16 1978 University l , Irvine l"'nr r ^ /i^9^. DARWINISM DARWINISM AN EXPOSITION OF TliS THEORY OF NATURAL SELECTION — I WITH SOME OF ITS APPLICATIOXS BY ALFEED EUSSEL WALLACE LL.D., F.L.S., ETC. WITH M/S^lWfW-iX^RATIQNS O. A. WKITING; Uniyersh Y V OF V Utah. Salt Lake City, - Utah. Hontion MACMILLAX AND CO. AND XEW YORK 1891 All rights reserved Presswork by John Wilson and Son, University Press. PEEFACE The present work treats the problem of the Origin of Species on the same general lines as were adopted by Darwin ; but from the standpoint reached after nearly thirty years of discussion, with an abundance of new facts and the advocacy of many new or old theories. While not attempting to deal, even in outlinCj with the vast subject of evolution in general, an endeavour has been made to give such an account of the theory of Natural Selec- tion as may enable any intelligent reader to obtain a clear conception of Darwin's work, and to understand something of the power and range of his great principle. Darwin Avrote for a generation which had not accepted evolution, and which poured contempt on those who upheld the derivation of species from species by any natural law of descent. He did his work so well that " descent with modification" is now universally accepted as the order of nature in the organic world; and the rising generation of naturalists can hardly realise the novelty of this idea, or that their fathers considered it a scientific heresy to be condemned rather than seriously discussed. The objections now made to Darwin's theory apply, solely, to the particular means by which the change of species has been brought about, not to the fact of that change. The objectors seek to minimise the agency of natural selection and to subordinate it to laws of variation, of use and disuse, of intelligence, and of heredity. These views and objections 3bo:i VI PREFACE are urged with much force and more confidence, and for the most i)art by the modern school of kdjoratory naturalists, to whom the peculiarities and distinctions of species, as such, their distribution and their affinities, have little interest as compared with the problems of histology and embryology, of physiology and morphology. Their work in these depart- ments is of the greatest interest and of the highest importance, but it is not the kind of work which, by itself, enables one to form a sound judgment on the questions involved in the action of the law of natural selection. These rest mainly on the external and \ital relations of species to species in a state of nature — on what has been well termed by Semper the "physiology of organisms," rather than on the anatomy or physiology of organs. It has always been considered a weakness in Dar^vin's work that he based his theory, primarily, on the evidence of variation in domesticated animals and cultivated plants. I have endeavoured to secure a firm foundation for the theory in the variations of organisms in a state of nature ; and as the exact amount and precise character of these variations is of paramount imj^ortance in the numerous problems that arise when we apply the theory to explain the facts of natiure, I have endeavoured, by means of a series of diagrams, to exhibit to the eye the actual variations as they are found to exist in a sufficient number of species. By doing this, not only does the reader obtain a better and more precise idea of variation than can be given l)y any number of tabular state- ments or cases of extreme individual variation, but we obtain a basis of fact by which to test the statements and objections usually put forth on the subject of specific variability ; and it will be found that, throughout the work, I have frecpiently to appeal to these diagrams and the facts they illustrate, just as Darwin Avas accustomed to appeal to the facts of variation among dogs and pigeons. PREFACE vil I have also made what appears to me an important change in the arrangement of the subject. Instead of treating first the comparatively difficult and unfamiliar details of variation, I commence with the Struggle for Existence, which is really the fundamental phenomenon on which natural selection depends, while the particular facts which illustrate it are comparatively familiar and very interesting. It has the further advantage that, after discussing variation and the effects of artificial selection, we proceed at once to explain how natural selection acts. Among the subjects of novelty or interest discussed in this volume, and which have important bearings on the theory of natural selection, are : (1) A proof that all specific characters are (or once have been) either useful in themselves or cor- related mth useful characters (Chap. VI) ; (2) a proof that natural selection can, in certain cases, increase the sterility of crosses (Chap. VII) ; (3) a fuller discussion of the colour relations of animals, with additional facts and arguments on the origin of sexual difierences of colour (Chaps. YIII— X) ; (4) an attempted solution of the difficulty presented by the occurrence of both very simple and very complex modes of securing the cross-fertilisation of plants (Chap. XI) ; (5) some fresh facts and arguments on the wind-carriage of seeds, and its bearing on the wide dispersal of many arctic and alpine plants (Chap. XII) ; (6) some new illustrations of the non- heredity of acquired characters, and a proof that the effects of use and disuse, even if inherited, must be overpowered by natural selection (Chap. XIV) ; and (7) a new argument as to the nature and origin of the moral and intellectual faculties of man (Chap. XV). Although I maintain, and even enforce, my differences from some of Darwin's views, my whole work tends forcibly to illustrate the overwhelming importance of Natural Selec- tion over all other agencies in the production of new species. Vlll PREFACE I thus take up Darwin's earlier position, from which he some- what receded in the later editions of his Avorks, on account of criticisms and objections which I have endeavoured to show are unsound. Even in rejecting that phase of sexual selection depending on female choice, I insist on the greater efficacy of natural selection. This is pre-eminently the Darwinian doctrine, and I therefore claim for my book the position of being the advocate of pure Darwinism. I wish to express my obligation to Mr. Francis Darwin for lending me some of his father's unused notes, and to many other friends for facts or information, which have, I believe, been acknowledged either in the text or footnotes. Mr. James Sime has kindly read over the proofs and given me many useful suggestions ; and I have to thank Professor Meldola, Mr. Hemsley, and Mr. E. B. Poulton for valuable notes or corrections in the later chapters in "which their special subjects are touched upon. GoDALMiNG, March 1889. CONTENTS CHAPTER I ORIGIN Definition of species — Special creation — The early transmutationists — Scientific opinion before Darwin — The prohJem before Darwin — The change of opinion effected by Darwin — The Darwinian theory — Pro- posed mode of treatment of the subject . . . Paiges 1-13 CHAPTEE II THE STRUGGLE FOR EXISTENCE Its importance — The struggle among plants— Among animals — Illustrative cases — Succession of trees in forests of Denmark — The struggle for existence on the Pampas — Increase of organisms in a geometrical ratio — Examples of rapid increase of animals — Rapid increase and wide spread of plants — Great fertility not essential to rapid increase — Struggle between closely allied species most severe — The ethical aspect of the struggle for existence . . . 14-40 CHAPTER III THE VARIABILITY OF SPECIES IN A STATE OF NATURE Importance of variability — Popular ideas regarding it— Variability of the lower animals — The variability of insects — Variation among lizards— CONTENTS A^'ariation among birds — Diagrams of bird -variation — Number of varying individuals — Variation in the mammalia — Variation in internal organs — Variations in the skull — Variations in the habits of animals— The variability of plants — Species which vary little — Con- cluding remarks ..... Pages 41-82 CHAPTER IV VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS The facts of variation and artificial selection — Proofs of the generality of variation — Variations of apples and melons— Variations of flowers — Variations of domestic animals — Domestic pigeons — Acclimatisation — Circumstances favourable to selection by man — Conditions favour- able to variation — Concluding remarks . . . 83-101 CHAPTER V NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST Effect of struggle for existence under unchanged conditions — The eJBFect under change of conditions — Divergence of character — In insects — In birds — In mammalia — Divergence leads to a maximum of life in each area — Closely allied species inhabit distinct areas — Adaptation to conditions at various periods of life — The continued existence of low- forms of life — Extinction of low types among the higher animals — Circumstances favourable to the origin of new species — Probable origin of the dippers — The importance of isolation — On the advance of organisation by natural selection — Summary of the first five chapters ....... 102-125 CHAPTER VI DIFFICULTIES AND OBJECTIONS Difficulty as to smallness of variations — As to the right variations occur- ring when required — The beginnings of important organs — Tlie mam- mary glands— The eyes of flatfish — Origin of the eye — Useless or non-adaptive characters — Recent extension of the region of utility in COXTEXTS xi plants — The same in animals — Uses of tails — Of the horns of deer — Of the scnle-ornamentation of reptiles — Instability of non-adaptive characters — Delbceiif's law — Xo "specific" character proved to be useless — The swamping effects of intercrossing — Isolation as prevent- ing intercrossing — Gulick on the effects of isolation — Cases in which isolation is ineffective ..... Pages 126-151 CHAPTEE YIl ON THE INFERTILITY OF CROSSES BET^YEEN DISTINCT SPECIES AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING Statement of the problem — Extreme susceptibility of the reproductive functions — Reciprocal crosses — Individual differences in respect to cross -fertilisation — Dimorphism and trimorphism ^-mong plants — Cases of the fertility of hybrids and of the infertility of moiigi-els — The effects of close interbreeding — Mr. Huth's objections — Fertile hybrids among animals — Fertility of hybrids among plants — Cases of sterility of mongrels — Parallelism beto'een crossing and change of conditions — Remarks on the facts of hybridity — Sterility due to changed conditions and usually correlated with other characters — Correlation of colour with constitutional peculiarities — The isolation of varieties by selective association — The influence of natural selection upon sterility and fertility — Physiological selection — Summary and concludiuct remarks ..... 152-186 CHAPTER VIII THE ORIGIN AND USES OF COLOI'R IN ANBL\LS The Darwinian theory threw new light on organic colour — The problem to be solved — The constancy of animal colour indicates utility — Colour and environment — Arctic animals white — Exceptions prove the rule — Desert, forest, nocturnal, and oceanic animals — General theories of animal colour — Variable protective colouriug — Mr. Poulton's experi- ments— Special or local colour adaptations — Imitation' of particular objects — How they have been produced — Special protective colouring of butterflies— Protective resemblance among marine animals— Pro- tection by terrifying enemies — Alluring coloration — The coloration of birds' eggs — Colour as a means of recognition — Summary of the preceding exposition — Influence of locality or of climate on colour — Concluding remarks .... 187-231 xn CONTEXTS CHAPTER IX WARNING COLORATION AND I^IIMICRY The skunk as an example of warning coloration — Warning colours among insects — Butterflies — Caterpillars — Mimicry — How mimicry has been produced — Heliconidse — Perfection of the imitation — Other cases of mimicry among Lepidoptera — Mimicry among protected groups — Its explanation — Extension of the principle— Mimicry in other orders of insects — Mimicry among the vertebrata — Snakes — The rattlesnake and the cobra — Mimicry among birds — Objections to the theory of mimicry — Concluding remarks on warning colours and mimicry Pages 232-267 CHAPTER X COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX Sex colours in the mollusca and crustacea^In insects — In butterflies and moths — Probable causes of these colours — Sexual selection as a supposed cause — Sexual coloration of birds — Cause of dull colours of female birds — Relation of sex colour to nesting habits— Sexual colours of other vertebrates — Sexual selection by the struggles of males — Sexual characters due to natural selection — Decorative plumage of males and its effect on the females — Display of decorative plumage by the males — A theory of animal coloration — The origin of accessory plumes — Development of accessory plumes and their display — The effect of female preference will be neutralised by natural selection — General laws of animal coloration — Concluding remarks . 268-300 CHAPTER XI THE SPECIAL COLOURS OF PLANTS : THEIR ORIGIN AND PURPOSE The general colour relations of plants— Colours of fruits— The meaning of nuts— Edible or attractive fruits— The colours of flowers— Modes of securing cross-fertilisation— The interpretation of the facts— Summary CONTENTS Xlll of additional facts bearing on insect fertilisation — Fertilisation of flowers by birds — Self-fertilisation of flowers — Difficulties and con- tradictions— Intercrossing not necessarily advantageous — Supposed evil results of close interbreeding — How the struggle for existence acts among flowers — Flowers the product of insect agency — Concluding remarks on colour in nature .... Pages 301-337 CHAPTER XII THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS The facts to be explained — The conditions which have determined dis- tribution— The permanence of oceans — Oceanic and continental areas — Madagascar and New Zealand — The teachings of the thousand- fathom line — The distribution of marsupials — The distribution of tapirs — Powers of dispersal as illustrated by insular organisms — Birds and insects at sea — Insects at great altitudes — The dispersal of plants — Dispersal of seeds by the wind — Mineral matter carried by the wind — Objections to the theory of wind-dispersal answered — Explanation of north temperate plants in the southern hemisphere — No proof of glaciation in the tropics — Lower temperature not needed to explain the facts — Coucludiug remarks .... 338-374 CHAPTEE XIII THE GEOLOGICAL EVIDENCES OF EVOLUTION What we may expect — The number of known species of extinct animals — Causes of the imperfection of the geological record — Geological evidences of evolution— Shells — Crocodiles — The rhinoceros tribe — The pedigree of the horse tribe — Development of deer's horns — Brain development — Local relations of fossil and living animals — Cause of extinction of large animals — Indications of general progress in plants and animals— The progressive development of plants— Possible cause of sudden late appearance of exogens— Geological distribu- tion of insects — Geological succession of vertebrata — Concluding remarks .,..■•• 375-409 CONTENTS CHAPTER XIV FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY Fundamental difficulties and objections — Mr, Herbert Spencer's factors of organic evolution — Disuse and effects of withdrawal of natural selection — Supposed effects of disuse among ^\'ild animals — Difficulty as to co-adaptation of parts by variation and selection — Direct action of the environment — The American school of evolutionists — Origin of the feet of the ungulates —Supposed action of animal intelligence — Semper on the direct influence of the environment — Professor Geddes's theory of variation in plants— Objections to the theory — On the origin of spines — Variation and selection overpower the effects of use and disuse — Supposed action of the environment in imitating varia- tions— Weismann's theory of heredity — The cause of variation — The non -heredity of acquired characters — The theory of instinct — Con- cluding remarks . . . . . Pages 410-444 CHAPTER XV DARWINISM APPLIED TO MAN General identity of human and animal structure — Rudiments and varia- tions showing relation of man to other mammals — The embryonic development of man and other mammalia — Diseases common to man and the lower animals — The animals most nearly allied to man — The brains of man and apes — External differences of man and apes — Summary of the animal characteristics of man — The geological anticiuity of man — The probable birthplace of man— The origin of the moral and intellectual nature of man — The argument from continuity — The origin of the mathematical faculty — The origin of the musical and artistic faculties — Independent proof thnt these faculties have not been developed by natural selection— The inter- pretation of the facts — Concluding remarks . . 445-478 INDEX 479-494 LIST OF ILLUSTRATIONS Portrait of Author ..... Frontispiece Map showing the 1000-fathom line . . To face page 349 FIG. PAGE 1. Diagram of Variations of Lacerta muralis . . 47 2. ,, Variation of Lizards . . . .48 3. ,, Variation of wings and tail of Birds . 53 4. ,, Variation op Dolicho^^yx oryziyorus . 55 5. ,, Variation of Agel^us phoeniceus . . 56 6. ,, Variation of Cardinalis virginianus , 58 7. „ Variation of tarsus and toes . . 60 8. ,, Variation of Birds in Leyden Museum . 61 9. ,, Variation of Icterus Baltimore . . 63 10. „ Variation of Agel^us ph(eniceus . . 64 11. „ Curves of Variation . . . .64 12. ,, Variation of Cardinalis yirginianus . 65 13. ,, Variation of Sciurus carolinensis . . 67 14. ,, Variation of skulls of "Wolf , . 70 15. ,, Variation of skulls of Ursus labiatus . 72 16. ,, Variation of skulls of Sus cristatus . 73 17. Primula veris (Cowslip). Yrom Daxv^m's Forms of Floivers . 157 18. Gazella scemmerringi (to show recognition marks) . . 219 19. Recognition marks of African Plovers (from Seebohm's Charadriddoe ....... 221 XVI LIST OF ILLUSTRATIONS no. PAGE 20. Recognition of GBdicnemus veiimiculatus and (E. senega- LENSis (from Seebohm's Charadriadce) . . . 223 21. Recognition of Cursorius chalcopterus and C. gallicus (from Seebohm's Charadriadce) .... 224 22. Recognition of Scolopax megala- and S. stenura (from Seebohm's Charadriadce) ..... 225 23. Methona psidii and Leptalis orise .... 241 24. Opthalmis lincea and Artaxa simulans (from the Official Narrative of the Voyage of the Challenger) . . . 247 25. Wings of Ituna Ilione and Thyridia megisto (from Pro- ceedings of the Entomological Society) .... 251 26. Mygnimia aviculus and Coloborhombus fasciatipennis . 259 27. Mimicking Insects from the Philippines (from Samper's Animal Life) ....... 260 28. Malva sylvestris and M. rotundifolia (from Lubbock's British Wild Flowers in JRelation to Insects) . . .311 29. Lythrum salicaria, three forms of (from Lubbock's British Wild Flowers iii Relation to Insects) . . , .312 30. Orchis pyramidalis (from Darwin's Fertilisation of Orchids) . 314 31. Humming-bird fertilising Marcgravia nepenthoides . 320 32. Diagram of mean height of Land and depth of Oceans 345 33. Geological development of the Horse tribe (from Huxley's American Addresses) . . . . . .388 34. Diagram illustrating the Geological Distribution of Plants (from Ward's Sketch of Palceohotany) . . . 402 35. Transformation of Artemia salina to A. Milhausenii (from Semper's Animal Life) ..... 426 36. Branchipus stagnalis and Artemia salina (from Semper's Animal Life) ....... 427 37. Chimpanzee (Troglodytes nicer) .... 454 CHAPTER I A^TIAT ARE "SPECIES," AND WHAT IS INIEANT BY THEIR " ORIGIN " Definition of species — Special creation — The early Transmutationists — Scientific opinion before Darwin — Tlie problem before Darwin — The change of opinion effected by Darwin — The Darwinian theory — Proposed mode of treatment of the subject. The title of Mr. Darwin's great work is — On the Origin of Species by means of Natural Selection and the Preservation of Favoured Races in the Struggle for Life. In order to ap- preciate fully the aim and object of this work, and the change which it has effected not only in natural history but in many other sciences, it is necessary to form a clear con- ception of the meaning of the term " species," to know what was the general belief regarding them at the time when Mr. Darwin's book first appeared, and to understand what he meant, and what was generally meant, by discovering their " origin." It is for want of this preliminary knowledge that the majority of educated persons who are not naturalists are so ready to accept the innumerable objections, criticisms, and difficulties of its opponents as proofs that the Darwinian theory is unsound, while it also renders them unable to ap- preciate, or even to comprehend, the vast change which that theory has effected in the whole mass of thought and opinion on the great question of evolution. The term " species " was thus defined by the celebrated botanist De Candolle : " A species is a collection of all the individuals which resemble each other more than they resemble anything else, which can by mutual fecundation © B DARWINISM CHAP. produce fertile individuals, and which reproduce themselves by i;enenitiou, in such a manner that we may from analogy suppose them all to have sprung from one single individual." And the zoologist Swainson gives a somewhat similar defini- tion : " A species, in the usual acceptation of the term, is an animal which, in a state of nature, is distinguished by certain peculiarities of form, size, colour, or other circumstances, from another animal. It propagates, 'after its kind,' individuals perfectly resembling the parent; its peculiarities, therefore, are permanent." ^ To illustrate these definitions we will take two common English birds, the rook (Corvus frugilegus) and the crow (Corvus corone). These are distinct species^ because, in the first place, they always differ from each other in certain slight peculiarities of structure, form, and habits, and, in the second place, because rooks always produce rooks, and crows produce crows, and they do not interbreed. It was therefore con- cluded that all the rooks in the world had descended from a single pair of rooks, and the crows in like manner from a single pair of crows, while it was considered impossible that crows could have descended from rooks or vice versa. The "origin" of the first pair of each kind was a mystery. Similar remarks may be applied to our two common plants, the sweet violet (Viola odorata) and the dog violet (Viola canina). These also produce their like and never produce each other or intermingle, and they were therefore each supposed to have sprung from a single individual whose " origin " was unknown. But besides the crow and the rook there are about thirty other kinds of bii'ds in various parts of the world, all so much like our species that they receive the common name of crows ; and some of them differ less from each other than does our crow from our i-ook. These are all species of the genus Corvus, and were therefore believed to have been always as distinct as they are now, neither more nor less, and to have each descended from one pair of ances- tral crows of the same identical species, which themselves had an unknown "origin." Of violets there are more than a hundred diff'erent kinds in various parts of the Avorld, all diffeiing very slightly from each other and forming distinct ^ Geography ami Class ijicalion 0/ Animals, p. 350. WHAT ARE SPECIES sjjecies of the genus Viola. But, as these also each produce their like and do not intermingle, it was believed that every one of them had always been as distinct from all the others as it is now, that all the indi^-iduals of each kind had descended from one ancestor, but that the " origin " of these hundred slightly differing ancestors was unknown. In the words of Sir John Herschel, quoted by Mr. Darwin, the origin of such sj^ecies was " the mystery of mysteries." The Early Transmutationists. A few great naturalists, struck by the very slight difference between many of these species, and the numerous links that exist between the most different forms of animals and plants, and also observing that a great many species do vary con- siderably in their forms, colours, and habits, conceived the idea that they might be all produced one from the other. The most eminent of these T\'riters was a great French natiu^alist, Lamarck, who published an elaborate work, the Philosophie Zoologique, in which he endeavoured to prove that all ani- mals whatever are descended from other species of animals. He attributed the change of species chiefly to the effect of changes in the conditions of life — such as climate, food, etc. — and especially to the desires and efforts of the animals them- selves to improve their condition, leading to a modification of form or size in certain parts, owing to the well-known physio- logical law that all organs are strengthened by constant use, while they are weakened or even completely lost by disuse. The arguments of Lamarck did not, however, satisfy naturalists, and though a few adopted the A'iew that closely allied species had descended from each other, the general belief of the educated public was, that each species was a " special creation " quite independent of all others ; while the great body cf naturalists equally held, that the change from one species to another by any known law or cause was impossible, and that the " origin of species " was an unsolved and probably insoluble problem. The only other important work dealing "s^ith the question was the celebrated Vestiges of Creation, published anonymously, but now acknowledged to have been ^vritten by the late Eobert Chambers. In this work the action of general laws was traced throughout the DARWINISM universe as a system of growth and development, and it was argued that the various species of animals and plants had been produced in orderly succession from each other by the action of unknown laws of development aided by the action of external conditions. Although this work had a consider- able effect in influencing public opinion as to the extreme improbability of the doctrine of the independent " special creation " of each species, it had little effect upon natural- ists, because it made no attempt to grapple with the problem in detail, or to show in any single case how the allied species of a genus could have arisen, and have preserved their numerous slight and apparently purposeless differences from each other. No clue whatever was afforded to a law Avhich should produce from any one species one or more slightly differing but yet permanently distinct species, nor was any reason given why such slight yet constant differences should exist at all. Scientific Opinion lefore Daruin. In order to show how little effect these -^Titers had upon the public mind, I will quote a few passages from the writings of Sir Charles Lyell, as representing the opinions of the most advanced thinkers in the period immediately preceding that of Darwin's work. When recapitulating the facts and arguments in favour of the invariability and permanence of species, he says : " The entire variation from the original type which any given kind of change can pro- duce may usually be effected in a brief period of time, after Avhich no further de^^ation can be obtained by continuing to alter the circumstances, though ever so gradually, indefinite divergence either in the way of improvement or deterioration being prevented, and the least possible excess beyond the defined limits being fatal to the existence of the individual." In another place he maintains that " varieties of some species may differ more than other species do from each other Avithout shaking our confidence in the reality of species." He further adduces certain facts in geology as being, in his opinion, "fatal to the theory of progressive development," and he explains the fact that there are so often distinct species in countries of similar climate and vegetation by WHAT ARE SPECIES " special creations " in each country ; and these conchisions were arrived at after a careful study of Lamarck's work, a full abstract of which is given in the earlier editions of the Principles of Geology } Professor Agassiz, one of the greatest naturalists of the last generation, went even further, and maintained not only that each species was specially created, but that it was created in the proportions and in the localities in which we now find it to exist. The following extract from his very instructive book on Lake Superior explains this view: "There are in animals jjeculiar adaptations which are characteristic of their species, and which cannot be supposed to have arisen from subordinate influences. Those which live in shoals cannot be supposed to have been created in single pairs. Those which are made to be the food of others cannot have been created in the same proportions as those which live upon them. Those Avhich are everywhere found in innumerable specimens must have been introduced in numbers capable of maintaining their normal proportions to those which live isolated and are comparatively and constantly fewer. For we know that this harmony in the numerical proportions between animals is one of the great laws of nature. The circumstance that species occur A\nthin definite limits where no obstacles prevent their ^dder distribution leads to the further inference that these limits Avere assigned to them from the beginning, and so we should come to the final conclusion that the order which prevails throughout nature is intentional, that it is regulated by the limits marked out on the first day of creation, and that it has been maintained unchanged through ages \dth. no other modifications than those which the higher intellectual powers of man enable him to impose on some few animals more closely connected with him." - These opinions of some of the most eminent and influential writers of the pre -Darwinian age seem to us, now, either altogether obsolete or positively absurd ; but they never- theless exhibit the mental condition of even the most advanced section of scientific men on the problem of the ^ These expressions occur in Chapter IX. of the earlier editions (to the ninth) of the Principles of Geology/. ^ L, Agassiz, Lake Siqjerior, p. 377. DARWINISM CHAP. nature and origin of species. They render it clear that, notwithstanding the vast knowledge and ingenious reasoning of Lamarck, and the more general exposition of the suhject by the author of the Vestiges of Creation^ the first step had not been taken towards a satisfactory explanation of the deriva- tion of any one species from any other. Such eminent naturalists as GeofFroy Saint Hilaire, Dean Herbert, Professor Grant, Yon Buch, and some others, had expressed their belief that species arose as simple varieties, and that the species of each genus w^ere all descended from a common ancestor ; but none of them gave a clue as to the law or the method by which the change had been effected. This was still " the great mystery." As to the further question — how far this common descent could be carried ; whether distinct families, such as crows and thrushes, could possibly have descended from each other; or, whether all birds, including such ^^ddely distinct types as ^\Tens, eagles, ostriches, and ducks, could all be the modified descendants of a common ancestor ; or, still further, whether mammalia, birds, reptiles, and fishes, could all have had a common origin ; — these questions had hardly come up for discussion at all, for it was felt that, while the very first step along the road of " transmutation of species " (as it was then called) had not been made, it was quite useless to speculate as to how far it might be possible to travel in the same direction, or where the road would ultimately lead to. The Problem before Darwin. It is clear, then, that what was understood by the " origin " or the " transmutation " of species before Darwin's work appeared, was the comparatively simple question whether the allied species of each genus had or had not been derived from one another and, remotely, from some common ancestor, by the ordinary method of reproduction and by means of laws and conditions still in action and capable of being thoroughly investigated. If any naturalist had been asked at that day whether, supposing it to be clearly- shown that all the different species of each genus had been derived from some one ancestral species, and that a full and complete explanation were to be given of how each minute difference in form, colour, or structure might have originated,' and how the WHAT ARE SPECIES several peculiarities of habit and of geographical distribution might have been brought about — whether, if this were done, the "origin of species " would be discovered, the great mystery solved, he would ^mdoubtedly have replied in the affirmative. He would probably have added that he never expected any such marvellous discovery to be made in his lifetime. But so much as this assuredly ^Ii\ Dar^vin has done, not only in the opinion of his disciples and admirers, but by the admissions of those who doubt the completeness of his explanations. For almost all their objections and difficidties apply to those larger differences which separate genera, families, and orders from each other, not to those which separate one species from the species to which it is most nearly allied, and from the remaining species of the same genus. They adduce such difficulties as the first development of the eye, or of the milk-producing glands of the mammalia ; the wonderful instincts of bees and of ants ; the complex arrangements for the fertilisation of orchids, and numerous other points of structure or habit, as not being satisfactorily explained. But it is evident that these peculiarities had their origin at a very remote period of the earth's history, and no theory, however complete, can do more than afford a probable conjecture as to how they were produced. Our ignorance of the state of the earth's siu-face and of the conditions of life at those remote periods is very great ; thousands of animals and plants must have existed of which we have no record ; while we are usually Avithout any information as to the habits and general life-history even of those of which we possess some fragmentary remains ; so that the truest and most complete theory would not enable us to solve all the difficult problems which the whole course of the development of life upon our globe presents to us. What we may expect a true theory to do is to enable us to comprehend and follow out in some detail those changes in the form, structure, and relations of animals and plants which are effected in short periods of time, geologically speaking, and which are now going on around us. AVe may expect it to explain satisfactorily most of the lesser and superficial differences which distinguish one species from another. AVe may expect it to throw light on the mutual relations of the DARWINISM animals and plants which live together in any one country, and to give some rational account of the phenomena presented by their distrilmtion in different parts of the world. And, lastly, we may expect it to explain many difficulties and to harmonise many incongruities in the excessively complex affinities and relations of living things. All this the Darwinian theory undoubtedly does. It shows us how, by means of some of the most universal and ever-acting laws in nature, new species are necessarily produced, while the old species become extinct ; and it enables us to understand how the continuous action of these laws during the long periods with which geology makes us acquainted is calculated to bring about those greater differences presented by the distinct genera, families, and orders into which all living things are classified by naturalists. The differences which these present are all of the same luiture as those presented by the species of many large genera, but much greater in anwiuit ; and they can all be explained by the action of the same general laws and by the extinction of a larger or smaller number of intermediate species. Whether the distinctions between the higher groups termed Classes and Su])-kingdoms may be accounted for in the same way is a much more difficult question. The differ- ences which separate the mammals, birds, reptiles, and fishes from each other, though vast, yet seem of the same nature as those which distinguish a mouse from an elephant or a swallow from a goose. But the vertebrate animals, the mollusca, and the insects, are so radically distinct in their whole organisation and in the very plan of their structure, that objectors may not unreasonably doubt whether they can all have been derived from a common ancestor by means of the very same laws as have sufficed for the differentiation of the various species of birds or of reptiles. The Change of Opinion effected hij Darwin. The point I wish especially to urge is this. Before Darwin's work appeared, the great majority of naturalists, and almost without exception the whole literary and scientific world, held firmly to the belief that species were realities, and had not been derived from other species by any process accessible to us ; the different species of crow and of violet WHAT ARE SPECIES were believed to have been always as distinct and separate as they are now, and to have originated by some totally unknown process so far removed from ordinary reproduction that it was usually spoken of as " special creation." There was, then, no question of the origin of families, orders, and classes, because the very first step of all, the " origin of species," was believed to be an insoluble problem. But now this is all changed. The whole scientific and literary world, even the whole educated public, accepts, as a matter of common knowledge, the origin of species from other allied species by the ordinary process of natural birth. The idea of special creation or any altogether exceptional mode of production is absolutely extinct ! Yet more : this is held also to apply to many higher groups as well as to the species of a genus, and not even Mr. Darwin's severest critics venture to suggest that the primeval bird, reptile, or fish must have been " specially created." And this vast, this totally unprecedented change in public opinion has been the result of the work of one man, and was brought about in the short space of twenty years ! This is the answer to those who continue to maintain that the "origin of species" is not yet discovered ; that there are still doubts and difficulties ; that there are divergencies of structure so great that we cannot understand how they had their beginning. AVe may admit all this, just as we may admit that there are enormous difficulties in the way of a complete comprehension of the origin and nature of all the parts of the solar system and of the stellar universe. But Ave claim for DarA\in that he is the Newton of natural history, and that, just so surely as that the discovery and demonstration by Xewton of the law of gravita- tion established order in place of chaos and laid a sure founda- tion for all future study of the starry heavens, so surely has DarAAan, by his discovery of the law of natural selection and his demonstration of the great principle of the preserva- tion of useful variations in the strua ole for life, not onlv thrown a flood of light on the process of develoj^ment of the whole organic world, but also established a firm foundation for all future study of nature. In order to show the view Dar^nn took of his own work, and what it was that he alone claimed to have done, the concluding passage of the introduction to the Origin of 10 DARWINISM CHAP. Species should be carefully considered. It is as follows : "Although much remains obscure, and will long remain obscure, I can entertain no doubt, after the most deliberate and dispassionate judgment of which I am capable, that the view which most naturalists until recently entertained and which I formerly entertained — namely, that each species has been independently created — is erroneous. I am fully con- vinced that species are not immutable ; but that those belonging to Avhat 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 im- portant, but not the exclusive, means of modification." It should be especially noted that all Avhich is here claimed is now almost universally admitted, Avhile the criticisms of Darwin's works refer almost exclusively to those numerous questions which, as he himself says, " will long remain obscure." The Darwinian Theory. As it ^vill be necessary, in the foUo^Wng chapters, to set forth a considerable body of facts in almost every department of natural history, in order to establish the fundamental propositions on which the theory of natural selection rests, I propose to give a preliminary statement of what the theory really is, in order that the reader may better appreciate the necessity for discussing so many details, and may thus feel a more enlightened interest in them. Many of the facts to be adduced are so novel and so curious that they are sure to be appreciated by every one who takes an interest in nature, but unless •the need of them is clearly seen it may be thought that time is being wasted on mere curious details and strange facts which have little bearing on the question at issue. The theory of natural selection rests on two main classes of facts which apply to all organised beings without exception, and which thus take rank as fundamental principles or laws. The first is, the power of rapid multiplication in a geometrical progression ; the second, that the offspring always vary slightly from the parents, though generally very closely resembling WHAT ARE SPECIES 11 them. From the first fact or law there follows, necessarily, a constant straggle for existence ; because, while the offspring always exceed the parents in number, generally to an enormous extent, yet the total number of living organisms in the world does not, and cannot, increase year by year. Consequently every year, on the average, as many die as are born, plants as well as animals ; and the majority die premature deaths. They kill each other in a thousand different ways ; they starve each other by some consuming the food that others want ; they are destroyed largely by the powers of natiu-e — by cold and heat, by rain and storm, by flood and fire. There is thus a perpetual struggle among them which shall live and which shall die ; and this struggle is tremendously severe, because so few can possibly remain alive — one in five, one in ten, often only one in a hundred or even one in a thousand. Then conies the cpiestion, Why do some live rather than others ? If all the individuals of each species were exactly alike in every respect, we could only say it is a matter of chance. But they are not alike. We find that they vary in many different ways. Some are stronger, some swifter, some hardier in constitution, some more cunning. An obscure colour may render concealment more easy for some, keener sight may enable others to discover prey or escape from an enemy better than their fellows. Among plants the smallest differences may be useful or the reverse. The earliest and strongest shoots may escape the slug ; their greater vigour may enable them to flower and seed earlier in a wet autumn ; plants best armed with spines or hairs may escape being devoured ; those whose flowers are most conspicuous may be soonest fertilised by insects. We cannot doubt that, on the whole, any beneficial variations will give the possessors of it a greater probability of lining through the tremendous 'ordeal they have to undergo. There may be something left to chance, but on the whole the fittest icill survive. Then Ave have another important fact to consider, the principle of heredity or transmission of variations. If we grow plants from seed or breed any kind of animals year after year, consuming or gi^Hing away all the increase we do not wish to keep just as they come to hand, our plants or animals Avill continue much the same ; but if every year we 12 DARWINISM carefully save the best seed to sow utkI the finest or brightest coloured animals to breed from, we shall soon find that an improvement will take place, and that the average quality of our stock will be raised. This is the way in which all our fine garden fruits and vegetables and flowers have been pro- duced, as well as all our s])lendid breeds of domestic animals ; and they have thus become in many cases so different from the wild races from which they originally sprang as to be hardly recognisable as the same. It is therefore proved that if any particular kind of variation is preserved and bred from, the variation itself goes on increasing in amount to an enormous extent ; and the bearing of this on the question of the origin of species is most important. For if in each generation of a given animal or plant the fittest survive to continue the breed, then whatever may be the special peculiarity that causes " fitness " in the particular case, that peculiarity will go on increasing and strengthening so long as it is nsefal to the species. But the moment it has reached its maximum of usefulness, and some other quality or modifica- tion would help in the struggle, then the individuals which vary in the new direction will survi^'e ; and thus a species may be gradually modified, first in one direction, then in another, till it differs from the original parent form as much as the greyhound differs from any wild dog or the cauliflower from any wild plant. But animals or plants which thus differ in a state of nature are always classed as distinct species, and thus we see how, by the continuous survival of the fittest or the preservation of favoured races in the struggle for life, new species may be originated. This self-acting process which, by means of a few easily demonstrated groups of facts, brings about change in the organic world, and keeps each species in harmony Avith the conditions of its existence, will appear to some persons so clear and simple as to need no further demonstration. But to the great majority of naturalists and men of science endless di'tficulties and ol)jections arise, owing to the wonderful variety of animal and vegetable forms, and the intricate relations of the different species and groups of species with each other ; and it was to answer as many of these objections as possible, and to show that the more we know of nature the more we WHAT ARE SPECIES 13 fiiid it to harmonise with the development hypothesis, that Darwin devoted the whole of his life to collecting facts and making experiments, the record of a portion of which he has given us in a series of twelve masterly volumes. Proposed Mode of Treatment of the Suhjed. It is e\adently of the most ^^ital importance to any theory that its foundations should be absolutely secure. It is therefore necessarj^ to show, by a '^^dde and comprehensive array of facts, that animals and plants do perpetually vary in the manner and to the amount requisite ; and that this takes place in A\nld animals as well as in those which are domesti- cated. It is necessary also to prove that all organisms do tend to increase at the great rate alleged, and that this increase actually occurs, under favourable conditions. We have to prove, further, that variations of all kinds can be increased and accumulated by selection ; and that the struggle for existence to the extent here indicated actually occurs in nature, and leads to the continued preservation of favourable variations. These matters will be discussed in the four succeeding chapters, though in a somewhat dilierent order — the struggle for existence and the power of rapid multiplication, which is its cause, occupying the first place, as comprising those facts which are the most fundamental and those which can be perfectly explained without any reference to the less generally understood facts of variation. These chapters will be followed by a discussion of certain difficulties, and of the vexed cpiestion of hybridity. Then will come a rather full account of the more important of the complex relations of organisms to each other and to the earth itself, which are either fully explained or greatly elucidated by the theory. The concluding chapter vn\\ treat of the origin of man and his relations to the lower animals. CHAPTER II THE STRUGGLE FOR EXISTENCE Its importance — The struggle among plants — Among animals — Illustrative cases — Succession of trees in forests of Denmark — The struggle for existence on the Pampas — Increase of organisms in a geometrical ratio — Examples of great powers of increase of animals — Rapid increase and wide spread of plants — Great fertility not essential to rapid increase — Struggle between closely allied species most severe — The ethical aspect of the struggle for existence. There is perhaps no phenomenon of nature that is at once so important, so universal, and so little understood, as the struggle for existence continually going on among all organ- ised beings. To most persons nature appears calm, orderly, and peaceful. They see the birds singing in the trees, the insects hovering over the flowers, the squirrel climbing among the tree-tops, and all living things in the possession of health and vigour, and in the enjoyment of a sunny existence. But they do not see, and hardly ever think of, the means by which this beauty and harmony and enjoyment is brought about. They do not see the constant and daily search after food, the failure to obtain which means weakness or death ; the con- stant eff*ort to escape enemies ; the ever-recurring struggle against the forces of nature. This daily and hourly struggle, this incessant warfare, is nevertheless the very means by which much of the beauty and harmony and enjoyment in nature is produced, and also affords one of the most important elements in bringing about the origin of species. We must, therefore, devote some time to the consideration of its various aspects and of the many curious phenomena to which it gives rise. It is a matter of common observation that if weeds are allowed to grow unchecked in a garden they will soon destroy CHAP. II THE STRUGGLE FOR EXISTENCE 15 a number of the flowers. It is not so commonly kno^\Ti that if a garden is left to become altogether wild, the weeds that first take possession of it, often covering the whole surface of the ground with two or three different kinds, will themselves be supplanted by others, so that in a few years many of the original flowers and of the earliest weeds may alike have disappeared. This is one of the very simplest cases of the struggle for existence, resulting in the successive displacement of one set of species by another ; but the exact causes of this displacement are by no means of such a simple nature. All the plants concerned may be perfectly hardy, all may grow freely from seed, yet when left alone for a number of years, each set is in turn driven out by a succeeding set, till at the end of a considerable period — a century or a few centuries perhaps — hardly one of the plants which first monopolised the ground would be found there. Another phenomenon of an analogous kind is presented by the different behaviour of introduced ^^^ld plants or animals into countries apparently quite as well suited to them as those which they naturally inhabit. Agassiz, in his work on Lake Superior, states that the roadside weeds of the north- eastern United States, to the number of 130 species, are all European, the native weeds having disappeared westwards ; and in New Zealand there are no less than 250 species of naturalised European plants, more than 100 species of which have spread widely over the country, often displacing the native vegetation. On the other hand, of the many hundreds of hardy plants which produce seed freely in our gardens, very few ever run "s^nld, and hardly any have become common. Even attempts to naturalise suitable plants usually fail ; for A. de Candolle states that several botanists of Paris, Geneva, and especially of Montpellier, have soA\ai the seeds of many hundreds of species of hardy exotic plants in what appeared to be the most favourable situations, but that, in hardly a single case, has any one of them become naturalised.^ Even a plant like the potato — so widely cultivated, so hardy, and so well adapted to spread by means of its many-eyed tubers — has not established itself in a ^vild state in any part of Europe. It would be thought that Australian plants would easily run ^ Geographie. Botaniqiie, p. 798. 16 DARWINISM chap. wild in New Zealand. But Sir Joseph Hooker informs us that the late Mr. Bidwell habitually scattered Australian seeds during his extensive travels in New Zealand, yet only two or three Australian plants appear to have established themselves in that country, and these only in cultivated or newly moved soil. These few illustrations sufficiently show that all the plants of a country are, as De CandoUe says, at war with each other, each one struggling to occupy ground at the expense of its neighbour. But, besides this direct competiticjii, there is one not less j^owerful arising from the exposure of almost all plants to destruction by animals. The buds are destroyed by birds, the leaves by caterpillars, the seeds by wee\ils ; some insects bore into the trunk, others burrow in the t^Wgs and leaves ; slugs devour the young seedlings and the tender shoots, wire- worms gnaw the roots. Herbivorous mammals devour many species bodily, while some uproot and devour the buried tubers. In animals, it is the eggs or the very young that suffer most from their various enemies ; in plants, the tender seedlings when they first appear above the ground. To illustrate this latter point Mr. DarA^nn cleared and dug a piece of ground three feet long and two feet wide, and then marked all the seedlings of weeds and other plants which came up, noting what became of them. The total number was 357, and out of these no less than 295 were destroyed by slugs and insects. The direct strife of plant ^\iib. plant is almost equally fatal when the stronger are allowed to smother the weaker. When turf is mown or closely browsed by animals, a number of strong and weak plants live together, because none are allowed to grow much beyond the rest ; but Mr. Darwin found that when the plants which compose such turf are allowed to grow up freely, the stronger kill the weaker. In a plot of tui'f three feet by four, twenty distinct species of plants Avere found to be growing, and no less than nine of these perished altogether when the other species were allowed to grow up to their full size.^ But besides having to protect themselves against competing plants and against destructive animals, there is a yet deadlier ^ The Origin of Sjjecies, p. 53. n THE STRUGGLE FOR EXISTENCE 17 enemy in the forces of inorganic nature. Each species can sustain a certain amount of heat and cold, each requires a certain amount of moisture at the right season, each wants a proper amount of light or of direct sunshine, each needs certain elements in the soil ; the failure of a due proportion in these inorganic conditions causes weakness, and thus leads to speedy death. The struggle for existence in plants is, therefore, threefold in character and infinite in complexity, and the result is seen in their curiously irregular distribution over the face of the earth. Not only has each country its distinct plants, but every valley, every hillside, almost every hedgerow, has a different set of plants from its adjacent valley, hillside, or hedgerow — if not always different in the actual species yet very different in comparative abundance, some which are rare in the one being common in the other. Hence it happens that slight changes of conditions often produce great changes in the flora of a country. Thus in 1740 and the two following years the larva of a moth (Phalsena graminis) committed such destruction in many of the meadows of Sweden that the grass Avas greatly diminished in quantity, and many plants which were before choked by the grass sprang up, and the ground became variegated with a multi- tude of different species of flowers. The introduction of goats into the island of St. Helena led to the entire destruction of the native forests, consisting of about a hundred distinct species of trees and shrubs, the young plants being devoured by the goats as fast as they grew up. The camel is a still greater enemy to woody vegetation than the goat, and Mr. Marsh believes that forests would soon cover' considerable tracts of the Arabian and African deserts if the goat and the camel were removed from them.^ Even in many parts of our own country the existence of trees is dependent on the absence of cattle. Mr. Darwin observed, on some extensive heaths near Farnham, in Surrey, a few clumps of old Scotch firs, but no young trees over hundreds of acres. Some portions of the heath had, however, been enclosed a few years before, and these en- closures were crowded with young fir-trees growing too close together for all to live ; and these were not sown or planted, nothing having been done to the ground beyond enclosing it ^ The Earth as Modified hy Human Action, p. 51. 0 18 Darwinism chap. so as to keep out cattle. On ascertaining this, Mr. Darwin was so much suri)rise(l that he searched among the heather in the unenclosed parts, and there he found multitudes of little trees and seedlings which had been perpetually browsed down by the cattle. In one square yard, at a point about a hundred yards from one of the old clumps of firs, he counted thirty- two little trees, and one of them had twenty-six rings of growth, showing that it had for many years tried to raise its head above the stems of the heather and had failed. Yet this heath was very extensive and very barren, and, as Mr. Darwin remarks, no one would ever have imagined that cattle would have so closely and so effectually searched it for food. In the case of animals, the competition and struggle are more obvious. The vegetation of a given district can only support a certain number of animals, and the different kinds of plant-eaters will compete together for it. They will also have insects for their competitors, and these insects will be kept down by birds, which will thus assist the mammalia. But there will also be carnivora destroying the herbivora ; while small rodents, like the lemming and some of the ff eld- mice, often destroy so much vegetation as materially to affect the food of all the other groups of animals. Droughts, floods, severe winters, storms and hurricanes will injure these in various degrees, but no one species can be diminished in numbers without the effect being felt in various complex ways by all the rest. A few illustrations of this reciprocal action must be given. Illustrative Cases of the Struggle for Life. Sir Charles Lyell observes that if, by the attacks of seals or other marine foes, salmon are reduced in numbers, the consequence will be that otters, living far inland, will be deprived of food and will then destroy many young birds or quadrupeds, so that the increase of a marine animal may cause the destruction of many land animals hundreds of miles away. Mr. Darwin carefully observed the effects produced by planting a few hundred acres of Scotch fir, in Staff"ordshire, on part of a very extensiv^e heath which had never been cultivated. After the planted portion was about twenty-five years old he observed that the change in the native vegetation II THE STRUGGLE FOR EXISTENCE 19 was greater than is often seen in passing from one quite ditierent soil to another. Besides a great change in the pro- portional numbers of the native heath-plants, twelve species which could not be found on the heath flomished in the plantations. The effect on the insect life must have been still greater, for six insectivorous birds which were very common in the plantations were not to be seen on the heath, which was, however, frequented by two or three different species of insectivorous birds. It would have recpiired continued study for several years to determine all the differences in the organic life of the two areas, but the facts stated by Mr. Dar^vin are sufficient to show how great a change may be effected by the introduction of a single kind of tree and the keeping out of cattle. The next case I will give in Mr. Dar^^in's own words : " In several paits of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this ; for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state ; and Azara and Rengger have shown that this is caused by the greater numbers, in Paraguay, of a certain fly which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects woidd probably increase ; and this would lessen the number of the navel- frecjuenting flies — then cattle and horses would become feral, and this would greatly alter (as indeed I have observed in parts of South America) the vegetation : this again would largely affect the insects, and this, as we have just seen in Staffordshire, the insectivorous birds, and so onward in ever- increasing circles of complexity. Xot that under nature the relations will ever be as simple as this. Battle ^nthin battle must be continually recurring Avith varying success; and yet in the long run the forces are so nicely balanced, that the face of nature remains for a long time uniform, though assuredly the merest trifle would give the Wctory to one organic being over another."^ ^ The Origin of Species, p. 56. 20 DARWINISM CHAi>. Such cases as the above may perhaps l)e thought excep- tional, but there is good reason to l)elieve that they are by no means rare, but are iUustrations of what is going on in every part of the world, only it is very difficult for us to trace out the complex reactions that are everywhere occurring. The general impression of the ordinary observer seems to be that wild animals and plants live peaceful lives and have few troubles, each being exactly suited to its place and surroundings, and therefore having no difficulty in maintain- ing itself. Before showing that this view is, everywhere and always, demonstrably untrue, we will consider one other case of the complex relations of distinct organisms adduced by Mr. Darwin, and often quoted for its striking and almost eccentric character. It is now well known that many flowers require to be fertilised by insects in order to produce seed, and this fertilisation can, in some cases, only be effected by one particular species of insect to which the flower has become specially adapted. Two of our common plants, the wild heart's- ease (Viola tricolor) and the red clover (Trifolium pratense), are thus fertilised by humble-bees almost exclusively, and if these insects are prevented from visiting the flowers, they j^roduce either no seed at all or exceedingly few. Now it is known that field-mice destroy the combs and nests of humble-bees, and Colonel Newman, who has paid great attention to these insects, believes that more than two- thirds of all the humble-bees' nests in England are thus destroyed. But the number of mice dei:)ends a good deal on the number of cats ; and the same observer says that near villages and towns he has found the nests of humble-bees more numerous than elseAvhere, which he attributes to the number of cats that destroy the mice. Hence it follows, that the abundance of red clover and wild heart's-ease in a district will depend on a good supply of cats to kill the mice, which would otherwise destroy and keep down the humble-bees and prevent them from fertilising the flowers. A chain of connection has thus been found between such totally distinct organisms as flesh-eating mammalia and sweet- smelling flowers, the abundance or scarcity of the one closely corresponding to that of the other ! The f(jllowing account of the struggle between trees in the forests of Denmark, from the researches of M. Hansten- II THE STRUGGLE FOR EXISTENCE 21 Blangsted, strikingly illustrates our subject."^ The chief com- batants are the beech and the birch, the former being every- where successful in its invasions. Forests composed wholly of birch are now only found in sterile, sandy tracts ; every- where else the trees are mixed, and wherever the soil is favourable the beech rapidly drives out the birch. The latter loses its branches at the touch of the beech, and devotes all its strength to the upper part where it towers above the beech. It may live long in this way, but it succumbs ultimately in the fight — of old age if of nothing else, for the life of the birch in Denmark is shorter than that of the beech. The writer believes that light (or rather shade) is the cause of the superiority of the latter, for it has a gi'eater development of its branches than the birch, which is more open and thus allows the rays of the sun to pass through to the soil below, while the tufted, bushy top of the beech preserves a deep shade at its base. Hardly any young plants can grow under the beech except its own shoots ; and while the beech can flourish under the shade of the birch, the latter dies im- mediately under the beech. The birch has only been saved from total extermination by the facts that it had possession of the Danish forests long before the beech ever reached the country, and that certain districts are unfavourable to the growth of the latter. But wherever the soil has been enriched by the decomposition of the leaves of the birch the battle begins. The birch still flourishes on the borders of lakes and other marshy places, where its enemy cannot exist. In the same way, in the forests of Zeeland, the fir forests are dis- appearing before the beech. Left to themselves, the firs are soon displaced by the beech. The struggle between the latter and the oak is longer and more stubborn, for the branches and foliage of the oak are thicker, and oflPer much resistance to the passage of light. The oak, also, has greater longevity ; but, sooner or later, it too succumbs, because it cannot develop in the shadoAv of the beech. The earliest forests of Denmark were mainly composed of aspens, with which the birch was apparently associated ; gradually the soil was raised, and the climate grew milder ; then the fir came and formed large forests. This tree ruled for centuries, and then ceded the ^ See Xature, vol. xxxi. p. 63, 22 DARWINISM chap. first place to the holm-oak, which is now giving way to the beech. Aspen, birch, fir, oak, and beech appear to be the steps in the struggle for the survival of the fittest among the forest-trees of Denmark. It may be added that in the time of the Eomans the beech was the principal forest-tree of Denmark as it is now, while in the much earlier bronze age, represented by the later remains found in the peat bogs, there were no beech-trees, or very few, the oak being the prevailing tree, while in the still earlier stone period the fir was the most abundant. The beech is a tree essentially of the temperate zone, having its northern limit considerably southward of the oak, fir, birch, or aspen, and its entrance into Denmark was no doubt due to the amelioration of the climate after the glacial epoch had entirely passed away. We thus see how changes of climate, which are continually occurring owing either to cosmical or geographical causes, may initiate a struggle among plants which may continue for thousands of years, and which must profoundly modify the relations of the animal world, since the very existence of innumerable insects, and even of many birds and mammals, is dependent more or less completely on certain species of plants. The Struggle for Existence on the Pampas. Another illustration of the struggle for existence, in which both plants and animals are implicated, is afforded by the pampas of the southern part of South America. The absence of trees from these vast plains has been imputed by Mr. Darwin to the supposed inability of the tropical and sub- tropical forms of South America to thrive on them, and there being no other source from which they could obtain a supply ; and that explanation was adopted by such eminent botanists as Mr. Ball and Professor Asa Gray. This explanation has always seemed to me unsatisfactory, because there are ample forests both in the temperate regions of the Andes and on the whole west coast down to Terra del Fuego; and it is inconsistent with what Ave know of the rapid variation and adaptation of species to new conditions. What seems a more satisfactory explanation has been given by Mr. Edwin Clark, a civil engineer, who resided nearly two years in the country and 11 THE STRUGGLE FOR EXISTENCE 23 paid much attention to its natural history. He says : " The peculiar characteristics of these vast level plains which descend from the Andes to the great river basin in unbroken monotony, are the absence of rivers or water-storage, and the periodical occurrence of droughts, or ' siccos,' in the summer months. These conditions determine the singular character both of its flora and fauna. " The soil is naturally fertile and favourable for the growth of trees, and they grow luxuriantly wherever they are pro- tected. The eucalyptus is covering large tracts wherever it is enclosed, and willows, poplars, and the fig surround every estancia when fenced in. "The open plains are covered with droves of horses and cattle, and overrun by numberless wild rodents, the original tenants of the pampas. During the long periods of drought, Avhich are so great a scourge to the country, these animals are starved by thousands, destroying, in their efforts to live, every vestige of vegetation. In one of these ' siccos,' at the time of my visit, no less than 50,000 head of oxen and sheep and horses perished from starvation and thirst, after tearing deep out of the soil every trace of vegetation, including the wiry roots of the pampas-grass. Under such circumstances the existence of an unprotected tree is impossible. The only plants that hold their own, in addition to the indestructible thistles, grasses, and clover, are a little herbaceous oxalis, pro- ducing viviparous buds of extraordinary vitality, a few poisonous species, such as the hemlock, and a few tough, thorny dwarf- acacias and wiry rushes, which even a starving rat refuses. " Although the cattle are a modern introduction, the numberless indigenous rodents must always have effectually prevented the introduction of any other species of plants ; large tracts are still honeycombed by the ubiquitous biscacho, a gigantic rabbit ; and numerous other rodents still exist, in- cluding rats and mice, jmmpas-hares, and the great nutria and carpincho (capybara) on the river banks. "^ Mr. Clark further remarks on the desperate struggle for existence Avhich characterises the bordering fertile zones, where rivers and marshy plains permit a more luxuriant and varied vegetable and animal life. After describing how the ^ A Visit to South America, 1878 ; also Nature, vol. xxxi. pp. 263-339. 24 DARWINISM chap. river sometimes rose 30 feet in eight hours, doing immense destruction, and the abundance of the larger carnivora and large reptiles on its banks, he goes on : " But it was among the flora that the principle of natural selection was most prominently displayed. In such a district — overrun 'with rodents and escaped cattle, subject to floods that carried away whole islands of botany, and especially to droughts that dried up the lakes and almost the river itself — no ordinary plant could live, even on this rich and watered alluvial debris. The only plants that escaped the cattle were such as were either poisonous, or thorny, or resinous, or indestructibly tough. Hence we had only a great development of solanums, talas, acacias, euphorbias, and laurels. The buttercup is replaced by the little poisonous yellow oxalis with its viviparous buds ; the passion-flowers, asclepiads, bignonias, convolvuluses, and climb- ing leguminous plants escape both floods and cattle by climb- ing the highest trees and towering overhead in a flood of bloom. The ground plants are the portulacas, turneras, and Oenotheras, bitter and ephemeral, on the bare rock, and almost independent of any other moisture than the heavy dews. The pontederias, alismas, and plantago, with grasses and sedges, derive protection from the deep and brilliant pools ; and though at first sight the ' monte ' doubtless impresses the traveller as a scene of the wildest confusion and ruin, yet, on closer examination, we found it far more remarkable as a manifestation of harmony and law, and a striking example of the marvellous power which plants, like animals, possess, of adapting themselves to the local peculiarities of their habitat, whether in the fertile shades of the luxuriant ' monte ' or on the arid, parched-up plains of the treeless pampas." A curious example of the struggle between plants has been communicated to me by Mr. John Ennis, a resident in New Zealand. The English water-cress grows so luxuriantly in that country as to completely choke up the rivers, sometimes leading to disastrous floods, and necessitating great outlay to keep the stream open. But a natural remedy has now been found in planting willows on the banks. The roots of these trees penetrate the bed of the stream in every direction, and the water-cress, unable to obtain the requisite amount of nourishment, gradually disappears. II THE STRUGGLE FOR EXISTENCE 25 Increase of Organisms in a Geometrical Batio. The facts which have now been adduced, sufficiently prove that there is a continual competition, and struggle, and war going on in nature, and that each species of animal and plant affects many others in complex and often unexpected ways. AVe will now proceed to show the fundamental cause of this struggle, and to prove that it is ever acting over the whole field of nature, and that no single species of animal or plant can possibly escape from it. This results from the fact of the rapid increase, in a geometrical ratio, of all the species of animals and plants. In the lower orders this increase is especially rapid, a single flesh-fly (Musca carnaria) producing 20,000 larvae, and these growing so Cjuickly that they reach their full size in five days ; hence the great Swedish naturalist, Linnaeus, asserted that a dead horse would be devoured by three of these flies as quickly as by a lion. Each of these larvae remains in the pupa state about five or six days, so that each parent fly may be increased ten thousand-fold in a fortnight. Supposing they went on increasing at this rate during only three months of summer, there would result one hundred millions of millions of millions for each fly at the commencement of summer, — a number greater probably than exists at any one time in the whole world. And this is only one species, while there are thousands of other species increasing also at an enormous rate ; so that, if they were unchecked, the whole atmosphere would be dense ^Wth flies, and all animal food and much of animal life would be destroyed by them. To prevent this tremendous increase there must be incessant war against these insects, by insectivorous birds and reptiles as well as by other insects, in the larva as Avell as in the perfect state, by the action of the elements in the form of rain, hail, or drought, and by other unknown causes ; yet we see nothing of this ever-present war, though by its means alone, perhaps, we are saved from famine and pestilence. Let us now consider a less extreme and more familiar case. AVe possess a considerable number of birds Avhich, like the redbreast, sparrow, the four common titmice, the thrush, and the blackbird, stay with us all the year round. These lay on an average six eggs, but, as several of them have 26 DARWINISM chap. two or more broods a year, ten will be below the average of the year's increase. Such birds as these often live from fifteen to twenty years in confinement, and we cannot suppose them to live shorter lives in a state of nature, if unmolested ; but to avoid j^ossible exaggeration we will take only ten years as the average duration of their lives. Now, if we start with a single pair, and these are allowed to live and breed, unmolested, till they die at the end of ten years, — as they might do if turned loose into a good-sized island with ample vegetable and insect food, but no other competing or destructive birds or quadrupeds — their luimbers would amount to more than twenty millions. But we know very w^ell that our bird population is no greater, on the average, now than it was ten years ago. Year by year it may fluctuate a little according as the winters are more or less severe, or from other causes, but on the whole there is no increase. What, then, becomes of the enormous surplus population annually produced? It is evident they must all die or be killed, somehow ; and as the increase is, on the average, about five to one, it follows that, if the average number of birds of all kinds in our islands is taken at ten millions — and this is probably far under the mark — then about fifty millions of birds, including eggs as possible birds, must annually die or be destroyed. Yet we see nothing, or almost nothing, of this tremendous slaughter of the innocents going on all around us. In severe winters a few birds are found dead, and a few feathers or mangled remains show us where a wood-pigeon or some other bird has been destroyed by a hawk, but no one would imagine that five times as many birds as the total number in the country in early spring die every year. No doubt a considerable proportion of these do not die here but during or after migration to other countries, but others which are bred in distant countries come here, and thus l)alance the account. Again, as the average number of young jjroduced is four or five times that of the parents, we ought to have at least five times as many birds in the country at the end of summer as at the Ijeginning, and there is certainly no such enormous disproportion as this. The fact is, that the destruction commences, and is probably most severe, with nestling birds, which are often killed by heavy rains or blown away by severe storms, or left to die of hunger if either of II THE STRUGGLE FOR EXISTENCE 27 the parents is killed ; while they offer a defenceless prey to jackdaws, jays, and magpies, and not a fev/ are ejected from their nests by their foster-brothers the cuckoos. As soon as they are fledged and begin to leave the nest great numbers are destroyed by buzzards, sparrow-hawks^ and shrikes. Of those Avhich migrate in autumn a considerable proportion are probably lost at sea or other"vvise destroyed before they reach a place of safety ; while those which remain with us are greatly thinned by cold and starvation during severe winters. Exactly the same thing goes on with every species of wild animal and plant from the lowest to the highest. All breed at such a rate, that in a few years the progeny of any one species would, if allowed to increase unchecked, alone monopolise the land ; but all alike are kept within bounds by various destructive agencies, so that, though the numbers of each may fluctuate, they can never permanently increase except at the expense of some others, which must proportionately decrease. Cases shovAng the Great Poivers of Increase of Animals. As the facts now stated are the very foundation of the theory we are considering, and the enormous increase and perpetual destruction continually going on rec[uire to be kept ever present in the mind, some direct evidence of actual cases of increase must be adduced. That even the larger animals, which breed comparatively slowly, increase enormously when placed under favourable conditions in new countries, is shown by the rapid spread of cattle and horses in America. Columbus, in his second voyage, left a few black cattle at St. Domingo, and these ran 'v\41d and increased so much that, twenty-seven years afterwards, herds of from 4000 to 8000 head were not uncommon. Cattle were afterwards taken from this island to ]\Iexico and to other parts of America, and in 1587, sixty- five years after the conc|uest of Mexico, the Spaniards exported 64,350 hides from that country and 35,444 from St. Domingo, an indication of the vast numbers of these animals which must then have existed there, since those captured and killed could have been only a small portion of the whole. In the pampas of Buenos Ayres there were, at the end of the last century, about twelve million cows and three million horses, besides great numbers in all other parts 28 DARWIXISM chap. of America where open })astures offered suitable conditions. Asses, about fifty years after their introduction, ran wild and midtiplied so amazingly in Quito, that the Spanish traveller UUoa describes them as being a nuisance; They grazed together in great herds, defending themselves with their mouths, and if a horse strayed among them they all fell upon him and did not cease biting and kicking till they left him dead. Hogs were turned out in St. Domingo by Columbus in 1493, and the Spaniards took them to other places where they settled, the result being, that in about half a century these animals were found in great numbers over a large part of America, from 25° north to 40° south latitude. More recently, in New Zealand, pigs have multiplied so greatly in a wild state as to be a serious nuisance and injury to agriculture. To give some idea of their numbers, it is stated that in the province of Nelson there were killed in twenty months 25,000 wild pigs.^ Now, in the case of all these animals, Ave know that in their native countries, and even in America at the present time, they do not increase at all in numbers ; therefore the Avhole normal increase must be kept down, year by year, by natural or artificial means of destruction. Rapid Increase and Wide Spread of Plants. In the case of plants, the power of increase is even greater and its effects more distinctly visible. Hundreds of square miles of the plains of La Plata are now covered with two or three species of European thistle, often to the exclusion of almost every other plant ; but in the native countries of these thistles they occupy, excej^t in cultivated or waste ground, a very subordinate part in the vegetation. Some American ])lants, like the cotton-weed (Asclepias curassavica), have now become common weeds over a large portion of the tropics. White clover (Trifolium repens) spreads over all the temperate regions of the world, and in New Zealand is exterminating many native species, including even the native flax (Phormium ^ Still more remarkable is the increase of rahhits both in New Zealand and Australia. No less than seven millions of rabbit-skins have been exported from the former country in a single year, their value bein^ £67,000. In both countries, sheep-runs have been greatly deteriorated in value by the abundance of rabbits, which destroy the herbage ; and in some cases they have had to be abandoned altogether. 11 THE STRUGGLE FOR EXISTENCE 29 tenax), a large plant with iris-like leaves 5 or 6 feet high. Mr. AV. L. Travers has paid much attention to the effects of introduced plants in New Zealand, and notes the following species as being especially remarkable. The common knot- grass (Polygonum aviculare) grows most luxuriantly, single plants covering a space 4 or 5 feet in diameter, and send- ing their roots 3 or 4 feet deep. A large sub-aquatic dock (Eumex obtusifolius) abounds in every river-bed, even far up among the mountains. The common sow-thistle (Sonchus oleraceus) grows all over the country up to an elevation of 6000 feet. The water-cress (Nasturtium officinale) grows wdth amazing vigour in many of the rivers, forming stems 12 feet long and f inch in diameter, and completely choking them up. It cost £300 a year to keep the Avon at Christchurch free from it. The sorrel (Rumex acetosella) covers hundreds of acres Avith a sheet of red. It forms a dense mat, exterminating other plants, and preventing cultiva- tion. It can, however, be itself exterminated by sowing the ground Avith red clover, Avhich Avill also vanquish the Polygonum aviculare. The most noxious Aveed in NeAV Zealand appears, hoAvever, to be the Hypochaeris radicata, a coarse yelloAv-floAvered composite not uncommon in our meadoAvs and Avaste places. This has been introduced Avith grass seeds from England, and is very destructive. It is stated that excellent pasture AA^as in three years destroyed by this Aveed, Avhich absolutely displaced every other plant on the ground. It groAvs in every kind of soil, and is said even to drive out the Avhite clover, Avhich is usually so poAverful in taking possession of the soil. In Australia another composite plant, called there the Cape- Aveed (Cryptostemma calendulaceum), did much damage, andAvas noticed by Baron Yon Hugel in 1833 as "an un exterminate Aveed " ; but, after forty years' occupation, it Avas found to give AA^ay to the dense herbage formed by lucerne and choice grasses. In Ceylon Ave are told by Mr. ThAvaites, in his Emimera- tion of Ceylon Plants, that a plant introduced into the island less than fifty years ago is helping to alter the character of the A^egetation up to an elcA'ation of 3000 feet. This is the Lantana mixta, a verbenaceous plant introduced 30 DARWINISM chap. from the West Indies, M'hich a})pears to have found in Ceylon a soil and climate exactly suited to it. It now covers thousands of acres with its dense masses of foliage, taking complete possession of land where cultivation has been neglected or abandoned, preventing the growth of any other plants, and even destroying small trees, the tops of which its subscandent stems are able to reach. The fruit of this plant is so accept- able to frugivorous birds of all kinds that, through their instru- mentality, it is spreading rapidly, to the complete exclusion of the indiii;enous vegetation where it becomes established. Great Fertility not essential to Bapid Increase. The not uncommon circumstance of slow-breeding animals being very numerous, shows that it is usually the amount of destruction which an animal or plant is exposed to, not its rapid multiplication, that determines its numbers in any country. The passenger-pigeon (Ectopistes migratorius) is, or rather was, excessively abundant in a certain area in North America, and its enormous migrating flocks darkening the sky for hours have often been described ; yet this bird lays only two eggs. The fulmar petrel is supposed to be one of the most numerous birds in the world, yet it lays only one egg. On the other hand the great shrike, the tree-creeper, the nut-hatch, the nut-cracker, the hoopoe, and many other birds, lay from four to six or seven eggs, and yet are never abundant. So in plants, the abundance of a species bears little or no relation to its seed-producing power. Some of the grasses and sedges, the wild hyacinth, and many buttercups occur in immense profusion over extensive areas, although each plant produces comparatively few seeds ; while several species of bell-flowers, gentians, pinks, and mulleins, and even some of the composite, which produce an abundance of minute seeds, many of which are easily scattered by the wind, are yet rare species that never spread beyond a very limited area. The above-mentioned passenger-pigeon affords such an excellent example of an enormous bird-population kept up by a comparatively slow rate of increase, and in spite of its complete helplessness and the great destruction which it suffers from its numerous enemies, that the folloA\ing account of one of its breeding-places and migrations by the celebrated II THE STRUGGLE FOR EXISTENCE 31 American naturalist, Alexander "Wilson, will be read with interest : — " Not far from Shelby ville, in the State of Kentucky, about five years ago, there was one of these breeding- places, which stretched through the Avoods in nearly a north and south direction, was several miles in breadth, and was said to be upwards of 40 miles in extent. In this tract almost every tree was furnished with nests wherever the branches could accommodate them. The pigeons made their first appearance there about the 10th of April, and left it altogether with their young before the 25th of May. As soon as the young were fully grown and before they left the nests, numerous parties of the inhabitants from all parts of the adjacent country came with waggons, axes, beds, cooking utensils, many of them accompanied by the greater part of their families, and encamped for several days at this immense nursery. Several of them informed me that the noise was so great as to terrify their horses, and that it was difficult for one person to hear another Avithout bawling in his ear. The ground was strewed with broken limbs of trees, eggs, and young squab pigeons, which had been j^recipitated from above, and on which herds of hogs were fattening. Hawks, buzzards, and eagles were sailing about in great numbers, and seizing the squabs from the nests at pleasure ; while, from 20 feet upwards to the top of the trees, the view through the woods presented a perpetual tumult of crowding and fluttering multitudes of pigeons, their wdngs roaring like thunder, mingled with the frequent crash of falling timber ; for now the axemen were at work cutting down those trees that seemed most crowded with nests, and contrived to fell them in such a manner, that in their descent they might bring down several others ; by which means the falling of one large tree some- times produced 200 squabs little inferior in size to the old birds, and almost one heap of fat. On some single trees upwards of a hundred nests were found, each containing one squab only ; a circumstance in the history of the bird not generally known to naturalists.^ It was dangerous to walk ^ Later observers liave proved that two eggs are laid and usually two young produced, but it may be that in most cases only one of these comes to maturity. 32 DARWINISM chap. under these flying and fluttering millions, from the frequent fall of large branches, broken down by the weight of the multitudes above, and which in their descent often destroyed numbers of the birds themselves; Avhile the clothes of those engaged in traversing the woods were completely covered with the excrements of the pigeons. '• These circumstances were related to me by many of the most respectable part of the community in that quarter, and were confirmed in part by what I myself witnessed. I passed for several miles through this same breeding place, Avhere every tree was spotted with nests, the remains of those above described. In many instances I counted upwards of ninety nests on a single tree ; but the pigeons had abandoned this place for another, 60 or 80 miles off", towards Green Kiver, where they were said at that time to be equally numerous. From the great numbers that were constantly passing over our heads to or from that quarter, I had no doubt of the truth of this statement. The mast had been chiefly consumed in Kentucky; and the pigeons, every morn- ing a little before sunrise, set out for the Indiana territory, the nearest part of which was about sixty miles distant. Many of these returned before ten o'clock, and the great body generally appeared on their return a little after noon. I had left the public road to visit the remains of the bieeding-place near Shelbyville, and was traversing the woods with my gun, on my way to Frankfort, when about ten o'clock the pigeons which I had observed flying the greater part of the morning northerly, began to return in such immense numbers as I never before had Avitnessed. Coming to an opening by the side of a creek, where I had a more uninterru^^ted view, I was astonished at their appearance : they were flying with great steadiness and rapidity, at a height beyond gunshot, in several strata deep, and so close together that, could shot have reached them, one discharge could not have failed to bring down several individuals. From right to left, as far as the eye could reach, the breadth of this vast procession ex- tended, seeming everywhere equally crowded. Curious to determine how long this appearance would continue, I took out my watch to note the time, and sat down to observe them. It Avas then half-past one ; I sat for more than an hour, but IT THE STRUGGLE FOR EXISTENCE 33 instead of a diminution of this prodigious procession, it seemed rather to increase, both in numbers and rapidity ; and anxious to reach Frankfort before night, I rose and went on. About four o'clock in the afternoon I crossed Kentucky Eiver, at the town of Frankfort, at which time the living torrent above my head seemed as numerous and as extensive as ever. Long after this I observed them in large bodies that continued to pass for six or eight minutes, and these again were followed by other detached bodies, all moving in the same south-east direction, till after six o'clock in the evening. The great breadth of front which this mighty multitude preserved Avould seem to intimate a corresponding breadth of their breeding- place, which, by several gentlemen who had lately passed through part of it, was stated to me at several miles." From these various observations, Wilson calculated that the number of birds contained in the mass of pigeons which he saw on this occasion was at least two thousand millions, while this was only one of many similar aggregations known to exist in various parts of the United States. The picture here given of these defenceless birds, and their still more defenceless young, exposed to the attacks of numerous rapacious enemies, brings vividly before us one of the phases of the unceasing struggle for existence ever going on ; but when we consider the slow rate of increase of these birds, and the enormous population they are nevertheless able to maintain, we must be convinced that in the case of the majority of birds which multiply far more rapidly, and yet are never able to attain such numbers, the struggle against their numerous enemies and against the adverse forces of nature must be even more severe or more continuous. Struggle for Life heticeen closely allied Animals and Plants often the most severe. The struggle we have hitherto been considering has been mainly that between an animal or plant and its direct enemies, whether these enemies are other animals which devour it, or the forces of nature which destroy it. But there is another kind of struggle often going on at the same time between closely related species, which almost always terminates in the destruction of one of them. As an example of w^hat is D 34 DARWINISM chap. meant, the missel-thrush has increased in numbers in Scotland during the last thirty years, and this has caused a decrease in the numbers of the closely allied song-thrush in the same country. The black rat (Mus rattus) was the common rat of Europe till, in the beginning of the eighteenth century, the large brown rat (Mus decumanus) appeared on the Lower Volga, and thence spread more or less rapidly till it overran all Europe, and generally drove out the black rat, which in most parts is now comparatively rare or quite extinct. This invad- ing rat has now been carried by commerce all over the world, and in New Zealand has completely extirpated a native i-at, which the Maoris allege they brought mth them from their home in the Pacific ; and in the same country a native fly is being supplanted by the European house-fly. In Russia the small Asiatic cockroach has driven away a larger native species ; and in Australia the imported hive-bee is exterminating the small stingless native bee. The reason why this kind of struggle goes on is apparent if we consider that the allied species fill nearly the same place in the economy of nature. They require nearly the same kind of food, are exposed to the same enemies and the same dangers. Hence, if one has ever so slight an advantage over the other in procuring food or in avoiding danger, in its rapidity of multiplication or its tenacity of life, it will increase more rapidly, and by that very fact ^WU cause the other to decrease and often become altogether extinct. In some cases, no doubt, there is actual war between the two, the stronger killing the weaker ; but this is by no means necessary, and there may be cases in which the weaker species, physically, may prevail, by its power of more rapid multiplication, its better withstanding vicissitudes of climates, or its ^-eater cunning in escaping the attacks of the common enemies. The same principle is seen at work in the fact that certain mountain varieties of sheep will starve out other mountain varieties, so that the two cannot be kept together. In plants the same thing occurs. If several distinct varieties of wheat are sown together, and the mixed seed resown, some of the varieties which best suit the soil and climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a few years supplant the other varieties. ir THE STRUGGLE FOR EXISTENCE 35 As an effect of this principle, Ave seldom find closely allied species of animals or plants living together, but often in distinct though adjacent districts where the conditions of life are somewhat different. Thus we may find coAvslips (Primula veris) growing in a meadow, and primroses (P. vulgaris) in an adjoining wood, each in abundance, but not often intermingled. And for the same reason the old turf of a pasture or heath consists of a great variety of plants matted together, so much so that in a patch little more than a yard square Mr. DarAvin found twenty distinct species, belonging to eighteen distinct genera and to eight natural orders, thus shoAving their extreme diversity of organisation. For the same reason a number of distinct gi^asses and clovers are soAvn in order to make a good laAA^n instead of any one species ; and the quantity of hay produced has been found to be greater from a variety of very distinct grasses than from any one species of grass. It may be thought that forests are an exception to this rule, since in the north- temperate and arctic regions Ave find extensive forests of pines or of oaks. But these are, after all, exceptional, and characterise those regions only AA'here the climate is little favourable to forest vegetation. In the tropical and all the AA^arm temperate parts of the earth, Avhere there is a sufficient supply of moisture, the forests present the same A^ariety of species as does the turf of our old pastures ; and in the ecjuatorial virgin forests there is so great a variety of forms, and they are so thoroughly intermingled, that the traA^eller often finds it difficult to discoA^er a second specimen of any particular species AA^hich he has noticed. EA^en the forests of the temperate zones, in all favourable situations, exhibit a considerable variety of trees of distinct genera and families, and it is only AA^hen AA'e approach the outskirts of forest vegetation, Avhere either drought or A\inds or the severity of the AAdnter is adverse to the existence of most trees, that we find extensive tracts monopolised by one or two species. Even Canada has more than sixty different forest trees, and the Eastern United States a hundred and fifty ; Europe is rather j^oor, containing about eighty trees only ; Avhile the forests of Eastern Asia, Japan, and Manchuria are exceedingly rich, about a hundred and seA^enty species being already known. And in all these countries the trees groAv inter- 36 . DARWINISM chap. mingled, so that in every extensive forest we have a consider- able variety, as may be seen in the few remnants of our primitive woods in some parts of Popping Forest and the New Forest. Among animals the same law prevails, though, owing to their constant movements and power of concealment, it is not so readily observed. As illustrations we may refer to the wolf, ranging over Europe and Northern Asia, while the jackal inhabits Southern Asia and Northern Africa ; the tree- porcu})ines, of which there are two closely allied species, one inhabiting the eastern, the other the western half of North America ; the common hare (Lepus timidus) in Central and Southern Europe, while all Northern Europe is inhabited by the variable hare (Lepus variabilis) ; the common jay (Garrulus glandarius) inhabiting all Europe, while another species (Garrulus Brandti) is found all across Asia from the Urals to Japan ; and many species of birds in the Eastern United States are replaced by closely allied species in the west. Of course there are also numbers of closely related species in the same country, but it will almost always be found that they frequent difierent stations and have somewhat different habits, and so do not come into direct competition with each other ; just as closely allied plants may inhabit the same districts, when one prefers meadows the other woods, one a chalky soil the other sand, one a damp situation the other a dry one. With plants, fixed as they are to the earth, we easily note these peculiarities of station ; but with wild animals, which we see only on rare occasions, it requires close and long-continued observation to detect the peculiarities in their mode of life which may prevent all direct competition between closely allied species dwelling in the same area. The Ethical Aspect of the Struggle for Existence. Our exj^osition of the i)henomena presented by the struggle for existence may be fitly concluded by a few remarks on its ethical aspect. Now that the war of nature is better knoAvn, it has been dwelt upon by many writers as presenting so vast an amount of cruelty and pain as to be revolting to our instincts of humanity, while it has jn-oved a stumbling-block in the way of those who would fain believe in an all-^\ise and II THE STRUGGLE FOR EXISTENCE 37 benevolent ruler of the universe. Thus, a brilliant AVTiter says : " Pain, grief, disease, and death, are these the inventions of a loving God ? That no animal shall rise to excellence except by being fatal to the life of others, is this the law of a kind Creator? It is useless to say that pain has its benevolence, that massacre has its mercy. Why is it so ordained that bad should be the raAv material of good ? Pain is not the less pain because it is useful ; murder is not less murder because it is conducive to development. Here is blood upon the hand still, and all the perfumes of Arabia will not sweeten it."i Even so thoughtful a ^^Titer as Professor Huxley adopts similar views. In a recent article on " The Struggle for Existence " he speaks of the myriads of generations of herbiv- orous animals which " have been tormented and devoured by carnivores " ; of the carnivores and herbivores alike " subject to all the miseries incidental to old age, disease, and over-multi- plication " ; and of the " more or less enduring suffering," which is the meed of both vancjuished and victor. And he concludes that, since thousands of times a minute, were our ears sharp enough, we should hear sighs and groans of pain like those heard by Dante at the gate of hell, the world cannot be governed by what we call benevolence. "-^ Now there is, I think, good reason to believe that all this is greatly exaggerated ; that the supposed " torments " and " miseries " of animals have little real existence, but are the reflection of the imagined sensations of cultivated men and Avomen in similar circumstances ; and that the amount of actual sufl'ering caused by the struggle for existence among animals is altogether insignificant. Let us, therefore, endeavour to ascertain what are the real facts on which these tremendous accusations are founded. In the first place, we must remember that animals are entirely spared the pain w^e suff'er in the anticipation of death — a pain far greater, in most cases, than the reality. This leads, probably, to an almost perpetual enjoyment of their lives ; since their constant watchfulness against danger, and even their actual flight from an enemy, will be the enjoyable ^ Wiuwood Reade's Martyrdom of Man, p. 520. ^ Nineteenth Century, February 1888, pp. 162, 163. 38 DARWINISM chap. exercise of the powers and faculties they possess, unmixed with any serious dread. There is, in the next place, much evidence to show that violent deaths, if not too prolonged, are painless and easy ; even in the case of man, whose nervous system is in all probability much more susceptible to pain than that of most animals. In all cases in which persons have escaped after being seized by a lion or tiger, they declare that they suffered little or no pain, j^hysical or mental. A well-known instance is that of Livingstone, who thus describes his sensations when seized by a lion : " Starting and looking half round, I saw the lion just in the act of springing on me. I was upon a little height ; he caught my shoulder as he sprang, and we both came to the ground below together. Growling horribly close to my ear, he shook me as a terrier-dog does a rat. The shock produced a stupor similar to that which seems to be felt by a mouse after the first shake of the cat. It causes a sort of dreaminess, in which there ivas no sense of pain or feeling of terror, though I was quite conscious of all that was happening. It was like what patients partially under the influence of chloroform describe, who see all the operation, but feel not the knife. This singular condition was not the result of any mental process. The shake annihilated fear, and allowed no sense of horror in looking round at the beast." This absence of pain is not peculiar to those seized by wild beasts, but is equally produced by any accident which causes a general shock to the system. Mr. Whymper describes an accident to himself during one of his preliminary explorations of the Matterhorn, when he fell several hundred feet, bounding from rock to rock, till fortunately embedded in a snow-drift near the edge of a tremendous precipice. He declares that while falling and feeling blow after blow, he neither lost consciousness nor suffered pain, merely thinking, calmly, that a few more blows would finish him. We have therefore a right to conclude, that when death follows soon after any great shock it is as easy and painless a death as possible ; and this is certainly what happens Avhen an animal is seized by a beast of prey. For the enemy is one Avhich hunts for food, not for pleasure or excitement ; and it is doubtful Avhether any carnivorous animal in a state of nature begins to seek after THE STRUGGLE FOR EXISTENCE prey till driven to do so by hunger. When an animal is caught, therefore, it is very soon devoured, and thus the first shock is followed by an almost painless death. Neither do those which die of cold or hunger suffer much. Cold is generally severest at night and has a tendency to produce sleep and painless extinction. Hunger, on the other hand, is hardly felt during periods of excitement, and when food is scarce the excitement of seeking for it is at its greatest. It is probable, also, that when hunger presses, most animals will devour anything to stay their hunger, and will die of gradual exhaustion and weakness not necessarily painful, if they do not fall an earlier prey to some enemy or to cold.^ NoAV let us consider what are the enjoyments of the lives of most animals. As a rule they come into existence at a time of year when food is most plentiful and the climate most suitable, that is in the spring of the temperate zone and at the commencement of the dry season in the tropics. They grow vigorously, being supplied with abundance of food ; and when they reach maturity their lives are a continual round of healthy excitement and exercise, alternating with complete repose. The daily search for the daily food employs all their faculties and exercises every organ of their bodies, while this exercise leads to the satisfaction of all their j^hysical needs. In our own case, we can give no more perfect definition of happiness, than this exercise and this satisfaction ; and we must therefore conclude that animals, as a rule, enjoy all the happiness of which they are capable. And this normal state of happiness is not alloyed, as ^vith us, by long periods — whole lives often — of poverty or ill-health, and of the un- satisfied longing for pleasures which others enjoy but to which we cannot attain. Illness, and what answers to poverty in animals — continued hunger — are quickly followed by unantici- pated and almost painless extinction. Where we err is, in giving to animals feelings and emotions which they do not possess. To us the very sight of blood and of torn or mangled limbs is painful, while the idea of the suffering implied by it ^ The Kestrel, which usually feeds on mice, birds, and frogs, sometimes stays its hunger with earthworms, as do some of the American buzzards. The Honey-buzzard sometimes eats not only earthworms and slugs, but even corn ; and the Buteo borealis of North America, whose usual food is small mammals and birds, sometimes eats crayfish. 40 DARWINISM chap, ii is heartrending. We have a horror of all violent and sudden death, because we think of the life full of promise cut short, of hopes and expectations unfulfilled, and of the grief of mourning relatives. But all this is quite out of place in the case of animals, for whom a violent and a sudden death is in every way the best. Thus the poet's picture of "Nature red in tooth and claw AVith ravine " is a picture the evil of which is read into it by our imaginations, the reality being made up of full and happy lives, usually terminated by the quickest and least painful of deaths. On the whole, then, we conclude that the popular idea of the struggle for existence entailing misery and pain on the animal world is the very reverse of the truth. What it really brings about, is, the maximum of life and of the enjoy- ment of life with the minimum of suffering and pain. Given the necessity of death and reproduction — and without these there could have been no progressive development of the organic world, — and it is difficult even to imagine a system by which a greater balance of happiness could have been secured. And this view was evidently that of Darwin himself, who thus concludes his chapter on the struggle for existence : " When we reflect on this struggle, we may console ourselves with the full belief that the war of nature is not incessant, that no fear is felt, that death is generally promjit, and that the vigorous, the healthy, and the hapj^y survive and multiply." CHAPTER III THE VARIABILITY OF SPECIES IN A STATE OF NATURE Importance of variability — Popular ideas regarding it — Variability of the lower animals— Tlie variability of insects — Variation among lizards — Variation among birds — Diagrams of bird-variation — Number of varying individuals — Variation in the mammalia — Variation in internal organs — Variations in the skull — Variations in the habits of Animals — The Variability of plants — Species which vary little — Concluding remarks. The foundation of the Darwinian theory is the variability of species, and it is quite useless to attempt even to underetand that theory, much less to appreciate the completeness of the proof of it, unless we first obtain a clear •conception of the nature and extent of this variability. The most frequent and the most misleading of the objections to the efficacy of natural selection arise from ignorance of this subject, an ignorance shared by many naturalists, for it is only since Mr. Darwin has taught us their importance that varieties have been systematically collected and recorded ; and even now very few collectors or students bestow upon them the attention they deserve. By the older naturalists, indeed, varieties — especially if numerous, small, and of frequent occurrence — were looked upon as an unmitigated nuisance, because they rendered it almost impossible to give precise definitions of species, then considered the chief end of systematic natural history. Hence it was the custom to describe what was supposed to be the " typical form " of species, and most collectors were satisfied if they possessed this typical form in their cabinets. Now, however, a collection is valued in proportion as it contains illustrative specimens of all the varieties that occur in each species, and in some cases these 1/ 42 DARWINISM have been carefully described, so that we possess a consider- able mass of information on the subject. Utilising this in- formation we will now endeavour to give some idea of the nature and extent of variation in the species of animals and plants. It is very commonly objected that the widespread and constant variability which is admitted to be a characteristic of domesticated animals and cultivated plants is largely due to the unnatural conditions of their existence, and that we have no proof of any corresponding amount of variation occurring in a state of nature. Wild animals and plants, it is said, are usually stable, and when variations occur these are alleged to be small in amount and to afi'ect superficial characters only ; or if larger and more imj^ortant, to occur so rarely as not to afford any aid in the supposed formation of new sj^ecies. This objection, as will be sho"\\Ti, is utterly unfounded ; but as it is one which goes to the very root of the problem, it is necessary to enter at some length into the various proofs of variation in a state of nature. This is the more necessary because the materials collected by Mr. Darwin bearing on this question have never been published, and comparatively few of them have .been cited in The Origin of Species ; while a considerable body of facts has been made kno^\^l since the publication of the last edition of that work. VariaUUty of the Loiver Animals. Among the lowest and most ancient marine organisms are the Foraminifera, little masses of living jelly, apparently structureless, but which secrete beautiful shelly coverings, often perfectly symmetrical, as varied in form as those of the mollusca and far more complicated. These have been studied with great care by many eminent naturalists, and the late Dr. W. B. Carpenter in his great work — the Introduction to the Study of the Foraminifera — thus refers to their variability: " There is not a single species of plant or animal of which the range of variation has been studied by the collocation and comparison of so large a number of specimens as have passed under the review of Messrs. AVilliamson, Parker, Rupert Jones, and myself in our studies of the types of this group ;" and he states as the result of this extensive comparison of Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 43 specimens : " The range of variation is so great among the Foraminifera as to include not merely those differential char- acters which have been usually accounted specific, but also those upon which the greater part of the genera of this group have been founded, and even in some instances those of its orders." ^ Coming now to a higher group — the Sea- Anemones — Mr. P. H. Gosse and other writers on these creatures often refer to variations in size, in the thickness and length of the tentacles, the form of the disc and of the mouth, and the character of surface of the column, while the colour varies enormously in a great number of the species. Similar variations occur in all the various groups of marine invertebrata, and in the great sub-kingdom of the mollusca they are especially numerous. Thus, Dr. S. P. Woodward states that many present a most perplexing amount of variation, resulting (as he supposes) from supply of food, variety of depth and of saltness of the water ; but we know that many variations are c^uite inde- pendent of such causes, and we will now consider a few cases among the land-mollusca in which they have been more care- fully studied. In the small forest region of Oahu, one of the Sandwich Islands, there have been found about 175 species of land-shells represented by 700 or 800 varieties ; and we are told by the Rev. J. T. Gulick, who studied them carefully, that "we frequently find a genus represented in several successive valleys by allied species, sometimes feeding on the same, some- times on different plants. In every such case the valleys that are nearest to each other furnish the most nearly allied forms ; and a fidl set of the varieties of each species ]ji'esents a minute gradation of forms between the more divergent types found in the more widely separcded localities.'' In most land-shells there is a considerable amount of varia- tion in colour, markings, size, form, and texture or striation of the surface, even in specimens collected in the same locality. Thus, a French author has enumerated no less than 198 varieties of the common wood -snail (Helix nemoralis), while of the equally common garden -snail (Helix hortensis) ninety varieties have been described. Fresh-water shells are also ^ Foraminifera, preface, p. x. 44 DARWINISM subject to great variation, so that there is much uncertainty as to the lumiber of species; and vai'iations are especially frequent in the Planorbid?e, which exhibit many eccentric deviations from the usual form of the species — deviations which must often affect the form of the living animal. In Mr. Ingersoll's Eeport on the Recent Mollusca of Colorado many of these extra- ordinary variations are referred to, and it is stated that a shell (Helisonia trivohis) abundant in some small ponds and lakes, had scarcely two specimens alike, and many of them closely resembled other and altogether distinct species.^ The Variability of Insects. Among Insects there is a large amount of variation, though very few entomologists devote themselves to its investigation. Our first examples will be taken from the late Mr. T. Vernon AYoUaston's book, On the J ^aviation of Species, and they must be considered as indications of very widespread though little noticed phenomena. He speaks of the curious little carabideous beetles of the genus Notiophilus as being " extremely unstable both in their sculpture and hue ; " of the common Calathus mollis as having " the hind wings at one time amj^le, at another rudimentary, and at a third nearly obsolete ; " and of the same irregularity as to the wings being characteristic of many Orthoptera and of the Homopterous Fulgoridse. Mr. Westwood in his Modern Classification of Insects states that " the species of Gerris, Hydrometra, and Yelia are mostly found perfectly apterous, though occasionally with full-sized wings." It is, however, among the Lepidoptera (butterflies and moths) that the most numerous cases of variation have been observed, and every good collection of these insects affords striking examples. I Avill first adduce the testimony of Mr. Bates, who speaks of the butterflies of the Amazon valley exhibiting innumerable local varieties or races, while some species showed great individual variability. Of the beautiful Mechanitis Polymnia he says, that at Ega on the Upper Amazons, " it varies not only in general colour and pattern, but also very considerably in the shape of the wings, especially in the male sex." Again, at St. Paulo, Ithomia ^ United States Geological Survey of the Territories^ 1874. Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 45 Orolina exhibits four distinct varieties, all occurring together, and these differ not only in colour but in form, one variety being described as having the fore wings much elongated in the male, while another is much larger and has "the hind wings in the male different in shape." Of Heliconius Numata Mr. Bates says: "This species is so variable that it is difficult to find two examples exactly alike," while " it varies in structure as well as in colours. The A\angs are sometimes broader, some- times narrower ; and their edges are simple in some examples and festooned in others." Of another species of the same genus, H. melpomene, ten distinct varieties are described all more or less connected by intermediate forms, and four of these varieties were obtained at one locality, Serpa on the north bank of the Amazon. Ceratina Ninonia is another of these very unstable species exhibiting many local varieties which are, however, incomplete and connected by intermediate forms ; while the several species of the genus Lycorea all vary to such an extent as almost to link them together, so that Mr. Bates thinks they might all fairly be considered as varieties of one species only. Turning to the Eastern Hemisphere we have in Papilo Severus a species which exhibits a large amount of simple variation, in the presence or absence of a pale patch on the upper wings, in the brown submarginal marks on the lower wings, in the form and extent of the yelloAv band, and in the size of the specimens. The most extreme forms, as well as the intermediate ones, are often found in one locality and in company with each other. A small butterfly (Terias hecabe) ranges over the whole of the Indian and Malayan regions to Australia, and everywhere exhibits great variations, many of which have been described as distinct species ; but a gentle- man in Australia bred two of these distinct forms (T. hecabe and T. ^siope), with several intermediates, from one batch of caterpillars found feeding together on the same plant. ^ It is therefore very probable that a considerable number of supposed distinct species are only individual varieties. Cases of variation similar to those now adduced among butterflies might be increased indefinitely, but it is as well to note that such important characters as the neuration of the ^ Proceedings of the Entomological Society of London, 1875, p. vii. 46 DARWINISM chap. winpjs, on which generic and family distinctions are often established, are also subject to variation. The Rev. R. P. Murray, in 1872, laid before the Entomological Society examples of such variation in six species of butterflies, and other cases have been since described. The larvae of butter- flies and moths are also very variable, and one observer recorded in the Proceedings of fJie Entomological Society for 1870 no less than sixteen varieties of the caterpillar of the bedstraw hawk-moth (Deilephela galii). Variation among Lizards. Passing on from the lower animals to the vertebrata, we find more abundant and more definite evidence as to the extent and amount of individual variation. I will first give a case among the Reptilia from some of Mr. Darwin's un- published MSS., which have been kindly lent me by Mr. Francis Dar\\in. "M. Milne Edwards {Annates des Sci. Nat, 1 ser., tom. xvi. p. 50) has given a curious table of measurements of four- teen specimens of Lacerta muralis ; and, taking the length of the head as a standard, he finds the neck, trunk, tail, front and hind legs, colour, and femoral pores, all varying wonder- fully ; and so it is more or less with other species. So ap- parently trifling a character as the scales on the head affording almost the only constant characters." As the table of measurements above referred to would give no clear conception of the nature and amount of the variation without a laborious study and comparison of the figures, I have endeavoured to find a method of presenting the facts to the eye, so that they may be easily grasped and appreciated. In the diagram opposite, the comparative variations of the different organs of this species are given by means of variously bent lines. The head is represented by a straight line because it presented (apparently) no variation. The body is next given, the specimens being arranged in the order of their size from No. 1, the smallest, to No. 14, the largest, the actual lengths being laid down from a base line at a suitable distance below, in this case two inches below the centre, the mean length of the body of the fourteen specimens being two inches. The respective lengths of the neck, legs, and toe of Ill DIAGRAM OF VARIATIOX 47 7 5 5 7 9 77 J 3 6 7 9 11 M The lengths in the table are given in millimetres, which are here reduced to inches for the means. Fig. 1.— Variations of Lacerta muralis. 48 DARWINISM Lacerta ocellata tsi — ..Neck ..Body ..Hind Legs ^ Tail Lacerta uiridis N- .... Neck ■(-- —Body M , Hind Legs m^^a^-. rrr Lacerta agilis M- Neck M -Body ..J^ind Legs Lacerta muralis -- ..Ueck _Body _Jtfind Legs . T^:i Lacerta ue/ox 1 Neck M ■(.-. Body Hind Legs Tail Lacerta deserti W ..Neck .. Body ..Hind Legs ^^^KM^Tai! Length of Head )i {^■^■^■1 taken as the standard'' in each of the abouc -named species Fia. 2.— Variation of Lizards. ill VARIABILITY OF SPECIES IN A STATE OF NATURE 49 each specimen are then laid down in the same manner at convenient distances apart for comparison ; and- Ave see that their variations bear no definite relation to those of the body, and not much to those of each other. With the exception of No. 5, in which all the parts agree in being large, there is a marked independence of each part, shown by the lines often curving in opposite directions ; which proves that in those specimens one part is large while the other is small. The actual amount of the variation is A'ery gieat, ranging from one-sixth of the mean length in the neck to considerably more than a fourth in the hind leg, and this among only fourteen examples Avhich happen to be in a particular museum. To prove that this is not an isolated case, Professor Milne Edwards also gives a table shoAving the amount of variation in the museum specimens of six common species of lizards, also taking the head as the standard, so that the comparative variation of each part to the head is giA^en. In the accompany- ing diagi^am (Fig. 2) the A-ariations are exhibited by means of lines of A^arying length. It Avill be understood that, hoAvever much the specimens A'aried in size, if they had kept the same proimiions, the variation line Avould haA-e been in every case reduced to a point, as in the neck of L. velox aa hich exhibits no A^ariation. The diiTerent proportions of the Aariation lines for each species may show a distinct mode of variation, or may be merely due to the small and differing number of specimens ; for it is certain that AA'hateA'er amount of variation occurs among a fcAv specimens aaiII be greatly increased when a much larger number of specimens are examined. That the amount of variation is large, may be seen by comparing it Avith the actual length of the head (given beloAv the diagram) which Avas used as a standard in determining the variation, but AA^hich itself seems not to have A-aried.^ Variation among Birds. Coming noAv to the class of Birds, Ave find much more copious evidence of A^ariation. This is due partly to the fact that Ornithology has perhaps a larger body of devotees than any other branch of natural history (except entomology) ; to the moderate size of the majority of birds ; and to the circum- ^ Ann, des Set. Nat., torn. xvi. p, 50. E 50 DARWINISM cbai*. stance that the form and dimensions of the wings, tail, beak, and feet offer the Ijest generic and specific characters and can all be easily measured and compared. The most systematic observations on the individual variation of birds have been made by Mr. J. A. Allen, in his remarkable memoir : " On the Mammals and Winter Birds of East Florida, with an examina- tion of certain assumed specific characters in Birds, and a sketch of the Bird Faunae of Eastern North America," published in the Bulletin of the Museum of ComjMvative Zoology at Harvard College, Cambridge, Massachusetts, in 1871. In this work exact measurements are given of all the chief external parts of a large number of species of common American birds, from twenty to sixty or more specimens of each species being measured, so that we are able to determine with some precision the nature and extent of the variation that usually occurs. Mr. Allen says : " The facts of the case show that a variation of from 15 to 20 per cent in general size, and an equal degree of variation in the relative size of different parts, may be ordinarily expected among specimens of the same species and sex, taken at the same locality, while in some cases the variation is even greater than this." He then goes on to show that each part varies to a considerable extent independently of the other parts ; so that when the size varies, the proportions of all the parts vary, often to a much greater amount. The wing and tail, for example, besides varying in length, vary in the pro- portionate length of each feather, and this causes their outline to vary considerably in shape. The bill also varies in length, width, depth, and curvature. The tarsus varies in length, as does each toe separately and independently ; and all this not to a minute degree requiring very careful measurement to detect it at all, but to an amount easily seen without any measurement, as it averages one-sixth of the whole length and often reaches one -fourth. In twelve species of common perching birds the wing varied (in from twenty-five to thirty specimens) from 1 4 to 2 1 per cent of the mean length, and the tail from 13*8 to 23 '4 per cent. The variation of the form of the wing can be very easily tested by noting which feather is longest, which next in length, and so on, the respective feathers being indicated by the numbers 1, 2, 3, etc., com- Ill VARIABILITY OF SPECIES IX A STATE OF NATURE 51 mencing -^dth the outer one. As an example of the irregular variation constantly met with, the following occurred among twenty -five specimens of Dendraeca coronata. Numbers bracketed imply that the corresponding feathers were of equal length.^ Eelatrt: Lengths of Prbiary Wing Feathers of dendr.eca coronata. Longest. Second in Third in Fourth in Fifth in Sixth in Length. Length. Length. Length. Length. 2 3 1 4 5 6 3 2 4 1 0 6 3 4 1 5 6 7 I ] 4 1 5 6 7 \ ] 5 6 / ,Q 9 Here Ave have five very distinct proportionate lengths of the Aving feathers, any one of which is often thought sufficient to characterise a distinct species of bird ; and though this is rather an extreme case, Mr. Allen assures us that "the com- parison, extended in the table to only a few species, has been carried to scores of others ^yiih. similar results." Along with this variation in size and proportions there occurs a large amount of variation in colour and markings. " The difference in intensity of colour between the extremes of a series of fifty or one hundred specimens of any species, collected at a single locality, and nearly at the same season of the year, is often as great as occurs between truly distinct species." But there is also a great amount of indiWdual variability in the markings of the same species. Birds having the plumage varied with streaks and spots differ exceedingly in diflerent individuals of the same species in respect to the size, shape, and number of these marks, and in the general aspect of the plumage resulting from such variations. "In the common 1 See Winter Birds of Florida, p. 206, Table F. 52 DARWINISM chap. song sparrow (Melospiza melodia), the fox-coloured sparrow (Passerella iliaca), the swamp sparrow (Melospiza palustris), the black and white creeper (Mniotilta varia), the water-wagtail (Seiiirus novieboracencis), in Turdns fuscescens and its allies, the difference in the size of the streaks is often very considerable. In the song sparrow they vary to such an extent that in some cases they are reduced to narrow lines ; in others so enlarged as to cover the greater part of the breast and sides of the body, sometimes uniting on the middle of the breast into a nearly continuous patch." Mr. Allen then goes on to particularise several species in which such variations occur, giving cases in Avhich two speci- mens taken at the same place on the same day exhibited the two extremes of coloration. Another set of variations is thus described : " The white markings so common on the wings and tails of birds, as the bars formed by the white tips of the greater wing-coverts, the Avhite patch occasionally present at the base of the primary quills, or the white band crossing them, and the white patch near the end of the outer tail- feathers are also extremely liable to variation in respect to their extent and the number of feathers to which, in the same species, these markings extend." It is to be especially noted that all these varieties are distinct from those which depend on season, on age, or on sex, and that they are such as have in many other species been considered to be of specific value. These variations of colour could not be presented to the eye without a series of carefully engraved plates, but in order to bring Mr. Allen's measurements, illusti^ting variations of size and proportion, more clearly before the reader, I have prepared a series of diagrams illustrating the more important facts and their bearings on the Dar^vinian theory. The first of these is intended, mainly, to show the actual amount of the variation, as it gives the true length of the wing and tail in the extreme cases among thirty specimens of each of three species. The shaded portion shows the minimum length, the unshaded portion the additional length in the maximum. The point to be specially noted here is, that in each of these common species there is about the same amount of variation, and that it is so great as to be obvious at a glance. Ill DIAGRAM OF VARIATION 53 g> O & o 05 03 0:3 I I i I I i I U I I "^ 5 O -i. "t^ ^ .5 54 DARWINISM chap. There is here no question of " minute " or " infinitesimal " variation, which many people suppose to be the only kind of variation that exists. It cannot even be called small ; yet from all the evidence we now j^ossess it seems to be the amount which characterises most of the common species of birds. It maybe said, however, that these are the extreme variations, and only occur in one or two individuals, while the great majority exhibit little or no difference. Other diagrams will show that this is not the case ; but even if it were so, it would be no objection at all, because these are the extremes among thirty specimens only. We may safely assume that these thirty specimens, taken by chance, are not, in the case of all these species, exceptional lots, and therefore we might expect at least two similarly varying specimens in each additional thirty. But the number of individuals, even in a very rare sj^ecies, is probably thirty thousand or more, and in a common species thirty, or even three hundred, millions. Even one individual in each thirty, varying to the amount shown in the diagram, would give at least a million in the total population of any common bird, and among this million many would vary much more than the extreme among thirty only. We should thus have a vast body of individuals varying to a large extent in the length of the wings and tail, and offering ample material for the modification of these organs by natural selection. We "vvill now proceed to show that other parts of the body vary, simultaneously, but independently, to an equal amount. The first bird taken is the common Bob-o-link or Rice-bird (Dolichonyx oryzivorus), and the Diagram, Fig. 4, exhibits the variations of seven important characters in twenty male adult specimens.^ These characters are — the lengths of the body, wing, tail, tarsus, middle toe, outer toe, and hind toe, being as many as can be conveniently exhibited in one diagram. The length of the body is not given by Mr. Allen, but as it forms a convenient standard of comparison, it has been obtained by deducting the length of the tail from the total length of the birds as given by him. The diagram has been constructed as follows : — The twenty specimens are first arranged in a series according to the body-lengths (which may be con- ^ See Table I, p. 211, of Allen's Winter Birds of Florida. Ill DIAGRAM OF VARIATION 55 r 5 10 15 Fig. 4.— Dolichonyx oryzivorus. 20 Males. 20 56 DARWINISM CHAP. 15 10 15 20 25 SO 85 40 15 10 15 20 25 20 35 40 FiQ. 5. — Agelaius phoeniceus. 40 Males. Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 57 sidered to give the size of the bird), from the shortest to the longest, and the same number of A^ertical lines are drawn, numbered from one to twenty. In this case (and wherever practicable) the body-length is measured from the lower line of the diagram, so that the actual length of the bird is exhibited as well as the actual variations of length. These can be well estimated by means of the horizontal line drawn at the mean between the two extremes, and it ^ill be seen that one-fifth of the total number of specimens taken on either side exhibits a very large amDunt of variation, which would of course be very much gi^eater if a hundred or more specimens were compared. The lengths of the vring, tail, and other parts are then laid down, and the diagram thus exhibits at a glance the comparative variation of these parts in every specimen as well as the actual amount of variation in the twenty specimens ; and we are thus enabled to arrive at some important con- clusions. AYe note, first, that the variations of none of the parts follow the variations of the body, but are sometimes almost in an opposite direction. Thus the longest wing corresponds to a rather small body, the longest tail to a medium body, while the longest leg and toes belong to only a moderately large body. Again, even related parts do not constantly vary together but present many instances of independent variation, as sho^\Ti by the want of parallelism in their respective variation-lines. In No. 5 (see Fig. 4) the wing is very long, the tail moderately so ; while in No. 6 the Aving is much shorter while the tail is considerably longer. The tarsus presents comparatively little variation ; and although the three toes may be said to vary in general together, there are many divergencies ; thus, in passing from Xo. 9 to No. 10, the outer toe becomes longer, while the hind toe becomes considerably shorter ; while in Nos, 3 and 4 the middle toe varies in an opposite way to the outer and the hind toes. In the next diagram (Fig. 5) we have the variations in forty males of the Red-winged Blackbird (Agelteus phoeniceus), and here we see the same general features. One-fifth of the whole number of specimens offer a large amoimt of variation either below or above the mean ; while the wangs, tail, and head vary quite indej^endently of the body. The wing and tail too, 58 DARWINISM CHAP. 16 10 15 20 35 SO 15 10 15 20 25 80 Fig. 6.— Cardinalis virginianus. 31 Males. Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 59 though showing some amount of correlated variation, yet in no less than nine cases vary in opposite directions as compared ^vith the preceding species. The next diagram (Fig. 6), showing the variations of thirty- one males of the Cardinal bird (Cardinalis virginianus), exhibits these features much more strongly. The amount of variation in proportion to the size of the bird is very much greater ; while the variations of the wing and tail not only have no correspondence with that of the body but very little mth each other. In no less than twelve or thirteen instances they vary in opposite directions, while even where they correspond in direction the amount of the variation is often very dispropor- tionate. As the proportions of the tarsi and toes of birds have great influence on their mode of life and habits and are often used as specific or even generic characters, I have prepared a diagram (Fig. 7) to show the variation in these parts only, among twenty specimens of each of four species of birds, four or five of the most variable alone being given. The extreme divergence of each of the lines in a vertical direction shows the actual amount of variation ; and if we consider the small length of the toes of these small birds, averaging about three-quarters of an inch, we shall see that the variation is really very large ; while the diverging curves and angles show that each part varies, to a great extent, independently. It is evident that if we compared some thousands of individuals instead of only twenty, we should have an amount of independent variation occurring each year which wQuld enable almost any modification of these important organs to be rapidly efifected. In order to meet the objection that the large amount of variability here shown depends chiefly on the observations of one person and on the birds of a single country, I have examined Professor Schlegel's Catalogue of the Birds in the Leyden Museum, in which he usually gives the range of variation of the specimens in the museum (which are commonly less than a dozen and rarely over twenty) as regards some of their more important dimensions. These fully support the statement of ]\Ir. Allen, since they show an equal amount of variability when the numbers compared are 60 DARWINISM CO CO Q. s: o ^,j^ o o CO i^ o ^1 Qi o CS 0:3 Tarsus Middle Toe. Outer Toe_ Hind Toe Tarsus Middle Toe. Outer Toe Hind Toe §■ i5 '^ Tarsus Middle Toe Outer Toe Hind Toe Tarsus. Middle Toe. Outer Toe Hind Toe • 1 2 3 From Table G. in Allen's Birds of Florida. Fig. 7.— Variation of Tarsus and Toes, Ill DIAGRAM OF VARIATION "81 Phonygama atra Oriolus galbula Pica caudata 2 :^" Semeioptera iuallacei iiij I I I I I I 1 1 • I.. Pyrrhocorax alpinus I I . Fig. 8. — Variation of Birds in Leyden Museum. 62 DARWINISM chap. sufficient, which, however, is not often the case. The accompanying diagram exhibits the actual differences of size in five organs which occur in five species taken almost at random from this catalogue. Here, again, we j^erceive that the variation is decidedly large, even among a very small number of specimens ; while the facts all show that there is no ground whatever for the common assumption that natural species consist of individuals which are nearly all alike, or that the variations which occur are " infinitesimal " or even "small." The proportionate Number of Individuals which present a considerable amount of Variation. The notion that variation is a comparatively exceptional phenomenon, and that in any case considerable variations occur very rarely in proportion to the number of individuals which do not vary, is so deeply rooted that it is necessary to show by every possible method of illustration how completely opposed it is to the facts of nature. I have therefore prepared some diagrams in which each of the individual birds measured is represented by a spot, placed at a proportionate distance, right and left, from the median line accordingly as it varies in excess or defect of the mean length as regards the particular part compared. As the object in this set of dia- grams is to show the number of individuals which vary con- siderably in proportion to those which vary little or not at all, the scale has been enlarged in order to allow room for placing the spots without overlapping each other. In the diagram opposite twenty males of Icterus Baltimore are registered, so as to exhibit to the eye the proportionate number of specimens which vary, to a greater or less amount, in the length of the tail, wing, tarsus, middle toe, hind toe, and bill. It will be noticed that there is usually no very great accumulation of dots about the median line which shows the average dimensions, but that a considerable number are spread at varying distances on each side of it. In the next diagram (Fig. 10), showing the variation among forty males of Agelseus phoeniceus, this approach to an equable spreading of the variations is still more apparent; while in Fig. 12, where fifty -eight specimens of Cardinalis Ill VARIABILITY OF SPECIES IK A STATE OF XATURE 63 virginianus are registered, we see a remarkable spreading out of the spots, showing in some of the characters a tendency to segregation into two or more groups of indi\iduals, each vary- ing considerably from the mean. In order fully to appreciate the teaching of these diagrams, VARIATION OF ICTERUS BALTIMORE. 20. (^ Tdil. • • Whg. Tarsus • •• •• Middle Toe. • ••• ^ • • • • Hindi Toe. Bill, ::: Bill. • •• Length, t..i.. Width. Fig. 9. we must remember, that, whatever kind and amount of varia- tions are exhibited by the few specimens here compared, would be gTeatly extended and brought into symmetrical form if large numbers — thousands or millions — Avere sub- jected to the same process of measurement and registration. We know, from the general law which governs variations from a mean value, that with increasing numbers the range 64 DARWINISM CHAP. VARIATION OF 40 MALES OF AGEL/EUS PpCENIGEUS. Length] of Bill. • o • •••••o«« Total Length of Bird. • •• • •• •• •• ••. • •••• •••••••••••••• Length of Tail. • •• ••• • ••• •• •••• Length of Wing. • • ••• Amount of ••••••• • Variation . BILL ^ LENGTH 1 TAIL. 4- 4 WING. 1 Fig. 10. of variation of each part would increase also, at first rather rapidly and then more slowly ; while gaps and irregularities Curves of Variation Fig. 11. would be gradually filled up, and at length the distribution of the dots would indicate a toleral)ly regular curve of double cuivutui-e like those shown in Fig. 11. The great divergence Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 65 of the clots, when even a few specimens are compared, shows that the curve, with high numbers, would be a flat one like the lower curve in the illustration here given. This being the case it would follow that a very large proportion of the total number of individuals constituting a species would diverge considerably from its average condition as regards each part or organ ; and as we know from the previous diagrams of variation (Figs. 1 to 7) that each part varies to a considerable extent, inde- pendently, the materials constantly ready for natural selection CARDINAUS VIRGINIANUS. 58 specimens. Florida. Tail. Length ••• •••••••••• Wing. of Bird. •V. •oooo •• •••••e«* ••• ••••e«o« • •o >••• • • ••e • •• • (From Allen's Birds of Florida, p. 281) Fig. 12 to act upon are abundant in quantity and very varied in kind. Almost any combination of variations of distinct parts will be available, where required ; and this, as w^e shall see further on, obviates one of the most weighty objections which have been urged against the efficiency of natural selection in pro- ducing new species, genera, and higher groups. Variation in the Mammalia, Owing to the generally large size of this class of animals, and the comparatively small number of naturalists who study them, large series of specimens are only occasionally examined DARWINISM CHAP. and compared, and thus the materials for determining the question of their variability in a state of nature are compara- tively scanty. The fact that our domestic animals belonging to this group, especially dogs, present extreme varieties not surpassed even by pigeons and poultry among birds, renders it almost certain that an equal amount of variability exists in the \nld state ; and this is confirmed by the example of a species of squirrel (Sciurus carolinensis), of which sixteen specimens, all males and all taken in Florida, were measured and tabulated by ]VIr. Allen. The diagram here given shows, that, both the general amount of the variation and the independent variability of the several members of the body, accord completely with the variations so common in the class of birds ; while their amount and their independence of each other are even greater than usual. Variation in the Internal Organs of Animals. In case it should be objected that the cases of variation hitherto adduced are in the external parts only, and that there is no proof that the internal organs vary in the same manner, it will be advisable to show that such varieties also occur. It is, however, impossible to adduce the same amount of evidence in this class of variation, because the great labour of dissecting large numbers of specimens of the same species is rarely undertaken, and we have to trust to the chance observations of anatomists recorded in their regular course of study. It must, however, be noted that a very large proportion of the variations already recorded in the external parts of animals necessarily imply corresponding internal variations. When feet and legs vary in size, it is because the bones vary ; when the head, body, limbs, and tail change their proportions, the bony skeleton must also change; and even when the vdng or tail feathers of birds become longer or more numerous, there is sure to be a corresponding change in the bones which support and the muscles which move them. I will, however, give a few cases of variations which have been directly observed. Mr. Frank E. Beddard has kindly communicated to me some remarkable variations he has observed in the internal Ill DIAGRAM OF VARIATION 67 15 20 25 80 32 5 :") 75 20 25 80 82 Fio. 13.— Sciurus carolinensis. 32 specimens. Florida. 68 DARWINISM chap. organs of a species of earthworm (Perionyx excavatus). The normal characters of this sj^ecies are — Setfe forming a complete row round each segment. Two pairs of spermatheca^, — spherical pouches without diverticulse — in segments 8 and 9. Two })airs of testes in segments 1 1 and 1 2. Ovaries, a single pair in segment 13. Oviducts open by a common pore in the middle of segment 14. Vasa deferentia open separately in segment 18, each furnished at its termination Avith a large prostate gland. Between two and three hundred specimens were examined, and among them thirteen sjiecimens exhibited the following marked variations : — (1) The number of the spermathecge varied from two to three or four pairs, their position also varying. (2) There were occasionally two pairs of ovaries, each with its own oviduct; the external apertures of these varied in position, being upon segments 13 and 14, 14 and 15, or 15 and 16. Occasionally when there was only the normal single oviduct pore present it varied in position, once occurring on the 10th, and once on the 11th segment. (3) The male generative pores varied in position from segments 14 to 20. In one instance there were two pairs instead of the normal single pair, and in this case each of the four apertures had its own prostate gland. Mr. Beddard remarks that all, or nearly all, the above variations are found normaUij in other genera and species. AVhen we consider the enormous number of earthworms and the comparatively very small number of individuals ex- amined, we may be sure, not only that such variations as these occur with considerable frequency, but also that still more extraordinary deviations from the normal structure may often exist. The next example is taken from Mr. Darwin's unpublished MSS. Ill VARIABILITY OF SPECIES IX A STATE OF XATURE 69 " In some species of Shrews (Sorex) and in some field-mice (Ar^^cola), the Eev. L. Jenyns {Ann. Nat. Hist, vol. vii. pp. 267, 272) found the proportional length of the intestinal canal to vary considerably. He found the same variability in the number of the caudal vertebrae. In three specimens of an Ar^'icola he found the gall-bladder having a very different degree of development, and there is reason to believe it is sometimes absent. Professor Owen has sho"«Ti that this is the case with the gall-bladder of the giraffe." Dr. Crisp {Proc. Zool. Soc, 1862, p. 137) found the gall- bladder present in some specimens of Cer\Tis superciliaris while absent in others ; and he found it to be absent in three giraffes which he dissected. A double gall-bladder was found in a sheep, and in a small mammal preserved in the Hunterian Museum there are three distinct gall-bladders. The length of the alimentary canal varies gi^eatly. In three adult giraffes described by Professor Owen it was from 124 to 136 feet long: one dissected in France had this canal 211 feet long ; while Dr. Crisp measured one of the extraordinary length of 254 feet, and similar variations are recorded in other animals. 1 The number of ribs varies in many animals. Mr. St. George Mivart says: "In the highest forms of the Primates, the niunber of true ribs is seven, but in Hylobates there are some- times eight pairs. In Semnopithecus and Colobus there are generally seven, but sometimes eight pairs of true ribs. In the Cebidse there are generally seven or eight pairs, but in Ateles sometimes nine" (Proc. Zool. Soc, 1865, p. 568). In the same paper it is stated that the number of dorsal vertebrae in man is normally twelve, very rarely thirteen. In the Chimpanzee there are normally thirteen dorsal vertebrae, but occasionally there are fourteen or only twelve. Variations in the Skull. Among the nine adult male Orang-utans, collected by myself in Borneo, the skulls differed remarkably in size and proportions. The orbits varied in vridth and height, the cranial ridge was either single or double, either much or little developed, and the zygomatic apertui'e varied considerably in 1 Froc. Zool. Soc, 1864, p. 6i. 70 DARWINISM 2 3 4 5 6 7 8 0)0 Fio. 14.— Variation of Skull of Wolf. 10 specimens Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 71 size. I noted particularly that these variations bore no necessary relation to each other, so that a large temporal muscle and zygomatic aperture might exist either with a large or a small cranium ; and thus was explained the curious difference between the single-crested and the double-crested skulls, which had been supposed to characterise distinct species. As an instance of the amount of variation in the skulls of fully adult male orangs, I found the ^Wdth between the orbits externally to be only 4 inches in one specimen and fully 5 inches in another. Exact measurements of large series of comparable skulls of the mammalia are not easily found, but from those available I have prepared three diagrams (Figs. 14, 15, and 16), in order to exhibit the facts of variation in this very important organ. The first shows the variation in ten specimens of the c;)mmon wolf (Canis lupus) from one district in North America, and we see that it is not only large in amount, but that each part exhibits a considerable independent variability. ^ In DiagTam 15 we have the variations of eight skulls of the Indian Honey-bear (Ursus labiatus), as tabulated by the late Dr. J. E. Gray of the British Museum. For such a small number of specimens the amount of variation is very large — from one-eighth to one-fifth of the mean size, — while there are an extraordinary number of instances of inde- pendent variability. In Diagram 16 we have the length and Avidth of twelve skulls of adult males of the Indian aWM boar (Sus cristatus), also given by Dr. Gray, exhibiting in both sets of measurements a variation of more than one-sixth, combined with a very considerable amount of independent variability.- The few facts now given, as to variations of the internal parts of animals, might be multiplied indefinitely by a search through the voluminous ^vritings of comparative anatomists. But the evidence already adduced, taken in conjunction with the much fuller evidence of variation in all external organs, leads us to the conclusion that wherever variations are looked for among a considerable number of individuals of the more ^ J. A. Allen, on Geogi-aphical Variation amonf:^ North American Mammals, Bull. U. S. Geol. and Geog. Survey, vol. ii. p. 314 (1876). 2 Proc. Zool. Soc. Lond., 1864, p. 700, and 1868, p. 28. 72 DARWINISM CHAP. Length Mean UK in. Widths Mean TV^ in Palate, (length) Mean 6'A in. Orbits...... (Width) Mean 2K in Nose^ (Width) Mean 2: 1 Q "i ^ R R 7 {From Table by Dr. J.E. Gray. P.Z.S. 1864. p. 700.) Fio. 15,— Variation of S skulls (Ursus labiatus). DIAGRAM OF VARIATIOX 73 ^ CO ?s -^ So a Co ^ CO "^ S P 00 CO CO si a; I 74 DARWINISM chap. common species they are sure to be found ; that they are everywhere of considerable amount, often reaching 20 per cent of the size of the part implicated ; and that they are to a great extent independent of each other, and thus aflford almost any combination of variations that may be needed. It must be particularly noticed that the whole series of variation-diagrams here given (except the three which illustrate the number of varying individuals) in every case represent the actual amount of the variation, not on any reduced or enlarged scale, but as it were life-size. Whatever number of inches or decimals of an inch the species varies in any of its parts is marked on the diagrams, so that with the help of an ordinary divided rule or a pair of compasses the variation of the different parts can be ascertained and compared just as if the specimens themselves were before the reader, but with much greater ease. In my lectures on the Dar-\dnian theory in America and in this country I used diagrams constructed on a different plan, equally illustrating the large amount of independent A'ariability, but less simple and less intelligible. The present method is a modification of that used by Mr. Francis Galton in his researches on the theory of variability, the upper line (showing the variability of the body) in Diagrams 4, 5, 6, and 13, being laid dovni on the method he has used in his experi- ments with sweet-peas and in pedigree moth-breeding.^ I be- lieve, after much consideration, and many tedious experiments in diagram-making, that no better method can be adopted for bringing before the eye, both the amount and the peculiar features of individual variability. Variations of the Habits of Animals. Closely connected w^ith those variations of internal and external structure which have been already described, are the changes of habits Avhich often occur in certain individuals or in whole species, since these must necessarily depend upon some corresponding change in the brain or in other parts of the organism ; and as these changes are of great importance in relation to the theory of instinct, a few examples of them will be now adduced. ^ See Trans. Entomological Society of London^ 1887, p. 24. Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 75 The Kea (Nestor notabilis) is a curious parrot inhabiting the mountain ranges of the Middle Island of New Zealand. It belongs to the family of Brush-tongued parrots, and naturally feeds on the honey of flowers and the insects which frequent them, together with such fruits or berries as are found in the region. Till quite recently this comprised its whole diet, but since the country it inhabits has become occupied by Europeans it has developed a taste for a carnivorous diet, Avith alarming results. It began by picking the sheepskins hung out to dry or the meat in process of being cured. About 1868 it was first observed to attack living sheep, which had frequently been found with raw and bleeding wounds on their backs. Since then it is stated that the bird actually burrows into the living sheep, eating its way down to the kidneys, which form its special delicacy. As a natural consequence, the bird is being destroyed as rapidly as possible, and one of the rare and curious members of the New Zealand fauna will no doubt shortly cease to exist. The case affords a remark- able instance of how the climbing feet and powerful hooked beak developed for one set of purposes can be applied to another altogether diff"erent purpose, and it also shows how little real stability there may be in what appear to us the most fixed habits of life. A somewhat similar change of diet has been recorded by the Duke of Argyll, in which a goose, reared by a golden eagle, was taught by its foster -parent to eat flesh, which it continued to do regularly and apparently with great relish.^ Change of habits appears to be often a result of imitation, of which Mr. Tegetmeier gives some good examples. He states that if pigeons are reared exclusively -with small grain, as wheat or barley, they will starve before eating beans. But when they are thus starving, if a bean-eating pigeon is put among them, they follow its example, and thereafter adopt the habit. So fowls sometimes refuse to eat. maize, but on seeing others eat it, they do the same and become excessively fond of it. Many persons have found that their yellow crocuses were eaten by sparrows, while the blue, purple, and white coloured varieties were left untouched ; but Mr. Teget- meier, who gTOws only these latter colours, found that after ^ Xature, vol. xix. p. 554. DARWINISM CHAP. two years the si)arrows began to attack them, and thereafter destroyed tliem quite as readily as the yellow ones ; and he believes it was merely because some bolder sparrow than the rest set the example. On this subject Mr. Charles 0. Abbott well remarks : " In studying the habits of our American birds — and I suppose it is true of birds everywhere — it must at all times be remembered that there is less stability in the habits of birds than is usually supposed ; and no account of the habits of any one species will exactly detail the various features of its habits as they really are, in every portion of the terri- tory it inhabits." ^ Mr. Charles Dixon has recorded a remarkable change in the mode of nest-building of some common chaffinches Avhich were taken to New Zealand and turned out there. He says : " The cup of the nest is small, loosely put together, apparently lined with feathers, and the walls of the structure are prolonged for about 18 inches, and hang loosely down the side of the supporting branch. The whole structure bears some re- semblance to the nests of the hangnests (Icteridse), with the exception that the cavity is at the top. Clearly these New Zealand chaffinches were at a loss for a design when fabricat- ing their nest. They had no standard to work by, no nests of their own kind to copy, no older birds to give them any instruc- tion, and the result is the abnormal structure I have just described."^ These few examples are sufficient to show that both the habits and instincts of animals are subject to variation ; and had we a sufficient number of detailed observations we should probably find that these variations were as numerous, as diverse in character, as large in amount, and as independent of each other as those which we have seen to characterise their bodily structure. The VanaUlity of Plants. The variability of plants is notorious, being proved not only by the endless variations which occur whenever a species is largely grown by horticulturists, but also l>y the great difficulty that is felt by botanists in determining the limits of species in ^ Nature, vol. xvi. p. 163 ; and vol. xi. p. 227. 2 Ibid., vol. xxxi. (1885), p. 533. Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 77 many large genera. As examples we may take the roses, the brambles, and the willows as well illustrating this fact. In Mr. Baker's Revision of the British Roses (published by the Linnean Society in 1863), he includes under the single species, Rosa canina — the common dog-rose — no less than twenty -eight named varieties distinguished by more or less constant characters and often confined to special localities, and to these are referred about seventy of the species of British and continental botanists. Of the genus Rubus or bramble, five British species are given in Bentham's Handbook of the British Flora, while in the fifth edition of Babington's Manual of British Botany, published about the same time, no less th3.n fort i/ five species are described. Of willows (Salix) the same two works enumerate fifteen and thirtij-one species respectively. The hawkweeds (Hieracium) are ecjually puzzling, for while Mr. Bentham admits only seven British species. Professor Babing- ton describes no less than thirty-two, besides several named varieties. A French botanist, Mons. A. Jordan, has collected numerous forms of a common little plant, the spring whitlow -gTass (Draba verna) ; he has cultivated these for several successive years, and declares that they preserve their peculiarities un- changed ; he also says that they each come true from seed, and thus possess all the characteristics of true species. He has described no less than fifty-two such species or permanent varieties, all found in the south of France ; and he urges botanists to follow his example in collecting, describing, and cultivating all such varieties as may occur in their respective districts. Xow, as the plant is very common almost all over Europe and ranges from Xorth America to the Himalayas, the number of similar forms over this ^Wde area would prob- ably have to be reckoned by hundreds if not by thousands. The class of facts now adduced must certainly be held to prove that in many large genera and in some single species there is a very large amount of variation, which renders it quite impossible for experts to agree upon the limits of species. We ^vill now adduce a few striking cases of individual variation. The distinguished botanist. Alp. de Candolle, made a special study of the oaks of the whole world, and has stated some 78 DARWINISM remnrkable facts as to their variability. He declares that on the same branch of oak he has noted the following vai'iations : (1) In the length of the petiole, as one to three ; (2) in the form of the leaf, being either elliptical or obovoid ; (3) in the margin l)eing entire, or notched, or even pinnatifid ; (4) in the ex- tremity being acute or blunt ; (5) in the base being sharp, l)lunt, or cordate ; (6) in the surface being pubescent or smooth ; (7) the perianth varies in depth and lobing ; (8) the stamens vary in number, independently ; (9) the anthers are mucronate or blunt; (10) the fruit stalks vary greatly in length, often as one to three; (11) the number of fruits varies; (12) the form of the base of the cup varies ; (13) the scales of the cup vary in form; (14) the proportions of the acorns vary ; (15) the times of the acorns ripening and falling vary. Besides this, many species exhibit well-marked varieties Avhich have been described and named, and these are most numerous in the best-known species. Our British oak (Quercus robur) has twenty -eight varieties ; Quercus Lusitanica has eleven ; Quercus calliprinos has ten ; and Quercus coccifera eight. A most remarkable case of variation in the parts of a common flower has been given by Dr. Hermann Miiller. He examined two hundred flowers of Myosurus minimus, among which he found thirty-five different proportions of the sej^als, petals, and anthers, the first varying from four to seven, the second from two to five, and the third from two to ten. Five sepals occurred in one hundred and eighty-nine out of the two hundred, but of these one hundred and five had three petals, forty-six had four petals, and twenty-six had five petals ; but in each of these sets the anthers varied in number from three to eight, or from two to nine. We have here an example of the same amount of " independent variability " that, as we have seen, occurs in the various dimensions of birds and mammals ; and it may be taken as an illustration of the kind and degree of variability that may be expected to occur among small and little specialised flowers.^ In the common wind-flower (Anemone nemorosa) an almost equal amount of variation occurs ; and I have myself gathered ^ Nature, vol. xxvi. p. 81. Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 79 in one locality flo"svers varying from J inch to If inch in diameter ; the bracts varying from Ih inch to 4 inches across; and the petaloid sepals either broad or narrow, and varying in number from five to ten. Though generally pure white on their upper surface, some specimens are a full pink, while others have a decided bluish tinge. Mr. Dar^vin states that he carefully examined a large number of plants of Geranium pha^um and G. pyrenaicum (not perhaps truly British but frequently found wild), which had escaped from cultivation, and had spread by seed in an open planta- tion ; and he declares that " the seedlings varied in almost every single character, both in their flowers and foliage, to a degree which I have never seen exceeded ; yet they could not have been exposed to any great change of their conditions. "^ The folloAving examples of variation in important parts of plants were collected by Mr. Darmn and have been copied from his unpublished MSS. : — " De Candolle {3Iem. S.oc. Phjs. de Geneve, torn. ii. part ii. p. 217) states that Papaver bracteatum and P. orientale present indifferently two sepals and four petals, or three sepals and six petals, which is sufficiently rare ^nth other species of the genus." " In the Primulaceae and in the great class to which this family belongs the unilocular ovarium is free, but M. Dubury {Mem. Soc. Phijs. de Geneve, tom. ii. p. 406) has often found individuals in Cyclamen hederaefolium, in which the base of the ovary was connected for a thii'd jmrt of its length vnth the inferior part of the calyx." " M. Aug. St. Hilaire (Sur la Gynobase, Mem, des Miis. d'Hist. Nat, tom. x. p. 134), speaking of some bushes of the Gomphia olesefolia, which he at first thought formed a quite distinct species, says : ' Yoila done dans un meme individu des loges et un style qui se rattachent tantot a un axe vertical, et tantot a un gynobase ; done celui-ci n'est qu'un axe veri- table ; mais cet axe est deprime au lieu d'etre vertical." He adds (p. 151), 'Does not all this indicate that nature has tried, in a manner, in the family of Rutacese to produce from a single multilocular ovary, one-styled and symmetrical, several unilocular ovaries, each with its own style.' And he ^ Animals and Plants under Domestication, vol. ii. p. 258. 80 DARWINISM subsequently shows that, in Xanthoxyhmi monogynum, ' it often happens that on the same plant, on the same panicle, we find flowers with one or with two ovaries ;' and that this is an important character is shown by the Rutacese (to which Xanthoxylum belongs), being placed in a group of natural orders characterised by having a solitary ovary." " De Candolle has divided the Criiciferae into five sub-orders in accordance with the position of the radicle and cotyledons, yet Mons. T. Gay {Ann. des Scien. Nat, ser. i. torn. \ni. p. 389) found in sixteen seeds of Petrocallis Pyrenaica the form of the embryo so uncertain that he could not tell whether it ought to be placed in the sub-orders 'Pleurorhizee' or 'Notorhizee' ; so again (p. 400) in Cochlearia saxatilis M. Gay examined twenty-nine embryos, and of these sixteen were \dgorously ' pleurorhizees,' nine had characters intermediate between pleuro- and notor- hizees, and four were pure notorhizees." "M. Raspail asserts (Aim. des Scien. Nat, ser. i. tom. v. p. 440) that a grass (Nostus Borbonicus) is so eminently variable in its floral organisation, that the varieties might serve to make a family with sufficiently numerous genera and tribes — a remark which shows that important organs must be here variable." Species which vary little. The preceding statements, as to the great amount of variation occurring in animals and plants, do not prove that all species vary to the same extent, or even vary at all, but, merely, that a considerable number of species in every class, order, and family do so vary. It ^\\\\ have been observed that the examples of great variability have all been taken from common species, or species which have a ^^'ide range and are abundant in indi\aduals. Now Mr. Darwin concludes, from an elaborate examination of the floras and faunas of several distinct regions, that common, "smle ranging species, as a rule, vary most, while those that are confined to special districts and are therefore comparatively limited in number of individuals vary least. By a similar comparison it is shown that species of large genera vary more than species of small genera. These facts explain, to some extent, why the opinion has been so prevalent that variation is very limited in amount and exceptional in character. For HI A^ARI ABILITY OF SPECIES IN A STATE OF NATURE 81 naturalists of the old school, and all mere collectors, were interested in species in proportion to their rarity, and would often have in their collections a larger number of specimens of a rare species than of a species that was very common. Now as these rare species do really vary much less than the common species, and in many cases hardly vary at all, it was very natural that a belief in the fixity of species should prevail. It is not, however, as we shall see presently, the rare, but the common and widespread species which become the parents of new forms, and thus the non-variability of any number of rare or local species offers no difficulty whatever in the way of the theory of evolution. Conclvding Bemarh. We have now sho^^Ti in some detail, at the risk of being tedious, that individual variability is a general character of all common and ^\^despread species of animals or plants ; and, further, that this variability extends, so far as we know, to every part and organ, whether external or internal, as well as to every mental faculty. Yet more important is the fact that each part or organ varies to a considerable extent inde- pendently of other parts. Again, we have shown, by abundant evidence, that the variation that occurs is very large in amount — usually reaching 10 or 20, and sometimes even 25 per cent of the average size of the varying part; while not one or two only, but from 5 to 10 per cent of the speci- mens examined exhibit nearly as large an amount of variation. These facts have been brought clearly before the reader by means of numerous diagrams, drawn to scale and exhibiting the actual variations in inches, so that there can be no pos- sibility of denying either their generality or their amount. The importance of this full exposition of the subject will be seen in future chapters, when Ave shall freciuently have to refer to the facts here set forth, esj^ecially when we deal with the various theories of recent writers and the criticisms that have been made of the Darwinian theory. A full exposition of the facts of variation among wild animals and plants is the more necessary, because compara- tively few of them were published in Mr. Darwin's works, while the more important have only been made known since G 82 DARWINISM the last edition of Tlip Origin of S2)ecies was prepared ; and it is clear that i\Ir. Darwin himself did not fully recognise the enormous amount of variability that actually exists. This is indicated by his frequent reference to the extreme slowness of the changes for which variation furnishes the materials, and also by his use of such expressions as the following : "A variety when once formed must again, perhaps after a long interral of time, vary or present individual differences of the same favourable nature as before" (Origin, p. 66). And again, after speaking of changed conditions " affording a better chance of the occurrence of favourable variations," he adds : " Unless such occur natural selection can do nothing" (Origin, p. 64). These exj^ressions are hardly consistent mth the fact of the constant and large amount of variation, of every part, in all directions, which evidently occurs in each genera- tion of all the more abundant species, and which must afford an ample supply of favourable variations whenever required ; and they have been seized uj^on and exaggerated by some ^n^ters as proofs of the extreme difficulties in the way of the theory. It is to show that such difficulties do not exist, and in the full conviction that an adequate knowledge of the facts of variation affords the only sure foundation for the Dar^vinian theory of the origin of species, that this chapter has been -written. CHAPTEK IV VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS The facts of variation and artificial selection — Proofs of the generality of variation — Variations of apples and melons — Variations of flowers- Variations of domestic animals — Domestic pigeons — Acclimatisation — Circumstances favourable to selection by man — Conditions favour- able to variation — Concluding remarks. HA\nNG SO fully discussed variation under nature it will be unnecessary to devote so much space to domesticated animals and cultivated plants, especially as Mr. Darwin has published two remarkable volumes on the subject where those who desire it may obtain ample information. A general sketch of the more important facts -sWll, however, be given, for the purpose of showing how closely they correspond vdth those described in the preceding chapter, and also to point out the general principles which they illustrate. It will also be necessary to explain how these variations have been increased and accumulated by artificial selection, since we are thereby better enabled to understand the action of natural selection, to be discussed in the succeeding chapter. The fads of Variation and Artificial Selection. Every one knows that in each litter of kittens or of puppies no two are alike. Even in the case in which several are exactly alike in colours, other difterences are always perceptible to those who observe them closely. They will differ in size, in the proportions of their bodies and limbs, in the length or texture of their hairy covering, and notably in their disposition. They each possess, too, an individual 84 DARWINISM countenance, almost as varied when closely studied as that of a human being ; not only can a shepherd distinguish every shee}> in his flock, but we all know that each kitten in the successive families of our old favourite cat has a face of its own, with an expression and individuality distinct from all its brothers and sisters. Now this individual variability exists among all creatures whatever, Avhich we can closely observe, even when the two parents are very much alike and have been matched in order to preserve some special breed. The same thing occurs in the vegetable kingdom. All plants raised from seed differ more or less from each other. In every bed of flowers or of vegetables we shall find, if we look closely, that there are countless small differences, in the size, in the mode of growth, in the shape or colour of the leaves, in the form, colour, or markings of the flowers, or in the size, form, colour, or flavour of the fruit. These differences are usually small, but are yet easily seen, and in their extremes are very considerable ; and they have this important quality, that they have a tendency to be reproduced, and thus by careful breeding any particular variation or group of varia- tions can be increased to an enormous extent — apparently to any extent not incompatible with the life, growth, and re- production of the plant or animal. The way this is done is by artificial selection, and it is Very important to understand this process and its results. Suppose we have a plant with a small edible seed, and we want to increase the size of that seed. AVe grow as large a quantity of it as possible, and when the crop is ripe we carefully choose a few of the very largest seeds, or we may by means of a sieve sort out a quantity of the largest seeds. Next year we sow only these large seeds, taking care to give them suitable soil and manure, and the result is found to be that the average size of the seeds is larger than in the first crop, and that the largest seeds are now somewhat larger and more numerous. Again so^^^ng these, we obtain a further slight increase of size, and in a very few years we obtain a greatly improved race, which will always produce larger seeds than the unimproved race, even if cultivated without any special care. In this way all our fine sorts of vegetables, fruits, and flowers have been obtained, all our choice breeds IV VARIATION UNDER DOMESTICATION 85 of cattle or of poultry, our wonderful race-horses, and our endless varieties of dogs. It is a very common but mistaken idea that this improvement is due to crossing and feeding in the case of animals, and to imj^roved cultivation in the case of plants. Crossing is occasionally used in order to obtain a combination of qualities found in two distinct breeds, and also because it is found to increase the constitutional vigour ; but every breed possessing any exceptional quality is the result of the selection of variations occurring year after year and accumulated in the manner just described. Purity of breed, with repeated selection of the best varieties of that breed, is the foundation of all improvement in our domestic animals and cultivated plants. Proofs of the Generality of Variation. Another very common error is, that variation is the exception, and rather a rare exception, and that it occurs only in one direction at a time — that is, that only one or two of the numerous possible modes of variation occur at the same time. The experience of breeders and cultivators, however, proves that variation is the rule instead of the exception, and that it occurs, more or less, in almost every direction. This is shoTNTi by the fact that different species of plants and animals have required different kinds of modification to adapt them to our use, and we have never failed to meet ^x\th variation in that jKirtictdar direction, so as to enable us to accumulate it and so to produce ultimately a large amount of change in the required direction. Our gardens furnish us with numberless examples of this property of plants. In the cabbage and lettuce we have found variation in the size and mode of growth of the leaf, enabling us to produce by selection the almost innumerable varieties, some with solid heads of foliage quite unlike any plant in a state of nature, others vnth curiously wrinkled leaves like the savoy, others of a deep purple colour used for pickling. From the very same species as the cabbage (Brassica oleracea) have arisen the broccoli and cauliflower, in which the leaves have undergone little alteration, while the branching heads of flowers gTOw into a compact mass forming one of our most delicate vegetables. The brussels sprouts are another form of the same plant, in 86 DARWINISM chap. which the whole mode of growth has been altered, numerous little heads of leaves being produced on the stem. In other varieties the ribs of the leaves are thickened so as to become themselves a culinary vegetable ; while, in the Kohlrabi, the stem grows into a turnip-like mass just above ground. Now all these extraordinarily distinct plants come from one original species which still grows wild on our coasts ; and it must have varied in all these directions, otherwise variations could not have been accumulated to the extent we now see them. The flowers and seeds of all these j^lants have remained nearly stationary, because no attempt has been made to accumulate the slight variations that no doubt occur in them. If now we turn to another set of plants, the turnips, radishes, carrots, and potatoes, we find that the roots or under- ground tubers have been wonderfully enlarged and improved, and also altered in shape and colour, while the stems, leaves, flowers, and fruits have remained almost unchanged. In the various kinds of peas and beans it is the pod or fruit and the seed that has been subjected to selection, and therefore greatly modified ; and it is here very important to notice that while all these plants have undergone cultivation in a great variety of soils and climates, with diff"erent manures and under diff'erent systems, yet the flowers have remained but little altered, those of the broad bean, the scarlet-runner, and the garden-pea, being nearly the same in all the varieties. This shows us how little change is produced by mere cultivation, or even by variety of soil and climate, if there is no selection to preserve and accumulate the small variations that are con- tinually occurring. When, however, a great amount of modifi- cation has been eff'ected in one country, change to another country produces a decided effect. Thus it has been found that some of the numerous varieties of maize produced and cultivated in the United States change considerably, not only in their size and colour, but even in the shape of the seed when grown for a few successive years in Germany.^ In all our cultivated fruit trees the fruits vary immensely in shape, size, colour, flavour, time of ripening, and other qualities, while the leaves and flowers usually diff*er so little that they are hardly distinguishable except to a very close observer. ^ Darwin, Animals and Plants under Domestication, vol. i. p. 322. lATION UXDER DOMESTICATION 87 Farmtions of A2)ples and of Melons. The most remarkable varieties are afforded by the apple and the melon, and some account of these vnW. be given as illustrating the effects of slight variations accumulated by selection. All our apples are known to have descended from the common crab of our hedges (Pyrus malus), and from this at least a thousand distinct varieties have been produced. These differ greatly in the size and form of the fruit, in its colour, and in the texture of the skin. They further differ in the time of ripening, in their flavour, and in their keeping properties ; but apple trees also differ in many other ways. The foliage of the diflerent varieties can often be clistingurshed by peculiarities of form and colour, and it varies considerably in the time of its appearance ; in some hardly a leaf appears till the tree is in full bloom, while others produce their leaves so early as almost to hide the flowers. The flowers differ in size and colour, and in one case in structure also, that of the St. Yalery apple having a double calyx ^x\ih. ten divisions, and fourteen styles with oblique stigmas, but without stamens or corolla. The flowers, therefore, have to be fertilised Avith the pollen from other varieties in order to produce fruit. The pips or seeds differ also in shape, size, and colour ; some varieties are liable to canker more than others, while the AVinter Majetin and one or two others have the strange con- stitutional peculiarity of never being attacked by the mealy bug even when all the other trees in the same orchard are in- fested with it. All the cucumbers and gourds vary immensely, but the melon (Cucumis melo) exceeds them all. A French botanist, M. Xaudin, devoted six years to their study. He found that previous botanists had described thirty distinct species, as they thought, which were really only varieties of melons. They differ chiefly in their fruits, but also very much in foliage and mode of growth. Some melons are only as large as small plums, others weigh as much as sixty-six pounds. One variety has a scarlet fruit. Another is not more than an inch in diameter, but sometimes more than a yard in length, twisting about in all directions like a serpent. Some melons are exactly like cucumbers ; and an Algerian variety, when ripe, 88 DARWINISM chap. cracks and falls to pieces, just as occurs in a wild gourd (C. momordica).^ Variations of Flowers. Turning to flowers, we find that in the same genus as our currant and gooseberry, which we have cultivated for their fruits, there are some ornamental species, as the Ribes sanguinea, and in these the flowers have been selected so as to produce deep red, pink, or white varieties. When any particular flov^er be- comes fashionable and is grown in large quantities, variations are always met with sufficient to produce great varieties of tint or marking, as shown by our roses, auriculas, and geraniums. When varied leaves are required, it is found that a number of plants vary sufficiently in this direction also, and we have zonal geraniums, variegated ivies, gold and silver marked hollies, and many others. Variations of Domestic Animal-; Coming now to our domesticated animals, we find still more extraordinary cases ; and it appears as if any special quality or modification in an animal can be obtained if we only breed it in sufficient quantity, watch carefully for the required varia- tions, and carry on selection with patience and skill for a sufficiently long period. Thus, in sheep we have enormously increased the wool, and have obtained the power of rapidly forming flesh and fat ; in cows we have increased the produc- tion of milk ; in horses we have obtained strength, endurance, or speed, and have greatly modified size, form, and colour ; in poultry we have secured various colours of i)lumage, increase of size, and almost perpetual egg-laying. But it is in dogs and pigeons that the most marvellous changes have been eff'ected, and these require our special attention. Our various domestic dogs are believed to have originated from several distinct wild species, because in every part of the world the native dogs resemble some wild dogs or wolves of the same country. Thus perhaps several species of wolves and jackals were domesticated in Aery early times, and from breeds derived from these, crossed and improved by selection, ^ These facts are taken from Darwin's Domesticated A nimals avd Cultivated Plants, vol. i. pp. 359, 360, 392-401 ; vol. ii. pp. 231, 275, 330. IV VARIATION UNDER DOMESTICATION 89 our existing dogs have descended. But this intermixture of distinct species will go a very little way in accounting for the peculiarities of the different breeds of dogs, many of which are totally unlike any wild animal. Such is the case with grey- hounds, bloodhounds, bulldogs, Blenheim spaniels, terriers, pugs, turnspits, pointers, and many others ; and these differ so greatly in size, shape, colour, and habits, as well as in the form and proportions of all the different parts of the body, that it seems impossible that they could have descended from any of the known Avild dogs, wolves, or allied animals, none of which differ nearly so much in size, form, and proj^ortions. We have here a remarkable proof that variation is not con- fined to superficial characters — to the colour, hair, or external appendages, when we see how the entire skeletons of such forms as the greyhound and the bulldog have been gradually changed in opposite directions till they are both completely unlike that of any known wild animal, recent or extinct. These changes have been the result of some thousands of years of domestication and selection, different breeds being used and preserved for different purposes ; but some of the best breeds are kno^^^l to have been improved and perfected in modern times. About the middle of the last century a new and im- proved kind of foxhound was produced ; the gTeyhound was also greatly improved at the end of the last century, while the true bulldog was brought to perfection about the same period. The Newfoundland dog has been so much changed since it was first imported that it is now quite unlike any existing native dog in that island. ^ Domestic Pigeons. The most remarkable and instructive example of variation produced by human selection is afforded by the various races and breeds of domestic pigeons, not only because the varia- tions produced are often most extraordinary in amount and diverse in character, but because in this case there is no doubt whatever that all have been derived from one wild species, the common rock-pigeon (Columba livia). As this is a very important point it is well to state the evidence on which the belief is founded. The ^Wld rock-j^igeon is of a slaty-blue ^ See Darwin's Animals and Plants under Domestiadion, vol. i. pp. 40-42. 90 DARWINISM chap. colour, the tail has a dark band across the end, the wings have two black bands, and the outer tail-feathers are edged viith. white at the base. No other wild pigeon in the world has this combination of characters. Kow in every one of the domestic varieties, even the most extreme, all the above marks, even to the white edging of the outer tail-feathers, are sometimes found perfectly ~ developed. When birds belonoiniir to two distinct breeds are crossed one or more times, neither of the parents being blue, or having any of the above-named marks, the mongrel offspring are very apt to acquire some of these characters. Mr. Darwin gives instances which he observed himself. He crossed some white fantails with some black barbs, and the mongrels were black, brown, or mottled. He also crossed a barb with a spot, which is a white bird with a red tail and red spot on the forehead, and the mongrel offspring Avere dusky and mottled. On now crossing these two sets of mongrels with each other, he obtained a bird of a beautiful blue colour, with the barred and white edged tail, and double-banded wings, so as almost exactly to resemble a ^vild rock -pigeon. This bird was descended in the second generation from a pure white and pure black bird, both of which when unmixed breed their kind remarkably true. These facts, well known to ex- perienced pigeon -fanciers, together Avith the habits of the birds, which all like to nest in holes, or dovecots, not in trees like the great majority of wild pigeons, have led to the general belief in the single origin of all the different kinds. In order to afford some idea of the great differences which exist among domesticated pigeons, it will be well to give a brief abstract of Mr. Darwin's account of them. He divides them into eleven distinct races, most of which have several sub-races. Eace I. Pouters. — These are especially distinguished by the enormously enlarged crop, which can be so inflated in some birds as almost to conce*al the beak. They are very long in the body and legs and stand almost ui)right, so as to present a very distinct appearance. Their skeleton has become modified, the ribs being broader and the vertebrae more numerous than in other pigeons. IV VARIATION UXDER DOMESTICATION 91 Race II. Carriers. — These are large, long- necked birds, with a long pointed beak, and the eyes surrounded with a naked carunculated skin or wattle, which is also largely developed at the base of the beak. The opening of the mouth is unusually ^vide. There are several sub -races, one being called Dragons. Race III. Bunts. — These are very large-bodied, long-beaked pigeons, with naked skin round the eyes. The Avings are usually very long, the legs long, and the feet large, and the skin of the neck is often red. There are several sub-races, and these differ very much, forming a series of links between the wild rock-pigeon and the carrier. Race IV. Barbs. — These are remarkable for their very short and thick beak, so unlike that of most pigeons that fanciers compare it with that of a bullfinch. They have also a naked carunculated skin round the eyes, and the skin over the nostrils swollen. Race V. Fantails. — Short-bodied and rather small-beaked pigeons, \d\h an enormously developed tail, consisting usually of from fourteen to forty feathers instead of twelve, the regular number in all other pigeons, wild and tame. The tail spreads out like a fan and is usually carried erect, and the bird bends back its slender neck, so that in highly -bred varieties the head touches the tail. The feet are small, and they walk stiffly. Race Y1. Turbits and Owls. — These are characterised by the feathers of the middle of neck and breast in front spreading out irregularly so as to form a frill. The Turbits also have a crest on the head, and both have the beak exceedingly short. Race YII. TuiiMers. — These have a small body and short beak, but they are specially distinguished by the singular habit of tumbling over backwards during flight. One of the sub -races, the Indian Lotan or Ground tumbler, if slightly shaken and placed on the ground, ^Y\\\ immediately begin tumbling head over heels until taken up and soothed. If not taken up, some of them will go on tumbling till they die. 92 DARWINISM Sonic Englisli tumhlers arc almost equally persistent. A writer, (jiioted by ]\Ir. Darwin, says that these birds generally begin to tumble almost as soon as they can fly; "at three months old they tumble w^ell, but still fly strong; at five or six months they tumble excessively ; and in the second year they mostly give -up flying, on account of their tuml^ling so much and so close to the ground." Some fly round with the flock, throwing a clean summersault every few yards till they are obliged to settle from giddiness and exhaustion. These are called Air- tumblers, and they commonly throw from twenty to thirty summersaults in a minute, each clear and clean. I have one red cock that I have on two or three occasions timed by my watch, and counted forty summer- saults in the minute. At first they throw a single summer- sault, then it is double, till it becomes a continuous roll, which puts an end to flying, for if they fly a few yards over they go, and roll till they reach the ground. Thus I had one kill herself, and another broke his leg. ]\Iany of them turn over only a few inches from the ground, and will tumble tAvo or three times in flying across their loft. These are called House -tumblers from tumbling in the house. The act of tumbling seems to be one over w^hich they have no control, an involuntary movement which they seem to try to prevent. I have seen a bird sometimes in his struggles fly a yard or tAvo straight upwards, the impulse forcing him backwards while he struggles to go forwards." ^ The Short-faced tumblers are an improved sub-race which have almost lost the power of tumbling, but are valued for possessing some other characteristics in an extreme degree. They are very small, have almost globular heads, and a very minute beak, so that fanciers say the head of a perfect bird should resemble a cherry with a barleycorn stuck in it. Some of these weigh less than seven ounces, whereas the wild rock- pigeon weighs about fourteen ounces. The feet, too, are very short and small, and the middle toe has twelve or thirteen instead of fourteen or fifteen scutellse. They have often only nine primary wing-feathers instead of 'ten as in all other pigeons. ^ Mr. Brent in Journal of Horticulture, 1861, p. 76 ; quoted by Darwin, Animals and Plants under Domestication, vol. i. p. 151. IV VARIATION UNDER DOMESTICATION 93 Eace VIII. Indian Frill-had. — In these birds the beak is very short, and the feathers of the whole body are reversed or turn backwards. Race IX. Jacohin. — These curious birds have a hood of feathers almost enclosing the head and meeting in front of the neck. The ^Wngs and tail are unusually long. ' Race X. Trumpeter. — Distinguished by a tuft of feathers curling forwards over the beak, and the feet very much feathered. They obtain their name from the peculiar voice unlike that of any other pigeon. The coo is rapidly repeated, and is continued for several minutes. The feet are covered -with feathers so large as often to appear like little mngs. Race XL comprises Laughers, Frill-hades, Nuns, Spots, and SwaUmvs. — They are all very like the common rock -pigeon, but have each some slight peculiarity. The Laughers have a peculiar voice, supposed to resemble a laugh. The Xuns are white, with the head, tail, and primary "wing-feathers black or red. The Spots are white, with the tail and a spot on the forehead red. The Swallows are slender, white in colour, with the head and ^dncis of some darker colour. Besides these races and sub-races a number of other kinds have been described, and about one hundred and fifty varieties can be distinguished. It is interesting to note that almost every part of the bird, whose variations can be noted and selected, has led to variations of a considerable extent, and many of these have necessitated changes in the plumage and in the skeleton quite as gTeat as any that occur in the numerous distinct species of large genera. The form of the skull and beak varies enormously, so that the skulls of the Short -faced tumbler and some of the Carriers differ more than any wild pigeons, even those classed in distinct genera. The breadth and number of the ribs vary, as well as the processes on them ; the number of the vertebrae and the length of the sternum also vary ; and the perforations in the sternum vary in size and shape. The oil gland varies in development, and is sometimes absent. The number of the wing-feathers varies, and those of the tail to an enormous extent. The proportions of the leg and feet and the number 94 DARWINISM chap. of the scntellpe also vary. The eggs also varj' somcAvhat in size and shape ; and the amount of downy clothing on the young bird, "svhen first hatched, differs very considerably. Finally, the attitude of the body, the manner of walking, the mode of flight, and the voice, all exhibit modifications of the most remarkable kind.^ AccUmaiisation. A very important kind of variation is that constitutional change termed acclimatisation, which enables any organism to become gradually adapted to a different climate from the parent stock. As closely allied species often inhabit different countries possessing very different climates, we should expect to find cases illustrating this change among our domesticated animals and cultivated plants, A few examples will therefore be adduced showinf;- that such constitutional variation does occur. Among animals the cases are not numerous, because no systematic attempt has been made to select varieties for this special quality. It has, however, been observed that, though no European dogs thrive well in India, the Newfoundland dog, originating from a severe climate, can hardly be kept alive. A better case, perhaps, is fm^nished by merino sheep, w^hich, when imported directly from England, do not thrive, Avhile those Avhich have been bred in the intermediate climate of the Cape of Good Hope do much better. When geese w^ere first intro- duced into Bogota, they laid feAv eggs at long intervals, and few of the young sur\dved. By degrees, hoAvever, the fecundity improved, and in about twenty years became equal to what it is in Europe. According to Garcilaso, Avhen fowls AA^ere first introduced into Peru they Avere not fertile, AA^hereas now they are as much so as in Europe. Plants furnish much more important eAddence. Our nurserymen distinguish in their catalogues varieties of fruit- trees Avhich are more or less hardy, and this is especially the case in America, Avhere certain varieties only will stand the severe climate of Canada. There is one variety of pear, the Forelle, Avhich both in England and France Avithstood frosts ^ This account of domestic pigeons is greatly condensed from Mr. Darwin's work already refen-ed to. IV VARIATION UXDER DOMESTICATION 95 that killed the flowers and buds of all other kinds of pears. Wheat, which is grown over so large a portion of the world, has become adapted to special climates. Wheat imported from India and sown in good wheat soil in England produced the most meagre ears; while wheat taken from France to the West Indian Islands produced either wholly barren spikes or spikes furnished with two or three miserable seeds, while AVest Indian seed by its side yielded an enormous harvest. The orange was very tender when first introduced into Italy, and continued so as long as it was i)ropagated by grafts, but when trees were raised from seed many of these were found to be hardier, and the orange is now perfectly acclimatised in Italy. Sweet-j^eas (Lathyrus odoratus) imported from England to the Calcutta Botanic Gardens produced few blossoms and no seed ; those from France flowered a little better, but still produced no seed, but plants raised from seed brought from Darjeeling in the Himalayas, but originally derived from England, flower and seed profusely in Calcutta. ^ An observation by Mr. Darwin himself is perhaps even more instructive. He says: "On 24th May 1864 there was a severe frost in Kent, and two rows of scarlet runners (Phaseolus multiflorus) in my garden, containing 390 plants of the same age and equally exposed, were all blackened and killed except about a dozen plants. In an adjoining row of Fulmer's dwarf bean (Phaseolus vulgaris) one single plant escaped. A still more severe frost occurred four days after- wards, and of the dozen plants which had previously escaped only three survived ; these were not taller or more vigorous than the other young plants, but they escaped completely, with not even the tips of their leaves browned. It was im- possible to behold these three plants, with their blackened, withered, and dead brethren all around them, and not see at a glance that they differed widely in their constitutional power of resisting frost.' " The preceding sketch of the variation that occurs among domestic animals and cultivated plants shows how wide it is in range and how great in amount ; and we have good reason to believe that similar variation extends to all organised beings. In the class of fishes, for examj^le, we have one kind which has ^ Animals and Plants under Domestication, vol. ii. pp. 307-311, DARWINISM been long domesticated in the East, the gold and silver carps ; and these present great variation, not only of colour but in the form and structure of the fins and other external organs. In like manner, the only domesticated insects, hive bees and silk- worm moths, present numbers of remarkable varieties which have been produced by the selection of chance variations just as in the case of plants and the higher animals. Circumstances favourable to Selection by Man. It may be supposed, that the systematic selection which has been employed for the purpose of improving the races of animals or plants useful to man is of comparatively recent origin, though some of the different races are known to have been in existence in very early times. But Mr. Darwin has l^ointed out, that unconscious selection must have begun to produce an eTect as soon as plants were cultivated or animals domesticated by man. It would have been very soon observed that animals and plants produced their like, that seed of early wheat produced early wheat, that the offspring of very swift dogs were also swift, and as every one would try to have a good rather than a bad sort this would necessarily lead to the slow but steady improvement of all useful plants and animals subject to man's care. Soon there would arise distinct breeds, owing to the varying uses to Avhich the animals and plants were put. Dogs would be wanted chiefly to hunt one kind of game in one part of the country and another kind else- where ; for one purpose scent would be more important, for another swiftness, for another strength and courage, for yet another watchfulness and intelligence, and this would soon lead to the formation of very distinct races. In the case of vegetables and fruits, different varieties would be found to succeed best in certain soils and climates ; some might be preferred on account of the cpiantity of food they produced, others for their sweetness and tenderness, while others might be more useful on account of their ripening at a particular season, and thus again distinct varieties would be established. An instance of unconscious selection leading to distinct results in modern times is afforded by two flocks of Leicester sheep which both originated from the same stock, and were then bred pure for upwards of fifty years by two gentlemen, Mr. Buckley IV VARIATION UNDER DOMESTICATION 97 and Mr. Bm^gess. Mr. Youatt, one of the greatest authorities on breeding domestic animals, says: " There is not a suspicion existing in the mind of any one at all acquainted with the subject that the owner of either of them has deviated in any one instance from the pure blood of Mr. Bakewell's original flock, and yet the difference between the sheep possessed by these two gentlemen is so great that they have the appearance of being quite ditferent varieties." In this case there was no desire to deviate from the original breed, and the diiference must have arisen from some slight difference of taste or judg- ment in selecting, each year, the parents for the next year's stock, combined perhaps ^vith some direct effect of the slight differences of climate and soil on the two farms. Most of our domesticated animals and cultivated plants have come to us from the earliest seats of civilisation in Western Asia or Egypt, and have therefore been the subjects of human care and selection for some thousands of years, the result being that, in many cases, we do not know the Avild stock from which they originally sprang. The horse, the camel, and the common bull and cow are nowhere found in a ^vi\d state, and they have all been domesticated from remote antiquity. The original of the domestic fowl is still wild in India and the Malay Islands, and it was domesticated in India and China before 1400 B.C. It was introduced into Europe about 600 B.C. Several distinct breeds were known to the Romans about the commencement of the Christian era, and they have since spread all over the civilised world and been subjected to a vast amount of conscious and unconscious selection, to many varieties of climate and to differences of food ; the result being seen in the wonderful diversity of breeds which differ quite as remarkably as do the different races of pigeons already described. In the vegetable kingdom, most of the cereals — wheat, barley, etc. — are unknown as truly wild plants ; and the same is the case ^Yith. many vegetables, for De Candolle states that out of 157 useful cultivated plants thirty-two are quite un- known in a wild state, and that forty more are of doubtful origin. It is not improbable that most of these do exist wild, but they have been so profoundly changed by thousands of years of cultivation as to be quite unrecognisable. The H 98 DARWINISM peach is unknown in a wild state, unless it is derived from the common almond, on which i)oint there is much difference of oi)inion among Itotanists and horticulturists. The immense antiquity of most of our cultivated plants sufficiently exi)lains the apparent absence of such useful productions in Australia and the Cape of Good Hope, not- withstanding that they both possess an exceedingly rich and varied flora. These countries having been, until a com- paratively recent period, inhabited only by uncivilised men, neither cultivation nor selection has been carried on for a sufficiently long time. In North America, however, where there was evidently a very ancient if low form of civilisation, as indicated by the remarkable mounds, earthworks, and other prehistoric remains, maize was cultivated, though it was probably derived from Peru ; and the ancient civilisation of that country and of Mexico has given rise to no fewer than thirty-three useful cultivated plants. Conditions favourable to the production of Variaiians. In order that plants and animals may be improved and modified to any considerable extent, it is of course essential that suitable variations should occur with tolerable frequency. There seem to be three conditions which are especially favour- able to the production of variations: (1) That the particular species or variety should be kept in very large numbers; (2) that it should be spread over a Avide area and thus subjected to a considerable diversity of physical conditions ; and (3) that it should be occasionally crossed with some distinct but closely allied race. The first of these conditions is perhaps the most important, the chance of variations of any partic- ular kind being increased in proportion to the quantity of the original stock and of its annual offspring. It has been re- marked that only those breeders who keep large flocks can effect much improvement ; and it is for the same reason that pigeons and fowls, w^hich can be so easily and rapidly increased, and which have been kept in such large numbers by so great a number of persons, have produced such strange and numer- ous varieties. In like manner, nurserymen who grow fruit and flowers in large quantities have a great advantage over private amateurs in the production of new varieties. IV VARIATION UNDER DOMESTICATION 99 Although I believe, for reasons which will be given further on, that some amount of variability is a constant and necessary property of all organisms, yet there appears to be good evidence to show that changed conditions of life tend to increase it, both by a direct action on the organisation and by indirectly affecting the reproductive system. Hence the extension of civilisation, by favouring domestication under altered conditions, facilitates the process of modification. Yet this change does not seem to be an essential condition, for nowhere has the production of extreme varieties of plants and flowers been carried farther than in Japan, where careful selection continued for many generations must have been the chief factor. The effect of occasional crosses often results in a great amount of variation, but it also leads to instability of character, and is therefore very little employed in the pro- duction of fixed and well-marked races. For this purpose, in fact, it has to be carefully avoided, as it is only by isolation and pure breeding that any specially desired qualities can be in- creased by selection. It is for this reason that among savage peoples, whose animals run half w\\d, little improvement takes place ; and the difficulty of isolation also ex|3lains why distinct and pure breeds of cats are so rarely met Avith. The wide dis- tribution of useful animals and plants from a very remote epoch has, no doubt, been a powerful cause of modification, because the particular breed first introduced into each country has often been kept pure for many years, and has also been subjected to slight differences of conditions. It ^Yil\ also usually have been selected for a somewhat different purpose in each locality, and thus very distinct races would soon originate. The important physiological effects of crossing breeds or strains, and the part this plays in the economy of nature, will be explained in a future chapter. Concluding Remarks. The examples of variation now adduced — and these might have been almost indefinitely increased — ^y\\\ suffice to show that there is hardly an organ or a quality in plants or animals which has not been observed to vary ; and further, that when- ever any of these variations have been useful to man he has 100 DARWINISM chap. been able to increase them to a marvellous extent by the sinn)lc j)rocess of always preserving the best varieties to breed from. Along with these larger variations others of smaller amount occasionally appear, sometimes in external, sometimes in internal characters, the very bones of the skeleton often changing slightly in form, size, or number ; but as these secondary characters have been of no use to man, and have not been specially selected by him, they have, usually, not been developed to any great amount except Avhen they have been closely dependent on those external characters which he has largely modified. As man has considered only utility to himself, or the satisfaction of his love of beauty, of novelty, or merely of something strange or amusing, the variations he has thus pro- duced have something of the character of monstrosities. Not only are they often of no use to the animals or plants them- selves, but they are not unfrequently injurious to them. In the Tumbler pigeons, for instance, the habit of tumbling is sometimes so excessive as to injure or kill the bird ; and many of our highly-bred animals have such delicate constitutions that they are very liable to disease, while their extreme peculiarities of form or structure would often render them quite unfit to live in a \vild state. In plants, many of our double flowers, and some fruits, have lost the power of pro- ducing seed, and the race can thus be continued only by means of cuttings or gTafts. This peculiar character of domestic productions distinguishes them broadly from wild species and varieties, which, as will be seen by and by, are necessarily adapted in every part of their organisation to the conditions under which they have to live. Their importance for our present inquiry depends on their demonstrating the occurrence of incessant slight variations in all parts of an organism, with the transmission to the ofi'spring of the special characteristics of the parents ; and also, that all such slight variations are capable of being accumulated by selection till they present very large and important divergencies from the ancestral stock. We thus see, that the evidence as to variation afforded by animals and plants under domestication strikingly accords with that which we have proved to exist in a state of nature. IV VARIATION UNDER DOMESTICATION 101 And it is not at all surprising that it should be so, since all the species were in a state of nature when first domesticated or cultivated by man, and whatever variations occur must be due to purely natural causes. Moreover, on comparing the variations which occur in any one generation of domesticated animals with those which we know to occiu* in wild animals, Ave find no evidence of greater individual variation in the former than in the latter. The results of man's selection are more striking to us because we have always considered the varieties of each domestic animal to be essentially identical, Avhile those Avhich we observe in a wild state are held to be essentially diverse. The greyhound and the spaniel seem wonderful, as varieties of one animal produced by man's selection ; while we think little of the diversities of the fox and the wolf, or the horse and the zebra, because we have been accustomed to look upon them as radically distinct animals, not as the results of nature's selection of the varieties of a common ancestor. CHAPTER V NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST Effect of struggle for existence under unchanged conditions — The effect under change of conditions — Divergence of character — In insects — In birds — In mammalia — Divergence leads to a maximum of life in each area — Closely allied species inhabit distinct areas — Adaptation to conditions at various periods of life — The continued existence of low forms of life — Extinction of low types among the higher animals — Circumstances favourable to the origin of new species— Probable origin of the dippers — The importance of isolation — On the advance of organi- sation by natural selection — Summary of the first five chapters. In the preceding chapters we have accumulated a body of facts and arguments which will enable us now to deal with the very core of our subject — the formation of species by means of natural selection. Vie have seen how tremendous is the struggle for existence always going on in nature owing to the great powers of increase of all organisms ; we have ascertained the fact of variability extending to every part and organ, each of which varies simultaneously and for the most part independ- ently ; and we have seen that this variability is both large in its amount in proportion to the size of each part, and usually affects a considerable proportion of the individuals in the large and dominant species. And, lastly, we have seen how similar variations, occurring in cultivated plants and domestic animals, are capable of being perpetuated and accumulated by artificial selection, till they have resulted in all the wonderful varieties of our fruits, flowers, and vegetables, our domestic animals and household pets, many of which differ from each other far more in external characters, habits, and instincts than do species in CHAP. V NATURAL SELECTION 103 a state of nature. We have now to inquire whether there is any analogous process in nature, by which wild animals and plants can be permanently modified and new races or new species produced. Effect of Struggle for Existence under Unchanged Conditions. Let us first consider what will be the eflfect of the struggle for existence upon the animals and plants which we see around us, under conditions which do not perceptibly vary from year to year or from century to century. We have seen that every species is exposed to numerous and varied dangers throughout its entire existence, and that it is only by means of the exact adaptation of its organisation — including its instincts and habits — to its surroundings that it is enabled to live till it produces off"spring w^hich may take its place when it ceases to exist. We have seen also that, of the whole annual increase only a very small fraction survives ; and though the survival in indi- vidual cases may sometimes be due rather to accident than to any real superiority, yet we cannot doubt that, in the long run, those survive which are best fitted by their perfect organisa- tion to escape the dangers that surround them. This " survival of the fittest" is what Darwin termed "natural selection," l)ecause it leads to the same results in nature as are produced by man's selection among domestic animals and cultivated plants. Its primary effect Avill, clearly, be to keep each species in the most perfect health and vigour, with every part of its organisation in full harmony AWth the conditions of its existence. It prevents any possible deterioration in the organic world, and produces that appearance of exuberant life and enjoyment, of health and beauty, that aff'ords us so much })leasure, and which might lead a superficial observer to suppose that peace and quietude reigned throughout nature. The Effect under cluuiged Conditions. But the very same process which, so long as conditions re- main substantially the same, secures the continuance of each species of animal or plant in its full perfection, will usually, under changed conditions, bring about whatever change of structure or habits may be necessitated by them. The changed conditions to which we refer are such as we know have occurred 104 DARWINISM throughout all geological time and in every part of the world. Land and water have l^cen continually shifting their positions ; some regions are undergoing subsidence with diminution of area, others elevation Avith extension of area ; dry land has been converted into marshes, while marshes have been drained or have even been elevated into i)lateaux. Climate too has changed again and again, either through the elevation of mountains in high latitudes leading to the accumulation of snow and ice, or by a change in the direction of winds and ocean currents produced by the subsidence or elevation of lands which connected continents and divided oceans. Again, along with all these changes have come not less important changes in the distribution of species. Vegetation has been greatly modified by changes of climate and of altitude ; while every union of lands before separated has led to extensive migrations of animals into new countries, disturbing the balance that before existed among its forms of life, leading to the extermina- tion of some species and the increase of others. When such physical changes as these have taken place, it is evident that many species must either become modified or cease to exist. When the vegetation has changed in character the herbivorous animals must become able to live on new and perhaps less nutritious food ; while the change from a damp to a dry climate may necessitate migration at certain periods to escape destruction by drought. This will expose the species to new dangers, and require special modifications of structure to meet them. Greater swiftness, increased cunning, nocturnal habits, change of colour, or the power of climbing trees and living for a time on their foliage or fruit, may be the means adopted by different species to bring themselves into harmony with the new conditions ; and by the continued survival of those individuals, only, which varied sufficiently in the right direction, the necessary modifications of structure or of func- tion would be brought about, just as surely as man has been able to breed the greyhound to hunt by sight and the fox- hound by scent, or has produced from the same wild plant such distinct forms as the cauliflower and the brussels sprouts. We will now consider the special characteristics of the changes in species that are likely to be effected, and how far they agree with what we observe in nature. V NATURAL SELECTION 105 Divergence of Character. In specie.s Avhicli have a wide range the struggle for exist- ence will often cause some individuals or groups of individuals to adopt new habits in order to seize upon vacant places in nature where the struggle is less severe. Some, living among extensive marshes, may adopt a more aquatic mode of life ; others, living where forests abound, may become more arboreal. In either case we cannot doubt that the chano-es of structure needed to adaj^t them to their new habits would soon be brought about, because we know that variations in all the external organs and all their separate parts are very abundant Lind are also considerable in amount. That such divergence of character has actually occurred we have some direct evidence. Mr. Dar^^^n informs us that in the Catskill IMountains in the United States there are two varieties of wolves, one with a light greyhound-like form which pursues deer, the other more bulky ^vith shorter legs, which more frecpiently attacks sheep. ^ Another good example is that of the insects in the island of Madeira, many of which have either lost their wings or have had them so much reduced as to be useless for flight, while the very same species on the continent of Europe possess fully develoj^ed wings. In other cases the wingless Madeira species are distinct from, but closely allied to, ^nnged species of Europe. The explanation of this change is, that Madeira, like many oceanic islands in the temperate zone, is much exposed to sudden gales of wind, and as most of the fertile land is on the coast, insects which flew much would be very liable to be blown out to sea and lost. Year after year, therefore, those individuals which had shorter mngs, or which used them least, were preserved ; and thus, in time, terrestrial, wingless, or im- perfectly winged races or species have been produced. That this is the true explanation of this singular fact is proved by much corroborative evidence. There are some few flower- frequenting insects in Madeira to Avhom wings are essential, and in these the Anngs are somewhat larger than in the same species on the mainland. AVe thus see that there is no general tendency to the abortion of -vWngs in JVIadeira, but that it is simply a case of adaptation to new conditions. Those insects ^ Oriyin of Species, p. 71. 106 DARWINISM to whom wings were not absolutely essential escaped a serious danger by not using them, and the wings therefore became reduced or were completely lost. But when they were essential they were enlarged and strengthened, so that the insect could battle against the winds and save itself from destruction at sea. Many flying insects, not varying fast enough, would be destroyed before they could establish themselves, and thus we may exjilain the total absence from Madeira of several Avhole families of winged insects which must have had many oppor- tunities of reaching the islands. Such are the large groups of the tiger-beetles (Cicindelidte), the chafers (Melolonthidse), the click-beetles (Elateridse), and many others. But the most curious and striking confirmation of this portion of Mr. Darwin's theory is afforded by the case of Kerguelen Island. This island was visited by the Transit of Venus expedition. It is one of the stormiest places on the globe, being subject to almost perpetual gales, while, there being no wood, it is almost entirely without shelter. The Eev. A. E. Eaton, an experienced entomologist, was naturalist to the expedition, and he assiduously collected the few insects that were to be found. All were incapable of flight, and most of them entirely without wings. They included a moth, several flies, and numerous beetles. As these insects could hardly have reached the islands in a wingless state, even if there were any other known land inhabited by them — which there is not — we must assume that, like the Madeiran insects, they were originally winged, and lost their power of flight because its j^ossession was injurious to them. It is no doubt due to the same cause that some butterflies on small and exposed islands have their wings reduced in size, as is strikingly the case with the small tortoise-shell butterfly (Vanessa urticiv) inhabiting the Isle of Man, which is only about half the size of the same species in England or Ireland ; and Mr. Wollaston notes that Vanessa callirhoe — a closely allied ►South European form of our red-admiral butterfly — is perma- nently smaller in the small and bare island of Porto Santo than in the larger and more wooded adjacent island of Madeira. A A'cry good example of comparatively recent divergence of character, in accordance Avith new conditions of life, is afforded by our red grouse. This bird, the Lagopus scoticus of V NATURAL SELECTION 107 naturalists, is entirely confined to the British Isles. It is, however, very closely allied to the willow grouse (Lagopus albus), a bird which ranges all over Europe, Northern Asia, and North America, but which, unlike our species, changes to white in winter. No difference in form or structure can be detected between the two birds, but as they differ so decidedly in colour — our species being usually rather darker in ^Wnter than in summer, while there are also slight differences in the call-note and in habits, — the two species are generally con- sidered to be distinct. The differences, however, are so clearly adaptations to changed conditions that we can hardly doubt that, during the early part of the glacial period, when our islands were united to the continent, our grouse was identical with that of the rest of Europe. But when the cold passed away and our islands became permanently separated from the mainland, with a mild and equable climate and very little snoAV in winter, the change to white at that season became hurtful, rendering the birds more conspicuous instead of serving as a means of concealment. The colour was, there- fore, gradually changed by the process of variation and natural selection ; and as the birds obtained ample shelter among the heather which clothes so many of our moorlands, it became useful for them to assimilate with its brown and dusky stems and withered flowers rather than with the snow of the higher mountains. An interesting confirmation of this change having really occurred is afforded by the occasional occurrence in Scotland of birds with a considerable amount of white in the "svinter plumage. This is considered to be a case of reversion to the ancestral type, just as the slaty colours and banded wings of the wild rock-pigeon sometimes reappear in oiu* fancy breeds of domestic pigeons.^ The principle of " divergence of character " pervades all nature from the lowest groups to the highest, as may be well seen in the class of birds. Among our native species Ave see it well marked in the different species of titmice, pijjits, and chats. The great titmouse (Parus major) by its larger size and stronger bill is adapted to feed on larger insects, and is even said sometimes to kill small and weak birds. The smaller and weaker coal titmouse (Parus ater) has adopted a ^ Yarrell's British Birds, fourtli edition, vol. iii. p. 77. 108 DARWINISM more vegetarian diet, eating seeds as well as insects, and feeding on the gronnd as \yell as among trees. The delicate little blue titmouse (Parus coeruleus), with its very small bill, feeds on the minutest insects and grubs which it extracts from cre^^ces of bark and from the buds of fruit-trees. The marsh titmouse, again (Parus palustris), has received its name from the low and marshy localities it frequents ; while the crested titmouse (Parus cristatus) is a northern bird frequenting especially pine forests, on the seeds of which trees it partially feeds. Then, again, our three common pipits — the tree-pipit (Anthus arboreus), the meadow-pipit (Anthus pratensis), and the rock-pipit or sea-lark (Anthus obscurus) have each occupied a distinct place in nature to which they have become sj^ecially adapted, as indicated by the different form and size of the hind toe and claw in each species. So, the stone-chat (Saxicola rubicola), the whin-chat (S. rubetra), and the wheat-ear (S. oenanthe) are all slightly divergent forms of one type, with modifications in tlie shaj^e of the wing, feet, and bill adapting them to slightly different modes of life. The whin-chat is the smallest, and frequents furzy commons, fields, and lowlands, feeding on worms, insects, small molluscs, and berries ; the stone-chat is next in size, and is especially active and lively, frequenting heaths and uplands, and is a permanent resident with us, the two other sj^ecies being migrants ; while the larger and more conspicuous wheat-ear, besides feeding on grubs, beetles, etc., is able to capture flying insects on the wing, something after the manner of true flycatchers. These examples sufficiently indicate how divergence of character has acted, and has led to the adaptation of numerous allied species, each to a more or less special mode of life, with the variety of food, of habits, and of enemies which must necessarily accompany such diversity. And Avhen we extend our inquiries to higher groups we find the same indications of divergence and special adaptation, often to a still more marked extent. Thus we have the larger falcons, which prey upon birds, while some of the smaller species, like the hobby (Falco subbuteo), live largely on insects. The true falcons capture their prey in the air, while the hawks usually seize it on or near the ground, feeding on hares, rabbits, squirrels, grouse, pigeons, and poultry. Kites and buzzards, on the V NATURAL SELECTION 109 other hand, seize their prey upon the ground, and the former feed largely on reptiles and offal as well as on birds and quadrupeds. Others have adopted fish as their chief food, and the osprey snatches its prey from the water with as much facility as a gull or a petrel ; while the South American caracaras (Polyborus) have adopted the habits of vultures and live altogether on carrion. In every great group there is the same divergence of habits. There are ground-pigeons, rock- pigeons, and wood-pigeons, — seed-eating pigeons and fruit- eating pigeons ; there are carrion-eating, insect-eating, and fruit-eating crows. Even kingfishers are, some aquatic, some terrestrial in their habits ; some live on fish, some on insects, some on reptiles. Lastly, among the primary divisions of birds we find a purely terrestrial group — the Ratita?, including the ostriches, cassowaries, etc. ; other great groui^s, including the ducks, cormorants, gulls, penguins, etc., are aquatic ; while the bulk of the Passerine birds are aerial and arboreal. The same general facts can be detected in all other classes of animals. In the mammalia, for example, we have in the common rat a fish-eater and flesh-eater as well as a grain-eater, Avhich has no doubt helped to give it the power of spreading over the world and driving away the native rats of other countries. Throughout the Kodent tribe we find everywhere aquatic, terrestrial, and arboreal forms. In the weasel and cat tribes some live more in trees, others on the ground ; squirrels have diverged into terrestrial, arboreal, and flying species ; and finally, in the bats we have a truly aerial, and in the whales a truly ac{uatic order of mammals. We thus see that, beginning with diff'erent varieties of the same species, we have allied species, genera, families, and orders, with similarly divergent habits, and adaptations to different modes of life, indicating some general princi])le in nature which has been operative in the development of the organic world. But in order to be thus operative it must be a generally useful principle, and Mr. Darmn has very clearly shown us in what this utility consists. Divergence leads to a Maximum of Organic Forms in each Area. Divergence of character has a double purpose and use. In the first place it enables a species which is being overcome 1/ 110 DARWINISM by rivals, or is in process of extinction by enemies, to save itself 1)y adopting new habits or by occu})ying vacant j^l^ices in nature. This is the immediate and obvious efl'ect of all the numerous examples of divergence of character which we have pointed out. But there is another and less obvious result, which is, that the greater the diversity in the organisms inhabiting a country or district the greater Avill be the total amount of life that can be supported there. Hence the continued action of the struggle for existence will tend to bring about more and more diversity in each area, which may be shown to be the case by several kinds of evidence. As an example, a piece of turf, three feet by four in size, was found by Mr. Darwin to contain twenty species of plants, and the.-e twenty species belonged to eighteen genera and to eight orders, showing how greatly they differed from each other. Farmers find that a greater quantity of hay is obtained from ground sown \Wth a variety of genera of grasses, clover, etc., than from similar land sown with one or two species only ; and the same principle applies to rotation of crops, plants differing very mdely from each other giving the best results. So, in small and uniform islands, and in small ponds of fresh water, the plants and insects, though few in number, are found to be wonderfully varied in character. The same principle is seen in the naturalisation of plants and animals by man's agency in distant lands, for the species that thrive best and establish themselves permanently are not only very varied among themselves but differ greatly from the native inhabitants. Thus, in the Northern United States there are, according to Dr. Asa Gray, 260 naturalised flower- ing plants which belong to no less than 162 genera ; and of these, 100 genera are not natives of the United States. So, in Australia, the rabbit, though totally unlike any native animal, has increased so much that it probably outnumbers in in- dividuals all the native mammals of the country ; and in New Zealand the rabbit and the pig have ecjually multiplied. Darwin remarks that this "advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as that of the i)hysiological division of labour in the organs of the same body. No physiologist doubts that a V NATURAL SELECTION 111 stomach adupted to digest vegetable matter alone, or flesh alone, draws more nutriment from these substances. So, in the general economy of any land, the more Avidely and perfectly the animals and plants are diversified for different habits of life, so Avill a greater number of individuals be capable of there supporting themselves." ^ The most closely allied Species inhabit distinct Areas. One of the curious results of the general action of this principle in nature is, that the most closely allied species — those whose differences though often real and important are hardly j^erceptible to any one but a naturalist — are usually not found in the same but in Avidely separated countries. Thus, the nearest allies to our European golden plover are found in North America and East Asia ; the nearest ally of our European jay is found in Japan, although there are several other species of jays in Western Asia and North Africa; and though we have several species of titmice in England they are not very closely allied to each other. The form most akin to our blue tit is the azure tit of Central Asia (Parus azureus) ; the Parus ledouci of Algeria is very near our coal tit, and the Parus lugubris of South- Eastern Europe and Asia Minor is nearest to our marsh tit. So, our four species of wild pigeons — the ring-dove, stock- dove, rock-pigeon, and turtle-dove — are not closely allied to each other, but each of them belongs, according to some ornithologists, to a separate genus or subgenus, and has its nearest relatives in distant parts of Asia and Africa. In mammalia the same thing occurs. Each mountain region of Europe and Asia has usually its own species of A\ald sheep and goatj and sometimes of antelope and deer ; so that in each region there is found the greatest diversity in this class of animals, while the closest allies inhabit cjuite distinct and often distant areas. In plants we find the same phenomenon prevalent. Distinct species of columbine are found in Central Europe (Aguilegia vulgaris), in Eastern Europe, and Siberia (A glandulosa), in the Alps (A. Alpina), in the Pyrenees (A. pyrenaiea), in the Greek mountains (A. ottonis), and in Corsica (A. Bernardi), but rarely are two ^ Origin of Species, p. 89. 112 DARWINISM chap. species found in the same area. 80, each p.art of the world has its own peculiar forms of pines, firs, and cedars, but the closely allied species or varieties are in almost every case inlribitants of distinct areas. Exami)les are the deodar of the Himalayas, the cedar of Lebanon, and that of North Africa, all very closely allied but confined to distinct areas; and the numerous closely - allied species of true pine (genus Pinus), which almost always inhabit different countries or occupy different stations. We will now consider some other modes in which natural selection will act, to adapt organisms to changed conditions. Adaptation to Conditions at Various Periods of Life. It is found, that, in domestic animals and cultivated plants, variations occurring at any one period of life reappear in the offspring at the same period, and can be perpetuated and increased by selection ^vithout modifying other parts of the organisation. Thus, variations in the caterpillar or the cocoon of the silkworm, in the eggs of poultry, and in the seeds or young shoots of many culinary vegetables, have been accumulated till those parts have become greatly modified and, for man's purposes, improved. Owing to this fact it is easy for organisms to become so modified as to avoid dangers that occur at any one period of life. Thus it is that so many seeds have Ijecome adapted to various modes of dissemination or protection. Some are winged, or have down or hairs attached to them, so as to enable them to be carried long distances in the air ; others have curious hooks and prickles, which cause them to be attached firmly to the fur of mammals or the feathers of birds ; while others are buried within sweet or juicy and brightly coloured fruits, which are seen and devoured by bii'ds, the hard smooth seeds passing through their bodies in a fit state for germination. In the struggle for existence it must benefit a plant to have increased means of dispersing its seeds, and of thus having young plants pro- duced in a greater variety of soils, aspects, and surroundings, with a greater chance of some of them escaping their numerous enemies and arriving at maturity. The various differences referred to would, therefore, be brought about by variation and survival of the fittest, just as surely as the length and quality V NATURAL SELECTION 113 of cotton on the seed of the cotton-plant have been increased by man's selection. The larvse of insects have thus been wonderfully modified in order to escape the numerous enemies to whose attacks they are exposed at this period of their existence. Their colours and markings have become marvellously adapted to conceal them among the foliage of the j^l^nt they live upon, and this colour often changes completely after the last moult, when the creature has to descend to the ground for its change to the pupa state, during which period a bro^ATi instead of a green coloiu* is protective. Others have acquired curious attitudes and large ocelli, which cause them to resemble the head of some reptile, or they have curious horns or coloured ejectile processes which frighten away enemies ; while a great number have acquired secretions which render them offensive to the taste of their enemies, and these are always adorned •\vdth very conspicuous markings or brilliant colours, which serve as a sign of inedibility and prevent their being needlessly attacked. This, however, is a portion of the very large sub- ject of organic colour and marking, which will be fully dis- cussed and illustrated in a separate chapter. In this way every possible modification of an animal or plant, whether in colour, form, structure, or habits, which would be serviceable to it or to its progeny at any period of its existence, may be readily brought about. There are some curious organs which are used only once in a creature's life, but which are yet essential to its existence, and thus have very much the appearance of design by an intelligent designer. Such are, the gTeat jaws possessed by some insects, used ex- clusively for opening the cocoon, and the hard tip to the beak of unhatched birds used for breaking the eggshell. The increase in thickness or hardness of the cocoons or the eggs being useful for protection against enemies or to avoid accidents, it is probable that the change has been very gradual, because it would be constantly checked by the necessity for a corresponding change in the young insects or birds enabling them to overcome the additional obstacle of a tougher cocoon or a harder eggshell. As we have seen, however, that every part of the organism appears to be varying independently, at the same time, though to different I 114 DARWINISM chap. amounts, there seems no reason to believe that the necessity for two or more coincident variations would prevent the required change from taking place. The Continued Existence of Low Forms of Life. Since species are continually undergoing modifications giving them some superiority over other species or enabling them to occupy fresh places in nature, it may be asked — Why do any low forms continue to exist ? Why have they not long since been imj^roved and developed into higher forms ? The answer, probably, is, that these low forms occupy places in nature which cannot be filled by higher forms, and that they have few or no competitors ; they therefore continue to exist. Thus, earthworms are adapted to their mode of life better than they would be if more highly organised. So, in the ocean, the minute foraminifera and infusoria, and the larger sponges and corals, occupy places which more highly developed creatures could not fill. They form, as it were, the base of the great structure of animal life, on which the next higher forms rest; and though in the coiu-se of ages they may undergo some changes, and diversification of form and structure, in accordance with changed conditions, their essential nature has probably remained the same from the very dawn of life on the earth. The low aquatic diatomaceae and confervas, together with the lowest fungi and lichens, occupy a similar position in the vegetable kingdom, filling places in nature which would be left vacant if only highly organised plants existed. There is, therefore, no motive power to destroy or seriously to modify them ; and they have thus probably persisted, under slightly varying forms, through all geological time. Extinction of Lower Types among the Higher Animals. So soon, however, as we approach the higher and more fully developed groups, we see indications of the often re- peated extinction of lower by higher forms. This is shown by the great gaps that separate the mammalia, birds, reptiles, and fishes from each other ; while the lowest forms of each are always few in number and confined to limited areas. Such NATUKAL SELECTION 115 are the lowest mammals — the echidna and ornithorhynchus of Australia; the lowest birds — the apteryx of New Zealand and the cassowaries of the New Guinea region ; while the lowest fish — the amphioxus or lancelet, is completely isolated, and has apparently survived only by its habit of burrow- ing in the sand. The great distinctness of the carnivora, ruminants, rodents, whales, bats, and other orders ot mammalia ; of the accipitres, pigeons, and parrots, among birds ; and of the beetles, bees, flies, and moths, among insects, all indicate an enormous amount of extinction among the comparatively low forms by which, on any theory of evolution, these higher and more specialised groups must have been preceded. Circumstances favourable to the Origin of New Species hy Natural Selection. We have already seen that, when there is no change in the physical or organic conditions of a country, the effect of natural selection is to keep all the species inhabiting it in a state of perfect health and full development, and to preserve the balance that already exists between the different groups of organisms. But, whenever the physical or organic condi- tions change, to however small an extent, some correspond- ing change will be produced in the flora and fauna, since, considering the severe struggle for existence and the complex relations of the various organisms, it is hardly possible that the change should not be beneficial to some species and hurtful to others. The most common effect, therefore, '^^il] be that some species will increase and others mil diminish : and in cases where a species was already small in numbers a further diminution might lead to extinction. This would afford room for the increase of other species, and thus a considerable readjustment of the proportions of the several species might take place. When, however, the change was of a more important character, directly affecting the existence of many species so as to render it difficult for them to maintain themselves mthout some considerable change in structure or habits, that change would, in some cases, be brought about by variation and natural selection, and thus new varieties or new species might be formed. We have to consider, then, which 116 DARWINISM are the species thnt would be most likely to be so modified, while others, not becoming modified, would succumb to the changed conditions and become extinct. The most important condition of all is, undoubtedly, that variations should occur of sufficient amount, of a sufficiently diverse character, and in a large number of individuals, so as to afford ample materials for natural selection to act upon ; and this, we have seen, does occur in most, if not in all, large, wide-ranging, and dominant species. From some of these, therefore, the new species adapted to the changed conditions Avould usually be derived ; and this would especially be the case when the change of conditions was rather rapid, and when a correspondingly rapid modification could alone save some species from extinction. But when the change was very gradual, then even less abundant and less widely distributed sj^ecies might become modified into new forms, more especially if the extinction of many of the rarer species left vacant places in the economy of nature. Prohahle Origin of the Dippers. An excellent example of how a limited group of species has been able to maintain itself by adaptation to one of these "vacant places" in nature, is aflforded by the curious little birds called dippers or water- ouzels, forming the genus Cinclus and the family Cinclidae of naturalists. These birds are something like small thrushes, with very short \Wngs and tail, and very dense plumage. They frequent, exclusively, mountain torrents in the northern hemisphere, and obtain their food entirely in the water, consisting, as it does, of water- beetles, caddis -worms and other insect -larvae, as well as numerous small fresh-water shells. These birds, although not far removed in structure from thrushes and ^ATens, have the extraordinary power of flying under water; for such, ac- cording to the best observers, is their process of diving in search of their prey, their dense and somewhat fibrous plumage retaining so much air that the water is prevented from touching their bodies or even from wetting their feathers to any great extent. Their powerful feet and long curved claws enable them to hold on to stones at the bottom, and thus to retain their position while picking up insects, shells, V NATURAL SELECTION 117 etc. As they frequent chiefly the most rapid and boisterous torrents, among rocks, waterfalls, and huge boulders, the water is never frozen over, and they are thus able to live during the severest mnters. Only a very few species of dipper are known, all those of the old world being so closely allied to our British bird that some ornithologists consider them to be merely local races of one species ; while in North America and the northern Andes there are two other species. Here then we have a bird, which, in its whole structure, shows a close affinity to the smaller typical perching birds, but which has departed from all its allies in its habits and mode of life, and has secured for itself a place in nature where it has few competitors and few enemies. We may well suppose, that, at some remote period, a bird which was perhaps the common and more generalised ancestor of most of our thrushes, warblers, AATens, etc., had spread widely over the great northern continent, and had given rise to numerous varieties adapted to special conditions of life. Among these some took to feeding on the borders of clear streams, picking out such larvae and molluscs as they could reach in shallow water. When food became scarce they would attempt to pick them out of deeper and deeper water, and while doing this in cold weather many would become frozen and starved. But any which possessed denser and more hairy plumage than usual, which was able to keep out the water, would survive ; and thus a race would be formed which would depend more and more on this kind of food. Then, following up the frozen streams into the mountains, they would be able to live there during the winter ; and as such places afforded them much protection from enemies and ample shelter for their nests and young, further adaptations would occur, till the wonderful power of diving and flying under water was acquired by a true land-bird. That such habits might be acquired under stress of need is rendered highly probable by the facts stated by the well- known American naturalist. Dr. Abbott. He says that "the water -thrushes (Seiurus sp.) all wade in water, a-nd often, seeing minute mollusca on the bottom of the stream, plunge both head and neck beneath the surface, so that often, for 118 DARWINISM chap. several seconds, a large part of the body is submerged. Now these birds still have the plumage pervious to water, and so are liable to be drenched and sodden ; but they have also the faculty of giving these drenched feathers such a good shaking that flight is practicable a moment after leaving the water. Certainly the water -thrushes (Seiurus ludovicianus, S. aurica- pillus, and S. noveboracensis) have taken many preliminary steps to becoming as aquatic as the dipper ; and the winter- ^^Ten, and even the Maryland yellow -throat are not far behind."! Another curious example of the way in which species have been modified to occupy new places in nature, is afforded by the various animals which inhabit the water- vessels formed by the leaves of many epiphytal species of Bromelia. Fritz Midler has described a caddis-fly larva which lives among these leaves, and which has been modified in the pupa state in accordance with its surroundings. The pupae of caddis-flies inhabiting streams have fringes of hair on the tarsi to enable them to reach the surface on leaving their cases. But in the species inhabiting bromelia leaves there is no need for s^vimming, and accordingly we find the tarsi entirely bare. In the same plants are found curious little Entomostraca, very abundant there but found nowhere else. These form a new genus, but are most nearly allied to Cy there, a marine type. It is believed that the transmission of this species from one tree to another must be effected by the young Crustacea, which are very minute, clinging to beetles, many of which, both teri'estrial and aquatic, also inhabit the bromelia leaves ; and as some water- beetles are knoAvn to frequent the sea, it is perhaps by these means that the first emigrants established themselves in this strange new abode. Bromelise are often very abundant on trees grooving on the water's edge, and this would facilitate the tran- sition from a marine to an arboreal habitat. Fritz Midler has also found, among the bromelia leaves, a small frog bearing its eggs on its back, and having some other peculiarities of structure. Several beautiful little aquatic plants of the genus Utricularia or bladder -wort also inhabit bromelia leaves ; and these send runners out to neighbouring plants and thus spread themselves with great rapidity. ^ Nature, vol. xxx. p. 30. V . NATURAL SELECTION 119 The Importance of Isolation. Isolation is no doubt an important aid to natm^al selection, as shown by the fact that islands so often present a number of peculiar species ; and the same thing is seen on the two sides of a gTcat mountain range or on opposite coasts of a continent. The importance of isolation is twofold. In the first place, it leads to a body of individuals of each species being limited in their range and thus subjected to uniform condi- tions for long spaces of time. Both the direct action of the enWronment and the natural selection of such varieties only as are suited to the conditions, will, therefore, be able to produce their fidl effect. In the second place, the jDrocess of change will not be interfered with by intercrossing with other individuals ^vhich are becoming adapted to somewhat different conditions in an adjacent area. But this question of the swamping eflects of intercrossing ^nll be considered in another chapter. Mr. Dar^vin was of opinion that, on the whole, the largeness of the area occupied by a species was of more importance than isolation, as a factor in the production of new species, and in this I quite agree mth him. It must, too, be remembered, that isolation will often be produced in a continuous area whenever a species becomes modified in accordance with varied conditions or diverging habits. For example, a wide-ranging species may in the northern and colder part of its area become modified in one direction, and in the southern part in another direction ; and though for a long time an intermediate form may continue to exist in the intervening area, this ^vill be likely soon to die out, both because its numbers will be small, and it ^^^ill be more or less pressed upon in varying seasons by the modified varieties, each better able to endure extremes of climate. So, when one portion of a terrestrial species takes to a more ai'boreal or to a more aquatic mode of life, the change of habit itself leads to the isolation of each portion. Again, as \^411 be more fully exj^lained in a future chapter, any difference of habits or of haunts usually leads to some modi- fication of colour or marking, as a means of concealment from enemies ; and there is reason to believe that this difference will be intensified by natural selection as a means of identification 120 DARWINISM chap. and recognition by members of the same variety or incipient species. It has also been observed that each differently coloured variety of wild animals, or of domesticated animals which have run wild, keep together, and refuse to pair with individuals of the other colours ; and this must of itself act to keep the races separate as completely as physical isolation. On the Advance of Organisation hy Natural Selection. As natural selection acts solely by the preservation of use- ful variations, or those which are beneficial to the organism under the conditions to Avhich it is exposed, the result must necessarily be that each species or group tends to become more and more improved in relation to its conditions. Hence we should expect that the larger groups in each class of animals and plants — those which have persisted and have been abundant throughout geological ages — would, almost necessarily, have arrived at a high degree of organisation, both physical and mental. Illustrations of this are to be seen everywhere. Among mammalia we have the carnivora, which from Eocene times have been becoming more and more specialised, till they have culminated in the cat and dog tribes, which have reached a degree of perfection both in structure and intelligence fully equal to that of any other animals. In another line of development, the herbivora have been specialised for living solely on vegetable food till they have culminated in the sheep, the cattle, the deer, and the antelopes. The horse tribe, commencing with an early four-toed ancestor in the Eocene age, has increased in size and in perfect adaptation of feet and teeth to a life on open plains, and has reached its highest per- fection in the horse, the ass, and the zebra. In birds, also, we see an advance from the imperfect tooth-billed and reptile- tailed birds of the secondary epoch, to the wonderfully developed falcons, crows, and swallows of our time. So, the ferns, lycopods, conifers, and monocotyledons of the palaeozoic and mesozoic rocks, have developed into the marvellous wealth of forms of the higher dicotyledons that now adorn the earth. But this remarkable advance in the higher and larger groups does not imply any universal law of progress in organisation, because we have at the same time numerous examples (as has been already pointed out) of the persistence of lowly organised V NATURAL SELECTION 121 forms, and also of absolute degradation or degeneration. Ser- pents, for example, have been developed from some lizard-like type which has lost its limbs ; and though this loss has enabled them to occupy fresh places in nature and to increase and flourish to a marvellous extent, yet it must be considered to be a retrogression rather than an advance in organisation. The same remark Avill apply to the whale tribe among mammals ; to the blind amphibia and insects of the great caverns ; and among plants to the numerous cases in which flowers, once specially adapted to be fertilised by insects, have lost their gay corollas and their special adaj^tations, and have become degraded into 'sWnd-fertilised forms. Such are our plantains, our meadow burnet, and even, as some botanists maintain, our rushes, sedges, and grasses. The causes which have led to this degeneration will be discussed in a future chapter ; but the facts are undisputed, and they show us that although variation and the struggle for existence may lead, on the whole, to a continued advance of organisation ; yet they also lead in many cases to a retrogTession, when such retrogression may aid in the preservation of any form under neAv conditions. They also lead to the j^ersistence, with slight modifications, of numerous lowly organised forms which are suited to places Avhich higher forms could not fully occupy, or to conditions under which they could not exist. Such are the ocean depths, the soil of the earth, the mud of rivers, deep caverns, subterranean waters, etc. ; and it is in such places as these, as well as in some oceanic islands which competing higher forms have not been able to reach, that we find many curious relics of an earlier world, which, in the free air and sunlight and in the great continents, have long since been driven out or exter- minated by higher types. Summary of the first Five Chaptei^s. We have now passed in review, in more or less detail, the main facts on which the theory of " the origin of species by means of natural selection " is founded. In future chapters we shall have to deal mainly with the application of the theory to explain the varied and complex phenomena presented by the organic world ; and, also, to discuss some of the theories put forth by modern 'vvriters, either as being more fundamental than 122 DARWINISM that of Darwin or as supplementary to it. Before doing this, however, it will be well briefly to summarise the facts and arguments already set forth, because it is only by a clear comprehension of these that the full importance of the theory can be appreciated and its further applications understood. The theory itself is exceedingly simple, and the facts on which it rests — though excessively numerous individually, and coextensive with the entire organic world — yet come under a few simple and easily understood classes. These facts are, — first, the enormous powers of increase in geometrical progres- sion possessed by all organisms, and the inevitable struggle for existence among them ; and, in the second place, the occurrence of much individual variation combined with the hereditary transmission of such variations. From these two great classes of facts, which are universal and indisputable, there necessarily arises, as Darmn termed it, the " preservation of favoured races in the struggle for life," the continuous action of which, under the ever-changing conditions both of the inorganic and organic universe, necessarily leads to the formation or development of new species. But, although this general statement is complete and indis- putable, yet to see its applications under all the complex conditions that actually occur in nature, it is necessary always to bear in mind the tremendous power and universality of the agencies at work. AVe must never for an instant lose sight of the fact of the enormously rapid increase of all organisms, which has been illustrated by actual cases, given in our second chapter, no less than by calculations of the results of un- checked increase for a few years. Then, never forgetting that the animal and plant popidation of any country is, on the whole, stationary, we must be always trying to realise the ever-recurring destruction of the enormous annual increase, and asking ourselves what determines, in each individual case, the death of the many, the survival of the few. We must think over all the causes of destruction to each organism, — to the seed, the young shoot, the growing plant, the full-grown tree, or shrub, or herb, and again the fruit and seed ; and among animals, to the egg or new-born young, to the youthful, and to the adults. Then, we must always bear in mind that what goes on in the case of the individual or family group we may V NATURAL SELECTION 123 observe or think of, goes on also among the millions and scores of millions of individuals which are comprised in almost every species ; and must get rid of the idea that clmnce determines which shall live and which die. For, although in many individual cases death may be due to chance rather than to any inferiority in those which die first, yet we cannot possibly believe that this can be the case on the large scale on which nature works. A plant, for instance, cannot be in- creased unless there are suitable vacant places its seeds can grow in, or stations where it can overcome other less vigorous and healthy plants. The seeds of all plants, by their varied modes of dispersal, may be said to be seeking out such places in which to gi^ow ; and we cannot doubt that, in the long run, those individuals whose seeds are the most numerous, have the greatest jDOwers of dispersal, and the greatest vigour of growth, will leave more descendants than the individuals of the same species which are inferior in all these respects, although now and then some seed of an inferior individual may chance to be carried to a spot where it can grow and sur\dve. The same rule will apply to every period of life and to every danger to which plants or animals are exposed. The best organised, or the most healthy, or the most active, or the best protected, or the most intelligent, aWII ine\itably, in the long run, gain an advantage over those which are inferior in these qualities ; that is, the fittest will survive, the fittest being, in each particular case, those which are superior in the special qualities on which safety depends. At one period of life, or to escape one kind of danger, concealment may be necessary ; at another time, to escape another danger, s^Wftness ; at another, intel- ligence or cunning ; at another, the power to endure rain or cold or hunger ; and those which possess all these faculties in the fullest perfection vd]l generally survive. Having fully grasped these facts in all their fulness and in their endless and complex results, we have next to consider the phenomena of variation, discussed in the third and fourth chapters ; and it is here that perhaps the greatest difficulty will be felt in appreciating the full importance of the evidence as set forth. It has been so generally the practice to speak of variation as something exceptional and comparatively rare — as an abnormal deviation from the uniformity and stability of the 124 DARWINISM characters of a species — and so few even among naturalists have ever compared, accurately, considerable numbers of individuals, that the conception of variability as a general characteristic of all dominant and widespread species, large in its amount and affecting, not a few, but considerable masses of the individuals which make up the species, will be to many entirely new. Equally important- is the fact that the vari- ability extends to every organ and every structure, external and internal ; while perhaps most important of all is the independent variability of these several parts, each one vary- ing without any constant or even usual dependence on, or correlation with, other parts. No doubt there is some such correlation in the differences that exist between species and species — more developed wings usually accompanying smaller feet and vice versa — but this is, generally, a useful adaptation which has been brought about by natural selection, and does not apply to the individual variability which occurs within the species. It is because these facts of variation are so important and so little understood, that they have been discussed in what will seem to some readers wearisome and unnecessary detail. Many naturalists, however, will hold that even more evidence is required ; and more, to almost any amount, could easily have been given. The character and variety of that already adduced will, however, I trust, convince most readers that the facts are as stated ; while they have been drawn from a sufficiently wide area to indicate a general principle through- out nature. If, now, we fully realise these facts of variation, along with those of rapid multiplication and the struggle for existence, most of the difficulties in the way of comprehending how species have originated through natural selection will disappear. For whenever, through changes of climate, or of altitude, or of the nature of the soil, or of the area of the country, any species are exposed to new dangers, and have to maintain themselves and provide for the safety of their offspring under new and more arduous conditions, then, in the variability of all parts, organs, and structures, no less than of habits and intelh'gence, we have the means of producing modifications which will certainly bring the species into harmony with its V NATURAL SELECTION 125 new conditions. And if Ave remember that all such physical changes are slow and gradual in their operation, we shall see that the amount of variation which we know occurs in every new generation aWII be quite sufficient to enable modification and adaptation to go on at the same rate. Mr. Dar^vin was rather inclined to exaggerate the necessary slowness of the action of natural selection ; but with the knowledge we now possess of the great amount and range of individual variation, there seems no difficulty in an amount of change, cjuite equivalent to that which usually distinguishes allied species, sometimes taking place in less than a centiury, should any rapid change of conditions necessitate an equally rapid adaptation. This may often have occiu'red, either to im- migrants into a new land, or to residents whose country has been cut off by subsidence from a larger and more varied area over which they had formerly roamed. When no change of conditions occurs, species may remain unchanged for very long periods, and thus produce that appearance of stability of species which is even now often adduced as an argument against evolution by natural selection, but which is really quite in harmony mth it. On the principles, and by the light of the facts, now briefly summarised, we have been able, in the present chapter, to indicate how natural selection acts, how divergence of char- acter is set up, how adaptation to conditions at various periods of life has been effected, how it is that low forms of life continue to exist, what kind of circumstances are most favourable to the formation of new species, and, lastly, to what extent the advance of organisation to higher types is produced by natural selection. We will now pass on to con- sider some of the more important objections and difficulties which have been advanced by eminent natui-alists. CHAPTER VI DIFFICULTIES AND OBJECTIONS Difficulty as to smallness of variations — As to the right variations occur- ring when required — The beginnings of important organs — The mam- mary glands — The eyes of flatfish — Origin of the eye — Useless or non-adaptive characters — Recent extension of the region of utility in plants— The same in animals — Uses of tails — Of the horns of deer — Of the scale-ornamentation of reptiles — Instability of non-adaptive characters — Delbceufs law — No "specific" character proved to be useless — The swamping effects of intercrossing — Isolation as prevent- ing intercrossing— Gulick on the efiects of isolation — Cases in which isolation is ineffective. In the present chapter I propose to discuss the more obvious and often repeated objections to Darwin's theory, and to show how far they affect its character as a true and sufficient explanation of the origin of species. The more recondite difficulties, affecting such fundamental questions as the causes and laws of variability, will be left for a future chapter, after we have become better acquainted with the applications of the theory to the more important adaptations and correlations of animal and plant life. One of the earliest and most often repeated objections was, that it was difficult " to imagine a reason why variations tend- ing in an infinitesimal degree in any special direction should be preserved," or to believe that the complex adaptation of living organisms could have been produced " by infinitesimal beginnings." Now this term " infinitesimal," used by a well- known early critic of the Origin of Species, was never made use of by Darwin himself, who spoke only of variations being "slight," and of the "small amount" of the variations that might be selected. Even in using these terms he undoubtedly afforded CHAP. VI DIFFICULTIES AXD OBJECTIONS 127 grounds for the objection above made, that such small and slight variations could be of no real use, and would not determine the siu'-^'ival of the individuals j^ossessing them. We have seen, however, in our third chapter, that even Darwin's terms were hardly justified; and that the variability of many im- portant species is of considerable amount, and may very often be properly described as large. As this is found to be the case both in animals and jolants, and in all their chief groups and subdivisions, and also to apply to all the separate joarts and organs that have been compared, we must take it as proved that the average amount of variability presents no difficulty whatever in the way of the action of natural selection. It may be here mentioned that, up to the time of the prepara- tion of the last edition of The Origin of Species, Dar^-in had not seen the work of Mr. J. A. Allen of Harvard University (then only just published), which gave us the first body of accu- rate comparisons and measurements demonstrating this large amount of variability. Since then eWdence of this nature has been accumulating, and we are, therefore, now in a far better position to appreciate the facilities for natural selection, in this respect, than was Mr. Darwin himself. Another objection of a similar nature is, that the chances are immensely against the right variation or combination of variations occurring just when required ; and further, that no variation can be perpetuated that is not accompanied by several concomitant variations of dependent parts — greater length of a wing in a bird, for example, would be of little use if unaccompanied by increased volume or contractility of the muscles which move it. This objection seemed a very strong one so long as it was supposed that variations occmTed singly and at considerable intervals ; but it ceases to have any weight now we know that they occur simultaneously in various parts of the organism, and also in a large proportion of the in- dividuals which make up the species. A considerable number of indi^-iduals vr\\\ therefore, every year possess the required combination of characters ; and it may also be considered probable that when the two characters are such that they always act together, there will be such a correlation between them that they will frequently Tarij together. But there is another consideration that seems to show that this coincident 128 DARWINISM chap. variation is not essential. All animals in a state of nature are kept, by the constant struggle for existence and the survival of the fittest, in such a state of perfect health and usually superabundant vigour, that in all ordinary circumstances they possess a surplus power in every important organ — a surplus only drawn upon in cases of the direst necessity when their very existence is at stake. It follows, therefore, that any additional power given to one of the component parts of an organ must be useful — an increase, for example, either in the wing muscles or in the form or length of the wing might give some increased powers of flight ; and thus alternate variations — in one generation in the muscles, in another generation in the 'sving itself — might be as effective in permanently impro\ang the powers of flight as coincident variations at longer intervals. On either supposition, however, this objection appears to have little weiiiht if we take into consideration the lar^e amount of coincident variability that has been shown to exist. The Beginnings of Important Organs. We now come to an objection which has perhaps been more frequently urged than any other, and Avhich Dar\vin himself felt to have much weight — the first beginnings of im- portant organs, such, for example, as A\^ngs, eyes, mammary glands, and numerous other structures. It is urged, that it is almost impossible to conceive how the first rudiments of these could have been of any use, and, if not of use they could not have been preserved and further developed by natural selection. Now, the first remark to be made on objections of this nature is, that they are really outside the question of the origin of all existing species from allied species not very far removed from them, which is all that Darwin undertook to prove by means of his theory. Organs and structures such as those above mentioned all date back to a very remote past, when the world and its inhabitants were both very different from what they are now. To ask of a neAv theory that it shall reveal to us exactly what took place in remote geological ei)Ochs, and how it took place, is unreasonable. The most that should be asked is, that some probable or possible mode of origination should be pointed out in some at least of these VI DIFFICULTIES AND OBJECTIONS 129 difficult cases, and this Mr. Darwin has done. One or two of these may be briefly given here, but the whole series should be carefully read by any one who wishes to see how many curious facts and observations have been required in order to elucidate them ; whence we may conclude that further know- ledge will probably throw light on any difficulties that still remain.^ In the case of the mammary glands Mr. Darmn remarks that it is admitted that the ancestral mammals were allied to the marsupials. Now in the very earliest mammals, almost before they really deserved that name, the young may have been nourished by a fluid secreted by the interior surface of the marsupial sack, as is believed to be the case with the fish (Hippocampus) whose eggs are hatched within a some- what similar sack. This being the case, those individuals which secreted a more nutritious fluid, and those whose young were able to obtain and swallow a more constant supply by suction, Avould be more likely to live and come to a healthy maturity, and would therefore be preserved by natural selec- tion. In another case which has been adduced as one of special difficulty, a more complete explanation is given. Soles, turbots, and other flatfish are, as is well known, unsym- metrical. They live and move on their sides, the under side being usually difl'erently coloured from that which is kept uppermost. Now the eyes of these fish are curiously distorted in order that both eyes may be on the upper side, where alone they would be of any use. It was objected by Mr. Mivart that a sudden transformation of the eye from one side to the other was inconceivable, Avhile, if the transit were gi^adual the first step could be of no use, since this would not remove the eye from the lower side. But, as Mr. Darwin shows by reference to the researches of Malm and others, the young of these fish are quite symmetrical, and during their growth exhibit to us the whole process of change. This begins by the fish (owing to the increasing depth of the body) being un- able to maintain the vertical position, so that it falls on one side. It then twists the lower eye as much as possible towards the upper side ; and, the whole bony structure of the head being at ^ See Origin of Species, pp. 176-198. K 130 DARWINISM chap. this time soft and flexiWe, the constant repetition of this effort causes the eye L;Ta(hially to move round the head till it comes to the upi)er side. Now if we sui)pose this process, which in the young is completed in a few days or M'eeks, to have been spread over thousands of generations (hiring the devehjpment of these fish, those usually surviving whose eyes retained more and more of the position into which the young fish tried to twist them, the change becomes intelligible ; though it still remains one of the most extraordinary cases of degeneration, by which symmetry — which is so universal a characteristic of the higher animals — is lost, in order that the creature may be adapted to a new mode of life, whereby it is enabled the better to escape danger and continue its existence. The most difficult case of all, that of the eye — the thought of which even to the last, Mr. Darwin says, " gave him a cold shiver " — is nevertheless shown to be not unintelligible ; granting of course the sensitiveness to light of some forms of nervous tissue. For he shows that there are, in several of the lower animals, rudiments of eyes, consisting merely of pigment cells covered with a translucent skin, which may possibly serve to distinguish light from darkness, but nothing more. Then we have an optic nerve and j^igment cells ; then we find a hollow filled with gelatinous substance of a convex form — the first rudiment of a lens. Many of the succeeding steps are lost, as would necessarily be the case, owing to the great advantage of each modification which gave increased distinctness of -sdsion, the creatures possessing it inevitably sui'viving, while those below them became extinct. But we can well understand how, after the first step was taken, every variation tending to more complete vision would be preserved till we reached the perfect eye of birds and mammals. Even this, as we know, is not absolutely, but only relatively, perfect. Neither the chromatic nor the spherical aberration is absolutely corrected; while long- and short- sightedness, and the various diseases and imperfections to Mdiich the eye is liable, may be looked upon as relics of the imperfect condition from which the eye has been raised by variation and natural selection. These few examples of difficulties as to the origin of remark- able or complex organs must suffice here ; but the reader who wishes further information on the matter may study carefully VI DIFFICULTIES AND OBJECTIONS 131 the whole of the sixth and seventh chapters of the last edition of The Origin of Species, in which these and many other cases are discussed in considerable detail. Useless or non-adaptive Characters. Many naturalists seem to be of opinion that a considerable number of the characters which distinguish species are of no service whatever to their possessors, and therefore cannot have been j^roduced or increased by natural selection. Professors Bronn and Broca have urged this objection on the continent. In America, Dr. Cope, the well-known palseontologist, has long since put forth the same objection, declaring that non-adaptive characters are as numerous as those which are adaptive ; but he differs completely from most who hold the same general opinion in considering that they occur chiefly " in the characters of the classes, orders, families, and other higher groups;" and the objection, therefore, is quite distinct from that in which it is urged that " specific characters " are mostly useless. More recently, Professor G. J. Romanes has urged this difficulty in his paper on "Physiological Selection" (Journ. Linn. Sac, vol. xix. pp. 338, 344). He says that the characters "which serve to distinguish allied species are frec[uently, if not usually, of a kind with which natural selection can have had nothing to do," being without any utilitarian significance. Again he speaks of " the enormous number," and further on of " the innumerable multitude " of specific peculiarities which are useless ; and he finally declares that the question needs no further arguing, " because in the later editions of his works Mr. Darwin freely acknowledges that a large proportion of specific distinctions must be conceded to be useless to the species presenting them." I have looked in vain in Mr. Dar^vin's works to find any such acknowledgment, and I think Mr. Romanes has not sufficiently distinguished between " useless characters " and "useless specific distinctions." On referring to all the passages indicated by him I find that, in regard to specific characters, IVIr. Darwin is very cautious in admitting inutility. His most pronounced "admissions" on this question are the follow- ing : " But when, from the nature of the organism and of the conditions, modifications have been induced which are 132 DARWINISM unimportant for the welfare of the species, they may be, and apparently often have been, transmitted in nearly the same state to numerous, ofhericise modified, descendants'' (Origin, p. 175). The words I have here italicised clearly show that such characters are usually not " s})ecific," in the sense that they are such as distinguish s})ecies from each other, but are found in numerous allied species. Again : " Thus a large yet undefined extension may safely be given to the direct and indirect results of natural selection ; but I now admit, after reading the essay of Nageli on plants, and the remarks by various authors with respect to animals, more especially those recently made by Professor Broca, that in the earlier editions of my Origin of Species I perhaps attributed too much to the action of natural selection or the survival of the fittest. I have altered the fifth edition of the Origin so as to confine my remarks to adaptive changes of structure, hut I am amcinced, from the light gained during even the last few years, that very many strtictures which noiv appear to us useless, ivill hereafter he pvved to he usefid, and icill therefore cor.w ivithin the range of natural selection. Nevertheless I did not formerly consider sufficiently the existence of structures which, as far as we can at present judge, are neither beneficial nor injurious ; and this I believe to be one of the greatest oversights as yet detected in my Avork." Now it is to be remarked that neither in these passages nor in any of the other less distinct expressions of opinion on this question, does Darwin ever admit that "specific characters " — that is, the particular characters which serve to distinguish one species from another — are ever useless, much less that "a large proportion of them " are so, as Mr. Eomanes makes him "freely acknowledge." On the other hand, in the passage which I have italicised he strongly expresses his view that much of what we suppose to be useless is due to our ignorance ; and as I hold myself that, as regards many of the supposed useless characters, this is the true explanation, it may be well to give a brief sketch of the progress of knoAV- ledge in transferring characters from the one category to the other. We have only to go back a single generation, and not even the most acute botanist could have suggested a reasonable use, for each species of plant, of the infinitely varied forms, sizes, vr DIFFICULTIES AND OBJECTIONS 133 and colours of the flowers, the shapes and arrangement ci the leaves, and the numerous other external characters of the whole plant. But since Mr. Darwin showed that plants gained both in vigoiu* and in fertility by being crossed with other individuals of the same species, and that this crossing was usually eff'ected by insects which, in search of nectar or pollen, carried the pollen from one plant to the flowers of another plant, almost every detail is found to have a purpose and a use. The shape, the size, and the colour of the petals, even the streaks and spots with which they are adorned, the position in which they stand, the movements of the stamens and pistil at various times, esjjecially at the period of, and just after, fertilisation, have been proved to be strictly adaptive in so many cases that botanists now believe that all the external characters of flowers either are or have been of use to the species. It has also been shown, by Kerner and other botanists, that another set of characteristics have relation to the pre- vention of ants, slugs, and other animals from reaching the flowers, because these creatures would devour or injure them without eff"ecting fertilisation. The spines, hairs, or sticky glands on the stem or flower-stalk, the curious hairs or processes shutting up the flower, or sometimes even the extreme smoothness and polish of the outside of the petals so that few insects can hang to the part, have been shown to be related to the possible intrusion of these "unbidden guests. "^ And, still more recently, attempts have been made by Grant Allen and Sir John Lubbock to account for the innumerable forms, textures, and groupings of leaves, by their relation to the needs of the plants themselves ; and there can be little doubt that these attempts will be ultimately successful. Again, just as flowers have been adapted to secure fertilisation or cross-fertilisation, fruits have been developed to assist in the dispersal of seeds ; and their forms, sizes, juices, and colours can be shown to be specially adapted to secure such dispersal by the agency of birds and mammals ; while the same end is secui'ed in other ^ See Kerner's Floicers and their Unbidden Guests for numerous other structures and peculiarities of plants which are shown to be adaptive and useful. 134 DARWINISM cases by downy seeds to be wafted through the air, or by hooked or sticky seed-vessels to be carried away, attached to skin, wool, or feathers. Here, then, we have an enormous extension of the region of utility in the vegetable kingdom, and one, moreover, which includes almost all the specific characters of plants. For the species of plants are usually characterised either by differences in the form, size, and colour of the flowers, or of the fruits ; or, by peculiarities in the shape, size, dentation, or arrange- ment of the leaves ; or by peculiarities in the spines, hairs, or down with which various parts of the plant are clothed. In the case of plants it must certainly be admitted that " specific " characters are pre-eminently adaptive ; and though there may be some which are not so, yet all those referred to by Darwin as having been adduced by various botanists as useless, either pertain to genera or higher groups, or are found in some plants of a species only — that is, are individual variations not specific characters. In the case of animals, the most recent wide extension of the sphere of utility has been in the matter of their colours and markings. It was of course always known that certain creatures gained protection by their resemblance to their normal surroundings, as in the case of white arctic animals, the yellow or brown tints of those living in deserts, and the green hues of many birds and insects surrounded by tropical vegetation. But of late years these cases have been greatly increased both in number and variety, especially in regard to those which closely imitate special objects among which they live ; and there are other kinds of coloration Avhich long appeared to have no use. Large numbers of animals, more especially insects, are gaudily coloured, either with vivid hues or with striking patterns, so as to be very easily seen. Now it has been found, that in almost all these cases the creatures possess some special quality Avhich prevents their being attacked by the enemies of their kind whenever the peculiarity is known ; and the brilliant or conspicuous colours or markings serve as a warning or signal flag against attack. Large numbers of insects thus coloured are nauseous and inedible; others, like wasps and bees, have stings; others are too hard to be eaten by small birds ; while snakes with VI DIFFICULTIES AXD OBJECTIOXS 135 poisonous fangs usually have some characteristic either of rattle, hood, or unusual colour, which indicates that they had better be left alone. But there is yet another form of coloration, which consists in special markings — bands, spots, or patches of white, or of bright colour, which vary in every species, and are often concealed when the creature is at rest but displayed when in motion, — as in the case of the bands and spots so frequent on the wings and tails of birds. Now these specific markings are believed, with good reason, to serve the purpose of enabling each species to be quickly recognised, even at a distance, by its fellows, especially the parents by their young and the two sexes by each other ; and this recognition must often be an important factor in securing the safety of individuals, and therefore the wellbeing and continuance of the species. These interesting peculiarities ^vill be more fidly described in a future chapter, but they are briefly referred to here in order to show that the most common of all the characters by which species are distinguished from each other — their colours and markings — can be shown to be adaptive or utilitarian in their nature. But besides colour there are almost always some structural characters which distinguish species from species, and, as re- gards many of these also, an adaptive character can be often discerned. In birds, for instance, we have differences in the size or shape of the bill or the feet, in the length of the wing or the tail, and in the proportions of the several feathers of which these organs are composed. All these differences in the organs on which the very existence of birds depends, which determine the character of flight, facility for running or climbing, for inhabiting chiefly the ground or trees, and the kind of food that can be most easily obtained for themselves and their offspring, must surely be in the highest degree utilitarian ; although in each individual case Ave, in our ignorance of the minutiae of their life-history, may be quite unable to see the use. In mammalia specific differences other than colour usually consist in the length or shape of the ears and tail, in the proportions of the limbs, or in the length and quality of the hair on different parts of the body. As regards the ears and tail, one of the objections by Professor 136 DARWINISM Bronn relates to this very point. He states that the length of these organs differ in the various species of hares and of mice, and he considers that this difference can be of no service whatever to their possessors. But to this objection Darwin replies, that it has been shown by Dr. Schobl that the ears of mice "are supplied in an extraordinary manner with nerves, so that they no doubt serve as tactile organs." Hence, when we consider the life of mice, either nocturnal or seeking their food in dark and confined places, the length of the ears may be in each case adapted to the particular habits and surroundings of the species. Again, the tail, in the larger mammals, often serves the purpose of driving off flies and other insects from the body ; and when we consider in how many parts of the world flies are injurious or even fatal to large mammals, we see that the peculiar characteristics of this organ may in each case have been adapted to its requirements in the particular area where the species was developed. The tail is also believed to have some use as a balancing organ, which assists an animal to turn easily and rapidly, much as our arms are used when running ; while in whole groups it is a prehensile organ, and has become modified in accordance with the habits and needs of each species. In the case of mice it is thus used by the young. Darwin informs us that the late Professor Henslow kept some harvest- mice in con- finement, and observed that they frequently curled their tails round the branches of a bush placed in the cage, and thus aided themselves in climbing ; Avhile Dr. Giinther has actually seen a mouse suspend itself by the tail {Origin, p. 189). Again, Mr. Lawson Tait has called attention to the use of the tail in the cat, squirrel, yak, and many other animals as a means of preserving the heat of the body during the nocturnal and the winter sleep. He says, that in cold weather animals with long or bushy tails will be found lying curled up, with their tails carefully laid over their feet like a rug, and with their noses buried in the fur of the tail, Avhich is thus used exactly in the same way and for the same purpose as we use respirators.^ Another illustration is furnished by the horns of deer which, especially when very large, have been supposed to be ^ Nature, vol. xx. p. 603. VI DIFFICULTIES AXD OBJECTIONS 137 a danger to the animal in passing rapidly through dense thickets. But Sir James Hector states, that the wapiti, in Xorth America, throws back its head, thus placing the horns along the sides of the back, and is then enabled to rush through the thickest forest Avith great rapidity. The brow- antlers protect the face and eyes, while the widely spreading horns prevent injury to the neck or flanks. Thus an organ which was certainly developed as a sexual weapon, has been so ouided and modified durino; its increase in size as to be of use in other ways. A similar use of the antlers of deer has been observed in India. ^ The various classes of facts now referred to serve to show us that, in the case of the two higher groups — mammalia and birds — almost all the characters by which species are distinguished from each other are, or may be, adaptive. It is these two classes of animals which have been most studied and whose life-histories are supposed to be most fully known, yet even here the assertion of inutility, by an eminent naturalist, in the case of two important organs, has been sufficiently met by minute details either in the anatomy or in the habits of the groups referred to. Such a fact as this, together with the extensive series of characters already enumerated which have been of late years transferred from the " useless " to the " useful " class, should convince us, that the assertion of " inutility " in the case of any organ or peculiarity which is not a rudiment or a correlation, is not, and can never be, the statement of a fact, but merely an expression of our ignorance of its purpose or origin.- 1 Nature, vol. xxxviii. p. 328. - A very remarkable illustration of function in an apparently useless ornament is given by Semper. He says, "It is known that tlie skin of reptiles encloses the body with scales. These scales are distinguished by very various sculpturiugs, highly characteristic of the different species. Irrespective of their systematic signiticauce they appear to be of no value in the life of the animal ; indeed, they are viewed as ornamental without regard to the fact that they are microscopic and much too delicate to be visible to other animals of their own species. It might, therefore, seem hopeless to show the necessity for their existence on Darwinian principles, and to prove that they are physiologically active organs. Nevertheless, recent investigations on this point have furnished evidence that this is possible. " It is known that many reptiles, and above all the snakes, ca.st off the whole skin at once, whereas human beings do so by degrees. If by anj' accident they are prevented doing so, they infallibly die, because the old 138 DARWINISM Instability of Non- adaptive Characters. One very weighty objection to the theory that specific characters can ever be wholly useless (or wholly uncon- nected with useful organs by correlation of growth) appears to have been overlooked by those who have maintained the frequency of such characters,- and that is, their almost necessary instability. Darwin has remarked on the extreme variability of secondary sexual characters — such as the horns, crests, plumes, etc., which are found in males only, — the reason being, that, although of some use, they are not of such direct and vital importance as those adaptive characters on which the wellbeing and very existence of the animals depend. But in the case of wholly useless structures, skin has groAVTi so tough and hard that it hinders the increase in volume which is inseparable from the growth of the animal. The casting of the skin is induced by the formation on the surface of the inner epidermis, of a layer of very fine and equally distributed hairs, which evidently serve the purpose of mechanically raising the old skin by their rigidity and position. These hairs then may be designated as casting hears. That they are destined and calculated for this end is evident to me from the fact established by Dr. Braun, that the casting of the sliells of the river cray-fish is induced in exactly the same manner by the formation of a coating of hairs which mechanically loosens the old skin or shell from the new. Now the researches of Braun and Cartier have shown that these casting hairs — which serve the same purpose in two groups of animals so far apart in the systematic scale — after the casting, are partly transformed into the concentric stripes, sharp spikes, ridges, or warts which ornament the outer edges of the skin-scales of reptiles or the cai'apace of crabs." ^ Professor Semper adds that this example, with many others that might be quoted, shows that we need not abandon the hope of exjilaining morphological characters on Darwinian principles, although their nature is often difficult to understand. During a recent discussion of this question in the pages of Nature, Mr. St. George Mivart adduces several examples of what he deems useless specific characters. Among them are the aborted index finger of the lem urine Potto, and the thumbless hands of Colobus and Ateles, the " life-saving action " of either of which he thinks incredible. These cases suggest two remarks. In the first place, they involve (jeneric, not specific, characters ; and the three genera adduced are somewhat isolated, implying considerable antiquity and the extinction of many allied forms. Tliis is important, because it afibrds ample time for great changes of conditions since the structures in question originated ; and without a knowledge of these changes we can never safely assert that any detail of structure could not have been useful. In the second jilace, all three are cases of aborted or rudimentary organs ; and these are admitted to be explained by non-use, leading to diminution of size, a further reduction being brought about by the action of the principle of economy 1 The Natural Conditions of Existence as tliey affect Animal Life, p. 19, VI DIFFICULTIES AND OBJECTIONS 139 which are not rudiments of once useful organs, we cannot see what there is to ensure any amount of constancy or stability. One of the cases on which Mr. Romanes lays great stress in his paper on "Physiological Selection" (Journ. Linn. Soc, vol. xix. p. 384) is that of the fleshy appendages on the corners of the jaw of Normandy pigs and of some other breeds. But it is expressly stated that they are not constant ; they appear "frequently," or "occasionally," they are "not strictly inherited, for they occur or fail in animals of the same litter ; " and they are not always symmetrical, sometimes appearing on one side of the face alone. Now whatever may be the cause or explanation of these anomalous appendages they cannot be classed with "specific characters," the most essential features of which are, that they are symmetrical, of growth. But, when so reduced, the rudiment might be inconvenient or even hurtful, and then natural selection would aid in its complete abortion ; in other words, the abortion of the part would be useful, and would therefore be subject to the law of survival of the fittest. The genera Ateles and Colobus are two of the most purely arboreal types of monkeys, and it is not difficult to conceive that the constant use of the elongated fingers for climbing from tree to tree, and catching on to branches while making great leaps, might require all the nervous energy and muscular growth to be directed to the fingers, the small thumb remaining useless. The case of the Potto is more difficult, both because it is, presumably, a more ancient type, and its actual life- history and habits are completely unknown. These cases are, therefore, not at all to the point as proving that positive specific characters — not mere rudiments characterising whole genera — are in any case useless. Mr. Mivart further objects to the alleged rigidity of the action of natural selection, because wounded or malformed animals have been found which had evidently lived a considerable time in their imperfect condition. But this simply proves that they were living under a temporarily favourable environ- ment, and that the real struggle for existence, in their case, had not yet taken place. We must surely admit that, when the pinch came, and when perfectly formed stoats were dying for Avant of food, the one-footed animal, referred to by Mr. Mivart, would be among the first to succumb ; and the same remark will apply to his abnormally toothed hares and rheumatic monkeys, which might, nevertheless, get on very well under favourable conditions. The struggle for existence, iinder which all animals and plants have been developed, is intermittent, and exceedingly irregular in its incidence and severity. It is most severe and fatal to the young ; but when an animal has once reached maturity, and especially when it has gained experience by several years of an eventful existence, it may be able to maintain itself under conditions which would be fatal to a young and inexperienced creature of the same species. The examples adduced by Mr. Mivart do not, therefore, in any way impugn the hardness of nature as a taskmaster, or the extreme severity of the recurring struggle for existence. ^ 1 See Nature, vol. xxxix. p. 127. 140 DARWINISM that they are inherited, and that they are constant. Ad- mitting that this peculiar appendage is (as Mr. Komanes says rather confidently, " we happen to know it to be ") wholly useless and meaningless, the fact would be rather an argument against specific characters being also meaningless, because the latter never have the characteristics which this particular variation possesses. These useless or non-adaptive characters are, apparently, of the same nature as the " sports " that arise in our domestic productions, but which, as Mr. Darwin says, without the aid of selection would soon disappear ; while some of them may be correlations with other characters which are or have been useful. Some of these correlations are very curious. Mr. Tegetmeier informed Mr. Darwin that the young of white, yellow, or dun-coloured pigeons are born almost naked, whereas other coloured pigeons are born well clothed with down. Now, if this difference occurred betAveen wild species of different colours, it might be said that the nakedness of the young could not be of any use. But the colour ^Wth which it is correlated might, as has been shown, be useful in many ways. The skin and its various appendages, as horns, hoofs, hair, feathers, and teeth, are homologous parts, and are subject to very strange correla- tions of growth. In Paraguay, horses with curled hair occur, and these always have hoofs exactly like those of a mule, while the hair of the mane and tail is much shorter than usual. Now, if any one of these characters were useful, the others correlated with it might be themselves useless, but would still be tolerably constant because dependent on a useful organ. So the tusks and the bristles of the boar are correlated and vary in development together, and the former only may be useful, or l)oth may be useful in uneciual degrees. The difficulty as to how individual differences or sports can become fixed and perpetuated, if altogether useless, is evaded by those who hold that such characters are exceedingly common. Mr. Komanes says that, ui)on his theory of physiological selec- tion, "it is quite intelligible that when a varietal form is differentiated from its parent form by the bar of sterility, any little meaningless ])eculiarities of structure or of instinct shmild at first he alloived to arise, and that they should then he allowed to ^perpetuate tlieniselves by heredity," until they are finally VI DIFFICULTIES AND OBJECTIOKS 141 eliminated by disuse. But this is entirely begging the ques- tion. Do meaningless peculiarities, which we admit often arise as spontaneous variations, ever perpetuate themselves in all the individuals constituting a variety or race, without selec- tion either human or natural ? Such characters present them- selves as unstable variations, and as such they remain, unless preserved and accumulated by selection ; and they can there- fore never become " specific " characters unless they are strictly correlated "vrith some useful and important peculiarities. As bearing upon this question we may refer to what is termed Delboeuf's laAv, which has been thus briefly stated by Mr. Murphy in his Avork on HaUt and Intelligence, p. 241. " If, in any species, a number of individuals, bearing a ratio not infinitely small to the entire number of births, are in every generation born with a particular variation which is neither beneficial nor injurious, and if it is not counteracted by reversion, then the proportion of the new variety to the original form will increase till it approaches indefinitely near to equality." It is not impossible that some definite varieties, such as the melanic form of the jaguar and the bridled variety of the guille- mot are due to this cause ; but from their very nature such varieties are unstable, and are continually reproduced in varying proportions from the parent forms. They can, therefore, never constitute species unless the variation in question becomes beneficial, when it Avill be fixed by natural selection. Darwin, it is true, says — " There can be little doubt that the tendency to vary in the same manner has often been so strong that all the individuals of the same species have been similarly modified without the aid of any form of selection." ^ But no proof whatever is ofi"ered of this state- ment, and it is so entirely opposed to all we know of the facts of variation as given by Darwin himself, that the important word " all " is probably an oversight. On the whole, then, I submit, not only has it not been proved that an " enormous number of specific peculiarities " are useless, and that, as a logical result, natural selection is " not a theory of the origin of species," but only of the origin 1 Origin of Species, p. 72. 142 DARWINISM CHAP. of adaptations which are usuall}^ common to many species, or, more commonly, to genera and families ; but, I urge further, it has not even been proved that any truly " specific " characters — those which either singly or in combination dis- tinguish each si)ecies from its nearest allies — are entirely un- ada])tive, useless, and meaningless ; while a great body of facts on the one hand, and some weighty arguments on the other, alike prove that sjiecific characters have been, and could only have been, developed and fixed by natural selection because of their utility. We may admit, that among the great number of variations and sports which continually arise many are altogether useless without being hurtful ; but no cause or influence has been adduced adequate to render such characters fixed and constant throughout the vast number of individuals which con- stitute any of the more dominant species.^ The Sicamjwig Effects of Intercrossing. This supposed insuperable difficulty was first advanced in an article in the North British Review in 1867, and much attention has been attracted to it by the acknowledgment of Mr. Darwin that it proved to him that "single variations," or what are usually termed " sports," could very rarely, if ever, be perpetuated in a state of nature, as he had at first thought might occasionally be the case. But he had always considered that the chief part, and latterly the whole, of the materials with which natural selection works, was afi'orded by individual variations, or that amount of ever fluctuating variability which exists in all organisms and in all their parts. Other writers have urged the same objection, even as against individual variability, apparently in total ignorance of its amount and range ; and quite recently Professor G. J. Romanes has adduced ^ Darwin's latest expression of opinion on this question is interesting, since it shows that he was inclined to return to his earlier view of the general, or universal, utility of specific characters. In a letter to Semper (30th Nov. 1878) he writes : " As our knowledge advances, very slight differences, con- sidered by systematists as of no importance in structure, are continually found to be functionally imjiortant ; and I have been especially struck with this fact in the case of plants, to which my observations have, of late years, been confined. Tlierefore it seems to me rather rash to consider slight differences between rejjresentative species, for instance, those inhabiting the different islands of the same archipelago, as of no functional importance, and as not in any way due to natural selection " {Life of Darwin, vol. iii. p. 161). VI DIFFICULTIES AXD OBJECTIOXS 143 it as one of the diflSculties which can alone be overcome by his theory of physiological selection. He urges, that the same variation does not occur simultaneously in a number of individuals inhabiting the same area, and that it is mere assumption to say it does ; Avhile he admits that " if the assumption Avere granted there would be an end of the present difficulty ; for if a sufficient luimber of individuals were thus simultaneously and similarly modified, there need be no longer any danger of the variety becoming swamped by intercrossing.'' I must again refer my readers to my third chapter for the proof that such simultaneous variability is not an assumption but a fact ; but, even admitting this to be proved, the problem is not altogether solved, and there is so much misconception regarding variation, and the actual process of the origin of new species is so obscure, that some further discussion and elucidation of the subject are desirable. In one of the j^reliminary chapters of Mr. Seebohm's recent work on the Charadriidce, he discusses the differentiation of species ; and he expresses a rather widespread view among naturalists AA'hen, speaking of the swamping effects of inter- crossing, he adds : " This is unquestionably a very grave difficulty, to my mind an absolutely fatal one, to the theory of accidental variation." And in another passage he says : " The simultaneous appearance, and its repetition in successive genera- tions, of a beneficial variation, in a large number of indiAiduals in the same locality, cannot possibly be ascribed to chance." These remarks appear to me to exhibit an entire misconception of the facts of variation as they actually occur, and as they have been utilised by natural selection in the modification of species. I have already shoAATi that every part of the organism, in common species, does vary to a very considerable amount, in a large number of indi^-iduals, and in the same locality ; the only point that remains to be discussed is, whether any or most of these variations are "beneficial." But every one of these variations consists either in increase or diminution of size or power of the organ or faculty that varies ; they can all be divided into a more effective and a less effective group — that is, into one that is more beneficial or less beneficial. If less size of body would be beneficial, then, as half the variations in size are above and half belpw the mean or existing standard of the species, there 144 DARWINISM chap. would be ample beneficial variations ; if a darker colour or a longer beak or wing were required, there are always a con- siderable number of individuals darker and lighter in colour than the average, with longer or with shorter beaks and wings, and thus the beneficial variation must always be present. And so Avith every other part, organ, function, or habit ; because, as variation, so far as we know, is and always must be in the two directions of excess and defect in relation to the mean amount, whichever kind of variation is wanted is always present in some degree, and thus the difficulty as to " beneficial " variations occurring, as if they were a special and rare class, falls to the ground. No doubt some organs may vary in three or perhaps more directions, as in the length, breadth, thickness, or curva- ture of the bill. But these may be taken as separate varia- tions, each of which again occurs as " more " or " less "; and thus the " right " or " beneficial " or " useful " variation must always be present so long as any variation at all occurs ; and it has not yet been proved that in any large or dominant species, or in any part, organ, or faculty of such species, there is no variation. And even were such a case found it would prove nothing, so long as in numerous other species variation was shown to exist ; because we know that great numbers of species and groups throughout all geological time have died out, leaving no descendants ; and the obvious and sufficient explanation of this fact is, that they did not vary enough at the time when varia- tion was required to bring them into harmony ^dth changed conditions. The objection as to the "right" or "beneficial" variation occurring when required, seems therefore to have no wei2;ht in view of the actual facts of variation. Isolation to ])revent Intercrossing. Most writers on the subject consider the isolation of a portion of a species a very important factor in the formation of new species, while others maintain it to be absolutely essential. This latter view has arisen from an exaggerated opinion as to the poAver of intercrossing to keep down any variety or incipient species, and merge it in the parent stock. But it is evident that this can only occur with varieties which are not useful, or which, if useful, occur in very small numbers; and from this kind of variations it is clear that VI DIFFICULTIES AND OBJECTIONS 145 new species do not arise. Complete isolation, as in an oceanic island, will no doubt enable natural selection to act more rapidly, for several reasons. In the first place, the absence of competition will for some time allow the new immigrants to increase rapidly till they reach the limits of subsistence. They will then struggle among themselves, and by survival of the fittest mil quickly become adapted to the new conditions of their environment. Organs which they formerly needed, to defend themselves against, or to escape from, enemies, being no longer required, would be encumbrances to be got rid of, while the power of appropriating and digesting new and varied food would rise in importance. Thus we may explain the origin of so many flightless and rather bulky birds in oceanic islands, as the dodo, the cassowary, and the extinct moas. Again, while this process was going on, the complete isolation would prevent its being checked by the immigration of new competitors or enemies, which would be very likely to occur in a continuous area ; while, of course, any intercrossing with the original unmodified stock would be absolutely pre- vented. If, now, before this change has gone very far, the variety spreads into adjacent but rather distant islands, the somewhat different conditions in each may lead to the development of distinct forms constituting what are termed representative species ; and these we find in the separate islands of the Galapagos, the West Indies, and other ancient groups of islands. But such cases as these A\all only lead to the production of a few peculiar species, descended from the original settlers which happened to reach the islands ; whereas, in wide areas, and in continents, we have variation and adaptation on a much larger scale ; and, whenever important physical changes de- mand them, \Wth even gTeater rapidity. The far greater complexity of the environment, together mth the occurrence of variations in constitution and habits, w\\\ often allow of effective isolation, even here, producing all the results of actual physical isolation. As we have already explained, one of the most frequent modes in which natural selection acts is by adapting some individuals of a species to a somewhat different mode of life, whereby they are able to seize upon unappropriated places in nature, and in so doing they become practically L 146 DARWINISM chap, isolated from their parent form. Let us suppose, for example, that one portion of a species usually living in forests ranges into the open plains, and finding al)un(lance of food remains there permanently. So long as the strnggle for existence is not exceptionally severe, these two portions of the species may remain almost unchanged ; but suppose some fresh enemies are attracted to the plains by the presence of these new immi- grants, then variation and natural selection would lead to the preservation of those individuals best able to cope with the difficulty, and thus the open country form would become modified into a marked variety or into a distinct sj^ecies ; and there would evidently be little chance of this modifica- tion being checked by intercrossing with the parent form which remained in the forest. Another mode of isolation is brought about by the variety — either owing to hal^its, climate, or constitutional change — breeding at a slightly different time from the parent species. This is known to produce complete isolation in the case of many varieties of plants. Yet another mode of isolation is brought about by changes of colour, and by the fact that in a wild state animals of similar colours prefer to keep together and refuse to pair with individuals of another colour. The probable reason and utility of this habit will be explained in another chapter, but the fact is well illustrated by the cattle which have run wild in the Falkland Islands. These are of several difi*erent colours, but each colour keeps in a separate herd, often restricted to one part of the island ; and one of these varieties — the mouse-coloiu-ed — is said to breed a month earlier than the others ; so that if this variety inhabited a larger area it might very soon be estab- lished as a distinct race or species.^ Of course where the change of habits or of station is still greater, as when a ter- restrial animal becomes sub-aquatic, or when aquatic animals come to live in tree -tops, as with the frogs and Crustacea described at p. 118, the danger of intercrossing is reduced to a minimum. Several writers, however, not content with the indirect effects of isolation here indicated, maintain that it is in itself a cause of modification, and ultimately of the origination of ^ See Variation of Animals and Plants, vol. i. p. 86. \l DIFFICULTIES AND OBJECTIONS 147 new species. This was the keynote of Mr. Vernon Wollaston's essay on "Variation of Species," published in 1856, and it is adopted by the Iter. J. G. Gulick in his paper on " Diversity of Evolution under one Set of External Conditions " (Journ. Linn. Soc. ZooL, vol. xi. p. 496). The idea seems to be that there is an inherent tendency to variation in certain divergent lines, and that when one portion of a species is isolated, even though under identical conditions, that tendency sets up a divergence which carries that portion farther and farther away from the original species. This view is held to be supported by the case of the land shells of the Sandwich Islands, which certainly present some very remarkable phenomena. In this comparatively small area there are about 300 species of land shells, almost all of which belong to one family (or sub- family), the Achatinellidae, found nowhere else in the world. The interesting point is the extreme restriction of the species and varieties. The average range of each species is only five or six miles, while some are restricted to but one or two square miles, and only a very few range over a whole island. The forest region that extends over one of the mountain -ranges of the island of Oahu, is about forty miles in length and five or six miles in breadth ; and this small territory furnishes about 175 species, represented by 700 or 800 varieties. Mr. Gulick states, that the vegetation of the different valleys on the same side of this range is much the same, yet each has a molluscan fauna differing in some degree from that of any other. " AVe frequently find a genus represented in several successive valleys by allied species, sometimes feeding on the same, sometimes on different plants. In every such case the valleys that are nearest to each other furnish the most nearly allied forms ; and a full set of the varieties of each species presents a minute gradation of forms between the more divergent types found in the more Avidely separated localities." He urges, that these constant differences cannot be attributed to natural selection, because they occur in different valleys on the same side of the mountain, where food, climate, and enemies are the same ; and also, because there is no greater difference in passing from the rainy to the dry side of the mountains than in passing from one valley to 148 DARWINISM another on the same side an eqnal distance apart. In a very- lengthy paper, i)resented to the Linnean Society last year, on "Divergent Evohition tlnough Cumulative Segregation," Mr. Gulick endeavours to work out his views into a complete theory, the main point of which may perhaps be indicated by the follo\Wng passage : " No two portions of a species possess exactly the same average character, and the initial differences are for ever reacting on the environment and on each other in such a way as to ensure increasing divergence in each successive generation as long as the individuals of the two groups are kept from intercrossing."^ It need hardly be said that the views of ]\Ir. Darwin and myself are inconsistent vnth. the notion that, if the environment were absolutely similar for the two isolated portions of the species, any such necessary and constant divergence would take place. It is an error to assume that what seem to us identical conditions are really identical to such small and delicate organisms as these land molluscs, of whose needs and difficulties at each successive stage of their existence, from the freshly-laid egg up to the adult animal, Ave are so profoundly ignorant. The exact proportions of the various species of plants, the numbers of each kind of insect or of bird, the peculiarities of more or less exj^osure to sunshine or to wind at certain critical epochs, and other slight differences which to us are absolutely immaterial and un- recognisable, may be of the highest significance to these humble creatures, and be quite sufficient to require some slight adjustments of size, form, or colour, which natural selection will bring about. All we know of the facts of variation leads us to believe that, without this action of natural selection, there would be produced over the whole area a series of inconstant varieties mingled together, not a distinct segregation of forms each confined to its own limited area. Mr. Darwin has shown that, in the distribution and modification of species, the biological is of more importance than the physical environment, the struggle with other organisms being often more severe than that with the forces of nature. This is particularly e\adent in the case of plants, many of which, when protected from competition, thrive in a ^ Journal of the Linnean Society, Zoology, vol. xx. p. 215. VI DIFFICULTIES AND OBJECTIONS 149 soil, climate, and atmosphere widely different from those of their native habitat. Thus, many alpine plants only found near perpetual snow thrive well in our gardens at the level of the sea ; as do the tritomas from the sultry plains of South Africa, the yuccas from the arid hills of Texas and Mexico, and the fuchsias from the damp and dreary shores of the Straits of Magellan. It has been well said that plants do not live where they like, but where they can ; and the same remark will apply to the animal world. Horses and cattle run Avild and thrive both in Xorth and South America ; rabbits, once con- fined to the south of Europe, have established themselves in our own country and in Australia ; while the domestic fowl, a native of tropical India, thrives well in every part of the temperate zone. If, then, we admit that when one portion of a species is separated from the rest, there will necessarily be a slight difference in the average characters of the two portions, it does not follow that this difference has much if any effect upon the characteristics that are developed by a long period of isolation. In the first place, the difference itself will necessarily be very slight unless there is an exceptional amount of variability in the species ; and in the next place, if the average characters of the sj^ecies are the expression of its exact adaptation to its Avhole environment, then, given a precisely similar environment, and the isolated portion will inevitably be brought back to the same average of characters. But, as a matter of fact, it is impossible that the environment of the isolated portion can be exactly like that of the bulk of the species. It cannot be so physically, since no two separated areas can be absolutely alike in climate and soil ; and even if these are the same, the geogi^aphical features, size, contoiu", and relation to winds, seas, and rivers, would certainly differ. Biologically, the differences are sure to be considerable. The isolated portion of a species ^Wll almost always be in a much smaller area than that occupied by the species as a whole, hence it is at once in a different position as regards its own kind. The proportions of all the other species of animals and plants are also sure to differ in the two areas, and some species will almost always be absent in the smaller Avhich are present in the larger country. These differences will act and react on 150 DARWINISM the isolated portion of the species. The struggle for existence will differ in its severity and in its incidence from that which aff'ects the bulk of the species. The absence of some one insect or other creature inimical to the young animal or plant may cause a vast diff'erence in its conditions of existence, and may necessitate a modification of its external or internal characters in quite a different direction from that Avhich happened to be present in the average of the individuals which were first isolated. On the whole, then, we conclude that, while isolation is an important factor in eff'ecting some modification of species, it is so, not on account of any effect produced, or influence exerted by isolation per se, but because it is always and necessarily accompanied by a change of environment, both physical and biological. Natural selection will then begin to act in adapting the isolated portion to its new conditions, and will do this the more quickly and the more effectually because of the isolation. We have, however, seen reason to believe that geographical or local isolation is by no means essential to the diff'erentiation of species, because the same result is brought about by the incipient species acquiring different habits or frequenting a different station ; and also by the fact that difl'erent varieties of the same species are known to prefer to pair with their like, and thus to bring about a physiological isolation of the most effective kind. This part of the subject will be again referred to when the very difficult problems presented by hybridity are discussed. ^ Cases in which Isolation is Ineffective. One objection to the views of those who, like Mr. Gulick, believe isolation itself to be a cause of modification of species deserves attention, namely, the entire absence of change where, ^ In Mr. Gulick's last paper {Journal of Linn. Soc. Zool.,\ol. xx. pp. 189- 274) he discusses tlie various forms of isolation above referred to, under no less than thirty-eight dilferent divisions and subdivisions, with an elaborate terminology, and he argues that these Avill frequently bring aliout divergent evolution without any change in tlie environment or any action of natural selection. The discussion of the problem here given will, I believe, sufficiently expose the fallacy of his contention ; but his illustration of the varied and often recondite modes by which practical isolation may be brought about, may help to remove one of the popular difficulties iu the way of the action of natural selection in the origination of species. VI DIFFICULTIES AND OBJECTIONS 151 if this were a vera causa, we should expect to find it. In Ireland we have an excellent test case, for we know that it has been separated from Britain since the end of the glacial epoch, certainly many thousand years. Yet hardly one of its mammals, reptiles, or land molluscs has undergone the slightest change, even although there is certainly a distinct difference in the environment both inorganic and organic. That changes have not occurred through natural selection, is perhaps due to the less severe struggle for existence owing to the smaller number of competing species ; but, if isolation itself were an efficient cause, acting continuously and cumula- tively, it is incredible that a decided change should not have been produced in thousands of years. That no such change has occurred in this, and many other cases of isolation, seems to prove that it is not in itself a cause of modification. There yet remain a number of difficulties and objections relating to the question of hybridity, which are so important as to require a separate chapter for their adequate discussion. CHAPTER VII ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING Statement of the problem — Extreme susceptibility of the reproductive functions — Reciprocal crosses — Individual differences in respect to cross - fertilisation — Dimorphism and trimorphism among plants — Cases of the fertility of hybrids and of the infertility of mongrels — The effects of close inter-brceding — Mr, Huth's objections — Fertile hybrids among animals— Fertility of hybrids among plants — Cases of sterility of mongrels — Parallelism between crossing and change of conditions — Remarks on the facts of hybridity — Sterility due to changed conditions and usually correlated •\vith other characters — Correlation of colour with constitutional peculiarities — The isolation of varieties by selective association — The influence of natural selection upon sterility and fertility — Physiological selection — Summary and concluding remarks. One of the greatest, or perhaps we may say the greatest, of all the clilhciilties in the way of accepting the theory of natural selection as a complete explanation of the origin of species, has been the remarkable difference between varieties and species in respect of fertility when crossed. Generally speaking, it may be said that the varieties of any one species, however different they may be in external appearance, are perfectly fertile when crossed, and their mongrel offspring are equally fertile when bred among themselves ; while distinct species, on the other hand, however closely they may resemble each other externally, are usually infertile when crossed, and their hybrid offspring absolutely sterile. This used to be considered a fixed law of nature, constituting the absolute test and criterion of a species as distinct from a variety ; and so long as it was believed that species were separate creations, or CHAP. VII OX THE IX FERTILITY OF CROSSES 153 at all events had an origin quite distinct from that of varieties, this law could have no exceptions, because, if any two species had been found to be fertile when crossed and their hybrid offspring to be also fertile, this fact would have been held to prove them to be not species but varieties. On the other hand, if two varieties had been found to be infertile, or their mongrel offspring to be sterile, then it would have been said : These are not varieties but true species. Thus the old theory led to ine^^table reasoning in a circle ; and what might be only a rather common fact was elevated into a law which had no exception:^. The elaborate and careful examination of the whole subject by Mr. Darwin, who has brought together a vast mass of evidence from the experience of agTiculturists and horti- culturists, as well as from scientific experimenters, has demon- strated that there is no such fixed laAv in natiu'e as was formerly supposed. He shows us that crosses between some varieties are infertile or even sterile, while crosses between some species are quite fertile ; and that there are besides a number of cuiious phenomena connected with the subject Avhich render It impossible to believe that sterility is anything more than an incidental property of species, due to the extreme delicacy and susceptibility of the reproductive powers, and dependent on physiological causes we have not yet been able to trace. Nevertheless, the fact remains that most species which have hitherto been crossed produce sterile hybrids, as in the well-known case of the mule ; while almost all domestic varieties, when crossed, produce offspring which are perfectly fertile among themselves. I will now endeavour to give such a sketch of the subject as may enable the reader to see some- thing of the complexity of the problem, referring him to ]\Ir. Darv.in's Avorks for fuller details. Extreme SuscepHhilitij of the Eeproductive Functions. One of the most interestins; facts, as showing; how sus- ceptible to changed conditions or to slight constitutional changes are the reproductive powers of animals, is the very general difficulty of getting those which are kept in confine- ment to breed ; and this is frequently the only bar to domesticating Avild species. Thus, elephants, bears, foxes. 154 DARWINJSM chap. and numbers of species of rodents, very rarely breed in confinement ; while other species do so more or less freely. Hawks, vultures, and owls hardly ever breed in confinement ; neither did the falcons kept for hawking ever breed. Of the numerous small seed- eating birds kept in aviaries, hardly any breed, neither do parrots. Gallinaceous birds usually breed freely in confinement, but , some do not ; and even the guans and curassows, kept tame by the South American Indians, never breed. This shows that change of climate has nothing to do ^vith the phenomenon ; and, in fact, the same species that refuse to breed in Europe do so, in almost every case, when tamed or confined in their native countries. This inability to reproduce is not due to ill -health, since many of these creatures are perfectly vigorous and live very long. With oui' true domestic animals, on the other hand, fertility is perfect, and is very little affected by changed conditions. Thus, we see the common fowl, a native of tropical India, living and multiplying in almost every part of the world ; and the same is the case with our cattle, sheep, and goats, our dogs and horses, and especially mth domestic pigeons. It therefore seems probable, that this facility for breeding under changed conditions was an original ])roperty of the species which man has domesticated — a property which, more than any other, enabled him to domesticate them. Yet, even with these, there is evidence that great changes of conditions affect the fertility. In the hot valleys of the Andes sheep are less fertile ; while geese taken to the high plateau of Bogota were at first almost sterile, but after some generations recovered their fertility. These and many other facts seem to show that, mth the majority of animals, even a slight change of conditions may produce infertility or sterility ; and also that after a time, when the animal has become thoroughly acclimatised, as it were, to the new conditions, the infertility is in some cases diminished or altogether ceases. It is stated by Bechstein that the canary was long infertile, and it is only of late years that good breeding birds have become common ; but in this case no doubt selection has aided the change. As showing that these phenomena depend on deep-seated causes and are of a very general nature, it is interesting VII ON THE INFERTILITY OF CROSSES 155 to note that they occur also in the vegetable kingdom. Allowing for all the circumstances which are known to prevent the production of seed, such as too great luxuriance of foliage, too little or too much heat, or the absence of insects to cross-fertilise the flowers, Mr. Dar^\4n shows that many species which gi'ow and flower with us, apparently in perfect health, yet never produce seed. Other plants are affected by very slight changes of conditions, producing seed freely in one soil and not in another, though apparently growing equally Avell in both ; while, in some cases, a difference of position even in the same garden produces a similar res alt. ^ Reciprocal Crosses. Another indication of the extreme delicacy of the adjustment between the sexes, which is necessary to produce fertility, is afforded by the behaviour of many species and varieties when reciprocally crossed. This will be best illustrated by a few of the examples furnished us by Mr. Dar\\in. The two distinct species of plants, Mirabilis jalapa and M. longiflora, can be easily crossed, and will produce healthy and fertile hybrids when the pollen of the latter is applied to the stigma of the former plant. But the same experimenter, Kolreuter, tried in vain, more than two hundred times during eight years, to cross them by applying the pollen of M. jalapa to the stigma of M. longiflora. In other cases two plants are so closely allied that some botanists class them as varieties (as with. Matthiola annua and ^I. glabra), and yet there is the same great difference in the result when they are reciprocally crossed. Individual Differences in respect to Cross-Fertilisation. A still more remarkable illustration of the delicate balance of organisation needful for reproduction, is afforded by the individual differences of animals and plants, as regards both their power of intercrossing A\ith other individuals or other species, and the fertility of the offspring thus produced. Among domestic animals, Darwin states that it is by no means rare to find certain males and females which will not breed ^ Darwin's Animals and Plants under Domestication, vol. ii. pp. 163-170. 156 DARWINISM chap. together, though both are known to be perfectly fertile with other males and females. Cases of this kind have occurred among horses, cattle, pigs, dogs, and pigeons ; and the experiment has been tried so frequently that there can be no doubt of the fact. Professor G. J. Romanes states that he has a number of additional cases of this individual incom- patibility, or of absolute sterility^ between two individuals, each of which is perfectly fertile with other individuals. During the numerous exj^eriments that have been made on the h}'1);idisation of plants similar peculiarities have been noticed, some individuals being capable, others incapable, of being crossed with a distinct species. The same individual peculiarities are found in varieties, species, and genera. Kolreuter crossed five varieties of the common tobacco (Nicotiana tabacum) with a distinct species, Nicotiana glutinosa, and they all yielded very sterile hybrids ; but those raised from one variety were less sterile, in all the experiments, than the hybrids from the four other varieties. Again, most of the species of the genus Nicotiana have been crossed, and freely produce hybrids ; but one species, N. acuminata, not particularly distinct from the others, could neither fertilise, nor be fertilised by, any of the eight other species experimented on. Among genera we find some — such as Hippeastrum, Crinum, Calceolaria, Dianthus — almost all the species of which will fertilise other species and produce hybrid offspring ; while other allied genera, as Zejjhyranthes and Silene, notwithstanding the most persevering efforts, have not produced a single hybrid even between the most closely allied species. Dimorphism and Trimorphism. Peculiarities in the reproductive system aff'ecting indi- viduals of the same species reach their maximum in what are called heterostyled, or dimorphic and trimorphic floAvers, the i)henoniena presented by which form one of the most lemarkable of Mr. Dar^vin's many discoveries. Our common cowslip and primrose, as well as many other species of the genus Primula, have two kinds of flowers in about equal I)roportions. In one kind the stamens are short, being situated about the middle of the tube of the corolla, while the vii OX THE INFERTILITY OF CROSSES 157 style is long, the globular stigma appearing just in the centre of the open flower. In the other kind the stamens are long, appearing in the centre or throat of the floAver, while the style is short, the stigma being situated halfway down the tube at the same level as the stamens in the other form. These two forms have long been known to florists as the "pin-eyed" and the "thrum -eyed," but they are called by Darwin the long-styled and short-styled forms (see woodcut). Long-styled form. Short-styled foiin. Fig, 17.— Primula veris (Cowslip). The meaning and use of these different forms was quite unknown till Darwin discovered, first, that cowslips and primroses are absolutely barren if insects are prevented from visiting them, and then, what is still more extraordinary, that each form is almost sterile when fertilised by its own pollen, and comparatively infertile when crossed mth any other plant of its own form, but is perfectly fertile when the pollen of a long -styled is carried to the stigma of a short -styled plant, or vice versa. It wiU be seen, by the figures, that the arrangement is such that a bee \nsiting the flowers ^s^\\ carry the pollen from the long anthers of the short -styled form to the stisnia of the loni<- styled form, while it Avould never reach the stigma of another plant of the short- styled form. 158 DARWINISM chap. But an insect visiting, first, a long-styled plant, "would deposit the pollen on the stigma of another plant of the same kind if it were next \'isited ; and this is i)roha})ly the reason why the wild short-styled phmts Avere found to he almost always most productive of seed, since they must be all fertilised by the other form, whereas the long- styled plants might often be fertilised by their own form. The whole arrangement, however, ensures cross-fertilisation ; and this, as Mr. Darwin has shown by copious experiments, adds both to the vigour and fertility of almost all plants as well as animals. Besides the primrose family, many other plants of several distinct natural orders pr-esent similar phenomena, one or two of the most curious of which must be referred to. The beautiful crimson flax (Linum grandiflorum) has also two forms, the styles only differing in length ; and in this case Mr. Darwin found by nuinerous experiments, which have since been repeated and confirmed by other observers, that each form is ab3 )]'itoly sterile with pollen from another plant of its own form, but abundantly fertile when crossed with any plant of the other form. In this case the pollen of the two forms cannot be distinguished under the microscope (whereas that of the two forms of Primula differs in size and shape), yet it has the remarkable property of being absolutely powerless on the stigmas of half the plants of its owii species. The crosses between the opposite forms, which are fertile, are termed by Mr. Darwin "legitimate," and those between similar forms, which are sterile, "illegitimate"; and he remarks that we have here, within the limits of the same species, a degree of sterility which rarely occurs except between plants or animals not only of different species but of different genera. But there is another set of plants, the trimorphic, in which the styles and stamens have each three forms — long, medium, and short, and in these it is possible to have eighteen different crosses. By an elaborate series of experiments it was shown that the six legitimate unions — that is, when a plant was fertilised by pollen from stamens of length corresponding to that of its style in the two other forms — were all abundantly fertile ; while the twelve illegitimate unions, when a plant was fertilised by pollen from stamens of a different length from its vn ON THE INFERTILITY OF CROSSES 159 own style, in any of the three forms, were either comparatively or wholly sterile. ^ We have here a wonderful amount of constitutional difference of the reproductive organs within a single species, greater than usually occurs A^thin the numerous distinct species of a genus or group of genera ; and all this diversity appears to have arisen for a purpose which has been obtained by many other, and apparently simpler, changes of structure or of function, in other plants. This seems to show us, in the first place, that variations in the mutual relations of the repro- ductive organs of different individuals must be as frequent as structural variations have been shown to be ; and, also, that sterility in itself can be no test of specific distinctness. But this point ^^all be better considered when ^ve have further illustrated and discussed the complex phenomena of hybridity. Cases of the Fertility of Hybrids, and of the Infertility of Mongrels. I now propose to adduce a few cases in which it has been proved, by experiment, that hybrids between two distinct species are fertile inter se; and then to consider why it is that such cases are so few in number. The common domestic goose (Anser ferus) and the Chinese goose (A. cygnoides) are very distinct species, so distinct that some naturalists have placed them in different genera ; yet they have bred together, and Mr. Eyton raised from a pair of these hybrids a Ijrood of eight. This fact was confirmed by j\lr. Darwin himself, who raised several fine birds from a pair of hybrids which were sent him.'- In India, according to Mr. Blyth and Captain Hutton, whole flocks of these hybrid geese are kept in various parts of the country where neither of the pure i:)arent species exists, and as they are kept for profit they must certainly be fully fertile. Another equally striking case is that of the Indian humped and the common cattle, species which differ osteologically, and also in habits, form, voice, and constitution, so that they are by no means closely allied ; yet Mr. Darwin assures us that he ^ For a full account of these interestiBg facts and of the various problems to which they give rise, the reader must consult Darwin's volume on The Different Forms of Flowers in Plants of the same Species, chaps, i.-iv. * See Nature, vol. xxi. p. 207. 160 DARWINISM chap. has received decisive evidence that the hybrids between these are perfectly fertile inter se. Dogs have been frequently crossed with wolves and with jackals, and theii- hybrid ottspring have been found to be fertile inter se to the third or fourth generation, and then usually to show some signs of sterility or of deterioration. The wolf and dog may be originally the same species, but the jackal is certainly distinct ; and the appearance of infertility or of weak- ness is probably due to the fact that, in almost all these experi- ments, the ofispring of a single pair — themselves usually from the same litter — were bred in-and-in, and this alone sometimes produces the most deleterious effects. Thus, Mr. Low in his great work on the Domesticated Animals of Great Britain, says : "If we shall breed a pair of dogs from the same litter, and unite again the offspring of this pair, we shall produce at once a feeble race of creatures ; and the process being repeated for one or two generations more, the family will die out, or be incapable of propagating their race. A gentleman of Scotland made the experiment on a large scale with certain foxhounds, and he found that the race actually became monstrous and perished utterly." The same writer tells us that hogs have been made the subject of similar experiments : " After a few generations the victims manifest the change induced in the system. They become of diminished size ; the bristles are changed into hairs ; the limbs become feeble and short ; the litters diminish in frequency, and in the number of the young produced ; the mother becomes unable to nourish them, and, if the experiment be carried as far as the case will allow, the feeble, and frequently monstrous offspring, will be incapable of being reared up, and the miserable race will utterly perish. "^ These precise statements, by one of the greatest authorities on our domesticated animals, are sufficient to show that the fact of infertility or degeneracy appearing in the oflspring of hybrids after a few generations need not be imputed to the fact of the first parents being distinct species, since exactly the same phenomena appear when individuals of the same species are bred under similar adverse conditions. But in almost all the experiments that have hitherto been made in crossing distinct species, no care has been taken to avoid close inter- ^ Low's Domesticated Animals of Great Britain, Introduction, p. Ixiv. VII ON THE IXFERTILITY OF CROSSES 161 breeding by securing several hybrids from quite distinct stocks to start with, and by having two or more sets of experi- ments carried on at once, so that crosses between the hybrids produced may be occasionally made. Till this is done no experiments, such as those hitherto tried, can be held to prove that hybrids are in all cases infertile inter se. It has, however, been denied by Mr. A. H. Huth, in his interesting work on The Marriage of Near Kin, that any amount of breeding in-and-in is in itself hurtful ; and he quotes the evidence of numerous breeders whose choicest stocks have always been so bred, as well as cases like the Porto Santo rabbits, the goats of Juan Fernandez, and other cases in which animals allowed to run wild have increased prodigiously and continued in perfect health and vigour, although all derived from a single pair. But in all these cases there has been rigid selection by which the weak or the infertile have been eliminated, and mth such selection there is no doubt that the ill effects of close interbreeding can be prevented for a long time ; but this by no means proves that no ill effects are pro- duced. Mr. Huth himself quotes M. Allie, M. Aube, Stephens, Giblett, Sir John Sebright, Youatt, Druce, Lord Weston, and other eminent breeders, as finding from experience that close interbreeding does produce bad effects ; and it cannot be supposed that there would be such a consensus of opinion on this point if the e^il were altogether imaginary. Mr. Huth argues, that the evil results which do occur do not depend on the close interbreeding itself, but on the tendency it has to perpetuate any constitutional weakness or other hereditary taints ; and he attempts to prove this by the argu- ment that " if crosses act by ^artue of being a cross, and not by virtue of removing an hereditary taint, then the greater the difference between the two animals crossed the more beneficial will that act be." He then shows that, the wider the difference the less is the benefit, and concludes that a cross, as such, has no beneficial effect. A parallel argument would be, that change of air, as from inland to the sea-coast, or from a low to an elevated site, is not beneficial in itself, because, if so, a change to the tropics or to the polar regions should be more beneficial. In both these cases it may well be that no benefit would accrue to a person in perfect health ; but then there is no M 162 DARWINISM such thing as " perfect health " in man, and probably no such thing as absolute freedom from constitutional taint in animals. The experiments of Mr. Darwin, showing the great and immediate good effects of a cross between distinct strains in plants, cannot be explained away ; neither can the innumerable arrangements to secure cross-fertilisation by insects, the real use and purport of which will be discussed in our eleventh chapter. On the whole, then, the evidence at our command proves that, whatever may be its ultimate cause, close inter- breeding does usually produce bad results ; and it is only by the most rigid selection, whether natural or artificial, that the danger can be altogether obviated. Fertile Hyhrids among Animals. One or two more cases of fertile hybrids may be given before we pass on to the corresponding experiments in plants. Professor Alfred Newton received from a friend a pair of hybrid ducks, bred from a common duck (Anas boschas), and a pintail (Dafila acuta). From these he obtained four ducklings, but these latter, when grown up, proved infertile, and did not breed again. In this case we have the results of close inter- breeding, with too great a difference between the original species, combining to produce infertility, yet the fact of a hybrid from such a pair producing healthy offspring is itself noteworthy. Still more extraordinary is the foUoMang statement of Mr. Low : "It has been long kno'WTi to shepherds, though ques- tioned by naturalists, that the progeny of the cross between the sheep and goat is fertile. Breeds of this mixed race are numerous in the north of Europe." ^ Nothing appears to be known of such hybrids either in Scandinavia or in Italy ; but Professor Giglioli of Florence has kindly given me some useful references to works in which they are described. The following extract from his letter is very interesting : " I need not tell you that there being such hybrids is now generally accepted as a fact. Bulfon {Sujrplements, tom. iii. p. 7, 1756) obtained one such hybrid in 1751 and eight in 1752. Sanson (La Cidhire, vol. vi. p. 372, 1865) mentions a case observed in the Yosges, France. Geoff. St. Hilaire {Hist Nat. G4n. des reg. org., vol. iii p. ^ Low's Domesticated Animals, p. 28. VII ON THE INFERTILITY OF CROSSES 163 163) was the first to mention, I believe, that in different parts of South America the ram is more usually crossed with the she-goat than the sheep with the he-goat. The well-known 'pellones' of Chile are produced by the second and third generation of such hybrids (Gay, ' Hist, de Chile,' vol. i. p. 466, Agriculture, 1862). Hybrids bred from goat and sheep are called 'chabin' in French, and 'cabruno' in Spanish. In Chile such hybrids are called 'carneros lanudos'; their breed- ing inter se appears to be not always successful, and often the original cross has to be recommenced to obtain the proportion of three-eighths of he-goat and five-eighths of sheep, or of three- eighths of ram and five-eighths of she-goat ; such being the reputed best hybrids." With these numerous facts recorded by competent observers we can hardly doubt that races of hybrids between these very distinct species have been produced, and that such hybrids are fairly fertile inter se; and the analogous facts already given lead us to believe that whatever amount of infertility may at first exist could be eliminated by careful selection, if the crossed races were bred in large numbers and over a considerable area of country. This case is especially valuable, as showing how careful we should be in assuming the infertility of hybrids when experiments have been made with the progeny of a single pair, and have been continued only for one or two generations. Among insects one case only appears to have been recorded. The hybrids of two moths (Bombyx cynthia and B. arrindia) were proved in Paris, according to M. Quatrefages, to be fertile inter se for eight generations. Fertility of Hybrids among Plants. Among plants the cases of fertile hybrids are more numerous, owing, in part, to the large scale on which they are grown by gardeners and nurserymen, and to the greater facility with which experiments can be made. Darwin tells us that Kolreuter found ten cases in which two plants considered by botanists to be distinct species were quite fertile together, and he there- fore ranked them all as varieties of each other. In some cases these were grown for six to ten successive generations, but after a time the fertility decreased, as we saw to be the case in 164 DARWINISM animals, and presumably from the same cause, too close inter- breeding. Dean Herbert, who carried on experiments ^\ath great care and skill for many years, found numerous cases of hybrids which were perfectly fertile inter se. Crinum capense, fertilised by three other species — C. pedunculatum, C. canaliculatum, or C. defixum — all very distinct from it, produced perfectly fertile hybrids ; while other species less different in appearance were quite sterile with the same C. capense. All the species of the genus Hippeastrum produce hybrid offspring which are invariably fertile. Lobelia syphylitica and L. fulgens, two very distinct species, have produced a hybrid which has been named Lobelia speciosa, and w^hich reproduces itself abundantly. Many of the beautiful pelargoniums of our greenhouses are hybrids, such as P. ignescens from a cross between P. citrinodorum and P. fulgidum, which is quite fertile, and has become the parent of innumerable varieties of beautiful plants. All the varied species of Calceolaria, how- ever different in appearance, intermix with the greatest readi- ness, and the hybrids are all more or less fertile. But the most remarkable case is that of two species of Petunia, of which Dean Herbert says : "It is very remarkable that, although there is a great difference in the form of the flower, especially of the tube, of P. nyctanigenseflora and P. phoenicea the mules between them are not only fertile, but I have found them seed much more freely with me than either parent. . . . . From a pod of the above-mentioned mule, to which no pollen but its own had access, I had a large batch of seed- lings in which there was no variability or difference from itself ; and it is evident that the mule planted by itself, in a congenial climate, would reproduce itself as a species ; at least as much deserving to be so considered as the various Calceo- larias of different districts of South America. "^ Darwin was informed by Mr. C. Xoble that he raises stocks for grafting from a hybrid between Rhododendron ponticum and R catawbiense, and that this hybrid seeds as freely as it is possible to imagine. He adds that horticulturists raise large beds of the same hybrid, and such alone are fairly treated ; for, by insect agency, the several individuals are freely ^ A7naryllidacece, by tlie Hon. and Rev. William Herbert, p. 379. VII ON THE INFERTILITY OF CROSSES 165 crossed A\4th each other, and the injurious influence of close interbreeding is thus prevented. Had hybrids, when fairly treated, always gone on decreasing in fertility in each suc- cessive generation, as Gartner believed to be the case, the fact would have been notorious to nurserymen. ^ Cases of Sterility of Mongrels. The reverse phenomenon to the fertility of hybrids, the sterility of mongrels or of the crosses between varieties of the same species, is a comparatively rare one, yet some undoubted cases have occurred. Gartner, who believed in the absolute distinctness of species and varieties, had two varieties of maize — one dwarf with yellow seeds, the other taller M-ith red seeds ; yet they never naturally crossed, and, when fertilised artificially, only a single head produced any seeds, and this one only five grains. Yet these few seeds were fertile ; so that in this case the first cross was almost sterile, though the hybrid when at length produced was fertile. In like manner, dis- similarly coloured varieties of Yerbascum or mullein have been found by two distinct observers to be comparatively infertile. The two pimpernels (Anagallis arvensis and A. coerulea), classed by most botanists as varieties of one species, have been found, after repeated trials, to be perfectly sterile when crossed. No cases of this kind are recorded among animals ; but this is not to be wondered at, when we consider how very few experiments have been made ^\^th natural varieties; while there is good reason for belie\ang that domestic varieties are exceptionally fertile, partly because one of the conditions of domestication was fertility under changed conditions, and also because long continued domestication is believed to have the efl'ect of increasing fertility and eliminating whatever sterility may exist. This is shown by the fact that, in many cases, domestic animals are descended from two or more distinct species. This is almost certainly the case with the dog, and probably with the hog, the ox, and the sheep ; yet the various breeds are now all perfectly fertile, although we have every reason to suppose that there would be some degree of infer- tility if the several aboriginal species were crossed together for the first time. ^ Origin of Species, p. 239. 166 DARWINISM chap. Parallelism between Crossing and Change of Conditions. In the whole series of these phenomena, from the beneficial effects of the crossing of different stocks and the evil effects of close interbreeding, up to the partial or complete sterility induced by crosses between species belonging to different genera, we have, as Mr. Darwin j^oints out. a curious parallelism with the effects produced by change of physical conditions. It is well known that slight changes in the conditions of life are beneficial to all living things. Plants, if constantly grown in one soil and locality from their own seeds, are greatly benefited by the importation of seed from some other locality. The same thing happens with animals ; and the benefit we our- selves experience from "change of air "is an illustration of the same phenomenon. But the amount of the change Avhich is beneficial has its limits, and then a greater amount is injurious. A change to a climate a few degrees warmer or colder may be good, while a change to the tropics or to the arctic regions might be injurious. Thus we see that, both slight changes of conditions and a slight amount of crossing, are beneficial; while extreme changes, and crosses between individuals too far removed in structure or constitution, are injurious. And there is not only a parallelism but an actual connection between the two classes of facts, for, as Ave have already shown, many species of animals and plants are rendered infertile, or altogether sterile, by the change from their natural conditions which occurs in confinement or in cultivation ; while, on the other hand, the increased vigour or fertility which is invariably pro- duced by a judicious cross may be also eflfected by a judicious change of climate and surroundings. We shall see in a subse- quent chapter, that this interchangeability of the beneficial effects of crossing and of new conditions, serves to explain some very puzzling phenomena in the forms and economy of flowers. Remarks on the Fads of Hyhridity. The facts that have now been adduced, though not very numerous, are sufficiently conclusive to prove that the old belief, of the universal sterility of hybrids and fertility of mongrels, is incorrect. The doctrine that such a universal VII ox THE INFERTILITY OF CROSSES 167 law existed was never more than a plausible generalisa- tion, founded on a few inconclusive facts derived from domesticated animals and cultiA'ated plants. The facts were, and still are, inconclusive for several reasons. They are founded, primarily, on what occurs among animals in domestication ; and it has been shown that domestication both tends to increase fertility, and Avas itself rendered possible by the fertility of those particular species being little affected by changed conditions. The exceptional fertility of all the varieties of domesticated animals does not prove that a similar fertility exists among natural varieties. In the next place, the generalisation is founded on too remote crosses, as in the case of the horse and the ass, the two most distinct and widely separated species of the genus Eciuus, so distinct indeed that they have been held by some naturalists to form distinct genera. Crosses between the two species of zebra, or even between the zebra and the cpagga, or the cpiagga and the ass, might have led to a very different result. Again, in pre- Darwinian times it was so universally the practice to argue in a circle, and declare that the fertility of the offsj^ring of a cross proved the identity of species of the parents, that experi- ments in hybridity were usually made between very remote species and even between species of different genera, to avoid the possibility of the reply : "They are both really the same species ;" and the sterility of the hybrid offspring of such remote crosses of com^se served to strengthen the popular belief. Xow that we have arrived at a different standpoint, and look upon a species, not as a distinct entity due to special creation, but as an assemblage of individuals which have become somewhat modified in structure, form, and constitution so as to adapt them to slightly different conditions of life ; which can be differentiated from other allied assemblages ; which reproduce their like, and which usually breed together — we require a fresh set of experiments calculated to determine the matter of fact, — whether such species crossed vriih their near allies do always produce offspring which are more or less sterile inter se. Ample materials for such experiments exist, in the numerous " representative species " inhabiting distinct areas on a continent or different islands of a group ; or even 168 DARWINISM chap. in those found in the same area but frequenting somewhat (lifterent stations. To carry out these experiments with any satisfactory result, it will be necessary to avoid the evil effects of confinement and of too close interbreeding. If birds are experimented with, they should be allowed as much liberty as possible, a plot of ground with trees and b.ushes being enclosed with wire netting overhead so as to form a large open aviary. The species experimented with should be obtained in con- siderable numbers, and by two separate persons, each making the opposite reciprocal cross, as explained at p. 155. In the second generation these two stocks might be themselves crossed to prevent the evil effects of too close interbreeding. By such experiments, carefully carried out with different groups of animals and plants, Ave should obtain a l)ody of facts of a character now sadly wanting, and without which it is hopeless to expect to arrive at a complete solution of this difficult problem. There are, hoAvever, some other aspects of the question that need to be considered, and some theoretical views Avhich require to be carefully examined, haA-ing done Avhich Ave shall be in a condition to state the general con- clusions to Avhich the facts and reasonings at our command seem to point. Sterilitjj due fo chanr/ed Conditions and usually correlated with other Characters, espcchdhj trith Colour. The evidence already adduced as to the extreme suscep- tibility of the reproductive system, and the curious irregu- larity Avith Avhich infertility or sterility apj^ears in the crosses between some varieties or species Avhile quite absent in those betAveen others, seem to indicate that sterility is a charac- teristic Avhich has a constant tendency to appear, either by itself or in correlation Avith other characters. It is knoAvn to be especially liable to occur under changed conditions of life ; and, as such change is usually the starting-point and. cause of the deA'elopment of ncAv species, Ave haA^e already found a reason Avhy it should so often appear Avhen species become fully differentiated. In almost all the cases of infertility or sterility betAA-een varieties or species, we have some external differences Avith VII ON THE INFERTILITY OF CROSSES 169 which it is correlated ; and though these differences are sometimes slight, and the amount of the infertility is not always, or even usually, proportionate to the external dif- ference between the two forms crossed, we must believe that there is some connection between the two classes of facts. This is especially the case as regards colour ; and j\Ir. Darmn has collected a body of facts which go far to prove that colour, instead of being an altogether trifling and un- important character, as was supposed by the older natural- ists, is really one of great significance, since it is un- doubtedly often correlated "sWth important constitutional differences. Now colour is one of the characters that most usually distinguishes closely allied species ; and when we hear that the most closely allied species of plants are infertile together, while those more remote are fertile, the meaning usually is that the former differ chiefly in the colour of their flowers, while the latter differ in the form of the flowers or foliage, in habit, or in other structural characters. It is therefore a most curious and suggestive fact, that in all the recorded cases, in which a decided infertility occurs between varieties of the same species, those varieties are distinguished by a difference of colour. The infertile varieties of Yerbascum were white and yellow flowered respectively ; the infertile varieties of maize were red and yellow seeded; while the infertile pimpernels were the red and the blue flowered varieties. So, the differently coloured varieties of hollyhocks, though grown close together, each reproduce their own colour from seed, showing that they are not capable of freely intercrossing. Yet Mr. Darwin assures us that the agency of bees is necessary to carry the pollen from one plant to another, because in each flower the pollen is shed before the stigma is ready to receive it. ^Ye have here, therefore, either almost complete sterility between varieties of different colours, or a prepotent effect of jDollen from a flower of the same colour, bringing about the same result. Similar phenomena have not been recorded among animals ; but this is not to be Avondered at when we consider that most of our pure and valued domestic breeds are characterised by definite colours which constitute one of theii* 170 DARWINISM chap. distinctive marks, and they are, therefore, seldom crossed with these of another colour ; and even when they are so crossed, no notice would be taken of any slight diminution of fertility, since this is liable to occur from many causes. "We have also reason to believe that fertility has been increased by long domestica- tion, in addition to the fact of the original stocks being exceptionally fertile ; and no experiments have been made on the differently coloured varieties of wild animals. There are, however, a number of very curious facts showing that colour in animals, as in plants, is often correlated with constitutional differences of a remarkable kind, and as these have a close relation to the subject we are discussing, a brief summary of them will be here given. Correlation of Colour ivith Constitutional Peculiarities. The correlation of a white colour and blue eyes in male cats with deafness, and of the tortoise-shell marking Mdth the female sex of the same animal, are two well-known but most extraordinary cases. Equally remarkable is the fact, com- municated to DarAvin by Mr. Tegetmeier, that white, yellow, pale blue, or dun pigeons, of all breeds, have the young birds born naked, while in all other colours they are well covered Avith down. Here we have a case in which colour seems of more jjhysiological importance than all the varied structural differences between the varieties and breeds of pigeons. In Virginia there is a plant called the paint-root (Lachnanthes tinctoria), which, when eaten by pigs, colours their bones pink, and causes the hoofs of all but the black varieties to drop off; so that black pigs only can be kept in the district.^ Buckwheat in flower is also said to be injurious to white pigs but not to black. In the Tarentino, black sheep are not injured by eating the Hypericum crispum — a species of St. John's-wort — which kills white sheep. White terriers suffer most from distemper , white chickens from the gapes. White-haired horses or cattle are subject to cutaneous diseases from which the dark coloured are free ; while, both in Thuringia and the West Indies, it has been noticed that white or pale coloured cattle are much more troubled by tiies than are those which are brown or black. The same law even extends ^ Origin of Species, sixth edition, p. 9. VII ox THE IXFERTILITY OF CROSSES 171 to insects, for it is found that silkworms which produce white cocoons resist the fungus disease much better than do those which produce yellow cocoons, i Among plants, we have in North America gi'een and yellow-fruited plums not affected by a disease that attacked the purple-fruited varieties. Yellow- fleshed peaches suffer more from disease than white-fleshed kinds. In Mauritius, white sugar-canes were attacked by a disease from which the red canes were free. White onions and verbenas are most liable to mildew ; and red-flowered hyacinths were more injui'ed by the cold during a severe "winter in Holland than any other kinds. "^ These curious and inexplicable correlations of colour with constitutional peculiarities, both in animals and plants, render it probable that the correlation of colour M-ith infertility, which has been detected in several cases in plants, may also extend to animals in a state of nature ; and if so, the fact is of the highest importance as thro^ving light on the origin of the infertility of many allied species. This will be better understood after considering the facts which vriW be now described. The Isolation of Varieties bij Selective Association. In the last chapter I have shown that the importance of geographical isolation for the formation of new species by natural selection has been greatly exaggerated, because the ^ In the Medico-Chirurgical Transactions, vol. liii. (1870), Dr. Ogle has adduced some curious physiological facts bearing on the presence or absence of white colours in the higher animals. He states that a dark pigment in the olfactory region of the nostrils is essential to perfect smell, and that this pigment is rarely deficient except when the whole animal is pure white, and the creature is then almost without smell or taste. Ke observes that there is no proof that, in any of the cases given above, the black animals actually eat the poisonous root or plant ; and that the facts are readily imderstood if the senses of smell and taste are dependent on a pigment which is absent in the white animals, who therefore eat what those gifted with normal senses avoid. This explanation however hardly seems to cover the facts. We cannot sup- pose that almost all the sheep in the world (which arc mostly white) are without smell or taste. The cutaneous disease on the white patches of hair on horses, the special liability of white terriers to distemper, of white chickens to the gapes, and of silkworms which produce yellow silk to the fungus, are not explained by it. The analogous facts in plants also indicate a real con- stitutional relation with colour, not an affection of the sense of smell and taste only. - For all these facts, see Aninials and Plants under Domestication, vol. it pp. 335-338. 172 DARWINISM very change of conditions, which is the initial power in starting such new forms, leads also to a local or stational segregation of the forms acted upon. But there is also a very powerful cause of isolation in the mental nature — the likes and dislikes — of animals ; and to this is probably due the fact of the comparative rarity of hybrids in a state of nature. The differently coloured herds of cattle in the Falkland Islands, each of which keeps separate, have been already mentioned ; and it may be added, that the white variety seem to have already developed a physiological peculiarity in breed- ing three months earlier than the others. Similar facts occur, however, among our domestic animals and are well kno^vn to breeders. Professor Low, one of the greatest authorities on our domesticated animals, says : " The female of the dog, when not under restraint, makes selection of her mate, the mastiff selecting the mastiff, the terrier the terrier, and so on." And again : " The Merino sheep and Heath sheep of Scotland, if two flocks are mixed together, each Avill breed with its own variety." Mr. Darmn has collected many facts illustrating this point. One of the chief pigeon-fanciers in England informed him that, if free to choose, each breed would prefer pairing with its own kind. Among the wild horses in Para- guay those of the same colour and size associate together; while in Circassia there are three races of horses which have received special names, and which, when living a free life, almost always refuse to mingle and cross, and mil even attack one another. On one of the Faroe Islands, not more than half a mile in diameter, the half-^\dld native black sheep do not readily mix with imported white sheep. In the Forest of Dean, and in the New Forest, the dark and pale coloured herds of fallow deer have never been known to mingle ; and even the curious Ancon sheep of quite modern origin have been observed to keep together, separating them- selves from the rest of the flock Avhen put into enclosures with other sheep. The same rule applies to birds, for Dar^vin was informed by the Eev. W. I). Fox that his flocks of Avhite and Chinese geese kept distinct.^ This constant preference of animals for their like, even in the case of slightly different varieties of the same species, is evidently ^ Animals and Plants under Domestication, vol. ii. pp. 102, 103. VII ON THE INFERTILITY OF CROSSES 173 a fact of great importance in considering the origin of species by natural selection, since it shows us that, so soon as a slight differentiation of form or colour has been effected, isolation will at once arise by the selective association of the animals themselves; and thus the great stumbling-block of "the swamping effects of intercrossing," which has been so pro- minently brought forward by many naturalists, will be com- pletely obviated. If now we combine with this fact the correlation of colour with important constitutional peculiarities, and, in some cases, mth infertility ; and consider, further, the curious parallelism that has been sho^vn to exist between the effects of changed conditions and the intercrossing of varieties in producing either an increase or a decrease of fertility, we shall have obtained, at all events, a starting-point for the production of that infertility which is so characteristic a feature of distinct species when intercrossed. All we need, now, is some means of increasing or accumulating this initial tendency ; and to a discussion of this problem we ^Wll therefore address ourselves. The Infl.uence of Xatural Selection upon Sterility and Fertility. It will occur to many persons that, as the infertility or sterility of incipient species would be useful to them when occupying the same or adjacent areas, by neutralising the effects of intercrossing, this infertility might have been in- creased by the action of natural selection ; and this will be thought the more probable if we admit, as we have seen reason to do, that variations in fertility occur, perhaps as frequently as other variations. Mr. Darwin tells us that, at one time, this appeared to him probable, but he found the problem to be one of extreme complexity ; and he was also influenced against the view by many considerations which seemed to render such an origin of the sterility or infertility of species when intercrossed very improbable. The fact that species which occupy distinct areas, and which nowhere come in contact Avith each other, are often sterile when crossed, is one of the difficulties ; but this may perhaps be overcome by the consideration that, though now isolated, they may, and often must, have been in contact at their origination. More important is the objection that natural selection could not 174 DARWINISM chap. possibly have produced the diiference that often occurs between reciprocal crosses, one of these being sometimes fertile, M'hile the other is sterile. The extremely different amounts of infertility or sterility between different species of the same genus, the infertility often bearing no proportion to the difference between the species crossed, is also an important objection. But none of these objections would have much weight if it could be clearly shown that natural selection is able to increase the infertility variations of in- cipient species, as it is certainly able to increase and develop all useful variations of form, structure, instincts, or habits. Ample causes of infertility have been shown to exist, in the nature of the organism and the laws of correlation; the agency of natural selection is only needed to accumulate the effects produced by these causes, and to render their final results more uniform and more in accordance with the facts that exist. About twenty years ago I had much correspondence and discussion with Mr. Darwin on this question. I then believed that I was able to demonstrate the action of natural selection in accumulating infertility ; but I could not convince him, owing to the extreme complexity of the process under the conditions which he thought most probable. I have recently returned to the question ; and, with the fuller knowledge of the facts of variation we now possess, I think it may be shown that natural selection h, in some probable cases at all events, able to accumulate variations in infertility between incipient species. The simplest case to consider, will be that in which tAvo forms or varieties of a species, occupying an extensive area, are in process of adaptation to somewhat different modes of life within the same area. If these two forms freely intercross with each other, and produce mongrel offsj^ring which are quite fertile inter se, then the further differentiation of the forms into two distinct species will be retarded, or perhaps entirely prevented ; for the offspring of the crossed unions will be, perhaps, more vigorous on account of the cross, although less perfectly adapted to the conditions of existence than either of the pure breeds ; and this would certainly estab- lish a powerful antagonistic influence to the further differentia- tion of the two forms. VII ON THE INFERTILITY OF CROSSES 175 Now, let us suppose that a partial sterility of the hybrids between the two forms arises, in correlation with the different modes of life and the slight external or internal peculiarities that exist between them, both of which we have seen to be real causes of infertility. The result will be that, even if the hybrids between the two forms are still freely produced, these hybrids will not themselves increase so rapidly as the two pure forms; and as these latter are, by the terms of the problem, better suited to their conditions of life than are the hybrids between them, they will not only increase more rapidly, but Mali also tend to supplant the hybrids altogether whenever the struggle for existence becomes exceptionally severe. Thus, the more complete the sterility of the hybrids the more rapidly will they die out and leave the two parent forms pure. Hence it ^WU follow that, if there is greater infertility between the two forms in one part of the area than the other, these forms "sWll be kept more pure wherever this greater infertility prevails, will therefore have an advantage at each recurring period of severe struggle for existence, and will thus ultimately supj^lant the less infertile or completely fertile forms that may exist in other portions of the area. It thus appears that, in such a case as here supposed, natural selection would preserve those portions of the two breeds which were most infertile with each other, or whose hybrid offspring were most infertile ; and would, therefore, if variations in fertility continued to arise, tend to increase that infertility. It must particularly be noted that this effect would result, not by the preservation of the infertile variations on account of their infertility, but by the inferiority of the hybrid offspring, both as being fewer in numbers, less able to continue their race, and less adapted to the conditions of existence than either of the piu-e forms. It is this inferiority of the hybrid offspring that is the essential point ; and as the number of these hybrids will be per- manently less where the infertility is greatest, therefore those portions of the two forms in which infertility is greatest will have the advantage, and will ultimately survive in the struggle for existence. The differentiation of the two forms into distinct species, with the increase of infertility between them, would be 176 DARWINISM chap. greatly assisted by two other important factors in the I)ioblem. It has already been shown that, with each modification of form and habits, and especially Avith modifica- tions of c(j1ovu', there arises a disinclination of the two forms to pair together ; and this would produce an amount of isolation which would greatly assist the specialisation of the forms in adaptation to their different conditions of life. Again, evidence has been adduced that change of conditions or of mode of life is a potent cause of disturbance of the reproductive system, and, consequently, of infertility. We may therefore assume that, as the two forms adopted more and more different modes of life, and perhaps acquired also decided peculiarities of form and coloration, the infertility between them would increase or become more general ; and as we have seen that every such increase of infertility would give that portion of the species in which it arose an advantage over the remaining portions in which the two varieties were more fertile together, all this induced infertility would main- tain itself, and still further increase the general infertility be- tween the two forms of the species. It follows, then, that specialisation to separate conditions of life, difterentiation of external characters, disinclination to cross-unions, and the infertility of the hybrid produce of these unions, would all proceed ^;flri pa.ssu, and would ultimately lead to the production of two distinct forms having all the characteristics, physiological as well as structural, of true species. In the case now discussed it has been supposed, that some amount of general infeitility might arise in correlation Avith the different modes of life of two varieties or incipient species. A considerable body of facts already adduced renders it probable that this is the mode in which any widespread infertility would arise ; and, if so, it has been shown that, by the influence of natural selection and the known laws which affect varieties, the infertility would be gradually increased. But, if we suppose the infertility to arise sporadically within the two forms, and to affect only a small proportion of the individuals in any area, it ^\i\\ be difficult, if not impossible, to show that such infertility would have any tendency to increase, or would produce any but a VII ox THE INFERTILITY OF CROSSES 177 prejudicial effect. If, for example, five per cent of each form thus varied so as to he infertile with the other form, the result would be hardly percept iljle, because the individuals which formed cross-unions and produced hybrids would con- stitute a very small portion of the whole species ; and the hybrid offspring, being at a disadvantage in the struggle for existence and being themselves infertile, would soon die out, while the much more numerous fertile portion of the two forms would increase rapidly, and furnish a sufficient number of pure-bred offspring of each form to take the place of the somewhat inferior hybrids between them whenever the struggle for existence became severe. We must suppose that the normal fertile forms would transmit their fertility to their progeny, and the iew infertile forms their infertility ; but the latter would necessarily lose half their proper increase by the sterility of their hybrid offspring whenever they crossed with the other form, and when they bred with their own form the tendency to sterility would die out except in the very minute proportion of the five per cent (one-twentieth) that chance would lead to pair together. Under these circumstances th-e incipient sterility between the two forms woidd rapidly be eliminated, and could never rise much above the numbers which were produced by sporadic variation each year. It was, probably, by a consideration of some such case as this that Mr. Darwin came to the conclusion that infertility arising between incipient species could not be increased by natural selection ; and this is the more likely, as he was always disposed to minimise both the frequency and the amount even of structural variations. We have yet to notice another mode of action of natural selection in favouring and perpetuating any infertility that may arise between two incipient species. If several distinct species are undergoing modification at the same time and in the same area, to adapt them to some new conditions that have arisen there, then any species in which the structural or colour differences that have arisen between it and its varieties or close allies were correlated with infertility of the crosses between them, would have an advantage over the corre- sponding varieties of other species in which there was no such N 178 DARWINISM chap. l^hysiological peculiarity. Thus, incipient species -which were infertile together would have an advantage over other incipient si)ecies which were fertile, and, whenever the struggle for existence became severe, would prevail over them and take their place. Such infertility, being correlated with constitutional or structural differences, would j^robably, as already suggested, go on increasing as these differences increased ; and thus, by the time the new species became fully differentiated from its parent form (or brother variety) the infertility might have become as well marked as we usually find it to be between distinct species. This discussion has led us to some conclusions of the greatest importance as bearing on the difficult problem of the cause of the sterility of the hybrids between distinct species. Accept- ing, as highly probable, the fact of variations in fertility occurring in correlation with variations in habits, colour, or structure, we see, that so long as such variations occurred only sporadically, and affected but a small proportion of the in- dividuals in any area, the infertility could not be increased by natural selection, but would tend to die out almost as fast as it was produced. If, however, it was so closely correlated with physical variations or diverse modes of life as to affect, even in a small degree, a considerable proportion of the individuals of the two forms in definite areas, it would be preserved by natural selection, and the portion of the varying species thus affected Avould increase at the expense of those portions which were more fertile when crossed. Each further variation towards infertility between the two forms would be again preserved, and thus the incipient infertility of the hybrid offspring might be increased till it became so great as almost to amount to sterility. Yet further, Ave have seen that if several competing species in the same area were being simultaneously modified, those between whose varieties infertility arose would have an advantage over those whose varieties remained fertile inter se, and would ultimately sup- plant them. The preceding argument, it will be seen, depends entirely upon the assumption that some amount of infertility char- acterises the distinct varieties which are in process of differentiation into species ; and it may be objected that of VII ON THE INFERTILITY OF CROSSES 179 such infertility there is no proof. This is admitted ; but it is urged that facts have been adduced which render such infertility probable, at least in some cases, and this is all that is required. It is by no means necessary that all varieties should exhibit incipient infertility, but only some varieties ; for we know that, of the innumerable varieties that occur but few become developed into distinct species, and it may be that the absence of infertility, to obviate the effects of inter- crossing, is one of the usual causes of their failure. All I have attempted to show is, that iclien incipient infertility does occur in correlation with other varietal differences, that in- fertility can be, and in fact must be, increased by natural selection ; and this, it appears to me, is a decided step in advance in the solution of the problem. ^ 1 As this argument is a rather difficult one to follow, while its theoretical importance is very great, I add here the following briefer exposition of it, in a series of propositions ; being, with a few verbal alterations, a copy of what I wrote on the subject about twenty years back. Some readers may find this easier to follow than the fuller discussion in the text : — Can Sterility of Hybrids have been Produced by Natural Selection 1 1. Let there be a species which has varied into two forms each adapted to certain existing conditions better than the parent form, which they soon supplant. 2. If these two forms, which are supposed to coexist in the same district, do not intercross, natural selection will accumulate all favourable variations till they become well suited to their conditions of life, and form two slightly differiug species. 3. But if these tivo forms freely intercross with each other, and produce hybrids, which are also quite fertile inter se, then tlie formation of the tAvo distinct races or species will be retarded, or perhaps entirely prevented ; for the offspring of the crossed unions will be more vigorous owing to the cross, although less ackqiied to their conditions of life than either of the pure breeds. 4. Now, let a partial sterility of the hybrids of some considerable propor- tion of these two forms arise ; and, as this would probably be due to some special conditions of life, we may fairly suppose it to arise in some definite portion of the area occupied by the two forms. 5. The result will be that, in that area, the hybrids (although continually produced by first crosses almost as freely as before) will not themselves increase so rapidly as the two pure forms ; and as the two jiure forms are, by the terms of the problem, better suited to their several conditions of life than the hybrids, they will inevitably increase more rapidly, and will continually tend to supplant the hybrids altogether at every recurrent severe struggle for existence. 6. We may fairly suppose, also, that as soon as any sterility appears some disinclination to cross unions will appear, and this will further tend to the diminution of the production of hybrids. 180 DARWINISM chap. Physiological Selection. Another form of infertility has been suggested by Professor G. J. Romanes as having aided in bringing about the char- acteristic infertility or sterility of hybrids. It is founded on the fact, already noticed, that certain individuals of some species possess what may be termed selective sterility — that is, while fertile with some individuals of the species they are sterile ^vith others, and this altogether independently of any differences of form, colour, or structure. The phenomenon, in the only form in which it has been observed, is that of "in- fertility or absolute sterility between two individuals, each of which is perfectly fertile with all other individuals;" but Mr. Romanes thinks that "it would not be nearly so remarkable, or physiologically improbable, that such incompatibility should run through a whole race or strain."^ Admitting that this may be 7. In the other part of the area, however, wliere hybridism occurs with perfect freedom, hybrids of various degi-ees may increase till they equal or even exceed in number the pure sj^ecies — that is, the incipient species will be liable to be swamped by intercrossing. 8. The first result, then, of a partial sterility of crosses appearing in one part of the area occupied by the two forms, Avill be — that the great majority of the individuals will there consist of the two pure forms only, while in the remaining part these will be in a minority, — which is the same as saying that the new pliysiolofjical variety of the two forms will be better suited to the conditions of existence than the remaining portion which has not varied physiologically. 9. But when the struggle for existence becomes severe, that variety which is best adapted to the conditions of existence always supplants that which is imperfectly adapted ; therefore, hy natural selection the varieties which are sterile when crossed will become established as the only ones. 10. Now let variations in the amount of sterility and in the disinclination to crossed unions continue to occur — also in certain parts of the area : exactly the same result must recur, and the progeny of this new physiological variety will in time occupy the whole area. 11. There is yet another consideration that would facilitate the process. It seems probable that the sterility variations would, to some extent, concur with, and perhaps depend upon, the specific variations ; so that, just in propor- tion as the two forms diverged and became better adapted to the conditions of existence, they would become more sterile when intercrossed. If this were the case, then natural selection would act with doiible strength ; and those which were better adapted to survive both structurally and physiologically would certainly do so. ^ Cases of this kind are referred to at p. 155. It must, however, be noted, that such sterility in first crosses appears to be equally rare between different species of the same genus as between individuals of the same species. Mules and other hybrids are freely produced between very distinct species, but are VII ON THE INFERTILITY OF CROSSES 181 so, though we have at present no evidence whatever in support of it, it remains to be considered whether such physio- logical varieties could maintain themselves, or whether, as in the cases of sporadic infertility already discussed, they would necessarily die out unless correlated viith. useful characters. Mr. Romanes thinks that they would persist, and urges that " whenever this one kind of variation occurs it cannot escape the preserving agencf/ of physiological selection. Hence, even if it be granted that the variation which affects the re- jDroductive system in this particular way is a variation of comparatively rare occurrence, still, as it must always be preserved whenever it does occur, its influence in the manu- facture of specific types must he cumnlative,'' The very positive statements which I have italicised would lead most readers to believe that the alleged fact had been demonstrated by a careful working out of the process in some definite supposed cases. This, however, has nowhere been done in Mr. Romanes' paper ; and as it is the Adtal theoretical point on which any possible value of the new theory rests, and as it appears so opposed to the self-destructive effects of simple infertility, which we have already demonstrated when it occurs between the intermingled portion of two varieties, it must be carefully examined. In doing so, I will suppose that the required variation is not of "rare occurrence," but of considerable amount, and that it appears afresh each year to about the same extent, thus giving the theory every possible advantage. Let us then suppose that a given species consists of 100,000 individuals of each sex, with only the usual amount of fluctuating external variability. Let a physiological variation a-rise, so that 10 per cent of the whole number — 10,000 individuals of each sex — while remaining fertile inter se become cjuite sterile with the remaining 90,000. This peculiarity is not correlated ^vit]l any external differences of themselves infertile or quite sterile ; and it is this infertility or sterility of the hybrids that is the characteristic — and was once thought to be the criterion — of species, not the sterility of their first crosses. Hence we should not expect to find any constant infertility in the first crosses between the distinct strains or varieties that formed the starting-point of new species, but only a slight amount of infertility in their mongrel offspring. It follows, that Mr. Romanes' theory of Physiological Selection — whith assumes sterility or in- fertility between first crosses as tlie fundamental fact in the origin of species — does not accord with the general i)heuomena of hybridism in nature. 182 DARWINISM chap. form or colour, or "vvith inherent peculiarities of likes or dislikes leading to any choice as to the pairing of the two sets of individuals. We have now to inquire, What would be the result 1 Taking, first, the 10,000 pairs of the physiological or abnormal variety, w^e find that each male of these might pair with any one of the Avhole 100,000 of the opposite sex. If, therefore, there was nothing to limit their choice to particular individuals of either variety, the probabilities are that 9000 of them would pair with the opposite variety, and only 1000 with their own variety — that is, that 9000 would form sterile unions, and only one thousand Avould form fertile unions. Taking, next, the 90,000 normal individuals of either sex, we find, that each male of these has also a choice of 100,000 to pair with. The probabilities are, therefore, that nine- tenths of them — that is, 81,000 — would pair with their normal fellows, while 9000 would pair with the opposite abnormal variety forming the above-mentioned sterile unions. Now, as the number of individuals forming a species remains constant, generally speaking, from year to year, we shall have next year also 100,000 pairs, of which the two physiological varieties will be in the proportion of eighty-one to one, or 98,780 pairs of the normal variety to 1220 ^ of the abnormal, that being the proportion of the fertile unions of each. In this year we shall find, by the same rule of probabilities, that only 15 males of the abnormal variety will pair with their like and be fertile, the remaining 1205 forming sterile unions with some of the normal variety. The follow- ing year the total 100,000 pairs will consist of 99,984 of the normal, and only 1 6 of the abnormal variety ; and the prob- abilities, of course, are, that the whole of these latter A\dll pair with some of the enormous preponderance of normal individuals, and, their unions being sterile, the physiological variety will become extinct in the third year. If now in the second and each succeeding year a similar proportion as at first (10 per cent) of the physiological variety is produced afresh from the ranks of the normal variety, the same rate of diminution Avill go on, and it will be found that, ^ The exact number is 1219 'SI, but the fractious are omitted for clearness. VII ox THE INFERTILITY OF CROSSES 183 on the most favourable estimate, the physiological variety can never exceed 12,000 to the 88,000 of the normal form of the species, as shown by the follo^ving table : — 1st Year. 10,000 of physiological variety to 90,000 of normal variety. 2d „ 1,220 + 10,000 again produced. 3d „ 16 + 1,220 + 10,000 do. = 11,236 4th „ 0+ 16+ 1,220 + 10,000 do. -11,236 5th „ 0+16+ 1,220 + 10,000 = 11,236 and so on for any number of generations. In the preceding discussion we have given the theory the advantage of the large proportion of 10 per cent of this very exceptional variety arising in its midst year by year, and we have seen that, even under these favourable conditions, it is unable to increase its numbers much above its starting-point, and that it remains wholly dependent on the continued renewal of the variety for its existence beyond a few years. It appears, then, that this form of inter -specific sterility cannot be increased by natural or any other knoAvn form of selection, but that it contains within itself its own principle of destruction. If it is proposed to get over the difficulty by postulating a larger percentage of the variety annually arising within the species, we shall not affect the law of decrease until we approach equality in the numbers of the tw^o varieties. But 'with any such increase of the physiological variety the species itself woukV inevitably suffer by the large projDor- tion of sterile unions in its midst, and would thus be at a great disadvantage in competition "s^ith other species which were fertile throughout. Thus, natural selection will always tend to Aveed out any species with too great a tendency to sterility among its own members, and will therefore prevent such sterility from becoming the general characteristic of vary- ing species, which this theory demands should be the case. On the whole, then, it appears clear that no form of infertility or sterility between the individuals of a species, can be increased by natural selection unless correlated with some useful variation, while all infertility not so correlated has a constant tendency to effect its own elimination. But the opposite property, fertility, is of vital importance to every species, and gives the offspring of the individuals which possess it, in consequence of their superior numbers, a greater 184 DARWINISM chap. chance of survival in the battle of life. It is, therefore, directly under the control of natural selection, which acts both by the self-preservation of fertile and the self-destruction of infertile stocks — except always where -correlated as above, when they become useful, and therefore subject to be increased by natural selection. Sumwiary and Concluding Remarks on Hyhridity. The facts which are of the greatest importance to a com- prehension of this very difficult subject are those which show the extreme susceptibility of the reproductive system both in plants and animals. We have seen how both these classes of organisms may be rendered infertile, by a change of conditions which does not affect their general health, by captivity, or by too close interbreeding. AVe have seen, also, that infertility is frequently correlated with a difference of colour, or with other characters ; that it is not proportionate to divergence of structure ; that it varies in reciprocal crosses between pairs of the same species ; while in the cases of dimorphic and tri- morphic plants the different crosses between the same pair of individuals may be fertile or sterile at the same time. It appears as if fertility depended on such a delicate adjustment of the male and female elements to each other, that, unless constantly kejit up by the preservation of the most fertile individuals, sterility is always liable to arise. This preservation always occurs within the limits of each species, both because fertility is of the highest importance to the continuance of the race, and also because sterility (and to a less extent infertility) is self-destructive as well as injurious to the species. So long therefore as a species remains undivided, and in occupation of a continuous area, its fertility is kept up by natural selection ; but the moment it becomes separated, either by geographical or selective isolation, or by diversity of station or of habits, then, while each portion must be kept fertile inter se, there is nothing to prevent infertility arising between the two separated portions. As the two |)ortions will necessarily exist under somcAvhat different conditions of life, and will usually have acquired some diversity of form and colour --both which circumstances we know to be either the cause of infertility or to be correlated with it, — the fact of VII ox THE IXFERTILITY OF CROSSES 185 some degree of infertility usually appearing between closely allied but locally or physiologically segregated species is exactly Avhat we should expect. The reason why varieties do not usually exhibit a similar amount of infertility is not difficult to explain. The popular conclusions on this matter have been drawn chiefly from what occurs among domestic animals, and we have seen that the very first essential to their becoming domesticated was that they should continue fertile under changed conditions of life. During the slow process of the formation of new A^arieties by conscious or unconscious selection, fertility has always been an essential character, and has thus been invariably preserved or increased ; while there is some e^adence to show that domestication itself tends to increase fertility. Among plants, wild species and varieties have been more frequently experimented on than among animals, and we accordingly find numerous cases in which distinct species of plants are perfectly fertile when crossed, their hybrid offspring being also fertile inter se. We also find some few examples of the converse fact — varieties of the same species which when crossed are infertile or even sterile. The idea that either infertility or geographical isolation is absolutely essential to the formation of new species, in order to prevent the swam^oing effects of intercrossing, has been shoA\Ti to be unsound, because the varieties or incipient species will, in most cases, be sufficiently isolated by having adopted different habits or by frequenting different stations ; Avhile selective association, which is known to be general among distinct varieties or breeds of the same species, M'ill produce an effective isolation even when the two forms occupy the same area. From the various considerations now adverted to, Mr. Darwin arrived at the conclusion that the sterility or in- fertility of species with each other, whether manifested in the difficulty of obtaining first crosses between them or in the sterility of the hybrids thus obtained, is not a constant or necessary result of specific difference, but is incidental on unknown peculiarities of the reproductive system. These peculiarities constantly tend to arise under changed conditions owing to the extreme susceptibility of that system, and they 186 DARWINISM chap, vii are usually correlated with variations of form or of colour. Hence, as fixed differences of form and colour, slowly gained by natural selection in adaptation to changed conditions, are what essentially characterise distinct species, some amount of infertility between species is the usual residt. Here the problem was left by Mr. Dar^vin ; but we have shown that its solution may be carried a step further. If we accept the association of some degi^ee of infertility, however slight, as a not unfrequent accompaniment of the external differences Avhich always arise in a state of nature between varieties and incipient species, it has been shown that natural selection luis power to increase that infertility just as it has power to increase other favourable variations. Such an in- crease of infertility will be beneficial, whenever new species arise in the same area with the parent form ; and we thus see how, out of the fluctuating and very unequal amounts of infer- tility correlated with physical variations, there may have arisen that larger and more constant amount which appears usually to characterise well-marked species. The great body of facts of which a condensed account has been given in the present chapter, although from an experi- mental point of view very insufficient, all point to the general conclusion we have now reached, and afford us a not unsatis- factory solution of the great problem of hybridism in relation to the origin of species by means of natural selection. Further experimental research is needed in order to complete the elucidation of the subject ; but until these additional facts are forthcoming no new theory seems required for the explanation of the phenomena. CHAPTER ^^n THE ORIGIN AND USES OF COLOUR IN ANBIALS The Darwinian theory threw new light on organic colour — The problem to be solved — The constancy of animal colour indicates utility — Colour and environment — Arctic animals white — Exceptions prove the rule — Desert, forest, nocturnal, and oceanic animals — General theories of animal colour — Variable protective colouring — Mr. Poulton's experi- ments—Special or local colour adaptations — Imitation of particular objects — How they have been produced — Special protective colouring of butterflies — Protective resemblance among marine animals — Pro- tection by ten-ifying enemies — Alluring coloration — The coloration of birds' eggs — Colour as a means of recognition — Summary of the preceding exposition — Influence of locality or of climate on colour — Concluding remarks. A3I0NG the numerous applications of the Darwinian theory in the interpretation of the complex phenomena presented by the organic world, none have been moie successful, or are more interesting, than those which deal with the colours of animals and plants. To the older school of naturalists colour was a trivial character, eminently unstable and untrustworthy in the determination of species ; and it appeared to have, in most cases, no use or meaning to the objects which displaj^ed it. The bright and often gorgeous coloration of insect, bird, or flower, was either looked upon as having been created for the enjoy- ment of mankind, or as due to unknown and perhaps undis- coverable laws of natiure. But the researches of ]\Ir. Darwin totally changed oiu* point of yievr in this matter. He showed, clearly, that some of the colours of animals are useful, some hurtful to them ; and he believed that many of the most brilliant colours were developed by sexual choice ; while hi^ great general principle, that all 188 DARWINISM chap. the fixed characters of organic beings have been developed under the action of the law of utility, led to the inevitable conclusion that so remarkable and conspicuous a character as colour, which so often constitutes the most obvious distinction of species from species, or group from group, must also have arisen from survival of the fittest, and must, therefore, in most cases have some relation to the wellbeing of its possessors. Continuous observation and research, carried on by multitudes of observers during the last thirty years, have shown this to be the case ; but the problem is found to be far more complex than was at first supposed. The modes in which coloiu- is of use to different classes of organisms is very varied, and have probably not yet been all discovered ; while the infinite variety and marvellous beauty of some of its developments are such as to render it hopeless to arrive at a complete and satisfactory explanation of every individual case. So much, however, has been achieved, so many curious facts have been explained, and so much light has been throAvn on some of the most obscure phenomena of nature, that the subject deserves a prominent l)lace in any account of the Darwinian theory. The Frohlem to he Solved. Before dealing Avith the various modifications of colour in the animal world it is necessary to say a few words on colour in general, on its prevalence in nature, and how it is that the colours of animals and plants require any special explanation. AVhat we term colour is a subjective phenomenon, due to the constitution of our mind and nerA^ous system; while, objectively, it consists of light-vibrations of different wave-lengths emitted by, or reflected from, various objects. Every visible object must be coloured, because to be visible it must send rays of light to our eye. The kind of light it sends is modified by the molecular constitution or the surface texture of the object. Pigments absorb certain rays and reflect the remainder, and this reflected portion has to our eyes a definite colour, according to the portion of the rays constituting white light which are absorbed. Interference colours are produced either by thin films or by very fine striae on the surfaces of bodies, Avhich cause rays of certain wave-lengths to neutralise each other, leaving the remainder to produce the effects of colour. Such VIII ORIGIN AND USES OF COLOUR IN ANIMALS 189 are the colours of soap-bubbles, or of steel or glass on which extremely fine lines have been ruled ; and these colours often produce the eftect of metallic lustre, and are the cause of most of the metallic hues of birds and insects. As colour thus depends on molecular or chemical constitution or on the minute surface texture of bodies, and, as the matter of which organic beings are composed consists of chemical com- pounds of great complexity and extreme instability, and is also subject to innumerable changes during growth and development, we might naturally expect the phenomena of colour to be more varied here than in less complex and more stable compounds. Yet even in the inorganic world Ave find abundant and varied colours ; in the earth and in the water ; in metals, gems, and minerals ; in the sky and in the ocean ; in sunset clouds and in the many-tinted rainbow. Here we can have no cpiestion of n^e to the coloured ol)ject, and almost as little perhaps in the vivid red of blood, in the brilliant colours of red snow and other low alg^e and fungi, or even in the universal mantle of green which clothes so large a portion of the earth's surface. The presence of some colour, or even of many brilliant colours, in animals and plants would recpiire no other explanation than does that of the sky or the ocean, of the ruby or the emerald — that is, it would require a pui-ely physical explanation only. It is the wonderful indi\aduality of the colours of animals and plants that attracts our attention — the fact that the colours are localised in definite patterns, sometimes in accordance A\dth structural characters, sometimes altogether indef)endent of them ; while often difi'ering in the most striking and fantastic manner in allied species. We are thus compelled to look upon colour not merely as a physical but also as a biological characteristic, which has been diflferentiated and specialised by natural selection, and must, therefore, find its explanation in the principle of adaptation or utility. The Constancy of Animal Colour indicates Utility. That the colours and markings of animals have been acquired under the fundamental law^ of utility is indicated by a general fact which has received very little attention. As a rule, colour and marking are constant in each species of wild animal, while, in almost every domesticated animal, there arises 190 DARWINISM OHAP. great variability. We see this in our horses and cattle, our dogs and cats, our pigeons and poultry. Now, the essential difference l)etween the conditions of life of domesticated and wild animals is, that the former are protected by man, while the latter have to protect themselves. The extreme variations in colour that inmiediately arise under domestication indicate a tendency to vary in this Avay, and _the occasional occurrence of white or })iebald or other exceptionally coloured individuals of many species in a state of nature, shows that this tendency exists there also ; and, as these exceptionally coloured in- dividuals rarely or never increase, there must be some con- stant power at work to keep it in check. This power can only be natural selection or the survival of the fittest, which again implies that some colours are useful, some injurious, in each particular case. With this principle as our guide, let us see how far we can account both for the general and special colours of the animal world. Colour and Environment. The fact that first strikes us in our examination of the colours of animals as a whole, is the close relation that exists between these colours and the general environment. Thus, white prevails among arctic animals ; yellow or brown in desert species ; while green is only a common colour in tropical ever- green forests. If we consider these cases somewhat carefully we shall find, that they afford us excellent materials for forming a judgment on the various theories that have been suggested to account for the colours of the animal world. In the arctic regions there are a number of animals which are wholly white all the year round, or which only turn white in muter. Among the former are the polar bear and the American polar hare, the snowy owl and the Greenland falcon ; among the latter the arctic fox, the arctic hare, the ermine, and the ptarmigan. Those which are permanently white remain among the snow nearly all the year round, while those which change their colour inhabit regions which are free from snow in summer. The obvious explanation of this style of coloration is, that it is protective, serving to conceal the herbivorous species from their enemies, and enablingcarnivorous animals to approach their prey unperceived. Two other explanations have, how- VIII ORIGIN AKD USES OF COLOUR IN AXIMALS 191 ever, been suggested. One is, that the prevalent white of the arctic regions has a direct effect in producing the white colour in animals, either by some photographic or chemical action on the skin or by a reflex action through vision. The other is, that the white colour is chiefly beneficial as a means of checking radiation and so j^reserving animal heat during the severity of an arctic A^nter. The first is part of the general theory that colour is the eflect of coloured light on the objects — a pure hypothesis which has, I believe, no facts whatever to support it. The second suggestion is also an hypothesis merely, since it has not been proved by experiment that a white colour, per se, independently of the fur or feathers which is so coloured, has any effect whatever in checking the radiation of low-grade heat like that of the animal body. But both alike are sufficiently disproved by the interesting exceptions to the rule of white coloration in the arctic regions, which exceptions are, nevertheless, quite in harmony with the theory of pro- tection. Whenever we find arctic animals which, from whatever cause, do not require protection by the white colour, then neither the cold nor the snow-glare has any effect upon their coloration. The sable retains its rich bro^vn fur throughout the Siberian winter ; but it frequents trees at that season and not only feeds partially on fruits or seeds, but is able to catch birds among the branches of the fir-trees, Avith the bark of which its colour assimilates. Then we have that thoroughly arctic animal, the musk-sheep, which is brown and conspicuous ; but this animal is gi-egarious, and its safety depends on its association in small herds. It is, therefore, of more im- portance for it to be able to recognise its kind at a distance than to be concealed from its enemies, against which it can well protect itself so long as it keeps together in a compact body. But the most striking example is that of the common raven, which is a true arctic bird, and is found even in mid-^^dnter as far north as any known bird or mammal. Yet it always retains its black coat, and the reason, from our point of view, is obvious. The raven is a powerful bird and fears no enemy, while, being a carrion-feeder, it has no need for concealment in order to approach its prey. The colour of the raven and of the musk -sheep are, therefore. 192 DARWINISM chai, both inconsistent with any other theory than that the whito colour of arctic animals has been acquired for concealment, and to that theory l)oth afford a strong support. Here we have a striking exam})le of the excei)tion proving the rule. In the desert regions of the earth we find an even more general accordance of colour with surroundings. The lion, the camel, and all the desert antelopes have more or less the colour of the sand or rock among which they live. The Egyptian cat and the Pampas cat are sandy or earth coloured. The Australian kangaroos are of similar tints, and the original colour of the wild horse is supposed to have been sandy or clay coloured. Birds are equally well protected by assimilative hues ; the larks, quails, goatsuckers, and grouse which abound in the North African and Asiatic deserts are all tinted or mottled so as closely to resemble the average colour of the soil in the districts they inhabit. Canon Tristram, who knows these regionis and their natural history so well, says, in an often quoted passage : "In the desert, where neither trees, brushwood, nor even undulations of the surface afford the slightest protection to its foes, a modification of colour which shall be assimilated to that of the surrounding country is absolutely necessary. Hence, without exception, the upper plumage of every bird, whether lark, chat, sylvain, or sand-grouse, and also the fur of all the smaller mammals, and the skin of all the snakes and lizards, is of one uniform isabelline or sand colour." Passing on to the tropical regions, it is among their evergreen forests alone that we find whole groups of birds whose ground colour is green. Parrots are very generally green, and in the East we have an extensive group of green fruit-eating pigeons; while the barbets, bee -eaters, turacos, leaf- thrushes (Phyllornis), white- eyes (Zosterops), and many other groups, have so much green in their plumage as to tend greatly to their concealment among the dense foliage. There can be no doubt that these colours have been acquired as a protection, when we see that in all the temperate regions, where the leaves are deciduous, the ground colour of the great majority of birds, especially on the upper surface, is a rusty brown of various shades, well corresponding with the bark, withered leaves, ferns, and bare thickets among which vni ORIGIN AND USES OF COLOUR IN ANIMALS 193 they live in autumn and winter, and especially in early spring when so many of them build their nests. Nocturnal animals supply another illustration of the same rule, in the dusky colours of mice, rats, bats, and moles, and in the soft mottled plumage of owls and goatsuckers which, while almost equally inconspicuous in the twilight, are such as to favour their concealment in the daytime. An additional illustration of general assimilation of colour to the surroundings of animals, is furnished by the inhabitants of the deep oceans. Professor Moseley of the Challenger Ex23edition, in his British Association lecture on this subject, says : " Most characteristic of pelagic animals is the almost crystalline transparency of their bodies. So perfect is this trans- parency that very many of them are rendered almost entirely invisible when floating in the watei', while some, even when caught and held up in a glass globe, are hardly to be seen. The skin, nerves, muscles, and other organs are absolutely hyaline and transparent, but the liver and digestive tract often remain opaque and of a yellow or brown colour, and exactly resemble when seen in the Avater small pieces of floating seaweed." Such marine organisms, however, as are of larger size, and either occasionally or habitually float on the surface, are beautifully tinged Avith blue above, thus harmonising with the colour of the sea as seen by hovering birds ; while they are white below, and are thus invisible against the wave-foam and clouds as seen by enemies beneath the surface. Such are the tints of the beautiful nudibranchiatc mollusc, Glaucus atlanticus, and many others. General Theories of Animal Colour. ^Ye are now in a position to test the general theories, or, to speak more correctly, the popular notions, as to the origin of animal coloration, before proceeding to apply the principle of utility to the explanation of some among the many extraordinary manifestations of colour in the animal world. The most generally received theory undoubtedly is, that brilliancy and variety of colour are due to the direct action of light and heat ; a theory no doubt derived from the abundance of bright - coloured birds, insects, and flowers which are brought from tropical regions. There are, however, 0 194 DARWINISM two strong arguments iigainst this theory, ^^'e have already seen how generally bright coloration is wanting in desert animals, 3'et here heat and light are both at a maximum, and if these alone Avere the agents in the production of colour, desert animals should be the most brilliant. Again, all naturalists who have lived in tropical regions know that the proportion of bright to dull .coloured species is little if any greater there than in the temperate zone, while there are many tropical groups in which bright colours are almost en- tirely unknown. No part of the world presents so many brilliant birds as South America, yet there are extensive families, containing many hundreds of species, which are as plainly coloured as our average temperate birds. Such are the families of the bush-shrikes and ant-thrushes (Formicariidas), the tyrant-shrikes (Tyrannid^), the American creepers (Den- drocolaptida?), together with a large proportion of the wood- warblers (Mniotiltidfe), the finches, the A\Tens, and some other groups. In the eastern hemisphere, also, we have the babbling- thrushes (Timaliidai), the cuckoo-shrikes (Campephagidse), the honey-suckers (Meliphagid^), and several other smaller groups which are certainly not coloured above the average standard of temperate birds. Again, there are many families of birds which spread over the whole world, temperate and tropical, and among these the tropical species rarely present any exceptional brilliancy of colour. Such are the thrushes, goatsuckers, hawks, plovers, and ducks ; and in the last-named group it is the temperate and arctic zones that afford the most brilliant coloration. The same general facts are found to prevail among insects. Although tropical insects present some of the most gorgeous coloration in the whole realm of nature, yet there are thousands and tens of thousands of species which are as dull coloured as any in our cloudy land. The extensive family of the carnivorous ground-beetles (Carabidae) attains its greatest brilliancy in the temperate zone; while by far the larger proportion of the great families of the longicorns and the weevils, are of obscure colours even in the tropics. In butter- flies, there is undoubtedly a larger proportion of brilliant colour in the tropics ; but if we compare families which are almost ec[ually developed over the globe — as the Picridae or VIII ORIGIN AND USES OF COLOUR IN ANIMALS 195 whites and yellows, and the Satyridae or ringlets — we shall find no great disproportion in colour between those of temperate and tropical regions. The Tarious facts which have now briefly been noticed are sufficient to indicate that the light and heat of the sun are not the direct causes of the colours of animals, although they may favour the production of colour when, as in tropical regions, the persistent high temperature favours the develop- ment of the maximum of life. We will now consider the next suggestion, that light reflected from surrounding coloured objects tends to produce corresponding colours in the animal world. This theory is founded on a number of very curious facts which prove, that such a change does sometimes occur and is directly dependent on the colours of surrounding objects ; but these facts are comparatively rare and exceptional in their nature, and the same theory will certainly not apply to the in- finitely varied colours of the higher animals, many of which are exposed to a constantly varying amount of light and colour during their active existence. A brief sketch of these dependent changes of colour may, however, be advantageously given here. Variable Protective Colouring. There are two distinct kinds of change of colour in animals due to the colouring of the environment. In one case the change is caused by reflex action set up by the animal seeing the colour to be imitated, and the change produced can be altered or repeated as the animal changes its position. In the other case the change occurs but once, and is probably not due to any conscious or sense action, but to some direct in- fluence on the surface tissues while the creature is undergoing a moult or change to the pupa form. The most striking example of the first class is that of the chameleon, which changes to white, brown, yellowish, or gi'een, according to the colour of the object on which it rests. This change is brought about by means of two layers of pigment cells, deeply seated in the skin, and of bluish and yellowish colours. By suitable muscles these cells can be forced upwards so as to modify the colour of the skin, which, 196 DARWINISM chap. when they are not brought into action, is a dirty white. These animals are excessively sluggish and defenceless, and the power of changing their colour to that of their immediate sur- roundings is no doubt of great service to them. Many of the flatfish are also capable of changing their colour according to the colour of the bottom they rest on ; and frogs have a similar power to a limited extent. Some Crustacea also change colour, and the power is much developed in the Chameleon shrimp (Mysis Chamseleon) which is gray when on sand, but brown or green when among brown or green seaweed. It has been proved by experiment that when this animal is blinded the change does not occur. In all these cases, therefore, we have some form of reflex or sense action by which the change is produced, probably by means of pigment cells beneath the skin as in the chameleon. The second class consists of certain larvae, and pupse, which undergo changes of colour when exposed to differently coloured surroundings. This subject has been carefully investigated by Mr. E, B. Poulton, who has communicated the results of his experiments to the Eoyal Society.^ It had been noticed that some species of larvae which fed on several different plants had colours more or less corresponding to the particular plant the individual fed on. Numerous cases are given in Professor Meldola's article on " Variable Protective Colouring" {Proc. Zool. Soc, 1873, p. 153), and while the general green coloration was attributed to the presence of chlorophyll beneath the skin, the particular change in corre- spondence to each food-jjlant was attributed to a special function which had been developed by natural selection. Later on, in a note to his translation of Weissmann's Theorij of Descent, Professor Meldola seemed disposed to think that the variations of colour of some of the species might be phytophagic — that is, due to the direct action of the differently coloured leaves on which the insect fed. Mr. Poulton's experiments have thrown much light on this question, since he has conclusively proved that, in the case of the sphinx cater- pillar of Smerinthus ocellatus, the change of colour is not due to the food but to the coloured light reflected from the leaves. ^ Proceedings of the Hoyal Societ}/, 'No. 2i3, 1886 ; Transactions of the Royal Society, vol. clxxviii. B. pp. 311-441. VIII ORIGIX AXD USES OF COLOUR IX ANIMALS 197 This Avas sho^vn by feeding two sets of larvae on the same plant but exposed to differently coloured siuToundings, obtained by se^Wng the leaves together, so that in one case only the dark upper surface, in the other the whitish under surface was exposed to view. The result in each case was a corresponding change of colour in the larvae, confirming the experiments on different individuals of the same batch of l2LTVi