAPTER III Struggle for Existence 76

CHAPTER IV Natural Selection; or the Survival op the Fittest 93

CHAPTER V Laws of Variation ^45

CHAPTER VI Difficulties of the Theory i/S

CONTENTS

CHAPTER VII PAGE

Miscellaneous Objections to the Theory of Natural Selection 219

CHAPTER VIII Instinct 262

CHAPTER IX Hybridism 298

CHAPTER X On the Imperfection of the Geological Record . 353

CHAPTER XI On the Geological Succession of Organic Beinss , 364

CHAPTER XII Geographical Distribution 395

CHAPTER XIII Geographical Distribution — continued .... 427

CHAPTER XIV

Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs .... 450

CHAPTER XV Recapitulation and Conclusion 499

GLOSSARY 531

INDEX 541

INTRODUCTORY NOTE

Charles Robert Darwin, horn at Shrcii'sbiiry, England, on February 12, 1809, came of a family of remarkable intellectual distinction which is still sustained in the present generation. His father tvas a successful physician zvith remarkable powers of observation, and his grandfather was Erasmus Danvin, the well- knozvn author of "The Botanic Garden." He went to school at Shrewsbury, zvhere he failed to profit from the strict classical curriculum there in force; nor did the regular professional courses at Edinburgh University, zvhere he spent two years study- ing medicine, succeed in rousing his interest. In 1827 he was entered at Christ's College, Cambridge, to study for the B. A. degree, preparatory to entering the Church; hut zvhile there his friendship with Henslow, the professor of botany, led to his enlarging his general scientific knozvledge and finally to his joining the expedition of the "Beagle" in the capacity of natural- ist. From this Darzmn returned after a voyage of five years with a vast first-hand knozvledge of geology and zoology, a reputation as a successful collector, and, most important of all, with the germinal ideas of his theory of evolution. The next few years were spent in working up the materials he had col- lected; hut his health gave signs of breaking, and for the rest of his life he suffered constantly, but without complaint. With extraordinary courage and endurance he took up a life of seclusion and methodical regularity, and accomplished his colossal results in spite of the most severe physical handicap. He had married in 1839, and three years later he withdrczv from London to the little village of Dozvn, about sixteen miles out, zvhere he spent the rest of his life. His custom, zvhich zvas almost a method, was to work till he was on the verge of complete collapse, and then to take a holiday just sufficient to restore him to zvorking condition.

As early as 1842 Darwin had thrown into rough form the out- lines of his theory of evolution, hut the enormous extent of the inve'stigations he engaged in for the purpose of testing it led to a constant postponing of publication. Finally in June, 1S5S, A. R. Wallace sent him a manuscript containing a statement of an identical theory of the origin of species, which had been

5

6 INTRODUCTORY NOTE

arrived at entirely independently. On the advice of Lyell, the geologist, and Hooker, the botanist, Wallace's paper and a letter of Darivin's of the previous year, in ivhich he had outlined his theory to Asa Gray, zvere read together on July I, 1858, and published by the Linncean Society. In November of the follow- ing year "The Origin of Species" was published, and the great battle was begun between the old science and the new. This work was followed in 1868 by his "Variation of Animals and Plants under Domestication," that in turn by the "Descent of Man" in i8yi, and that again by "The Expression of the Emo- tions in Man and Animals." Each of these books was the elabo- ration or complement of a section of its predecessor. The later years of Darwin's life were chieAy devoted to botanical research, and resulted in a series of treatises of the highest scientific value. He died at Down on April ig, 1882, and is buried in Westminster Abbey.

The idea of the evolution of organisms, so far from originating with Darwin, is a very old one. Glimpses of it appear in the ancient Greek philosophers, especially Empedocles and Aristotle; modern philosophy from Bacon onward shows an increasing definiteness in its grasp of the conception; and in the age pre- ceding Darivin's, Buffon, Erasmus Darwin, and Lamarck had given it a fairly concrete expression. As we approach the date of the publication of "The Origin of Species" adherence to the doctrine not only by naturalists but by poets, such as Goethe, becomes comparatively frequent; and in the six years before the joint announcement of Darwin and Wallace, Herbert Spencer had been supporting and applying it vigorously in the field of psychology.

To these partial anticipations, however, Darwin oiued little. When he became interested in the problem, the doctrine of the fixity of species was still generally held; and his solution occurred to him mainly as the result of his own observation and thinking. Speaking of the voyage of the "Beagle," he says, "On my return home in the autumn of 1836 I immediately began to prepare my journal for publication, and then saw how many facts indicated the common descent of species. ... In July (1837) I opened my first note-book for facts in relation to the Origin of Species, about which I had long reflected, and never ceased working for the next twenty years. . . . Had been greatly struck from about

INTRODUCTORY NOTE 7

the month of previous March on character of South American fossils, and species on Galapagos Archipelago. These facts (especially latter) origin of all my views." Again, "In October (1838), that vs fifteen months after I had begun my systematic inquiry, I happened tr> read for amusement 'Malthus on Popu- lation,' and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued ob- servation of the habits of animals and plants, it at once struck me that under these circumstances favorable variations would tend to be preserved, and unfavorable ones to be destroyed. The result of this zvould be the formation of new species. Here then I had at laist got a theory by which to work."

From these statements by Darwin himself we can see how far it is from being the case that he merely gathered the ripe fruit of the labors of his predecessors. All progress is continuous, and Darwin, like other men, built on the foundations laid by others; but to say this is not to deny him originality in the only vital -sense of that word. And the importance of his contribution — in verifying the doctrine of descent, in interpreting and apply- ing it, and in revealing its bearings on all departments of the investigation of nature — is proved by the fact that his work opened a new epoch in science and philosophy. As Huxley said, "Whatever be the ultimate verdict of posterity upon this or that opinion which Mr. Darwin has propounded; whatever adumbra- tions or anticipations of his doctrines may be found in the writ- ings of his predecessors ; the broad fact remains that, since the publication and by reason of the publication of 'The Origin of Species' the fundamental conceptions and the aims of the students of living Nature have been completely changed."

The present year (1909) has seen the celebration of the hun- dredth anniversary of Darwin's birth and the fiftieth anniversary of the publication of his great work. Among the numerous ex- pressions of honor and gratitude which the world of science has poured upon his memory, none is more significant than the vol- ume on "Darwin and Modern Science" which has been issued by the press of his old University of Cambridge. In this are col- lected nearly thirty papers by the leaders of modern science dealing with the influence of Darwin upon various fields of thought and research, and with the later developments and modi- fications of his conclusions. Biology, in many different depart-

8 INTRODUCTORY NOTE

ments. Anthropology, Geology, Psychology, Philosophy, Soci- ology, Religion, Language, History, and Astronomy are all repre- sented, and the mere enumeration suggests the colossal nature of his achievement and its results.

Yet the spirit of the man was almost as wonderful as his work. His disinterestedness, his modesty, and his absolute fairness were not only beautiful in themselves, but remain as a proof of the importance of character in intellectual labor. Here is his own frank and candid summing up of his abilities: "My success as a man of science, whatever this may have amounted to, has been determined, as far as I can judge, by complex and diversified mental qualities and conditions. Of these, the most important have been — the love of science — unbounded patience in long re- flecting over any subject — industry in observing and collecting facts — and a fair share of invention as zvell as of common sense. With such moderate abilities as I possess, it is truly surprising that I should have influenced to a considerable extent the belief of scientific men on some important points."

AN HISTORICAL SKETCH

OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES

PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS WORK

I WILL here give a brief sketch of the progress of opinion on the Origin of Species. Until recently the great majority of naturalists believed that species were immutable productions, and had been separately created. This view has been ably maintained by many authors. Some few naturalists, on the other hand, have believed that species undergo modification, and that the existing forms of life are the descendants by true generation of pre-existing forms. Passing over allu- sions to the subject in the classical writers,* the first author who in modern times has treated it in a scientific spirit was Buffon. But as his opinions fluctuated greatly at different periods, and as he does not enter on the causes or means of the transformation of species, I need not here enter on details.

*Aristotle, in his ' Physicae Auscultationes ' (lib. 2, cap. 8, s. 2). after remarking that rain does not fall in order to make the corn grow, any more than it falls to spoil the farmer's corn when threshed out of doors, applies the same argument to organisation; and adds (as translated by Mr. Clair Grace, who first pointed out the passage to mc), "So what hinders the dif- ferent parts [of the body] from having this merely accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating the food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to the other parts in which there appears to exist an adaptation to an end. Wheresoever, therefore, all things together (that is all the parts of one whole) happened like as if they were made for the sake of something, these were preserved, having been appro- priately constituted by an internal spontaneity; and whatsoever things w;ere not thus constituted, perished, and still perish." We here sec tiie principle of natural selection shadowed forth, but how little Aristotle fully compre- hended the principle, is shown by his remarks on the formation of the teeth.

9

10 HISTORICAL SKETCH

Lamarck was the first man whose conclusions on the subject excited much attention. This justly-celebrated nat- uralist first published his views in 1801 ; he much enlarged them in 1809 in his 'Philosophic Zoologique,' and subse- quently, in 1815, in the Introduction to his 'Hist. Nat. des Animaux sans Vertebres.' In these works he upholds the doctrine that species, including man, are descended from other species. He first did the eminent service of arousing at- tention to the probability of all change in the organic, as well as in the inorganic world, being the result of law, and not of miraculous interposition. Lamarck seems to have been chiefly led to his conclusion on the gradual change of species, by the difficulty of distinguishing species and varieties, by the almost perfect gradation of forms in certain groups, and by the analogy of domestic productions. With respect to the means of modification, he attributed something to the direct action of the physical conditions of life, something to the crossing of already existing forms, and much to use and disuse, that is, to the efifects of habit. To this latter agency he seems to attribute all the beautiful adaptations in nature; — such as the long neck of the giraffe for browsing on the branches of trees. But he likewise believed in a law of progressive de- velopment; and as all the forms of life thus tend to progress, in order to account for the existence at the present day of simple productions, he maintains that such forms are now spontaneously generated.*

Geoffroy Saint-Hilaire, as is stated in his 'Life,' written by his son, suspected, as early as 1795, that what we call species are various degenerations of the same type. It was

* I have taken the date of the first publication of Lamarck from Isid. Geoffroy Saint-Hilaire]s ('Hist. Nat. Generale,' torn, ii, p. 405, 1859) excellent history of opinion on this subject. In this work a full account is given of Buffon's conclusions on the same subject. It is curious how largely my grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous grounds of opinion of Lamarck in his ' Zoonomia ' (vol. i. pp. 500-510), pub- lished in 1794. According to Isid. Geoffroy there is no doubt that Goethe was an extreme partisan of similar views, as shown in the Introduction to a work written in 1794 and 1795, but not published till long afterwards: he has pointedly remarked (' Goethe als Naturforscher,' von Dr. Karl Meding, s. 34) that the future question for naturalists will be how, for instance, cattle got their horns, and not for what they are used. It is rather a singu- lar instance of the manner in which similar views arise at about the same time, that Goethe in Germany, Dr. Darwin in England, and Geoffroy Saint- Hilaire (as we shall immediately see) in France, came to the same conclu- sion on the origin of species, in the years 1794-5.

HISTORICAL SKETCH U

not until 1828 that he published his conviction that the same forms have not been perpetuated since the origin of all things. Geoffroy seems to have relied chiefly on the condi- tion of life, or the "monde ambiant" as the cause of change. He was cautious in drawing conclusions, and did not believe that existing species are now undergoing modification; and, as his son adds, "C'est done un probleme a reserver entierement a I'avenir, suppose meme que I'avenir doive avoir prise sur lui."

In 1813, Dr. W. C. Wells read before the Royal Society 'An Account of a White female, part of whose skin re- sembles that of a Negro' ; but his paper was not published until his famous * Two Essays upon Dew and Single Vision' appeared in 1818. In this paper he distinctly recognises the principle of natural selection, and this is the first recognition which has been indicated ; but he applies it only to the races of man, and to certain characters alone. After remarking that negroes and mulattoes enjoy an immunity from certain trop- ical diseases, he observes, firstly, that all animals tend to vary in some degree, and, secondly, that agriculturists improve their domesticated animals by selection ; and then, he adds, but what is done in this latter case ''by art, seems to be done with equal efficacy, though more slowly, by nature, in the formation of varieties of mankind, fitted for the country which they inhabit. Of the accidental varieties of man, which would occur among the first few and scattered inhab- itants of the middle regions of Africa, some one would be better fitted than the others to bear the diseases of the coun- try. This race would consequently multiply, while the others would decrease; not only from their inability to sustain the attacks of disease, but from their incapacity of contending with their more vigorous neighbours. The colour of this vigorous race I take for granted, from what has been already said, would be dark. But the same disposition to form varie- ties still existing, a darker and a darker race would in the course of time occur : and as the darkest would be the best fitted for the climate, this would at length become the most prevalent, if not the only race, in the particular country in which it had originated." He then extends these same views to the white inhabitants of colder climates. I am indebted

12 HISTORICAL SKETCH

to Mr. Rowley, of the United States, for having called my attention, through Mr. Brace, to the above passage in Dr. Wells' work.

The Hon. and Rev. W. Herbert, afterwards Dean of Man- chester, in the fourth volume of the ' Horticultural Trans- actions,' 1822, and in his work of the 'Amaryllidacese' (1837, pp. 19, 339), declares that "horticultural experiments have established, beyond the possibility of refutation, that botanical species are only a higher and more permanent class of varieties." He extends the same view to animals. The Dean believes that single species of each genus were created in an originally highly plastic condition, and that these have produced, chiefly by intercrossing, but likewise by variation, all our existing species.

In 1826 Professor Grant, in the concluding paragraph in his well-known paper ('Edinburgh Philosophical Journal,' vol. xiv. p. 283) on the Spongilla, clearly declares his belief that species are descended from other species, and that they become improved in the course of modification. This same view was given in his 55th Lecture, published in the ' Lancet ' in 1834.

In 1831 Mr. Patrick Matthew published his work on 'Naval Timber and Arboriculture,' in which he gives precisely the same view on the origin of species as that (presently to be alluded to) propounded by Mr. Wallace and myself in the 'Linnean Journal,' and as that enlarged in the present volume. Unfortunately the view was given by Mr. Matthew very brief- ly in scattered passages in an Appendix to a work on a differ- ent subject, so that it remained unnoticed until Mr. Matthew himself drew attention to it in the 'Gardener's Chronicle,' on April 7th, i860. The differences of Mr. Matthew's view from mine are not of much importance: he seems to consider that the world was nearly depopulated at successive periods, and then re-stocked; and he gives as an alternative, that new forms may be generated " without the presence of any mould or germ of former aggregates." I am not sure that I under- stand some passages; but it seems that he attributes much influence to the direct action of the conditions of life. He clearly saw, however, the full force of the principle of natural selection.

HISTORICAL SKETCH 13

The celebrated geologist and naturalist, Von Buch', in his excellent 'Description Physique des Isles Canaries' (1836, p. 147), clearly expresses his belief that varieties slowly be- come changed into permanent species, which arc no longer capable of intercrossing.

Rafinesque, in his 'New Flora of North America,' pub- lished in 1836, wrote (p. 6) as follows : — " All species might have been varieties once, and many varieties are gradually becoming species by assuming constant and peculiar charac- ters;" but farther on (p. 18) he adds, "except the original types or ancestors of the genus."

In 1843-44 Professor Haldeman (' Boston Journal of Nat. Hist. U. States,' vol. iv. p. 468) has ably given the arguments for and against the hypothesis of the development and modi- fication of species : he seems to lean towards the side of change.

The 'Vestiges of Creation' appeared in 1844. In the tenth and much improved edition (1853) the anonymous author says (p. 155) : — " The proposition determined on after much consideration is, that the several series of animated beings, from the simplest and oldest up to the highest and most re- cent, are, under the providence of God, the results, first, of an impulse which has been imparted to the forms of life, ad- vancing them, in definite times, by generation, through grades of organisation terminating in the highest dicotyledons and vertebrata, these grades being few in number, and generally marked by intervals of organic character, which we find to be a practical difficulty in ascertaining affinities ; second, of another impulse connected with the vital forces, tending, in the course of generations, to modify organic structures in accordance with external circumstances, as food, the nature of the habitat, and the meteoric agencies, these being the ' adaptations ' of the natural theologian." The author ap- parently believes that organisation progresses by sudden leaps, but that the effects produced by the conditions of life are gradual. He argues with much force on general grounds that species are not immutable productions. But I cannot see how the two supposed " impulses " account in a scientific sense for the numerous and beautiful co-adaptations which we see throughout nature; I cannot see that we thus gain

14 HISTORICAL SKETCH

any insight how, for instance, a woodpecker has become adapted to its peculiar habits of life. The work, from its powerful and brilliant style, though displaying in the earlier editions little accurate knowledge and a great want of scientific caution, immediately had a very wide circulation. In my opinion it has done excellent service in this country in calling attention to the subject, in removing prejudice, and in thus preparing the ground for the reception of analogous views.

In 1846 the veteran geologist M. J. d'Omalius d'Halloy published in an excellent though short paper ('Bulletins de I'Acad. Roy. Bruxelles,' torn. xiii. p. 581) his opinion that it is more probable that new species have been produced by descent with modification than that they have been separately created: the author first promulgated this opinion in 1831.

Professor Owen, in 1849 ('Nature of Limbs,' p. 86), wrote as follows: — "The archetypal idea was manifested in the flesh under diverse such modifications, upon this planet, long prior to the existence of those animal species that actually exemplify it. To what natural laws or secondary causes the orderly succession and progression of such organic phenomena may have been committed, we, as yet, are igno- rant." In his Address to the British Association, in 1858, he speaks (p. li.) of "the axiom of the continuous operation of creative power, or of the ordained becoming of living things." Farther on (p. xc), after referring to geographical distribution, he adds, "These phenomena shake our confi- dence in the conclusion that the Apteryx of New Zealand and the Red Grouse of England were distinct creations in and for those islands respectively. Always, also, it may be well to bear in mind that by the word ' creation ' the zoologist means ' a process he knows not what' " He amplifies this idea by adding that when such cases as that of the Red Grouse are " enumerated by the zoologist as evidence of dis- tinct creation of the bird in and for such islands, he chiefly expresses that he knows not how the Red Grouse came to be there, and there exclusively ; signifying also, by this mode of expressing such ignorance, his belief that both the bird and the islands owed their origin to a great first Creative Cause." If we interpret these sentences given in the same Address,

HISTORICAL SKETCH IS

one by the other, it appears that this eminent philosopher felt in 1858 his confidence shaken that the Apteryx and the Red (Grouse first appeared in their respective homes, "he knew not how," or by some process "he knew not what."

This Address was delivered after the papers by Mr. Wal- lace and myself on the Origin of Species, presently to be referred to, had been read before the Linnean Society. When the first edition of this work was published, I was so com- pletely deceived, as were many others, by such expressions as " the continuous operation of creative power," that I included Professor Owen with other palaeontologists as being firmly convinced of the immutability of species ; but it appears (' Anat. of Vertebrates,' vol. iii. p. 796) that this was on my part a preposterous error. In the last edition of this work I inferred, and the inference still seems to me perfectly just, from a passage beginning with the words " no doubt the type- form," &r. (Ibid. vol. i. p. XXXV.). that Professor Owen admitted that natural selection may have done something in the formation of a new species; but this it appears (Ibid. vol. iii. p. 798) is inaccurate and without evidence. I also gave some extracts from a correspondence between Professor Owen and the Editor of the * London Review,' from which it appeared manifest to the Editor as well as to myself, that Professor Owen claimed to have promulgated the theory of natural selection before I had done so; and I expressed my surprise and satisfaction at this announcement ; but as far as it is possible to understand certain recently published pas- sages (Ibid. vol. iii. p. 798) I have either partially or wholly again fallen into error. It is consolatory to me that others find Professor Owen's controversial writings as difficult to understand and to reconcile with each other, as I do. As far as the mere enunciation of the principle of natural selection is concerned, it is quite immaterial whether or not Professor Owen preceded me, for both of us, as shown in this histori- cal sketch, were long ago preceded by Dr. Wells and Mr. Matthews.

M. Isidore Geofifroy Saint-Hilaire, in his lectures delivered in 1850 (of which a Resume appeared in the 'Revue et Mag. de Zoolog.,' Jan. 1851), briefly gives his reason for believing that specific characters "sont fixes, pour chaque cspcce, tant

16 HISTORICAL SKETCH

qu'elle se perpetue ou milieu des memes circonstances : ils se modifient, si les circonstances ambiantes viennent a changer." "En resume, I'cbservaiion des animaux sauvages demontre deja la variabilite limitee des especes. Les experiences sur les animaux sauvages devenus domestiques, et sur les ani- maux domestiques redevenus sauvages, la demontrent plus clairement encore. Ces memes experiences prouvent, de plus, que les dififerences produites peuvent etre de valeur generique." In his ' Hist. Nat. Generale' (torn ii. p. 340, 1859) he amplifies analogous conclusions.

From a circular lately issued it appears that Dr. Freke, in 1851 ('Dublin Medical Press,' p. 322), propounded the doc- trine that all organic beings have descended from one pri- mordial form. His grounds of belief and treatment of the subject are wholly different from mine; but as Dr. Freke has now^ (1861) published his Essay on the 'Origin of Spe- cies by means of Organic Affinity,' the difficult attempt to give any idea of his views would be superfluous on my part.

Mr. Herbert Spencer, in an Essay (originally published in the 'Leader,' March, 1852, and republished in his 'Essays,' in 1858), has contrasted the theories of the Creation and the Development of organic beings with remarkable skill and force. He argues from the analogy of domestic productions, from the changes which the embryos of many species under- go, from the difficulty of distinguishing species and varie- ties, and from the principle of general gradation, that species have been modified; and he attributes the modification to the change of circumstances. The author (1855) has also treated Psychology on the principle of the necessary acquire- ipent of each mental power and capacity by gradation.

La 1852 M. Naudin, a distinguished botanist, expressly stated, in an admirable paper on the Origin of Species ('Revue Horticole,' p. 102; since partly republished in the 'Nouvelles Archives du Museum,' tom. i. p. 171), his belief that species are formed in an analogous manner as varieties are under cultivation ; and the latter process he attributes to man's power of selection. But he does not show how selec- tion acts under nature. He believes, like Dean Herbert, that species, when nascent, were more plastic than at present. He lays weight on what he calls the principle of finality;

HISTORICAL SKETCH 17

"puissance mysterieuse, indeterminee ; fatalite pour Ics uns ; pour les autres, volonte providentielle, dont Taction inces- sante sur les etres vivants determine, a toutes les epoques de I'existence du monde, la forme, le volume, et la duree de chacun d'eux, en raison de sa destinee dans I'orde de choses dont il fait partie. C'est cette puissance qui harmonise chaque membre a I'ensemble, en I'appropriant a la fonction qu'il doit remplir dans I'organisme general de la nature, fonc- tion qui est pour lui sa raison d'etre."*

In 1853 a celebrated geologist, Count Keyserling ('Bulletin de la Soc. .Geolog./ 2nd Sen, torn. x. p. 357), suggested that as new diseases, supposed to have been caused by some miasma, have arisen and spread over the world, so at certain periods the germs of existing species may have been chem- ically afifected by circumambient molecules of a particular nature, and thus have given rise to new forms.

In this same year, 1853, Dr. Schaaffhausen published an excellent pamphlet ('Verhand. des Naturhist. Vereins der Preuss. Rheinlands,' &c.), in which he maintains the devel- opment of organic forms on the earth. He infers that many species have kept true for long periods, whereas a few have become modified. The distinction of species he explains by the destruction of intermediate graduated forms. "Thus living plants and animals are not separated from the extinct by new creations, but are to be regarded as their descendants through continued reproduction."

A well-known French botanist, M. Lecoq, writes in 1854 ('Etudes sur Geograph. Bot.,' torn. i. p. 250), "On voit que nos recherches sur la fixite ou la variation de I'espece, nous conduisent directement aux idees emises, par deux hommes justement celebres, Geoffroy Saint-Hilaire et Goethe." Some

* From references in Bronn's ' Untersuchungen iiber die Entwickclungs- Gesetze,' it appears that the celebrated botanist and palxontologist Unger published, in 1852, his belief that species undergo development and modifi- cation. Dalton, likewise, in Pander and Dalton's work on Fossil Sloths, ex- pressed, in 1 82 1, a similar belief. Similar views have, as is well known, been maintained by Oken in his mystical ' Natur-Philosophie.' From other references in Godron's work ' Sur I'Espece,' it seems that Bory St. Vincent, Burdach, Poiret, and Fries, have all admitted that new species arc con- tinually being produced.

I may add, that of the thirty-four authors named in this Historical Sketch, who believe in the modification of species, or at least disbelieve in separate acts of creation, twenty-seven have written on special branches of natural history or geology.

18 HISTORICAL SKETCH

other passages scattered through M. Lecoq's large work, make it a little doubtful how far he extends his views on the modification of species.

The 'Philosophy of Creation' has been treated in a mas- terly manner by the Rev. Baden Powell, in his 'Essays on the Unity of Worlds,' 1855. Nothing can be more striking than the manner in which he shows that the introduction of new species is "a regular, not a casual phenomenon," or, as Sir John Herschel expresses it, "a natural in contradistinction to a miraculous process."

The third volume of the 'Journal of the Linnean Society' contains papers, read July ist, 1858, by Mr. Wallace and my- self, in which, as stated in the introductory remarks to this volume, the theory of Natural Selection is promulgated by Mr. Wallace with admirable force and clearness.

Von Baer, towards whom all zoologists feel so profound a respect, expressed about the year 1859 (see Prof. Rudolph Wagner, 'Zoologisch-Anthropologische Untersuchungen,* 1861, s. 51) his conviction, chiefly grounded on the laws of geographical distribution, that forms now perfectly distinct have descended from a single parent-form.

In June, 1859, Professor Huxley gave a lecture before the Royal Institution on the 'Persistent Types of Animal Life.' Referring to such cases, he remarks, "It is diflicult to com- prehend the meaning of such facts as these, if we suppose that each species of animal and plant, or each great type of organisation, was formed and placed upon the surface of the globe at long intervals by a distinct act of creative power; and it is well to recollect that such an assumption is as un- supported by tradition or revelation as it is opposed to the general analogy of nature. If, on the other hand, we view 'Persistent Types' in relation to that hypothesis which sup- poses the species living at any time to be the result of the gradual modification of pre-existing species a hypothesis v/hicli, though unproven, and sadly damaged by some of it.s supporters, is yet the only one to which physiology lends any countenance; their existence would seem to show that the amount of modification which living beings have undergone during geological time is but very small in relation to the whole series of changes which they have sufifered."

HISTORICAL SKETCH 19

In December, 1859, Dr. Hooker published his 'Introduction to the Australian Flora.' In the first part of this great work he admits the truth of the descent and modification of spe- cies, and supports this doctrine by many original observa- tions.

The first edition of this work was published on November 24th, 1859, and the second edition on January 7th, i860.

INTRODUCTION

When on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the organic beings in- habiting South America, and in the geological relations of the present to the past inhabitants of that continent. These facts, as will be seen in the latter chapters of this volume, seemed 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 specu- late 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 (1859) nearly finished; but as it will take me many 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 natural history of the Malay archipelago, has arrived at almost exactly the same general conclusions that I have on the origin of species. In 1858 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 im- perfect. 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

21

22 INTRODUCTION

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 is here impossible.

I much regret that want of space prevents my having the satis- faction of acknowledging the generous assistance which I have received from very many naturalists, some of them personally unknown to me. I cannot, however, 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 con- clusion that species have 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 justly excites our admiration. Naturalists continually refer to external conditions, such as climate, food, etc., as the only possible cause of variation. In one 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 mistletoe, which draws its nourish- ment from certain trees, which has seeds that must be trans- ported 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

INTRODUCTION 23

account for the structure of this parasite, with its relations to several distinct organic beings, by the effects of external condi- tions, or of habit, or of the volition of the plant itself.

It is, therefore, of the highest importance to gain a clear in- sight 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 com- monly 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 pos- sible; and, what is equally or more important, we shall see how great is the power of man in accumulating by his Selection suc- cessive slight variations. I will then pass on 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, how- ever, be enabled to discuss what circumstances are most favour- able 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 increase, will be considered. 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, conse- quently, there is a frequently recurrent 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 con- ditions 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 Diver-

24 INTRODUCTION

gence of Character. In the next chapter I shall discuss the com- plex and little known laws of variation. In the five succeeding chapters, the most apparent and gravest difficulties in accepting the theory will be given : namely, first, the difficulties of transi- tions, or how a simple being or a simple organ can be changed and perfected into a highly developed being or into an 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 through- out time; in the twelfth and thirteenth, their geographical distri- bution throughout space ; in the fourteenth, their classification or mutual affinities, both when mature and in an embryonic condi- tion. In the last chapter I shall give a brief recapitulation of the whole work, and a few concluding remarks.

No one ought to feel surprise at much remaining as yet unex- plained in regard to the origin of species and varieties, if he make due allowance for our profound ignorance in regard to the mutual relations of the many 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 dis- passionate judgment of which I am capable, that the view which most naturalists until recently entertained, and which I formerly entertained — namely, that each species has been independently created — is erroneous. I am fully convinced that species are not immutable ; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that Natural Selection has been the most important, but not the exclusive, means of modification.

ORIGIN OF SPECIES

CHAPTER I Variation under Domestication

Causes of variability — Effects of habit and the use or disuse of parts- Correlated variation — Inheritance — Character of domestic varie- ties— Difificulty of distinguishing between varieties and species — Origin of domestic varieties from one or more species — Domestic pigeons, their differences and origin — Principles of selection, an- ciently followed, their efifects — Methodical and unconscious selection — Unknown origin of our domestic productions — Circum- stances favourable to man's power of selection

CAUSES OF VARIABILITY

WHEN we compare 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. And if 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, we are driven to conclude that this great varia- bility is 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 had been exposed under nature. There is, also, some probability in the view propounded by Andrew Knight, that this variability may be partly connected with excess of food. It seems clear that organic beings must be exposed during several genera- tions to new conditions to cause any great amount of varia- tion; and that, when the organisation has once begun to vary, it generally continues varying for many generations. No case is on record of a variable organism ceasing to vary wnder cultivation. Our oldest cultivated plants, such as

25

26 ORIGIN OF SPECIES

wheat, still yield new varieties : our oldest domesticated ani- mals are still capable of rapid improvement or modification.

As far as I am able to judge, after long attending to the subject, the conditions of life appear to act in two ways, — • directly on the whole organisation or on certain parts alone, and indirectly by affecting the reproductive system. With re- spect to the direct action, we must bear in mind that in every case, as Professor Weismann has lately insisted, and as I have incidentally shown in my work on 'Variation under Domesti- cation,' there are two factors; namely, the nature of the organism, and the nature of the conditions. The former seems to be much the more important; for nearly similar variations sometimes arise under, as far as we can judge, dissimilar conditions; and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform. The effects on the offspring are either definite or indefinite. They may be considered as definite when all or nearly all the offspring of individuals exposed to certain conditions during several generations are modified in the same manner. It is extremely difificult to come to any con- clusion in regard to the extent of the changes which have been thus definitely induced. There can, however, be little doubt about many slight changes, — such as size from the amount of food, colour from the nature of the food, thick- ness of the skin and hair from climate, etc. Each of the endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same cause were to act uniforml}'^ during a long series of generations on many individuals, all probably would be modified in the same manner. Such facts as the complex and extraordinary out-growths which variably follow from the insertion of a minute drop of poison by a gall-producing insect, show us what singular modifications might result in the case of plants from a chemical change in the nature of the sap.

Indefinite variability is a much more common result of changed conditions than definite variability, and has prob- ably played a more important part in the formation of our domestic races. We see indefinite variability in the endless slight peculiarities which distinguish the individuals of the same species, and which cannot be accounted for by inher-

VARIATION UNDER DOMESTICATION 27

ilance from either parent or from some more remote ances- tor. Even strongly-marked differences occasionally appear in the young of the same litter, and in seedlings from the same seed capsule. At long intervals of time, out of millions of individuals reared in the same country and fed on nearly the same food, deviations of structure so strongly pro- nounced as to deserve to be called monstrosities arise; but monstrosities cannot be separated by any distinct line from slighter variations. All such changes of structure, whether extremely slight or strongly marked, which appear amongst many individuals living together, may be considered as the indefinite effects of the conditions of life on each individual organism, in nearly the same manner as the chill affects dif- ferent men in an indefinite manner, according to their state of body or constitution, causing coughs or colds, rheumatism, or inflammation of various organs.

With respect to what I have called the indirect action of changed conditions, namely, through the reproductive sys- tem of being affected, we may infer that variability is thus induced, partly from the fact of this system being extremely sensitive to any change in the conditions, and partly from the similarity, as Kolreuter and others have re- marked, between the variability which follows from the crossing of distinct species, and that which may be ob- served with plants and animals when reared under new or unnatural conditions. Many facts clearly show how eminently susceptible the reproductive system is to very slight changes in the surrounding conditions. Nothing is more easy than to tame an animal, and few things more diffi- cult than to get it to breed freely under confinement, even when the male and female unite. How many animals there are which will not breed, though kept in an almost free state in their native country ! This is generally, but erroneously, attributed to vitiated instincts. Many cultivated plants dis- play the utmost vigour, and yet rarely or never seed ! In some few cases it has been discovered that a very trifling change, such as a little more or less water at some particular period of growth, will determine whether or not a plant will produce seeds. I cannot here give the details which I have collected and elsewhere published on this curious subject;

28 ORIGIN OF SPECIES

but to show how singular the laws are which determine thei reproduction of animals under confinement, I may mention that carnivorous animals, even from the tropics, breed in this country pretty freely under confinement, with the excep- tion of the plantigrades or bear family, which seldom pro- duce young; whereas carnivorous birds, with the rarest ex- ceptions, hardly ever lay fertile eggs. Many exotic plants have pollen utterly worthless, in the same condition as in the most sterile hybrids. When, on the one hand, we see domes- ticated animals and plants, though often weak and sickly, breeding freely tmder 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 repro- ductive system so seriously affected by unperceived causes as to fail to act, we need not be surprised at this system, when it does act under confinement, acting irregularly, and pro- ducing offspring somewhat unlike their parents. I may add, that as some organisms breed freely under the most unnat- ural conditions (for instance, rabbits and ferrets kept in hutches), showing that their reproductive organs are not easily affected ; so will some animals and plants withstand domestication or cultivation, and vary very slightly — per- haps hardly more than in a state of nature.

Some naturalists have maintained that all variations are connected with the act of sexual reproduction; but this is certainly an error; for I have given in another work a long list of "sporting plants," as they are called by gardeners; — that is, of plants which have suddenly produced a single bud with a new and sometimes widely different character from that of the other buds on the same plant. These bud-varia- tions, as they may be named, can be propagated by grafts, offsets, etc., and sometimes by seed. They occur rarely under nature, but are far from rare under culture. As a single bud out of the many thousands, produced year after year on the same tree under uniform conditions, has been known suddenly to assume a new character ; and as buds on distinct trees, growing under different conditions, have some- times yielded nearly the same variety — for instance, buds on peach-trees producing nectarines, and buds on common roses

VARIATION UNDER DOMESTICATION 29

producing moss-roses — we clearly see that the nature of the condition is of subordinate importance in comparison with the nature of the organism in determining each particular form of variation — perhaps of not more importance than the nature of the spark, by which a mass of combustible matter is ignited, has in determining the nature of the flames.

EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED VARIATION; INHERITANCE

Changed habits produce an inherited efifect, as in the pe- riod of the flowering of plants when transported from one climate to another. With animals the increased use or dis- use of parts has had a more marked influence; thus 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 skele- ton, than do the same bones in the wild duck; and this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parents. The great and inherited development of the udders in cows and goats in countries where they are habitually milked, in com- parison with these organs in other countries, is probably another instance of the effects of use. Not one of our do- mestic animals can be named which has not in some country drooping ears ; and the view which has been suggested that the drooping is due to disuse of the muscles of the ear, from the animals being seldom much alarmed, seems probable.

Many laws regulate variation, some few of which can be dimly seen, and will hereafter be briefly discussed. I will here only allude to what may be called correlated variation. Important changes in the embryo or larva will probably en- tail changes in the mature animal. In monstrosities, the correlations between quite distinct parts are very curious; and many instances are given in Isidore Geoff roy 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 cats which are entirely white and have blue eyes are generally deaf ; but it has been lately stated by Mr. Tait that this is confined to the males. Colour and constitutional pecu-

30 ORIGIN OF SPECIES

liarities go together, of which many remarkable cases could be given amongst animals and plants. From facts collected by Heusinger, it appears that white sheep and pigs are in- jured by certain plants, whilst dark-coloured individuals es- cape : Professor Wyman has recently communicated to me a good illustration of this fact; on asking some farmers in Virginia how it was that all their pigs were black, they in- formed him that the pigs ate the paint-root (Lachnanthes), which colored their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the "crackers" (i.e. Virginia squatters) added, "we select the black members of a litter for raising, as they alone have a good chance of living.'' 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 pecu- liarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of cor- relation.

The results of the various, unknown, or but dimly under- stood laws of variation are infinitely complex and diversified. It is well worth whde carefully to study the several treatises on some of our old cultivated plants, as on the hyacinth, potato, even the dahlia, etc. ; and it is really surprising to note the endless points of structure and constitution in which the varieties and sub-varieties differ slightly from each other. The whole organisation seems to have become plastic, and departs in a slight 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, are 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; that like produces like is his fundamental be- lief: doubts have been thrown on this principle only by theo- retical writers. When any deviation of structure often

I

VARIATION UNDER DOMESTICATION 31

appears, and we see it in the 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, etc., appearing in several members of the same family. If strange and rare deviations of structure are really 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 for the most part unknown. No one can say why the same peculiarity in dif- ferent individuals of the same species, or in different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or 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 importance to us, that peculiarities ap- pearing in the males of our domestic breeds are often trans- mitted, either exclusively or in a much greater degree, to the 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 reappear in the offspring at a corresponding age, though sometimes earlier. In many cases this could not be otherwise ; thus the inherited pecu- liarities in the horns of cattle could appear only in the off- spring 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

32 ORIGIN OF SPECIES

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 the primary cause which may have acted on the ovules or on the male element; in nearly the same manner as the increased length of the horns in the offspring from a short-horned cow by a long-horned bull, though appearing late in life, is clearly due to the male element.

Having alluded to the subject of aversion, I may here refer to a statement often made by naturalists — namely, that our domestic varieties, when run wild, gradually but invariably 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 M-as, and so could not tell whether or not nearly perfect re- version had ensued. It would be necessary, in order to pre- vent the effects of intercrossing, that only a single variety should have been turned loose in its new home. Neverthe- less, as our varieties certainly do occasionally 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 definite 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 ex- periment 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

CHARACTER OF DOMESTIC VARIETIES 33

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 escu- lent vegetables, for an unlimited number of generations, would be opposed to all experience.

CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF

DISTINGUISHING BETWEEN VARIETIES AND

SPECIES; ORIGIN OF DOMESTIC

VARIETIES FROM ONE OR

MORE SPECIES

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 often have a somewhat mon- strous character; by which I mean, that, although differing 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 an- other, and more especially when compared with the species under 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), domes- tic races of the same species differ from each other in the same manner as do the closely allied species of the same genus in a state of nature, but the differences in most cases are less in degree. This must be admitted as true, for the domestic races of many animals and plants have been ranked by some competent judges as the descendants of aboriginally distinct species, and by other competent judges as mere varieties. If any well-marked distinction existed between a domestic race and a species, this source of doubt would not so perpetually recur. It has often been stated that domestic races do not differ from each other in characters of generic value. It can be shown that this statement is not correct;

B— HC XI

34 ORIGIN OF SPECIES

but naturalists differ much in determining what characters are of generic value; all such valuations being at present empirical. When it is explained how genera originate under nature, it will be seen that we have no right to expect often to find a generic amount of difference in our domesticated races.

In attempting to estimate the amount of structural differ- ence between allied domestic races, we are soon involved in doubt, from not knowing whether they are 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 bulldog, which we all know propagate their kind truly, were the offspring of any single species, then such facts would have great weight in making us doubt about the immuta- bility of the many closely allied natural species — for in- stance, 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 a small part of the difference is due to their being descended from distinct species. In the case of strongly marked races of some other domesticated species, there is presumptive or even strong evidence, that all are descended from a single wild stock.

It has often been assumed that man has chosen for domes- tication 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 ani- mal, whether it would vary in succeeding generations, and whether it would endure other climates? Has the little variability of the ass and goose, or the small power of en- durance of warmth by the reindeer, or of cold by the com- mon 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 genera-

CHARACTER OF DOMESTIC VARIETIES 35

tions under domestication, they would on an average vary 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, it is not possible to come to any definite con- clusion, whether they are descended from one or several wild species. The argument mainly relied on by those who believe in the multiple origin of our domestic animals is, that we find in the most ancient times, on the monuments of Egypt, and in the lake-habitations of Switzerland, much diversity in the breeds; and that some of these ancient breeds closely resemble, or are even identical with, those still ex- isting. But this only throws far backwards the history of civilisation, and shows that animals were domesticated at a much earlier period than has hitherto been supposed. The lake-inhabitants of Switzerland cultivated several kinds of wheat and barley, the pea, the poppy for oil, and flax; and they possessed several domesticated animals. They also carried on commerce with other nations. All this clearly shows, as Heer has remarked, that they had at this early age progressed considerably in civilisation ; and this again implies a long continued previous period of less advanced civilisation, during which the domesticated animals, kept by different tribes in different districts, might have varied and given rise to distinct races. Since the discovery of flint tools in the superficial formations of many parts of the world, all geologists believe that barbarian man existed at an enormously remote period and we know that at the present day there is hardly a tribe so barbarous, as not to have domesticated at least the dog.

The origin of most of our domestic animals will prob- ably for ever remain vague. But I may here state, that, looking to the domestic dogs of the whole world. I have, after a laborious collection of all known facts, come to the conclusion that several wild species of Canidc'c have been tamed, and that their blood, in some cases mingled together, flows in the veins of our domestic breeds. In regard to sheep and goats I can form no decided opinion. From facts communicated to me by Mr. Blyth, on the habits, voice, con- stitution, and structure of the humped Indian cattle, it is

36 ORIGIN OF SPECIES

almost certain that they are descended from a different abo- rigmal stock from our European cattle and some competent judges believe that these latter have had two or three wild progenitors, — whether or not these deserve to be called species. This conclusion, as well as that of the specific dis- tinction between the humped and common cattle, may, in- deed, be looked upon as established by the admirable re- searches of Professor Riitimeyer. With respect to horses, from reasons which I cannot here give, I am doubtfully inclined to believe, in opposition to several authors, that all the races belong to the same species. Having kept nearly all the English breeds of the fowl alive, having bred and crossed them, and examined their skeletons, it appears to me almost certain that all are the descendants of the wild Indian fowl, Gallus bankiva; and this is the conclusion of Mr. Blyth, and of others who have studied this bird in India. In regard to ducks, and rabbits, some breeds of which differ much from each other, the evidence is clear that they are 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 eleven wild species of sheep peculiar to Great Britain ! When we bear in mind that Britain has now not one peculiar mammal, and France but few distinct from those of Germany, and so with Hungary. Spain, etc., but that each of these kingdoms possesses several peculiar breeds of cattle, sheep, etc., we must admit that many domestic breeds must have originated in Europe; from whence other- wise could they have been derived? So it is in India. Even in the case of the breeds of the domestic dog throughout the world, which I admit are descended from several wild spe- cies, it cannot be doubted that there has been an immense amount of inherited variation ; for who will believe that animals closely resembling the Italian greyhound, the blood-

DOMESTIC PIGEONS 37

hound, the bull-dog, pug-dog, or Blenheim spaniel, etc. — so unlike all wild Canidae — ever existed in a state of nature? It has often been loosely said that all our races of dogs have been produced by the crossing of a few aboriginal spe- cies ; 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, etc., in the wild state, IMoreover, the possibility of making distinct races by crossing has been greatly exaggerated. ^lany cases are on record, showing that a race may be modified by occasional crosses, if aided by the careful selection of the individuals which present the desired character ; but to obtain a race intermediate between two quite distinct races, would be very difficult. Sir J, Sebright expressly experimented with this object and failed. The offspring from the first cross be- tween two pure breeds is tolerably and sometimes (as I have found with pigeons) quite uniform in character, and every- thing seems simple enough ; but when these mongrels are crossed one with another for several generations, hardly two of them are alike, and then the difficulty of the task becomes manifest.

BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES

AND ORIGIN

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 pig- eons, and some of them are very important, as being of con- siderable 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 as- tonishing. Compare the English carrier and the short-faced tumbler, and see the wonderful difference in their beaks.

38 ORIGIN OF SPECIES

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 vv^ide 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 long beak, has a very short and 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 short and conical beak, with a line of reversed feathers down the breast; and it has the habit of continually expand- ing, slightly, the upper part of the oesophagus. The Jacobin has the feathers so much rcA^ersed along the back of the neck that they form a hood; and it has, proportionally to its size, 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 the members of the great pigeon family : these feathers are kept expanded, and are 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 caudal and sacral vertebrae vary in number; 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 vari- able; so is the degree of divergence and relative size of tht

DOMESTIC PIGEONS 39

two arms of the furcula. The porportional 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 the wing and tail to each other and to the body; the relative length of the leg and foot; the number of scutellae on the toes, the development of skin between the toes, are all points of struct- ure 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, and in some breeds the voice and disposition, differs re- markably. Lastly, in certain breeds, the males and females have come to differ in a slight degree in 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 be ranked by him as well-defined species. Moreover, I do not believe that any ornithologist would in this case place the 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 would call them, could be shown him.

Great as are the differences between the breeds of the pigeon, I am fully convinced that the common opinion of naturalists is correct, namely, that all are 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 de- gree applical)le in other cases, I will here briefly give them. If the several breeds are not varieties, and have not pro- ceeded 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 pes-

40 ORIGIN OF SPECIES

sessed the characteristic enormous crop ? The supposed abo- riginal stocks must all have been rock-pigeons, that is, they did not breed or willingly perch 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 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 ex- terminated 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 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 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 difficult to get wild ani- mals to breed freely under domestication; yet on the hy- pothesis of the multiple origin of our pigeons, it must be assumed that at least seven or eight species were so thor- oughly domesticated in ancient times by half-civilised man, as to be quite prolific under confinement.

An argument of great weight, and applicable in several other cases, is, that the above-specified breeds, though agree- ing generally with the wild rock-pigeon in constitution, habits, voice, colouring, and in most parts of their structure, yet are certainly highly abnormal in other parts ; we may look in vain through the whole great family of Columbidje for a beak like that of the English carrier, or that of the short-faced tum- bler, 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-

DOMESTIC PIGEONS 41

civilised man succeeded in thorouj^hly 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 are improbable in the highest degree. Some facts in regard to the colouring of pigeons well de- serve consideration. The rock-pigeon is of a slaty-blue, with white loins;' but the Indian sub-species, C. intermedia of Strickland, has this part bluish. The tail has a terminal dark bar, with the outer feathers externally edged at the base with white. The wings have two black bars. Some semi-domes- tic breeds, and some 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, w'hen birds belonging to two or more distinct breeds are crossed, none of which are blue or have any of the above-specified marks, the mongrel off- spring are very apt suddenly to acquire these characters. To — give one instance out of several which I have observed : — I 1 crossed some white fantails, which breed very true, with some \ black barbs — and it so happens that blue varieties of barbs are so rare that I never heard of an instance in England; and the mongrels were black, brown, and mottled. I also crossed a barb with a spot, which is a white bird with a red tail and red spot on the forehead, and which notoriously breeds very true ; the mongrels were dusky and mottled. I then crossed one of the mongrel barb-fantails with a mongrel barb-spot, and they produced a bird of as beautiful a blue colour, with the white loins, double black wing-bar, and barred and white- / edged tail-feathers, as any wild rock-pigeon ! We can under- y* stand these facts, on the well-known principle of reversion to ancestral characters, if all the domestic breeds are descended from the rock-pigenn. But if we deny this, we must make one of the two following highly improbable suppositions. Either, first, 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

42 ORIGIN OF SPECIES

each separate breed there might be a tendency to revert to the very same colours and markings. Or, secondly, 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 no instance is known of crossed descendants reverting to an ancestor of foreign blood, removed by a greater number of generations. In a breed which has been crossed only once, the tendency to revert to any character derived from such a cross will nat- urally become less and less, as in each succeeding generation there will be less of the foreign blood; but when there has been no cross, and there is a tendency in the breed to revert to a character which was lost during some former genera- tion, 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 of reversion are often confounded together by those who have written on inheri- tance.

Lastly, the hybrids or mongrels from between all the breeds of the pigeon are perfectly fertile, as I can state from my own observations, purposely made, on the most distinct breeds. Now, hardly any cases have been ascertained with certainty of hybrids from two quite distinct species of animals being perfectly fertile. Some authors believe that long-continued domestication eliminates this strong tendency to sterility in species. From the history of the dog, and of some other do- mestic animals, this conclusion is probably quite correct, if applied to species closely related to each other. But to ex- tend it so far as to suppose that species, aboriginally as dis- tinct as carriers, tumblers, pouters, and fantails now are, should yield offspring perfectly fertile inter se, would be rash in the extreme.

From these several reasons, namely, — the improbability of man having formerly made seven or eight supposed species of pigeons to breed freely under domestication; — these sup- posed species being quite unknown in a wild state, and their not having become anywhere feral; — these species presenting certain very abnormal characters, as compared with all other Columbidse, though so like the rock-pigeon in most respects ; — the occasional re-appearance of the blue colour and various

DOMESTIC PIGEONS 43

black marks in all the breeds, both when kept pure and when crossed; — and lastly, the mongrel offspring being perfectly fertile ; — from these several reasons, taken together, we may safely conclude that all our domestic breeds are descended from the rock -pigeon or Columba livia with its geographical sub-species.

In favour of this view, I may add, firstly, that the wild C. livia 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. Sec- ondly, that, although an English carrier or a short-faced tumbler differs immensely in certain characters from the rock-pigeon, yet that, by comparing the several sub-breeds of these two races, more especially those brought from distant countries, we can make, between them and the rock-pigeon, an almost perfect series; so we can in some other cases, but not wath all the breeds. Thirdly, those characters which are mainly distinctive of each breed are in each eminently vari- able, 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 ; and the explanation of this fact will be obvious when we 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 ear- liest known record of pigeons is in the fifth Egyptian dy- nasty, 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 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 cross- ing 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 ex-

44 ORIGIN OF SPECIES

plaining the immense amount of variation which pigeons have undergone, will likewise be obvious when we treat of Selec- tion. We shall then, also, see how it is that the several 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, well knowing how truly they breed, I felt fully as much difficulty in believing that since they had been domesticated they had all proceeded from a common parent, as any naturalist could in coming to a similar conclusion in regard to the many species of finches, or other groups of birds, in nature. One circumstance has struck me much ; namely, that nearly all the breeders of the various domestic animals and the cultivators of plants, with whom I have 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. Ask, as I have asked, a celebrated raiser of Here- ford cattle, whether his cattle might not have descended from Longhorns, or both from a common parent-stock, 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 differ- ences 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 differ- ences 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

SELECTION BY MAN 45

he does of the intermediate links in the long lines of descent, yet admit that many of our domestic races are descended from the same parents — may they not learn a lesson of cau- tion, when they deride the idea of species in a state of nature being lineal descendants of other species?

PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND

THEIR EFFECTS

Let us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species. Some effect may be attributed to the direct and defi- nite action of the external conditions of life, and some to habit; but he would be a bold man who would account by such agencies for the differences between a dray- and race- horse, a greyhound and bloodhound, a carrier and tumbler pigeon. One of the most remarkable features in our domes- ticated races 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 sud- denly, or by one step ; many botanists, for instance, believe that the fuller's teasel, 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 sud- denly 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 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

46 ORIGIN OF SPECIES

breeds were suddenly produced as perfect and as useful as we now see them ; indeed, in many cases, we know that this has not been their history. The key is man's power of accumu- lative selection : nature gives successive variations ; man adds them up in certain directions useful to him. In this sense he may be said to have made for himself useful breeds.

The great power of this principle of selection is not hypo- thetical. It is certain that several of our eminent breeders have, even within a single lifetime, modified to a large extent their 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 organi- sation as something 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 agri- culturists than almost any other individual, and who was him- self a very good judge of animals, speaks of the principle o£ 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 Somer- ville, 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." In Sax- ony 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 been exported to almost every quarter of the world. The improvement is by no means generally due to crossing different breeds ; 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 ordi-

SELECTION BY MAN 47

nary 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 be- come 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 has not been so in several 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 flor- ists' flowers, when the flowers of the present day are com- pared 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 ani- mals this kind of selection is, in fact, likewise followed; for hardly any one is so careless as to breed from his worst animals.

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 com- parison with the leaves and flowers of the same set of vari-

48 ORIGIN OF SPECIES

eties. 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, — I speak after careful observation, — per- haps never, the case. The law of correlated variation, the im- portance of which should never be overlooked, will ensure some differences ; but, as a general rule, it cannot be doubted 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 refer- ences to works of high antiquity, in which the full impor- tance of the principle is acknowledged. In rude and bar- barous periods 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 nur- serymen. The principle of selection I find distinctly given in an ancient Chinese encyclopaedia. Explicit rules are laid down bj some of the Roman classical writers. From pas- sages in Genesis, it is clear that the colour of domesticated animals was at that early period attended to. Savages now sometimes cross their dogs with wild canine animals, to im- prove 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 states that good domestic breeds are highly valued by the negroes in the interior of Africa who have not associated with Europeans. Some of these facts do not show actual selection, but they show that the

UNCONSCIOUS SELECTION 49

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.

UNCONSCIOUS SELECTION

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 of the kind in the country. But, for our purpose, a form 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 nat- urally tries to get as good dogs as he can, and afterwards breeds from his own best dogs, but he has no wish or expec- tation of permanently altering the breed. Nevertheless we may infer that this process, continued during centuries, would improve and modify any breed, in the same way as Bakewell, Collins, etc., by this very same process, only carried on more methodically, did greatly modify, even during their lifetimes, the forms and qualities of their cattle. Slow and insensible changes of this kind can never be recognised unless actual measurements or careful drawings of the breeds in question have been made long ago, which may serve for comparison. In some cases, however, unchanged, or but little changed individuals of the same breed exist in less civilised districts, where the breed has been less improved. There is reason to believe that King Charles' 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 foxhound : but what concerns us is. that the change has been effected unconsciously and gradually, and yet so effectually, that, though the old Spanish pointer cer- tainly came from Spain, Mr. Borrow has not seen, as I am

so ORIGIN OF SPECIES

informed by him, any native dog in Spain like our pointer. By a simple process of selection, and by careful training, English racehorses have come to surpass in fleetness and size the parent Arabs, so that the latter, by the regulations for the Goodwood Races, are favoured in the weights which 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 various old treatises of the former and present state of carrier and tumbler pigeons in Britain, India, and Persia, we can 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 unconscious, in so far that the breeders could never have expected, or even wished, to produce the result which ensued — namely, the pro- duction 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 sus- picion 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 dift'erent 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.

UNCONSCIOUS SELECTION 51

whether or not sufficiently distinct to be ranked at their first appearance as distinct varieties, and whether or not two or more species or races have become blended together by cross- ing, 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 sur- prise expressed in horticultural works at the wonderful skill of gardeners, in having produced such splendid results from such poor materials; but the art has been simple, and, as far as the final result is concerned, has been followed almost un- consciously. It has consisted in always cultivating the best- known variety, sowing its seeds, and, when a slightly better variety chanced to appear, selecting it, and so onwards. But the gardeners of the classical period, who cultivated the best pears which 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 pre- served the best varieties they could anywhere find.

A large amount of change, thus slowly and unconsciously accumulated, explains, as I believe, the well-known fact, that in a 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 standard of usefulness to man, we can understand how it is that neither Australia, the Cape of Good Hope, nor any other region in- habited 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

52 ORIGIN OF SPECIES

perfection comparable with that acquired by 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, indi- viduals of the same species, having slightly different consti- tutions 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 why the varieties kept by savages, as has been remarked by some authors, have more of the character of true species than the varieties kept in civilised countries.

On the view here given of the important part which selec- tion 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 ex- ternally visible ; and indeed he rarely cares for what is inter- nal. He can never act by selection, excepting on variations 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 some- what 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 in- dividuals of other and distinct breeds, in which as many as

UNCONSCIOUS SELECTION S3

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 fancy any novelty, however slight, in one's own possession. Nor must the value which would formerly have been set on any slight differences in the individuals of the same species, be judged of by the value which is now set on them, after several breeds have fairly been established. It is known that with pigeons many slight variations now occasionally appear, but these are rejected as faults or deviations from the stand- ard of perfection in each breed. The common goose has not given rise to any marked varieties ; hence the Toulouse and the common breed, which differ only in colour, that most fleeting of characters, have lately been exhibited as distinct at our poultry-shows.

These views appear to explain what has sometimes been noticed — namely, that we know hardly anything 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 a distinct 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 animals slowly spread in the immediate neighbourhood. But they will as yet hardly have a distinct name, and from being only slightly valued, their history will have been disregarded. When further improved by the same slow and gradual process, they will spread more widely, and will be recognised as something distinct and valu- able, and will then probably first receive a provincial name. In semi-civilised countries, with little free comnnmication, the spreading of a new sub-breed would be a slow process. As soon as the points of value are once 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 dis-

54 ORIGIN OF SPECIES

trict than in another, according to the state of civilisation 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.

CIRCUMSTANCES FAVOURABLE TO MAN's POWER OF SELECTION

I will now say a few words on the circumstances, favour- able, or the reverse, to man's power of selection. A high de- gree of variability is obviously favourable, as freely giving the materials for selection to work on; not that mere indi- vidual differences are not amply sufficient, with extreme care, to allow of the accumulation of a large amount of modifica- tion in almost any desired direction. But as variations mani- festly 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. Hence, number is of the highest importance for success. On this principle Marshall formerly remarked, with respect to the sheep of parts of Yorkshire, "as they generally belong to poor people, and are mostly in small lots, they never can be improved." On the other hand, nurserymen, from keeping large stocks of the same plant, are generally far more successful than amateurs in raising new and valuable varieties. A large number of individuals of an animal or plant can be reared only where the conditions for its propagation are favourable. When the individuals are scanty, all will be allowed to breed, whatever their quality may be, and this will effectually prevent selec- tion. But probably the most important element is that the animal or plant should be so highly valued by man, that the closest attention is paid to even the slightest deviations in its qualities or structure. 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 to this plant. No doubt the straw- berry had always varied since it was cultivated, but the slight varieties had been neglected. As soon, however, as gar- deners picked out individual plants with slightly larger, ear- lier, or better fruit, and raised seedlings from them, and again

CIRCUMSTANCES FAVOURABLE TO SELECTION 55

picked out the best secdlin.^-s and bred from them, then (with some aid by crossing distinct species) those many admirable varieties of the strawberry were raised which have appeared during the last half-century.

With animals, facility in preventing crosses is an important element 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 improved and kept true, though mingled in the same aviary ; and this circumstance must have largely favoured the formation of new breeds. Pigeons, I may add, can be propa- gated 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 easily matched, and, although so much valued by women and children, we rarely see a distinct breed long kept up ; such breeds as we do sometimes see are almost always imported from some other country. 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; for recently in certain parts of Spain and of the United States this animal has been surprisingly modified and improved by careful selection ; 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 dis- play of distinct breeds ; but the goose, under the conditions to which it is exposed when domesticated, seems to have a sin- gularly inflexible organisation, though it has varied to a slight extent, as I have elsewhere described.

Some authors have maintained that the amount of variation in our domestic productions is soon reached, and can never afterwards be exceeded. It would be somewhat rash to as-

56 ORIGIN OF SPECIES

sert that the limit has been attained in any one case ; for al- most all our animals and plants have been greatly improved in many w^ays within a recent period ; and this implies variation. It vi^ould be equally rash to assert that characters now in- creased to their usual limit, could not, after remaining fixed for many centuries, again vary under new conditions of life. No doubt, as Mr. Wallace has remarked with much truth, a limit will be at last reached. For instance, there must be a limit to the fleetness of any terrestrial animal, as this will be determined by the friction to be overcome, the weight of body to be carried, and the power of contraction in the mus- cular fibres. But what concerns us is that the domestic vari- eties of the same species differ from each other in almost every character, which man has attended to and selected, more than do the distinct species of the same genera. Isi- dore Geoffroy St. Hilaire has proved this in regard to size, and so it is with colour and probably with the length of hair. With respect to fleetness, which depends on many bodily char- acters, Eclipse was far fleeter, and a dray-horse is incom- parably stronger than any two natural species belonging to the same genus. So with plants, the seeds of the different varieties of the bean or maize probably differ more in size, than do the seeds of the distinct species in any one genus in the same two families. The same remark holds good in re- gard to the fruit of the several varieties of the plum, and still more strongly with the melon, as well as in many other anal- ogous cases.

To sum up on the origin of our domestic races of animals and plants. Changed conditions of life are of the highest importance in causing variability, both by acting directly on the organisation, and indirectly by affecting the reproductive system. It is not probable that variability is an inherent and necessary contingent, under all circumstances. The greater or less force of inheritance and reversion determine whether variations shall endure. Variability is governed by many unknown laws, of which correlated growth is probably the most important. Something, but how much we do not know, may be attributed to the definite action of the conditions of life. Some, perhaps a great, effect may be attributed to the increased use or disuse of parts. The final result is thus

CIRCUMSTANCES FAVOURABLE TO SELECTION 57

rendered infinitely complex. In some cases the intercrossing of aboriginally distinct species appears to have played an im- portant part in the origin of our breeds. When several breeds have once been formed in any country, their occa- sional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds ; but the im- portance of crossing has been much exaggerated, both in re- gard to animals and to those plants which are propagated by seed. With plants which are temporarily propagated by cut- tings, buds, &c., the importance of crossing is immense ; for the cultivator may here disregard the extreme variability both of hybrids and of mongrels, and the sterility of hybrids; but plants not propagated by seed are of little importance to us, for their endurance is only temporary. Over all these causes of Change, the accumulative action of Selection, whether applied methodically and quickly, or unconsciously and slowly but more efficiently, seems to have been the pre- dominant Power.

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 each country vary more frequently 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.

B

EFORE 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 properly, a long catalogue of dry facts ought to be given ; but these I shall reserve for a future work. Nor shall I here discuss the various definitions which have been given of the term species. No one defini- tion has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Gen- erally the term includes the unknown element of a distinct act of creation. The term "variety" is almost equally diffi- cult to define ; but here community of descent is almost uni- versally 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 consid- erable deviation of structure, generally injurious, or not use- ful to the species. 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 plants on Alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited for at least a few generations ? and in this case I presume that the form would be called a variety. It may be doubted whether sudden and considerable devi-

58

INDIVIDUAL DIFFERENCES SB

ations of structure such as we occasionally see in our domes- tic productions, more especially with plants, are ever perma- nently propagated in a state of nature. Almost every part of every organic being is so beautifully related to its complex conditions of life that it seems as improbable that any part should have been suddenly produced perfect, as that a com- plex machine should have been invented by man in a perfect state. Under domestication monstrosities sometimes occur which resemble normal structures in widely different animals. Thus pigs have occasionally been born with a sort of pro- boscis, and if any wild species of the same genus had nat- urally possessed a proboscis, it might have been argued that this had appeared as a monstrosity ; but I have as yet failed to find, after diligent search, cases of monstrosities resem- bling normal structures in nearly allied forms, and these alone bear on the question. If monstrous forms of this kind ever do appear in a state of nature and are capable of reproduc- tion (which is not always the case), as they occur rarely and singly, their preservation would depend on unusually favour- able circumstances. They would, also, during the first and succeeding generations cross with the ordinary form, and thus their abnormal character would almost inevitably be lost. But I shall have to return in a future chapter to the pres- ervation and perpetuation of single or occasional variations.

INDIVIDUAL DIFFERENCES

The many slight differences which appear in the oft'spring from the same parents, or which it may be presumed have thus arisen, from being observed in the individuals of the same species inhabiting the same confined locality, may be called individual dift'erences. No one supposes that all the individuals of the same species are cast in the same actual mould. These individual differences are of the highest im- portance for us, for they are often inherited, as must be familiar to every one ; and they thus aft'ord materials for natural selection to act on and accumulate, in the same man- ner as man accumulates in any given direction individual dif- ferences in his domesticated productions. These individual differences generally aft'ect what naturalists consider unim-

60 ORIGIN OF SPECIES

portant 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, some- times vary in the individuals of the same species. I am con- vinced that the most experienced naturalist would be sur- prised at the number of the cases of variability, even in im- portant 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 being pleased at finding variability in important characters, and that there are not many men who will laboriously examine inter- nal and important organs, and compare them in many speci- mens of the same species. It would never have been expected that the branching of the main nerves close to the great cen- tral ganglion of an insect would have been variable in the same species ; it might have been thought that changes of this nature could have been effected only by slow degrees; yet Sir J. 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 philosoph- ical naturalist, I may add, has also shown that the muscles in the larvae of certain insects are far from uniform. Authors sometimes argue in a circle when they state that important organs never vary; for these same authors practically rank those parts as important (as some few naturalists have hon- estly confessed) which do not vary; and, under this point of view, no instance will ever be found of an important part varying; but under any other point of view many instances assuredly can be given.

There is one point connected with individual differences, which is extremely perplexing : I refer to those genera which have been called "protean" or "polymorphic," in which the species present an inordinate amount of variation. With re- spect to many of these forms, hardly two naturalists agree whether to rank them as species or as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects and of Brachiopod shells. In most poly- morphic genera some of the species have fixed and definite characters. Genera which are polymorphic in one country seem to be, with a few exceptions, polymorphic in other coun-

INDIVIDUAL DIFFERENCES 61

tries, and likewise, judging from Brachiopod shells, at former periods of time. These facts are 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, at least in some of these polymorphic genera, variations which are of no service or disservice to the species, and which con- sequently have not been seized on and rendered definite by natural selection, as hereafter to be explained.

Individuals of the same species often present, as is known to every one, great differences of structure, independently of variation, as in the two sexes of various animals, in the two or three castes of sterile female or workers amongst insects, and in the immature and larval states of many of the lower animals.

There are, also, cases of dimorphism and trimorphism, both with animals and plants. Thus, Mr. Wallace, who has lately called attention to the subject, has shown that the females of certain species of butterflies, in the Ma- layan archipelago, regularly appear under two or even three conspicuously distinct forms, not connected by intermediate varieties. Fritz Miiller has described analogous but more extraordinary cases with the males of certain Brazilian Crustaceans: thus, the male of a Tanais regularly occurs under two distinct forms; one of these has strong and dif- ferently shaped pincers, and the other has antennae much more abundantly furnished with smelling-hairs. Although in most of these cases, the two or three forms, both with animals and plants, are not now connected by intermediate gradations, it is probable that they were once thus connected. Mr. Wallace, for instance, describes a certain butterfly which presents in the same island a great range of varieties con- nected by intermediate links, and the extreme links of the chain closely resemble the two forms of an allied dimorphic species inhabiting another part of the Malay archipelago. Thus also with ants, the several worker-castes are generally quite distinct; but in some cases, as we shall hereafter see, the castes are connected together by finely graduated varie- ties. So it is, as I have myself observed, with some dimor- phic plants. It certainly at first appears a highly remarkable fact that the same female butterfly should have the power

62 ORIGIN OF SPECIES

of producing at the same time three distinct female forms and a male ; and that an hermaphrodite plant should produce from the same seed-capsule three distinct hermaphrodite forms, bearing three different kinds of females and three or even six different kinds of males. Nevertheless these cases are only exaggerations of the common fact that the female produces offspring of two sexes which sometimes differ from each other in a wonderful manner.

DOUBTFUL SPECIES

The forms which possess in some considerable degree the character of species, but which are so closely similar to other forms, or are so closely linked to them by intermediate gra- dations, 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 for a long time ; for as long, as far as we know, as have good and true species. Practically, when a naturalist can unite by means of intermediate links any two forms, he treats the one as a variety of the other; ranking the most common, but sometimes the one first described, as the spe- cies, and the other as the variety. But cases of great diffi- culty, which I will not here enumerate, sometimes arise in deciding whether or not to rank one form as a variety of another, even when they are closely connected by interme- diate links; nor will the commonly-assumed hybrid nature of the intermediate forms 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 sup- pose either that they do now somewhere exist, or may for- merly 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 ma- jority of naturalists, for few well-marked and well-known

DOUBTFUL SPECIES 63

varieties can be named which have not been ranked as spe- cies by at least some competent judges.

That varieties of this doubtful nature are far from un- common 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 differ- ence 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 the 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, geographical races ! Mr. Wallace, in several valuable papers on the various animals, especially on the Lepidoptera, inhabiting the islands of the great Ma- layan archipelago, shows that they may be classed under four heads, namely, as variable forms, as local forms, as geo- graphical races or sub-species, and as true representative species. The first or variable forms vary much within tho limits of the same island. The local forms are moderately constant and distinct in each separate island; but when all from the several islands are compared together, the differ- ences are seen to be so slight and graduated, that it is im- possible to define or describe them, though at the same time the extreme forms are sufficiently distinct. The geo- graphical races or sub-species are local forms completely fixed and isolated; but as they do not differ from each other

64 ORIGIN OF SPECIES

by strongly marked and important characters, "there is no possible test but individual opinion to determine which of them shall be considered as species and which as varieties." Lastly, representative species fill the same place in the nat' ural economy of each island as do the local forms and sub- species; but as they are distinguished from each other by a greater amount of difference than that between the local forms and sub-species, they are almost universally ranked by naturalists as true species. Nevertheless, no certain cri- terion can possibly be given by which variable forms, local forms, sub-species, and representative species can be recognised.

Many years ago, when comparing, and seeing others com- pare, the birds from the closely neighbouring islands of the Galapagos archipelago, 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 characterised as varieties in Mr. Wollaston's admirable work, but which would certainly 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 experienced ornithologists consider our British red grouse as only a strongly-marked race of Nor- wegian species, whereas the greater number rank it as an undoubted species peculiar to Great Britain. A wide dis- tance between the homes of two doubtful forms leads many naturalists to rank them as distinct species ; but what dis- tance, it has been well asked, will suffice; if that between America and Europe is ample, will that between Europe and the Azores, or Madeira, or the Canaries, or between the sev- eral islets of these small archipelagos, be sufficient?

Mr. B. D. Walsh, a distinguished entomologist of the United States, has described what he calls Phytophagic varieties and Phytophagic species. Most vegetable-feeding insects live on one kind of plant or on one group of plants; some feed indiscriminately on many kinds, but do not in consequence vary. In several cases, however, insects found living on different plants, have been observed by Mr. Walsh

DOUBTFUL SPECIES 65

to present in their larval or mature state, or in both states, slight, though constant differences in colour, size, or in the nature of their secretions. In some instances the males alone, in other instances both males and females, have been observed thus to differ in a slight degree. When the differ- ences are rather more strongly marked, and when both sexes and all ages are affected, the forms are ranked by all entomologists as good species. But no observer can deter- mine for another, even if he can do so for himself, which of these Phytophagic forms ought to be called species and which varieties. Mr. Walsh ranks the forms which it may be supposed would freely intercross, as varieties ; and those which appear to have lost this power, as species. As the differences depend on the insects having long fed on distinct plants, it cannot be expected that intermediate links connect- ing the several forms should now be found. The naturalist thus loses his best guide in determining whether to rank doubtful forms as varieties or species. This likewise neces- sarily occurs with closely allied organisms, which inhabit distinct continents or islands. When, on the other hand, an animal or plant ranges over the same continent, or in- habits many islands in the same archipelago, and presents different forms in the different areas, there is always a good chance that intermediate forms will be discovered which will link together the extreme states ; and these are then degraded to the rank of varieties.

Some few naturalists maintain that animals never present varieties; but then these same naturalists rank the slightest difference as of specific value ; and when the same identical form is met with in two distinct countries, or in two geologi- cal formations, they believe that two distinct species are hid- den under the same dress. The term species thus comes to be a mere useless abstraction, implying and assuming a sep- arate act of creation. It is certain that many forms, consid- ered by highly-competent judges to be varieties, resemble species so completely in character, that they have been thus ranked by other highly-competent judges. But to discuss whether they ought to be called species or varieties, before any definition of these terms has been generally accepted, is

vainly to beat the air.

c— lie XI

66 ORIGIN OF SPECIES

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 in the attempt to determine their rank ; but space does not here per- mit me to discuss them. Close investigation, in many cases, v^^ill no doubt bring naturalists to agree how to rank doubt- ful forms. Yet it must be confessed that it is in the best known countries that we find the greatest number of them. 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 attracts his attention, varieties of it will almost universally be found recorded. These varieties, moreover, will often be 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 almost universally considered by other botanists to be varieties ; and in this country the highest botanical authori- ties and practical men can be quoted to show that the sessile and pedunculated oaks are either good and distinct species or mere varieties.

I may here allude to a remarkable memoir lately published by A. de CandoUe, on the oaks of the whole world. No one ever had more ample materials for the discrimination of the species, or could have worked on them with more zeal and sagacity. He first gives in detail all the many points of struc- ture which vary in the several species, and estimates numeri- cally the relative frequency of the variations. He specifies above a dozen characters which may be found varying even on the same branch, sometimes according to age or develop- ment, sometimes without any assignable reason. Such char- acters are not of course of specific value, but they are, as Asa Gray has remarked in commenting on this memoir, such as generally enter into specific definitions. De Candolle then goes on to say that he gives the rank of species to the forms that differ by characters never varying on the same tree, and never found connected by intermediate states. After this discussion, the result of so much labour, he emphatically re- marks: "They are mistaken, who repeat that the greater part of our species are clearly limited, and that the doubtful

DOUBTFUL SPECIES 67

species are in a feeble minority. This seemed to be true, so long as a genus was imperfectly known, and its species were founded upon a few specimens, that is to say, were pro- visional. Just as we come to know them better, intermediate forms flow in, and doubts as to specific limits augment." He also adds that it is the best known species which present the greatest number of spontaneous varieties and sub-varieties. The Quercus robur has twenty-eight varieties, all of which, excepting six, are clustered round three sub-species, namely, Q. pedunculata sessiliflora, and pubescens. The forms which connect these three sub-species are comparatively rare; and, as Asa Gray again remarks, if these connecting forms which are now rare, were to become wholly extinct, the three sub- species would hold exactly the same relation to each other, as do tlie four or five provisionally admitted species which closely surround the typical Quercus robur. Finally. De Candolle admits that out of the 300 species, which will be enumerated in his Prodromus as belonging to the oak family, at least two-thirds are provisional species, that is, are not known strictly to fulfil the definition above given of a true species. It should be added that De Candolle no longer be- lieves that species are immutable creations, but concludes that the derivative theory is the most natural one, "and the most accordant with the known facts in palaeontology, geo- graphical botany and zoology, of anatomical structure and classification."

When a young naturalist commences the study of a group of organisms quite unknown to him, he is at first much per- plexed in determining what differences to consider as specific, and what as varietal; 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 doubt- ful forms. His 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 differ- ence 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

S8 ORIGIN OF SPECIES

impressions. As he extends the range of his observations, he will meet with more cases of difliculty ; for he will encounter a greater number of closely-allied forms. But if his observa- tions be widely extended, he will in the end generally be able to make up his own mind; 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 he comes to study allied forms brought from countries not now continuous, in which case he cannot hope to find inter- mediate links, he will be compelled to trust almost entirely to analogy, and his difificulties will 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 by 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 the highest importance for us, as being the first steps towards such slight varieties as are barely thought worth recording in works on natural his- tory. And I look at varieties which are in any degree more distinct and permanent, as steps towards more strongly- marked and permanent varieties; and at the latter, as lead- ing to sub-species, and then to species. The passage from one stage of difference to another may, in many cases, be the simple result of the nature of the organism and of the different physical conditions to which it has long been ex- posed ; but with respect to the more important and adaptive characters, the passage from one stage of difference to an- other, may be safely attributed to the cumulative action of natural selection, hereafter to be explained, and to the effects of the increased use or disuse of parts. A well-marked vari- ety may therefore be called an incipient species ; but whether this belief is justifiable must be judged by the weight of the various facts and considerations to be given throughout this work.

It need not be supposed that all varieties or incipient

DOMINANT SPECIES VARY MOST 69

species attain the rank of species. They may 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, and with plants by Gaston de Saporta. If a variety were to flourish so as to exceed in numbers the parent species, it w'ould 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 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 dif- ferences, is also applied arbitrarily, for convenience' sake.

WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES

VARY MOST

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 a future work the discussion of these diffi- culties, and the tables 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 neces- sarily 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.

Alphonse de Candolle and others have shown that plants

70 ORIGIN OF SPECIES

which have very v^^ide ranges generally present varieties ; and this might have been expected, as they are exposed to diverse physical conditions, and as they come into competi- tion (which, as we shall hereafter see, is an equally or more important circumstance) with different sets of organic beings. But my tables further show that, in any limited country, the species which are the 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 considera- tion from wide range, and to a certain extent from com- monness), oftenest 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 which range widely, are the most diffused in their own country, and are the most numerous in individuals, — which oftenest produce well-marked varie- ties, or, as I consider them, incipient species. And this, per- haps, 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 spe- cies which are already dominant will be the most likely to yield offspring, which, though in some slight degree modi- fied, still inherit those advantages that enabled their parents to become dominant over their compatriots. In these re- marks on predominance, it should be understood that refer- ence is made only to the forms which come into competition with each other, and more especially to the members of the same genus or class having nearly similar habits of life. With respect to the number of individuals or commonness of species, the comparison of course relates only to the members of the same group. One of the higher plants may be said to be dominant if it be more numerous in individuals and more widely diffused than the other plants of the same country, which live under nearly the same conditions. A plant of this kind is not the less dominant because some conferva inhabiting the water or some parasitic fungus is infinitely more numerous in individuals, and more widely diffused. But if the conferva or parasitic fungus exceeds its allies in the above respects, it will then be dominant within its own class.

SPECIES OF LARGER GENERA VARIABLE 71

SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY

MORE FREQUENTLY THAN THE SPECIES OF THE

SMALLER GENERA

If the plants inhabiting a country, as described in any Flora, be divided into two equal masses, all those in the larger genera (i.e., those including many species) being placed on one side, and all those in the smaller genera on the other side, the former will be found to include a some- what larger number of the very common and much diffused or dominant species. This 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 larger 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 generally have 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 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 wher- ever many closely related species (i.e., species of the same genus) have been formed, many varieties or incipient spe- cies ought, as a general rule, to be now forming. Where m.any large trees grow, we expect to find saplings. Where many species of a genus have been formed through varia- tion, circumstances have been favourable for variation; and

72 ORIGIN OF SPECIES

hence we might expect that the circumstances would gener- ally 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 tVi'O 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 presented 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 least genera, with from only one to four species, are altogether excluded from the tables. These 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 holds true, 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: inas- much 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, decline, 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 certainly holds good.

RESEMBLE VARIETIES 73

MANY OF THE SPECIES INCLUDED WITHIN THE LARGER

GENERA RESEMBLE VARIETIES IN BEING VERY CLOSELY,

BUT UNEQUALLY, RELATED TO EACH OTHER, AND

IN HAVING RESTRICTED RANGES

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 when in- termediate 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 anal- ogy 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 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 be- tween the species is often exceedingly small. I have en- deavoured 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 manu- facturing, many of the species already manufactured still to a certain extent resemble varieties, for they differ from each other by less than the usual amount of difference.

Moreover, the species of the larger 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 sec- tions, or lesser groups. As Fries has well remarked, little groups of species are generally clustered like satellites around other species. And what are varieties but groups of forms, unequally related to each other, and clustered round

74 ORIGIN OF SPECIES

certain forms — that is, round their parent-species? Un- doubtedly 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, we shall see how this may be explained, and how the lesser dif- ferences between varieties tend to increase into the greater differences between species.

There is one other point which is worth notice. Varieties generally have much restricted ranges : this statement is in- deed 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 would be reversed. But there is reason to believe that the 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. Wat- son 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 "j provinces ; whereas, the species to which these varieties belong range over 14 "3 provinces. So that the ac- knowledged varieties have nearly the same restricted aver- age range, as have the 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.

SUMMARY

Finally, varieties cannot be distinguished from species, — except, first, by the discovery of intermediate linking forms ; and, secondly, by a certain indefinite amount of difference between them; for two forms, if differing very little, are generally ranked as varieties, notwithstanding that they cannot be closely connected; but the amount of difference

SUMMARY 75

considered necessary to give to any two forms the rank of species cannot be defined. 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 varie- ties. In large genera the species are apt to be closely, but unequally, allied together, forming little clusters round other species. Species very closely allied to other species appar- ently have restricted ranges. In all these respects the spe- cies of large genera present a strong analogy with varieties. And we can clearly understand these analogies, if species once existed as varieties, and thus originated; whereas, these analogies are utterly inexplicable if species are independent creations.

We have, also, seen that it is the most flourishing or dom- inant species of the larger genera within each class which on an average yield the greatest number of varieties ; and varie- ties, as we shall hereafter see, tend to become converted into new and distinct species. Thus the larger genera 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.

CHAPTER III Struggle for Existence

Its bearing on natural selection — The term used in a wide sense — Geometrical ratio of increase — Rapid increase of naturalized animals and plants — Nature of the checks to increase — Competi- tion 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 indi- viduals 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.

B

EFORE 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: in- deed 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 in- stance, 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 organic being to another being, been perfected? We see these beautiful co-adaptations most plainly in the wood- pecker and the mistletoe; and only a little less plainly in the humblest parasite which clings to the hairs of a quad- ruped 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

_76

STRUGGLE FOR EXISTENCE 77

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 con- stitute 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, variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the 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 viseful, is preserved, by the term Natural Selection, in order to mark its relation to man's power of selection. But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient. We have seen that man by selection can certainly produce great re- sults, 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 immeasurably superior to man's feeble efforts, as the works of Nature are to those of Art.

We vv^ill now discuss in a little more detail the struggle for existence. In my future work this subject will be treated, as it well deserves, at 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, evirlcntly the result of his great horticultural knowledge. Nothing is

78 ORIGIN OF SPECIES

easier than to admit in words the truth of the universal struggle for Hfe, 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, 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 con- stantly destroying life; or we forget how largely these song- sters, 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.

THE TERM, STRtTGGLE FOR EXISTENCE, USED IN A LARGE SENSE

I should premise that I use this term in a large and meta- phorical 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 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 only one of an average 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 mistletoe 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 para- sites grow on the same tree, it languishes and dies. But several seedling mistletoes, growing close together on the same branch, may more truly be said to struggle with each other. As the mistletoe is disseminated by birds, its exist- ence depends on them; and it may metaphorically be said to struggle with other fruit-bearing plants, in tempting the

GEOMETRICAL RATIO OF INCREASE 79

birds to devour and thus disseminate its seeds. In these sev- eral senses, which pass into each other, I use for conveni- ence sake the general term of Struggle for Existence.

GEOMETRICAL RATIO OF INCREASE

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, other- wise, 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 an- other 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 can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increas- ing, 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 less than a thousand years, there would literally not be standing-room for his progeny. Lin- naeus 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 ele- phant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable mini- mum rate of natural increase ; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this

80 ORIGIN' OF SPECIES

be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive, 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 in- credible. So it is with plants: cases could be given of intro- duced 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, which are now the commonest over the wide plains of La Plata, cloth- ing square leagues of surface almost to the exclusion of everv other plant, 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 discover)-. In such cases, and endless others could be given, no one sup- poses, that the fertility of the animals or plants has been suddenly and tem.porarily increased in any sensible degree. The obvious explanation is that the conditions of life have been higlilv favourable, and that there has constantlv been less destruction of the old and young, and that nearly all the young have been enabled to breed. Their geometrical ratio of increase, the result of which never fails to be surprising, simplv explains their extraordinarily rapid increase and wide diffusion in their new homes.

In a state of nature almost ever}' full-gro\sTi plant annually produces seed, and amongst animals there are very few which do not annually pair. Hence we may confidently as- sert, that all plants and animals are tending to increase at a geometrical ratio, — that all would rapidly stock ever}- station in which they could anyhow exist, — and that this geomet- rical tendencv to increase must be checked bv destruction at

GEOMETRICAL RATIO OF INCREASE 81

some period of life. Our familiarity with the larger domes- tic animals tends, I think, to mislead us : we see no great destruction falling on them, but we do not keep in mind 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 pro- duce 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 hun- dreds of eggs, and another, like the hippobosca, a single one; but this difference does not determine how many indi- viduals of the two species can be supported in a district. A large number of eggs is of some importance to those spe- cies which depend on a fluctuating amount of food, for it allows them rapidly to increase in number. But the real im- portance 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 num- ber 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 suf- fice 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 ani- mal or plant depends only indirectly on the number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the fore- going considerations always in mind — never to forget that every single organic being may be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably

82 . ORIGIN OF SPECIES

falls either on the young or old, during each generation or at recurrent intervals. Lighten any check, mitigate the de- struction ever so little, and the number of the species will almost instantaneously increase to any amount.

NATURE OF THE CHECKS TO INCREASE

The causes which check the natural tendency of each spe- cies to increase are most obscure. Look at the most vig- orous species; by as much as it swarms in numbers, by so much will it tend to increase still further. We know not exactly what the checks are even in a single instance. Nor will this surprise any one who reflects how ignorant we are on this head, even in regard to mankind, although so incom- parably better known than any other animal. This subject of the checks to increase has been ably treated by several authors, and I hope in a future work to discuss it at con- siderable length, more especially in regard to the feral ani- mals of South America. Here I will make only a few re- marks, 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, it appears that the seedings 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 357 no less than 295 were destroyed, chiefly by slugs and in- sects. 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 gradually kill the less vigorous, though fully grown plants ; thus out of twenty spe- cies growing on a little plot of mown turf (three feet by four) nine species perished, from the other species being al- lowed to grow up freely.

The amount of food for each species of course gives the extreme limit to which each can increase; but very fre-

NATURE OF THE CHECKS TO INCREASE 83

quently 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, al- though hundreds of thousands of game animals are now annually shot. On the other hand, in some cases, as with the elephant, none are destroyed by beasts of prey; for even the tiger in India most rarely dares to attack a j-oung ele- phant protected by its dam.

Climate plays an important part in determining the aver- age numbers of a species, and periodical seasons of extreme cold or drought seem to be the most effective of all checks. I estimated (chiefly from the greatly reduced numbers of nests in the spring) that the winter of 1854-5 destroyed four- fifths of the birds in my own grounds ; and this is a tremen- dous 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, acts directly, it will be the least vigorous individuals, 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 suft"ering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these ene- mies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers; and

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as each area is already fully stocked with inhabitants, the other species must 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 fa- voured, 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, de- creases northwards; hence in going northwards, or in as- cending 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 v/e 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 clearly see in the prodigious number of plants which in our gardens can perfectly well endure our climate, but which never became naturalised, for they can- not compete with our native plants nor resist destruction by our native animals.

When a species, owing to highly favoured circumstances, increases inordinately in numbers in a small tract, epidemics — at least, this seems generally to occur with our game ani- mals— often ensue; and here we have a limiting check inde- pendent 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 fa- cility of diffusion amongst the crowded animals, been dis- proportionally 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 indi- viduals 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 sea- son, 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

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from a few wheat or other such plants in a garden : I have in this case lost every single seed. This view of the neces- sity 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 exist; and that of some social plants being social, that is abounding in individuals, even on the extreme verge of their range. For in such cases, we may believe, that a plant could exist only where the condi- tions 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 intercrossing, and the ill effects of close interbreeding, no doubt come into play in many of these cases; but I will not here enlarge on this subject.

COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS

TO EACH OTHER IN THE STRUGGLE

FOR EXISTENCE

Many cases are on record showing how complex and un- expected are the checks and relations between organic beings, which have to struggle together in the same coun- try. I will give only a single instance, which, though a simple one, 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 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 pass- ing 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 com- mon in the plantations, which were not to be seen on the heath; and the heath was frequented by two or three dis- tinct insectivorous birds. Here we see how potent has been

86 ORIGIN OF SPECIES

the effect of the introduction of a single tree, nothing what- ever else having been done, with the exception of the land having 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 en- closed, and self-sown firs are now springing up in multitudes, so close together that all cannot live. 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 per- petually 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 en- closed, 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 efifectually 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 num- ber in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would lessen the number of the navel-frequenting flies —

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then cattle and horses would become feral, and this would certainly greatly alter (as indeed I have observeu in parts of South America) the vegetation: this again would largely affect the insects; and this, as we have just seen in Stafford- shire, the insectivorous birds, and so onwards in ever-in- creasing circles of complexity. Not that under nature the relations will ever be as simple as this. Battle within battle must be continually recurring with varying success; and yet in the long-run the forces are so nicely balanced, that the face of nature remains for long periods of time uniform, though assuredly the merest trifle would 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, 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 is never visited in my garden by insects, and consequently, from its peculiar structure, never sets a seed. Nearly all our orchidaceous plants absolutely require the visits of insects to remove their pollen-masses and thus to fertilise them. I find from experi- ments that humble-bees are almost indispensable to the fer- tilisation of the heartsease (Viola tricolor), for other bees do not visit this flower. I have also found that the visits of bees are necessary for the fertilisation of some kinds of clover; for instance, 20 heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but 20 other heads protected from bees produced not one. Again, 100 heads of red clover (T. pratense) produced 2,700 seeds, but the same number of protected heads produced not a single seed. Humble-bees alone visit red clover, as other bees cannot reach the nectar. It has been suggested that moths may fertilise the clovers; but I doubt whether they could do so in the case of the red clover, from their weight not being suflkient to depress the wing petals. Hence we may infer as highly probable that, if the whole genus of humble-bees

88 ORIGIN OF SPECIES

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 measure upon the number of field-mice, which destroy their combs and nests ; and Col. Newman, who has long attended to the habits of humble-bees, believes that "more than two-thirds of them are thus destroyed all over Eng- land." Now the number of mice is largely dependent, as every one knows, on the number of cats; and Col. 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 ani- mal in larje numbers in a district might determine, through the intervention first of mice and then of bees, the fre- quency 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 will concur in deter- mining 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 virgin forest. What a struggle must have gone on during long centuries between the sev- ' eral kinds of trees, 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, all feeding on each other, or on the trees, 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 fall

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to the ground according to definite laws; but how simple is tlie problem where each shall fall compared to that of the action and reaction of the innumerable plants and animals which have determined, in the course of centuries, the pro- portional 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 likewise sometimes the case with those which may be strictly said to struggle with each other for existence, as in the case of locusts and grass- feeding quadrupeds. But the struggle will almost invariably 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 varie- ties of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest soon de- cided: for instance, if several varieties of wheat be sown together, and the mixed seed be resown, some of the varie- ties 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 supplant the other varieties. To keep up a mixed stock of even such extremely close varieties as the variously-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 number 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 varie- ties of any of our domestic plants or animals have so ex- actly the same strength, habits, and constitution, that the original proportions of a mixed stock (crossing being pre- vented) could be kept up for half-a-dozen generations, if they were allowed to struggle together, in the same manner as beings in a state of nature, and if the seed or young were not annually preserved in due proportion.

90 ORIGIN OF SPECIES

STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE SAME SPECIES

As the species of the same genus usually have, though by- no means invariably, much similarity in habits and consti- tution, and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the 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 cli- mates ! In Russia the small Asiatic cockroach has every- where driven before it its great congener. In Australia the imported hive-bee is rapidly exterminating the small, stingless native bee. One species of charlock has been known to supplant another species; 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 ; 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 mose essential yet often hidden manner, to that of all the other organic beings, with M'^hich 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 with 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

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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, it may be suspected that the chief use of the nutriment in the seed is to favour the growth of the seedlings, whilst struggling with other plants growing all around.

Look at a plant in the midst of its range, why does it not double or quadruple its numbers? We know that it can per- fectly 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 wish in imagination to give the plant the power of increasing in number, we should have to give it some ad- vantage over its competitors, or over the animals which prey 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 wc have reason to believe that only a few plants or animals range so far, that they are de- stroyed exclusively by the rigour of the climate. 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 indi- viduals of the same species, for the warmest or dampest spots.

Hence we can see that when a plant or animal is placed in a new country amongst new competitors, the conditions of its life will generally be changed in an essential manner, although the climate may be exactly the same as in its former home. If its average numbers are to increase in its new home, we should have to modify it in a different way to what we should have had to do 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 imagination to give to any one

92 ORIGIN OF SPECIES

species an advantage over another. Probably in no single instance should we know what to do. This ought to con- vince us of our ignorance on the mutual relations of all organic beings ; a conviction as necessary, as it is difificult to acquire. All that we can do, is to keep steadily in mind that each organic being is striving to increase in a geomet- rical 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 inces- sant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.

CHAPTER IV Natural Selection ; or the Survival of the Fittest

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 inter- crosses between individuals of the same species — Circumstances favourable and unfavourable to the results of 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 — Advance in organisation — Low forms preserved — Convergence of Character — Indefinite multiplication of species — Summary.

HOW will the struggle for existence, briefly discussed in the last chapter, act in regard to variation? Can the principle of selection, v^'hich we have seen is so potent in the hands of man, apply under nature? I think we shall see that it can act most efficiently. Let the endless number of slight variations and individual differences occur- ring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind ; as well as the strength of the hereditary tendency. Under domestication, it may be truly said that the whole organisation becomes in some degree plastic. But the variability, which we almost universally meet with in our domestic productions, js not directly pro- duced, as Hooker and Asa Gray have well remarked, by man ; he can neither originate varieties, nor prevent their occur- rence ; he can only preserve and accumulate such as do occur. Unintentionally he exposes organic beings to new and chang- ing conditions of life, and variability ensues; but similar changes of conditions might and do occur under nature. Let it also 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; and conse-

93

94 ORIGIN OF SPECIES

quently what infinitely varied diversities of structure might be of use to each being under changing conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other vari- ations useful in some wzy to each being in the great and com- plex battle of life, should occur in the course of many suc- cessive generations? If such do occur, can we doubt (re- membering 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 injuri- ous would be rigidly destroyed. This preservation of favour- able individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions.

Several writers have misapprehended or objected to the term Natural Selection. Some have even imagined that nat- ural selection induces variability, whereas it implies only the preservation of such variations as arise and are beneficial to the being under its conditions of life. No one objects to agriculturists speaking of the potent effects of man's selec- tion; and in this case the individual differences given by nature, which man for some object selects, must of necessity first occur. Others have objected that the term selection im- plies conscious choice in the animals which become modified; and it has even been urged that, as plants have no volition, natural selection is not applicable to them! In the literal sense of the word, no doubt, natural selection is a false term ; but who ever objected to chemists speaking of the elective affinities of the various elements? — and yet an acid cannot strictly be said to elect the base with which it in preference combines. It has been said that I speak of natural selection as an active power or Deity; but who objects to an author speaking of the attraction of gravity as ruling the movements

NATURAL SELECTION 95

of the planets? Every one knows what is meant and is im- plied by such metaphorical expressions ; and they are almost necessary for brevity. So again it is difficult to avoid per- sonifying the word Nature; but I mean by Nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as ascertained by us. With a little familiarity such superficial objections will be forgotten.

We shall best understand the probable course of natural selection by taking the case of a country undergoing some slight physical change, for instance, of climate. The propor- tional numbers of its inhabitants will almost immediately un- dergo a change, and some species will probably become ex- tinct. We may conclude, from what we have seen of the in- timate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of the inhabitants, independently of the change of climate itself, would seriously affect the others. If the country were open on its borders, new forms would certainly immigrate, and this would likewise seriously disturb the rela- tions of some of the former inhabitants. Let it be remem- bered 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 are'a been open to immigration, these same places would have been seized on by intruders. In such cases, slight modifications, which in any way favoured the individuals of any species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would have free scope for the work of improvement.

We have good reason to believe, as shown in the first chap- ter, that changes in the conditions of life give a tendency to increased variability; and in the foregoing cases the con- ditions have changed, and this would manifestly be favour- able to natural selection, by affording a better chance of the occurrence of profitable variations. Unless such occur, nat- ural selection can do nothing. Under the term of "vari-

96 ORIGIN OF SPECIES

ations," it must never be forgotten that mere individual dif- ferences are included. As man can produce a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selec- tion, but far more easily from having incomparably longer time for action. Nor do I believe that any great physical change, as of climate, or any unusual degree of isolation to check immigration, Is necessary in order that new and un- occupied places should be left, for natural selection to fill up by 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 species would often give it an ad- vantage over others; and still further modifications of the same kind would often still further increase the advantage, as long as the species continued under the same conditions of life and profited by similar means of subsistence and de- fence. No country can be named in which all the native in- habitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of them could be still better adapted or improved; for in all- countries, the natives have been so far conquered by natural- ised productions, that they have allowed some foreigners to take firm possession of the land. And as foreigners have thus in every country beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted the intruders.

As man can produce, and certainly has produced, a great result by his methodical and unconscious means of selection, what may not natural selection effect? Man can act only on external and visible characters : Nature, if I may be allowed to personify the natural preservation or survival of the fit- test, cares nothing for appearances, except in so far as they are 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: Na- ture only for that of the being which she tends. Every selected character is fully exercised by her, as is implied by the fact of their selection. Man keeps the natives of many climates in the same country; he seldom exercises each se-

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lected character in some peculiar and filling 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 de- stroy 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 the eye or to be plainly useful to him. Under nature, the slight- est differences of structure or constitution may well turn the nicely-balanced scale in the struggle for life, and so be pre- served. How fleeting are the wishes and efforts of man ! how short his time ! and consequently how poor will be his results, compared with those accumulated by Nature during whole geological periods? Can we wonder, then, that Na- ture'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, the slightest variations; rejecting those that are bad, preserving and add- ing up all that are good; silently and insensibly working, whenever and wherever opportunity offers, at the improve- ment of each organic being in relation to its organic and in- organic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages, and then so imperfect is our view into long- past geological ages, that we see only that the forms of life are now different from what they formerly were.

In order that any great amount of modification should be effected in a species, a variety when once formed must again, perhaps after a long interval of time, vary or present indi- vidual differences of the same favourable nature as before ; and these must be again preserved, and so onwards step by step. Seeing that individual differences of the same kind perpetually recur, this can hardly be considered as an unwar- rantable assumption. But whether it is true, we can judge

D— lie XI

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only by seeing how far the hypothesis accords with and ex- plains the general phenomena of nature. On the other hand, the ordinary belief that the amount of possible variation is a strictly limited quantity is likewise a simple assumption.

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, 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 num- bers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey — so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence natural selection might be 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 par- ticular colour would produce little effect: we should remem- ber how essential it is in a flock of white sheep to destroy a lamb with the faintest trace of black. We have seen how the colour of the hogs, which feed on the "paint-root" in Virginia, determines whether they shall live or die. In plants, the down on the fruit and the colour of the flesh are considered by botanists as characters of the most trifling im- portance : yet we hear from an excellent horticulturist. Down- ing, 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.

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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., have no doubt produced some direct effect. It is also necessary to bear in mind that, owing to the law of cor- relation, when one part varies, and the variations are accu- mulated through natural selection, other modifications, often of the most unexpected nature, will ensue.

As we see that those variations which, under domestica- tion, appear at any particular period of life, tend to reappear in the offspring at the same period; — for instance, in the shape, size, and flavour of 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 oi 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 selec- tion, 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 con- cern the mature insect; and these modifications may effect, through correlation, the structure of the adult. So, con- versely, modifications in the adult may affect the structure of the larva ; but in all cases natural selection will ensure that they shall not be injurious: for if they were so, the species w'ould become extinct.

Natural selection will modif)^ the structure of the 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 whole community; if the community 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

100 ORIGIN OF SPECIES

works of natural history, I cannot find one case which will bear investigation. A structure used only once in an ani- mal's 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 unhatched birds, used for breaking the egg. It has been asserted, that of the best short-beaked tumbler-pigeons a greater number 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