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(SR ati aha oe te. c Sas bes MAO mba Rhy wyte mia wre nee eee! e mn nls PRLEE RIC GC bee alen'e whe wie x é wat” # ‘ £ j 3 # } THE ORIGIN OF SPECIES. ta4 cf - yu s i y J { a 4 # + a4 #' fa 4 a ¢ 4 pty fd, # oA a Ba j ; . Annan NIRARARARARARAR Ae 4 - , ’ \ ‘ “ But with regard to the material world, we can at least go so far as this—we can perceive that events are brought about not by insulated interpositions of Divine power, exerted in each particular case, but by the establishment of general laws.” WHEWELL: Bridgewater Treatise. “The only distinct meaning of the word ‘natural’ is stated, fixed, or settled ; since what is natural as much requires and presupposes an intel- ligent agent to render it so, z.¢., to effect it continually or at stated times, as what is supernatural or miraculous does to eftect it for once.” BUTLER: Analogy of Revealed Religion. “To conclude, therefore, let no man out of a weak conceit of sobriety, or an ill-applied moderation, think or maintain, that a man can search too far or be too well studied in the book of God’s word, or in the book of God’s works; divinity or philosophy; but rather let men endeavour an endless progress or proficience in both.” Bacon : Advancement of Learning. Down, Beckenham, Kent, First Edition, November 24th, 1859. Siath Edition, Jan. 1872. aa THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION, OR THE PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE. By CHARLES DARWIN, M.A. F.BS, &e. SIXTH EDITION, WITH ADDITIONS AND CORRECTIONS. (ELEVENTH THOUSAND.) LONDON: JOHN MURRAY, ALBEMARLE STREET. 1872. The right of Translation ts reserved. BY THE SAME AUTHOR, THE DESCENT OF MAN AND SELECTION IN RELA- TION TO SEX. Fighth Thousand. 2 vols. 8vo. 248. Morray, 1871, THE VARIATION OF ANIMALS AND PLANTS UNDER DOMESTICATION. With Illustrations. Third Thousand. 2 vols. 8vo. 28s. Morray, 1868, A NATURALIST’S VOYAGE ROUND THE WORLD; or, A JOURNAL OF RESEARCHES INTO THE NATURAL HISTORY AND GEOLOGY OF THE Countrtss visited during the voyage of H.MLS. ‘ Beagle,’ under the command of Captain FirzRoy, R.N. Eleventh Thousand. Post 8vo. 9s. Morray. ON THE VARIOUS CONTRIVANCES sy waice BRITISH AND FOREIGN ORCHIDS ARE FERTILISED BY INSECTS; and on the Goop EFFEcts OF CrossInc. With Woodcuts. Post 8vo. 9s. Murray. ON THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS. SmirH, ELper, & Co. GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS. SmitH, ELper, & Co. GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA. Smrtu, Exper, & Co. A MONOGRAPH OF THE CIRRIPEDIA. With numerous Illustrations. 2 vols. 8vo. Ray Soctety. HarDWICKE. ON THE MOVEMENTS AND HABITS OF CLIMBING PLANTS. With Woodcuts. Witurams & NorGATE, FACTS AND ARGUMENTS FOR DARWIN. By Frirz Mutter. From the German, with Additions by the Author, Translated by W. S. Datias, F.L.S. With Illustrations. Post 8vo. 6s. Mougray. LONDON: PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET, AND CHARING CROSS. a CONTENTS. PW ADDITIONS AND CORRECTIONS, TO THE SIXTH EDITION .. Page xi-xii HISTORICAL SKETCH ..we ce ee ewe ee wee XIX XI INTRODUCTION ee oe ee oo oe oe ee ee oe ee ee 1—4 CHAPTER I. VARIATION UNDER DOMESTICATION. Causes of Variability — Effects of Habit and the use or disuse of Parts — Correlated Variation — Inheritance —Character of Domestic Varieties — Difficulty of distinguishing between Varieties and Species — Origin of Domestic Varieties from one or more Species — Domestic Pigeons, their Differences and Origin— Principles of Selection, anciently fol- lowed, their Effects — Methodical and Unconscious Selection — Un- known Origin of our Domestic Productions — Circumstances favour- able to Man’s power of Selection i rr rs) NTS CHAPTER IL VARIATION UNDER NATURE. Variability — Individual differences — Doubtful species — Wide ranging, much diffused, and common species, vary most — Species of the larger genera in each country vary more frequently than the species of the smaller genera— Many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, wepT\G and in having restricted ranges... $8 se we cork? CHAPTER III. m STRUGGLE FOR EXISTENCE. a att Its bearing on natural selection — The term used in a wide sense—Geome- trical ratio of increase — Rapid increase of naturalised animals and plants — Nature of the checks to increase — Competition universal— Effects of climate — Protection from the number of individuals — Complex relations of all animals and plants throughout nature — Struggle for life most severe between individuals and varieties of the same species: often severe between species of the same genus — The relation of organism to organism the most important of all rela- tionS «nue ueeeteeneelnlw 48-61 ¥ vi CONTENTS, CHAPTER IV. NaTurAL SELECTION; OR THE SURVIVAL OF THE FITTEsT, z Natural Selection — its power compared with man’s selection — its power on characters of trifling importance —its power at all ages and on both sexes — Sexual Selection — On the generality of intercrosses between individuals. of the same species — Circumstances favourable and unfa- vourable to the results of Natural Selection, namely, intercrossing, isolation, number of individuals — Slow action — Extinction caused by Natural Selection—Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation— Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent — Explains the grouping of all organic beings — Advance in organisation — Low forms preserved '—Convergence of character—Indefinite multiplication of species — Summary ek ews eee eee” | Pare 62-105 CHAPTER V. LAWS OF VARIATION. Effects of changed conditions— Use and disuse, combined with natural selection; organs of flight and of vision — Acclimatisation — Correlated variation — Compensation and economy of growth — False correlations — Multiple, rudimentary, and lowly organised structures variable— Parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable — Species of the same genus vary in an analogous manner — Reversions to long-lost characters—-Summary .. .. ~- 106-132 CHAPTER VI. DIFFICULTIES OF THE THEORY. Difficulties of the theory of descent with modification — Absence or rarity of transitional varieties — Transitions in habits of life — Diversified habits in the same species — Species with habits widely different from those of their allies — Organs of extreme perfection — Modes of transition — Cases of difficulty — Natura non facit saltum — Organs of small importance —Organs not in all cases absolutely perfect — The law of Unity of Type and of the Conditions of Existence embraced by the theory of Natural Selection .. 2. «2 « «+ 133-167 CONTENTS. Vil. CHAPTER VII. MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION. Longevity — Modifications not necessarily simultaneous — Modifications apparently of no direct service—Progressive development — Characters of small functional importance, the most constant—Supposed incom- petence of natural selection to account for the incipient stages of useful structures—Causes which interfere with the acquisition through natural selection of useful structures—Gradations of structure with changed functions—Widely different organs in members of the same class, developed from one and the same source—Reasons for disbeliev- ing in great and abrupt modifications .. .. .. ~. Page 168-204 CHAPTER VIII. INSTINCT. Instincts comparable with habits, but different in their origin — Instincts graduated — Aphides and ants — Instincts variable — Domestic in- stincts, their origin — Natural instincts of the cuckoo, molothrus, ostrich, and parasitic bees —Slave-making ants — Hive-hee, its cell- making instinct— Changes of instinct and structure not necessarily simultaneous —Difficulties of the theory of the Natural Selection of instincts — Neuter or sterile insects — Summary oe ee 205-234 CHAPTER IX. HyYBRIDISM. Distinction between the sterility of first crosses and of hybrids — Sterility various in degree, not universal, affected by close interbreeding, re- moved by domestication — Laws governing the sterility of hybrids — Sterility not a special endowment, but incidental on other differences, not accumulated by natural selection — Causes of the sterility of first crosses and of hybrids — Parallelism between the effects of changed conditions of life and of crossing — Dimorphism and Trimorphism — Fertility of varieties when crossed and of their mongrel offspring not universal — Hybrids and mongrels compared independently of their fertility —Summary «6 «6 oe oe wee we we 234-263 Will CONTENTS. CHAPTER X. On THE IMPERFECTION OF THE GEOLOGICAL RECORD. On the absence of intermediate varieties at the present day— On the ' nature of extinct intermediate varieties; on their number — On the lapse of time, as inferred from the rate of denudation and of deposition — On the lapse of time as estimated by years— On the poorness of our paleontological collections — On the intermittence of geological formations — On the denudation of granitic areas — On the absence of intermediate varieties in any one formation — On the sudden appearance of groups of species On their sudden appearance in the lowest known fossiliferous strata— Antiquity of the habitable earth oe wee oe ewe we we we Swe Page 264-289 CHAPTER XI. On THE GEOLOGICAL SUCCESSION oF ORGANIC BEINGS. On the slow and successive appearance of new species — On their different rates of change — Species once lost do not reappear — Groups of species follow the same general rules in their appearance and disappearance as do single species — On Extinction — On simultaneous changes in the forms of life throughout the world — On the affinities of extinct species to each other and to living species — On the state of develop- ment of ancient forms — On the succession of the same types within the same areas — Summary of preceding and present chapter 290-315 CHAPTER XII. GEOGRAPHICAL DISTRIBUTION. Present distribution cannot be accounted for by differences in physical conditions — Importance of barriers — Affinity of the productions of the same continent — Centres of creation — Means of dispersal, by changes of climate and of the level of the land, and by occasional means —Dispersal during the Glacial period — Alternate Glacial periods in the north and south ne ee we we we we we ee 816-342 CHAPTER XIII. GEOGRAPHICAL DIstrinutTion—continued. Distribution of fresh-water productions — On the inhabitants of oceanic islands — Absence of Batrachians and of terrestrial Mammals — On the relation of the inhabitants of islands to those of the nearest main- land — On colonization from the nearest source with subsequent modi- fication — Summary of the.last and present chapter .. +. 343-362 a —_™ CONTENTS. 1x CHAPTER XIV. MouruaL AFFINITIES oF OrGANIC Bernas: MorPHoLoeY : EMBRYOLOGY : RUDIMENTARY ORGANS. CLASSIFICATION, groups subordinate to groups — Natural system — Rules and difficulties in classification, explained on the theory of descent with modification — Classification of varieties — Descent always used in classification — Analogical or adaptive characters — Affinities, general, complex, and radiating — Extinction separates and defines groups — MorpPHoLocy, between members of the same class, between parts of the same individual — Empryonoey, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age — RUDIMENTARY ORGANS; their origin explained — Summary ve ee ee we ee we Tee Page 863-403 CHAPTER XV. RECAPITULATION AND CONCLUSION. Recapitulation of the objections to the theory of Natural Selection — Recapitulation of the general and special circumstances in its favour — Causes of the general belief in the immutability of species — How far the theory of Natural Selection may be extended — Effects of its adoption on the study of Natural History — Con- cluding remarks...) ww) wee ewe nee we 404-429 GLOSSARY OF SCIENTIFIC TERMS INDUS “ae we oo sc rf Gh ws 6 oF 6% 45 . 443 INSTRUCTION TO BINDER. en oO eeeaearas The Diagram to front page 90, and to face the latter part of the Volume. ADDITIONS AND CORRECTIONS TO THE SIXTH EDITION. ———$*Oo Numerous small corrections have been made in the last and present editions on various subjects, according as the evidence has become somewhat stronger or weaker. The more important corrections and some additions in the pre- sent volume are tabulated on the following page, for the convenience of those interested in the subject, and who possess the fifth edition. The second edition was little more than a reprint of the first. The third edition was largely corrected and added to, and the fourth and fifth still more largely. As copies of the present work will be sent abroad, it may be of use if I specify the state of the foreign editions. The third French and second Ger- man editions were from the third English, with some few of the additions given in the fourth edition. A new fourth French edition has been translated by Colonel Moulinié ; of which the first half is from the fifth English, and the latter half from the present edition. A third German edition, under the superintendence of Professor Victor Carus, was from the fourth English edition ; a fifth is now preparing by the same author from the present volume. The second American edition was from the English second, with a few of the additions given in the third; and a third American edition has been printed from the fifth English edition. The Italian is from the third, the Dutch and three Russian editions from the second English edition, and the Swedish from the fifth English edition. Xil Additions and Corrections. ean, he wae: Chief Additions and Corrections. Page Page 100 68 | Influence of fortuitous destruction on natural selection. 158 101 | On the convergence of specific forms. 220 | 142 | Account of the Ground-Woodpecker of La Plata modified. 225 | 145 | On the modification of the eye. 230 149 | Transitions through the acceleration or retardation of the period of reproduction. 231 150 | The account of the electric organ of fishes added to. 233 151 | Analogical resemblance between the eyes of Cephalopods and Vertebrates. 234 | 153 | Claparéde on the analogical resemblance of the hair-claspers of the Acaride. 248 | 162 | The probable use of the rattle to the Rattle-snake. 248 | 163 | Helmholtz on the imperfection of the human eye. 255 | 168 | The first part of this new chapter consists of portions, in a much modified state, taken from chap. iv. of the former editions. The latter and larger part is new, and relates chiefly to the supposed incompetency of natural selection to account for the incipient stages of useful structures. There is also a discussion on the causes which prevent in many cases the acquisition through natural selection of useful structures. Lastly, reasons are given for dis- believing in great and sudden modifications. Gradations of character, often accompanied by changes of function, are likewise here incidentally considered. 268 | 214 | The statement with respect to young cuckoos ejecting their foster-brothers confirmed. 270 | 215 | On the cuckoo-like habits of the Molothrus. 307 | 240 | On fertile hybrid moths. 319 | 248 | The discussion on the fertility of hybrids not having been ac- quired through natural selection condensed and modified. 326 | 252 | On the causes of sterility of hybrids, added to and corrected. 377 284 | Pyrgoma found in the chalk. 402 | 801 | Extinct forms serving to connect existing groups. 440 | 328 | On earth adhering to the feet of migratory birds. 463 | 343 | On the wide geographical range of a species of Galaxias, a fresh-water fish. 505 | 873 | Discussion on analogical resemblances, enlarged and modified. 516 | 382 | Homological structure of the feet of certain marsupial animals. 518 | 384 | On serial homologies, corrected. 520 | 3885 | Mr. E. Ray Lankester on morphology. 521 | 387 | On the asexual reproduction of Chironomus. 541 | 401 | On the origin of rudimentary parts, corrected. 547 | 405 | Recapitulation on the sterility of hybrids, corrected. 552 | 409 | Recapitulation on the absence of fossils beneath the Cam- brian system, corrected. 568 | 421 | Natural selection not the exclusive agency in the modi- fication of species, as always maintained in this work. 572 | 424 | The belief in the separate creation of species generally held by naturalists, until a recent period. a AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES, PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS WORK. I witt here a give a brief sketch of the progress of opinion on the Origin of Species. Until recently the great majority of naturalists believed that species were immutable productions, and had been separately created. This view has been ably maintained by many authors. Some few naturalists, on the other hand, have believed that species undergo modification, and that the existing forms of life are the descendants by true generation of pre-existing forms. Passing over allusions to the subject in the classical writers,* the first author who in modern times has treated it in a scientific spirit was Buffon. But as his opinions fluctuated greatly at different periods, and as he does not enter on the causes or means of the transforma- tion of species, I need not here enter on details. Lamarck was the first man whose conclusions on the subject excited much attention. This justly-celebrated naturalist first pub- lished his views in 1801; he much enlarged them in 1809 in his ‘Philosophie Zoologique,’ and subsequently, in 1815, in the Intro- duction to his ‘ Hist. Nat. des Animaux sans Vertébres.’ In these * Aristotle, in his ‘ Physice Auscultationes’ (lib. 2, cap. 8, s. 2), after remarking that rain does not fall in order to make the corn grow, any more than it falls to spoil the farmer’s corn when threshed out of doors, applies the same argument to organisation; and adds (as translated by Mr. Clair Grece, who first pointed out the passage to me), “So what hinders the different parts [of the body] from having this merely accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for mas- ticating the food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to the other parts in which there appears to exist an adaptation toanend. Wheresoever, there- fore, all things together (that is all the parts of one whole) happened like as if they were made for the sake of something, these were preserved, having been appropriately constituted by an internal spontaneity; and whatsoever things were not thus constituted, perished, and still perish.” We here see the principle of natural selection shadowed forth, but how little Aristotle fully comprehended the principle, is shown by his remarks on the formation of the teeth, Xiv . EHlistorical Sketch. ce Re works he upholds the doctrine that all species, including man, are descended from other species. He first did the eminent service of arousing attention to the probability of all change in the organic, as well as in the inorganic world, being the result of law, and not of miraculous interposition. Lamarck seems to have been chiefly led to his conclusion on the gradual change of species, by the difficulty of distinguishing species and varieties, by the almost perfect gradation of forms in certain groups, and by the analogy of domestic preduc- tions. With respect to the means of modification, he attributed something to the direct action of the physical conditions of life, something to the crossing of already existing forms, and much to use and disuse, that is, to the effects of habit. ‘To this latter agency he seems to attribute all the beautiful adaptations in nature ;—such as the long neck of the giraffe for browsing on the branches of trees. But he likewise believed in a law of progressive development; and as all the forms of life thus tend to progress, in order to account for the existence at the present day of simple productions, he maintains that such forms are now spontaneously generated. * Geoffroy Saint Hilaire, as is stated in his ‘ Life,’ written by his son, suspected, as early as 1795, that what we call species are various degenerations of the same type. It was not until 1828 that he published his conviction that the same forms have not been perpetuated since the origin of all things. Geoffroy seems to have relied chiefly on the conditions of life, or the “ monde ambiant” as the cause of change. He was cautious in drawing conclusions, and did not believe that existing species are now undergoing modifica- tion; and, as his son adds, “ C’est donc un probleme & réserver entigrement & Vavenir, supposé méme que l’avenir doive avoir prise sur lui.” * I have taken the date of the first publication of Lamarck from Isid. Geoffroy Saint Hilaire’s (‘ Hist. Nat. Generale,’ tom. ii. p. 405, 1859) excel- lent history of opinion on this subject. In this work a full account is given of Buffon’s conclusions on the same subject. It is curious how largely my grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous grounds of opinion of Lamarck in his ‘ Zoonomia’ (vol. i. pp. 500-510), published in 1794. According to Isid. Geoffroy there is no doubt that Goethe was an extreme partisan of similar views, as shown in the Intro- duction to a work written in 1794 and 1795, but not published till long afterwards: he has pointedly remarked (‘ Goethe als Naturforscher,’ von Dr. Karl Meding, s. 34) that the future question for naturalists will be how, for instance, cattle got their horns, and not for what they are used. It is rather a singular instance of the manner in which similar views arise at about the same time, that Goethe in Germany, Dr. Darwin in England, and Geoffroy Saint Hilaire (as we shall immediately see) in France, came to the same conclusion on the origin of species, in the years 17 94-5. ne Sa le aa a ee | oe tea tea omens army Historical Sketch. XV In 1813, Dr. W. C. Wells read before the Royal Society ‘An Account of a White Female, part of whose skin resembles that of a Negro’; but his paper was not published until his famous ‘ 'l'wo Essays upon Dew and Single Vision’ appeared in 1818. In this paper he distinctly recognises the principle of natural selection, and this is the first recognition which has been indicated ; but he applies it only to the races of man, and to certain characters alone. After remarking that negroes and mulattoes enjoy an immunity from certain tropical diseases, he observes, firstly, that all animals tend to vary in some degree, and, secondly, that agriculturists improve their domesticated animals by selection ; and then, he adds, but what is done in this latter case “ by art, seems to be done with equal efficacy, though more slowly, by nature, in the formation of varieties of mankind, fitted for the country which they inhabit. Of the acci- dental varieties of man, which would occur among the first few and scattered inhabitants of the middle regiotts of Africa, some one would be better fitted than the others to bear the diseases of the country. ‘This race would consequently multiply, while the others would decrease ; not only from their inability to sustain the attacks of disease, but from their incapacity of contending with their more vigorous neighbours. ‘The colour of this vigorous race I take for granted, from what has been already said, would be dark. But the same disposition to form varieties still existing, a darker and a darker race would in the course of time occur: and as the darkest would be the best fitted for the climate, this would at leneth become the most prevalent, if not the only race, in the particular country in which it had originated.” He then extends these same views to the white inhabitants of colder climates. I am indebted to Mr. Rowley, of the United States, for having called my atten- tion, through Mr. Brace, to the above passage in Dr. Well’s work. The Hon. and Rev. W. Herbert, afterwards Dean of Manchester, in the fourth volume of the ‘ Horticultural Transactions,’ 1822, and in his work on the ‘ Amaryllidacez ’ (1887, p. 19, 889), declares that “horticultural experiments have established, beyond the possibility of refutation, that botanical species are only a higher and more per- manent class of varieties.” He extends the same view to animals. The Dean believes that single species of each genus were created in an originally highly plastic condition, and that these have produced, chiefly by intercrossing, but likewise by variation, all our existing species. In 1826 Professor Grant, in the concluding paragraph in his well known paper (‘Edinburgh Philosophical J ournal,’ vol. xiv. p. 283) on the Spongilla, clearly declares his belief that species are ES Xvi Historical Sketch. descended from other species, and that they become improved in the course of modification. This same view was given in his 55th Lecture, published in the ‘ Lancet’ in 1834. In 1831 Mr. Patrick Matthew published his work on ‘Naval Timber and Arboriculture,’ in which he gives precisely the same view on the origin of species as that (presently to be alluded to) propounded by Mr. Wallace and myself in the ‘Linnean Journal,’ and as that enlarged in the present volume. Unfortunately the view was given by Mr. Matthew very briefly in scattered passages in an Appendix to a work on a different subject, so that it remained unnoticed until Mr. Matthew himself drew attention to it in the ‘Gardeners’ Chronicle,’ on April 7th, 1860. The differences of Mr. Matthew’s view from mine are not of much importance: he seems to consider that the world was nearly depopulated at successive periods, and then re-stocked; and he gives as an alternative, that new forms may be generated “ without the presence of any mould or germ of former aggregates.” JI am not sure that I understand some passages; but it seems that he attributes much influence to the direct action of the conditions of life. He clearly saw, how- ever, the full force of the principle of natural selection. The celebrated geologist and naturalist, Von Buch, in his ex- cellent ‘Description Physique des Isles Canaries’ (18386, p. 147), clearly expresses his belief that varieties slowly become changed into permanent species, which are no longer capable of inter- crossing. Rafinesque, in his‘ New Flora of North America,’ published in 1836, wrote (p. 6) as follows:—“ All species might have been varieties once, and many varieties are gradually becoming species by assuming constant and peculiar characters;” but farther on (p. 18) he adds, “except the original types or ancestors of the genus.” In 1843-44 Professor Haldeman (‘Boston Journal of Nat. Hist. U. States,’ vol. iv. p. 468) has ably given the arguments for and against the hypothesis of the development and modification of species: he seems to lean towards the side of change. | The ‘ Vestiges of Creation’ appeared in 1844. In the tenth and much improved edition (1853) the anonymous author says _(p. 155):—The proposition determined on after much considera- | tion is, that the several series of animated beings, from the simplest ? and oldest up to the highest and most recent, are, under the provi- : dence of God, the results, first, of an impulse which has been | imparted to the forms of life, advancing them, in definite times, by : ) generation, through grades of organisation terminating in the , a Htistorical Sketch. XVII highest dicotyledons and vertebrata, these grades being few in number, and generally marked by intervals of organic character, which we find to be a practical difficulty in ascertaining affinities ; second, of another impulse connected with the vital forces, tending, in the course of generations, to modify organic structures in accor- dance with external circumstances, as food, the nature of the habitat, and the meteoric agencies, these being the ‘ adaptations’ of the natural theologian.” The author apparently believes that organisation progresses by sudden leaps, but that the effects produced by the conditions of life are gradual. He argues with much force on general grounds that species are not immutable productions. But I cannot see how the two supposed “impulses” account in a scientific sense for the numerous and beautiful co- adaptations which we see throughout nature; I cannot see that we thus gain any insight how, for instance, a woodpecker has become adapted to its peculiar habits of life. The work, from its powerful and brilliant style, though displaying in the earlier editions little accurate knowledge and a great want of scientific caution, imme- diately had a very wide circulation. In my opinion it has done excellent service in this country in calling attention to the sub- ject, in removing prejudice, and in thus preparing the ground for the reception of analogous views. In 1846 the veteran geologist M. J. d’Omalius d’Halloy pub- lished in an excellent though short paper (‘ Bulletins de l’Acad. Roy. Bruxelles, tom. xiii. p. 581), his opinion that it is more probable that new species have been produced by descent with modification than that they have been separately created: the author first promulgated this opinion in 1831. Professor Owen, in 1849 (‘ Nature of Limbs,’ p. 86), wrote as follows :—“ 'he archetypal idea was manifested in the flesh under diverse such modifications, upon this planet, long prior to the existence of those animal species that actually exemplify it. To what natural laws or secondary causes the orderly succession and progression of such organic phenomena may have been committed, we, as yet, are ignorant.” In his Address to the British Association, in 1858, he speaks (p. li.) of “the axiom of the continuous Operation of creative power, or of the ordained becoming of living things.” Farther on (p. xc.), after referring to geographical distri- bution, he adds, “These phenomena shake our confidence in the conclusion that the Apteryx of New Zealand and the Red Grouse of England were distinct creations in and for those islands respec- tively. Always, also, it may be well to bear in mind that by the word ‘creation’ the zoologist means ‘a process he knows not b xVili ‘Historical Sketch. what.” He amplifies this idea by adding, that when such cases as that of the Red Grouse are ‘enumerated by the zoologist ag evidence of distinct creation of the bird in and for such islands, he chiefly expresses that he knows not how the Red Grouse came to be there, and there exclusively ; signifying also, by this mode of expressing such ignorance, his belief that both the bird and the islands owed their origin to a great first Creative Cause.” If we interpret these sentences given in the same Address, one by the other, it appears that this eminent philosopher felt in 1858 his con- fidence shaken that the Apteryx and the Red Grouse first appeared in their respective homes, “he knew not how,” or by some process “he knew not what.” : This Address was delivered after the papers, by Mr. Wallace and myself on the Origin of Species, presently to be referred to, had been read before the Linnean Society. When the first edition of this work was published, I was so completely deceived, as were many others, by such expressions as “‘ the continuous operation of creative power,” that I included Professor Owen with other paleontologists as being firmly convinced of the immutability of species; but it appears (‘ Anat. of Vertebrates,’ vol. iii. p. 796) that this was on my part a preposterous error. In the last edition of this work I inferred, and the inference still seems to me perfectly just, from a passage beginning with the words “ no doubt the type-form,” &c. (ibid. vol. i. p. xxxv.), that Professor Owen admitted that natural selection may have done something in the formation of new species; but this it appears (Ibid. vol. iii. p. 798) is inaccurate and without evidence. I also gave some extracts from a correspondence between Professor Owen and the Editor of the ‘ London Review,’ from which it appeared manifest to the Editor as well as to myself, that Professor Owen claimed to have promulgated the theory of natural selection before I had done so; and I expressed my surprise and satisfaction at this announcement; but as far as it is possible to understand certain recently published passages (Ibid. vol. iil. p- 798), | have either partially or wholly again fallen into error. It is consolatory to me that others find Professor Owen's controver- sial writings as difficult to understand and to reconcile with each other, as Ido. As far as the mere enunciation of the principle of natural selection is concerned, it is quite immaterial whether or not Professor Owen preceded me, for both of us, as shown in this historical sketch, were long ago preceded by Dr. Wells and Mr. Matthews. M. Isidore Geoffroy Saint Hilaire, in his Lectures delivered in 1850 (of wRich a Résumé appeared in the ‘ Revue et Mag. de ce oo, Se TH Se Oe ee ae ae ay ey Do @ Fa eM So ™m™ Se fS- Historical Sketch. xix Zoolog.,’ Jan. 1851), briefly gives his reason for believing that specific characters “ sont fixés, pour chaque espéce, tant quelle se perpétue au milieu des mémes circonstances: ils se modifient, si les circonstances ambiantes viennent & changer.” ‘“ En résumé, lobservation des animaux sauvages démontre déja la variabilité limitée des especes. Les expériences sur les animaux sauvages devenus domestiques, et sur les animaux domestiques redevenus sauvages, la démontrent plus clairement encore. Ces mémes expé- riences prouvent, de plus, que les différences produites peuvent étre de valeur générique.” In his ‘ Hist. Nat. Générale’ (tom. ii. p. 430, 1859) he amplifies analogous conclusions. From a circular lately issued it appears that Dr. Freke, in 1851 (‘ Dublin Medical Press,’ p. 322), propounded the doctrine that all organic beings have descended from one primordial form. His grounds of belief and treatment of the subject are wholly different from mine; but as Dr. Freke has now (1861) published his Essay on ‘the Origin of Species by means of Organic Affinity,’ the diffi- cult attempt to give any idea of his views would be superfluous on my part. Mr. Herbert Spencer, in an Essay (originally published in the ‘Leader,’ March 1852, and republished in his ‘Essays’ in 1858), has contrasted the theories of the Creation and the Development of organic beings with remarkable skill and force. He argues from the analogy of domestic productions, from the changes which the embryos of many species undergo, from the difficulty of dis- tinguishing species and varieties, and from the principle of general gradation, that species have been modified; and he attributes the modification to the change of circumstances. The author (1855) has also treated Psychology on the principle of the necessary acquirement of each mental power and capacity by gradation. In 1852 M. Naudin, a distinguished botanist, expressly stated, in an admirable paper on the Origin of Species (‘ Revue Horticole,’ p. 102; since partly republished in the ‘Nouvelles Archives du Muséum,’ tom. i. p. 171), his belief that species are formed in an analogous manner as varieties are under cultivation; and the latter process he attributes to man’s power of selection. But he does not show how selection acts under nature. He believes, like Dean Herbert, that species, when nascent, were more plastic than at present. He lays weight on what he calls the principle of finality, “puissance mystérieuse, indéterminée; fatalité pour les uns ; pour les autres, volonté providentielle, dont l’action incessante sur les €tres vivants détermine, & toutes les époques de Vexistence du monde, la forme, le volume, et la durée de chacun d’eux, en raison b 2 xX Historical Sketch. —n “de sa destinée dans ordre de choses dont il fait partie. C'est cette puissance qui harmonise chaque membre a Vensemble en l’appro- priant a la fonction qwil doit remplir dans l’organisme général de la nature, fonction qui est pour lui sa raison d’étre.” * In 1853 a celebrated geologist, Count Keyserling (‘Bulletin de la Soc. Geolog.,’ 2nd Ser., tom. x. p. 357), suggested that as new diseases, supposed to have been caused by some miasma, have arisen and spread over the world, so at certain periods the germs of existing species may have been chemically affected by circum- ambient molecules of a particular nature, and thus have given rise to new forms. In this same year, 1853, Dr. Schaaffhausen published an ex- ‘cellent pamphlet (‘Verhand. des Naturhist. Vereins der Preuss. Rheinlands,’ &c.), in which he maintains the progressive develop- ment of organic forms on the earth. He infers that many species have kept true for long periods, whereas a few have become modi- ‘fied. The distinction of species he explains by the destruction of intermediate graduated forms. ‘Thus living plants and animals are not separated from the extinct by new creations, but are to be regarded as their descendants through continued reproduction.” A. well-known French botanist, M. Lecog, writes in 1854 (‘Etudes sur Géograph. Bot.,’ tom. i. p. 250), “On voit que nos ‘recherches sur la fixité ou la variation de l’espéce, nous conduisent directement aux idées émises, par deux hommes justement célébres, “Geoffroy Saint-Hilaire et Goethe.” Some other passages scattered through M. Lecoq’s large work, make it a little doubtful how far he extends his views on the modification of species. The ‘Philosophy of Creation’ has been treated in a masterly manner by the Rev. Baden Powell, in his ‘ Essays on the Unity of Worlds,’ 1855. Nothing can be more striking than the manner in which he shows that the introduction of new species is “a regular, * From references in Bronn’s ‘ Untersuchungen iiber die Entwickelungs- Gesetze,’ it appears that the celebrated botanist and paleontologist Unger published, in 1852, his belief that species undergo development and modifi- cation. D’Alton, likewise, in Pander and Dalton’s work on Fossil Sloths, expressed, in 1821, a similar belief. Similar views have, as is well known, been maintained by Oken in his mystical ‘ Natur-Philosophie.’ From other references in Godron’s work ‘Sur |’Espéce,’ it seems that Bory St. Vincent, Burdach, Poiret, and Fries, have all admitted that new species are continu- ally being produced. I may add, that of the thirty-four authors named in this Historical Sketch, who believe in the modification of species, or at least disbelieve in separate acts of creation, twenty-seven have written on special branches of natural history or geology. oo ee eae cps eae cr rr bento | = i ~ ee Historical Sketch. xxi not a casual phenomenon,” or, as Sir John Herschel expresses it, ‘“‘o natural in contradistinction to a miraculous process.” The third volume of the ‘ Journal of the Linnean Society ’ con- tains papers, read July Ist, 1858, by Mr. Wallace and myself, in which, as stated in the introductory remarks to this volume, the theory of Natural Selection is promulgated by Mr. Wallace with admirable force and clearness. Von Baer, towards whom all zoologists feel so profound a respect, expressed about the year 1859 (see Prof. Rudolph Wagner, ‘ Zoolo- gisch-Anthropologische Untersuchungen,’ 1861, s. 51) his convic- tion, chiefly grounded on the laws of geographical distribution, that forms now perfectly distinct have descended from a single parent-form. In June, 1859, Professor Huxley gave a lecture before the Royal Institution on the ‘ Persistent 'l'ypes of Animal life.’ Referring to such cases, he remarks, “It is difficult to comprehend the meaning of such facts as these, if we suppose that each species of animal and plant, or each great type of organisation, was formed and placed upon the surface of the globe at long intervals by a distinct act of creative power; and it is well to recollect that such an assumption is as unsupported by tradition or revelation as it is opposed to the general analogy of nature. If, on the other hand, we view ‘ Persistent Types’ in relation to that hypothesis which supposes the species living at any time to be the result of the gradual modification of pre-existing species—a hypothesis which, though unproven, and sadly damaged by some of its supporters, is yet the only one to which physiology lends any countenance ; their existence would seem to show that the amount of modification which living beings have undergone during geological time is but very small in relation to the whole series of changes which they have suffered.” In December, 1859, Dr. Hooker published his ‘Introduction to the Australian Flora.’ In the first part of this great work he admits the truth of the descent and modification of species, and supports this doctrine by many original observations. The first edition of this work was published on November 24th, 1859, and the second edition on January 7th, 1860, ORIGIN OF SPECIES. INTRODUCTION. WHEN on board H.M.S. ‘Beagle,’ as naturalist, I was much struck with certain facts in the distribution of the organic beings inhabit- ing South America, and in the geological relations of the present to the past inhabitants of that continent. These facts, as will be seen in the latter chapters of this volume, seemed to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. After five years’ work I allowed myself to speculate on the subject, and drew up some short notes ; these I enlarged in 1844 into a sketch of the conclusions, which then seemed to me probable: from that period to the present day I have steadily pursued the same object. I hope that I may be excused for entering on these personal details, as I give them to show that I have not been hasty in coming toa decision. My work is now (1859) nearly finished; but as it will take me many more years to complete it, and as my health is far from strong, I have been urged to publish this Abstract, I have more especially been induced to do this, as Mr. Wallace, who is now studying the natural history of the Malay archipelago, has arrived at almost exactly the same general conclusions that I have on the origin of species. In 1858 he sent me a memoir on this subject, with a request that I would forward it to Sir Charles Lyell, who Sent it to the Linnean Society, and it is published in the third volume of the Journal of that Society. Sir C. Lyell and Dr. Hooker, who both knew of my work—the latter having read my sketch of 1844—honoured me by thinking it advisable to publish, with Mr. Wallace’s excellent memoir, some brief extracts from my manu- scripts. This Abstract, which I now publish, must necessarily be im- perfect. I cannot here give references and authorities for my B 2 Introduction. several statements; and I must trust to the reader reposing some confidence in my accuracy. No doubt errors will have crept in, though I hope I have always been cautious in trusting to good authorities alone. I can here give only the general conclusions at which 1 have arrived, with a few facts in illustration, but which, I hope, in most cases will suffice. No one can feel more sensible than I do of the necessity of hereafter publishing in detail all the facts, with references, on which my conclusions have been grounded ; and I hope in a future work to do this. For I am well aware that scarcely a single point is discussed in this volume on which facts cannot be adduced, often apparently leading to conclusions directly opposite to those at which I have arrived. A fair result can be obtained only by fully stating and balancing the facts and argu- ments on both sides of each question; and this is here impossible. I much regret that want of space prevents my having the satis- faction of acknowledging the generous assistance which I have received from very many naturalists, some of them personally un- known to me. I cannot, however, let this opportunity pass without expressing my deep obligations to Dr. Hooker, who, for the last fifteen years, has aided me in every possible way by his large stores of knowledge and his excellent judgment. In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the con- clusion that species had not been independently created, but had descended, like varieties, from other species. Nevertheless, such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified, so as to acquire that perfection of structure and coadaptation which justly excites our admiration. Naturalists continually refer to external conditions, such as climate, food, &c., as the only possible cause of variation. In one limited sense, as we shall hereafter see, this may be true; but it is preposterous to attribute to mere external conditions, the structure, for instance, of the woodpecker, with its feet, tail, beak, and tongue, so admirably adapted to catch insects under the bark of trees. In the case of the mistletoe, which draws its nourishment from certain trees, which has seeds that must be transported by certain birds, and which has flowers with separate sexes absolutely requiring the agency of certain insects to bring pollen from one flower to the other, it is equally preposterous to account for the structure of this parasite, with its relations to several distinct organic beings, by the effects Liutroductton. 3 of external conditions, or of habit, or of the volition of the plant itself. It is, therefore, of the highest importance to gain a clear insight into the means of modification and coadaptation. At the commence- ment of my observations it seemed to me probable that a careful study of domesticated animals and of cultivated plants would offer the best chance of making out this obscure problem. Nor have I been disappointed ; in this and in all other perplexing cases I have invariably found that our knowledge, imperfect though it be, of variation under domestication, afforded the best and safest clue. I may venture to express my conviction of the high value of such studies, although they have been very commonly neglected by naturalists. From these considerations, I shall devote the first chapter of this Abstract to Variation under Domestication. We shall thus see that a large amount of hereditary modification is at least possible ; and, what is equally or more important, we shall see how great is the power of man in accumulating by his Selection successive slight variations. I will then pass on to the variability of species in a state of nature; but I shall, unfortunately, be compelled to treat this subject far too briefly, as it can be treated properly only by giving long catalogues of facts. We shall, however, be enabled to ‘discuss what circumstances are most favourable to variation. In the next chapter the Struggle for Existence amongst all organic beings throughout the world, which inevitably follows from the high geometrical ratio of their increase, will be considered. This is the doctrine of Malthus, applied to the whole animal and vegetable kingdoms. As many more individuals of each species are born than an possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the com- plex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form. This fundamental subject of Natural Selection will be treated at some length in the fourth chapter; and we shall then see how Natural Selection almost inevitably causes much Extinction of the less improved forms of life, and leads to what I have called Diver- gence of Character. In the next chapter I shall discuss the complex and little known laws of variation. In the five succeeding chapters, the most apparent and gravest difficulties in accepting the theory will be given: namely, first, the difficulties of transitions, or how a BQ 4 Introduction. ————— simple being or a simple organ can be changed and perfected into: a highly developed being or into an elaborately constructed organ s. secondly, the subject of Instinct, or the mental powers of animals ; thirdly, Hybridism, or the infertility of species and the fertility of varieties when intercrossed ; and fourthly, the imperfection of the Geological Record. In the next chapter I shall consider the geo-. logical succession of organic beings throughout time ; in the twelfth and thirteenth, their geographical distribution throughout space ; in the fourteenth, their classification or mutual affinities, both when mature and in an embryonic condition. In the last chapter I shall cive a brief recapitulation of the whole work, and a few concluding remarks. No one ought to feel surprise at much remaining as yet un- explained in regard to the origin of species and varieties, if he make due allowance for our profound ignorance in regard to the mutual relations of the many beings which live around us. Who can ex- plain why one species ranges widely and is very numerous, and. why another allied species has a narrow range and is rare? Yet these relations are of the highest importance, for they determine the present welfare, and, as I believe, the future success and modification of every inhabitant of this world. Still less do we know of the mutual relations of the innumerable inhabitants of the world during the many past geological epochs in its history. Although much) remains obscure, and will long remain obscure, I can entertain no: doubt, after the most deliberate study and dispassionate judgment of which I am capable, that the view which most naturalists until recently entertained, and which I formerly entertained—namely, that each species has been independently created—is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal) descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that Natural Selection has been the most important, but not the exclusive, means Of modification. Cuap. 1. Variation under Domestication. 5 a te me ae me ee ee ene CHAPTER I. VARIATION UNDER DOMESTICATION. Causes of Variability — Effects of Habit and the use or disuse of Parts — Correlated Variation — Inheritance —Character of Domestic Varieties — Difficulty of distinguishing between Varicties ana Species — Origin of Domestic Varieties from one or more Species — Domestic Pigeons, their Differences and Origin — Principles of Selection, anciently fol- lowed, their Effects — Methodical and Unconscious Selection — Un- known Origin of our Domestic Productions — Circumstances favour- able to Man’s power of Selection. Causes of Variability. \WueEN we compare the individuals of the same variety or sub- variety of our older cultivated plants and animals, one of the first points which strikes us is, that they generally differ more from each other than do the individuals of any one species or variety in a state of nature. And if we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all ages under the most different climates and treat- ment, we are driven to concludo that this great variability is due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent-species had been exposed under nature. ‘There is, also, some probability in the view propounded by Andrew Knight, that this variability may be partly connected with excess of food. It seems clear that organic beings must be exposed during several oenerations to new conditions to cause any great amount of varia- tion; and that, when the organisation has once begun to vary, it cenerally continues varying for many generations. No case is on record of a variable organism ceasing to vary under cultivation. Our oldest cultivated plants, such as wheat, still yield new varie- ties: our oldest domesticated animals are still capable of rapid improvement or modification. As faras I am able to judge, after long attending to the subject, the conditions of life appear to act in two ways,—directly on the whole organisation or on certain parts alone, and indirectly by affecting the reproductive system. With respect to the direct © Variation under Domestication. Crap. I. action, we must bear in mind that in every case, as Professor Weismann has lately insisted, and as I have incidentally shown in my work on ‘ Variation under Domestication,’ there are two factors: namely, the nature of the organism, and the nature of the conditions. The former seems to be much the more important ; for nearly similar variations sometimes arise under, as far as we can judge, dissimilar conditions; and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform. The effects on the offspring are either definite or indefinite. They may be considered as definite when all or nearly all the offspring of individuals exposed to certain conditions during several generations are modified in the same manner. It is extremely difficult to come to any conclusion in regard to the extent of the changes which have been thus definitely induced. There can, however, be little doubt about many slight changes,—such as size from the amount of food, colour from the nature of the food, thickness of the skin and hair from climate, &c. Each of the endless variations which we see in the plumage of our fowls must have had some efficient cause ; and if the same cause were to act uniformly during a long series of cenerations on many individuals, all probably would be modified in the same manner. Such facts as the complex and extraordinary out-growths which invariably follow from the insertion of a minute drop of poison by a gall-producing insect, show us what singular modifications might result in the case of plants from a chemical change in the nature of the sap. . Indefinite variability is a much more common result of changed: conditions than definite variability, and has probably played a more: important part in the formation of our domestic races. We see: indefinite variability in the endless slight peculiarities which dis- tinguish the individuals of the same species, and which cannot be accounted for by inheritance from either parent or from some more remote ancestor. Even strongly-marked differences occasionally appear in the young of the same litter, and in seedlings from the same seed-capsule. At long intervals of time, out of millions of individuals reared in the same country and fed on nearly the same food, deviations of structure so strongly pronounced as to deserve: to be called monstrosities arise; but monstrosities cannot be: separated by any distinct line from slighter variations. All such changes of structure, whether extremely slight or strongly marked, which appear amongst many individuals living together, may be considered as the indefinite effects of the conditions of life on each individual organism, in nearly the same manner as a chill affects. different men in an indefinite manner, according to their state: Cuar. I. Variation under Domestication. f of body or constitution, causing coughs or colds, rheumatism, or inflammations of various organs. With respect to what I have called the indirect action of changed conditions, namely, through the reproductive system being affected, we may infer that variability is thus induced, partly from the fact of this system being extremely sensitive to any change in the con- ditions, and partly from the similarity, as Koireuter and others have remarked, between the variability which follows from the crossing of distinct species, and that which may be observed with plants and animals when reared under new or unnatural conditions. Many facts clearly show how eminently susceptible the reproduc- tive system is to very slight changes in the surrounding conditions. Nothing is more easy than to tame an animal, and few things more difficult than to get it to breed freely under confinement, even when the male and female unite. How many animals there are which will not breed, though kept in an almost free state in their native country! his is generally, but erroneously, attributed to vitiated instincts. Many cultivated plants display the utmost vigour, and yet rarely or never seed! In some few cases it has been discovered that a very trifling change, such as a little more or less water at some particular period of growth, will determine whether or not a plant will produce seeds. I cannot here give the details which 1 have collected and elsewhere published on this curious subject ; but to show how singular the laws are which determine the repro- duction of animals under confinement, J may mention that car- nivorous animals, even from the tropics, breed in this country pretty freely under confinement, with the exception of the planti- grades or bear family, which seldom produce young; whereas carnivorous birds, with the rarest exceptions, hardly ever lay fertile evos. Many exotic plants have pollen utterly worthless, in the same condition as in the most sterile hybrids. When, on the one hand, we see domesticated animals and plants, though often weak and sickly, breeding freely under confinement; and when, on the other hand, we see individuals, though taken young from a state of nature perfectly tamed, long-lived and healthy (of which I could cive numerous instances), yet having their reproductive system so seriously affected by unperceived causes as to fail to act, we need not be surprised at this system, when it does act under confinement, acting irregularly, and producing offspring somewhat unlike their parents. I may add, that as some organisms breed freely under the most unnatural conditions (for instance, rabbits and ferrets kept in hutches), showing that their reproductive organs are not easily affected; so will some animals and plants withstand domestication 8 Variation under Domestication. Crap. I. or cultivation, and vary very slightly—perhaps hardly more than in a state of nature. oo Some naturalists have maintained that all variations are con- nected with the act of sexual reproduction; but this is certainly an error; for I have given in another work a long list of sporting plants,” as they are called by gardeners ;—that is, of plants which have suddenly produced a single bud with a new and sometimes widely different character from that of the other buds on the same plant. These bud variations, as they may be named, can be pro- pagated by grafts, offsets, &c., and sometimes by seed. They occur rarely under nature, but are far from rare under culture. As a single bud out of the many thousands, produced year after year on the same tree under uniform conditions, has been known suddenly to assume a new character; and as buds on distinct trees, growing under different conditions, have sometimes yielded nearly the same variety—for instance, buds on peach-trees producing nectarines, and buds on common roses producing moss-roses—we clearly see that the nature of the conditions is of subordinate importance in comparison with the nature of the organism in determining each particular form of variation ;—perhaps of not more importance than the nature of the spark, by which a mass of combustible matter is ignited, has in determining the nature of the flames. Effects of Habit and of the Use or Disuse of Parts; Correlated Variation ; Inheritance, Changed habits produce an inherited effect, as in the period of the flowering of plants when transported from one climate to another. With animals the increased use or disuse of parts has had a more marked influence; thus I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild-duck ; and this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parents. The sreat and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison: with these organs in other countries, is probably another instance of the effects of use. Not one of our domestic animals can be named which has not in some country drooping ears; and the view which has been suggested that the drooping is due to the disuse of the muscles of the ear, from the animals being seldom much alarmed, seems probable, Many laws regulate variation, some few of which can be dimly Cuap. I. Variation under Domestication. 9 seen, and will hereafter be briefly discussed. I will here only allude to what may be called correlated variation. Important changes in the embryo or larva will probably entail changes in the maturé animal. In monstrosities, the correlations between quite distinct parts are very curious ; and many instances are given in Isidore Geoffroy St. Hilaire’s great work on this subject. Breeders believe that long limbs are almost always accompanied by an elongated head. Some instances of correlation are quite whimsical : thus cats which are entirely white and have blue eyes are generally deaf; but it has been lately stated by Mr. Tait that this is confined to the males. Colour and constitutional peculiarities go together, of which many remarkable cases could be given amongst animals and plants. From facts collected by Heusinger, it appears that white sheep and pigs are injured by certain plants, whilst dark- coloured individuals escape: Professor Wyman has recently com- municated to me a good illustration of this fact; on asking some farmers in Virginia how it was that all their pigs were black, they informed him that the pigs ate the paint-root (Lachnanthes), which coloured their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the “crackers” (i.e. Virginia squatters) added, ‘‘ we select the black members of a litter for raising, as they alone have a good chance of living.” Hair- less dogs have imperfect teeth: long-haired and coarse-haired animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet. Hence if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly modify unintentionally other — parts of the structure, owing to the mysterious laws of correlation. The results of the various, unknown, or but dimly understood laws of variation are infinitely complex and diversified. It is well worth while carefully to study the several treatises on some of our old cultivated plants, as on the hyacinth, potato, even the dahlia, &c.; and it is really surprising to note the endless points of structure and constitution in which the varieties and sub-varieties differ slightly from each other. The whole organisation seems to have become plastic, and departs in a slight degree from that of the parental type. Any variation which is not inherited is unimportant for us. But the number and diversity of inheritable deviations of structure, both those of slight and those of considerable physiological impor- tance, are endless. Dr. Prosper Lucas’s treatise, in two large volumes, is the fullest and the best on this subject. No brecder IO Variation under Domestication. CHAP. 1. doubts how strong is the tendency to inheritance ; that like pro- duces like is kis fundamental belief: doubts have been thrown on this principle only by theoretical writers. When any deviation of structure often appears, and we see it in the father and child, we cannot tell whether it may not be due to the same cause having acted on both; but when amongst individuals, apparently exposed to the same conditions, any very rare deviation, due to some extraordinary combination of circumstances, appears in the parent —say, once amongst several million individuals—and it reappears in the child, the mere doctrine of chances almost compels us to. attribute its reappearance to inheritance. Every one must have heard of cases of albinism, prickly skin, hairy bodies, &c., appearing. in several members of the same family. If strange and rare deviations of structure are really inherited, less strange and com- moner deviations may be frecly admitted to be inheritable. Perhaps the correct way of viewing the whole subject would be, to. look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly. . The laws governing inheritance are for the most part unknown. No one can say why the same peculiarity in different individuals of the same species, or in different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or more remote ances-. tor; why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not exclusively to. the like sex. It is a fact of some importance to us, that peculiarities. appearing in the males of our domestic breeds are often transmitted, either exclusively or in a much greater degree, to the males alone. A much more important rule, which I think may be trusted, is that, at whatever period of life a peculiarity first appears, it tends to re-appear in the offspring at a corresponding age, though some- times earlier. In many cases this could not be otherwise; thus the inherited peculiarities in the horns of cattle could appear only in the offspring when nearly mature; peculiarities in the silk- worm are known to appear at the corresponding caterpillar or cocoon stage. But hereditary diseases and some other facts make me believe that the rule has a wider extension, and that, when there is no apparent reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared in the parent. I believe this rule to be of the highest importance in explaining the: laws of embryology. ‘These remarks are of course confined to the first appearance of the peculiarity, and not to the primary cause: Cuap. I. Variation under Domestiu ation. It which may have acted on the ovules or on the male element ; in nearly the same manner as the increased length of the horns in the offspring from a short-horned cow by a long-horned bull, though appearing late in life, is clearly ¢ ne to the male element. Having alluded to the subject of reversion, I may here refer to a statement often made by naturalists—namely, that our domestic — varieties, when run wild, gradually but invariably revert in charac- ter to their aboriginal stocks. Hence it has been argued that no deductions can be drawn from domestic races to species in a state of nature. I have in vain endeavoured to discover on what decisive facts the above statement has so often and so boldly been made. ‘There would be great difficulty in proving its truth: we may safely conclude that very many of the most strongly marked domestic varie- ties could not possibly live in a wild state. In many cases we do not know what the aboriginal stock was, and so could not tell whether or not nearly perfect reversion had ensued. It would be necessary in order to prevent the effects of intercrossing, that only a single variety should have been turned loose in its new home. N everthe- less, as our varieties certainly do occasionally revert in some of their characters to ancestral forms, it seems to me not improbable that if we could succeed in naturalising, or were to cultivate, during many generations, the several races, for instance, of the cabbage, in very poor soil (in which case, however, some effect would have to be attributed to the definite action of the poor soil), that they would, to a large extent, or even wholly, revert to the wild aboriginal stock, Whether or not the experiment would succeed, is not of great importance for our line of argument ; for by the experiment itself the conditions of life are changed. If it could be shown that our domestic varieties manifested a strong tendency to reversion,—that is, to lose their acquired characters, whilst kept under the same conditions, and whilst kept in a considerable body, so that free intercrossing might check, by blending together, any slight deviations in their structure, in such case, I grant that we could deduce nothing from domestic varieties in regard to species. But there is not a shadow of evidence in favour of this view: to assert that we could not breed our cart and race-horses, long and short-horned cattle, and poultry of various breeds, and esculent vegetables, for an unlimited number of generations, would be opposed to all experience. 12 Character of Domestic Varteties. Cuap. I, Character of Domestic Varieties: difficulty of distinguishing between Varieties und Species; origin of Domestic Varicties Srom one or more Species. When we look to the hereditary varicties or races of our domestic animals and plants, and compare them with closely allied species, we generally perceive in each domestic race, as already remarked, less uniformity of character than in true species. Domestic races often have a somewhat monstrous character; by which I mean, that, although differing from each other, and from other species of the same genus, in several trifling respects, they often differ in an extreme degree in some one part, both when compared one with another, and more especially when compared with the species under nature to which they are nearest allied. With these exceptions ‘(and with that of the perfect fertility of varieties when crossed,—a subject hereafter to be discussed), domestic races of the same species differ from each other in the same manner as do the closely-allied species of the same genus in a state of nature, but the differences in most cases are less in degree. This must be admitted as trtie, for the domestic races of many animals and plants have been ranked by some competent judges as the descendants of aborigi- nally distinct species, and by other competent judges as mere varieties. If any well marked distinction existed between a ‘domestic race and a species, this source of doubt would not so perpetually recur. It has often been stated that domestic. races ‘do not differ from each other in characters of generic value. It can be shown that this statement is not correct ; but naturalists differ much in determining what characters are of generic value; all such valuations being at present empirical. When it is explained -how genera originate under nature, it will be seen that we have no right to expect often to find a generic amount of difference in our domesticated races, In attempting to estimate the amount of structural difference between allied domestic races, we are soon involved in doubt, from not knowing whether they are descended from one or several parent species. This point, if it could be cleared up, would be interesting ; if, for instance, it could be shown that the greyhound, bloodhound, terrier, spaniel, and bull-dog, which we all know propagate their kind truly, were the offspring of any single species, then such facts would have great weight in making us doubt about the immutability of the many closely allied natural species—for instance, of the many foxes—inhabiting different quarters of the world. Ido not believe, as we shall presently see, that the whole e Cuar. 1 Character of Domestic Varieties. 13, —— amount of difference between the several breeds of the dog has been. produced under domestication ; I believe that a small part of the difference is due to their being descended from distinct species. In the case of strongly marked races of some other domesticated species, there is presumptive or even strong evidence, that all are descended from a single wild stock. It has often been assumed that man has chosen for domestica- tion animals and plants having an extraordinary inherent tendency to vary, and likewise to withstand diverse climates. I do not dispute that these capacities have added largely to the value of most of our domesticated productions ; but how could a savage possibly know,, when he first tamed an animal, whether it would vary in succeeding eenerations, and whether it would endure other climates? Has. the little variability of the ass and goose, or the small power of endurance of warmth by the reindeer, or of cold by the common camel, prevented their domestication? I cannot doubt that if other animals and plants, equal in number to our domesticated. productions, and belonging to equally diverse classes and countries, were taken from a state of nature, and could be made to breed for an equal number of generations under domestication, they would on an average vary as largely as the parent species of our existing domesticated productions have varied. In the case of most of our anciently domesticated animals and plants, it is not possible to come to any definite conclusion, whether: they are descended from one or several wild species. The argument mainly relied on by those who believe in the multiple origin of our domestic animals is, that we find in the most ancient times, on the monuments of Egypt, and in the lake-habitations of Switzerland, much diversity in the breeds; and that some of these ancient breeds closely resemble, or are even identical with, those still exist- ing. But this only throws far backwards the history of civilisation, and shows that animals were domesticated at a much earlier period than has hitherto been supposed. ‘The lake-inhabitants of Swit-- zerland cultivated several kinds of wheat and barley, the pea, the poppy for oil, and flax; and they possessed several domesticated animals. They also carried on commerce with other nations. All. this clearly shows, as Heer has remarked, that they had at. this. early age progressed considerably in civilisation; and this again implies a long continued previous period of less advanced civilisation,. during which the domesticated animals, kept by different tribes in different districts, might have varied and given rise to distinct races.. ‘ince the discovery of flint tools in the superficial formations of many parts of the world, all geologists believe that barbarian man: 14 Character of Domestic Varieties. Cuap. I, existed at an enormously remote period ; and we know that at the present day there is hardly a tribe so barbarous, as not to have domesticated at least the dog. The origin of most of our domestic animals will probably for ever remain vague. But I may here state, that, looking to the domestic dogs of the whole world, I have, after a laborious collection of all known facts, come to the conclusion that several wild species of Canide have been tamed, and that their blood, in some cases mingled together, flows in the veins of our domestic breeds. In regard to sheep and goats I can form no decided opinion. From facts communicated to me by Mr. Blyth, on the habits, voice, con- stitution, and structure of the humped Indian cattle, it is almost certain that they are descended from a different aboriginal stock from our European cattle ; and some competent judges believe that these latter have had two or three wild progenitors,—whether or not these deserve to be called species. ‘This conclusion, as well as that of the specific distinction between the humped and common cattle, may, indeed, be looked upon as established by the admirable re- searches of Professor Riitimeyer. With respect to horses, from reasons which I cannot here give, [am doubtfully inclined to believe, ‘in opposition to several authors, that all the races belong to the same species. Having kept nearly all the English breeds of the fowl alive, having bred and crossed them, and examined their skeletons, it appears to me almost certain that all are the descendants of the wild Indian fowl, Gallus bankiva; and this is the conclusion of Mr. Blyth, and of others who have studied this bird in India. In regard to ducks and rabbits, some breeds of which differ much from each other, the evidence is clear that they are all descended from the common wild duck and rabbit. The doctrine of the origin of our several domestic races from ‘several aboriginal stocks, has been carried to an absurd extreme by ‘some authors. ‘They believe that every race which breeds true, let the distinctive characters be ever so slight, has had its wild proto- type. At this rate there must have existed at least a score of species of wild cattle, as many sheep, and several goats, in Europe alone, and several even within Great Britain. One author believes that there formerly existed eleven wild species of sheep peculiar to Great Britain! When we bear in mind that Britain has now not one peculiar mammal, and France but few distinct from those of Germany, and so with Hungary, Spain, &c., but that each of these kingdoms possesses several peculiar breeds of cattle, sheep, &c., we must admit that many domestic breeds must have originated in Europe; for whence otherwise could they have been derived? So it is Cua. I. Domestic Pigeons. 1m in India. Even in the case of the breeds of the domestic dog through- out the world, which I admit are descended from several wild spe- cies, it cannot be doubted that there has been an immense amount of inherited variation; for who will believe that animals closely resembling the Italian greyhound, the bloodhound, the bull-dog, pug-dog, or Blenheim spaniel, &c.—so unlike all wild Canida— ever existed in a state of nature? It has often been loosely said that all our races of dogs have been produced by the crossing of a few aboriginal species; but by crossing we can only get forms in some degree intermediate between their parents ; and if we account for our several domestic races by this process, we must admit the former existence of the most extreme forms, as the Italian grey- hound, bloodhound, bull-dog, &c., in the wild state. Moreover, the possibility of making distinct races by crossing has been greatly exaggerated. Many cases are on record, showing that a race may be modified by occasional crosses, if aided by the careful selection of the individuals which present the desired character; but to obtain a race intermediate between two quite distinct races, would be very difficult. Sir J. Sebright expressly experimented with this object, and failed. The offspring from the first cross between two pure breeds is tolerably and sometimes (as I have found with pigeons) quite uniform in character, and everything seems simple enough ; but when these mongrels are crossed one with another for several generations, hardly two of them are alike, and then the difficulty of the task becomes manifest. Breeds of the Domestic Pigeon, their Differences and Origin. Believing that it is always best to study some special group, I have, after deliberation, taken up domestic pigeons. I have kept every breed which I could purchase or obtain, and have been most kindly favoured with skins from several quarters of the world, more especially by the Hon. W. Elliot from India, and by the Hon. C. Murray from Persia. Many treatises in different languages have been published on pigeons, and some of them are very important, as being of considerable antiquity. I have associated with several eminent fanciers, and have been permitted to join two of the London Pigeon Clubs. The diversity of the breeds is something astonishing. Compare the English carrier and the short-faced tumbler, and see the wonderful difference in their beaks, entailing corresponding differences in their skulls. The carrier, more especially the male bird, is also remarkable from the wonderful development of the carunculated skin about the head; and this is accompanied by creatly elongated eyelids, very large external orifices to the nostrils, 10 Domestic Pigeons. Cuap. |, and a wide gape of mouth. The short-faced tumbler has a beak in outline almost like that of a finch; and the common tumbler has the singular inherited habit of flying at a great height in a compact flock, and tumbling in the air head over heels. ‘The runt is a bird of great size, with long massive beak and large feet ; some of the sub-breeds of runts have very long necks, others very long wings and tails, others singularly short tails. ‘Ihe barb is allied to the carrier, but, instead of a long beak, has a very short and broad one. ‘The pouter has a much elongated body, wings, and legs ; and its enormously developed crop, which it glories in inflating, may well excite astonishment and even laughter. ‘The turbit has a short and conical beak, with a line of reversed feathers down the breast ; and it has the habit of continually expanding, slightly, the upper part of the cesophacus. he Jacobin has the feathers so much reversed along the back of the neck that they form a hood; and it has, pro- portionally to its size, elongated wing and tail feathers. The trumpeter and laugher, as their names express, utter a very different coo from the other breeds. ‘The fantail has thirty or even forty tail-feathers, instead of twelve or fourteen—the normal number in all the members of the great pigeon family : these feathers are kept expanded, and are carried so erect, that in good birds the head and tail touch: the oil-gland is quite aborted. Several other less distinct breeds might be specified. In the skeletons of the several breeds, the development of the bones of the face in length and breadth and curvature differs enor- mously. ‘he shape, as well as the breadth and length of the ramus of the lower jaw, varies in a highly remarkable manner. ‘The caudal and sacral vertebra vary in number; as does the number of the ribs, together with their relative breadth and the presence of processes. ‘The size and shape of the apertures in the sternum are highly variable ; so is the degree of divergence and relative size of the two arms of the furcula. The proportional width of the gape of mouth, the proportional length of the eyelids, of the orifice of the nostrils, of the tongue (not always in strict correlation with the length of beak), the size of the crop and of the upper part of the cesophagus ; the development and abortion of the oil-gland; the number of the primary wing and caudal feathers; the relative length of the wing and tail to each other and to hp body ; the relative length of the lez and foot; the number of scutella on the toes, the development “of skin between the toes, are all points of structure which are variable. The period at which the perfect plumage is acquired varies, as does the state of the down with which — the nestling birds are clothed when hatched. The shape and size SS “2 be iat td Cuap. I Domestic Pigeons. 17 of the eggs vary. The manner of flight, and in some breeds the voice and disposition, differ remarkably. Lastly, in certain breeds, the males and females have come to differ in a slight degree from each other. Altogether at least a score of pigeons might be chosen, which, if shown to an ornithologist, and he were told that they were wild birds, would certainly be ranked by him as well-defined species. Moreover, I do not believe that any ornithologist would in this case place the English carrier, the short-faced tumbler, the runt, the barb, pouter, and fantail in the same genus; more especially as in each of these breeds several truly-inherited sub-breeds, or species, as he would call them, could be shown him. Great as are the differences between the breeds of the pigeon, T am fully convinced that the common opinion of naturalists is correct, namely, that all are descended from the rock-pigeon (Columba livia), including under this term several geographical races or sub-species, which differ from each other in the most trifling respects. As several of the reasons which have led me to this belief are in some degree applicable in other cases, I wiil here briefly give them. If the several breeds are not varieties, and have not proceeded from the rock-pigeon, they must have descended from. at least seven or eight aboriginal stocks; for it is impossible to make the present domestie breeds by the crossing of any lesser number: how, for instance, could a pouter be produced by crossing two breeds unless one of the parent-stocks possessed the characteristic enormous crop? ‘The supposed aboriginal stocks must all have been rock-pigeons, that is, they did not breed or willingly perch on trees. But besides C. livia, with its gecgraphical sub-species, only two or three other species of rock-pigeons are known; and these have not any of the characters of the domestic breeds, Hence the supposed aboriginal stocks must either still exist in the countries where they were originally domesticated, and yet be unknown to ornithologists ; and this, considering their size, habits, and remark- able characters, seems improbable; or they must have become extinct in the wild state. But birds breeding on precipices, and sood fliers, are unlikely to be exterminated; and the common rock-| pigeon, which has the same habits with the domestic breeds, has not been exterminated even on several of the smaller British islets, or on the sores of the Mediterranean. Hence the supposed exter- —_ st ¥ . Se risers ok fate many species hay Ing eee habits with the rock- pig Sa very rash assumption. Morever, the several above- re a enone oh a eal not ) tore, em must have been carried back C 18 Domestic Pigeons. Cuap. Ii again into their native country; but not one has become wild or feral, though the dovecot-pigeon, which is the rock-pigeon in a very slightly altered state, has become feral in several places. Again, all recent experience shows that it is difficult to get wild animals to. breed freely under domestication; yet, on the hypothesis of the multiple origin of our pigeons, it must be assumed that at least seven or eight species were so thoroughly domesticated in ancient: times by half-civilised man, as to be quite prolific under con- finernent. An argument of great weight, and applicable in several other cases, is, that the above-specified breeds, though agreeing generally with the wild rock-pigeon in constitution, habits, voice, colouring,, and in most parts of their structure, yet are certainly highly abnor- mal in other parts; we may look in vain through the whole great family of Columbide for a beak like that of the English carrier, or that of the short-faced tumbler, or barb; for reversed feathers like: those of the Jacobin; for a crop like that of the pouter; for tail- feathers like those of the fantail. Hence it must be assumed not: only that half-civilised man succeeded in thoroughly domesticating several species, but that he intentionally or by chance picked out extraordinarily abnormal species; and further, that these very species have since all become extinct or unknown. So many strange contingencies are improbable in the highest degree. Some facts in regard to the colouring of pigeons well deserve consideration. The rock-pigeon is of a slaty-blue, with white loins; but the Indian sub-species, C. intermedia of Strickland, has this part bluish, The tail has a terminal dark bar, with the outer feathers externally edged at the base with white. The wings have: two black bars. Some semi-domestic breeds, and some truly wild breeds, have, besides the two black bars, the wings chequered with black. These several marks do not occur together in any other species of the whole family. Now, in every one of the domestic breeds, taking thoroughly well-bred birds, all the above marks, even to the white edging of the outer tail-feathers, sometimes concur’ perfectly developed. Moreover, when birds belonging to two or more distinct breeds are crossed, none of which are blue or have any of the above-specified marks, the mongrel offspring are very apt suddenly to acquire these characters. To give one instance out of several which I have observed :—I crossed some white fantails, which breed very true, with some black barbs—and it so happens that blue varieties of barbs are so rare that I never heard of an instance’ in England ; and the mongrels were black, brown, and mottled. I also crossed a barb with a spot, which is a white bird with a red Cua. I. Domestic Pigeons, — IQ tail and red spot on the forehead, and which notoriously breeds very true; the mongrels were dusky and mottled. I then crossed one of the mongrel barb-fantails with a mongrel barb-spot, and they pro- duced a bird of as beautiful a blue colour, with the white loins, double black wing-bar, and barred and white-edged tail- feathers, as any wild rock-pigeon! We can understand these facts, on the well-known principle of reversion to ancestral characters, if all the domestic breeds are descended from the rock-pigeon. But if we deny this, we must make one of the two following highly im- probable suppositions, Either, first, that all the several imacined aboriginal stocks were coloured and marked like the rock-pigeon, although no other existing species is thus coloured and marked, so that in each separate breed there might be a tendency to revert to the very same colours and markings. Or, secondly, that each breed, even the purest, has within a dozen, or at most within a score, of generations, been crossed by the rock-pigeon: 1 say within a dozen or twenty generations, for no instance is known of crossed descendants reverting to an ancestor of foreign blood, removed by a greater number of generations. In a breed which has been crossed only once, the tendency to revert to any character derived from such a cross will naturally become less and less, as in each succeed- ing generation there will be less of the foreign blood; but when there has been no cross, and there is a tendency in the breed to revert to a character which was lost during some former generation, this tendency, for all that we can see to the contrary, may be transmitted undiminished for an indefinite number of generations. These two distinct cases of reversion are often confounded together by those who have written on inheritance. Lastly, the hybrids or mongrels from between all the breeds of the pigeon are perfectly fertile, as I can state from my own obser- vations, purposely made, on the most distinct breeds. Now, hardly any cases have been ascertained with certainty of hybrids from two quite distinct species of animals being perfectly fertile. Sorne authors believe that long-continued domestication eliminates this strong tendency to sterility in species. From the history of the dog, and of some other domestic animals, this conclusion is pro- bably quite correct, if applied to species closely related to each other. But to extend it so far as to suppose that species, aborizi- nally as distinct as carriers, tumblers, pouters, and fantails now are, should yield offspring perfectly fertile inter sé, would be rash in the extreme. From these several reasons, namely,—the improbability of man having formerly made seven or eight supposed species of pigeons to c 2 20 Domestic Pigeons. Cuap. I, — breed freely under domestication ;— these supposed species being quite unknown in a wild state, and their not having become any- where feral ;—these species presenting certain very abnormal cha- yacters, as compared with all other Columbide, though so like the yock-pigeon in most respects ;—the occasional re-appearance of the blue colour and various black marks in all the breeds, both when kept pure and when crossed ;—and lastly, the mongrel off- spring being perfectly fertile;—from these several reasons, taken together, we may safely conclude that all our domestic breeds are descended from the rock-pigeon or Columba livia with its geogra- phical sub-species. In favour of this view, I may add, firstly, that the wild C. livia has been found capable of domestication in Europe and in India; and that it agrees in habits and in a great number of points of struc- ture with all the domestic breeds. Secondly, that, although an English carrier or a short-faced tumbler differs immensely in certain characters from the rock-pigeon, yet that, by comparing the several sub-breeds of these two races, more especially those brought from distant countries, we can make, between them and the rock-pigeon, an almost perfect series; so we can in some other cases, but not with all the breeds. Thirdly, those characters which are mainly dis- tinctive of each breed are in each eminently variable, for instance the wattle and length of beak of the carrier, the shortness of that of the tumbler, and the number of tail-feathers in the fantail; and the explanation of this fact will be obvious when we treat of Selec- tion. Fourthly, pigeons have been watched and tended with the utmost care, and loved by many people. They have been domesti- cated for thousands of years in several quarters of the world; the earliest known record of pigeons is in the fifth Augyptian dynasty, about 38000 B.c., as was pointed out to me by Professor Lepsius ; but Mr. Birch informs me that pigeons are given in a bill of fare in the previous dynasty. In the time of the Romans, as we hear from Pliny, immense prices were given for pigeons; “nay, they are come to this pass, that they can reckon up their pedigree and race.” Pigeons were much valued by Akber Khan in India, about the year 1600; never less than 20,000 pigeons were taken with the court. “The monarchs of Iran and Turan sent him some very rare birds ;” and, continues the courtly historian, “His Majesty by crossing the breeds, which method was never practised before, has improved them astonishingly.” About this same period the Dutch were as eager about pigeons as were the old Romans. The paramount importance of these considerations in explaining the immense amount of vari- ation which pigeons have undergone, will likewise be obvious when Cuap. L Domestic Pigeons. 21 we treat of Selection. We shall then, also, see how it is that the several breeds so often have a somewhat monstrous character, It is also a most favourable circumstance for the production of dis- tinct breeds, that male and female pigeons can be easily mated for life; and thus different breeds can be kept together in the same aviary. I have discussed the probable origin of domestic pigeons at some, yet quite insufficient, length; because when I first kept pigeons and watched the several kinds, well knowing how truly they breed, I felt fully as much difficulty in believing that since they had been domesticated they had all proceeded from a common parent, as any naturalist could in coming to a similar conclusion in regard to the many species of finches, or other groups of birds, in nature. One circumstance has struck me much; namely, that nearly all the breeders of the various domestic animals and the cultivators of plants, with whom I have conversed, or whose treatises I have read, are firmly convinced that the several breeds to which each has at- tended, are descended from so many aboriginally distinct species. Ask, as I have asked, a celebrated raiser of Hereford cattle, whether his cattle might not have descended from Long-horns, or both from @ common parent-stock, and he will laugh you to scorn. 1 have never met a pigeon, or poultry, or duck, or rabbit fancier, who was not fully convinced that each main breed was descended from a dis- tinct species. Van Mons, in his treatise on pears and apples, shows how utterly he disbelieves that the several sorts, for instance a Ribston-pippin or Codlin-apple, could ever have proceeded from the seeds of the same tree. Innumerable other examples could be given. The explanation, I think, is simple: from long-continued study they are strongly impressed with the differences between the several races; and though they well know that each race varies slightly, for they win their prizes by selecting such slight differences, yet they ignore all general arguments, and refuse to sum up in their minds slight differences accumulated during many successive eene- rations. May not those naturalists who, knowing far less of the laws of inheritance than does the breeder, and knowing no more than he does of the intermediate links in the long lines of descent, yet admit that many of our domestic races are descended from the same parents—may they not learn a lesson of caution, when they deride the idea of species in a state of nature being lineal descendant3 of other species ? a : 22 Selection by Man. Car. I, Principles of Selection anciently followed, and their Effects, Let us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species, Some effect may be attributed to the direct and definite action of the external conditions of life, and some to habit; but he would be a bold man who would account by such agencies for the differences between a dray and race horse, a greyhound and bloodhound, a car- rier and tumbler pigeon. One of the most remarkable features in our domesticated races is that we see in them adaptation, not indeed to the animal’s or plant’s own good, but to man’s use or fancy. Some variations useful to him have probably arisen suddenly, or by one step; many botanists, for instance, believe that the fuller’s teasel, with its hooks, which cannot be rivalled by any mechanical con- trivance, is only a variety of the wild Dipsacus; and this amount of change may have suddenly arisen in a seedling. So it has pro- bably been with the turnspit dog; and this is known to have been the case with the ancon sheep. But when we compare the dray- horse and race-horse, the dromedary and camel, the various breeds of sheep fitted either for cultivated land or mountain pasture, with - the wool of one breed good for one purpose, and that of another breed for another purpose; when we compare the many breeds of dogs, each good for man in different ways; when we compare the game-cock, so pertinacious in battle, with other breeds so little quar- relsome, with “everlasting layers” which never desire to sit, and with the bantam so small and elegant; when we compare the host of agricultural, culinary, orchard, and flower-garden races of plants, most useful to man at different seasons and for ditferent purposes, or so beautiful in his eyes, we must, I think, look further than to mere variability. We cannot suppose that all the breeds were suddenly produced as perfect and as useful as we now see them; indeed, in many cases, we know that this has not been their history. The key is man’s power of accumulative selection: nature gives succes- sive variations; man adds them up in certain directions useful to him. In this sense he may be said to have made for himself useful breeds. | The great power of this principle of selection is not hypothetical. It is certain that several of our eminent breeders have, even within a single lifetime, modified to a large extent their breeds of cattle and sheep. In order fully to realise what they have done, it is almost necessary to read several of the many treatises devoted to this sub-— ject, and to inspect the animals. Breeders habitually speak of an animal’s organisation as something plastic, which they can model al’ Cuar. L Selection by Man. 23 almost as they please. If I had space I could quote numerous pas- saces to this effect from highly competent authorities, Youatt, who was probably better acquainted with the works of agriculturists than almost any other individual, and who was himself a very good judge of animals, speaks of the principle of selection as “ that which enables the agriculturist, not only to modify the character of his flock, but to change it altogether. It is the magician’s wand, by means of which he may summon into life whatever form and mould he pleases.” Lord Somerville, speaking of what breeders have done for sheep, says:—‘It would seem as if they had chalked out upon a wall a form perfect in itself, and then had given it existence.” In Saxony the importance of the principle of selection in regard to merino sheep is so fully recognised, that men follow it as a trade: the sheep are placed on a table and are studied, like a picture by a connoisseur ; this is done three times at intervals of months, and the sheep are each time marked and classed, so that the very best may ultimately be selected for breeding. What English breeders have actually effected is proved by the enormous prices given for animals with a good pedigree; and these have been exported to almost every quarter of the world. The improvement is by no means generally due to crossing different breeds; all the best breeders are strongly opposed to this practice, except sometimes amongst closely allied sub-breeds. And when a cross has been made, the closest selection is far more indispensable even than in ordinary cases. If selection consisted merely in sepa- rating some very distinct variety, and breeding from it, the principle would be so obvious as hardly to be worth notice; but its import- ance consists in the great effect produced by the accumulation in one direction, during successive generations, of differences absolutely inappreciable by an uneducated eye—differences which I for one have vainly attempted to appreciate. Not one man in a thousand has accuracy of eye and judgment sufficient to become an eminent breeder. If gifted with these qualities, and he studies his subject for years, and devotes his lifetime to it with indomitable perse- verance, he will succeed, and may make great improvements; if he wants any of these qualities, he will assuredly fail. Few would readily believe in the natural capacity and years of practice requisite to become even a skilful pigeon-fancier. The same principles are followed by horticulturists; but the vari- ations are here often more abrupt. No one supposes that our choicest productions have been produced by a single variation from the aboriginal stock. We have proofs that this has not been so in -several cases in which exact records have been kept; thus, to give u 24 Methodical Selection. Cuap. §, es very trifling instance, the steadily-increasing size of the common gooseberry may be quoted. We see an astonishing improvement in many florists’ flowers, when the flowers of the present day are com- pared with drawings made only twenty or thirty years ago, | When arace of plants is once pretty well established, the seed-raisers do. not pick out the best plants, but merely go over their seed-beds, and pull up the “rogues,” as they call the plants that deviate from the proper standard. With animals this kind of selection is, in fact, likewise followed; for hardly any one is so careless ag to. breed from his worst animals. In regard to plants, there is another means of observing the accumulated effects of selection—namely, by comparing the diver- sity of flowers in the different varieties of the same species in the flower-garden; the diversity of leaves, pods, or tubers, or whatever part is valued, in the kitchen-garden, in comparison ‘with the flowers of the same varieties; and the diversity of fruit of the same species in the orchard, in comparison with the leaves and flowers of the same set of varieties. See how different the leaves of the cabbage are, and how extremely alike the flowers; how unlike the flowers of the heartsease are, and how alike the leaves; how much the fruit of the different kinds of gooseberries differ in size, colour, shape, and hairiness, and yet the flowers present very slight diffe- rences,. It is not that the varieties which differ largely in some one point do not differ at all in other points; this is hardly ever,—I speak after careful observation,—perhaps never, the case. The law of correlated variation, the importance of which should never be overlooked, will ensure some differences ; but, as a general rule, it cannot be doubted that the continued selection of slight variations, either in the leaves, the flowers, or the fruit, will produce races dif~ fering from each other chiefly in these characters, It may be objected that the principle of selection has been reduced to methodical practice for scarcely more than three-quarters of a century; it has certainly been more attended to of late years, and many treatises have been published on the subject; and the result has been, in a corresponding degree, rapid and important. But it is very far from true that the principle is a modern discovery. I could give several references to works of high antiquity, in which: the full importance of the principle is acknowledged. In rude and barbarous periods of English history choice animals were often im- ported, and laws were passed to prevent their exportation: the: destruction of horses under a certain size was ordered, and this may be compared to the “ roguing ” of plants by nurserymen. ‘The prin- ciple of selection I find distinctly given in an ancient Chinese ency~ Cuap. I. Unconscious Selection. 25, a ee clopedia. Explicit rules are laid down by some of the Roman classical writers. From passages in Genesis, it is clear that the colour of domestic animals was at that early period attended to. Savages now sometimes cross their dogs with wild canine animals, to improve the breed, and they formerly did so, as is attested by passages in Pliny. The savages in South Africa match their draught cattle by colour, as do some of the Esquimaux their teams of dogs. Livingstone states that good domestic breeds are highly valued by the negroes in the interior of Africa who have not associ- ated with Europeans. ome of these facts do not show actual selection, but they show that the breeding of domestic animals was carefully attended to in ancient times, and is now attended to by the lowest savages. It would, indeed, have been a strange fact, had. attention not been paid to breeding, for the inheritance of good and bad qualities is so obvious. Unconscious Selection. At the present time, eminent breeders try by methodical selection, with a distinct object in view, to make a new strain or sub-breed, superior to anything of the kind in the country. But, for our pur-- pose, a form of Selection, which may be called Unconscious, and. which results from every one trying to possess and breed from the: best individual animals, is more important. ‘Thus, a man who intends keeping pointers naturally tries to get as good dogs as he: can, and afterwards breeds from his own best dogs, but he has no. wish or expectation of permanently altering the breed. Neverthe- less we imay infer that this process, continued during centuries, would improve and modify any breed, in the same way as Bake-- well, Collins, &e., by this very same process, only carried on more methodically, did greatly modify, even during their lifetimes, the- forms and qualities of their cattle. Slow and insensible changes of this kind can never be recognised unless actual measurements or careful drawings of the breeds in question have been made long ago, which may serve for comparison. In some cases, however, un-- changed, or but little changed individuals of the same breed exist in less civilised districts, where the breed has been less improved. There is reason to believe that King Charles's spaniel has been un- consciously modified to a large extent since the time of that monarch. Some highly competent authorities are convinced that the setter is directly derived from the spaniel, and has probably been slowly altered from it. It is known that the English pointer has been greatly changed within the last century, and in this case the change has, it is believed, been chiefly effected by crosses with the foxhound :: 26 — Unconsctous Selection. Crap. I, but what concerns us is, that the change has been effected uncon- sciously and gradually, and yet so effectually, that, though the old Spanish pointer certainly came from Spain, Mr. Borrow has not seen, as I am informed by him, any native dog in Spain like our pointer. By a similar process of selection, and by careful training, English recehorses have come to surpass in fleetness and size the parent Arabs, so that the latter, by the regulations for the Goodwood Races, are favoured in the weights which they carry. Lord Spencer and others have shown how the cattle of England have increased in weight and in early maturity, compared with the stock formerly kept in this country. By comparing the accounts given in various -old treatises of the former and present state of carrier and tumbler pigeons in Britain, India, and Persia, we can trace the stages through ‘which they have insensibly passed, and come to differ so greatly ‘from the rock-pigeon. Youatt gives an excellent illustration of the effects of a course of -selection, which may bé considered as unconscious, in so far that the ‘breeders could never have expected, or even wished, to produce the result which ensued—namely, the production of two distinct strains. The two flocks of Leicester sheep kept by Mr. Buckley -and Mr. Burgess, as Mr. Youatt remarks, “ have been purely bred from the original stock of Mr. Bakewell for upwards of fifty years. ‘here is not a suspicion existing in the mind of any one at all acquainted with the subject, that the owner of either of them has -deviated in any one instance from the pure blood of Mr. Bakewell’s flock, and yet the difference between the sheep possessed by these two gentlemen is so great that they have the appearance of being quite different varieties.” If there exist savages so barbarous as never to think of the inherited character of the offspring of their domestic animals, yet any one animal particularly useful to them, for any special purpose, would be carefully preserved during famines and other accidents, to which savages are so liable, and such choice animals would thus generally leave more offspring than the inferior ones; so that in this case there would be a kind of unconscious selection going on. We see the value set on animals even by the barbarians of Tierra del Fuego, by their killing and devouring their old women, in times of dearth, as of less value than their dogs. | In plants the same gradual process of improvement, through the occasional preservation of the best individuals, whether or not ‘sufficiently distinct to be ranked at their first appearance as distinct varieties, and whether or not two or more Species or races have become blended together by crossing, may plainly be recognised in cuap. I. Unconscious Selection. 27 the increased size and beauty which we now see in the varieties of the heartsease, rose, pelargonium, dahlia, and other plants, when compared with the older varieties or with their parent-stocks. No one would ever expect to get a first-rate hearsease or dahlia from the sced of a wild plant. No one would expect to raise a first-rate melting pear from the seed of the wild pear, though he might succeed from a poor seedling growing wild, if it had come from a carden-stock. The pear, though cultivated in classical times, appears, from Pliny’s description, to have been a fruit of very inferior quality. I have seen great surprise expressed in horti- cultural works at the wonderful skill of gardeners, in having pro- duced such splendid results from such poor maicrials ; but the art has been simple, and, as far as the final result is concerned, has been followed almost unconsciously. It has consisted in always cultivating the best known variety, sowing its seeds, and, when a slightly better variety chanced to appear, selecting it, and so on- wards. But the gardeners of the classical period, who cultivated the best pears which they could procure, never thought what splendid fruit we should eat; though we owe our excellent fruit, in some small degree, to their having naturally chosen and preserved the best varieties they could anywhere find. A large amount of change, thus slowly and unconsciously ac- cumulated, explains, as I believe, the well-known fact, that in a number of cases we cannot recognise, and therefore do not know, the wild parent-stocks of the plants which have been longest cul!ti- vated in our flower and kitchen gardens. If it has taken centuries or thousands of years to improve or modify most of our plants up to their present standard of usefulness to man, we can understand how it is that neither Australia, the Cape of Good Hope, nor any other region inhabited by quite uncivilised man, has afforded us a single plant worth culture. It is not that these countries, so rich in species, do not by a strange chance possess the aboriginal stocks of any use- ful plants, but that the native plants have not been improved by continued selection up to a standard of perfection comparable with that acquired by the plants in ‘countries anciently civilised. In regard to the domestic animals kept by uncivilised man, it should not be overlooked that they almost always have to struggle lor their own food, at least during certain seasons. And in two countries very differently circumstanced, individuals of the same species, having slightly different constitutions or structure, would oiten succeed better in the one country than in the other; and thus by a process of “natural selection,” as will hereafter be more fully explained, two sub-breeds might be formed. ‘This, perhaps, partly 28 Unconscious Selection. Cuap. 5 explains why the varieties kept by savages, as has been remarked by some authors, have more of the character of true species than the- varieties kept in civilised countries. On the view here given of the important part which selection by man has played, it becomes at once obvious, how it 1s that our domestic races show adaptation in their structure or in their habits. to man’s wants or fancies. We can, I think, further understand the frequently abnormal character of our domestic races, and likewise: their differences being so great in external characters, and relatively so slight in internal parts or organs. Man can hardly select, or only with much difficulty, any deviation of structure excepting such as is externally visible; and indeed he rarely cares for what is. internal, He can never act by selection, excepting on variations. which are first given to him in some slight degree by nature. No. ’ man would ever try to make a fantail till he saw a pigeon with a tail developed in some slight degree in an unustial manner, or a pouter till he saw a pigeon with a crop of somewhat unusual size; and the more abnormal or unusual any character was when it first appeared, the more likely it would be to catch his attention. But to use such an expression as trying to make a fantail, is, I have no: doubt, in most cases, utterly incorrect. The man who first selected a pigeon with a slightly larger tail, never dreamed what the descend-: ants of that pigeon would become through long-continued, partly unconscious and partly methodical, selection. Perhaps the parent- bird of all fantails had only fourteen tail-feathers somewhat expanded,, like the present Java fantail, or like individuals of other and distinct breeds, in which as many as seventeen tail-feathers have been counted. Perhaps the first pouter-pigeon did not inflate its crop: much more than the turbit now does the upper part of its ceso- phagus,—a habit which is disregarded by all fanciers, as it is not one of the points of the breed. Nor let it be thought that some great deviation of structure would be necessary to catch the fancier’s eye: he perceives ex- tremely small differences, and it is in human nature to value any novelty, however slight, in one’s own possession. Nor must the value which would formerly have been set on any slight differences in the individuals of the same species, be judged of by the value which 1S now set on them, after several breeds have fairly been established. It is known that with pigeons many slight variations: now occasionally appear, but these are rejected as faults or devia- tions from the standard of perfection in each breed. The commom goose has not given rise to any marked varieties; hence the Tou- Jouse and the common breed, which differ only in colour, that mos’ cuar.I. Circumstances favourable to Selection. 29 fleeting of characters, have lately been exhibited as distinct at our poultry-shows. . _ These views appear to explain what has sometimes been noticed —namely, that we know hardly anything about the origin or history of any of our domestic breeds. But, in fact, a breed, like a dialect of a language, can hardly be said to have a distinct origin. A man preserves and breeds from an individual with some slight deviation of structure, or takes more care than usual in matching his best animals, and thus improves them, and the improved animals slowly spread in the immediate neighbourhood. But they will as yet hardly have a distinct name, and from being only slightly valued, their history will have been disregarded. When further improvea by the same slow and gradual process, they will spread more widely, and will be recognised as something distinct and valuable, and will then probably first receive a provincial name. In semi-civilised countries, with little free communication, the spreading of a new sub-breed would be a slow process. As soon as the points of value are once acknowledged, the principle, as I have called it, of un- conscious selection will always tend,—perhaps more at one period than at another, as the breed rises or falls in fashion,—perhaps more in one district than in another, according to the state of civilisation of the inhabitants,—slowly to add to the characteristic features of the breed, whatever they may be. But the chance will be infinitely small of any record having been preserved of such slow, varying, and insensible changes. Circumstances fuvourable to Mun’s Power of Selection. I will now say a few words on the circumstances, favourable, or the reverse, to man’s power of selection. A high degree of vari- ability is obviously favourable, as freely giving the materials for selection to work on; not that mere individual differences are not amply sufficient, with extreme care, to allow of the accumulation of a large amount of modification in almost any desired direction. But as variations manifestly useful or pleasing to man appear only occasionally, the chance of their appearance will be much increased by a large number of individuals being kept. Hence, number is of the highest importance for success. On this principle Marshall formerly remarked, with respect to the sheep of parts of Yorkshire, “as they generally belong to poor people, and are mostly am small lots, they never can be improved.” On the other hand, hurserymen, from keeping large stocks of the same plant, are gener- ally far more successful than amateurs in raising new and valuable varieties, A large number of individuals of an animal or plant can 30 Circumstances favourable to Selection, Car. 1. be reared only where the conditions for its propagation are favour- able. When the individuals are scanty, all will be allowed to breed, whatever their quality may be, and this will effectually prevent, selection. But probably the most important element is that the animal or plant should be so highly valued by man, that the closest attention is paid to even the slightest deviations in its qualities or structure. Uniess such attention be paid nothing can be effected, I have seen it gravely remarked, that it was most fortunate that the strawberry began to vary just when gardeners began to attend to this plant. No doubt the strawberry had always varied since it was cultivated, but the slight varieties had been neglected. Ag: soon, however, as gardeners picked out individual plants with slightly larger, earlier, or better fruit, and raised seedlings from them, and again picked out the best seedlings and bred from them, then (with some aid by crossing distinct species) those many admirable varieties of the strawberry were raised which have ap- peared during the last half-century. With animals, facility in preventing crosses is an important element in the formation of new races,—at least, in a country which is already stocked with other races. In this respect en- closure of the land plays a part. Wandering savages or the in- habitants of open plains rarely possess more than one breed of the same species. Pigeons can be mated for life, and this is a great convenience to the fancier, for thus many races may be improved and kept true, though mingled in the same aviary; and this cir- cumstance must have largely favoured the formation of new breeds. Pigeons, I may add, can be propagated in great numbers and at a very quick rate, and inferior birds may be freely rejected, as when killed they serve for food. On the other hand, cats, from their nocturnal rambling habits, cannot be easily matched, and, although so much valued by women and children, we rarely see a distinct breed long kept up; such breeds as we do sometimes see are almost always imported from some other country. Although I do not doubt that some domestic animals vary less than others, yet the rarity or absence of distinct breeds of the cat, the donkey, peacock, goose, &c., may be attributed in main part to selection not having been brought into play: in cats, from the difficulty in pairing them ; m donkeys, from only a few being kept by poor people, and little attention paid to their breeding; for recently in certain parts of Spain and of the United States this animal has been surprisingly modified and improved by careful selection: in peacocks, from not being very easily reared and a large stock not kept: in geese, from. being valuable only for two purposes, food and feathers, and more cua. Circumstances favourable to Selection. 31 aving been felt in the display of distinct: onditions to which it is exposed ngularly inflexible organisa- extent, as I have elsewhere: especially from no pleasure h breeds; but the goose, under the cond! when domesticated, seems to have a 51 tion, though it has varied to a slight described. tee Some authors have maintained that the amount of variation in our domestic productions is soon reached, and can never afterwards. pe exceeded. It would be somewhat rash to assert that the limit: has been attained in any one case; for almost all our animals and. plants have been greatly improved in many ways within a recent period ; and this implies variation. It would be equally rash to assert that characters now increased to their utmost limit, could not, after remaining fixed for many centuries, again vary under new conditions of life. No doubt, as Mr. Wallace has remarked with much truth, a limit will be at last reached. For instance, there must be a limit to the fleetness of any terrestrial animal, as. this will be determined by the friction to be overcome, the weight. of body to be carried, and the power of contraction in the muscular fibres. But what concerns us is that the domestic varicties of the: same species differ from each other in almost every character, which man has attended to and selected, more than do the distinct species of the same genera. Isidore Geoffroy St. Hilaire has proved this in regard to size, and so it is with colour and probably with the length of hair. With respect to fleetness, which depends on many bodily characters, Kclipse was far fleeter, and a dray-horse is incomparably stronger than any two natural species belonging to the same genus. So with plants, the seeds of the different varieties of the- bean or maize probably differ more in size, than do the seeds of the distinct species in any one genus in the same two families. The same remark holds good in regard to the fruit of the several. varieties of the plum, and still more strongly with the melon, as. well as in many other analogous cases. To sum up on the origin of our domestic races of animals and plants. Changed conditions of life are of the highest importance in causing variability, both by acting directly on the organisation, and indirectly by affecting the reproductive system. It is not probable: that variability is an inherent and necessary contingent, under all circumstances. ‘The greater or less force of inheritance and rever- Sion determine whether variations shall endure. Variability is. governed by many unknown laws, of which correiated growth is probably the most important. Something, but how much we do. not know, may be attributed to the definite action of the conditions of life. Some, perhaps a great, effect may be attributed to the 32 | ~— Summary of Selection. Crap. I, increased use or disuse of parts. The final resuit is thus rendered infinitely complex. In some cases the intercrossing of aboriginally distinct species appears to have played an important part in the origin of our breeds. When several breeds have once been formed in any country, their occasional intercrossing, with the aid of selec- tion, has, no doubt, largely aided in the formation of new sub- breeds; but the importance of crossing has been much exaggerated, both in regard to animals and to those plants which are pro- yagated by seed. With plants which are temporarily propagated by cuttings, buds, &c., the importance of crossing is immense; for the cultivator may here disregard the extreme variability both of hybrids and of mongrels, and the sterility of hybrids; but plants not propagated by seed are of little importance to us, for their endurance is only temporary. Over all these causes of Change, the accumulative action of Selection, whether applied methodicaily and quickly, or unconsciously and slowly but more efficiently, seems to shave been the predominant Power. Cuap. Il. Variation under Nature. 33 CHAPTER II. VARIATION UNDER NATURE. Variability — Individual differences — Doubtful species — Wide ranging, much diffused, and common species, vary most — Species of the larger genera in each country vary more frequently than the species of the smaller genera— Many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges. Lrrorn applying the principles arrived at in the last chapter to organic beings in a state of nature, we must briefly discuss whether these latter are subject to any variation. To treat this subject properly, a long catalogue of dry facts ought to be given; but these I shall reserve for a future work. Nor shall I here discuss the various definitions which have been given of the term species. No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation, The term “variety” is almost equally difficult to define; but here community of descent is almost universally implied, though it can rarely be proved. We have also what are called monstrosities; but they graduate into varieties. By & monstrosity I presume is meant some considerable deviation of structure, generally injurious, or not useful to the species. Some authors use the term “ variation” in a technical sense, as implying a modification directly due to the physical conditions of life; and “variations” in this sense are supposed not to be inherited ; but Who can say that the dwarfed condition of shells in the brackish Waters of the Baltic, or dwarfed plants on Alpine summits, or the thicker fur of an animal from far northwards, would not in’ some cases be inherited for at least a few generations? and in this case I presume that the form would be called a variety. it may be doubted whether sudden and considerable deviations of structure such as we occasionally see in our domestic productions, more especially with plants, are ever permanently propagated in a state of nature. Almost every part of every organic being is so beautifully related to its complex conditions of life that it seems as D Individual Differences. Cuap. II, «(34 improbable that any part should have been suddenly produced perfect, as that a complex machine should have been invented by Under domestication monstrosities some- man in a perfect state. : times occur which resemble normal structures in widely different animals. Thus pigs have occasionally been born with a sort of proboscis, and if any wild species of the same genus had naturally possessed a proboscis, it might have been argued that this had appeared as a monstrosity 5 but I have as yet failed to find, after diligent search, cases of monstrosities resembling normal structures in nearly allied forms, and these alone bear on the question. If monstrous forms of this kind ever do appear in a state of nature and are capable of reproduction (which is not always the case), as they © occur rarely and singly, their preservation would depend on unusually favourable circumstances. ‘They would, also, during the first and succeeding generations cross with the ordinary form, and thus their abnormal character would almost inevitably be lost. But I shall have to return in a future chapter to the preservation and perpetuation of single or occasional variations, Individual Differences. The many slight differences which appear in the offspring from the same parents, or which it may be presumed have thus arisen, from being observed in the individuals of the same species in- habiting the same confined locality, may be called individual differences. No one supposes that all the individuals of the same species are cast in the same actual mould. These individual differences are of the highest importance for us, for they are often inherited, as must be familiar to every one; and they thus afford materials for natural selection to act on and accumulate, in the same manner as man accumulates in any given direction individual differences in his domesticated productions. These individual differences generally affect what naturalists consider unimportant parts; but I could show by a long catalogue of facts, that parts which must be called important, whether viewed under a physio- logical or classificatory point of view, sometimes vary i the individuals of the same species. I am convinced that the most experienced naturalist would be surprised at the number of the cases of variability, even in important parts of structure, which he could collect on good authority, as I have collected, during a course of years. It should be remembered that systematists arets from being pleased at finding variability in important characters, and _ that there are not many men who will laboriously examine internal and important organs, and compare them in many specimens of Cuap, IL Individual Differences. 38 the same species. It would never have been expected that the branching of the main nerves close to the great central ganglion of an insect would have been variable in the same species ; it might have been thought that changes of this nature could have beer effected only by slow degrees; yet Sir J. Lubbock has shown a decree of variability in these main nerves in Coccus, which may almost be compared to the irregular branching of the stem of a tree. This philosophical naturalist, I may add, has also shown that the muscles in the larve of certain insects are far from uniform. Authors sometimes argue in a circle when they state that important organs never vary ; for, these same authors practically rank those parts as important (as some few naturalists have honestly confessed) which do not vary ; and, under this point of view, no instance will ever be found of an important part varying; but under any other point of view many instances assuredly can be given. There is one point connected with individual differences, which is extremely perplexing: I refer to those genera which have been called “ protean ” or “ polymorphic,” in which the species present an inor- dinate amount of variation. With respect to many of these forms, hardly two naturalists agree whether to rank them as species or as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects and of Brachiopod shells. In most polymorphic genera some of the species have fixed and definite characters. Genera which are polymorphic in one country seem to be, with a few exceptions, polymorphic in other countries, and like- wise, judging from Brachiopod shells, at former periods of time. These facts are very perplexing, for they seem to show that this kind of variability is independent of the conditions of life. I am inclined to suspect that we see, at least in some of these polymorphic genera, Variations which are of no service or disservice to the species, and which consequently have not been seized on and rendered definite by natural selection, as hereafter to be explained. Individuals of the same species often present, as is known to every one, great differences of structure, independently of variation, as in the two sexes of various animals, in the two or three castes of sterile females or workers amongst insects, and in the immature and larval states of many of the lower animals. There are, also, cases i" dimorphism and trimorphism, both with animals and plants. has shown ee wae eas ate’y called attention to the subject, Malaves _ , a ele an certain species of butterflies, in the yan archipelago, recularly appear under two or even three Pi ase fos eb conneed by trate varie as des analogous but more extraordinary cases D 2 / 36 Doubtful Species. Cuap. Il. with the males of certain Brazilian Crustaceans: thus, the male. of a Tanais regularly occurs under two distinct forms; one of these has strong and differently shaped pincers, and the other has antennss much more abundantly furnished with smelling-hairs, Although in most of these cases, the two or three forms, both with animals and plants, are not now connected by intermediate gradations, it. is probable that they were once thus connected. Mr. Wallace, for instance, describes a certain butterfly which pre- sents in the same island a great range of varieties connected by intermediate links, and the extreme links of the chain closely resemble the two forms of an allied dimorphic species inhabiting another part of the Malay archipelago. Thus also with ants, the several worker-castes are generally quite distinct; but in some cases,. as we shall hereafter see, the castes are connected tegether by finely. graduated varieties. So it is, as I have myself observed, with some- dimorphic plants. It certainly at first appears a highly remarkable fact that the same female butterfly should have the power of pro- ducing at the same time three distinct female forms and a male; and that an hermaphrodite plant should produce from the same seed-capsule three distinct hermaphrodite forms, bearing three: different kinds of females and three or even six different kinds of males. Nevertheless these cases are only exaggerations of the common fact that the female produces offspring of two sexes which. sometimes differ from each other in a wonderful manner. Doubtful Species. The forms which possess in some considerable degree the cha- racter of species, but which are so closely similar to other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to rank them as distinct species, are in several respects the most important for us. We have every reason to believe that many of these doubtful and closely allied forms have permanently retained their characters for a long time; for as long, as far as we know, as have good and true species. Practically, when a naturalist can unite by means of intermediate links any two forms, he treats the one as a variety of the other} ranking the most common, but sometimes the one first described, as the species, and the other as the variety. But cases of, great difficulty, which I will not here enumerate,. sometimes arise in deciding whether or not to rank one form as 2 variety of another, even when they are closely connected by inter- mediate links; nor will the commonly-assumed hybrid nature of A Cup. IL. Doubtful Species. 37 the intermediate forms always remove the difficulty. In very many cases, however, one form 1s ranked ag a variety of another, not because the intermediate links have actually been found, but because analogy leads the observer to suppose either that they do now somewhere exist, or may formerly have existed ; and here a wide door for the entry of doubt and conjecture is opened. Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judg- ment and wide experience seems the only guide to follow. We must, however, in many cases, decide by a majority of naturalists, for few well-marked and well-known varieties can be named which have not been ranked as species by at least some competent judges, | That varieties of this doubtful nature are far from uncommon cannot be disputed. Compare the several floras of Great Britain, of rance, or of the United States, drawn’ up by different botanists, and see what a surprising number of forms have been ranked by one botanist as good species, and by another as mere varieties. Mr. H. C. Watson, to whom I lie under deep obligation for assistance of all kinds, has marked for me 182 British plants, which are cenerally considered as varieties, but which have all been ranked by botanists as species; and in making this list he has omitted many trifling varieties, but which nevertheless have been ranked by some botanists as species, and he has entirely omitted several highly polymorphic genera. Under genera, including the most polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112,—a difference of 139 doubtful forms! Amongst animals which unite for each birth, and which are highly Locomotive, doubtful forms, ranked by one zoologist as a species and by another as a variety, can rarely be found within the same country, but are common in separated areas. How many of the birds and insects in North America and Europe, which differ very slightly from each other, have been ranked by one eminent naturalist as undoubted species, and by another as varieties, or, as they are often called, geographical races!) Mr. Wallace, in several valuable papers on the various animals, especially on the Lepi- doptera, inhabiting the islands of the great Malayan archipelago, shows that they may be classed under four heads, namely, as vari- uble forms, as local forms, as geographical races or sub-species, and s true representative species. The first or variable forms vary much within the limits of the same island. he local forms we moderately constant and distinct in each separate island; but Vaen all from the several islands are compared together, the dif- 38 , Doubtful Species. Cap, 13, ferences are seen to be so slight and graduated, that it is impossible to define or describe them, though at the same time the extreme forms are sufficiently distinct. The geographical races or sub-species are local forms completely fixed and isolated ; but as they do not differ from each other by strongly marked and important characters, “there is no possible test but individual opinion to determine which of them shall be considered as species and which as varieties.” Lastly, representative species fill the same place in the natural economy of each island as do the local forms and sub-species; but as they are distinguished from each other by a greater amount of | difference than that between the local forms and sub-species, they are almost universally ranked by naturalists as true species. Never- theless, no certain criterion can possibly be given by which variable forms, local forms, sub-species, and representative species can be recognised. | co Many years ago, when comparing, and seeing others compare, the birds from the closely neighbouring islands of the Galapagos archi- pelago, one with another, and with those from the American main- land, I was much struck how entirely vague and arbitrary is the distinction between species and varieties. On the islets of the little Madeira group there are many insects which are charac- terized as varieties in Mr. Wollaston’s admirable work, but which would certainly be ranked as distinct species by many entomologists. Even Ireland has a few animals, now generally regarded as varieties, but which have been ranked as species by some zoologists. Several experienced ornithologists consider our British red grouse as only a strongly-marked race of a Norwegian species, whereas the greater number rank it as an undoubted species peculiar to Great Britain, A wide distanée between the homes of two doubtful forms leads many naturalists to rank them as distinct species; but what distance, it has been well asked, will suffice; if that between America and Europe is ample, will that between Europe and the Azores, or Madeira, or the Canaries, or between the several islets of these small archipelagos, be sufficient ? Mr. B. D. Walsh, a distinguished entomologist of the United States, has described what he calls Phytophagic varieties and Phy- tophagic species. Most vegetable-feeding insects live on one kind of plant or on one group of plants; some feed indiscriminately on many kinds, but do not in consequence vary. In several cases, however, insects found living on different plants, have been observed by Mr. Walsh to present in their larval or mature state, or in both States, slight, though constant differences in colour, size, or in the / ) Crs Cuap, II. Doubtful Species. : ne - nature of their secretions. Tn some instances the males aione, in other instances both males and females, have been observed thus to differ in a slight degree. When the differences are rather more strongly marked, and when both sexes and all ages are affected, the forms are ranked by all entomologists as good species. But no observer can determine for another, even if he can do.so for himself, which of these Phytophagic forms ought to be. called species and which varietics. Mr. Walsh ranks the forms which it may be supposed would freely intercross, as varieties 5 and those which appear to have lost this power, as species. As the differences depend on the insects having long fed on distinct plants, it cannot be expected that intermediate inks connecting the several forms should now be found. The naturalist thus loses his best guide in deter- mining whether to rank doubtful forms as varieties or species. This likewise necessarily occurs with closely allied organisms, which ‘nhabit distinct continents or islands. When, on the other hand, an animal or plant ranges over the same continent, or inhabits many islands in the same archipelago, and presents different forms in the different areas, there is always a‘. good chance that intermediate forms will be discovered which will link together the extreme states; and these are then degraded to the rank of varieties. Some few naturalists maintain that animals never present varie- tics; but then these same naturalists rank the slightest difference as of specific: value; and when the same identical form is met with in two distant countries, or in two geological formations, they believe that two distinct species are hidden under the same dress. The term species thus comes to be a mere useless abstraction, 1m- plying and assuming a separate act of creation. It is certain that many forms, considered by highly-competent judges to be varieties, resemble species so completely in character, that they have been thus ranked by other highly-competent judges. But to discuss whether they ought to be called species or varieties, before any definition of these terms has been generally accepted, is vainly to beat the air. Many of the cases of strongly-marked varieties or doubtful species well deserve consideration ; for several interesting lines of argument, from geographical distribution, analogical variation, hybridism, &c., have been brought to bear in the attempt to determine their rank ; but space does not here permit me to discuss them. Close investi- gation, In many cases, will no doubt bring naturalists to agree how to rank doubtful forms. Yet it must be confessed that it is in the best known countries that we find the greatest number of them. T have been struck with the fact, that if any animal or plant ina state of nature be highly useful to man, or from any cause closely 40 a Doubtful Species. Cuap. II. attracts his attention, varieties of it will almost universally be found recorded. ‘These varieties, moreover, will often be ranked by some authors as species. Look at the common oak, how closely it has been studied; yet a German author makes more than a dozen species out of forms, which are almost universally considered by other botanists to be varieties; and in this country the highest botanical authorities and practical men can be quoted to show that the sessile and pedunculated oaks are either good and distinct specieg | or mere varieties. . I may here allude to a remarkable memoir lately published by A. de Candolle, on the oaks of the whole world. No one ever had more ample materials for the discrimination of the species, or could have worked on them with more zeal and sagacity. He first gives in detail all the many points of structure which vary in the several species, and estimates numerically the relative frequency of the variations. He specifies above a dozen characters which may be ‘ found varying even on the same branch, sometimes according to . age or development, sometimes without any assignable reason. Such characters are not of course of specific value, but they are, as Asa Gray has remarked in commenting on this memoir, such as generally enter into specific definitions. De Candolle then goes on to say that he gives the rank of species to the forms that differ by characters never varying on the same tree, and never found con- nected by intermediate states. After this discussion, the result of so much labour, he emphatically remarks: “ They are mistaken, who repeat that the greater part of our species are clearly limited, and that the doubtful species are in a feeble minority. This seemed to be true, so long as a genus was imperfectly known, and its species were founded upon a few specimens, that is to Say, Were provisional. Just as we come to know them better, intermediate forms flow in, and doubts as to specific limits augment.” He also adds that it is the best known species which present the greatest number of spon-. taneous varieties and sub-varieties. Thus Quercus robur has twenty- — eight varieties, all of which, excepting six, are clustered round three Be panes, namely, Q. pedunculata, sessiliflora, and pubescens. he forms which connect these three sub-species are comparatively rare; and, as Asa Gray again remarks, if these connecting forms, which are now rare, were to become wholly extinct, the three sub- species would hold exactly the same relation to each other, as do the four or five provisionally admitted species which closely sur- round the typical Quercus robur, Finally, De Candolle admits that out of the 800 species, which will be enumerated in his Pro- dromus as belonging to the oak family, at least two-thirds are SL ™ (uae. IL Doubtful Spectres. AI se provisional species, that is, are not known strictly to fulfil the defi- nition above given of a true species. It should be added that De Candolle no longer believes that species are immutable creations, but concludes that the derivative theory is the most natural one, ‘and the most accordant with the known facts in palxontology, geo- eraphical botany and zoology, of anatomical structure and classifi- cation.” When a young naturalist commences the study of a group of organisms quite unknown to him, he is at first much perplexed in determining what differences to consider as specific, and what as varietal; for he knows nothing of the amount and kind of variation to which the group is subject ; and this shows, at least, how very cenerally there is some variation. But if he confine his attention to one class within one country, he will soon make up his mind how to rank most of the doubtful forms. His general tendency will be to make many species, for he will become impressed, just like the pigeon or poultry fancier before alluded to, with the amount of difference in the forms which he is continually studying; and he has little general knowledge of analogical variation in other croups and in other countries, by which to correct his first impres- sions. As he extends the range of his observations, he will meet with more cases of difficulty; for he will encounter a greater number of closely-allied forms. But if his observations be widely extended, he will in the end generally be able to make up his own mind ; but he will succeed in this at the expense of admitting much variation,—and the truth of this admission will often be disputed by other naturalists. When he comes to study allied forms brought from countries not now continuous, in which case he cannot hope to find intermediate links, he will be compelled to trust almost entirely to analogy, and his difficulties will rise to a climax. Certainly no clear line of demarcation has as yet been drawn between species and sub-species—that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank of species: or, again, between sub-species and well-marked varieties, or between lesser varieties and individual dif- ierences. ‘These differences blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage, Hence I look at individual differences, though of small interest to the systematist, as of the highest importance for us, as being the first steps towards such slight varieties as are barely thought worth recording in works on natural history. And I look at varieties which are in any degree more distinct and permanent, as steps 42 Dominant Species vary mosi. Cuap, I. towards more strongly-marked and permanent varieties ; and at the latter, as leading to sub-species, and then to species. ‘The passage from one stage of difference to another may, in many cases, be the simple result of the nature of the organism and of the different physical conditions to which it has long been exposed; but with respect to the more important and adaptive characters, the passage from one stage of difference to another, may be safely attributed to | the cumulative action of natural selection, hereafter to be explained, ; and to the effects of the increased use or disuse of parts. A well- marked variety may therefore be called an incipient species; but whether this belief is justifiable must be judged by the weight of the various facts and considerations to be given throughout this work. It need not be supposed that all varieties or incipient species attain the rank of species. They may become extinct, or they may endure as varieties for very long periods, as has been shown to be the case by Mr. Wollaston with the varieties of certain fossil land- shells in Madeira, and with plants by Gaston de Saporta. Ifa variety were to flourish so as to exceed in numbers the parent ‘species, it would them rank as the species, and the species as the variety ; or it might come to supplant and exterminate the parent species; or both might co-exist, and both rank as independent species. But we shall hereafter return to this subject. From these remarks it will be seen that I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in } comparison with mere individual differences, is also applied arbi- trarily, for convenience’ sake. an Wide-ranging, much-diffused, and common Species vary most. Guided by theoretical considerations, I thought that some in- teresting results might be obtained in regard to the nature and relations of the species which vary most, by tabulating all the varieties in several well-worked floras. At first this seemed a simple task; but Mr. H. C. Watson, to whom I am much indebted for valuable advice and assistance on this subject, soon convinced me that there were many difficulties, as did subsequently Dr. Hooker, even in stronger terms. I shall reserve for a future work the discussion of these difficulties, and the tables of the proportional numbers of the varying species. Dr. Hooker permits me to add, that after having carefully read my manuscript, and examined the — Cua. Il. Dominant Species vary most. 43 ( tables, he thinks that the following statements are fairly well esta- plished. ‘Che whole subject, however, treated as 1b necessarily here ‘s with much brevity, is rather perplexing, and allusions cannot be avoided to the “struggle for existence,” “divergence of character, and other questions, hereafter to be discussed. . Alphonse de Candolle and others have shown that plants wach have very wide ranges generally present varieties ; and this might have been expected, as they are exposed to diverse physical condi- tions, and as they come into competition (which, as we shall here- after see, is an equally or more important circumstance) with different sets of organic beings. But my tables further show that, in any limited country, the species which are the most common, that is abound most in individuals, and the species which are most widely diffused within their own country (and this is a different consideration from wide range, and to a certain extent from com- monness), oftenest give rise to varicties sufficiently well-marked to have been recorded in botanical works. Hence it is the most flourishing, or, as they may be called, the dominant species,—those- which range widely, are the most diffused in their own country, and are the most numerous in individuals,—which oftenest produce well-marked varieties, or, as I consider them, incipient species. And this, perhaps, might have been anticipated; for, as varieties, in order to become in any degree permanent, necessarily have to struggle with the other inhabitants of the country, the species which are already dominant will be the most likely to yield offspring, which, though in some slight degree modified, still inherit those advantages that enabled their parents to become dominant over their compatriots. In these remarks on predominance, it should be understood that reference is made only to the forms which come into competition with each other, and more especially to the mem- bers of the same genus or class having nearly similar habits of life. With respect to the number of individuals or commonness of species, the comparison of course relates only to the members of the same group. One of the higher plants may be said to be dominant if it be more numerous in individuals and more widely diffused than the other plants of the same country, which live under nearly the same conditions. A plant of this kind is not the less dominant because some conferva inhabiting the water or some parasitic fungus is infinitely more numerous in individuals, and more widely diffused. But if the conferva or parasitic fungus exceeds its a in the above respects, it will then be dominant within its own Clg ds», 44 Species of Larger Genera variable, Cuar. Il, Species of the Larger Genera in each Country vary more frequently than the Species of the Smaller Genera. If the plants inhabiting a country, as described in any Flora, be divided into two equal masses, all those in the larger genera (i.e., those including many species) being placed on one side, and all those in the smaller genera on the other side, the former will be found to include a somewhat larger number of the very common and much diffused or dominant species. This might have been antici- pated ; for the mere fact of many species of the same genus inhabit- ing any country, shows that there is something in the organic or inorganic conditions of that country favourable to the genus; and, consequently, we might have expected to have found in the larger genera, or those including many species, a larger proportional number of dominant species. But so many causes tend to obscure this result, that I am surprised that my tables show even a small majority ‘on the side of the larger genera. I will here allude to only two causes of obscurity. Fresh-water and salt-loving plants generally have very wide ranves and are much diffused, but this seems to be connected with the nature of the stations inhabited by them, and -has little or no relation to the size of the genera to which the species belong. Again, plants low in the scale of organisation are generally much more widely diffused than plants higher in the scale; -and here again there is no close relation to the size of the genera. The cause of lowly-organised plants ranging widely will be discussed in our chapter on Geographical Distribution. From looking at species as only strongly-marked and well-defined varieties, I was led to anticipate that the species of the larger genera -in each country would oftener present varieties, than the species of the smaller genera; for wherever many closely related species (i.¢., ‘species of the same genus) have been formed, many varieties or incipient species ought, as a general rule, to be now forming. Where many large trees grow, we expect to find saplings. Where many species of a genus have been formed through variation, cir- cumstances have been favourable for variation ; and hence we might expect that the circumstances would generally be still favourable to variation. On the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one chaving few. | To test the truth of this anticipation I have arranged the plants of twelve countries, and the coleopterous insects of two districts, into ‘two nearly equal masses, the species of the larger genera on one Cusr. Ul. Species of Larger Genera variable. AS. side, and those of the smaller genera on the other side, and it has invariably proved to be the case that a larger proportion of the: species on the side of the larger genera presented varieties, than on the side of the smaller genera. Moreover, the species of the large cenera which present any varieties, invariably present a larger averaze number of varieties than do the species of the small genera. Both these results follow when another division is made, and when all the least genera, with from only one to four species, are altogether excluded from the tables. These facts are of plain signification on the view that species are only strongly-marked and permanent varieties ; for wherever many species of the same genus have been formed, or where, if we may use the expression, the manufactory of species has been active, we ought generally to find the manufactory still in action, more especially as we have every reason to believe: the process of manufacturing new species to be a slow one. And this certainly holds true, if varieties be looked at as incipient species 3. for my tables clearly show as a general rule that, wherever many species of a genus have been formed, the species of that genus present a number of varieties, that is of incipient species, beyond the average, It is not that all large genera are now varying much, and are thus increasing in the number of their species, or that no- small genera are now varying and increasing; for if this had been so, it would have been fatal to my theory ; inasmuch as geology plainly tells us that small gencra have in the lapse of time often increased greatly in size; and that large genera have often come to. their maxima, declined, and disappeared. All that we want to show is, that, where many species of a genus have been formed, on an average many are still forming; and this certainly holds good. Many of the Species included within the Larger Genera resemble Vurieties in being very closely, but unequally, related to each other, and in having restricted ranges. There are other relations between the species of large genera and. their recorded varieties which deserve notice. We have seen that there is no infallible criterion by which to distinguish species and well-marked varieties ; and when intermediate links have not been found between doubtful forms, naturalists are compelled to come to: a determination by the amount of difference between them, judging by analogy whether or not the amount suffices to raise one or both to the rank of species. Hence the amount of difference is one very important criterion in settling whether two forms should be ranked. as species or varieties. Now Fries has remarked in regard to plants,. and Westwood in regard to insects, that in large genera the amount. 46 Species of Larger Genera Cuap, Ii, of difference between the species is often exceedingly small. I have endeavoured to test this numerically by averages, and, as far as my imperfect results go, they confirm the view. I have also consulted some sagacious and experienced observers, and, after deliberation, they concur in this view. In this respect, therefore, the species of the larger gencra resemble varieties, more than do the species of the smaller genera. Or the case may be put in another way, and it may be said, that in the larger genera, in which a number of varie- ties or incipient species greater than the average are now manufac- turing, many of the species already manufactured still to a certain extent resemble varieties, for they differ from each other by less than the usual amount of difference. Moreover, the species of the larger genera are related to each other, in the same manner as the varieties of any one species are elated to each other. No naturalist pretends that all the species of a genus are equally distinct from each other; they may generally be divided into sub-genera, or sections, or lesser groups. As Fries has well remarked, little groups of species are generally clustered like satellites around other species. And what are varieties but groups of forms, unequally related to each other, and clustered round certain forms—that is, round their parent-species. Undoubtedly there is one most important point of difference between varieties and species; namely, that the amount of difference between varie- ties, when compared with each other or with their parent-species, is much less than that between the species of the same genus. But when we come to discuss the principle, as I call it, of Divergence of Character, we shall see how this may be explained, and how the lesser differences between varieties tend to increase into the greater differences between species. There is one other point which is worth notice. Varieties gene- rally have much restricted ranges: this statement is indeed scarcely more than a truism, for, if a variety were found to have a wider range than that of its supposed parent-species, their denomi- nations would be reversed. But there is reason to believe that the species which are very closely allied to other Species, and in so far resemble varieties, often have much restricted ranges. For instance, Mr. H. C. Watson has marked for me in the well-sifted London Catalogue of plants (4th edition) 68 plants which are therein ranked as species, but which he considers as so clasely allied to other species as to be of doubtful value: these 63 reputed species range on an average over 6°9 of the provinces into which Mt. Watson has divided Great Britain. Now, in this same Catalogue, 53 acknow- ledged varieties are recorded, and these range over 7'7 provinces; Cup. IL. vesemble Variettes. 47 whereas, the species to which these varieties belong range over 14°3 provinces. So that the acknowledged varieties have nearly the same restricted average range, as have the closely allied forms, marked for me by Mr. Watson as doubtful species, but which are almost universally ranked by British botanists as good and true species. Summary. Finally, varieties cannot be distinguished from species,—except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference between them ; for two _ forms, if differing very little, are generally ranked as varieties, not- withstanding that they cannot be closely connected; but the amount of difference considered necessary to give to any two forms the rank of species cannot be defined. In genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties. In large genera the species are apt to be closely, but unequally, allied together, forming little clusters round other species. Species very closely allied to other species apparently have restricted ranges. In all these respects the species of large genera present a strong analogy with varieties. And we can clearly understand these analogies, if species once existed as varieties, and thus originated; whereas, these analogies are utterly inexplicable if species are independent creations. We have, also, seen that it is the most flourishing or dominant species of the larger genera within each class which on an average yield the greatest number of varieties; and varieties, as we shall hereafter see, tend to become converted into new and distinct species. Thus the larger genera tend to become larger ; and through- out nature the forms of life which are now dominant tend to become still more dominant by leaving many modified and dominant descendants. But by steps hereafter to be explained, the larger cencra also tend to break up into smaller genera. And thus, the forms of life throughout the universe become divided into groups subordinate to groups. 48 Struggle for Existence. Crap. Ul. CHAPTER III. STRUGGLE FoR EXISTENCE. Its bearing on natural selection — The term used in a wide sense—Geome- trical ratio of increase — Rapid increase of naturalised animals and plants — Nature of the checks to increase — Competition universal— Effects of climate — Protection from the number of individuals — Complex relations of all animals and plants throughout nature — Struggle for life most severe between individuals and varieties of the same species: often severe between species of the same genus — The relation of organism to organism the most important of all relations, BeroRE entering on the subject of this chapter, I must make a few preliminary remarks, to show how the struggle for existence bears on Natural Selection. It has been seen in the last chapter that amongst organic beings in a state of nature there is some individual variability : indeed I am not aware that this has ever been disputed. It is immaterial for us whether a multitude of doubtful forms be called species or sub-species or varieties ; what rank, for instance, the two or three hundred doubtful forms of British plants are entitled to hold, if the existence of any well-marked varieties be admitted. But the mere existence of individual variability and of some few well-marked varieties, though necessary as the foundation for the work, helps us but little in understanding how Species arise in nature. How have all those exquisite adaptations of one part of the organisation to another part, and to the conditions of life, and of one organic being to another being, been perfected? We see these beautiful co-adaptations most plainly in the woodpecker and the misletoe ; and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle which dives through the water: in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world. Again, it may be asked, how is it that varieties, which I have called incipient species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species? How do those groups of species, which constitute what are called distinct Guar. IIL Struggle for Existence. AQ senera, and which differ from each other more than do the species of the same genus, arise? All these results, as we shall more fully see in the next chapter, follow from the struggle for life. Owing to this struggle, variations, however slight, and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the terrn Natural Selection, in order to mark its relation to man’s power of selection. But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power inces- santly ready for action, and is as immeasurably superior to man’s iceble efforts, as the works of Nature are to those of Art. We will now discuss in a little more detail the struggle for existence. In my future work this subject will be treated, as it well deserves, at greater length. The elder De Candolle and Lyell have largely and philosophically shown that all organic beings are exposed to severe competition. In regard to plants, no one has treated this subject with more spirit and ability than W. Herbert, Dean of Manchester, evidently the result of his great horticultural knowledge. Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult—at least I have found it so—than constantly to bear this conclusion in mind. Yet unless it be thoroughly engrained in the mind, the whole economy of nature, with every fact on distribution, rarity, abundance, extinc- tion, and variation, will be dimly seen or quite misunderstood. We behold the face of nature bright with gladness, we often see superabundance of food ; we do not see or we forget, that the birds Which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these Songsters, or their ecgs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that, thouch food may be now superabundant, it is not so at all seasons of each recurring year, E 50 Geometrical Ratio of [ncrease. _ Cup. IIE. The Term, Struggle for Huistence, used in a large sense. I should premise that I use this term in a large and metaphorical sense including dependence of cone being on: another, and including (which is more important) not only the life of the individual, but success in leaving progeny. ‘l'wo canine animals, in a time of dearth, may be truly said to struggle with each other which shall get food and live. Buta plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which annually produces a thousand seeds, of which only one on an average comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The misletoe is dependent on the apple and a few other trees, but can. only in a far-fetched sense be said to struggle with these trees, for, if too many of these parasites grow on the same tree, it languishes and dies, But several seedling misletoes, growing close together on the same branch, may more truly be said to struggle with each other. As the misletoe is disseminated by birds, its existence depends on them; and it may metaphorically be said to struggle with other fruit-bearing plants, in tempting the birds to devour and thus disseminate its seeds. In these several senses, which pass into each other, I use for convenience’ sake the general term of Struggle for Existence. Geometrical Ratio of Increase. A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must sufier destruction during some period of its life, and during some seasot or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, oF with the physical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them. There is no exception to the rule that every organic being. . Cuap. III. Geometrical Ratio of [ncrease. 51 naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in less than a thousand years, there would literally not be standing-room for his progeny. lLinneus has calculated that if an annual plant produced only two seeds—and there is no plant so unproductive as this—and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive, descended from the first pair. But we have better evidence on this subject than mere theoretical calculations, namely, the numerous recorded cases of the astonish- ingly rapid increase of various animals in a state or nature, when circumstances have been favourable to them during two or three following seasons, Still more striking is the evidence from our domestic animals of many kinds which have run wild in several parts of the world; if the statements of the rate of increase of slow-breeding cattle and horses in South America, and latterly in Australia, had not been well authenticated, they would have been incredible. So it is with plants; cases could be given of introduced plants which have become common throughout whole islands in a period of less than ten years. Several ofthe plants, such as the cardoon and a tall thistle, which are now the com- monest over the wide plains of La Plata, clothing square leagues of surface almost to the exclusion of every other plant, have been introduced from Europe; and there are plants which now range in India, as I hear from Dr. Falconer, from Cape Comorin to the Himalaya, which have been imported from America since its dis- covery. In such cases, and endless others could be given, no one supposes, that the fertility of the animals or plants has been suddenly and temporarily increased in any sensible degree. The obvious explanation is that the conditions of life have been highly favourable, and that there has consequently been less destruction of the old and young, and that nearly all the young have been enabled to breed. ‘Their geometrical ratio of increase, the result of which never fails to be Surprising, simply explains their extraordinarily rapid increase and wide diffusion in their new homes. E 2 52 Geometrical Ratio of Increase. © Case. Il, In a state of nature almost every full-grown plant annually produces seed, and amongst animals there are very al which do not annually pair. Hence we may confidently asset t, that all plants and animals are tending to increase at a geometrical ratio,— that all would rapidly stock every station In which they could any how exist,—and that this geometrical tendency to increase must be checked by destruction at some period of life. Our famiharity with the larger domestic animals tends, J think, to mislead us : We see no great destruction falling on them, but we do not keep in mind that thousands are annually slaughtered for food, and that in a state of nature an equal number would have somehow to be disposed of. The only difference between organisms which annually produce eggs or seeds by the thousand, and those which produce extremely few, is, that the slow-breeders would require a few more years to people, under favourable conditions, a whole district, let it be ever so large. The condor lays a couple of eggs and the ostrich a score, and yet in the same country the condor may be the more numerous of the two; the Fulmar petrel lays but one egg, yet it is believed to be the most numerous bird in the world. One fly deposits hundreds of eggs, and another, like the hippobosca, a single one ; but this difference does not determine how many individuals of the two species can be supported in a district. A large number of eggs is of some importance to those species which depend on a fluctua- ting amount of food, for it allows them rapidly to increase in number. But the real importance of a large number of eggs or seeds is to make up for much destruction at some period of life; and this period in the great majority of cases is an early one. If an animal can in any way protect its own eggs or young, a small number may be produced, and yet the average stock be fully kept up; but if many eggs or young are destroyed, many must be produced, or the species will become extinct. It would suffice to keep up the full number of a tree, which lived on an average for a thousand years, if a single seed were produced once in a thousand years, supposing that this seed were never destroyed, and could be ensured to germinate in a fitting place. So that, in all cases, the average number of any animal or plant depends only indirectly on the number of its eggs or seeds. | In looking at Nature, it is most necessary to keep the foregoing considerations always in mind—never to forget that every single organic being may be said to be striving to the utmost to increase in numbers ; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals. Lighten any Cuar. Ul Nature of the Checks to [ucrease. 53 — —_—_-— check, mitigate the destruction ever so little, and the number of the species will almost instantaneously increase to any amount. Nature of the Checks to Increase. The causes which check the natural tendency of each species to increase are most obscure. Look at the most vigorous species; by as much as it swarms in numbers, by so much will it tend to increase still further. We know not exactly what the checks are even in a Single instance. Nor will this surprise any one who reflects how ignorant we are on this head, even in regard to mankind, although so incomparably better known than any other animal. This subject of the checks to increase has been ably treated by several authors, and I hope in a future work to discuss it at considerable length, more especially in regard to the feral animals of South America, Here I will make only a few remarks, just to recall to the reader’s mind some of the chief points. Eges or very young animals seem generally to suffer most, but this is not invariably the case. With plants there is a vast destruction of secds, but, from some observations which I have made it appears that the seedlings suffer most from germinating in ground already thickly stocked with other plants. Seedlings, also, are destroyed in vast numbers by various enemies ; for instance, on a piece of ground three feet long and two wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of 357 no less than 295, were destroyed, chiefly by slugs and insects. If turf which has long been inewn, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown plants; thus out of twenty species growing on a little plot of mown turf (three feet by four) nine species perished, from the other species being allowed to grow up freely. The amount of food for each species of course gives the extreme limit to which each can increase ; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species. Thus, there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin. If hot one head of game were shot during the next twenty years in England, and, at the same time, if no vermin were destroyed, there would, in all probability, be less game than at present, although hundreds of thousands of game animals are now annually shot. Cn the other hand, in some cases, aS with the elephant, none are 54 Nature of the Checks to Increase. Cuap. II, destroyed by beasts of prey; for even the tiger in India most rarely dares to attack a young elephant protected by its dam. Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought seem to be the most effective of all checks. I estimated (chiefly from the greatly reduced numbers of nests in the spring) that the winter of 1854-5 destroyed four-fifths of the birds in my own grounds; and this is a tremendous destruction, when we remember that ten per cent. is an extraordinarily severe mortality from epidemics with man. . The action of climate seems at first sight to be quite independent of the struggle for existence; but in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist on the same kind of food. Even when climate, for instance extreme cold, acts directly, it will be the least vigorous individuals, or those which have got least food through the advancing winter, which will suffer most. When we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing ; and the change of climate being conspicuous, we are tempted to attribute the whole effect to its direct action. But this is a false view ; we forget that each species, even where it most abounds, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers ; and as each area is already fully stocked with inhabi- tants, the other species must decrease. When we travel south- ward and see a species decreasing in numbers, we may feel sure that the cause lies quite as much in other species being favoured, as in this one being hurt. So it is when we travel northward, but in a somewhat lesser degree, for the number of species of all kinds, — and therefore of competitors, decreases northwards ; hence in going northwards, or in ascending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do in proceeding southwards or in descending a mountain. When we reach the Arctic regions, or snow-capped summits, or absolute deserts, the struggle for life is almost exclusively with the elements. That climate acts in main part indirectly by favouring other species, we clearly see in the prodigious number of plants which in our gardens can perfectly well endure our climate, but which never become naturalised, for they cannot compete with our native plants nor resist destruction by our native animals. Cua. IL. Struggle for Existence. 55 When a species, owing to highly favourable circumstances, increases inordinately in numbers in a small tract, epidemics—at least, this seems generally to occur with our game animals—often ensue; and here we have a limiting check independent of the struggle for life. But even some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through facility of diffusion amongst the crowded animals, been disproportionally favoured : and here comes in a sort of struggle between the parasite and its prey. On the other hand, in many cases, a large stock of individuals of the same species, relatively to the numbers of its enemies, is absolutely necessary for its preservation. ‘Thus we can easily raise plenty of corn and rape-seed, &c., in our fields, because the seeds are in great excess compared with the number of birds which feed on them; nor can the birds, though having a superabundance of food at this one season, increase in number proportionally to the supply of seed, as their numbers are checked during winter; but any one who has tried, knows how troublesome it is to get seed from a few wheat or other such plants in a garden: I have in this case lost every single seed. This view of the necessity of a large stock of the same species for its preservation, explains, I believe, some singular facts in nature such as that of very rare plants being sometimes extremely abundant, in the few spots where they do exist ; and that of some social plants being social, that is abounding in individuals, even on the extreme verge of their range. For in such cases, we may believe, that a plant could exist only where the conditions of its life were so favourable that many could exist tozether, and thus save the species from utter destruction. I should add that the good effects of intercrossing, and the ill effects of close interbreeding, no doubt come into play in many of these cases; but I will not here enlarge on this subject. Complex Relations of all Animals and Plants to each other in the Struggle for Existence. Many cases are on record showing how complex and unexpected are the checks and relations between organic beings, which have to struggle together in the same country. I will give only a single instance, which, though a simple one, interested me. In Stafford- shire, on the estate of a relation, where I had ample means of investigation, there was a large and extremely barren heath, which had never been touched by the hand of man; but several hundred acres of exactly the same nature had been enclosed twenty-five years previously and planted with Scotch fir. The change in the bi a 56 Struggle for Existence, Cuap, IL. native vegetation of the planted part of the heath was most remarkable, more than is generally seen in passing from one quite: different soil to another; not only the proportional numbers of the: heath-plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the heath. The effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations, which were not to be seen on the- heath ; and the heath was frequented by two or three distinct insectivorous birds. Here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception of the land having been enclosed, so that: cattle could not enter. But how important an element enclosure is, I plainly saw near Farnham, in Surrey. Here there are extensive heaths, with a few clumps of old Scotch firs on the distant hill- tops: within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live. When I ascertained that these young trees had not been sown or planted, I was so much sur- prised at their numbers that I went to several points of view, whence I could examine hundreds of acres of the unenclosed heath,. and literally I could not see a single Scotch fir, except the old: planted clumps. But on looking closely between the stems of the heath, I found a multitude of seedlings and little trees which had been perpetually browsed down by the cattle. In one square yard, at a point some hundred yards distant from one of the old clumps,. I counted thirty-two little trees; and one of them, with twenty- six rings of growth, had, during many years tried to raise its head above the stems of the heath, and had failed. No wonder that, as. soon as the land was enclosed, it became thickly clothed with vigorously growing young firs. Yet the heath was so extremely barren and so extensive that no one would ever have imagined that. _ cattle would have so closely and effectually searched it for food. Here we see that cattle absolutely determine the existence of the Scotch fir ; but in several parts of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious. instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly, which lays its. eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably, by other parasitic insects, Hence, if certain Cnap. III. Struggle for Existence. 57 insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would lessen the number of the navel-frequenting flies—then cattle and horses would become feral, and this would certainly greatly alter (as indeed I have obser- ved in parts of South America) the vegetation: this again would largely affect the insects; and this, as we have just seen in Stafford- shire, the insectivorous birds, and so onwards in ever-increasing circles of complexity. Not that under nature the relations will ever be as simple ag this. Battle within battle must be con- tinually recurring with varying success; and yet in the long-’ run the forces are so nicely balanced, that the face of nature remains for long periods of time uniform, though assuredly the merest trifle would give the victory to one organic being over another. Never- theless, so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life! Iam tempted to give one more instance showing how plants and animals, remote in the scale of nature, are bound together by a web of complex relations. I shall hereafter have occasion to show that the exotic Lobelia fulgens is never visited in my garden by insects, and consequently, from its peculiar structure, never sets a seed. Nearly all our orchidaceous plants absolutely require the visits of insects to remove their pollen-masses and thus to fertilise them. T find from experiments that humble-bees are almost indispensable to the fertilisation of the heartsease (Viola tricolor), for other bees do hot visit this flower. Ihave also found that the visits of bees arc necessary for the fertilisation of some kinds of clover : for instances. 20 heads of Dutch clover (‘Trifolium repens) yielded 2,290 seeds, but 20 other heads protected from bees produced not one. Again, 100 heads of red clover (‘T. pratense) produced 2,700 sceds, but the same number of protected heads produced not a single seed. Humble-bees alone visit ted clover, as other bees cannot reach fhe nectar. It has been suggested that moths may fertilise the clovers; but I doubt Whether they could do so in the case of the red clover, from their weight not being sufficient to depress the wing-petals. Hence we may infer as highly probable that, if the whole genus of humble-bees became extinct or very rare in England, the heartsease and red clover would become very rare, or wholly disappear. The number of humble-bees in any district depends in a creat measure on the number of field-mice, which destroy their combs and nests ; and Col. Newman, who has long attended to the habits of humble-bees, * 58 Struggle for Existence. Cup. HL, believes that “more than two-thirds of them are thus destroyed all over England.” Now the number of mice is largely dependent, as every one knows, on the number of cats; and Col. Newman says, “Near villages and small towns I have found the nests of humble- bees more numerous than elsewhere, which I attribute to the num- ber of cats that destroy the mice.” Hence it is quite credible that the presence of a feline animal in large numbers in a district might dotermine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district ! In the case of every species, many different checks, acting at different periods of life, and during different seasons or years, pro- bably come into play ; some one check or some few being generally the most potent; but all will concur in determining the average number or even the existence of the species. In some cases it can be shown that widely-different checks act on the same species in different districts. When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their pro- portional numbers and kinds to what we call chance. But how false a view is this! Every one has heard that when an American forest is cut down, a very different vegetation springs up; but it has been ‘observed that ancient Indian ruins in the Southern United States, which must formerly have been cleared of trees, now display the same beautiful diversity and proportion of kinds as in the surround- ing virgin forest. What a struggle must have gone on during long centuries between the several kinds of trees, each annually scattering its seeds by the thousand; what war between insect and insect— between insects, snails, and other animals with birds and beasts of prey—all striving to increase, all feeding on each other, or on the trees, their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees! Throw up a handful of feathers, and all fall to the ground according to definite laws ; but how simple is the problem where each shall fall compared to that of the action and reaction of the innumerable plants and animals which have determined, in the course of cen- turies, the proportional numbers and kinds of trees now growing on the old Indian ruins! The dependency of one organic being on another, as of a parasite on its prey, lies generally between beings remote in the scale of nature. ‘This is likewise sometimes the case with tliose which may ‘be strictly said to struggle with each other for existence, as in the ease of locusts and grass-feeding quadrupeds. But the struggle will almost invariably be most severe between the individuals of the same species, for they frequent the same districts, require the same Guar. IL. Struggle for Existence. Cry food, and are exposed to the same dangers. In the case of varieties of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest soon decided: for instance, -¢ several varieties of wheat be sown together, and the mixed seed be regown, some of the varieties which best suit the soil or climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a few years supplant the other varie- ties. ‘To keep up a mixed stock of even such extremely close varic- ties as the variously-coloured sweet-peas, they must be each year harvested separately, and the seed then mixed in due proportion, otherwise the weaker kinds will steadily decrease in number and disappear. So again with the varieties of sheep: it has been asserted that certain mountain-varieties will starve out other mountain- varieties, so that they cannot be kept together. The same result has followed from keeping together different varieties of the medicinal leech. It may even be doubted whether the varieties of any of our domestic plants or animals have so exactly the same strength, habits, and constitution, that the original proportions of a mixed stock (crossing being prevented) could be kept up for half-a-dozen cenerations, if they were allowed to struggle together, in the same manner as beings in a state of nature, and if the seed or young were not annually preserved in due proportion. Struggle for Life most severe between Individuals and Varieties of the same Species. As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the species of distinct genera. We sce this in the recent extension over parts of the United States of one species of swallow having caused the decrease of another species. The recent increase of the missel-thrush in parts of Scotland has caused the decrease of the song-thrush. How frequently we hear of one species of rat taking the place of another species under the most different climates! In Russia the small Asiatic cockroach has everywhere driven before it its great congener. In Australia the imported hive-bee is rapidly exterminating the small, stingless native bee. One species of char- lock has been known to supplant another species; and so in other cases. We can dimly see why the competition should be most severe between allied forms, which fill nearly the same place in the economy oie gaestas ‘melee m no one case could we precisely say why 1g victorious over another in the great battle of life. 60 Struggle for Existence. Crap. IIE, A corollary of the highest importance may be deduced from the foregoing remarks, namely, that the structure of every organic being is related, in the most essential yet often hidden manner, to that of all the other organic beings, with which it comes into competition for food or residence, or from which it has to escape, or on which it preys. This is obvious in the structure of the teeth and talons of the tiger ; and in that of the legs and claws of the parasite which clings to the hair on the tiger’s body. But in the beautifully plumed seed of the dandelion, and in the flattened and fringed legs of the water-beetle, the relation seems at first confined to the elements of air and water. Yet the advantage of plumed seeds no doubt stands in the closest relation to the land being already thickly clothed with other plants ; so that the seeds may be widely distributed and fal] on unoccupied ground. In the water-beetle, the structure of itg legs, so well adapted for diving, allows it to compete with other aquatic insects, to hunt for its own prey, and to escape Serving as prey to other animals, The store of nutriment laid up within the seeds of many plants seems at first sight to have no sort of relation to other plants. But from the strong growth of young plants produced from such seeds, as peas and beans, when sown in the midst of long grass, it may be suspected that the chief use of the nutriment in the seed is to favour the growth of the seedlings, whilst struggling with other plants growing vigorously all around, Look at a plant in the midst of its range, why does it not double or quadruple its numbers? We know that it can perfectly well withstand a little more heat or cold, dampness or dryness, for else- where it ranges into slightly hotter or colder, damper or drier dis- tricts. In this case we can clearly see that if we wish in imagination to give the plant the power of increasing in number, we should have to give it some advantage over its competitors, or over the animals which prey on it. On the confines of its geographical range, a change of constitution with respect to climate would clearly be an advantage to our plant ; but we have reason to believe that only a few plants or animals range so far, that they are destroyed exclusively by the rigour of the climate. Not until we reach the extreme confines of life, in the Arctic regions or on the borders of an utter desert, will competition cease. The land may be extremely cold or dry, yet there will be competition between some few species, or between the individuals of the same species, for the warmest or dampest spots. Hence we can see that when a plant or animal is placed in a new country amongst new Competitors, the conditions of its life will generally be changed in an essential manner, although the. climate Cwar, IL. Struggle for Existence. 61 — may be exactly the same as in its former home. If its average num- bers are to increase in its new home, we should have to modify it in 9 different way to what we should have had to do in its native country ; for we should have to give it some advantage over a different set of competitors or enemies. It is good thus to try in imagination to give to any one species an advantage over another. Probably in no single instance should we know what to do. ‘This ought to convince us of our ignorance on the mutual relations of all organic beings; a conviction as necessary, as it is difficult to acquire. All that we can do, is to keep steadily in mind that each organic being is striving to increase in a geometri- cal ratio; that each at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life and to suffer great destruction. When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply. 62 Natural Selection. Cuap, IV, CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITrest, Natural Selection —its power compared with man’s selection — its power ~ on characters of trifling importance —its power at all ages and on both sexes — Sexual Selection — On the generality of intercrosses between individuals of the same species — Circumstances favourable and unfa- vourable to the results of Natural Selection, namely, intercrossing, isolation, number of individuals — Slow action — Extinction caused by Natural Selection—Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation — Action of Natural Selection, through Divergence of Character, and Extinction, on the de- scendants from a common parent—Explains the grouping of all organic beings — Advance in organisation — Low forms preserved — Convergence of character — Indefinite multiplication of species — Summary. How will the struggle for existence, briefly discussed in the last chapter, act in regard to variation? Can the principle of selection, which we have seen is so potent in the hands of man, apply under nature? I think we shall see that it can act most efficiently. Let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind; as well as the strength of the hereditary tendency. Under domestication, it may be truly said that the whole organisation becomes in some degree plastic. But the variability, which we almost universally meet with in our domestic productions, is not directly produced, as Hooker and Asa Gray have well remarked, by man; he can neither originate varieties, nor prevent their occurrence ; he can only preserve and accumulate such as do occur. Unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues ; but similar changes of conditions might and do occur under nature. Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have Z Natural Selection. 63 Cuap. iV. Oe undoubtedly occurred, that other variations useful in some way to cach being in the great and complex battle of life, should occur in the course of many successive generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly de- stroyed. This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be affected by natural selec- tion, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions. Several writers have misapprehended or objected to the term Natural Selection. Some have even imagined that natural selection induces variability, whereas it implies only the preservation of such: variations as arise and are beneficial to the being under its con- ditions of life. No one objects to agriculturists speaking of the potent effects of man’s selection; and in this case the individual differences given by nature, which man for some object. selects, must of necessity first occur. Others have objected that the term selection implies conscious choice in the animals which become modified; and it has even been urged that, as plants have no voli- tion, natural selection is not applicable to them! In the literal sense of the word, no doubt, natural selection is a false term; but who ever objected to chemists speaking of the elective affinities of the various elements ?—and yet an acid cannot strictly be said to elect the base with which it in preference combines. It has been said that I speak of natural selection as an active power or Deity ; but who objects to an author speaking of the attraction of oravity as ruling the movements of the planets? Every one knows what is meant and is implied by such metaphorical expressions ; and they are almost necessary for brevity. So again it is difficult to avoid personifying the word Nature; but I mean by Nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as ascertained by us. With a little familiarity such superficial objections will be forgotten. We shall best understand the probable course of natural selection by taking the case of a country undergoing some slight physical change, for instance, of climate. The proportional numbers of its + 64 Natural Selection. Cuap, IV, inhabitants will almost immediately undergo a change, and some species will probably become extinct. We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of the inhabitants, independently of the change of climate itself, would seriously affect the others. If the country were open on its borders, new forms would certainly immigrate, and this would likewise seriously disturb the relations of some of the former inhabitants. Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up, if some of the original inhabitants were in some manner modified ; for, had the area been open to immigration, these same places would have been seized on by intruders. In such cases, slight modifica- tions, which in any way favoured the individuals of any species, by better adapting them to their altered conditions, would tend to be preserved ; and natural selection would have free scope for the work of improvement. We have good reason to believe, as shown in the first chapter, that changes in the conditions of life give a tendency to increased variability; and in the foregoing cases the conditions have changed, and this would manifestly be favourable to natural selection, by aifording a better chance of the occurrence of profitable variations. Unless such occur, natural selection can do nothing. Under the term of “ variations,” it must never be forgotten that mere indivi- - dual differences are included. As man can produce a great result with his domestic animals and piants by adding up in any given © direction individual differences, so could natural selection, but far more easily, from having incomparably longer time for action. Nor do I believe that any great physical change, as of climate, or any un- usual degree of isolation to check immigration, is necessary in order that new and unoccupied places should be left, for natural selec- tion to fill up by improving some of the varying inhabitants. For as all the inhabitants of each country are strugeling together with nicely balanced forces, extremely slight modifications in the struc- ture or habits of one species would often give it an advantage over others ; and still further modifications of the same kind would often still further increase the advantage, as long as the species continued under the same conditions of life and profited by similar means of subsistence and defence. No country can be named in which all cpap. IV Natural Selection. 65 —__. the native inhabitants are now so periectly adapted to each other and to the physical conditions under which they live, that none of them could be still better adapted or improved ; for in all countries, the natives have been so far conquered by naturalised productions, that they have allowed some foreigners to take firm possession of the land. And as foreigners have thus in every country beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted the intruders. As man can produce, and certainly has produced, a great result by his methodical and unconscious means of selection, what may not natural selection effect ? Man can act only on external and visible characters: Nature, if I may be allowed to personify the natural preservation or survival of the fittest, cares nothing for appearances, except in so far as they are useful to any being. She can act on every internal organ, on every shade of constitutional difference, en the whole machinery of life. Man selects only for his own good: Nature only for that of the being which she tends. Every sclected character is fully exercised by her, as is implied by the fact of their selection. Man keeps the natives of many climates in the same country; he seldom exercises each selected character in some peculiar and fitting manner; he feeds a long anda short beaked pigeon on the same food; he does not exercise a long-backed or long-legged quadruped in any peculiar manner ; he exposes sheep with long and short wool to the same climate. He does not allow the most vigorous males to struggle for the females. He does not rigidly destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his productions. He often begins his selection by some half-monstrous form; or at least by some modification prominent enough to catch the eye or to be plainly useful to him. Under nature, the slightest differences of structure or constitution may well turn the nicely-balanced scale in the struggle for life, and so be preserved. How fleeting are the wishes and efforts of man! how short his time! and consequently how poor will be his results, compared with those accumulated by Nature during whole geological periods! Can we wonder, then, that Nature’s productions should be far “ truer” in character than man’s productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear t of far higher workmanship ? It may metaphorically be said that natural selection is daily and hourly scrutinising, throughout the world, the slightest variations ; rejecting those that are bad, preserving and adding up all that are F Le stamp 66: Natural Selection. Cnap, IV. — good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages, and then so imperfect is our view into: long-past geological ages, that we see only that the forms of life are now different from what they formerly were. In order that any great amount of modification should be effected in a species, a variety when once formed must again, perhaps after a long interval of time, vary or present individual differences of the same favourable nature as before; and these must be again pre- served, and so onwards step by step. Seeing that individual differences of the same kind perpetually recur, this can hardly be considered as an unwarrantable assumption. But whether it is true, we can judge only by seeing how far the hypothesis accords — with and explains the general phenomena of nature. On the other hand, the ordinary belief that the amount of possible variation is a strictly limited quantity is likewise a simple assumption. Although natural selection can act only through and for the good of each being, yet characters and structures, which we are apt to consider as of very trifling importance, may thus be acted on. When we see leaf-eating insects green, and bark-feeders mottled-grey ; the alpine ptarmigan white in winter, the red-grouse the colour of heather, we must believe that these tints are of service to these birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would increase in countless humbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey—so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence natural selection might be effective in giving the proper colour to each kind of grouse, and in keeping that colour, when once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any particular colour would produce little effect: we should remember how essential it is in a flock of white sheep to destroy a lamb with the faintest trace of black. We have seen how the colour of the hogs, which feed on the ‘‘paint-root ” in Virginia, determines whether they shall live or die. In plants, the down on the fruit and the colour of the flesh are con- sidered by botanists as characters of the most trifling importance * yet we hear from an excellent horticulturist, Downing, that in the United States smooth-skinned fruits suffer far more from a beetle, a Curculio, than those with down; that purple plums suffer far Cuap. IV. Natural Selection. 67 more from a certain disease than yellow plums; whereas another disease attacks yellow-fleshed peaches far more than those with other coloured flesh. If, with all the aids of art, these slight differ- ences make a great difference in cultivating the several varieties, assuredly, in a state of nature, where the trees would have to struggle with other trees and with a host of enemies, such differ- ences would effectually settle which variety, whether a smooth or downy, a yellow or purple fleshed fruit, should succeed. . In looking at many small points of difference between species, which, as far as our ignorance permits us to judge, seem quite unimportant, we must not forget that climate, food, &c., have no doubt produced some direct effect. It is also necessary to bear in mind that, owing to the law of correlation, when one part varies, and the variations are accumulated through natural selection, other modifications, often of the most unexpected nature, will ensue. As we see that those variations which, under domestication appear at any particular period of life, tend to reappear in the offspring at the same period ;—for instance, in the shape, size, and flavour of the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silkworm ; in the eggs of poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly adult ;—so in a state of nature, natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of variations profitable at that age, and by their inheritance at a corresponding age. If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater diffi- culty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees. Natural selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which concern fhe mature insect; and these modifications may affect, through correlation, the structure of the adult. So, conversely, modifications in the adult may affect the structure of the larva; but in all cases natural selection will ensure that they shall not be injurious: for if they were so, the Species would become extinct. Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation to the young. In social animals it will adapt the structure of each individual for the benefit of the whole community ;, if the community profits by the selected change. What natural selection cannot do, is to modify the strue- ture of one Species, without giving it any advantage, for the good of F 2 68 Natural Selection. Cuap. LV, another species; and though statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation. A structure used only once in an animal’s life, if of high importance to it, might be modified to any extent by natural selection ; for instance, the great jaws possessed by certain Insects, used exclusively for opening the cocoon—or the hard tip to the beak of unhatched birds, used for breaking the egg. It has been asserted, that of the best short-beaked tumbler-pigeons a greater number perish in the egg than are able to get out of it; so that fanciers assist in the act of hatching. Now if nature had to make the beak of a full-grown pigeon very short for the bird’s own advan- tage, the process of modification would be very slow, and there would be simultaneously the most rigorous selection of all the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably perish ; or, more delicate and more easily broken shells might be selected, the thickness of: the shell being known to vary like every other structure. It may be well here to remark that with all beings there must be much fortuitous destruction, which can have little or no influence on the course of natural selection. or instance a vast number of eggs or seeds are annually devoured, and these could be modified through natural selection only if they varied in some manner which protected them from their enemies. Yet many of these eggs or seeds would perhaps, if not destroyed, have yielded individuals better adapted to their conditions of life than any of those which happened to survive. So again a vast number of mature animals aud plants, whether or not they be the best adapted to their con- ditions, must: be annually destroyed by accidental causes, which would not be in the least degree mitigated by certain changes of structure or constitution which would in other ways be beneficial to ' the species. But let the destruction of the adults be ever so heavy, if the number which can exist in any district be ‘not wholly kept down by such causes,—or again let the destruction of egos or seeds be so great that only a hundredth or a thousandth part are developed, —yet of those which do survive, the best adapted individuals, sup- posing that there is any variability in a favourable direction, will tend; to propagate their kind in larger numbers than the less well adapted. : If the numbers: be wholly kept down by the causes just indicated, as will often have been the case, natural selection will be powerless in certain beneficial directions; but this is no valid objection to its efficiency at other times and in other ways; for we are far from having any reason to suppose that many species ever Cap. LV. Serualt Selection. 69 underco modification and improvement at the same time in the : oO same area, Sexual Selection. Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, so no doubt it will be under nature. Thus it is rendered possible for the two sexes to be modified through natural selection in relation to different habits of life, as is sometimes the case; or for one sex to be modified in relation to the other sex, as commonly occurs, This leads me to say a few words on what I have called Sexual Selection. This form of selection depends, not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful com- petitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. But in many cases, victory depends not so much on general vigour, as on having special weapons, confined to the male sex, A hornless stag or spurless cock would have a poor chance of Icaving numerous offspring. Sexual selection, by always allowing the victor to breed might surely give indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, in nearly the same manner as does the brutal cocktighter by the care- ful selection of his best cocks. How low in the scale of nature the law of battle descends, I know not; male alligators have been described as fighting, bellowing, and whirling round, like Indians in a war-dance, for the possession of the females; male salmons have been observed fighting all day long; male stag-beetles sometimes bear wounds from the huge mandibles of other males; the males of certain hymenopterous insects have been frequently seen by that inimitable observer M. Fabre, fighting for a particular female who sits by, an apparently unconcerned beholder of the struggle, and then retires with the conqueror. ‘The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons. The males of carnivorous animals are already well armed; though to them and to others, special means of defence may be given through means of sexual selection, as the mane to the lion, and the hooked jaw to the male salmon; lor the shield may be as important for victory, as the sword or spear. Amongst birds, the contest is often of a more peaceful charaeter, 70 Sexual Selection. Cuap. IV, All those who have attended to the subject, believe that there is the severest rivalry between the males of many species to attract, by singing, the females. The rock-thrush of Guiana, birds of paradise, and some others, congregate ; and successive males display with the most elaborate care, and show off in the best manner their gorgeous plu- mage; they likewise perform strange antics before the females, which, standing by as spectators, at last choose the most attractive partner, Those who have closely attended to birds in confinement well know that they often take individual preferences and dislikes: thus Sir R, Heron has described how a pied peacock was eminently attractive to all his hen birds. I cannot here enter on the necessary details; but if man can in a short time give beauty and an elegant carriage to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect. Some well- known laws, with respect to the plumage of male and female birds, in comparison with the plumage of the young, can partly be ex- plained through the action of sexual selection on variations occurring at different ages, and transmitted to the males alone or to both sexes at corresponding ages; but I have not space here to enter on this subject. Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection: that is, by individual males having had, in suc- vessive generations, some slight advantage over other males, in their weapons, means of defence, or charms, which they have transmitted to their male offspring alone. Yet, I would not wish to attribute all sexual differences to this agency : for we see in our domestic animals peculiarities arising and becoming attached to the male sex, which apparently have not been augmented through selection by man. The tuft of hair on the breast of the wild turkey-cock cannot be of any use, and it is doubtful whether it can be ornamental in the eyes of the female bird ;—indeed, had the tuft appeared under domestica- tion, it would have been called a monstrosity. Illustrations of the Action of Natural Selection, or the Survival of the Fittest. In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness 5 Cua. IV. | Natural Selection. 7T and let us suppose that the fleetest prey, a deer for instance, had from any change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf was hardest pressed for food. Under such circumstances the swiftest and slimmest wolves would have the best chance of surviv- ing, and so be preserved or selected,—provided always that they retained strength to master their prey at this or some other period of the year, when they were compelled to prey on other animals. I can seo ho more reason to doubt that this would be the result, than that man should be able to improve the fleetness of his greyhouuds by careful and methodical selection, or by that kind of unconscious selection which follows from each man trying to keep the best dogs without any thought of modifying the breed. I may add, that, according to Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains in the United States, one with a light grey- hound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd’s flocks. It should be observed that, in the above illustration, 1 speak of the slimmest individual wolves, and not of any single strongly- marked variation having been preserved. In former editions of this work I sometimes spoke as if this latter alternative had frequently occurred. I saw the great importance of individual differences, and this led me fully to discuss the results of unconscious selection by man, which depends on the preservation of all the more or less valuable individuals, and on the destruction of the worst. I saw, also, that the preservation in a state of nature of any occasional deviation of structure, such as a monstrosity, would be a rare event ; and that, if at first preserved, it would generally be lost by subse- quent intercrossing with ordinary individuals. Nevertheless, until reading an able and valuable article in the ‘ North British Review’ (1867), I did not appreciate how rarely single variations, whether slight or strongly-marked, could be perpetuated. The author takes the case of a pair of animals, producing during their lifetime two hundred offspring, of which, from various causes of destruction, only two on an average survive to pro-create their kind. This is rather an extreme estimate for most of the higher animals, but by no means 80 for many of the lower organisms. He then shows that if a single individual were born, which varied in some manner, giving it twice as good a chance of life as that of the other individuals, yet the chances would be strongly against its survival. Supposing it te survive and to breed, and that half its young inherited the favour- able variation ; still, as the Reviewer goes on to show, the young 72 Illustrations of the Action of CHap, 2) eee would have only a slightly better chance of surviving and breeding ; and this chance would go on decreasing in the succeeding Senerar- tions. The justice of these remarks cannot, I think, be disputed. If, for instance, a bird of some kind could procure its food more: easily by having its beak curved, and if one were born with its beak strongly curved, and which. consequently flourished, nevertheless, there would be a very poor chance of this one individual perpetuat- ing its kind to the exclusion of the common form; but there can hardly be a doubt, judging by what we see taking place under domestication, that this result would follow from the preservation during many generations of a large number of individuals with more or less strongly curved beaks, and from the destruction of a still larger number with the straightest beaks. It should not, however, be overlooked that certain rather strongly marked variations, which no one would rank as mere. individual differences, frequently recur owing to a similar organisation being similarly acted on,—of which fact numerous instances cou'd be given with our domestic productions, In such cases, if the varying individual did not actually transmit to its offspring its newly-acquired character, it would undoubtedly transmit to them, as long as the existing conditions remained the same, a still stronger tendency to vary in the same manner. ‘There can also be little doubt that the tendency to vary in the same manner has often been so strong thas all the individuals of the same species have been similarly modified without the aid of any form of selection. Or only a third, fifth, or tenth part of the individuals may have been thus affected, of which fact several instances could be given. Thus Graba estimates that about one-fifth of the guillemots in the Faroe Islands consist of a variety so well marked, that it was formerly ranked as a distinct species under the name of Uria lacrymans. In cases of this kind, if the variation were of a beneficial nature, the original form would soon be supplanted by the modified form, through the survival of the fittest. . To the effects of intercrossing in eliminating variations of all kinds, I shall have to recur; but it may be here remarked that most animals and plants keep to their proper homes, and. do not needlessly wander about; we see this even with migratory birds, which almost always return to the same spot, Consequently: eacn newly-formed variety would generally be at first local, as seems to be the common tule with varieties in a state of nature; so that similarly modified individuals would soon exist in a small body — together, and would often breed together, If the new variety were snecessful in its battle for life, it would slowly spread from a centiab snap. IV. Natural Selection. 73, district, competing with and conquering the unchanged individuals on the margins of an ever-increasing circle. It may be worth while to give another and more complex illus~ tration of the action of natural selection. Certain plants excrete- sweet juice, apparently for the sake of eliminating something in- jurious from the sap: this is effected, for instance, by glands at the pase of the stipules in some Leguminose, and at the backs of the leaves of the common laurel. This juice, though small in quantity, is greedily sought by insects; but their visits do not in any way benefit the plant. Now, let us suppose that the juice or nectar was excreted from the inside of the flowers of a certain number of plants: of any species. Insects in seeking the nectar would get dusted with. pollen, and would often transport it from one flower to another. The flowers of two distinct individuals of the same species would thus get crossed; and the act of crossing, as can be fully proved,,. vives rise to vigorous seedlings, which consequently would have the. best chance of flourishing and surviving. The plants which produced flowers with the largest glands or nectaries, excreting most nectar, would oftenest be visited by insects, and would oftenest be crossed ; and so in the long-run would gain the upper hand and form a local variety. The flowers, also, which had their stamens and pistils. placed, in relation to the size and habits of the particular insect which visited them, so as to favour in any degree the transportal of the pollen, would likewise be favoured. We might have taken the- case of insects visiting flowers for the sake of collecting pollen in- stead of nectar; and as pollen is formed for the sole purpose of fertilisation, its destruction appears to be a simple loss to the plant ; yet if a little pollen were carried, at first occasionally and then habitually, by the pollen-devouring insects from flower to flower, and a cross thus effected, although nine-tenths of the pollen were- destroyed, it might stiil be a great gain to the plant to be thus robbed; and the individuals which produced more and more pollen, and had larger anthers, would be selected. When our plant, by the above process long continued, had been: rendered highly attractive to insects, they would, unintentionally on their part, recularly carry pollen from flower to flower; and that they do this effectually, I could easily show by many striking facts. twill give only one, as likewise illustrating one step in the separa-- nen of the sexes of plants. Some holly-trees bear only male flowers,. vole nave ao Stamens oan. a rather small quantity of flowers; these have ; / ae a ter oly-trees peut only female: shriveled anthers, is ahd SizeC ae u, and four sraNons with 8, 1 not a grain of pollen can be detected. Illustrations of the Action of Cuap. IV. a 74 Having fund a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers, taken from different branches, under the microscope, and on all, without exception, there were a few pollen-grains, and on some a profusion. As the wind had get for several days from the female to the male tree, the pollen could not thus have been carried. The weather had been cold and boisterous, and therefore not favourable to bees, nevertheless every female flower which I examined had been effectually fertilised by the bees, which had flown from tree to tree in search of nectar. But to return to our imaginary case: as soon as the plant had been rendered so highly attractive to insects that pollen was regularly carried from flower to flower, another process might commence. No naturalist doubts the advantage of what has been called the “ physio- logical division of labour ;” hence we may believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in another flower or on another plant. In plants under culture and. placed under new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent ; now if we suppose this to occur in ever so slight a degree under nature, then, as pollen is already carried regularly from flower to flower, and as a more complete sepa- ration of the sexes of our plant would be advantageous on the prin- ciple of the division of labour, individuals with this tendency more and more increased, would be continually favoured or selected, until at last a complete separation of the sexes might be effected. It would take up too much space to show the various steps, through dimorphism and other means, by which the separation of the sexes in plants of various kinds is apparently now in progress; but I may add that some of the species of holly in North America, are, according to Asa Gray, in an exactly intermediate condition, or, as he expresses it, are more or less diceciously polygamous. | Let us now turn to the nectar-feeding insects; we may suppose the plant, of which we have been slowly increasing the nectar by. continued selection, to be a common plant; and that certain insects depended in main part on its nectar for food. I could give many facts showing how anxious bees are to save time: for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which with a very little more trouble, they can enter by the mouth. Bearing such facts in mind, it may be believed that under certain circumstances individual differences in the curvature or length of the proboscis, &¢c., too slight to be appreciated by us, might profit a bee or other insect, so that — certain individuals would be able to obtain their food more quickly Cua. IV. Natural Selection. 75 than others; and thus the communities to which they belonged would flourish and throw off many swarms inheriting the same peculiarities. The tubes of the corolla of the common red and incarnate clovers (Trifolium pratense and incarnatum ) do not ona hasty glance appear to differ in length ; yet the hive-bee can easily suck the nectar out of the incarnate clover, but not out of the common red clover, which is visited by humble-bees alone; so that whole fields of the red clover offer in vain an abundant supply of precious nectar to the hive-bee. That this nectar is much liked by the hive-bee is certain ; for I have repeatedly seen, but only in the autumn, many hive-bees sucking the flowers through holes bitten in the base of the tube by humble-bees. The difference in the length of the corolla in the two kinds of clover, which determines the visits of the hive-bee, must be very trifling; for I have been assured that when red clover has been mown, the flowers of the second crop are somewhat smaller, and that these are visited by many hive-bees. I do not know whether this statement is accu- rate; nor whether another published statement can be trusted, namely, that the Ligurian bee, which is generally considered a mere variety of the common hive-bee, and which freely crosses with it, is able to reach and suck the nectar of the red clover. Thus, in a country where this kind of clover abounded, it might be a great advantage to the hive-bee to have a slightly longer or differently constructed proboscis. On the other hand, as the fertility of this clover absolutely depends on bees visiting the flowers, if humble- bees were to become rare in any country, it might be a great advantage to the plant to have a shorter or more deeply divided corolla, so that the hive-bees should be enabled to suck its flowers. Thus I can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted to each other in the most perfect manner, by the con- tinued preservation of all the individuals which presented slight deviations of structure mutually favourable to each other. Tam well aware that this doctrine of natural selection, exempli- fied in the above imaginary instances, is open to the same objections Which were first urged against Sir Charles Lyell’s noble views on “the modern changes of the earth, as illustrative of geology ;” but We now seldom hear the agencies which we see still at work, spoken of as trifling or insignificant, when used in explaining the excavation of the deepest valleys or the formation of long lines of inland clifls, Natural selection acts only by the preservation and accumu- ‘ation of small inherited modifications, each profitable to the pre- ‘served being; and ag modern geology has almost banished such 76 On the Intercrossing of Individuals. Cuar. iv, LT views as the excavation of a great valley by a single diluvial wave,. so will natural selection banish the belief of the continued creation, of new organic beings, or of any great and sudden modification in their structure. On the Intercrossing of Individuals. I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of course obvious that two individuals must always (with the exception of the curious and not well-understood cases of parthenogenesis) unite for each birth ; but in the case of hermaphrodites this is far from obvious. Nevertheless there is reason to believe that with all hermaphrodites. two individuals, either occasionally or habitually, concur for the reproduction of their kind. This view was long ago doubtfully - suggested by Sprengel, Knight and Kélreuter. We shall presently see its importance ; but I must here treat the subject with extreme brevity, though I have the materials prepared for an ample dis- cussion. All vertebrate animals, all insects, and some other large: groups of animals, pair for each birth. Modern research has much diminished the number of supposed hermaphrodites, and of real hermaphrodites a large number pair; that is, two individuals regularly unite for reproduction, which is all that concerns us, But still there are many hermaphrodite animals which certainly do not habitually pair, and a vast majority of plants are hermaphro- dites. What reason, it may be asked, is there for supposing in these cases that two individuals ever concur in reproduction? As it is impossible here to enter on details, I must trust to some general considerations alone, In the first place, I have collected So large a body of facts, and made so many experiments, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross: between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the off- spring; and on the other hand, that close interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a general law of nature that no organic being fertilises itself for a perpetuity of generations; but that a cross with another individual is occasionally—perhaps at long intervals of time— indispensable. On the belief that this is a law of nature, we can, I think, under= stand several large classes of facts, such as the following, whicli on any other view are inexplicable, Every hybridizer knows how unfavourable exposure to wet is to the fertilisation of a flower, yet ; Cap. IV. On the Intercrossing of Individuals. 77 what a multitude of flowers have their anthets ue seigmas “uly exposed to the weather! If an occasional ine Te wae spensable, notwithstanding that the plant’s own anthers and pistil stand so near each other as almost to ensure self-fertilisation, the fullest freedom for the entrance of pollen from another individual will explain the above state of exposure of the organs. Many flowers, on the other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family ; but these almost invariably present beautiful and curious adaptations in relation to the visits of insects. So necessary are the visits of bees to many papilionaceous flowers, that their fertility is greatly dimi- nished if these visits be prevented. Now, it is scarcely possible for insects to fly from flower to flower, and not to carry pollen from one to the other, to the great good of the plant. Insects act like a camel-hair pencil, and it is sufficient, to ensure fertilisation, just to touch with the same brush the anthers of one flower and then the stigma of another; but it must not be supposed that bees would thus produce a multitude of hybrids between distinct species ; for if a plant’s own pollen and that from another species are placed on the same stigma, the former is so prepotent that it invariably and completely destroys, as has been shown by Girtner, the influence of the foreign pollen. '» When the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other towards it, the contrivance seems adapted solely to ensure self-fertilisation ; and no doubt it is useful for this end: but the agency of insects is often required to cause the stamens to spring forward, as Kélreuter has shown to be the case with the barberry ; and in this very genus, which seems to have a special contrivance for self-fertilisation, it is well known that, if closely-allicd forms or varieties are planted near each other, it is hardly possible to raise pure seedlings, so largely do they naturally cross. In numerous other cases, far from self-fertilisation being favoured, there are special contrivances which effecixally prevent the stigma receiving pollen from its own flower, as I could show from the works of Sprengel and others, as well as from my own observations: for instance, in Lobelia fulgens, there is a really beautiful and elaborate contrivance by which all the infinitely humerous po.len-eranules are Swept out of the conjoined anthers of each flower, before the stigma of that individual flower is ready to receive them; and as this flower is never visited, at least in my sarden, by insects, it never sets a seed, though by placing pollen trom one flower on the stigma of another, I raised plenty of seed- lings. Another species of Lobelia, which is visited by bees, seeds 78 On the Intercrossing of Individuals. Cuar.1V freely in my garden. In very many other cases, though there is no special mechanical contrivance to prevent the stigma receiving pollen from the sume flower, yet, as Sprengel, and more recently Hildebrand, and others, have shown, and as I can confirm, either the anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower is ready, so that these so-named dichogamous plants have in fact separated sexes, and must habitually be crossed. So it is with the reciprocally dimorphic and trimorphic plants previously alluded to. How strange are these facts! How strange that the pollen and stigmatic surface of the same flower, though placed so close together, as if for the very purpose of self-fertilisation, should be in so many cases mutually useless to each other? How simply are these facts ex- plained on the view of an occasional cross with a distinct individual being advantageous or indispensable! If several varieties of the cabbage, radish, onion, and of some other plants, be allowed to seed near each other, a large majority of the seedlings thus raised turn out, as I have found, mongrels: for instance, I raised 283 seedling cabbages from some plants of different varieties growing near each other, and of these only 78 were true to their kind, and some even of these were not perfectly true. Yet the pistil of each cabbage-flower is surrounded not only by its own six stamens, but by those of the many other flowers on the same plant ; and the pollen of each flower readily gets on its own stigma without insect-agency; for I have found that plants carefully protected from insects produce the full number of pods. How, then, comes it that such a vast number of the seedlings are mon- grelized? It must arise from the pollen of a distinct variety having a prepotent effect over the flower’s own pollen; and that this is part of the general law of good being derived from the intercrossing of distinct individuals of the same species. "When distinct species are crossed the case is reversed, for a plant’s own pollen is almost always prepotent over foreign pollen; but to this subject we shall return in a future chapter. In the case of a large tree covered with innumerable flowers, it may be objected that pollen could seldom be carried from tree t0 tree, and at most only from flower to flower on the same tree; and flowers on the Same tree can be considered as distinct individuals only in a limited sense. I believe this objection to be valid, but that nature has largely provided against it by giving to trees a strong tendency to bear flowers with Separated sexes. When the _ are separated, although the male and female flowers may be produced on the same tree, pollen must be regularly carried from Cuar.IV. On the Intercrossing of Individuals. 79 flower to flower ; and this will give a better chance of pollen being occasionally carried from tree to tree. That trees belonging to all Orders have their sexes more often separated than other plants, I find to be the case in this country; and at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the United States, and the result was as I anticipated. On the other hand, Dr. Hooker informs me that the rule does not hold good in Australia; but if most of the Australian trees are dichogamous, the same result would follow as if they bore flowers with separated sexes. I have made these few remarks on trees simply to call attention to the subject. Turning for a brief space to animals: various terrestrial species are hermaphrodites, such as the land-mollusca and earth-worms ; but these all pair, As yet I have not found a single terrestrial animal which can fertilise itself. This remarkable fact, which offerg so strong a contrast with terrestrial plants, is intelligible on the view of an occasional cross being indispensable; for owing to the nature of the fertilising element there are no means, analogous to the action of insects and of the wind with plants, by which an occasional cross could be effected with terrestrial animals without the concurrence of two individuals, Of aquatic animals, there are many self-fertilising hermaphrodites ; but here the currents of water offer an obvious means for an occasional cross. As in the case of flowers, I have as yet failed, after consultation with one of the highest authorities, namely, Professor Huxley, to discover a single hermaphrodite animal with the organs of reproduction so perfectly enclosed that access from without, and the occasional influence of a distinct individual, can be shown to be physically impossible. Cirripedes long appeared to Ine to present, under this point of view, a case of great difficulty ; but I have been enabled, by a fortunate chance, to prove that two. individuals, though both are self-fertilisng hermaphrodites, do: sometimes cross. It must have struck most naturalists as a strange anomaly that, both with animals and plants, some species of the same family and even of the same genus, though agreeing closely with each other in their whole organisation, are hermaphrodites, and some unisexual. But if, in fact, all hermaphrodites do occasionally intercross, the difference between them and unisexual species is, as far as function 1S concerned, very small. _ From these several considerations and from the many special facts which I have collected, but which I am unable here to give, 1t appears that with animals and plants an occasional intercross between distinct individuals is a very general, if not universal, law of nature, 80 Circumstances favourable to the — Car. IV, Circumstances favourable for the proaenan of new forms through Natural Selection. This is an extremely intricate subject. A great amount ot varia- bility, under which term individual differences are always included, will evidently be favourable. A large number of individuals, by civing a better chance within any given period for ue appearance of profitable variations, will compensate for a lesser amount ot variability in each individual, and is, I believe, a highly important element of success. Though Nature grants long periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving to seize on each place in the economy of nature, if any one species does not become modi- fied and improved in a corresponding degree with its competitors, it will be exterminated. Unless favourable variations be inherited by some at least of the offspring, nothing can be effected by natural selection. The tendency to reversion may often check or prevent the work; but as this tendency has not prevented man from forming by selection numerous domestic races, why should it prevail against natural selection ? In the case of methodical selection, a breeder selects for some definite object, and if the individuals be allowed freely to intercross, his work will completely fail. ‘But when many men, without intending to alter the breed, have a nearly common standard of perfection, and all try to procure and breed from the best animals, improvement surely but slowly follows from this unconscious process of selection, notwithstanding that there is no separation of selected individuals. Thus it will be under nature; for within a confined area, with some place in the natural polity not perfectly occupied, all the individuals varying in the right direction, though in different degrees, will tend to be preserved. But if the area be large, its several districts will almost certainly present different conditions of life; and then, if the same species undergoes modifi- cation in different districts, the newly-formed varieties will intercross on the confines of each. But we shall see in the sixth chapter that intermediate varieties, inhabiting intermediate districts, will in the » Joug run generally be supplanted by one of the adjoining varieties. Intercrossing will chiefly affect those animals which unite for each birth and wander much, and which do not breed at xate. Hence with animals of thi varieties, will generally be confined t I find to be the case. only occasionally, inten sink ait Mite ibis a very quick S nature, for instance, birds, 0 separated countries; and this With hermaphrodite organisms which cross and likewise with animals which unite for each Cap. IV. Results of Natural Selection. 8i birth, but which wander little and can increase at a rapid rate, a new and improved variety might be quickly formed on any one spot, and might there maintain itself in a body and afterwards spread, so that the individuals of the new variety would chiefly cross together. On this principle, nurserymen always prefer saving seed from a large body of plants, as the chance of intercrossing is thus lessened. Evey with animals which unite for each birth, and which do not ] ropagate rapidly, we must not assume that free intercrossing would always eliminate the effects of natural selection; for I can bring forward a considerable body of facts showing that within the same area, two varieties of the same animal may long remain distinct, from haunting different stations, from breeding at slightly different seasons, or from the individuals of each variety preferring to pair tovether. Intercrossing plays a very important part in nature by keeping the individuals of the same species, or of the same variety, true and uniform in character. It will obviously thus act far more efficiently with those animals which unite for each birth ; but, as already stated, we have reason to believe that occasional intercrosses take place with all animals and plants. Even if these take place only at long inter- vals of time, the young thus produced will gain so much in vigour and fertility over the offspring from long-continued self-fertilisation, that they will have a better chance of surviving and propagating their kind; and thus, in the long run, the influence of crosses, even at rare intervals, will be great. With respect to organic beings extremely low in the scale, which do not propagate sexually, nor conjugate, and which cannot possibly intercross, uniformity of cha- racter can be retained by them under the same conditions of life, only through the principle of inheritance, and through natural selec- tion which will destroy any individuals departing from the proper™ type. Ifthe conditions of life change and the form undergoes modifi- cation, uniformity of character can be given to the modified oftspring, solely by natural selection preserving similar favourable variations. Isolation, also, is an important element in the modification of Species through natural selection. Ina confined or isolated area, if hot very large, the organic and inorganic conditions of life will generally be almost uniform; so that natural selection will tend to modily all the varying individuals of the same species in the same manner, Intercrossing with the inhabitants of the surrounding dis- tricts Will, also, be thus prevented. Moritz Wagner has lately pub- lished an interesting essay on this subject, and has shown that the S-rvice rendered by isolation in preventing crosses between newly- G 82 Circumstances favourable to the Cuar. IV, —— ably greater even than I supposed. But d I can by no means agree with this d isolation are necessary elements for , i The importance of isolation is like ~ ee of new agen any ph sical change in the conditions wise great in preventing, alter any pay che immiorat euch as of climate, elevation of the land, &c., the Immigration of Detter adapted orgaitisms ; and thus new places nn natural economy of the district will be left open to be ed up by the modification of the old inhabitants. Lastly, isolation will give time for a new variety to be improved at a slow rate ; and this may some- times be of much importance. If, however, an isolated area be very small, either from being surrounded by barriers, or from having very peculiar physical conditions, the total number of the inhabitants will be small; and this will retard the production of new species through natural selection, by decreasing the chances of favourable variations arising. The mere lapse of time by itself does nothing, either for or against natural selection. I state this because it has been erroneously asserted that the element of time has been assumed by me to play an all-important part in modifying species, as if all the forms of life were necessarily undergoing change through some innate law. Lapse of time is only so far important, and its importance in this respect is great, that it gives a better chance of beneficial variations arising, and of their being selected, accumulated, and fixed. It likewise tends to increase the direct action of the physical conditions of life, in relation to the constitution of each organism. If we turn to nature to test the truth of these remarks, and look at any small isolated area, such as an oceanic island, although the number of species inhabiting it is small, as we shall see in our chapter on Geographical Distribution ; yet of these species a very large proportion are endemic,—that is, have been produced there, and nowhere else in the world. Hence an oceanic island at first sight seems to have been highly favourable for the production of new species, But we may thus deceive ourselves, for to ascertain whether a small isolated area, or a large open area like a continent, has been most favourable for the production of new organic forms, we ought to make the comparison within equal times; and this we are incapable of doing. Although isolation is of great importance in the production of new species, on the whole I am inclined to believe that largeness of area is still more importaint, especially for the production of species which shall prove capable of enduring for a long period, and of spreading widely. ‘T'hroughout a great and open area, not only will there be@ formed varieties is prob from reasons already assigne naturalist, that migration an sas Cnap. IV. Results of Natural Selection. 83 better chance of favourable variations, arising from the large number of individuals of the same species there supported, but the conditions of life are much more complex from the large number of already existing species; and if some of these many species become modified and improved, others will have to be improved in @ corresponding degree, or they will be exterminated. Each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area, and will thus come into competition with many other forms. Moreover, great areas, though now continuous, will often, owing to former oscillations of level, have existed in a broken condition ; so that the good effects of isolation will generally, to a certain extent, have concurred. Finally, I conclude : that, although small isolated areas have been in some respects highly favourable for the production of new species, yet that the course of modification will generally have been more rapid on large areas ; and what is more important, that the new forms produced on large areas, which already have been victorious over many competitors, will be those that will spread most widely, and will give rise to the greatest number of new varieties and species. They will thus play 4 more important part in the changing history of the organic world. In accordance with this view, we can, perhaps, understand some facts which will be again alluded to in our chapter on Geographical Distribution ; for instance, the fact of the productions of the smaller continent of Australia now yielding before those of the larger Kuropxo-Asiatic area. Thus, also, it is that continental productions have everywhere become so largely naturalised on islands. On a small island, the race for life will have been less severe, and there will have been less modification and less extermination. Hence, we can understand how it is that the flora of Madeira, according to Oswald Heer, resembles to a certain extent the extinct tertiary flora of Europe. All fresh-water basins, taken together, make a small area compared with that of the sea or of the land. Consequently, the competition between fresh-water productions will have been less severe than elsewhere ; new forms will have been then more slowly produced, and old forms more slowly exterminated. And it is in iresh-water basins that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh water we find nithrbyches an Lape eee the wel th certain ectent a epi OsHnER, which, like fossils, connect to a — These someone fome be, te os - ne md have endured a ms mos may de called living fossils ; they » present day, from having inhabited a confined G 2 Circumstunces favourable to the Crap. IV. ee 84 area, and from having been exposed to less varied, and therefore less severe, competition. : intri subject its, To sum up, as far as the extreme intricacy of the subject permits, the circumstances favourable and unfavourable for the production of new species through natural selection. I conclude that for terrestrial productions a large continental area, which has undergone many oscillations of level, will have been the most favourable for the pro- duction of many new forms of life, fitted to endure for a long time and to spread widely. Whilst the area existed as a continent, the inhabitants will have bee numerous in individuals and kinds, and will have been subjected to severe competition. When converted by subsidence into large separate islands, there will still have existed many individuals of the same species on each island: intercrossing on the confines of the range of each new species will have been checked: after physical changes of any kind, immigration will have been prevented, so that new places in the polity of each island will have had to be filled up by the modification of the old inhabitants; and time will have been allowed for the varieties in each to become well modified and perfected. When, by renewed elevation, the islands were reconverted into a continental area, there will again have been very severe compevition: the most favoured or improved varieties will have been enabled to spread: there will have been much extinction of the less improved forms, and the relative propor tional numbers of the various inhabitants of the reunited continent will again have been changed ; and again there will have been a fair field for natural selection to improve still further the inhabitants, and thus to produce new species. | That natural selection generally acts with extreme slowness I fully admit. It can act only when there are places in the natural polity of a district which can be better occupied by the modification of some of its existing inhabitants. The occurrence of such places will often depend on physical changes, which generally take place very slowly, and on the immigration of better adapted forms being pre- vented. As some few of the old inhabitants become modified, the mutual relations of others will often be disturbed ; and this will create new places, ready to be filled up by better adapted forms; but all this will take place very slowly. Although all the indi- viduals of the same species differ in some slight degree from each other, wb would often be long before differences of the right nature in various parts of the organisation might occur. The result would often be greatly retarded by free intercrossing. Many will exclaim that these several causes are amply sufficient to neutralise the power of natural selection. I do not believe so. But I do believe that Crap. 1Y, Results of Natural Selection. 85 natural selection will generally act very slowly, only at long intervals of time, and only on a few of the inhabitants of the same region, I further believe that these slow, intermittent results accord well with what geology tells us of the rate and manner at which the in- habitants of the world have changed. Slow though the process of selection may be, if feeble man can do much by artificial selection, I can see no limit to the amount of change, to the beauty and complexity of the coadaptations between all organic beings, one with another and with their physical con- ditions of life, which may have been effected in the long course of time through nature’s power of selection, that is by the survival af the fittest. Extinction caused by Natural Selection. This subject will be more fully discussed in our chapter on Geology ; but it must here be alluded to from being intimately con- nected with natural selection. Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure. Owing to the high geometrical rate of increase of ali organic beings, each area is already fully stocked with inhabit- ants; and it follows from this, that as the favoured forms increase in number, so, generally, will the less favoured decrease and become are. Rarity, as geology tells us, is the precursor to extinction. We can see that any form which is represented by few individuals will run a good chance of utter extinction, during great fluctuations in the nature of the seasons, or from a temporary increase in the number of its enemies. But we may go further than this; for, as new forms are produced, unless we admit that specific forms can go on indefi- nitely increasing in number, many old forms must become extinct. ‘That the number of specific forms has not indefinitely increased, geology plainly tells us; and we shall presently attempt to show why it is that the number of species throughout the world has not become immeasurably great. We have seen that the species which are most numerous in indi- viduals have the best chance of producing favourable variations within any given period. We have evidence of this, in the facts stated in the second chapter, showing that it is the common and diffused or dominant species which offer the greatest number of recorded varieties. Hence, rare species will be less quickly modified or improved within any given period; they will consequently be beaten in the race for life by the modified and improved descendants of the commoner species. -From these several considerations I think it inevitably follows, 86. Extinction by Natural Selection. Car. IV, —_— that as new species in the course of time are formed nove natural selection, others will become rarer and rarer, and nally extinct. The forms which stand in closest competition with those undergoing modification and improvement, will naturally suffer most. And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms,—varieties of the same species, and Species of the same genus or of related genera,—which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other ; consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them. We see the same process of extermination amongst our domesticated, productions, through the selection of improved forms by man. Many curious instances could be given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds. In Yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these “were swept away by the short- horns” (I quote the words of an agricultural writer) “as if by some murderous pestilence.” Divergence of Character. The principle, which I have designated by this term, is of high importance, and explains, as I believe, several important facts. In the first place, varieties, even strongly-marked ones, though having somewhat of the character of species—as is shown by the hopeless doubts in many cases how to rank them—yet certainly differ far less from each other than do good and distinct species. Neverthe- less, according to my view, varieties are species in the process of formation, or are, as I have called them, incipient species. How, then, does the lesser difference between varieties become augmented. into the greater difference between species? ‘Uhat this does habitu- ally happen, we must infer from most of the innumerable species throughout nature presenting well-marked differences ; whereas varieties, the supposed prototypes and parents of future well-marked species, present slight and ill-defined differences. Mere chance, as we may call it, might cause one variety to differ in some character from its parents, and the offspring of this v i ariety again to differ from its parent in the very s bee ame character and in a ereater degree ; but this alone would never account for go habitual and large a degree of difference as that between the species of the same genus. As has always been my practice, I have soucht light on this ee er eat ee Cap. LV. Divergence of Character. 87 head from our domestic productions. We shall here find something analogous. It will be admitted that the production of races so different as short-horn and Hereford cattle, race and cart horses, the several breeds of pigeons, &c., could never have been effected by the mere chance accumulation of similar variations during many successive generations. In practice, a fancier is, for instance, struck by a pigeon having a slightly shorter beak; another fancier is struck by a pigeon having a rather longer beak; and on the acknowledged principle that “ fanciers do not and will not admire a medium standard, but like extremes,” they both go on (as has actually occurred with the sub-breeds of the tumbler-pigeon) choosing and breeding from birds with longer and longer beaks, or with shorter and shorter beaks. Again, we may suppose that at an early period of history, the men of one nation or district required swifter horses, whilst those of another required stronger and bulkier horses. The early differences would be very slight; but, in the course of time, from the continued selection of swifter horses in the one case, and of stronger ones in the other, the differences would become greater, and would be noted as forming two sub-breeds. Ultimately, after the lapse of centuries, these sub-breeds would become converted into two well-established and distinct breeds. As the differences became greater, the inferior animals with interme- diate characters, being neither very swift nor very strong, would not have been used for breeding, and will thus have tended to dis- appear. Here, then, we see in man’s productions the action of what may be called the principle of divergence, causing differences, at first barely appreciable, steadily to increase, and the breeds to diverge in character, both from each other and from their common parent, But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply most efficiently (though it was a long time before I saw how), from the simple circumstance that the more diversified the descendants from any one ‘species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers. We can clearly discern this in the case of animals with simple habits. Take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. If its natural power of increase be allowed to act, it can succeed in increasing (the country not undergoing any change in conditions) only by its varying descendants seizing on places at present occupied by other animals: some of them, for $8 Divergence of Character. Crap, IV, —— —-: ‘nstance, being enabled to feed on new kinds of prey, either dead or alive; some: inhabiting new stations, climbing trees, frequenting od live: °. or alive ; ing less carnivorous. The more . ‘haps becom water, and some perhap Jovte af a diversified in habits and structure the descendants of our carnivo- yous animals become, the more places they will be enabled to occupy. What applies to one animal will apply throughout all time to all animals—that is, if they vary—for otherwise natural selection can effect nothing. So it will be with plants. It has been experi~ mentally proved, that if a plot of ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of plants and a greater weight of dry herbace can be raised in the latter than in the former case. The same has been found to hold good when one variety and several mixed varieties of wheat have been sown on equal spaces of ground, Hence, if any one species of grass were to go on varying, and the varieties were continually selected which differed from each other in the same manner, though in a very slight degree, as do the distinct species and genera of grasses, a greater number of individual plants of this species, including its modified descendants, would succeed in living on the same piece of ground. And we know that each species and each variety of grass is annually sowing almost countless seeds; and is thus striving, as it may be said, to the utmost to increase in number. Consequently, in the course of many thousand generations, the most distinct varieties of any one species of grass would have the best chance of succeeding and of increasing in numbers, and thus of supplanting the less distinct varieties; and varicties, when rendered very distinct from each other, take the rank of species. _ The truth of the principle that the greatest amount of life can be supported by great diversification of structure, is seen under many natural circumstances. In an extremely small area, especially if freely open to immigration, and where the contest between indivi- dual and individual must be very Severe, we always find great diversity in its inhabitants. For instance, I found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, Supported twenty species. of plants, and these belonged to eighteen genera and to eight orders, which shows how much these plants differed from each other. So it is with the plants and insects on small and uniform islets: also in small ponds of fresh water. Farmers find that they can raise most food by a rotation of plants belonging to the most different orders: nature follows what may be called a simultaneous rotation. Most of the animals and plants which live close round any small Cuap. IV. Divergence of Character. 89 - piece of ground, could live on it (supposing its nature not to be in any way peculiar), and may be said to be striving to the utmost to live there; but, it is seen, that where they come into the closest competition, the advantages of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders. The same principle is seen in the naturalisation of plants through man’s agency in foreign lands. It might have been expected that the plants which would succeed in becoming naturalised in any jand would generally have been closely allied to the indigenes; for these are commonly looked at as specially created and adapted for their own country. It might also, perhaps, have been expected that naturalised plants would have belonged to a few groups more especially adapted to certain stations in their new homes. But the case Is very different; and Alph. de Candolle has well remarked, in his great and admirable work, that floras gain by naturalisation, proportionally with the number of the native genera and species, far more in new genera than in new species, To give a single instance: in the last edition of Dr. Asa Gray’s ‘Manual of the Flora of the Northern United States, 260 naturalised plants are enumerated, and these belong to 162 genera. We thus see that these naturalised plants are of a highly diversified nature. They differ, moreover, to a large extent, from the indigenes, for out of the 162 naturalised genera, no less than 100 genera are not there indi- genous, and thus a large proportional addition is made to the genera now living in the United States, By considering the nature of the plants or animals which have in any country struggled successfully with the indigenes, and have there become naturalised, we may gain some crude idea in what manner some of the natives would have to be modified, in order to gain an advantage over their compatriots; and we may at least infer that diversification of structure, amounting to new generic differences, would be profitable to them. The advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as that of the physiological division of labour in the organs of the same individual body—a subject so well elucidated by Milne Edwards, No physiologist «loubts that a stomach adapted to digest vegetable matter alone, or flesh alone, draws most nutriment from these substances. So in the general economy of any land, the more widely and perfectly the animals and plants are diversified for different habits of life, so will Results of the Action of Cuap, IV, gO a greater number of individuals be capable of there supporting ‘themselves. A set of animals, with their organisation but little | diversified, could hardly compete with a set more perfectly diversified in structure. It may be doubted, for instance, whether the Austra- lian marsupials, which are divided into groups differing but little. from each other, and feebly representing, as Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and rodent mam- mals, could successfully compete with these well-developed orders. In the Australian mammals, we see the process of diversification in an early and incomplete stage of development, The Probable Effects of the Action of Natwral Selection through Divergence of Character and Extinction, on the Descendants of a Common Ancestor. After the foregoing discussion, which has been much compressed, we may assume that the modified descendants of any one species. will succeed so much the better as they become more diversified in structure, and are thus enabled to encroach on places occupied by other beings. Now let us see how this principle of benefit being derived from divergence of character, combined with the principles. of natural selection and of extinction, tends to act. The accompanying diagram will aid us in understanding this. rather perplexing subject. Let A to L represent the species of a genus large in its own country; these species are supposed to resemble each other in unequal degrees, as is so generally the case in nature, and as is represented in the diagram by the letters: __ standing at unequal distances. I have said a large genus, because _ as we saw in the second chapter, on an average more species vary in _ large genera than in small genera; and the varying species of the large genera present a greater number of varieties. We have, also, seen that the species, which are the commonest and the most widely diffused, vary more than do the rare and restricted species. Let (A) _ be a common, widely-diffused, and varying species, belonging to a genus large in its own country. The branching and diverging: dotted lines of unequal lengths proceeding from (A), may represent ” its varying offspring, The variations are supposed to be extremely ~ Slight, but of the most diversified nature ; they are not supposed all to appear simultancously, but often after long intervals of time; nor are they all supposed to endure for equal periods. Only those variations which are in some way profitable will be preserved or naturally selected. And here the importance of the. principle of benefit derived from divergence of character comes in; for this will generally lead to the most different or divergent variations (repre= oe Lol Rar eed By oe a ahd Cuap. IV. Natural Selection. Or Pe sented by the outer dotted lines) being preserved and accumulated by natural selection. When a dotted line reaches one of the hori- zontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to form it into a fairly well-marked variety, such as would be thought worthy of record in a systematic work. The intervals between the horizontal lines in the diagram, may represent each a thousand or more generations. After a thousand generations, species (A) is supposed to have produced two fairly - well-marked varieties, namely a! and m4. These two varieties will generally still be exposed to the same conditions which made their parents variable, and the tendency to variability is in itself heredi- tary; consequently they will likewise tend to vary, and commonly in nearly the same manner as did their parents. Moreover, these two varieties, being only slightly modified forms, will tend to inherit those advantages which made their parent (A) more numerous than most of the other inhabitants of the same country; they will also partake of those more general advantages which made the genus to which the parent-species belonged, a large genus in its own country. And all these circumstances are favourable to the production of new varieties, If, then, these two varieties be variable, the most divergent of their variations will generally be preserved during the next thousand generations. And after this interval, variety a! is supposed in the diagram to have produced variety a?, which will, owing to the prin- ciple of divergence, differ more from (A) than did variety a}. Variety m' is supposed to have produced two varieties, namely m? and s*, differing from each other, and more considerably from their common parent (A). We may continue the process by similar steps for any length of time; some of the varieties, after each thousand generations, producing only a single variety, but in a more and more modifiéd condition, some producing two or three varieties, and some failing to produce any. Thus the varieties or modified descendants of the common parent (A), will generally go on increasing in number and diverging in character. In the diagram the process is represented up to the ten-thousandth generation, and under a condensed and simplified form up to the fourteen-thousandth generation. But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular, nor that it goes on continuously ; it is far more probable that each form remains for long periods unal- tered, and then again undergoes modification. Nor do I stippose ri Q2 Results of the Action of Cua® LV, — that the most divergent varieties are invariably preserved: a medium form may often long endure, and may or may not produce more than one modified descendant; for natural selection will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings; and this will depend on infinitely complex relations. But as a general rule, the more diversified in structure the descendants from any one species an. be rendered, the more places they will be enabled to seize on, and the more their modified progeny will increase. In our diagram the line of succession is broken at regular intervals by small num- bered letters marking the successive forms which have become sufficiently distinct to be recorded as varieties. But these breaks are imaginary, and might have been inserted anywhere, after inter- vals long enough to allow the accumulation of a considerable amount. -of divergent variation. As all the modified descendants from a common and widely- diffused species, belonging to a large genus, will tend to partake ef the same advantages which made their parent successful in life, they will generally go on muitiplying in number as well as diverging in character: this is represented in the diagram by the several divergent branches proceeding from (A). ‘The modified offspring from the later and more highly improved branches in the lines of descent, will, it is probable, often take the place of, and so «destroy, the earlier and less improved branches: this is represented in the diagram by some of the lower branches not reaching to the upper horizontal lines. In some cases no doubt the process of ancdification wili be confined to a single line of descent, and the number of modified descendants will not be increased; although the amount of divergent modification may have been augmented. This case would he represented in the diagram, if all the lines pro- ceeding from (A) were removed, excepting that from a'toa™, In the same way the English race-horse and English pointer have appa- rently both gone on slowly diverging in character from their original stocks, without either having given off any fresh branches or races. After ten thousand generations, species (A) is supposed to have produced three forms, a, f/?, and m, which, from having diverged in character during the successive generations, will have come to differ largely, but perhaps unequally, from each other and from their common parent. If we suppose the amount of change be- tween each horizontal line in our diagram to be excessively small, these three forms may still be only well-marked varieties; but we have only to suppose the steps in the process of modification to be amore numerous or greater in amount, to convert these three forms _ Crap. IV. Natural Selection. 93 ‘nto doubtful or at last into well-defined species. Thus the diagram. illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing spe- cies. By continuing the same process for a greater number of gene- rations (as shown in the diagram in a condensed and simplified manner), we get eight species, marked by the letters between a” and m4, all descended from (A). Thus, as I believe, species are multiplied and genera are formed. In a large genus it is probable that more than one species would vary. In the diagram I have assumed that a second species (1) has produced, by analogous steps, after ten thousand generations, either two well-marked varieties (w" and 21°) or two species, according to: the amount of change supposed to be represented between the hori- zontal lines. After fourteen thousand generations, six new species, marked by the letters 1 to z'4, are supposed to have been produced. In any genus, the species which are already very different in cha-- racter from each other, will generally tend to produce the greatest number of modified descendants; for these will have the best. chance of seizing on new and widely different places in the polity of nature: hence in the diagram I have chosen the extreme species (A), and the nearly extreme species (1), as those which have largely varied, and have given rise to new varieties and species. The other nine species (marked by capital letters) of our original genus, may for long but unequal periods continue to transmit unaltered de- scendants; and this is shown in the diagram by the dotted lines. unequally prolonged upwards. But during the process of modification, represented in the dia- gram, another of our principles, namely that of extinction, will have played an important part. As in each fully stocked country natural selection necessarily acts by the selected form having some advan- tage in the struggle for life over other forms, there will be a constant tendency in the improved descendants of any one species to sup- plant and exterminate in each stage of descent their predecessors. and their original progenitor. For it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitution, and structure. Hence all the intermediate forms between the earlier and later states, that is between the less and more improved states. of the same species, as well as the original parent-species itself, will generally tend to become extinct. So it probably will be with many whole collateral lines of descent, which will be conquered by later and improved lines. If, however, the modified offspring of a species get into some distinct country, or become quickly adapted to some 94 Results of the Action of Cap. IV, quite new station, in which offspring and progenitor do not come into competition, both may continue to exist. : ? If, then, our diagram be assumed to represent a considerable amount of modification, species (A) and all the earlier varieties will have become extinct, being replaced by eight new species (a™ to m) ; and species (1) will be replaced by six (** to z*) new species, But we may go further than this. The original species of our genus were supposed to resemble each other in unequal degrees, as is so generally the case in nature; species (A) being more nearly related to B, OC, and D, than to the other species ; and species (1) more to G, H, K, L, than to the others. These two species (A) and (1) were also supposed to be very common and widely diffused species, so that they must originally have had some advantage over most of the other species of the genus. Their modified descendants, fourteen in number at the fourteen-thousandth generation, will probably have inherited some of the same advantages: they have also been modified and improved in a diversified manner at each stage of descent, so as to have become adapted to many related places in the natural economy of their country. It seems, therefore, extremely probable that they will have taken the places of, and thus exterminated, not only their parents (A) and (1), but likewise some of the original species which were most nearly related to their parents. Hence very few of the original species will have trans- mitted offspring to the fourteen-thousandth generation. We may suppose that only one (F), of the two species (E and F) which were least closely related to the other nine original species, has trans- mitted descendants to this late stage of descent. The new species in our diagram descended from the original eleven species, will now be fifteen in number. Owing to the diver- gent tendency of natural selection, the extreme amount of difference in character between species a and z' will be much greater than that between the most distinct of the original eleven species. The new species, moreover, will be allied to each other in a widely dif- ferent manner. Of the eight descendants from (A) the three marked a’, q*, p“', will be nearly related from having recently branched off from a”; 64, and f'4, from having diverged at an earlier period from a”, will be in some degree distinct from the three first-named. species; and lastly, o', e', and m*, will be nearly related one to the other, but, from having diverged at the first commencement of the process of modification, will be widely different from the other five species, and may constitute a sub-genus or a distinct genus. ‘The six descendants from (1) will form two sub-genera or genera. But as the original species (I) differed largely from (A), standing Cuap. IV. Natural Selection. 95 nearly at the extreme end of the original genus, the SIX descendants from (1) will, owing to inheritance alone, differ considerably from the eight descendants from (A); the two groups, moreover, are supposed to have gone on diverging in different directions. The intermediate species, also (and this is a very important considera~ tion), which connected the original species (A) and (1), have all become, excepting (F), extinct, and have left no descendants. Hence the six new species descended from (I), and the eight de- scended from (A), will have to be ranked as very distinct genera, or even as distinct sub-families. Thus it is, as I believe, that two or more genera are produced by descent with modification, from two or more species of the same genus. And the two or more parent-species are supposed to be descended from some one species of an earlier genus. In our dia- gram, this is indicated by the broken lines, beneath the capital letters, converging in sub-branches downwards towards a single point; this point represents a species, the supposed progenitor of our several new sub-genera and genera, It is worth while to reflect fora moment on the character of the new species F’4, which is supposed not to have diverged much in character, but to have retained the form of (F), either unaltered or altered only in a slight degree. In this case, its affinities to the other fourteen new species will be of a curious and circuitous nature. Being descended from a form which stood between the parent-species (A) and (1), now supposed to be extinct and unknown, it will be in some degree intermediate in character between the two groups descended from these two species. But as these two groups have gone on diverging in character from the type of their parents, the new species (¥™) will not be directly intermediate between them, but rather between types of the two groups; and every naturalist will be able to call such cages before his mind. In the diagram, each horizontal line has hitherto been supposed to represent a thousand generations, but each may represent a million or more generations; it may also represent a section of the Successive strata of the earth’s crust including extinct remains. We shall, when we come to our chapter on Geology, have to refer again to this subject, and I think we shall then sce that the diagram throws light on the affinities of extinct beings, which, though gene- rally belonging to the same orders, families, or genera, with those now living, yet are often, in some degree, intermediate in character between existing groups; and we can understand this fact, for the extinct species lived at various remote epochs when the branching lines of descent had diverged less, 96 Results of the Action of Crap, —— I see no reason to limit the process of modification, as now ex- plained, to the formation of genera alone, If, in the diagram, we suppose the amount of change represented by each successive group of diverging dotted lines to be great, the forms marked a" to p'4, those marked b™ and /4, and these marked o to m™, will form three very distinct genera. We shall also have two very distinct; genera descended from (1), differing widely from the descendants of (A). These two groups of genera will tius form two distinct families, or orders, according to the amount of divergent modifica- tion supposed to be represented in the diagram. And the two new families, or orders, are descended from two species of the original genus, and these are supposed to be descended from some still more ancient and unknown form. We have seen that in each country it is the species belonging to the larger genera which oftenest present varieties or incipient species. ‘This, indeed, might have been expected ; for, as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group shows that its species have inherited from a common ancestor some advantage in common. Hence, the struggle for the production of new and modified descendants will mainly lie between the larger groups which are all trying to increase in number. One large group will slowly conquer another large group, reduce its numbers, and thus lessen its chance of further variation and improvement. Within the same large group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of Nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. Small and broken groups and sub-groups will finally disappear. Looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up, that is, which have as yet suffered least extinction, will, for a long period, continue to increase. But whichsgroups will ultimately prevail, no man can _ predict; for we know that many groups, formerly most extensively developed, have now become extinct. Looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants ; and consequently that, of the species living at any one period, extremely few will transmit descendants to a remote futurity. I shall have to return to this subject in the chapter on Classification, but I may add that as, according to this view, extremely few of the Onap. IV. Natural Selection. 97 more ancient species have transmitted descendants to the present day, and, as all the descendants of the same species form a class, we © can understand how it is that there exists so few classes in each main division of the animal and vegetable kingdoms. Although few of the most ancient species have left modified descendants, yet, at remote geological periods, the earth may have been almost as well peopled with species of many genera, families, orders, and classes, as at the present time. On the Degree to which Organisation tends to advance. Natural Selection acts exclusively by the preservation and accu- mulation of variations, which are beneficial under the organic and inorganic conditions to which each creature is exposed at all periods of life. The ultimate result is that each creature tends to become more and more improved in relation to its conditions. This im- provement inevitably leads to the gradual advancement of the organisation of the greater number of living beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each other’s satisfaction what is meant by an advance in organisation. Amongst the vertebrata the degree of intellect and an approach in structure to man clearly come into play. It might be thought that the amount of change which the various parts and organs pass through in their development from the embryo to maturity would suffice as a standard of comparison ; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less perfect, so that the mature animal cannot be called higher than its larva. Von Baer’s standard seems the most widely applicable and the best, namely, the amount of differentiation of the parts of the same organic being, in the adult state as I should be inclined to add, and their specialisa- tion for different functions; or, as Milne Edwards would express it, the completeness of the division of physiological labour. But we shall see how obscure this subject is if we look, for instance, to fishes, amongst which some naturalists rank those as highest which, like the sharks, approach nearest to amphibians ; whilst other naturalists rank the common bony or teleostean fishes as the highest, inasmuch as they are most strictly fish-like, and differ most from the other vertebrate classes. We see still more plainly the obscurity of the subject by turning to plants, amongst which the standard of intel- lect is of course quite excluded; and here some botanists rank those plants as highest which have every organ, as sepals, petals, stamens, and pistils, fully developed in each flower; whereas other botanists, H 98 On the Degree to which. Cuap. IV. probably with more truth, look at the plants which have their several organs much modified and reduced in number as the highest. If we take as the standard of high organisation, the amount of. differentiation and specialisation of the several organs in each being when adult (and this will include the advancement of the brain for intellectual purposes), natural selection clearly leads towards this standard : for all physiologists admit that the specialisation of organs, inasmuch as in this state they perform their functions better, is an advantage to each being; and hence the accumulation of variations — tending towards specialisation’ is within the scope of natural selec- tion. On the other hand, we can see, bearing in mind that all organic beings are striving to increase at a high ratio and to seize on every unoccupied or less well occupied place in the economy of nature, that it is quite possible for natural selection gradually to fit a being to a situation in which several organs would be superfluous: or useless : in such cases there would be retrogression in the scale of ° organisation. Whether organisation on the whole has actually advanced from the remotest geological periods to the present day will be more conveniently discussed in our chapter on Geological Succession. But it may be objected that if all organic beings thus tend to. rise in the scale, how is it that throughout the world a multitude of the lowest forms still exist; and how is it that in each great class some forms are far more highly developed than others? Why have not the more highly developed forms everywhere supplanted and exterminated the lower? Lamarck, who believed in an innate and inevitable tendency towards perfection in all organic beings, seems: to have felt this difficulty so strongly, that he was led to suppose that new and simple forms are continually being produced by spon- taneous generation. Science has not as yet proved the truth of this belief, whatever the future may reveal. On our theory the continued existence of lowly organisms offers no difficulty; for natural selection, or the survival of the fittest, does not necessarily include progressive development—it only takes advantage of such variations as arise and are beneficial to each creature under its com- plex relations of life. And it may be asked what advantage, as far as we can see, would it be to an infusorian animalcule—to an in- testinal worm—or even to an earth-worm, to be highly organised. If it were no advantage, these forms would be left, by natural selec- tion, unimproved or but little improved, and might remain for indefinite ages in their present lowly condition. And geology tells us that some of the lowest forms, as the infusoria and rhizopods, tae Cuap. IV. Organisation tends to aavance. — 99 have remained for an enormous period in nearly their present state. But to suppose that most of the many now existing low forms have not in the least advanced since the first dawn of life would be _extremely rash; for every naturalist who has dissected some of the beings now ranked as very low in the scale, must have been struck with their really wondrous and beautiful organisation. Nearly the same remarks are applicable if we look to the different crades of organisation within the same great group; for instance, in the vertebrata, to the co-existence of mammals and fish— amongst mammalia, to the co-existence of man and the ornithorhynchus— amongst fishes, to the co-existence of the shark and the lancelet (Amphioxus), which latter fish in the extreme simplicity of its structure approaches the invertebrate classes. But mammals and fish hardly come into competition with each other; the advance- ment of the whole class of mammals, or of certain members in this class, to the highest grade would not lead to their taking the place of fishes. Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires aérial respiration ; so that warm-blooded mammals when inhabiting the water lie under a disadvantage in having to come continually to the sur- face to breathe. With fishes, members of the shark family would not tend to supplant the lancelet; for the lancelet, as I hear from Fritz Miiller, has as sole companion and competitor on the barren sandy shore of South Brazil, an anomalous annelid. The three lowest orders of mammals, namely, marsupials, edentata, and rodents, co-exist in South America in the same region with nume- rous monkeys, and probably interfere little with each other. Although organisation, on the whole, may have advanced and be still advancing throughout the world, yet the scale will always present many degrees of perfection; for the high advancement of certain whole classes, or of certain members of each class, does not at all necessarily lead to the extinction of those groups with which they do not enter into close competition, In some cases, as we shall hereafter see, lowly organised forms appear to have been pre- served to the present day, from inhabiting confined or peculiar stations, where they have been subjected to less severe competition, and where their scanty numbers have retarded the chance of favour- » able variations arising, Finally, I believe that many lowly organised forms now exist throughout the world, from various causes. In some cases varia- tions or individual differences of a favourable nature may never have arisen for natural selection to act on and accumulate. In no case, probably, has time sufficed for the utmost possible amount of H 2 100 Convergence of Character. Cuap. IV, i development. In some few cases there has been what we must call retrogression of organisation. But the main. cause lies in the fact that. under very simple conditions of life a high organisation would be of no service,—possibly would be of actual disservice, as being of a more delicate nature, and more liable to be put out of order and injured. Looking to the first dawn of life, when all organic beings, as we may believe, presented the simplest structure, how, it has been asked, could the first steps in the advancement or differentiation of parts have arisen? Mr. Herbert Spencer would probably answer that, as soon as simple unicellular organism came by growth or division to be compounded of several cells, or became attached to any supporting surface, his law “that homologous units of any order become differentiated in proportion as their relations to inci- dent forces’ become different” would come into action, But as we have no facts to guide us, speculation on the subiect is almost useless. It is, however, an error to suppose that there would be no struggle for existence, and, consequently, no natural selection, until many forms had been produced: variations in a single species inhabiting an isolated station might be beneficial, and thus the whole mass of individuals might be modified, or two distinct forms might arise. But, as I remarked towards the close of the Introduction, no one ought to feel surprise at much remaining as yet unexplained on the origin of species, if we make due allowance for our profound igno- vance on the mutual relations of the inhabitants of the world at the present time, and still more so during past ages. Convergence of Character. Mr. H. C. Watson thinks that I have overrated the importance of divergence of character (in which, however, he apparently believes), and that convergence, as it may be called, has likewise played a part. If two species, belonging to two distinct though allied genera, had both produced a large number of new and diver- gent forms, it is conceivable that these might approach each other , so closely that they would have all to be classed under the same ~ genus ; and thus the descendants of two distinct genera would con- verge intoone. Butit would in most cases be extremely rash to attri- bute to convergence a close and general similarity of structure in the modified descendants of widely distinct forms. The shape of a crystal is determined solely by the molecular forces, and it is not surprising that dissimilar substances should sometimes assume the same form; but with organic beings we should bear in mind that the form of ~~ OS Vesa CO! Cuap. IV. Convergence of Character. IOI —_ each depends on an infinitude of complex relations, namely on the variations Which have arisen, these being due to causes far too | intricate to be followed out,—on the nature of the variations which have been preserved or selected, and this depends on the surround- ing physical conditions, and in a still higher degree on the sur- rounding organisms with which each being has come Into competi- tion,—and lastly, on inheritance Gn itself a fluctuating element) from innumerable progenitors, all of which have had their forms deter- mined through equally complex relations. It is incredible that the descendants of two organisms, which had originally differed in a marked manner, should ever afterwards converge so closely as to lead to a near approach to identity throughout their whole organi- sation. If this had occurred, we should meet with the same form, independently of genetic connection, recurring in widely separated cseological formations; and the balance of evidence is opposed to any such an admission. Mr. Watson has also objected that the continued action of natural selection, together with divergence of character, would tend to make an indefinite number of specific forms. As far as mere inorganic con- ditions are concerned, it seems probable that a sufficient number of species would soon become adapted to all considerable diversities of heat, moisture, &c.; but I fully admit that the mutual relations of organic beings are more important; and as the number of species in any country goes on increasing, the organic conditions of life must become more and more complex. Consequently there seems at first sight no limit to the amount of profitable diversification of structure, and therefore no limit to the number of species which might be produced. We do not know that even the most prolific area is fully stocked with specific forms: at the Cape of Good Hope and in Australia, which support such an astonishing number of species, many European plants have become naturalised. But geology shows us, that from an early part of the tertiary period the number of species of shells, and that from the middle part of this same period the number of mammals, has not greatly or at all increased. What then checks an indefinite increase in the number of species? The amount of life (I do not mean the number of specific forms) supported on an area must have a limit, depending 80 largely as it does on physical conditions ; therefore, if an area be inhabited by very many species, each or nearly each species will be represented by few individuals; and such species will be liable to extermination from accidental fluctuations in the nature of the seasons or in the number of their enemies. The process of extermi- nation in such cases would be rapid, whereas the production of new 102 Natural Selection. Cuap. IV. ————. species must always be slow. Imagine the extreme case of as many species as individuals in England, and the first severe winter or very dry summer would exterminate thousands on thousands of species. Rare species, and each species will become rare if the number of species in any country becomes indefinitely increased, will, on the principle often explained, present within a given period few favourable variations; consequently, the process of giving birth to new specific forms would thus be retarded. When any species becomes very rare, close interbreeding will help to exterminate it; authors have thought that this comes into play in accounting for the deterioration of the Aurochs in Lithuania, of Red Deer in Scot- land, and of Bears in Norway, &c. Lastly, and this I am inclined to think is the most important element, a dominant species, which has already beaten many competitors in its own home, vill tend to spread and supplant many others. Alph. de Candolle has shown that those species which spread widely, tend generally to spread very widely ; consequently, they will tend to supplant and exter- minate several species in several areas, and thus check the inordinate increase of specific forms throughout the world. Dr. Hooker has recently shown that in the 8.E. corner of Australia, where, appa- rently, there are many invaders from different quarters of the globe, the endemic Australian species have been greatly reduced in number. How much weight to attribute to these several considerations I will not pretend to say; but conjointly they must limit in each country the tendency to an indefinite augmentation of specific forms. Summary of Chapter. If under changing conditions of life organic beings present indivi- dual differences in almost every part of their structure, and this cannot be disputed; if there be, owing to their geometrical rate of | increase, a severe struggle for life at some age, season, or year, and this certainly cannot be disputed ; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of life, causing an infinite diversity in structure, con- stitution, and habits, to be advantageous to them, it would be a most extraordinary fact if no variations had ever occurred useful to each being’s own welfare, in the same manner as so many variations have occurred useful to man. But if variations useful to any organic being ever do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance, these will tend to produce offspring similarly characterised. This principle of pre- Cuap. IV. Summary. 103 ee al of the fittest, I have called Natural Selection. It leads to the improvement of each creature 1n relation to its organic and inorganic conditions of life ; and consequently, in most cases, to what must be regarded as an advance in organisation. Nevertheless, low and simple forms will long endure if well fitted for their simple conditions of life. - a Natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg, seed, or young, as easily | as the adult. Amongst many animals, sexual selection will have civen its aid to ordinary selection, by assuring to the most vigorous and best adapted males the greatest number of offspring. Sexual selection will also give characters useful to the males alone, in their struggles or rivalry with other males ; and these characters will be transmitted to one sex or to both sexes, according to the form of inheritance which prevails. Whether natural selection has really thus acted in adapting the various forms of life to their several conditions and stations, must be judged by the general tenor and balance of evidence given in the following chapters. But we have already seen how it entails extinction; and how largely extinction has acted in the world’s history, geology plainly declares. Natural selection, also, leads to divergence of character; for the more organic beings diverge in structure, habits, and constitution, by so much the more can a large number be supported on the same area,—of which we see proof by looking to the inhabitants of any small spot, and to the productions naturalised in foreign lands. Therefore, during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in numbers, the more diversified the de- scendants become, the better will be their chance cf success in the battle for life. Thus the small differences distinguishing varieties of the same species, steadily tend to increase, till they equal the greater differences between species of the same genus, or even of distinct genera, ~ We have seen that it is the common, the widely-diffused, and widely-ranging species, belonging to the larger genera within each class, which vary most; and these tend to transmit to their modified oy sbring that superiority which now makes them dominant in i" - jn countries, Natural selection, as has just been remarked, improved and ints of character and to much extinction of the less saturs of er aeirmae forms of life. On these principles, the between the oie the generally well-defined distinctions organic beings in each class throughout servation, or the surviv 104 Natural Selection. - Cuap. LY, the world, may be explained. It is a truly wonderful fact—the wonder of which we are apt to overlook from familiarity—that all animals and all plants throughout all time and space should be related to each other in groups subordinate to groups, in the manner which we everywhere behold—namely, varieties of the same species most closely related, species of the same genus less closely and unequally related, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. The several subordinate groups in any class cannot be ranked in a single file, but seem clustered round points, and these round other points, and so on in almost endless cycles. If species had been independently created, no explanation would have been possible of this kind of classification ; but it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram. The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during former years may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have at all times overmastered other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was young, budding twigs ; and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear the other branches; so with the species which lived during long-past geolo- gical periods, very few have left living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these fallen branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only in a fossil state. As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we Cuap. IV. Summary. 10S. _ — occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large pranches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many @ feebler branch, so by generation I pelieve it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications. 106 Laws of Variation. Cuap. V. CHAPTER V. LAWS OF VARIATION. Effects of changed conditions — Use and disuse, combined with natural selection; organs of flight and of vision — Acclimatisation — Correlated variation — Compensation and economy of growth — False correlations — Multiple, rudimentary, and lowly organised structures variable — Parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable — Species of the same genus vary in an analogous manner — Reversions to long-lost characters — Summary. I HAVE hitherto sometimes spoken as if the variations—so common and multiform with organic beings under domestication, and in a lesser degree with those under nature—were due to chance. This, of course, is a wholly incorrect expression, but it serves to acknow- ledge plainly our ignorance of the cause of each particular variation. Some authors believe it to be as much the function of the repro- ductive system to produce individual differences, or slight deviations of structure, as to make the child like its parents. But the fact of variations and monstrosities occurring much ‘more frequently under domestication than under nature, and the greater variability of species having wide ranges than of those with restricted ranges, lead to the conclusion that variability is generally related to the condi- tions of life to which each species has been exposed during several successive generations. In the first chapter I attempted to show that changed conditions act in two ways, directly on the whole organisa- tion or on certain parts alone, and indirectly through the reproductive system. In all cases there are two factors, the nature of the organism, which is much the most important of the two, and the nature of the conditions. The direct action of changed conditions leads to definite or indefinite results. In the latter case the organi- sation seems to become plastic, and we have much fluctuating vari- ability. In the former case the nature of the organism is such that it yields readily, when subjected to certain conditions, and all, or nearly all the individuals become modified in the same way. It is very difficult to decide how far changed conditions, such as of climate, food, &¢., have acted in a definite manner. There is Cuar. V. Laws of Vartation. 107 reason to believe that in the course of time the effects have been ereater than can be proved by clear evidence. But we may safely conclude that the innumerable complex co-adaptations of structure, which we see throughout nature between various organic beings, cannot be attributed simply to such action. In the following cases the conditions seem to have produced some slight definite effect : i, Forbes asserts that shells at their southern limit, and when living in shallow water, are more brightly coloured than those of the same species from further north or from a greater depth ; but this certainly does not always hold good. Mr. Gould believes that birds of the same species are more brightly coloured under a clear atmos- phere, than when living near the coast or on islands; and Wollaston is convinced that residence near the sea affects the colours of insects. Moquin-Tandon gives a list of plants which, when growing near the sea-shore, have their leaves in some degree fleshy, though not else- where fleshy. These slightly varying organisms are interesting in as far as they present characters analogous to those possessed by the species which are confined to similar conditions, When a variation is of the slightest use to any being, we cannot tell how much to attribute to the accumulative action of natural selection, and how much to the definite action of the conditions of life. Thus, it is well known to furriers that animals of the same species have thicker and better fur the further north they live; but who can tell how much of this difference may be due to the warmest-clad indivi- duals having been favoured and preserved during many generations, and how much to the action of the severe climate? for it would appear that climate has some direct action on the hair of our domes- tic quadrupeds, Instances could be given of similar varieties being produced from the same species under external conditions of life as different as can well be conceived ; and, on the other hand, of dissimilar varieties being produced under apparently the same external conditions, Again, innumerable instances are known to every naturalist, of species Keeping true, or not varying at all, although living under the most opposite climates. Such considerations as these incline me to lay less weight on the direct action of the surrounding con- ditions, than on a tendency to vary, due to causes of which we are quite ignorant, In one sense the conditions of life ma variability, either directly or indirect] natural selection ; for the conditions det variety shall survive, clearly see that the two y be said, not only to cause y, but likewise to include ermine whether this or that But when man is the selecting agent, we elements of change are distinct ; variability — 108 Effects of Use and Disuse. Cuap. V: is in some manner excited, but it is the will of man which accumu-. lates the variations in certain directions ; and it is this latter agency which answers to the survival of the fittest under nature. Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection. From the facts alluded to in the first chapter, I thirtk there can be no doubt that use in our domestic animals has strengthened and enlarged certain parts, and disuse diminished them; and that such modifications are inherited. Under free nature, we have no standard of comparison, by which to judge of the effects of long- continued use or disuse, for we know not the parent-forms; but many animals possess structures which can be best explained by the effects of disuse. As Professor Owen has remarked, there is no greater anomaly in nature than a bird that cannot fly; yet there are several in this state. The logger-headed duck of South America can only flap along the surface of the water, and has its wings in “nearly the same condition as the domestic Aylesbury duck: it is a remarkable fact that the young birds, according to Mr. Cunning- ham, can fly, while the adults have lost this power. As the larger ground-feeding birds seldom take flight except to escape danger, it is probable that the nearly wingless condition of several birds, now inhabiting or which lately inhabited several oceanic islands, tenanted by no beast of prey, has been caused by disuse. The ostrich indeed | inhabits continents, and is exposed to danger from which it cannot escape by flight, but it can defend itself by kicking its enemies, as efficiently as many quadrupeds. We may believe that the proge- nitor of the ostrich genus had habits like those of the bustard, and. that, as the size and weight of its body were increased during suc- cessive generations, its legs were used more, and its wings less, until they became incapable of flight. Kirby has remarked (and I have observed the same fact) that the anterior tarsi, or feet, of many male dung-feeding beetles are often broken off; he examined seventeen specimens in his own collection, and not one had even a relic left. In the Onites apelles the tarsi are so habitually lost, that the insect has been described as not having them. In some other genera they are present, but in a rudimentary condition. In the Ateuchus or sacred beetle of the Egyptians, they are totally deficient. The evidence that accidental mutilations can be inherited is at present not decisive; but the remarkable cases observed by Brown-Séquard in guinea-pigs, of the inherited effects of operations, should make us cautious in denying Cuar. V. Effects of Use and Disuse. 109 this tendency. Hence it will perhaps be safest to look at the entire absence of the anterior tarsi in Ateuchus, and their rudimentary condition in some other genera, not as cases of inherited mutilations, put as due to the effects of long-continued disuse ; for he Et y dung-feeding beetles are generally found with their tarsi lost, this must happen early in life; therefore the tarsi cannot be of much importance or be much used by these insects. a In some cases we might easily put down to disuse modifications of structure which are wholly, or mainly, due to natural selection. Mr. Wollaston has discovered the remarkable fact that 200 beetles, out of the 550 species (but more are now known) inhabiting Madeira, are so far deficient in wings that they cannot fly; and that, of the twenty-nine endemic genera, no less than twenty-three have all their species in this condition! Several facts,—namely, that beetles in many parts of the world are frequently blown to sea and perish; that the bectles in Madeira, as observed by Mr. Wollaston, lie much concealed, until the wind lulls and the sun ‘shines; that the proportion of wingless beetles is larger on the exposed Desertas than in Madeira itself; and especially the extra- ordinary fact, so strongly insisted on by Mr. Woilaston, that certain large groups of beetles, elsewhere excessively numerous, which absolutely require the use of their wings, are here almost entirely absent ;—these several considerations make me believe that the wingless condition of so many Madeira beetles is mainly due to the action of natural selection, combined probably with disuse. For during many successive generations each individual beetle which flew least, either from its wings having been ever so little less perfectly developed or from indolent habit, will have had the best chance of surviving from not being blown out to sea; and, on the other hand, those beetles which most readily took to flight would oftenest have been blown to sea, and thus destroyed. The insects in Madeira which are not ground-feeders, and which, as certain flower-feeding coleoptera and lepidoptera, must habitually use their wings to gain their subsistence, have, as Mr. Wollaston suspects, their wings not at all reduced, but even enlarged. This is quite compatible with the action of natural selection. For when a new insect first arrived on ¢he island, the tendency of natural selection to enlarge or to reduce the Wings, would depend on Netra « greater number of individuals were saved by successfully a i . . +s ; 1 -_ - with the w inds, or by giving up the attempt and rareiy or avetbe ing. As with mariners shipwrecked near a coast, it would have been be ; ne sui eal : ae for the sood Swimmers if they had been able to : urther, whereas it would have been better for the bad 110 ' Effects of Use and Disuse. Cuap, V, —-— swimmers if they had not been able to swim at all and had stuck to the wreck. : : The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. In South America, a bur- rowing rodent, the tuco-tuco, or Ctenomys, is even more subter- ranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that they were frequently blind. One which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane. As frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would aid the effects of disuse. It is well known that several animals, belonging to the most different classes, which inhabit the caves of Carniola and of Ken- tucky, are blind. In some of the crabs the foot-stalk for the eye remains, though the eye is gone ;—the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, their loss may be attributed to disuse. In one of the blind animals, namely, the cave-rat (Neotoma), two of which were captured by Professor Silliman at above half a mile distance from the mouth of the cave, and there- fore not in the profoundest depths, the eyes were lustrous and of large size; and these animals, as I am informed by Professor Silli- man, after having been exposed for about a month to a graduated light, acquired a dim perception of objects. It is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar climate; so that, in accordance with the old view of the blind animals having been separately created for the American and European caverns, very close similarity in their organisation and affinities might have been expected. This is certainly not the case if we look at the two whole faunas; and with respect to the insects alone, Schiddte has remarked, “ We are accordingly prevented from considering the entire phenomenon in any other light than something purely local, and the similarity which is exhibited in a few forms between thé Mammoth cave (in Kentucky) and the caves in Carniola, otherwise than as a very plain expression of that analogy which subsists Cuap. V. Effects of Use and Disuse. LII generally between the fauna of Europe and of x oh america” On my view we must suppose that American an _ ae most cases ordinary powers of vision, slowly migrate rd veooee ee. generations from the outer world into the deeper an eeper me cesses of the Kentucky caves, as did European animals into the caves of Europe. We have some evidence of this gradation of habit ; for, as Schiddte remarks, “ We accordingly look upon the subterranean faunas as small ramifications which have penetrated into the earth from the geographically limited faunas of the adja- cent tracts, and which, as they extended themselves into darkness, have been accommodated to surrounding circumstances. Animals not far remote from ordinary forms, prepare the transition from light to darkness. Next follow those that are constructed for twi- light ; and, last of all, those destined for total darkness, and whose formation is quite peculiar.” These remarks of Schiodte’s, it should be understood, apply not to the same, but to distinct species. By the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have effected other changes, such as an increase in the length of the antenne or palpi, as a compensation for blindness. Notwithstand- ing such modifications, we might expect still to see in the cave- animals of America, affinities to the other inhabitants of that con- tinent, and in those of Europe to the inhabitants of the European continent. And this is the case with some of the American cave- animals, as I hear from Professor Dana ; and some of the European cave-insects are very closely allied to those of the surrounding country. It would be difficult to give any rational explanation of the affinities of the blind cave-animals to the other inhabitants of the two continents on the ordinary view of their independent creation, ‘That several of the inhabitants of the caves of the Old and New Worlds should be closely related, we might expect from the well-known relationship of most of their other productions. As * blind species of Bathyscia is found in abundance on shady rocks ar from caves, the loss of vision in the cave-species of this one Sate probably had no relation to its dark habitation ; for it becotne “ . a aot already deprived of vision should readily thalmus) o dark caverns. Another blind genus (Anoph- ollers this remarkable peculiarity, that the species, as er Mr. Mu | 7 saves observes, have not as yet been found anywhere except Arnerica oat 1m the several caves of Europe and ible that the progenitors of these ished with eyes, may formerly II2 Acclimatisation. Cuap. V, have ranged over both continents, and then have become extinct, excepting in their present secluded abodes. Far from feeling sur- prise that some of the cave-animals should be very anomalous, as Agassiz has remarked in regard to the blind fish, the Amblyopsis, and as is the case with the blind Proteus with reference to the reptiles of Europe, I am only surprised that more wrecks of ancient life have not been preserved, owing to the less severe competition to which the scanty inhabitants of these dark abodes will have been exposed. Acclimatisation. Habit is hereditary with plants, as in the period of flowering, in the time of sleep, in the amount of rain requisite for seeds to germi- nate, &c., and this leads me‘ to say a few words on acclimatisation. As it is extremely common for distinct species belonging to the same genus to inhabit hot and cold countries, if it be true that all the species of the same genus are descended from a single parent-form, acclimatisation must be readily effected during a long course of descent. It is notorious that each species is adapted to the climate of its own home: species from an arctic or even from a temperate region cannot endure a tropical climate, or conversely. So again, many succulent plants cannot endure a damp climate. But the degree of adaptation of species to the climates under which they live is often overrated. We may infer this from our frequent in- ability to predict whether or not an imported plant will endure our climate, and from the number of plants and animals brought front different countries which are here perfectly healthy. We have reason to believe that species in a state of nature are closely limited in their ranges by the competition of other organic beings quite as much as, or more than, by adaptation to particular climates. But whether or not this adaptation is in most cases very close, we have evidence with some few plants, of their becoming, to a certain extent, naturally habituated to different temperatures ; that is, they become acclimatised : thus the pines and rhododendrons, raised from seed collected by Dr. Hooker from the same Species growing at different heights on the Himalaya, were found to ‘possess in this country different constitutional powers of resisting cold. Mr. Thwaites informs me that he has observed similar facts in Ceylon ; analogous observations have been made by Mr. H. C. Watson on Kuropean species of plants brought from the Azores to England ; and I could give other cases. In regard to animals, several authentic instances could be adduced of species having largely extended, within historical times, their range from warmer to cooler latitudes, LS —“—i—s al io] Acclimatisation. 113 Cuar. V. ene but we do not positively know that these animals were strictly adapted to their native climate, though 2 all prenary cases we assume such to be the case; nor do we know an they have subsequently become specially acclimatised a” t a new homes, so as to be better fitted for them than they were at first. As we may infer that our domestic animals were originally chosen by uncivilised man because they were useful and because they bred readily under confinement, and not because they were subsequently found capable of far-extended transportation, the common and ex- traordinary capacity in our domestic animals of not only withstand- ing the most different climates, but of being perfectly fertile (a far severer test) under them, may be used as an argument that a large proportion of other animals now in a state of nature could easily be brought to bear widely different climates. We must not, how- ever, push the foregoing argument too far, on account of the pro- bable origin of some of our domestic animals from several wild stocks; the blood, for instance, of a tropical and arctic wolf may perhaps be mingled in our domestic breeds. The rat and mouse cannot be considered as domestic animals, but they have been trans- ported by man to many parts of the world, and now have a far wider range than any other rodent; for they live under the cold climate of Faroe in the north and of the Falklands in the south, and on many an island in the torrid zones. Hence adaptation to any special climate may be looked at as a quality readily grafted on an innate wide flexibility of constitution, common to most animals. On this view, the capacity of enduring the most different climates by man himself and by his domestic animals, and the fact of the extinct elephant and rhinoceros having formerly endured a glacial climate, whereas the living species are now all tropical or sub- tropical in their habits, ought not to be looked at as anomalies, but ws examples of a very common flexibility of constitution, brought, under peculiar circumstances, into action. How much of the acclimatisation of species to any peculiar climate 18 due to mere habit, and how much to the natural selection both ae paving ae innate constitutions, and how much to has some influence T mn st real roth fenandoce aia f custom incessant advice a st believe, both from analogy and from the Encyclopedias of “China, to be vers a a y tnane ie ancien mals from one district to another And az it isn Pikes tio nan should have succeeded in selecting HS EOS not Likely tat man witl § so many breeds and sub-breéds f 1 constitutions specially fitted for their own districts, the result must, I think : ust, I think, be due to habit. On the other hand, natural selec- I and conversely 5 114 Correlated Variation. Czar. V. tion would inevitably tend to preserve those individuals which were porn with constitutions best adapted to any country which they inhabited. In treatises on many kinds of cultivated plants, certain varicties are said to withstand certain climates better than others; this is strikingly shown in works on fruit-trees published in the United States, in which certain varieties are habitually recom- mended for the northern and others for the southern States; and as most of these varieties are of recent origin, they cannot owe their constitutional differences to habit. The case of the Jerusalem artichoke, which is never propagated in England by seed, and of which consequently new varieties have not been produced, has even been advanced, as proving that acclimatisation cannot be effected, for it is now as tender as ever it was! The case, also, of the kidney- bean has been often cited for a similar purpose, and with much greater weight; but until some one will sow, during a score of generations, his kidney-beans so early that a very large proportion are destroyed by frost, and then collect seed from the few survivors, with care to prevent accidental crosses, and then again get seed from these seedlings, with the same precautions, the experiment cannot be said to have been tried. Nor let it be supposed that differences in the constitution of seedling kidney-beans never appear, for an account has been published how much more hardy some seedlings are than others; and of this fact I have myself observed striking instances. On the whole, we may conclude that habit, or use and disuse, have, in some cases, played a considerable part in the modification of the constitution and structure; but that the effects have often been largely combined with, and sometimes overmastered by, the natural selection of innate variations. Correlated Variation. I mean by this expression that the whole organisation is so tied together during its growth and development, that when slight variations in any one part occur, and are accumulated through natural selection, other parts become modified. This is a very im- portant subject, most imperfectly understood, and no doubt wholly different classes of facts may be here easily confounded together. We shall presently see that simple inheritance often gives the false appearance of correlation. One of the most obvious real cases is, that variations of structure arising in the young or larve naturally tend to affect the structure of the mature animal. The several parts of the body which are homologous, and which, at an early embryonic period, are identical in structure, and which are neces- Cuap. V. Correlated Variation. 115 sarily exposed to similar conditions, seem eminently liable to vary ina like manner: we see this in the right and left sides of the body varying in the same manner ; in the front and hind legs, and even in the jaws and limbs, varying together, for the lower jaw is believed by some anatomists to be homologous with the limbs, These tendencies, I do not doubt, may be mastered more or less completely by natural selection: thus a family of stags once existed with an antler only on one side; and if this had been of any great use to the breed, it might probably have been rendered permanent by selection. Homélogous parts, as has been remarked by some authors, tend to cohere; this is often seen in monstrous plants: and nothing is more common than the union of homologous parts in normal structures, as in the union of the petals into a tube. Hard parts ‘seem to affect the form of adjoining soft parts; it is believed by some authors that with birds the diversity in the shape of the pelvis causes the remarkable diversity in the shape of their kidneys. Others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child. In snakes, according to Schlegel, the form of the body and the manner of swallowing determine the position and form of several of the most important viscera. The nature of the bond is frequently quite obscure. M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain malcon- formations frequently, and that others rarely, co-exist, without our being able to assign any reason. What can be more singular than the relation in cats between complete whiteness and blue eyes with deafness, or between the tortoise-shell colour and the female SEX 5 orin pigeons between their feathered feet and skin betwixt the outer toes, or between the presence of more or less down on the young pigeon when first hatched, with the future colour of its plumage ; Tertoh are pation pehween fia hair and teeth in the naked With ree ai . A pre no doubt homology comes into play? be nentdentet 4 . ‘ er case of correlation, I think it can hardly abnormal. in ‘ a ; e two orders of mammals which are most Edentata (armadill aml OVENS: VIE Menace, (whales) and whole the most ulloes, scaly ant-eaters, &c.), are likewise on the abnormal in their teeth ; but there are so many exceptions t i é oe F ae. dalue. 0 this rule, as Mr. Mivart has remarked, that it has I know of no case bet laws of correlation an therefore of y ter adapted to show the importance cf the ve d Variation, independently of utility and atural selection, than that of the difference between I 2 116 Correlated Variation. Cuar. V2 the outer and inner flowers in some Compositous and Umbelliferous: plants. Every one is familiar with the difference between the ray and central florets of, for instance, the daisy, and this difference is often accompanied with the partial or complete abortion of the reproductive organs. But in some of these plants, the seeds also differ in shape and sculpture. These differences have sometimes been attributed to the pressure of the involucra on the florets, or to their mutual pressure, and the shape of the seeds in the ray- florets of some Composite countenances this idea; but with the Umbellifere, it is by no means, as Dr. Hooker informs me, the species with the densest heads which most frequently differ in their inner and outer flowers. It might have been thought that the development of the ray-petals by drawing nourishment from the reproductive organs causes their abortion ; but this can hardly be the sole cause, for in some Composite the seeds of the outer and inner florets differ, without any difference in the corolla. Possibly these several differences may be connected with the different flow of nutriment towards the central and external flowers: we know, at least, that with irregular flowers, those nearest to the axis are most subject to peloria, that is to become abnormally symmetrical. I may add, as an instance of this fact, and as a striking case of correlation, that in many pelargoniums, the two upper petals in the central flower of the truss often lose their patches of darker colour ; and when this occurs, the adherent nectary is quite aborted ; the central flower thus becoming peloric or regular. When the colour is absent from only one of the two upper petals, the nectary is not quite aborted but is much shortened. With respect to the development of the corolla, Sprengel’s idea that the ray-florets serve to attract insects, whose agency is highly advantageous or necessary for the fertilisation of these plants, is highly probable ; and if so, natural selection may have come into play.. But with respect to the seeds, it seems impossible that their differ- ences in shape, which are not always correlated with any difference in the corolla, can be in any way beneficial: yet in the Umbelli- feree these differences are of such apparent importance—the seeds being sometimes orthospermous in the exterior flowers and ccelo- spermous in the central flowers, — that the elder De Candolle founded his main divisions in the order on such characters. Hence modifications of structure, viewed by systematists as of high value, may be wholly due to the laws of variation and correlation, without being, as far as we can judge, of the slightest service to the species. We may often falsely attribute to correlated variation structures which are common to whole groups of species, and which in truth cuar.V. Compensation and Economy of Growth. 117 are simply due to inheritance ; for an ancient Pee may fave acquired through natural selection some one Moa cation ae ture, and, after thousands of generations, some other an " e- yendent modification ; and these two modifications, having een, transmitted to a whole group of descendants with diverse habits, would naturally be thought to be in some necessary Manner CO related. Some other correlations are apparently due to the manner in which natural selection can alone act. For instance, Alph. de Candolle has remarked that winged seeds are never found in fruits which do not open: I should explain this rule by the impossibility of seeds gradually becoming winged through natural selection, unless the capsules were open; for in this case alone could the seeds, which were a little better adapted to be wafted by the wind, gain an advantage over others less well fitted for wide dispersal. Compensation and Economy of Growth. The elder Geoffroy and Goethe propounded, at about the same time their law of compensation or balancement of growth; or, as Goethe expressed it, “in order to spend on one side, nature is forced to economise on the other side.” I think this holds true to a certain extent with our domestic productions: if nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it is difficult to get a cow to give much milk and to fatten readily. The same varieties of the cabbage do not yield abundant and nutritious foliage and a copious supply of oil-bearing seeds. When the seeds in our fruits become atrophied, the fruit itself gains largely in size and quality. In our poultry, a large tuft of feathers on the head is generally accompanied by a diminished. comb, and a large beard by diminished wattles. With species in a state of nature it can hardly be maintained that the law is of universal application; but many good observers, more ‘especially botanists, believe in its truth. I will not, however, here give any instances, for I see hardly any way of distinguishing between the elfects, on the one hand, of a part being largely developed through natural selection and another and adjoining part being reduced by this same process or by disuse, and, on the other hand, the actual Withdrawal of nutriment from one part owing to the excess of growth in another and adjoining part. 1 suspect, also, that some of the cases of compensation which have been advanced, and likewise some other facts, may be merged ue ie general principle, namely, that natural selection is ani che, ed ne ° eoononiise every part of the organisation. If ang ditions of life a structure, b8fore useful, becomes 118 Multiple and Rudimentary Cuar, V. less useful, its diminution will be favoured, for it will profit the individual not to have its nutriment wasted in building up an useless structure. I can thus only understand a fact with which I was much struck when examining cirripedes, and of which many analogous instances could be given: namely, that when a cirripede is parasitic within another cirripede and is thus protected, it loses more or less completely its own shell or carapace. This is the case with the male Ibla, and in a truly extraordinary manner with the Proteo- lepas: for the carapace in all other cirripedes consists of the three highly-important anterior segments of the head enormously deve- loped, and furnished with great nerves and muscles; but in the parasitic and protected Proteolepas, the whole anterior part of the head is reduced to the merest rudiment attached to the bases of the prehensile antenne. Now the saving of a large and complex structure, when rendered superfluous, would be a decided advantage: to each successive individual of the species ; for in the struggle for life to which every animal is exposed, each would have a better chance of supporting itself, by less nutriment being wasted. Thus, as I believe, natural selection will tend in the long run to reduce any part of the organisation, as soon as it becomes, through changed habits, superfluous, without by any means causing some other part to be largely developed in a corresponding degree. And, conversely, that natural selection may perfectly well succeed in largely developing an organ without requiring as a necessary com- pensation the reduction of some adjoining part. Multiple, Rudimentary, and Lowly-organised Structures are Variable. It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both with varieties and species, that when any part or organ is: repeated many times in the same individual (as the vertebre in snakes, and the stamens in polyandrous flowers) the number is variable ; whereas the same part or organ, when it occurs in lesser numbers, is constant. The same author as well as some botanists. _ have further remarked that multiple parts are extremely liable to vary in structure. As “vegetative repetition,” to use Prof. Owen's expression, is a sign of low organisation, the foregoing statements accord with the common opinion of naturalists, that beings which stand low in the scale of nature are more variable than those which are higher. I presume that lowness here means that the several parts of the organisation have been but little specialised for particular functions ; and as long as the same part has to perform diversified work, we can perhaps see why it should remain variable, that is,. &&&=Qa— pigeon, I presume that no one will doubt that this is a case oj -yeversion, and not of a new yet analogous variation appearing in the several breeds. We may, I think, confidently come to this conclusion, because, as we have seen, these coloured marks are eminently liable to appear in the crossed offspring of two distinct and differently coloured breeds; and in this case there is nothing in the external conditions of life to cause the reappearance of the . slaty-blue, with the several marks, beyond the influence of the mere act of crossing on the laws of inheritance. : No doubt it is a very surprising fact that characters should re- appear after having been lost for many, probably for hundreds of generations. But when a breed has been crossed only once by some other breed, the offspring occasionally show for many generations a tendency to revert in character to the foreign breed—some say, for a dozen or even a score of generations. After twelve generations, the proportion of blood, to use a common expression, from one ancestor, is only 1 in 2048; and yet, as we see, it is generally believed that a tendency to reversion is retained by this remnant of foreign blood. In a breed which has not been crossed, but in which both parents have lost some character which their progenitor pos- sessed, the tendency, whether strong or weak, to reproduce the lost character might, as was formerly remarked, for all that we can see to the contrary, be transmitted for almost any number of gener- ations. When a character which has been lost in a breed, reappears after a great number of generations, the most probable hypothesis is, not that one individual suddenly takes after an ancestor removed by some hundred generations, but that in each successive generation the character in question has been lying latent, and at last, under unknown favourable conditions, is developed. With the barb-pigeon, for instance, which very rarely produces a blue bird, it is probable that there is a latent tendency in each generation to produce blue plumage. The abstract improbability of such a ten- dency being transmitted through a vast number of generations, is not greater than that of quite useless or rudimentary organs beits similarly transmitted. A mere tendency to produce a rudiment is indeed sometimes thus inherited, | As all the species of the same genus are supposed to be descended from a common progenitor, it might be expected that they would occasionally vary in an analogous manner; so that the varieties of two or more species would resemble each other, or that a variety of one species would resemble in certain characters another and distinct species,—this other species being, according to our view; only a well-marked and permanent variety. But characters exclu- Analogous Variations. 127 CHAP. V. sively due to analogous variation would probably be of an unim- portant nature, for the preservation of alk saneeeeEe enon 4 characters will have been determined through natur it 4 accordance with the different habits of the species. might further be expected that the species of the same genus. wen d sical sionally exhibit reversions to long lost characters. Ss, However, we do not know the common ancestor of any natural group, we cannot distinguish between reversionary and analogous characters. If, for instance, we did not know that the parent rock-pigeon was not feather-footed or turn-crowned, we could not have told, whether such characters in our domestic breeds were reversions or only analogous variations; but we might have inferred that the blue colour was a case of reversion from the number of the markings, which are correlated with this tint, and which would not probably have all appeared together from simple variation. More especially we might have inferred this, from the blue colour and the several marks so often appearing when differently coloured breeds are crossed. Hence, although under nature it must generally be left doubtful, what cases are reversions to formerly existing characters, and what are new but analogous variations, yet we ought, on our theory, sometimes to find the varying offspring of a species assuming characters which are already present in other members of the same group. And this undoubtedly is the case. The difficulty in distinguishing variable species is largely due to the varieties mocking, as it were, other species of the same genus. A considerable catalogue, also, could be given of forms intermediate between two other forms, which themselves can only doubtfully be ranked as species; and this shows, unless all these closely allied forms be considered as independently created species, that they have in varying assumed some of the characters of the others. But the best evidence of analogous variations is afforded by parts or organs which are generally constant in character, but which occa- sionally vary So as to resemble, in some degree, the same part or organ in an allied species. I have collected a long list of such ens ; - here, as before, I lie under the great disadvantage of not tainly Seca op them. I can only repeat that such cases cer- Twill he and seem to me very remarkable. \ ffectine 7 vever, give one curious and complex case, not indeed as species - ne “portant character, but from occurring in several under nature _ genus, partly under domestication and partly - itis acase almost certainly of reversion. ‘The ass sometimes has very distinct transverse bars on its lezs. like th on the legs of the zebra: ; lts legs, ike those 5 © zebra: it has been asserted that these are plainest 128 Distinct Species present Crap. Ve a in the foal, and, from inquiries which I have made, I believe this. to be true. ‘I'he stripe on the shoulder is sometimes double, and; is very variable in length and outline. A white ass, but not an albino, has been described without either spinal or shoulder stripe : and these stripes are sometimes very obscure, or actually quite lost, ‘n dark-coloured asses. The koulan of Pallas is said to have been seen with a double shoulder-stripe. Mr. Blyth has seen a specimen of the hemionus with a distinct shoulder-stripe, though it properly has none; and I have been informed by Colonel Poole that the foals of this species are generally striped on the legs, and faintly on the shoulder. The quagga, though so plainly barred like a zebra over the body, is without bars on the legs ; but Dr. Gray has figured one specimen with very distinct zebra-like bars on the hocks. With respect to the horse, I have collected cases in England of the spinal stripe in horses of the most distinct breeds, and of all colours: transverse bars on the legs are not rare in duns, mouse- duns, and in one instance in a chestnut : a faint shoulder-stripe may sometimes be seen in duns, and I have seen a trace in a bay horse. My son made a careful examination and sketch for me of a dun Belgian cart-horse with a double stripe on each shoulder and with leg-stripes; I have myself seen a dun Devonshire pony, and a small dun Welsh pony has been carefully described to me, both with three parallel stripes on each shoulder. In the north-west part of India the Kattywar breed of horses is so generally striped, that, as I hear from Colonel Poole, who exa- mined this breed for the Indian Government, a horse without stripes is not considered as purely-bred. ‘The spine is always striped ; the legs are generally barred; and the shoulder-stripe, which is some- times double and sometimes treble, is common; the side of the face, moreover, is sometimes striped. The stripes are often plainest in the foal; and sometimes quite disappear in old horses. Colonel Poole has seen both gray and bay Kattywar horses striped when first foaled. I have also reason to suspect, from information given me by Mr. W. W. Edwards, that with the English race-horse the spinal stripe is much commoner in the foal than in the full-grown animal. I have myself recently bred a foal from a bay mare (ofl- spring of a Turcoman horse and a Flemish mare) by a bay English race-horse; this foal when a week old was marked on its hinder quarters and on its forehead with numerous, very narrow, dark, zebra-like bars, and its legs were feebly striped: all the stripes soon disappeared completely. Without here entering on further details, I may state that I have collected cases of leg and shoulder stripes — in horses of very different breeds in various countries from Britain te y = \ Cuar. V Analogous Variations. 129 astern China; and from Norway in the now woe vey — pelago in the south. In all parts of the wor ese pe i far oftenest ‘n duns and mouse-duns ; by the term dun a large range of colour is included, from one between brown and black to a close approach to cream-colour. ; oP arn aware that Colonel Hamilton Smith, who has written on this subject, believes that the several breeds of the herse are descended from several aboriginal species—one of which, the dun, was striped; and that the above-described appearances are all due to ancient crosses with the dun stock. But this view may be safely rejected ; for it is highly improbable that the heavy Belgian cart- horse, Welsh ponies, Norwegian cobs, the lanky Kattywar race, &c., inhabiting the most distant parts of the world, should all have been crossed with one supposed aboriginal stock. Now let us turn to the effects of crossing the several species of the horse-genus. Rollin asserts, that the common mule from the ass and horse is particularly apt to have bars on its legs; accord- ing to Mr. Gosse, in certain parts of the United States about nine out of ten mules have striped legs. I once saw a mule with its legs so much striped that any one might have thought that it was a hybrid-zebra; and Mr. W. C. Martin, in his excellent treatise on the horse, has given a figure of a similar mule. In four coloured drawings, which I have seen, of hybrids between the ass and zebra, the legs were much more plainly barred than the rest of the body ; and in one of them there was a double shoulder-stripe. In Lord Morton’s famous hybrid from a chestnut mare and male quagga, the hybrid, and even the pure offspring subsequently produced from the same mare by «# black Arabian sire, were much more plainly barred across the legs than is even the pure quagga. Lastly, and this is another most remarkable case, a hybrid has been figured by Dr. Gray (and he informs me that he knows of a second case) from the ass and the hemionus ; and this hybrid, though the ass only occasionally has stripes on his legs and the hemionus has none and has not even a shoulder-stripe, nevertheless had all four Pi mare an bad three short shoulder-stripes, like those on the rane re an. Welsh ponies, and even had some zebra-like pes on the sides of its face. With respect to this last fact, I was S ay — . © convinced that not even a stripe of colour appears from what is monly called chance, that I was led solely from the occurrence e@ ace- 1 o 7 . Colonel e Stripes on this hybrid from the ass and hemionus to ask oo1e whether such face-stripes ever occurred in the emi- nently striped K 7 neat y 5 iped Kattyw ar breed of horses, and was, as we have scen Auswered in the affirmative, : EK 130 Distinct Species present Cap. V. ——_—. - What now are we to say to these several facts? We see several distinct species of the horse-genus becoming, by simple variation, striped on the legs like a zebra, or striped on the shoulders like an ass. In the horse we see this tendency strong whenever a dun tint appears—a tint which approaches to that of the general colouring of the other species of the genus. The appearance of the stripes is not accompanied by any change of form or by any other new character, We see this tendency to become striped most strongly displayed in, hybrids from between several of the most distinct species. Now observe the case of the several breeds of pigeons: they are descended from a pigeon (including two or three sub-species or geographical races) of a bluish colour, with certain bars and other marks; and when: any breed assumes by simple variation a bluish tint, these bars and: other marks invariably reappear; but without any other change of form or character. When the oldest and truest breeds of various colours are crossed, we see a strong tendency for the blue tint, and bars and marks to reappear in the mongrels. I have stated that the most. probable hypothesis to account for the reappearance of very ancient characters, is—that there is a tendency in the young of each succes- sive generation to produce the long-lost character, and that this. tendency, from unknown causes, sometimes prevails. And we have just seen that in several species of the horse-genus the stripes are either plainer or appear more commonly in the young than in the ‘old. Call the breeds of pigeons, some of which have bred true for centuries, species ; and how exactly parallel is the case with that of the species of the horse-genus! For myself, I venture confidently to look back thousands on thousands of generations, and I see an animal striped like a zebra, but perhaps otherwise very differently constructed, the common parent of our domestic horse (whether or not it be descended from one or more wild stocks) of the ass, the hemionus, quagga, and zebra. He who believes that each equine species was independently created, will, I presume, assert that each species has been ereated with a tendency to vary, both under nature and under domestication, in this particular manner, so as often to become striped like the other species of the genus; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in their stripes, not their own parents, but other species of the genus. To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere mockery and deception ; I would almost ag soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, Cuap. V. Analogous Variations. 131 but had been created in stone so as to mock the shells living on the ae ry.—Our ignorance of the laws of variation is profound, Not in one case out of a hundred can we pretend to assign any reason why this or that part has varied. But whenever we have the means of instituting a comparision, the same laws appeah to have acted in producing the lesser differences between varieties of the same species, and the greater differences between species of the same genus. Changed conditions generally induce mere fluctuating variability, but sometimes they cause direct and definite effects ; and these may become strongly marked in the course of time, though we have not sufficient evidence on this head. Habit in producing constitutional peculiarities and use in strengthening and disuse in weakening and diminishing organs, appear in many cases to have been potent in their effects. Homologous parts tend to vary in the same manner, and homologous parts tend to cohere. Modifi- cations in hard parts and in external parts sometimes affect softer and internal parts, When one part is largely developed, perhaps it tends to draw nourishment from the adjoining parts; and every part of the structure which can be saved without detriment will be saved. Changes of structure at an early age may affect parts sub- sequently developed ; and many cases of correlated variation, the nature of which we are unable to understand, undoubtedly occur. Multiple parts are variable in number and in structure, perhaps arising from such parts not having been closely specialised for any particular function, so that their modifications have not been closely checked by natural selection. It follows probably from this same cause, that organic beings low in the scale are more variable than those standing higher in the scale, and which have their whole organisation more specialised, Rudimentary organs, from being useless, are not regulated by natural selection, and hence are variable, Specific characters—that is, the characters which have come to differ since the several species of the same genus branched Yom ‘ common parent—are more variable than generic cha- differct a ie have long been inherited, and have not fered to a SS same period. In these remarks we have re- have Seen ° ot S or organs being still variable, because they in the oa! _ and thus come to differ ; but we have also seen individual ; for apter that the Same principle applies to the whole ah In a district where many species of a genus are ae there has been much former variation and been actively at workin that distsiet od aa poole forms as é and amongst these species, K 2 differentiatio 132 Laws of Variation. Cuap. V. we now find, on an average, most varieties. Secondary sexual characters are highly variable, and such characters differ much in the species of the same group. Variability in the same parts of the organisation has generally been taken advantage of in giving secon- dary sexual differences to the two sexes of the same species, and specific differences to the several species of the same genus. Any part or organ developed to an extraordinary size Or In an extra- ordinary manner, in comparison with the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state. But when a species with any extraordinarily- developed organ has become the parent of many modified descen- dants—which on our view must be a very slow process, requiring a long lapse of time—in this case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a manner it may have been developed. Species inheriting nearly the same constitution from a common parent, and exposed to similar influences, naturally tend to present analogous variations, or these same species may occasionally revert to some of the characters of their ancient progenitors. Although new and important modifica- tions may not arise from reversion and analogous variation, such modifications will add to the beautiful and harmonious diversity of nature. Whatever the cause may be of each slight difference between the offspring and their parents—and a cause for each must exist—we have reason to believe that it is the steady accumulation of bene- ficial differences which has given rise to all the more important modifications of structure in relation to the habits of each species. Cuap. VI. D ificult les of the Naieh 133 _-----—— CHAPTER VI. DIFFICULTIES OF THE THEORY. Difficulties of the theory of descent with modification oo Absence or rarity of transitional varieties — Transitions in habits of life — Diversified habits in the same species — Species with habits widely different from those of their allies — Organs of extreme perfection — Modes of transition — Cases of difficulty — Natura non facit saltum — Organs of small importance —Organs not in all cases absolutely perfect — The law of Unity of Type and of the Conditions of Existence embraced by the theory of Natural Selection. Lona before the reader has arrived at this part of my work, a crowd of difficulties will have occurred to him. Some of them are so serious that to this day I can hardly reflect on them without being in some degree staggered ; but, to the best of my judgment, the number are only apparent, and those that are real are greater not, I think, fatal to the theory. These difficulties and objections may be classed under the follow- ing heads :—First, why, if species have descended from other species by fine gradations, do we not everywhere see innumerable tran- sitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined ? Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modifi- cation of some other animal with widely-different habits and structure? Can we believe that natural selection could produce, on the one hand, an organ of trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, an organ so wonderful as the eye? selena? ~ ee be acquired and modified through natural Selection ¢ at shall we say to the instinct which leads the bee to make cells, and which has practically anticipated the discoveries of profound mathematicians ? Fourthly, how ¢ sterile and produe crossed, their fe an we account for species, when crossed, being teing sterile offspring, whereas, when varieties are rtility is unimpaired ? ry) = n . Ihe two first heads will here be discussed ; some miscellaneous 134 Absence or Rarity Cuap. VI, _ objections in the following chapter ; Instinct and Hybridism in the i ters. Meiers or Rarity of Transitional Varieties.—As natural selection acts solely by the preservation of profitable modifications, each new form will tend in a fully-stocked country to take the place of, and finally to exterminate, its own less improved parent-form and other less-favoured forms with which it comes into competition. Thus extinction and natural selection go hand in hand. Hence, if we look at each species as descended from some unknown form, both the parent and all the transitional varieties will generally have been exterminated by the very process of the formation and per- fection of the new form. But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth? It will be more convenient to discuss this question in the chapter on the Imperfection of the Geological Record ; and I will here only state that I believe the answer mainly lies in the record being incomparably less perfect than is generally supposed. The crust of the earth is a vast museum; but the natural collections have been imperfectly made, and only at long intervals of time. But it may be urged that when several closely-allied species inhabit the same territory, we surely ought to find at the present time many transitional forms. Let us take a simple case: in travelling from north to south over a continent, we generally meet at successive intervals with closely allied or representative species, evidently filling nearly the same place in the natural economy of the land. ‘These representative species often meet and interlock ; and as the one becomes rarer and rarer, the other becomes more and more frequent, till the one replaces the other. But if we compare these species where they intermingle, th ey are generally as absolutely distinct from each other in every detail of structure as are specimens taken from the metropolis inhabited by each. By my theory these allied species are descended from a common parent; and during the process of modification, each has become adapted to the conditions of life of its own region, and has supplanted and exterminated its original parent-form and all the transitional varieties between its past and present states. Hence we ought not to expect at the present time to meet with numerous transitional varieties in each region, though they must have existed there, and may be embedded there in a fossil condition. But in the intermediate region, having intermediate conditions of life, why do we not now find closely- linking intermediate varieties? This difficulty for a long time eo § e« e e e I of Transitional Varieties. ; 3 5 ee But I think it can be in large part ex- yar. VI. _ quite confounded me. ! { . . . . Ss ae tremely cautious in inferring, In the first place we should be ex y . it has been continuous because a mare M oslosy would tas ne to believe that most ring a lon g ; vontimonts hove been broken up into islands even during the later rr a, h islands distinct species might have tertiary periods; and in suc eh ¢ intermediate been separately formed without the possibility of mterm th varieties existing in the intermediate zones. By changes a ee form of the land and of climate, marine areas now continuous must often have existed within recent times in a far less continuous and uniform condition than at present. But I will pass over this way of escaping from the difficulty ; for I believe that many perfectly defined species have been formed on strictly continuous areas ; though I do not doubt that the formerly broken condition of areas now continuous, has played an important part in the formation of new species, more especially with freely-crossing and wandering animals, In looking at species as they are now distributed over a wide area, we generally find them tolerably numerous over a large territory, then becoming somewhat abruptly rarer and rarer on the confines, and finally disappearing. Hence the neutral territory between two representative species is generally narrow in comparison with the territory proper to each. We see the same fact in ascending mountains, and sometimes it is quite remarkable how abruptly, as Alph. de Candolle has observed, a common alpine species disappears. ‘The same fact has been noticed by E. Forbes in sounding the depths of the sea with the dredge. ‘To those who iook at climate and the physical conditions of life as the all- important elements of distribution, these facts ought to cause surprise, as climate and height or depth graduate away insensibly. But when we bear in mind that almost every species, even in its metropolis, would increase immensely in numbers, were it not for other competing species; that nearly all either prey on or serve as prey for others; in short, that each organic being is either directly or Indirectly related in the most important manner to other organic beings,—we see that the range of the inhabitants of any country by no means exclusively depends on insensibly changing physical heh ee ah ange Parton the presence of other species, on Snés Competition: y whieh it 1s destroyed, or with which it comes petition; and as these species are already defined. objects not blending one into another by insensible gradations, the range f any one species, depending ag it does on the . 7 © False © ' 3 e range of others, will tend to be sharply defined, Moreover, each gs ic ; pecies on the confines 136 Absence or Rarity Cuap, VIL of its range, where it exists in lessened numbers, will, during fluctu- ations in the number of its enemies or of its prey, or in the nature of the seasons, be extremely liable to utter extermination ; and thus its geographical range will come to be still more sharply defined. As allied or representative species, when inhabiting a.continuous area, are generally distributed in such a manner that each has a wide range, with a comparatively narrow neutral territory between them, in which they become rather seddenly rarer and rarer; then, as varieties do not essentially differ from species, the same rule will probably apply to both; and if we take a varying species inhabiting a very large area, we shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone. The intermediate variety, consequently, will exist in lesser numbers from inhabiting a narrow and lesser area; and practically, as far as. I can make out, this rule holds good with varieties in a state of nature. I have met with striking instances of the rule in the case of varieties intermediate between well-marked varieties in the genus. Balanus. And it would appear from information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that generally, when. varieties intermediate between two other forms occur, they are much rarer numerically than the forms which they connect. Now, if we: may trust these facts and inferences, and conclude that varieties linking two other varieties together generally have existed in lesser numbers than the forms which they connect, then we can understand why intermediate varieties should not endure for very long periods = —why, as a general rule, they should be exterminated and disappear,. sooner than the forms which they originally linked together. For any form existing in lesser numbers would, as already remarked, run a greater chance of being exterminated than one existing in large numbers; and in this particular case the inter-- mediate form would be eminently liable to the inroads of closely- allied forms existing on both sides of it. But it is a far more. important consideration, that during the process of further modifi- cation, by which two varieties are supposed to be converted and perfected into two distinct Species, the two which exist in larger numbers, from inhabiting larger areas, will have a great advantage: over the intermediate variety, which exists in smaller numbers. in a narrow and intermediate zone. For forms existing in larger numbers will have a better chance, within any given period, of presenting further favourable variations for natural selection to seize on, than will the rarer forms which exist in lesser numbers. Hence, the more common forms, in the race for life, will tend to- beat and supplant the less common forms, for these will be more: Cuar. VI. of Transitional Varieties. 137 slowly modified and improved. It is the same principe wns 7 I believe, accounts for the common species in eee ce a ane _ shown in the second chapter, presenting on an aver age ¢ - er number of well-marked varieties than do the rarer spews may illustrate what I mean by supposing three variguee of s neep to : kept, one adapted to an extensive mountainous in 5 : Secon! to a comparatively narrow, hilly tract ; and a an ° ne wie plains at the base; and that the inhabitants “ a jryins . ; r equal steadiness and skill to improve their stocks by se ee ion : i chances in this case will be strongly in favour of the great holders on the mountains or on the plains, improving their breeds more quickly than the small holders on the intermediate narrow, hilly tract; and consequently the improved mountain or plain breed will soon take the place of the less improved hill breed ; and thus the two breeds, which originally existed in greater numbers, will come into close contact with each other, without the interposition of the supplanted, intermediate hill-variety. 'o sum up, I believe that species come to be tolerably well- defined objects, and do not at any one period present an inextricable chaos of varying and intermediate links: first, because new varie- tics are very slowly formed, for variation is a slow process, and natural selection can do nothing until favourable individual differences or variations occur, and until a place in the natural polity of the country can be better filled by some modification of some one or more of its inhabitants. And such new places will depend on slow changes of climate, or on the occasional immigration of new inhabitants, and, probably, in a still more important degree, on some of the old inhabitants becoming slowly modified, with the: new forms thus produced and the old ones acting and reacting on each other. So that, in any one region and at any one time, we ought to see only a few species presenting slight modifications of structure in some degree permanent ; and this assuredly we do see. Secondly, areas now continuous must often have existed within the recent period as isolated portions, in which many forms, more especially amongst the classes which unite for each birth and wander much, may have separately been rendered sufliciently i. ” zane as TePRESONUALANS: species. In this case, inter- ciate varieties between the several representative species and = common parent, must formerly have existed within each ta etn wl hanes see ee oes of that they will no longer be found ma nares en unatae, > S na living state. Thirdly, wl ~ y, When two or more varieties have been formed in different 138 Transitions of Organic Beings. Cuap. VI, portions of a strictly continuous area, intermediate varieties will, it is probable, at first have been formed in the intermediate zones, but they will generally have had a short duration. For these inter- mediate varieties will, from reasons already assigned (namely from what we know of the actual distribution of closely allied or repre- sentative species, and likewise of acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they tend to connect. From this cause alone the intermediate varieties will be liable to accidental extermination ; and during the process of further modification through natural selection, they will almost certainly be beaten and supplanted by the forms which they connect ; for these from existing in greater numbers will, in the aggregate, present more varieties, and thus be further improved through natural selection and gain further advantages. Lastly, looking not to any one time, but to all time, if my theory be true, numberless intermediate varieties, linking closely together all the species of the same group, must assuredly have existed; but the very process of natural selection constantly tends, as has been so often remarked, to exterminate the parent-forms and the inter- mediate links. Consequently evidence of their former existence could be found only amongst fossil remains, which are preserved, as we shall attempt to show in a future chapter, in an extremely im- perfect and intermittent record. : On the Origin and Transitions of Organic Beings with peculiar dabits and Structure.—It has been asked by the opponents of such views as I hold, how, for instance, could a land carnivorous animal have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted 2? It would be easy to show that there now exist carnivorous animals presenting close intermediate grades from strictly terrestrial to aquatic habits ; and as each exists by a struggle for life, it is clear that each must be well adapted to its place in nature. Look at the Mustela vison of North America, which has webbed feet, and which resembles an otter in its fur, short legs, and form of tail. During the summer this animal dives for and preys on fish, but during the long winter it leaves the frozen waters, and preys, like other pole-cats, on mice and land animals. If a different case had been taken, and it had been asked how an insectivorous quadruped could possibly have been converted into a flying bat, the question would have been far more difficult to answer. Yet I think such difficulties have little weight. Here, as on other occasions, I lie under a heavy disadvantage, for, out of the many striking cases which I have collected, I can give only one or two instances of transitional habits and structures in Cuap. VL. Transitions of Organic Beings. 139 —_——— ersified habits, either constant or occa- And it seems to me that nothing less es is sufficient to lessen the difficulty in allied species; and of div sional, in the same species. than a long list of such cas any particular case like that of the bat. Look at the family of squirrels; here we have the finest gra- dation from animals with their tails only slightly flattened, and from others, as Sir J. Richardson has remarked, with the posterior part of their bodies rather wide and with the skin on their flanks yather full, to the so-called flying squirrels ; and flying squirrels have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to clide through the air to an astonishing distance from tree to tree. We cannot doubt that each structure is of use to each kind of squirrel in its own country, by enabling it to escape birds or beasts of prey, to collect food more quickly, or, as there is reason to believe, to lessen the danger from occasional falls. But it does not follow from this fact that the structure of each squirrel is the best that it is possible to conceive under all possible conditions. Let the clirnate and vegetation change, let other competing rodents or new beasts of prey immigrate, or old ones become modified, and all analogy would lead us to believe that some at least of the squirrels would decrease in numbers or become exterminated, unless they also became modified and improved in structure in a corresponding manner. Therefore, I can see no difficulty, more especially under changing conditions of life, in the continued preservation of indi- viduals with fuller and fuller flank-membranes, each modification being useful, each being propagated, until, by the accumulated eflects of this process of natural selection, a perfect so-called flying squurel was produced. _ Now look at the Galeopithecus or so-called flying lemur, which lormerly was ranked amongst bats, but is now believed to belong to the Insectivora. An extremely wide flank-membrane stretches trom, the corners of the jaw to the tail, and includes the limbs ve the elongated fingers. ‘This flank-membrane is furnished with Soe eine ee A though no graduated links of structure, fitted the other Tasecti = “tf sei now connect the Galeopithecus with such links formerly, ‘ist ; ied that ty in eae aa e ‘hat same Tiahiner as mith (he : , and that each was developed in the of structure having be —_ perlectly gliding squirrels ; each grade . ¢ been useful to it Jor ¢ 2 any insuperable difficulty in f zee possessor. Nor can I see connected fingers and el in further believing that the membrane been greatly lengthened by naturel eel | Galeopithecus might have atural selection: and this, as far as the 140 Transitions of Organic Beings. Cuap. VIL organs of flight are concerned, would have converted the animal into a bat. In certain bats in which the wing-membrane extends from the top of the shoulder to the tail and includes the hind-legs, we perhaps see traces of an apparatus originally fitted for gliding through the air rather than for flight. If about a dozen genera of birds were to become extinct, who would have ventured to surmise that birds might have existed which used their wings solely as flappers, like the logger-headed, duck (Micropterus of Eyton) ; as fins in the water and as front-legs. on the land, like the penguin; as sails, like the ostrich; and func- tionally for no purpose, like the Apteryx? Yet the structure of each of these birds is good for it, under the conditions of life to which it is exposed, for each has to live by a struggle; but it is not necesarily the best possible under all possible conditions. It must not be inferred from these remarks that any of the grades of wing- structure here alluded to, which perhaps may all be the result of disuse, indicate the steps by which birds actually acquired their perfect power of flight; but they serve to show what diversified means of transition are at least possible. Seeing that a few members of such water-breathing classes as the Crustacea and Mollusca are adapted to live on the land; and seeing that we have flying birds and mammals, flying insects of the most diversified types, and formerly had flying reptiles, it is con- ceivable that flying-fish, which now glide far through the air, slightly rising and turning by the aid of their fluttering fins, might have been modified into perfectly winged animals. If this had been effected, who would have ever imagined that in an early transitional state they had been the inhabitants of the Open occan, and had used their incipient organs of flight exclusively, as far as we know, to escape being devoured by other fish ? When we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we should bear in mind that animals displaying early transitional grades of the structure will seldom have survived to the present day, for they will have been supplanted by their successors, which were gradually rendered more perfect through natural selection. Furthermore, we may conclude that transitional states between structures fitted for very different habits of life will rarely have been developed at an early period in great numbers and under many subordinate forms. Thus, to return to our imaginary illustration of the flying-fish, it does not seem probable that fishes capable of true flight would have been developed. under many subordinate forms, for taking prey of many kinds in Tmany ways, on the land and in the water, until their organs of flight. Cuar. VI. Transitions of Organic Beings. IAI had come to a high stage of perfection, so as to have given them a decided advantage over other animals in the battle for life. Hence the chance of discovering species with transitionai grades of struc- ture in a fossil condition will always be less, from their having existed in lesser numbers, than in the case of species with fully developed structures. I will now give two or three instances both of diversified and of changed habits in the individuals of the same species. In either case it would be easy for natural selection to adapt the structure of the animal to its changed habits, or exclusively to one of its several habits. It is, however, difficult to decide, and immaterial for us, whether habits generally change first and structure after- wards; or whether slight modifications of structure lead to changed habits ; both probably often occurring almost simultaneously. Of cases of changed habits it will suffice merely to allude to that of the many British insects which now feed on exotic plants, or exclu- sively on artificial substances. Of diversified habits innumerable instances could be given: I have often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America, hovering over one spot and then proceeding to another, like a kestrel, and at other times standing stationary on the margin of water, and then dashing into it like a kingfisher at a fish. In our own country the larger © titmouse (Parus major) may be seen climbing branches, almost like a creeper; it sometimes, like a shrike, kills small birds by blows on the head; and I have many times seen and heard it hammering the seeds of the yew on a branch, and thus breaking them like a nuthatch. In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching, almost like a whale, insects in the water. As we sometimes see individuals following habits different from those proper to their species and to the other species of the same genus, we might expect that such individuals would occasionally give rise to new species, having anomalous habits, and with their Structure either slightly or considerably modified from that of their type. And such instances occur in nature. Can a more striking instance of adaptation be given than that of a woodpecker for climbing trees and seizing insects in the chinks of the bark ? Yet in North America there are woodpeckers which feed largely on fruit, and others with elongated wings which chase insects on the wing. On the plains of La Plata, where hardly a tree grows, there is a woodpecker (Colaptes campestris) which has two toes before and two behind, a long pointed tongue, pointed tail-feathers, sufficiently stiff to support the bird in a vertical position on a post, but not so 142 Transitions of Organic Beings. Cua. VI. stiff as in the typical woodpeckers, and a straight strong beak. The beak, however, is not so straight or so strong as in the typical woodpeckers, but it is strong enough to bore ‘into wood. Hence this Colaptes in all the essential parts of its structure is a wood- pecker. Even in such trifling characters as the colouring, the harsh tone of the voice, and undulatory flight, its close blood- relationship to our common woodpecker is plainly declared ; yet, as I can assert, not only from my own observations, but from those of the accurate Azara, in certain large districts it does not climb trees, and it makes its nest in holes in banks! In certain other districts, however, this same woodpecker, as Mr. Hudson states, frequents trees, and bores holes in the trunk for its nest. I may mention as another illustration of the varied habits of this genus, that a Mexican Colaptes has been described by De Saussure as boring holes into hard wood in order to lay up a store of acorns, Petrels are the most aérial and oceanic of birds, but in the quiet sounds of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its astonishing power of diving, in its manner of swim- ming and of flying when made to take flight, would be mistaken by any one for an auk or a grebe; nevertheless it is essentially a petrel, but with many parts of its organisation profoundly modified in relation to its new habits of life; whereas the woodpecker of La Plata has had its structure only slightly modified. In the case of the water-ouzel, the acutest observer by examining its dead body would never have suspected its sub-aquatic habits; yet this bird, which is allied to the thrush family, subsists by diving—using its wings under water, and grasping stones with its feet. All the members of the great order of Hymenopterous insects are terrestrial, excepting the genus Proctotrupes, which Sir John Lubbock has discovered to be aquatic in its habits; it often enters the water and dives about by the use not of its legs but of its wings, and remains as long as four hours beneath the surface; yet it exhibits no modi- fication in structure in accordance with its abnormal habits. He who believes that each being has been created as we now see it, must occasionally have felt surprise when he has met with an animal having habits and structure not in agreement. What can be plainer than that the webbed feet of ducks and geese are formed for swimming? Yet there are upland geese with webbed feet which rarely go near the water; and no one except Audubon has seen the frigate-bird, which has all its four toes webbed, alight on the surface of the ocean. On the other hand, grebes and coots are eminently aquatic, although their toes are only bordered by membrane. What seems plainer than that the long toes, not furnished with membrane * Car. VI. Organs of extreme Perfection. 143 ne of the Grallatores are formed for walking over swamps and floating plants ?—the water-hen and landrail are members of this order, yet the first is nearly as aquatic as the coot, and the second nearly as terrestrial as the quail or partridge. In such cases, and many others could be given, habits have changed without a corresponding change of structure. The webbed feet of the upland goose may be said to have become almost rudimentary in function, though not in structure. In the frigate-bird, the deeply scooped membrane between the toes shows that structure has begun to change. He who believes in separate and innumerable acts of creation may say, that in these cases it has pleased the Creator to cause a being of one type to take the place of one belonging to another type; but this seems to me only re-stating the fact in dignified language. He who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers; and that if any one being varies ever so little, either in habits or structure, and thus gains an advantage over some other inhabitant of the same country, it will seize on the place of that inhabitant, however - different that may be from its own place. Hence it will cause him no surprise that there should be geese and frigate-birds with webbed feet, living on the dry land and rarely alighting on the water ;. that there should be long-toed corncrakes, living in meadows in- stead of in swamps ; that there should be woodpeckers where hardly a tree grows; that there should be diving thrushes and diving Hymenoptera, and petrels with the habits of auks. Organs of extreme Perfection and Complication. To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first. said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vow populi, vow Det, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such varia- tions should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye 144 Organs of extreme Perfection. Cuap. VI, —— could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory, How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated ; but I may remark that, as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it docs not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility. In searching for the gradations through which an organ in any species has been perfected, we ought to look exclusively to its lineal progenitors; but this is scarcely ever possible, and we are forced to iook to other species and genera of the same group, that is to the collateral descendants from the same parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted in an unaltered or little altered condition. But the state of the same organ in distinct classes may incidentally throw light on the steps by which it has been perfected. The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells and covered by translucent skin, but without any lens or other refractive body. We may, however, according to M. Jourdain, descend even a step lower and find aggregates of pigment-cells, apparently serving as organs of vision, without any nerves, and resting merely on sarcodic tissue. Kyes of the above simple nature are not capable of distinct vision, and serve only to distinguish light from darkness. In certain star- fishes, small depressions in the layer of pigment which surrounds the nerve are filled, as described by the author just quoted, with transparent gelatinous matter, projecting with a convex surface, like the cornea in the higher animals. He suggests that this serves not to form an image, but only to concentrate the luminous rays and render their perception more easy. In this concentration of the rays we gain the first and by far the most important step towards _ the formation of a true, picture-forming eye; for we have only to place the naked extremity of the optic nerve, which in some of the lower animals lies deeply buried in the body, and in some near the surface, at the right distance from the concentrating apparatus, and an image will be formed on it. In the great class of the Articulata, we may start from an optic nerve simply coated with pigment, the latter sometimes forming a sort of pupil, but destitute of a lens or other optical contrivance. With insects it is now known that the numerous facets on the cornea of their great compound eyes form true lenses, and that the cones include curiously modified nervous filaments. But these Cuar. VI. Organs of extreme Perfection. 148 organs in the Articulata are so much diversified that Miiller formerly made three main classes with seven subdivisions, besides a fourth main class of aggregated simple eyes. When we reflect on these facts, here given much too briefly, with respect to the wide, diversified, and oraduated range of structure in the eyes of the lower animals; and when we bear in mind how small the number of all living forms must be in comparison with those which have become extinct, the difficulty ceases to be very great in believing that natural selection may have converted the simple apparatus of an optic nerve, coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the Articulate Class. He who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this volume that large bodies of facts, otherwise inexplicable, can be explained by the theory of modification through natural selection; he ought to admit that a structure even as perfect as an eagle’s eye might thus be formed, although in this case he does not know the transitional states, It has been objected that in order to modify the eye and still preserve it as a perfect instrument, many changes would have to be effectec simultaneously, which,-it is assumed, could not be done througr. natural selection ; but as I have attempted to show in my work on the variation of domestic animals, it is not necessary to suppose that the modifications were all simultaneous, if they were extremely slight and gradual. Different kinds of modification would, also, serve for the same general purpose: as Mr. Wallace has remarked, “if a lens has too short or too long a focus, it may be amended either by an alteration of curvature, or an alteration of density ; if the curvature be irregular, and the rays do not converge to a point, then any increased regularity of curvature will be an improvement. So the contraction of the iris and the muscular movements of the eye are neither of them essential to vision, but only improvements Which might have been added and perfected at any stage of the construction of the instrument.” Within the highest division of the animal kingdom, namely, the Vertebrata, we can start from an eye so simple, that it consists, as in the lancelet, of a little sack of transparent skin, furnished with a nerve and lined with pigment, but destitute of any other apparatus. In fishes and reptiles, as Owen has remarked, “the range of gradations of dioptric structures 1S very great.” It is a significant fact that even in man, according to the high authority of Virchow, the beautiful crystalline lens is formed in the embryo by an accumulation of epidermic cells, lying in a sack-like fold of the skin; and the vitreous body is formed we 146 Modes of Transttion. Cuap. VI. from embryonic sub-cutaneous tissue. ‘To arrive, however, at @ just conclusion regarding the formation of the eye, with all its mar- vellous yet not absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but I have felt the difficulty far too keenly to be surprised at others hesitating to extend the principle of natural selection to so startling a length. It is scarcely possible to avoid comparing the eye with a telescope. We know that this instrument has been perfected by the long- continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. But may not this inference be presumptuous? Have we any right to assume that the Creator works by intellectual powers like those of man? If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further we must suppose that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparent layers; and carefully preserving each which, under varied circum- stances, in any way or in any degree, tends to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million; each to be preserved until a better one is produced, and then the old ones to be all destroyed. In living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man ? \ * Modes of Transition. If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, suc- cessive, slight modifications, my theory would absolutely break down. But I can find out no such case. No doubt many organs exist of which we do not know the transitional grades, more espe- cially if we look to much-isolated species, round which, according to the theory, there has been much extinction. Or again, if we take Rete. ee oA aio. % Cuap. VI. Modes of Transition. 147 an organ common to all the members of a class, for in this latter case the organ must have been originally formed at a remote period, since which all the many members of the class have been developed ; and in order to discover the early transitional grades through which the organ has passed, we should have to look to very ancient ances- tral forms, long since become extinct. We should be extremely cautious in concluding that an organ could not have been formed by transitional eradations of scme kind. Numerous cases could be given amongst the lower animals of the same organ performing at the same time wholly distinct functions; thus in the larva of the dragon-fly and in the fish Cobites the alimentary canal respires, digests, and excretes. In the Hydra, the animal may be turned inside out, and the exterior surface will then digest and the stomach respire. In such cases natural selection might specialise, if any advantage were thus gained, the whole or part of an organ, which had previously performed two functions, for one function alone, and thus by insensible steps greatly change its nature. Many plants are known which regularly produce at the same time differently constructed flowers; and if such plants were to produce one kind alone, a great change would be effected with comparative suddenness in the character of the species. It is, how- ever, probable that the two sorts of flowers borne by the same plant were originally differentiated by finely graduated steps, which may still be followed in some few cases. Again, two distinct organs, or the same organ under two very different forms, may simultaneously perform in the same individual the same function, and this is an extremely important means of transition: to give one instance,—there are fish with gills or branchiw that breathe the air dissolved in the water, at the same time that they breathe free air in their swimbladders, this latter organ being divided by highly vascular partitions, and having a ductus pneumaticus for the supply of air. To give another instance from the vegetable kingdom : plants climb by three distinct means, by spirally twining, by clasping a support with their sensitive tendrils, and by the emission of aérial rootlets; these three means Ha ee found in distinct groups, but some few species exhibit pi bi Shana even all three, combined in the same indivi- nae eh ee Be cases one of the two organs might readily be Si sdiicr ta ta ected so as to perform all the work, being aided ite agh progress of modification by the other organ; and then cr organ might be modified for some other and quite distinct purpose, or be wholly obliterated. The illustration of the swimbladder in fishes is a good one, eB fey 148 | Modes of Transition. Cuap. VI. because it shows us clearly the highly important fact that an organ: originally constructed for one purpose, namely, flotation, med be converted into one for a widely different purpose, namely, respiration.. The swimbladder has, also, been worked in as an accessory to the auditory organs of certain fishes. All physiologists admit that the swimbladder is homologous, or “ ideally similar” in position and structure with the lungs of the higher vertebrate animals: hence there is no reason to doubt that the swimbladder has actually been converted into lungs, or an organ used exclusively for respiration. : According to this view it may be inferred that all vertebrate animals with true lungs are descended by ordinary generation from an ancient and unknown prototype, which was furnished with a floating apparatus or swimbladder. We can thus, as I infer from Owen’s interesting description of these parts, understand the strange fact that every particle of food and drink which we swallow has to pass over the orifice of the trachea, with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed. In the higher Vertebrata the branchie have wholly disappeared—but in the embryo the slits on the sides of the neck and the loop-like course of the arteries still mark their former position. But it is conceivable that the now utterly lost branchia might have been gradually worked in by natural selection for some distinct purpose: for instance, Landois has shown that the wings. of insects are developed from the trachex; it is therefore highly probable that in this great class organs which once served for respiration have been actually converted into organs for flight. In considering transitions of organs, it is so important to bear in mind the probability of conversion from one function to another, that I will give another instance. Pedunculated cirripedes have two minute folds of skin, called by me the ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs until they are hatched within the sack. These cirripedes have no branchie, the whole surface of the body and of the sack, together with the small frena, serving for respiration. The Balanide or sessile cirripedes, on the other hand, have no ovigerous frena, the eges lying loose at the bottom of the sack, within the well-enclosed shell; but they have, in the same relative position with the frena,. large, much-folded membranes, which freely communicate with the circulatory lacunz of the sack and body, and which have been considered by all naturalists to act as branchie. Now I think no one will dispute that the ovigerous frena in the one family are strictly homologous with the branchiz of the other family ; indeed, Cap. VI. Modes of Transition. 149 they graduate into each other. Therefore it need not be doubted that the two little folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly aided in the act of respiration, have been gradually converted by natural selection into branchie, simply through an increase in their size and the obliteration of their adhesive glands. If all pedunculated cirripedes had become extinct, and they have suffered far more extinction than have sessile cirripedes, who would ever have imagined that the branchiw in this latter family had originally existed as organs for preventing the ova from being washed out of the sack ? ‘here is another possible mode of transition, namely, through the acceleration or retardation of the period of reproduction. ‘This has lately been insisted on by Prof. Cope and others in the United States. It is now known that some animals are capable of repro- duction at a very early age, before they have acquired their perfect characters ; and if this power became thoroughly well developed in a species, it seems probable that the adult stage of development would sooner or later be lost; and in this case, especially if the larva differed much from-the mature form, the character of the species would be greatly changed and degraded. Again, not a few animals, after arriving at maturity, go on changing in character during nearly their whole lives. With mammals, for instance, the form of the skull is often much altered with age, of which Dr. Murie has given some striking instances with seals; every one knows how the horns of stags become more and more branched, and the plumes of some birds become more finely developed, as they grow older. Prof. Cope states that the teeth of certain lizards ‘change much in. shape with advancing years; with crustaceans not only many trivial, but some important parts assume a new character, as recorded by Fritz Miller, after maturity. In all such cases,—and many could be given,—if the age for reproduction were retarded, the character of the species, at least in its adult state, would be modified; nor is it improbable that the previous and earlier stages of development would in some cases be hurried through and finally lost. Whether species have often or ever been modified through this comparatively sudden mode of transition, I can form no opinion; but if this has occurred, it is probable that the differences between the young and the mature, and between — the mature and the old, were primordially acquired by graduated steps. 150 Difficulties of the Lheory Cuap. VE. Special Difficulties of the Theory of Natural Selection. Although we must be extremely cautious in concluding that any organ could not have been produced by successive, small, tran- sitional gradations, yet undoubtedly serious cases of difficulty occur. One of the most serious is that of neuter insects, which are often differently constructed from either the males or fertile females ; but this case will be treated of in the next chapter. The electric organs of fishes offer another case of special difficulty; for it is impossible to conceive by what steps these wondrous organs have been produced. But this is not surprising, for we do not even know of what use they are. In the Gymnotus and Torpedo they no doubt serve as powerful means of defence, and perhaps for securing prey; yet in the Ray, as observed by Matteucci, an analogous organ in the tail manifests but little electricity, even when the animal is greatly irritated; so little, that it can hardly be of any use for the above purposes. Moreover, in the Ray, besides the organ just referred to, there is, as Dr. R. M‘Donnell has shown, another organ near the head, not known to be electrical, but which appears to be the real homologue of the electric battery in the Torpedo. It is generally admitted that there exists between. these organs and ordinary muscle a close analogy, in intimate structure, in the distribution of the nerves, and in the manner in which they are acted on by various reagents. It should, also, be especially observed that muscular contraction is accompanied by an electrical discharge ; and, as Dr. Radcliffe insists, “in the electrical apparatus of the torpedo during rest, there would seem to be a charge in every respect like that which is met with in muscle and nerve during rest, and the discharge of the torpedo, instead of being peculiar, may be only another form of the discharge which attends upon the action of muscle and motor nerve.” Beyond this we can- not at present go in the way of explanation; but as we know so little about the uses of these organs, and as we know nothing about the habits and structure of the progenitors of the existing electric fishes, it would be extremely bold to maintain that no serviceable transitions are possible by which these organs might have been gradually developed. These organs appear at first to offer another and far more serious difficulty ; for they occur in about a dozen kinds of fish, of which several are widely remote in their affinities. When the same organ is found in several members of the same class, especially if in members having very different habits of life, we may generally attribute its presence to inheritance from a common ancestor ; and Cuap. VI. of Natural Selection. I51 its absence in some of the members to loss through disuse or natural selection. So that, if the electric organs had been inherited from some one ancient progenitor, we might have expected that all electric fishes would have been specially related to each other; but this is far from the case. Nor does geology at all lead to the belief that most fishes formerly possessed electric organs, which their modified descendants have now lost. But when we look at the subject more closely, we find in the several fishes provided with electric organs, that these are situated in dif- ferent parts of the body,—that they differ in construction, as in the arrangement of the plates, and, according to Pacini, in the process or means by which the electricity is excited—and lastly, in being supplied with nerves proceeding from different sources, and this is perhaps the most important of all the differences. Hence in the several fishes furnished with electric organs, these cannot be considered as homologous, but only as analogous in function. Con- sequently there is no reason to suppose that they have been inherited from a common progenitor ; for had this been the case they would have closely resembled each other in all respects. Thus the difficulty of an. organ, apparently the same, arising in several remotely allied species, disappears, leaving only the lesser yet still great difficulty ; namely, by what graduated steps these organs have been developed in each separate group of fishes. The luminous organs which occur in a few insects, belonging to widely different families, and which are situated in different parts of the body, offer, under our present state of ignorance, a difficulty almost exactly parallel with that of the electric organs. Other similar cases could be given; for instance in plants, the very curious contrivance of a mass of pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently the same in Orchis and Asclepias,—genera almost as remote as is possible amongst flowering plants ; but here again the parts are not homologous. In all cases of beings, far removed from each other in the scale of organisation, which are furnished with similar and peculiar organs, it will be found that although the general appearance and function of the organs may be the same, yet fundamental differences between them can always be detected. For instance, the eyes of cephalopods or cuttle-fish and of vertebrate animals appear wonderfully alike ; and in such widely sundered groups no part of this resemblance can be due to inheritance from a common progenitor. Mr. Mivart has advanced this case as one of special difficulty, but I am unable to see the force of his argument. An organ for vision must be formed of transparent tissue, and must include some sort of lens for 152 Dufficulites of the Theory Cuap. VI, throwing an image at the back of a darkened chamber. Beyond this superficial resemblance, there is hardly any real similarity between the eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen’s admirable memoir on these organs in the Cephalopoda. It is impossible for me here to enter on details, but I may specify a few of the points of difference. The crystalline lens in the higher cuttle-fish consists of two parts, placed one behind the other like two lenses, both having a very different structure and disposition to what occurs in the vertebrata. The retina is wholly different, with an actual inversion of the elemental parts, and-with a large nervous ganglion included within the membranes of the eye. ‘The relations of the muscles are as different as it is pos- sible to conceive, and so in other points. Hence it is nota little difficult to decide how far even the same terms ought to be employed in describing the eyes of the Cephalopoda and Vertebrata. It is, of course, open to any one to deny that the eye in either case could ~ have been developed through the natural selection of successive, slight variations; but if this be admitted in the one case, it is clearly possible in the other; and fundamental differences of struc- ture in the visual organs of two groups might have been anti- cipated, in accordance with this view of their manner of formation. As two men have sometimes independently hit on the same invention, so in the several foregoing cases it appears that natural selection, working for the good of each being, and taking advan- tage of all favourable variations, has produced similar organs, as far as function is concerned, in distinct organic beings, which owe none of their structure in common to inheritance from a common progenitor. Fritz Miiller, in order to test the conclusions arrived at in this volume, has followed out with much care a nearly similar line of argument. Several families of crustaceans include a few species, possessing an air-breathing apparatus and fitted to live out of the water. In two of these families, which were more especially examined by Miiller, and which are nearly related to each other, the species agree most closely in all important characters ; namely in their sense-organs, circulating system, in the position of the tufts of hair within their complex stomachs, and lastly in the whole structure of the water-breathing branchie, even to the microscopical hooks by which they are cleansed. Hence it might have been expected that in the few species belonging to both families which live on the land, the equally-important air-breathing apparatus would have been the same; for why should this one apparatus, given for the same purpose, have been made to differ, Cuap. VI. of Natural Selection. 153 whilst all the other important organs were closely similar or rather identical. Fritz Miiller argues that this close similarity in so many points of structure must, in accordance with the views advanced by me, be accounted ‘for by inheritance from a common progenitor. But as the vast majority of the species in the above two families, as well as most other crustaceans, are aquatic in their habits, it is improbable in the highest degree, that their common progenitor should have been adapted for breathing air. Muller was thus led carefully to examine the apparatus in the air-breathing species ; and he found it to differ in each in several important points, as in the position of the orifices, in the manner in which they are opened. and closed, and in some accessory details. Now such differences are intelligible, and might even have been expected, on the suppo- sition that species belonging to distinct families had slowly become adapted to live more and more out of water, and to breathe the air, or these species, from belonging to distinct families, would have differed to a certain extent, and in accordance with the principle that the nature of each variation depends on two factors, viz. the nature of the organism and that of the surrounding con- ditions, their variability assuredly would not have been exactly the same. Consequently natural selection would have had different materials or variations to work on, in order to arrive at the same functional result; and the structures thus acquired would almost necessarily have differed. On the hypothesis of separate acts of éreation the whole case remains unintelligible. his line of argument seems to have had great weight in leading Fritz Miiller to accept the views maintained by me in this volume. Another distinguished zoologist, the late Professor Claparéde, has argued in the same manner, and has arrived at the same result. He shows that there are parasitic mites (Acaridze), belonging to distinct sub-families and families, which are furnished with hair- claspers. These organs must have been independently developed, as they could not have been inherited from a common progenitor ; and in the several groups they are formed by the modification of the fore-legs,—of the hind-legs,—of the maxilla or lips,—and of appendages on the under side of the hind part of the body. In the foregoing cases, we see the same end gained and the same function performed, in beings not at all or only remotely allied, by organs In appearance, though not in development, closely similar. On the other hand, it is a common rule throughout nature that the same end should be gained, even sometimes in the case of closely- related beings, by the most diversified means. How differently 154 Difficulties of the Theory Car. VI. constructed is the feathered wing of a bird and the membrane- covered wing of a bat; and still more so the four wings of a butter- fly, the two wings of a fly, and the two wings with the elytra of a beetle. Bivalve shells are made to open and shut, but on what a number of patterns is the hinge constructed,—from the long row of neatly interlocking teeth in a Nucula to the simple ligament of a Mussel! Seeds are disseminated by their minuteness,—by their capsule being converted into a light balloon-like envelope,—by being embedded in pulp or flesh, formed of the most diverse parts, and rendered nutritious, as well as conspicuously coloured, so as to attract and be devoured by birds,—by having hooks and grapnels of many kinds and serrated awns, so as to adhere to the fur of quadrupeds,—and by being furnished with wings and plumes, as different in shape as they are elegant in structure, so as to be wafted. by every breeze. I will give one other instance ; for this subject of the same end being gained by the most diversified means well deserves attention. Some authors maintain that organic beings have been formed in many ways for the sake of mere variety, almost like toys in a shop, but such a view of nature is incredible. With plants having separated sexes, and with those in which, though hermaphrodites, the pollen does not spontaneously fall on the stigma, some aid is necessary for their fertilisation. With several kinds this is effected by the pollen-grains, which are light and incoherent, being blown by the wind through mere chance on to the stigma; and this is the simplest plan which can well be conceived. An almost equally simple, though very different, plan occurs in many plants in which a symmetrical flower secretes a few drops of ngctar, and is consequently visited by insects; and these carry the pollen from the anthers to the stigma. From this simple stage we may pass through an inexhaustible number of contrivances, all for the same purpose and effected in essentially the same manner, but entailing changes in every part of the flower. The nectar may be stored in variously shaped recep- tacles, with the stamens and pistils modified in many ways, some- times forming trap-like contrivances, and sometimes capable of neatly adapted movements through irritability or elasticity. From such structures we may advance till we come to such a case of extraordinary adaptation as that lately described by Dr. Oriiger in the Coryanthes. This orchid has part of its labellum or lower lip hollowed out into a great bucket, into which drops of almost pure water continually fall from two secreting horns which stand above it; and when the bucket is half full, the water overflows by a spout on one side. The basal part of the labellum stands over the bucket, and is itself hollowed out into a sort of chamber with two ees Cuap. VI. of Natural Selection. 155, lateral entrances; within this chamber there are curious fleshy ridges. The most ingenious man, if he had not witnessed what takes place, could never have imagined what purpose all these parts serve. But Dr. Criiger saw crowds of large humble-bees visiting the gigantic flowers of this orchid, not in order to suck nectar, but to gnaw off the ridges within the chamber above the bucket; in doing this they frequently pushed each other into the bucket, and their wings being thus wetted they could not fly away, but were compelled to crawl out through the passage formed by the spout or overflow. Dr. Criiger saw a “continual procession” of bees thus crawling out of their involuntary bath. The passage is narrow, and is roofed over by the column, so that a bee, in forcing its way out, first rubs its back against the viscid stigma and then against the viscid glands of the pollen-masses. The pollen-masses are thus glued to the back of the bee which first happens to crawl out through the passage of a lately expanded flower, and are thus carried away. Dr. Criiger sent me a flower in spirits of wine, with a bee which he had killed before it had quite crawled out with a pollen-mass still fastened to its back. When the bee, thus provided,. flies to another flower, or to the same flower a second time, and is pushed by its comrades into the bucket and then crawls out by the passage, the pollen-mass necessarily comes first into contact with the viscid stigma, and adheres to it, and the flower is fertilised. Now at last we see the full use of every part of the flower, of the water-secreting horns, of the bucket half full of water, which prevents the bees from flying away, and forces them to crawl out through the spout, and rub against the properly placed viscid pollen- masses and the viscid stigma. The construction of the flower in another closely allied orchid, namely the Catasetum, is widely different, though serving the same end; and is equally curious. Bees visit these flowers, like those of the Coryanthes, in order to gnaw the labellum ; in doing this they inevitably touch a long, tapering, sensitive projection, or, as I have called it, the antenna. This antenna, when touched, transmits a sensation or vibration to a certain membrane which is instantly ruptured ; this sets free a spring by which the pollen-mass is shot forth, like an arrow, in the right direction, and adheres by its viscid extremity to the back of the bee. The pollen-mass of the male plant (for the sexes are separate in this orchid) is thus carried to the flower of the female plant, where it is brought into contact with the stigma, which is viscid enough to break certain elastic threads, and retaining the pollen, fertilisation is effected. How, it may be asked, in the foregoing and in innumerable other 156 Organs of little [Importance Cup, VI instances, can we understand the graduated scale of complexity and the multifarious means for gaining the same end. The answer no doubt is, as already remarked, that when two forms vary, which already differ from each other in some slight degree, the variability will not be of the same exact nature, and consequently the results obtained through natural selection for the same general purpose will not be the same. We should also bear in mind that every highly developed organism has passed through many changes; and that each modified structure tends to be inherited, so that each modi- fication will not readily be quite lost, but may be again and again further altered. Hence the structure of each part of each species, for whatever purpose it may serve, is the sum of many inherited changes, through which the species has passed during its successive — adaptations to changed habits and conditions of life. Finally then, although in many cases it is most difficult even to conjecture by what transitions organs have arrived at their present state; yet, considering how small the proportion of living and known forms is to the extinct and unknown, I have been astonished how rarely an organ can be named, towards which no transitional grade is known to lead. It certainly is true, that new organs appearing .as if created for some special purpose, rarely or never appear in any being ;—as indeed is shown by that old, but somewhat exaggerated, — canon in natural history of “ Natura non facit saltum.” We meet with this admission in the writings of almost every experienced naturalist; or as Milne Edwards has well expressed it, Nature is prodigal in variety, but niggard in innovation. Why, on the theory of Creation, should there be so much variety and so little real novelty? Why should all the parts and organs of many independent beings, each supposed to have been separately created for its proper place in nature, be so commonly linked together by ‘graduated steps? Why should not Nature take a sudden leap from structure to structure? On the theory of natural selection, we can clearly understand why she should not; for natural selection acts only by taking advantage of slight successive varia- tions; she can never take a great and sudden leap, but must advance by short and sure, though slow steps. Organs of little apparent Importance, as affected by Natural Selection. As natural selection acts by life and death,—by the survival of the fittest, and by the destruction of the less well-fitted indi- viduals,—I have sometimes felt great difficulty in understanding the origin or formation of parts of little importance; almost as Cuap. VI. affected by Natural S election. 157 great, though of a very different kind, as in the case of the most perfect and complex organs. In the first place, we are much too ignorant in regard to the whole economy of any one organic being, to say what slight modifi- cations would be of importance or not. In a former chapter I have civen instances of very trifling characters, such as the down on fruit and the colour of its flesh, the colour of the skin and hair of quadrupeds, which, from being correlated with constitutional differences or from determining the attacks of insects, might assuredly be acted on by natural selection. ‘The tail of the giraffe looks like an artificially constructed fly-flapper ; and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better fitted, for so trifling an object as to drive away flies; yet we should pause before being too positive even in this case, for we know that the distribution and existence of cattle and other animals in South America absolutely depend on their power of resisting the attacks of insects: so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a great advantage. It is not that the larger quadrupeds are actually destroyed (except in some rare cases) by flies, but they are incessantly harassed and their strength reduced, so that they are more subject to disease, or not so well enabled in a coming dearth to search for food, or to escape from beasts of prey. Organs now of trifling importance have probably in some cases been of high importance to an early progenitor, and, after having been slowly perfected at a former period, have been transmitted to existing species in nearly the same state, although now of very slight use; but any actually injurious deviations in their structure would of course have been checked by natural selection. Seeing how important an organ of locomotion the tail is in most aquatic animals, its general presence and use for many purposes in so many land animals, which in their lungs or modified swimbladders betray their aquatic origin, may perhaps be thus accounted for. A well-developed tail having been formed in an aquatic animal, it might subsequently come to be worked in for all sorts of purposes, —as a fly-flapper, an organ of prehension, or as an aid in turning, as in the case of the dog, though the aid in this latter respect must be slight, for the hare, with hardly any tail, can double still more quickly. In the second place, we may easily err in attributing importance to characters, and in believing that they have been developed | 158 Organs of little Lmportance Crap. VI. —... through natural selection. We must by no means overlook the effects of the definite action of changed conditions of life,—of go- called spontaneous variations, which seem to depend in a quite subordinate degree on the nature of the conditions,—of the ten- dency to reversion to long-lost characters,—of the complex laws of growth, such as of correlation, compensation, of the pressure of one part on another, &c.,—and finally of sexual selection, by which characters of use to one sex are often gained and then transmitted more or less perfectly to the other sex, though of no use to this sex. But structures thus indirectly gained, although at first of no advantage 10 a species, may subsequently have been taken advan- tage of by its modified descendants, under new conditions of life and newly acquired habits. If green woodpeckers alone had existed, and we did not know that there were many black and pied kinds, I dare say that we should have thought that the green colour was a beautiful adapta- tion to conceal this tree-frequenting bird from its enemies; and consequently that it was a character of importance, and had been acquired through natural selection; as it is, the colour is probably in chief part due to sexual selection. A trailing palm in the Malay Archipelago climbs the loftiest trees by the aid of exquisitely con- structed hooks clustered around the ends of the branches, and this contrivance, no doubt, is of the highest service to the plant ; but as we see nearly similar hooks on many trees which are not climbers, and which, as there is reason to believe from the distribution of the thorn- bearing species in Africa and South America, serve as a defence against browsing quadrupeds, so the spikes on the palm may at first have been developed for this object, and subsequently have been improved and taken advantage of by the plant, as it underwent further modification and became aclimber. The naked skin on the head of a vulture is generally considered as a direct adaptation for wallowing in putridity ; and so it may be, or it may possibly be due to the direct action of putrid matter; but we should be very cautious in drawing any such inference, when we see that the skin on the head of the clean-feeding male Turkey is likewise naked. The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition, and no doubt they facilitate, or may be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals, We are profoundly ignorant of the cause of each slight variation Cuap. VI. affected by Natural Selection. << 256 or individual difference ; and we are immediately made conscious of this by reflecting on the differences between the breeds of our domesticated animals in different countries,—more especially im the less civilised countries where there has been but little methodical selection. Animals kept by savages in different countries often have to struggle for their own subsistence, and are exposed to a certain extent to natural selection, and individuals with slightly different constitutions would succeed best under different climates. With cattle susceptibility to the attacks of flies is correlated with colour, as is the liability to be poisoned by certain plants; so that even colour would be thus subjected to the action of natural selection. Some observers are convinced that a damp climate affects the growth of the hair, and that with the hair the horns are correlated. Mountain breeds always differ from lowland breeds; and a mountainous country would probably affect the hind limbs from exercising them more, and possibly even the form of the pelvis; and then by the law of homologous variation, the front limbs and the head would probably be affected. The shape, also, of the pelvis might affect by pressure the shape of certain parts of the young in the womb. The \aborious breathing necessary in high regions tends, as we have good reason to believe, to increase the size of the chest ; and again correlation would come into play. The effects of lessened exercise together with abundant food on the whole organisation is probably still more important; and this, as H. von Nathusius has lately shown in his excellent Treatise, is apparently one chief cause of the great modification which the breeds of swine have undergone. But we are far too ignorant to specu- late on the relative importance of the several known and unknown causes of variation; and I have made these remarks only to show that, if we are unable to account for the characteristic differences of our several domestic breeds, which nevertheless are generally admitted to have arisen through ordinary generation from one or a few parent-stocks, we ought not to lay too much stress on our ignorance of the precise cause of the slight analogous differences between true species. Utilitarian Doctrine, how far true: Beauty, how acquired. The foregoing remarks lead me to say a few words on the protest lately made by some naturalists, against the utilitarian doctrine that every detail of structure has been produced for the good of its possessor, ‘They believe that many structures have been created for the sake of beauty, to delight man or the Creator (but this latter point is beyond the scope of scientific discussion), or for the 160 Utilitarian Doctrine how far true: Cuar. VI. sake of mere variety, a view already discussed. Such doctrines, jf true, would be absolutely fatal to my theory. I fully admit that many structures are now of no direct use to their possessors, and may never have been of any use to their progenitors; but this does not prove that they were formed solely for beauty or variety. No. doubt the definite action of changed conditions, and the various causes of modifications, lately specified, have all produced an effect, probably a great effect, independently of any advantage thus gained. But a still more important consideration is that the chief part of the organisation of every living creature is due to inherit- ance; and consequently, though each being assuredly is well fitted for its place in nature, many structures have now no very close and direct relation to present habits of life. Thus, we can hardly believe that the webbed feet of the upland goose or of the frigate- bird are of special use to these birds; we cannot believe that the similar bones in the arm of the monkey, in the fore-leg of the horse, in the wing of the bat, and in the flipper of the seal, are of special use to these animals. We may safely attribute these structures to inheritance. But webbed feet no doubt were as useful to the progenitor of the upland goose and of the frigate-bird, as they now are to the most aquatic of living birds. So we may believe that the progenitor of the seal did not possess a flipper, but a foot with five toes fitted for walking or grasping; and we may further venture to believe that the several bones in the limbs of the monkey, horse, and bat, were originally developed, on the principle of utility, probably through the reduction of more numerous bones in the fin of some ancient fish-like progenitor of the whole class. It is scarcely possible to decide how much allowance ought to be made for such causes of change, as the definite action of external conditions, so-called spontaneous varia- tions, and the complex laws of growth; but with these important exceptions, we may conclude that the structure of every living creature either now is, or was formerly, of some direct or in- direct use to its possessor. 3 With respect to the belief that organic beings have been created beautiful for the delight of man,—a belief which it has been pro- nounced is subversive of my whole theory,—I may first remark that the sense of beauty obviously depends on the nature of the mind, irrespective of any real quality in the admired object; and that the idea of what is beautiful, is not innate or unalterable. We see this, for instance, in the men of different races admiring an entirely different standard of beauty in their women. If beautiful objects had been created solely for man’s gratification, it ought to Cuap. VI. Beauty how acquired. 161 be shown that before man appeared, there was less beauty on the face of the earth than since he came on the stage. Were the beau- tiful volute and cone shells of the Eocene epoch, and the gracefully sculptured ammonites of the Secondary period, he that man might ages afterwards admire them 1n his cabinet? Few objects are more beautiful than the minute siliceous cases of the diato- maces: were these created that they might be examined and admired under the higher powers of the microscope? ‘The beauty in this latter case, and in many others, is apparently wholly due to symmetry of growth. flowers rank amongst the most beautiful productions of nature; but they have been rendered conspicuous in contrast with the green leaves, and in consequence at the same time beautiful, so that they may be easily observed by insects, I have come to this conclusion from finding it an invariable rule that when a flower is fertilised by the wind it never has a gaily-coloured corolla. Several plants habitually produce two kinds of flowers ; sne kind open and coloured so as to attract insects; the other closed, not coloured, destitute of nectar, and never visited by insects. Hence we may conclude that, ifinsects had not been developed on the face of the earth, our plants would not have been decked with beau- tiful flowers, but would have produced only such poor flowers as we see on our fir, oak, nut and ash trees, on grasses, spinach, docks, and nettles, which are all fertilised through the agency of the wind. A similar line of argument holds good with fruits; that a ripe straw- berry or cherry is as pleasing to the eye as to the palate,—that the gaily-coloured fruit of the spindle-wood tree and the scarlet berries of the holly are beautiful objects,—will be admitted by every one. But this beauty serves merely as a guide to birds and beasts, in order that the fruit may be devoured and the manured seeds dis- seminated: I infer that this is the case from having as yet found no exception to the rule that seeds are always thus disseminated when embedded within a fruit of any kind (that is within a fleshy or pulpy envelope), if it be coloured of any brilliant tint, or ren- dered conspicuous by being white or black. On the other hand, I willingly admit that a great number of male animals, as all our most gorgeous birds, some fishes, reptiles, and mammals, and a host of magnificently coloured butterflies, have been rendered beautiful for beauty’s sake; but this has been effected through sexual selection, that is, by the more beautiful males having been continually preferred by the females, and not for the delight of man. So it is with the music of birds. We may infer from all this that a nearly similar taste for beautiful colours and for musical sounds runs through a large part of the animal M 162 Utilitarian Doctrine how far trie, Cuar. Vi. kingdom. When the female is as beautifully coloured as the male, which is not rarely the case with birds and butterflies, the cause apparently lies in the colours acquired through sexual selection having been transmitted to both sexes, instead of to the males alone. How the sense of beauty in its simplest form—that is, the reception of a peculiar kind of pleasure from certain colours, forms, and sounds—was first developed in the mind of man and of the lower animals, is a very obscure subject. The same sort of difficulty is presented, if we enquire how it is that certain flavours and odours give pleasure, and others displeasure. Habit in all these cases appears to have come to a certain extent into play; but there must be some fundamental cause in the constitution of the nervous system in each species. Natural selection cannot possibly produce any modification in a species exclusively for the good of another species; though throughout nature one species incessantly takes advantage of, and profits by, the structures of others. But natural selection can and does often produce structures for the direct injury of other animals, as we see in the fang of the adder, and in the ovipositor of the ichneumon, by which its eggs are deposited in the living bodies of other insects. If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection. Although many statements may be found in works on natural history to this effect, I cannot find even one which seems to me of any weight. It is admitted that the rattlesnake has a poison-fang for its own defence, and for the destruction of its prey; but some authors suppose that at the same time it is furnished with a rattle for its own snjury, namely, to warn its prey. I would almost as soon believe that the cat curls the end of its tail when preparing to spring, in order to warn the doomed mouse. It is a much more probable view that the rattlesnake uses its rattle, the cobra expands its frill, and the puff-adder swells whilst hissing so loudly and harshly, in order to alarm the many birds and beasts which are known to attack even the most venomous species. Snakes act on the same principle which makes the hen ruffle her feathers and expand her wings when a dog approaches her chickens; but I have not space here to enlarge on the many ways by which animals endeavour to frighten away their enemies, Natural selection will never produce in a being any structure more injurious than beneficial to that being, for naturai selection cuar. Vi Utilitarian Doctrine how far true. 163 acts solely by and for the good of each. No organ will be formed, as Paley has remarked, for the purpose of causing pain or for doing an injury to its possessor. If a fair balance be struck between the good and evil caused by each part, each will be found on the whole advantageous. After the lapse of time, under changing conditions of life, if any part comes to be injurious, it will be modified ; or if it be not so, the being will become extinct as myriads have become extinct. Natural selection tends only to make each organic being as perfect as, or slightly more perfect than, the other inhabitants of the same country with which it comes into competition, And we see that this is the standard of perfection attained under nature. The endemic productions of New Zealand, for instance, are perfect one compared with another; but they are now rapidly yielding before the advancing legions of plants and animals introduced from Europe. Natural selection will not produce absolute perfection, nor do we always meet, as far as we can judge, with this high standard under nature. The correction for the aberration of light is said by Miiller not to be perfect even in that most perfect organ, the human eye. Helmholtz, whose judgment no one will dispute, after describing in the strongest terms the wonderful powers of the human eye, adds these remarkable words: “That which we have discovered in the way of inexactness and imperfection in the optical machine and in the image on the retina, is as nothing in comparison with the incongruities which we have just come across in the domain of the sensations. One might say that nature has taken delight in accu- mulating contradictions in order to remove all foundation from the theory of a pre-existing harmony between the external and internal worlds.” If our reason leads us to admire with enthusiasm a multitude of inimitable contrivances in nature, this same reason tells us, though we may easily err on both sides, that some other contrivances are less perfect. Can we consider the sting of the bee as perfect, which, when used against many kinds of enemies, cannot be withdrawn, owing to the backward serratures, and thus inevitably causes the death of the insect by tearing out its viscera ? If We look at the sting of the bee, as having existed in a remote Progenitor as a boring and serrated instrument, like that in so many members of the same great order, and that it has since been modi- fied but not perfected for its present purpose, with the poison origi- nally adapted for some other object, such as to produce galls, since en We can perhaps understand how it is that the use of the rea — so often cause the insect’s own death: for if on ole the power of stinging be useful to the social community, mM 2 164 Summary. Cuap. VI. it will fulfil all the requirements of natural selection, though it may cause the death of some few members. If we admire the truly wonderful power of scent by which the males of many insects find their females, can we admire the production for this single purpose of thousands of drones, which are utterly useless to the community: for any other purpose, and which are ultimately slaughtered by their industrious and sterile sisters? It may be difficult, but we ought to admire the savage instinctive hatred of the queen-bee, which urges her to destroy the young queens, her daughters, as soon as they are born, or to perish herself in the combat; for undoubtedly this is for the good of the community ; and maternal love or ma- ternal hatred, though the latter fortunately is most rare, is all the same to the inexorable principle of natural selection. If we admire the several ingenious contrivances, by which orchids and many other plants are fertilised through insect agency, can we consider as equally perfect the elaboration of dense clouds of pollen by our fir-trees, so that a few granules may be wafted by chance on to the ovules ? Summary: the Law of Unity of Type and of the Conditions of Existence embraced by the Theory of Natural Selection. We have in this chapter discussed some of the difficulties and objections which may be urged against the theory. Many of them are serious; but I think that in the discussion light has been thrown on several facts, which on the belief of independent acts of creation are utterly obscure. We have seen that species at any one period ‘are not indefinitely variable, and are not linked together by a multitude of intermediate gradations, partly because the process of natural selection is always very slow, and at any one time acts only on a few forms; and partly because the very process of natural selection implies the continual supplanting and extinction of pre- ceding and intermediate gradations. Closely allied species, now living on a continuous area, must often have been formed when the area was not continuous, and when the conditions of life did not insensibly graduate away from one part to another. When two varieties are formed in two districts of a continuous area, an inter- mediate variety will often be formed, fitted for an intermediate zone; but from reasons assigned, the intermediate variety will usually exist in lesser numbers than the two forms which it connects; consequently the two latter, during the course of further modification, from existing in greater numbers, will have a great advantage over the less numerous intermediate variety, and will thus generally succeed in supplanting and exterminating it. Cay. VI. Summary. 165 We have seen in this chapter how cautious we should be in con- cluding that the most different habits of life could not graduate into each other; that a bat, for instance, could not have been formed by natural selection from an animal which at first only glided through the air. | ae : We have seen that a species under new conditions of life may change its habits; or it may have diversified habits, with some very unlike those of its nearest congeners. Hence we can under- stand, bearing in mind that each organic being is trying to live wherever it can live, how it has arisen that there are upland geese with webbed feet, ground woodpeckers, diving thrushes, and _petrels with the habits of auks. Although the belief that an organ so perfect as the eye could have been formed by natural selection, is enough to stagger any one; yet in the case of any organ, if we know of a long series of gradations in complexity, each good for its possessor, then, under changing conditions of life, there is no logical impossibility in the acquirement of any conceivable degree of perfection through natural selection. In the cases in which we know of no intermediate or transitional states, we should be extremely cautious in concluding that none can have existed, for the metamorphoses of many organs show what wonderful changes in function are at least possible. For instance, a swimbladder has apparently been converted into an air- breathing lung. The same organ having performed simultaneously very different functions, and then having been in part or in whole specialised for one function; and two distinct organs baving per- formed at the same time the same function, the one having been perfected whilst aided by the other, must often have largely facili- tated transitions, ) 3 | We have seen that-in two beings widely remote from each other in the natural scale, organs serving for the same purpose and in external appearance closely similar may have been separately and independently formed ; but’ when such organs are closely examined, essential differences in their structure can almost always be detected ; and this naturally follows from the principle of natural selection. On the other hand, the common rule throughout nature is infinite diversity of structure for gaining the same end; and this again naturally follows from the same great principle. In many cases we are far too ignorant to be enabled to assert that & partior organ is so unimportant for the welfare of -a species, that modifications in its structure could not have been slowly accumu- dated by means of natural selection, In many other cases, modifi- 166 Summary. Cuap. VI. cations are probably the direct result of the laws of variation or of growth, independently of any good having been thus gained. But even such structures have often, as we may feel assured, been subsequently taken advantage of, and still further modified, for the good of species under new conditions of life. We may, also, believe that a part formerly of high importance has frequently been re- tained (as the tail of an aquatic animal by its terrestrial descend+ ants), though it has become of such small importance that it could not, in its present state, have been acquired by means of natural selection. Natural selection can produce nothing in one species for the exclusive good or injury of another; though it may well produce parts, organs, and excretions highly useful or even indispensable, or again highly injurious to another species, but in all cases at the same time useful to the possessor. In each well-stocked country natural selection acts through the competition of the inhabitants, and consequently leads to success in the battle for life, only in accordance with the standard of that particular country. Hence: the inhabitants of one country, generally the smaller one, often yield to the inhabitants of another and generally the larger country. For in the larger country there will have existed more individuals. and more diversified forms, and the competition will have been severer, and thus the standard of perfection will have been rendered. higher. Natural selection will not necessarily lead to absolute perfection ; nor, as far as we can judge by our limited faculties, can absolute perfection be everywhere predicated. On the theory of natural selection we can clearly understand the full meaning of that old canon in natural history, “ Natura non facit saltum.” This canon, if we look to the present inhabitants alone of the world, is not strictly correct; but if we include all those of past times, whether known or unknown, it must on this. theory be strictly true. It is generally acknowledged that all organic beings have been formed on two great laws—-Unity of Type, and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure which we see in organic beings of the same class, and which is quite independent of their habits of life. On my theory, unity of type is explained by unity of descent. The expression of conditions of existence, so often insisted on by the illustrious Cuvier, is fully embraced by the principle of natural selection. For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life; or by Cuap. VI. SUMMALH « 167 having adapted them during past periods of time: the adaptations being aided in many cases by the increased use or disuse of parts, being affected by the direct action of the external conditions of life, and subjected in all cases to the several laws of growth and variation. Hence, in fact, the law of the Conditions of Existence is the higher law; as it includes, through the inheritance of former variations and adaptations, that of Unity of Type. 168 Miscellaneous Objections to the —_- Curar. VI, an CHAPTER VII, : MISCELLANEOUS OBJECTIONS TO THE THEORY OF NatTurat, SELECTION. Longevity — Modifications not necessarily simultaneous — Modifications apparently of no direct service—Progressive development — Characters of small functional importance, the most constant—Supposed incom- petence of natural selection to account for the incipient stages of useful structures—Causes which interfere with the acquisition through natural selection of useful structures—Gradations of structure with changed functions—Widely different organs in members of the same class, developed from one and the same source—Reasons for disbeliey- ing in great and abrupt modifications, I witu devote this chapter to the consideration of various mis- cellaneous objections which have been advanced against my views, as some of the previous discussions may thus be made clearer ; but it would be useless to discuss all of them, as many have been made by writers who have not taken the trouble to understand the subject. Thus a distinguished German naturalist has asserted that the weakest part of my theory is, that I consider all organic beings as imperfect: what I have really said is, that all are not as perfect as they might have been in relation to their conditions ; and this is shown to be the case by so many native forms in many quarters of the world having yielded their places to intruding foreigners. Nor can organic beings, even if they were at any one time perfectly adapted to their conditions of life, have remained so, when their conditions changed, unless they themselves likewise changed; and no one will dispute that the physical conditions of each country, as well as the numbers and kinds of its inhabitants, have undergone many mutations. A critic has lately insisted, with some parade of mathematical accuracy, that longevity is a vreat advantage to all species, so that he who believes in natural selection “« must arrange his genealogical tree” in such a manner that all the descendants have longer lives than their progenitors! Cannot our critic conceive that a biennial plant or one of the lower animals might range into a cold climate and perish there every winter; and yet, owing to advantages Cuap. VII. Theory of Natural Selection. 169 gained through natural selection, survive from year to year by means of its seeds or ova? Mr. E. Ray Lankester has recently discussed this subject, and he concludes, as far as its extreme com- plexity allows him to form a judgment, that longevity 1s generally related to the standard of each species in the scale of organisation, as well as to the amount of expenditure in reproduction and in general activity. And these conditions have, it is probable, been largely determined through natural selection. — It has been argued that, as none of the animals and plants of Egypt, of which we know anything, have changed during the last three or four thousand years, so probably have none in any part of the world. But, as Mr. G. H. Lewes has remarked, this line of argument proves too much, for the ancient domestic races figured on the Egyptian monuments, or embalmed, are closely similar or even identical with those now living; yet all naturalists admit that such races have been produced through the modification of their original types, The many animals which have remained unchanged since the commencement of the glacial period, would have been an incomparably stronger case, for these have been exposed to great changes of climate and have migrated over great distances; whereas, in Egypt, during the last several thousand years, the conditions of life, as far as we know, have remained absolutely uniform. The fact of little or no modification having been effected since the glacial period would have been of some avail against those who believe in an innate and necessary. law of. development, but is powerless against the doctrine of natural selection or the survival of the fittest, which implies that when variations or individual differences of a beneficial nature happen to arise, these will be preserved ; but this will be effected only under certain favourable circumstances, The celebrated paleontologist, Bronn, at the close of his German translation of this work, asks, how, on the principle of natural selection, can a variety live side by side with the parent-species ? If both have become fitted for slightly different habits of life or conditions, they might live together ; and if we lay on one side polymorphic Species, in which the variability seems to be of a peculiar nature, and all mere temporary variations, such as size, albinism, &c., the more permanent varieties are generally found, as far as I can discover, inhabiting distinct stations,—such as high land or low land, dry or moist districts, Moreover, in the case of animals which wander much about and cross freely, their varieties seem to be generally confined to distinct regions, Bronn also insists that distinct species never differ from each other 170 Miscellaneous Objections to the — Cuar. VII. in single characters, but in many parts ; and he asks, how it always comes that many parts of the organisation should have been modi- fied at the same time through variation and natural selection ? But there is no necessity for supposing that all the parts of any being have been simultaneously modified. The most striking modifications, excellently adapted for some purpose, might, as was formerly remarked, be acquired by successive variations, if slight, first in one part and then in another ; and as they would be trans- mitted all together, they would appear to us as if they had been simultaneously developed. ‘The best answer, however, to the above objection is afforded by those domestic races which have been modified, chiefly through man’s power of selection, for some special purpose. Look at the race and dray horse, or at the greyhound and mastiff. Their whole frames and even their mental characteristics have been modified ; but if we could trace each step in the history of their transformation,—and the latter steps can be traced,—we should not see great and simultaneous changes, but first one part and then another slightly modified and improved. Even when selection has been applied by man to some one character alone, —of which our cultivated plants offer the best instances,—it will invariably be found that although this one part, whether it be the flower, fruit, or leaves, has been greatly changed; almost all the other parts have been slightly modified. ‘This may be attributed. partly to the principle of correlated growth, and partly 1o so-called spontaneous variation. ) | A much more serious objection has been urged by Bronn, and recently by Broca, namely, that many characters appear to be of no service whatever to their possessors, and therefore cannot have been influenced through natural selection. Bronn adduces the length of the ears and tails in the different species of hares and mice,—the complex folds of enamel in the teeth of many animals, and a multitude of analogous cases. With respect to plants, this subject has been discussed by Nageli in an admirable essay. He admits that natural selection has effected much, but he insists that the families of plants differ chiefly from each other in morphological characters, which appear to be quite unimportant for the welfare of the species. He consequently believes in an innate tendency towards progressive and more perfect development. He specifies the arrangement of the cells in the tissues, and of the leaves on the axis, as cases in which natural selection could not have acted. ‘lo these may be added the numerical divisions in the parts of the flower, the position of the ovules, the shape of the seed, when not of any use for dis- semination, &c. Cuar. VI. Theory of Natural Selection. I7E There is much force in the above objection. Nevertheless, we ought, in the first place, to be extremely cautious in pretending to decide what structures now are, or have formerly been, of use to each species. In the second place, it should always be borne in mind that when one part is modified, so will be other parts, through certain dimly seen causes, such as an increased or diminished flow of nutriment to a part, mutual pressure, an early developed part affecting one subsequently developed, and so forth, —as well as: through other causes which lead to the many mysterious cases of correlation, which we do not in the least understand. These: agencies may be all grouped together, for the sake of brevity, under the expression of the laws of growth. In the third place, we have: to allow for the direct and definite action of changed conditions of life, and for so-called spontaneous variations, in which the nature of the conditions apparently plays a quite subordinate part. Bud- variations, such as the appearance of a moss-rose on a commcu rose, or of a nectarine on a peach-tree, offer good instances of spon- taneous variations ; but even in these cases, if we bear in mind the power of a minute drop of poison in producing complex galls, we: ought not to feel too sure that; the above variations are not the effect. of some local change in the nature of the sap, due to some change in the conditions. There must be some efficient cause for each slight individual difference, as well as for more strongly marked variations which occasionally arise; and if the unknown cause were to act persistently, it is almost certain that all the individuals of the species would be similarly modified. In the earlier editions of this work I under-rated, as it now seems. probable, the frequency and importance of modifications due to spontaneous variability. But it is impossible to attribute to this cause the innumerable structures which are so well adapted to the habits of life of each species. I can no more believe in this, thar that the well-adapted form of a race-horse or greyhound, which before the principle of selection by man was well understood, excited so much surprise in the minds of the older naturalists, can thus be explained. “ may be worth while to illustrate some of the foregoing remarks. W ith respect to the assumed inutility of various parts and organs, it is hardly necessary to observe that even in the higher and best- known animals many structures exist, which are so highly developed that no one doubts that they are of importance, yet their use has not been, or has only recently been, ascertained. As Bronn gives the length of the ears and tail in the several species of mice as instances, though trifling ones, of differences in structure which cam 172 Miscellaneous Objections to the — Cuar. VII, be of no special use, I may mention that, according to Dr. Schobl, the external ears of the common mouse are supplied in an extra- ordinary manner with nerves, so that they no doubt serve as tactile organs; hence the length of the ears can hardly be quite unim- portant, We shall, also, presently see that the tail is @ highly useful prehensile organ to some of the species; and its use would be much influenced by its length. With respect to plants, to which on account of Nageli’s essay I shall confine myself in the following remarks, it will be admitted that the flowers of orchids present a multitude of curious structures, which a few years ago would have been considered as mere morpho- logical differences without any special function; but they are now known to be of the highest importance for the fertilisation of the species through the aid of insects, and have probably been gained through natural selection. No one until lately. would have imagined that in dimorphic and trimorphic plants the different lengths of the stamens and pistils, and their arrangement, could have been of any service, but now we know this to be the case. In certain whole groups of plants the ovules stand erect, and in others they are suspended; and within the same ovarium of some few plants, one ovule holds the former and a second ovule the latter position. These positions seem at first purely morphological, or of no physiological signification ; but Dr. Hooker informs me that within the same ovarium, the upper ovules alone in some cases, and in other cases the lower ones alone are fertilised; and he suggests that this probably depends on the direction in which the pollen-tubes: enter the ovarium, If so, the position of the ovules, even when one is erect and the other suspended within the same ovarium, would follow from the selection of any slight deviations in position which favoured their fertilisation, and the production of seed. Several plants belonging to distinct orders habitually produce flowers of two kinds,—the one open of the ordinary structure, the other closed and imperfect. ‘These two kinds of flowers sometimes differ wonderfully in structure, yet may be seen to graduate into each other on the same plant. The ordinary and open flowers can be intercrossed ; and the benefits which certainly are derived from this process are thus secured. The closed and imperfect flowers are, however, manifestly of high importance, as they yield with the utmost safety a large stock of seed, with the expenditure of wonder- fully little pollen, The two kinds of flowers often differ much, as just stated, in structure. The petals in the imperfect flowers almost always consist of mere rudiments, and the pollen-grains are reduce in diameter, In Ononis columne five of the alternate stamens are 5 he be Cuar. VIL. Theory of Natural Selection. 173 rudimentary ; and in some species of Viola three stamens are in this state, two retaining their proper function, but being of very small size. In six out of thirty of the closed flowers In an Indian violet (name unknown, for the plants have never produced with me perfect flowers), the sepals are reduced from the normal number of five to three. In one section of the Malpighiacew the closed flowers, according to A.de Jussieu, are still further modified, for the five stamens which stand opposite to the sepals are all aborted, a sixth stamen standing opposite to a petal being alone developed ; and this stamen is not present in the ordinary flowers of these species; the style is aborted ; and the ovaria are reduced from three to two. Now although natural selection may well have had the power to prevent some of the flowers from expanding, and to reduce the amount of pollen, when rendered by the closure of the flowers superfluous, yet hardly any of the above special modifications can have been thus determined, but must have followed from the laws of growth, including the functional inactivity of parts, during the pro- gress of the reduction of the pollen and the closure of the flowers. It is so necessary to appreciate the important effects of the laws of growth, that I will give some additional cases of another kind, namely of differences in the same part or organ, due to differences in relative position on the same plant. In the Spanish chestnut, and in certain fir-trees, the angles of divergence of the leaves differ, according to Schacht, in the nearly horizontal and in the upright branches. In the common rue and some other plants, one flower, usually the central or terminal one, opens first, and has five sepals and petals, and five divisions to the ovarium; whilst all the other flowers on the plant are tetramerous. In the British Adoxa the uppermost flower generally has two calyx-lobes with the other organs tetramerous, whilst the surrounding flowers generally have three calyx-lobes with the other organs pentamerous. In many Composite and Umbellifere: (and in some other plants) the circum- ferential flowers have their corollas much more developed than those of the centre ; and this seems often connected with the abortion of the reproductive organs. It isa more curious fact, previously referred to, that the achenes or seeds of the circumference and centre sometimes differ greatly in form, colour, and other characters. In Carthamus and some other Composite the central achenes alone are furnished with a pappus; and in Hyoseris the same head yields achenes of three different forms. In certain Umbellifere the ex- terior seeds, according to Tausch, are orthospermous, and the central one coelospermous, and this is a character which was considered by De Candolle to be in other species of the highest systematic im- 174 “Miscellaneous Objections to the ~ Cuar. VIL portance. Prof. Braun mentions a Fumariaceous genug, in which the flowers in the lower part of the spike bear oval, ribbed, one- seeded nutlets; and in the upper part of the spike, lanceolate, two- valved, and two-seeded siliques. In these several cases, with the exception of that of the well developed ray-florets, which are of service in making the flowers conspicuous to insects, natural selection can- not, as far as we can judge, have come into play, or only in a quite subordinate manner. All these modifications follow from the relative position and inter-action of the parts ; and it can hardly be doubted that if all the flowers and leaves on the same plant had been sub- jected to the same external and internal condition, as are the flowers and leaves in certain positions, all would have been modified in the | same manner. In numerous other cases we find modifications of structure, which are considered by botanists to be generally of a highly import- ant nature, affecting only some of the flowers on the same plant, or occurring on distinct plants, which grow close together under the same conditions. As these variations seem of no special use to the plants, they cannot have been influenced by natural selection. Of their cause we are quite ignorant; we cannot even attribute them, as in the last class of cases, to any proximate agency, such as relative position. I will give only a few instances. It is so common to observe on the same plant, flowers indifferently tetra- merous, pentamerous, &c., that I need not give examples; but as numerical variations are comparatively rare when the parts are few, I may mention that, according to De Candolle, the flowers of Papaver bracteatum offer either two sepals with four petals (which is the common type with poppies), or three sepals with six petals. The manner in which the petals are folded in the bud is in most groups a very constant morphological character; but Professor Asa Gray states that with some species of Mimulus, the exstivation is almost as frequently that of the Rhinanthidew as of the Antirrhi- nidez, to which latter tribe the genus belongs. Aug. St. Hilaire gives the following cases: the genus Zanthoxylon belongs to a division of the Rutacez with a single ovary, but in some species flowers may be found on the same plant, and even in the same panicle, with either one or two ovaries. In Helianthemum the capsule has been described as unilocular or 3-locular; and in H. mutabile, “Une lame, plus ou moins large, s’étend entre le pericarpe et le placenta.” In the flowers of Saponaria officinalis, Dr. Masters has observed instances of both marginal and free central placentation. Lastly, St. Hilaire found towards the southern ex- treme of the range of Gomphia olesformis two forms which he did Cuar. VIL. Theory of Natural Selection. 175 not at first doubt were distinct species, but he subsequently Saw them growing on the same bush; and he then adds, “Voila done dans un méme individu des loges et un style qui se rattachent tantot & un axe verticale et tantét & un gynobase.” We thus see that with plants many morphological changes may be attributed to the laws of growth and the inter-action of parts, independently of natural selection. But with respect to Nageli’s doctrine of an innate tendency towards perfection or progressive development, can it be said in the case of these strongly pro- nounced variations, that the plants have been caught in the act of | progressing towards a higher state of development? On the con- trary, I should infer from the mere fact of the parts in question differing or varying greatly on the same plant, that such modi- fications were of extremely small importance to the plants them- selves, of whatever importance they may generally be to us for our classifications. The acquisition of a useless part can hardly be said to raise an organism in the natural scale; and in the case of the imperfect, closed flowers above described, if any new prin- ciple has to be invoked, it must be one of retrogression rather than of progression ; and so it must be with many parasitic and degraded animals. We are ignorant of the exciting cause of the above specified modifications; but if the unknown cause were to act almost uniformly for a length of time, we may infer that the result would be almost uniforra; and in this case all the individuals of the species would be modified in the same manner. From the fact of the above characters being unimportant for the welfare of the species, any slight variations which occurred in them would not have been accumulated and augmented through natural selection. A structure which has been developed through long- continued selection, when it ceases to be of service to a species, generally becomes variable, as we see with rudimentary organs ; for it will no longer be regulated by this same power of selection. But when, from the nature of the organism and of the conditions, modifications have been induced which are unimportant for the Welfare of the species, they may be, and apparently often have been, transmitted in nearly the same state to numerous, otherwise modified, descendants. It cannot have been of much importance — sve ae of mammals, birds, or reptiles, whether they mitted pra es am feathers, or scales; yet hair has been trans- al dea i ne #9 mammals, feathers to all birds, and scales to std ih ei engin whatever it may be, which is com- Seomophenibe yi p omms, 1s ranked by us as of high systematic ? onsequently is often assumed to be of high vital 176 Miscellaneous Objections to the —- Curae. VIL ee importance to the species. Thus, as I am. inclined to believe, morphological differences, which we consider as important—such as the arrangement of the leaves, the divisions of the flower or of the ovarium, ‘the position of the ovules, &c.—first appeared in many cases as fluctuating variations, which sooner or later became con- stant through the nature of the organism and of the surrounding conditions, as well as through the intercrossing of distinct indivi- duals, but not through natural selection ; for as these morphological characters do not affect the welfare of the species, any slight devia- tions in them could not have been governed or accumulated through this latter agency. It is a strange result which we thus arrive at, namely that characters of slight vital importance to the species, are the most important to the systematist ; but, as we shall here- after see when we treat of the genetic principle of classification, this is by no means so paradoxical as it may at first appear. Although we have no good evidence of the existence in organic beings of an innate tendency towards progressive development, yet this necessarily follows, as I have attempted to show in the fourth chapter, through the continued action of natural selection. For the best definition which has ever been given of a high standard of organisation, is the degree to which the parts have been specialised or differentiated ; and natural selection tends towards this end, inas- much as the parts are thus enabled to perform their functions more efficiently. A distinguished zoologist, Mr. St. George Mivart, has recently collected all the objections which have ever been advanced by myself and others against the theory of natural selection, as pro- pounded by Mr. Wallace and myself, and has illustrated them with admirable art and force. When thus marshalled, they make a formidable array ; and as it forms no part of Mr. Mivart’s plan to give the various facts and considerations opposed to his conclusions, no slight effort of reason and memory is left to the reader, who may wish to weigh the evidence en both sides. When discussing special cases, Mr. Mivart passes over the effects of the increased use and disuse of parts, which I have always maintained to be highly im- portant, and have treated in my ‘ Variation under Domestication’ at greater length than, as I believe, any other writer. He likewise often assumes that I attribute nothing to variation, independently of natural selection, whereas in the work just referred to I have collected a greater number of well-established cases than can be found in any other work known tome. My judgment may not be trustworthy, but after reading with care Mr. Mivart’s book, and Cuar. VII. Theory of Natural Selection. 177 comparing each section with what I have said on a same head, I never before felt so strongly convinced of the general truth of the conclusions here arrived at, subject, of course, in so intricate a subject, to much partial error. : All Mr. Mivart’s objections will be, or have been, considered in the present volume. ‘The one new point which appears to have struck many readers is, “that natural selection 1s Incompetent to account for the incipient stages of useful structures.” This subject is intimately connected with that of the gradation of characters, often accompanied by a change of function,—for instance, the con- version of a swim-bladder into lungs,—points which were discussed in the last chapter under two headings, Nevertheless, I will here consider in some detail several of the cases advanced by Mr. Mivart, selecting those which are the most illustrative, as want of space prevents me from considering all. The giraffe, by its lofty stature, much elongated neck, fore-legs, head and tongue, has its whole frame beautifully adapted for browsing on the higher branches of trees. It can thus obtain food beyond the reach of the other Ungulata or hoofed animals inhabiting the same country; and this must be a great advantage to it during dearths. The Niata cattle in 8. America show us how small a difference in structure may make, during such periods, a great differ- ence in preserving an animal’s life. These cattle can browse as well as others on grass, but from the projection of the lower jaw they cannot, during the often recurrent droughts, browse on the twigs of trees, reeds, &c., to which food the common cattle and horses are then driven ; so that at these times the Niatas perish, if not fed by their owners. Before coming to Mr. Mivart’s objections, it may be well to explain once again how natural selection will act in all ordinary cases. Man has modified some of his animals, without necessarily having attended to special points of structure, by simply preserving and breeding from the fleetest individuals, as with the race-horse and greyhound, or as with the game-cock, by breeding, from the victorious birds. So under nature with the nascent girafie, the individuals which were the highest browsers and were able during dearths to reach even an inch or two above the others, will often have been preserved; for they will have roamed over the Meo search of food. That the individuals of the same ce ‘i a liffer slightly in the relative lengths of all their parts vinci in many works of natural history, in which careful TSA AU ETS are given. These slight proportional differences, due nk hi idagy and variation, are not of the slightest use or 4 é lost species. But it will have been otherwise with N 178 Miscellaneous Objections to the Crap. VIL, the nascent giraffe, considering its probable habits of life ; for those. individuals which had some one part or several parts of their bodies. rather more elongated than usual, would generally have survived, hese will have intercrossed and left offspring, either inheriting the. same bodily peculiarities, or with a tendency to vary again in the: same manner; whilst the individuals, less favoured in the same: respects, will have been the most liable to perish. We here see that there is no need to separate single pairs, as man does, when he methodically improves a breed: natural selection will preserve and thus separate all the superior individuals, allowing them freely to intercross, and will destroy all the inferior indivi- duals. By this process long-continued, which exactly corresponds. with what I have called unconscious selection by man, combined no- doubt in a most important manner with the inherited effects of the: increased use of parts, it seems to me almost certain that an ordinary; hoofed quadruped might be converted into a giraffe. To this conclusion Mr. Mivart brings forward two objections. One is that the increased size of the body would obviously require: an increased supply of food, and he considers it as “very problemati- cal whethes the disadvantages thence arising would not, in times of scarcity, more than counterbalance the advantages.” But as the: giraffe does actually exist in large numbers in 8. Africa, and as- some of the largest antelopes in the world, taller than an ox, abound: there, why should we doubt that, as far as size is concerned, inter- mediate gradations could formerly have existed there, subjected as. now to severe dearths. Assuredly the being able to reach, at each stage of increased size, to a supply of food, left untouched by the: other hoofed quadrupeds of the country, would have been of some advantage to the nascent giraffe. Nor must we overlook the fact,. that increased bulk would act as a protection against almost all beasts of prey excepting the lion; and against this animal, its tall neck,—and the taller the better,—would, as Mr. Chauncey Wright has remarked, serve as a watch-tower. © It-is from this cause, as Sir 8. Baker remarks, that no animal is more difficult to stalk than the giraffe. This animal also uses its long neck as a means of offence: or defence, by violently swinging its head armed with stump-like~ horns. ‘The preservation of each species can rarely be determined by any one advantage, but by the union of all, great and small. Mr. Mivart then asks (and this is his second objection), if natural selection be so’potent, and if high browsing be so great an advan- tage, why has not any other hoofed quadruped acquired a long neck and lofty stature, besides the giraffe, and, in a lesser degree, the camel, guanaco, and macrauchenia? Or, again, why has not any Cuap, VIL Theory of Natural Selection. 179 member of the group acquired a long proboscis? With respect to S. Africa, which was formerly inhabited by numerous herds of the giraffe, the answer is not difficult, and can best be given by an illustration. In every meadow in England in which trees grow, we see the lower branches trimmed or planed to an exact level by the browsing of the horses or cattle; and what advantage would it be, for instance, to sheep, if kept there, to acquire slightly longer necks? In every district some one kind of animal will almost certainly be able to browse higher than the others; and it is almost equally certain that this one kind alone could have its neck elongated for this purpose, through natural selection and the effects of increased use. In §. Africa the competition for browsing on the higher branches of the acacias and other trees must be between giraffe and giraffe, and not with the other ungulate animals. Why, in other quarters of the world, various animals belonging to this same order have not acquired either an elongated neck or a proboscis, cannot be distinctly answered; but it is as un- reasonable to expect a distinct answer to such a& question, as why some event in the history of mankind did not occur in one country, whilst it did in another. We are ignorant with respect to the conditions which determine the numbers and range of each species; and we cannot even conjecture what changes of structure would be favourable to its increase in some new country. We can, however, see in a general manner that various causes might have interfered with the development of a long neck or proboscis. Tc reach the foliage at a considerable height (without climbing, for which hoofed animals are singularly ill-constructed) implies greatly increased bulk of body; and we know that some areas support singularly few large quadrupeds, for instance §. America, though it is so luxuriant ; whilst §. Africa abounds with them to an un- paralleled degree. Why this should be so, we do not know; nor why the later tertiary periods should have been much more favour- able for their existence than the present time. Whatever the 180 Miscellaneous Objections to the Cap. VII. some species are much more variable than others. Even if the fitting variations did arise, it does not follow that natural selection would be able to act on them, and produce a structure which ap- parently would be beneficial to the species. For instance, if the number of individuals existing in a country 1s determined chiefly through destruction by beasts of prey,—by external or internal parasites, &c.,—as seems often to be the case, then natural selection will be able to do little, or will be greatly retarded, in modifying any particular structure for obtaining food. Lastly, natural selec- tion is a slow process, and the same favourable conditions must long endure in order that any marked effect should thus be pro- duced. Except by assigning such general and vague reasons, we. cannot explain why, in many quarters of the world, hoofed quadru- peds have not acquired much elongated necks or other means for browsing on the higher branches of trees. Objections of the same nature as the foregoing have been advanced by many writers. In each case various causes, besides the general ones just indicated, have probably interfered with the acquisition through natural selection of structures, which it is thought would be beneficial to certain species. One writer asks, why has not the ostrich acquired the power of flight? But a moment’s reflection will show what an enormous supply of food would be necessary to give to this bird of the desert force to move its huge body through the air. Oceanic islands are inhabited by bats and seals, but by no terrestrial mammals; yet as some of these bats are peculiar species, they must have long inhabited their present homes. Therefore Sir C. Lyell asks, and assigns certain reasons in answer, why have not seals and bats given birth on such islands to forms fitted to live on the land? But seals would necessarily be first converted into terrestrial carnivorous animals of considerable size, and bats into terrestrial insectivorous animals; for the former there would be no prey; for the bats ground-insects would serve as food, but these would already be largely preyed on by the reptiles or birds, which first colonise and abound on most oceanic islands. Gradations of structure, with each stage beneficial to a changing species, will be favoured only under certain peculiar conditions. A. strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted into an animal so thoroughly aquatic as to brave the open ocean. But seals would not find on oceanic islands the conditions favourable to their gradual reconversion into a terrestrial form, Bats, as formerly shown, probably acquired their wings by at first gliding through the air from tree to tree, like the so-called flying-squirrels, Car. VIL Theory of Natural Selection. ISI for the sake of escaping from their enemies, or for avoiding falls ; but when the power of true flight had once been acquired, it would never be reconverted back, at least for the above purposes, into the less efficient power of gliding through the air. Bats might, indeed, like many birds, have had their wings greatly reduced in size, or completely lost, through disuse; but in this case it would be necessary that they should first have acquired the power of running quickly on the ground, by the aid of their hind legs alone, so as to compete with birds or other ground animals; and for such a change a bat seems singularly ill-fitted. These conjectural remarks have been made merely to show that a transition of structure, with each step beneficial, is a highly complex affair; and that there is nothing strange in a transition not having occurred in any particular case. Lastly, more than one writer has asked, why have some animals had their mental powers more highly developed than others, as such development would be advantageous to all? Why have not apes acquired the intellectual powers of man? Various causes could be assigned ; but as they are conjectural, and their relative probability cannot be weighed, it would be useless to give them. A definite answer to the latter question ought not to be expected, seeing that no one can solve the simpler problem why, of two races of savages, one has risen higher in the scale of civilisation than the other; and this apparently implies increased brain-power. We will return to Mr. Mivart’s other objections. Insects often resemble for the sake of protection various objects, such as green or decayed leaves, dead twigs, bits of lichen, flowers, spines, excrement of birds, and living insects; but to this latter point I shall here- after recur. The resemblance is often wonderfully close, and is not confined to colour, but extends to form, and even to the manner in which the insects hold themselves. The caterpillars which project motionless like dead twigs from the bushes on which they feed, offer an excellent instance of a resemblauce of this kind. The cases of the imitation of such objects as the excrement of birds, are rare and exceptional. On this head, Mr. Mivart remarks, “ As, according to Mr. Darwin’s theory, there is a constant tendency to indefinite variation, and as the minute incipient variations will be in all directions, they must tend to neutralize each other, and at first to form such unstable modifications that it is difficult, if not impossible, to see how such indefinite oscillations of infinitesimal beginnings can ever build up a sufficiently appreciable resemblance to a leaf, bamboo, or other object, for Natural Selection to seize upon and perpetuate.” 182 Miscellaneous Objections to the _ Cutar. VU, But in all the foregoing cases the insects in their original state no doubt presented some rude and accidental resemblance to an object commonly found in the stations frequented by them. Nor is this at all improbable, considering the almost infinite number of surrounding objects and the diversity in form and colour of the hosts of insects which exist. As some rude resemblance is neces- sary for the first start, we can understand how it is that the larger and higher animals do not (with the exception, as far as T know, of one fish) resemble for the sake of protection special objects, but only the surface which commonly surrounds them, and this chiefly in colour. Assuming that an insect originally happened to resemble in some degree a dead twig or a decayed leaf, and that it varied slightly in many ways, then all the variations which rendered the insect at all more like any such object, and thus favoured its escape, would be preserved, whilst other variations would be neglected and ultimately lost; or, if they rendered the insect at all less like the imitated object, they would be eliminated. There would indeed be force in Mr. Mivart’s objection, if we were to attempt to account for the above resemblances, independently of natural selection, through mere fluctuating variability ; but as the case stands there is none. Nor can I see any force in Mr. Mivart’s difficulty with respect to “the last touches of perfection in the mimicry ;” as in the case given by Mr. Wallace, of a walking-stick insect (Ceroxylus jaceratus), which resembles “a stick grown over by a creeping moss or jungermannia.” So close was this resemblance, that a native Dyak maintained that the foliaceous excrescences were really moss. Insects are preyed on by birds and other enemies, whose sight is probably sharper than ours, and every grade in resemblance which aided an insect to escape notice or detection, would tend towards its preservation; and the more perfect the resemblance so much the better for the insect. Considering the nature of the differ- ences between the species in the group which includes the above Ceroxylus, there is nothing improbable in this insect having varied in the irregularities on its surface, and in these having become more or less green-coloured ; for in every group the characters which differ in the several species are the most apt to vary, whilst the generic characters, or those common to all the species, are the most constant. , The Greenland whale is one of the most wonderful animals jn the world, and the baleen, or whale-bone, one of its greatest pecu- liarities. The baleen consists of a row, on each side, of the upper Crap. VII. Theory of Natural Selection. 183 jaw, of about 300 plates or lamina, which stand close together transversely to the longer axis of the mouth. Within the main row there are some subsidiary rows: The extremities and inner margins of all the plates are frayed into stiff bristles, which clothe the whole gigantic palate, and serve to strain or sift the water, and thus to secure the minute prey on which these great animals subsist. ‘The middle and longest lamina in the Greenland whale is ten, twelve, or even fifteen feet in length; but in the different species of Cetaceans there are gradations in length; the middle lamina being in one species, according to Scoresby, four feet, in another three, in another eighteen inches, and in the Balenoptera rostrata only about nine inches in length. The quality of the whale-bone also differs in the different species. With respect to the baleen, Mr. Mivart remarks that if it “ had once attained such a size and development as to be at all useful, then its preservation and augmentation within serviceable limits would be promoted by natural selection alone. But how to obtain the beginning of such useful development?” In answer, it may be asked, why should not the early progenitors of the whales with baleen have possessed a mouth constructed something like the lamellated beak of a duck? Ducks, like whales, subsist by sifting the mud and water; and the family has sometimes been called Criblatores, or sifters. I hope that I may not be misconstrued into saying that the progenitors of whales did actually possess mouths ‘amellated like the beak of a duck. I wish only to show that this 3s not incredible, and that the immense plates of baleen in the Greenland whale might have been developed from such lamellae by finely graduated steps, each of service to its possessor. The beak of a shoveller-duck (Spatula clypeata) is a more beau- tiful and complex structure than the mouth of a whale. ‘The upper mandible is furnished on each side (in the specimen examined by me) with a row or comb formed of 188 thin, elastic lamellae, obliquely bevelled so as to be pointed, and placed transversely to the longer axis of the mouth. They arise from the palate, and are attached by flexible membrane to the sides of the mandible. Those standing towards the middle are the longest, being about one-third of an inch in length, and they project *14 of an inch beneath the edge. At their bases there is a short subsidiary row of obliquely transverse lamella. In these several respects they resemble the plates of baleen in the mouth of a whale. But towards the extremity of the beak they differ much, as they project inwards, instead of straight downwards. The entire head of the shoveller, though incomparably less bulky, is about one-eighteenth of the 184 MMiscellaneous Oljections to the Cuap. VIL. — length of the head of a moderately large Balenoptera rostrata, in which species the baleen is only nine inches long; so that if we were to make the head of the shoveller as long as that of the Balenoptera, the lamella would be six inches in length,—that is, two-thirds of the length of the baleen in this species of whale. The lower mandible of the shoveller-duck is furnished with lamella of equal length with those above, but finer; and in being thus fur- nished it differs conspicuously from the lower jaw of a whale, which is destitute of baleen. On the other hand, the extremities of these lower lamella are frayed into fine bristly points, so that they thus curiously resemble the plates of baleen. In the genus Prior, a ‘member of the distinct family of the Petrels, the upper mandible alone is. furnished with lamelle, which are well developed and project beneath the margin; so that the beak of this bird resembles in this respect the mouth of a whale. From the highly developed structure of the shoveller’s beak we may proceed (as I have learnt from information and specimens sent to me by Mr. Salvin), without any great break, as far as fitness for sifting is concerned, through the beak of the Merganetta armata, anda in some respects through that of the Aix sponsa, to the beak of the common duck. In this latter species, the lamelle are much coarser than in the shoveller, and are firmly attached to the sides of the mandible ; they are only about 50 in number on each side, and do not project at all beneath the margin. They are Square-topped, and are edged with translucent hardish tissue, as if for crushing food. The edges of the lower mandible are crossed by numerous fine ridges, which project very little. Although the beak is thus very inferior as a sifter to that of the shoveller, yet this bird, as every one knows, constantly uses it for this purpose. There are other species, as [ hear from Mr. Salvin, in which the lamelle are considerably less developed than in the common duck ; but I do not know whether they use their beaks for sifting the water, Turning to another group of the same family. In the Egyptian goose (Chenalopex) the beak closely resembles that of the common duck; but the lamella are not so numerous, nor so distinct from each other, nor do they project so much inwards ; yet this goose, as I am informed by Mr. E. Bartlett, “uses jts bill like a duck by throwing the water out at the corners.” Its chief food, however, is grass, which it crops like the common goose. In this latter bird, the lamella of the upper mandible are much coarser than in the common duck, almost confluent, about 27 in number on each side, and terminating upwards in teeth-like knobs, The palate is also covered with hard rounded knobs. The edges of the lower a Cuap, VII. Theory of Natural Selection. 18s mandible are serrated with teeth much more prominent, coarser, and sharper than in the duck. The common goose does not sift the water, but uses its beak exclusively for tearing or cutting herbage, for which purpose it is so well fitted, that it can crop grass closer than almost any other animal. ‘here are other species of geese, as I hear from Mr. Bartlett, in which the lamelle are less developed than in the common goose. We thus see that a member of the duck family, with a beak constructed like that of the common goose and adapted solely for grazing, or even a member with a beak having less well-developed lamelle, might be converted by small changes into a species like the Egyptian goose,—this into one like the common duck,—and, lastly, into one like the shoveller, provided with a beak almost exclusively adapted for sifting the water; for this bird could hardly use any part of its beak, except the hooked tip, for seizing or tearing solid food. The beak of a goose, as I may add, might also be con- verted by small changes into one provided with prominent, recurved teeth, like those of the Merganser (a2 member of the same family), serving for the widely different purpose of securing live fish. Returning to the whales. ‘The Hyperoodon bidens is destitute of true teeth in an efficient condition, but its palate is roughened, according to Lacepede, with small, unequal, hard points of horn. There is, therefore, nothing improbable in supposing that some early Cetacean form was provided with similar points of horn on the palate, but rather more regularly placed, and which, like the knobs on the beak of the goose, aided it in seizing or tearing its food. If so, it will hardly be denied that the points might have been con- verted through variation and natural selection into lamellw as well- developed as those of the igyptian goose, in which case they would have been used both for seizing objects and for sifting the water ; then into lamellex like those of the domestic duck ; and so onwards, until they became as well constructed as those of the shoveller, in which case they would have served exclusively as a sifting appa- ratus. rom this stage, in which the lamella would be two-thirds of the length of the plates of baleen in the Balenoptera rostrata, gradations, which may be observed in still-existing Cetaceans, lead us onwards to the enormous plates of baleen in the Greenland whale. Nor is there the least reason to doubt that each step in this scale might have been as serviceable to certain ancient Ceta- ceans, with the functions of the parts slowly changing during the progress of development, as are the gradations in the beaks of the different existing members of the duck-family. We should bear in mind that each species of duck is subjected to a severe "186 Miscellaneous Objections to the — Cuar. VIL. = epee atc) = struggle for existence, and that the structure of every part of its frame must be well adapted to its conditions of life. The Pleuronectidx, or Flat-fish, are remarkable for their asym- metrical bodies. They rest on one side,—in the greater number of species on the left, but in some on the right side; and occasionally reversed adult specimens occur. The lower, or resting-surface, re- sembles at first sight the ventral surface of an ordinary fish: it is of a white colour, less developed in many ways than the upper side, with the lateral fins often of smaller size. But the eyes offer the most remarkable peculiarity ; for they are both placed on the upper side of the head. During early youth, however, they stand oppo- site to each other, and the whole body is then symmetrical, with both sides equally coloured. Soon the eye proper to the lower side begins to glide slowly round the head to the upper side; but does not pass right through the skull, as was formerly thought to be the case. It is obvious that unless the lower eye did thus travel round, it could not be used by the fish whilst lying in its habitual position on one side. ‘The lower eye would, also, have been liable to be abraded by the sandy bottom. ‘That the Pleuronectidz are admirably adapted by their flattened and asymmetrical structure ‘or their habits of life, is manifest from several species, such as soles, flounders, &c., being extremely common. The chief ad- vantages thus gained seem to be protection from their enemies, and facility for feeding on the ground. The different members, however, of the family present, as Schiddte remarks, “a long series of forms exhibiting a gradual transition from Hippoglossus pinguis, which does not in any considerable degree alter the shape in which it leaves the ovum, to the soles, which are entirely thrown to one side.” Mr. Mivart has taken up this case, and remarks that a sudden Spontaneous transformation in the position of the eyes is hardly eonceivable, in which I quite agree with him. He then adds: “if the transit was gradual, then how such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the individual is, indeed, far from clear. It seems, even, that such an incipient transformation must rather have been inju- rious.”. But he might have found an answer to this objection in the excellent observations published in 1867 by Malm. The Pleuronectide, whilst very young and still symmetrical, with their eyes standing on opposite sides of the head, cannot long retain a vertical position, owing to. the excessive depth of their bodies, the small size of their lateral fins, and to their being destitute: of a ¢ Cuar. VIL. Theory of Natural Selection. 187 swimbladder. Hence soon growing tired, they fall to the ‘bottom on one side. Whilst thus at rest they often twist, as Malm observed, the lower eye upwards, to see above them; and they © do this so vigorously that the eye is pressed hard against the upper part of the orbit. The forehead between the eyes conse- quently becomes, as could be plainly seen, temporarily contracted in breadth. On one occasion Malm saw a young fish raise and - depress the lower eye through an angular distance of about seventy degrees. . We should remember that the skull at this early age is cartila- ginous and flexible, so that it readily yields to muscular action. It is also known with the higher animals, even after early youth, that the skull yields and is altered in shape, if the skin or muscles be permanently contracted through disease or some accident. With long-eared rabbits, if one ear lops forwards and downwards, its weight drags forward all the bones of the skull on the same side, of which I have given a figure. Malm states that the newly-hatched young of perches, salmon, and several other symmetrical fishes, have the habit of occasionally resting on one side at the bottom; and he has observed that they often then strain their lower eyes so as to look upwards; and their skulls are thus rendered rather crooked. ‘These fishes, however, are soon able to hold themselves in a vertical position, and no permanent effect is thus produced. With the Pleuronectide, on the other hand, the older they grow the more habitually they rest on one side, owing to the increasing flatness of their bodies, and a permanent effect is thus produced on the form of the head, and on the position of the eyes. Judging from analogy, the tendency to distortion would no’ doubt be increased through the principle of inheritance. Schiodte believes, in opposition to some other naturalists, that the Pleuronectida are not quite symmetrical even in the embryo; and if this be so, we could understand how it is that certain species, whilst young, habitually fall over and rest on the left side, and other species on the right side. Malm adds, in confirmation of the above view, that the adult Trachypterus arcticus, which is not a member of the Pleuronectide, rests on its left side at the bottom, and swims diagonally through the water; and in this fish, the two sides of the head are said to be somewhat dissimilar. Our great authority on Fishes, Dr. Giinther, concludes his abstract of Malm’s paper, by remarking that “the author gives a very simple explanation of the abnormal condition of the Pleuronectoids.” We thus see that the first stages of the transit of the eye from one side of the head to the other, which Mr. Mivart considers would _—— 188 Miscellaneous Objections to the Cap. VIE. be injurious, may be attributed to the habit, no doubt bene- ficjal to the individual and to the species, of endeavouring to look upwards with both eyes, whilst resting on one side at the bottom. We may also attribute to the inherited effects of use the fact of the mouth in several kinds of flat-fish being bent towards the lower surface, with the jaw bones stronger and more effective on this, the eyeless side of the head, than on the other, for the sake, as Dr. Traquair supposes, of feeding with ease on the ground, Disuse, on the other hand, will account for the less deve- loped condition of the whole inferior half of the body, including the lateral fins; though Yarrell thinks that the reduced size of these fins is advantageous to the fish, as “‘ there is so much less room for their action, than with the larger fins above.” Perhaps the lesser number of teeth in the proportion of four to seven in the upper halves of the two jaws of the plaice, to twenty-five to thirty in the lower halves, may likewise be accounted for by disuse. From the colourless state of the ventral surface of most fishes and of many other animals, we may reasonably suppose that the absence of colour in flat-fish on the side,,whether it be the right or left, which is undermost, is due to the exclusion of light. But it can- not be supposed that the peculiar speckled appearance of the upper side of the sole, so like the sandy bed of the sea, or the power in some species, as recently shown by Pouchet, of changing their colour in accordance with the surrounding surface, or the presence of bony tubercles on the upper side of the turbot, are due to the action of the light. Here natural selection has probably come into play, as well as in adapting the general shape of the body of these fishes, and many other peculiarities, to their habits of life. We should keep in mind, as I have before insisted, that the inherited effects of the increased use of parts, and perhaps of their disuse, will be strengthened by natural selection. For all spontaneous varia- tions in the right direction will thus be preserved ; as will those individuals which inherit in the highest degree the effects of the increased and beneficial use of any part. How much to attribute in each particular case to the effects of use, and how much to natural selection, it seems impossible to decide. I may give another instance of a structure which apparently owes its origin exclusively to use or habit. The extremity of the tail in some American monkeys has been converted into a wonderfully perfect prehensile organ, and serves as a fifth hand. A reviewer who agrees with Mr. Mivart in every detail, remarks on this struc- ture : . It is impossible to believe that in any number of ages the first slight incipient tendency to grasp could preserve the lives of Crap, VII. Theory of Natural Selection. 189 the individuals possessing it, or favour their chance of having and of rearing offspring.” But there is no necessity for any such belief. Habit, and this almost implies that some benefit great or small is thus derived, would in all probability suffice for the work. Brehm saw the young of an African monkey (Cercopithecus) clinging to the under surface of their mother by their hands, and at the same time they hooked their little tails round that of their mother. Professor Henslow kept in confinement some harvest mice (Mus messorius) which do not possess a structurally prehensile tail; but he frequently observed that they curled their tails round the branches of a bush placed in the cage, and thus aided themselves in climbing. I have received an analogous account from Dr. Giinther, who hag seen a mouse thus suspend itself. If the harvest mouse had been more strictly arboreal, it would perhaps have had its tail rendered structurally prehensile, as is the case with some members of the same order. . Why Cercopithecus, considering its habits whilst young, has not become thus provided, it would be difficult to say. It is, however, possible that the long tail of this monkey may be of more service to it as a balancing organ in making its prodigious leaps, than as a prehensile organ. The mammary glands are common to the whole class of mam- mals, and are indispensable for their existence; they must, there- fore, have been developed at an extremely remote period, and we can know nothing positively about their manner of development. Mr. Mivart. asks: “Is it conceivable that the young of any animal was ever saved from destruction by accidentally sucking a drop of scarcely nutritious fluid from an accidentally hypertrophied cuta- neous gland of its mother? And even if one was so, what chance was there of the perpetuation of such a variation?” But the case is not here put fairly. It is admitted by most evolutionists that mammals are descended from a marsupial form; and if so, the mammary glands will have been at first developed within the mar- supial sack. In the case of the fish (Hippocampus) the eggs are hatched, and the young are reared for a time, within a sack of this nature; and an American naturalist, Mr. Lockwood, believes from what he has seen of the development of the young, that they are nourished by a seeretion from the cutaneous glands of the sack. Now with the early progenitors of mammals, almost before they deserved to be thus designated, is it not at least possible that the young might have been similarly nourished? And in this case, the individuals which secreted a fluid, in some degree or manner the most nutritious, so as to partake of the nature of. milk, would $ re ‘ 190 Miscellaneous Objections to the Car. VIL. in the long run have reared a larger number of well-nourished off- spring, than would the individuals which secreted a poorer fluid; and thus the cutaneous, glands, which are the homologues of the mammary glands, would have been improved or rendered more effective. 1¢ accords with the widely extended principle of speciali- sation, that the glands over a certain space of the sack should have become more highly developed than the remainder ; and they would then have formed a breast, but at first without a nipple, as we see in | the Ornithorhyncus, at the base of the mammalian series. Through what agency-the glands over a certain space became more highly specialised than the others, I will not pretend to decide, whether in part through compensation of growth, the effects of use, or of natural selection. The development of the mammary glands would have been of no service, and could not have been effected through natural-selection, unless the young at the same time -were able to partake of the secretion, ‘There is no greater difficulty in understanding how young mammals have instinctively learnt to suck the breast, than in understanding how unliatched chickens have learnt to break the — egg-shell by tapping against it with their specially adapted beaks ; or how a few hours after leaving the shell they have learnt to pick up grains of food. In such cases the most probable solution seems to be, that the habit was at first acquired by practice at a moré advanced age, and afterwards transmitted to the: offspring at an earlier age. But the young kangaroo is said not to suck, only to cling to the nipple of its mother, who has the power of injecting milk into the mouth of her helpless, half-formed offspring. On this head Mr, Mivart remarks: “ Did:no special provision exist, the young one must infallibly be choked by the intrusion of the milk into the windpipe. But there zs a special provision. The larynx is so elongated that it rises up into the posterior end of the nasal passage, and is thus enabled to give free entrance to the air for the lungs, while the milk passes harmlessly on each side of this elon- gated larynx, and so safely attains the gullet behind it.” Mr. Mivart' then asks how did natural selection remove in the adult kangaroo (and in most other mammals, on the assumption that they are descended from a marsupial form), “this at least perfectly innocent . and harmless structure?” It may be suggested in answer that the voice, which is certainly of high importance to many animals, could hardly have been used with full force as long ag the larynx entered’ the nasal passage ; and Professor Flower has suggested to me that this structure would have greatly interfered with an animal swallow- ing solid food, | : Cuap. VIL Theory of Natural Selection. IO} We will now turn for a short space to the lower divisions of the animal kingdom. The Echinodermata (star-fishes, sea-urchins, &c.) are furnished with remarkable organs, called pedicellariz, which consist, when well developed, of a tridactyle forceps—that is, of one: formed of three serrated arms, neatly fitting together and placed on the summit of a flexible stem, moved by muscles. These forceps can seize firmly hold of any object; and Alexander Agassiz has. seen an Echinus or sea-urchin rapidly passing particles of excrement from forceps to forceps down certain lines of its body, in order that its shell should not be fouled. But there is no doubt that besides. removing dirt of all kinds, they subserve other functions; and one of these apparently is defence. With respect to these organs, Mr. Mivart, as on so many pre- vious occasions, asks: “ What would be the utility of the jirst rudimentary beginnings of such structures, and how could such incipient buddings have ever preserved the life of a single Echinus ?” He adds, “ not even the swdden development of the snapping action could have been beneficial without the freely moveable stalk, nor could the latter have been efficient without the snapping jaws, yet no minute merely indefinite variations could simultaneously evolve these complex co-ordinations of structure ; to deny this seems to do no less than to affirm a startling paradox.’ Paradoxical as this. may appear to Mr. Mivart, tridactyle forcepses, immovably fixed. at the base, but capable of a snapping action, certainly exist on some star-fishes; and this is intelligible if they serve, at least in part, as a means of defence. Mr, Agassiz, to whose great kindness- I am indebted for much information on the subject, informs me that there are other star-fishes, in which one of the three arms of the forceps is reduced to a support for the other two; and again, other genera in which the third arm is completely lost. In Echino- neus, the shell is described by M. Perrier as bearing two kinds of pedicellariz, one resembling those of Echinus, and the other those of Spatangus; and such cases are always interesting as affording the means of apparently sudden transitions, through the abortion of one of the two states of an organ. With respect to the steps by which these curious organs have been evolved, Mr. Agassiz infers from his own researches and those of Miiller, that both in star-fishes and sea-urchins the pedicellariz: must undoubtedly be looked at as modified spines. This may be inferred from their manner of development in the individual, as well as from a long and perfect series of gradations in different species and genera, from simple granules to ordinary spines, to perfect tridactyle pedicellarix, The gradation extends even to 192 Miscellaneous Objections to the Cur. VII. —_ the manner in which ordinary spines and the pedicellarie with their supporting calcareous rods are articulated to the shell. In certain genera of star-fishes, “the very combinations needed to show that the pedicellarie are only modified branching spines ‘ may be found. Thus we have fixed spines, with three equi-distant, serrated, moveable branches, articulated to near their bases; and higher up, on the same spine, three other moveable branches, Now when the latter arise from the summit of a spine they form in fact a rude tridactyle pedicellaria, and such may be seen on the same spine together with the three lower branches. In this case she identity in nature between the arms of the pedicellariz and the moveable branches of a spine, is unmistakeable. It is generally admitted that the ordinary spines serve as a protection; and if so, there can be no reason to doubt that those furnished with serrated and moveable branches likewise serve for the same purpose; and they would thus serve still more effectively as soon as by meeting together they acted as a prehensile or snapping apparatus. Thus every gradation, from an ordinary fixed spine to a fixed pedicellaria, would be of service, In certain genera of star-fishes these organs, instead of being fixed or borne on an immovable support, are placed on the summit of a flexible and muscular, though short, stem ; and in this case they probably subserve some additional function besides defence. | In the sea-urchins the steps can be followed by which a fixed spine becomes articulated to the shell, and is thus rendered moveable. I wish I had space here to give a fuller abstract of Mr. Agassiz’s interesting observations on the development of the pedicellarie. All possible gradations, as he adds, may likewise be found between the pedicellaris of the star-fishes and the hooks of the Ophiurians, another group of the Echinodermata ; and again between the pedi- cellariz of sea-urchins and the anchors of the Holothuriz, also belonging to the same great class. Certain compound animals, or zoophytes as they have been termed, namely the Polyzoa, are provided with curious organs called avicularia. These differ much in structure in the different species. In their most perfect condition, they curiously resemble the head and beak of a vulture in miniature, seated on a neck and cap- able of movement, as is likewise the lower jaw or mandible. In one species observed by me all the avicularia on the same branch often moved simultaneously backwards and forwards, with the lower jaw widely open, through an angle of about 90°, in the course of five seconds; and their movement caused the whole polyzoary to ‘Cuap. VII. Theory of Natural Selection. 193 tremble. When the jaws are touched with a needle they seize it so firmly that the branch can thus be shaken. Mr. Mivart adduces this case, chiefly on account of the supposed difficulty of organs, namely the avicularia of the Polyzoa and the pedicellaria of the Echinodermata, which he considers as “ essen- tially similar,” having been developed through natural selection in widely distinct divisions of the animal kingdom. But, as far as structure is concerned, I can see no similarity between tridactyle pedicellariz and avicularia. The latter resemble somewhat more closely the chele or pincers of Crustaceans ; and Mr. Mivart might have adduced with equal appropriateness this resemblance as a special difficulty ; or even their resemblance to the head and beak of a bird. The avicularia are believed by Mr. Busk, Dr. Smitt, and Dr. Nitsche—naturalists who have carefully studied this group—to -be homologous with the zooids and their cells which compose the .zoophyte ; the moveable lip or lid of the cell corresponding with the lower and moveable mandible of the avicularium. Mr. Busk, how- ever, does not know of any gradations now existing between a zooid and an avicularium. It is therefore impossible to conjecture by what serviceable gradations the one could have been converted into the other: but it by no means follows from this that such grada- tions have not existed. As the chela of Crustaceans resemble in some degree the avicu- laria of Polyzoa, both serving as pincers, it may be worth while to show that with the former a long series of serviceable gradations still exists. In the first and simplest stage, the terminal segment of a limb shuts down either on the square summit of the broad penultimate segment, or against one whole side; and is thus enabled to catch hold of an object; but the limb still serves as an organ of locomotion. We next find one corner of the broad penultimate segment slightly prominent, sometimes furnished with irregular teeth; and against these the terminal segment shuts down. By an increase in the size of this projection, with its shape, as well as that of the terminal segment, slightly modified and improved, the pincers are rendered more and more perfect, until we have at last an instru- ment as efficient as the chele of a lobster; and all these gradations can be actually traced. Besides the avicularia, the Polyzoa possess curious organs called vibracula. These generally consist of long bristles, capable of movement and easily excited. In one species examined by me the vibracula were slightly curved and serrated along the outer margin; and all of them on the same polyzoary often moved simul- taneously ; so that, acting like long oars, they swept a branch rapidly O 194 Miscellaneous Objections to the Cuap. VII. across the object-glass of my microscope. When a branch wag placed on its face, the vibracula became entangled, and they made violent efforts to free themselves. They are supposed to serve as. a defence, and may be seen, as Mr. Busk remarks, “to sweep slowly and carefully over the surface of the polyzoary, removing what might be noxious to the delicate inhabitants of the cells when their tentacula are protruded.” The avicularia, like the vibracula, probably serve for defence, but they also catch and kill small living animals, which it is believed are afterwards swept by the currents within reach of the tentacula of the zooids. Some species are provided with avicularia and vibracula ; some with avicularia alone, and a few with vibracula alone. It is not easy to imagine two objects more widely different in appearance than a bristle or vibraculum, and an avicularium like the head of a bird ; yet they are almost certainly homologous and have been developed from the same common source, namely a zooid with its cell. Hence we can understand how it is that these organs graduate in some cases, as 1 am informed by Mr. Busk, into each other. Thus with the avicularia of several species of Lepralia, the moveable mandible is so much produced and is so like a bristle, that the presence of the upper or fixed beak alone serves to determine its avicularian nature. The vibracula may have been directly developed from the lips of the cells, without having passed through the avicularian stage; but it seems more probable that. they have passed through this stage, as during the early stages of the transformation, the other parts of the cell with the included zooid could hardly have disappeared at once. In many cases the vibracula have a grooved support at the base, which seems to repre- sent the fixed beak; though this support in some species is quite absent, This view of the development of the vibracula, if trust- worthy, is interesting ; for supposing that all the species provided with avicularia had become extinct, no one with the most vivid imagination would ever have thought that the vibracula had originally existed as part of an organ, resembling a bird’s head or an irregular box or hood. It is interesting to see two such widely different organs developed from a common origin; and as the moveable lip of the cell serves as a protection to the zooid, there is no difficulty in believing that all the gradations, by which the lip became con- verted first into the lower mandible of an avicularium and then into an elongated bristle, likewise served as a protection in different ways and under different circumstances. In the vegetable kingdom Mr. Mivart only alludes to two cases, bes Cuap. VIL. Theory of Natural Selection. 195 namely the structure of the flowers of orchids, and the movements of climbing plants. With respect to the former, he says, “ the explanation of ‘their origin is deemed thoroughly unsatisfactory— utterly insufficient to explain the incipient, infinitesimal beginnings of structures which are of utility only when they are considerably developed.” AsI have fully treated this subject in another work, I will here give only a few details on one alone of the most striking peculiarities of the flowers of orchids, namely their pollinia. A pollinium when highly developed consists of a mass of pollen-grains, affixed to an elastic foot-stalk or caudicle, and this to a little mass of extremely viscid matter. The pollinia are by this means trans- ported by insects from one flower to the stigma of another. In some orchids there is no caudicle to the pollen-masses, and the grains are merely tied together by fine threads; but as these are not confined to orchids, they need not here be considered; yet I may mention that at the base of the orchidaceous series, in Cypri- pedium, we can see how the threads were probably first developed. In other orchids the threads cohere at one end of the pollen-masses; and this forms the first or nascent trace of a caudicle. That this is the origin of the caudicle, even when of considerable length and highly. developed, we have good evidence in the aborted pollen- grains which can sometimes be detected embedded within the central and solid parts. With respect to the second chief peculiarity, namely the little mass of viscid matter attached to the end of the caudicle, a long series of gradations can be specified, each of plain service to the plant. In most flowers belonging to other orders the stigma se- cretes a little viscid matter. Now in certain orchids similar viscid matter is secreted, but in much larger quantities by one alone of the three stigmas ; and this stigma, perhaps in consequence of the copious secretion, is rendered sterile. When an insect visits a flower of this kind, it rubs off some of the viscid matter and thus at the same time drags away some of the pollen-grains. From this simple condition, which differs but little from that of a multitude of common flowers, there are endless gradations,—to species in which the pollen-mass terminates in a very short, free caudicle,—to others in which the caudicle becomes firmly attached to the viscid matter, with the sterile stigma itself much modified. In this latter case we have a pollinium in its most highly developed and perfect con- dition. He who will carefully examine the flowers of orchids for himself will not deny the existence of the above series of cradations —from a mass of pollen-grains merely tied together by threads, with the stigma differing but little from that of an ordinary flower, 0 2 * 196 Miscellaneous Objections to the = Cur. VILL to a highly complex pollinium, admirably adapted for transportal by insects; nor will he deny that all the gradations in the several species are admirably adapted in relation to the general structure of each flower for its fertilisation by different insects. In this, and in almost every other case, the enquiry may be pushed further backwards; and it may be asked how did the stigma of an ordinary flower become viscid, but as we do not know the full history of any one group of beings, it is as useless to ask, as it is hopeless to attempt answering, such questions. We will now turn to climbing plants. These can be arranged in a long series, from those which simply twine round a support, to those which I have called leaf-climbers, and to those provided with tendrils. In these two latter classes the stems have generally, but not always, lost the power of twining, though they retain the power of revolving, which the tendrils likewise possess. The gradations from leaf-climbers to tendril-bearers are wonderfully close, and certain plants may be indifferently placed in either class. But in ascending the series from simple twiners to leaf-climbers, an impor- tant quality is added, namely sensitiveness to a touch, by which means the foot-stalks of the leaves or flowers, or these modified and converted into tendrils, are excited to bend round and clasp the touching object. He who will read my memoir on these plants will, I think, admit that all the many gradations in function and structure between simple twiners and tendril-bearers are in each case beneficial in a high degree to the species. or instance, it is clearly a great advantage to a twining plant to become a leaf-. climber; and it is probable that every twiner which possessed leaves with long foot-stalks would have been developed into a leaf- climber, if the foot-stalks had possessed in any slight degree the requisite sensitiveness to a touch. As twining is the simplest means of ascending a support, and forms the basis of our series, it may naturally be asked how did plants acquire this power in an incipient degree, afterwards to be improved and increased through natural selection. The power of twining depends, firstly, on the stems whilst young being extremely flexible (but this is a character common to many plants which are not climbers); and, secondly, on their continually bending to all points of the compass, one after the other in succession, in the same order. By this movement the stems are inclined to all sides, and are made to move round and round. As soon as the lower part of of a stem strikes against any object and is stopped, the upper part still goes on bending and revolving, and thus necessarily twines round and up the support. The revolving movement ceases after Cuap. VIL Theory of Natural Selection. 197 the early growth of each shoot. AS in many widely separated families of plants, single species and single genera possess the power of revolving, and have thus become twiners, they must have independently acquired it, and cannot have inherited it from a common progenitor. Hence I was led to predict that some slight tendency to a movement of this kind would be found to be far from uncommon with plants which did not climb; and that this had afforded the basis for natural selection to work on and improve. When I made this prediction, I knew of only one imperfect case, namely of the young flower-peduncles of a Maurandia which revolved slightly and irregularly, like the stems of twining plants, but without making any use of this habit. Soon afterwards Fritz Miiller discovered that the young stems of an Alisma and of a Linum,—plants which do not climb and are widely separated in the natural system,—revolved plainly, though irregularly; and he states that he has reason to suspect that this occurs with some other plants. These slight movements appear to be of no service to the plants in question ; anyhow, they are not of the least use in the way of climbing, which is the point that concerns us. Nevertheless we can see that if the stems of these plants had been flexible, and if under the conditions to which they are exposed it had profited them to as- cend to aheight, then the habit of slightly and irregularly revolving might have been increased and utilised through natural selection, until they had become converted into well-developed twining species. With respect to the sensitiveness of the foot-stalks of the leaves and flowers, and of tendrils, nearly the same remarks are applicable as in the case of the revolving movements of twining plants. As a vast number of species, belonging to widely distinct groups, are endowed with this kind of sensitiveness, it ought to be found ina nascent condition in many plants which have not become climbers. This is the case: I observed that the young flower-peduncles of the above Maurandia curved themselves a little towards the side which was touched. Morren found in several species of Oxalis that the leaves and their foot-stalks moved, especially after exposure to a hot sun, when they were gently and repeatedly touched, or when the plant was shaken. I repeated these observations on some other species of Oxalis with the same result ; in some of them the movement was distinct, but was best seen in the young leaves; in others it was extremely slight. It is a more important fact that according to the high authority of Hofmeister, the young shoots and leaves of all plants move after being shaken; and with climbing plants it is, as we know, only during the early stages of growth that the foot-stalks and tendrils are sensitive. 198 Miscellaneous Objections to the —_ Cuap. VIL, —————_ It is scarcely possible that the above slight movements, due to a touch or shake, in the young and growing organs of plants, can be of any functional importance to them. But plants possess, in obedience to various stimuli, powers of movement, which are of manifest importance to them; for instance, towards and more rarely from the light,—in opposition to, and more rarely in the direction of, the attraction of gravity. When the nerves and muscles of an animal are excited by galvanism or by the absorption of strychnine, the consequent movements may be called an incidental result, for the nerves and muscles have not been rendered specially sensitive to these stimuli. So with plants it appears that, from having the power of movement in obedience to certain stimuli, they are excited in an incidental manner by a touch, or by being shaken. Hence there is no great difficulty in admitting that in the case of leaf- climbers and tendril-bearers, it is this tendency which has been taken advantage of and increased through natural selection. It is, however, probable, from reasons which I have assigned in my memoir, that this will have occurred only with plants which had already acquired the power of revolving, and had thus become twiners. I have already endeavoured to explain how plants became twiners, namely, by the increase of a terdency to slight and irregular revolving movements, which were at first of no use to them ; this _ Movement, as well as that due to a touch or shake, being the inci- dental result of the power of moving, gained for other and bene- ficial purposes. Whether, during the gradual development of climbing plants, natural selection has been aided by the inherited effects of use, I will not pretend to decide; but we know that certain periodical movements, for instance the so-called sleep of plants, are governed by habit. I have now considered enough, perhaps more than enough, of the cases, selected with care by a skilful naturalist, to prove that natural selection is incompetent to account for the incipient stages of useful structures; and I have shown, as I hope, that there is no great difficulty on this head.